INTRODUCTION
Coal, a combustible organic rock composed primarily of carbon, hydrogen, and oxygen. Coal is burned to produce energy and is usedto manufacture steel. It is also an important source of chemicals used to make medicine, fertilizers, pesticides, and other products. Coal comes from ancient plants buried over millions of years in Earth's crust, its outermost layer.Coal,petroleum, natural gas, and oil shale are all known as fossil fuels because they come from the remains of ancient life buried deep in the crust.
Coal is rich in hydrocarbons (compounds made up of the elements hydrogen and carbon). All life forms contain hydrocarbons, and in general, material that contains hydrocarbons is called organic material. Coal originally formed from ancient plants that died, decomposed, and were buried under layers of sediment during the Carboniferous Period, about 360 million to 290 million years ago. As more and more layers of sediment formed over this decomposed plant material, the overburden exerted increasing heat and weight on the organic matter. Over millions of years, these physical conditions caused coal to form from the carbon, hydrogen, oxygen, nitrogen, sulfur, and inorganic mineral compounds in the plant matter. The coal formed in layers known as seams.
Plant matter changes into coal in stages. In each successive stage, higher pressure and heat from the accumulating overburden increase the carbon content of the plant matter and drive out more of its moisture content. Scientists classify coal according to its fixed carbon content, or the amount of carbon the coal produces when heated under controlled conditions. Higher grades of coal have a higher fixed carbon content.
HISTORY OF COAL
Early humans used wood, straw, and dried dung for fuel. One of the earliest known references to coal was made by Greek philosopher and scientist Aristotle, who referred to charcoallike rock found in Thrace (a region on the northeastern shore of the Aegean Sea) and in northeastern Italy. Although authentic records are unavailable, historians believe coal was first used commercially in China. Reports indicate the Fu-shun mine in northeastern China provided coal for smelting copper and for casting coins around 1000 bc .
Coal cinders found among Roman ruins in England suggest that the Romans harnessed energy from coal before ad 400. The written records of the monk Reinier of Liège from the early 13th century describe workers mining black earth in Europe. Blacksmiths used this black earth as fuel for metalworking. Other historical records contain numerous references to coal mining in England, in Scotland, and in continental Europe throughout the 13th century.
In the early 18th century the demand for coal escalated when English iron founders John Wilkinson and Abraham Darby used coal, in the form of coke, to manufacture iron. An almost constant demand for coal was created by metallurgical and engineering developments, most notably the invention of the coal-burning steam engine by Scottish mechanical engineer James Watt in 1769.
Until the American Revolution (1775-1783), most of the coal consumed by the American colonies was imported from England or Nova Scotia. Wartime shortages and the need to manufacturemunitions spurred the formation of small American coal-mining companies that mined Virginia's Appalachian bituminous field and other deposits. By the early 1830s U.S. mining companies had emerged throughout the Appalachian region and along the Ohio, Illinois, and Mississippi rivers. The construction of the first practical locomotive in 1804 in England by British engineer Richard Trevithick sparked a tremendous demand for coal. The growth of the railroad industry and the subsequent rise of the steel industry in the 1800s spurred enormous growth in the coal industry in the United States and Europe.
The widespread use of petroleum as a fuel before, during, and after World War I (1914-1918) eventually reduced the demand for coal. The change from coal to oil as fuel in warships (particularly in the United States and British navies) in the early 1900s, the switch in the railway industry to diesel-electric locomotive engines in the 1940s and 1950s, and increasing use of natural gas as a heating fuel all contributed to a decline in coal production. In the 1980s and 1990s, petroleum continued to supplant coal in industry and was increasingly used in oil-fired power plants. Still, electric utilities continued to burn large amounts of coal to produce electricity.
MODERN USES OF COAL
Eighty-six percent of the coal used in the United States is burned by electric power plants to produce electricity. When burned, coal generates energy in the form of heat. In a power plant that uses coal as fuel, this heat converts water into steam, which is pressurized to spin the shaft of a turbine . This spinning shaft drives a generator that converts the mechanical energy of the rotation into electric power
Coal is also used in the steel industry. The steel industry uses coal by first heating it and converting it into coke , a hard substance consisting of nearly pure carbon. The coke is combined with iron ore and limestone. Then the mixture is heated to produce iron. Other industries use different coal gases given off during the coke-forming process to make fertilizers, solvents, medicine, pesticides, and other products.
Fuel companies convert coal into easily transportable gas or liquid fuels. Coal-based vapor fuels are produced through the process of gasification. Gasification may be accomplished either at the site of the coalmine or in processing plants. In processing plants, the coal is heated in the presence of steam and oxygen to produce synthesis gas, a mixture of carbon monoxide, hydrogen, and methane used directly as fuel or refined into cleaner-burning gas.
