IN DEGREE PROJECT MATERIALS SCIENCE AND ENGINEERING,SECOND CYCLE, 30 CREDITS

, STOCKHOLM SWEDEN 2018

Process Simulation of Plasma Gasification for Landfill Waste

BOON HAU TAN

KTH ROYAL INSTITUTE OF TECHNOLOGYSCHOOL OF INDUSTRIAL ENGINEERING AND MANAGEMENT

TableofContents

ListofFigures.....................................................................................................................I

ListofTables.....................................................................................................................II

ABSTRACT........................................................................................................................III

1.INTRODUCTION.............................................................................................................11.1Background.....................................................................................................................11.2Objective........................................................................................................................41.3Scope..............................................................................................................................4

2.LITERATUREREVIEW......................................................................................................52.1Finefraction...................................................................................................................52.2AspenPlusModelling.....................................................................................................62.3PlasmaGasification......................................................................................................122.4TarCracking..................................................................................................................142.5Melting.........................................................................................................................16

3.METHODOLOGY...........................................................................................................173.1PlasmaGasificationProcess.........................................................................................173.2PlasmaGasificationModel...........................................................................................183.2.1Feedstockproperties.............................................................................................183.2.2Drying....................................................................................................................203.2.3Pyrolysis.................................................................................................................213.2.4CharCombustionandGasification........................................................................213.2.5Melting..................................................................................................................223.2.6PlasmaTarCracking..............................................................................................22

3.3ASPENPlusModeldescription.....................................................................................243.3.1Assumptions..........................................................................................................283.3.2BoundaryConditions.............................................................................................28

3.4ParameterStudies........................................................................................................29

4.RESULTSANDDISCUSSION...........................................................................................314.1VerificationandValidationofModel...........................................................................314.1.1SyngasCompositionPreandPostTarCracking....................................................32

4.2MassandEnergyBalance.............................................................................................334.3ResultsofParameterStudies.......................................................................................354.3.1EffectofER............................................................................................................354.3.2EffectofPreheatedAirTemperature....................................................................37

5.CONCLUSION...............................................................................................................40

6.FUTUREWORK............................................................................................................41

7.REFERENCE..................................................................................................................42

I

ListofFiguresFigure1.Theroleofplasmagasificationinacirculareconomy.(Power,2018)......................3Figure2.PlasmagasificationmodelofEPJModel(Minutilloetal.,2009)...............................9Figure3.Atwostageplasmagasificationprocess(Materazzietal.,2016)...........................12Figure4.TarCrackingModel(Fourcaultetal.,2010)............................................................15Figure5.Comparisonofrequiredheatforincinerationandgasification(Lietal.,2007)......16Figure6.Schematicoftwostageplasmagasificationmodel.................................................17Figure7.Simplifiedschematicoftheplasmagasificationmodel..........................................20Figure8.FlowsheetofplasmagasificationprocessinAspenPlus.........................................24Figure9.Comparisonbetweensyngascompositionbeforeandaftertarcracking...............32Figure10.MassandEnergyBalancediagramforplasmagasificationsystem......................33Figure11.Sankeydiagramshowingflowofenergyforplasmagasificationsystem.............34Figure12.Piechartshowingdistributionofenergyoutput...................................................35Figure13.EffectofincreasingERonsyngascompositionatTpreheatair=873K...............36Figure14.EffectofincreasingERonLHVandCGE................................................................36Figure15.Heatofpartialcombustionrequiredfromair(ER=0.208)atdifferent

Tpreheatair...................................................................................................................38Figure16.GraphshowinglowestpossibleERatdifferentTpreheatair...............................38Figure17.EffectofTpreheataironLHVandCGEaccordingtolowestpossibleERateach

Tpreheatair...................................................................................................................39

II

ListofTablesTable2.MaterialCompositionofRDF...................................................................................18Table3.UltimateandproximateanalysesofRDF.................................................................19Table4.AnalysesofmetalcontentsofRDF...........................................................................19Table5.Charcombustionandgasificationreactionsconsidered..........................................22Table6.Reactionsconsideredfortarcracking......................................................................23Table7.DescriptionofunitoperationblocksinASPENPlusflowsheet.................................26Table8.AspenPlusmaterialstreams....................................................................................27Table9.AspenPlusheatstreams...........................................................................................28Table9.Boundaryconditionsforplasmagasificationmodel................................................29Table10.ParametersthatwerestudiedintheASPENPlusmodel........................................30Table11.ComparisonofcompositionbetweenAspenPlusModelandSimilarSetup..........31Table12.MassbalanceofAspenPlusmodel........................................................................34Table13.EnergybalanceofAspenPlusmodel......................................................................34

III

ABSTRACT

ThegrowingamountoflandfillwastewithintheEUcouldposeaprobleminthefutureshould

therenotbeanyeffectivetreatmentmethods.Thisstudyaimstoinvestigatetheperformance

oflandfillwasteinaplasmagasificationprocessbysimulatingtheprocessinASPENPlus.The

investigationisfocusedontheenergyrecoverypotentialofRDFbasedoncompositionand

heatingvalueofsyngas,andcoldgasefficiency(CGE).Theplasmagasificationsystemconsists

ofashaftgasifierandaseparatetarcrackingreactorwherehightemperatureplasmaisused

forconversionoftarcompoundsconsideredinthemodel,whicharetolueneandnaphthalene.

Inaddition,themodelisdividedintofivesections,namelydrying,pyrolysis,chargasification,

meltingandtarcracking.Massandenergybalanceofthesystemwasperformedtobetter

understand the system.The results show that theplasmagasification systemwasable to

produce a syngaswith a LHVof 4.66MJ/Nm3while improving syngas yieldby attaining a

highercontentofhydrogen.Thus,theplasmatarcrackingoftarcompoundscanachievea

cleansyngasandimprovesyngasyield.ParameterstudyoneffectofERshowthatsyngashas

higherheatingvalueandCGEatlowerER.Ontheotherhand,preheatedaircanhelprecover

energyfromthesystemwhileloweringtheERrequiredforthechargasificationprocessto

meettheheatdemandfrompartialcombustion.Thefindingsimpliedthatlandfillwastehas

energypotentialbyusingasuitabletreatmentprocesssuchasplasmagasification.

Keywords:landfillwaste,RDF,plasmagasification,syngas

1

1.INTRODUCTION1.1Background

Landfillhasbeentraditionallythesolutionforallthewastesproducedpriortoinventionof

modernandadvancedtreatmentmethods.Theconsequenceistheexistenceofanestimated

150,000to500,000landfillsiteswithinEuropethatwillhaverepercussionsforyearstocome

(Hoglandetal.,2010).Throughoutthedecades,wastemanagementhasprogressgradually

towardsamoresustainablepathwithfocusonreductioninconsumption,recycleofwaste

products,andwastevalorization.Stateof theart technology is requiredtoreduce landfill

wastewhichcanopenupotheropportunities.

Pertaining to landfilling, ithasbeen included in ‘Roadmap toaResourceEfficientEurope’

documentedbyEuropeanUnion(EU)thatlandfillingshouldbethefinaloptionandgradually

be ruled out as a solution as part of their aim by 2020 tomanagewaste resources in a

sustainablemanner(Commission,2011).Thishasledtotwoinnovativeconceptsknownas

EnhancedWasteManagement(EWM)andEnhancedLandfillMining(ELFM)thatattemptto

address issues surrounding landfill waste. EWM emphasizes on reduction in usage and

recycling,which rulesout landfillingas the finaldestinationbut as abuffer storage tobe

processedafter.Ontheotherhand,ELMisanapproachforwastevalorization,wherewaste

frombotholdandnewlandfillsareusedassourceofmaterialsandfuelforenergyrecovery

process (Bosmanset al., 2013). Such conceptswill beable tounlockpotential in thevast

amountofwastethathavebeenburiedandlefttoonceagainbepreciousresourcesthatcan

beutilize forvariousapplications.Furthermore,notonlyextravagantcostsofremediation

canbe avoided, the landused for landfilling can attain a higher valueby commercialized

explorationandactivities.Aprojected0,1-1trillioneurointhenext5decadeswillberequired

as remediation costs for the EU-28 while the Flemish Public Waste Agency, OVAM was

reportedtohavespent80millioneurotoremediatejust5problematiclandfills,byexcavating

landfilledmaterial,transportingandre-landfillinginamodernsanitarylandfill(Jonesetal.,

2013).Thereishoweverinsufficientandlessefficientusageofbudgettoperformsuchascale

ofprojectformajorityofstatesinEU(Vautmans,2015).

