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Chinese Journal of Catalysis 34 (2013) 80–100 available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/chnjc Review Factors in mass cultivation of microalgae for biodiesel ZHU Junying, RONG Junfeng, ZONG Baoning * SINOPEC Research Institute of Petroleum Processing ARTICLE INFO ABSTRACT Article history: Received 28 October 2012 Accepted 1 December 2012 Published 20 January 2013 Biofuel from microalgae is a long term strategy to solve the energy crisis. It is a new area of biologi‐ cal engineering and process engineering that consists of the isolation and characterization of micro‐ algae species, mass cultivation of microalgae, harvesting and post‐processing. The successful mass cultivation of microalgae is one of its main challenges. Several factors influencing the mass cultiva‐ tion of microalgae are discussed, such as microalgae species, metabolic mechanism, culture condi‐ tions and the photobioreactor. This paper will help the development of biofuels from microalgae and its photobioreactor. © 2013, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved. Keywords: Microalgae Biofuel Photobioreactor Lipid content Mass cultivation 1. Introduction Energy is the basic driving force for the development of so‐ ciety and economy. The rising demand for energy threatens the availability of sustainable energy for future generations. Fossil fuel is a non‐renewable resource that is getting exhausted. De‐ veloping new means of biofuel production as a renewable green energy is becoming increasing important [1,2]. The first generation of biofuels were mainly extracted from food and oil crops, which include starch and corn. The use of cellulose, such as crop stalks, as the raw material to extract biofuel is consid‐ ered the second generation of biofuels. Biofuels from microal‐ gae are considered the third generation of biofuels. It has marked advantages over the previous two generations and it has been widely studied [3–5]. The technology of biofuels from microalgae mainly com‐ prises four areas, which are isolation and characterization of microalgae species, mass cultivation of microalgae, harvesting, and post‐processing. It is a new area that integrates biological engineering and process engineering, which still needs much study. Several factors are important in the development of bio‐ fuels from microalgae, among which its mass cultivation plays a key role. Many species have a high lipid content, including Chlorophyta and Bacillariophyta, such as Chlorella, Scenedesmus, and Phaeodactylum. However, the growth rate and lipid content of microalgae under mass cultivation conditions are significantly lower than those grown in the laboratory. This may be due to that optimal conditions are provided in the la‐ boratory, and these may not be the same in mass cultivation, thereby influencing the growth rate and lipid content. There‐ fore, it is very important to study how to maintain microalgae in a state of high growth rate and lipid content. The study of the factors, such as isolation of microalgae species, metabolic mechanism, culture conditions and the photobioreactor, can improve the development of biofuels from microalgae. Further studies and summaries of previously obtained results are im‐ portant for optimal microalgae biomass production, optimal photobioreactor design, the development of catalysts for the conversion of lipids to biodiesel and for understanding the pollutant formation chemistry of microalgae‐derived biofuels. * Corresponding author. Tel: +86‐10‐82368011; E‐mail: [email protected] This work was supported by the National Basic Research Program of China (973 Program, 2012CB224803). DOI: 10.1016/S1872‐2067(11)60497‐X

Alga Biodiesel Review

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ChineseJournalofCatalysis34(2013)80–100 

 

a v a i l a b l e   a t  www. s c i e n c e d i r e c t . c om  

j o u r n a l   h omep a g e :  www. e l s e v i e r. c om / l o c a t e / c h n j c  

Review 

Factorsinmasscultivationofmicroalgaeforbiodiesel

ZHUJunying,RONGJunfeng,ZONGBaoning*SINOPECResearchInstituteofPetroleumProcessing

A R T I C L E I N F O  

A B S T R A C T

Articlehistory:Received28October2012Accepted1December2012Published20January2013

Biofuelfrommicroalgaeisalongtermstrategytosolvetheenergycrisis.Itisanewareaofbiologi‐calengineeringandprocessengineeringthatconsistsoftheisolationandcharacterizationofmicro‐algaespecies,masscultivationofmicroalgae,harvestingandpost‐processing.Thesuccessfulmasscultivationofmicroalgaeisoneofitsmainchallenges.Severalfactorsinfluencingthemasscultiva‐tionofmicroalgaearediscussed,suchasmicroalgaespecies,metabolicmechanism,culturecondi‐tionsand thephotobioreactor.Thispaperwillhelp thedevelopmentofbiofuels frommicroalgaeanditsphotobioreactor.

©2013,DalianInstituteofChemicalPhysics,ChineseAcademyofSciences.PublishedbyElsevierB.V.Allrightsreserved.

Keywords:MicroalgaeBiofuelPhotobioreactorLipidcontentMasscultivation

 

1. Introduction

Energyisthebasicdrivingforceforthedevelopmentofso‐cietyandeconomy.Therisingdemandforenergythreatenstheavailabilityofsustainableenergyforfuturegenerations.Fossilfuelisanon‐renewableresourcethatisgettingexhausted.De‐veloping new means of biofuel production as a renewablegreenenergyisbecomingincreasingimportant[1,2].Thefirstgenerationofbiofuelsweremainlyextractedfromfoodandoilcrops,whichincludestarchandcorn.Theuseofcellulose,suchascropstalks,astherawmaterialtoextractbiofuelisconsid‐eredthesecondgenerationofbiofuels.Biofuels frommicroal‐gae are considered the third generation of biofuels. It hasmarked advantages over the previous two generations and ithasbeenwidelystudied[3–5].

The technology of biofuels from microalgae mainly com‐prises four areas, which are isolation and characterization ofmicroalgaespecies,masscultivationofmicroalgae,harvesting,andpost‐processing.Itisanewareathatintegratesbiologicalengineering and process engineering, which still needsmuch

study.Severalfactorsareimportantinthedevelopmentofbio‐fuelsfrommicroalgae,amongwhichitsmasscultivationplaysakey role. Many species have a high lipid content, includingChlorophyta and Bacillariophyta, such as Chlorella, Scenedes‐mus, andPhaeodactylum. However, the growth rate and lipidcontent of microalgae under mass cultivation conditions aresignificantly lower than those grown in the laboratory. Thismaybedue to thatoptimalconditionsareprovided in the la‐boratory, and thesemaynotbe the same inmass cultivation,thereby influencing the growth rate and lipid content. There‐fore,itisveryimportanttostudyhowtomaintainmicroalgaeinastateofhighgrowthrateandlipidcontent.Thestudyofthefactors, such as isolation of microalgae species, metabolicmechanism, culture conditions and the photobioreactor, canimprovethedevelopmentofbiofuelsfrommicroalgae.Furtherstudies and summariesofpreviouslyobtained results are im‐portant for optimal microalgae biomass production, optimalphotobioreactor design, the development of catalysts for theconversion of lipids to biodiesel and for understanding thepollutantformationchemistryofmicroalgae‐derivedbiofuels.

*Correspondingauthor.Tel:+86‐10‐82368011;E‐mail:zongbn.ripp@sinopec.comThisworkwassupportedbytheNationalBasicResearchProgramofChina(973Program,2012CB224803).DOI:10.1016/S1872‐2067(11)60497‐X

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2. Microalgaespecies

2.1. Microalgae

Microalgae are microscopic photosynthetic, free living or‐ganisms of several microns (Chlorella only 35 μm). Theythrive in diverse aquatic habitats, which include freshwater,brackish (<3.5% salt), marine (3.5% salt), and hypersaline(>3.5% salt) environmentswith awide rangeof temperatureand pH. Algae are typically subdivided into microalgae andmacroalgae according to size. Unlike microalgae, macroalgaehave cells organized into structures resembling the leaves,stems,androotsofhigherplants,andsomeareaslongas60m.Microalgae can alsobe subdivided into twobroad categories:the prokaryotic cyanobacteria and the true eukaryoticmicro‐algae.Cyanobacteria,oftenreferredtoastheblue‐greenalgae,havechlorophyllaanddonotpossesschloroplastsoranyothersuchorganelles.Thesehaveahighproteincontentasmuchas70%ofdrymassandalowfatcontentofapproximately5%[6].Thekindsofeukaryoticmicroalgaeareenormousandtherearetens of thousands of species, such as Chlorophyta, Bacillari‐ophytaandXanthophyta.Theseareexceedingrichinlipidandprotein, and are considered the main microalgae species forbiofuels.

Microalgaehaveseveraladvantagesasamaterialforbiofu‐els to replacepetroleum‐based fuels.Microalgaegrowrapidlyandmanykindsarerichinlipid(20%50%).Thecultivationofmicroalgaedoesnotentail a landconflictwithagriculture forfoodbecause itcanutilizewastewaterandnon‐potablesalinewater that cannot be used by conventional agriculture. ThefixationofCO2bymicroalgalphotosynthesisisoneofthemostpromising method for CO2 sequestration from flue gas. Themicroalgaebiomasscanproducebiofuels includinggreendie‐sel,greengasoline,aviationfuels,ethanol,andmethaneaswellasvaluableco‐products.Thebiochemicalmechanismofphoto‐synthesis inmicroalgae is similar to that inplants,butdue totheir simple structure, microalgae are particularly efficientconvertersof solarenergy.Theydonothave supportand re‐productive structures, andwhen providedwith light and nu‐trients,themicroalgalcellscanusemostoftheenergytrappedforbiomassgrowth.

