Concept of Super Critcal

April 11, 2023 PMI Revision 00 1

CONCEPT OF SUPER CRITICAL

CYCLE


• What is importance • History of this technology• Super Critical cycle details

Presentation Outline


PERCAPITA ELECTRIC POWER CONSUMPTION

COUNTRY PERCAPITA ELECTRICPOWER CONSUMPTION –KWH

INDIA 513CHINA 773CANADA 16413USA 13040MEXICO 1439NORWAY 24033SWITZERLAND 7346FRANCE 7069UNITED KINGDOM 5968SPAIN 4072RUSSIA 5108

ITALY 4610SWEDEN 15244GERMANY 6406TURKEY 1259JAPAN 7749

These are collected from Ststistics Organisation for Economic Cooperation and Development of I.E.A.


Emerging Market Requirements For Utility Units

High Reliability & Availability Highest economically achievable plant

efficiency and heat rate Suitable for differing modes of operation Suitable for different quality of fuel Ability to operate under adverse grid

conditions / fluctuations Minimum emission of Pollutants Lowest life cycle cost


Thermal Power GenerationHigher cycle efficiency for :• Conservation of fuel resources

• Reduction of Atmospheric Pollutants - SOX

& NOX

• Reduction in CO2 emission (linked to global warming)

• Better economy in power generation where fuel costs are high and pollution control requirements are stringent


GROWTH OF UNIT SIZES IN INDIA

RATING YEAR OF INTRODUCTION

60/70MW 1965

110/120MW 1966

200/210MW 1972

250MW 1991

500MW 1979

660MW Commg

800 MW PROPOSAL STAGE


AS THE UNIT SIZES GREW, BOILER SIZES SUPPLYING STEAM TO SUCH TURBINES HAVE ALSO INCREASED

UNIT STEAM SHO SHO/RHOSIZE FLOW PRESSURE TEMPERATURE

(T/H.) (KG/CM2) (DEG. C)

30MW 150 63 490

60/70MW 260 96 540

110/120MW 375 139 540/540

200/210MW 690 137/156 540/540

250MW 805 156 540/540

500MW 1670 179 540/540

600MW 2100 255 540/568

800 MW 2565 255 568/596


Major Sulzer/Combustion EngineeringInnovations for Fossil Utility Boilers

• First Sulzer Boiler

• First Pulverized Coal Fired Utility Boiler

• Tangential Firing

• First Commercial Monotube Steam Generator

• Controlled Circulation

• First Commercial Supercritical Monotube Steam Generator

1841

1912

1927

1931

1942

1954

Year of Introduction


Major Sulzer/Combustion EngineeringInnovations for Fossil Utility Boilers

• MHI Adopted as Monotube Technology Licensee

• Highest Temperature and Pressure Supercritical Boiler

• Combined Circulation - Supercritical

• Largest Oil/Gas Fired Supercritical Steam Generator

• Controlled Circulation Plus

• Sliding Pressure Supercritical

1957

1960

1964

1970

1978

1980

Year of Introduction


Fuels for Steam Power PlantsCoal & Lignite:

• Abundant availability• Lower cost• Will continue as the main fuels in many

countries


Cycle EfficiencyHigher efficiency can be realised with

• Higher live steam parameters • Adoption of double reheat cycle• Reduction in condenser absolute pressure


Measures to improve Plant Efficiency and / or Heat Rate

Boiler side measures :

Minimum RH spray

Minimum SH spray (if tapped off before feed heaters)

Minimum flue gas temperature at AH outlet

Minimum excess air at AH outlet

Minimum unburnt Carbon loss

Reduced auxiliary power consumption


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

00

1

2

3

4

5

6

7

8

9

10

HP / RH outlet temperature [deg. C]Pressure [bar]

Increase of efficiency [%]


Approximate improvement in Cycle Efficiency

Pressure increase : 0.005 % per bar

Temp increase : 0.011 % per deg K


500 MW Steam GeneratorCoal Consumption and Emissions

SubcriticalUnit

SupercriticalUnit

Coal Saving t/year Base 68800

CO2 Reduction t/year Base 88270

SO2 Reduction t/year Base 385

Basis:

Cycle Efficiency % Base +1.0

No. of operatinghrs.

