Supercritical boiler technology for future market conditions
Joachim Franke and Rudolf Kral
Siemens Power Generation
Presented at Parsons Conference 2003
Oct. 2003
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1
Introduction
The requirements for environmental protection and operating economy in future steam power
plants make high efficiency levels and operating flexibility a matter of course not only in the EU but
also in increasing measure around the world. Existing technologies have currently enabled fulfillment of these requirements by pulverized-coal-fired power plants and in part also by power plants
with circulating fluidized bed (CFB) combustion systems.
Higher efficiencies can be achieved only along the path of higher steam temperatures and pressures.
2
State of the art
Power plants operating at supercritical pressure and high steam temperatures were already being
constructed in the 1950s (Fig.1). The 1960s saw a series of supercritical plants in the U.S. (such
as those equipped with the universal pressure boiler) and in the last twenty years supercritical
plants were used exclusively in Germany and Japan. The latter were designed for sliding-pressure
operation and thus also
fulfill the requirements
for
high
operating
Eddystone
Chemische Werke
Plant
Philo Nr. 6
flexibility and high plant
Nr. 1
Hüls
efficiencies at part load.
(Fig.2).
Comission Date
1956
1957
1959
To date, CFB power
plants have been used
especially for smaller
power output levels,
generally
with
drum
boilers. Plants up to 350
MW are in the meantime
already in operation and
several plants equipped
with Benson1 boilers
have
also
been
constructed. Supercritical plants for ratings
above 400 MW are
planned.
Electrical Output
MW
85
125
325
Steam Flow
t/h
260
306
907
Main Steam
bar/°C
304 / 600
321 / 621
357 / 649
Reheater 1
bar/°C
109 / 560
82 / 565
76 / 565
Reheater 2
bar/°C
32 / 560
13 / 537
19 / 565
Figure 1: World’s first supercritical Power Plants
Power plants operating at supercritical steam pressure have already demonstrated their operational capabilities and high availability over decades. The transition to steam temperatures of
600°C and higher is now a further major development step, which decisively affects many aspects
of the design of the power plant, especially of the boiler. Whether the transition to these high steam
temperatures is economical also depends not only on the choice of main steam pressure, reheat
pressure and feedwater temperature, but also on the range of fuel.
1
Benson is a registered trademark of Siemens AG
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To date, the focus
was on material development for the
superheaters
and
the
thick-walled
components for high
steam temperatures.
However, investigations indicate that
the wall heating surfaces can become
the limiting components for further
increases in steam
parameters.
One
reason for this is the
increasing fraction
of superheater heat
to be transferred
with
increasing
steam parameters.
3
-
Power Plant
Output
[MW]
Design Pre ssure *)
[bar]
Steam Temperature
Boiler Outlet [°C]
Year of
Commisioning
Avedorevaerket 2
Boxberg
Skaerbaekvaerket
Lippendorf
Nordjyllandsvaerket
Aghios Dimitrios
Schkopau
Neckar 2
Rostock
Hemweg
Meri Pori
Staudinger 5
Fynsvaerket
415
915
410
2 x 930
410
350
2 x480
340
550
660
550
550
430
332
285
310
285
310
242
285
285
285
261
240
285
275
582 / 600
545 / 580
582 / 580 / 580
554 / 583
582 / 580 / 580
540 / 540
545 / 562
545 / 568
545 / 562
540 / 540
540 / 540
545 / 562
540 / 540
2001
2000
1999
1999
1998
1998
1997
1997
1994
1994
1994
1992
1992
Europe
1050
700
1000
1000
500
1000
600
700
450
285
275
280
275
275
275
293
275
270
2001
2000
1998
1997
1995
1994
1993
1992
1991
Japan
Tachibanawan
Tachibanawan 1
Haramachi 2
Matsuura 2
Nanao Ota
Shinchi
Noshiro
Hekinan 2
Shin Miyazu
605 /
570 /
604 /
598 /
570 /
542 /
542 /
543 /
541 /
613
595
602
596
595
567
567
569
569
*) max. allow able
working pressure
at boiler outlet
Figure 2: Large Supercritical BENSON Boilers in Europe and Japan References
Effect on design
Size of heat exchange surfaces
Higher steam temperatures automatically diminish the temperature differences between the flue
gas and steam, with relatively large superheater and reheater heating surfaces as a consequence.
