Optical investigations on particles evolution and flame properties during pulverized coal combustion in O2/N2 and O2/CO2 conditions

The evolution of soot and coal/char as well as the flame properties, during pulverized coal combustion in O-2/N-2 and O-2/CO2 conditions, were experimentally studied using optical diagnostics. The jet flames of pulverized coal particles were produced by using an entrained-flow reactor with optical access, and the oxygen concentrations of the gas carrying pulverized coal particles were varied between 0 and 100%. A combination of laser-induced incandescence (LII), laser-induced fluorescence (LIF), elastic laser scattering (ELS), and radiative emission detection methods was employed to characterize the spatial distributions of soot, polycyclic aromatic hydrocarbons (PAHs), coal/char, and the flame temperature. The measured results revealed that ELS signal intensity declined accompanied with the appearance of LIF and LII signals. An evolution pattern of PAHs and soot was investigated to be similar to those found in gaseous hydrocarbon diffusion flames. An abrupt expansion of the pulverized coal particle jet was observed from ELS measurement at the position where coal particle jet was ignited, which was likely attributed to the more intensive dispersion of small particles caused by a rapid elevation of the flame temperature. As the oxygen concentration increased, the ignition distance of pulverized coal particle jet became shorter and the flame temperature increased. The PAHs and soot-loaded regions were shrunk and the sooting region shifted to upstream position of the flame. Nonetheless, replacing N-2 with CO2 tended to increase the ignition distance but lower the flame temperature. The peak LII signal, corresponding to the maximum soot volume fraction, increased to a maximum and then decreased with oxygen concentration and the intensity for the O-2/N-2 case was far larger than for the O-2/CO2 case. However, the LIF signal intensities exhibited an apparent decrease with increasing oxygen concentration, and they were nearly consistent when N-2 was replaced with CO2, which may imply that the effect of suppressing radical formation on PAHs evolution in CO2 atmosphere was negligible.