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What is Coal?


Coal is a black to brown organic sedimentary rock formed from compressed plants, leaves, roots, bark, and tree debris that accumulated in slow-moving, fresh-water swamps or “bogs.” It is a combustible rock mostly made of carbon hydrogen, nitrogen, oxygen, and sulfur. The fossil fuel is found in layers of sedimentary rock in the earth and is an important economic deposit used mostly to generate electricity. In Wyoming, it is also used for industrial purposes such as heating bentonite kilns, making cement and steel, and at trona processing facilities. Coal miners prefer to mine deposits close to the surface and close to railroads for hauling coal to the power plants. Coal mines are usually in rural areas because coal cannot be mined beneath cities, towns, power lines, rivers, or other human-made features. Coal is mined both underground and on the surface in open-pit mines. Some underground mines are as deep at 3,000 feet. Surface coal mines in Wyoming use the largest excavators and trucks in the nation.

How was it Formed?

Coal formed when dead, swampy plant materials were compressed together to form “peat” in moist, spongy bogs. This dead plant material was later submerged and covered by overlying sediments. Over time, this buried bog was compressed and modified by heat and pressure to produce chemical and physical changes to the peat. Oxygen was then released, and mostly carbon remained. Over geologic time, this layer became coal. Wyoming coal is 50 million to 130 million years old.

What Makes up Coal?

Coal Chunk

Coal differs from other rocks because it consists of organic carbon rather than mineralized rocks. Coal is made up of “macerals,” which are composed of the different types of woody material (bark, twigs, leaves, branches) that originally fell into the swamp. Macerals are the building blocks of coal, just as minerals make up rocks. These macerals determine the internal chemistry of the coal and how much energy is locked in different types of coal.

What are the Four Types of Coal?

Based on how much heat and pressure were applied to the peat, the resulting coal can be classified into four different types: lignite, subbituminous, bituminous, and anthracite. These different coals can be identified by their color, maceral type, heat content, and shininess. Lignite is a soft, dull coal, often brown, with the least amount of stored energy because it had the least amount of pressure and temperature applied. Subbituminous coal is black and harder than lignite, but is not very shiny. Bituminous coal is black, shiny, and has a substantial amount of heat value. Anthracite is the hardest, and this black shiny coal has the highest density of any coal type. Subbituminous and bituminous coals are the best power plant coals because they contain the most energy. All coal forms except anthracite are found in Wyoming.