STEAM POWER PLANT / THERMAL POWER PLANT

STEAM POWER PLANT, Thermal Power Plant

1. SNITS (JNTUH) – B.Tech / M.Tech (Thermal) UNIT-I: STEAM POWER PLANT THERMAL AND NUCLEAR POWER PLANT Dr. S. VIJAYA BHASKAR M.Tech ( Mech ), Ph.D ( Mech ), Ph.D (Mgmt) Professor in Mechanical Engineering

2. Unit-I: Syllabus Steam power plant: Introduction General layout of steam power plant Modern coal fired steam power plant Power plant cycle Fuel Handling, Combustion equipment, Ash handling and Dust collectors. 2

3. CLASSIFICATION OF POWER PLANTS 3 Based on the form of energy converted into electrical energy, the power plants are classified as 1) Steam Power Plants 2) Diesel Power Plants 3) Gas Power Plants 4) Hydro-electric Power Plants 5) Nuclear Power Plants

4. STEAM POWER PLANT 4 A Steam Power Plant converts the chemical energy of the fossil fuels (coal, oil, gas) or fissile fuels (Uranium, Thorium) into electrical energy. Steam Power Plant basically works on Rankine cycle . Steam is produced in the boiler by utilizing the heat of fuel combustion; thus steam is expanded through the steam turbines. The Steam turbine drives the generator which converts mechanical energy of the turbine into electrical energy.

5. STEAM POWER PLANT 5

6. 6 Modern Steam Power Plant – Lay out

7. Modern Steam Power Plant

8. 8 Boiler 2. Super heater 3. Reheater 4. Air preheater 5. Economizer 6. Steam Turbine 7. Condenser 8. Cooling Tower 9. Circulating water pump 10. Boiler Feed Pump 11. Generator 12. Water Treatment Section 13.Chimney and Control Room Major Components of Steam Power Plant

9. Steam Power Plant Layout 9 The Modern Steam Power Plant Mainly Divided Into Four Circuits. 1) Coal And Ash Circuit. 2) Air And Gas Circuit. 3) Feed Water And Steam Flow Circuit. 4) Cooling Water Circuit.

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11. Coal And Ash Circuit 11 It comprises of coal handling equipment and ash handling equipment. Coal is supplied to the boiler from the storage point by coal handling equipment and Steam is generated in the boiler. Ash resulting from the combustion of coal is removed to the ash storage yard through ash handling equipment.

12. Air And Gas Circuit 12 Air is blown to the combustion chamber by induced draught fan or forced draught fan or both. The dust present in the air is removed by dust catching device or precipitator.

13. 13 AIR AND GAS CIRCUIT The Induced Draught (ID) fan helps in sucking the exhaust gases and exhausting them through the chimney into the atmosphere. The heat in the exhaust gases is partly extracted by passing (i) through the Economiser to preheat the water and then (ii) through the Air Preheater for heating up the incoming air.

14. ID and FD Fan 14

15. Feed Water And Steam Flow Circuit 15 The high pressure steam produced in the boiler is used to operate the turbine. The exhaust steam is condensed in the condenser. The condensate is heated in the feed heaters using the steam trapped from different points of turbine and finally the condensate is pumped into the boiler.

16. 16 FEED WATER AND STEAM FLOW CIRCUIT During this process a part of steam and water is lost by passing through different components and this is compensated by supplying additional feed water This feed water should be purified before supply to prevent scaling of the tubes of the boiler .

17. Cooling Water Circuit 17 The exhaust steam entering the condenser is cooled by circulating the cooling water. The cooling water supply to the condenser helps in maintaining a low pressure in it. The cooling water may be taken from any source like river, lake or alternatively, same water may be cooled and circulated again. Based on these, this circuit is divided 1) Open system 2) Closed system

18. 18 Open System : Water from any natural source (river, sea) is used to for condensation of steam in this open system. Water from the upstream of the river is taken and circulated in the condenser. The heated water is let down into the stream of river in this system.