Hrs./year 8000 8000


Steam generation details


Supercritical Cycles• Initially adopted in the late fifties and sixties • Higher Steam temperature employed on some units • Unit sizes also witnessed an increasing trend


Enthalpy Variations vs Pressure and Boiler Load

Sliding Pressure Supercritical DesignSliding Pressure Supercritical Design


Operating Experience

The first generation supercritical units

• Experienced increased forced outages

• Witnessed reduced plant reliability and availability


Comparison of Subcritical and SupercriticalCycle Availability (NERC)

0

2

4

6

8

10

12

14

EFOR %

Plant (Super) 13.347 12.077 9.668 7.685 7.534 7.482

Plant (Sub) 10.405 9.439 8.16 6.793 7.103 7.013

Blr (Super) 8.441 7.285 5.823 4.872 4.434 4.023

Blr (Sub) 5.928 5.464 4.344 3.811 3.926 4.018

1982-1984 1985-1987 1988-1990 1991-1993 1994-1996 1997


Increased outages were caused by

• Inadequate experience while extrapolating to the new designs and the increased unit sizes.

• Inadequate knowledge of high temperature materials.


The increased outages led to :

• Reversal of steam pressures to subcritical range

• Lowering of steam temperatures to 540 Deg C


Current Trends in Steam Parameters

• 1980s : Pressure increased from 175-180

bar to 225 bar; temp mostly around

540 Deg C

• 1990 : Pressures raised to 285 bar;temp

raised to 565-580-600 Deg C

• 300 bar & 620 Deg C not unusual today

• 255 bar 568/568 Deg C commonly used presently


Implications of higher steam parameters on boiler design

Boiler type

Materials

Reliability and Availability


Types of boilers

Drum type

Once-through type


Drum type boiler

Steam generation takes place in furnace water walls

Fixed evaporation end point - the drum

Steam -water separation takes place in the drum

Separated water mixed with incoming feed water


Drum type boiler Natural Circulation Boiler

Circulation thru water walls by

thermo-siphon effect Controlled Circulation Boiler

At higher operating pressures

just below critical pressure levels,

thermo-siphon effect supplemented

by pumps


Once Through Boiler-Concept


THE CONCEPT

The mass flow rate thru’ all heat transfer circuits

from Eco. inlet to SH outlet is kept same except at

low loads wherein recirculation is resorted to

protect the water wall system


COTROLLED CIRCULATION (Vs) ONCE THRU’

CC OT


Once Through Boiler-Concept


Once Through Boiler

• Once -through flow through all sections of

boiler (economiser, water walls &

superheater)

• Feed pump provides the driving head

• Suitable for sub critical & super critical

pressures


Once-thru Boiler

Major differences from Drum type boiler :

• Evaporator system• Low load circulation system• Separator


Once -thru BoilerEvaporator system :• Formed by a number of parallel tubes• Tubes spirally wound around the furnace to

reduce number of tubes and to increase the mass flow rate thru’ the tubes

• Small tube diameter• Arrangement ensures high mass velocity thru the

tubes


Once -thru Boiler - Furnace Wall


Furnace ArrangementFurnace Arrangement

VERTICAL TYPE

SPIRAL TYPE


ONCE - THROUGH OPERATING RANGE


Once -thru Boiler Low load circulation system :

At part loads once -thru flow not adequate to cool the tubes

To maintain required mass velocities boiler operates on

circulating mode at low loads

Excess flow supplied by feed pump or a dedicated circulating

pump


LOW LOAD SYSTEM WITH CIRC. PUMP


LOW LOAD SYSTEM WITH HEAT EXCHANGER


Once - thru’ BoilerLow load circulation system :

The excess flow over the once-thru flow separated in separator and– Returned to the condenser thru’ a heat

exchanger

or– Recirculated back to the boiler directly

by the dedicated circulating pump


Once -thru BoilerSeparator :

Separates steam and water during the

circulating mode operation

Runs dry during once-thru flow mode

Smaller in size compared to drum in a

drum type boiler


Typical Separator sizes

Number of separators 2 4

Inside diameter approx mm 850 600

Thickness mm 95 70


Once -thru Boiler

Advantages: Better suited for sliding pressure operation

Steam temperature can be maintained over wider load range

under sliding pressure

Quick response to load changes

Shorter start up time

Higher tolerance to varying coal quality

Suitable for sub critical & super critical pressures


Sliding Pressure Operation


Advantages of sliding pressure operation: Lower thermal stresses in the turbine during load changes.

