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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,⁎
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