Bharat Heavy Electricals Limited, Tiruchirapalli
Supercritical Boiler and
Once through
Steam Generators
Design Aspects
HRDC 1st June 2009
Bharat Heavy Electricals Limited, Tiruchirapalli
Contents
Emerging Market Requirements
Sub critical Vs Supercritical Boilers
Once Through Boilers Major systems
Features of 800 MW Once Through Boiler
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Emerging market requirements
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Fuels for Steam Power Plants
Coal & Lignite:
Abundant availability
Lower cost
Will continue as the main fuels in many countries
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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
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Bharat Heavy Electricals Limited, Tiruchirapalli
Thermal Power Generation
Higher 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
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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
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Parameters for increasing Cycle Efficiency in
PC-fired Units
Increasing main steam pressure
Increasing superheat and reheat temperatures
Adopting double reheat
Increasing the vacuum of condenser
Increasing final feed water temperature.
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Current Trends in Steam Parameters
1980s : Pressure increased from 175-180 bar to 225
bar; temp mostly around 540 Deg.C
1990s : Pressures raised to 285 bar; Temp raised to
565 Deg.C
300 bar & 620 Deg.C not unusual today
255 bar 568/596 Deg C commonly used presently
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Bharat Heavy Electricals Limited, Tiruchirapalli
Heat Rate Improvement
Parameters at Turbine Inlet (bar/oC / oC)
% Improvement In Station Heat Rate
170 / 538 / 538
Base
170 / 538 / 565
0.5%
170 / 565 / 565
1.3%
246 / 538 / 538
1.6%
246 / 538 / 565
2.1%
246 / 565 / 565
3.0%
246 / 565 / 598
3.6%
306 / 598 / 598
5.0%
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Reduction in Coal consumption and CO2 emissions
Cycle parameters
Heat rate
improvement
Savings in annual
coal consumption
Reduction in annual
CO2 emissions
Unit
500 MW
660 MW
800 MW
Bar/0C/0C
170/538/538
246/538/565
246/565/598
Base
2.1
3.6
tons
Base
56000
96000
Base
61600
105600
tons
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Growth of unit sizes in India
Rating
Year of Introduction
60/70MW
1965
110/120MW
1966
200/210MW
1972
250MW
1991
500MW
1979
660MW
2004
800 MW
2006
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Increase in unit sizes to match increase in Cycle
parameters
Unit
Size
Steam
Flow
(t/h.)
SHO
Pressure
(Kg/cm2)
SHO/RHO
Temperature
(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
660MW
2120
256
568/596
800MW
2592
256
568/596
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Subcritical
Vs
Supercritical Boilers
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Steam generation process
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Rankine cycle
The Carnot Cycle is theoretically most efficient, but it is having practical
difficulties.
For steam power plant, practical thermal cycle was suggested by Rankine,
called Ideal cycle or Rankine cycle.
T
4
T1
p1
3
T2
p2
3-3 BFP raises pressure from p2 to p1
3-4 Heating In feed heaters & economizer
4 -1 Heating In boiler
1-2 Work done in Turbine from p1 to p2
2-3 Heat reduction in condenser
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Rankine cycle with Superheater
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Methods of increasing Rankine Cycle
Efficiency
Raising supply temperature by super heating.
Increasing the inlet temperature will raise the heat
supply to the cycle more than the heat rejection.
Raising inlet pressure of steam :
Increasing the pressure will mean increase in
saturation temperature at which steam evaporates thus
increasing the average temperature (T1) of heat
addition.
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(Contd..)
Dropping the final pressure (or temperature) at which heat is rejected.
Regenerative Heating : Heating the feed water pumped to Boiler by bleeding
steam from turbine.
Reheat Cycle : Reheating of steam in boiler after it has already expanded in
HP Turbine will avoid moisture formation in LP Turbine. Also, more heat
content of steam before IP Turbine, will improve efficiency.
