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Title Hydrothermal treatment followed by enzymatichydrolysis and hydrothermal carbonization as meansto valorise agro- and forest-based biomass residues

Author(s) Wikberg, Hanne; Grönqvist, Stina; Niemi, Piritta;Mikkelson, Atte; Siika-aho, Matti; Kanerva, Heimo;Käsper, Andres; Tamminen, Tarja

Citation BioresourceTechnology. Elsevier. Vol. 235 (2017), pages 70-78

Date 2017URL DOI: 10.1016/j.biortech.2017.03.095Rights Pre-print version of the article. This article may be

downloaded for personal use only.

1IntroductionDuetotheneedtofindalternativerenewablesourcestoreplacefossilrawmaterialsfortheproductionoffuelsandchemicals,newwaystoexploitlignocellulosicbiomassarebeingextensivelystudied.Atthesametimethe

amountoflignocellulosicsavailablefornewapplicationsislimited.Thus,thechallengeofincreasingtheusageoflignocellulosicbiomassasafeedstockforvariousapplicationse.g.bioenergyandbiochemicalscanbeansweredbythe

utilizationofunexploitedrawmaterialsources.

Varioustypesofside-streamsaregeneratedinagricultureandforestindustry.Mostofthesestreamsarepoorlyutilisedorcombustedforenergy.Thechallengesinthecost-efficientutilizationoftheseside-streamsforadded

valueproductsaretheirheterogeneity,seasonalavailabilityandtheirremotelocations(LeboreiroandHilaly,2011;Lietal.,2016;Mosieretal.,2005).Toovercomethesecomplexitiesmobileandflexibleprocessingstepssuitablefor

variousrawmaterialsareneeded.Pre-treatmentstepsthatcanfractionateandsortdifferentrawmaterialstreamsenableutilizationofwiderrawmaterialbasisindifferentapplications.Itis,however,criticaltorecognizethatall

biomassfractionsarenotequallysuitableforconversionintobiofuels,biochemicalsorbiopower(Lietal.,2016).Thequalityofbiomasswillaffectthematerialhandling,conversionefficiencyandthusproductyieldandevenproduct

quality.Thus,thebasisforefficientandsustainableutilizationofbiomassside-streamsisinthedeepunderstandingofthebiomasschemistry,structureandthefactorsaffectingthesuitabilityofbiomassforvariousprocesses.

Numerous biotechnical as well as thermochemical processing routes for biomass have been suggested over the years. E.g. enzymatic hydrolysis of polysaccharides of plant biomass into monomeric sugars followed by

Hydrothermaltreatmentfollowedbyenzymatichydrolysisandhydrothermalcarbonizationasmeanstovaloriseagro-andforest-basedbiomassresidues

HanneWikberga,⁎

hanne.wikberg@vtt.fi

StinaGrönqvista

PirittaNiemia

AtteMikkelsona

MattiSiika-ahoa

HeimoKanervaa

AndresKäsperb

TarjaTamminena

aVTTTechnicalResearchCentreofFinlandLtd,P.O.Box1000,FI-02044VTT,Espoo,Finland

bBiogoldOY,Lossi19A,12616Tallinn,Estonia

⁎Correspondingauthor.

Abstract

Thesuitabilityofseveralabundantbutunderutilizedagroandforestbasedbiomassresiduesforhydrothermaltreatmentfollowedbyenzymatichydrolysisaswellasforhydrothermalcarbonizationwasstudied.The

selectedapproachesrepresentsimplebiotechnicalandthermochemicaltreatmentroutessuitableforwetbiomass.Basedontheresults,thehydrothermalpre-treatmentfollowedbyenzymatichydrolysisseemedtobemost

suitable for processing of carbohydrate rich corn leaves, corn stover, wheat straw and willow. High content of thermally stable components (i.e. lignin) and low content of ash in the biomass were advantageous for

hydrothermalcarbonizationofgrapepomace,coffeecake,Scotspinebarkandwillow.

Keywords:Hydrothermalcarbonization;Hydrothermaltreatment;Enzymehydrolysis;Lignocellulose;Biomass

fermentationhasbeenclaimedtobeoneofthemostpromisingapproachtoproducerenewablechemicals,e.g.ethanol,withhighyields(HendriksandZeeman,2009;Jørgensenetal.,2007;SunandCheng,2002).Forenzymatic

hydrolysis of lignocellulosic biomass, a pre-treatment of some sort is a pre-requisite in order to achieve high saccharification yields (Hendriks and Zeeman, 2009). The purpose of the pre-treatment is to make the cell wall

polysaccharidesmoresusceptibleforenzymes,asintheirnativestatelignocellulosicmaterialsareforthemostpartresistanttohydrolyticenzymes.Themostcommonpre-treatmentmethodusedpriortohydrolysisoflignocellulosic

material issteamexplosion (Pielhopetal.,2016;SunandCheng,2002).Anothercommonlyusedsteampre-treatment ishydrothermal treatment.Thechemicalchangescausedby thesemethodsarenearly similar,and themain

differencebetweenthemisthatsteamexplosionisusuallycarriedoutinhighdrymattercontent,whereasmorewaterispresentinhydrothermaltreatment(Kallioinen,2014).Duetothesetreatmentscelluloseinbiomassbecomes

moreaccessibletoenzymatichydrolysisasmostofhemicellulosesaresolubilizedintotheliquidphase(Mosieretal.,2005).Ahydrothermaltreatmentisapotentoptionforpre-treatmentoflignocellulosebiomassduetothesimplicity

ofthereactorneededandtheuseofwaterassolvent.

