INTRODUCTION The Farakka Super Thermal Power Station (F.S.T.P.S) spread across over 4000 odd acres with the townships scattered in the two districts of Murshidabad & Malda is a 1600MW plant(200x3+500x2).The foundation stone was laid on 29.12.81 by late Smt. Indira Gandhi. Its first unit was synchronized on 1/1/1986 and its last (fifth) unit on 07/03/1994.SIXTH unit 500 MW is under erection by M/S BHEL

The source of coal is the nearby Rajmahal coalfields in Bihar and water is obtained from the Farakka feeder canal. The plant supplies power to West Bengal as well as the neighboring states .

FARAKKA SUPER THERMAL POWER PROJECT - AN OVERVIEW

LOCATION: - STATE : - WEST BENGAL DISTRICT : - MURSHIDABAD

SOURCES OF RAW MATERIAL : - COAL - RAJMAHAL COAL FIELD, JHARKHAND WATER- FARAKKA FEEDER CANAL.

GOVT. APROVAL OF STAGE I [ 3*200MW ] MARCH ‘79THE PROJECT :- STAGE II [ 2*500MW ] SEPTEMPER’89

CAPACITY : - 1600MW STAGE I 3*200 MW STAGE II 2*500 MW

PROJECT COST: - STAGE I [ 3*200MW ] STAGE II [ 2*500MW ] Rs.3184.22

Crores

TOTAL AREA: - PLANT 2129 Acres ASH POND 1894 Acres 4398 AcresTOWNSHIP 375 Acres

TOTAL MANPOWER : - 1750 (31-03-2002)

POWER BENEFICIARY STATE AND ITS DISTRIBUTION:- W.B - 33.12 BIHAR - 18.25ORISSA - 14.69 D.V.C - 6.50

SIKKIM- 0.99 JHARKHAND- 2.61 OTHER REGION- 23.92MANUFACTURE OF THE MAJOR EQUIPTMENTS:-

STAGE I BOILER – BHEL Maker, CORNER FIRED TYPETURBINE & GENERATOR – BHEL MAKECOOLING SYSTEM – OPEN CYCLE.

STAGE II BOILER – Ansaldo Italy front & rear fired type .TURBINE & GENERATOR – BHEL MAKECOOLING SYSTEM – OPEN CYCLE.

Coal to Electricity …. Basics

In a thermal power plant, the thermal energy of the superheated steam is used to drive the generator which in turn produces electricity. The superheated steam is produced from any fossil fuel like coal, crude oil etc. Since coal is available in plenty and in cheap, we use coal as the primary fuel in the thermal power plants. The coal is pulverized, grinded and powdered before it is used in the boiler. The pulverization of coal increases the surface area of the coal which helps in the smooth combustion

Coal

Chemical Energy(Coal)

Super Heated

Steam

Pollutants

Thermal Energy(Steam)

Turbine Torque

Heat Loss in Condenser

Mechanical Energy

Electrical Energy

Alternating current in Stator

Mech. Energy Loss

ASH

HeatLoss

Elec. Energy Loss

of the coal. The coal is feed to the boiler where it is fired to produce heat. Initially, a small quantity of crude oil is used for the purpose. The water is passed through the pipes fitted in the walls of the boiler. The water is heated to produce steam, which is further reheated to produce superheated steam. The superheated steam at an exceedingly high temp and pressure drives the turbine. The turbine rotates following which the generator which is coupled to the turbine also rotates producing electricity. Thus, the chemical energy of the coal is utilized to produce steam. The thermal energy of the steam is converted to the mechanical energy of the rotating turbines which is converted to the electrical energy in the generator.

C OAL HANDLING PLANT(CHP) For a pulverized coal-fired boiler, the coal is transported from the nearby Lalmatia mines via the Merry-Go-Round (MGR) system. It is then handled at the CHP (Coal handling Plant) and supplied to the main plant at about 20mm size. From there through coal Bunkers, coal feeders and mills (for pulverization) it is supplied to the burners through a piping system. Heated air, called primary air, is passed through the pulverizers to pick up the fine particles of coal and carry them to the furnace.

