The effect of NO and NO2 on the partial oxidation of methane:: experiments and modeling

Partial oxidation of methane promoted by NO or NO2 has been studied at long residence times experimentally and computationally. Highly diluted fuel rich CH4 + O-2 + N-2 mixtures with varying concentrations of NOx added were reacted in an uncoated tubular flow reactor at a pressure of 120 kPa-a and temperatures ranging from 773 to 923 K. The residence time was up to 55 s. Products of the partial oxidation were measured by gas chromatographs and NOx gases with a chemiluminescence analyzer. The impact of heterogeneous reactions on the reactor tube wall was analyzed by comparing the experiments and modeling. It was demonstrated that the influence of the surface reactions at 823 K is less pronounced than that at 1100 K. It has been observed experimentally and confirmed computationally that, under the prevailing conditions, the promoting effect of both oxides of nitrogen (NO and NO2) is remarkably similar. Key rate-controlling reactions were revealed by the sensitivity analysis. The similarity of the sensitivity functions for different methane + oxygen + NOx mixtures has been revealed. The comparison of the calculated temporal profiles shows that irrespective of whether NO or NO2 is originally present in the mixture, the concentrations of these two oxides approach unique limiting values in approximately 0.5 s at 823 K. This peculiarity of the partial oxidation of methane promoted by NO or NO2 at long residence times is discussed in terms of the concurrence of the reactions of H and alkyl radicals relative to HO2 and alkyl peroxy radicals. The importance of the reactions of these radicals with NO and NO2 in a detailed reaction mechanism has been demonstrated. (c) 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved.