Coal is a type of rock formed by the chemical element carbon mixed with other substances. It is one of the main sources of energy. In 1990, for example, coal supplied 27.2% of the world's commercial energy.
Coal was formed, mainly, when the extensive forests of ferns and giant horsetails that populated the Earth 300 million years ago, in the Carboniferous period of the Paleozoic era, died and were buried in the swamps in which they lived. As the soil is a mixture of water and mud, very poor in oxygen, the usual putrefaction did not occur and, little by little, large amounts of dead plants accumulated, and due to the combined action of pressure and temperature, the matter organic was transformed into carbon.
Depending on the pressures and temperatures that have formed them, we distinguish different types of coal: peat, lignite, coal (bituminous coal) and anthracite. The higher the pressures and temperatures, the more compact and carbon-rich coal is produced, with a higher calorific value.
The coal comes from forests buried in ancient times. The firewood in these forests, basically composed of carbon, oxygen and hydrogen, has lost these last two elements over time, leaving only the first in a fossil state. The most frequent forms of coal extraction are two: underground mining and open air extraction.