Pyrolysis of low grade biogenic feedstock with in-situ sorption of … · Pyrolysis of low grade...
Transcript of Pyrolysis of low grade biogenic feedstock with in-situ sorption of … · Pyrolysis of low grade...
Pyrolysisoflowgradebiogenicfeedstockwithin-situsorptionofChlorineforemissionreduction
YashaDave
ThesistoobtaintheMasterofScienceDegreein
EnergyEngineeringandManagement
Supervisors:Prof.DieterStapfProf.JorgeManuelFigueiredoCoelhodeOliveira
ExaminationCommittee
Chairperson:Prof.FranciscoManueldaSilvaLemosSupervisor:Prof.JorgeManuelFigueiredoCoelhodeOliveiraMemberoftheCommittee:Prof.JoãoCarlosMouraBordado
November2017
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Acknowledgements
IwouldtakethisopportunitytothankalmightyGodforshoweringHisblessingsuponme,inthisjourneyofmymaster
studies.Ialsothankmyparentsandfriendsforsupportingmeinallmydecisionsandmotivatingmetoworkhard.It
wasmypleasureandhonourtostudyinthemasterschoolprogramofEITInnoEnergywhichhasprovidedmethis
opportunitytostudyintwogooduniversitiesofEurope.
IamgratefultoMr.MarcoTomasiMorgano,myadvisorwhothoughtIwouldbesuitableforworkingonthisproject.
Heisasourceofmotivationformebecauseoftheeffortanddedicationheputsinhiswork,thisconstantlykeptme
focusedonmywork.Ialsothankhimforhistime,patienceandvaluableremarks.
Ialsothankthehard-workingstaffofInstituteofTechnicalChemistry(ITC),KarlsruheInstituteofTechnology(KIT)
withaspecialmentionofMr.FrankRichterwhohelpedmeunderstandtheexperimentalsetupandgavemevaluable
advicethroughoutmythesis.IexpressspecialgratitudetowardsMrs.MonikaSchleinkoferwhoanalysedmysamples
andhelpedmesubmitmyworkontime.Mr.PatrickSchieberperformedGCanalysisformysamplesandhenceI
thankhimforhistimeandexpertise.
IwouldalsoliketothankMr.HansLeiboldandProf.DieterStapfforlettingmeusethefacilitiesofITCandforthe
warmwelcomeinthePyrolysisgroup.
Iwouldliketospeciallymentioncontributionofafriend,Mr.BhargavRavindranathforproofreadingmythesis.
Finally,Ithankmysupervisorsfrommyhomeuniversities,Prof.JorgeOliveira(IST,Lisbon),Prof.WojciechNowak
(AGH-UST,Krakow),Ms.AgataMlonkaMedrala(AGH-UST,Krakow)fortheirtimeandsuggestions.
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AbstratoAutilizaçãocomercialdebiomassacomorecursodeenergiarenováveltemvindoaaumentarnaúltimadécada.O
desperdíciodediferentescomposiçõesoriginabiomassadebaixaqualidadeque,apósautilização térmica,pode
ajudaraumamelhordisposição.AstecnologiastérmicasincluemCombustão,PiróliseeGasificação.Odesafiomais
crucialparaessastecnologiaségarantirqueasemissõesestejamabaixodos limitesderegulaçãorespectivos.As
emissõesdeenxofreegásácidoforamamplamentediscutidaseadessulfurizaçãodegasesdecombustão(FGD)
mostrou uma implementação bem sucedida em vários casos. Devido à composição diversa e, eventualmente, a
naturezadasemissões,amatéria-primadebiomassarequerométododedisposiçãodefonteespecífica.
Comopartedestetrabalhode investigação,aPirólisedeLododeEsgotocomsorção in-situdeCloro/Enxofre foi
avaliadausandosorventesàbasedeCálcioeSódio.Ainvestigaçãotambémexaminaaeficiênciadeutilizarsorção
in-situemcomparaçãocomousodeumequipamentoseparadodelimpezadegás,apósumestudocuidadosoda
literaturaeobtençãoderesultadosexperimentais.OCloroestápresenteemquantidadesignificativanofenode
Trigo,portanto,requerumapiróliseparacomparaçãocomasemissõesdapirólisedasLamasdeEsgoto.Estudos
sobre a utilização de sorção com Óxido de Cálcio (CaO) e Carbonato de Hidrogénio de Sódio (NaHCO3) foram
conduzidosusandoPlantaFixaePirólisedeParafusoIntegrada(STYX).
OsresultadosexperimentaisreflectemumasorçãonotáveldeCleSnaconfiguraçãoSTYX;NoReactordeCamaFixa
foramencontradasproblemasdereprodutibilidadedosresultadosdesejados.Orendimentodosprodutosdepirólise
dependedediferentescondiçõescomoanaturezadamatéria-prima,atemperaturadeoperação,otipodereactor,
otempoderesidência,etc.Acomparaçãodoequilíbriodemassaentreutilizaçãocomesemabsorventenesteestudo
demonstrouumaumentodafracçãodegásPermanente,aumentoligeironaquantidadedecarvão,diminuiçãoda
fracçãoorgânicaefracçãoaquosadosprodutoslíquidosobtidos.
Palavras-chave:Emissãodecloro,Reatordepirólisedeparafusointegrado,Lododeesgoto,Sorçãoin-situ.
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AbstractCommercialattentiononBiomass,asanenergyresourcehasincreasedinthepastdecade.Wasteofdifferentnature
andoriginrepresentslowgradebiomasswhichuponthermalutilisationcanaidinbetterdisposal.Thermalutilization
technologiesareCombustion,PyrolysisandGasification.Mostcrucialchallengeforthesetechnologiesistoensure
theemissionsarebelowtherespectiveregulationlimits.Sulphurandacidgasemissionshavebeendiscussedwidely
and Flue Gas Desulphurization (FGD) has shown successful implementation in many cases. Due to the diverse
compositionandeventuallynatureofemissions,biomassfeedstockrequiressourcespecificdisposalmethod.
Aspartofthisresearchwork,SewageSludgePyrolysiswithin-situsorptionofChlorine/Sulphurwasevaluatedusing
CalciumandSodiumbasedsorbents.Theworkalsoexaminesefficiencyofusingin-situsorptionascomparedtousing
aseparategascleaningequipment,aftercarefulstudyofliteratureandobtainedexperimentalresults.Chlorineis
present insignificantamount inWheatStraw,hence it isalsopyrolyzedforcomparisonwiththeemissions from
SewageSludgepyrolysis.SorptionstudieswithCalciumOxide(CaO)andSodiumHydrogenCarbonate(NaHCO3)were
conductedusingFixedBedandIntegratedScrewPyrolysis(STYX)plant.
TheresultsofexperimentalcampaignreflectednotablesorptionofClandSinSTYXconfiguration;intheFixedBed
Reactor issues of reproducibility of desired results were encountered. Yield of pyrolysis products depends on
conditionslikenatureoffeedstock,temperatureofoperation,reactortype,residencetimeetc.TheMassBalance
comparisonbetweencasesofwithandwithoutsorbentuse,inthisstudy,showedincreaseoffractionofPermanent
gas,slightincreaseinthecharamount,decreaseintheorganicandaqueousfractionofliquidproductsobtained.
Keywords:Chlorineemission,IntegratedScrewPyrolysisReactor,SewageSludge,In-SituSorption.
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TableofContentsAcknowledgements.........................................................................................................................................................i
Abstrato..........................................................................................................................................................................ii
Abstract.........................................................................................................................................................................iii
TableofContents..........................................................................................................................................................iv
Abbreviations................................................................................................................................................................vi
ListofFigures...............................................................................................................................................................vii
ListofTables..................................................................................................................................................................ix
1. ObjectiveoftheWork...........................................................................................................................................1
1.1 Introduction......................................................................................................................................................1
1.2 ResearchObjective............................................................................................................................................5
1.3 ThesisOutline....................................................................................................................................................5
2. UnderstandingBiomassanditsThermochemicalConversion..............................................................................7
2.1 BiomassCharacteristics.....................................................................................................................................7
2.2 BiomassComposition........................................................................................................................................8
2.2.1 WoodandNon-WoodChemistry.............................................................................................................9
2.2.2 Moisturecontent:...................................................................................................................................10
2.2.1 Mineralogy.............................................................................................................................................11
2.2.2 ElementalcompositionofOrganicmatter.............................................................................................11
2.3 ThermochemicalConversionProcesses..........................................................................................................12
2.3.1 Combustion............................................................................................................................................13
2.3.2 Gasification.............................................................................................................................................13
2.3.3 Pyrolysis..................................................................................................................................................14
3. SewageSludge:CharacteristicsandDisposal......................................................................................................16
3.1 Sewagesludge.................................................................................................................................................16
3.2 TreatmentofSewage......................................................................................................................................17
3.3 CompositionofSewageSludge.......................................................................................................................18
3.4 Disposal...........................................................................................................................................................21
3.5 EULegislationsforSewageSludgehandlinganddisposal..............................................................................25
4. ChlorineemissionsfromBiomassThermalConversion......................................................................................26
4.1 ReleaseofChlorinefromdifferentfeedstock.................................................................................................26
4.2 ChlorineemissionsfromSewageSludgePyrolysis..........................................................................................30
5. UseofSorbentsforChlorineCapture..................................................................................................................31
5.1 RemovalofHClandmetalchlorides...............................................................................................................31
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5.2 AlkaliandAlkalineearthmetalsassorbents...................................................................................................32
6. ExperimentalSetUp............................................................................................................................................36
6.1 FixedBedReactor...........................................................................................................................................36
6.1.1 ExperimentalProcedure.........................................................................................................................37
6.2 ScrewPyrolysisReactor(STYX).......................................................................................................................38
6.2.1 FeedingSystem......................................................................................................................................40
6.2.2 SequentialExtractionandFiltrationunit................................................................................................40
6.2.3 CondensationAssembly.........................................................................................................................41
6.2.4 OnlineGasAnalysis................................................................................................................................41
6.2.5 ExperimentalProcedure.........................................................................................................................41
6.3 FeedstockProperties.......................................................................................................................................42
6.4 ChemicalAnalysisoftheProducts..................................................................................................................45
7. ResultsandDiscussion.........................................................................................................................................46
7.1 FixedBedReactor...........................................................................................................................................46
7.1.1 MassBalance..........................................................................................................................................47
7.1.2 SorbentEfficiencyComparisonforSSandWS.......................................................................................51
7.2 STYXExperimentalReactor.............................................................................................................................55
7.2.1 MassBalance..........................................................................................................................................56
7.2.2 SorbentPerformance.............................................................................................................................58
7.2.3 PermanentGasCombustion...................................................................................................................59
8. Conclusions..........................................................................................................................................................62
9. FutureWork........................................................................................................................................................63
References....................................................................................................................................................................64
vi
Abbreviations1. BIGCC:IntegratedGasificationCombinedCycle
2. CHP:CombinedHeatandPower
3. EEA:EuropeanEconomicArea
4. ESP:ElectroStaticPrecipitator
5. EU:EuropeanUnion
6. FBR:FixedBedReactor
7. FGD:FlueGasDesulphurisation
8. FID:Flameionizationdetector
9. GC:GasChromatography
10. GC-MS:GasChromatography-MassSpectroscopy
11. GHG:GreenHouseGases
12. IGCC:IntegratedGasCombinedCycle
13. MSW:MunicipalSolidWaste
14. NS:NoSorbent
15. PAH:PolycyclicAromaticHydrocarbon
16. PCB:Poly-chlorinatedBiphenyl
17. PCDD:Polychlorinateddibenzodioxins(dioxins)
18. P.Gas:PermanentGas
19. SFG:SimulatedFluegas
20. SS:SewageSludge
21. STYX:(IntegratedPyrolysisPlant)ScrewPyrolysisReactorwithintegratedhotgasfiltration
22. TCD:ThermalConductivitydetector
23. WBA:WorldBiomassAssociation
24. WS:WheatStraw
vii
ListofFiguresFigure1:TotalEnergySupplyintheWorld,2014[1]....................................................................................................1
Figure2:GrossFinalConsumptionofEnergyResourcesin2014[1].............................................................................2
Figure3:GrossFinalConsumptioncomparingtop10Countries,2014[1]...................................................................2
Figure4:Wastegenerationbyeconomicactivitiesandhouseholds,EU-28,2014(%),Eurostat..................................3
Figure5:Electricitygeneratedfromrenewableenergysources,EU-28,2005-2015,Eurostat.....................................4
Figure6:SourcesofBiomassfeedstock[1]....................................................................................................................7
Figure7:BasicClassificationofBiomassFeedstock.......................................................................................................8
Figure8:Distributionofthethreemostcommoncomponentsoflignocellulosicbiomassdrymatter[13]................8
Figure9:PolymericStructureofCellulose[15]..............................................................................................................9
Figure10:PolysaccaharideunitsofHemicellulose[9]...................................................................................................9
Figure11:Thefourmainmono-lignolscomposingLigninstructure[19]....................................................................10
Figure12:H/CvsO/CgraphofBiomassFuels[12]......................................................................................................12
Figure13:AccumulatedexperienceinbiomassgasificationintermsofnumberofprojectsandMW[27]...............14
Figure14:Detailedwastewatertreatmentprocess[36]............................................................................................17
Figure15:ClassificationofSewageSludgetreatment.................................................................................................17
Figure16:TypicalDryingCurveforSewageSludge[35]..............................................................................................18
Figure17:ThermalwastetreatmentinGermany,2012(StatistischesBundesamtWiesbaden2015)........................22
Figure18:EEASewagesludgedisposalbyprocessused(%oftotalmass),EUROSTAT2015.....................................23
Figure19:PercentagedistributionofdisposalmethodsinGermanregionalstatesfor2011(Umweltbundesamt)..24
Figure20:AlkalimetalandChloroatoms(mmol/100gfuel)inBiomass[8]...............................................................27
Figure21:PossiblereactionpathforKreleaseduringdevolatilizationandcombustionofannualcrops[47]...........29
Figure22:PossiblereactionpathforClreleaseduringdevolatilizationandcombustionofannualcrops[47]..........29
Figure23:Limeconversionfordifferenttemperaturewithrespecttotime[79].......................................................33
Figure24:Performanceofsorbentswiththepollutantpartialpressures[87]............................................................35
Figure25:Laboratoryscalefixedbedreactor..............................................................................................................36
Figure26:Reactoroutsideandinsideview.................................................................................................................37
Figure27:FlowdiagramoftheBenchscalePyrolysisReactor(STYX)..........................................................................39
Figure28:SchematicdiagramofHotgasFiltrationAssembly[9]................................................................................41
Figure29:Benchscaleexperimentalsetup(actualpicturesfromKIT).......................................................................42
Figure30:ClandSinFeed...........................................................................................................................................45
Figure31:OverallMassBalanceforSS........................................................................................................................47
Figure32:OverallMassBalanceforWSPyrolysis........................................................................................................48
Figure33:Volume%DistributionofP.Gas,SS+NaHCO3.............................................................................................49
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Figure34:Vol%CO2Released......................................................................................................................................49
Figure35:IncreaseinP.GasforSSandNaHCO3.........................................................................................................50
Figure36:P.GasdecreaseinthecaseofSS+CaO........................................................................................................50
Figure37:Tabulationofg-CO2releasedperKgfeed....................................................................................................51
Figure38:NaHCO3PerformanceinthecaseofSS.......................................................................................................52
Figure39:StandarddeviationforthecaseofSSandNaHCO3.....................................................................................53
Figure40:PerformanceandStandardDeviation,SS+CaO...........................................................................................54
Figure41:PerformanceofNaHCO3incaseofWS.......................................................................................................54
Figure42:ComparisonofFeedstockwithrespecttoClandSemissionsandtheirsorption......................................55
Figure43:MassBalanceandYielddistributionofexperimentsusingSTYX................................................................57
Figure44:MassBalanceWSforSTYXexperiments.....................................................................................................57
Figure45:ComparisonofChlorineYieldforSSandWSforSTYX................................................................................58
Figure46:ComparisonofSulphurYieldforSS.............................................................................................................59
Figure47:Permanentgas,vol%...................................................................................................................................59
Figure48:HydrocarbonshareofthePermanentgasinFigure47...............................................................................60
ix
ListofTablesTable1:TypicalRangeofMineralmatter,wt%[12]....................................................................................................11
Table2:ThermochemicalConversionProcesses.........................................................................................................12
Table3:Bios-Bioenergyreport,2012[24,26].............................................................................................................13
Table4:ComparisonofCalorificValuesofSewageSludgeandBiomasswithcoal[41]..............................................19
Table5:BasiccharacteristicsandelementalcompositionofSewageSludge[43].......................................................19
Table6:OrganicandInorganiccomponentsofSewageSludge[43]...........................................................................20
Table7:RangeofvaluesformajorheavymetalspresentinSludge[40,49]..............................................................21
Table8:UseandDisposalofSludgebasedonmethodused[50]...............................................................................22
Table9:DisposalmethodsforsewagesludgeinEUMemberStatesaspercentage[52]............................................24
Table10:Airemissionlimitvaluesasperthe2001WasteIncinerationDirective......................................................25
Table11:CompositionofSorbentsused[83]..............................................................................................................34
Table12:Sorbentproperties[88]...............................................................................................................................35
Table13:BreakthroughpointforChlorine[88]...........................................................................................................35
Table14:ReactorSpecifications..................................................................................................................................39
Table15:UltimateandProximateanalysis,HeatingValueofdriedsewagesludge....................................................43
Table16:AshAnalysisofsewagesludge.....................................................................................................................43
Table17:EstimatedPhosphorousRecyclingPotentialinGermany.............................................................................43
Table18:MetalconcentrationinthisstudyandcomparisonwithGermanpermissibleamount[37].......................44
Table19:UltimateandProximateanalysis,HeatingValueofWheatStraw...............................................................44
Table20:AshAnalysisofWheatStraw........................................................................................................................45
Table21:SummaryofExperimentsinFBRsystem......................................................................................................46
Table22:ListofExperimentsperformedonSTYX.......................................................................................................56
Table23:TabulatedvaluesofSO2andHClemissionsfromCombustion.....................................................................61
1
1. ObjectiveoftheWork1.1 Introduction
Growing energy demand and climate change have led to complex sustainability concerns. The green-house gas
emissions and dwindling fossil fuel reserves and their costs have led to widespread research and commercial
attentiononBiomassasanenergyresource.Currently,developingcountriesarethehighestconsumersofBioenergy,
traditionallyincookingandheatingpurposes,whilemodernusageoftheseresourcescanhelpinprovidingenergy
accessandsecuritytotheworld.TheavailabilityoffeedstockintheformofMSW,agriculturalwaste,Organicwaste
etc.isnotaconcernbutmoderntechnologiesarenotaccessibleandaffordabletomostofthedevelopingcountries.
