sustainable landfill foundation - gas modeling - literature review methane from lanfills

LITERATUREREVIEW:METHANEFROMLANDFILLSMETHODSTOQUANTIFYGENERATION,OXIDATIONANDEMISSIONHansOonk April2010

Fabianusstraat127333BDApeldoorn

TheNetherlandswww.oonkay.nl

LITERATUREREVIEW:METHANEFROMLANDFILLSMETHODSTOQUANTIFYGENERATION, OXIDATIONANDEMISSIONFinalreportDate: April2010Author: HansOonk,OonKAY! Incooperationwith:

For: SustainableLandfillFoundation

c/oNVAfvalzorgHoldingPOBox21566ZGAssendelft

Numberofpages: 75ThecopyrightofthisreportlieswithOonKAY!Allrightsarethereforereserved.Fortherightsandobligationsofcontractorandcontractant,pleasebereferredtothegeneralconditionsofOonKAY!,orthespecificconditionsundertheagreementconcludedbetweenparties.Thecontractantisallowedtosubmitthisreporttodirectstakeholders.

Oonkay,2010

Literaturereview:methanefromlandfills

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CONTENTSummaryandconclusions............................................................................................5Chapter1:Introduction................................................................................................8Chapter2:Modelingmethaneemissions..................................................................10

2.1Introduction..................................................................................................102.2Modelinglandfillgasformation...................................................................10

2.2.1General..............................................................................................102.2.2Calculatingmethanegeneration.......................................................122.2.3Availablegenerationmodels............................................................182.2.4Characterizationofgenerationmodels............................................212.2.5Evaluationofgenerationmodels......................................................24

2.3Methanecontent,recovery..........................................................................292.4Methaneoxidation.......................................................................................30

2.4.1Processesofmethaneoxidation.......................................................302.4.2Methodsformodelingoxidation......................................................312.4.3Evaluationofmodelsformethaneoxidation....................................33

2.5Accuracyofmodeledmethaneemission.....................................................342.6Conclusionsmodelling..................................................................................36

Chapter3:Measuringemissions................................................................................393.1Introduction.........................................................................................................393.2Availablemethods................................................................................................40

3.2.1Soilcoremeasurements....................................................................403.2.2Closedchambermeasurements.......................................................413.2.3Micrometeorologicalmethods.........................................................423.2.4Massbalancemethods/Transectmeasurements.............................433.2.5Tracerplumemeasurements............................................................463.2.6Plumemeasurements.......................................................................473.2.7Qualitativeemissionmeasurements................................................48

3.3Evaluationofmethods.................................................................................493.3.1General..............................................................................................493.3.2Accuracy............................................................................................523.3.3Equipment.........................................................................................543.3.4Constraints........................................................................................54


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3.3.5Costs..................................................................................................553.4Measuringmethaneoxidation.....................................................................55

3.4.1Introduction......................................................................................553.4.2Samplingmethods............................................................................553.4.3Analysisandinterpretation...............................................................56

3.5Conclusionsmeasuringmethaneemissionsandoxidation..........................573.5.1Methaneemissions...........................................................................573.5.2Methaneoxidation............................................................................57

Chapter4.Estimatingemissionsbasedonrecoveredamountsofmethane.............594.1General.........................................................................................................594.2Prerequisites.................................................................................................594.3Application....................................................................................................59

Chapter5:Improvingqualityoflandfillmethaneemissioninventories....................615.1Qualityofanemissioninventory..................................................................615.2Improvingmethodstoquantifylandfillmethane........................................62

5.2.1Harmoniseandimprovemethaneemissionmodels........................625.2.2Improveandvalidatemeasurementmethods..................................635.2.3Defineatieredapproach..................................................................645.2.4Knowledgetransfer...........................................................................65

5.3Impactofimprovements..............................................................................65Symbols......................................................................................................................67References..................................................................................................................68


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SUMMARYANDCONCLUSIONSMethanefromlandfillsisconsideredamajorsourceofgreenhousegases,bothinEUandworldwide.Emissionreductionfromlandfillsisamongstthemostfeasibleandcosteffectivemeasurestoreducegreenhousegasemissions.TheEuropeanLandfillDirectiveobligesEuropeanlandfilloperatorstoreducemethaneemissions.TheEPRTRregulationrequireslandfilloperatorstoreportmethaneemissionsannuallytothecompetentauthorities.Landfilloperatorscanquantifymethaneemissionsusinganemissionmodeloranappropriatemeasurementmethod.Differentemissionmodelsgiveverydifferentresults,evenwhenthesamedataareentered.Emissionsmeasurementmethodsaregenerallyconsideredinsufficientlyaccurate.Thisisnodesirablesituation,sinceitishardtoassessboththeimpactofmeasurestakenbylandfilloperatorsandpoliciesdevelopedbyregulators.TheSustainableLandfillFoundation(SLF)iscommittedtominimizationoflandfillmethaneemissions.Sinceatthemomentbothmodelapproachesanddirectmeasurementofemissionsarenotyetconsideredaccurateenough,SLFcommissionedOonKAY!toperformacomprehensiveandcriticalreviewwasperformedofbothavailablemodelsandmeasurementmethodswiththefollowingobjectives: Aliteraturereviewonmethodsforquantificationofannualaveragemethane

emissionsfromanindividuallandfill. Evaluationofthemethods,a.o.whetherthemethodsmeettheminimumstan

dardsasdescribedinIPCCorEPRTRguidancedocuments. Discussionofoptionsforimprovementandpotentialdirectionsforharmoniza

tion.C O N C L U S I O N S O N M O D E L L I N G Methaneemissionscanbecalculatedfrommethanegeneration,methanerecoveryandoxidation.Thereareseveralmodelsavailablethatdescribegeneration,suchastheIPCCmodel,theTNOmodelandGasSim.TheFrenchEPRTRmodelismuchsimplerandmightbejustaseffective.ThesemodelswillproducereasonableresultsforMSWdominatedbyhouseholdwaste,landfilledinWesternEurope.TheaccuracyofthesemodelsforothertypesofwasteorindifferentregionsinEuropeislimited.Oxidationismoredifficulttodescribe,thanmethanegeneration,duetothescarcityofavailableinformationonactualoxidationunderfieldconditions.TheIPCCdefaultvalueof10%seemsalowguess,leavingroomforimprovement.Modeledapproachestoestimatemethaneoxidation,basedona.o.toplayerdesignandclimateconditionsareindevelopment.Howevertheseapproachesstilllackfullscalevalidation.

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Intheend,modeledmethaneemissionsarehighlyuncertain,duetoapropagationoferrors,whichishighlyunfavorable.Errorsof2535inmethanegenerationresultinerrorsof6585%inmethaneemissions.Anidealmethaneformationoremissionmodeldoesntexistandeachofthemodelshastheirprosandcons.Combiningstrengthsofdifferentmodelscouldbeonewayforharmonization(seebelow).C O N C L U S I O N S O N M E A S U R E M E N T S Themaindifficultyinmeasuringmethaneemissionsfromlandfillsisthespatialandtemporalvariabilityofemissions,incombinationwiththesizeofamodernlandfill.Severalmethodsaredevelopedandtestedtomeasuremethaneemissionsfromlandfills.Howeveratthemomentthereisnosinglemethod,thatiswidelyrecognizedasthepreferredmethodtomeasureannualaveragemethaneemissions.Closedchambermethodsarethemostfrequentlyappliedmeasurementmethod.However,thereisagrowingagreementthattheytendtounderestimateemissions,evenwhenprescribedproceduresarefollowedforgridwisemeasurementsandapplicationofgeostatisticalmethodsforinterpolation.The1Dmassbalancemethodandboththemobileandstaticplumetracermeasurementsaremethodsthatpromiseacceptableaccuracyatrelativelowcost.Claimedaccuracyofmethodsisintheorderofmagnitudeof25%,ontheconditionsthatthemeasurementstayswithinthepredefinedconstraints.Howeverthisclaimisquestionable.Itrequiresmoremeasurementintercomparisonsandmeasurementsinsituationswithcontrolledmethanerelease,toconfirmthatthisaccuracycanbeclaimedwithconfidence.Formeasuringannualaverageemissions,daytodayandseasonalvariationshavetobedealtwithand4to6onedaymeasurementswillberequired.Themostaccuratemethodtoquantifymethaneoxidationismeasurementandinterpretationof13Cintheplume.Alsothismethodisatdiscussionandmostrecentinsightsindicatethatitmightunderestimatemethaneoxidation.1Dmassbalancemeasurementsmightbeanalternative.HoweverboththemeasurementofCH4andCO2emissionsusingthistechniqueandtheestimationofmethaneoxidationfromashiftinCH4/CO2ratioisnotwidelyacknowledgedasareliablemethod.I M P R O V I N G M E T H O D S Ingeneral,thequalityofanemissioninventorydependsontheperspectiveforwhichtheemissioninventoryisused.QualitycriteriafornationalinventoriesofgreenhousegasestoUNFCCCdifferfromqualitycriteriafordataonindividualcompaniesintheframeworkofEPRTR.Fordatausedinalegalcontext(e.g.toverifywhetheracompanycompliestoitsemissionlimits),againdifferentqualitycriteriaexist:intheendtheyhavetobeconvincingincourt.

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Methodstoquantifymethaneemissionsmightbeimprovedby Modelharmonizationandvalidation.Itisverywellpossibletotakethe

strengthsofeverymodelandcombinethemintoaharmonizedversion.Thedegreeofcomplexityofsuchamodelshouldbeinbalancewithitsexpectedaccuracy.Harmonizationdoesntnecessarilyimplymoreaccuratemodels.Formoreaccuracy,fieldvalidationisrequired;

Improvementandvalidationofmeasurementmethodse.g.bytestingmethodsinsituationswithacontrolledreleaseofaknownamountofmethane.Improvementshouldalsoimplycostreductionandproliferation,a.o.bytakingmethodsawayfromtheresearchphaseandhandknowledgeovertospecializedcompanies.

Definitionoftieredapproachesforquantificationofemissions,allowingemissionmeasurements,ratherthanmodelingemissions.Landfillownersshouldbeallowedtoapplyhighertieredmethodstoquantifyemissions;

Transferofknowledgeofbothmodelingandmeasuringmethaneemissionstolandfillowners,nationalgovernmentsandlocallegislativeauthorities.

AharmonizedmodeloratieredapproachwillbeacceptableforEPRTR.ForapplicationinmakingnationalestimatesandreportingthemtoUNFCCC,suchamodelshouldbethoroughlyvalidated.Whenmethaneemissionlimitvalueshavetobeenforced,modelsandtheirinaccuraciesshouldbethoroughlyvalidated.Whenemissionlimitvaluesaretobeenforcedbymeasurement,methodsshouldbeacceptedbetweenpeersandtheaccuracyshouldbewellassessed.Testingmethodsincontrolledreleasetestsundervaryingconditionsseemstobeastrongtoolinthis.


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CHAPTER1:INTRODUCTIONWhenwasteislandfilled,theorganicfractioninthewaste(allmaterialsfromabiogenicsource,suchasfoodandgardenresidues,textiles,paper)slowlydecomposes.Inthisprocess,landfillgasisformed,amixtureofmethaneandcarbondioxide1.Theemissionofmethanecontributessignificantlytogreenhousegasemissions.TotalEuropeanemissionsareestimatedtobeabout2%oftotalgreenhousegasof5000Mtonperyear(EEA,2009).LandfillmethaneemissionsareconsideredoneofthemaindrawbacksoflandfillingofsolidwasteandabatementofmethaneemissionsfromlandfillsisanimportantdriverforcurrentEUWastepolicy.Inthelastdecade,attentiontomethaneemissionsfromlandfillshasgrownsignificantly.Effortsofbothnationsandindividuallandfillsarecloselymonitored.NationalauthoritieshavetheobligationtoquantifylandfillmethaneemissionsandsubsequentlyreportemissionstoUNFCCC.IndividuallandfillshavetoreportemissionswithintheframeworkofEPRTR.P R O B L E M Tofulfillreportingobligationsasdescribedabove,severalmethodsaredevelopedtoquantifyannualaveragemethaneemissionsfromlandfills.SeveralmodelsaredevelopedintheframeworkofthereportingobligationstoUNFCCCandEPRTR.Howeverforanindividuallandfilldifferentmodelsresultinemissionestimatesthatarehighlyvariable.Soatafirstglance,modelsdontseemreliableandaccurateenoughtoenforcelimitvaluesformethaneemissions.Analternativetomodelingismeasuringemissions.Forthispurpose,severalmethodsaredevelopedandtestedinthepasttwodecades.Butatthemomentthereisnoagreementonwhatmethodsarebestapplicable,andnosinglemethodisgenerallyacceptedassufficientlyaccurateandstillcosteffective.Thelackofpropertoolsforestimatingmethaneemissionsisnodesirablesituation.Itishardtoassessboththeimpactofmeasurestakenbylandfilloperatorsandpoliciesdevelopedbypolicy.Asaresult,localmeasuresandnationalpoliciesforreductionoflandfillmethanecouldstillbemoreeffective.TheSustainableLandfillFoundation(SLF)iscommittedtominimizationoflandfillmethaneemissions.Sinceatthemomentbothmodelapproachesanddirectmeasurementofemissionsarenotyetconsideredaccurateenough,SLFcommissionedOonKAY!toperformacomprehensiveandcriticalreviewwasperformedofbothavailablemodelsandmeasurementmethodswiththefollowingobjectives:1Carbondioxideemissionsfromlandfillsstemfromashortcarboncycle.Upongrowthoftheorganicmaterialscarbondioxideissequestratedandthetotalcycleofgrowth,useandfinallydecompositiontakesplaceoveraintervalofseveralmonthstomaximumseveraldecades.Thisisveryshortcomparedtothetimeintervalofgrowth,useanddecompositionofmaterialsfromfossilorigin.ThereforeCO2emissionsfromsuchshortcycleare0bydefinition.

