Design and analysis of biodiesel production from algae grown

CleanTechnEnvironPolicy(2010)12:239–254DOI10.1007/s10098-009-0215-6

ORIGINALPAPER

Designandanalysisofbiodieselproductionfromalgaegrownthroughcarbonsequestration

GracePokoo-AikinsÆAhmedNadimÆ

MahmoudM.El-HalwagiÆVladimirMahalec

Received:18October2008/Accepted:13March2009/Publishedonline:31March2009ÓSpringer-Verlag2009

AbstractThispaperaddressesthedesignandtechno-economicanalysisofanintegratedsystemforthepro-ductionofbiodieselfromalgaloilproducedviathesequestrationofcarbondioxidefromthe?uegasofapowerplant.Theproposedsystemprovidesanef?cientwaytothereductioningreenhousegasemissionsandyieldsalgaeasapotentialalternativetoedibleoilscur-rentlyusedforbiodieselproduction.Algaecanbeprocessedintoalgaloilbyvariouspathways.Thealgaloilcanthenbeusedtoproducebiodiesel.A?owsheetoftheintegratedsystemissynthesized.Then,processsimulationusingASPENPlusiscarriedouttomodelatwo-stagealkalicatalyzedtransesteri?cationreactionforconvertingmicroalgaloilofChlorellaspeciestobiodiesel.Costesti-mationiscarriedoutwiththeaidofICARUSsoftware.Furthereconomicanalysisisperformedtodeterminepro?tabilityofthealgaloiltobiodieselprocess.Theresultssuggestthat,forthealgaloiltobiodieselprocessanalyzedinthisstudy,factorssuchaschoosingtherightalgalspe-cies,usingtheappropriatepathwayforconvertingalgaetoalgaloil,sellingtheresultingbiodieselandglycerolatafavorablemarketsellingprices,andattaininghighlevelsofprocessintegrationcancollectivelyrenderalgaloiltobeacompetitivealternativetofood-basedplantoils.KeywordsProcessintegrationÁProcesssimulationÁBiodieselÁTransesteri?cationÁMicroalgaloil

G.Pokoo-AikinsÁM.M.El-Halwagi(&)

DepartmentofChemicalEngineering,TexasA&MUniversity,3122TAMU,CollegeStation,TX77843,USAe-mail:El-Halwagi@tamu.edu

A.NadimÁV.Mahalec

DepartmentofChemicalEngineering,McMasterUniversity,1280MainStreetWest,Hamilton,ONL8S4L9,Canada

Introduction

Biodieselisatransportationfuelthathasgrownimmenselyinpopularityoverthepastdecade.Withthedwindlingreservesoffossilfuels,http://wendang.chazidian.commonsourcesforbiodieselfeedstockincludesoy,sun?ower,saf?ower,canola,http://wendang.chazidian.comtelytherehasbeengrowingcontroversyabouttheuseofpotentialfoodsourcesfortheproductionoffuel.Inattempttoaddresstheseconcerns,researchershaveturnedtheirfocusfromthepopularfeedstockandarecurrentlyinvestigatingtheuseofalternative,non-foodrelatedfeedstocksuchasoilfromalgae.

Algaearealargeanddiversegroupofsimpleplant-likeorganisms,rangingfromunicellartomulticellarforms.Thesecellshavetheabilitytoconvertcarbondioxidetobiomassthatcanfurtherbeprocesseddown-streamtoproducebiodiesel,fertilizerandotherusefulproducts.Photosyntheticgrowthofalgaerequirescarbondioxide,waterandsunlight.Temperatureshouldbeintherangeof20–30°Cinordertohavegoodgrowingcondi-tions.Algaealsoneedotherinorganicnutrientslikephosphorusandnitrogeninordertogrow.Thefactthatmicroalgaegrowinaqueoussuspensions,allowsformoreef?cientaccesstoH2O,CO2andothernutrientswhichexplainsthepotentialfortheproductionofmoreoilperunitareathanothercropscurrentlyused.Thechemicalcompositionofalgaediffersbasedonspecies.Algaehaveseveralcharacteristicsthatcausethemtobeacandidatebiodieselfeedstockthatdeservesseriousinvestigation.

