steam generators for the next generation of power plants
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Steam Generators for the Next Generation of Power Plants
This article first appeared in VGB Power Tech 12/99 1 of 12
Steam Generators for the Next Generation of Power Plants
Aspects of Design and Operating Performance
Dr. J. Franke, R. Kral and E. Wittchow
Siemens AG, Power Generation Group KWU
Introduction
The requirements to be met by the next generation of power plants are subject to various criteria
depending on regional considerations. Whereas efficiency together with environmental protection,
availability and power generating costs head the list of priorities in the highly industrialized
countries, investment costs and financing are becoming increasingly important factors in the
growth countries. Steam generators, as the most costly component and the component of
fundamental importance to power plant availability, play a significant role in both cases. Against
this background, Table 1 summarizes various development tasks which may give rise to new
design and operating solutions for future steam generators.
Table 1: Development tasks for steam generators.
Adaptation to process with high power plant efficiency
Improved materials for supercritical steam conditions Minimization of exhaust gas loss Utilization of exhaust gas heat for heating condensate and feed waterMeeting increasingly stringent requirements for operating behaviour
Low part loads with high steam temperatures Start-up process with low service life consumption despite shorter start-up times Low material stress even with large and rapid load changes Minimization of NOx-emissions with simultaneous increase in combustion efficiencyReducing investment costs
Simplified combustion chamber tubing Reduced and simplified start-up system Optimized thermodynamic design
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Steam Generators for the Next Generation of Power Plants
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Components Material Temperature at 105
creep resistance at100 mm/(s.t.p.)
Membrane wall 13CrMo4 47CrMoVTiB9 10
HCM 12
515 C580 C600 C
Superheater tubes X3CrNiMoN17 13
Esshete 1250TP 347 H FGAlloy 617Alloy 625
630 C
640 C655 C 690 C 740 C
Headers P 91E 911 / NF 616
NF 12TP 347 H FG
Alloy 617 modified
590 C615 C645 C655 C
700 C
Table 2:Materials for steam generators with high steam temperatures.
Adapting the Steam Generator to the Power Plant Process
Figure 1 shows the correlation between main steam conditions (MS conditions) and the semi-net
heat rate of a steam power plant. The semi-net heat rate in this context is the net heat ratecorrected for the auxiliary power requirement for the turbine-driven feed pump. The lines of equal
heat rate on this diagram refer to a 700 MW unit with single reheat and a condenser pressure of
0.04 bar. Intervals between lines of equal heat rate correspond to 100 kJ/kWh.
When considering Fig. 1 we are
confronted by the following
question: given the same heat
rate, for example, should main
steam parameters of 220
bar/610C or 300 bar/580C be
selected? This question regarding
the correct main steam
parameters is answered by the
degree of material stress
sustained by the most highly stressed thick-walled component, i.e. the main steam header.
Studies performed in this field have produced interesting results, as discussed below.
Possible materials for future
steam generators are listed in
Table 2. This consists of materials
which are either already proven,
are currently being developed or
are under discussion. Figure 2shows the 105 hours creep fatigue
values, based on VdTV material
specifications, for a number of
materials which are suitable for
main steam headers. The
information supplied for the
material NF12, a ferritic steel with a 12 % chrome content, which is still in development, is based
on a published anticipated value of "100 N/mm at 650C (105
h)". Straight line curves for the
Main steam pressure upstream of turbine [bar]
540
400
360
320
280
240
200
160
Main steam temperature upstream of turbine [C]
560 580 600 620 640 660 680 700 720
100kJ/kWh
Figure 1:Lines of constant heat consumption (half net) for various main steam conditions.
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Steam Generators for the Next Generation of Power Plants
This article first appeared in VGB Power Tech 12/99 3 of 12
Figure 2:Creep resistance (105 h) of several highly heat-resistantmaterials for steam generators.
Creep resistance [MPa]
500
Temperature [C]
550 600 650 700 750 800
200
180
160
140
120
100
80
60
40
20
X20
P91
NF616
NF12
TP 347H FG
Alloy 617
1 2 3 4 5 6
1
6
5
4
3
2
same level of material stress, i.e. same fatigue life, are plotted for these materials in Fig. 3. and
provide information on possible MS pressure and MS temperature combinations at the turbine
inlet. These values apply to MS headers with a 1.8 ratio of outside to inside diameter, a ligament
efficiency of 0.8, and take appropriate design margins for pressure and temperature into account.The different gradients of the straight line curves reflect the corresponding profiles of the material
strength curves in the relevant
temperature range in a simplified
form; other sizes of headers result
in a minor parallel displacement of
the straight line curves for the
same level of material stress. On
the basis of this, the level of
material stress sustained by a P91
header, for example, is the same
at main steam conditions of 250
bar/595C and 350 bar/568C.