COAL UNLOADING :

At NTPC end, Coal is unloaded in the Track hoppers. Track Hopper, normally of 200-250m length. After placing the wagons on the hopper, electrical pulse is given to the

pneumatic device for opening the bottom doors. Coal rushes down due to its own weight. When the track hopper is empty, it takes only 20 seconds for unloading

one wagon i.e. 60 Tons. This coal falls on a ‘T’ table, over which a paddle feeder runs and pushes

the coal on to a conveyor.

COAL HANDLING SYSTEM EQUIPMENT :

Plough feeder/ paddle feeder vibrating screen Crushers, Conveyor belt,

Idlers, Pulleys, Drive Unit, Take-ups, Skirt board, Scrapper, Stacker-cum-reclaimer, Magnetic separators, Motorized tripper.

COAL-HANDLING SYSTEM:

Coal handling system is the arrangement for transferring of coal from coal-wagons to coal-bunker or coal stock yard.

It also provide arrangement for separation of material impurities, crushing of coals, Coal sampling etc.

In the coal handling system of NTPC stations, three coal paths are normally available

Path A - direct conveying of coal from track hopper to boiler bunkers.Path B - from track hopper to stockyardPath C - from stockyard to boiler bunkers.

The storage facilities at the stockyards have been provided only for crushed coal.

COAL-HANDLING PROCESS:

From the track hopper 200mm coal is transferred to the conveyor via paddle feeder and finally sent to the crusher house.

In the Crusher House the vibrating screen sends the coal having size of less than 20mm to belt feeder through the bypass chute bypassing the crusher and sends the coal of more than 20mm size to the crusher.

Farakka Borhait Pokhra Lalmatia

STEAM GENERATION

BOILING STEAM GENERATION

Boiling & consequent steam generation is a quite familiar process. In brief , as we began to heat water, it goes on absorbing heat at constant pressure and is evident by rise in the temp. A stage reaches when water begins to boil and there is no rise in temp., at this stage steam is formed, whichcontinues to be so at the same temp. unless and until pressure changes. The first stage of heat has ;dueto the latent heat. Thus, thermodynamically speaking, boiling may be considered a special case of adding heat to the working substance in a constant-pressure, constant temp. process..As the pressure rises ,latent heat decreases and a stage is reached at (221.06 bar ), when thelatent heat become zero, this pressure began termed as critical pressure. .Further , once the steam is formed & does not have any traces of water i.e. is dry & saturated ,we keep the pressure constant while heat is being added, the temperature of steam will begin to rise, theheat expended being known as superheat.As the working pressure rises, the specific weight of the water goes on decreasing while that of steam goes on rising till at critical pressure it becomes equal both for steam and water . .Because of specific weight, differential between water & steam at a given pressure, there existsa different head, which ever a pretty long range of pressure provides a good means for circulation during steam generation. Depending upon working pressure i.e. below critical , the steam generatorsare designated as subcritical or super-critical.Whereas, for a greater part of the range in subcritical pressure it is possible to have naturalcirculation during evaporation, but above supercritical pressure circulation is forced one. In the highersubcritical pressure range also due to the reason of economy, forced or assisted circulation may beemployed.

CIRCULATION SYSTEMS :

GENERALModern boilers may be with or without the drum. The boilers working below the sub-criticalpressure are generally provided with drums, or a small separating vessel (high pressure nearing critical) in its place irrespective of the type of the type of circulation employed. The drum-acts as reservoir forwater & saturated steam and also provides means and arrangements for separation and purification of steam. The term circulation generally with drum type of boilers applies to the

movement of fluid fromthe drum to the combustion zone and back to the drum. The feed water to the drum in any case reachesthe drum from the boiler feed pump via the economizer, the drum - less boilers work above criticalpressures & there is straight transformation to steam without passing through the boiling stageinvolving latent heat of evaporation. Some designs in this type of boilers provide a small mixing or separating vessel which deals with the water drops particularly during starting.It is essential to provide an adequate flow of water and /or of water steam mixture for anefficient transfer of heat from furnace to the working fluid and to prevent 'burn outs'. This isirrespective of the mode of circulation being used.

