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WELCOMETO
PRESENTATION ON
SUPERCRITICAL BOILERBy
OPERATION TEAM
APML,TIRODA
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Introduction to Supercritical Technology
What is Supercritical Pressure ?
Critical point in water vapour cycle is a thermodynamic state where there is no clear distinction between liquid and gaseous state of water. Water reaches to this state at a critical pressure above 22.1 MPa and 374 oC.
Rankine Cycle Subcritical Unit
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1 - 2 > CEP work2 - 3 > LP Heating3 - 4 > BFP work4 - 5 > HP Heating5 – 6 > Eco, WW6 – 7 >
Superheating7 – 8 > HPT Work8 – 9 > Reheating9 – 10 > IPT Work10–11 > LPT Work11 – 1 >
Condensing
Rankine Cycle Supercritical Unit
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1 - 2 > CEP work
2 – 2s > Regeneration
2s - 3 > Boiler
Superheating
3 – 4 > HPT expansion
4 – 5 > Reheating
5 – 6 > IPT & LPT Expansion
6 – 1 > Condenser Heat
rejection
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Absolute Absolute PressurePressure
(Bar)(Bar)
Saturation Saturation TemperatureTemperature
((ooC)C)
Latent HeatLatent Heat(K J/Kg.)(K J/Kg.)
5050150150200200221221
264264342342366366374374
1640164010041004592592
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VARIATION OF LATENT HEAT WITH PRESSURE
Nucleate boiling is a type of boiling that takes place when the surface temp is hotter than the saturated fluid temp by a certain amount but where heat flux is below the critical heat flux. Nucleate boiling occurs when the surface temperature is higher than the saturation temperature by between 40C to 300C.
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Departure from Nucleate Boiling
PRESSURE(ksc)
DE
NS
ITY
WATER
STEAM
175 224
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Supercritical Boiler Water Wall
Rifle Tube And Smooth Tube
Natural Circulation Vs. Once Through System
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From CRH Line
From FRS Line
Boiler Recirculation Pump
Economizer Phase 1
Economizer Phase 2
LTRH
LTSH4430C
FRH
Platen Heater
Mixer Header
FSH
To HP Turbine To IP
Turbine
Separator
Bottom RingHeader
2830C
3260C
4230C
4730C
4620C5340C
5260C
5710C
5690C
3240C
2800C
NRV
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Feed water control
In Drum type Boiler Feed water flow control by Three element controller1.Drum level2.Ms flow3.Feed water flow.
Drum less Boiler Feed water control by 1.Load demand2.Water/Fuel ratio(7:1)3.OHD(Over heat degree)
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Difference of Subcritical(500MW) and Supercritical(660MW)
COMPARISION OF SUPER CRITICAL & SUB CRITICAL
DESCRIPTION SUPERCRITICAL
(660MW)
SUB-CRITICAL
(500MW)
Circulation Ratio 1 Once-thru=1
Assisted Circulation=3-4
Natural circulation= 7-8
Feed Water Flow Control
-Water to Fuel Ratio
(7:1)
-OHDR(22-35 OC)
-Load Demand
Three Element Control
-Feed Water Flow
-MS Flow
-Drum Level
Latent Heat Addition Nil Heat addition more
Sp. Enthalpy Low More
Sp. Coal consumption Low High
Air flow, Dry flu gas loss Low High
Continue..
DESCRIPTION SUPERCRITICAL
(660MW)
SUB-CRITICAL
(500MW)
Coal & Ash handling Low High
Pollution Low High
Aux. Power Consumption
Low More
Overall Efficiency High
(40-42%)
Low
(36-37%)
Total heating surface area Reqd
Low
(84439m2)
High
(71582m2)
Tube diameter Low High
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Continue..
DESCRIPTION SUPERCRITICAL
(660MW)
SUB-CRITICAL
(500MW)
Material / Infrastructure
(Tonnage)
Low
7502 MT
High
9200 MT
Start up Time Less More
Blow down loss Nil More
Water Consumption Less More
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Water Wall Design
WATER WALL ARRANGEMENT
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Bottom spiral & top vertical tube furnace arrangement
Once through design feature is used for boiler water wall design
The supercritical water wall is exposed to the higher heat flux
Spiral tube wall design (wrapped around the unit) with high mass flow & velocity of steam/water mixture through each spiral
Higher mass flow improves heat transfer between the WW tube and the fluid at high heat flux.
