SUMMER TRAINING

REPORT21 st JUNE to 31 st JULY

SUBMITED BY:-Rohit Juyal08/ME/94BSATIM, FBD.

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ABOUT THE COMPANY

NTPC, the largest power Company in India, was setup in 1975 to accelerate power development in the country. It is among the world’s largest and most efficient power generation companies. In Forbes list of World’s 2000 Largest Companies 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 coal based power stations (23,395 MW), 7 gas based power stations (3,955 MW) and 4 power stations in Joint Ventures (1,794 MW). The company has power generating facilities in all major regions of the country. It plans to be a 75,000 MW company by 2017.

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NTPC has gone beyond the thermal power generation. It has diversified into hydro power, coal mining, power equipment manufacturing, 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 share on 31 Mar 2008 in the total installed capacity of the country was 19.1% and it contributed 28.50% of the total power generation of the country during 2007-08. NTPC has set new benchmarks for the power industry both in the area of power plant construction and operations. With its experience and expertise in the power sector, NTPC is extending consultancy services to various organizations in the power business. It provides consultancy in the area of power plant constructions and power generation to companies in India and abroad. In November 2004, NTPC came out with its Initial Public Offering (IPO) consisting of 5.25% as fresh issue and 5.25% as offer for sale by Government of India. NTPC thus became a listed company with Government holding 89.5% of the equity share capital and rest held by Institutional Investors and

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Public. The issue was a resounding success. NTPC is among the largest five companies 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 well on its way to realize its vision of being "A world class integrated power major, powering India's growth, with increasing global presence".

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Thermal power station employs a great number of equipment performing number of complexity processes. The ultimate aim being the production of electricity. In order to have stable generating condition always a balance has to be achieved so that heat input is equal to electricity output and losses. BTPS is designed by the central water and power commission.

Approval capacity

750 MW

Installed capacity

705 MW

Location

New 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 Power Plant

Selection of site for a thermal power plant is one of the most important aspects which an effect the efficiency of a thermal power plant. For the overall economy of the plant, the following points should be considered while selecting a site:-

Availability of coal: - Steam power station should be located near mines so that minimum cost of fuel is maintained. However if such a plant is to be installed at such a place where coal is not available, the adequate facilities should be made for transportation of coal. Like the railway lines which are provided in such case of BTPS.

Availability of water: - A large amount of water is necessary for the condenser therefore such a plant should 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 well

interconnected with roads or and railway system for the efficient transportation of material.

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

Nearness of the road centers: - This is very important

factor when considering dc transmission. If an ac system is employed than this factor is less important because ac can be transmitted at high voltage with consequent reduce transmission cause. That is why; it is possible to install the plant away from the load centers provided other conditions are favorable.

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Ash disposal facilities: - The quantity of ash to handle is as large as 1500 to 2000 tons per day. The disposal of large quantities of ash from the power station can be into the river, see or lake or can be brought into useful purposes like in the manufacturing of bricks as done by BTPS.

Distance from populated areas: - Since there is a lot ash from the power station there will be lot smokes fumes that make the surrounding of plant very hazardous.

BMD

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FUNCTIONING

Functioning of thermal power plant:

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

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Detailed process of power generation in aThermal power plant:

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

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

3) Steam Turbine: The steam generated in the boiler is sent through a steam turbine. The turbine has blades that rotate when high velocity steam flows across them. This rotation of turbine blades is used to generate electricity.

4) Generator: A generator is connected to the steam turbine. When the turbine rotates, the generator produces electricity which is then passed on to the power distribution systems.

5) Special mountings: There is some other equipment like the economizer and air pre-heater. An economizer uses the heat from the exhaust gases to heat the feed water. An air pre-heater heats the air sent into the combustion chamber to improve the efficiency of the combustion process.

6) Ash collection system: There is a separate residue and ash collection system in place to collect all the waste

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materials from the combustion process and to prevent them from escaping into the atmosphere. Apart from this, there are various other monitoring systems and instruments in place to keep track of the functioning of all the devices. This prevents any hazards from taking place in the plant.

