(For BE Mechanical Engineering Students) (As per New Syllabus of Leading Universities) Dr SRamachandran, ME, PhD, Professor and Research Head Faculty of Mechanical Engineering SATHYABAMA UNIVERSITY Jeppiaar Nagar, Chennai - 600 119 AIR WALK PUBLICATIONS (Near All India Radio) 80, Karneeshwarar Koil Street Mylapore, Chennai - 600 004 Ph: 2466 1909, 94440 81904 Email: aishram2006@gmailcom, airwalk800@gmailcom wwwairwalkpublicationscom
First Edition : 20-06-2004 Second Edition : April 2016 and
POWER PLANT ENGINEERING SYLLABUS Unit 1: Coal Based Thermal Power Plants Rankine Cycle - Improvisations - Layout of modern coal power plant, Supercritical Boilers, FBC boilers, Turbines, Condensors, Steam and Heat rate, Subsystems of Thermal power plants - Fuel and handling, Draught system, Feed water treatment, Binary cycles and Cogeneration systems Unit 2: Diesel, Gas Turbine and Combined Cycle Power Plants Otto, Diesel, Dual and Brayton Cycle - Analysis and Optimisation, Components of Diesel and Gas Turbine power plants Combined Cycle Power Plants Integrated Gasifier based Combined Cycle systems Unit 3: Nuclear Power Plant Basics of Nuclear Engineering, Layout and subsystems of Nuclear Power Plants, Working of Nuclear Reactors : Boiling Water Reactor (BWR), Pressurized Water Reactor (PWR), CANada Deuterium- Uranium reactor (CANDU), Breeder, Gas Cooled and Liquid Metal Cooled Reactors Safety measures for Nuclear Power plants Unit 4: Power from Renewable Energy Hydro Electric Power Plants - Classification, Typical Layout and associated component including Turbines Principle, Construction and working of Wind, Tidal, Solar Photo Voltaic (SPV), Solar Thermal, geo Thermal, Biogas and Fuel Cell power systems Unit 5: Energy, Economic and Environmental issues of Power Plants Power tariff types, Load distribution parameters, load curve, Comparison of site selection criteria, relative merits & demerits, Capital & Operating Cost of different power plants Pollution control technologies including Waste Disposal Options for Coal and Nuclear Power Plants
Contents I Coal Based Thermal Power Plants 1 11 Rankine Cycle (Simple Steam Power Cycle) 11 12 Improvisations - Modern Trends in Rankine Cycle Improvements 112 121 Reheat Cycle 112 122 Advantages (or) effects of Re-heating 115 123 Disadvantages 116 13 Methods of Reheating 116 131 Gas Reheating 116 132 Live-Steam Reheating 117 133 Combined gas and live steam reheater 118 14 Regenerative Cycle (Bleeding Cycle) 129 141 Advantages of Regenerative cycle 140 15 Layout of Modern Coal Power Plant (or) Layout of Steam Power Plant 144 151 Coal and ash Circuit 145 152 Air and Flue gas circuit 145 153 Feed water and steam flow circuit 146 154 Cooling Water Circuit 147 16 Selection of Site For A Steam Power Plant 147 17 Super Critical Boilers 150 172 Benson Boiler 153 173 Loefler Boiler 156 174 Velox boilers 157 18 Fluidized Bed Combustion (FBC) Boilers 159
2 181 Conversion of Oil fired boilers to fluidized bed boilers 162 182 Types of Fluidised Bed Combustion system 163 (i) Bubbled fluidised bed combustion system 163 (ii) Circulating Fluidised Bed Combustion system (CFBC) 166 (iii) Pressurised Fluidised Bed Combustion system 168 19 Steam Turbines 172 191 Types of steam turbine 173 192 Impulse Turbine 173 193 Reaction turbine 177 1931 Velocity diagram for reaction turbine blade 178 1932 Degree of reaction 179 110 Steam Condensers 180 1101 Jet condensers 180 1102 Surface condenser 181 111 Capacity of a Steam Power Plant 184 112 Sub System of Thermal Power Plant 185 1121 Boiler Accessories 185 11113 Cooling tower1104 111131 Type of Cooling towers1105 111132 Atmospheric (or) natural draft cooling towers 1105 111133 Mechanical Draft Cooling Towers1107
3 111134 Air Cooled or Dry-type Cooling System1108 112 Boiler Mountings 1112 113 Fuel and Ash Handling 1114 1131 Handling of coal 1114 1132 Ash handling and dust collecting system1124 114 Draught 1134 1141 Classification of Draught 1135 11411 Natural draught 1135 11412 Artificial draught 1137 1142 Forced draught 1138 1143 Induced draught1139 1144 Balanced draught1140 115 Stockers 1141 1151 Overfeed stokers1142 1152 Under feed stokers 1146 1153 Pulveriser1148 11531 Unit (or) Direct system1150 11532 BIN (or) Central System1152 116 Feed Water Treatment1153 1161 Need for feed water treatment 1153 1162 Arrangements for Feed Water Treatment 1154 117 Binary Vapour Cycle 1160 118 Waste Heat Recovery / Cogeneration System 1168 Waste Heat Boilers 1169
4 2 Diesel, Gas Turbine and Combined Cycle Power Plants 21 Otto Cycle 21 22 Diesel Cycle 29 23 Mixed/Dual Cycle 217 24 Brayton Cycle 229 25 Diesel Engine Power Plant 233 26 Types of Diesel Power Plants 235 27 Layout of Diesel Power Plant 237 28 Diesel Engine Used For Diesel Power Plants 244 281 Selection of Engine Type 245 282 Super charging 247 29 Gas Turbine Power Plant 249 291 Gas turbine 249 210 Classification of Gas Turbine Power Plants 250 211 Layout of Open Cycle Gas Turbine Power Plant 252 212 Working of Gas Turbine Power Plant 253 213 Fuels For Gas Turbines 254 2131 Fuel qualities 255 214 Gas Turbine Materials 256 215 Open and Closed Cycles 258 2151 Open cycle gas turbine 258 2152 Closed cycle gas turbine 260 216 Reheating, Regeneration and Intercooling 262 2161 Intercooling 263 2162 Reheating 264
5 2163 Regeneration 265 217 Combined Power Cycles 266 218 Combined Gas Turbine and Diesel Cycles 270 219 Integrated Gasifier Based Combined Cycle (IGCC) System 272 3 Nuclear Power Plant 31 Introduction 31 32 Basics of Nuclear Engineering 31 33 Radioactivity 32 34 Nuclear Reactions 37 35 Nuclear Fission and Chain Reaction 38 36 Layout of Nuclear Power Plant 310 37 Site Selection For A Nuclear Power Plant 312 38 Nuclear Reactor 314 39 Boiling Water Reactor (BWR) 318 310 Pressurized Water Reactor (PWR) 319 311 CANada Deuterium Uranium (CANDU) reactor 321 312 Gas Cooled Reactor 324 313 Fast Breeder Reactor 326 314 Liquid Metal Cooled Reactor 327 315 Safety Measures For Nuclear Power Plant 329 4 Power From Renewable Energy 41 Hydel Power Plants 41 42 Essential Elements of Hydel Power Plant 42 43 Site Selection For Hydel Power Plant 421 44 Working of Hydro Electric Power Plant 424
