Coking

Coke is a fuel with few impurities and a high carbon content, usually made from coal. It is the solid carbonaceous material derived from destructive distillation of low-ash, low-sulfur bituminous coal. Cokes made from coal are grey, hard, and porous. While coke can be formed naturally, the commonly used form is man-made. The form known as petroleum coke, or pet coke, is derived from oil refinery coker units or other cracking processes.

Coke is used in preparation of producer gas which is a mixture of carbon monoxide (CO) and nitrogen (N₂). Producer gas is produced by passing air over red-hot coke. Coke is also used to manufacture water gas.

Historical sources dating to the 4th century describe the production of coke in ancient China. The Chinese first used coke for heating and cooking no later than the ninth century. By the first decades of the eleventh century, Chinese ironworkers in the Yellow River valley began to fuel their furnaces with coke, solving their fuel problem in that tree-sparse region.

In 1589 a patent was granted to Thomas Proctor and William Peterson for making iron and steel and melting lead with "earth-coal, sea-coal, turf, and peat". The patent contains a distinct allusion to the preparation of coal by "cooking". In 1590 a patent was granted to the Dean of York to "purify pit-coal and free it from its offensive smell". In 1620 a patent was granted to a company composed of William St. John, Robert Follensbee and other knights, mentioning the use of coke in smelting ores and manufacturing metals. In 1627 a patent was granted to Sir John Hacket and Octavius de Strada for a method of rendering sea-coal and pit-coal as useful as charcoal for burning in houses, without offense by smell or smoke.

In 1603 Hugh Plat suggested that coal might be charred in a manner analogous to the way charcoal is produced from wood. This process was not put into practice until 1642, when coke was used for roasting malt in Derbyshire; previously, brewers had used wood, as uncoked coal cannot be used in brewing because its sulfurous fumes would impart a foul taste to the beer. It was considered an improvement in quality, and brought about an "alteration which all England admired"—the coke process allowed for a lighter roast of the malt, leading to the creation of what by the end of the 17th century was called pale ale.

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