July11 performance

Applied Energy xxx (2012) xxx–xxx

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Applied Energy

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Performance analysis of municipal solid waste gasification with steamin a Plasma Gasification Melting reactor

Qinglin Zhang a,⇑, Liran Dor b, Lan Zhang a, Weihong Yang a, Wlodzimierz Blasiak a

a Energy and Furnace Technology Division, Royal Institute of Technology, Brinellvägen 23, S-10044 Stockholm, Swedenb Environmental Energy Resources Ltd., 7 Jabotinski St., 52520 Ramat-Gan, Israel

a r t i c l e i n f o a b s t r a c t

Article history:Received 24 August 2011Received in revised form 11 March 2012Accepted 12 March 2012Available online xxxx

Keywords:PlasmaGasificationMSWSimulationOptimizingSteam

0306-2619/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.apenergy.2012.03.028

⇑ Corresponding author. Tel.: +46 8 790 6545; fax:E-mail address: [email protected] (Q. Zhang).

Please cite this article in press as: Zhang Q et areactor. Appl Energy (2012), http://dx.doi.org/1

Plasma Gasification Melting (PGM) is a novel gasification technology which offers a promising treatmentof low-heating-value fuels like municipal solid waste (MSW), medical waste (MW) and other types ofwaste. By considering the differences in pyrolysis characteristics between cellulosic fractions and plasticsin MSW, a semi-empirical model was developed to predict the performance of the PGM process. The mea-sured results of MSW air and steam gasification in a PGM demo-reactor are demonstrated and comparedwith the model predicted results. Then, the effects of dimensionless operation parameters (ER, PER, andSAMR) are discussed. It was found that all three numbers have positive effects on system cold gas effi-ciency (CGE). The reasons can be attributed to promoted tar cracking by enhanced heat supply. Theeffects of PER and ASME on syngas LHV are also positive. The influence of ER on syngas pyrolysis canbe divided into two parts. When 0.04 < ER < 0.065, the effect of ER is on LHV positive; when0.065 < ER < 0.08, the effect of ER is positive. This phenomenon was explained by two contradictoryeffects of ER. It is also found that interactions exist between operation parameters. For example, increas-ing PER narrows the possible range of ER while increasing SAMR broadens possible ER range. Detailextents for those operation parameters are demonstrated and discussed in this paper. Finally, the optimalpoint aiming at obtaining maximum syngas LHV and system CGE are given.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

The increment of municipal solid waste (MSW) yield givesprominence to sustainable waste disposal. Among various methodsof waste disposal, gasification is one of the promising technologies.During gasification, the chemical energy inside MSW can be recov-ered through production of a combustible syngas. Meanwhile, thevolume of solid waste can be sharply reduced [1]. Compared todirect incineration, MSW gasification prevented largely dioxinformation and reduced thermal NOx formation due to low temper-ature and the reduction condition. Moreover, Moreover, the vol-ume of produced syngas was much lower than that of flue gasfrom incineration. The reduction of gaseous volume produced po-sitive reflects in a decreasing size of gas cleaning equipment [2].The state-of-art of MSW gasification technology was summarizedby Thomas [3].

If additional sensible heat is provided to the gasification pro-cess, the efficiency of gasification can be increased [4]. Meanwhile,other benefits like higher syngas quality, better system stability,and lower tar yield can be obtained [5,6]. When the temperatureof gasification residual reaches its melting temperature, the solid

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l. Performance analysis of mun0.1016/j.apenergy.2012.03.028

residual would be melted and form vitrified slag. In that case, cor-rosion and emission by retaining heavy metals (with the exceptionof mercury, zinc and lead, which can vaporize at high temperaturesand be retained in fly ash and syngas [7]) would be prevented sincethey were trapped by slag [8–11]. Based on above studies, a newMSW gasification technology called Plasma Gasification Melting(PGM) has been developed. In this technology, MSW gasificationand plasma melting of gasification residual are achieved in a singlefixed-bed reactor by a continuous one-step process. By applyingPGM technology, benefits like less investment and operation cost,reduced emissions, and overall environmental friendliness can beachieved.

Steam is a widely used gasification agent which affects energyand mass balance of the gasification process. The previous experi-mental study on the characteristics of steam added gasification [6],[12] showed that the addition of steam favors the formation ofH2 and CO2, and restrains the CO formation by water–gas andwater–gas-shift reactions. Total syngas yield will decline sincethe addition of steam decreases the temperature inside the fixed-bed. It was also discovered that the steam temperature has apositive effect on both syngas LHV and syngas yield, so high-temperature steam feeding is more favorable for gasification.

