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353685130-CCGT-Combined-Cycle-Gas-Turbine.ppt
- 1. Combined Cycle Gas Turbine Combined Cycle Gas Turbine. CCGT Power Plant Abbas A M Al Fardan
- 2. Combined Cycle Gas Turbine What is the CCGT? A combined cycle gas turbine power plant, frequently identified by CCGT shortcut, is essentially an electrical power plant in which a gas turbine and a steam turbine are used in combination to achieve greater efficiency than would be possible independently. The gas turbine drives an electrical generator. The gas turbine exhaust is then used to produce steam in a heat exchanger (steam generator) to supply a steam turbine whose output provides the means to generate more electricity. However the Steam Turbine is not necessarily, in that case the plant produce electricity and industrial steam which can be used for heating or industrial purpose.
- 3. Combined Cycle Gas Turbine Basic Gas Turbine Information •Main Gas Turbine Manufactures: General Electrics, Simens Westinghouse & Alstom •Approximately Cost per MW – 0.7mln E •Efficiency approx 40% for gas turbine however in the CCGT plant the efficiency is 50-60% (even higher for cogenerated plant) •Low Green Gas Emission C02, NOx & SOx •Chepear comparing to other technology e.g. CCS •Lifetime 30-40 years
- 4. Combined Cycle Gas Turbine How it works? 220kV Tabert Substation 110kV Clahane Substation
- 5. Combined Cycle Gas Turbine CCGT Fuel Available in KSA Natural Gas. Resources available in KSA Synthetic Gas from coal. Resources not available in KSA Fuel Oil. Resources available in KSA Biogas from forestry, domestic and agricultural waste. Resources not available in KSA
- 6. Combined Cycle Gas Turbine CCGT Plants Conventional or Cogeneration Variable CCGT High Efficiency Cogeneration Transmission Network Lower Impact Higher Impact Power Losses Less power losses Higher Power Losses Heat Market Required Not Required Fuel consumption -33% +33% CO2 Emission -67% +67% Water Consumption -30% +30% Capital Cost per kW delivered 630 1200
- 7. Combined Cycle Gas Turbine grid Grid Code Grid Code contains general conditions and rules for general application. The specification and conditions for each application are adjust individually. Those information are included in Grid Connection Offer & Agreement between developer and Transmission Operator TSO. •Client (Requires connection) and TSO must implement Grid Code specification during each stages of the project, for project above 10MW •TSO may be disconnected or terminated the Grid Connection Agreement if the Grid Code is not implemented by client. •The Implementation of the Grid Code may have significant impact on the cost of the Grid Connection •ESB Networks Electrical Safety Rules must be implemented
- 8. Combined Cycle Gas Turbine Grid Constraints •Capacity of the transmission lines • Small Infrastructures of the High Voltage Lines •Distance from Energy Load Centres (West Coast) • High Cost of Design and planning permission for Shallow Connection, significantly for OHL 220kV •Planning Restrictions regarding OHL Construction
- 9. Combined Cycle Gas Turbine Grid Connection Costs Variable Cost Gas & Steam Turbine Generator 210’000’000 2 bay 110kV/220kV Substation 4’420’000 220kV OHL 710’000/km (12km) 110kV OHL 320’000/km (15km) Buried Cable 500MVA (optional) 2’150’000/km Total Cost 227’740’000
- 10. Combined Cycle Gas Turbine 10 Gas Turbine Basics • Gas Turbines – Types – How They Work – Applications – Components of Plant – Flow Paths – Operation
- 11. Combined Cycle Gas Turbine 11 Gas Turbine Applications • Simple Cycle • Combined Cycle • Cogeneration
- 12. Combined Cycle Gas Turbine 12 Types of Gas Turbine Plants • Simple Cycle – Operate When Demand is High – Peak Demand – Operate for Short / Variable Times – Designed for Quick Start-Up – Not designed to be Efficient but Reliable • Not Cost Effective to Build for Efficiency • Combined Cycle – Operate for Peak and Economic Dispatch – Designed for Quick Start-Up – Designed to Efficient, Cost-Effective Operation – Typically Has Ability to Operate in SC Mode
- 13. Combined Cycle Gas Turbine 13 The energy contained in a flowing ideal gas is the sum of enthalpy and kinetic energy. Pressurized gas can store or release energy. As it expands the pressure is converted to kinetic energy. Principles of Operation • Open Cycle Also referred to as simple cycle) Link to picture
- 14. Combined Cycle Gas Turbine 14 Brayton Cycle – Gas Turbine Cycle
- 15. Combined Cycle Gas Turbine 15 Thermodynamic Fundamentals • Pressure Ratio & CT Components
- 16. Combined Cycle Gas Turbine 16 Combustion or Gas Turbine
- 17. Combined Cycle Gas Turbine 17 Principles of Operation Compressor • As air flows into the compressor, energy is transferred from its rotating blades to the air. Pressure and temperature of the air increase. • Most compressors operate in the range of 75% to 85% efficiency. Combustor • The purpose of the combustor is to increase the energy stored in the compressor exhaust by raising its temperature. Turbine • The turbine acts like the compressor in reverse with respect to energy transformation. • Most turbines operate in the range of 80% to 90% efficiency.
- 18. Combined Cycle Gas Turbine 18 Principles of Operation Overall Energy Transformations (Thermal Efficiency) • Useful Work = Energy released in turbine minus energy absorbed by compressor. The compressor requires typically approximately 50% of the energy released by the turbine. • Overall Thermal Efficiency = Useful Work/Fuel Chemical Energy *100 Typical overall thermal efficiencies of a combustion turbine are 20% - 40%.
