Physical Properties of Particulate Matter Emitted from Combustion of Coals of Various Ranks in O2/N2 and O2/CO2 Environments

This work examined the particulate emissions from pulverized coals burning under either conventional or oxyfuel combustion conditions. Oxyfuel combustion is a process that takes place in O-2/CO2 environments, which are achieved by removing nitrogen from the intake gases and recirculating large amounts of flue gases into the boiler; this is done to moderate the high temperatures caused by the elevated oxygen partial pressure therein. In this study, combustion took place in a laboratory laminar-flow drop-tube furnace (DTF) in environments containing various mole fractions of oxygen in either nitrogen or carbon dioxide background gases. A bituminous coal, a sub-bituminous coal, and a lignite were burned at a DTF temperature of 1400 K. Trimodal ash particle size distributions were observed with peaks in the submicrometer region (similar to 0.2 mu m), as well as in the supermicrometer region (similar to 5 mu m and >10 mu m). Both submicrometer and supermicrometer particulate emission yields of all three coals were typically lower in O-2/CO2 than in O-2/N-2 environments. Emission yields typically increased with increasing oxygen concentration in the furnace, with an exception noted at moderate oxygen mole fractions (20%-30%) in CO2, where significant amounts of unburned carbon were detected. Submicrometer particulate yields were found to be comparable in the effluents of all three coals, independently of their ash contents, whereas supermicrometer particulate yields were nearly analogous to the ash contents of the three coals. Scanning electron microscopy (SEM) revealed that submicrometer particles were spherical, whereas supermicrometer particles were often of irregular shapes, fractured spheres, and spheres with small particles attached to their surface.