Biodiesel from microalgaeIntroductionPotential of microalgal biodieselMicroalgal biomass productionRaceway pondsPhotobioreactorsComparison of raceways and tubular photobioreactorsAcceptability of microalgal biodieselEconomics of biodiesel productionImproving economics of microalgal biodieselBiorefinery based production strategyEnhancing algal biologyPhotobioreactor engineeringConclusionReferencesResearch review paperBiodiesel from microalgaeYusuf Chisti⁎Institute of Technology and Engineering, Massey University, Private Bag 11 222, Palmerston North, New ZealandAvailable online 13 February 2007AbstractContinued use of petroleum sourced fuels is now widely recognized as unsustainable because of depleting supplies and thecontribution of these fuels to the accumulation of carbon dioxide in the environment. Renewable, carbon neutral, transport fuels arenecessary for environmental and economic sustainability. Biodiesel derived from oil crops is a potential renewable and carbonneutral alternative to petroleum fuels. Unfortunately, biodiesel from oil crops, waste cooking oil and animal fat cannot realisticallysatisfy even a small fraction of the existing demand for transport fuels. As demonstrated here, microalgae appear to be the onlysource of renewable biodiesel that is capable of meeting the global demand for transport fuels. Like plants, microalgae use sunlightto produce oils but they do so more efficiently than crop plants. Oil productivity of many microalgae greatly exceeds the oilproductivity of the best producing oil crops. Approaches for making microalgal biodiesel economically competitive withpetrodiesel are discussed.© 2007 Elsevier Inc. All rights reserved.Keywords: Biofuels; Biodiesel; Microalgae; Photobioreactors; Raceway pondsContents1. Introduction ...................................................... 2952. Potential of microalgal biodiesel ........................................... 2963. Microalgal biomass production ............................................ 2973.1. Raceway ponds ................................................ 2973.2. Photobioreactors ............................................... 2984. Comparison of raceways and tubular photobioreactors ................................ 3005. Acceptability of microalgal biodiesel ......................................... 3006. Economics of biodiesel production .......................................... 3017. Improving economics of microalgal biodiesel. .................................... 3027.1. Biorefinery based production strategy ..................................... 3027.2. Enhancing algal biology ............................................ 3027.3. Photobioreactor engineering .......................................... 3038. Conclusion ...................................................... 304References ......................................................... 304Biotechnology Advances 25 (2007) 294 – 306www.elsevier.com/locate/biotechadv⁎Tel.: +64 6 350 5934; fax: +64 6 350 5604.E-mail address: [email protected]/$ - see front matter © 2007 Elsevier Inc. All rights reserved.doi:10.1016/j.biotechadv.2007.02.0011. IntroductionMicroalgae are sunlight-driven cell factories thatconvert carbon dioxide to potential biofuels, foods,feeds and high-value bioactives (Metting and Pyne,1986; Schwartz, 1990; Kay, 1991; Shimizu, 1996,2003; Borowitzka, 1999; Ghirardi et al., 2000; Akker-man et al., 2002; Banerjee et al., 2002; Melis, 2002;Lorenz and Cysewski, 2003; Metzger and Largeau,2005; Singh et al., 2005; Spolaore et al., 2006; Walteret al., 2005). In addition, these photosynthetic micro-organisms are useful in bioremediation applications(Mallick, 2002; Suresh and Ravishankar, 2004; Kalinet al., 2005; Munoz and Guieysse, 2006) and asnitrogen fixing biofertilizers Vaishampayan et al.,2001). This article focuses on microalgae as a potentialsource of biodiesel.Microalgae can provide several different types ofrenewable biofuels. These include methane produced byanaerobic digestion of the algal biomass (Spolaor e et al.,2006); biodiesel derived from microalgal oil (Roessleret al., 1994; Sawayama et al., 1995; Dunahay et al., 1996;Sheehan et al., 1998; Banerjee et al., 2002; Gavrilescuand Chisti, 2005); and photobiologically pro ducedbiohydrogen (Ghirardi et al., 2000; Akkerman et al.,2002; Melis, 2002; Fedorov et al., 2005; Kapdan andKargi, 2006). The idea of using microalgae as a source offuel is not new (Chist i, 1980–81; Nagle and Lemke,1990; Sawayama et al., 1995), but it is now being takenseriously because of the escalating price of petroleumand, more significantly, the emerging concern aboutglobal warming that is associated with burning fossilfuels (Gavrilescu and Chisti, 2005).Biodiesel is produced currently from plant andanimal oils, but not from microalgae. This is likely tochange as several companies are attempting to com-mercialize microalgal biodiesel. Biodiesel is a provenfuel. Technology for producing and using biodiesel hasbeen known for more than 50 years (Knothe et al., 1997;Fukuda et al., 2001; Barnwal and Sharma, 2005;Demirbas, 2005; Van Gerpen, 2005; Felizardo et al.,2006; Kulkarni and Dalai, 2006; Meher et al., 2006). Inthe United States, biodiesel is produced mainly fromsoybeans. Other sources of commerc ial biodieselinclude canola oil, animal fat, palm oil, corn oil, wastecooking oil (Felizardo et al., 2006; Kulkarni and Dalai,2006), and jatropha oil (Barnwal and Sharma, 2005).The typically used process for commercial production ofbiodiesel is explained in Box 1. Any future productionof biodiesel from microalgae is expected to use the sameprocess. Production of methyl esters, or biodiesel, frommicroalgal oil has been demonstrated (Belarbi et al.,Box 1Biodiesel productionParent oil used in making biodiesel consists oftriglycerides (Fig. B1) in which t hree fatty acidmolecules are esterified with a molecule of glycerol.In making biodiesel, triglycerides are reacted withmethanol in a reaction known as transesterification oralcoholysis. Transestrification produces methyl estersof fatty acids, that are biodiesel, and glycerol (Fig. B1).The reaction occurs stepwise: triglycerides are firstconverted to diglycerides, then to monoglycerides andfinally to glycerol.Fig. B1. Transesterification of oil to biodiesel. R1–3arehydrocarbon groups.Transesterification requires 3 mol of alcohol for eachmole of triglyceride to produce 1 mol of glycerol and3 mol of methyl esters (Fig. B1). The