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United States Department of Agriculture

Agricultural Research Service

Research Project: VEGETABLE OIL-BASED FUELS, ADDITIVES AND COPRODUCTS

Location: Bio-oils Research Unit

Title: A comprehensive evaluation of the cetane numbers of fatty acid methyl esters

Author
item Knothe, Gerhard

Submitted to: Fuel
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 10, 2013
Publication Date: November 28, 2013
Citation: Knothe, G.H. 2014. A comprehensive evaluation of the cetane numbers of fatty acid methyl esters. Fuel. 119:6-13.

Interpretive Summary: Biodiesel is an alternative to diesel fuel obtained from petroleum. It is produced from vegetable oils such as soybean oil or other sources such as animal fats and waste frying oils by means of a chemical reaction called transesterification. An important fuel property of such biodiesel fuels is the so-called cetane number (CN). The CN is related to the combustion behavior of the fuel. It is conceptually similar to the octane number used for gasoline fuels. In this work, it is described what the CNs of individual components of biodiesel are, how these CNs are affected by the nature of these components, and how the overall CN of a biodiesel fuel can be determined from the CNs of the individual components. These results lead to a better understanding of biodiesel performance. Therefore, the results are of direct interest to biodiesel researchers and producers and also affect other parties interested in this fuel.

Technical Abstract: The cetane number (CN) is a prime indicator of the quality of diesel fuels, including those derived from renewable resources such as biodiesel. While many effects of compound structure are known or have been postulated, experimental data have not always been available for the various ester components of biodiesel and a comprehensive evaluation, including some structural features not yet experimentally investigated such as double bond configuration and position, is lacking. In this work, existing CN data of fatty esters are complemented by studying C18 esters with differing double bond positions and double bond configurations. For the first time, CNs of neat trans fatty acid methyl esters, methyl elaidate (methyl 9(E)-octadecenoate) and methyl linolelaidate (methyl 9(E),12(E)-octadecenoate), were determined as were the CNs of the C18:1 positional isomers methyl petroselinate (methyl 6(Z)-octadecenoate) and methyl asclepate (methyl 11(Z)-octadecenoate). The CNs of the positional and geometric isomers of methyl oleate are close to the CN of methyl oleate. These data are evaluated in connection with other previously determined CN data. Furthermore, the applicability of CN data is evaluated using an extensive collection of CN determinations for methyl oleate as “standard” fatty acid (methyl) ester. The average CN value for methyl oleate is in the range of 56-58 with a CN of 57 suggested for calculating purposes. Similarly, a range could be established for each CN of other fatty materials. The data are also useful for predicting CNs of compounds for which CNs are not readily available or easily determinable. The relation to minimum CNs prescribed in biodiesel standards is also discussed.

Last Modified: 7/28/2014
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