On-site gasification is accomplished by controlled, incomplete burning of an underground coal bed while adding air and steam. To do this, workers ignite the coal bed, pump air and steam underground into the burning coal, and then pump the resulting gases from the ground. Once the gases are withdrawn, they may be burned to produce heat or generate electricity. Or they may be used in synthetic gases to produce chemicals or to help create liquid fuels.
Liquefaction processes convert coal into a liquid fuel that has a composition similar to that of crude petroleum. Coal can be liquefied either by direct or indirect processes. However, because coal is a hydrogen-deficient hydrocarbon, any process used to convert coal to liquid or other alternative fuels must add hydrogen. Four general methods are used for liquefaction: (1) pyrolysis and hydrocarbonization, in which coal is heated in the absence of air or in a stream of hydrogen; (2) solvent extraction, in which coalhydrocarbons are selectively dissolved and hydrogen is added to produce the desired liquids; (3) catalytic liquefaction, in which hydrogenation takes place in the presence of a catalyst; and (4) indirect liquefaction, in which carbon monoxide and hydrogen are combined in the presence of a catalyst.
COAL FORMATION
Coal is a sedimentary rock formed from plants that flourished millions of years ago when tropical swamps covered large areas of the world. Lush vegetation, such as early club mosses, horsetails, and enormous ferns, thrived in these swamps. Generations of this vegetation died and settled to the swamp bottom, and over time the organic material lost oxygen and hydrogen, leaving the material with a high percentageof carbon. Layers of mud and sand accumulated over the decomposed plant matter, compressing and hardening the organic material as the sediments deepened. Over millions of years, deepening sediment layers, known as overburden, exerted tremendous heat and pressure on the underlying plant matter, which eventually became coal.
Before decayed plant material forms coal, the plant material forms a dark brown, compact organic material known as peat. Although peat will burn when dried, it has a low carbon and high moisture content relative to coal. Most of coal's heating value comes from carbon, whereas inorganic materials, such as moisture and minerals, detract from its heating value. For this reason, peat is a less efficient fuel source than coal. Over time, as layers of sediment accumulate over the peat, this organic material forms lignite , the lowest grade of coal. As the thickening geologic overburden gradually drives moisture from the coal and increases its fixed carbon content, coal evolves from lignite into successively higher-graded coals: subbituminous coal, bituminous coal, and anthracite . Anthracite, the highest rank of coal, has nearly twice the heating value of lignite.
Coal formation began during the Carboniferous Period (known as the first coal age ), which spanned 360 million to 290 million years ago. Coal formation continued throughout the Permian , Triassic , Jurassic , Cretaceous , and Tertiary Periods (known collectively as the second coal age ), which spanned 290 million to 1.6 million years ago. Coals formed during the first coal age are older, so they are generally located deeper in Earth's crust. The greater heat and pressures at these depths produce higher-grade coals such as anthracite and bituminous coals. Conversely, coals formed during the second coal age under less intense heat and pressure are generally located at shallower depths. Consequently, these coals tend to be lower-grade subbituminous and lignite coals.
COMPONENTS OF COAL
Coal contains organic (carbon-containing) compounds transformed from ancient plant material. The original plant material was composed of cellulose , the reinforcing material in plant cell walls; lignin, the substance that cements plant cells together; tannins , a class of compounds in leaves and stems; and other organic compounds, such as fats and waxes. In addition to carbon, these organic compounds contain hydrogen, oxygen, nitrogen, and sulfur. After a plant dies and begins to decay on a swamp bottom, hydrogen and oxygen (and smaller amounts of other elements) gradually dissociate from the plant matter, increasing its relative carbon content.
Coal also contains inorganic components, known as ash . Ash includes minerals such as pyrite and marcasite formed from metals that accumulated in the living tissues of the ancient plants. Quartz, clay , and other minerals are also added to coal deposits by wind and groundwater. Ash lowers the fixed carbon content of coal, decreasing its heating value.
COAL DEPOSITS AND RESERVES
Although coal deposits exist in nearly every region of the world, commercially significant coal resources occur only in Europe, Asia, Australia, and North America. Commercially significant coal deposits occur in sedimentary rock basins, typically sandwiched as layers called beds or seams between layers of sandstone and shale.