6 45 Classification of Hydraulic Turbines 426 451 Impulse turbine 427 4511 Working of a Pelton Wheel 430 452 Reaction turbine 430 4521 Francis Turbine 431 4522 Axial Flow Reaction Turbines 432 46 Working Principle of A Kaplan Turbine 433 47 Turbine Governing 435 471 Working of oil pressure governor 436 48 Selection of Turbine 438 49 Micro Hydel Development 450 491 Components of Micro hydel power plant 451 492 Power from a micro hydro plant 453 493 Suitable conditions for micro-hydro power 454 494 Turbines for micro hydro power 454 410 Turgo Turbine 455 4101 Working of Turgo Turbine 455 4102 Load factor 456 4103 Load control governors 457 4104 Electrical power from micro hydro plant 457 4105 Economics of micro hydel plant 458 4106 Low cost grid connection 460 4107 Advantages of Micro hydro plant 462 411 Wind Energy and Wind Power 462 4111 Advantages of Wind energy 463 4112 Disadvantages of Wind energy 463
7 4113 Characteristic of a good wind power plant site 463 4114 Wind-Electrical generating power plant 464 4115 Wind Turbine Types 465 4116 Types of wind mills 467 4117 Performance of wind machine 469 412 Tidel Power Plant 470 413 Pumped Storage 476 4131 Types of pumped storage plant 477 414 Solar Power Plants 481 4142 Solar Central Receiver system 487 Heliostats (Mirrors) 488 415 Solar Photovoltaics (SPV) 490 416 Geo Thermal Power Plant 494 4161 Geothermal sources 496 4162 Hydrothermal convective system 497 417 Ocean Energy Conversion (OTEC) Plant 4102 4171 Types of OTEC4105 4172 Closed-cycle OTEC System 4105 4173 Open-Cycle OTEC system4106 4174 Hybrid OTEC System4106 4175 Important points about OTEC 4107 418 Biogas Power Plant 4109 419 Fuel Cell 4115 420 MHD Power Plant 4118 1 Open cycle MHD 4120
8 2 Closed cycle MHD4122 3 Closed Cycle MHD with Liquid metal 4123 5 Energy, Economic and Environmental Issues of Power Plants 51 Economics in Plant Selection 51 52 Important Terms and Definitions 53 53 Economics of Power Generation 510 531 Land, building and equipment cost and installation cost 511 532 Depreciation cost 512 (i) Straight line method 512 (ii) Sinking fund method 513 (iii) Diminishing value method 515 533 Insurance 516 534 Management cost 516 535 Operating cost 516 536 Total cost 516 Customer charges 517 537 Cost of power generation 517 54 Power Tariffs 518 541 Aim of tariffs 518 542 Selection of tariffs 518 543 Types of tariffs 519 1 Flat demand rate 519 2 Straight line meter rate 520 3 Step meter rate 521
9 4 Block meter rate 522 5 Two-part tariff or Hopkinson demand rate 523 6 Three-part tariff (or) Doherty rate 523 7 Wright demand rate 524 55 Choice of Power Plant and its Site 527 56 Electric Load (or) Power Distribution System 530 57 Load Duration Curves 534 571 Important terms and definition 534 (i) Load curve 534 (ii) Residential load 534 (iii) Industrial load 535 (iv) Municipal load 535 (v) Irrigation load 535 (vi) Traction load 535 (vii) Commercial load 535 (a) Residential load curve 535 (b) Industrial load curve for one shift 536 (c) Municipal load curve 536 (d) Traction load curve 537 (e) Commercial load curve 538 (f) Load duration curve 538 58 Pollution and its Control 540 581 Air Pollution by Thermal Power Plants 540 582 Control of Atmospheric Pollution by Thermal Power Plants 544
10 583 Water Pollution By Thermal Power Plant and its Control 549 584 Instrument used to monitor pollution 550 5841 CO 2 recorders 550 5842 Automatic controls for feed water 552 5843 Automatic combustion control 554 585 Indian Boiler Act 556 586 Boiler Inspection 557 587 Boiler Performance 559 5871 Boiler Testing 561 5872 Boiler Trial 562 588 Boiler safety regulations as per Indian Boiler Act 567 59 Nuclear Waste Disposal and Safety 568
Chapter - I COAL BASED THERMAL POWER PLANTS 11 RANKINE CYCLE (Simple Steam Power Cycle) Rankine Cycle - Improvisations - Layout of modern coal power plant, Supercritical Boilers, FBC boilers, Turbines, Condensors, Steam and Heat rate, Subsystems of Thermal power plants - Fuel and handling, Draught system, Feed water treatment, Binary cycles and Cogeneration systems Rankine cycle is the theoretical cycle on which the steam turbine (engine) works Boiler Refer the process (4) to (1): Feed water is passing to the boiler Heat is added to the water in the boiler The water gets heated and becomes dry saturated steam (or) super heated steam Turbine Refer the process (1) to (2): The high pressure steam is expanding in the turbine, thus work is produced ie The turbine rotates The steam leaves the turbine as low pressure steam Condenser: Condenser is used to convert the low pressure steam into water Refer the process (2) to (3) The low pressure steam is passing through condenser where heat is liberated from the steam So the steam becomes water To cool the steam, separate cooling water is circulated through condenser from the cooling tower This cooling water and the steam will not mix together in most of the condensers
12 Power Plant Engineering - wwwairwalkpublicationscom High Pressure Water (4) (4) Boiler High Pressure Steam (1) Q in W in Pump Steam Turbine W out (3) Low Pressure Steam (2) Condenser Low Pressure Water (3) (2) Low Pressure Steam Q out Fig:11 (a) Pump Refer the process (3) to (4) The water leaving condenser is pumped to the boiler by pump Usually, pump work is neglected since it is very small work when compared to turbine work output W = h -h p 4 3 T 4 3 = 1 4 p p Q s = h1-h4 p =p 2 3 p =p 1 4 Fig:11 (b) s = s 1 2 1 2 W = h -h T 1 2 S
Coal Based Thermal Power Plants 13 1-2 Turbine work Turbine work output (Isentropic expansion in Turbine) W T h 1 h 2 kj/kg Turbine power m h 1 h 2 kw where m Mass flow rate of steam in kg/sec h 1 and h 2 can be taken from steam table for p 1 and p 2 respectively (p 1 high pressure (or) boiler pressure (or) inlet to turbine pressure) (p 2 low pressure (or) condenser pressure) Also, we can use Mollier diagram to find h 1 and h 2 2-3 Constant pressure condensation Q 2 Heat rejected h 2 h 3 kj/kg Q 2 in kw m h 2 h 3 kw h 3 h f at low pressure p 2 3-4: Pump Work W pump W p h 4 h 3 kj/kg v f p 1 p 2 kj/kg where v f for p 2 from steam table p 1 and p 2 in kpa Pump power m W p Net Work W net W T W p W T If W p is negligible
