1. INDUSTRIAL TRAINING REPORT
NATIONAL THERMAL POWER CORPORATION
REPORT SUBMITTED BY:
Rajan kumar choudhary
Scholar No. 091116012
Maulana Azad National Institute Of Technology
I hereby declare that this project is being submitted in fulfilment of
the VOCATIONAL TRAINING PROGRAMME in NTPC Sipat, and
is the result of self done work carried out by me under the guidance of
various Engineers and other officers.
I further declare that the structure and content of this project are
original and have not been submitted before for any purpose.
RAJAN KUMAR CHOUDHARY
SCHOLAR NO. 091116012
B.TECH. (6TH SEMESTER)
MAULANA AZAD NATIONAL INSTITUTE OF TECHNOLOGY
BHOPAL (MADHYA PRADESH), 462051
THERMAL POWER PLANTS
In thermal power stations, mechanical power is produced by a heat
engine that transforms thermal energy, often from combustion of a fuel,
into rotational energy. Most thermal power stations produce steam,
and these are sometimes called steam power stations. Not all thermal
energy can be transformed into mechanical power, according to the
second law of thermodynamics. Therefore, heat loss to the environment
is always there. If this loss is employed as useful heat, for industrial
processes or district heating, the power plant is known as a
cogeneration power plan or CHP (combined heat and power) plant. In
countries where district heating is common, there are dedicated heat
plants called heat-only boiler stations. In thermal power stations,
mechanical power is produced by a heat engine that transforms thermal
energy, often from combustion of a fuel, into rotational energy.
COAL FIRED THERMAL POWER PLANTS
More than half of the electricity generated in the world and three-fourth
generated in India is by using coal as primary fuel.
The function of the coal fired thermal power plant is to convert the chemical
energy available in the coal to Electricity. Several steps are involved in
transforming the energy stored in coal to usable electricity that powers almost
all the amenities of our modern lifestyle.
The conversion from coal to electricity takes place in three stages:
The first conversion of energy takes place in the boiler. Coal is burnt in the
boiler furnace to produce heat. Carbon in the coal and Oxygen in the air
combine to produce Carbon Dioxide and heat.
The second stage is the thermodynamic process:
1. The heat from combustion of the coal boils water in the boiler to produce
steam. In modern power plant, boilers produce steam at a high pressure
2. The steam is then piped to a turbine.
3. The high pressure steam impinges and expands across a number of sets of
blades in the turbine.
4. The impulse and the thrust created rotate the turbine.
5. The steam is then condensed and pumped back into the boiler to repeat
In the third stage, rotation of the turbine rotates the generator rotor to produce
electricity based on Faraday’s Principle of Electromagnetic Induction.
NATIONAL THERMAL POWER
NTPC Limited is the largest power generation company in India. Forbes
Global 2000 for 2011 ranked it 348th
in the world. It is an Indian public sector
company listed on the Bombay Stock Exchange although at present the
Government of India holds 84.5% of its equity. With a current generating
capacity of 39,174 MW, NTPC has embarked on plans to become a 75,000 MW
company by 2017. It was founded on November 7, 1975.
NTPC’s core business is engineering, construction and operation of power
generating plants and providing consultancy to power utilities in India and
The total installed capacity of NTPC in India is as follows at present:
NO. OF PLANTS CAPACITY (MW)
Coal 15 25,375
Gas/Liquid Fuel 7 3,955
Total 22 29,330
Owned By JVs
Coal & Gas 5 3,364
Total 27 32,694
NTPC has been operating its plants at high efficiency levels. Although the
company has 18.10% of the total national capacity, it contributes 28.60% of
total power generation due to its focus on high efficiency.
Coal Based Power Stations:
With 15 coal based power stations, NTPC is the largest thermal power generating company in
the country. The company has a coal based installed capacity of 25,375 MW.
