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.