SUMMER INTERNSHIP REPORT
REPORTON DETAILED STUDY OF COMBINED CYCLE GAS POWER GENERATION
& COMPARATIVE STUDY WITH COAL POWER PLANT 1st June2011-25th
July2011
SUBMITTED BY: PENUGONDA HARDEEP DAPARTHY SRI SATYA CHANAKYA
B.Tech Power system engineering University of Petroleum &
Energy Studies
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ACKNOWLEDGEMENT
It is a great honor to take the responsibility to thank the
distinguished personalities who have always accorded top priority
to reform motivation and to carry out our mini project
successfully.
We owe our sincere gratitude to Mr soumitra ray, Plant Head,
REL-SPS for giving us this opportunity.
With respectful regards we would like to thank our guide
Mr.Chetan Tanki ,HEAD(O&M), REL-SPS for his excellent guidance
and support throughout our Internship.
We also express our profound thanks to all engineers in O&M
, REL-SPS for helping us having a live exposure.
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TABLE OF CONTENTS I. BRIEF DETAILS OF THE PLANT PART I OVERVIEW
OF COMBINED CYCLE POWER PLANT Page No
1. INTRODUCTION TO COMBINED CYCLE 2. GT POWER PLANT 3.
COMPRESSOR 4. COMBUSTION CHAMBER 5. GAS TURBINE 6. GT SUPPORT
SYSTEM 7. HRSG 8. GT FUELS 9. STEAM TURBINE 10. CONDENSER 11.
DEARATOR 12. COOLING TOWER 13. EMISSION CONTROL 14. SWITCH YARD
6 8 9 11 12 19 27 31 33 34 38 39 40 41
PART- II3
15. COMPARATIVE STUDY OF COAL THERMAL POWER PLANTS AND CCPP
15.1. INTRODUCTION 15.2. NEED FOR COMPARISON 15.3. METHODOLOGY 15.4
STEPS FOR CALCULATIONS 16. COMPARISON 16.1. PRINCIPLE OF OPERATION
16.2. BASED ON FUEL 16.3. BASED ON FUEL COST 16.4. BASED ON INITIAL
SET UP 16.5. BASED ON RUNNING COST 16.6. BASED ON PLF 16.7. BASED
ON FUEL HANDLING 17. BASED ON AUXILIARIES 18. BASED ON EFFECIENCY
19. BASED ON EMISSION CONTROL 20. ADVANTAGES OF COMBINED CYCLE 21.
CONCLUSIONS III. REFERENCES 45 45 47 48 51 52 53 54 55 57 58 58 59
63 64 65 66 66
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1.
INTRODUCTION TO COMBINED CYCLE
A gas fired combined cycle power plant also known as, combined
cycle gas turbine power plant combines the strengths of two thermal
cycles in an ideal fashion, electricity production with gas turbine
along with the steam turbine. It is simply known as CCGT combined
cycle gas turbine. A combined cycle power plant operates on the
combination of two basic cycles. They are
Brayton cycle (Gas turbine) Rankine cycle (steam turbine) These
two cycles are in series during the operation. The first cycle is
the Brayton cycle (gas turbine) thereafter comes the use of Rankine
cycle( steam turbine).
Therefore, simply a combined cycle power plant = Brayton cycle +
Rankine cycle A combined cycle is basically a closed cycle in which
there is a loss of only a minimum amount of heat, as the heat from
the outlet of Gas turbine is utilized in the Steam turbine and the
main objective is to utilize all the available heat energy in a
power system. Hence a combined cycle power plant can achieve more
efficiency when compared to that of other thermal power plants. A
combined cycle encompasses a large range of capabilities for both
50 Hz and 60 Hz operating frequencies. The gas cycle efficiency can
be calculated using the equation:5
Ncc = Nb + Nr (Nb*Nr) Where, Nb - Brayton gas turbine cycle
efficiency Nr Rankine cycle efficiency The efficiency that is
obtained theoretically is not perfect and contains some operational
losses. Hence the efficiency obtained will be less than
theoretically obtained The following diagram explains the basic
operation of a combined cycle power plant with step wise
description.
Figure 1
Energy flow diagram of combined cycle
1.1 ADVANTAGES OF COMBINED CYCLE:6
The combined cycle power plant is more beneficial than the
conventional coal thermal power plant. The following are some
advantages that are incurred with the use of combined cycle power
plant operation.
