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Coal is a fossil fuel extracted from the ground by deep mining, coal mining (open-pit mining or strip mining). It is a readily combustible black or brownish-black sedimentary rock. It is composed primarily of carbon and hydrocarbons, along with assorted other elements, including sulfur. Often associated with the Industrial Revolution, coal remains an enormously important fuel and is the most common source of electricity world-wide. In the United States, for example, the burning of coal generates over half the electricity consumed by the nation.



Etymology and folklore

Coal is thought ultimately to derive its name from the Old English col but this actually meant charcoal at the time; coal was not mined prior to the late Middle Ages; i.e. after ca. 1000 AD. Mineral coal was referred to as sea-coal, either because it was found on beaches occasionally having fallen from the exposed coal seams above, or because it was easier to transport by sea rather than on the very poor road system (in London, England there is still a sea coal road/lane where the coal merchants conducted their business).

It is associated with the astrological sign Capricorn. It is carried by thieves to protect them from detection and to help them to escape when pursued. It is an element of a popular ritual associated with New Year's Eve. To dream of burning coals is a symbol of disappointment, trouble, affliction and loss, unless they are burning brightly, when the symbol gives promise of uplifting and advancement.

Santa Claus is said to leave a lump of coal instead of Christmas presents in the stockings of naughty children.

Composition and creation

Coal consists of more than 50 percent by weight and more than 70 percent by volume of carbonaceous material (including inherent moisture). Coal is formed from plant remains that have been compacted, hardened, chemically altered, and metamorphosed by heat and pressure over geologic time. Much coal was formed from ancient plants that grew in swamp ecosystems. When such plants died, their biomass was deposited in anaerobic, aquatic environments where low oxygen levels prevented their decay and oxidation (rotting and release of carbon dioxide). Successive generations of this type of plant growth and death formed thick deposits of unoxidized organic matter that were subsequently covered by sediments and compacted into carbonaceous deposits such as peat or bituminous or anthracite coal. Evidence of the types of plants that contributed to carbonaceous deposits can occasionally be found in the shale and sandstone sediments that overlie coal deposits, and with special techniques, within the coal itself. The greatest coal-forming time in geologic history was during the Carboniferous era (280 to 345 million years ago).

Types of coal

As geological processes apply pressure to peat over time, it is transformed successively into:

  • Lignite - also referred to as brown coal, is the lowest rank of coal and used almost exclusively as fuel for steam-electric power generation. Jet is a compact form of lignite that is sometimes polished and has been used as an ornamental stone since the Iron Age.
  • Sub-bituminous coal - whose properties range from those of lignite to those of bituminous coal and are used primarily as fuel for steam-electric power generation.
  • Bituminous coal - a dense coal, usually black, sometimes dark brown, often with well-defined bands of bright and dull material, used primarily as fuel in steam-electric power generation, with substantial quantities also used for heat and power applications in manufacturing and to make coke.
  • Anthracite - the highest rank, used primarily for residential and commercial space heating.


Coal rail cars in Ashtabula, Ohio
Coal rail cars in Ashtabula, Ohio

Coal as fuel

See also Clean coal

Coal is primarily used as a solid fuel to produce heat through combustion.

World coal consumption is about 5,800 million short tons annually, of which about 75% is used for electricity production. The region including China and India uses about 1,700 million tons annually, forecast to exceed 3,000 million tons in 2025. [1] The USA consumes about 1,100 million tons of coal each year, using 90% of it for generation of electricity. Coal is the fastest growing energy source in the world, with coal use increasing by 25% for the three-year period ending in Dec. 2004 (BP Statistical Energy Review June 2005).

When coal is used in electricity generation, it is generally pulverized and then burned. The heat produced is used to create steam, which is then used to spin turbines which turn generators and create electricity. Approximately 40% of the Earth's current electricity production is powered by coal, and the total known deposits recoverable by current technologies are sufficient for 300 years' use at current rates (see World Coal Reserves, below).


High prices of oil and natural gas are leading to increased interest in "Btu Conversion" technologies such as coal gasification, methanation and liquefaction.

In the past, coal was converted to make coal-gas, which was piped to customers to burn for illumination, heating, and cooking. At present, the safer natural gas is used instead. South Africa still uses gasification of coal for much of its petrochemical needs.

Gasification is also a possibility for future energy use, as it generally burns hotter and cleaner than conventional coal, can spin a more efficient gas turbine rather than a steam turbine, and makes capturing carbon dioxide for later sequestration much easier.


