Module-2.pptx Electric Power Generation and Economics

Steam Power Plant

1. 1 2 course: ELECTRICAL POWER GENERATION AND ECONOMICS Course code:bEE405A 4th Semester Course Coordinator: Prof. Renuka R.Thakai Department of Electrical and Electronics Engineering S.G.Balekundri Institute of Technology, Belagavi 1

2. 1 2 Department of Electrical and Electronics Engineering S.G.Balekundri Institute of Technology, Belagavi MODULE -2 STEAM POWER PLANT,DIESEL POWER PLANT AND GAS POWER PLANT Syllabus: Steam Power Plants: Introduction, Efficiency of steam plants, Merits and demerits of plants, selection of site. Working of steam plant, Power plant equipment and layout, Steam turbines, Fuels and fuel handling, Fuel combustion and combustion equipment, Coal burners, Fluidized bed combustion, Combustion control, Ash handling, Dust collection, Draught systems, Feed water, Steam power plant controls, plant auxiliaries. Diesel Power Plant: Introduction, Merits and demerits, selection of site, elements of diesel power plant, applications. Gas Turbine Power Plant: Introduction Merits and demerits, selection of site, Fuels for gas turbines, Elements of simple gas turbine power plant, Methods of improving thermal efficiency of a simple gas power plant, Closed cycle gas turbine power plants. Comparison of gas power plant with steam and diesel power plants.

3. The use of steam power started when it was first used in locomotives invented by James Watt. Thereafter , steam power is used to rotate the prime mover of electric generator and it is known as steam power plant . In this process heat energy is converted into mechanical energy and then to electrical energy through turbine–generator system. Heat energy may be obtained by the proper combustion of a commercial fuel such as coal, gas, oil etc. Since abundant availability with reasonably no cost, water is used to generate steam, which readily conveyed through pipes, in a boiler by burning fuel in furnace. Steam power plants are also called thermal power plants . The prime movers of steam power plant may be operated either in noncondensing or condensing. In the noncondensing operation, the steam is exhausted from the prime movers and is discharged at atmospheric pressure or at greater than atmospheric pressure. Whereas in condensing plant, the prime movers exhaust discharge steam into a condenser in which the pressure is less than atmospheric and steam is converted to water. This is most commonly used in modern age power plants.

4. Merits & Demerits of Steam Power plant Merits ➢ Fuel used is cheaper ➢ Less space is required when compared with hydro electric plant. ➢ Cheaper in initial cost in comparison with other power plants. ➢ Cheaper in production cost when compared with diesel power plants. ➢ Such plants can be installed at any place irrespective of the existence of fuels, whereas hydro electric power plants can be installed only where water is available. ➢ Such plants can be located near to load centers. ➢ They provide the stable output

5. Demerits ➢ High maintenance & operating costs ➢ Pollution of atmosphere due to fumes & residues from pulverized fuels . ➢ Requirement of water in huge quantity. ➢ Handling of coal & disposal of ash is quite difficulty Troubles from smoke & heat from the plant ➢ Costlier in operating cost when compared with hydroelectric plants.

6. Site Selection: 1.Supply of Fuel (Coal) The thermal plant should be located near the coal mines so that the transportation cost is minimum. Although, if the thermal power plant is to be installed at a place where coal is not available near the site, then care should be taken that adequate facilities exist for the transportation of coal. 2.Water Availability Since in a thermal power plant, huge amount of water is required for the operation. Hence, a thermal power plant should be located near a river or canal to ensure the continuous supply of water. 3. Transportation Facilities As a modern thermal power plant requires transportation of material (ex. coal) and machinery. Hence, the power plant should be well connected to the other parts of the country by rail, road, etc. so that adequate transportation facilities are available.

7. 4.Type and Cost of Land The thermal power should be located at a place where land is cheap and the further extension is possible. As in a thermal power plant, heavy equipment are to be installed, therefore the bearing capacity of the land should be adequate. 5.Near to the Load Centres The thermal power plant should be located near to the centres of the load, so that the transmission cost is reduced. It is more important if DC supply system is adopted rather than AC supply system. 6.Distance from the Populated Area The thermal power plant should be located at a considerable distance from the populated area. Because, a large amount of coal is burnt in a thermal power station, which produces smoke and fumes that pollute the surrounding environment and may have adverse effects on the health.