As higher tube wall temperatures also mean an increased tendency to fouling, corresponding
heating surface reserves must be provided.
Feedwater temperature has a large effect on the size of the heating surfaces in the cooler flue-gas
path. Values of 290°C to 300°C or higher are necessary for high-efficiency plants. As on the one
hand the flue-gas temperature downstream of the economizer is set in the design case at roughly
400°C – the temperature window for DeNOx – and on the other hand the water outlet temperature
from the economizer is limited to avoid steaming, the upstream superheaters must absorb more
heat with increasing feedwater temperature. At higher steam conditions, especially at increasing
reheat pressures, the exhaust steam temperatures from the HP section of the turbine and thus the
reheat inlet temperatures also increase. While these temperatures are still approx. 320°C at a design main steam temperature of 540°C, they already increase to over 350°C in a 600°C main
steam temperature design and even up to over 420°C in a 700°C design. This considerably decreases the temperature difference to the flue gas, with the consequence of still larger heating
surfaces in the reheaters.
Under consideration of a cost-effective heating surface design, feedwater temperatures should not
exceed 300°C, and HP exhaust steam pressures should lie in the range of 60 bar.
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-
End of evaporation
The location of the separator determines the location of the end of the evaporator on startup and
at low load in recirculation mode. Usually the separator is configured such that its temperature is
slightly superheated at the lowest once-through load point. Design of the boiler for high steam
temperatures and pressures leads to this being already the case in lower areas of the furnace
walls instead of as from the outlet first pass or in the boiler roof. The reason for this is the increasing degree of superheat and correspondingly decreasing fraction of evaporation in the heat input
to the HP section with increasing steam parameters. At a load of 40%, the degree of superheat in
a 540°C boiler is
approx.
27%,
and
this
in100
creases to 39%,
90
HPSuperheating
Superheating
HP
for example, in a
27
32
80
design for 700°C
39
70
main
steam
temperature
60
Q
(Fig. 3 and Fig.
50
Preheatingand
and
Preheating
58
[%]
53
4). As the highly
Evaporation
Evaporation
46
40
loaded heating
30
surface
area
must lie
up20
RHSuperheating
Superheating
RH
stream of the
15
15
15
10
separators
for
0
1
2
3
reasons
of
Steam conditions
evaporator coolT HP/RH
700 / 720 °C
540 / 560 °C
600 / 620 °C
ing
and
the
p HP
350 bar
250 bar
290 bar
separator thus
MHP
476 kg/s
600 kg/s
543 kg/s
cannot
be
moved arbitrarily
Figure 3: Heat Flow Distribution in Variable Pressure Operation
toward the burnat 40% Load
ers, a significantly larger degree of superheat will result at the lowest once-through operating point (Fig.5).
This considerably increases the downward step of the steam temperatures on the transition to recirculation mode. In order to extensively prevent this temperature change, the transition from
once-through to recirculation mode must be placed at a very low load point, requiring recirculation
mode only for startup. Whereas for boilers with spiral wound tubing the minimum load in once
through operation is in the range of 30% to 40%, an evaporator based on the "Benson Low Mass
Flux"[1] design with vertical rifled tubes enables loads to below 20%.
