Metallurgical Coal

Metallurgical Coal Metallurgical coal is also called ‘met coal’ or ‘coking coal. It is a bituminous coal which allows the production of a coke suitable to support a blast furnace (BF) charge. It is distinguished by the strong low-density coke produced when the coal is heated in a low oxygen (O2) environment or in absence of air to reduce mineral impurities (e.g. less sulphur, phosphorus). On heating, the coal softens, and volatile components evaporate and escape through pores in the mass. On cooling, the resultant coke has swollen, becoming a larger volume. The strength and density of coke is particularly important when it is used in a BF, as the coke supports part of the ore and flux burden inside the BF. Metallurgical coal possesses the ability to soften and re-solidify into a coherent, porous mass, when heated from 300 deg C to 550 deg C in the absence of air in a confined space. The conversion from coal to coke occurs in chambers called coke ovens where the volatiles from the coal escape, leaving behind what is referred to as metallurgical coke, which reaches a temperature of around 1,000 deg C to 1200 deg C before being removed from the ovens. The coking cycle is normally dependent on several parameters. Coke is used primarily as a fuel and a reducing agent in a BF. The gross calorific value (CV) of the metallurgical coal is greater than 5700 kcal/kg on an ash?free but moist basis. It presents unique plastic properties during carbonization which in turn produces a porous solid, high in carbon (C) coke. Metallurgical coals, when heated at a moderate rate in the absence of air, undergo complex and continuous changes in chemical composition and physical character. During carbonization, most bituminous coals, except those bordering...

Bituminous coal

Bituminous coal Bituminous coal is an organic sedimentary rock formed by diagenetic and sub metamorphic compression of peat bog material. It is also called as black coal. It is often referred to as soft coal. However, this designation is a layman’s term and has little to do with the hardness of the rock. Bituminous coal is by far the largest group and is characterized as having lower fixed carbon (C) 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. Usually, bituminous coal comes from fairly old coal deposits (around 300 million years old).The energy density of this coal is relatively high, therefore, releases a significant amount of energy when burned. Bituminous coal is defined as a medium?rank coal with either a gross calorific value (CV) on a moist, ash?free basis of not less than 24 mega joules per kilogram (MJ/kg) and with a Vitrinite mean Random Reflectance less than 2.0 %, or with a gross CV on a moist, ash?free basis of less than 24 MJ/kg provided that the Vitrinite mean random reflectance is equal to, or greater than 0.6 %. Bituminous coals are agglomerating and have a higher volatile matter (VM) and lower C content than anthracite coal. This coal is originated by coalification of plant matter deposited in sequences dominated by clastic sediments under diagenetic conditions (thermal and pressure mode) of a given coal basin. Coalification proceeded under geologic time scale. In various coal basins (coal seams) coal matter differs in regard of different primary composition of plant matter and sedimentary environment. Composition of coal (e.g. elemental composition, VM etc.) and mean reflectance of vitrinite reflect final stage of coal metamorphism of a given sedimentary basin. Bituminous...

Anthracite Coal

Anthracite Coal Anthracite coal derives its name from the Greek word ‘anthrakít?s’, literally meaning ‘coal-like’.  It is frequently being referred as hard coal and is one of the four types of coals. Other types of coals are lignite coal, sub- bituminous coal and bituminous coal. Since anthracite coal 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’ coal. 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. Geology and mining of anthracite coal Anthracite coal was formed from bituminous coal when great pressures had developed in the folded rock. Transformation of the bituminous coal into anthracite is called ‘Anthracitization’. It was formed during the Carboniferous Age, when the dense green vegetation that thrived during the tropical climate of the time fossilized. It is the oldest and cleanest type of coal. It is the rarest and most mature coal. It is a hard, compact variety of coal. It has the highest ranking amongst all the four types of coals. It has undergone the most metamorphosis. It has the highest fixed carbon content and the least impurities. It has the highest energy density amongst all types of coal. The formation of anthracite coal is shown in Fig 1. Fig 1 Formation of anthracite coal Anthracite coal normally occurs in old geological formations which have spent the longest time underground. It is the rarest and most mature coal which accounts for only around 1 % of the world’s total coal reserves. The major reserves of the anthracite coal are...

Lignite Coal

Lignite Coal Lignite coal is a natural resource which is readily available. It is often referred to as brown coal. It has some special characteristics which make it different from other coals. Lignite coal 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 (C) content amongst all types of coals. It is mined all around the world and is mainly used as a fuel for steam and electric power generation. Since it is not economical to transport lignite coal, it is not traded extensively on the world market when compared with higher grades of coal. Large reserves of lignite coal are available in limited areas of the world. Australia, USA and China have the major reserves of lignite coal. Germany has the largest number of power plants based on the lignite coal. In USA, most of the reserves are located in the North Dakota province while in India, the lignite coal reserves are in Neyveli in Tamil Nadu and in Rajasthan. Around 17 % of the world’s coal reserves are lignite coal. As the world’s oil and gas reserves decline, other sources have become attractive. That is why there is a sustained interest in the use of lignite coal. 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. 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. Coal is a complex combination of organic matter and inorganic ash formed over eons from successive layers of fallen vegetation....

Drying Technologies of Lignite Coals Jul20

Drying Technologies of Lignite Coals...

Drying Technologies of Lignite Coals Coals are generally ranked as anthracite, bituminous, sub-bituminous, and lignite, with anthracite being the oldest and lignite the youngest in the age. As coal ages, its moisture content decreases and heating value increases. The lignite coal is often being referred to as brown coal. It is considered to have the lowest rank, lowest carbon (C) content and highest moisture content. Moisture content in lignite coals can be even 60 % or more. Lignite coals are usually shallow buried facilitating its easy open mining. These coals besides high moisture content also have high volatile content and low calorific value (CV) with easy spontaneous ignition. High moisture content is the main restraint for the application of lignite coals. Moisture content of coal causes many difficulties during processing, storage, transport, grinding, and combustion. The high moisture content considerably reduces the CV and combustion efficiency of the coal. It also results into higher heat loss in the exhaust gas. In the combustion of lignite coals, the important part of the energy is consumed to evaporate the moisture inside the coal. The combustion of the high moisture content coal creates several problems such as the additional energy consumption for the moisture evaporation, the insufficient combustion and the additional exhaust discharge etc. Moisture content of the lignite coals can be classified into the following three types. Surface moisture – It is also known as external moisture. The moisture adheres to the surface of coal particulates or in the bigger capillary cavities. It is the moisture, which can be removed by the coal drying in air at ambient temperature (around 25 deg C). It depends on water conditions in deposit. Inherent moisture – It is a naturally combined part of the coal deposit. It is also...