19. 19 Closed System : When enough quantity of water is not available the same water will be used in the condenser. The heated water from the condenser is cooled by cooling tower or spray pond and recirculated. Make up water is added periodically to cover the water/ steam loss.

20. Depends on Choice of Steam Conditions 20

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22. 22 Effect of Steam Temperature on Cost and Efficiency

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25. 25 Fuels and Coal Handling Equipments Factors affecting the Selection of Equipment 1. Plant Fuel Rate 2. Plant Location in respect of Fuel Shipping 3. Storage area Available These aspects become important in view of the high operating costs ( 50 -60% for the fuel purchase and handling)

26. Fuels and Coal handling Equipments (contd.) Means of Coal Shipment 1. Transportation by sea or river 2. Transportation by rail 3. Transportation by ropeways 4. Transportation by road, and 5. Transportation of coal by pipe line Advantages of Transport by Pipeline i . Simplicity in installation and increased safety in operation ii. More economical when large volumes need to be transported over long distances iii. Continuous operation and not affected by climate and weather iv. High degree of reliability v. loss due to theft and pilferage eliminated v. Man power requirement is low 26

27. Fuels and Coal handling Equipments (contd..) Requirement of Good Coal Handling Plant 1. Should operate with minimum maintenance . 2. Should be reliable 3. Simple in design and to operate 4. Require minimum of operatives 5. Should be able to deliver requisite quantity of coal at destination during peak demand 6. There should be minimum wear in running the equipment due to abrasive action of the coal particles 27

28. 28 Advantages of Mechanical Handling: Higher Reliability Less Labour required Economical for medium and Large capacity plants Operation is Easy and Smooth Can be easily started and economically adjusted as per need Minimized labour and hence easier management and control of the plant Reduced health hazards as less labour involved Losses in transport – minimized. Mechanical Handling vs Manual handling

29. 29 Disadvantages of Mechanical Handling: 1. Needs continuous and timely attention for maintenance and repairs (Preventive vis-à-vis Breakdown Maintenance) 2. Higher Capital Cost 3. A part of Power is used up for running the equipment.  Reduced net power output . Mechanical Handling vs Manual handling

30. 30 Coal Delivery  Unloading  Preparation Transfer  Out door Storage  Covered Storage  In-plant Handling  Weighing and Measuring  Furnace Firing Stages of Coal Handling Equipments Process 1. Coal Delivery 2. Unloading 3. Preparation 6.Covered Storage 5.Out door Storage 4. Transfer 7. In-plant Handling 8. Weighing and Measuring 9.Furnace Firing  

31. 31 Various Stages of Coal handling Coal Delivery Unloading Preparation Transfer Outdoor Storage Covered Storage Inplant Handling. Weighing and measuring Furnace Firing

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33. 33 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.

34. 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. Unloading 34

35. 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. Unloading 35

36. 36 Lift Truck with Scoop iv. Unloading Bridges v. Self unloading Boats

37. 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. Preparation 37

38. Fuel/Coal Handling Equipment After preparation coal is transferred to the dead storage by means of the following systems. 1. Belt conveyors 2. Screw conveyors 3. Bucket elevators 4. Grab bucket elevators 5. Skip hoists 6. Flight conveyor 38

39. 39 1. Belt Conveyor Average Belt Speed: 60 – 100 m / minute Load Carrying Capacity of Belt: 50 – 100 t / h Distance Covered (Typical): 400 m Inclination of Drive: 20deg. Note: Used in Medium and Large Power Plants.

