Coal is a naturally occurring black rock consisting primarily of the element carbon. It is one of the most important fossil fuels, used to heat buildings, to generate electricity, and to power trains and ships. Coal is the most plentiful of the fossil fuels, compared to petroleum and natural gas. There are still large reserves of coal available in the ground. Burning coal releases large amounts of carbon dioxide and other pollutants, which is one reason levels of carbon dioxide in the atmosphere have increased steadily over the past two centuries.
Origins of Coal
Coal is a fossil fuel, a name that comes from the way it is formed. Much of the world's coal began to form from plants that lived during the Carboniferous Period, between 359 and 299 million years ago. When plants living in wet, swampy areas died, they were covered by wet soil and water. As the soil on top of them grew thicker, the dead plants were compressed. No air could reach them, so they could not decay as dead things normally do. Instead, they got hotter as they were compressed, and the combination of pressure and heat slowly converted them into fossil fuels. First they became a soft material called peat, which is itself a fossil fuel widely used in places such as Ireland. As the peat was further compressed, it became harder and transformed into brown coal, also called lignite. As more time passed, the lignite compressed into bituminous or soft coal, and finally became the hardest coal, anthracite.
Coal has formed at many times in the past, most abundantly during the Carboniferous Period and later during the Upper Cretaceous Period, about 100 million years ago. The processes that formed coal continue today in areas where peat forms.
Because of the way it formed, by plants being layered with soil, coal is often found between layers of sedimentary rock. Sedimentary rock is formed when sand, silt, clay, and similar materials are packed together under heavy pressure. In some cases, the coal layers may lie at or very near Earth’s surface. In other cases, they may be buried thousands of feet underground. Coal seams usually range from no more than 3 to 200 feet (1 to 60 meters) in thickness. The location and configuration of a coal seam determines the method by which the coal will be mined.
Composition of Coal
Coal is classified according to its heating value and according to the percentage of carbon it contains. For example, anthracite contains the highest proportion of pure carbon (about 86–98%) and has the highest heat value (13,500 to 15,600 Btu/lb; British thermal units per pound) of all forms of coal. Bituminous coal generally has lower concentrations of pure carbon (from 46–86%) and lower heat values (8,300 to 15,600 Btu/lb). Bituminous coals are often subdivided on the basis of their heat value, being classified as low, medium, and high volatile bituminous and subbituminous. Lignite, the poorest of the true coals in terms of heat value (5,500 to 8,300 Btu/lb) generally contains about 46 to 60 percent pure carbon. All forms of coal also contain other elements present in living organisms, such as sulfur and nitrogen, that are very low in absolute numbers but that have important environmental consequences when coals are used as fuels.
Properties and Reactions
By far the most important property of coal is that it burns. When the pure carbon and hydrocarbons found in coal burn completely, only two products are formed: carbon dioxide and water. During this chemical reaction, a relatively large amount of heat energy is released.
In the nineteenth century coal was a major source of energy for heating homes and other buildings, running ships and trains, and in many industrial processes. Coal is somewhat smoky, so during the twentieth century natural gas and other power sources became more popular for use in homes and vehicles. In the twenty-first century, the most significant use of coal is powering generators in electrical power plants. Some coal is still used to manufacture coke, a pure carbon fuel generated by heating coal and used in steel manufacture.
Coal is extracted from Earth using one of two major methods: subsurface or surface (strip) mining.
Subsurface mining digs coal from deep below Earth's surface. The first step in subsurface mining is to dig vertical tunnels into Earth until the coal seam is reached. Horizontal tunnels are then constructed off the vertical tunnel. Miners go deep into the earth to dig out the coal and carry it back up to daylight. Mines are dangerous places to work, prone to collapsing and trapping miners.
Surface mining can be used when a coal seam is close enough to Earth’s surface to allow the overburden (top layers of rock or soil) to be removed easily and inexpensively. In such cases, the first step is to strip off all of the overburden to reach the coal itself. The coal is then scraped out by huge power shovels, some capable of removing up to 100 cubic meters at a time. Strip mining is a far safer form of coal mining for coal workers, but it presents a number of environmental problems. In most instances, an area that has been strip-mined is terribly scarred. Restoring the area to its original state can be a long and expensive procedure. In addition, any water that comes in contact with the exposed coal or overburden may become polluted and require treatment.
Environmental Problems Associated with Burning Coal
Burning coal results in a great deal of air pollution. Even in the best of circumstances, it produces large amounts of carbon dioxide. Carbon dioxide is a greenhouse gas. Increasing amounts of carbon dioxide in the atmosphere are causing Earth's surface temperature to rise. If carbon and hydrocarbons do not burn completely, they form other polluting compounds, such as carbon monoxide, hydrogen, and other types of pure carbon, such as soot. Soot is a black dust that settles out of the air and deposits an unattractive coating on homes, cars, buildings, and other structures. Carbon monoxide is a toxic gas and may cause illness or death in humans and other animals. Oxides of sulfur and nitrogen react with water vapor in the atmosphere and then settle out in the air as acid rain, a serious environmental problem
Coal often contains a small percentage of mineral matter: quartz, calcite, or perhaps clay minerals. These components do not burn readily and so become part of the ash formed during combustion. This ash then either escapes into the atmosphere or is left in the combustion vessel and must be discarded. Sometimes coal ash also contains significant amounts of lead, barium, arsenic, or other elements. Whether airborne or in bulk, coal ash can therefore be a serious environmental hazard.
Coal is a nonrenewable resource, meaning it is not replaced easily or readily. Coal was formed over millions of years, so once it is used up, humans cannot wait for more to form. When Earth's supply of coal is depleted, there will be no more. There is still a fair amount of coal in the ground. In 2006, British Petroleum estimated that there were at least 140 years of known coal reserves still available, although these numbers may not be based on accurate data.
China is the world's top coal producer, followed by the United States and India. In the United States, the largest coal-producing states are Montana, North Dakota, Wyoming, Alaska, Illinois, and Colorado. U.S. coal production declined between 2006 and 2012 as coal-fired power plants closed down in favor of less polluting plants powered by natural gas. In China, however, the use of coal was still growing in 2012. Environmental opposition to coal was growing as well.
Conversion of Coal
A number of processes have been developed by which solid coal can be converted to a liquid or gaseous form for use as a fuel. Conversion has a number of advantages. In a liquid or gaseous form, the fuel may be easier to transport. Also, the conversion process removes a number of impurities (such as sulfur) from the original coal that have environmental disadvantages.
One of these conversion methods is known as gasification. In gasification, crushed coal is forced to react with steam and either air or pure oxygen. The coal is converted into a complex mixture of gaseous hydrocarbons with heat values ranging from 100 Btu to 1000 Btu. One day it may be possible to construct gasification systems within a coal mine, making it much easier to remove the coal in a gaseous form from its original seam.
In the process of liquefaction, solid coal is converted to a petroleum-like liquid that can be used as a fuel for motor vehicles and other applications. On the one hand, both liquefaction and gasification are attractive technologies in the United States because of its very large coal resources. On the other hand, the wide availability of raw coal means that expensive new technologies have been unable to compete economically with the natural product.