There are basically three main units 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 governor valve

19. Super heater

2. Cooling water pump 11. High pressure turbine

20. Forced draught fan

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 extraction pump

16. Pulverized fuel mill

25. Precipitator

8. Condensor 17. Boiler drum 26. Induced draught fan

9. Intermediate pressure turbine

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 pulverized fuel mill (16). There it is mixed with preheated air (24) driven by the forced draught fan (20). The hot air-fuel mixture is forced at high pressure into the boiler where it rapidly ignites. Water of a high purity flows vertically up the tube-lined walls of the boiler, where it turns into steam, and is passed to the boiler drum, where steam is separated from any remaining water. The steam passes through a manifold in the roof of the drum into the pendant superheated (19) where its temperature and pressure increase rapidly to around 200 bar and 540ｰC, sufficient to make the tube walls glow a dull red. The steam is piped to the high pressure turbine (11), the first of a three-stage turbine process. A steam governor valve (10) allows for both manual control of the turbine and automatic set-point following. The steam is exhausted from the high pressure turbine, and reduced in both pressure and temperature, is returned to the boiler reheater (21). The reheated steam is then passed to the intermediate pressure turbine (9), and from there passed directly to the low pressure turbine set (6). The exiting steam, now a little above its boiling point, is brought into thermal contact with cold water (pumped in from the cooling tower) in the condensor (8), where it condenses rapidly back into water, creating near vacuum-like conditions inside the condensor chest. The condensed water

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is then passed by a feed pump (7) through a desecrator (12), and pre-warmed, first in a feed heater (13) powered by steam drawn from the high pressure set, and then in the economizer (23), before being returned to the boiler drum. The cooling water from the condensor is sprayed inside a cooling tower (1), creating a highly visible plume of water vapor, before being pumped back to the condensor (8) in cooling water cycle. The three turbine sets are sometimes coupled on the same shaft as the three-phase electrical generator (5) which generates an intermediate level voltage (typically 20-25 kV). This is stepped up by the unit transformer (4) to a voltage more suitable for transmission (typically 250-500 kV) and is sent out onto the three-phase transmission system (3). Exhaust gas from the boiler is drawn by the induced draft fan (26) through an electrostatic precipitator (25) and is then vented through the chimney stack (27).

Steam Generator or Boiler

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

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Pulverized coal is air-blown into the furnace from fuel nozzles at the four corners and it rapidly burns, forming a large fireball at the center. The thermal radiation of the fireball heats the water that circulates through the boiler tubes near the boiler perimeter. The water circulation rate in the boiler is three to four times the throughput and is typically driven by pumps. As the water in the boiler circulates it absorbs heat and changes into steam at 700 ｰF (370 ｰC) and 3,200 psi (22.1MPa). It is separated from the water inside a drum at the top of the furnace. The saturated steam is introduced into superheat pendant tubes that hang in the hottest part of the combustion gases as they exit the furnace. Here the steam is superheated to 1,000 ｰF (540 ｰC) to prepare it for the turbine.

The steam generating boiler has to produce steam at the high purity, pressure and temperature required for the steam turbine that drives the electrical generator. The generator includes the economizer, the steam drum, the chemical dosing equipment, and the furnace with its steam generating tubes and the super heater 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 bag house) and the flue gas stack.

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Schematic diagram of a coal-fired power plant steam generator

For units over about 210 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.

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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 tubes which are surrounded by water. Depending on where the tubes are horizontal or vertical. They are further classified as horizontal or vertical tube boilers. They may be internally or externally fed. An internally fed has grate or combustion chamber enclosed with in the boiler shell. In an externally fed boiler the setting, including furnace and grates is separate and distant from boiler shell. A fire tube boiler is simple compact and rugged in construction. Its initial cost is low. A vertical fire tube boiler occupies less floor space. However they are economical only for low pressure and are there for available in small sizes having steam capacity of about 15000kg/hr.

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

Water Tube Boiler

In this boiler water flows inside the tubes and hot gases flows outside the tubes are interconnected to common water channels and to steam outlet. Water tube boilers are classifieds as vertical full. The number of drum may be one or more.

The circulation of water in the boiler may be natural or forced through the action pumps. Forced circulation has the advantage 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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Boiler Furnace and Steam Drum

Once water is 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 down comers to the lower inlet water wall headers. From the inlet headers the water rises through the water walls and is eventually turned into steam due to the heat being generated by the burners located on the front and rear water walls (typically). As the water is turned into steam/vapor in the water walls, the steam/vapor once again enters the steam drum.