8. Working of steam power plants : Steam power plants operates on the Rankin cycle. Coal is burnt in the boiler which converts water into steam. The steam is expanded in turbine which produces mechanical power driving the alternator coupled to the turbine The steam after expansion in turbine is usually condensed in condenser to fed to boiler again Working of steam power plants may be divided into four parts Coal and ash handling arrangement Steam generating plant Steam turbine Alternator Feed water Cooling arrangement

9. 1.Coal and ash handling plant : The coal is transported to the power station by road or rail and is stored in the coal storage plant. From the coal storage plant, coal is delivered to the coal handling plant where it is pulverized (i.e., crushed into small pieces) in order to increase its surface exposure, thus promoting rapid  combustion  without using large quantity of excess air. The pulverized coal is fed to the boiler by belt conveyors. The coal is burnt in the boiler and the ash produced after the complete combustion of coal is removed to the ash handling plant and then delivered to the ash storage plant for disposal. The removal of the ash from the boiler furnace is necessary for proper burning of coal. 2.Steam generating plant : The steam generating plant consists of a boiler for the production of steam and other auxiliary equipment for the utilisation of flue gases . Boiler: The heat of combustion of coal in the boiler is utilised to convert water into steam at high temperature and pressure. The flue gases from the boiler make their journey through super­heater , economiser , air pre-heater and are finally exhausted to atmosphere through the chimney.

10. Superheater :   The steam produced in the boiler is wet and is passed through a superheater where it is dried and superheated (i.e., steam temperature increased above that of boiling point of water) by the flue gases on their way to chimney. Superheating provides two principal benefits . Firstly, the overall efficiency is increased. Secondly, too much condensation in the last stages of turbine (which would cause blade corrosion) is avoided. The superheated steam from the superheater is fed to steam turbine through the main valve. Economiser :   An economiser is essentially a feed water heater and derives heat from the flue gases for this purpose. The feed water is fed to the economiser before supplying to the boiler . The economiser extracts a part of heat of flue gases to increase the feed water temperature.

11. Air preheater:   An air preheater increases the temperature of the air supplied for coal burn­ing by deriving heat from flue gases. Air is drawn from the atmosphere by a forced draught fan and is passed through air preheater before supplying to the boiler furnace. The air preheater extracts heat from flue gases and increases the temperature of air used for coal combustion . The principal benefits of preheating the air are : increased thermal efficiency and increased steam capacity per square metre of boiler surface . 3.Steam turbine :  The dry and super heated steam from the super heater is fed to the steam turbine through main valve. The heat energy of steam when passing over the blades of turbine is converted into mechanical energy. After giving  heat energy  to the turbine, the steam is exhausted to the condenser which condenses the exhausted steam by means of cold water circulation. 4.Alternator:   The steam turbine is coupled to an alternator. The alternator converts mechanical energy of turbine into electrical energy. The electrical output from the alternator is delivered to the bus bars through transformer,  circuit breakers  and isolators.

12. 5.Feed water.   The condensate from the condenser is used as feed water to the boiler. Some water may be lost in the cycle which is suitably made up from external source. The feed water on its way to the boiler is heated by water heaters and economizer. This helps in raising the overall efficiency of the plant. 6.Cooling arrangement :  In order to improve the efficiency of the plant, the steam exhausted from the turbine is condensed* by means of a condenser. Water is drawn from a natural source of supply such as a river , canal or lake and is circulated through the condenser. The circulating water takes up the heat of the exhausted steam and itself becomes hot. This hot water coming out from the condenser is discharged at a suitable location down the river. In case the availability of water from the source of supply is not assured throughout the year, cooling towers are used . During the scarcity of water in the river, hot water from the condenser is passed on to the cooling towers where it is cooled. The cold water from the cooling tower is reused in the condenser.

13. Steam Turbines: Steam turbines are have higher thermodynamic efficiency. The basic construction of steam turbine is simple. There is no need of piston rod mechanism and side valves,no fly wheel is needed. Steam turbines can be built in large sizes. The steam turbines are of two types. 1.Impulse turbine 2.Reaction turbine. Impulse turbine: An impulse turbine is a type of steam turbine that converts thermal energy into mechanical energy through the use of high-velocity jets of steam. It was invented by French engineer Victor Gustave Girard in the 19th century and has since been used in various applications, including power generation and propulsion systems.

14. An impulse turbine is a type of steam turbine that uses the impact force of high-velocity steam jets to generate mechanical energy. It operates on the principle of Newton’s third law of motion, which states that every action has an equal and opposite reaction. One of the advantages of impulse turbines is their simplicity in design and operation, making them suitable for use in smaller power generation systems. They also have high efficiency, as they can convert up to 70% of thermal energy into mechanical energy. Additionally , impulse turbines can operate at high speeds, which is advantageous for applications such as aircraft propulsion systems.