If the lines plotted in Fig. 1 and the straight line curves for ferritic chrome steels shown in Fig. 3
are now combined in Fig. 4, it is possible to mark the main steam conditions for every material
which will produce the lowest heat rate for a certain header size assuming the same level of
material stress. Combination of these points then produces a curve with optimum steam conditions
for these chrome steels.
This method was used to ascertain
the optimum main steam conditions
for the individual material groups
(Fig. 5). Allowing for the material
strength values given in Fig. 2, the
optimum main steam conditions are
within the indicated range.
Austenitic steel was included here
for reference purposes only as it is
now rarely used for thick-walled
Figure 3:Lines of equal material stressfor given main steam headerdimensions.
Main steam pressureupsteam of turbine[bar ]
540
400
360
320
280
240
200
160
Main steam temperatureupst ream of turbine [C]
560 580 600 620 640 660 680 700 720
TP347H FGNF12NF 616P 91X 20
Al loy 617
Do/D i =1.8f v =0.8
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Steam Generators for the Next Generation of Power Plants
This article first appeared in VGB Power Tech 12/99 4 of 12
components because of its unfavorable characteristics with regard to operating flexibility. The
same method can also be used in principle for the main steam line and produces two interesting
results:
Main steam pressures of around 300 bar upstream of the turbine should not be exceeded even
with the newly developed chrome steels. This also applies to high-temperature projects which
necessitate use of nickel-based alloys, such as the Advanced 700C PF Power Plant which is
being discussed as part of the THERMIE project.
Taking investment costs for the HP feed heater train and for a major part of the steam
generator into consideration, cost-effective main steam pressures are below the optimum
values given in Fig. 5.
The optimum main steam
conditions identified in
these studies will then
produce the net
efficiencies shown in
Fig. 6 subject to further
advances in the field of
materials development.
Net efficiencies in the
region of 50 % are
feasible in conjunction
Figure 4:Optimum main steam conditions for ferritic chromium steelswith given main steam header dimensions.
Figure 5:Optimum main steam conditionswith given main steam header dimensions.
540
360
320
280
240
200560 580 600 620 640 660
100kJ/kWh
Main steam pressure upstream of turbine [bar]
Main steam temperature upstream of turbine [C]
E911/NF616 NF 12P 91X 20
Da/Di =1.8f v =0.8
Optimummain steamconditions
Main steam pressure upstream of turbine [bar]
540
360
320
280
240
200
Main steam temperature upstream of turbine [C]
560 580 600 620 640 660 680 700 720
Ni-basedmaterial
Austenitic
X 20
P 91
E 911/NF 616
NF 12
TP 347H FG
Alloy 617
Ferritic
Figure 6:Measures for increasing efficiencyof steam power plants.
X20
51
50
49
48
47
46
45
44
43
42
41
P 91 E 911/ NF 616
NF 12 N i-b asedX20
167 bar538/538C
270 bar580/600C
285 bar600/620C
300 bar625/640C
300 bar700/720C
250 bar540/560C
Fu rth er d evel opm ent of m ater ial s Furt her develop ment of pro cessesand components
1.3
0.6
0.7
1.6
1.5
0,8
0,40,6
Doublereheat
Auxiliarypowerrequire-ments
Boilerefficiency
Pressurelosses(verticaltubing
Water /steamcycle
Waste heatutilizationin steamgenerator
Steamturbineefficiency
0,6
Fuel: bitumino us coal
Condenser pressure : 0.04 bar
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Steam Generators for the Next Generation of Power Plants
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with appropriate process engineering measures and component improvements.
It is also necessary to examine the arrangement of heating surfaces in the flue gas path. High-
temperature corrosion and steam-side scaling become increasingly significant in heat exchangertubes at steam temperatures above 600 C. Measurements published by the CEGB in 1988 show
that maximum corrosion occurs in
austenitic materials at wall
temperatures of between 650 and
700C and that rates of material
thinning rise with increasing flue gas
temperatures (Fig. 7). Cost
considerations are therefore not
always the only criterion on which to
base the arrangement of final
superheater heating surfaces in the
flue gas path.
