Applied Energy xxx (2013) xxx–xxx

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Applied Energy

journal homepage: www.elsevier .com/locate /apenergy

A thermodynamic analysis of solid waste gasification in the PlasmaGasification Melting process

0306-2619/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.apenergy.2013.03.054

⇑ Corresponding author. Tel.: +46 8 790 6545; fax: +46 8 207 681.E-mail address: qinglin@kth.se (Q. Zhang).

Please cite this article in press as: Zhang Q et al. A thermodynamic analysis of solid waste gasification in the Plasma Gasification Melting procesEnergy (2013), http://dx.doi.org/10.1016/j.apenergy.2013.03.054

Qinglin Zhang a,⇑, Yueshi Wu a, Liran Dor b, 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., 21 HaMelacha St., 48091 Rosh Ha’ayin, Israel

h i g h l i g h t s

� A thermodynamic analysis was conducted to evaluate the characteristics of the PGM.� Energy recovery using gas furnaces is suggested due to high total energy and exergy.� Gas turbines are not recommended due to high tar yield.� Increasing heat to the PGM is beneficial for cold gas energy and exergy efficiencies.� A small steam addition is beneficial for PGM energy and exergy efficiencies.

a r t i c l e i n f o

Article history:Received 25 September 2012Received in revised form 20 March 2013Accepted 22 March 2013Available online xxxx

Keywords:GasificationPlasmaMSWEnergyExergy

a b s t r a c t

Plasma Gasification Melting is a promising technology for solid waste treatment. In this work, a thermo-dynamic analysis has been conducted to evaluate the advantages and limitations of the PGM technology.According to the characteristics of the PGM, the whole process was divided into four sections such as dry-ing, pyrolysis, char gasification and inorganics melting. The energy and exergy in each section has beencalculated. According to different usage of syngas, two kinds of energy and exergy efficiencies are defined.The results show that the PGM process produces a tar-rich syngas. When considering the raw syngas(syngas with tar), the energy and exergy efficiency of PGM process is very high. The effects of operatingconditions on the thermodynamic performance of the PGM process have been analyzed. Considering theenergy and exergy of clean syngas, it is beneficial to increase sensible heat input to the PGM system.However, high sensible heat input or high steam injection is not suggested when considering the energyand exergy efficiency of raw syngas.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Municipal solid waste (MSW) is one of the main by-products ofhuman society, but at the same time, it is also a potential energysource which has attracted more and more attention over theyears. The waste-to-energy conception has become one of the mostpopular topics in the energy field. Among various waste-to-energytechnologies, gasification is recognized as a promising method[1–3].

Gasification is generally an endothermic process. The heat re-quired for gasification can be provided by either partial combus-tion of feedstock or external heat sources. It has been confirmedthat the use of an external heat source can increase both the energyand the exergy efficiency of gasification. Furthermore, it has beenshown that the heating value of syngas can also be enhanced

[4–6]. In recent years, the usage of thermal plasma in gasificationhas gained increasing interest [7–9]. By using plasma gasification,the solid feedstock can be decomposed into two products: a com-bustible syngas, and an inert vitreous slag. It was declared thatplasma gasification exhibits remarkable environmental advantagesfor both air emission and slag toxicity control [10,11]. However,since plasma gasification is a new technology, the knowledgeabout performance and characteristics of plasma gasification arestill not enough. There are still disputes about the efficiency, espe-cially the energy and exergy efficiency of plasma gasification.