- 19. Combined Cycle Gas Turbine 19 Gas Turbine Applications • Simple Cycle
- 20. Combined Cycle Gas Turbine 20 Simple Cycle Power Plant Westinghouse 501D5 – 340 MW
- 22. Combined Cycle Gas Turbine 22 Combined Cycle Plant Design GT PRO 13.0 Drew Wozniak 1512 10-13-2004 23:27:31 file=C:Tflow13MYFILES3P 0 70.gtp Net Power 95959 kW LHV Heat Rate 7705 BTU/kWh p[psia], T[F], M[kpph], Steam Properties: Thermoflow - STQUIK 4.717 m Fogger 1X GE 6581B 2 X GT 33781 kW 12.54 p 90 T 30 %RH 944 m 4327 ft elev. 12.39 p 68 T 948.7 m Natural gas 18.58 m 96 T 77 T LHV 369671 kBTU/h 149.2 p 684 T 143.2 p 2072 T 967.3 m 12.93 p 1034 T 1934.6 M 73.85 %N2 13.53 %O2 3.233 %CO2+SO2 8.497 %H2O 0.8894 %Ar 1031 T 1934.6 M 1031 897 569 568 538 534 481 419 326 268 268 T 1934.6 M 30813 kW 0.1296 M FW 1.694 p 120 T 222.1 M 120 T Natural gas 0 M 122 T 292.6 M 122 T 17.19 p 220 T 29.58 M 17.19 p 220 T 29.65 M LPB 29.65 M 292.6 M 203.6 p 373 T 292.6 M IPE2 203.6 p 383 T 36.75 M IPB 199.7 p 460 T 36.75 M IPS1 195.8 p 500 T 36.75 M IPS2 924.2 p 472 T 251.1 M HPE2 910.5 p 523 T 251.1 M HPE3 910.5 p 533 T 248.6 M HPB1 879.8 p 954 T 248.6 M HPS3 850 p 950 T 248.6 M 879.8 p 954 T 6.89 M 183 p 375 T 70 M V4 26.36 M 195.8 p 597 T V8 6.89 M
- 23. Combined Cycle Gas Turbine 23 Gas Turbine Components Compressor – Combustor - Turbine
- 24. Combined Cycle Gas Turbine 24 Gas Turbine Components & Systems (cont’d) • Combustion System – Silo, Cannular, Annular – Water, Steam, DLN • Turbine – Multiple Shaft, Single Shaft – Number of Stages – Material and Manufacturing Processes Exhaust System Simple Cycle Stack Transition to HRSG Generator Open-Air cooled TEWAC Hydrogen Cooled Starting Systems Diesel Motor Static
- 25. Combined Cycle Gas Turbine 25 Combustion Turbine Fuels • Conventional Fuels – Natural Gas – Liquid Fuel Oil • Nonconventional Fuels – Crude Oil – Refinery Gas – Propane • Synthetic Fuels – Chemical Process – Physical Process
- 26. Combined Cycle Gas Turbine 26 GE Combustion Turbine Comparisons
- 27. Combined Cycle Gas Turbine 27 Parameter Heavy Duty Aero-Derivative Capital Cost, $/kW Lower Higher Capacity, MW 10 - 330 5 – 100 Efficiency Lower Higher Plan Area Size Larger Smaller Maintenance Requirements Lower Higher Technological Development Lower Higher Advanced Heavy-Duty Units Advanced Aero derivative Units Gas Turbine Types
- 28. Combined Cycle Gas Turbine 28 Gas Turbine Major Sections • Air Inlet • Compressor • Combustion System • Turbine • Exhaust • Support Systems
- 29. Combined Cycle Gas Turbine 29 Gas Turbine Barrier Inlet Filter Systems
- 30. Combined Cycle Gas Turbine 30 Gas Turbine Pulse Inlet Filter System
- 33. Combined Cycle Gas Turbine 33 Gas Turbine Compressor Rotor Assembly
- 35. Combined Cycle Gas Turbine 35 Gas Turbine Cut Away Side View
- 36. Combined Cycle Gas Turbine 36 Gas Turbine Combustor Arrangement
- 38. Combined Cycle Gas Turbine 38 GE LM2500 Aero-derivative Gas Turbine Compressor Compressor Turbine Section Power Turbine Section
- 40. Combined Cycle Gas Turbine 40 FT4 Gas Turbine – Gas Generator Compressor)
- 41. Combined Cycle Gas Turbine 41 FT4 Gas Turbine – Gas Generator (Compressor)
- 42. Combined Cycle Gas Turbine 42 FT4 Gas Turbine – Free Turbine
- 43. Combined Cycle Gas Turbine 43 FT4 Gas Turbine – Free Turbine Gas Path
- 44. Combined Cycle Gas Turbine 44 FT4 Gas Generator Performance
- 45. Combined Cycle Gas Turbine 45 FT4 Free Turbine Performance
- 46. Combined Cycle Gas Turbine 46 Aero-derivative Versus Heavy Duty Combustion Turbines • Aero-derivatives – Higher Pressure Ratios and Firing Temperatures Result in Higher Power Output per Pound of Air Flow – Smaller Chilling/Cooling Systems Required – Compressor Inlet Temperature Has a Greater Impact on Output and Heat Rate – Benefits of Chilling/Cooling Systems are More Pronounced
- 47. Combined Cycle Gas Turbine 47 Typical Simple Cycle CT Plant Components • Prime Mover (Combustion Turbine) • Fuel Supply & Preparation • Emissions Control Equipment • Generator • Electrical Switchgear • Generator Step Up Transformer • Starting System (Combustion Turbines) • Auxiliary Cooling • Fire Protection • Lubrication System
- 48. Combined Cycle Gas Turbine 48 Typical Peaking Plant Components Lube Oil System GSU Generator Fire Protection Starting Engine Switchgear / MCC
- 49. Combined Cycle Gas Turbine 49 Combining the Brayton and Rankine Cycles • Gas Turbine Exhaust used as the heat source for the Steam Turbine cycle • Utilizes the major efficiency loss from the Brayton cycle • Advantages: – Relatively short cycle to design, construct & commission – Higher overall efficiency – Good cycling capabilities – Fast starting and loading – Lower installed costs – No issues with ash disposal or coal storage • Disadvantages – High fuel costs – Uncertain long term fuel source – Output dependent on ambient temperature
- 50. Combined Cycle Gas Turbine 50 Picture courtesy of Nooter/Eriksen How does a Combined Cycle Plant Work?
- 52. Combined Cycle Gas Turbine 52 Combined Cycles Today • Plant Efficiency ~ 58-60 percent – Biggest losses are mechanical input to the compressor and heat in the exhaust • Steam Turbine output – Typically 50% of the gas turbine output – More with duct-firing • Net Plant Output (Using Frame size gas turbines) – up to 750 MW for 3 on 1 configuration – Up to 520 MW for 2 on 1 configuration • Construction time about 24 months • Engineering time 80k to 130k labor hours • Engineering duration about 12 months • Capital Cost ($900-$1100/kW) • Two (2) versus Three (3) Pressure Designs – Larger capacity units utilize the additional drums to gain efficiency at the expense of higher capital costs
- 53. Combined Cycle Gas Turbine 53 Combined Cycle Efficiency • Simple cycle efficiency (max ~ 44%*) • Combined cycle efficiency (max ~58-60%*) • Correlating Efficiency to Heat Rate (British Units) h= 3412/(Heat Rate) --> 3412/h = Heat Rate* – Simple cycle – 3412/.44 = 7,757 Btu/Kwh* – Combined cycle – 3412/.58 = 5,884 Btu/Kwh* • Correlating Efficiency to Heat Rate (SI Units) h= 3600/(Heat Rate) --> 3600/h = Heat Rate* – Simple cycle – 3600/.44 = 8,182 KJ/Kwh* – Combined cycle – 3600/.58 = 6,207 KJ/Kwh* • Practical Values – HHV basis, net output basis – Simple cycle 7FA (new and clean) 10,860 Btu/Kwh (11,457 KJ/Kwh) – Combined cycle 2x1 7FA (new and clean) 6,218 Btu/Kwh (6,560 KJ/Kwh) *Gross LHV basis
- 54. Combined Cycle Gas Turbine 54 Gas Turbine Generator Performance Factors that Influence Performance – Fuel Type, Composition, and Heating Value – Load (Base, Peak, or Part) – Compressor Inlet Temperature – Atmospheric Pressure – Inlet Pressure Drop • Varies significantly with types of air cleaning/cooling – Exhaust Pressure Drop • Affected by addition of HRSG, SCR, CO catalysts – Steam or Water Injection Rate • Used for either power augmentation or NOx control – Relative Humidity
- 57. Combined Cycle Gas Turbine 57 Cogeneration Plant • A Cogeneration Plant – Power generation facility that also provides thermal energy (steam) to a thermal host. • Typical thermal hosts – paper mills, – chemical plants, – refineries, etc… – potentially any user that uses large quantities of steam on a continuous basis. • Good applications for combined cycle plants – Require both steam and electrical power
- 58. Combined Cycle Gas Turbine 58 Major Combined Cycle Plant Equipment • Combustion Turbine (CT/CTG) • Steam Generator (Boiler/HRSG) • Steam Turbine (ST/STG) • Heat Rejection Equipment • Air Quality Control System (AQCS) Equipment • Electrical Equipment
- 59. Combined Cycle Gas Turbine 59 Heat Recovery Steam Generator (HRSG)
- 61. Combined Cycle Gas Turbine Primary to Secondary to End-Use Energy Secondary Energy Utilization Deviceor System Final Useful Energy Transformation Transportation Distribution Primary Energy Losses Losses
