WP1 Gas conditioning and grid operation

DELIVERABLE 1.1.1

Upgrading of Biogas to Biomethane with the

Addition of Hydrogen from Electrolysis

Prepared by: Dadi Sveinbjörnsson and Ebbe Münster, PlanEnergi

Reviewed by: Nabin Aryal, DGC and RasmusBoBramstoftPedersen,DTU

Date: 18.09.2017

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Contents1 Introduction.........................................................................................................................................3

1.1 Motivation..................................................................................................................................3

1.2 Thescopeofthereport..............................................................................................................4

1.3 Gastypes.....................................................................................................................................4

1.3.1 Biogas.....................................................................................................................41.3.2 Hydrogen................................................................................................................41.3.3 Biomethane............................................................................................................5

2 Methanationofbiogas........................................................................................................................6

2.1 Catalyticmethanationofbiogas.................................................................................................6

2.2 Biologicalmethanationofbiogas................................................................................................9

3 Plantdescriptionsanddatasheetsforexistingbiogasmethanationplants.....................................11

3.1 HaldorTopsøe:CatalyticmethanationinFoulum,Denmark....................................................11

3.2 Etogas&Audi:CatalyticmethanationinWerlte,Germany......................................................14

3.3 MeGa-stoRE:CatalyticmethanationinLemvig,Denmark........................................................17

3.4 Electrochaea:BiologicalmethanationinAvedøre,Denmark...................................................18

3.5 MicrobEnergy&Audi:BiologicalmethanationinAllendorf,Germany....................................23

3.6 Otherbiogasmethanationplants.............................................................................................26

References..................................................................................................................................................27

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1 Introduction

1.1 MotivationProducing biomethane (also called synthetic natural gas, SNG) by combining hydrogen produced viaelectrolysis and biogas produced via anaerobic digestion can be an effective way of lowering CO2emissions in energy systems with a high penetration of renewable electricity generation. This canfurthermorebeaneffectivemethodforindirectstorageofexcesselectricityproductionfromfluctuatingsources,suchaswindturbinesandphotovoltaics,byallowingfortheconversionofintermittentelectricalenergytostorablechemicalenergy.Thisscheme,whichfrequentlygoesbythenameof“power-to-gas”,opensupthepossibilityofusingthevaststoragecapacityofthenaturalgassystemtoindirectlystoreexcessrenewableelectricityproduction.Thebiomethanecanbeusedinthetransportsectordirectlyandalso be reformed to liquid vehicle fuels, if desired. Figure 1.1 shows how biogas methanation withhydrogenadditioncanactasanintegratorbetweendifferentsectorsoftheenergysystem.

Figure1.1Asystem-levelschematicshowinghowbiogasmethanationwithhydrogenadditioncanfitintotheenergysystemandhowitcanserveasasystemintegratorbetweentheelectricity,gasandtransportsectors.Notethatthemethanationunitismarkedas“Sabatierreactor”inthefigure.Theonlyinputtothebiogasreactorinthefigureismanure,butsludgeandstrawcanalsobeusedasinputs.Figurereproducedinslightlyalteredformfrom[1,2].

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ProducinggasfrombiomassandrenewableelectricityandinjectingittothenaturalgasnetworklowersthenetCO2emissionsofthegassector.Thehydrogenadditiontobiogasalsoleadstoamoreefficientutilizationofthe(finite)biomassresourcesconsumedintheprocess,asallcontentsofthebiogasareusedforproducingbiomethane(ascomparedtoconventionalbiogasupgrading,wheretheCO2fractionofthebiogasisdiscarded).

1.2 ThescopeofthereportThis report is confined to the description of methanation of biogas with hydrogen addition. Biogasproductionandhydrogenproductionarenotthemainsubjectsofthisreport,andtheseprocessesareonly briefly touchedupon in the text. Biomethaneproductionwithout hydrogen addition, i.e. by CO2removalfrombiogasviaprocessessuchasaminescrubbing,isnotincludedinthisreport.BiomethanecanalsobeproducedviamethanationofsyngasfromthermalgasificationorviamethanationofCO2fromothercarbonsources(e.g.capturedCO2fromindustrialpointsources).Thesemethodsofbiomethaneproductionarenotcoveredinthisreport.

Areviewofbiogasupgrading,includingdescriptionsofbiogasupgradingwithouthydrogenaddition,hasbeenpreparedbyK.HjulerandN.AryalaspartoftheFutureGasresearchproject[3].Agoodtechnologicalandeconomicreviewofbiomethaneproductionwithhydrogenaddition,includingthemethodsexcludedinthisreport,hasbeencarriedoutbyGötzetal.[4].Examplesofreportsdescribingthetechnologyandperspectivesforbiomethaneproductionfrombiogasmethanationinclude[5–13].Aquickoverviewofallpower-to-gasdemonstrationprojectsinEuropeisprovidedin[14].

1.3 Gastypes

1.3.1 BiogasBiogasisproducedviatheprocessofanaerobicdigestionofwetbiomass.Inthisprocess,microorganismsfermentorganicmaterialintoamixtureofmethaneandcarbondioxideintheabsenceofoxygen.Thisprocessusuallytakesplaceinthetemperatureintervalof35-55°C.Typicalfeedstockfortheproductionofbiogasiswetbiomass,suchasanimalmanurefromagricultureorsludgefromwastewatertreatmentplants.

Theresultingbiogas istypicallycomposedof50-70%methane(CH4),30-50%carbondioxide(CO2)andtraceamountsofothergases.Oneofthetracegasesistypicallyhydrogensulfide(H2S),whichmustberemoved before the methanation process. Other trace gases may include NH3, hydrogen, oxygen,nitrogen,CO,VOCandsiloxanes.Theanaerobicdigestionprocessyieldsanitrogen-richdigestateasabi-product,whichcanbeusedasfertilizer.