Microalgaecaneitherbeautotrophicorheterotrophic.Theformer require inorganic compounds suchasCO2, salts andalightenergysourceforgrowthandthelatterarenonphotosyn‐thetic[7–10].Somemicroalgaearemixotroph,suchasChlorel‐la, which has the ability to both perform photosynthesis andacquireexogenousorganicnutrients.Microalgalcultivationnotonlyprovidesbiofuelsbutalsoprovidesgreenhousegasreduc‐tionasitutilizesalargeamountofCO2duringthecultivation.Moreover,wastewatercanbeused toculturemicroalgae.Theheterotrophicormixotrophicgrowthcapabilitiesofmicroalgalstrainsareattractiveattributessincetheseallowthemtogrowboth in light and in dark conditions. The addition of externalcarbonsourcescanhelpthegrowthofmicroalgaeandtheac‐cumulationoflipids.Apotentiallyseriousdisadvantageofadd‐ing an external carbon source, especially an organic carbonsource, is thepossibilityof increasedcontaminationbyunde‐

siredmicrobes.Thereare twogeneral resources frommicroalgae thatcan

be utilized: biofuels and co‐products [5,11,12]. Biofuels frommicroalgae include lipids, H2, isoprenoids, carbohydrates, al‐cohols(eitherdirectlyorbybiomassconversion),andmethane(from anaerobic digestion). Lipids, mainly in the form of tri‐acylglycerols(TAGs),aredistributedinthecellmembraneandintracellular organelle membrane, and parts of these containcarbohydrates, and they are similar to petroleum. Besides li‐pids,co‐productsarealsovaluable.Theseincludeproteinsandpigments,whichcanbeusedinmanyfields,e.g.,pharmaceuti‐cals (therapeutic proteins, secondary metabolites), food sup‐plements,andmaterials fornanotechnology.Therehavebeenmanydiscussionsontheuseofmicroalgaebiomasstoproducefuel and non‐fuel co‐products. In our opinion, the approachshouldnotbeinflexible.Areasonableapproachtodecidewhichfuelproductsandadditionalco‐productstomakeistoidentifythe optimalmicroalgae species according to the environmentso that cellular metabolism is geared towards the products,which simplifies the characterization and possible develop‐mentforproducts,ratherthantobefixedaboutthetypeoffuelorco‐product.

2.2. Isolationandcharacterizationofmicroalgaespecies

Manymicroalgal species are available from culture collec‐tionssuchasUTEX(CultureCollectionofAlgaeattheUniversi‐tyofTexasatAustin,Texas)withabout3000speciesandCCMP(Provasoli‐GuillardNationalCenterforCultureofMarinePhy‐toplankton at the Bigelow Laboratory for Ocean Sciences inWestBoothbayHarbor,Maine)withmore than2500species.However, many speciesmay have lost some of their originalproperties,suchasmatingcapabilityorversatilityduetobeingcultivatedforseveraldecadesinthecollections.Therefore,itisimportanttoobtainversatileandrobustspecies formasscul‐tivation.Further, tocollect the informationoncultivationandspeciesdevelopmentforthemasscultivationofmicroalgae,itisrecommendedthattheisolatedspeciesbescreenedtodevelopbaseline data on regional environmental variability [13]. Toprovidethelargestpossiblerangeinmetabolicversatility,newspeciesshouldbeisolatedfromawidevarietyofenvironmentsranging fromfreshwater,brackishwaters,marine,andhyper‐saline environments to a soil environment, aswell as from awidevarietyof theirsymbioticassociationswithotherorgan‐isms. Detailed information should be available for potentiallyvaluable new species, such as required time, specific habitat,growthconditions, germinationconditions, and temporal suc‐cession.Inourresearch,ithasbeenfoundthatmanypotential‐lyvaluablenewspeciesarenotsuitableformasscultivationforbiofuels, although they grow rapidly and are rich in lipid. Si‐nopecGroupcollaborateswiththeChineseAcademyofScienc‐esinthestudyofthemicroalgaeincludingisolationandchar‐acterization ofmicroalgae specieswith high growth rate andlipid content in sea water, freshwater, and coastal zone anddesertareas.Somemicroalgaespeciesareculturedtoprovidebiomassforbiofuels.Itisalsopossiblethatgeneticengineeringachievementsinthelaboratorycanbeappliedtothesespecies.

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Theidealscreeningshouldcoverthreemajorareas:growthphysiology,metabolite products, and species robustness. Thegrowth physiology of microalgae encompasses a number ofparameterssuchasmaximumspecificgrowthrate,maximumcell density, tolerance to environmental variables (tempera‐ture, pH, CO2 levels, etc), and variability between in situ andlaboratoryperformance.Screeningof themetaboliteproductshastoincludenotonlythemetabolitecompositionandcontent,but also the productivity of the cells formetabolites and thevaluable cellular contents of proteins, lipids, and carbohy‐drates. The rapid lipid analysis of strainswould be also veryhelpful. An ideal analyticalmethod should be able to providefatty acid profiles and identify neutral and polar lipids. Fur‐thermore,metabolitesexcreted into thegrowthmediumhavebeen largely ignored, but these may prove to be valuableco‐products. For themass cultivationof a selectedmicroalgalspecies, it is also important to consider robustness,which in‐cludeparameterssuchascultureconsistency,resilience,com‐munity stability, and susceptibility to predators present in agivenenvironment.Moreover,itisnecessarytoperformsmallscale simulation of mass cultivation conditions to determinetherobustnessoftheselectedspecies.Theprojectontheinves‐tigationof the technologyofbiofuels frommicroalgae carriedoutbySinopecGroupandtheChineseAcademyofScienceshasbegunthecultureofmicroalgaeinphotobioreactorslargerthan100L.

2.3. Biosynthesisandregulationoflipids

Triacylglycerols(TAGs)arethemajorsourceofbiofuelsasthesearethemainstoredneutralcompoundinmanymicroal‐gaeunderstressconditions,suchashighlightornutrientstar‐vation [14–17].Somemicroalgal speciesnaturallyaccumulatelarge amounts of TAGs (30%60% of dry weight), and havephotosyntheticefficiencyandlipidproductpotentialthatareatleast an order of magnitude higher than those of terrestrialcropplants.However,thesynthesismechanismsof fattyacidsandTAGsinmicroalgaearenotknown.

TheKennedypathwayisbelievedtobethemajorpathwayfor the accumulation of TAGs in plants andmicroalgae. It in‐volvesdenovofattyacidsynthesisinthestromaofplastidsandsubsequent incorporation of the fatty acid into the glycerolbackbone,andleadstoTAGsviathreesequentialacyltransfersfromacylCoAintheendoplasmicreticulum.TheconversionofacetylCoAtomalonylCoA,catalyzedbyacetylCoAcarboxylase(ACCase),isthecommitmentstepandalsothefirststepinfattyacid synthesis. Thus, it has been propounded that enhancingtheactivityofACCasewouldbehelpfulfortheaccumulationoflipids inmicroalgae.However, the resultsweredifferent, andexcessive production of ACCase, which catalyses a keymeta‐bolicstepinthebiosynthesisoflipid,didnotleadtoincreasedlipid content. There are several steps between the first stepcatalyzedbyACCaseand the stepof synthesizingTAGs in theKennedy pathway, and the decreased activity of ACCase inthesestepsmaybethereason[18,19].Thereareseveralpath‐waysforTAGssynthesisbymicroalgae.Therelativecontribu‐tionsofindividualpathwaystooverallTAGsformationdepend

onenvironmentalandcultureconditions.Alternativepathwaysthat convertmembrane lipids and/or carbohydrates to TAGshavebeendemonstratedinplantsandyeast,whichperforminan acyl CoA‐independent way, but these have not yet beenfound inmicroalgae [20,21]. It iswell known that fatty acidsare common precursors for the synthesis of bothmembranelipidsandTAGs,butthereareseveralaspectsthatneedeluci‐dation, includingthedistributionof theprecursors inthetwodistinct destinations, the inter‐conversion between the twotypes of lipids, and information on regulation at the geneticlevel.

Itisachallengetoextrapolateinformationlearnedaboutli‐pidbiosynthesisandregulationfromlaboratoryspeciestospe‐ciesculturedonalargescale.Itisalsonotknowniftheabilitytocontrolthefateoffattyacidsvariesamongmicroalgaltaxo‐nomicgroupsandevenbetweenisolatesorstrainsofthesamespecies,thatistosay,whetherthebasallipid/TAGscontentisanintrinsicpropertyof individualspecies.Similarly, it isdiffi‐cult to use information about lipid biosynthesis in plants formicroalgae[22].Thus,thestudyoffattyacidandlipidsynthesisinordertoidentifythekeygenes/enzymesandnewpathwaysinmicroalgae species is very important for biofuels, as is thestudyofthegenesinvolvedinlipidmetabolism.

