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7/31/2019 Overiew of Comb Cycle Rev 6.0_Part 2
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Combined Cycle Power Generation
-An Introduction
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Combined
Cycle
PowerGeneration
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One Gas
Turbine andOne SteamTurbine.
[without anyadditionalfuelconsumption]
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Fully firedcombinedcycle
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District
heating andprocess heatfor industry.
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Bypass Stack /
Waste Heat Recovery Systems
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GTHRSG
+
ST
3%
MISC
LOSSES
100% FUEL
INPUT
OUTPUT
30 %
67%
OUTPUT
16 %
3% MISC
LOSSES
34%
CONDENSER
LOSSES
14% TO
STACK
Combined Cycle Heat Flow
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HRSG - Classification
HRSG
VERTICAL HORIZONTAL
DRUMTYPE
ONCETHROUGH
UNFIRED SUPPLEMENTRY FIRED
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Heat Recovery
Steam Generator[HRSG]
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Typical two stage Combined Cycle
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Water Steam Path
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VERTICAL HRSG
Flue gas flow -vertical
Water/ steam flow inhorizontal finned tubes
Small foot print area Added circulation
Good cycling capability
Replacement of tubeseasily possible
Easily access forinspections
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VERTICAL HRSG
Flue gas flow -vertical
Water/ steam flow inhorizontal finned tubes
Small foot print area Added circulation
Good cycling capability
Replacement of tubeseasily possible
Easily access forinspections
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Horizontal HRSG
Flue gas flow-horizontal
Water/ steam flow invertical finned tubes
Large foot print area
Natural circulation
In cycling duty problemsin superheater/ reheatersections.
Replacement of tubes notpossible
Access for inspections isdifficult
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SINGLE PRESSURE NON REHEAT
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DOUBLE PRESSURE NON REHEAT CYCLE
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DUAL PRESSURE CC PLANT
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TRIPLE PRESSURE CC PLANT
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Temperature
Profile in aWaste HeatRecoveryBoiler
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Pressure Stages in WHRB
Criteria :Maximum available Temperature at WHRB inlet
Provision of Supplementary firing in WHRB
Station owners own economic evaluation ofHeat rate,
Efficiency,
Power output,Extra investment,
Added O&M Cost
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Pressure Stages in WHRB
Output (%) Plant Eff. (%)
Single pr., Non reheat - 4.7 - 4.7
Two pr., Non reheat - 1.0 - 1.0
Three pr., Non reheat Base BaseThree pr., Reheat + 0.7 + 0.7
Limiting Factor:
Saturation temperature at particular pressure
Possible degree of Superheat
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Combined Cycle Parameters
PINCH POINT:
Difference between flue gas and water/steam
temperature at evaporator section
It is the minimum differential temperature
between gas and water/steam in the Boiler
Lower pinch point results in linear rise in cycle
efficiency
Lower pinch point results in exponential rise in
boiler heat transfer area
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PINCH POINT:Every 10C decrease in Pinch Point, results inq HP steam flow increases by 4.6 %
q Steam turbine output increases by 3.4 %
q Heat rate improves by 20 kcal/kWHrq Efficiency improves by 0.52 %
Optimum breakeven is 10 C, with regard to
Heat Transfer area of Evaporator
Pinch point at Dadri (HP Ckt) is 11.6C
(LP ckt) is 8.0 C
Combined Cycle Parameters
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Optimum Approach & Pinch Point
Triple Pressure Levels Reduces Irreversibility &
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Triple Pressure Levels Reduces Irreversibility &Increasing Heat Transfer
Triple Pressure CC Plant
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Triple Pressure CC Plant
T i l P HRSG With SCR
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Triple Pressure HRSG With SCR
SCR
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SCR
Suitable temperature range 300 to 400 oC.
Segments having honeycomb patterns containing
catalyst is arranged within HRSG.
Ammonia slip is a concern, requires sophisticatedcontrol system for controlling injection.
Excessive Size and Weight.
Costly as compared to primary methods.
Sensitive to fuels containing more than 1000 ppm of
sulfur.
P i i l f D NO th SCR
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Principle of DeNOx thru SCR
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Post Combustion Pollution Control
SCR: NOx is converted into nitrogenand water vapour by injecting
ammonia in presence of a catalyst.
SCONOx: Single catalyst for removal
of CO, NOx, VOCs, SO2 and requires
no chemical injection.
Energy balance in diff scenario
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Energy balance in diff scenario
Config GTOutput
GTLosses
HRSGLossesStacklossesCondenser losses STLossesSToutput
Single
pr.
37.6 0.5 0.2 11.4 29.9 0.3 20.1
Doublepr.
37.6 0.5 0.3 8.2 32.1 0.3 21.0
Triple
pr.
