Dilution effects on the combustion and extinction of opposed-jet syngas diffusion flames

The dilution effects on the combustion, emissions, and extinction characteristics of opposed-jet syngas diffusion flames were studied in this paper. A model of one-dimensional counterflow syngas diffusion flames was constructed with constant strain-rate formulations, which used detailed chemical kinetics and thermal and transport properties with flame radiation calculated by statistic narrow band radiation model. The dilution effects from three diluents (CO₂, H₂O, N₂) on flame structures, NO emission, and extinction limits of H₂/CO synthetic mixture flames were discussed. It was found that the maximum flame temperature is decreased the most with CO₂ as the diluents, which leads to a significant reduction of NO formation from Zeldovich route. However, CO-rich syngas flame with H₂O dilution has the lowest NO production rate, due to a reduced reaction rate of NNH+O→NH+NO, but highest flame temperature at extinction due to the competition between chemical effect and radiation effect. The flammability maps were also constructed with strain rates and dilution gases percentages as the coordinates. Besides the inert effect of diluents, the chemical effect of H₂O contributes to higher flame temperature, while the radiation effect of H₂O and CO₂ plays an important role in the flame extinction at low strain rates.