CO2 Enhanced Steam Gasification of Biomass Fuels-nawtec16-1949.pdf

Proceedings of NAWTEC16

16th Annual North American Waste-to-Energy Conference

May 19-21, 2008, Philadelphia, Pennsylvania, USA

NAWTEC16-1949

16th North American Waste to Energy Conference-May 2008

CO 2 Enhanced Steam Gasification of Biomass Fuels

HeidiC.ButtermanandMarcoJ.Castaldi*

DepartmentofEarthandEnvironmentalEngineering(HKSM)

ColumbiaUniversity,NewYork,N.Y.10027

Abstract

Thecurrentstudyinvolvesanexperimentalinvestigationofthedecompositionofvariousbiomass

feedstocksandtheirconversiontogaseousfuelssuchashydrogen.Thesteamgasificationprocessresultedinhigher

levelsofH 2 andCOforvariousCO 2 inputratios.WithincreasingratesofCO 2 introducedintothefeedstream,

enhancedcharconversionandincreasedCOlevelswereobserved.WhileCH 4 evolutionwaspresentthroughoutthe

gasificationprocessatconsistentlylowconcentrations,H 2 evolutionwasatsignificantlyhigherlevelsthoughitwas

detectedonlyatelevatedgasificationtemperatures:above500 o Cfortheherbaceousandnonwoodsamplesand

above650 o Cforthewoodbiomassfuelsstudied.

ThebiomassfeedstockswerestudiedthroughtheuseofThermoGravimetricAnalysis(TGA),Gas

Chromatography,Calorimetry,AtomicAbsorptionSpectrophotometry(AAS),andtheScanningElectron

MicroscopewithEnergyDispersiveXRayAnalysis(SEM/EDX).Thechemicalcompositionofthevarious

biomassfuelsandtheircombustionandgasificationashresidues,inadditiontothemassdecayandgaseous

evolutionbehaviorwereinvestigatedasafunctionoftemperature.

Thethermaltreatmentofbiomassfuelsinvolvespyrolysisandgasificationwithcombustionoccurringat

thehighertemperatures.Inthegasificationenvironment,whencombustionprocessesareoccurring,gaseous

componentsevolvefromthefuelandreactwithoxygeneitherreleasedfromthebiomassstructureitself,orfromthe

injectedsteamandCO 2 .Thesehightemperaturereactionsareresponsiblefortheenhancedburnoutofthecarbon

(charcoal)structurethatisproducedduringthelowtemperaturepyrolyticbreakdownofthebiomass.Sincethe

lignocellulosicbiomasscomponenttypicallyfoundinU.S.MSWisgreaterthan50%,techniquestoenhancethe

thermaltreatmentofbiomassfeedstockscanalsoaidintheprocessingofMSW.

GasevolutionasafunctionoftemperaturewasmonitoredforH 2 ,CH 4 ,CO 2 andCOforseveralbiomass

fuelsthatincludedwoods,grassesandotherlignocellulosicsamples.Theseincludedoak,sugarmaple,poplar,

spruce,whitepine,Douglasfir,alfalfa,cordgrass,beachgrass,maplebark,pineneedles,bluenoblefirneedles,

pecanshells,almondshells,walnutshells,wheatstraw,andgreenolivepit.TheTGAmassdecaycurvesshowed

similarbehaviorforthewoods,grassesandagriculturalresidues,wheremostofthemasslossoccurredbefore

500oC.Mostfeedstocksexhibited2constantmassstepsthoughseveralexhibitedathirdwithcompletedmassloss

by900 o 1000 o C.Twodistinctmassdecayregimeswerefoundtocorrelatewellwithtwodistinctgasevolution

regimesexhibitedinthecurvesforCO,H 2 andCH 4 .Mostofthemasslossoccurredduringpyrolysis,withthe

remainingdegradationtoashorcharoccurringinthehightemperaturegasificationregime.

