The differentiated effect of NO and NO2 in promoting methane oxidation

It has long been recognised that nitric oxide (NO) and nitrogen dioxide (NO2) can promote the oxidation of methane and other hydrocarbons under various fuel-lean and fuel-rich conditions and a range of temperatures and pressures conditions. However, despite the ease with which NO and NO2 interconvert under the oxidation conditions, the reactions responsible for initiating the oxidation process for methane are different. This is demonstrated experimentally by the different temperature characteristics of the sensitised reaction and supported by kinetic modelling. In this study, the effect of NO/NO2-sensitised oxidation of methane was experimentally studied with two fuel-lean mixtures, viz. 2.5% methane-in-air and 0.05% methane-in-air mixtures, at atmospheric pressure, over temperature ranges of 823-948 K, and 8731023 K, respectively. Kinetic modelling of the experimental results showed satisfactory agreement and an exhaustive sensitivity analysis was conducted. It was shown through brute-force sensitivity analysis that the difference in key reactions in both NO and NO2 system leads to the observed trend: the effect of NO starts at lower temperatures while NO2 appears to be more potent at higher temperatures. Another salient point emerging from the sensitivity analysis is that the initiation reaction in NO-promoted system was identified to be CH3O2+ NO = CH3O+ NO 2; for NO2-sensitised case, CH4+ NO2 = CH3 + HONO is the most important initiator at lower temperatures while NO2 + O = NO + O-2 and NO2 + HO2 = HONO + O-2 appear to be of prime importance at higher temperatures. (C) 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved.