A critical review of NH3–H2 combustion mechanisms

NH3 is a good carrier of H2 and does not emit CO2 during combustion; thus, it is attracting attention as a renewable energy carrier with the goal of reducing greenhouse gas emissions and global carbon neutrality. For practical applications, blending with H2 has been found to be effective in improving the shortcomings of the low reactivity and high NOx emissions of NH3. Therefore, NH3–H2 is a promising fuel for application. NH3–H2 combustion mechanisms are relevant for investigating N transfer pathways, optimizing operating parameters, and promoting the development of NH3–H2 combustion. Unfortunately, current mechanisms have weaknesses, such as unclear key reactions and inaccurate numerical simulations, and still need to be further optimized in under more comprehensive conditions. This review summarizes various mechanisms and the fundamental kinetic studies proposed in recent years for NH3–H2 combustion. Subsequently, the prediction of laminar burning velocity, ignition delay time, combustion species, and NOx emissions is reviewed. In particular, critical sensitive reactions that have a generalized impact on predicted results are summarized, suggesting that optimizing the kinetic parameters of key intermediate (N2Hx, H2NO) reactions may be the development direction for more accurate numerical simulations in the future. Finally, this review summarizes the trends and challenges for future optimization of NH3–H2 mechanisms, emphasizing the need for theory to support the kinetic coefficients of key low-temperature reactions and for more experimental data to compare predicted NOx.