Radical Concentrations and Prompt NO Formation in Hydrocarbon-Air Premixed Flames

Hydroxyl radical concentrations in laminar, premixed CH4-air flat-flames were measured by a conventional absorption method in the flame temperature range T=2200–1700 K. The OH mole fractions in the reaction zone P OH were found to be insensitive to the flame temperature and the maximum P OH to be about 6×10-3 at T=2000 K in a stoichiometric flame. The rate constant for the prompt NO formation, CH+N2→HCN+N2NO (4), is modified as k 4=1.2 ×1012 exp (-13600/R T) by the relation between the amount of prompt NO and the saturation ion current I s. It is also shown from the measurements of NOx, unburnt hydrocarbons H.C., and I s, that the chemical kinetics are not significantly influenced by the flame temperature, and NO emission is reduced by reactions among NO, H.C., and N-containing intermediate species in low-temperature flames, even in stoichiometric and fuel-lean mixtures.