37154010 Ntpc Project Report (1)

8/8/2019 37154010 Ntpc Project Report (1)

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SUMMER TRAINING REPORT

21st JUNE to 31st JULY

SUBMITED BY:-

Rohit Juyal08/ME/94BSATIM, FBD.

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ABOUT THE COMPANY

NTPC, the largest power Company in India, was setup in1975 to accelerate power development in the country. It isamong the worlds largest and most efficient powergeneration companies. In Forbes list of Worlds 2000 LargestCompanies for the year 2007, NTPC occupies 411th place.

A View of Badarpur Thermal Power Station, New Delhi

NTPC has installed capacity of 29,394 MW. It has 15 coalbased power stations (23,395 MW), 7 gas based powerstations (3,955 MW) and 4 power stations in Joint Ventures(1,794 MW). The company has power generating facilities inall major regions of the country. It plans to be a 75,000 MWcompany by 2017.

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NTPC has gone beyond the thermal power generation. It hasdiversified into hydro power, coal mining, power equipmentmanufacturing, oil & gas exploration, power trading &distribution. NTPC is now in the entire power value chain and

is poised to become an Integrated Power Major. NTPC's shareon 31 Mar 2008 in the total installed capacity of the countrywas 19.1% and it contributed 28.50% of the total powergeneration of the country during 2007-08. NTPC has set newbenchmarks for the power industry both in the area of powerplant construction and operations. With its experience andexpertise in the power sector, NTPC is extending consultancyservices to various organizations in the power business. Itprovides consultancy in the area of power plantconstructions and power generation to companies in Indiaand abroad. In November 2004, NTPC came out with itsInitial Public Offering (IPO) consisting of 5.25% as fresh issueand 5.25% as offer for sale by Government of India. NTPCthus became a listed company with Government holding89.5% of the equity share capital and rest held byInstitutional Investors and Public. The issue was a

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resounding success. NTPC is among the largest fivecompanies in India in terms of market capitalization.

Recognizing its excellent performance and vast potential,Government of the India has identified NTPC as one of the

jewels of Public Sector 'Navratnas'- a potential global giant.Inspired by its glorious past and vibrant present, NTPC is wellon its way to realize its vision of being "A world classintegrated power major, powering India's growth, withincreasing global presence".

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Thermal power station employs a great number of equipmentperforming number of complexity processes. The ultimate aim beingthe production of electricity. In order to have stable generatingcondition always a balance has to be achieved so that heat input isequal to electricity output and losses. BTPS is designed by the central

water and power commission.

Approval capacity

750 MW

Installed capacity

705 MW

LocationNew Delhi

Coal source Jharia coal fields CCL (Central Coal Fields Ltd.)

BCCL (Bharat Cooking Coals Ltd.)

ECL (Eastern Coal Fields Ltd.)

Water source

Agra canal

Beneficing state

New Delhi

Size of units

3*95 and 2*210

Commissioning of units

Unit 1 95 MW 1973-74

Unit 2 95 MW 1974-75

Unit 3 95 MW 1974-75

Unit 4 210 MW 1978-79

Unit 5 210 MW 1981-82

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Site Selection For PowerPlant

Selection of site for a thermal power plant is one of themost important aspects which an effect the efficiencyof a thermal power plant. For the overall economy ofthe plant, the following points should be consideredwhile selecting a site:-

Availability of coal: - Steam power station should belocated near mines so that minimum cost of fuel is

maintained. However if such a plant is to be installed atsuch a place where coal is not available, the adequatefacilities should be made for transportation of coal. Likethe railway lines which are provided in such case ofBTPS.

Availability of water: - A large amount of water isnecessary for the condenser therefore such a plantshould be located at the bank of the river or near a

canal like the Agra canal in case of BTPS.

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A coal-fired Thermal Power Plant

Means of transportation: - The plant should be wellinterconnected with roads or and railway system forthe efficient transportation of material.

Land properties: - The land should be cheap and thereshould be enough land surrounding the plant so thatthere may be option of expanding. The land should alsobe able to withstand the load of heavy equipments.

Nearness of the road centers: - This is very importantfactor when considering dc transmission. If an acsystem is employed than this factor is less important

because ac can be transmitted at high voltage withconsequent reduce transmission cause. That is why; itis possible to install the plant away from the loadcenters provided other conditions are favorable.

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Ash disposal facilities: - The quantity of ash to handle isas large as 1500 to 2000 tons per day. The disposal oflarge quantities of ash from the power station can beinto the river, see or lake or can be brought into useful

purposes like in the manufacturing of bricks as done byBTPS.

