Toward reliable high-temperature NH3-NO interaction kinetics: Oxygen-free measurements and evaluation of the NH2+NO branching ratio

Abstract

The interaction between NH₃ and NO plays a crucial role in controlling NOx formation during ammonia combustion. However, experimental data and mechanistic understanding under practical combustion temperatures and pressures remain limited. In this study, laminar burning velocities (LBVs) and ignition delay times (IDTs) of NH₃/NO mixtures under oxygen-free conditions were systematically investigated. Experiments were conducted using a combustion vessel and a shock tube, covering a broad range of temperatures (1500–2800 K), pressures (1.5–10 atm), and equivalence ratios (ϕ = 1.0–2.0). Key findings reveal that LBVs peak at 31 cm/s (ϕ = 1.4, 373 K, 1 atm) but decrease significantly with pressure (24 cm/s at 5 atm). IDTs shorten abruptly above 1630 K at 10 atm (<100 μs). Kinetic analyses demonstrate that the reactions NH₂ + NO → Products plays a dominant role in both flame propagation and ignition over the studied temperature range, due to their natures of chain branching and chain termination, with an even more pronounced influence during ignition. The pressure-dependent reaction NNH + M = N₂ + H + M exhibits a significant pressure-dependent effect on flame propagation but has almost no influence on ignition. Through a careful evaluation of the branching ratio between NH₂ + NO = NNH + OH and NH₂ + NO = N₂ + H₂O, the kinetic mechanism was optimized using the rate constants recommended by Klippenstein [Proc. Combust. Inst. 36 (2017) 77–111], enabling the developed kinetic model to reasonably predict the newly obtained experimental data. These findings reveal the reaction mechanism governing the interactions between NH₃ and NO, providing a solid theoretical foundation for achieving low-NOx ammonia combustion under practical operating conditions.

Novelty and significance statement: Understanding the interaction between NH₃ and NO is essential for advancing low-NOx ammonia combustion technologies, especially under fuel-rich conditions where NH₃–NO reactions play a crucial role in NOx reduction. This work systematically investigates the interaction between NH₃ and NO during ignition and flame propagation across a wide range of temperatures and pressures, achieving the first implementation of oxygen-free NH₃/NO ignition experiments in a shock tube. The results demonstrate the dominant role of the two competing reactions NH₂ + NO = NNH + OH and NH₂ + NO = N₂ + H₂O in both ignition and flame propagation characteristics due to their roles as chain branching and chain termination reactions, respectively. These measurements establish highly sensitive validation targets for its kinetic chemistry. The updated kinetic model exhibits outstanding predictive performance across broad combustion regimes, offering critical insights into NOx-reducing ammonia combustion and providing foundational support for ammonia combustion applications.