Calcined petroleum coke is used as a raw material in Aluminium smelting. It is produced by heating up sized green petroleum coke in a rotary or shaft calciner.
The resulting calcinate is used to produce carbon products like activated carbon and synthetic graphite, as well as asphalts and chemicals. It also has a low metal and sulfur content, protecting industrial equipment and the environment.
The raw coke that comes directly out of the coker is known as green petroleum coke (GPC). To make carbon anodes for aluminum smelting, GPC must be further processed by calcination. This transforms the unorganized, electrically non-conductive structure into a much more orderly form with acceptable low impurity levels for anode production.
Calcining is a highly energy-intensive process. Fortunately, fuel grade petcoke is rich in carbon and has high heating value. It is therefore used to generate much of the heat in the calcination process.
The calcination process takes place in an electrothermal fluidized bed kiln with integrated waste heat utilization. The kiln includes the coke charger and electrodes (black), gas cleaning system with nitrogen supply (yellow-gold) and the combustor/boiler, where waste heat is recovered (purple).
The petroleum coke production process converts a pitch pyrolysis material into the solid
Coke calcining produces a product with lower porosity, higher bulk density and lower reactivity, all important properties of the carbon for aluminum anodes. In addition, the calcining process is energy efficient as it utilizes heat generated during the calcining cycle in waste heat recovery systems to dry the green coke and to cool the calcined coke.
Most smelters use a blend of different types of calcined coke to meet their operating needs. This is based on the specific anode recipe and the operating issues of each smelter, such as ash content and anode density. Blending systems at smelters range from relatively simple weigh belt systems to complex concentration strategies that combine the highest VBD coke with coarser fractions of the aggregate recipe in order to maximize anode density.
After refining crude oil, the heavy fractions are coked in a delayed coking unit to produce petroleum coke (petcoke). The petcoke produced is unprocessed and known as green coke. This coke is heated further in a rotary or shaft kiln to remove residual volatile hydrocarbons and increase its carbon content. The resulting calcined coke is used in the manufacture of graphite electrodes for aluminium smelting and other carbon products such as carbon brushes. It is also a possible fuel for fuel cells that generate electricity.
The kiln consists of a steel shell lined with high-temperature refractory brick. During the firing process, combustible gases escape the kiln through vents along the cylinder wall. These off-gases are known as lean gas and consist of a mixture of hydrocarbons, CO2, water and coke dust. The smelter can use lean gas to help control the temperature distribution in the kiln. This reduces the amount of time needed to fire the kiln and prevents caking on the kiln lining.
There are a large number of complex physical and chemical changes in the calcining process. These include the decomposition and polymerization of the combustible gases. Moreover, the volatile matter is burning out and the fixed carbon content rises. As a result, the calcined petroleum coke has better real density and mechanical strength and heat endurance.
The cooled coke is then used in metallurgy to produce primary aluminum and steel, as well as in cement. It is also used as a fuel in power generation. The cooling process also reduces the sulfur content of the coke.
During the cooling process, waste heat of the calcining process is recovered through a heat exchanger. The heat exchanger consists of an external and internal tube. The ends of the tubes are held together by ring pipes. The model of the heat exchanger consists of the two ring pipes, the inner and external tubes, and the region between the tubes and thermal barriers.
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