Effect of trace amount of Nitric Oxide (NO) addition on ammonia autoignition in a rapid compression machine

Nitric oxide (NO) is an important product and a major intermediate species in ammonia combustion. This paper investigates the impact of NO on ammonia oxidation in a rapid compression machine. Ignition delay times are measured under stoichiometric and lean (φ = 0.5) conditions with 0 to 2000 ppm NO addition at 25 - 40 bar and 1080 - 1200 K. Due to a small fraction of NO converted to nitrogen dioxide (NO 2 ) during the reactant preparation, the investigation can be considered as simultaneous doping with NO and NO 2 where the NO 2 can be quantified using simulation. The addition of NO (and the converted NO 2 ) consistently promotes ammonia autoignition where the ignition delay can be reduced by up to a factor of 6. This NO promotion behaviour was consistent across the pressures investigated. A kinetic model was developed using recent updates on elementary reactions, which improves the simulation compared to existing models. Analysis using the developed model indicated that NO addition enhances the HO 2 production via reaction H 2 NO + O 2 = HNO + HO 2 , and HNO + O 2 = NO + HO 2 , which provide an abundant supply of HO 2 for reaction NO + HO 2 = NO 2 + OH and keep high OH production all the way to autoignition. The impact of pure NO is also isolated using simulation. The results indicate that NH 3 is unique in that higher NO invariably leads to shorter ignition delay which does not show the saturation effect as previously reported for hydrocarbon/NO and H 2 /NO cases.