Under environmental stress conditions, such as nutrientsstarvationandhighlight,microalgaequicklystoptheirdivisionandaccumulateTAGsasthemainstoragecompound[14–17].This is almost the default pathway to synthesize and depositTAGs to form lipid bodies in cytosol under environmentalstress. The pathway of TAGs synthesismay playmore activeanddiverserolesinresponsetostress.Understress,excessiveelectrons that accumulate in the photosynthetic electrontransport chain may induce the over‐production of reactiveoxygenspecies,whichmayinturncausetheinhibitionofpho‐tosynthesis anddamagemembrane lipids, proteins andothermacromolecules.TheexcesselectronscanbeconsumedduringtheTAGssynthesispathway.ThesynthesisofaC18fattyacidconsumes approximately 24 NADPH from the electrontransportchain,whichistwicethatrequiredfortheformationofa carbohydrateorproteinmoleculeof thesamemass.Thisreduces the over‐produced electrons in the transport chainunder high light or other stresses [23]. Conversely, the TAGssynthesispathwayisusuallyassociatedwithsecondarycarot‐enoid synthesis in microalgae [24,25]. The molecules (e.g.β‐carotene and lutein) produced in the pathway are seques‐tered in cytosolic lipid bodies, which can prevent or reduceexcessivelight fromstrikingthechloroplastunderstressesbyperipheral distribution. TAGs synthesis can also utilize phos‐phatidylcholine, phatidylethanolamine and galactolipids ortoxic fatty acids excreted from themembrane system as acyldonors,therebyservingasamechanismtodetoxifymembranelipidsanddepositthemintheformofTAGs.Thenitrogencon‐centration influences the lipid content of microalgae. A lowconcentration of nitrogen reduces the protein content inmi‐croalgae and increases the lipid and carbohydrate contents.With Dunaliella tertiolecta cultured in a nitrogen‐starvationmedium, the accumulation of total lipid occurred on the fifthday,andwashigherthaninanormalmedium[26].Theaccu‐

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mulationof lipidpercellwasmore in thenitrogen‐starvationmedium than in thenormalmedium, and it happenedon thethird day and it had achieved amaximum of more than fivetimesbythefourthday.Althoughtheaccumulationratesweredifferent in the twomedia, the composition of the lipidwerealmostthesame[26].

Currently, near‐complete genome informationof somemi‐croalgal species are orwill shortly become available. A largescaleEST(ExpressedSequenceTag)sequencingofmicroalgaewill providebetterknowledgeongenesdifferently expressedunderdifferentlipidproductionconditionstogiveinformationaboutphotosyntheticcarbonpartitioningandlipidsynthesisinmicroalgae.Basedonsuchinformation,metabolicengineeringthrough geneticmanipulation represents yet another promis‐ing strategy for producing biofuels. The available approachesinclude randomand targetedmutagenesis and gene transfor‐mation. The cloning and transformingof genes that influencelipid synthesis or improve robustness in selected microalgalspecies that have proven amenable to mass cultivation willenhance the overall performance and sustainable products ofTAGsorotherlipids.Geneticengineeringcanbeusedtoregu‐late lipid metabolism to increase the microalgae content byenhancing the fatty acid synthesis pathway, regulating theTAGssyntheticbypass,inhibitingcompetitiveanddegradationpathways, as well as enhancing the lipid composition[18,19,27]. However, genome functional annotations have in‐dicatedthatsomeaspectsofTAGsaccumulationanddegrada‐tion pathways are species‐specific and these are essentiallyunknownthusfar.Itisworthnotingthattheecologicalsecurityof microalgae species during genetic engineering should re‐ceivemoreattention.

3. Factorsinfluencingthegrowthandlipidcontentofmicroalgae

Microalgaearepotentialvaluablematerialsforbiofuels.Mi‐croalgaewith sequenced genomes and transgenic capabilitiesare suitable for studying cellular processes to provide infor‐mationonthebasiccellularprocessesandregulationinvolvedinthesynthesisofthebiofuelprecursors,whichwillbeusefulforthecultivation.Moreover,microalgaethatcangrowwellinthelaboratorymaybesuitableforlargescalecultivationunderdifferentenvironments.However, this isnottrueofallmicro‐algae.Therefore,thereisuncertaintyaboutwhethertheinfor‐mationobtainedfromthelaboratorymodelspeciescanbeap‐pliedtomasscultivationoutdoors.Numerousfactorsinfluencethe cultivation ofmicroalgae, including light, nutrient supply,CO2,pH, temperatureandO2[28–31]. It is important toapplythe factors that influence the cultivation ofmicroalgae in thelaboratory tomass cultivationoutdoors.Thegrowth rate andlipidcontentofmicroalgaehaveacloserelationshipwithlight,nitrogen,phosphorus,andtemperature[15,32,33].Thecondi‐tionsforgrowthandlipidaccumulationaredifferentwithdif‐ferentspecies.Therefore,theoptimummasscultivationcondi‐tions tobeused formicroalgae shouldbe those that increasegrowth rate and lipid content,which are themain factors af‐fectingthebiofuelprocess.

3.1. Light

The intensity,wavelengthand frequencyof lightaffect thephotosyntheticefficiencyofmicroalgae[34].Thelightintensityiscriticalbecausemicroalgaegrowonlywhenthe intensity ishigher than the light compensation point. It is better for thelightintensitytobelowerthanlightsaturation.Microalgaeandplants have two photosystems: photosystem I with peak ab‐sorptionat680nmandphotosystemIIwithpeakabsorptionat700 nm. The absorptivity of light with different wavelengthsvaryformicroalgaeandplants.Forexample, the lightabsorp‐tivity order of Chlorella is red light, followed by yellow andglaucomalight[35].Lightanddarkcyclealsostronglyinfluencethegrowthandphotosyntheticefficiencyofmicroalgae. Ithasbeensuggestedthatwhenthefrequencyofthelight/darkcycleincreasestohigherthan1Hz, thephotosyntheticefficiencyofmicroalgae is improved [36]. This phenomena suggested thatadding baffles to a flat photobioreactor helps to enhance thephotosyntheticefficiencyofmicroalgae[37,38].Naturallightisalwaysusedforthemasscultivationofmicroalgaebecauseitisfree.SolarradiationisplentifulinChina.Itcanreachmorethan280W/m2peryearforsomezonesand120W/m2peryearformostzones[39].Thenaturallightconversionefficiencyofmi‐croalgae can reach 3%11%, which becomes higher underartificial conditions. Natural light has a full light spectrum,whichisgoodforcultivation.Oneofthedisadvantagesofnatu‐rallightisthedifficultyofitscontrol,anditistoohighonsunnydaysespeciallyatnoonandtoolowonrainydays.

3.2. Nutrients

Many elements have to be provided for the growthofmi‐croalgae, suchascarbon(C),oxygen (O),hydrogen(H),nitro‐gen (N), potassium (K), calcium (Ca), magnesium (Mg), iron(Fe),sulfur(S),phosphorus(P),andtraceelements.Themajornutrientsarecarbon,oxygen,hydrogen,nitrogen,phosphorus,andpotassium.Thefirstthreeareobtainedfromwaterandairandthelatterthreehavetobeabsorbedfromthecultureme‐dium.During cultivation,N andPbecome limiting.Theybothplayaroleincontrollingthegrowthratioandlipidproductionofmicroalgae.Therefore,theratioofNandPisoftenusedasanimportant indicator, with too high a value meaning P re‐strictionand too lowa value showing that the supplyofN isfallingshort.

Nitrogen is one of the essential elements for the growth,development, reproduction, and other physiological activitiesofmicroalgae.Thenitrogen sourceand concentrationalso af‐fect theaccumulationof lipid inmicroalgae.Usually,ammoni‐umsalts,nitrates,urea, etc. areusedasnitrogensources, buttheir absorption rates and utilization are different [40]. Ex‐periments showed the absorption and utilization of nitrogenhave the followingorder: ammonia> urea>nitrate >nitrite.This isbecauseammonia isdirectlyusedtosynthesizeaminoacidwhiletheothernitrogensourceshavetobeconvertedtoammonia to synthesize amino acid [41,42]. It also has beenfound thatmicroalgaegrowwellwithureaandnitrate.Usingthenitrogen sources of urea,NaNO3 andNH4HCO3, themaxi‐

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mumgrowth rateofChlorellawas found tobewithurea andthe maximum lipid content was found to be with NaNO3.Therefore, if the cost is not a problem, it is better to chooseNaNO3 as the nitrogen source from a consideration of theproductivityand lipidcontent [43].On increasingtheconcen‐trationofN,thegrowthrateofmicroalgaefirst increasedandthendecreased,whichindicatedthatahighconcentrationofNinhibitedthegrowthrate[43,44].Apossiblereasonisthatmi‐croalgaegrowsofastwithabundantnitrogenthatPisdepleted,andthustheratioofNtoPgetsunbalanced[45].Inotherre‐searches, the maximum growth rate of Dunaliella tertiolectawasfoundtobeattheconcentrationof23mmol/LNaNO3andthegrowthratedidnot increasewiththeconcentrationupto46mmol/L[26].ThegrowthrateofDunaliellatertiolectawasalmost the samewith the concentrations of 1mmol/LNH4Cland2.3mmol/LNaNO3anddecreasedon increasing thecon‐centrationofNH4Cl.AlthoughNO3hastobereducedtoNH4+tobe used by microalgae, a high concentration of NH4+ is notsuitableforgrowth.Thiswhichmaybeduetothattherespira‐tion of microalgae is adversely affected by NH4+ in too highconcentration[26].

