Chemical kinetics and numerical simulation of NO emission characteristics in CH4/NH3/air flame

Abstract

Methane–ammonia fuel has emerged as a prominent focus in the realms of combustion and power generation, drawing considerable attention. Despite numerous experiments and simulations, the adoption of large-scale detailed mechanisms poses challenges for computational fluid dynamics (CFD) simulations in predicting pollutant emissions accurately. Moreover, the distinctive high NO emission associated with ammonia-based fuels remains a critical concern. This study addresses these issues by reducing three detailed mechanisms and delving into the chemical kinetic processes of the resulting reduced mechanism concerning laminar burning velocity and pollutant emissions. A micro gas turbine model was employed for a three-dimensional simulation of emission control. To enhance predictive capabilities, one-dimensional simulation was utilized to calculate the laminar burning velocity and species concentration of CH₄/NH₃/air flame. The rate of production (ROP) analysis and sensitivity analysis were employed to investigate the impact of radicals and crucial reactions on NO emission. This study explored the characteristics of premixed and non-premixed combustion modes in the micro gas turbine model. Additionally, a strategy involving air-staging injection to mitigate NO emissions was proposed. The findings highlight the superior predictive performance of the reduced mechanism (Okafor 26 species) in both laminar burning velocity and NO emission. Under fuel-rich conditions, the H and OH radical pool significantly influences NO formation, with the optimized H + CH₃(+M) = CH₄(+M) revealing more consistent predictions. Non-premixed combustion yields higher NO emissions when Φ < 0.8, while premixed combustion prevails at Φ = 0.8–1.2. The spatial non-uniformity of the local equivalence ratio of CH₄/NH₃/air swirl flame leads to NO generation in regions with lower local equivalence ratios. Notably, employing air-staging injection at Φ_overall = 0.96 effectively reduces NO emissions from 3879 ppm to 2132 ppm.