KIET Group of Institutions, Ghaziabad

Department of Mechanical Engineering

B.Tech. VII sem (ME)

Power Plant Engineering (RME071)

Unit-2
Steam Power Plant
Topic: Coal Handling

COAL HANDLING

Coal handling is the prime task in the steam based power plant. Coal handling
equipment is one of the major components of plant cost. The various steps
involved in coal handling are as follows:
(1) Coal delivery (2) Unloading
(3) Preparation (4) Transfer
(5) Outdoor storage (6) Covered storage
(7) In plant handling (8) Weighing and measuring
(9) Feeding the coal into furnace.

Fig.1: Coal handling sequence

(1) Coal Delivery. The coal from the supply points is delivered by ships or
boats to power stations situated near to sea or river. The coal is supplied by rail
or trucks to the power stations which are situated away from the sea or river.
The transportation of coal by trucks is used if the railway facilities are not
available.

(2) Unloading. The type of equipment to be used for unloading the coal
received at the power station depends on how coal is received at the power
station. If 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 if
the lift trucks with scoop are used. In case the coal is brought by railway
wagons, ships or boats, the unloading may be done by car shakes, rotary car
dumpers, cranes, grab buckets and coal accelerators. Rotary car dumpers
although costly are quite efficient for unloading closed wagons.

(3) Preparation. 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, sizers, driers and magnetic separators.

(4) Transfer. After preparation, the coal is transferred to the dead storage by
means of the following systems:
1. Belt conveyors. 2. Screw conveyors.
3. Bucket elevators. 4. Grab bucket elevators.
5. Skip hoists. 6. Flight conveyor.

Fig.2: Belt conveyor

(4.1) Belt conveyor- Fig. 2 shows a belt conveyor. It consists of an endless belt
moving over a pair of end drums (rollers). At some distance a 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. It is used in
medium and large power plants. 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. The average speed of belt
conveyors varies between 200-300 r.p.m. This conveyor is preferred over other
types.
Advantages of belt conveyor
1. Its operation is smooth and clean.
2. It requires less power as compared to other types of systems.
3. Large quantities of coal can be discharged quickly and continuously.
4. Material can be transported on moderates inclines.

Fig.3: Screw Conveyor

(4.2) Screw conveyor- It consists of an endless helicoid screw fitted to a shaft

(Fig. 3). 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. It suffers from the drawbacks that the power consumption is high and
there is considerable wear of screw. Rotation of screw varies between 75-125
r.p.m.

(4.3) Bucket elevator- It consists of buckets fixed to a chain (Fig. 4). The
chain moves over two wheels. The coal is carried by the buckets from bottom
and discharged at the top.

Fig.4: Bucket Elevator

(4.4) Grab bucket elevator- It lifts and transfers coal on a single rail or track
from one point to the other. The coal lifted by grab buckets is transferred to
overhead bunker or storage. This system requires less power for operation and
requires minimum maintenance.
The grab bucket conveyor can be used with crane or tower as shown in Fig.
4.6. Although the initial cost of this system is high but operating cost is less.

(4.5) Skip hoist- It consists of a vertical or inclined hoistway a bucket or a car

guided by a frame and a cable for hoisting the bucket. The bucket is held in up
right position. It is simple and compact method of elevating coal or ash. Fig. 5
shows a skip hoist.

Fig.5: Skip Hoist Fig.6: Flight Conveyor

(4.6) Flight conveyor- It consists of one or two strands of chain to which steel
scraper or flights are attached’, which scrap the coal through a trough having
identical shape. This coal is discharged in the bottom of trough. It is low in first
cost but has large energy consumption. There is considerable wear.
Skip hoist and bucket elevators lift the coal vertically while Belts and flight
conveyors move the coal horizontally or on inclines.
Fig. 6 shows a flight conveyor. Flight conveyors possess the following
advantages and disadvantages.
Advantages-
(i) They can be used to transfer coal as well as ash.
(ii) The speed of conveyor can be regulated easily.
(iii) They have a rugged construction.
(iv) They need little operational care.
Disadvantages-
(i) There is more wear due to dragging action.
(ii) Power consumption is more.
(iii) Maintenance cost is high.
(iv) Due to abrasive nature of material handled the speed of conveyors is
low (10 to 30 m/min).

(5) 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.
Also when the prices are low, the coal can be purchased and stored for future
use. The amount of coal to be stored depends on the availability of space for
storage, transportation facilities, the amount of coal that will whether away and
nearness to coal mines of the power station.
Usually coal required for one month operation of power plant is stored in case
of power stations situated at longer distance from the collieries whereas coal
need for about 15 days is stored in case of power station 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.
The coal received at the power station is stored in dead storage in the form of
piles laid directly on the ground.
The coal stored has the tendency to whether (to combine with oxygen of air)
and during this process coal loss some of its heating value and ignition quality.
Due to low oxidation the coal may ignite spontaneously. This is avoided by
storing coal in the form of piles which consist of thick and compact layers of
coal so that air cannot pass through the coal piles. This will minimize the
reaction between coal and oxygen. The other alternative is to allow the air to
pass through layers of coal so that air may remove the heat of reaction and
avoid burning. In case the coal is to be stored for longer periods the outer
surface of piles may be sealed with asphalt or fine coal.
The coal is stored by the following methods:

(5.1) Stocking the coal in heats- The coal is piled on the ground up to 10-12 m
height. The pile top should be given a slope in the direction in which the rain
may be drained off. The sealing of stored pile is desirable in order to avoid the
oxidation of coal after packing an air tight layer of coal. Asphalt, fine coal dust
and bituminous coating are the materials commonly used for this purpose.

(5.2) Under water storage- The possibility of slow oxidation and spontaneous
combustion can be completely eliminated by storing the coal under water. Coal
should be stored at a site located on solid ground, well drained, free of standing
water preferably on high ground not subjected to flooding.

(6) 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.

(7) 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 weighters 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.
The important factor considered in selecting fuel handling systems is as
follows:
(i) Plant flue rate
(ii) Plant location in respect to fuel shipping
(iii) Storage area available.

DEWATERING OF COAL
Excessive surface moisture of coal reduces and heating value of coal and
creates handling problems. The coal should therefore be dewatered to produce
clean coal. Cleaning of coal has the following advantages:
(i) Improved heating value.
(ii) Easier crushing and pulverising
(iii) Improved boiler performance
(iv) Less ash to handle.
(v) Easier handling.
(vi) Reduced transportation cost.