Kinetic Modeling of the Oxidation of Ammonia in Flames

Abstract

We have investigated theoretically a variety of burner-stabilized and freely propagating NH₃/₂ and NH₃/H₂/₂. flames. The agreement between theory and experiment is generally good except, perhaps, for very rich flames, where it appears that the kinetic model may be missing important pyrolysis steps. Nevertheless, the important NO and N₂ formation reactions can be identified. In lean flames, nitric oxide is produced primarily through the nitroxyl (HNO intermediate, formed either by the reaction of NH₂. with oxygen atoms or NH with hydroxyl and oxygen molecules. Similarly, in lcan flames conversion of NO to N₂. is by reaction with NH₂ or NH, forming NNH or N₂.O as intermediates. Under richer conditions NH₂.and NH are rapidly converted to nitrogen atoms, and the extended Zel'dovich mechanism becomes responsible for NO formation and the conversion of NO to N₂. In addition to our analysis of the nitric oxide production and destruction paths, we find from a sensitivity analysis that the most important reaction in dutcrmining the flame speed under stoichiometric conditions is the chain branching reaction H | O₃.⇌2OH+0. Other properties of the stoichiometric NH₃-o₂ flame worthy of mention are (I) a local maximum in the temperature profile (higher than the adiabatic Rame temperature) occurs in the interior of the flame, and (2) the flame speed is almost independent of pressure From 0.1 to 10 atmospheres.