1.3.2 HydrogenHydrogenfromanexternalsourceisneededforthemethanationoftheCO2fractionofthebiogas.Purehydrogen is obtained by electrolyzingwater. There are threemain types of electrolysis technologies;alkalineelectrolysis,protonexchangemembraneelectrolysis(PEM)andsolidoxideelectrolysis(SOEC).Alkalineelectrolysisisamatureandwidelyusedtechnologyinindustry.PEMandSOECelectrolysisarestillmainlyusedonasmallerscaleinresearchanddemonstrationprojects.EspeciallytheSOECtechnology

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hasalargepotentialofbecomingsuperiortoalkalineelectrolysisduetoitshigherenergyefficiencyandfutureprospectsforlowcosts[15,16].

AlkalineandPEMelectrolysistakesplaceatmodesttemperaturesof60-100°Candgenerateexcessheatas a byproduct. SOEC electrolysis is also called “high-temperature electrolysis” and operates attemperaturesof650-800°C.TheelectricalenergyinputrequiredfortheSOECprocesscanbeloweredincaseapartoftheenergyisavailableassteamfromotherprocesses,whichoffersaverygoodsynergywiththeexothermicprocessofbiogasmethanation.

1.3.3 BiomethaneThe terms “synthetic natural gas” or “substitute natural gas” (SNG) are frequently used for gas fromchemicaland/orbiologicalprocessesthatissuitableforinjectionintotheexistinggasgridasasubstituteforconventionalnaturalgasoffossilorigin.Inanewinternationalstandardforbiogasupgrade(EN16723-1), the term “biomethane” is used instead of SNG. In this standard, biomethane is defined as “gascomprisingprincipallymethane,obtainedfromeitherupgradingofbiogasormethanationofbiosyngas”.Inthisreport,wefollowtheterminologyoftheEN16723-1standardandrefertomethanatedbiogasas“biomethane”.

Themethodsforupgradingbiogastobiomethanecanbedividedintotwocategories:

a) BiogasupgradingbyremovaloftheCO2fractionofthebiogasbyphysicalorchemicalprocesses.b) MethanationofbiogasbyreactingtheCO2 fractionof thebiogaswithhydrogenfromanother

source.

Onlytheprocessesinvolvingmethanationreactionsareincludedinthisreport,i.e.categorya)isexcludedhere.Byutilizingthewholecontentofthebiogas,insteadofdiscardingtheCO2fraction,theCO2emissionsfromtheprocessareloweredandthe(finite)biomassfeedstockforthebiogasproductionisusedinamoreefficientway[17].

Beforeinjectiontoagasgrid,biomethanemustcomplywiththerequirementsofthegridconcerninggascompositionandquality.OneoftherequirementsconcernstheWobbeindex,whichisameasureofthecombustionenergyofagas.ThetypicalWobbeindexfornaturalgasfromtheNorthseaisaround54.9MJ/Nm3 [18]. Other requirements for injection to the gas distribution grid concern e.g. themoisturecontent(dewpoint)andthesulfurcontentsofthebiomethane.

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Figure1.2TheWobbeindexofupgradedbiogasasafunctionofCH4content.Hereitisassumedthatthegascontains1%atmosphericairandthattheremainingfractionofthegasisCO2.Thedashed,redlineshowsthelowerlimitforthepermittedWobbeindexofgasintheDanishgastransmissionanddistributionsystem.Thedashed,greenlineshowsthelowerlimitfortheWobbeindexofnaturalgasthatfollowstheH-gasstandard.Figurefrom[19].

2 Methanationofbiogas

2.1 CatalyticmethanationofbiogasThemethanationofbiogastakesplaceviathefollowingchemicalreaction,wheretheCO2contentsofthebiogasreactwithhydrogentoformmethane:

CO# + 4H# ⟶ CH( + 2H#O + ΔH (Eq.1)

ThereactionisexothermicwithΔH=-164.9kJ/mol.Theprocessusuallytakesplaceoveranickel-basedcatalyst.Intheprocess,theexistingCH4fractionofthebiogasremainsunchangedwhiletheCO2fractionofthebiogasismethanised.

Themethanationprocessofbiogasconsistsoftworeactionsteps.First,areversewater-gasshiftreactiontakesplacetoformCObyreactingCO2andhydrogen:

CO# + H# + ΔH ⟶ CO + H#O (Eq.2)

ThisreactionisendothermicwithΔH=+41.5kJ/mol.ThelatterreactionistheSabatierreaction,wheremethaneisformedbyreactingCOandhydrogen:

CO + 3H# ⟶ CH( + H#O + ΔH (Eq.3)

TheSabatierreactionisexothermicwithΔH=-206.2kJ/mol.

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Figure 2.1A schematic of the process for themethanation of biogas using hydrogen from alkaline electrolysis,showingthestoichiometryofthegasflows.TheamountofmethaneinthebiogasisdenotedwithxtotakeintoaccountthattheratioofmethaneandCO2inbiogasvariesbasedonthefeedstock.Themethaneinthebiogasgoesunalteredthroughthemethanationreactor.Excessheatfromtheelectrolysisand/orthemethanationcanbeutilizedintheanaerobicdigestionprocess,orpossiblyforotherindustrialprocessesordistrictheating.Waterformedinthemethanationreactioncanbeusedintheelectrolyzer.Itisnotstrictlynecessarytoincludebothahydrogenstorageandabiogasstorage,asthisdependsontheoperatingstrategyoftheplant.

Figure2.2AschematicoftheprocessformethanationofbiogasusinghydrogenfromSOECelectrolysis,showingthestoichiometryofthegasflows.TheamountofmethaneinthebiogasisdenotedwithxtotakeintoaccountthattheratioofmethaneandCO2inbiogasvariesbasedonthefeedstock.Excessheatfromthemethanationcanbeutilizedintheanaerobicdigestionprocess,orpossiblyforotherindustrialprocessesordistrictheating.Themethanationreactorcanprovidesteamto theelectrolyzer, therebysupplying itwith its requiredwaterandheat inputs.Theexternalwaterinputcanbeturnedintosteambyutilizingwasteheatfromthemethanationprocess.Themethaneinthebiogasgoesunalteredthroughthemethanationreactor.Itisnotstrictlynecessarytoincludebothahydrogenstorageandabiogasstorage,asthisdependsontheoperatingstrategyoftheplant.Figurepartlybasedonafigurefrom[6].

