Ignition and combustion characteristics of coal particles under high-temperature and low-oxygen environments mimicking MILD oxy-coal combustion conditions

The experiments of coal particles combustion were conducted to mimic moderate and intense low-oxygen dilution (MILD) oxy-coal combustion on a diffusion-flamelet-based Hencken burner under different O-2/CO2 and O-2/N-2 environments with the coflow temperatures of 1400-1800 K and oxygen concentrations of 5%-20%. The images of coal particles combustion flame were captured by a digital camera to determine the ignition standoff distances of coal particles. The radiation spectrums of burning coal particles at different coflow temperatures and oxygen concentrations were measured by a fiber optic spectrometer and the temperatures of burning coal particles were deduced with the two-color method. It is found that either increasing the coflow temperature or the oxygen concentration leads to brighter flame luminosity, shorter ignition standoff distance and higher burning particles temperature. The ignition standoff distance of coal particles is increased by 0.07-0.49 cm when carbon dioxide is used as diluent gas instead of nitrogen. The temperatures of the burning coal particles under O-2/CO2 environments are lower by 51-185 K than those under O-2/N-2 environments at the same conditions due to the combined physicochemical properties of CO2 with higher heat capacity and the endothermic reaction of char gasification. Moreover, a method based on the coal particles temperature uniformity coefficient is proposed to quantitatively identify the combustion modes of MILD coal combustion and MILD oxy-coal combustion. All coal combustion flames are visible under both O-2/CO2 and O-2/N-2 environments, and the MILD coal combustion flames under high-temperature and low-oxygen concentration conditions are weaker and more uniform with dark red color.