Coal

Coal is a brown or black rock of sedimentary biochemical origin that is used as a fossil fuel and for industrial purposes. Coal is formed from decaying plants and the bodies of animals that form layers of organic material. The decaying matter goes through stages that release water, oxygen, and nitrous gases to form coal, which is mostly carbon, hydrogen, and oxygen. The proportions of these elements determine the type of coal. The stages in which coal is formed are: peat, lignite, soft bituminous, bituminous, and anthracite. In the prehistoric Carboniferous period (coal age), vast swamps and shallow seas covered much of the surfaces of the continents. During these geologic times, great beds of coal were formed from decaying vegetable matter. Imprints of prehistoric ferns and other life forms have been found in coal buried deep in the Earth. Today, coal is still being formed in peat bogs. There are large peat bogs in areas such as Ireland and Finland that are exploited as a fuel source.

When peat is buried for millions of years by sediments of sand, shale, and sometimes limestone, the weight of the sedimentary overburden will subject the peat to enough pressure to harden it into lignite; eventually, it turns into semibituminous coal, which is brown. If peat is subjected to sufficient pressures by sedimentary rock, the compaction will change it into bituminous coal, which is soft, slick, and black in color. It is the most common form of coal. About 30 ft. of peat is needed to form a 1 ft. thick coal seam. The highest grade of coal is anthracite, which has a glassy luster and is usually considered a metamorphic rock. It is about 95 percent carbon. In comparison, petroleum is about 85 percent carbon.

All forms of coal contain carbon. Peat has the least amount of carbon and anthracite has the most. Consequently, peat smokes the most when burned and gives off the least amount of heat. It also leaves behind the most ash. Anthracite, with the most carbon, burns slowly and gives off the greatest amount of heat with the least amount of ash. However, because it is hard to burn, it is not used to produce electricity.

Deposits of coal are found on every continent and in their continental shelves. Coal deposits can be found in thick beds or in seams or veins that are only a fraction of an inch thick. The largest sources of coal are the United States, China, and Russia, followed by Germany, Poland, India, South Africa, and Australia. The United States and Canada have about half of the world's known coal supplies.