Theadvantagesofusingalgaeforbiodieselproductioninclude:

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240?Nocompetitionforlandwithcrops?Nocompetitionwiththefoodmarket

?

Abilitytogrowinwaterwithhighlevelsofsaltsothereisnoadditionaldemandoffreshwater.Also,areaswithsalinegroundwaterthathasnootherusefulapplica-tionscanbetargeted

?Overalluselesswaterthanoilseeds

?

Highoilyield:algae(oftheaquaticspecies)requirelesslandforgrowththanbiodieselfeedstockfromterrestrialplantsbecausetheyarecapableofproducingmoreoilperhectare(Chisti2008a).Table1showsthepotentialgallonsofoilperacreperyearfromdifferentcrops.Furthermore,theoilcontentinalgae(perdryweight)canreachashighas80%(Chisti2008a).Itisworthnotingthattheoilfrommicroalgaecanbeextractedwithyieldsupto80–90%(Grimaetal.1994;Fajardoetal.2007;Belarbietal.2000).

?

Ef?cientsequestrationofCO2:anotherreasonwhymicroalgaeareattractiveisthatCO2(ofabouthalfoftheofdryalgaeweight)isneededforgrowth(Chisti2008a).CO2isacommonindustrialpollutant,thusmicroalgaecancontributetoreducingatmosphericCO2byconsumingCO2wastesfromindustrialsourcessuchaspowerplants.

Thereareninemajorgroupsofalgaewhicharecya-nobacteria(Cyanophyceae),greenalgae(Chlorophyceae),diatoms(Bacillariophyceae),yellow-greenalgae(Xanto-phyceae),goldenalgae(Chrysophyceae),redalgae(Rhodophyceae),brownalgae(Phaeophyceae),dino?ag-ellates(Dinophyceae)and‘pico-plankton’(PrasinophyceaeandEustigmatophyceae)(Huetal.,2008).Oftheseninegroups,thegreenalgaearethelargesttaxonomicgroup.Microalgaehavebeenknowntosurviveunderawiderangeofconditions.Underoptimalconditions,microalgaehavelipidcontentbetween5and20%dryweightwhileunderunfavorableconditionslipidcontentincreasesbetween20and50%(Huetal.2008).Hence,itisidealtocultivatemicroalgaeunderoptimalconditionsandlaterexposethemtounfavorableconditionsinordertoincreaselipidcontent.

Table1Gallonsofoilperacreperyear(Chisti2008a)GallonsofoilperacreperyearCorn18Soybeans48Saf?ower83Sun?ower102Rapeseed127

OilPalm635

Microalgae

5,000–15,000

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G.Pokoo-Aikinsetal.

Laboratoryexperimentsutilizinggreenalgae,diatoms,andoleaginousspeciesfromothereukaryotictaxashowthatthemicroalgaehaveoilcontentof26,23,and27%dryweight,respectively,underoptimalconditionsand46,38,and45%dryweight,respectivelyunderstressconditions(Huetal.2008).Dependingonthespeciesofmicroalgae,oilcontentcanbefurtherincreasedbylimitingcertainnutrientssuchasnitrogen,phosphorusorsulfur.Forexample,limitingsulfurcontentcanincreaselipidcontentinChlorellasp.(Otsuka1961).