The main purpose of this paper is to provide an energy andexergy analysis for one of these plasma gasification processescalled Plasma Gasification Melting (PGM). It is the continuationand developments of our previous experimental and numericalstudies about the characteristics of the PGM process [12–15]. Aprocess simulating model which was used for performance analy-sis and syngas composition prediction [13] is adopted and updatedto perform energy and exergy analysis. Special attention was given

s. Appl

Nomenclature

AbbreviationCGE cold gas efficiencyCGEE cold gas exergy efficiencyER equivalence ratioLHC light hydrocarbonsLHV lower heating valueMSW municipal solid wastePE polyethylenePER plasma energy ratioPGM Plasma Gasification and MeltingPP polypropylenePVC polyvinyl chlorideSAMR Steam air mass ratio

Symbolscp heat capacity (J kg�1 K�1)h specific enthalpy (J kg�1)eh molar enthalpy (J mol�1)heva evaporation enthalpy of water (J kg�1)_M mass flow rate (kg s�1)~_M molar flow rate (mol s�1)P power (W)R universal gas constant (J mol�1 K�1)~s molar entropy (J mol�1 K�1)T temperature (K)xi mass fraction of species i~xi molar fraction of species ie specific exergy (J kg�1)

~e molar exergy (J mol�1)g efficiency

Subscript0 standard stateair airash ashcel cellulosic speciesch chemicaldry dryen energyex exergygas gas phaseH2O moisturei ith time stepin inletMSW municipal solid wasteout outletph physicalpla plasticsplasma plasmapri primary pyrolysissec secondary pyrolysissolid solid phasesteam steamsyngas syngastar tar

Fig. 1. Typical schematics of a PGM processing chamber.

2 Q. Zhang et al. / Applied Energy xxx (2013) xxx–xxx

to the influence of different operating parameters on process en-ergy and exergy efficiency.

2. The PGM process and modeling

2.1. The PGM process

The PGM is a new plasma gasification technology developedand owned by Environmental Energy Resources Ltd. The typicalschematics of a PGM processing chamber are shown in Fig. 1.MSW or other types of solid waste are fed into the reactor by air-tight feeding system at the upper part of the processing chamber.Thermal plasma torches are placed near the bottom of the process-ing chamber, and high temperature plasma air (also known as theprimary gasification agents) of above 5000 �C is fed into the cham-ber from these plasma torches. The high temperature plasma jetssupply the necessary heat to vitrify the inorganics of the feedstock.In order to ensure the highest gasification efficiency, secondarygasification agents (air and steam) are fed from nozzles aroundplasma nozzles. After the air and steam mixing, the first and sec-ond agents flow into the process chamber. Reactions related tofixed-bed gasification occur in the waste column, and finally pro-duces a combustible gas mixture known as syngas. The main com-bustible species in the syngas are CO, H2 and LHCs. At the syngasexit, the gas temperature is about 200–400 �C. By using the PGMtechnology, multiple objectives such as waste elimination, energyrecovery and benign slag product can be achieved in one singleprocess chamber.

By using the PGM technology, the following benefits can beexpected:

� The syngas lower-heating-value from PGM can reach up to10 MJ/Nm3. The syngas can be used as good fuel or chemical-engineering materials.

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� The energy efficiency of the PGM technology is higher than tra-ditional gasification.� The PGM technology provides more than 95% volume reduction

of raw MSW.� Most heavy metals can be trapped in the molten slag [16,17].

After cooling down, the slag can be used as constructionmaterial.� Lower pollutant emission due to the reduction environment.

f solid waste gasification in the Plasma Gasification Melting process. Appl

Table 1MSW proximate and ultimate analyses.

Proximate analysisMoisture 20.0%Fixed carbon 10.7%Volatile 77.6%Ash 11.7%

Ultimate analysis (in dry basis)Carbon 50.5%Hydrogen 5.6%Oxygen 30.7%Nitrogen 1.1%Chlorine <0.1%Sulfur 0.3%

Fig. 2. Simplified scheme of the PGM process model.

Q. Zhang et al. / Applied Energy xxx (2013) xxx–xxx 3

2.2. Feedstock properties

The feedstock used in this study is MSW collected in Israel. Theproximate and ultimate analyses of the MSW are given in Table 1.In reality, the size of MSW particles varies from 1 to 100 mm.

