Methane and Methyl Propanoate High-Temperature Kinetics

The biodiesel surrogate, methyl propanoate (MP), is more reactive than methane. Mixtures of the two can be used to control combustion initiation in various combustion systems. Reported here is a shock tube study of the influence of chemical interactions resulting from mixing the two fuels on observable combustion properties, such as global chemical time scales and species time histories. Experiments are carried out at pressures of about 4, 7.4, and 10 atm covering a temperature window of 1000 to 1500 K. Using direct laser absorption, CO time histories during MP pyrolysis are obtained. The CO absorbance is further used to determine pyrolysis times by means of which the effect of temperature on MP pyrolysis is probed. Reactivity differences are first examined with the fuel concentration maintained at 3% and then with the oxygen concentration fixed at 10%. The evidence of chemical interactions during ignition is observed through a reduction of methane ignition delay times caused by MP addition. The influence is nonlinear, with the result that ignition delay times of blends of 50% of each fuel are much closer to the ignition delay times of MP, the more reactive fuel. This is understood to result from the rapid generation of radicals during MP oxidation which further react with methane in low-activation energy elementary reactions, such as OH which reacts almost barrier-less. With respect to CO formation during MP pyrolysis, the presence of methane is not observed to significantly influence the pyrolysis time, indicating limited radical withdrawal by methane during the propanoate pyrolysis as it is the case during oxidation when the chemical interactions are accentuated by the exchange of oxygen-mediated radical formation. The measured data are compared with two model predictions, showing reasonable agreement for the ignition data and discrepancies with respect to the pyrolysis data.