What Is Delayed Coking?

Delayed coking units convert heavy hydrocarbons into lighter, more valuable ones. As a byproduct, they also produce coke, a solid material that is more carbon than hydrocarbon. As of 2014, there were more than 70 delayed coking units in the U.S. with a combined capacity of 2.7 million barrels per day. [1]

Why Refineries Use Delayed Coking

The main advantage of having a delayed coking unit is the ability to purchase heavier, discounted crude oils and convert them into transportation fuels. Without a coker, a refinery is limited to lighter crudes that command a higher price.

Why It Is Called "Delayed" Coking

The name comes from how the reaction works. Feed is heated rapidly in a charge heater, which initiates the cracking reaction. But the completion of the reaction is delayed until the feed reaches the coke drum, where the coke solidifies.

What Happens to the Coke

Refinery coke is typically sold as a fuel for steam generation (electricity production) and cement plants. For every 1,000 tons of cement produced, roughly 75 to 115 tons of coke are consumed as fuel. [1] Coke is often blended with coal. It has a higher heating value than coal (around 14,000 BTU/lb versus 9,000 to 12,500 BTU/lb for coal) and a much lower ash content (around 0.5% versus 10 to 20% for coal), which reduces ash disposal costs. [2]

References

[1] Handbook of Petroleum Refining Processes, 4th Ed., Meyers, Chapter 11.2

[2] American Coal Ash Association - Coal Ash FAQ