POLITECNICO DI MILANO POLO TERRITORIALE DI PIACENZA School of Industrial and Information Engineering Master of Science in Energy Engineering for an Environmentally Sustainable World “Sewage sludge disposal routes: thermal treatments and energy recovery” Supervisor: Prof. ing. Stefano Consonni Cosupervisor: ing. Marco Gabba Master Graduation Thesis by: Priscilla Aradelli Student ID number: 817969 Giacomo Cantù Student ID number: 817978 A. Y. 2014/2015
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POLITECNICO DI MILANO
POLO TERRITORIALE DI PIACENZA
School of Industrial and Information Engineering
Master of Science in Energy Engineering for an Environmentally Sustainable
World
“Sewage sludge disposal routes: thermal treatments and energy recovery”
Supervisor: Prof. ing. Stefano Consonni Cosupervisor: ing. Marco Gabba
Master Graduation Thesis by: Priscilla Aradelli
Student ID number: 817969 Giacomo Cantù
Student ID number: 817978
A. Y. 2014/2015
1
Table of contents
Table of contents ...................................................................................................................................... 1
List of Tables ............................................................................................................................................. 4
List of Figures ........................................................................................................................................... 5
Abstract .................................................................................................................................................... 1
Keywords .............................................................................................................................................. 1
Sommario ................................................................................................................................................. 2
Parole chiave ........................................................................................................................................ 2
Motivation, goals and new findings ......................................................................................................... 3
1 Introduction ..................................................................................................................................... 5
1.1 Problem definition ................................................................................................................... 5
1.2 Sludge production data ............................................................................................................ 5
1.3 Directives .................................................................................................................................. 9
1.3.1 Landfill .............................................................................................................................. 9
1.3.2 Use in agriculture ........................................................................................................... 10
1.3.3 Incineration .................................................................................................................... 12
1.4 Sludge as a valuable waste ..................................................................................................... 14
2 Sludge sources, treatments and characterization.......................................................................... 16
2.1 Sources ................................................................................................................................... 16
2.1.1 Pre-treatment ................................................................................................................. 17
2.1.2 Primary sludge ................................................................................................................ 17
2.1.3 Secondary sludge ........................................................................................................... 18
2.1.4 Mixed sludge .................................................................................................................. 19
2.1.5 Tertiary sludge ................................................................................................................ 19
2.1.6 Digested and stabilized sludge ....................................................................................... 20
2.1.7 Raw Sludge ..................................................................................................................... 20
2.1.8 Industrial sludge ............................................................................................................. 20
2.1.9 Different sewage sludge types comparison ................................................................... 21
2.2 Treatments ............................................................................................................................. 22
2.2.1 Stabilization .................................................................................................................... 22
2
2.2.2 Thickening ...................................................................................................................... 24
2.2.3 Dewatering ..................................................................................................................... 26
2.2.4 Conditioning ................................................................................................................... 30
2.2.5 Drying ............................................................................................................................. 31
2.3 Characterization ..................................................................................................................... 37
2.3.1 Proximate analysis .......................................................................................................... 37
2.3.2 Ultimate analysis ............................................................................................................ 39
2.3.3 Lower Heating Value determination .............................................................................. 41
3 Sludge Recovery and Disposal Routes ............................................................................................ 43
3.1 Waste hierarchy ..................................................................................................................... 43
3.1.1 Waste hierarchy definition ............................................................................................. 43
3.1.2 Waste hierarchy and sludge disposal routes ................................................................. 44
3.2 Material Recovery .................................................................................................................. 46
3.2.1 Nutrients in sewage sludge ............................................................................................ 46
3.2.2 Landspreading or Agricultural use ................................................................................. 46
3.2.3 Phosphorus recovery ...................................................................................................... 48
3.2.4 Material recovery from Ash ........................................................................................... 50
3.3 Energy recovery ...................................................................................................................... 51
3.3.1 Biogas production .......................................................................................................... 52
3.3.2 Mono-incineration ......................................................................................................... 52
3.3.3 Co-incineration ............................................................................................................... 58
3.3.4 Pyrolysis .......................................................................................................................... 62
3.3.5 Gasification ..................................................................................................................... 70
3.3.6 Wet oxidation ................................................................................................................. 75
3.4 Current situation and future trends of disposal routes in EU ................................................ 75
4 Sludge thermal treatments SWOT analysis .................................................................................... 85
4.1 Mono-incineration ................................................................................................................. 85
4.2 Co-incineration ....................................................................................................................... 86
4.3 Pyrolysis .................................................................................................................................. 87
4.4 Gasification ............................................................................................................................. 89
4.5 Summary and comparison ..................................................................................................... 90
5 Preliminary calculations on biogas production .............................................................................. 95
3
6 Sludge Incineration Models ............................................................................................................ 97
6.1 Mono-Incineration ................................................................................................................. 97
6.1.1 Necessary conditions for self-sufficient combustion ..................................................... 97
6.1.2 Determination of sludge dry matter content fed in the dryer for an auto-thermal process
102
6.1.3 Energy recovery possibilities ........................................................................................ 109
6.1.4 ASPEN model of energy recovery ................................................................................. 112
6.2 Co-incineration in WtE ......................................................................................................... 118
6.2.1 Model and analysis ....................................................................................................... 118
7 Sludge Pyrolysis and Gasification Models .................................................................................... 121
7.1 ASPEN ................................................................................................................................... 121
7.2 Pyrolysis step model ............................................................................................................. 121
7.3 Digested sludge model ......................................................................................................... 123
7.3.1 Pyrobustor® model ....................................................................................................... 123
7.3.2 IDA Tobl plant model .................................................................................................... 125
7.4 Raw primary sludge Model .................................................................................................. 127
7.4.1 Pyrobustor® model ....................................................................................................... 128
7.4.2 IDA Tobl plant model .................................................................................................... 128
7.5 Summary of data and results ............................................................................................... 130
8 Primary energy consumption of different scenarios ................................................................... 136
9 Conclusions .................................................................................................................................. 141
APPENDIX 1 .......................................................................................................................................... 141
Pyrobustor: IDA TOBL plant by ARA Pustertal, San Lorenzo di Sebato (BZ) .................................... 143
Introduction ................................................................................................................................. 143
Process description ...................................................................................................................... 143
APPENDIX 2 .......................................................................................................................................... 153
Pyrobio: Synecom plant, in Pedrengo (BG) ...................................................................................... 153
Introduction ................................................................................................................................. 153
Process description ...................................................................................................................... 154
Implementation ............................................................................................................................ 156
References ............................................................................................................................................ 162
4
List of Tables
Table 1: Estimated sewage sludge production in selected countries all around the world. ................... 5
Table 2: Amounts of sewage sludge and specific sewage sludge production for population equivalent
(p.e.) in EU-27. Source Eurostat [4]. ........................................................................................................ 7
Table 3: Limits for metals in sludge. ....................................................................................................... 11
Table 4: Limits for agronomic and microbiological parameters. ........................................................... 11
Table 5: Limits for soil analysis. .............................................................................................................. 12
Table 6: Daily and 30 minutes average emission limit values. ............................................................... 12
Table 7: Average emission limit values obtained with 1-hour sampling period. ................................... 13
Table 8: Average emission limit values obtained with 8 hours sampling period. .................................. 13
Table 9: emission limit values in the wastewater from flue gases cleaning. ......................................... 14
Table 10: Composition of different kind of sludge [15]. ........................................................................ 21
Table 11: Thickening technology comparison [18]. ............................................................................... 26
Table 12: Dewatering technologies comparison [18]. ........................................................................... 29
Table 13: Comparison of different dewatering processes. .................................................................... 30
Table 14: Summary of advantages and disadvantages of indirect dryer types. .................................... 35
Table 15: Heating media and drying apparatuses [10]. ......................................................................... 36
Table 16: Sludge proximate compostions found in literature. .............................................................. 39
Table 17: Sludge Ultimate compositions found in literature. ................................................................ 40
Table 18: Sludge ultimate composition from IREN data. ....................................................................... 41
Table 19: LHV of IREN Sludge calculated with the described procedure. .............................................. 41
Table 20: Comparison of calculated HHV with literature value. ............................................................ 42
Table 21: Range for reference LHV values for sewage sludge [52]. ....................................................... 42
Table 22: Disposal routes and material and energy recovery possibilities. ........................................... 45
Table 23: Fraction of sewage sludge’s disposal routes in EU member states. ...................................... 77
Table 24: Results of calculation of Biogas energy for anaerobic digestion of raw primary sludge. ...... 96
Table 25: Results of calculation of Biogas energy for anaerobic digestion of raw mixed sludge. ......... 96
Table 26: Considered sludge types compostitions and LHV. ................................................................. 98
Table 27: Dry matter content for 900 °C flame temperature. ............................................................... 98
Table 28: mono-incineration results (combustion air temperature 650 °C). ....................................... 104
Table 29: Dewatering limits for different technologies. ...................................................................... 104
Table 30: Comparison of Zurich plant Outotec data and calculation results. ...................................... 110
Table 31: Mono-incineration energy recovery results summary. ........................................................ 111
Table 32: Aspen mono-incineration model results summary for digested and raw primary sludge... 116
Table 33: Power fluxes and efficiencies. .............................................................................................. 117
Table 34: Co-incineration of digested and raw mixed sludge effect on WtE outputs. ........................ 120
Table 35: Pyrolysis syngas yield literature data. .................................................................................. 122
Table 36: Experimental data for syngas composition. ......................................................................... 122
Table 37: Hypothesis assumed to perform the pyrolysis model. ......................................................... 125
Table 38: Summary of design specifications used in the ARA Pustertal Model for digested sludge. .. 127
Table 39: Summary of design specification used in IDA Tobl plant model for Raw primary sludge. .. 128
5
Table 40: Summary of data, input and results of pyrolysis-based process model. Part 1. .................. 131
Table 41: Summary of data, input and results of pyrolysis-based process model. Part 3. .................. 132
Table 43: Legend for Figure 61 and Figure 62. ..................................................................................... 133
Table 42: Summary of primary energy consumption calculation. ....................................................... 139
List of Figures
Figure 1: Sewage sludge production data for EU member state (year 2010). Source Eurostat [4] ......... 6
Figure 2: Sludge Production trend in years in some EU member states. Source Eurostat [4]. ................ 8
Figure 3: Macro-steps of sludge lifecycle. .............................................................................................. 16
Figure 4: Sludge occurrence relative to treatment phase [10]. ............................................................. 17
Figure 5: Primary sludge in pretreatments flowsheet. .......................................................................... 18
Figure 6 : Typical wastewater treatment process [13]. ......................................................................... 19
Figure 7: Digestion reactions scheme. .................................................................................................. 22
Figure 8: Dewatering centrifuge scheme. .............................................................................................. 27
Figure 9: Belt filter press dewatering in treatments chain. ................................................................... 28
Figure 10: Solar drying of sludge [23]. ................................................................................................... 33
Figure 11: Conveyor belt dryer configuration. ....................................................................................... 34
Figure 12: Example of a Disk Dryer (source: Hosokawa Micron [27]). ................................................... 35
Figure 13: Scheme containing Rotary disc for sludge drying [28] .......................................................... 36
Figure 14: Example of proximate analysis determined by means of TGA [30]. ..................................... 38
Figure 15: Waste hierarcy definitions. ................................................................................................... 43
Figure 16: Connection between Waste hierarchy and sludge disposal routes. ..................................... 44
Figure 17: Sludge landspreading. ........................................................................................................... 47
Figure 18: Global distribution of explored raw phosphate reserves as of 2013 [56]. ........................... 49
Figure 19: The Puerto Rico fluid bed incineration plant. ....................................................................... 54
Figure 20: A typical cross-section of a fluid bed..................................................................................... 55
Figure 21: Hot blast stove or Cowper stove, on the left, and flue-gas-through-tube (FGTT). ............... 56
Figure 22: Outotec Sewage Sludge Incineration Plant 100. ................................................................... 57
Figure 23: Pyrolysis in a biomass particle [79] ....................................................................................... 62
Figure 24: Pyrolysis plant scheme [79] ................................................................................................... 63
Figure 25: Temperature effect of products yields for fast (A) and slow (B) pyrolysis [34]. ................... 66
Figure 26: The effect of moisture content on the yields of pyrolysis products [32].. ............................ 69
Figure 27: Rotary kiln reactor [92]. ........................................................................................................ 69
Figure 28: C-H-O diagram of the gasification process [29]. ................................................................... 71
Figure 29: Disposal routes in new and old EU member states. ............................................................. 78
Figure 30: Sewage Sludge Disposal Routes in EU member States. ........................................................ 79
Figure 31: Change in disposal routes expected for year 2020 with respect to current situation. Reference
for current situation: Table 23; Reference for year 2020: [8]. ............................................................... 81
Figure 32: Predicted disposal routes share in EU-15, EU-12 and EU-27 for 2020. ................................ 82
6
Figure 33: Mono-incineration SWOT analysis. ....................................................................................... 90
Figure 34: Co-incineration SWOT analysis. ............................................................................................ 91
Figure 35: Pyrolysis SWOT analysis. ....................................................................................................... 92
Figure 36: Gasification SWOT analysis. .................................................................................................. 93
Figure 37: Scheme of WWTP and sludge types produced. .................................................................... 97
Figure 38: Raw primary sludge flame temperature with dry matter at different preheating. .............. 99
Figure 39: Raw mixed sludge flame temperature with dry matter at different preheating. ................. 99
Figure 40: Digested sludge flame temperature with dry matter at different preheating. .................. 100
Figure 41: Comparison of different minimum dry matter content with different air preheating. ...... 100
Figure 42: Dry matter and preheating temperature chart for 900 °C flame tempertaure .................. 101
Figure 43: Dry matter and preheating temperature chart for 900 °C flame tempertaure .................. 101
Figure 44: Dry matter and preheating temperature chart for 900 °C flame tempertaure .................. 102
Figure 45: Sludge Mono-Incineration self-sufficient combustion scheme. ......................................... 103
Figure 46: Raw primary sludge results for auto-thermal incineration. ................................................ 106
Figure 47: Raw mixed sludge results for auto-thermal incineration. .................................................. 107
Figure 48: Digested sludge results for auto-thermal incineration. ...................................................... 108
Figure 49: Raw mixed sludge results for mono-incineration energy recovery plant. .......................... 109
Figure 50: Raw primary sludge results for mono-incineration energy recovery plant. ....................... 109
Figure 51: Digested sludge results for mono-incineration energy recovery plant. ............................. 111
Figure 52: Aspen mono-incineration model flowsheet. ...................................................................... 115
Figure 53: Pyrobustor scheme and data [125]. .................................................................................... 123
Figure 54: IDA Tobl plant configuration [125]. ..................................................................................... 125
Figure 55: Aspen Flowsheet of IDA Tobl plant model for digested sludge. ........................................ 126
Figure 56: Aspen Flowsheet of IDA Tobl plant model for raw primary sludge. .................................. 129
Figure 57: Energy Balance in the Pyrolysis model fed by digested sludge. ......................................... 130
Figure 58: Energy Balance in the Pyrolysis model fed by raw sludge. ................................................. 130
Figure 59: Schematic overview of the IDA Tobl Aspen model with results for Digested Sludge ......... 134
Figure 60: Schematic overview of the IDA Tobl Aspen model with results for Raw Primary Sludge .. 135
Figure 61: CASE AD+TCP INC plant configuration. ............................................................................... 136
Figure 62: CASE TCP ONLY INC plant configuration. ............................................................................ 136
Figure 63: CASE AD+TCP PYRO plant configuration. ............................................................................ 137
Figure 64: CASE TCP ONLY PYRO plant configuration. ......................................................................... 137
Figure 65: View of the IDA Tobl plant within its landscape ................................................................. 143
Figure 66: Drawing of Digestion facilities at IDA TOBL, San Lorenzo di Sebato. .................................. 145
Figure 67: P&I of Gas Engines present at Ida Tobl Plant. ..................................................................... 146
Figure 68: Picture of the Belt Dryer in operation at Ida Tobl Plant...................................................... 147
Figure 69: Picture of the Bio-Filter in operation at Ida Tobl Plant. ...................................................... 148
Figure 70: 3D Draw of the Pyrobustor technology present at Ida Tobl. .............................................. 148
Figure 71: Inside view of the pytolysis chamber of the Pyrobustor .................................................... 149
Figure 72: Inside view of Pyrobustor and Piping. ................................................................................. 149
Figure 73: P&I screenshot of Pyrobustor during the operation at Ida Tobl Plant. .............................. 150
Figure 74: Heat exchanger oil-flue gases to recover heat released by the combustion ...................... 151
7
Figure 75: Overview of the sludge thermal disposal scheme for Ida Tobl plant.................................. 152
Figure 76: Input Biomass composed by Industrial Sludge, wood chips, paper .................................... 154
Figure 77: Pyro-gasification reactor ..................................................................................................... 155
Figure 78: Flowsheet of Synecom Pyrobio Plant .................................................................................. 157
8
1
Abstract
Sewage sludge management has been getting continuously increasing attention in EU; directives and
good environmental practices prescribe to switch from the actual disposal routes. After having assessed
the extent of the issue and performed an overview of the current situation, the present thesis aims at
comparing the energetic performance of established and innovative thermal conversion routes, the
disposal macro-category identified as the main increasing one in the next future. A SWOT analysis of
the candidate energy recovery processes is included, as preliminary study before modeling. Then,
several investigations, comprising also Aspen models, regarding sludge energy behavior in the different
routes have been developed. The sludge characteristics, namely composition and lower heating value,
have been taken in consideration all along the work. The concept of Waste Hierarchy has been
representing a constant in the technologies evaluation. Co-incineration of sludge in a waste-to-energy
plant is a viable option, although the R1 index is decreased, and the actually available capacity has to
be considered for the sludge amount to dispose of. Dewatered sludge (25% dry solid) incineration can
be auto-thermal, if preheated air temperature is adjusted according to the type of sludge, ranging from
ambient temperature to 650 °C. Sludge feeding in mono-incineration plant can lead to a specific net
electric power production of 0.49 kWh/kg of dry raw sludge and 0.26 kWh/kg of dry digested sludge,
generated by means of a heat-recovery steam cycle. The incineration plants inadequacy to small-scale
application, due to both economic and environmental reasons, could promote innovative technologies
development for sludge management. Pyrolysis-based model simulation results show that, although
energy recovery is still quite far from being achieved, the process has the capability of disposing of raw
primary sludge without supplementary fuel consumption, while for digested sludge, anyway, the
consumption is contained. This result, together with the possibility of wide improvements through
process optimization and better knowledge gaining, makes pyrolysis a promising low energy and
environmentally sustainable thermal route for sludge disposal.
Keywords Sewage Sludge – Energy Recovery – Disposal – Pyrolysis – Incineration – Waste Hierarchy
2
Sommario
In Europa, la gestione dei fanghi di depurazione attira attenzione sempre crescente; direttive e buona
pratiche ambientali prescrivono di trovare soluzioni diverse dalle modalità di smaltimento attualmente
perseguite. Dopo aver valutato l'entità del problema ed eseguito una panoramica della situazione
attuale, la presente tesi si propone di confrontare le performance energetiche di percorsi di
conversione termica, sia consolidati che innovativi, essendo questa stata identificata come la principale
macro-categoria di smaltimento/recupero in aumento nel prossimo futuro.
Un’analisi SWOT dei processi candidati a recupero di energia è inclusa, come studio preliminare alla
modellazione. Segue lo sviluppo di alcuni studi, che comprendono anche modelli in Aspen PLUS,
riguardanti il comportamento energetico dei fanghi nei diversi percorsi. Le caratteristiche dei fanghi,
vale a dire composizione e potere calorifico inferiore, sono state prese in considerazione lungo tutto il
lavoro. La nozione di gerarchia dei rifiuti ha inoltre rappresentato una costante nella valutazione delle
tecnologie. Il co-incenerimento dei fanghi in un termovalorizzatore di rifiuti è una valida opzione,
sebbene l’indice R1 venga ridotto, e la capacità effettivamente disponibile debba essere considerata
per comprendere la quantità di fanghi che può essere smaltita. L’incenerimento di fanghi disidratati (al
25% di sostanza secca) può essere auto-termico, se la temperatura di preriscaldamento dell’aria è
regolata in base al tipo di fango, da temperatura ambiente fino a 650 °C. L’utilizzo di fanghi in un
impianto di mono-incenerimento può portare a una produzione di potenza elettrica specifica lorda di
0.49 kWh/kg di fanghi secchi, se grezzi, e di 0.26 kWh/kg di fanghi secchi, se digeriti, generata mediante
un ciclo a vapore a recupero. L’inadeguatezza degli impianti di incenerimento in applicazioni di piccola
scala, dovuta a ragioni sia economiche che ambientali, potrebbe favorire lo sviluppo di tecnologie più
innovative per la gestione dei fanghi. I risultati della simulazione del modello di pirolisi mostrano che,
sebbene il processo sia ancora abbastanza lontano dal produrre un output di energia netto, esso
prensenta la capacità di smaltire fanghi grezzi senza l’utilizzo di combustibile supplementare, mentre
per i fanghi digeriti tale consumo è comunque contenuto. Questo risultato, insieme alla possibilità di
ampi miglioramenti ottenibili attraverso ottimizzazione e una più profonda conoscenza del processo,
rende la pirolisi un’opzione di smaltimento promettente, in qualità di trattamento termico a bassa
energia, e sostenibile in termini ambientali.
Parole chiave Fanghi di depurazine – Recupero di Energia – Smaltimento – Pirolisi – Incenerimento– Gerarchia dei
rifiuti
3
Motivation, goals and new findings
The present thesis has been elaborated in the settings of a LEAP activity commissioned by IREN
ambiente in 2015.
Laboratorio Energia & Ambiente Piacenza is a specialized laboratory in the field of highly efficient, low
environmental impact energy technology, supported by Politecnico di Milano [1].
IREN ambiente is the branch of IREN group in charge of waste collection, the design and management
of waste treatment and disposal plants and in the renewable energies sector [2].
The consulting activity, whose reference scientific director is prof. Roberto Canziani, consisted in the
technological evaluation of sewage sludge treatments and disposal.
Therefore, we feel compelled to thank LEAP and IREN ambiente, without which it would not have been
possible to develop this work.
The need to investigate the topic rises from the concern on sewage sludge sanitary problems, by now
not solved by the disposal options currently adopted: restriction on both landfilling and landspreading
practices imposed by directives, for reasons of environment quality preservation, together with a bad
public perception, of the latter in particular because of the involvement in food production, make them
unsufficient to dispose of the overall amount of sludge. The study of thermal treatments has become
mandatory, primarily to meet the sludge disposal requirement, with the obtainable 90% volume
reduction or zero-waste status, but also in the perspective of a further energy recovery possibility.
This thesis goal is to get an insight in the main thermal conversion process of sewage sludge, identifying
technological limits and opportunities, and with particular care on primary energy consumption while
comparing the different options. Strictly related to this, the additional purpose of this work is to get a
deeper, although still preliminary, knowledge of the slow pyrolysis process, to whom most of efforts
has been dedicated, and that consists in the main contribute of this work to scientific research.
Additionally, this work tries to assess whether digestion and biogas production represent absolute
benefits or, instead, detriments to the subsequent thermal treatment and disposal, a urgent question,
although not yet investigated, especially for plant operators.
The economic analysis has not been performed, despite it could be extremely useful, and is strongly
suggested as future work.
To conclude, this thesis has developed a comprehensive overview of sewage sludge issues and
treatments, a more technological evaluation – beyond the purely managerial perspective of previous
works – of sludge thermal treatments, and attempted to get a classification of the different routes
according to the waste hierarchy levels.
4
5
1 Introduction
1.1 Problem definition
Sewage sludge management is becoming an issue of growing importance, mainly because of the
mandatory growing interest on environmental health and quality preservation. In the European Union,
more and more strict directives are being introduced, so that sludge management methods involving
storage, first, and land spreading of as it is sludge then, are progressively being replaced by more pro-
ecological routes, involving valuable raw material or energy recovery. Sludge management deals with
not only environmental, but also technological issues; other constraints are, as always, energy use and
costs minimization; therefore, it is of primary importance to find the optimal mix of disposal or recovery
methods that allows solving the problem.
This chapter presents an analysis of the production of sludge in Europe and a report of the main
European and Italian directives to give an insight of sludge management issue and its extent.
1.2 Sludge production data
The world’s population is increasing and concentrating in urban centers. This trend is particularly
intense in developing countries, where an additional 2.1 billion people are expected to be living in cities
by 2030 [3]. These cities produce billions of tons of waste every year, including sludge and wastewater.
The fate of these wastes is very different depending on the local context: they can be collected or not,
treated or not, used directly, indirectly or end without beneficial use. In literature, data on these waste
streams is scarce and scattered; however, data for sewage sludge production of some countries,
selected for the data availability, are reported in Table 1.
Country Sewage sludge [thousands of ton DM/year] Year Source
EU-27 9906 2005 [4]
USA 6514 2004 [5]
China 2966 2006 [5]
Japan 2000 2006 [5]
Korea Rep 1900 - [6]
Iran 650 2008 [5]
Turkey 580 2004 [5]
Canada 550 2008 [5]
Brazil 372 2005 [5]
Australia and New Zeland 360 2008 [5]
Jordan 300 2008 [5]
Table 1: Estimated sewage sludge production in selected countries all around the world.
6
Comprehensive reviews and assessments at global level are missing, with only few exceptions as the
case of European countries for which EUROSTAT [4] is used as main reference for data on sewerage
and sludge.
Figure 1 shows the most recent but also quite complete data published by EUROSTAT regarding year
2010. As for year 2010, in many case EUROSTAT data present some lack for some years and/or
countries.
Figure 1: Sewage sludge production data for EU member state (year 2010). Source Eurostat [4]
In fact, data mainly form year 2005 and nearly years are collected in Table 2, since it is the period with
the highest concentration of data. This table is used as reference also by Kelessidis et al. [7], to try to
assess the recent situation in sludge production trend together with qualitative considerations and
reasonable expectations. According to Kelessidis et al. [7], during the last decades, the implementation
of Urban Waste Water Treatment (UWWT) Directive 91/271/EC forced EU-15 countries (old Member
States) to improve their wastewater collecting and treatment systems. As a result, an almost 50%
increase of annual sewage sludge production in EU-15 was noticed, from 6.5 million tons dry solids (DS)
in 1992 [4] to 8.7 million tons DS in 2005 [4]. On the other hand, the annual sewage sludge production
in EU-12 (new Member States) was estimated to be 1.1 million tons DS in 2005 [4], resulting to a total
amount of 9.9 million tons DS for EU-27 (all Member States) in 2005.
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Country Year Sewage Sludge Production
[thousands of ton DM/year]
Specific sewage sludge
production [kg/p.e./year]
Germany 2005 2170 26.3
UK 2005 1771 29.5
Spain 2005 1121 26
France 2004 1059 17
Italy 2005 1053 18.1
Netherlands 2005 348 22
Austria 2006 254 30.8
Sweden 2005 210 23.3
Portugal 2007 189 18
Finland 2005 148 28.2
Denmark 2007 140 26
Greece 2005 115 10.5
Belgium 2004 103 10.8
Ireland 2005 60 14.6
Luxembourg 2003 14 27.8
EU-15 8755 21.9
Poland 2005 486 12.7
Hungary 2004 184 18.2
Czech Republic 2005 172 16.8
Romania 2005 68 3.1
Lithuania 2005 66 19.1
Slovakia 2005 56 10.5
Bulgaria 2005 42 5.4
Estonia 2005 29 22.1
Latvia 2005 27 12.5
Slovenia 2005 14 6.8
Cyprus 2005 7 11.1
Malta 2005 0.1 0.1
EU-12 1151.1 11.5
EU-27 9906.1 16.7
Table 2: Amounts of sewage sludge and specific sewage sludge production for population
equivalent (p.e.) in EU-27. Source Eurostat [4].
As shown in Table 2, Germany is the first sludge producer, followed by the United Kingdom, France,
Italy and Spain, which generate altogether nearly 75% of the European sewage sludge. All other
countries produce less than 350 000 ton of DS each. This situation roughly reflects the demography of
8
each country. To eliminate the demography effect, the sludge production in the European Union per
population equivalent and per year are also reported in Table 2. According to these data, among the
EU-15 states, Greece produces the lowest amount of sludge per inhabitant (around 10 kg/p.e./year),
whereas Denmark is the most important producer with 30 kg/p.e./year.
Figure 2: Sludge Production trend in years in some EU member states. Source Eurostat [4].
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1500
2000
2500
3000
2004 2005 2006 2007 2008 2009 2010 2011 2012tho
usa
nd
s o
f to
ns
of
Dry
Mat
ter
Sludge production trend in Spain
9
In Kelessidis et al. [7] study, it is observed that the implementation of UWWT Directive by EU-12
countries is going to cause a significant increase of annual sewage sludge production in EU during the
following years, exceeding 13 million tons DS up to 2020. However, data form EUROSTAT (Figure 2)
evidence that sludge production may not increase for sure in any case. Figure 2 shows that trends in
sludge production in time can differ a lot country by country: in the period between 2004 and 2013, in
Spain sludge production shown a big increase, while Netherlands trend seems constant, and Germany
decreased its production.
Looking to the near future, it is possible to refer at the European Commission (EC) [8] study performed
in 2008. A baseline scenario for the period to 2020 is developed: this scenario assumes that no change
is made to the Sewage Sludge Directive, and it extrapolates from the current situation and current
developments at EU level and in the Member States its forecasts of future sludge production.
In terms of overall sludge production, the following trends were identified for the EU27 [8]:
The population of the EU will grow slowly, from about 499 million in 2010 to just under 514
million in 2020 (according to Eurostat projections)
While industrial production will grow, process improvements, pollution prevention and
improved on-site treatment will reduce sludge coming from industry
The level of sewage connection and wastewater treatment will continue to increase across the
EU27, meaning more sewage sludge being produced which will need proper management.
According to this trends and to all data collected and presented in this chapter, it seems that two
different situation are expected for EU-15 and EU-12 in the next years. For EU-15, which register a high
percentage of population (80%) connected to WWTP [9], sludge production will be more or less
constant or even slightly decreasing, while for EU-12, an increase in sludge production is expected,
since they are forced to increase sewerage collection and treatment. A specific sewage sludge
production of 11.5 [kg/p.e./year] for EU-12 against 21.9 for EU-15 remarks the actual gap between new
and old member states, that is expected to be reduced in the future.
1.3 Directives
1.3.1 Landfill
The European normative of reference for sewage sludge landfill is the Council Directive 1999/31/EC of
26 April 1999, on landfill of waste in general. Its stated objective is: “[…] by way of stringent operational
and technical requirements on the waste and landfills, to provide for measures, procedures and
guidance to prevent or reduce as far as possible negative effects on the environment. In particular the
pollution of surface water, groundwater, soil and air, and on the global environment, including the
greenhouse effect, as well as any resulting risk to human health, from landfilling of waste, during the
whole life-cycle of the landfill”.
10
The Directive imposes the Member States to develop a national strategy for the implementation of the
reduction of biodegradable waste going to landfill, and fixes the correspondent target percentage of
amount reduction for the following years.
The corresponding Italian normative that implements the European Directive is the Minister Decree
27/09/2010.
1.3.2 Use in agriculture
The EU sewage sludge directive.
Directive 86/278/EEC, 12 June 1986, on the protection of the environment, in particular the soil, when
sewage sludge is used in agriculture, aims to (a) regulate the agricultural use of sewage sludge by
avoiding deleterious effects on soil, vegetation, plants and livestock, and at the same time (b) promote
sound sludge use practices. The directive contains limit values for heavy metals in soil and sludge, and
for the amounts of heavy metals that may be applied to soil annually. Sewage sludge use is prohibited
insofar if the soil concentration of one or more heavy metals exceeds the limit values set by the
directive. The member states are required to institute measures ensuring that these limit values are
not exceeded for sewage sludge use.
The directive stipulates that sewage sludge must be treated before being used as fertilizer. However,
the use of untreated sewage sludge is permitted insofar when the sludge is washed down or buried in
the soil. The directive furthermore stipulates that on pastures and fields used for forage cultivation, as
well as during the vegetation period of fruit and vegetable crops, a waiting period prior to sewage
sludge application must be observed.
The directive also requires the member states to maintain a register that regularly reports on the
amounts of sewage sludge produced and used for agricultural purposes, as well as the composition and
characteristics of this sludge, with attention on pH and the metals content [10].
The Italian sewage sludge directive
Legislative Decree no. 99/1992, 27 January 1992 Implementation of Directive 86/278/EEC, allows the
use of sludge in agriculture only if sludge:
• has been treated;
• is likely to have a fertilizing effect and/or soil amendment and correction of the land;
• do not contain toxic and harmful substances and/or persistent, and/or bio-accumulative in
concentrations harmful to land, crops, animals, humans and the environment in general.
The sludge can be applied to and in land in doses no higher than 15 tons of dry matter per hectare in
the three-year period, provided that the soils have the following characteristics:
• Cation Exchange Capacity (CEC) greater than 15 meq/100 g;
• pH between 6.0 and 7.5;
In case of pH less than 6 and CEC under 15, the quantities are halved; in case of pH higher than 7.5 the
quantities may be increased by 50% and the sludge from food processing industry can be used in
maximum amounts up to 3 times higher.
11
Moreover, the sludge from food processing industry can be used in maximum amounts up to 3 times
higher, provided that the concentrations of heavy metals content does not exceed the limit of more
than one fifth.
The decree also provides for the cases in which sewage sludge use is prohibited, namely:
- in flooded soils, subject to flooding and/or natural floods, waterlogged or aquifer outcrops, or
landslides in place;
- on terrain that slopes more than 15% (if the DS is less than 30%);
- on soils with pH less than 5;
- on soils with CEC less than 8 meq/100 g;
- on land for pasture, with grass pasture, fodder, also intercropped with other crops in the 5
weeks before grazing or harvesting of forage;
- on land for fruits and vegetables cultivation, whose product are normally in direct contact with
the ground and are usually eaten raw, in the 10 months preceding the harvest and during the
harvest itself;
- when it has been established that there is still a danger to the health of humans and/or animals
and/or to protect the environment.
The application of liquid sludge with the technique of spray irrigation is also prohibited.
Great care must be paid on pH, CEC and metals content in the soil analysis, while the sludge analysis
should comprise dry substance; organic carbon; degree of humification; total nitrogen; total potassium;
cadmium, chrome, mercury, nickel, lead, copper, zinc; salmonella.
The quality parameters are reported in Table 3, Table 4 and Table 5.
Parameter Limit
Cadmium ≤20 mg/kgds
Total Chromium ≤1000 mg/kgds
Mercury ≤10 mg/kgds
Nickel ≤300 mg/kgds
Lead ≤750 mg/kgds
Copper ≤1000 mg/kgds
Zinc ≤2500 mg/kgds
Arsenic ≤10 mg/kgds
Table 3: Limits for metals in sludge.
Parameter Limit
Organic Carbon ≥20%ds
Total Nitrogen ≥1.5%ds
Total Phosphorus ≥0.4%ds
Salmonella ≤1000 MPN/gds
Table 4: Limits for agronomic and microbiological parameters.
12
Parameter Limit
Cadmium ≤1.5 mg/kgds
Mercury ≤1 mg/kgds
Nickel ≤75 mg/kgds
Lead ≤100 mg/kgds
Copper ≤100 mg/kgds
Zinc ≤300 mg/kgds
Table 5: Limits for soil analysis.
The normative also prescribes:
• adoption of specific provisions concerning the use of sewage sludge from the agro-food
sector, with particular reference to the storage capacity required in relation to the seasonal
nature of agricultural production and the level of treatment/stabilization to be ensured
before using them;
• changing the amounts of financial guarantees for waste recovery operations, limited to
storage operations of sludge intended for use in agriculture, with particular reference to
those arising from agro-food sector;
• adoption of specific provisions concerning the use of sewage sludge from the treatment
plants of waste water which also treat waste.
1.3.3 Incineration
Differently from the sludge use in agriculture, a norm dedicated to sludge incineration does not exist.
Therefore, the legislation is the general one for waste incineration. In Italy, the reference legislation is
Legislative Decree 133/05, on the implementation of Directive 2000/76/EC on the incineration of
waste. Sludge coming from wastewater treatment is included as non-hazardous waste which can be
utilized as a fuel or for other means to generate energy. This type of activity, in simplified authorization
system, is subject to a series of constraints for the plant, the characteristics of the sludge to be treated
and the emissions [11]. The Legislative Decree 133/05 fixes the following atmospheric emission limits.
Daily 30 minutes
Total particulate 10 mg/Nm3 30 mg/Nm3
Organic substances in the form of gas and vapour,
expressed as total organic carbon (TOC) 10 mg/Nm3 20 mg/Nm3
Inorganic chlorine compounds, in the form of gas and
vapour, expressed as hydrochloric acid (HCI) 10 mg/Nm3 60 mg/Nm3
Inorganic fluorine compounds, in the form of gas and
vapour, expressed as hydrofluoric acid (HF) 1 mg/Nm3 4 mg/Nm3
Sulphur oxides expressed as sulphur dioxide (SO2) 50 mg/Nm3 200 mg/Nm3
Nitrogen oxides expressed as nitrogen dioxide (NO2) 200 mg/Nm3 400 mg/Nm3
Table 6: Daily and 30 minutes average emission limit values.