Distance from populated areas: - Since there is a lotash from the power station there will be lot smokesfumes that make the surrounding of plant veryhazardous.

BMD

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FUNCTIONING

Functioning of thermal power plant:

In a thermal power plant, one of coal, oil or natural gas isused to heat the boiler to convert the water into steam. Thesteam is used to turn a turbine, which is connected to agenerator. When the turbine turns, electricity is generatedand given as output by the generator, which is then suppliedto the consumers through high-voltage power lines.

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Detailed process of powergeneration in aThermal power plant:

1) Water intake: Firstly, water is taken into the boilerthrough a water source. If water is available in a plenty inthe region, then the source is an open pond or river. Ifwater is scarce, then it is recycled and the same water isused over and over again.

2) Boiler heating: The boiler is heated with the help ofoil, coal or natural gas. A furnace is used to heat the fuel andsupply the heat produced to the boiler. The increase intemperature helps in the transformation of water into steam.

3) Steam Turbine: The steam generated in the boiler issent through a steam turbine. The turbine has blades thatrotate when high velocity steam flows across them. Thisrotation of turbine blades is used to generate electricity.

4) Generator: A generator is connected to the steamturbine. When the turbine rotates, the generator produceselectricity which is then passed on to the power distributionsystems.

5) Special mountings: There is some other equipmentlike the economizer and air pre-heater. An economizer usesthe heat from the exhaust gases to heat the feed water. Anair pre-heater heats the air sent into the combustionchamber to improve the efficiency of the combustionprocess.

6) Ash collection system: There is a separate residueand ash collection system in place to collect all the wastematerials from the combustion process and to prevent them

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from escaping into the atmosphere. Apart from this, thereare various other monitoring systems and instruments inplace to keep track of the functioning of all the devices. Thisprevents any hazards from taking place in the plant.

There are basically three mainunits of a thermal power plant:

1. Steam Generator or Boiler

2. Steam Turbine

3. Electric Generator

Typical Diagram of a Coal based Thermal Power Plant

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1. Cooling tower 10. Steam governorvalve

19. Super heater

2. Cooling water pump 11. High pressureturbine

20. Forced draughtfan

3. Transmission line (3-phase)

12. Deaerator 21. Reheater

4. Unit transformer (3-phase)

13. Feed heater 22. Air intake

5. Electric generator (3-phase)

14. Coal conveyor 23. Economizer

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6. Low pressure turbine 15. Coal hopper 24. Air preheater

7. Condensate extractionpump

16. Pulverized fuel mill 25. Precipitator

8. Condensor 17. Boiler drum 26. Induced draughtfan

9. Intermediate pressureturbine

18. Ash hopper 27. Chimney Stack

Coal is conveyed (14) from an external stack and ground to a

very fine powder by large metal spheres in the pulverizedfuel mill (16). There it is mixed with preheated air (24) drivenby the forced draught fan (20). The hot air-fuel mixture isforced at high pressure into the boiler where it rapidlyignites. Water of a high purity flows vertically up the tube-lined walls of the boiler, where it turns into steam, and ispassed to the boiler drum, where steam is separated fromany remaining water. The steam passes through a manifoldin the roof of the drum into the pendant superheated (19)

where its temperature and pressure increase rapidly toaround 200 bar and 540C, sufficient to make the tube wallsglow a dull red. The steam is piped to the high pressureturbine (11), the first of a three-stage turbine process. Asteam governor valve (10) allows for both manual control ofthe turbine and automatic set-point following. The steam isexhausted from the high pressure turbine, and reduced inboth pressure and temperature, is returned to the boilerreheater (21). The reheated steam is then passed to theintermediate pressure turbine (9), and from there passeddirectly to the low pressure turbine set (6). The exitingsteam, now a little above its boiling point, is brought intothermal contact with cold water (pumped in from the coolingtower) in the condensor (8), where it condenses rapidly backinto water, creating near vacuum-like conditions inside thecondensor chest. The condensed water is then passed by a

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feed pump (7) through a desecrator (12), and pre-warmed,first in a feed heater (13) powered by steam drawn from thehigh pressure set, and then in the economizer (23), beforebeing returned to the boiler drum. The cooling water from

the condensor is sprayed inside a cooling tower (1), creatinga highly visible plume of water vapor, before being pumpedback to the condensor (8) in cooling water cycle. The threeturbine sets are sometimes coupled on the same shaft as thethree-phase electrical generator (5) which generates anintermediate level voltage (typically 20-25 kV). This isstepped up by the unit transformer (4) to a voltage moresuitable for transmission (typically 250-500 kV) and is sentout onto the three-phase transmission system (3). Exhaustgas from the boiler is drawn by the induced draft fan (26)

through an electrostatic precipitator (25) and is then ventedthrough the chimney stack (27).