Processingbiomassofdiverseoriginsbyhydrothermalcarbonization(HTC)hasalsogainedalotofinterestduringrecentyears(FunkeandZiegler,2010;Huetal.,2010;Libraetal.,2011;Titiricietal.,2015;Titiriciand

Antonietti,2010;Wikbergetal.,2015a).ByHTCbiomasscanbeconvertedtocoal likematerial inanenergy-efficientcarbonizationprocessatmoderate temperatures (∼180–260°C).Thevariouschemical reactions inHTC,e.g.

hydrolysis,dehydration,decarboxylation,polymerizationandcondensationresultinasolidcarbonaceousmaterial,aliquidphaseconsistingofsmallmolarmassdegradationproductsandaminorgasphase(FunkeandZiegler,2010).

Thesolidcarbonaceousmaterial,definedashydrocharinthetext,consistsofparticlesofdifferentshapes,sizeandcomposition(SevillaandFuertes,2009;TitiriciandAntonietti,2010).Duetothediversepropertiesofhydrochar,

severalpossibleenduseshavebeenproposedinmanyareasincludingenvironmental,catalytic,electronicandbiologicalapplications(Titiricietal.,2012).

HemicelluloseandcelluloseareamongthemoststudiedrawmaterialsinacidichydrothermalconditionsofHTC(Falcoetal.,2011;SevillaandFuertes,2009).Undertheseconditions,waterbreakstheglycosidicbondsof

hemicellulosedegradingitintosugarmonomers,whichfurtherdegradeintofurfuralsandothercompounds,including5-HMF(hydroxylmethylfurfural).HMFformspolymericcarbonaceousparticlesandsimultaneouslydecomposes

into lowmolecularweight acidsby time (Falco et al., 2011; Titirici et al., 2008). Cellulose requires more severe conditions to break the glycosidic bonds. The conversion can occur either via hydrolysis to glucose or via direct

transformation through intramolecular condensation, dehydration and decarboxylation (Falco et al., 2011; Sevilla and Fuertes, 2009; Wikberg et al., 2016). Lignin is far more stable in higher temperatures than cellulose and

hemicelluloseandthusitisonlyslightlyalteredinHTC(Dinjusetal.,2011;Kangetal.,2012;LuandBerge,2014;Wikbergetal.,2015b).Theuseofcrudebiomassinsteadofpurebiomasscomponentswouldmakethehydrochar

productioneconomicallymorefeasible.SeveralstudiesconcerningtheuseofcomplexlignocellulosicbiomassesinHTChaverecentlybeenpublished(Hoekmanetal.,2011;Lynametal.,2015;Mummeetal.,2011;Poerschmannetal.,

2014;Rezaetal.,2014,2013a,2013b).

Inthiswork,theprocessabilityofseveralabundant,butunderutilizedagroandforestbasedbiomassresiduesbyhydrothermaltreatmentfollowedbyenzymatichydrolysisaswellasbyhydrothermalcarbonizationwasstudied.

Theselectedpaths,representingsimplebiotechnicalandthermochemicalroutesarebothsuitableforwetbiomasses,havingthustheadvantagethatnodryingisneededpriortoprocessing.Theprocessingrouteswerecomparedin

termsofsuitabilityforthedifferentbiomassfractions.

2Materialsandmethods2.1Rawmaterials

NineEuropeanbiomassresidueswereusedinthisstudy,i.e.cornstover(leavesandstalksofcornplantsleftinthefieldafterharvest),cornleaves(leavesaroundthecorncob),coffeecake(roastedcoffeebeansfromwhich

thearomashavebeenextractedwithhotwatertoproducecoffeeconcentrate i.e. instantcoffee),grapepomace(solidremainsofgrapesaftermechanicalandwaterextraction inwinemaking),greenhouseresidues(plantwaste

resultingfromthemaintenanceandrenewalofpublicandprivategreenspaces),Salixwillow,Scotspinebark,wheatstrawandbrewersspentgrain(BSG).

Thecornstover,cornleavesandgreenhouseresiduesweremilledassuch(inwetstage)usingagardenwastemill(BoschAXT23TC,RobertBoschGmbH,Stuttgart,Germany)priortoanalysisandprocessing.Willowstalks,

Scotspinebarkandwheatstrawweredriedat<70°inaflat-beddryingwagonwithforcedventilationpriortoshreddingusingasingle-shaftshredder(LindnerMicromat2000,Lindner-RecyclingtechGmbH,Spittal,Austria)witha

15mmscreen.Otherfractionswereusedassuchifnototherwisementionedlaterinthetext.

