How Coal is Formed- A Long Story10/12/2010
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 is a result of the fossilization of organic matter, mainly trees. This process, measured in terms of geological time, takes place over several million years and follows a very specific sequence:
• Everything begins with a marsh on the border of a sedimentary basin (i.e. a lagoon or a lake).
• Tectonic activity raises sea levels, submerging and killing off vegetation.
• Plant debris accumulates and becomes covered with layers of mud and sand in a process known as sedimentation. This shelters the debris from the air and slows down the rotting process.
• Vegetation grows back... until the next flooding.
The sedimentary basin gradually sinks under the weight of the sediments and the layers of dead plants are subjected to rising temperatures, leading progressively to their transformation.
Cellulose, the main component of wood, goes through different phases. The first stage of sedimentation turns it into peat, then lignite, then hard coal, and finally anthracite. Anthracite has the highest carbon content. In Indonesia, where the geothermal gradient is very high, anthracite lies close to the surface. However, the deposits in the Moscow basin have never gone beyond the lignite stage - it is too cold there!
The Different Types of Coal
There are several different types of coal. It comes in different sizes and is ranked according to the percentage of carbon and volatile matter (bitumen and wood residue) it contains.
• Anthracite is 95% pure carbon and 5% volatile matter. It is an excellent fuel that is still used for domestic heating.
• Lean to rich coal, contains 88-92% carbon and 10-30% volatile matter. This coal is used to make coke, a coal concentrate used in metallurgy.
• Rich to flaming coal, is 80-88% carbon and 30-40% volatile matter. It is burnt in industrial boilers.
• Lignite has 65-75% carbon and 50% volatile matter. It is a mediocre fuel because it is very damp. It is used in industrial boilers.
• Peat is plant matter that has undergone relatively little change - it is not a type of coal, dtrictly speaking. Its carbon content is 55% and it is made up entirely of volatile matter. It is a poor fuel that was once used throughout Europe in the form of dried bricks for heating purposes. It is no longer widely used except in a few regions (such as Ireland).
History Recorded in Geological Time
The most favorable period for the formation of coal was 360-290 million years ago, during the Carboniferous period (which means "coal-bearing"). However, lesser amounts continued to form in some parts of the earth during all subsequent periods, in particular the Permian period (290-250 million years ago), and throughout the Mesozoic era, 250-65 million years ago.
The accumulating plant matter buried during the Tertiary period, i.e. less than 65 million years ago, is generally less developed- it often comes in the form of lignite, which still contains a lot of volatile matter (bitumen and wood residue) and has a low carbon content.
However, there is also some high quality coal from the Tertiary period, coal that matured early, heated by plate tectonics. Examples of this include Paleocene coal (65-55 million years), found in Columbia and Venezuela and Miocene coal (20 million years), found in Indonesia.
Finally, recent accumulations (from 10,000 years ago to today) are very rich in fibrous debris, making peat, in which the shapes of branches and roots can still be seen. This material was not buried deep enough to contain basic carbon.