Research review paper Biodiesel from microalgae Yusuf Chisti ��� Institute of Technology and Engineering, Massey University, Private Bag 11 222, Palmerston North, New Zealand Available online 13 February 2007 Abstract Continued use of petroleum sourced fuels is now widely recognized as unsustainable because of depleting supplies and the contribution of these fuels to the accumulation of carbon dioxide in the environment. Renewable, carbon neutral, transport fuels are necessary for environmental and economic sustainability. Biodiesel derived from oil crops is a potential renewable and carbon neutral alternative to petroleum fuels. Unfortunately, biodiesel from oil crops, waste cooking oil and animal fat cannot realistically satisfy even a small fraction of the existing demand for transport fuels. As demonstrated here, microalgae appear to be the only source of renewable biodiesel that is capable of meeting the global demand for transport fuels. Like plants, microalgae use sunlight to produce oils but they do so more efficiently than crop plants. Oil productivity of many microalgae greatly exceeds the oil productivity of the best producing oil crops. Approaches for making microalgal biodiesel economically competitive with petrodiesel are discussed. �� 2007 Elsevier Inc. All rights reserved. Keywords: Biofuels Biodiesel Microalgae Photobioreactors Raceway ponds Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 295 2. Potential of microalgal biodiesel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 296 3. Microalgal biomass production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 297 3.1. Raceway ponds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 297 3.2. Photobioreactors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 298 4. Comparison of raceways and tubular photobioreactors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 300 5. Acceptability of microalgal biodiesel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 300 6. Economics of biodiesel production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 301 7. Improving economics of microalgal biodiesel. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 302 7.1. Biorefinery based production strategy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 302 7.2. Enhancing algal biology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 302 7.3. Photobioreactor engineering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303 8. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 304 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 304 Biotechnology Advances 25 (2007) 294���306 www.elsevier.com/locate/biotechadv ��� Tel.: +64 6 350 5934 fax: +64 6 350 5604. E-mail address: Y.Chisti@massey.ac.nz. 0734-9750/$ - see front matter �� 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.biotechadv.2007.02.001

1. Introduction Microalgae are sunlight-driven cell factories that convert 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 Walter et al., 2005). In addition, these photosynthetic micro- organisms are useful in bioremediation applications (Mallick, 2002 Suresh and Ravishankar, 2004 Kalin et al., 2005 Munoz and Guieysse, 2006) and as nitrogen fixing biofertilizers Vaishampayan et al., 2001). This article focuses on microalgae as a potential source of biodiesel. Microalgae can provide several different types of renewable biofuels. These include methane produced by anaerobic digestion of the algal biomass (Spolaore et al., 2006) biodiesel derived from microalgal oil (Roessler et al., 1994 Sawayama et al., 1995 Dunahay et al., 1996 Sheehan et al., 1998 Banerjee et al., 2002 Gavrilescu and Chisti, 2005) and photobiologically produced biohydrogen (Ghirardi et al., 2000 Akkerman et al., 2002 Melis, 2002 Fedorov et al., 2005 Kapdan and Kargi, 2006). The idea of using microalgae as a source of fuel is not new (Chisti, 1980���81 Nagle and Lemke, 1990 Sawayama et al., 1995), but it is now being taken seriously because of the escalating price of petroleum and, more significantly, the emerging concern about global warming that is associated with burning fossil fuels (Gavrilescu and Chisti, 2005). Biodiesel is produced currently from plant and animal oils, but not from microalgae. This is likely to change as several companies are attempting to com- mercialize microalgal biodiesel. Biodiesel is a proven fuel. Technology for producing and using biodiesel has been 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). In the United States, biodiesel is produced mainly from soybeans. Other sources of commercial biodiesel include canola oil, animal fat, palm oil, corn oil, waste cooking oil (Felizardo et al., 2006 Kulkarni and Dalai, 2006), and jatropha oil (Barnwal and Sharma, 2005). The typically used process for commercial production of biodiesel is explained in Box 1. Any future production of biodiesel from microalgae is expected to use the same process. Production of methyl esters, or biodiesel, from microalgal oil has been demonstrated (Belarbi et al., Box 1 Biodiesel production Parent oil used in making biodiesel consists of triglycerides (Fig. B1) in which three fatty acid molecules are esterified with a molecule of glycerol. In making biodiesel, triglycerides are reacted with methanol in a reaction known as transesterification or alcoholysis. Transestrification produces methyl esters of fatty acids, that are biodiesel, and glycerol (Fig. B1). The reaction occurs stepwise: triglycerides are first converted to diglycerides, then to monoglycerides and finally to glycerol. Fig. B1. Transesterification of oil to biodiesel. R1���3 are hydrocarbon groups. Transesterificationrequires3 mol ofalcohol foreach mole of triglyceride to produce 1 mol of glycerol and 3 mol of methyl esters (Fig. B1). The reaction is an equilibrium. Industrial processes use 6 mol of methanol for each mole of triglyceride (Fukuda et al., 2001). This large excess of methanol ensures that the reaction is driven in the direction of methyl esters, i.e. towards biodiesel. Yield of methyl esters exceeds 98% on a weight basis (Fukuda et al., 2001). Transesterification is catalyzed by acids, alkalis (Fukuda et al., 2001 Meher et al., 2006) and lipase enzymes (Sharma et al., 2001). Alkali-catalyzed transesterification is about 4000 times faster than the acid catalyzed reaction (Fukuda et al., 2001). Consequently, alkalis such as sodium and potassium hydroxide are commonly used as commercial catalysts at a concentration of about 1% by weight of oil. Alkoxides such as sodium methoxide are even better catalysts than sodium hydroxide and are being increas- ingly used. Use of lipases offers important advantages, but is not currently feasible because of the relatively high cost of the catalyst (Fukuda et al., 2001). Alkali- catalyzed transesterification is carried out at approxi- mately60��Cunderatmosphericpressure, as methanol boils offat 65 ��C at atmospheric pressure. Under these conditions, reaction takes about 90 min to complete. A higher temperature can be used in combination with higher pressure, but this is expensive. Methanol and oil do not mix, hence the reaction mixture contains two liquid phases. Other alcohols can be used, but methanol is the least expensive. To prevent yield loss (continued on next page) 295 Y. Chisti / Biotechnology Advances 25 (2007) 294���306