Coal is a sedimentary rock of organic origin, black or dark brown. It is used mainly as a fossil fuel because of its high calorific value because it has a majority carbon content. 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.
Composition of coal
The carbon element appears in coal in a percentage higher than 50% by weight and 70% by volume. The water it contains (moisture) is variable and comes from the one that was trapped at the time of coal formation.
Other constituents of coal are mineral matter (various silicates) and carbonate minerals (siderite, calcite and aragonite). Pyrite is a mineral with sulfur very common in coals. There are small amounts of metals, such as iron, uranium, cadmium and, in very small quantities, gold.
Methane is a gas found in coal mines and can be the source of dangerous explosions in underground mines. In this specific context you will be & rsquo; usually called gray.
Origin of coal
It is believed that most of the current coal was formed during the Carboniferous period (280 to 345 million years ago) of the Primary Era. Also in the Permian, Triassic and Jurassic large deposits were formed. In the Cretaceous, lignite was formed. There is currently peat formation in the peat bogs.
The coal from Europe, Asia, and North America was formed mainly in the coal-based tropical vegetation.
Southern hemisphere coal was formed with cold climate vegetation (tundra). The old plants through the geological changes were compacted, hardened, chemically altered and undergoing a process of metamorphosis by high temperature and pressure.
Northern hemisphere coal was formed in wetland ecosystems called carboniferous forests. When plants die and accumulate in aquatic environments with little oxygen (anaerobic media), they undergo bacterial degradation. For the formation of coal it is necessary that these conditions have a sufficient duration of time, and without erosion in sedimentary conditions.
History of the use of coal
The British Isles (especially rich in coal 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 has been proven that it was a component of funerary pyres and by 200 BC there is evidence, in the same area, of commercial activity and of being used to dry cereals. Under Roman domination there are mentions of the sporadic use of coal, but until the Middle Ages it did not acquire relevant importance.
The first coal used was simply collected from the beach, when this source was exhausted it had to go to coal 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, the fossil energy of coal was converted into mechanical energy.
In the 20th century, when coal became quite expensive, liquid fossil fuels (petroleum products) began to be preferred for transport, and from the middle of the century the use of natural gas was increasing in favor of oil and of coal in industry and obtaining energy. However, even in the 21st century, coal is used to obtain thermal energy (heat) and electricity in industrial boilers and thermal power stations.
Currently the main problems that it presents are pollution and sustainability, since it is a natural resource in the process of exhaustion.
Use of coal
The coal is used mainly as a primary source of heat in industrial boilers and for obtaining electricity in the coal combustion chambers (fixed bed or fluid bed) of the thermoelectric plants. It is, then, mainly a fuel, which can be classified within fossil fuels. 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, among which are, for example, cement kilns and the production of coking coal from hard coal to produce steel.
Two technologies with great prospects for the future are gasification and coal liquefaction. The first is older, was already used in the eighteenth century to obtain what was then called water gas and currently has the interest of producing gaseous fuel synthetic fingers (synthetic natural gas, hydrogen, etc.), which claim to be more easy to store and transport, in addition to being more respectful with the environment, than solid charcoal.
Coal liquefaction began in Germany during the Second World War, so as not to depend on the other countries for oil and its derivatives, since they had no oil deposits but coal mines. As the oil runs out in the world, this technique, direct or indirect, is becoming more advantageous every day, and also allows producing less polluting fuels and designed to be more suitable for use in the automotive industry. The liquid fuel obtained by liquefying coal has twice the calorific power of the coal used to make it.
Like other fossil fuels, coal, when burning, emits mainly carbon dioxide (CO2) air, an atmospheric pollutant that is considered the main greenhouse gas. In addition, its extraction is increased by radioactive radon from the air and, depending on how it is done, the soil and water can be contaminated for wastewater that is not properly treated.
A coal-fired thermoelectric plant also produces nitrogen oxides (NO and NO2) and sulfur dioxide (SO2), which cause acid rain. Nitrogen oxides are also toxic. Sulfur dioxide can be reduced considerably by desulfurizing the gases in the limestone stack (CaCO3), but then important 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. Plaster, 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.
Last review: June 12, 2016