This is the second chapter of Power Plant Engineering authored by PRIME in the Philippines. The topic in this chapter is Fuels and Combustion.Full description
Power Plant EngineeringFull description
power plant engineering by pk nag for mechanical engineers
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Power Plant Engineering 1.1
Power plant engineering deals with the study of energy, its sources and utilization of energy for power generation. The power is generated by prime movers (example Hydraulic turbines, steam turbines, diesel engines). Large amount of power is generated using prime movers in a site or layout called power plants, where all the equipments and machineries required for power generation is located. Energy: Energy may be defined as the capacity to do work. Energy exists in various forms, such as Mechanical Energy, thermal energy, electrical energy, solar energy etc. Electricity is the only form of energy, which is easy to produce, easy to transport, easy to use and easy to control. Electricity consumption per capita is the index of the living standard people of a place or country i.e. the utilization of energy is an indication of the growth of the nation. 1.2 Power and Power Plant: Power is primarily associated with mechanical work and electrical energy. Therefore, power can be defined as the rate of flow of energy and can state that a power plant is a unit built for production and delivery of a flow of mechanical or electrical energy. In a common usage, a machine or assemblage of equipment that produces and delivers a flow of mechanical or electrical energy is a power plant. Hence an internal combustion engine is a power plant; a water wheel is a power plant, etc. However, what we generally mean by the term power plant is that assemblage of equipment, permanently located on some chosen site which receives raw energy in the form of a substance capable of being operated on in such a way as to produce electrical energy for deliver from the power plant. 1.3 Sources of energy The basic sources of energy for power generation are coal, oil, nuclear fuels and gas. These sources are known as “conventional sources of energy”. These sources of energy will one day be used up and are exhaustible. The most reassuring and promising energy, which is abundant in supply and is inexhaustible is “Non – conventional sources of energy” such as solar, wind, tidal, geothermal etc. Energy resources can be broadly classified as follows
Energy Resources Conventional sources of energy energy (Or) Non – renewable sources of energy
or Renewable source of energy
Examples: Fuels like coal, oil, energy Natural gas, nuclear fuels etc. electric
Examples:Sun, wind, From
Power etc. 1.3.1 Non – renewable sources: Most of the energy we use are from source like coal, oil, natural gas and nuclear fuels. These primary energy sources are called Non – renewable sources because once they have been used up, they cannot be replaced. 1.3.2 Renewable sources: Sources of energy that can be used over and over again are called renewable sources. These sources can be used to produce electricity. Some of the renewable sources are: • • • •
Energy Energy Energy Energy
from from from from
the sun (Heat and light energy) the wind (Kinetic energy) the waves and tides (Kinetic energy) earth’s core (Geothermal energy)
1.4 Classification of power plants A power plant makes use of any one of the energy sources to produce power. Depending on the type of energy source the power plants are classified as • • • • • •
Thermal power plant (It makes use of coal) Internal combustion engine plants (makes use of petrol or diesel) Gas turbine power plant (makes use of a permanent gas) Nuclear power plant (makes use of nuclear fuels) Solar power plant (makes use of the suns radiation heat) Tidal power plant (makes use of the power of tides in the sea)
• • •
Hydro electric power plant (makes use of the potential energy of water) Wind power (makes use of energy available in wind) Geothermal power plant (makes use of heat energy available under the ground)
1.5 Working principle of Steam power plants 1.5.1 Introduction Steam power plant is also known as Thermal power plant. A steam power plant converts the chemical energy of the fossil fuels (coal, oil, gas) into mechanical / electrical energy. This is achieved by raising the steam in the boilers, expanding it through the turbines and coupling the turbines to the generators which convert mechanical energy into electrical energy as shown in fig. 1.1. The following two purposes can be served by a steam power plant: 1. To produce electric power 2. To produce steam for industrial purposes besides producing electric power. The steam may be used for varying purposes in the industries such as textiles, food manufacture, paper mills, sugar mills and refineries.
Fig. 1.1. Production of Electric energy by steam power plant 1.5.2 Classification of steam power plants The steam power plants may be classified as follows: 1. Central stations
2. Industrial power stations or captive power stations 1. Central stations. The electrical energy available from these stations is meant for general sale to the customers who wish to purchase it. Generally, these stations are condensing type where the exhaust steam is discharged into a condenser instead of into the atmosphere. In the condenser the pressure is maintained below the atmospheric pressure and the exhaust steam is condensed. 2. Industrial power stations or captive power stations. This type of power station is run by a manufacturing company for its own use and its output is not available for general sale. Normally these plants are non-condensing because a large quantity of steam (low pressure) is required for different manufacturing operations. In the condensing steam power plants the following advantages accrue: (i) (ii)
The amount of energy extracted per kg of steam is increased (a given size of the engine or turbine develops more power). The steam, which has been condensed into water in the condenser, can be recirculated to the boilers with the help of pumps.
In a non – condensing steam power plants a continuous supply of fresh feed water is required which becomes a problem at places where there is a shortage of pure water. 1.5.3 Layout of modern steam power plant A steam power plant must have the following equipments: • • • •
A furnace to burn the fuel. Steam generator or boiler containing water. Heat generated in the furnace is utilized to convert water into steam. Main power unit such as an engine or turbine to use the heat energy of steam and perform work. Piping system to convey steam and water.
