高燒失硅藻土的炭化處理及吸附特性研究

I英文翻譯HydrothermalcarbonizationofmicroalgaeAbstractHydrothermalcarbonizationisaprocessinwhichbiomassisheatedinwaterunderpressuretocreateacharproduct.Withhigherplants,thechemistryoftheprocessderivesprimarilyfromlignin,celluloseandhemicellulosecomponents.Incontrast,greenandblue-greenmicroalgaearenotlignocellulosicincomposition,andthechemistryisentirelydifferent,involvingproteins,lipidsandcarbohydrates(generallynotcellulose).Employingrelativelymoderateconditionsoftemperature(ca.200℃),time(<1h)andpressure(<2MPa),microalgaecanbeconvertedinanenergyefficientmannerintoanalgalcharproductthatisofbituminouscoalquality.Potentialusesfortheproductincludecreationofsynthesisgasandconversionintoindustrialchemicalsandgasoline;applicationasasoilnutrientamendment;andasacarbonneutralsupplementtonaturalcoalforgenerationofelectricalpower.1.IntroductionWiththeapexoftheworld’spetroleumproductionfromknownreserveshavingalreadybeenachievedorwillbeattainedintheverynearfuture,increasedutilizationofcoalasanenergysourceseemsacertainty.Asidefromformidablehealthproblemsassociatedwithincreasedatmosphericparticulateandheavymetalcontents,coal,likepetroleum,isafossilfuelandburningmassivelyincreasedquantitiesofcoalwillgreatlyexacerbatetheveryseriousproblemofglobalwarming.Incontrast,combustionofbiomassthathasnotbeenstoredforeonsinsubterraneanreservoirsreleasescarbondioxidethatisnot“new”totheearth’satmosphereandconstitutesa“carbonneutral”event.Greenandblue-green(cyanobacteria)microalgaehavebeenontheearthformillionsofyearsanddiffersubstantiallyfromhigherplants.Theyaresingle-celledmicroorganismsthatliveinaquaticenvironments,andallcomponentsnecessaryforlifeandprocreationarelocatedwithinasinglecell.Inhigherterrestrialplants,specializedcellswithspecificfunctionsarerequiredthatmakeuproots,stems,flowersandotherfunctionalparts.Cellulose,hemicelluloseandligninoftenprovidestructuralsupportforthesespecializedcellsandarepresentinsignificantquantities.Incontrast,microalgaeandcyanobacteriaarenotlignocellulosicincompositionbutarecomprisedofproteins,lipids,non-cellulosiccarbohydrates,andnucleicacids.Varioushydrothermalprocessingmethodshavebeenreported.Allenjoythesignificantadvantagethatstartingbiomassdoesnotneedtobedry,andthesignificantenergyinputrequiredtoremovewaterbyevaporationiseliminated.Hydrothermalgasificationisthemostthermallysevereandhasbeenconductedbothwithoutcatalystat400–800℃[1]andinthepresenceofNiandRucatalystsat350–400℃[2].Gaseousproductsincludehydrogen,methane,andcarbondioxide,andthisprocesshasalsobeenextendedtomicro-algae[3].Hydrothermalliquefaction,generallyconductedat250–450℃[4],providesliquidbio-oilsaswellasgaseousproductsandhasalsobeenextendedtomicroalgae[5].Themildestreactionconditionsintermsoftemperatureandpressureareemployedinhydrothermalcarbonization(HTC).Lignocellulosicsubstrateshavebeenextensivelyexamined[6]asreactantsattemperaturesfrom170to250℃overaperiodofafewhourstoaday,andthisprocesshasbeenthesubjectofarecenttutorialreview[7].Theprocesstakesplaceeffectivelyonlyinwater,isexothermic,andproceedsspontaneously.Twoproductstreamsarecreatedthatareisolatedbyfiltration:1)aninsoluble,charproductand2)watersolubleproducts.Ingeneral,thedesiredobjectiveofincreasingthecarbon-to-oxygenratio(commonlyreferredtoas“carbonization