Furnace Design and Size is given by
Coal and Ash Quality
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3000
FEGT = IDT - 50 K
kJ/kg
FEGT = Furnace exit gas temperature
IDT = Initial deformation temperature of ash
2600
2200
1800
Furnace Design
and Size is given
by Coal and Ash
Quality
Corresponding
full load steam
pressure:
350 bar
290 bar
250 bar
190 bar
∆h Evaporation
at 40% Load
(sliding pressure)
1000
0
100
200
bar
Zones of Evaporation (at Part Load)
Full Load Steam Conditions
190 bar / 535 °C / 535 °C
Full Load Steam Conditions
250 bar / 540 °C / 560 °C
Full Load Steam Conditions
350 bar / 700 °C / 720 °C
Full Load Steam Conditions
290 bar / 600 °C / 620 °C
Figure 4: Increasing steam conditions lead to different evaporator designs
Siemens AG . Power Generation
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T [°C]
4000
800
3900
703 °C / 358 bar
476 kg/s
Superheater Outlet
3800
3700
603 °C / 300 bar
543 kg/s
Reheater
3600
540 °C
200 bar
3500
3400
750
700
544 °C / 261 bar
600 kg/s
650
600
3300
Water Wall
Outlet
3200
Roof
3100
3000
2900
550
Water Wall
Outlet
500
100% Load
Roof
2800
Nose
2700
Nose
2600
2500
450
2400
h [kJ/kg]
2300
2200
2100
2000
400
1900
Evaporator
Inlet
1800
1700
Evaporator
Inlet
1600
350
1500
1400
300
1300
1200
Economiser Inlet
1100
250
100% Load
1000
900
200
800
Economiser Inlet
40% Load
700
150
600
500
100
400
0
20
40
60
80
100
120
140
160
180
200 220
p [bar]
240
260
280
300
320
340
360
380
400
Figure 5: Water and Steam Temperatures in the h-p Diagram
Water walls
The water walls in boilers for subcritical steam conditions are generally configured as evaporators.
At increasing steam temperatures and pressures, the fraction of evaporator heating surfaces decreases, with the result that parts of the water walls must also be configured as superheaters, i.e.
downstream of the separator.
In the highly loaded furnace area, spiral-wound evaporator tubing is usually used with smooth
tubes and high mass fluxes – approx. 2000 – 2500 kg/m³s. As spiral-wound furnace tubing of this
type is not self-supporting, it is reinforced with support straps which are welded to the tube wall
with support blocks.
High steam parameters also lead to higher material loading in the evaporator. The previously existing design reserves are no longer available, with the result that a detailed stress analysis is required for the design of the evaporator tubing in each case. As a result of the requisite large wall
thicknesses, the design of highly loaded heating surface areas is in part no longer determined by
the primary stresses due to internal pressure but rather by the secondary stresses due to reSiemens AG . Power Generation
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strained thermal expansion. The higher evaporator temperatures also result in increasing temperature differences between the tubes and support straps on startup and shutdown. This in turn
leads to longer startup times, especially on cold start.
The "Benson Low
Æ Low mass flux design with natural
Mass Flux" design
developed
by
circulation characteristic
SIEMENS with design mass fluxes
Æ Simple, cost-effective manufacture and
of approx. 1000
assembly of water walls
kg/m²s and below
and with vertical
Æ Simpler maintenance e. g. for tube damage
rifled
evaporator
tubes requires no
Æ No stresses due to thermal expansion
additional support
because welded-on support straps
structure and thus
are eliminated
also does not impair plant flexibility
Æ Reduced auxiliary power consumption
in spite
of wall
outlet
temperaÆ Reduced slagging
tures of approx.
500°C
and
above.(Fig.6).
In a design for Figure 6: Vertically-Tubed Furnace for BENSON Boilers
main steam temPrinciple and Characteristics
peratures
of
600°C and above, the creep strengths of the wall materials commonly used to date such as
13CrMo44 (T12) are no longer sufficient, necessitating the transition to new developments such
as 7CrMoVTiB1010 (T24) or HCM2S (T23). This is already the case at steam pressures of 300
bar and above for lower design temperatures. Looking at primary stresses the creep strengths of
these materials, which require no post-welding heat treatment, permit steam temperatures up to
530°C in the furnace walls depending on main steam pressure, but the corrosion resistance and
secondary stresses limit these values down to 500°C. Main steam temperatures of 630°C at moderate steam pressures are thus achievable as regards the walls.
At higher steam temperatures, materials such as HCM12 or T92 are required which must be heattreated after welding. In order to minimize the manufacturing expenditure in such a design, the
erection welds on evaporator tubes must be reduced to the absolute minimum possible. This is
currently feasible only with vertical tubing. The relatively complex welds in the corners for spiralwound furnace tubing are eliminated and the individual wall segments are welded together only at
the fins. Welding of tubes may become necessary only in the horizontal plane. Solutions are also
available for this which minimize expenditure on heat treatment on erection.