2.2Hydrothermalpre-treatmentfollowedbyenzymatichydrolysisThehydrothermalpre-treatmentutilisedinthisworkwasahotpressurizedwaterpre-treatmentperformedinalaboratoryscalefixedbedflow-throughreactor.Thereactorhadatotalvolumeof6L,withanelectricheaterand

liquidphasecirculationpump.Foreachpre-treatmenttrial550–1000g(wetbasis)materialwasloadedintoreactorandwaterwasaddedinordertoobtaintheinitialDM:waterrateabout1:6to1:7w/w,andthepHwasadjustedto12

with5MNaOH.ThepHwasadjustedtothisleveltoensurethatthefinalpHafterthetreatmentwouldremainabovepH4asunwanteddegradationofhemicelluloseisknowntotakeplaceatlowerpH.Thesubstantialreleaseofacidic

compounds,especiallyaceticacidfrombiomassinthetreatmentconditionsevidentlyneutralizestheamountsofalkaliused.Liquidphasewaspumpedthroughthepackedmaterialbedatacirculationflowrateof1L/min. Before

processstartthereactortemperaturewasraisedto85-–90°Candheldfor30–45minforbetterwetting.Reactiontimemeasurementwasstartedatthemomentwhenthetemperaturebuild-uphadbeendeveloped.Theaverageheating

ratewas10-–12°C/min,thusapre-treatmenttimeof15minat190°Crequiredatotaltimeof30min.Afterthereactionwasstopped,liquidwasdrivenoutfromthebottomreliefvalveandcooleddowninadouble-pipeheatexchanger.

Thepressurewasreleased,thereactorcoverwasopenedandsolidphaseremovedfromreactor.Bothliquidandsolidfractionswereweighed,andpHanddrymattercontentofextractwasmeasured.Bothsolidandliquidfractions

werestoredinfreezer(-(−18°C)priortofurtheruse.

Enzymatichydrolysisofbothuntreatedbiomasssamplesandsamplesprocessedbythehydrothermaltreatmentwascarriedouttoevaluatetheeffectofthehydrothermaltreatment.Theuntreatedmaterialswereair-driedand

milledwithaWileymill (2mmsieve) inorder togethomogenoussamples.Thepre-treatedsampleswereusedas suchexcept thegrapepomace,pinebarkandwillowsamples,whichweredriedandmilledwithaWileymillas

describedabovetoobtainmorehomogenoussamples.Enzymatichydrolysiswasperformedintriplicatesat1%consistency(á50mgovendrybiomass),at45°CandpH5.0(100mMsodiumacetatebufferincluding0.02%sodiumazide

asbiocide).TheenzymesusedinthehydrolysiswereCelluclast1.5LandNovozym188(Novozymes,Bagsvaerd,Denmark)andtheenzymedosagewas10FPU/gofdrymatter forCelluclastand200nkatofβ-glucosidase/gofdry

matter.

2.3HydrothermalcarbonizationHTCofthestudiedbiomassside-streamswascarriedoutina2LHastelloyC276stirredautoclavereactor(AmarEquipmentsPVT.Ltd.,India)equippedwithanelectricalheaterband,temperaturecontrol,digitalandanalog

pressureindicator,internalwatercoolingcoilandPCcontrolleddatalogger.Pressurisednitrogenwasintroduced(∼35bar)toflushandleaktestthereactor.Nitrogenwasventedbeforereaction.Thereactionsequenceconsistedof

reactorheatupto260°C(∼60min),holdupfor360min,andwatercoolingtoambienttemperaturebeforeventingtheresidualpressureandopeningthereactor.ThecarbonaceousslurrywasweighedandfilteredinaBücherfunnel

usingqualitativefilterpaperMN616(Macherey-Nagel).CharwasrinsedinaBüchnerfunnelwith∼1Lofdeionisedwaterandthendriedinovenat105°Covernight.

2.4Analyses2.4.1Compositionanalysisofrawmaterialsbeforeandafterhydrothermalpre-treatment

Thechemicalcompositionsofthematerialsweredeterminedbothbeforeandafterthehydrothermalpre-treatment.Priortocompositionanalyses,theair-driedmaterialsweregroundusingaFritschPulverisette14mill.Theextractiveswere

determinedgravimetricallyafterextractionwithheptane.Todeterminethecarbohydrateandlignincomposition,theheptane-extractedmaterialswerehydrolyzedwithsulphuricacidandtheresultingmonosaccharidesweredeterminedbyHPAECwith

pulseamperometricdetection(DionexICS3000equippedwithCarboPacPA1column)accordingtoNRELmethod(Hausalo,1995;Sluiteretal.,2008).

Thepolysaccharidecontentinthesampleswascalculatedfromthecorrespondingmonosaccharidesusingananhydrocorrectionof0.88forpentosesand0.9forhexoses.Acidsolublelignininthehydrolysatewasdetectedat215and280nmusing

equationdescribedbyGoldschmid(Goldschmid,1971).

