Coal is a solid fossil fuel found in sedimentary rock layers. It formed from the remains of swamp plants that died 300 to 400 million years ago. They were buried in sedimentary layers and compressed by intense heat and pressure over millions of years. Coal starts out as peat; a type of precursor to coal. Then it becomes lignite, a very high-moisture type of young coal. Coal is mostly carbon, but it also contains some sulfur and trace amounts of toxic mercury and radioactive materials. We extract coal from the ground by digging or mining.
There are different types of coal based on how long and how far down the initial plant material is buried. Peat is partially decayed plant material and it accumulates when dead plant material exceeds the rate of the material’s bacterial decay. It has very high moisture content.
Lignite is also called brown coal. It is the youngest of all the coal types and has only 25 to 35 percent carbon. It is used almost exclusively for electric power plants, where it is burned to produce steam. It has a low energy yield and so is not usually transported internationally or even locally. Lignite is often used in power plants close to where it is mined. Lignite can have up to 66 percent moisture content, making it an inefficient, low-grade fuel source.
Sub-bituminous coal is in a more advanced stage of development than lignite, and is 35 to 45 percent carbon. It has properties in between lignite and bituminous coal and is used primarily for electricity production. Bituminous coal is used extensively as a fuel source because it has a high energy yield. There are large supplies of it worldwide, so it is readily available for use. Bituminous coal has 45 to 86 percent carbon, and is commonly used as a source of electric power and in the steel industry.
Anthracite is a very hard, glossy black coal. It has low moisture content and is highly desirable for fuel because of its high energy content. It has the highest carbon content (86 to 98 percent) and the fewest impurities of all coal types.
The map shows the major coal producing areas of the U.S. They include the Appalachian Mountain area of the east, areas near the Gulf of Mexico, and the Intermountain West. The most plentiful type of coal in the U.S. is bituminous coal. Note that the lignite coal found in the Gulf of Mexico area is a low-grade coal that is not as commercially profitable as higher-grade coal forms, like bituminous coal. Thus we do not tap, or use, the coal in this area as much as the much higher quality coal of the Appalachians and the Intermountain West.
