Effects of hydrogen addition to fuel on soot formation and NO_x emission in diffusion flames

A numerical study of soot formation and NO_x emission in axisymmetric coflow laminar methane/hydrogen-air diffusion flame was conducted using detailed gas-phase chemistry and complex thermal and transport properties. To investigate the impact of hydrogen addition, three cases were studied, namely, Case 1, maintaining a constant volume flow of fuel. Case 2, maintaining the fuel heat release rate unchanged, and the fuel nozzle diameter unchanged. Case 3, maintain the fuel heat release rate unchanged, and the fuel jet velocity constant. The results show that, in all three cases, with increasing level of H₂ addition, the flame height decreased, the reaction zone length shortened, residence time shortened, therefore, soot surface growth was inhibited, soot formation reduced, and NO_x emissions reduced. Effects of hydrogen addition on soot formation and NO_x emissions in Case 2 and Case 3 were much worse than that in Case 1. The results also show that while in reducing soot formation and NO_x emission, effect of hydrogen addition in Case 3 was better than that in Case 2.