N‐Stresscanincreaselipidcontent.Thisisusuallyappliedtogetmorebiofuelsinthemicroalgaeprocess.Thepossiblerea‐sonisthatthecontentofadenosinemonophosphatedeaminase(AMPD) increases under N‐stress, enhancing the catalysis ofadenosinemonophosphate (AMP) to inosine monophosphate(IMP)andammonia.Becausemostofisocitratedehydrogenase(ICDH)inmitochondrionisdehydrogenasedependentonAMP,thereducingofAMPwoulddepressorevencompletelyinhibittheactivityofICDH[46].CitricacidcaneitherbecatalysedtoacetylCoAbycitratelyase,orinthecitricacidcycle.ICDHisanenzymeinthecitricacidcycle,andtheinhibitionoftheactivityof ICDHwill increase the production of acetyl CoA. Applyingstress in the form of limiting nutrients, esp. N and P, can in‐creaselipidcontent.However,thisstressapplicationalsocur‐tailsthegrowthratio,andthusmaylowertheamountofbio‐massandleadtoareducedoveralllipidproduction.

Phosphorusisanotheressentialelementforthecultivationofmicroalgae.Phosphate,hydrogenphosphate,andsoon,playanimportantroleinthemetabolicprocessesofmicroalgae,aswellasthesuccessionofphytoplanktoninaquaticecosystems.It takes part in many metabolic processes, such as signaltransduction,energyconversionandphotosynthesis.Themet‐abolicmechanismsofP in thedifferent formsaredifferent inmicroalgae. Orthophosphate ismost easily absorbed and sig‐nificantly promotes the growth of microalgae [47]. Within arange,thegrowthrateofmicroalgaeincreaseswithincreasingconcentrationofP,andtheoppositeoccurswhentheconcen‐trationistoohigh,whichmaybeduetothatthechangingN/Pinhibitsthecelldivisionofmicroalgae[48].

3.3. CO2andpH

CO2isoneofthereactantsandalsooneofthelimitingfac‐tors in thephotosynthesisofmicroalgaeandplants.Thepho‐tosynthesisofmicroalgaerequiresacertainCO2concentration,and themaximumphotosynthetic efficiency is often achieved

withCO2concentrationsfrom1%to5%(byvolume).Increas‐ingCO2 levelscan improvephotosyntheticefficiency,which isconsistentwithahigherCO2concentrationleadingtoahigherbiomass of microalgae [49]. Adding NaHCO3 to the mediumduring the cultivation of microalgae not only supply CO2 topromoteproductivity,butalsocanbeusedasabufferingagenttocontrolthepH[43].TheconversionefficiencyofsolarenergyisimprovedbysupplementingCO2tothemediumofNitzschiaclosteriumtoprovidesufficientenergyforphotosynthesis[50].ThephotosynthesisofNitzschiaclosteriumisaffectedbyahighCO2concentrationintwoways:increasingthebindingofCO2toRubisco (Ribulose bisphosphate carboxylase oxygenase) sitesthereby enhancing carboxylation, and inhibiting the pho‐torespiration activity to increase the net photosynthetic effi‐ciency [49,50]. In addition, the photosynthetic efficiency ofphotosystemIIriseswithincreasingCO2levelsandmorelightenergy is captured and transformed to chemical energy [51].Thiswould explainwhy high CO2 levels enhanced the photo‐synthetic efficiencyofmicroalgae.CO2 levels in fluegases arerelativelyhighandareabletomeettheneedforCO2ofmicro‐algae.Soitisverymeaningfultoutilizefluegases,whichwouldreducegreenhousegasemissionaswellasthecostintheeco‐nomics of biofuels frommicroalgae.WhenChlorella sp. (wildtype) and its mutant were cultured with a continuousCO2‐enriched gas (2%, 10% and 25% CO2) and flue gas (ap‐proximately25%CO2,4%O2,0.008%NO,and0.009%SO2),themaximumgrowthratewasfoundwithChlorellasp.(wild‐type)with2%CO2andthegrowthrateofthewildtypewashigherthanthatofthemutant[52].ItwasalsofoundthatbothChlo‐rellasp.(wildtype)anditsmutantgrewfasterwiththefluegasthanwiththeCO2‐enrichedgas.ThiswasbecauseNOabsorbedinthemediumcanbeconvertedtoNO2andthenoxidizedtoNO3,whichcanbeutilizedasanitrogensource.ThepresenceofNOx in flue gasdidnot inhibit the growthofmicroalgae. Ademonstrationbasewith400m2hasbeenbuilttoculturemi‐croalgaeinShijiazhuangbySinopecGroup,whichdirectlyusedthe flue gas froma petroleum refinery, and the cultivationofChlorellawassuccessful.

ThepHisanothermainfactorinfluencingtheabundanceofinorganiccarbon(DTC).WhenthepHisbelow5,themajorityofDICisCO2.CO2andHCO3areequalundertheconditionofpH=6.6.ItisalmostallHCO3whenthepHis8.3[53].There‐fore,thepHshouldbecontrolledduringcultivationtoenhancetheabsorbabilityandutilizationofCO2bymicroalgae.ThepHcanalsodirectlyaffectthepermeabilityofthecellandthehy‐dronium forms of the inorganic salt, and indirectly influencetheabsorptionof the inorganicsalt.CO2 in theculture iscon‐sumed by the microalgae during photosynthesis, thereby in‐creasingthepHofthemedium.Therefore,substanceslikehy‐drochloricacidandaceticacidhavetobeaddedtocontrolthepHtokeepthepHfromincreasingtoomuchtobebeyondthetoleranceofthemicroalgae.Comparedwithhydrochloricacid,aceticacidhastheadvantagethatitnotonlyadjustthepHval‐uebutalso isusedasacarbonsourcetoenhancethegrowthrateofmicroalgae[54].

3.4. Otherfactors

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OtherfactorsliketemperatureandO2arealsoimportantforthegrowthofmicroalgae.Thetolerancerangeoftemperatureis different for different species. Freshwater microalgae, forinstance, Chlorella and Scenedesmus, are able to adapt to thetemperature in the rangeof 535 oC,with the optimum tem‐perature of 2530 oC, which should be controlled during themasscultivationofmicroalgae.Formicroalgaeandplants,O2isthenecessarygasforrespirationandalsothereleasedgasfromphotosynthesis.Muchdissolvedoxygen releasedas abyprod‐uctofphotosynthesisisaccumulatedinthemedium,whichcanlead toahighdissolvedoxygencontent that can threaten thesurvivalofthemicroalgae.Attentionshouldbepaidtothedis‐solvedoxygen content in the cultivationofmicroalgae,whichcan oxidize one or more enzymes and affect the electrontransmissionchain,andinhibitthephotosynthesisprocess.

4. Photobioreactors

Photobioreactorsare thecriticalequipment in thebiofuelsfrommicroalgaeprocess,andareoneoftechnicalbottlenecks.Before an analysis of the photobioreactor configuration, gen‐eraldesignconsiderationsarepresentedsothatphotobioreac‐tordesignscanbeevaluatedandcomparedeffectively.Theaimofphotobioreactordesignistoachieveoptimalmasstransfer,light transfer and transmission with low cost. The followingaspects should be considered in thedesign. A growth systemthat rely on artificial light is not often considered because ofenergyefficiencyandcosts.Adesignprincipleforphotobiore‐actordesignistomaximizethesurfaceareatovolumeratiotoprovide sufficient light. How tomaximize the surface area tovolumeratioisthemajorfactorconsideredinthedesigns.CO2isnecessaryforphotosynthesisandshouldbesuppliedinsuffi‐cientquantityforthemicroalgae.Ahighdissolvedoxygencon‐tent does not favor photosynthesis, which limits the size ofphotobioreactors. Amajor disadvantage of open ponds is theloss ofwater to the atmosphere by evaporation, especially inwater‐poorareas.Thisproblemalmostdoesnotexistinclosedphotobioreactors. Controlling the temperature inopenphoto‐bioreactors is relatively easy due to evaporation, but this isdifficult for closed photobioreactors. The cultivation locationandmodehavetobechosenaccordingtothenaturalenviron‐mentalconditions. Inall cases, thedesignprinciple forphoto‐bioreactordesigns is tomaximize the surface tovolumeratioandreducethecosts.

