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Bituminous coal is a relatively hard coal containing a tar-like substance called bitumen. It is of better quality than lignite coal but of poorer quality than anthracite coal.
Bituminous coal is an organic sedimentary rock formed by diagenetic and submetamorphic compression of peat bog material.
Bituminous coal has been compressed and heated so that its primary constituents are the macerals vitrinite, exinite, etc. The carbon content of bituminous coal is around 60-80%, the rest is composed of water, air, hydrogen, and sulphur which have not been driven off from the macerals.
The heat content of bituminous coal ranges from 21 to 30 million Btu/ton (24 to 35 MJ/kg) on a moist, mineral-matter-free basis.
Bituminous coal is usually black, sometimes dark brown, often with well-defined bands of bright and dull material. Bituminous coal seams are stratigraphically identified by the distinctive sequence of bright and dark bands and are classified accordingly as either “dull, bright-banded” or “bright, dull-banded” and so on.
Bituminous coals are graded according to vitrinite reflectance, moisture content, volatile content, plasticity and ash content. Generally, the highest value bituminous coals are those which have a specific grade of plasticity, volatility and low ash content, especially with low carbonate, phosphorus and sulphur.
Plasticity is vital for coking and steel making, where the coal has to behave in a manner which allows it to mix with the iron oxides during smelting. Low phosphorus content is vital for these coals, as phosphorus is a highly deleterious element in steel making.
Coking coal is best if it has a very narrow range of volatility and plasticity. This is measured by the Free Swelling Index test. Tar content, volatile content and swelling index are used to select coals for coke blending.
Volatility is also critical for steel making and power generation, as this determines the burn rate of the coal. High volatile content coals, while easy to ignite often are not as prized as moderately volatile coals; low volatile coal may be difficult to ignite although it will contain more energy per unit volume. The smelter must balance the volatile content of the coals to optimise the ease of ignition, burn rate, and energy output of the coal.
Low ash, sulphur, and carbonate coals are prized for power generation because they do not produce much boiler slag and they do not require as much effort to scrub the flue gases to remove particulate matter. Carbonates are deleterious as they readily stick to the boiler apparatus. Sulphide contents are also deleterious in some fashion as this sulphur is emitted and can form smog, acid rain and haze pollution. Again, scrubbers on the flue gases aim to eliminate particulate and sulphur emissions.
When used for many industrial processes, bituminous coal must first be “coked” to remove volatile components. Coking is achieved by heating the coal in the absence of oxygen, which drives off volatile hydrocarbons such as propane, benzene and other aromatic hydrocarbons, and some sulfur gases. This also drives off a considerable amount of the contained water of the bituminous coal.
Coking coal is blended with uncoked coal for power generation. The primary use for coking coal is in the manufacture of steel, where carbon must be as volatile-free and ash-free as possible.
Extensive but low-value coals of Jurassic age extend through the Surat Basin in Australia, formed in an intracratonic sag basin, and contain evidence of dinosaur activity in the numerous ash plies. These coals are exploited in Queensland from the Walloon Coal Measures which are up to 15m thick of sub-bituminous to bituminous coals suited for coking, steam-raising and oil cracking.
Coals of Triassic age are known from the Clarence-Moreton and Ipswich Basins, near Ipswich, Australia and the Esk Trough. Coals of this era are rare, and many contain fossils of flowering plants. Some of the best coking coals are Australian Triassic coals, although most economic deposits have been worked out.
The second largest deposits of the world’s bituminous coal is contained within Permian strata in Russia and also in the Bowen Basin in Queensland, Australia, as well as in the Sydney Basin and Perth Basin where thicknesses in excess of 300m are known. Current reserves and resources are projected to last for over 200 years.
Australia exports the vast majority of its coal for coking and steel making in Japan. Certain Australian coals are the best in the world for these purposes, requiring little to no blending. Some bituminous coals from the Permian and Triassic in Australia are also the most suitable for cracking into oil.
Vast deposits of oil shale exist in the Permian sediments of Queensland.
Much North American coal was created when swamps created organic material faster than it could decay, before the orogenies that created the Appalachian Mountains during the Carboniferous epoch, which is subdivided in American literature into the Mississippian and Pennsylvanian eras after the two main coal-bearing time periods.
Bituminous coal is mined in the Appalachian region, primarily for power generation. Mining is done via both surface and underground mines. Pocahontas bituminous coal at one time fueled half the world’s navies and today stokes steel mills and power plants all over the globe.
While coal mining is an important part of Appalachia’s economy, many miners are afflicted with black lung disease.