UNIT_1_PPE

8/11/2019 UNIT_1_PPE

1/22

UNIT-I

Power PlantsINTRODUCTION

Power plants are used for the generation of electric power.

In 1950. the total installed capacity in Tamil Nadu was only 156 MWwhich has been increased to about 5473 MW in 1990.

On all India basis, the figures in the same period were 2301 MW and 6393MW respectively.

In India, the per capita consumption of electricity was only 12 kWh in1950 which has increased to about 280 kWh in 1990 while in Japan, ithas gone as high as 5633 kWh.

CLASSIFICATION OF POWER PLANTSPower plants can be mainly classified as follows:

1. Steam power plant

2. Nuclear power plant3. Gas turbine power plant4. Diesel power plant5. Hydro power plant6. Power from alternate sources of energyIn the remaining part of this chapter, a description of the above power plants and

alternate sources of energy are provided.

STEAM POWER PLANTThe layout of a steam power plant is given in Fig

Steam from the boiler is taken to the turbine through the steam pipe fitted withan expansion joint.

The joint provides a flexible connection to prevent any crack in the steam pipewhich is subjected to expansion and contraction due to the variation of temperature.

From the turbine, the steam enters a condenser, details of which are shown inFig. In the condenser, the exhaust steam from the turbine is condensed due towhich a high vacuum is produced.

Due to the vacuum, the power output and the thermal efficiency of the turbineare considerably in creased.

In the condenser, cooling water is circulated by a pump through the water tubesto condense the exhaust steam.

The cooling water at the outlet becomes hot and it is taken to a cooling pond or a cooling tower to cool and to recalculate the same water if the power plant isnot located on the bank of a river or a lake.

8/11/2019 UNIT_1_PPE

2/22

The condensate from the condenser before entering the boiler is subjected to thefollowing treatments.1. Removal of air and oxygen2. Preheating the feed water in different stages using low pressure heater (LPH) deaerator and high pressure heater (HPH).Air and oxygen are removed at the air ejector and the deaerator. In case, air and oxygenare not removed from the feed water, the vacuum cannot he main tained in the condenser,resulting in loss of power and thermal efficiency.Factors to be Considered in the Selection of a- Site for a Steam Power Plant1. The location of the plant should be at a minimum distance from the load centre

(consumer) to avoid transmission losses.2. Availability of water is a desirable factor.3. The water should be preferably free from salt to reduce the cost for water treatment.4. The soil should be satisfactory for a strong foundation.5. The site should be away from the thickly populated area to reduce the effect of

pollution.

8/11/2019 UNIT_1_PPE

3/22

NUCLEAR POWER PLANT A nuclear power plant is very similar to a conventional steam power plant

except for the furnace. The nuclear reactor becomes the furnace in thiscase.

It has been estimated that complete fission of 1 kg of uranium producesheat energy equivalent to 4500 tons of coal or 1700 tons of oil.

Some of the important commercial reactors commonly used for power generationare given be low:1. Boiling water reactor (BWR)2. Pressurized water reactor (PWR)3. Gas cooled reactor (GCR)

Boiling Water Reactor A simple boiling water reactor is shown in Fig. 17.2. Due to nuclear fission of the fuel uranium, large amount of heat is

produced. The nuclear reaction and thereby the temperature is controlled by

moderators. The coolant used here is water which absorbs the heat produced in the

reactor. Water evaporates and steam is generated in the reactor itself. In this type of power plant. there is no need for a separate boiler. The steam produced in the reactor is used to run the turbine coupled with a

generator from which we get the electrical power. The steam after expansion i the turbine is condensed in the condenser. The

condensate after getting heated in several feed water heaters is pumpedagain into the reactor by means of feed pump.

In the reactor, the thermal shielding reduces the heat loss and th thick concrete shielding prevents external

8/11/2019 UNIT_1_PPE

4/22

radiation. In the primary loop, the pressuriser maintains a high pressure in the water in the

range of 150 bar. Due to the high pressure of water in the reactor, the water doesnot boil.

8/11/2019 UNIT_1_PPE

5/22

The coolant gets heated in the reactor and the hot water goes to the boiler andtransfers the heat to the feed water in the boiler in the secondary loop.

The feed water evaporates and becomes steam and runs a turbo generator fromwhich power is obtained. Functions of various parts of the reactor are the same asthose of a boiling water reactor.

