Most abundant form of coal. It is dark brown to black and has a relatively high heat value. Widely abundant and with the broadest range of commercial uses, it has long been used for steam generation in electric power plants and industrial boiler plants. Certain varieties are also used to make coke, a hard substance of almost pure carbon that is important for smelting iron ore. One major problem is that burning large quantities of bituminous coal that has a medium to high sulfur content contributes to air pollution and produces acid rain. Seealso subbituminous coal.
Learn more about bituminous coal with a free trial on Britannica.com.
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 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). Bulk density typically runs 833 kg/m³ (52 lb/ft³).
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 will contain 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.
Coking coal is used in the manufacture of steel, where carbon must be as volatile-free and ash-free as possible.
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.
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.