2

Solidwastebasedenergyrecoveryviaincinerationhasbeenacommonmethodusedtotreat

municipalsolidwastebutalsoassourceofelectricityandheat.Wasteincinerationcombust

solidwastetypicallyintheformasrefusedderivedfuel(RDF)toobtainamoresustainable

operation aswell as to achieve a higher calorific value thus a higher electrical efficiency.

Conversionofmunicipal solidwaste (MSW) intoRDF involvespossiblemechanical and/or

manualhandling toobtainmorehomogeneouspropertieswhichenhances calorific value.

However, process and treatment varies according to landfill sites because of variation in

wastepropertiesandcompositions.Thedownsideofincinerationbeingacompleteoxidative

combustionprocessistheneedforexpensivefluegastreatmentequipmentduetopresence

of SOx andNOx, disposal problemswith fly ash andbottomash thatmay causepotential

hazard in leachingofheavymetals should itbe reused.Advanced thermal treatment that

turns bottom ash into environmentally friendly products is available but consumes a

significantamountofenergythatreducesenergyrecoveryefficiency.

Gasificationwhichisapartialoxidationprocessoffersanotheralternativethermochemical

processtotreatsolidwastesbyconvertingintosynthesisgas(syngas)thathascalorificvalue

and can be used as a fuel for various industries such as chemical industry and power

generation.Differentgasificationagentssuchasair,oxygencanbeusedtoproducesyngasof

higher calorific value using the latter. There has been development in using plasma

technologywithgasificationwherethehighenergydensityfromplasmacanenhancereaction

rate and at a higher temperature,materials can bemelted into slag. Plasma gasification

resultsinsyngaswithprimarilycarbonmonoxideandhydrogen.Atextremehightemperature,

tarandcharcanbebrokendownthushavingacleanersyngaswithoutexpensivegascleaning

facilities.Meanwhile,hazardousinorganicmaterialscanbevitrifiedinthemoltenslagwithout

leaching issues. Thus, the gasification process can play an important role in achieving a

circulareconomyasillustratedinFigure1.

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Figure1.Theroleofplasmagasificationinacirculareconomy.(Power,2018)

StudieshasbeenmuchonMSW,ratherlessonlandfillwaste.Landfillwastewhichareburied

foryearsanddecadesundergoesdecompositionwhichmakespropertiesdifficulttopredict

anditincreaseheterogeneityofthewaste.Thedecompositionincreasessoilandfinefraction

whichmayaffecttheenergyrecoverypotentialandalsoadditionalpre-treatmentprocessis

required before being used as a fuel. Preliminary studies have been conducted through

experiments,modelling,andsimulationbutstillrequirefurtherinvestigationastherearestill

uncertaintiesinthelandfillwastebeforeinvestinginlargescaleplants.

Therehavebeenstudiesconductedoncharacterizationoflandfillwasteusingvariousplasma

gasificationtechnologiessuchasfixedbed,fluidizingbedandentrainedflow.However,there

arenotmanystudiesontwostageplasmagasificationprocesscomprisingofastand-alone

gasifierandplasmatarconverter.Presenceof tar in theproductgasmakes itdifficult for

productgastobeusedandadditionalcostsmayberequiredtocleanupthegasforpractical

usage.Aseparateplasmatarconverterhasbetterpotentialinachievinghighertarconversion

rate. Tars contained in the product gas can contribute to an increase in heating value of

productgas.Tarisoftennottakenintoaccountinsimulationofgasificationduetocomplexity

oftarcompoundsandcrackingprocess.Hence,thisstudyintendstoinvestigatethepotential

of landfillwaste in energy recovery in termsof compositionofproduct gas, energy value

4

consideringtarcompoundsintheprocessandtheefficiencyofthegasificationprocess.Firstly,

someliteraturestudieswillbedonetoreviewtheproblematiccontentofthelandfillwaste.

Next, some previous investigations onmodelling of gasification using ASPEN Plus will be

reviewed,followedbyastudyonusageofplasmagasificationandapplicationsoftarcracking.

Then,gasificationprocesswillbebrokendownindistinctmodelsbeforebuildingaASPEN

Plusmodelbasedontheschematicofthegasificationprocess.Usingthemodel,massand

energybalancewillbeperformedbeforeconductingparameterstudies.

1.2Objective

The objective of this study is to understand the energy and mass balance of plasma

gasificationsystemforalandfillwastebasedontheprocessmodelling,aswellaspredictthe

plasma gasification performance in terms of efficiency and syngas quality from its

composition.

1.3Scope

ThescopeofthisprojectinvolvesdevelopingthemodelforplasmagasifierusingASPENPlus

by breaking down into individual sections. The model will then be verified by using the

experimentaldataobtainedfromapilotscaleplasmagasifierofsimilarconstruction.Finally,

simulationoflandfillwastegasificationshallbeperformedtoobtaintheoptimumconditions

for the gasification process by carrying out parameter study to investigate the effect of

differentparametersintermsoftheefficiencyofprocessandcompositionofthesyngas.

5

2.LITERATUREREVIEW

2.1Finefraction

Earlier studies on energy recovery potential of landfill wastewere carried out at various

locations.Duetonon-homogeneouspropertiesoftheexcavatedwaste,theyarerequiredto

bepre-processedviaprocesses suchascrushing, sievingandmanual sortingbeforebeing

utilizedasRDF.Themainconcerninlandfillwasteisthesignificantamountoffinefraction

that ispartlydue to thedecompositionofwastewhichwouldaffect thecalorificvalueof

syngasproducedasshowninsomestudies.

In a study done at REMO landfill located atHouthalen-Helchteren, Belgium characterized

eachcategoryofmaterialinthewastepostsievingprocessshowedthatthefinefraction(

6

between24-40yearsoldweremechanicallyseparateddefinedwithsizebelow20mmand

30mmwere investigatedof itsproperties suchasBiochemicalMethanePotential (BMP),

volatilesolidsandtotalsolids.ItwasdiscoveredthatBMPincreaseswithsizefraction.Besides,

finefractionincreaseswithageduetogreaterdecompositionofthewaste(Monkareetal.,

2016).(Kaartinenetal.,2013)suggesteddryingonthewastetoremovefinefractionattached

oncalorificfraction.Inthestudy,thewastewastreatedviamanualsortingandmechanical

treatment.However,investigatedcalorificvaluewereofdifferentsizefractions,largerthan

20mmand70mmformanuallysortedportionandmechanicallytreatedportionrespectively,

theobtainedcalorificvaluewasrathersimilar(20–25MJ/kg).Similarfindingsby(Hoglandet

al.,2004)infinefractionswithcalorificvaluebetween0–1MJ/kg,whileincoarsefraction,

itisfeasibleforenergyrecoverywithahighercalorificvalue.Thus,dryingappearstobeable

toimprovethecalorificvalueofthewastewhilemakingprocessmoreefficientaslessenergy

iswastedonheatingupwater.

2.2AspenPlusModelling

Numerousstudiesonmodellingandsimulationofgasificationprocesshasbeenperformed

usingtheprocesssimulationsoftware,AdvancedSystemforProcessEngineering(ASPEN)Plus

sinceitisamorecosteffectivewaytostudythecharacteristicofdifferentfeedstocksused

comparedtoapilotplantscale.Parameterstudiescanalsobedonetoinvestigateeffectof

differentparametersontheprocess.StudiesconductedusingAspenwerewidelyappliedfor

differentfeedstockandtechnology.Twodifferentmethodsthatcanbeusedtosimulatethe

modelinAspen,whichareequilibriumandkinetic.Equilibriumbasedmodelaresimulatedby

minimization of Gibbs free energywhile kinetic basedmodel takes into account reaction

kineticsthatoccur.Equilibriummodelcanbeusedasamodelthatprovidesabroadpicture

ofhowaparticularfeedstockbehavesinaprocessbeforedetailedstudiesarecarriedoutby

consideringmoreaspects.

