India ranks seventh as the world’s energy producer accounting for about 2.5% of the world’s total annual energy production, and world’s fifth largest energy consumer with about 3.5% of the global primary energy demand (IEA, 2007; Planning Commission, Govt. of India, 2007). Despite being among the largest energy producer, India is a net importer of energy, largely due to huge imbalance between energy consumption and production. About 30% of India’s total primary energy need is being met by petroleum oil, of which 76% is imported. India’s transportation fuel require­ments are unique in the world. India consumes almost five times more diesel fuel than gasoline, whereas all other countries in the world use more gasoline than diesel fuel (Khan et al., 2009). Thus, search for alterna­tives to diesel fuel is of special importance in India. Bioalcohols are unsuitable substitutes for diesel engines, because of their low cetane numbers (CNs) along with poor energy content per unit biomass (Bhattacharyya and Reddy, 1994; Rao and Gopalkrishnan, 1991). There­fore, biodiesel is the only option to fulfill the require­ments in future.

BIODIESEL

Biodiesel is chemically monoalkyl esters of long — chain fatty acids derived from vegetable oils or animal fats. The history of using vegetable oil as an alternative fuel dates back to 1900, when Rudolph Diesel used pea­nut oil as fuel in the World Exhibition in Paris. It was found that vegetable oils, in general, have acceptable CNs and calorific values comparable with the conven­tional diesel. However, the major problem with the direct use of vegetable oils as fuel of compression igni­tion engine is their high viscosity, which interferes with the fuel injection and atomization contributing to incomplete combustion, nozzle clogging, injector coking, severe engine deposits, ring sticking and gum formation leading to engine failure (Knothe, 2005; Meher et al., 2006; Singh and Rastogi, 2009). Therefore, vegetable oils need to be modified to bring their combustion-related properties closer to those of diesel fuel. One possible method to overcome the problem of high viscosity of vegetable oils is their chemical modification to esters, what is nowadays called as "biodiesel".

Biodiesel has emerged as the most suitable alterna­tive to petroleum diesel fuel owing to its ecofriendly characteristics and renewability (Krawczyk, 1996). It burns in conventional diesel engines with or without any modifications while reducing pollution (100% less sulfur dioxide, 37% less unburned hydrocarbons, 46% less carbon monoxide, and 84% less particulate matter) in comparison to the conventional diesel fuel (McMillen et al., 2005). The basic feedstocks for the production of first-generation biodiesel were mainly edible vegetable oils like soybean, rapeseed, sunflower and safflower. The use of first-generation biodiesel has generated a lot of controversy, mainly due to their impact on global food markets and food security for diverting food away from the human food chain. The second-generation biodiesel was produced by using nonedible oil sources like used frying oil, grease, tallow, lard, karanja, jatropha and mahua oils (Alcantara et al., 2000; Francis and Becker 2002; Canakci and Gerpen, 1999; Dorado et al., 2002; Ghadge and Raheman, 2006; Mittelbach, 1990). Nevertheless, the cost of biodiesel production is still a major obstacle for large-scale commercial exploitation, mainly due to the high feed cost of vegetable oils (Lang et al., 2001). Moreover, the first- as well as the second-generation biodiesel based on terrestrial plants initiate land clearing and potentially compete with net food produc­tion (Chisti, 2008; Marsh, 2009). The focus of researchers has now been shifted to the next generation biodiesel. The third-generation biodiesel is both promising and different; it is based on simple microscopic organisms that live in water and grow hydroponically, i. e. microalgae.

The possibilities of biodiesel production from edible oil resources in India is almost impossible, as primary need is to first meet the demand of edible oil that is being imported. India accounts for 9.3% of the world’s total oil seed production and contributes as the fourth largest edible oil producing country. Even then, about 46% of edible oil is imported for catering the domestic needs (Jain and Sharma, 2010). So the nonedible oil resources like Jatropha, pongamia, mahua, etc. seem to be the only possibility for biodiesel production in the country. The Government of India has duly realized the impor­tance of biodiesel and introduced a nationwide program under the National Biodiesel Mission in 2003 with the aim of achieving a target of meeting 13.4 Mt of biodiesel (@ 20% blending) from Jatropha curcas by 2012, and to achieve the target about 27 billion of planting materials are required to be planted over 11.2 million hectares of land (Planning Commission, Govt. of India, 2003). At the current rate of consumption, if all petroleum — derived transport fuel is to be replaced with biodiesel from Jatropha oil, Jatropha would need to be grown over an area of 384 million hectares, which is more than 100% of the geographic area of India (Khan et al.,

2009) . Therefore, India must find additional, reliable, cost-effective and sustainable feedstock for biodiesel production. In this context, biodiesel from microalgae seems to be a suitable substitute for diesel fuel in the long run.