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P R O J E C T O B J E C T I V E S Aliteraturereviewonmethodsforquantificationofannualaveragemethane

emissionsfromanindividuallandfill. Evaluationofthemethods,a.o.whetherthemethodsmeettheminimumstan

dardsasdescribedinIPCCorEPRTRguidancedocuments. Discussionofoptionsforimprovementandpotentialdirectionsforharmoniza

tion.T H I S R E P O R T Thisreportcontainsthefindingsonthereview.Chapter2givesinformationonmodeledemission.Chapter3describesprogressindevelopingmeasurementmethods.Chapter4givessomeinformationonyetanothermethodtoquantifyemissions,basedonrecoveredamountsofmethane.Chapter2,3and4discussmethods,irrespectiveofthecontextinwhichtheyareused.Thiscontext(UNFCCCEPRTRorenforcingemissionlimits)isofimportanceconsideringpossibleimprovementsinbothmodelsandmethodsinchapter5.Differencesindefinitionofthequalityofamethodresultinslightlydifferentwaysaheadforeachapplication.


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CHAPTER2:MODELINGMETHANEEMISSIONS

2.1INTRODUCTIONMethaneemissionsmightbeobtainedfrommodels.Methaneemissionsaregenerallycalculatedfromthemethanemassbalance:

emissions=generationrecoveryoxidation (eq.1)Wheremethanegenerationiscalculatedas

methanegeneration=LFGgeneration*methanecontent (eq.2)Whenmodelingmethaneemissions,mostofthediscussionsareaboutmodelingmethaneorlandfillgasformation.Therearenumerousmodelsaround,mostofthebasedonafirstorderdecaymodeloramultiphasemodel.Modelingoxidationhasreceivedlessattention.Inmostcases10%ofmethanefluxthroughthetoplayerisassumedtobeoxidized.Howevermorerecentlyotherwaystodealwithoxidationarebeingdeveloped.Thischaptergivesanoverviewofallpartsofthemethanemassbalanceasdescribedinequations1and2.

2.2MODELINGLANDFILLGASFORMATION2.2.1GENERAL Whenwasteislandfilled,theorganicmatterinthewasteisconvertedtolandfillgas.Landfillgasisamixtureofmethane(4560%),carbondioxide(4055%)andtracecomponents(H2S,mercaptanes,organicestersandothervolatilehydrocarbons,allofthemgivinglandfillgasitscharacteristicsmell).Biodegradationoforganicmatterproceedsinanumberofsteps.AgeneraldescriptionofconsecutivestepswasproposedbyFarquharandRovers(1973).Thedegradationoforganicmaterialwasbythemasasequentialprocessofhydrolysisofthesolidorganicmaterials(e.g.hemicellulose,cellulose)intolargersolubleorganicmolecules,subsequentfermentationofthesematerials,yieldingorganicacidsandfinallymethanogenesis.Organicmaterialisnotasinglecomponent,butconsistsofabroadspectrumofmoleculeswithvaryingdegradability.Smallermolecules,suchassimplesugarsandfatsareeasilydegraded.Hemicelluloseisalsorelativelyeasilyconverted,cellulosesomewhatslower,aslongasitisaccessibleforenzymesandbacteria.Ligninhoweverisresistanttobiodegradationunderanaerobicconditions2andlignincanshield2Anaerobic(nooxygenpresent)conditionsareeprerequisiteformethaneformation.Underaerobic(oxygenrich)conditionswastemightbiodegrade,butthisprocessonlyyieldsCO2.

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cellulose,thuspreventingitfrombiodegradation.AccordingtoChandleretal.(1980)arelationshipexistsbetweenlignincontentandthemaximumbiodegradabilityoforganicmaterialunderanaerobicconditions,asindicatedinthefigurebelow.

FIGURE 1: RELATIONSHIP BETWEENFRACTION OF ORGANIC WASTE ULTIMATELY CONVERTED AND THE LIGNINRATION OF THE WASTE (1= WHEAT STRAW, 2 = CORN STALKS, 3 = CORN LEAVES, 4 = PURPLE LOOSESTRIFE, 5 = SEAWEED, 6 = WATER HYACINT, 7 = CORN FLOUR, 8 = NEWSPAPER, 9 = ELEPHANT MANURE, 10= CHICKEN MANURE, 11 = PIGSMANURE, 12 EN13= COW DUNG;CHANDLER ET AL., 1980).Sonotallorganicmaterialcanbeconvertedtolandfillgas.Andinpracticenoteverythingthatcanbeconvertedwillbeconverted,simplybecauseconditionsinpartsofthewasteinhibitbiologicalactivity.Therearemanypossibilitieswhydegradationisinhibited,e.g.becausewasteislocallytoodryorbecausethewastewasfrozenuponlandfillingandtemperaturessubsequentlystaytoolow.Itisalsopossiblethatthewastehasexcesswater,leadingtostagnantsaturatedzonesinthewaste,wherethefirsttwostepsofbiodegradationarefastandresultinadropofpH,thuslimitingmethanogenesis.Sothemethaneformationpotentialisgenerallybasedonthetotalamountoforganicmaterial,correctedfor(i)theamountoforganicmaterialthatdoesnotdegradeunderanaerobicconditionsand(ii)theamountthatdoesntdegradebecauseconditionsarenotfavorable.Thefirstamountisdefinedbythewastecomposition.Thesecondpartisdeterminedbylandfilldesignandoperationandismostlikelyalsoinfluencedbyclimateconditions.H I S T O R Y O F L A N D F I L L G A S M O D E L I N G Attemptstomodellandfillgasformationstemfromtheearly80s.Inthosedaysmethaneemissionswasnotyetrecognizedasapotentialproblem;howeveronewasawareoftheenergeticpotentialofthelandfillgasandeagertoexploitthisalternativeenergysource.Sothefirstlandfillgasformationmodelsweremadetohelpdeterminethesizeoflandfillgasrecoveryprojects:howmuchgasisformed,whatareexpectationsforthenext10yearsandwhichpartofitcanberecovered?

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Sincethemid90smodelingemphasisshiftstoquantificationofmethaneemissions,firstonanationalscale(intheframeworkofobligationofcountriestoreportgreenhousegasemissionstoUNFCCC)andafterwardsaswellonalandfillbylandfillbasis(intheframeworkofEPRTR).Thecallforimprovedaccuracy,transparencyandthedesireforbenchmarkemissions(comparisonofmethaneemissionsbetweennationsorbetweenlandfills,comparisonofUNFCCCreportedemissionsbyacountryandemissionsreportedbyindividuallandfillsinthiscountry)resultedinanumberofemissionmodelsthatcanbeconsideredstateoftheart.2.2.2CALCULATINGMETHANEGENERATIOND E T E R M I N I N G M E T H A N E P O T E N T I A L Themethanepotential,L0,istheamountofmethanethatisproducedthroughoutthelifetimeofthewaste.InmostgenerationmodelsL0istheamongstthemostimportantparameters.Asdescribedabove,landfillgasandmethaneareproducedupondecompositionoforganicpartsofthewaste.Anoftenapproachfordeterminingmethanegenerationfrombiodegradationisbasedon: (CH2O)nnCH4+nCO23 (eq.3)Inwhich(CH2O)nistheapproximatecompositionoforganicmatterinthewaste.Themethanepotentialorthelandfillgasgenerationpotentialisgenerallydescribedasproportionaltotheproductofamountofwastelandfilled(W)andtheconcentrationoforganiccarbon(DOC4,5)inthewaste.Howeverdescribedinchapter2.1,notallorganicmaterialisconverted.Partofit(lignin,cellulosecoveredbylignin)isnotdegradableunderanaerobicconditions.Anotherpartsimplydoesntdegradebecauseconditionsinthewasteareunfavorablefordegradation.SowhencalculatingL0,afactorDOCfisintroducedthatdescribesthepartofDOCthatultimatelyisconvertedtolandfillgas.ThemethanepotentialpertonofwastedependsonthemethaneconcentrationinthelandfillgasandL0isultimatelycalculatedas6:

3Thereactionequationsuggestsalandfillgascompositionof50%methaneand50%CO2.Inrealitymethaneconcentrationsaresomewhathigher,duetobiodegradationofcomponentswithahigherH/Oratio.PartoftheCO2producedisalsodissolvedandreleasedasCO32inthewaterphaseinthelandfill(theleachate).4Afulloverviewofsymbolsusedisgivenattheendofthisreport.5Pleasenotethedifferencebetweenorganiccarbonanddryorganicmatter.DOCgenerallyreferstotheamountofCinthe(CH2O)nanddryorganicmattercontainsabout40%DOC.6Modeldescriptionsseemtodifferinthisaspect,butonacloserlooktheyareallthesame.E.g.IPCCcalculatesmethaneasF*16/12*DOC*DOCf,whichissimilartoequationabove.TNOcalculateslandfillgasformationpotential(inm3hr1)as1,87*DOC*DOCf.AssumingafractionFinthelandfillgasandadensityofmethaneof0,72kg/m3,L0isobtainedofF*1,87/0,72*DOC*DOCf.Afvalzorgdoesntbaseitscalculationonorganiccarbon(DOC),butondryorganicmatter(DOM)andcalculates

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L0=1,33*F*DOC*DOCf (eq.4)InmostmodelsDOCfisconsideredaconstantvalueinbetween0,4and0,7,dependingonthemodel.DOCisgenerallycalculatedonthebasisofwastecomposition,eitherthroughitsorigin(DOCvaluesforhouseholdwaste,industrialwaste,etc.),orfromitsmicrocomposition(%putrescibles,%paper,%textiles,etc.).S I M P L E F I R S T O R D E R D E C A Y M O D E L Landfillgasgenerationisoftendescribedasafirstorderprocessoravariationofthis.Afirstorderdecayprocessimpliesarelativelargeamountoflandfillgasbeingformedimmediatelyafterdeposition,graduallybeingreducedintime.Characteristicofafirstorderdecayprocessisafixedhalftimeoflandfillgasgeneration.Whenahalftimeof7yearsisassumed,methanegenerationafter7yearsis50%oftheinitialgeneration(inkgmethaneperyear),after14years25%,after21years12,5%andsoon.Inafirstordermodelmethanegeneration(CH4f)intimefromaancertainamountofwaste(W),landfilledinasingleyear,isdescribedas:

CH4f=W*L0*kekt (eq.5)InwhichL0isthemethanegenerationpotentialofthewaste,kistherateconstantofbiodegradationandtisthetimeelapsedsincelandfillingofthewaste.Alandfillgenerallyconsistsofwastedepositedinanumberofyears.Methanegenerationfromsuchalandfilliscalculatedasthesumofmultipleequations(asinequation5),eachdescribingthemethanebeingformedfromthewastelandfilledinoneyearofoperation.Eitheraspreadsheetprogramisusedforthiscalculation,orthecalculationispartofalargermathematicalprogram.N.B.Landfillgasgenerationisoftendescribedasafirstorderprocess(oravariationofthis).Fromamechanisticpointofviewitisnot.Inafirstorderreaction(wellknowninchemistryandphysics)areactanthasachancetoreactinthenexthour,dayoryear,andthischanceisindependentoftheamountofreactantstillavailable.Firstorderreactionscanbecharacterizedbytheirhalflife,whichisthetimeinwhich50%oftheoriginalamountofreactanthasreacted.Anexampleofafirstorderreactionisradioactivedecay.Thechancethatamoleculeofplutonium(238Pu)fallsbacktouranium(234Ur)inthenextyearisafixedone.Thehalflifeof238Puisabout88years,independentonactualplutoniumconcentration.Thehalftimeofbiodegradation,t1/2,canbecalculatedfromkthrough:

t1/2=0,693/k (eq.6)I P C C R E V I S E D E Q U A T I O N Aproblemwithfirstordermodelsasdescribedaboveisthatitisanapproximation.Themethodyieldsamethanegenerationforeachyearasadiscretevalue,ratherthanacontinuousdecliningamount.Figure2illustratesthis.Asaresultanunderestimationofmethanegenerationisobtained,comparedtothecontinuouscurve.Thislandfillgasformationas0,75*DOM*DOCf.HoweverwhenassumingDOMcontains40%DOC,theAfvalzorgmodelandTNOmodelareinagreement.