The general layout of the thermal power plant consists of mainly 4 circuits as shown in fig. 1.2. The four main circuits are: 1. 2. 3. 4.
Coal and ash circuit Air and gas circuit Feed water and steam flow circuit Cooling water circuit
Coal and ash circuit: - This circuit consists of coal storage, ash storage, coal handling and ash handling systems. The handling system consists of belt conveyors, screw conveyors etc. Coal arrives at the storage yard and after necessary handling, passes on to the furnaces through the fuel feeding device. Ash resulting from combustion of coal collects at the back of the boiler and is removed to the ash storage yard through ash handling equipment. The Indian coal contains 30 to 40% of ash and a power plant of 100MW produces normally 20 to 25 tones of hot ash per hour.
Fig.1.2 Layout of Steam Power plant Air and gas circuit: - This circuit consists of air filter, air preheater, dust collector and chimney. Air is taken in from the atmosphere to the air preheater through the action of a forced or induced draught fan or by using both. The dust from the air is removed by means of using air filter before supplying to the combustion chamber. The exhaust gases carrying sufficient quantity of heat and ash are passed through air preheater where the exhaust heat of the gases is given to the air and then it is passed through dust 5
collectors where most of the dust is removed before exhausting the gases to the atmosphere through chimney. Feed water and steam flow circuit: - This circuit consists of boiler feed pump, boiler, turbine and feed heaters. The steam generated in the boiler is fed to the steam prime mover to develop the power. The steam coming out of prime mover is condensed in the condenser and then fed to the boiler with the help of pump. The condensate is heated in the feed heaters using the steam tapped from different points of the turbine. The feed heaters may be of mixed type or indirect heating type. Some of the steam and water is lost passing through different components of the system, therefore, feed water is supplied from external source to compensate this loss. The feed water supplied from external source is passed through the purifying plant to reduce the dissolved salts to an acceptable level. The purification is necessary to avoid the scaling of the boiler tubes. Cooling water circuit: - This circuit consists of circulating water pump, cooling water pumps and cooling tower. The quantity of cooling water required to condense the steam is considerably large and it is taken from lake, sea or river. The cooling water is taken from the upper side of the river, it is passed through the condenser and heated water is discharged to the lower side of the river. Such system of cooling water supply is possible if adequate cooling water is available through the year. This system is known as open system. When the adequate water is not available, then the water coming out from the condenser is cooled either in cooling pond or cooling tower. The cooling is effected by partly evaporating the water. This evaporative loss (this includes evaporation and carryover) is nearly 2 to 5 % of the cooling water circulated in the system. To compensate the evaporative loss, the water from the river is continuously supplied. When the cooling water coming out of the condenser is cooled again and supplied to the condenser, then the system is known as closed system. When the water coming out from the condenser is discharged to river downward side directly, the system is known as open system. Open system is economical than closed system provided adequate water is available throughout the year. The different components, which are used in thermal power plants, are listed below: Boiler, steam turbine, generator, condenser, cooling towers, circulating water pump, boiler feed pump, induced/forced draught fans, ash precipitators etc. 1.5.4 Working of the thermal power plant
Steam is generated in the boiler of the thermal power plant using the heat of the fuel burned in the combustion chamber. The steam generated is passed through steam turbine where part of its thermal energy is converted into mechanical energy, which is further used for generating electric power. The steam coming out of the steam turbine is condensed in the condenser and the condensate is supplied back to the boiler with the help of the feed pump and the cycle is repeated. The function of the boiler is to generate the steam. The function of condenser is to condensate the steam coming pout of steam turbine at low pressure. The function of the steam turbine is to convert part of heat energy of steam into mechanical energy. The function of the pump is to raise the pressure of the condensate from the condenser pressure (0.015 bar) to boiler pressure (200 bar). The other components like economiser, superheater and steam feed heaters (steam from different points of turbine is fed to the heaters to heat the condensate to a higher temperature) are used in the primary circuit to increase the overall efficiency of the thermal power plant. 1.5. 5 Characteristics of steam power plant: • • • • • •
Higher efficiency Lower cost Ability to burn coal especially high ash content, inferior coals Reduced environmental impact in terms of air pollution Reduced water requirement Higher reliability and availability
1.5.6 Advantages (merits) of thermal power plant 1. The initial cost of construction of the plant is low compared to hydro electric plant 2. The power plant may be located near the load centre, so that the cost of transmission and the losses due to transmission are considerably reduced. 3. The quantity of water in hydroelectric plant depends on nature, such as rain and rivers. This is not so in the case of thermal power plants. 4. The construction and commissioning of thermal power plant takes lesser period when compared to hydro electric power plant. 1.5.7. Disadvantages (demerits) of thermal power plant 1. The fuel (coal or oil) used in thermal power plant will one day get exhausted since it is a non renewable source of energy that is used.
