Temperature and NOx distribution characteristics of coal particles under high-temperature and low-oxygen environments simulating MILD oxy-coal combustion conditions

A diffusion-flamelet-based Hencken burner was used to produce moderate and intense low-oxygen dilution oxycoal combustion (MILD-OCC) of coal particles under O-2/CO2 environments with the coflow temperatures of 1000-1400 K and oxygen concentrations of 2.86-8.75%. The flame temperature distributions were measured by a R-type thermocouple coupled with temperature continuous recorder. The NOx was captured by a self-made air extraction sampling device and measured by UV spectrophotometer. Three influencing factors including fuel-air ratio of methane, secondary air and particle size of pulverized coal on temperature and NOx distributions were experimentally researched. It is found that increasing the fuel-air ratio and the volume of secondary air leads to lower peak temperature, more uniform temperature distribution, lower NOx and stronger NO-reburning effect in flame. To a certain extent, decreasing the particle size of pulverized coal causes lower ignition delay, higher release rate of volatiles and little change in NOx emissions, further reducing the particle size of pulverized coal results in higher NOx in flame. Moreover, the mathematical criterion of MILD-OCC state is deduced based on criterion of MILD coal combustion. The research has important reference value for in-depth understanding of MILD-OCC regime and the generation mechanism of fuel NOx.