A comprehensive evaluation of kinetic reaction mechanisms for NO and N2O emissions during NH3/H2 combustion across multi-temperature regimes

Abstract

Ammonia is an alternative green fuel to achieve carbon neutrality. However, the emissions of N₂O and NO_x are the major challenges for broad applications. Understanding N₂O and NO_x formation and reduction at various temperatures is crucial for developing ammonia combustion technologies in equipment across multi-temperature regimes. This study comprehensively investigates the performance of 12 kinetic reaction mechanisms in predicting N₂O and NO emissions at various temperatures. A significant variation is observed among different kinetic reaction mechanisms in the prediction of N₂O and NO emissions across temperatures. The kinetic reaction mechanisms proposed by Zhang, Nakamura and Okafor exhibit the highest predictive accuracy at low, medium and high temperatures, respectively. Based on sensitivity analysis, under all conditions and kinetic reaction mechanisms, the reaction H + O₂<=>O + OH consistently exhibits high positive sensitivity coefficients for the mole fractions of N₂O and NO. Meanwhile, the reactions N₂O + H<=>N₂+OH and NH₂+NO<=>N₂+H₂O exhibit large negative sensitivity coefficients for the mole fractions of N₂O and NO, respectively. In addition, N₂O and NO are primarily produced by NO and HNO via the reactions NH + NO<=>N₂O + H and HNO + OH<=>NO + H₂O, HNO + O₂<=>NO + HO₂, respectively, while N₂O is primarily consumed via reactions with O and H radicals. Notably, the radicals NH and NH₂ play critical roles in the emissions of N₂O and NO. Furthermore, the sensitivity coefficients of key reactions vary significantly across different kinetic reaction mechanisms, highlighting the need for further investigation. This study provides valuable insights and a foundation for future development and refinement of ammonia combustion kinetic mechanisms.