15. Working Principle of Impulse Turbine The working principle of an impulse turbine is based on the conversion of the kinetic energy of a high-velocity jet of steam into Mechanical Energy. The turbine consists of a stationary set of nozzles that directs the steam onto a set of blades mounted on a rotor. The blades of impulse turbine are designed similarly to the  Kaplan turbine  to redirect the flow of steam, causing the rotor to rotate. When the steam passes through the nozzle, it is accelerated to a very high velocity, creating a high-speed jet of steam. The steam jet then impacts the blades of the rotor, causing it to rotate due to the force of the steam. The steam is then exhausted from the turbine through the exhaust system.

16. Reaction turbines In the  reaction turbine , the  rotor  blades themselves are arranged to form convergent  nozzles . This type of turbine makes use of the reaction force produced as the steam accelerates through the nozzles formed by the stator. Steam is directed onto the rotor by the fixed vanes of the  stator . It leaves the stator as a jet that fills the entire circumference of the rotor. The steam then changes direction and increases its speed relative to the speed of the blades . A pressure drop occurs across both the stator and the rotor, with steam accelerating through the stator and decelerating through the rotor, with no net change in steam velocity across the stage but with a decrease in both pressure and temperature, reflecting the work performed in the driving of the rotor.

17. Fuel Handling System: Coal delivery equipment is one of the major components of plant cost. The various steps involved in coal handling are as follows: Coal delivery . 2. Unloading 3 . Preparation 4 . Transfer 5. Outdoor storage 6 . Covered storage 7 . In plant handling 8. Weighing and measuring 9. Feeding the coal into furnace.

18. Coal delivery: The coal from supply points is delivered by ships or boats to power stations situated near to sea or river whereas coal is supplied by rail or trucks to the power stations which are situated away from sea or river. The transportation of coal by trucks is used if the railway facilities are not available . Unloading: The type of equipment to be used for unloading the coal received at the power station depends on how coal is received at the power station. If coal delivered by trucks, there is no need of unloading device as the trucks may dump the coal to the outdoor storage. Coal is easily handled if the lift trucks with scoop are used. In case the coal is brought by railways wagons, ships or boats, the unloading may be done by car shakes, rotary car dumpers, cranes, grab buckets and coal accelerators. Rotary car dumpers although costly are quite efficient for unloading closed wagons. Preparation: When the coal delivered is in the form of big lumps and it is not of proper size, the preparation (sizing) of coal can be achieved by crushers, breakers, sizers, driers and magnetic separators.

19. iv)Transfer After preparation coal is transferred to the dead storage by means of the following systems. Belt conveyors Screw conveyors Bucket elevators Grab bucket elevators Skip hoists Flight conveyor Pulverized coal storage in Bunker Periodically a power plant may encounter the situation where coal must be stored for sometimes in a bunker, for instance during a plant shut down. The bunker, fires can occur in dormant pulverized coal from spontaneous heating within 6 day of loading. This time can be extended to 13 days when a blanket of CO2 is piped into the top of the bunker. The perfect sealing of the bunker from air leakage can extend the storage time as two months or more. The coal in the bunker can be stored as long as six months by expelling air from above the coal with the use of CO2 and then blanketing of all sources of air

20. Fuel Combustion and Combustion Equipment: Fuel is burned in confined space called furnace. An efficient combustion of fuel is essential for economical working of power plant. In case of firing unpulverised coal in combustion furnaces, two methods are used. 1. Hand Firing 2.Stoker(or mechanical ) firing. A stoker is a fuel firing device which receives fuel by gravity, carries it into the furnace for combustion and after combustion discharges ash at appropriate point. In case of firing pulverised coal in combustion furnaces, two methods are used . Unit system Central (or storage system)

21. Hand Firing: This method of firing is simple. Requiring no capital investment. This method can be used in small installations. Adjustment for the supply of air are to be made every time coal is fed to furnace. This method of fuel firing is discontinuous process, and there is a limit to the size of furnace which can be efficiently fired by this method. While burning coal the total area of air openings varies from 30 to 50% of the total grate area. Hand fired grates are made up of cast iron . Stoker Firing : In this method of firing coal is carried into the furnace for combustion and ash formed after combustion is discharged at appropriate point. Stokers are designed for meeting specific requirement of fuels. It is possible to burn caking, non-caking and non clinking fuels.

