Physicochemical Effects of Water Vapor and CO2 on the NOx Formation during Moderate and Intense Low-Oxygen Dilution Oxy-Coal Combustion

Abstract

The influence of water vapor (H₂O) volume fraction on the NO_x formation and reduction during MILD oxy-coal combustion (MILD-OCC) was investigated by using the plane diffusion flame pulverized coal combustion experimental system. The physicochemical effects of H₂O and CO₂ on NO_x formation during MILD-OCC were quantitatively separated by individually modifying the physicochemical properties of H₂O or CO₂. Numerical analysis was conducted on the effects of H₂O volume fraction on the combustion and NO_x formation characteristics under different coflow conditions during MILD-OCC. The results indicate that CO₂’s molar specific heat capacity is the main reason for the increased release time of volatile nitrogen. The effects of CO₂’s molar heat capacity and thermal conductivity are significant, delaying the volatile release time by approximately 38 and 11%, respectively. The dominant effects of H₂O’s thermal conductivity, radiative absorption coefficient, and mass diffusion coefficient of species in H₂O cause the combustion temperature to gradually decrease as H₂O volume fraction increases at 20%O₂. The gasification reaction weakens as the H₂O volume fraction increases under low-oxygen conditions, the endothermic reaction decreases, the combustion temperature rises, the CO concentration decreases, the NO_x reduction weakens, and the NO_x concentration gradually increases. The NO_x concentrations at 1473, 1673, and 1873 K decrease by approximately 12, 8, and 4%, respectively, when the H₂O volume fraction is increased from 0 to 40% at 20%O₂. Compared to the absence of H₂O, the NO_x concentrations at 10%O₂ and 5%O₂ with a 40% H₂O volume fraction increase by approximately 12 and 7%, respectively, at 1873 K.