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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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