Mineralogical Transformations in Coal Feedstocks during Carbon Conversion, Based on Packed-Bed Combustor Tests

Handpicked carbon-rich particles (coal or char), mineral-rich particles (heated stones), and bonded stone aggregates (clinkers) from the upper and lower parts of a pilot-scale packed-bed combustor bed, designed to simulate conversion or combustion processes, were subjected to mineralogical and inorganic geochemical studies, in order to evaluate the reactions that may occur during carbon conversion at a particle-by-particle scale. The mineral matter of the carbon-rich particles was found to have higher proportions of carbonate minerals and kaolinite than the mineral-rich (stone) particles in the coal feedstocks used in the study, and to form phases such as anorthite, gehlenite, pyrrhotite, troilite, and vaterite, as well as mullite and cristobalite, in the high-temperature parts of the combustor column. The clinkers and the heated stones from the hightemperature parts of the bed both contain similar minerals (quartz, mullite, cristobalite, K-feldspar, and hematite), but the clinkers have higher proportions of anorthite and amorphous material. From the study, it is suggested that melting of Ca-rich ash remaining after destruction of the organic matter in the carbonrich particles takes place at high temperature (up to 1250uC) in the combustor bed. Melting of Ca-rich ash from the carbon-rich particles, with a high base to acid ratio, represents a key process in clinker formation. Anorthite crystallizes from this melt as it cools. Before cooling, the melt may envelop still-intact heated stone and, in some cases, char particles, allowing the resulting anorthite-bearing glass to bond those particles together and form clinkers within the combustor ash. The mineralogical changes observed in the individual particles in this study complement those identified from analysis of composite feed coal and ash samples and, thus, provide a key to better understanding ash formation and clinker formation processes associated with the inherently heterogeneous, coarse coal feedstocks used in some conversion or combustion systems.