Excess Air, Schmidt Number, and NOx Formation in Laminar Jet Flames

Studying NOx formation is essential for reducing nitric oxide emissions from gas-fired burners. In the case of laminar jet diffusion flames, the temperature profiles in the inner rich core, outer lean, and the plume regions can affect the extent of thermal NOx formation. The temperature and oxygen profiles in the plume region depend upon the extent of air entrained by the jet containing the fuel stream. In our previous works, the concept of excess air in jet diffusion flames was introduced and it was shown that excess air percentage for any fuel in the jet depends only upon the Schmidt number of the fuel jet, if one assumes equal diffusion coefficients for all species. In the current article, it is shown that NOx production is affected by excess air and, hence, Schmidt number. Thus, under identical flame temperatures of two different fuels, the difference in Schmidt number can cause differences in NOx emissions. A desired Schmidt number can be achieved by selecting a combination of fuels and inerts. Computational results from our model are compared with experimental data in the literature.