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underestimationisdependingontheassumedvalueofk,isabout3,5%fork=0,1y1andincreaseswhenkincreases.

FIGURE2:ERROR IN CALCULATED METHANE GENERATION IN A CONVENTIONAL FIRSTORDER MODELTheLandgemmodelofUSEPA(Reinhartetal.,2005)minimizesthediscrepancybyapplicationoftheconventionalmodelper1/10thofayear.ThereasonforthisisthatintheUSAmoreandmorelandfillbioreactorsareexpectedtoberealized,wherewastedegradationisenhancedbyleachaterecirculation.Duetothehighkvaluesencounteredhere(Reinhardetal.,2005,expecthalflivesjustinexcessof2y)theerrorintheconventionalfirstordermodelisincreased.ToaccommodateLandgemforlandfillbioreactors,Landgemisadaptedtocalculateper1/10thofayear.IPCC(2006)comesupwithamoreaccurateequation,basedonintegratingtheactualgenerationcurve.Theactualequationismorecomplicatedasequation1andcanbedescribedas:ThediscussionwithinIPCCwas,whetherthedifferencebetweenbothmodelsissuchaproblem,becausetheerrorisrelativelysmallincomparisontoothermodelerrors.Butmoreimportant,firstorderdecaymodelparametersarevalidated(Oonketal.,1994;Vogtetal.,1997)assumingtheconventionalfirstordermodel.UponvalidationavalueforDOCfwasobtainedthatis3,5%higherthantheDOCfthatwouldhavebeenobtainedwhenamoreaccuratedescriptionwasapplied.Sorunningaconventionalfirstordermodel,usingmodelparametersvalidatedforthismodeldoesresultinanaccurateestimationofmethanegeneration.ThisiswhyIPCCconsidersconventionalfirstordermodelsequivalenttotheIPCCrevisedequation.MU L T I P H A S E M O D E L Themultiphasemodelisanotherelaborationofthefirstordermodel(Hoeks,1983).Themultiphasemodeldescribesthate.g.kitchenwastedegradesmuchfasterthanwoodorpaper.Generallyinmultiphasemodelsthreefractionsaredistinguished:fast,moderateandslowdegradingwaste,eachwiththeirownhalftimeofbiodegradation.

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Thefirstapproachassumesdegradationofdifferenttypesofwastetobecompletelydependentoneachother.Sothedecayofwoodisenhancedduetothepresenceoffoodwaste,andthedecayoffoodwasteissloweddownduetothewood.Thesecondapproachassumesdegradationofdifferenttypesofwasteisindependentofeachother.Wooddegradesaswood,irrespectivewhetheritisinanalmostinertsolidwastedisposalsite(SWDS)orinaSWDSthatcontainslargeamountsofmorerapidlydegradingwastes.Inrealitythetruthwillprobablybesomewhereinthemiddle.Howeverthemultiphaseapproachrequiresdetailedinformationofcarboncontentandcarbonqualityofnumerouswastecategories.Usuallyreliableinformationisnotoronlypartlyavailable.Moreovertherehasbeenlittleresearchperformedtoidentifythebetteroneofbothapproaches(OonkandBoom,1995;Scharffetal.,2003)andthisresearchwasnotconclusive.S I M P L I F I E D M O D E L S Anumberofmodelingapproachesexist,thatgivelessdetailthanthefirstordermodeldescribedabove.Ingeneralthesearemodelsdevelopedintheearlydaysoflandfillgasgenerationmodeling,developedforsituationswherelittleornoinformationwasavailableonamount,ageandcompositionofthewaste.Sinceelaboratedfirstorderandmultiphasemodelsarenowsoreadilyavailable,thesesimplermodelscannolongerbeconsideredstateoftheartorgoodpractice(IPCC,2000,2006).Inordertobecomplete,thesemodelsareonlybrieflymentionedhere. Directdecaymodel.Inadirectdecaymodel,thewholemethanepotentialof

thewastelandfilledisassumedtobereleasedatonesinglemoment(IPCC,1996);

Inazeroordermodel,wasteisassumedtoformafixedamountofmethane/landfillgaseitherforafixednumberofyearsorforeternity(e.g.Peeretal.,1992).Thiszeroordermodelwasuntilthemidst1990sfrequentlyappliedindesignoflandfillgasrecovery(Vogtetal.,1997);

Atriangularmodel(Halvadakis,1983)issimilartoazeroordermodel,butcombinedwithalinearincreaseingenerationinthefirstyearandalineardecreaseinthefinalyearsoflandfillgasgeneration;

ASchollCanyonmodelisasimplifiedfirstordermodel(Emcon,1980).Assumingannualamountsofwastedepositedandwastecompositiontobeequalthroughouttheexploitationperiod,asimplifiedequationisobtained.

MO R E F U N D A M E N T A L M O D E L I N G A P P R O A C H E S Asdescribedinchapter2.1,generationofmethanefromorganicmatterinthewasteactuallyprogressesinacomplexorderofreactionsteps,firstenzymesbreakapartthesolidorganicmacromoleculestosmallermolecules,thatarefurtherprocessedmicrobiologically.Anumberofscientistsareworkingonamorefundamentalunderstandingoftheseprocessesandtrytomodeltheoverallkinetics.AnoverviewofactivitiesisgivenbyLamborn(2005).Howeveruntilnow,thesemodelsdonotproducereliableestimatesoflandfillgasgenerationfromrealbatchesofwaste(Beaven,2008).Mostlikelymethanegenerationinactuallandfillsisgovernedbyitsheterogeneityandchanceplaysanimportantrole.


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L A G T I M E Inasimplefirstorderdecaymodelitisassumedthatmethanegenerationstartsimmediatelyafterdepositionofthewaste.Inrealitythisishowevernotthecase.Mostlikelyittakesseveralmonthstoayear(Gregoryetal.,2003;Bergman,1995;KmpferandWeissenfels,2001;Barlaz,2004;IPCC,2006)beforeallmicrobiologicalprocesseshavestartedupandmethanegenerationpeaks.Severalapproachesexisttodescribemethanegenerationinthisinitialperiodmoreaccurately.Themostsimpleapproachisintroductionofalagtime.Methanegenerationisassumedzeroduringacertaintime(e.q.6months)andafterwardsmethanegenerationisdescribedasanormalfirstorderdecayprocess.Inthiswaythereisstilladiscontinuityatthemomentwhenlagtimefinishesandmethanegenerationincreasesfromzerotoitsmaximumvalueinoneday.Thereforeotherapproachesaredevelopedaswell,describingaslowincreaseofmethanegenerationinthefirstmonths,afterwhichfirstorderdecaygraduallytakesover(Findikakisetal.,1988;Keely,1994;VanZantenandScheepers,1995).M E T H A N E C O R R E C T I O N F A C T O R ( M C F ) Methanegenerationonlyoccursinpartsofthelandfillthatarestrictlyanaerobic.Inrealitymanylandfillswillnotbecompletelyanaerobic.Duetoa.o.windactivityandchangesinambientpressurepartsofalandfillmightcontainoxygen,especiallywhenalandfillislesswellmanaged(nowastecompactation,nodailycovers,morethinorpermeabletemporarycovers)andatolderlandfillswhereinternalpressureduetogasproductionisreduced.Inthesepartsmethanegenerationisinhibitedandaerobicdecayoforganicwaste(notleadingtomethane)mighttakeover.Onewaytodealwithaerobiczonesinthewasteistheintroductionofamethanecorrectionfactor(MCF),describingthepartofthelandfillthatisnotentirelyanaerobicandfromwhichnomethaneisgenerated.C L I M A T E C O R R E C T I O N : A M B I E N T T E M P E R A T U R E P R E C I P I T A T I O N ClimateinEuropeishighlydiversandmethanegenerationfromhouseholdwasteinFinlandwillbedifferentfromgenerationinItaly.Methanegenerationisinfluencedbyclimateandmainlybytemperatureandprecipitationandthishasimpactonboththedecayrateofwaste(thehalflife)andamountofmethaneultimatelygeneratedpertonofwaste(L0).Figure3describesclimatezonesinEuropeandwithrespecttomethanegeneration(andalsomethaneoxidation,seechapter2.4)atleast4zonescouldbedistinguished:(i)subarcticandhighland,(ii)humidoceanic,(iii)humidcontinentaland(iv)subtropical.


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FIGURE 3: CLIMATEZONES INEUROPE7Itisknownthatwastetemperaturehasaneffectonthespeedofmethanegeneration(Hartzetal.,1982;Gendebienetal.,1992)butmaybealsoontheamountofmethanebeingformed.Howeverambienttemperaturehaslittledirecteffectonthetemperatureofthedeeperwaste.Buttheremightbeanindirecteffect.Initialstagesofwastedecompositiontakeplaceimmediatelyafterdepositionandmaybeevenalreadyinthebinandduringcollectionandtransportandambienttemperatureinthisperiodmighthavealonglastingeffectonmethanegenerationinthelandfill.Forexample,wasteproduced,collectedandlandfilledintheNordiccountriesinwinterwillbelargelyfrozen.Asaresultinitialstagesofwastedecompositionwillbeseriouslyhamperedandtemperaturesinthewastewillremainlow,comparedtowastelandfilledelsewhereinEurope.Theimpactofmoistureinthewasteonwastedecompositioniswidelyrecognized.Howeveritspreciseimpactisstilltopicofscientificdiscussion.Accordingtosome,wastedecompositionisenhancedbyincreasedmoisturecontentofthewaste,untilanoptimummoisturecontentisreached.Accordingtoothersmovementofmoistureinthewasteisimportant(KlinkandHam,1982).Toomuchstagnantmoistureinthewasteisevenreportedtoinhibitwastedecomposition(OonkandWoelders,1999;Wensetal.,2001).Moisturemovementspreadsmethanogenicactivitythroughoutthewasteandavoidslocalbuildupofinhibitingcomponents.Ifthefirst7Fromhttp://printablemaps.blogspot.com/2008/09/mapofclimatezonesineurope.html


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istrue,wastedecompositionisfavoredbymoisturecontentofthewaste;ifthelatteristrue,wastedecompositionisfavoredbyprecipitation(Alexanderetal.,2005)andamorepermeabletoplayer.Ofcoursethereisacorrelationbetweenmoisturecontentofthewasteandprecipitationseveralmodelssimplyassumeacorrelationbetweenthemoisturecontentofthewasteandthemeanannualprecipitation(ChianandDeWalle,1979;McDougalandPyrah,2003).2.2.3AVAILABLEGENERATIONMODELSThemethodsofcalculatingmethanegenerationinthepreviousparagrapharemeredescriptionshowacertainpotentialofmethaneisreleased.Forapracticalapplication,thesedescriptionsdonotsuffice.Thereforemodelsaredevelopedthatenablecalculationofmethanegenerationinaspecificyearfromlandfilledwaste.Inputparametersinthesegenerationmodelsistheamountofwastelandfilledineachyearofexploitationandinmostmodelsalsoaspecificationofthewaste.ThemodelitselfsubsequentlycalculatesDOC,DOCf,L0andcalculatesthewaythismethanepotentialisreleasedthroughouttheyears.Inthepastyearsanumberofmodelshavebecomeavailable.Mostofthemconsistofaspreadsheetprogram.Someofthemareexecutables8.Themostwidelyappliedmodelsareamongsttheonesreviewedbelow.Somemodels(e.g.Calmin,theFinnishEPRTRmodel)areconsideredinthisevaluation,becauseoftheycontaininterestingfeaturesthatmightdeservefollowup.TheIPCCmodel(tobeobtainedfromIPCC,2010)isdevelopedbyaninternationalteamofexperts,andisintendedtogiveguidancetonationalauthoritiesinthequantificationofmethaneemissionsfromalllandfillsinacountry.Butthemodelitselfcanalsobeusedforindividuallandfills.ThemodelitselfisfreewareandcanbedownloadedfromtheIPCCwebsite.WithintheIPCCprocess,transparencyisofutmostimportanceandthemethodisdescribedindetailbyIPCC(2006).Inputofthemodelisamountofwasteperyearandaclassificationofthecompositionofthewasteinoriginofthewaste(householdwaste,industrialwaste,etc.).Alternativelythemodelalsoallowsforawastecompositionoption,wherewastecanbedefinedin%foodwaste,%paper,%wood,etc.Thechoiceexistsbetweenafirstorderdecaymodel(theIPCCrevisedequation)andamultiphasemodel(alsobasedontheIPCCrevisedequation)andthedefaultlagtimeof6monthscanbeadapted.TheIPCCmodelaccommodatesfor4differentclimateregions:wetborealortemperate;dryborealortemperate;wettropicalanddrytropical.Theclimateconditionschosenaffectthechosenkvalue.TheTNOmodel(Oonketal.,1994)isthefirstmodel,wheremodelparameterswerebasedonrealdataoflandfillgasgenerationatalargergroupoflandfills.MethaneandCO2emissionmeasurementswereusedtovalidatethemodel(Oonketal.,1995,Scharffetal.,2003).Bothafirstorderandamultiphasemodelweremade,thatdescribelandfillgasgenerationasafunctionofamountofwastedepositedfromdif8anexecutable(file)causesacomputer"toperformindicatedtasksaccordingtoencodedinstructions".