1. Singrauli Uttar Pradesh 2,000
2. Korba Chhattisgarh 2,600
3. Ramagundam Andhra Pradesh 2,600
4. Farakka West Bengal 2,100
5. Vindhyachal Madhya Pradesh 3,760
6. Rihand Uttar Pradesh 2,500
7. Kahalgaon Bihar 2,340
8. NCTPP, Dadri Uttar Pradesh 1,820
9. Talcher Kaniha Orissa 3,000
10. Feroze Gandhi, UnchaharUttar Pradesh 1,050
11. Talcher Thermal Orissa 460
12. Simhadri Andhra Pradesh 1,500
13. Tanda Uttar Pradesh 440
14. Badarpur Delhi 705
15. Sipat Chhattisgarh 2,980
Coal Based Joint Ventures:
(Owned by JVs)
1. Durgapur West Bengal 120
2. Rourkela Orissa 120
3. Bhilai Chhattisgarh 574
4. Kanti Bihar 110
5. IGSTPP, Jhajjar Haryana 500
BASIC POWER PLANT CYCLE
The Rankine cycle is a cycle that converts heat into work. The heat is supplied
externally to a closed loop, which usually uses water. This cycle generates about
80% of all electric power used throughout the world, including virtually all
solar thermal, biomass, coal and nuclear power plants. It is named after William
John Macquorn Rankine, a Scottish polymath. The Rankine cycle is the
fundamental thermodynamic underpinning of the steam engine.
There are four processes in the Rankine cycle; the processes are identified by
number in the diagram above:
Process 1-2: The working fluid is pumped from low to high pressure, as
the fluid is a liquid at this stage the pump requires little input energy.
Process 2-3: The high pressure liquid enters a boiler where it is heated at
constant pressure by an external heat source to become a dry saturated
vapour. The input energy required can be easily calculated using  or h-s
chart or enthalpy-entropy chart also known as steam tables.
Process 3-4: The dry saturated vapour expands through a turbine,
generating power. This decreases the temperature and pressure of the
vapour, and some condensation may occur. The output in this process can
be easily calculated using the Enthalpy-entropy chart or the steam tables.
Process 4-1: The wet vapour then enters a condenser where it is
condensed at a constant pressure to become a saturated liquid.
In an ideal Rankine cycle the pump and turbine would generate no entropy and
hence maximize the net work output. Processes 1-2 and 3-4 would be
represented by vertical lines on the T-S diagram and more closely resemble that
of the Carnot cycle. The Rankine cycle shown here prevents the vapor ending
up in the superheat region after the expansion in the turbine.
Reasons for Considering Rankine Cycle as an Ideal Cycle
For Steam Power Plants:
1) It is very difficult to build a pump that will handle a mixture of liquid and
vapour at state 1’ (refer T-s diagram) and deliver saturated liquid at state 2’. It is
much easier to completely condense the vapor and handle only liquid in the
2) In the rankine cycle, the vapor may be superheated at constant pressure from
3 to 3” without difficulty. In a Carnot cycle using superheated steam, the
superheating will have to be done at constant temperature along path 3-5.
During this process, the pressure has to be dropped. This means that heat is
transferred to the vapor as it undergoes expansion doing work. This is difficult
to achieve in practice.
Variations of the basic Rankine cycle:
The overall thermodynamic efficiency (of almost any cycle) can be increased by
raising the average heat input temperature of that cycle.
Increasing the temperature of the steam into the superheat region is a simple
way of doing this. There are also variations of the basic Rankine cycle which
are designed to raise the thermal efficiency of the cycle in this way; two of these
are described below.
Rankine cycle with reheat
In this variation, two turbines work in series. The first accepts vapor
from the boiler at high pressure. After the vapor has passed through the first
turbine, it re-enters the boiler and is reheated before passing through a second,
lower pressure turbine. Among other advantages, this prevents the vapor from
condensing during its expansion which can seriously damage the turbine blades,
and improves the efficiency of the cycle, as more of the heat flow into the cycle
occurs at higher temperature.
0mixed with the fluid at 4 (both at the same pressure) to end up with the
saturated liquid at 7. This is called "direct3
+. contact heating". The Regenerative Rankine cycle (with minor variants) is
commonly used in real power stations.
Another variation is where 'bled steam' from between turbine stages is sent to
feedwater heaters to preheat the water on its way from the condenser to the
boiler. These heaters do not mix the input steam and condensate, function as an
ordinary tubular heat exchanger, and are named "closed feed water heaters".
The regenerative features here effectively raise the nominal cycle heat input
temperature, by reducing the addition of heat from the boiler/fuel source at the
relatively low feedwater temperatures that would exist without regenerative
feedwater heating. This improves the efficiency of the cycle, as more of the heat
flow into the cycle occurs at higher temperature.
THERMAL EFFICIENCY OF A POWER PLANT
The efficiency of the power plant can be calculated by using the
EFFICIENCY = (WT - WP ) / QH
WP = work done by pump.
WT = work done by turbine.
QH = heat energy given to the boiler.