Gas Cooled Reactor The schematic diagram of a gas cooled reactor is shown in Fig. 17.2(b). In this, gas CO is employed as coolant and the heat carried by the gas from the

reactor is either used for steam generation in the secondary circuit like pressurizedwater reactor or is directly used as the working fluid in a gas turbine plant.Usually the gas used is CO and graphite is the moderator.

C O gas gets heated in the reactor and loses its heat to the superheater, evaporator and economiser tubes in the secondary loop. The cooled gas is recirculated again in the primary loop by means of a gas blower.

The superheat ed steam is expanded in the turbine to run the generator to produceelectrical power.

Advantages of a Nuclear Power Plant1. Very large amount of heat is liberated by a very small quantity of fuel2. Suitable for large power generation3. Cost of fuel transportation and storage is less.Disadvantages

Installation cost is very high.2. Availability of nuclear fuel is scarce and cost is high.3. Large number of trained and qualified personnel are required to oper ate the plant.4. Maintenance cost is higher.5. We have the problems involved in waste disposal and also the risk of radiationhazards.

8/11/2019 UNIT_1_PPE

6/22

GAS TURBINES Gas turbines are used mainly for electric power generation and also in jet engines

of aircrafts and in turbochargers of internal combustion (IC) engines. It has limited application in marine engines. Gas turbines have the unique

advantage of using any type of fuel, i.e. solid, liquid or gas.

Gas turbines operate either on an open cycle or in a closed cycle.Working of an Open Cycle Single Stage Gas Turbine

A simple open cycle gas turbine is represented in Fig. 17.3(a). It consists of a compressor, a combustion chamber and a turbine. The compressor and turbine co by a common shaft with a suitable flange. Air from the atmosphere is taken and compressed to a pressure ratio ranging from

2-8 before assing to the combustion chamber where the fuel is injected. The fuel burns and the temperature is raised at constant pressure. Then, it passes

to the turbine where it expands to its original pressure before being exhausted toatmosphere. Fig. 17.3 (b).

Advantages of Gas Turbines1. Possibility to use any type of fuel.2. Compact size, less weight and low space requirement.3. Simple foundation and low installation cost.

8/11/2019 UNIT_1_PPE

7/22

4. Less requirement of lubrication oil, water, etc.5. Vibration is less.

Disadvantages of Gas Turbines1. High operating temperature in the combustion chamber and in the turbine. So we need

special high temperature alloys.2. Thermal efficiency is very low in the case of simple gas turbine due to hightemperature of about 450C in the waste exhaust gases.and also for cooling.

Methods to Improve the Thermal Efficiency of a Single Stage Gas Turbine1.By using a regenerator to heat the compressed air before entering the

combustion chamber as in Fig. 17.4 thereby making use of the heat in the exhaust gases beforeleaving to the atmosphere.

2. By using a multistage compressor with intercooling to reduce the work of compression.3. By using a multistage turbine to reduce the temperature of exhaust gases beforeleaving the turbine.

Closed Cycle Gas Turbine The closed cycle plant can use some stable gas with a higher specific heat as the

working medium. Instead of burning the fuel directly in the air steam, an externally fired

combustion chamber or furnace is used and heat is transferred to the workingmedium through a heat exchanger.

Intercooler is also provided to improve the overall efficiency of compressions. As a multistage turbine is used, the temperature of exhaust gases leaving the

turbine is considerably reduced resulting in a higher thermal efficiency. The regenerator preheats the gas before entering the furnace. By these provisions,

the thermal efficiency is further increased to about 30 per cent.

8/11/2019 UNIT_1_PPE

8/22

The closed cycle has the following advantages:

1. Flexibility as to the type of fuel.2. Uncontaminated working medium, and hence maintenance is easier.3. Possibility of using a gas having better thermal properties as the working medium. Byusing an inert gas with high specific heat, the unit will become compact.

DIESEL POWER PLANT The layout of a diesel power plant is given in Fig. 17.6. Multicylinder 2-stroke turbocharged Diesel engines are used in power plants. In

turbocharged engine, The atmospheric air is compressed by a compressor run by an exhaust driven gas

turbine and the compressed air is taken inside the cylinder.Due to this, mass of air

intake and amount of fuel burnt will be considerably increased giving rise toincreased output power and higher thermal efficiency.

