Towards a kinetic understanding of the NOx promoting-effect on ignition of coalbed methane: A case study of methane/nitrogen dioxide mixtures

Nitrogen dioxide (NO2) is an important impurity in coal-bed methane (CBM) and a dominant component of NOx pollution in practical engines. Its promoting effect on methane ignition has been studied in the current experimental and kinetic study. Ignition delay times of NO2/CH4/O-2/Ar mixtures, with blending ratios of NO2:CH4 of 30:70, 50:50 and 70:30 for stoichiometric methane mixtures were measured in a shock tube. Experiments cover a range of pressures (1.2-10.0 atm) and temperatures (933-1961 K). Under all tested pressures, NO2 addition promotes the reactivity of methane and reduces the global activation energy at all pressures, and these effects are most significant for the mixtures with highest NO2 concentrations, at the highest pressures and at the lowest temperatures. To simulate the experimental measurements, five literature NOx sub-mechanisms were integrated with AramcoMech 1.3. The simulations demonstrate that, for the mixtures with low levels of NOx concentrations, the five models agree well with the experimental ignition delay times. For the mixtures with high NOx content, however, all five models are unable to reproduce the measured data, and the level of disagreement increases with increasing NO2 concentration. An updated mechanism is proposed, based on modifications made as a result of sensitivity and reaction flux analyses performed to quantitatively determine the chemical reasons for NO2 promoting methane ignition. The results indicate that, NO2 addition perturbs the branching ratio of key reaction pathways by affecting the structure of the free radical pool at the initial ignition stage of methane oxidation. A new reaction cycle via the following sequence of reactions (C) over dotH(3) + NO2 double left right arrow CH3(O) over dot + NO, CH3(O) over dot + M double left right arrow CH2O + (H) over dot + M, NO2 + (H) over dot double left right arrow NO + (O) over dotH, and CH4 + (O) over dotOH double left right arrow (C) over dotH(3) + H2O is proposed to explain the observed effect of NO2 addition on the promotion of methane ignition. (C) 2016 Elsevier Ltd. All rights reserved.