DRY QUENCHING is a process of cooling a hot solid sub stance without using a liquid in direct contact with the substance to be quenched. The sensible heat in most of the cases can thereby economically be converted into useful energy and the quality of the product improved. This process has been applied and devel oped primarily for the quenching of coke as it is discharged from coke ovens at temperatures of I,800° to 2,000° F. It was for a long time common practice to sprinkle this hot coke with water or to submerge it in water to reduce its temperature sufficiently to prevent it from re-igniting and to facilitate handling and transpor tation. This method, however, has several disadvantages. The coke takes up water, thereby considerably reducing its heating value. The wet quenching vapours are corrosive and affect steel and concrete construction, thereby largely increasing maintenance cost. They also carry coke dust high into the air and scatter it widely over the plant and the community.
The hot coke is conveyed to the dry quencher in more or less regular intervals depending upon the schedule of discharge of the coke ovens. It is introduced through the top door of the coke con tainer which holds a number of loads. Before a new load of coke is charged, an equal amount of cooled coke is withdrawn from the bottom of the apparatus. The coke, therefore, is allowed to re main in the container for several hours gradually reaching lower cooling zones. The cooling gas is circulated by means of a fan con tinuously operating and flows through the coke mass in an upward direction, thereby performing the cooling in accordance with the counterflow principle.
The inert mixture of gas is formed when starting up the plant. A small portion of coke is converted into a mixture of carbon monoxide and carbon dioxide whereby the oxygen of the air within the system is completely consumed. The cooling gas therefore consists mainly of these products of combustion and nitrogen. Care must be taken that two doors are never open at the same time in order to prevent air entering the system causing combustion of coke. As long as only one door is open at a time, the pressure in the system adjusts itself to atmospheric pressure at that point, but no gas is pushed out nor air sucked in. However, on discharging cooled coke an equal volume of air enters the system.
The specific heat of coke has been determined by many investi gators and can be considered as sufficiently established for prac tical use. It depends upon the temperature, the ash content and the amount of volatile matter still contained in the coke. The thermal efficiency of a dry quenching plant taking the inlet and outlet temperatures of the coke as limits is about 90 to 95%; there is no loss due to combustible matter in the ash, stack waste gases, etc., as is the case in direct fired boilers; the only loss is through radiation. Dry quenched coke has a higher thermal value than wet quenched coke owing to absence of water. Considerable improvement in operation of gas-producers, water-gas plants, blast furnaces, foundry cupolas and domestic furnaces is the benefit resulting from the use of dry quenched coke. Breeze from this coke can still be considered as a high quality fuel, while wet quenched coke breeze due to high moisture is very likely to cause trouble in furnace operation. The dry-quenching process may also be adapted for the purpose of cooling other hot products of the chemical industry such as calcium carbide, lime, cement clinker, etc., saving the sensible heat and improving the quality of the product. (H. D. S.)