A Common Type of Hard Coal with Thermal and Metallurgical Uses
Bituminous and sub-bituminous coal represents more than 90 percent of all the coal consumed in the United States. When burned, the coal produces a high, white flame. Bituminous coal is so-called because it contains a tar-like substance called bitumen. There are two types of bituminous coal: thermal and metallurgical.
Types of Bituminous Coal
- Thermal coal, sometimes called steaming coal, is used to power plants that produce steam for electricity and industrial uses. Trains that run on steam sometimes are fueled with "bit coal," a nickname for bituminous coal.
- Metallurgical coal, sometimes referred to as coking coal, is used in the process of creating coke necessary for iron and steel production. Coke is a rock of concentrated carbon created by heating bituminous coal to extremely high temperatures without air. This process of melting the coal in the absence of oxygen to remove impurities is called pyrolysis.
Characteristics of Bituminous Coal
Bituminous coal contains moisture up to approximately 17 percent. About 0.5 to 2 percent of the weight of bituminous coal is nitrogen. Its fixed carbon content ranges up to approximately 85 percent, with ash content up to 12 percent by weight.
Bituminous coal can be categorized further by the level of volatile matter it contains high-volatile A, B, and C, medium-volatile, and low-volatile. Volatile matter includes any material that is liberated from the coal at high temperature. In the case of coal, volatile matter may include sulfur and hydrocarbons.
- Heating value: Bituminous coal provides approximately 10,500 to 15,000 BTU per pound as mined.
- Availability: Bituminous coal is abundant. More than half of all available coal resources are bituminous.
- Mining locations: In the U.S., bituminous coal can be found in Illinois, Kentucky, West Virginia, Arkansas (Johnson, Sebastian, Logan, Franklin, Pope, and Scott counties), and locations east of the Mississippi River.
Bituminous coal lights on fire easily and can produce excessive smoke and soot -- particulate matter -- if burned improperly. Its high sulfur content contributes to acid rain.
Bituminous coal contains the mineral pyrite, which serves as a host for impurities such as arsenic and mercury. Burning the coal releases trace mineral impurities into the air as pollution. During combustion, approximately 95 percent of bituminous coal's sulfur content gets oxidized and released as gaseous sulfur oxides.
Hazardous emissions from bituminous coal combustion include particulate matter (PM), sulfur oxides (SOx), nitrogen oxides (NOx), trace metals such as lead (Pb) and mercury (Hg), vapor-phase hydrocarbons such as methane, alkanes, alkenes and benzenes, and polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans, commonly known as dioxins and furans. When burned, bituminous coal also releases hazardous gasses such as hydrogen chloride (HCl), hydrogen fluoride (HF) and polycyclic aromatic hydrocarbons (PAHs).
Incomplete combustion leads to higher levels of PAHs, which are carcinogenic. Burning bituminous coal at higher temperatures reduces its carbon monoxide emissions. Therefore, large combustion units and well-maintained ones generally have lower pollution output. Bituminous coal has slagging and agglomerating characteristics.
Bituminous coal combustion releases more pollution into the air than sub-bituminous coal combustion, but due to its greater heat content, less of the fuel is required to produce electricity. As such, bituminous and sub-bituminous coals produce approximately the same amount of pollution per kilowatt of electricity generated.
In the early 20th century, bituminous coal mining was an exceptionally dangerous job, taking an average of 1,700 coal miners lives annually. During that same time period, approximately 2,500 workers per year were left permanently disabled as a result of coal mining accidents.
Tiny particles of waste bituminous coal that are left over after preparation of commercial grade coal are called "coal fines." Fines are light, dusty, and difficult to handle, and traditionally were stored with water in slurry impoundments to keep them from blowing away.
New technologies have been developed to reclaim fines. One approach uses a centrifuge to separate the coal particles from slurry water. Other approaches bind the fines into briquettes that have low moisture content, making them suitable for fuel use.
Ranking: Bituminous coal ranks second in heat and carbon content compared with other types of coal, according to ASTM D388 - 05 Standard Classification of Coals by Rank.