When experts develop estimates of the world's coal supply, they distinguish between coal reserves and resources . Reserves are coal deposits that can be mined profitably with existing technology—that is, with current equipment and methods. Resources are an estimate of the world's total coal deposits, regardless of whether the deposits are commercially accessible. Exploration geologists have found and mapped the world's most extensive coal beds. At the beginning of 2001, global coal reserves were estimated at 984.2 billion metric tons, in which 1 metric ton equals 1,016 kg (2,240 lb). These reserves occurred in the following regions by order of importance: the Asia Pacific, including Australia, 29.7 percent; North America, 26.1 percent; Russia and the countries of the former Union of Soviet Socialist Republics (USSR), 23.4 percent; Europe, excluding the former USSR, 12.4 percent; Africa and the Middle East, 6.2 percent; and South and Central America, 2.2 percent.
Coal deposits in the United Kingdom, which led the world in coal production until the 20th century, extend throughout parts of England, Wales, and southern Scotland. Coalfields in western Europe underlie the Saar and Ruhr valleys in Germany, the Alsace region of France, and areas of Belgium. Coalfields in central Europe extend throughout parts of Poland, the Czech Republic, and Hungary. The most extensive and valuable coalfield in eastern Europe is the Donets Basin, between the Dnieper and Don rivers (in parts of Russia and Ukraine). Large coal deposits in Russia are being mined in the Kuznetsk Basin in southern Siberia. Coalfields underlying northwestern China are among the largest in the world. Mining of these fields began in the 20th century.
In 2006 estimates of total U.S. coal reserves were approximately 17 billion metric tons. At the beginning of the 21st century production amounted to about 980 million metric tons each year.
COAL MINING
Coal mining is the removal of coal from the ground. The mining method employed to extract the coal depends on the following criteria: (1) seam thickness, (2) the overburden thickness, (3) the ease of removal of the overburden, (4) the ease with which a shaft can be sunk to reach the coal seam, (5) the amount of coal extracted relative to the amount that cannot be removed, and (6) the market demand for the coal.
The two types of mining methods are surface mining and underground mining. In surface mining the layers of rock or soil overlying a coal seam are first removed after which the coal is extracted from the exposed seam. In underground mining, a shaft is dug to reach the coal seam. Currently, underground mining accounts for approximately 60 percent of the world recovery of coal.
ENVIRONMENTAL ISSUES
Coal mining is the removal of coal from the ground. The mining method employed to extract the coal depends on the following criteria: (1) seam thickness, (2) the overburden thickness, (3) the ease of removal of the overburden, (4) the ease with which a shaft can be sunk to reach the coal seam, (5) the amount of coal extracted relative to the amount that cannot be removed, and (6) the market demand for the coal.
The two types of mining methods are surface mining and underground mining. In surface mining the layers of rock or soil overlying a coal seam are first removed after which the coal is extracted from the exposed seam. In underground mining, a shaft is dug to reach the coal seam. Currently, underground mining accounts for approximately 60 percent of the world recovery of coal.
Surface mining has resulted in a great deal of damage to the landscape. Many surface mines have removed acres of vegetation and altered topographic features, such as hills and valleys, leaving soil exposed for erosion. Longwall mining, which allows the mine to collapse, results in widespread land subsidence, or sinking. Coal and rock waste, often dumped indiscriminately during surface and underground mining processes, weathers rapidly, producing acid drainage . Acid drainage contains sulfur-bearing compounds that combine with oxygen in water vapor to form sulfuric acid . In addition, weathering of coalmine waste can produce alkaline compounds, heavy metals, and sediments. Acid drainage, alkaline compounds, heavy metals, and sediment leached from mine waste into groundwater or washed away by rainwater can pollute streams, rivers, and lakes.
Today, enterprises in many countries must secure government permits before mining for coal. In the United States, mining companies must submit plans detailing proposed methods for blasting, road construction, land reclamation, and waste disposal. New land reclamation methods, driven by stringent laws and regulations, require coal mining companies to restore strip-mined landscapes to nearly premined conditions.
BURNING OF COAL
The burning of coal produces environmentally harmful emissions. Some gases produced from burning coal, such as carbon dioxide, are known as greenhouse gases because they trap the Earth's heat like the roof of a greenhouse and may contribute to global warming. Other emissions from coal combustion can lead to air and water pollution.
Some major outcomes are :
(1) The Greenhouse Effect : Earth absorbs much of the heat energy radiated from the Sun. The planet then reradiates this heat back into the atmosphere. Carbon dioxide and some other gases that are naturally present in the atmosphere prevent much of the heat from escaping back into space, maintaining Earth at a temperature that can support life. These gases are known as greenhouse gases because they trap the Sun's heat in much the same way as the glass roof of a botanical greenhouse. However, the immense quantity of fossil fuels burned during the world's rapid industrialization over the last 200 years has raised levels of carbon dioxide in the atmosphere by about 28 percent. This dramatic increase in atmospheric carbon dioxide, coupled with continuing depletion of the world's forests, which absorb carbon dioxide, has led many scientists to predict a heating of the atmosphere on a global scale. Such a global warming could disrupt weather patterns, cause the polar ice caps to melt, and possibly lead to other environmental problems.