Steam Generator or Boiler

The boiler is a rectangular furnace about 50 ft (15 m) on aside and 130 ft (40 m) tall. Its walls are made of a web ofhigh pressure steel tubes about 2.3 inches (60 mm) indiameter.

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Pulverized coal is air-blown into the furnace from fuel nozzlesat the four corners and it rapidly burns, forming a largefireball at the center. The thermal radiation of the fireballheats the water that circulates through the boiler tubes near

the boiler perimeter. The water circulation rate in the boileris three to four times the throughput and is typically drivenby pumps. As the water in the boiler circulates it absorbsheat and changes into steam at 700 F (370 C) and 3,200psi (22.1MPa). It is separated from the water inside a drumat the top of the furnace. The saturated steam is introducedinto superheat pendant tubes that hang in the hottest part ofthe combustion gases as they exit the furnace. Here thesteam is superheated to 1,000 F (540 C) to prepare it forthe turbine.

The steam generating boiler has to produce steam at thehigh purity, pressure and temperature required for thesteam turbine that drives the electrical generator. Thegenerator includes the economizer, the steam drum, thechemical dosing equipment, and the furnace with its steamgenerating tubes and the super heater coils. Necessarysafety valves are located at suitable points to avoid

excessive boiler pressure. The air and flue gas pathequipment include: forced draft (FD) fan, air preheater(APH), boiler furnace, induced draft (ID) fan, fly ashcollectors (electrostatic precipitator or bag house) and theflue gas stack.

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Schematic diagram of a coal-fired power plant steamgenerator

For units over about 210 MW capacity, redundancy of keycomponents is provided by installing duplicates of the FDfan, APH, fly ash collectors and ID fan with isolatingdampers. On some units of about 60 MW, two boilers perunit may instead be provided.

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Boilers can be classified as water tube boiler and fire tube boiler.

Fire Tube Boiler

In this boiler, products of combustion pass through tubeswhich are surrounded by water. Depending on where thetubes are horizontal or vertical. They are further classified ashorizontal or vertical tube boilers. They may be internally orexternally fed. An internally fed has grate or combustionchamber enclosed with in the boiler shell. In an externallyfed boiler the setting, including furnace and grates isseparate and distant from boiler shell. A fire tube boiler issimple compact and rugged in construction. Its initial cost islow. A vertical fire tube boiler occupies less floor space.However they are economical only for low pressure and arethere for available in small sizes having steam capacity ofabout 15000kg/hr.

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External View of an Industrial Boiler at Badarpur Thermal PowerStation, New Delhi

Water Tube Boiler

In this boiler water flows inside the tubes and hot gasesflows outside the tubes are interconnected to common waterchannels and to steam outlet. Water tube boilers areclassifieds as vertical full. The number of drum may be oneor more.

The circulation of water in the boiler may be natural orforced through the action pumps. Forced circulation has theadvantage of:

1) Lesser weight of boiler and cheaper foundation.

2) Lighter tube.

3) Freedom from scaling problem.

4) Uniform heating of all parts.

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5) Increased efficiency of boiler.

6) Better control of temperature.

7) Quicker response to local changes.

8) Greater freedom in configuration of furnace, tube etc.

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The steam/vapor is passed through a series of steam andwater separators and then dryers inside the steam drum.

The steam separators and dryers remove the water dropletsfrom the steam and the cycle through the water walls isrepeated. This process is known as natural circulation.

The boiler furnace auxiliary equipment includes coalfeed nozzles and igniter guns, soot blowers, water lancingand observation ports (in the furnace walls) for observationof the furnace interior. Furnace explosions due to anyaccumulation of combustible gases after a trip out areavoided by flushing out such gases from the combustion

zone before igniting the coal.The steam drum (as well as the super heater coils and

headers) have air vents and drains needed for initial startup. The steam drum has an internal device that removesmoisture from the wet steam entering the drum from thesteam generating tubes. The dry steam then flows into thesuper heater coils.

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Geothermal plants need no boiler since they usenaturally occurring steam sources. Heat exchangers may beused where the geothermal steam is very corrosive orcontains excessive suspended solids. Nuclear plants also boil

water to raise steam, either directly passing the workingsteam through the reactor or else using an intermediateheat exchanger.

