Between 200 and 300 million years ago, long before there were any reptiles, birds or mammals, in many parts of the world there existed warm and human climatic conditions. These conditions favored the growth of huge tropical ferns and giant trees, which grew and died in vast swamp areas. The dead plants fell into the boggy waters, which tended to exclude oxygen and bacteria, so that they only partially decomposed to produce a peat-like material. This fossilized plant debris was the beginning of coal, but only the beginning.
Vegetation continued to grow for many generations and centuries, forming vast, thick peat beds which were later to turn into coal. After a time the areas of swamp gradually became submerged by shallow seas, where they were covered by sediment. these sediments would later become sedimentary rock. This cycle of swamp followed by submersion was often repeated a number of times, so that a sequence of horizontal bands of peat and inorganic, sedimentary rock was built up. This formed the first stage, called the biochemical stage. Coal formation occurred in other geologic periods as well.
Subsequently, the bands of peat were altered by the action of pressure and temperature during the second, or geochemical stage, to form the various kinds of coal found today. As much as a 20-fold reduction in the thickness of the original plant deposits sometimes occurred. During the course of time these horizontal coal seams were further altered as they became folded, tilted and eroded. Much of this action was due to the motion of the continents, as mountains formed and were then worn away. The study of fossils is called paleontology. The creation of coal from fossils is part of geology.
The partitioning between gas and solid is controlled by the volatility and chemistry of the individual elements. Virtually 100 percent of the radon gas present in feed coal is transferred to the gas phase and is lost in stack emissions. In con-trast, less volatile elements such as thorium, uranium, and the majority of their decay products are almost entirely retained in the solid combustion wastes. Modern power plants can recover greater than 99.5 percent of the solid combustion wastes. The average ash yield of coal burned in the United States is approximately 10 weight percent. Therefore, the concentration of most radioactive elements in solid combustion wastes will be approximately 10 times the concentration in the original coal. Figure 2 illustrates that the uranium concentration of most fly ash (10 to 30 ppm) is still in the range found in some granitic rocks, phosphate rocks, and shales. For example, the Chattanooga Shale that occurs in a large portion of the Southeastern United States contains between 10 and 85 ppm U.
There are two primary methods of mining coal, surface mining and underground mining. There are over 1,000 surface mines and more than 1,000 underground mines in the US. Underground mining is more difficult and requires more miners, but much of our best coal is underground. Mining has become much safer and more efficient over the years. In 1980 there were over 220,000 coal miners in the country. Today there are fewer than 100,000. But while 1980 production was about 800,000 tons, today we produce over 1 billion tons with fewer than half the number of miners.
In recent years surface mining in Wyoming has mushroomed, making that state the leading coal producer at almost 300,000 tons per year. West Virginia and Kentucky, the traditional leaders, with both underground and surface mines, each produce around 170,000 tons.