In all cases, it can be stated that the problems in the design of the water walls increase disproportionately with increasing steam pressures. A reduction of main steam pressure from 350 bar to
250 bar reduces the efficiency of a 700°C plant by 0.7 percentage points but it also reduces the
wall outlet temperature from 540°C to 500°C and makes a design with materials without post weld
heat treatment possible. Main steam pressures far above 250 bar should therefore be avoided,
also in plants with high steam temperatures.
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-
Evaporator/superheater dividing point
At high steam parameters the water walls can no longer be designed entirely as an evaporator.
The transition from evaporator walls to superheater walls then lies above the furnace. This transition must be designed so as to minimize the temperature differences between the evaporator and
superheater sections of the walls which automatically result on water filling after shutdown, especially on water filling after an emergency shut down. Values of up to 80 K represent no cause for
concern. For higher values such as can occur at very high steam conditions as well as in large
furnaces, a flexible connection, not necessarily welded gas-tight, should also be taken into consideration for this transition.
-
Superheater heating surfaces
For steam temperatures up to approx. 550°C, all heating surfaces can be constructed of ferritic or
martensitic materials, while at 600°C austenitic materials are necessary for the final superheater
heating surfaces for both the HP section of the boiler as well as the reheater. In addition to the
strength parameters, corrosion behavior on the flue-gas and oxidation behavior on the steam
sides is especially determinative for material selection. Fig.7, Superheater materials for high temperatures, shows a selection of available materials. With regard to strength parameters, construction of superheater heating surfaces for steam temperatures up to 650°C is currently already feasible with austenitic steel materials. The corrosion resistance of the available materials however
reduces the design limits to about 630°C.
Maximum HP Steam Temperature limited by
Approved by
Creep Rupture Strength*
Corrosion
X3CrNiMoN1713
595
580
EN
AC66
605
620
VdTÜV
Esshete
615
580
VdTÜV / BS
TP 347 H (FG)
620
600
VdTÜV / ASME
MITI
Super 304H (FG)
635
600
ASME / MITI
NF 709
645
620
MITI
HR 3C
630
630
VdTÜV / ASME
MITI
Save 25
655
630
under development / MITI
Alloy 617 A130
685
720
under
development
* 100 MPa at Steam Temperature +35K
Figure 7: Available Superheater Tube Materials
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-
Thick-walled components
In the first steam generators with very high steam temperatures, austenitic materials were used for
the hot headers and connecting lines. However, the poor thermoelastic behavior – low thermal
conductivity, high thermal expansion – render these materials unsuitable for boilers which are implemented in power plants with a large number of load changes and minimum startup times.
The development of chromium steels such as P91, P92 or E911 has enabled steam temperatures
up to 620°C without the use of austenitic materials for thick-walled components. More recent developments such Main steam pressure upstream of turbine [bar]
360
as NF12 and
Save 12 could
NF 12
extend the limits
of implementation
E 911/
at moderate main
NF 616
320
steam pressures
TP 347H FG
P 91
up to 650°C in the
near future.
With regard to the
thick-walled components,
especially for the main
steam headers, it
proves that the
main steam pressures
should
more likely lie below 300 bar for
optimum component
utilization
(Fig.8) [2].
280
Alloy 617
X 20
240
Ferritic
200
540
560
580
Austenitic
600
620
640
Ni-based
material
660
680
700
720
Main steam temperature upstream of turbine [°C]
Figure 8: Optimum Main Steam Conditions with given
Main Steam Header Dimensions
-
Effect on operation
Power plants which are designed for fast load changes and short and frequent starts must necessarily be operated in sliding-pressure mode. Only then does the material loading of the turbine remain acceptable: in sliding-pressure operation– usually between full load and 40% load - the temperature curve in the turbine remains nearly constant over the entire load range. These advantages for the turbine contrast with disadvantages for the boiler. For example, the temperatures in
the water walls decrease from full load to part load by approx. 100 K. Due to their magnitude and
the ordinarily larger wall thicknesses at the elevated steam parameters, the temperature changes
during start up and load variations place increased requirements on the design of the thick-walled
components such as multiple parallel passes, but also on the design of the tube walls, such as
vertical tubing, in order to achieve similar startup times and load change rates to those in plants
with conventional steam parameters.
