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

• New ignition data of NO2/CH4/O2/Ar mixtures were provided.
• Literature models are inapplicability in predicting reactivity for CH4/NO2 mixtures.
• A modified model capturing well the new data and literature data was supposed.
• Free radical pool during induced time is strongly perturbed by NO2 addition.
• This study is valuable to develop the combustors and organize recycling of NOx.

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/O2/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 ĊH3 + NO2 ⇔ CH3Ȯ + NO, CH3Ȯ + M ⇔ CH2O + Ḣ + M, NO2 + Ḣ ⇔ NO + ȮH, and CH4 + ȮH ⇔ ĊH3 + H2O is proposed to explain the observed effect of NO2 addition on the promotion of methane ignition.