40. Belt Conveyor 40

41. 41 2. Bucket Conveyor

42. Bucket Conveyor/elevators move bulk material vertically much like the conveyor belt.   Buckets are attached to a rotating belt and fill with the material at the bottom of the elevator then move it to a designated point. When the bucket reaches this point, it discharges the contents, returns to the start point, and begins the process again. Buckets prevent spillage with their upright position design. 2. Bucket Conveyor 42

43. 43 3. Screw Conveyor

44. 44 4. Grab Bucket Conveyor Capacity ~ 50 tonnes/ h

45. 45 5. Flight Conveyor Capacity: 10 – 100 tonnes /h Inclination with horizl ; to 35 deg Max speed~ 30 m/min

46. 46 Skip Hoist

47. Coal Storage 47

48. 48 Lay out of a Fuel handling Equipment

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50. 50 Outline of Coal Handling Equipment

51. COMBUSTION A Fuel is basically a s o u r ce of heat . The u s ual m eth o d of p r o d uci n g heat f ro m fu el is b y th e p r ocess of Comb u s t io n . I t is a c h e m ical re a c tio n be twe e n th e Fuel a n d th e Oxi d a nt .

52. COMBUST I ON EQUIPMENTS The combustion efficiency depends on: Fr e ely ig n ited F r esh cha r ge of fu e l Steady combustion for obtai n i n g the d esired a m ount of heat r e l e as e . A d e q uate combustion space Quant i ty o f air supply for complete combustion The method of air supply Combustion equipment includes h e aters, o v ens, stoves, furn a c es, fi r eplac e s, d r yer s, st o ker s , b urners , and m any m o r e .

53. BASIC REQUIREMENT OF COMBUST I ON EQUIPMENTS

54. 54 FIRING SYSTEM The firing system is mainly classified into two types Hand firing 2) Stoker firing. Hand firing: This is the simplest method of fuel firing. The combustion efficiency is very low, when compared to others. Due to lower combustion efficiency it cannot be used in Modern power plants.

55. Method of fuel firing : Hand firing This is a simple method of firing coal into the furnace. It requires no capital investment. It is used for smaller plants. Adjustments are to be made every time for the supply of air when fresh coal is fed into furnace. Hand fired grates: used to support the fuel bed and admit air for combustion. While burning coal the total area of air openings varies from 30 to 50% of the total grate area. The grate area required for an installation depends upon various factors such as its heating surface, the rating at which it is to be operated and the type of fuel burnt by it. The width of air openings varies from 3 to 12 mm. The construction of the grate should be such that it is kept uniformly cool by incoming air. It should allow ash to pass freely. Hand fired grates are made up of cast iron .

56. 56 Stoker Firing A stoker is a power- operated fuel feeding mechanism using a grate. This method of firing is used for burning solid coal on a grate. Stoker are classified as follows: 1) overfeed stokers 2) underfeed stokers

57. Classification of Stokers 57

58. 58 Over feed stokers In the overfeed mechanism a forced draught fan slightly pressurizes the atmosphere air before it enters under the bottom of the grate . The fuel bed receives fresh coal on top surface The ignition zone lies between green coal and incandescent coke. The air gets heated up as it follows through the grate openings where as the grate gets cooled. This warm air gets additional heat energy by further passing through a layer of hot ashes .

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60. Primitive Method of solid Combustion Primary Air Secondary Air Flame Green Coal Incandescent coke Grate CO+CO2+N2+H2 VM+CO+CO2+N2+H2 O2+CO2+N2+H2O ASH

61. OVER FEED STOKERS 61 Different types of over feed stokers: These type of stokers are used for large capacity boiler installations where the coal is burnt without pulverization . 1)Travelling grate stoker a) Chain grate type b) Bar grate type 2) Spreader stoker

62. CHAIN GRATE STOKER 62 A chain grate stoker consists of an endless chain which forms a support for the fuel bed. The chain surface is made of a series of cast iron links connected by pins .

63. CHAIN GRATE STOKER 63 The chain travels over two sprocket wheels located at the front and at the rear of the furnace. The front sprocket is connected to a variable speed drive mechanism Speed range 15 cm/ min to 50 cm/ min

64. Chain grate stoker The chain travels over two sprocket wheels, one at the front and one at the rear of furnace. The traveling chain receives coal at its front end through a hopper and carries it into the furnace. The speed of grate (chain) can be adjusted to suit the firing condition. The air required for combustion enters through the air inlets situated below the grate. The stokers are suitable for low ratings because the fuel must be burnt before it reaches the rear of the furnace.