13
Cadmium and its compounds, expressed as Cadmium (Cd)
0.05 mg/Nm3 total Thallium and its compounds, expressed as Thallium (Tl)
Mercury and its compounds, expressed as Mercury (Hg)
Antimony and its compounds, expressed as Antimony (Sb) 0.05 mg/Nm3
Arsenic and its compounds, expressed as Arsenic (As)
0.05 mg/Nm3 total
Lead and its compounds, expressed as Lead (Pb)
Chromium and its compounds, expressed as Chromium (Cr)
Cobalt and its compounds, expressed as Cobalt (Co)
Copper and its compounds, expressed as Copper (Cu)
Manganese and its compounds, expressed as Manganese (Mn)
Nickel and its compounds, expressed as Nickel (Ni)
Vanadium and its compounds, expressed as Vanadium (V)
Table 7: Average emission limit values obtained with 1-hour sampling period.
Dioxins and Furans (PCDD + PCDF) 0.1 mg/Nm3
Polycyclic aromatic hydrocarbons (PAH) 0.01 mg/Nm3
Table 8: Average emission limit values obtained with 8 hours sampling period.
The carbon monoxide emission limit values in the flue gases, excluding the start-up and shutdown
phases, has been set at:
50 mg/Nm3 as daily average value;
100 mg/Nm3 as an average value of 30 minutes, in a period of 24 hours or, in case of non-
complete compliance with the limit, the 95% of the mean values over 10 minutes does not
exceed the value of 150 mg/Nm3.
The competent authority may grant derogations for waste incineration plants using fluidized bed
technology, provided that the permit foresees an emission limit value for carbon monoxide (CO) of not
more than 100 mg/Nm3 as an hourly average value.
All the emission limits are expressed with respect to the following reference conditions:
Temperature=273.15 K
Pressure=101.3 kPa
Dry gas
Oxygen content in flue gases=11%.
The Legislative Decree sets also the pollutants concentration limits in the plant wastewater from waste
gases cleaning, as in Table 9.
14
Total suspended solids 95% 100%
30 mg/l 45 mg/l
Mercury and its compounds, expressed as Mercury (Hg) 0.03 mg/l
Cadmium and its compounds, expressed as Cadmium (Cd) 0.05 mg/l
Thallium and its compounds, expressed as Thallium (TI) 0.05 mg/l
Arsenic and its compounds, expressed as Arsenic (As) 0.15 mg/l
Lead and its compounds, expressed as Lead (Pb) 0.2 mg/l
Chromium and its compounds, expressed as Chromium (Cr) 0.5 mg/l
Copper and its compounds, expressed as Copper (Cu) 0.5 mg/l
Nickel and its compounds, expressed as Nickel (Ni) 0.5 mg/l
Zinc and its compounds, expressed as Zinc (Zn) 1.5 mg/l
Dioxins and furans (PCDD + PCDF) 0.3 mg/l
Polycyclic aromatic hydrocarbons (PAHs) 0.0002 mg/l
Table 9: emission limit values in the wastewater from flue gases cleaning.
1.4 Sludge as a valuable waste
Legislative Decree no. 152/2006, 3 April 2006, on environment norms, in Article 127, provides that
sewage sludge from the treatment of wastewater, identified as “special waste”, must be subjected to
the discipline of the waste, when applicable. Therefore, sludge must be re-used whenever the reuse is
appropriate. The decree states that waste must be recovered or disposed of without endangering
human health and without using processes or methods which could harm the environment, without
determining risk to water, air, soil and fauna and flora; without causing a nuisance through noise or
odors; without damaging the landscape and sites of particular interest, protected in accordance with
current legislation. Furthermore, it must be taken into account that with the purpose of a proper waste
management, public authorities favor the reduction of the final disposal of waste by:
• reuse and recycling;
• other forms of recovery to obtain secondary raw material from waste;
• the adoption of economic measures and forecasting of contract provisions conditions,
requiring the use of the materials recovered from the waste in order to promote the
market for such materials;
• the use of waste as a means to generate energy.
While it is impossible to associate reuse practice to sludge, as it is not a good with a define scope but
just a by-product of the water treatment process, it is possible to recycle it: land spreading is a way for
recycling the compounds of agricultural value present in sludge to land. Sludge recovery to obtain
secondary raw materials such a phosphorus and compost, and energy recovery are also viable options.
These aspects are presented in the sections on material recovery (paragraph 0), and energy recovery
(paragraph 3.3). Being the main topic of this work, the latter is analyzed in more detail in the
subsequent chapters.
15
16
2 Sludge sources, treatments and characterization
2.1 Sources
The source of sludge is any plant in which is required to purify a water stream before sending it to a
river/lake/sea or using it as drinkable water or for sanitary purposes. Therefore, sludge can be defined
as the by-product of the water clean-up process.
There are three main source categories of sludge:
SEWAGE SLUDGE: sludge originating from the treatment of urban wastewater.
INDUSTRIAL SLUDGE: originating from the treatment of industrial wastewater.
SLUDGE FROM DRINKING WATER PURIFICATION.
Sludge originated in the treatment of urban wastewater consists in domestic or in a mixture of domestic
with industrial wastewater and/or run-off rainwater, while industrial sludge comes only from the
purification of water used in industrial processes.
When drinking water is produced, it has to be treated before its consumption. The amount of sludge
generated from drinking water treatment is significantly lower than that generated from wastewater
treatment. Also industrial sludge accounts for a minor amount and it can be very different depending
on the industrial process considered.
In section 1.1, data from Eurostat [4] are referred to the total amount of sludge, although it is stated
that this data consider mainly the sewage sludge. Since sewage sludge is clearly the largest contribution
to the total sludge production, together with a lack in data on the other two sources categories, it is
reasonable to analyze data from sewerage only.
According to what previously said, the most important source of sludge is the one produced in the
sewage treatments plant and only this source of sludge is investigated.
The treatments on which the sludge undergoes are briefly described.
Figure 3: Macro-steps of sludge lifecycle.
The place where wastewater is treated and where the sewage sludge is produced consequently is called
Waste Water Treatment Plant (WWTP).
Sewage sludge is a generic term that provides no indication of the origin and/or type of sludge involved.
Each of the various types of sludge has a specific designation, depending on the juncture in the
purification process at which the sludge is generated. Figure 4 shows the juncture in a sewage
treatment plant purification process at which the various types of sludge are generated.
17
Figure 4: Sludge occurrence relative to treatment phase [10].
A brief description of each pretreatment and sludge type follows, with reference to European
Commission technical report: Disposal and recycling routes for sewage sludge [12] and Sludge
Management in Germany [10].
2.1.1 Pre-treatment
Pre-treatment consists of various physical and mechanical operations, such as screening, sieving, blast
cleaning, oil separation and fat extraction. Pre-treatment allows the removal of voluminous items,
sands and grease. The residues from pretreatments are not considered sludge. They are disposed of in
landfills.
2.1.2 Primary sludge
Primary sludge is produced following primary treatment. This step consists of physical or chemical
treatments to remove matter in suspension (e.g. solids, grease and scum).
The most common physical treatment is sedimentation. Sedimentation is the removal of suspended
solids from liquids by gravitational settling. Sedimentation is usually considered first because it is a
simple and cost-effective method.
Another physical treatment is flotation. Air is introduced into the wastewater in the form of fine
bubbles, which attach themselves to the particles to be removed. The particles then rise to the surface
and are removed by skimming.
Chemical treatments are coagulation and flocculation, used to separate suspended solids when their
normal sedimentation rates are too slow to provide effective clarification. Coagulation is the addition
and rapid mixing of a coagulant to neutralize charges and collapse the colloidal particles so that they
can agglomerate and settle. Flocculation is the agglomeration of the colloidal particles that have been
subjected to coagulation treatment.
18
The color of primary sludge ranges from greyish black to greyish brown to yellow. Sludge mainly
contains easily recognizable debris such as toilet paper. After being removed from the system without
being treated, it putrefies rapidly and emits an unpleasant odor.
Figure 5: Primary sludge in pretreatments flowsheet.
2.1.3 Secondary sludge
Secondary sludge (also called waste activated sludge), which occurs after biological treatment, is
generated by microbial growth, is usually brownish in color, and is far more homogenous than primary
sludge. After being removed from the system, secondary sludge is digested more rapidly than in the
case of primary sludge.
The active agents in these systems are microorganisms, mostly bacteria, which need the available
organic matter to grow. The techniques employed are lagooning, bacterial beds, activated sludge as
well as filtration or biofiltration processes.
The lagooning technique exploits a bacterial population development in a lagoon, which converts
organic matter into CO2 and biomass. Oxygen is fed into the system via the photosynthetic activity of
microphytes (unicellular algae) or macrophytes (plants), although an alternative technique consists of
artificial aeration of the lagoon. In practice, water is passed through several lagoons, each reaching a
higher level of de-pollution. This technique is suitable for WWTPs with large site areas.
In bacterial beds, the effluent is in contact with bacteria, which are attached to a support.
In activated sludge, bacteria are kept in suspension in the vessel in aerobic conditions. At the end of
the process, the treated water has to be decanted off in order to separate the cleaner water from the
activated sludge. This treatment generates another type of sludge, which is recirculated in the system
called return activated sludge, and is not an output stream of the WWT plant.
19
Figure 6 : Typical wastewater treatment process [13].
The amount of sewage sludge produced from the activated sludge process is directly proportional to
the amount of wastewater treated. The total sludge production consists of the sum of primary sludge
from the primary sedimentation tanks as well as waste activated sludge from the bioreactors. The
activated sludge process produces about 70–100 kg/ML of waste activated sludge (that is kg of dry
solids produced per ML of wastewater treated; 1 mega liter (ML) is 103 m3). A value of 80 kg/ML is
regarded as being typical [14]. In addition, about 110–170 kg/ML of primary sludge are produced in the
primary sedimentation tanks which most - but not all - of the activated sludge process configurations
use [14].
2.1.4 Mixed sludge
The primary and secondary sludge described above can be mixed together generating a type of sludge
referred to as mixed sludge.
2.1.5 Tertiary sludge
Tertiary sludge is generated when carrying out tertiary treatment. It is an additional process to
secondary treatment and is designed to remove remaining unwanted nutrients (mainly nitrogen and
phosphorus) through high performance bacterial or chemical processes. These treatments are
necessary when a high level of depollution is required.
Nitrogen consumes oxygen when a nitrification reaction takes place in the natural environment. It is
toxic under its ammoniac or nitrate phase, and is responsible of eutrophication. The nitrogen removal
is a biological process leading to the production of N2. Each step is carried out by specific bacteria,
which need different conditions to grow.
Physical-chemical processes for phosphorous removal consist of chemical precipitation using additives
followed by sedimentation; they increase the quantity of sludge produced by an activated sludge plant
by about 30%. Biological treatments employ specific microorganisms, which are able to store
phosphorus. It accumulates within the bacteria enabling its removal from the rest of sludge.
20
The precipitation process is usually carried out in conjunction with primary or biological sewage
treatment, rather than in a structurally separate treatment system. Hence tertiary sludge often occurs
not separately, but rather mixed with primary or secondary sludge. Tertiary sludge color is determined
by the acting reactions, whereby the chemical properties of tertiary sludge differ considerably from
those of primary and secondary sludge. Tertiary sludge is normally stable and does not emit an
unpleasant odor.
2.1.6 Digested and stabilized sludge
After water treatment, additional treatments need to be performed, in order to:
- reduce its water content,
- stabilize its organic matter and reduce the generation of odors,
- reduce its pathogen load,
- reduce its volume and global mass.
Several treatments can be applied, and the obtained sludge is considered as a new type:
Digested sludge (sludge that undergoes an anaerobic sludge stabilization process)
Stabilized sludge (sludge that undergoes a chemical or biological sludge stabilization process).
2.1.7 Raw Sludge
When sludge not undergoes a digestion process, it is called raw sludge. Raw sludge comprises primary,
secondary and tertiary sludge in any given mixture that occurs at a sewage treatment plant. Raw sludge
is untreated sludge prior to stabilization.
2.1.8 Industrial sludge
As stated above, industrial sludge is originated from the treatment of industrial wastewater only.
2.1.8.1 Pulp and paper industry
Composition of pulp and paper industry sludge depends on the paper production process. Using virgin
wood, fiber generates a liquid effluent mainly loaded with lignin and cellulose, therefore containing a
higher level of stable organic matter. On the contrary, recycling of waste paper induces additional steps
such as de-inking and bleaching, and therefore generates a so-called deinking sludge, containing
coloring agents and chemicals. Recycled paper usually generates a greater amount of sludge than when
using virgin wood fibers.
Pulp and paper sludge is therefore a mixture of cellulose fibers, ink and mineral components. Inks is
produced by using heavy metals. Their usage has however been greatly reduced in the last 20 years,
reducing their level in sludge. The higher content of cellulose fibers makes the nitrogen availability
lower than in the case of urban sludge. As a consequence, nitrogen is released more slowly into the soil
after application, reducing the risk of leaching to groundwater [12].
21
2.1.8.2 Tannery Sludge
Leather manufacturing generates liquid and solid wastes originated from the different steps in the
transformation of the mammalian skin into leather, performed by using several reactive products.
Liquid effluents contain collagen fixed to tanning agents and heavy metals originated from the reactive
products used during the tanning process. Sludge composition varies according to the specific process
performed on site. As tannery wastewater is rich in proteins, nitrogen content in the sludge is higher
than in the case of urban sludge, and therefore of interest for landspreading. However, heavy metal
(especially chromium) content may prevent their use in agriculture [12].
2.1.9 Different sewage sludge types comparison
Each kind of treatment has a specific impact on the composition of sewage sludge:
Primary sludge
Biological sludge
Mixed Sludge
Digested sludge
Dry matter (DM) g/l 12 8 10 30
Volatile matter (VM) % DM 65 77 72 50
pH % VM 6 7 6.5 7
C% % VM 51.5 53 51 49
H% % VM 7 6.7 7.4 7.7
O% % VM 35.5 33 33 35
N% % VM 4.5 6.3 7.1 6.2
S% % VM 1.5 1 1.5 2.1
C/N - 11.4 8.7 7.2 7.9
P % DM 2 2 2 2
K % DM 0.8 0.8 0.8 0.8
Al % DM 0.2 0.2 0.2 0.2
Ca % DM 10 10 10 10
Fe % DM 2 2 2 2
Mg % DM 0.6 0.6 0.6 0.6
Fat % DM 18 10 14 10
Protein % DM 24 34 30 18
Fibres % DM 16 10 13 10
Table 10: Composition of different kind of sludge [15].
As can be noticed, sewage sludge contains both compounds of agricultural value and pollutants.
Compounds of agricultural value include organic matter, nitrogen, phosphorus and potassium, and to
a lesser extent, calcium, sulfur and magnesium. Pollutants are usually divided between heavy metals,
organic pollutants and pathogens.
22
2.2 Treatments
2.2.1 Stabilization
2.2.1.1 Anaerobic digestion
The aim of stabilization is the reduction of biological and chemical reactions to a minimum. Anaerobic
digestion is one of the oldest and most widely used processes for wastewater sludge stabilization.
Concentrated organic and inorganic sludge matter is decomposed microbiologically in the absence of
oxygen and converted to methane and inorganic products. The main benefits from digestion are the
stabilization of sewage sludge, volume reduction and biogas production [16].
Anaerobic digestion involves several successive stages of chemical and biochemical reactions involving
enzymes and a mixed culture of microorganisms. The process comprises three general degradation
phases: hydrolysis, acidogenesis and methanogenesis, according to Figure 7.
Figure 7: Digestion reactions scheme.
Anaerobic digestion process is very sensitive to environmental factors, which therefore need to be
controlled properly and carefully. The most important are: temperature, pH, alkalinity, and presence
of toxic and inhibitor compounds.
For what concerns temperature, the anaerobic digestion process is operated either in the mesophilic
(around 35-40 °C), or thermophilic (53-57 °C) temperature ranges. The main advantages of thermophilic
treatment are higher sludge treatment capacity and a better sludge dewatering result with a higher
hygienic quality of the treated sludge. The disadvantages are higher energy costs and lower
supernatant quality due to dissolved solids. Thermophilic digestion has caused more odor
inconvenience and the process stability is weaker compared to mesophilic digestion. For this reason,
23
most anaerobic digesters are designed to operate in the mesophilic temperature range (www.iea-
biogas.net).
Methanogens bacteria are extremely sensitive to pH, working well when pH is 6.8-7.2, with the neutral
being the optimum. Volatile acids produced during acidogenesis tend to reduce the pH, but the
reduction is normally countered by methanogens, which also produce alkalinity in the form of carbon
dioxide, ammonia, and bicarbonate. The best way to increase pH and buffering capacity in a digester is
by the addition of sodium bicarbonate or lime.
The most important thing that has to be ensure in the anaerobic digester sizing is that the bacteria have
sufficient time to reproduce and metabolize volatile solids. The key parameters are:
- the solids retention time, SRT, which is the average time the solids are held in the digester,
expressed in kg of solid in the digester over kg of solid withdrawn daily,
- the hydraulic retention time, HRT, which is the average time the liquid sludge is held in the
digester, expressed as digester volume over daily sludge volume flow rate.
A decrease in SRT decreases the extent of reactions: because a portion of the bacterial population is
removed with each withdrawal of digested sludge, the rate of cell growth must at least match cell
removal to maintain the system in steady state. Otherwise, the population of bacteria in the digester
declines and the process eventually fails (washout). Therefore, a minimum SRT is essential to ensure
that bacteria are being produced at the same rate at which they are withdrawn daily. Several solutions
have been developed in order to have SRT>HRT, so that the organic and useful matter is retained for
longer time, with more time available for reactions. Wastewater sludge processing [17] recommends
solid retention time of between 20 and 25 days, with the minimum being 10 days.
The most commonly used anaerobic digester is equipped with heating and mixing devices.
Biogas is taken from the top point of the digester. Generation of biogas is a direct result of the
destruction of volatile solids, with a specific gas production for wastewater sludge that generally ranges
from 0.4 to 1.1 m3/kg of volatile solids destroyed, according to [17], [18] and (www.iea-biogas.net).
Specific gas production values will be closer to the high end of this range if the sludge contains a higher
percentage of fats and grease as long as adequate SRT is provided for these slow-metabolizing
materials. Gas production is dependent on the used substrate quality and amount of volatile solid (VS)
organic material. In addition, the biological activity and mixing conditions have a significant effect.
Primary sludge has a much higher biogas potential than activated sludge.
It is fundamental to underline that the values reported above, however, indicate the gross biogas
production: part of the total amount has to be used to get the electric power needed to run the process
itself. An average value of a sludge anaerobic digestion plant electric consumption is 250 kWh/ton DS
[18].
Biogas conversion in electricity shows efficiencies that range from 25% (for small scale plants of less
than 100 kW) to 45% (for more than 500 kW plants) (www.iea-biogas.net).
To assess the digester performance, the organic matter degradation can be used: for example, a
degradation of 50 % of organic matter is considered as good performance.
A healthy digestion process produces a gas with about 65 to 75% methane, 30 to 35% carbon dioxide,
and very low levels of water, nitrogen, hydrogen, and hydrogen sulfide, but more in general the
24
methane fraction is between 58 and 64%. The heating value of digester gas is approximately 23
MJ/Nm3.
2.2.1.2 Aerobic digestion
Sludge can be stabilised, as an alternative to anaerobic digestion, by long-term aeration in aeration
tank that biologically destroys volatile solids.
Aerobic digestion is the biochemical oxidative stabilization of wastewater sludge in open or closed
tanks that are separate from the liquid process system. The aerobic digester operates on the same
principles as the activated sludge process. Air or oxygen can be supplied by surface aerators or by
diffusers; other equipment may include sludge recirculation pumps and piping, mixers and scum
collection baffles [19].
Aerobic digestion produces sludge suitable for various disposal options.
Aerobic stabilisation can be realised by increasing the retention time at the biological treatment up to
25 days with a good oxygen supply [18]. This process does not need any special competence beyond
the normal operation of a wastewater treatment plant, as operation is relatively easy [20].
It is possible to apply other aerobic stabilization methods, for example, aerobic thermophilic
stabilization, which is designed for medium-size and large plants, and allows to reach 100% pathogens
destruction [19]. A constant mesophilic or thermophilic temperature and a good oxygen supply
guarantee that aerobic stabilization takes place.
The aerobic digestion process allows basic fertilizer value of sludge recovering in a larger extent, with
respect to the anaerobic process [20].
Usually, the investment costs are lower than for anaerobic digestion plant [19], [19], but the drawbacks
of the aerobic digestion process are the high cost due to energy intensive aeration and that no biogas
is produced [18]. Moreover, aerobic digested sludge has poorer mechanical dewatering characteristics
[20].
2.2.2 Thickening
The sludge that comes out of wastewater treatment has a water content of between 97% and 99.5%.
Sludge thickening allows increasing the dry solids (DS) content of sludge by reducing the water content
with low energy input, and is particularly attractive because considerable volume reduction is achieved
even with a relatively small dry solid increase. Thickened sludge is still pumpable. Sludge thickening can
be applied both as a pre-treatment for digestion and as a pre-treatment for dewatering in wastewater
treatment plants that operate without digestion.
In sludge thickening, like in sludge dewatering, inorganic or organic flocculants aid chemicals (usually
polymers) are used, although they are not strictly necessary.
The flocculant aids need specific mixing, storage and feeding conditions. Optimising polymer dosing
and mixing obviously helps to improve the thickening result, although it is not recommended to use
flocculant aids for the thickening of primary sludge.
The achieved DS content, energy consumption and chemical consumption vary with the type of sludge.
25
2.2.2.1 Gravity thickening
Gravity thickening is the easiest way to reduce the water content of sewage sludge with low energy
consumption, with the gravity tank operation similar to a settling one.
Sludge is pumped directly to a circular tank equipped with a slowly rotating rake mechanism, which
breaks the junction between the sludge particles and therefore increases settling and compaction.
The incoming sludge flow is directed to the central cone of the tank. Settled sludge is collected at the
bottom of the tank and pumped out from the bottom outlet pipe to the next treatment step, which
could be a digestion, dewatering or a secondary (mechanical) thickening.
With gravity thickening, the total sludge volume can be reduced by even 90% from the original volume;
this method consumes very little energy.
Gravity thickening normally requires its own basin, usually circular and made from concrete, with a
typical diameter between 8 m and 20 m. Sometimes it can be carried out inside the primary or
secondary clarifier but the total reached sludge DS content is smaller and the risk of anaerobic
conditions is higher compared to conventional gravity thickening.
All types of sludge can be thickened by gravity. Digested sludge is often dewatered directly. Sometimes
there is no thickening; the sludge is pumped directly to sludge dewatering, as some dewatering devices
are also able to dewater sludge with very high water content and separate thickening is not always
necessary.
2.2.2.2 Flotation thickening
Flotation thickening is used for light and fluffy sludge, such as waste activated sludge, as gravity
thickening works well with heavy sludge and it is not so effective in this kind of applications, while other
sludge types are difficult to thicken by flotation because they are heavy and tend to settle.
Flotation uses tiny air bubbles that attach themselves to sludge particles, making them lighter than the
surrounding liquid and thus buoying them to the surface where they are scraped off as thickened
sludge. Air is introduced under pressure to recycled effluent, which is then mixed with the incoming
sludge.
According to [16], the float concentration is hardly predictable, and depends on the height of the float
above the water line.
2.2.2.3 Mechanical thickening
Mechanical thickening is used especially for excess sludge thickening. It needs flocculant aid and
electrical energy. The flocculant aid is fed in a flocculation reactor with a stirrer to ensure good mixing
and stable flocks. The mechanical thickening methods can be operated continuously, and (especially
for medium size plants) in shifts, but in these cases, a buffer tank is required.
Mechanical thickening is typical for large and medium-size wastewater treatment plants and as pre-
treatment for direct dewatering without digestion [18].
Typical cleaning procedures must be carried out approximately every two weeks. There are no
particular environmental or safety issues with the different mechanical thickening methods.
Examples are screw, drum, belt and centrifuge thickening and their different performances, again
according to [18], are reported below.
26
Technology Screw Drum Belt Centrifuge
DS content 4-7% 5-7% 5-7% 5-7%
Polymer
consumption 2-6 g/kg DS 2-6 g/kg DS 2-6 g/kg DS 1-1.5 g/kg DS
Energy
consumption Low Low Low High
Maintenance Low Low Low Low
Capacity and
remarks 20-100 m3/h 10-70 m3/h 24-180 m3/h
5-200 m3/h use without
polymers possible
Table 11: Thickening technology comparison [18].
Mechanical thickening has much higher operational costs but the reachable DS content is also higher.
2.2.3 Dewatering
The sludge dewatering process consists in increasing the dry solids content of the sludge with different
types of equipment. The difference between sludge thickening is the degree of dry solid content
increase: after thickening solid concentrations are less than 15%, while after dewatering are more than
15%.
The dewatering process always requires the use of at least some flocculants aid that keeps the excess
sludge flocculated in the dewatering unit. Sometimes, coagulation chemicals such as iron or aluminum
salts are also added in order to enhance the efficiency of flocculant aids (polymers) and reduce the
consumption of them in sludge dewatering.
After dewatering, the dry solids content of the sludge is usually between 19% and 30%. Depending on
the dewaterabilty, it is possible to reach a dry solid content of up to 40%. After reaching the maximum
DS content with dewatering, the water left in the sludge is bound in the cells and can be reduced only
with sludge drying.
It must be remarked as biological phosphorus removal reduces the dewaterability of the sludge.
2.2.3.1 Lagoon
Lagoons are large holes in the ground where sludge is pumped and allowed to evaporate. The process
is obviously extremely slow.
2.2.3.2 Sand beds
Drying beds are shallow ponds with sand bottoms and tile drains. Sludge is pumped to the beds at a
depth of 15-30 cm. In the first step, free water is drained through the sludge into the sand and out tile
drains. In the second step, further dewatering is achieved through evaporation.
The time required for dewatering ranges from several weeks to several months.
Sand beds perform better with sludge with a low biological fraction.
27
2.2.3.3 Centrifuge
The decanter centrifuge with its continuous feed and sludge output is the standard centrifuge type.
The key elements are the bowl, which includes cylindrical and conical sections, the conveyor screw
inside the bowl and the drive units to rotate them. The casing surrounding the bowl acts as a protective
and noise suppression barrier, and channels the dewatered sludge cake and separated clarified liquid
out from the unit.
Sludge is pumped through a central pipe into the rotating bowl and, because of centrifugal force, hugs
the bowl inside walls. The heavier solids sink to the inner bowl wall and the lighter liquid remains pooled
on the outside.
Dewatered sludge cake is discharged out from the bowl through a port located in the small diameter
end of the conical section. A small difference in the rotational speed between the bowl and the
conveyor allows the accumulated sludge cake to roll, thicken further and be transported from the
cylindrical section up the cone for discharge. The clarified liquid outlet ports include adjustable height
overflow weirs, with which the liquid level inside the bowl can be adjusted. Centrifuges can be arranged
both in co-current and in countercurrent design.
Figure 8: Dewatering centrifuge scheme.
Centrifuges are usually used for dewatering digested or aerobically stabilized sludge, but it is also
possible to dewater other types of sludge. The process is compact and closed, tidy and reliable, and
models with small capacity are now available.
The dewatering result mainly depends on the type of sludge. Primary sludge is much easier to dewater
than a mixture of primary and excess sludge, aerobically stabilized or digested sludge, although primary
sludge has higher torque requirement and potential for material erosion than excess sludge.
Centrifuges are able to dewater primary sludge to a dry solid content of about 32-40%; a mixture of
primary and excess sludge to about 26-32%; aerobically stabilized sludge to 18-24%; and digested
sludge to a DS content of about 22-30%.
2.2.3.4 Belt filter press
Belt filters use positive pressure to force water to pass through a fabric, in a continuous process.
28
The process is composed of three steps: chemical conditioning, gravity drainage to a non-fluid
consistency, and compaction in a pressure and shear zone. After chemical conditioning, a distribution
system evenly applies the mixture onto the gravity feed belt and the filtrate from the gravity zone is
collected and piped to a drain system. Further dewatering occurs as the sludge is squeezed between
the two porous belts. The pressure increases as sludge passes through a wedge zone and enters the
high-pressure stage. The belts proceed around several drums of decreasing diameter to maximize the
shearing action and increase the pressure.
Figure 9: Belt filter press dewatering in treatments chain.
The dewatering result is little lower than with centrifuges.
Belt filter presses are often used for digested sludge; it is also possible to dewater thickened sludge
with no intermediate digestion step. It is not recommended, however, to dewater sludge that has not
been thickened with this technique.
2.2.3.5 Chamber filter press
A chamber filter press consists of a series of filter chambers containing filter plates supported in a
frame. The sludge is fed in a batch manner, which is a disadvantage compared to belt filter press.
Loaded filter chambers are forced together with hydraulic rams. The sludge is squeezed in few seconds
by up to 60 bar pressure in the press. The dewatered sludge is then discharged from chambers by
opening the filter plate and shaking cloth or plate.
The dewatering result of chamber filter presses mainly depends on the characteristics of the sludge
and its conditioning. With organic flocculant aids, the dewatering results are similar to centrifuges.
It is possible to use milk of lime (15-25 kg/m³) and iron chloride (5-12 kg/m³) for conditioning. In this
case, filter cloths with permeability are needed, the air has to be cleaned by an acid washer, and
hydrochloric acid is needed for cleaning the filter cloths at certain intervals. With lime dewatering,
results of over 40% DS are possible; however, in this case, there is 30-50% of lime inside. Milk of lime
has a hygienization effect, which enables the use of sludge in agriculture in certain countries.
Chamber filter press dewatering can be applied for primary or excess sludge, possibly after thickening
and digestion, and with different types of wastewater treatment processes. It is particularly good in
handling inorganic suspended solids and chemical sludge.
29
2.2.3.6 Hydraulic press
The hydraulic press belongs to the innovative solutions of sludge handling and it can be considered to
be worth especially when the dewatering properties are poor and/or high dry solids content is needed.
The hydraulic press is designed as a rotating cylinder piston system with hydraulic drive. Between the
bottom of the cylinder and the piston, there are flexible drainage elements, which allow the filtrate to
drain out of the press interior. The pressing process consists of the following steps: sludge feeding,
dewatering by a cyclic press and bulking loops, and the discharge of the filter cake. Continuous
operation consists of several impulse filling cycles. The dewatering steps are repeated until the required
dewatering is reached.
The dry solids content of the dewatered sludge usually ranges from 25% to 40%.
Hydraulic presses are usually used for digested sludge, but it is also possible to dewater other sludge
types. The suitable dry solid content of a suspension to be treated varies between 2% and 10% DS. This
type of equipment is much more expensive than belt filter presses or centrifuges and therefore usually
suitable mainly for large wastewater treatment plants.
A summary of the considered dewatering techniques is reported in the Table 12 from [18].
Technology Centrifuge Belt filter
press
Chamber filter press Hydraulic
press Polymer
conditioning
Lime
conditioning
Dewatering
result
aerobically
stabilized 18-24% 15-22% 18-24% 28-35% 20-35%
digested 22-30% 20-28% 22-30% 30-40% 20-35%
Flocculant aid
consumption
4-14 g/kg
DS
4-12 g/kg
DS
5-12 g/kg
DS
15-25
kg/m3
5-12 g/kg
DS
Energy consumption High Low Medium Medium Medium
Automatic and
continuous Yes/Yes Yes/Yes No/No No/No Yes/No
Investment costs Medium Medium Very high Very high Very high
Applications All sizes
plants
All sizes
plants Large plants Large plants Large plants
Table 12: Dewatering technologies comparison [18].
Finally in Table 13, is reported a comparison, made by [12], focused on the main advantages and
disadvantages if different dewatering technologies.
30
Technology Advantages Disadvantages
Drying beds
Easy to operate Land requirement
Adapted to small WWTP Weather dependency
Functions throughout the year Risk of odors
Low operation costs Workforce requirements
High DM content reached
Centrifuging
Continuous operation Specialized maintenance
Compact Sludge texture
Possible automation Noise
High energy consumption
High investment costs
Filter belt
Continuous operation Limited water content reduction
Easy to perform Cleaning water consumption
Moderate investment costs Supervision necessary
Filter press
High water content reduction Discontinuous operation
Structure of the sludge Low productivity
Possible automation Consumption of mineral conditioner
Supervision necessary
High investment costs
Table 13: Comparison of different dewatering processes.
2.2.4 Conditioning
2.2.4.1 Chemical conditioning
Chemical conditioning involves the addition of reagents to the sludge, in order to achieve coagulation
of colloidal or super-colloidal particles and their subsequent flocculation with reduction of the finely
dispersed phase. Either inorganic or organic chemicals or a combination of both can be used. Examples
of inorganic conditioners are lime and ferrous sulfate, and organic ones are polymers, in particular,
polyelectrolytes.
The primary objective of conditioning is to increase particle size by bringing together and combining
the smaller particles into cohesive large particles that carry less water. The sludge particles carry a net
negative charge. Due to the similar surface charge, repulsive forces dominate over a certain distance
from the particles’ surface. On the other hand, also attractive forces are present because of Van der
Waals forces. The conditioners action is intended to reduce the sludge particles surface charge through
the addition of counter-ions, so that the attractive force starts to dominate.
Milk of lime is typically fed to non-thickened sludge with a low DS content, before dewatering with a
chamber filter press, or it is mixed with the sludge before thickening and stabilization.
Milk of lime and iron can be used for dewatering as flocculant aid with chamber filter presses.
Stabilisation with calcium oxide is usually applied on sludge with high DS content, 20–40%, which means
dewatered sludge.
31
However, conditioning has the disadvantage that reject water quality changes and calcium carbonate
accumulates in pipes.
2.2.4.2 Thermal conditioning
Heating sludge alters its surface properties and ruptures the microbial cells. This process releases
chemicals and some of the water bound within flocs or inside the cells and makes sludge easier to
dewater. The advantages of heat conditioning are excellent sludge dewatering characteristics, no
requirement for chemical conditioners, sludge stabilization and pathogen destruction achieved
simultaneously. Moreover, if this process is applied before digestion, higher biogas production is
possible.
The sludge dewatering characteristics can also be improved with freezing. During freezing, the
advancing ice front rejects and pushes the solids until they contact with each other and form larger
particles. As the ice thaws, the particles retain their new compact sizes and shapes. This process
converts the sludge solids to a more granular form, enhancing water drainage through the solids and
must be very slow, so that the water within the cells is allowed to crystallize and squeeze the solid into
compact granules.
2.2.5 Drying
Drying commonly describes the process of thermally removing volatile substances (moisture) to yield
a solid product. Moisture held in loose chemical combination, present as a liquid solution within the
solid or even trapped in the microstructure of the solid, which exerts a vapor pressure less than that of
pure liquid is called bound moisture. Moisture in excess of bound moisture is called unbound moisture.
When a wet solid is subjected to thermal drying, two processes occur simultaneously:
1. Transfer of energy (mostly as heat) from the surrounding environment to evaporate the surface
moisture.
2. Transfer of internal moisture to the surface of the solid and its subsequent evaporation due to
process 1.
The rate at which drying is accomplished is governed by the rate at which the two processes proceed
[21].
According to [12], “partial drying” enables reaching a DM content of 30 to 45%, at which it is possible
auto-combust the sludge. Those processes inhibit the re-growth of bacteria, mainly because of the
reduced moisture level, which may be reached. If sludge is dried to more than 90% DM, the process is
called “total drying”.
Dried sewage sludge has a number of advantages over wet sludge that stems directly from the
treatment process. Sludge drying is preferable for the following reasons [10]:
interstitial water is eliminated, and the volume of the sludge is further;
stabilization and disinfection is obtained when DM exceeds 90%;
the calorific value of the sludge is increased, before thermal oxidation;
it allows spreading using techniques similar to those used for mineral fertilizers;
it reduces the transportation costs.
32
The main drawback of drying is the additional energy needed for drying. Energy requirements for drying
are much higher than dewatering when comparing volume of extracted water. Therefore, in most
cases, drying takes place after a dewatering phase: the key factor for subsequent thermal treatment is
increasing the calorific value. In many cases, the level of DM achieved through mechanical dewatering
does not allow for self-sustaining sludge incineration so for this reasons additional drying is necessary
for sludge incineration. The most energy efficient method in this regard is to dry the sludge at the
incineration site using a method involving waste heat recovery.
Sewage sludge drying uses a tremendous amount of energy, as residual sludge water is evaporated
using thermal energy. In this process, the drying gradient is determined by the intended use of the
sludge.
Recent studies [22] have shown that the integrated process of drying and incineration is much more
convenient than a process without drying, both in terms of production of emissions and both in terms
of consumption of conventional fuel.
For spontaneous incineration (without an auxiliary combustion system) in sewage sludge mono-
incineration plants, dewatering and drying of raw sludge to a total solids of 35% DM are normally
sufficient. The counterpart minimum value for digested sludge is 45 to 55% DM.
