38729456 Super Critical Boiler Technology for Future Market Conditions

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H30-K0018-X-X-7600 Textseite, engl. 2001-09 D97 H:\BENSON\Homepage\Parsons_2003.doc Page 1 of 13

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 powerplants make high efficiency levels and operating flexibility a matter of course not only in the EU butalso in increasing measure around the world. Existing technologies have currently enabled fulfill-ment of these requirements by pulverized-coal-fired power plants and in part also by power plantswith circulating fluidized bed (CFB) combustion systems.

Higher efficiencies can be achieved only along the path of higher steam temperatures and pres-sures.

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. (suchas those equipped with the universal pressure boiler) and in the last twenty years supercriticalplants were used exclusively in Germany and Japan. The latter were designed for sliding-pressureoperation and thus alsofulfill the requirementsfor high operatingflexibility and high plantefficiencies at part load.(Fig.2).

To date, CFB powerplants have been usedespecially for smaller

power output levels,generally with drumboilers. Plants up to 350MW are in the meantimealready in operation andseveral plants equippedwith Benson1 boilershave also beenconstructed. Supercriti-cal plants for ratingsabove 400 MW areplanned.

Power plants operating at supercritical steam pressure have already demonstrated their opera-tional capabilities and high availability over decades. The transition to steam temperatures of600C and higher is now a further major development step, which decisively affects many aspectsof the design of the power plant, especially of the boiler. Whether the transition to these high steamtemperatures is economical also depends not only on the choice of main steam pressure, reheatpressure and feedwater temperature, but also on the range of fuel.

1 Benson is a registered trademark of Siemens AG

Plant

19 / 56513 / 53732 / 560Reheater 2 bar/C

76 / 56582 / 565109 / 560Reheater 1 bar/C

357 / 649321 / 621304 / 600Main Steam bar/C

907306260Steam Flow t/h

32512585Electrical Output MW

195919571956Comission Date

Eddystone

Nr. 1Philo Nr. 6

Chemische Werke

Hls

Figure 1: Worlds first supercritical Power Plants


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To date, the focuswas on material de-velopment for thesuperheaters andthe thick-walled

components for highsteam temperatures.However, investiga-tions indicate thatthe wall heating sur-faces can becomethe limiting compo-nents for furtherincreases in steamparameters. Onereason for this is theincreasing fractionof superheater heat

to be transferredwith increasingsteam parameters.

3 Effect on design

- Size of heat exchange surfaces

Higher steam temperatures automatically diminish the temperature differences between the fluegas 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, correspondingheating surface reserves must be provided.

Feedwater temperature has a large effect on the size of the heating surfaces in the cooler flue-gaspath. Values of 290C to 300C or higher are necessary for high-efficiency plants. As on the onehand the flue-gas temperature downstream of the economizer is set in the design case at roughly400C the temperature window for DeNOx and on the other hand the water outlet temperaturefrom the economizer is limited to avoid steaming, the upstream superheaters must absorb moreheat with increasing feedwater temperature. At higher steam conditions, especially at increasingreheat pressures, the exhaust steam temperatures from the HP section of the turbine and thus thereheat inlet temperatures also increase. While these temperatures are still approx. 320C at a de-sign main steam temperature of 540C, they already increase to over 350C in a 600C mainsteam temperature design and even up to over 420C in a 700C design. This considerably de-

creases the temperature difference to the flue gas, with the consequence of still larger heatingsurfaces in the reheaters.

Under consideration of a cost-effective heating surface design, feedwater temperatures should notexceed 300C, and HP exhaust steam pressures should lie in the range of 60 bar.

Avedorevaerket 2

Boxberg

Skaerbaekvaerket

Lippendorf

Nordjyllandsvaerket

Aghios DimitriosSchkopau

Neckar 2

Rostock

Hemweg

Meri Pori

Staudinger 5

Fynsvaerket

Tachibanawan

Tachibanawan 1

Haramachi 2

Matsuura 2

Nanao Ota

Shinchi

Noshiro

Hekinan 2

Shin Miyazu

415

915

410

2 x 930

410

3502 x480

340

550

660

550

550

430

1050

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285

310

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242285

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545 / 580

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Power PlantOutput

[MW]Design Pressure *)

[bar]

Steam Temperature

Boiler Outlet [C]

Year of

Commisioning

*) max. allowableworking pressure

at boiler outlet

Japan

Europe

Avedorevaerket 2

Boxberg

Skaerbaekvaerket

Lippendorf

Nordjyllandsvaerket

Aghios DimitriosSchkopau

Neckar 2

Rostock

Hemweg

Meri Pori

Staudinger 5

Fynsvaerket

Tachibanawan

Tachibanawan 1

Haramachi 2

Matsuura 2

Nanao Ota

Shinchi

Noshiro

Hekinan 2

Shin Miyazu

415

915

410

2 x 930

410

3502 x480

340

550

660

550

550

430

1050

700

1000

1000

500

1000

600

700

450

332

285

310

285

310

242285

285

285

261

240

285

275

285

275

280

275

275

275

293

275

270

582 / 600

545 / 580

582 / 580 / 580

554 / 583

582 / 580 / 580

540 / 540545 / 562

545 / 568

545 / 562

540 / 540

540 / 540

545 / 562

540 / 540

605 / 613

570 / 595

604 / 602

598 / 596

570 / 595

542 / 567

542 / 567

543 / 569

541 / 569

2001

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1999

1998

19981997

1997

1994

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1994

1992

1992

2001

2000

1998

1997

1995

1994

1993

1992

1991

Power PlantOutput

[MW]Design Pressure *)

[bar]

Steam Temperature

Boiler Outlet [C]

Year of

Commisioning

*) max. allowableworking pressure

at boiler outlet

Japan

Europe

Figure 2: Large Supercritical BENSON Boilers in Europe and Japan -References


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15

58

27

15

53

32

15

46

39

0

10

20

30

40

50

60

70

80

90

100

Q

[%]

1 2 3

RH SuperheatingRH Superheating

Preheating and

Evaporation

Preheating and

Evaporation

HP SuperheatingHP Superheating

540 / 560 C250 bar600 kg/s

600 / 620 C

290 bar543 kg/s

700 / 720 C350 bar476 kg/s

Steam conditions

T HP/RHp HPMHP

15

58

27

15

53

32

15

46

39

0

10

20

30

40

50

60

70

80

90

100

Q

[%]

1 2 3

RH SuperheatingRH Superheating

Preheating and

Evaporation

Preheating and

Evaporation

HP SuperheatingHP Superheating

540 / 560 C250 bar600 kg/s

600 / 620 C

290 bar543 kg/s

700 / 720 C350 bar476 kg/s

Steam conditions

T HP/RHp HPMHP

- End of evaporation

The location of the separator determines the location of the end of the evaporator on startup andat low load in recirculation mode. Usually the separator is configured such that its temperature isslightly 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 furnacewalls instead of as from the outlet first pass or in the boiler roof. The reason for this is the increas-ing degree of superheat and correspondingly decreasing fraction of evaporation in the heat inputto the HP section with increasing steam parameters. At a load of 40%, the degree of superheat ina 540C boiler isapprox. 27%,and this in-creases to 39%,for example, in adesign for 700Cmain steamtemperature(Fig. 3 and Fig.

4). As the highlyloaded heatingsurface areamust lie up-stream of theseparators forreasons ofevaporator cool-ing and theseparator thuscannot bemoved arbitrarilytoward the burn-

ers, a signifi-cantly 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 re-circulation mode. In order to extensively prevent this temperature change, the transition fromonce-through to recirculation mode must be placed at a very low load point, requiring recirculationmode only for startup. Whereas for boilers with spiral wound tubing the minimum load in oncethrough operation is in the range of 30% to 40%, an evaporator based on the "Benson Low MassFlux"[1] design with vertical rifled tubes enables loads to below 20%.

