The coupling influence of chemical/physical effects of NH3 on the NO formation in the premixed CH4–NH3 flames

Abstract

The individual and coupling chemical/physical effects of NH3 addition on the NO formation of the CH4 premixed combustion process are investigated using the numerical simulation and the detailed chemical mechanism. The roles of chemical and physical effects of NH3 component in NO formation routes and major NO-related reactions of CH4–NH3 premixed flames are identified at different equivalence ratios (ϕ) and blending ratios (ηNH3). The results show that, with the increased NH3 addition, the continuously increased peak NO concentration at the fuel-lean condition is predominated by the chemical effects of NH3, while the variation of peak NO concentration at the fuel-rich condition can be ascribed to both chemical and physical effects of NH3. Additionally, for the pure CH4 fuel or the CH4/NH3 blends with the low NH3 addition, the NO/NO2 conversion process is always significant to the NO concentration variations at ϕ = 0.8. Furthermore, with the NH3 addition, the enhanced importance of the NO/NO2 conversion and the HNO route to the NO formation are attributed to the physical and chemical effects respectively at ϕ = 0.8. By contrast, with the NH3 addition at ϕ = 1.2, the NO formation is predominated by the HNO route, the thermal route and the NNH route, while their contribution variations are all dominated by chemical effects of NH3. Specifically, the HNO route becomes increasingly significant to the NO production, while the thermal route and the NNH route play the more paramount roles in the NO consumption in the CH4–NH3 flame at ϕ = 1.2. Besides, with the increased NH3 addition up to the medium level, the considerably decreased contribution of the thermal NO pathway is mainly attributed to physical effects of NH3, while the improved HNO pathway is undoubtedly dominated by chemical effects. Furthermore, the NO/NO2 conversion process always plays a significant role in the NO formation with the increased NH3 addition, and its contribution is increased initially due to physical effects but decreased subsequently thanks to chemical effects. In addition, it is also confirmed that the NO destruction can be accelerated by the increased NHi radicals with the NH3 addition, while the C–N kinetics could be important to the NO formations in the CH4–NH3 flame considering the certain contribution of CH3+HNONO + CH4 to the NO production.