Withthegrowinginterestingrowingalgaeforenergyapplications,differentopinionshavebeenexpressed.Theopinionsrangefromconcernstoskepticismabouttheenergyef?ciency,scaleup,andeconomicfeasibilityofmicroalgalusefortransportationfuelsandotherenergyneeds(e.g.,Anslow2008;Sweeney2008;Reijnders2008)topositiveassessmentofitsef?ciencyandfutureindustrialapplicationsinproducingbiodieselmeetingASTMstan-dards(e.g.,Chisti2007,2008a,b;MiaoandWu2006).Thegrowthofalgaerequirescarbondioxideasoneofthemainnutrientsneeded.ThereisanopportunitytosequesterCO2byusing?uegasemissionsfromindustrialsourcesastheCO2feedforalgaecultivation.Theobjectiveofthispaperistodevelopatechno-economicanalysisofaprocessforsequesteringCO2from?uegasintogrowingalgaewhichprovideslipidsthatareprocessedtoproducebiodiesel.Acombinationofsystemsynthesis,simulation,integration,andanalysisisusedtoassessthetechnicalandeconomicperformanceoftheprocess.Acasestudyissolvedtodiscussthevariousmetricsoftheprocess.

Problemstatement

Theproblemtobeaddressedinthepapermaybestatedasfollows:

Givenanindustrialsource(e.g.,powerplant)whichproduces?uegas(?owrateMandcompositionZ),itisdesiredtosequesterCO2fromthe?uegastogrowalgaewhichistobeprocessedtoproducebiodiesel.Thepaperwilldevelopasystemsapproachforthealternativeprocesspathsandperformatechno-economicanalysistodeterminetheoptimaldesignofa?uegastobiodieselsystemthroughthecultivationofalgae.Thepaperwillalsoprovideananalysisofthetechnicalandeconomicmetricsoftheaforementionedsteps.Systemoverview

Theoverallsystemiscomposedoftwomainsections:anupstreamprocessingsectionwhichisaimedatsequesteringtheCO2,growingthealgae,andproducingthelipidsandadownstreamprocessingsectionwhichincludesthe

Designandanalysisofbiodieselproductionfromalgae241

pretreatmentofthelipidsfollowedbytransesteri?cationthenseparationand?nishingtoyieldthebiodiesel.Figure1illustratesthesekeysteps.Algaeselection

Thechoiceofalgaespeciesshouldaddressspeci?ccharacteristicsthatallowtheuseof?uegasastheCO2source.Muchresearchhasbeendoneonthetoleranceofdifferentspeciesto?uegases.Severalspecieswerefoundtobesuitableforthegrowthofalgaeusing?uegas.OneofthesemanyspeciesisChlorellaspecies.Hanagataetal.(1992)foundthatChlorellaistoleranttoCO2concen-trationsofupto40%byvolume.Sungetal.(1999)reportedthatchlorellagrewinconditionsofupto40°C.TheseresultsindicatethatChlorellaisagoodchoiceforthisstudy.

Inthiswork,theChlorellaspeciesischosen.TheoilcontentofChlorellatypicallyrangesbetween28and32%dryweight(Chisti2007)butcanreach46%dryweightunderstressconditions(Huetal.2008)and55%dryweightwhengrownheterotrophically(http://wendang.chazidian.comrmationaboutthefattyacidcompositionsofvariousmicroalgae(namelythegreenalgaeintheclassesChlorophyceaeandPrasinophyceae)waspublishedin1992(Dunstanetal.1992).ChlorellaisintheclassChloro-phyceaandthefattyacidcompositionsofthreeChlorellaspecieswerelisted.Chlorellasp.(CS-195)wasusedinthisanalysisbecauseofitspotentialeaseforuseinsimulation.ItisinterestingtonotethattheChlorellaprotothecoides(CS-41)compositionincludesthesamefattyacidspresentintheChlorellasp.chosen(Dunstanetal.1992)butinslightlydifferentproportions.AnotherreasonwhyChlo-rellasp.waschosenistheavailabilityofinformationaboutitsgrowth,harvestingandextraction.Feedstockproduction

Algaecanbecultivatedviaanopensystemoraclosedsystem.Racewaypondsarethemostcommerciallyusedopensystemforgrowingalgae.Photobioreactorsareaclosedsystemforalgaecultivation.BothracewaypondsandphotobioreactorsaredescribedbyChisti(2007).Twosystemsareconsideredinthiswork:theuseofanopenpondsystemversustheBio-Kingsystem(CleanTech

2008)thatusesareactortocultivatealgae.TheBio-KingprocessisutilizedbyacompanyinTheNetherlands.Methodsforharvestingincludecentrifugation,?ltration,and?occulation.Centrifugationisexpensivebutalsooneofthemosteffectivewaystoharvestalgae.TheAlfaLavalPXseriescentrifugeswillbeusedtoharvestthealgae.Centrifugationwillresultinthealgaebeing30%solidwith70%moisturecontent.Asaresult,furtherdryingisrequired.

Dryingisconsideredtobethemostenergyintensivepartofthisprocess.Therearemanywaystodrythewetpasteslurrythatcomesoutofthecentrifuge.Asmentionedtheslurrycontains30%solidswiththeremaining70%water.Totryandsaveonenergycostsforthisprocess,thedryingwillbedoneusingexcess?uegas.

Extractionisthe?nalstepintheprocessingofalgaeforuseinbiodieselproduction.Algaloilcanbeextractedeitherphysically,chemicallyorboth.Anexpeller/presscanbeusedtophysicallyextractalgaloil.TheBK-oilpressiscapableofprocessing20kg/handwillbeusedforthisstudy.

Chlorellaisanalgalspeciesthatcontainsanywherefrom29to32%lipids(oilcontent).ForthisstudyitisassumedthatChlorellaisabout30%oilandforsensitivityanalysispurposes,thecostofproducingalgaloilassuming50%oilcontentwillalsobeevaluated.Costofproducingalgaloil

AnanalysiswasconductedassumingaprocessthatutilizedtheBio-KingBioreactorforgrowingthealgae,centrifu-gationforharvesting,excess?uegasfordrying,andtheBio-Kingoilpressforextraction.Economicandsensitivityanalyseswereconductedforthisprocessandusedforestimatingarangeofcostsforproducingoilfromalgae.Speci?cally,twofactorswerevaried:oilcontentinthealgaeandperformance.Twooilcontentsareconsidered:30and50%(drybasisoilinalgae).Thesecondfactoristheperformanceofthedryingandextractionunits.Forthehigh-performancecase,lowcostofelectricity($0.05/kWh),highproduction(100ton/dayplant),andtheuseofheatintegrationindryingusingthehot?uegaseswereassumed.Forthelow-performancecase,highcostofelectricity($0.20/kWh),lowproduction(1ton/day),andnoheatintegrationfordryingwasassumed.Thecost

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242

Table2Estimatedcostsforproducingalgaloil

OilcontentUnits

30%

50%Lowperformance1.140.63$/lbHighperformance0.210.07$/lbAverage

0.68

0.35

$/lbestimatesarepresentedinTable2.Thesecostsareusedlaterintheeconomicanalysisoftheproductionofbiodiesel.

Biodieselprocessdescription

MiaoandWu(2006)haveshownthataspeciesofChlo-rella(Chlorellaprotothecoides)canbeusedtoproducebiodieselthatmeetsASTMstandards.Themicroalgaeweregrownheterotrophicallytoincreasetheoilcontentfrom14.6%dryweightto55.2%dryweight.Acidtransesteri?cationwasusedsincetheacidvalueforthealgaloilwasreportedas8.97mgKOH/g.Thebiodieselyieldwasapproximately70%at50°Candconditionsof60%H2SO4catalyst,5hreactiontime,160rpm,9.12gmicroalgaloil,and30–1methanoltooilratio(MiaoandWu2006).

