New insights into the NH3/N2O/Ar system: Key steps in N2O evolution

Abstract

Understanding the formation and evolution of N2O in NH3-fueled combustion has implications for pollutant control and for developing a more accurate kinetic model of NH3. A previous study on an NH3/N2O/N2 mixture showed a significant discrepancy between experimental measurements and modeling predictions. The importance of the highlighted reaction NH2 + N2O = N2H2 + NO, whose rate constant remains substantially uncertain, is still an open question. In the present work, two NH3/N2O/Ar mixtures, referred to as fuel-lean and fuel-rich cases, respectively, were experimentally investigated using JSRs over 800–1225 K under atmospheric pressure. Various diagnostic techniques, including synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS), gas chromatography (GC), and Fourier transform infrared spectroscopy (FTIR), were employed to detect the detailed product species. The key reaction NH2 + N2O was calculated for the first time, and the kinetic model was updated based on the theoretical calculation and interpretation of the experimental observations. The results showed that the rate constants of NxHy + N2O reactions, including NH2 + N2O and NH + N2O, were significantly overestimated in previous models. This results in the non-dominant role for this reaction class, even in the scenario where N2O appears in large amounts. On the contrary, the thermal dissociation of N2O (N2O (+M) = N2 + O (+M)) controlled the reactivity of the title system due to its chain-initiated role that triggered the consumption of NH3. The fate of amine radical (NH2) was critical to determine the nitrogen chemistry in this system. Instead of being oxidated, NH2 was converted mainly via amine-amine steps, which implied that the title system had particular implications for understanding the NH3 pyrolysis or oxidation under fuel-rich conditions.