2. It cannot be used as peak load plant, as its part load efficiency decreases very rapidly with decreasing load. 3. The transportation of fuel is a major problem for power plants located away from coal fields. 4. The cost of power generation is considerably high compared to hydroelectric power plant. 5. The smoke produced by the burning fuel when exhausted into the atmosphere causes air pollution. 6. The life of thermal power plant according to the Electricity supply act is 25 years and that of hydroelectric plant is 35 years. The efficiency decreases to less than 10% after its life period. Hydroelectric power plant can have a life of even 100 to 125 years. 7. The turbines in thermal power plants run at a speed of 3000 to 4000 rpm and they require special material and rigid construction as compared to hydro electric plant which has a low running speed of 300 to 400 rpm. 1.6 Gas Turbine Power plant A gas turbine power plant may be defined as one “in which the principal prime mover is of the turbine type and the working medium is a permanent gas”. A simple gas turbine plant consists of the following: 1. 2. 3. 4. 5. 6.
compressor Intercooler Regenerator Combustion chamber Gas turbine Reheating unit
1. Compressor: In gas turbine plant, the axial and centrifugal flow compressors are used. In most of the gas turbine power plant, two compressors are used. One is low pressure compressor and the other is high pressure compressor. In low pressure compressor, the atmospheric air is drawn into the compressor through the filter. The major part of the power developed by the turbine (about 66%) is used to run the compressor. This low pressure air goes to the high pressure compressor through the intercooler. Then the high pressure air goes into the regenerator. 2. Intercooler:
The intercooler is used to reduce the work of the compressor and it is placed in between the high pressure and low pressure compressor. Intercoolers are generally used when the pressure ratio is very high. The energy required to compress the air is proportional to the air temperature at inlet. The cooling of compressed air in intercooler is generally done by water. 3. Regenerator: Regenerators are used to preheat the air which is entering into the combustion chamber to reduce the fuel consumption and to increase the efficiency. His is done by the heat of the hot exhaust gases coming out of the turbine. 4. Combustion chambers Hot air from regenerator flows to the combustion chambers and the fuel like coal, natural gas or kerosene are injected into the combustion chamber. After the fuel injection, the combustion takes place. This high pressure, high temperature products of combustion are passed through the turbine.
Fig.1.3 Layout of Gas turbine power plant 5. Gas turbine Two types of gas turbines are used in gas turbine plant. 1. High pressure turbine 2. Low pressure turbine
1. High Pressure turbine In the beginning, the starting motor runs the compressor shaft. The burnt gases (product of combustion) expand through the high pressure turbine. It is important to note that when the turbine shaft rotates it infact drives the compressor shaft which is couples to it. Now, the high pressure turbine runs the compressor and the starting motor is stopped. About 66% of the power developed by the turbine is used to run the compressor and only 34% of the power developed is used to generate electric power. 2. Low pressure turbine The purpose of the low pressure turbine is to produce electric power. The shaft of the LPT is coupled with the generator. The burnt fuel (gases) after leaving the HPT is again sent to a combustion chamber where it further undergoes combustion. Even if there is any left out unburnt fuel from the previous turbine it gets fully burnt in the combustion chamber. The burnt gases run the low pressure turbine (LPT). The shaft of the turbine is directly coupled with the generator for producing electricity. The exhaust hot gases after leaving the LPT passes through the regenerator before exhausted through the chimney into the atmosphere. The heat from the hot gases is used to preheat the air leaving the HPC before it enters the combustion chamber. This preheating of the air improves the efficiency of the combustion chamber. 6. Reheating unit In this unit, the additional fuel is added to the exhaust gases coming out from the high pressure turbine, and the reheated combustion products goes into the low pressure turbine. 1.6.1 Working of Gas Turbine Power plant The working of gas turbine plant is shown in fig.1.3. The atmosphere air is drawn into the low pressure compressor through the air filter and it is compressed. The compressed low pressure air goes into the high pressure compressor through the intercooler. Here, the heat of the compressed air is removed. Then the high pressure compressed air goes into the combustion chamber through the regenerator. In the combustion chamber, the fuel is added to the compressed air and the combustion of the fuel takes place.
The product of the combustion goes into the high pressure turbine. The exhaust of the high pressure turbine goes to another combustion chamber and the additional fuel is added and it goes to the low pressure turbine. After the expansion in the low pressure turbine, the exhaust is used to heat the high pressure air coming to the combustion chamber through the regenerator. After that, the exhaust goes to the atmosphere. 1.6.2..Advantages of Gas turbine power plant 1. For a gas turbine plant, Natural gas is a very suitable fuel. It would be ideal to install gas turbine plants near the site where natural gas is readily available. 2. Gas turbine plants can work economically for short running hours. 3. Storage of fuel requires less area and handling is easy. 4. Gas turbine plant is small and compact in size as compared to steam power plants. 5. It can be started quickly and can be put on load in a very short time. 6. The cost of maintenance is less. 7. It is simple in construction. There is no need for boiler, condenser and other accessories as in the case of steam power plants. 8. The gas turbine can operate at high speed since there are no reciprocating parts 9. Cheaper fuel such as kerosene, paraffin, benzene and powdered coal (cheaper than petrol and diesel) can be used. 10. Gas turbine plants can be used in water scarcity areas 11. Less pollution and less water is required. 1.6.3 Disadvantages of gas turbine power plant 1. 66% of the power developed is used to drive the compressor; the gas turbine unit has a low thermal efficiency 2. The running speed of the gas turbine is in the range of (40, 000 to 1, 00,000 rpm) and the operating temperature is as high as 2000 0C, for this reason special metals and alloys have to be used for the various parts of the turbine. 3. Special cooling methods are required for cooling the turbine blades. 4. It is difficult to start a gas turbine as compared to a diesel engine in a diesel power plant 5. The life of a gas turbine plant is upto 10 years, after which its efficiency decreases to less than 10 percent. 1.6.4 Applications of gas turbine power plant • • •
To drive generators and supply loads in steam, diesel or hydro plants To work as combination plants with conventional steam boilers Thermal process industries 11
• • •
Petro chemical industries Power generation (used for peak load and as stand by unit) Aircraft and ships for their propulsion. They are not suitable for automobiles because of their very high speeds.