- 62. Combined Cycle Gas Turbine Outline • Electricity Basics • Electricity from Fossil Fuels • Co-generation and Tri-generation • Economics
- 63. Combined Cycle Gas Turbine Electricity Basics • Electricity can be either direct current (DC) or alternating current (AC) • In AC current, the voltage and current fluctuate up and down 60 times per second in North America and 50 times per second in the rest of the world • The power (W) in a DC current is equal to current (amps) x voltage (volts): P=VI • The power in an AC current is equal to the product of the root mean square (RMS) of the fluctuating current and voltage if the current and voltage are exactly in phase (exactly tracking each other): P=Vrms x Irms • The standard electricity distribution system consists of 3 wires with the current in each wire offset by 1/3 of a cycle from the others, as shown in the next figure
- 64. Combined Cycle Gas Turbine Three-phase AC Current -1.0 0.0 1.0 0 5 10 15 20 Time (ms) Voltage
- 65. Combined Cycle Gas Turbine Two Pole Synchronous Generator Source: EWEA
- 66. Combined Cycle Gas Turbine • Electricity demand continuously varies, and power utilities have to match this variation as closely as they can by varying their power production. The following distinctions are made: • Base_load power plants: these are plants that run steadily at full load, with output equal to the typical minimum electricity demand during the year. Plants (such as coal or nuclear) that cost a lot to build but are cheap to operate (having low fuel costs) are good choices • Peaking powerp lants: these are plants that can go from an off state to full power within an hour or so, and which can be scheduled based on anticipated variation in demand (natural gas turbines or diesel engines would be a common choice) • Spinning reserve: these are plants that are on but running at part load – this permits them to rapidly (within a minute) vary their output, but at the cost of lower efficiency (and so requires greater fuel use in the case of fossil fuel power plants).
- 67. Combined Cycle Gas Turbine Electricity from Fossil Fuels • Pulverized coal • Integrated Gasification/Combined Cycle (IGCC) • Natural gas turbines and combined cycle • Diesel and natural gas reciprocating engines • Fuel cells
- 68. Combined Cycle Gas Turbine Technical issues related to electricity from fossil fuels • Full load efficiency • Part-load efficiency • Rates of increase of output • Impact of temperature on output • Auxiliary energy use
- 69. Combined Cycle Gas Turbine Generation of electricity from a conventional, pulverized-coal power plant steam High-Pressure Boiler Generator electricity out to cooling tower or cold river water and/or cogeneration cooling water return flow Condenser Pump Steam Turbine water condensate sequestered CO out 2 CO up the stack 2 CO2 air (O ) 2 fossil fuel in Source: Hoffert et al (2002, Science 298, 981-987)
- 70. Combined Cycle Gas Turbine The upper limit to the possible efficiency of a power plant is given by the Carnot efficiency: η = (Tin-Tout)/Tin So, the hotter the steam supplied to the steam turbine, the greater the efficiency. Hotter steam requires greater pressure, which requires stronger steel and thicker walls. so there is a practical limit to the achievable Carnot efficiency (and actual efficiencies are even lower)
- 71. Combined Cycle Gas Turbine Coal power plant operating temperatures and efficiencies • Typical: 590ºC, 35% efficiency • Best today: > 600ºC, 42-44% efficiency • Projected by 2020: 720ºC, 48-50% efficiency
- 72. Combined Cycle Gas Turbine Integrated Gasification Combined Cycle (IGCC) • This is an alternative advanced coal power plant concept • Rather than burning pulverized solid coal, the coal is heated to 1000ºC or so at high pressure in (ideally) pure oxygen • This turns the coal into a gas that is then used in a gas turbine, with heat in the turbine exhaust used to make steam that is then used in a steam turbine • Efficiencies of ~ 50% are expected, but are much lower at present
- 73. Combined Cycle Gas Turbine Generation of electricity with natural gas • Simple-cycle power generation • Combined-cycle power generation • Simple-cycle cogeneration • Combined-cycle cogeneration
- 74. Combined Cycle Gas Turbine Simple-cycle turbine • Has a compressor, combustor, and turbine proper • Because hot gases rather than steam are produced, it is not restricted in temperature by the rapid increase in steam pressure with temperature • Thus, the operating temperature is around 1200ºC
- 75. Combined Cycle Gas Turbine Simple-cycle gas turbine and electric generator FUEL EXHAUST SHAFT COMPRESSOR TURBINE INTAKE AIR GENERATOR ELECTRICITY COMBUSTOR Source: Williams (1989, Electricity: Efficient End-Use and New Generation Technologies and Their Planning Implications, Lund University Press)
- 76. Combined Cycle Gas Turbine Efficiency of generating electricity using natural gas • One might expect a high efficiency from the gas turbine, due to the high input temperature (and the resulting looser Carnot limit) • However, about half the output from the turbine has to be used to compress the air that is fed into it • Thus, the overall efficiency is only about 35% in modern gas turbines
- 77. Combined Cycle Gas Turbine Turbine efficiency vs turbine size (power) 0 5 10 15 20 25 30 35 40 45 0 50 100 150 200 250 Turbine Power (MW) Turbine Efficiency (%)
- 78. Combined Cycle Gas Turbine Efficiency and cost of a simple-cycle gas turbine with and without water injection 0 5 10 15 20 25 30 35 40 45 50 0 20 40 60 Turbine Power (MW) Turbine Efficiency (%) 0 100 200 300 400 500 600 Cost (euros/kW) Without water injection With water injection Cost
- 79. Combined Cycle Gas Turbine Due to the afore-mentioned high operating temperature of the gas turbine, the temperature of the exhaust gases is sufficiently hot that it can be used to either: Make steam and generate more electricity in a steam turbine (this gives combined cycle power generation). Or: provide steam for some industrial process that can use the heat, or to supply steam for district heating (this gives simple cycle cogeneration)
- 80. Combined Cycle Gas Turbine Combined-cycle power generation using natural gas INTAKE AIR STEAM TURBINE ELECTRICITY CONDENSER COOLING TOWER STEAM EXHAUST WATER PUMP HEAT RECOVERY STEAM GENERATOR FUEL SHAFT COMPRESSOR TURBINE GENERATOR ELECTRICITY COMBUSTOR Source: Williams (1989, Electricity: Efficient End-Use and New Generation Technologies and Their Planning Implications, Lund University Press)
- 81. Combined Cycle Gas Turbine Simple-cycle cogeneration EXHAUST WATER PUMP HEAT RECOVERY STEAM GENERATOR PROCESS STEAM FUEL SHAFT COMPRESSOR TURBINE INTAKE AIR GENERATOR ELECTRICITY COMBUSTOR Source: Williams (1989, Electricity: Efficient End-Use and New Generation Technologies and Their Planning Implications, Lund University Press)
- 82. Combined Cycle Gas Turbine The energy can be cascaded even further, as follows: • Gas turbine → steam turbine → useful heat as steam from the steam turbine (combined cycle cogeneration), or • Gas turbine → steam turbine → steam → hot water (also combined cycle cogeneration), or • Gas turbine → steam → hot water
- 83. Combined Cycle Gas Turbine Combined-cycle cogeneration STEAM TURBINE ELECTRICITY CONDENSER COOLING TOWER STEAM EXHAUST WATER PUMP HEAT RECOVERY STEAM GENERATOR FUEL SHAFT COMPRESSOR TURBINE INTAKE AIR GENERATOR ELECTRICITY COMBUSTOR PROCESS STEAM Source: Williams (1989, Electricity: Efficient End-Use and New Generation Technologies and Their Planning Implications, Lund University Press)