Today, many industrial countries are working to reduce emissions of greenhouse gases. One proposal is to establish a system requiring companies that create greenhouse gases to pay to emit carbon dioxide above a specified level. This payment could take several forms, including (1) purchasing the rights to pollute from a company with carbon dioxide emissions below the specified level; (2) purchasing forests, which absorb carbon dioxide, and keeping them from being developed; or (3) paying to upgrade a plant in a developing country, thus lowering that plant's carbon dioxide emissions.
(2) Acid Rain : Another environmental problem is acid rain , which forms from sulfur contained in coal. As coal burns, the sulfur combines with oxygen in the air to form sulfur dioxide. As sulfur dioxide is released into the atmosphere, this compound reacts with atmospheric moisture, forming sulfuric acid. This acidic moisture eventually falls back to Earth in the form of precipitation known as acid rain. Environmental studies indicate that acid rain damages crops and forests as well as streams, lakes, and rivers.
The U.S. Clean Air Act, implemented in 1970 and revised in 1970 and 1990, is the federal law regulating air pollution in the United States. This legislation has significantly reduced emissions of sulfur oxides, known as acid gases. For example, the Clean Air Act requires facilities such as coal-burning power plants to burn low-sulfur coal. High-grade coals (coals with a higher heating value) generally contain more sulfur than low-grade coals such as lignite and subbituminous coal. Therefore, certain processes have been developed to remove sulfur-bearing compounds from high-grade coal prior to burning. The Clean Air Act also requires use of pollution-trapping equipment such as air scrubbers (devices installed inside plant smokestacks to remove sulfur dioxide from coal emissions). In addition, revisions to the Clean Air Act in 1990 established a system that allows coal-burning power plants to buy and sell sulfur emission permits with one another. This system tries to establish a financial incentive to lower sulfur emissions by rewarding power plants that reduce emissions below federal levels. Power plants that cut their sulfur emissions below the permitted levels can sell permits to burn coal to companies that exceed federal levels. Companies that reduce emissions reap financial rewards while polluters must pay an extra cost to pollute.
(3) Fly Ash : The burning of coal releases ashes known as fly ash into the atmosphere. Fly ash contains toxic metals such as arsenic and cadmium. In the United States the Clean Air Act requires that fly ash be removed from coal emissions. As a result, antipollution devices such as air scrubbers, baghouses, and electrostatic precipitators are used to trap these pollutants. Baghouses work like giant vacuum cleaners, drawing coal emissions through giant fabric bags that trap the fly ash inside. Electrostatic precipitators use discharge electrodes (electrically charged parts of an electric circuit) to trap ash particles. In an electrostatic precipitator the electrodes are located between long, positively charged collection plates. As the fly ash passes between these collection plates, the discharge electrodes give each particle a negative charge. These negatively charged particles are then attracted to and held by the positively charged collection plates.
CONCLUSION : OPTING FOR CLEAN TECHNOLOGIES
Since 1986 the United States government and private industry have been working together to develop cleaner and more efficient ways to harness the energy in coal. This joint effort, known as the Clean Coal Technology Demonstration Program, includes several technologies, such as fluidized bed coal combustion, furnace sorbent injection, and advanced flue-gas desulfurization.
Fluidized bed coal combustion burns coal in a limestone bed that transfers heat to water, generating steam. This steam is pressurized and used to turn a turbine shaft, which subsequently drives an electric generator. The limestone absorbs sulfur dioxide emitted by the coal, thus reducing the amount of acid gases released during combustion.
A process called furnace sorbent injection removes acid gas from coal emissions at less cost than expensive scrubbers. A sorbent is a highly absorbent material, such as powdered limestone. It is injected into furnaces, where the powdered limestone reacts with the acid gases emitted by the burning coal. The used powder is siphoned away through the furnace out take and is captured (with fly ash) in a baghouse or electrostatic precipitator.
A process called advanced flue-gas desulfurization also removes acid gas from burning coal without expensive scrubbers. Emissions from burning coal are piped into a container called an absorber, where the acid gases react with an absorbing solution (such as a mixture of lime, water, and oxygen). This reaction forms gypsum , a soft white mineral valuable as an ingredient in cement.