Any thermal power plant should be able to maintain a high
efficiency of the cycle, as it will indicate the fraction of heat being
utilized usefully. There are several ways of increasing the thermal
efficiency of the power plant. Some of the ways are mentioned below:
a) An increase in the initial pressure of the steam can raise the
efficiency of the power plant.
b) The efficiency of the plant can be increased by raising the
initial temperature of the steam without raising the steam pressure.
c) Intermediate re-heating of the steam improves the thermal
efficiency of the plant. An increase in the initial pressure of the steam
increases the efficiency, but the wetness fraction of such a steam also
grows at the end of expansion. Such a high wetness can cause wear of
the blades of the last stages of a steam turbine. Therefore, the steam
from boiler after partial expansion in the first stage of turbine is fed to
d) Thermal efficiency of the plant can be increased by carrying
out regenerative heating of the feed water. Such heating of the water is
carried out by using the heat of steam partly tapped from the turbine.
RAW MATERIALS USED FOR POWER
Basic Raw Materials Used Are:
a. LDO (Light Diesel Oil): It is used for ignition purpose
b. HFO (Heavy Furnace Oil): It is used to raise the
temperature inside the furnace up to the ignition
temperature of coal. When ignition temperature is
reached, combustion of coal starts. HFO is highly
viscous in nature.
c. Pulverised Coal: The most important fuel for thermal power
generation is coal. Coal is converted into pulverised coal by
mills. It is a mixture of carbon, sulphur, hydrogen, oxygen,
Beside producing steam from water in the boiler it is alse used for
condensing vapour in the condenser. The water used for condensing
purpose is the demineralised water
There are 4 types of air used in NTPC:
a. Primary Air: This is used to remove moisture from coal and to
transport coal from mill to coal nozzles. This is done with the
help of P.A Fans.
b. Secondary Air: This is also known as combustion air as it helps
in combustion. It provides oxygen and extra air (over fire air)
from reduction processes during combustion. It is preheated to
help in combustion.
GENERAL LAYOUT OF POWER PLANT:
1. Cooling tower 10. Steam governor valve 19. Superheater
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. Economiser
6. Low pressure turbine 15. Coal hopper 24. Air preheater
7. Boiler feed pump 16. Pulverised fuel mill 25. Precipitator
8. Condenser 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 pulverised 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
superheater (19) where its temperature and pressure increase rapidly to around,
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 is then passed by a
feed pump (7) through a deaerator (12), and pre-warmed, first in a feed heater
(13) powered by steam drawn from the high pressure set, and then in the
economiser (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 21 kV). This is stepped up by the unit transformer (4) to a voltage
more suitable for transmission (typically 410 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 (7).
There are four main circuits in any thermal power plant and these are:
1. Coal & Ash Circuit: This circuit deals mainly with feeding the boiler
with coal for combustion purposes and taking care of the ash that is
generated during the combustion process and includes equipment that is
used to handle the transfer of coal and ash.
2. Air & Gas Circuit: We know that air is one of the main components of
the fire triangle and hence necessary for combustion. Since lots of coal is
burnt inside the boiler it needs a sufficient quantity of air which is
supplied using either forced draught or induced draught fans. The exhaust
gases from the combustion are in turn used to heat the ingoing air through
a heat exchanger before being let off in the atmosphere. The equipment
which handles all these processes fall under this circuit.
3. Feed Water & Steam Circuit: This section deals with supplying of
steam generated from the boiler to the turbines and to handle the outgoing
steam from the turbine by cooling it to form water in a condenser so it
can be reused in the boiler plus making good any losses due to
4. Cooling Water Circuit: This part of the thermal power plant deals with
handling of the cooling water required in the system. Since the amount of
water required to cool the outgoing steam from the boiler is substantial, it
is either taken from a nearby water source such as a river, or it is done
through evaporation if the quantity of cooling water available is limited.
Capacity : 3 X 220 MW Stage-I
2 X 500 MW Stage-II
1 X 500 MW Stage-III
Water Source : From Hasdeo right bank canal
Coal Mines : Gevra Mines of SECL Korba.
Coal Trans : By dedicated MGR (34.8 Kms)
1) COAL HANDLING PLANT (C.H.P): It is the place
where everything associated with coal is taken care of, right from its
arrival from the coal mines, to its treatment and finally it being fed to the
boiler. For the plant at Sipat, the coal is provided by the Gevra mines
under the S.E.C.L, with the help of a merry-go-round (MGR).When the
coal is supplied at the CHP, the coal is moved along the track hopper
towards the crusher, where the lumps of coal are crushed into 20 mm
sized particles, from where they may be stored in the stack-yard, or sent
to the bunkers before being fed into the boilers.