Onecharacteristicofbiomasssamplesisthehighlyvariablenatureofthemineralcomposition.SEM/EDX

analysesindicatedhighlevelsofpotassium,magnesiumandphosphorusintheashresidue.Thedevitrificationand

embrittlementofthequartzfurnaceandbalancerodswereattributedtothehighmineralcontentofmanyofthe

biomassfeedstocks,withthehighalkalineoxidelevelsofthegrassesbeingparticularlydestructive.Whilemineral

contentmayexertabeneficialeffectthroughenhancedcharreactivitywiththepossibilityforamorethorough

processingofthefeedstock,thepotentialforcorrosionandslaggingwouldnecessitatethejudiciousselectionand

possiblepretreatmentofbiomassfuels.Amajoradvantageofthermaltreatmentthroughgasificationpriorto

combustionistheabilitytoremovemanyofthecorrosivevolatilesandashelementssuchaspotassium,sodiumand

chlorinetoavertdamagetotheprocessequipment.

1. Introduction

Risingenergydemands,heightened

awarenessofman'simpactontheglobalclimate

system,andtheinherentinstabilityofenergy

geopoliticshaveledtoagreaterawarenessofthe

importanceofdevelopingsourcesofenergythatcan

eitheraugmentthecapacityorreplacetheuseof

fossilfuels.Oneofthemorepromisingrenewable

energysourcesisbiomassfuels.Theycanaddress

bothenergysecurityand,beingacarbonneutral

energysource,environmentalsustainabilityand

corporateaccountability.Thoughcurrentlylessthan

3%oftheU.S.energyproductionisthroughtheuse

ofrenewableenergysources,aboutonethirdofthis

isattributabletowoods,grasses,forestrywastesand

agriculturalresidues.Biomassfeedstockcapacityhas

beencalculatedashavingthepotentialtosupply5%

ofthenation'spowerby2030,whilebiomassderived

fuelsofferevenmorepromiseinbeingabletomeet

20%ofthedomestictransportationdemand(1).

Currentenergystudiespredictadoublingofenergy

consumptionwithinthenext3040years.

Heightenedpublicawarenessoftheburningoffossil

fuelsandtheirconnectiontovisibleevidenceof

climatechangehasledtolegislationlimitingCO 2

emissionsthathavefarreachingimpactinthe

industrial,commercialandpublicsectors.

Asenergyproductivity,efficiencyand

supplybecomemorecriticaltoeconomicviability

andasglobalemissionsstandardsbecomemore

stringent,thecapabilitiesofexistingtechnologies

willneedtoevolvetomeetmoredemanding

engineeringconstraints.Sustainabilityandenergy

securitywilldriveengineeringdesignstoincorporate

moreefficientprocessesandequipment.These

designswillstrivetomeetazeroemissionenergy

conversionpolicyasweheadtowardsazerowaste

atomeconomy.Companiescurrentlyengagedin

energyintensiveindustriesarelookingatrenewable

resourcestechnologiesthatwillenablethemtobe

moreenergyefficientandeconomicallycompetitive.

Manyofthesecompanies,suchasthoseinthe

lumberindustry,havechosengasificationtechnology

forwoodresiduetoenergyandseetheoperational

shiftasalongtermstrategicinvestment.

Globally,economicfactorshavedriventhe

useofindigenousfuelstosatisfytheneedforpower

andenergyinemergingeconomies.Biomasshas

takenamoreprominentroleforuseastransportation

andindustrialfuelsinadditiontothetraditionaluse

forhouseholdcookingandlight.Inthesedeveloping

nations,useofbiomassascookingfuelin

combinationwithcookingstovetechnologytransfer

hasledtodecreasedparticulateemissionsandindoor

COlevels,acorrespondingdecreasedincidenceof

respiratoryinfectionandmoresustainablelocal

economies.Emergingeconomieshavebeen

consideringthealternativeuseofvariousbiomass

sourcesasfuelratherthanfood,andtheimportance

oflandpreservationinitiativesthatcouplebiomass

harvestingwithsustainableagroforestry.Policy

formulationthatincludeseducationofindividualsin

theseemergingeconomiesisessential.Theyneedto

bemadeawareofthebenefitsofalternativelyusing

biomassresiduesasfueltosupportsustainable

energyproductionratherthanexclusivelyfor

traditionalsoilnutrientreplenishment.

