The presence of NO 2 in premixed flames

Abstract

Previous findings that a substantial proportion of the NO x in the gases withdrawn from a premixed CH 4 or H 2 flame by a suction probe may be present as NO 2 are confirmed. Strong evidence is provided, however, that this result is an artifact of the probe system and that the proportion of NO 2 actually present in the flame is quite small. The evidence is of two kinds: (1) A 50-reaction kinetic model of reactions of the flame radicals was used to calculate the concentrations of NO and NO 2 emerging from the probe. The results showed first, that the NO 2 /NO ratio calculated for the most rapid sampling procedure used experimentally is close to that measured but second, that the ratio is independent of the initial NO 2 /NO ratio assumed to be present in the flame. This indicates that the measured NO 2 concentration in the sample gas bears little relation to the NO 2 concentration in the flame. (2) A laser-resonance fluorescence technique was used to measure NO 2 in situ in the flame. The NO 2 was also measured simultaneously by the probe technique. The true concentration of NO 2 in the flame as indicated by fluorescence was considerably less than that recorded by the probe technique. It is concluded that the results of probe measurements as hitherto conducted do not reflect even approximately the true NO 2 /NO ratios in the flames; the true ratios are in fact much smaller than those indicated by the probe technique.