Impact of third-body colliders on ammonia pyrolysis and oxidation: Detailed kinetic modeling and mechanistic insights

A major challenge in the chemical kinetics of ammonia is the quantification of the role of the bath gas in pressure-dependent reactions, in the full operating space. To this purpose, this work systematically investigates the impact of third-body colliders on ammonia and ammonia/hydrogen pyrolysis and oxidation chemistry, through an integrated workflow: after incorporating recent high-level theoretical calculations into a comprehensive detailed kinetic model, the key pressure-dependent reaction rates were parametrized through a fitting procedure, obtaining average errors below 3% compared to the starting theoretical values, while explicitly accounting for collider-specific behavior. Validation against experimental data highlighted the impact of major colliders on ignition delay times, species profiles, and laminar flame propagation. It was found that recombination reactions involving NH3, HNO, and HO2 are significantly affected by the bath gas composition, including ammonia itself as a collider, which is often ignored in most kinetic models. Species profiles in both pyrolysis and oxidation conditions showed significant sensitivity to the collider-specific effects: specifically, ammonia third-body effect in the recombination reaction H+O2(+M)→HO2(+M) was found to play a major role in the inhibition of H2 oxidation, confirming the previous hypotheses. On the other hand, laminar flame speeds exhibited a lower sensitivity, with deviations typically within experimental uncertainties. Finally, the impact of mixture rules in the kinetic predictions was assessed by considering the four pressure-dependent reactions for which theoretical data on ammonia-related collision efficiencies are currently available. It was found that adopting a more accurate reduced-pressure mixture rule instead of a linear mixing, important deviations in the pressure-dependent rate constants at higher pressures were observed, yet with a moderate effect on macroscopic observables like ignition delay time and laminar flame speeds.