In ball and race mill the coal passes between the rotating elements again and again until it is pulverized to the desired degree of fineness. The coal is crushed between two moving surfaces namely balls and races. A worm and gear drove the upper stationary race, and the lower rotating race holds the balls between them. The raw coal supplied falls on the inner side of the races. The moving balls and races catch coal between them to crush it into a fine powder. For crushing of coal, the force needed is applied with the help of springs. The hot air supplied picks up the coal dust as it flows between the balls and races; then enters the classifier. Oversized coal particles are returned for grinding. The coal particles of required dimension are discharged from the top of the classifier.
In this ball and race mill, coal is pulverized by a combination of’ crushing, impact, and attrition between the grinding surfaces.
The advantages of ball and race mill are as follows:
- Lower capital cost
- Lower electricity power consumption
- Lower workspace required
- Lower weight.
However, in ball and race mill there is greater wear as compared to other coal pulverizers. The usage of pulverized coal as a fuel has now become the standard method of firing in many huge boilers.
Preparation of pulverized coal as fuel with an intermediate bunker is shown in below Fig. 1. The fuel moves to the automatic balance and then goes to the feeder and ball mill through which hot air is blown. Air heater dries the pulverized coal and carries it from the ball mill to the separator.
The air fed to the ball mill is heated in the air heater. In the separator dust (finely pulverized coal) is separated from large coal particles which are returned to the ball mill for regrinding. The dust moves to the cyclone. Most of the dust from cyclone sent to bunker. The remaining dust is mixed with air and fed to the burner.
Coal is ground in low-speed ball tube mill. It is filled to 20-35% of its volume. With steel balls having a diameter varies from 30-60 mm. The steel balls crush and ground the lumps of coal. The average speed of rotation of tube or drum is about 18 to 20 R.P.M.
Using pulverized coal has the following advantages:
- Pulverized coal becomes easy to burn a wide variety of coal. Low-grade coal can be burnt easily.
- Powdered coal has more heating surface area. They permit rapids and high rates of combustion.
- Pulverized coal firing requires a low percentage of excess air.
- By using pulverized coal, the rate of combustion can be adjusted easily to meet the varying load.
- The system is free from clinker troubles.
- Pulverized coal can utilize highly preheated air (of the order of 370°C) successfully which promotes rapid flame propagation.
- As the fuel pulverizing equipment is located outside the furnace, therefore it can be repaired without cooling the unit down.
- High temperature can be produced in the furnace.
Using pulverized coal has the following disadvantages:
- Pulverized coal requires additional equipment to pulverize the coal. The initial and maintenance cost of the equipment is high.
- Pulverized coal firing produces fly ash which requires a separate fly ash removal equipment.
- The furnace for this type of firing has to be carefully designed to withstand for burning the pulverized fuel because combustion takes place while the fuel is in suspension.
- The flame temperatures are high and conventional types of refractory-lined furnaces are inadequate. It is desirable to provide water cooled walls for the safety of the furnaces.
- There are more chances of the explosion as coal burns like a gas.
- Pulverized fuel fired furnaces designed to burn a particular type of coal can not be used to any other type of coal with the same efficiency.
- The particle size of coal used in pulverized coal furnace is limited to 70 to 100 microns.