Oxidation of dimethyl ether and its interaction with nitrogen oxides

The oxidation of dimethyl ether (DME) under flow reactor conditions has bee n studied experimentally and in terms of a detailed chemical kinetic model. The experiments were performed at atmospheric pressure in the temperature range 600-1500 K and at different air/fuel ratios. Of particular interest w as the interaction of DME with nitrogen oxides. The results show that the o xidation of DME occurs readily at temperatures above 1000 K, largely indepe ndent of the stoichiometry. Addition of NO under stoichiometric and fuel-ri ch conditions does not affect the oxidation chemistry for DME, but above 11 00 K a minor amount of the NO is reduced to HCN and N-2 in reburn-type reac tions. Addition of NO or NO2 under oxidizing conditions significantly enhan ces the oxidation rate of DME and shifts the temperature for onset of oxida tion to lower values, a phenomenon similar to that of NOx-sensitized oxidat ion of hydrocarbons. The proposed chemical kinetic model provides a good description of DME oxid ation in the absence of nitrogen oxides. Under the conditions of the presen t study, the conversion of DME proceeds mainly through the high-temperature mechanism, with little importance of the intermediate peroxy species. In t he presence of NO or NO2, the reaction CH3 + NO2 reversible arrow CH3O + NO , followed by dissociation of CH3O, readily provides H atoms and thereby pr omotes the oxidation. At lower temperatures the mechanism involves CH3OCH2O 2 and CH3O2 radicals. While the effect of NOx generally is described satisf actorily by the model, deviations at lower temperatures may indicate inadeq uacies in the reaction subset for these peroxy species.