Furnace Design and Size is given byCoal and Ash Quality

Figure 3: Heat Flow Distribution in Variable Pressure Operationat 40% Load


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Furnace Designand Size is givenby Coal and AshQuality

FEGT = IDT - 50 K

FEGT = Furnace exit gas temperatureIDT = Initial deformation temperature of ash

Zones of Evaporation (at Part Load)

Full Load Steam Conditions190 bar / 535 C / 535 C




h Evaporationat 40% Load(sliding pres-sure)

0 100 200 bar

1000

1800

2200

2600

3000

kJ/kg

Correspondingfull load steampressure:350 bar290 bar250 bar190 bar

Figure 4: Increasing steam conditions lead to different evaporator designs


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Figure 5: Water and Steam Temperatures in the h-p Diagram

100

150

200

250

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650

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0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400p [bar]

h[

kJ/kg]

T [C]

700

750

800

EvaporatorInlet

Nose

Water WallOutlet

Superheater Outlet

Economiser Inlet

100% Load

Roof

40% Load

EvaporatorInlet

Nose

Water WallOutlet

Economiser Inlet

Roof

703 C / 358 bar476 kg/s

603 C / 300 bar543 kg/s

544 C / 261 bar600 kg/s

Reheater

100% Load

540 C200 bar

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 de-creases, 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/ms. As spiral-wound furnace tubing of thistype is not self-supporting, it is reinforced with support straps which are welded to the tube wallwith support blocks.

High steam parameters also lead to higher material loading in the evaporator. The previously ex-isting design reserves are no longer available, with the result that a detailed stress analysis is re-quired for the design of the evaporator tubing in each case. As a result of the requisite large wallthicknesses, the design of highly loaded heating surface areas is in part no longer determined bythe primary stresses due to internal pressure but rather by the secondary stresses due to re-


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strained thermal expansion. The higher evaporator temperatures also result in increasing tem-perature differences between the tubes and support straps on startup and shutdown. This in turnleads to longer startup times, especially on cold start.

The "Benson Low

Mass Flux" designdeveloped bySIEMENS with de-sign mass fluxesof approx. 1000kg/ms and belowand with verticalrifled evaporatortubes requires noadditional supportstructure and thusalso does not im-pair plant flexibility

in spite of walloutlet tempera-tures of approx.500C andabove.(Fig.6).In a design formain steam tem-peratures of600C and above, the creep strengths of the wall materials commonly used to date such as13CrMo44 (T12) are no longer sufficient, necessitating the transition to new developments suchas 7CrMoVTiB1010 (T24) or HCM2S (T23). This is already the case at steam pressures of 300bar and above for lower design temperatures. Looking at primary stresses the creep strengths ofthese materials, which require no post-welding heat treatment, permit steam temperatures up to

530C in the furnace walls depending on main steam pressure, but the corrosion resistance andsecondary stresses limit these values down to 500C. Main steam temperatures of 630C at mod-erate steam pressures are thus achievable as regards the walls.

At higher steam temperatures, materials such as HCM12 or T92 are required which must be heat-treated after welding. In order to minimize the manufacturing expenditure in such a design, theerection welds on evaporator tubes must be reduced to the absolute minimum possible. This iscurrently feasible only with vertical tubing. The relatively complex welds in the corners for spiral-wound furnace tubing are eliminated and the individual wall segments are welded together only atthe fins. Welding of tubes may become necessary only in the horizontal plane. Solutions are alsoavailable 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 dispropor-tionately with increasing steam pressures. A reduction of main steam pressure from 350 bar to

250 bar reduces the efficiency of a 700C plant by 0.7 percentage points but it also reduces thewall outlet temperature from 540C to 500C and makes a design with materials without post weldheat treatment possible. Main steam pressures far above 250 bar should therefore be avoided,also in plants with high steam temperatures.

Simple, cost-effective manufacture andassembly of water walls

Simpler maintenance e. g. for tube damage

No stresses due to thermal expansionbecause welded-on support strapsare eliminated

Reduced auxiliary power consumption

Reduced slagging

Low mass flux design with naturalcirculation characteristic

Figure 6: Vertically-Tubed Furnace for BENSON BoilersPrinciple and Characteristics


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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 transi-tion 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, espe-cially on water filling after an emergency shut down. Values of up to 80 K represent no cause forconcern. For higher values such as can occur at very high steam conditions as well as in largefurnaces, a flexible connection, not necessarily welded gas-tight, should also be taken into consid-eration for this transition.

- Superheater heating surfaces

For steam temperatures up to approx.550C, all heating surfaces can be constructed of ferritic ormartensitic materials, while at 600C austenitic materials are necessary for the final superheaterheating surfaces for both the HP section of the boiler as well as the reheater. In addition to thestrength 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 tem-peratures,shows a selection of available materials. With regard to strength parameters, construc-tion of superheater heating surfaces for steam temperatures up to 650C is currently already fea-sible with austenitic steel materials. The corrosion resistance of the available materials howeverreduces the design limits to about 630C.