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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 the water droplets from the steam and the cycle through the water walls 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 super heater coils and headers) have air vents and drains needed for initial startup. The steam drum has an internal device that removes moisture from the wet steam entering the drum from the steam generating tubes. The dry steam then flows into the super heater coils.

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Geothermal plants need no boiler since they use naturally occurring steam sources. Heat exchangers may be used where the geothermal steam is very corrosive or contains excessive suspended solids. Nuclear plants also boil water to raise steam, either directly passing the working steam through the reactor or else using an intermediate heat exchanger.

Fuel Preparation System

In 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 pulverize may be ball mills, rotating drum grinders, or other types of grinders.

Some power stations burns 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 1000C before being pumped through the furnace fuel oil spray nozzles.

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Boiler Side of the Badarpur Thermal Power Station, New Delhi

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.

Fuel Firing System and Igniter System

From the pulverized coal bin, coal is blown by hot air through the furnace coal burners at an angle which imparts a swirling motion to the powdered coal to enhance mixing of

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the coal powder with the incoming preheated combustion air and thus to enhance the combustion.

To provide sufficient combustion temperature in the furnace before igniting the powdered coal, the furnace temperature is raised by first burning some light fuel oil or processed natural gas (by using auxiliary burners and igniters provide for that purpose).

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

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

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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 Disposal

At the bottom of every boiler, a hopper has been provided for collection of the bottom ash from the bottom of the furnace. 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 Storage

Since there is continuous withdrawal of steam and continuous return of condensate to the boiler, losses due to blow-down and leakages have to be made up for so as to maintain 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 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 water demineralising treatment plant (DM).

A DM plant generally consists of cation, anion and mixed bed exchangers. The final water from this process consists essentially of hydrogen ions and hydroxide ions which is the chemical composition of pure water. The DM water, being very pure, becomes highly corrosive once it

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absorbs oxygen from the atmosphere because of its very high affinity for oxygen absorption.

A DM plant generally consists of cation, anion and mixed bed exchangers. The final water from this process consists essentially of hydrogen ions and hydroxide ions which is the chemical composition of pure water. The DM water, being very pure, becomes highly corrosive once it absorbs oxygen from the atmosphere because of its very high affinity for oxygen absorption.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 atmospheric air. DM water make-up is generally added at the steam space of the surface condenser (i.e., the vacuum side). This arrangement not only sprays the water but also DM water gets deserted, with the dissolved gases being removed by the ejector of the condenser 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 basic requirement of power plant station. The boiler treatment plant (WTP) is meeting this requirement. The water, which is used in the boiler circuit, must be in pure form to avoid corrosion of boiler tubes, scale formation on the inside surface of various pressure parts and avoid silica carry one to turbine. The main objective of WTP is to remove all impurities from the water being sent to boiler in order that the steam generated is pure and boiler can give uninterrupted service.

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

Clarifloculator

To the chlorinator raw water chemicals are added in the form of solutions and violent turbulence chemicals are adequately mixed in a flush mix. The water is then led to central chamber of Clarifloculator having rotary type of arrangement. The water from outer than led into clear water storage tank. The accumulated sedge at the bottom is then pumped out in slurry form.

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Demineralization

The filtered water as processed above is having by now only dissolved salts as impurities in the form of ions. This water is then pushed through a strongly acidic cation exchange resins in hydrogen form. The resin is chemically similar to plastic but is treated in such a way so as to confer ion exchange properties. All cation periling in water are exchanged for hydrogen ion only. The result is that the neutral 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 chloride sulphate nitrate etc.

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

Carbonate and bicarbonate ions are mostly decomposed under such strong acid medium to carbon dioxide. The rest decomposed into carbonic acid.

The anion exchange resin bed but packed with strongly basic resins.

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 making steam for thermal plant the other fields where bulk amount of water is used are:

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

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

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

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Cooling towers

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

Types of Cooling Tower

Forced Draft Cooling Tower:- Motor driven fans are located at the base i.e. ground level, below air into the tower from the sides. The top of the tower is open to the air vapour discharge. The main drawback in this type of tower is that exit velocity is low and this result in reciprocating hot air into the fan intake. Thus the efficiency of the tower is reduced. The disadvantage of forced draft cooling tower can be summarized as:-

1) High velocity from the fan located at the base makes a difficult to distribute air evenly over the whole of packing.