Klasonlignincontenti.e.theinsolubleresiduefromthehydrolysiswasdeterminedgravimetrically.TheKlasonlignincontentwascorrectedforproteinandash.ElementalanalysiswascarriedoutforbothKlasonligninandthewholematerial

usingFLASH2000seriesanalyser(ThermoScientific).ProteincontentsofthewholematerialandKlasonligninweredeterminedbymultiplyingthenitrogencontentwith6.25.AshcontentofKlasonligninwascalculatedbysubtractingthecontentsof

nitrogen,carbon,oxygen,hydrogenandsulphurfrom100%.Ashcontentofthewholematerialwasmeasuredgravimetricallyafterburningthesamplesat550°Cfor23h.

2.4.2CompositionanalysisofliquidfractionsfromhydrothermaltreatmentTheliquidsampleswerefiltratedpriortoanalysisinordertoremoveallsolidparticles.Thesolidsremovedwereincludedinthemassbalancecalculations.

Thealkalinesolubleligninwasdetectedat280nmandtheobtainedvaluewascorrectedbysubtractingabsorbenciesofHMFandfurfural.TheabsorbenciesforfurfuralandHMFwerecalculatedbasedonanalysedconcentrationsintheliquids

(Rochaetal.,2012).Furfuraland5-HMF(5-Hydroxymethyl-2-furaldehyde)concentrationswerequantifiedusingaFlexar™HPLC-RI(PerkinElmer,Waltham,MA,USA)systemequippedwithanAminexcolumn(HPX-87H,300mm×7.8mm,Bio-Rad,CA,

USA)underisocraticconditions(40°C)ataflowrateof0.5mL/minof2.5mMH2SO4(PatrickandKracht,1985).Theresultswerecalculatedbyusingcorrespondingstandards:Furfural(Sigma-Aldrich)and5-HMF(Sigma-Aldrich).

Totalcarbohydratesintheliquidfractionsweredeterminedafteraone-stepsulphuricacidhydrolysis(4%w/wsulphuricacidat120°Cfor50min)byHPAEC(DionexICS-3000)withpulseamperometricdetectionusingCarboPacPA1column

accordingtoHausalo(Hausalo,1995).

2.4.3Analysisofdegreeofenzymatichydrolysis

Thereleaseofsolublesugarsintheenzymatichydrolysiswasmonitoredat0,24and48hbyanalysingthereducingsugarsusingthedinitrosalicylicacid(DNS)method(Bernfeld,1955)scaleddowntoa96-wellmicrotiterplate.Glucosewasused

asstandard.Thesolubilizedsugarsmeasuredasglucoseequivalentswereconvertedtopolymericcarbohydratesbymultiplyingwithafactorof0.9.

2.4.4AnalysesofHTChydrocharsandliquidfractionsElementalcomposition(C,H,NandO)oftheHTCsampleswasanalysedusingFLASH2000seriesanalyzer.Theashcontentwasanalysedasdescribedabove.Totalorganiccarbon(TOC)oftheliquidfractionswasmeasuredusingShimadzuTOC

analyzer(TOC-Vcsh,ShimadzuCorporation)afterfiltration.

HeatingvaluesforbiocharsandliquidfractionswerecalculatedaccordingtoReed(1979).

3Resultsanddiscussion3.1Rawmaterialcharacterisation

Thechemicalcompositions (i.e. carbohydrates, lignin,extractives,proteinandash)of theselectedninebiomasses representingsidestreams from forestryandagriculture, i.e. cornstover,corn leaves,coffeecake,grape

pomace,brewersspentgrain(BSG),greenhouseresidues,willow,Scotspinebarkandwheatstrawwereanalysed.Klasonlignincontentwascorrectedforbothashandproteinasmostofthesampleswereknowntocontainthese

components.

Thechemicalcompositionanalysesrevealedthattheselectedsamplesformedaratherheterogeneousgroupwhereeachbiomassfractionhadauniquechemicalcomposition(Table1).Cornstover,cornleaves,BSG,wheat

strawandwillowwererichincarbohydrates,whereascoffeecakehadespeciallyhighextractivecontent.Greenhouseresiduescontainedasignificantamountofash,whichprobablycamefromtothepresenceofsoilandsandinthe

material.Scotspinebarkandgrapepomacehadhighlignincontents.BSGwasrichinprotein.

Table1Chemicalcompositionsofthestudiedbiomassesbeforeandafterhydrothermalpre-treatmentandtheendpHofthetreatment.Forthechemicalcomposition,theleft-handcolumnrepresentsthevaluebefore

thepre-treatmentandtheright-handcolumnafterthetreatment.