The common growth systems used are open and closedphotobioreactors. The advantages and disadvantages of thesetwo systems are compared in Table 1. Each system hastradeoffsbetween thekeydesignparameters.The fullunder‐standingofthesewouldhelpthedevelopmentandinnovationofphotobioreactors.Inpractice,theenergyconsumptionintheprocess isconsiderable,whichisthecore issueinthe innova‐tionofnewphotobioreactors. Several representativephotobi‐oreactorsarediscussedbelow.

4.1. Openphotobioreactors

Raceway ponds are the most common open systems for

commercial photobioreactors. They are usually constructedeitheras singleorasgroupsofchannelsbuiltby joining indi‐vidual raceways together with a depth of 1530 cm. Paddle‐wheelsareoftenusedtodrivethewatercontinuouslyaroundthe circuit tousemixing toprevent the cells fromdepositing,therebyincreasingtheexposureofthemicroalgaetolightandCO2.Othertypesofmixingsystems,suchaspumpsandairlifts,have also been proposed. Open raceway pondswere used inthe treatment of industrial wastewater in the United States,Israelandothercountriesfromthe1960sto1970s,andwereappliedtoculturemicroalgaeusedforhealthpurposes,suchasSpirulina,inChina,theUnitedStates,andJapanfromthe1980sto 1990s [55,56]. Open pond racewayswere the focus in thewellknown“AquaticSpeciesProgram”conductedbytheUnit‐edStatesDepartmentofEnergy.The1000m2pondswere lo‐cated inNewMexico and good researchwas achieved in thistest site. However, the disadvantages of open pond racewaysemergeoutdoors,andthisshouldbeconsideredinthedesign,mainlyhowthisaffectsthefactorsincludinglightandmediumdepth.

Firstofall,lightisthemajorfactorthatlimitsthephotobio‐reactors.Manymethodshavebeentriedtoincreasetheexpo‐sureofthecellstolight.Theflowvelocityisanimportantfactortopreventthecellsfromdepositiontoincreasetheexposureofmicroalgae to lightandCO2. It is chosenbasedon thesinkingrateofthecellsandthemediumdepth.Sotheappropriateflowratesfordifferentphotobioreactorsarenecessaryforthemasscultivation.Avelocityof10to30cm/sisfoundeffective.High‐er velocities are preferred, but it would consume too muchenergy[57].Addingequipmentstostirthemediumisanothereffectivemethodtoincreasetheflowrate[58].Providingaddi‐tional light, especially artificial light, can enhance photosyn‐theticefficiency,but theenergyconsumptionishighandveryexpensive.Hsiehetal. [59]employed transparentrectangularchambers(TRCs)toconductlightdeepintoanopentankpho‐tobioreactortoimprovethephotosyntheticefficiencyofmicro‐algae. This is an effectivemethodwith low cost. Figure1 de‐picts thephotobioreactorconsistingofanopentankwithdif‐ferentarrangementsofTRCs.Inthisproposedphotobioreactor,TRCsmadeoftransparentacrylicprovidedalargeareaofillu‐mination by conducting irradiance deep into the culture andredistributing light inside the tank.This increased thesurface

Table1Comparisonofopenandclosedmicroalgaecultivationphotobioreactorsystems.

Parameter Opensystem ClosedsystemBiomassconcentration low,0.1‒0.5g/L,

highharvestingcostshigh,2‒8g/L,

lowharvestingcostsSpacerequired high lowConstructioncosts low highContaminationrisk high lowWaterlosses high almostnoneCO2‐losses high lowBiomassquality difficulttocontrol easytocontrolWeatherdependence high lowRepeatability low highPeriodofculture long,

approx.6‒8weeksrelativelyshort,

approx.2‒4weeks

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areatovolumeratioofthephotobioreactor,andimprovedlightutilizationbythemicroalgae,especiallyathighcelldensityanddeep depth. In our research, the TRCswere in an open tankwith the samevolume, shown inFig.1.TRC3has thehighestilluminationsurface‐to‐volumeratioanditgavethehighestcelldensity (3.745 g/L) and biomass productivity (0.34 g/(L·d)).Thetotalbiomass fromTRC3was56%more thanastandardopentanksystemwithoutTRCs.Manymethodshavebeencon‐sideredtoenhancethelightinanopenpondraceway,buttherearestillnoeffectivesolutions.

Themediumdepth,which is in the range of 1530 cm, isanotherkeyfactorinfluencingthemasscultivationofmicroal‐gaeinopenpondraceways.Thishasacloserelationshipwithlighttransmittance.Intheinitialphase,thedensityofmicroal‐gaeislow,andthelightcanbetransportedtoagreatdepthtopromote the growth and photosynthetic efficiency, which islimitedbythedensity[60].Enhancingthelightsupplyispossi‐ble by reducing layer thickness. For example, a thickness of110cmshouldbepresentformicroalgaeindeepopenpondraceways,whichisachievedbyraisingthebottombylessthanorequalto5cmsothatathinlayerisprovided[61].Thepho‐tosynthesisbythemicroalgaecanbeincreased,butthepropor‐tionofthebottomraisedisnotclear.Thedepthofopenpondraceways affects construction cost.With a depth less than15cm, theconstructionandoperationof largepondsystemsbe‐comedifficult, andwhen this ismore than50 cm, the cost ofconstructionbecomesprohibitiveanditisverydifficulttogetahighcelldensity.

Therearesomeotherproblemsinthecultivationofmicro‐algaeinopenpondraceways.Itisdifficulttocontroltheenvi‐ronmentalconditionsincludingCO2,temperature,light,andpH.For instance, the optimum light intensity ofChlorella is 430klx,buttherecanbeupto80120klxatnoon.Photoinhibitioneasily happens under this high intensity, especiallywhen thedensityofmicroalgaeislow.CO2isanotherlimitingfactorbutitis relatively easy to solve by bubbling. It is difficult to com‐pletelyapplyopenpondsystemstoculturemicroalgaebecausetheserequirehighlyselectiveenvironmentsduetotheinherentthreatofcontaminationandpollutionfromotherspecies.

4.2. Closedphotobioreactors

Although open systems have many advantages, they alsohave many problems in the mass cultivation of microalgae.Thus,closedsystemshavebecomethechoiceforbiofuelspro‐

duction. The volumetric productivity in closed photobioreac‐torsismorethan30timeshigherthanthatinopenpondrace‐ways [62]. Owing to the highermass productivity, harvestingcosts can be significantly reduced. Numerous types of closedphotobioreactorshavebeendesignedtobestcontrolthecondi‐tionsinmasscultivation.Thesecanbedividedintothreemaincategories: tubular, column, and flat plate photobioreactors.Table2listssomeclosedphotobioreactorswithsolarandarti‐ficialradiationthathaveovercomesomeshortcomingsinopenphotobioreactors.However, theseclosedphotobioreactorsarestillnotverysuitabletouseformasscultivationandneedmoredesignwork.

Lightpathisanimportantdesignfactorforclosedphotobi‐oreactors.Inacertainrange,ashorterlightpathleadstomoreproductivity.Asshown inTable2, themaximumproductivitywasgainedinthethin‐layerphotobioreactorwiththeshortestlightpath.Lightpath,whichcanbeconsideredasthedepthofthemedium, directly affects the probability ofmicroalgae ex‐posure to light. Therefore, microalgae grow well where thelightpathisshort,andslowlywherethelightpathislongdueto the severe light attenuation. Chlorophyta was cultured inphotobioreactorswithlightpathsof3and6cm.Theproductiv‐itywiththe3cmlightpathwashigherby50%thanthatwiththelightpathof6cm[73].Inaddition,thelightpathalsoaffectsthelipidcontent.Thelipidcontentofmicroalgaeculturedwiththe3cmlightpathwashigherthanthatculturedwiththe6cmlightpath[73].However, light intensity that is toohigh isnothelpfulformorelipidinthemicroalgae.WhenPyramidomonassp. andChlorophyceaeL‐4wereculturedunderdifferent lightintensities,thelipidcontentwasreducedasthelightintensityincreased,anditdecreasedfrom14.17%to5.94%forPyrami‐domonassp.andfrom14.17%to3.40%forChlorophyceaeL‐4[74].Thelightpathmayactintwowaystoaffectthelipidcon‐tent.One is thatmicroalgaegrow fastwitha short lightpath,whichleadstonutritionstresswhenthereisnomoresupply,andthusthelipidcontentincreases.Theotheristhatphotoin‐hibition often occurs under high light intensity with a shortlightpath,whichaffectsthegrowthrateandlipidcontent.

Severaltypicalclosedphotobioreacorsarediscussedbelow. Tubular photobioreactors canbe alignedhorizontally, ver‐

tically, inclined or as a helix. The diameters of the tubes aregenerally2.55.0cm[64,75].ThelengthofthetubesdependsonpotentialO2accumulation,CO2depletionandotherfactors,whichlimitsthescaleoftubularphotobioreactors.Toincreasethe scale, the tubes have to be arrayed horizontal fence‐like,

 Fig.1.Opentankwithdifferentsizesoftransparentrectangularchambersinserted[59]. 

