Devolatilization and ash comminution phenomena of sewage ... · Besides, sewage sludge, due to its biogenic nature, is a promising substitute of fossil fuels in the light of the increasing

R. Solimene1, M. Urciuolo

1, A. Cammarota

1, R. Chirone

1, P. Salatino

1,2

1. Istituto di Ricerche sulla Combustione - C.N.R., Napoli - ITALY

2. Dipartimento di Ingegneria Chimica - Università egli Studi di Napoli Federico II - ITALY

1. Abstract

Wet sewage sludge has been characterized during fluidized bed combustion with reference to

devolatilization behavior and ash comminution phenomena with the aid of different and

complementary experimental protocols. Devolatilization/pyrolysis and char bun-out processes

have been analyzed by visual observation and by the time series of gaseous species measured

at the exhaust. Primary fragmentation pointed out the formation of a significant amount of

relatively large fragments as a function of the initial size of fuel particle.

2. Introduction

Sewage sludge management is one of the most important environmental problems.

Construction of new biological municipal wastewater treatment plants has resulted in a

continuous increase in the volume of sewage sludge generated, nevertheless a feasible

solution to the disposal of the expected enormous quantities of sludge must be found. The

restrictive environmental legislation that limits more and more the landfilling of this

biodegradable waste and the progressive decrease of the use of sludge in agriculture, that can

only be considered under very well controlled conditions due to the presence of heavy metals

and pathogens, indicates an ever more increasing interest in sludge thermal processes, in

particular, in sludge incineration. Combustion and co-combustion are the mostly viable

strategies to dispose sewage sludge [1]. Besides, sewage sludge, due to its biogenic nature, is

a promising substitute of fossil fuels in the light of the increasing concern to CO2 emissions.

Fluidized bed combustion/gasification has been indicated as one of the best option, due to

operation flexibility and to high efficiency and low pollutant emissions achieved with

different biogenic fuels, used either alone or in combination with fossil fuels [2-6]. The

success of fluidized bed combustion technology for sewage sludge can be attributed, apart

from minor specific advantages, by considering the great volume reduction of the produced

ash and the destruction of organic micro-pollutants and pathogens [1,7]. However even if

several bed units are in operation for sewage sludge combustion, fundamental work on the

comprehension of basic mechanisms (e.g. sludge drying, release and combustion of volatiles,

combustion of the high-ash content char) taking place during fuel conversion has received

considerably less attention and requires additional investigations [8-15].

The present paper aims at contributing to a better understanding of devolatilization and ash

comminution of wet sewage sludge under fluidized bed combustion conditions with the aid of

different and complementary experimental protocols. Devolatilization/pyrolysis and char

burn-out processes have been analyzed by visual observation and by the time series of

gaseous species measured at the exhaust. Primary particle fragmentation has been

Devolatilization and ash comminution phenomena of sewage

sludge during fluidized bed combustion

1

Processes and Technologies for a Sustainable Energy

characterized in terms of number and dimension of the mm-sized generated fragments as a

function of the initial size of the fuel particle.

3. Experimental

A stainless steel atmospheric bubbling fluidized bed combustor 41 mm ID and 1 m high was

used for devolatilization, fragmentation and combustion experiments (Fig. 1A). A 2 mm thick

perforated plate with 55 holes (ID 0.5 mm) disposed in a triangular pitch was used as gas

distributor. A 0.6 m high stainless steel column for gas preheating and mixing was placed

under the distributor. Two semicylindrical 2.2 kW electric furnaces were used for heating the

fluidization column and the preheating section. Bed temperature, measured by means of a

chromel-alumel thermocouple placed 4 mm above the distributor, was regulated by a PID

controller. The freeboard was kept unlagged in order to minimize fines post-combustion in

this section. Gases were fed to the column via high-precision digital mass flowmeters. The

fluidization column top section was left open to the atmosphere. A stainless steel circular

basket could be inserted from the top in order to retrieve fragmented and un-fragmented

particles from the bed. The tolerance between the column walls and the basket was limited to

reduce as much as possible the amount of particles left in the bed when pulling out the basket.

