Green coke calcination

Raw coal NMP-extract Residue Green coke Calcined coke... 

The green cokes were calcined by placing a weighed amount of green coke into an alumina tube. The tube was fitted with end caps to allow for a constant purge of nitrogen. The alumina tube was then inserted into a high-temperature furnace and the temperature raised to about 1000°C for a period between 30 and 60 minutes. The furnace was turned off, cooled to room temperature, and the product recovered to determine the calcined coke yield. 

Polarized light photomicrographs were taken of the green and calcined cokes, as well as their corresponding test graphites. The untreated extract cokes are characterized by very small amsotropic domains on the order of 3 microns or less. This type of optical structure is believed to be highly desirable for nuclear graphite applications. 

Removal of volatile matter to about 0.5 wt% can be accomplished by calcining in a rotary kiln, rotary hearth, or vertical shaft calciner All of these processes heat green coke to temperatures in excess of 1000°C where shrinkage and subsequent densification take place. The volatile components are comprised primarily of methane, ethane, hydrogen, and hydrogen sulfide gases which can be employed as fuel for process heat. 

The coke for synthetic graphite is also calcined before further processing. It is calcined until a sample of —35 + 100 mesh material shows an electrical resistivity of less than 0.05 ohm-inch. The same type of calciners are used that are used to calcine coke for the aluminum industry (29). The calcined coke retains a major portion of the sulfur present in the green petroleum coke (1). Low-sulfur, green coke is preferred. 

In the proprietary rotary hearth method of calcining coke, green coke is fed to the perimeter of the rotating circular table and gently moved toward the center of the hearth in a circular path by stationary rabbles. A combustion zone above the coke bed, formed by liberated volatiles, supplies the heat necessary for calcination. A rotating hearth furnace can also be equipped with energy efficient features, such as steam generation and air preheat. 

Petroleum coke calcining is a process whereby green or raw petroleum coke is thermally upgraded to remove associated moisture and volatile combustible matter (VCM) and to otherwise improve critical physical properties, e.g., electrical conductivity and real density (JL ) The calcining process is essentially a time-temperature function the most important variables to control are heating rate, VCM to air ratio and final temperature. To attain the calcined coke properties necessary for its end use by the amorphous carbon or graphite industries, the coke must be heat treated to temperatures of 1200-1350°C (2200-2500°F), or higher, to refine its crystalline structure. 

The eventual quality of the calcined coke is directly related to the particular characteristics/quality of the green coke fed to the calciner. While calcination cannot improve upon certain quality limits inherent in the green coke, potential quality can be lost by improper calcining, e.g., by using incorrect heating rates and/or atmospheric conditions. 

The last claim has certainly been verified, but it has some negative aspects. The higher carbon yields are realized because the green coke fines introduced to the calciner exit as calcined product. 

Properties of Calcined Coke. Table II shows the properties of calcined cokes A, B, C, D and F by the new and traditional methods. Regardless of the green coke type, the thermal expansion coefficient of calcined coke by the new method was lower than that of traditional calcination method. Porosity values for calcined cokes by the new method have a higher value, regardless of the coke type, than... 

We found the latter factor-voids to be important. Experimental results showed that when green coke was calcined under the new methods, and the derived calcined coke was observed by scanning electron microscopy (Figure 2) and its pore size distribution was measured by mercury porosimetry (Figure 3), microcracks of significant sizes (1 to 60 microns) were developed. This was an important contribution to the reduction of the thermal expansion coefficients of the calcined coke processed under the new method. 

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