Coal is classified into four main types, depending on the amount of carbon, oxygen, and hydrogen present. The higher the carbon content, the more energy the coal contains.
Lignite is the lowest rank of coal, with a heating value of 4,000–8,300 British thermal units (Btu) per pound. Lignite is crumbly and has high moisture content. Most lignite mined in the United States comes from Texas. Lignite is mainly used to produce electricity. It contains 25–35 percent carbon. About seven percent of the coal mined in 2006 was lignite.
Subbituminous coal typically contains less heating value than bituminous coal (8,300–13,000 Btu per pound) and more moisture. It contains 35–45 percent carbon. Forty-four percent of the coal mined in 2006 in the U.S. was sub-bituminous.
Lignite is the lowest rank of coal, with a heating value of 4,000–8,300 British thermal units (Btu) per pound. Lignite is crumbly and has high moisture content. Most lignite mined in the United States comes from Texas. Lignite is mainly used to produce electricity. It contains 25–35 percent carbon. About seven percent of the coal mined in 2006 was lignite.
Subbituminous coal typically contains less heating value than bituminous coal (8,300–13,000 Btu per pound) and more moisture. It contains 35–45 percent carbon. Forty-four percent of the coal mined in 2006 in the U.S. was sub-bituminous.
Bituminous coal was formed by added heat and pressure on lignite. Made of many tiny layers, bituminous coal looks smooth and sometimes shiny. It is the most abundant type of coal found in the United States and has two to three times the heating value of lignite. Bituminous coal contains 11,000–15,500 Btu per pound. Bituminous coal is used to generate electricity and is an important fuel for the steel and iron industries. It contains 45–86 percent carbon. Almost half of the coal mined in 2006 was bituminous coal.
Anthracite was created where additional pressure combined with very high temperature inside the earth. It is deep black and looks almost metallic due to its glossy surface. It is found primarily in 11 northeastern counties of Pennsylvania. Like bituminous coal, anthracite coal is a big energy producer, containing nearly 15,000 Btu per pound. It contains 86–97 percent carbon. Less than one percent of coal mined in 2008 was anthracite.