1.6.5 Classification of Gas Turbine Gas turbines may be broadly classified as: (i) Open cycle gas turbine (ii) Closed cycle gas turbine Open cycle gas turbine In the open cycle gas turbine, fig.1.4, air is drawn into the compressor from the atmosphere. The compressed air is heated by directly burning the fuel in the air at constant pressure inside the combustion chamber. The high pressure hot gases from the combustion chamber drive the turbine and the power is developed when the turbine shaft rotates. Gas turbines are not self starting. A starting motor drives the compressor till fuel is injected inside the combustion chamber, once the turbine starts gaining speed the starting motor is disengaged. Part of the power developed by the gas turbine (about 60%) is used to drive the compressor and the remaining is used to drive a generator or other machinery.
Fig. 1.4 Open cycle gas turbine In the open cycle, system, the working fluid i.e. air and the fuel must be replaced continuously as they are exhausted into the atmosphere. Thus the entire flow comes from the atmosphere and is returned to the atmosphere, hence it is called “open cycle”. Closed cycle gas turbine
In this the compressed air from the atmosphere is heated in air heater (heat exchanger). Heat is added to the air heater from some external source (oil or coal) at constant pressure. High pressure working fluid expands through the turbine and power is developed. The exhaust working fluid is cooled in a pre cooler before the same fluid is sent into the compressor again.
Fig. 1.4 Closed cycle gas turbine In a closed cycle gas turbine the same working fluid is continuously circulated. The fuel required for adding heat from an external source can be any fuel ranging from kerosene, to heavy oil and even peat and coal slurry without reducing the efficiency. 1.7 Diesel power plant 1.7.1 Introduction This is a fossil fuel plant since diesel is a fossil fuel. Diesel engine power plants are installed where supply of coal and water is not available in sufficient quantity. (i) (ii) (iii) (iv) (v)
These plants produce the power in the range of 2 to 50 MW. They are used as standby sets for continuity of supply such as hospitals, telephone exchanges, radio stations, cinema theatres and industries. They are suitable for mobile power generation and widely used in railways and ships. They are reliable compared to other plants. Diesel power plants are becoming more popular because of difficulties experienced in construction of new hydel plants and thermal plants
1.7.2 Layout of Diesel Power plant The essential components of diesel power plant are
• Diesel engine • Air filter and super charger • Engine starting system • Fuel system • Lubrication system • Cooling system • Governing system • Exhaust system Diesel engine: This is the main component of a diesel power plant. The engines are classified as two stroke engine and four stroke engines. Engines are generally directly coupled to the generator for developing power. In diesel engines, air admitted into the cylinder is compressed. At the end of compression stroke, fuel is injected. The fuel is burned and the burning gases expand and do work on the piston. The shaft of the engine is directly coupled to the generator. After the combustion, the burned gases are exhausted to the atmosphere. Air filter and supercharger The air filter is used to remove the dust from the air which is taken by the engine. Air filters may be of dry type, which is made up of felt, wool or cloth. In oil bath type of filters, the air is swept over a bath of oil so that dust particles get coated. The function of the supercharger is to increase the pressure of the air supplied to the engine and thereby the power of the engine is increased. Engine starting system Diesel engine used in diesel power plants is not self starting. Engine starting system includes air compressor and starting air tank. This is used to start the engine in cold conditions by supplying the air. Fuel system It includes the storage tank, fuel pump, fuel transfer pump, strainers and heaters. Pump draws diesel from the storage tank and supplies it to the small day tank through the filter. Day tank supplies the daily fuel need for the engine. The day tank is usually placed high so that diesel flows to engine under gravity. Diesel is again filtered before being injected into the engine by the fuel injection pump. The fuel injection system performs the following functions.
• • • • • •
Filter the fuel Meter the correct quantity of the fuel to be injected Time the injection process Regulate the fuel supply Secure fine atomization of fuel oil Distribute the atomized fuel properly in the combustion; chamber.
The fuel is supplied to the engine according to the load on the plant. Lubrication system It includes oil pumps, oil tanks, coolers and pipes. It is used to reduce the friction of moving parts and reduce wear and tear of the engine parts such as cylinder walls and piston. Lubrication oil which gets heated due to the friction of the moving parts is cooled before recirculation.
In the lubrication system the oil is pumped from the lubricating oil tank through the oil cooler where the oil is cooled by the cold water entering the engine. The hot oil after cooling the moving parts return to the lubricating oil tank.