A combined cycle power plant handling is easy when compared to
coal thermal power plant
The efficiencies that are obtained in a combined cycle are
better when compared to other means of power generation.
The waste heat evolved from the gas turbine is further utilized
in steam generation from the HRSG, thus become highly beneficial
economically also.
The pollution contents evolved are also less when compared to
coal power plants thus keeping the environment safe.
2. GAS TURBINE POWER PLANT PRINCIPLE OF OPERATION: A gas turbine
converts the kinetic energy of the air and fuel into the mechanical
energy. 2.1 ADVANTAGES:
The condition from no load to full load is achieved in few
minutes and not in hours. Initial set up cost is easy The gas
turbine produces more useful power from the same unit size and
weight. The gas turbine has high efficiency.
2.2 DISADVANTAGES:
The system is dependent on external means as considerable
quantity of cooling water is required for pre cooling.
The response to the load variation is poor comparatively.7
3. COMPRESSOR3.1 DESCRIPTION: A compressor is a rotating device
which is mainly used to compress the air which is taken from the
air inlet system. A compressor consists of rotor and stator,
likewise in turbine, in which the rotor blades rotate and each row
of rotor blades are followed by the row of stator blades. 3.2
OPERATION: A compressor uses air as its working medium. It
compresses the air and increases its pressure energy. Later it will
discharge air in to the combustion chamber. It works on the
conservation of energy principle. It imposes pressure energy the
air. 3.3 TYPES: There are two types of compressor. They are
centrifugal type and axial flow type compressors. CENTRIFUGAL
COMPRESSOR: Centrifugal compressor consists of an impeller and a
diffuser. The impeller imparts high kinetic energy to the air
whereas the diffuser converts that high kinetic energy into
pressure energy. The pressure ratio of 2 to 3 can be obtained for a
single stage compressor, where as it can be up to 20 for a three
stage compressor. The compressors can have single or double inlet.
The single inlet compressors can have an air flow of 15 to
300m3/min where as for double inlet compressors the air flow can be
above 300 m3/min. the efficiency of a centrifugal compressor may be
80-90%. The efficiency of a multi stage compressor is generally
less than the single stage compressor on account of more pressure
losses. AXIAL FLOW COMPRESSOR: The axial flow compressor consists
of a series of rotor and stator stages with the circumferences
gradually decreasing along with the air flow direction. The blades
are fixed on the rotor and rotor blade is fixed on the shaft. The
shaft blades are fixed on the stator casing. The air flows along
the
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axis of the rotor. The kinetic energy is given to the air as it
passes through the rotor and part of it is converted into the
pressure. The no of stages required for maintaining a pressure
ratio 5 are more than 16. An air filter is imparted before the air
enters in to the compressor for the purpose of cleaning because the
deposition of dust on the rotor blades reduces the efficiency of
the compressor. Advantages:
The axial flow compressors have high isentropic efficiency when
compared to the centrifugal type compressors.
These have high flow rate and can handle more air flow for same
weight and size of machine.
3.4 COMPONENTS OF COMPRESSOR: The compressor is having 16
stages, in which two extractions are made at 5 th stage and 10th
stage for blow off system. A compressor contains following parts in
it. STATOR: The stator consists of the stationery or fixed blades
which are located just after the row of rotor blades. Thus each set
of stator blades with rotor blades constitute a stage. Likewise,
there are 16 stages in the compressor. ROTOR: The rotor is the
rotating device on which the blades are fixed. The blades are the
moving blades which are helpful in increasing the pressure of the
air, which will be discharged into the gas turbine. INLET SECTION
The front portion of the compressor consists of comparatively large
size of blades which takes air from the inlet section. The total
compressor has 16 stages.
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DISCHARGE SECTION: The discharge section of the compressor is
tapered towards the gas turbine inorder to increase the pressure of
the inlet air. BLOW OFF SYSTEM: The blow system is mainly designed
when the turbine runs less than its rated speed. This is done
inorder to avoid the phenomenon of surge in the compressor. It
consists of a tank, from which the air is blown off through two
lines, one from the 5th stage of compressor and other from the 10th
stage of compressor. These two lines are provided with some valves
and filters before admitting the air in to the compressor. 4.