Pollutantsfromcombustionandtheir

atmospherictransportisaveryactiveresearchtopic

inthefieldofclimatologicalmodeling.Gasification,

ratherthancombustion,offerstheopportunityto

controlthelevelofthesegaseousandparticulate

emissionsleadingtolowerconcentrationsofsoot

particlesandaerosols.Thesootcreatedfromthe

combustionofdiesel,coalandwoodfuelsresultsin

bothpositivedarkparticlewarmingandnegative

shinyparticlecoolingoftheatmosphere.Particulate

transportandatmosphericchemistryaswellas

aerosolemissionsneedtobeconsideredwhen

developingregionalandglobalscaleclimatemodels.

Biomassburningaerosolswereobservedtobemore

easilyprecipitatedoverAfricathanothercontinents

suchasSouthAmerica.Thisfinding,coupledwith

thefactthatAfricaproducesmostoftheglobal

biomassburningemissions,isessentialindeveloping

localclimatemodels.Heightenedconcernforthe

consequencesofglobalwarmingandthe

consequencesofregionallyproducedbutglobally

dispersedpollutanttransporthasledtotechnology

transfertodevelopingnations.Thishasenabled

themtomoreefficientlycombustfuelsoriscurrently

helpingthemtofindalternativemeans,suchas

gasificationtechnology,toproduceadequatesupplies

ofenergy,powerandheatinaneconomicallyviable

andenvironmentallysustainableway,thatwould

ultimatelybegloballybeneficial.

Comparedtoatypicalfossilfuel,the

complexlignocellulosicstructureofbiomassismore

difficulttogasifyorcombust.Thenatureofthe

mineralimpuritiesinconjunctionwiththepresence

ofvariousinorganicspecies,aswellassulfurand

nitrogencontainingcompounds,adverselyimpacts

thebenignthermalprocessingoftheoxygenated

hydrocarbonstructureofthebiomass.While

combustionofbiomassfeedstocksresultsinfuel

boundnitrogenandsulfurbeingconvertedtoNO x

andSO x ,steamgasificationinvolvesthermal

treatmentunderareducingatmosphereresultingin

fuelboundnitrogenreleaseasN 2 andfuelbound

sulfurconversiontoH 2 Sthatismoreeasilyremoved

bymeansofadsorptionbeds.Unlikecombustion,the

gasificationprocessismoreenergyintensive.

Carefulengineeringoftheprocessisnecessaryifthe

resultistoproduceratherthanconsumeasignificant

amountofenergyorpowerasaresultofthethermal

treatment.

Introductionofsteamasareactantinfluent

durnggasificationhasbeenshowntoenhanceH 2

productionduringthegasificationofavarietyof

fuelsthatincludecoal,biomass,andmunicipalsolid

waste(2,3,4).IntroductionofCO 2 hasbeenshown

(5)toenhancetheCOproductionduringhigh

temperaturesteamgasificationwhiledepressingthe

H 2 andCH 4 .BoththesteamandCO 2 havebeen

showntoincreasethecharreactivity(6,7,8)through

amodificationofthecharporestructureandsurface

activity.Biomassgasificationusingsteamcanresult

inincreasedH 2 concentrationinthesynthesisgas

withhigherconcentrationscorrespondingtohigher

gasificationtemperaturesinthe7001100 o Crange.If

aCH 4 fuelstreamisthepreferredproductthen

carefulmonitoringoftheprocesstooperatebelow

600 o CwouldpermitoptimizingCH 4 production

whileminimizingtheH 2 stream.Ageneral

800 o Cwithasignificantlyhighervalueforthe

residueCH 4 molefraction.Theligninandcellulose

structuralcomponentsofbiomassfeedstockseach

havetheirowncollectionofcharacteristic

mechanismswhosereactionsarecoupledand

influencedbythemineralimpuritiesandthe

interactionoftheCO 2 withthethermal

decompositionproducts.Thethermodynamically

favoredproductsofsteamgasificationatthese

elevatedtemperaturesareCOandH 2 .Biomassfuels

highinlignincontentwereobservedtoproduce

highercharyieldsfollowingpyrolysisuptoabout

450 o C.Throughsubsequentthermaltreatmentat

elevatedtemperatures,enhancedproductionof

gaseousproductswaspossiblethroughinjectionof

H 2 OandCO 2 .Thisimprovedtarandchar

conversiontovolatilestranslatestoeconomicand

environmentaladvantagesthroughlesswasteresidue

fortreatmentandminimizationofhighlyalkalineash

residueresponsibleforcorrosivedamagetothe

processequipment.Largeashvolumedueto

oxidizedmineralsmoreprevalentinthegrassy

feedstocks,whensubjectedtothehighgasification

temperatures,canleadtomeltingandagglomeration

andthusslaggingasaresultofthermaltreatment.

