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.