Microwave-assisted pyrolysis of microalgae for biofuel production

Microwave-assistedpyrolysisofmicroalgaeforbiofuelproduction

ZhenyiDua,YecongLia,XiaoquanWanga,YiqinWana,b,QinChena,ChenguangWanga,XiangyangLina,c,YuhuanLiua,b,PaulChena,RogerRuana,b,

a

CenterforBiore ningandDepartmentofBioproductsandBiosystemsEngineering,UniversityofMinnesota,1390EcklesAve.,St.Paul,MN55108,UnitedStatesBiomassEnergyCenterandStateKeyLaboratoryofFoodScience,NanchangUniversity,Nanchang330031,Chinac

CollegeofBiologicalScienceandTechnology,FuzhouUniversity,Fuzhou350002,China

b

articleinfoabstract

ThepyrolysisofChlorellasp.wascarriedoutinamicrowaveovenwithcharasmicrowavereceptionenhancer.Theresultsindicatedthatthemaximumbio-oilyieldof28.6%wasachievedunderthemicro-wavepowerof750W.Thebio-oilpropertieswerecharacterizedwithelemental,GC–MS,GPC,FTIR,andthermogravimetricanalysis.Thealgalbio-oilhadadensityof0.98kg/L,aviscosityof61.2cSt,andahigherheatingvalue(HHV)of30.7MJ/kg.TheGC–MSresultsshowedthatthebio-oilsweremainlycom-posedofaliphatichydrocarbons,aromatichydrocarbons,phenols,longchainfattyacidsandnitrogenatedcompounds,amongwhichaliphaticandaromatichydrocarbons(accountfor22.18%ofthetotalGC–MSspectrumarea)arehighlydesirablecompoundsasthoseincrudeoil,gasolineanddiesel.Theresultsinthisstudyindicatethatfastgrowingalgaeareapromisingsourceoffeedstockforadvancedrenewablefuelproductionviamicrowave-assistedpyrolysis(MAP).

Ó2011ElsevierLtd.Allrightsreserved.

Articlehistory:

Received26October2010

Receivedinrevisedform15January2011Accepted17January2011

Availableonline22January2011Keywords:

Microwave-assistedpyrolysisMicroalgaeChlorellasp.Bio-oil

1.Introduction

Fossilfuelsareproducedfromunsustainableresourcesandtheirusescontributesigni cantlytogreenhousegas(GHG)emis-siontotheenvironment.Biomassisanabundantandpotentiallycarbon-neutralenergysourcewidelyavailableontheearth(Mo-hanetal.,2006;McKendry,2002).Biomassfeedstockcanbecon-vertedintosolid,liquid,andgaseousproductsthroughvariousthermochemicalprocessesincludingpyrolysis(Huberetal.,2006).Inpyrolysis,organicsinbiomassarethermallyconvertedtobio-oil,combustiblegases,andbiochar(BridgwaterandPea-cocke,2000;Yaman,2004).Whilemostoftraditionalslowandfastpyrolysisprocessesuse xedand uidizedbedreactorswhoseheatingisprovidedbyheatedsurface,sands,etc.(MeierandFaix,1999;CzernikandBridgwater,2004;Mohanetal.,2006),otherslookedintoalternativeheatingmethodssuchasmicrowaveheat-ing.Thenewmicrowave-assistedpyrolysis(MAP)processthatwedevelopedoffersseveraladvantagesovertraditionalprocesses,includinguniforminternalheatingoflargebiomassparticles,easeofcontrol,andnoneedforagitationor uidizationandhencelessparticles(ashes)inthebio-oil.StudiesofMAPofwood(Miuraetal.,2004),cornstover(Yuetal.,2007;Wanetal.,2009),riceCorrespondingauthorat:CenterforBiore ningandDepartmentofBioproductsandBiosystemsEngineering,UniversityofMinnesota,1390EcklesAve.,St.Paul,MN55108,UnitedStates.Tel.:+16126251710;fax:+16126243005.

E-mailaddress:ruanx001@umn.edu(R.Ruan).

