Numerical Investigation on the Flame Structure and CO/NO Formations of the Laminar Premixed Biogas–Hydrogen Impinging Flame in the Wall Vicinity

Abstract

The near-wall flame structure and pollutant emissions of the laminar premixed biogas-hydrogen impinging flame were simulated with a detailed chemical mechanism. The spatial distributions of the temperature, critical species, and pollutant emissions near the wall of the laminar premixed biogas–hydrogen impinging flame were obtained and investigated quantitatively. The results show that the cold wall can influence the premixed combustion process in the flame front, which is close to the wall but does not touch the wall, and results in the obviously declined concentrations of OH, H, and O radicals in the premixed combustion zone. After flame quenching, a high CO concentration can be observed near the wall at equivalence ratios (φ) of both 0.8 and 1.2. Compared with that at φ = 1.0, more unburned fuel is allowed to pass through the quenching zone and generate CO after flame quenching near the wall thanks to the suppressed fuel consumption rate near the wall and the excess fuel in the unburned gases at φ = 0.8 and 1.2, respectively. By isolating the formation routes of NO production, it is found that the fast-rising trend of NO concentration near the wall in the post flame region at φ = 0.8 is attributed to the NO transportation from the NNH route primarily, while the prompt NO production accounts for more than 90% of NO generation in the wall vicinity at φ = 1.2. It is thus known that, thanks to the effectively increased surface-to-volume ratio, the premixed combustion process in the downsized chamber will be affected more easily by the amplified cooling effects of the cold wall, which will contribute to the declined combustion efficiency, increased CO emission, and improved prompt NO production.