Coal is a combustible compact black or brownish black sedimentary rock normally occurring in rock strata in layers or veins called coal beds or coal seams. It is formed from vegetation, which has been consolidated between other rock strata and altered by the combined effects of pressure and heat over millions of years to form coal seams. The harder forms can be regarded as metamorphic rock because of its exposure to elevated temperature and pressure. Fig 1 shows the geological processes for the formation of coal.
Fig 1 Geological processes for the formation of coal
Coal is normally classified for convenience into four categories. These categories are (i) lignite or brown coal, (ii) sub bituminous coal, (iii) bituminous coal, and (iv) anthracite.
Lignite coal is a natural resource which is readily available. It is frequently referred to as brown coal. It has some special characteristics which make it different from other coals. It is a soft, brown, combustible, sedimentary rock formed from naturally compressed peat. It is considered to be the lowest rank of coal due to its relatively low heat content. It has lowest carbon content amongst all types of coals. It is mined all around the world and is mainly used as a fuel for steam production and electric power generation.
Bituminous coal is by far the largest group and is characterized as having lower fixed carbon and higher volatile matter than anthracite coals. It is the type of coal which is most widely used in the world today. Bituminous coal is the second highest quality of coal (below anthracite) and the most abundant type. Normally, bituminous coal comes from fairly old coal deposits (around 500 million years old).The energy density of this coal is relatively high and hence it releases a significant amount of energy when burned.
Anthracite coal derives its name from the Greek word ‘anthrakítēs’, literally meaning ‘coal-like’. It had been subjected to the intense pressure and heat. It is the most compressed and hardest coal available. Being a hard coal, it contains greater potential to produce heat energy than softer, geologically ‘newer’ coals. As per ISO 11760:2005, anthracite coal is defined as the coal, synonymous with high-rank coal, having a mean random vitrinite reflectance, equal to or greater than 2.0 % but less than 6.0 %, or, preferably, a mean maximum reflectance, , less than 8.0 % for geologically unaltered coal.
Coal is composed primarily of carbon along with varying amounts of other elements mainly hydrogen, oxygen, nitrogen and sulphur. The quality of each coal deposit is determined by temperature and pressure and by the length of time in formation, which is referred as its ‘organic maturity. It is determined by (i) varying types of vegetation from which the coal originated, (ii) depths of burial, (iii) temperatures and pressures at these depths, and (iv) length of time the coal has been forming in the deposit. Fig 2 shows the depth of burial and the age of different types of coals.
Fig 2 Formation of coal
The process of coal formation from organic compounds includes two distinct stages namely (i) biochemical, and (ii) geochemical. In the biochemical stage the plant material had first converted into peat and then to lignite while in the geochemical stage the lignite had first converted into the bituminous coal and then to anthracite.
Coal was formed from prehistoric plants, in marshy environments, some tens or hundreds of millions of years ago. The presence of water restricted the supply of oxygen and allowed thermal and bacterial decomposition of plant material to take place, instead of the completion of the carbon cycle. Under these conditions of anaerobic decay, in the so-called biochemical stage of coal formation, a carbon-rich material called ‘peat’ was formed. In the subsequent geochemical stage, the different time-temperature histories led to the formations of coal of widely differing properties. These formations of coal are lignite (65 % to 72 % carbon), sub-bituminous coal (72 % to 76 % carbon), bituminous coal (76 % to 90 % carbon), and anthracite (90 % to 95 % carbon).
The process of conversion of organic substances into coal involves three phases consisting of microbiology, chemistry and physics. During the microbiology phase plant remains sink in a body of stagnant water. Mud and sand are deposited on top. The flow of oxygen is cut off. Anaerobic bacteria convert lignin and cellulose into humus. During the phase of chemistry, humic acid develops. Polymerization takes place. The pH level drops. The resulting brown substances are high in acid. Conversion into peat begins. During the phase of physics the peat is compressed and dehydrated under the weight of sands and glaciers. The conversion to lignite coal is reached at a water content of less than 75 %. Increasing depth, pressure and temperature determine the carburization over the course of time.
Initially the peat is converted into lignite or ‘brown coal’. Lignite is the coal type with low organic maturity. In comparison to other coals, lignite is quite soft and its colour can range from dark black to various shades of brown. Over many more millions of years, the continuing effects of temperature and pressure produces further change in the lignite, progressively increasing its organic maturity and transforming it into the range known as ‘sub-bituminous’ coal. Further chemical and physical changes occur until the coal became harder and blacker, forming the ‘bituminous’ or ‘hard’ coal. Under the right conditions, the progressive increase in the organic maturity can continue, finally forming anthracite.
Coal is a complex combination of organic matter and inorganic ash formed over eons from successive layers of fallen vegetation. The degree of change undergone by a coal as it matures from peat to anthracite is known as coalification. Coalification has an important bearing on the physical and chemical properties of coal and is referred to as the ‘rank’ of the coal. The ranks of coals, from those with the least carbon to those with the highest carbon, are lignite, sub-bituminous, bituminous and anthracite. Low rank coals are typically softer, friable materials with a dull and earthy appearance.
Coals are classified by rank according to their progressive alteration in the natural metamorphosis from lignite to sub bituminous coal to bituminous coal and to anthracite. Ranking is determined by the degree of transformation of the original plant material to carbon. Coal rank depends on the volatile matter, fixed carbon, inherent moisture, and oxygen, although no one parameter defines rank. Typically coal rank increases as the amount of fixed carbon increases and the amount of volatile matter decreases. High-rank coals are high in carbon and hence have high heat value, but are low in hydrogen and oxygen. Low-rank coals are low in carbon but high in hydrogen and oxygen content.
