Bituminous coal

Bituminous coal, or black coal, is a type of coal containing a tar-like substance called bitumen or asphalt. Its coloration can be black or sometimes dark brown; often there are well-defined bands of bright and dull material within the seams. It is typically hard but friable. Its quality is ranked higher than lignite and sub-bituminous coal, but lesser than anthracite. It is the most abundant rank of coal, with deposits found around the world, often in rocks of Carboniferous age. Bituminous coal is formed from sub-bituminous coal that is buried deeply enough to be heated to 85 °C (185 °F) or higher.

Bituminous coal is used primarily for electrical power generation and in the steel industry. Bituminous coal suitable for smelting iron (coking coal or metallurgical coal) must be low in sulfur and phosphorus. It commands a higher price than other grades of bituminous coal (thermal coal) used for heating and power generation.

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 coal is a particular rank of coal, as determined by the amount and type of carbon present in the coal and the amount of energy it can produce when burned. It is higher in rank than sub-bituminous coal but lower in rank than anthracite. Bituminous coal is the most abundant rank of coal.

Coal rank is based on several characteristics of the coal. The fixed carbon content refers to the percentage of the coal that is neither moisture, nor ash, nor volatile matter. When evaluated on a dry, mineral-matter-free basis, the fixed carbon content is the fraction of the coal that is not volatile organic matter. An agglomerating coal is a coal that softens when heated, forming a hard, gray, porous coke that resists crushing. Vitrinite reflectance is a measure of how reflective a polished surface of an average particle of vitrinite in the coal is. It is determined by how much of the carbon has condensed to an aromatic form from the heat and pressure of deep burial.

In the United States, bituminous coal is defined as agglomerating coal yielding at least 10,500 Btu/lb (24,400 kJ/kg) of energy on combustion (on a moist, mineral-matter-free basis), with a fixed carbon content less than 86% (on a dry, mineral-matter-free basis.) Coal with a higher fixed carbon content is classified as anthracite, while agglomerating coal yielding less than 10,500 Btu/lb (24,400 kJ/kg) or nonagglomerating coal yielding less than 11,500 Btu/lb (26,700 kJ/kg) is classified as sub-bituminous coal. In the international market, bituminous coal is defined as coal with a vitrinite reflectance between 0.5 and 1.9. Vitrinite reflectance is also routinely measured for U.S. coal as a check on its rank classification

Bituminous coal is dark brown to black, hard, but friable. It is commonly composed of thin bands of alternating bright and dull material. Though bituminous coal varies in its chemical composition, a typical composition is about 84.4% carbon, 5.4% hydrogen, 6.7% oxygen, 1.7% nitrogen, and 1.8% sulfur, on a weight basis. Its bank density (the density of a coal seam prior to breaking up during mining) is about 1346 kg/m³ (84 lb/ft³) while the bulk density of extracted coal is up to 833 kg/m³ (52 lb/ft³). Bituminous coal characteristically burns with a smoky flame and softens and swells during combustion. It gets its name from this tendency to form a softened, sticky mass when heated, which reflects the presence of bitumen (mineral tar) in the coal.

Though almost all agglomerating coal is of bituminous rank, some bituminous coal is not agglomerating. Non-agglomerating bituminous coal includes cannel coal and boghead coal. These are nonbanded and nonreflective, and break with a conchoidal fracture. Both are sapropelic, in contrast with most bituminous coal, which is humic (composed of decayed woody tissue of plants). Cannel coal is composed mostly of plant spores, while boghead coal is composed mostly of nonspore algal remains.