Althoughtherehasbeenashiftintheregimeofutilizationofbioresources,butatthesametimesomearguments
havebeenraisedaboutthreebroadissues:Firstly,ifBioenergytranslatestonetGHGsavings.Secondly,iflargescale
bioenergyexploitationadverselyaffectsfoodsecurity.Lastly,environmentalill-effectsofusingBioenergy.
Figure1:TotalEnergySupplyintheWorld,2014[1]
TheTotalEnergysuppliedasdefinedbyWBA[1]intermsofenergycontentofthefuel,comparesthetotalproduction
ofenergysourcesincludingimports,exportsandstoragefacilities.Theshareoffossilfuelisthehighestasshownin
Figure1.,Asiahasthehighesttotalenergysupplyintheworldalongwiththehighestrenewableenergysupply.The
Africancontinent,duetoitslargeuseofbiomassandhydropower,providesalmosthalfofalltheenergysupplyby
renewableenergysources.Incomparison,Europehasonly10.3%shareofrenewableresourcesinitsenergysupply
statistics[1].TheGrossfinalenergyconsumptiondefinedbyWBAis,thesumofthefollowingis[1]:
𝐓𝐨𝐭𝐚𝐥𝐅𝐢𝐧𝐚𝐥𝐂𝐨𝐧𝐬𝐮𝐦𝐩𝐭𝐢𝐨𝐧 + 𝐄𝐥𝐞𝐜𝐭𝐫𝐢𝐜𝐢𝐭𝐲𝐚𝐧𝐝𝐇𝐞𝐚𝐭𝐂𝐨𝐧𝐬𝐮𝐦𝐩𝐭𝐢𝐨𝐧 + 𝐋𝐨𝐬𝐬𝐞𝐬𝐢𝐧𝐝𝐢𝐬𝐭𝐫𝐢𝐛𝐮𝐭𝐢𝐨𝐧𝐚𝐧𝐝𝐭𝐫𝐚𝐧𝐬𝐦𝐢𝐬𝐬𝐢𝐨𝐧
2
Where,
TotalFinalConsumptionistheenergythatisspentintheendusessector,calculatedusingtheenergycontentofthe
fuelandElectricityandHeatconsumptionistheheatandelectricitygeneratedinthepowerplants.Figure2below
clearlyshowsthatAfricahasamajorpartofenergysuppliedbyBiomass,whereasOceaniahasthelowestshareof
biomassintheprimarysupplyofenergy.ThemainsourceofenergyinAfricaandpoorpartsofAsia,forcookingis
woodandotheragriculturalresidues.Theselow-gradefuelsposeathreattohealthandenvironmentboth.
Figure2:GrossFinalConsumptionofEnergyResourcesin2014[1]
Figure3:GrossFinalConsumptioncomparingtop10Countries,2014[1]
3
China consumes the highest amount of energywhich accounts to 80 EJ,with global consumption of renewable
resourcesaccountingforasignificantshare.InFigure3itisimportanttonotethattherenewableresourceuseisnot
justintheelectricitysectorbutinheatingandtransportationtoo.
Figure4:Wastegenerationbyeconomicactivitiesandhouseholds,EU-28,2014(%),Eurostat
Figure4andFigure5showthestatisticsofwastegenerationandutilizationintheelectricitysectorinEU-28.Since
thereisalotofattentiongiventominimizingwasteandutilizingBiomassenergy,theshareofBiomassinelectricity
generation has increased over the years 2005-2015 (Figure 5). It is evident that a visible portion of the waste
generatediswastewaterwhichis9.1%,henceitisrequiredthatthiswateristreatedensuringthatcontaminants
are well under the legal limit, by products are re-used and nutrients are recovered. Sewage sludge disposal
technologiesarecurrentlydiscussedandimplementedintheEUbecauseoftheharmfuleffectscausedbylandfilling
sewagesludge.Thelandfillgasandtheleachatesaretoxicfortheenvironment,plantsandgroundwatertable.One
ofthemosttechnologicallymatureprocessesofsludgeutilizationisanaerobicdigestion,whichproducesbiogas.One
ofthekeyconcernswiththistechnology isthatthedigestatematterafterdigestionstillhasenergyandnutrient
contentthatcanberecoveredusingthermalutilizationtechniques.Pyrolysisisoneofthethermalprocessesused
forutilizationofenergyfromtheorganiccontentpresentinthesludge;aspyrolysisisendothermicinnatureitshould
benotedthatahighenergyyielddoesnotcorrespondtohigherefficiencybecausetheproductsgettheirenergy
fromtheexternalheatofpyrolysisthatisbeingsupplied.
4
Figure5:Electricitygeneratedfromrenewableenergysources,EU-28,2005-2015,Eurostat.
Pyrolysis ispreferredovergasificationand incinerationbecauseof itsversatilityandalso theefficientuseof the
productsobtained,forexample,thepyrolysisoilsuponfurtherprocessingcanbeusedastransportfuels[2].The
productsofsewagesludgepyrolysiscanbeutilizedinsoilqualityenhancement(stabilizedbiochar)andthefluegas
canbecombustedtobeusedinenergygenerationapplications[3].However,thereareconcernsrelatedtorelease
oftoxicpolychlorinatedcompoundsandacidicgaseslikeHClandH2Sinthefluegas.Therefore,suitablemethods
shouldbeemployedpostorpre-utilizationsothattheemissionsarewithinthesafelimitsandtheenergyutilization
efficiencyiseconomicallyviable.Theconstraintsofusingsewagesludgeforthermaltreatmentsarenotlimitedonly
topresenceofpathogensandheavymetalsbutincludealsothecorrosiveeffectsofsomecompoundsformedduring
pyrolysisandtheacidicgases influegas.Alkalimetals inthesewagesludgeformmetalchloridesresponsiblefor
corrosionandfoulingofthereactorandhenceevenifchlorineispresentinconsiderablysmallamountsinthesludge,
itcanformmultiplecompoundsdependingonprocessconditionsthatarehazardousforenvironmentandhealth
[4].Ithasalsobeenreportedthattorrefaction(lowtemperaturepyrolysis)inthetemperaturerangeof200-350°C
causestheemissionofnitrogen-containingcompoundslikeN2O,NO,NO2,NH3andHCN,amongwhichNOandHCN
arethemostcommonones[5].Gascleaningandfeedpre-treatmentmethodsareprevalentinBiomasstoenergy
applicationsbutifthefeedismixedwithasorbentthatcancapturethechlorineinbiomasstoconcentrateitinthe
ash, it can prove to be an interesting alternative. Use of suitable compounds as sorbents like kaolin, bauxite,
limestoneetc.havebeenreportedtoreleasechlorinefromalkalimetalchloridesandhenceavoiddepositformation
[6].Thermodynamiccalculationsonsyngas,carriedoutbyJosephLeeetal.[7]inthetemperaturerangeof300-
1500K,Pressure0.1-11MPaandinitialcontaminant(HClandH2S)concentrationbetween1-10000ppmresultedin
CaO,K2CO3,Na2CO3andNaHCO3tobethefourbestcandidatesamong12othersforeffectivechlorineremovalat
moderatetohightemperatures.
5
1.2 ResearchObjective
Theobjectiveofthisstudywastoevaluatetheso-calledIn-situsorptionofChlorine/Sulphurbyco-feedingBiomass
andsorbentinaPyrolysisprocess.Toattainthisobjective,laboratoryscaleandbenchscale(STYX)experimentswere
conductedusingSewageSludge(SS)andWheatStraw(WS)asfeedandfindingasuitablesorbenttoreducechlorine
andsulphuremissions.Thisisrelevantbecauseitisevidentthatwastegenerationwillincreaseandhenceitisthe
need of the hour to invest in technologies for efficient utilization of it, without compromising environmental
regulations. Sewage sludge combustion ispracticed in someEuropean countriesbut if theprocessandprinciple
modificationscanleadtoenergygenerationwithoutincreasingtoxicemissionsthentheprocesswillbeadvantageous
inanoveralllifecycleaspect.Presenceofchlorineinthesewagesludgecatalysessomecrucialprocesses:promotes
themobilityofinorganiccompoundslikePotassium,Sodiumetc.,Clcanformgasphasemetalchlorides,whichare
stableinnature,HClandothercarboncompoundsaretheprecursorsfordioxinformationandthisoccursmainlyin
thepostcombustionzone[8].
1.3 ThesisOutline
Thisresearchworkispresentedin2sections,firstistheLiteratureReviewandsecondistheExperimentalCampaign
withresultsanddiscussion.
The thesis continues inChapter 2,marking the beginning of Literature review. It consists of three sub chapters
describing the basic concepts of biomass properties, composition and structure, Thermochemical Conversion
processesrespectively.
Chapter3 is dedicated for cradle to gravedescriptionof SewageSludge,divided in to5 sub chapters. Since the
feedstockexaminedinthisthesisissewageSludge,knowingitscharacteristics,composition,treatmentetc.isvital
fortheselectionofsuitablesorbent.TheGermanandEUregulationsaresummarizedinthelastsubchapter4.5,so
thatthequantificationofresultsisrelevantandasperthelimitsmentioned.
Theevaluationoffactorsresponsibleforchlorineemission,Pyrolysisprinciplesspecifictosewagesludgeandtheir
comparisonwithcombustion,arediscussedelaboratelyinChapter4.Thisprovidesreasoningfortheresearchwork
conductedastherootcauseofemissionisbeingaddressed.
Thelastchapter intheliteraturereview,Chapter5,ofthisworkmentionsthesignificanceandroleofarangeof
sorbentsstudiedinliteratureforsorptionofClandSincombustionandpyrolysis.Thechaptergivesaninsightfor
selection of sorbent to conduct this study. The extensive literature review focusing on use of sorbent, their
characteristics and efficiency for combustion and pyrolysis, biomass feedstock composition and its effect on
thermochemicalconversiontechnologiesaredescribedindetail.
6
ThesecondsectionofthesisstartsfromChapter6,whichdescribesboththeexperimentalsetups:fixedbedreactor
and bench-scale Screw Pyrolysis (STYX1) reactor (developed by Institute of Technical Chemistry (ITC), Karlsruhe
Institute of Technology (KIT), Germany). Experimental procedure to evaluate performance of sorbents for both
SewageSludge(SS)andWheatStraw(WS)ispresentedseparately.
Lastly,theexperimentalresultsarepresentedintheChapter7forbothreactorconfigurations.Thesorbentstested
arecompared,overallmassbalanceiscalculatedanddepictedingraphs.Comparisonoftheresultsproducedinthis
study and the observations from literature is conducted, providing probable reasons for some deviations. The
experiencesandlearningfromtheconductofexperimentsandresultanalysispavedwayforfutureworkandoutlook
whicharealsolistedinthislastchapter.
1STYXstandsfortheriveroftheGreekmythology.ItistheriveroverwhichCharon,theferryman,transportsthesoulsofthedeadfromtheEarthtoanewlifeintheUnderworld.
7
2. UnderstandingBiomassanditsThermochemicalConversion
2.1 BiomassCharacteristics
Figure6:SourcesofBiomassfeedstock[1]
Biomass isaverygeneral termandcanbedescribedasallorganicmatter (excluding the fossil fuels,which took
millionsofyearstogettransformedbeneaththeearth’ssurface[9]) inwhichtheenergyderivedfromtheSunis
storedintheformofchemicalbonds,alsoincludingwastefromagriculture,Industryandmunicipality[10].Itisan
abundant raw resource, which requires processing depending on its use and composition. Biomass is a non-
homogeneoustypeofresourcebecauseitscompositionishighlyvariabledependingonthesource,location,climate,
seasonandotherfactors.ThebasicelementsbondedtogetherintheformofBiomassareC,HandO
AccordingtoWBA2017report[1],intheyear2014,10.3%ofallsupplyofenergyintheworldwasfromBiomass.As
showninFigure6,themajorsourceofBiomassisforestrythatcontributesto87%ofallthefeedstock.Thequality
andutilityoftheBiomassdependsonthecompoundsthatmakeitup.McKendry[10]classifiedBiomassas:Woody
Plants,HerbaceousPlants,AquaticplantsandManure. In therecentdecades,waste toenergyapplicationshave
gainedmomentumandthusMSW,Sewagesludge,Deadanimals,etc.canalsobeclassifiedunderBiomass.Theuse
ofBiomassisbroadlyobtainingChemicals,EnergyorFuels,wheredifferentconversionprocessesareusedtogetthe
8
desiredproduct.AsshowninFigure7below,Biomassfeedstockcanbeofavarietyoftypesanddifferentresearchers
categorizeditbasedontheiranalysis[10–12].
Figure7:BasicClassificationofBiomassFeedstock
ForestProducts:Wood,deadtreesandshrubs,SawdustandBark
WastesandResidues:Organicwastefrom-Municipality,Industry,Hospitals,Agriculture.Manureandsewagesludge
canalsobeincludedhere.
EnergyandFoodCrops:Woodycrops,Grasses,StarchandSugarCrops,Oilseedcrops,Grains.
PlantandAnimalOrigin:AquaticanimalsandPlants,Algae,deadanimalsetc.
2.2 BiomassComposition
Biomassconsistsprimarilyofthreetypesofmacromolecules:Cellulose,HemicelluloseandLignin.Thefirsttwoare
carbohydrateswhereasLigninisanimportantpartofthecellwallofvascularplants,ferns,clubmossesandhasnon-
sugary polymeric units. For ease of understanding the diverse components of Biomass, one can classify its
physicochemicalproperties,whicharementionedinthenextsubsection.
Figure8:Distributionofthethreemostcommoncomponentsoflignocellulosicbiomassdrymatter[13]
9
2.2.1 WoodandNon-WoodChemistry
LignocellulosicBiomassconsistsofCelluloseasmajorconstituent.Celluloseisawater-insolublepolymercomposed
ofglucoseunits(>10 000),whicharelinkedbyβ-(1-4)-glycosidicbonds[14].
Figure9:PolymericStructureofCellulose[15]
HemicellulosesareheterogeneouspolysaccharidesmadeupofpolymersofPentoses(Xylose,Arabinose),Hexoses
(Mannose, Glucose, Galactose) and sugar acids [16]. Hemicellulose is also composed by a number of different
pentoseandhexosemonosaccharidesandittendstobemuchshorterinlengththancellulose,withthemolecular
structureslightlybranched[17].AsshowninFigure9,alllignocellulosicBiomassesfollowthegeneralcomposition
butitisarangeofpercentagebecauseofvariabilityinthecarbohydratecontent,whichalsoleadstodifferencein
yieldsofBiofuelsandeconomics[13].WhenBiomass isusedforproductionofBiofuelsorBio-power,theoverall
energycontentofthefeedisaveryimportantparameterforeconomicsandyield.Thecarbohydratecontentpresent
inthefeed isthemainsourceofenergycontentbut it isnotexplicitlyusedasaspecification inthermochemical
conversionprocesses.