An important thing to be noted is that, before feeding the coal to be
fired, we employ light diesel oil (LDO) and heavy furnace oil (HFO) to
fire the boiler and create a stable flame. For this we employ oil guns,
which are placed near the boilers, which release oil for being fired.
Thus, the function of the CHP is to improve the heating value of coal,
and to make its handling easier. Coal is supplied through conveyor belt
2) MILL: The coal particles are ground into finer sized granules.
The coal which is stored in the bunker is sent into the mill, which is
primarily a ball type, in which a drum contains a ball, and when the drum
rotates the ball also does, and this causes the coal particles caught in
between to be ground.
After grinding, the coal is then passed through a desired size of
mesh, so that any coal particle not properly ground is not allowed
through. Then the coal is forced by a blast of air coming from the primary
air fans to enter the boiler. Coal is fed to the mills from the bunkers via
the raw coal feeders.
Another type of mill is the ball and race mill, in which the coal passes
between the rotating elements again and again until it has been pulverized
to the desired degree of fineness. However, there is greater wear in this
mill as compared to other pulverisers.
INTERIOR OF BOWL MILL
Bowl Mill Structure
(3) BOILER: A boiler is the central component of a power plant,
and it is the unit where the steam required for driving the turbine is
Boilers are categorized according to several parameters. They may be
classified on the basis of the presence of a drum, on the no. of passes, on
the type of firing used to burn the fuel, on the type of tubing used, and so
The components of a boiler and their functions are given below:
a) DRUM: It is a type of storage tank much higher than the level at
which the boiler is placed, and it is also a place where water and steam
are separated. First the drum is filled with water coming from the
economizer, from where it is brought down with the help of down-
comers, entering the bottom ring headers. From there they enter the riser,
which carries the water (which now is a liquid-vapor mixture), back to
the drum. Now, the steam is sent to be superheated.
b) SUPER HEATERS: The steam is then sent for superheating. This
takes place in three stages. In the first stage, the steam is sent to a simple
super heater, known as the low temperature super heater, after which the
second stage consists of several divisional panels. The final stage
involves further heating in a Platen super heater, after which the steam is
released for driving the turbine. After the HP stage of the turbine the
steam is re-heated and then again released.
Super heated steam also has several merits such as increased working
capacity, ability to increase the plant efficiency, lesser erosion and so on.
It is also of interest to know that while the super heater increases the
temperature of the steam, it does not change the pressure. There are
different stages of superheaters besides the sidewalls and extended
sidewalls. The first stage consists of LTSH(low temperature superheater),
which is conventional mixed type with upper & lower banks above the
economiser assembly in rear pass. The other is Divisional Panel
Superheater which is hanging above in the first pass of the boiler above
the furnace. The third stage is the Platen Superheater from where the
steam goes into the HP turbine through the main steam line. The outlet
temperature & pressure of the steam coming out from the super-heater is
540 degrees Celsius & 147 kg/cm2
c) WATER WALLS: The water from the bottom ring header is then
transferred to the water walls, where the first step in the formation of
steam occurs. This steam then enters the drum.
d) ECONOMIZER: The economizer is a tube-shaped structure
which contains water from the boiler feed pump. This water is heated up
by the hot flue gases which pass through the economizer layout, which
then enters the drum. The economizer is usually placed below the second
pass of the boiler, below the Low Temperature Superheater. As the flue
gases are being constantly produced due to the combustion of coal, the
water in the economizer is being continuously being heated up, resulting
in the formation of steam to a partial extent. Economiser tubes are
supported in such a way that sagging, deflection & expansion will not
occur at any condition of operation.
A de-aerator is a device that is widely used for the removal of air and
other dissolved gases from the feed water to steam-generating boilers.
Most de-aerators are designed to remove oxygen down to levels of 7
ppb by weight (0.005 cm³/L) or less.
There are two basic types of de-aerators, the tray-type and the spray-
The tray-type (also called the cascade-type) includes a vertical
domed de-aeration section mounted on top of a horizontal cylindrical
vessel which serves as the de-aerated boiler feed water storage tank.
The spray-type consists only of a horizontal (or vertical)
cylindrical vessel which serves as both the de-aeration section and the
boiler feed water storage tank.