Waste incineration plants handle dewatered, partly dried and fully dried sewage sludge. For power
plants, sewage sludge with a solids content ranging from 20 to 35% dry residue is normally used for
incineration purposes. Such plants have coal grinding systems that allow for integrated sewage sludge
drying. Fully dried sludge can also be used in power plants. Sewage sludge in cement plants needs to
be both dewatered and fully dried.
The choice of drying method and of the heating medium for a particular situation depends, however,
on numerous parameters, such as integration into the process as a whole, the desired end-product
characteristics, as well as economic and particularly ecological considerations.
2.2.5.1 Solar drying Solar drying, which, as the name suggests, dries sewage sludge using solar energy, has come into
greater use in recent years.
This process entails heating the sludge and then drying it in a greenhouse-like construction [13]. The
drying of sludge using solar energy requires a considerable amount of land and may give rise to an odor
problem that is difficult to solve.
In recent years, thermal drying has received much attention and is becoming a major sludge-processing
technology.
33
Figure 10: Solar drying of sludge [23].
2.2.5.2 Direct Thermal Drying In direct dryers (also known as convective dryers), it is required an intensive contact between gas
(usually air or flue gas) and sludge. The vapor generated by the drying process is a mixture of water
vapor, air and the gases expelled from the sludge. This vapor requires subsequent scrubbing. In the
interest of avoiding odor emissions and endangering the health of nearby residents, dust particles are
filtered out of the vapor before it is released into the atmosphere through bio-filters.
Direct dryers are:
Rotary-drum
Belt dryer
Flash dryer.
Belt dryer
According to Handbook of drying [13] and to the manufacturer ANDRITZ ®[24], conveyor belt dryer
presents high flexibility in the sludge outlet dry percentage and is particularly attractive for applications
in which the drying air is heated by waste heat at low temperature. Other important advantages are its
modular structure, simple design and high availability.
The conveyor dryer is conceptually very simple. Product is carried through the dryer on conveyors and
hot air is forced through the bed of product.
34
The air enters in the system in different sections with a temperature around 125°C, releases the thermal
power needed to dry the sludge to the desired extent and exits at a temperature of about 80°C [25].
As this temperature is still relatively high, part of the air stream is recycled to the dryer to reduce the
energy demand to heat the make-up air that is at ambient temperature.
Figure 11: Conveyor belt dryer configuration.
2.2.5.3 Indirect Thermal Drying In indirect drying systems (also known as contact dryers), the necessary heat is provided by a steam
generator, or by a thermal oil apparatus that uses oil as a heating medium. The heat in contact dryers
is transferred between a hot dryer surface and the sludge, whereby the heating medium and sludge
are kept separate. The advantage of this technology is that it prevents the vapor from mixing with the
heating medium, and this in turn facilitates subsequent purification of the two substance flows. Contact
dryers normally achieve solids content ranging from 65 to 80%. The only impurities in the water that is
evaporated by the drying process are leakage air and trace amounts of volatile gases. Virtually all of the
steam can condense out of the vapor, and the remaining gases are then deodorized by the boiler.
The indirect drying system has the advantage of producing minimal amounts of vapors and is therefore
easy to manage. The drying rate of indirect dryers may be lower than that of direct dryers because the
latter can operate at much higher temperatures.
Indirect dryers are:
Rotary-Disc dryer
Rotary-Tray dryer
Thin-film dryer
Report on Biomass Drying Technologies [26] analyzes and compares different indirect dryer types that
use steam as drying medium. Results of the study are summarized in the Table 14.
35
Dryer type Requires small
material? Requires
uniform size? Ease of heat
recovery Fire hazard Steam use
Rotary dryer No No Difficult High Can use steam
Flash dryer Yes No Difficult Medium None
Disk dryer No No Easy Low Saturated
steam
Cascade dryer No Yes Difficult Medium None
Superheated steam dryer
Yes No Easy Low Excess steam
produced
Table 14: Summary of advantages and disadvantages of indirect dryer types.
Rotary-Disc dryer
The disk dryer layout can be seen in Figure 12. The sludge is fed via the top inlet and moved by the
rotating arms from one heated tray to another, in a zigzag path until it exits at the bottom as a dried
and pelletized product with up to 95% total solid content. The dryer trays are hollow and are heated
by condensing steam or thermal oil. The sludge can be uniformly spread on the heated surface with its
layer thickness controlled properly. Hence, particularly uniform drying is achieved in such a dryer.
Figure 12: Example of a Disk Dryer (source: Hosokawa Micron [27]).
According to Haarslev Rotadisc® technology (Figure 13), used in the Zaragoza plant that treats paper
sludge, the dryer operating pressure is normally around 5 bar [28].
In addition, the reported scheme suggests that the evaporated moisture from the sludge has to be
treated in a scrubber for particles removal, and, as it is still at a temperature of 125°C, it can allow a
further heat recovery. The condensed steam at the dryer exit, with a temperature of 147°C, returns to
the waste heat recovery boiler, considering that it is subcooled of 5°C with respect to the saturation
temperature.
36
Figure 13: Scheme containing Rotary disc for sludge drying [28]
2.2.5.4 Heating medium choice
Various heating media can be used for sludge dryers.
Table 15, according to [10], lists the heating media and the correspondent drying systems in which they
can be used.
Heating medium Drying apparatus
Flue gas Drum dryer
District heating power plant flue gas Fluidized bed dryer
Air Drum and belt dryers
Steam Thin layer, disc, fluidized bed dryers
Pressurized water Thin layer, disc, fluidized bed dryers
Thermal oil Thin layer, disc, fluidized bed dryers
Solar energy Solar dryer
Table 15: Heating media and drying apparatuses [10].
37
2.3 Characterization
Sludge, as biomass, contains a large number of complex organic compounds, moisture, and a small
amount of inorganic impurities known as ash. The organic compounds comprise four principal
elements: carbon (C), hydrogen (H), oxygen (O), and nitrogen (N). It also have small amounts of chlorine
(Cl) and sulfur (S), and heavy metals [29].
It must be underlined, as in [15], that the structural and chemical composition, and the behavior on
thermal conditions of sewage sludge, highly depends on the pollution load of effluent to be treated,
and/or also on the technical and design features of the waste water treatment process, as well as on
the sludge treatment (stabilization technology).
Each sludge disposal path, from agricultural reuse to incineration, but also landfill, necessarily needs
information about the composition of the sludge, and its energy content, mainly in the case of thermal
utilization.
2.3.1 Proximate analysis
Proximate analysis gives the composition of the biomass in terms of gross components such as moisture
(M), volatile matter (VM), ash (ASH), and fixed carbon (FC).
The moisture and ash determined in proximate analysis refer to the same moisture and ash determined
in ultimate analysis (2.3.2). However, the fixed carbon in proximate analysis is different from the carbon
in ultimate analysis: in proximate analysis, it does not include the carbon in the volatile matter and is
often referred to as the char yield after devolatilization.
The volatile matter of a fuel is the condensable and non-condensable vapor released when the fuel is
heated. Its amount depends on the rate of heating and the final temperature at which it is heated. For
the determination of volatile matter, the fuel is heated to a standard temperature and at a standard
rate in a controlled environment.
Faster heating rates may yield higher volatile matter content, but that is not considered the volatile
matter of the fuel’s proximate analysis.
Ash is the inorganic solid residue left after the fuel is completely burned. It is composed of silica,
aluminum, iron, calcium, magnesium, titanium, sodium, and potassium. Strictly speaking, this ash does
not represent the original inorganic mineral matter in the fuel, as some of the ash constituents can
undergo oxidation during burning.
Fixed carbon (FC) in a fuel is determined as:
FC 1 M VM ASH
This represents the solid carbon in the biomass that remains in the char in the pyrolytic process after
devolatilization.
A reference standard procedure to obtain the proximate analysis is the ASTM D3172-89.
As an alternative, an extremely useful tool is the Thermo-gravimetric Analysis or Analyzer (TGA), which
allows recording the weight loss of a sample subjected to a predetermined temperature program.
Another advantage of this technique is the limited mass of sample needed (few mg). The TG apparatus
38
gives the rate of change in the weight of the fuel sample continuously, and thus, from the measured
weight loss in time, the fuel’s moisture, volatile mater, and ash content can be determined, while the
fixed carbon is obtained by difference. Moreover, TG analysis provides additional information on
reaction mechanisms, kinetic parameters, thermal stability, and heat of reaction.
The thermal program followed and explained in the [30] work provides for an initial heating from Tamb
up to 105 °C, followed by a 10 minutes isotherm at this temperature, so as to ensure the total loss of
moisture contained in the sample . Subsequently, the sample is heated up to 900 °C. The sample heating
until this step is run in N2 environment, to avoid oxidation. A 10 minutes isotherm at 900 °C in air
follows, to assess the amount of ash.
Figure 14: Example of proximate analysis determined by means of TGA [30].
In the [31] study, the proximate analysis provides the following results and allows to reach the following
considerations: considering that sewage sludge from wastewater treatment plants contains 70-80%
moisture on total weight, a temperature around 80-90 °C is sufficient to get matter with less than 10%
water content.
Thermogravimetry provides information about VOCs release modalities. Experimental analysis proved
that considering the total sludge VOCs content, around 90% of this is released heating up to a
temperature of 400 °C. Considering an average sludge VOCs content around 55-65%, referred to dry
matter, heating sewage sludge up to 400 °C, a product with less than a half of its starting weight is
obtained, with massive benefits also for the final disposal issues.
Examples of sludge proximate compositions considered in literature are reported in Table 16.
39
Author
PROXIMATE COMPOSITION [dry basis]
Volatile solid Fixed carbon Ash
Xiong et al. [32] 45.5% 6.9% 47.6%
Sun et al. [33] 39.9% 1.9% 58.3%
Gao et al. [34] 62.8% 13.1% 24.1%
Yuan et al. [35] 39.7% 4.6% 55.7%
Huang et al. [36] 40.1% 3.1% 56.8%
Pokorna et al. [37]
54.6% 20.0% 25.4%
42.1% 8.6% 49.3%
51.7% 19.4% 28.8%
Shen et al. [38] 61.3% 16.1% 22.6%
Han et al. [39] 46.5% 5.0% 48.5%
Inguanzo et al. [40] 73.0% 1.1% 25.9%
Sanchez et al. [41] 59.2% 8.4% 32.4%
Beneroso et al. [42] 74.5% 10.1% 15.4%
Hossain et al. [43] 54.3% 8.9% 36.8%
Karaca et al. [44] 55.5% 8.9% 35.6%
Zhang et al. [45] 41.5% 5.9% 52.6%
Xie et al. [46] 68.6% 16.4% 15.0%
Table 16: Sludge proximate compostions found in literature.
2.3.2 Ultimate analysis
The composition of the hydrocarbon fuel is expressed in terms of its basic elements except for its
moisture, M, and inorganic constituents:
C H O N S ASH M 100%
C, H, O, N, and S are the weight percentages of carbon, hydrogen, oxygen (obtained by subtraction),
nitrogen, and sulfur, respectively, in the fuel. The moisture or water in the fuel is expressed separately
as M. Thus, hydrogen or oxygen in the ultimate analysis does not include the hydrogen and oxygen in
the moisture, but only the hydrogen and oxygen present in the organic components of the fuel.
The ultimate analysis provides many informations that are essentially based on the ratio between the
main elements and allow to define the better sludge use. In fact, in general, high LHV corresponds to
high C and H contents, while the opposite holds for high O and N contents. In addition, N, S and Cl are
particularly important for pollutants emissions and corrosion, fouling and slagging phenomena.
Experimental determination of the ultimate analysis is covered by ASTM standard D3176-89.
The method used by [31] for the ultimate analysis determination is the CHNS analyzer: this device
provokes the flash combustion of the sample to analyze, which converts all the present substances in
40
combustion products. Combustion gases are then sent to a chromatographic column providing their
separation and then to a thermal conductivity analyzer whose output signal is proportional to each
component concentration.
Examples of sludge ultimate composition found in literature are reported in Table 17.
Author
ULTIMATE COMPOSITION [dry basis]
C H N S O ASH
Xiong et al. [32] 22.2% 1.9% 4.3% 10.0% 13.9% 47.6%
Sun et al. [33] 16.2% 2.6% 3.4% 0.0% 19.5% 58.3%
Gao et al. [34] 36.5% 5.9% 7.0% 0.8% 25.7% 24.1%
Huang et al. [36] 20.5% 3.4% 3.5% 0.6% 15.2% 56.8%
Nowicki et al. [47] 30.7% 4.4% 3.7% 0.9% 27.2% 33.1%
Pokorna et al. [37]
40.0% 6.0% 8.0% 0.7% 19.9% 25.4%
28.0% 4.0% 3.5% 1.0% 14.2% 49.3%
39.0% 5.6% 6.0% 3.0% 17.6% 28.8%
Shen et al. [38] 32.4% 4.2% 3.3% 0.9% 36.7% 22.6%
Han et al. [39] 28.6% 4.3% 1.9% 1.0% 15.7% 48.5%
Inguanzo et al. [40] 35.7% 5.2% 3.5% 0.7% 25.4% 29.5%
Sanchez et al. [41] 37.4% 5.3% 6.6% 0.9% 17.5% 32.4%
Beneroso et al. [42] 43.8% 6.1% 9.7% 0.1% 24.9% 15.4%
Hossain et al. [43] 35.0% 4.8% 3.5% 0.0% 19.9% 36.8%
Karaca et al. [44] 34.1% 4.3% 5.3% 1.0% 19.7% 35.6%
Xie et al. [46] 45.2% 6.3% 5.2% 0.0% 28.3% 15.0%
Table 17: Sludge Ultimate compositions found in literature.
In the models developed in the present work (chapters 6 and 7), different types of sewage sludge,
subjected to different treatments, have been considered. Their compositions have been evaluated as
the average between the ones of selected waste water treatment plants in Parma and Reggio Emilia
area (data given by IREN [48]).
In particular, the plants considered for Raw Primary Sludge are Langhirano (PR) and Praticello (RE); S.
Martino (RE) and Guastalla (RE) for Raw Mixed Sludge; Mancasale (RE), Felino (PR) for Digested Sludge.
The compositions are reported in Table 18.
41
Type of sludge ULTIMATE COMPOSITION [dry basis]
C H N S O ASH
Raw primary 43.4% 6.0% 6.9% 1.2% 19.4% 23.2%
Raw mixed 35.9% 5.0% 7.0% 1.0% 22.0% 29.3%
Digested 30.2% 4.2% 4.6% 0.8% 15.1% 45.1%
Table 18: Sludge ultimate composition from IREN data.
2.3.3 Lower Heating Value determination The considered LHV of the dry matter represents the average of the values resulting from four
reference equations:
1) according to a publication on the Asian Journal [49] , the following is a correlation that suits
best sewage sludge:
𝐻𝐻𝑉𝑑𝑟𝑦 [𝑘𝐽
𝑘𝑔𝑑𝑟𝑦] = 430.2 ∙ 𝐶 − 186.7 ∙ 𝐻 − 127.4 ∙ 𝑁 + 178.6 ∙ 𝑆 + 184.2 ∙ 𝑂 − 2379.9
𝐿𝐻𝑉𝑑𝑟𝑦 [𝑀𝐽
𝑘𝑔𝑑𝑟𝑦] = 𝐻𝐻𝑉𝑑𝑟𝑦 [
𝑀𝐽
𝑘𝑔𝑑𝑟𝑦] − 2.442 ∙ 9 ∙ 𝐻
2) according to a publication on Technology and innovative options for sludge journal [50], from
the value of 23 MJ/kgdaf, the LHV of dry matter is found knowing the amount of ashes in IREN
data compositions
𝐿𝐻𝑉𝑑𝑟𝑦 [𝑀𝐽
𝑘𝑔𝑑𝑟𝑦] = 23 ∙ (1 − 𝐴𝑆𝐻)
3) Sludge Engineering [16] provides the value of HHVdry for each type of sludge and the LHVdry is
found using the hydrogen fraction of the IREN sludge;
4) LHVdry computed as in 3) from the HHVdry given in the study by Manara et al. [15].
Considering the ultimate compositions reported above, the resulting LHV of the three kinds of sludge
is as follows.
Type of sludge LOWER HEATING VALUE [MJ/kg] dry basis
[49] [50] [16] [15] AVERAGE
Raw primary 16.75 17.67 21.68 - 18.70
Raw mixed 14.37 16.27 14.91 16.44 15.50
Digested 11.26 12.62 10.08 10.70 11.17
Table 19: LHV of IREN Sludge calculated with the described procedure.
To assess the goodness of this procedure, the value of HHV given in the work of Inguanzo et al. [40] has
been compared to the one calculated as the average of the chosen correlations from the ultimate
composition given in the paper.
42
Inguanzo et al.
ULTIMATE COMPOSITION [dry basis] HHV [MJ/kg] dry basis
C H N S O ASH Given in the paper Computed
35.7% 5.2% 3.5% 0.7% 25.4% 29.5% 16.6 16.8
Table 20: Comparison of calculated HHV with literature value.
Moreover, the LHV values obtained from the procedure are similar to the ones found in literature.
Reference Year Sludge type LHV [MJ/kg] dry basis
Min Mean Max
BREF [51] 2006 Raw 14.12 15.73 17.34
Digested 9.34 10.74 12.14
Table 21: Range for reference LHV values for sewage sludge [52].
43
3 Sludge Recovery and Disposal Routes
Once treated, sludge can be recycled or disposed of using three main routes: recycling to agriculture
(land spreading), undergoing thermal treatments (Mono-incineration, Co-incineration, Gasification and
Pyrolysis) or landfilling. Each recycling or disposal route has specific inputs, outputs and impacts.
It is now a fact that is important to investigate further, in order to discover novel trends in sewage
sludge handling and to make the existing ones economically viable. Nevertheless, in order to reach a
zero-landfill sludge management solution, it is necessary to define new criteria and parameters for
sewage sludge collection and disposal routes. A focus on industrial symbiosis could represent a first
approach to this issue: a cross-sectorial approach could lead to exploitation of novel and alternative
value chains with strong connections to waste hierarchy [53].
3.1 Waste hierarchy
3.1.1 Waste hierarchy definition
The waste management hierarchy indicates an order of preference for action to reduce and manage
waste, and is usually presented diagrammatically in the form of a pyramid [54]. The hierarchy captures
the progression of a material or product through successive stages of waste management, and
represents the latter part of the life-cycle for each product. The aim of the waste hierarchy is to extract
the maximum practical benefits from products and to generate the minimum amount of waste. The
proper application of the waste hierarchy has several benefits. It prevents emissions of greenhouse
gases, reduces pollutants, saves energy, conserves resources, creates jobs and stimulates the
development of green technologies. Waste Hierarchy definitions are taken from Article 3 of the revised
Waste Framework Directive 2008/98/EC:
Figure 15: Waste hierarcy definitions.
44
Prevention means measures taken before a substance, material or product becomes waste,
that reduce: (a) the quantity of waste, including through the re-use of products or the extension
of the life span of products; (b) the adverse impacts of the generated waste on the environment
and human health; or, (c) the content of harmful substances in materials and products.
Re-use means any operation by which products or components that are not waste are used
again for the same purpose for which they were conceived.
Preparing for re-use means checking, cleaning or repairing operations, by which products or
components of products that have become waste are prepared, so that they can be re-used
without any other pre-processing.
Recycling means any recovery operation by which waste materials are reprocessed into
products, materials or substances whether for the original or other purposes. It includes the
reprocessing of organic material, but not energy recovery or the reprocessing into materials
that are to be used as fuels or for backfilling operations.
Recovery means any operation the principal result of which is that waste replaces other
materials which would otherwise have been used to fulfil a particular function, or waste is
prepared to fulfil that function, in the plant or in the wider economy.
Other Recovery is not specifically defined in the revised Waste Framework Directive, although
‘energy recovery’ is referenced as an example.
Disposal means any operation which is not recovery, even where the operation has a secondary
consequence, the reclamation of substances or energy.
It can be assumed by their exclusion in the definition of recycling, that processing of wastes into
materials to be used as fuels or for backfilling can be considered ‘other recovery’.
3.1.2 Waste hierarchy and sludge disposal routes
In Figure 16, different disposal routes are connected to one or more possible steps of the waste
hierarchy in which they can be placed according to the path followed during the disposal.
Figure 16: Connection between Waste hierarchy and sludge disposal routes.
45
Agriculture Use is a way of material recovery, consisting in recycling to land the compounds of
agricultural value present in sludge; however, there are many constraints in this practice defined by
local, national and international directives.
Thermal treatments may lead to recover some by-products such as char and tar after a pyrolysis, to
energy production from gasification syngas or directly from sludge incineration or they can be only a
thermal disposal without energy recovery options.
Sending sludge directly to landfill is clearly a pure disposal processes.
During a recovery or disposal routes sludge can be transformed and so ‘residues’ of the processes may
have been generated eventually. The ‘Residues’ are a new waste for which the hierarchy must be
considered.
In Table 22 different intermediate processes that may lead the route to be classified as material
recovery, energy recovery or disposal are presented, for each route.
Disposal Routes
Intermediate Processes
Biogas
Production
Phosphorous
Production
Phosphorous
Recovery
form ash
Thermal/
electrical
energy
production
Nutrients
Recovery
Bio-Fuels
production
Landspreading x x - - x -
Thermal
Treatment
Mono-
Incineration
x x
x
x
- -
Co-
Incineration - - -
Gasification
& Pyrolysis x x x
Landfill x x - - - - Table 22: Disposal routes and material and energy recovery possibilities.
Every route can handle digested sludge, so biogas production is viable for all of them, and also
phosphorous production from sludge has no restrictions due to type of route. It must be noticed that
for land spreading, if phosphorous is removed, the remaining sludge will have less nutrient property.
Ash resulting from incineration process, for example, may be a vector for further material recovery
through the phosphorous production, but this processes is possible only if sludge is incinerated, gasified
or pyrolyzed alone, without the mixing with other biomasses or wastes.
46
3.2 Material Recovery
Dried sludge can be converted into artificial lightweight aggregates, slags or bricks for the construction
industry. Different properties and destinations for these materials depend on different process
variables and operating conditions. For the use of dewatered sludge, the production of Portland
cement injecting the sludge directly into cement kilns seems the most appealing one. The major
elements present in Portland cement are in fact Ca, Si, Al and Fe, which match reasonably well with
sewage sludge composition. Sludge can be exploited in construction industry in other different forms
such as dried sludge powder or incinerated ash. Many technically feasible processes have been studied
and tested, but most of the techniques are not economically viable because of a high production cost,
with respect to market price.
3.2.1 Nutrients in sewage sludge
Depending on its origin and dewatering gradient, sewage sludge contains varying amounts of nutrients
such as nitrogen, phosphorous and potassium. For instance, 100 tons of wet sludge with 5% dry
substance contains 190 kg of nitrogen, 55 kg of which is ammonium-N, 195 kg of phosphate and 30 kg
of potassium [10], as average.
The bonding structure of the phosphorous contained in sewage sludge depends on factors such as the
phosphorus precipitation method used by the sewage treatment plant. Depending on whether a
chemical or biological phosphorous precipitation method is used, the 60 to 80% of phosphorous occurs
in an inorganic form, and around 1 to 38% of it is water soluble [10].
The actual phyto-availability of phosphorous is determined by various factors such as soil and fertilizer
pH and sewage sludge iron and aluminum content. As an unfavorable phosphorous-iron ratio can
greatly reduce phyto-availability [10], during the treatment process biological phosphorous
precipitation rather than chemical phosphorous precipitation should be used for sewage sludge
intended to be used as fertilizer. In this case, its actual nutrient content (which often deviates greatly
from mean content data) should be taken into consideration and factored into nutrient balance
assessments.
3.2.2 Landspreading or Agricultural use
Sewage sludge is one of the most commonly used and regularly controlled secondary raw material
fertilizers that has the capacity to meet part of the nutrient requirements of crops.
However, sewage sludge fertilizer is also a pollution sink for harmful sewage components from
households, businesses and diffuse sources, concerning whose environmental impact too little is
known. The extent of the possible soil, plant, groundwater, and surface-water pollution resulting from
these sources is difficult to determine, even in cases where relatively small amounts of sewage sludge
are used.
Only sewage sludge from municipal sewage treatment plants can be used as fertilizer for conventional
farm crops.
47
Sewage sludge can be sent directly to landspreading or eventually landspreaded after a composting
process. During composting, the organic solids in sludge are transformed into a stable, pathogen-free,
humus-like material rich in carbon, nitrogen and phosphorous. Composting usually involves blending
dewatered sludge with other organic material such as wood chips, yard trimmings or straw. Properly
composted sludge is an excellent source of organic and inorganic nutrients for horticultural and
agricultural plants, and is often used as a soil amendment. In conclusion, composted sludge will end for
sure in landspreading as final disposal destination, so it is associated to that disposal route when data
on the past and current situation on sludge disposal routes aree mentioned in this work.
Landspreading is a way for recycling the compounds of agricultural value present in sludge to land. All
sludge types (liquid, semi-solid, solid or dried sludge) can be spreaded on land. However, the use of
each of them induces practical constraints on storage, transport and spreading itself.
The sludge production from a given WWTP is more or less constant throughout the year, but the use
on farmland is seasonal [12]. Therefore, storage capacity must be available on the WWTP or on the
farm, either separately or in combination with animal slurry when allowed by the national regulations.
Average storage duration is about 6 months.
Storage on fields may also be practically observed. This however should only be performed shortly
before spreading, and with solid and stabilized sludge in order to reduce risks of leaching.
Figure 17: Sludge landspreading.
European Commision document of 2011 [12] describes sludge storage systems. Liquid sludge may be
stored in concrete tanks (mostly for small WWTP) or lagoons. It can be pumped to be transported.
Semi-solid sludge may be stored on a platform, which must be waterproof, or in tanks. Sludge pits may
48
also be found. As in most cases this type of sludge cannot be pumped, sludge has to be conveyed by
using specific hauling equipment such as grabs. Odors may arise when sludge is handled to be
conveyed. The structure of solid sludge enables storage on piles, and its handling implies the use of a
crane or a tractor.
Dried sludge does not present any specific constraint. If sludge however is pulverulent, storage must
be monitored in order to prevent any explosion and emission of particles to air.
Transportation is the most expensive aspect of this route. It is possible to use tankers for liquid sludge
or articulated trucks for other sludge types.
Sludge can be applied to the fields by using trailer tank or umbilical delivery system and may be applied
by surface spreading (it is however of importance to reduce the formation of aerosols to reduce the
risk of odor nuisance) or directly injected into the soil. Dried sludge may be supplied by using the same
equipment as for solid mineral fertilizers. The spreading equipment has also to be adapted to the type
of sludge.
Culture types, soil occupation, accessibility of the field, meteorological conditions influence
landspreading. Mostly, the practice can be performed at two times in the year: at the end of summer,
after harvesting, or in spring, before ploughing and sowing.
The cost of this route may be cheaper than other disposal routes.
However, the presence of pollutants in sludge implies that the practice should be carefully done and
monitored. To this purpose, in some countries, codes of practice and spreading schemes have been
established, summarizing the regulatory obligations. Periods for spreading, types of culture, adequate
record keeping are described in order to manage the sanitary and environmental risks.
3.2.3 Phosphorus recovery
Chemical analysis of sewage sludge shows its high content in potentially valuable phosphorus.
Phosphate demand is high for the manufacture of fertilizers, animal feed and detergents. The number
of phosphate geological reserves is limited, and once phosphor enters rivers or sees is no longer
economically recoverable.
Sufficient phosphorous remains in currently exploited continental phosphorous reserves for worldwide
use for around 360 years. However, the quality of this phosphorous is declining, particularly for raw
phosphate that is obtained from sediment reserves, owing to increasing contamination from toxic
heavy metals (mainly cadmium: up to 147 mg per kg of phosphorous) and radionuclei (mainly uranium:
up to 687 mg/kg of phosphorous) and the consequent environmental and health risks [10].
Worldwide phosphate fertilizer demand is set to increase by two per cent annually (i.e. around four
million tons a year), with around 90% of this demand stemming from Asia and North America [55]. The
most important drivers of this trend are world population growth and efforts on the part of developing
nations to achieve a high standard of living.
Around 90% of phosphorous reserves are controlled by only five nations, and nearly half of the world’s
proven continental phosphorous reserves are located in Africa (Figure 18).
35% of proven phosphorous reserves are located in China and the US, which themselves need a large
amount of phosphorous, thus these reserves are available for global trading to only a limited extent.
49
Figure 18: Global distribution of explored raw phosphate reserves as of 2013 [56].
Combination of thermo-chemical treatments allow the phosphorus solubilization process, which
releases the element to a supernatant. These processes produce mainly calcium phosphate and
magnesium ammonium phosphate (MAP). The first chemical is readily recyclable in industries, since it
is the same substance found in mined phosphate. The second one, which is generally referred to as
struvite, is a good fertilizer due to its slow release properties, and is applicable directly on the soil.
When it comes to substance recycling for electro-thermal phosphorous manufacturing purposes, the
molar Fe:P ratio needs to be lower than 0.2 [10]. On the other hand, recycling substances from sewage
treatment plants that use biological phosphorous precipitation has proven to be very cost effective.
Further research in this domain is currently ongoing via various research projects.
Wet chemical processes using magnesium ammonium phosphate (MAP) as a precipitate, as well as
thermal metallurgical processes, are regarded as being particularly promising [10]. The MAP process
allows for the recovery of around 40 to 70% of the phosphorous contained in wastewater treatment
plant sewage input, and allows for production of a low pollution nitrogen phosphate fertilizer, as well
as a highly suitable raw material for fertilizer manufacturing, both of which are outstanding particularly
owing to their good phyto-availability. However, the residual organic content of MAP fertilizers is
relatively high, depending on the gradient of the subsequent purification process.
Although thermal-metallurgical processes are more technically complex than MAP precipitation, they
allow:
recovery of more than 90% of the phosphorous in wastewater treatment plant sewage input;
concurrent use of the thermal energy in sewage sludge;
elimination of the organic pollutants in sludge during incineration.
Most of these methods are still studied only at laboratory or pilot scale, because of their high energy
and economic cost. In fact, the cost of phosphorus recovery is estimated as 22 times higher than the
50
cost of mined phosphorus [53]. In addition, on one hand the progressive exhaustion of phosphorus
mines will result in a global rise in phosphorus price in next decades, and, on the other hand, limits on
sewage sludge landfilling will rise sludge disposal cost, resulting in a higher industrial appeal for these
processes over upcoming decades.
3.2.4 Material recovery from Ash
The output of sewage sludge incineration process is Incinerated Sewage Sludge Ash (ISSA).
As for common sewage sludge, the major elements in ISSA are silica, aluminum, calcium, iron and
phosphor [53]. The difference in trace elements composition is due to the partial or complete
volatilization of metals such as mercury, cadmium, antimony and lead during combustion. However,
these metals presence depends on combustion process and is highly variable in literature. Ash mean
particle diameters range from 8 to 263 µm, with particle sizes ranging from submicron to around 700
µm. The pH of sludge ash can vary between 6 and 12, with a general alkaline behavior.
ISSA is commonly landfilled with high disposal costs and environmental impact. Since it is basically
waterless, it can be recycled in more ways than common sewage sludge, particularly for what concerns
the construction industry.
3.2.4.1 Production of Sintered materials
Production of sintered materials is favored by ISSA elemental composition. In fact, during sintering, the
formation of a liquid phase highly reduces the temperature and time necessary to create sintered
products [53]. Sintering is a step involved in most of the ceramic industry processes.
Possibilities for sewage sludge ash recycle involve the production of:
Bricks, tiles and pavers, substituting clay with ISSA;
Lightweight aggregates, which reduce concrete density and improve thermal insulation (these
materials are of high value because of scarcity of natural alternatives);
Glass-ceramics;
Lightweight aerated cementitious materials.
3.2.4.2 Phosphorus recovery
Options for recovering phosphor from sewage sludge as exits from wastewater treatment plants have
high disadvantages due to high water and organic matter content.
Phosphorous recovery from sewage sludge ash is also possible, but sewage sludge needs to be
incinerated separately owing to the fact that it contains relatively high phosphorous concentrations, as
well as manageable levels of pollutants such as heavy metals.
ISSA is dry and in form of powder, and this greatly simplifies phosphate extraction processes.
Furthermore, incineration does not lower sewage sludge fertilizing potential, while phosphate is
thermally stable up to high temperatures. This means, it does not volatilize during incineration at 800-
900 °C.
Phosphorous recovery from sewage sludge ash is a viable option for country were mono-incineration
of sludge has an important share on the sludge disposal routes such as German. All of Germany’s
sewage sludge is incinerated separately (around 2 million tons of dry mass annually) via mono-
51
incineration, around 66,000 tons of phosphorous could potentially be recovered from the residual ash
[10]. This represents around 55% of agricultural use of mineral phosphorous in the country.
Most promising methods for phosphor recovering are recovery by acid leaching, recycling of acid
insoluble ISSA residue and thermal methods. As for processes described for sewage sludge, these
methods encounter high energy and chemical costs. In literature it is supposed they will become more
attractive as both phosphate prices and ISSA disposal costs continue to rise.
3.2.4.3 Other recycling and recovery options
Zhang et al. [57] considered untreated ISSA a good amending material, thanks to minor nutrients
concentration. A focus put on these nutrients solubility and release rates [58] highlights some limits to
this solution. Moreover, heavy metal content limits direct application to soil in many countries.
Lin et al. [59] studied the combination of ISSA with Ca(OH)2 or cement for soil stabilization applications.
ISSA has also been used as mineral filler in asphalt production replacing limestone [60].
3.3 Energy recovery
Very briefly, the various options for the recovery of energy from sewage sludge, or better from its
organic compounds, can be subdivided into six groups:
1. Biogas production
2. Mono-incineration with energy recovery
3. Co-incineration in WtE, coal-fired power plants and in cement plants
4. Pyrolysis
5. Gasification
6. Wet oxidation
Several of these treatment options are already applied in practice (mainly biogas production, mono-
incineration, co-incineration), while others are still in the research phase (pyrolysis, gasification and
wet oxidation).
As highlighted in section 3.1.2, thermal treatments can be classified as material recovery, energy
recovery or even disposal. For MSW, to assess whether a process represents an energy recovery
application or not, the R1 index has been defined in the directive 2008/98/EC:
𝑅1 =𝐸𝑃 − (𝐸𝐹 + 𝐸𝐼)
0.97 ∙ (𝐸𝑊 + 𝐸𝐹)
In which:
- EP is the energy produced yearly in the form of heat or electricity;
- EF is the energy fed yearly to the system with fuels, contributing to the production of EP;
- EW is the yearly energy contained in waste, on the base of its LHV;
- EI is energy yearly imported, other than EW and EF;
- 0.97 is a factor accounting for energy loss due to ash and radiations.
52
All the energies must be expressed in terms of primary energy, multiplying electric energy by a factor
of 2.6 (38.5% efficiency) and thermal energy by a factor 1.1 (efficiency 90.9%).
To get the status of energy recovery, R1 must be greater than 0.6, for plants authorized before 2009,
and greater than 0.65 for post 2009 plants. However, unfortunately, in Guidelines on the interpretation
of te R1 efficiency formula, it is clearly stated that: “The R1 formula does not apply for co-incineration
plants and facilities dedicated to the incineration of hazardous waste, hospital waste, sewage sludge or
industrial waste” [61].
In the next paragraphs, the various options are further discussed and assessed.
Energy recovery from sludge is what this thesis is highly more focused on. The review presented in this
chapter is important for the subsequent thermal conversion process models in chapter 6 and 7.
3.3.1 Biogas production
Production of biogas from sewage sludge is already applied worldwide on small, medium, and large
scales. In this case, biogas is intended only as the gas produced in the anaerobic digestion process.
It must be underlined how the biogas production from sludge, or the sludge digestion, on one hand,
does not represent a final disposal option, as the digestate (digested sludge) must be disposed of, still.
On the other hand, it allows further energy recovery possibilities, although it will affect the choice and
the performance of the subsequent treatment.
To underline the previous considerations, the anaerobic digestion process description is already been
dealt with in the treatment section (section 2.2.1.1), to which the biogas production refers.
Brief calculations about sludge digestion are reported in chapter 5.
3.3.2 Mono-incineration
3.3.2.1 Literature review
Incineration of sewage sludge is aimed at a complete oxidation at high temperature of the organic
sludge compounds, also including the toxic organic ones. The process can be applied to either
mechanically dewatered sludge or dried sludge.
Potential environmental problems related to sludge incineration are the emissions of pollutants carried
in the exhaust gases to the atmosphere and with the quality of the ashes. However, there is a lot of
standard technology available to abate the gaseous emissions very efficiently, so that the stringent air
quality standards can be met [62].
However, since, in general, the incineration process deals with large quantities of polluted exhaust
gases, the costs of an efficient and adequate gas treatment system are very high. This is the main reason
that sludge incineration is rather expensive.
The ash quality, especially with respect to heavy metals in the ash, is not a real environmental problem.