14 Power Plant Engineering - wwwairwalkpublicationscom Thermal Efficiency: It is the ratio of net work done to the heat supplied W net cycle or rankine or thermal Q supply 4-1 Heat Supplied in Boiler: Q supply (Constant pressure heat supply) Q supply h 1 h 4 kj/kg Q supply in kw m h 1 h 4 kw Specific steam consumption SSC (or) steam flow rate per kw 3600 W net kg kwhr Work ratio W net W T Problem 11: A steam turbine receives steam at 15 bar and 350C and exhausts to the condenser at 006 bar Determine the thermal efficiency of the ideal rankine cycle operating between these two limits Neglect the pump work Solution p 1 15 bar; t 1 350C; p 2 006 bar Boiler pressure From Mollier diagram, Condenser pressure h 1 31475 kj; h 2 22641 kj/kg; h 3 h f for 006 bar 1515 kj/kg h 4 h 3 W p h 3 1515 kj/kg [ W p is negligible ] W net W T h 1 h 2 31475 21881 9594 kj/kg Q s h 1 h 4 h 1 h 3 31475 1515 2996 kj/kg [ h 4 h 3 ]
Coal Based Thermal Power Plants 15 h Mollier Diagram h =31475kJ/kg 1 1 350 o c 1 5 bar h =21881kJ/kg 2 2 006 bar Fig:12 s rankine W net Q s 9594 2996 32023 % Problem 12: In a rankine cycle, the steam flows to turbine as saturated steam at a pressure of 35 bar and the exhaust pressure is 02 bar Determine (using steam table only) (i) pump work (ii) the turbine work (iii) the rankine efficiency (iv) the condenser heat flow (v) the dryness fraction at the end of expansion The mass flow rate of steam is 95 kg/sec Solution p 1 35 bar dry saturated; p 2 02 bar; m 95 kg/sec To Find h 1 h 1 h g for 35 bar 2802 kj/kg To Find x 2 s 1 s g for 35 bar = 6123 kj/kg K Isentropic expansion So, s 1 s 2
16 Power Plant Engineering - wwwairwalkpublicationscom So, s 2 6123 kj/kg K T But at 02 bar, s g 7909 kj/kg K p=p=35bar 1 4 1 Since s 2 6123 s g, 7909 it is wet steam at exit of turbine 4 3 p=p=02bar 2 3 2 So, s 2 s f x 2 s fg for 02 bar Fig:13 S 6123 0832 x 2 7077 x 2 0748 Dryness fraction at the exit of turbine x 2 0748 To Find h 2 h f 2515 ; h fg 23584 for 02 bar h 2 h f x 2 h fg for 02 bar 2515 0748 23584 201472 kj/kg To Find h 3 h 3 h f3 2515 for 02 bar To Find h 4 v f3 0001017 m 3 /kg for 02 bar h 4 h 3 W p W p v f3 p 1 p 2 0001017 35 02 10 2 354 kj/kg h 4 2515 354 25504 kj/kg
Coal Based Thermal Power Plants 17 To Find Pump Work W p 354 kj/kg Pump power m W p 354 95 3363 kw To Find Turbine Work W T h 1 h 2 2802 201472 78728 Turbine power m W T 95 78728 74792 kw To Find Rankine Efficiency rankine W net Q s Q s h 1 h 4 2802 25504 254696 kj/kg W net W T W p 78728 354 78374 kj/kg rankine 78374 030772 30772% 254696 To Find Condenser Heat Flow Q rej Q rej h 2 h 3 201472 2515 176322 kj/kg Q rej in kw m h 2 h 3 95 176322 16751 kw Problem 13: Steam at 10 MPa and degree of super heat of 90C is supplied to a rankine cycle The condenser pressure is 10 KPa For mass rate of flow of 1 kg/sec determine (i) Power output (ii) thermal Solution h 1 3100 kj/kg; h 2 1975 kj/kg from Mollier diagram
18 Power Plant Engineering - wwwairwalkpublicationscom h 3 h f3 for 01 bar 1918 kj/kg W p h 4 h 3 v f3 p 1 p 2 v f3 0001010 for 01 bar h 4 h 3 v f3 p 1 p 2 1918 0001010 100 01 10 2 20189 kj/kg [ 10 2 for making bar into kpa] To Find W T, W p and Q s W T h 1 h 2 3100 1975 1125 kj/kg W p v f3 100 01 10 2 0001010100 01 10 2 101 kj/kg W net W T W p 1125 101 1115 kj/kg Q s h 1 h 4 3100 20189 289811 kj/kg Power output m W net 1 1115 1115 kw thermal W net Q s 1115 289811 3847% T 1 90 o c h h = 3100kJ/kg 1 10 0ba r 40 1 c o o 90 c 4 p 2=01bar 3 2 h = 1975kJ/kg 2 2 311 c o 01 ba r Fig:14 (a) s Fig:14 (b) s
Coal Based Thermal Power Plants 19 Problem 14: Dry saturated steam at 15 bar is supplied to a rankine cycle where exhaust pressure is 1 bar Find (a) thermal (rankine), steam consumption per kw, carnot (b) If the exhaust pressure is reduced to 02 bar by introducing a jet condenser, then determine % increase in rankine efficiency and % decrease in SSC (Apr 96-Madras University) Solution Given: p 1 p 4 15 bar; p 2 p 3 1 bar; Initially dry saturated rankine W net Q supply W T W p Q s W T h 1 h 2 W p h 4 h 3 v f p 1 p 2 Q s h 1 h 4 Find h 1, h 2, h 3 and h 4 h 1 h g for p 1 15 bar 2790 kj/kg from steam table (or) from Mollier chart, hs diagram T p = p 1 4 h 1 - h 4 p =p 1 4 1 h =2790 1 h 1 5bar 1 1 bar W P=h 4-h 3 4 3 p =p 2 3 s=s 1 2 2 Fig:15 (a) W =h -h T 1 2 s h =2340 2 2 Saturated Curve S 1=S2 Fig:15 (b) s
110 Power Plant Engineering - wwwairwalkpublicationscom 15 bar line will cut saturated curve at (1) Draw vertical line from (1) This vertical line will cut the 1 bar line at (2) h 2 2340 kj/kg h 3 h f (for 1 bar from steam table) = 4175 kj/kg v f 0001043 m 3 /kg (for 1 bar) h 4 h 3 v f p 1 p 2 h 4 h 3 v f p 1 p 2 4175 0001043 15 1 10 2 4189602 kj/kg [15 bar and 1 bar are multiplied by 10 2 to make KPa 1 bar 100 KPa ] To Find W T, W p and Q s W T h 1 h 2 2790 2340 450 kj/kg W p v f p 1 p 2 0001043 15 1 10 2 14602 kj/kg Q s h 1 h 4 2790 4189602 237104 kj/kg To Find rankine rankine W net Q s W T W p Q s 450 14602 237104 1892% To Find Specific Steam Consumption (SSC) SSC 3600 3600 803 kj/kwhr W net 44854
Coal Based Thermal Power Plants 111 To Find carnot T carnot T max T min T max For 15 bar, t sat t max 1983 C 273 p=p= 15bar 1 4 1 4713 K 4 For 1 bar, t sat t min 9963 C 273 37263 K 3 p=p=02bar 2 3 Fig:16 (a) 2 S carnot 4713 37263 4713 20936% Case (b) When p 2 02 bar h 3 h f for 02 bar 2515 kj v f3 0001017 m 3 /kg (for 02 bar) h 4 h 3 v f3 p 1 p 2 h 4 2515 0001017 15 02 10 2 253005 kj/kg W T h 1 h 2 2790 2120 670 kj/kg W p v f3 p 1 p 2 0001017 15 02 10 2 151 kj/kg W net W T W p 670 151 6685 kj/kg Q s h 1 h 4 2790 253005 2536995 kj/kg New rankine rankine W net Q s 6685 2536995 2635 %
112 Power Plant Engineering - wwwairwalkpublicationscom Increase in rankine efficiency 2635 1892 01892 New SSC 393% SSC 3600 W net 3600 6685 539 kg/kwhr h h = 2790 1 h = 2120 2 1 2 15 ba r 02 bar Decrease in SSC 803 539 803 3294% Fig:16 (b) s 12 IMPROVISATIONS - MODERN TRENDS IN RANKINE CYCLE IMPROVEMENTS Rankine Cycle - Reheating and Regenerative cycle 121 Reheat Cycle: If the dryness fraction of steam leaving the turbine is less than 088, then corrosion and erosion of turbine blades occur To avoid this situation, reheat is used In the reheat cycle, the expansion of steam takes place in one (or) more turbines Steam is expanded in the HP turbine first, then it is reheated The reheated steam is again expanded in the LP turbine Reheat cycle gives small increase in cycle efficiency It increases the net work output Reheating means heating the steam between turbine stages (between HP stage and LP stage) P 1 - Boiler pressure; P 2 P 3 = Reheat pressure; P 4 = Condenser pressure; T 1 = boiler temperature (or) superheat temperature; T 3 = Reheat temperature
