Bituminous coal or black coal is a relatively soft coal containing a tarlike substance called bitumen. It is of higher quality than lignite coal but of poorer quality than Anthracite. It is usually formed as a result of high pressure on lignite.
Bituminous coal is an organic sedimentary rock formed by diagenetic and sub metamorphic compression of peat bog material.
Bituminous coal has been compressed and heated so that its primary constituents are macerals vitrinite, exinite, and so on. The carbon content of bituminous coal is around 60-80%; the rest is composed of water, air, hydrogen, and sulfur, which have not been driven off from the macerals.
The heat content of bituminous coal ranges from 21 million 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.
Bank density is approximately 1346 kg/m³ (84 lb/ft³). bBulk density typically runs to 833 kg/m³ (52 lb/ft³).
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Bituminous coals are graded according to vitrinite reflectance, moisture content, volatile content, plasticity and ash content. Generally, the highest value bituminous coals have a specific grade of plasticity, volatility and low ash content, especially with low carbonate, phosphorus, and sulfur.
Plasticity is vital for coking as it represents its ability to gradually form specific plasticity phases during the coking process, measured by coal dilatation tests. Low phosphorus content is vital for these coals, as phosphorus is a highly deleterious (damaging) 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. Volatile content and swelling index are used to select coals for coke blending as well.
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 contains more energy per unit volume. The smelter must balance the volatile content of the coals to optimize the ease of ignition, burn rate, and energy output of the coal.
Low ash, sulfur, 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. Sulfide contents are also deleterious in some fashion as this sulfur is emitted and can form smog, acid rain and haze pollution. Again, scrubbers on the flue gases aim to eliminate particulate and sulfur emissions.
Smithing coal is a type of high quality bituminous coal ideally suited for use in a coal forge. It is as free from ash, sulfur, and other impurities as possible.[1] The constituents of the coal should be as follows:[2]
| Constituent | Percentage |
|---|---|
| Sulfur | Not over 1% |
| Ash | Not over 7% |
| Carbon | Not less than 70% |
| Moisture | Not over 12% |
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 used in the manufacture of steel, where carbon must be as volatile-free and ash-free as possible.
Coking coal is heated to produce coke, a hard, grey, porous material which is used to blast in furnaces for the extraction of iron from the iron ore.
Bituminous coal in the United States is between 100 to 300 million years old[3].
Much North American coal was created in subsiding areas adjacent to the Appalachian Mountains during the Pennsylvanian subperiod. A vast network of swamps covered large parts of North America at this time and much of the organic material created in these wetlands accumulated to form thick layers of peat (the precursor to coal) that were buried faster than they could decay.
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.
The second largest deposits of the world's bituminous coal are contained within Permian strata in Russia. Australian deposits in the Bowen Basin in Queensland, the Sydney Basin and Perth Basin are Permian coal, where thicknesses in excess of 300 m 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.
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.
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.
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