Coal can also be converted into liquid fuels like gasoline or diesel by several different processes. The Fischer-Tropsch process of indirect synthesis of liquid hydrocarbons was used in Nazi Germany, and for many years by Sasol in South Africa - in both cases, because those regimes were politically isolated and unable to purchase crude oil on the open market. Coal would be gasified to make syngas (a balanced purified mixture of CO and H2 gas) and the syngas condensed using Fischer-Tropsch catalysts to make light hydrocarbons which are further processed into gasoline and diesel. Syngas can also be converted to methanol: which can be used as a fuel, fuel additive, or further processed into gasoline via the Mobil M-gas process.

A direct liquefaction process Bergius process (liquefaction by hydrogenation) is also available but has not been used outside Germany, where such processes were operated both during World War I and World War II. SASOL in South Africa has experimented with direct hydrogenation. Several other direct liquefaction processes have been developed, among these being the SRC-I and SRC-II (Solvent Refined Coal) processes developed by Gulf Oil and implemented as pilot plants in the United States in the 1960's and 1970's.[2]

Yet another process to manufacture liquid hydrocarbons from coal is low temperature carbonization (LTC). Coal is coked at temperatures between 450 and 700°C compared to 800-1000° for metalurgical coke. These temperatures optimize the production of coal tars richer in lighter hydrocarbons than normal coal tar. The coal tar is then further processed into fuels. The process was developed by Lewis Karrick, an oil shale technologist at the U.S. Bureau of Mines in the 1920s.[3]

All of these liquid fuel production methods release CO2 carbon dioxide in the conversion process. CO2 sequestration is proposed to avoid releasing it into the atmosphere. As CO2 is one of the process streams, sequestration is easier than from flue gasses produced in combustion of coal with air, where CO2 is diluted by nitrogen and other gases.

Coal liquefaction is one of the backstop technologies that limit escalation of oil prices. Estimates of the cost of producing liquid fuels from coal suggest that domestic US production of fuel from coal becomes cost-competitive with oil priced at around 35 USD per barrel [4], (break-even cost), which is well above historical averages - but is now viable due to the spike in oil prices in 2004-2005. [5].

Among commercially mature technologies, advantage for indirect coal liquefaction over direct coal liquefaction are reported by Williams and Larson (2003). Estimates are reported for sites in China where break-even cost for coal liquefaction may be in the range between 25 to 35 US$/barrel of oil.

Coking and use of coke

Main article: Coke (fuel)

Coke is a solid carbonaceous residue derived from low-ash, low-sulfur bituminous coal from which the volatile constituents are driven off by baking in an oven without oxygen at temperatures as high as 1,000 °C (2,000 °F) so that the fixed carbon and residual ash are fused together. Coke is used as a fuel and as a reducing agent in smelting iron ore in a blast furnace. Coke from coal is grey, hard, and porous and has a heating value of 24.8 million Btu/ton (29.6 MJ/kg). Byproducts of this conversion of coal to coke include coal-tar, ammonia, light oils, and "coal-gas".

Petroleum coke is the solid residue obtained in oil refining, which resembles coke but contains too many impurities to be useful in metallurgical applications.

Harmful effects of coal burning

Combustion of coal, like any other compound containing carbon, produces carbon dioxide (CO2), along with varying amounts of sulfur dioxide (SO2) depending on where it was mined. Sulfur dioxide reacts with water to form sulfurous acid. If sulfur dioxide is discharged into the atmosphere, it reacts with water vapor and is eventually returned to the Earth as acid rain.

Emissions from coal-fired power plants represent the largest source of artificial carbon dioxide emissions, according to most climate scientists a primary cause of global warming. Many other pollutants are present in coal power station emissions. Some studies claim that coal power plant emissions are responsible for tens of thousands of premature deaths annually in the United States alone. Modern power plants utilize a variety of techniques to limit the harmfulness of their waste products and improve the efficiency of burning, though these techniques are not widely implemented in some countries, as they add to the capital cost of the power plant. To eliminate CO2 emissions from coal plants, carbon sequestration has been proposed but is not yet in large-scale use.

Coal also contains many trace elements, including arsenic and mercury, which are dangerous if released into the environment. Coal also contains low levels of uranium, thorium, and other naturally-occurring radioactive isotopes whose release into the environment may lead to radioactive contamination.[6][7] While these substances are trace impurities, if a great deal of coal is burned, significant amounts of these substances are released.

If coal liquefaction or gasification is used to make petrochemicals, a great deal of carbon dioxide is produced in the process. If a carbon tax was introduced and sufficient CO2 was not captured, the economics of such processes would be significantly less attractive. However, if sequestration or some other process were used to dispose of this by-product, fuels produced from this process would be less polluting. Some process do not have a much greater total impact on carbon dioxide levels than ones refined from petroleum. Others may be less polluting still. Research in this field is ongoing.

