Ignition-promoting effect of NO2 on methane, ethane and methane/ethane mixtures in a rapid compression machine

Autoignition delay times of stoichiometric methane, ethane and methane/ethane mixtures doped with 100 and 270ppm of NO2 have been measured in a RCM in the temperature range 900–1050K and pressures from 25 to 50bar. The measurements show that addition of NO2 to CH4/O2/N2/Ar and CH4/C2H6/O2/N2/Ar mixtures results in a significant reduction in the autoignition delay time and that the ignition-promoting effect of NO2 increases substantially with increasing temperature, from ∼20% to more than a factor of two over the range of temperature studied.

Addition of NO2 to C2H6/O2/N2/Ar mixtures results in only a modest reduction in ignition delay time over the range of pressure and temperature measured. Computations with an updated chemical mechanism show good agreement with the measurements for undoped methane, but overpredict the delay times for undoped ethane and underestimate the effects of replacing 10% methane by ethane.

For NO2-containing mixtures, the model predicts the observed trend in decreasing delay time with increasing NO2 fraction. However, the computations tend to overestimate the effect of NO2 addition on ignition, particularly for C2H6 mixtures. Analysis of the reaction mechanism for the effects of NO2 addition to methane mixtures indicates that the ignition-promoting effect of NO2 is related to the appearance of new conversion channels for CH3 and CH3OO, i.e., NO2+CH3→NO+CH3O and NO+CH3OO→NO2+CH3O, generation of chain-initiating OH radicals through NO/NO2 interconversion, i.e., NO2+H→NO+OH and NO+HO2→NO2+OH, and to the direct initiation step CH4+NO2→CH3+HNO2.

Analyses further show that the formation of CH3NO2 via CH3+NO2(+M)↔CH3NO2(+M) essentially inactivates NO2. This reaction limits the promoting effect of NO2 at lower temperatures and higher pressures, where stabilization of CH3NO2 is favored, explaining the experimentally observed trends.