Thedesign,integration,andeconomicassessmentoftheprocessarebasedontheprocedureshowninFig.2.Thisapproachisbasedonwell-establishedproceduresintheareasofprocesssynthesis,simulation,andintegration.Inthisworkthealgaloilistransesteri?edtobiodieselinacontinuousprocess.Biodieselcanbeproducedbyoneofthreecommonroutes.Theyare:acidcatalyzedtrans-esteri?cation,basecatalyzedtransesteri?cationoracidcatalyzedesteri?cationoffeedstocktofattyacidsandthentoalkylesters(NBB2008).Basecatalyzedtransesteri?-cationisthewell-establishedmeansofprocessingbiodieselandtheoverwhelmingoptionusedinindustryforeco-nomicandtechnicalreasons.Rashidetal.(2008)producedmethylestersfromsun?oweroilutilizingNaOHcatalystat1wt%concentrationina6:1methanoltooilratioat60°Catyieldsof97.1%.Georgogiannietal.(2008)reportedmethylesteryieldsfromtheprocessingofsun?oweroilof90%forconditionsof60°C,7:1methanoltooilmolarratioand1wt%NaOHascatalyst.Intheresultsanddiscussionitwaslaterstatedthat‘‘thehighestconversiontoester(93–98%)wasobservedataratioof6:1’’(Georgogiannietal.2008).RashidandAnwar(2008)foundthatbiodieselcouldbeproducedfromsaf?oweroilwithyieldsupto98%forbase-catalyzedtransesteri?cationutilizingsodiummeth-oxidecatalystat1wt%concentration,60°C,and6:1methanoltooilratioandyieldsofabove90%couldbe

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G.Pokoo-Aikinsetal.

achievedforthesameconditionswiththeexceptionoftheuseofNaOHasacatalyst.Mekaetal.(2007)alsosyn-thesizedbiodieselfromsaf?oweroilandfoundthatat60°Cfor6:1methanoltooilratioand1wt%NaOHcatalyst,yieldof96%couldbeobtained.LeungandGuo(2006)performedexperimentsutilizingneatcanolaoilandusedfryingoilandfoundthatforexperimentsexploringdif-ferentparameters,atemperatureof60°Cwasoptimumforareactiontimeof20minusedfryingoil,thatestercontentwashighest(98%)forcanolaoilforanalcoholtooilratioof6:1(correspondingtoayieldof94%)andthatforthethreealkalicatalystexploredsodiumhydroxidewasthecheapestandhadanoptimumconcentrationof1.0forneatcanolaoiland1.1wt%forusedfryingoil.Foonetal.(2004)inexploringthekineticsofthetransesteri?cationofpalmoil,performedexperimentsutilizingbase-catalyzedtransesteri?cationandfoundthatformationofmethylesterswasfastestforNaOHat60°Cfortheparametersexplored.Conversionsabove97%werereported.

Leevijit

Designandanalysisofbiodieselproductionfromalgaeetal.(2008)utilizedalkalicatalyzedtransesteri?cationina6-stagereactortoafattyacidmethylesterproductfrompalmoil.NaOHwasusedat60°Cinamethanoltooilratioof6:1.Gerpenetal.(2004)describesthevariouspossibleroutesforbiodieselproductionincludingalkali-transesteri?cation.

Forthisreason,base-catalyzedtransesteri?cationisusedinthisinvestigation.Pretreatmentisrequiredforfeedstockwithhighfreefattyacid(FFA)content(i.e.greaterthan1%,suchaswastecookingoils)aswellasfeedstockwithsubstantialamountsofimpurities(suchassomealgaloils).FeedstockwithFFAcontentof1wt%orlessaregenerallyrequiredforbase-catalyzedtransesteri?cation.ThealgaloilusedfortheprocessingofbiodieselinthisworkisassumedtohaveonlytraceamountsofimpuritiesandtohaveFFAcontentof0.05wt%thereforenopretreatmentisrequired.

Ingeneral,thebiodieselprocessinthisworkconsistsofsevensections:?Feedstockcomposition

?Two-stagetransesteri?cation?FAMEandglycerolseparation?Methanolrecovery?Alkaliremoval

?Waterwashing(FAMEpuri?cation)?

