Training Session4 - Heat Recovery Steam Generators
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Transcript of Training Session4 - Heat Recovery Steam Generators
8/10/2019 Training Session4 - Heat Recovery Steam Generators
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Industrial Application of Gas
Turbines (IAGT)
Introduction to HRSGs
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HRSG
Heat Recovery Steam Generator
• Also referred to as waste heatboiler
• Cools hot gases – mostcommonly the exhaust of a gasturbine
• Generates steam and regainsenergy
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HRSG• Critical link between the gas turbine and steam turbine in combined
cycle and cogeneration plants
Combined Cycle Plant
Cogeneration Plant
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FUNDAMENTAL PARTS OF HRSG
Four Basic HRSG Components
• Evaporators (gas to wetsteam heat exchanger)
•Economizers (gas to waterheat exchanger)
• Superheaters/Reheaters (gasto dry steam heatexchanger)
• Preheaters (gas towater/glycol/air etc. heatexchanger)
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EVAPORATOR
• Vaporize water and producesteam
• Water/steam circulates fromlower drum to steam drum
• Steam exits from the steamdrum after passing throughsteam separating equipment
• Water level must be
carefully maintained
Lower
Drum
Steam
Drum
Gas
Flow
Steam
Feedwater
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ECONOMIZERS
• Preheats water prior toentry into the steam drum
• Desirable to prevent steamfrom forming in theeconomizer
Gas
Flow
To
Evaporator Feedwater
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SUPERHEATERS/REHEATERS
• Saturated steam fromevaporator is sent tosuperheater to produce drysteam
• Dry steam is required forsteam turbines
Gas
Flow
SH
Steam
Dry Steam
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HRSG TYPES
Three (3) Main Types
•NATURAL CIRCULATION HRSGs•FORCED CIRCULATION HRSGs
•ONCE THROUGH HRSGs
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NATURAL CIRCULATION HRSG
• Typically horizontal gas flowand vertical tubes
• Tube bundles typically growthermally down
• For gas turbines less than 50MW, evaporator is shipped tosite in single pieces
• For larger gas turbines theevaporator is shipped inmultiple sections
SH Steam Feedwater
Economizer Evaporator
Superheater
Gas
Flow
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INTEGRAL STEAM DRUM
• Evaporator shipped to site insingle section (up to approx.400,000 lbs)
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SEPARATE STEAM DRUM
• Evaporator shipped to site in multiple sections
• Increased field erection costs
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FORCED CIRCULATION HRSG
• Typically vertical gas flow andhorizontal tubes
• Steam/water mixturecirculation through evaporatortubes and to/from drum with apump
• Historically common in Europe
due to small footprint
Economizer
Evaporator
Superheater
Feedwater
Circulation
Pump
Gas Flow
SH
Steam
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ONCE THROUGH HRSG
• Typically vertical gas flow andhorizontal tubes
• OTSGs eliminate the need forsteam drums
• Phase change from water tosteam is free to movethroughout the bundle
Gas Flow
Economizer
Evaporator
Superheater
Feedwater
SH Steam
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HRSG DESIGN PHILOSOPHY
• Exchange heat from the exhaust gas to the fluid at the highesttemperature difference available
• Accomplished by making the exhaust gas and the fluid (steam/water)temperature gradients as nearly parallel to each other as possible
HRSG Temperature Profile - Unfired
Steam Production = 66,850 lb/hr
430
517
700
240
495
485
0
100
200
300
400
500
600
700
800
900
0 10 20 30 40 Number of Rows
T e m p e r a t u r e ( F )
GasSteam/Water
Pinch = 20 F
Approach = 10 F
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SINGLE VS MULTIPLE PRESSRE HRS• Adding additional pressure levels in the HRSG can increase the
amount of heat that can be recovered from the exhaust gas• As the saturation temperatures are lower at successive pressures, the
stack temperature can be lowered
Dual Pressure HRSG Temperature Profile
HP Steam Production = 75000 lb/h
LP Steam Production = 100000 lb/h
635
660
846
331410
375
495
475
375
230
0
100
200
300
400
500
600
700
800
900
1000
0 10 20 30 40 50 60 70
Number of Rows
T e m p e r a t u r e ( F
Gas
HP Steam/Water LP Steam/Water
HP Pinch = 165
HP Approach = 20 F
Common Economizer
390
LP Approach = 35 F
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HRSGs
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HRSGs vs Conventional Boilers
Differences between HRSGs and Conventional boilers
• HRSGs use exhaust from a gas turbine as a heat source and do notneed a dedicated firing system (burner, fan, motor etc.)