Turbine: After the boiler the most vital unit is the turbine, which works on
the steam generated from the boiler. Thus a turbine employed in a thermal
power plant is a steam turbine. The initial steam is admitted ahead of the
blading via two main stop and control valve combinations. The turbine unit of
any thermal power plant is not a single stage operation, rather it consists of
a) High Pressure Turbine Stage (HPT Stage): This stage takes
place immediately after the Platen super heater stage. This is the first
stage of the turbine operation.
b) Intermediate Pressure Turbine Stage (IPT Stage): After the
HPT stage, the steam gets saturated and, consequently, gets cooled. It is,
therefore, first sent back to the boiler unit to be reheated, after which it is
sent to the IPT stage. Its section is of double flow construction with
horizontally split casings.
c) Low Pressure Turbine Stage (LPT Stage): After the IPT, the
steam gets cooled to an intermediate extent, thus directly entering the
LPT, where it gets saturated. Its casing is of the three-shell design. After
this stage the water enters the condenser, which is connected to a
condensate extraction pump.
The shaft of the turbine is connected to the generator. The purpose of
the generator is to convert the mechanical shaft energy it receives from
the turbine into electrical energy. Steam turbine driven AC synchronous
generators (alternators) are of two or four pole designs. Large generators
have cylindrical rotors with minimum heat dissipation surface and so they
have forced ventilation to remove the heat. Such generators generally use
an enclosed system with air or hydrogen coolant. The gas picks up the
heat from the generator and gives it up to the circulating water in the heat
Every turbine, except the LPT, has a stop valve and a regulating valve
attached to it. The stop valve is used to stop the flow of steam, whenever
required, whereas the regulating valve is also a kind of a flow controlling
device. Each turbine also has an inlet and an outlet pipe for the steam to
enter and exit, respectively. Between the HPT-IPT combine and the IPT-
LPT combine is attached a bearing assembly. It is constructed using a
cross around pipe.
After the steam leaves the turbine, it enters the condenser . The
condenser is meant to receive the steam from the turbine, condense it and
to maintain a pressure at the exhaust lower than the atmospheric pressure.
The functions of each of these auxiliary units are self-explanatory.
ASH HANDLING AND DISPOSAL:
Any power plant which does not have a proper planning for handling
of ash and its subsequent disposal may not be able to get the necessary
clearance from the authorities, as it may be detrimental to the safety of
There are two types of ash handling methods: dry ash handling and
wet ash handling. Dry ash handling is carried out by storing the ash
deposited in large pits, where in the wet ash handling method, the ash is
deposited into large reservoirs or ponds.
Fly ash is captured from the flue gas by using electrostatic
precipitators, which are located at the outlet of the furnace and before the
ID fans. Also at the bottom of every boiler, a hopper is 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 from the
furnace. There is an arrangement for the crushing of the clinkers and for
conveying the crushed clinkers and bottom ash to the storage site.
ASSOCIATED SYSTEMS IN A POWER PLANT
There are several systems in a power plant which assist the main units
to carry out their functions properly:
1) PA FANS: The primary air fans are used to carry the pulverized
coal particles from the mills to the boiler. They are also used to
maintain the coal-air temperature. The specifications of the PA fan used
at the plant under investigation are: axial flow, double stage, reaction
fan. A PA fan uses 0.72% of plant load for a 500 MW plant.
2) FD FANS: The forced draft fans, also known as the secondary air
fans are used to provide the secondary air required for combustion, and
to maintain the wind box differential pressure. Specifications of the FD
fans are: axial flow, single stage, impulse fan.
FD fans use 0.36% of plant load for a 500 MW plant.
The main purpose of an ID fan is to suck the flue gas through all the
above mentioned equipments and to maintain the furnace pressure. ID
fans use 1.41% of plant load for a 500 MW plant.
3) SCANNER AIR FAN: Scanner air fan is used to provide air to the
scanner. For a tangentially fired boiler, the vital thing is to maintain a
stable ball of flame at the centre. A scanner is used to detect the flame,
to see whether it is proper and stable. The fan is used to provide air to
the scanner, and it is a crucial component which prevents the boiler
4) BOILER FEED PUMP: The auxiliary component which consumes
the maximum amount of power earmarked for such purposes is the
boiler feed pump.
5) AIR PRE-HEATERS: Air pre-heaters are used to take heat from
the flue gases and transfer it to the incoming air. They are of two types:
6) ELECTROSTATIC PRECIPITATORS: They are used to
separate the ash particles from the flue gases. In this the flue gas is
allowed into the
there are several
placed at a
from each other.
gases enter, a
causes the gas
ionize and stick to the plates, whereas the ash particles fall down and
are collected in a hopper attached to the bottom of the ESP. The flue gas
is allowed to cool down and is then released to the ID fan to be sent to
7) COOLING TOWERS: Cooling towers are used to remove the
heat from the condensers. In this cooling water is discharged to the
condenser with the help of a cooling water pump. This water enters the
condenser through several tubes. Steam entering the condenser from the
turbine after expansion further loses heat and condenses, while the
water circulating inside the tube gains heat and goes back to the cooling
tower. Inside the tower is a cooling fan which takes the heat from this
batch of water, which is then sent back again for the cycle to be
repeated. It is hence known as a regenerating cycle.