Previous studiesusingequilibriumbasedmodel showed reasonableoutcomeas validated

againstexperimentaldataandotherstudies.CrucialparameterssuchasEquivalenceRatio

(ER),gasificationtemperatureandsteamadditionareamongothersthecommonparameters

7

beinginvestigatedbecauseofthesignificantinfluenceontheenergyrecoverypotentialinthe

producedgas.(Niuetal.,2013)simulatedgasificationofmunicipalsolidwasteinabubbling

fluidizedbedusingtheminimizationofGibbsfreeenergymethodandinvestigatedtheeffect

ofEquivalenceRatio(ER),gasificationtemperature,moisturecontent,steam-wasteratio,and

percentageofoxygeninair.Itwasdiscoveredthatforahighersyngasyieldcanbeobtained

athighergasificationtemperature,anoptimalERof0.3at800°C,higherpercentageatlower

temperature.Pre-dryingoffeedstocktoalowermoisturetopreventextraenergytoconvert

watertosteamwhilehighersteam-wasteratiofacilitatesforconversiontoH2.Comparably,

a circulating fluidized bed gasifierwas simulated and the preheated air temperaturewas

investigatedover a range25 – 825°C showedan increase in syngas heating valuedue to

increaseinH2andCO.Furthermore, lesservolumeofair isrequiredtoachieveagasifier’s

operatingtemperaturecanhelptoreducethesizeofreactorwhich inturn isasavingsto

capitalcost.PreheatedairwasfoundtobeeffectiveatlowerER,wherealimitof0.35was

recommended(Dohertyetal.,2009).Similarly,influenceofERwasalsostudiedby(Lietal.,

2013)inasimulationwhereanoptimalpreheatedairtemperatureat600°CwithERof0.4

wasalsofoundouttobeimportantforthegasificationprocess.Meanwhile,anincreaseofER

ledtoadecreaseinlowerheatingvalue(LHV)ofsyngas.Plasmagasificationofthreetypesof

biomass, coffee husks, vines pruning and forest residues for hydrogen production were

investigated.ERwasvariedbetween0.1–0.6anditwasfoundthatoxidationreactionswas

moreprominentduetohigheroxygencontentthusdecreasingH2yield.Atthesametime,N2

contentrisescausedbyincreasedinamountofair.Steamusedasgasifyingagentincreased

H2 yieldbypromotingwatergas shift and steam reformingdue to increasedwater vapor

partialpressure(Favasetal.,2017).Gasifiertemperaturewasvariedfrom900–2000°Cand

itwasdiscovered thatCOyield increasedbutH2andCO2was theoppositeaswater shift

reactionsareexothermic.

AfixedbedgasifierthatusedMSWasfeedstockandresultswerecomparedtoexperimental

data.Themodelwasusedtostudytheeffectofair-fuelratioandgasifiertemperature.Results

showedthatoptimumair-fuelratiowasaround0.3andathighergasifiertemperature,yield

ofCOincreased(Begumetal.,2014).Adowndraftgasifierintegratedwithapowergeneration

unitfordifferentbiomassfuelscomprisedofdifferentwoodswassimulatedwithamodelin

8

AspenPlususingREquilreactortocomparethegasifierefficiencies.Simulationresultswere

comparedwithexperimentaldataandCH4hadahugevariancepossiblyduetoverysimple

modelbuilt.Averagecalorificvalueobtainedforthebiomassfuelsisabout18MJ/kg(Keche

etal.,2014).

Apartfromair,steamisalsousedasagasifyingagentalongwithairtoachieveaproductgas

withhigheryieldofhydrogen.PerformanceforgasificationofMSW,foodwasteandpoultry

wastewerecomparedtoexperimentaldata.Parameterstudiesleadtofindingsthatshowed

highertemperaturefavorsCOandH2productionandsteamadditionatanoptimumrange

0.15–0.3 for steam-to-biomass ratiocanenhanceH2yield (Ramzanetal.,2011).Another

modelbasedonminimizingGibbsfreeenergywasdesignedtopredictsyngascomposition

usingsteamgasificationofdifferentbiomassforFischer-Tropschsynthesis.Itwasobserved

that increase in gasification temperature from 750 – 950 °C increases yield of CO but

decreasestheyieldofCO2andCH4(Palaetal.,2017).Endothermicreactionsarefavorableas

temperatureincreasesdrivingchemicalreactiontotherightside.Meanwhile,steamaddition

benefitedH2andCO2yield.

Therearealsostudiesthatintegratesthegasificationprocesswithotherprocessessuchas

powergeneration.PlasmagasificationofMSWandplasticsolidwasteatdifferentblending

ratiowassimulatedby(MazzoniandJanajreh,2017)foranIntegratedPlasmaGasification

CombinedCycle(IPGCC)tostudytheeffectofvaryingoxygenratioandsteamratioinplasma

gas.Thestudysuggestedthatathighersteamratioresultedinincreasedhydrogencontent

insyngas,whilehigheroxygenratioleadtolowersyngasyieldcausedbyformationofH2O

andCO2.SimilarplasmagasificationmodelusingGibbsfreeenergyequilibriumapproachwas

used tosimulatedifferent industrialwastesuchas shredded tires,plywoodwithdifferent

gasifyingagent,airandsteam.Electricalefficiencyfromthegasproductswasstudiedbyan

IPGCC. Steam addition contributed to the increase in hydrogen yield(Valmundsson and

Janajreh,2011).However,thereisalimitationinamountofsteamascoolingeffectmaytake

placethusreducingefficiency.AmodelcalledEquipPlasmaJet(EPJ)modelasshowninFigure

2 was developed for estimation of syngas production of a plasma gasification

9

process(Minutilloetal.,2009),which ispartofanIntegratedPlasmaGasification/FuelCell

systemwas developed to predict the electrical efficiency. The EPJmodel determined the

compositionofgasbyminimizingGibbsfreeenergy.Electricalefficiencyoffuelcellobtained

wasabout33%whichhigherthanconventionalincinerationtechnologyatabout20%(Galeno

etal.,2011).TheEPJmodelwasalsousedforanotherintegratedsystemwithaCombined

Cycle system. Different blending of different wastes was investigated together with

combination of different gasifying agents such as air, oxygen and steam for a plasma

gasificationprocess.ItwasdiscoveredthatablendofequalamountofMSWandhazardous

wastecouldachieveaplantefficiencyof24.3%byusingpureairasplasmagas(Mazzoniet

al.,2017).

Figure2.PlasmagasificationmodelofEPJModel(Minutilloetal.,2009).

Kineticbasedmodelswerestudiedconsideringmoredetailedaspectsoftheprocess.There

aresomebuiltinfunctionwithinASPENforinputofkineticparameters.Kineticscanalsobe

included via customized FORTRAN subroutines codes to represent desired model. A

comparison between Gibbs energy minimization model and kinetic model was done by

(Eikelandetal.,2015)toinvestigatethecompositiondifferenceinthesyngascomposition.

10

Reactionratestosimulateactualreactionsinthegasifierweredefinedinthekineticmodel.

Considerationofrateofformationandresidencetimearefactorsthatwillaffecttheresultin

akineticmodel.Bothmodelsshoweddifferentoutcomesfromvaryingfactorssuchassteam

flowrate,reactiontemperatureandresidencetime.Resultscanvarybecauseofthekinetics

consideredasseeninanothermodelwhereafluidizedbedreactorforbiomassgasification

wassimulatedtakingintoaccounttarformationandcrackingbyincludingtarkineticsintothe

modelallowingvariationinfeedstockandgasifyingagentsuchasoxygenandsteam.After

comparingthemodelwithexperimentaldata,itwasconcludedthatmodelperformancecan

beenhancedbyconsideringtarformationandkinetics(KaushalandTyagi,2017).Comparably,

afluidizedbedreactorusedforbiomassgasificationprocesswasalsosimulatedthatfactored

in reaction kinetics and hydrodynamic parameters representing fluidized reactions at the

sametimeconsideringkineticsofchargasification.Validationofmodelwasdoneagainsta

lab scale experimental data. Both gasification temperature and steam-to-biomass ratio

contributed toahigherproductionofhydrogen.ERwas found tobeoptimumat0.23,of

whichhigherthanthatwoulddecreasethecarbonconversionefficiency(NikooandMahinpey,

2008).

A semi detailed kineticmodel was used to simulate a biomass gasification of wood in a

bubbling fluidized bed. Empirical correlations and reaction kinetics of pyrolysis,

hydrodynamics parameters were applied to obtain more accurate results while also

consideringtarformationinthemodelbecauseitisoneofthecrucialparttocommercialize

gasification(Beheshtietal.,2015).Studyshowedthermalcrackingoftarisbetteratincreasing

ER. Gasification temperature increase led to an increase in yield of H2 and CO because

endothermicreactionsarefavorableathighertemperature.Steamadditionalsocontributed

tohigherhydrogenyield.