2.3. Model description

In this work, a steady state model of the PGM process was used.The model was developed with the process simulating softwareAspen Plus. In order to demonstrate the phenomena at differentstages of the PGM process, the whole PGM process is schematizedinto four sub-models: drying, pyrolysis, char gasification and plas-ma melting. Mass and energy balances were considered individu-ally in each zone. The simplified scheme of the PGM gasificationmodel is shown in Fig. 2. More details about the model have beenpublished previously [13–15].

2.3.1. DryingThe flow sheet diagram of the drying sub-model is shown in

Fig. 3a. Raw MSW firstly meets the syngas from the pyrolysis zone,and the drying happens in the reactor named DRY-REA. The heat ofdrying is considered as the potential evaporation heat of water. Theenergy balance of heat exchanger is described as:X

i

_Mi

Z Tsyngas-in

Tsyngas-out

cp;idT ¼ _MMSW-dry

Z TMSW-out

TMSW-in

cp;MSW-drydT

þ _MH2O

Z Tsyngas-out

TMSW-in

cp;H2OdT þ heva=MH2O

!ð1Þ

2.3.2. PyrolysisPyrolysis behavior is very important for the PGM process.

Firstly, the pyrolysis gaseous products directly flow through thedrying zone, and a major contributor of the syngas. Secondly, tarproduction is a serious problem in updraft fix-bed gasification pro-cesses like the PGM process [18,19], while tar formation mainly oc-curs in the pyrolysis section. As a result, accurately simulating ofthe pyrolysis behavior is one of the key factors for a successfulPGM model.

The heterogeneous MSW composition is the reason for the com-plicated modeling of pyrolysis. According to the main pyrolysischaracteristics, combustible components in MSW can be dividedinto two main groups: cellulosic fractions (wood, paper, vegetationand cardboard) and plastics (PE, PP, PVC and rubber) [20]. Due tothe complexity in both reaction paths and products generated,the detailed kinetics of pyrolysis is still unclear. However, thetwo-step pyrolysis model is reported to be a good approach forfixed-bed processes where pyrolysis occurs at low heating rates

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[21–23]. In this work, the two-step pyrolysis model is applied forboth cellulosic species and plastics:

Cellulosic species! aGascel;pri þ bTarcel;pri þ cAshþ dC ð2Þ

Plastic species! a0Gaspla;pri þ b0Tarpla;pri þ c0Ashþ dC ð3Þ

Tarcel;pri ! eTarsec þ fGascel;sec ð4Þ

Tarpla;pri ! e0Tarsec þ f0Gaspla;sec ð5Þ

Yields and composition of cellulosic group pyrolysis and sec-ondary tar cracking, including the composition of produced gasesand tars are taken from Hla [24]. Yields and compositions of pri-mary pyrolysis of plastics are taken from Williams and Williams[25]. No literature data was found for the secondary pyrolysis ofplastic mixture, so the yield of primary tar cracking of the plasticgroup is calculated from elementary balance. To simplify the mod-el, all LHCs except CH4 are considered as C2H4. The composition ofsecondary tar is assumed to be benzene.

2.3.3. Char gasificationThe char gasification zone involves a large number of reactions.

In the PGM process, the temperature in this zone can reach up to1000 �C. The order of magnitude of gases residence time in the chargasification zone is 101 s.

f solid waste gasification in the Plasma Gasification Melting process. Appl

Fig. 3. Flow sheet diagrams of four sub-models. (a) Drying, (b) pyrolysis, (c) char gasification and (d) melting.

4 Q. Zhang et al. / Applied Energy xxx (2013) xxx–xxx

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Fig. 3. (continued)

Table 2Standard chemical exergy, standard formation enthalpy and standard formationentropy of gaseous species.