- 84. Combined Cycle Gas Turbine Cogeneration system with production of steam and hot water FUEL ELECTRICITY GAS TURBINE GENERATOR EXHAUST GAS STEAM HEAT RECOVERY STEAM GENERATOR HEAT EXCHANGER HOT WATER EXHAUST GAS Source: Malik (1997, M. Eng Thesis, U of Toronto)
- 85. Combined Cycle Gas Turbine • State-of-the-art natural gas combined-cycle (NGCC) systems have electricity generation efficiencies of 55-60%, compared to a typical efficiency of 35% for single-cycle turbines • However, NGCC systems are economical only in sizes of 25-30 MW or greater, so for smaller applications, only the less efficient simple-cycle systems are used • Thus, a number of techniques are being developed to boost the electrical efficiency of simple gas turbines to 42-43%, with one technique maybe reaching 54-57%
- 86. Combined Cycle Gas Turbine In cogeneration applications, the overall efficiency (counting both electricity and useful heat) depends on how much of the waste heat can be put to use. However, overall efficiencies of 90% or better have been achieved
- 87. Combined Cycle Gas Turbine Reciprocating engines • These have pistons that go back and forth (reciprocate) • Normally they use diesel fuel – so these are the diesel generators normally used for backup or emergency purposes • However, they can also be fuelled with natural gas, with efficiencies as high as 45%
- 88. Combined Cycle Gas Turbine Fuel cells • These are electrochemical devices – they generate electricity through chemical reactions at two metal plates – an anode and a cathode • Thus, they are not limited to the Carnot efficiency • Operating temperatures range from 120ºC to 1000ºC, depending on the type of fuel cell • All fuel cells require a hydrogen-rich gas as input, which can be made by processing natural gas or (in the case of high- temperature fuel cells) coal inside the fuel cells
- 89. Combined Cycle Gas Turbine Fuel cells (continued) • Electricity generation efficiencies using natural gas of 40-50% are possible, and 90% overall efficiency can be obtained if there is a use for waste heat • In the high-T fuel cells, the exhaust is hot enough that it can be used to make steam that can be used in a steam turbine to make more electricity • An electrical efficiency of 70% should be possible in this way – about twice that of a typical coal-fired.
- 90. Combined Cycle Gas Turbine Several such cells would be placed next to each other to form a fuel cell stack. DC Power Eelectron flow Nitrogen Oxidized Fuel (H O) Fuel distribution plate Negative ions Positive ions CA THO DE ELECTRO LYTE Fuel (H ) 2 Air (Mostly N + O ) 2 2 2 or Cross section of a single fuel cell.
- 91. Combined Cycle Gas Turbine United Technologies Company 200-kW phosphoric acid fuel cell that uses natural gas as a fuel. Source: www.utcfuelcells.com 1=fuel processor, 2=cell stack, 3=power conditioner, 4=electronics and controls
- 92. Combined Cycle Gas Turbine Solid Oxide Fuel Cell / Gas Turbine System SOFC = Solid Oxide Fuel cell AC,FC = Air & Fuel compressor CB = Catalytic burner GT = Gas turbine HRSG = Heat recovery steam generator HE = Heat exchanger 847 C o GT-2 1079 C o GT-1 CB 1290 C o SOFC 985 C o Turbine Exhaust To heat load From heat load HE-3 Pump 25 C o 224 C o 509 C o 440 C o 468 C o HRSG M 448 C o 526 C o HE-2 738 C o HE-1 25 C o Air Fuel FC AC 301 C o 236 C o 25 C o
- 93. Combined Cycle Gas Turbine Electrical efficiency vs. load 0 10 20 30 40 50 60 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Relative Load Electrical Efficiency (%) Combined cycle SOFC Micro-turbine 3.5 MW gas turbine 7.52 MW gas turbine Reciprocating engine
- 94. Combined Cycle Gas Turbine Figure 3.11b Relative electrical efficiency vs. load 0.0 0.2 0.4 0.6 0.8 1.0 1.2 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Relative Load Relative Efficiency Combined cycle SOFC 7.52 & 3.5 MW gas turbines and micro-turbine Reciprocating engine
- 95. Combined Cycle Gas Turbine Summarizing the preceding slides and other information, • Natural gas combined-cycle has the highest full-load efficiency (55-60%) and holds its efficiency well at part load • Reciprocating engines have intermediate full-load efficiencies (40-45%) and load their efficiencies well at part load • Gas turbines and micro-turbines have low full-load efficiencies (typically 25-35%, but ranging from 16% to 43%) and experience a substantial drop at part load • Fuel cells using natural gas have intermediate full-load efficiency (40-45%) but this efficiency increases at part load
- 96. Combined Cycle Gas Turbine Capital Costs Today • Pulverized coal power plant with state-of- the-art pollution controls: $1200-1400/kW • Natural gas combined cycle: $400-600/kW in mature markets, $600-900/kW in most developing countries • Reciprocating engines: $600-1200/kW • Fuel cells: $3000-5000/kW
- 98. Combined Cycle Gas Turbine Cogeneration is the simultaneous production of electricity and useful heat – basically, take the waste heat from electricity generation and put it to some useful purpose. Two possible uses are to feed the heat into a district heating system, and to supply it to an industrial process
- 99. Combined Cycle Gas Turbine Figure 3.12 Proportion of electricity produced decentrally (overwhelmingly as cogeneration) 0 10 20 30 40 50 60 Denmark Netherlands Finland Russia Germany Canada China Chile Portugal USA Percent Decentralized Electricity Generation
- 100. Combined Cycle Gas Turbine Technical issues • Impact of withdrawing useful heat on the production of electricity • Ratio of electricity to heat production • Temperature at which heat is supplied • Electrical, thermal and overall efficiencies • Marginal efficiency of electricity generation
- 101. Combined Cycle Gas Turbine Four efficiencies for cogeneration: • The electrical efficiency – the amount of electricity produced divided by the fuel use (later I’ll need to call this the direct electrical efficiency) • The thermal efficiency – the amount of useful heat provided divided __by the fuel use • The overall efficiency – the sum of the of two • The effective or marginal efficiency of electricity generation – explained later
- 102. Combined Cycle Gas Turbine Impact of withdrawing heat • In simple-cycle cogeneration, capturing some of the heat in the hot gas exhaust does not reduce the production of electricity, but the electrical production is already low • In cogeneration with steam turbines, the withdrawal of steam from the turbine at a higher temperature than would otherwise be the case reduces the electricity production • The higher the temperature at which we want to take heat, the more that electricity production is reduced
- 103. Combined Cycle Gas Turbine Example of the tradeoff between production of useful heat and loss of electricity production using steam turbine cogeneration 0.05 0.10 0.15 0.20 0.25 0.30 0.35 80 100 120 140 160 180 200 220 240 Steam Temperature (o C) Loss of electricity as a fraction of heat withdrawn Source: Bolland and Undrum (1999, Greenhouse Gas Control Technologies, 125-130, Elsevier Science, New York)
- 104. Combined Cycle Gas Turbine Thus, to maximize the electricity production, we want to be able to make use of heat at the lowest possible temperature. If the heat is to be provided to buildings, that means having well insulated buildings that can be kept warm with radiators that are not very hot
- 105. Combined Cycle Gas Turbine The alternative to cogeneration is the separate production of heat and electricity. The effective efficiency in generating electricity is the amount of electrical energy produced divided by the extra fuel used to produce electricity along with heat compared to the amount of fuel that would be used in producing heat alone. The extra amount of fuel required in turn depends on the efficiency with which we would have otherwise have produced heat with a boiler or furnace.