0960-8524/$-seefrontmatterÓ2011ElsevierLtd.Allrightsreserved.doi:10.1016/j.biortech.2011.01.055

straw(Huangetal.,2010),coffeehulls(Domínguezetal.,2007),pinesawdust(Wangetal.,2009),andwheatstraw(Budarinetal.,2009)havebeenrecentlyreported,andsuggestthatMAPisahighlyscalabletechnologysuitablefordistributedconversionofbulkybiomass.

Microalgaeasanalternativebiofuelsourcehavegainedmuchattentionthesedaysbecausetheyhavenumerousadvantagescom-paredwithlignocellulosicfeedstocks:(1)theyhavehigherbiomassproduction,5–30timesofoilcropsperunitsurfacearea(Schenketal.,2008);(2)theydonotcompetewithtraditionalagriculturalresourcesastheycanbecultivatedonnon-arablelandoronwaste-water(Chenetal.,2009);(3)theyareexceedinglyrichinoil,over60%byweightofdrybiomassinsomespecies(GouveiaandOliveira,2009).Todate,variousthermochemicalconversionroutesofmicro-algaehavebeeninvestigated(Chenetal.,2009).Therearefewre-portsonpyrolysisofmicroalgae.MiaoandWu(2004)performedfastpyrolysisofautotrophicandheterotrophicChlorellaprototheco-idesandreportedbio-oilyieldsof16.6%and57.2%,respectively.Inaddition,thebio-oilsobtainedhadbetterqualitythanthosefromwoodintermsofbio-oilviscosity,densityandheatingvalue.Panetal.(2010)investigatedpyrolysisofNannochloropsissp.residuewithandwithoutthepresenceofHZSM-5catalystandobtainedbio-oilrichinaromatichydrocarbonsfromcatalyticpyrolysis.

Inthisstudy,MAPofChlorellasp.wasconductedunderdifferentmicrowavepowerlevels.Detailedcompositionalcharacterizationandcomparisonswerecarriedoutwithelemental,gaschromatog-raphy–massspectrometry(GC–MS),gelpermeationchromatogra-

Z.Duetal./BioresourceTechnology102(2011)4890–48964891

phy(GPC),Fouriertransforminfrared(FTIR)spectroscopy,andthermogravimetric(TG)analysis.Thecomponentsofthegaseousbyproductswerealsoanalyzedwithgaschromatography(GC).2.Methods2.1.Materials

Chlorellasp.,awild-typealgaestrain,wasscreenedfromlocalfreshwaterinMinnesota,USA,andthencultivatedinapilot-scale1300Lphotobioreactor lledwithTris–Acetate-Phosphorus(TAP)media(Harris,1989).Thephotobioreactorwassetupinthegreen-houselocatedontheSaintPaulcampusattheUniversityofMinne-sota,TwinCities,wheretheaveragesunshinedurationinMayofSaintPaulwas14–15hperdayandthetemperature uctuatedbe-tween22°Cto30°C,andstayedaround25°Cmostofthetime.Whenthebiomassreachedaround1g/L,asemi-continuoushar-vestingregimen,inwhich450Loftheculturevolumewashar-vestedfollowedbysupplementingwiththesamevolumeoftapwaterenrichedwithTAPmedia,wascarriedout.Algaepastewithawatercontentof85–90%wasobtainedafter occulationand l-tration,andthensubjectedtonaturaldryingtoconstantweight.ThemaincharacteristicsofdryChlorellasp.arelistedinTable1.2.2.Pyrolysis