Figure10:PolysaccaharideunitsofHemicellulose[9]
10
Lignin falls in the category of most abundant compounds on Earth after Cellulose and Chitin. It is a complex
compound,hydrophobicandaromatic,withhighlycross-linkedunitsofhydroxyl-phenylpropaneunits,thesebeing
thephenollikestructuresactingasmonomericbase.TheimportantfunctionofLigninistoimpartstructuralstrength,
sealingthewaterconductingsystemthatlinksrootswithleaves,anditalsoprotectsplantsagainstdegradation[18,
19]
Figure11:Thefourmainmono-lignolscomposingLigninstructure[19]
Thenon-woodycompoundspresentinbiomassarebroadlyclassifiedasSaccharides,LipidsandProteins.Theseare
thenaturallyoccurringelementsinplantsandanimals.Saccharidesinthelivingorganismsprovideenergyandalso
act as key codemolecules in important functions of the body [19]. Lipids are a heterogeneous class of organic
compoundsthatareinsolubleinwaterbutsolubleinnon-aqueoussolventslikeChloroform,alcoholetc.Theyare
naturallypresentinplantsandanimalsusedfordirectproductionofBiodiesel[14].Proteinsarethemostimportant
compoundsconsistingofaminoacids,whichareneededbybiologicalcellsforfunctioning.Aminoacidsof20different
typesformthestructureofproteinsandbasedonthemfurtherthermochemicalprocessescanhavelimitations.The
presenceofaminegroupsinproteinsadverselyaffectsthethermochemicaltechnologieslikepyrolysis,gasification
etc.becausethenitrogenleadstoacidicgasescausingpollutionandcorrosion[20].
2.2.2 Moisturecontent:
TherearetwotypesofmoisturepresentinBiomass,namely:IntrinsicMoistureandExtrinsicMoisture[10].Asthe
namesuggests,whenmoistureispresentnaturallythenitisintrinsicwhereaswhenitoccursbecauseofclimateand
storage conditions then it is called extrinsic moisture. Moisture content has a different effect on the process
dependingonthedesiredendproductandthetechnologyused.Forexample,thermochemicalprocessesrequirelow
moisturecontentbiomass(lessthan40%)whilebiochemicaltechnologieslikefermentationanddigestionfavorhigh
moisturesaturatedbiomassfeedstock[19].Hydrothermalprocessingtechnologiesaredevelopedforaddressingthe
11
highmoisturebiomassforthermalprocessing.Oneofthemaindisadvantagesofmoisturepresenceinbiomassisthe
increaseoftransportationcoststhatadverselyaffectstheeconomics[21].
2.2.1 Mineralogy
Theagriculturebasedbiomasscanhaveseveralfactorsresponsibleforthemineralcontentlikesoilquality,useof
fertilizersetc.Ingeneral,themineralmatterpresentinbiomassisofinorganicnaturebutsometimessomeorganic
compoundsmayalsobepresentdependingontheextentofcontaminationfromindustrialprocesses.Themineral
matterrepresentsthemajorconstituentofpostprocessingresidues likeAsh.Alkalimetals,suchassodium(Na),
potassium(K),calcium(Ca),phosphorous(P)andmagnesium(Mg),arepresent indifferentformsandconvertto
differentcompoundsdependingonthetypeofthermalprocess.Theyalsoreactwithsilica(SiO2)toproduceasticky
andmobileliquidphase,whichleadstoblockageinBoilers[9].Thepostprocessingresiduescanbeutilisedinother
applicationsdependingonthecomposition,ifitisrichinN,P,Kthenitcanbeusedasfertilizer,butiftoxicmetalsare
presentthentheycanbeamajorlimitationforfurtheruse[19].TheTable1showstypicalrangesinpercentageof
differentbiomassfeedstockforMineralmatterpresentanditisclearthatthevaluesvarydrastically(2%woodytrees
tomorethan45%formanureandsewage),thisdeterminesthesuitabilityofthefeedforthetreatmentandprocesses
thatcanbeconsidered.
Table1:TypicalRangeofMineralmatter,wt%[12]
Feedstock Minimum(%) Maximum(%) Average(%)
WoodTrees 0.1 6 2
EnergyCrops(grasses) 1.1 17 6
CerealStraw 1.3 20 7
CerealHusks 1 20 9
Sewage 21 74 49
Manure 11 74 49
Ashhasanegativeeffectonthermochemicalconversionasitreplacescarbohydratesthatarevaluableandhence
theconvertiblebiomasscontentdecreases.Whenpyrolysisisusedforthermochemicalconversionofbiomass,the
ashspecificationislessthan1%,whichclearlyshowsthedisadvantageofhavinglargepercentagesofashasitcan
taketheplaceofvaluablecarbohydrates[13].
2.2.2 ElementalcompositionofOrganicmatter
Theorganicmatterelementalcompositionisdeterminedbyusingtwoimportantmethods(whichweredeveloped
forthecoal industry)UltimateandProximateanalysis.Biomassisheatedto700°Cinaninertatmosphere,which
helpsinclassificationofbiomassintovolatiles,fixedcarbon,ashandfreemoisture.Thevolatilesandmoistureare
releasedleavingbehindthefixedcarbonresidue,andthisistheproximateanalysisofBiomass[22].Itisimportant
12
toknowthecontentofvolatilesandfixedcarbonastheyareimportantparametersforignitionandthermochemical
conversionpotentialofthefuel.Ultimateanalysisisdonetofindtheelementalcompositionofvolatilematterand
fixedcarbon.ThemajorelementsdeterminedareC,H,N,SandO;NandScontentisveryimportantastheyarethe
causeofenvironmentalpollution.Generally,biomasshaslowScontent(upto1%)exceptSewagesludge,Blackliquor
andsomemarinealgaewhereScontentismorethan1%,andinsomecasessewagesludgecanalsoleadto6%or
highersulphurcontent.OneofthedisadvantagesofbiomassisthatithasahigherNcontentcomparedtocoal,10-
12%forsomealgae,sewagesludgeandsomeseedandseedcakes[12].Figure12,showstheVanKrevelenatomic
H/CtoO/Cdiagramfordifferentbiomasstypes;CarbonandHydrogencontenthelpinestimatingthecalorificvalue
offuelandOdeterminesthelossesandCO2emissionduringprocessing.Carbontocarbonbondpossessmoreenergy
thanC-OandC-Hbondsinthecaseofthermochemicalconversion[10]
2.3 ThermochemicalConversionProcesses
Pre-processing is required for economical use of Biomass in thermochemical processes. These include Density
increasebycompaction,thermaltreatmentthroughTorrefaction,Sizereductionbycrushingandgrinding(although
uniformsizecannotbeobtained,itcanassuretobeinaspecificsizerange)[23].
Figure12:H/CvsO/CgraphofBiomassFuels[12]
Table2:ThermochemicalConversionProcesses
ThermochemicalProcessingTechnology Products
Combustion Heat,Steam,Electricity
Gasification Heat,Steam,Electricity,Methane,Hydrogen
Pyrolysis Biogas,Bio-oil,Charcoal/Bio-char
HydrothermalProcessing Biogas,Bio-oil,Charcoal
13
ThermochemicalconversionofBiomasstoproducefuelandCHPunitshavebeenwidelyresearchedgivingriseto
commercialscaleplantsinmanydevelopedpartsoftheworld.Pyrolysisisoneofthemethodsofthermaltreatment
inwhichthecarbonaceousmatterdecomposesintheabsenceofoxygen.Theproductsofthisprocessconsistofoil,
gasandcharofdifferent compositionsdependingon theprocess (pyrolysis) conditions.Biomassappropriate for
pyrolysisislignocellulosic,chemicallycomposedofcelluloseandlignin,whichformsthebasehardstructureofthe
plantmatterandHemicellulosebindstheligninandcellulose.Lignocellulosicbiomassgenerallyusedforpyrolysis
are-Cropresidues,Forestresidue,OrganicMSWandsewagesludge[12].
2.3.1 Combustion
Itisthemostwidelyproventechnologyforproductionofheatandpowerininstallationsbetweenafewkilowatts
andmore than 100MW [24]. Combustion is burning biomass using an oxidant, leading to a series of complex
exothermicreactionsbetweenthebiomassfuelandoxidant.Thecommercialviabilityofthisprocessisduetothe
high level technicalmaturity and considerable heat production, achieving economic feasibility [25]. The current
researchincombustionofbiomassdealswithoptimizationoffurnacedesign,increaseincontrolofcombustionand
overall efficiency. Early research on the commercial implementation of this process concluded that biomass
combustionoccursviafourbasicstages:Drying,devolatilization,Combustionofvolatilematter,Combustionofchar
[25].
Table3:Bios-Bioenergyreport,2012[24,26]
Year/growthrate Approximate
Primaryenergy
consumption,
PJ(petajoule)per
annum
ApproximateTurnoverofBiomassCombustionplants,
MillionEurosperannum
Turnoverof
Smallscale
plants
Turnoverof
Mediumscale
plants
TotalTurnover
2008 3800 3500 2800 6300
2020 7700 9800 6700 16500
Expectedgrowthratefrom
2008to2020
100% 180% 140% 160%
2.3.2 Gasification
Biomassgasification isburningbiomass fuel to combustiblegasesusinga limited supplyofoxygenonanyother
oxidantlikeCO2orsteam.ThegasthusobtainediscomposedbyHydrogen(12-20%),Carbonmonoxide(17-22%),
Methane (2-3%), Carbondioxide ( 9-15%),water vapour,Nitrogen andother impurities dependingonoperating
conditionsandthetypeofgasifier[10].Biomassgasificationalsoinvolvescomplexchemicalreactionstakingplace
insideagasifier,whichhasfourseparatezones:Dryingzone,Pyrolysiszone,PartialCombustionzoneandReduction
14
zone[24].TheFigure13belowshowsthatintotal24biomassgasificationplantshavebeenconstructedandsupplied
bytenmajorcompaniesfromfourcountries(Sweden,Finland,GermanyandAustria).
Figure13:AccumulatedexperienceinbiomassgasificationintermsofnumberofprojectsandMW[27]
2.3.3 Pyrolysis
Biomasspyrolysishasbeenexploredthroughouttheworldforhundredsofyearsandwithtimeandtechnological
progress better control over the process and products has been attained. Biomass Pyrolysis can operate in two
differentmodesdependingontheresidencetimeandheatingrateofthefeedinthereactor:FastPyrolysisandSlow
Pyrolysis.Slowpyrolysisistheconventionalprocess,whichyieldsamajorportionofcharandlowlevelofliquids,
whereasinfastpyrolysishighliquidyieldscanbeobtained[20].TheheatingratesinSlowpyrolysiscanbeseveral
degrees perminute, temperature 500°Cor less [28] and residence time is a fewminutes [29]. Flashpyrolysis is
anothertermforfastpyrolysis,whichischaracterizedbyreactordesignscapableofprovidingshortandintensive
heatfluxtosmallsizedbiomassparticles.Thesolidandvapourresidencetimesareveryshortandthetemperature
canrangefrom450-550°C[30].Theheatingrateforfastpyrolysisisbetween1000°C/sto10,000°C/s[29]andfor
gettinghighliquidyieldsthetemperatureisbetween500-520°C[30].Fastpyrolysisyieldsamixtureofvapoursand
gases,vapourscanbefurthercondensedtoseparatetheoilsandaerosols.Theshareofliquidsdominateswith50-
80%, for fast pyrolysis of lignocellulosic biomass, the remainder being char and gaseswith approximately equal
proportions[29–31].Somepyrolyticprocessesarecarriedoutintheintermediateregimeofslowandfastpyrolysis
andtermedasIntermediatePyrolysis.Someoftheconstraintsthatlimittheuseofbothfastandslowpyrolysisare:
15
Feedpreparationandpre-treatmentaccordingtosize,dryingandgrinding,capitalcosts,productquality,scalability,
lowenergyefficiency.Atpresent,therearenoreactordesignsreportedthatcanovercometheselimitations[12].
SomeofthefactorsplayingakeyroleinthermalutilisationofBiomassaredescribedbelow:
1. Particlesize:Inthecaseofslowpyrolysisthereisnorequirementforrapidheatingandhenceitcantakeparticle
sizeupto50mmwhereasinFastpyrolysis,dependingonthetypeofreactorused,theappropriateparticlesize
changes.Forablativepyrolysis,itsdesigncanprocessbiomassofsizesupto20mmasaresultofconstantshear
ontheparticle,butforotherreactorsthefeedparticlesizeshouldbebetween0.5-6mmindiameter[12].
2. MoistureContentintheFeed:Itisoneofthefactorsthatcausesanincreaseinthecapitalinvestmentbecause
dryingisrequiredtoremovewater,leadingtodecreaseintheprocessefficiencyasthelatentheatofvaporisation
ofwatercannotberecoveredeasily.Ithasbeenreportedthattypicallythemoisturecontentinharvestedstraw
is18%[32]andinwoodchips55%[33]hencerequiringdryingsystems.Thecurrentpyrolysisreactordesignsrun
withafeedmoisturecontentoflessthan10%[2].
3. EnergyEfficiency:Theenergyrequirementdependsonfeedquality,typeandmoisturecontentastheprocesses
ofdryingandgrindingaredependentonit.Thecostandtheprocessefficiencyareaffectedmostbydrying,asit
needsmorethan10%oftheenergyvalueofrawBiomasswhenwoodyfeedstocksareused[12].Othertypesof
restrictivefactorsareheatlosses,whichcanbecausedincaseofexternalheatingsources.Ifthemixingisalso
improper thenheat transferwill bepoor andwill result innon-uniform temperatureprofile along the cross-
sectionofreactor.Theuseoffossilfuelsforthedemandoffeedpreparation,heatingratesetcdecreasesthe
overalloutputandthereforethecurrentmaximumefficiencyobtainedfromPyrolysishasseveralrestrictions.
4. ScalabilityandEconomics:Thedesignofthereactorisanimportantlimitationinscalability,especiallyforthose
thathavea criticalheating surfacearea tovolume ratio.When thesedesignsareused foran increased feed
volumetheyareunabletoprovidetherequiredareaforoptimumheattransfer.Somefacilitieshaveincreased
theircapacitybyusingmodularplantsbutthiscanbeexpensiveifthedesigniscostintensive.Capitalcostsare
highforaplantwithcomplicateddesignandrequirealotofauxiliaryequipmentfordryingandgrindingthefeed.
In most of the currently operational pyrolysis methods, the vapours have to be extracted, condensed and
separated;thisalongwiththegashandlingsystemsincreasethecostsignificantly[12].
5. ProductQuality:Inmostofthelignocellulosicbiomasspyrolysis,thequalityoftheoilisofconcernbecauseofthe
presenceofoxygen,waterandlowpH.Thesefactorsmaketheoildifficultforupgradingtouseastransportfuel
and as feedstock for refineries. Theoil can also contain some finedust and charcoal particles thatwerenot
separatedandhenceitmustbecleaned.Thewatercontentintheoilisanindicatorofthemoisturecontentof
thefeed[12].
16
3. SewageSludge:CharacteristicsandDisposal
3.1 Sewagesludge
WastewatertreatmentfacilitiesgenerateSewageSludge,whichdiffersinpropertiesdependingonthetypeofplant,
physical and chemical properties of waste water used as feed. According to the European Council Directive
86/278/EEC,Sewagesludgecanbedefinedas:
a)Residual sludge fromsewageplants that treatdomesticorurbanwastewatersand fromothersewageplants
treatingwastewatersofacompositionlikedomesticandurbanwastewaters;
b)Residualsludgefromseptictanksandothersimilarinstallationsforthetreatmentofsewage;
c)Residualsludgefromsewageplantsotherthanthosementionedina)andb).
Sewage contains a large amount ofwater but after it undergoes treatment the particulatematter and colloidal
substancesareconcentratedtoformsludge[34].Thus,sludgecontainsasubstantialfractionofwaterandstudies
haveshownthatitcanbeupto90%oftotalwetweight[35].Presenceofwaterinhighamountcanhindertreatment
processes likethermochemicalconversion,hencethesludgeobtained isdehydratedusingsuitablemethods.The
unitoperationsthususedtoreducewatercontentofsludgearedependentonthetypeofmoisture.Waterinsludge
can be of four types: Free moisture (which can be removed by mechanical processes like thickening and
compression), CapillaryMoisture (removed by thermal drying), Adhesive or surfacemoisture (also removed by
thermal drying), Interstitial or chemically bondedwater (which canonly be separatedby changing the chemical
structureofsludgeparticles[35–37]).Ingeneral,73-84%ofmoistureispresentindewateredsewagesludge.Before
sendingsludgefordryingitisrequiredthatallthesolidsareaggregatedandhenceflocculantsareused,likeLime,
saltsoftrivalentFeorAl[37].Hence,onecanfindthereasonsforinhomogeneityofSewagesludge,tremendous
differencesintheconcentrationsofitscomponentsareobservedleadingtodifficultiesindeterminingordefininga
standardcompositionforsewagesludge(mainlycomposedoforganiccompounds)[38].
