Chemical Kinetics Study on Ignition Characteristics of Biodiesel Surrogates 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.