- 106. Combined Cycle Gas Turbine For example, suppose that we have a cogeneration system with an electrical efficiency of 25% and an overall efficiency of 80%. Then, the thermal efficiency is 80%-25%=55% - we get 55 units of useful heat from the 100 units of fuel. If the alternative for heating is a furnace at 80% efficiency, we would have required 68.75 units of fuel to produce the 55 units of heat. Thus, the extra fuel use in cogeneration is 100-68.75=31.25 units, and the effective electricity generation efficiency is 25/31.25=80%. I call this the marginal efficiency, because it is based on looking at things on the margin (this is a concept from economics).
- 107. Combined Cycle Gas Turbine With a little algebra, it can be shown that the marginal efficiency is given by nmarginal = nel/(1-nth/nb) where nel and nth are the electrical and thermal efficiencies of the cogeneration system, and nb is the efficiency of the boiler or furnace that would otherwise be used for heating
- 108. Combined Cycle Gas Turbine Marginal efficiency of electricity generation in cogeneration (ηel = efficiency of the alternative, central power plant for electricity generation) 0.0 0.5 1.0 1.5 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Thermal Efficiency Marginal Efficiency of Electricity Generation Boiler efficiency = 0.8 Boiler efficiency = 0.9 ηel=0.25 ηel=0.40 ηel=0.55
- 109. Combined Cycle Gas Turbine Key points • For a given thermal efficiency, the effective electrical efficiency is higher the higher the direct electrical efficiency • However, very high effective electrical efficiencies can be achieved even with low direct electrical efficiencies if the thermal efficiency is high – that is, if we can make use of most of the waste heat • To get a high thermal efficiency requires being able to make use of low- temperature heat (at 50-60ºC), as well as making use of higher temperature heat
- 110. Combined Cycle Gas Turbine Electricity:heat ratio • Because the marginal electricity generation efficiency in cogeneration is generally much higher than the efficiency of a dedicated central powerplant, there is a substantial reduction in the amount of fuel used to generate electricity when cogeneration is used • Thus, we would like to displace as much inefficient central electricity generation as possible when cogeneration is used to supply a given heating requirement • This in turn requires that the electricity-to-heat production ratio in cogeneration be as large as possible • (Remember – none of the gains that we’ve talked about occur if we can’t use the waste heat produced by cogeneration)
- 111. Combined Cycle Gas Turbine Electricity : heat output ratio in cogeneration 0.0 0.5 1.0 1.5 2.0 Electricity:Useful-Heat Ratio Back- pressure steam turbine Micro- turbine Gas turbine/ heat recovery steam generator Fuel cell/ heat recovery steam generator Gas turbine/ back-pressure steam turbine Fuel cell/ gas turbine/ back-pressure steam turbine Reciprocating engine/ heat recovery steam generator
- 112. Combined Cycle Gas TurbineFigure 3.17 Dependence of overall savings through cogeneration on the electricity:heat ratio and on the central powerplant efficiency, assuming a 90% overall efficiency for cogeneration and 90% efficiency for the alternative heating system 0 10 20 30 40 50 60 0.0 0.5 1.0 1.5 2.0 Electricity:Heat Ratio Per cent Overall Savings η=0.6 η=0.5 η=0.4
- 114. Combined Cycle Gas Turbine Issues related to the cost of electricity: • Capital cost, interest rate, lifespan • Fuel cost (impact of depends on efficiency) • Fixed and variable operation & maintenance costs • Baseload vs peaking costs • Transmission line costs and transmission losses • Amount of backup capacity
- 115. Combined Cycle Gas Turbine Capital cost of natural gas combined cycle cogeneration plants 0 400 800 1200 1600 0 100 200 300 400 500 Electrical Capacity (MW) Cost (US$/kW)
- 116. Combined Cycle Gas Turbine Amortization of capital cost: CRF x Ccap / (8760 x CF) units: $/kWh where CRF = i /(1-(1+i)-N) is the cost recovery factor _i = interest rate _N = financing time period Ccap = capital cost ($/kW) 8760 is the number of hours in a year CF= capacity factor (annual average output as a fraction of capacity)
- 117. Combined Cycle Gas Turbine Fuel contribution to the final cost: Cfuel ($/GJ) x 0.0036 (GJ/kWh) / efficiency The cost of electricity from less efficient power plants will be more sensitive to the cost of fuel than the cost of electricity from efficient power plants, but more efficient power plants will tend to have greater capital cost
- 118. Combined Cycle Gas Turbine Typical overnight capital costs and best efficiencies • Pulverized coal: $1200-1400/kW,η= 0.45-0.48 • IGCC: $1400-2600/kW today, η= 0.41-0.55 $1150-1400/kW hoped for, future • NGCC: $400-600/kW, η = 0.55-0.60 • Reciprocating engine: $600-1200/kW,η=0.40-0.46 • Micro-turbine: $1800-2600/kW, η= 0.23-0.27 • Fuel cells: $3000-5000/kW, η= 0.35-0.45 $1000-1500/kW hoped for, future • NGCC/FC hybrid: $2000-3000/kW, η= 0.70-0.80
- 119. Combined Cycle Gas Turbine Cost of electricity from coal and natural gas 0 2 4 6 8 10 12 14 16 0.2 0.4 0.6 0.8 1 Capacity Factor Electricity Cost (cents/kWh) Coal, $1500/kW, η=0.5, $4/GJ Coal, $1000/kW, η=0.4, $4/GJ NG, $2000/kW, η=0.7, $10/GJ NG, $500/kW, η=0.4, $10/GJ
- 120. Combined Cycle Gas Turbine Cost of heat from boilers, electricity with or without cogeneration, and heat from cogeneration 0 1 2 3 4 5 6 7 8 9 2 4 6 8 10 Fuel Cost ($/GJ) Heat or Electrictiy Cost (cents/kWh) Heat cost Electricity cost, dedicated facility Gross electricity cost, cogen facility Net electricity cost, cogen facility
- 121. Combined Cycle Gas Turbine Cost of electricity from central coal (at $2/GJ) and from natural gas (at $10/GJ) 0 2 4 6 8 10 12 14 Central baseload coal Central peaking coal Central peaking NG Onsite peaking NG Peaking NG cogen Cost of Electricity (cents/kWh)
- 122. Combined Cycle Gas Turbine Water requirements • Most thermal power plants use water to cool the condenser of a steam turbine and for other, minor, purposes • There are two approaches: a once-through cooling system a recirculating system in a cooling tower • Water use by power generation represents the largest or second largest use of water in most countries (with irrigation sometimes being a larger use)
- 123. Combined Cycle Gas Turbine • In once-through systems, the water is returned to the source (but at a warmer temperature). Large volumes of water are needed – not available in arid regions • In a recirculating systems, water that has removed heat from the condenser is sprayed through a cooling tower, where it is cooled by evaporation, then returns to the condenser • This consumes water, but the amount that is withdrawn from the water source (lakes, rivers or groundwater) is smaller than in once- through systems
- 124. Combined Cycle Gas Turbine Typical water requirements • Steam turbines (as in coal power plants) Once through: 80-190 liters withdrawn per kWh of __generated electricity, ~ 1 liter / kWh consumed Recirculating: 1-3 liters/kWh withdrawn 1-2 liters/kWh consumed • Natural gas combined cycle Once through: 30 liters/kWh withdrawn ~ 0.4 liters/kWh consumed Recirculating: 0.9 liters/kWh withdrawn 0.7 liters/kWh consumed
