An experimental, theoretical, and kinetic modeling study of post-flame oxidation of ammonia

The post -flame oxidation rate of ammonia was investigated in a novel atmospheric pressure flow reactor at temperatures of 1280 +/- 16 K and as a function of residence time and mixture composition (1-10% O2, dry and moist). The experimental results, as well as selected data from literature, were analyzed using an updated detailed chemical kinetic model. The medium temperature, very lean conditions enhance the importance of reactions of the nitroxyl (HNO) intermediate. High-level theory was used to calculate the rate constant for HNO + NH2, indicating that this step is significantly faster than values used in literature. Furthermore, a trajectory based approach was used to determine collision efficiencies for selected bath gases for HNO + M. The experimental results show that the NH3 oxidation rate increases with temperature and O2 concentration, while the presence of water vapor slightly inhibits reaction. Formation of NO and N2O was strongly promoted at higher levels of O2. Modeling results agreed well with the measurements, except at the lowest level of O2. The predicted oxidation rate of NH3 was shown to result from a delicate balance between chain branching and terminating steps involving NH2, H2NO, and HNO. Recent theoretical work on reactions of these species by Klippenstein and coworkers and Stagni et al. was instrumental in improving modeling predictions. After initiation, NO reached a pseudo -steady-state level, where the pathways to NO were largely balanced by the NH2 + NO reaction. Nitric oxide was partly oxidized to NO2, with the NH2 + NO2 reaction responsible for most of the N2O formation. Novelty and significance statement: This study provides the first detailed kinetic analysis of the lean postflame oxidation of ammonia, based on time -resolved flow reactor data in a novel reactor. In addition to the post -flame oxidation rate of ammonia, data for formation of NO and N2O were compared with modeling predictions. The medium temperature, very lean conditions enhance the importance of reactions of the HNO and H2NO intermediates. Inclusion in the model of results from recent high-level theoretical work, including present calculations for HNO + NH2 and HNO + M, was crucial for capturing the observed behavior. It is argued that the post -initiation steady-state NH3 oxidation rates constitute important data for model validation, along with ignition delays and laminar flame speeds.