Coking is the process of heating coal in the absence of oxygen to a temperature above 600 °C (1,112 °F) to drive off the volatile components of the raw coal, leaving behind a hard, strong, porous material with a high carbon content called coke.
It is the ability to withstand this crushing force, in addition to its high energy content and rapid combustion, that makes coke ideal for use in blast furnaces.
[1] In heterogeneous catalysis, the process is undesirable because the clinker blocks the catalytic sites.
Coking is characteristic of high temperature reactions involving hydrocarbon feedstocks.
Acidic catalysts are thus especially prone to coking because they are effective at generating carbocations (i.e., alkylating agents).