Bluff-body MILD combustion of large-proportion semicoke-blend under various secondary air velocities

Abstract

Secondary air jet velocity is strongly related to flue gas entrainment in semicoke-blend MILD combustion, which exerts a critical influence on the turbulence–chemistry interaction and combustion characteristics. To extend the application of moderate or intense low oxygen dilution (MILD) combustion to tangential fired boilers, this paper numerically investigates the effect of secondary air velocity (Vsec) on the turbulent flow and chemical reaction interaction behaviors of large-proportion semicoke and bituminous coal co-firing to establish the bluff-body MILD combustion regime. The results show that increasing Vsec abates the peak temperature by enlarging radiative heat transfer and enhances flue gas recirculation. The MILD regime first occurs at Vsec = 51.8 m/s and then extends quickly from 0.4 m to 1.2 m with further increasing Vsec. Increasing Vsec decreases the maximum of the turbulent Damköhler number Dat and increases the minimum of Karlovitz number Ka, implying that the predominant role of turbulent mixing gradually is transferred into the comparable role. The local-MILD regime (1 < Dat < 10) is enhanced significantly and an ideal MILD combustion regime occurs at a velocity of 158.6 m/s, i.e., Dat≈1, and Ka≫1, denoting a slow-chemistry regime. Meanwhile, the maximum heterogeneous Damköhler number Da,C-O2 decreases from 1.09 to 0.86, while the maximum Da,C-CO2 and Da,C-H2O decrease from 0.013 to 0.007 and from 0.003 to 0.0018, respectively, hinting that the char-O2 reaction is controlled by diffusion/kinetics, char-CO2 and char-H2O reactions are determined by kinetics, and they all proceed toward the kinetics-controlled regime with increasing Vsec. Low NOx emissions can be captured and coupled with small Dat by increasing Vsec, and weakening the transitional MILD regime (10 < Dat < 30) is of great importance when establishing the MILD regime.