Pilot-scale combustion tests were carried out on coals with low (13.5%) and high (39.6%) ash percentages from, the Highveld Coalfield of South Africa, using the ALS-ACIRL 100-200 kW Combustion Test Facility. The mineral-matter in the feed coals and also the chemistry and mineralogy of the ash deposits from different parts of the test furnace were analyzed using quantitative X-ray diffraction and X-ray fluorescence techniques, and the results were used to develop an understanding of the mineralogical processes taking place in the different parts of the pulverized-fuel combustion system: Low-temperature oxygen, plasma ashing and quantitative X-ray diffractometry showed that the mineral matter of both coals contained abundant kaolinite, lesser but still very significant proportions of quartz and carbonates (ankerite and calcite), and minor proportions of pyrite and other minerals. A small but significant proportion of nonmineral Ca also occurs in the organic matter, especially of the low-ash coal. The chemical composition of the combustion products taken from the different sampling points in the test facility (slagging panels, furnace ash, and fouling probes etc.) was overall very similar for each coal sample. However, the percentages of the different crystalline minerals, in the combustion products showed a wide range of variation at the different sampling points. Anorthite, derived from interaction between Ca and the aluminosilicate components, was formed from both coals at temperatures above 1300 degrees C, mostly as part of sintered aggregates that built up and became detached from the slagging panel in the highest temperature part of the combustion system. Anhydrite was formed by interaction of Ca with SO2; lime and periclase were also formed from the low-ash coal, where insufficient sulfur was available for complete sulfate development.