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whichimprovestheutilizationoflandbutincreasestheopera‐tioncost.Thewashingofthewalloftubularphotobioreactorsisanotherdifficultproblem.Thishasaclosedrelationshipwithlightpermeability.Atpresent,mechanicalcleaningisthemainmethod to do the wash. Although tubular photobioreactorshave many disadvantages, they are considered suitable foroutdoormass cultivation due to a large surface area to lightthat can allow a high density ofmicroalgae. The productivityreached830mg/(L·d) in theworks inTable2. Inaddition tothe light path, other factors should be considered when de‐signing the photobioreactor, such as the ratio of culture vol‐ume.Chlorellavulgariswasculturedinhelixtubeswith3.12cmdiameter and straight tubeswith3.2 cmdiameter. Themaxi‐mumproductivitiesofthehelixtubesandstraighttubeswere40and600mg/(L·d),respectively[63,66].Thelightpathsandculture conditions were similar in the two photobioreactors,butthemaximumproductivityofthestraighttubeswasalmost15timeshigherthanthatofthehelixtubes.Thiswasbecauseout of the 230 L culture volume, only 150 Lwas in the helixtubeandunder lightandtherestwasin thestoragetank,de‐creasingthetotalproductivity.

The main types of column photobioreactors are the bub‐bling, airlift and stirring types. They aremadeof transparentmaterialswithlowcosts,suchasglass,plasticandpolyethylene[76,77]. Column photobioreactors are similar to the conven‐tional fermentation tank, and the main difference is that theformerneedsaninternalorexternal light.Airliftcolumnpho‐tobioreactors use a draft tube to mix the medium. The lightutilizationefficiencyofstirredcolumnphotobioreactorsisrela‐tivelylow.Columnphotobioreactorsaregenerally22.5mhighwith 20‒50 cm diameter. The simple materials used help to

reduce costs, and mixing by CO2 bubbling is another way tomaximizeCO2captureandtoreducemixingcosts.However,itisdifficult toobtainahighcelldensity incolumnphotobiore‐actors and this limits the scale. The columnphotobioreactorsused to culture microalgae by Sinopec Group in Beijing areshowninFig.2.Itwasfoundthatwhenthedensityishigh,lightlimitsthegrowthofmicroalgae.

Flat plate photobioreactors are horizontal or verticallyaligned[71,78]andisinfluencedbyplacement,lightpath,anddissolved oxygen concentration. They have attracted muchattentionduetothelargesurfaceareaexposedtoilluminationandthehighdensityofphotoautotrophiccellsobserved.Whenflatplatephotobioreactorsarehorizontal,thesurfacereceivesmore light to enhance photosynthetic efficiency but they re‐quiremorespaceandare liabletophotoinhibition.Spaceand

 Fig.2.Schematicofcolumnphotobioreactor. 

Table2TexturalpropertiesoffreshCZsamplespreparedbydifferentmethods.

Photobioreactor Species Light Productivity Ref.Helical tubularphotobioreactor, columndiameter3.12cm,culturevolume230L,CO2supplied

Chlorellavulgaris artificialadiation maximumproductivity,40mg/(L·d) 63Chlorellaemersonii artificialadiation maximumproductivity,41mg/(L·d)

Helical tubular photobioreactor, diameter 1.6 cm, cul‐turevolume21L,CO2supplied

Spirulinaplatensis artificialadiation maximumproductivity,400mg/(L·d) 64

Horizontaltube,diameter2.8cm,culturevolume500L,CO2supplied

Scenedesmusbliquus solarradiation maximumproductivity,284mg/(L·d);averageproductivity,11.31g/(m2·d)

65

Verticaltubularphotobioreactor,200cmhigh,diameter3.2cm,culturevolume1.4L,CO2supplied

Dunaliellatertiolecta artificialadiation maximumproductivity,830mg/(L·d) 66Chlorellavulgaris artificialadiation maximumproductivity,600mg/(L·d)

Verticaltubularphotobioreactor,63cmhigh,diameter6cm,culturevolume0.26L,CO2supplied

Anabaenavariabilis artificialadiation maximumproductivity,750mg/(L·d) 67

Verticaltubularphotobioreactor,75cmhigh,diameter7.5cm,culturevolume3.0L,CO2supplied

Aphanothececroscop‐ica

artificialadiation maximumproductivity,770mg/(L·d) 68

Vertical tubular photobioreactor, diameter 7 cm, cul‐turevolume0.8L,CO2supplied

Chlorellasp. artificialadiation maximumproductivity,640mg/(L·d) 52

Verticalcolumn,300cmhigh,diameter16cm,culturevolume50L,CO2supplied

Chlorellasp. solarradiation maximumproductivity,520mg/(L·d);averageproductivity,360mg/(L·d)

52

Verticaltubularphotobioreactor,36cmhigh,diameter10cm,culturevolume2L,CO2supplied

Chlorellasp. artificialadiation maximumproductivity,1.9g/L;averageproductivity,111.8mg/(L·d)

69

Flat,lightpath17cm,culturevolume60L,CO2supplied Chlorellazofingiensis solarradiation maximumproductivity,41.3mg/(L·d)inMay

70

maximumproductivity,58.4mg/(L·d)inNovember

Flat, lightpath10cm, culturevolume200L,CO2 sup‐plied

Nannochloropsissp. solarradiation averageproductivity,240mg/(L·d) 71

Thin‐layer photobioreactor, 224 m2, 28 m long, lightpath0.60.7cm,CO2supplied

Chlorellasp. solarradiation averageproductivity,4.3g/(L·d) 72

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lightefficiencycanbeimprovedwhenflatplatephotobioreac‐tors are vertically aligned, but thesehave tobemadeof highcost rigidmaterials. It has been considered, by analogy withsolarpanels,forflatplatephotobioreactorstoautomaticallytilttothezenithangleofthesuntomaximizetheamountofsolarradiationintercepted.However,thescalabilityofsuchsystemshaveproveddifficult.Highintensityradiationhasbeenreport‐ed to bemore efficient formicroalgal cultivation. Indeed, thephotosyntheticefficiencyofverticalflatplatephotobioreactorshasbeenhigherthanoptimaltiltreactors,reachingthevalueof20%[79].This isduetothefactthatlowirradiancelevelsre‐sultinhigherphotosyntheticefficiencyandthetoohighirradi‐ancelevelsinoptimaltiltreactorsinhibitedthephotosynthesis.The lightpathswere in the rangeof1.53.0 cm,which limitsthe culture volume.Therefore, it is necessary to optimize therelationshipamongthearealproductivity,volumetricproduc‐tivity,andtotalproductivity for flatplateandotherphotobio‐reactors.The lengthof the lightpath inaplate reactordeter‐minesitsarealvolume.Thevolumetricproductivitywashigh‐est in the shortest light path, and lowest in the longest lightpath reactor, as shown in Table 3. The highest volumetricproductivitywas foundwith the shortest lightpathof1.3 cm[71]. Volumetric productivity, therefore, hasmeaning only inrelationtothelightpath.Arealproductivity,incontrast,hasanoptimal light path at which the productivity or product ismaximal.Theaccumulationofdissolvedoxygenlimitsthescaleof flat plate photobioreactors. Closed photobioreactors com‐monlymakeuseof an externalpower supply to ensure suffi‐cientmasstransfercapacity.Toattainthesamemasstransfercapacity,53W/m3areneeded ina flatplatephotobioreactor,40W/m3inabubblingcolumnand24003200W/m3inatub‐ularphotobioreactor[80].

AthinlayerphotobioreactorisshowninFig.3.Thisphoto‐bioreactorismadeoftransparentmaterialsformaximumsolarenergycapture,andathin layerofdensecultureflowsacrosstheculturearea.Douchaetal.[81]reportedthattheinclinationoftheareawas1.1%2.5%,theflowvelocityofthemicroalgaesuspensionwaspreferably30150cm/s,andthethicknessofthesuspensionwas518mm.Theproductivityofthemicroal‐gaeChlorella sp.was 38.2 g/(m2·d) or 4.3 g/(L·d),whichwashigherthanthoseinthetubular,column,andflatplatephoto‐bioreactors. This productivity was obtained using batch feedcycles in a thin layer photobioreactor with a 224m2 culturearea(length28m,slope1.7%),anda67mmlayerofculture[72,82].The layerof culture is thin, so the area forCO2masstransfer is very important, esp. with the use of flue gas. Tominimize the loss of dissolved CO2 in themicroalgal suspen‐sion,aretention tankanddistribution tubewitha thick layershould supply CO2. The productivities presented in the re‐searches indicated good prospect for using thin layer culturetechnology for the production of biomass as a feedstock forbiofuels.However,therearestillsomeshortcomingshinderingthedevelopmentof the thin layerphotobioreactor.Forexam‐ple,duetothethinnessofthelayer,photoinhibitionofmicro‐algaeeasilyhappenswhenthedensityisnothigh,andthetotalculturevolumeislow.Theconceptoftheuseofathinlayertocultureahighconcentrationofmicroalgaeisanewdesignforaphotobioreactor.Rongetal.[83]inventedaphotobioreactortoculture microalgae that included collecting tank, conveyingequipment, and culture area. In this photobioreactor, theme‐diumflowthroughthecultureareawithathinlayer,andit issuitable for the growthof highdensitymicroalgae on a largescaleandlowcosts.Congetal.[61]devisedanequipmentthatprovidesathinlayerforthephotobioreactorwithathicklayerlikeopenpondraceways,therebyphotosyntheticefficiencyandproductivitywere increased. A patent by Gorny et al. [84] in2011andexhibitedinFig.4providedaflow‐throughphotobi‐oreactorcontainingat leastone thermoplasticmulti‐wall, andoneinnerandtwoormoreendcaps.Themediumflowedinthemulti‐wallsheetswithhighphotosyntheticefficiency.UVlightfromthesunwasfilteredoutbythereactorwalls,thetemper‐aturewas controlled, water in the system did not evaporate,andCO2fromfluegascanbefedtoincreasetheyield.