Improved Utilization of Exhaust Heat
Feedwater temperatures will remain at around 280C to 300C in future. Because of
thermodynamic considerations and the problems associated with the dew point of sulfuric acid,
this places strict limits on any further reduction in the flue gas temperature to below 120C which
can be achieved through use of larger air heater heat exchange surfaces. One potential solution to
this problem is the heat recovery system in which the flue gases are cooled to 80 C directly
upstream of the flue gas desulfurization plant. With this process, either the flow of flue gases
through the air heater is decreased or the flow of air increased. In both cases, the slopes of the
temperature curves plotted for flue gas and air converge because some of the heat output istransferred to the water-steam cycle (Fig. 8). In a coal-fired steam generator without a high-dust
DeNOx system, this can be implemented with a gasside air heater bypass with feedwater and
condensate heating surfaces. However, a hot gas recirculation system is useful in steam
generators with high-dust DeNOx systems in order to prevent fouling of the feedwater and
condensate heating surfaces with corrosive ammonium hydrogen sulfate, which is difficult to
remove.
Material thinning rate [10-9
m/h]
1400
580 600 620 640 660
1200
1000
800
60
50
40
30
20
10
0
Gas temper atur e [C]
Tube wall temperature [C]
640 700 C
Figure 7:High temperature corrosionin austenitic heat exchange surfaces.
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Steam Generators for the Next Generation of Power Plants
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This heat recovery system allows efficiency improvements of up to 0.6 percentage points in hard-
coal-fired power plants and in excess of 1 percentage point in lignite-fired plants due to the larger
specific flue gas flows. It
should be borne in mind,however, that this system is
only appropriate for power
plants which do not use flue
gas heat to reheat the flue
gases downstream of the flue
gas desulfurization system,
i.e. where the flue gases are
discharged via the cooling
tower.
Design and Process Engineering Measures to Improve Operating Performance
Use of rifled tubes for water walls of the combustion chamber can significantly improve operating
performance. Rifled tubes have two important advantages over smooth tubes:
1. At pressures below 200 bar, heat transfer is so efficient that the tubes are safely cooled even at
extremely low mass flow densities. The difference between smooth tubes and rifled tubes is
particularly evident in terms of their impact on tube wall temperatures in areas of high heat flux
in the burner region, for example (Fig. 9).
2. The amount of heat transferred from the inner wall surface to the fluid is also higher in the
pressure range between 210 and 220 bar that is unfavorable for heat transfer. Given the sameboundary conditions, the same wall temperatures as in a smooth tube are achieved at about
half the mass flow density.
Use of rifled tubes for the water walls therefore allows the "BENSON minimum load" to be reduced
from the previous value of 35 % (smooth tubes) to 20 %. This permits the operating range with
high main steam temperatures to be extended downwards without necessitating additional control
and changeover actions. Thanks to this low "BENSON minimum load", night-time or weekend
shutdowns with their associated increased life expenditure are no longer necessary, even for
Combustionair
Fluegas
340 C 380 C
120 C
125 C
80 C
93 C
HPheater
42 C
38 C
Air heater
LPheater
Bypassheater
400
300
200
100
Temperature [C]
Air heater,Bypass heater
Flue gascooler
Indu-ced-draft
Flue gas
Air
Feedwater
Condensate
Flue gascooler
Figure 8:Exhaust gas heat recovery systemConfiguration and temperature profile in air heater.
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Steam Generators for the Next Generation of Power Plants
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intermediate peaking duty. This is an important advantage particularly for the future generation of
high-temperature plants.
A "BENSON minimum load" of 20 % simultaneously means that the mass flow rate through theevaporator during startup can be reduced to 20 %. Transition to BENSON mode operation can
therefore already take place at 20 % load; rapid elevation of main steam temperature up to the
necessary conditions for turbine rolloff for a warm or a hot start takes less time and entails lower
startup losses than previously.
A superheater bypass is worth thinking about again for high-temperature plants. This reduces dips
in main steam temperature during the initial startup phase and supports the main steam
temperature setpoint controller while the plant is being run up to temperature. This reduces
material stresses throughout the power plant unit during startup.
Instrumentation and Control Measures for Improved Operating Performance
There is also still scope for I&C measures to significantly enhance the steam generator's operatingperformance. Out of the measures listed in Table 3, this report will be looking at only the newly
developed predictive load-margin computer and the combustion diagnostics system.
The load-margin computers used to date make insufficient allowance for the significant thermal
inertia of thick-walled components during startup, and stress limits are frequently exceeded. Wall
temperature measurements are also subject to considerable time lags.
