Challenges in Kinetic modeling of ammonia pyrolysis

Ammonia pyrolysis reactions have implications for its ignition and oxidation in engines and gas turbines. In the present work, the chemistry of ammonia pyrolysis is investigated by kinetic modeling and theory. Rate constants for key reactions are carefully evaluated based on available experimental and theoretical results. The high pressure limit k${}_{1,\infty }$ for NH${}_2$ + H (+M) $\rightleftarrows$ NH${}_3$ (+M) (R1) is calculated to be essentially the collision frequency, indicating that dissociation of ammonia in combustion processes will be at or close to the low pressure limit even at engine and gas turbine conditions. The chemical kinetic model is validated against reported shock tube measurements of NH${}_3$, NH${}_2$, and NH in ammonia pyrolysis. Predictions are in good agreement with observations for dilute conditions ($\le$ 0.5% NH${}_3$), but the model appears to underpredict the NH${}_3$ consumption rate at longer times in less dilute mixtures. At short reaction times, thermal dissociation of NH${}_3$, together with the NH${}_3$ + H reaction, controls conversion. At longer times, secondary reactions involving NH${}_2$ and NH become important due to their impact on the radical pool. Predictions become sensitive to formation and consumption of diazene (tHNNH and cHNNH). Several of the key steps in the ammonia pyrolysis mechanism are radical-radical reactions that are difficult to measure accurately and challenging to calculate theoretically, and a more comprehensive experimental characterization is desirable to support further model development.