MILD-Oxy Combustion Characteristics of Pulverized Coal Affected by Oxidizer O2 Concentrations under Dry and Wet Recycle Modes

Moderate or intense low-oxy dilution oxy-fuel (MILD-oxy)combustioncan synergistically reduce the emissions of CO2 and NO x , and its industrial implementation is carriedout under dry and wet recycle modes. In this work, a series of numericalsimulations on pulverized coal MILD-oxy combustion under the typicaldry and wet recycle modes are performed, aiming at fully evaluatingthe combustion behaviors affected by the oxidizer O-2 concentration(21-30%) under different recycle modes. The results show thatincreasing O-2 concentration weakens the in-furnace fluegas recirculation and promotes the combustion intensity, while a moreuniform temperature distribution is obtained under dry recycle modethan under wet recycle mode. The diffusion/kinetics-controlled MILDregime is realized as the O-2 concentration increases. Nevertheless,the diffusion effect is enhanced due to the accelerated reaction rateunder a high O-2 level. The gasification reaction is strengthenedwith the increased O-2 concentration. Compared with wetrecycle mode, the burnout of coal char is slowed down under dry recyclemode because of the lower burnout rate. The fuel-NO conversion ratiois significantly reduced with the increased O-2 concentration,while wet recycle mode shows more potential in achieving low NO x emission than dry recycle mode. This workis expected to provide new insights into the comprehensive understandingof MILD-oxy combustion.