A Chemkin Modelling Study of the Ignition and Combustion Characteristics of NH₃ in Air in the Presence of H₂

NH3 has been proposed as a hydrogen carrier and clean, zero-carbon transport fuel for combustion in spark ignition engines. However, NH3 as a fuel suffers from several drawbacks including high auto-ignition temperature, low flame propagation speed, and narrow flammability limits. The Centre for Energy at The University of Western Australia, in collaboration with National Institute of Clean-and-low-carbon Energy of China, is developing an innovative technology to partially dissociate NH3 in-situ to produce NH3/H2/N2 mixtures before the mixed fuel is injected into the engine cylinder. The presence of H2 in the mixed fuel is expected to improve the ignition quality and combustion performance of NH3. Using CHEMKIN modelling this study examined the ignition and combustion characteristics of NH3 in the presence of H2. The 1D steady-state laminar adiabatic flame and homogenous closed reactor models were used to predict the laminar flame speed, flame structure, ignition delay time, and adiabatic flame temperature. A sensitivity analysis was also performed to identify the key elementary reactions determining the role of H2 in promoting ignition and combustion. The results showed that increasing H2 concentration in the fuel mixture reduced the ignition delay time and increased the laminar flame speed significantly. It was found that the presence of H2 in the fuel enhanced the production of key radicals including OH, H, O, NH2, and HNO during ignition and combustion of NH3, thus promoting the ignition and accelerating the flame speed.