8/11/2019 UNIT_1_PPE

9/22

Due to turbo charging, the operating temperature of the engine is increased. So.the lubricating oil coming out of the engine should be cooled in an oil cooler.

The cooling water from the engines is normally cooled in a spray tank andrecirculated. Due to high capacity, the engine is started by using com pressed air.

HYDRO POWER PLANT In a hydro power plant, the potential energy of water stored in a dam is made use

of in running a water turbine coupled to an electrical generator. It is esti mated that bout 23 per cent of the total electric power in the world comes

from hydro power. In Tamil Nadu. the total generation of power from hydroelectric plants amounts to

1950 MW and in all India level, it amounts to about 18000 MW.Layout of a Hydro Power Plant

The layout of a hydro power plant is given in Fig. 17.7. The water from the dam is brought to the water turbine by a large diameter

penstock pipe. The penstock pipe is made of steel or reinforced concrete. It is desirable to eliminate sharp bends in the penstock pipe to avoid the loss of

head and special anchoring.

Depending upon the load on the turbine, the amount of water needed is controlledautomatically by a valve operated by a centrifugal governor.

In case the amount of water is suddenly reduced or stopped by the governor mechanism. water coming down with a high velocity will produce turbulenceresulting in a water hammer in the pipe.

The penstock pipe may be damaged due to the water hammer. To prevent this, asurge tank is provided. From the turbine, water is allowed to pass through a drafttube to the tail race.

The tail race is the water path leading the discharge water from the turbine to theriver or canal.

8/11/2019 UNIT_1_PPE

10/22

MHD ( Magneto Hydro Dynamic ) -STEAM POWER PLANT The maximum steam temperature and pressure being fixed by metallurgical

considerations, the minimum temperature by the ambient conditions, and with theoptimum degree of regeneration and number of reheats, the ceiling for theconversion efficiency of a conventional thermal power station is somewhere near

45%. There is a great deal of world-wide interest to achieve a higher conversionefficiency and hence, fuel economy, by converting heat directly to electricityeliminating the link process of producing mechanical energy via steam.

The magneto hydrodynamic (MHD) power generation seems to be the most promising for a utility system.

The maximum limiting temperature for turbine blades being 750 800 C, theMHD generator is capable of tapping the vast potential offered by modernfurnaces, which can reach temperatures of more than 2500 K, and up to 3000 K with preheating of air.

Principle of MHD Power Generation Faraday s law of electromagnetic induction states that when a conductor and amagnetic field move relative to each other, an electric voltage is induced in theconductor.

The conductor may be a solid, liquid or gas. In an MHD generator, the hot ionizedgas replaces the copper windings of an alternator.

When a gas is heated to high temperatures, the valence electrons of the excitedatoms move on to higher quantized orbits and ultimately, at certain energy levelsthey fly off and become free electrons.

For a gas to be conducting, a certain number of free electrons must be presentalong with an equal number of ions and the main body of neutral atoms.

Since a very high temperature is required to ionize a gas (thermal ionization)which cannot be endured by the materials available, the hot gas is seeded with analkali metal, such as cesium or potassium (K or KOH) having a low ionization

potential (energy needed to ionize one g mol of atoms) before the gas enters theMHD duct.

An adequate electrical conductivity of the order of 10 mho/m can thus be realizedat somewhat lower temperatures in the range 22002 700 C.

A simple view of the MHD generator is shown inFig The duct throughwhich the electrically conducting ionized gas flows has two sides

supporting a

8/11/2019 UNIT_1_PPE

11/22

strong transverse magnetic field of 4 5 tesla (1 tesla = i gauss) at right anglesto the flow and the other sides forming the faces of electrodes which are joinedthrough an electrical circuit.

As the hot ionized gas or plasma enters the MHD duct, due to the effect of thestrong magnetic field and the consequent Lorentz force, there is a decrease in thekinetic energy of the plasma, and the electrons and ions get deposited on theopposite electrodes.

The power generated per unit length is approximately proportional to cru B wherec-is the electrical conductivity, u is the velocity of the gas, B is the magnetic fieldstrength and p is the density.

The power produced being dc, the conversion to ac is done by an inverter. Figure

3.31 shows the principal components of a typical MHD plant and its cycle of operations on T s diagram.