Inner wall temperature [ C]
0.2
400
380
360
340
320
300
280
Steam fraction [-]
0.4 0.6 0.80.3 0.5 0.7
Burner range
Smooth tube
35
700
Rifled tube
20
200
Fluid
Minimum BENSONload [%]
Mass flux [kg/ms]
Ex ternal h eat f lu x [ kW/m ] 250
Pressure [bar] 700
Figure 9:Tube wall temperatures in burner area at low loads.
Time [min]50250
Main steamtemp eratur e [C]
Temperaturedifference [K]
40
-40
500
200
Lower limit curve
Upper limit curve
Max. allowable
temperature difference
Figure 10:Typical startup behaviour on cold startwith forecasting load margin computer.
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Steam Generators for the Next Generation of Power Plants
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The predictive load-margin
computer uses a computer
model which predicts future
differential temperature andthermal stresses from the
measured variables steam
temperature, steam
pressure and steam mass
flow for set time intervals
(Fig. 10), producing a
continuously updated
forecast. The maximum
permitted temperature is
calculated from the results of this forecast by means of parameter variation. The temperature
computed in this way is used to control the setpoints for steam temperature, pressure and firing
rate resulting in a straightforward startup and shutdown strategy with minimized material stresses.
Not only the water-steam cycle but also the combustion process is likely to see further
improvements in operating performance. The keyword here is combustion diagnostics.
Besides visual observation of the flame pattern, the quality of the combustion process has always
been evaluated to date by measurement of input parameters - air flow and delivery rate of coal
feeder system - and by analysis of the flue gases (O2, CO, NOx) at the steam generator outlet.
Combustion processes are optimized during the planning phase using simulated computer models
and by specialist teams during commissioning and subsequent operation. None of these methods,
however, has ever involved acquisition and subsequent evaluation of metrological data on the
combustion process.
The combustion diagnostics system developed by Siemens now fills this gap (Fig. 11). Special
cameras - one camera per burner - measure the spectral lines of the gas in the furnace. A
software program uses this data to compute temperatures and gas concentrations, so producing
an analysis of the combustion process.
The major advantages this gives the operator are self-evident:
The commissioning phase for the combustion system, which generally takes several months to
complete, can be shortened considerably.
Table 3:I & C measures for improved operating behaviour.
Forecasting load margin computerSteam temperature behaviour is calculated in advance with a model, from
which setpoints for steam temperatures pressure and output are determined.Condensate throttling for frequency stabilizationStep changes in unit load are transferred to the steam generator only as acontinuous load change.New feedwater controlAccounting for evaporator storage behaviour prevents unnecessarytemperature changes at the evaporator outlet on load changes.Improved main steam pressure control on start-upSmooth transition from pressure increase to maintaining constant pressureprevents temperature fluctuations.Combustion diagnosisMeasurement and evaluation of spectral lines in combustion chamber enablesdetermination of flame temperatures and gas concentrations and analysis ofcombustion process.
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Steam Generators for the Next Generation of Power Plants
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Burner and classifier settings are easily adjusted to match the burnout behavior of different
types of coal; material thinning of furnace walls resulting from CO spiking is avoided.
Low NOx operation, even during dynamic processes, reduces costs for NH3 and catalyst
consumption. It may also be possible to reduce the total excess air requirement because all burners can now
be supplied with the correct air flow with greater precision than previously.
The suitability of this innovative
technology has already been
demonstrated in a number of
power plants. As the next step it is
planned to integrate this
combustion diagnostics system
into the combustion control
system. This would allow it to
initiate automatic actions to control
air distribution if changes in the
combustion process are required.
Reduced Investment Costs
Increased power plant efficiency achieved by raising main steam parameters and/or by installing a
heat recovery system entails a higher level of investment. However, this additional outlay can be
offset by various cost-reducing factors, some of which will be discussed below.
One interesting cost-cutting option is the provision of vertical tubing for water walls of the
combustion chamber (Fig. 12). Membrane walls of this type with rifled tubes are considerablyeasier and therefore more cost effective to manufacture and install than water walls with spiral-
wound, smooth tubing. This is in addition to the operating advantages discussed earlier and
illustrated in Fig. 10.
The BENSON boiler with vertical tubing, which has attracted major interest worldwide, is supplied
by BENSON licensees with the usual function-based warranties. An expert's report commissioned
by a bank in connection with the financing of a specific project also rates this concept positively in
comparison with conventional water wall designs.
