The yield of calcined coke per ton of green coke varies from 75 to 85 wt% (dry basis) depending on the technology used.
The worldwide calcination capacities distribute to the calciner types: rotary kiln calciner about 85% and rotary hearth calciner and shaft kiln calciner each about 7–8.
6.1.2.3.2.1 Rotary Kiln Calciner
The rotary kiln calciner (Figure 6.1.2.8) consists of a rotating heated steel tube (e) with a refractory lining. Because of the gradient and the rotation of the tube, the green coke descends in the calciner, and heat flow rises in countercurrent. The calcined coke leaving the kiln is discharged into a rotary cooler (f). The flue gases are cooled in a waste heat boiler (i) and cleaned in an electrostatic precipitator (j).
Figure 6.1.2.8 Schematic of the rotary kiln calciner. (a) Screen. (b) Crusher. (c) Weigh feeder. (d) Green coke sampling. (e) Rotary kiln. (f) Cooler. (g) Sampling calcined coke. (h) Incinerator. (i) Heat exchanger. (j) Electrostatic precipitator (ESP). (k) Stack.
Figure 6.1.2.9 Schematic of the rotary hearth calciner. (a) Feed bin. (b) Rotary hearth. (c) Calcinate cooler. (d) Waste heat boiler. (e) Electrostatic precipitator. (f) Stack. (g) Sampling point calcined coke.
6.1.2.3.2.2 Rotary Hearth Calciner
The rotary hearth calciner (Figure 6.1.2.9) was originally developed by the Wise Coal and Coke Company. Green coke charged at the periphery of the hearth travels around the calciner (b) in a series of concentric paths. At the completion of each revolution, the coke is moved by a rabble arm set in the roof into the next concentric path. This process is repeated, until at the final rabble, the coke is discharged into the soaking pit. From the soaking pit the coke can be discharged into a rotary cooler (c). During normal operation the retention time for the coke in the hearth and soaking pit is approximately two hours [26].
The advantage of rotary hearth is low demand for area.
6.1.2.3.2.3 Shaft Kiln Calciner
In shaft kiln technology, the coke flows by gravity through multiple vertical shafts with flue wall surrounding it. Residence time is longer compared with rotary kiln (24–36 hours for shaft vs. < 1 hour for rotary kilns), resulting in slower heat up time. Shaft kilns have an advantage when processing coke containing high quantity of fines and high VCM.
6.1.2.4. Uses and Economic Aspects
Worldwide annual consumption of industrial carbon was about 25 × 106 t/a in 1994, excluding metallurgical coke (see Table 6.1.2.5 and Figure 6.1.2.10); main products are carbon anodes and carbon black. Two thirds of this margin is calcinate from petroleum coke [6, 7, 27, 28].
Table 6.1.2.5 World Production of Industrial Carbon 1994.
| Carbon products | Production (t/a) | Average product value ($/t) | Market value (109 $/a) |
|---|---|---|---|
| From petroleum coke | |||
| Carbon anodes | 9 × 106 | 700 | 6.3 |
| Söderberg anodes | 4 × 106 | 500 | 2.0 |
| Graphite electrodes | 1.1 × 106 | 5500 | 6.05 |
| Reduction medium/TiO2 process | 2 × 106 | 600 | 1.2 |
| Partly from petroleum coke | |||
| Carbon cathodes | 6 × 105 | 1300 | 0.78 |
| From other products | |||
| Carbon black | 8 × 106 | 1500 | 12 |
| Activated carbons | 5 × 105 | 3000 | 1.5 |
| Carbon construction materials | 1 × 105 | 800 | 0.8 |
| Electrographite powder | 8 × 104 | 1500 | 0.12 |