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Figure2.1andFigure2.2showtheschematicprocesslayoutforthemethanationprocessusingalkalineelectrolysisandSOECelectrolysis,respectively.NotethatwhenproducingonemoleofCH4,theoverallreactionalsoyieldstwomolesofH2O.Thiswatercanbefedtotheelectroyzerforhydrogenproduction,therebycoveringhalfofitswaterdemand.Thisholdstrueregardslessofwhichelectrolysistypeisused.Thissaveswaterresourcesandpossiblecostsofwatertreatmentpriortotheelectrolysis.InthecaseofSOEC,thiswatercanbeprovidedassteam,therebyreducingtheneedforexternalenergyinputforthehigh-temperatureelectrolysis.

The methanation process typically takes place at tempertures in the range of 250-550°C. In thisexothermicprocess,between8.5%and15%oftheenergyoutputofthereactioncomesintheformofheatatthereactiontemperature[16].Thishigh-gradeheatcane.g.beutilizedtoprovidesteamforhightemperature (SOEC) electrolysis. Alternatively (and less exergy efficiently), the heat can be utilized inlower-gradeapplicationssuchastheanaerobicdigestionprocess(whichtypicallytakesplacearound50°C)orforinjectioninadistrictheatingnetwork(typicallyat70-90°C).Inanycase,theutilizationoftheexcessheatisimportanttomaximizetheoverallenergyefficiencyofthemethanationprocess.

Figure 2.3Gas volume and energy flows of the biogasmethanation process, scaled for an electrolyzerwith anelectricityinputof10MWandanassumedefficiencyof84%.Itisassumedthatthebiogasiscomposedof65%CH4and 35% CO2.Hydrogen and CO2 are injected in the ratio 4:1, corresponding to the chemical equation of themethanationreaction.Thecalculationassumesa100%conversionrateofCO2andH2toCH4.Inthecalculation,itisassumedthatallgastypestakeupidenticalvolume,aspertheidealgaslaw.Itshouldperhapsbementionedthatthisdataisnotbasedonanyexistingplant;theseareonlycalculations.Figurereproducedfrom[1].

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Foreffectiveintegrationoffluctuatingelectricityproductionfromwindandphotovoltaics,itisbeneficialtobeabletoregulatetheoperationofthemethanationprocess.Theproductionoftheelectrolyzercanberegulatedfromzeroto100%onthetimescaleofafewsecondsassumingawarm-start,i.e.thattheelectrolyzer iskeptat itsoperatingtemperaturealsoduringzeroproduction.Theproductionfromthemethanationunitcanberegulatedonthetimescaleofaround15minutes.Theoutputofthebiogasplantcan,however,onlyberegulatedonthetimescaleofhoursordays,dependingonthestoragefacilitiesoftheplant.At least twopossible regulation strategies canbe considered for amethanationplant; onewhere biogas is stored and the electrolysis and methanation processes are regulated based on theelectricity supply (and spot prices), and one where hydrogen is stored and the methanation unit isoperatedcontinuously.Theoptimalrelativescalingof thebiogasplant,electrolyzer,methanationunitandstorageswillbedifferentforthesetwostrategies.

IntheForskELresearchprojectMeGa-stoRE[20],carriedoutbyAarhusUniversity,DTUandothers,Thegasvolumeandenergyflowsofthemethanationprocesswerecalculatedwiththeassumptionthatthewholesystemisscaledtofitanelectrolyzerwith10MWofelectricityinput.TheresultisshowninFigure2.3.

2.2 BiologicalmethanationofbiogasBiogas can bemethanised usingmicroorganisms called archaea,which also play an important role inbiogas production via anaerobic digestion. Archaea are single-celledmicrobes that are in somewayssimilartobacteria,butareclassifiedasaseparatedomainoflife.Amongthepropertiesofarchaeathatseparatethemfromotherlifeformsisthefactthattheycanuseawidearraycompounds,organicandinorganic,asenergysources.Certaintypesofarchaea,namedhydrogenotrophicmethanogens,produceCH4usingCO2asacarbonsourceandhydrogenasareducingagent.Thisprocess,calledmethanogenesis,isabiologicalequivalentofthechemicalbiogasmethanationprocessdiscussedinsubsection2.1:

CO# + 4H# ⟶ CH( + 2H#O + ΔH (Eq.4)

whereΔH=-164.9kJ/mol.ThebiologicalmethanationprocessishighlycontrollablebecausethemicrobesperformtheconversiononlywhenCO2andhydrogenareavailableandstayinactiveatothertimes.ThemicrobeshavelessstringentrequirementsonthepurityoftheinletgasesthantheSabatierprocess,buttoohighoxygen levelsareharmful to themicrobes.Theprocess takesplaceat40-70°C,which isverysimilartothetemperatureoftheanaerobicdigestionprocess(biogasformation)andconsiderablylowerthanfortheSabatierprocess.

The required hydrogen and CO2 ratio in themethanogenesis process is the same as in the chemicalmethanation of biogas. Therefore, hydrogen (from electrolysis)must be added to the system for fullutilization of the CO2 in the biogas. The ratio of hydrogen to CO2 injection shown inFigure2.3alsoholdsforthebiologicalmethanationprocess.

Themethane-generatingmicrobescanbeplacedeitherinsidethebiogasreactor(in-situ)orinaseparatebiologicalmethanation reactor (ex-situ). Eitherway, the hydrogen can be injected directlywhere the

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microbesarelocated.Incasethemicrobesarelocatedin-situ,thewholeprocessofbiogasformationandmethanationtakesplacewithinasingleanaerobicdigester,withouttheneedforanextrareactor.

The biologicalmethanation process has been known formore than 100 years, but has only recentlyreachedthedemonstrationlevel.Insections3.4and3.5belowtwodemonstrationplantsoftheex-situdesignaredescribed.

The in-situ design has the obvious advantage of simplicity and lower investments. It also solves theproblemofutilizationofthelowtemperatureheatsurplusoftheprocessinanelegantway.However,itfaces practical problems because of the limited solubility of H2 in water. This problem can be partlyovercomebycarefulinjection,wheretheH2bubblesizeisminimized,combinedwithextensivestirringofthereactor.Butthelattercausesincreasedelectricitydemandandhighlevelsofbackmixingofthegases[4].Forthesereasons,itisdifficulttoachievegoodeffectivenessfortheconversion(CH4%>90)inaneconomicmanner.TheseconsiderationsareconfirmedbyexperimentscarriedoutbyAarhusUniversityattheFoulumResearchStationina1200m3biogasreactor[21].

