Role of Impurities

Coal is a composite material with organic and inorganic constituents; its structure can be viewed as consisting of hydroaromatic structures with aromaticity in­creasing from low-rank to high-rank coals (Wender, 1976). The heteroatoms oxygen, nitrogen, and sulfur are associated with the coal in varying amounts. The sulfur is divided into organic and inorganic, with the organic sulfur being distributed throughout the entire coal. The inorganic sulfur is mainly associated with iron disulfide (pyrite or marcasite) and with the sulfates in weathered coals. Elemental sulfur has also been observed in coals although it is not a common occurrence (Yurovskii, 1960). Nitrogen in coal is found mainly in ring positions. Oxygen is present in phenolic hydroxyl, open ethers, and ring ethers; it is also an important constituent of the clay minerals. Lower-rank coals are rich in oxygen. The inorganic constituents of coal are mainly the commonly occurring minerals illite, kaolinite, mixed-layer clay, quartz, calcite, and pyrite. Other minerals occur in smaller amounts like siderite, ankerite, dolomite, feldspar, gypsum, marcasite, and sphalerite. Szomolnokite, coquimbite, and other sulfates are detected in partially oxidized coals (Rao Prasada et al., 1973). Minor minerals like rutile, anatase, cassiterite, etc. have been detected in several coals (Harris et al., 1981). Trace elements appear in range down to parts per million and practically cover the whole Periodic Table. The direct determination of the chemical forms of the trace elements in coal is quite difficult.