Methods of coal are
1. COAL DELIVERY
2. UNLOADING
3. PREPARATION
4. TRANSFER
5. OUTDOOR STORAGE•
6. COVERED STORAGE
7. INPLANT HANDLING
8. WEIGHING & MEASURING
9. FURNACE FIRING
(i). Coal delivery
The coal is delivered by ships or boats to power station situated near to river or sea. If coal is situated away from sea or river coal is supplied by rail or trucks.
(ii). Unloading
The type of equipment to be used for unloading the coal received at the power station depends on how coal is received buckets and coal accelerators. Rotary car dumpers although costly are quite efficient for unloading closed wagons.
(iii). Preparation
When the coal delivered is in the form of big lumps and it is not of proper size, the preparation (sizing) of coal can be achieved by crushers, breakers, sizes driers and magnetic separators at the power station. In case the coal is brought by railway wagons, ships or boats, the unloading may be done by car shakes, rotary car dumpers, cranes, and grab.
(iv). Transfer
The following systems are used to transfer the coal to the dead storage.
1. Belt conveyor
2. Screw conveyor
3. Bucket elevator
4. Grab bucket elevators
5. Skip hoists
6. Flight conveyor
(a). Belt conveyor
It consists of an endless belt, which is moving over a pair of end drums (rollers). At some distance, supporting roller is provided at the center.
The belt is made, up of rubber or canvas. Belt conveyor is suitable for the transfer of coal over long distances.
The initial cost of the system is not high and power consumption is also low.
The inclination at which coal can be successfully elevated by belt conveyor is about 20.
Average speed of belt conveyors varies between 200-300 r.p.m. This conveyor is preferred than other types.
Advantages of belt conveyor
Operation is smooth & clean.
Requires less power than other types.
Large quantities of coal can be discharged quickly and continuously.
Material can be transported on moderate inclines.
(b). Screw conveyor
It consists of an endless helicoids screw fitted to a shaft.
The screw while rotating in a trough transfers the coal from feeding end to the discharge end.
This system is suitable, where coal is to be transferred over shorter distance and space limitations exist.
The initial cost of the system is low. Rotation of screw varies between 75-125 r.p.m.
(v). Storage of coal.
It is desirable that sufficient quantity of coal should be stored.
Storage of coal gives protection against the interruption of coal supplies when there is delay in transportation of coal or due to strikes in coal mines.
(vi). In Plant Handling.
From the dead storage the coal is brought to covered storage (Live storage) (bins or bunkers).
In plant handling may include the equipment such as belt conveyors, screw conveyors, bucket elevators etc. to transfer the coal.
Weigh lorries hoppers and automatic scales are used to record the quantity of coal delivered to the furnace.
(vii). Coal weighing methods
Weigh lorries, hoppers and automatic scales are used to weigh the quantity coal.
The commonly used methods to weigh the coal are as follows
(i) Mechanical (ii) Pneumatic (iii) Electronic.
The Mechanical method works on a suitable lever system mounted on knife edges and bearings connected to a resistance in the form of a spring of pendulum. The Pneumatic weightier use a pneumatic transmitter weight head and the corresponding air pressure determined by the load applied.
The Electronic weighing machines make use of load cells that produce voltage signals proportional to the load applied.
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