Effects of methane addition on flame temperature, equilibrium products and thermodynamic properties of ammonia-air combustion

Abstract

This study presents a comprehensive thermodynamic and numerical equilibrium analysis of ammonia-methane-air combustion across a wide range of conditions, including equivalence ratios (phi = 0.3-2.0), pressures (1-60 atm), and unburned mixture temperatures (300-800 K). The work investigates how methane blending and equivalence ratio control influences the equilibrium combustion characteristics of ammonia-a carbon-free but kinetically limited fuel-using a validated equilibrium model benchmarked against NASA CEA and GASEQ. Results reveal that methane addition consistently increases adiabatic flame temperature and specific heat capacity, enhancing energy release and combustion intensity. However, this comes at the cost of higher thermal NO formation due to elevated post-flame temperatures. In contrast, ammonia-rich blends yield greater H2O and entropy while suppressing CO2 emissions and radiative heat transfer, owing to their lower flame temperatures and product molar mass. The analysis shows that product composition, thermodynamic properties, and dissociation behavior are highly sensitive to equivalence ratio, pressure, and reactant temperature. Entropy increases monotonically with equivalence ratio due to greater product diversity in fuel-rich regimes, even as internal energy and enthalpy decline. The equilibrium composition transitions smoothly from complete oxidation products (CO2, H2O) to dissociation-dominated species (CO, H2, radicals) under rich conditions. These findings establish a thermodynamic foundation for optimizing ammonia-methane blends in low-carbon combustion systems, offering critical insight into energy release, product formation, and emissions-related behavior. The results support the development of advanced fuel strategies for engines, turbines, and industrial burners targeting cleaner energy solutions.