Combustion chemistry of ammonia/C1 fuels: A comprehensive kinetic modeling study

The application of ammonia (NH3) as a fuel could contribute to mitigating global warming and achieving carbon neutrality by mid-century. To enhance the reactivity of NH3 combustion, cofiring NH3 with hydrogen or C1 fuels like syngas, methanol and methane is proposed. In previous work, we studied the combustion chemistry of NH3/ H2 mixtures using a comprehensive kinetic model. In this work, we expanded our comprehensive kinetic model to cover NH3/syngas, NH3/methanol and NH3/methane mixtures. Rate constants of critical cross reactions be- tween C- and N-containing species were evaluated based on previous experimental and theoretical studies for selection and incorporation in the present model. Experimental data for both NOx/C1 and NH3/C1 fuels were selected from literature to test the present model, including speciation data measured in shock tubes, flow re- actors, jet-stirred reactors, burner-stabilized flames, and the global parameters like ignition delay times and laminar flame speeds. The kinetic model validation conditions cover temperatures of 473-2000 K, pressures of 0.04-100 atm, and equivalence ratios of 0.04-116. In general, the present model adequately captures the selected experimental data. The present model can not only be used to predict the combustion of NH3/C1 fuel mixtures, but is also capable to predict the mutual interaction of NOx/C1 fuels. It is found that the rate constants between C-containing species and H2-related radicals are generally faster than (or close to) those between C- and N-containing species. At high temperatures, H2-related radicals (i.e. H, O, OH) have higher concentrations than reactive N-related radicals (NH, NH2, NO2). Therefore, under these conditions, C-containing species are more likely to react with H2-related radicals rather than N-related ones, making the C/N cross reactions less prominent. However, under low- and intermediate-temperature conditions, the concentrations of H and O radicals become lower, while those of NH2 and NO2 become higher, making the cross reactions between C- and N-containing species possible to compete with C/H cross reactions. The present model can be used as the base chemistry model for NH3/larger hydrocarbon fuels.