Dilution effects analysis on NO_x emissions of opposed-jet H₂/CO syngas diffusion flames

Syngas is a promising alternative fuel for stationary power generation due to cleaner combustion than convectional fossil fuels. During the gasification processes, the by-products of CO₂, H₂O, or N₂ may be present in the syngas mixture to control the flame temperature and emissions. Several studies indicated that syngas with dilutions is capable of reducing pollutant emissions such as NO_x emissions. This work applied a numerical model of opposed-jet diffusion fames to explore the dilution effects on NO_x formation and differentiate the inert effect, thermal/diffusion effect, chemical effect, and radiation effect from CO₂, H₂O, or N₂ dilutions. The numerical study was performed by a revised OPPDIF program coupling with narrowband radiation model and detail chemical mechanism. The dilution effects on NO_x formation were analyzed by comparing the realistic and hypothetical cases. Regardless the diluent types, the inert effect is the main cause to reduce NO production, followed by chemical effect and radiation effect. The thermal/diffusion effect may promote NO formation because the preferential diffusion due to different diffusivities between diluents and syngas magnifies the reaction rate locally. CO₂ dilution reduces NO by radiation effect at low strain rate, and contributes NO reduction by chemical effect at high strain rate. At the same dilution percentage, CO₂ dilution reduces NO production the most, followed by H₂O and N₂. Besides the thermal/diffusion effect, the chemical effect of H₂O enhances NO production through thermal route and reburn route.