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

Knothe, Gerhard
Fuel 2014 v.119 pp. 6-13
biodiesel, chemical bonding, diesel fuel, fatty acid methyl esters, isomers, oleic acid, prediction
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.