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Trends in Operating Pressures
Increased Operating Pressures are employed to
improve Thermal Power Plant Cycle Efficiency
Unit size increase has traditionally been associated
with pressure increase
Higher Unit Sizes (500 MW and above ) currently
employ pressure cycles around 170 ata and above
HRDC 1st June 2009
Bharat Heavy Electricals Limited, Tiruchirapalli
Boiler Parameters
Raichur
Bellary
Ramagundam
210MW
500MW
500 MW
680
1625
1675
Steam flow at SHO
t/h
Steam pressure at SHO
kg/Sq.Cm(g) 155
178
178
Steam temperature at SHO
Deg.C
540
540
540
Reheat steam flow
t/hr.
597.2
1389.76
1387.2
Steam temperature at RHO Deg.C
540
540
540
Feed water temperature
247.5
254.6
253.7
Deg.C
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Boiler Water Circulating Systems
Choice of Circulating system depends on Operating Pressure
The density difference between water and steam provides the
driving force for the Circulating fluid
Higher pressures units warrant pumps to ensure circulation
(Natural circulation not adequate)
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Types of Circulation
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Natural Circulation
Controlled Circulation
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Types of boilers
Drum type
Once-through type
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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
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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
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210 MW Boiler Typical Arrangement
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500 MW Boiler Typical Arrangement
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500 MW Arrangement Of Circulating System
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Comparison Of 210 & 500 MW Boilers
210 MW
500MW
Furnace
Double arch
Single arch
Upper furnace
Open
Panellete
Type of circulation
Natural
Controlled
Water wall tubing
Plain tubes
Rifled
(63.5mm)
tubes(51mm)
2 Trisector
2 Trisector (or)
Air heaters
4 Bisector
Mill arrangement
Front or Rear
Both sides or
Rear
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Multiple-lead Rifled Tube
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QUEST FOR EFFICIENCY IMPROVEMENT
Since the time thermal power stations have been
engineered, there is a quest for efficiency improvement.
And supercritical parameters (Pressure above 225Kg/cm2
and temperature above 374.15 C) is an effort in that
direction.
HRDC 1st June 2009
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SC Steam generator
Boiler Steam Pressure
above the critical point
T
Critical Point
221 bar
Temperature
1-2 Pumping Process
2-3 Heat addition Rad.
Heating in the Boiler
3-4 Expansion process in
Turbine
4
4-1 Heat reduction in
Condenser
S
Entropy
Simple Supercritical cycle
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1st
June 2009
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Why Supercritical Pressure
The purpose of having high inlet steam pressure for turbine has
already been discussed in this presentation.
A Boiler operating at a pressure above critical point is called
SUPERCRITICAL BOILER
A point where boiling water and dry saturated lines meet so that
associated latent heat is zero, this point is called Critical Point.
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Pressure range
Sub critical
: Below 221 bar
Super critical
: 221 and above
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CRITICAL CONDITION
Definition
CRITICAL is a thermodynamic expression describing the state
of a substance beyond which there is no clear distinction
between the liquid and gaseous phase.
The critical pressure & temperature for water are
Pressure = 225.56 Kg / cm2
Temperature = 374.15 C
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Once Through Boiler-Concept
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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
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SUPERCRITICAL BOILER
Supercritical pressure boiler has no drum and heat absorbing surface being,
in effect, one continuous tube, hence called once through Supercritical
pressure boilers.
The water in boiler is pressurized by Boiler Feed Pump, sensible heat is added
in feed heaters, economizer and furnace tubes, until water attains saturation
temperature and flashes instantaneously to dry saturated steam and super
heating commences.
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Circulation Systems
STEAM TO TURBINE
STEAM TO TURBINE
SH
SH
DRUM
EVAPORATOR
ECO.
EVAPORATOR
SEPERATING
VESSEL
ASSISTED
CIRCULATION TYPE
Drum Type
FEED
FEED
ECO.
CIRCULATION PUMP
(LOW LOAD &
START-UP)
Once-through
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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
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Operating Pressure Range
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Once-thru Boiler
Major differences from Drum type boiler :
Evaporator system
Low load circulation system
Separator
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Evaporator system
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Once -thru Boiler
Evaporator 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
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Spiral Tube Arrangement
Features
Reduced number of
tubes with pitch.
Increased mass flow.
Mass flow rate can be
selected by number of
tubes.
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Once -thru Boiler - Furnace Wall
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Furnace Wall Designs
Spiral Wall Configuration
Vertical Wall Configuration
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Spiral Furnace
Windbox Panel
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Vertical Furnace Wall Design
Vertical tube furnace walls will provide all the
operational benefits of the currently popular spiral
design while significantly reducing the cost
and
construction time for the furnace and providing some
reduction in pressure drop.