In our previous work, experimental test has been performedand analysis has been carried out to study the characteristics of a

icipal solid waste gasification with steam in a Plasma Gasification Melting

Nomenclature

AbbreviationCGE cold gas efficiencyER equivalence ratioLHCs light hydrocarbonsLHV lower heating valueMSW municipal solid wasteMW medical wastePER plasma energy ratioPGM Plasma Gasification MeltingSAMR steam air mass ratio

SymbolsCp heat capacity (J kg�1 �C�1)h thermal enthalpy of plasma air (J kg�1)L latent heat (J kg�1)LHV lower heating value (J kg�1)

_m mass flow rate (kg h�1)P power (W)T temperature (�C)Y extent of primary tar crackingx mass fraction

Subscriptsair airash ashi species iMSW municipal solid wastepla plasmapyr pyrolysissteam steamstoic stoichiometric reaction

Table 1Proximate and ultimate analysis of MSW.

Proximate analysisMoisture 20.0%Fixed carbon (dry basis) 10.7%Volatile (dry basis) 77.6%Ash (dry basis) 11.7%

Ultimate analysisCarbon C 47.9%Hydrogen H 6.0%Nitrogen N 1.2%Chlorine Cl <0.1%Sulfur S 0.3%Oxygen O 32.9%

2 Q. Zhang et al. / Applied Energy xxx (2012) xxx–xxx

trial PGM reactor [13]. Several test runs were performed at differ-ent operation conditions where both air and steam are used as gas-ification agents. For each test run, the temperature and pressuredistribution inside the reactor, as well as syngas composition, weremeasured. Due to the limitation of test condition, it is not practicalto test all possible operation conditions by experimental measure-ment. For further understanding of the gasification characters ofMSW in the PGM reactor, it is necessity of develop an accuratemodel to predict the performance of PGM process under variousoperating conditions, and determine the optimal operating condi-tions according to the desired target.

Process simulation is an important tool which has been widelyapplied in various energy-engineering processes. For gasification,various models have been developed. For most models, a globalchemical equilibrium was assumed [14–19]. The equilibriummight be available for entrained-flow, fluidized-bed and downdraftfixed-bed gasification, but not an appropriate approach for updraftfixed-bed gasification. Firstly, in updraft fixed-bed gasification pro-cess, the pyrolysis gases go straight out of the reactor. The chemi-cal equilibrium model cannot correctly predict the yields ofpyrolysis. Secondly, the equilibrium model always underestimatesthe yield of light hydrocarbons from gasification [18]. Vittorio triedto simulating fixed-bed coal gasification by using several individ-ual reactors, and consider the whole gasification process as anassembly of there reactors. In his work the pyrolysis process issimply assumed as a constant yield reaction, the influence of pyro-lysis temperature on pyrolysis yields is not considered [20]. Whenmodeling MSW gasification, the pyrolysis mechanism is morecomplicated than that of coal and biomass, because the composi-tion of MSW is complex. In a common MSW sample, the mass frac-tion of volatile species is 60–80%. An accurate simulation of thepyrolysis is the key for a successful simulation of MSW gasificationin a fixed-bed gasifier. However, very few works has been found onthis topic.

In this study, a semi-empirical model for the PGM process ofMSW is developed using Aspen Plus. Results from the test runsof air and steam gasification inside the PGM reactor are demon-strated, and compared with the predicted results. The effects ofoperating parameters such as air feeding rate, steam feeding rateand plasma power on characteristics of MSW gasification in thePGM process are discussed. The interactions between operatingparameters are also considered from view points of both energyand chemical equilibrium. Finally, the optimal operation condi-tions by considering highest syngas lower heating value (LHV)and cold gas efficiency (CGE) are suggested.

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2. Methodology

2.1. Feedstock

The feedstock used in this study is MSW collected in Israel. Themain components of this MSW are paper, wood, cloth vegetationmaterial, plastics, rubber and debris. The proximate and ultimateanalysis was performed for a sample of this MSW, and the resultsare shown in Table 1.

2.2. The PGM reactor

A PGM demonstration reactor has been built up in NorthernIsrael, with a capacity of 12–20 tons of MSW per day. The PGMreactor is generally a moving-bed counter current updraft gasifier,with a melting chamber placed at its bottom. The scheme of thereactor is shown in Fig. 1. Air is fed into the melting chamberthrough plasma torches at high speed, and forms high temperatureplasma jets which melt the inorganic components which fall fromthe fixed-bed. Then, air with residual heat mixes with steam fedthrough steam nozzles placed at the side wall of the melting cham-ber, and flows into the fixed-bed. The feeding rates of air and steamare controlled by central control system. Feedstock is fed into thereactor from airtight feeding chambers located at the top of thereactor. MSW is fed intermittently every half an hour.

In order to measure the temperature distribution inside thereactor, thermocouples are placed along the gasifier shaft. Addi-tionally, a probe is placed in the syngas outlet to obtain syngassamples, which are sent to a gas analyzer for composition analysis.

icipal solid waste gasification with steam in a Plasma Gasification Melting

Fig. 1. Scheme of the PGM reactor.

Fig. 2. Scheme of PGM gasification process.

Q. Zhang et al. / Applied Energy xxx (2012) xxx–xxx 3

Detailed information about the reactor has been published previ-ously [13].