Because of the high temperatures applied in the incineration process and the composition of the
inorganic compounds in the sludge, the heavy metals are very well-immobilized and resistant to
leaching. This ash has to be disposed of or can be used as a source for the production of building
materials.
53
The energy produced in the incineration process can be used for the drying of the mechanically
dewatered sludge cake prior to the incineration process and/or can be used for the production of
electricity. Currently, sludge incineration processes are increasingly focused on the recovery of energy
from the sludge in the form of heat (steam) or electricity. The amount of energy that can be obtained
strongly depends upon the water content of the sludge and the modification and performance of the
incineration, mechanical dewatering, and drying processes. Incineration of sludge is applied worldwide,
currently, more and more in combination with energy recovery. The process is mainly applied on a large
scale.
Combustion is the currently used thermal treatment method for sludge energetic valorization. The
amount of sludge incinerated in many EU countries before 2004 had already reached the percentage
of 20% of the sludge produced, while in the USA and Japan the percentage had reached the 25% and
55% respectively [63]. Wet or dry sludge combustion (with a 41-65 wt% content of dry material) can be
effectively introduced in fluid bed combustion reactors [64]. Modern fluidized bed incinerators have
become more and more attractive, both in terms of capital and operating costs, in comparison to the
conventional multiple hearth type [22].
Incineration technology is the controlled combustion of waste with the recovery of heat to produce
steam that in turn produces power through steam turbines [8]. Incineration still remains the most
attractive disposal method for sewage sludge in Europe, especially in most industrialized countries.
Moreover, EC [8] predict a slight increase of incineration on the share of sludge disposal methods for
countries in which the share of incineration is already high, like Germany with 50%. Moreover, for
country in which incineration share is almost zero, it is expected a considerable increase in the
incineration share.
Having in mind the strict limitations concerning both sludge landfilling and agricultural reuse,
incineration is expected to play a key role in the long term [65].
The advantages of incineration can be summarized as:
Large reduction of sludge volume.
Thermal destruction of pathogens and odors minimization.
Recovery of renewable energy.
The drawback of incineration is that it is the route used for sludge minimization rather than for a
complete disposal, since 30 wt% of the dry solids remain finally as ash. Combustion ash is a potential
hazardous waste due to its content of heavy metals. Additional expenses are thus required for ash
handling and disposal [66], although there are opportunities for ash utilization in the production of
construction materials [65].
Another major constraint in the widespread use of incineration is the public concern about possible
harmful emissions. However, new technologies for controlling gaseous emissions, such as the one
studied in [65], can minimize the adverse effects mentioned beforehand. Further reduction of the
combustion gas cleaning costs would give incineration considerable advantages in future [51].
54
3.3.2.2 Technology selection and description
According to BREF document [51], the BAT for sludge incineration prescribes a fluidized bed furnace
(FBF), since it shows a higher combustion efficiency and lower flue gas volume (lower excess air
requirements). In addition, to be BAT, the energy required for the sewage sludge drying must be
provided by heat recovered from the incineration to the extent that additional combustion support
fuels are not generally required for the normal operation.
Various options are available for the flue gas heat recovery. As main examples, flue gas heat can be
used to preheat combustion air, even until the technological limit of 650 °C, and then to dry the sludge
in a lower temperature heat recovery; alternatively, it can be exploited to produce steam in a boiler
and the steam produced is used for sludge drying.
The most important use of waste heat is primary recovery to reduce or eliminate auxiliary fuel
requirements for combustion. The most common form of primary recovery is for preheating of the
combustion air to the system [67]. Supplementary fuel consumption depends on two factors: the heat
content of the feed material and the heat content of the combustion air, which depends on how intense
heat recovery is in the heat exchanger. The greater the solids content and the greater the combustion
air temperature, the lower the auxiliary fuel requirement.
Water Environment Federation of Virginia (USA) [67] shows that fluid bed incinerators can take
advantage of preheat temperatures as high as 650 °C. An example of application is the Puerto Rico
incineration plant (Figure 19), while in Figure 20 a scheme of the incinerator with the air box is shown.
Figure 19: The Puerto Rico fluid bed incineration plant.
Also in Japan MITSUBISHI HEAVY INDUSTRIES ® [68], fluidized bed type sewage sludge incinerator with
combustion air heated to 650 °C is used all around the Japan country in different scales ranging from 5
55
ton/d to 300 t/d. The same source also underlines that circulation in the fluidized bed is not necessary
when dealing with incineration of sludge at temperature higher than 850°C.
Figure 20: A typical cross-section of a fluid bed.
A conventional Ljungstrom type air preheater is not adequate for such high temperatures, for which
the use of ceramic materials for the heat exchanger may be required [69].
For this application a gas-gas ceramic recuperative heat exchanger, as the Cowper stoves type (Figure
21, left), can be selected, although it is a costly component. An alternative to Cowper stove can be
represented by the so-called flue-gas-through-tube (FGTT) for which ceramic material is no more
required and as it can be made of stainless steel or alloy 20 (Figure 21, right).
56
Figure 21: Hot blast stove or Cowper stove, on the left, and flue-gas-through-tube (FGTT).
In most recuperators, a hot, dirty flue gas stream flows through tubes, while combustion air passes
over in multiple, cross-counterflow passes [67]. The axial (straight) flow of dirty gas through the tubes
solves several problems. Because particulate matter in gas is carried parallel to the tube wall, abrasive
impingement and erosion is minimized. Further, vertical tubes do not offer areas on which ash can
collect and they minimize damage from thermal expansion.
In a typical FGTT recuperator, insulating refractory linings are typically required for the entire casing
and hot face on the tube sheets. Dense, abrasion-resistant refractories help avoiding erosion from
particulates in flue gas. Vapor barriers or coatings on interior carbon steel surfaces minimize acid attack
on flue gas plenums.
What makes the investment costs of the recuperative air preheater effective is the auxiliary fuel saving:
Wastewater solids incineration text [67] shows that preheating combustion air to 650 °C reduces
auxiliary fuel requirements to levels from 5 to 35% of those without preheat. It is also shown that it
takes approximately 23.5% of the energy in the furnace exhaust flue gases to preheat combustion air
to 650 °C, which means cooling exhaust flue gases to approximately 540 to 600°C. This range is well
within the capability of the equipment and current heat exchanger designs.
Since the limit in flue gas cooling is set to a minimum of about 180 to 200 °C [51, 67] because of acid
condensation problems, a further (secondary) heat recovery is possible.
A variety of heat recovery systems that can take advantage of energy in exhaust flue gases from
fluidized bed systems downstream of the primary air preheater exists. Among them, again to eliminate
auxiliary fuel consumption, the heat recovered can be used to dry the sludge to the desired extent.
However, following this route for flue heat recovery, further energy is hardly recoverable.
An example of incineration facilities with net energy output is the Outotec Sewage Sludge Incineration
Plant 100 [70] (Figure 22), completed in 2015 for the Disposal and Recycling Department of Zürich. It is
57
designed to handle all the sewage sludge produced in the Zürich area, which amounts to 100,000 metric
tons a year, and is Switzerland’s largest thermal sewage sludge treatment plant.
Sludge, dried to a certain extent, is burned in a fluidized bed incinerator, whose combustion air is not
preheated. The flue gas thermal energy is exploited in a waste heat recovery boiler, in which
superheated steam is produced. The steam produced generates electrical power in a steam turbine. At
the turbine outlet, part of the steam is sent to the disc dryer where the sludge has to be dried, while
the rest is used for the district heating. After the waste heat recovery boiler, the flue gases are treated
and purified in the air pollution control line.
The fluidized bed incinerator technology is the one provided in the BREF document [51], and described
above.
According to Wastewater Solids Incineration Systems [67], the waste heat recovery boiler is of water
tube type. In general, the usual inlet temperature of waste gases is between 540 and 980 °C, depending
on the type of incinerator, the presence of an afterburner, and of the air preheater.
Figure 22: Outotec Sewage Sludge Incineration Plant 100.
Hot gases are in contact with the outside surface of the tubes and boiler water and steam are in contact
with the inside surface of the tubes.
The boiler exit, metal surface of the casing and tube walls temperature must be maintained at a value
greater than the flue gas acid dew point, which can be as high as 120 to 180 °C, to prevent corrosion.
The range of boiler sizes ranges from approximately 2300 to 23000 kg/h of steam. In the Outotec plant,
the steam pressure is 60 bar, and the superheating temperature is 450°C.
The choice of the heating medium for the drying process is the steam since it is already available at the
plant site, so, it is necessary to use an indirect dryer, in particular of rotary disc type (section 2.2.5).
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3.3.3 Co-incineration
3.3.3.1 Literature review
Co-incineration consists of using already existing installations, usually the ones designed for municipal
waste incineration, limiting additional investment. The technique is especially attractive if the
incinerator is settled near the WWTP [12].
The advantages of co-firing include the use of the available capacity of the combustion plant with well-
trained and experienced personnel to handle it, providing a reliable short term disposal opportunity
especially in cases where obtaining approval for the construction of a sewage sludge incinerator is long
and tedious. In addition, modern coal power stations are currently equipped with a flue gas cleaning
facility, which should be able to handle the expected increase in emissions during co-firing with sewage
sludge.
A drawback of co-incineration is that it precludes recovery of the phosphorous in sewage sludge from
the generated ash (chapter 3.2.3).
For what concerns sludge co-incineration in Waste to Energy plant, the objective is to reduce the
combined costs of incinerating sludge and solid wastes. The process can generate sufficient heat energy
necessary for drying the sludge, supporting the combustion of solid wastes and sludge, and generating
process steam, if desired, without the use of auxiliary fossil fuels. Sludge pre-drying enables the solid
contents of the sludge to correspond to that of the waste, which is around 55–65% [12], to increase
the heating values of the sludge and enable auto-thermal combustion.
Furthermore, co-firing of sewage sludge with MSW could exploit the capacities of several MSW
incinerators available with modern flue gas cleaning technology. However, it is thought that most of
the MSW incinerators currently operate at full capacity and may not provide the opportunity for co-
incineration with sewage sludge: in these conditions, new MSW incinerators could be planned with co-
firing [71].
If the calorific value of the sludge is similar to that of municipal wastes, sludge can easily be added to
the waste. When sludge is dry, it must be carefully mixed to the waste, to avoid accidents, such as
explosions, during combustion.
It is also possible to introduce thickened sludge, reducing the treatment costs (dewatering and/or
drying costs). In this case, however, a reduced calorific value implies to restrain the proportion to waste
(about 20% of the tonnage). There are different techniques for injecting the sludge: sludge can be
mixed before the combustion with the waste, injected under pressure in the furnace or at the exit of
the combustion chamber.
According to the BREF for waste incineration [51], sewage sludge is sometimes incinerated with other
wastes in grate municipal waste incineration plants. For co-firing with municipal wastes in existing
waste incinerators, no extra approval is required.
The investment costs are much lower than in the case of mono-incineration, as the process only needs
a modification of an existing installation (sludge injection system, eventually sludge treatment on site).
Werther and Ogada [71] reported several examples of sludge and MSW co-firing plants in the world,
with both grate multiple hearth and fluidized bed technologies.
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Lin et al. [72] verified the feasibility of wet and semi-dried sludge co-incineration with MSW through
computational fluid dynamic analysis. Different amounts of sludge in the co-firing blend were tested.
They concluded that, owing to higher moisture content than that of MSW, the addition of wet sludge
greatly delays the ignition point, therefore is recommended to add no more than 10% of wet sludge to
the blend; for semi-dried sludge, instead, 20% is acceptable. The addition of wet sludge also delays the
devolatilization onset, shortens the devolatilization stage and advances the char burning, showing a
deeper effect with respect to semi-dried sludge. Consequently, they assessed that with lower moisture
content and higher LHV, semi-dried sludge is more appropriate for co-incineration with MSW in grate
furnace incinerator.
Besides the co-incineration in WtE plant, on which the present work is focused, sewage sludge can also
be incinerated together with coal and in other processes, mainly cement kilns.
For co-incineration of sewage sludge with coal, the addition of component for fuel preparation and
modification of the combustion and pollutant control system have to be considered. In most
applications, sludge must be pre-dried, and often milled. Dried sludge also requires special handling
during transportation, feeding and storage, since the dusts show high potential for self-ignition and are
explosive.
The sludge high contents of nitrogen, sulphur and heavy metals are likely to lead to an increase in the
pollutants emissions (NOx, SO2 and heavy metals), leading to more strict emission limits than normally
applicable in coal power plants.
As described by Werther and Ogada [71], in pulverized coal power plants, the coal is pulverized so that
90% of the particles are smaller than 75 µm diameter, and carried in an air suspension to be fed into
the combustion chamber. Several burner and firing modes are used to enable high efficiency
combustion and low emissions. The sewage sludge for co-combustion is dried, pulverized and
pneumatically fed to the burners, pre-blended with coal and fed together, or fed separately. The sludge
incineration takes place at high temperature, and the ash from sludge and coal is removed in a molten
form.
When burnt with bituminous coal, the boilers can tolerate sewage sludge with water contents of up to
10%; with brown coal, sludge moisture contents of up to 40-50% are acceptable, because the boilers
are designed to operate with relatively highly moist fuels.
Also sludge co-combustion with coal in fluidized bed furnaces has been studied. Leckner et al. [73]
investigated co-combustion of sewage sludge with coal or wood in a circulating fluidized bed (CFB)
boiler, focusing on emissions of trace metals. The study shows that EU emission limits are not exceeded
for practically interesting sludge energy fractions and how the ashes are enriched by trace elements
with increasing share of sludge.
In the Otero et al. [74] work, the co-combustion of several types of sewage sludge, also pyrolyzed, with
coal has been analyzed through TG-MS. They conclude that co-combustion with coal may provide an
attractive option for the disposal and utilization of a renewable waste resource such as sewage sludge
in an economic and environmentally safe manner. They show how, in mixtures of coal and sewage
sludge, the combustion starts at lower temperatures with respect to coal combustion, because of the
60
early volatiles liberation from the sludge, and this could mean a great advantage when attempting to
initiate the combustion process. On the other hand, the mixture may decrease the unburned solids
generated in the combustion kilns as for sludge combustion ends at lower temperatures than that of
coal. Nevertheless, it is necessary to know very well the behavior of both combustible materials
together and alone to avoid uncontrolled combustion problems at low temperatures: this could be
solved simply by having separate feeding systems. This would not occur in the case of using pyrolyzed
instead of fresh sewage sludge, as its combustion, although less energetic, is more similar to that of
coal in terms of temperature range of combustion.
Calvo et al. [75] studied the kinetic behavior of sludge co-combustion with coal in parallel with TGA:
their results show that, as maximum amount of sludge in the blend is 10%, the kinetic is analogous to
the combustion of coal alone and that TG represents an easier and well explicating tool for the process.
Folguera et al. [76] work has the objective of studying the combustion of bituminous coal, three types
of sewage sludge and their blends by TG, evaluating the interactions between the blend components.
Several comprehensible differences were found between the combustion profiles of the samples. In
general, the regions of organic matter combustion of sludge shift to lower temperatures than the
corresponding ones for coal; the temperature ranges of these regions are broader for sludge than coal;
and these regions are more complex for sludge (two or more peaks) than for coal (one peak). The
sludge-coal blends show an intermediate behavior between sludge and coal, which may be predicted
from the weighted sum of the blend components.
Stasta et al. [77], instead, study sludge co-firing in cement kilns from an energy, environmental and
economic points of view, assessing that co-firing of sewage sludge in cement works using excess heat
can be considered as one of the most appropriate solutions of sludge treatment both for WWTPs and
cement works.
Pre-dried sludge (90% DS) can be co-fired with coal in main firing or secondary firing stages, although
experience shows that co-firing of sewage sludge in the secondary stage results in incomplete
combustion of the sludge particles and higher CO emissions.
Important advantages of this kind of application is the possibility to use the waste heat generation for
sludge drying, analogously to mono-incineration process. Moreover, ash from sludge incineration have
a similar composition to the one of raw material ash, and are bound to clinker, avoiding its disposal
needs. A significant amount of raw material can be saved, in the proportion of 1/3 saved tons per sludge
ton used.
However, the maximal sewage sludge feed rate should not be more than 5% of the clinker production
capacity of the cement plant [76]. The important restriction of the sludge/coal ratio is the emission of
harmful substances with the heavy metals and dust. Their concentrations in the flue gas should meet
the environmental regulations. Obviously, other factors influencing the co-combustion process is the
change of physical and thermal properties of the fuel: heating value, moisture content and ash
composition. These influence the thermal output, the amount of air required for combustion, the
volume of the flue gases, dust concentration and particle [64].
It is also possible to co-fire sludge in clay brick manufacture.
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3.3.3.2 Technology selection
Sewage sludge co-incineration in waste to energy plant appears to be the most interesting co-
incineration route, because the LHV is quite similar to the MSW one, making the required drying extent
lower with respect to the case of co-firing with coal, and sludge milling is not necessary. Moreover, due
to the strong increase of Renewable Energies in the last years, the use of coal-fired power stations is
getting more and more irregular.
The case of MSW has been also preferred with respect to the co-incineration in cement kiln, brick
manufacture or other processes, as the co-incineration feasibility and amount of sludge effectively
disposed of depends greatly on the local situation and the availability of this kind of facilities in the
wastewater treatment neighborhood or in the sludge management zone of interest, and the particular
technology present.
It must also be considered that technological and spatial requirements for a sewage sludge acceptation
and co-incineration are not present in every facility.
For the waste to energy plant technology description, the reference is the the BREF for waste
incineration [51]. Here, for conciseness, only the main modifications of the plant, required for sludge
co-incineration adaptation are presented.
According to the the BREF for waste incineration [51], when sewage sludge is added to MSW it is often
the feeding techniques that represent a significant proportion of the additional investment costs.
The following three supply technologies can be used:
dried sewage sludge (90% DS) is blown as dust into the furnace;
drained sewage sludge (20-30% DS) is supplied separately through sprinklers into the
incineration chamber and distributed on a grate. The sludge is integrated into the bed material
by overturning the waste on the grates;
drained, dried or semi-dried (50-60% DS) sludge is mixed with the remaining waste or fed
together into the incineration chamber. This can occur in the waste bunker through targeted
doses by the crane operator, or controlled in a feeding hopper by pumping dewatered sludge
into the hopper or by spreading systems into the bunker.
The feeding option chosen in the PAI plant, taken as reference, is the third one, with sludge that has to
be dried to 65% DS, at least for a portion of the sludge delivered. The other part is fed through the
second option, for the furnace temperature control.
Operational experiences show up to 20% sludge in mass, otherwise sewage sludge can clump together
and not burn out. In addition, if the sludge ratio is too high (e.g.>10%.), high fly ash content or unburnt
material in bottom ash may occur. Another issue is the risk of dried sludge sifting on the grate.
The sludge dryer is the main and most evident component that needs to be added to the facility. As
shown in the section 2.2.5 of this work, sludge drying can be achieved through several different
technologies. However, since the plant produces steam, the natural choice is to bleed it from the
turbine and use it for drying, either through an indirect dryer (for example a disk dryer), or through a
convective dryer in which the drying air is heated by the steam. The second option is the one applied
62
to the PAI plant in Parma. In this case also the addition of the steam-air heat exchanger has to be
considered. Choosing the steam to perform the sludge drying also involves the installation of a bleeding
system at an appropriate section of the steam turbine and piping.
Although the pollutant emissions are expected to worsen, as reported in literature, to a certain extent,
the flue gas treatment line is not supposed to require significant modifications.
3.3.4 Pyrolysis
3.3.4.1 Introduction and explanation of the processes
Pyrolysis is the thermal decomposition of fuel into liquids, gases, and char (solid residue) in the absence
of oxygen, in an inert atmosphere.
The absence, or a very limited amount, of oxidizing agent does not permit gasification to an appreciable
extent. The difference from combustion is that the products still have a certain LHV, are more refined
and can be used in a more efficient way [78].
From a chemical point of view, pyrolysis consists in large hydrocarbon molecules breakdown into small
molecules of methane, hydrogen, carbon monoxide, carbon dioxide, steam, phenol, acetic acid,
benzene, …
This breakdown can be subdivided into a primary and a secondary one:
Figure 23: Pyrolysis in a biomass particle [79]
The pyrolysis process may be represented by a generic reaction such as:
𝐶𝑛𝐻𝑚𝑂𝑝 + ℎ𝑒𝑎𝑡 → ∑ 𝐶𝑎
𝑙𝑖𝑞
𝐻𝑏𝑂𝑐 + ∑ 𝐶𝑥
𝑔𝑎𝑠
𝐻𝑦𝑂𝑧 + ∑ 𝐶
𝑠𝑜𝑙
+ 𝐻2𝑂
The scheme of pyrolysis plant is reported below.
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Figure 24: Pyrolysis plant scheme [79]
Pyrolysis products are solid, liquid and gaseous, and the production of each of them is enhanced in
defined ranges of design conditions.
The solid product is char; the liquid is bio-oil, a black and tarry fluid, made of water, phenolic
compounds, complex hydrocarbons, oxygen. The gas products are distinguished between primary
gases, which are the non-condensable gases produced by the primary cracking, and secondary gases,
that are the non-condensable gases produced by the secondary cracking of condensable gases out of
the primary breakdown. Gaseous products after the secondary cracking are made mainly of CO2, CO,
CH4, C2H6.
All the pyrolysis products can have a potential use. With the suitable composition, both the gaseous
and liquid products can be used as fuel or as feedstock for chemicals production. The solid product can
be used more likely in agriculture or as adsorbent, than for further energy recovery [80]; according to
Agrafioti et al. [81], biochar is getting the attention of both the political and scientific community due
to its potential to improve soil productivity, remediate contaminated soils and mitigate climate change;
it is environmentally resistant and holds potential for carbon sequestration, soil conditioning and
adsorbent production [82].
Main factors determining different product distributions and characteristics are process temperature,
residence time in the reactor, heating rate, pressure, turbulence, reactor type and configuration and
raw materials’ characteristics (sludge type and pretreatment, ash and volatiles content) and feed rate
[78].
Temperature range varies from 300 °C to 900 °C and depends on residence time. Optimum process
parameters depend on experimental scale and specific technique.
Process variables differ depending on the final product desired. Even though pyrolysis is generally
aimed to the production of liquid products via liquefaction, other two routes optimize the production
of solid products (carbonization) or biogas (gasification) [79].
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It should be noted that the liquid product from pyrolysis can be easily stored and transported, while
the gaseous products, as well as syngas from gasification, need to be used on site for further energy
production [78].
Pyrolysis can be performed in a very large variety of ways, depending on the set design conditions, but
three different categories can be identified:
Slow Pyrolysis: conventional or slow pyrolysis is characterized by slow biomass heating rates, low
temperatures and lengthy gas and solids residence times. Gas residence time may be greater than five
seconds while that of the biomass can range from minutes to days. Depending on the system, heating
rates are about 0.1 to 2 °C/s and prevailing temperatures are less than 400-500 °C [83]. During
conventional pyrolysis, the biomass is slowly devolatilized; hence, tar and char are the main products.
After the primary reactions have occurred, re-polymerization or recombination reactions are allowed
to take place [84].
- Flash Pyrolysis: it is characterized by moderate temperatures exits (400-600 °C) and rapid
heating rates (>2 °C/s). Vapor residence times are usually less than two seconds. Compared to
slow pyrolysis, considerably less char and gas are produced. However, the tar and oil products
are maximized.
- Fast Pyrolysis: the only difference between flash and fast pyrolysis (more accurately defined as
thermolysis) is heating rates and hence residence times and products derived. Heating rates
are between 200 and 105 °C/s and the prevailing temperatures are usually higher than 550 °C.
Due to the short vapor residence time, products are high quality, ethylene-rich gases that could
subsequently be used to produce alcohols or gasoline. Notably, the production of char and tar
is considerably less during this process [84].
However, pyrolysis temperature can also be set at a much higher temperature, with respect to the 600
°C of the previous descriptions, as it can be seen in the following section (3.3.4.2), with temperatures
usually typical of gasification process, until 1000 °C.
The effect of heating rate is explained in the work of Sadaka et al. [84], which states that the yield of
volatile products (gases and liquids) increases with increasing heating rate while solid residue
decreases. The effect of heating rate can be viewed as the effect of temperature and residence time.
As the heating rate is increased, the residence time of volatiles at low or intermediate temperatures
decreases. Most of the reactions that favor tar conversion to gas occur at higher temperatures. At low
heating rates, the volatiles have sufficient time to escape from the reaction zone before significant
cracking can occur. Heating rate is a function of the feedstock size and the type of pyrolysis equipment.
The rate of thermal diffusion within a particle decreases with increasing particle size, thus resulting in
lower heating rate. Liquid products are favored by pyrolysis of small particles at high heating rates and
high temperature, while char is maximized by pyrolysis of large particles at low heating rates and low
temperatures, as mentioned earlier.
Accounting for the considerations above reported, operating parameters of a pyrolyzer are adjusted to
meet the requirement of the final product of interest.
Tentative design norms for heating in a pyrolyzer include the following:
65
To maximize char production, use a slow heating rate (<0.01-2.0 °C/s), a low final temperature,
and a long gas residence time.
To maximize liquid yield, use a high heating rate, a moderate final temperature (450-600 °C),
and a short gas residence time.
To maximize gas production, use a slow heating rate, a high final temperature (700-900 °C), and a long
gas residence time [29, 78].
Depending on the feedstock and on how the process is carried out, sludge pyrolysis can be either a
material recovery (production of syngas or oil as fuel or feedstock for chemicals production, char mainly
as adsorbent), an energy recovery (when the products are used to produce energy) or a disposal option
(neither valuable products, nor energy are produced). Therefore, to the intrinsic complexity of the
process, a complexity also in terms of classification, which reflects also in the creation of standards and
proper norms, is added.
3.3.4.2 Literature review
Inguanzo et al. [40] investigated the pyrolysis of sewage sludge, carried out in a laboratory furnace, and
pyrolysis conditions, like heating rate and final pyrolysis temperature, influence on the characteristics
of the resulting gases, liquids and solid residues. Temperature was varied from 450 and 850 °C, while
the heating rates considered were 5 and 60 °C/min. It was found that increasing the pyrolysis
temperature, the solid fraction yield decreases and the gas fraction yield increases, while that of the
liquid fraction remains almost constant. Furthermore, the effect of the heating rate was found to be
significant only at low final pyrolysis temperatures. Both oils and gases produced in the pyrolysis
showed relatively high overall heating values (over 20 MJ/kg), comparable to some conventional fuels,
revealing the potentiality of these products as fuels.
In the work of Gao et al. [34], dried sludge pyrolysis was analyzed through TG-FTIR-MS; the main gases
identified by FTIR analysis were CH4, CO2, CO, H2, and organic volatile compounds such as aldehydes,
acids, alcohols and phenols. Temperature was varied between 450 and 650 °C, with heating rates of 8
°C/min (slow pyrolysis (B)) and 100 °C/min (fast pyrolysis (A)).
Results of these experimentations are shown in Figure 25, and confirm the increase of gas amount
while temperature rises. More specifically, H2, CO, CH4 concentrations increase, while CO2 decreases,
showing the same trend identified by[40]. On the contrary, as expected, solid products reduce as
temperature gets higher.
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Figure 25: Temperature effect of products yields for fast (A) and slow (B) pyrolysis [34].
With the higher heating rate, the maximum tar yield obtained was 46.14% at the temperature of 550
°C.
Sanchez et al. [85] studied the effect of pyrolysis temperature, varied from 350 to 950 °C, on the oil
product characteristics. More than 100 different compounds presence was identified in tar.
Quantification of the main compounds showed that sewage sludge pyrolysis oils contain significant
quantities of potentially high-value hydrocarbons such as mono-aromatic hydrocarbons and phenolic
compounds; it was demonstrated that, as the temperature of pyrolysis increases, the concentration of
mono-aromatic hydrocarbons in the oils also increases. The trend of the different product yield with
increasing temperature from literature was also confirmed.
Nowicki et al. [47] estimated the compositions of pyrolysis products through TG-MS and atom balance
calculations, at different process stages, from ambient temperature to 1000 °C, with a constant heating
rate of 10 °C/min.
In the work of Karaca et al. [44], high temperature (1000°C) pyrolysis was tested for thermal conversion
of the sludge into syngas, at a 10 °C/min heating rate. The generated syngas essentially included 25
wt% of H2, with CO (14 wt%), CO2 (27 wt%), CH4 (10 wt%), C2H4 (2 wt%), C2H6 (1 wt%) and other
compounds (21 wt%), resulting in 9 MJ/Nm3 heating value. Experiments indicated that around 80% of
the energy in sewage sludge could be recovered and converted into syngas, highlighting pyrolysis in
such conditions as a sustainable process for energy recovery.
In Sun et al. [33] study, sewage sludge was pyrolyzed in a fixed bed reactor, using composite alumina
(CA) as catalyst. The effects of temperature (from 400 to 600 °C) and CA additive ratio on the products
were investigated. The product yields and component distribution of non-condensable gas were more
sensitive to the change of temperature, and the maximum liquid yield of 48.44 wt%, with the maximum
usable energy of 3.87 MJ/kg of sludge were observed at 500 °C with 1/5 CA/SS (mass ratio). The
67
presence of CA could strengthen secondary cracking and interaction among primary products from
different organic compounds and reduce the content of oxygenated compounds.
In the study of Han et al. [39] about sludge fast pyrolysis, from 500 °C to 900 °C, the products yield
trend with temperature is confirmed. Comparing two different sludge, one biophysically dried and one
thermally dried, they prove that fast pyrolysis of BDS facilitates syngas and char formation more than
TDS. For the yielded syngas, the thermal conversion of BDS was characterized by high H2 and CH4
content.
Huang et al. investigated sewage sludge fast pyrolysis in a drop tube furnace. They aim at understanding
the effects of pyrolysis temperature and sweeping gas flow rate (SGFR) on the yields and chemical
composition of pyrolysis products. The maximum bio-oil yield reached 45.3% at 500 °C and a SGFR of
300 mL/min. They found that chemical composition of the bio-oil significantly depends on the pyrolysis
temperature: at low temperatures, the main species are alkenes, alkanes, long-chain fatty acids and
esters, aliphatic nitriles and amides; at high temperatures, aliphatic and thermally labile organooxygen
species were mainly cracked to gaseous products, while the organonitrogen species tended to form
aromatic species. They state that, because of its high nitrogen content, the sewage sludge bio-oil is not
suitable for use as fuel feedstock, but can be used as chemical feedstock.
Pokorna et al. [37] studied flash pyrolysis at 500 °C to evaluate the production of pyrolysis oil from
three types of sewage sludge. The maximum oil yield was 43.1%, and the water content in bio-oils
obtained from secondary sludge was relatively low. Results showed that pyrolytic bio-oils of studied
sludge dominantly contained fatty acids and nitrogenous compounds, with potential added value, while
the fraction of aromatic was low. Obtained solids had high ash content and low calorific value, making
them unattractive for use in incineration, but the estimated chemical features allow them to be
potentially used as adsorbents.
Also Alvarez et al. [80] stated that the maximum oil yield in flash pyrolysis is obtained at 500 °C; they
also assessed that the char fraction retains most of the heavy metals contained in the sludge.
Other relevant studies about oil products, conducted at similar temperatures, are in Shen et al. [38]
and Lozano et al. [86], that stated that bio-oil LHV ranges from 28 and 32 MJ/kg.
Zielinska et al. [87] evaluated that also initial sewage sludge properties, together with pyrolysis
temperature, affect significantly the characteristics and composition of sewage sludge-based bio-chars,
but the effect is hardly predictable. In particular, important characteristic of the bio-char regards:
chemical composition, as char can be a valuable source of mineral substances for soil microorganisms,
specific surface area, and porosity: the aim is to assess its suitability of the use in agriculture.
Results show how the biochar produced at the lowest temperature (500 °C) was characterized by
similar pH of the initial sewage sludge. An increase in pyrolysis temperature up to 600 °C caused a
significant increase in pH (up to 11.0). It was also observed that the ash content in biochar is higher in
relation to the sewage sludge, and an increase in pyrolysis temperature from 500 °C to 700 °C further
increases the ash quantity. In addition, it was found that higher pyrolysis temperatures promote the
formation of biochar with a higher contribution of nutrients. Based on the surface properties of sewage
68
sludge, it is not possible to predict the surface area of biochar, but it may be concluded that higher
surface area of the sewage sludge corresponds to more developed surface area of biochar.
The work of Agrafioti et al. [81] shows that using a heating rate of 17°C/min for the pyrolysis of a
dewatered sludge, with a residence time of 30 min, the temperature that maximizes the char yield is
300 °C; the produced char is found to have good leaching properties, and can be used in agriculture.
In the same framework, Yuan et al. [35] and Hossain et al. [43] studied the effect of pyrolysis
temperature (from 300°C to 700°C) on the produced biochar properties.
Dominguez et al. [88] carried out the pyrolysis of a wet sewage sludge as it is produced in the water
treatment plant, as an alternative to the usual pyrolysis of dried sludge. Their purpose is to study the
feasibility of performing drying, pyrolysis and gasification of wet sewage sludge in a single thermal
process at high temperatures (1000 °C), aiming at maximizing the production of a H2-rich fuel. In fact,
under conditions of high temperature, long residence time and high heating rates, the natural moisture
of the sludge is converted during the process into steam, which gives rise to the partial gasification of
the sludge and the reforming of the organic vapors at an early stage. In addition, homogeneous
reactions between non-condensable gases are also favored, especially the water gas shift reaction. To
observe the effect of the moisture content in the sludge, the experiments were run at different
moisture levels. Moreover, they studied and compared an anaerobically digested sludge and an
aerobically digested one. Their results show that the highest char yield was obtained from the pyrolysis
of the anaerobically digested sludge (L), while the highest oil and gas yields correspond to the sludge
obtained in the aerobic process (V), in agreement with the higher volatile matter content of V with
respect to L. Moreover, as aerobic digestion produces a greater degradation of the components than
anaerobic digestion, it was found that the more degraded the compounds are, the easier it is for them
to volatilize, which results in a decrease in char yield and an increase in the yield of volatiles upon
pyrolysis. Pyrolysis of the L-sludge produced a gas with a higher H2 concentration and a lower CO
concentration than that obtained from the pyrolysis of the V-sludge. The presence of water in the
sludge increases the production of gases and contributes to the formation of gases at lower
temperatures than when the pyrolysis is carried out on dry sludge. The steam generated during the
treatment reacts with both the vapors (steam reforming) and the solid residue (steam gasification)
produced, resulting in an increase in the hydrogen production.
Also Xiong et al. [32] tested sewage sludge with different moisture pyrolysis at 1000°C (Figure 26). The
large amount of steam generated by the high moisture content of sewage sludge at high temperature
not only increased the production of hydrogen rich fuel gas, but also reduced the solid yield due to the
steam gasification and steam reforming reactions. However, they show that the increase in the
production of H2 was insignificant as the moisture content increased from 47% to 80%, which indicates
that the steam involved in the reactions has a saturation point.
69
Figure 26: The effect of moisture content on the yields of pyrolysis products [32]..
The same mechanism of reaction was shown in the work of Zhang et al. [89], that analyzed pyrolysis of
wet sludge between 600 °C and 1000 °C.
Yu et al. [90] studied microwave-assisted pyrolysis and compared the effect of six different catalysts,
which showed the effect of a faster sludge temperature rise in the process and in syngas composition.
In their study, Zhang et al. [91] performed a co-pyrolysis of sewage sludge and biomass (rice husk).
Special experimental conditions (vacuum reactor, long contact time and high temperature) were
applied. Synergetic effects for this process were observed. Sewage sludge provided more CO2 and H2O
during co-pyrolysis, promoting intense CO2-char and H2O-char gasification, which benefited of the
increase of gas yield and lower heating value.
Zajec [92] master thesis deals with the slow pyrolysis process in a rotary kiln reactor, with an integrated
small size gas burner. A scheme of the reactor is in Figure 27.
Figure 27: Rotary kiln reactor [92].
70
Although the studied feedstock is beech, and not sludge, this work is particularly interesting for the
present study purposes because of design process parameters. Being a slow pyrolysis, the maximum
temperature in the reactor is 450 °C and the residence time is 2 h. The process results to produce all
the three products of pyrolysis. The syngas composition was obtained from a gas chromatograph
analysis, and the evaluated syngas LHV was 5.92 MJ/kg, in accordance with the values in literature. The
estimated efficiency of the pyrolysis reactor is 0.68.
Many studies, mostly of experimental nature, have been dedicated to sludge pyrolysis kinetic [93], [94],
[95], [96], [97], but for the complexity and the case-by-case dependency are not reported here in detail.
Samolada et al. [82] performed the evaluation of three thermal technologies as potential sludge-to-
energy valorization methods. Pyrolysis was identified to be a promising sludge treatment method. One
of the main reasons supporting this conclusion is that pyrolysis is a zero waste method having a greater
potential in the solution of the wastewater problem, compared to other methods, and is characterized
by lower and acceptable gas emission. Sludge pyrolysis is an innovative process that can convert both
raw and digested sludge into useful bioenergy in the form of oil and gas and forming bio-char as a
byproduct. However, also a barrier for pyrolysis viability is identified: challenge of finding markets for
the solid and liquid products. Char for use as a fertilizer, for soil amendment or absorbent would help
in improving the economics of these systems.