COMBINED CYCLE PLANTS The maximum steam temperature in a power cycle does not exceed 600 C,

although the temperature in a dry bottom pulverized coal furnace is about 1300C.

By superposing a high temperature power plant as a topping unit to the steam plant, a higher energy conversion efficiency from fuel to electricity can beachieved, since the combined plant operates through a higher temperature range.

Combined plants may be of the following types:(i) Gas turbine steam turbine plant(ii) MHD steam plant(iii) Thermionic steam plant(iv) Thermoelectric steam plar

GAS TURBINE-STEAM TURBINE POWER PLANT The air standard cycle for a gas turbine power plant is the Brayton cycle which,

like Rankine cycle, also consists of two reversible adiabatics and two reversible

8/11/2019 UNIT_1_PPE

12/22

isobars, but unlike Rankine cycle the working fluid does not undergo phasechange.

A gas turbine plant can be either open or closed. Simple, open gas turbine plant isshown in Fig. 3.6.

Since the product of combustion is the working fluid which produces power by

doing work on the blades of the gas turbine, it is an internal combustion plant.However, unlike the reciprocating internal combustion engine, the gas turbine is asteady flow device and the blades are always subjected to the highest gastemperature.

To limit the maximum gas temperature to about 1200 K at inlet to the turbineconsistent with the materialused, a high air-fuel ratio is used.

The disadvantages of a gas turbine power plant in a utility system are thefollowing:

1. Large compressor work input, since the power required to drive the compressor isconsiderably higher than that required by a pump for the same pressure rise. Thecompressor thus consumes a large part of the work produced by the turbine.2. Large exhaust loss, since the exhaust gas temperature is quite high and also the massflow rate of gas is large due to high air fuel ratio used.3. Machine inefficiencies, since with the decrease in compressor efficiency (i the work input to the compressor increases and with the decrease in turbine efficiency ( the work output from the turbine decreases. At certain values of i and 17 a situation may arisewhen the compressor consumes more power than what the turbine develops. So, themachine efficiencies of the compressor and the turbine have to be high enough to yield

justifiable net work output.4. Low cycle efficiency, due to the large exhaust loss, large compressor work andmachine inefficiencies.5. Costly fuel, since the cost of kerosene and other fuels used is much higher than that of coal. Its availability is also not always guaranteed.Due to the above factors, the cost of power generated by a stationary gas turbine plant for a utility system is high. However, a gas turbine plant offers certain advantages also, asgiven below:

8/11/2019 UNIT_1_PPE

13/22

I. Less installation cost2. Less installation time3. Quick starting and stopping4. Fast response to load changesSo, a gas turbine plant is often used as a peaking unit for certain hours of the day when

the energy demand is high. A large steam plant designed to meet peak loads wouldoperate at an uneconomical load factor during most of the year.

Thermodynamics of Brayton-Rankine Combined Cycle Plant Let us consider two cyclic power plants coupled in series, the topping plant

operating on Brayton cycle and the bottoming one operating on Rankine cycle(Fig. 3.7).

Helium gas may be the working fluid in the topping plant and water in the bottoming plant. As shown in Section 3.4.1, the overall efficiency of thecombined plant is given by Eq. (3.6)

8/11/2019 UNIT_1_PPE

14/22

neglecting the pump work. As inlet temperatures to gas turbine keep increasing (due to the use of better

material and blade cooling), the importance of supplementary firing diminishesfurther. However, supplementary firing may provide increased operating and fuelflexibilities in CC plants, which may fall into the following two categories.

1. Combined cycle plants with limited supplementary firing

Supplementary firing raises the temperature of the exhaust gas to 800 to 900 C.Relatively high flue gas temperature raises the condition of steam (84 bar, 525 C),thereby improving the efficiency of the steam cycle. Following is given an example(Kehlhoffer 1991):Combined MHD-Steam Power Plant

If the gas entering the MHD duct at about 3000 C could be expanded to theambient temperature of 30 C, the Carnot efficiency would have reached 90%.Unfortunately, the MHD power output is restricted because by the time the gas

8/11/2019 UNIT_1_PPE

15/22

temperature falls to 2000 C the electrical conductivity becomes very low with theelectrons combining with ions to form neutral atoms, and the generator then ceases tooperate satisfactorily. Therefore, the MHD generator is used as a topping unit and theMHD exhaust at about 2000 C is utilized in raising steam to drive turbine and generateelectricity in a conventional steam power

8/11/2019 UNIT_1_PPE

16/22

8/11/2019 UNIT_1_PPE

17/22

In the closed cycle scheme, helium (or argon) gas seeded with cesium is heated ina nuclear reactor, passed into the MHD duct and then into the steam generatingsystem (Fig. 3.34). A gas turbine plant can also be used as a bottoming unit (Fig.3.35).