Another problemof the in-situ design is that themethanation process removes CO2 from the biogasreactor.ThisaltersthepartialpressuresandpHlevelsinthebiogasreactor,whichaffectstheequilibriumofthechemicalreactionsthattakeplaceduringbiogasformation.Thebiogasmethanationprocesscanthushaveanegativeinfluenceonthebiogasformationprocess.

Several researchprojects aimatovercoming theproblems relating to the in-situ design.One idea forachievingthisisbymixingH2withrecirculatedliquidfromthereactorinaseparatetank.TheH2passesametallicdiffusorfollowedbyaceramicsponge[22].Asecondprojectisworkingwiththeformationofabiofilm ina tricklebedreactor,which is relevant forsewagewater [23].A thirdproject is injectingH2throughgaspermeablemembranes,whichisrelevantforallliquidbiogasreactors[24].

Apossibleoutcomeofthepresentendeavorsinthefieldofin-situbiologicalmethanationofbiogascouldbeasuggestionforaconcept,wherethebulkofthemethanationiscarriedoutbyarelativelysimpleH2

injection,andthefinalconditioningofthegasforinjectioninthenaturalgasgridismadebyatraditionalbiogasupgradingtechnology.Inthisway,mostoftheadvantagesofthePtGprocesscanbemaintainedinaflexibleandeconomicfeasibleway.

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3 Plantdescriptionsanddatasheetsforexistingbiogasmethanationplants

3.1 HaldorTopsøe:CatalyticmethanationinFoulum,DenmarkA small demonstration plant for the catalytic methanation of biogas is located at the AgrigculturalResearchCentreofAarhusUniversityinFouluminNorthernJutland,Denmark.Thedemonstrationplantwasbuilt as a part of project called ‘El upgradedbiogas’ coordinatedbyHaldor Topsøe andwith theparticipationofAarhusUniversity,HMNNaturgas,NaturgasFyn,EnergiMidt,theDanishGasTechnologyCenter,Xergi,Cemtecfonden,EaEnergianalyseandPlanEnergi.Theprojectstartedin2013andendsmid-year2017.Thetotalbudgetoftheprojectis39.3millionDKK(5.28million€)and66%ofthisamountwasfundedthroughtheEUDPgrantprogramoftheDanishEnergyAgency.Thefollowingdescriptionanddatasheetforthedemonstrationplantisbasedonthereports[25,26].

The demonstration plant has a 50 kW SOEC electrolyzer for hydrogen production. The electrolyzerincludes8stackswith75cellseach.Thestacksarecontainedintwounitswith4stackseach.Theoperatingtemperatureoftheelectrolyzerisaround725°C.Theaimistooperatetheelectrolyzeratthermoneutralvoltage, i.e.atavoltagewherenoexcessheat isgenerated.Tocompensate for theslowlydecreasingperformanceof theelectrolysiscellsduring their lifetime, theoperating temperature isplannedtobeslightlyincreasedduringtheplant’slifetime.

The methanation plant receives biogas from Aarhus University’s on-site biogas plant. The biogas iscomposedof43%CO2,57%CH4and small amountsoforganic sulfur compounds,oxygenandothers.Thesecompoundsareharmful for themethanationcatalystand therefore theplant includesabiogasclean-upunitforremovingthempriortothemethanation.

The methanation unit is designed to yield 10 Nm3/h of biomethane. A proprietary nickel-basedmethanationcatalystfromHaldorTopsøeisused.Thereactiontakesplaceat280°C.Themethanationprocess is designed such that the resulting gas should be at least 96% pure CH4. The product gas isconditionedbydryingittoadewpointoflessthan-8°Candcompressingitto40bar(forinjectiontothenaturalgasdistributiongrid).TheLHVconversionefficiencyof(biogas+hydrogen)tobiomethaneiscloseto80%.Thebulkoftheremaining20%oftheenergyinputsisconvertedtoheat,withtheprocessyieldingusableheatbothatthereactiontemperatureandatlowertemperatures.

Toreachthedesignyieldof10Nm3/hofbiomethanewhenmethanatingbiogaswith43%CO2content,the50kWSOECmustprovide17.3Nm3/hofhydrogen.Thishydrogenproductioncorrespondstoalowerheating value (LHV) electricity-to-hydrogen efficiency of 96.5%. Such high efficiency values can beobtainedaccordingtoHaldorTopsøe’scalculations, inpartbecausetheexcesshigh-temperatureheatfromthemethanationprocessisusedforevaporatingthewaterusedintheelectrolyzer(seeFigure3.1).Thisheatinputisnottakenintoaccountinthecalculatedefficiencyof96.5%.Becauseofthesteaminput,theelectrolyzerdoesnotconsumeenergyforevaporatingliquidwater.

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HaldorTopsøehascarriedoutcalculationsforafullscalemethanationplantyielding7.5millionNm3/yearofbiomethane,whichequalsapprox.940Nm3/hfor8000annualoperatinghours.ThecalculatedLHVefficiency for the total conversion of biogas + electricity to biomethane is 76.3%. The LHV biogas +electricityconversionefficiencytoheat (forusebiogasproductionand/ordistrictheating) is14%.Thetotal calculated LHV efficiency of (biogas + hydrogen) to (biomethane + heat) is therefore 90.3%. Animportantpartinreachingsuchhighefficienciesisthesynergyeffectofutilizinghigh-temperatureheatfromthemethanationasaninputfortheelectrolyzer.

Early results from the plant operation showed a H2 production of 16.3 Nm3/h. This equals an LHVelectricity-to-hydrogenefficiencyof91.0%.However,thefullintegrationofsteamgenerationandusehasnotbeenimplementedatthedemonstrationplant.Thesameearlyresultsshowedthattheproductgasisofsufficienclyhighqualityforgridinjection,containing97.7%CH4,1.4%H2,0.9%N2andtraceamountsofCO2.ThiswillbemeasuredinmoredetailbytheDanishGasTechnologyCenterlaterintheproject.

