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Origins of Coal
Coal is a carbon-rich rock which also contains small amounts of hydrogen and oxygen, with lesser amounts of nitrogen, sulphur and other elements. It also contains some water and inorganic matter which forms an incombustible residue, known as ash. Coal is formed over millions of years from organic matter which has been altered by decay and by various amounts of heat and pressure.
Coal advances through various stages during its formation. A gradual increase in temperature and pressure results in a decrease in water content and therefore an increase in carbon content. Coal formation begins with lignite (or brown coal) and continues through the ranks of sub-bituminous coal, bituminous coal and anthracite (all known as black coal).
Interlayered with other sedimentary rocks, coal forms beds ranging from less than a millimetre to many metres thick. Whether a particular seam is economic to mine depends on the quality, quantity and thickness of the beds.
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Coal in Australia
Most coal in Australia was deposited during the Permian Age, about 250 million years ago. There are also important Triassic, Jurassic and Cretaceous deposits.
Australia has economic reserves of an estimated 50 billion tonnes of black coal, found predominantly in New South Wales and Queensland. About 200 million tonnes of black coal is mined each year, with about 70% of this exported. Coal is Australia's largest earner of export revenue. In 1998, Australia sold more than 160 million tonnes of black coal overseas, making it the world's largest exporter. Australian coal generally has a high energy content, and low sulphur and heavy metal content, limiting emissions of sulphur dioxide and carbon dioxide. Brown coal, or lignite, is found mostly in Victoria, and is mined for domestic power use.
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Mining
Coal can be mined by either open cut or underground mining methods.
Open cut mining normally involves the use of blasting of large areas of overburden (or dirt) to loosen this material that is above the coal. The overburden is then usually removed by a dragline which uses buckets capable of carrying about 45 cubic metres of material. The uncovered coal is then removed by a truck and shovel fleet.
As mining progresses, the coal seams deepen, and mining through underground methods may become the more economic option. The most common modern method of mining large coal seams is through longwall mining, which can mine coal blocks extending beyond 200 metres in width. The equipment is placed at the end of a developed block of coal and a shearer cuts the coal from side to side until the block is mined out. A conveyor is used to transport the coal to the surface. The longwall is then moved on to the next pre-developed panel.
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Processing
Black coal may be crushed and screened to reduce it to a consistent size, with further processing unnecessary for use. However, in Australia, it is usually washed to reduce ash and improve overall quality. Washing involves immersing the crushed coal in a liquid of high specific gravity in which coal floats and can be recovered while the heavier rock and minerals sink and are discarded. Brown coal is often dried and compressed into briquettes before being used in power generation.
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Uses
The two predominant types of coal mines in Queensland and New South Wales are thermal and coking coal.
Thermal coal is used for electricity generation in power stations, where it is crushed and burnt to heat steam-generating boilers. It is also used by the cement and other industries.
Coking coal is used to produce coke when heated in the absence of air. Coke is a porous solid made up of carbon and ash. Good quality coke is hard, has a high crushing strength, and is mainly used to produce iron and steel. By-products of coke production include organic products used in plastics production and tar.
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