8) CHIMNEY: These are tall RCC structures with single & multiple
flues. Here, for I & II we have 1 chimney, for unit III there is 1 chimney
& for units & V there is 1 chimney. So number of chimneys is 5 and the
height of each is 275 metres.
9) COAL BUNKER: These are in process storage used for storing
crushed coal from the coal handling system. Generally, these are made
up of welded steel plates. Normally, these are located on top of mills to
aid in gravity feeding of coal.
10) REHEATER: The function of reheater is to reheat the steam
coming out from the high pressure turbine to a temperature of 540
degrees Celsius. It is composed of two sections: the rear pendant section
is located above the furnace arc & the front pendant section is located
between the rear water hanger tubes & the Platen super-heater section.
A boiler is a closed vessel in which water or other fluid is heated. The heated or
vaporized fluid exits the boiler for use in various processes or heating
The Basic Theory Behind All
Volume of one unit mass of steam is thousand times that of water. When water
is converted to steam in a closed vessel the pressure will increase. Boiler uses
this principle to produce High Pressure steam.
Conversion of Water to Steam Evolves in three stages:
Heating the water from cold condition to boiling point or saturation
temperature – Sensible Heat Addition.
Water boils at saturation temperature to produce steam – Latent Heat
Heating steam from saturation temperature to higher temperature called
Superheating to increase the power plant output and efficiency.
TANGENTIAL TYPE BOILER
In a tangential firing system the coal is pulverized in coal mills and is carried by
primary air to the furnace through coal pipes. The mills are usually a constant
airflow mill and have a specific output in mass of coal ground depending on
coal properties like hardness, moisture, and fineness which affect the mill
output. In direct tangential firing systems, the pulverized coal from the coal
mills is directly taken to the furnace. The total quantity of coal to be pulverized
for a specified size of boiler at a designed efficiency will depend on the calorific
value of coal. The secondary air required for combustion is sent into the furnace
through a wind box housing the coal nozzles, oil guns, and the secondary air
nozzles. Behind the coal nozzles there are fuel-air dampers which are used for
keeping the flame front away from the coal nozzles by at least one meter from
the tip. This is required to prevent the coal nozzle tips from getting burnt due to
radiation from coal flame. The flame front is predominantly affected by the
volatile matter in coal and the fuel air damper is modulated for controlling the
flame front. As the fuel air dampers are opened, more secondary air goes
through this damper and physically pushes the flame front away. However,
when the flame front is already away from the nozzle tip, the fuel air damper
needs to be closed fully.
Boiler Fittings And Accessories:
Safety Valve: It is used to relieve pressure and prevent possible
explosion of a boiler.
Water Level Indicators: They show the operator the level of fluid in
the boiler, also known as a sight glass, water gauge or water column is
Bottom Blowdown Valves: They provide a means for removing solid
particulates that condense and lie on the bottom of a boiler. As the
name implies, this valve is usually located directly on the bottom of the
boiler, and is occasionally opened to use the pressure in the boiler to
push these particulates out.
Continuous Blowdown Valve: This allows a small quantity of water to
escape continuously. Its purpose is to prevent the water in the boiler
becoming saturated with dissolved salts. Saturation would lead to
foaming and cause water droplets to be carried over with the steam - a
condition known as priming. Blowdown is also often used to monitor
the chemistry of the boiler water.
Flash Tank: High pressure blowdown enters this vessel where the
steam can 'flash' safely and be used in a low-pressure system or be
vented to atmosphere while the ambient pressure blowdown flows to
Automatic Blowdown/Continuous Heat Recovery System: This
system allows the boiler to blowdown only when makeup water is
flowing to the boiler, thereby transferring the maximum amount of heat
possible from the blowdown to the makeup water. No flash tank is
generally needed as the blowdown discharged is close to the
temperature of the makeup water.