Higher rank coals are normally harder and stronger and frequently have a black and vitreous luster. The relative amount of moisture, volatile matter, and fixed carbon content varies from one to the other end of the coalification series. The moisture and volatile matter decrease with enhancement of rank while carbon content increases i.e., carbon content is lowest in peat and highest in anthracite.
Classification of different types of coal into practical categories for use at an international level is difficult. Divisions between coal categories vary between classification systems, both national and international, based on calorific value, volatile matter content, fixed carbon content, caking and coking properties, or combination of two or more of these criteria.
The International Coal Classification of the UN Economic Commission for Europe (UN/ECE) recognizes two broad categories of coals. This classification is adopted by International Energy Agency (IEA). These two broad categories are (i) hard coal, and (ii) brown coal. Hard coal is the coal of gross calorific value greater than 5736 kcal/kg (kilocalories per kilogram) on an ash free but moist basis and with a mean random reflectance of vitrinite of at least 0.6. Brown coal a non agglomerating coal with a gross calorific value less than 5636 kcal/kg containing more than 31 % volatile matter on a dry mineral matter free basis.
Brown coal normally is further subdivided into (i) sub bituminous coal which is having a gross calorific value between 4165 kcal/kg and 5736 kcal/kg, and (ii) lignite which is having a gross calorific value less than 4165 kcal/kg. Hard coals are also of two types. These are (i) coking coal which is the hard coal with quality that allows the production of coke suitable to support burden in a blast furnace, and (ii) non coking coal which is also known as thermal coal, boiler coal, or steam coal. Non coking coal is defined as all other types of hard coal which do not have coking properties.
The term ‘coking coal’ is used to designate certain types of bituminous coals which, when heated at high temperatures (over 1,000 deg C) in the absence of air (carbonization), soften, liquefy, and then re-solidify into a hard but porous mass known as coke, used mainly in the production of hot metal in a blast furnace. Coking coals have specific properties which allow for the formation of coke. Only bituminous coals possess such properties, and in varying degrees.
Coking coal is also referred as ‘metallurgical (met) coal’. In fact, metallurgical coal is a wider term since it also includes all those coals which are used in steelmaking and foundry. Coking coal is a naturally occurring sedimentary rock found within the Earth’s crust. Categories of metallurgical coal include hard coking coal, semi-hard coking-coal or semi-soft coking coal, and coal for pulverized coal for injection (PCI).
Hard coking coal is used in the coke ovens. It is a necessary input in the production of strong coke. The semi-soft coking coal (also known as medium / weak coking coal) is used in the coal blends along with hard coking coal. Coal for PCI is used for its heat value and injected directly into the blast furnace as a replacement for coke. Coal for PCI reduces the consumption of coke per ton of hot metal as it replaces coke as a source of heat and thus reduces the requirement of higher quality, higher cost coking coal.
To be useful in the metallurgical industry, non-coking coal is required to meet three criteria namely (i) its usefulness as reducing agent, (ii) low in sulphur and ash, and (iii) high heat value. To be a useful reducing agent, the coal must have very high carbon content. To keep the iron reasonably pure, the coal is required to have low contents of sulphur and ash. To provide ample heat, the coal is required to have a high content of fixed carbon and a high heat value. Non coking coal has three uses in ironmaking. It is mainly used (i) for pulverized coal injection (PCI) into blast furnace, (ii) for the production of direct reduced iron (DRI) in coal based direct reduction processes, and (iii) for ironmaking by Corus, Finex and other iron smelting processes. Non coking coal needed for these processes has more narrowly defined qualities than the non-coking coal used for power generation. Fig 3 shows comparison of different types of coals.
Fig 3 comparison of different types of coals
Utilization of coal
There are four major pathways for coal utilization. The principal process by which coal is used is combustion which involves burning the coal in air to liberate thermal energy (heat). The heat is used as such for comfort or to carry out many industrial processes which require high temperatures. It is also used to generate steam for use in electric power plants. The second pathway is carbonization which is the heating of coal to high temperatures in the absence of air. It is used for the production of coke for the blast furnace process. The third pathway is the use of carbon of the coal for the reduction of ores besides using its heating value. In this pathway, coal is used for direct reduction of iron ores. The fourth pathway is the conversion of coals. Conversion of coal is carried out by various chemical processes to transform coal into gaseous or liquid fuels, called synthetic fuels. Common to all these pathways is prior mining of the coal, its preparation (processing) and its transportation to the consumers.
Different types of coal also have different uses, as shown in Fig 4. From lignite to anthracite all the members of the series are widely used as fossil fuel in different industries worldwide. The most significant use of coal is in electric power generation, steel production, cement manufacturing, and as a liquid fuel. Steam coal, which is also known as thermal coal or boiler coal, is mainly used in electric power generation. Coking coal, which is also known as metallurgical coal, is mainly used in the production of iron and steel. Coal used for pulverized coal injection into blast furnace has more narrowly defined qualities than steam coal used in electric power generation.
Fig 4 Types and uses of coals
Other important users of coal include alumina refineries, paper manufacturers, and the chemical and pharmaceutical industries. Several chemical products can be produced from the by-products of coal. Refined coal tar is used in the manufacture of chemicals, such as creosote oil, naphthalene, phenol, and benzene. Ammonia gas recovered from coke ovens is used to manufacture ammonium salts, nitric acid and agricultural fertilizers.