”)hasbeenaccomplishedbyendeavoringtosplitoffcarbondioxide[8–10].Thismechanismisundesirablebecause,withlossofcarbondioxide,carbonisdepletedaswellasoxygen,andcreationofgaseousproductscausesevengreaterreactionpressuresthatincreasecomplexity/costofreactionequipment.NopublishedtechnicalreportsofalgalspeciesbeingsubjectedtoHTChavebeenfound.Theprincipalobjectiveofthepresentworkwastofocusonthecharproductandtoobtainahighlevelofcarbonizationandyield,whilesimultaneouslyminimizingprocessingtime.Hopefully,relativelybriefreactiontimescanbeemployedinabatchmodethatwouldsuggestthepotentialforcontinuousprocessing.Thisisregardedasbeingimperativeifthetechnologyistohavelongertermpracticalimpact.Microalgaeshouldbeexcellentbiomasssubstratesforthispurposebecausetheirsmallsizewillfacilitaterapidthermaltransfertoprocessingtemperatures.Asecondaryobjectivewastoaccomplishcarbonizationbyamechanismotherthanbylossofcarbondioxide,andthenon-cellulosiccarbohydratecompositionofmicroalgaemayallowdehydrationtooccuratrelativelymoderatetemperaturesinthemannerofsolublebiomasssubstrates,e.g.,glucose[11].Reactionparametersexaminedincludedtime,temperature,andalgalconcentration.PotentialcatalystsfortheHTCprocesswithmicroalgaewerealsoevaluated.CompositionsandenergycontentsofresultingalgalcharproductsweredeterminedandcomparedwithanaturalcoalandacharobtainedbyHTCofalignocellulosicbiomasssubstrate.2.Experimental2.1.MaterialandmethodsElementalanalyses,heatsofcombustion,ash,andcarbonatedeterminationsforthevariousproductswereperformedbyGalbraithLaboratories,Inc.(Knoxville,TN).SEManalyseswereperformedattheUniversityofMinnesotaImagingCenter,CollegeofBiologicalSciences,St.Paul,MN.2.1.1.AlgaeChlamydomonasreinhardtii(CC-125wildtypemt+137c)wasobtainedfromTheChlamydomonasResourceCenter(supportedbytheNationalScienceFoundation)attheUniversityofMinnesota.Thealgawasinoculatedinto20Lglasscarboyscontaining18LofTAPmedium[12].Synechocystissp.StrainPCC6803[N-1]wasobtainedfromtheAmericanTypeCultureCollection(Manassas,VA)andusedtoinoculatea20Lglasscarboycontaining18LofBG-11medium[13].Theinoculatedcarboyswereplacedwithinfluorescentlightrings,producing5960cdandspargedwithaircontaining5%carbondioxideforseveraldaysuntilthecellcountreachedaplateauasdeterminedusingahemocytometer.Algaewereharvestedbycentrifugation(8000×gat22℃for15min).Whencrossflowfiltrationwasemployed,thealgawasresuspendedin1Lofwateranddiafilteredagainst8LofwaterbypassagethroughanAmershamBiosciencesCFP-2-E-5AHollowFiberCartridgeusingaperistalticpumpandaflowrateof10L/min.centrifugatepasteswerefreeze-driedinordertoemployaccurateandreproduciblemassesinexperiments.AphanizomenonflosaquaewaspurchasedfromKlamathLake,Inc.(KlamathFalls,OR).Spirulinaspp.AndChlorellaspp.werebothpurchasedasfood-gradematerialsfromalocalhealthfoodstore;thespraydriedmaterialswereutilizedasreceived.Dunaliellasalinacontaining2%β-carotenewasafood-gradeproductobtainedfromAlibabaInc.andwasusedasreceived.2.1.2.OthermaterialsOxalicacid,citricacid,andmetalsaltadditiveswerepurchasedfromAldrichChemicalInc.(Milwaukee,WI)andAlfaAesar(WardHill,MA).2.1.3.ReactorThereactoremployedwasa450mLstirredstainlesssteelreactorpurchasedfromParrInstruments,Inc.