37.6 0.5 0.3 8.2 32.0 0.3 21.1
Triple,reheat
37.6 0.5 0.3 8.6 31.0 0.3 21.7
Fi d T b
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Finned Tubes
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Single Shaft CC Plant
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Single Shaft CC Plant
Single Shaft Plant Arrangement
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Single Shaft Plant Arrangement
Multi-shaft Arrangement
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Multi-shaft Arrangement
Single-shaft Arrangement
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Single-shaft Arrangement
Si l Sh f C bi d C l Pl
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Single Shaft Combined Cycle Plant
Simplified Plant design and operation
Lower initial investment
Unitized design means problems faced inmulti shaft configuration employing Triple
Pressure Reheat ST are absent.
Si l Sh ft CC U it D t N d
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Single Shaft CC Units Dont Need
Main steam non return valves
Cold Reheat isolation valves
Cold Reheat Balancing valves
Reheater relief valves
Hot Reheat stop/ control valvesLow pressure Non return valves
Steam headers
C t R d ti i Si l Sh ft U it lt f
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Cost Reduction in Single Shaft Unit results from:
Reduction in number of Electric generators,step up transformers, and high voltagefeeders.
Civil works Single building, Reducedbuilding height
Reduction in number of valves and lengthof piping.
Elimination of Bypass stack and diverterdamper.
Limitations of Single Shaft Units
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Limitations of Single Shaft Units
Need for higher starting power
Less Operating Flexibility ;
Option of phased construction and
commissioning not available.
Si l Sh ft U it C fi ti
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Single Shaft Unit Configuration
GT- Generator Clutch ST
GT ST Generator
GT Generator Clutch ST config
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GT-GeneratorClutchST config.
Steam turbine must be moved to remove andservice the generator Rotor.
Startup of gas Turbine is independent of Steamturbine
SSS clutch allows load operation of Gas Turbinein case of outage of the ST
Gas turbine and steam turbines have their ownthrust and journal bearings
GTSTGenerator Configuration
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GT ST Generator Configuration
Axial steam turbine exhaust is not
possible Steam turbine shares the cold end
thrust / journal bearing of Gas Turbine
Auxiliary steam is required for cooling ofsteam turbine during startup
Outage of ST necessarily lads to outage of the
whole power train.
Gas Turbine is accessible formaintenance only after cool-down ofcomplete power train.
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C bi d C l O ti
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Combined Cycle Options
STEAM CYCLE:
Single pressureTwo pressure
Three pressure
Reheat
Non-reheat
DEAERATION:
Deaerating condenser
Deaerator/evaporatorintegral with WHRB
HRSG DESIGN:
Natural circulationevaporator
Forced circulationevaporator
Unfired
Supplementary fired
NOx CONTROL:
Water InjectionSteam Injection
SCR ( NOx and/or CO)
Dry Low NOx
Combustion
C bi d C l O i
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CONDENSER:
Water cooled (once through system)
Water cooled (evaporative cooling tower)
Air Cooled condenser)
FUEL:
Natural gas
Distillate oil
Ash bearing oil
Low Btu coal and oil-derived gas
Multiple fuel system
Combined Cycle Options
Cost Distribution for a CC Power Plant
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(2GTs + 2HRSGs + 1ST)
bine, Aux equipment : 26piping + auxiliary equipment: 1urbine + generator+piping+condenser:
and supervisory equipment+transformeineering: 6 %+ supervision: 18 %
Cost Distribution for a CC Power Plant
Bypass Stack /
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Bypass Stack /Waste Heat Recovery Systems
Power Enhancement Methods
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It is Site specific and dependant on:Site ambient temperature
Level of desired output enhancement
Anticipated Operational hours
Water availability
Structure of Power Purchase Agreement
Allowable plant emission
Owners own economic evaluation factors
for plant output, heatrate, O&M costs.
Power Enhancement Methods
Coal fired Vs Combined Cycle
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Coal fired Vs. Combined Cycle
0.6
0.4
0.55
0.73
0.46
0.35
0.65
1.37
0
0.2
0.4
0.6
0.8
1
1.2
1.4
C
ostofInstallation
O&MCost
O&MStaff
FuelRequirement
Landrequirement
WaterRequirement
GestationPeriod
PlantEfficency
Thermal
Comb. Cycle
Comparison of Availability Reliability
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Comparison of Availability, ReliabilityAvailability = [Total Operating Hours Planned outage-Forced outage]/ Total Operating Hours
Reliability = [Total Operating Hours Forced outage]/ Total Operating Hours
Typical Figures
Plant types Availability Reliability
Gas turbine (gas) 88-95 97-99
Steam Turbine (coal) 82-89 94-97
Comb cycle (gas) 86-93 95-98
Nuclear 80-89 92-98
Diesel generator 90-95 96-98
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