Steam Generators for the Next Generation of Power Plants

8/4/2019 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


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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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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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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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



E911/NF616 NF 12P 91X 20

Da/Di =1.8f v =0.8

Optimummain steamconditions


540

360

320

280

240

200


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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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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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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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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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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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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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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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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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

Documents

Steam Generators for the Next Generation of Power Plants