Evaluation of Ignition Improvers for Methane Autoignition

Abstract

The objective of this study was to estimate the efficiency of methane autoignition promotion by testing different ignition improvers including hydrogen, H2, hydrogen peroxide, H2O2, ozone,O3 and dimethyl ether, DME, (CH3)2O. This was accomplished by computing ignition delays for CH4/O2/Ar or N2 mixtures of various compositions, concentrations of the promoters, pressures and temperatures. Ignition delay times for additive-free mixtures were used for tuning a methane oxidation mechanism consisting of 185 reversible elementary reactions between 32 species. A selection of the reaction rate parameters available in the standard databases was made to optimize the agreement between simulation and experimental results for one particular set of test conditions (reference mixture) by refining the rate parameters of the most sensitive stages revealed by sensitivity analysis. The agreement achieved between model predictions and shock tube experimental data is very good. To investigate the effect of dimethyl ether on methane autoignition, the mechanism was extended up to 301 reactions and 53 species to predict the autoignition delay at high pressures. Methane autoignition promotion is proven to be much stronger for H2O2, O3 and (CH3)2O additives than for H2. The mechanism of ignition acceleration is attributed mainly to the dominant role of the O and OH-radicals generated by the rapid decomposition of hydrogen peroxide and ozone, and to the peroxide intermediates and low temperature branching, in the case of dimethyl ether.