Duetothephysicalandchemicaltreatmentprocessesinvolved,sludgetendstoconcentrateheavymetalsandpoorly
biodegradabletraceorganiccompoundsaswellaspotentiallypathogenicorganisms(viruses,bacteria,fungietc.)
present in waste water streams. It is also rich in nutrients, such as nitrogen and phosphorous, containing vital
compoundsandorganicmatterthatisusefulwhensoilsaredepletedoraresubjectedtoerosion.Thispropertyof
thesludgeenablesspreadingofthiskindofwasteonagriculturallandasafertilizer[38].Researchershaveexplored
differentmethodsofSewagesludgedisposalbutthereisnoagreementastowhichisthemostappropriatemethod.
17
However,recentresearchhasshownthatenergyrecoverywilldominatethetreatmentmethodsinthefuture[39].
ThemethodsofdisposalinEUarealsonothomogeneousdependingonthelevelofimplementationofdirectives
andnumberofhouseholdsconnectedtothesewers[40].
3.2 TreatmentofSewage
Figure14:Detailedwastewatertreatmentprocess[36]
AsshowninFigure14,wastewateristreatedusingdifferentmethodstargetedatdifferentconstituents(seelistof
sewage sludge composition in Table 5 and Table 6) to be removed ormodified within the safe limits. Physical
processesincludeSedimentation,floatation;ChemicalprocesseslikeCoagulation,Flocculation;Biologicalmethods.
The individual waste water treatment procedures are further distinguished as Primary, Secondary and Tertiary
methodsdependingontheextentofremovalofcontaminants(followingtheEUregulations)[36](Figure15).
Figure15:ClassificationofSewageSludgetreatment.
18
Figure16:TypicalDryingCurveforSewageSludge[35]
Thedryingcurve,sketchedinFigure16,isanimportantcharacteristicofsewagesludgetreatment.Asmentionedin
theprevioussection,waterisassociatedwithsewagesludgeinfourseparateways,sowhenitissubjectedtodrying,
sewagesludgeshowstwofallingrate intervalsafterthe initialconstantdryingrateperiod.This isbecauseofthe
difference in thewaywater is bound to sewage sludge, in the initial constant rate period the freemoisture is
evaporated.Whileinterstitialwaterisremovedinthefirstfallingrateperiods,thesurfacewaterisevaporatedinthe
secondperiod[41].
3.3 CompositionofSewageSludge
Sewagesludgeisacomplexmixtureofconstituents;organic,inorganicandwidevarietyofmicro-organisms.Hence,
asdiscussedintheprevioussection,differenttreatmentmethodsareusedtoensurethatregulationsaremet.Also,
itshouldbenotedthatsewagesludgeindryformhasahighcalorificvalue,whichiscomparabletofossilcoals[42].
Basedonextensiveresearchandreviewpapers,HassanandWangetal.reportedacomparisonofcalorificvalues,
showninTable4[41].Thepresenceofundigestedorganics,suchaspaper,plantresidues,oils,faecalmaterial,isone
ofthecausesofpollutionandtoxicityassociatedwithsewagesludge,becauseitcontainshighlycomplexmolecules
ofphenolic,aromatic,aliphaticstructuresandpolycyclicaromatichydrocarbons(microorganicpollutants)[39–43].
Theinorganiccompoundspresentintheliquidsarederivedfromsoilandsyntheticpolymershavinganthropogenic
roots[44].Therefore,thecompositionofthesewagesludgesamplesobtainedfromdifferenttreatmentplantscan
varyevenifthesamewastewatertreatmentproceduresareemployed[45].Table5showsthatdriedsewagesludge
canbearichsourceofenergywhencomparedtolignitecoalandbiomass(onaverage).However,Sewagesludge
19
hasahighNcontent,which itgets fromproteins,peptides,acidetc.whereas theScontentof sewagesludge is
significantlyhigherthantheBiomassaverage,butcomparabletoLignite[41].
Table4:ComparisonofCalorificValuesofSewageSludgeandBiomasswithcoal[41]
Fuel HHV,drybasis(MJ/Kg) (wt%,drybasis) (wt%,dryashfree
basis)
Volatile
Matter
Ash N S
WoodPellet 18.30-19.60 82 17.4 1.5 0.9
Lignite 11.80-21.90 42.62 26.23 1.49 1.93
Bituminouscoal 25.40-33.15 35.5 6.37 1.59 0.55
WheatStraw 16 77.04 9.07 1.06 0.12
SewageSludge 11.10-22.10 48.41 43.99 7.15 1.41
Therearediverseways inwhichthesewagesludge isprocessedbeforethermochemicalconversionandtheyare
DigestedanddryrawSS,AnaerobicallydigestedanddrySS,chemicalandactivatedsludge,etc.Ithasbeenreported
thatanaerobicallydigestedandthermallydriedSSismostwidelyusedforpyrolysisexperimentsbecausethistype
ofSSisproducedinhighcapacityurbanwastewatertreatmentplants[39].Oneofthebasicpurposesfordryingthe
sewage sludge is that the resultant particles have good fluid-dynamic properties and hence can be used in
applicationslikefluidizationwhereparticlesizeisacrucialparameterforoperation[46].
Table5:BasiccharacteristicsandelementalcompositionofSewageSludge[43]
Constituent A B1 B2 C D
DryMatter(DM),g/l 12 9 7 10 30
VolatileMatter,%DM 65 67 77 72 50
CalorificValue,KWh/t
DM
4200 4100 4800 4600 3000
pH,VM 6 7 7 6.5 7
C,%VM 51.5 52.5 53 51 49
H,%VM 7 6 6.7 7.4 7.7
O,%VM 35.5 33 33 33 35
S,%VM 1.5 1 1 1.5 2.1
N,%VM 4.5 7.5 6.3 7.1 6.2
20
Sewagesludgealsocontainsavarietyofheavymetals,whichoriginateinindustrialwastewater,runoffandcorrosion
ofthesewersystem.Ithasbeenreportedthatapproximately50-80%oftheheavymetalcontentinthewastewater
isconcentratedinthesewagesludgebydifferenttreatmentmethods[47].AsshowninTable6,K,Al,Ca,Mgcontent
insewagesludgeiscomparabletothatofCl;ononehandmetalslikeAlandCatendtoretainClwhereasonthe
otherhandithasbeenreportedthatthereleaseofchlorineisdependentonKcontent[48].
Table6:OrganicandInorganiccomponentsofSewageSludge[43]
Constituent A B1 B2 C D
Protein,%DM 24 36 34 30 18
Fibers,%DM 16 17 10 13 10
Fat,%DM 18 8 10 14 10
P,%DM 2 2 2 2 2
Cl,%DM 0.8 0.8 0.8 0.8 0.8
Ca,%DM 10 10 10 10 10
K,%VM 0.3 0.3 0.3 0.3 0.3
Al,%VM 0.2 0.2 0.2 0.2 0.2
Fe,%DM 2 2 2 2 2
Mg,%DM 0.6 0.6 0.6 0.6 0.6
HeavymetalslikeTin,Lead,Cobalt,Cadmium,Chromium,Nickeletc.arethemajorelementsforrejectionofSewage
sludgeinagriculturalpurposes.Iftheyarepresentinhumanbodiesbymakingwayfromfoodchain,theycancreate
detrimentaleffectstohealth.Itisdifficulttogeneralizeanytreatmentmethodbecausethecontentofheavymetals
variessignificantlydependingontheoriginsite.Itwasreported(Table7)thatCd,Ni,Tiifpresentinlowlevelsare
safeascomparedtoCr,Cu,Pbwhicharegenerallypresentinatoxicrange[49].(Table7,onnextpage)
21
Table7:RangeofvaluesformajorheavymetalspresentinSludge[40,49]
Metal DrySludge(mg/Kg)
TypicalRange MedianValue
Tin 2.6-329 14
Lead 13-26,000 500
Cadmium 20-40 30
Cobalt 11.3-2490 30
Nickel 2-5300 80
Copper 84-17,000 800
Iron 1000-154,000 17,000
Molybdenum 0.1-214 4
Mercury 0.6-5.6 6
3.4 Disposal
Studiesconductedin2001,reportedthattherewere50,000wastewatertreatmentplantsworkingintheEuropean
Uniongeneratingabout7.9milliontonnesofdrysolids.By2006,thisnumberincreasedto8.3milliontonnesdry
solidperyear,thisclearlyshowsthatimplementationoftheEUdirectivewillleadtoanincreaseintheamountof
sludge[50].Sewagesludgeisaverychallengingwastetobemanagedbecauseoftheinflatedcostsandenvironmental
problemsassociatedwithit.Thereareseveralconstraintsinutilizingitforagriculture,althoughitisfeasiblefroma
policyperspective.Thepresenceofheavymetalsandpathogensrestrictstheuseofsewagesludgeasthequalityis
highlyvariable,whichleadstohighstandardcontrolandtreatmentmeasure.Itistheneedofthehour,forthepolicy
makerstofindabalancebetweenpreferredpolicyandsustainabledevelopmentinenvironmentalperspective.There
arefourmethodstohandlesewagesludge:Landfilling,Composting,andmorethan60%ofitisutilizedinagriculture
becauseofthepresenceofnutrientslikeP,Netc.[50,51].CountrieslikeAustria,Netherlands,Germany,Slovenia
useincinerationastheirmajortoolfordisposalwhile,Malta,Italy,Romaniausecontrolledlandfillforthesame,as
shown in Figure18.Biologicalmethodsof conversionof sewage sludge touseful product canbeAnaerobic and
aerobicdigestionandcomposting;butconstraintslikeodour,qualitycontrol,monitoringandheavymetalcontent
make itdifficult tobeused.Although thesemethodshelp inphaseseparationof sewagesludge, theyalsoneed
dewatering.
22
Table8:UseandDisposalofSludgebasedonmethodused[50].
Method Examples Constraints
LandBased Agriculture,Forestry Qualityandvariability,Impact,Vulnerability
Landfill MonoandCo-Disposal Leachate,Gasemissions,Potentialresourceloss
Thermal Incineration,Gasification HighCosts,Ashdisposal,Emissions,PublicPerception
Figure17:ThermalwastetreatmentinGermany,2012(StatistischesBundesamtWiesbaden2015)
Incinerationalsohas its shareof criticismbecauseof several reasons, it is said that incineration is justaway to
minimize the sludge but it cannot completely dispose it. The ash produced by incineration is classified under
hazardouswasteandmustbecarefullyhandledanddisposedinspeciallandfills.Thecostofthetechnologyisalso
high and requires precise information of the calorific value, pre-treatment and should comply with emission
standards.ThepresenceofpollutantslikePCB,PAH,PCDD,etc.requirecarefulinvestigation,otherwisethesludge
willfallunderhazardouswastecategory[50].
23
Figure18:EEASewagesludgedisposalbyprocessused(%oftotalmass),EUROSTAT2015
Thecostincreaseoftheincinerationprocessisduetothehighwatercontentofthesewagesludgeandtheremoval
ofwatercorrespondstoanenergyrequirement.Table9belowshowsthemethodsofsewagesludgedisposalused
bysomeoftheEUcountries.Somecountriespreferincinerationoverlandusebecauseoftheharmfuleffectsbutat
thesametimetheyshouldtakecareoftheashdisposalfromincineration.AsshownintheFigure17,Germanyused
thermaltreatmentmethodtodisposemostofitswastegenerated(totalof24.2Milliontonnes)in2012.Also,sewage
sludgehasbeenincreasingdisposedinmostsustainablemethodinGermanstates,asreportedbyUmweltBundesamt
intheyear2011[38](Figure19).
24
Figure19:PercentagedistributionofdisposalmethodsinGermanregionalstatesfor2011(Umweltbundesamt)
Table9:DisposalmethodsforsewagesludgeinEUMemberStatesaspercentage[52]
Whereitshouldbenotedthat,(i)3outof16federalstatesintendtostopagriculturalsludgeuse,(ii)Whilein2004,therewasstill9%ofsludgerecycledtoagriculture,
itdecreasedto3%in2005.In2000,otheroutletsinclude27%aslandfillcoverand53%forlandscaping.
Country Yearofdata Agriculture Landfill Incineration Other
Germany 2003 30 3 38 29(i)
Austria 2005 18 1 47 34
Denmark 2002 55 2 43
France 2002 62 16 20 3
Poland 2000 14 87 7
Netherlands 2006 0 60 40
Finland 2000 12 6 80(ii)
Belgium,BrusselsRegion 2002 32 2 66
25
3.5 EULegislationsforSewageSludgehandlinganddisposal
1. Earlyin1975,thememberstateswererequiredtohaveenvironmentallyfriendlywaysofdisposalandwaste
preventionandmanagement.
2. TheSewagesludgedirective-86/278/EEC laidrules for theuseofsewagesludge inagriculturalactivitiesby
definingthevaluesofpermissiblelimitsofheavymetalslikeCd,Cu,Hg,Ni,Pb,Zninthesludge.Itdirectedthe
memberstatestousesludgeinagriculturalactivitiesbyfirsttreatingitusingbiological,chemicalorthermal
treatmentssothattheextentoffermentationandhealthhazardsareminimised[53].
3. In1991theEUalsosetuprulesforhandlinghazardouswasteandtheUrbanWastewaterDirective-91/271/EEC
wasamended to98/15/EC, tobeapplicable from2005. This amendmentensured stricter rules andquality
standardforwastewaterwitharticle14statingthatmemberstatesshouldensurethatsewagesludgeshould
no longer be disposed in water bodies and surface waters. It also directed that sludge should be re-used
wheneverappropriate[49,54].
4. In 2005 the EuropeanUnion also declared the implementation of theDirective, approved in 2000, for the
reductionofdioxinsby90%emittedduringincineration[40].
5. In2001,theEUsetupstrictemissionlimitsforthefollowingcomponentsemittedduringincinerationofwaste
[55].
Table10:Airemissionlimitvaluesasperthe2001WasteIncinerationDirective.
Component Incinerators,mg/m3 CementKilnsmg/m3
TotalOrganicCarbon 10 10
TotalDust 10 30
HCl 10 10
CO 50
HF 1 1
SO2 50 50
DioxinsandFurans 0.1ng/m3 0.1
Hg 0.05 0.05
Cd,Ti(total) 0.05 0.05
Sb,As,Pb,Cr,Co,Cu,Mn,Ni,V
(total)
0.5 0.5
NO/NO2 Plants>6t/h200
Plants<6t/h400
ExistingPlants800
Newplants500
26
4. ChlorineemissionsfromBiomassThermalConversion
4.1 ReleaseofChlorinefromdifferentfeedstock
Chlorinecanbepresentindifferentformsdependingontheoriginandnatureoffuel.Incoal,theconcentrationof
Clvariesfrom50-2000mg/Kgwhereastheoriginisfromthegroundwaterthatpercolatesthroughlayersbelowthe
surfaceduringittheformation[55].ThepresenceofClincoalwasstudiedbydifferentresearchersbuttheproblem
is very complicatedbecauseofpoordataagreementobtainedbydifferentmethods.Chlorine ispresent inboth
inorganicandorganicformincoal.ThemineralchlorinepresentincoalisintheformofNaClandoxychlorides[55].
ItwasalsoassumedbyCrossleyetal.[56]thatwatersolublechlorineexistsincoalasNaCl,KCl,CaCl2,MgCl2etc.
WhileorganicchlorinemakesupthemainpartofthetotalClincoal.TheorganicClconsistsoftwoforms:1.Water-
insolubleClorganiccompounds,whereCliscovalentlybondedwithorganicmatterofcoal,2.Partlyorfullywater-
solubleClwhichissorbedontheporesurfaceofcoalorganicmatter[55].Inthecaseofcoalpyrolysis,Clisreleased
in the formofHClmostly in thetemperaturerangeof400-600°C.This releasedHClcanreadily reactwithmetal
impurities (likeCa) to form inorganicandorganic chlorine functionalities.Upon, further increaseof temperature
thesecanreleaseHClagain[57].Coalcombustionandthehotfluegaswerestudiedextensivelyinthelate1980sto
find suitable solutions for the undesirable and toxic components present in the flue gas. The presence of alkali
vapoursinthefluegaswasagraveconcernandhenceitsformationwasstudiedforcoalcombustionatatemperature
of800°C.ForthispurposeKaolinitewasusedasasorbenttoremovealkalichloridesfromfluegas[58].Inthisstudy
the adsorption of NaCl and KCl vapours on kaolinite (under nitrogen and simulated flue gas atmospheres) was
studied.Theauthorsproposeamodelwhichsuggestssurfaceadsorptionanddiffusionthroughboththesaturated
productlayerandporesoftheactivekaolinite.TheconclusionwasthatKaoliniteisaneffectivesorbentforremoving
vapoursofbothNaClandKClfromthesyngasproducedbycoalgasificationandcombustion.