Substance ~e0;i (J mol�1) ~h0;i (J mol�1) ~s0;i (J mol�1 K�1)

H2O 9500 �241,845 188.715CO 275,100 �110,541 197.548H2 236,100 0 130.595CO2 19,870 �393,546 213.736CH4 831,650 �74,831 186.188C2H4 1,361,100 52,283 219.827N2 720 0 191.511O2 3970 0 205.043

Q. Zhang et al. / Applied Energy xxx (2013) xxx–xxx 5

At this condition, the chemical reactions occurring in this zonecan be regarded as at chemical equilibrium. In this model, theproducts from the char gasification zone are calculated byminimizing the system Gibbs free energy.

The flow sheet diagram of the char-gasification sub-model isshown in Fig. 3c. All reactions involved in char gasification happenin the Gibbs reactor named GASI-REA. Then, gaseous and solidproducts are separated in the separator GASI-SEP. A heat streamis specified afterwards so as to consider the temperature differencebetween gas and solid phases.

2.3.4. Plasma melting and vitrificationThe inorganic components (ash) of the MSW are melted and vit-

rified due to the oxygen content of the plasma gas and by the hightemperature plasma jets in the plasma melting section. In thiswork, the melting of inorganics is assumed to happen in the reactornamed MELTING. The heat needed for melting is provided by themixture of plasma and secondary air. According to the originalcomposition of the MSW, the heat capacity of the inorganics is cal-culated as following:

cp;ash ¼Xn

i¼1

xicp;i ð6Þ

The composition of the inorganic components is assumed as amixture of 50% SiO2, 30% Al2O3 and 20% CaO. The melting latentheat of the inorganics is calculated similarly to that of the heatcapacity.

2.4. Exergy calculation

In this work, the exergy analysis is carried out in each sub-model by calculating the global exergy input and output. Theexergy involved in the PGM process including chemical exergyEch and physical exergy Eph.

For the gaseous stream, the chemical exergy is calculated as:

~ech;gas ¼X

i

~xi~e0;i þ RT0

Xi

~xi lnð~xiÞ ð7Þ

~eph;gas ¼X

i

~xi~eph;i ð8Þ

~eph;i ¼ ð~hi � ~h0;iÞ � T0ð~si � ~s0;iÞ ð9Þ

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where ~e0;i,~h0;i and ~s0;i are the standard chemical exergy (J mol�1),standard formation enthalpy (J mol�1) and standard formation en-tropy (J mol�1 K�1) of gaseous components. The values of ~e0;i

~h0;i

and ~s0;i of main gaseous species are listed in Table 2.Due to the heterogeneous composition of MSW, it is not practi-

cal to calculate its chemical exergy by its constructional species.Szargut and Styrylska [26] introduced an empirical correlationfor the specific chemical exergy of solid fuels. This correlationhas been used by other researchers for exergy analysis of solid fuelgasification [27]. In this work, it is chosen due to its positive feed-back from previous works:

ech;MSW ¼ LHVMSW

�1:044þ 0:016 xH

xC� 0:3493 xO

xC1þ 0:0531 xH

xC

� �þ 0:0493 xN

xC

1� 0:4124 xOxC

ð10Þ

Similarly, the chemical exergy of various tars are simulatedwith another correlation for liquid fuels [28]:

ech;tar ¼ LHVtar 1:0401þ0:1728xH

xCþ0:0432

xO

xCþ0:2196

xS

xC1�2:0628

xH

xC

� �� �ð11Þ

2.5. Energy and exergy efficiencies

Generally, the energy efficiency of a gasification process is oftendefined as the cold-gas-efficiency (CGE). For the PGM process, theCGE is written as:

f solid waste gasification in the Plasma Gasification Melting process. Appl

Table 4Comparison between measured and simulated results.

