Coal: Encyclopedia II - Coal - Composition

Coal - Composition

Carbon forms more than 50 percent by weight and more than 70 percent by volume of coal (this includes inherent moisture). This is dependent on coal rank, with higher rank coals containing less hydrogen, oxygen and nitrogen, until 95% purity of carbon is achieved at Anthracite rank and above. Graphite formed from coal is the end-product of the thermal and diagenetic conversion of plant matter (50% by volume of water) into pure carbon.

Coal usually contains a considerable amount of incidental moisture, which is the water trapped within the coal in between the coal particles. Coals are usually mined wet and may be stored wet to prevent spontaneous combustion, so the carbon content of coal is quoted as both a 'as mined' and on a 'moisture free' basis.

Lignite and other low-rank coals still contain a considerable amount of water and other volatile components trapped within the particles of the coal, known as its macerals. This is present either within the coal particles, or as hydrogen and oxygen atoms within the molecules. This is because coal is converted from carbohydrate material such as cellulose, into carbon, which is an incremental process (see below). Therefore coal carbon contents also depend heavily on the degree to which this cellulose component is preserved in the coal.

Other constituents of coals include mineral matter, usually as silicate minerals such as clays, illite, kaolinite and so forth, as well as carbonate minerals like siderite, calcite and aragonite. Iron sulphide minerals such as pyrite are common constituents of coals. Sulphate minerals are also found, as is some form of salt, trace amounts of metals, notably iron, uranium and cadmium, and rarely gold.

Methane gas is another component of coal, produced not from bacterial means but from methanogenesis. Methane in coal is dangerous as it can cause coal seam explosions especially in underground mines, and may cause the coal to spontaneously combust. It is, however, a valuable by-product of some coal mining, serving as a significant source of natural gas.

Coal composition is determined by specific coal assay techniques, and is performed to quantify the physical, chemical and mechanical behaviour of the coal, including whether it is a good candidate for coking coal.

Adapted from the Wikipedia article "Composition", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki