Process Description :
Coal is converted to coke in large coke oven batteries like the one pictured. The coking process consists of heating coal in the absence of air to drive off the volatile compounds; the resulting coke is a hard, but porous carbon material that is used for reducing the iron in the blast furnace. The modern by-product coke oven recovers volatile chemicals in the form of coke oven gas, tars, and oils.

Pusher side of a by-product coke oven battery.

Equipment :
A coke oven battery consists of 10-100 individual coke ovens with batteries of 45 or more. The long thin coking chambers (Height 6-22 ft., Length: 3-52 ft, Width: 1-2 ft) alternate with the heating chambers so that there are heating chambers on both sides of each coking chamber to provide even heating. This alternating row of heating and coking chambers sits atop regenerative chambers that recover the flue gas heat for preheating the combustion air. The burners/regenerators are alternated on a 15-30 minute cycle determined by the need to maintain temperature control within the coking chambers. On top of the coke oven battery are coal charging cars. On the pusher side of the battery is the large pusher/leveler that levels the input charge and then pushes the hot coke out of the oven. On the coke side of the oven, quench cars take the incandescent coke to the quenching system.

Combustion Technology :
Various burner, flue, and regenerator configurations are employed. Practically all heating systems can be grouped into two general classes, the gun-flue type (shown) and the under-jet type. In the gun flue type, the gas is introduced through a horizontal gas duct extending the length of each wall, below the oven floor line. Short connecting ducts lead vertically upward to a replaceable nozzle brick at the bottom of each of the vertical heating flues. The heat in the flue is transferred to the oven and the hot flue gases are drawn back down through the regenerator section. The firing is reversed every 30 minutes, more frequently on high production batteries.

Energy Cnsumption Statistics :
Coke production has been declining due to the associated environmental clean-up issues. Over 25 million tons of metallurgical coal are used to produce 18 million tons of coke. Each ton of coke produced requires 1.37 tons of coal. In the wet quench process, 0.6 million Btu of steam is required per ton of coke for chemical recovery. In the dry quench process, enough heat is recovered in quenching to provide all of the steam requirements of the process and provide some steam for export as well. The energy required for the coking process, virtually all of which comes from the recovered coke oven gas itself or blast furnace gas which ultimately derives from the coke equals 3.2 million Btu per ton of coke produced. Approximately 40% of the coke oven gas produced is returned to the coke ovens to supply this energy.
About 68% of the input coal energy goes into the coke itself. Coke oven gas (exported, i.e. the 60% not needed by the coking process itself) equals 12% of the input coal energy. Recovered tars, oils, and other chemicals contain about 12% of the input energy, and about 8% is consumed as fuel for the process.