Coal
Coal is a fossil fuel extracted from the ground by underground mining or open-pit mining. It is a readily combustible black or brownish-black sedimentary rock. It is composed primarily of carbon along with assorted other elements, including sulfur. Often associated with the Industrial Revolution, coal remains an enormously important fuel and is the largest single source of electricity world-wide. In the United States, for example, the coal power plants generate 50% of the electricity produced. In the UK in 2004, coal supplies 28% of electricity production.
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[edit] 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.
[edit] Uses
[edit] Fuel
Coal is primarily used as a solid fuel to produce heat through combustion.
World coal consumption is about 5,800 million short tons (5.3 petagrams) annually, of which about 75% is used for electricity production. The region including China and India uses about 1,700 million short tons (1.5 Pg) annually, forecast to exceed 3,000 million short tons (2.7 Pg) in 2025. The USA consumes about 1,100 million short tons (1.0 Pg) 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 December 2004 (BP Statistical Energy Review, June 2005).
[edit] Gasification
High prices of oil and natural gas are leading to increased interest in "BTU Conversion" technologies such as coal gasification, methanation, liquefacation, and solidification.
Coal gasification breaks down the coal into its components, usually by subjecting it to high temperature and pressure, using steam and measured amounts of oxygen. This leads to the production of carbon dioxide and oxygen, as well as other gaseous compounds.
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.
[edit] Liquefaction
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.
[edit] Coking
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.
[edit] Environmental impact
Combustion of coal, like any other compound containing carbon, produces carbon dioxide (CO2) and nitrogen oxides (NOx) along with varying amounts of sulfur dioxide (SO2) depending on where it was mined. Sulfur dioxide reacts with oxygen to form sulfur trioxide (SO3), which then reacts with water to form Sulfuric Acid. Sulfuric Acid is returned to the Earth as a form of acid rain.
Emissions from coal-fired power plants represent the largest source of carbon dioxide emissions, a primary cause of global warming. Since the carbon content of coal is much higher than oil, burning coal is a more serious threat to global temperatures. 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.
[edit] Coal fires
There are hundreds of coal fires burning around the world. Those burning underground can be difficult to locate and many cannot 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. See also Mine fire.
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. 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.
[edit] World coal reserves
It has been estimated that, as of 1996, there is around one exagram (1 × 1015 kg) of total coal reserves 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" does not 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. British Petroleum, in its annual report 2005, estimated at 2004 end, there were 909,064 million tons of proved coal reserves worldwide, or 164 years reserve to production ratio.
