Quenching distance, wall heat flux and CO/NO thermochemical states in the wall vicinity of laminar premixed biogas-hydrogen impinging flame

Abstract

The quenching distance, wall heat flux and pollutant thermochemical states of the laminar premixed biogas-hydrogen impinging flame at ϕ = 0.8, 1.0 and 1.2 were investigated using the numerical simulation with detailed chemical kinetics and transport parameters. With the 10% H2 addition, the biogas composed of 75% CH4 and 25% CO2 was used as fuel in the study. Results show that PeQ ≈5 is obtained for the laminar premixed biogas-hydrogen impinging flame at ϕ = 0.8, 1.0 and 1.2. The smaller PeQ than that of CH4 flame is resulted from the CO2 increasing the flame thickness, as well as the decelerated flame propagation due to the strong stretch effects. The quenching distance of biogas-hydrogen flame is codetermined by intrinsic characteristics of heat transfer process and effects of equivalence ratio and flame stretch on the flame propagation. The wall heat flux of laminar premixed biogas-hydrogen impinging flame is maximum at ϕ = 1.0, followed by ϕ = 1.2 and 0.8. At ϕ = 0.8 and 1.2, the location difference between the quenching point and peak wall heat flux of premixed biogas-hydrogen impinging flame is ascribed to the oxidization of escaped unburned fuel near the wall, and the worse field synergy of temperatue and velocity near the quenching point. Additionally, it is found that the fast NO accumulation near the wall at ϕ = 0.8 is ascribed to the transportation of NO generated by NNH route and the effective prompt NO production, while that at ϕ = 1.2 is resulted from the prompt NO production primarily. In the wall vicinity, the low temperature shift of NO production are resulted from the much faster characteristic time scale of heat transfer, while the NNH route and prompt route play the dominant roles in this low-temperature shifts of NO production at ϕ = 0.8 and 1.2, respectively. This means the increased importance of prompt and NNH routes on the NO emission in the downsized combustor.