No biogas or hydrogen storage facilities are included in the demonstration plant. The feasibility ofincludingsuchstorageswillbeinvestigatedasapartofupscalingtoalargermethanationplant.

Figure3.1AschematicofthesystemlayoutofthecatalyticbiogasmethanationfacilityinFoulum.

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Table3.1AdatasheetforthecatalyticbiogasmethanationfacilityinFoulum.Thenumbersarebasedonactualplantconfigurationandoperationdata,andnotjustdesignfigures.Nocostinformationisavailableforthissmalldemoplant.Datatype Value Unit Ref. Electrolysis Electrolyzertype SOEC [25]Electrolysiselectricityinputcapacity 50 kW [25]Hydrogenoutput(designvalue) 17.3 Nm3/h [25]Oxygenoutput(designvalue) 8.65 Nm3/h [25]Operatingtemperature 725 °C [25]Start-uptime,coldstart 10-11 Hours [25]Start-uptime,hotstart 0-100%inseconds [25] Methanation Reactortype Boilingwatertype [25]CO2source Directlyfrombiogas [25]Hydrogeninput 17.3 Nm3/h [25]Biogasinput(57%CH4,43%CO2) 10 Nm3/h [25]CO2input 4.3 Nm3/h [25]Biomethaneoutput(designvalue) 10 Nm3/h [25]BiomethaneCH4content(measured) 97.7 % [26]BiomethaneH2content(measured) 1.4 % [26]BiomethaneN2content(measured) 0.9 % [26]BiomethaneCO2content(measured) Traceamount [26]Operatingpressure1 “Mediumpressure” [25]Operatingtemperature 280 °C [25] Efficiency Electrolysisefficiency(LHV),electricitytoH2(measured)2

91.0 % [25]

Electrolysisefficiency(LHV),electricitytoH2(designvalue)3

96.5 % [25]

Methanationefficiency,hydrogen+biogastobiomethane4

79.1 %

Total efficiency, biogas + electricity tobiomethane(designvalue)

76.3 % [25]

Usefulheatoutput(LHV),biogas+electricitytoheat(designvalue)

14.0 % [25]

Totalenergyefficiency(LHV),el.+biogastobiomethane+heat(designvalue)

90.3 % [25]

1Nonumericaldataavailable.2Basedonearlyresults,presentedin[25],with50kWelectricityinput,32Nm3/hsteaminputandahydrogenproductionof16.3Nm3/h.3Basedontheplant’sdesignvaluesof50kWelectricityinput,steaminput(fromthemethanationprocess)andahydrogenproductionof17.3Nm3/h.4Calculatedbasedonanelectrolysisefficiencyof95.5%andatotalefficiencyof76.3%.

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3.2 Etogas&Audi:CatalyticmethanationinWerlte,Germany

The largest biogas methanation plant in operation in the world (as of October 2016) is a catalyticmethanation plant located inWerlte, Lower Saxony,Germany [27–30]. The plantwas constructed byEtogas (formerly called Solarfuel) for theGerman automakerAudi, in collaborationwith the researchinstitutionsFraunhofer IWESandZSWandtheenergycompanyEWE.ThegasandenergyflowsoftheplantareshowninFigure3.2.Table3.2isadatasheetwithtechnicalspecificationsfortheplant.NotethatallnumbersinTable3.2aredesignvaluesandnotmeasurementvaluesfromtheoperationoftheplant.

Theplanthas three2MWalkalineelectrolyzerunits forhydrogenproduction, i.e.a totalelectrolyzercapacityof6MW.Theenergyefficiencyoftheelectrolysisprocessis70%andthemaximumhydrogenoutput is 1310 Nm3/h. The plant has a hydrogen storage capacity corresponding to the maximumelectrolyzeroutputofonehour.Thehydrogenisstoredinpressurizedvessels.

ThemethanationplantislocatedatthesiteofalargebiogasplantwherebiogasisalsoupgradedbyCO2removal(aminescrubbing)forinjectiontothenaturalgasnetwork.Theannualbiomethaneinjectiontothegridfromthisprocessisapproximately3millionNm3[27].Around340Nm3/hCO2areremovedfromthebiogasintheaminescrubbingprocess.ThisisusedastheCO2feedstockforthebiogasmethanation.Themethanationunitthereforedoesnotdirectlyreceivebiogas,butonlytheCO2fractionafter ithasbeenseparatedfromthemethaneinthebiogas.

The biogasmethanation unit produces biomethane fromhydrogen and CO2 according to the processdescribedinsection2.1.Ittakes1300Nm3/hofhydrogenand325Nm3/hofCO2asinputsandyields325Nm3/hofbiomethane.Theenergyefficiencyofthemethanationprocessisaround78%,withanadditional10%energy output in the formof heat. In theWerlte plant, this excess heat is utilized in the biogasproduction(anaerobicdigestion)andthebiogasupgrading(aminescrubbing)processes.Theoutputgasis composed of >91%methane, <5% hydrogen and <6% CO2. The biomethane from themethanationprocessfulfillstheGermanrequirementsforinjectiontothegasgrid(althoughpropaneinjectionmayberequiredtofulfillallregulations,dependingonwhereinthegasgridtheinjectiontakesplace,accordingtoDVFWG685).

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Figure3.2AschematicofthesystemlayoutofthebiogasmethanationfacilityinWerlte.

Theplantisexpectedtohave4000annualfullloadhours,whichyieldsaproductionof1000metrictons(1.5millionNm3)ofbiomethane.ThemotivationforAudi toproducebiomethanefromelectricityandbiomass is that this low-emission biomethane compensates for the CO2 emissions of their gas-fueledvehicles, that consumenatural gasof fossil origin. Thebiomethaneproduction from theWerlteplantwouldsufficetopower1500AudiA3e-tronbiomethanecars,assumingthatthecarsaredriven15000km/yearonaverage[28].AccordingtoAudi,thismakesupfor2800tons/yearofCO2thatareemittedfromthesecars[30].