65. CHAIN GRATE STOKER – ADVANTAGES & DISADVANTAGES 65 ADVANTAGES: 1) Simple in construction 2) Low initial cost. 3) Self-cleaning stoker . 4) The rate of heat release can be controlled just by controlling chain speed . 5) Rate of heat release is high per unit volume of furnace. DISADVANTAGES: 1) cannot be used for high capacity boilers. 2) temperature of pre-heated air limited to 180 C 3) clinker troubles are very common. 4) loss of coal in the form of fine particles - carried away with the ash.

66. Spreader stoker 66 Spreader stoker mechanism involves throwing (spreading) the coal uniformly on the grate . The grate may be of stationary or moving type, with air openings for admitting the air. The selection of coal size is very important for a spreader stoker -- the coal size should be in between 6 cm to 36 cm . The spreader stoker is mostly used for steam capacities of 9.5 to 50 kg/sec. (34 to 180 tonnes/hr) It can burn a wide variety of coals from high ranking bituminous to lignite .

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68. Spreader stoker 68

69. Spreader Stoker 69 Advantages: 1) A wide variety of coal can be burnt. 2) The clinkering problems can be reduced by spreading action. 3) High temperature preheated air can be used. 4) Volatile matter is removed by burning coal in suspension. 5) Good response to load fluctuation . 6) Low running cost . Disadvantage: Difficult to operate with varying sizes of coal with varying moisture content. Fly ash is a major problem. Fuel loss due to suspension and exhaust gases.

70. UNDER FEED STOKERS 70 1) This underfeed mechanism is best situated for bituminous and semi bituminous coals. 2) In this underfeed mechanism the fuel is fed from underneath the fire and moves upwards gradually. 3) The air entering through the grate opening comes in contact with raw fuel and mixes with the volatile matter released from raw fuel and enters into the combustion chamber.

71. 71 Principle of Underfeed Stokers

72. 72

73. UNDER FEED STOKERS 73 Different types of under feed stokers. 1) single retort stoker 2) multi-retort stoker. Single retort stoker The single retort stoker consists of a trough shaped retort to which the fuel (coal) is fed by a reciprocating ram or screw conveyor. The capacity of this stoker ranges from 100 to 2000 kg of coal burned per hour.

74. MULTI RETORT STOKER 74 The coal falling from the hopper is pushed forward during the inward stroke of stoker ram. The distributing rams (pushers) then slowly move the entire coal bed down the length of stoker. The slope of stroke helps in moving the fuel bed downwards. The primary air enters the fuel bed from main wind box situated below the stoker. Partly burnt coal moves on to the extension grate. The air entering from the main wind box into the extension grate wind box is regulated by an air damper.

75. MULTI RETORT STOKER 75 The number of retorts may vary from 2 to 20 with coal burning capacity of 300 to 2000 kg/hr.

76. MULTI RETORT STOKER - ADVANTAGES 76 1) High thermal efficiency when compared with chain grate stoker. 2) The grate is self cleaning . 3) Part load efficiency is high with multiple retort system. 4) high combustion rate . 5) wide variety of coals can be used. 6) best suitable for non-clinkering, high volatile and low ash content coals.

77. MULTI RETORT STOKER DISADVANTAGES 77 1) Initial cost is high. 2) Large building area is required. 3) Clinker troubles are usually present. 4) Low grade fuels with high ash content cannot be burnt economically.

78. Pulverized coal-fired boiler 78 A pulverized coal-fired boiler is an industrial or utility boiler that generates thermal energy by burning pulverized coal (also known as powdered coal or coal dust since it is as fine as face powder in cosmetic makeup) that is blown into the firebox. The basic idea of a firing system using pulverised fuel is to use the whole volume of the furnace for the combustion of solid fuels. Coal is ground to the size of a fine grain, mixed with air and burned in the flue gas flow. Biomass and other materials can also be added to the mixture.

