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Vertical Coal Ball Mill Design,Coal Mill Pulverizer

Vertical Coal Mill

Coal Grinding Mill

Inputs and output of coal mill are: Coal is first crushed by coal crushers to a size of 20 to 25 mm and is then fed to coal bunkers. Coal feeders feed this crushed coal continuously to coal mill as per requirement. Coal gets ground by grinding elements in stages. Coal particles get re-circulated around four times before achieving required fineness. Classifier allows finer particles to escape mill outlet while coarse particles are returned to mill for further grinding.

Primary air is supplied by P.A. fan through air pre heaters to coal mill or in some boilers, P.A. fan pressurize hot air received from air pre heaters and then supply to mills. Hot primary air drives moisture from coal and acts as carrier to transport pulverized coal to the boiler through pipes.

Output is pulverized coal having required fineness. For complete combustion coal should have fineness of 75% through 200 mesh and 99.5% through 50 mesh. Primary air + coal mixture flowing through each coal pipes should have minimum velocity of 18 m/sec, Temp. Around 80 degrees C, and air/coal ratio of 1.8 -2. Fuel flow through pipes be balanced within range of +/ -10%. Finer is the coal, better is the flow distribution. Higher velocity with inferior distribution, will allow less time to burn the coal particles within furnace. Combustion may take place in upper part of furnace, and increase temperatures at furnace exit, super heaters, re heaters etc.


Vertical Coal Ball Mill Design

In Indian power stations, coal mills are of ball and race design or Raymond vertical spindle with rolls and bull rings. These mills are designed to pulverize coal from size 25mm to fineness of 70% through 200 mesh. Hence it is essential to supply coal to mills with correct size and correct quantity by coal feeders. Coal mills have to use extra power when higher size coal is fed to the mill. Similarly correct quantity of primary air needs to be supplied to mill so as to maintain air to coal ratio of 1.8 -2 in mill outlet pipes. For 30MT capacity mill 55 MT of primary air needs to be supplied per hour.


Coal Mill Performance

Gravimetric coal feeders can ensure correct and measured quantity of coal to coal mills, hence are preferred. Similarly to supply correct quantity of primary air, accurate measurement of flow is essential i.e. within +/- 2%. Hence venturies are preferred for flow measurement. Periodic calibration however is necessary. Many times it is observed that the operator supplies more primary air with the ratio of 2.2 to 2.5 or more to avoid high quantity of reject through pyrite hopper. In doing so, primary air convey heavier particles also in addition to finer particles to mill outlet. Instead of getting recirculated for further grinding these particles find way to burners, affecting combustion and flow distribution. Coal particles escape as un burnt along with flue gases or delayed combustion occurs at pendent super heaters causing high enough temperature to form slag. Operator thinks that it is the best mill as there is no problem of spillage of coal as reject and sharing more loads (handling more tonnage of coal). In one of the boiler, it is observed slag getting formed on probe during its travel in furnace, bellow Goose neck within minutes.


Hence keeping to coal ratio more than 1.8 works satisfactorily as far as boiler loading is concerned but is detrimental for combustion for following reasons.

Coal Mill Performance

1. Reduction in coal retention time in coal mill for fine grinding.
2. Increase in velocity through classifier to carry coarse particles to coal pipes causing poor fuel flow balance.
3. Poor flame stability at lower loads.
4. Higher flue gas temperature leaving air pre heater.
5. Increased furnace exit gas temperature as delayed combustion occurs in upper part of furnace.
6. Higher slagging in furnace.
7. Increased wear on coal pipes and coal burners.


Hence it is utmost essential to supply air only 1.8 times coal flow only. Solution to avoid coal spillage is not high primary air flow but reducing the throat gap. Velocity of 38 to 40 m/sec is required at throat of the mill to avoid coal spilling as reject. It will also maintain correct velocity of air and coal mixture in coal pipes.

Vane wheel be installed with correct annular gap to achieve this velocity. Moreover the air flow is directed towards centre of mill to achieve primary classification and also necessary turbulence to dry out coal. Further classification i.e. secondary classification can be achieved by classifier vanes set at equal and correct angle.


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