Hard coal is a mineral deposit used as fuel. It is a black or sometimes greyish-black dense solid with a shiny, semi-gloss or matte surface. As to the origin of coal, there are two main points of view. According to the first, coal was formed as a result of plants decaying over millions of years. However, this process has not always lead to the formation of coal deposits. The fact is that oxygen supply should be limited so that the decaying plants would not release carbon into the atmosphere. The swamps represent an environment that is suitable for this process. Stagnant water with a minimum content of oxygen prevents bacteria from destroying the plants completely. Acids are released at a certain point, bringing the operation of bacteria to a complete stop. Peat is thus formed, which is first transformed into brown coal, then into bituminous coal, and finally into anthracite coal. The formation of coal is subject to another important factor: due to the motion of the Earth's crust the layer of peat must be covered by other layers of soil. Thus coal is formed under pressure, in elevated temperatures, and in the absence of water and gases. There is another version, too. It assumes that coal is the result of transfer of carbon from the gaseous state into the crystalline state. It is based on the fact that Earth's bowels may contain large amounts of carbon in the gaseous state. During cooling, it deposits in the form of coal. The simplest method of coal mining is known since ancient times and is first recorded in China and Greece. In Russia, coal was seen for the first time by Tsar Peter I, in the area of what is the Town of Shahti today, in 1696. In 1722 expeditions were started to explore coal deposits in the territory of Russia. At this time people started using coal in salt production, blacksmithing and for heating homes. There are two basic methods of coal mining: the open and the closed method. The method of mining depends on the depth of the layer. If the deposits are 100 meters deep, the open method of mining is used (the layer of soil above the site is removed, that is, a quarry or cut is formed). If the deposits lie deeper, mines with special underground tunnels are made. Incidentally, coal is generally formed at a depth of 1.9 miles or more. But as a result of movement of Earth's strata, the layers are raised up closer to the Earth's surface, or lowered at deeper levels. Coal is deposited in the form of beds and lenticular deposits. The structure is layered or granular. An average coal bed is about 2 meters thick. Coal is not merely a useful mineral; it is also a congregation of high-molecular compounds with high carbon content, also containing water and volatile substances, with a small amount of mineral impurities.
1. The chemical composition of coal includes:
2. The material composition of coal includes:
These figures are given in percentage; the exact composition depends on the location of deposits and climate conditions. In order to understand the subject matter, the quality of coal is determined by several important indicators. Firstly, the degree of effective humidity (less moisture means better energetic properties). Water content in coal is 4 – 14 per cent, which gives the calorific value of 10-30 MJ/kg. Secondly, it is the ash content of coal. Ash is formed due to the presence of mineral impurities in coal and is determined by the output of the residue after burning the coal at 800 C. Coal is considered fit for use if the ash after combustion accounts for 30 per cent or less of the residue. In contrast to brown coal, bituminous coal contains no humic acids which have been transformed into carboids (dense carbon compounds). Accordingly, the density and carbon content in bituminous coal is higher than in brown coal.
3. Physical composition and properties of coal:
|Density (specific gravity)||1,2-1,5||gr/сm3|
|The coefficient of refraction||1,82-2,04||-|
As to the properties, the following types of hard coal are distinguished: shiny coal (vitrain), semi-bright coal (clarain), matt coal (drogen) and wavy coal (fusain). In terms of the degree of enrichment coal is divided into coal concentrate, middlings, and slurry. Concentrates are used in boilers and to produce electric power. Middlings are used to serve the needs of iron and steel industry. Slurry is suitable for the production of briquettes and for retail sale for household needs.
4. Classification of coal according to the size of pieces:
|Slab (Плитный)||П||more 100 mm|
|a Large (Крупный)||К||50..100 mm|
|Walnut (Орех)||О||25..50 mm|
|Small (Мелкий)||М||13..25 mm|
|Seed (Семечко)||С||6..13 mm|
|Dust (Штыб)||Ш||less 6 mm|
|Ordinary (Рядовой)||Р||0-300 mm|
The main technological properties of coal are the agglutinating value and the coking ability. The agglutinating value is the ability to coal to form melted residue when heated (without air feed). Coal acquires this property during the stages of its formation. The coking ability refers to the ability of coal to form coke, a porous lump material, under certain conditions and at high temperatures. This ability adds an extra value to coal. During the coal formation, changes occur in its carbon content, the amount of oxygen, hydrogen and volatile substances decreases, and the calorific value changes, too.
5. Classification of coal by brands:
|Classification of coal by brand:||Designation|
|Lean Caking(Отощенные спекающиеся)||ОС|
|Half anthracite (Полуантрациты)||ПА|
|Coke fat (Коксовые жирные)||КЖ|
Application: Long-flame and gas coal brands are commonly used in boilers since they can burn without air feed. Gas fat coals and fat coals are used in iron and steel industry for the production of steel and cast iron. Lean caking coal, lean coal and low-caking coal is used to produce electricity as they have a high calorific value. At the same time the burning of these coals is associated with certain technological difficulties. The areas of application of hard coal are very extensive; at the beginning of production of coal in Russia it was used primarily for home heating and in blacksmithing. Currently there are many areas where coal is used, for example, in the iron and steel industry. Here high temperatures are needed in order to melt metal, and therefore use of the coke type of coal is required. The chemical industry uses coal for coking and to further obtain coke oven gas which is used to obtain hydrocarbons. During the process of refinery of hydrocarbons, toluene, benzene and other substances are obtained, thanks to which linoleum, lacquers, paints and other products are produced. Coal is also used as a heat source both by the population and to obtain energy in thermal power stations. Certain amounts of carbon soot (quality soot is obtained from gas and fat coal) are also produced during the heating of coal which are then used to produce rubber, printing inks, drawing ink, plastic and so on.