Green petroleum coke is the solid carbon byproduct of oil refining. It has high moisture and volatile matter content, but it can be changed to calcined petroleum coke (CPC) with a rotary kiln calcining process.
CPC is used in the primary aluminum industry as a recarburizer and in graphite electrode production. It is also a useful blending fuel in conventional cyclone and PC boilers.
Petroleum coke (often referred to as “petcoke”) is a carbonaceous solid byproduct of oil refinery cracking processes. It may come from delayed or fluid coker units and can be pulverized for use in power plants or thermally processed into fuel grade or anode grade calcined petroleum coke for the aluminum smelting industry.
Unlike coal, petcoke is composed primarily of carbon and has no specific chemical formula. Its impurities include residual hydrocarbons and elemental forms of sulfur, nitrogen, nickel, and vanadium.
In recent years, smelters have faced challenges obtaining access to sufficient quality low S, low metals fuel grade calcined petroleum coke. As a result, many smelters are blending different types of coke to meet their needs. This has increased the complexity and cost of smelter operations. It has also changed the way in which environmental risks associated with coke calcining are assessed. This risk characterization evaluates the exposure concentrations of particulate matter and gaseous pollutants from the emissions generated by a fuel-grade coke calcining facility.
The primary use of calcined petcoke is in the production of carbon anodes for aluminum smelting. Calcined petcoke is preferred by smelters due to its low impurity levels, ready availability, and relatively low cost.
Anode grade calcined petroleum coke is obtained from sponge or “green” petcoke which has been processed to remove residual volatile hydrocarbons by heating in a rotary kiln. The resulting coke is a dark grey to black, with low sulfur and metal content and a high bulk density.
GPC quality for anode grade calcined coke can vary widely, as shown in the figure below. Generally, smelters prefer GPC with lower VM content and higher vibrated bulk density (VBD), but the relationship can be more complicated for specific GPC sources. Smelters are increasingly willing to relax specifications for V and S in the face of higher CPC prices. The relaxation of specifications can allow for a reduction in CPC costs and may help to mitigate the impact of higher CPC prices on smelter margins.
The ability to blend calcined petroleum coke (CPC) is important for smelters that process both shaft and rotary kiln/rotary hearth CPC. The different porosity and bulk density profiles of these types of cokes can result in significant pitch level variations during fraction preparation. This can lead to significant operational problems and increased capital expenditures.
The blending process involves feeding sized coke into a rotary kiln and raising it to temperatures in the range of 1200-1260°C. During this heat treatment the material dehydrates, devolatalises, and re-stabilizes with improved crystalline structure and density.
The CPC produced in a delayed coking refinery depends on the crude oil that is used for coking. Refineries that run heavy crude oil typically produce more GPC due to the higher API gravity and S levels of the heavy crudes they use for coking. This extra GPC can be used as a fuel in power generation and cement production. It can also be calcined to produce anode and graphite artifact grades of carbon.
In a high-profile speech in March 2014, Chinese Premier Li Keqiang declared a war on pollution, putting environmental protection on par with economic development. But the exact nature of this war and its impact on calciners is unclear.
As sulfur dioxide (SO
Most rotary kiln plants have a scrubber installed in their coke ovens to meet air quality requirements. However, the scrubbers are not always operating to the best specifications.
The scrubbing process can be affected by the SO
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