Biomassfeedstocksofferthepromiseof

newsustainablesourcesforchemicals,energyand

power.Nevertheless,thegreatvariabilityin

chemicalcompositionresultsinawidevarietyofgas

evolutionprofiles,heatcontent,andpotentialfor

corrosivebehavior.ThroughtheuseofGas

Chromatography,ThermogravimetricAnalysisand

ScanningElectronMicroscopywithEnergy

DispersiveXRayAnalysis,alongwithchemical

informationthatcancharacterizethedistributionof

lignocellulosicstructuralcomponents,wehave

beguntointerpretthegasevolutionasafunctionof

composition,massdecayandtemperature.

Ultimatelyabetterunderstandingofthethermal

processingofthevariousbiomassfuelswillresultin

amoreinformedpredictionastotheenergy

productionpotentialforthevariousfeedstocks.The

currentstudyexaminedCO 2 injectionasatechnique

toenhancecharburnoutbycreatingmorereactive

charsthatresultinmorecompletethermaltreatment.

Thermalprocessingtechniquesthatcanmore

completelyconvertbiomassfuelsaretransferableto

MSWprocessingsincemorethanhalfofthemassof

municipalwasteisbiogenicinorigin.

2. Experimental Setup

2.1 Gasification System.Figure1showsa

schematicdiagramofthegasificationtestfacilityin

theCastaldiCombustionandCatalysisLabat

ColumbiaUniversity,HenryKrumbSchoolof

Mines.ItconsistsofanInstrumentSpecialists

TemperatureProgrammerInterface/Thermal

Analyzerthat,throughAcquisitionsoftware,can

characteristicofthefuelstreamproducedthrough

gasificationisacleanstreamwithaminimumoftar,

sootandparticulates.Biomassgasificationresultsin

aproducergaswhosemaincomponentsareCO,H 2

andCH 4 .OthervolatilecomponentsincludeCO 2 ,

acetaldehyde,aceticacid,phenol,formaldehyde,

formicacidandacetone.Sincethegasification

processwasperformedunderhighlevelsofN 2

dilution,manyofthesespecieswerenotdetectable

sincetheywerepresentinthepartspermillionrange.

Atgasificationtemperaturesinthe700

1200 o Crange,thedominantreactionsthatgovern

biomasssteamgasificationarethesteamreforming

reaction

C+H 2 O ã CO+H 2

H=+131.3kJ/mol,

(1)

andtheBoudouardreaction

C+CO 2 ã 2CO

H=+172.5kJ/mol,

(2)

inadditiontothepyrolyticcleavageand

condensationreactionsthatbreakdownthebiomass

latticestructure,thedecompositionreactionsofthe

oxygenatedmineralsreleasingO 2 ,theoxidationof

thechar

C+ ď O 2 (frombiomassstructure) ã CO

H=─110.5kJ/mol,

(3)

andthereversewatergasshiftreaction(RWGS)

H 2 +CO 2 ã CO+H 2 O

H=+41.2kJ/mol,

(4)

toreleaseCOandCO 2 asthechardegradesfroma

hydrocarbonskeletontoamineralashresidue.

ThoughH 2 evolutionappearedtocommencefor

manyrunsshortlyafter500 o C,apronounced

depressionintheH 2 concentrationwasnotclearly

discernableuntilafter700 o C.Ameasurable

enhancementofCOevolutionwithCO 2 recycleanda

pronouncedincreaseinCOconcentrationduetothe

Boudouardreactionwasobservedforallfeedstocks.

Thestrongestsignalappearedfollowingpyrolysisin

thegasificationregime.Atmuchhighergasification

temperaturesabove1050 o C,H 2 dissociation

competeswithH 2 formationtendingtodecreasethe

netyieldofmolecularH 2 .Manyoftherapiddrops

ingasevolutionconcentrationswereattributableto

theexhaustionofbiomassatthehightemperatures,

particularlyforthehigherCO 2 ratios.