Coal Based Thermal Power Plants 113 3 Reheater 2 6 Boiler 1 HPT LPT Pump 5 Condenser Fig:17 T 1 6 5 Fig:18 (a) 2 3 4 S h 6 1 p =C 5 1 T 1 p =C 4 3 p 2 =p =C 4 Fig:18 (b) 3 T 3 s Note: If T 3 is not given, then we can assume T 3 T 1
114 Power Plant Engineering - wwwairwalkpublicationscom h 1, h 2, h 3, h 4 - Take from mollier chart h s diagram) (or) from steam table h 5 h f for condenser pressure W p h 6 h 5 v f P 1 P 2 100 [ v f sp volume of fluid at condenser pressure] W T h 1 h 2 h 3 h 4 W p h 6 h 5 W net net work W T W P Q s heat supplied h 1 h 6 h 3 h 2 thermal W net Q s The ordinary Rankine cycle efficiency can be increased by increasing the pressure and temperature of the steam entering into the turbine When the initial pressure increases, the expansion ratio in the turbine also increases, and the steam becomes quite wet at the end of expansion This is not desirable because the increased moisture content of the steam causes corrosion in the turbine blades and so increase the losses Due to this, the nozzle and blade is decreased In reheat cycle, the steam is taken out from the turbine and it is heated by the flue gases in the boiler The main purpose of reheating is to increase the dryness fraction of steam passing through the lower stages of the turbine The dryness fraction of steam coming out from the turbine should not fall below 088 By using the
Coal Based Thermal Power Plants 115 reheat cycle, the specific steam consumption decreases and thermal also increases The increase in thermal due to reheat depends upon the ratio of reheat pressure to original pressure of steam The reheat pressure is generally kept within 20% of the initial pressure of steam: At low pressure, the of the cycle is reduced It is preferred for only high capacity plants, (or) 50,000 kw and the steam pressure range is 100 kgf/cm 2 ab 122 Advantages (or) effects of Re-heating Due to reheating, net work done increases Due to reheating, heat supply increases Due to reheating, thermal efficiency increases Due to reheating, the turbine exit steam dryness fraction increases - so moisture decreases - so blade erosion becomes minimum - so life of the turbine will be increased It reduces the fuel consumption upto 4 to 5% The size of the low pressure turbine blades can be reduced It reduces the steam flow of 15 to 20% with corresponding reductions in boiler, turbine and feed water heating equipments It also reduces the pumping power Less costly materials are used for lower steam pressures and temperatures to obtain required thermal efficiency It has higher thermal
116 Power Plant Engineering - wwwairwalkpublicationscom It has reduced feed pump power The condenser and boiler sizes are small The turbine has a very long life 123 Disadvantages: This cycle is more expensive than simple Rankine cycle It occupies more space The second stage turbine blade s design is complicated and expensive The operation and control reheat rankine cycle is complicated At light loads, the superheated steam will overheat the blades To avoid this, the feed water should be sprayed on the blades 13 Methods of Reheating: (a) Gas reheating (b) Live-steam reheating (c) Combined gas live steam reheater 131 Gas Reheating The steam taken from the high pressure turbine (HP) turbine is sent back to the reheater to reheat the steam to its initial throttle temperature The reheater is normally placed after the superheater so that the superheater receives the flue gases first and then the reheater receives flue gases Since the reheater should operate at much smaller temperature difference between the gas and steam, the counter flow heat exchanger is used for reheating
Coal Based Thermal Power Plants 117 To make the steam to be reheated to its initial throttle temperature in this gas reheating system, the following disadvantages are faced 1 Long and large pipe connections are required and hence cost is more and the pressure drop becomes higher Boiler To condenser Super heater HP LP Reheater Fig:19 Steam Reheating with Flue Gases 2 For piping system, the expansion and contraction allowances should be given 3 The amount of steam stored within the piping and reheater may cause considerable rise in turbine speed and it leads to accident in case of failure of emergency control 132 Live-Steam Reheating: The live steam reheating circuit is shown in Fig110 The high pressure steam from the superheater is used for reheating the steam coming out from the HP turbine in a specially designed reheater
118 Power Plant Engineering - wwwairwalkpublicationscom Boiler High Pressure Steam for Reheating HPT LPT Super Heater Live Steam Reheater Condensate Fig:110 Steam Reheating with Live Steam The advantages of live-steam reheating over gas heating are given here 1 The operation of this reheating system is simple 2 The reheater can be placed near the HP turbine and minimise extra pipe fittings 3 The control of temperature is easy since varying combustion condition will not affect the live steam reheater performance 4 Wet steam can also be reheated 5 More than one reheating can be used since the piping requirements are less 133 Combined gas and live steam reheater The combined gas and live steam reheater system is shown in Fig111 Live steam reheating system does not allow the steam to be reheated to its initial throttle temperature By using combined reheating system, this problem can be solved The live steam reheating system is
Coal Based Thermal Power Plants 119 Boiler To condenser HPT SH LPT Gas Reheater Live Steam Reheater Condensate Fig:111 Combined Live Steam and Reheating System placed in series with the gas reheater The steam extracted from HP turbine is first sent to live steam reheater and then to gas reheater as shown in fig In order to maintain a constant final temperature, the supply of live steam to first reheater is thermostatically controlled Problem 15: Steam at 90 bar, 480C is supplied to a steam turbine The steam is reheated to its original temperature by passing it through a reheater at 12 bar The condenser pressure is 007 bar Steam flow rate is 1 kg/sec Determine (a) network output; (b) thermal [neglect the pressure loss in reheating and boiler The expansion is isentropic (FAQ) Given: p 1 boiler pressure 90 bar; T 1 boiler temperature 480C T 3 Reheat temperature T 1 given 480C
