SUPERCRITICAL BOILER

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By

Ashvani Shukla

C&I

BGR ENERGY

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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.

RANKIN CYCLE SUBCRITICAL UNIT

1 - 2 > CEP work 2 - 3 > LP Heating 3 - 4 > BFP work 4 - 5 > HP Heating 5 – 6 > Eco, WW 6 – 7 >

Superheating 7 – 8 > HPT Work 8 – 9 > Reheating 9 – 10 > IPT Work 10–11 > LPT Work 11 – 1 > Condensing3

RANKINE CYCLE SUPERCRITICAL UNIT

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 4

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Absolute Absolute PressurePressure

(Bar)(Bar)

Saturation Saturation TemperatureTemperature

((o o C)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 controller

1.Drum level 2.Ms flow 3.Feed water flow.

Drum less Boiler Feed water control by 1.Load demand 2.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=1Assisted Circulation=3-4Natural 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 moreSp. Enthalpy Low More

Sp. Coal consumption Low HighAir flow, Dry flu gas loss Low High

CONTINUE..

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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

CONTINUE..DESCRIPTION SUPERCRITICAL

(660MW)SUB-CRITICAL

(500MW)Material / Infrastructure

(Tonnage)Low

7502 MTHigh

9200 MTStart up Time Less More

Blow down loss Nil More

Water Consumption Less More

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Water Wall Design

WATER WALL ARRANGEMENT 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. 16

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SPIRAL VS VERTICAL WALL

VERTICAL WALL Less ash deposition on

wall Less mass flow More number of tubes More boiler height for

same capacity No uniform heating of

tubes and heat transfer in all tubes of WW

SPIRAL WALL

More ash deposition More fluid mass flow Less number of tubes Less boiler height Uniform 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 COMPARISONDescriptionDescription 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

11Type 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 cycleExpected 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 AVT30 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 cyclecycle

Standard value <0.15 Standard value <0.15 µµS /cm in the cycleS /cm in the cycleExpected 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 controlcontrol

By maintaining feed water quality By maintaining feed water quality andandBy 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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ECONOMYHigher 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

6,775,79

3,74

5,744,81

2,764,26

3,44

1,47

3,372,64

0,75

2,421,78

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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 Challenges More 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.

THANK YOU