Power plant training report

7/31/2019 Power plant training report

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Submitted To: Submitted By:

MECHNICAL DEPT. Ravinder Pal

25095


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Acknowledgement

The tree of knowledge grows best when it has sturdy roots and the strength of them

is clearly dependent upon our intentions. During the journey of knowledge we

meet certain people whoplay a pivotal role in our development and its a privilege

to thank them for the same. So I would take this opportunity in expressing

gratitude towards my mentor and guide in this period of vocational training, Mr.Summit Chaurasia. It would have been extremely difficult to cover this course

without his able guidance. Then Im obliged to thank Mr. Pawan Tiwari sir whotook the pains and interest in explaining the nicks of the thermal power plant.

Im ever thank full to my parents and of course god. In fact, manypeople have contributed to this report and I would love to express my gratitude to

all of them, like Mr. Anil Awasthi, Mr. Chandra Prakash, Mr. Bharat Patel and

many more.

Ravinder Pal


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Abstract

A thermal power station is a power plant in which the prime mover is steam

driven. Water is heated, turns into steam and spins a steam turbine which drives an

electrical generator. After it passes through the turbine, the steam is condensed in a

condenser and recycled to where it was heated; this is known as a Rankine cycle.

The greatest variation in the design of thermal power stations is due to the different

fuel sources. Some prefer to use the term energy center because such facilities

convert forms of heat energy into electricity. Some thermal power plants also

deliver heat energy for industrial purposes, for district heating, or for desalination

of water as well as delivering electrical power. A large part of human CO2

emissions comes from fossil fueled thermal power plants; efforts to reduce these

outputs are various and widespread. At present 54.09% or 93918.38 MW (Data

Source CEA, as on 31/03/2011) of total electricity production in India is from Coal

Based Thermal Power Station. A coal based thermal power plant converts the

chemical energy of the coal into electrical energy. This is achieved by raising the

steam in the boilers, expanding it through the turbine and coupling the turbines tothe generators which converts mechanical energy into electrical energy.


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Contents

Project


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Report

.


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Objectives:-

To learn the basic working of thermal power plants. To learn about various components of the same. To develop the understanding of the operation and maintenance of

turbines.


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Plant Layout


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Products and specifications

Following two are the main products in a thermal power plant

ElectricityElectricity is produced at approximately 15.5 KV after which it is stepped

up to 220 KVfor reduction in losses due to transmission. Then it is connected to

the grid for supply.

AshAsh is the byproduct of coal after its combustion. It can be categorized in two

parts:-

1. Fly ash, which is sold to cement manufacturing organizations like Diamondand

Satna. Earlier they were given away to the same, but since posses certainvalue,

theyre now being sold to them which generates revenues up to twenty lakhs.

2. Ash slurry, it is a waste product which is generally provided to construction

companies for road-filling etc.


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Product flow Chart

Procedure for production of electricity is based on modified Rankine cycle. The

four process ofRankine cycle as used in thermal power plants are as follows:-

Heat addition in boiler. Adiabatic expansion in turbines

Heat rejection in condenser and Adiabatic compression in boiler feed pumps.

This may seem to be a simple enough process, but every step employs various

circuits toaccomplish the required conditions for the fore told steps. Certain

circuits are as follows

Fuel and Ash Circuit.

Air and Gas Circuit.

Feed water and Steam Circuit.

Cooling Water Circuit

Various methods are employed to increase the efficiency of classical rankine cycle byadding devices like air-preheater, economizer, superheater etc


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The input at boiler is the DM water and pulverized coal with air. The DM water is

prepared in the water treatment plant facility where it is deionized and deareated. It

prepared in the scale of neutral liquid i.e. 7ph, although, slightly basic nature is

used.

The coal is prepared at coal handling plant, where it first arrives in wagons. The

coal is taken out from wagons with the help of a machine known as wagon tippler.

The coal is the picked and sent to crushers, where it crushed and then to bunkers.