120 Power Plant Engineering - wwwairwalkpublicationscom p 4 condenser pressure 007 bar From mollier chart, h 1 3330 kj/kg h 2 2805 kj/kg h 6 a p =90 1 o T 1= 480 2 b p =12 3 h 3 3440 kj/kg h 4 2360 kj/kg 5 p =007 c 4 h 5 h f for 007 1634 Fig:112 (a) s h 6 h 5 v f p a p c 100 v f for condenser pressure h 6 1634 0001007 90 007 100 1724625 W T h 1 h 2 h 3 h 4 3330 2805 3440 2360 1605 kj/kg W p h 6 h 5 1724625 1634 90625 kj/kg W net W T W p 15959375 kj/kg Q s h 1 h 6 h 3 h 2 3330 1724625 3440 2805 37925375 kj/kg 37925375 kj/kg thermal W net Q s 15959375 37925375 04208
Coal Based Thermal Power Plants 121 Extra Compare network done & efficiency of the above cycle with that of the simple cycle The simple cycle is shown here h 1 3330 kj/kg h 2 2030 kj/kg h 3 h f for condenser pressure 1634 kj/kg h 4 h 3 v f P a P c 100 [v f for condenser pressure P c ] h 4 1724559 W T h 1 h 2 1300 kj/kg W P h 4 h 3 90559 kj/kg W net W T W p 1290944 kj/kg Q s h 1 h 4 31575441 kj/kg thermal Q s W net 31575441 1290944 h o T 1=480 thermal 40884 % Note: reheat cycle p =9 0 a o 1 W net 15959375 kj/kg thermal 4208% 4 p =007 c 2 Because of reheating, work output increases and also thermal efficiency 3 Fig:112 (b) s
122 Power Plant Engineering - wwwairwalkpublicationscom Problem 16: In the reheat cycle, steam at 150 bar and 550C enters into the HP turbine The condenser pressure is 01 bar The moisture content at condenser inlet is 5% Determine (a) reheat pressure; (b) cycle efficiency; (c) steam flow rate per KW [FAQ] Solution Hint: condenser inlet is condition (4) x 4 095 Assumption T 3 T 1 h 1 3455 kj/kg h 2 2785 kj/kg h 3 3590 kj/kg h 6 1 p =150bar 2 T 1 1 p =01 bar 5 Fig:112 (c) 4 4 T 3 = 550 o C 3 x=095 4 s h 4 2460 kj/kg h 5 h f for condenser pressure v c 1918 kj/kg h 6 h 5 v f p 1 p 4 100 [v f is for condenser pressure 01 bar] h 6 1918 0001010 150 01 100 h 6 2069399 kj/kg (a) P 3 reheat pressure 1275 bar W T h 1 h 2 h 3 h 4 3455 2785 3590 2460 1800 kj/kg
Coal Based Thermal Power Plants 123 W p h 6 h 5 2069399 1918 151399 kj/kg W net W T W p 17848601 kj/kg Q s h 1 h 6 h 3 h 2 3455 2069399 3590 2785 405306 Q s 405306 kj/kg (b) thermal (c) SSC q s W net 044037 1 1 3600 W net 17848601 3600 201696 kg kw hr Problem 17: In a reheat rankine cycle, the condenser pressure is 75 bar The boiler temperature and reheat temperature are 500C The moisture content at any stage should not exceed 15 % Determine (a) Boiler pressure; (b) reheat pressure; (c) work done; (d) thermal Solution x 2 x 4 085 [moisture is 15%] 100 Condenser Pressure P c 75 KPa 0075 bar From mollier chart h 4 2215 kj/kg h 1 T =T = 550 c 1 3 3 o h 3 3460 kj/kg P 1 h 2 2540 kj/kg h 1 2990 kj/kg h 5 h f for p c 16865 kj/kg 6 5 2 P =0 075 4 4 Fig:112 (d) x =x =085 2 4 s
124 Power Plant Engineering - wwwairwalkpublicationscom h 6 h 5 v f p 1 p 4 100 h 6 16865 00010075 340 0075 100 h 6 2039049 kj/kg (a) boiler pressure P 1 340 bar; (b) reheat pressure P 2 38 bar W T h 1 h 2 h 3 h 4 2990 2540 3460 2215 1695 kj/kg W p h 6 h 5 2039049 16865 352549 kj/kg (c) W net W T W p 1695 352549 16597451 kj/kg Q s h 1 h 6 h 3 h 2 2990 2039049 3460 2540 Q s 37060951 kj/kg (d) the Q s W net 4478 % Problem 18: Steam at a pressure of 10 MPa, 500C is supplied to a reheat rankine cycle After expansion in the HPT, the steam is reheated at an optimum pressure to an optimum temp The moisture content at LPT exit should not exceed 15% Network done is 1600 KJ/Kg Determine (a) heat supply per kg; (b) thermal Condenser pressure is 7 kpa Solution Hint: Dont assume T 1 T 3 because steam is reheated to optimum temperature P 2 is not given, reheat temperature is not given
Coal Based Thermal Power Plants 125 P c P 4 pressure = 007 bar condenser The moisture content m in LP turbine is 15% (ie) x 3 085 h 1 3370 kj/kg h 4 2215 kj/kg h 5 h f for 007 bar h 6 1 P =1 00bar 5 1 2 T = 500 c 1 o P =007bar 4 Fig:113 4 T 3 3 x=085 2 s h 5 1634 kj/kg h 6 h 5 v f p 1 p 4 100 1734629 v f for p c 1634 0001007 100 007 100 h 6 1734629 W T h 1 h 2 h 3 h 4 W p h 6 h 5 1734629 1634 100629 kj/kg W net 1600 kj/kg h 1 h 2 h 3 h 4 h 6 h 5 W net W T W P ; W T W net W p 1600 10063 W T 1610063 kj/kg W T h 1 h 2 h 3 h 4 h 1 h 4 h 3 h 2 3370 2210 h 3 h 2
126 Power Plant Engineering - wwwairwalkpublicationscom h 3 h 2 4500626 KJ/Kg Q s h 1 h 6 h 3 h 2 3370 17346295 4500626 Q s 36465997 KJ/Kg thermal W Q s the 43876% 1600 36465997 043876 Problem 19: In an reheat rankine cycle steam at 3 MPa, 450C is supplied to a HPT The reheat temperature is 450C Condenser pressure is 4 KPa The HPT expansion is limited with dry & saturation Determine (a) reheat pressure; (b) net work; (c) the Solution From Mollier Chart h 4 2510 kj/kg h 1 3345 kj/kg h 1 450 o c 3 h 2 2720 kj/kg h 3 3385 kj/kg h 5 h f for P c P 4 1214 kj/kg 6 1 p =30bar 5 2 Dry & Saturation p =004 bar 4 Fig:114 4 s h 6 h 5 v f p 1 p 4 100 h 6 1214 0001004 30 004 100 h 6 12440798 kj/kg
Coal Based Thermal Power Plants 127 (a) reheat pressure 25 bar W T h 1 h 2 h 3 h 4 3345 2720 3385 2510 1500 kj/kg W p h 6 h 5 12440798 1214 300798 kj/kg W net W T W p 1500 300798 149699 kj/kg Q s h 1 h 6 h 3 h 2 3345 12440798 3385 2720 388559 kj/kg the W Q s 149699 388559 03853 the 3853% Problem 110: A steam power plant uses the Reheat cycle Steam Inlet to turbine 150 bar, 550C Reheat at 40 bar to 550C Condenser pressure is at 01 bar Using mollier diagram, find (i) the dryness fraction of steam at exit of turbine (ii) cycle efficiency (iii) specific steam consumption SSC Solution From mollier chart, T p =150 bar 1 550 o c 1 3 h 1 3465 kj/ kg; h 2 3065 kj/kg; h 3 3565 kj/kg h 4 2300 kj/kg; x 4 088 p =40 bar 2 6 p 3=01 bar 5 4 Fig:115 (a) 2 x 4 S h 5 h f at 01 bar from steam table 1918 kj/kg
128 Power Plant Engineering - wwwairwalkpublicationscom Pump work W p W p v f P 1 P 4 100 0001 [ 150 01 100] 1499 kj/kg W p h 6 h 5 1499 h 1 3 550 o c h 6 h 5 1499 1918 1499 20679 kj/kg 6 5 p =150 bar 1 2 p =40 bar 2 P =01 bar 3 4 x 4 s Heat supplied Q s Fig:115 (b) Q s h 1 h 6 h 3 h 2 3465 20683 3565 3065 375817 kj/kg Turbine Work W T W T h 1 h 2 h 3 h 4 3465 3065 3565 2300 1665 kj/kg Net work donew net W net W T W P 1665 1499 165001 kj/kg cycle W net Q s 165001 375817 Specific steam consumption SSC 3600 3600 W net 165001 0439 439% 2182 kg/kwhr