22. Mechanical stokers are commonly used to feed solid fuels into the furnace in medium and large size power plants . The various advantages of stoker firing are as follows : 1. Large quantities of fuel can be fed into the furnace. Thus greater combustion capacity is achieved 2 . Poorer grades of fuel can be burnt easily. 3 . Stoker save labour of handling ash and are self-cleaning. 4 . By using stokers better furnace conditions can be maintained by feeding coal at a uniform rate . 5. Stokers save coal and increase the efficiency of coal firing. Mechnical stokers are of two types underfeed stokers and overfeed stokers Incase of underfeed stokers ,fuel is supplied in to the furnace below the point of air admission. In case of overfeed stoker, coal is fed on to the grate above the point of air admission.

23. Pulverized Coal Firing : In pulverized fuel firing system, the coal is powdered and then charged into the combustion chamber with the help of hot air current. The main purpose of pulverizing coal is to increase the surface area of exposure to the combustion process, which results in faster and efficient combustion. In burning the pulverized coal, the secondary air required for the complete combustion of fuel is supplied separately to the combustion chamber. The resulting turbulence in the combustion chamber helps for uniform mixing of fuel and air. The air required to carry the pulverized coal and dry it before entering the combustion chamber is termed the Priming Air, and the air supplied separately for complete combustion is termed the Secondary Air. Pulverized coal firing systems are universally adopted far large scale power plants. Pulverised Fuel Burning System,there are two common methods of pulverized fuel burning systems 1.Unitsystem 2 . Central or Bin system

24. Coal burners: The coal burners are employed to fire the pulverized coal along with primary air in to the furnace. The secondary air is admitted separately below the burner, around the burner or elsewhere in the furnace. The main requirement for a good coal burner is capability of producing uniform and stable flame with almost complete combustion of the fuel. Coal burners can be classified according to the design and by their arrangement in the furnace . In one type of firing, say in opposed firing the burners are placed on the opposite walls of the furnace and they fire directly against each other causing intimate mixing of the fuel and air. In second type of firing cross firing, the burners fire in the vertical direction and in horizontal direction and the fuel streams intersect with each other. I n third type of firing burners are built into the furnace walls at the corners. They inject the air-fuel mixture tangentially to an imaginary circle in the centre of furnace. As the flames intercept, it leads to a swirling action. This produces sufficient turbulence in the furnace for complete combustion. Hence in such burners, there is no need to produce high turbulence within the burners. Tangential burners give fast and high heat release rates

25. Fluidized bed combustion: Fluidization is a new method of mixing fuel and air for obtaining combustion. A fluidized bed may be defined as the bed of solid particles behaving as a fluid. It operates on the following principle: When an evenly distributed air is passed upward through a finely divided bed of solid particles at low velocity, the particles remain undisturbed but if the velocity is steadily increased, a stage is reached when the individual particles are suspended in the air stream. If the air velocity is further increased, the bed becomes highly turbulent and rapid mixing of particles occur which appear like formation of bubbles in a boiling liquid and a bed is said to be fluidized . The velocity of air causing fluidization depends on a given particle size, range and density . In fluidized bed combustion, rapid mixing ensures uniformity of temperature. The main advantage of such a combustion system is that municipal waste, sewage plant sludge, biomass, agricultural waste (such as rice straw) and other high moisture fuels can be used for generating heat . A fluidized furnace has an enclosed space with a base having openings to admit air.

26. Crushed coal, ash and either crushed dolomite or limestone is mixed in the bed furnace and high velocity combustion air is then passed through the bed, entering from the furnace bottom. With the steady increase in the velocity of air, a stage will be reached when the pressure drop across the bed becomes equal to the weight per unit cross section of the bed and this critical velocity is called the minimum fluidizing velocity . With the further increase in velocity of air, the bed will expand and allow passage of additional air, in the form of bubbles. When the air velocity becomes 3-5 times the critical velocity, the bed resembles a violently boiling liquid. The evaporator tubes of boiler are directly immersed in the fluidized bed and the tubes, being in direct contact with the burning coal particles, produce very high heat transfer rates.

27. Ash handling: Belt Conveyor System: In this system, the ash is made to fall through a water seal over the belt conveyor in order to cool it down and then carried to a dumping site over the belt . This is a continuous handling and the power consumption is low . It can deliver around 3 tonnes of ash per hour with a speed of 0.3 m/minute . Life of belt is about 5 years in normal working conditions. This system of ash handling is employed in small power plants.

