A compact kinetic reaction mechanism for NH3/H2 flames

Ammonia (NH3) has been considered a potential fuel for energy production to achieve zero carbon emissions. However, several challenges must be addressed to ensure its widespread use and safety. The current work focuses on developing a kinetic reaction mechanism that not only accurately predicts laminar flame speeds and the emissions from NH3 and NH3/H2 flames across various conditions but also ensures seamless applicability in Computational Fluid Dynamics (CFD) simulations, particularly in scenarios involving turbulent flows, such as swirl burners or complex engine chamber conditions. Using code Optima++, the rate parameters of the San Diego NH3 mechanism (only 21 species and 64 reactions) were optimised against a large collection of laminar burning velocity data, and concentration data measured in jet-stirred reactors and burner-stabilised stagnation flame experiments to develop a compact, yet robust model for CFD simulations. Due to its small size, the mechanism lacks important chemical pathways, so the requirement for physically realistic rate coefficients had to be sacrificed in order to achieve the best possible predictivity for practical applications. The mechanism has been tested for 70/30 vol% NH3/H2 mixtures in CFD simulations of a general swirl burner against experimentally measured concentrations. Its predictions demonstrated good qualitative and often quantitative agreement with the experimental data for NO, N2O, and NO2 emissions, and NH3 slip in the whole equivalence ratio range, while allowing accelerated simulations compared to other leading mechanisms.