Coal Based Thermal Power Plants 129 14 REGENERATIVE CYCLE: (Bleeding Cycle) Assume 1 kg of steam is expanded in the turbine Before complete amount of steam is expanded, some amount of steam (m kg) is extracted (this process is called bleeding) and utilized for heating the feedwater So remaining amount of steam 1 m kg is completely expanded in the turbine and condensing in condenser In the regenerator, m kg of high temperature steam and 1 m kg of condensate are passing Heat transfer from steam to condensate (feed water) takes place So this process increases the enthalpy of feed water Thus the heat supplied to boiler will be reduced Re-generation means heating the feed water by steam taken from the turbine The steam is exhausted (or) bled from the turbine at several locations (before exhaust) and is supplied to regenerator (feed water heater) to heat the feed water Extracting the steam in the turbine before exhaust is called bleeding 1kg 7 Boiler 1kg 1 T Pump 2 mkg 2 bleeding mkg 3 (1-m)kg Condenser 1kg 6 Heat Exchanger (or) Regenerator (or) Feed Water Heater (1-m)kg (1-m)kg 5 Pump 1 4 Fig:116
130 Power Plant Engineering - wwwairwalkpublicationscom Energy - balance Assume 1 kg of working fluid is circulated Energy in = Energy out m mass of steam bled mass of steam circulated Energy entering regenerator = Energy leaving regenerator m h 2 1 m h 5 1 h 6 m h 2 h 5 mh 5 h 6 m h 2 h 5 h 6 h 5 mass of steam bled mass of steam circulated m h 6 h 5 kg of steam bled h 2 h 5 kg of steam circulated Take h 1, h 2, h 3 from mollier diagram h s diagram) (or) from steam table h 4 h f for condenser pressure p 3 p 4 h 6 h f for regenerator pressure p 6 p 2 p 5 h 5 h 4 v f p 6 p 4 10 2 [ p is in bar; p 10 2 kpa] [v f for condenser pressure P c P 3 P 4 ] h 7 h 6 v f p a p b 10 2 [p b for regenerator pressure p b and v f for P b W T 1 h 1 h 2 1 m h 2 h 3
Coal Based Thermal Power Plants 131 h 1 T 1 7 5 6 mkg 1 kg p =C (1-m ) (1-m) p =C a b p =C c 1kg 2 (1-m) 3 h 7 1kg 5 6 4 1 1kg mkg 2 (1-m) (1-m) 3 4 Fig:117 (a) s Fig:117 (b) s W P h 5 h 4 1 m 1 h 7 h 6 W net W T W P Q s h 1 h 7 thermal W net Q s Problem 111: A steam turbine plant equipped with a single regenerative feed water heating operates with the following data Initial pressure 165 bar; Initial super heat 93C; Extraction pressure 2 bar; exhaust pressure 005 bar Compare regenerative and non-regenerative cycle for (a) the ; (b) network; (c) SSC [Frequenty Asked University Questions] Solution Given data: P a P 1 165 bar t sat 1 2029 C Degree of superheat 93C t 1 t sat 1 t 1 2959 C
132 Power Plant Engineering - wwwairwalkpublicationscom P b regenerative pressure 2 bar P 2 P c condenser pressure 005 bar Case (a) non-regenerative (simple cycle) From mollier chart, h p =165 bar 1 1 o T 1=2959 c h 1 3035 kj/kg; 4 h 2 2100 kj/kg; h 3 h f for P 2 1378 kj/kg p =005 2 2 h 4 h 3 v f P 1 P 2 100 3 v f for P 2 h 4 1378 0001005 165 005 100 h 4 13945 kj kg W T 1 h 1 h 2 3035 2100 935 kj/kg W p h 4 h 3 1394532 1378 16532 W p 16532 W net W T W p 93335 kj/kg Fig:118 (a) s Q s h 1 h 4 3035 13945 2895568 kj/kg thermal W net Q s 3223%
1 1 SSC 3600 W net 93335 Case (b) regenerative cycle Coal Based Thermal Power Plants 133 3600 385708 kg/kwhr h 1 3035 kj/kg h 1 o T 1=296 c h 2 2610 kj/kg h 3 2100 kj/kg h 4 h f forp c 7 1kg 6 p =165 bar=p a 1 mkg p =2bar b 1kg 2 (1-m) 1378 kj/kg h 5 h 4 v f P b P c 100 5 (1-m ) 4 p =005 c 3 Fig:118 (b) s h 5 1378 0001005 2 005 100 137996 kj/kg h 6 h f for 2 bar P b h 6 5047 kj/kg h 7 h 6 v f P a P b 100 h 7 5047 0001061 165 2 100 h 7 50624 kj/kg W T 1 h 1 h 2 1 m h 2 h 3 3035 2610 1 m 2610 2100 m h 6 h 5 5047 137996 h 2 h 5 2610 137996 0148
134 Power Plant Engineering - wwwairwalkpublicationscom m 0148 kg of steam bled kg of steam circulated W T 3035 2610 1 0148 2610 2100 859345 kj/kg W p h 5 h 4 1 m 1 h 7 h 6 137996 1378 1 0148 1 50624 5047 171 kj/kg W p 171 kj/kg W net W T W p 85764 kj/kg Q s h 1 h 7 3035 50624 252876 kj/kg the W Q s 03392 3392% SSC 1 W net 3600 4198 kg/kwhr Simple cycle W net 93335 kj/kg Regenerative cycle W net 85764 kj/kg the 32116 % the 3392% Note: Work out put slightly decreases and efficiency increases Problem 112: A steam turbine operates on a simple regenerative cycle Steam is supplied dry saturated at 40 bar and exhausted to condenser pressure of 007 bar The condensate is pumped to a pressure of 35 bar at which it is mixed with bled steam from the turbine at 35 bar The resulting water at saturation is then pumped to the boiler For
Coal Based Thermal Power Plants 135 the ideal cycle, calculate (a) the amount of steam bled per kg of supply steam and (b) the of the plant, neglecting pump work (FAQ) Solution h 1 2800 kj/kg h 2 2380 kj/kg h 3 1880 kj/kg h p =40bar a p =35bar b 1 2 h 4 h f for p c 1634 kj/kg 76 45 p =00 7 c 3 h 4 h 5 Neglecting pump work h 6 h f for p b 35 bar 5843 kj/kg h 6 h 7 Neglecting pump work Fig:119 s m h 6 h 5 5843 1634 h 2 h 5 2380 1634 01899 kg of steam bled kg of steam circulated W T 1 h 1 h 2 1 m h 2 h 3 2800 2380 1 01899 2380 1880 W T W net 825058 KJ/Kg [ W p is neglected ] Q s h 1 h 7 2800 5843 22157 kj/kg
136 Power Plant Engineering - wwwairwalkpublicationscom the W Q s 37237 % Problem 113: A ideal regenerative cycle operates with steam supplied at 30 bar and 400C and condenser pressure of 010 bar For this cycle, find (a) W T in KJ/Kg; (b) cycle efficiency; (c) steam rate in Kg/KW hr The feed water heater can be assumed to be direct contact type which operates at 5 bar (FAQ) 30 bar 400 o c 1kg 1 Turbine Boiler 5 bar m kg 5 bar 2 01 bar 3 condenser Heater 7 6 4 Pump 5 Pump 1kg (1-m)kg Fig:120 Solution h 1 3230 kj/kg h 2 2800 kj/kg h 3 2195 kj/kg h 4 h f for P c 1918 kj/kg
Coal Based Thermal Power Plants 137 T 7 6 30 bar 1kg 5 bar mkg 1 T 1 = 400 o c 1kg 2 (1-m) kg h 7 1kg p = 6 a 3 0bar p = 5 ba r b (m kg) 1 1kg 2 (1-m) 5 4 01 bar (1-m) kg Fig:120 (a) 3 S 5 (1-m) p c= 0 1 4 Fig:120 (b) 3 s h 5 h 4 v f P b P c 100 h 5 1918 0001010 5 01 100 h 5 192295 kj/kg h 6 h f for P b 6401 kj/kg h 7 h 6 v f P a P b 100 h 7 6401 0001093 30 5 100 h 7 64283 kj/kg m h 6 h 5 6401 19229 h 2 h 5 2800 19229 01717 kg of steam kg of steam circulated W T h 1 h 2 1 m h 2 h 3 3230 2800 1 01717 2800 2195 W T 931107 kj/kg
138 Power Plant Engineering - wwwairwalkpublicationscom W P 1 m h 5 h 4 h 7 h 6 1 01717 19229 1918 64283 6401 314 kj/kg W net W T W p 927965 kj/kg Q s h 1 h 7 3230 64283 2587168 kj/kg the W Q s 3587% SSC 1 1 3600 W net 92797 3600 388 kj/kwhr Problem 114: A steam turbine plant, working on a single stage of regenerative feed heating receives steam at 3 MPa and 300C The turbine exhausts to a condenser at 15 KPa while the bled steam is at 300 KPa Assuming that the cycle uses actual regenerative cycle, calculate the thermal efficiency of cycle Compare this value with a rankine cycle operating between same boiler and condenser pressures (FAQ) h 1 2990 kj/kg; h 2 2540 kj/kg; h 3 2115 kj/kg; h 4 h f forp c 226 kj/kg h 5 h 4 v f p b p c 100 h 5 226 0001014 3 015 100 h 5 22629 kj/kg h 6 h f for P b 5615 kj/kg h 7 5615 0001074 30 3 100 h 7 5644 kj/kg
