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 NH 3 , HNO, and HO 2 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 + O 2 ( + M ) → HO 2 ( + M ) was found to play a major role in the inhibition of H 2 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. • Systematic characterization of third-body collider effects in ammonia combustion. • Impact of rate constant parametrization assessed on kinetic predictions. • Key role of ammonia itself as a collider in inhibiting H 2 oxidation. • Collider-specific rate constants significantly affects species formation. • Advanced mixture rules essential for accurate rate constants at higher pressures.