How Coal Works

How Coal Forms

Coal is a sedimentary organic rock that contains a lot of carbon -- between 40 and 90 percent carbon by weight. Coal is formed by ancient plants and animals accumulating in moist peat bogs. As plants die off in a wet area, they pile up into peat. It takes between 4,000 and 100,000 years for one meter of peat to accumulate. This process happens best in river deltas or coastal plains.

Over time, these peat seams are compressed by further deposits and the carbon content of the coal is concentrated. The older the coal gets, generally, the harder and blacker it gets. There are four "ranks" of coal: lignite, subbituminous, bituminous, and anthracite, from lowest to highest. Rank is determined by energy content and chemical composition. The ranks are really on a continuum from low carbon/low energy content to high carbon/high energy content.

• The youngest coal is not even coal yet -- peat. Peat is a traditional fuel in parts of the world, like Ireland, where it is cut from the earth, dried, and burned for heat. The energy content of peat is quite low.
  
• Young coal is called lignite, and is soft and brown, not much different than dried peat. Lignite has a low energy content, typically about 13 million Btu per ton. The carbon content is low also, around 40 percent. Lignite is typically used only when higher grades of coal are not available or affordable, such as in Poland.   In the US, only North Dakota and Texas use lignite.
  
• Subbituminous coal is common in the US. It has an energy content of about 18 million Btu per ton, and is used mostly in coal-fired power plants.
  
• Bituminous coal is the most widespread form in the US. It dates from the carboniferous era, about 300 million years ago, and is high in energy content, averaging 24 million Btu per ton. Bituminous and subbituminous account for most coal use in America.
  
• The hardest coal, anthracite, is found mostly in Pennsylvania, but most supplies of anthracite there have been exhausted. The energy content is high, around 23 million Btu per ton, but it tends to have a high sulfur content. It is more than 90 pecent carbon.

Coal can be formed in salt water or fresh water areas. High-sulfur coal was formed in salt water swamps that were covered by sea water. Bacteria in the swamp converted sulfate in the sea water into pyrite that became part of the coal. Low-sulfur coal deposits were developed primarily under fresh-water conditions.

Coal deposits in the Eastern US date back mainly to the Pennsylvanian period of the Earth's geologic history, about 300 million years ago, long before the age of dinosaurs. By contrast, most of the coal in the West is younger, formed less than 140 million years ago in the Cretaceous period, when dinosaurs were alive, and in the subsequent Tertiary period, when they became extinct.

Coal is present in 38 States, lying under 13 percent of the land area of the United States. As can be seen on the map of coal fields in the U.S., bituminous coal comes mostly from the Appalachian Basin and the Midwest, while the Western coals are mostly subbituminous.

Finding coal is typically a simple matter. In the West, where coal seams are not far underground, rocks called "clinker" are found on the surface. Clinker is made when exposed coal seams are ignited by prairie fires, which turns rocks and minerals into a sort of slag. If clinker is found on the ground, a coal seam is bound to be underneath. Sometimes, How Coal worksthe coal seam itself is visible. In truth, there is so much coal already known about that exploration is unnecessary.