Fuel Preparation System

In coal-fired power stations, the raw feed coal from the coalstorage area is first crushed into small pieces and thenconveyed to the coal feed hoppers at the boilers. The coal isnext pulverized into a very fine powder. The pulverize maybe ball mills, rotating drum grinders, or other types ofgrinders.

Some power stations burns fuel oil rather than coal. Theoil must kept warm (above its pour point) in the fuel oilstorage tanks to prevent the oil from congealing andbecoming unpumpable. The oil is usually heated to about

1000C before being pumped through the furnace fuel oilspray nozzles.

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Boiler Side of the Badarpur Thermal Power Station, NewDelhi

Boilers in some power stations use processed natural gas astheir main fuel. Other power stations may use processednatural gas as auxiliary fuel in the event that their main fuelsupply (coal or oil) is interrupted. In such cases, separate gasburners are provided on the boiler furnaces.

Fuel Firing System and Igniter

System

From the pulverized coal bin, coal is blown by hot air throughthe furnace coal burners at an angle which imparts a swirlingmotion to the powdered coal to enhance mixing of the coalpowder with the incoming preheated combustion air andthus to enhance the combustion.

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To provide sufficient combustion temperature in thefurnace before igniting the powdered coal, the furnacetemperature is raised by first burning some light fuel oil orprocessed natural gas (by using auxiliary burners and

igniters provide for that purpose).

Air Path

External fans are provided to give sufficient air forcombustion. The forced draft fan takes air from theatmosphere and, first warming it in the air preheater forbetter 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 slightlynegative pressure in the furnace to avoid backfiring throughany opening. At the furnace outlet, and before the furnacegases are handled by the ID fan, fine dust carried by theoutlet gases is removed to avoid atmospheric pollution. Thisis an environmental limitation prescribed by law, andadditionally minimizes erosion of the ID fan.

Auxiliary Systems

Fly Ash Collection

Fly ash is captured and removed from the flue gas byelectrostatic precipitators or fabric bag filters (or sometimesboth) located at the outlet of the furnace and before theinduced draft fan. The fly ash is periodically removed fromthe collection hoppers below the precipitators or bag filters.Generally, the fly ash is pneumatically transported to

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storage silos for subsequent transport by trucks or railroadcars.

Bottom Ash Collection and Disposal

At the bottom of every boiler, a hopper has been providedfor collection of the bottom ash from the bottom of thefurnace. This hopper is always filled with water to quenchthe ash and clinkers falling down from the furnace. Somearrangement is included to crush the clinkers and forconveying the crushed clinkers and bottom ash to a storagesite.

Boiler Make-up Water Treatment Plantand Storage

Since there is continuous withdrawal of steam andcontinuous return of condensate to the boiler, losses due toblow-down and leakages have to be made up for so as tomaintain the desired water level in the boiler steam drum.For this, continuous make-up water is added to the boiler

water system. The impurities in the raw water input to theplant generally consist of calcium and magnesium saltswhich impart hardness to the water. Hardness in the make-up water to the boiler will form deposits on the tube watersurfaces which will lead to overheating and failure of thetubes. Thus, the salts have to be removed from the waterand that is done by water demineralising treatment plant(DM).

A DM plant generally consists of cation, anion and

mixed bed exchangers. The final water from this processconsists essentially of hydrogen ions and hydroxide ionswhich is the chemical composition of pure water. The DMwater, being very pure, becomes highly corrosive once itabsorbs oxygen from the atmosphere because of its veryhigh affinity for oxygen absorption.

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A DM plant generally consists of cation, anion andmixed bed exchangers. The final water from this processconsists essentially of hydrogen ions and hydroxide ionswhich is the chemical composition of pure water. The DM

water, being very pure, becomes highly corrosive once itabsorbs oxygen from the atmosphere because of its veryhigh affinity for oxygen absorption.

The capacity of the DM plant is dictated by the type andquantity of salts in the raw water input. However, somestorage is essential as the DM plant may be down formaintenance. For this purpose, a storage tank is installedfrom which DM water is continuously withdrawn for boilermake-up. The storage tank for DM water is made frommaterials not affected by corrosive water, such as PVC. The

piping and valves are generally of stainless steel.Sometimes, a steam blanketing arrangement or stainlesssteel doughnut float is provided on top of the water in thetank to avoid contact with atmospheric air. DM water make-up is generally added at the steam space of the surfacecondenser (i.e., the vacuum side). This arrangement not onlysprays the water but also DM water gets deserted, with thedissolved gases being removed by the ejector of thecondenser itself.