Coal Based Thermal Power Plants 139 m h 6 h 5 5615 22629 h 2 h 5 2540 22629 0145 kg of steam bled kg of steam circulated W T h 1 h 2 1 m h 2 h 3 2990 2540 1 0145 2540 2115 813426 kj/kg W p 1 m h 5 h 4 h 7 h 6 1 0145 22629 226 5644 5615 315 kj/kg W net W T W p 81028 kj/kg Q s h 1 h 7 2990 56434 24256 kj/kg the W net Q s 3341% Simple Rankine cycle h 1 2990 kj/kg h 2 2115 kj/kg h 3 226 kj/kg h 4 22903 kj/kg h 7 6 p =30bar a p =3bar b 1 (m kg) o T 1=300 c 1kg 2 (1-m) kg W T h 1 h 2 2990 2115 5 p =015bar c 3 875 kj/kg 4 Fig:121 (a) s W p h 4 h 3
140 Power Plant Engineering - wwwairwalkpublicationscom 22903 226 h 1 o T 1=300 c 303 kj/kg p =30ba r a 1kg W net W T W P 87197 kj/kg 4 Q s h 1 h 4 2990 22903 p =015 bar b 2 276097 kj/kg the W net Q s 87197 276097 03158 3 Fig:121 (b) s 3158% Note Mass rate of steam bled m m [ m kg of steam circulated /s ] mass of steam bled m mass of steam circulated kg of steam bled kg of steam circulated kg of steam circulated sec So mass rate of steam bled kg of steam bled sec 141 Advantages of Regenerative cycle m m 1 Heat supplied to boiler becomes reduced 2 The heating process in the boiler approaches the reversible process
Coal Based Thermal Power Plants 141 3 Since feed water temperature is high, the range of temperature in the boiler is minimum It reduces the thermal stresses produced in the boiler 4 Thermal efficiency is increased since the average temperature of heat addition to the cycle is increased 5 Due to bleeding in the turbine, erosion of turbine due to moisture is reduced 6 Condenser can be a smaller size This type of heating arrangement gives the efficiency equivalent to the Carnot cycle efficiency This type of arrangement cannot be used in practice because steam becomes too wet in the later stages of the turbine In actual practice, the advantage of regenerative heating principle is taken by bleeding a part of steam from the turbine at certain stages of expansion and it is used for heating the feed water in the separate feed heaters This arrangement does not reduce the dryness fraction of remaining steam passing through the turbine There are different methods of using the bled steam for heating the feed water as discussed here Methods: (a) Direct contact heaters The steam bleed from the different points of turbine is mixed directly with the feed water to increase the temperature of feed water and the steam mixed with feed water is extracted with the help of the pump and supplied to the boiler
142 Power Plant Engineering - wwwairwalkpublicationscom To boiler m 1 kg 1kg p b p 1 m 2 kg p 2 (1-m 1 -m 2 )kg Feed (1-m 1) Feed Heater heater condenser Pump 1 Pump 2 Pump 3 The main disadvantage is that The pump has to work with hot feed water So this system is normally not used in practice (b) Drain pump method: Fig:122 Direct Contact Heaters (1-m-m) 1 2 kg To B oiler 1kg p b condenser m kg 1 m kg 2 1kg (1- m 1 ) p 1 (1- m 1 ) p 2 (1-m 1 -m 2) kg m kg 1 m kg 1 m kg 2 m kg 2 Fig:123 In this method, the feed water is heated with indirect contact of bled steam in the heat exchangers The bled condensate is extracted by the drain pump discharges into the feed pipe line This method also suffers from the same disadvantage as mentioned in the direct contact heaters arrangement
Coal Based Thermal Power Plants 143 (b) All drains to hot well: All drains to hot well method is shown in Fig124 (1-m -m )kg 1 2 To boiler m 1 kg m 2 kg condenser p C 1kg p C p 1 1kg p 2 p a Feed heater Feed heater m 2 kg 1kg (1-m 1 -m )kg 2 In this method, the condensate of bled steam coming out from the indirect heat exchangers is fed to the hotwell The total condensate (Condensate coming from condenser + Condensate from bled steam) from the hot well is pumped to the boiler through the regenerative feed heaters (c) Cascade System: m kg 1 Fig:124 All Drain to Hot Well (1-m -m )kg 1 2 To boiler p b m kg 1 m kg 2 Condenser p c 1kg 4kg 1kg p a p a Feed Heater m kg 1 (m +m )kg 1 2 Fig:125 Cascade System
144 Power Plant Engineering - wwwairwalkpublicationscom In this method, the condensate of bled steam coming out from the first heat exchanger is passed through the second heat exchanger and lastly to hot well as shown in Fig125 15 LAYOUT OF MODERN COAL POWER PLANT (OR) LAYOUT OF STEAM POWER PLANT In steam power plant, the water is converted into steam and the steam expanded in a turbine to produce kinetic energy which is converted into mechanical energy The steam power plant has four major circuits by which the layout can be studied in detail These are Ash Storage Hot Ash or Slag Handling Air from atmosphere FD Fan To Atmosphere Chimney Coal handling Coal Preparation Fuel Superheater Preheated air Control Air Preheater Valve Economiser Pump IDFan Dust Collector Turbine Generator Turbine Exhaust Hot Water Cooling Water In Cooling Tower Coal Storage Boiler Feed Pump High Pressure Heater Deaerator Low Pressure Heater Fig:126 Steam Power Plant or Thermal Power Plant
Coal Based Thermal Power Plants 145 1 Coal and ash circuit 2 Air and flue gas circuit 3 Feed water and steam circuit 4 Cooling water circuit 151 Coal and ash Circuit: This circuit consists of coal delivery, preparation of coal, handling of coal to the boiler furnace,ash handling and ash storage The coal which is received from the mines are stored in coal storage This raw coal is sized by crushers and then this prepared coal is transferred to the boilers In the boiler, the coal is burnt and converted into ash This ash is usually quenched to reduce the temperature, corrosion and dust content Then it is stored at ash storage 152 Air and Flue gas circuit Air from Atmosphere To Atmosphere FD F an Chimney Super Heater Preheated Air Air Preheater IDFan Dust Collector Economizer Fig:127 Air and Flue Gas Circuit This circuit consists of forced draught fan, air-preheater, boiler furnace, super heater, economiser, dust collector, induced draught (ID) fan and chimney
