The Ideal Vegetable Oil-based Biodiesel Composition: A Review of Social, Economical and Technical Implications

摘要

Though a considerable number of publications about biodiesel can be found in literature, several problems remain unsolved, encompassing economical, social, and technical issues. Thus, the biodiesel industry has come under attack by some environmental associations, and subsidies for biofuel production have been condemned by some governments. Yet, biodiesel may represent a truly competitive alternative to diesel fuel, for which fuel tax exemption and subsidies to energetic crops are needed. Biodiesel must increase its popularity among social movements and governments to constitute a valid alternative of energy source. In this sense, the use of nonedible oils to produce biodiesel is proposed in the present review. Moreover, the compromise of noninterference between land for energetic and food purposes must be addressed. Concerning technical issues, it is important to consider a transesterification optimization, which is missing or incomplete for too many vegetable oils already tested. In most cases, a common recipe to produce biodiesel from any raw material has been adopted, which may not represent the best approach. Such strategy may fit multifeedstock biodiesel plant needs but cannot be accepted for oils converted individually into biodiesel, because biodiesel yield will most likely fail, increasing costs. Transesterification optimization results depend on the chemical composition of vegetable oils and fats. Considering "sustainable" vegetable oils, biodiesel from Calophyllum inophyllum, Azadirachta indica, Terminalia catappa, Madhuca indica, Pongamia pinnata, and Jatropha curcas oils fits both current biodiesel standards: European EN 14214 and US ASTM D 6751 02. However, none of them can be considered to be the "ideal" alternative that matches all the main important fuel properties that ensure the best diesel engine behavior. In search of the ideal biodiesel composition, high presence of monounsaturated fatty acids (as oleic and palmitoleic acids), reduced presence of polyunsaturated acids, and controlled saturated acids content are recommended. In this sense, C18:1 and C16:1 are the best-fitting acids in terms of oxidative stability and cold weather behavior, among many other properties. Furthermore, genetic engineering is an invaluable tool to design oils presenting the most suitable fatty acid profile to provide high quality biodiesel. Finally, most published research related to engine performance and emissions fails in using a standard methodology, which should be implemented to allow the comparison between tests and biofuels from different origin. In conclusion, a compromise between social, economical, and technical agents must be reached.