As microalgae possess a simple cellular structure, their capability to efficiently convert solar energy into chemical energy is high. The production of oil per unit area of land from selected microalgae is around 30 times greater than that of ter­restrial plants. The scenario thus looks promising for the production of biodiesel from the microalgal oil. There are various steps involved—from the stage of cul­tivation of microalgae to the final stage of production of bio-oil or biodiesel. The intermediate steps include harvesting, dewatering, concentration, and extraction of microalgal oil. The composition of fatty acids and other constituents present in plant and animal oils varies considerably from microalgal oil. In addition to triglycerides and free fatty acids, microalgal oil contains hydrocarbons, sterols, wax and sterol esters, and free alcohols that cannot be saponified. The major components in microalgae include carbohydrates, proteins, and lipids. In gen­eral, the lipid content of microalgal biomass increases when they are deprived of certain nutrients (nitrogen and silicon). However, the deprivation of nitrogen and silicon does not necessarily favor all species viz. Euglena, Nannochloropsis strains where cell division has been found to be blocked. There are certain species (such as Escherichia coli and Saccharomyces cerevisiae) that can be converted to oleaginous species (microbes that can accumulate more than 20% of their cellular dry weight in lipid) by genetic engineering. Although there are several species of microalgae, only a few have been explored with respect to their potential for high biomass yield and lipid content. The FAME content in the biodiesel should have a minimum value of 96.5%, as per the recommendation of EN 14103 (Sarin et al., 2009). However, biodiesel synthesized from only a few of the microalgal species has fulfilled the minimum criteria of ester content in biodiesel as specified by the EN. The reason for this may be attributed to the presence of unsaponifiable constit­uents in the microalgae. Microalgal species not fulfilling the minimum specified criteria of ester content limits their suitability for biodiesel production. However, microalgal oil can be converted to bio-oil by pyrolysis or thermochemical catalytic liquefaction. The bio-oil can be further upgraded by chemical or physical means. While the chemical upgradation includes processes such as catalytic esterifica­tion, catalytic hydroprocessing, and catalytic cracking, the physical upgradation can be done by char removal, hot vapor filtration, liquid filtration, or solvent addi­tion (Xiong et al., 2011). The present status for the production of biodiesel and bio-oil from microalgae is cost intensive. However, the major advantage that the microalgae provide is their growth in aquatic environments and their noncompeti­tiveness with terrestrial plants. India and many other countries have a vast coastal area where microalgae can be grown, cultured, and harvested. The other important benefit of microalgae is the suitability of some of the species in wastewater. Thus, microalgae cultured with wastewater will have the dual benefit: of the production of oil and the treatment/disposal of wastewater. Numerous strains of microalgae are available in nature, and several of these species may be explored for their feasibility to be cultured as oleaginous species. The future seems bright for micro­algae to provide a future alternative to the other fuels.

8.2 CONCLUSION

The oil extracted from microalgae consists of polar lipids and neutral lipids. The neutral lipids consist of triglycerides, free fatty acids, hydrocarbons, sterols, wax and sterol esters, and free alcohols. Because only triglycerides and free fatty acids are saponifiable, they must be separated from the others so as to convert them to biodiesel by esterification or transesterification. The synthesis of biodiesel/bio-oil from microalgae involves several steps, including selection of an appropriate species among a large diversity of species of microalgae (around 300,000). For the production of bio-oil, a thermochemical method is adopted for the preparation of fuel from microalgae through pyrolysis, direct combustion, or thermochemical l iquefaction, wherein the organic compound is thermally decomposed at high temperature in the absence of oxygen. A high yield of bio-oil (97.05%) was obtained through liquefaction of Dunaliella tertiolecta. Thermochemical catalytic liquefaction has an advantage over pyrolysis or direct liquefaction, in that a low nitrogen content is present. A high oxygen content has been observed, which requires deoxygenation of the bio-oil. Other compounds are also formed along with bio-oil, such as bio-char, gases, and ash, all of which lower the calorific value of the fuel. The amount of biodiesel obtained from microalgal oil can be enormous The fatty acid composition of the feedstock has been found to play a significant role in the composition of the biodiesel. A trade-off between the oxidation stability and low-temperature proper­ties of biodiesel has been observed and hence a balance between the two must be maintained.

ACKNOWLEDGMENTS

Bhaskar Singh is grateful to the Council of Scientific and Industrial Research (CSIR) New Delhi, India for the award of Research Associateship.

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