Effects of radiation reabsorption on the flame speed and NO emission of NH3/H2/air flames at various hydrogen ratios

Abstract

Ammonia has attracted extensive attention in the direct combustion applications due to its carbon–neutral properties. In this study, the numerical simulations of NH3/H2/air one-dimensional premixed laminar flame under the stoichiometric condition with different H2 ratios (η = XH2/(XNH3 + XH2) = 0.2–0.7) were conducted to explore the influence of radiation reabsorption on laminar flame speed and NO emission. Also the effect of radiation reabsorption on NO generation and emission in NH3/H2/air mixture when η = 0.6 with different equivalence ratios was discussed. The results showed that the radiation reabsorption effect on flame speed decreased monotonically with the H2 ratio increasing from 0.2 to 0.7, and the maximum effect was 17%. The effect on flame speed was regulated by the combination of direct radiation effect and chemical effect, which was influenced by reaction R257 NH2 + NO = NNH + OH via OH radical when η = 0.2–0.7 and ϕ = 1.0. The promotion effect of radiation reabsorption on NO generation and emission decreased monotonically with the increasing H2 proportion. The enhancement of NO generation caused by radiation reabsorption was mainly controlled by R222 N + OH<=>NO + H. On the other hand, R220 N + NO<=>N2 + O, R240 NH + NO = N2O + H and R242 NH + NO = N2 + OH affected the NO emission in the post-flame zone. At η = 0.6 and ϕ = 0.65–1.5, the promotion of NO generation by radiation reabsorption was non-linear, which was mainly affected by R257, R241 NH + NO = N2O + H and R222 reactions. The NO emission was mainly influenced by R241 and R233 in the post-flame zone at ϕ = 0.65–1.05. And R241, R242, R256 NH2 + NO = N2 + H2O and R257 in the post-flame zone dominated the NO emission when ϕ > 1.05.