The sensitizing effects of NO2 and NO on methane low temperature oxidation in a jet stirred reactor Proceedings of the Combustion Institute

The oxidation of neat methane (CH4) and CH4 doped with NO2 or NO in argon has been investigated in a jet-stirred reactor at 107 kPa, temperatures between 650 and 1200 K, with a fixed residence time of 1.5 s, and for different equivalence ratios (Φ), ranging from fuel-lean to fuel-rich conditions. Four different diagnostics have been used: gas chromatography (GC), chemiluminescence NOx analyzer, continuous wave cavity ring-down spectroscopy (cw-CRDS) and Fourier transform infrared spectroscopy (FTIR). In the case of the oxidation of neat methane, the onset temperature for CH4 oxidation was above 1025 K, while it is shifted to 825 K with the addition of NO2 or NO, independently of equivalence ratio, indicating that the addition of NO2 or NO highly promotes CH4 oxidation. The consumption rate of CH4 exhibits a similar trend with the presence of both NO2 and NO. The amount of produced HCN has been quantified and a search for HONO and CH3NO2 species has been attempted. A detailed kinetic mechanism, derived from POLIMI kinetic framework, has been used to interpret the experimental data with a good agreement between experimental data and model predictions. Reaction rate and sensitivity analysis have been conducted to illustrate the kinetic regimes. The fact that the addition of NO or NO2 seems to have similar effects on promoting CH4 oxidation can be explained by the fact that both species are involved in a reaction cycle interchanging them and whose result is 2CH3+ O2= 2CH2O + 2H. Additionally, the direct participation of NO2 in the NO2+ CH2O = HONO + HCO reaction has a notable accelerating effect on methane oxidation.