28. 2. Pneumatic System : In this system, air is employed as the medium for driving the ash through a pipe over long distances. The ash is passed on to the crushers from the boilers and then into the conveying pipe. Air is sucked through the delivery end which makes the ash to flow into the separators where the ash is collected in ash hoppers. The dusty air is filtered and exhausted to atmosphere through the exhaust fan. The system can handle 5-30 tonnes of ash per hour. The advantages of this system are that it can carry ash through long distances and dust menace is reduced. Disadvantages of this system are that large amount of wear and tear results in the conveying pipe, high labour and maintenance charges and noisy operation. This system is usually employed for disposal of fly ash.

29. 3 . Hydraulic System: In this system, a stream of water carries ash along with it in a closed channel and disposes it off to the proper site. This system can be used for large capacity power plants where the ash is to be disposed off over long distances. This is a healthy, clean, dustless and completely enclosed system . This system can also handle molten ash breaking it into small sizes. Hydraulic systems are of two types namely : ( i ) High pressure system, and ( ii) Low pressure system. High pressure system operates intermittently whereas low pressure system is continuous one. In a high pressure system the hoppers below the boilers are fitted with water nozzles both at the top and on sides. The top nozzles quench the ash while the side ones provide the driving force for the ash. The ash and water then flow along a trough to a receiving hopper or sump where the ash is separated from the water. The water is used again while the ash is sent out through carriages. High pressure system is more expensive to install and operate.

30. In a low pressure system a trough or drain is provided below the boilers and water is made to flow through the trough. The ash directly falls into the trough and is carried by the water to the sumps and tanks. In the sump, the water and ash are made to pass through a screen in order to separate them out. The water is pumped back to the trough and used again while the ash is removed through carrier units and sent out to the dumping site. 4. Steam Jet System: This system employs jets of high pressure blowing in the direction of ash travel through a conveying pipe in which the ash from the boiler ash hoppers is fed. Chilled iron or nickel alloy are used in lining the pipe as ash is abrasive and travels at a high speed. This system is useful where installation of other ash handling plants and conveyors is not possible because of lack of space or where the path of travel of ash is not straight. This system is employed in small and medium size plants. Steam consumption is about 110 kg per tonne of material conveyed.

31. Dust collection: The exhaust gases leaving the boiler contain particles of solid matter in suspension-smoke, dust, soot, fly ash or carbon as material called ‘cinder’. The quantity of these solid particles largely depends upon the method of fuel firing . Flue dust is greatest with pulverised fuel and spreader stoker firing systems and is much less with underfeed stoker systems . In case of pulverized fuel firing, 60 to 80 per cent of the total ash produced in the furnace, escapes through the chimney as flue dust. Removal of dust from the exhaust gases is very important . Gas cleaning devices make use of certain physical/electrical properties of the particular matter of the gas stream. Basically , gas cleaning devices called the dust collectors may be classified into mechanical and electrical ones (electrostatic precipitators). Mechanical dust collectors have efficiency increasing with load while the efficiency of electrostatic precipitators falls with the increase in load. Combination of the two, giving constant efficiency characteristic, is often employed. Heavier dust particles are removed by mechanical dust collectors and finer particles are eliminated by electrostatic precipitator.

32. Mechanical dust collectors can be further classified as wet and dry dust collectors. In wet type units, dust is washed away from the flue gases by spraying water on it. This system is usually not used because it needs large amounts of water. The principles employed in case of dry type mechanical dust collectors are shown in Fig . In Fig. (a) the dust area is increased causing reduction in gas velocity and thus settles out heavier dust particles. In Fig. (b) there is a sudden change in the direction of flow of the gas resulting in the settling out of the heavier particles which cannot flow along the gas. In Fig. (c) the dust particles strike the baffles placed in the path of the exhaust gases and the dust particles will settle out

33. Electrostatic Precipitator: It essentially consists of two sets of electrodes which are completely insulated from each other and a high voltage electrostatic field is maintained across them. One set, called the emitting or discharge electrode, is in the form of thin wires and the other set is called the collecting electrode. The emitting or discharge electrodes are placed in the centre of a pipe in case of tubular type precipitator (or midway between two plates in case of plate type precipitator) and are connected to negative polarity of HVDC source (25 to 100 kV) while the collecting electrodes are connected to the positive polarity of the source and are earthed. High electrostatic field thus set up between the two sets of electrodes. The dust particles in the gas acquire negative charge and are attracted to the electrodes connected to the positive polarity (collecting electrodes) and get deposited there. The deposited dust is made to fall off the electrodes when rapped mechanically.

34. Draught systems : In a boiler, the combustion of the fuel requires supply of sufficient quantity of air and removal of exhaust gases and this is achieved by draught system. The circulation of air is caused by a difference in pressure, known as draught. Thus the draught is the difference in pressure between the two points i.e., atmosphere and inside the boiler. It is measured in mm of water. A differential in draught is required to cause flow of air or gas through the boiler setting Types of Draught : i . Natural Draught: The natural draught is provided by the action of chimney or stack and is used only in small boilers. Its intensity depends upon the average temperature difference between the flue gases within the chimney and the outside air (the gases within the chimney are at a higher temperature than that of the surrounding air) and also on the height of the chimney above the level of the furnace grate. Its intensity is also affected by weather conditions and boiler operating conditions. Chimney, in addition of providing natural draught, helps in reducing air pollution too, as it delivers the products of combustion and fly ash to a high altitude.

35. ii. Mechanical Draught : Artificial or mechanical draught is provided when the natural draught caused by a chimney is not sufficient or where a certain draught is required to be maintained irrespective of weather conditions or boiler operating conditions . In case of large steam boilers where economizers and air preheaters are employed, the exit temperature of the flue gases is sufficiently lowered and also the volume of air required is tremendously high. In such cases the height of the chimney to cause the required draught may be excessive in height and cost. Under such conditions, mechanical draught is essential. The boilers with mechanical draught do not require a chimney of such a height as is necessary with natural draught. In a mechanical draught system, the movement of air is due to the action of a fan. A mechanical draught may consist of induced draught or forced draught or both.

36. The fans, in all these cases, have high efficiency aero foil blades inclined backward to the direction of rotation. In an induced draught system, the blower is installed near the base of the chimney and the burnt gases are sucked out of the boiler, reducing the pressure inside the boiler to less than atmospheric one . This induces fresh air to enter the furnace. In case of a forced draught system the blower is installed near the base of the boiler. In this system of draught the air pressure throughout the system is above the atmospheric pressure and air is forced to flow through the system (furnace, economiser , air pre-heater and chimney).

37. Feed water: The steam coming out of turbine is condensed and the condensate is fed back to the boiler as feed water. Some water may be lost due to blow-down, leakage etc. and to make up these losses additional water, called the make-up water, is required to be fed to the boiler. The make-up water in a modern thermal plant is about 1-4%. In a large size plant this quantity may be around a few hundred tonnes per hour. The source of boiler feed water is generally a river or lake which may contain suspended and dissolved impurities, dissolved gases etc. The suspended impurities include mud, silt, clay and silica either in suspension or in colloidal form.

38. The dissolved impurities include carbonates, bicarbonates, sulphates and chlorides of calcium, magnesium and sodium, iron oxide, silica. Dissolved gases include oxygen and carbon dioxide. These impurities in feed water may lead to scale formation, corrosion, carry over and embrittlement in boiler and other apparatus. Scale formation reduces the heat transmission through the heating surfaces and causes the overheating of the boiler tubes and the shell plates. Corrosion produces pits, grooves and cracks or a general wastage of material. It is necessary to heat and purify the water before feeding to the boiler. 1 . Improves the overall efficiency of the plant, 2 . Removes dissolved oxygen and carbon dioxide , 3. Causes precipitation of other impurities carried by steam and condensate outside the boiler and 4. Avoids thermal stresses owing to entry of cold water into the boiler.

39. The feed water is heated, put under pressure and then further heated so that its temperature approaches and pressure exceeds that of water in the boiler. The water is treated for removal of suspended and soluble solids and removal of gases. The various methods used for water treatment are: 1 . Mechanical (sedimentation and filtration), 2 . Thermal (distillation and deaerative heating), and 3. Chemical (lime treatment, soda treatment, lime soda treatment, zeolite treatment and demineralisation ).

40. Steam power plant controls: Until few years back, even in case of large power plants the various controls used to be accomplished manually on the basis of instrument readings. However, now the various controls involved in the power plant operation have been completely automated resulting in: ( i ) Increased labour productivity, (ii) Improvement in the safety of operation and reliable functioning of the various instruments and equipment. A number of controls, such as the boiler, turbine and generator unit are provided in a steam power plant so as to maintain the best condition at all loads. Turbine governing is affected by throttling the steam at the main valve (thus reducing the steam pressure and, hence mass flow also) or by reducing only the steam mass flow by cutting off one or more nozzles through which the seam enters the blades. The first method of governing, known as throttle governing or qualitative governing , is used in case of small turbines and the second method of governing, known as nozzle governing or cut-off governing or quantitative governing is used for large turbines.