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Table3.2DatasheetwithdesigndataforthebiogasmethanationplantinWerlte,Germany.Datatype Value Unit Ref. Electrolysis Electrolyzertype Alkaline [28]Electrolyzerunitelectricityinputcapacity 3unitswith2 MW/unit [28]Electrolysistotalelectricityinputcapacity 6 MW [28]Hydrogenoutput 1310 Nm3/h [28]Oxygenoutput 655 Nm3/h [28]Operatingtemperature 80 °C [28]Regulationability(foreachelectrolyzer) 30-100 %ofmax. [28]Hydrogenstoragecapacity 6 MWh [28] Methanation Reactortype Fixed-bed [28]CO2source BiogasCO2removal

(aminescrubbing) [28]

Hydrogeninput 1300 Nm3/h [28]CO2input 325 Nm3/h [28]Biomethaneoutput 325 Nm3/h [28]BiomethaneCH4content >91 % [28]BiomethaneH2content <5 % [28]BiomethaneCO2content <6 % [28]Operatingpressure 8-10 bar [28]Operatingtemperature 250-550 °C [4] Efficiency Electrolysisefficiency(HHV) 70 % [29]Methanationefficiency 77 % [28,29]TotalefficiencyelectricitytoBiomethane 54 % [28]Usefulheatoutputfrommethanation 10 % [28]Methanationwasteheatutilization Biogasproduction,

biogasCO2removal(aminescrubbing)

[28]

Totalenergyefficiency(el.tobiomethane+heat)

64 % [28]

Otherinformation Plannedannualfullloadhours 4000 hours [28]Plannedannualbiomethaneproduction 1’500’000 Nm3 [28]Startofoperation June,2013 [28] Costs Nocostinformationavailable

17

3.3 MeGa-stoRE:CatalyticmethanationinLemvig,DenmarkAbiogasmethanationresearchanddemonstrationprojectcalledMeGa-stoREwascarriedoutfrom2013-2015,withademonstrationfacilityatabiogasplantinLemviginWestern-Jutland,Denmark[1,2,20].TheprojectpartnerswereDTUMechanicalEngineering,AarhusUniversityinHerning,Lemvigbiogas,GreenHydrogenandElplatek.TheprojectwasfundedbytheForskELgrantprogrammeoftheDanishelectricityandgasTSO,Energinet.dk.

Intheproject,averysmallbiogasmethanationpilotplant,dimensionedforanbiomethaneoutputof1Nm3/hour,wasconstructedandtested.Accordingtotheprojectpartners,thisbiogasmethanationplantisthefirstthatdirectlymethanisesbiogaswithoutfirstseparatingitsmethaneandCO2contents[20].Theplantusesanovelsulfurcleaningandone-stepmethanationreactor,designedasapartoftheproject.

TheMeGa-stoRE project report [20] contains bothmeasurement data from the pilot plant aswell asassumptionsandcalculationsforalarger(10MW)plantthathasnotyetbeenconstructed.Theavailablemeasurement data for the pilot plant is shown in Table 3.3. Unfortunately, no data is available onelectrolysis,ontheeconomicsorontheenergyefficiencyofthecurrentpilotplantintheMeGa-stoREprojectreports.

Itisnotexplicitlymentionedintheprojectreportsifthehydrogenwasproducedviaelectrolysison-site,oriftheysimplypurchasedcompressedhydrogengasonbottlesforthepilotplant.Duetothelackofdiscussion of electrolysis in the reports, and due to the small hydrogen flow rates of 1.4 Nm3/hour(correspondingtoanelectrolyzercapacityofonly6-7kW),thelatterseemsmoreplausible.Asuccessorproject,“MeGa-stoRE,UpgradingandUpscaling”,hasbeenfundedbyForskELandrunsfrom2015until2018.Inthatproject,hydrogenwillbeproducedusinga250kWalkalineelectrolyzer.

Table3.3DatasheetfortheMeGa-stoREbiogasmethanationdemonstrationplantinLemvig,Denmark.Allnumbersinthetablearebasedonmeasurementresultspresentedin[20].NodatawasavailableontheefficiencyoreconomicsofthecurrentpilotplantintheMeGa-stoREproject.Datatype Value UnitMethanationreactortype Newdesign CO2source Directlyfrombiogas Biogasinput(65%CH4,35%CO2) 0.72 Nm3/hHydrogeninput 1.0 Nm3/hBiomethaneCH4content >97 %BiomethaneH2content <3 %BiomethaneCO2content <2 %Operatingpressure 8 barOperatingtemperature 260-310 °CReactorvolume 0.0004 m3Startofoperation 4thquarterof2014

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3.4 Electrochaea:BiologicalmethanationinAvedøre,DenmarkThelargestbiologicalmethanationprojectinDenmarkislocatedattheBiofoswastewatertreatmentplantinAvedøre,justoutsideofCopenhagen.TheplantwasconstructedbythecompanycalledElectrochaea,withAudi,Hydrogenics,NEASEnergy,HMNNaturgas,BiofosandInseroaspartners.Thisdemonstrationproject received funding from the ForskEL grant programme of Energinet.dk under the project nameBioCat[31].Theprojectstartedin2014.Thetotalprojectbudgetis49.9millionDKK(6.7million€)and55%ofthisamountwasfundedbyForskEL.ThefinalreportoftheBioCatresearchprojectisexpectedtobepublishedinthespringof2017.

Amuchsmallerproof-of-conceptplant,alsosupportedbyForskELwassetupbyElectrochaeaandAarhusUniversityatAarhusUniversity’sAgricultureResearchCentreinFoulum,Denmarkpriortothisproject.

AttheplantinAvedøre,twelvealkalineelectrolyzermodulesfromtheprojectpartnerHydrogenics,withatotalelectricityinputcapacityof1.2MW,areusedforproducinghydrogen.Theelectrolyzersconsume5.5kWh/Nm3H2andhaveatotalhydrogenoutputcapacityof200Nm3/h.ThisequalsaLHVelectricitytohydrogen efficiency of 51%. The pressure of the output hydrogen is 12-13 bar. Each of the twelveelectorlyzerscanoperatewitha40%-100%load.Theelectrolyzerspowercanberampedupanddownonthetimescaleofminutesandcouldinprinciplebeusedforprovidingcapacityregulationservicestothegrid(althoughthishasnotbeentestedintheBioCatproject).