Anothermodelonfluidizedbedgasificationofwoodwassimulatedbyincorporatingexternal

Fortran code into Aspen Plus blocks to customize the model. Reaction kinetics and

hydrodynamicparameterswereconsideredinthemodeltobetterrepresentreactionsinthe

fluidizedbedgasifier.Themodelthenwasusedtoexamineeffectsofdifferentparameters

suchasair-fuelratio,steam-fuelratioandgasifiertemperature.Increaseamountofairnot

11

only reducesyieldof combustible fraction,H2andCObutalso introducedmoreN2which

decreasestheoverallcalorificvalueofsyngas.Steam-fuelratioincreasehelpedinproducing

moreH2andCOfacilitatedbymoresteamreformingreactions.Carbonconversionimproved

byincreasinggasifiertemperatureshownintemperaturerange600–1000°C(Begumetal.,

2013).

Break down of different reaction zones in a gasification process allows more in depth

reactionstobeconsideredalthoughthezonesmayoverlapinanactualprocess.(Paulsetal.,

2016)developedamodel for a gasificationofwoodybiomass in abubbling fluidizedbed

taking intoaccounthydrodynamics,gasificationkinetics,extensivepyrolysisreactions,and

tarformationkinetics.Amorerealisticapproachconsideringmoreempiricalcorrelationsfor

pyrolysisandchargasificationresultedinmoreaccurateresultscomparedtooriginalmodel

by(NikooandMahinpey,2008)intermsofH2andCOcomposition.However,CO2andCH4

compositionpredictionwere lessaccurateduetheirsmallcomposition.Detailedmodelof

woodgasificationwasdevelopedbybreakingdownintoreactionzonesfordrying,pyrolysis,

secondaryreactionsandchargasificationbyincorporatingFortransub-modelsintoexisting

modelwhileatthesametimeconsideringmorecompoundsinthereactions.Themodelalso

includedsyngascleaningsystemforremovalofinorganicsandparticles,andwatertreatment.

Resultsobtainedwasthesyngascomposedofmajorcompounds21%H2and42%CO,with

calorificvalue12.5MJ/kg(Francoisetal.,2013).Anotherdetailedmodelofdualfluidizedbed

forbiomassgasificationbyimplementingchemicalreactionsfordifferentreactionzoneswith

external Fortran file to simulate operation of a fluidized bed reactor. The model also

consideredtarformationbygroupinginto4differentgroupsoftar,namelybenzene,phenol,

naphthaleneandtoluene(Abdelouahedetal.,2012).ItwasconcludedthatpredictionofCO,

CO2andH2yieldisdependentonthekineticofthewater-gasshiftreaction,whileresultof

methaneandtarcorrespondswellwithexperimentaldata.Hence,asmoredetailssuchastar

compoundsandchargasificationareconsidered,thepredictionofthemodelcanbemore

accurate.

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2.3PlasmaGasification

StudiesusingplasmagasificationfortreatmentoffeedstockssuchasbiomassandRDFhave

shownthatacleanerproductcanbeproducedforvariousapplications.Thehightemperature

from the plasma torch helps to promote tar cracking, converting light hydrocarbons into

combustible gases. Presence of tar poses operational challenges such as erosion and

corrosioninthegasifier.Inordertoachieveacleanersyngas,atwostagefluidizedbedwhich

comprisedofgasifierforprimarygasificationandaseparateplasmareactorfortarcracking

wasdesignedtoovercomesomeissuessuchastaremissionandashslagging.Asecondstage

reactorforplasmatarcrackingshowedthatresidualtarsandcharwereabletobecrackedas

observedintheabsenceoforganiccarbondownstreamoftheplasmareactorasshownin

Figure 3. Aromatic system can be destabilized at high temperature by thermal activation

influences the tar conversion (Materazzietal., 2014). Sootproduced is alsominimalwith

moreeffectivetardecomposition.Furthermore,thesyngaspostplasmareactorwasricherin

H2andCOthanpostgasifierashighertemperatureenhancesreactionssuchaswater-gasshift

reaction(Materazzietal.,2016).Secondaryoxygenasthermalcrackingsourcewascompared

anditwasfoundthatplasmacrackingcanprovidemoreindependentcontrolfortarcracking.

Figure3.Atwostageplasmagasificationprocess(Materazzietal.,2016).

13

AtestreactorwasusedtoassessplasmagasificationofRDFconsistedofpaper,wood,plastic

andorganicmaterialwhichwerepelletized for feeding into the system.Plasma torchcan

preventtemperaturefluctuationsfromthetimedependentexothermicreactionsbyallowing

independentcontrolofheatinput.Slagproducedfromthereactorwasalsotestedandwas

determined to be suitable as secondary building materials according to Flemish

legislation(Lemmensetal.,2007).

TheGasplasmaprocess, a two-stage thermal treatment system for thermal conversionof

wastedevelopedbyAdvancedPlasmaPowerwassimulatedinASPENPlusforvariouswaste

materialssuchaslandfillminedwaste,MSWandindustrial.Syngasexitingplasmaconverter

showed negligible content of benzene which can be a good indicator for tar removal

effectiveness.Calcia-alumina-silicarichslagasobtainedfromtheirgasificationprocesswent

throughleachingteststoinvestigatesuitabilityasaggregatematerialsanditwasfoundthat

pollutantlevelswerefarbelowhazardouslimit(Rayetal.,2012).

Plasmagasification is seen tohave thepotential to treat solidwastebecauseofability to

drasticallyreducewastevolumeandharmfulimpuritiesthusisbeingusedforvarioussolid

wastefromdifferentsources.Syngasproducedfromplasmagasificationofsolidwastefor

highpurityH2productionsinaH2recoverysystem.Dioxinslevelmeasurementswellbelow

regulatory standards of Korea, USA and EU indicated that plasma treatmentwas able to

decomposeorganicscontainedinwaste.Carbonconversionefficiencyof97%wasachieved

suggestingthatplasmatreatmentcanbeefficientintreatmentofwaste(Byunetal.,2011).

DCplasmatorchwithH2O/Arwasusedforplasmagasificationofbiomassandwasteresulted

insyngaswithhighcontentofH2andCOandnegligibletarcontent.Sufficientresidencetime

andoptimumtemperatureinthereactorleadtolowCO2concentrationdespiteCO2being

used as oxidizing agent (Hlina et al., 2014). Plasma gasification application was also

investigatedontreatmentof solidwaste fromUnitedStatesAirForceBasicExpeditionary

Airfield Resources Base in a small reactor where major products obtained are H2 and

CO(Vaidyanathanetal.,2007).MixtureofMSWandrawwoodarepre-treatedusingsteam

14

mechanicalheattreatment(SMHT)wereusedafeedstockforaplasmagasificationreactor.

TheSMHTturnedMSWintorefusedderivedbiomass,andmixturewithrawwoodresultedin

acompost-likeformwhichismoresuitableforgasification(Shieetal.,2014).Studyshowed

thatincreaseintemperatureandadditionofsteamcontributedtotheincreaseofH2yield

whichsubsequentlyincreasedtheenergyyieldofsyngas.

2.4TarCracking

BesidesthemainproductgasessuchasCOandH2,gasificationalsoresultsintarswhichcould

jeopardizetheprocessbyproblemssuchaserosionandcorrosiononturbinesandengines

thusmakingsyngasthatisproducedfromagasificationprocessundesirablefordirectusage.

C10H8andC7H8werechosentorepresent tarcompounds inastudytomodel thethermal

removaloftarsathightemperatureusingplasmatorch.Tarcrackingmodelisasshownin

Figure 4. Themathematicalmodelwas based on a CSTRmodel that considered different

kinetics of tar cracking process. Comparison between equilibriummodel that is basedon

Gibbsenergyminimizationmethodandkineticmodelshowedthattarconversionwasnear

completion forbothcases.Sensitivityanalysison influenceof incomingtemperature from

gasifier showed that increase in gas temperature from gasifier would increase the

temperatureofgasexitingtarreactor.Tarcompositioncanaffectthecompositionofsyngas,

anditwasshownatdifferentoverallC10H8contentwhereathighercontentledtoarisein

CO and H2 composition. However, it also increases the soot content in the output gas

(Fourcaultetal.,2010).

15

Figure4.TarCrackingModel(Fourcaultetal.,2010).

