ASH FORMATION DURING PULVERIZED SUBBITUMINOUS COAL COMBUSTION .2. INORGANIC TRANSFORMATIONS DURING MIDDLE AND LATE STAGES OF BURNOUT

Two subbituminous coals were burned in a down-fired combustor of 745 MJ/h.m3 capacity. Transmission electron microscopy, computer-controlled scanning electron microscopy, and X-ray diffraction were used to study changes of inorganic matter in aerodynamically sized char samples collected at different degrees of burnout. The previous paper in this series1 showed that the coals underwent 33 %-50 % burnout of combustible matter between introduction to the combustor and the first sampling port. During this stage, inorganic interactions consisted primarily of vaporization and condensation of inorganics that were originally associated with the organic portion of the coals. The present paper deals with subsequent changes that occur in two stages up to 99.9 % combustible burnout. Between approximately 50 % and 90 % burnout, mineral and ash particle decomposition, fragmentation, and coalescence are the primary interactions, dominated by coalescence to form new calcium-rich phases. Submicron mineral matter that was not exposed at a burning char surface underwent no change in morphology up to these residence times. As burnout increased to 99.9 %, the changes in the supermicron inorganic particles were similar but less dramatic, although some sulfation of the smallest particles occurred, and sodium interacted with aluminosilicates because of the lower temperatures. However, because of the longer residence times, the submicron mineral matter in the remaining char particles flowed through char micropores to form coalesced inorganic globules and rivulets within the pores.