Coal and Coke

Coal

— Coal is a pit fuel formed from parts of ancient plants underground without oxygen. The international name of carbon comes from the Latin word carbo (coal). Coal was the first kind of pit fuel used by man. It allowed bringing about the industrial revolution, which in turn contributed to the development of the coal industry by providing it with more modern technology.

On average, combustion of one kilogram of this fuel results in emission of 2.93 kg of CO2 and allows to generate 6.67 kW•h of power or, at an efficiency of 30 %, 2.0 kW•h of electricity. In 1960, coal accounted for about half the world's energy production. By 1970, its share had fallen to one-third. Increasing use of coal is observed in periods of high prices for oil and other energy products.

Formation of coal

Abundant accumulation of plant matter is required for the formation of coal. Starting from the Devonian period (about 416 million years ago), organic matter had been accumulating in ancient peat bogs to form pit coals under anaerobic conditions. Most commercial deposits of pit coal date from this period, although there are younger deposits. The age of the oldest coals is estimated to be about 300-400 million years.

Coal is formed when decaying plant material builds up faster than its bacterial decomposition takes place. The ideal conditions for this process are created in swamps, where oxygen-depleted standing water prevents the vital functions of bacteria and thus protects plant material from complete destruction. At a certain stage of the process, evolving acids start to prevent further bacterial activity. As a result, a feedstock for coal formation — peat — originates. If it is then buried under other sediments, peat is exposed to compression and converted into coal losing water and gases.

Under the action of a 1 km-thick stratum pressure, a 20-meter layer of peat gives a 4-meter layer of brown coal. If burial depth of the plant material reaches up to 3 miles, the same peat layer turns into a 2- meter coal bed. At a greater depth of about 6 kilometers and at higher temperatures, a 20-meter layer of peat transforms into a 1.5-meter anthracite seam.

Coal seams were experiencing uplift and folding as a result of the Earth's crust movement. The raised zones were disintegrating due to erosion or spontaneous combustion over time, while the lowered zones remained unchanged in vast shallow basins where coal was at least 900 meters from the surface. The formation of the thickest coal seams is associated with those areas of the Earth's crust that were subject to a gradual tectonic subsidence at a rate of peat accumulation on their surface for a considerable time — millions of years. At some places, such as Hat Creek (Canada), the thickness of one coal seam can reach 500 m and more.

Types of coal

Anthracite

Anthracite, the oldest type of the pit coals, has the highest degree of coalification. It is characterized by a high density and lustre. Anthracite contains 95 % carbon. It is used as a high-energy solid fuel with an energy value 6800-8350 kcal/kg. Anthracite has the greatest heat of combustion value, but is difficult to ignite. It is formed from hard coal at an elevated pressures and temperatures and at depths of about 6 kilometers.

Hard coal

Hard coal is a sedimentary rock formed by deep decomposition of plant residues (tree ferns, equisetums, lycopodiums and the first gymnosperms). Most coal deposits were formed in the Paleozoic era, mainly in the Carboniferous period, about 300–350 million years ago. By its chemical composition, coal is a mixture of high molecular weight polycyclic aromatic compounds with a high mass content of carbon, as well as water and volatile substances with small amounts of mineral impurities that give ash after coal combustion. Pit coals differ in the ratio of constituent components, which determines their calorific value. A number of organic compounds in hard coal are carcinogenic.

Depending on coal grade, carbon content in coal varies from 75 % to 95 %. This type of coal contains up to 12 % moisture (3-4 % fixed moisture) and has a higher heat of combustion as compared to brown coal. It contains up to 32 % volatile matter, which results in a higher flammability. Hard coal is formed from brown coal at depths of about 3 kilometers.

Brown coal

Brown coal is a solid pit coal formed from peat. The youngest type of coals contains 65-70 % carbon and is brown in colour. It is used as a domestic fuel and a chemical feedstock. This coal has a high water content (43 %) and therefore a low heat of combustion. It also contains a large number of volatile substances (up to 50 %). Brown coal is formed from dead organic matter under pressure load and at high temperatures, at the depths of about 1 kilometer.

Coal mining

Methods of coal mining depend on the depth of its occurrence. Open-pit coal mining is used, if the depth of the coal seam does not exceed 100 meters. There are also cases when it becomes more profitable to excavate coal deposits by the underground mining method as the coal quarry deepens. Coal mines are used to extract coal from the depths. In the deepest mines of the Russian Federation, coal is excavated from the depths just over 1,200 meters. Besides coal, many types of mineral resources possessing consumer relevance are also found in coal-bearing strata. These resources include enclosing rocks used as raw materials for the construction industry, groundwater, coalbed methane, rare and trace elements, including precious metals and their compounds. For example, some coals are enriched in germanium.

Coal marking

Like oil and gas, coal is an organic substance which has been exposed to slow decomposition caused by biological and geological processes. Plant remains constitute the coal basis. Depending on the degree of conversion and the relative amount of carbon in coal, the following four types of coal are differentiated: brown coal, hard coal, anthracite and graphite. There is a slightly different classification in Western countries: lignite, sub-bituminous coal, bituminous coal, anthracite and graphite respectively.

Coal marking — is established with the aim of rational industrial use of coal. The coals are divided into grades and technological groups, the subdivision being based on the parameters that characterize coal behavior during thermal exposure. The Russian classification differs from the Western one.

Russian classification

For some basins, the following intermediate grades are introduced that differ from those specified in the above table:

• gas fatty GZh (ГЖ)
• сoke fatty KZh (КЖ)
• сoke 2 (K2)
• low-caking (CC).

Coals are divided into groups by their agglomeration capacity; a number indicating the lower value of the plastimetry layer thickness for these coals, e. g. G6 (Г6), G17 (Г17), KZh14 (КЖ14), etc., is added to the grade letter designation to specify the corresponding technological group.

By size of lumps produced after excavating, coal is classified into the following classes: P (П, slabby) over 100 mm, K (large) 50–100 mm; O (hazelnut-sized) 25–50 mm, M (small) 13–25 mm; C (pea-sized) 6–13 mm, Sh (Ш, culm) 0–6 mm; P (run-of-mine) 0 - 200 mm, career 0–300 mm.

Coal grades D (Д) and G (Г) can burn without blasting, and this feature allows using the grades in boilers. Coal grades СС, Т or ОС can be used in power generation. As a rule, grades G (Г) and Zh (Ж) are used in the ferrous metallurgy.

Other classifications

German classification is based on elemental analysis results (expressed as percentages).

Proven coal reserves