Impact of nitric oxide (NO) on ammonia/hydrogen autoignition in a rapid compression machine

Nitric oxide (NO) is an important combustion by-product of ammonia and hydrogen and plays a major role in ammonia oxidation. This work investigates the impact of NO on the ignition delay time (IDT) of stoichiometric ammonia/hydrogen mixtures in a rapid compression machine. NH 3 /H 2 blends containing 5% and 10% hydrogen molar fractions of the fuel were studied with 0–2000 ppm NO addition at 12.5–40 bar and 930–1200 K. Due to NO–NO 2 conversion during mixture preparation, the results are effectively for the combined NO/NO 2 additions where the share of NO 2 is numerically determined and accounted for its impact on autoignition. The results show that NO produces a weak, non-monotonic impact on NH 3 /H 2 autoignition, where the mixture’s reactivity can even be lower than the case without NO addition. This impact is in drastic difference to what has been observed in NO/NH 3 , H 2 /NH 3 , and H 2 /NO systems, where a promotion effect is generally observed. A kinetic model was developed which reproduced the observed effects and compared favourably with existing models. Analysis using the developed model found that adding NO/NO 2 promotes NH 2 + NO x pathways, where both chain branching and chain termination reactions are involved, over the reaction with H 2 , via NH 2 + H 2 = NH 3 + H, which solely promotes NH 3 /H 2 autoignition. Furthermore, the branching ratio of the NH 2 + NO pathway favours the chain-terminating NH 2 + NO = N 2 + H 2 O, whereas that for the NH 2 + NO 2 pathway favours NH 2 + NO 2 = H 2 NO + NO that promotes OH formation and enhances the reactivity . By isolating the impact of pure NO (without NO 2 conversion) through modelling, it suggests that pure NO would be inhibitive for autoignition while the NO 2 fraction promotes the reactivity, with the combined effects resulting in the observed weak promotion effect.