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Vertical Wall Windbox
Only a Few Bends at the
Top and Bottom
Straight Tubes
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Spiral Walls
Furnace walls are not self supporting because the tubes are
inclined.
External support strap system is needed
Fabrication and installation are more difficult.
Above aspects tend to increase the cost.
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Spiral vs. Vertical Wall Comparison
Spiral Furnace System Applicable for
all size units
Benefits from averaging of lateral heat
absorption variation (each tube forms a
part of each furnace wall)
Simplified inlet header arrangement
Large number of operating units
Use of smooth bore tubing
throughout entire furnace wall
system
One material utilized throughout entire
waterwall system
No individual tube orifices Less
maintenance & pluggage potential
Vertical Furnace Wall System Limited
to larger capacity units (>600 MW
depending on fuel)
Less complicated windbox openings
Traditional furnace water wall support
system
Elimination of intermediate furnace wall
transition header
Less welding in the lower furnace wall
system
Easier to identify and repair tubes leaks
Lower water wall system pressure
drop thereby reducing required feed
pump power
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Vertical Wall Design - Advantages
The tubes are self supporting.
Transition
headers
at
spiral/vertical
interface
are
avoided.
Ash hopper tubing geometry simplified
Corners are easier to form
Reduced pressure drop, auxiliary power
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Start-up and Low load system
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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
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ONCE - THROUGH OPERATING RANGE
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LOW LOAD SYSTEM WITH CIRC. PUMP
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Once - thru Boiler
Low 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
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Once -thru Boiler
Separator :
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
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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
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Sliding Pressure Supercritical Operation
Pressure operation mode at boiler outlet
4350
1
psig)
3625
2900
2
2175
1450
725
1. Constant Pressure Operation
2. Modified Sliding Pressure Operation
3. Pure Sliding Pressure Operation
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Once -thru Boiler
Requirements :
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
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Super Critical (Once thru) boiler enables :
Peak power generation with better TG
Quicker response to TG load changes
Better heat rate of lower loads
efficiency levels
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Typical General Arrangement of Once Through
Steam Generator
Separators
Collecting
vessel
LTRH
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Indian Coal fired 800 MW
Steam Generator
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800 MW Boiler Parameters
Flow, t/h
Pressure,
kg/cm2(g)
Temp. 0C
Main Steam
2585
255
568
Reheat steam
2061
59.53
596
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Fuel Analysis
Proximate Analysis
Design Coal
Total Moisture
12
Ash
32 (Max.38)
Volatile Matter
24
Fixed Carbon
32
HHV
kcal/kg
4100
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Features of Boiler
Pulverised coal fired
Once through, evaporator suitable for
variable pressure operation
Single reheat
Tilting Tangential firing System
Dry Bottom
Balanced draft furnace
Side mill layout
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Features of 800 MW OT Boiler (Contd.)
Dry Bottom
Balanced draft furnace
High performance Bowl Mills
Two Nos. of Axial FD Fans
Two Nos. of Axial PA Fans
Two Nos. of Axial ID Fans
ESP to meet outlet concentration
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Side View of Boiler Arrangement
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Plan View of Boiler Arrangement
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Spiral Tube Arrangement
Features
Reduced number of
tubes with pitch.
Increased mass
flow.
Mass flow rate can
be selected by
number of tubes.
