Chemical Kinetics Study on Ignition Characteristics of Biodiesel Surrogates

Kazunari Kuwahara; Koryu Nakahara; Yoshimitsu Wada; Jiro Senda; Yasuyuki Sakai; Hiromitsu Ando

doi:10.4271/2011-01-1926

Methyl butanoate (MB) and methyl decanoate (MD) are surrogates for biodiesel fuels. According to computational results with their detailed reaction mechanisms, MB and MD indicate shorter ignition delays than long alkanes such as n-heptane and n-dodecane do at an initial temperature over 1000 K. The high ignitability of these methyl esters was computationally analyzed by means of contribution matrices proposed by some of the authors.

Due to the high acidity of an α-H atom in a carbonyl compound, hydroperoxy radicals are generated out of the equilibrium between forward and backward reactions of O₂ addition to methyl ester radicals by the internal transfer of an α-H atom in the initial stage of an ignition process. Some of the hydroperoxy methyl ester radicals can generate OH to activate initial reactions.

MB has an efficient CH₃O formation path via CH₃ generated by the β-scission of an MB radical which has a radical site on the α-C atom to the carbonyl group. MB has also other CH₃O formation paths via some of fragmental oxygenated radicals. Therefore, the CH₃O concentration is remarkably high in a thermal ignition preparation phase. The rich CH₃O decomposes into CH₂O and H, and then H combines with O₂ into HO₂. This exothermic reaction, H + O₂ + M = HO₂ + M, plays a key role in promoting initial heat release.

MD has efficient paths for initial heat release starting from the O₂ addition to some of fragmental methyl ester radicals and ending in the OH formation via the internal transfer of an α-H atom. These paths considerably contribute not only to promoting initial heat release but also to generating OH in the initial stage of an ignition process.

In conclusion, these mechanisms for the high ignitability are caused by a common local structure of methyl ester molecules, a carbonyl group in the molecule.

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Chemical Kinetics Study on Ignition Characteristics of Biodiesel Surrogates 2011-01-1926

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