Application of ACM in handling coal

  The Air Classifier Mill is an air swept mechanical impact mill designed to grind a variety of materials to D97 < 20 µm. Its durability, reliability and precision make it one of the most popular industrial workhorses on the market. It can be used to process coal and help improve energy efficiency. Here we describe how this is achieved.

l Ball and Tube Mills

l Operation

Ball and Tube Mills

A ball mill is a type of pulverizer that consists of a horizontal rotating cylinder, up to three diameters in length, containing a large number of rolling or cascading steel balls, pebbles or rods.

A tube mill is a rotating cylinder with a maximum length of five diameters for the fine comminution of ores, rocks and other such materials; material mixed with water is fed into the chamber from one end and discharged as a slurry from the other end.

Operation

The primary air input to the ball and tube mill has a dual function. It is used for drying and as a fuel delivery medium and is adjusted to regulate mill throughput. Under control of the pulverized fuel outlet temperature requirements, cold and hot air dampers are adjusted to achieve the correct primary air temperature. In addition to increasing the temperature of the coal in the mill for drying and better grinding, the same air is used as a conveying medium to move the pulverized coal out of the mill: it advances through the annular space tube between the fixed trunnion tube and the rotating hot air to the classifier. The coal containing air passes through the double cone static classifier with adjustable classifier blades, separating the pulverized fuel and the coarse particles into the required fineness. The pulverized fuel continues to burn towards the coal burner. The coarse particles rejected in the classifier are returned to the mill for another grinding cycle.

To avoid blowing too much coal out of the mill, only a portion of the primary air proportional to the boiler load demand passes through the mill. In addition, to ensure that the pulverized fuel has sufficient velocity to avoid settling in the ducts, an additional amount of primary air is fed into the mixing box on the raw coal return circuit. The bypass air from the primary air duct into the mill makes a significant contribution to the drying of the raw coal through the flash drying effect, in addition to collecting the pulverized fuel from the mill outlet for transport to the classifier.

The tube mill output (in response to boiler load demand) is controlled by regulating the primary air flow. This regulation is very fast by removing pulverized fuel from the mill; comparable to oil fired response, but requires coal level to be maintained in the mill. The control circuit monitors the coal level in the mill and controls the speed of the raw coal feeder to maintain it. Maintaining the coal level in the mill provides a built-in buffer of pulverized fuel capacity to cope with brief interruptions in the raw coal circuit.

The mill is pressurized and its airtightness is ensured by a pressurized chamber around the rotating trunnion filled with pressurized sealing air. Sealed air is vented from the pressurized chamber to the mill to maintain separation between the pulverized fuel in the mill and the external atmosphere. Insufficient or non-existent seal air will allow the powdered fuel to escape to the atmosphere. On the other hand, excessive leakage of seal air into the mill can affect the mill outlet temperature. Therefore, the seal air is controlled by a locally controlled damper to maintain just enough differential pressure for sealing.

Conclusion

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