ThepyrolysisofbiomasswascarriedoutinaPanasonicNN-SD787Smicrowaveovenwiththemaximumincidentpowerof1250Watafrequencyof2450MHz.TheschematicdiagramofexperimentalapparatusisshowninFig.1.Asbiomassispoormicrowaveabsorbent,mixingitwithmicrowaveabsorptionenhancersshouldimprovebiomassheating(Menéndezetal.,2002;Domínguezetal.,2007).Sinceahigherabsorber/biomassra-tioleadstohigherenergyconsumptionperunitbiomass,itisimportanttodeterminetheminimumamountofabsorbertocre-atetherequiredpyrolysisconditions.Thecharproducedfrompyrolysisofbiomassfeedstockisanexcellentmicrowaveabsorber.PartialrecyclingofthecharinacontinuousMAPprocessisex-pectedtorecoversomeheatandimprovemicrowaveabsorptionandhenceenergyef ciency.Inourstudy,thelowesteffectiveratioofabsorber(char)tobiomasswas1:5determinedthroughpreli-minaryexperiments.Forconsistentcomparison,eachsamplewaspreparedbyblending30galgaebiomasswith6gsolidcharinthisstudy.Thecharforthe rstexperimentatacertainpowerlevelwasobtainedbymixingChlorellasp.withactivatedcarbon(approximately1.5mmdiameterÂ3mm),whichwaseasilysepa-ratedfromthesolidresidueafterpyrolysis.Thecharforthesubse-quentexperimentswasgleanedfrompreviousexperimentsatthesamepowerlevel.Afterthesamplepreparation,themixturewasplacedina500mLquartz ask,whichwasthensubjectedtomicrowavetreatmentwithnitrogenusedasinertcarriergasata owrateof500mL/mintomaintainanoxicatmospherebeforeandduringtheexperiment.Throughoutthepyrolysisprocess,theevolutionofreactiontemperaturewasmonitoredwithaninfrared

Table1

Characteristicsofdriedalgaebiomass.Proximateanalysisa(wt.%)Elementalanalysisb(wt.%)Moisture13.7C49.70Volatile

68.4H6.98FixedCarbon10.1N10.92Ash

7.8

Oc

24.60

aWetbasis.bDrybasis.

c

Calculatedbydifference,O(%)=100–C–H–N–Ash.

opticalpyrometer,andthe naltemperaturewasmeasuredbyinsertingathermocoupleintothesampleimmediatelyattheendofreaction.Meanwhile,thecondensablevolatileswerecontinu-ouslycollectedusing vecondenserswithcoolingwatertempera-turearound0–2°C,andthenon-condensablegasesinagasbag.Thereactiontimewassetfor20minwhennoappreciablevolatileswereobservedlateron.Thesolidandliquidfractionyieldswerecalculatedfromtheweightofeachfraction,whilethegasyieldwascalculatedbydifferencebasedonthemassbalance.Allexper-imentswereperformedintriplicatetodeterminetheuncertaintiesintheexperimentalresults.

2.3.Bio-oilandgaseousproductsanalysis

Thebio-oilpropertieswerecharacterizedwithelemental,GC–MS,GPC,FTIR,andTGanalysis,depictedasfollows:

Theviscosityofthebio-oilwasmeasuredbyaRVASuper4Vis-coAnalyzer(NewportScienti cPtyLtd.,Australia).

Theelementalanalysiswasperformedwithanelementalana-lyzer(CE-440,ExerterAnalyticalInc.,USA).

Thecomponentsoftheliquidproductwerespeci edusinganAgilent7890–5975Cgaschromatography/massspectrometerwithaHP-5MScapillarycolumn.Heliumwasemployedasthecarriergasata owrateof1.2mL/min.Theinjectionsizewas1lLwithasplitratioof1:10.Theinitialoventemperaturewas40°Cheldfor3minandthenincreasedto290°Catarateof5°C/min,andheldat290°Cfor5min,whiletheinjectoranddetectorweremaintainedatconstanttemperatureof250°Cand230°C,respec-tively.Thecompoundswereidenti edbycomparingtheirmassspectrawiththosefromtheNationalInstituteofStandardsandTechnology(NIST)massspectraldatalibrary.

GPCanalysisofbio-oilwasperformedusingaVarianPolarisHPLCsystemequippedwithoneOligo-PoreGPCcolumn(polysty-rene-divinyl-benzenecopolymer,300Â7.5mm)at35°C.Inthissystem,tetrahydrofuran(THF)wasusedastheeluentata owrateof1mL/min,andadifferentialrefractometerwasusedasthedetec-tor.Bio-oilsamplesweredissolvedinTHFataconcentrationof10mg/mLandmolecularweightcalibrationwasperformedbyninepolystyrenestandardsinthemolecularweightrangeof162–2900.TheFTIRspectrawerecollectedinaNicoletSeriesIIMagna-IRSystem750spectrometer,equippedwithaliquidnitrogencooledmercurycadmiumtelluride(MCT)detector.TheoilwasdepositedbetweentwoNaCldisks.Thespectralrangewasselectedat400–650cmÀ1,witharesolutionof4cmÀ1.

