Biodiesel from microalgae

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.

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, Shimizu, 2003, Borowitzka, 1999, Ghirardi et al., 2000, Akkerman 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 microorganisms 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 commercialize 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., 2000) although the product was intended for pharmaceutical use.