By-product coking is classified as a true distillation process because all the components of the coal are recovered. One of the chief differences between by-product ovens and the beehive ovens is the source of heat. Instead of burning part of the coking coal, as in the beehive oven, the by-product oven is heated by an external fire. Several types of coking ovens are used in this process,but the basic design is similar for all types, and they differ chiefly in the method of heat application. The three best known types are the Semet-Solvay, the Otto-Hoffman, and the Koppers.

The coking chambers (ovens) into which the coal is charged are rectangular and range from 30 to 42 feet in length, from 6 to 14 feet in height, and from 12 to 22 inches in width. These chambers are constructed with silica fire-brick to withstand the extreme temperatures of the coking process. Between each oven is a series of heating flues which are arranged so as uniformly to heat the entire side of the oven.

The individual by-product coke oven operates alternately, but each oven is started and stopped at different times, so that the operation of the entire block continually produces gas of good average composition. A charge of finely crushed coal is dropped from a larry car through charging holes (usually four) in the top and into the oven, where the walls are at approximately 1100 C.

The surface of the coal in the oven is levelled and the charging holes are covered. At the top and at the ends of the coking chambers are openings for collection of the volatile matter as it evolves during the heating cycle. These openings are fitted to off-take pipes that carry the gases to chemical recovery units where the by-products of coking are obtained. The coking coal, which entered the retort (oven) as individual particles, begins to soften at about 400 C, and as the temperature rises the coal fuses into a plastic mass. Depending on the volatile matter content, the coal expands or contracts during the plastic range and gas evolution period until it resolidifies at about 500 C.

At the end of the coking cycle (approximately 17 hours), both end doors of the chamber are opened, and a "pusher" mechanism shoves the block of incandescent coke into a quenching car. The car then advances to a spray system that rapidly cools the coke to prevent combustion. After quenching, the coke is crushed and screened for proper sizing to meet specific applications.

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Last updated February 1, 2006