The material has to stand up to temperatures above 2200 C and the corrosiveatmospheres of alkali-seeded gases.

The duct wall will also need to be an electrical insulator at these temperatures.Materials used are magnesium oxide, strontium zirconate and hafnia. Electrodesin the dc MHD generator perform the same function as brushes in a conventionaldc generator. Tungsten or carbon electrodes have been used.

Electrodes are often segmented to reduce energy losses due to Hall effect Toreduce the power consumption of these electromagnets, cryogenic or superconducting coils at liquid helium temperatures have been suggested.

8/11/2019 UNIT_1_PPE

18/22

The only fuel which has better characteristics than coal is char, which containsalmost no hydrogen and, in general, results in a 25% increase in the performance

of the generator.

8/11/2019 UNIT_1_PPE

19/22

8/11/2019 UNIT_1_PPE

20/22

SELECTION OF POWER PLANTThe following factors are considered for the selection of power plant1. Depending on the load requirement2. Availability of fuel and water 3. Availability of fuel storage facility.4. Transportation facility.5. Availabilitv land6. Environmental conditions7. Efficiency of the plant8. Capacity of the plant.9. Distance from the load center.10. Life of the plant.11. Availability of time period for power production.12. Cost of the fuel used.13. Nature of losses.14. Requirement of labours.15. Depending on the noise of vibration.

8/11/2019 UNIT_1_PPE

21/22

UNIT-I

LAYOUT OF POWER PLANT PART-A

1. Name the four major circuits in steam power plant.2. What consists of air and flue gas circuit ?3. What consists of feed water and steam flow circuit in steam power plant ?4. What consists of cooling water circuit and coal and ash circuit in steam power

plant?5. What is the main purpose of the reservoir?6. What is the main purpose of the dam?7. Why trash rack is used?8. What is the use of surge tank?9. What is the function of Fore bay?10. Explain about penstock?11. What is the use of spill ways?12. Write about prime movers?13. What are the uses of air filter and superchargers in diesel engine power plant?14. What is the use of draft tube?15. What is the function of cooling system in Diesel power plant?16. What consists of lubrication system in diesel engine power plant?17. Name different types of MILD generators?18. What is the working principle of Magneto Hydro Dynamic power plant?19. What is the purpose of intercooler in gas turbine power plant?20. What is the use of Regenerator in gas turbine power plant?21. What is the difference between open cycle and closed cycle gas turbine?22 What is the use of moderator in Nuclear power plant?23. What is the purpose of control rods?24. What is the use of coolant and name some coolants used in Nuclear power plant?25. Name some reflectors generally used and their uses in Nuclear power plant?26. Name two combined power cycles?27. Define turbo charging in combined gas turbine and diesel cycles?

PART-B1. Explain the layout of steam power plant with neat sketch?2. Explain the working of condenser used in steam power plant with a neat sketch?3. What are the types of nuclear power plants? Also discuss advantages and

disadvantages of nuclear power plant?4. Explain boiling water reactor briefly with a neat sketch?5. Explain layout of pressurized water reactor with neat sketch?6. Explain layout of gas cooled reactor with neat sketch?7. Explain open cycle single stage gas turbine?8. Explain the working of closed cycle gas turbine?9. Explain the layout of diesel power plant with neat sketch?10. Explain the layout of hydro power plant with neat sketch?11. Explain types of water turbines?12. Explain the principle and types of magneto hydro dynamic power plant?

8/11/2019 UNIT_1_PPE

22/22

13. Explain open cycle MHD?14. Explain closed cycle MHD?15. Explain closed cycle MHD with liquid metal?16. Explain combined gas turbine and steam cycle?17. Explain combined gas turbine and diesel cycle?