TYPES OF BOILER CIRCULATION SYSTEMS :

The boiler circulation systems have been designated in different ways by the variousmanufacturers. Of course they are similar as for as basic principles are concerned and differ in minordetails and nomenclature. The various classification in vogue are given below.1 SOME DESIGNERS HAVE THE FOLLOWING CLASSIFICATIONS FOR TYPES OF CIRCULATIONS :

a) Natural circulation:- In this type, no external pumping device is used for the movement of thefluid. The difference in densities in contents of fluids in downcomers from the drum and risers in thefurnace is used to effect the movement of fluids. This type of circulation is employed in most of theutility boilers and in fact in India excepting one unit at Trombay power station all boilers work onnatural circulation.

b) Positive circulation ::- In this type , use of pumps is made for movement of fluid through thecombustion zone or complete heat transfer circuits. This type of circulation is further divided into

.I) Assisted circulation : The term is generally used in case of boilers having drums and working below critical pressures. Circulating water pumps are used in between the bottom headers of downcomers and risers to overcome frictional losses and the consequent movement of water and watersteam mixture.500 Mw BHEL boiler in NTPC Farakka has this type of circulation where CC pump is used in downcomers.

II)Forced circulation ::- This term is generally used for movement of fluid in boilers working above critical pressures. These boilers are of 'Once-through' type. The pump forces movement of fluidthrough all the heating zones viz. Economizers , tubes in combustion zones and superheaters. A smallseparating or mixing vessel may be provided for removal of moisture from steam and pumping back tothe circuit.

CIRCULATION RATIO : It is essential to maintain a certain amount of flow of water to the steam generatingcircuits in commensurate with the amount of steam generated from them, in order to prevent `Burn-outs' and `On-load Corrosion'. The ratio by weight of the water fed to the steam-generating circuits tothe steam actually generated (Kg water : Kg steam ) is called `Circulation Ratio'. Taking circuit shownin Fig. 1.8 as an example for a unit period of time 5 kg water is admitted to risers (which will getconverted into mixture of water and steam during its passage through the furnace) and 1 kg of steam istaken out of the drum, the value of circulation ratio will be five. The remaining 4 Kg of water will berecirculated in the system. To compensate for 1 Kg. Of steam taken out, 1 Kg. Of water will have to beadded to the drum, which will enter the risers alongwith the water under recirculation.

INTRODUCTION OF BOILER AT NTPC FARAKKA:

The 200 MW unit boilers (3 Nos.) are Natural Circulation, Dry bottom, Single drum, Tangentially fired, Balanced draft, Radiant reheat type direct corner fired pulverized coal system.

The description of these technical terms is –

(a) Natural Circulation – The flow of water & steam within the boiler circuit is called Circulation. If circulation is caused by density difference (without assistance of any pump), the boiler is said to have natural circulation.

(b) Dry Bottom – In a dry-bottom furnace, ash or slag is removed in the

solid or dry state. The exit temperature of gases leaving the furnaces must be below the ash fusion temperature.

(c) Single Drum –There is only one drum i.e. steam drum in boiler circuit. There is no mud drum (also called water drum) in our unit.

(d) Tangentially Fired & Direct corner fired –In tangential fired boilers with corner burners, flame jets directed tangentially to an imaginary circle 1-2.5 m in diameter in the furnace center. Both fuel & air are projected horizontally from the corners of the furnace along lines tangent to a vertical cylinder (Imaginary) at the center of the furnace. Intensive mixing occurs where the stream meet. A rotating motion, similar to that of cyclone, is imparted to the flame body, which spreads out & fills the furnace area.

This arrangement helps in avoiding clinkering of water walls due to intense thermal loading near those walls in Corner Firing with Encountering Flame Jets burner arrangement.

(e) Balanced Draft –In balance draft (draught) the pressure in furnace is slightly negative gauge pressure to ensure that any leakage would be inward.

(f) Radiant reheat type – In our boiler reheater is placed in the radiant zone of the furnace near the rear water wall to absorb heat by radiation.

(g) Pulverized coal system -In pulverized coal system, coal is first ground to dust like size in a mill & powdered coal is then carried in a stream of air (PA) to be fed through burners into the furnace.