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ferentorigin(householdwaste,industrialwaste,etc.).Themodelitselfexistsasapublicationonpaper,butaspreadsheetversionisavailableondemand(Oonk,2010).

FIGURE 4: VALIDATION OF TNOMODEL. COMPARISON OF CALCULATED AND MEASURED LANDFILL GASGENERATION. BLUE DOTS ARE ESTIMATED FROM LANDFILL GAS RECOVERY (OONK ET AL, 1994), OPEN DOTS BLUE DOTSAREFROMEARLY 90SMEASUREMENTS (OONK AND BOOM, 1995), RED DOTSAREFROMEARLY 2000SEMISSION MEASUREMENTS (SCHARFF ET AL., 2003)GasSimLiteisdevelopedbyGolderAssociates(2010)fortheEnvironmentAgencyofEnglandandWales.GasSimquantifiesalllandfillgasrelatedproblemsofalandfill,rangingfrommethaneemissions,effectsofutilizationoflandfillgasonlocalairqualitytolandfillgasmigrationviathesubsoiltoadjacentbuildings.Atthemoment(March2010)GasSim2.1isthelatestversionandiscommerciallyavailable;howeveraliteversion1.5isavailableasfreeware,andisdesignedtohelpoperatorswiththeirpollutioninventory,.GasSimisanexecutableanddefaultvaluesused,algorithmsappliedandassumptionsmadearesomewhatmorehiddenintheprogram.InformationishowevernotconfidentialandstaffofGolderAssociatesarewillingtoprovidemoreinformationondemand(Gregory,2010).GasSimisbasedonUKwastestatisticsandstartsfromhemicellulosesandcellulosecontentinthevariouswastefractions.ForeachwastefractionaDOCfisassumed,basedonresearchbyNorthCarolinaStateUniversity(Gregory,2010).LandgemisamodeldevelopedforandmadeavailablebyUSEPA(2010).Itisafirstorderdecaymodel,withseparatedefaultvaluesforkconventionalregions,aridregionsandforenhanceddegradationcells9.Themostrecentversionofthemodelisthe3.02version,datedMay2005.ThemathematicsofLandgemissometimesdescribedsomewhatconfusinglyas(notetheW/10),9Incellsforenhancedbiodegradation(bioreactors)landfillgasformationisacceleratedbyrecirculationofleachate.

0

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0 500 1000 1500 2000 2500 3000

calculated formation (m3 hr-1)

meas

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CH4fT=(W/10)*L0*f(T) (eq.7)butinthiswaythemodelcalculatesmethaneemissionsper1/10thofayear(forthispurposeTisalsoexpressedintenthsofayear,Reinhartetal,2005).Thereasonforthisistoavoidinaccuracieswhenkvaluesareusedinexcessof0,1y1.TheAfvalzorgmodelisdevelopedbyNVAfvalzorgintheNetherlands.Itisbasedonacombinationofliterature(asaccumulatedinthe2006IPCCmodel)andownexperienceswithlandfillgasgenerationandmeasuredemissionsattheAfvalzorgsitesatNauerna,BraambergenenWieringermeer.Themodelitselfisamultiphasemodelandisintendedtogiveamorerealisticprognosisofmethanegenerationatlandfillswithlittleornohouseholdwastedeposited.Themodelitselfisfreewareandavailableondemand(Scharff,2010).TheFrenchEPRTRmodel(Ademe,2003)isasimplifiedfirstorderdecaymodel.Themodeldescribesmethanegenerationof4.8kg(6.6m3)pertonwasteperyearinthefirst5yearsafterlandfilling;2.4kgpertonwasteperyearthe5yearsafter,1,3kgpertonwasteperyearinthe2nddecadeand0,6kgpertonwasteperyearinthe3rddecadeafterlandfilling.Formoderatelydecomposablewaste(e.g.nonhazardousindustrialwaste;householdwastethatismilledorcomposted),methaneformationis50%ofthesevalues.Themodelisnotavailableasaspreadsheet,butconsistsofasimplefillintable.TheFinnishEPRTRmodel(Petj,2010)isamultiphasemodelwithmodelparametersinlinewiththeIPCCmodelforwetboralortemperateregions.ThemodelitselfiscompletelyinFinnish,whichmakesittoughertoevaluatehereanddifficulttoapplyforlandfilloperatorsoutsideFinland.ThemodelitselfhoweverisinterestingbecausethedefinitionofwastestreamsisbasedontheEWCcodes.Thisconnectstothesystemofwasteregistrationatlandfillsandreducesproblemswithwastedefinition.Calminisnogenerationmodel,howeveritservesasimilarfunctionandforreasonsofclarityitisdiscussedinthischapter.CalminisdevelopedbyresearchersinUSAbyorderofauthoritiesinCalifornia,andquantifiesmethaneemissionsinanewandinterestingapproach.Atthemomentthebetaversionisavailableondemand(Spokas,2010).ThemodelintendstoprovideanimprovedmethodforquantificationoflandfillmethaneemissionsfortheCaliforniagreenhousegasinventory.Calminisnotbasedonthemethanemassbalanceasdescribedinequation1.Insteaditcalculatesmethanediffusionthroughthetoplayerandmethaneoxidationinthetoplayer,ultimatelyyieldingamethaneemission.Thismethaneemissionisafunctionofthetoplayerscompositionandthelocationofthelandfillontheglobe.Forthelatterpurpose,thelandfillscoordinatesaretranslatedintoclimateconditions,processesarecalculatedforeachdayintheyearandsubsequentlyemissionsareaveraged.Weakpointofthemodelistheassumptionthatemissionstakeplacethroughdiffusion.AsaresultthemodelapplicabilityofCalminmightbelimitedonlandfillswherelargepartofemissionstakeplacethroughpreferentialchannels.CalminanditsoutcomearevalidatedinanumberofclosedchambermeasurementsontwoCalifornianlandfills.Howeverasindicatedinchapter3,closedchambermea


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surementscannotbeconsideredareliablemeasurementmethod,sincetheytendtomissmethaneemissionsthroughpreferentialchannels.Inthisrespect,modelandvalidationmethodseemtohavesimilarflawsandtheCalminitselfmightgiveagoodestimateofthepartofmethanethatisemittedthroughdiffusion.2.2.4CHARACTERIZATIONOFGENERATIONMODELSSotherearenumerouslandfillgasgenerationmodelsaround.Howeveralllandfillgasgenerationmodelconsistoftwoparts: adescriptionofthetotalmethanepotential,L0,whichisthetotalamountof

landfillgaswhichisformedduringthelifetimeofthelandfill; afunctionf(t),thatdescribeshowthispotentialisreleasedovertime.Sowhich

fractionofthetotalmethanepotentialisreleasedinthe1st,2nd,3rdyearandsoon.

TotalmethanegenerationinyearT(CH4gT)canbedescribedasfollows:CH4gT=W*L0*f(T) (eq.8)

Thefunctionf(t)isinmostmodelsafirstorderdecaymodel,amultiphasemodeloravariationofthis.R A T E C O N S T A N T O F B I O D E G R A D A T I O N ( K ) , H A L F L I F E O F M E T H A N E G E N E R A T I O N Inmanyevaluationsofmodelparameters(e.g.KhleWeidemeierandBogon,2009),mostattentionispaidtothehalflifeofmethanegeneration(ortherateconstantofbiodegradation,k).However,inmanycases,theoutcomeisnotthatsensitiveforassumedhalflifeorassumingmultiphasedegradation,ratherthanfirstorderdegreedegradation.ThisisillustratedinthemodelcalculationinFigure5.Inthisexample,ofkonmethanegenerationislimitedtoabout20%,forkbetween0,07and0,14(halflivesof5tot1years).Achangeinkonlyresultsinachangeoftimewhenmethaneisassumedtobereleased.Shorterhalflivesorhighervaluesofkimplythatthemethanepotentialisreleasedsomewhatearlier,moreduringadimmediatelyafterexploitation.Longerhalflivesimplyashiftinmethanegenerationtotheperiodafterexploitation.Attheverylowendofk(halflivesassumedinexcessof15years),resultinamethanegenerationthatisbothreducedduringexploitationandalsoafterwards(comparedtotheassumedgenerationwithk=0,1/y).Onlyonaverylogterm,thiswillbecompensatedbyanincreasedmethanegeneration.Studiesindicatingverylonghalflivesofmethanegeneration(e.g.underaridconditions,Atabietal.,2009)shouldbeconsideredwithgreatcare.Inthesecasesreducedmethanegasformationcanbecausedbybothareducedrateofbiodegradationaswellasareducedmethanegenerationpotential(L0,seebelow)andthedifferencebetweenbothcanonlybeobserveddecadesafterclosure.


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FIGURE 5: EFFECT OF ASSUMED RATECONSTANT OF BIODEGRADATION ON METHANE GENERATION (METHANE GENERATION COMPARED TO GENERATION, CALCULATED ASSUMING K=0,1/Y).CALCULATION PERFORMED WITH THE AFVALZORGMODELFOR A LANDFILL WHERESIMILAR AMOUNTS OF WASTE ARE DUMPED DURING 20 YEARS.M E T H A N E G E N E R A T I O N P O T E N T I A L , L 0 Comparedtok,L0(themethanegenerationovertimefromatonofwaste),hasreceivedconsiderablylessattention.TherearetwowaystoquantifyL0.OnewayisadirectestimationofthemethaneorlandfillgasgenerationpotentialinafieldvalidationastheTNOmodelandthemodelbyVogtetal.(1997.Fromalargerdatasetofwastecharacteristicsatonehand,andmethanegenerationattheotherhand,L0canbeobtainedbylinearregression.AssumingavalueforDOC,DOCfissubsequentlyestimated.)Theothermethodistoquantifyitfromequation4,whereDOCgenerallyisestimatedfromwasteanalysesandDOCfisobtainedfromliterature.Mostgenerationmodelsin2.2.2arebuiltthisway.