2.3. Operation parameters

In this study, three dimensionless characteristic numbers areused to characterize the operating parameters of the PGM air andsteam gasification.

Plasma flow supplies heat for gasification in the PGM process.The amount of plasma heat is characterized by plasma energy ratio(PER), which is defined as:

PER ¼ Ppla

LHVMSW � _mMSWð1Þ

The equivalence ratio (ER) is commonly used to indicate quan-titatively the extent of combustion in the combustion/gasificationprocesses:

ER ¼ ð _mair= _mMSWÞð _mair= _mMSWÞstoic

ð2Þ

Steam–air mass ratio (SAMR) is a dimensionless parameterwhich was usually used to characterize the steam feeding rate inair and steam gasification process. It was used in this work asthe third dimensionless parameter.

3. Numerical model

A steady state model of the PGM gasification process was devel-oped using Aspen Plus. Considering the real process in the PGMreactor, the model schematized the PGM process into four differentsections: drying, pyrolysis, char gasification and combustion, andplasma melting. Moisture, volatiles, fixed-carbon and ash were re-moved from feedstock in these sections, respectively. The simpli-fied scheme of the model is shown in Fig. 2. The following modelassumptions are used in this work:

� The system is zero dimensional. Material properties like tem-perature (of gas phase and solid phase), gas composition andsolid composition in each zone is expressed by ‘‘mean’’ values,which are calculated from the mass and energy balance.

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� The flow of solid is from top to the end, while the gas flow isfrom the bottom to the top. No reflux for each phase is allowed.� The ash-free fuel is composed of C, H and O. The gas-phase spe-

cies included in this model are CO, H2, CO2, H2O, CH4, C2H4, O2,N2 and tars (including primary tar from cellulosic group, pri-mary tar from plastic group and secondary tar).� The heat loss of each section is calculated from the measured

temperature layout of gasifier wall surface and the gasifierstructure.

The flow sheet of the model is shown in Fig. 3. The blocks shownin this figure is summarized and described in Table 2.

3.1. Drying

In the drying section, raw MSW is heating up by hot syngas anddecomposed into dry MSW and steam. The energy balance of heatexchanger is described as:

icipal solid waste gasification with steam in a Plasma Gasification Melting

Fig. 3. GPM gasification flow sheet.

Table 2Description of unit operation blocks used in the model.

Block ID Block type Description

DRYER Heat exchanger Exchanging heat between raw MSW and hot syngasSEPARATR Splitter Separating dry MSW and steam in the drying sectionMIXER2 Mixer Mixing syngas and steam from drying sectionMIX1 Mixer Mixing dry MSW and gaseous products from the gasification sectionPRI-PYRO Stoichiometric reactor Stoichiometric reactor for the primary pyrolysisSEC-PYRO Stoichiometric reactor Stoichiometric reactor for the primary tar crackingPYR-SEPR Separator Separating gas phase and solid phase in the pyrolysis sectionSTEAM-HT Heater Heating up the fed steam to 1273 KCHAR-GAS Gibbs reactor Gibbs reactor for char gasificationSEPARATI Splitter Separating gaseous and solid products from gasification sectionHEATER1 Heater/cooler Adjust the temperature difference between gaseous and solid products from gasification sectionHEATER2 Heater/coolerPLASMA-G Heater Heating up the primary air using plasma generatorMELT-H Cooler Calculating the heat usage in the solid residual meltingHEATEX2 Heat exchanger Exchanging heat between primary air and slag, ensuring the slag temperature at the outlet is constant (1773 K)MELTING Stoichiometric reactor Simulating the melting process of solid residualAIR-MIX Mixer Mixing primary and secondary airHEATLOSS Cooler Simulating the system heat loss

4 Q. Zhang et al. / Applied Energy xxx (2012) xxx–xxx

Pi

_miR Tsyngas-in

Tsyngas-outCp;idT ¼ _mMSW-dry

R TMSW-outTMSW-in

Cp;MSW-drydT

þ _msteamR TMSW-out

TMSW-inCp;steamdT þ Lsteam

� �ð3Þ

Considering the impact of heat gradient inside MSW particles,the average temperature of dried MSW at the DRYER outlet is setto 120 �C. The heat capacity of MSW is calculated using the corre-lation given by IGT [21].

3.2. Pyrolysis

The heterogeneous MSW composition determines the compli-cation of pyrolysis. According to the pyrolysis characteristics, stud-ied MSW composition can be divided into two main groups:cellulosic species (wood, paper, vegetation and cardboard) and

Please cite this article in press as: Zhang Q et al. Performance analysis of munreactor. Appl Energy (2012), http://dx.doi.org/10.1016/j.apenergy.2012.03.028

plastics (PE, PP, PVC and rubber). In this model, the pyrolysis ofeach group was simulated separately. For cellulosic group, thetwo-step pyrolysis model [22] was applied. The feedstock firstundergoes primary pyrolysis, and decomposes into primary gas,primary tar and char. Then, the primary tar undergoes secondarycracking, and further decomposed into secondary tar and second-ary gas. The yields of the primary pyrolysis products, includingthe composition of produced gases and tars are taken from litera-ture [23]. To simplify the model, all light hydrocarbons except CH4

are considered as C2H4. The composition of secondary tar is as-sumed to be benzene.