41. Maintenance of proper vacuum in the condenser, enough circulating water, a number of pumps, oil pressure for control of circuits, steam bleeding if any and the heater and feed-water control are other requirements for the turbine. In case of an isolated generating unit, increase in load causes reduction in the speed of the unit and hence reduction in frequency. However , in case of generator connected to infinite bus-bars the load shared by the unit can be adjusted by adjusting the turbine speed. In this case frequency remains constant. In general, centralized control is employed for modern steam power plants, the boiler and turbine control being at one place in the turbine room and the generator and feeder controls in the control room, in some cases all controls are centralized in one room, called the control room.

42. Plant auxiliaries:  The equipment’s which help in the proper functioning of the plant are called plant auxiliaries. The various plant auxiliaries can be grouped under the subheadings of boiler auxiliaries, coal and ash auxiliaries, turbo-alternator auxiliaries and miscellaneous ones.  The boiler auxiliaries depend upon the type of fuel firing employed. In the case of stoker firing, stoker drives are essential along the forced and induced draught fans, boiler feed pumps, secondary air fans, air preheaters, soot blowers etc. In the case of pulverized fuel firing, the various auxiliaries are primary air fans, forced and induced draught fans, feed pumps, pulverized fuel conveyors and feeders, pulverizing mills, exhausters, air heaters and soot blowers etc. For oil firing the auxiliaries are fuel oil pumps and associated auxiliaries.  Coal and ash auxiliaries include wagon tipplers, elevators, skip hoists, conveyors, cranes, pumps, exhausters etc. Turbo-alternator auxiliaries include circulating water pumps, condensate extraction pumps, governor control, evaporator, sludge and distillate pumps, ventilating fans, oil pumps, oil purifier, exciter, exciter field rheostat, turning gear etc.

43.  Miscellaneous auxiliaries include air compressors, water and fire service pumps, workshop machinery and equipment.  All the above plant auxiliaries can be divided into two categories namely essential or continuous auxiliaries and nonessential or non-continuous auxiliaries. The essential auxiliaries are those which are associated with the running of a unit and whose loss would cause an immediate reduction in the output of the unit. The non-essential auxiliaries are those which may be put out of operation for some time.

44. Diesel Electric Power Plant Introduction to Diesel Electric Power Plant: A generating station in which diesel engine is used as the prime mover for the generation of electrical energy is known as diesel power station . In a diesel power station Diesel engine is used as the prime mover. The diesel burns inside the engine and the products of this combustion act as the “working fluid” to reduce mechanical energy. The diesel engine drives the alternator which converts mechanical energy into electrical energy. As the generation cost is considerable due to high price of diesel, therefore, such power stations are only used to produce small power. Although steam power stations and hydro-electric plants are invariably used to generate bulk power at cheaper cost, yet diesel power stations are finding favour at places where demand of power is less, sufficient quantity of coal and water is not available and the transportation facilities are inadequate. These plants are also used as standby sets for continuity of supply to important points such as hospitals , radio stations, cinema houses and telephone exchanges.

45. Selection of Site for Diesel Electric Power Plant: The following factors should be considered while selecting a site for diesel electric power plant: 1. Distance from the load center – The site should be as near to the load center as possible in order to avoid transmission costs and losses. 2. Availability of land – The land should be available at cheap rate to keep the capital cost of the plant to the reasonable. 3. Availability of fuel – The fuel should be easily available and at reasonable rate . 4. Availability of transportation facilities – The transportation facilities should be available. 5. Availability of water – Water should be available in sufficient quantity for cooling purposes. 6. Distance from populated area – The site should be away from thickly populated area because of noise and nuisance caused from exhaust.

46. Merits and Demerits of Diesel Electric Power Plant: Diesel electric power plants have many advantages over other types of power plants , as given below: 1. The design and installation of such power plants is very simple. 2. Such plants can be located at any place. 3. Such plants can be quickly procured, installed and commissioned. 4. The layout, design and construction of foundations and buildings for such power plants are simple and cheap. 5. Such plants require less space for fuel storage and are free from ash handling problems. 6. Such plants can respond to varying loads without any difficulty. 7. Such plants occupy less space because of minimum auxiliaries. 8. The quantity of water required for cooling is limited. 9. Such plants can be started and put on load quickly.

