What Coal Is

  • Coal is a dark brown or black solid fossil fuel that was formed tens to hundreds of millions of years ago by the partial decomposition of land vegetation under increased pressure and temperature in an oxygen-free environment.

The three fossil fuelscoal, petroleum, and natural gas were formed in a similar way by heat and pressure, but petroleum and natural gas were formed from plants and animals that lived in oceans. This caused them to become a liquid (petroleum) or a gas (natural gas).

Stock Coal Pile
Coal: There are four different types of coal: anthracite, bituminous, subbituminous and lignite.
  • Some coal in the US were formed during the CarboniferousCarboniferous gets its name from carbon, the basic element in coal and other fossil fuels. Period of the Paleozoic Era (between 359 and 299 million years ago).
  • There are four types of coal in the world:
Carbon Content and Heat Values by Coal Type
Type of Coal Carbon Content Heat Value (BTUs per pound)
Anthracite 86 to 97 percent nearly 15,000
Bituminous 45 to 86 percent 10,500 to 15,000
Subbituminous 35 to 45 percent 8,300 to 13,000
Lignite 25 to 35 percent 4,000 to 8,300


A British thermal unit, or BTU, is a measure of heat (thermal) energy. It is the amount of energy needed to cool or heat one pound of water by one degree Fahrenheit (1⁰ F).

The higher the number of BTUs, the more heat is produced. Lignite has a lower BTU level than any other type of coal; therefore, it burns faster and contains less heat energy than harder coals (anthracite, bituminous, and subbituminous).

  • During the first part of the Cenozoic Era, which began about 65 million years ago, the last of the seas that had covered North Dakota drained away.
    • Layer after layer of rotted trees and other vegetation had piled up in swamps over millions of years.
      • Pressure from the heavy weight and heat from the earth eventually turned these mats of vegetation into coal.
      • Approximately ten feet of decayed vegetation make up one foot of coal.
Stages of Coal Formation Characteristics
Stages of Coal Formation Characteristics
Stage 1 – Peat Fibrous, soft, spongy substance in which plant remains are recognizable; contains large amounts of water; burns with a long flame and a lot of smoke; used as a soil conditioner and as an absorbent for fuel and oil spills on land.
Stage 2 – Lignite The youngest coal; formed when peat was subjected to increased vertical pressure; dark brown in color; contains traces of plants; used mainly for electric power generation.
Stages 3, 4, 5 – Subbituminous, bituminous, anthracite Progressively older and harder coals; found deeper underground than lignite; mined differently from the way lignite is mined; not found in North Dakota.


  • The kind of coal that was formed in North Dakota is called lignite.
    • LigniteThe word lignite was derived from Latin – lignum = wood;ite is a suffix used in scientific and technical terms. is a soft, crumbly coal with a high moisture content. It burns fast and does not give off as much heat as harder coals.
    • Lignite is good for power production because it is not usable for other purposes, like producing metals.