L0=1,33*F*DOC*DOCf (eq.4)V A L U E S F O R M O D E L P A R A M E T E R S Table1referstohouseholdwasteorMSW.Mostoftheexperienceswithlandfillgasgenerationcomesfromlandfillrecoveryonthistypeofwaste;thelargescalevalidationstudiesbyOonketal.(1994)andVogtetal.(1997)areperformedonthistypeofwaste;mostoftheemissionmeasurementsareperformedonlandfillswithMSW.HoweverinEurope,landfillingofMSWismoreandmorediscouragedandasaresultthenonmunicipalsolidwastebecomesmoreandmoreimportantformethaneformation.Table2describeshowmethaneemissionsfromindustrialwastearehandledbyvariousmodels.ItiswellknownthatindustrialwastecancontainawiderangeofDOC.ExamplesofwasteswithoutanysignificantDOCarewastesfromthesteelindustryorasbestoswastes.Incountrieswherebiodegradablewastesaretoalargeextentbannedfromlandfillsanaverageindustrialwastecarboncontentmaynolongerbeappropriate.TheFinnishapproachdefiningDOCforeachwasteintheEuropeanWasteCataloguecannotbepresentedinTable2.TheEuropeanWasteCata

0

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loguehasapproximately800entries.ThismayneverthelessbeamoresuitableapproachforlandfillsacceptingwasteswithlowDOC.Sincetheyarebasedonextensivefieldstudies,theTNOmodelandthemodelbyVogtetal(1997)mightbeconsideredasbestguesses.TheTNOmodelcanbeconsideredrepresentativeforhumidoceanicregionandthemodelofVogtetal.(obtainedforlandfillsinCalifornia)asbeingrepresentativeforlandfillsinmoresubtropicalconditions,whereconditionsaremoredry,landfillgenerationmightbeinhibited,resultinginlongerhalftimesandultimatelyalsoareducedconversionoforganicmaterialtolandfillgas(henceareducedDOCfandL0).Table1:Comparisonofmodelsformethanepotential(kgmethanepertonwaste)andhalflifesforbiodegradationforhouseholdwasteorMSW

L0 (kg/ton) halflife(year) remarkIPCCmodel 631 1223(slow)2,3

7(moderate)24(fastdegradable)2

MSWEurope

TNOmodel 60 7 DutchHHWGasSim 514 15(slow)

9(moderate)6(fastdegradable)

HHWUK

Landgem 122(CAA)572(inventory)5

14(conventional)635(arid)6

MSWUSA

Afvalzorg 3945 23(slow)7(moderate)3(fastdegradable)

DutchHHW

EPRTR(Fr) 55 510 HHWFranceEPRTR(Fi) 65 23(slow)

14(moderate)3,5(fastdegradable)

HHWFinland

Vogtetal.(1997) 44 17 MSWCalifornia1valueforbulkMSW2valuesforwetborealandtemperateregions.Fordryregionsandtropicalconditionsotherkvaluesaresuggested;3differenthalflivesspecifiedforpaperlikematerialsandwoodlikematerials;4sumofmethaneemissionsinthe1st100yearsafterlandfillingof1tonof19802010100%householdwaste,assumingnorecoveryand0%oxidation,ascalculatedusingGasSimLite5CAAdefaultsarebasedonrequirementsforUSlandfills,asspecifiedintheCleanAirAct.InventorydefaultsarebasedonresultsofaninventorybyUSEPA6aridreferstoregionswithlessthan625mm(25inch)rainfallperyear.Conventionalreferstononaridregions.ComparedtotheTNOmodel,theIPCCmodelandtheFinnishEPRTRmodelhasaboutthesameL0.TheaveragehalftimeofthehalftimesoftheIPCCmodelisaboutthesameasthehalftimeoftheTNOmodel.Applicationofbothmodelswill


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giveaboutthesameresult,whenappliedtoalandfill10.GasSimwillgiveabout20%lessmethanegeneration.Afvalzorgabout3035%lessmethaneemissions.Landgem(inventoryL0andconventionalhalftime)however,hasahigherL0butasubstantiallylongerhalftime.AsaresultinitialmethanegenerationmightbecomparabletoIPCC,TNOorGasSim).Onthelongerterm,Landgemwillmostlikelyoverestimateemissions.TABLE 2: COMPARISON OF MODELSFOR METHANE POTENTIAL AND HALFLIFES FOR BIODEGRADATION FOR INDUSTRIAL WASTE

L0 (kg/ton) halflife (year) IPCCmodel 50 1223(slow)1,2

7(moderate)14(fastdegradable)1

TNOmodel 50 7 GasSim 263 15(slow)


Landgem notspecified notspecified Afvalzorg 3639 23(slow)


EPRTR(Fr) 28 510 1valuesforwetborealandtemperateregions.Fordryregionsandtropicalconditionsotherkvaluesaresuggested;2differenthalflifesspecifiedforpaperlikematerialsandwoodlikematerials;3sumofmethaneemissionsinthe1st100yearsafterlandfillingof1tonof19802010100%industrialwaste,assumingnorecoveryand0%oxidation,ascalculatedusingGasSimLite2.2.5EVALUATIONOFGENERATIONMODELSApartfromtheaforementionedvalidationeffortsoftheTNOmodel(Oonketal.,1994;OonkandBoom,1995;Scharffetal.,2003)andthevalidationstudyofVogtetal.,(1997),therehavebeenseveralotherattemptstovalidateformationoremissionmodels,forexample: EhrigandScheelhase(1999)interpretedrecoveredamountsofmethaneat

Germanlandfill.Onthebasisofthesedatatheysuggestamethanegeneration

10IthastobenotedthattheIPCCmodelendsupatassimilarL0butinadifferentway.IPCCcombinesarelativehighvalueofDOCwithalowvalueofDOCfandF.Inthiswayoneendsupatsimilarmethanegenerationpotential.HoweverthesameDOCandDOCfarealsoatthebasisofanotherimportanteffectoflandfilling:theamountoforganiccarbonthatissequestratedinthelandfill.AsaresultofthesamerelativehighDOCandlowDOCf,IPCCendsupwithamuchhigheramountofcarbonsequestratedase.g.theAfvalzorgmodel.


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ofabout2,5kgmethanepertonwaste11,10yearsafterclosureofthesite.KhleWeidemeierandBogon(2008)reinterpretthesedataandconcludeabestfitisobtainedassuminganL0of80kgmethanepertonwaste12andahalflifeofbiodegradationof3,5to6years.

Fellneretal.(2003)comparesmodeledmethanegenerationwithactualmethanegenerationforlandfillsdescribedinliterature.UnfortunatelytheyonlyvalidateamodelbyTabasaranandRettenberger(1987)withaL0of93kgmethanepertonwasteandahalftimeofbiodegradationof20years.Theconclusionwasthatthemodeloverestimatesgenerationinmostofthecases.

ScharffandJacobs(2005)comparedtheoutcomeofanumberofmodels(a.o.TheTNOmodel,theAfvalzorgmodel,Landgem,GasSimandazeroordermodels)withmeasuredemissionsatthreeDutchlandfills.Forindividuallandfills,differencesbetweenmodelswasenormous(differencebetweenthelowestandhighestestimationwasmorethanafactor10.Inonecaseevenafactor20).AccordingtoScharffandJacobs,thisisinindicationthatcurrentmodelsgivenoreliablemethaneemission.Methaneemissionmeasurementsarealsouncertain;howeverdiscrepancybetweendifferentmeasuredemissionsismuchlessasthedifferencebetweenmodeledemissions.

Fredenslundetal.(2007)compare4generationmodels(Landgem,IPCCmodel,GasSimandtheAfvalzorgmodel)atalandfillsiteinDenmark.Hugedifferencesareobservedbetweenmodels,withhighestgenerationinLandgemandlowestgenerationandlowestgenerationforGasSimandtheAfvalzorgmodel.Alsowithinamodel,resultsarehighlydependingonspecificassumptions.Onbasisofthiscomparison,KhleWeidemeierandBogon(2008)concludeitisquestionablewhethergenerationsmodelsarereliable.

Thompsonetal.(2009)validatedanumberofgenerationmodelsinacomparisonwithrecoveryatCanadianlandfills.Thisarticlehoweverhastobeinterpretedwithcare,sinceitappearstobeerratic13.

Therearealsosomeeffortstovalidatemodelsinveryaridortropicalzones.AlthoughsimilarclimatesarenotfoundinEU,thesestudiesareofinterest,sincetheyillustratetheeffectofclimateonmethanegeneration: Atabietal.,(2009)validateLandgemforalandfillinIraninextremearidcondi

tionsinacomparisonwithrecoveredamountsoflandfillgas.Landgem,assumingarateconstantofbiodegradationof0,02/y(ahalftimeofbiodegradationof35years)gavegooddescriptionoflandfillgasgeneration.Asdescribedinchap

11Interpretation:EhrigandScheelhaseconcludegasformationtenyearsafterclosureofthesite


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ter2.2.3,itislikelythatreducedlandfillgasformationhereistheresultofbothareducedmethanegenerationpotential(L0)andareducedrateofbiodegradation(k)

Machadoetal.(2009)measuredthemethanegenerationpotentialinwastesamplesofdifferentageataBrazilianlandfill.ReductioninthispotentialcouldexplainedbyafirstorderdecayprocesswithaL0ofabout70kgmethanepertonwasteandahalflifeofbiodegradation,of3.5years.

Wangyaoetal.(2009)describerapiddecompositionofwasteonalandfillinThailand.Methaneemissionsweremeasuredin2008and2009.Despitethehighheterogeneityofemissions,areductioninarithmeticmeanemissionswasobserved,fromwhichahalflifeofbiodegradationofabout2yearswasestimated.Thishighrateofbiodegradationisrelatedtoboththenatureofthewaste(containingalotofputrescibles)andclimate.

Howeverinterestingthesevalidationsmayseem,theyareallbasedononetofewlandfills.areillustrativeforproblemsencounteredwhentryingtofindasuitablemodelforthespecificemissionsituation.AsillustratedfortheTNOmodelinFigure4,theuncertaintyinagenerationmodelislarge,anddependingonthequalityofwastedata,thechanceexiststhatgenerationisoverorunderestimatedby2550%.Thismakesithardtodrawconclusionsonthebasisofexperiencesatonlyoneorafewlandfills.Forapropervalidationofemissionsamuchlargersetofobservationsisrequired,beforeonecanconcludewhetheramodelisonaverageagoodpredictorofmethanegeneration.Inthisreport,evaluationofmodelsisnotonlyrelatedtoaccuracy.Indicatorsasscientificbasis,transparencyandvalidatedarealsoused.Theyindicatewhethermodelassumptionsmadeareclearandinlinewithscience.TheevaluationissummarizedinTable3andexplainedinmoredetailintheparagraphsbelow.Availability:Allmodelsarefreeware.ThisincludesGasSimLite,whichisthefreewareversionofGasSim.GasSimLiteenableslandfillownerstofulfilltheirreportingobligationsintheframeworkofEPRTR.InTable3a++meansthatthemodelcanbedownloadedfromtheweb.A4meansavailableondemand.Aormeansthatusershavetodoconsiderableeffortstoobtainaversionofthemodel.Easeofoperationreferstotherequiredexpertiseoftheuserwiththespecificmodelandthecomplexityofchoicesrequiredbytheuser.Numberofdifferentmanipulations/actionsbeforearesultisderived.IncaseofGasSimaisgiven,alsobecausethemodelrequiresinformationthatisnotusedincalculatingmethaneemissions.TheFinnishEPRTRmodelgeta,becauseitissetinFinnishandthereforelesseasytooperatefornonFinnishlanguage.Transparencyreferstoaproperdescriptionofthemodel,modelparametersused,assumptionsmadeandeffortsdonedovalidatetheoutcomeofthemodel.AspreadsheetbasedmodelisinitselfmoretransparentthanexecutablesasGasSimandCalmin,sincethemethodofcalculationanddefaultvaluesusedcanbetracedback.Forliteraturereferencesrelatedtotransparency,seethedescriptionofthemodelschapter2.2.3).