AnotherstudyontarthermalcrackingusingaCSTRmodelalsopredictedsimilaroutcomeof

tarconversionofC10H8andC7H8taking inaccountreactionkinetics.Thestudyshownthat

increasedinplasmatorchpowernotonlyincreasedtarconversionbutalsoincreasedtheLHV

ofthesyngas.CharparticlesconsideredinthismodelalsocontributedhighercontentofCO

andH2.OverFiringAiradditionalsoenhancedtarconversionbutaffectsthetemperaturedue

torise inoxidation,decrease inLHVand increaseofsootcontent insyngas(Mariasetal.,

2016).

AtwostageFluidizedBedplasmagasificationprocesswasusedtostudytarevolutionusing

RDFasfeedstock.Comparisonoftarcontentpost-FluidizedBedGasifierandpost-plasmatar

reactor showed high conversion of tar, where in equilibrium model showed complete

conversionwhileplasmathermalmodelshowedbetween95–99%conversionofdifferent

tar compounds (Materazzi et al., 2014). The system showed an effective method of gas

cleaning,higherconversionofcarbonandhigheryieldofCOandH2.

16

2.5Melting

An investigation of fly ashmelting from incineration ofMSW using Differential Scanning

CalorimeterandDifferentialThermographicAnalysistodeterminetheheatcapacityandheat

ofmelting for fly ash at the same time using a thermodynamicmodel to determine the

theoreticalrequiredheatthatisusedforcomparison.MeltingofflyashfromMSWgenerally

occursabove1300°C.DuetothehighcontentofCaointheashsample,theexperimental

heatofmeltingrangefrom1.4–1.8MJ/kg(Lietal.,2007).Itwasalsopredictedthatheatof

meltingusinggasification is lowerthan incinerationprocessasshowninFigure5canvary

accordingtooperatingconditionssuchascalorificvalueofwasteandtemperatureofgasifier.

Figure5.Comparisonofrequiredheatforincinerationandgasification(Lietal.,2007).

AnotherstudywasconductedontheinfluenceofadditionofbiomassashtoMSWflyashon

themelting characteristics. Investigation showedmelting of pureMSW took place above

1400°C but with addition of biomass ashes, themelting temperature reduced. The total

energyrequiredtomeltashisthesensibleheatrequiredtoraisethetemperatureofashto

themeltingtemperatureandthelatentheatoffusionforashtochangephasefromsolidto

liquid.Themodeldevelopedtopredictthemeltingheatprojectedtheheatintherangefrom

1.65–2.65MJ/kg(Alhadj-Mallahetal.,2015).

17

3.METHODOLOGY

3.1PlasmaGasificationProcess

Aplasmagasificationcomprisedofafixedbedupdraftgasifierandaplasmatarreactorwas

developedbyScanArcPlasmaTechnologiesABinHofors,Swedenusedtotreatsolidwasteis

showninFigure6.Solidwasteisfedfromthetopofthegasifierwhilepreheatedairorsteam

asgasifyingagentisfedfromthebottomsideofthereactor.Theproductfromgasifierwhich

containsvolatilegases,moistureandtargoestoaseparateplasmatarreactorwhereahigh

temperatureplasmaisusedfortarcracking.Acleansyngasisproductattheendwhichis

suitableforvariousapplications.Meanwhile,asimplifiedschemeofthemodelisshownin

Figure7,whichwillbeusedfordevelopmentoftheASPENPlusmodel.

Figure6.Schematicoftwostageplasmagasificationmodel.

18

3.2PlasmaGasificationModel

3.2.1Feedstockproperties

The feedstock used for the purpose of this study is excavationwaste from a landfill site

locatedatMont-Saint-Guibert,Belgium.Thewastewasprocessedbysorting,screeningand

millingthatresultedinapowder-likefractionthatconsistsofRDFandinertmaterials.The

compositionoftheRDFisshowninTable1whiletheultimateandproximateanalysesare

showninTable2.

Table1.MaterialCompositionofRDF

Component,wt%

Plastics 21.46

Wood 11.90

Paperandcardboard 2.38

Textilesandfibres 0.94

Combustiblefraction 21.15

Metals 5.49

Inert(soil,glass,etc.) 36.69

19

Table2.UltimateandproximateanalysesofRDF

ProximateAnalysis,wt% UltimateAnalysis,wt%(db)

Moisturecontent(ar) 30.00 Carbon 39.94

FixedCarbon(db) 11.60 Hydrogen 5.50

Volatilematter(db) 48.40 Nitrogen 1.50

Ash(db) 40.00 Oxygen 11.00

Chlorine 1.77

Sulphur 0.29

LHVofrawRDF(MJ/kg) 11.848

Table3.AnalysesofmetalcontentsofRDF

Metalscontent(mg/kg)

Silica(Si) 160000 Lead(Pb) 441

Aluminium(Al) 17700 Boron(B) 13.1

Calcium(Ca) 29800 Cadmium(Cd) 5.24

Iron(Fe) 21600 Cobalt(Co) 9.68

Potassium(K) 4950 Copper(Cu) 187

Magnesium(Mg) 2470 Chromium(Cr) 124

Manganese(Mn) 271 Mercury(Hg) 1.53

Sodium(Na) 2910 Molybdenum(Mo) 3.04

Phosphorus(P) 831 Nickel(Ni) 28.8

Titanium(Ti) 2090 Vanadium(V) 27.2

Arsenic(As) 5.66 Zinc(Zn) 1200

Barium(Ba) 378

20

Figure7.Simplifiedschematicoftheplasmagasificationmodel.

3.2.2Drying

Landfillwastecontainsasignificantamountofwater.Thefunctionofdryingsub-modelisto

reducethemoisturethatiscontainedinthewetfeedstockwherethetemperatureisbetween

100–150°C.Theprocessofmoistureremovaloccursbytheexchangingheatwiththesyngas

that is flowing upwards from the lower section of the gasifier. Heat is used to heat up,

evaporateandsuperheatsteam,andalsotoheatupthefeedstockasshownin(3).Themass

balanceofthedryingblockisshowninequation(2)

𝑚+,,-./012 = 𝑚456 + 𝑚809./:;,(2)

𝑄 = 𝑚+,,-./012 ∙ 1 − 𝑋809./:;, ∙ 𝐶B ∙ 𝑇0:/ − 𝑇9D + 𝑚+,,-./012 ∙ 𝑋809./:;, ∙ (ℎ./,G8,IJKL − ℎ./,G8,IMN)(3)

21

3.2.3Pyrolysis

Pyrolysisalsoknownasde-volatilizationoccursbyheatingwithoutpresenceofoxygenthat

leadstodecompositionintotwomaincomponents,namelyvolatilematterandchar.Volatile

matter considered as shown in (4) consists of mainly gases such as𝐻Q, 𝐶𝑂, 𝐶𝑂Q, 𝐶𝐻S , tar

(𝐶TU𝐻V,𝐶W𝐻V,𝐶X𝐻X),whilecharisprimarilyfixedcarbonandash,wherecompositioncanbe

obtained from proximate and ultimate analysis of landfill waste. Process of pyrolysis is

sustainedbyheatfluxfromthegasificationatthelowerpartofgasifier.Thesub-modelfor

pyrolysiscanbewritteninaone-stepglobalreaction(4).

𝑅𝐷𝐹 → 𝐺𝑎𝑠(𝐻Q, 𝐶𝑂, 𝐶𝑂Q, 𝐶𝐻S, 𝐻𝐶𝑙, 𝐻Q𝑆) + 𝑇𝑎𝑟(𝐶TU𝐻V,𝐶W𝐻V,𝐶X𝐻X) + 𝐶ℎ𝑎𝑟(4)

Itisexpectedthatthetemperatureofupperzoneishighthustarcanexitthegasifierasvapor

andenterthetarcrackingreactor.

3.2.4CharCombustionandGasification

Theprocessofdryingandpyrolysisresultincharasaproductandcharisbeingtreatedas

fixedcarbonandash.Charwillundergogasificationandpartialcombustioninthepresence

ofairasgasifyingagenttoform𝐶𝑂and𝐶𝑂Q.Partialcombustionisrequiredasgasification

reactionsbeingexothermicreactionswillsupplytherequiredheatfordrying,pyrolysisand

meltingprocess.Reactionsconsideredinairgasificationareshownintable4.Thisprocess

willapplytheGibbsfreeenergytheorywherephaseandchemicalequilibriumisassumed

becauseof thehigh temperature fromthechar combustion.Change inGibbs freeenergy

definesifareactionisspontaneousorotherwise.WhentryingtodetermineGibbsfreeenergy

atafixedtemperatureandpressure,itcanbewrittenas(5).Inordertoobtainequilibrium

composition,Gibbsfreeenergyhastobeminimizedandfinallyachieve(6).