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

3) It is the experience that there is rapid growth of fungus inside cooling tower which leads to reduction in the removal of droplets from the air. Thus the efficiency gets reduced.

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Induced Draft Cooling Tower: - Induced draft is preferred over the forced draft. In an induced draft fan, the fan is located at the top and air enters from the openings located at ground level. Air, mixed with vapour, is discharged through a form stack located at the top of the tower, most of the discharged higher in the atmosphere there by dispersing to a great distance from tower. There is cylindrical RCC structure supported on RCC columns. Hot water taken to the top of the tower by steel pipes and discharged on the packaging with distribution system of preheats of hole in the tubes. Eliminators of asbestos are provided at the top to arrest the droplets. The fan is located at the top to draw air from the cylinder for dispersion.

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TMD

Steam Turbine

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Steam turbines are used in all of our major coal fired power stations to drive the generators or alternators, which produce electricity. The turbines themselves are driven by steam 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 the turbine. The turbine normally consists of several stages with each stage consisting of a stationary blade (or nozzle) and a rotating blade. 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 rotating blades convert the kinetic energy into

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forces, caused by pressure drop, which results in the rotation of the turbine shaft. The turbine shaft is connected to a generator, which produces the electrical energy. The rotational 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 turbines are split into three separate stages, the first being the High Pressure (HP), the second the Intermediate Pressure (IP) and the 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 is returned to the boiler to be re-heated to its original temperature 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 across the stage. There are two measures for pressure drop, the pressure ratio and the percent reaction. Pressure ratio is the pressure at the stage exit divided by the pressure at the stage entrance. Reaction is the percentage isentropic enthalpy drop across the rotating blade or bucket compared to 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 the one steam turbine. A single shaft or several shafts coupled together may be used. Either way, the principles are the same for all steam turbines. The configuration is decided by the use to which the steam turbine is put, co-generation or pure electricity production. For cogeneration, the steam pressure is highest when used as process steam and at a lower pressure when used for the secondary function of electricity production.

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Nozzles and Blades

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Steam enthalpy is converted into rotational energy as it passes through a turbine stage. A turbine stage consists of a stationary 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 rotating blades convert the kinetic energy into impulse and reaction forces caused by pressure drop, which results in the rotation of the turbine shaft or rotor.

Steam turbines are machines which must be designed, manufactured and maintained to high tolerances so that the design power output and availability is obtained. They are subject to a number of damage mechanisms, with two of the most important being:

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

Solid Particle Erosion: - The entrainment of erosive materials from the boiler in the steam causes wear to the turbine blades.

Cogeneration Cycles

In cogeneration cycles, steam is typically generated at a higher temperature and pressure than required for a particular industrial process. The steam is expanded through a turbine to produce electricity and the resulting extractions at the discharge are at the temperature and pressure required by the process.

Turbines can be condensing or non-condensing design typically 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 towards higher levels of each continues.

There are now a considerable number of co-generation steam turbines with initial steam pressures in the 8.63 to 10 MPa range and steam temperatures of 482 to 510 oC.

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 the turbine rotors. A journal bearing consists of two half-cylinders that enclose the shaft and are internally lined with Babbitt, a metal alloy usually consisting of tin, copper and antimony;

2) Thrust bearings axially locate the turbine rotors. A thrust bearing is made up of a series of Babbitt lined pads that run against a locating disk attached to the turbine rotor. High-pressure oil is injected into the bearings to provide lubrication. The oil is carefully filtered to remove solid particles. Specially designed centrifuges remove any water from the oil.

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Shaft Seals

The shaft seal on a turbine rotor consist of a series of ridges and groves around the rotor and its housing which present a long, 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 leaking steam is collected and returned to a low-pressure part of the steam circuit.

Turning Gear

Large steam turbines are equipped with "turning gear" to slowly rotate the turbines after they have been shut down and while they are cooling. This evens out the temperature distribution around the turbines and prevents bowing of the rotors.

Vibration

The balancing of the large rotating steam turbines is a critical component in ensuring the reliable operation of the plant. Most large steam turbines have sensors installed to measure the movement of the shafts in their bearings. This condition monitoring can identify many potential problems and allows the repair of the turbine to be planned before the problems become serious.

