Coal is classified based on 1) How much carbon it contains and 2) The amount of heat and pressure it has endured. Harder coals have usually undergone more heat and pressure than softer coals. Harder coals have less moisture and burn easier, like firewood.
Different parts of the world classify coal differently. Germany, for instance, has eight classifications. In the U.S. coal usually falls into one of four categories. According to the American Coal Foundation they are:
- Lignite (soft): This type of coal has the least amount of carbon. However, it is also the most abundant. Most of the coal found in Texas is lignite. This grade of coal is mainly used for electricity generation.
- Sub-bituminous (medium-soft): This grade is also used for electricity generation. Sub-bituminous coal is harder than lignite. Most reserves are found in the western U.S. and Alaska.
- Bituminous (medium-hard): This is the most plentiful type of coal in the U.S. Bituminous coal is used to produce electricity and coke, a byproduct used in steel mills.
- Anthracite (hard): This type of coal has the highest carbon content. It is classified as a metamorphic rock, whereas the three softer varieties are classified as sedimentary rocks. Anthracite is primarily used as a heating fuel for homes. Most anthracite coal in the United States is found in Pennsylvania.
