An experimental and computational study of methyl ester decomposition pathways using shock tubes

The high-temperature decomposition of three simple methyl esters: methyl acetate, methyl propionate and methyl butanoate, were studied behind reflected shock waves using tunable diode laser absorption of CO2 near 2.7 mu m. CO2 yield measurements were made over the range of temperatures 1260-1653 K, pressures of 1.4-1.7 atm and reactant concentrations of 2-3%, with the balance Ar. The CO2 absorption strengths near 2.7 mu m are approximately 50 to 1000 times stronger than the bands near 2.0 and 1.55 mu m, respectively, and offer opportunities for significantly more sensitive and accurate combustion measurements than previous absorption work using CO2 bands at shorter wavelength. The experiments provide the first laser-based time-history measurements of the CO2 yields during pyrolysis of these bio-diesel surrogate fuels in a shock tube. Model predictions for CO2 yields during methyl butanoate pyrolysis at high temperatures, using the detailed reaction mechanisms of [E. M. Fisher, W. J. Pitz, H. J. Curran, C. K. Westbrook, Proc. Combust. Inst. 28 (2000) 1579-1586.] and others, are significantly lower than those measured in this study. However, an improved methyl butanoate model which extends the recent theoretical work of [L.K. Huynh, A. Violi, J. Org. Chem. 73 (2008) 94-101.] provides substantially improved predictions of CO2 yields during methyl butanoate pyrolysis. As earlier mechanisms predicted low yields of CO2 from methyl butanoate decomposition, these new findings imply that existing bio-diesel fuel models, which rely on the rapid formation of two oxygenate radicals from methyl esters (rather than a single non-reactive CO2 molecule) to account for the tendency for soot reduction, may have to be revisited. (C) 2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved.