Effect of steam addition on turbulence-chemistry interaction behaviors of pulverized coal MILD-oxy combustion

Abstract The enrichment of steam is closely associated with flue gas recirculation in the pulverized coal MILD-oxy combustion, which exerts significant impacts on the turbulence-chemistry interaction and combustion behaviors. The present work numerically evaluates the effect of steam addition on the interaction behaviors between turbulent mixing and chemical reaction of pulverized coal MILD-oxy combustion through investigating the Damkohler number Da in the main reaction zone. The results show that the addition of steam affects the flame temperature by enhancing thermal radiation and weakens the internal flue gas recirculation. While maintaining the slow chemistry regime in main reaction zone, the turbulent Damkohler number Dat increases and Karlovitz number Ka decreases with an increase in steam concentration, respectively, which reveal that high concentration steam dilution governs MILD-oxy combustion by turbulent mixing rather than chemical kinetics. Meanwhile, the ranges of the surface Damkohler number Das-O2, Das-CO2 and Das-H2O are within 15–45, 0.06–0.65, 0.02–0.20, respectively, which indicate that the Cs-O2 reaction is governed by diffusion while the Cs-CO2 reaction and Cs-H2O reaction are determined by diffusion-kinetics. As the steam concentration increases, the heterogeneous reactions proceed towards the diffusion-controlled regime. The proportion of char consumed by oxidation reactions increases due to the corresponding weakening of gasification reactions, which leads to the less formation of synthesis gas. The findings can provide deep insight into the interaction between turbulence and chemistry of pulverized coal MILD-oxy combustion under wet recycle.