Coal is a sedimentary rock used as a fossil fuel, black in color, very rich in carbon. Formed from plant matter through a transformation process known as carbonization. It is usually located under a layer of slate and on a layer of sand and clay. Most coal is believed to have formed during the Carboniferous Age (280 to 345 million years ago).

There are basically four types of coal: anthracite, hard coal, lignite and peat.

Thermal power plants transform coal into heat energy to generate electrical energy or for other uses. Depending on the purity of the coal, more or less atmospheric pollutants are generated, acting directly in acid rain.

Historically, coal was the energy source that drove, worldwide, the first phase of industrialization. But at the beginning of the 20th century, oil gradually replaced coal in the United States and later in the most industrialized European countries. However, in the long term and compared to oil and natural gas, coal has improved its position as the most available source of fossil energy worldwide, as there are enough reserves to last more than 200 years, even if production increases.

The most common uses of coal are: electricity production, steelmaking, and heating in the residential, commercial and service sectors.

There are numerous varieties of charcoal, which can be classified according to characteristics such as:

■ Humidity.

■ Percentage of non-combustible mineral matter (ash).

■ The heating power.

■ Flammability, in connection with the percentage of volatile elements.

Elemental analysis is a chemical test that provides the mass fraction of each of the five elements that primarily make up all types of coal: carbon (C), nitrogen (N), oxygen (O), hydrogen (H), sulfur ( S).

Most coal-producing countries have their own classification of coal types, however for international trade the American classification (ASTM) is the most used.