- 125. Combined Cycle Gas Turbine Bottom line: • More efficient power plants, such as natural gas combined cycle power plants, use less water per kWh of generated electricity than less efficient power plants • The water requirements can be a constraining factor in arid regions • It is possible to use air rather than water to cool the condenser, but then the efficiency drops
- 126. Combined Cycle Gas Turbine 126 Section 3.1 – Steam Turbine Fundamentals Overview • Hero Reaction Turbine – 120 B.C. • First Practical Turbine – 1884, C. Parsons • First Power Plant – 7.5 kw – 1890 • Reaction, Impulse and Velocity-Compounded • Reheat Steam – 1930’s • Last 100 years Turbine is the key element in generating electricity • Turbines run Generators, Pumps, Fans, etc. • Today up to 1,500 MW
- 128. Combined Cycle Gas Turbine 128 Fundamentals Coal, Natural Gas, Nuclear, Biofuel, Waste Fuel Energy Transfer Steam Turbine Fundamentals
- 129. Combined Cycle Gas Turbine 129 Section 3.1 – Steam Turbine Fundamentals Reaction Turbines Newton’s third law of motion – For every action there is an equal and opposite reaction. Narrowing Steam Path Narrowing Steam Path Steam Turbine Fundamentals
- 130. Combined Cycle Gas Turbine 130 Section 3.1 – Steam Turbine Fundamentals Impulse Turbines Steam / Gas Flow Fixed Vanes Moving Blades Steam Turbine Fundamentals
- 131. Combined Cycle Gas Turbine 131 Section 3.1 – Steam Turbine Fundamentals Reaction – Impulse Comparison
- 132. Combined Cycle Gas Turbine 132 Section 3.1 – Steam Turbine Fundamentals Velocity-Compounded Turbine Velocity compounding is a form of staging which by dividing the work load over several stages results in improved efficiency and a smaller diameter for the blade wheels due to a reduction in Ideal blade speed per stage. Inlet Pressure Inlet Velocity P = 1 V Steam Turbine Fundamentals
- 133. Combined Cycle Gas Turbine 133 Section 3.1 – Steam Turbine Fundamentals Turbine Components - Blades Reaction Impulse Steam Turbine Fundamentals
- 134. Combined Cycle Gas Turbine 134 Section 3.1 – Steam Turbine Fundamentals Turbine Diaphragms Diaphragms contain the fixed blades Steam Turbine Fundamentals
- 135. Combined Cycle Gas Turbine 135 Section 3.1 – Steam Turbine Fundamentals Steam Turbine Casing Steam Turbine Fundamentals
- 136. Combined Cycle Gas Turbine 136 Section 3.1 – Steam Turbine Fundamentals Turbine Rotor Steam Turbine Fundamentals
- 137. Combined Cycle Gas Turbine 137 Section 3.1 – Steam Turbine Fundamentals Turbine Shaft and Casing Seals
- 138. Combined Cycle Gas Turbine 138 Section 3.1 – Steam Turbine Fundamentals Turbine Types Straight HP Tandem HP Tandem LP Steam Turbine Fundamentals
- 139. Combined Cycle Gas Turbine 139 Section 3.1 – Steam Turbine Fundamentals Turbine – Multiple Sets Steam Turbine Fundamentals
- 140. Combined Cycle Gas Turbine 140 Section 3.2 – Steam Turbine Design Overview Classification by; • Type – Reaction or Impulse • Steam Temperature and Pressure • Configuration – Compound, Tandem Compound, Cross Compound • Reheat • Output – MW • Structural Elements Steam Turbine Fundamentals
- 141. Combined Cycle Gas Turbine 141 Section 3.2 – Steam Turbine Design Turbine Design - Basics Steam Turbine Fundamentals
- 142. Combined Cycle Gas Turbine 142 Section 3.2 – Steam Turbine Design Materials • Blades • Stainless Steel – 403 & 422 (+Cr) • 17-4 PH steel (+ Ti) • Super Alloys • Rotor • High “Chrome – Moley” Steel – Cr-Mo-V • Low “Ni Chrome Steel – Ni-Cr-Mo-V Steam Turbine Fundamentals
- 143. Combined Cycle Gas Turbine A steam turbine is a device that extracts thermal energy from pressurized steam and uses it to do mechanical work on a rotating output shaft. Its modern manifestation was invented by Sir Charles Parsons in 1884. Steam Turbine may also be define as a device which converts heat energy of to the steam to the mechanical energy which finally converted into electrical energy. 143
- 144. Combined Cycle Gas Turbine Because the turbine generates rotary motion, it is particularly suited to be used to drive an electrical generator – about 90% of all electricity generation in the United States, is by use of steam turbines. The steam turbine is a form of heat engine that derives much of its improvement in thermodynamic efficiency through the use of multiple stages in the expansion of the steam, which results in a closer approach to the ideal reversible process. 144
- 145. Combined Cycle Gas Turbine The modern steam turbine was invented in 1884 by Sir Charles Parsons, whose first model was connected to a dynamo that generated 7.5 kW (10 hp) of electricity. The Parsons turbine also turned out to be easy to scale up. Parsons had the satisfaction of seeing his invention adopted for all major world power stations, and the size of generators had increased from his first 7.5 kW set up to units of 500MW capacity. 145
- 146. Combined Cycle Gas Turbine Steam turbines are made in a variety of sizes ranging from small <0.75 kW units used as mechanical drives for pumps, compressors and other shaft driven equipment, to 1,500 MW turbines used to generate electricity. There are several classifications for modern steam turbines. 146
- 147. Combined Cycle Gas Turbine WORK IN A TURBINE VISUALIZED 147
- 148. Combined Cycle Gas Turbine Further the steam turbine is based upon Rankine cycle • An ideal Rankine cycle operates between pressures of 30 kPa and 6 MPa. The temperature of the steam at the inlet of the turbine is 550°C. Find the net work for the cycle and the thermal efficiency. – Wnet=Wturbine-Wpump OR Qin-Qout – Thermal efficiency hth=Wnet/Qin – Net work done is converted into power output of turbine. 148
- 149. Combined Cycle Gas Turbine Ideal Rankine Cycle This cycle follows the idea of the Carnot cycle but can be practically implemented. 1-2 isentropic pump 2-3 constant pressure heat addition 3-4 isentropic turbine 4-1 constant pressure heat rejection 149
- 150. Combined Cycle Gas Turbine CLASSIFICATION OF STEAM TURBINE Classification of steam turbines may be done as following: 1. According to action of steam (a) Impulse turbine (b) Reaction turbine (c) Combination of both 2. According to direction of flow: (a) Axial flow turbine (b) Radial flow turbine 3. According to number of stages (a) Single stage turbine (b) Multi stage turbine 150
- 151. Combined Cycle Gas Turbine (4). According to number of cylinders (a) Single cylinder turbine (b) Double cylinder turbine (c) Three cylinder turbine (5) According to steam pressure at inlet of Turbine: (a) Low pressure turbine (b) Medium pressure turbine. (c) High pressure turbine (d) Super critical pressure turbine. 151
- 152. Combined Cycle Gas Turbine Description of common types of Turbines. The common types of steam turbine are 1. Impulse Turbine. 2. Reaction Turbine. The main difference between these two turbines lies in the way of expanding the steam while it moves through them. In the impulse turbine, the steam expands in the nozzles and it's pressure does not alter as it moves over the blades. In the reaction turbine the steam expanded continuously as it passes over the blades and thus there is gradual fall in the pressure during expansion below the atmospheric pressure. 152