Coal, a combustible organic rock composed primarily of carbon, hydrogen, and oxygen. Coal is burned to produce energy and is usedto manufacture steel. It is also an important source of chemicals used to make medicine, fertilizers, pesticides, and other products. Coal comes from ancient plants buried over millions of years in Earth's crust, its outermost layer.Coal,petroleum, natural gas, and oil shale are all known as fossil fuels because they come from the remains of ancient life buried deep in the crust.
Coal is rich in hydrocarbons (compounds made up of the elements hydrogen and carbon). All life forms contain hydrocarbons, and in general, material that contains hydrocarbons is called organic material. Coal originally formed from ancient plants that died, decomposed, and were buried under layers of sediment during the Carboniferous Period, about 360 million to 290 million years ago. As more and more layers of sediment formed over this decomposed plant material, the overburden exerted increasing heat and weight on the organic matter. Over millions of years, these physical conditions caused coal to form from the carbon, hydrogen, oxygen, nitrogen, sulfur, and inorganic mineral compounds in the plant matter. The coal formed in layers known as seams.
Plant matter changes into coal in stages. In each successive stage, higher pressure and heat from the accumulating overburden increase the carbon content of the plant matter and drive out more of its moisture content. Scientists classify coal according to its fixed carbon content, or the amount of carbon the coal produces when heated under controlled conditions. Higher grades of coal have a higher fixed carbon content.
HISTORY OF COAL
Early humans used wood, straw, and dried dung for fuel. One of the earliest known references to coal was made by Greek philosopher and scientist Aristotle, who referred to charcoallike rock found in Thrace (a region on the northeastern shore of the Aegean Sea) and in northeastern Italy. Although authentic records are unavailable, historians believe coal was first used commercially in China. Reports indicate the Fu-shun mine in northeastern China provided coal for smelting copper and for casting coins around 1000 bc .
Coal cinders found among Roman ruins in England suggest that the Romans harnessed energy from coal before ad 400. The written records of the monk Reinier of Liège from the early 13th century describe workers mining black earth in Europe. Blacksmiths used this black earth as fuel for metalworking. Other historical records contain numerous references to coal mining in England, in Scotland, and in continental Europe throughout the 13th century.
In the early 18th century the demand for coal escalated when English iron founders John Wilkinson and Abraham Darby used coal, in the form of coke, to manufacture iron. An almost constant demand for coal was created by metallurgical and engineering developments, most notably the invention of the coal-burning steam engine by Scottish mechanical engineer James Watt in 1769.
Until the American Revolution (1775-1783), most of the coal consumed by the American colonies was imported from England or Nova Scotia. Wartime shortages and the need to manufacturemunitions spurred the formation of small American coal-mining companies that mined Virginia's Appalachian bituminous field and other deposits. By the early 1830s U.S. mining companies had emerged throughout the Appalachian region and along the Ohio, Illinois, and Mississippi rivers. The construction of the first practical locomotive in 1804 in England by British engineer Richard Trevithick sparked a tremendous demand for coal. The growth of the railroad industry and the subsequent rise of the steel industry in the 1800s spurred enormous growth in the coal industry in the United States and Europe.
The widespread use of petroleum as a fuel before, during, and after World War I (1914-1918) eventually reduced the demand for coal. The change from coal to oil as fuel in warships (particularly in the United States and British navies) in the early 1900s, the switch in the railway industry to diesel-electric locomotive engines in the 1940s and 1950s, and increasing use of natural gas as a heating fuel all contributed to a decline in coal production. In the 1980s and 1990s, petroleum continued to supplant coal in industry and was increasingly used in oil-fired power plants. Still, electric utilities continued to burn large amounts of coal to produce electricity.
MODERN USES OF COAL
Eighty-six percent of the coal used in the United States is burned by electric power plants to produce electricity. When burned, coal generates energy in the form of heat. In a power plant that uses coal as fuel, this heat converts water into steam, which is pressurized to spin the shaft of a turbine . This spinning shaft drives a generator that converts the mechanical energy of the rotation into electric power
Coal is also used in the steel industry. The steel industry uses coal by first heating it and converting it into coke , a hard substance consisting of nearly pure carbon. The coke is combined with iron ore and limestone. Then the mixture is heated to produce iron. Other industries use different coal gases given off during the coke-forming process to make fertilizers, solvents, medicine, pesticides, and other products.
Fuel companies convert coal into easily transportable gas or liquid fuels. Coal-based vapor fuels are produced through the process of gasification. Gasification may be accomplished either at the site of the coalmine or in processing plants. In processing plants, the coal is heated in the presence of steam and oxygen to produce synthesis gas, a mixture of carbon monoxide, hydrogen, and methane used directly as fuel or refined into cleaner-burning gas.
