Coal is a fossil fuel extracted from the ground either by underground 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 since it was found washed up on beaches occasionally.
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. It is suspected that 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).
Coal as fuel
Coal is primarily used as a solid fuel to produce heat through combustion.
When coal is used in electricity generation, the 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).
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. Gasification is also a possibility for future energy use, as it generally burns hotter and cleaner than conventional coal.
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 catylists 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.
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. (http://www.rexresearch.com/karrick/karric~1.htm)
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  (http://www.findarticles.com/p/articles/mi_m0CYH/is_15_6/ai_89924477), (break-even cost), which is well above historical averages – but is now viable due to the spike in oil prices in 2004-2005.  (http://www.coalpeople.com/old_coalpeople/march03/tiny_tomorrow.htm).
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.
Harmful effects of coal burning
Combustion of coal, like any other carbon containing compond, 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. In addition, emissions from coal-fired power plants are a major contributor to acid rain in some countries. 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 (see  (http://www.ornl.gov/info/ornlreview/rev26-34/text/colmain.html) and  (http://greenwood.cr.usgs.gov/energy/factshts/163-97/FS-163-97.html) for numbers and details). While these substances are trace impurities, if a great deal of coal is burned, significant amounts of these substances are released.
Types of coal
As geological processes apply pressure to peat over time, it is transformed successively into lignite, subbituminous coal, bituminous coal, and finally anthracite.
Lignite, also referred to as brown coal, is the lowest rank of coal and used almost exclusively as fuel for steam-electric power generation. It is brownish-black and has a high inherent moisture content, sometimes as high as 45 percent. The heat content of lignite ranges from 9 to 17 million Btu/ton (10 to 20 MJ/kg) on a moist, mineral-matter-free basis. The heat content of lignite consumed in the United States averages 13 million Btu/ton (15 MJ/kg), on the as-received basis (i.e., containing both inherent moisture and mineral matter).
Sub-bituminous coal is a coal whose properties range from those of lignite to those of bituminous coal and are used primarily as fuel for steam-electric power generation. It may be dull, dark brown to black, soft and crumbly at the lower end of the range, to bright, jet-black, hard, and relatively strong at the upper end. Subbituminous coal contains 20 to 30 percent inherent moisture by weight. The heat content of subbituminous coal ranges from 17 to 24 million Btu per ton on a moist, mineral-matter-free basis. The heat content of subbituminous coal consumed in the United States averages 17 to 18 million Btu/ton (20 to 21 MJ/kg), on the as-received basis (i.e., containing both inherent moisture and mineral matter). A major source of subbituminous coal in the United States is the Powder River Basin in Wyoming.
Bituminous coal is 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. Bituminous coal is the most abundant coal in active U.S. mining regions. Its moisture content usually is less than 20 percent. The heat content of bituminous coal ranges from 21 to 30 million Btu/ton (24 to 35 MJ/kg) on a moist, mineral-matter-free basis. The heat content of bituminous coal consumed in the United States averages 24 million Btu/ton (28 MJ/kg), on the as-received basis (i.e., containing both inherent moisture and mineral matter).
Anthracite is the highest rank of coal; used primarily for residential and commercial space heating. It is hard, brittle, and black lustrous coal, often referred to as hard coal, containing a high percentage of fixed carbon and a low percentage of volatile matter. The moisture content of fresh-mined anthracite generally is less than 15 percent. The heat content of anthracite ranges from 22 to 28 million Btu/ton (26 to 33 MJ/kg) on a moist, mineral-matter-free basis. The heat content of anthracite coal consumed in the United States averages 25 million Btu/ton (29 MJ/kg), on the as-received basis (i.e., containing both inherent moisture and mineral matter). Note: Since the 1980s, anthracite refuse or mine waste has been used for steam electric power generation. This fuel typically has a heat content of 15 million Btu/ton (17 MJ/kg) or less.
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 2,000 Â°F (1,000 Â°C) 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 petroleum refining, which resembles coke but contains too many impurities to be useful in metallurgical applications.
Jet is a compact form of lignite that is sometimes polished and has been used as an ornamental stone since the Iron Age.
World coal reserves
It has been estimated that, as of 1996, there are around one exagram (1 Ã— 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 natual 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 2032. “Peak” doesn’t mean coal will disappear, but defines the time after which no matter efforts are expended coal production will begin to decline in quantity and energy content. [ (http://www.fromthewilderness.com/free/ww3/052504_coal_peak.html)]
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) ( (http://www.eia.doe.gov/emeu/iea/res.html)). 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) ( (http://www.iea.org/dbtw-wpd/bookshop/add.aspx?id=144)). 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.