For the plastics, then, no available pyrolysis model was found.However, the experimental results of plastic pyrolysis show thatthe main products from plastics pyrolysis are gas, liquid and char.Meanwhile, the mass yield of liquid also demonstrates a decreasingtrend with increasing pyrolysis temperature [24,25]. From this

icipal solid waste gasification with steam in a Plasma Gasification Melting

Table 3Components and compositions of primary pyrolysis products.

Cellulosic group Plastics

Gas yield (kg/kg dry MSW) 0.248 0.148Gas compositions (vol.%)H2 33.3 7.8CO 6.0 –CO2 59.5 9.8CH4 1.2 13.8C2H4 – 68.6

Tar yield (kg/kg dry MSW) 0.491 0.839Tar elemental composition (mass%)C 53.4 87.4H 7.9 12.6O 38.7 –

Char yield (kg/kg dry MSW) 0.261 0.013Char composition (mass%)C 47.3 100.0Ash 52.7 –

Table 4Comparison between measured and predicted results of air and steam gasification inthe PGM reactor (dry basis).

Case number 1 2 3 4

Operation parametersER 0.060 0.060 0.052 0.048PER 0.118 0.118 0.118 0.128SAMR 0.389 0.556 0.452 0.490Steam temperature (�C) 1000 1000 1000 1000

Measured resultsSyngas yield (Nm3/kg MSW) 1.36 1.38 1.26 1.29Syngas LHV (MJ/Nm3) 8.23 8.43 8.24 8.70H2 volume fraction (%) 19.23 19.23 20.92 26.87CO volume fraction (%) 15.46 12.50 14.51 15.74LHCs volume fraction (%) 8.15 9.27 8.09 7.69H2/CO 1.24 1.53 1.45 1.70

Predicted resultsSyngas yield (Nm3/kg MSW) 1.27 1.32 1.16 1.14Syngas LHV (MJ/Nm3) 8.48 8.70 8.05 8.38H2 volume fraction (%) 19.37 20.64 21.94 23.08CO volume fraction (%) 16.65 15.48 16.29 16.35LHCs volume fraction (%) 8.22 8.69 7.05 7.29H2/CO 1.16 1.33 1.35 1.41

Q. Zhang et al. / Applied Energy xxx (2012) xxx–xxx 5

viewpoint, the pyrolysis of plastics can also be simulated using asimilar two step pyrolysis model. In this work, the products fromprimary pyrolysis of plastics are taken from Williams and Williams[24]. No literature data is found for the secondary pyrolysis of plas-tic mixture, so the yield of secondary pyrolysis of the plastic groupis calculated from elementary balance. The detailed componentsand compositions of primary pyrolysis products are listed in Table3.

The extent of secondary cracking of tar is sensitive to both pyro-lysis temperature and residence time. According to the previousexperiments for the demonstration PGM reactor, the residencetime for pyrolysis gas in the pyrolysis zone is around 10 s, whichis long enough for the secondary tar cracking [26,27]. In that case,the influence of residence time on tar cracking can be ignored. Inthis model, the extent of primary tar cracking is controlled by pyro-lysis temperature [27]:

Y ¼ 1� expð�0:0058ðTpyr � T0ÞÞ ð4Þ

where T0 = 500 �C.The combustion values of MSW and tars are calculated based on

their elementary compositions, using the empirical correlation gi-ven by Boie [28].

3.3. Char gasification and combustion

Char coming from the pyrolysis zone meets and reacts with gas-ification agents in the char gasification and combustion section. Atthe same time, homogeneous gas phase reactions also take place.When high temperature steam is injected into this section, as wedid in the test runs, the reaction system will become more com-plex. Firstly, the injection of high temperature steam activatesthe water shift reaction and water–gas shift reaction, which visiblyaffects the chemical equilibrium in this section. Secondly, theintroductions of additional mass and energy affects the mass andenergy balance inside PGM reactor. It is not practical to accuratelysimulate all reactions which occur in this section. Instead, theGibbs free energy theory is applied in this section. The productsof this section are calculated by minimizing the system Gibbsenergy.

3.4. Plasma melting

The inorganic components (ash) of the MSW coming from thegasification and combustion zone were melted by high tempera-ture plasma air in the plasma melting section.

The composition of the inorganic components is assumedaccording to the original composition of the MSW. Based on the

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assumed composition, the heat capacity of the inorganics is calcu-lated as following:

Cp;ash ¼Pni¼1

xiCp;i ð5Þ

The melting latent heat of the inorganics is calculated similarlyto that of the heat capacity. The heat loss of the plasma meltingprocess is set to 30% of the total plasma energy, which is summa-rized from the tested temperature distribution inside the meltingchamber.