Carbohydrates(%) Klasonlignin(%) Acid-solublelignin(%) Totallignin(%) Protein(%) Extractives(%) Ash(%) Notanalysed(%) EndpH

Prior After Prior After Prior After Prior After Prior After Prior After Prior After Prior After

Grapepomace 16.2 16.8 41.8 44.2 6.5 2.2 48.3 46.4 11.9 8.8 7.5 14.4 3.9 9.5 12.1 4.3 7.0

Coffeecake 32.9 30.3 18.2 18.9 0.8 0.8 19.0 19.7 14.1 12.1 28.9 27.6 0.4 5.1 4.7 5.2 7.7

Brewer'sspentgrain 43.2 43.8 11.1 9.1 5.0 3.0 16.1 12.1 24.4 13.4 7.4 12.3 4.1 10.8 4.7 7.7 9.4

Greenhouseresidues 25.2 27.6 24.4 26.1 3.0 2.3 27.4 28.4 8.7 9.7 2.3 2.7 26.0 28.2 10.5 3.4 6.4

Cornstover 48.6 51.4 12.9 9.9 3.9 2.9 16.9 12.8 8.0 5.2 1.5 1.8 6.6 11.7 18.4 17.1 8.5

Cornleaves 64.9 55.7 6.7 9.4 3.4 3.5 10.1 12.8 9.4 7.3 2.0 2.7 2.8 4.4 10.8 17.1 4.8

Pinebark 39.0 35.9 36.1 39.3 1.5 0.8 37.7 40.1 1.6 1.1 4.5 4.9 2.4 6.1 14.9 11.9 6.1

WheatStraw 57.1 56.3 18.2 14.3 1.7 1.4 19.9 15.7 1.9 1.6 1.7 1.4 8.6 11.5 10.9 13.6 6.7

Willow 48.5 49.1 23.6 28.0 1.9 1.9 27.2 29.9 2.4 2.9 1.4 1.7 1.9 4.4 18.4 12.0 4.3

3.2HydrothermaltreatmentfollowedbyenzymatichydrolysisInthisstudy,thepolysaccharidesintheselectedbiomassfractionsweremademoreaccessibleforenzymatichydrolysisbyhydrothermalpre-treatment.Inthetreatment,thebiomassstructureisopenedupforthefollowing

enzymatichydrolysisashemicellulosesareremoved.Steamtreatmentshavealsobeenshowntohaveasignificanteffectondistributionandchemicalstructureoflignin(Marchessault,1991;PereiraRamos,2003).

Theyieldsofdrymatterafterhydrothermaltreatmentvariedsignificantly.Forinstance,thesolubilizationofBSGinthetreatmentwasashighas52%,whereasforcoffeegroundandgreenhouseresiduesitwasonly10and

14%,respectively(Fig.1,darkergreypartofthecolumns).ThesolubilizationofdifferentcomponentswasmostlikelyenhancedbythealkalinepH(pH12)atthebeginningofthepre-treatment,ashemicelluloses,ligninandlipidsare

knowntodissolveinalkalineconditions.ThepHdropped(Table1)inthecourseofthepre-treatment,mostprobablyduetothereleaseofacids,e.g.viadeacetylation,weakeningthusthedissolvingeffectsofthealkali.Thechangein

pHduringthepre-treatmentvariedbetweenmaterials,whichwaspresumablycausedbythedifferentchemicalcompositions,especiallyacetylationofhemicellulose.

Interestingly,theoverallchemicalcompositionsdidnotchangedramaticallyduetothehydrothermaltreatmentindicatingthatthepre-treatmentdidnotselectivelydissolvecertaincomponentsbutaffectedallofthem(Table1,

Fig.2).Somemorenotablechangeswere,however,observed.Eventhoughtherelativeamountofcarbohydrateswasnotablydeceraseddecreasedonlyforcornleaves,theanalysisrevealedthatthehemicellulosesweresolubilisedmore

readilythancelluloseduringthepre-treatment(resultsnotshown).Incornleavesthecarbohydratecontentdecreasedfrom65to56%.Anotablechangewasevenseenintheglucosecontent(datanotshown),whichwasprobablydue

to the presence of starch in the material, as the corn leaves also contained residual corn kernels. With corn leaves having the highest carbohydrate content and rather high degree of solubilization, i.e. 45%, the mass balance

calculationsrevealedthatasignificantamountofcarbohydratescouldberecoveredfromtheliquidfractionafterhydrothermaltreatment(Fig.2).TheproteincontentofBSGwasdecreasedfrom24to13%.Eventhoughthemajorityof

proteinsinBSGarewater-insolublestorageproteins,thehighproteinsolubilitycanbeexplainedbythefactthatBSGcontainsalsoproteinsthataresolubleinwaterandalkalinesolutions(Celusetal.,2006).Additionally,duetothe

complexandlinkedstructureofBSG,itispossiblethatsomeproteinsarereleasedfromthestructureasothercomponentsaredissolvedtothewaterphase(Ohra-Ahoetal.,2016).Hydrothermaltreatmentofgrapepomaceandpine

bark,havingthehighestlignincontents,resultedinratherhighsolubilizationandthuswithliquidfractionshavinghighlignincontents(Fig.2).Thetotalamountofcomponentsdetectedaslignininthesolidandliquidfractionswas

somewhathigherafterhydrothermaltreatment.Thiscanbeexplainedbytanninsthataredissolvedfromthematerialduringhotwaterextractionandare,aslignin,detectedat280nm.