X3CrNiMoN1713 595

615

605

635

580

580

620

600

AC66

Esshete

Super 304H (FG)

645 620NF 709

630 630HR 3C

EN

VdTV

VdTV / ASMEMITI

ASME / MITI

MITI

under

development

TP 347 H (FG)

Save 25

Alloy 617 A130

620

655

685

600

630

720

VdTV / BS

VdTV / ASMEMITI

under develop-

ment / MITI

Approved byMaximum HP Steam Temperature limited by

Creep Rupture Strength* Corrosion

* 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 forthe hot headers and connecting lines. However, the poor thermoelastic behavior low thermalconductivity, high thermal expansion render these materials unsuitable for boilers which are im-

plemented 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 temperaturesup to 620C without the use of austenitic materials for thick-walled components. More recent de-velopments suchas NF12 andSave 12 couldextend the limitsof implementationat moderate mainsteam pressuresup to 650C in thenear future.

With regard to thethick-walled com-ponents, espe-cially for the mainsteam headers, itproves that themain steam pres-sures shouldmore likely lie be-low 300 bar foroptimum compo-nent utilization

(Fig.8) [2].

- Effect on operation

Power plants which are designed for fast load changes and short and frequent starts must neces-sarily be operated in sliding-pressure mode. Only then does the material loading of the turbine re-main acceptable: in sliding-pressure operation usually between full load and 40% load - the tem-perature curve in the turbine remains nearly constant over the entire load range. These advan-tages 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 andthe ordinarily larger wall thicknesses at the elevated steam parameters, the temperature changesduring start up and load variations place increased requirements on the design of the thick-walledcomponents such as multiple parallel passes, but also on the design of the tube walls, such asvertical tubing, in order to achieve similar startup times and load change rates to those in plantswith conventional steam parameters.

With increasing steam parameters, the degree of superheat at the outlet of the evaporator sectionsof the water walls at the lowest once-through load point also increases. A high degree of superheat

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 8: Optimum Main Steam Conditions with givenMain Steam Header Dimensions


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leads to a temperature reduction at the evaporator end and in the superheaters in the transition torecirculation 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 recircu-lation and use of the uppermost burner levels. The higher the steam temperatures and pressuresbecome, 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 alsoresults in changes in startup behavior at high steam parameters. On warm and hot startup in recir-culation mode, the achievable hot steam temperatures are below the values required by the tur-bine. The earliest possible transition to once-through operation is necessary in order to shortenstartup time, as full main steam temperatures are also already possible at low load in this operatingmode.

High feedwater temperatures can restrict the sliding-pressure range in plants with very high mainsteam pressures. In order to prevent the economizer from approaching the evaporation point at lowload, 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 com-ponents, as, not least for reasons of cost, the decision for advanced materials is not made until thereserves of lower quality materials become insufficient. This also increases the requirements oncontrol quality: temperature deviations from the design value, such as on load changes, must bekept to a minimum. The conventional cascade controller is no longer sufficient for superheat tem-perature control;concepts such astwo-loop feedbackcontrol or observerfeatures providesignificantly bettercontrol quality.

Special attentionmust be given tofeedwater control.Conventional sys-tems which employonly simple delaymodules to accountfor the dynamicdifferences be-tween heat releaseby the fuel andheat absorption by

the evaporatortubes usually leadto large tempera-ture fluctuations atthe evaporator out-let on loadchanges. New con-trol concepts which account for effects such as those of changes in the evaporator inlet tempera-

Evaporator outlet temperature

0 250 500 750 1000 1250 1500

Previous feedwatercontrol concept

New feedwatercontrol conceptwith allowance for

- inlet enthalpy- storage of thermal

energy

s

Time

400

410

420

430

440

460

470

450

Figure 9: Comparison of Feedwater Control ConceptsLoad reduction from 100% to 50%


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ture or the thermal storage capacity of the tube wall in the form feed forward control (Fig.9) in-crease control quality decisively and thus minimize the use of more expensive, higher-quality mate-rials.

For high degrees of superheat at the lowest once-through load point, the transition from recircula-

tion mode to once-through operation and back can no longer take place without delay due to therelatively large temperature change; the control must be adapted accordingly for a sliding transi-tion.

