The calcination process is an advanced technology for transforming petroleum coke. It raises material temperatures in a rotary kiln to de-moisturises and devolatalises it while imparting desirable crystal structure, bulk density, and electrical conductivity.
It also increases volume and true density of green coke [1]. A high-quality carbon source, calcined petroleum coke is used as negative electrodes in lithium batteries, anode in aluminum electrolysis.
The calcination process is a thermal conversion of coke. The coke is heated above its melting point in a controlled atmosphere, which removes volatiles and moisture. This allows for the transformation of the coke into a metallurgical product with improved crystal structure, appearance and electrical conductivity.
Currently, GPC used for anode production must be calcined to meet minimum quality requirements. Calcination transforms the structure of GPC and eliminates most of the volatiles and water present in raw material coke, resulting in reduced reactivity to air and carbon dioxide (CO2), increased apparent and real density, and higher mechanical strength.
The calcination process takes place in a cylindric rotary kiln. Wet green coke is placed in the kiln and is slowly conveyed through a heated reaction zone via a rotary drum. The kiln rotates to provide consistent temperatures throughout the process. The calcinate is discharged into a rotary cooling drum to be quenched. The calcinate is sized to be acceptable for anode making and is conveyed from the rotary kiln to a storage silo or bunker.
The calcination of petroleum coke is typically carried out in large cylindric rotary kilns equipped with gas or oil burners. The wet green coke is slowly conveyed through the furnace by rotation where it is heated up to temperatures of between 1200 and 1250 degC, thereby de-moisturises, devolatalises and stabilises with improved crystalline structure. The off-gases contain a significant amount of hydrocarbons which are partially burnt in the kiln and supply a significant proportion of the energy required for this process.
Various internals such as flights, dams and bed disturbers can be used to customize the kiln in order to achieve specific objectives like improving agglomeration or dehydration. In addition, various temperature controls as well as retention or residence time are critical to achieving an optimal quality of the calcined coke product.
Testing petcoke is important because it ensures that its metal, sulfur and carbon contents are at a low enough level to protect refining equipment and the environment. It also allows for compliance with increasingly strict industrial emissions regulations.
During the calcination process, air emissions are produced. These are in the form of particulate matter (PM) and gaseous pollutants. PM is emitted into ambient air and has been shown to be associated with adverse respiratory health effects in humans. The emission of gaseous pollutants has also been linked to respiratory health effects.
Independent coke calcining plants receive GPC from oil refineries and blend it into fuel grade petcoke, which contains 9-14% volatile matter (VM). They heat this material in rotary kilns to temperatures of 1200-1350°C (2192-2460°F), removing most of the VM. This produces calcined petroleum coke that has a consistent crystalline structure and electrical conductivity properties suitable for use in aluminum anode production. This coke is called RPC or CPC. It is used by the graphite electrode industry and for a variety of industrial applications. Several methods have been developed to reduce the sulfur content of GPC. These involve the desorption of organic sulfur from the carbon skeleton or dissolving inorganic sulfur into a solution.
Past procedure has been to rely on the combustion of volatiles to provide a substantial proportion of heat for the calcining operation. This is achieved with a fuel burner at the coke discharge end of the kiln, which is fired intermittently for possibly recurring periods as needed to raise the coke bed to calcining temperature at the beginning of its travel toward the gas outlet end.
However, it has been found that a substantially more satisfactory operation is obtainable with the present invention, whereby the calcination of the petroleum coke is accomplished without the use of any combustible fuel and solely by the application of oxygen supplied to the coke bed by means of air introduced through a series of openings or nozzles, here represented by the tuyeres 27a, 27b, etc. through 27n. Increases in air supply to the tuyeres have the major effect of raising the coke bed calcining temperature, while decreases in such supply correspondingly lower the calcination temperature Tc.
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