On-site gasification is accomplished by controlled, incomplete burning of an underground coal bed while adding air and steam. To do this, workers ignite the coal bed, pump air and steam underground into the burning coal, and then pump the resulting gases from the ground. Once the gases are withdrawn, they may be burned to produce heat or generate electricity. Or they may be used in synthetic gases to produce chemicals or to help create liquid fuels.
Liquefaction processes convert coal into a liquid fuel that has a composition similar to that of crude petroleum. Coal can be liquefied either by direct or indirect processes. However, because coal is a hydrogen-deficient hydrocarbon, any process used to convert coal to liquid or other alternative fuels must add hydrogen. Four general methods are used for liquefaction: (1) pyrolysis and hydrocarbonization, in which coal is heated in the absence of air or in a stream of hydrogen; (2) solvent extraction, in which coalhydrocarbons are selectively dissolved and hydrogen is added to produce the desired liquids; (3) catalytic liquefaction, in which hydrogenation takes place in the presence of a catalyst; and (4) indirect liquefaction, in which carbon monoxide and hydrogen are combined in the presence of a catalyst.
COAL FORMATION
Coal is a sedimentary rock formed from plants that flourished millions of years ago when tropical swamps covered large areas of the world. Lush vegetation, such as early club mosses, horsetails, and enormous ferns, thrived in these swamps. Generations of this vegetation died and settled to the swamp bottom, and over time the organic material lost oxygen and hydrogen, leaving the material with a high percentageof carbon. Layers of mud and sand accumulated over the decomposed plant matter, compressing and hardening the organic material as the sediments deepened. Over millions of years, deepening sediment layers, known as overburden, exerted tremendous heat and pressure on the underlying plant matter, which eventually became coal.
Before decayed plant material forms coal, the plant material forms a dark brown, compact organic material known as peat. Although peat will burn when dried, it has a low carbon and high moisture content relative to coal. Most of coal's heating value comes from carbon, whereas inorganic materials, such as moisture and minerals, detract from its heating value. For this reason, peat is a less efficient fuel source than coal. Over time, as layers of sediment accumulate over the peat, this organic material forms lignite , the lowest grade of coal. As the thickening geologic overburden gradually drives moisture from the coal and increases its fixed carbon content, coal evolves from lignite into successively higher-graded coals: subbituminous coal, bituminous coal, and anthracite . Anthracite, the highest rank of coal, has nearly twice the heating value of lignite.
Coal formation began during the Carboniferous Period (known as the first coal age ), which spanned 360 million to 290 million years ago. Coal formation continued throughout the Permian , Triassic , Jurassic , Cretaceous , and Tertiary Periods (known collectively as the second coal age ), which spanned 290 million to 1.6 million years ago. Coals formed during the first coal age are older, so they are generally located deeper in Earth's crust. The greater heat and pressures at these depths produce higher-grade coals such as anthracite and bituminous coals. Conversely, coals formed during the second coal age under less intense heat and pressure are generally located at shallower depths. Consequently, these coals tend to be lower-grade subbituminous and lignite coals.
COMPONENTS OF COAL
Coal contains organic (carbon-containing) compounds transformed from ancient plant material. The original plant material was composed of cellulose , the reinforcing material in plant cell walls; lignin, the substance that cements plant cells together; tannins , a class of compounds in leaves and stems; and other organic compounds, such as fats and waxes. In addition to carbon, these organic compounds contain hydrogen, oxygen, nitrogen, and sulfur. After a plant dies and begins to decay on a swamp bottom, hydrogen and oxygen (and smaller amounts of other elements) gradually dissociate from the plant matter, increasing its relative carbon content.
Coal also contains inorganic components, known as ash . Ash includes minerals such as pyrite and marcasite formed from metals that accumulated in the living tissues of the ancient plants. Quartz, clay , and other minerals are also added to coal deposits by wind and groundwater. Ash lowers the fixed carbon content of coal, decreasing its heating value.
COAL DEPOSITS AND RESERVES
Although coal deposits exist in nearly every region of the world, commercially significant coal resources occur only in Europe, Asia, Australia, and North America. Commercially significant coal deposits occur in sedimentary rock basins, typically sandwiched as layers called beds or seams between layers of sandstone and shale.
