Coal Handling Unit in Thermal Power Plant
The coal handling may change slightly from plant to plant, which may depend upon plant type and type of fuel used, still Figure shows various stages In coal handling.
(i) Delivery of coal
- Coal may be delivered by sea or river, rail or road. Selection of suitable method of coal supply from the coal mines to power station depends system capacity in tonnes per hour, location upon of the plant w.r.t. rail or water facilities available and location of outside storage and overhead coal bunkers.
(ii) Unloading
- In unloading the choice of equipment will depend on how the coal is received.
(iii) Preparation
- Preparation of coal before feeding to the combustion chamber becomes necessary if unsized coal is brought to the site and sizing is desirable for the purposes of storage or firing.
A coal preparation plant may include the following:
(a) Crushers
(b) Sizers
(c) Dryers
(d) Magnetic separators
(iv) Transfer
- It is the process of carrying the coal from unloading point to the storage site from where it is discharged to the firing equipment. It may require one or more than one equipment depending on local conditions. Equipment used for this purpose may be one more of the following:
(a) Belt conveyors
(b) Screw conveyors
(c) Bucket elevators
(d) Grab bucket conveyors
(e) Skip hoists
(f) Flight conveyors
(v) Outdoor storage (Dead storage)
- The amount of coal to be stored depends upon available space for storage, transportation facilities, amount of coal that will weather away and nearness of the power plant to coal mines.
- The usual practice is to store coal required for one month of operation of power station in case it is situated at a comparatively longer distance from the collieries whereas coal needed for about 15 days is stored in case of power stations situated near to collieries.
- Storage of coal for longer periods is not advantageous because, it blocks the capital and results in deterioration of the quality of coal.
- Coal received at the power station is stored in dead storage in the form of piles laid directly the ground.
(vi) Indoor (live) storage
- Such a storage constitutes coal requirements of the plant for a day. The live storage can be provided with bunkers and coal bins.
(vii) Inplant handling
- It is the process of coal handling between the final storage to the firing equipment. In case of simple stroker firing only chutes may be required to feed the coal from storage bunker to the firing units.
- In addition to this, gates and valves may be included in the system to control the flow according to load on the plant.
- The pulverized fuel firing system would require equipments such as chutes, pulverizing mills, feeders, weighing equipment and many others for inplant handling.
- Equipments used for inplant handling are the same as used for coal transfer.
(viii) Coal weighing
- Cost of coal is the major running cost of the plant. Hence, it is necessary to weigh coal at unloading point and also that used as feed to the individual boilers.
- A correct measurement of coal enables one to have an idea of total quantity of coal delivered at the site and also whether or not proper quantity has been burned as per load on the plant.
The system consists of :
(a) Weigh bridge
(b) Belt scale
(c) Automatic recording system
(ix) Furnace firing
- Fuel is burned in a confined space called furnace. An efficient combustion of fuel is essential for economical working of power plant.
- In case of firing unpulverized coal in combustion furnaces, two general methods, namely hand firing and stroker (or mechanical) firing are available.
- A stroker is a fuel firing device which receives fuel by gravity, carries it into the furnace for combustion and after combustion discharges the ash at the appropriate point.
- In case of pulverized coal two delivery systems, namely unit system and central (or storage) system are available. Coal is carried into the furnace for combustion and ash formed after combustion is discharged at appropriate point.