Coking

coking [′kok·iŋ]

(chemical engineering)

Destructive distillation of coal to make coke

A process for thermally converting the heavy residual bottoms of crude oil entirely to lower-boiling petroleum products and by-product petroleum coke.

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Coking 

an industrial method of processing natural fuels (mainly pit coal) by heating to 950°–1050°C in the absence of air. The main product of coking is coke.

Coking began during the 18th century when total destruction of forests for the production of charcoal, which was originally used for the production of pig iron, appeared likely and it was necessary to replace charcoal with mineral fuels. A blast furnace was first operated using coke in England in 1735. The coke was burned out in piles, by a method analogous to that used for the production of charcoal. At the end of the 18th century, coking was performed in partly closed chambers and, beginning in 1830, in closed chambers, in which case the volatile by-products were burned. The use of the volatile by-products began in the 1870’s, and improvements in the methods of heating coke ovens were achieved. The coking process was developed in its modern form by the beginning of the 20th century, and only its improvement has taken place since then.

The coking of coal takes place in several stages. Heating to 250°C leads to the elimination of moisture and the evolution of the products of decomposition, CO and CO². Then, at 300°C, there is evolution of a small quantity of light tar and so-called pyrogenetic moisture, formed by the decomposition of oxygen-containing compounds that are components of coal. At approximately 350°C, the coal softens and passes into a doughy, plastic state. Intense decomposition of coal occurs in the melt, with the evolution of the primary products (primary gas and primary tar), which have complex compositions. Heavy carbonaceous residues from the decomposition of coal undergo sintering at about 500°C to give a solid, porous material, semicoke. Upon further heating, semicoke loses the remaining volatile materials, mainly hydrogen, and shrinks, which leads to its cracking. Above 700°C, semicoke is converted into coke. However, the primary products of decomposition undergo pyrolysis on contact with the hot walls and dome of the oven and are converted into secondary products. The predominant components of the gas become H₂ (50 percent by volume) and CH₄ (25 percent by volume), and the organic by-products undergo aromatization. The secondary products are collected and used as valuable raw material in the chemical industry.

Coking in a coke oven takes place in layers, in which case the layer temperature decreases from the heated oven walls (above 1000°C) toward the center of the charge. The layer composition also changes in the same direction (starting at the wall), from coke through semicoke, coal in the plastic state, and dry coal to raw coal. The coking is considered complete when the continuously increasing coke layers meet in the center of the oven. At the end of coking, the “coke cake” is divided by a seam or fissure running parallel to the chamber walls, as a result of shrinkage, and each half of the “cake” is divided into large and small pieces by cracks running at right angles to the walls. Coking requires 13–18 hours. The finished coke is discharged from the oven by a coke pushing machine and is conveyed into a quenching wagon, where the incandescent coke is cooled (quenched) with water or an inert gas (by the “wet” or “dry” method).

The technology of coking is being continuously perfected by increasing the oven chamber size and mechanization of oven operation. Coke ovens are loaded with predried and preheated (200°C) charge. New, continuous coking methods are being developed, based on the formation of briquettes from liquefied coal in the plastic state, followed by the ignition of the briquettes.

REFERENCES

Spravochnik koksokhimika, vol. 2. Moscow, 1965.
Taits, E. M. Svoistva kamennykh uglei i protsess obrazovaniia koksa. Moscow, 1961.
Syskov, K. I., and Iu. G. Korolev. Koksokhimicheskoe proizvodstvo. Moscow, 1969.

D. D. ZYKOV