3.3.4.3 Technology selection
Since pyrolysis process can be run in an extremely large variety of ways, the technologies under study
are many, and mostly at the experimental stage, and no standards are yet available for this kind of
process, a proper technology overview is not present here. During the development of this study, two
pyrolysis facilities, one at a purely experimental stage (Pyrobio®) and a more established one
(Pyrobustor®) have been visited. The two reports that describes them are in the Appendixes, to weigh
not the discussion down excessively.
3.3.5 Gasification
3.3.5.1 Introduction and explanation of the processes
In general, gasification is the conversion of solid or liquid feedstock into useful and convenient gaseous
fuel (syngas) or materials that can be burned to release energy or used for production of value-added
chemicals. Gasification packs energy into chemical bonds in the product gas; it adds hydrogen to and
strips carbon away from the feedstock to produce gases with a higher hydrogen-to-carbon (H/C) ratio
[29]. Therefore, in comparison to sludge pyrolysis, gasification partitions most of the feedstock
potential energy into a single syngas stream, which can be prepared as an engine fuel using simpler
means than those needed for bio-oil [98].
According to Biomass gasification and pyrolysis [29], the process typically include four steps:
- Drying
71
- Thermal decomposition or pyrolysis
- Partial combustion of some gases, vapors and chars
- Gasification of decomposition products.
Gasification process consists, in practice, of a partial oxidation process, conducted with different
gasifying agents, such as air, oxygen, and steam [78]. Gasifying agents react with solid carbon and
heavier hydrocarbons to convert them into low-molecular-weight gases, like CO and H2. The choice of
the gasifying agent and the amount fed deeply affect the syngas composition and, therefore, the
heating value.
If air is used for gasification, the product is a mixture of CO, CO2, H2, CH4, N2 and tar, which has a low
heating value of about 5 MJ/Nm3 [78], leading to difficulty in combustion, particularly in a gas turbine.
If oxygen is used as a gasifying agent, N2 is absent from the gas product, and the syngas heating value
can reach about 10 to 12 MJ/Nm3. Although the use of oxygen as a gasifying agent is costly compared
to air, a better-quality fuel gas can compensate for such extra cost [78].
A ternary diagram (Figure 28) of carbon, hydrogen, and oxygen demonstrates the conversion paths of
formation of different products in a gasifier.
In the case of oxygen as gasifying agent, the conversion path moves toward the oxygen corner, leading
to a lowering of hydrogen content and an increase in carbon-based compounds (CO and CO2) in the
product gas. The relative quantities of CO and CO2 depend on the amount of oxygen fed: if it is low,
there is most of CO, and, moving from highly sub-stoichiometric conditions toward the stoichiometric
amount of oxygen, the CO2 amount increases more and more, and the process moves from gasification
to proper combustion [29].
Typical values of equivalence ratio found in literature range from 0.12 to 0.4 [99], [100].
Figure 28: C-H-O diagram of the gasification process [29].
72
If steam is used as the gasification agent, the path moves toward the hydrogen corner. Then the
product gas contains more hydrogen per unit of carbon, resulting in a higher H/C ratio. Some of the
intermediate reaction products like CO and H2 also help to gasify the solid carbon [29]. The use of steam
maximizes the methane and hydrocarbon contents in the mixed gas, with a resulting heating value that
can be as high as 15 to 20 MJ/Nm3 [78].
In general, it is implicit that operating conditions have to be optimized in order to maximize the H2 and
minimize to CO2 amounts in syngas, for the best LHV and product quality.
A significant gasification issue is the presence of tar, which is the liquid formed during the pyrolysis
phase, through the condensation of condensable gases. Since the liquid products from pyrolysis cannot
be fully utilized, the residual tar exists in the final gas product, and, being a sticky liquid, creates a great
deal of difficulty in industrial use of the gasification products.
The gasification temperature is typically not less than 700-900°C [16], [98] with the exact value
depending on the biomass specifically used, gasifying agent and amount, reactor type.
According to Sludge engineering [16], gasification works best if sludge is dried to over 90% dry solid,
but also dewatered sludge can be used (even 25% dry solid). In this case, however, additional heat has
to be provided for sludge drying.
Depending on the operating conditions, sewage sludge gasification can be an exothermic or
endothermic process [101].
A fundamental parameter of the gasification process is the Cold Gas Efficiency, CGE: it represents the
gasification process efficiency and is defined as follows.
𝐶𝐺𝐸 =�̇�𝑠𝑦𝑛𝑔𝑎𝑠 ∙ 𝐿𝐻𝑉𝑠𝑦𝑛𝑔𝑎𝑠
�̇�𝑠𝑙𝑢𝑑𝑔𝑒 ∙ 𝐿𝐻𝑉𝑠𝑙𝑢𝑑𝑔𝑒
3.3.5.2 Literature review
Sludge, with two different compositions, gasification with air in a fixed bed reactor and equivalence
ratio, oxygen concentration and air temperature effects on syngas parameters have been studied by
Werle [99]. The results show that, increasing the equivalence ratio from 0.12 to 0.18, the syngas LHV
increases, but a further increase in the equivalence ratio, until 0.27, produces the expected decrease
in syngas LHV, because of the dilution with N2. An increase in the oxygen concentration, even if small,
in the medium leads to an increase in the gasification temperature, enhancing the formation of lighter
species in the gas, finally increasing the syngas LHV. The increase in preheating temperature, from 50
to 250 °C, is found to provide the heat necessary to support the endothermic reactions of the process,
resulting again in a syngas LHV increase.
Nipattummakul et al. [102] studied the effect of steam to carbon ratio in high temperature (900 °C)
steam gasification of wastewater sludge. Peak value of syngas yield, energy yield, and hydrogen yield
was obtained at S/C ratio of 5.62 (given in mol/mol). The reason for this peak value behavior is
attributed to the presence of two competing reactions: increase in the steam flow rate increases the
steam concentration inside the reactor to accelerate the involved steam reactions, but decreases the
73
residence time in the reactor, which consequently decreases the time available for steam-involved
reactions.
Jayaraman et al. [103] investigated the dried sludge (93.5% dry solid) behavior in combustion, pyrolysis
and gasification processes through TG-MS method. For gasification, the final temperature is 1100 °C,
and it was performed with a blend of steam and oxygen as gasifying agent. The results indicate that an
efficient conversion process to produce syngas is achieved with temperature between 850 and 950 °C.
In the Choi et al. work [104], steam/oxygen gasification of dried sewage sludge (95% dry solid) was
performed in a two-stage gasifier, with the addition of activated carbon, to produce an H2-rich and tar-
free syngas. The reactors temperature was 800 °C, and the gasifying medium was preheated at 450 °C.
The activated carbon addition allowed to obtain a tar-free syngas and also helped in NH3 lowering. An
increase in the steam to fuel ratio, varied from 0.52 to 0.9, produced an increase of the H2 content in
syngas and of CGE.
Choi et al. [105] also studied the effect of additives to enhance tar cracking and lower NH3 presence in
syngas for the air blown gasification of dried sludge. The equivalence ration was set at 0.36.
Moon et al. [106] studied the effect on hydrothermal treatment on sewage sludge performance in
gasification. The hydrothermal treatment is explained in the paper. The gasification was performed
with steam, with a steam to fuel ratio of 2.4, and the gasification temperature was varied between 700
and 800 °C. It was shown that, increasing the gasification temperature, the gas yield increases. They
assess that after hydrothermal treatment of sewage sludge, the gas yield and heating value of product
gas obtained from steam gasification improved.
Nowicki et al. [107] studied the steam and CO2 gasification of char produced in sewage sludge pyrolysis,
with different gasification temperature and steam to fuel ratio, and evaluated the kinetic parameters.
They show that gasification reactions start at lower temperature for steam gasification with respect to
CO2, and that temperatures between 700 and 900 °C are necessary to achieve conversion within
reasonable time.
Gil-Lalaguna et al. [100] performed a comparison between air-steam gasification in a fluidized bed
reactor of sludge and of sludge pyrolytic char. The range of temperature considered was 700-850 °C.
Their work shows how char gasification led to an improvement in the gas yield - calculated on a dry and
ash-free basis - due to the increased concentration of carbon in the organic fraction of the solid after
the pyrolysis step, with an increase in the average CO yield, although the carbon fraction in the residue
is higher for char gasification. The reduction in the fraction of carbon forming tar is another advantage
of char gasification over the direct gasification of sewage sludge. The CGE is similar for the two
feedstock.
In their other work Gil-Lalaguna et al. [101] the sludge pyrolysis and produced char gasification have
been studied as a route for full energy recovery from sewage sludge. The pyrolysis temperature was
set at about 530 °C and the obtained char yield was 51%. The results show that the energy contained
in the product gases from pyrolysis and char gasification is not enough to cover the energy consumption
for thermal drying of sewage sludge. Additional energy could be obtained from the calorific value of
the pyrolysis liquid, but some of its properties must be improved facing towards its use as fuel. The
74
energy contained in the product gas of sewage sludge gasification, instead, is enough to cover the
energy demand for both the sewage sludge thermal drying and the gasification process itself.
Fan et al. [108] show how the presence of formic acid as catalyst increases the syngas yield and
hydrogen yield of supercritical water gasification, as formic acid acts as an acid hydrolysis agent and an
effective hydrogenating agent that facilitates rapid hydrolysis of carbohydrates to produce small
molecules and effectively suppresses polymerization.
Gong et al. [109] studied the effect of reactant composition, in terms of C, H and O content, in
dewatered sludge gasification in supercritical water (at 400°C and 22.1 MPa), with a residence time of
60 min. They show that: an increase in C/H2O ratio produce an increase in gas production; char amount
in the solid residue increases with increasing C/H; increasing C/O, the PAH formation increases. In
conclusion, they state that it is possible to optimize the reaction process and control the composition
of gasification products by adjusting the reactant C/H/O ratios, through addition of appropriate
amounts of carbon, hydrogen and oxygen containing substances.
In this perspective, much research has been dedicated to sludge co-gasification with other feedstocks.
Smolinski et al. [110] studied the air-steam co-gasification of sludge with coal, with 20 and 40% of
sludge in the blend, at a temperature of 700 °C. It is shown that the hydrogen content in syngas
decreases if the amount of sludge in the blend is increased.
Recently Hu et al. [111] study, catalytic co-gasification of wet sludge with pine sawdust in a fixed bed
reactor is considered. The catalyst used was NiO/MD (modified dolomite); the gasification temperature
was varied from 600 °C and 900 °C. The use of the catalyst was found to be effective for tar reduction.
The optimal amount of pine sawdust in the blend, varied from 0% to 100%, resulted to be 40%, with a
gasification temperature of 900 °C.
Le Rong et al. [112] assessed the toxicity of ash from the co-gasification of sludge with woody biomasses
in a fixed bed gasifier.
Zhu et al. [113] studied the dried sludge gasification with air combined to syngas combustion. The
gasification equivalence ratio was set to 0.35, the gasification temperature was 800 °C and they state
that gasification process was self-sustaining. The syngas was burned in a down-flow combustor, with
air staging, with a maximum temperature of 1150 °C; the obtained combustion efficiency was 99.2%.
The provided equivalence ratio for the combustion reductive zone resulted to be a crucial parameter
for NOx emissions. In Lumley et al. [98] work, several thermochemical conversion technologies have
been analyzed, from the perspective of small urban WWTPs, and, among them, air-blown gasification
was found to be the most suitable approach. They designed and simulated a gasification-based
generating system in ASPEN Plus, to determine net electrical and thermal outputs. As a result, air-blown
gasification was found to convert sludge to electricity with an efficiency greater than 17% (about triple
the efficiency of electricity generation using anaerobic digester gas), with the possibility to offset up to
1/3 of the electrical demands of a typical WWTP. It is also concluded that a gasification-based power
system can be economically feasible for WWTPs with raw sewage flows above 0.093 m3/s, providing a
meaningful profit over an alternative thermal drying and landfill disposal.
75
3.3.6 Wet oxidation Wet oxidation is the reaction between the organic substance and the oxygen in the aqueous phase (dry
concentration in the incoming sludge <10%) at high pressure and temperature. The reaction often
occurs in the presence of catalysts. The reaction products depend on the content of the sludge, but in
general are carbon monoxide, carbon dioxide, nitrogen, in different forms depending on the catalysts
presence and type (in the absence of catalyst the prevalent form is ammonia nitrogen), sulphates,
originated from organic sulfur, phosphates from phosphorus-containing compounds.
In the absence of catalysts, high partial oxidation of organic compounds occurs (volatile acids,
aldehydes, ketones are also present).
Depending on the temperature and pressure used, wet oxidation is classified into two types:
- Subcritical wet oxidation, which takes place at subcritical conditions of below 374 °C and a
pressure of 10 MPa;
- Supercritical wet oxidation, occurring at a temperature and pressure above the supercritical
point of water (374 °C and 22.1 MPa) [71], [114].
One of the most obvious advantages of wet oxidation is that dewatering of sewage sludge before
oxidation is not necessary. Although a large scale subcritical wet oxidation system for sewage sludge is
available [78], supercritical wet oxidation has not yet been fully commercialized, even after over 20
years of technology development [78]. Several small supercritical wet sludge oxidation plants have
been reported in the United States, Sweden and Japan [78].
According to IREN [48], that made a preliminary study to consider the wet oxidation to dispose of sludge
from Parma and Reggio Emilia area, the following drawbacks are identified:
- Structural complexity and management
- High investment costs
- High operating costs, in the case of sludge from plants other than that of the seat of the basin
served by the installation of wet oxidation treatment
- The concentration of metals in the solid residue can force the disposal of the material in
landfills for hazardous waste
- Land use is significant
- There are very few wet oxidation plants dedicated to the treatment of municipal sludge
- The costs are 30% higher than those of other thermal treatments, and it may increase in case
of treatment of the dedicated liquid stream of the wet oxidation process.
As consequence of these drawbacks, the wet oxidation disposal routes is not considered in the model
section of this work.
3.4 Current situation and future trends of disposal routes in EU
In this paragraph, the share of the three main sewage sludge’s disposal routes (landfill, agricultural use
and thermal treatment) are analyzed for each EU member states.
76
Data from EUROSTAT [4] specifies also the fraction of sewage sludge produced that is sent to
composting processes, but here, as stated in paragraph 3.2.2, that fraction is considered together with
land spreading applications, and goes under the “Agricultural Use” disposal route, since also compost
produced from sewage sludge is re-used for agricultural purposes.
Instead, the term “thermal treatment” in connection with sewage sludge pertains to all routes
mentioned in paragraph 3.3, with exception of biogas production from anaerobic digestion, which is an
intermediate process and not a final disposal method. In fact, the “thermal treatment” voice takes into
account incineration at mono-incineration plants (including gasification installations), at coal fired
power plants and cement plants, and in waste incineration facilities.
It was not possible to investigate a further distinction between the different thermal treatment
technologies. Moreover, the search for alternative sewage sludge treatment and disposal methods, as
pyrolysis based processes, has intensified only in recent years [53] and data regarding that routes are
not yet available or are included in the thermal treatment route too.
In Table 23: Fraction of sewage sludge’s disposal routes in EU member states, fraction of sewage
sludge’s disposal routes in EU member states are reported according mainly to EUROSTAT data [4],
with exception of Germany and Poland for which specific studies on the sludge management strategies
are present in literature [115, 116].
Data for Switzerland, Croatia, Iceland, Turkey, and Bosnia and Herzegovina are not available neither in
EUROSTAT database, nor in literature on the topic. Due to missing data for some year and country, for
different countries, different time of data (from 2005 to 2013) are reported in table.
Results for EU-15 and EU-12 are calculated as a weighted average of disposal routes fractions. The
weight was the sludge production over a year for the target country. Also for EU-27 the same procedure
is applied using EU-15 and EU-12 as starting point for the calculation.
In Figure 30Errore. L'origine riferimento non è stata trovata. and Figure 29, the data collected in Table
23 are reported graphically. On the horizontal axis member states are reported in order of sludge
production: form left to right states are ordered form the biggest producer to the smallest. Under the
voice “Other”, present just in Poland, goes the fraction of sludge used for land reclamation.
77
Country Landfill Agricultural
Use
Thermal
Treatment
Production
[10^3 ton
DM/y]
Source Year
Germany 0% 46% 54% 2170 [115] 2013
UK 5% 70% 15% 1771
[4]
2010
Spain 4% 65% 15% 1121 2009
France 7% 73% 20% 1059 2007
Italy 42% 45% 3% 1053 2010
Netherlands 0% 0% 100% 348 2009
Austria 5% 49% 46% 254 2007
Sweden 3% 57% 0% 210 2008
Portugal 10% 90% 0% 189 2008
Finland 0% 95% 5% 148 2005
Denmark 6% 59% 16% 140 2007
Greece 55% 4% 35% 115 2007
Belgium 0% 15% 85% 103 2010
Ireland 5% 69% 0% 60 2007
Luxembourg 0% 78% 12% 14 2009
EU-15 9% 60% 30% 8755 Calculated -
Poland 17% 25% 2% 486 [116] 2009
Hungary 30% 59% 1% 184
[4]
2009
Czech Republic 15% 78% 2% 172 2009
Romania 80% 20% 0% 68 2010
Lithuania 2% 98% 0% 66 2010
Slovakia 15% 65% 0% 56 2005
Bulgaria 60% 40% 0% 42 2009
Estonia 20% 78% 2% 29 2009
Latvia 0% 52% 0% 27 2009
Slovenia 15% 2% 61% 14 2009
Cyprus 0% 82% 0% 7 2009
Malta 100% 0% 0% 0.1 2009
EU-12 24% 49% 2% 1151.1 Calculated -
EU-27 11% 59% 27% 9906.1 Calculated -
Table 23: Fraction of sewage sludge’s disposal routes in EU member states.
78
Figure 29: Disposal routes in new and old EU member states.
In Figure 30, where the sum of the sludge disposed via the three main routes, plus “other” routes, does
not match with the totality of sludge produced, the remaining part is labeled as “No Data”.
Landfill9%
Agricultural Use61%
Thermal Treatment
30%
EU-15 8755 [10^3 ton/year]
Landfill24%
Agricultural Use49%
Thermal Treatment
2%
Other25%
EU-12 1151 [10^3 ton/year]
Landfill11%
Agricultural Use59%
Thermal Treatment
27%
Other3%
EU-27 9906 [10^3 ton/year]
79
Figure 30: Sewage Sludge Disposal Routes in EU member States.
Figure 29 and Figure 30 show that share of disposal routes can differ a lot country by country and it
results in even completely different policies: Netherlands and Belgium thermally diposed nearly 100%
of the sludge produced, while in Portugal and Finland respectively 90% and 100% is recovered in
agricultural use. Intermediate situation are present in countries such as Germany and Austria, which
dipose nearly half of the production in agriculture, and half in thermal treatments.
For all EU-15 countries, with exception of Italy and Greece, the landill routes accounts for less than 5%.
Differently, in EU-12 countries landfill is the most common route: its fraction accounts for 100% in
Malta, 80% in Romania, 60% in Bulgaria and more than 15% for all other EU-12 states, with exception
of Lithuania, but for which more than 60% of data are missing.
To have an idea for the near future, it is possible to refer to the European Commission (EC) [8] study
performed in 2008, already taken as reference in section 1.1.
The following major trends are expected to influence the spreading of sludge on land:
There will be a general phasing out of sludge being sent to landfill, due to EC restrictions on
organic waste going to landfill as well as public disapproval: it is estimated that by 2020 there
will be no significant amounts of sludge going regularly to landfill in the EU-27.
Sludge treatments before its recycling to land, as anaerobic digestion and other biological
treatments, like composting, will increase. The use of raw sludge will no longer be acceptable.
Restrictions on types of crops being allowed to receive treated sludge will potentially increase.
Semi-voluntary and voluntary quality management programs, such as the ones in place in
England and Sweden to increase the safety of sludge use on food chain crops will be introduced.
Increased attention will be paid to recovery of organic nutrients, including those in sludge.
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
Ge
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Spai
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SewageSludge Disposal Routes in EU member States
Landfill Agricultural Use Thermal Treatment Other No Data
80
The main alternative to spreading sludge on land is likely to be incineration, with energy
recovery for sludge produced at sites where land suitable for recycling is unavailable. This will
be the case in particular where population densities are high and public opposition (e.g. to odor
problems) makes it more difficult to recycle to land; it will be seen also where animal manures
are over-abundant.
Sludge management will be also influenced by developments related to climate change policy and
renewable energy, leading to:
Increased attention to climate change and mitigation of greenhouse gas emissions and thus
recognized additional benefits of sludge applications to soils.
Increased treatment of sludge with energy recovery through anaerobic digestion, incineration
or other thermal treatment, with ash recycling. There may be increased production and
utilization of biogas from sewage sludge, as well as some production of alcohols and other fuels
directly from sewage sludge using pyrolysis and gasification.
Increased application of sludge to fuel crops such as miscanthus, hybrid poplars and other non-
food energy crops.
In the European Commission (EC) [8] study, also predictions for new share in disposal routes of sludge
for any member states are presented. The expected change in percentage of each disposal route is
shown in Figure 31. It is obtained by comparing the EC predictions for year 2020 to the current situation
discussed above and reported in Table 23.
The share of landfill, in case of country with an actual high one as Romania, Bulgaria and Hungary, will
be drastically reduced. For example, Bulgaria will pass from 60% of sludge disposal in landfill to 30%, in
favor of both agricultural re-use and thermal treatment routes. Also for the main sludge producers
within the EU-15 states, an increase in thermal treatments is expected, both for countries with almost
0% landfill, and for Italy, currently landfilling 40% of sludge. For the first, the increase in thermal
treatments will substitute agricultural use, while for Italy it will be mainly at the expense of landfill.
81
Figure 31: Change in disposal routes expected for year 2020 with respect to current situation.
Reference for current situation: Table 23; Reference for year 2020: [8].
-50% -40% -30% -20% -10% 0% 10% 20% 30% 40% 50%
G E R M A N Y
U K
S P A I N
F R A N C E
I T A L Y
N E T H E R L A N D S
A U S T R I A
S W E D E N
P O R T U G A L
F I N L A N D
D E N M A R K
G R E E C E
B E L G I U M
I R E L A N D
L U X E M B O U R G
E U - 1 5
P O L A N D
H U N G A R Y
C Z E C H R E P U B L I C
R O M A N I A
L I T H U A N I A
S L O V A K I A
B U L G A R I A
E S T O N I A
L A T V I A
S L O V E N I A
C Y P R U S
M A L T A
E U - 1 2
E U - 2 7
CHANGE IN DISPOSAL ROUTES EXPECTED FOR YEAR 2020
Thermal Treatments Landfill Agricultural Use
82
Figure 32: Predicted disposal routes share in EU-15, EU-12 and EU-27 for 2020.
Landfill5%
Agricultural Use52%
Thermal Treatment
43%
EU-15 Share of disposal routes expected for 2020
Landfill18%
Agricultural Use39%
Thermal Treatment
17%
Other26%
EU-12 Share of disposal routes expected for 2020
Landfill8%
Agricultural Use51%
Thermal Treatment
37%
Other4%
EU-27 Share of disposal routes expected for 2020
83
In conclusion, looking at Figure 32 and comparing to the current situation share, depicted in Figure 29,
it can be immediately seen that, globally for all EU-27, an increase of thermal treatments is expected:
it will reach nearly 40% of the total share by year 2020. Consequently, the share of landfill and
agricultural use will be reduced globally in EU.
Therefore, among all the fact depicted in this paragraph, it is evident that the study of sludge
management must be focused on the “thermal treatments” route by investigating both innovative
solutions, such as pyrolysis based processes, and established ones, as mono-incineration and co-
incineration applications.
84
85
4 Sludge thermal treatments SWOT analysis
The evaluation of the four thermal technologies (mono-incineration, co-incineration, pyrolysis and
gasification), as potential sludge-to-energy valorization methods, is performed.
The SWOT (Strengths, Weaknesses, Opportunities, Threats) analysis is an extremely useful tool for
understanding and decision-making, for all sorts of situations in business and organizations. Although
it is usually associated to marketing and business decision making, SWOT analysis is a powerful model
for other many different situations, and in this study it is used for project planning and project
management [117]. SWOT analysis is usually applied for preliminary evaluations.
It involves the collection of information about internal and external factors that have, or may have, an
impact on the evolution of the project. It provides a list, referring to this case, of technology's Strengths
and Weaknesses (internal factors), as indicated by an analysis of its resources and capabilities, plus a
list of the Threats and Opportunities (external factors), identified by an analysis of its environment
[118].
SWOT has been proved, by UNEP (United Nations Environmental Protection), to be a useful planning
tool to understand the Strengths, Weaknesses, Opportunities and Threats of both processes and plans
[82].
4.1 Mono-incineration
STRENGHT
Nearly complete elimination of the organic materials due to a combustion process that takes
place in a controlled environment, where excess air and temperature are monitored [82].
Possible utilization of the ashes obtained since there are opportunities for ash utilization in the
production of construction materials [65].
Volume reduction of 90% and the efficient production of a useful heat or electric energy [82].
Possibility to use the existing emissions control systems already available for waste incineration
plants [62].
Established technology, especially in some European countries [8].
Sludge quality not essential [82].
No need for extensive sludge storage [82].
WEAKNESSES
Incineration process can be energy deficient depending on the characterization of the incoming
sludge [10, 82].
86
Dewatering of the sludge at least at 20% moisture content is necessary to make the mono-
incineration process feasible [10, 82].
Far from Zero Waste method, since 30 wt% of the dry solids remain finally as ash. Combustion
ash is a potential hazardous waste due to its content of heavy metals. Additional expenses are
thus required for ash handling and disposal [66].
Necessity to face air pollution problems (NOx and SO2 emissions) managing them with air
pollution control devices [82].
Manage environmental issues, like the greenhouse effect, since production of GHG (CO2)
emissions occurs [82].
High cost due to emission control systems, flue gas cleaning and ash disposal (heavy metals)
[82].
Large scale application for attractive economics [62, 82].
OPPORTUNITIES
Possibility to easily substitute or integrate other conventional fuel (coal, other biomass) in the
operation of the plant [82].
Flexibility in the waste heat exploitation: according to the energy market variability during the
day and during the year there is the possibility to exploit the flue gases heat to provide district
heating or for internal uses in the plant such as sludge drying.
THREATS
Strong public opposition: the major constraint in the widespread use of incineration is the
public concern about possible harmful emissions [82].
Unstable economic environment/price of competitive fuels [82].
4.2 Co-incineration
STRENGHT
Nearly complete elimination of the organic materials due to a combustion process that takes
place in a controlled environment were excess air and temperature are monitored [82].
Possible utilization of the ashes obtained since there are opportunities for ash utilization in the
production of construction materials [65].
Volume reduction of 90% and the efficient production of a useful heat or electric energy [82].
Possibility to use the existing emissions control systems that is already available for waste
incineration plants [62].
Established technology, especially in some European countries [8].
87
Sludge quality not essential [82].
No need for extensive sludge storage [82].
WEAKNESSES
Need for sludge drying, even to very high level of dryness for certain applications [71].
Increase in heavy metal content in ash [73].
Necessity to face air pollution problems (NOx and SO2 emissions) managing it with air pollution
control devices. [82]
Manage environmental issues, like the greenhouse effect, since production of GHG (CO2)
emissions occurs. [82]
No phosphorous recovery possibility from ash (section 3.2.3).
OPPORTUNITIES
Possibility to easily substitute or integrate other conventional fuel (coal, other biomass) in the
operation of the plant [82].
Possibility to exploit the available capacity of already existing combustion plants, with well-
trained and experienced personnel to handle it [71].
Avoid plant construction huge investment costs [71].
Flue gas cleaning system already in place.
THREATS
Strong public opposition [82].
Technological limits on the sludge amount and quality in the fuel mix [51].
4.3 Pyrolysis
STRENGHT
Zero waste process [82].
Better control of heavy metal emissions with respect to incineration. Pyrolysis flue gases will
need less treatment to meet emission limits than incineration [64, 119].
Possible conversion of all sludge biomass fraction into useful energy.
Volume reduction of 90% and the efficient production of a sterile carbon char [82].
Reduced GHG emissions [82].
Typical pyrolysis plants are more compact, compared to incineration plants [82].
88
Potential marketable products [82].
WEAKNESSES
A better understanding of sludge pyrolytic thermal degradation has to be reached: it is a
complex process and a number of consecutive and parallel reactions are involved; the
mechanistic insights on the behaviors of dried sludge pyrolysis and detailed investigation on
the pyrolysis products at different working conditions are not very clear [34].
In many cases dewatering/thickening of the sludge is required in order to avoid problems such
as additional energy consumption for pyrolysis, higher amount of liquids in the products and a
change in products composition due to high water partial pressure [34, 82].
New technology, few commercial applications [82].
High investment costs: viability is proven only in large scale plants (> 20000 tons/yr) [82].
Lack in products standardization [82].
Byproducts (Char) difficult commercialization: the heating value of the chars is low (near to 5
MJ/kg of HHV), making it generally unattractive for incineration or any other energetic
valorization. Moreover, the high heavy metal content in char may require costly flue gas
treatments and also limits char landfilling possibility [82, 119].
OPPORTUNITIES
Turn a waste into a valuable raw material: high added value products [82].
Funding opportunities (green activity)[82].
Possible “Char Market” and valorization of char: char is usually the main byproduct of sewage
sludge pyrolysis for liquid production.
Potential replacing of sludge with bio-char for agricultural purposes: bio-char, is
getting the attention of both the political and scientific community due to its potential
to improve soil productivity, remediate contaminated soils and mitigate climate
change [81].
Use of Bio-Char for adsorbent production: for the removal of pollutants such as H2S
or NOx in gaseous streams [82].
THREATS
Unstable economic environment: the barrier to pyrolysis application is the economic viability
of the system and the relative complexity of the processing equipment [82].
Lack in environmental standards and BATs (Best Available Technology) [82].
89
4.4 Gasification
STRENGHT
Integrated technology [82].
Higher efficiency of energy recovery [82], [98].
Most of the energy converted into a single stream (syngas) [98].
Production of an inert solid waste [16].
Lower amount of gas produced with respect to combustion [78].
Reduced environmental emissions [78].
Complete sludge energy recovery in the case of combined pyrolysis and gasification of pyrolysis
char [100].
Potential co-feeding with biomass [109], [111], [112].
Reduced GHG emissions [78].
High energy efficiency and carbon balance [82].
Syngas can be used for CHP or as second generation fuel [82].
Marketable products [82].
WEAKNESSES
Ash disposal problems (heavy metals) [112].
Dewatering and/or drying is needed [16].
Complexity of the technology [82].
Heavy organic pollutant compounds in the exhaust stream [78], [29].
Extensive gas cleaning for syngas applications [82].
High investment and operation costs [82].
OPPORTUNITIES
Turn a waste into energy [82].
Production of a renewable syngas or a chemical feedstock [78].
Funding opportunities (green activity)[82].
Economic feasibility [98].
THREATS
Unstable economic environment [82].
Lack in environmental standards and BATs (Best Available Technology) [82].
90
4.5 Summary and comparison
Figure 33: Mono-incineration SWOT analysis.
Strengths
1. Nearly complete elimination of the organic materials
2. Possible utilization of the ashes obtained
3. Volume reduction of 90% 4. Existing emissions control systems 5. Established technology 6. Sludge quality not essential 7. No need for extensive sludge storage
Weaknesses
1. Incineration process can be energy deficient
2. Dewatering of the sludge is required 3. Air pollution problems (NOx and SO2
emissions) 4. Far from Zero Waste method 5. Production of GHG (CO2) emissions 6. High cost due to emission control
systems and flue gas cleaning ash disposal (heavy metals)
7. Large scale application for attractive economics
Opportunities 1. Substitute or integrate other
conventional fuel in the operation of
the plant
2. Flexibility in the waste heat
exploitation
Threats 1. Strong public opposition 2. Unstable economic environment/
price of competitive fuels.
MONO-INCINERATION
91
Figure 34: Co-incineration SWOT analysis.
Strengths
1. Nearly complete elimination of the organic materials
2. Possible utilization of the ashes obtained
3. Volume reduction of 90% 4. Existing emissions control systems 5. Established technology 6. Sludge quality not essential 7. No need for extensive sludge storage
Weaknesses
1. Incineration process can be energy deficient
2. Sludge drying often required 3. Air pollution problems (NOx and SO2
emissions) 4. Production of GHG (CO2) emissions
5. Increase in heavy metal content in ash.
6. No phosphorous recovery
Opportunities 1. Substitute or integrate other
conventional fuel in the operation of
the plant
2. Exploit already available combustion
capacity
3. Lower investment
4. Flue gas cleaning already in place
Threats
1. Strong public opposition 2. Technological limit on the sludge
amount and quality in the fuel mix to be burned.
3. Unstable economic environment/ price of competitive fuels.
CO-INCINERATION
92
Figure 35: Pyrolysis SWOT analysis.
Strengths
1. Zero waste process 2. Control of heavy metal emissions 3. Possible conversion of all sludge
biomass fraction into useful energy 4. Volume reduction of 90% 5. Reduced GHG emissions 6. Typical pyrolysis plants are more
compact, compared to incineration plants
7. Potential marketable products
Weaknesses
1. A better understanding of sludge pyrolytic thermal degradation has to be reached
2. In many cases dewatering/thickening of the sludge is required
3. New technology, few commercial applications
4. High investment costs 5. Lack in products standardization 6. By-products (char) difficult
commercialization
Opportunities
1. Turn a waste into a valuable raw material
2. Funding opportunities (green activity)
3. Possible “Char Market” and valorisation of char
Threats
1. Unstable economic environment 2. Lack in environmental standards and
BATs (Best Available Technology)
PYROLYSIS
93
Figure 36: Gasification SWOT analysis.
Strengths
1. Integrated technology 2. High energy conversion efficiency 3. Single product stream 4. Reduced GHG and other pollutants
emissions 5. Co-feeding with biomass possibility 6. Potential marketable product
Weaknesses
1. Complex technology 2. Dewatering/drying of the sludge is
required 3. Tar problems 4. Gas cleaning required 5. High investment and operation
costs
Opportunities 1. Turn a waste into a valuable raw
material/energy 2. Funding opportunities (green
activity)
Threats 1. Unstable economic environment 2. Lack in environmental standards
and BATs (Best Available Technology)
GASIFICATION
94
95
5 Preliminary calculations on biogas production
Since the models developed and described in chapter 6 and 7 present also the purpose of comparing
the energy performances of different sludge types, the present section has the aim of evaluating,
before entering in more complex computation, the amount of biogas and energy produced by digested
sludge, which also explains its lower energy content (LHV).
The values of biogas production from sludge anaerobic digestion in literature range from 0.4 to 1.1
Nm3/kg VS reduced.
Considering raw mixed sludge digestion, using the composition of the sludge before (raw mixed sludge)
and after (digested sludge) digestion, and knowing that the ash mass does not vary during the digestion
process, it is easy to compute the volatile solid reduction amount, which results to be 0.54 kg of lost VS
per kg of dry digested sludge. If the digestion of raw primary sludge is considered, the result is 0.94 kg
of lost VS per kg of dry digested sludge. A mean value of 0.75 Nm3/kg VS reduced as a gross biogas
production, and an average value of electric power consumption of 900 kJ/kg of dry organic matter fed
are assumed. The latter is converted into primary energy through a factor of 2.6 (conversion efficiency
from primary to electric energy of 38.5%), to find the amount of biogas used for the plant consumption.
Consequently, the biogas production per kg of dry digested sludge is 0.29 Nm3 in the case of raw mixed
sludge digestion, and 0.55 Nm3 in raw primary sludge case. These results will be useful for the sludge
types comparison in the incineration and pyrolysis models.
In order to reach a better understanding of the energy performance of raw and digested sludge and
develop a more complete comparison, the amount of energy produced in the form of biogas during
digestion and the amount of energy left in the exiting sludge have to be evaluated. The results are
reported in the following tables, for raw primary and raw mixed sludge digestion.
As can be seen, the considered biogas lower heating value, as well as the electric consumption, have
been assumed equal for the two primary sludge types digestion, as they result from an average of the
values found in literature. This was done for simplicity, and only to give an idea of the digestion process,
but it is not true in principle. Instead, the values obtained for volatile solid reduction are consistent
with literature, and the consequent lower biogas production for raw mixed sludge digestion is
reasonable.
96
Raw Primary
Basis 1 kg raw dry
No digestion LHV raw dry 18.7 MJ
Primary Energy IN 18.7 MJ primary
Digestion LHV biogas 23 MJ/Nm3
LHV digested dry 11.17 MJ
Volatile solid reduction 0.49 kg VS red
Digested sludge amount 0.51 kg digested dry
Gross biogas production 0.75 Nm3/kg VS red
0.36 Nm3
Dry ash free mass fraction 0.77 kg daf
Electric consumption 900 kJ/kg raw daf
Net Primary ENERGY in biogas 6.56 MJ primary
Primary Energy in sludge 5.74 MJ primary
Total Primary Energy OUT 12.30 MJ primary
Table 24: Results of calculation of Biogas energy for anaerobic digestion of raw primary sludge.