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TABLE 3: SUMMARY OF EVALUATIONOF METHANE GENERATION MODELS1)

1)Inthistable++meansverygoodandmeansverypoor.Ifagenerationmodelscoresorlessononeoftheevaluationparameters,usersofthemodelshouldbewellawareofthelimitsofthemodel.Requiredinput.Moredetailofinputisconsideredhereanadvantage.Itallowsamoreaccurateprognosisoflandfillgasgeneration,sinceitmightbringtheflexibilitytoincorporatecircumstancesthatarespecificforthislandfill.Themodelitselfisevaluatedinamorepositiveway,whenthewaytheinputparameterscanbedefinedisinlinewiththetypeofinformationavailableatthelandfill.Sowastecanbespecifiedaccordingtoitssource(householdwaste,officeswaste,commercialwaste,etc.,asintheTNO,Afvalzorgmodel)ratherthanitscomposition(putrescibles,paper,plastics,etc.asintheIPCCmodel).Specificationaccordingtoitssourceispreferred,sinceitconnectstothewayinformationisavailableatthelandfill.GasSimandtheFinnishEPRTRmodelgivethepossibilitytobothchangetheamountsofwasteperwastecategorie,butontopalsoaccommodateschangesincompositionofthewastestreams.Soalandfilloperatorcancalculatetheeffectofbothlesshouseholdwasteandachangeinhouseholdwastecomposition,e.g.duetoareducedpapercontent.IncaseofLandgemandtheFrenchEPRTRmodel,littleornoroomexiststospecifywastecompositiondetailofinputisconsideredtoolowforanaccuratemodel.Scientificbasisreferstowhetheramodelcanbeconsideredstateoftheartfromascientificpointofviewandtransparancyreferstohowclearassumptionare.TheIPCCmodel,TheTNOmodelandtheAfvalzorgmodelcanbeconsideredstateoftheart.GasSimseemstobestateoftheartaswell,butisgivenaneutralvalue,becausethescientificbasiscannotbeevaluatedduetolackoftransparency.TheFrenchEPRTRmodelisverysimpleincomparisontoothermodels.HoweveritsL0andhalflifeareinlinewithothermodels,anditsoutcomewillbeaboutthesame.Thereis

IPCC

TNO

mod

el

GasSim

Land

gem

Afvalzorg

Calmin

EPRTR(

Fr)

EPRTR(

Fi)

operationalavailabil ity ++ + ++ ++ + + + +easeofoperation + + + + 0 ++ requiredinput 0 + + 0 + 0 0 +

performancescientificbasis + + 0 0 + + +transparancy ++ + 0 0 0 + validated 0 + 0 0 0 0 0

constraintswastechanges + 0 + + +climatezones 0/+ 0 +

accuracy 0 0 0 0 0 0


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noevidencethatitsprognosisofmethanegenerationwillbelessaccuratethanothermodelprognoses.ThescientificbasisofLandgemisconsideredless,becauseofthehighvalesforL0assumedinthemodel.Calmingetsforthemomentanegativeevaluation,sincethediscussiononCalminsapproachisstillpending.ValidatedmodelTheTNOmodelismostextensivelyvalidated.Themodelparametersthemselvesaredeterminedinacomparisonwithlandfillgasrecoveryat9landfills.TheresultinggenerationisvalidatedinacomparisonwithmeasurementsoflandfillgasemissionsDutchon25Dutchlandfills,usinga1Dmassbalancemethod.TheAfvalzorgmodelisvalidatedinamorelimitedeffort,usingexperiencesfromoneDanishandthreeDutchlandfills.LandgemisbasedontheresultsofthevalidationofVogtetal.(1997).Calminisvalidatedinacomparisonwithresultsofclosedchambermeasurementson2Californianlandfills.Howeverasconcludedinchapter3,closedchambersmeasurementscannotbeconsideredareliablemethodtomeasuremethaneemissions.TheIPCCmodelisnotvalidateditself,butisforalargepartbasedontheTNOmodelandusesacomparableL0(althoughcalculatedinadifferentway).Validationoftheothermodelsisunclear.Wastechanges.TheIPCC,TNO,GasSimandAfvalzorgmodelcanhandlechangesinwastecomposition.ThedefaultvaluesintheTNOmodelhoweverseemtobeabitoutdated.TheapproachintheIPCCandGasSimatonehandandAfvalzorgmodelandTNOmodelattheotherhanddiffers:InIPCCandLandgemthecompositionofthewastecanbedefined(amountofputrescibles,paper,plastics,etc.).IntheAfvalzorgandTNOmodelchangesinoriginofthewastecandefined(e.g.amountofhouseholdwaste,officeswaste,commercialwaste,etc.).Asaresult,theAfvalzorgmodelismoresuitedtodealwithchangesinoriginofthewaste,wheretheIPCCmodelchangesinthecompositionofe.g.householdwaste.GasSimandtheFinnishEPRTRmodelaccommodatebothchangesinoriginofthewasteandchangesinthecompositionofeachindividualstream.TheFrenchEPRTRmodeldoesaccommodateforchangesinwaste,butitsassumptionofa50%reductioninmethanegenerationisquiterough.Landgemdoesnotaccommodateforchangesinwastecomposition.InCalminwastecompositionisassumednottobeofinfluenceonmethaneemissions.Applicabilitytovariousclimatezones.Asdescribedinchapter2.2,climatehasimpactonmethanegenerationandboththeamountofmethanegeneratedpertonofwaste,andthespeedatwhichthisisgeneratedisinfluencedbyclimate.MostmodelshoweveraremadeandvalidatedfornorthwesternEurope(sothepartindicatedinFigure3ashumidoceanic)andhavetobeconsideredlessaccuratewhenappliedtootherregionsinEurope.OutofallmodelsevaluatedonlytheIPCCmodelandLandgemdistinguishsomewhatbetweenclimatezones(thesetwomodelsonlytheeffectofwetanddryonhalflifeofmethanegeneration,somethanegenerationinlandfillsinthesouthofEuropewillbesomewhatdelayed).TheaccuracyinTable3referstotheaccuracyfortypesofwasteandclimateconditionsforwhichthemethodisdeveloped.ApartfortheTNOmodel,whichisvalidatedforwastelandfilledintheNetherlandsintheperiodupto2000,thereislittle


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ornoinformationavailableonthebasisofwhichmethodsmutuallycanbecompared.Ingeneralmostgenerationmodelsarebuiltonreasonableassumptionsitisimpossibletoconcludethatonesetofreasonableassumptionsyieldsamoreaccurateresultthantheotherset.Havingsaidthis,theIPCCmodel,GasSimandtheFinnishandFrenchEPRTRmodelseemtobeinfairtogoodagreementwiththeTNOmodelforhouseholdwaste/MSW.SoforhumidoceaniczoneandforwastedominatedbyMSWthesemodelswontbetoofaroff.TheAfvalzorgmodelseemstounderestimatemethanegenerationfromMSW,butthestrengthofthismodelliesinlandfillswithwastefromothersources.OntheotherhandassumptionsinLandgemandCalminseemtobelessreasonable.Themethanepotential,L0,inLandgemseemsratherhigh(alsoincomparisonwiththevalidationstudyofVogtetal.,1997)andthereforeLandgemwillmostlikelyoverestimategenerationatmostlandfills.Calminonlymakesanimplicitprognosisoflandfillgasgeneration.Thisgenerationexcludesmethaneemittedthroughshortcutsandthereforeresultsinanunderestimationofmethaneformationatlandfills,wherelargepartofmethaneisemittedthroughtheseshortcuts.

2.3METHANECONTENT,RECOVERYTheamountofmethanerecoveredisgenerallycalculatedfromtheamountoflandfillgasrecoveredandthemethanecontent:

methanerecovery=landfillgasrecovery*methanecontent(eq.9)L A N D F I L L G A S R E C O V E R Y Themostaccuratewaytoobtaintheamountoflandfillgasrecoveredisbycontinuouslymeasuringtheflowoflandfillgastoutilizationand/orflare,byusingaturbinemeter.Themeasurementhastobecorrectedfortemperature,pressureandmoisturecontent,sopressureandtemperaturehastobemeasuredandmoisturecontentcanbecalculatedfromtemperature,assumingfullsaturationofthegas.Whenamountofgasisnotmetered,landfillgasrecoverymightbeestimatedfromenergyproduction,e.g.assuming1,8kWhproducedperm3oflandfillgasextracted.Howeverthisestimateoflandfillgasrecoveryismuchlesaccurateasameteredrecovery.Thereisnoaccuratewaytoestimatetheamountoflandfillgasrecovered.Asindicatedinchapter4inthisreport,25%to75%recoveryefficiencycanbeexpected,whenthesystemoflandfillgasrecoverycanbeconsideredstateoftheart.IPCCgivesadefaultrecoveryefficiencyof20%forsystemswithoutanyfurtherspecification(IPCC,2006).


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M E T H A N E C O N T E N T Methanecontentofthelandfillgasgeneratedisasomewhatneglectedparameter.Someemissionmodels(astheIPCCmodelandGasSim)assumeadefaultvalueof50%.Howeveractualmeasurementsindicatethataveragemethanecontentinlandfillswillbemoreintheorderofmagnitudeof57%(OonkandBoom,1995).Methanecontentarerelativelyeasymeasuredusingfromtimetotime(dailytoweekly)asimpleFIDanalyzerinthetotalamountofrecoveredgas.Onbasisofthesemeasurementsweighedannualaveragemethaneconcentrationcanbecalculated.

2.4METHANEOXIDATION2.4.1PROCESSESOFMETHANEOXIDATIONWhenmethanepassesthroughthetoplayer,itentersanoxygencontainingzonewherebacteriacanconvertpartofthemethanetoproduceCO2.Thisprocess,normallyreferredtoasmethaneoxidationcanbedescribedas:

CH4+2O2CO2+2H2O (eq.10)Thereareseveralfactorsthatcontroltheamountofmethanebeingoxidized,themostimportantonesbeing: Thehomogeneityatwhichmethaneisemitted.Atlandfills,largepartoftheme

thaneisreleasedthroughshortcuts.Theseshortcutsarealltypesofcracksandrupturesatthesurfaceorsubsurface,butalsogaswellsordrainagepipesthatarenotwellsealedorareleaking.Asaresultmethaneemissionsarehighlyheterogeneous(seealsochapter3.1)andmethaneoxidationathotspotsismostlikelymuchlessthanoxidationmethanethatisemittedinamorehomogeneousway;

Thefluxofhomogeneouslyemittedmethane(theflowofmethanefromthebulkofthewastetothebottomofthetoplayer).Whenthisfluxincreases,diffusionofoxygenintothetoplayerisreducedandmethaneoxidationitselfaswell(Scheutzetal.,2009a);

Theporosityofthetoplayer.Increasedporosityimpliesatonehandamorehomogeneousmethaneemission.Attheotherhand,oxygendiffusionintothetoplayerisenhanced.Soincreasedporosityisadvantageoustomethaneoxidation.Waterlogginginperiodswithhighprecipitationdecreasesporosity(Gebertetal.,2009);

Thewatercontentofthetoplayer.Bacterianeedmoisturetobeactiveandbacteriologicalactivityisfavoredbymoisture.Howevertoomuchwatermightblockthepores.Sothereisanoptimumwatercontentofthetoplayer(Brjessonetal.,1997,Cabral,2004)

Thetemperatureofthecoverlayer,whichiscloselyconnectedtoambienttemperatures.Athighertemperaturesbacteriabecomemoreactive.Every10oCtemperatureincreasemeansabout24foldincreaseinmethaneoxidation(Gebert,2007).


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Asaresultofitsmoistureandtemperaturedependency,methaneoxidationdependsonaverageweatherconditions.Soitisclimatedependent.Methaneoxidationisdescribedtobeatitsmaximumintemperatetowarmconditionswithlimitedexcessrainfall.Methaneoxidationismostlikelylessincolderclimatesandunderwarmbutdryconditions(e.g.Abichouetal.,2010).Foraspecifictoplayeritisalsodependingontheseason;methaneoxidationisinwinterislessthaninsummer.ThisisobservedinNordiccountries,asDenmark(ChristophersenandKjeldsen,1999),Sweden(MauriceandLagerkvist,1997;Brjessonetal.,2007),Belgium(Boeckxetal.,1996)eninnorthernpartsofUSA(Czepieletal.,1996b).2.4.2METHODSFORMODELINGOXIDATIONA P P L I C A T I O N O F S I M P L E D E F A U L T S : IPCC(2006)acknowledgesthelackofreliablefieldmeasurementsonoxidationandthereforeproposeacareful10%defaultvalueforwellmanagedlandfills.This10%defaultismeanttobeaconservativefirstguess,leavingroomforimprovement.Sincetheywerepublished,theIPCCdefaultvaluedrewalotofdiscussionandproposalsforimprovement: Brjessonetal.(2007,seeFigure6)performedmeasurementsatafewlandfills

inSweden.Somelandfillswerestillinoperationwhileotherswererecentlyclosed.Themeasurementmethodisbasedon13Cofthemethaneintheplume,amethodwhichcanbeconsideredasoneofthemorereliablemethodstoquantifymethaneoxidation(seechapter2.4.2).Brjessonetal.explicitlypayattentiontoimproveddefaultvaluesformethaneoxidationandpropose10%foractiveand20%forclosedlandfills.


