The highest grade of coal, anthracite or hard coal, was exposed to extremely high temperatures during its formation. It is a dark, brilliant black and burns most cleanly (produces the fewest pollutants per unit of heat released) of all the types of coal because it is not contaminated by large amounts of sulfur. Anthracite also has the highest heat-produc­ing capacity of any grade of coal. Most of the an­thracite in the United States is located east of the Mississippi River, particularly in Pennsylvania.

    Coal is usually found in underground layers, called seams that vary from 2.5 cm (1 inch) to more than 30 m (100 ft) in thickness. Because they are easily located, geologists think that most or all of Earth's major coal deposits have probably been identified. Scientists working with coal are there­fore concerned less about finding new deposits than about the safety and environmental problems asso­ciated with coal, to be discussed shortly.

Coal Mining

The two basic types of coal mines are surface, or open-pit, mines and subsurface (underground) mines. The type of mine chosen depends on the location of the coal bed relative to the surface as well as on surface contours. If the coal bed is within 10 m (100ft) or so of the surface, open-pit, or strip) mining is usually done. This pro­cess involves using bulldozers, giant power shovels, and wheel excavators to remove the ground cover­ing the coal seam. The coal is then scraped out of the ground and loaded into railroad cars or trucks. Approximately 60 percent of the coal mined in the United States is obtained by strip mining.

    When the coal is deeper in the ground or runs deep into the Earth from an outcrop on a hillside, it is mined underground. Subsurface mining ac­counts for approximately 40 percent of the coal mined in the United States. There are several types of underground mining: drift mining, slope mine, and shaft mining. When a coal outcrop occurs on a hillside, miners simply tunnel into the hill. Such a mine is called a drift mine. A coal deposit located a little too deep in the ground for strip mining is usually reached with a slope mine, in which tin coal is hauled out of the ground through a sloping shaft. Coal deposits located very deep underground are mined by digging a vertical shaft down as much as 150 m (500 ft) or even deeper; this is known as shaft mining.

Coal Reserves

Coal, the most abundant fossil fuel in the world, i^ found primarily in the Northern Hemisphere. The largest coal deposits are in North America, Russia, and China, but deposits are also found in the Arctic islands, Western Europe, India, South Africa, Australia, and eastern South Amer­ica. The United States has 25.6 percent of the world's coal supply in its massive deposits.

Duing the Carboniferous Period, approximately 300 million years ago, the land masses that were to be­come our present-day South America, Africa, and Australia were joined, making up a massive conti­nent. Known as Gondwanaland, this continent was located near the South Pole. Al­though the climate in the tropical and temperate regions of the Earth was mild, much of Gondwanaland was covered by ice sheets, and because few plants grew there, coal never formed. At this same time, however, much of Europe, North America, and Asia were located closer to the equator, where the warm climate promoted lush vegetation. Since the Carboniferous Period, the continents have sep­arated and migrated to their present locations.

    According to the World Resources Institute, known world coal reserves could last for several hundred years at the present rate of consumption.

Additional coal resources that are currently too expensive to develop have the potential to provide enough coal to last for a thousand or more years (at current consumption rates).

Safety and Environmental Problems Associated with Coal

Although we usually focus on the environmental problems caused by mining and burning coal, there are also significant human safety and health risks in the mining process itself. Underground mining is an extremely dangerous occupation. According to the Department of Energy, during the 2Oth century more than 90,000 American coal miners have died in mining accidents. Those nut killed or maimed in accidents have an increased risk of cancer and black lung disease, a condition in which the lungs arc coated with inhaled coal dust so that the exchange of oxygen between the lungs and the blood is severely restricted. It is esti­mated that these diseases are responsible for the deaths of at least 4,000 miners in the United States each year.

Environmental   Impacts   of the   Mining   Process

Coal mining, especially strip mining, has substan­tial effects on the environment. In strip mining, vegetation and topsoil are completely removed, causing a loss of habitat for plants and animals and increasing soil erosion and water pollution. Over time, as the coal is removed, strip mining lowers the surface of the ground. Because it is extremely expensive to reclaim land chat has been strip mined, the mines are usually left as large open pits. Acid and toxic mineral drainage from the mine and the removal of topsoil prevent most plants from naturally recolonizing, the land. Few tree species will grow on land that is badly disturbed 'as a result of coal mining, called coal spoils. Some sites are so severely dam­aged that only a few types of herbs will grow there. Coal spoils can he restored to prevent further degradation and to make the land productive for other purposes, but restoration is extremely expensive.

