There are four types of coal:
Each has characteristics that make it more or less suitable for different end uses. In general, coal of all geological composition is characterized by end use as either “steam coal” or “metallurgical coal.” Steam coal is used by electricity generators and by industrial facilities to produce steam, electricity or both. Metallurgical coal is refined into coking coal which is used in the production of steel. Heat value and sulfur content, the two most important coal characteristics, determine the best end use of particular types of coal. The heat value of coal is commonly measured in British thermal unit (Btu) per pound of coal. Sulfur content can vary from seam to seam and sometimes within each seam. Coal combustion produces sulfur dioxide, the amount of which varies depending on the chemical composition and the concentration of sulfur in the coal. Also, ash is the inorganic residue remaining after the combustion of coal. As with sulfur content, ash content varies from seam to seam. Ash content is an important characteristic of coal because electric generating plants must handle and dispose of ash following combustion. Moisture content of coal varies by the type of coal, the region where it is mined, and the location of coal within a seam. In general, high moisture content decreases the heat value and increases the weight of the coal, thereby making it more expensive to transport. Moisture content in coal, as sold, can range from approximately 5% to 30% of the coal’s weight. When some types of coal are super-heated in the absence of oxygen, they form a hard, dry, caking form of coal called “coke.” Steel production uses coke as a fuel and reducing agent to smelt iron ore in a blast furnace.