How coke is made

Blend the coals

Coke plants do not bake one coal. They mix several bituminous coals until the blend gives off the right amount of gas as it cooks, because that decides how strong the finished lumps will be. A volatile content of 26 to 29 percent is considered good for the job.

Seal it in an oven

The blended coal is loaded into a chamber of firebrick and shut off from the air. In the old beehive ovens, a dome about 4 meters wide, the coal lay in a layer 60 to 90 centimeters deep. Ovens are built in rows, called batteries, with walls shared between them.

Bake out the volatiles

With no air to burn it, the coal cannot catch fire. It just cooks, and water, coal gas and tar are driven out as vapor. The cooking front creeps through the charge from the outside in, and a beehive oven needed two to three days to finish.

1,000 to 1,100 °C

Quench the red-hot cake

The glowing coke is pushed out and cooled, usually with a water spray or by air. Water-quenched coke soaks up 3 to 6 percent of its mass in water in its pores.

Screen and sort the lumps

Lump size matters. A blast furnace is a tall stack of ore and fuel, and gas has to rise through it, so the coke must be strong enough not to crush under the load. Strength is checked with crush tests before it goes to the furnace.