COMBUSTION CHAMBER 4.1 PROCESS: The combustion is the process in
which a substance is oxidized to give heat which is an important
factor for high output in a power plant. As the gas turbine is a
continuous flow process the combustion has to occur continuously.
In a combustion chamber chemical reaction takes place between the
fuel and the air which yields many products, out of which some are
used for turbine expansion where the rest will be discharged in to
the atmosphere. In a combined cycle power plants, generally two
combustion chambers are located on each side after the compressor,
which exhausts in to the turbine. There are 8 burners in each
combustion chamber. 4.2 CLASSIFICATION Generally combustion
chambers are of different types. The mostly used types are silo
type and axial type. But the mostly used combustion chambers are
silo type chambers which are elevated sideways to the gas turbine.
These are generally two in number.
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5. GAS TURBINE 5.1 INTRODUCTION: Gas turbines are relatively new
in generation of electricity. The first practical gas turbine used
to ran for electricity generation was used in 1939 at neuchatel,
switzerland and was developed by Brown bovery company. Gas turbines
are classified into 2 types based on their application. They are 1)
air craft engine 2) land based gas turbine The aircraft engine
finds its application in air craft engines, as the name indicates
where as the land based gas turbine finds its application in power
plants. Generally one will assume that gas turbines use gas as its
fuel. How ever, a gas turbine uses compressor to suck in the air
and compress, a combustor to add fuel to it and a turbine to expand
and to give a rated output. Gas turbines ar IC engines. For a gas
turbine to expand the steam ,there must be a pressure ratio
developed along its ends. So for creating a pressure ratio there
should install the air compressor before it compresses the air
adiabatically (no exchange of heat between system and the
surroundings). Generally ,more stages of a turbine are always
preferred in a gas power plant as this reduce the stresses on the
blades. But if, after the compressor the air is directly fed to the
turbine for its rotation, the turbine may rotate, but the energy
output is almost same as that of the compressor output. So there is
no useful output of work from the turbine. For getting the higher
output from the same input, the working fluid should be expanded to
raise its temperature. This could be done by the combustion chamber
where the working fluid is expanded by increasing its temperature .
then if it fed to the turbine the output obtained will be more than
the inout, the net output will be the useful work. The performance
of the Gas Turbine mainly depends on the 2 factors. They are
Efficiency of the components in it Turbine working
temperature11
For the cycle to perform more efficiently, the pressure ratio
should be maintained as 20:1 and the turbine inlet temperature be
1350k.
Fig: Gas turbine in power plant 5.2 FEATURES OF GAS TURBINES:
The gas turbines have the following opearational features. They are
1. The gas turbines produce a large amount of useful work from the
relatively small input2. The mechanical life is long when compared
to apiston driven engine.
3. The start up time to a full load for a gas turbine is in
minutes VS for a steam turbine 4. Gas turbines can operate
utilizing various types of fuels. But generally natural gas is been
used in it.5. Atmospheric air is typically the working fluid for
the gas turbine and doesnot require any
coolant for basic power generation.
5.3 BRAYTON CYCLE:A gas turbine is basically not simply a
turbine. It comprises of 3 different sections. Altogether
constitutes the gas turbine. The 3 sections are12
Compressor Combustion chamber Turbine
Gas turbines may operate either on a closed or on an open cycle.
The majority of gas turbines currently in use, operate on the open
cycle in which the working fluid, after completing the cycle is
exhausted to the atmosphere. The air fuel ratio used in these gas
turbines is approximately 60:1. The ideal cycle for gas turbine is
Brayton Cycle or Joule Cycle. The Brayton cycle was developed in
the year 1876. This cycle is of the closed type using a perfect gas
with constant specific heats as a working fluid. This cycle is a
constant pressure cycle and is shown in Fig. 9.24. On P-V diagram
and in Fig. 9.25 on T- diagram. This cycle consists of the
following processes: The cold air at 3 is fed to the inlet of the
compressor where it is compressed along 3-4 and then fed to the
combustion chamber where it is heated at constant pressure along
4-1. The hot air enters the turbine at 1 and expands adiabatically
along 1-2 and is then cooled at constant pressure along 2-3.
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5.4 TYPES OF GAS TURBINE: MAIN ELEMENTS:1) ROTOR:
It is the rotating element in the gas turbine, up on which
blades are fixed on its pheriphery.