Glycerolpuri?cation

Feedstockcomposition

ThefeedstockinthisworkisalgaloilfromChlorellasp.andischaracterizedintermsofthecompositionoftheindividualfattyacidsandtriglycerides.TheFFAcontentisassumedtobe0.05wt%andthuspretreatmentisnotnecessary.BasedonthedatapresentedbyDunstanetal.(1992)forChlorellasp.(CS-195),thefattyacidcompo-sitionisdistributedsuchthatthetotalweightpercentis0.05.Fortheremaining99.95wt%,thesamedata(Dunstanetal.1992)aredistributedsuchthatitencom-passesthetriglyceridecomposition.Forsimpli?cation,eachtriglycerideisrepresentedascontainingthreeiden-ticalcomponentfattyacids,althoughinrealitynumerouspossiblecombinationsexistforthefattyacidscomprisingeachtriglyceride.

SincetheASPENPlussimulationsoftwareonlyhasthethermodynamicdataandotherinformationforalimitednumberoffattyacidsandthecorrespondingtriglyceridesandmethylestersthatarefoundinplantoils,fatsandalgaloils(MyintandEl-Halwagi2009),mostofthecomponentsofthealgaloilfeedstockwereenteredmanuallyusingtheuser-de?nedmethodandthestructuresofeachcompoundwereconstructedusingISISsoftware.

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Two-stagetransesteri?cation

Theoverallreactionbetweenthetriglycerides(algaloil)andmethanolisgivenby:

OO ==H2CO

C

R1

H2C

OH

C

R1

OO Catalyst

==R2

+ 3 CH3OH

OH

+

C

R2

OO

==H2R3

H2OHC

R3

TriglycerideMethanol

GlycerolFattyAcidAlkylEster

Consequently,onemoleculeofeachtriglycerideinthealgaloilreactswiththreemoleculesofmethanoltoproducethreemoleculesofmethylesters,thebiodie-selproduct,andonemoleculeofglycerol(Gerpenetal.2004).

Basedonseveralstudiesofalkali-catalyzedtranseste-ri?cation,thereactionwillbecarriedoutatthetemperatureneartheboilingpointofthealcohol(60°Cformethanol).Amolarratioof6:1,alcohol:oil,isalsocon?rmedtobetheoptimalratiobynumerousstudies(MaandHanna1999;Tapasvietal.2005;Meheretal.2006;MyintandEl-Halwagi2009).Inthisstudy,thetemperatureof60°C,methanolasthealcohol,amolarratioof6:1methanoltooil,andNaOHasthebasecat-alystaretheconditionsusedasaresultofcomprehensiveliteraturereviewmentionedabove‘‘Biodieselprocessdescription’’.

Inthe?rstreactor,sodiumhydroxidewithaconcen-trationof1.0wt%ofthefeedalgaloilwasused.TheconcentrationofNaOHfortheunreactedoilsuggestedinpatentdocumentsbyWimmer(1995)andTanakaetal.(1981)forthesecondreactoris0.2wt%ofinletoils.Fortheprocesswhere97.7%conversionisassumedthrougheachreactor,noadditionalNaOHisneededinthesec-ondreactor.Fortheprocesswhere70%conversionisassumedthrougheachreactor,additionalNaOHthatisonly0.14wt%ofinletoilsisneededinthesecondreactorasaresultofmassbalancecalculations,tobringthetotalNaOHto1.0wt%oftheinlettothesecondreactor.Thepurityofalgaloilisassumedtobe99.95wt%whiletheFFAcontentwasassumedtobe0.05wt%.

Inordertoincreasetheconversionofthealgaloil,twotransesteri?cationreactionsareconductedinsequence.ConversionoffeedstockhavebeendocumentedbyTanakaetal.(1981)toreachupto99.5wt%usingthistwo-stepprocess.Inthiswork,theconversionissettothesamepercentineachreactor.Inthe?rstscenario,theconversionthrougheachreactorissetat97.7%andinthesecondscenariotheconversionthrougheachreactorissetat70%.Thereactionproductsbiodieselandglycerol

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