3.5. Dimensionless operation parameters

In this study, three dimensionless characteristic numbers areused to characterize the operating parameters of the PGM air andsteam gasification.

Plasma flow supplies heat for gasification in the PGM process.The amount of plasma heat is characterized by plasma energy ratio(PER), which is defined as:

PER ¼ Ppla

LHVMSW � _mMSWð6Þ

The equivalence ratio (ER) is commonly used to indicate quan-titatively the extent of combustion in the combustion/gasificationprocesses:

ER ¼ ð _mair= _mMSWÞð _mair= _mMSWÞstoic

ð7Þ

Steam–air mass ratio (SAMR) is a dimensionless parameterwhich was usually used to characterize the steam feeding rate inair and steam gasification process. It was used in this work asthe third dimensionless parameter:

SAMR ¼_msteam

_mairð8Þ

4. Results and analysis

4.1. Measured results of air and steam gasification

The measured results [13] of air and steam gasification of MSWin the PGM demo-reactor are shown in Table 4. Results are shown

icipal solid waste gasification with steam in a Plasma Gasification Melting

Fig. 4. Effect of PER on (a) gasification and pyrolysis temperature, (b) syngascomposition and tar yield, (c) total syngas yield and syngas LHV.

6 Q. Zhang et al. / Applied Energy xxx (2012) xxx–xxx

in terms of syngas yield, syngas LHV and the ratio of H2 and CO vol-ume fractions (H2/CO). Despite the fact that the number of casesare very limited, some effects of operation parameter can still befound. For example, the measured results for cases 1 and 2 showthat increasing SAMR is beneficial for syngas production. The re-sults of cases 2 and 3 demonstrate a negative effect of decreasingER on syngas yield and LHV. In order to further understand thecharacter of air/steam gasification in the PGM process, more resultsfrom model prediction could be used.

The predicted results at the same operation parameters as thatof test measurement are also shown in Table 4. By comparing thepredicted results with the measured results, it was found thatthe results from modeling are in the acceptable ranges for analyz-ing the character of PGM process. It has to be point out that the cal-culation method for syngas LHV in this work is slightly updatedfrom the one we used in the previous publication [13]. The LHCsare assumed to be composed of CH4 and C2H4, instead of the pre-vious assumption that LHCs were pure CH4. The new assumptionis supported by Ponzio et al. [5].

4.2. Effect of plasma power

The high-temperature plasma air injection is the most impor-tant significance of the PGM process. It supplies heat for the melt-ing of the inorganic component of MSW. After that, the residualheat provide sensible heat to gasification. In this way, the powerof plasma affects the energy equilibrium of the whole gasificationprocess, and directly influences the temperature profile, syngascomposition, tar yield and stability of the gasification process. Aserious of cases are simulated to investigate the effect of PER ongasification characters in PGM process. In these cases, the valuesof ER and SAMR are set to 0.06 and 0.389 respectively, which aretestified as ‘‘reasonable’’ values for PGM air and steam gasificationby previous test runs. The value of PER varies from 0.098 to 0.137.

The effect of PER on the average temperature in the gasificationand pyrolysis zone is illustrated in Fig. 4a. It was found that bothgasification temperature and pyrolysis temperature increase linearwith PER. This is easy to understand since increasing PER enhancethe average temperature of feeding air, and increases the heat sup-ply for gasification and pyrolysis.

Fig. 4b shows the syngas composition, as well as tar yield withdifferent PER. All the gaseous species are shown in volume frac-tions on dry basis, and tar is shown by tar-MSW mass ratio ondry basis. It was found that the volume fractions of all combustiblegaseous increase with PER, while the trends of CO2 and N2 areopposite. The increment of combustible gases is mainly due to pro-moted tar cracking by increasing pyrolysis temperature. At thesame time, the total yields of incombustible gases like CO2 andN2 do not vary much. Considering the increasing of combustiblegases with PER, the decreasing trends of CO2 and N2 volume frac-tions are understandable.

Fig. 4c shows the effects of PER on syngas yield and syngas LHV,where both results are calculated on dry basis. It was found thatboth the syngas yield and syngas LHV increase with PER. This isnot hard to understand since the increase of combustible gas yieldsby tar cracking is profitable for both quantity and quality of syngas.When PER increase from 0.098 to 0.137, the syngas yield increasesfrom 0.96 to 1.08 Nm3/kg MSW. At the same time the syngas LHVvaries from 7.32 to 9.31 MJ/Nm3. It seems the positive effects ofPER provides a possible method for not only solving the tar prob-lem, but also increase the syngas yield and quality. However, ithas to be noticed that beneficial effects of increasing PER is notunlimited. As we can see in Fig. 4, the temperature inside thereactor also increases with PER. A high PER value may leads tothe formation of a high temperature zone in the combustionand gasification section. Too high temperature challenges the

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thermostability of the reactor wall. Furthermore, the low-melt-ing-point components in the solid residual like SiO2 may be meltedin the gasification and combustion section if the temperature is toohigh. The partial melting of solid residual will dramatically decreasethe void fraction in the fixed-bed, and leads to the occurrence ofbridging. It can be found from Fig. 4 that when PER = 0.26, the aver-age temperature in the gasification section has reached 1330 �C.This temperature is already too high for an engineering application.