47. However, the diesel electric power plants have some drawbacks as given below: 1. Operating cost, due to high cost of diesel oil as fuel, is very high. 2. Maintenance and lubrication cost is also high as compared in the case of other plants. 3. Diesel plants cannot supply overloads continuously whereas steam plants can work under 25% overload continuously. 4. Diesel unit’s capacity is limited. These cannot be constructed in large size. 5. Noise from the exhaust is a serious problem. 6. Their useful life is very short (say, about 10 years).

48. Application : 1. Central Power Stations: Diesel power plants are installed where supply of coal and water is not available in sufficient quantity or where power is to be generated in small quantity. Power stations of this type in common use are of capacities up to 10 MW. 2. Standby Power Stations: Diesel power plants may be used as standby plants where continuity of power supply is essential such as in hospitals, telephone exchanges, radio stations, cinemas etc. 3. Peak Load Plants: Diesel power plant can be employed to supply the peak load on the power system while the base load is supplied by a nuclear or hydro power plant. The base load factor will thus be improved and cost of electrical energy per unit will be reduced..

49. 4. Emergency Plants: A small diesel power plant may be installed in a large power station to supply essential auxiliaries in case of failure of main supply. Arrangements can be made to start the diesel plant automatically. 5. Private Power Plant For Small Industries: Diesel electric power plants can be employed as private industrial plants for supply of electric power specially if the capacity requirements are within the limits, set by the size of the diesel units available.

50. Elements of diesel power plant: Engine: This is the main component of the power plant which develops power. The diesel engines employed for diesel electric power plants may be four or two stroke engines. In a 4-stroke engine the complete cycle of operations is performed in four strokes namely suction, compression, expansion (working or power) and exhaust strokes and two revolutions of the engine. Air Intake System : . Engine Air Intake including Air Filters, Ducts and Supercharger (Integral) with the Engine : Air intake system is provided to supply necessary air to engine for fuel combustion. Air requirements of large diesel electric power plants are considerable (about 4-8 m3 per kWh). The air filters are provided to remove dust and other suspended impurities from the air to be supplied to the engine. The supercharger is usually employed to increase the pressure of intake air above atmospheric one in order to develop an increased power output. The air is drawn from outside the engine room and delivered to the inlet manifold through ducts or pipes and filters.

51. Fuel Supply System: Engine Fuel System including Fuel Storage Tanks, Fuel Transfer Pumps, Strainers, Heaters and Connecting Pipe Work: Fuel transfer pumps are required to transfer fuel from delivery point to storage tanks and from storage tanks to daily consumption tanks and then to engine. Strainers are provided to remove suspended impurities and thus ensure clean fuel supply to the engine. Heaters are required to heat the oil specially during winter season. Exhaust System : Engine Exhaust System including Silencers and Connecting Ducts: This system is provided to discharge the engine exhaust to the atmosphere outside the building . The exhaust manifold connects the engine cylinder exhaust to the exhaust pipe provided with a muffler to reduce pressure in exhaust line and eliminate most of the noise which may result if the waste gases were discharged directly into the atmosphere.

52. Cooling System: Engine Cooling System including Cooling Pumps, Cooling Towers or Spray Ponds, Water Treatment or Filtration Plant and Connecting Pipe: It is known that the heat generated by the burning of fuel in the engine cylinder is partially converted into useful work. The remainder is wasted as heat in the outgoing exhaust gases and in heating the engine, and if not removed may disintegrate the lubricating oil film on the cylinder walls and damage the cylinder liners , heads, walls, piston and rings In the forced water cooling cold water is sent through the cylinder jacket with the help of a pump . The hot water is cooled in spray pond and is re-circulated. The water used for cooling engine cylinder is softened by water treatment or water filtration plant in order to avoid formation of scales etc., in it.

53. Lubricating System: Engine Lubricating Oil System including Lubricating Oil Pumps , Oil Tanks, Filters, Coolers, Purifiers and Connecting Pipe Work: The life of the engine and its efficiency depend largely on its lubrication system. The parts of the engine requiring lubrication are piston and cylinders, gears, crankshaft and connecting rod, bearings etc. Piston and cylinder require special lubricating oil. The forced feed lubrication is mostly employed. In this system of lubrication the lubrication oil is drawn from the sump by means of a pump and is passed through a strainer and then through a filter in order to remove impurities .

54. Engine Starting System: Engine Starting including Battery, Starter, Compressed Air Supply etc.:This system is provided to rotate the engine initially, while starting , until the firing starts and the unit runs under its own power. Small sets are usually started manually by handles but for sets of large capacity, say above 75 kW, the compressed air system is mostly employed for starting diesel engines. Battery driven motors can also be used for starting the diesel engine sets