Thermal Power Plant Cercs. Catalonia



Coal is a fossil fuel, a sedimentary rock of organic origin. Its thermodynamic properties allow obtaining a large amount of heat energy during its combustion process. The energy obtained is called fossil energy. It comes in black or dark brown.

It is a limited natural resource. Generation is a process of several million years, while the consumption of this energy resource is very fast. For this reason, it is considered a non-renewable energy source.

Its high calorific value is due to the fact that it is mainly made of carbon.

Coals can be classified by the percentage of carbon they contain, which is related to the percentage of moisture and impurities. According to this criterion, peat, lignite, coal and anthracite can be distinguished.

What is the origin of the coal?

Fossil coal was formed from parts of ancient plants. To a large extent, of bitumen masses dumped on the planet's surface, subjected to:

  • Metamorphism. Result of the descent to great underground depths.
  • High temperatures.
  • Absence of oxygen.

Most of today's coal is believed to have formed during the  Carboniferous periods (280 to 345 million years ago) of the Primary Age. Also in the Permian, Triassic and Jurassic large deposits were formed. Lignite formed in the Cretaceous.

There is currently peat formation in peatlands.

Coal from Europe, Asia, and North America was formed primarily in the coal from tropical vegetation.

Southern hemisphere

Coal from the southern hemisphere was formed with cold climate vegetation (tundra).

Ancient plants through geological changes were compacted, hardened, chemically altered. They underwent a process of metamorphosis due to high temperature and pressure.

North Hemisphere

Northern hemisphere coal formed in wetland ecosystems called carboniferous forests.

When plants die and accumulate in aquatic environments and  marshy areas with little oxygen (anaerobic media) they undergo bacterial degradation. For the formation of coal, these conditions must have a sufficiently long duration. No erosion in sedimentary conditions.

What types of coal are there?

There are many different types. The coal classification is made according to certain characteristics: flammability, calorific value, percentage of non-combustible materials and humidity.

CoalThe most used classification for international trade is as follows:

  • Anthracite.
  • Bituminous.
  • Sub-Bituminous.
  • Lignite.

However, most producing countries have their own classification. For example, the European one. In the European classification, the main categories of coal are based on the percentage of carbon they contain. These are its variants:

  • Turba (50 a 55 %)
  • Lignite (55 to 75%)
  • Subbituminous coal or black lignite
  • Coal (75 to 90%)
  • Anthracite (90 to 95%): the one with the highest proportion of carbon
  • Graphite: pure carbon. This variety is not used as fuel.


Anthracite is the one that warms the most. It is characterized by its high density and shine. Contains 95% carbon. It is used as a high-calorie solid fuel (calorific value of 6800-8350 kcal / kg).

It has the highest calorific value, but it is not very flammable.

It is made of coal with increasing pressure and temperature at depths of approximately 6 km.

Bituminous coal

Bituminous coal is a relatively hard coal that contains bitumen, between lignite and anthracite.

The calorific content of the bituminous ranges from 21 to 30 million Btu / t (24 to 35 MJ / kg).


Lignite is a mineral coal that is formed by compression of the peat. Compression makes it a crumbly substance. Traces of plant matter can still be recognized in brown coal.

It is black or brown. It often has a texture similar to that of the wood from which it comes.

What is the composition of the coal?

Coal is a sedimentary rock made up of:

  • Organic elements. In a majority proportion.
  • Mineral substances.
  • Water. Humidity is variable. It comes from the one that was trapped at the time of its formation.
  • Gases. Located in submicroscopic pores.

The carbon element appears in a percentage greater than 50% by weight and 70% by volume.

The composition and structure of an element transformed into carbon depend on the particular process of carbogenesis that it has undergone. In this sense, for the characterization of a coal reference is made to its petrographic composition. The petrographic composition is the proportion and distribution of their respective macerals and constituent minerals.

In this way, for example, humic and sapropelic coals are distinguished.

The humic compounds are mainly made of vitrinite and give rise to the so-called ligno-coal or natural series (since most of the coals belong to it).

Methane gas

Methane is a gas found in coal mines and can be the source of dangerous explosions in underground mines. In this specific context it is often called a firedamp.

What is coal used for? Applications

Coal is mainly used in two applications:

  • As a primary source of heat in industrial boilers.
  • For obtaining electricity. Thermoelectric plants.

75% of the world's coal is used to produce electricity. The global energy efficiency of coal plants is not very high, around 25% -27%.

It also has other more minor uses, including, for example, cement kilns and the production of coking coal from coal to produce steel.

Gasification and liquefaction of coal

Two technologies with great prospects for the future are:

  • gasification.
  • liquefaction of coal.

The first one is older. It was already used in the 18th century to obtain what was then called water gas. It is currently interested in producing synthetic finger gas fuels (synthetic natural gas, hydrogen, etc.). These gases are intended to be easier to store and transport, as well as more environmentally friendly, than solid coal.

Liquification began in Germany during World War II. The motivation was the desire not to depend on other countries for oil and its derivatives. Germany had no oil fields but did have coal mines.

As oil runs out in the world, this technique, direct or indirect, becomes more and more advantageous every day. It also allows the production of less polluting fuels. Fuels designed to be more suitable in the automotive industry.

The liquid fuel obtained by liquefying coal has twice the heat output as the coal used to make it.

What influence does coal have on the environment?

Like other fossil fuels, coal, when burned, mainly emits carbon dioxide (CO 2 ) into the air. Carbon dioxide is the main greenhouse gas. It favors the greenhouse effect. Furthermore, its extraction increases the radioactive radon in the air. Depending on how the extraction is carried out, the soil and water can be contaminated for wastewater that is not properly treated.

A coal thermoelectric power station also produces nitrogen oxides (NO and NO 2 ) and sulfur dioxide (SO 2 ). Acid rain causing gases. Nitrogen oxides are also toxic.

Sulfur dioxide can be greatly reduced by desulfurizing the flue gases with limestone (CaCO 3 ), but then significant gypsum residues and more carbon dioxide are produced. This process is not usually done if it is not required by law, due to its high economic cost.

Gypsum, ash and other solid particles emitted into the atmosphere can be reduced with filters. The plant also increases the natural series of atmospheric radioactive, mainly those of the radon family.

History of the use of coal

The British Isles (especially rich in this fossil fuel and where the industrial revolution began) are the first place studied where the use of this fossil fuel is detected.

There, in the third millennium BC, it was found to be a component of funeral pyres. Around 200 BC there is evidence, in the same area, of commercial activity and of being used to dry cereals.

Under Roman rule there are mentions of the sporadic use of coal, but it was not until the Middle Ages that it became relevant.

First extraction of the mineral

The first coal used was simply collected from the beach, when this source was exhausted it had to be transferred to mining. It began to be used massively with the first applications of the steam engine, both in industry and transport, especially in trains and ships. In these cases, fossil energy from coal was converted into mechanical energy.

In the 20th century, when it became quite expensive, liquid fossil fuels (derived from petroleum) began to be preferred for transportation. From the middle of the century, the use of natural gas increased in favor of oil and coal in industry and in obtaining energy. However, even in the 21st century, coal is used for obtaining thermal energy (heat) and electricity in industrial boilers and thermal power plants.

Currently the main problems it presents are pollution and sustainability, since it is a natural resource in the process of depletion.


Published: July 12, 2016
Last review: April 24, 2020