"Our ability to produce clean liquid fuels from our own coal supplies strikes at the heart of those who would hold us hostage to imported oil... and because these fuels are clean, we also show that coal use does not mean compromising our commitment to the environment."
--Robert Gentile, Assistant Secretary for Fossil Energy, US Department of Energy
Coal is classified into four main types, or ranks (lignite, subbituminous, bituminous, anthracite), depending on the amounts and types of carbon it contains and on the amount of heat energy it can produce. The rank of a deposit of coal depends on the pressure and heat acting on the plant debris as it sank deeper and deeper over millions of years. For the most part, the higher ranks of coal contain more heat-producing energy.
Lignite is the lowest rank of coal with the lowest energy content. Lignites tend to be relatively young coal deposits that were not subjected to extreme heat or pressure. Lignite is crumbly and has high moisture content. About eight percent of the coal produced in the United States is lignite, and most of it comes from Texas and North Dakota. Lignite is mainly burned at power plants to generate electricity.
Subbituminous coal has a higher heating value than lignite. Subbituminous coal typically contains 35-45 percent carbon, compared to 25-35 percent for lignite. Most subbituminous coal in the U.S. is at least 100 million years old. Over 40 percent of the coal produced in the United States is subbituminous.
Bituminous coal contains 45-86 percent carbon, and has two to three times the heating value of lignite. Bituminous coal was formed under high heat and pressure. Bituminous coal in the United States is between 100 to 300 million years old. It is the most abundant rank of coal found in the United States, accounting for about half of U.S. coal production. Bituminous coal is used to generate electricity and is an important fuel and raw material for the steel and iron industries.
Anthracite contains 86-97 percent carbon and its heating value is slightly lower than bituminous coal. Anthracite is very rare in the United States. The only anthracite mines in the United States are located in northeastern Pennsylvania.
Coal reseves are beds of coal still in the ground waiting to be mined. The United States has the world's largest known coal reserves, about 275 billion short tons. This is enough coal to last over two hundred years at today's level of use.
Coal production is the amount of coal that is mined and sent to market. The United States produces over a billion short tons of coal each year, over 1/5 of the world's coal. Coal is mined in 27 states. Wyoming mines the most coal, followed by West Virginia, Kentucky, Pennsylvania, and Texas. Coal is mainly found in three large regions, the Appalachian Coal Region, the Interior Coal Region, and Western Coal Region (includes the Powder River Basin).
Although a great variety of mineral deposits were formed during the late Paleozoic, the fossil fuels (coal, oil, and gas) are particularly significant.
Coal occurs in all post-Devonian systems. In Northern Hemisphere continents, it is particularly characteristic of the Late Carboniferous (Pennsylvanian). Thick deposits of Pennsylvanian coal occur in the Appalachians, the Illinois basin, and the industrial heartland of Europe. Some sequences of Pennsylvania strata may include many coal beds, as in West Virginia, where 117 different layers have been named. In the anthracite district of western Pennsylvania, orogenic compression has partially metamorphosed coal into an exceptionally high-carbon, low-volatile variety that is prized for its industrial uses. Permian coal seams are found in China, Russia, India, South Africa, Antarctica, and Australia.
Gasification is a promising coal technology.
Coal is sold in tons.
Scattered records of the use of coal as a fuel date from at least 1100 BCE. However, coal was not used widely until the Middle Ages, when small mining operations in Europe began to supply it for forges, smithies, lime-burners, and breweries. The invention of firebricks in the late 1400s, which made chimneys cheap to build, helped create a home heating market for coal. Despite its drawbacks (smoke and fumes), coal was firmly established as a domestic fuel by the 1570s. By that time, production in England was high enough that exports were thriving. Some of that coal eventually went to the American colonies.
The total amount of coal consumed in the United States in all the years before 1800 was an estimated 108,000 tons, much of it imported. The U.S. market for coal expanded slowly and it was not until 1885 that the young and heavily forested nation burned more coal than wood. However, the arrival of the industrial revolution and the development of the railroads in the mid-nineteenth century inaugurated a period of generally growing production and consumption of coal that continues to the present time. Today, the United States extracts coal in enormous quantities. In 1998 U.S. production of coal reached a record 1.12 billion short tons and was second worldwide after China. U.S. 2000 production was 1.08 billion short tons.
From 1885 through 1951, coal was the leading source of energy produced in the United States. Crude oil and natural gas then vied for that role until 1982. Coal regained the position of the top resource that year and again in 1984, and has retained it since. At 23 quadrillion Btu in 2000, coal accounted for nearly a third of all energy produced in the country.
Over the past several decades, coal production shifted from primarily underground mines to surface mines. In addition, the coal resources of Wyoming and other areas west of the Mississippi River underwent tremendous development.
Coal also occurs in Cenozoic rocks. Most Cenozoic coal is of the lignitic or sub-bituminous variety, but because of its low content of sulfur, it is extensively used. The coal beds of the Paleocene Fort Union Formation are mined in the Dakotas, Wyoming, and Montana. They represent the largest recoverable fossil fuel deposits in the United States. Scattered coal beds, mostly of Eocene age, are exploited along the Pacific coast of the United States.
The Tertiary stratigraphic succession of California is impressive, but also imposing are sections of Tertiary rocks far to the north in Alaska and the arctic islands of Canada. Coal found associated with these strata, along with fossil spores and pollen, indicates that a temperature or even warmer climate prevailed in these northern lands. Along the coastal mountains and Aleutian chain, Tertiary sands and shales are interspersed with layers of pyroclastics and lava flows. Then, as now, the area of present-day Alaska was tectonically active.