Petroleum coke (petcoke) is a carbonaceous solid residual byproduct of the oil refining coking process. As crude oil is refined, lighter fractions or products, such as gasoline and jet fuel, are driven off leaving a residual oil of relatively little value. In refineries with cokers, this residual oil is processed further to yield additional amounts of light products, along with petroleum coke. Over 75% of the petcoke produced is considered to be fuel grade and has about 15% higher heating value than coal.
Some 40 to 60 percent of the sulfur in the oil feedstock remains in the coke, which means that the sulfur content of this refinery byproduct is usually quit high. While the actual amount of sulfur in the coke will vary depending on the sulfur in the crude oil entering the refinery, the sulfur content of coke typically ranges from four to eight percent – much greater than even high-sulfur coal. Therefore, control of sulfur emissions is very important when using as a fuel.
What is Fluidized Bed Combustion?
Fluidized bed combustion (FBC) is a technology used in the design of "clean coal” power plants. It evolved from efforts to find a combustion process able to control pollutant emissions without external emission controls (such as scrubbers). The technology suspends solid fuels on upward-blowing jets of air during the combustion process. The result is a turbulent mixing of gas and solids. The tumbling action, much like a bubbling fluid, provides more effective chemical reactions and heat transfer.
FBC technology controls emissions in two ways. First, the fuel is burned at temperatures of 1,400 to 1,700 degrees F, which is well below the threshold where nitrogen oxides form. Second, sulfur dioxide (a priority gaseous pollutant that is generated when sulfur from high sulfur coal or petcoke is burned in the presence of oxygen) is captured inside the combustion chamber by burning the fuel with lime or dolomite. Sulfur dioxide is removed via this reaction: 2CaO + 2 SO2 + O2 = 2CaSO4.
The mixing action of the fluidized brings the flue gases into contact with the sulfur-absorbing chemical. More than 95 percent of the sulfur pollutants in coal can be captured inside the boiler using this technology.
There are two primary residues that remain after petcoke is combusted in a CFB steam generator: calcined limestone residue (commonly known as fly ash) and calcined limestone sand (also known as bed ash).
Fly ash is a generic term used to define fine residue that is removed from stack gases using various types of air quality control equipment. However, all fly ash is not equal. Petcoke fly ash is unique because there is virtually no ash from petcoke combustion. The residue that remains after petcoke is burned consists primarily of lime, calcium carbonate, and calcium sulfate.
Bed ash is the coarse, solid, particulate matter that sinks to the bottom of the fluidized bed combustion chamber and is periodically removed. Petcoke bed ash has a similar chemical composition to fly ash, but is produced in gradations ranging from fine sand to small aggregate.