Coalto Elect

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POWER PLANT LAYOUT & COAL TO ELECTRICITY

BY

P. SRINIVASA, B.E(Mech), MHRM.

DEPUTY DIRECTOR, PSTI-NPTI


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COAL TO ELECTRICITY

How Electricity is produced ?

Thermal Power Stations burn fuels and use theresultant heat to raise steam which drive the turbogenerator.The fuel may be `fossil'

Coal -lignite, bituminous coal, anthracite coal,

Oil or

Natural Gas or

it may be fissionable (Uranium).

Whichever fuel is used, the objective is same i.e., toconvert heat into mechanical energy in the turbine,which is coupled to a generator and to convert thatmechanical energy into electricity by rotating amagnet inside a set of windings of the generator.


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A 200 MW POWER PLANT CONSUMES ABOUT 2500TONNES OF COAL EVERY DAY.

ENERGY AVAILABLE IN 1 lb OF URANIUM = 1000TONS OF COAL.

Cv OF COAL = 4500 to 5000 kcal/kgCv OF LIGNITE = AROUND 2000 kcal/kg

Cv of fuel oil is 10,000 kcal/kg


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THE HEIGHT OF A BOILER HOUSE OF A 210 MW UNIT IS 64

METRES

AND HEIGHT OF CHIMNEY IS 220 METRES.

THE HEIGHT OF A BOILER HOUSE OF A 500 MW UNIT IS

ABOUT 89 METRES AND HEIGHT OF CHIMNEY IS 275

METRES.

IN A TOWER TYPE BOILER, EVEN FOR A 210 MW UNIT,

THE HEIGHT OF A BOILER HOUSE IS ABOUT 89 METRES.

A 210 MW POWER PLANT CONSUMES ABOUT 2500 TONSOF COAL EVERY DAY.

THE BOTTOM ASH PRODUCED IS ABOUT 175 TONS.


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THE ORDER IN WHICH DIFFERENT FECILITIES ARRANGED IN A T.P.S. :

COAL HANDLING PLANT

OIL HADLING PLANTWATER TREATMENT PLANT

ASH HANDLING PLANT

CHIMNEYS

I.D. FANS

ELECTRO STATIC PRECIPITATOR

F.D. FANS

MILLS

FURNACE

BUNKER BAYCONTROL ROOM

TURBINE FLOOR

GENERATING & UNIT TRANSFORMERS

SWITCH YARD


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In a coal-fired power station the basic raw

materials are Coal & Air.

The coal, brought to the station by trains

or by other means, travels from the coal

handing plant by conveyor belt and to thecoal bunkers, from where it is fed to the

pulverising mills which grind it as fine as

face powder.


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The finely powdered coal mixed with pre-heated,

air is then blown into the boiler by a fan called

Primary Air Fan where it burns, more like a gasthan like a solid in the conventional domestic or

industrial grate, with additional amount of air

called secondary air supplied by a Forced Draft

Fan.

As the coal has been ground finely, the resultant

ash is also a fine powder. Some of it binds

together to from lumps which fall into the ashpits at the bottom of the furnace.


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Coal handling plant


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The water quenched ash from the bottom of the

furnace is conveyed to pits for subsequent

disposal or sale. Most of ash, still in fine particle form is carried

out of the boiler to the precipitators as dust,

where it is trapped by electrodes charged with

high voltage electricity.

The dust is then conveyed by water to disposal

areas or to bunkers for sale while the cleaned

flue gasses pass on through I.D.Fan to be

discharged from the chimney.


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Ash Handling system


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Meanwhile the heat released from the coal has been

absorbed by the many kilometers of tubing which line theboiler walls.

Inside the tubes is the boiler feed water which istransformed by the heat into steam at high pressure andtemperature

The steam, super heated in Super Heater passes to theturbine where it is discharged through nozzles on theturbine blades.

Just as the energy of the wind turns the sails of the wind-mill, so the energy of steam, striking the blades, makes

the turbine rotate. Coupled to the end of the turbine is the rotor of the

generator a large cylindrical magnet so that when theturbine rotates the rotor turns with it.


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The rotor is housed inside the stator havingheavy coils of copper bars in which electricity isproduced through the movement of the magnetic

field created by the rotor.

The electricity passes from the stator windings tothe step-up transformer which increases its

voltage so that it can be transmitted efficientlyover the power line of the grid.

The steam that has given up its heat energy ischanged back into water in a condenser so that

it is ready for re-use. The condenser containsmany kilometers of tubing through which coldwater is constantly pumped.


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The steam passing around the tubes loosesheat and is rapidly changed back to water. But

the two lots of water (i.e., boiler feed waterandcooling water) must never mix.

The cooling water is drawn from the river/sea,

but the boiler feed water must be absolutelypure, far purer than the water that we drink, if itis not to damage the boiler tubes. The chemistryat a power station is largely the chemistry ofwater.

a condenser, which by rapidly changing thesteam back into water creates a vacuum.


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To condense the large quantities of steam, huge

and continuous volume of cooling water is

essential. In most of the power stations thesame water is to be used over and over again.

So the heat which the water extracts from the

steam in the condenser is removed by pumping

the water out to the cooling towers. The cooling towers are simple concrete shells

acting as huge chimneys creating a draught

(natural/mechanically assisted by fans) of air.

The water is sprayed out at top of the towers

and as it falls into the pond beneath, the upward

draught of air-cools it.


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The cold water in thepond is then recirculatedby pumps to thecondensers. Inevitably,however, some of thewater is drawn upwardsas a vapour by thedraught and it is thiswater which forms thefamiliar white cloudswhich emerge from the

towers seen sometimes.