SensorCombustionAnalysis
Standard Control NewMeasurementBoiler
Closed-Loop Control
OperationObservationEvaluation
Air ControlDamper
Figure 11:Combustion DiagnosisOptical Measurement with Combustion Analysis.
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Steam Generators for the Next Generation of Power Plants
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Mass flux reductionfrom 2000 to 1000 kg/m2sflow characteristicas in drum boilers:increased heat input to an ind ividualtube increases throughp ut in that tube
Cost-effective fabricationand assembly
Minim um BENSON outpu t: 20%
Simple startup system for20% evaporator throughp ut
Reduced slaggingon combustion chamber walls
Figure 12:Vertical tube combustion chamber for BENSON steam generatorsPrinciple and characteristics.
Turbine Turbine
35% minim umBENSON output
20% minimu mBENSON output
Figure 13:Startup systems for BENSON steam generators.
Moreover, use of rifled tubes - regardless of whether the water walls are spirally or vertically tubed
- means that the startup system can be dimensioned for a 20 % evaporator flow rate. Circulating
pumps are no longer necessary provided that an adequate water inventory can be stored for thestartup procedure. Since it is known from previous experience that steam pressure is generally
between 60 and 120 bar prior to a warm or hot start, cheap, rugged centrifugal pumps of standard
design and dimensioned for pressures of up to 130 bar can be used in other cases, irrespective of
the type of evaporator tubing and the BENSON minimum load, instead of expensive circulating
pumps with wet-rotor motors. These pumps, which are installed in a secondary loop, merely have
to be fitted with an additional safety valve (Fig. 13).
A standard feature of two-pass steam generators of American or Japanese design is the inclusion
of widely spaced platen walls to form part of the furnace heating surfaces. Only when the flue
gases come into contact with heating surfaces with a transversal spacing of less than 300 to
400 mm is it necessary for the average flue gas temperature to have decreased to around 50 K
below the ash softening point. By bringing the central European approach into line with this design
philosophy it would be possible to raise the temperature of the flue gases as they enter the platen
heating surfaces, thereby reducing investment costs. One particular reason for the apparent
feasibility of this approach is that improved combustion due to finer coal pulverization and more
highly differentiated admixture of air is known to reduce the tendency of the platen heating
surfaces to become clogged with slag.
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Steam Generators for the Next Generation of Power Plants
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Other potential methods of reducing costs which have already been proposed by other parties are
included only briefly here:
Avoidance of excessive design margins
Economizer with externally ribbed tubes Simplified platform design and construction
Single-train air and flue gas path
Warranties to be restricted to most frequently burned coal types (e.g. acceptance of load
restrictions when burning adverse types of coal)
Summary
The purpose of this paper is to demonstrate that state-of-the-art steam generator technology still
has considerable potential for further development which can be exploited for the steam
generators of the next generation of power plants (Fig. 14). This applies not only to the steam
generators themselves but also to their integration into the power plant process.
Finned economizertubes
Optimum combination ofmain steam pressure andmain steam temperature
Startup systemfor 20% output
Combustiondiagnosis
Vertical tubecombustionchamber
Rifled combustionchamber tubes
Additional implemen-tation of intelligentI&C systems
Heat recoverysystem
Figure 14:Features of modern steam generatorsSummary.
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Steam Generators for the Next Generation of Power Plants
This article first appeared in VGB Power Tech 12/99 12 of 12
References
|1| Naoi, H., Ohgami, M., Mimura, H., Fujita, T.: Mechanical properties of 12Cr-W-CO ferritic
steels with high creep rupture strength. Materials for Advanced Power Engineering 1994.Liege October 3 - 6, 1994
|2| Meadowcroft, D. B.: An introduction to fireside corrosion experience in the Central
Electricity Generating Board.
Werkstoffe und Korrosion 39, 45 - 48 (1988)
|3| Griem, H., Khler, W. and Schmidt, H.: Heat Transfer, Pressure Drop and Stresses in
Evaporator Water Walls - From Experiment to Design.
VGB Kraftwerkstechnik 79 (1999), Vol. 1, p.
|4| Franke, J., Khler, W. and Wittchow, E.: Evaporator Designs for BENSON Boilers, State of
the Art and Latest Development Trends.
VGB Kraftwerkstechnik 73 (1995), Number 4.
|5| Franke, J., Cossmann, R. and Huschauer, H.: BENSON Steam Generator with Vertically-
Tubed Furnace, Large-Scale Test under Operating Conditions Demonstrates Safe Design.
VGB Kraftwerkstechnik 73 (1993) Vol. 4, pp. 353 - 359
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