While the context of rav’s question has been explained, there is still the broader question of “What the hell is coker?”. Well this has nothing to do with Somerset, but still it is a word... Briefly, a 'coker' converts heavy crude oil into fuel-grade petroleum coke (not to be confused with the stuff you put in your stove!).
Most oil refineries were designed to process mainly light liquid feedstock and were not well able to digest heavy crude. As the lighter, sweeter crudes have been used first, the quality of crude oil available has declined while final product fuel specifications have become more demanding to meet environmental regulations. As a result, many refineries have added, or are adding, ‘delayed coker’ facilities.
This enables cleaner, more efficient processing of the cheaper heavy crude oil feedstocks, allowing the refinery to produce cleaner, higher-valued fuels at a lower cost. The refinery hits two birds with one stone by increasing its production of higher margin products, and improving its environmental conditions.
If you really want more technical data try this link:
http://www.lloydminsterheavyoil.com/upgraddecoke.htm