Cooling Water System


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CONSIDERATIONS IN THE LOCATION OF THERMAL POWER STATIONS

The preparation of feasibility report for a thermal station requires studyunder two headings, viz. Area Selection and Site Selection.

a) Supply of raw materials, which in the case of thermal power stations arecoal and water, are of extreme importance.

b) Transport facilities to haul the raw materials, viz. Coal in this case andthe capital equipment

c) Transmission of power produced to the load centers.

d) A labour force of size and quality required but this will not be of overriding consideration. In our country the migration of labour from one place toanother does not pose very difficult problems.

e) Means of disposal for any trade effluents or byproducts. In case ofthermal stations both the flue gases and the ash are effluents and themeans for their disposals are to be thought of.


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Climatic conditions:

The climatic conditions also play a part in area selection;as in the case of thermal power stations these affect not

only the capital cost of structures and machines etc. Butalso the economics of generation during normal running.

Station Layout

The station layout is the arrangement of all the plantequipment, engineering systems and buildings which arenormally accommodated within the site boundary. This isdivided in to two areas.

First, the main station buildings which contain the major

plant items and systems such as boiler and turbinegenerator & second the auxiliary supporting systems andservices such as and handling plant, oil handling plant,ash handling plant, cooling water pumps etc. Which arelocated around the site outside the main buildings.


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The lay out of turbo-generator and the auxiliariesplus the coal crushing equipment for the boilerare generally located in an integrated building so

as to achieve the following objectives: Proper flow of process fluids steam, condensate,

air, coal and oil

Economy of space

Ease of installation/operation/maintenance Avoidance of interference

Clear passages and easy access to various floorsand equipment

Proper safety precautions Streamline locations for good appearance


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General Arrangement:

Normally the concept in India is to have turbo generator and itsauxiliaries located in one big hall called turbo generator hall orT.G.bay.

Adjacent to this bay is an electrical bay comprising of control room,electrical switchgear and other minor auxiliaries.

The next bay generally consists of coal bunkers and coal mills.Sometimes coal bunkers and electrical bay overlap as is in the caseof Badarpur 3X100 MW layout.

Sometimes the mill bay is taken on the other side of the boiler forvarious reasons.

It may be pointed out here that station layout is not an exact sciencebut combination of art and science where the personal liking and

considerations weigh even more than the purely economicalconsideration.

A proper balance has to be struck between these two to have anacceptable lay out.

Proper lay out goes a long way in providing necessary comforts to

the operation and maintenance staff.


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Turbine Lay Out:

Once the basic concept of general arrangement is

agreed to and the decision between transverse andlongitudinal is taken the more detailed studies arerequired to finalize turbine lay out.

This consists of proper location of the following major

equipment 1) turbo generator ii) condensate pumps iii)circulating water pumps iv) LP heaters v) HP heaters vi)boiler feed pumps vii) oil coolers viii) stator water coolingsystem and various local control panels.


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Turbine operating floor should be in line with themain control room as far as possible so as to

have easy and direct access between the two.This floor should have as little obstruction on thefloor as possible.

Openings are left in the floor for approaching

equipment that is located underneath.

Generally this takes in to consideration thelength of HP heater shell which may have to be

removed for periodic maintenance. Anothermajor decision in TG bay is to have thebasement floor or not.


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Location of Deaerator:

Deaerator location has certain basic process

requirements with respect to the boiler feedpump.

Each boiler feed pump is designed to operateunder a particular net positive suction head and

this has to be provided by increasing thedearator level to particular height.

Moreover relative location of dearator withrespect of the boiler feed pump should be

chosen so as to have minimum BFP suction line.Provision has to be made in its foundation totake care of these requirements.


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Mill Bay

Coal from the coal yard is crushed to 25mm size

and conveyed through a conveyor gallery to the

transfer points. From the transfer point another

pair of conveyor belts carry the coal to the

bunkers.

Conventionally the milling in station with 210

MW units are housed in the space available

between the boiler and the power house in a line

and ducting have been conveniently done withsix mills.


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Piping layout

Besides equipment lay out station lay out engineer hasto keep in view the pipe routing not only for high-

pressure piping but also for low-pressure piping. Pipesare supported on brackets located in trenches as well asthe tunnels so as to have minimum interference betweenthem and the adjoining equipment.

Electrical Cables:

Provision has to be made in the station lay out for layingup electrical cables. For this generally two cablesgalleries are provided in electrical bay. In the bay,generally cables are taken in small cable trenches andwith pipes near the motors. In boiler area, they are beingtaken in cable tunnels and trenches. Cables may be laidover head also in addition to laying them in trenches.


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Ventilation:

For proper ventilating in the whole building windows areprovided but they are not sufficient and hence are

supplemented by having exhaust fans either on the sidesor in the roof. Space has to be provided for theventilation equipment in the main building as far aspossible.

Layout Aids:

The lay out engineer has to work in close co-operationwith the engineers of electrical as well as civil designside. Sometimes he may have to change his lay out ifthere are certain civil engineering constraints and alsocertain electrical requirements. Generally templates areused to see the various alternative locations ofequipment and these alternatives are studied withrespect to their positive and negative points.


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General Considerations:

In any plant lay out care has to be taken to

provide the basic necessities such as toilets andwash rooms, drinking water, rest rooms,engineers cabin, small stores, passages andstairs.

Generally lift is provided on two ends of the mainbuilding with a provision for an additional goodslift somewhere in between. This leads to easytransfer to heavy equipment from one floor toanother. With the higher capacity units andadditional lift for reaching the top of the boiler isalso provided.


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THANK YOU

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