PHYSICAL STRUCTURE CHANGES OF CANADIAN COALS DURING COMBUSTION

Three coals were combusted in the Alberta Research Council laminar flow combustor in order to understand the changes in the physical structure 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 reactivity. The subbituminous coal and the high-volatile bituminous coal were very reactive, with burnouts of 95% and 88% achieved under stable operating conditions. The low-volatile bituminous coal was relatively unreactive. It was not possible to achieve a stable flame with the burnout decreasing below 50% in less than 1 h. Direct comparison of the partially burnt samples from the three coals was difficult because of the different reactivities. The subbituminous and high-volatile bituminous coals burned so rapidly that it was not possible to collect samples below 70% burnout. Conversely, it was not possible to generate samples of low-volatile bituminous coal char at burnouts above 72%. The subbituminous 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 actually increased in size up to 60% burnout, followed by a slight decrease. Surface area analysis of the subbituminous coal indicated a large surface area contained in micropores. At high levels of burnout (above 90%), the surface area decreased. The same behavior was observed for the high-volatile bituminous coal. While the low-volatile bituminous coal also showed this large increase in surface area, the decrease occurred at about 50% burnout, much earlier than for the other coals. Results of mercury porosimetry tests on the partially burnt samples revealed a significant change in the pore volume for both the subbituminous and high-volatile bituminous coals, while no large changes were observed for the low-volatile bituminous sample. It was difficult to draw any conclusions from the porosimetry results due to the different particle size of the chars and wide variance in the measurements.