With increasing steam parameters, the degree of superheat at the outlet of the evaporator sections
of the water walls at the lowest once-through load point also increases. A high degree of superheat
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leads to a temperature reduction at the evaporator end and in the superheaters in the transition to
recirculation mode. The separators are therefore moved as far as possible toward the burner zone.
Operating measures to reduce the degree of superheat are increased excess air, flue-gas recirculation and use of the uppermost burner levels. The higher the steam temperatures and pressures
become, the more important is the lowest possible load point in once-through operation, so that the
once-through/recirculation mode transition need be traversed only on startup.
The large degree of superheat in the separator at the lowest once-through operating point also
results in changes in startup behavior at high steam parameters. On warm and hot startup in recirculation mode, the achievable hot steam temperatures are below the values required by the turbine. The earliest possible transition to once-through operation is necessary in order to shorten
startup time, as full main steam temperatures are also already possible at low load in this operating
mode.
High feedwater temperatures can restrict the sliding-pressure range in plants with very high main
steam pressures. In order to prevent the economizer from approaching the evaporation point at low
load, the pressure must be already fixed below 50% load or still higher depending on the design.
Increasing steam parameters also decrease the design reserves of nearly all pressure part components, as, not least for reasons of cost, the decision for advanced materials is not made until the
reserves of lower quality materials become insufficient. This also increases the requirements on
control quality: temperature deviations from the design value, such as on load changes, must be
kept to a minimum. The conventional cascade controller is no longer sufficient for superheat temperature
control;
concepts such as Evaporator outlet temperature
two-loop feedback
Previous feedwater
470
control concept
control or observer
°
features
provide
New feedwater
significantly better
460
control concept
control quality.
with allowance for
- inlet enthalpy
450
Special
attention
- storage of thermal
energy
must be given to
440
feedwater control.
Conventional systems which employ
430
only simple delay
modules to account
420
for the dynamic
differences
between heat release
410
by the fuel and
heat absorption by
400
the
evaporator
s 1500
0
250
500
750
1000
1250
tubes usually lead
Time
to large temperature fluctuations at
Figure 9: Comparison of Feedwater Control Concepts
the evaporator outLoad reduction from 100% to 50%
let
on
load
changes. New control concepts which account for effects such as those of changes in the evaporator inlet temperaSiemens AG . Power Generation
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ture or the thermal storage capacity of the tube wall in the form feed forward control (Fig.9) increase control quality decisively and thus minimize the use of more expensive, higher-quality materials.
For high degrees of superheat at the lowest once-through load point, the transition from recirculation mode to once-through operation and back can no longer take place without delay due to the
relatively large temperature change; the control must be adapted accordingly for a sliding transition.
4
Other effects
Design of the tube walls in particular is impeded by the high steam temperatures and pressures.
The design parameters should be selected as best as possible so as not to necessitate the use of
materials for which heat treatment must be performed after welding. A significant aspect for this is
selection of the fuel. Coals with low ash deformation temperatures require large furnaces, associated with high heat input to the walls. A 100K lower ash deformation temperature leads in a comparable boiler concept to a temperature increase at the wall outlet of about 25K. Because of this for
the
currently
Wall exit temperature °C
available wall materials
without
post-welding heat
600
treatment, the ash
deformation temperature for a
600°C boiler may
not be much lower
than
1200°
TFD = 700°C
(Fig.10).
500
The implementation of flue-gas
recirculation
–
extraction of the
flue gases if possible upstream of
the air heater in
order to reduce
the negative effect
on
exhaust-gas
temperature– can
shift the limits to
higher steam parameters.
pFD = 350 bar
TFD = 600°C
pFD = 300 bar
A617
TFD = 540°C
pFD = 250 bar
P92
7CrMoVTiB1010
13CrMo44
400
1100
1200
1300
Ash deformation temperature °C
Figure 10: Design Limits for Water Wall Materials
Steam generators for power plants with high steam parameters and hence high plant efficiencies
are consequently also designed for high boiler efficiencies. The lowest possible exhaust-gas temperatures – 115°C to 110°C can be achieved depending on the coal – and lower excess air are
prerequisites for this. Both of these factors lead to an increased heat input to the evaporator and
thus impede the design of the wall heating surfaces.