4.3. Effect of ER

For a traditional gasifier, the energy needed for feedstock heat-ing up, pyrolysis and char gasification is mainly from the partialcombustion of char. The equivalence ratio (ER) for traditional gas-ifier should be around 0.3 to fulfill the need of energy. For PGM airand steam gasification, heat can be supplied by plasma and hightemperature steam, so the ER for a PGM gasifier will be much lower(0.04–0.10). It is worthwhile to study the influence of ER on theperformance of a PGM gasifier.

icipal solid waste gasification with steam in a Plasma Gasification Melting

Q. Zhang et al. / Applied Energy xxx (2012) xxx–xxx 7

Theoretically, the effects of ER on gasification process should beconsidered from two aspects. On one side, higher ER provides morechemical heat by combustion. It is known that increased heat sup-ply is beneficial for both syngas yield and LHV value, so this effectof ER is positive. On the other side, higher ER means more combus-tion in the reactor, which will consume some combustible gases.Additionally, the increasing N2 introduced into the reactor dilutesthe content of combustible gases. From this point of view, the ERalso has negative effects on syngas production. The final influenceof ER on PGM process should be a combination of these twoaspects.

A group of simulations with different ER was carried out tostudy the exact influence of ER on PGM process. The values ofSAMR and PER are set to 0.389 and 0.118, respectively. The valueof ER varies from 0.04 to 0.08.

Fig. 5a shows the syngas composition and tar yield with differ-ent ER value. It was found that when ER increases, the volume frac-tions of CH4, C2H4 and N2 increase, and the volume fraction of H2

and tar yield decrease. The volume fraction of CO first increasesand then decreases. An opposite trend was found for CO2 volumefraction. The increase of CH4 and C2H4 volume fractions can beunderstand as the result of positive effect of ER on tar cracking,while the increase of N2 and decrease of H2 volume fractions arethe results of negative effects of ER. The variations of CO and CO2

volume fractions are affected by both aspects.Fig. 5b shows the variation of syngas LHV and system CGE with

increasing ER. It was found that the influence of ER on syngas LHVcan be divided into two parts. When ER increases from 0.04 to 0.07,the syngas LHV increases from 6.11 to 8.63 MJ/Nm3. The positiveeffect of ER is dominant. When ER increases from 0.07 to 0.08,the syngas LHV keeps almost constant. It seems negative effect ofER starts to appear in this range, and counterbalances the positiveeffect. For system CGE, however, the effect of increasing ER is posi-tive in all ER range.

Fig. 5. Effect of ER on (a) syngas composition and tar yield, (b) syngas LHV andsystem CGE.

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4.4. Effect of SAMR

The feeding of high temperature steam influences the PGM pro-cess from two aspects. Firstly, steam is involved in chemical reac-tions such as water–gas reaction and water gas shift reaction. Inthat case, it influences the chemical equilibrium of the PGM sys-tem. Secondly, the high temperature steam changes the total massand energy flow inside the reactor, and influence the energy bal-ance of the system.

As an example, the effect of different SAMR on syngas composi-tion and tar yield for ER = 0.06 and PER = 0.118 is shown in Fig. 6. Itwas found that the most important effect of increasing SAMR is thevariation of H2, CO, and CO2 volume fractions. When the SARMincreases from 0 to 0.67, the volume fraction of H2 in syngas in-creases from 9.5% to 21.3%. The volume fraction of CO2 increasessimilarly from 12.4% to 20.2%, while the volume fraction of CO de-creases from 26.8% to 14.9%. The similar trends were also reportedby other researchers [6–12]. This phenomenon is the result of pro-moted water–gas shift reaction (CO + H2O M H2 + CO2) by increas-ing steam feeding rate. It was also found that the yield of tar showna slight decreasing trend when the SAMR increases. at the sametime, the CH4 and C2H4 volume fractions increase slightly. It isbelieve that this phenomenon is the result of steam preheating,which introduces extra energy to the PGM system, and increasesthe global temperature of pyrolysis. It was reported by Lewiset al. [29] that the critical steam temperature for supporting energysupply in steam-only gasification process is above 1200 �C. In PGMprocess, due to the heat supply from plasma air and char combus-tion, the critical steam temperature should be reduced. It was im-plied by the tar decreasing that the critical steam temperature atthe analyzed condition is lower than 1000 �C. As a result of the ex-tra heat supply by high-temperature steam, the syngas yield andLHV increase slowly with SAMR. It was also found that the effectof SAMR on syngas composition weakens with increasing SAMR.The effect is most remarkable when SAMR varies from 0 to 0.1.