CharacteristicallydifferentCH 4 behaviorwas

observedforthewoodsandgrassesascomparedto

theagriculturalresidues.Whiletheoptimum

methaneproductionforthewoodandherbaceous

feedstocksoccurredintheintervalbetween500

600 o C,thatfortheresiduesoccurredbetween600

regulatethetemperatureandheatingrateofthe

quartzfurnaceintheDupont951Thermogravimetric

Analyzer.ThecarrierflowconsistsofUHPN 2 and

BoneDryCO 2 whoseflowratesareregulatedby

meansoftwoGilmontGF1060rotameters.Akd

Scientific780100syringepumpfeedsdistilledwater

intoastainlesssteelsteamgeneratorthatproduces

slightlysuperheatedsteam(~110─120 o C)whose

temperatureismonitoredbyanOmegadigitalEtype

thermocouplereadoutpriortoenteringthefurnace.

Thetotalfeedflow(steam+N 2 +CO 2 )was

maintainedat90mL/min.Thevolumetric%CO 2 fed

intothelinevariedfrom050%whilethelevelof

waterintroducedwaskeptatroomtemperature

saturationvalues.Gaseoussteamwasintroduced

intothefurnacebymeansofasidearmthrough

whichflowedthesteamandthatportionoftheN 2

andCO 2 remainingafterafractionwasdivertedfor

purgeflowtopreventdepositionontotheTGA

electronics.Thegasificationprocesswasrunwith

excessH 2 OandCO 2 toensurethatthebiomasswas

thelimitingagentinthesteamgasificationreactions.

Thebiomasssamplesitsinaninertplatinum

pansuspendedoneitheraquartzorceramicrod.Gas

evolutiondataisrecordedasafunctionof

temperaturewiththemassdecayasafunctionof

temperaturedisplayedgraphicallyinrealtimeonthe

TPI/TGAAcquisitionsoftware.Textfilesare

exportedtoaspreadsheetforprocessing.The

incomingreactantsandbiomassproductsof

gasificationexitthefurnaceandenteranicewater

condensationcolumnthatremovesanymoisturefrom

thegasevolutionproductspriortoenteringthe

Agilent3000Amicrogaschromatograph.The

carriergasesforthe4channelmicroGCareUHPHe

andUHPAr.Thechromatographsensitivityand

resolutionisinthesingledigitppmVrange.TheGC

chromatogramsaregeneratedbyAgilentCerity

softwareandthedataisstoredinaspreadsheetfor

processing.

Figure 1.Schematicofexperimentalapparatus

2.2 Methodology of Gasification Testing.The

woodsampleswerepreparedbydrillingcoreswithin

planksofuntreatedwoodtoproducedrysawdust.

EvolvedGasesw/oH 2 O

GCData

Aquisition

GasChromatograph

CoolingWater

H 2 OTrap

Condensation

Column

CO 2

Rotameter

SyringePump

water

N 2

Rotameter

InletThermocouple

Steam

Generator

DigitalTemp .Readout

OutletThermocouple

Steam,N 2 ,CO 2

N 2 ,CO 2

Thermo GravimetricAnaly zer

(TGA)

Gases

exit

furnace

DataAcquisitionandAnalysis

T=f(t)

Variac

CO 2

N 2

TemperatureProgrammerInterface

ThermalAnalyzer

Theherbaceousfeedstockswereairdried,groundby

mortarandpestleandthenballmilled.Theamount

ofmechanicalprocessingwasdonesoastominimize

thechemicaldegradationofthefeedstock.Evidence

pyrolysisbetweenabout275400 o Cduringwhich

timethebiomassstructuralcomponents

depolymerizationandcondensationreactions.With

increasingtemperatureduringlowtemperature

gasificationfrom400600 o C,thelignocellulosic

structureevolvesintoacarbonlatticestructureasthe

pyrolyticcharundergoesdecompositionand

carbonization.Finally,hightemperaturecombustion

reactionsbetween700850 o Ccontinuethechar

burnoutwiththeaidofoxygenbroughtinbythe

steamandoxygenreleasefromthebiomassstructure

itself.By1000 o Callofthebiomasssamplesappear

tohavecompletedtheirmassburnouttoresidual

charandash.SamplesexposedtoCO 2 enhanced

steamgasificationhadlowerresidualcharfractions.

