Effects of methane addition on ammonia flame heat release characteristics

Abstract

Blending ammonia with methane for combustion is a promising low-carbon strategy that overcomes the inherently low reactivity and slow flame speed of ammonia. In this work, a reduced mechanism for ammonia/methane combustion is employed, and the fundamental combustion characteristics of premixed ammonia/methane/air flames are investigated via numerical simulations, with a focus on heat release, flame speed, and NO formation under various blending ratios and initial pressures. The results indicate that the addition of methane significantly enhances flame propagation and thermal characteristics. The laminar flame speed, adiabatic flame temperature, and peak net heat release rate all increase monotonically with an increasing methane blending ratio. Sensitivity analysis reveals that the elementary reaction H + O₂O + OH is dominant under all investigated conditions. An increase in initial pressure reduces the laminar burning velocity by promoting pressure-dependent chain-termination reactions. The NO concentration exhibits a non-monotonic behavior with methane addition, reaching its peak at a methane blending ratio of approximately 0.5–0.7. Rate of production analysis shows that whereas excessive amounts or fuel-rich conditions promote radical-driven NO reduction, leading to a decrease in net NO. These findings provide fundamental insights into the chemical kinetics governing NH₃/CH₄ co-firing and offer guidance for optimizing low-emission combustion systems.