Production
Volatile constituents of the coal—including water, coal-gas, and coal-tar—are driven off by baking in an airless furnace or oven at temperatures as high as 2,000 °C (3,600 °F) usually around 1000–1100 °C (1832–2012 °F) though. This fuses together the fixed carbon and residual ash. Most modern facilities have "by-product" coking ovens. Today, the volatile hydrocarbons are mainly used, after purification, in a separate combustion process to generate energy. Non by-product coking furnaces or coke furnaces (ovens) burn the hydrocarbon gases produced by the coke-making process to drive the carbonization process.
Bituminous coal must meet a set of criteria for use as coking coal, determined by particular coal assay techniques. These include moisture content, ash content, sulfur content, volatile content, tar, and plasticity. This blending is targeted at producing a coke of appropriate strength (generally measured by coke strength after reaction (CSR), while losing an appropriate amount of mass. Other blending considerations include ensuring the coke doesn't swell too much during production and destroy the coke oven through excessive wall pressures.
The greater the volatile matter in coal, the more by-product can be produced. It is generally considered that levels of 26-29% of volatile matter in the coal blend are good for coking purposes. Thus different types of coal are proportionally blended to reach acceptable levels of volatility before the coking process begins.
Natural coke is formed when a coal seam is intersected by a volcanic intrusion. These intrusions heat the surrounding coal in an anoxic atmosphere producing coke in a zone (usually several meters) around the intrusion. However, the coke is of wildly varying strength and ash content and is generally considered unsaleable except in some cases as a thermal product—as it has lost its volatile matter it has lost the ability to be coked again.
Read more about this topic: Coke (fuel)
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