In fossil-fueled power plants,steam generator refers to a furnace that burns the fossil fuel to boil water to generate steam. The steam generating boiler has to produce steam at the high purity, pressure and temperature required

Schematic Diagram of 200 MW Boiler

for the steam turbine that drives the electrical generator. A fossil fuel steam generator includes an economizer, a steam drum, and the furnace with its steam generating tubes and superheater coils. Necessary safety valves are located at suitable points to avoid excessive boiler pressure. The air and flue gas path equipment include: forced draft (FD) fan, air preheater (APH), boiler furnace, induced draft (ID) fan, fly ash collectors (electrostatic precipitator or baghouse) and the flue gas stack..For units over about 200 MW capacity, redundancy of key components is provided by installing duplicates of the FD fan, APH, fly ash collectors and ID fan with isolating dampers. On some units of about 60 MW, two boilers per unit may instead be provided.

Boiler furnace and Steam drum

Once water inside the boiler or steam generator, the process of adding the latent heat of vaporization or enthalpy is underway. The boiler transfers energy to the water by the chemical reaction of burning some type of fuel.The water enters the boiler through a section in the convection pass called the economizer.

From the economizer it passes to the steam drum. Once the water enters the steam drum it goes down the downcomers to the lower inlet waterwall headers. From the inlet headers the water rises through the waterwalls(due to density difference) and is eventually turned into steam due to the heat being generated by the burners located on the front and rear waterwalls (typically). As the water is turned into steam/vapor in the waterwalls, the steam/vapor once again enters the steam drum. The steam/vapor is passed through a series of steam and water separators and then dryers inside the steam drum. The steam separators and dryers remove water droplets from the steam and the cycle through the waterwalls is repeated. This process is known as natural circulation.

The boiler furnace auxiliary equipment includes coal feed nozzles and igniter guns, soot blowers, water lancing and observation ports (in the

furnace walls) for observation of the furnace interior. Furnace explosions due to any accumulation of combustible gases after a trip-out are avoided by flushing out such gases from the combustion zone before igniting the coal.

The steam drum (as well as the superheater coils and headers) have air vents and drains needed for initial startup. The steam drum has internal devices that removes moisture from the wet steam entering the drum from the steam generating tubes. The dry steam then flows into the superheater coils.

Superheater

Fossil fuel power plants can have a superheater and/or reheater section in the steam generating furnace. In a fossil fuel plant, after the steam is conditioned by the drying equipment inside the steam drum, it is piped from the upper drum area into tubes inside an area of the furnace known as the superheater, which has an elaborate set up of tubing where the steam vapor picks up more energy from hot flue gases outside the tubing and its temperature is now superheated above the saturation temperature. The superheated steam is then piped through the main steam lines to the valves before the high pressure turbine.

ReheaterPower plant furnaces may have a reheater section containing tubes heated by hot flue gases outside the tubes. Exhaust steam from the high pressure turbine is rerouted to go inside the reheater tubes to pickup more energy to go drive intermediate or lower pressure turbines.

Fuel preparation systemIn coal-fired power stations, the raw feed coal from the coal storage area is first crushed into small pieces and then conveyed to the coal feed hoppers at the boilers. The coal is next pulverized into a very fine powder. The pulverizers may be ball mills, rotating drum grinders, or other types of grinders.Some power stations burn fuel oil rather than coal. The oil must kept warm (above its pour point) in the fuel oil storage tanks to prevent the oil from congealing and becoming unpumpable. The oil is usually heated to about 100°C before being pumped through the furnace fuel oil spray nozzles.Boilers in some power stations use processed natural gas as their main fuel. Other power stations may use processed natural gas as auxiliary fuel in the

event that their main fuel supply (coal or oil) is interrupted. In such cases, separate gas burners are provided on the boiler furnaces.

Air path

External fans are provided to give sufficient air for combustion. The forced draft fan takes air from the atmosphere and, first warming it in the air preheater for better combustion, injects it via the air nozzles on the furnace wall.The induced draft fan assists the FD fan by drawing out combustible gases from the furnace, maintaining a slightly negative pressure in the furnace to avoid backfiring through any opening. At the furnace outlet, and before the furnace gases are handled by the ID fan, fine dust carried by the outlet gases is removed to avoid atmospheric pollution. This is an environmental limitation prescribed by law, and additionally minimizes erosion of the ID fan.