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 ofmaterials for which heat treatment must be performed after welding. A significant aspect for this isselection of the fuel. Coals with low ash deformation temperatures require large furnaces, associ-

ated with high heat input to the walls. A 100K lower ash deformation temperature leads in a com-parable boiler concept to a temperature increase at the wall outlet of about 25K. Because of this forthe currentlyavailable wall ma-terials withoutpost-welding heattreatment, the ashdeformation tem-perature for a600C boiler maynot be much lowerthan 1200(Fig.10).

The implementa-tion of flue-gasrecirculation extraction of theflue gases if pos-sible upstream ofthe air heater inorder to reducethe negative effecton exhaust-gastemperature canshift the limits to

higher steam pa-rameters.

Steam generators for power plants with high steam parameters and hence high plant efficienciesare consequently also designed for high boiler efficiencies. The lowest possible exhaust-gas tem-peratures 115C to 110C can be achieved depending on the coal and lower excess air areprerequisites for this. Both of these factors lead to an increased heat input to the evaporator andthus impede the design of the wall heating surfaces.

400

500

600

1100 1200 1300

Ash deformation temperature C

Wall exit temperature C

TFD = 540CpFD = 250 bar

TFD = 600CpFD = 300 bar

TFD = 700C

pFD = 350 bar

13CrMo44

7CrMoVTiB1010

P92

A617

Figure 10: Design Limits for Water Wall Materials


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The high tube wall temperatures of the superheater heating surfaces as well as lower excess airand low-NOx firing systems increase the corrosion problem. For the selection of supeheater mate-rials the resistance to scale formation from the flue-gas atmosphere and steam is therefore just asimportant as creep resistance.

4.1 Special aspects for CFB

The advantages of CFB technology are uncontested for low-grade fuels or for fuels with widelyfluctuating quality as well as for low exhaust-gas emissions without post-combustion control meas-ures. CFB plants up to capacities of 350 MWe are currently in operation. However, only once-through operation with high steam conditions render CFB technology serious competition for pul-verized-coal firing. A plant for approx. 460 MWe with steam parameters of 560C/580C and 265bar was developed in an EU research program. The BENSON "Low Mass Flux" design was se-lected 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 tem-perature variations between the evaporator tubes are extensively prevented, as non-uniform heatinputs 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 isparallel, thus eliminating the need for elaborate water/steam distribution.

The suitability of this evaporator system for sliding-pressure operation also fulfills all requirementsfor 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 boil-ers. Increasing exhaust-gas temperatures downstream of gas turbines as well as the increasingrequirements on flexibility of a combined-cycle plant with frequent starts also make the use ofonce-through systems interesting here. Elimination of the drum on the one hand increases operat-

ing flexibility and on the other hand is a noticeable cost aspect. In the Cottam combined-cycleplant, a heat-recovery steam generator with a once-through evaporator based on the Benson "LowMass Flux" design was constructed for the first time and runs successfully in commercial operationsince Sept.1999. This evaporator concept is characterized by extremely low mass fluxes which stilllie far below those of fired boilers.


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5 Summary and outlook

Steam temperatures of 600C to 620C are currently possible as a result of efforts in materialsdevelopment. 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 nec-essary for the evaporator in particular in order to give boilers for high-temperature plants similarflexibility to that of previous once-through boilers.

The Low Mass Flux Design provides an evaporator concept which meets the new requirementsand which permits further development to higher steam parameters for pulverized-coal-fired boil-ers and for boilers with circulating fluidized bed firing as well as for heat-recovery steam genera-tors downstream of gas turbines.

A further increase in steam temperatures appears possible in the next years with continuous ma-terials development, but without using nickel based materials not more than 10K to 20K. From thecurrent standpoint, the jump to 700C will not take place until the next decade. However, from an

economic perspective, the high steam temperatures will only be selected given correspondinglycompetitive materials prices and if, among other things, the appropriate main steam and reheatpressures are selected and the fuel ranges are limited.

References

[1] J. Franke and R. KralInnovative Boiler Design to Reduce Capital Cost and Construction TimePower-Gen 2002

[2] J. Franke, R. Kral and E. WittchowSteam Generators for the Next Generation of Power PlantsVGB Power Tech 12/99

38729456 Super Critical Boiler Technology for Future Market Conditions

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