A basket mesh of 0.8mm was used, so that the bed material could easily pass through the net

openings. A stainless steel probe was inserted from the top of the column in order to convey a

fraction of the exit gases directly to gas analyzers. A high efficiency cellulose filter was

inserted in the line to avoid particle entrainment into the analyzers. The probe, 2 mm ID, was

positioned 0.6 m above the distributor, approximately along the column axis. Data from the

analyzers were logged and further processed on a PC equipped with a data acquisition unit. A

quartz atmospheric bubbling fluidized bed combustor was also used. The geometrical features

of the fluidization column were the same of stainless steel column as well as the heating

system. An observation rectangular window (about 5x10cm) was proposely designed along

the reactor lateral insulation in order to visualize a part both of fluidized bed and of freeboard.

The visual observation was accomplished by a high-resolution video camera.

Quartz sieved in the size range 150-200m as bed material (bed inventory:180g) and a wet

sewage sludge, whose properties are reported in table 1, were used. The sludge was stabilized

1) gas preheating section;

2) electrical furnaces;

3) ceramic insulator;

4) gas distributor;

5) thermocouple;

6) fluidization column;

7) steel basket;

8) manometer;

9) digital mass flowmeters;

10) air dehumidifier (silica gel).

TC

1

2

3

4

air/nitrogen

4

55

6 7

1) gas preheating section;

2) gas distributor;

3) quartz fluidization column;

4) electrical furnaces;

5) ceramic insulator;

6) observation window;

7) Digital video camera;

A B

Fig. 1 Experimental apparatus. A) stainless steel fluidization column apparatus; B) quartz

fluidization column apparatus

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Ischia, June, 27-30 - 2010

and conditioned by inorganic and organic materials. Almost spherical particles (1-3cm) and

air or technical-grade nitrogen as fluidizing gas were used.

Experiments were carried out at fluidization velocity of 0.5m/s by injecting a single fuel

particle into the bed kept at 850 °C from the top of the column. After pyrolysis or combustion

process was complete, the resulting char or ash was retrieved by means of the basket in order

to investigate the number and the size of the produced fragments. Fragments smaller than

about 0.8 mm were lost through the basket net openings. However, these small fragments

were more likely to be generated by attrition rather than by fragmentation. The experiment

was repeated to collect a statistically significant number of fragments. Video recordings of the

combustion of fuel particles were also performed in the quartz bubbling fluidized bed

combustor. The experiments were characterized in terms of the initial size of the fuel particle.

4. Results and discussion

4.1. Phenomenology

Figure 2 reports some snapshots captured during the combustion of a 2cm sewage sludge

particle in the quartz atmospheric bubbling fluidized bed combustor. The upper part of the

bed and the first part of the freeboard are dark red and brighter red, respectively, in the

images. The origin of time scale corresponds to the time at which the fuel particle approaches

the bed surface. It can be observed that: i) the particle appears black (particle temperature

smaller than bed temperature) and often remains nearby bed surface during all the

devolatilization process which lasts about 210s; ii) volatile matter flames are absent; iii) bed

material partially covers fuel particle also when it is on the top of the bed (t=43.68s,

t=111.48s); iv) fuel particle become brighter when char burn-out takes place; v) at the end of

char burn-out, the particle appears only slightly darker of bed material (t=287.84). Taking into

account that devolatilization and drying mainly occur in parallel, the absence of volatiles

flames can be mainly due to the large amount of moisture present in the fuel particle which

reduces both the particle temperature during devolatilization and the heating value of the

released volatile matter. In these conditions, the volatile matter combustion takes place

homogenously mainly in the freeboard without hot spots (flameless combustion). It is also

worth to noting that the diagnostic techniques - the flame period and flame extinction time

methods - based on the detection of volatile flames around the devolatilizing fuel particles by

visual observation or UV detector, can not be applied to this kind of solid fuel. Instead, the