Auxiliary systems ASH HANDLING

Fly ash collection Fly ash is captured and removed from the flue gas by electrostatic precipitators or fabric bag filters (or sometimes both) located at the outlet of the furnace and before the induced draft fan. The fly ash is periodically removed from the collection hoppers below the precipitators or bag filters. Generally, the fly ash is pneumatically transported to storage silos for subsequent transport by trucks or railroad cars.

Bottom ash collection and disposalAt the bottom of the furnace, there is a hopper for collection of bottom ash. This hopper is always filled with water to quench the ash and clinkers falling down from the furnace. Some arrangement is included to crush the clinkers and for conveying the crushed clinkers and bottom ash to a storage site.

Boiler make-up water treatment plant and storageSince there is continuous withdrawal of steam and continuous return of condensate to the boiler, losses due to blowdown and leakages have to be made up to maintain a desired water level in the boiler steam drum. For this,

continuous make-up water is added to the boiler water system. Impurities in the raw water input to the plant generally consist of calcium and magnesium salts which impart hardness to the water. Hardness in the make-up water to the boiler will form deposits on the tube water surfaces which will lead to overheating and failure of the tubes. Thus, the salts have to be removed from the water, and that is done by a water demineralising treatment plant (DM). A DM plant generally consists of cation, anion, and mixed bed exchangers. Any ions in the final water from this process consist essentially of hydrogen ions and hydroxide ions, which recombine to form pure water. Very pure DM water becomes highly corrosive once it absorbs oxygen from the atmosphere because of its very high affinity for oxygen.The capacity of the DM plant is dictated by the type and quantity of salts in the raw water input. However, some storage is essential as the DM plant may be down for maintenance. For this purpose, a storage tank is installed from which DM water is continuously withdrawn for boiler make-up. The storage tank for DM water is made from materials not affected by corrosive water, such as PVC. The piping and valves are generally of stainless steel. Sometimes, a steam blanketing arrangement or stainless steel doughnut float is provided on top of the water in the tank to avoid contact with air. DM water make-up is generally added at the steam space of the surfaccondenser(i.e., the vacuum side). This arrangement not only sprays the water but also DM water gets deaerated, with the dissolved gases being removed by an air ejector attached to the condenser.

ELECTROSTATIC PRECIPITATOR

It is a device which captures the dust particles from the flue gas thereby reducing the chimney emission.Precipitators function by electrostatically charging the dust particles in the gas stream. The charged particles are then attracted to and deposited on plates or other collection devices. When enough dust has accumulated, the collectors are shaken to dislodge the dust, causing it to fall with the force of gravity to hoppers below. The dust is then removed by a conveyor system for disposal or recycling .

TYPICAL SPECIFICATIONS FOR A 200MW UNITS

NUMBER OF PASS PER BOILER 4NUMBER OF FIELD IN EACH PASS 6EFFICIENCY 99.9%PRESSURE DROP 20 mmWCGAS FLOW RATE 312.7 Cu.m/secINLET TEMPERATURE 136 DEGREEVELOCITY OF GAS AT ELECTRODE 0.839 m/secTOTAL TREATMENT TIME 32.18 SEC

THEORY OF PRECIPITATIONElectrostatic precipitation removes particles from the exhaust gas stream of Boiler combustion process. Six activities typically take place:

Ionization - Charging of particles Migration - Transporting the charged particles to the collecting surfaces Collection - Precipitation of the charged particles onto the collecting

surfaces Charge Dissipation - Neutralizing the charged particles on the collecting

surfaces Particle Dislodging - Removing the particles from the collecting surface

to the hopper

Particle Removal - Conveying the particles from the hopper to a disposal point

Components of ESP

• Discharge Electrodes • Power Components • Precipitator Controls • Rapping Systems • Purge Air Systems • Flue Gas Conditioning • Emitting Electrodes• Collecting Electrodes• High Voltage Equipment• Rapping Mechanism• Hoppers• Heaters• ALI• Gas Distribution Screen • Segregating Gates