Investigating auto-ignition characteristics and kinetic modeling of NH3/ CH4 mixtures using an RCM

Ammonia (NH3) has shown promise as a carbon-neutral fuel, and blending NH3 with methane (CH4) can improve its combustion characteristics. This study investigated the auto-ignition characteristics and kinetic modeling of NH3/CH4 mixtures using a rapid compression machine (RCM). Ignition delay times were measured under various conditions, including three different NH3/CH4 ratios, pressures of 15 and 25 bar, temperatures ranging from 910 to 1272 K, and equivalence ratios of 0.5 and 1.0. The results showed the addition of CH4 significantly promoted NH3 ignition. An intriguing phenomenon of pre-ignition pressure rise was noted in blends containing 25 % CH4, which cannot be replicated by previous models. Time-resolved species measurements were conducted using a fast sampling system coupled with gas chromatography. The measurements revealed distinct early consumption of CH4 compared to NH3 in blends exhibiting pre-ignition pressure rise. A kinetic model was developed for NH3/ CH4 combustion with emphasis on key "C-N" reaction pathways constrained by experimental results. Sensitivity and reaction path analyses highlighted the significance of these "C-N" reactions in determining the varying ignition characteristics. Additional simulations systematically assessed factors influencing auto-ignition behavior in NH3/CH4 mixtures. Notably, CH4 concentration emerged as the primary parameter affecting the ignition behavior. Lower CH4 concentrations led to NH3 chemistry primarily controlling the overall ignition process, which could cause early CH4 oxidation and potential gradual pre-ignition pressure rise. Conversely, higher CH4 content facilitated its oxidation to trigger the oxidation of NH3, thereby narrowing the time gap between the initiation of consumption for the two fuels.