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Spiral Water wall Tubing
Lateral Heat Flux Profile
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Transition Area
Vertical Wall Tubes
Vertical Tube
Forgings
Spiral Wall Tubes
Spiral Tube Forgings
35 mm Vertical Tube Forgings
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Sliding Pressure Supercritical Design
Spiral to Vertical Transition Area - Load Transfer
Support
Fingers
SPIRAL WALL SUPPORT
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Sliding Pressure Supercritical Design
Spiral Wall
Windbox
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Supercritical Boiler with Vertical wall
Unit Mwe:
750
Max. Continuous Rating: 5,560,000 lb/hr (2522 t/hr)
SH Outlet Press: 3800 psig (262 bar)
SH Outlet Temp: 1054F (568C)
RH Outlet Temp: 1105F (596 C)
Fuel: Sub-bituminous (PRB)
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FRONT WALL
RIFLED TUBING
SIDE WALL
RIFLED TUBING
Vertical Wall Sliding
Pressure Supercritical Design
SCREEN TUBES
SMOOTH TUBING
HANGER TUBES
SMOOTH TUBING
ARCH
RIFLED TUBING
1 1/4 (31.8 mm)O.D. Tubing
1 1/8 (28.5 mm)O.D. Tubing
SIDE WALL
RIFLED TUBING
REAR WALL
RIFLED TUBING
FRONT WALL
RIFLED TUBING
SMOOTH TUBING
FROM THIS ELEVATION
ALL WALLS
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Boiler Pressure Part Materials
Tubing Oxidation Temperature Limits
MATERIAL
ASME ALLOY
OXIDATION LIMIT
Carbon Steel
SA-178C/ D
454C (850F)
Carbon-1/2 Mo
SA-209 T-1A
482C (900F)
1 Cr-1/2 Mo
SA-213 T-12
552C (1025F)
2-1/4 Cr-1 Mo
SA-213 T-22
593C (1100F)
2-1/4 Cr-1.6W-V-Cb
SA-213 T-23
593C (1100F)
9 Cr-1 Mo-V
SA-213 T91
649C (1200F)
9 Cr-2W
SA-213 T92
649C (1200F)
18 Cr-8 Ni
SA-213 TP304H
760C (1400F)
18 Cr-10 Ni-Cb
SA-213 TP347H
760C (1400F)
18 Cr-9 Ni-3Cu-Cb-N
SA-213 Super304H
760C (1400F)
25 Cr-20 Ni-Cb-N
SA-213 HR3C
760C (1400F)
SA-210 A-1/ C
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Materials used in various pressure parts of
conventional boilers
Area of application
Material
ASME specification
Tubes
Pipes
Drum
Carbon steel / Low
alloy steel
SA 299
Water walls,
Economizer
Carbon Steel
SA192
SA210 Gr.A1
SA210 Gr.C
SA106 Gr.B
SA106 Gr.C
SH and RH
1 Cr Mo
SA213 T11
SA335 P11
2 Cr 1 Mo
SA213 T22
SA335 P22
9 Cr 1 Mo V
SA213 T91
SA335 P91
18 Cr 8 Ni
SA213 TP304 H
18 Cr 10 Ni Cb
SA213 TP347 H
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New materials for high temperature
A. Evaporators
T12
T23 and T24 : For 600 oC (SHO) /620 oC (RHO)
B. Superheaters and Reheaters
T91 : for 550 oC (SHO)/570 oC (RHO).
P91 : Upto 270 bar and 580 oC (SHO)
P92 : Upto 290 bar and 600 oC (SHO)
Austenitic steels Super 304H/TP347 HFG :
For
temperature 600oC (SHO)/620oC (RHO)
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Materials in typical 800MW
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Boiler Equipment Upgrades
500 MW DRUM TYPE
ELIMINATE
DIVISION PANELS
FLAT-BOTTOM
PENDANT
SECTIONS
SINGLE FIREBALL
FOR LARGE
CAPACITY UNITS
SQUARE-UP
FURNACE
50 HOPPER
SLOPE
800 MW Once Thru Supercritical
INCORPORATE ONCE-THRU
CIRCULATION SYSTEM, INCLUDING
VERTICAL SEPARATOR
REVERSE SLOPE
OF CONVECTIVE
PASS FLOOR
SMOOTH WATERWALL TUBE IN
SPIRAL CONFIGURATION OR
RIFLED WATERWALL TUBE IN
VERTICAL CONFIGURATION
WINDBOX TILT DRIVE
SYSTEM UPGRADE
TWO (2) REGENERATIVE
TRI-SECTOR AIRHEATERS IN LIEU OF
FOUR (4) BISECTOR
AIR-HEATERS
REPLACE RP MILLS
WITH HP MILLS
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Summary
CE/Sulzer (later ALSTOM) mono tube technology
Sliding pressure in water walls over load range (rifled tubing
affording water wall protection).
Conventional vertical water wall construction
Simplified startup system
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HRDC 1st June 2009