79. Pulverized coal-fired boiler 79 Coal contains mineral matter which is converted to ash during combustion. The ash is removed as bottom ash and fly ash. The bottom ash is removed at the furnace bottom. This type of boiler dominates the electric power industry, providing steam to drive large turbines. Pulverized coal provides the thermal energy which produces about 50% of the world's electric supply.

80. Pulverized Coal Power Plants- Current Technologies 80 Pulverized coal power plants are broken down into three categories; subcritical pulverized coal (SubCPC) plants, supercritical pulverized coal (SCPC) plants, and ultra-supercritical pulverized coal (USCPC) plants. The primary difference between the three types of pulverized coal boilers are the operating temperatures and pressures. Subcritical plants operate below the critical point of water (647.096 K and 22.064 MPa). Supercritical and ultra-supercritical plants operate above the critical point. As the pressures and temperatures increase, so does the operating efficiency.

81. Requirements of Pulverised Fuel Firing 81 There are two requirements which are must for Pulverised Coal to burn successfully in a furnace. 1) Presence of large quantities of Fine particles of coal usually that would pass enough through a 200-mesh to ensure Spontaneous Ignition because of their large surface to volume ratio. 2) Presence of minimum quantity of Coarser particles to ensure High Combustion Efficiency.

82. 82 ELEMENTS OF PULVERIZED COAL SYSTEM

83. Pulverized Coal Power Plant (PCPP) 83

84. ADVANTAGES OF PCPP 84 1) Any grade ofcoal can be used efficiently because it is powdered before use. 2) Higher boiler efficiency due to complete combustion. 3) Flexible method and can respond well for sudden change in demand . 4) Fan power required is low . 5) Free from clinker problem. 6) Volume of the furnace required is less. 7) No major problem in ash handling .

85. ADVANTAGES contd. 85 8) The system works successfully with or in combination with gas and oil . 9) No moving parts in the furnace are subjected to high temperatures. 10) Much smaller quantity of air is required as compared to the stoker firing. 11) The external heating surfaces are free from corrosion. 12) It is possible to use highly preheated secondary air (350 deg. C) which helps in rapid flame propagation.

86. DISADVANTAGES OF PCPP 86 1) Additional investment for coal preparation unit/ plant. 2) Extra power is needed for pulverising coal. 3) Maintenance cost is more which depends on quality of coal. 4) Due to very high temperature, maintenance of furnace walls is difficult. 5) More space is required. 6) Special equipment is required to start the system.

87. Pulverized coal Preparation using Pulverizer /Mill Coal is pulverized (powdered) to increase its surface area ( and therefore exposure) thus permitting rapid combustion . The pulverized coal is obtained by grinding the raw coal in pulverizing mills. Various types of pulverizing mills are: Ball mill Ball and race mill Hammer mill Bowl mill Essential functions of pulverizing mills are : Drying of the coal Grinding Separation of particles of a desired size. Coal pulverizing mills reduce coal to powder by any (or all) action such as Impact, Abrasion and Crushing 

88. Ball and race mill for pulverizing coal This is a low speed unit in which grinding pressure is maintained by adjustable springs. The coal passes between the two rotating elements again and again until it has been pulverized to desired degree of fineness.

89. Ball Mill

90. Pulverised Fuel Handling 90

91. Pulverized Coal Firing

92. pulverized fuel plants Basically , pulverized fuel plants may be divided into two systems based on the method used for firing the coal : Unit System or Direct System Bin System or Central System Unit or Direct System : This system works as follows : Coal from bunker drops on to the feeder. Coal is dried in the feeder by passage of hot air. The coal then moves to a mill for pulverizing . A fan supplies primary air to the pulverizing mill . Pulverized coal and primary air are mixed and sent to a burner where secondary air is added .