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PAM

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Water treatment plant

The availability of suitable supply of water both for cooling

purpose and for boiler feed make up in the basicrequirement of power plant station. The boiler treatmentplant (WTP) is meeting this requirement. The water, which isused in the boiler circuit, must be in pure form to avoidcorrosion of boiler tubes, scale formation on the insidesurface of various pressure parts and avoid silica carry oneto turbine. The main objective of WTP is to remove allimpurities from the water being sent to boiler in order thatthe steam generated is pure and boiler can giveuninterrupted service.

The major constituents of all natural waters consist of salt ofsodium, potassium, calcium and magnesium together withbio carbonate, sulphate, chloride and nitrate ion.

Clarifloculator

To the chlorinator raw water chemicals are added in the formof solutions and violent turbulence chemicals are adequatelymixed in a flush mix. The water is then led to centralchamber of Clarifloculator having rotary type ofarrangement. The water from outer than led into clear waterstorage tank. The accumulated sedge at the bottom is thenpumped out in slurry form.

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Demineralization

The filtered water as processed above is having by now onlydissolved salts as impurities in the form of ions. This water is

then pushed through a strongly acidic cation exchangeresins in hydrogen form. The resin is chemically similar toplastic but is treated in such a way so as to confer ionexchange properties. All cation periling in water areexchanged for hydrogen ion only. The result is that theneutral salts are connected to their corresponding acids.

2RH + Ca(NO3)2 R2Ca + 2HNO3

or

RH + KCl RK + HCl

i.e. resin in H2 form + impurities in the form of chloridesulphate nitrate etc.

Resin in cation form + strong acids of free mineral acidity.

Carbonate and bicarbonate ions are mostly decomposedunder such strong acid medium to carbon dioxide. The restdecomposed into carbonic acid.

The anion exchange resin bed but packed with strongly basicresins.

ROH + HCl RCl + H2O

H2SO4 R2SO4 + H2O

HNO3 RNO3

i.e. Resin in hydrogen form + anions in acid form exchanged resins + water.

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Circulating Water System

Water requirements: - apart from water usage in makingsteam for thermal plant the other fields where bulk amountof water is used are:

1) Bulk requirement of water in thermal power plant forthe purpose of cooling the steam in condenser. Therequirement for this purpose is of the order of 1.5 to 2.0of installation.

2) The second place where large amount of water is usedis in sluicing out ash that is produce by burning thecoal.

3) Also in the coal handling plant water is used forseparation of dust.

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Cooling towers

Cooling tower is important components of thermal powerstation where a limited supply to make up water is available.Cooling towers thus provide flexibility for selection of sitesfor thermal power station even though capital investmentand running costs are generally on the high side.

Types of Cooling Tower

Forced Draft Cooling Tower:- Motor driven fansare located at the base i.e. ground level, below air into thetower from the sides. The top of the tower is open to the airvapour discharge. The main drawback in this type of tower isthat exit velocity is low and this result in reciprocating hot airinto the fan intake. Thus the efficiency of the tower isreduced. The disadvantage of forced draft cooling tower canbe summarized as:-

1) High velocity from the fan located at the base makes adifficult to distribute air evenly over the whole ofpacking.

2) Low height, low velocity of air low wind velocitygenerally results in reciprocating of hot air which leadsto rise in temp and reduction and efficiency.

3) It is the experience that there is rapid growth of fungusinside cooling tower which leads to reduction in theremoval of droplets from the air. Thus the efficiencygets reduced.

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Induced Draft Cooling Tower: - Induced draftis preferred over the forced draft. In an induced draft fan,the fan is located at the top and air enters from the openingslocated at ground level. Air, mixed with vapour, isdischarged through a form stack located at the top of thetower, most of the discharged higher in the atmospherethere by dispersing to a great distance from tower. There iscylindrical RCC structure supported on RCC columns. Hotwater taken to the top of the tower by steel pipes anddischarged on the packaging with distribution system ofpreheats of hole in the tubes. Eliminators of asbestos are

provided at the top to arrest the droplets. The fan is locatedat the top to draw air from the cylinder for dispersion.

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TMD

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Steam Turbine

Steam turbines are used in all of our major coal fired powerstations to drive the generators or alternators, whichproduce electricity. The turbines themselves are driven bysteam generated in 'Boilers' or 'Steam Generators' as

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they are sometimes called.

