Comparison of species profiles between O 2 and NO 2 oxidizers in premixed methane flames

Abstract

The profiles of the species H, OH, CH, NH, CN, NCO, NO 2, and CH 3O are compared in a series of five premixed stoichiometric 15-torr CH 4/O 2/NO 2/N 2 flames with NO 2 comprising between 0% and 40% of the oxidizer. Relative species concentrations were measured by laser-induced fluorescence (LIF) and these results are compared with calculations using measured temperature profiles. The reaction mechanism of Miller and Bowman incorrectly predicts the standoff from the burner in flames containing more than 20% NO 2; addition of several reactions involving NO 2 and HONO produces excellent agreement with experiment for most species. The reaction CH 3 + NO 2 → CH 3O + NO is found to be particularly important in the reaction mechanism. LIF profiles of CH 3O show this species to be present in far larger quantities in the NO 2 supported flames than in the CH 4/O 2 system. The nitrogen-containing intermediates CN, NCO, and NH are all overpredicted by a factor of two in the 40% NO 2 flame relative to the 10% NO 2 flame. This indicates an inaccuracy in either the reburn reactions or the fuel nitrogen chemistry when large amounts of NO are present. The kinetic modeling shows that in the 40% NO 2 flame, the dominant pathway to N 2 formation is through N 2O, which is produced primarily by the reaction of NCO with NO. Comparison of emission profiles of NO 2∗ for the various flames indicates that the appearance of an orange-yellow luminous zone at the base of NO 2 supported flames is caused by thermal excitation of NO 2, not by a chemiluminescence mechanism.