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CHD

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Coal Handling Plant

Coal is delivered by highway truck, rail, and barge or collier ship. Some plants are even built near coal mines and coal is delivered by conveyors. A large coal train called a "unit train" may be a kilometers (over a mile) long, containing 60 cars with 100 tons of coal in each one, for a total load of 6,000 tons. A large plant under full load requires at least one coal delivery this size every day. Plants may get as many as three to five trains a day, especially in "peak season", during the summer months when power consumption is high. A large thermal power plant such as the Badarpur Thermal Power 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 eliminate problems with coal freezing in bottom dump cars. The unloader includes a train positioner arm that pulls the entire train to position each car over a coal hopper. The dumper clamps an individual car against a platform that swivels the car upside down to dump the coal. Swiveling couplers enable the 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", which relies on air pressure from the engine plus a "hot shoe" on each car. This "hot shoe" when it comes into contact with a "hot rail" at the unloading trestle, shoots an electric charge through the air dump apparatus and causes the doors on the bottom of the car to open, dumping the coal through the opening in the trestle. Unloading one of these trains takes anywhere from an hour to an hour and a half. Older unloader may still use manually operated bottom-dump rail cars and a "shaker" attached to dump the coal. Generating stations adjacent to a mine may receive coal by conveyor belt or massive diesel electric- drive trucks.Coal is prepared for use by crushing the rough coal to pieces less than 2 inches (50 mm) in size. The coal is then transported from the storage yard to in-plant storage silos by rubberized conveyor belts at rates up to 4,000 tons/hour.

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

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mixes them with primary combustion air which transports the coal to the furnace and preheats the coal to drive off excess moisture content. In plants that do not burn pulverized coal, the larger 2 inch pieces may be directly fed into the silos which then feed the cyclone burners, a specific kind of combustor that can efficiently burn larger pieces of fuel.

Run-Of-Mine (ROM) Coal

The coal delivered from the mine that reports to the Coal Handling Plant is called Run-of-mine, or ROM, coal. This is the raw material for the CHP, and consists of coal, rocks, middlings, minerals and contamination. Contamination is usually introduced by the mining process and may include machine parts, used consumables and parts of ground engaging tools. ROM coal can have a large variability of moisture and maximum particle size.

Coal Handling

Coal needs to be stored at various stages of the preparation process, and conveyed around the CHP facilities. Coal handling 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 the CHP. ROM coal is delivered with large variations in production rate of tones per hour (tph). A ROM stockpile is used to allow the wash plant to be fed coal at lower, constant rate.

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Coal Handling Division of Badarpur Thermal Power Station, New Delhi

A simple stockpile is formed by machinery dumping coal into a pile, either from dump trucks, pushed into heaps with bulldozers or from conveyor booms. More controlled stockpiles are formed using stackers to form piles along the length of a conveyor, and declaimers to retrieve the coal when required for product loading, etc.

Taller and wider stockpiles reduce the land area required to store a set tonnage of coal. Larger coal stockpiles have a reduced rate of heat lost, leading to a higher risk of spontaneous combustion.

Stacking

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Traveling, lugging boom stackers that straddle a feed conveyor are commonly used to create coal stockpiles. Stackers are nominally rated in tph (tones per hour) for capacity and normally travel on a rail between stockpiles in the stockyard. A stacker can usually move in at least two directions typically: horizontally along the rail and vertically by luffing its boom. Luffing of the boom minimizes dust by reducing the height that the coal needs to fall to the top of the stockpile. The boom is luffed upwards as the stockpile height grows.

Wagon Tripler at Badarpur Thermal Power Station, New Delhi

Some stackers are able to rotate by slewing the boom. This allows a single stacker to form two stockpiles, one on either side of the conveyor. Stackers are used to stack into different patterns, such as cone stacking and chevron

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stacking. Stacking in a single cone tends to cause size segregation, with coarser material moving out towards the base. Raw cone ply stacking is when additional cones are added next to the first cone. Chevron stacking is when the stacker travels along the length of the stockpile adding layer upon layer of material. Stackers and Reclaimers were originally manually controlled manned machines with no remoteControl. Modern machines are typically semi-automatic or fully automated, with parameters remotely set.