93. Bin or Central System:   Coal from bunker is fed by gravity to a dryer where hot air is admitted to dry the coal. D ry coal is then transferred to the pulverizing mill. Pulverized coal then moves to a cyclone separator where transporting air is separated from coal. Primary air is mixed with coal at the feeder and supplied to the burner. Secondary air is supplied separately to complete the combustion

94. ADVANTAGES OF UNIT SYSTEM 94 1) Simple layout and easy operation. 2) It requires less space. 3) It is cheaper when compared with central system. 4) Less maintenance. 5) Simple coal transportation system. 6) Direct control of combustion from the pulveriser is possible. 7) Better control over fuel feed rate.

95. DISADVANTAGES OF UNIT SYSTEM 95 1) Less flexible when compared to central system. 2) With the load factor in common practice, the total capacity of all the mills must be higher than for the control system. 3) Any fault in the coal preparation unit may stop the entire steam generating system. 4) Excessive wear and tear of the fan blades as it handles air and coal particles.

96. CENTRAL SYSTEM 96

97. ADVANTAGES OF CENTRAL SYSTEM 97 1) More flexible because the quantity of fuel and air can be controlled separately. 2) More reliable. 3) No problem of excessive wear of fan blades. 4) Less labour is required. 5) Low power consumption per tonne of coal handled.

98. DISADVANTAGES OF CENTRAL SYSTEM 98 1) High initial cost. 2) It requires large space area. 3) Possibility of fire and explosion hazards. 4) Driers are necessary. 5) Operation and maintenance costs are high when compared to unit system of same capacity. 6) More number of auxiliaries.

99. Various types of burners are used for combustion of pulverized coal. Burners 99

100. Contd.

101. Burners 101

102. Various types of burners are used for combustion of pulverized coal. Pulverized Fuel Burners 102

103. Long Flame (U-Flame) Burner 103

104. Long Flame (U-Flame) Burner  Long Flame (U-Flame) Burner: In this burner, air and coal mixture travels a considerable distance thus providing sufficient time for complete combustion

105. Turbulent Burner 105

106. Tangential Burners 106

107. Cyclone Burner 107 In this system, the coal and air will be mixed due to cyclonic whirling action The coal is crushed into tiny powder in addition to pulverized coal The ash is easily collected due to cyclonic action

108. Cyclone Burner 108

109. Cyclone Burner 109

110. Cyclone Burner: In this system, the cyclonic action whirls coal and air against the wall of the furnace to facilitate thorough mixing of coal and air. Advantage of this burner is that it can also use crushed coal in addition to pulverized coal thus providing an option. When crushed coal is used, ash is collected in molten form for easy disposal.

111. ADVANTAGES OF CYCLONE BURNER 111 1) Costly pulverisers are not required. Instead, simple coal crushing equipment can be used. 2) By using forced draught fan it can be operated with small quantities of excess air . 3) It can burn low grades of coal effectively. 4) High temperatures are obtained. 5) Boiler fouling (sticking) problems can be reduced as all the incombustibles are retained in the cyclone burner. 6) Boiler efficiency is increased .

112. Ash Handling Systems 112

113. 113 Ash Disposal Equipment. General Layout of Ash handling and Dust Collection System

114. Mechanical Handling System 114

115. 115 Hydraulic System

116. 116 Ash Disposal Equipment contd. Low Pressure System

117. 117 Below the boiler nozzles(top and sides), are fixed

118. Advantages 118

119. 119 Pneumatic or Vacuum Extraction Ash Handling System

120. 120

121. 121

122. Ash handling Equipment-Desired Characteristics 122 Enough capacity to cope up with the volume of ash being produced Able to handle large clinkers (stony residue), boiler refuse, soot etc. with little personal attention of the workmen. Able to handle hot and wet ash effectively and with good speed. Possible to action of the ashes while minimize corrosive or abrasive economical in erection and operation. noiseless Should be possible to add additional units

123. Equipment commonly used for Ash Handling 123 Bucket Elevator Bucket Conveyor Belt Conveyor Pneumatic Conveyor Hydraulic Sluicing Equipment Trollies or Rail cars etc.

