Submitted to: Energy and Fuels
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/3/2008
Publication Date: 2/19/2008
Citation: Knothe, G.H. 2008. "Designer" biodiesel: optimizing fatty ester composition to improve fuel properties. Energy and Fuels. 22:1358-1364. 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. The major components of biodiesel are materials called alkyl esters of fatty acids and are present as a mixture. Therefore, the fuel properties of the individual fatty acid esters are the overall properties representing a mix of the properties of the individual components. Since not all properties of the individual components are advantageous, biodiesel has technical problems such as poor low-temperature properties, storage stability and nitrogen oxides in exhaust emissions. Solutions to one problem often aggravate another problem. This work now discusses how the individual properties of fatty esters can be used to devise a composition for biodiesel that addresses these problems simultaneously without aggravating any of the aforementioned problems. Several possible components are suggested for biodiesel that may be suitable for addressing these problems.
Technical Abstract: Biodiesel is a domestic and renewable alternative with the potential to replace some of the petrodiesel market. It is obtained from vegetable oils, animal fats or other sources with a significant content of triacylglycerols by means of a transesterification reaction. The fatty acid profile of biodiesel thus corresponds to that of the parent oil or fat and is a major factor influencing fuel properties. Besides being renewable and of domestic origin, advantages of biodiesel compared to petrodiesel include biodegradability, higher flash point, reduction of most regulated exhaust emissions, miscibility in all ratios with petrodiesel, compatibility with the existing fuel distribution infrastructure and inherent lubricity. Technical problems with biodiesel include oxidative stability, cold flow and increased NOx exhaust emissions. Solutions to one of these problems often entail increasing the problematic behavior of another property and have included the use of additives or modifying the fatty acid composition, either through physical processes such as winterization or through genetic modification. Methyl oleate has been proposed as a suitable major component of biodiesel in this connection. In this work, the properties of methyl oleate are examined and compared to other potential major components of biodiesel with favorable properties. For example, methyl palmitoleate has advantages compared to methyl oleate, especially with regards to low-temperature properties. Other materials that are examined in this connection are short-chain (C8-C10) saturated esters, with only C10 esters appearing suitable. It is also suggested that to obtain biodiesel fuel with favorable properties, it is advantageous for the fuel to consist of only one major component in as high a concentration as possible.