InthecaseofBiomass,chlorinecontentvariesfromlessthan200mg/Kgtoamaximumof7000mg/Kg;pyrolysisof
woodybiomassleadstocompletereleaseofchlorineat350°C,whichconfirmedtheresultsofstudiesstatingthat
the fractionof chlorine released ishigher for lowCl contentbiomass [8,59].Theextentofpresenceof chlorine
determinesitsreleasebehaviourasitwasshowedinaresearchthatwithbiomassofmoderatealkalicontent,the
increaseofCalciumcontentseemstobemoreeffectiveindecreasingtheHClemissionsthanincreasingtheKcontent
[5].ThermalutilizationstudiesusingdifferenttypeofBiomasslikestraw,wood,agriculturalresiduesetc.suggested
thatchlorineisreleasedasseveraltypesofcompoundsbecauseofthedifferenceintheoriginoffeed.Inthecaseof
combustionorgasificationofstrawandsewagesludgeseparately,increasingtheexcessaircoefficientledtoincrease
ofchlorineemissionsviaKClorNaClformationontheotherhand,additionofKaolinincreasesthereleaseofHCland
significantlyreducestheformationofKClinstrawcombustion[60].Pectinisamajorcomponentoftheprimarycell
27
wallofplants,whichactsasamethyldonorfortheformationofChloromethane(CH3Cl)byabioticconversion[61].
The emission of CH3Cl during biomass combustion has been reported to be either a free radical process during
combustionofcelluloseor thecoal-charcatalysedreactionofmethanolwithHClproducedduringpyrolysis [61].
ResearchersalsoconductedstudyofchlorinereleaseasafunctionofpectincontentvsCl/Pectinratioandfoundthat
concentrationofpectininthebiomassisnotaratelimitingstepandotherorganiccompoundscanalsoactasCH3
donors[60].TheinorganicchlorinereleasedduringbiomasscombustionismajorlyintheformofHClandparticulate
chlorine[5].Thepredictedchlorinatedcompoundsdependonthetemperatureofpyrolysisorcombustion.Bjorkman
etal.[8]reportedthatthedistributionofchlorinebetweensolidliquidandgaseousstateisdependentontheprocess
conditions, dominantly pressure and temperature. Also, they stated that composition of gaseous mixture after
pyrolysisorgasificationdependsonthetemperature,ifitisbelow600°CthenHClisthedominantproductbutabove
800°CKClandNaCldominates(meltingpointofKClis770°CandthatofNaClis801°C[5,8].Chlorineemissionduring
pyrolysis of some selectedBiomass feedstock: Sugarcane trash, switch grass, Lucerne, straw(rape) and synthetic
wastewerestudiedunderpyrolysisconditions[8]andtheyreleasedintothegasphase,between20-50%ofallthe
chlorinecontentat400°C,exceptstraw(rape).Theauthorsalsomadeaveryimportantpointclear,thattherewas
nosignificantdifferenceintheemissionofchlorine(mixtureofinorganicandorganicchlorides)fromBiomassand
syntheticwaste.Ithasbeenfoundthatformajorityofbiomassfeedstockthetorrefactionandpyrolysis(upto700°C)
chlorinereleaseisintheformofHCl[5].Chlorineoccursintheformofalkalimetalsaltsinthebiomassandhence
canreadilyconvertintovapourformduringthepyrolysisandgasificationprocesses.MostcommonisHClwhichcan
furtherformcompoundslikeNH4ClandNaClcausingfouling;hotcorrosionofgasturbinebladeswhichcanoccur
withconcentrationsofchlorineandalkalievenaslowas0.024ppm[62].Thepresenceofpotassiuminbiomassas
showninFigure20ishighenoughforthecompletebindingofchlorineasKCl[8].
Figure20:AlkalimetalandChloroatoms(mmol/100gfuel)inBiomass[8]
28
Thereleaseofchlorinefromthebiomassmatrixdependslargelyontheparticlesizeandheatingrate;ithasalsobeen
shownthatthepyrolysisofamixtureofKClwithchlorinefreebiomassleadtoreleaseofchlorine(30-50%)evenat
temperatures below 400°C, this is because of reaction between KCl and the carboxylic groups in biomass [63].
Another observation about release of chlorine from biomass with considerable silicon content is the reaction
betweenKClandsteam(formedbydryingofbiomass),sincetheequilibriumofthisreactionislargelyaffectedbythe
presenceofacidicSiO2,atatemperatureof400°CthispathofreleaseofchlorineasHClissignificant[8].
𝟐𝐊𝐂𝐥 + 𝐧𝐒𝐢𝐎𝟐 + 𝐇𝟐𝐎 𝐠 → 𝐊𝟐𝐎(𝐒𝐢𝐎𝟐)𝐧 + 𝟐𝐇𝐂𝐥(𝐠)
Thisalsoshowsthateveniftheoriginofchlorineinbiomassisofinorganicnatureitcanreleaseatatemperature
lessthanthemeltingpointofthesalt[8,63].InthecaseofhighchlorinebiomasslikecornStover,morethan50wt%
ofchlorineandSulphurwerereleasedbelow500°C.SinceSulphurisalsoassociatedwiththeorganicmatrixofthe
biomasswhichdecomposesat500°ChencethereleaseofSulphuratconsiderablylowtemperaturewasobserved(S
exists inarangeofoxidationstatesfrom-2to+4bothorganicand inorganic innature[64])while inthecaseof
chlorine it ismostlythe ionexchangereaction leadingtoHCl formation [47].RecentpyrolysisstudiesofClandS
releaseusingtwodifferentreactorconfigurations(RotaryandFixedbedreactor)showedthat20%ofClwasreleased
fromstrawatatemperatureof250°Cand64%at350°C,thelowtemperaturereleaseofClwasattributedtopresence
ofCH3donors[59].Zintletal.[65]performedreactionsofKClandwoodinthetemperaturerangeof200-700°Cand
proposedthat the initial lowtemperaturechlorinereleasewasaresultofareactionbetweenKClandcarboxylic
groups(shownbelow).
𝐊𝐂𝐥 𝐬 + 𝐑 − 𝐂𝐎𝐎𝐇 𝐬 → 𝐑 − 𝐂𝐎𝐎𝐊 𝐬 + 𝐇𝐂𝐥 𝐠 − − − 𝟏
TounderstandthereleaseofchlorinefromstrawandcornStoverpyrolysisinanitrogenatmosphere,between200-
1050°C,inasystematicmanner,Jensenetal.reportedtwo-stepprocessofchlorinerelease[47,63]:
1. 60%Clreleasebetween200-400°C,(reaction1)thisstepdependsonthefunctionalgroupspresentinthe
organicmatrix.
2. The rest between 700-900°C, (reaction 2) aluminosilicates reacts with alkali metals (at moderate
temperature this step is kinetically limited and at temperature above700°C it competeswith the alkali
chlorideevaporation).InthereactionbelowY(s)canbesilica,aluminaoracombinationofboth.
𝐇𝟐𝐎 + 𝟐𝐍𝐚𝐂𝐥 + 𝐱𝐘 𝐬 ↔ 𝐍𝐚𝟐𝐎. 𝐱𝐘 𝐬, 𝐥 + 𝟐𝐇𝐂𝐥 − − − 𝟐
29
Figure21:PossiblereactionpathforKreleaseduringdevolatilizationandcombustionofannualcrops[47]
As reportedearlier,Potassium isoneof thesourceofdeposit formation in reactors.Acloser lookat therelease
mechanismasshownintheFigure21,showsthatitisacomplexprocesswhichisrelatedtothecontentofSi,S,Clin
thebiomass.Theorganic fractionofKpresent inthebiomass isreleasedduringthedevolatilizationstageandat
elevatedtemperatureKClundergoessublimation.Thecharburnoutstagecan leadto formationofKsilicatesor
alumino silicates. The alkali release in to the gas phase is slow and limited till 600°C because of the diffusional
resistanceofferedbyintactorganicmatrix[47].Ithasalsobeenshowedthattheincreaseofalkalimetalcontentof
biomassdecreasestheamountsofthecombustiongeneratedemissionsofchlorine.Hence,theHClemissionsfrom
thecombustionofbiomassareinverselyproportionaltotheiralkalitochlorineratio[5].
Figure22:PossiblereactionpathforClreleaseduringdevolatilizationandcombustionofannualcrops[47]
30
4.2 ChlorineemissionsfromSewageSludgePyrolysis
Themotiveofusingthermo-chemicalmethodsfordisposalofsewagesludgeistoextracttheusableenergyfromit
andreducetheharmfuleffectsontheenvironment.Inthecaseofforsewagesludgedisposal,combustionhasbeen
researchedandimplementedextensivelyinEurope.Eachthermaldisposaltechniquehasitsownadvantagesand
disadvantagesbutasmentionedinchapter4,sewagesludgeishighlyvariableintermsofcomposition,henceitisa
crucialfactorforchoosingthetechnologytobeused.Combustionisoneofthemostresearchedmethodconcerning
sewagesludgeasitreducesthedisposalvolumeandcompletedestructionofpathogens.Butthemaindrawbackof
usingcombustionisthegenerationofhazardousairpollutantsinthefluegas,combustordesignsarehencemade
takingcareofthechlorinecontentofsludge[66].Co-incinerationisoneofthesuggestedwaystodealwithsewage
sludgeasitcanbeusedincombinationwithotherfuelslikecoalandMSWetc.togenerateenergy[48].Ithasalso
beenfoundthatco-incineratingMSWandsewagesludgereducesthecostoftheprocessbecausesufficientenergy
canbeproducedfordryingthesludgefromMSW[66].Approximately30wt%ofthedrysolidsremainfinallyasash
incombustionofsewagesludgeandthisitdoesnotcontributesignificantlytocompletedisposal[48].Thekinetics
ofpyrolysissuggeststhatreactionconfigurationandresidencetimearecrucialindeterminingthefinalresidueand
pollutants inthefluegas.This isbecauseprimary(rawmaterialdecomposition)andsecondaryreactionsaretwo
basicstepsoccurringduringpyrolysiswheresecondaryreactions(primaryvolatilesreactwiththechar)arearesult
ofhigh residence timeandhigh temperatures [67].Pyrolysishasemergedasanefficientway for sewagesludge
handlingbecauseitproduceslessemissionsbythevirtueofitsprocessconditions,theheavymetalemissionisnilin
thegasphaseastheyarecollectedinthecharwhichisalsoknownasbio-char[68].Sewagesludgecompositionis
veryimportantparameterfordeterminingthethermodynamicfeasibilityofthereactionsleadingtoemissionofHCl.
OnesuchobservationwasreportedbyMatsudaetal.[69]andconfirmedbyKassmannetal.[70]whentheycarried
outextensivethermodynamicstudiesconsideringallthepossiblereactionpathways.Theyreportedthatpresenceof
SO3affectstheformationofHClfrommetalchloridesaccordingtothefollowingreaction[69]:
𝐌𝐂𝐥𝐱 +𝐱𝟐𝐇𝟐𝐎 +
𝐱𝟐𝐒𝐎𝟐 →
𝐱𝟐𝐌𝟐/𝐱𝐒𝐎𝟑 + 𝐱𝐇𝐂𝐥
Chlorine and Sulphur emission reduction was studies since early 1990s as the acidic gases posed threat to
environment and the reactors. Calcium based sorbents have been extensively studied for this purpose andwet
scrubbingisreportedtobeabetterchoicewhenthebiomasshashighmetalcontent[62].
31
5. UseofSorbentsforChlorineCaptureChlorineisamicroelementpresentinthehighestquantitywhencomparedtoothermicroelements.Itisabsorbed
asCl-anionbytherootsoftheplantsandthenassimilatedintheleavesandstem.Researchershaveconcludedthat
biomasshavinghighpercentageofClreleaseslowerfractionofClthanbiomasswithlowpercentageofCl.Studies
havealsorevealedthatchlorineemissionsfrompyrolysiscanbeoriginatingfrombothorganicandinorganiccontents
ofthebiomass.Also,thereleaseisstronglydependentontheinorganicconstituentslikealkaliandalkalineearth
metals in thebiomass [71].Anothercompound released in the formofCH3Cl is thecauseofClemissions in the
pyrolysisofwoodyandleafybiomassstudiedbyHamiltonandco-workers[61].TheyshowedthatthePectinpresent
intheleavesactsasaCH3donorandplaysakeyroleinClemissions,thiskindofreleasestartsatatemperatureof
150°Candincreasesupto300°C.Also,pectinisnottheonlycomponentforClrelease,theyconcludedthatCH3Cl
emissions during combustionofwoodybiomass can also beoriginated from the reactionof Cl-with lignin [61].
Chlorineemissionsfrompyrolysisofbiomassintheformofhydrogenchlorideormetalchloridescanbecapturedby
using sorbents. These chemical substances undergo chemical reactions with chlorine or by physical adsorption
removeschlorinefromthefluegas.Basedonthedesiredpropertiesofsorbentslikefastrateofadsorption,high
loadingcapacity,irreversibleadsorption,costetc.studieshavebeenconductedtocheckthecaptureefficiencyand
processcalculations.
5.1 RemovalofHClandmetalchlorides
In the early 1980s researchers observed the presence of alkali metal compounds in the vapour phase during
gasificationorcombustion[72].Thesecompoundscausefurtherprocesscomplicationsanddamageforexample,
corrosioninthepostprocessutilization.IfcontentofchlorineinBiomassishigh,thenitcanformdepositionsofalkali
chloridesonthewallsoftheboilerwhichslowsdowntheheattransfer.ChlorineintheformofHCl,influegaswhich
is utilized for combined cycle process, can cause corrosion of the turbine parts [58]. Since a sizeable portion of
chlorineinthefluegasisassociatedwithalkalimetals,reportswerepublishedillustratingbenchscaleandlabscale
experimentsunderdifferentreactorandprocesssetup.
Aluminosilicatesbecamepopularascatalystsbecauseoftheirhighsurfaceareaandporosity,henceKaolinitewas
investigated as a sorbent for removal of alkali chlorides fromhot flue gases. Theexperimentusednitrogenand
simulatedfluegasenvironmenttocaptureNaClat800°C.Mathematicalmodelssuggestedthatthe initialrateof
adsorption isdirectlyproportional to thealkali concentration in thebulkgas, also it isnearly the same forboth
environments considered. The adsorption is irreversible and depends on the gas composition. As in a SFG
environmentonlysodiumwasretained,unlikewithN2atmosphere,itwasproposedthattheadsorbedNaClwould
havereactedwithKaoliniteinthepresenceofwatertoformnepheliteandHClvapours[58].Sorbentsthatcontain
ahighpercentageofSilicacouldadsorb irreversibly just thealkaliandnotchlorine, releasing itasHCl.Activated
bauxiteandemathliteinthediameterrangeof2.4-3.4-mmwereusedforalkaliremoval,theyalsodemonstrated
32
thattheproductgaschlorineconcentrationwasunaffectedorwasreducedtohalfinthebestcasescenario,whileK
andNaremovalefficiencywere98%and92%respectively[73].
5.2 AlkaliandAlkalineearthmetalsassorbents
Alkalimetalsorbentsareincreasinglyusedforhalideremoval,commonlyHClremovalresultingfromthermochemical
conversionofBiomass.ThesemetalsorbentsformsaltslikeNaCl,KCletc.onreactionwithhalidesandhenceprove
tobeuseful[74].Similarly,alkalineearthmetals(BaO,CaO,MgO)alsoshowedtobethermodynamicallyfeasiblefor
removalofhalides[75].
HClformedduringpyrolysisisfoundtoberemovedmostefficientlyinthetemperaturerangeof500-550°Cbecause
ofthechemicalequilibriumconditionsbetweentheconstituentgasesandsolidsinvolved[76].Thesorbentselection
also depends on cost, hence sodium based compounds like sodium bicarbonate, sodium carbonate, Ca(OH)2,
Mg(OH)2 and their calcined versionsofCaO,MgOare reported tobeuseful and inexpensive. Experimentsusing
calcium based sorbents have shown 80% removal of HCl [77]. Ketov et al. (1968) [78] showed that there is an
optimumtemperaturerange,whichleadstomaximumCaCO3toCaCl2conversion(dependingonthekindoflime
taken),thatwasfoundtobebetween540-550°C.Onpilotscaletheuseofslakedlimeparticlesasasorbentforflue
gasdemonstrated.Theresultsweremonitoredatatemperatureof260-400°C,HClretentionwasintherangeof40-
100%which increasedbasedonthetemperature increase,watercontent inthegasandwithdecreasingparticle
diameter from11-39µm.The limeconversion ratewas rapid in thebeginningasmostof theHCl in thegaswas
absorbed, after some time it decreased and then gradually became constant (depending on other reaction
parameters)asshownbyFigure23[79].Duoetal.(1994)[80]studiedthereactionbetweenCaCO3andHClanddue
to the lowvalueof chemicalpotential concluded that the reactionwas slow,aswell as the sorbent conversion.