TGandDerivativethermogravimetric(DTG)analysiswereper-formedwithaPerkinElmerTG/DTA6300inbothnitrogenandairatmospheres.Sampleswereheatedfrom30°Cto700°Cwithaheatingrateof30°C/min.Thegas owratewas20mL/min.

ThegaseousproductswereanalyzedbyaVarianMicro-GCCP4900/thermalconductivitydetector(TCD)witha5AmolecularsievecolumnandaPPQcolumn.Thetemperaturesofinjectoranddetectorweremaintainedat110°C.Theoventemperaturesof5AmolecularsieveandPPQcolumnwerekeptat80°Cand150°C,respectively.3.ResultsandDiscussion

3.1.Temperaturepro les

Thetemperaturepro lesofpyrolysisdeterminedbytheinfra-redpyrometeratdifferentmicrowavepowerlevelsarepresentedinFig.2.Accuratemeasurementoftheevolutionofthetempera-tureduringtheprocesswasverydif cult(Domínguezetal.,2003),andhencethetemperaturepro lesofthesamplesshowninFig.2onlyservethepurposeofcomparisonsamongdifferent

Table2

The naltemperaturesunderdifferentmicrowavepowersmeasuredbyathermocouple.

Microwavepower(W)Finaltemperature(°C)

500

462±29

750

569±42

1000600±27

1250627±17

28.6%at750W,andthendecreasedgradually.Theyieldofgasin-creasedovertherangeofmicrowavepowerstudied,whilethatofthechardecreasedfrom500Wto750W,andthenremainedal-mostconstant.Theunderlyingreasonisthatthesamplewasnotpyrolyzedadequatelybelow750W,anditmightjustreachcompletedecompositionat750Wandthemaximumoilyield

Z.Duetal./BioresourceTechnology102(2011)4890–48964893

wasobtained.Beyond750Wthedecreaseinoilyieldandincreaseingasyieldmaybecausedbythesecondarycrackingofoilvaporsintoincondensablegases.Inaddition,constantcharyieldinthepowerrangeof750W-1250Wispossiblyarisingfromthefactthatthedecompositionofthesamplewascompleteortherewasabal-ancebetweendecompositionofsolidsandformationofchar-likecarbonaceousmaterialthroughrepolymerization.Thewaterphaseyieldremainedvirtuallyconstantatabout21%inthestudiedpowerrange.AsshowninFig.3,therewasatradeoffbetweenheatingrateandpyrolysistemperature.Lowerpowerwithlowerheatingratealwaysledtotheformationofhigheryieldofchar(WilliamsandBesler,1996),whilehigherpowerwithhigherpyro-lysistemperaturefavoredgasi cationreactionswhichthusde-creasedtheyieldofbio-oil.Theseresultsaresimilartothosereportedintheliterature(Sßensözetal.,2006;Panetal.,2010;Is-lametal.,2010).Thevariationsaremainlyduetothecomposi-tionaldifferencesoffeedstockandthespeci ccharacteristicsofthemicrowavepyrolysissystem.Inthisstudy,750Wwastheopti-mumpowertoobtainbio-oilproductfromMAPofChlorellasp.3.3.Analysisofbio-oils

3.3.1.Physicalpropertiesandelementalanalysisofbio-oil

Thecharacteristicsofthealgalbio-oilincomparisonwithwoodbio-oilsanddieselfuelareshowedinTable3.Thebio-oilfromChlorellasp.hadaloweroxygencontent,highercarbon,hydrogencontentandHHVthanbio-oilproducedfromlignocellulosicmate-rials.ThesevaluesareclosetotheresultsofMiaoandWu(2004).Algalbio-oilhasalowerdensitythanlignocellulosicbio-oil,andaviscosityinthetypicalrangeofwoodbio-oil.Thepresenceofnitro-genbases,includingindole,pyridine,amides,ammonia,etc.,ren-deredthealgalbio-oilpHalkaline(9.7),whichisverydifferentfromthatforlignocellulosicbio-oil(typically2–3).However,theelementalcompositionandHHVofthebio-oilfromChlorellasp.arestillnotcomparableto(quitedifferentfrom)thoseoffossiloil.3.3.2.GC–MScharacterizationofbio-oil