Raw Mixed
Basis 1 kg raw dry
No digestion LHV raw dry 15.5 MJ
Primary Energy IN 15.5 MJ primary
Digestion LHV biogas 23 MJ/Nm3
LHV digested dry 11.17 MJ
Volatile solid reduction 0.35 kg VS red
Digested sludge amount 0.65 kg digested dry
Gross biogas production 0.75 Nm3/kg VS red
0.26 Nm3
Dry ash free mass fraction 0.71 kg daf
Electric consumption 900 kJ/kg raw daf
Net Primary ENERGY in biogas 4.39 MJ primary
Primary Energy in sludge 7.24 MJ primary
Total Primary Energy OUT 11.63 MJ primary
Table 25: Results of calculation of Biogas energy for anaerobic digestion of raw mixed sludge.
97
6 Sludge Incineration Models
6.1 Mono-Incineration
6.1.1 Necessary conditions for self-sufficient combustion
This section is intended to present a preliminary evaluation of the required dry matter content of sludge
(DM%) that allows to reach sufficient flame temperature, for different sludge types and compositions.
Sewage sludge mono-incineration facilities are operated at temperatures ranging from 850 to 950 °C
[10]; temperatures below 850 °C can result in odor emissions, and at temperatures above 950 °C ash
sintering, or sand melting (in case a Fluidized Bed Furnace is used) can occur.
The temperature that is reached during incineration depends on the energy content and quantity of
the sewage sludge being used, as well as by the amount of available combustion air. In this study, a
flame temperature of 900 °C is fixed to be sure to fulfill the minimum requirements defined by the
European legislation.
By law Directive 2000/76/EC [120] order to guarantee complete waste combustion, the Directive
requires all plants to keep the incineration or co-incineration gases at a temperature of at least 850 °C
for at least two seconds.
Referring to the following scheme of a wastewater treatment plant, the considered sludge types are
the following:
Raw primary sludge
Raw mixed sludge (part from primary clarifier, part biologically treated)
Digested mixed sludge
Figure 37: Scheme of WWTP and sludge types produced.
98
The main sludge characteristic that affects the flame temperature is the Lower Heating Value. To
evaluate the LHV, on a dry basis, the composition of the different sludge types has been evaluated as
the average between the ones of selected waste water treatment plants in Parma and Reggio Emilia
area (data given by IREN [48]).
In particular, the plants considered for Raw Primary Sludge are Langhirano (PR) and Praticello (RE); S.
Martino (RE) and Guastalla (RE) for Raw Mixed Sludge; Mancasale (RE), Felino (PR) for Digested Sludge.
The considered LHV of the dry matter results from the average of the values resulting from four
reference equations, as explained in the section dedicated to LHV analysis (section 2.3.3).
Type of sludge ULTIMATE COMPOSITION [dry basis] LHV of dry
matter [MJ/kg] C H N S O ASH
Raw primary 43.4% 6.0% 6.9% 1.2% 19.4% 23.2% 18.7
Raw mixed 35.9% 5.0% 7.0% 1.0% 22.0% 29.3% 15.5
Digested 30.2% 4.2% 4.6% 0.8% 15.1% 45.1% 11.17
Table 26: Considered sludge types compostitions and LHV.
To ensure that the quantity of oxygen fed is sufficient for combustion in each condition of humidity,
the oxygen content in flue gas has been fixed at 6% (BREF for Waste Incineration 2006 [51]),
corresponding to an excess air value between 0.45 and 1.7, depending on the moisture content in the
fuel.
Results
The effect of air preheating temperature is shown in the following graphs (Figure 38, Figure 39 and
Figure 40).
Results of dry matter percentages to generate a flame temperature of 900°C, at different combustion
air temperature, are summarized in Table 27.
Types of sludge Preheated Air Temperature [°C]
25 350 650
Raw primary 31.4% 24.5% 20.0%
Raw mixed 35.1% 28.0% 23.3%
Digested 50.2% 38.7% 31.4%
Table 27: Dry matter content for 900 °C flame temperature.
99
Figure 38: Raw primary sludge flame temperature with dry matter at different preheating.
Figure 39: Raw mixed sludge flame temperature with dry matter at different preheating.
0100200300400500600700800900
10001100120013001400150016001700180019002000
0% 20% 40% 60% 80% 100%
T fl
ame
[°C
]
DM%
RAW PRIMARY SLUDGET flame vs sludge DM% content
T air = 25°C T air = 350°C T air = 650°C T flame required
0100200300400500600700800900
10001100120013001400150016001700180019002000
0% 20% 40% 60% 80% 100%
T fl
ame
[°C
]
DM%
RAW MIXED SLUDGET flame vs sludge DM% content
T air = 25°C T air = 350°C T air = 650°C T flame required
100
Figure 40: Digested sludge flame temperature with dry matter at different preheating.
It can be noticed that for digested sludge the effect of air preheat is more evident, since the minimum
dry matter ranges from 50% when using ambient air to about 30% with the maximum preheat (650 °C);
for raw sludge, both primary and mixed, instead, it can be seen a lower variation, from 33% to 20%.
Figure 41 compares the different types of sludge behavior in mono-incineration.
Figure 41: Comparison of different minimum dry matter content with different air preheating.
0100200300400500600700800900
10001100120013001400150016001700180019002000
0% 20% 40% 60% 80% 100%
T fl
ame
[°C
]
DM%
DIGESTED SLUDGET flame vs sludge DM% content
T air = 25°C T air = 350°C T air = 650°C T flame required
15,00%
25,00%
35,00%
45,00%
55,00%
65,00%
25 75 125 175 225 275 325 375 425 475 525 575 625
min
imu
m D
M%
T preheat air [°C]
Minimum DM% to reach 900°C
raw primary raw mixed digested
101
Figure 42: Dry matter and preheating temperature chart for 900 °C flame tempertaure
for raw primary sludge.
Figure 43: Dry matter and preheating temperature chart for 900 °C flame tempertaure
for raw mixed sludge.
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
25 75 125 175 225 275 325 375 425 475 525 575 625
min
imu
m D
M%
T preheat air [°C]
Minimum DM% to reach 900°C - RAW PRIMARY
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
25 75 125 175 225 275 325 375 425 475 525 575 625
min
imu
m D
M%
T preheat air [°C]
Minimum DM% to reach 900°C - RAW MIXED
Combustion without auxiliary fuels
Combustion with auxiliary fuels
Combustion without auxiliary fuels
Combustion with auxiliary fuels
102
Figure 44: Dry matter and preheating temperature chart for 900 °C flame tempertaure
for digested sludge.
Figure 42, Figure 43 and Figure 44 represent the dry matter content and air preheating temperature
zones that allow to reach 900 °C combustion with or without auxiliary fuel. Again, it can be seen as the
raw primary and mixed sludge have an analogous behavior. The difference of minimum dry matter
content is almost constant (about 3.5%) and it depends mainly on the different LHV of dry matter and,
in a minor extent, on the required air quantity. Raw mixed sludge requires a lower amount of
stoichiometric air, as it carries a larger amount of oxygen (due to the biological treatments) and this
mitigates the effect of the lower LHV. The effect of ash heat loss instead is much smaller.
For digested sludge, the effect of air preheat is much stronger, because, since the LHV is lower, the
heat content of air represents a heavier contribute to what enters in the energy balance.
6.1.2 Determination of sludge dry matter content fed in the dryer for an auto-thermal
process
Based on the considerations done in section 3.3.2.2, an Excel model of the system and the considered
components has been created. The aim is to find the minimum dry matter of sludge fed to the dryer
that makes the process auto-thermal and self-sufficient.
It is important to underline that the combustion air temperature is not influent in this study since
preheating of air exploits heat of flue gases, which is an heat flux internal to the process.
A scheme of the process used for the study is reporte in Figure 45.
First, the dry matter to be fed to the furnace that allows reaching a flame temperature of 900 °C is set
according to results obtained in Necessary conditions for self-sufficient combustion, changed according
to the different inlet temperature of the sludge (80 °C instead of 25 °C). In this phase, the air
15,00%
25,00%
35,00%
45,00%
55,00%
65,00%
25 75 125 175 225 275 325 375 425 475 525 575 625
DM
%
T preheat air [°C]
Minimum DM% to reach 900°C - DIGESTED
103
temperature has been fixed to 650 °C, according to section 3.3.2.2, while the oxygen fraction in the wet
flue gas is again 6%. The air and flue gas amount are found using the sludge composition given by IREN
[48] and reported in section 6.1.1. All the heat exchanger efficiencies are assumed to be 0.9 for
simplicity.
Figure 45: Sludge Mono-Incineration self-sufficient combustion scheme.
With the energy balance at the air preheater, the flue gas temperature at the outlet is computed.
Fixing the minimum flue gas temperature at the stack at 200 °C to avoid problems of acid condensation,
as prescribed in [67], with an energy balance at the flue gas/air heat exchanger, the amount of drying
air air that can be heated up to 125 °C, with reference to [25], can be calculated. Since the air at the
dryer exit has a fixed temperature of 88 °C and it is still far from the saturation, the 80% is recycled to
the flue gas/air heat exchanger [25].
The two fluxes of dry air and air humidity are separated for the best clearness.
Considering an air relative humidity at the heat exchanger inlet of 50%, the amount of water carried by
air is found and it is used for the water mass balance in the dryer to find the humidity in the air exiting
stream. As the amount of dry matter at the dryer inlet is the target of these calculations, an initial guess
is used.
The heat required by the drying process is computed as follows, considering an evaporation
temperature of 83 °C:
�̇�𝑑𝑟𝑦𝑖𝑛𝑔 = �̇�ℎ𝑒𝑎𝑡 𝑠𝑙𝑢𝑑𝑔𝑒 + �̇�ℎ𝑒𝑎𝑡 𝑤𝑎𝑡𝑒𝑟 + �̇�ℎ𝑒𝑎𝑡 𝑣𝑎𝑝𝑜𝑢𝑟 + �̇�𝑒𝑣𝑎 + �̇�𝑙𝑜𝑠𝑠
�̇�ℎ𝑒𝑎𝑡 𝑠𝑙𝑢𝑑𝑔𝑒 = �̇�𝑠𝑙𝑢𝑑𝑔𝑒 ∙ 𝑐𝑝 𝑠𝑙𝑢𝑑𝑔𝑒 ∙ (𝑇𝑜𝑢𝑡 − 𝑇𝑖𝑛)𝑠𝑙𝑢𝑑𝑔𝑒
Where Tin sludge is 25 °C and Tout sludge is 80 °C; the sludge heat capacity is computed as:
𝑐𝑝 𝑠𝑙𝑢𝑑𝑔𝑒 = 𝐷𝑀 ∙ 1.95 + (1 − 𝐷𝑀) ∙ 4.18 𝑘𝐽 𝑘𝑔𝐾⁄
�̇�ℎ𝑒𝑎𝑡 𝑤𝑎𝑡𝑒𝑟 = �̇�𝑒𝑣𝑎 ∙ 𝑐𝑝 𝑤𝑎𝑡𝑒𝑟 ∙ (𝑇𝑒𝑣𝑎 − 𝑇𝑜𝑢𝑡 𝑠𝑙𝑢𝑑𝑔𝑒)
104
�̇�ℎ𝑒𝑎𝑡 𝑣𝑎𝑝𝑜𝑢𝑟 = �̇�𝑒𝑣𝑎 ∙ 𝑐𝑝 𝑣𝑎𝑝𝑜𝑢𝑟 ∙ (𝑇𝑜𝑢𝑡 𝑎𝑖𝑟 − 𝑇𝑒𝑣𝑎)
�̇�𝑒𝑣𝑎 = �̇�𝑒𝑣𝑎 ∙ (ℎ𝑠𝑣(𝑇𝑒𝑣𝑎) − ℎ𝑠𝑙(𝑇𝑒𝑣𝑎))
Qloss is assumed to be 0.116 MJ/kgdry.
The heat required by sludge drying must be equal to the heat provided by the drying air: imposing this
condition in Excel solver, the target sludge minimum DM%, previously guessed, can be found.
Results
The main results are summarized in the Table 28.
Types of sludge DM% in combustor minimum DM% in dryer
Raw primary 18.8% 16.2%
Raw mixed 21.9% 18.7%
Digested 29.7% 24.3%
Table 28: mono-incineration results (combustion air temperature 650 °C).
It can be noticed how the minimum sludge dry matter that allows to reach the energy self-sufficiency
is very low, especially for raw sludge.
Wastewater Solids Incineration Systems [67] shows how with an air temperature of 648 °C (40%
excess), the feed material, digested sludge, with a LHV of dry matter of approximately 14 MJ/kg, is
burned autogenously, or thermally self-supporting, at a solid content of 27%. This reference value has
to be compared with the result of minimum dry matter in combustion found with the model.
The difference between the reported results are due to difference in sludge composition, and to
process parameters, such as combustion air quantity, heat exchangers efficiency assumptions.
The results previously reported have also to be compared with the achievable dewatering, which,
according to Sludge Engineering [16], depends on whether the sludge is primary or secondary, and it’s
not much affected by digestion. The dewatering limit are reported in Table 29, for the main dewatering
technologies.
TYPES OF SLUDGE maximum dry matter content (DM%) with dewatering
Sand Bed Filter Press Belt Press Centrifuge
RAW PRIMARY
SLUDGE NA 45% 38% 35%
RAW MIXED
SLUDGE NA 38% 28% 27%
DIGESTED
SLUDGE 50.0% 41% 27% 27%
Table 29: Dewatering limits for different technologies.
105
As can be seen, all the dewatering technologies, can easily reach the required minimum dry matter
content before the dryer for all the sludge types. In addition, even the DM% before the furnace can be
reached with dewatering, which would mean that drying is not necessary.
Usually, dewatered sludge delivered to a plant for disposal is considered to have a dry matter content
of 25%, which is sufficient for a good combustion of raw sludge, while digested sludge needs a further
drying.
It must be considered, however, that digested sludge shows worse performance in the present analysis
because part of its energy content has been used for the biogas production. According to the
calculations of chapter 5, the biogas production of 1 kg of wet digested sludge to be dried and burned
is 0.07 Nm3, considering raw mixed sludge is digested, and 0.13 Nm3 considering raw primary.
In the hypothesis of receiving a sludge at 25% dryness, if the drying is avoided, part of the flue gas
energy is left available for other purposes. As well, in the case in which the sludge is dried from 25%
dryness instead that from 15.5% (raw primary sludge example), the flame temperature would be higher
than 900 °C, which again means more energy available than required.
The results of the study “Determination of sludge dry matter content fed in the dryer for auto-thermal
combustion” are reported below in the plant scheme, for all the considered sludge types.
The results of minimum dry matter in combustion reported in the table above are consistent with the
values from literature.
In particular, the publication Sewage sludge management in Germany [10] states that for spontaneous
incineration (without an auxiliary combustion system) in sewage sludge mono-incineration plants,
dewatering and drying of raw sludge to a total solids of 35% dry residue are normally sufficient. The
counterpart minimum value for digested sludge is 45 to 55% dry residue, since digestion leaves behind
a lesser amount of organic material for incineration.
In Biosolid treatment processes [121] is written: «self sustained combustion without supplementary
fuel is often possible with dewatered raw sludge having a DM% more than 30%».
A strong air preheating allows obtaining auto-thermal combustion with an even lower amount of dry
matter.
106
Figure 46: Raw primary sludge results for auto-thermal incineration.
107
Figure 47: Raw mixed sludge results for auto-thermal incineration.
108
Figure 48: Digested sludge results for auto-thermal incineration.
6.1.3 Energy recovery possibilities
Assessed the sludge characteristics required for auto-thermal incineration, it is interesting to
investigate the possibility to produce an energy output. With this purpose, the Zurich plant
technology by Outotec, described in the section 3.3.2.2, has been chosen.
First, a preliminary excel sheet has been created to verify and compare data provided by
OUTOTEC on operational results of the Zurich Plant. The Excel Model follow the scheme
represented in Figure 49. For these calculations, raw mixed sludge type is used as input in the
model.
Figure 49: Raw mixed sludge results for mono-incineration energy recovery plant.
Results of this preliminary comparison are reported in Table 30, where it is also possible to look
at data on energy production, while the results for the other sludge types are in Figure 50 and
Figure 51.
Figure 50: Raw primary sludge results for mono-incineration energy recovery plant.
sludge in boiler flow rate kg/s 2.5
parameter Zurich plant data Model
Throughput t/y 100000 95277.0
h/y 7000 7000.0
mass flow rate in dryer kg/s 4.0 3.8
Sludge dryer
Inlet DS content 22% 30% 22%
mass flow rate dry matter kg/s 0.9 1.2 0.8
Outlet DS content 35% 45% 33%
mass flow rate out dryer kg/s 2.5 2.6 2.5
Water evaporation kg/h (5000) 5306.1 4761.9 4611.0
Steam consumption kg/h 7000.0 7468.4
kWh/kg h2o 1.02 1.14 1.02
Fluidized bed incinerator
Fluidized air flow (STP) Sm3/h 16000.0
kg/s 5.7 7.1
alpha 2.3 2.2 2.8
Oxygen content vol% dry 7% 11% 8.8%
Flue gas flow outlet (STP) Sm3/h 26500.0
kg/s 8.8 9.6
kg/kg tq 3.5 3.3 3.8
Temperature °C 870.0 950.0 900.4
Heat recovery boiler
Steam temperature °C 450.0 450.0
Steam pressure bar 60.0 60.0
Steam generation t/h 9.0 7.5
kg vap/kg dry sludge in comb 2.9 2.1 2.5
Steam turbine and generator set
Electrical power output kWel 900.0 866.7
kJ/kg dry 1030.9 756.0 1037.0
data hp
Table 30: Comparison of Zurich plant Outotec data and calculation results.
Figure 51: Digested sludge results for mono-incineration energy recovery plant.
The main results are also summarized in Table 31.
Types of sludge DM% in combustor minimum DM% in
dryer
Electric power production [kJ/kg
dry]
Raw primary 29.8% 19.2% 1262.9
Raw mixed 33.4% 22.1% 1037
Digested 48.0% 30.4% 815.8
Table 31: Mono-incineration energy recovery results summary.
In this case, the minimum dry matter before the furnace, required for a good combustion, is
much higher than the values found in the previous section, because in this case any air
preheating is not present. The minimum dry matter before the dryer is higher as well, as this
does not represent a mere disposal but a true energy recovery option, and electrical power is
produced.
Again, the fact that digested sludge has already produced useful energy in the form of biogas
must be taken into account. Following the procedure already explained in the previous section,
the amount of biogas produced for kg of digested sludge to be dried and burned is 0.09 Nm3,
considering raw mixed sludge digestion, and 0.17 Nm3, considering raw primary.
Observing the result of energy production, it can be seen as it decreases with decreasing sludge
LHV, as could be expected. This is because the value of energy production is given per kg of dry
matter; in the schemes, instead, the basis is 1 kg wet, with the dry matter content resulting from
the computation, and, therefore, different for each sludge type. Being the minimum dry matter
content higher for lower LHV sludge, also the burner air and flue gas mass flow are higher. This
results in a higher amount of steam and energy produced, at fixed steam and turbine conditions.
If the received sludge, of each type, is considered to have a dry matter content of 25%, typical
result of dewatering process, raw sludge is able to produce more energy than evaluated in this
discussion, while digested sludge probably does not even reach the target 900 °C combustion.
In addition, with reference to Table 29, digested sludge is also slightly harder to be dewatered.
These considerations lead to the conclusion that digested sludge is not particularly suitable for
this application, as it is; changes in the process parameters, or at least a slight air preheating
addition, would be worth to make also digested sludge energy content more exploitable.
In this sense, an Aspen model for sludge mono-incineration energy recovery, described in
section 6.1.4, has been developed: the feeding sludge dry matter will be imposed to 25% for
each sludge type, so that the results could be compared in an immediate way. It is also useful to
understand whether the Excel model results approach accuracy at a satisfactory level or not.
6.1.4 ASPEN model of energy recovery
6.1.4.1 Aspen
Aspen Plus is a popular tool that has been developed by Aspentech to design and simulate many
kinds of industrial processes. This software can predict flow rates, compositions and properties
of the streams, the operating conditions and the sizes for the equipment. There are two main
modes in which the software can be run: sequential modular (SM), which solves each unit
operation in a certain sequence, and equation oriented (EO), which requires the user to insert
equations that are then simultaneously solved. For the present study the sequential modular
mode was selected.
The Stream Class is another important parameter to be set. It describes the type of stream that
will be used in the simulation; the selection of certain stream classes allows the edition of
multiple substreams, depending on the kind of component modeled.
MIXCINC stream class was chosen for the simulation, as the process includes conventional gas
and liquid phases, conventional solid phase (for solid carbon and sulfur), as well as non-
conventional solid phase (for sludge and ashes).
Subsequently the components of the process were defined. All the reactants and final products,
as well as the intermediate products of the different steps have to be specified.
The Aspen Plus has two different kinds of database from which the components can be defined,
alongside physical and chemical properties: enterprise databases or legacy databases. The
enterprise databases were available in the version used for the present study.
The gaseous and liquid components are considered conventional components, thus are easily
defined in the database by their chemical name. This kind of component enters the streams in
the MIXED sub-stream. Examples of conventional components are H2, O2, N2 and H2O.
The solids can be either conventional or non-conventional. Conventional solids have been widely
studied and used in experiments and processes, their standard properties are known and
defined and thus they appear in the CISOLID sub-stream. Examples of conventional solids are
graphite (solid carbon) and sulfur. Non-conventional solids are not standardized compounds,
whose properties are not known. For this reason they have to be defined by the user and are
grouped in the NC sub-stream. Non-conventional solids guidelines for modeling in the Aspen
environment are in [122].
The Aspen Plus software provides different correlations that can be adopted to set the
properties of the solid. This category is used to define certain kinds of coals, fuels or other
compounds, by setting two main algorithms, one for enthalpy and one for density. Sewage
sludge belongs to this sort of solids and was defined, in the case of the present study, through
the DCOALIGT algorithm for density and HCOALGEN algorithm for enthalpy. As their ID suggests,
these algorithms are suitable for the characterization of coal and, more in general, of
carbonaceous fuels. If more specific correlations for the feed used are not available to the user,
these are certainly the most appropriate.
DCOALIGT refers to IGT (Institute of Gas and Technology) Coal Density Model, HCOALGEN is a
General Coal Enthalpy Model and includes a number of different correlations. The user can
choose among various relationships by setting four different option codes.
By setting the option codes to 6-1-1-1 for enthalpy, the correlation is based on a user-input value
for the heat of combustion (HCOMB). For sewage sludge, the value of HCOMB was equated to
the LHV of sludge calculated as shown in the paragraph 2.3.3: Aspen requires the wet sludge
LHV (with moisture content condition at the inlet of DECOMP), but reported on a dry basis, as
explained in the Aspen user guide [123].
Both density and enthalpy correlations require the input of PROXIMATE, ULTIMATE and SULFUR
analyses, in order to calculate physical and chemical properties.
Another parameter that has to be set, in order for the simulation to run properly, is the physical
property method. Indeed, Aspen Plus calculates physical properties for each component, by
means of the method chosen, which comprises an ensemble of equations of state (EoS).
Because the system deals with multiple phases, as well as conventional and non-conventional
solids, the Ideal Gas method cannot be chosen, thus the Peng Robinson – Boston Mathias
modified (PR-BM) method was selected.
6.1.4.2 Configuration
The fed sludge type is considered raw primary, as it appears more likely to be able to produce a
energy output. The wet sludge mass flow rate in input to the plant is 1980 kg/h, in order to
compare in a more homogeneous way the results of the pyrolysis model of chapter 7. The layout
of the plant, depicted in Figure 52, modeled in ASPEN, considers the incineration of sludge,
previously dried in an indirect dryer, without any auxiliary fuel support in normal operating
condition. Moreover, an energy recovery section aimed to produce electricity through a heat-
recovery steam cycle is included in the model.
First the stream WETSLD, at ambient conditions, with 75% moisture content is sent to the drying
section, that is composed of a RYIELD reactor, a HEATER and a SSPLIT, and exploits the heat of
the steam cycle condenser. The moisture content is reduced to 66% in DRYSLD stream.
The DRYSLD stream enters in the DECOMP block, used to decompose non-conventional material
into singleton molecules (C, S, O2, N2, H2), ash, and the decomposition heat is imported into
incineration reactor called COMB. For DECOMP block the RYIELD Reactor is used because
stoichiometry and kinetics are unknown or unimportant, but a yield distribution is known. The
mass yields of the RYIELD reactor DECOMP are determined and set using a calculator block
starting from data of the ultimate analysis.
The stream DRYSLD2 made of elements is fed to the block COMB where combustion is simulated
using a RGIBBS reactor: reactor with phase equilibrium or simultaneous phase and chemical
equilibrium, calculating phase equilibrium for solid solutions and vapor-liquid-solid systems.
Together with DRYSLD2, in the COMB block the preheated combustion air COMBAIR is fed after
being heated in the APH block by means of the stream FLUEGAS2. APH is simulated with a
MHeatX model. Mass flow of AMBAIR is determined by setting a Design Specification in order to
obtain an oxygen molar fraction of 6% on a wet basis in the stream PRODUCTS. The temperature
of air at outlet of APH is determined by setting a Design Specification in order to reach 900 °C as
combustion temperature: that is the temperature of the stream PRODUCTS leaving the block
COMB.
Ashes in the stream named ASH are separated by the gaseous components in the block SSPLIT
named ASHSEP: this model combines material streams and divides the resulting stream into two
or more streams according to their phases. Downstream this component the ashes are cooled,
since they should leave the COMB at a lower temperature (300 °C) with respect to the one of
the gaseous products, and the resulting heat stream (Q-ASH) is sent back to the COMB reactor.
Gaseous products contained in the stream FLUEGAS1 enter in a HeatX block called HRSG with
the aim of generate steam at 450 °C and 6 MPa (STEAM), according to what described in section
6.1.3.
The stream STEAM enters in a TURBINE block, defined by an isentropic efficiency of 0.8 and a
mechanical-electrical efficiency of 0.95. The flux is discharged at a pressure of 0.3 MPa and sent
to COND block modeled as a HEATER. The reason of the high value of the turbine discharge
pressure is the heat requirement for the dryer.
Downstream a PUMP is used to increase the pressure of the water fed to the Heat Recovery
Steam Generator to 6 MPa. The steam flow rate that HRSG block is able to generate is related
to the constraint on the final flue gas temperature of at least 200 °C: a Design specification varies
the steam flow rate in order to satisfy it.
Note that the pressure of all feed streams, with exception of the steam cycle, and unit operation
blocks were set to 1 bar (i.e. no pressure drop in the system).
Figure 52: Aspen mono-incineration model flowsheet.
6.1.4.3 Results
Results for digested sludge and raw primary sludge are reported in Table 32. Raw mixed sludge
has not been modeled for conciseness and because of its intermediate properties and results.
parameter digested raw
primary
Sludge characteristics
LHV of dry matter [MJ(kg] 11,17 18,7
mass flow rate in dryer [kg/h] 1980 1980
Inlet DS content 25% 25%
mass flow rate dry matter [kg/h] 495 495
Sludge dryer
Outlet DS content 33% 33%
mass flow rate out dryer [kg/h] 1500 1500
water evaporation [kg/h] 480 480
Heat requirement [kWh/kg evap] 0,87 0,87
sludge outlet temperature [°C] 110 110
Fluidized bed incinerator
air mass flow [kg/h] 3702 5098
air preheated temperature [°C] 453 63
flue gas temperature [°C] 900 900
flue gas flow [kg/h] 4984 6488
oxygen content [vol% (wet)] 6% 6%
ash temperature [°C] 300 300
ash mass flow [kg/h] 223 114
Heat recovery boiler
steam temperature [°C] 450 450
steam pressure [bar] 60 60
steam generation [kg/h] 1128 2095
flue gas temperature [°C] 455 225
Steam turbine and generator set
gross electrical power output [kWel]
162 301
isoentropic efficiency 0,8 0,8
outlet pressure [bar] 3 3
steam outlet temperature [°C] 138 138
exhaust flue gas temperature [°C] 200 200
input result
Table 32: Aspen mono-incineration model results summary for digested and raw primary sludge.
As anticipated in the previous section (6.1.3) comments on results, digested sludge requires a
strong air preheating (453 °C) in order to reach 900 °C combustion, while for raw primary sludge
63 °C are enough. The results of the Aspen model are worse than the ones obtained in the Excel
calculation, and this shows how their approximation.
In the case of digested sludge, the high extent of preheating has the consequence that part of
the flue gas energy is not available for steam production, which results to be about half of the
raw primary case. Also the higher amount of flue gas (both because of higher air/fuel ratio and
lower ash content) and the higher flue gas temperature contribute to the higher amount of
steam produced for raw primary.
In the following table (Table 33) the energy fluxes and efficiencies of the plant section are
reported.
digested raw primary
msludge,wet*LHVsludge,wet in dryer [MW] 0,529 1,565
msludge,wet*LHVsludge,wet in comb [MW] 0,885 1,858
Q in steam cycle [MW] 0,873 1,630
Gross electric power [MW] 0,162 0,301
Pump electric power [MW] 0,004 0,005
Auxiliaries consumption [% of gross power] 20% 20%
Net electric power [MW] 0,13 0,24
Specific net electric power [kWh/kg dry] 0,26 0,49
eta th=Q in steam cycle/ (msludge,wet*LHVsludge,wet )in comb 0,99 0,88
eta steam cycle= (P el out/Q in) steam cycle 0,18 0,18
Pel net/(msludge,wet*LHVsludge,wet )in comb 0,15 0,13
eta el gross= Pel gross/(msludge,wet*LHVsludge,wet )in dryer 0,31 0,19
eta el net= Pel net/(msludge,wet*LHVsludge,wet )in dryer 0,24 0,15
Table 33: Power fluxes and efficiencies.
On 1 kg of dry sludge basis, the production of net electric power is 0.49 kWh/kgdry for raw
primary, and 0.26 kWh/kgdry for digested sludge.
The digested sludge efficiencies are, unexpectedly, higher than for raw primary: this must be
due to the much lower LHV in input. As expected, the steam cycle efficiency is the same for the
two feedstocks cases, since the thermodynamic points are equal.
Moreover, the considerations made on the different energy content and performances of sludge
types due to previous biogas production are still valid. Consequently, an analysis of the primary
energy consumption for two different routes accounting for the biogas production is run: it can
be found in chapter 8.
6.2 Co-incineration in WtE
6.2.1 Model and analysis
The co-incineration of sludge in a waste to energy plant has been simply modeled in Excel, in
order to assess the effect of treating sludge, which means how the plant output are affected.
Three scenarios have to be compared: the first is the scenario without sludge, only burning
waste; in the second one, half of the sludge is dried and part is burned as it is; in the third one,
all the sludge is dried.
6.2.1.1 Plant description (first scenario)
The reference waste to energy plant is Parma PAI. Its nominal capacity is 130 000 t/y of wastes,
and 71.4 MW of power, operating 8000 h/y, 4000 in summer and 4000 in winter; the waste LHV
is considered to be 15.803 MJ/kg. The combustion efficiency is 0.9.
Integration boilers working with natural gas are present, with an overall thermal power of 19
MW, of which 17.2 are used.
The heat generated by the waste (or waste and sludge, in the subsequent scenarios) combustion
is used to generate steam that runs a turbine to produce electric power. During summer, all the
steam is used for electricity, while in winter part of it is sent to the district heating through a
turbine bleed at 1.5 bar. The summer electric power is 17.3 MW, while the winter one is 12.5
MW, with 40 MW of district heating.
6.2.1.2 Second scenario: half sludge dried
The total amount of sludge to be treated is 50 000 t/y, with the 25% dry matter content. To dry
only half of the sludge is sufficient to bleed steam at 1.5 bar, as the turbine bleed is already in
place for the district heating. The steam is used to heat air that properly dries the sludge. The
estimated energy requirement of the drying is 1.02 kWh per kg of removed water.
The sludge drying is pushed up to get the 65% dry matter.
The amount of steam to perform the drying is computed as follows:
�̇�𝑠𝑡𝑒𝑎𝑚 =𝑒𝑛𝑒𝑟𝑔𝑦 𝑟𝑒𝑞𝑢𝑖𝑟𝑒𝑑 ∙ �̇�𝑒𝑣𝑎𝑝
(ℎ𝑣𝑠 − ℎ𝑙𝑠)@1.5 𝑏𝑎𝑟
During summer, the steam used for sludge drying involves a lower electric power production, as
the steam does not expand and produce work from 1.5 bar to 0.1 bar.
𝑃𝑒𝑙 𝑚𝑖𝑠𝑠𝑒𝑑 = �̇�𝑠𝑡𝑒𝑎𝑚 ∙ (ℎ𝑏𝑙𝑒𝑒𝑑 − ℎ𝑜𝑢𝑡 𝑡𝑢𝑟𝑏) ∙ 𝜂𝑒𝑙 ∙ 𝜂𝑜
Where ηel is assumed to be 0.985 and ηo 0.99; to compute hout_turb, the isentropic efficiency is
considered 0.85.
During winter, that steam is not used to produce heat power for the district heating, while the
electric production is not reduced.
𝑃𝑡ℎ 𝑚𝑖𝑠𝑠𝑒𝑑 = �̇�𝑠𝑡𝑒𝑎𝑚 ∙ (ℎ𝑣𝑠 − ℎ𝑙𝑠)@1.5 𝑏𝑎𝑟
Since the district heating requirement must always be provided, it is necessary to produce the
missed thermal power through the integrative boiler. The needed amount of natural gas is:
�̇�𝑛𝑔 𝑖𝑛𝑡 =𝑃𝑡ℎ 𝑚𝑖𝑠𝑠𝑒𝑑
𝜂𝑐𝑜𝑚𝑏 ∙ 𝐿𝐻𝑉𝑛𝑔
Another important consideration is that if the sludge is burned in the waste to energy plant, a
certain amount of waste cannot be treated, in the hypothesis that the plant capacity is full. The
missing treatment of waste means missed gate fee.
The amount of waste that in this scenario cannot be burned, exceeding the plant capacity, can
be estimated in the following way:
�̇�𝑤𝑎𝑠𝑡𝑒 𝑚𝑖𝑠𝑠𝑒𝑑 =�̇�𝑠𝑙𝑢𝑑𝑔𝑒 65% ∙ 𝐿𝐻𝑉𝑠𝑙𝑢𝑑𝑔𝑒 65% + �̇�𝑠𝑙𝑢𝑑𝑔𝑒 25% ∙ 𝐿𝐻𝑉𝑠𝑙𝑢𝑑𝑔𝑒 25%
𝐿𝐻𝑉𝑤𝑎𝑠𝑡𝑒
However, it is more common the situation in which the waste to energy capacity is not fully
exploited because the actual amount of waste delivered is lower. In this case, the co-incineration
of sludge is obviously advantageous, because it also allows running the plant at a higher load,
closer to the nominal one.
The technological limit in co-incinerating sludge in a waste to energy plant is that the maximum
amount of sludge on the grate (only the drained sludge), is the 15% of the total solid burned.
�̇�𝑠𝑙𝑢𝑑𝑔𝑒,65%
�̇�𝑠𝑙𝑢𝑑𝑔𝑒,65% + �̇�𝑤𝑎𝑠𝑡𝑒∙ 100 = 7% ≤ 15%
6.2.1.3 Third scenario: all sludge dried
Since the amount of sludge to be dried is doubled, the drying air conditions of the previous
scenario are no longer sufficient. As the amount of drying air must not be changed, its
temperature must be increased and the 1.5 bar bleed is not enough. Then, to perform the
drying, another bleed at 45 bar is considered.
The amount of steam at 45 bar is computed in the same way as described above.
In this case, in winter the missed power production is not only thermal, because of 1.5 bar steam,
but also electric, due to the new steam bleed.
The evaluation of the missed power production and missed treatable waste are analogous to
the previous case.
The technological limit is not exceed also in this scenario:
�̇�𝑠𝑙𝑢𝑑𝑔𝑒,65%
�̇�𝑠𝑙𝑢𝑑𝑔𝑒,65% + �̇�𝑤𝑎𝑠𝑡𝑒∙ 100 = 14% ≤ 15%
For all the scenarios, also the R1 indicator for energy recovery has been calculated
Results
Results of sludge co-incineration in the waste to energy plant are summarized in Table 34.
parameter m.u. Only MSW
MSW+ half sludge dried;
half sludge dewatered
MSW+ all sludge dried
Natural gas consumption
t/y 6113 6756 6756
1,5 bar bleed steam t/y 0 23379 23379
MW 0 1.81 1.81
45 bar bleed steam t/y 0 0 24964
MW 0 0 1.81
Produced electricity -summer
MWel 17.57 17.30 16.70
GWh/y 70.29 69.20 66.78
Produced electricity -winter
MW 12.50 12.50 11.90
GWh/y 50.00 50.00 47.58
District heating MWth 41.81 40 40
GWh/y 167.23 160 160
Treatable waste
digested raw
primary digested
raw primary
MW_LHV 71.40 68.43 65.16 67.12 63.85
t/y 130000 124583 118627 122205 116249
R1 Index - 0.67 0.64 0.64 0.62 0.62
Table 34: Co-incineration of digested and raw mixed sludge effect on WtE outputs.
