Relations between ash yield and chemical and mineral composition of coals

Abstract

Some relations between the ash yield (db) and the mineral and chemical composition of coals and their high-temperature ashes were studied. Low-ash coals (< 10 wt%) are enriched in moisture, volatile matter, fixed carbon and minerals such as montmorillonite, Fe, Al and Mg oxyhydroxides, dolomite, witherite and Fe sulfates, while their ashes are abundant in MgO, CaO, Na 2O and TiO 2. High-ash coals (> 20 wt%) have increased contents of H, N, O, S and minerals such as quartz, illite, calcite, pyrite and gypsum, while their ashes are enriched in SiO 2, Fe 2O 3 and K 2O. Medium-ash coals (10–20 wt%) have high concentrations of C and Al 2O 3 (ash basis) and minerals such as kaolinite, mica and siderite, and show the lowest contents of plagioclases and K-feldspars. The low-ash coals are enriched in ash-forming elements associated with probable authigenic minerals and organics, whereas higher-ash coals demonstrate an enrichment in ash-forming elements associated with probable detrital minerals. Various processes related to changes in ash yield and inorganic matter during coalification are discussed. The material supply and leaching behaviour of different minerals, phases and elements during peat formation and conversion to coal as well as the various plant constituents and regional, depositional and paleoenvironmental conditions are responsible for the most essential changes in the mineral and chemical composition or ash yield of coals.