Fig. 1. 5 Diesel Power plant Cooling system The temperature of the burning fuel inside the engine cylinder is in the order of 15000 C to 20000 C. In order to lower this temperature, water is circulated around the engine. The water envelopes (water jacket) the engine, the heat from the cylinder, piston, combustion chamber etc, is carried by the circulating water. The hot water leaving the jacket is passed through the heat exchanger. The heat from the heat exchanger is carried away by the raw water circulated through the heat exchanger and is cooled in the cooling tower. Governing system It is used to regulate the speed of the engine. This is done by varying the fuel supply according to the engine load.
Exhaust system The exhaust gases coming out of the engine is very noisy. In order to reduce the noise a silencer (muffler) is used. 1.7.3 Working of Diesel Power Plant The air and fuel mixture act as a working medium in diesel engine power plant. The atmosphere air enters inside the combustion chamber during the suction stroke and the fuel is injected through the injection pump. The air and fuel is mixed inside the engine and the charge is ignited due to high compression inside the engine cylinder. The basic principle in diesel engine is that, the thermal energy is converted into mechanical energy and this mechanical energy is converted into electrical energy to produce the power by using generator or alternator. 1.7.4 Applications of Diesel Engines in Power Field The diesel electric power plants fiel d.
are ch iefly used in the follo wing
(a) Peak load plant: Diesel plants can be used in combination with thermal or hydro-plants as peak load units. They can be easily started or stopped at a short notice to meet the peak demand. (b) Mobile plant: Diesel plants mounted on trailers can be used for temporary or emergency purposes such as for supplying power to large civil engineering works. (c) Standby unit: If the main unit fails or cannot cope up with the demand, a diesel plant can supply the necessary power. For example, if water available in a hydro-plant is not adequately available due to less rainfall, the diesel station can operate in parallel to generate the short fall in power. (d) Emergency plant: During power interruption in a vital unit like a key industrial plant or a hospital, a diesel electric plant can be used to generate the needed power. (e) Nursery station: In the absence of main grid, a diesel plant can be installed to supply power in a small town. In course of time, when electricity from the main grid becomes available in the town, the diesel unit can be shifted to some other area which needs power on a small scale. Such a diesel plant is called a "nursery station". (f) Starting stations: Diesel units can be used to run the auxiliaries (like FD and ID fans, BFP, etc.) for starting a large steam power plant. 17
(g) Central stations :Diesel electric plants can be used as central station where the capacity required is small 1.7.5 Advantages and disadvantages of diesel power plant Following are the advantages of dies
el el ectri c station s.
1. It is easy to design and install these electric stations. 2. They are easil y avail able in st andard capacities . 3. They can r espond to lo ad change s without much difficulty. 4. There are les s standby losse s. 5. They o ccupy less space . 6. They can be started and stopped quickly. 7. They require less cooling w ater . 8. Capital cost is less. 9. Less operating and supervisin g staf f required. 10. High efficiency of energy conversion from fuel to ele ctricity. 11. Efficiency at part loads is also highe r. 12. Less of civil engineering wo rk i s re qui red. 13. They can be located nea r the load cent re. 14. There i s no a sh handling probl em. 15. Easier lubric ation system. 1.7.6 Disadvant generation . 1. 2. 3. 4. 5. 6. 7.
age s in in stalling diesel units
for po we r
High operating cost. High maintenance and lubrication cost . Capac ity is restri cted . Cannot be of very big siz e. Noise problem. Cannot supply overload . Unhygienic emis sions. The life of the diesel power plant is less (7 to 10 years) as compared to that of a steam power plant which has a life span of 25 to 45 years. The efficiency of the diesel plant decreases to less than 10% after its life period.
1.8 Hydroelectric power plant 1.8.1 Working principle Hydroelectric power plant (Hydel plant) utilizes the potential energy of water stored in a dam built across the river. The potential energy of the stored water is converted into kinetic energy by first passing it through the penstock pipe. The kinetic energy of water is then converted into mechanical 18
energy in a water turbine. The turbine is coupled to the electric generator. The mechanical energy available at the shaft of the turbine is converted into electrical energy by means of the generator. Because gravity provides the force which makes the water fall, the energy stored in the water is called gravitational potential energy.
Fig. 1.6 Principle of Hydro electric plant 1.8.2 Layout of Hydro electric power plant Fig.1.7 shows the schematic representation of a Hydro electric power plant. The main components are • • • • • • • • • • •
Water reservoir Dam Spillway Gate Pressure tunnel Surge tank Penstock Water turbine Draft tube Tail race level Power house
Water reservoir: In a reservoir the water collected from the catchment area during rainy season is stored behind a dam. Catchment area gets its water from rains and streams. Continuous availability of water is a basic 19
necessity for a hydroelectric power plant. The level of water surface in the reservoir is called Head water level. The water head available for power generation depends on the reservoir height. Dam: the purpose of the dam is to store the water and to regulate the out going flow of water. The dam helps to store all the incoming water. It also helps to increase the head of the water. In order to generate a required quantity of power, it is necessary that a sufficient head is available. Spillway: Excess accumulation of water endangers the stability of dam construction. Also in order to avoid the overflow of water out of the dam especially during rainy seasons spillways are provided. This prevents the rise of water level in the dam. Spillways are passages which allow the excess water to flow to a different storage area away from the dam. Gate: A gate is used to regulate or control the flow of water from the dam. Pressure tunnel: It is a passage that carries water from the reservoir to the surge tank. Surge tank: A surge tank is a small reservoir or tank in which the water level rises or falls due to sudden changes in pressure. There may sudden increase of pressure in the penstock pipe due to sudden backflow of water, as load on the turbine is reduced. This sudden rise of pressure in the penstock pipe is known as water hammer.
