Submitted to: Fuel Processing Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/29/2004
Publication Date: 3/20/2005
Citation: Knothe, G.H. 2005. Dependence of biodiesel fuel properties on the structure of fatty acid alkyl esters. Fuel Processing Technology. 86(10):1059-1070. Interpretive Summary: Biodiesel is an alternative diesel fuel derived from vegetable oils such as soybean oil or other sources such as animal fats and waste frying oils. It is a mixture of compounds called fatty acid methyl esters. Fatty acids, in form of triacylglycerols (triglycerides) are the major components of fats and oils. Therefore, biodiesel derived from fats and oils has the same composition of fatty acids as the parent oil or fat. However, to obtain biodiesel, the oil or fat is subjected to a reaction called transesterification with a chemical class of compounds known as alcohols. As a result, biodiesel consists not only of fatty acid moieties but also of moieties derived from the alcohols. Both moieties contribute to the fuel properties of biodiesel. This paper places new perspectives on some essential fuel properties such as ignition, cold flow, stability upon exposure to air, viscosity (thickness of a liquid) and lubricity. Some new results on how the various moieties in biodiesel affect these properties are compared to existing results. Generally, biodiesel compares well with conventional petroleum-derived diesel fuel with regard to these fuel properties.
Technical Abstract: Biodiesel, defined as the mono-alkyl esters of vegetable oils or animal fats, is an "alternative" diesel fuel that is becoming accepted in a steadily growing number of countries around the world. Since the source of biodiesel varies with the location and other sources such as recycled oils are continuously gaining interest, it is important to possess data on how the various fatty acid profiles of the different sources can influence biodiesel fuel properties. The properties of the various individual fatty esters that comprise biodiesel determine the overall fuel properties of the biodiesel fuel. In turn, the properties of the various fatty esters are determined by the structural features of the fatty acid chain and the alcohol moieties that comprise a fatty ester. Structural features that influence the physical and fuel properties of a fatty ester molecule are chain length, degree of unsaturation, and branching of the chain. Important fuel properties of biodiesel that are influenced by the fatty acid profile and in turn the structural features of the various fatty esters comprising it are cetane number and ultimately exhaust emissions, cold flow, oxidative stability, viscosity, and lubricity.