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FIGURE 6: CORRELATIONBETWEEN SOIL TEMPERATUREAND METHANE OXIDATION ON SWEDISH LANDFILLS (BRJESSON ET AL., 2007) KhleWiedemeijerandBogon(2008)reviewincludingmethaneoxidationon

thebasisofaliteraturesurveyinscientificjournalsandavailablegreyliteratureonthistopic(o.a.GermanresearchonmethaneoxidationatlandfillsformechanicallybiologicallypretreatedwasteandaninterpretationofthemeasurementsofOonkandBoom,1995andScharffetal.,2003).TheyultimatelyconcludethereisnosolidbasisforthedefinitionofmoreaccuratedefaultvaluesandthereforetheyproposetostayclosetotheIPCCdefaultvalues(10%whenmethanefluxishigherthan1,5gCH4m2hr1and15%oxidationwhenthefluxislower),butmentionthisismostlikelyanunderestimation.

Chantonetal.(2009)alsoputtheIPCCdefaultvalueatdiscussioninareviewthatlimitsitselftopeerreviewedliterature.Theyconcludethatonly1outof10measurementsresultinavalueoflessthan10%.Averageofallavailablemeasurementsis35%.IthastobenotedthatChantonetal.(2009)arenotcriticaltowardsanyofthemeasurementsandsimplymakeanaverageofallavailablemeasurements,boththereliableonesaswellastheonesperformedwithlessreliablemethods(e.g.manymeasurementsareperformedusingfluxchambers,amethodknownforitsinaccuracyonlargersurfaces).

GasSimgivestheopportunityeithertochoosethe10%IPCCdefault,ortouseyourownvalue.WhenthelatterischosenGasSimproducesadefaultof25%oxidation,exceptfor10%ofthemethanethatisemittedthroughpreferentialchannels.Itisunclearonwhatinformationthisisbasedupon.

Oonk(2010)reviewsavailableliterature.Importantconclusionisthatlargepartofmethaneisemittedthroughpreferentialchannelsandthepercentagethatis


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emittedinamorehomogeneouswaydeterminesmethaneoxidation.Howeverthereisamaximumoxidationcapacityinperm2peryear.Hesuggestsa1030%oxidationforDutchlandfillsinexploitationand2040%oxidationforclosedlandfillswithamaximumof510kgCH4/m2/yr.

AlsoinAustraliathe10%IPCCdefaultisatdiscussion(Dever,2010).Deverconfirmstheimportanceofshortcutsandindicatesthatactualoxidationwillbesignificantlyinfluencedbytheseshortcuts.

MO D E L E D A P P R O A C H E S Themostelaboratedmodelofmethaneoxidationisperformedintheframe

workofCalmin(Spokasetal.,2009;Abichouetal.,2010).Methaneoxidationisdeterminedonthebasisofthecompositionofthetoplayerandclimateconditions.Themodelitselfultimatelyproducesamaximummethaneoxidationinkg/m2/yrandwhenthefluxofmethanetothetoplayerisbelowthismaximum,methaneemissionisassumedtobezero.Ultimatelymethaneoxidationismuchhigherthanthe1035%mentionedaboveandisintheorderofmagnitudeof75%.ThishighvalueiscausedbytheassumptioninCalminthatallmethaneisemittedhomogeneously.Asdescribedbefore,largepartofmethaneisemittedthroughshortcutsandhotspotsandoxidationhereismostlikelylowornegligible.Forlandfillswhereshortcutsandhotspotsplayaroleinemissions,Calminwilloverestimatemethaneoxidation.

TheCLEARgroup(aninternationalgroupofleadingexpertsonmethaneoxidation14)discussesimprovementofquantificationofmethaneoxidation.Somemembersofthegrouphaveproposedadraftmodelinwhichmethaneoxidationiseitherlimitedbytheamountofmethanethatishomogeneouslyemittedorthemaximumoxidationcapacityofthetoplayer.Bothparametersareestimatedasafunctionofmethaneflux,toplayermaterial,porosity,moisturecontentandambienttemperatureandthelowestofbothisactualmethaneoxidation.Thedraftmodelwillbediscussed,revisedanddefinedinmoredetailbythewholeCLEARgroupduringthenextmonths(Scharff,2010b).

2.4.3EVALUATIONOFMODELSFORMETHANEOXIDATIONMajorproblemindefiningandevaluatingmodelsformethaneoxidationisthelackoffielddata.Mostofthemeasurementsthatareavailablearedoneusingclosedchambersandthismethodmostlikelyoverestimatesmethaneoxidation(seechapter3.4).MorerecentevaluationsofavailableinformationallyieldeddefaultvaluesintheorderofmagnitudeoftheIPCCdefaultvalueof10%.Mostlikelylandfillsitesinoperationhavelessmethaneoxidationthanclosedlandfillsites.Methaneoxidationisgenerallyexpressedasapercentageofthemethanefluxfromthebulkofthetoplayer.Butmostlikely,thereisalsoamaximummethaneoxidation,whenexpressedingm2hr1.Sobeyondthismaximum,thefixedpercentagemightleadtoanunderestimation.Theprinciplesoutlinedinthedraftmodelformethaneoxidation,being14FormoreinformationonCLEAR,seehttp://ch4ox.lmem.us/


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discussedbytheCLEARgroup(Scharff,2010b),mightresultinthebestguessformethaneoxidationatthemoment.D I F F E R E N T C L I M A T E Z O N E S Althoughitisgenerallyacknowledgedthatclimateandseasonhasimpactonmethaneoxidation,thisimpactisnotincludedinmostoftheevaluationsformethaneoxidation.AnexceptionisCalmin.HoweverCalminassumesallmethaneemissionstotakeplacehomogeneously.Itunderestimatesmethaneemissionsfromlandfills,whereshortcutsandhotspotsareimportantpathwaysformethaneemissions.ThedraftCLEARoxidationmodeldoescorrectfortheimpactofambienttemperature.Thecurrentversiondoesnotyetcorrectfortheeffectofprecipitation.A C C U R A C Y Theaccuracyofmethaneoxidationisunclear.E.g.IPCC(2006)doesntgiveguidanceonthistopic.InterpretationofemissionmeasurementsfromOonkandBoom(1995)givevaluesformethaneoxidationinbetween10and30%forlandfillsinexploitationand10to60%forclosedlandfills.GasSimgivestheopportunitytopickarealisticmethaneoxidationof25%andarangeoferrorfrom10to40%.Bothuncertaintyrangesareobtainedforcountriesinahumidoceanicclimate.Methaneoxidationinsubarcticorhighlandregionscanbeexpectedless,duetoonaveragecoldertemperatures.Methaneoxidationinsubtropicalregionscanbeexpectedlessbecauseofrelativedryconditionsofthetopsoil.Methaneinlandfillsinhumidcontinentalregionscanbeexpectedlessaswell,partiallybecauseofthelongerandcolderwintersandmoredrysummers.

2.5ACCURACYOFMODELEDMETHANEEMISSIONTheaccuracyofmodeledmethaneemissionisafunctionoftheaccuracyofmodeledmethanegeneration,theaccuracyofmethanerecoveryandtheaccuracyinmethaneoxidation.Sincemethaneemissionisobtainedasadifferencebetweengenerationandthesumofextractionandoxidation,theaccuracyoftheoverallresultisquitepoor.Outofallavailableemissionmodels,GasSimpaysmostattentiontoaccuracyoftheestimatedemission.InGasSimanaccuracydistributionofallinputvariablesandmodelparameterscanbeintroduced.InaMonteCarloanalysis15,1to99%confidenceintervalsarecalculatedformethaneemission.IthastobenotedthatsuchaMonteCarloanalysisonlyquantifiestheeffectofknowninaccuracies.Therearealsounknownmodelinaccuracies,e.g.theinherentinaccuracyoftheassumptionthat15InaMonteCarloanalysisallparametersarevariedatrandom,withinthedefineddistributionofaccuracy.Subsequentlymethaneemissionsarecalculated.Thiscalculationisrepeated100times,everytimewithadifferentrandomchoiceofparameters.Resultisaprobabilitydistributionofmethaneemissions(1%chancemethaneemissionsarelessthanxkg/yr;5%thattheyarelessthanykg/yrto99%theyarelessthanzkg/yr).


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methanegenerationcanbedescribedthroughafirstorderormultiphasemodel(eveninthetheoreticalcasethattheexactamount,compositionandrateofbiodegradationofthewasteisknown,thereisstilluncertaintyaboutmethanegenerationbecauseitisnotclearwhetherthemodelisanexactdescriptionofhowmethaneisbeingformed).UnfortunatelyGasSimdoesnotspecifyanydefaultuncertainty,soitrequiresanexperiencedandknowledgeableusertomakeuseofthisknowledge.Inallothermodels,accuracyhastobecalculatedbyhandandbypropagationoferrors.Ingeneraltheminimumandmaximummethaneemissioncanbecalculatedasfollows:

CH4emin=CH4gmin(Rmax*Fmin)OXmax (eq.10)CH4emax=CH4gmax(Rmin*Fmax)OXmax (eq.11)

IPCC(2006)givesguidancetoestimatingtheaccuracyinmethaneemissionthrougherrorpropagationandthismethodcanbeappliedtoothermodelsaswell.AlthoughtheIPCCmethodologyismadeforestimationofmethaneemissionsfromalllandfillsinacountry,guidanceisalsoapplicabletoindividuallandfills.AccordingtoIPCCtheerrorinlandfillgasgenerationpertonofwasteconsistsof theerrorinamountoforganiccarboninthewaste(20%whenbasedonIPCC

defaultvalues,10%whenbasedonregularsamplingandanalysis); fractionoforganiccarbonthatactuallydecomposes(20%whenbasedonIPCC

defaultvalues,10%whenbasedonexperimentaldataforreallandfillsoverlongertimeperiods);

anerrorinthemethanecorrectionfactor(10%formanagedlandfills)and anerrorintheassumedmethanecontentofthelandfillgasformed(5%);Totalsumoferrorsinmethanegenerationpertonofwaste,accordingtoIPCC,rangesbetween35%and55%,dependingonlocalinformationavailable.HoweverasdescribedinIPCC(2000),someoftheparametersaremutuallydependentandtotalerrormightbelessthantheonespecified.E.g.organiccarboncontentisknownwithlimitedaccuracyandthesamegoesforthefractionoforganiccarbonthatactuallydecomposesandthemethanecorrectionfactor.Theproductofthethreeistheamountoflandfillgasthatisproducedpertonofwaste,andthisoneisknownmoreaccuratelythanthesumofuncertaintyofallthreefactorssuggest.Soactualuncertaintyinamountofmethaneproducedpertonofwastewillbelessthanthe35to55%andmightbe20to40%.


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TABLE 4: INDICATION OF UNCERTAINTIES IN METHANE MASS BALANCEgoodcase badcase

amountofwaste 1 2%:weighed waste 20%:estimatedbasedonlandfillvolume

L0 20%:accuratewastedescription,humidoceanicclimatezone

40%:noaccuratewastedescription,otherclimatezone

modeluncertainty 5 10% 10%methanerecovery 10%:measured 25%:estimatedmethanecontent 5%:measured 10%:estimatedmethaneoxidation 150%:otherclimatezone 250%:humidoceaniccli

matezoneTable5givesanexampleoftheresultofasimpleerrorpropagationofmethaneemissionsfromalandfill.Evenwithmodestassumptionsonaccuracyofmethanegenerationandotherfactorsinvolved,theinaccuracyinmethaneemissioninthisexampleturnsouttobe65%.Asaruleofthumb,inaccuracyinmethaneemissionincreaseswhentheefficiencyoflandfillgasrecoveryincreases.TABLE 5: EXAMPLE OF PROPAGATION OF ERRORS WHENCALCULATING METHANE EMISSIONSFROM THEMETHANE MASS BALANCE (LANDFILLIS CHOSEN IN SUCH A WAY THAT BEST GUESS METHANE EMISSIONIS 100 KG/Y)

minimum mean maximumLFGgeneration(m3/y) 300 (30%) 428 557(+30%)methanecontent (vol%) 50 54 58methanegeneration (kg/y) 108 167 232methanerecovery (kg/y) 61 (+10%) 56 50(10%)methaneoxidation (kg/y) 12 (25%) 11(10%) 18(10%)methaneemission (kg/y) 35 100 164

Howeveronanindividuallandfill,knowledgeofthelocalsituation,e.g.onthequalityoflandfillgasextractionmightimprovetheaccuracyconsiderably.Inthisexample,theminimumvaluewouldimplyover50%recoveryefficiency,wherethemaximumemissionwouldimplyjustover20%efficiencyoflandfillgasrecovery.Anexpertjudgmentofthequalityoftherecoverysystem,andeffortsdoneinthepasttooptimizerecoveryshouldhelptoseewhatrangeinrecoveryvalueisrealistic(seealsochapter4).Basedonthis,theerrorinmodeledemissioncouldbereduced.

