A comparative study of oxidation of pure ammonia and ammonia/dimethyl ether mixtures in a jet-stirred reactor using SVUV-PIMS

Oxidation of pure ammonia or ammonia/dimethyl ether (NH3/DME) mixtures in O2/Ar in a jet-stirred reactor (JSR) at atmospheric pressure was experimentally studied using a synchrotron vacuum ultraviolet photoionization mass spectrometer (SVUV-PIMS) and a gas chromatograph (GC) to analyse the reaction intermediates and product species. Particular attention was given to the effect of DME addition on NH3 → NH2, a critical step that initiates NH3 oxidation and subsequent formation of nitrogen oxides. The initial mole fraction of NH3 was set to 0.01, and for the NH3/DME mixture, the NH3/DME molar ratio was set to 1. The experiments were performed at two equivalence ratios of ϕ = 1.0 and 0.2 in the temperature range of 500 – 1100 K. A kinetic model for NH3/DME oxidation was developed and kinetic analysis was performed. It was found that the addition of DME significantly promotes the NH3 to NH2 pathway and the mechanisms of this promotion effect are different at low and high temperatures. At a low temperature (∼ 600 K), NH3 → NH2 is mainly promoted by OH radicals produced from the DME low-temperature chain-branching reactions, while at a high temperature (∼ 1000 K), mainly by OH radicals produced from the reactions consuming HO2 and H radicals, which were formed from DME high temperature oxidation. At T > 800 K with ϕ = 0.2, the addition of DME promotes the formation of nitrogen oxides, however, at a low temperature of 600 K, the addition of DME only promotes the formation of N2O and NO2 but not NO. A reaction pathway analysis indicates that the oxidation of NO into NO2 at low temperature is promoted by HO2 and CH3OCH2O2, the intermediate products from the low temperature oxidation of DME.