146 Power Plant Engineering - wwwairwalkpublicationscom The air from the atmosphere is forced into the circuit by a forced draught fan This air is preheated in the air preheater by flue gases This pre heated air is supplied to the furnace where this air is converted into flue gases This flue gases pass over the boiler tubes by which the water is converted into steam Then this flue gas is passed to the super heater where the steam is converted into super heated steam Then it is passed to the economiser to heat the feed water and to the air preheater The dust in the flue gases are collected by the dust collector and then left to atmosphere through chimney 153 Feed water and steam flow circuit: Superheater Control Valve Economiser Pump Turbine Generator Turbine Exhaust Hot Water Boiler Feed Pump High Pressure Heater Deaerator Low Pressure Heater Fig:128 Feed Water And Steam Flow Circuit This circuit consists of feed pump, economiser, boiler drum super heater, turbine and condenser From the hot well, the feed water is pumped to the economiser where the water is preheated by the flue gases This preheated water is supplied to the boiler drum In the boiler drum, the pre heated water is converted into steam by burning of coal The steam raised in boiler is passed through the super heater where the steam is converted into super
Coal Based Thermal Power Plants 147 heated steam The super heated steam is expanded in turbine which is coupled with generator The expanded steam is then passed through the condenser in which the steam is converted into water and this water is recirculated 154 Cooling Water Circuit This circuit consists of a pump, condenser and cooling tower In the condenser, cold water is circulated to condense the steam in to waterthe steam is condensed by loosing its latent heat to the circulating cold water By this, the Turbine Exhaust Steam Hot Water Cooling Water in circulating water is heated This hot water is cooled at the cooling tower, where the water is sprayed in the form of droplet through nozzles The cold air enters the cooling tower from the bottom which cools the sprayed hot water The cooled water is collected in the cooling pond and the same is re circulated again and again To compensate the water lost due to vapourisation, the make up water is added to the pond by means of a pump 16 SELECTION OF SITE FOR A STEAM POWER PLANT The following consideration should be taken while selecting the site for a steam power plant 1 Availability of raw materials Huge quantity of coal and fuel are required to run a steam (thermal) power plant Therefore, it is important to Cooling Tower water Fig:129 Cooling Water Circuit
148 Power Plant Engineering - wwwairwalkpublicationscom locate the plant as near as possible to the coal fields to reduce the transportation cost If it is not possible to locate the plant near the coal field, then it should be located near the railway station or near to a port 2 Ash disposal facilities As a huge quantity of coal is burnt, this results in a huge quantity of ash too The ash handling problem is more serious as compared to handling of coal because it comes out very hot and is very corrosive If not disposed properly it will result in environmental pollution and other hazards Therefore there must be sufficient space to dispose this large quantity of ash 3 Nature of land The land should have good bearing capacity about 1 MN/m 2 as it has to withstand the dead load of plant and force transmitted to the foundation due to working of heavy machinery 4 Cost of land Large area is required to build a thermal power plant, therefore the land price should be affordable (cheap) For eg: Large plant in the heart of city will be very costly 5 Availability of water Water is the working fluid in a steam power plant, and a large quantity of water is converted to steam in order to run the turbine It is important to locate the plant near the water source to fulfill its water demand through out the year
Coal Based Thermal Power Plants 149 6 Size of the plant The capacity of the plant decides the size of the plant, large plant requires large area and the smaller plant requires considerably smaller area Therefore, the size of the plant and its capacity play an important role in selection of site 7 Availability of workforce During construction of plant, enough labour is required The labour should be available at the proposed site at cheap rate 8 Transportation facilities Availability of proper transportation is another important consideration for the selection of site as a huge quantity of raw materials (coal & fuel) through out the year and heavy machinery are to be brought to the site during the installation 9 Load centre The plant must be near to the load centre to which it is supplying power in order to decrease transmission loss and minimize transmission line cost 10 Public problems The plant should be away from the town or city in order to avoid nuisance from smoke, ash, heat and noise from the plant 11 Future extension A choice for future extension of the plant should be made in order to meet the power demand in future
150 Power Plant Engineering - wwwairwalkpublicationscom 17 SUPER CRITICAL BOILERS Super critical boiler is a boiler that operates at super critical pressure (high pressure) to increase the efficiency of the plant and to reduce the cost of electricity production Normally, water tube boilers are generally preferred for high pressure In this water tube boilers, the water is circulated through tubes and their external surfaces are exposed to the flue gases It is the most economical cycle The working steam pressure range is 125 bar to 300 bar and temperature is 510C to 600C Usually sub-critical boiler consists of three distinct sections as preheater, evaporator and superheater And in case of supercritical boiler, only preheater and super heater are required Generally super critical boilers are used for more than 300 MW power plants Advantages of super critical boilers: 1 The amount of scale formation is less since the velocity of water through pipes are more 2 In this method, light weight tubes with better heating surface arrangement can be used It occupies less space The cost for foundation, time for erection are very less 3 All parts of the system are heated uniformly So there is no danger of over heating 4 The differential expansion is reduced due to uniform temperature throughout structure So there is no leakages of gas (or) air 5 The flexibility is more