(379d) An Updated Chemical Kinetic Model of NH3 and NH3/H2 Oxidation Using Reaction Mechanism Generator (RMG)

Due to the pressing need for global carbon neutrality and the mitigation of climate change, ammonia has emerged as a viable renewable energy carrier and a potential substitute for traditional fossil fuels in propulsion and power generation systems. However, owing to the suboptimal ignition characteristics and high NOx emissions, its blending with H₂ has been found to be efficient for technical applications. Although the chemical reaction mechanisms of pure NH₃ have been explored widely, large discrepancies and ambiguous sources still persist in thermo-kinetic parameters across literature models, hindering comprehensive model prediction for experimental measurements over a wide range of conditions. Our study compares all species thermodynamic properties and reactions which rate coefficients are in significant disagreement in the recent literature models. In particular, the difference in ΔG⁰_f of several N-species such as N₂H₃ at 1000 K can exceed 50 kJ/mol, and the rate constant of primary fuel oxidation reaction (NH₃+O₂=NH₂+HO₂) exhibits a discrepancy of up to ~ 4 orders of magnitude at 1000 K between literature models published in the last five years. After rigorous evaluation on the thermo-kinetic parameters, we provide the available recommended values or conduct theoretical calculations at the CCSD(T)-F12/cc-pVTZ-F12 level of theory. Based on the updated kinetics and thermodynamics database, we use the Reaction Mechanism Generator (RMG) software to generate a new comprehensive NH₃ and NH₃/H₂ oxidation model, which is thoroughly benchmarked against literature experimental observations of pure NH₃ and NH₃/H₂ mixtures. The generated kinetic model has better or comparable prediction performance without any modifications nor fittings in the thermo-kinetic parameters, while the prediction of NOx and N₂O formation still needs to be refined. Such a model facilities a reasonable interpretation for the reported data, revealing that H₂ addition can boost NH₃ reactivity at different conditions. From the chemical effect perspective, this feature is primarily due to the enhanced production of reactive radicals such as H, OH and HO₂ as H₂ is added. However, H₂ addition to NH₃ oxidation may elevate NOx emissions compared to pure NH₃ oxidation, which can be attributed to the more competitive NO formation than DeNOx-related reactions involving the aforementioned reactive radicals.