Fig. 1.7 Layout of Hydro electric Power plant
A surge tank is introduced between the dam and the turbine and serves the following purposes: 1. To reduce the distance between the free water surface in the dam and the turbine, thereby reducing the water hammer effect. Otherwise, penstock will be damages by the water effect. 2. To serve as a supply tank to the turbine when the water in the pipe is accelerated during increased load conditions and as a storage tank when the water is decelerating during reduced load conditions. Penstock: Penstock pipe is used to bring water from the dam to the hydraulic turbine. Penstock pipes are made up of steel or reinforced concrete. The turbine is installed at a lower level from the dam. Penstock is provided with a gate valve at the inlet to completely close the water supply. It has a control valve to control the water flow rate into the turbine. Water turbine or hydraulic turbine (Prime mover): The hydraulic turbine converts the energy of water into mechanical energy. The mechanical energy (rotation) available on the turbine shaft is coupled to the shaft of an electric generator and electricity is produced. The water after performing the work on turbine blade is discharged through the draft tube. The prime movers which are in common use are Pelton wheel, Kaplan turbine, Francis turbine. Draft tube: Draft tube is connected to the outlet of the turbine. It converts the kinetic energy available in the water into pressure energy in the diverging portion. Thus, it maintains a pressure of just above the above the atmospheric at the end of the draft tube to move the water into a tail race. Water from the tail race is released for irrigation purposes. Tail race level: Tail race is a water path to lead the water discharged from the turbine to the river or canal. The water held in the tail race is called Tail race water level. Power House: The power house accommodates the water turbine, generator, transformer and control room. As the water rushes through the turbine, it spins the turbine shaft, which is coupled to the electric generator. The generator has a rotating electromagnet called a rotor and a stationary part called a stator. The rotor creates a magnetic field that produces an electric charge in the stator. The charge is transmitted as electricity. The step up transformer increases the voltage of the current coming from the stator. The electricity is distributed through power lines.
1.8.3 Classification of Hydro electric power plant Hydro electric power plants are usually classified according to the available of head of water • High head power plants • Medium head power plants • Low head power plants High head power plants: When the operating head of water exceeds 70 meters, the plant is known as High head power plant. Pelton wheel turbine is the prime mover used. Medium head power plants: When the water ranges from 15 to 70 meters, then the power plant is known as Medium head power plant. It uses Francis Turbine. Low head power plants: When the head is less than 15 meters, the plant is named as Low head power plant. It uses Francis or Kaplan turbine as prime mover. 1.8.4 Advantages of hydro electric power plant 1. Water source is perennially available. No fuel is required to be burnt to generate electricity. It is aptly termed as 'the white coal'. Water passes through turbines to produce work and downstream its utility remains undiminished for irrigation of farms and quenching the thirst of people in the vicinity. 2. The running costs of hydropower installations are very low as compared to thermal or nuclear power stations. 1n thermal stations, besides the cost of fuel, one has to take into account the transportation cost of the fuel also. 3. There is no problem with regards to the disposal of ash as in a thermal station. The problem of emission of polluting gases and particulates to the atmosphere also does not exist. Hydropower does not produce any greenhouse effect, cause the pernicious acid rain and emit obnoxious NO. 4. The hydraulic turbine can be switched on and off in a very short time. In a thermal or nuclear power plant the steam turbine is put on turning gear for about two days during start-up and shut-down. . 5. The hydraulic power plant is relatively simple in concept and selfcontained in operation. Its system reliability is much greater than that of other power plants. 6. Modern hydropower equipment has a greater life expectancy and can easily last 50 years or more. This can be compared with the effective life of about 30 years of a thermal or nuclear station.
