PHYSICAL STRUCTURE CHANGES OF CANADIAN COALS DURING COMBUSTION

Three coals were combusted in the Alberta Research Council laminar flo w combustor in order to understand the changes in the physical structu re that occur during pulverized coal combustion. A subbituminous (coal A), and both high-volatile (coal B) and low-volatile bituminous (coal C) coals were chosen to examine coals of different rank and reactivit y. The subbituminous coal and the high-volatile bituminous coal were v ery reactive, with burnouts of 95% and 88% achieved under stable opera ting conditions. The low-volatile bituminous coal was relatively unrea ctive. It was not possible to achieve a stable flame with the burnout decreasing below 50% in less than 1 h. Direct comparison of the partia lly burnt samples from the three coals was difficult because of the di fferent reactivities. The subbituminous and high-volatile bituminous c oals burned so rapidly that it was not possible to collect samples bel ow 70% burnout. Conversely, it was not possible to generate samples of low-volatile bituminous coal char at burnouts above 72%. The subbitum inous coal showed a continuous decrease in particle size with burnout. The high-volatile bituminous coal showed a significant size decrease only before 70% burnout, whereas the low-volatile bituminous coal actu ally increased in size up to 60% burnout, followed by a slight decreas e. Surface area analysis of the subbituminous coal indicated a large s urface area contained in micropores. At high levels of burnout (above 90%), the surface area decreased. The same behavior was observed for t he high-volatile bituminous coal. While the low-volatile bituminous co al also showed this large increase in surface area, the decrease occur red at about 50% burnout, much earlier than for the other coals. Resul ts of mercury porosimetry tests on the partially burnt samples reveale d a significant change in the pore volume for both the subbituminous a nd high-volatile bituminous coals, while no large changes were observe d for the low-volatile bituminous sample. It was difficult to draw any conclusions from the porosimetry results due to the different particl e size of the chars and wide variance in the measurements.