The first consideration that has to be made is that the sludge drying leads to both natural gas
consumption, to the maximum exploitation of the integration boilers, and a reduction in
electricity production. The district heating requirement, as explained before in the procedure, is
fulfilled in all the cases.
The drying of the total amount of sludge worsens all the energy outputs with respect to the case
of only half of the sludge drying; the electric power production, in particular, is the 3% less in
summer and 4.8% less in winter than when only half of the sludge is dried. This is due to the
constraint of performing the additional steam bleeding at a high pressure to have a sufficiently
high temperature of the drying air, whose flow rate cannot be varied; allowing the air flow rate
to vary, adding another dryer, or changing the dryer type, would probably make the comparison
change. Moreover, the effect of the expectable worse combustion conditions of the half sludge
dried-half dewatered case, not considered in this discussion, should be taken into account.
In addition, the amount of waste and sludge (in kg/h, as received) fed to the combustor to keep
the overall 71.4 MW_LHV, could be excessively high.
For how the calculations have been run, the sludge type does not affect the energy inputs and
outputs. The only difference between the two sludge types is the lower amount of treatable
waste for raw primary, with respect to digested sludge, in both the studied scenario. The reason
is simply given by the higher LHV of raw primary sludge.
The R1 index results to be affected by sludge co-firing: since the use of its definition is not proper
when dealing with sludge, it was reported only to give an idea of the recovery. Other indexes
shall have to be used.
7 Sludge Pyrolysis and Gasification Models
7.1 ASPEN
For any information about Aspen and for settings in the property section of Aspen, refer to
paragraph 6.1.4, since all hypothesis for property calculations for sludge and chemical species
considered are unvaried with respect to the Aspen model of mono-incineration.
The model proposed is intended to represent the entire process of sludge thermal disposal used
at IDA Tobl plant, owned by ARA Pustertal (described in APPENDIX 1), to obtain energy input
and output streams to compare the process with other thermal routes from the energetic point
of view, since they are the performance indexes considered in this work. To do so, first an Aspen
model of the slow pyrolysis, which is the innovative and unconventional part of the sludge
thermal treatment, has been built. The slow pyrolysis model was needed to assess the quantity
and composition of products. Successively, additional components that valorize the pyrolysis
products and complete the Pyrobustor® process to make it feasible, such as sludge oxidation
chamber and natural gas support burners, were added in the Aspen model in order to fully
represent the entire sludge thermal disposal at IDA Tobl plant.
In IDA Tobl plant sludge of the WWTP located there, together with raw sludge coming from
neighbourhood WWTPs, is anaerobically digested and then dewatered in loco. Consequently,
the sludge considered as input for the pyrolysis model is a digested sludge and its composition
and lower heating value are assumed to be the same of the sludge used as reference for
“Digested Sludge” in this thesis (see section 412.3.3). This choice is partly due to the
unavailability of data regarding the plant, and partly due to the necessary consistency between
models of different thermal treatments: in fact the same sludge compositon was used for the
models and calculations on sludge mono-incineration and co-incineration (chapter 6).
7.2 Pyrolysis step model
The first step in Aspen, when dealing with NC components, is to decompose the NC stream into
singleton molecules (C, S, O2, N2, H2, ASH): for DECOMP block the RYIELD Reactor was used, as
the yield distribution is known. The mass yields of the reactor DECOMP are determined and set
using a calculator block starting from data of the sludge ultimate analysis.
The
The resulting stream is sent to PYRO block to complete the pyrolysis process.
As it is common for a slow pyrolysis process, the solid residence time is long (hours), however it
is not reasonable to assume thermodynamic equilibrium inside the reactor because of the low
value of temperature (350 °C): for this reason, it was not possible to model the pyrolysis with
the Aspen component RGIBBS. Consequently, it was necessary to use a RYIELD reactor for the
PYRO block, that made necessary to provide some data on the yield of products. According to
Kim et al. [119], that show that syngas composition vary mainly with temperature and not with
sludge composition, syngas yield value is assumed as an average of all values found in literature
concerning sludge pyrolysis at temperatures in the range of 300-400 °C (0.12 kgsyngas/kgsludge).
Author T [°C] Syngas
Yield Author T [°C]
Syngas
Yield
Agrafioti et al. [81] 300 0,19 Kim et al. [119]
300 0,16
Hossain et al. [43] 300 0,14 300 0,11
Waheed et al. [124] 350 0,20 Zajec [92] 300 0,08
Huang et al. [36] 400 0,08 Inguanzo et al. [40] 400 0,10
Beneroso et al. [42] 400 0,16 Nowicki et al. [47]
300 0,03
Sanchez et al. [85] 350 0,20 400 0,06
Gao et al. [34] 400 0,10 Shen et al. [38] 300 0,05
400 0,15 Sun et al. [33] 400 0,12
Kim et al. [119] 300 0,14 Yuan et al. [35] 300 0,09
Mean value 355,6 0,12
Table 35: Pyrolysis syngas yield literature data.
To complete the input data required by the PYRO block it was necessary to define also the
gaseous species present in syngas (CO2, CO, H2 and CH4 are considered) and its molar
composition. Again, this procedure is performed based on a literature review of experimental
slow pyrolysis processes run under conditions similar to the ones of IDA Tobl facility facility.
Table 36 shows the reference studies, including also other biomasses, for syngas composition.
Type of Biomass
Authors
Pyrolysis Temperature
Solid residence
time
Syngas molar composition [%vol]
Syngas LHV
°C min CO2 CO H2 CH4 MJ/kg
Dry sludge
Sun et al. [33]
400 40 82% 6% 6% 6% 2.05
Gao et al. [34]
450 60 78% 8% 10% 4% 2.11
Inguanzo et al. [40]
350 70 78% 14% 0% 9% 2.75
Dry MSW
Beneroso et al. [42]
400 30 68% 29% 0% 2% 2.60
Beech Zajec [92] 350 100 46% 42% 3% 9% 5.92
Table 36: Experimental data for syngas composition.
The choice done for the model is the syngas composition for Zajec [92]: although he studied
another biomass type, the reactor type (tubular rotary reactor) and the process design
parameters, temperature and residence time in particular (for IDA Tobl plant solid residence
time in the whole pyrobustor is 150 min), are analogous to the one of the present study. In
addition, the feedstock composition does not show a great effect on the syngas composition,
because the gas yield is small (0.12 kgsyngas/kgsludge).
Since the liquid yield at 350 °C is expected to be low, it is assumed a unique non-conventional
stream (CHAR-TAR) to collect all the fraction of sludge not volatilized during the pyrolysis step.
The yield of CHAR-TAR stream is calculated by difference with respect to syngas yield. The CHAR-
TAR ultimate analysis is the result of an atomic balance performed in Aspen using a calculator
block:
𝐴𝑡𝑜𝑚𝑠, 𝑖𝐶𝐻𝐴𝑅−𝑇𝐴𝑅 = 𝐴𝑡𝑜𝑚𝑠, 𝑖𝐷𝑅𝑌𝑆𝐿𝐷 − 𝐴𝑡𝑜𝑚𝑠, 𝑖𝑆𝑌𝑁𝐺𝐴𝑆
It is obvious that using this procedure and changing the sludge composition in input, the amount
and composition of syngas predicted by the model will remain unvaried, while the CHAR-TAR
ultimate composition will change as consequence. It can be noticed that H2O is missing in the
vapor phase: it is assumed that moisture content present in sludge, as it is low (10%), remains
bounded in the liquid phase inside the CHAR-TAR stream. For this stream, the lower heating
value is calculated in Aspen with its General Coal Enthalpy Model that includes a number of
different correlations. This is done when the properties for the NC component in HCOALGEN are
set through the option code 1-1-1-1 for enthalpy.
When all the required inputs are set, Aspen is able to calculate the heat required to perform
pyrolysis. The stream exiting the pyrolysis reactor, composed of both conventional gaseous
components and non-conventional solid, is separated in the Cyclone component into the two
streams, called respectively SYNGAS and CHAR-TAR.
7.3 Digested sludge model
7.3.1 Pyrobustor® model Pyrobustor® consists of a two chambers rotating kiln: in the first chamber the sludge is pyrolyzed
between 300-400 °C (this part is modeled as described above), while in the directly following
second chamber the pyrolysis char and tar are partially oxidized at 625 °C by sub-stoichiometric
air flow. The syngas produced in the first chamber pass inside a pipe through the oxidation zone
where it is heated to 565 °C. A natural gas support burner is installed to provide the heat for the
pyrolysis in the first chamber.
Figure 53: Pyrobustor scheme and data [125].
All these parts were modeled in the Aspen environment: CHAR-TAR stream is first decomposed
to conventional components into DECOMP2 block (RYIELD) and then gasified in the CH-GASIF
block (RGIBBS), whose gaseous products enters the SUPPCOMB block (RGIBBS) together with
natural gas and air to complete the oxidation process. The syngas pipe is modeled using a
HEATER component, in the hypothesis that its composition does not change while being heated.
The DECOMP2 block uses a calculator to set the yield distribution of C, S, O2, N2, H2 and ASH,
starting from the ultimate analysis of the CHAR-TAR inlet stream. The outlet stream of
conventional components is sent to the CH-GASIF reactor modeled with a RGIBBS Aspen
component since it is assumed that the process is able to reach equilibrium at 625 °C.
The stream OX-AIR is made of air at ambient temperature and its flow rate is set in a calculator
block according to a fixed value of Equivalence Ratio (ER = 0.3), defined as actual air/fuel ratio
over stoichiometric air/fuel ratio. The value of 0.3 is in the range of the ones used in literature
for sludge gasification (see paragraph 3.3.5). In this way, changing the characteristics of the
sludge in input to the process or the pyrolysis conditions before, the extent of the partial
oxidation will remain constant by changing the amount of OX-AIR.
The equilibrium species considered for the RGIBBS reactor CH-GASIF are CO2, CO, WATER, SO2,
NO2, NO, N2, S, H2, H2S, C2H4, C2H2, CH4, C2H6, C3H8.
Notice that solid carbon is not present in the list of possible products, since its volatilization
process is not calculated based on equilibrium as for all the other species, but it is imposed using
a calculator block to take into account the constraint of 3% wt of Carbon in the discharged ashes.
Part of the heat produced in the CH-GASIF block is sent to the HEATER component called HX,
which is in charge of heating-up syngas to 565 °C, and the rest is provided to the pyrolysis
process. Downstream this component the ashes are cooled, since they should be discharged at
a lower temperature (300 °C) with respect to the one of the gaseous products, and the resulting
heat stream (Q-ASH) is sent to pyrolysis.
Immediately after the gasification reactor, the ashes are separated by the gaseous phase in a
CYCLONE component. The GASIFSYN stream enters in the support burner, called SUPPCOMB
and modeled using a RGIBBS reactor, together with natural gas (SUPP-NG) and air (AIRSUPPC)
streams. After the SUPPCOMB, an HEATER component (HX2) is placed to cool-down the CH-OX-
FG stream to 384 °C: the heat produced in HX2 is sent to pyrolysis.
All the heat streams directed to pyrolysis are sent in a MIXER called QMIX together with the heat
requirements from the pyrolysis itself. The resulting stream of QMIX block is called Q-PYRO. If
Q-PYRO is equal to zero means that pyrolysis heat requirement is fulfilled.
The SUPP-NG flow rate is calculated according to a design specification in which it is varied until
the target of QPYRO = 0±100 W is achieved. The air amount to the support burner is varied in
another design specification such that the combustion temperature reaches 891 °C, as indicated
by the data provided.
The hypothesis made for the pyrolysis-based process simulation are summarized in Table 37,
with the aim of clarifying the procedure and underlining the unavoidable limits.
List of hypothesis
Pyrolysis
1) Sludge composition does not influence syngas yield and composition,
with respect to temperature and residence time.
2) Char and tar are considered as a unique stream, with calculated
ultimate composition; the molecular composition has not been neither assumed nor evaluated.
3) All the moisture present in sludge is considered in char-tar stream
(it is not considered to volatilize because of its low amount).
Pyrobustor
4) Everything is considered at atmospheric pressure.
5) In the char-tar gasification process, the gaseous species are considered
to the at the equilibrium condition.
6) The amount of C in char-tar reacted in gasification is fixed such that
there is 3% C in the discharged ash.
7) The pyrolysis syngas composition is not considered to change while
being heated by char-tar gasification.
8) No wall heat loss are considered.
Table 37: Hypothesis assumed to perform the pyrolysis model.
7.3.2 IDA Tobl plant model To assess final results, as energy inputs and outputs, it is necessary to extend the boundaries of
the model to the whole IDA Tobl facility, schematically represented in Figure 54. New
components are added: a post-combustion chamber that burn pyrolysis syngas, gaseous
products exiting the Pyrobustor and supplementary fuel (natural gas), and a dryer that, using
the heat of post combustor chamber, is responsible for the sludge drying to the 90% of dry
matter. The Aspen flowsheet of the entire ARA Pustertal Model is reported in Figure 55.
Figure 54: IDA Tobl plant configuration [125].
Figure 55: Aspen Flowsheet of IDA Tobl plant model for digested sludge.
As described above, four different streams are fed to the POSTCOMB block: HSYNGAS, FGOX,
PC-NG, PC-AIR. The POSTCOMB is modeled using a RGIBBS reactor, since the oxidation reaction
at 900 °C with a large excess air is considered at equilibrium. In the POSTCOMB reactor adiabatic
conditions are imposed.
Composition and mass flow rates of pyrolysis syngas (HSYNGAS) and of products exiting the
Pyrobustor (FGOX) were calculated from the Pyrolysis and Pyrobustor models (paragraphs 7.2
and 7.3.1). Natural gas (PC-NG) and oxidation air (PC-AIR) mass flow rates are set by design
specifications. The first is varied to fulfill the constraint on the heat duty at the DRYER block,
while the second is varied in order to reach the temperature of 900 °C in the post-combustion
chamber.
The combustion products (FG) are cooled down to 165 °C in HeatX block called DRYER
representing the energy demand needed for drying, that corresponds to 1700 kW, as stated by
the plant operator and confirmed in an Aspen Model for the belt dryer. In the dryer model, the
temperatures are the same as suggested in section 2.2.5; exhaust air exiting the dryer is in part
(80%) recirculated, since it is not saturated, and mixed with make-up air that accounts for 20%
by mass of total air fed to the dryer.
Design specifications for Digested Sludge
n. target varying
variable value tolerance m.u. variable
1 T fg supp-comb 891 1 °C air mass flow supp-
comb
2 Qsupp-comb + Qchartar-
gas -Qpyrolysis 0 0.1 kW
natural gas mass flow supp-comb
3 T fg post-comb 900 0.1 °C air mass flow
post-comb
4 Q dryer 1700 1 kW natural gas mass flow post-comb
Table 38: Summary of design specifications used in the ARA Pustertal Model for digested sludge.
7.4 Raw primary sludge Model
It is now interesting to investigate the behavior of the model when the plant is fed with a
different type of sludge. In particular, raw primary sludge is considered in this section.
As for the previous case, the composition and lower heating value considered for the input
sludge in this model are the same of the reference for “Raw Primary Sludge” in this work
(paragraph 2.3.3). The change of input sludge, according to hypothesis assumed in the pyrolysis
model, will cause no change in syngas yield and composition. Therefore the Pyrolysis model is
unchanged with respect to the digested sludge model, while some significant modifications were
necessary for the Pyrobustor and IDA Tobl plant models.
7.4.1 Pyrobustor® model First, it was made the attempt to run the Aspen model with same assumptions and
values described in the previous paragraphs, and the results was that the gasification the Pyrobustor was supplying more heat than what required by the pyrolysis zone. This result
was obtained despite the natural gas fed at Pyrobustor’s support combustor, calculated by design specification set to balance pyrolysis heat, was zero. Consequently, the Pyrobustor’s support combustor has been eliminated from the Aspen model (as can be seen in the Aspen
flowsheet reported in
Figure 56) and the gasification syngas has been sent directly to the post-combustor without
undergoing any intermediate oxidation.
7.4.2 IDA Tobl plant model However, it is necessary to set another design specification to ensure that heat needed for the
pyrolysis is provided: the temperature of the char-tar gasification is varied (increased) until the
thermal balance is achieved. Proceeding with the IDA Tobl plant model applied to raw primary
sludge, it turns out that also supplementary fuel at post-combustor is not necessary (“PC-NG
stream has 0 flow rate”): for digested sludge feeding case, it was varied in order to satisfy the
constraint of 1700 kW at the DRYER block. The model is, therefore, modified, since also without
natural gas the heat available after the POST-COMB unit exceeds the 1700 kW. The new design
specification that allows to exactly match the DRYER demand is related to exhaust gas
temperature, which can be cooled less with respect to the case of digested sludge.
The design specifications used for raw primary are summarized in Table 39.
Design specifications for Raw Primary Sludge
n. target varying
variable value tolerance m.u. variable
1 Qsupp-comb + Qchartar-gas -
Qpyrolysis 0 0.1 kW T char-tar gasification
2 Q dryer 1700 1 kW T exhaust gases
3 T fg post-comb 900 0.1 °C air mass flow
post-comb
Table 39: Summary of design specification used in IDA Tobl plant model for Raw primary sludge.
Figure 56: Aspen Flowsheet of IDA Tobl plant model for raw primary sludge.
7.5 Summary of data and results
In Figure 57 and Figure 58, the energy balances for the slow pyrolysis process, based on results
generated by the Aspen simulation for both digested and raw sludge, are represented in Sankey
diagrams. As expected, the fraction of energy that continues in the syngas stream is much less
than the one of char-tar. It can be noticed that the heat required for raw primary sludge pyrolysis
is lower than for digested sludge one, both on absolute terms (0.25 vs 0.4 MW), but especially
as percentage of the total energy input to the process (9% vs 21%).
Figure 57: Energy Balance in the Pyrolysis model fed by digested sludge.
Figure 58: Energy Balance in the Pyrolysis model fed by raw sludge.
Results of the digested and raw primary fed pyrolysis-base models are summarized in Errore.
L'origine riferimento non è stata trovata., Table 40 and Table 41, together with the provided
data of the real facility and the chosen input values.
Parameter
Data of the plant
(digested sludge)
Digested sludge
Raw sludge
sludge in pyrobust
or
flow rate [kg/h] 550 550 550
DM 90% 90% 90%
dry basis
C
not available
30.2% 43.4%
H 4.2% 6.0%
N 4.6% 6.9%
Cl 0.0% 0.0%
S 0.8% 1.2%
O 15.1% 19.3%
ASH 45.1% 23.2%
LHV [MJ/kg] 11.17 18.7
pyrolysis
Solid residence time [min] 150 150 150
T pyrolysis [°C] 350 350 350
syngas
mass yield not
available 0.12 0.12
Cp @350° [kJ/kg K] not
available 1.21 1.21
mass flow rate [kg/h] not
available 66 66
molar composition
CO2
not available
46% 46%
CO 42% 42%
H2 3% 3%
CH4 9% 9%
Q pyrolysis [kW] not
available 398 217
char-tar
mass flow rate [kg/h] not
available 484 484
Cp [kJ/kg K] not
available 0.42 0.4
LHV [MJ/kg] not
available 13.11 19.75
dry basis
C
not available
29.50% 44.73%
H 4.65% 6.73%
N 5.31% 7.96%
Cl 0% 0%
S 0.92% 1.38%
O 7.58% 12.43%
ASH 52.04% 26.77%
DM not
available 88.64% 88.64%
data input results
Table 40: Summary of data, input and results of pyrolysis-based process model. Part 1.
Parameter
Data of the plant (digested sludge)
Digested sludge
Raw sludge
char-tar gasification
T gasification [°C] 625 625 642
equivalence ratio not
available 0.3 0.37
air mass flow rate [kg/h] not
available 602.4 1100.0
char-tar gasification
syngas
molar composition
H2
not available
22.9% 20.4%
N2 45.5% 49.2%
H2O 5.1% 4.1%
CH4 2.9% 1.3%
CO 15.3% 16.5%
CO2 8.0% 7.5%
H2S 0.3% 0.3%
other HC
trace 0.6%
ash mass flow rate [kg/h] not
available 230 118.4
Carbon mass fraction in ash 3% 3% 3%
T syngas [°C] 565 565 565
support combustor
air mass flow [kg/h] not
available 596 -
natural gas mass flow [kg/h] not
available 44.5 -
supp-combustor
gases
T fg supp-comb [°C] 891 891 -
O2 fraction (v) not
available 0.02% -
T fg supp-comb. Cool [°C]
384 384 -
Qsupp-comb + Qchartar-gas -Qpyrolysis [kW] not
available 0.007 0
post-combustor
air mass flow [kg/h] not
available 5571 6873
natural gas mass flow [kg/h] not
available 4.7 0
post combustor
gases
T fg post-comb [°C] 900 900 900
mass flow rate [kg/h] 8100 7139 8406
O2 fraction (v) not
available 11.2% 12.3%
dryer Q dryer [kW] 1700 1700 1700
T fg after dryer [°C] 165 165 277
total natural gas consumption [kg/h] 60 49.2 0
Table 41: Summary of data, input and results of pyrolysis-based process model. Part 3.
In both cases the molar oxygen fraction in the flue gases exiting the post-combustor is very high
(11.2% for digested and 12.3% for raw), meaning that a large excess of air is used. This was due
to the constraints on flue gas temperature at outlet of post-comb and heat required from dryer:
if 900 °C cannot be exceeded (mainly due to downstream heat exchangers design reasons), using
reasonable values for excess air, the flue gases mass flow rate would not be enough to satisfy
the constraint of providing 1700 kW. In practice, the design specification increases the air
amount far above the stoichiometric value, to subsequently increase the flue gases flow rate. As
an overall result, a big portion of air (not required for oxidation) is just heated form 25 °C to the
final exhaust temperature at the stack (165 °C for Digested and 277 °C for Raw Primary).
From a thermodynamic point of view, this design choice made for IDA Tobl plant is questionable:
the stack losses are too high. Probably, it should be better to set typical values of oxygen fraction
in flue gases (3% vol.) and do not provide all the required heat using the flue gases stream, but
provide the remaining part with a natural gas additional burner directly for the dryer. In this
case, the natural gas burned directly at dryer should be less than the extra natural gas actually
consumed to satisfy the dryer constraint with just flue gases and high stack losses.
A schematic view of the whole thermal plant is provided in Figure 59 (for Digested sludge) and
Figure 60 (for Raw Sludge) at the end of this chapter together with the main results of mass and
energy balances computed by Aspen.
sludge
diathermic oil
flue gases M wet mass flow rate kg/h
air DM dry matter content %wt wet basis
natural gas T temperature °C
ash LHV lower heating value dry MJ/kg
Pyrolysis syngas Q thermal power kW
air + evaporated moisture
Gasification syngas
Char-tar
Table 42: Legend for Figure 59 and Figure 60.
Figure 59: Schematic overview of the IDA Tobl Aspen model with results for Digested Sludge
Figure 60: Schematic overview of the IDA Tobl Aspen model with results for Raw Primary Sludge
8 Primary energy consumption of different scenarios
A comparison in the primary energy utilization for the disposal of digested and raw sludge can be
performed. Actually, it corresponds to the investigation of a route in which raw primary sludge is
directly fed to the thermal conversion facility (Incineration plant or Pyrobustor), against another route
in which raw primary previously undergoes anaerobic digestion and biogas production, and its
digestate is thermally converted through incineration or pyrolysis. The two paths are defined in the
figure, .
Figure 61: CASE AD+TCP INC plant configuration.
Figure 62: CASE TCP ONLY INC plant configuration.
Figure 63: CASE AD+TCP PYRO plant configuration.
Figure 64: CASE TCP ONLY PYRO plant configuration.
With reference to the chapter 5, the net biogas production from 495 kg/h of dry digested sludge fed to
the Pyrobustor or to the incineration plant is 274.5 Nm3/h. To obtain such amount of digested sludge
flow rate, 962 kg/h of dry raw primary has to be digested (this calculation is according to procedure
described in chapter 5). As reported in Table 41, for the PYRO case, the natural gas total consumption
is 49.2 kg/h, while in all the other cases no natural gas is consumed. The electric energy consumption
of the dryer and Pyrobustor, in the PYRO case, is considered to be the same for both sludge types and
equal to 0.245 MW, while in the INC case it is already considered in the value of net electricity
production.
To get the net difference of primary energy utilization between CASE AD+TPC and CASE TCP ONLY, the
following equation is used:
𝑃𝑟𝑖𝑚𝑎𝑟𝑦 𝐸𝑛𝑒𝑟𝑔𝑦𝑜𝑢𝑡 = 𝑃𝑟𝑖𝑚𝑎𝑟𝑦 𝐸𝑛𝑒𝑟𝑔𝑦𝑜𝑢𝑡,𝑏𝑖𝑜𝑔𝑎𝑠 + 𝑃𝑟𝑖𝑚𝑎𝑟𝑦 𝐸𝑛𝑒𝑟𝑔𝑦𝑜𝑢𝑡,𝑓𝑙𝑢𝑒 𝑔𝑎𝑠𝑒𝑠
− 𝑃𝑟𝑖𝑚𝑎𝑟𝑦 𝐸𝑛𝑒𝑟𝑔𝑦 𝑖𝑛 ,𝑛𝑎𝑡.𝑔𝑎𝑠
Where:
𝑃𝑟𝑖𝑚𝑎𝑟𝑦 𝐸𝑛𝑒𝑟𝑔𝑦 𝑖 = �̇�𝑖 [𝐿𝐻𝑉𝑖 + 1.1 𝐶𝑝𝑖 (𝑇𝑖 − 25)]
The amount of flue gases that has to be considered for raw primary calculation must be relative to 962
kg/h. While the LHV energy is already primary energy, the thermal energy (in this case the only the one
in flue gases) must be converted in primary energy through a multiplying factor of 1.1 (thermal
efficiency 0.909), and the electric energy through the multiplying factor of 2.6 (electric efficiency 0.385).
When the exhaust gases have an higher temperature with respect to 165 °C of the AD+TCP PYRO case,
considered as the minimum temperature for flue gas cooling, the possibility to further recover energy
from the hotter gases (until 165 °C) is considered. With this assumption, flue gas represent a primary
energy output, but only in the case in which their heat is actually exploited, which means availability of
equipment and thermal user presence.
The results of all the four scenarios are summarized in Table 43.
Apparently, the disposal with both biogas production and thermal conversion shows a lower primary
energy consumption.
The results, however, are affected by the assumed values for: gross specific biogas production,
anaerobic digestion electricity consumption, biogas LHV. A small change in these parameters could
make the only thermal conversion case more attractive. To resolve this problem, the values of the
specific facility under study must be used, thing that was not possible in this discussion.
For the PYRO case, the amount of natural gas consumption, resulting from the simulation, has a deep
effect as well; it must be taken into account that it results from a no loss case, and it is supposed to be
higher in reality (the declared value of the IDA Tobl plant is 60 kg/h, and not 49 kg/h). This would lead
again to better performance of CASE TCP ONLY PYRO, than the one evaluated.
In the comparison between the two thermal conversion processes, it results that the incineration
facility shows a lower primary energy consumption, mainly because of the production of electricity.
PYRO INC
AD+TPC TPC
ONLY AD+TPC
TPC ONLY
Wet sludge in Pyrobustor flow rate kg/h 1980 3850 1980 3850
Moisture Content - 75% 75% 75% 75%
Dry Sludge in Pyrobustor flow rate kg/h 495 962 495 962
Dry Raw Sludge flow rate kg/h 962 962 962 962
Moisture Content - 75% 75% 75% 75%
Wet Raw Sludge flow rate kg/h 3850 3850 3850 3850
Raw Sludge LHV [dry basis] MJ/kg 18.7 18.7 18.7 18.7
Raw Sludge LHV wet MJ/kg 2.875 2.875 2.875 2.875
Primary Energy in Raw Sludge MW 3.07 3.07 3.07 3.07
Net specific biogas production Nm3/kg dig
dry 0.55 0 0.55 0
Net Biogas flow rate Nm3/h 274.49 0 274.49 0
Biogas LHV MJ/Nm3 23 - 23 -
Primary Energy out Biogas MW 1.75 0 1.75 0
Natural Gas total consumption kg/h 49 0 0 0
Natural Gas LHV MJ/kg 44 - - -
Primary Energy in Natural Gas MW 0.60 0 0 0
Electricity consumption MW 0.25 0.25 0 0
Primary energy in electricity MW 0.64 0.64 0 0
Net Electricity production MW 0 0 0.13 0.47
Net Primary Energy out electricity MW 0 0 0.34 1.21
Flue Gas mass flow rate kg/h 7145 16345 4984 12615
T Flue Gas °C 165 277 200 200
T ref °C 165 165 165 165
Cp Flue Gas kJ/kgK 1.1 1.1 1.1 1.1
Primary Energy out Flue Gas MW 0.00 0.62 0.06 0.15
Net Primary Energy consumption MW 2.56 3.09 0.92 1.71
Table 43: Summary of primary energy consumption calculation.
9 Conclusions
In the sections 1.2 and 3.4, where EU data are analyzed, it is shown that sludge production will increase
for new member states and, for all the EU-27 states, that the thermal treatment disposal route is
continuously increasing and is expected to replace a big portion of the agricultural use and landfill
actual share in the near future. In addition, the legislative framework, indirectly, opens the road at the
thermal treatments. Directive 1999/31/EC on landfill together with the Waste Framework Directive
2008/98/EC (that defines the waste hierarchy), contribute in zeroing the landfill route, while, in
consequence of the Directive 86/278/EC on the protection of the environment and soils, it turns out
that not all sludge is suitable to be reused in agriculture for fertilizers recovery.
According to the expected trends, the important role that thermal treatments will have for the disposal
of sludge it is evident. It is therefore important to study the performances of the main thermal routes
in order to select the best process in connection with the waste hierarchy.
The most important parameter to determine the energy recovery possibilities is the LHV of the dry
matter present in the sludge. By applying recent correlations developed for the sludge at data of some
representative WWTPs in the Parma and Reggio Emilia area, together with a review of literature data,
it is found that the LHV of dry matter for Raw Primary, Raw Mixed and Digested Sludge are respectively
of 18.7, 15.5 and 11.2 MJ/kg.
The energy recovery from sludge, even if it is previously dewatered, is not an easy task, essentially
because of its moisture content, largely higher then every other biomass type. The drying energy needs
have a great effect on the overall energy balance of any sludge thermal treatment: it is found that
around 1 kWh of thermal energy is needed for each kg of moisture evaporated.
In this work both the traditional and established thermal processes (Mono-incineration, Co-incineration
in WtE) and the innovative ones (Pyrolysis and Gasification) have been studied and modeled using
Aspen Plus.
From the energy recovery point of view, models of the traditional thermal treatments show and
confirm their advantages with respect to Pyrolysis and Gasification:
Dewatered sludge incineration is always auto-thermal if preheated air temperature is adjusted
according to the type of sludge, ranging from ambient temperature to 650 °C.
Dispose of sludge in mono-incineration plant lead to a specific net electricity production of 0.49
kwh/kg of dry raw sludge and 0.26 kWh/kg of dry digested sludge, generated by means of a
heat-recovery steam cycle.
Co-incineration of sludge in a WtE plant (using a feed with a ratio 2.6 kg_waste /1
kg_dewatered sludge) is not subject of significant variations in the R1 index, which diminishes
of 3-5% points, depending on the configuration. R1 was taken just as a performance indicator
because in any case being the sludge considered a special waste by the legislation, it will not
enter in the calculation for the achievement of energy recovery status.
However, qualitative considerations on mono-incineration made through the SWOT analysis tool,
highlighted that the high cost due to fluidized bed combustor, emission control systems and ash
disposal make mostly large-scale application to have attractive economics.
Co-incineration in already existing WtE instead, do not present the problem of investment costs, but it
cannot be considered a long-term solution, since the capacity of the existing plant will be saturated
soon. Moreover, material recovery from ashes is not feasible in co-incineration.
Differently, pyrolysis and gasification innovative processes are suitable for the small scale and they
could be applied near the WWTPs since they are characterized by a more compact configuration,
compared to incineration plants. Furthermore, they could reach the status of “zero waste process”,
because of phosphorous recovery possibility and ash re-utilization as construction materials.
To make also quantitative comparisons between incineration and pyrolysis, the whole thermal
conversion plant (TCP) visited at IDA Tobl in S. Lorenzo was modeled in the Aspen environment.
The selected technology is a slow pyrolysis at 350 °C and have a long solid residence time leading to
12% of syngas yield. The process requires a dried sludge (90% of dry matter). Model simulation results
show that, although energy recovery cannot be achieved, the process has the capability of disposing of
raw primary sludge without supplementary fuel consumption, providing the energy needs for drying
and pyrolysis from pyrolysis products thermal valorization. In fact, the pyrolysis syngas produced is
burned together with a second syngas stream produced by char and tar gasification.
Finally, the integration of biogas production, by means of raw primary sludge anaerobic digestion (AD),
upstream the TCP is investigated. It results that if the TCP is fed with digested sludge (CASE AD+TCP), it
is necessary to make use of natural gas (0.1 kg of NG/ kg of dry sludge) to fuel the energy needs of the
process. However, the previous biogas production must have relevance in the analysis, and a
comparison of the two cases (TCP ONLY and AD+TCP) primary energy consumption is performed. Its
result is a moderately lower primary energy consumption of the AD+TCP case, but great care must be
paid in the assessment of the input values chosen for the calculation, primarily the ones relative to the
anaerobic digestion process, as they could lead to a result or another.
An economic analysis would be useful in order to be able to choose between the two cases, and it is
suggested as future work, as there was not the possibility to significantly develop it in the present thesis
setting.
Similar scenarios were considered for the mono-incineration of raw and digested sludge. By comparing
them on the same small scale (disegned to serve WWTP of a small province) with pyrolysis-based
thermal disposal, it results that the seconds are still needing a long attention and research to reach
energetic performance of the mono-incineration.
In general, it is difficult to compare sludge pyrolysis and gasification to conventional thermal treatments
and to find an absolute solution, as the innovative technologies are present in many different forms
and a BAT definition is not available. It is incorrect to extend the conclusion made on this particular
pyrolysis-based process to all the others; it should be necessary to evaluate each technology case-by-
case.
APPENDIX 1
Pyrobustor: IDA TOBL plant by ARA Pustertal, San Lorenzo di Sebato (BZ) Introduction The IDA TOBL plant [125] owned by the ARA Pustertal company is located in San Lorenzo di Sebato (BZ),
Italy. It is the only waste water treatment plant in central Europe housed inside a cavern. Its
construction started in 1991 and it has been operating since 1996 [125].
ARA Tobl serves 14 communities in the Puster Valley and has a catchment area of 1 150 km2, with 130
000 equivalent inhabitants.
To reduce volume and mass (of 88% and 93% respectively) of the treated sludge, a dryer, first in 1999,
and then a thermal valorization plant in 2005 have been added.
This need has been driven by different reasons:
the will of avoiding the dependence on Po Valley landfill and the increasing costs for the treated
sludge disposal, which otherwise was the only possible solution
possibility of exploit thermal energy from the thermo-valorization of dried sludge process.
the saving of primary energy
The drying plant is performed by means of hot air in a belt dryer, while the thermal treatment plant is
based on the Pyrobustor® technology developed by EISENMANN® [126].
Figure 65: View of the IDA Tobl plant within its landscape
Process description The sludge coming from the first clarifiers (primary sludge) is mechanically thickened by a rotating
drum, and sent to the secondary clarifiers. The pre-thickened sludge is digested into an anaerobic
digestion plant (AD). During the digestion process, sludge is stabilized since microorganisms transform
the organic compounds into Biogas (composed on molar basis by 65% CH4, 30% CO2 and water vapor
for the remaining part [125]) It’s energetic content is partly exploited and used for thermal heating of
digestion chambers and for factory heating. The remaining part of Biogas not used for this purpose is
collect in a storage tank and then used to produce electrical power in three internal combustion engines
(150 kW each). The thermal power discharged by the engines is recovered and used to heat-up the the
galleries. It must be notice that to increase the performances of the digestion process, the plant
operator choose to add cheesy whey bought from elsewhere. Therefore it is difficult to evaluate the
performance in biogas production due to the sludge only.
The digestate from the digester, together with the sludge from the others waste water treatment plants
is then dried in a belt dryer, which substitutes since 2008 the Vomm Turbo-technology (owned by
VOMM® Impianti e Processi Spa) replaced both for security and performances reasons [125]. The belt
dryer use circulating air heated in a heat exchanger where pass diathermic oil that extract heat from
the flue gases of the downstream thermal conversion plant (TCP). Thanks to the TCP plant more than
55% of natural gas necessary for the drying process is saved with respect to the case without TCP
installed (pre 2005): 60 kg/h of natural gas are consumed instead of 132 kg/h.