Case number 1 2 3 4

Measured resultsSyngas yield (Nm3 kg�1 MSW) 1.36 1.38 1.26 1.29Syngas LHV (MJ Nm�3) 8.23 8.43 8.24 8.70H2/CO 1.24 1.53 1.45 1.70

Predicted resultsSyngas yield (Nm3 kg�1 MSW) 1.27 1.32 1.16 1.14Syngas LHV (MJ Nm�3) 8.48 8.70 8.05 8.38H2/CO 1.16 1.33 1.32 1.41

6 Q. Zhang et al. / Applied Energy xxx (2013) xxx–xxx

gen;CGE ¼_Msyngas � LHVsyngas

_MMSW � LHVMSW þ Psteam þ Pplasma

� 100% ð12Þ

where Psteam denotes the power needed to produce high tempera-ture steam, and Pplasma is the power of plasma generators.

Due to the relatively high tar yield in the syngas, the most effi-cient usage of the syngas is direct combustion in gas furnace. Inthat case, tar in the syngas can be directly combusted. So for thePGM process, the energy efficiency can also be defined as the totalenergy efficiency:

gen ¼_Msyngas � LHVsyngas þ _Mtar � LHVtar

_MMSW � LHVMSW þ Psteam þ Pplasma

� 100% ð13Þ

In this work, both definitions of energy efficiency will beconsidered.

Similarly, the exergy efficiency of the PGM process can be de-fined as the cold-gas-exergy-efficiency (CGEE) or total exergyefficiency:

gex;CGE ¼~_Msyngas � ~ech;syngas

_MMSW � ech;MSW þ _Msteam � eph;steam þ Pplasma

� 100% ð14Þ

gex ¼~_Msyngas � ~ech;syngas þ _Mtar � ech;tar

_MMSW � ech;MSW þ _Msteam � eph;steam þ Pplasma

� 100% ð15Þ

3. Results and analysis

3.1. Comparison with measured results

A demonstration PGM plant was constructed in Yblin, Israel,with a designed capacity of 12–20 tons of MSW per day. From2007, a series of test runs has been performed in the demonstra-tion PGM plant, so as to study the characteristics of the PGM pro-cess. Some typical results at different operation conditions werepublished in by Zhang [12]. In this work, these experimental mea-sured results are used to validate the numerical model. The opera-tion parameters of compared cases are shown in Table 3.

The simulated results of syngas yield and composition are com-pared with experimental results, in terms of syngas yield, syngasLHV and the ratio of H2 and CO volume fractions (H2/CO). Both re-sults are shown in Table 4.

The simulated results fit well with the measured results in bothsyngas yield and its composition. The H2/CO values are slightlyunderestimated in all cases, but they are still in acceptable range.This model is acceptable to predict the performance of PGMprocess.

3.1.1. Energy and exergy balance of the base caseBased on the operation experience of previous runs, the opera-

tion condition in case 1 is a rather stable condition, at which thePGM process can keep stable operation. In order to better under-stand the thermo-dynamic characteristics of the PGM process,

Table 3Operation parameters of the test runs.

Case number 1 2 3 4

Plasma power (kw) 240 240 240 260Primary air feeding rate (kg h�1) 120 120 120 130Secondary air feeding rate (kg h�1) 60 60 35 13Secondary air feeding temperature (�C) 25 25 25 25Steam feeding rate (kg h�1) 70 100 70 70Steam temperature (�C) 1000 1000 1000 1000

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the case 1 is chosen as an example to demonstrate the energyand exergy balance during the PGM process.

Fig. 4 shows the Sankey diagram of the energy flow of the basecase. It provides visible evidence for some characters of the PGMprocess of MSW. For instance, the chemical energy transfer fromMSW to syngas mainly happens in the pyrolysis and char gasifica-tion sections, while about 80% of total chemical energy transfer oc-curs during pyrolysis. The sensible heat input rate from plasma andhigh temperature steam is about 10% of the chemical energy trans-fer rate. It can also be found that the total energy efficiency gen isabout 94.4%, which is quite high compared to other technology.However, the chemical enthalpy of tar is 1236.3 kW, which isabout 46% of total chemical enthalpy in the syngas. The gen,CGE va-lue in the base case is only 50.8%. This demonstrates quite low en-ergy efficiency for clean syngas usage in gas turbines and gasengines.