𝑑𝐺.d./,8 = 𝑑𝐻.d./,8 − 𝑇𝑑𝑆.d./,8(5)

𝑑𝐺.d./,8 = 0(6)

22

Table4.Charcombustionandgasificationreactionsconsidered.

Reactions Type

R1 𝐶 + 𝐶𝑂Q → 2𝐶𝑂(Boudouardreaction) Endothermic

R2 𝐶 + 𝑂Q → 𝐶𝑂Q(Combustion) Exothermic

R3 𝐶 + 0.5𝑂Q → 𝐶𝑂(Combustion) Exothermic

3.2.5Melting

Ashproduced fromchar gasification consistsof inorganic componentsof the feedstock is

melted by the heat from partial oxidation process. The high temperature obtained from

combustionissufficienttomelttheashintoslagthatflowsoutatthebottomofthegasifier.

Ashcanbeassumedtoconsistsof70%𝑆𝑖𝑂Q,13%𝐶𝑎𝑂,9%𝐹𝑒Q𝑂k8%𝐴𝑙Q𝑂k,according to

metalcontentsinTable3.Heatcapacityofashcanbecalculatedaccordingtoequation(7)

(Zhangetal.,2013)andLatentheatof fusionofash isalsocalculated inasimilarmanner

basedonvaluesfrom(Fontetal.,2017),(Lietal.,2007),(Patnaik,2003).

𝑐B,G.n = 𝑥9𝑐B,9D9pT (7)

3.2.6PlasmaTarCracking

Tar inthefixedbedupdraftgasifier iscarriedoverfromthegasifiertotheplasmareactor

alongwiththegasesproducedfrompyrolysisandchargasificationprocess.Inthepyrolysis

processattemperaturerange200–500°C,tarisproducedandleavesupwardsintocooler

regionsabovewhileexchangingheatwiththefeedstock,thusconversionoftarintogasesis

low,thushighamountoftarispresentinthegas(ValderramaRiosetal.,2018).Inthismodel,

3majorcompoundsof tarswhicharenaphthalene, toluene,andbenzeneareconsidered.

Naphthalenewas selected since it is often found asmain compound in tertiary tar from

biomassandwastegasificationand it isdifficult tocrackdue to its structureasPolycyclic

Aromatic Hydrocarbon. For benzene, it is commonly used to represent primary tarwhile

23

Tolueneformedathightemperaturehasastablearomaticstructurefoundintars(Mariaset

al., 2016). Due to very high temperature of plasma, it is assumed that reaction reaches

equilibrium very fast. Products are obtained by minimizing Gibbs energy. Reactions

consideredinthetarcrackingreactorareshowninTable5wheremodelwasalsobasedona

fixedbedreactor.

Table5.Reactionsconsideredfortarcracking.

Reaction References

R4 𝐶TU𝐻V + 4𝐻Q𝑂 → 4𝐶𝑂 + 5𝐻Q (Jess,1996)

R5 𝐶W𝐻V + 𝐻Q → 𝐶X𝐻X + 𝐶𝐻S (Jess,1996)

R6 𝐶X𝐻X + 5𝐻Q𝑂 → 5𝐶𝑂 + 6𝐻Q + 𝐶𝐻S (Virketal.,1974)

R7 𝐶𝐻S + 𝐻Q𝑂 → 𝐶𝑂 + 3𝐻Q (Nozahic,2008)

R8 𝐻Q +12𝑂Q → 𝐻Q𝑂

(Turns,1996)

R9 𝐶𝑂 +12𝑂Q → 𝐶𝑂Q

(PetersenandWerther,2005)

R10 𝐶𝑂 + 𝐻Q𝑂 → 𝐶𝑂Q + 𝐻Q (PetersenandWerther,2005)

R11 𝐶𝑂Q + 𝐻Q → 𝐶𝑂 + 𝐻Q𝑂 (PetersenandWerther,2005)

R12 𝐶𝐻S + 2𝑂Q → 𝐶𝑂Q + 2𝐻Q𝑂 (PetersenandWerther,2005)

R13 𝐶𝐻S +12𝑂Q → 𝐶𝑂 + 2𝐻Q

(Turns,1996)

R14 𝐶X𝐻X +152𝑂Q → 6𝐶𝑂Q + 4𝐻Q𝑂

(Turns,1996)

R15 𝐶X𝐻X + 3𝑂Q → 6𝐶𝑂 + 3𝐻Q (PetersenandWerther,2005)

R16 𝐶W𝐻V + 9𝑂Q → 7𝐶𝑂Q + 4𝐻Q𝑂 (Turns,1996)

24

Figure8.FlowsheetofplasmagasificationprocessinAspenPlus.

3.3ASPENPlusModeldescription

ThecompleteASPENPlusmodel isshowninFigure8,whiledescriptionofblocks,streams

usedareshowninTable6,Table7,Table8respectively.Thefeedstockproperties,proximate

andultimateanalyseswasassignedtonon-conventionalstream‘RDFF’isfedintothemodel

andfirstmeetsthegasmixturefromthelowerpartofgasifierandincreaseintemperature

of stream ‘RDFF2’ by exchanging heat in block ‘HEATEX1’ with stream ‘GASMIX1’ before

moving to the block ‘DRYING’ where feedstock is converted into dry basis according to

moisturecontentandthenthestream‘WETRDF’isseparatedintheblock‘’SEPARATE’.The

drystream‘DRYRDF’isthenbrokendownintoelementarycomponentsintheblock‘PYRO’

according to a calculator based on the ultimate analysis. The stream ‘DECOMPRO’which

contains ash is separated into ‘ASH’ and stream ‘DECOMPRO2’, where block ‘PYRO2’

simulatesde-volatilizationprocessintovolatilegasesalongwithcharandtarestimatedbased

onreferencefrom(Zainietal.,2018)and(DiBlasi,2004).

25

Heatstreamsfordrying‘Q-DRYING’,decomposition‘Q-DECOMP’,pyrolysis‘Q-PYRO’andchar

gasification ‘Q-CHAR’ are connected between blocks to contain the enthalpy within the

system as source of heat for each process are dependent on the char combustion and

gasificationprocess.Thestreams‘ASH’and‘CHAR’separatedfromtheblock‘SEPARAT’goes

intotheGibbsblock‘CHARGAS’forcharcombustionandgasificationprocesswithgasifying

agent‘AIR1’ofwhichitsmassflowrateiscalculatedbasedonaspecificERthatiswithinthe

rangeoftypicalgasificationprocessandabletosupplysufficientheatfordrying,pyrolysis,

andmeltingwhiletakingintoaccounttheheatlossfromthegasifierstream‘Q-LOSS’.

Productofcharcombustionandgasification‘CHARPRO’containingashisseparatedinblock

‘SEPARAT2’wherestream‘ASH2’goingtothe‘MELTING’block.Productgasesonlystream

‘CHARPRO2’moves up the gasifier andmeet with stream ‘PYROCHAR’. An external heat

streamisaddedtotheblock‘MELTING’becausethemeltingheatrequiredformeltingash

intoslagiscalculatedmanually.

Thestream‘GASMIX2’goesintotheGibbsblock‘TARCRACK’forplasmatarcrackingprocess

withplasmaairheatedwithheatinput‘ELECTRIC’consideringaheatlossstream‘Q-LOSS3’

attheblock‘PLASTORC’while‘Q-LOSS2’istheheatlossfromthetarcrackingreactorfrom

thesensibleheatoftheoutput.Theresultingproductstream‘HOTSYN’proceedsforcooling

processatblock‘AIREX’whereenergylossisrecoveredtopreheatairforchargasification.

Theoutput‘HOTSYN2’thengoestotheblock‘H2OEX’toheatupwaterintosaturatedsteam

ifitisrequired.Thefinaloutputofsyngasstreamis‘COLDSYN’.

26

Table6.DescriptionofunitoperationblocksinASPENPlusflowsheet.