Avarietyofclosedphotobioreactorswereintroducedabove,which can overcome the limitations of open pond raceways.However,therearestillsomedrawbackstoculturingmicroal‐

Table3Arealandvolumetricproductivityofaverticalflatplateglassphotobi‐oreactor,asaffectedbythelightpath[71].

Lightpath(cm)Arealproductivity

(g/(m2·d))Volumetricproductivity

(g/(L·d))

1.3 5.5 0.8462.6 7.25 0.5585.2 9.25 0.35510.0 12.10 0.23917.0 10.05 0.118

Multi-wall Sheet (s) 

Fig.4.Schemeof flow‐through thermoplasticmulti‐wall sheet photo‐bioreactor[84]. 

Fig.3.Schematicofthephotobioreactor.(1a,1b)Culturearea;(2)Re‐tentiontank;(3)Circulationpump;(4)CO2storagetank.M‒aerationairAS‒algalsuspensionflow.CC‒connectingchannel;CD‒carbondioxide;DT‒distributiontube;RW‒rainwater[81]. 

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gaeona largescale.Thephotobioreactors toculturemicroal‐gaeforbiofuelsproductsrequirehighbiomassproductivityperareaandminimalcosts.Thebestreactortypeshouldhavesim‐plestructure,easyoperation, lowcosts, lowenergyconsump‐tionandthatcanbeadaptedtothelocalenvironment.

4.3. Combinationsofopenand/orclosedsystems

Some investigators have suggested that combinations ofopen and/or closed systems are themost effective configura‐tion for mass cultivation. The combinations can be betweenopen and/or closed systems, and between autotrophic andheterotrophic cultivation. Open pond raceways can provideabundantmicroalgaewithalowdensity,whichisthenculturedin closed photobioreactors at a high density. Two stage pro‐cessesareusedtoincreasetheproductivityandlipidcontentofthemicroalgae.Ahighcelldensityisobtainedinthefirststepinclosed photobioreactorswith a controlled environment. Next,the microalgae are exposed to nutrient deprivation by beingtransferredtoopensystemstoincreasethelipidcontent.Hanetal.[85]combinedopenandclosedsystems,whichincludedthreereactorsfromtheinsidetotheoutside.Theinsidewasaclosedphotobioreactorwithartificial light tokeepmicroalgaeintheexponentialgrowthphase.Thenextwasanopenphoto‐bioreactorwithartificialorsolarradiationtoculturemicroal‐gae fromthe insidereactor.Theoutsidewasusedto increasethe lipid content under nutrient stress. Liu et al. [86] used asimilar combination of open pound raceway and flat platephotobioreactor(orbubblingcolumn),inwhichthebiomassofNannochloropsiswasincreasedby75%.Thecapitalandoper‐ating costs for the combined systems are likely to be signifi‐cantlyhigherthanforonereactor,aswellasthelandrequire‐ments.

4.4. Computationalfluiddynamics

Toimprovethedevelopmentofphotobioreactors,thechal‐lengeistheefficientdesignof largescaleproductionfacilities.Computationalfluiddynamics(CFD)isattractiveforthedesignofoptimallargeandintegratedsystems,bymakingitpossibleto estimate the productivities of the photobioreactor beforeconducting cultivation experiments. There is no need to con‐structanumberofactualphotobioreactors.However,therearestillmore aspects that have to be incorporated in themodel,suchastemperaturechangeandtheconcentrationsofCO2andnutrients.Satoetal.[87]appliedacomputermodelsystemtoevaluatetheeffectsoftheamountsofemittedO2,CO2fixationamountandthegrowthcurveofmicroalgae,andobtainedtheoptimal culture conditions. Slegers et al. [88] gavemore evi‐denceforthevalidityofthesimulation.Toinvestigatetheeffectof location, variable sunlight, and reactor layout on biomassproductioninsinglestandingandparallelpositionedflatpan‐els, amodelapproachwasused topredict theyearlyproduc‐tion of Phaeodactylum tricornutum and Thalassiosira pseu‐donanaintheNetherlands,France,andAlgeria.Themodelap‐proach found that vertically placed and east‐west orientatedsinglepanelsproducedthemostbiomassandshadinganddif‐

fused light penetration between parallel panels had a strongeffect on the productivity in parallel flat panels. This was inagreement with reports in the literature [88]. Although thisapproach is highly desirable for its low cost andmany otherbenefits,itisalsowellknownthatCFDsimulationsneedtobevalidatedbeforeuse,anditisdifficulttoinvestigateallthede‐cisionvariablesofanexperimentalwork.

The costs and energy consumption of the process are thekey factors in the economics that inhibit the development ofbiofuelsfrommicroalgae.CFDsimulationscanreducethecostandshortenthedevelopmentcycleofphotobioreactors.Itcanalsobeusedtoevaluatetheenergyconsumptionintheprocess,andcansuggestcombinationswithotherenergysources,suchas wind energy and electrical energy (using the microalgaebiomass), to form a complete and recyclable culture system.Themasscultivationofmicroalgaecanabsorb theCO2 in fluegases to reduce the release of greenhouse gas. It also utilizewastewater,whichhelptoprotecttheenvironmentandreducethecostofbiofuelsfrommicroalgae.

5. Conclusions

Biofuelscanbeobtainedfromavarietyofsources,butmi‐croalgaeisofparticularinterestasoneofthemostpromisingsourcesofbiomass forbiofuels.Biofuels frommicroalgaehasbeen demonstrated to have broad application prospects, butthese currently still remain at the exploratory stage. This re‐viewunderlines several aspects involved in themass cultiva‐tion of microalgae, including microalgae species, metabolicmechanism, culture conditions, and photobioreactors, to helpthe development of biofuels from microalgae. The followingaspectsneedtobefurtherdeveloped.Microalgaespecieshavetobescreenedtogetgoodrobustnessformasscultivationus‐ingamaturesystem.Themetabolicmechanismofmicroalgaehastoberegulatedto increasethegrowthrateandlipidcon‐tentof themicroalgaeundermasscultivationconditionswithlowcosts,highefficiencyandeasyoperation.Thecombinationofmass cultivation and the use of flue gas or wastewater toreduce the costshas tobe studied, and themetabolicmecha‐nisms of NOx and SOx in microalgae need to be investigated.Photobioreactors,which affect the photosynthesis ofmicroal‐gae for the initial conversion of sunlight into stored energy,shouldreceivemoreattention,andspecialphotobioreactorsforthemasscultivationofmicoalgaethatovercometheproblemsofthecostsandenergyconsumptionareneeded.

References

[1] AmaroHM,GuedesAC,MalcataFX.ApplEnerg,2011,88:3402[2] MinEZ,YaoZhL.ProgrChem,2007,19:1050[3] AptKE,BehrensPW.JPhycol,1999,35:215[4] Rodolfi L, Zittelli G C, Bassi N, Padovani G, Biondi N, Bonini G,

TrediciMR.BiotechnolBioeng,2008,102:100 [5] BrennanL,OwendeP.RenewSustEnergRev,2010,14:557[6] YangS,ChenChY,ZhaoShL,NiuYH,LiL.YunnanChemTechnol,

2006,33(3):49[7] WangJ,YangSL,CongW,CaiZhL.ChinJProcEng,2003,3:141[8] de‐BashanLE,AntounH,BashanY.FEMSMicrobiolEcol,2005,54:

Page 11: Alga Biodiesel Review

ZHUJunyingetal./ChineseJournalofCatalysis34(2013)80–100

 

197[9] WenJ,SunY,LiBSh,ZhuRX,JiangYJ,WenHCh,ZhangYK.Mi‐

crobiolChina,2010,37:1721[10] Perez‐GarciaO,EscalanteFME,de‐BashanLE,BashanY.Water

Res,2011,45:11[11] ChenGQ,JiangY,ChenF.FoodChem,2007,104:1580[12] SongDH,HouLJ,ShiDJ.ChinJBiotechnol,2008,24:341[13] AndersenRA,KawachiM.TraditionalMicroalgaeIsolationTech‐

niques, In: Algal Culturing Techniques. Burlington: Elsevier Aca‐demicPress,2005.Chapter6,83

[14] LiYQ,HorsmanM,WangB,WuN,LanCQ.ApplMicrobiolBio‐technol,2008,81:629

[15] Khozin‐GoldbergI,CohenZ.Phytochemistry,2006,67:696 [16] LynnSG,KilhamSS,KreegerDA,InterlandiSJ.JPhycol,2000,36:

510[17] RaoAR,DayanandaC, SaradaR, ShamalaTR,RavishankarGA.