Hand holes: They are steel plates installed in openings in "header" to
allow for inspections & installation of tubes and inspection of internal
Steam Drum Internals: A series of screen, scrubber & cans (cyclone
Low- Water Cutoff: It is a mechanical means (usually a float switch)
that is used to turn off the burner or shut off fuel to the boiler to
prevent it from running once the water goes below a certain point. If a
boiler is "dry-fired" (burned without water in it) it can cause rupture or
Surface Blowdown Line: It provides a means for removing foam or
other lightweight non-condensible substances that tend to float on top
of the water inside the boiler.
Circulating Pump: It is designed to circulate water back to the boiler
after it has expelled some of its heat.
Feedwater Vheck Valve or Clack Valve: A non-return stop valve in
the feedwater line. This may be fitted to the side of the boiler, just
below the water level, or to the top of the boiler.
Top Feed: A check valve (clack valve) in the feedwater line, mounted
on top of the boiler. It is intended to reduce the nuisance of limescale.
It does not prevent limescale formation but causes the limescale to be
precipitated in a powdery form which is easily washed out of the
Desuperheater Tubes or Bundles: A series of tubes or bundles of
tubes in the water drum or the steam drum designed to cool
superheated steam. Thus is to supply auxiliary equipment that doesn't
need, or may be damaged by, dry steam.
Chemical Injection Line: A connection to add chemicals for
controlling feedwater pH.
Most boilers produce steam to be used at saturation temperature; that is,
saturated steam. Superheated steam boilers vaporize the water and then
further heat the steam in a super-heater. This provides steam at much higher
temperature, but can decrease the overall thermal efficiency of the steam
generating plant because the higher steam temperature requires a higher flue
gas exhaust temperature. There are several ways to circumvent this problem,
typically by providing an economizer that heats the feed water, a combustion
air heater in the hot flue gas exhaust path, or both. There are advantages to
superheated steam that may, and often will, increase overall efficiency of
both steam generation and its utilisation: gains in input temperature to a
turbine should outweigh any cost in additional boiler complication and
expense. There may also be practical limitations in using wet steam, as
entrained condensation droplets will damage turbine blades.
Super-heater operation is similar to that of the coils on an air conditioning
unit, although for a different purpose. The steam piping is directed through
the flue gas path in the boiler furnace. The temperature in this area is
typically between 1,300–1,600 degree Celsius (2,372–2,912 °F). Some super-
heaters are radiant type; that is, they absorb heat by radiation. Others are
convection type, absorbing heat from a fluid such as a gas. Some are a
combination of the two types. Through either method, the extreme heat in the
flue gas path will also heat the super-heater steam piping and the steam
within. While the temperature of the steam in the super-heater rises, the
pressure of the steam does not: the turbine or moving pistons offer a
continuously expanding space and the pressure remains the same as that of
A steam turbine is a mechanical device that extracts steam energy from
pressurized steam, & converts it into useful mechanical work. The simplest
turbines have one moving part, a rotor assembly, which is a shaft or drum with
blades attached. Moving fluid acts on the blades, or the blades react to the flow,
so that they move and impart rotational energy to the rotor.
TYPES OF TURBINE:
IMPULSE TURBINE: These turbines change the direction of flow of a
high velocity fluid or gas jet. The resulting impulse spins the turbine and
leaves the fluid flow with diminished kinetic energy. There is no pressure
change of the fluid or gas in the turbine rotor blades (the moving blades),
as in the case of a steam or gas turbine, all the pressure drop takes place
in the stationary blades (the nozzles). Before reaching the turbine, the
fluid's pressure head is changed to velocity head by accelerating the fluid
with a nozzle. Pelton wheels and de Laval turbines use this process
exclusively. Impulse turbines do not require a pressure casement around
the rotor since the fluid jet is created by the nozzle prior to reaching the
blading on the rotor. Newton's second law describes the transfer of
energy for impulse turbines.
REACTION TURBINE: These turbines develop torque by reacting to
the gas or fluid's pressure or mass. The pressure of the gas or fluid
changes as it passes through the turbine rotor blades. A pressure casement
is needed to contain the working fluid as it acts on the turbine stage(s) or
the turbine must be fully immersed in the fluid flow (such as with wind
turbines). The casing contains and directs the working fluid and, for water
turbines, maintains the suction imparted by the draft tube. Francis
turbines and most steam turbines use this concept. For compressible
working fluids, multiple turbine stages are usually used to harness the
expanding gas efficiently. Newton's third law describes the transfer of
energy for reaction turbines.