(Moline,IL).Heatingwasappliedtothereactorusinganinductionheatingsystem(availablefromLCMiller,Co.,MontereyPark,CA).2.2.Hydrothermalcarbonizationreactions2.2.1.AlgalreactionsexemplifiedwithC.reinhardtii(7.5%solids,2.3wt%oxalicacid,203℃,2h)Freeze-driedC.reinhardtii(16.2g),oxalicacid(0.37g;2.3wt%),and200mLofdistilledwaterweretransferredintoa500mLround-bottomedflaskandshakenvigorously.Toensurethatthematerialwasdispersedadequately,thecontentswerepouredintoablenderandagitatedathighspeedfor1min.Thecontentswerethenpouredintothe450mLreactor,with208.4g(96%)beingtransferred.Whilestirringat60rpm,theautoclavewasheatedinductivelyto203℃for2h.Finalpressurewas1.65MPa.Aftercoolingtoroomtemperature(22℃)overnight,theresidualpressurewas0.41MPa.GasintheheadspaceofthereactorwasreleasedintoaTedlargascollectionbagandanalyzedusingaPrimadBQuadrupoleMassSpectrometer(availablefromThermofisher,VernonHills,IL)usingGasworksSoftware(version2.0).Carbondioxidewasthepredominantgas,withsomecarbonmonoxidealsobeingdetected.Ammoniagaswasnotdistinguishablebythismethod.Vacuumfiltrationofthereactionmixtureseparatedtheblackenedcharproductfromthewatersolubleproducts.Thefilteredsolidwaswashedwellwithwaterandfreeze-dried.Thecharproductweighed6.15g(39%massyieldbasedonalgacharged).Thecharandfreeze-driedstartingalgaweresubmittedforelementalanalysisandheatofcombustiondetermination:startingC.reinhardtii,%C=51.6,%H=7.9,%N=9.8,%S=0.6andheatofcombustion=18.04MJkg-1;algalcharfromC.reinhardtii,%C=72.7,%H=9.7,%N=5.2,%S=<dlandheatofcombustion=31.58MJkg-.Withalignocellulosicprairiegrass(LittleBlueStem)LittleBlueStem[Schizachyriumscoparium(Michx.)Nash]growninmonoculturewasreceivedfromC.LehmanintheDepartmentofEcology,Evolution&BehaviorattheUniversityofMinnesota.Thematerialwasthoroughlydryandbrownincolor.Thegrasshadalignincontentof20%andapoly-saccharidecontentderivedfromthefollowingmon-saccharidesobtainedonhydrolysisandlistedindecreasingquantity:glucose,xylose,galactose,arabinoseandmannose.ThedriedmaterialwasinitiallygroundusingaWileymill,thenaThomasmillemployinga0.05mmscreen.Theresultingfinepowdercontainedparticleswithalengthaxisofabout1mm.Intoa500mLround-bottomedflaskwerecharged29.25gofthefinelydividedgrass,0.67gofoxalicacid,and263mLofdistilledwater(achievinga10%solidsconcentration).Thiswastransferred(97%transfer)intothe450mLreactorandallowedtostir(60rpm)andhydrateovernight.Heatingto200℃wasconductedusingaheatingmantlefor17h.Whencool,theresidualpressurewas0.69MPaandthemixturewasvacuumfiltered.Thefiltratewasclearandyellowincolor.Thefilteredsolidwaswashedwellwithwater,frozenat-20℃andfreeze-dried.Thesomewhatfibrous,brownfilteredproductweighed16.29g(57%yieldbasedongrasscharged).Analysisofthestartingprairiegrass:%C=46.6,%H=6.2,%N=<0.5%andheatofcombustion=17.92MJkg-1.Analysisofthelignocellulosicchar:%C=62.3,%H=5.6,%N=<0.5%andheatofcombustion24.38MJkg-1.3.CalculationsComputationsforthefollowingsectionswereconductedusingexperimentaldetailsoftheHTCofC.reinhardtiiofSection.1.Comparisonofenergyinput/outputforcombustionofC.reinhardtiianditsalgalcharproduct(Section4.3.2)3.1.1.CombustionofC.reinhardtiiThecentrifugatepastehadanalgalconcentrationof10wt%,and10kgofpastewereconsideredinthecomputations.FromSteamTables[14],inordertoremove9kgofwaterfrom10kgofpaste:Hsteam@373K-H295K=2.68-0.09=2.59MJkg-1.For9kgofwatertheenergyrequiredis9×2.59=23.31MJ.Heatofcombustionofthedryalga=18.04MJkg-1,andtheoverallenergybalanceisanetlossof18.04-23.31=-5.27MJ.3.1.2.CombustionofalgalcharfromC.reinhardtiStep1:Heat10kgofcentrifugatepastefrom295Kto476Kunderpressurewithnovaporizationinthepressurizedsystem:Enthalpiesofsaturatedliquids(hf)=0.86MJkg-1at476Kand0.09MJkg-1at295K;Dhf=0.86-0.09=0.77MJkg-1;assumingtheheatcapacityofthe1kgofdryalgapresenttobeabout50%thatofwater:△Hstep1=0.77(9kg)＋0.5(0.77)(1kg)=7.31MJ.Step2:Filter10kgofwetalgalchartoobtain0.63kgofmoistcharand9.37kgoffiltrate:Nosignificantenergyinput.Step3:Drythe0.63kgofmoistalgalchartoremove0.23kgofwaterat373K:△Hvaporization=2.26MJkg-1,andfor0.23kg×2.26MJkg-1=0.52MJ.Totalheatloadfor0.4kgofdryalgalchar:7.31+0.52=7.83MJ.Totalheatloadfor1.0kgofdryalgalchar:7.83÷0.4=19.57MJ,andtheoverallenergybalanceisanetgainof31.58-19.57=12.01MJ.3.2.Carbonaccounting(forSection4.3.3)3.2.1.ReactantFreeze-driedstartingalgaweight(Section2.2.1)=15.55g(actuallytransferredintothereactor)havinga%C=51.9,providing8.07gofcarboninthestartingalgasubstrate.3.2.2.ProductsFreeze-driedalgalcharweight=6.15ghavinga%C=72.7,providing4.47gofcarbonor55%ofthestartingcarboninthecharproduct.Theaqueousfiltratevolumewas192mLandhada%solids=3.94;thebrownsolidsolutesweighed7.56gandpossesseda%C=48.1(carbonateanalysiswas<0.03%),providing3.64gofcarbonintheaqueousfiltrateor45%ofthestartingcarbon.Assumingallthegasintheheadspaceofthereactorwascarbondioxide,thepressureat22℃(295K)was0.41MPa＋0.10MPa=0.51MPaandoccupiedca.250mL.ApplyingtheUniversalGasLaw(rearrangedtocomputethenumberofmolesofcarbondioxide)withtheappropriateconstant(R=8.31mLMPaK-1mol-1):Numberofmoles=pressure×volume÷R×Temperature(295K)=(0.51)(250)÷(8.31)(295)=0.05moleofcarbondioxidehavingamolecularweightof44g/moleand%C=27.3whichcomputesto0.60gofcarboninthecarbonintheheadspaceCalculationoftheamountofcarbondioxidedissolvedinthe192mLofwaterat22℃(solubilityofcarbondioxideinwaterat22℃=0.16g/100gat1atm[15])provides0.31g@%C=27.3or0.08gofcarbondissolvedinthewater.Thecombinedquantityofcarbondioxideintheheadspaceanddissolvedinthewaterwas0.68gor8%ofthestartingcarbon.4.Resultsanddiscussion4.1.EffectofmetalsaltadditivesandacidsAnearlyreport[16]examiningthehydrothermalcarbonizationofsucrosefocusedonthedevelopmentofturbidityinthepresenceofvariousmetalsaltadditivesat100–120℃.Ofthemetalsaltsreportedtohaveahighdegreeofinfluenceonturbiditydevelopment,onlyCaCl2andMgCl2wereenviron-mentallyacceptableasadditives;oxalicacidwasalsoreportedtobehighlyeffective.Anotherreport[17]indicatedthatferrousionandironoxidenanoparticleswereeffectivecatalystsinHTCofinsolublebiomass.TheseadditivesandcitricacidwereexaminedwithD.salinaat15%solids,200℃,andfor2h;additiveconcentrationinallcaseswas0.54molpercentwhichwasequivalentto2.0wt%ofCaCl2examinedintheinitialadditiveexperiment.Carbonizationlevelsasindicatedby%Cvalueswereessentiallythesameinallcasesincludingacontrolexperimentwithnoadditive.Similarly,%massyieldsrangedfrom37to40%.Therefore,noclearlysuperioradditiveswereidentifiedthatgavesignificantlyincreasedlevelsofcarbonizationormassyields.However,sinceanacidicpHhadbeenreported[6]toprovidelesscarbondioxideproduct,eithercitricoroxalicacid(2–3wt%)wasaddedinsubsequentexperiments.4.2.DesignedexperimentwithD.salinaD.Salinawasofparticularinterestbecauseitcangrowwellinwatercontainingrelativelyhighconcentrationsofdissolvedsalts.Inthisenvironment,predatoryeffectsofbacteriaaregreatlyreduced,andexpensivesterilizationmeasuresarenotrequiredforalgalproduction.Inordertoexaminetheimportanceandinterdependenceofsuspectedkeyvariables,athree-variable,two-levelfactorialexperimentwithreplicatedcenterpointswasconducted.Variablesoftemperature,timeandalgalconcentrationwereexaminedsimultaneously.Temperaturesexaminedwere:190,200,and210℃;reactiontimes:0.50,1.25,and2.00h;and%solidslevels:5,15,and25%algabyweight.TheresultsaregiveninTable1.Alinearregressionequationwasdevelopedfromtheorthogonalfactorialdesign:%CarbonRecovered=51.54-1.375X1-0.375X2＋9.875X3whereX1=dimensionlesstemperature;X2=dimensionlesstime;andX3=dimensionless%solids.Analysisofvariance(ANOVA)showedthat%solidswassignificantatthe99%confidencelevel,andtemperaturewassignificantatthe95%confidencelevel.Timewasnotstatisticallysignificantwhichhaspositiveimplicationsforscalingfrombatchtocontinuousprocessingmethodsbyemployingreactiontimesevenlessthan30min.Furtherstatisticalanalysisindicatedthatthesimplelinearmodeldidnotexplainallthevariationintheexperimentaldata.Thesignificantinteractionbetweentemperatureandtimeatthe95%levelindicatedanon-linearresponsesurface.Overall,theseresultssuggestedthattheprocessingwindowwasrelativelywide.ThedataofTable1aredepictedgraphicallyinFig.1A–C.Fig.1Aisaplotof%carbonrecoveredinthealgalcharatdifferenttemperaturesandtimes,anddarkercoloredregionsrepresenthigher%carbonyields.Thetopographicalresponsesurfaceisfairly“flat”inthatnotmuchischangingwithregardtothesetwoinputvariables.Bycontrast,Fig.1BandCthatinvolvealgalconcentration(%solids)withtemperatureandtime,respectively,showconsiderablechangeoccurringinbothplotsduetothesignificantimportanceofconcentrationofthealga.Furthermore,Fig.1BandCindicatethatanundesirable“overcooking”isindicatedbothatthehighestreactiontemperatureandlongestreactiontime.Theseobservationssuggestthatcontinuousprocessesmightbedevelopedbyemployingevenhigher%solidsattemperaturesoflessthan200℃withreactiontimesoflessthan30mintoprovideacceptablecharproductsintermsofcarbonizationandyield.4.3.Summaryevaluationofgreenandblue-greenmicroalgaeinHTCSeveralstrainsofalgaewereexaminedforcharproductionunderavarietyofexperimentalconditions.Elementalanalysesofbothstartingfreeze-driedalgaandalgalcharsaregiveninTable2forcomparison.Oneobservationfromthetablewasthatusefulcarbonizationlevelsincharsderivedfromblue-greenbacteriacouldbeobtained,e.g.,withAphanizomenonflos-aquaeandSyn-echocystis,buttheyieldsofcharsweresignificantlylowerthanthoseobtainedwithgreenmicroalgae.Thismaybeattributedtothereducedstrengthofabacterialcellwallrelativetoaplantcellwallandthatthecontentsofthebacteriaweremoreextensivelylysed,possiblyprovidinglessmaterialavailableforcharformation.Cellwallcompositionmaynotbetheonlyfactorinfluencingformationofchar,however,asD.Salinathatdoesnotpossessacellwallbutonlyacellmembraneofmostlylipidcomposition[18]providedacharwithahighdegreeofcarbonizationandingoodyield.4.4.AlgalcharscomparedwithcoalandalignocellulosiccharThissectionisdevotedtocomparisonofthecharsobtainedfromC.reinhardtiiasdescribedinSection2.2.1andfromD.Salinapreparedat15%solids,200℃,3h,with2.3%oxalicacid.ThenaturalcoalwasaPowderRiverBasinCoalobtainedasapulverizedpowderfromXcelEnergy,Inc.