Environmental Impacts of Burning Coal

Burning any fossil fuel releases carbon dioxide, CO₂, into the atmosphere. You may recall from the discussion of the carbon cycle in Chapter 5 that a natural equilibrium exists between the CO₂ in the atmos­phere and the CO₂ dissolved in the oceans. Cur­rently we are releasing so much CO₂ into the atmos­phere through our consumption of fossil fuels that the Earth's CO₂ equilibrium has been disrupted. Because the concentration of CO₂ in the atmos­phere is increasing arid CO_Z prevents heat from escaping from the planet, the temperature of Earth may be affected. An increase of a few degrees in global temperature caused by higher levels of CO₂ and other greenhouse gases may not seem very serious at first glance, but a closer look reveals that such an increase would be catastrophic. For example, as polar ice began to melt, sea levels would rise, flooding coastal areas and placing them at higher risk for storm damage. Other serious environmental consequences of global climate change are considered. The CO₂ problem is made more serve by the burning of coal than by the burning at other fossil fuels, because coal burning releases more CO₂ per unit of heat produced.

    Coal burning also contributes more of other air pollutants than does the combustion of either oil or natural gas. Bituminous coal contains sulfur and nitrogen that, when burned, are released into the atmosphere as sulfur oxides (SO₂ and SO₃ and ni­trogen oxides (NO, NO₂, and N₂O). Both sulfur and nitrogen oxides form acids when they react with water, and when these reactions occur in the Earth's atmosphere, acid precipitation (including acid rain) results. The combustion of coal is partly responsible for acid precipitation, which seems to be particularly prevalent downwind from coal-burning power plants. Normal rain is slightly acidic (pH 5.6), but in some areas the pH of acid rain has been measured at 2.1, equivalent to that of lemon juice (see Appendix I for a review of pH). Acid precipitation is a factor in some of the forest de­cline that has been documented worldwide.

    Although it is relatively easy to identify and measure pollutants in the atmosphere, it is difficult to trace their exact origins. They are dispersed by air currents and are often altered as they react chemically with other pollutants in the air. Even so, it is clear that some nations suffer the damage of acid rain caused by pollutants produced in other countries, and as a result acid precipitation has be­come an international issue. The severe environ­mental repercussions of acid rain are considered in detail.

    It is possible to reduce sulfur emissions associ­ated with the combustion of coal by installing desulphurization systems, or scrubbers, in smokestacks; these decrease the amount of sulfur released in the air by 90 percent or more. By federal law, all coal-burning power plants built in the United States since 1978 must have scrubbers. Installing and maintaining scrubbers is extremely expensive; for example, the estimated cost for scrubber installa­tion in older coal-burning power plants in the United States alone is about $10 billion.

    Furthermore, the scrubbers themselves give rise to another environmental problem: disposal of the copious sludge produced by the scrubbing. A large power plant may produce enough sludge annually to cover 2.6 square km (1 square mile) of land 0.1 meter (1 foot) deep. Currently, most power plants place the sludge in holding ponds and landfills.

    The 1990 Clean Air Act states that the nation's 111 dirtiest coal-burning power plants must cut sulfur dioxide emissions by 1995. This would result in a total annual decrease of 5 million metric tons nationwide. (In compari­son, the U.S. total sulfur oxide emissions in 1989 exceeded 20 million metric tons per year.) These

power plants may continue to use high-sulfur coal and get a two-year extension of the 1995 deadline if they commit to buying scrubbers. In the second phase of the Clean Air Act, more than 200 additional power plants will be imposed after the year 2000.

    While CO₂ emissions remain a significant problem, new methods for burning coal (called dean coal technologies) are being developed that will not contaminate the atmosphere with sulfur oxides and will significantly reduce nitrogen oxide production. Clean coal technologies include coal ossification (considered shortly, in the discussion of synfuels) and fluidized bed combustion.

Fluidized bed combustion mixes crushed coal with particles of limestone in a strong air current during combustion. This coal-burning process has greater efficiency and several additional advan­tages. Because fluidized-bed combustion takes place at a lower temperature than regular coal burning, fewer nitrogen oxides are produced. (Higher tem­peratures cause atmospheric nitrogen and oxygen to combine, forming nitrogen oxides.) Also, be­cause the sulfur in coal reacts with the calcium in limestone to form calcium sulfate, which then pre­cipitates out, sulfur is removed from the coal during the burning process, so scrubbers are not needed to remove it after combustion.

    Fluidized bed combustion is being tested at sev­eral large power plants in the United States, and the conversion of other plants to this method will probably begin before the year 2000. The 1990 Clean Air Act provides incentives for utility com­panies to convert to clean coal technologies.