4.5. Interaction between operating parameters

PER, ER and SAMR generalize main operating parameters duringPGM process. However, the effects of different operating parame-ters are not independent, and internal connections exist betweenthese parameters. For deep understanding of the PGM process,these interactions between operating parameters should beconsidered.

4.5.1. Interactions between ER and PERIn the PGM process, the required heat for MSW gasification

comes from two sources: the sensible heat of plasma air and chem-ical heat from char combustion. In other words, the energy equilib-rium of PGM gasification is controlled by both PER and ER. From

Fig. 6. Effect of SAMR on syngas composition and tar yield.

icipal solid waste gasification with steam in a Plasma Gasification Melting

Fig. 7. Definition of possible operation extent of PER and ER in the PGM process.

Fig. 8. Distributions of syngas LHV and system CG

8 Q. Zhang et al. / Applied Energy xxx (2012) xxx–xxx

Please cite this article in press as: Zhang Q et al. Performance analysis of munreactor. Appl Energy (2012), http://dx.doi.org/10.1016/j.apenergy.2012.03.028

this point of view, the effects of PER and ER are connected to eachother. When study the energy equilibrium of the PGM process, theinteraction between PER and ER should be considered. The SAMRvalue was set to 0.389 in this study.

Fig. 7 shows the delimitation of possible operation extent of PERand ER in the PGM process. Three curves are defined to restrict thelogical area for PGM:

� ERpla,min shows the minimum of ER requested for generatingplasma flow. In PGM process, air is used as the carrier of sensi-ble heat from plasma generators. The relationship between PERand ERpla,min is linear. The gradient of the ERpla,min denotes theratio between MSW LHV and the thermal enthalpy of plasmaair:

k ¼ ð _mair= _mMSWÞstoicLHVMSW

hplað9Þ

E in region 1. (a) Syngas LHV, (b) system CGE.

icipal solid waste gasification with steam in a Plasma Gasification Melting

Fig. 9. Delimitation of possible operation extent of SAMR and ER in the PGMprocess.

Q. Zhang et al. / Applied Energy xxx (2012) xxx–xxx 9

� ERgasif,min shows the ER needed for complete gasification (i.e. nosolid carbon residual and enough temperature for gasificationand pyrolysis). In this work, the request for complete gasifica-tion is satisfied when the syngas temperature at the outlet ishigher than 120 �C.� ERtem,max shows the maximum of ER to prevent too high tem-

perature in the char combustion and gasification section. If thistemperature is too high, the wall material of the reactor mightbe damaged. In this study, the maximum of the temperature isset to 1300 �C.� PERmel,min shows the minimum of PER required for melting all

the solid residual.

Four different regions are divided by these curves:

� Region 1: In this region, the PGM process can operate normally.The energy supplied by plasma and char combustion is enoughfor MSW gasification to take place. The temperature in the gas-ification and combustion zone is not too high to damage thereactor wall.� Region 2: In this region, the energy supplied by plasma, char

combustion and High temperature steam is not enough for sup-porting MSW gasification.� Region 3: In this region, the temperature of the char combustion

and gasification section is higher than 1300 �C. In other words,the temperature in char combustion and gasification sectionmay damage the reactor wall.� Region 4: In this region, the energy supplied by plasma flow is

not enough for melting of solid residual from MSW gasification.

It can be found from Fig. 7 that when PER is less than 0.045, theplasma energy is not enough for entirely melting of inorganic com-ponents in MSW. When PER increases from 0.045 to 0.13, the en-ergy require for inorganic components melting is satisfied. Theextent of available ER is limited by ERtem,max and ERgasif,min. In otherwords, the minimum of available ER is restricted by entire energysupply, and the maximum of available ER is controlled by gasifica-tion and combustion temperature. When PER further increasesfrom 0.13 to 0.14, the lower limit of ER does not exist anymore,which means the energy supply is enough for PGM even the sec-ondary air feeding is set to 0. If PER is higher than 0.14, the PGMis not available because the temperature at the char gasificationand combustion section is too high. Generally speaking, the avail-able PER extent is 0.045–0.14. Increase of PER narrows the varia-tion range of ER.

The distribution of syngas LHV, as well as system CGE in region1 is demonstrated in Fig. 8. It was found that the maximum syngasLHV in region 1 is about 9.5, while the minimum is about 4.0. It hasbeen discussed previously that the LHV variation is mainly causedby thermal cracking of primary tar. The large difference betweenmaximum and minimum syngas LHV illustrates that the extentof tar cracking is a very important factor which determines thequality of syngas in PGM process. Furthermore, it is obvious thatthe effect of PER on syngas LHV is stronger than that of ER. The po-sitive effect of ER on syngas LHV is due to promoted primary tarcracking caused by chemical heat from combustion. However,the ER still have some negative effects on syngas LHV. For example,increased combustion by increasing ER consumes some combusti-ble gases in syngas. Additionally, the introduced N2 also dilutes thecontents of combustible gases. These negative effects somehowweaken the positive effect of ER. So the maximum LHV was foundin the area with highest PER value. The dependence of LHV on ERand PER has been confirmed by previous running of the pilotPGM reactor.

