Petroleum coke is the residual solid, carbon-rich material left over after all the useful products are removed from crude oil. It is produced by the following refinery processing steps:
- After feeding crude oil into an Atmospheric Distillation Unit (to recover diesel and naphtha) and then taking the heaviest portion of the crude oil from the Atmospheric Distillation Unit, the atmospheric bottoms are fed to a…
- …Vacuum Distillation Unit (where the oil is boiled at ~750oF at very low pressure to extract additional light ends for further processing) and then…
- …by heating this “vacuum residuum” to ~900oF in a Delayed Coking Unit (where the vacuum resid cracks into smaller molecules such as diesel and naphtha), and then…
- …by injecting the heater effluent into a coke drum (where the cracked light fraction flows up and over to a fractionation tower, and the heavy material is held in the drum to soak for 12-24 hours) and then…
- …by periodically taking the coke drum out of service (there is an art to this – generally coke drums are turned around every 14-20 hours) the solid material, known as petroleum coke, is removed from the drum.
Compared to coal, petroleum coke (or pet coke) is generally higher in sulfur, higher in noble metals such as vanadium, very hard (it has a low Hardgrove Grindability Index, or HGI), higher in ash melting point and it has very little inherent moisture. There are several pet coke grades, but we are interested in the very lowest quality, known as fuels grade or green petroleum coke. Fuels grade coke has various physical forms such as shot or sponge or fluid depending upon the crude source and the coking technology employed.
As the name implies, this fuels grade coke can be used in place of coal. However, because of its very high sulfur content, its high vanadium content, and its low HGI, many coal-burning plants have been reluctant to use it.
We need to be aware of how production rates are quoted. Process engineers for coke plants generally provide coke production rates on a dry basis and coke sales are generally quoted on an as-received basis. Delayed coke has about a 6-10% moisture content because water is used to cut the coke from the drum. Fluid coke has a lower as-received moisture content, because water is not used to remove the coke.