Anotherstudybysameauthors,forIGCCFuelgascleaningsorbentalsoconfirmedtheaboveresultsoflowconversion
ofCaCO3at400°C,whiletheyalsotestedNa2CO3,NaHCO3andNa2CO3.10H20[81].TheconcentrationofCO2affects
theCabasedsorbentsshowingbetterresultsinoxygenblownthanairblownfuelgases,whileitdoesnotaffectthe
Na based sorbents in the temperature range of 300-600°C. Na2CO3.10H2Owas found to be better thanNa2CO3
becauseofhighporosityofitsdehydratedcompound[81].
AstudyofbindingofHClwiththesorbentparticleswasdonebyClausandco-workers;thesorbentsusedwereslaked
LimeandLimestone.Thebindingcapacitywasdependentonchemicalequilibriumofsolidandgasabove500°C,it
wasindependentoftheparticlesizeandslightlydependentonspecificsurfacearea.Inthetemperaturerangeof
500-600°Candbelow150°Cthebindingcapacitywashighest,almostfullconversionoflimetoCaCl2wasobserved.
Thekineticswasdependentonthediffusioninsidethesolidparticlesandfollowedunreactedgrain-coremodel[82].
33
Figure23:Limeconversionfordifferenttemperaturewithrespecttotime[79]
Somefixedbedreactorstudiesweredoneinthecontextofchlorineremovalandthereactionkinetics;onesuggested
thatthetestedalkaliandalkalineearthmetalsorbentslikeNa2CO3,CaCO3,MgOetc.couldreducetheHClvapour
concentration from10-3 to 10-6 at a temperatureof 500°C and space velocityof 3000h-1. The reaction kinetics
followedfirstorderwithrespecttoinitialHClconcentration[76].AnotherstudyofHClremovalinfixedbedusing
NaHCO3,CaCO3,Ca(OH)2,Mg(OH)2andAl2O3wasconductedatatemperatureof550°C,showingsignificantreduction
of lessthan2wt%indownstreamascomparedtofullysaturatedupstreamend[83].SorbentECl1asdepictedin
Table11,showedhigheradsorptioncapacity,probablybecauseofthecompositionandstructure,asthereactive
component was 87 wt% while in ECl2 it is 11 wt%. The combination of Dolomite and Silica as a catalyst for
decompositionofpyrolysis tar showedgoodefficiencyevenathigh temperature [84].Karlssonetal. (1981) [85]
studiedCa(OH)2asasorbentinfixedbedreactorfrom150-400°C,,confirmingthatthereactionfollowsfirstorder
kinetics,aspreviouslymentionedbyothers,alsotheavailablemaximumCa(OH)2forthesorptionwasaround55%.
Amagazinereport[86]publishedin2014,showedtheadvantagesofretrofittingacidgasremovalsystemsoftwo
wastetoenergyplantsinGermany.TheoldsystemsusedCa(OH)2assorbentwhichwasreplacedwithNaHCO3.The
resultsshowedbetterefficiencyofremovalofHClwithcomparableeconomicfeasibility.
34
Table11:CompositionofSorbentsused[83]
Sorbent
ECl1 ECl2
MainComponents NaHCO3,CaCO3,
Ca(OH)2,Mg(OH)2:87%
Ca(OH)2,11%,Al2O3,89%
Preparation Drymixing Wetimpregnation
BulkDensity(g/cm3) 0.66 0.73
SurfaceArea(m2/g) 3.24 127.89
AveragePorediameter(Å) 247.80 47.34
DryinjectionofCalciumbasedsorbentsinlaboratoryscalefurnaceexperimentsatgastemperaturesof600-1000°C
wereconductedtochecktheHClcapture.Thesorbentswerepowderedcalciumformate(CF),calciummagnesium
acetate(CMA),calciumpropionate(CP),calciumoxide(CX),andcalciumcarbonate(CC).Thesorbentsfluidizedina
streamofairwereintroducedinthefurnaceconcurrently,showingrelativeutilizationof80%.Calciumcarbonate
andcalciumoxideweretheinexpensiveandlowporositysorbents,theyperformedwellwithCaCO3utilizationof
54%atmidtemperaturerangeandCaOof80%inthelowesttemperatureofinvestigation.Calciumsaltsvolatilizein
thetemperaturerangeof300-460°Ctoformcalciumcarbonate,whichisstableupto700°CandaboveitformsCaO
andliberatesCO2.TheremainingCaCO3reactswithHClasfollows[77]:
𝐂𝐚𝐂𝐎𝟑 𝐬 + 𝟐𝐇𝐂𝐥 𝐠 → 𝐂𝐚𝐂𝐥𝟐 𝐬 + 𝐂𝐎𝟐 𝐠 + 𝐇𝟐𝐎 𝐠
Thethermodynamicstudiesweredone inthe2000stocomparesorbentsandtheirefficiencybyvaryingprocess
parameters.Nicolaetal.[87]conductedincinerationexperiment,toremovepollutantsinfluegasinanin-ductdry
removal set up to compare Ca and Na based sorbents by establishing theoretical limits achieved by them,
thermodynamically.Theycheckedthelimitingvaluesofequilibriumvapourpressuresofthepollutants(withboth
NaHCO3andCa(OH)2)byvarying the temperature from100-600°Candkept themolar ratiobetweenamountof
sorbentinjectedandtheamountofpollutantsinthegas,constant.Thesecondcasekeptthetemperatureconstant
andvariedthemolarratiofrom0-1.2.AsshowninFigure24,NaHCO3issuperiortoCa(OH)2intheentiretemperature
rangeconsideredbecausebicarbonateallowstoobtaintheoreticallimitsforHClatcomparativelylowervalue(six
ordersofmagnitude)thanwithLime.NaHCO3wasfoundtobeeffectiveevenat600°CreducingHCltoNaCl.When
themolarratioofsodiumbicarbonatewasincreasedthedecreasingvalueofHClinthegasshowedthatitisbetter
thanLime[87].
35
Figure24:Performanceofsorbentswiththepollutantpartialpressures[87]
Anotherstudywasconductedbythesameauthorsinafixedbedreactorsystemwiththereactionconditions:550°C,
spacevelocity3000h-1andinletHClconcentrationof1000mg/m3.AsshowninTable13,thebreakthroughpointfor
sorbentE1wasthebestforthetemperatureof550°Canditwasabletoreducetheoutletconcentrationto1mg/m3,
whichisneartothedesirablelimitoflessthan1mg/m3[88].
Table12:Sorbentproperties[88]
Sorbent Compositionwt% Surface
Area(m2/g)
Porevolume
(mL/g)
Pore
diameter(Å)
HClremoval
E1 MgO,30%;MMt,70% 136.20 0.20 89.80
E2 Commercialcatalyst 127.90 0.15 47.30
E3 MgO,50%;MMt,50% 12.10 0.05 300.8
E4 MgO,70%;MMt,30% 16.40 0.07 289.2
Table13:BreakthroughpointforChlorine[88]
Sorbent E1 E2 E3 E4
Breakthroughtime(h) 7.20 4.00 2.10 2.00
Breakthroughchlorine
content(%)
8.60 3.60 3.20 3.00
Saturationchlorine
content(%)
48.3 32.15 19.70 19.20
36
6. ExperimentalSetUpThischapterprovidesthedetaileddescriptionofthereactorconfigurationusedfortheexperiments,theprocess
conditions,feedcharacteristicsandIntegralMassBalanceequation.
Thepyrolysisexperimentswereconductedusingtwodifferentreactorconfigurations:FixedbedbatchReactorand
ScrewPyrolysisReactor(STYX).ThefeedusedwasSewageSludge,obtainedfromawastewatertreatmentfacilityin
Germany,havingparticlesizeintherangeof4-8mm.Wheatstrawwasalsousedtocarryoutsameexperimentsas
alternativefeedforcomparisonpurposes.Thefeedstocksampleswereanalysedbyultimateandproximateanalysis
accordingtoGermanstandards.Themetal,halogensandothercompounds inashwerealsoanalysedasperDIN
22022-1andDIN51729-1standards.Thenextsubheadingwilldiscussboththereactorconfigurationindetailalong
withtheexperimentalprocedure.
6.1 FixedBedReactor
Thefixedbedreactorsetupusedforpyrolysisexperimentsconsistedofacylindricalreactormadeupofstainless
steelcoiledwithtubesallrounditsoutersurface.Thesecoilscarriednitrogengaswhichwasinjectedfromthetop.
Themantle,asshownintheFigure25below,wasthesourceofheating,temperaturerampwasprogrammedupto
500°Candheldconstantuntiltheexperimentwascompleted.
Figure25:Laboratoryscalefixedbedreactor
37
Threepositionswerechosenformeasuringthetemperatureinsidethereactor(oneinthemidofthereactorand
twoon the sides)using thermocouples.Thepyrolysis liquid containing somevolatilematterwas condensedand
collectedbeneaththereactor,withthehelpofacondenser(maintainedat7-10°C)havingparalleltubesinsideand
amixture of ethylene glycol andwater as the cooling fluid. The next stage involved capture of aerosols, using
ElectrostaticPrecipitator(ESP),fromthevolatilegaseousmixturecomingfromthecondensationstage.Anabsorber
columnwasusedwithNaOHsolutiontoremoveHClvapoursfromthefluegasbeforeitwassenttoagasanalyser
andsubsequentlyouttotheatmosphere.ThechangeinpHofthissolutionindicatedthepresenceofHClinthegas
streamenteringthecolumn.
Figure26:Reactoroutsideandinsideview
6.1.1 ExperimentalProcedure
PyrolysisoftheSewagesludge(Klärschlamm)orofWheatStraw(Weizenstroh)wascarriedoutwithandwithout
theuseofasorbent.ThesorbentsusedinthisstudywereSodiumHydrogenCarbonate(NaHCO3)andCalciumoxide
(CaO),variedwithrespecttothemolarratioof feed/sorbent.Thetemperatureofthereactorwasmaintainedat
500°Candthetemperatureoftheovenwas670°C.Thefeedstockineachexperimentwasweighedto100gprecision,
mixedwithsuitableamountofsorbent(basedonmolarcalculations)andthenintroducedtothereactor.Awashing
solutionofNaOH0.1Mwasusedintheabsorbercolumn,measuringitsinitialpHvalue.Theemptycondensateglass
bottlewasconnectedtotheendofthecondenser.Thecoolingsystemwasswitchedonforthecondenserwiththe
38
temperaturesetas9°C.Thenitrogenconnectiontubewasscrewedtightlyandtheflowratecontrolledasdesired,
generally5 litersperminute(L/min).Thevoltagevaluefortheelectrostaticprecipitatorwasnotmorethan7kV
initially,butoncethegasstartedfillingitcouldbeincreasedtoamaximumof14kV.Afterthereactorreached500°C,
itwasallowedtorunfor15-20minandthentheovenshutoff.ThepHchangeoftheabsorbercolumnsolutionwas
monitored.Aftertheexperimentalsetupcooledovernight,thefilledcondensatebottleandthecharwereweighed
forthenecessarymassbalance.
6.2 ScrewPyrolysisReactor(STYX)
ThebenchscaleScrewPyrolysisReactorwithintegratedhotgasfiltration(STYX2)wasdevelopedbytheInstituteof
TechnicalChemistry(ITC),KarlsruheInstituteofTechnology(KarlsruherInstitutfürTechnologie),inGermany.Itwas
usedinthestudyofintermediatepyrolysisoflowgradebiogenicresiduesinanisothermalreactionenvironment.It
consistedof a feeding system, through screw reactor, char collectiondrum, condensationunits andgas analysis
systems.Thesignificanceofusingascrewreactoristhatitenablesthefeedtohaveawell-definedresidencetimeby
virtueofitsdesign.Thefeedingzonewaspurgedwithnitrogensothatoxygenwasremovedfromthebulkofbiomass
feed. The reactor contained twomain units, first the screw conveyor and second the sequential extraction and
filtrationunit.Thermocoupleswerelocatedthroughoutthelengthofthereactoratfixedpositionstomeasurethe
temperatureandcontrolitwhenrequired.Sincethereactorassemblyhasafiltrationunit,therawvapoursgenerated
wereextractedfromthereactorandthesolidparticlescollectedseparatelyaschar.Afterfiltration,thecleangas
mixturewasextractedthroughapipe,whichwaspositionedinsidetheoventomaintainthesametemperatureas
the reactor. This is necessary to avoid any possibility of condensation or cracking of the pyrolysis vapours. The
condensation assembly consisted of two parallel condensers maintained at 15°C and an ESP (Electrostatic
Precipitator), which provided capture of aerosols and thus removed the last residues of tar. The flow rate,
temperatureandabsolutepressureofthepermanentgaswasdeterminedbyusingaflowmeter,athermocouple
and a manometer, respectively. A gas analysis device by Swedish company, ABB, was used to determine the
composition of the permanent gas in volume% (methane, carbon-dioxide, carbon-monoxide, oxygen). For this
purpose,asmallpartofthegaswaswithdrawnassampleandpumpedintotheassembly.Thestreamofpermanent
gaswassenttoatorchviaasuctiontrain.
ThereactorwasconstructedofsteelalloyEN1.4571andcouldhandleupto15kg/hofsewagesludge(thermalinput
45kWTH).Isothermalconditionsinsidethereactorweremaintainedandcontrolledbythermocoupleslocatedonthe
bedofthescrewandinthefilterunits.Thereactortemperaturewasheldconstantwiththetemperaturecontrol
system that controlled theheatingelements fora totalheating capacityof40kW.The reactorwasdivided in7
segments. Themaximumnumber of filtration elements could be 14, in this study therewere 6 filter cartridges
2STYXstandsfortheriveroftheGreekmythology.ItistheriveroverwhichCharon,theferryman,transportsthedeathsoulsfromtheEarthtoanewlifeintheUnderworld.
39
presentinthe2,4and6segmentsofthereactor.Thedescriptionofindividualvitalcomponentsoftheexperimental
apparatusisdescribedinthenextsections.
ThespecificationsofthereactorassemblyaretabulatedbelowinTable14:
Table14:ReactorSpecifications
HeatedLength 2m
Diameter 0.15m
ResidenceTime(inthisstudy) 7.5min
Numberoffilterelements 14
Numberofsegmentsinthereactor 7
Temperature 500°C(maximum600°C)
Flowrate(inthisstudy) 2kg/h(maximumis10
kg/h)
Lengthoffilterelements 200mm
Diameteroffilterelements 60mm
MaximumElectricPower 40kW
RelativePressuredrop Ca.-2….-20mbarrel.
Figure27:FlowdiagramoftheBenchscalePyrolysisReactor(STYX)
40
6.2.1 FeedingSystem
Theplantwasdesignedforprocessingdifferenttypeandgradeofbiomass.Inthisstudy,SSandWSwereused,which
havedifferentcomposition,particlesizeandmoisturecontent.Hence,thefeedingunitconsistedofalock-hopper
andadosingscrewconveyor.Thefeedingsystemwasprovidedwithfourindependentinlets,whichenablesinjecting
multiplefeedstocksandadditives(inthiscase,sorbent).
Thelock-hopperisthecrucialelement,consistingoftwomixers.Thefirstonehadaverticalaxisandwasattached
totheplugsuchthatitwasdirectlyintroducedinthelock-hopper.Thesecondonewasplacedatthesamelevelof
thedosingscrewwaslocatedinthechamberbehindthelock-hopper.Thesemixersweredrivenbyelectricmotors
foratotalpowerof100W.Thescrewconveyorfunctionedasdosingdeviceanddidnotcontainashaft.Thedosing
protocol(correlationbetweenrotationspeedandflowrate)waspreviouslycalculatedandwasprovidedbysetting
therotationalspeedofthemotordrivingthescrewconveyor
6.2.2 SequentialExtractionandFiltrationunit
ThepresenceoffiltersinsidethereactorwasoneofthestrikingfeaturesoftheSTYXused.Theinclusionofsucha
filtrationhasadvantageslikeparticlefreecondensates,whichmakesthemstableandlesspronetoageing.Also,it
avoidsthecloggingandfoulingofthepipelines,whichisamajorconstraintinthermalprocessingofbiomass.
The filtrationunitwas locatedabove the screwconveyoranddirectly inside the reactor. Itwasdivided in seven
segmentsandeachsegmentaccommodated two filter candles. Thehotgas filtrationunitwasequippedwithan
automaticonlinere-cleaningsystemtomaintainsuitablepressuredropamongthefilters.Thefiltercandleswere
constructed with a coarse-grained support made of silicon carbide associated with fine alumino-silicate filter
membrane;candleshadtwoopenings:oneforthehotvapoursthatareextractedfromthereactorandtheother
sideworkedastheinletforre-cleaninggas(heatednitrogenwasusedforre-cleaning).ReferringtoFigure28,there
weretwogascollectionsections:oneforthecleangasandtheotherforthenitrogenusedtocleanthefilters.The
filter candleswere placed perpendicular to the screw axis. The raw pyrolysis gaswas sucked from the reaction
chambertothefilterswhilethecleanedvapoursweresenttothecondensationunit.