Coal fires

There are hundreds of coal fires burning around the world.[8] Those burning underground can be difficult to locate and many can not be extinguished. Fires can cause the ground above to subside, combustion gases are dangerous to life, and breaking out to the surface can initiate surface wildfires.

Coal seams can be set on fire by spontaneous combustion or contact with a mine fire or surface fire. A grass fire in a coal area can set dozens of coal seams on fire.[9] [10] Coal fires in China burn 120 million tons of coal a year, emitting 360 million metric tons of carbon dioxide. This amounts to 2-3% of the annual worldwide production of CO2 from fossil fuels, or as much as emitted from all of the cars and light trucks in the United States. [11] [12]

In the United States , a trash fire was lit in the borough landfill located in an abandoned Anthracite strip mine pit in the portion of the Coal Region called Centralia, Pennsylvania from 1962. It burns underground today, 40 years later.

The reddish siltstone rock that caps many ridges and buttes in the Powder River Basin (Wyoming), and in western North Dakota is called porcelanite, which also may resemble the coal burning waste "clinker" or volcanic "scoria." [13] Clinker is rock that has been fused by the natural burning of coal. In the case of the Powder River Basin approximately 27 to 54 billion metric tons of coal burned within the past three million years. [14] Wild coal fires in the area were reported by the Lewis and Clark expedition as well as explorers and settlers in the area. [15]

The Australian Burning Mountain was originally believed to be a volcano, but the smoke and ash comes from a coal fire which may have been burning for 5,000 years.[16]

World coal reserves

It has been estimated that, as of 1996, there is around one exagram (1 × 1015 kg) of total coal reserves economically accessible using current mining technology, approximately half of it being hard coal. The energy value of all the world's coal is well over 100,000 quadrillion Btu (100 zettajoules). There probably is enough coal to last for 300 years. However, this estimate assumes no rise in population, and no increased use of coal to attempt to compensate for the depletion of natural gas and petroleum. A recent (2003) study by scientist Gregson Vaux, which takes those factors into account, estimates that coal could peak in the United States as early as 2046, on average. "Peak" doesn't mean coal will disappear, but defines the time after which no matter what efforts are expended coal production will begin to decline in quantity and energy content. The disappearance of coal will occur much later, around the year 2267, assuming all other factors do not change, which they naturally will.[17]

US coal regions.
US coal regions.

The United States Department of Energy uses estimates of coal reserves in the region of 1,081,279 million short tons, which is about 4,786 BBOE (billion barrels of oil equivalent) [18]. The amount of coal burned during 2001 was calculated as 2.337 GTOE (gigatonnes of oil equivalent), which is about 46 MBOED (million barrels of oil equivalent per day) [19]. At that rate those reserves will last 285 years. As a comparison natural gas provided 51 MBOED, and oil 76 MBD (million barrels per day) during 2001.

See also

External links

Wikimedia Commons has media related to:


  • Robert H. Williams and Eric D. Larson (December 2003). A comparison of direct and indirect liquefaction technologies for making fluid fuels from coal, Energy for Sustainable Development, VII: 103-129, also [20]
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  2. ^  Cleaner Coal Technology Programme (October 1999). "Technology Status Report 010: Coal Liquefaction." Department of Trade and Industry (UK).
  3. ^  "" Accessed September 9, 2005.
  4. ^  "Diesel Fuel News: Ultra-clean fuels from coal liquefaction: China about to launch big projects - Brief Article." Accessed September 9, 2005.
  5. ^  "Welcome to Coal People Magazine." Accessed September 9, 2005.
  6. ^  "Coal Combustion." Accessed September 9, 2005.
  7. ^  "Radioactive Elements in Coal and Fly Ash, USGS Factsheet 163-97." Accessed September 9, 2005.
  8. ^  "Sino German Coal fire project." Accessed September 9, 2005.
  9. ^  "Committee on Resources-Index." Accessed September 9, 2005.
  10. ^  "" Accessed September 9, 2005.
  11. ^  "EHP 110-5, 2002: Forum." Accessed September 9, 2005.
  12. ^  "Overview about ITC's activities in China." Accessed September 9, 2005.
  13. ^  "North Dakota's Clinker." Accessed September 9, 2005.
  14. ^  "BLM-Environmental Education- The High Plains." Accessed September 9, 2005.
  15. ^  "" Accessed September 9, 2005.
  16. ^  "Burning Mountain Nature Reserve." Accessed September 9, 2005.
  17. ^  "The Peak in U.S. Coal Production." Accessed September 9, 2005.
  18. ^  "International Energy Annual 2003: Reserves." Accessed September 9, 2005.
  19. ^  "IEA Publications Bookshop." Accessed September 9, 2005.
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