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How Coal Is Formed
3 minute read • Graham Lumley • Last update March 2024
What is coal?
Coal is a sedimentary rock with a black or brownish-black color. It is found in underground deposits called coal beds or coal seams. Coal is primarily composed of carbon and hydrocarbons along with varying amounts of other elements, chiefly hydrogen, sulfur, oxygen, and nitrogen.
Due to it’s chemical makeup, coal is highly combustible. As a result, coal is used around the globe as an energy source to fuel electricity generation. The burning of coal produces significant pollutants and greenhouse gases that contribute to global warming.
How is coal formed?
Coal is formed from the remains of plants that lived millions of years ago. It starts with the plants absorbing energy from the sun through a process called photosynthesis. Over time, when these plants die, their remains get buried under layers of soil and other debris.
Over millions of years, the plants are transformed into coal through a series of biological, chemical, and physical changes. This process, known as coalification, occurs in several stages:
- Accumulation of plant material: Coal formation begins in swampy areas where waterlogged conditions prevent the decay of dead plant material. Over time, layers of plant debris accumulate. These areas are often referred to as peat bogs or peat swamps.
- Formation of peat: As new plant material adds to the layers, the weight and pressure increase on the lower layers. This pressure, along with microbial activity (in the initial stages), begins to compact the material, driving out some of the water and gases. The material at this stage is called peat, a brownish-black material that contains about 60% carbon.
- Burial under sediments: Geological changes or rising water levels can bury the peat under layers of sediment. As the peat layers get buried deeper, they are subjected to increased temperature and pressure. This stage significantly slows down the microbial activity and starts the chemical transformation of peat into coal.
- Coalification: Buried peat undergoes further chemical changes and physical compaction due to the heat and pressure. This process increases the carbon content and decreases the oxygen, hydrogen, and nitrogen content. Moisture content also decreases during coalification. Over millions of years, this process transforms peat into coal.
Types of coal
Throughout Earth’s geological history, various processes such as tectonic movements can further bury coal seams deeper, subjecting them to even greater pressures and temperatures. This can enhance the quality of coal, transforming it into higher ranks.
The quality and characteristics of coal that eventually forms depend on the original plant material, the conditions under which it was buried, and the duration and intensity of the heat and pressure it was subjected to. This long process means that coal we use today began forming around 300 to 400 million years ago during the Carboniferous period, a time when the earth was covered with swampy forests.
Different types or ranks of coal include anthracite, bituminous, sub-bituminous, and lignite. These ranks depend on the amount of carbon they contain and the heat energy they can produce. The rank of coal is determined by the pressure and heat that acted on the plants over a long time.
Anthracite
The highest rank of coal containing 86% to 97% carbon. It has the highest heating value among all coal types. However, it makes up less than 1% of coal mining in the United States. Anthracite is mainly used by the metals industry and is mined in northeastern Pennsylvania.
Bituminous
The most common type of coal in the United States. It contains 45% to 86% carbon and is between 100 million and 300 million years old. Bituminous coal accounted for about 45% of total U.S. coal production in 2021. It is used to generate electricity and is important for making coking coal used in the iron and steel industry. The top five states for bituminous coal production are West Virginia, Pennsylvania, Illinois, Kentucky, and Indiana.
Subbituminous
Subbituminous coal contains 35% to 45% carbon and has a lower heating value than bituminous coal. It is at least 100 million years old. About 46% of total U.S. coal production in 2021 was subbituminous, with Wyoming and Montana being the major producers. Some sub-bituminous coal is also mined in Alaska, Colorado, and New Mexico.
Lignite
The lowest rank of coal, containing 25% to 35% carbon. It has the lowest energy content among coal types. Lignite deposits are relatively young and were not subjected to extreme heat or pressure. It is crumbly and has a high moisture content, which reduces its heating value. Lignite accounted for 8% of total U.S. coal production in 2021.
North Dakota and Texas are the primary producers, with smaller amounts coming from Louisiana, Mississippi, and Montana. Lignite is mostly used to generate electricity, but there is a facility in North Dakota that converts lignite to synthetic natural gas for use in natural gas pipelines in the eastern United States.
Is coal renewable?
Coal is not considered a renewable resource because it takes hundreds of millions of years to form. That means we can consume more coal than the Earth can produce and hypothetically can eventually run out of coal to burn.
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