Control range of RH temp is extended.

Reduced pressure level at lower loads prolongs the life

span of the components.

Overall reduction in power consumption and improved heat

rate.


Once -thru BoilerRequirements : Stringent water quality

Sophisticated control system

Low load circulation system

Special design to support the spiral furnace wall weight

High pressure drop in pressure parts

Higher design pressure for components from feed pump

to separator


Advanced CyclesEffect on Boiler Components

• Evaporator (Furnace) walls• Superheaters• Thickwalled boiler components• Steam piping


Furnace walls

• Increased operating pressure increases the medium temperatures.

• Increased regenerative feed heating increases the fluid temp entering.

• Larger furnaces required for NOX reduction, increase SH steam temperature at furnace wall outlet.


Superheaters

• Tube metal temperatures in final sections increase with outlet steam temperature.

• Susceptibility for high temperature corrosion.

• Susceptibility to steam side oxidation


Thick walled components• Higher pressure & temperature lead to

increased thickness of :– Shells of separator, start-up system

components, SHO header..– Main steam piping.

• Higher thickness results in larger temperature gradients across walls.


• Varying combustion and fouling behaviour of different coals within a wide range of coals cause varying heat release and heat absorption in the furnace

• Benson boiler principle compensates these effects by shifting of the final evaporation point without diminishing efficiency

Changed heat release in the furnace

by varying coal qualities


Definition of Supercritical DesignEvaporator pressure (MCR) 222 bar Supercritical

Design

Source: Siemens


• Varying combustion and fouling behaviour of different coals within a wide range of coals cause varying heat release and heat absorption in the furnace

• Benson boiler principle compensates these effects by shifting of the final evaporation point.

Changed heat absorption in furnace due to changes in coal

quality


Fixed evaporation end point

• For a drum type boiler the flue gases at the combustion chamber outlet can not be cooled below a certain value.

• Dimensioning of the heating surfaces of boilers having fixed evaporation end point must be done precisely.

• Generation of steam and spraying quantity in the SH change substantially if the operating point deviates from the design point.


First Fire to Turbine Synch,

Minute without Bypass System First Fire to Turbine Synch,Minute with Bypass System

Hot Start Up, after 2 hr shutdown 40 30 Warm Start Up, after 8 hr shutdown 65 45 Cold Start Up, after 36 hr shutdown 130 90

Faster Start-up Time with Supercritical Design



Hot Start Up, after 2 hr shutdown 40 30 Warm Start Up, after 8 hr shutdown 65 - 90 45 - 70 Cold Start Up, after 36 hr shutdown 180 - 260 140 - 220

Once - Thru

Drum


Present Trend in IndiaUnit Size MW

SHO flow(t/hr)

SHO pr. (Kg/Sq.cm)

SHOT (°C)

RHOT (°C)

660 2100 255 568 596

800 2565 255 568 596




Hot Start Up, after 2 hr shutdown 40 30 Warm Start Up, after 8 hr shutdown 65 45 Cold Start Up, after 36 hr shutdown 130 90

Faster Start-up Time with Supercritical Design



Hot Start Up, after 2 hr shutdown 40 30 Warm Start Up, after 8 hr shutdown 65 - 90 45 - 70 Cold Start Up, after 36 hr shutdown 180 - 260 140 - 220

Once - Thru

Drum


Typical Parameters SH Outlet

Steam Temp.,°F

RH OutletSteam Temp.

°F Drum Type

3% per minute (30%-100% load) +/- 10 +/- 15 5% per minute (50% - 100% load) +/- 35 +/- 40 Once-Through

3% per minute (30% - 100% load) +/-10 +/-10 5% per minute (50% - 100% load) +/-10 +/-12

Note:Above values are based on sliding pressure mode and a 5 minute load ramp.

Tighter Control of Steam Temperatures


THANK YOU

Concept of Super Critcal

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