Feature Report: Coal, Still a Growing Energy Resource

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How Is Coal Formed – A Process Spanning Eras

Like oil and natural gas, coal is a fossil fuel. It started forming over 350 million years ago, through the transformation of organic plant matter. 

Coal was formed more than 300 million years ago. © THINKSTOCK

It all starts with a swamp on the edge of a sedimentary basinDepression in the Earth's crust found on land or in the ocean that collects large quantities of sediment..., such as a lagoon or a lake. Tectonic activity raises sea levels, covering and killing vegetation. Plant debris accumulates and is buried under layers of mud and sand in a process known as sedimentation. This protects the debris from the air and slows down the decomposition process. The vegetation grows back, until the next flooding.

95% That's the carbon content of anthracite coal

The sedimentary basin gradually sinks under the weight of the sediments, and the layers of dead plants are subjected to rising temperatures that gradually “cook” them, leading to their transformation. The different stages of sedimentation turn cellulose, the main component of wood, from peat to ligniteRock whose properties are somewhere between peat and coal. It has a carbon content of about 70 to 75%... (brown coalCoal is ranked by its degree of transformation or maturity, increasing in carbon content from...), then sub-bituminous coal, followed by bituminous coal and, finally, anthraciteA type of coal that is 95% pure carbon. It is an excellent fuel.. Anthracite has the highest carbon content (See Infographic: "From Forests to Coal: A Slow Transformation").

Geological Time For The Formation of Coal

The most favorable conditions for the formation of coal occurred 360 million to 290 million years ago, during the Carboniferous (“coal-bearing”) Period. However, lesser amounts continued to form in some parts of the Earth during all subsequent periods, in particular the Permian (290 million to 250 million years ago), and throughout the Mesozoic Era (250 million to 65 million years ago).

The accumulated plant matter buried during the Tertiary Era — less than 65 million years ago — is generally less mature. It is often in the form of lignite, which still contains a high content of volatile matter (bitumen and decayed wood) and has a lower carbon content. However, there is also some higher rank coal from the Tertiary Era, coal that matured early, heated by plate tectonics. Examples of this include Paleocene coal (65 to 55 million years ago), found in Columbia and Venezuela, and Miocene coal (20 million years ago), found in Indonesia. In Indonesia, where the geothermalDescribes the technology used to tap subsurface heat to produce energy... gradient is very high, anthracite lies close to the surface.

Coal began forming 360 million years ago

However, the deposits in the Moscow Basin have never gone beyond the lignite stage. It’s too cold!

Finally, recent accumulations (from 10,000 years ago to today) are very rich in fibrous debris known as peat, in which the shapes of branches and roots can still be discerned. This material was not buried deep enough to contain elemental carbon.

The Different Types of Coal

There are several different types of coal. They are ranked according to their carbon and volatile matter content.