- 153. Combined Cycle Gas Turbine PRESSURE-VELOCITY DIAGRAM FOR A TURBINE NOZZLE ENTRANCE HIGH THERMAL ENERGY HIGH PRESSURE LOW VELOCITY STEAM INLET EXIT LOW THERMAL ENERGY LOW PRESSURE HIGH VELOCITY STEAM EXHAUST PRESSURE VELOCITY 153
- 154. Combined Cycle Gas Turbine Simple impulse Turbine. It the impulse turbine, the steam expanded within the nozzle and there is no change in the steam pressure as it passes over the blades 154 NOZZLE STEAM CHEST ROTOR
- 156. Combined Cycle Gas Turbine PRESSURE-VELOCITY DIAGRAM FOR A MOVING IMPULSE BLADE VELOCITY PRESSURE TURBINE SHAFT DIRECTION OF SPIN ENTRANCE HIGH VELOCITY STEAM INLET REPRESENTS MOVING IMPULSE BLADES EXIT LOW VELOCITY STEAM EXHAUST 156
- 157. Combined Cycle Gas Turbine Reaction Turbine In this type of turbine, there is a gradual pressure drop and takes place continuously over the fixed and moving blades. The rotation of the shaft and drum, which carrying the blades is the result of both impulse and reactive force in the steam. The reaction turbine consist of a row of stationary blades and the following row of moving blades. The fixed blades act as a nozzle which are attached inside the cylinder and the moving blades are fixed with the rotor as shown in the figure. 157
- 158. Combined Cycle Gas Turbine When the steam expands over the blades there is gradual increase in volume and decrease in pressure. But the velocity decreases in the moving blades and increases in fixed blades with change of direction. Because of the pressure drops in each stage, the number of stages required in a reaction turbine is much greater than in a impulse turbine of same capacity. It also concluded that as the volume of steam increases at lower pressures therefore the diameter of the turbine must increase after each group of blade rings. 158
- 159. Combined Cycle Gas Turbine REACTION TURBINE PRINCIPLE STEAM CHEST ROTOR 159
- 161. Combined Cycle Gas Turbine PRESSURE-VELOCITY DIAGRAM FOR A MOVING REACTION BLADE TURBINE SHAFT DIRECTION OF SPIN ENTRANCE HIGH PRESSURE HIGH VELOCITY STEAM INLET REPRESENTS MOVING REACTION BLADES EXIT LOW PRESSURE LOW VELOCITY STEAM EXHAUST PRESSURE VELOCITY 161
- 163. Combined Cycle Gas Turbine .Compounding in Steam Turbine. The compounding is the way of reducing the wheel or rotor speed of the turbine to optimum value. It may be defined as the process of arranging the expansion of steam or the utilization of kinetic energy or both in several rings. There are several methods of reducing the speed of rotor to lower value. All these methods utilize a multiple system of rotors in series keyed on a common shaft, and the seam pressure or jet velocity is absorbed in stages as the steam flows over the blades. 163
- 164. Combined Cycle Gas Turbine Different methods of compounding are: 1.Velocity Compounding 2.Pressure Compounding 3.Pressure Velocity Compounding. These are explained in detail as given below: 164
- 165. Combined Cycle Gas Turbine Velocity Compounding: There are a number of moving blades separated by rings of fixed blades. All the moving blades are keyed on a common shaft. When the steam passed through the nozzles where it is expanded to condenser pressure, it's Velocity becomes very high. This high velocity steam then passes through a series of moving and fixed blades When the steam passes over the moving blades it's velocity decreases. The function of the fixed blades is to re-direct the steam flow without altering it's velocity to the following next row moving blades where a work is done on them and steam leaves the turbine with a low velocity as shown in diagram. 165
- 166. Combined Cycle Gas Turbine VELOCITY COMPOUNDED TURBINE 166
- 168. Combined Cycle Gas Turbine These are the rings of moving blades which are keyed on a same shaft in series, are separated by the rings of fixed nozzles. The steam at boiler pressure enters the first set of nozzles and expanded partially. The kinetic energy of the steam thus obtained is absorbed by moving blades. The steam is then expanded partially in second set of nozzles where it's pressure again falls and the velocity increase the kinetic energy so obtained is absorbed by second ring of moving blades. This process repeats again and again and at last, steam leaves the turbine at low velocity and pressure. During entire process, the pressure decrease continuously but the velocity fluctuate as shown in diagram. 168 Pressure Compounding:
- 171. Combined Cycle Gas Turbine This method of compounding is the combination of two previously discussed methods. The total drop in steam pressure is divided into stages and the velocity obtained in each stage is also compounded. The rings of nozzles are fixed at the beginning of each stage and pressure remains constant during each stage as shown in figure. The turbine employing this method of compounding may be said to combine many of the advantages of both pressure and velocity staging By allowing a bigger pressure drop in each stage, less number stages are necessary and hence a shorter turbine will be obtained for a given pressure drop. 171 Pressure velocity compounding
- 172. Combined Cycle Gas Turbine PRESSURE-VELOCITY COMPOUNDED IMPULSE TURBINE CURTIS STAGE NOZZLE, MOVING BLADE, FIXED BLADE, AND MOVING BLADE MOVING BLADE NOZZLE FIXED BLADE MOVING BLADE RATEAU STAGE – NOZZLE & MOVING BLADE MOVING BLADE NOZZLE PRESSURE VELOCITY 172
- 174. Combined Cycle Gas Turbine These types include condensing, non-condensing, reheat, extraction and induction. Condensing turbines are most commonly found in electrical power plants. These turbines exhaust steam in a partially condensed state, typically of a quality near 90%, at a pressure well below atmospheric to a condenser. Non-condensing or back pressure turbines are most widely used for process steam applications. The exhaust pressure is controlled by a regulating valve to suit the needs of the process steam pressure. These are commonly found at refineries, heating units, pulp and paper plants, and desalination facilities where large amounts of low pressure process steam are available. 174 Steam supply and exhaust conditions
- 175. Combined Cycle Gas Turbine Reheat turbines are also used almost exclusively in electrical power plants. In a reheat turbine, steam flow exits from a high pressure section of the turbine and is returned to the boiler where additional superheat is added. The steam then goes back into an intermediate pressure section of the turbine and continues its expansion. Extracting type turbines are common in all applications. In an extracting type turbine, steam is released from various stages of the turbine, and used for industrial process needs or sent to boiler feedwater heaters to improve overall cycle efficiency. Extraction flows may be controlled with a valve, or left uncontrolled. Induction turbines introduce low pressure steam at an intermediate stage to produce additional power. 175