From bunkers the coal moves on to mills and is finely grounded to a pulverizedform and the fed to the boiler. Then this coal is fed to the boiler and combustion

takes place. The energy of the combustion is helpful in transforming the water into

the steam. This steam is then used to drive the turbine, the turbine shaft drives the

generator. Hence electricity is developed.

The other product, which is ash, is fed into the ash treatment plant and flue gasses

are expelled in the atmosphere.


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Production Process

In a coal based power plant coal is transported from coal mines to the power plant

by railway in wagons or in a merry-go-round system. Coal is unloaded from the

wagons to a moving underground conveyor belt. This coal from the mines is of no

uniform size. So it is taken to the Crusher house and crushed to a size of 25mm.

From the crusher house the coal is either stored in dead storage( generally 20 days

coal supply) which serves as coal supply in case of coal supply bottleneck or to the

live storage(8 hours coal supply) in the raw coal bunker in the boiler house. Raw

coal from the raw coal bunker is supplied to the Coal Mills by a Raw Coal Feeder.The Coal Mills or pulverizer pulverizes the coal to 200 mesh size. The powdered

coal from the coal mills is carried to the boiler in coal pipes by high pressure hot

air. The pulverized coal air mixture is burnt in the boiler in the combustion zone.

Generally in modern boilers tangential firing system is used i.e. the coal nozzles/

guns form tangent to a circle. The temperature in fire ball is of the order of 1300

deg.C. The boiler is a water tube boiler hanging from the top. Water is converted to


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steam in the boiler and steam is separated from water in the boiler Drum. The

saturated steam from the boiler drum is taken to the Low Temperature Superheater,

Platen Superheater and Final Superheater respectively for superheating. The

superheated steam from the final superheater is taken to the High Pressure Steam

Turbine (HPT). In the HPT the steam pressure is utilized to rotate the turbine and

the resultant is rotational energy. From the HPT the out coming steam is taken to

the Reheater in the boiler to increase its temperature as the steam becomes wet at

the HPT outlet. After reheating this steam is taken to the Intermediate Pressure

Turbine (IPT) and then to the Low Pressure Turbine (LPT). The outlet of the LPT

is sent to the condenser for condensing back to water by a cooling water system.

This condensed water is collected in the Hotwell and is again sent to the boiler in a

closed cycle. The rotational energy imparted to the turbine by high pressure steam

is converted to electrical energy in the Generator.

Principal

Coal based thermal power plant works on the principal of Modified Rankine Cycle.


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Components of Coal Fired Thermal Power Station:

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 pulverizers may be ball

mills, rotating drum grinders, or other types of grinders.

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.

Boiler furnace and steam drum

Once water inside the boiler or steam generator, the process of adding the latent

heat of vaporization or enthalpy is underway. The boiler transfers energy to the

water by the chemical reaction of burning some type of fuel.

The water enters the boiler through a section in the convection pass called the

economizer. From the economizer it passes to the steam drum. Once the water

enters the steam drum it goes down the downcomers to the lower inlet waterwall

headers. From the inlet headers the water rises through the waterwalls and is

eventually turned into steam due to the heat being generated by the burners located

on the front and rear waterwalls (typically). As the water is turned into steam/vapor

in the waterwalls, the steam/vapor once again enters the steam drum. The

steam/vapor is passed through a series of steam and water separators and then

dryers inside the steam drum. The steam separators and dryers remove water

droplets from the steam and the cycle through the waterwalls is repeated. Thisprocess 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.


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Furnace explosions due to any accumulation of combustible gases after a trip-out

are avoided by flushing out such gases from the combustion zone before igniting

the coal.

The steam drum (as well as the superheater coils and headers) have air vents and

drains needed for initial startup. The steam drum has internal devices that removes

moisture from the wet steam entering the drum from the steam generating tubes.

The dry steam then flows into the superheate coils.