2) STATOR:
The stator consists of the stationery or fixed blades which are
located just after the row of rotor blades. Thus each set of stator
blades with rotor blades constitute a stage. Likewise, there are 4
stages in the gas turbine. COOLING ARRANGEMENT: The blades of the
gas turbine are cooled by the extractions from the compressor at
the stages 5 and 10. The air which is taken as extraction from
these stages 5.5 STARTING SEQUENCE OF GAS TURBINE: Initially the
turbine shaft should be lifted inorder to make it rotate. For that,
lift oil system will be used. Lift oil system lifts the turbine
shaft in to the air inorder to reduce the friction between the
shaft and the gear There are 4 bearings present in total .all these
bearings are providing with lub oil system. There is also a turning
gear for initial rotation of turbine and for its protection during
start up and shut down.
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Whenever turbine is under cold start up condition, for its
starting turning gear should be used to increase its speed from 0
rpm to 100rpm. This much speed will be attained with the help of
turning gear. Then start command: Before giving start command to
the turbine the following parameters should be checked initially.
1) Check whether the lub oil system is started or not (it should be
started).2) Lift oil system should be ready 3) Turning gear system
should be ready
4) Hydraulic oil system should be checked. 5) Air inlet damper
should be opened(if not, it should be opened)
If any of the above things gone wrong, then they will be
corrected first before giving start command to the turbine. After
attaining 100-120 rpm for turbine starting frequency converter
(SFC) come into action. This helps in increase the speed of the
turbine slowly and continuously till 2100 rpm thereafter it will
cut off. SFC takes its supply from the RYB phases of the station
transformer. After SFC comes into action, at 450 rpm, actual firing
will occur in the combustion chamber (the fuel valve will be opened
and the gas enters the combustion chamber.
5.6 How firing will occur? When gas enters the combustion
chamber, ignition gas control valve will be opened and a sudden
spark will be generated for a time of 2 sec only that too with the
help of spark ignition plug.
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When the speed reaches 500 rpm, lift oil support will cutoff.
But SFC is still working .when it attains the speed 600 rpm, main
oil valve (MOV) will be closed and now the diffusion gas enters and
gas quantity increases gradually based on the lifting rate of the
shaft. At 2100 rpm, SFC will cutoff but the turbine is self
sustainable to attain the required speed(3000rpm) at 2850 rpm, blow
out valve will be closed and finally the gas entering the turbine
can individually be responsible for attaining the speed of 3000
rpm. The important Parameters that are to be considered are 1)
Bearing temp and vibration 2) Lub oil pressure 3) Hydraulic oil
pressure. Total time taken to bring turbine from 0 rpm to 3000rpm
is normally 10-14 min. For synchronization the following parameters
should be taken into account 1) Matching the voltage It will be
done by the AVR 2) Matching the frequency It will be done by the
control valve 3) Phase angle matching It will be done by the
synchronoscope4) Phase sequence (it should be constant)
All the above mentioned parameters should be matched in the
sequential order as given above. 1) Speed controller 2) Load
control and load set
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Based on the load rise rate the gas flow will be regulated by
means of the gas control valve. But the load rise rate should be
normally maintained at 11 mw/min. Till now the turbine is under
load control mode 3) TETC mode ( Turbine Exhaust Temperature
Calculated): When the Igniter Gas Valve is 95% opened, then the
TETC will come into operation in the place of the load control. The
TETC itself calculates the flow based on the parameters like
temperature and pressure from time to time. 4) Compressor pressure
ratio limiter ( beta limiter): When frequency suddenly decreases
pressure will be decreased and there will be a back flow of gases
from the turbine which causes the surge (vibrations in the
machine), then b-limiter will reduce the load accordingly and close
the IGV 5) Load limiter: It will reduce the load to the rated load.
Mal operation of this will lead to decrease in the life of the
machine (plant). All the above discussed will come under Turbine
Governing system 6. GAS TURBINE SUPPORTING SYSTEMS 6.1
INTRODUCTION: A gas turbine support system includes the systems
which are employed for the efficient operation of gas turbine.
These may include fuel inlet system, air intake system etc. 6.2
FUEL INLET SYSTEM: The gas that is taken to GRS will be supplied to
the combustion chamber for the production of steam which will be
used to produce electricity. This will be done through the
pipelines.