Theidenti edcompoundswerecategorizedintothefollowinggroups:aliphatichydrocarbons,aromatichydrocarbons(includingbenzeneandbenzenealkylderivatives),nitrogenatedcompounds(includingnitriles,amidesandN-heterocycliccompounds),phe-nols,polycyclicaromatichydrocarbons(PAHs),andothers(suchasfattyacids,alcoholsandesters).Asemi-quantitativeanalysiswasperformedbycalculatingtherelativepercentageofareaofthechromatographicpeakswithresultsshowninTable4.Among

Table4

Relativeproportions(area%)ofthemaincompoundsofbio-oilobtainedunder750Wmicrowavepower.CategoriesAliphatics

Bicyclo[3.1.1]heptane,2,6,6-trimethyl-,(1.alpha.,2.beta.,5.alpha.)-2-Hexadecene,3,7,11,15-tetramethyl-,[R-[R ,R -(E)]]-Dodecane,2,6,10-trimethyl-1-Tridecene

Aromatics

Toluene

EthylbenzeneStyreneo-XyleneBenzene

Nitrogenatedcompounds

Indole

HexadecanamidePentadecanenitrile1H-Pyrrole,3-methyl-Phenols

Phenol,4-methyl-Phenol

Phenol,2-ethyl-PACs

NaphthaleneAnthracene

Others

n-HexadecanoicacidOleicAcid

Hexadecenoicacid,Z-11-9,12-Octadecadienoicacid,methylester

Unidenti ed

Compounds

Area/%15.192.021.830.550.496.992.341.020.910.720.5228.392.241.811.721.116.202.591.670.553.380.610.4817.905.044.732.090.1121.95

thesecompounds,hydrocarbonsarevaluablecomponentsinbio-oilfromthepointofviewoffuelapplication.Speci cally,aromatichydrocarbonsserveasimportantindustrialchemicalsandtrans-portationfueladditivestoincreaseoctanenumber.Table4showsthatbothaliphaticandaromatichydrocarbonswerehigherthanthoseobtainedfromotherbiomasses(Adametal.,2006;Wangetal.,2009;Zhangetal.,2009).Thismightresultfromthelargeramountoflipidsinmicroalgaebeingcrackedintohydrocarbonsduringpyrolysis.Phenolanditsalkylatedderivatives,whichareofgreatcommercialimportance,represented6.20%ofthebio-oil.N-containingcompoundsformedduringthedecompositionofproteinsinalgaecells,andtheymayaccountforpotentialNOx

Table3

ComparisonamongNo.2dieselfuel,bio-oilfromMAPofChlorellasp.andotherlignocellulosicfeedstocks.Properties

Bio-oilsChlorellasp.

Elementalcomposition(wt.%)CHNO

HHV(MJ/kg)fDensity(kg/L)pH

Viscosity,at40°C(cSt)

abcdefg

No.2dieselfueld

Pinechips(slowpyrolysis)a54.766.030.0939.1222.0–––

Wheatstraw(MAP)b58.96.851.1533.224.81.2––

Wood(fastpyrolysis)c56.46.20.137.316–191.22–3

25–1000

86.3113.27––430.83–

2.5–3.2

65.47.8410.2816.48e30.70.98g9.761.2

DerivedfromthereferenceSßensözandCan(2002).DerivedfromthereferenceBudarinetal.(2009).

DerivedfromthereferenceBridgwaterandPeacocke(2000).DerivedfromthereferenceTatandVanGerpen(1999).Calculatedbydifference.

Calculatedusingtheequation(Friedletal.,2005)HHV(MJ/kg)=(3.55ÂC2À232ÂCÀ2230ÂH+51.2ÂCÂH+131ÂN+20600)Â10À3.at30°C.

