Petroleum coke is a solid byproduct of the refining of crude oil. It is used to produce carbon electrodes for the aluminum industry, graphite electrodes for steel production and as fuel in power generation and cement kilns. In the United States, about 1.5 million tonnes (1.68 x 106 tons) of petroleum coke are utilized by power plants each year (Table 5.7). It is a low cost fossil fuel compared to coal.
Coke is the result of the decarboxylation of hydrocarbon fractions from the heavy residues of petroleum refining processes. The resulting coke, known as petcoke, can be used as an energy source because it has a high Btu/lb value and emits lower sulphur oxides and particulates than other fossil fuels.
A major function of coke in the steelmaking process is to absorb oxygen from molten iron to prevent oxidation, called rusting. The coke is a source of carbon needed to remove oxygen from the iron ore, called hematite and magnetite, in a furnace called a blast furnace. Without coke, it would be very difficult to extract pure iron from the ore.
Metallurgical coke, also referred to as needle coke, is a premium petroleum coke made by baking green petcoke to remove volatiles and moisture. This process is called “coking.” The resulting calcined petroleum coke is high in carbon and has good thermal conductivity, making it ideal for use as graphite electrodes in the aluminum smelting process. It can also be used as a reducing agent to strip oxygen from hematite and magnetite in the blast furnace to form molten iron.
Fuel-grade petroleum coke, also referred to as projectile or petroleum coke, is less dense than needle coke and has a more open structure. It can be highly anisotropic, i.e. well developed in the plane directions but not in a c direction, or it can be very isotropic and hard like fluid bed coke (Figure 7.8a). Fuel-grade coke is generally less expensive than needle coke.
The most common use of petroleum coke in the United States is as a substitute for coal in power plants. Coke can be burned in either pulverized or cyclone boilers. Cyclone boilers are designed for a crushed fuel and can accommodate a wide range of granule sizes. They tend to be more efficient than pulverized fuels and produce fewer particulate emissions.
Cofiring coke with opportunity fuels such as wood has been demonstrated to be a beneficial way of increasing fuel flexibility in a power plant. In the case of cyclone boilers, cofiring coke with urban wood can significantly increase the percentage of available carbon from the original fuel and reduce the sulfur content and the vanadium concentration in the ash. This can help to improve slagging and fouling characteristics, as well as decrease the load on the smelter. This type of cofiring is particularly attractive for coal-fired plants with a low sulfur and vanadium emission limit and a need to improve overall efficiency.
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