Thermal cracking is a process that manipulates long straight chains found in gas oils and other crude fractions, into shorter straight chain alkanes. The chemistry involves free radical reactions which are the key factor concerning the relatively low octane numbers of gas oils that undergo thermal cracking. Without this process, vacuum distillation residue (VDR) would essentially be a useless byproduct. Thermal cracking allows this residue to be converted into distillate fuels along with its primary goal – coke.

Thermal cracking was initially introduced in the early 1900s in order to produce more motor gasoline and high-octane gasoline for aircrafts. It wasn’t until the 1930s and 1940s, when catalytic cracking was introduced, that the petroleum industry seemed to lose its interest in thermal cracking. Today, there is still a desire for such a process, mainly in countries where the chief petroleum fuel in high demand is diesel fuel. Thermal cracking is also used for VDR with visbreaking and coking processes.

There are two (but technically three) main types of thermal cracking during coking, which include delayed coking and fluid coking. The third is known as flexi-coking, a derivative of fluid coking utilized to maximize the yield of distillate products. There is a notable market for this rejected carbon since coke has economic value as it can be used as fuel or as filler when producing anodes for the electrolysis of alumina.