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| Sunday, 3 July 2005 |
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Junior Observer |  |
| News Business Features Editorial
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Coal - a main source of energy Coal is an enormously important fuel and is the most common source of electricity worldwide. Coal is a fossil fuel, removed from the ground either by underground mining, open-pit mining or strip mining. It is a readily flammable black or brownish-black rock. Coal is composed of carbon and hydrocarbons, along with an assortment of other elements, including sulphur. Coal consists of more than 50 per cent by weight and more than 70 per cent by volume of carbonaceous material (including permanent moisture). Coal is formed from plant remains that have been compacted, hardened, chemically altered, and metamorphosed (change of form) by heat and pressure over geologic time. It is thought that coal was formed from ancient plants that grew in swamp ecosystems. When such plants died, their biomass was deposited in anaerobic (not requiring oxygen), 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 unoxidised 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 (slate - like stone) and sandstone sediments that overlie coal deposits, and with special techniques, within the coal itself. Coal is primarily used as a solid fuel to produce heat through combustion. When coal is used in electricity generation, it is generally crushed and then burned. The heat produced is used to create steam, which is then used to spin turbines that turn generators and create electricity. Nearly 40 per cent 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. In the past, coal was converted to make coal-gas, which was piped to customers to be used 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. Countries like South Africa still uses gasification of coal for much of its petrochemical needs. Coal can also be converted into liquid fuels like gasoline or diesel by several different processes. Coal would be gasified to make syngas (a balanced purified mixture of CO and H2 gas). 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. Harmful effects of coal burning Combustion of coal, like any other carbon containing compound, produces carbon dioxide (CO2), along with varying amounts of sulphur dioxide (SO2) depending on where it was mined. Sulphur dioxide reacts with water to form sulfurous acid. If sulphur dioxide is discharged into the atmosphere, it reacts with water vapour 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, and is a primary cause of global warming. Many other pollutants are present in coal power station emissions. Some foreign studies claim that coal power plant emissions are responsible for tens of thousands of premature deaths around the world annually. Emissions from coal-fired power plants are a major contributor to acid rain in some countries. Modern power plants use 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. *** World coal reserves Of the total coal reserves economically accessible, using current mining technology, half is 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. *** 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 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. |
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