41
Figure28:SchematicdiagramofHotgasFiltrationAssembly[9]
6.2.3 CondensationAssembly
Thecondensationunitconsistedoftwocondensersplacedinseries,leadingtoanElectrostaticprecipitator(ESP)at
theend.Asthemixturecomingtothecondenserhasrecoverablecomponents,thecondensationoccuredintwo
stages.Inthefirststage,heavyoilandtheaqueouscondensatewererecovered,theywerecollectedinaglassbottle
locatedoutsidethereactorasshowninFigure29,whileinthesecondstagethevapourswerecooleddownto15°C
andmovedtotheElectrostaticprecipitator(ESP).Thefunctionoftheelectrostaticprecipitatorwastoremovethe
aerosolsfromthenon-condensablegases(alsoknownaspermanentgases).Thecondensatesgetnaturallyseparated
inorganicandaqueousphases.Insomecases,whenitisdifficulttoseparatenaturallytheoilandaqueousphases,a
decantercouldbeused.
6.2.4 OnlineGasAnalysis
AftertheESPunitthegasenteredtheanalysisunitwherethecombustioncalorimeterCDW200wasusedtomeasure
density,WobbeindexandtheLHV;BINOS100-MmeasuredthevolumetricconcentrationsofCO,CO2;OXYNOS100
wasusedtomeasuretheconcentrationofmolecularoxygen(O2).Theconcentrationofhydrocarbons,particularly
methane,wasmeasuredbythegasanalysisfacilityusingaflameionizationdetector(FID),Agilent/HP6890GC,for
theprecision.
6.2.5 ExperimentalProcedure
Thesewagesludgewasfedthroughthefeedingunitandittransportedwiththescrewsinsidethereactor.Themass
flowrateofthefeedwassetto2kg/hinalltheexperiments.Themassflowrateofthefeedstockwasdetermined
beforeconductingtheexperimentsbasedonthedosingprotocol.About4kgoffeed(sewagesludgeorwheatstraw)
wastakenwithsuitableamountofsorbenttobepyrolyzedatatemperatureof500°C.SodiumHydrogencarbonate
(NaHCO3)wasusedasasorbenttoconductexperimentsusingsewagesludgeandwheatstrawandwasfedwiththe
42
biomassaftermanualmixing.Thenitrogenflowratewassetto6litresperminuteandthescrewrotationspeedset
accordingtothedosingcurve.Thefeedmovedalongtheaxiswiththescrew(whichprovideswelldefinedresidence
timeofthesolids)byadjustingitsrotation,leadingtoaresidencetimeof7.5minutes.Theamountofcharcollected
wasweighedwhilethecondensateswereseparatedandweighed.Thecompositionanddensityofthepermanent
gasesweremeasuredonline,sampleswerealsotakentobeanalysedbyGasChromatography(GC),forgettingthe
detailedgascomposition.
Figure29:Benchscaleexperimentalsetup(actualpicturesfromKIT)
6.3 FeedstockProperties
a)Sewagesludgewasthefeedofprimaryfocuswhilewheatstrawwasusedforcomparisonpurposesasitcontains
asignificantamountofchlorine.Theultimateandproximateanalysisofbothfeedsweredoneandareshownin
Table15andTable16below.ItcanbeclearlyseenthatthesewagesludgehasahighcontentofNitrogen,Sulphur
and Phosphorous, while the ash was rich in Silicon, Calcium and Phosphorous present as SiO2, CaO and P2O5
respectively.Chlorinecontentwasalsocomparativelyhigh,andit,arisesfromthetreatmentanddisinfectionprocess
ofthewastewater.Thelowheatingvaluewasattributedtoahighashcontentandlowfixedcarbonofthesewage
sludge[89].
43
Table15:UltimateandProximateanalysis,HeatingValueofdriedsewagesludge
UltimateAnalysiswt%ondrybasis Halogensmg/kgondrybasis
C H N S Cl F
30.10 4.27 4.95 1.32 1260 299
ProximateAnalysiswt%asreceivedbasis HeatingValueKJ/Kgasreceivedbasis
Moisture Ash(550°C) VolatileMatter FixedCarbon HHV LHV
10.30 39.40 47.60 2.80 11708 10630
Table16:AshAnalysisofsewagesludge
AshComposition,wt%ofashondrybasis
SiO2 CaO Na2O K2O Al2O3 MgO SO3 P2O5
29.7 13.30 0.40 1.30 10.40 2.10 5.40 15.50
TheGermanlaw(AbfKlärV2010)specifiesthepermissiblelimitofvariousmetalsinsewagesludge.Itisclearfrom
theTable18below that theamountofChromium,Copper,ManganeseandNickelwashigher than theallowed
standards.Therestofthemetalswerebelowthedesiredrequirements.Extensiveandelaborateresearchhasbeen
done on phosphorous recovery from sewage sludge and according to German Sewage Sludge statistics a large
amountofinorganicphosphorouscanberecovered[37].
Table17:EstimatedPhosphorousRecyclingPotentialinGermany
NatureofsewageSludge EstimatedPhosphorousRecovery,tonnes/year
IndustrialSewage 15,000
MunicipalSewageSludge 50,000
Manure 444,000
EstimatedPhosphorousDemandinGermany 170
44
Table18:MetalconcentrationinthisstudyandcomparisonwithGermanpermissibleamount[37]
Metalname SewageSludgeusedfor
thisstudy(mg/Kg)
AllowedinSoil(mg/Kg) AllowedinSewage
sludge(mg/Kg)
Lead 61 40-100 150
Antimony 7 - -
Copper 390 20-60 800
Mercury 0.60 0.1-1 2
Nickel 110 15-70 100
Chromium 230 30-100 120
Thallium 0.2 - 1.5
Manganese 740 - -
Cobalt 8 - -
Arsenic 6.3 - 18
Cadmium 1.2 0.4-1.5 3
Tin 20 - -
b)WheatStraw
WheatstrawwasusedforpyrolysisexperimentswithScrewreactorandcomparisonwiththeresultsfromSewage
sludgepyrolysis.ItisevidentfromTable20thattheashfromwheatstrawhadasignificantamountofPotassium,
whichhasbeenreportedtopromotetheformationofKClandcausedepositformationinsidereactors[63,64].The
ultimateandproximateanalysisof thewheatstrawusedas feed for this study (Table19) shows thatchlorine is
presentinsubstantialamountscomparedtoSulphur,onweightbasis.
Table19:UltimateandProximateanalysis,HeatingValueofWheatStraw
UltimateAnalysiswt%ondrybasis Halogensmg/Kgondrybasis
C H N S Cl F
43.60 5.80 0.55 0.04 1420 <10
ProximateAnalysiswt%asreceivedbasis HeatingValueKJ/Kgasreceivedbasis
Moisture Ash(550°C) VolatileMatter FixedCarbon HHV LHV
8.60 11.60 65.50 14.40 15260 13880
45
Table20:AshAnalysisofWheatStraw
AshComposition,wt%ofashondrybasis
SiO2 CaO Na2O K2O Al2O3 MgO SO3 P2O5
72.70 3.70 <0.10 11.30 0.50 1.00 1.30 1.10
Figure30:ClandSinFeed
6.4 ChemicalAnalysisoftheProducts
Samplesoftheproductswerecollecteddependingontheirphysicalstateandanalysismethodtobeused.Forliquids,
thecollectionwasdonesimultaneouslyduringtheexperiment,buttheseparationandanalysiswasdoneafterthe
experimenthasfinished.Thesolidproduct,char,wascollectedinadrum,weighedandsentforanalysisafterthe
experiment.GaseoussampleswerecollectedduringtheexperimentforGCanalysislater.Sewagesludgehasnon-
homogenouscompositionandhence reproducibilityof results isachallenge.Wheatstrawcompositiondoesnot
poseaproblem,butthecharobtainedundergoeschemicalchangesandinturngetsheatedup.Also,thenatureof
bio-oilsobtained fromwheatstrawandsewagesludgediffer inphysicalandchemicalproperties.Bothsolidand
liquidproductsarecharacterizedbyelementalandproximateanalysis.Theliquidproductsconsistoforganicand
aqueousphase,analysedbyGasChromatography-MassSpectroscopy(GC-MS),(Agilent5975CVLMSDwith6890
GC) and pH determination. The solid product, char, was characterised by ash and metal content analysis. The
permanentgaseswereanalysedonlinebyusingagasvolume%detectorandbyGCanalysisbyAgilent/HP6890GC
(TCDandFIDdetectors).
46
7. ResultsandDiscussionInthisstudy,insitusorptionofClandSisexaminedusingtwotypesofexperimentalapparatusforlowgradebiogenic
feedstock (in this case, SS and WS). The basic screening of suitable sorbents was done taking into account
observationsinliterature,thefeedcharacteristicsanddesiredlimitofemissionasperGermanStandards.Theeffect
andperformanceof the sorbentson reducing emissionswas evaluatedbasedon the amountof Cl and S in the
pyrolysisvapoursandchar.
Theexperimentscarriedoutusingtworeactorconfigurations(FixedbedreactorandSTYX)aresummarizedbelow.
Theresultsarediscussedintermsthefollowingpoints:
• Efficiencyofusinginsitusorptionforreductionofacidgasfrompyrolysisvapours.
• Reactorconfigurationanditsinfluenceonthesorptionperformanceofthesorbent.
• Massbalanceshowingdistributionoftheproductsobtainedinboththereactorconfigurations.
• Comparisonofcharobtainedfromusingandnotusingsorbent.
• Compositionofpyrolysisvapoursandpotentialofusingitforgasturbineoperation.
7.1 FixedBedReactor
Experimentsweredividedintwocategories:1.NoSorbentused2.Usingsorbent.Table21showsasummaryofthe
experimentsconductedatafixedtemperatureof500°Cinthefixedbedreactorsystem.
Table21:SummaryofExperimentsinFBRsystem
Feed(Biomass+Sorbent) Amount(Biomass+Sorbent),g Temperature
SewageSludge 100+0
500°C
SewageSludge+NaHCO3 100+(0.3,6,15,30)
SewageSludge+CaO 100+(26.1,52.3)
WheatStraw 100+0
WheatStraw+NaHCO3 100+30
CaOistestedinfixedbedreactoronlywhileNaHCO3wasusedassorbentbothinFBRandSTYXconfigurations.There
aretworeasonsforthisselection,firstistheextensiveliteraturereportedonefficiencyofbothsorbentsandNaHCO3
beingsuperiorinperformance.Secondis,theresultsobtainedduringexperimentsonFBRshowedlowreproducibility
in using both sorbents, but visible efficiency of NaHCO3 for Wheat Straw, in capturing Cl from 50% to 70%
approximately.
47
7.1.1 MassBalance
Themassbalanceofeachexperimentwascarriedout.Duetothesizeofthedesignofthecondenser,therecovery
oftheliquidwaschallenging.Therefore,thecondensateiscalculatedbydifference.Nevertheless,sincethemotive
of these experiments was to test the efficiency of Cl sorption, char and gas analysis were the most crucial
measurements.
OverallmassbalanceofsorbentandnosorbentcasefromFigure31,showsthatthefractionofgasproductsincrease
whensorbent isused.This isadesirable resultas thisgasaftercleaningcanbecombustedandused forenergy
generation.Moreover,theamountofSandClretainedincharallowsforconsideringsorptionasaneffectiveprocess.
Throughouttheexperiments,thebestcasewasselectedforsorbenttofeedmolarratiowhichis(NaorCa):Clmolar
=349.60
Figure31:OverallMassBalanceforSS
48
Figure32:OverallMassBalanceforWSPyrolysis
Theresultsofgasmeasurementsystemwhichgivesonlinevolume%ofCO2producedareshowninFigure33.The
comparison between CO2 release shows that significant amount is contributed by sorbent decomposition. A
comparisonismadebetweentheamountsofCO2releasedwhenCO2wasusedassorbentandwhennosorbentwas
used,showninFigure34.TheamountofCO2releasedinvol%ofP.Gasisfarlessthannosorbentcase.Thisisbecause
oftheoccurrenceoffollowingreaction:
𝐂𝐎𝟐 𝐠 + 𝐂𝐚𝐎(𝐬) → 𝐂𝐚𝐂𝐎𝟑(𝐬)
Hence,majorfractionofCO2iscapturedbythesorbent.Becauseofwhich,whileinthecaseofNaHCO3+SStheshare
ofP.Gasincreasesfrom71.03to95.35g/Kgfeed,inthecaseofCaO+SSitdecreasesfrom71.03to2.53g/Kgfeed,
referfigureFigure37.
49
Figure33:Volume%DistributionofP.Gas,SS+NaHCO3
Figure34:Vol%CO2Released
50
Figure35:IncreaseinP.GasforSSandNaHCO3
Figure36:P.GasdecreaseinthecaseofSS+CaO
51
Figure37:Tabulationofg-CO2releasedperKgfeed
7.1.2 SorbentEfficiencyComparisonforSSandWS
Twosorbents(NaHCO3andCaO)wereselectedforinsitusorptionpyrolysisexperimentswithSewageSludgeafter
careful screening of information from literature. The Sorbent/Feed mass ratio was set based on theoretical
calculationsandtheeffectofchangingthisratiowasexamined.AsshownintheFigure38below,thequantitiesof
sorbentused(NaHCO3) isvariedfrom0.3-30gwith100g(fixed)ofSewagesludge.Yaxisshowstheyieldofthe
desiredelementwithrespecttothechangeofsorbentamountandisdefinedasfollows:
𝐘𝐢𝐞𝐥𝐝𝐨𝐟𝐂𝐥𝐨𝐫𝐒 = 𝐀𝐦𝐨𝐮𝐧𝐭𝐫𝐞𝐭𝐚𝐢𝐧𝐞𝐝𝐢𝐧𝐭𝐡𝐞𝐜𝐡𝐚𝐫, 𝐠
𝐀𝐦𝐨𝐮𝐧𝐭𝐢𝐧𝐢𝐭𝐢𝐚𝐥𝐥𝐲𝐩𝐫𝐞𝐬𝐞𝐧𝐭𝐢𝐧𝐭𝐡𝐞𝐟𝐞𝐞𝐝, 𝐠
ChangingtheamountofNaHCO3used,affectedSandClcaptureindiverseways.ForClthetrendwasnotclearand
wasabruptintheextentofincreaseordecrease.WhileforS,initialincreasefrom0.3to6gdidnotshowanyeffect
butfurtherincreaseto15and30gofsorbentshowedanincreaseinyieldofSulphur.ThisincreaseinSyieldagrees
withliterature[89,90]previouslyreportedforthedrysorbentinjectiontotargetSO2removal.Accordingtosuch
expectations,anincreaseintheratioofNa/SorCa/SshouldincreasetheSulphurcapturebecauseofavailabilityof
moresurfaceareaforadsorption,duetoincreaseinsorbentamount.Accordingtootherstudies[91],Sulphurcapture
isverysensitivetotemperatureandismaximumintherangeof120-175°C, itdecreaseswithfurther increaseof
temperature inaNaHCO3-Ssystem. In-SitusorptionofCl, inFixedBedReactor (FBR)pyrolysisofSewageSludge
showedvaryingefficiency(from80%captureofClinchartoaslowas49%)eventhough3-5repetitionsofthesame
52
experimentwerecarriedout.WhiletheScaptureincharshowedamildincreasefromNoSorbent(NS)casetousing
asorbentcase.WhenNaHCO3wasusedassorbenttheScaptureincharincreasedfrom42%to52%.
Figure38:NaHCO3PerformanceinthecaseofSS
For the clear understanding of Cl capture with increase of sorbent amount, two modes of sorbent use were
considered.One,inwhichthesorbentandSSweremixedmanually,andtheother,inwhichseparatelayersofSSand
sorbentwereplacedinthereactoroneuponanother.Inbothmethods,thetrendwasnotasexpectedandreported
in literature.Thereasonforsuchabehaviour isnotclear.Somepossibilitiescanbetheoccurrenceofsecondary
reactionsbetweenNaHCO3andcharmatrix[47]therebydecreasingtheamountavailableforCladsorption(ithas
beenreported[90,91]thatahighstoichiometricamountofNaHCO3isrequiredforeffectivesorptionefficiencyof
Cl, typically more than 90%), the contacting pattern and flow regime between solid-gas components (low HCl
concentrationinincineratorshavereportedmodificationindesignofreactorsuchthatthecontactbetweenashand
fluegasisenhanced[66].Hence,furtheranalyticalandstructuralanalysisofthecharandliquidproductsneedsto
bedonetounderstandthecause.Sorbent/SSratiowasvariedfrom0.003to0.3 inthecaseoftheFBR-NaHCO3
system,whichshowedabruptandnon-conclusiveresultsforClsorptionprobablybecauseofunevendistributionof
sorbentevenaftermanualmixingortheanalysedcharsampletobenotarepresentativeone.However,Scapture
inthecharwasinitiallyconstantforSorbent/SSratiointherangeof0.003-0.06but,itincreasedsteadilywhenthe
sorbent amount was increased. Literature reports mentioned that in the case of NaHCO3 as sorbent higher
stoichiometricratiosaredesirableastheyshowbetterefficiency.
