Coal Handling in Thermal Power Plant

Coal requirements per day of a large thermal plant are very large. A 600 MW power plant handles about 7200 tons of coal per day. Therefore, one of the major requirement of a power plant is to reduce the cost of handling of coal from the point of its origin upto the furnace of boiler where it is burnt. Various steps involved in a coal handling system as shown in Figure A are :

1. Coal delivery
2. Unloading
3. Preparation of coal
4. Transfer of coal
5. Storage
6. Outdoor (a) dead storage (b) Indoor or live or active storage
7. Inplant handling
8. Weighing
9. Feeding mechanism of coal to furnace

Figure A

Coal Delivery :

1. The coal from mines is transported by ships or boats or by rail transport depending on the location of power plant and the cost of transport involved.
2. In certain cases the transport of coal by trucks is also used where the rail facilities are not available.
3. Transport of coal by rope ways is also used if the distance between the mines and power station is less than 10 km.

1. The equipment used for unloading of coal at site depends on the mode of transport used for transportation of coal to power plants.

In case the coal is delivered by ship the equipment used are :

1. Coal towers.
2. Unloading bridges.
3. Coal accelerators.
4. Portable conveyers.
5. Self unloading boats.

In case of coal is brought by railway wagons the equipments used are car shakes, rotary car dumpers for unloading from closed wagons, cranes, grab buckets and coal accelerators. 

If the coal is delivered by trucks, there is no need of unloading device as the trucks may dump the coal to the outdoor storage. Coal is easily handled by lift trucks with scoop as shown in Figure B.

The outdoor storage is provided for about 30 days to 80 days of coal requirement to cater for any delay in transportation of coal at site. The covered storage of coal, also called as active or live storage , are provided for supply coal to the furnace.

If the coal received from mines is of different sizes then it is not suitable for feeding into furnace of the boiler. In such a case, the coal is prepared into required size by crushers. The crushed coal is passed through sizers which removes the coal of bigger size than required.

After preparation of coal, it is transferred from the site of preparation of coal to dead storage by means of various equipments like belt conveyors, screw conveyors , grab bucket conveyors, bucket elevators, skip hoists and flight conveyors. Details of some of the equipments used for transfer of coal are as follows : 

(a) Belt conveyors : Figure C shows the section of a belt conveyor suitable for transporting large quantities of coal over long distances..

It consists of an endless belt of suitable material running over a pair of end drums. It is supported at regular intervals by a series of rollers called idlers, these being in turn supported on a conveyor frame. The return roller which supports the empty belt are plain rollers and these are spaced at large distances. The maximum inclination of belt conveyors for transport of coal is 20″. Il runs at a speed of 400 to 550 rpm. It can transport coal 50000 kgh to 1 Lakh kg/hr continuously. 

1. It has low cost and power consumption. 
2. It has smooth operation and it is clean. 
3. Maintenance is cheap 
4. Large quantity of coal can be handled continuously. 
5. Rate of coal transfer can be varied by varying the belt speed.
6. Coal can be transferred upto 20° incline. 

(b) Screw conveyor :

• Schematic diagram of a screw conveyor is shown in Figure C. It consists of an endless helicoid screw fitted over a shaft. The driving mechanism is connected at one end of the shaft while the other end is supported in a bearing. Diameter of screw varies 15 em to 50 cm and speed varies between 70 rpm to 120 rpm. It can transfer about 100 tons of coal per hour.
• The screw while rotating in a trough transfers the coal from one end to the other end. This type of conveyor is suitable for transfer of coal for short distances when the available space is limited. Though its cost is low but its power consumption and wear of screw are high.

1. It is cheap.
2. It requires small space.
3. It is simple and compact.
4. It can be made dust tight.

Disadvantages of screw conveyor :

1. High power consumption per ton of coal transfer.
2. It can be used for transfer of coal over the short distances (upto 30 m).
3. Wear and tear of screw is high. Therefore, its life is short. 

(c) Bucket elevators :

Sketch of a bucket elevator is shown in Figure B. This equipment carries the buckets fixed to a chain. The buckets are loaded with coal at the bottom and discharges at the top by centrifugal force. Therefore, coal is lifted in vertical or near vertical direction. 

The lift of coal is limited to about 30 m and it can lift coal at the rate of 60 tons per hour. The inclination of coal is limited to about 60° from the horizontal.

It lifts and transfers coal on a single rail or track from one point to the other. The coal lifted by the grab bucket is transferred to overhead bunker or storage. The grab bucket elevator can be used with crane or tower as shown in Figure F. 

This system requires less power for operation and minimum maintenance. 

The initial cost of the system is high but less operating cost. Being costly machine, it is only used when other arrangements are not possible. 

These elevators can lift coal upto 50 tonnes per hour.

Schematic diagram of a flight conveyor is shown in Figure G.

Flight conveyors are generally used for transfer of coal when filling of number of storage bins under the conveyor are required. 

It consists of one or two strands of chain to which steel scraper or flights are attached. The scraper scratches the coal through a trough having identical shape. This coal is discharged in the bottom of trough as shown. These conveyors can be used for carrying the coal horizontally or upto an inclination of 30°. It can discharge 10 to 100 tons of coal per hour.

It has low capital cost.

It can transfer both coal and ash.

It can vary easily the rate of coal discharge.

It has high energy consumption.

It has high wear and tear due to dragging action.

It has high maintenance cost.

(f) Skip hoist : Skip hoist is the simplest and oldest means of lifting coal or ash. It is used for high lifts, however its operation is intermittent.

The schematic diagram of a skip hoist is shown in Figure H. This arrangement of handling coal or ash is simple, cheap with low operation and maintenance cost.

The purpose of coal storage is : 

(i) To store the coal for a period of 30 to 80 days so that the plant is never required to be shut-down due to failure of supplies of coal from the place of origin or due to transport problems. 

(ii) Storage of coal permits some choice of date of purchase, it allows the purchaser to take the advantage of seasonal market fluctuations in prices of coal.

1. Outdoor coal storage : In the outdoor storage or dead storage, the coal is usually kept on ground in the form of a pile exposed to outside weather. The coal is required to be protected from deterioration and weathering. Coal has tendency to oxidise with liberation of heat. If the oxidation is rapid, it may lead to spontaneous combustion particularly with coals having high sulphur content due to reaction between sulphur, air and moisture. 

Due to this the coal is stored in following ways : 

(a) Stocking the coal in piles : Coal is stored in the form of piles upto a height of 10 to 12 m directly on the concrete ground to avoid the flow of air from bottom. Piles consist of thick and compact layers of coal so that air cannot pass through the coal piles.

Figure I