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Coke
an artificial hard fuel obtained from burning natural fuels (or products obtained from the processing thereof) at a high temperature (950-1,050°C) in the absence of air. Depending on the type of raw material used, cokes are classified as coking coal, electrode pitch coke and petroleum coke (petcoke). Most coke is produced from coal.
Coking coal is used mainly in blast furnaces for smelting iron (blast coke), where it functions simultaneously as a fuel and as a reductant of the iron ore. Coke is used in significantly smaller quantities in foundries (casting coke), ore agglomeration, the chemical industry, nonferrous metallurgy, etc.
The first coking coal was produced in the 18th century, when it became necessary to find a replacement for increasingly scarce charcoal. Coke was first used for industrial smelting in the UK in 1735. By 1970 global production of coke exceeded 300 million tonnes a year.
Coking coal takes the form of elongated grey lumps. Its true specific density is 1.8-1.95 g/m3, while its actual relative density allowing for porosity (50% on average) is 0.8-1.0 g/m3. Coke has a bulk density of 400-500 kg/m3. The calorific value of coke is around 29 MJ/kg (about 7,000 kcal/kg) and of its combustible content around 33 MJ/kg (about 8,000 kcal/kg).
The combustible content of coke contains around 96% carbon, with 0.8-1% being volatiles. Coke has a moisture content of 0.5% in dry quenching and 2-4% in wet quenching. Donetsk smelting coke has a sulphur content of 1.5-1.9%, while for Kuznetsk coal the figure is 0.4-0.5%. For casting coke this percentage should be less than 1.2%. The phosphorous content of coke used in the Bessemer iron-smelting process must be less than 0.015%. The ash content of smelting coke should be less than 9-10.5%. Higher percentages of these components in the coke leads to lower metal quality, increases the amount of coke and charge needed, and dramatically reduces the performance of the blast furnace.
By comparison, electrode pitch coke and petcoke have very low ash content, usually less than 0.3% (0.8% in coking coal). Electrode pitch coke is obtained by firing coal-tar pitch in silica chamber furnaces. Petcoke is formed by cracking and pyrolysis of refined oil products. Electrode pitch coke and petcoke are the main raw materials used in the production of electrodes.
