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OWOE - Fossil Fuels - What is coal?
  Coalification Process (Kentucky Geological Survey)
Coalification Process (Kentucky Geological Survey)
Coal Rank as Function of Time, Heat and Pressure (Kentucky Geological Survey)
Coal Rank as Function of Burial Depth (source Plant Fossils of West Virginia)
What is coal?

Coal is the compacted and preserved remains of plant matter that didn't totally decompose. Much of the coal in the United States was formed around 350 million years ago when the land was swampy and covered with forests of large ferns and trees. As the organic matter died, it settled to the bottom of the swamp and became buried under subsequent layers of plants and soil. Increased heat and pressure over time turned it first into peat and then into coal.

The amounts of heat and pressure experienced by the peat as it changed to coal increased the material density and removed oxygen. This process, called coalification, gave rise to different properties for the coal. The products of coalification are divided into four major categories based on the carbon content of the material, which is the key component of its heating value. The greater the carbon to oxygen ratio the harder the coal, the simpler the carbon containing molecules, and the more potential energy the coal contains.

The softest form of coal, containing the lowest percentage of carbon, is lignite, which has between 20-35% carbon content by mass. Lignite is the "youngest" form of coal and experienced the least temperature and pressure of burial. It is typically found closer to the earth's surface and is typically surface mined. Lignite deposits are found in Texas, Montana, North Dakota and the Gulf Coast region. Sub-bituminous and bituminous coal are progressively harder with higher carbon content. Sub-bituminous coal is found in a number of Western states and Alaska. Bituminous coal is the most commonly found type of coal in the U.S. with major deposits in the Appalachians, the Great Plains, and the Colorado Plateau.The hardest form of coal is anthracite, which has a carbon content between 80-96% carbon. With high carbon content and correspondingly few impurities, anthracite has the best heat quality and fewer by-products than other forms of coal. However, it is harder to ignite and the burning processs must be carefully managed to separate the ashes from the burning coal. Anthracite is a relatively rare form of coal which in the U.S. is found primarily in a small area of Pennsylvania.

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