Computed NO _x emission characteristics of opposed-jet syngas diffusion flames

This paper reported a numerical study on the NO _x emission characteristics of opposed-jet syngas diffusion flames. A narrowband radiation model was coupled to the OPPDIF program, which used detailed chemical kinetics and thermal and transport properties to enable the study of 1-D counterflow syngas diffusion flames with flame radiation. The effects of syngas composition, pressure and dilution gases on the NO _x emission of H ₂/CO synthetic mixture flames were examined. The analyses of detailed flame structures, chemical kinetics, and nitrogen reaction pathways indicate NO _x are formed through Zeldovich (or thermal), NNH and N ₂O routes both in the hydrogen-lean and hydrogen-rich syngas flames at normal pressure. Zeldovich route is the main NO formation route. Therefore, the hydrogen-rich syngas flames produce more NO due to higher flame temperatures compared to that for hydrogen-lean syngas flames. Although NNH and N ₂O routes also are the primary NO formation paths, a large amount of N ₂ will be reformed from NNH and N ₂O species. For hydrogen-rich syngas flames, the NO formation from NNH and N ₂O routes are lesser, where NO can be dissipated through the reactions of NH+NO→N ₂+OH and NH+NO→N ₂O+H more actively. At a rather low pressure (0.01atm), NNH-intermediate route is the only formation path of NO. Increasing pressure then enhances NO formation reactions, especially through Zeldovich mechanisms. However, at higher pressures (5-10atm), NO is then converted back to N ₂ through reversed N ₂O route for hydrogen-lean syngas flames, and through NNH as well for hydrogen-rich syngas flames. In addition, the dilution effects from CO ₂, H ₂O, and N ₂ on NO emissions for H ₂/CO syngas flames were studied. The hydrogen-lean syngas flames with H ₂O dilution have the lowest NO production rate among them, due to a reduced reaction rate of NNH+O→NH+NO. But for hydrogen-rich syngas flames with CO ₂ dilution, the flame temperatures decrease significantly, which leads to a reduction of NO formation from Zeldovich route.