Investigation of air-MILD and oxy-MILD combustion characteristics of semicoke and bituminous coal mixtures in a 0.3 MW fuel-rich/lean fired furnace

Abstract

To study the combination of fuel-rich/lean combustion with oxy-MILD (moderate and intense low-oxygen dilution) combustion for the reduction of NOx emissions by adopting carbon capture and storage, this paper numerically investigates the turbulence−chemistry interactions and NOx emission behaviours of semicoke mixtures under different secondary velocities Vsec (from air-fuel combustion to air-MILD combustion) and under different CO2 dilution levels (oxy-MILD combustion). The bias concentration ratio (BCR) hardly affects the establishment of air-MILD combustion, which is obtained at Vsec = 158.6 m/s. The oxy-MILD combustion, which is composed of small flamelets diffusing in large-scale eddies (turbulent Damköhler number Dat = 0.1–1), is enhanced, and the ignition and char burnout times are prolonged by increasing CO2 from 0 vol% to 79 vol%. The char oxidation reaction in the diffusion/kinetics-controlled regime and the char gasification reactions in the kinetics-controlled regime both shift towards the kinetics-controlled regime with increasing velocities and CO2 dilution, thus leading to lower burnout rates. The heterogeneous MILD regime of the fuel-lean side is more easily established for the air-MILD combustion, while that of the fuel-rich side is more easily established for oxy-MILD combustion. This is because the intensive turbulence increases the O2 dilution level in the fuel-lean jet, while a high concentration of CO2 reduces the oxygen diffusivity in the fuel-rich jet, which also leads to a larger XNO in the fuel-lean jet compared to the fuel-lean jet. The NOx concentration at the furnace outlet decreases significantly from air-fuel combustion to air-MILD combustion and further decreases as the oxy-MILD combustion is enhanced, along with relatively lower homogeneous and heterogeneous Da.