Tab. 1 Properties of tested fuel LHV (as received), kJ/kg 1245 Ash analysis (as received), %w

Proximate analysis (as received), %w Na 0.019

Moisture 77.8 Mg 0.081

Volatiles 14.5 Al 1.002

Fixed carbon 1.0 P 0.496

Ash 6.7 K 0.085

Ultimate analysis (dry basis), %w Ca 0.547

Carbon 34.7 Ti 0.009

Hydrogen 5.4 Cr 0.019

Nitrogen 5.1 Mn 0.006

Sulfur 0.5 Fe 0.328

Chlorine 1.2 Ni 0.062

Ash 30 Cu 0.025

Oxigen (by diff.) 24 Zn 0.023

3

Processes and Technologies for a Sustainable Energy

techniques consisting of the analysis of the time-resolved gas concentration profiles measured

during devolatilization/pyrolysis and char burn-out are still adequate.

4.2. Devolatilization/pyrolysis and char burn-out experiments

Assessment of fuel devolatilization/pyrolysis and char burn-out of a sewage sludge single

particle at 850°C was based on the analysis of the time series of gas concentration measured

at the exhaust. Figure 3 reports the time series of gas concentration (CO, CO2, SO2, NO, H2,

CH4) corresponding to a typical pyrolysis test of an unfragmented 22mm fuel particle. The

time-history of release of measured species is different: shorter for NO and CH4, longer for H2

and CO, whereas it has an intermediate behavior for CO2 and SO2. As a consequence, the

time-resolved CO2 concentration profile has been adopted to estimate the pyrolysis time

corresponding to the 95% of the total released amount which, in this case, results in 305s.

Figures 4 and 5 report the time series of gas concentration (CO, CO2, NO) corresponding to a

typical combustion test of an unfragmented and a fragmented 22mm fuel particle,

respectively. Adopting the time-resolved CO2 concentration profile, it has been evaluated

both devolatilization and complete burn-out time which are 250 and 358s for unfragmented

particle and 160 and 230s for fragmented particle, respectively. The comparison between

combustion and pyrolysis results (Fig. 3, 4 and 5) highlights: i) CO2 profile presents a

shoulder at the end of the process due to char-burn-out; ii) the devolatilization time is smaller

than pyrolysis time indicating that also without the presence of flames volatile matter

t=0s t=5.8s t=43.68s

t=287.84st=210.36s t=211.36s t=214s

t=111.48st=0s t=5.8s t=43.68s

t=287.84st=210.36s t=211.36s t=214s

t=111.48s

Fig. 2 Snapshots captured during devolatilization and char burn-out of a particle of wet

sewage sludge. Fluidizing gas: air; fluidization velocity: 0.25m/s; fuel particle size:

2cm; frame rate 25fps.

4

Ischia, June, 27-30 - 2010

combustion enables faster the

devolatilization process; iii) as

expected, particle fragmentation

reduces both devolatilization and

char burn-out time.

4.3. Primary fragmentation

experiments

The number of particle fragments

collected (multiplication factor) at

the end of pyrolysis as well as

combustion tests is reported in

figure 6A as a function of the

particle initial size. Results show

that in both oxidative and non-

oxidative atmosphere the number

of fragments generated by

comminution phenomena was

almost the same. In the

experimental conditions adopted,

there was a critical particle size

below which the sludge particle

do not fragment, 18-20mm.

Above this critical size, the

multiplication factor increases

exponentially. Figure 6B reports

the ratio between the largest

fragment size generated (with and

without particle fragmentation)

and the initial value of particle

size as a function of the particle

initial size. This ratio is quite

constant about 0.75 up to the

critical initial particle size (18-

20mm) then collapses because of

the fragmentation phenomena.