When experts develop estimates of the world's coal supply, they distinguish between coal reserves and resources . Reserves are coal deposits that can be mined profitably with existing technology—that is, with current equipment and methods. Resources are an estimate of the world's total coal deposits, regardless of whether the deposits are commercially accessible. Exploration geologists have found and mapped the world's most extensive coal beds. At the beginning of 2001, global coal reserves were estimated at 984.2 billion metric tons, in which 1 metric ton equals 1,016 kg (2,240 lb). These reserves occurred in the following regions by order of importance: the Asia Pacific, including Australia, 29.7 percent; North America, 26.1 percent; Russia and the countries of the former Union of Soviet Socialist Republics (USSR), 23.4 percent; Europe, excluding the former USSR, 12.4 percent; Africa and the Middle East, 6.2 percent; and South and Central America, 2.2 percent.
Coal deposits in the United Kingdom, which led the world in coal production until the 20th century, extend throughout parts of England, Wales, and southern Scotland. Coalfields in western Europe underlie the Saar and Ruhr valleys in Germany, the Alsace region of France, and areas of Belgium. Coalfields in central Europe extend throughout parts of Poland, the Czech Republic, and Hungary. The most extensive and valuable coalfield in eastern Europe is the Donets Basin, between the Dnieper and Don rivers (in parts of Russia and Ukraine). Large coal deposits in Russia are being mined in the Kuznetsk Basin in southern Siberia. Coalfields underlying northwestern China are among the largest in the world. Mining of these fields began in the 20th century.
In 2006 estimates of total U.S. coal reserves were approximately 17 billion metric tons. At the beginning of the 21st century production amounted to about 980 million metric tons each year.
COAL MINING
Coal mining is the removal of coal from the ground. The mining method employed to extract the coal depends on the following criteria: (1) seam thickness, (2) the overburden thickness, (3) the ease of removal of the overburden, (4) the ease with which a shaft can be sunk to reach the coal seam, (5) the amount of coal extracted relative to the amount that cannot be removed, and (6) the market demand for the coal.
The two types of mining methods are surface mining and underground mining. In surface mining the layers of rock or soil overlying a coal seam are first removed after which the coal is extracted from the exposed seam. In underground mining, a shaft is dug to reach the coal seam. Currently, underground mining accounts for approximately 60 percent of the world recovery of coal.
ENVIRONMENTAL ISSUES
Coal mining is the removal of coal from the ground. The mining method employed to extract the coal depends on the following criteria: (1) seam thickness, (2) the overburden thickness, (3) the ease of removal of the overburden, (4) the ease with which a shaft can be sunk to reach the coal seam, (5) the amount of coal extracted relative to the amount that cannot be removed, and (6) the market demand for the coal.
The two types of mining methods are surface mining and underground mining. In surface mining the layers of rock or soil overlying a coal seam are first removed after which the coal is extracted from the exposed seam. In underground mining, a shaft is dug to reach the coal seam. Currently, underground mining accounts for approximately 60 percent of the world recovery of coal.
Surface mining has resulted in a great deal of damage to the landscape. Many surface mines have removed acres of vegetation and altered topographic features, such as hills and valleys, leaving soil exposed for erosion. Longwall mining, which allows the mine to collapse, results in widespread land subsidence, or sinking. Coal and rock waste, often dumped indiscriminately during surface and underground mining processes, weathers rapidly, producing acid drainage . Acid drainage contains sulfur-bearing compounds that combine with oxygen in water vapor to form sulfuric acid . In addition, weathering of coalmine waste can produce alkaline compounds, heavy metals, and sediments. Acid drainage, alkaline compounds, heavy metals, and sediment leached from mine waste into groundwater or washed away by rainwater can pollute streams, rivers, and lakes.
Today, enterprises in many countries must secure government permits before mining for coal. In the United States, mining companies must submit plans detailing proposed methods for blasting, road construction, land reclamation, and waste disposal. New land reclamation methods, driven by stringent laws and regulations, require coal mining companies to restore strip-mined landscapes to nearly premined conditions.
BURNING OF COAL
The burning of coal produces environmentally harmful emissions. Some gases produced from burning coal, such as carbon dioxide, are known as greenhouse gases because they trap the Earth's heat like the roof of a greenhouse and may contribute to global warming. Other emissions from coal combustion can lead to air and water pollution.
Some major outcomes are :
(1) The Greenhouse Effect : Earth absorbs much of the heat energy radiated from the Sun. The planet then reradiates this heat back into the atmosphere. Carbon dioxide and some other gases that are naturally present in the atmosphere prevent much of the heat from escaping back into space, maintaining Earth at a temperature that can support life. These gases are known as greenhouse gases because they trap the Sun's heat in much the same way as the glass roof of a botanical greenhouse. However, the immense quantity of fossil fuels burned during the world's rapid industrialization over the last 200 years has raised levels of carbon dioxide in the atmosphere by about 28 percent. This dramatic increase in atmospheric carbon dioxide, coupled with continuing depletion of the world's forests, which absorb carbon dioxide, has led many scientists to predict a heating of the atmosphere on a global scale. Such a global warming could disrupt weather patterns, cause the polar ice caps to melt, and possibly lead to other environmental problems.
