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What are Coal and Coke? Steel Foundations

Coal is a mineral consisting mainly of fossilized carbon with small amounts of other substances such as sulfur, hydrogen, oxygen, nitrogen, tar, and volatile organic compounds (VOC).

Coal is extracted from the earth’s crust either by underground shaft mining or at ground level by open-pit mining. It comes in a variety of qualities based on how much heat it can produce. The highest quality is anthracite, followed by bituminous, then lignite. Coal is the largest source of energy worldwide for generating electricity, but it is also used as fuel in blast furnaces in the form of metallurgical coke. Some smaller blast furnaces can utilize charcoal as a carbon source, but the larger blast furnaces require the strength and durability of coke.

Metallurgical coke is produced by igniting bituminous coal under reduced oxygen conditions in specially designed coking ovens that consist of a set of connected smaller ovens. The coking process uses the heat generated by the ovens to remove volatile compounds and other impurities that are naturally found in the coal such as coke oven gas, tar, ammonium sulfate, benzol, toluol and naphtha.

Prior to being fed into the coking oven, the coal is crushed and blended. The combination of smaller coal pieces and the use of multiple small coking ovens increases the surface area of the coal that is exposed to the heat, which speeds the coking process.

At the coking oven, a larry car is used to fill each small oven with the blended coal. Then the coal is heated to 1,800°F for approximately 18 hours. During that time, the volatile compounds and impurities within the coal are removed and a pure carbon form called metallurgical coke remains. When exposed to oxygen, the hot coke will immediately ignite, and begin to burn. Because of this, when the coke is pushed from the oven into the rail car it must be quickly quenched to stop the burning process. The cooled coke is then dumped onto a coal wharf where it is taken to a facility to be screened and sized prior to being put into the blast furnace.

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