7. Due to its great ease of taking up and throwing off the load, the hydropower can be used as the ideal spinning reserve in a system mix of thermal, hydro and nuclear power stations. 8. Modern hydro-generators give high efficiency over a considerable range of load. This helps in improving the system efficiency. 9. Hydro-plants provide ancillary benefits like irrigation, flood control, afforestation, navigation and aqua-culture. 10. Being simple in design and operation, the hydro-plants do not require highly skilled workers. Manpower requirement is also low. 1.8.5 Disadvantages of Water Power 1. Hydro-power projects are capital-intensive with a low rate of return. The annual interest of this capital cost is a large part of the annual cost of hydropower installations. 2. The gestation period of hydro projects is quite large. The gap between the foundation and completion of a project may extend from ten to fifteen years. 3. Power generation is dependent on the quantity of water available, which may vary from season to season and year to year. If the rainfall is in time and adequate, then only the satisfactory operation of the plant can be expected. 4. Such plants are often far way from the load centre and require long transmission lines to deliver power. Thus the cost of transmission lines and losses in them are more. 5. Large hydro-plants disturb the ecology of the area, by way of deforestation, destroying vegetation and uprooting people. Strong public opinion against. Erection of such plants is a deterrent factor. The emphasis is now more on small, mini and micro hydel stations. 1.8.6 Hydro electric power plant in India Srisailam Hydel power plant – AP – 770 MW Upper sileru Hydor electric project – AP - 120 Kodayar hydro electric power plant – TN – 100 MW Iddiki hydel project – Kerala – 800 MW
1.9 Nuclear power plant 1.9.1. Introduction As large amounts of coal and petroleum are being used to produce energy, time may come when their reserves may not be able to meet the energy requirements. Thus there is tendency to seek alternative sources of energy. The discovery that energy can be liberated by the nuclear fission of materials like uranium (U), Plutonium (Pu), has opened up a new source of
power of great importance. The heat produced due to fission of U and Pu is used to heat water to generate steam which is used for running turbogenerator. It has been found that one kilogram of U can produce as much energy as can be produced by burning 4500 tonnes of high grade coal. This shows that nuclear energy can be successfully employed for producing low cost energy in abundance as required by the expanding and industrializing population of future. Some of the factors which go in favor of nuclear energy are as follows: 1. Hydro electric power is of storage type and is largely dependent of monsoons. The systems getting power from such plants have to shed load during the period of low rainfall. 2. Oil is mainly needed for transport, fertilizers and petrochemicals and thus cannot be used in large quantities for power generation. 3. Coal is available only in some parts of the country and transportation of coals requires big investments. 4. Nuclear power is partially independent of geographical factors, the only requirement being there should be reasonably good supply of water. Fuel transportation networks and larger storage facilities are not needed and nuclear power plant is a clean source of power which does not pollute the air if radioactive hazards are effectively prevented. 5. Large quantity of energy is released with consumption of only a small amount of fuel. 1.9.2 Nuclear fission The fuel inside the reactor is a metal called uranium. Uranium exists as an isotope in the form of U235, U234 and U238. Out of these isotopes U235 is more unstable. When a neutron is captured by a nucleus of an atom of U 235, it splits up roughly into two equal fragments and about 2.5 neutrons are released and a large amount of energy (nearly 200 million electron volts MeV) is produced. This is called fission process. The neutrons so produced are very moving neutrons and can be made to fission other nuclei of U 235 thus enabling a chain reaction to take place. When a large number of fission occurs, enormous amount of heat is produced. The following fig.1.8. shows the chain reaction
Fig. 1.8 Nuclear Fission It may be observed from the fig.1.8 that 2.5 neutrons are released in fission of each nucleus of U235, out of these one neutron is used to sustain the chain reaction, 0.9 neutrons is absorbed by U238 and becomes Pu239. The remaining 0.6 neutrons escape from the reactor. Moderators are provided to slow down the neutrons from the high velocities but not to absorb them. The moderators which are commonly used are ordinary water and Heavy water. The fig.1.9 shows how the reactor is put on and off.
Fig. 1.9 Control rods Control rods limit the number of fuel atoms that can split. They are made up of a material that absorbs neutrons. To turn on the reactor some rods are pulled out. The rods are made of boron or cadmium.
1.9.3 Main components of a nuclear power plant The main components of a nuclear power plant are 25
• • • • • • • • •
Nuclear fuel Nuclear reactor Steam generator Moderator Control rods Reflector Turbine Condenser Shielding
Nuclear Fuel: Fuel of a nuclear reactor should be fissionable material which can be defined as an element or isotope whose nuclei can be caused to undergo nuclear fission by nuclear bombardment and to produce a fission chain reaction. It can be one or all of the following U235, U233 and Pu239 Nuclear reactor: A nuclear reactor may be regarded as a substitute for the boiler furnace of a steam power plant. Heat is produced in the reactor due to nuclear fission of the fuel. During the fission process, the large amount of heat is liberated. This large amount of heat is absorbed by the coolant and it is circulated through the core. The various types of reactors used in nuclear power plant is 1. 2. 3.
Boiling water reactor Pressurised water reactor Fast breeder reactor
Steam generator: The heat liberated in the reactor is taken up by the coolant circulating through the core. The purpose of the coolant is to transfer the heat generated in the reactor core and use it for steam generation. Ordinary water or heavy water is a common coolant. Moderator: It is used to reduce the kinetic energy of fast neutrons into slow neutrons and to increase the probability of chain reaction. Graphite, heavy water and beryllium are generally used as moderator. A moderator should possess the following properties: 1. It should have high thermal conductivity 2. It should be available in large quantities in pure form 3. It should have high melting point in case of solid moderators and low melting point in case of liquid moderators. Solid moderators should also possess good strength and machinability. 4. It should provide good resistance to corrosion 5. It should be stable under heat and radiation 6. It should be able to slow down neutrons 26
Control rods: They regulate the rate of a chain reaction. They are made of boron, cadmium or other elements which absorb neutrons. Control rods should posses the following properties: 1. 2. 3. 4.
