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Coal blending is the process of mixing coals after coal has been mined to achieve quality attributes that are desirable for the coal’s intended application (e.g. steam generation, coking).[1] The quality attributes that are most important in blending will differ from one mine site to another and also depend on how the coal seams vary in quality and their final intended use. In thermal coals, quality attributes of interest often include ash, volatile matter, total Sulfur, and gross calorific value. For coking coals, additional attributes are sometimes considered including crucible swelling number, fluidity.
Characteristics of coking coals
Vitrinite reflectance - It is also known as mean maximum reflectance (MMR) and is used to determine the coal rank. It is measured by the percentage of light reflected off the vitrinite maceral at 500x magnification in oil immersion. The MMR values for the coking coals in case of low volatile coals are normally in the range of 1.42 % to 1.75 %, in case of medium volatile coals normally in the range of 1.05 % to 1.4 %, and in case of high volatile coals normally in the range of 0.70 % to 1.02 %.
Rank - It is measured through a process called petrography whereby the amount of light which is reflected off the coal is quantified as its ‘reflectance’ (R). The higher is the reflectance, the higher is the coal rank.
Hardness - The hardness of the coke produced increases the value of the coal. Coal can be classified as weak, semi-soft coking, semi-hard and hard coking coal.
Calorific value - It is the amount of chemical energy stored in a coal which is released as thermal energy upon its combustion. It is measured in kilo-calories. It is directly related to the coal rank.
Moisture content - It is determined by heating an air-dried coal sample at 105 deg C to 110 deg C under specified conditions until a constant weight is obtained.
Ash - It is left over inorganic residue after the coal is completely burnt.
Ash fusion temperature - Ash fusion temperature (AFT) is the temperature which characterizes the behaviour of ash as it is heated
Volatile matter - Volatile matter (VM) consists of substances other than moisture which are given off as gas and vapour during combustion.
Plasticity -It refers to the melting and bonding behaviour of the coal. It is the ability for coal to soften and become plastic when heated, and then to re-solidify into a coke.
Fluidity - Fluidity of coking coal is measured by Gieseler plastometer. In this test fine coal (not pulverized) is heated slowly and as it melts and passes through its plastic range, its fluidity is measured. Results are expressed as maximum fluidity in ‘dial divisions per minute’ (ddpm).
Maximum dilatation - Maximum dilatation test measures the expanding and contracting characteristics of the coal.
Gray-King coke type - The test determines the coking properties. It is a simple coking test
Free swelling index - Free swelling index (FSI) also tests the plastic properties of coal. It is also known as crucible swelling number (CSN).
Негізгі бет Ғылым және технология COAL BLENDING | PROPERTIES OF COOKING COAL | TYPES AND SOURCES OF COOKING COAL | BLENDING | CRUSHING
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