Theeffectofthehydrothermaltreatmentonenzymatichydrolysabilitywasestimatedbasedonthedifferenceinthedegreeofhydrolysisbeforeandafterthepre-treatment(Fig.3).Forcornleaves,cornstover,wheatstrawand

grapepomacethepre-treatmentwasclearlybeneficialfortheenzymatichydrolysisandenabledpracticallycompletehydrolysisofcarbohydrates.BSGandwillowcarbohydrateswerealsoefficientlyhydrolyzedafterthepre-treatment,

Fig.1Totalsolubilizationofbiomassesinsequentialhydrothermalpre-treatment(darkergreypartofthecolumns)andenzymatichydrolysis(lightergreypartofthecolumns).

Fig.2Carbohydrateandligninrecoveryinsolidandliquidfractionspriorandafterhydrothermalpre-treatment.

whereasthepre-treatmenthadasmallereffectonhydrolysabilityofpinebarkandcoffeecake.Thevaluesexceeding100%(Fig.3)aremost likelycausedby inaccuracies in thechemicalcompositionanalysis.Theanalyticalacid

hydrolysisstepwasnotoptimisedforeachmaterialandthussomesugarsmayhavedegradedorremainedunhydrolyzedandhavethusnotbeincludedinthetotalamountofcarbohydrates.Therefore,whentheamountofsugars

releasedfromthebiomassduringtheenzymatichydrolysisstepwascomparedwiththeanalysedamountofcarbohydrates,hydrolysisyieldsover100%couldbeobtained.

Whencountingboththedrymattersolubilizedintheinhydrothermaltreatmentandenzymatichydrolysis,thetotalsolubilizationofthematerialsrangedfrom28to78%(basedon48hhydrolysis)(Fig.1).Incasetheaimisto

producefermentablesugarsfromsolidfractioninmoreconcentratedsolutions,itcanbeconsideredthatthehighertheenzymaticsolubilizationiscomparedtothesolubilizationinthepre-treatment,thebetter.Ifhighamountsof

carbohydratesaredissolvedinthepre-treatment,itmayaffectthesugaryieldandprocesseconomy,asthesugarsmaybedifficulttorecovereconomicallyfromthediluteliquidphasecontainingamixtureofsolubilizedcompounds.

Consideringtheseaspects,thehydrothermalpre-treatmentwiththeusedconditionsseemedmostsuitableforcornstover,cornleaves,wheatstrawandwillow.Asthehydrolysisofwillowwasnotcomplete,likelyslightlyharsherpre-

treatmentconditionswouldbeneeded.

3.3HydrothermalcarbonizationHTCwasselectedastheotherprocessingrouteas it isasimpleandeasilyscalabletechnologybasicallysuitable foranyorganicrawmaterial.HTCcanbeusedtoconvertvariousbiomasses intobio-basedcarbonaceous

materialswithimprovedphysicalandchemicalpropertiestobeusedinspecificapplications.

TheapplicabilityoftheselectedbiomassfractionstotheHTCprocesswasevaluatedbasedonparametersdescribingthetotalfeasibility.Theyieldinevitablyaffectstheeconomicfeasibilityoftheprocess.Highcarboncontent

isadvantageousformanyapplicationsaimingatreplacingexistingfossil-basedcarbonmaterials.Additionally,highsolidcarbonrecoverylevelisimportantnotonlyfortheprocessefficiencybutfortheenvironmentalperspectives,as

wastefractionsareminimised.However,thefinalvalueofthehydrocharsissetbytheirfunctionalityinthepotentialapplicationswhichwasnotstudiedinthispaper.

Thebiomasseswerehydrothermallycarbonizedat260°Cfor6hunderacidicconditions.FairlyhighHTCprocessingtemperaturecombinedwithratherlongresidencetimewasselectedtopromotethefullconversionofthe

inhomogeneous biomasses into homogeneous hydrochars. The processing conditions were fixed in order to detect clear differences between the biomasses behaviour in the HTC processing. Further optimization of the process

conditionsforeachbiomassrelativetohydrocharyieldandqualityneedstobedoneseparately.

Therewerelargedifferencesinthehydrocharyieldsbetweentherawmaterials,asshowninTable2.Thehydrocharyieldsvariedfrom∼21wt%forcornstoverto∼55wt%forcoffeecake.Theobtainedyieldswerepartlylower

thantheyieldsreportedpreviouslyforsimilarbiomassestreatedinthesametemperaturerange(Hoekmanetal.,2011;Lynametal.,2015, ).Theobserveddifferencemaybeexplainedbythelongerresidencetimeusedinthisstudy.