Fig. 5 is the Sankey diagram of the exergy flow for the PGM pro-cess for the base case. It is found that the exergy loss in the pyro-lysis section reaches 150 kW, which is the highest in all foursections. This shows that the intense chemical energy transfer be-tween solid and gas during the pyrolysis process is strongly irre-versible. The exergy loss in the plasma melting section is122 kW, this phenomena fits the results in Fig. 4 because there isa large heat loss during plasma melting. Another exergy loss is inthe mixing of high temperature plasma flow with secondary air.The mixing process is strongly irreversible. Despite the exergy lossduring the PGM process, the total exergy efficiency gex for the basecase is 86.5%, while the cold gas exergy efficiency gex,CGE is 44.9%.

3.2. Parameter study

3.2.1. Influences of equivalence ratio on energy and exergy efficiencyIn a gasification process, the feeding of air provides an oxidizer

for char gasification. Meanwhile, it supplies heat by partial com-bustion of feedstock. The amount of air supply for gasification isusually represented by the equivalence ratio (ER):

ER ¼_Mair= _MMSW

� �_Mair= _MMSW

� �stoic

ð16Þ

In the PGM process, due to sensible heat supplied by thermalplasma and high temperature steam, the ER value is much lower(0.04–0.10) than that of conventional gasifiers (around 0.3). Fromthe view point of exergy efficiency, a low ER is desirable becausethe combustion process is irreversible, and it reduces the totalexergy inside the gasifier. In addition, a low ER value prevents dilu-tion of syngas from N2 and CO2. This thought is confirmed by thesimulated gen and gex values at different ER value. As an example,Fig. 6 shows the variation of gen and gex with ER, when the plasmapower is 240 kw, and the steam feeding rate is 70 kg/h. It is clearthat both gen and gex decrease with increasing ER, and decreasingtrends become more obvious with increasing ER.

Fig. 7 shows the variation of gen,CGE and gex,CGE with the sameconditions. It is found that the gen,CGE and gex,CGE first increase with

f solid waste gasification in the Plasma Gasification Melting process. Appl

Fig. 4. Sankey diagram of energy balance for the base case.

Fig. 5. Sankey diagram of exergy balance for the base case.

Q. Zhang et al. / Applied Energy xxx (2013) xxx–xxx 7

increasing ER, and reach their maximum at about ER = 0.13. If theER value further increases, both gen,CGE and gex,CGE show slightdecreasing trends.

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Together with the trends of gen and gex, This indicates a signif-icant reduction of tar yield with increasing ER. This result coincidewith the experimental results by Ponzio et al. [29], who carried out

f solid waste gasification in the Plasma Gasification Melting process. Appl

Fig. 6. Influence of ER on gen and gex.

Fig. 7. Influence of ER on gen,CGE and gex,CGE.

Fig. 8. Influence of PER on gen and gex.

Fig. 9. Influence of PER on gen,CGE and gex,CGE.

8 Q. Zhang et al. / Applied Energy xxx (2013) xxx–xxx

an experimental study on waste gasification in an updraft fixed-bed gasifier with highly preheated air and steam as agents. It is be-lieved that the decreasing of tar yield is mainly due to favored ther-mal cracking at increased environmental temperature. Thecracking produces combustible gases such as CO, H2 and LHCs,and this explains the increasing of gen,CGE and gex,CGE with ER whenER < 0.13. If the value of ER exceeds 0.13, the negative effects of ERdue to feedstock consumption exceed the tar cracking, so gen,CGE

and gex,CGE start to decrease, and gen and gex decrease faster.

3.2.2. Influences of plasma power on energy and exergy efficiencyUsing thermal plasma torches in the melting section is one

innovation of the PGM technology. These plasma torches not onlyprovide high temperatures for the melting of inorganic compo-nents in MSW, but also provide sensible heat for the MSW gasifica-tion. The providing of extra heat can largely increase the energyand exergy efficiency of gasification. However, the generation ofplasma flow needs large exergy input by electricity. As a result,the use of plasma torches has two contrary influences on energyand exergy efficiency.