(St.Paul,MN).ThelignocellulosiccharwastheproductfromtheprairiegrassofSection.4.1.SEMcomparisonsTheimagesshowninFig.2fornaturalcoalrevealedamaterialthatwassynthesizedinacompressivemode,undersubstantialpressureandheatwithonlyasomewhatflakysurfaceappearanceevidentathighmagnification.ThecharpreparedfromLittleBlueStemprairiegrasshadconsiderableresemblancetothestartingprairiegrass(notshown).ThismaybeattributedtothecellulosecontentofthestartingbiomassthatremainedunaffectedbyHTCconditionssincereactiontemperatureswere<220℃[19].CharderivedfromC.reinhardtii,ontheotherhand,wasconceivablyformedbyinitiallysisofcells,carbonizationofcomponentsinsolution(oratleastinaliquefiedstate),andagglomerationintolargerparticlesbybindingontowhateversolidstructuresremainedinthesuspension.Asaresult,thealgalcharpossessedamoretortuoussurfaceappearanceathighmagnification.4.4.2.Comparisonoftheelementalanalyses,heatsofcombustion,andashvaluesofnaturalcoal,charfromalignocellulosicsubstrate,andalgalcharsobtainedfromC.reinhardtiiandD.SalinaInTable3the%Cvaluesforthealgalcharsandthenaturalcoalmaterialwerecomparableandinthe66–73%range,whileonly62%Cwasobservedwiththelignocellulosicchardespitetheextendedreactionperiodof17h.Heatsofcombustionweregreatestwiththealgalcharswhichindicatedtheimportanceofnotonlyofcarbonbutalsohydrogenforenergycontent,sincethealgalcharspossessedsignificantlyincreasedamountsofhydrogenthaneithernaturalcoalorlignocellulosicchar.Nitrogenwasaninsignificantcomponentofbothnaturalcoalandlignocellulosiccharandsuggestedthatnaturalcoalprobablyoriginatedpredominantlyfromlignocellulosicvegetation.Inthealgalchars,however,higherlevelsofnitrogenwereobservedandmayanegativeissueifcombustionconditionsfavorformationofoxidesofnitrogen.Onthepositiveside,theverylowashvalueofcharderivedfromD.Salinamaybeasignificantattributeifthecharsareutilizedasacarbonsourceforconversionintosynthesisgas.Consideringthemostbasicuseforalgalchar,i.e.,burningitforitsenergycontent,acomparisonofenergyoutputsforburningC.ReinhardtiiitselfandthecharderivedfromitareeachsummarizedinTable4(CalculationsSection3.1).Notethatthestartingpointforeachmaterialwasthecentrifugateat10wt%concentration.Onemajorissuewithanyindustrialprocessinvolvingthecombustionofalgaeisremovalofwater.Inthetable,inordertoobtain1kgofdryalgafromacentrifugateat10%solids,9kgofwaterwererequiredtoberemovedfrom10kgofthecentrifugate.Thisrequired23.31MJandresultedinanetenergyinputintothesystemof5.27MJ.IncontrastwiththeHTCprocess,the10%solidsconcentrationwasthedesiredHTCreactionmediumandnoenergyinputwasinitiallyrequired.Toheatthesystemfromambientto203℃,7.31MJwererequired,andwithproperinsulationandtemperaturecontrol,nosignificantadditionalenergywasneededtomaintainreactiontemperaturefor2h.Thecharproductisolatedbyfiltrationwasmoistandrequired0.52MJtoobtain0.4kgofdrychar.Theoverallnetresultwasthattheprocessliberated12.01MJkgL1whichwasanimprovementof17.28MJoverburningthestartingalga.Itshouldalsobementionedthatnorecoveredheatwasconsideredinthishypotheticalprocess,andheatconservationmeasureswouldmostcertainlybeemployedinanyindustrialprocessresultinginadditionalenergyimprovement.4.4.3.CarbonaccountinginthealgalcharIftheclaimisvalidundertheexperimentalconditionsthatcarbonizationwasnotachievedbylossofcarbondioxide,itwasimportanttounderstandhowcarbonwasdistributedamongtheproductsofthereaction(CalculationsSection3.2).Fig.3summarizesthecarbonaccounting.Carbonwasdistributedasfollows:55%inthealgalchar,45%infreeze-driedsolutesintheaqueousfiltrate,and8%ascarbondioxidepresentintheheadspaceofthereactor(assumingallthegaswascarbondioxide)anddissolvedinthefiltrate.Asignificantamountofcarbonwaspresentinthesolutesinthefiltrates,andanimportantquestionwaswhetherasignificantconcentrationofcarbonateionwaspresentthatcouldbederivedfromcarbondioxide.Thiswasshownnottobethecase,asanalysisofthefiltrateindicated<0.03%carbonateswerepresent.Onepossibleexplanationforthesignificantamountofcarbonfoundinthefreeze-driedsolutesmaybeduetoMaillardreactionproductsandthecarboncontainedtherein.Thisreactionhasbeenreported[20]tocreateliterallyhundredsofheterocycliccompoundsthatwouldbesolubleinwaterorbeadsorbedontonanoparticlesofcarbonizedmaterialpresentintheaqueousfiltratethatwouldessentiallypassthroughaconventionalfilter.Animportantconclusionregardingcarbonizationmechanismthatcanbemadefromthisstudywasthatapropertallyofcarbondioxidewasobtained.Evenassumingthatallthegaseousproductsintheheadspaceofthereactoranddissolvedintheaqueousfiltratewerecarbondioxide,thelevelofthatproductwaslessthan10%ofthetotalproducts.Themostreasonablealternativeexplanation,thoughnotdirectlyproven,wasthatthepredominantmechanisticpathwayforcarbonizationwasdehydration.Anotherexplanation,thoughbelievedlessprobable,wasthattheoxygencontentwasincreasedinsolutespresentinthefiltrate(andthereforeremovedfromthecarbonizedchar)byfragmentationreactionsofcarbo-hydratepolyolmaterialspresentinthelyzedalgaandformationofwatersolubleoxidizedproductssuchascarboxylicacids,ketonesandaldehydes.Clarificationofreactionmechanismawaitsfurtherinvestigation.5.ConclusionsHydrothermalcarbonizationofmicroalgaeprovidedcharproductsofuniquecompositionandwithenergycontentsinthebituminouscoalrange.Processconditionswereremark-ablymild,e.g.,ca.200℃andtimesasbriefas0.50h,fordevelopingacceptablelevelsofcarbonizationandyieldsofalgalcharmaterials.TherelativelybriefreactiontimedemonstratedinbatchprocessingsuggestedthatacontinuousprocessmightbedevelopedfortheHTCprocessingofalgae.Somestrainsofcyanobacteriaalsoprovidedhighqualitycharsbutyieldswereonlyhalfthoseobtainedwithgreenmicroalgae.Nodefinitivecatalyticagentswereidentifiedthatsignificantlyacceleratedcarbonizationand/orenhancedyieldwithalgalsubstrates.Thefundamentalcarbonizationprocesswasshowntonotproceedbylossofcarbondioxide.ThemostAnimportantareaofactivefutureworkwillbetodefineutilityfortheaqueousfiltratebyproducts.Thesecomplexsolutionscontainconsiderablequantitiesofnitrogen-containingsolutesandarecurrentlybeingexaminedasnutrientmaterialsforbothhigherplantsandalgae.plausiblealternativepathwayproposed,thoughnotdirectlysubstantiated,wascarbonizationviaadehydrationroute.AdditionalimportantandpracticalcomparisonsofnaturalcoalandalgalchararecontainedinTable5.AcknowledgementFinancialassistancewasprovidedbytheBioTechnologyInstituteoftheUniversityofMinnesotaandtheInitiativeforRenewableEnergyandtheEnvironment(IREE)andisgrate-fullyacknowledged.Dr.KannanSeshadriof3Misalsothankedforplottingthedataofthedesignedexperiment.