Submitted to: Fuel Processing Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 30, 2006
Publication Date: June 19, 2007
Citation: Knothe, G.H. 2007. Some aspects of biodiesel oxidative stability. Fuel Processing Technology. 88:677-699. 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 made by a chemical reaction of the vegetable oil or animal fat with chemical compounds called alcohols. The resulting materials are also known not only as biodiesel but as fatty acid alkyl esters. The fatty acids, the prime components of the triglycerides in the parent vegetable oil or fat, are the same as found in biodiesel. Some of the fatty acids in oils and fats are known to react with oxygen in air, known as oxidation reactions. The same reactions can occur when these fatty acids are present in biodiesel. The products of these reactions can ultimately affect the fuel properties of biodiesel negatively. Factors promoting oxidation are elevated temperature, as well as presence of light or some extraneous materials. Therefore, it is important to possess a better understanding of how oxidation of biodiesel works, how it can be inhibited and how biodiesel can be analyzed for its oxidation status. This work addresses all these aspects and their interrelationships and is valuable to the producers of biodiesel.
Technical Abstract: Biodiesel, an "alternative" diesel fuel derived from vegetable oils, animals fats or used frying oils, largely consists of the mono-alkyl esters of the fatty acids comprising these feedstocks. One major technical issue facing biodiesel is its susceptibility to oxidation upon exposure to oxygen in ambient air. This susceptibility is due to its content of unsaturated fatty acid chains, especially those with bis-allylic methylene moieties. Oxidation of fatty acid chains is a complex process that proceeds by a variety of mechanisms. Besides presence of air, various other factors influence the oxidation process of biodiesel including presence of light, elevated temperature, extraneous materials such as metals which may be even present in the container material, peroxides, and antioxidants, as well as the size of the surface area between biodiesel and air. Approaches to improving biodiesel oxidative stability include the deliberate addition of antioxidants or modification of the fatty ester profile. This article details the various factors influencing biodiesel oxidative stability and their interaction. Resulting approaches to improving this property of biodiesel are related to these factors and the corresponding mechanisms.