ThebiologicalmethanationunitisdesignedbyElectrochaeaandwasconstructedbyasubcontractorfromtheNetherlands.Theplantusesanex-situreactor,meaningthatthehydrogeninjectionandmethanationdoesnottakeplaceinthebiogasplant(thewastewatertreatmentplant)itself.Theprimaryreasonsforthisaretheproblemswithin-situreactorsmentionedinsection2.2.Duringtheplantstartup,thearchaeamicroorganismswereinjectedinthereactor,whereittookthemthreeweekstomultiplytotheirfinalpopulation.Afterthisphase,thearchaeaperformthetaskofproducingmethanewheneverhydrogenandCO2areavailable.Themethanationprocesscanberampedupanddownalmostinstantlybyregulatingthegasflows.Thebiologicalprocessisthereforenotabottleneckintheregulationabilityoftheplant.Theenergyconversionefficiencyofthebiologicmethanationprocessfromhydrogentomethaneis84%,basedonmeasurementdatafromMarch2017.Themethanationtakesplaceatapressureof9barandatemperatureof63°C.

Thearchaeaarevery tolerant regardinggas impurities, suchasH2SandO2.Theproductgas fromthemethanationreactoris90-95%pureCH4.Inordertoincreasethemethanepurityabove97%(whichistherequiredvalueforinjectiontothedistributiongrid),thegasgoesthroughamembranecleaningunit.AnironspongeisusedforremovingH2Sfromthegas.Theresultinggasis98%CH4,2%H2,1%CO2and<40ppmH2O.Thegaswillbeinjectedtothe4barlocalgasdistributionnetworkofHMNNaturgas.Thishasnotbeentestedyet,butallconnectionsareinplaceandgridinjectionisplannedtostartsoon.

Duringtheproject,theplantwillbetestedbothonrawbiogas(65%CH4,35%CO2)andonpureCO2(fromanaminescrubbingbiogasupgradeprocess).Untilnow,theplanthasonlybeentestedwithrawbiogas,astheaminescrubbingunit isstillunderconstruction. Inthebiogasoperationmode,thebiomethaneoutput capacity is limited by the biogas injection capacity, and not by the electrolysis capacity. The

19

electrolysisthereforeonlyoperatesat46%oftheinstalledcapacitytoprovidethestoichiometricamountof hydrogen required for the methanation of 60 Nm3/h of biogas. In the CO2 operation mode, theelectrolysiscapacitywillbefullyutilizedtomethanate50Nm3/hofCO2.

Theplantdoesnotincludehydrogenorbiogasstoragefacilities,asthemethanationplantisrathersmallcomparedwiththebiogasplantatthesite.Hydrogenisproducedondemandandthesystemistestedbothwithcontinuousoperationandwithstartandstopeverynight.Thefinaloperationstrategyoftheplantistooperatetheelectrolysisonlywhentheelectricitypriceissufficientlylow.

Theelectrolysisprocessyieldsusableexcessheatamountingto20%oftheelectricalenergyinputandthemethanationprocessyieldsusableexcessheatcorrespondingtoaround15%oftheH2energyinput.Theexcessheatisextractedviaaheatexchangerandhasatemperatureofabout58°C.Thisisslightlytoolowformostdistrictheatingnetworks.Aheatpumpwouldthereforeberequiredtoboostthetemperaturetodistrictheatinglevels,iftheplant’sexcessheatshouldbefullyutilized.ThishasnotbeendoneattheplantinAvedøreduetoitssmallscalebutwillbeconsideredinthefutureforlargerplants.

The total capital expenses for this first-of-its-kind plant were around 3.85 million €. Electrochaea iscurrentlybuildingaseconddemonstrationplantinSolothurninSwitzerlandasapartoftheHorizon2020projectStore&Go[32].A10MW(electrolyzerenergyinput)methanationplantisconsideredafull-scaleplant by Electrochaea and the company has recently begunwork on a plant at this scale at anotherlocation.Electrochaea’sfuturecosttargetforafull-scaleplantisaround1million€.

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Figure3.3AschematicofthesystemlayoutofthebiogasmethanationfacilityinAvedørewhenbiogasisusedasafeedstockforthemethanationreactor.

Figure3.4AschematicofthesystemlayoutofthebiogasmethanationfacilityinAvedørewhenpureCO2isusedasafeedstockforthemethanationreactor.Thishasnotbeentestedyet,astheaminescrubbingfacilityatthesiteisstillunderconstruction.

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Table3.4DatasheetforthebiogasmethanationplantinAvedøre,Denmark.ThemeasurementvaluesinthedatasheetweremeasuredduringbiogasinjectionmodeinSeptember2016.Datatype Value Unit Ref. Electrolysis Electrolyzertype Alkaline [33]Electricityinputcapacity 1.2 MW [33]Electricityconsumption5(measured) 5.5 kWh/Nm3H2 [33]Waterconsumption(measured) 0.8 liters/h [33]Maximumhydrogenoutput(99%pureH2) 200 Nm3/h [33]Maximumoxygenoutput 100 Nm3/h [33]MaximumH2operatingpressure 13 bar [33]Partialloadoperationcapability 4-100 % [33]Datatype Value Unit Ref. Methanation Reactortype Separatereactor [33]CO2source Wastewatertreatment [34]Maximumhydrogeninput 200 Nm3/h [33]Maximumbiogasinput(35%CO2,65%CH4) 60 Nm3/h [33]MaximumCO2input(pureCO2stream) 50 Nm3/h [33]Maximumbiomethaneoutput 60 Nm3/h [33]BiomethaneCH4content(measured) >98 % [33]BiomethaneH2content(measured) <2 % [33]BiomethaneCO2content(measured) <1 % [33]BiomethaneH2Ocontent(measured) <40 ppm [33]Operatingpressure(measured) 9 bar [33]Operatingtemperature(measured) 63 °C [33]Excessheatoutputtemperature(measured) 58 °C [33] Efficiency6 Electrolyzerenergyefficiency(LHV),electricitytohydrogen(measured)7

51 % [33]

Electrolyzerenergyefficiency,electricitytoheat(measured)

20 % [33]

Methanationenergyefficiency(LHV),hydrogentobiomethane(measured)

84 % [33]

Methanationenergyefficiency,hydrogentoheat(measured)

15 % [33]

5At„beginningoflife“.Thisvaluewillprobablyslightlyincreaseoverthelifetimeoftheelectrolyzerduetocelldegradation.6Themethanecontentofbiogaspassesunalteredthroughthemethanationreactor.Theefficiencycalculationsthereforeonlytakeenergyinputintheformofhydrogenintoaccount.7Calculatedbasedonanelectricityconsumptionof5.5kWh/Nm3H2.

