Combustion of ammonia supported by oxygen, nitrous oxide or nitric oxide: Laminar flame propagation at low pressures in binary mixtures

Abstract

Measurements of burning velocity in binary mixtures of ammonia with oxygen, nitrous oxide and nitric oxide have been made at sub-atmospheric pressures. Mixtures were ignited in a closed vessel and flame speeds (S B) measured by schlieren photography with a drum camera. The stoichiometry of reaction and the effects upon the flame speed of (a) variation in the mixture composition over the whole range of flammability, (b) variation in the initial reactant pressure and (c) addition of inert diluents (helium, argon and nitrogen) have been investigated. Composition limits of flammability were also recorded. In each system, combustion of a balanced mixture led to nitrogen and water, very little nitric oxide being found. In rich mixtures, the excess ammonia decomposed and in lean mixtures, the excess nitrous or nitric oxide. Maximum burning velocities occurred at or near stoichiometric proportions except in the nitric oxide system, where a rich mixture burned fastest. (Parallel behaviour was found in hydrazine flames.) Burning velocities were not significantly affected by total pressure, in concordance with an overall kinetic expression of the second order. To obtain flame speeds relative to unburnt gases (S U), the adiabatic flame temperatures and the equilibrium compositions of the hot products were calculated for mixtures corresponding to (i) complete combustion and (ii) maximum burning velocity. Flames supported by oxygen burned fastest; flames supported by nitric oxide were hottest but burned least rapidly. This order is in accord with the known relative reactivities in low temperature reactions and the same pattern of speeds was repeated in the order of diminishing resistance to quenching by inert diluents: O 2 > N 2O > NO and in the width of the composition limits of flammability.