4894

Table5

FTIRfunctionalgroupsofthebio-oil.Frequencyrange(cmÀ1)3600–32003100–30103000–28002300–20001775–16501680–15751550–14901470–13251300–950

Groups

OAHstretchingNAHstretchingCAHstretchingCAHstretchingC NstretchingC@OstretchingC@CstretchingNAHbendingC@CstretchingCAHbendingCAOstretchingOAHbending

Z.Duetal./BioresourceTechnology102(2011)4890–4896

Classofcompounds

Phenols,AlcoholsAminesAromaticsAlkanesNitriles

Carboxylicacids,esters,ketonesAlkenesAmidesAromaticsAlkanes

Alcohols,phenols,esters

forlignocellulosicmaterial(Mullenetal.,2010;Hassanetal.,2009).Thelowermolecularweightandhigherhomogeneitymightresultfromthefactthatmicroalgaecontainnolignin,whichisthemajorsourceofphenolicoligomericspeciesinbio-oilfromligno-cellulosicbiomass.Thosepyrolyticligninmacromoleculars,whichconstitute25–30%ofthewholebio-oil,havemolecularweightrangefromseveralhundredtoashighas5000orhigher(Mohanetal.,2006).

FunctionalgroupcompositionalanalysiswascarriedoutusingFTIRspectrometry.Thefunctionalgroupsidenti edfromFTIRspectraareshowninTable5.AccordingtotheinterpretationofthemainbandsbySocrates(1994),FTIRfunctionalgroupsindi-catedthepresenceofalkanes,aromaticcompounds,nitriles,amides,phenol,andetc.Theseresultsarecomplementaryto

102(2011)4890–48964895

51%between200–350°C,whicharesimilartotheboilingpointrangeoflightnaphtha,heavynaphthaandmiddledistillate,respectively(Laresgoitietal.,2004).Theexistenceofhighamountofmiddledistillateindicatesthatbio-oilfrommicroalgaeisverypromisingaskeroseneanddieselfuels.Theresultalsoshowsthattherewasabout50%ofthebio-oilwithaboilingpoint<250°C.ThisfurtherdemonstratesthatcompoundsanalyzedbyGC–MSrepresentedafractionofthebio-oilsincehighmolecularweightcompoundscouldnotbegasi edandidenti edwithGC–MS.3.4.Analysisofgaseousproducts

ThepermanentgascollectedwascomprisedofH2,CO,CO2andgaseoushydrocarbons.Thequantitativeresultsofthefourmaincomponents(H2,CO,CO2andCH4)arepresentedinFig.5.Withtheincreaseofmicrowavepower,theCO2concentrationdecreasedsigni cantly,whiletheH2andCOconcentrationsincreasedgradu-ally.ThehighestconcentrationofH2+CO(syngas)was49.8%.Thetrendsofthecomponentswereinagreementwiththeliteratureandindicatedthatthefollowingreactionswerefavoredathighertemperatures(Wangetal.,2009;Domínguezetal.,2007):

CðsÞþCO2ðgÞ$2CODH298k¼173kJ=molCðsÞþH2O$COþH2DH298k¼132kJ=mol

4.Conclusions

Chlorellasp.waspyrolyzedinamicrowavecavity.Themicro-wavepowerof750Wwasfoundtobetheoptimummicrowavepowerasthemaximumbio-oilyieldof28.6%wasobtained.Severalanalysesindicatethatthealgalbio-oilexhibitabetterqualitythanlignocellulosicbio-oilsintermsofphysicalandchemicalproper-ties.Thealgalbio-oilwascharacterizedbylowoxygencontentwithaliphaticandaromatichydrocarbonsconstituting22.18%ofthetotalionchromatogramofGC–MS.However,furtherupgradingtoremoveNandOfrombio-oilisnecessarytomakeitsuitableasenginefuels.Acknowledgements

TheauthorsaregratefultoDOT/SunGrant,USDA/DOE,andUniversityofMinnesotaIREEandCenterforBiore ning,aswell

asChinaMOSTInternationalCooperationFund2009DFA61680,fortheir nancialsupportforthiswork.PartsofthisworkwerecarriedoutintheCharacterizationFacility,UniversityofMinne-sota,whichreceivespartialsupportfromNSFthroughtheMRSECprogram.

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