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 between 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 NO_x/C₁ and NH₃/C₁ fuels were selected from literature to test the present model, including speciation data measured in shock tubes, flow reactors, 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 NO_x/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.