Nitrogen-origin-determination in NOᵪformation under ammonia/methane/air co-combustion using a nitrogen-tagged reaction model

Abstract

This paper proposes a chemical reaction mechanism using the nitrogen element-tracking method for ammonia combustion to obtain the fuel-origin nitric oxides formation characteristics under the co-combustion of ammonia/methane/air diffusion flames. The proposed mechanism was verified by comparing the reaction rates of nitric oxide, nitrous oxide of the element-tracking method with those of the original reaction mechanism (non-tracking). Subsequently, the effects of the mole fraction of ammonia, strain rate of the flow, and preheating temperature of the oxidizer on the emission characteristics of nitric oxides in co-combustion of ammonia/methane/air diffusion flame were investigated. The results showed that the emission index of fuel-originated nitric oxide (N⁕O) increased slightly, whereas that of oxidizer-originated nitric oxide (NO) decreased with an increase in the ammonia content in the fuel. The emission index of N⁕O overcame that of NO with an increase in the ammonia concentration in the fuel, and N⁕O became the dominant molecule in the emission of ammonia/methane co-combustion. Additionally, the reaction path analysis highlighted that a decrease in flame temperature under high strain rate conditions causes an increase in N⁕O formation as well as the suppression of N⁕₂O decomposition.