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

To investigate the oxidation of ammonia (NH 3 )/hydrogen (H 2 ) mixtures at intermediate temperatures, this work has implemented jet-stirred reactor (JSR) oxidation experiments of NH 3 /H 2 mixtures at atmospheric pressure and over 800-1280 K. The H 2 content in the NH 3 /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 NH 3 and NH 3 /H 2 mixtures is also developed for this research, and validated against the present experimental data for pure NH 3 and NH 3 /H 2 mixtures, as well as those for pure NH 3 , H 2 /NO, H 2 /N 2 O, NH 3 /NO, NH 3 /NO 2 and NH 3 /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 NH 3 . Based on the model analysis, under the present experimental conditions, NH 3 + H = NH 2 + 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 HO 2 through a chain-propagation reaction. HO 2 is an important radical under the present intermediate-temperature conditions, which can convert NH 2 to OH via NH 2 + HO 2 = H 2 NO + OH; H 2 NO is then able to convert H to NH 2 and OH. In this reaction sequence, NH 2 and H 2 NO are chain carriers, converting HO 2 and H to two OH radicals. Since the OH radical is the dominant radical to consume NH 3 under the present conditions, the enhanced OH yield via H + O 2 = O + OH, NH 2 + HO 2 = H 2 NO + OH and H 2 NO + H = NH 2 +OH, with increasing H 2 content, promotes the consumption of NH 3 . For NO x formation, non-monotonous trends are observed by increasing the content of H 2 at the 99% conversion of NH 3 . These trends are determined by the competition between the dilution effects and the chemical effects of H 2 addition. Nitrogen related radicals, such as NH 2 , NH and N, decrease as H 2 increases, and this dilution effect reduces NO x formation. For chemical effects, the yields of oxygenated radicals, such as O, OH and HO 2 , are enhanced with increasing H 2 content, which results in enhancing effects on NO formation. For N 2 O formation, the enhanced oxygenated radicals (O, OH and HO 2 ) suppress its formation, while the enhanced NO promotes its formation.