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Chapter: Mechanical and Electrical - Power Plant Engineering - Steam Power Plants

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Steam Power Plants System

Coal needs to be stored at various stages of the preparation process, and conveyed around the CPP facilities. Coal handling is part of the larger field of bulk.

STEAM POWER PLANT

 

Coal needs to be stored at various stages of the preparation process, and conveyed around the CPP facilities. Coal handling is part of the larger field of bulk

 

Material handling, and is a complex and vital part of the CPP.

 

FUEL HANDLING SYSTEM

 

Coal delivery 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.     Implant handling

 

8.     Weighing and measuring

 

9.     Feeding the coal into furnace



i) Coal delivery

 

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

 

ii) 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 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 railways 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.

 

(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, sizers, driers and magnetic separators.

 

iv)Transfer

 

After preparation 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

 

Belt Conveyor


Figure 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 centre. 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 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 preferred than other types.

 

2. Screw Conveyor

 

It consists of an endless helicoid screw fitted to a shaft (figure). 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 consumption is high and there is considerable wear o screw. Rotation of screw varies between 75-125 r.p.m

 


3. Bucket elevator

 

It consists of buckets fixed to a chain (figure). The chain moves over two wheels. The coal is carried by the bucket from bottom and discharged at the top.


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 figure . Although the initial cost of this system is high but operating cost is less.


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 strike 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 is 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.

 

 

 

PULVERIZED COAL STORAGE

 

Periodically a power plant may encounter the situation where coal must be stored for sometimes in a bunker, for instance during a plant shut down. The bunker, fires can occur in dormant pulverized coal from spontaneous heating within 6 day of loading. This time can be extended to 13 days when a blanket of CO2 is piped into the top of the bunker. The perfect sealing of the bunker from air leakage can extend the storage time as two months or more. The coal in the bunker can be stored as long as six months by expelling air from above the coal with the use of CO2 and then blanketing of all sources of air. A control system used for storing the pulverized fuel in bunker is shown in figure.


Pulverized Fuel Handling System:

 

Two methods are in general use to feed the pulverized fuel to the combustion chamber of the power plant. First is ‘Unit System’ secoandis ‘Centralor Bin System ‘.

 

In unit system, each burner of the plant is fired by one or more pulverizers connected to the burners, while in the central system, the fuel is pulverized in the central plant and then disturbed to each furnace with the help of high pressure air current. Each type of fuel handling system consists of crushers, magnetic separators, driers, pulverizing mills, storage bins, conveyors and feeders.


 

 

Ball Mill

 

A line diagram of ball mill using two classifiers is shown in figure. It consists of a slowly rotating drum which is partly filled with steel balls. Raw coal from feeders is supplied to the classifiers from where it moves to the drum by means of a screw conveyor. As the drum rotates the coal get pulverized due to the combine impact between coal and steel balls. Hot air is introduced into the drum. The powdered coal is picked up by the air and the coal air mixture enters the classifiers, where sharp changes in the direction of the mixture throw out the oversized coal particles. The over-sized particles are returned to the drum. The coal air mixture from the classifier moves to the exhauster fan and then it is supplied to the burners.


 

 

Ball And Race Mills

 

In this mill the coal passes between the rotating elements again and again until it has been pulverized to desired degree of fineness. The coal is crushed between two moving surfaces, namely, balls and races. The upper stationary race and lower rotating race driven by a worm and gear hold the balls between them. The raw coal supplied falls on the inner side of the races. The moving balls and races catch coal between them to crush it to a powder. The necessary force needed for crushing is applied with the help of springs. The hot air supplied picks up the coal dust as it flows between the balls and races and then enters the classifier. Where oversized coal particles are returned for further grinding.

 

ASH HANDLING SYSTEM:

 

Boilers burning pulverized coal (PC) have bottom furnaces.   The  large  ash  particles  are

 

collected under the furnace in a water-filled ash hopper, Fly ash is collected in dust collectors with either an electrostatic precipitator or a baghouse. A PC boiler generates approximately 80% fly ash and 20% bottom ash. Ash must be collected and transported from various points of the plants as shown in figure. Pyrites, which are the rejects from the pulverizers, are disposed of with the bottom ash system. Three major factors should be considered for ash disposal systems.

 

1.     Plant site

 

2.     Fuel source

 

3.     Environmental regulation

 

Needs for water and land are important considerations for many ash handling systems. Ash quantities to be disposed of depend on the king of fuel source. Ash storage and disposal sites are guided by environmental regulations.



1 Ash Handling Equipment:

 

Mechanical means are required for the disposal of ash. The handling equipment should perform the following functions: 1. Capital investment, operating and maintenance charges of the equipment should be low. 2. It should be able to handle large quantities of ash. 3. Clinkers, shoot, dust etc. create troubles.The equipment should be able to handle them smoothly.4. The equipment used should remove the ash from the furnace, load it to the conveying system to deliver the ash to dumping site or storage and finally it should have means to dispose of the stored ash. 5. The equipment should be corrosion and wear resistant.



Hydraulic System

 

In this system, ash from the furnace grate falls into a system of water possessing high velocity and is carried to the sumps. It is generally used in large power plants. Hydraulic system is of two types, namely, low pressure hydraulic system used for intermittent ash disposal figure. Figure shows hydraulic system.

 

Water-Jetting System

 

Water jetting of ash is shown in figure. In this method a low pressure jet of water coming out of quenching nozzle is used to cool the ash. The ash falls into trough and is then removed.


Pneumatic System

 

In this system ash from the boiler furnace outlet falls into a crusher where a lager ash particles are crushed to small sizes. The ash is then carried by a high velocity air or steam to the point of delivery. Air leaving the ash separator is passed through filter to remove dust etc. So that the exhauster handles clean air which will protect the blades of the exhauster.


Mechanical system


Figure shows a mechanical ash handling system.

 

In this system ash cooled by water seal falls on the belt conveyor and is carried out continuously to the bunker.


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