Energy in the steam after it leaves the boiler is

converted into rotational energy as it passes through theturbine. The turbine normally consists of several stages witheach stage consisting of a stationary blade (or nozzle) and arotating blade. Stationary blades convert the potentialenergy of the steam (temperature and pressure) into kineticenergy (velocity) and direct the flow onto the rotatingblades. The rotating blades convert the kinetic energy into

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forces, caused by pressure drop, which results in the rotationof the turbine shaft. The turbine shaft is connected to agenerator, which produces the electrical energy. Therotational speed is 3000 rpm for Indian System (50 Hz)

systems and 3600 for American (60 Hz) systems.In a typical larger power stations, the steam turbinesare split into three separate stages, the first being the HighPressure (HP), the second the Intermediate Pressure (IP) andthe third the Low Pressure (LP) stage, where high,intermediate and low describe the pressure of the steam.After the steam has passed through the HP stage, it isreturned to the boiler to be re-heated to its originaltemperature although the pressure remains greatly reduced.

The reheated steam then passes through the IP stage and

finally to the LP stage of the turbine.A distinction is made between "impulse" and "reaction"turbine designs based on the relative pressure drop acrossthe stage. There are two measures for pressure drop, thepressure ratio and the percent reaction. Pressure ratio is thepressure at the stage exit divided by the pressure at thestage entrance. Reaction is the percentage isentropicenthalpy drop across the rotating blade or bucket comparedto the total stage enthalpy drop. Some manufacturers utilize

percent pressure drop across stage to define reaction.Steam turbines can be configured in many different

ways. Several IP or LP stages can be incorporated into theone steam turbine. A single shaft or several shafts coupledtogether may be used. Either way, the principles are thesame for all steam turbines. The configuration is decided bythe use to which the steam turbine is put, co-generation orpure electricity production. For cogeneration, the steampressure is highest when used as process steam and at alower pressure when used for the secondary function ofelectricity production.

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Nozzles and Blades

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Steam enthalpy is converted into rotational energy as itpasses through a turbine stage. A turbine stage consists of astationary blade (or nozzle) and a rotating blade (or bucket).

Stationary blades convert the potential energy of the steam(temperature and pressure) into kinetic energy (velocity)and direct the flow onto the rotating blades. The rotatingblades convert the kinetic energy into impulse and reactionforces caused by pressure drop, which results in the rotationof the turbine shaft or rotor.

Steam turbines are machines which must be designed,manufactured and maintained to high tolerances so that thedesign power output and availability is obtained. They are

subject to a number of damage mechanisms, with two of themost important being:

Erosion due to Moisture: -The presence of waterdroplets in the last stages of a turbine causes erosion to theblades. This has led to the imposition of an allowable limit ofabout 12% wetness in the exhaust steam;

Solid Particle Erosion: -The entrainment of erosivematerials from the boiler in the steam causes wear to theturbine blades.

Cogeneration Cycles

In cogeneration cycles, steam is typically generated at ahigher temperature and pressure than required for aparticular industrial process. The steam is expanded througha turbine to produce electricity and the resulting extractionsat the discharge are at the temperature and pressurerequired by the process.

Turbines can be condensing or non-condensing designtypically with large mass flows and comparably low output.

Traditionally, pressures were 6.21 MPa and below with

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temperatures 441oC or lower, although the trend towardshigher levels of each continues.

There are now a considerable number of co-generationsteam turbines with initial steam pressures in the 8.63 to 10

MPa range and steam temperatures of 482 to 510

o

C.

Bearings and Lubrication

Two types of bearings are used to support and locate the

rotors of steam turbines:

1) Journal bearings are used to support the weight of theturbine rotors. A journal bearing consists of two half-cylinders that enclose the shaft and are internally linedwith Babbitt, a metal alloy usually consisting of tin,copper and antimony;

2) Thrust bearings axially locate the turbine rotors. Athrust bearing is made up of a series of Babbitt linedpads that run against a locating disk attached to theturbine rotor. High-pressure oil is injected into thebearings to provide lubrication. The oil is carefullyfiltered to remove solid particles. Specially designedcentrifuges remove any water from the oil.

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Shaft Seals

The shaft seal on a turbine rotor consist of a series of ridgesand groves around the rotor and its housing which present along, tortuous path for any steam leaking through the seal.

The seal therefore does not prevent the steam from leaking,merely reduces the leakage to a minimum. The leakingsteam is collected and returned to a low-pressure part of thesteam circuit.