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The gas will enter the main path that leads to the combustion
chamber. There after it is passed through the ESV (Emergency Stop
Valve), there after there will be a bypass system provided (to
remove the gas during GT shutdown to the atmosphere). From there it
will be divided into the 2 streams, each provide with the pressure
gauge individually to check out the inlet pressure for combustion.
This whole system is in PREMIX mode to reduce the NOx emissions.
After that, the gas enters the pilot valve in two streams through
which it enters the two combustion chambers form the main path.
Before that there are two paths from the main path that leads to
the each combustion chamber. These two paths are provided with two
control valves respectively for premix mode and diffusion mode.
Basically there are two modes of operation, namely PREMIX mode
DIFFERENTIAL mode
In premix mode, the flame is an oxidizing flame; hence the
temperature will be less, which will reduce the NOx formation. In
this mode, air will be added to the gas before entering the
combustion chamber there by temperature can be controlled inside
the combustion chamber.( O2 is more in this mode). In diffusion
mode, first gas will enter the combustion chamber and thereafter
the air is mixed to it Hence premix mode is preferable generally.
But if TETC< 490c, the mode will automatically changes over from
PREMIX to DIFFUSION mode and also until the GT load is < 50 MW,
the system is in DIFFUSION MODE. AIR FILTERS: Air also plays an
important role in the combined cycle power plant as it supports the
phenomenon of combustion through the intake system.19
Air intake system consists of three pads namely, primary filter,
coallescer pads and fine filters which will reduce the solid dust
particles from the incoming air through the air receiver.primary
filter is mainly used to remove the large particles.coallescer pads
are used to remove the moisture form the air for the atmosphere
where as the fine filters are used mainly to remove the fine
particles of dust from the air. After the air receiver there will
be an air plenum which directs the air into the combustion chamber.
Before admitting to the compressor there will be a silencer to
remove the noise of the air. It contains the vertical baffles to
reduce the pressure across these baffles. Inside the air plenum,
there is an air damper at the inlet of the combustion chamber which
regulates the air flow into the combustion chamber according to the
load. From there the air will enter the combustion chamber through
the annular space in the combustion chamber. During the time of
start up, this diverted damper is opened slowly according to the
load.this diverted damper is hanged by means of two levers on its
one side.
6.2 LUB OIL SYSTEM: The lub oil is mainly used for protection of
bearing system. The lub oil avoids the wearing of the bearings
contact surfaces by reducing the friction between the contact
surfaces. The lub oil inturn cleans the surfaces there by removes
the deposits of dust on the inner surfaces. The lub oil is a
continuous system. For this there is a lub oil system associated
individually to each system. It consists of tank, AOP, MOP, lub oil
filters and lub oil coolers. Along with these there are also level
indicators and temperature and pressure maintaning devices and
valves associated with the system.
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Fig :
PARTS: OIL FILTER(STRAINER): These are mainly used to purify the
lub oil before entering the required system. In these filters,
there is a mesh like network which seperates the solid dust
particles from entering the system.
OIL COOLERS:
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These are employed to cool the lub oil after its use in the
system. For this cooling, the cooling water is circulated in the
tubes which take over the heat from the hot lub oil thus making it
cool and make it useful again. Generally two oil coolers are
employed in the lub oil system, one is in service and the other in
stand by position. Along with the above equipments, there are many
other accessories in the lub oil system which include, Auxiliary
oil pump, Main oil pump, Drain valve, Air vent, pressure relief
valve etc. Whereas the auxiliary oil pump is used to pump the oil
during startup when the pressure is not upto the mark. Pressure
relief valve is used to reduce the pressure which may cause
overheating of the system. 6.3 STARTING SEQUENCE OF LUBE OIL
SYSTEM: 1) Check the oil level in the oil reservoir (tank). If the
oil is above the minimum level then only the pump will start. The
level switch will take care of it. 2) When the oil level is above
the minimum level, then the AOP (Auxiliary Oil Pump) will start.3)
Gradually it will build up the pressure above the 2 bar and then
the MOP will start along
with the BFP. This will be done by the 1st pressure switch. 4)
Then both pumps will start running and when the pressure becomes
>3bar, AOP automatically trips. Then only the MOP will be
running.5) When MOP is running, if the pressure drops