Reclaiming

Tunnel conveyors can be fed by a continuous slot hopper or bunker beneath the stockpile to reclaim material. Front-end loaders and bulldozers can be used to push the coal into feeders. Sometimes front-end loaders are the only means of reclaiming coal from the stockpile. This has a low up-front capital cost, but much higher operating costs, measured in dollars per tone handled.

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Coal Storage Area of the Badarpur Thermal Power Station, New Delhi

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

Coal Sampling

Sampling of coal is an important part of the process control in the CHP. A grab sample is a one-off sample of the coal at a point in the process stream, and tends not to be very representative. 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 by size. These size ranges are also called grades. Dewatering screens are used to remove water from the product. Screens can be static, or mechanically vibrated. Screen decks can be made from different materials such as high tensile steel, stainless steel, or polyethylene.

Screening and Separation Unit of Coal Handling Division of a Thermal 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 incorporates permanent or electromagnets into the head pulley of a belt conveyor. The tramp iron clings to the belt as it goes around the pulley drum and falls off into a collection hopper or trough after the point at which coal is charged from the belt. The other type consists of permanent or electromagnets incorporated into a belt conveyor that is suspended above a belt 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 collection hopper or trough. Magnetic separators shall be used just ahead of the coal crusher, if any, and/or just prior to coal discharge to the in-plant bunker or silo fill system.

Coal Crusher

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

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Pulverizer

Most commonly used pulverizer is the Bowl Mill. The arrangement consists of 2 stationary rollers and a power driven baul in which pulverization takes place as the coal passes through the sides of the rollers and the baul. A primary air induced draught fan draws a stream of heated air through the mill carrying the pulverized coal into a stationary classifier at the top of the pulverizer. The classifier separates 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 no limitations on the combustion capacity.

3) Coal with higher ash percentage cannot be used without pulverizing because of the problem of large amount ash deposition after combustion.

4) Increased thermal efficiency is obtained through pulverization.

5) The use of secondary air in the combustion chamber along with the powered coal helps in creating turbulence and therefore uniform mixing of the coal and the air during combustion.

6) Greater surface area of coal per unit mass of coal allows faster combustion as more coal is exposed to heat and combustion.

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

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

9) Practically no ash handling problem.

10) The furnace volume required is less as the turbulence caused aids in complete combustion of the coal with minimum travel of the particles.

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The pulverized coal is passed from the pulverizer to the boiler by means of the primary air that is used not only to dry the coal but also to heat is as it goes into the boiler. The secondary air is used to provide the necessary air required for complete combustion. The primary air may vary anywhere from 10% to the entire air depending on the design of the boiler. The coal is sent into the boiler through burners. A very important and widely used type of burner arrangement is the Tangential Firing arrangement.

Tangential Burners-:

The tangential burners are arranged such that they discharge the fuel air mixture tangentially to an imaginary circle in the center of the furnace. The swirling action produces sufficient turbulence in the furnace to complete the combustion in a short period of time and avoid the necessity of producing high turbulence at the burner itself. High heat release rates are possible with this method of firing.

The burners are placed at the four corners of the furnace. At the Badarpur Thermal Power Station five sets of such burners are placed one above the other to form six firing zones. These burners are constructed with tips that can be angled through a small vertical arc. By adjusting the angle of the burners the position of the fire ball can be adjusted so as to raise or lower the position of the turbulent combustion region. When the burners are tilted downward the furnace gets filled completely with the flame and the furnace exit gas temperature gets reduced. When the burners are tiled upward the furnace exit gas temperature increases. A difference of 100 degrees can be achieved by tilting the burners.

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Ash Handling

The ever increasing capacities of boiler units together with their ability to use low grade high ash content coal have been responsible for the development of modern day ash handling systems. The widely used ash handling systems are:-

1) Mechanical Handling System

2) Hydraulic System

3) Pneumatic System

4) Steam Jet System

The Hydraulic Ash handling system is used at the Badarpur Thermal 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 water with high velocity through a channel and finally dumps into a sump. The hydraulic system is divided into a low velocity and high velocity system. In the low velocity system the ash from the boilers falls into a stream of water flowing into the sump. The ash is carried along with the water and they are separated at the sump. In the high velocity system a jet of water is sprayed to quench the hot ash. Two other jets force the ash into a trough in which they are washed away by the water into the sump, where they are separated. The molten slag 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 ash handling capacity, considerable distance can be traversed, absence of working parts in contact with ash.

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