Combustion chemistry of ammonia/hydrogen mixtures: Jet-stirred reactor measurements and comprehensive kinetic modeling

To investigate the oxidation of ammonia (NH3)/hydrogen (H-2) mixtures at intermediate temperatures, this work has implemented jet-stirred reactor (JSR) oxidation experiments of NH3/H2 mixtures at atmospheric pressure and over 800-1280 K. The H-2 content in the NH3/H-2 mixtures is varied from zero to 70 vol% at equivalence ratios of 0.25 and 1.0. Species identification and quantification are achieved by using Fourier transform infrared (FTIR) spectroscopy. A kinetic model for pure NH3 and NH3/H-2 mixtures is also developed for this research, and validated against the present experimental data for pure NH3 and NH3/H2 mixtures, as well as those for pure NH3, H2/NO, H-2/N2O, NH3/NO, NH3/NO2 and NH3/H-2 mixtures in literature. The model basically captures the experimental data obtained here, as well as in literature. Both measured and predicted results from this work show that H-2 blending enhances the oxidation reactivity of NH3. Based on the model analysis, under the present experimental conditions, NH3 + H = NH2 + H-2 proceeds in its reverse direction with increasing H-2 content. The H atom produced is able to combine with O-2 to produce either O and OH via a chain-branching reaction, or to yield HO2 through a chain propagation reaction. HO2 is an important radical under the present intermediate-temperature conditions, which can convert NH2 to OH via NH2 + HO2 = H2NO + OH; H2NO is then able to convert H to NH2 and OH. In this reaction sequence, NH2 and H2NO are chain carriers, converting HO2 and H to two OH radicals. Since the OH radical is the dominant radical to consume NH3 under the present conditions, the enhanced OH yield via H + O-2 = O + OH, NH2 + HO2 = H2NO + OH and H2NO + H = NH2 + OH, with increasing H-2 content, promotes the consumption of NH3. For NOx formation, non-monotonous trends are observed by increasing the content of H-2 at the 99% conversion of NH3. These trends are determined by the competition between the dilution effects and the chemical effects of H-2 addition. Nitrogen related radicals, such as NH2, NH and N, decrease as H-2 increases, and this dilution effect reduces NOx formation. For chemical effects, the yields of oxygenated radicals, such as O, OH and HO2, are enhanced with increasing H-2 content, which results in enhancing effects on NO formation. For N2O formation, the enhanced oxygenated radicals (O, OH and HO2) suppress its formation, while the enhanced NO promotes its formation. (c) 2021 The Combustion Institute. Published by Elsevier Inc. All rights reserved.