Turning Gear

Large steam turbines are equipped with "turning gear" toslowly rotate the turbines after they have been shut downand while they are cooling. This evens out the temperaturedistribution around the turbines and prevents bowing of therotors.

Vibration

The balancing of the large rotating steam turbines is acritical component in ensuring the reliable operation of theplant. Most large steam turbines have sensors installed tomeasure the movement of the shafts in their bearings. Thiscondition monitoring can identify many potential problemsand allows the repair of the turbine to be planned before theproblems become serious.

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CHD

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Coal Handling Plant

Coal is delivered by highway truck, rail, and barge or colliership. Some plants are even built near coal mines and coal isdelivered by conveyors. A large coal train called a "unittrain" may be a kilometers (over a mile) long, containing 60cars with 100 tons of coal in each one, for a total load of6,000 tons. A large plant under full load requires at least onecoal delivery this size every day. Plants may get as many asthree to five trains a day, especially in "peak season", duringthe summer months when power consumption is high. Alarge thermal power plant such as the Badarpur ThermalPower Station, New Delhi stores several million tons of coal

for use when there is no wagon supply.

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Modern unloader use rotary dump devices, which eliminateproblems with coal freezing in bottom dump cars. Theunloader includes a train positioner arm that pulls the entiretrain to position each car over a coal hopper. The dumperclamps an individual car against a platform that swivels thecar upside down to dump the coal. Swiveling couplers enablethe entire operation to occur while the cars are still coupled

together. Unloading a unit train takes about three hours.

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Shorter trains may use railcars with an "air-dump", whichrelies on air pressure from the engine plus a "hot shoe" oneach car. This "hot shoe" when it comes into contact with a

"hot rail" at the unloading trestle, shoots an electric chargethrough the air dump apparatus and causes the doors on thebottom of the car to open, dumping the coal through theopening in the trestle. Unloading one of these trains takesanywhere from an hour to an hour and a half. Older unloadermay still use manually operated bottom-dump rail cars and a"shaker" attached to dump the coal. Generating stationsadjacent to a mine may receive coal by conveyor belt ormassive diesel electric- drive trucks.

Coal is prepared for use by crushing the rough coal to piecesless than 2 inches (50 mm) in size. The coal is thentransported from the storage yard to in-plant storage silos byrubberized conveyor belts at rates up to 4,000 tons/hour.

In plants that burn pulverized coal, silos feed coalpulverizes (coal mill) that take the larger 2 inch pieces grindthem into the consistency of face powder, classify them, and

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mixes them with primary combustion air which transportsthe coal to the furnace and preheats the coal to drive offexcess moisture content. In plants that do not burnpulverized coal, the larger 2 inch pieces may be directly fed

into the silos which then feed the cyclone burners, a specifickind of combustor that can efficiently burn larger pieces offuel.

Run-Of-Mine (ROM) Coal

The coal delivered from the mine that reports to the CoalHandling Plant is called Run-of-mine, or ROM, coal. This isthe raw material for the CHP, and consists of coal, rocks,middlings, minerals and contamination. Contamination isusually introduced by the mining process and may includemachine parts, used consumables and parts of groundengaging tools. ROM coal can have a large variability ofmoisture and maximum particle size.

Coal Handling

Coal needs to be stored at various stages of the preparationprocess, and conveyed around the CHP facilities. Coalhandling is part of the larger field of bulk material handling,and is a complex and vital part of the CHP.

StockpilesStockpiles provide surge capacity to various parts of theCHP. ROM coal is delivered with large variations inproduction rate of tones per hour (tph). A ROM stockpile isused to allow the wash plant to be fed coal at lower,constant rate.

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stacking. Stacking in a single cone tends to cause sizesegregation, with coarser material moving out towards thebase. Raw cone ply stacking is when additional cones areadded next to the first cone. Chevron stacking is when the

stacker travels along the length of the stockpile adding layerupon layer of material. Stackers and Reclaimers wereoriginally manually controlled manned machines with noremoteControl. Modern machines are typically semi-automatic orfully automated, with parameters remotely set.

Reclaiming

Tunnel conveyors can be fed by a continuous slot hopper orbunker beneath the stockpile to reclaim material. Front-endloaders and bulldozers can be used to push the coal intofeeders. Sometimes front-end loaders are the only means ofreclaiming coal from the stockpile. This has a low up-frontcapital cost, but much higher operating costs, measured indollars per tone handled.

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Coal Storage Area of the Badarpur Thermal PowerStation, New Delhi

High-capacity stockpiles are commonly reclaimed usingbucket-wheel reclaimers. These can achieve very high rates.