From Fig. 8b it was found that the maximum CGE in region 1 isabout 0.62 and the minimum is about 0.22. The maximum CGE

Please cite this article in press as: Zhang Q et al. Performance analysis of munreactor. Appl Energy (2012), http://dx.doi.org/10.1016/j.apenergy.2012.03.028

appears when ER = 0.08 and PER = 0.10. The large difference ofCGE is also explained by the influence of the extent of tar cracking.The influences of PER and ER on CGE have similar intensity. Aninteresting phenomenon found in Fig. 8 (b) is that the effects ofER and PER on CGE shows a linear relation. It implies that the influ-ence of ER and PER can be further synthesized to a unified param-eter. The further correlation of ER and PER can be an interestingtopic of our future work.

4.5.2. Considering the oxygen equilibriumSteam and air are two popular gasification agents which supply

oxygen for the gasification process. As the material base of gasifica-tion, the oxygen supply directly influence the conversion of C duringgasification and combustion section. From this perspective, the ERand SAMR may also have internal connecting with each other.

Fig. 9 shows the delimitation of possible operation extent ofSAMR and ER in the PGM process at PER = 0.118. Three curvesare used to restrict the possible operation conditions for SAMRand ER: ERpla,min, ERgasif,min, and ERtem,max.

These curves defined 3 main regions with different operationconditions: In region 10, the PGM process can work continuously;In region 20, the energy supplied by plasma and char combustionis not enough for MSW gasification; In region 30, the temperatureof the char combustion and gasification section is too high. Itwas found that when SAMR increases, the maximum of possibleER increases and the minimum of possible ER in region 10 de-creases. Increase of SAMR means enhanced oxygen supply fromsteam. In that case, the oxygen equilibrium in the reactor is af-fected, and the requested air decreases. The increase of maximumER can be explained by the increases of total heat capacity withincreasing SAMR, which increases the uniformity of temperaturedistribution inside the reactor. This uniformity is also beneficialto syngas LHV because the temperature difference between gasifi-cation and pyrolysis will be reduced.

The distribution of syngas LHV in region 10 is demonstrated inFig. 10. It was found that the syngas LHV in region 10 varies from6.5 to 9.0 MJ/Nm3. The increase of SAMR has positive effects onsyngas LHV. This positive effect may be mainly due to the hightemperature of steam, which also introduce some heat into thePGM system. At the same time, the decreased temperature differ-ence between gasification and pyrolysis section by increasingSAMR enhances the potential of LHV increase by larger energy sup-ply. An interesting phenomenon found in Fig. 10 is that the effect ofincreasing ER on syngas LHV changes when SAMR is larger than0.55. In this area, the LHV first increase, and then start to decrease

icipal solid waste gasification with steam in a Plasma Gasification Melting

Fig. 10. Distributions of syngas LHV in region 10 .

10 Q. Zhang et al. / Applied Energy xxx (2012) xxx–xxx

when ER keep increasing. The maximum of LHV appears at aboutER = 0.055. This result illustrate that the positive aspect of ER effectby increasing chemical heat is not always dominant. The negativeaspects such as consumption of combustible gas and dilution fromN2 play important roles in high SAMR condition. The suggested ERin high SAMR condition is 0.055.

5. Conclusions

� A semi-empirical model for air and steam gasification of MSWin the PGM process has been built up.� The performance of PGM reactors with high-temperature steam

feeding is analyzed by both test measurement and model pre-diction. The effects of three dimensionless operation parame-ters are discussed. PER has positive effect for both syngasyield and syngas LHV. The main reason for this effect is thefavored tar cracking by increasing heat supply.� The ER has two contradictory effects on syngas LHV: the posi-

tive effect by increasing chemical heat and the negative effectby syngas combustion and N2 dilution. When ER is lower than0.065, the positive effect is dominant; When ER is larger than0.065, two effects counterbalance each other. The effect of ERon CGE is positive in the studied region.� The SAMR mainly influence the equilibrium of water–gas shift

reaction in the PGM process. Steam at 1000 �C can supply someheat for pyrolysis, so the SAMR also have slight positive effecton syngas yield and LHV.� Interactions exist between PER and ER. The available extent of

PER and ER is defined at air/steam gasification conditions. Thepossible range for PER at the studied condition is 0.045–0.14.Increase of PER narrows the variation range of ER. The optimalsyngas LHV can be obtained when the PER reaches it is maxi-mum. The effect of ER and PER on syngas CGE seems can be syn-thesized to a unified parameter.� The available extent of SAMR and ER is defined at PER = 0.118.

Increasing SAMR broadens the available range of ER. WhenSAMR > 0.6, the secondary air is not necessary anymore. Theoptimal syngas LHV can be obtained at SAMR = 0.8 and ER =0.055.

Please cite this article in press as: Zhang Q et al. Performance analysis of munreactor. Appl Energy (2012), http://dx.doi.org/10.1016/j.apenergy.2012.03.028

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