Twodistinctmassdecayregimesthatcorrelatewith

thetwodistinctlydifferentgasevolutionregimescan

beidentifiedinthedecompositioncurvewherethe

transitiontemperatureappearstobeinthevicinityof

400 o C.Themassdecaycurvesareforaslow

gasificationrateof10 o C/minandwouldbe

significantlydifferentasdiscussedbyBrunneretal.

(9)athigherheatingrates.

Thethermaldecompositionofthephenolic

andhighlycrosslinkedligninstructurefollowsavery

differentsetofreactionpathwaysthanthatofthe

polysaccharidecelluloseandhemicellulose

structures.Lignindecompositionbeginsmuchearlier

atabout225 o Candtakesmuchlongeruntilabout

625 o Ctocompletewhereascellulosedecomposition

beginslateratabout325 o Candoccursrapidlysothat

by425 o Cdecompositionisnearlycomplete.The

behaviorofthestructuralcomponentsismanifested

intheresultantdecompositionbehaviorofthe

biomassfeedstock.Thevariousfeedstocksshowed

decompositionratesthatwereintermediatebetween

thatofligninandcellulosethoughwecanexpectthat

residuesorgrasseshighinalkalinecontentmayalso

exhibitacatalyticeffectnotpresentinthe

decompositionofthepurestructuralcomponents.To

understandtheinfluenceoflignincontentonthe

volumeofcharproducedduringpyrolysis,the

thermaltreatmentwashaltedat400 o Cfollowing

pyrolysisbutpriortogasification.Grassesandbarks

thatwerelowerinlignincontentproducedcharsthat

werebetween20and30%lessbyweight.The

distinctionbetweencharvolumesproducedwere

greatestwhencomparingsamplesthatinvolvedno

CO 2 injection.

ofthisdegradationcouldbeseeninthedistinctly

differentgasevolutionprofilesexhibitedbythegreen

pineneedlesascomparedtothedriedpineneedles.

Noneofthegrassesorneedleswerewaterwashed

butusedasreceivedtomaintaintheoriginalmineral

contentofthesample.Onlythebeachgrasssample

neededcarefulinspectionandpretreatmentsothatit

couldbecleanedofallsandgrains.Alloftheresidue

samples,shellsandpits,werepulverizedwitha

coarsedrillbitwhilethericehullsweregroundwith

mortarandpestle.

Thefeedstocksincludedspruce,whitepine,

Douglasfir,poplar,sugarmaple,oak,alfalfa,

cordgrass,Americanbeachgrass,pineneedles,maple

bark,bluenoblefirneedles,wheatstraw,greenolive

pit,pecanshell,almondshell,walnutshell,peanut

shell,ricehullandcottonplant.Thetypicalsizeof

thesamplesrangedfrom2025mg.Theagricultural

residuesgasevolutionrunsusedsamplesthatwere

2730mg.TheDouglasfirsamplewas16mg,

poplarwas25mg,beachgrasswas34mgandthe

largepanbeachgrasssamplewas110mg.The

sampleshavetheirweightbothmeasuredinrealtime

bytheTPI/TAsoftwareand,forverification,each

sampleisweighedonaMettlerscaleaccurateto+/

0.1mg.

ThetotalincomingflowofCO 2 +N 2 +H 2 Ois

maintainedat90mL/min,withthesyringepump

regulatingthedistilledwaterflowrate,and

rotametersregulatingthebonedryCO 2 andUHPN 2

ratessoastomaintainthedesiredCO 2 concentration.

TherelativelysmallsampleintheTGApanandthe

relativelyhighflowrateofthefeedresultin

significantdilutionofthegasevolutionproducts

detectedbythemicoGC.Tomoreclearlyidentify

H 2 ,Arisusedasthecarriergasonthemolecular

sievecolumnusedtodetectH 2 ,CH 4 andCO.The

otherthreeGCcolumnsuseUHPHeasthecarrier

gas.CO 2 concentrationlevelsaredetectedusinga

porouslayeropentubularPLOTUcolumn.GC

samplingoccurredapproximatelyevery4minutes

andtheTPI/TAwassettocontrolthetemperature

rampofthefurnaceat10 o C/min.

3. Results and Discussion

3.1 Physical Observations. Themass

decompositioncurveforrepresentativewoods,

grassesandagriculturalresiduesappearsinFigure

2.Thelargestpercentmasslossoccursduring

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