Ingeneral,differencesintheyieldsarerelatedtothebiomasscompositionandtheusedreactiontemperatures(Hoekmanetal.,2011;Lynametal.,2015).Highertemperaturesresultinincreasedformationofcomponentsdepartingfrom

the solid fraction into the liquid- and gas phases thus decreasing the hydrochar yield. Lowering the temperature would increase the yield but would have a negative effect on the hydrochar quality, especially when processing

heterogeneousbiomasseshavinglargeoriginalparticlesize.Chemicalcompositionofthebiomass,solubilityofbiomasscomponentsintowaterandthereactionmechanismstakingplaceduringtheprocesshavealsoimpactonthe

yieldandthefinalcompositionoftheproducedfractions.Thus,theamountofcellulose,hemicelluloses,ligninandextractivesinthebiomasshasasubstantialeffectontheyields.Cornleavesandcornstoveraswellaswheatstrawhad

Fig.3Enzymatichydrolysisofcarbohydratesbeforeandafterhydrothermalpre-treatment.

2013b

lowestyieldsduetohighamountofcarbohydratesinthebiomass.Accordingtopreviousstudies,biomasswithhighercontentofhemicelluloseandwater-solubleextractivesresultsinlowerhydrocharyieldswhilehighamountoflignin

inthebiomasscontributestohigheryields(LuandBerge,2014;Lynametal.,2015;Rezaetal.,2013b).Hemicellulosesdegradeatreactiontemperaturesofapproximately200°Cwhileligninstructureisonlyslightlyalteredinconventional

HTCtemperatures.Consequently,highcontentoflignincontributedtohighhydrocharyieldsforgrapepomace(∼48%)andpinebark(∼50wt%).Thehighesthydrocharyieldwas,however,obtainedforcoffeecake.

Table2Yield,elementalcompositionincluding,ashcontentincludingashrecoveryandheatingvaluesforbiocharsbeforeandafterHTCaswellasTOCvaluesfortheliquidfractions.

Solidyield(%) C(%) H(%) O(%) N(%) Ash(%) Ashrecovery(%) Heatingvalue(MJ/kg) TOC(mg/L)

Prior After Prior After Prior After Prior After Prior After Prior After

Grapepomace 47.7 54.0 68.0 6.1 5.6 35.5 18.6 1.9 1.9 4.0 3.1 38 22 28 3867

Coffeecake 54.7 60.0 73.4 7.7 7.2 31.9 15.6 2.3 3.1 0.4 0.3 39 27 32 5762

Brewer'sspentgrain 34.5 50.0 69.3 6.6 6.7 39.1 15.4 3.9 3.8 4.1 3.7 31 21 30 5786

Greenhouseresidues 28.4 39.1 55.5 4.7 4.8 30.8 16.1 1.4 1.7 26.0 23.4 26 15 23 4568

Cornstover 21.1 46.5 66.2 5.7 5.2 39.3 15.9 1.3 2.3 6.6 9.4 28 19 27 8418

Cornleaves 28.7 46.8 72.2 6.3 5.3 44.4 15.8 1.5 2.5 2.8 1.6 17 19 30 3816

Scotspinebark 49.9 53.0 70.1 5.9 4.9 41.7 21.8 0.3 0.3 2.4 1.3 25 21 28 5429

WheatStraw 24.2 45.6 67.3 5.8 5.0 42.9 19.0 0.3 0.7 8.6 6.3 23 18 27 3507

Willow 40.3 49.3 69.5 6.0 5.0 45.3 21.4 0.4 0.6 1.9 0.7 15 19 28 4954

Thecoffeecakecontainedplentyofextractives,whichbasedonliteraturecontainlipophiliccompounds,suchasfreefattyacids(Pujoletal.,2013).Mostof thefattyacidsarereportedtoretain inHTCincreasingthusthe

hydrocharyield(Luetal.,2015).

ThereactionsoccurringinHTCresultinalossofcarbon,hydrogenandoxygenfromthebiomass.Table2presentstheelementalcompositionandashcontentbeforeandafterHTCaswellasthecomputationalheatingvalue.

Carbonisanimportantelementregardingmanyapplicationswherehydrocharscouldbeused,e.g.assoilamendmentorfuel.Hydrocharsproducedfromcoffeecake,cornleavesandpinebarkhadthehighestcarboncontentbeing

over70%.

Fig.4 presents thedistributionof carbon in thebiomass to solid, liquidandgasphases inHTC.Theamountof carbon in the solid fractionwasmeasuredusingelemental analysiswhile carbon in the liquid fractionwas

measuredusingTOCanalysis.Theremainingcarbonwasassumedtobeinthegasphase.Coffeecakeandpinebarktogetherwithgrapepomacehadthehighestsolidcarbonrecoverylevelsi.e.over60%.Theresultcorrelateswiththe

highoverallsolidyields.Lowsolidcarbonrecoverylevelsforcornstover,cornleavesandwheatstrawcorrelatedwithlowsolidyields.Thehighamountofcarbonintheliquidfractionofcornstovercanbeexplainedbythehigh

amountofdegradationproductssolubleinwaterintheusedconditions.Theproductsareknowntobemainlymonomericsugars(Chenetal.,2007;Rezaetal.,2013a).