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SPIRAL VS VERTICAL WALL
VERTICAL WALL
Less ash deposition on wall
Less mass flowMore number of tubesMore boiler height for
same capacityNo uniform heating of
tubes and heat transfer in all tubes of WW
SPIRAL WALL
More ash depositionMore fluid mass flowLess number of tubesLess boiler heightUniform heat transfer
and uniform heating of WW tubes
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Furnace Arrangement
VERTICAL TYPE
SPIRAL TYPE
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Supercritical Sliding Pressure Boiler Water Wall Design
Comparison of Vertical Wall and Spiral Wall
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Ash accumulation on wallsVertical water walls Spiral water walls
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Super Critical Boiler Materials
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Advanced Supercritical Tube Materials(300 bar/6000c/6200c)
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Material Comparison
DescriptionDescription 660 MW660 MW 500 MW500 MW
Structural SteelStructural Steel Alloy SteelAlloy Steel Carbon SteelCarbon Steel
Water wallWater wall T22T22 Carbon SteelCarbon Steel
SH CoilSH Coil T23, T91T23, T91 T11, T22T11, T22
RH CoilRH CoilT91,Super 304 T91,Super 304 HH T22, T91,T11T22, T91,T11
LTSHLTSH T12T12 T11T11
EconomizerEconomizer SA106-CSA106-C Carbon SteelCarbon Steel
Welding Joints (Pressure Parts)Welding Joints (Pressure Parts) 42,000 Nos42,000 Nos 24,000 Nos24,000 Nos
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Steam Water Cycle Chemistry Controls
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S. S.
No.No.ParameterParameter Sub CriticalSub Critical Super CriticalSuper Critical
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Type of Boiler Type of Boiler
water water
treatmenttreatment
LP and HP dosing. OrLP and HP dosing. Or All Volatile Treatment All Volatile Treatment
(Hydrazine + Ammonia)(Hydrazine + Ammonia)
No HP dosingNo HP dosing Combined water treatment (CWT). Combined water treatment (CWT).
22SilicaSilica < 20 ppb in feed water and steam, < 20 ppb in feed water and steam,
< 250 ppb in boiler drum< 250 ppb in boiler drumStandard value <15 ppb in the cycleStandard value <15 ppb in the cycle
Expected value <10 ppb in the cycleExpected value <10 ppb in the cycle
33pHpH 9.0 - 9.5 for feed, steam & 9.0 - 9.5 for feed, steam &
condensate,condensate,
9.0 9.0 –– 10.0 for Boiler drum 10.0 for Boiler drum
9.0 9.0 –– 9.6 for AVT(All volatile treatment) 9.6 for AVT(All volatile treatment)
8.0 8.0 –– 9.0 for CWT(Combine water 9.0 for CWT(Combine water
treatment)treatment)
44Dissolved Dissolved
Oxygen (DO)Oxygen (DO)< 7 ppb for feed.< 7 ppb for feed. < 7 ppb for feed in case of AVT< 7 ppb for feed in case of AVT
30 30 –– 150 ppb for feed in case of CWT 150 ppb for feed in case of CWT
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Cation (HCation (H++) )
ConductivityConductivity<0.20 <0.20 µµS/cm in the feed & steam S/cm in the feed & steam
cyclecycleStandard value <0.15 Standard value <0.15 µµS /cm in the cycleS /cm in the cycle
Expected value- <0.10 Expected value- <0.10 µµS /cm in the cycleS /cm in the cycle
66 (CPU)(CPU) CPU is optionalCPU is optional CPU is essential for 100% flow.CPU is essential for 100% flow.
77Silica and TDS Silica and TDS
controlcontrolBy maintaining feed water quality By maintaining feed water quality
andand
By operating CBDBy operating CBD
Blow down possible till separators are Blow down possible till separators are
functioning (upto 30% load). functioning (upto 30% load).
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Advantages of SC Technology
I ) Higher cycle efficiency means Primarily– less fuel consumption– less per MW infrastructure investments– less emission– less auxiliary power consumption– less water consumptionII ) Operational flexibility– Better temp. control and load change flexibility– Shorter start-up time– More suitable for widely variable pressure operation
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ECONOMY
Higher Efficiency (η%)
•Less fuel input.•Low capacity fuel handling system.•Low capacity ash handling system.•Less Emissions. Approximate improvement in Cycle Efficiency
Pressure increase : 0.005 % per barTemp increase : 0.011 % per deg K
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Increase of Cycle Efficiency due to Steam Parameters
300241
175 538 / 538
538 / 566
566 / 566
580 / 600
600 / 620
6,77
5,79
3,74
5,74
4,81
2,76
4,26
3,44
1,47
3,37
2,64
0,75
2,42
1,78
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1
2
3
4
5
6
7
8
9
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HP / RH outlet temperature [deg. C]Pressure [bar]
Increase of efficiency [%]
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Sub. vs. Supercritical Cycle Impact on Emissions
Plant Efficiency, %*
Plant Efficiency, %
Fuel Consumption/Total Emissionsincluding CO2
Subcritical Supercritical 34 - 37 37 - 41
Plant Efficiency, Btu / kw-hr 10,000 - 9,200 9,200 - 8,300
34%
Base
37%
Base-8%
41%
Base-17%
* HHV Basis
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Challenges of supercritical technology
Water chemistry is more stringent in super critical once through boiler.
Metallurgical ChallengesMore complex in erection due to spiral water
wall.More feed pump power is required due to
more friction losses in spiral water wall.Maintenance of tube leakage is difficult due to
complex design of water wall.Ash sticking tendency is more in spiral water
wall in comparison of vertical wall.
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