Roles of NH2 reactions in ammonia oxidation at intermediate temperatures: Experiments and chemical kinetic modeling

Abstract

In recent years, a number of experimental and numerical studies on chemical kinetics of ammonia (NH3) have been performed and NH3 chemical kinetic models have been proposed. However, as NH3 oxidation at intermediate temperatures (below 1400 K) has been investigated by reactor experiments, discrepancies between measurements and model-prediction have been reported. In addition, NH3 oxidation in extremely fuel-rich conditions has not been well studied. This study aims to further explore the chemical kinetics of NH3 oxidation at intermediate temperatures using a micro flow reactor with a controlled temperature profile (MFR). NH3 and H2 in the oxidation of NH3/O2/Ar mixtures at atmospheric pressure were measured using a gas chromatograph and a quadrupole mass spectrometer connected to the MFR. The maximum wall temperatures were varied in the range of 1100–1400 K, and six equivalence ratios (ϕ = 0.5–4.0) were examined. An updated reaction model for NH3 was developed by placing special emphasis on NH2 reactions. The revised model satisfactorily reproduced the experimental and literature data. Reaction analysis revealed that the main channel of the NH2 + HO2 reaction is chain-terminating, as proposed by Klippenstein et al., which was critically important to reproduce the measured profiles of NH3 in the MFR. Furthermore, the contribution of NH2 + NH2 = N2H2 + H2 to H2 production showed large differences among the models because of large uncertainty of its rate constant. Models that used larger rate constants for this reaction overestimated H2 production. Novelty and significance statementThis study addresses unrevealed intermediate temperature reactions in NH3 oxidation where large discrepancies between experiments and model predictions as well as among models have been reported. Species profiles of NH3 and H2 in NH3 oxidation at intermediate temperatures (1100–1400 K) over a wide range of equivalence ratio (0.5–4.0) was obtained using a micro flow reactor with a controlled temperature profile. The model developed in the present study (adopted the rate constants of NH2 reactions in the latest literatures) satisfactorily reproduced the experimental results as well as literature data. Branching reactions of NH2 + + HO2 and NH2 + NH2 are found to be crucial for the NH3 oxidation and H2 production at intermediate temperatures, which would enhance further improvement of the ammonia chemical kinetic model and moreover, help control ammonia combustion.