Generally,theHTCliquidfractioncontainsvariousorganiccompounds,mainlyacetic, lacticandformicacidsbutalsosugarsandphenols(Hoekmanetal.,2011).TheHTCliquidstreamcontaininghighamountofdissolved

carbonisapotentiallyproblematicwastefraction.Anaerobictreatment(Oliveiraetal.,2013;Poerschmannetal.,2014)andwetairoxidationhasbeensuggestionsforreducingitsorganicload(Baskyretal.,2014;Rezaetal.,2016).

Thebiomass fractions containeddifferent amounts of inorganics.Ash recovery inTable2 describes the amount of ash retaining in the hydrochar. The ash contentwas significantly reduced inHTC for all the biomasses.

Greenhouseresidueshadoriginallythehighestamountofashmainlyduetodirtinthesample.Loosesandandsoilwereprobablypartlyleachedoutfromthesampletothehotliquidfractionduringprocessing.Also,thelossofmassin

HTCcausessimultaneouslossoftheash(Rezaetal.,2013a).Thehightemperaturesusedinthisstudy,havebeenreportedtocausehigherreductionintheashrecoveryduetomoreporoushydrocharstructureenablingleachingofthe

entrappedorlooselybondedinorganics(Rezaetal.,2013a).

TheextentofreductionofoxygenandhydrogeninthecarbonizationprocesscanbedescribedbyaVanKrevelendiagram(Fig.5)wheremolarratiosofO/CandH/Cillustratethecarbonizationofeachbiomass(Ramkeetal.,

2009).Theconversionproceedsgraduallyfromtheupperrightcornerofthediagram,whichisatypicalbiomassregion,to lowerlefttoregionsresemblingligniticbrowncoal,browncoal,blackcoalandfinallyanthracite,asthe

carbonizationadvances.Theintensityofthecarbonization isdescribedbythe lengthofthevectorandit ismainlyaffectedbythedehydrationanddecarboxylationreactions inHTC.DehydrationduringHTCcanresult fromboth

chemicalandphysicalprocessesthroughtheeliminationofhydroxylgroupsorfromdewatering,whenwaterisejectedfromthebiomassduetotheincreasedhydrophobicityofthehydrochar(Rezaetal.,2014).Asaresult,thebiomass

iscarbonizedandtheO/CandH/Cmolarratiosaredecreased.ThehydrocharsinthisstudyexhibitedO/Cratiosof0.16–0.23andH/Cmolarratiosof0.84–1.18.Interestingly,thelengthoftheinput-outputvectorshowedthatcorn

leaveshadreactedmostintensivelyreachingtheareaofligniticblackcoal.HydrocharsfromcoffeecakeandBSGhadthelowestO/Cmolarratios(0.159and0.1667)indicatingremovalofcarboxylgroupsfromthebiomassextractives,

hemicellulose,andcelluloseintheHTCprocess.AtthesametimethelowestH/Cmolarratioswerefoundforwoodybiomasseswillowandpinebark,indicatingeliminationofhydroxylgroupsintheprocess.Theheatingvaluesor

hydrocharswerecalculatedbasedontheelementalcompositionreachingmaximumvalues≥30MJkg−1forcoffeecake,cornleavesandBSGcorrelatingwithhighcarboncontentandlowH/Cmolratio.Theincreaseinhydrochar

energydensitycomparedtorawbiomasswerecomparabletopreviouslyreportedresults(Hoekmanetal.,2011;Mummeetal.,2011).

Fig.4Carbondistributiontosolid,liquidandgasfractionsinHTC.

4ConclusionsTheobtainedresultsshowedthatallbiomassfractionswerenotequallysuitedforsaccarificationsaccharificationandhydrocharproduction.Ifaimingatfermentablesugarsthehydrothermalpre-treatmentpriortoenzymatic

hydrolysiswasmostsuitabletomaterialshavinghighcarbohydratecontent(i.e.cornleaves,cornstover,wheatstrawandwillow).Basedonyield,highcarboncontentandsolidcarbonrecoveryleveltheHTCroutewasmostsuitable

forbiomasseshavinghighcontentofthermallystablecomponents,suchasligninandlowcontentofash(i.e.coffeecake,grapepomace,Scotspinebarkandwillow).

AcknowledgementsThetechnicalanddatahandlingassistanceofRiittaAlander,JenniLehtonen,PäiviMatikainenandMaxNyströmfromVTTisgratefullyacknowledged.MirvaCederfromNaturalResourcesInstituteFinlandis

acknowledgedforTOCanalysis.

Prof.SylviaLarssonandGunnarKalénfromSLU(SwedishUniversityofAgriculturalSciences)aswellasMatthieuCamparguefromRAGTarewarmlythankedforprovidingtherawmaterials.

ThisprojecthasreceivedfundingfromtheEuropeanUnion’sHorizon2020ResearchandInnovationProgrammeundergrantagreementNo637020-MOBILEFLIP.

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Highlights

• Biomasscompositiondefinesitsfeasibilityforalternativeprocessroutes.

• Highcarbohydratecontentwasfavourableforthesaccharificationroute.

• Hydrothermalcarbonisationyieldswerehighforbiomasseswithhighlignincontents.

Graphicalabstract