53
Figure39:StandarddeviationforthecaseofSSandNaHCO3
ResultsobtainedagreedwithliteraturethatthesorptionreactionrateofCaOislowerforacidgasesascomparedto
NaHCO3.Chlorinecapturewasmarginal(69%to72%)whencomparedtoSulphurwhichincreasedfrom41%to76%.
WhenCaO/SSratiowasincreasedfrom0.26to0.52,theScaptureshowedsignificantincreaseasmoresorbentwas
availableforreaction.AnotherimportantresultinthiscaseisaffinityofCaOtowardsCO2capturewhencompared
tonosorbentcaseofSSpyrolysis,Figure34andFigure37
TheSulphurcaptureincreasedsignificantlywiththeamountofCaOincrease,fromapproximately42%to72.This
couldbeattributedtohighaffinityofalkaliandalkalineearthmetalstowardsSulphurcompounds,thealkalimetals
presentinthecharalsocontributetofixingofSulphurbyfavourablereactions[92].ReleaseofSulphurinpyrolysisis
thermodynamically favoured towards formation of reduced Sulphur compounds as the stable ones and not the
gaseous form of S [87]. In the range of 175-500°C most of the organically bound S is released following the
devolatilizationstageandifthesystemtemperaturereachesashighas1000°Cthencharburnoutcontributesto
furtherreleaseofS[47].Sulphurcaptureinbothfeedstockfollowthisreleasepatternmentionedinliteratureand
maximumamountofthereleasedSiscapturedatthepyrolysistemperature.Whenwheatstrawwasusedasfeed
forpyrolysisandNaHCO3usedassorbentintheFBRsystemthechlorineyieldwasvisiblyhigherandthesamewas
thecasewithS,whichisanexpectedresultandhasbeenreportedelsewhere[47,63,93].Thechlorinereleasein
wheatstrawwashigherthanSSandhencethesorbentcancapturemostofitinchar.Thiscausestheincreaseof
precisionwithwhichitcanbedetectedinthecharanalysisdonelater.Whenboththefeedstocksarecomparedin
54
termsofClandScaptureyield,forthecaseofNaHCO3assorbent,WSshowsbetteryieldforyieldofbothClandS
thanSS.
Figure40:PerformanceandStandardDeviation,SS+CaO
IntheFBR-WSsystem,onlyNaHCO3wastestedasthepreviousexperimentsshowedNaHCO3toberelativelybetter
sorbent.TheClcaptureincharincreasedfrom51%to69%,Scapturefrom25%to51%;whichisadecentefficiency
ascomparedtoFBR-SSsystem.
Figure41:PerformanceofNaHCO3incaseofWS
55
Standarddeviation isameasureoftheextentofdeviationordispersionofresultsobtainedfromperformingthe
sameexperimentmultipletimes.Inthiscase,itgivesaninsightontheabruptvaluesobtainedinthecaseofClcapture
forSSusingbothNaHCO3andCaO.Thisdeviationcanbebecauseofanalyticalerrorand inhomogeneity inchar
compositioninthesampletakenforanalysis.AnalyticalerrorcanbejustifiedbythelowcontentofClascompared
toSinthefeedstockwhichischallengingtobedeterminedwithprecisionusingtheavailableinstrumentsforanalysis
Figure42:ComparisonofFeedstockwithrespecttoClandSemissionsandtheirsorption
TheanalyticalerrorcanbejustifiedbythelowcontentofClcomparedtoSinthefeedstock,whichischallengingto
bedeterminedwithprecisionusingtheavailable instrumentsforanalysis(alsothefeedusedis100g inFBRand
approximately2kginSTYX,hencethebetterprecisionobservedinthelattercase).
7.2 STYXExperimentalReactor
Thein-situsorptionofClandSwereinvestigatedatthebench-scalescrewpyrolysisreactorSTYX,adoptingNaHCO3
assorbent.ThisselectionwasacombinationofobservationsfromthesmallscaleFBRexperimentsandtheextensive
literaturepublishedalready.Fourexperimentswerecarriedout(2repetitionofeachone).Theprocessconditions
areenlistedinTable22
56
Table22:ListofExperimentsperformedonSTYX
Feed(Biomass+Sorbent) Amount(Biomass+
Sorbent),g
Temperature Residence
Time(min)
FlowRateof
Feed
SewageSludge 4000+0
500°C 7.5 2kg/hSewageSludge+NaHCO3 4000+(1199.7)
WheatStraw 4000+0
WheatStraw+NaHCO3 4000+(1199.7)
Themassratioof feedtoSorbentwasheldconstantas itwas intheexperimentswithFBRconfiguration. Inthe
followingparagraphs,themassbalanceswillbefirstdiscussed,thenthesorbentperformanceevaluation.
7.2.1 MassBalance
TheresultsofthemassbalanceareshownintheFigure43.Thebalanceiswithrespecttothefeedwhichincludes
both the biomass feedstock and the sorbent. Two sets of experiment were conducted with both the biomass
feedstockseparately.BiomassfeedstockstudiedwereSewageSludge(SS)andWheatStraw(WS).Firstexperiment
wasdonetoknowhowmuchClisreleasedduringpyrolysisoftherespectivefeedstockwithoutsorbent.Thenboth
thefeedstocksweremixedwithNaHCO3andpyrolyzedatthesameprocessconditions.Thedepictionofthemass
balanceoftheexperimentsperformedintheIntegratedpyrolysisSTYXplantaredemarcatedasshowninFigure43
andFigure44.Theyieldsofsolid,liquidandgaseousproductsforbothcasesarecompared;duetoreleaseofCO2
fromthedecompositionofNaHCO3thegasfractionofSS+NaHCO3,ishigherthanthenosorbentexperimentand
leadtoanoverallbalanceof108wt%).Sorbentadditionincreasesthecrackingreactionsbetweenvolatilesandchar
leadingtobreakdownofhigherhydrocarbonstolighterones,eventuallyincreasingtheshareofgas[40].Also,the
increaseinP.GasforSS+NaHCO3andWS+NaHCO3iscompensatedbyadecreaseinthecontentofliquidproducts
(OrganicandAqueousfractions).Thereasonforthedecreaseintheliquidyieldforthecasewhensorbentisused
withboth the feedstocks, isattributed to thepresenceofalkalimetalsandalkalineearthmetals in the reaction
mixture.Thesespeciespromotesecondaryreactions(cracking)ofvolatileswiththechar[38,40,44].Inthecaseof
sewagesludgewithouttheutilizationofsorbent,theyieldofthecharwas52%.While,inthecaseofUtilizationof
the sorbent, yield of char decreased to 49.3%.Wheat straw showed slightly different response towards using a
sorbent.TheincreaseinP.Gasyieldwassignificant(from26.3%to32.6%)whileincreaseintheyieldofcharwas
marginal(36.5%to37).ItshouldbenotedthatsincethepyrolysisgasfromWSexperimentswerenotanalysedby
GC,theyellowcomponentinFigure44representsP.Gas+loss.
57
Figure43:MassBalanceandYielddistributionofexperimentsusingSTYX
Figure44:MassBalanceWSforSTYXexperiments
58
7.2.2 SorbentPerformance
Since release of chlorine compounds, in this study, mainly takes place between 350-550°C [94], Intermediate
pyrolysiswithhotgasfiltrationledtoanincreaseofP.Gasbecauseoflowheatratesandhighresidencetimeas
comparedtofastpyrolysis[95].UsingNaHCO3assorbentwithSewageSludge,increasedthecaptureofClinchar
from83%to93%approximatelyasshowninFigure45.
Figure45:ComparisonofChlorineYieldforSSandWSforSTYX
Asdiscussedpreviously,incaseofFBRClcaptureincharcouldnotbemeasuredwithhighprecisionbecauseofissues
relatedtomixingandsampling(lessClcontent),differentheatingprogramandtemperaturedistributioninthebulk
solid also contributed to thisproblem.Nevertheless, in caseof SSwhichhashighS content, sorbenthas shown
reliableperformanceofin-situsorption(from50%to68%)depictedinFigure46.Thepresenceofalkalimetalsinthe
reactorintheformofNaOHorNaHCO3reducestheemissionofH2Sintwoways.Firstistheoxidationofunstable
aliphaticandaromaticsulphurstomorestablesulphoxidesandsulphonicacidatatemperatureof250°C.Secondis
thefixationofsulphurintheformofin-organicsulfideandsulphateinchar,furtherreducingthereleaseofsulfur
intogasphase[92].Hence,thesereportsexplainthereductionofSinthegasphasewhensorbentisused,inthis
study.
59
Figure46:ComparisonofSulphurYieldforSS
7.2.3 PermanentGasCombustion
Inordertoevaluatethereductionoftheemissionsduetotheimplementationofthein-situsorption,calculationsof
permanentgascombustionwerecarriedout.ThepermanentgeneratedbysewagesludgepyrolysisinSTYXreactor,
wereanalysedusingGCbyAgilent/HP6890GC(FIDandTCDdetectors).ThecompositionoftheP.Gasisevaluated
intodetails.QuantitativeanalysisofN2,H2,CO,CO2andthehydrocarbonsuptoC4waspossible(seeFigure47).
Figure47:Permanentgas,vol%
60
Itwasalsoobservedthatshareofhydrocarbonscontributingtothecalorificvalue,increasedinthecaseofuseof
NaHCO3evenconsideringthatadditionalCO2 is released insuchsituation. (Note:Thepermanentgas is thenon-
condensablepartofthepyrolysisvapours.Uponcombustionofthispermanentgaswegetthefluegaswhichisbeing
analysedaspertheGermanregulations)
Figure48:HydrocarbonshareofthePermanentgasinFigure47
Combustionofpermanentgasusingexcessairaspertheguidelinesmentionedin17.BImSchV[96]limitstheHCl
andSO2emissionstobenotmorethan10and50mg/m3respectively.Italsopreciselymentionstheamountofoxygen
presentinthefluegas.Theoxygencontentpresentinthefluegasshouldbe11%andhencetherequiredamountof
air is re-calculatedbasedonthisvalue.Usingtheseguidelinesandtheequationbelow,combustionscalculations
weredone.GeneralcombustionequationsfortherangeofhydrocarbonsandH2Spresentinthepermanentgasare
shownbelowTable23.
𝐂𝐧𝐇𝐦 +𝐚𝐎𝟐 → 𝐛𝐂𝐎𝟐 + 𝐜𝐇𝟐𝐎
𝐇𝟐𝐒 + 𝟏. 𝟓𝐎𝟐 → 𝐒𝐎𝟐 +𝐇𝟐𝐎
61
Table23:TabulatedvaluesofSO2andHClemissionsfromCombustion
Experiment
Type
SO2emission(mg/m3) HClemission(mg/m3)
Calculatedin
thisStudy
17.BImSchV
regulation
(11%O2)
Calculatedin
thisStudy
17.BImSchV
regulation
(11%O2)
NoSorbent,SS 36044.49 17164.04 231.69 110.33
NaHCO3+SS 24867.20 11841.52 81.90 39.00
InTable23,theemissionsarereportedbothunderstoichiometricconditionsaswellasaftertheGerman17.BImSchV
regulation (11%O2) [96]. Emissions ofHCl are reducedby a factor 3.However, itwas not possible tomeet the
requiredemissionlimits.Nevertheless,thetargetappearsachievablebyimprovingthemixingqualityaswellasby
slightlyincreasingthesorbenttofeedratio.Ontheotherhand,theemissionsofSO2arefarawayfromtherequired
target.AlternativestrategiesfortheremovalofsulphurfromtheP.Gasneedtobeinvestigated.Inconclusion,the
co-feeding of sorbent for the reduction of Cl emission appears a suitable approach; however, optimization is
required.InthecaseofcombinedremovalofClandSfromtheP.Gas,thein-situsorptionapproachshowedlimited
results.
62
8. ConclusionsInthisthesis,PyrolysisofSewageSludge(SS)andWheatStraw(WS)wascarriedoutwithfocusonin-situsorptionof
ClandSusingSodiumHydrogencarbonate(NaHCO3)andCalciumOxide(CaO)assorbents.Thefollowinginformative
remarksandconclusionscanbedrawnfromthestudiesconducted:
a) Use of NaHCO3 and CaO as sorbents for Cl and S capture in the FBR showed varying results and issues of
reproducibility.ThestandarddeviationofClcapture incharshowedthatananalyticalerror is thepredicted
causeofthedwindlingvalues.TherelativeamountofClissmallascomparedtoSintheSSfeedstockwhichgives
risetodecreaseofprecisionwhilemeasurementinasmall-scaleexperiment(100gSSasfeed).
b) Higher stoichiometric ratio of Sorbent and Feed could lead to better capture of Cl and S, literature reports
mentionedthat,inthecaseofNaHCO3higherstoichiometricratiosaredesirableastheyshowbetterefficiency.
However,economicconsiderationsmayalsobeconsidered.
c) CalciumOxideassorbentforbothSSandWSshowedcompetitivesorptionofCO2andacidgases.Thecapture
ofSinthecharusingCaO,wasmoreefficientthanCl,forsewagesludge.Thisisbecauseofthehighamountof
SpresentinSS,whichenablesbetteranalysisofthecontentsinchar.CaOundergoesothercompetitivereactions
forexample,withCO2toformCaCO3atthereactionconditions,whichwasmentionedinliteraturebefore.When
CaO/SSratiowasincreasedfrom0.26to0.52,theScaptureshowedsignificantincreaseasmoresorbentwas
availableforreaction.
d) IntheFBR-WSsystem,onlyNaHCO3wastestedasthepreviousexperimentsshowedNaHCO3toberelatively
bettersorbent.TheClcaptureincharincreasedfrom51%to69%,Scapturefrom25%to51%;whichisadecent
efficiencyascomparedtoFBR-SSsystem.Hence,itwasconcludedthatNaHCO3duetoitssuperiorperformance,
willalsobetestedinSTYXexperimentsforcaptureofSandCl.
e) WhenSewageSludgeandwheatstrawisusedasfeedandSodiumHydrogenCarbonate(NaHCO3)assorbentin
STYX,chlorineremovaltrendisincreasingfromnosorbenttousingasorbent.InthecaseofSulphurwhichis
presentinveryhighamountinSSvisiblyhighSsorptionefficiencywasobserved.Hence,itwasconcludedthat
furtherstudiesmustbeconductedonNaHCO3+SSsystemforSTYXpyrolysisplanttogethigherefficiencyofCl
sorption.
f) CombustionCalculationsofpermanentgasfromSSobtainedfromtwostreams:WithNaHCO3assorbentand
without a sorbent, showed that the values of SO2 and HCl emissions are above the regulation limit. The
limitationsinHClemissionsappearachievablebyoptimizationoftheprocess.TheremovalofSulphurrequires
adifferent strategy. Further componentbalance from theanalysisofbio-oil andaqueousphasecan lead to
betterunderstandingoftheefficiencyofsorbentandthepathwaysofformationofClandScompounds.
63
9. FutureWorkFuture works related to the reduction Cl emissions should follow two main strategies. On the one hand, an
optimizationofthein-situsorptionisrequired:
• Improvementandoptimizationofthesorbent/feedratio
• Evaluationofmoreeconomicsorbents,suchasCaCO3ordolomite.
Moreover, the analysis of the bio-oilwill assess the chance of direct combustion of the pyrolysis vapours as an
alternativetothecombustionoftheP.Gas.InthecaseoflimitedClcontentintheliquids,theincreaseofthemass
flowwouldhelptoachievetheemissionstargets.
ThesecondstrategythatshouldbeinvestigatedconsistsofinvestigatingdifferentapproachesfortheremovalofCl
fromthepyrolysisvapoursorfromtheP.Gas.
Thedryinjectionofthesorbentinthegas-phase(entrainedflowsorption)appearstobeaninterestingalternative
fortheSTYXreactor.Thepresenceofthehightemperaturefilterswillremovethereactedsorbentfromthegas.The
sorbentwillberetainedonthesurfaceofthefiltergeneratinganadditionalopportunityforsorption.Thisapproach
appearsinterestinginboththecaseofdirectcombustionofthepyrolysisvapoursoroftheP.Gasaftercondensation
ofthebio-oil.
InthecaseofcombustionoftheP.Gas,theutilizationofanabsorbentafterthecondensationofthebio-oilmight
alsobeasuitableoption.Inthiscase,bothClandScanbereducedtoverylowlevels.However,anadditionalaqueous
wastestreamisgenerated;therefore,atechno-economiccomparisonofthedescribedoptionsismandatoryforthe
overalloptimizationoftheprocess.
64
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