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Totalenergyefficiency(LHV),electricitytobiomethane8

43 % [33]

Totalenergyefficiency,electricityto(biomethane+heat)3

70 % [33]

Otherinformation Plantramp-uptime,coldstart 60 min. [33]Plantramp-uptime,hotstart 15 min. [33]Plantshut-downtime <1 min. [33]Startofoperation April2016 [33]Investmentlifetime 20 years [33] Costs Electrolyzerinvestment 2.1 M€ [33]Methanationinvestment 1.75 M€ [33]Operatingexpenses Noinformation [33]

8Calculatedbasedontheelectrolyzerefficiencyandthemethanationefficiency.

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3.5 MicrobEnergy&Audi:BiologicalmethanationinAllendorf,GermanyThelargestbiologicalmethanationprojectinGermanyislocatedatabiogasplantinAllendorf(Eder)inHessen. The plant was constructed by MicrobEnergy, which is a subsidiary of Viessmann Group, incooperationwithAudi.LikewiththemethanationplantinWerlte,themotivationforAuditoinvestinthisproject is that the low-emission biomethane compensates for the CO2 emissions of their gas-fueledvehicles, thatconsumenaturalgasof fossilorigin. Theplant ispartofademonstrationprojectcalledBioPower2Gas,whichisexpectedtoreceivea30%subsidyoftheinvestmentcostsaccordingto[35].TheavailabledatafortheplantareshowninTable3.5.NotethatalldatainTable3.5aretheplant’sdesigndatabutnotmeasurementdatafromtheoperationoftheplant.

Theplanthasa300kWPEMelectrolyzerthatproduces60Nm3/hofhydrogen.Thehydrogenisinjectedintoanex-situ biologicalmethanation reactor togetherwithbiogas containing15Nm3/hofCO2. Thisyieldsanoutputof15Nm3/hbiomethane.Afterthemethanation,thebiomethanefulfills theGermanrequirementsforinjectionintothegasgrid.Theenergyefficiencyofthebiologicalmethanationprocessis75-80%[35].Theplantislocatedatabiogasplanthatproduces330Nm3/hofbiogas.Theplanthasabiogasupgradingfacility(withouthydrogenaddition)thatinjected175Nm3/hofbiomethaneintothegridbeforethemethanationplantwasbuilt.Themethanationplantonlytakesasmallfractionofthebiogasproduced at the plant as an input. The project partners have plans about doubling the biomethaneproductioncapacityofthemethanationplant[36].

Theelectrolysisandmethanationprocesscanberegulatedquickly.Thebiologicalmethanationprocesscanbefullystartedandstoppedin lessthanaminute.Whenthemicrobeshavetheinputgases,theyperformtheworkrequiredformethaneproduction,otherwisetheyaredormant.Animportantpartofthebusinessplanbehindtheplantistoprovideancillaryservicestothegrid;thesalesofsuchservicesareinfactplannedtocontributemoretotherevenueoftheplantthanthesalesofbiomethanetothegrid[35].

24

Figure3.5AschematicforthelayoutofthebiogasmethanationfacilityinAllendorf.

25

Table3.5DatasheetwithdesigndataforthebiogasmethanationplantinAllendorf,Germany.Datatype Value Unit Ref. Electrolysis Electrolyzertype PEM [36]Electricityinputcapacity 300 kW [36]Hydrogenoutput 60 Nm3/h [36]Oxygenoutput 30 Nm3/h [36]Operatingpressure 40 bar [36] Methanation Reactortype Separatereactor [36]CO2source Biogasplant [36]Hydrogeninput 60 Nm3/h [36]CO2input 15 Nm3/h [36]Biomethaneoutput 15 Nm3/h [36]BiomethaneCH4content >95 % [35]BiomethaneH2content <3 % [35]BiomethaneCO2content <3 % [35]Operatingpressure 5 bar [37]Operatingtemperature 40 °C [35]Regulation(start/stop) <1 minute [35] Efficiency Methanationefficiency 75-80 % [35] Otherinformation Startofoperation Spring2015 [36]Investmentlifetime 20 years [35] Costs No reliable cost data available. Some cost data is

presented in [35], but these costs are estimatesfrom2013foralargerversionoftheplant.

26

3.6 OtherbiogasmethanationplantsThefollowingisalistofotherbiogasmethanationplantsthattheauthorsareawareof,butaboutwhichtheyhavenofurtherdataorknowledgeatthispoint[38].Theseplantsareallverysmallcomparedwithmostofthemethanationplantscoveredintheprevioussections.

• DVGW:CatalyticmethanationinKarlsruhe,Germanyo AtestsetupoftheDVGWresearchcenterattheEnglerBunteInstituteofKarlsruhe

InstituteofTechnology.o Usesa200kWelectrolyzer.

• MicroPyros:BiologicalmethanationinStraubing,Germanyo LocatedatthewastewatertreatmentplantinStraubingo Atestsetupwithanbiomethaneproductionofonly0.4Nm3/h.

• Stedin:CatalyticmethanationinRozenburg,Netherlandso Asmalldemonstrationplant.o TheonlybiogasmethanationprojectoutsideGermanyandDenmarkknowntothe

authors.o Usesa10kWPEMelectrolyzer.

TheHorizon 2020 R&Dproject Store&Go [32],which started inMarch 2016, aims at setting up bothcatalytic and biologicalmethanation plants at three locations; Falkenhagen in Germany, Solothurn inSwitzerlandandPugliainItaly.Theseplantswillbeonademonstrationscale(<1MW)withtheaimofincreasingthetechnologyreadinesslevelofthemethanationprocesses.

Furthermore, a recent publication by Bailera et al. provides a listing of existing research anddemonstrationprojectsinthefieldofbiogasmethanation[39].

27

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