They should have adequate heat transfer properties They should be stable under heat and radiation They should be corrosion resistant They should be sufficient strong and should be able to shut down the reactor almost instantly under all conditions. 5. They should have sufficient cross sectional area for the absorption. Reflector: The neutrons produced during the fission process will be partly absorbed by the fuel rods, moderator, coolant or structural material etc. Neutrons left unabsorbed will try to leave the reactor core and will be lost. Such loss is minimized by surrounding the reactor core by a material called reflector which will send the neutrons back into the core. The returned neutrons can then cause more fission and improve the neutrons economy of the reactor. Generally the reflector is made up of graphite and beryllium. Turbine: The steam produced in the steam generator is passed to the turbine. Work is done by the expansion of stem in the turbine. Condenser: The exhaust steam from the turbine flows to the condenser where cooling water is circulated. The exhaust steam is condensed to water in the condenser by cooling. The condensate is pumped again into the steam generator by the feed pump. Shielding: The reactor is a source of intense radioactivity. These radiations are very harmful and shielding is provided to absorb the radioactive rays. A thick concrete shielding and a pressure vessel are provided to prevent the radiations escaped to atmosphere. 1.9.4 Working of a Nuclear Power plant The reactor of a nuclear power plant is similar to the furnace of steam power plant. The heat liberated in the reactor due to the nuclear fission of the fuel is taken up by the coolant circulating through the reactor core. Hot coolant leaves the reactor at top and then flows through the tubes of steam generator (boiler) and passes on its heat to the feed water. The steam produced is passed through the turbine and after work has been done by expansion of steam in the turbine, steam leaves the turbine and flows to condenser. Pumps are provided to maintain the flow of coolant, condensate and feed water. 1.9.5 Boiling water reactor (BWR) 27
In this reactor, enriched uranium (enriched uranium contains more fissionable isotope U235 than the naturally occurring percentage 0.7% as nuclear fuel and water is used as coolant. Water enters the reactor at the bottom. It takes up the heat generated due to the fission of fuel and gas converted into steam. Steam leaves the reactor at the top and flows into the turbine. Water also serves as moderator. India’s first nuclear power plant at Tarapur has two reactors (each of 200 MW capacity) of boiling water reactor type.
Fig. 1.10 Boiling water reactor 1. 9.6 Pressurised Water reactor (PWR)
Fig. 1.11 Pressurised Water Reactor A pressurised water nuclear plant is shown in fig. It uses enriched uranium as fuel. Water is used as coolant and moderator. Water passes through the reactor core and takes up the heat liberated due to nuclear fission of the fuel. In order that water may not boil (due to its low boiling point 2120 F at atmospheric conditions) and remain in liquid state, it is kept under a pressure of about 1200 p.s.i.g in the pressuriser. This enables water to take up more heat from the reactor. From the pressuriser, water flows to the steam generator where it passes on its heat to the feed water which in turn gets converted into steam. 1.9.7 Fast breeder reactor (FBR) In this reactor the core containing U235 is surrounded by a blanket (a layer of fertile material placed outside the core) or fertile material U 238. In this reactor no moderator is used. The fast moving neutrons liberated due to fission of U235 are absorbed by U238 which gets converted into fissionable material Pu239, Pu239 is capable of sustaining chain reaction. Thus the reactor is important because it breeds fissionable material from fertile material U238 available in large quantities. This reactor uses two liquid metal coolant circuits. Liquid sodium is used as primary coolant when circulated through the tubes of intermediate heat exchanger transfer its heat to secondary coolant sodium potassium alloy. The secondary coolant while flowing through the tubes of steam generator, transfer its heat to feed water. Fast breeder reactors are better than conventional reactor both from the point of view of safety and thermal efficiency. For India which already is fast advancing towards self reliance in the field of nuclear power technology, 29
the fast breeder reactor becomes inescapable in view of the massive reserves of thorium and the finite limits of its uranium resources. 1.9.8 Advantages of nuclear power plant 1. The fuel used in nuclear power plant is uranium; it does not release chemical or solid pollutants into the air during use. 2. Space required is less when compared with other power plants. 3. Fuel consumption is very less. 4. Fuel transportation cost is low and no large storage area for fuel is required. 5. The plant is not affected by weather conditions. The plant can function throughout the year (Hydel power plants depends on monsoon) 6. By using nuclear fuel we can conserve the fossil fuels like coal, oil, gas etc for other purposes. For example coal can be used to power steam engines, oil can be used for running vehicles, and gas be used for cooking. 7. Number of workers required is less. 8. Nuclear power plant is the only source which can meet the increasing demand of electricity. 9. A nuclear power plant uses much less fuel than a fossil fuel plant 1 metric ton of uranium fuel = 3 million metric tons of coal = 12 million barrels of oil
1.9.9 Disadvantages of nuclear power plant 1. Nuclear plants cost more to build than thermal or hydro electric power plants of the same capacity. 2. Radioactive wastes must be disposed carefully, otherwise it will adversely affect the health of workers and the environment as a whole. 3. Maintenance cost of the plant is high. 4. Not suitable for varying load conditions 5. Well trained persons are required to operate the plant. 1.9.10 Nuclear power stations in India • • • •
Tarapur Nuclear power station (Bombay) – Boiling water reactor – 200 MW Rana Pratap Sagar Nuclear power station (Kota in Rajasthan) – Two 200 MW Kalpakkam Nuclear power station – Two 235 MW – Pressurised water reactor Narora Nuclear power station (Uttar Pradesh) – Two 235 MW – CANDU reactor 30
Kakarpur Nuclear power plant (Gujarat) – 4 235 MW CANDU type reactor