Types of coal
Coal is classified based on the extent to which it has been altered from the plant material that it is derived from. The degree of alteration determines the rank of the coal. Rank is determined via proximate analysis, a series of procedures that assess the proportionate amounts of various constituents of coal, such as moisture (water), volatile matter, fixed carbon, and ash.
Coal is classified into four main ranks: lignite, subbituminous, bituminous, anthracite, depending on the amounts and types of carbon it contains and on the amount of heat energy it can produce. The rank of a deposit of coal depends on the pressure and heat acting on the plant debris as it sank deeper and deeper over millions of years.
Low rank coals, such as lignite and sub-bituminous coals, are typically softer, friable materials with a dull, earthy appearance. They are characterized by high moisture levels and low carbon content, and therefore a low energy content.
Higher rank coals are generally harder and stronger and often have a black vitreous luster. They contain more carbon, have lower moisture content, and produce more energy. Anthracite is at the top of the rank scale and has a correspondingly higher carbon and energy content and a lower level of moisture.
Peat is not defined as a coal, but it is an important early material formed in the coalification process. Peat is a soft organic material consisting of partly decayed plant matter together with deposited minerals. Peat forms in wetlands or peatlands, variously called bogs, moors, muskegs, pocosins, mires, and peat swamp forests. Peatlands are situated in wetland areas, primarily in the temperate and cold belt of the Northern Hemisphere, where large deposits developed from the gradual decomposition of plant matter under anaerobic conditions. Under the right conditions, peat is the earliest stage in the formation of coal.
Lignite, or brown coal, is the lowest rank of coal. It has a 46–60 percent fixed-carbon content, but the lowest heating value, 5,500–8,300 Btu/lb (5.8–8.8 million joules/lb) of all coals. Because of its low energy density, lignite coal is inefficient to transport and is not traded extensively on the world market compared with higher coal grades. It is often burned in power stations constructed very close to mines, so-called mine mouth power plants. Emissions from lignite coal fired plants are generally higher than for higher rank coal plants.
Lignite has a high content of volatile matter which makes it easier to convert into gas and liquid petroleum products than higher ranking coals. However, its high moisture content and susceptibility to spontaneous combustion can cause problems in transportation and storage.
Lignite coal that has been subjected to longer and deeper burial is converted to a darker and harder coal known as subbituminous coal.Subbituminous coal has a higher heating value than lignite. Subbituminous coal has a 46–60 percent fixed-carbon content and a heating value of 8,300–13,000 Btu/lb (8.8–13.7 million joules/lb). Although its heat value is lower, subbituminous coal coal generally has a lower sulfur content than other types, which makes it attractive for use because it is cleaner burning.
Bituminous coal is a soft coal that produces smoke and ash when burned, has a 46–86 percent fixed-carbon content and a heating value of 11,000–15,000 Btu/lb (11.6–15.8 million joules/lb). Bituminous coal is considered a sedimentary rock because is a product of the deep burial and compaction of plant and mineral matter. It 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.
Bituminous coal is the most abundant economically recoverable coal globally and the main fuel burned in steam turbine-powered electric generating plants. Some bituminous coals, known as metallurgical or coking coals, have properties that make them suitable for conversion to coke used in steel-making
The highest rank of coal is anthracite, a hard black coal that burns with little flame and smoke, has the highest fixed-carbon content, 86–98 percent, and a heating value of 13,500–15,600 Btu/lb (14.2–16.5 million joules/lb).
Other terms which refer to anthracite are blue coal, hard coal, stone coal, blind coal (in Scotland), Kilkenny coal (in Ireland), crow coal (or craw coal from its shiny black appearance), and black diamond.
Anthracite coal may be considered to be a transition stage between ordinary bituminous coal and graphite, produced by the more or less complete elimination of the volatile constituents of the former; and it is found most abundantly in areas that have been subjected to considerable earth-movements, such as the flanks of great mountain ranges. Anthracite coal is a product of metamorphism and is associated with metamorphic rocks, just as bituminous coal is associated with sedimentary rocks. For example, the compressed layers of anthracite that are deep mined in the folded (metamorphic) Appalachian Mountains of the Coal Region of northeastern Pennsylvania are extensions of the layers of bituminous coal that are strip mined on the (sedimentary) Allegheny Plateau of Kentucky and West Virginia, and Western Pennsylvania.
- Energy Information Administration, U.S. Department of Energy, Coal Basics 101, Accessed 2 January 2009.
- Kentucky Geological Survey, University of Kentucky, Coal, Accessed 2 January 2009.
- U.S. Department of the Interior, U.S. Geological Survey, Peat, 2007 Minerals Yearbook.
- Wikipedia Contributors, Coal, Wikipedia The Free Encyclopedia, Accessed 2 January 2009.
- World Coal Institute, Coal Types, Accessed 2 January 2009.