- 176. Combined Cycle Gas Turbine These arrangements include single casing, tandem compound and cross compound turbines. Single casing units are the most basic style where a single casing and shaft are coupled to a generator. Tandem compound are used where two or more casings are directly coupled together to drive a single generator. A cross compound turbine arrangement features two or more shafts not in line driving two or more generators that often operate at different speeds. A cross compound turbine is typically used for many large applications. 176 Casing or shaft arrangements
- 177. Combined Cycle Gas Turbine A two-flow turbine rotor. The steam enters in the middle of the shaft, and exits at each end, balancing the axial force. The moving steam imparts both a tangential and axial thrust on the turbine shaft, but the axial thrust in a simple turbine is unopposed. To maintain the correct rotor position and balancing, this force must be counteracted by an opposing force. Either thrust bearings can be used for the shaft bearings, or the rotor can be designed so that the steam enters in the middle of the shaft and exits at both ends. The blades in each half face opposite ways, so that the axial forces negate each other but the tangential forces act together. This design of rotor is called two-flow or double-exhaust. 177 Two-flow rotors
- 179. Combined Cycle Gas Turbine An ideal steam turbine is considered to be an isentropic process, or constant entropy process, in which the entropy of the steam entering the turbine is equal to the entropy of the steam leaving the turbine No steam turbine is truly isentropic, however, with typical isentropic efficiencies ranging from 20–90% based on the application of the turbine. The interior of a turbine comprises several sets of blades, or buckets as they are more commonly referred to. One set of stationary blades is connected to the casing and one set of rotating blades is connected to the shaft. The sets intermesh with certain minimum clearances, with the size and configuration of sets varying to efficiently exploit the expansion of steam at each stage. 179 Principle of operation and design
- 180. Combined Cycle Gas Turbine Schematic diagram outlining the difference between an impulse and a reaction turbine To maximize turbine efficiency the steam is expanded, doing work, in a number of stages. These stages are characterized by how the energy is extracted from them and are known as either impulse or reaction turbines. Most steam turbines use a mixture of the reaction and impulse designs: each stage behaves as either one or the other, but the overall turbine uses both. Typically, higher pressure sections are impulse type and lower pressure stages are reaction type. 180 Turbine efficiency
- 181. Combined Cycle Gas Turbine An impulse turbine has fixed nozzles that orient the steam flow into high speed jets. These jets contain significant kinetic energy, which is converted into shaft rotation by the bucket-like shaped rotor blades, as the steam jet changes direction. A pressure drop occurs across only the stationary blades, with a net increase in steam velocity across the stage. As the steam flows through the nozzle its pressure falls from inlet pressure to the exit pressure (atmospheric pressure, or more usually, the condenser vacuum). Due to this high ratio of expansion of steam, the steam leaves the nozzle with a very high velocity. The steam leaving the moving blades has a large portion of the maximum velocity of the steam when leaving the nozzle. The loss of energy due to this higher exit velocity is commonly called the carry over velocity or leaving loss. 181 Impulse turbines
- 182. Combined Cycle Gas Turbine 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 rotor. 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. 182 Reaction turbines
- 183. Combined Cycle Gas Turbine When warming up a steam turbine for use, the main steam stop valves (after the boiler) have a bypass line to allow superheated steam to slowly bypass the valve and proceed to heat up the lines in the system along with the steam turbine. Also, a turning gear is engaged when there is no steam to the turbine to slowly rotate the turbine to ensure even heating to prevent uneven expansion. After first rotating the turbine by the turning gear, allowing time for the rotor to assume a straight plane (no bowing), then the turning gear is disengaged and steam is admitted to the turbine, first to the astern blades then to the ahead blades slowly rotating the turbine at 10–15 RPM (0.17– 0.25 Hz) to slowly warm the turbine. 183 Operation and maintenance
- 184. Combined Cycle Gas Turbine Any imbalance of the rotor can lead to vibration, which in extreme cases can lead to a blade breaking away from the rotor at high velocity and being ejected directly through the casing. To minimize risk it is essential that the turbine be very well balanced and turned with dry steam - that is, superheated steam with a minimal liquid water content. If water gets into the steam and is blasted onto the blades (moisture carry over), rapid impingement and erosion of the blades can occur leading to imbalance and catastrophic failure. Also, water entering the blades will result in the destruction of the thrust bearing for the turbine shaft. 184
- 185. Combined Cycle Gas Turbine To prevent this, along with controls and baffles in the boilers to ensure high quality steam, condensate drains are installed in the steam piping leading to the turbine. Modern designs are sufficiently refined that problems with turbines are rare and maintenance requirements are relatively small. The steam turbine operates on basic principles of thermodynamics using the part of the Rankine cycle. Superheated vapor (or dry saturated vapor, depending on application) enters the turbine, after it having exited the boiler, at high temperature and high pressure. The high heat/pressure steam is converted into kinetic energy using a nozzle. Once the steam has exited the nozzle it is moving at high velocity and is sent to the blades of the turbine. 185
- 186. Combined Cycle Gas Turbine A force is created on the blades due to the pressure of the vapor on the blades causing them to move. A generator or other such device can be placed on the shaft, and the energy that was in the vapor can now be stored and used. The gas exits the turbine as a saturated vapor (or liquid- vapor mix depending on application) at a lower temperature and pressure than it entered with and is sent to the condenser to be cooled 186
- 187. Combined Cycle Gas Turbine To measure how well a turbine is performing we can look at its isentropic efficiency. This compares the actual performance of the turbine with the performance that would be achieved by an ideal, isentropic, turbine. When calculating this efficiency, heat lost to the surroundings is assumed to be zero. The starting pressure and temperature is the same for both the actual and the ideal turbines, but at turbine exit the energy content ('specific enthalpy') for the actual turbine is greater than that for the ideal turbine because of irreversibility in the actual turbine. 187 Isentropic turbine efficiency
- 188. Combined Cycle Gas Turbine The isentropic efficiency is found by dividing the actual work by the ideal work. where •h1 is the specific enthalpy at state one •h2 is the specific enthalpy at state two for the actual turbine •h2s is the specific enthalpy at state two for the isentropic turbine 188