TURBINES USED IN NTPC: Steam Turbines of the following make are
being used in NTPC:
KWU, Siemens (Germany)
ABB- Alstom (Germany)
GEC- Alstom (U.K)
Turbine Layout in NTPC
There are 3 types of Turbines used in Thermal Power Plants:
1) HP TURBINE:
Single Flow Cylinder
Thermal expansion up to 16mm
2) IP TURBINE:
Axially Split Design In Both Casings
Blade Shrouded & Inverted T-Root Design
3) LP TURBINE:
Pressure is Very Less
Three Shell Design
All Casings Axially Split
Exhaust Hood Spray Arrangement
2 Free Standing Blades in the end
ASH HANDLING SYSTEM
Types of ash generated:
1. Fly ash ( approx. 20%)
2. Bottom ash ( approx. 80%)
BOTTOM ASH SYSTEM:
The systems used in Bottom Ash System below a furnace are:
1. Intermittent type , water impounded hoppers & jet pump system
2. Continuous type dry bottom ash hopper and submerged scrapper
Intermittent Type, Water Impounded Hoppers & Jet
Flow diagram of hopper overflow water in jetFlow diagram of hopper overflow water in jet
pump systempump system
Sludge to ash
To ash water
P/p house by
BOTTOM ASH HANDLING SYSTEM
Continuous Type Dry Bottom Ash Hopper & Submerged
This system of ash disposal is entirely different from previous one. In
this system water along with the ash is collected in the hopper. Ash
absorbs the sufficient moisture, excess water is taken out with the help
of overflow tank, ash is taken into the scrapper conveyor, from where it
is send to grinder with the help of conveyor belt, then with the help of
trench it is send to the bottom ash transfer slurry pump, which leads it to
the ash slurry pump house from where it is send to the ash dykes.
Fly Ash Handling System
The fly ash handling system consists of two modes, namely:
1. Wet mode & 2.Dry mode
Ash evacuation from ESP hoppers:
Ash evacuation from ESP hopper can be either:
1.Vacuum evacuation with the help of vacuum pump
2.Pressure evacuation by evacuation compressor and air lock tank
Flow diagram of wet fly ash systemFlow diagram of wet fly ash system
Trench to A.S. P. House
The fly ash is collected in the ESP hopper are send into wetting head
where they are changed from dry to wet condition then these mixture is
subjected to air washer which is connected to vacuum pump which
sucks off the air and thus in turn create vacuum. Then finally it travels
from discharge pipe, seal box, through fly ash slurry trench into A.S.
They are used to separate the ash particles from the flue gases. In this the flue
gas is allowed into the ESP, where there are several metallic plates placed at a
certain distance from each other. When these gases enter, a very high
potential difference is applied, which causes the gas particles to ionize and
stick to the plates, whereas the ash particles fall down and are collected in a
hopper attached to the bottom of the ESP. The flue gas is allowed to cool
down and is then released to the ID fan to be sent to the chimney.
LOSSES DURING OPERATION &
MAINTAINANCE OF PLANT
It increases friction & resistance. It can be due
to Chemical deposits, Solid particle damage, Corrosion Pitting & Water
erosion. As a thumb rule, surface roughness of about 0.05 mm can lead to
a decrease in efficiency of 4%.
Turbine end Gland Leakages
About 2 - 7.5 kW is lost per stage if clearances are increased by
0.025 mm depending upon LP or HP stage.
Drag Loss: Due to difference in the velocities of the steam & water
particles, water particles lag behind & can even take different
trajectory leading to losses.
Sudden condensation can create shock disturbances & hence losses.
About 1% wetness leads to 1% loss in stage efficiency.
4)OFF DESIGN LOSSES:
Losses resulting due to turbine not operating with design terminal
Change in Main Steam pressure & temperature.
Change in HRH pressure & temperature.
Condenser Back Pressure
Convergent-Divergent nozzles are more prone to Off Design losses
then Convergent nozzles as shock formation is not there in
5)PARTIAL ADMISSION LOSSES:
In Impulse turbines, the controlling stage is fed with means of nozzle
boxes, the control valves of which open or close sequentially.
At some partial load some nozzle boxes can be partially open /
Shock formation takes place as rotor blades at some time are full of
steam & at some other moment, devoid of steam leading to
6)LOSS DUE TO EROSION OF LP LAST STAGE BLADES:
Erosion of the last stage blades leads to considerable loss of energy.
Also, It is the least efficient stage.
Erosion in the 10% length of the blade leads to decrease in 0.1% of
The Operational Efficiency of this thermal project is about
All the minor & major sections in the thermal project had been
visited & also understood to the best of my knowledge. I believe that
this training has made me well versed with the various processes in
the power plant.