BiorescourceTechnol,2007,98:560[18] YaoR,ChengLH,XuXH,ZhangL,ChenHL.ProgrChem,2010,22:

1221[19] ZhuShN,WangZhM,ShangChH,ZhouWZh,YangK,YuanZhH.

ProgrChem,2011,23:2169[20] Arabolaza A, Rodriguez E, Altabe S, Alvarez H, Gramajo H. Appl

EnvironMicrobiol,2008,74:2573[21] DahlqvistA,StahlU,LenmanM,BanasA,LeeM,SandagerL,Ronne

H,StymneS.ProcNatlAcadSci,2000,97:6487 [22] RiekhofWR,SearsBB,BenningC.EukaryotCell,2005,4:242[23] NationalAlgalBiofuelsTechnologyRoadmap.U.S.Departmentof

Energy,EnergyEfficiencyandRenewableEnergy,2010[24] Rabbani S, Beyer P, Von Lintig J, Hugueney P, Kleinig H. Plant

Physiol,1998,116:1239 [25] ZhekishevaM,BoussibaS,Khozin‐GoldbergI,ZarkaA,CohenZ. J

Phycol,2002,38:325 [26] ChenM,TangHY,MaHZ,HollandTC,NgKYS,SalleySO.Biore‐

sourceTechnol,2011,102:1649[27] FengGD,ChengLH,XuXH,ZhangL,ChenHL.ProgrChem,2012,

24:1413[28] EriksenNT,GeestT,IversenJJL.JApplPhycol,1996,8:345[29] ChengLH,ZhangL,ChenHL,GaoC.SepPurifTechnol,2006,50:

324[30] ChenGQ,ChenF.BiotechnolLett,2006,28:607[31] OyangZhR,WenXB,GengYH,MeiH,HuHJ,ZhangGY,LiYG.J

WuhanBotRes,2010,28:49[32] LiangY,MaiKS,SunShCh,YuDD.MariSci,2002,26:48[33] Solovchenko A E, Khozin‐Goldberg I, Didi‐Cohen S, Cohen Z,

MerzlyakMN.JApplPhycol,2008,20:245[34] RoháčekK,BartákM.Photosynthetica,1999,37:339[35] HuaRCh.Beijing:ChinaAgriculturePress,1986

[36] Janssen M, Slenders P, Tramper J, Mur L R, Wijffels R. EnzymeMicrobTech,2001,29:298

[37] CongW,SuZhF,XueShCh,YangChY.CNPatent101899385A.2010

[38] CongW,ZhangQH,XueShCh.CNPatent102260629A.2011[39] YinZQ.Beijing:ChinaElectricPowerPress,2008 [40] ZhangCh,ZouJZh.OceanoletLimnolSin,1997,28:599[41] LiB,OuLJ,LüSH.MarEnvironSci,2009,28:264[42] DingYC,GaoQ,LiuJY,YiYJ,LiuJG,LinW.ActaEcolSin,2011,31:

5307[43] WangLZh,WenHCh,ZouY,ZhouWW,XieTH,ZhangYK.Micro‐

biology,2010,37:336[44] YinCL,LiangY,ZhangQF.FisheriesSci,2008,27:27[45] KolberZ,ZehrJ,FalkowskiP.PlantPhysiol,1988,88:923[46] EvansCT,ScraggAH,RatledgeC.EurJBiochem,1983,132:609[47] LiYing,LüSH,XuN,XieLCh.JEcolSci,2005,24:314[48] WenShY,ZhaoDZh,ZhaoL,YangJH,ZhangFSh,WangL,GaoSh

G,SunD.JDalianMaritUniv,2009,35:118[49] WuY,SunJM,SunPH,ZhangDM.JFisheriesChin,2004,28:742[50] WangGD,ZhangLL,WuY,SunJM.JHydroecol,2008,1(2):35[51] ZhangQD,LuCM,FengLJ,LinShQ,KuangTY,BaiKZh.ChinBull

Bot,1996,38:77[52] ChiuSY,KaoCY,HuangTT,LinCJ,OngSC,ChenCD,ChangJS,

LinCS.BioresourceTechnol,2011,102:9135[53] YangB,ChuZhSh,JinXC,YanF,ZengQR.ChinEnvironSci,2007,

27:54[54] ZhuM,ZhangXCh,MaoYX.PeriodOceanUnivChin,2005,35:499[55] LiuJL,ZhangSL.ChinJBiotechnol,2000,16(2):119[56] WangQ,QuZM,LiuJL.GuangdongAgricSci,2011:180[57] SheehanJ,DunahayT,BenemannJ,RoesslerP,NationalRenewa‐

bleEnergyLab.,Golden,CO.:UnitedStates,1998[58] UgwuCU,OgbonnaJC,TanakaH.ProcessBiochem,2005,40:3406[59] HsiehCH,WuWT.BiochemEngJ,2009,46:300 [60] LiXW,LiYG,ShenGM,YangD.ChinJProcEng,2006,6:277[61] CongW,LiuM,BaoYL.CNPatent101948740A.2011[62] ChistiY.BiotechnolAdv,2007,25:294[63] ScraggAH, IllmanAM,CardenA,ShalesSW.BiomassBioenerg,

2002,23:67[64] TraviesoL,HallDO,RaoKK,Benítez F, SánchezE,BorjaR. Int

BiodeterBiodegr,2001,47:151[65] HulattCJ,ThomasDN.BioresourceTechnol,2011,102:6687[66] HulattCJ,ThomasDN.BioresourceTechnol,2011,102:5775[67] YoonJH,ChoiSS,ParkTH.BioresourceTechnol,2012,110:430[68] Jacob‐LopesE,ScoparoCHG,LacerdaLMCF,FrancoTT.Chem

EngProcess,2009,48:306[69] Rasoul‐Amini S, Montazeri‐Najafabady N, Mobasher M A, Ho‐

seini‐AlhashemiS,GhasemiY.ApplEnerg,2011,88:3354

GraphicalAbstract

Chin.J.Catal.,2013,34:80–100 doi:10.1016/S1872‐2067(11)60497‐X

Factorsinmasscultivationofmicroalgaeforbiodiesel

ZHUJunying,RONGJunfeng,ZONGBaoning*SINOPECResearchInstituteofPetroleumProcessing

Thisreviewpresentsthefactorsthatinfluencethemasscultivationofmicroalgaefor biofuels, such as microalgae species/strains, metabolic mechanism, cultureconditionsandthephotobioreactor.

Page 12: Alga Biodiesel Review

ZHUJunyingetal./ChineseJournalofCatalysis34(2013)80–100

 

[70] FengP,DengZ,HuZ,FanL.BioresourceTechnol,2011,102:10577[71] RichmondA,ZhangCW.JBiotechnol,2001,85:259[72] DouchaJ,LívanskýK.JApplPhycol,2009,21:111[73] ZhanW,SangM,LiAF,ZhangChW.RenewEnergResour,2010,

28(3):67[74] MiuJL,WangB,KanGF,DingY,JiangYH,HouXG,LiGY.MariSci,

2005,29:2[75] BorowitzkaMA.JBiotechnol,1999,70:313[76] Alías C B, López M C G M, Fernández F G A, Sevilla J M F,

SánchezJLG,GrimaEM.BiotechnolBioeng,2004,87:723[77] ChaeSR,HwangEJ,ShinHS.BioresTechnol,2006,97:322[78] HuQ,GutermanH,RichmondA.BiotechnolBioeng,1996,51:51[79] HuQ,FaimanD,RichmondA.JFermentBioeng,1998,85:230 [80] SierraE,AciénFG,FernándezJM,GarcíaJL,GonzálezC,MolinaE.

ChemEngJ,2008,138:136

[81] DouchaJ,LívanskýK.USPatent5981271.1999[82] DouchaJ,LívanskýK.JApplPhycol,2006,18:811[83] RongJF,HuangXG,ZhouXH.CNPatent101870950A.2010[84] GornyR,MasonJP,HiltonG,SchwarzP.WOPatent2011/034567

A2.2011[85] HanChM,YangJQ,ChenHT,ZhangH,LiuMSh.CNPatent101

735948A.2010[86] LiuTZh,ZhangW,ChenY,PengXW,ChenXL,ChenL.CNPatent

102206570A.2010[87] SatoT,YamadaD,HirabayashiS.EnergConversManage,2010,51:

1196[88] SlegersPM,WijffelsRH,vanStratenG,vanBoxtelAJB.ApplEn‐

erg,2011,88:3342[89] QiuChT,LinT,ZhangQL,XuHD,ChenYQ,GongMCh.Chin J

Catal,2011,32:1227