ASPENPlusName BlockID Description

HeatX HEATEX1 Heatexchangebetweenfeedstockandgasmixture

fromlowerpartofgasifier

AIREX Heatexchangebetweenairinputandhotsyngas

H20EX Heatexchangebetweenwaterinputandhotsyngas

Heater PLASTORC Increasesthetemperatureoftheplasmaair

RStoic DRYING Convertsfeedstockintodrybasisaccordingto

moisturecontent

PYRO2 Breakdownvolatilematterintovolatilegases

accordingtostoichiometry

MELTING Convertsashtoslag

RYield PYRO Convertsnon-conventionalstream‘RDFF’into

conventionalstream

RGibbs CHARGAS Simulateschargasificationprocessbyminimizing

Gibbsfreeenergy

TARCRACK SimulatestarcrackingprocessbyminimizingGibbs

freeenergy

Mixer MIX Combinesgasmixturefromchargasificationand

pyrolysisprocess

MIX1 Combinescharandash

MIX2 Combinesmoistureandgasmixturefromlowerpart

ofgasifier

SSplit SEPARAT Separatescharfromgasesandtar

SEPARAT1 Separatesashfromconventionalstream

SEPARAT2 Separatesashfromgasesfromchargasification

Sep2 SEPARATE Separatesmoisturefromfeedstock

27

Table7.AspenPlusmaterialstreams.

ASPENPlusStream StreamID Description

Material RDFF FeedstockflowRDF

RDFF2 HeatexchangeofRDFwithgasflow

WETRDF RDFmoisturecalculationusingstoichiometry

DRYRDF DryRDFwithoutmoisture

MOISTURE MoistureremovedfromRDF

DECOMPRO RDFdecomposedintoelementsaccordingtoproximate

andultimateanalysis

DECOPRO2 AshisseparatedfromdecomposedRDF

ASH Separatedashfromdecomposition

PYROPRO Productsformfrompyrolysisaccordingtostoichiometry

basedonexperiment

PYROCHAR Mixtureofgasesfrompyrolysisandchargasification

CHAR Charseparatedfromgasmixture

CHAR2 Mixtureofashandchar

CHARPRO Productfromchargasificationandcombustion

CHARPRO2 Chargasproductwithashseparated

ASH2 Ashseparatedfromchargases

SLAG Slagfrommeltedash

GAS+TAR Mixtureofgasesfrompyrolysisandchargasification

GASMIX1 Mixtureofgasesandmoisture

GASMIX2 GasmixesheatingincomingRDF

PLASAIR Heatedcarriergasforplasmaconverter

AIR2 Carriergasforplasmaconverter

COLDAIR Airinputforchargasificationandcombustion

AIR1 Preheatedairforchargasificationandcombustion

COLDH2O Waterinputforsteamgasificationwhenrequired

HOTSYN Productgasfromtarcracking

HOTSYN2 Productgasafterheatexchangeforpreheatedair

COLDSYN Productgasafterheatexchangewithwater

28

Table8.AspenPlusheatstreams.

ASPENPlusStream StreamID Description

Heat Q-DRYING Heatdutyformoistureremoval

Q-DECOMP HeatdutyfordecompositionofRDF

Q-PYRO Heatdutyforpyrolysis

Q-CHAR Netheatdutyfromchargasification

Q-LOSS Heatlossfromgasifier

ELECTRIC Electricpowerforplasmaconverter

Q-LOSS2 Heatlossfromtarcrackingreactor

Q-LOSS3 Heatlossfromplasmaconverter

3.3.1Assumptions

Thefollowingareassumptionsusedforthemodel:

• Steadystateandthereactionsreachchemicalequilibrium.

• Syngasproducedfrompyrolysisconsistsof𝐻Q, 𝐶𝑂, 𝐶𝑂Q, 𝐶𝐻S, 𝐻Q𝑆, 𝐻𝐶𝑙,tar(𝐶TU𝐻V,

𝐶W𝐻V)andchar(ascarbonandash)

• 𝐴 𝐹./091 = 6.5basedonempiricalformulausingcompositionofC,H,Ofrom

ultimateanalysis,𝐶v𝐻V𝑂 + 6.5 𝑂Q + 3.76𝑁Q → 5𝐶𝑂Q + 4𝐻Q𝑂 + 24.44𝑁Q

• Sootproductionisnotconsidered.

• Ashisspecifiedasnon-conventionalandnon-reactiveintheprocess.

3.3.2BoundaryConditionsTheoperatingconditionsfortheplasmagasificationmodelisshowninTable9.

29

Table9.Boundaryconditionsforplasmagasificationmodel.

RDFFeed(kg/hr) Flowrate(kg/hr) 1000 Pressure(bar) 1 Temperature(K) 298GasificationAir,𝑀yG.9G9; Flowrate(kg/hr) 1325.88 Pressure(bar) 1 Temperature(K) 873 EquivalenceRatio 0.208Gasifier Pressure(bar) 1 Temperature(K) 423-1773PlasmaAir,𝑀BzG.8GG9; Flowrate(kg/hr) 406.89 Pressure(bar) 1 Temperature(K) 298PlasmaTorch Power(kW) 500HeatLoss(%) ShaftGasifier 5 TarCrackingReactor 5 PlasmaConverter 30

3.4ParameterStudies

TwodifferentparameterswereinvestigatedusingtheASPENPlusmodel,namelyairflowrate

andpreheatedairtemperature,whichisshowninTable10.Dimensionlessnumberisusedto

betterrepresentthestudiedparameterofairflowrate.Amountofactualairperkilogramof

RDFcomparedtothestoichiometricamountcanberepresentedasequivalenceratio(ER):

𝐸𝑅 = |}~M~M/

(|}~M~M/)LJM(1)

30

Table10.ParametersthatwerestudiedintheASPENPlusmodel.

EffectofEREffectofPreheated

AirTemperature

GasificationAir,𝑀yG.9G9; (kg/h) 1300-1950 1325.88

PlasmaAir,𝑀BzG.8GG9; (kg/h) 406.88 406.88

PreheatedAirTemperature,𝑇B;,n,G/G9; (K) 873 373-1173

Plasmapower,𝑃BzG.8G(kW) 500 500

DimensionlessParameters

ER 0.2–0.3 0.208

31

4.RESULTSANDDISCUSSION

4.1VerificationandValidationofModel

ThemodeldevelopedusingAspenPlusisvalidatedandverifiedagainstasimilarsetupdone

by ScanArc Plasma Technologies AB.However, the feedstock is slightly differentwhere it

consistsofmixtureofwastessuchasmunicipalwaste,industrialwaste,hazardouswaste,car

tyres,computerscrapwhichmayresultindifferentcompositionsofsyngas.Thecomparison

isshowninTable11.

𝐿𝐻𝑉 = 𝐻𝐻𝑉 −𝑀 ∙ 𝐿(8)

𝐻𝐻𝑉 = TU.W∙TQ.X∙TUU

(9)

𝐶𝐺𝐸 = N}~∙|N}~∙|zG.8G0,;

(10)

Table11.ComparisonofcompositionbetweenAspenPlusModelandSimilarSetup

Model

(Mole%)

ScanArcData

(Mole%)

𝐶𝑂 18.2 20.9

𝐶𝑂Q 5.1 6.1

𝐻Q 18.9 15.9

𝐻Q𝑂 12.8 13.8

𝑁Q 44.6 43.1

LHV(MJ/Nm3) 4.66 4.34

CGE(%) 72.5 69.1

32

4.1.1SyngasCompositionPreandPostTarCracking

Figure9showsthecomparisonbetweenthegascompositionbeforeandafterthetarcracking

reactor.ThefigureshowedthatbothH2andCO2increasedfrom16.96%to18.90%and3.57%

to5.05%respectivelywhileCOandH2Ofellfrom18.64%to18.24%and15.54%to12.80%

correspondingly.BothCH4andtarcompoundsarecompletelyconverted.TheriseofH2 is

attributedtotheconversionoftarcompoundsaccordingto(R4),(R6),(R7),(R15),(R16)from

Table5.Thissuggeststhatplasmatarcrackingoflighthydrocarbonsuchastarcompoundsis

beneficial shouldasyngasofhigherH2yield isdesiredbesidescleanersyngasthatcanbe

utilisedwithoutmuchposttreatmentofsyngas.Bythesametoken,thedeclineinH2Ocould

berelatedtothedecompositionoftarcompoundsbymoistureinthegasthatexistassteam,

thus contributing to the increment of H2 during tar cracking. This is consistent with the

findings by (Materazzi et al., 2016). On the other hand, the drop in CO and rise in CO2

contradictswiththefindingsof(Materazzietal.,2014).Thiscouldbeassociatedbythe(R10)

wheretheactivationenergyislowerthan(R11)accordingto(Fourcaultetal.,2010)andthe

excessairthatissuppliedasplasmaair.