124. 124 Uses of Ash and Dust i . in the production of Cement ii. Production of concrete (20% Fly Ash And 30% Bottom Ash) iii. Treating acidic soils iv. Extraction (Recovery) of metals such as Al, Fe, Si, and Titanium from the ash

125. 125 Ash Disposal Equipment

126. General layout of ash handling system 126

127. Mechanical ash handling system 127

128. Hydraulic ash handling System 128 Low velocity

129. High velocity 129

130. Pneumatic ash handling system 130

131. DUST COLLECTORS 131 Dust collectors are grouped into two types 1.Mechanical Dust Collectors i ) Wet type dust collectors ii) Dry type dust collectors Gravitational separators Cyclone separators 2.Electrical Dust Collectors Rod type Plate type

132. Basic Principles of Mechanical Dust Collectors 132 Enlarging the duct cross sectional area to slow down the gas gives the heavier particles a chance to settle out. When a gas makes a sharp change in flow direction, the heavier particles tend to keep going in the original direction and so settle out. Impingement baffles have more effect on the solid particles than the gas, helping them to settle.

133. Mechanical Dust Collectors 133 i) Wet type dust collectors: Wet types called scrubbers operate with water sprays to wash dust from the air. Large quantities of wash water are needed for central station gas washing that this system is seldom is used. It also produces a waste water that may require chemical neutralization before it can be discharged into the central bodies of water. ii) Dry type dust collectors: These are commonly used dust collectors. a) Gravitational Separators: These collectors act by slowing down gas flow so that particles remain in a chamber long enough to settle in the bottom. They are not very suitable because of large chamber volume needed.

134. Cyclone (or) Centrifugal separator 134 The cyclone is a separating chamber where in high-speed gas rotation is generated for the purpose of centrifuging the particles from the carrying gases. There is an outer downward flowing vortex which turns into an inward flowing vortex. Involute inlets and sufficient velocity head pressures are used to produce the vortices. The factors which affect the performance are gas volume, particles loading, inlet velocity, temperature, diameter- to- height ratio of cyclone and dust characteristics.

135. Advantages and Disadvantages of Cyclone Separator 135 ADVANTAGES: Rugged in construction Maintenance costs are relatively low Efficiency increases with increase in load Easy to remove bigger size particles DISADVANTAGES: Requires more power than other collectors Incapable to remove dust and ash particles which remain in suspension with still air Less flexible High pressure loss Requires considerable head room and must be placed outside the boiler room

136. Electrical Dust Collector 136 The main elements of an electrostatic precipitator are: Source of high voltage Ionizing and collecting electrodes Dust removal mechanism Shell to house the elements

137. Electrical Dust Collector 137 The precipitator has two sets of electrodes , insulated from each other, that maintain an electrostatic field between them at high voltage The field ionizes dust particles that pass through it, attracting them to the electrode of opposite charge. The high voltage system maintains the negative potential of 30,000 to 60,000 volts with the collecting electrodes grounded. Accumulated dust falls of the electrode when it is rapped mechanically Wet type of unit removes dust by a water film flowing down on the inner side of the collecting electrode. These units have collection efficiency around 90%

138. Electrical Dust Collector 138 ADVANTAGES: Can effectively remove very small particles like smoke, mist and fly-ash Easy operation Draught loss is quite less Most effective for high dust loaded gas Maintenance charges are minimum The dust collected in dry form and can be removed either dry or wet DISADVANTAGES: Space requirement is more Need to protect the collector from sparking Running charges are high Capital cost of equipment is high

139. Efficiency of Dust Collectors 139 The collection efficiency of a dust separator is the amount of dust removed per unit weight of dust. Though dust collectors remove contaminants, they increase draught losses and hence the fan power. The absolute efficiency of a dust collector is the percentage of entering solids that will be removed by the collector.

140. Layout of Dust Collection System 140