5. Conclusions

Devolatilization/pyrolysis and

combustion processes of a wet

sewage sludge in fluidized bed

have been characterized by visual

observation and by the analysis of

the time-resolved gas concentration profiles measured at the exhaust. Visual observation of

wet sewage sludge combustion pattern highlights the absence of flames during the volatile

matter combustion. The time-resolved CO2 concentration profile has been adopted to estimate

time, s

0 100 200 300 400 500

CO

CH

4 H

2, p

pm

0

2000

4000

6000

8000

10000

12000S

O2 N

O, p

pm

0

50

100

150

200

250

CO

2,

%

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

CO, ppm

CH4, ppm

SO2, ppm

H2, ppm

CO2, %

NO, ppm

Fig. 3 Time-resolved CO, CH4, H2, SO2, NO and CO2

concentration during a pyrolysis experiment of

a unfragmented 22mm sludge particle.

time, s

0 100 200 300 400 500

CO

, p

pm

0

100

200

300

400

500

CO

2,

%

0

1

2

3

4

5

6

NO

, p

pm

0

50

100

150

200

250

300

CO, ppm

CO2, %

NO, ppm

Fig. 4 Time-resolved CO, NO and CO2 concentration

during a combustion experiment of a 22mm

unfragmented sewage sludge particle.

time, s

0 100 200 300 400 500

CO

, p

pm

0

100

200

300

400

500

CO

2,

%

0

1

2

3

4

5

6

NO

, p

pm

0

50

100

150

200

250

300

CO, ppm

CO2, %

NO, ppm

Fig. 5 Time-resolved CO, NO and CO2 concentra-

tions curves during a combustion experiment

of a 22mm fragmented sludge particle.

5

Processes and Technologies for a Sustainable Energy

the pyrolysis, devolatilization and fuel burn-out time highlighting that: i) devolatilization is

faster than pyrolysis; ii) fragmentation enables faster conversion processes. Particle

fragmentation occurs during devolatilization whenever the particle initial size overcomes 18-

20mm under the investigated operating conditions.

6. References

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2. La Nauze, R.D.: J. Inst. Energy, 60:66 (1987).

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6. Hartman, M., Svoboda, K., Pohoel, M., Trnka, O.: Ind. Eng. Chem. Res., 44:3432 (2005).

7. Mullen, J.F.: Chem. Eng. Prog., 88:50 (1992).

8. Ogada, T., Werther, J.: Fuel 75:617 (1996).

9. Cammarota, A., Chirone, R.: Proceeding of the 16th

International Conference on

Fluidized Bed Combustion, Reno, NV (U.S.A.), p.1201 (2001).

10. Chirone, R., Salatino, P., Scala, F., Solimene, R., Urciuolo, M.: Comb. Flame, 155:21

(2008).

11. Scott, S.A., Davidson, J.F., Dennis, J.S., Hayhurst, A.N.: Chem. Eng. Sci., 62:584 (2007).

12. Van de Velden, M., Baeyens, J., Douganb, B., McMurdo, A.: China Particuology, 5:247

(2007).

13. Khiari, B., Marias, F., Zagrouba, F., Vaxelaire, J.: J. of Cleaner Prod., 16:178 (2008).

14. Rink, K.K., Kozinski, J.A., Lighty, J.S.: Comb. Flame, 100:121 (1995).

15. Solimene, R., Urciuolo, M., Cammarota, A., Chirone, R., Salatino, P., Damonte, G.,

Donati, C., Puglisi, G.: Exp. Therm. Flu. Sci., 34:387 (2010).

d, mm

10 12 14 16 18 20 22 24 26 28

No

ut

/ N

in

1

10

100

Nitrogen

Air

d, mm

10 12 14 16 18 20 22 24 26 28

do

ut

/ d

in

0.4

0.6

0.8

1.0

Nitrogen

Air

A B

Fig. 6 Fragmentation data as a function of the particle initial size. A) Multiplication factor;

B) Ratio between the largest fragment size generated and the particle initial size.

6