Today, many industrial countries are working to reduce emissions of greenhouse gases. One proposal is to establish a system requiring companies that create greenhouse gases to pay to emit carbon dioxide above a specified level. This payment could take several forms, including (1) purchasing the rights to pollute from a company with carbon dioxide emissions below the specified level; (2) purchasing forests, which absorb carbon dioxide, and keeping them from being developed; or (3) paying to upgrade a plant in a developing country, thus lowering that plant's carbon dioxide emissions.
(2) Acid Rain : Another environmental problem is acid rain , which forms from sulfur contained in coal. As coal burns, the sulfur combines with oxygen in the air to form sulfur dioxide. As sulfur dioxide is released into the atmosphere, this compound reacts with atmospheric moisture, forming sulfuric acid. This acidic moisture eventually falls back to Earth in the form of precipitation known as acid rain. Environmental studies indicate that acid rain damages crops and forests as well as streams, lakes, and rivers.
The U.S. Clean Air Act, implemented in 1970 and revised in 1970 and 1990, is the federal law regulating air pollution in the United States. This legislation has significantly reduced emissions of sulfur oxides, known as acid gases. For example, the Clean Air Act requires facilities such as coal-burning power plants to burn low-sulfur coal. High-grade coals (coals with a higher heating value) generally contain more sulfur than low-grade coals such as lignite and subbituminous coal. Therefore, certain processes have been developed to remove sulfur-bearing compounds from high-grade coal prior to burning. The Clean Air Act also requires use of pollution-trapping equipment such as air scrubbers (devices installed inside plant smokestacks to remove sulfur dioxide from coal emissions). In addition, revisions to the Clean Air Act in 1990 established a system that allows coal-burning power plants to buy and sell sulfur emission permits with one another. This system tries to establish a financial incentive to lower sulfur emissions by rewarding power plants that reduce emissions below federal levels. Power plants that cut their sulfur emissions below the permitted levels can sell permits to burn coal to companies that exceed federal levels. Companies that reduce emissions reap financial rewards while polluters must pay an extra cost to pollute.
(3) Fly Ash : The burning of coal releases ashes known as fly ash into the atmosphere. Fly ash contains toxic metals such as arsenic and cadmium. In the United States the Clean Air Act requires that fly ash be removed from coal emissions. As a result, antipollution devices such as air scrubbers, baghouses, and electrostatic precipitators are used to trap these pollutants. Baghouses work like giant vacuum cleaners, drawing coal emissions through giant fabric bags that trap the fly ash inside. Electrostatic precipitators use discharge electrodes (electrically charged parts of an electric circuit) to trap ash particles. In an electrostatic precipitator the electrodes are located between long, positively charged collection plates. As the fly ash passes between these collection plates, the discharge electrodes give each particle a negative charge. These negatively charged particles are then attracted to and held by the positively charged collection plates.
CONCLUSION : OPTING FOR CLEAN TECHNOLOGIES
Since 1986 the United States government and private industry have been working together to develop cleaner and more efficient ways to harness the energy in coal. This joint effort, known as the Clean Coal Technology Demonstration Program, includes several technologies, such as fluidized bed coal combustion, furnace sorbent injection, and advanced flue-gas desulfurization.
Fluidized bed coal combustion burns coal in a limestone bed that transfers heat to water, generating steam. This steam is pressurized and used to turn a turbine shaft, which subsequently drives an electric generator. The limestone absorbs sulfur dioxide emitted by the coal, thus reducing the amount of acid gases released during combustion.
A process called furnace sorbent injection removes acid gas from coal emissions at less cost than expensive scrubbers. A sorbent is a highly absorbent material, such as powdered limestone. It is injected into furnaces, where the powdered limestone reacts with the acid gases emitted by the burning coal. The used powder is siphoned away through the furnace out take and is captured (with fly ash) in a baghouse or electrostatic precipitator.
A process called advanced flue-gas desulfurization also removes acid gas from burning coal without expensive scrubbers. Emissions from burning coal are piped into a container called an absorber, where the acid gases react with an absorbing solution (such as a mixture of lime, water, and oxygen). This reaction forms gypsum , a soft white mineral valuable as an ingredient in cement.
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