Detailed and reduced chemistry for methanol ignition

Abstract

Simplified chemical-kinetic mechanisms are sought that can provide agreement with measured shock-tube autoignition times and counterflow critical ignition conditions for methanol (CH 3OH) oxidation. Existing detailed chemistry over-predicts measured counterflow ignition temperatures by 100 K or more. It was found that the elementary step CH 3OH + HO 2 → CH 2OH + H 2O 2 most strongly affects the predictions. Increasing the pre-factor in the Arrhenius expression for the rate of this step from different available literature values by a factor ranging from 2 to 13, namely to 8 × 10 13 cm 3/(mol s), within existing uncertainty, produces agreement of predictions with experiment. Using this revised rate, unimportant steps are deleted from the San Diego mechanism to obtain a set of 26 irreversible elementary steps (augmented to 27 by including fuel dissociation to CH 3 + OH for high-temperature shock-tube conditions) that predict ignition nearly as well as the detailed mechanism. In this mechanism, the intermediate species CH 2OH, CH 3O, HCO, H, O, and OH accurately obey steady states, while the intermediates CH 2O, HO 2, H 2O 2, CO, and H 2 do not. The result is a six-step overall reduced mechanism that describes ignition well at the lower temperatures. At higher temperatures, the aforementioned fuel decomposition becomes important, increasing the six-step mechanism to a seven-step mechanism. Expressions for the reaction rates, branching ratios, and steady-state species concentrations in the six-step reduced mechanism are given to facilitate future methanol ignition computations. Higher alcohols, which are less dependent on HO 2 attack in ignition, are indicated to nevertheless possibly benefit from an increase of the rate of the corresponding step.