NOx reduction mechanism of a methane–air Smithells flame

As a simple-shaped laminar flame that can simulate conventional two-staged combustion devices, a methane–air Smithells flame, which is a peculiar Bunsen flames whose outer diffusion flame is separated from the inner premixed flame, was studied experimentally and numerically with detailed kinetics. In particular, the flame structure and the NOx emission characteristics of the Smithells flame were compared with those of the ordinary Bunsen flame under the same conditions, and its NOx reduction mechanism was investigated in detail. It was found experimentally that the Smithells flames have about 40% less total NOx emission than Bunsen flames. The numerically simulated flame structures are in good agreement with those of the experimental results, and the reduction rate of NOx emission also agreed well. From the numerical results it was found that the reduction is mainly caused by the suppression of Zeldovich NO due to the temperature decrease of the outer flame, and to the absence of the active production of O radicals at the flame base that are inherently formed by an interaction between the outer flame and the inner flame in the case of Bunsen flame. A secondary cause of the reduction is a deoxidization process from NO to N2 in the reduction atmosphere of the interconal regions of the Smithells flames, in which cooling by the outer tube wall promotes the conversion from NO to HNO in the reaction loop NO → HNO → NH → N → NO.