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The high tube wall temperatures of the superheater heating surfaces as well as lower excess air
and low-NOx firing systems increase the corrosion problem. For the selection of supeheater materials the resistance to scale formation from the flue-gas atmosphere and steam is therefore just as
important as creep resistance.
4.1
Special aspects for CFB
The advantages of CFB technology are uncontested for low-grade fuels or for fuels with widely
fluctuating quality as well as for low exhaust-gas emissions without post-combustion control measures. CFB plants up to capacities of 350 MWe are currently in operation. However, only oncethrough operation with high steam conditions render CFB technology serious competition for pulverized-coal firing. A plant for approx. 460 MWe with steam parameters of 560°C/580°C and 265
bar was developed in an EU research program. The BENSON "Low Mass Flux" design was selected as the evaporator concept. It fulfills the requirements of a fluidized bed to a special degree:
the tube orientation parallel to the flue gas/ash flow ensures low susceptibility to erosion, and temperature variations between the evaporator tubes are extensively prevented, as non-uniform heat
inputs are evened out by the natural circulation flow characteristic of the low mass flux design. It
also features an especially simple construction, as flow through all of the tubes in a single pass is
parallel, thus eliminating the need for elaborate water/steam distribution.
The suitability of this evaporator system for sliding-pressure operation also fulfills all requirements
for a power plant with regard to operating flexibility.
4.2
Combined-cycle plants
Heat-recovery steam generators downstream of gas turbines are usually designed as drum boilers. Increasing exhaust-gas temperatures downstream of gas turbines as well as the increasing
requirements on flexibility of a combined-cycle plant with frequent starts also make the use of
once-through systems interesting here. Elimination of the drum on the one hand increases operating flexibility and on the other hand is a noticeable cost aspect. In the Cottam combined-cycle
plant, a heat-recovery steam generator with a once-through evaporator based on the Benson "Low
Mass Flux" design was constructed for the first time and runs successfully in commercial operation
since Sept.1999. This evaporator concept is characterized by extremely low mass fluxes which still
lie far below those of fired boilers.
Siemens AG . Power Generation
Transmittal, reproduction, dissemination and/or editing of this document as well as utilization
of its contents and communication thereof to others without express authorization are
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5
Summary and outlook
Steam temperatures of 600°C to 620°C are currently possible as a result of efforts in materials
development. However, not only are new materials necessary for higher temperature ranges, but
further development was also necessary for the wall materials. On further temperature increases,
previous design concepts can no longer be adopted without modifications. New designs are necessary for the evaporator in particular in order to give boilers for high-temperature plants similar
flexibility to that of previous once-through boilers.
The Low Mass Flux Design provides an evaporator concept which meets the new requirements
and which permits further development to higher steam parameters for pulverized-coal-fired boilers and for boilers with circulating fluidized bed firing as well as for heat-recovery steam generators downstream of gas turbines.
A further increase in steam temperatures appears possible in the next years with continuous materials development, but without using nickel based materials not more than 10K to 20K. From the
current standpoint, the jump to 700°C will not take place until the next decade. However, from an
economic perspective, the high steam temperatures will only be selected given correspondingly
competitive materials prices and if, among other things, the appropriate main steam and reheat
pressures are selected and the fuel ranges are limited.
References
[1]
J. Franke and R. Kral
Innovative Boiler Design to Reduce Capital Cost and Construction Time
Power-Gen 2002
[2]
J. Franke, R. Kral and E. Wittchow
Steam Generators for the Next Generation of Power Plants
VGB Power Tech 12/99
Siemens AG . Power Generation
Transmittal, reproduction, dissemination and/or editing of this document as well as utilization
of its contents and communication thereof to others without express authorization are
prohibited. Offenders will be held liable for payment of damages. All rights created by patent
grant or registration of a utility model or design patent are reserved.
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