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

Mineralogical and inorganic geochemical studies have been carried out on composite samples of six different coarse-crushed (,120 mm) feed coals tested in a pilot-scale packed-bed combustor designed to simulate conversion and some fixed-bed combustion processes, and also on composite samples of the ashes produced. The mineral matter of the coals consisted mainly of quartz, kaolinite, and Ca-bearing carbonate minerals (calcite, dolomite, aragonite), with minor proportions of illite, pyrite, and several other phases, along with some Ca in an organically associated form. The base to acid ratio of the coal ashes ranged from 0.1 to 0.3, due mainly to variations in relative abundance of Ca-bearing carbonate minerals. The minerals in the feed coals reacted at temperatures of up to 1250uC in the packed-bed combustor to form cristobalite, mullite, anorthite, and other Ca-silicates, as well as anhydrite, iron oxides, and amorphous material. The proportions of the different crystalline phases in the ashes produced from each of the coals in the packed-bed combustor are related to the proportions of relevant minerals in the respective feed-coal materials. Comparisons of feed coal and packed-bed combustor ash mineralogy suggest that the cristobalite in the packed-bed combustor ashes mainly represents a high-temperature product derived from quartz in the feed coals. Mullite in the combustor ash is formed by high-temperature reactions from the mainly kaolinitic clay component. The proportions of anorthite and other Ca-silicates in the combustor ashes have been related to the proportions of Ca-bearing carbonate minerals and organically associated Ca in the feed coals. The proportion of iron-oxide minerals is broadly related to the proportion of pyrite in the respective coal samples. These observations provide a basis for assessing the mineralogical processes that take place during conversion of different Highveld coals, and possibly for predicting ash properties based on coal mineral matter characteristics.