Coal Sampling

Sampling of coal is an important part of the process controlin the CHP. A grab sample is a one-off sample of the coal at apoint in the process stream, and tends not to be veryrepresentative. A routine sample is taken at a set frequency,either over a period of time or per shipment.

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Screening

Screens are used to group process particles into ranges bysize. These size ranges are also called grades. Dewatering

screens are used to remove water from the product. Screenscan be static, or mechanically vibrated. Screen decks can bemade from different materials such as high tensile steel,stainless steel, or polyethylene.

Screening and Separation Unit of Coal Handling Division of aThermal Power Plant

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Magnetic Separation

Magnetic separators shall be used in coal conveying systems

to separate tramp iron (including steel) from the coal.Basically, two types are available. One type incorporatespermanent or electromagnets into the head pulley of a beltconveyor. The tramp iron clings to the belt as it goes aroundthe pulley drum and falls off into a collection hopper ortrough after the point at which coal is charged from the belt.

The other type consists of permanent or electromagnetsincorporated into a belt conveyor that is suspended above abelt conveyor carrying coal. The tramp iron is pulled from

the moving coal to the face of the separating conveyor,which in turn holds and carries the tramp iron to a collectionhopper or trough. Magnetic separators shall be used justahead of the coal crusher, if any, and/or just prior to coaldischarge to the in-plant bunker or silo fill system.

Coal Crusher

Before the coal is sent to the plant it has to be ensured thatthe coal is of uniform size, and so it is passed through coalcrushers. Also power plants using pulverized coal specify amaximum coal size that can be fed into the pulverizer and sothe coal has to be crushed to the specified size using thecoal crusher. Rotary crushers are very commonly used forthis purpose as they can provide a continuous flow of coal tothe pulverizer.

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Pulverizer

Most commonly used pulverizer is the Bowl Mill. The

arrangement consists of 2 stationary rollers and a powerdriven baul in which pulverization takes place as the coalpasses through the sides of the rollers and the baul. Aprimary air induced draught fan draws a stream of heated airthrough the mill carrying the pulverized coal into astationary classifier at the top of the pulverizer. The classifierseparates the pulverized coal from the un pulverized coal.

An external view of a Coal Pulverizer

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Advantages of Pulverized Coal:-

1) Pulverized coal is used for large capacity plants.

2) It is easier to adapt to fluctuating load as there are nolimitations on the combustion capacity.

3) Coal with higher ash percentage cannot be usedwithout pulverizing because of the problem of largeamount ash deposition after combustion.

4) Increased thermal efficiency is obtained through

pulverization.

5) The use of secondary air in the combustion chamberalong with the powered coal helps in creatingturbulence and therefore uniform mixing of the coaland the air during combustion.

6) Greater surface area of coal per unit mass of coalallows faster combustion as more coal is exposed toheat and combustion.

7) The combustion process is almost free from clinker andslag formation.

8) The boiler can be easily started from cold condition incase of emergency.

9) Practically no ash handling problem.

10)The furnace volume required is less as the turbulencecaused aids in complete combustion of the coal withminimum travel of the particles.

The pulverized coal is passed from the pulverizer to theboiler by means of the primary air that is used not only todry the coal but also to heat is as it goes into the boiler. The

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Ash HandlingThe ever increasing capacities of boiler units together withtheir ability to use low grade high ash content coal havebeen responsible for the development of modern day ashhandling systems. The widely used ash handling systemsare:-

1) Mechanical Handling System

2) Hydraulic System

3) Pneumatic System

4) Steam Jet System

The Hydraulic Ash handling system is used at the BadarpurThermal Power Station.

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Ash Handling System of a Thermal Power Plant

Hydraulic Ash Handling System

The hydraulic system carried the ash with the flow of waterwith high velocity through a channel and finally dumps into a

sump. The hydraulic system is divided into a low velocity andhigh velocity system. In the low velocity system the ash fromthe boilers falls into a stream of water flowing into the sump.

The ash is carried along with the water and they areseparated at the sump. In the high velocity system a jet ofwater is sprayed to quench the hot ash. Two other jets forcethe ash into a trough in which they are washed away by thewater into the sump, where they are separated. The moltenslag formed in the pulverized fuel system can also be

quenched and washed by using the high velocity system.The advantages of this system are that its clean, large ashhandling capacity, considerable distance can be traversed,b f ki i i h h

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37154010 Ntpc Project Report (1)

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