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Coal stockpile at the Bowie Mine #2, Delta County, Colorado, June 2004. Photo credit: Chris Carroll for the CGS.

Coal


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Coal extraction has a long and storied history in Colorado. The estimated value of Colorado coal production in 2017 was $623 million. Although coal production slightly increased in 2017, the overall decreasing trend in coal production is due primarily to the nationwide increased use of natural gas and renewable energy. The state fell from the 11th largest coal producer in 2015 to the 13th in the U.S. in 2016. Colorado coal is mostly bituminous and sub-bituminous—with both underground and surface mines currently in operation on the Western Slope—and is characterized as of high heat, low sulfur, low to medium ash, and low mercury content.

Coal is an organic clastic sedimentary rock composed mostly of ancient plant material. It is combustible because it contains carbonaceous material that burns. It consists of carbonized ancient plant matter that is solid, but brittle. Originally deposited as leaf-litter and plant remains in a fresh-water environment, the material first begins as peat, as in a peat bog. Over time it is compressed, dried, and modified by heat and pressure by sedimentary layers that cover it and by the proximity to the heat of the earth. This process is called ‘coalification’. The peat then undergoes diagenesis with depth of burial and tectonic forces. Coal, a black-to-brown rock, is the end product of this diagenetic process. The more complete the process, the higher the grade, or rank of the coal.

Texturally, coal is subdivided into four main classes: lignite, sub-bituminous, bituminous, and anthracite. This last class is the hardest coal and contains the most carbon. Lignite is the least dense coal with lower carbon content. This sedimentary rock is composed mostly of carbon, hydrogen, oxygen, nitrogen (volatile hydrocarbons), and lesser amounts of ash, sulfur, and trace elements.

In Colorado, anthracite is found in Gunnison and Pitkin counties, and lignite is found in Adams and Elbert counties. Bituminous and sub-bituminous coals make up the majority of Colorado’s coal resources, and are mined as clean low-sulfur but high-heat content coal products. Most of the electricity generated in Colorado comes from coal that is mined within the state. Coal is prolific in over 28 percent of Colorado’s surface area. Most of the mineable coal resources today are located on the Western Slope.