Figure9.Comparisonbetweensyngascompositionbeforeandaftertarcracking.

0

5

10

15

20

H2 CO CO2 H2O CH4 TAR

Mole%

Pre-TarCracking Post-TarCracking

33

4.2MassandEnergyBalance

Amassandenergybalancefortheplasmagasificationmodelwasdoneasshownin.while

Table12showsthematerialbalanceforthesystemandTable13showstheenergybalance

ofthesystem.Meanwhile,aSankeydiagramshowingtheflowofenergyisillustratedinFigure

11andthedistributionofenergyoutputisshowninFigure12.Theplasmapowersuppliedas

aninputtothesystemwasabout13%ofthetotalenergyinputandtherestcamefromthe

RDF.Thechemicalenergyofsyngascomprisedof68.36%,thesensibleheatwas24.30%,total

heatlossincludingheatinslagwas7.34%.Thus,thisgivesthesystemathermalefficiencyof

about92%.

Figure10.MassandEnergyBalancediagramforplasmagasificationsystem.

34

Table12.MassbalanceofAspenPlusmodel.

Input kg/h Output kg/h

RDF 1000 Syngas 2432.01

GasificationAir 1325.88 Slag 300.76

PlasmaAir 406.89

Total 2732.77 Total 2732.77

Table13.EnergybalanceofAspenPlusmodel.

Input MJ/h Output MJ/h

RDF 11848.00 Syngas(ChemicalEnergy) 9329.94

PlasmaPower 1800.00 SensibleHeatofGas 3316.25

Slag 65.77

HeatLosses

Gasifier 225.25

TarCrackingReactor 170.79

PlasmaConverter 540.00

Total 13648.00 Total 13648.00

Figure11.Sankeydiagramshowingflowofenergyforplasmagasificationsystem.

35

Figure12.Piechartshowingdistributionofenergyoutput.

4.3ResultsofParameterStudies

4.3.1EffectofER

As defined according to equation (1), ER is the ratio of actual air-fuel ratio against the

stoichiometricair-fuelratioforcompletecombustion.Inthisstudy,ERwasvariedfrom0.2–

0.3, where𝑀yG.9G9; is varied from 1300 – 1950 kg/h at a fixed𝑇B;,n,G/G9; at 873K. The

influenceofERonthecompositionofsyngasisshowninFigure13andontheLHVandCGEis

shown in Figure14. It canbeobserved that as ER increased,CO2 content rosewhileCO

decreased.Thisoccurreddue to increasedcombustion reactionwhereCO2was favoured.

Similarly, for reduction of H2 content where oxidation was favoured, thus increased the

formation of H2O. The increase of overall N2 content was attributed by the increased in

𝑀yG.9G9;andbeinginert.ThedropinCOandH2,atthesametimedilutionofN2,causedthe

LHVofsyngasandsubsequentlyCGEtodecline.TheLHVrangedfrom3.15–4.73MJ/Nm3

whileCGErangedfrom53.64–72.95%.Similartrendswereshownby(Favasetal.,2017),

(Begumetal.,2014),(Beheshtietal.,2015).

36

Figure13.EffectofincreasingERonsyngascompositionatT=873K

Figure14.EffectofincreasingERonLHVandCGE.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.200 0.220 0.240 0.260 0.280 0.300

Syngasco

mpo

sition(M

oleFractio

n)

ER

H2 CO CO2 N2 H2O

2

2.5

3

3.5

4

4.5

5

40%

50%

60%

70%

80%

0.200 0.220 0.240 0.260 0.280 0.300

LHV(M

J/Nm

3)

CGE

ER

CGE LHV

37

4.3.2EffectofPreheatedAirTemperature

Preheating air helps in increasing efficiency of gasifier by reducing energy required from

gasification.Thus,airinputorERcanbereducedallowingloweramountofpartialcombustion

whichmayleadtoincreasingamountofCO2.ReductioninERhelpsretainthesyngascalorific

valuewithinCO.Thepreheatedairtemperatureusedinthemodelis873K,whileairinputis

fixed at 1325.88 kg/h. Investigation on effect of𝑇B;,n,G/G9; on the model, the range of

temperaturefrom373–1173Kwasstudiedatthespecifiedflowrateforchargasificationand

combustion. Based on the boundary conditions fixed for the model, the heat of partial

combustionrequiredforallprocesseswasdeterminedandisplottedagainstthesensibleheat

frompreheatedairatthesaidrangeoftemperatureasshowninFigure15.Thegraphshows

thatas𝑇B;,n,G/G9;increases,theportionofheatrequiredfrompartialcombustionofchar

decreases,therebyloweringERfortheprocess.AsshowninFigure16,theminimumERat

higher𝑇B;,n,G/G9;decreases.Hence,thistranslatestoincreasingLHVandCGEasshownin

Figure17whereitshowstheminimumLHVandCGEforateach𝑇B;,n,G/G9;whichisableto

meettheheatdemandofgasification.Thisphenomenaisfoundtobesimilarwith(Mathieu

andDubuisson,2002)and(Dohertyetal.,2009).LHVwasfoundtobeintherangeof3.96–

5.03MJ/Nm3whileCGEwasintherangefrom64.19–76.25%.

38

Figure15.Heatofpartialcombustionrequiredfromair(ER=0.208)atdifferentT

Figure16.GraphshowinglowestpossibleERatdifferentT.

0%

20%

40%

60%

80%

100%

0

200

400

600

800

1000

1200

373 473 573 673 773 873 973 1073 1173

HeatofP

artia

lCom

bustion(M

J/h)

PreheatedAirTemperature(K)

Hgasification Hpreheatedair

0.17

0.18

0.19

0.2

0.21

0.22

0.23

0.24

0.25

373 473 573 673 773 873 973 1073 1173

ER

PreheatedAirTemperature(K)

LowestER

39

Figure17.EffectofTonLHVandCGEaccordingtolowestpossibleERateach

T.

60%

62%

64%

66%

68%

70%

72%

74%

76%

78%

3

3.5

4

4.5

5

5.5

273 473 673 873 1073 1273

CGE

LHV(M

J/Nm

3)

PreheatedAirTemperature(K)

LHV CGE

40

5.CONCLUSION

Thereisnodoubtthatlandfillwastemayposeahugechallengeinalongrun.However,with

advancementoftechnology,aprocesssuchasplasmagasificationcouldbethekeyintackling

theissue.Nevertheless,continuousinvestigationoflandfillwasteisrequiredtoenhanceour

understandingonthesubject.Hence,ASPENPlusisagoodplatformtostudytheperformance

oflandfillwasteinaplasmagasificationprocesswhiletakingintoaccountmassandenergy

requirements.Thestudywasabletoprovideanestimateofmaterialandenergyinputsfor

the process. Composition of syngas obtained showed that the application of plasma tar

crackingwas able to achieve a cleaner gaswith a higherH2 content and reasonable LHV

despite RDF containing high amount of ash. Theoutcome also depends on the feedstock

composition.Limitationsofthemodelsuchasfixedoperatingtemperaturesofblocklimits

thecompositionchanges.Parameterstudyshowedthatsimilartoothersetupsofgasification,

alowerERismorefavourableforasyngaswithhigherLHV.Ontheotherhand,preheating

air by recovering heat from the output is advantageous from overall thermal efficiency

perspective and also reducing the heat required from partial combustion of char. In

conclusion, plasma gasification has potential in treating landfill waste at the same time

contributingtowardsthecirculareconomy.

41

6.FUTUREWORK

Intermsofinputdata,duetolackofinformationoncharandtarcontent,bothcharandtar

contentwasbasedonliteraturestudies.However,bothcanbeaffectedbythepropertiesof

thefeedstock.Someactualexperimentaldatawouldgiveabetterestimationforthemodel.

From the aspect ofmodel, perhaps the char gasification and tar crackingmodels can be

performedusing kineticsmodel insteadof equilibriummodel and compare the results to

understandtheinfluenceofkinetics.Moreparameterscanbeinvestigatedusingthemodel,

suchaseffectofplasmapower,effectoftarcrackingtemperature.

42

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