Superheater:

Coal based power plants can have a superheater and/or reheater section in the

steam generating furnace. Nuclear-powered steam plants do not have such sections

but produce steam at essentially saturated conditions. In a coal based plant, after

the steam is conditioned by the drying equipment inside the steam drum, it is piped

from the upper drum area into tubes inside an area of the furnace known as thesuperheater, which has an elaborate set up of tubing where the steam vapor picks

up more energy from hot flue gases outside the tubing and its temperature is now

superheated above the saturation temperature. The superheated steam is then piped

through the main steam lines to the valves before the high pressure turbine.

Reheater:

Power plant furnaces may have a reheater section containing tubes heated by hot fluegases outside the tubes. Exhaust steam from the high pressure turbine is rerouted to go

inside the reheater tubes to pickup more energy to go drive intermediate or lowerpressure turbines. This is what is called as thermal power.

Fly ash collection:

Fly ash is captured and removed from the flue gas by electrostatic precipitators or

fabric bag filters (or sometimes both) located at the outlet of the furnace and before

the induced draft fan. The fly ash is periodically removed from the collection

hoppers below the precipitators or bag filters. Generally, the fly ash is

pneumatically transported to storage silos for subsequent transport by trucks or

railroad cars.

Bottom ash collection and disposal

At the bottom of the furnace, there is a hopper for collection of bottom ash. This

hopper is always filled with water to quench the ash and clinkers falling down


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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 tothe boiler, losses due to blowdown and leakages have to be made up to maintain adesired water level in the boiler steam drum. For this, continuous make-up water isadded to the boiler water system. Impurities in the raw water input to the plant generallyconsist of calcium and magnesium salts which impart hardness to the water. Hardnessin the make-up water to the boiler will form deposits on the tube water surfaces whichwill lead to overheating and failure of the tubes. Thus, the salts have to be removedfrom the water, and that is done by a water demineralising treatment plant (DM). A DMplant generally consists of cation, anion, and mixed bed exchangers. Any ions in thefinal water from this process consist essentially of hydrogen ions and hydroxide ions,which recombine to form pure water. Very pure DM water becomes highly corrosive

once it absorbs oxygen from the atmosphere because of its very high affinity for oxygen.The capacity of the DM plant is dictated by the type and quantity of salts in the

raw water input. However, some storage is essential as the DM plant may be down formaintenance. For this purpose, a storage tank is installed from which DM water iscontinuously withdrawn for boiler make-up. The storage tank for DM water is made frommaterials not affected by corrosive water, such as PVC. The piping and valves aregenerally of stainless steel. Sometimes, a steam blanketing arrangement or stainlesssteel doughnut float is provided on top of the water in the tank to avoid contact with air.DM water make-up is generally added at the steam space of the surface condenser(i.e., the vacuum side). This arrangement not only sprays the water but also DM watergets deaerated, with the dissolved gases being removed by an air ejector attached to

the condenser

Steam turbine-driven electric generator


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The steam turbine-driven generators have auxiliary systems enabling them to worksatisfactorily and safely. The steam turbine generator being rotating equipmentgenerally has a heavy, large diameter shaft. The shaft therefore requires not onlysupports but also has to be kept in position while running. To minimise the frictionalresistance to the rotation, the shaft has a number of bearings. The bearing shells, in

hich the shaft rotates, are lined with a low friction material like Babbitt metal. Oillubrication is provided to further reduce the friction between shaft and bearing surfaceand to limit the heat generated.

Barring gear

Barring gear (or turning gear) is the mechanism provided to rotate the turbinegenerator shaft at a very low speed after unit stoppages. Once the unit is tripped (i.e.,the steam inlet valve is closed), the turbine coasts down towards standstill. When itstops completely, there is a tendency for the turbine shaft to deflect or bend if allowed toremain in one position too long. This is because the heat inside the turbine casing tends

to concentrate in the top half of the casing, making the top half portion of the shaft hotterthan the bottom half. The shaft therefore could warp or bend by millionths of inches.This small shaft deflection, only detectable by eccentricity meters, would be

enough to cause damaging vibrations to the entire steam turbine generator unit when itis restarted. The shaft istherefore automatically turned at low speed (about one percentrated speed) by the barring gear until it has cooled sufficiently to permit a completestop.