2.6CONCLUSIONSMODELLINGModelingemissionsofmethanegenerallyrequiresmodelingofmethanegeneration,measuringlandfillgasrecoveryandassumingsomemethaneoxidation.Inthelastfewyearsdevelopmentofmethaneorlandfillgasgenerationmodelshavereceivedmostattentionandseemtohavedeveloped.Thereareseveralmodels


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available,suchastheIPCCmodel,theTNOmodelandGasSim,allbuiltupfromreasonableassumptions.Howeverduetoalackofvalidationonreallandfilldata,theseassumptionsmightresultinonlyanapparentaccuracy.TheFrenchEPRTRmodelismuchsimplerandmightbejustaseffective.AlltheaforementionedmodelsmightproducereasonableresultsforMSWdominatedbyhouseholdwaste,landfilledinWesternEurope.TheaccuracyofthesemodelsforothertypesofwasteorindifferentregionsinEuropeislimited: Theimpactofclimateonlandfillgasformationiswidelyrecognized.Climatewill

haveimpactonboththeamountofmethanethatisultimatelyreleasedpertonofwaste(L0)andthespeedatwhichmethaneisreleased(halflifevalues).TheimpactofclimateonL0isuntilnowneglectedinformationmodels.TheimpactofclimateonhalflifeisonlydescribedintheIPCCmodel,butinaveryrudimentaryway.Foranimproveddescription,4climatezonesinEuropecouldbedistinguished:(i)subarcticandhighland,(ii)humidoceanic,(iii)humidcontinentaland(iv)subtropical;

Asaresultofexistingpolicy,landfillingoforganicwasteismoreandmorediscouraged.ThischangeinwastecompositionalsorequiresimproveddefaultvaluesforL0.Itisalsopossiblethatthespeedandcompletenesswillbeaffectedatwhichorganicwastethatremainstobelandfilleddegrades;

Oxidationismoredifficulttodescribe,thanmethanegeneration.Knowledgeonoxidationisalsolimitedbyscarceinformationavailableonactualmethaneoxidationunderfieldconditions.TheIPCCdefaultvalueof10%hastobeconsideredasalowguess,aconservativevalue,leavingroomforimprovement.Actualmethaneoxidationisagaindependentonthedesignofthetoplayer,themethanefluxthroughthetoplayerandclimateconditions(precipitationandambienttemperature).Hotspotsandshortcutsformethaneemissionlimitmethaneoxidation,sinceatmanylandfillslargepartofmethanewillescapewithoutpassingtheoxidizingzoneinthetoplayer.Mostlikelyismethaneoxidation(expressedin%)somewhathigheratclosedlandfills,somewhatlessatlandfillsinexploitationandbecomesmoreorlessaconstantvalueinkg/m2/yrwhenmethanefluxtothetoplayerishigh(e.g.deeplandfills,withoutstateoftheartlandfillgasrecovery).Intheend,modeledmethaneemissionsarehighlyuncertain,evenwhenmethaneformationandoxidationcanbedescribedrelativelyaccurately.Thereasonforthisisthepropagationoferrors,whichishighlyunfavorable.Thisisbecausemethaneemissioniscalculatedasthedifferenceofthreeuncertainparameters.Anidealmethaneformationoremissionmodeldoesntexist.SuchanidealmodelshouldhavethetransparencyofIPCCmodel,thelevelofvalidationoftheTNOmodel,awasteinputmodulefornonhouseholdwasteofAfvalzorg,anuncertaintyanalysisasinGasSim,amorereliabledescriptionofoxidationasafunctionofclimateconditionsasinCalmin,butthenwithmorerealisticassumptionsonshortcutsasinthedraftoxidationmodeloftheCLEARworkinggroup.However,aslongasnoadditionalvalidationeffortsareperformed,oneshouldbeawarethatmodelswithmoresophistication,builtonevenmoreassumptionsmightonlygiveanimproved


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apparentaccuracy.SosimplicityasintheFrenchEPRTRmodelmightalsohaveitsbenefits.


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CHAPTER3:MEASURINGEMISSIONS

3.1IntroductionInthepastdecadestherehasbeenconsiderableinterestinmeasurementmethodsformethanefromlandfills.Inthisperiod,variousmethodsareproposed,developed,tested,improved.Howeveratthemomentthereisnosinglemethod,thatiswidelyrecognizedasthepreferredmethodtomeasureannualaveragemethaneemissions.Themaindifficultyinmeasuringmethaneemissionsfromlandfillsisthespatialandtemporalvariabilityofemissions,incombinationwiththesheersizeofamodernlandfill.Thespatialvariabilityofmethaneemissionsisreportedbyvariousresearchers.Emissionsatonespotcanbe1.000foldofemissionfromaspotlocatedafewmetersaway(Verschutetal.,1991).AccordingtoCzepieletal.(1996)thereisnocorrelationbetweenemissionataspotatthelandfillandtheemission6metersaway.Theyestimatethat50%ofemissionsisreleasedat5%ofthelandfillsurface;Bergamaschietal.(1998)estimatethat70%ofmethaneemissionsarereleasedthroughshortcuts.Figure7givesatypicaldistributionofdistributionofemissionsatalandfillandsimilarpatternsarepublishedbyNozhevnikovaetal.(1993),butsimilardistributionsarereportedbyOonketal.(2004),MackieandCooper(2009)andChantonetal(2010).RachorandGebert(2009)studiedvariationinemissionswithinthesquaremeterandevenatthissmallscaleemissionsprovedtobehighlyheterogeneous.

FIGURE 7: METHANEEMISSIONS FROMTHEKUCHINOLANDFILL SURFACE(NOZHEVNIKOVA ET AL., 1993)Changesinweathercauseatemporalvariability.Verschutetal.(1992)indicatepressurevariationstobeveryimportant.Czepieletal(1996)indicatethathigheremissionsareobtainedduringdayswithlowerpressure.AlsoScharffetal.(2003)reportacorrelationofmethaneemissionsandchangesinambientpressure.Rainfall,windandeventsinthegasextractionsystemareotheraspectsthathaveimpactonmethaneemissions.


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FIGURE 8: VARIABILITY INEMISSIONSAND CHANGES IN PRESSURE DROP. RECONSTRUCTED FROM A 1D MASSBALANCEMETHOD MEASUREMENT(SCHARFF ET AL., 2003)BasedondaytodayvariabilitiesasinFigure8,Scharffetal.(2003)estimate46daymeasurementsthroughouttheyeararerequiredtoobtainanaccurateannualaverageemissionestimate.Ontopofthedaytodayvariationmentionedabove,aseasonalvariationinmethaneemissionsisexpected,duetoaseasonalvariationintemperatureandmoisturecontent.Asdescribedinchapter2.4.1,averagemethaneemissionsinwinterissomewhathigherthanaverageemissionsinsummer,especiallyinNordiccountries.Soamethodtomeasureannualaveragemethaneemissionsshouldbeabletodealwiththetemporalandspatialfluctuationsasdescribedabove.

3.2Availablemethods3.2.1SOILCOREMEASUREMENTSMeasurementsinthetoplayermaygiveusefulmechanisticinformationaboutthefundamentalstepsleadingtomethaneemissions:diffusionandoxidation.Methaneandcarbondioxideconcentrationgradientsinthesoilmaygiveanindicationofmethaneandcarbondioxidediffusionthroughthelayer(Bogneretal.,1995);landfillsoilcoresmaybecollectedandtransportedtothelabfordeterminingbacteriologicalactivityofmethanotrophes.ThelatterisdonebyexposingthesoilsampletoahighconcentrationofCH4andmeasurethedecreaseoftheCH4concentrationintime,thusgivinganindicationoftheoxidationcapacityofthesoil.Theseexperimentsmaybecarriedoutatdifferenttemperaturesorsoilmoisturelevelsetc.tostudyimprovethemechanisticunderstandingofoxidation.ADVANTAGES AND DISADVANTAGES Theadvantageofsoilcoremeasurementsisthatitgivesinsightinthefundamentalstepsleadingtoemissions.Themethodhoweveralsohassomedisadvantages:it

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doesnottakeintoaccountemissionscausedbyconvectionanditsspatialandtemporalresolutionarelow(onegetsanimpressionofemissionandoxidationofaverysmallspotonasinglemoment).Besides,themethodologyisverylaborintensive.3.2.2CLOSEDCHAMBERMEASUREMENTSApplicationofclosedchamberismostfrequentappliedtomeasuremethaneemissionsfromlandfills.Itisappliedbymanyresearchgroupsaroundtheworld,bothformonitoringmethaneemissionsfromsmallerpartsofalandfill(e.g.testfieldsforenhancedmethaneoxidation)aswellasestimatingemissionsfromanentirelandfill.AnoverviewofselectedapplicationsofclosedchambersisgivenbyScheutzetal.(2009).Ingeneralterms,inaclosedchambermeasurementafluxboxisputonthelandfill,andtheincreaseofmethaneconcentrationsintheboxintimeismeasured.Themethanefluxiscalculatedfromtheincreaseofmethaneconcentrationintime,thevolumeoftheboxandthesurfacecapturedbythebox.Theareafewpitfalls,whenperformingaclosedchambermeasurement: Whenlandfillgasiscollectedwithinthebox,thepressureintheboxincreases.

Sowhenthemeasurementisperformedoveratoolongtime,landfillgasemissionfromthesurfaceencapsulatedbytheboxmightbeaffectedandmethanefluxisunderestimated;

Withvegetationonthelandfillsurface,sealingoftheboxtothesurfaceisofimportance.Anyleakageswilldisturbthemeasurement.Incaseofexcessivevegetation,mowingpriortothemeasurementmightbeanoption.Toimprovesealingsomeresearchprojectswherethesamespotismeasuredmultipletimesovertime,usefixedcollarswhicharemountedtothegroundonwhichclosedchamberscanbepositioned;

Whenvegetationispresent,themethodisnotsuitedformeasuringcarbondioxide.Thisisbecauseofdissimilationofcarbondioxidefromthevegetation.

Somevariationsonclosedchambermeasurementare: Dynamicboxes,openchannelsthroughwhichacontinuousairstreamisled.Us

ingamatchingpairofinletandoutletventilatorthepressureintheboxiskeptambientandlandfillgasemissionisnotinfluenced(Verschutetal,1991;HuberHumerandLechner,2001ab);

Fastboxmeasurements,usinganalyticalequipmentthatalreadycandetectafewppbincreaseinmethaneconcentration.Usingthisbox,asinglemeasurementtakeslessthanaminute.Asaconsequencethenumberofmeasurementthatcanbeperformedinonesingledayissignificantlyincreased(Oonketal.,2004).

G R I D W I S E M E A S U R E M E N T S Thelargestdrawbackofclosedchambermeasurementsisthesmallsurfaceareasampledpermeasurement.Inanattempttoobtainareliablemethaneemissionestimate,systematicsamplingstrategiesareproposed(BognerandScott,1995;Bour,2007;Long,2004;Rosevaeretal.(2004);Savanneetal.,1997;Spokasetal.,2006).Suchasamplingstrategyconsistsofsamplingatpointslocatedonasystematicgrid,


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sometimesfollowedbyapplicationofgeostatisticalmodels(Spokasetal.,2003).Typicaldistancesbetweenpointsonsuchasamplinggridare1060meters;themoremeasurementsmade,themoreaccuratetheresult.Anotheroptiontoimproveclosedchambermeasurementsistousequalitativesurveys(seechapter3.2.7)toidentifyhotspotsandsubsequentlydecreasethegriddistanceatplaceswherehotspotsofemissionsaresuspected.Thisdoeshoweverintroducetheissueofweighingthehotspotsmeasurementsandtheothermeasurementscorrectlytoobtainanoveralllandfillaverage.A D V A N T A G E S A N D D I S A D V A N T A G E S Closedchambermeasurementshaveanumberclearadvantages.Tostartwith,themethodiseasytounderstand,doesntrequireanalyticalequipmentbeyondacommonFIDoraIRanalyzer.Themethodisabletodetectsmallfluxesofmethaneandisnotsensitivetotopographicconstraintsorothersourcesofmethanenearthelandfill.Themethoditselfalsohascleardisadvantages.Themostimportantoneisthatonmanylandfillsmethaneemissionstakeplaceinsuchaheterogeneousway,thatclosedchambersdonotgiveareliableaveragemethaneemission.Thereisabigchancethathotspotsofmethaneemissionsaremissed,resultinginanunderestimationofem

sustainable landfill foundation - gas modeling - literature review methane from lanfills

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