• HRSGs typically do not use fans (draft is from gas turbine exhaust)• HRSGs generate steam at multiple pressure levels to improve heat
recovery efficiency
• Heat transfer is by convection rather than radiation
• HRSGs do not use membrane water walls• HRSGs use finned tubes to maximize heat transfer
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HRSGs vs Conventional Boilers
• Differences between HRSGs and Conventional boilers
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Finned Tubing
• Finned tubing is used to increase heat transfer
• Two types – solid fins and serrated fins
• Heat transfer can be adjusted by changing fin height, fin thickness,fin density, fin materials
• Fins are spiral wound onto tubes using various processes:
– Brazing
– Welding
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Finned Tubing
Serrated Finned Tube Solid Finned Tube
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Fabrication of HRSGs
• Large HRSGs typically made from pressure part modules referred toas “harps”
• Significant field assembly
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Erection of HRSGs
• Shipment of harps, cased or uncased sent to site• Modules stacked up to three (3) wide
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Erection of HRSGs
• Steam drum sent to site separately• Exhaust stack sent to site in multiple sections
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Fabrication of HRSGs
• Smaller HRSGs such as the OTSG maximize shop fabrication,minimizing field assembly
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DUCT BURNERS
• Add heat to the gas turbine exhaust stream• Exhaust gas typically has enough oxygen to sustain stable
combustion
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DUCT BURNER• Steam demand increases
without any change in the gasturbine exhaust
• Desired steam flow of finalsteam temperature cannot beachieved with the available heat
from the gas turbine• Gas turbine is completely down
but steam is still needed (Freshair firing)
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DUCT BURNER• Burners can be configured to burn a variety of different fuels
from natural gas to oil
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DUCT BURNER• Fresh air firing is used to produce steam when the gas turbine is
down
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DISTRIBUTION GRID
• Used to correct flowmaldistribution
• Variable porosity platesand turning vanes
commonly used• Typical gas side pressure
drop for a variableporosity plate rangesfrom 0.5 inches H2O to
3 inches H2O
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FLOW MODELIN• Gas flow distribution leaving the gas turbine is non-uniform
• Proper performance of the HRSG, duct burner and emissionequipment requires uniform flow and temperature profile
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NOx REMOVAL PROCES• Ammonia (NH3) is injected into the gas stream upstream of SCR
catalyst• Catalyst layer decomposes NOx (nitrous oxides, principally NO and
NO2) into harmless N2 and H2O
Superheater
Steam Out
SCR Catalyst
Economizer
Feedwater
Evaporator
NH3 Injection Grid
Gas Flow
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SCR LOCATIO
• SCR must be placed in theappropriate gas temperaturezone for maximum efficiency
• Typical medium temperature
catalyst maximumcontinuous temperature is800 F with excursions toapprox. 900 F
60
70
80
90
100
400 500 600 700 800
GAS TEMPERATURE, (°F)
DeNOx
EFF
ICIENCY,
(%).
Typical DeNOx Efficiency VS. Gas Temperature
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CORROSION IN HRSG
Water dewpoint corrosion
• Metal temperatures fall belowthe water dewpoint
• Can lead to acceleratedcorrosion
Acid dewpoint corrosion
• Trace quantities of sulphur infuel form sulphur trioxide (SO3)and combine with water to form
acids• Leaves deposits on fin tubes
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QUESTIONS ??????