Feedstocks for second-generation biodiesel: microalgae's biology and oil composition

LEONARDI PATRICIA INES; POPOVICH, CECILIA; DAMIANI, MARIA CECILIA

Biofuel

INTECH

Año: 2011;

The solar energy is an inexhaustible source, while other energy reserves, like fossil and nuclear fuels, are limited in quantity and are depleted as years go by. The first generation biofuels, produced from oil seeds and crops are a possible alternative but, they are limited in their capacity to provide all the energy demanded in the world. Therefore, new sources for the sustainable production of renewable energy are being looked for. This concern has promoted the keen interest in developing second generation biofuels produced from other feedstocks, such as microalgal oils. Microalgae live as single cells or colonies and vary a great deal with respect to their cell sizes, pigments, storage products, cell wall compositions and life cycles. This highly specialised group of micro-organisms has both the capability to adapt to diverse habitats and the ability to efficiently modify its lipid metabolism in response to changes in environmental conditions as well. Microalgae produce oils as an energy storage. Specifically, some species called oleaginous algae synthesize and accumulate substantial amounts of neutral lipids, mainly in the form of triacylglycerol (TAG), under diverse stress conditions. Oleaginous algae can be found in diverse taxonomic groups and their total lipid content and fatty acid composition may vary noticeably among individual species or strains within and between taxonomic groups. Unlike polar lipids found in membranes, TAGs as storage lipids are the best substrate to produce biodiesel. This biofuel is obtained by transesterification of oil or fat with a monohydric alcohol, yielding the corresponding mono-alkyl esters. Since transesterification maintains the relative ratio of fatty acids present in the feedstock, the profile of the fatty acid ethyl esters is a reflection of the feedstock fatty-acid composition. Biodiesel production from microalgae is technically feasible, but for an effective use of this renewable resource as biofuel, it is necessary to be able to modify microalgal growth conditions in order to obtain the desired lipid quantity and quality. Thus, it is important to have information about the various fatty-acid profiles of diverse microalga oils in order to evaluate their suitability as feedstocks for fuel-conversion processes. Unlike plants, oils of some microalga species have a significant amount of polyunsaturated fatty acids with four and more double bonds, which are valuable oils. This is a feature that limits the algal species that may be used for biodiesel production.             This chapter aims to provide an overview of the current status of research on microalgal feedstocks as regards biodiesel production. Since there are many species of microalgae with varied biological characteristics and lipid composition, a diversity of approaches for biodiesel production have been analysed. In this review the following relevant topics will be considered: 1) diagnostic characteristics of some microalgal main groups, such as Chlorophyceae, Eustigmatophyceae and Bacillariophyceae classes 2) triggering of lipid production, and 3) oil composition, i.e. lipid fractions, content of each lipid class and fatty acid composition of each fraction. In this context, how the latter might affect the biodiesel quality will be discussed. We hope this information provides a framework for future screening of oleaginous microalgae employed as feedstock for biofuel production.