The key input to the coal fired power plant is the coal. Coal in the form that is available from the mines is not suitable for efficient combustion in the boiler of the power plant. The pulverizing system prepares the coal for combustion.
There are basically three different types of pulverizing.
1. Attrition Type
Coal feeds onto a steel rotating table. Tapered, cylindrical, or spherical rollers on top of the table, under spring pressure, exert force on the coal particles on the table. When the table rotates, the coal passes between the rollers and the table and gets ground due to attrition. The hot air under pressure carries this ground coal upwards. Larger particles fall back to the grinding table. These coal particles then pass through a classifier where the large particles get separated due to centrifugal action and are returned back to the grinding table. Different manufacturers use different type of rollers. “Bowl mills” or Raymond mills use tapered rollers. MPS mill use tire type rollers and are called “roller mills.” The “ball and race mills” use spherical rollers. The main characteristics are:
- Lower investment costs than other types.
- Smaller capacity for each mill means a better degree of combustion flexibility.
- They require periodic roller changes as they wear out during pulverizing, increasing the maintenance cost.
- Harder coals require this type of pulverizers.
2. Impact and Attrition Type
Coal is fed into a rotating cylinder containing steel balls of around 50 mm diameter. When the cylinder rotates, the balls rotate along with the cylinder and fall down when they reach the top position. Coal is ground due to attrition and impact. Coal feeds into the cylinder axially at one side and discharges through the other side. Hot air or gas carries away the fine coal through the other end of the Tube. This called the “Tube mill” or “Ball mill.” These types of mills have:
- Initial costs that is higher.
- Maintenance that is considerably less.
- As the balls wear out during operation, the addition of fresh ball charges can be done while the mill is running.
- Can give a very high fineness coal with a high percentage of fines.
3. Impact or Hammer Type
This type consists of a rotating shaft inside a stationary cylinder . The shaft contains arms that impact the coal inside the cylinder. The impact powders the coal. The hot air carries the coal to the burners. This is more suitable for softer coals with a higher Grindability index. Periodic replacement of hammers because of wear is a requirement. Another version of this in Lignite fired power plants.This is in the form of a fan and doubles as a fan and a mill. The impellers act as the hammers. The main charectristics are:
- Lower costs.
- Suitable only for softer coals.
- Low fineness output.
Selection of the type of pulverizer depends on
- Grindability of the coal.
- Capital cost and maintenance cost is another criteria.
- Patent rights and licence agreements also decide which type of pulverizer the boiler suppliers recommend.