Condenser


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The surface condenser is a shell and tube heat exchanger in which cooling water iscirculated through the tubes. The exhaust steam from the low pressure turbine entersthe shell where it is cooled and converted to condensate (water) by flowing over thetubes as shown in the adjacent diagram. Such condensers use steam ejectors or rotarymotor-driven exhausters for continuousremoval of air and gases from the steam side to

maintain vacuum. For best efficiency, the temperature in the condenser must be kept aslow as practical in order to achieve the lowest possible pressure in the condensingsteam. Since the condenser temperature can almost always be kept significantly below100 C where the vapor pressure of water is much less than atmospheric pressure, thecondenser generally works under vacuum. Thus leaks of noncondensible air into theclosed loop must be prevented. Plants operating in hot climates may have to reduceoutput if their source of condenser cooling water becomes warmer; unfortunately thisusually coincides with periods of high electrical demand for air conditioning.

The condenser generally uses either circulating cooling water from a coolingtower to reject waste heat to the atmosphere, or once-through water from a river, lake orocean.

Feedwater heater

In the case of a conventional steam-electric power plant utilizing a drum boiler, thesurface condenser removes the latent heat of vaporization from the steam as it changesstates from vapour to liquid. The heat content (joules or Btu) in the steam is referred toas enthalpy. The condensate pump then pumps the condensate water through a Airejector condenser and Gland steam exhauster condenser. From there the condensategoes to the deareator where the condenstae system ends and the Feedwater systembegins

Preheating the feedwater reduces the irreversibilities involved in steamgeneration and therefore improves the thermodynamic efficiency of the system.Thisreduces plant operating costs and also helps to avoid thermal shock to the boiler metalwhen the feedwater is introduced back into the steam cycle.

Deaerator

A steam generating boiler requires that the boiler feed water should be devoid of air andother dissolved gases, particularly corrosive ones, in order to avoid corrosion of themetal. Generally, power stations use a deaerator to provide for the removal of air andother dissolved gases from the boiler feedwater. A deaerator typically includes avertical, domed deaeration section mounted on top of a horizontal cylindrical vesselwhich serves as the deaerated boiler feedwater storage tank.

Cooling tower

A cooling tower is a heat rejection device, which extracts waste heat to the atmospherethough the cooling of a water stream to a lower temperature. The type of heat rejectionin a cooling tower is termed evaporative in that it allows a small portion of the waterbeing cooled to evaporate into a moving air stream to provide significant cooling to the


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rest of that water stream. The heat from the water stream transferred to the air streamaises the airs temperature and its relative humidity to 100%, and this air is dischargedto the atmosphere. Evaporative heat rejection devices such as cooling towers arecommonly used to provide significantly lower water temperatures than achievable withair cooled or dry heat rejection devices, like the radiator in a car, thereby achieving

more cost-effective and energy efficient operation of systems in need of cooling.

The cooling towers are of two types: -

1. Natural Draft Cooling Tower2. Mechanized Draft Cooling Tower

i. Forced Draft cooling towerii. Induced Draft cooling toweriii. Balanced Draft cooling tower

Auxiliary systems

Oil system

An auxiliary oil system pump is used to supply oil at the start-up of the steam turbinegenerator. It supplies the hydraulic oil system required for steam turbines main inletsteam stop valve, the governing control valves, the bearing and seal oil systems, therelevant hydraulic relays and other mechanisms.

At a preset speed of the turbine during start-ups, a pump driven by the turbine mainshaft takes over the functions of the auxiliary system.


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Generator heat dissipation

The electricity generator requires cooling to dissipate the heat that it generates. Whilesmall units may be cooled by air drawn through filters at the inlet, larger units generallyrequire special cooling arrangements. Hydrogen gas cooling, in an oil-sealed casing, is

used because it has the highest known heat transfer coefficient of any gas and for itslow viscosity which reduces windage losses. This system requires special handlingduring start-up, with air in the chamber first displaced by carbon dioxide before fillingwith hydrogen. This ensures that the highly flammable hydrogen does not mix withoxygen in the air. The hydrogen pressure inside the casing is maintained slightly higherthan atmospheric pressure to avoid outside air ingress. The hydrogen must be sealedagainst outward leakage where the shaft emerges from the casing. Mechanical sealsaround the shaft are installed with a very small annular gap to avoid rubbing betweenthe shaft and the seals. Seal oil is used to prevent the hydrogen gas leakage toatmosphere. The generator also uses water cooling. Since the generator coils are at apotential of about 22 Kv and water is conductive, an insulating barrier such as Teflon is

used to interconnect the water line and the generator high voltage windings.Demineralized water of low conductivity is used.

Generator high voltage system

The generator voltage ranges from 11 kV in smaller units to 22 kV in larger units. Thegenerator high voltage leads are normally large aluminum channels because of theirhigh current as compared to the cables used in smaller machines. They are enclosed inwell-grounded aluminum bus ducts and are supported on suitable insulators. Thegenerator high voltage channels are connected to step-up transformers for connectingto a high voltage electrical substation (of the order of 115 kV to 520 kV) for furthertransmission by the local power grid. The necessary protection and metering devicesare included for the high voltage leads. Thus, the steam turbine generator and thetransformer form one unit. In smaller units, generating at 11 kV, a breaker is provided toconnect it to a common 11 kV bus system.

Other systems

Monitoring and alarm system

Most of the power plant operational controls are automatic. However, at times, manualintervention may be required. Thus, the plant is provided with monitors and alarmsystems that alert the plant operators when certain operating parameters are seriouslydeviating from their normal range.

Battery supplied emergency lighting and communication

A central battery system consisting of lead acid cell units is provided to supplyemergency electric power, when needed, to essential items such as the power plantscontrol systems, communication systems, turbine lube oil pumps, and emergency


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lighting. This is essential for a safe, damage-free shutdown of the units in an emergencysituation.

TURBINES

A steam turbine is a mechanical device that extracts and converts it into rotary motion.Its modern manifestation was invented by Parsons in 1884. It has almost completelyreplaced the greater thermal efficiency and higher motion, it is particularly suited to beused to drive an electricity generation in the world is by use of steam turbines.

TYPES

Steam turbines are made in a variety of sizes ranging from small


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reciprocating piston steam engine primarily because of its power-to-weight ratio.Because the turbine generates , electrical generator about 80% of all hp (


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Casing or shaft arrangements

These arrangements include single casing, tandem compound and cross compoundturbines. Single casing units are the most basic style where a single casing and shaftare coupled to a generator. Tandem compound are used where two or more casingsare directly coupled together to drive a single generator. A cross compound turbinearrangement features two or more shafts not in line driving two or more generators thatoften operate at different speeds. A cross compound turbine is typically used for manylarge applications.

Principal And design operation:-

An ideal steam turbine is considered to be an isentropic process, or constant entropyprocess, in which the entropy of the steam entering the turbine is equal to the entropy of

the steam leaving the turbine. No steam turbine is truly isentropic, however, withtypical isentropic efficiencies ranging from 20%-90% based on the application of theturbine. The interior of a turbine comprises several sets of blades, or buckets as theyare more commonly referred to. One set of stationary blades is connected to the casingand one set of rotating blades is connected to the shaft. The sets intermesh with certainminimum clearances, with the size and configuration of sets varying to efficiently exploitthe expansion of steam at each stage.

