The examples and perspective in this article deal primarily with the United States and do not represent a worldwide view of the subject. (February 2017)
Bituminous coal, or black coal, is a relatively soft coal containing a tar-like substance called bitumen or asphalt. Its quality is ranked higher than lignite and sub-bituminous coal, but poorer than anthracite. Formation is usually the result of high pressure being exerted on lignite. Its coloration can be black or sometimes dark brown; often there are well-defined bands of bright and dull material within the seams. These distinctive sequences, which are classified according to either "dull, bright-banded" or "bright, dull-banded", is how bituminous coals are stratigraphically identified.
Bituminous coal is an organic sedimentary rock formed by diagenetic and sub metamorphic compression of peat bog material. Its primary constituents are macerals: vitrinite and liptinite. The carbon content of bituminous coal is around 45-86%; the rest is composed of water, air, hydrogen, and sulfur, which have not been driven off from the macerals. Bank density is approximately 1,796 kg/m3 (112.1 lb/cu ft). Bulk density typically runs to 1,683 kg/m3 (105.1 lb/cu ft). The heat content of bituminous coal ranges from 24 to 35 MJ/kg (21 to 30 MMBtu/short ton) on a moist, mineral-matter-free basis.
Within the coal mining industry, this type of coal is known for releasing the largest amounts of firedamp, a dangerous mixture of gases that can cause underground explosions. Extraction of bituminous coal demands the highest safety procedures involving attentive gas monitoring, good ventilation and vigilant site management.
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 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, 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.
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. The constituents of the coal should be as follows:
|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 heated to produce coke, a hard, grey, porous material which is used in blast furnaces to extract iron from the iron ore.
Bituminous coal in the United States is between 100 and 300 million years old.
In Canada, the Western Canada Sedimentary Basin of Alberta and British Columbia hosts major deposits of bituminous coal that formed in swamps along the western margin of the Western Interior Seaway. They range in age from latest Jurassic or earliest Cretaceous in the Mist Mountain Formation, to Late Cretaceous in the Gates Formation. The Intermontane and Insular Coalfields of British Columbia also contain deposits of Cretaceous bituminous coal.
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 15 m 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 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.
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.