Extinction limit and NO/N2O formation of methanol/ammonia counterflow diffusion flames under elevated conditions

To address ammonia’s low reactivity, the interest in co-firing with methanol, a conventional green transportation fuel in internal combustion engines, is steadily growing. However, the effects of methanol on ammonia flame extinction and NO_x are unknown. In this study, we examine the influence of methanol addition on ammonia flame stability under elevated conditions, based on the counterflow diffusion flame via numerical simulations using detailed chemical kinetics and chemical explosive mode analysis. Our analysis shows that methanol addition significantly extends extinction strain rate and introduces a two-stage explosive mode structure under high strain rate conditions. At 1 atm, a small methanol addition (energy fraction of CH₃OH in fuels E_CH3OH=5%) increases the extinction strain rate by 13%, and with E_CH3OH = 50%, the extinction strain rate is nearly 4.75 times higher than that of pure ammonia. This is primarily attributed to the enhanced OH radical production, which shows a strong positive correlation with flame extinction limits. Moreover, elevated pressures and oxidizer preheating further expands the extinction strain rate. Pressure effects are primarily linked to increased local heat release rate despite lower radical mole fraction. Methanol has minimal influence on the pressure–extinction relationship. Furthermore, methanol blending slightly increases NO formation via enhanced HNO and fuel-N pathways but simultaneously reduces N₂O emissions due to accelerated radical-driven consumption. Methanol addition significantly alters the radical chemistry by introducing CH₂OH as a key intermediate, shifting OH formation from the conventional O₂ + H pathway to a new dominant route via O₂ + CH₂OH → HO₂ → OH. This work provides new insights into the extinction characteristics and nitrogen chemistry of NH₃/CH₃OH flames under realistic conditions, supporting the design of cleaner and more efficient ammonia-based energy systems.