During the drying process the water content of the sludge is decreased, on average, from 75% at the
inlet to 10% at the outlet.
Another aspect to consider is the management of the exhaust air coming from the drying plant (15 000
m³/h), since it main contains pollutants absorbed during the contact with sludge: the air is flowed into
a wet scrubber, where is treated and cooled with biologically treated water. Before being emitted in
the atmosphere the air is forced to pass in a 320 m² surface bio-filter. Into the bio-filter the air coming
from the thickeners (13 000 m³/h) is also biologically treated.
The dried sludge is sent to the Pyrobustor®, which consists in a two stage rotary kiln. The first stage is
a endothermic pyrolysis process at 300-400°C, while the second one is a exothermic gasification process
at 600-650°C. In the oxidation zone of the Pyrobustor® a supplementary burner, fed by natural gas, is
installed to ensure that in the pyrolysis zone enough heat is provided to maintain the proper
temperature for the pyrolysis process.
The ashes produced by the thermal treatment are valuable since they are recycled for construction’s
material production purposes.
Flue gas and pyrolysis gas are then oxidized at 900°C within a post combustion chamber, also in that
combustion chamber a supplementary firing, fed by natural gas, is installed. The heat recovery system
takes out the energy from the flue gas to be used within the air for drying process by means of
diathermic oil as heat exchange medium.
A bag filter house with a dry neutralization guarantees minimum emissions of acid gases and fine
particulate matter. The plant is equipped with “continuous clean gas monitoring” that ensure to fulfill
the ecological standards prescribed for of the plant.
Digester Two anaerobic digestion chambers, with a usable volume of 1800 m3 each, were built at the same time
of the wastewater plant, inside the mountain. They stabilized the sludge and produce a valuable biogas
at the same time. Residence time is of sludge inside the reactor is 30 days and it is heated. In addition
to sludge, in the digester, is fed also cheesy whey, which is bought by the company for the purpose of
enhance and accelerate the digestion process.
In year 2014 the has been able to produce 1 553 382 Nm3 of biogas, while around 25 139 ton/y of
stabilized sludge leave the digester with a moisture content of 75% is dewatered and stored before
being sent to drying plant. At that point its dry lower heating value is reduced to 10-12 MJ/kg, which
is a typical range for digested sludge.
Figure 66: Drawing of Digestion facilities at IDA TOBL, San Lorenzo di Sebato.
Reduction in lower heating value due to digestion will reduce the amount of heat recoverable in the
TCP downstream, however that part has been added to the plant 10 years later but probably, as
suggested by the IDA Tobl CEO Konrad Engl [125], in the design phase it would be possible to think to
skip sludge digestion.
Cogenerative Engines The biogas is collected in a gasometer of 135 m3 to compensate the biogas overproduction with respect
to engine consumption.
In 2014 the three gas engines, composing the power generation section of the plant, have been
operative for 8440 hours and according to the amount of biogas produced in the digester the flow rate
of biogas available for the engines turns out to be 200 kg/h, with a methane molar fraction of 65%. As
results of our calculations and as confirmed by the Piping&Instruments (P&I) diagram here reported,
the electrical power produced by each engine is around 140 kW. From any single engine are also
recovered 220 kW of thermal power, which is probably exploited for heating the digester.
Figure 67: P&I of Gas Engines present at Ida Tobl Plant.
Dryer Digested and dewatered sludge of the IDA Tobl plant, together with sludge from other 10 municipal
waste water treatment plants of the province, is fed into the drying plant.
The technology used is a belt dryer, provided by ANDRITZ SEPARATION®[24] by means of circulating
134 000 m3/h of air entering in the system in different sections with an average temperature of 134°C.
The air, which takes 1700 kW of thermal power from a close circuit of diathermic oil, is recirculated
(80%). The exhaust air that is not recirculated, is treated in a scrubber and in a bio-filter and finally sent
to the atmosphere.
The ANDRITZ SEPARATION belt drying system granulates the dewatered sludge in a mixer with sludge
that has already been dried [24]. The layer of material on the belt creates optimum conditions for
distribution of the drying air. This, in turn, is necessary for even heating and drying of the sewage sludge
during its residence time of 30-40 minutes. In addition it forms a filter medium for the air flowing onto
the granulate layer from above and thus prevents entrainment of dust. The low temperature of the
drying gases (< 150°C) and the low dust content in the system facilitate safe operation. The dried
material is not exposed to mechanical stress during the process and it is also pre-cooled before the
dryer discharge.
The technology presents the following advantages:
▪ The belt dryer is particularly attractive economically because it uses waste heat with a low
temperature.
▪ Modular structure and simple design
▪ High availability.
Figure 68: Picture of the Belt Dryer in operation at Ida Tobl Plant.
In the IDA Tobl dryer 2350 kg/h of moisture is evaporated, consequently the mass flow rate of sludge
pass from 2900 to 550 kg/h. At the end of the process sludge has a moisture content of less then 10%
as reported by data of 2014: a good result compared to the VOMM turbo-dryer, previously in operation
in the plant, which had never reduced moisture content lower than 20%.
Air purification The 15 000 m3/h of exhaust air at 64°C from the dryer pass through a wet scrubber where, by means
of 15 l/s of water, is cooled down to approx. 30°C and its NH3 content is reduced from 400-40 ppm.
Than it is mixed with air coming from the post-thickening (13 000 m3/h) and it is treated together with
it in a bio-filter where NH3 is further reduced to less than 10 ppm and also dust, HCl, HF, H2S, NOx, SOx
are captured from the air before being emitted in atmosphere.
Figure 69: Picture of the Bio-Filter in operation at Ida Tobl Plant.
Pyrobustor® In year 2014 just 3666 tons of dried sludge over the 5500 tons exiting from the dryer is treated in the
TCP section, which has been in operation for 7995 hours in year 2014. Hence, an average mass flow of
sludge of 460 kg/h with 10% of moisture content is fed into the Pyrobustor®. However it was sized to
550 kg/h.
Figure 70: 3D Draw of the Pyrobustor technology present at Ida Tobl.
In response to rising landfill disposal costs, EISENMANN has developed Pyrobustor® to reduce waste
mass through the thermal treatment of sewage sludge [126]. Through a process of pyrolysis and
oxidation, dried sewage sludge is converted into usable heat energy and inert ash suitable for disposal
in local landfills or reuse in industrial processes. This technology also offers a significant reduction in
energy consumption and substantially lowers the cost of disposal [126]. Its advantages are:
Mass Reduction
Reduced disposal costs
No complex pre-treatment required
Significant energy savings
Compact design
From a storage tank, the dry sludge granulates are dosed into the Pyrobustor® through an infinitely
variable, water-cooled screw conveyor. In the first chamber, the material is pyrolyzed at 350°C. In the
directly following second chamber, the pyrolysis char and tar are gasified at 625°C to inert ash with
3%wt of carbon. Helically arranged transport and mixing blades (Figure 71) are responsible for the
transport inside the Pyrobustor®.
Figure 71: Inside view of the pytolysis chamber of the Pyrobustor
Figure 72: Inside view of Pyrobustor and Piping.
The flue gases formed in the gasification process pass the ring gap between the main tube and the
pyrolysis tube in counter current of the material and thus deliver the process heat required for the
pyrolysis. Then they leave the Pyrobustor® (at 384°C). After exiting the Pyrobustor®, the flue gases
loaded with dust are guided across a cyclone, where a large part of the dust particles carried along are
separated and then disposed of via a lock. Then finally they enter in a post-combustion chamber.
While the syngas formed during pyrolysis, passing in a close pipe across the combustion section, reach
a temperature of 565°C and is sent directly to the post-combustion chamber.
The inert portion of the sewage sludge that remains in the form of ash falls into the outer tube of the
Pyrobustor®, at the end of the combustion part, and is transported to the ash disposal system via
transport and mixing blades.
The ash is conditioned so that it can be disposed of on any domestic refuse dump of dump category 1.
ARA Pustertal, however, found a much better solution that is even more environmentally friendly. The
residual product is used as filler in a brickwork.
Figure 73: P&I screenshot of Pyrobustor during the operation at Ida Tobl Plant.
Post-combustion and heat recovery The actual flue gas purification is performed in a pre-combustion chamber by combusting the pyrolysis
and oxidation gases generated in the Pyrobustor®. The combustion at 900 °C is characterized by a gas
residence time of 2 seconds.
In the heat recovery system (heat exchanger flue gas-diathermic oil) that follows, the 8100 kg/h of hot
flue gases are cooled from 900°C to 164°C. The heat that amount for 1700 kW is exchanged to heat 175
m3/h of thermal oil that heats the air for the drying process in a close circuit where oil is cooled down
from 195°C to 175°C.
Both oxidative part of Pyrobustor® and post-combustion chamber are equipped with support burners
that burn together 60 kg/h of natural gas at design condition of 550 kg/h of sludge fed.
Instead, to provide the thermal power for the drying process, without using TCP, it would be necessary
to burn 132 kg/h of natural gas: it turn out that 55% of primary energy necessary is saved using the TCP
to treat sludge.
Figure 74: Heat exchanger oil-flue gases to recover heat released by the combustion
Flue gas treatment line The fabric filter that removes fine dust from the flue gases In addition, adsorbents (12,7 kg/h of
bicarbonate) are blended into the flue gases before they enter the filter to separate acid gas and bond
any heavy metals. The ashes collected downstream the fabric filter results to be 28 kg/h. Following dust
removal, an induced draught ventilator transports the cleaned flue gases that have been cooled to
approx. 164 °C into the stack.
Figure 75: Overview of the sludge thermal disposal scheme for Ida Tobl plant.
APPENDIX 2
Pyrobio: Synecom plant, in Pedrengo (BG)
Introduction The plant has been installed by Synecom to dispose of 1 t/h of industrial sludge (with a minor part of
waste paper and wood) for Italcanditi, company which operates in the agri-food sector. The same
technology can be used also for waste water treatment sludge, as it has been happening in the case of
Fismes (FRANCE) from 2012.
The pyrolysis is performed using the Finaxo Environment patent n° 0309592 for organic’s pyrolysis using
steel balls (INOX AISI 310S with diameter of 20 mm), heated in an external loop, as indirect medium to
transfer heat in co-current with the organic matter to treat.
The installation has dimensions of 100 m2 x 7 m and occupies about 380 m2 including entrance and
ancillary areas. It could work 7500 hours/year.
This process is named PYROBIO and it is classified as a fast pyrolysis (few seconds at high temperature,
850 °C). In fact, as reported by the plant operator during the visit, at the first contact between balls and
organic matter, more than 80% of volatile matter of the biomass is gasified.
The pyrolysis gases, in particular carbon residue present, also undergo thermal cracking leaving the
pyrolysis reactor, thus becoming essentially non-condensable combustible gas. The heating mode, in
direct contact with the solid at high temperature (average about 700 °C), allows a rapid kinetic heating,
promoting the formation of gas at the expense of the production of char.
Therefore, the process can be called pyro-gasification: it is a pyrolysis because of the absence of any
oxidizing gent and it is a gasification since only syngas is produced (no tar, no char).
Innovations and advantages of the PYROBIO process, according to Finaxo Environment [127] are:
Avoid the presence of an heat exchanger because energy required for the pyrolysis reactions
is carried out inside the pyrolysis reactor, mixing metal balls (preheated) to the load;
The possibility to burn the char to complete the supply of heat required for the pyrolysis
reaction, avoiding the delicate issue of the future use of the char exiting from conventional
pyrolysis processes. The combustion of coke takes place separately from the pyrolysis reactor,
thus allowing not mixing the exhaust gases from the combustion of the char with the pyrolysis
gas. (However, this possibility of burning char is not exploited in Pedrengo Plant since,
essentially, char is not produced by this flash pyrolysis.);
The cracking of the tar avoids problems related to fouling and allows the use of the syngas in a
gas engine;
Possibility to treat the waste at source, avoiding the collection and transport and thus enabling
energy recovery;
The use of steel balls as a carrier of heat transfer allows the construction of systems of any size
with a good performance.
In addition, this technology exploits all advantages of the pyrolysis with respect to other solutions for
waste disposal, for instance the incineration: absence of dioxins emissions, no production of
contaminated ashes and flexibility of operation.
Process description
Input characterization A storage area is present at the beginning of the process because the overall plant works for 5 hours
per day.
The dry input to the process of PYROBIO (250 kgdry/h) is a set of sludge, paper and wood. The sludge,
which is previously digested in a digester already present within the company, is mixed with paper and
wood. Mixing the sludge with paper and wood has the advantage of decreasing the moisture content
of the sludge from 80% to 75 % (paper and wood have 20 % humidity) and increase its lower heating
value (LHV).
Data from the plant operator indicates that paper and wood accounts for 25% of the dry matter of the
mixture. Paper and wood , before being mixed with the sludge, are shredded to reach the size of about
500 nm3 , in a suitable shredder that consumes 55 kW of electrical power and works for 4 hours, while
for the subsequent 32 is stopped, referring to the operating hours.
Figure 76: Input Biomass composed by Industrial Sludge, wood chips, paper
Pyrolyzer In the pyrolysis reactor the sludge is fed continuously while the balls come into very close batch of 1.5
minutes, so the reactor can be considered as a PFR (Plug Flow Reactor).
Finaxo states that the ratio between balls and wet sludge is 7.57 by volume, and 33.3 by mass. However,
there are based on a sludge at 95% dry content. Since in the case of the analyzed plant the sludge after
dryer is instead at 85%, the mass ratio is increased a little bit.
The mix of balls and sludge is moved along the reactor by means of a rotary screw.
The sludge goes in the interstices between the balls and in this way, there is a very efficient exchange
of heat. The balls at the entrance of the pyrolysis reactor have a temperature of 850 °C.
Considering a sludge residence time of 12 minutes, the number batches of balls in the reactor is 8. The
steel balls are heated-up in 6 natural gas fired ovens, which could be fed also by syngas. The ovens are
in parallel since the time needed to heat up a batch of balls is 9 minutes (at normal operation).
In a single pass sludge is converted into syngas and ash, without the presence of char. The ashes are
equal to about 10% of the dry input and contain 1% of carbon. The process is conducted in total absence
of oxygen.
Both balls and pyrolysis products are considered to exit the reactor at 450 °C.
Figure 77: Pyro-gasification reactor
To create an anaerobic environment at the beginning of the process and to avoid air entrainment and
product gas leakage during the operation, a depression in the reactor is created using an inert fluid
(nitrogen locks) to achieve a relative pressure inside of 2-5 mbar.
Dryer A screw mixing brings the biomass produced to the dryer, which has two stages: in the first, the sludge
is brought to 30 % dry, in the second to 85%. The dryer uses diathermic oil heated in a heat exchanger
by the hot flue gases from the natural gas fired ovens.
The diathermic oil passes in the external part of the dryer, while the sludge and part of the flue gas out
of the heat exchanger are sent to the internal part. This has the aim of avoiding the condensation of
vapor on the cold sludge at the drying beginning.
Syngas The syngas obtained from the pyrolysis, according to the information given by plant operator, has more
or less the following molar composition:
N2: 10%
H2: 10%
CH4: 10%
CO: 25%
CO2: 30%
H2O: 15%
With a LHV of 6.35 MJ/kg.
The syngas produced is aspirated by a fan and sent to a cyclone for dust removal, where the process is
so fast that the temperature decrease is negligible. Right after the cyclone, the syngas is cooled by
means of a vapor compression refrigeration cycle, from its 450°C until 15-90 °C, depending on the
operating conditions.
At the end, the syngas is sent to a cogenerative engine.
Cogenerative Engine The cogenerative engine is rated for 200 kW of maximum electric power, assuming 0.35 as electric
efficiency. It could be used to recover also heat, with a conventional thermal efficiency of 0.9, but this
possibility is not exploited in the actual conditions.
No syngas storage is present since it is produced in very short batches so that its stream is nearly
continuous in the collection pipes that feed the engine.
Implementation
To assess the goodness of the process, a simplified calculation has been performed using an Excel sheet.
Most of the data has been provided by the Synecom CEO during the plant visit as indicative values,
while for missing ones usual values in literature have been assumed.
Main results from mass and energy balances are reported in the following simplified flow diagram.
Figure 78: Flowsheet of Synecom Pyrobio Plant
158
Characterization DATA:
�̇�𝑠𝑙𝑢𝑑𝑔𝑒 = 1000 𝑘𝑔/ℎ ; 𝑀𝐶 𝑠𝑙𝑢𝑑𝑔𝑒 = 0,75 [
𝑘𝑔𝑚𝑜𝑖𝑠𝑡𝑢𝑟𝑒
𝑘𝑔 𝑠𝑙𝑢𝑑𝑔𝑒] ; 𝑀𝐶
𝑤𝑜𝑜𝑑 = 0,2 [𝑘𝑔𝑚𝑜𝑖𝑠𝑡𝑢𝑟𝑒
𝑘𝑔 𝑤𝑜𝑜𝑑];
𝐷𝐿𝐻𝑉𝑑𝑖𝑔𝑒𝑠𝑡𝑒𝑑 𝑠𝑙𝑢𝑑𝑔𝑒 = 11 𝑀𝐽/𝑘𝑔 ; 𝐷𝐿𝐻𝑉𝑝𝑎𝑝𝑒𝑟 = 21,4 𝑀𝐽/𝑘𝑔 ; 𝐷𝐿𝐻𝑉𝑤𝑜𝑜𝑑 = 18 𝑀𝐽/𝑘𝑔 ;
Sludge:
�̇�𝑑𝑟𝑦 𝑠𝑙𝑢𝑑𝑔𝑒 = (1 − 𝑀𝐶𝑠𝑙𝑢𝑑𝑔𝑒) ∙ �̇�𝑠𝑙𝑢𝑑𝑔𝑒 = 250 𝑘𝑔/ℎ
𝐷𝐿𝐻𝑉𝑠𝑙𝑢𝑑𝑔𝑒 = 𝐷𝐿𝐻𝑉𝑤𝑜𝑜𝑑+𝑝𝑎𝑝𝑒𝑟 ∙ 0,25 + 𝐷𝐿𝐻𝑉𝑑𝑖𝑔𝑒𝑠𝑡𝑒𝑑 𝑠𝑙𝑢𝑑𝑔𝑒 ∙ (1 − 0,25) = 13,175 𝑀𝐽/𝑘𝑔
Paper + wood:
�̇�𝑑𝑟𝑦 𝑤𝑜𝑜𝑑+𝑝𝑎𝑝𝑒𝑟 = 0,25 ∙ �̇�𝑑𝑟𝑦 𝑠𝑙𝑢𝑑𝑔𝑒 = 62,5 𝑘𝑔/ℎ
�̇�𝑤𝑜𝑜𝑑+𝑝𝑎𝑝𝑒𝑟 =�̇�𝑑𝑟𝑦 𝑤𝑜𝑜𝑑+𝑝𝑎𝑝𝑒𝑟
1 − 𝑀𝐶𝑤𝑜𝑜𝑑= 78,125 𝑘𝑔/ℎ
𝐷𝐿𝐻𝑉𝑤𝑜𝑜𝑑+𝑝𝑎𝑝𝑒𝑟 = 𝐷𝐿𝐻𝑉𝑝𝑎𝑝𝑒𝑟 ∙ 0,5 + 𝐷𝐿𝐻𝑉𝑤𝑜𝑜𝑑 ∙ 0,5 = 19,7 𝑀𝐽/𝑘𝑔
Digested Sludge:
𝑚 ̇ 𝑑𝑟𝑦 𝑑𝑖𝑔𝑒𝑠𝑡𝑒𝑑 𝑠𝑙𝑢𝑑𝑔𝑒
= �̇�𝑑𝑟𝑦 𝑠𝑙𝑢𝑑𝑔𝑒 − �̇�𝑑𝑟𝑦 𝑤𝑜𝑜𝑑+𝑝𝑎𝑝𝑒𝑟 = 187,5 𝑘𝑔/𝑠
𝑚 ̇ 𝑑𝑖𝑔𝑒𝑠𝑡𝑒𝑑 𝑠𝑙𝑢𝑑𝑔𝑒
= �̇�𝑠𝑙𝑢𝑑𝑔𝑒 − �̇� 𝑤𝑜𝑜𝑑+𝑝𝑎𝑝𝑒𝑟 = 921,88 𝑘𝑔/𝑠
𝑀𝐶 𝑑𝑖𝑔𝑒𝑠𝑡𝑒𝑑𝑠𝑙𝑢𝑑𝑔𝑒
= 1 −
�̇�𝑑𝑟𝑦 𝑑𝑖𝑔𝑒𝑠𝑡𝑒𝑑𝑠𝑙𝑢𝑑𝑔𝑒
𝑚 ̇ 𝑑𝑖𝑔𝑒𝑠𝑡𝑒𝑑 𝑠𝑙𝑢𝑑𝑔𝑒
= 0,7966 [𝑘𝑔𝑚𝑜𝑖𝑠𝑡𝑢𝑟𝑒
𝑘𝑔𝑑𝑖𝑔𝑒𝑠𝑡𝑒𝑑 𝑠𝑙𝑢𝑑𝑔𝑒]
Shredder Here is computed the Power consumed by the shredder as if it works continuously during plant
operation:
𝑃𝑒𝑙,𝑠ℎ𝑟𝑒𝑑𝑑𝑒𝑟 = 55 [𝑘𝑊] ∙4 [ℎ]
32 [ℎ] + 4 [ℎ]= 6,11 𝑘𝑊
Dryer DATA:
𝑀𝐶 𝑖𝑛
𝑑𝑟𝑦𝑒𝑟
= 0,75 ; 𝑀𝐶 𝑜𝑢𝑡
𝑑𝑟𝑦𝑒𝑟
= 0,15 ; 𝐶𝑝 𝑑𝑟𝑦
𝑠𝑙𝑢𝑑𝑔𝑒
= 2 [𝑘𝐽
𝑘𝑔 𝐾] ; 𝑇𝑜𝑢𝑡 𝑑𝑟𝑦𝑒𝑟 = 80°𝐶 ;
𝐶𝑝𝐻2𝑂 = 4,186 [𝑘𝐽
𝑘𝑔 𝐾] ; 𝜆 = 2500 [
𝑘𝐽
𝑘𝑔 𝐻2𝑂 𝑒𝑣𝑎] ; 𝑇𝑎𝑚𝑏 = 25°𝐶 ; 𝐶𝑝𝑜𝑖𝑙 = 2,3 [
𝑘𝐽
𝑘𝑔 𝐾] ;
𝑇𝑜𝑖𝑙,𝑖𝑛 𝑑𝑟𝑦𝑒𝑟 = 350 °𝐶 ; 𝑇𝑜𝑖𝑙,𝑜𝑢𝑡 𝑑𝑟𝑦𝑒𝑟 = 30 °𝐶 ; 𝑇𝑓𝑔 𝑖𝑛 𝑑𝑟𝑦𝑒𝑟 = 110°𝐶; 𝑇𝑓𝑔 𝑜𝑢𝑡 𝑑𝑟𝑦𝑒𝑟 = 80 °𝐶 ;
159
�̇� 𝑠𝑙𝑢𝑑𝑔𝑒𝑜𝑢𝑡 𝑑𝑟𝑦𝑒𝑟
= �̇�𝑑𝑟𝑦 𝑠𝑙𝑢𝑑𝑔𝑒 ∙1
1 − 𝑀𝐶𝑜𝑢𝑡= 294,12 𝑘𝑔/ℎ
�̇�𝐻2𝑂 𝑒𝑣𝑎 = �̇� 𝑠𝑙𝑢𝑑𝑔𝑒𝑖𝑛 𝑑𝑟𝑦𝑒𝑟
− �̇� 𝑠𝑙𝑢𝑑𝑔𝑒𝑜𝑢𝑡 𝑑𝑟𝑦𝑒𝑟
= 705,88 𝑘𝑔/ℎ
𝐶𝑝 𝑠𝑙𝑢𝑑𝑔𝑒𝑖𝑛 𝑑𝑟𝑦𝑒𝑟
= 𝐶𝑝 𝑑𝑟𝑦𝑠𝑙𝑢𝑑𝑔𝑒
∙ (1 − 𝑀𝐶𝑖𝑛) + 𝐶𝑝𝐻2𝑂 ∙ 𝑀𝐶𝑖𝑛 = 3,64 [𝑘𝐽
𝑘𝑔 𝐾]
�̇�𝑟𝑒𝑞𝑢𝑖𝑟𝑒𝑑 𝑑𝑟𝑦𝑒𝑟 =
[�̇�𝑒𝑣𝑎 ∙ 𝜆 + �̇� 𝑠𝑙𝑢𝑑𝑔𝑒𝑖𝑛 𝑑𝑟𝑦𝑒𝑟
∙ 𝐶𝑝 𝑠𝑙𝑢𝑑𝑔𝑒𝑖𝑛 𝑑𝑟𝑦𝑒𝑟
∙ (𝑇 𝑜𝑢𝑡𝑑𝑟𝑦𝑒𝑟
− 𝑇𝑎𝑚𝑏)]
3600= 497 𝑘𝑊
�̇�𝑜𝑖𝑙 = 7508 𝑘𝑔/ℎ iteratively found
�̇�𝑜𝑖𝑙 = �̇�𝑜𝑖𝑙 ∙ 𝐶𝑝𝑜𝑖𝑙 ∙ (𝑇𝑜𝑖𝑙,𝑖𝑛 𝑑𝑟𝑦𝑒𝑟 − 𝑇𝑜𝑖𝑙,𝑜𝑢𝑡 𝑑𝑟𝑦𝑒𝑟) ∙ 𝜂 = 475 𝑘𝑊
�̇�𝑓𝑙𝑢𝑒 𝑔𝑎𝑠𝑒𝑠𝑑𝑟𝑦𝑒𝑟
= �̇�𝑟𝑒𝑞𝑢𝑖𝑟𝑒𝑑 𝑑𝑟𝑦𝑒𝑟 − �̇�𝑜𝑖𝑙 = 22 𝑘𝑊
�̇�𝑓𝑙𝑢𝑒 𝑔𝑎𝑠𝑒𝑠 =
�̇�𝑓𝑙𝑢𝑒 𝑔𝑎𝑠𝑒𝑠𝑑𝑟𝑦𝑒𝑟
∙ 3600
𝜂 ∙ 𝐶𝑝 𝑓𝑙𝑢𝑒𝑔𝑎𝑠𝑒𝑠
∙ (𝑇𝑓𝑔 𝑖𝑛 𝑑𝑟𝑦𝑒𝑟 − 𝑇𝑓𝑔 𝑜𝑢𝑡 𝑑𝑟𝑦𝑒𝑟)= 2595.5 𝑘𝑔/ℎ
Pyrolizer DATA:
�̇�𝑏𝑎𝑙𝑙𝑠 = 2352 𝑘𝑔/ℎ ; 𝑚𝑎𝑠ℎ/𝑚𝑑𝑟𝑦 𝑠𝑙𝑢𝑑𝑔𝑒 = 0,1 ; 𝑇 𝑖𝑛𝑏𝑎𝑙𝑙𝑠
= 450°𝐶 ; 𝐶𝑝𝑏𝑎𝑙𝑙𝑠 = 0,5 [𝑘𝐽
𝑘𝑔 𝐾] ; 𝜂 = 0.9;
𝑇 𝑜𝑢𝑡𝑏𝑎𝑙𝑙𝑠
= 𝑇 𝑜𝑢𝑡𝑠𝑦𝑛𝑔𝑎𝑠
= 𝑇𝑜𝑢𝑡𝑎𝑠ℎ
≑ 𝑇𝑝𝑦𝑟 = 850°𝐶 ; Syngas Composition 𝑥𝑖 ;
�̇�𝑏𝑎𝑙𝑙𝑠
�̇�𝑠𝑙𝑢𝑑𝑔𝑒 = 7.997
�̇�𝑏𝑎𝑙𝑙𝑠 =
�̇�𝑏𝑎𝑙𝑙𝑠 ∙ 𝐶𝑝𝑏𝑎𝑙𝑙𝑠 ∙ (𝑇 𝑖𝑛𝑏𝑎𝑙𝑙𝑠
− 𝑇𝑝𝑦𝑟) ∙ 𝜂
3600= 117.6 𝑘𝑊
�̇�𝑒𝑣𝑎 =
�̇� 𝑠𝑙𝑢𝑑𝑔𝑒𝑜𝑢𝑡 𝑑𝑟𝑦𝑒𝑟
∙ 𝑀𝐶 𝑠𝑙𝑢𝑑𝑔𝑒𝑜𝑢𝑡 𝑑𝑟𝑦𝑒𝑟
∙ 𝜆
3600= 27.6 𝑘𝑊
160
�̇�ℎ𝑒𝑎𝑡−𝑢𝑝𝑠𝑙𝑢𝑑𝑔𝑒
=
=
�̇� 𝑠𝑙𝑢𝑑𝑔𝑒𝑜𝑢𝑡 𝑑𝑟𝑦𝑒𝑟
∙ 𝐶𝑝 𝑠𝑙𝑢𝑑𝑔𝑒𝑜𝑢𝑡 𝑑𝑟𝑦𝑒𝑟
∙ (100 − 𝑇 𝑜𝑢𝑡𝑑𝑟𝑦𝑒𝑟
) + (�̇� 𝑑𝑟𝑦 𝑠𝑙𝑢𝑑𝑔𝑒
∙ 𝐶𝑝 𝑑𝑟𝑦 𝑠𝑙𝑢𝑑𝑔𝑒
+ �̇� 𝑠𝑙𝑢𝑑𝑔𝑒𝑜𝑢𝑡 𝑑𝑟𝑦𝑒𝑟
∙ 𝑀𝐶 𝑠𝑙𝑢𝑑𝑔𝑒𝑜𝑢𝑡 𝑑𝑟𝑦𝑒𝑟
∙ 𝐶𝑝𝑣𝑎𝑝) ∙ (𝑇𝑝𝑦𝑟 − 100)
3600= 61 𝑘𝑊
�̇�𝑝𝑦𝑟𝑜𝑙𝑦𝑠𝑖𝑠𝑟𝑒𝑎𝑐𝑡𝑖𝑜𝑛
= �̇�𝑏𝑎𝑙𝑙𝑠 − �̇�𝑒𝑣𝑎 − �̇�ℎ𝑒𝑎𝑡−𝑢𝑝𝑠𝑙𝑢𝑑𝑔𝑒
= 29 𝑘𝑊
�̇�𝑎𝑠ℎ = 𝑚𝑎𝑠ℎ
𝑚𝑠𝑙𝑢𝑑𝑔𝑒 ∙ �̇� 𝑠𝑙𝑢𝑑𝑔𝑒
𝑜𝑢𝑡 𝑑𝑟𝑦𝑒𝑟
= 25 𝑘𝑔/ℎ
𝑚 ̇ 𝑠𝑦𝑛𝑔𝑎𝑠 = �̇� 𝑠𝑙𝑢𝑑𝑔𝑒𝑜𝑢𝑡 𝑑𝑟𝑦𝑒𝑟
− �̇�𝑎𝑠ℎ = 269 𝑘𝑔/𝑠
𝑀𝑀𝑠𝑦𝑛𝑔𝑎𝑠 = ∑ 𝑥𝑖 ∙ 𝑀𝑀𝑖
𝑛
𝑖=1
= 27,5 𝑘𝑔/𝑘𝑚𝑜𝑙
𝑦𝑖 = 𝑥𝑖 ∙𝑀𝑀𝑖
𝑀𝑀𝑚𝑖𝑥
𝐿𝐻𝑉𝑠𝑦𝑛𝑔𝑎𝑠 = ∑ 𝑦𝑖 ∙ 𝐿𝐻𝑉𝑖
𝑛
𝑖=1
= 6,35 𝑀𝐽/𝑘𝑔
Syngas Cooling
DATA: 𝑇 𝑜𝑢𝑡𝑐𝑜𝑜𝑙𝑖𝑛𝑔
= 50 °𝐶 ; 𝐶𝑝𝑠𝑦𝑛𝑔𝑎𝑠 = 1.39 [𝑘𝐽
𝑘𝑔 𝐾] ; COP = 2 ;
�̇�𝑐𝑜𝑜𝑙𝑖𝑛𝑔 =
�̇�𝑠𝑦𝑛𝑔𝑎𝑠 ∙ 𝐶𝑝𝑠𝑦𝑛𝑔𝑎𝑠 ∙ (𝑇𝑝𝑦𝑟 − 𝑇 𝑜𝑢𝑡𝑐𝑜𝑜𝑙𝑖𝑛𝑔
)
3600= 41 𝑘𝑊
𝑃 𝑒𝑙,𝑟𝑒𝑓𝑟𝑖𝑔𝑒𝑟𝑎𝑡𝑜𝑟
=�̇�𝑐𝑜𝑜𝑙𝑖𝑛𝑔
𝐶𝑂𝑃= 21 𝑘𝑊
Oven
DATA:
𝑇 𝑜𝑢𝑡𝑓𝑙𝑢𝑒 𝑔𝑎𝑠𝑒𝑠
= 850 °𝐶 ; 𝐶𝑝 𝑓𝑙𝑢𝑒 𝑔𝑎𝑠𝑒𝑠
= 1.1 [𝑘𝐽
𝑘𝑔 𝐾] ; 𝐿𝐻𝑉 𝑛𝑎𝑡𝑢𝑟𝑎𝑙
𝑔𝑎𝑠= 44 𝑀𝐽/𝑘𝑔; 𝑇𝑟𝑒𝑓 = 25 °𝐶;
161
�̇� 𝑛𝑎𝑡𝑢𝑟𝑎𝑙 𝑔𝑎𝑠
=�̇�𝑏𝑎𝑙𝑙𝑠 ∙ 3600
𝐿𝐻𝑉 𝑛𝑎𝑡𝑢𝑟𝑎𝑙 𝑔𝑎𝑠
∙ (1 + 0.01) − �̇� 𝑓𝑙𝑢𝑒 𝑔𝑎𝑠𝑒𝑠
∙ 𝐶𝑝 𝑓𝑙𝑢𝑒 𝑔𝑎𝑠𝑒𝑠
∙ (𝑇 𝑜𝑢𝑡𝑓𝑙𝑢𝑒 𝑔𝑎𝑠𝑒𝑠
− 𝑇𝑟𝑒𝑓)= 62.5 𝑘𝑔/ℎ
�̇�𝑎𝑖𝑟 = �̇� 𝑓𝑙𝑢𝑒 𝑔𝑎𝑠𝑒𝑠
− �̇� 𝑛𝑎𝑡𝑢𝑟𝑎𝑙 𝑔𝑎𝑠
= 2532 𝑘𝑔/ℎ
𝛼 =�̇�𝑎𝑖𝑟
�̇� 𝑛𝑎𝑡𝑢𝑟𝑎𝑙 𝑔𝑎𝑠
= 40.51 𝑘𝑔𝑎𝑖𝑟/𝑘𝑔𝑛𝑎𝑡𝑢𝑟𝑎𝑙 𝑔𝑎𝑠
�̇�𝐿𝐻𝑉 𝑠𝑦𝑛𝑔𝑎𝑠 =�̇�𝑠𝑦𝑛𝑔𝑎𝑠 ∙ 𝐿𝐻𝑉𝑠𝑦𝑛𝑔𝑎𝑠
3600 = 475 𝑘𝑊
Flue gases-oil heat exchanger DATA: everything is known; the following equation is used for the iterations.
�̇�𝑓𝑙𝑢𝑒 𝑔𝑎𝑠𝑒𝑠𝐻𝑋
= �̇� 𝑓𝑙𝑢𝑒 𝑔𝑎𝑠𝑒𝑠
∙ 𝐶𝑝 𝑓𝑙𝑢𝑒 𝑔𝑎𝑠𝑒𝑠
∙ (𝑇 𝑜𝑢𝑡𝑓𝑙𝑢𝑒 𝑔𝑎𝑠𝑒𝑠
− 𝑇𝑓𝑔 𝑖𝑛 𝑑𝑟𝑦𝑒𝑟) ∙ 𝜂
= �̇�𝑜𝑖𝑙 ∙ 𝐶𝑝𝑜𝑖𝑙 ∙ (𝑇 𝑜𝑖𝑙𝑖𝑛 𝑑𝑟𝑦𝑒𝑟
− 𝑇 𝑜𝑖𝑙𝑜𝑢𝑡 𝑑𝑟𝑦𝑒𝑟
)
Generator
DATA:
𝜂𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐 = 0.35 ; 𝜂𝑡ℎ𝑒𝑟𝑚𝑎𝑙 = 0.9 ;
𝑃𝑒𝑙,𝑒𝑛𝑔𝑖𝑛𝑒 = 𝜂𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐 ∙ �̇�𝐿𝐻𝑉 𝑠𝑦𝑛𝑔𝑎𝑠 = 166 𝑘𝑊
�̇�𝑢𝑠𝑒𝑓𝑢𝑙 = 𝜂𝑡ℎ𝑒𝑟𝑚𝑎𝑙 ∙ (�̇�𝐿𝐻𝑉 𝑠𝑦𝑛𝑔𝑎𝑠 − 𝑃𝑒𝑙,𝑒𝑛𝑔𝑖𝑛𝑒) = 385 𝑘𝑊
162
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