Turbine efficiency


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To maximize turbine efficiency the steam is expanded, doing work, in a number ofstages. These stages are characterized by how the energy is extracted from them andare known as either impulse or reaction turbines.Most steam turbines use a mixture of the reaction and impulse designs: each stagebehaves as either one or the other, but the overall turbine uses both.

Typically, higher pressure sections are impulse type and lower pressure stagesare reaction type.

Impulse turbines

An impulse turbine has fixed nozzles that orient the steam flow into high speed jets.These jets contain significant kinetic energy, which the rotor blades, shaped likebuckets, convert into shaft rotation as the steam jet changes direction. A pressure dropoccurs across only the stationary blades, with a net increase in steam velocity acrossthe stage. As the steam flows through the nozzle its pressure falls from inlet pressure tothe exit pressure (atmospheric pressure, or more usually, the condenser vacuum). Due

to this higher ratio of expansion of steam in the nozzle the steam leaves the nozzle witha very high velocity. The steam leaving the moving blades has a large portion of themaximum velocity of the steam when leaving the nozzle. The loss of energy due to thishigher exit velocity is commonly called the "carry over velocity" or "leaving loss".

REATION TURBINES

In the reaction turbine, the rotor blades themselves are arranged to form

convergent nozzles. This type of turbine makes use of the reaction force produced

as the steam accelerates through the nozzles formed by the rotor. Steam is directedonto the rotor by the fixed vanes of the stator. It leaves the stator as a jet that fills

the entire circumference of the rotor. The steam then changes direction and

increases its speed relative to the speed of the blades. A pressure drop occurs

across both the stator and the rotor, with steam accelerating through the stator and

decelerating through the rotor, with no net change in steam velocity across the

stage but with a decrease in both pressure and temperature, reflecting the work

performed in the driving of the rotor


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Operation and maintenance

When warming up a steam turbine for use, the main steam stop valves (after the boiler)have a bypass line to allow superheated steam to slowly bypass the valve and proceedto heat up the lines in the system along with the steam turbine. Also, a turning gear isengaged when there steam to the turbine to slowly rotate the turbine to ensure evenheating to prevent uneven expansion. After first rotating the turbine by the turning gear,allowing time for the rotor to assume a straight plane (no bowing), then the turning gear

is d the turbine, first to the astern blades then to the ahead blades slowly rotating theturbine at 10 to 15 RPM to slowly warm the turbine.Problems with turbines are now rare and maintenance requirements are relatively small.

Any imbalance of the rotor can lead to vibration, which in extreme cases can lead to ablade letting go and punching straight through the casing. It is, however, essential thatthe turbine be turned with dry steam - that is, superheated steam with a minimal liquidwater content. If water gets into the steam and is blasted onto the blades (moisturecarryover), rapid impingement and erosion of theblades can occur leading to imbalance and catastrophic failure. Also, water entering theblades will result in the destruction of the thrust bearing for the turbine shaft. To preventthis, along with controls and baffles in the boilers to ensure high quality steam,

condensate drains are installed in the steam piping leading to the turbine.


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Speed regulation

The control of a turbine with a governor is essential, as turbines need to be run upslowly, to prevent damage while some applications (such as the generation ofalternating current electricity) require precise speed control. Uncontrolled acceleration ofthe turbine rotor can lead to an overspeed trip, which causes the nozzle valves thatcontrol the flow of steam to the turbine to close. If this fails then the turbine may

continue accelerating until it breaks apart, often spectacularly. Turbines are expensiveto make, requiring precision manufacture and special quality materials. During normaloperation in synchronization with the electricity network, power plants are governed witha five percent droop speed control. This means the full load speed is 100% and the no-load speed is 105%. This is required for the stable operation of the network withouthunting and drop-outs of power plants. Normally the changes in speed are minor.

Adjustments in power output are made by slowly raising the droop curve by increasingthe spring pressure on a centrifugal governor. Generally this is a basic systemrequirement for all power plants because the older and newer plants have to becompatible in response to the instantaneous changes in frequency without dependingon outside communication.


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Power plant training report

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