A reduced kinetic mechanism for ammonia/hydrogen mixtures with alleviated stiffness and high accuracy tailored for high-fidelity numerical simulations

Abstract

Co-firing ammonia with hydrogen is recognized as a promising energy strategy for achieving a carbon-neutral and sustainable future. It offers potential advantages over the combustion of pure hydrogen/air or pure ammonia/air individually in practical applications. High-fidelity simulations, such as Direct Numerical Simulations and Large-Eddy Simulations are essential for a better understanding of ammonia/hydrogen flames. Such simulations require the availability of an affordable but accurate kinetic mechanism with alleviated stiffness. In this study, a reduced kinetic mechanism comprising 17 transported species, 10 quasi-steady-state species, and 180 reactions was developed, derived from a comprehensive mechanism (the NUIG-2023 mechanism with 39 species and 306 reactions) while maintaining high accuracy across a wide range of validation cases. The reduced mechanism was extensively compared with the original one and with experimental datasets for predictions of ignition delay times, laminar flame speeds, species mole fraction profiles, and S-curves. Moreover, the obtained integrated fuel consumption rates and species profiles of NO, NO₂, and N₂O were compared for hydrogen-ammonia stratification. The analysis has been repeated for a variety of ammonia/hydrogen mixtures. As a result, the reduced mechanism demonstrated excellent capability to accurately replicate the fundamental combustion characteristics of ammonia/hydrogen flames for a wide range of conditions. The calculation time using the reduced mechanism in the simulations of stratified flame with an explicit time integration is approximately 5.5 times faster than that of the full NUIG-2023 mechanism, which requires additionally an implicit scheme to handle chemical stiffness. This work paves the way for future investigations of NH₃/H₂ flames by DNS and LES in practically relevant configurations.