Coal

I

Introduction

Coal, solid fuel of plant origin. In remote geological times, and particularly in the Carboniferous period, between 345 and 280 million years ago, much of the world was covered with luxuriant vegetation growing in swamps. Many of these plants were types of ferns, some as large as trees. This vegetation died and became submerged under water, where it gradually decomposed. As decomposition took place, the vegetable matter lost oxygen and hydrogen atoms, leaving a deposit with a high percentage of carbon. In this way peat bogs were formed. As time passed, layers of sand and mud settled from the water over some of the peat deposits. The pressure of these overlying layers, movements of the Earth's crust, and, sometimes, volcanic heat, acted to compress and harden the deposits, thus producing coal.

Various types of coal are recognized, according to their fixed carbon content. Peat, the first stage in the formation of coal, has a low fixed carbon content and a high moisture content. The carbon content is greater in lignite, the lowest rank of coal. Bituminous coal has even more carbon and a correspondingly higher heating value. Anthracite coal has the highest carbon content and heating value. Coal may be transformed by further pressure and heat into graphite, which is almost pure carbon. Other components of coal are volatile hydrocarbons, sulphur and nitrogen, and the minerals that remain as ash when the coal is burned.

Some products of coal combustion have detrimental effects on the environment. Burning coal produces carbon dioxide, among other byproducts. Some scientists believe that, owing to the widespread use of coal and other fossil fuels, the amount of carbon dioxide in the Earth's atmosphere could increase to such an extent that changes in the Earth's climate will occur (see Global Warming; Greenhouse Effect). Also, sulphur and nitrogen in the coal form oxides during combustion that can contribute to the formation of acid rain. Acid rain is the result of a series of complex reactions involving chemicals and compounds from many industrial, transport, and natural sources. Sulphur dioxide (SO₂) emissions from new coal-fired facilities are now controlled in many countries. As a result, sulphur dioxide emissions have dropped in those countries even though coal use has increased.

All ranks of coal have some economic value. For centuries peat has been used as a fuel for open fires, and more recently peat and lignite have been made into briquettes for burning in furnaces. Electric utilities and general industry use bituminous coal. Steel producers use coke, or metallurgical coal, a fuel that is almost pure carbon, produced by distilling coal (heating it strongly in the absence of air, so that it does not burn).

The process of producing coke yields a number of chemical byproducts, including coal tar, which are used in the manufacture of many other products. Coal was also used, from the early 19th century to the World War II era, for the production of fuel gas, and coal liquefaction techniques were used to produce liquid oil products. Manufacture of fuel gas and other products from coal diminished as natural gas became widely available. In the 1980s, however, industrialized nations again became interested in gasification and new clean coal technology (CCT). Indeed, coal liquefaction supplies all of the Republic of South Africa's oil needs.

II

Clean Coal Technologies

CCTs are a new generation of advanced coal utilization processes, some of which may be commercially viable early in the 21st century. A wide variety of CCTs exist, but all of them alter the basic structure of coal before, during, or after combustion. In this way they reduce emission of impurities, such as sulphur and nitrogen oxide, and increase the efficiency of energy production. They include improved methods of cleaning coal, fluidized bed combustion, integrated gasification combined cycle, and advanced flue-gas desulphurization.

III

Location of Deposits

Coal is found in nearly every region of the world, but deposits of present commercial importance are confined to Europe, Asia, Australia, and North America (see Mining).

Great Britain, which led the world in coal production until the 20th century, has deposits in southern Scotland, England, and Wales. In western Europe, important coalfields are found throughout the Alsace region of France, in Belgium, and in the Saar and Ruhr valleys in Germany. Central European deposits include those of Poland, the Czech Republic, and Hungary. The most extensive and valuable coalfield in the former Soviet Union is that of the Donets Basin between the Dnepr and Don rivers; large deposits have also recently been exploited in the Kuznetsk Basin in western Siberia. The coalfields of north-western China are among the largest in the world.

The coal reserves of the United States are divided into six major regions, only three of which are mined extensively. The most productive region is the Appalachian field, which includes parts of Pennsylvania, West Virginia, Kentucky, Tennessee, Ohio, and Alabama. In the Midwest one large field covers most of Illinois and sections of Indiana and Kentucky. A thick field extends from Iowa through Missouri, Kansas, and Oklahoma. These three regions produce most of the coal mined in the United States. There are large deposits of lignite and subbituminous coal in North Dakota, South Dakota, and Montana. Subbituminous and bituminous coal deposits are scattered throughout Wyoming, Utah, Colorado, Arizona, and New Mexico. The Pacific Coast and Alaska have small reserves of bituminous coal. Almost all the anthracite in the United States is in a small area around Scranton and Wilkes-Barre, in Pennsylvania. The best bituminous coal for coking purposes comes from the Middle Atlantic states.

Estimates of world coal reserves vary widely. According to the World Energy Council, recoverable world reserves of anthracite, bituminous, and subbituminous coal in the late 1980s exceeded 1.2 trillion tonnes. Of this recoverable coal, China held about 43 per cent, the United States 17 per cent, the former Soviet Union 12 per cent, South Africa 5 per cent, and Australia 4 per cent. The World Coal Institute has estimated that, at 1998 levels of production, coal reserves are likely to last about another 200 years. The Institute's data, based on statistics released by BP Amoco, show that South and Central America have 7.8 billion tonnes of coal reserves; Western Europe 25.8 billion tonnes; Australasia 47.3 billion tonnes; Africa and the Middle East 61.4 billion tonnes; China 62.2 billion tonnes; other countries in Asia (excluding the former Soviet Union republics) 74.9 billion tonnes; Eastern Europe and the former Soviet Union (both in Asia and Europe) 113.3 billion tonnes; and North America 116.7 billion tonnes. See also Energy Supply, World.