In this work, the amount of plasma energy input is expressed bydimensionless plasma energy ratio (PER), which is defined as:

PER ¼ Pplasma

LHVMSW � _MMSW

ð17Þ

Fig. 8 shows the variation of gen and gex with PER, at ER = 0.06and steam feeding rate equal to 70 kg/h. It is found that the influ-ence of PER on gen and gex seems slightly negative. A possible rea-son for this phenomenon is the large increase of energy and exergyinput to the PGM system with increasing PER. As was demon-strated in section 3.1.2, the plasma flow undergoes large energy

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and exergy loss in the plasma melting process. Since the generationof plasma flow consumes a large amount of electricity, it is not sug-gested to use very high plasma power for the gas furnace applica-tion of PGM.

Fig. 9 shows the variation of gen,CGE and gex,CGE with PER. It isfound that when PER increases from 0.078 to 0.109, the gen,CGE in-creases from 45% to 55%, and the gex,CGE increases from 40% to48.5%. This result fits previous experimental results from a demon-stration PGM processing chamber [12]. Similarly to the effect of ER,it is believed that the increase of gen,CGE and gex,CGE with increasingPER is mainly due to favored char gasification and tar cracking byenhanced reaction temperatures.

3.2.3. Influences of steam feeding rate on energy and exergy efficiencyIn the PGM process, high temperature steam is usually used as

additional agent to promote char gasification. Meanwhile, it is alsoused as a carrier of sensible heat. In this work, the amount of steamfeeding rate is expressed by steam air mass ratio:

SAMR ¼_Msteam

_Mair

ð18Þ

Fig. 10 shows the variation of gen and gex with SAMR, atER = 0.06 and PER = 0.094. It is found that both gen and gex demon-strate increasing trends when SAMR increases from 0 to 0.2.According to previous study results [15], the dramatic increasingof gen and gex is caused by incomplete gasification of char due toa deficiency of agent supply. The injection of high temperaturesteam largely enhances the char gasification ratio, and the com-plete gasification of char can be achieved at about SAME = 0.2.When SAME continues increasing from 0.2, the gen and gex startto decrease slightly with increasing PER.

f solid waste gasification in the Plasma Gasification Melting process. Appl

Fig. 10. Influence of SAMR on gen and gex.

Fig. 11. Influence of SAMR on gen,CGE and gex,CGE.

Q. Zhang et al. / Applied Energy xxx (2013) xxx–xxx 9

The variation of gen,CGE and gex,CGE with SAMR at ER = 0.06 andPER = 0.094 is shown in Fig. 11. Generally, the effects of SAMR ongen,CGE and gex,CGE are positive. It is believed that this is caused byfavored steam reforming of tar. Meanwhile, the positive effects ofSAMR on char gasification can also be found when the SAMR is be-low 0.2.

4. Conclusions

Energy and exergy analysis for the PGM process is performedbased on a process simulating model. Two kinds of energy andexergy efficiency are defined. For the base case, the gen,CGE = 50.8%,and the gex,CGE = 449%. When the LHV of tar is included in the effi-ciency calculation, then gen = 94.4%, and gex = 86.5%. It is suggestedto directly use the syngas from The PGM in the gas furnace to ob-tain high energy and exergy efficiency.

A parameter study was performed to investigate the influencesof plasma power, air feeding rate and steam feeding rate on the en-ergy and the exergy efficiency of the PGM process. Generally, at thereasonable operating conditions, the effects of all three parametersare positive for gen,CGE and gex,CGE. However, when consider totalenergy efficiency gen and exergy gex, it is not suggested to use highER, PER or SAMR. From either definition of energy and exergy effi-ciency, a small amount of steam injection is beneficial.

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