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Advanced Ultra-SupercriticalPower Plant (700 to 760C)Design for Indian Coal

Technical PaperBR-1884

Authors:

P.S. Weitzel, PE

J.M. TanzoshB. Boring

Babcock & Wilcox

Power Generaton Group, Inc.

Barberton, Ohio, U.S.A.

N. Okita

T. Takahashi

N. Ishikawa

Toshiba Corporaton

Tokyo, Japan

Presented to:

Power-Gen Asia

Date:

October 3-5, 2012

Locaton:

Bangkok, Thailand

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2 Babcock & Wilcox Power Generation Group

to note is the High Performance Steam System (HPSS)

sponsored by the U.S. Department of Energy (U.S. DOE) in

the mid-1990s and conducted by Solar Turbines to develop

a 1500F/1500 psig steam turbine for cogeneration. A 4 MW

steam turbine, throttle control valve, and once-through steam

generator was constructed and operated at full temperature

conditions [5]. By making an assessment of the previous

A-USC component testing already conducted, and then

completing the further testing of newer component materi-als, the industry and future owner-operators will be able to

reduce the risks for proceeding to the prototype plant phase.

The next step to take is the planning, design, fabrication,

construction and operation of the lead prototype +700C dem-

onstration plants. Demonstrating the commercial viability

and success in meeting the value required in the market place

is the next milestone. Proving the capability of the supply

chain, actually performing the plant installation, and placing

the equipment in the control of the personnel that operate

and perform maintenance will demonstrate whether or not

the risks are acceptable to the industry.

The designs for A-USC boilers started with the currently

acceptable congurations which are two-pass or tower style

arrangements [6, 7]. The earliest proposed designs have been

employed for about the last half century. An important need

is the recognition to carefully consider the arrangement of

the steam generator and the location in the plant arrangement

relative to the steam turbine because of the need to use high

cost nickel alloy steam leads. The steam generator steam

outlets would be moved closer to the steam turbine inlets

by physical repositioning or by using non-conventional ar-

rangements. There have been designs proposed to arrange

the boiler to have horizontal gas ow or to put part of the

steam turbine up near the top of the steam generator close

to the superheater outlet to shorten the high energy steampiping. Traditionally, the power industry is very conserva-

tive and reluctant to accept radical changes to designs and

procedures before adequate testing and demonstrations are

conducted to prove the economics and technical benets.

However, there appears to be some recognition in the power

generation community of the need for a new arrangement

paradigm. There are earlier steam generator design arrange-

ments that employ some of these proposed features.

Steam generator materials development

The advancement to 700C steam temperatures for coal

ring represents an increase of 166C (300F) above the av-erage predominant operating experience. The current state-

of-the-art plant uses 600C (1112F) technology, which is not

widely applied so that vast industry experience is lacking

or limited in the knowledge base. The introduction of the

700C technology has to overcome the reluctance to adopt

A-USC plants by conducting development programs that test

and demonstrate that the risk is acceptable for a very capital

intensive industry. Pioneering technology introductions such

as American Electric Power Philo 6 (31MPa, 621C, 565C,

538C) in 1957, and Philadelphia Electric Eddystone 1 (34.5

MPa, 649C, 565C, 565C ) in 1959 can provide valuable

lessons in meeting challenges [8, 9]. The low height of the

Philo 6 steam generator should be noted as it provides an

example of a conguration that is extremely different than

currently accepted boiler congurations, and has the feature

of reducing the length of steam lead run to the steam turbine.

See Figure 1.

B&W PGG is a member in the consortium for the U.S.

DOE/OCDO Materials Development Program for A-USCalong with other suppliers and research organizations. The

major aspects of this program are to perform work tasks in:

conceptual design and economic analysis, mechanical prop-

erties, steam side oxidation, reside corrosion, weldability,

ease of fabrication, coatings, and design data and rules.

As an example, the program developed a new formula for

calculating material thickness, Appendix A-317, adopted by

Section I of the American Society of Mechanical Engineers

(ASME) code, and the code case acceptance of alloy INCO

740 nickel for pipe and tube in the ASME I Code. This and

other new materials allow for improved design performance

of a +700C steam generator [6]. There is also a DOE/OCDO

program associated with the development of steam turbine

materials with participation of major turbine suppliers.

Welding development

The DOE/OCDO Boiler Materials Program, has de-

veloped the necessary welding procedure and weldment

property data for several new alloys. Dissimilar alloy welds

for many tubing combinations were performed and tested.

For thicker sections representing pipe and headers, large

plates and pipes have been welded and procedures qualied

in thicknesses never previously needed for boiler service.

Nickel alloy plates of 617, 230 and the new INCO 740 ma-terial for boilers were the selected candidates. Nickel alloy

282 has been more recently included in the program and

work is in progress to gain experience and develop welding

procedures for ASME Code acceptance.

Fig. 1 AEP Philo 6 universal pressure steam generator,

B&W Contract UP-1.

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Babcock & Wilcox Power Generation Group 3

Materials selection

The materials shown in Table 1 are available for applica-

tion in steam generators designed to ASME Section I, except

for Haynes 282 which is being prepared for submittal of a

code case. Alloy 740H was approved in Code Case 2702.

For current design studies, the materials chosen are carbon

steel, T12, T22, T92, 347HFG and 740H. Nickel alloys

617 and 230 are also candidates, but have lower allowable

stress properties than 740H. For economic reasons, 740H

presently has the advantage for tube and pipe selection where

lower weight will be required. This material has the highest

strength, as well as very good steam side and reside cor-

rosion resistance, at a price per weight comparable to the

other candidate alloys.

The allowable stress values for materials listed in Table

1 are graphically depicted in Figure 2.

Steam generator confguration

New boiler arrangements have been proposed that pri-

marily change the steam lead terminal point on the steamgenerator. There has been some consideration for placing the

boiler partially in the ground and/or raising the steam turbine

pedestal. Some have considered dividing the turbine; for ex-

ample, the high pressure (HP) and intermediate pressure (IP)

sections could be located at the higher elevation and the low

pressure (LP) section could be located at the conventional

pedestal condenser location. One designer has proposed a

boiler design that lays the boiler down with a horizontal gas

ow and the steam turbine immediately at the side.

For U.S. coals the B&W PGG design arrangement for

the conventional 600C boiler is a two-pass type (Carolina)

with parallel gas path biasing for reheat steam temperature

control, shown in Figure 3. The two-pass boiler is considered

to have the following advantages:1. shorter steel structure than a tower design,

2. time savings of parallel construction sequence,

3. less complicated high temperature tube sections support,

4. more economical to erect, and

5. less sootblowing required to clean the pendant surfaces

than high temperature horizontal surfaces in the tower

design.

The tower design is considered to have the following

advantages:

1. better gas ow distribution resulting in lower tube

metal upset temperatures,

2. wider tube spacing allowing high fuel ash removal to

a single furnace hopper,3. more drainable heating surface,

4. steam lead outlets positioned closer to the steam

turbine,

5. increased ability to handle internal oxidation exfolia-

tion with distribution along the tubes, and

6. ability to successfully re high fouling brown lignite.

The modied tower combines features of both designs:

the structure is shorter than the standard tower design, and

steam leads are shorter and nearer to the steam turbine.

B&W PGG is also developing a modied tower for A-

USC (a folded tower similar to the two-pass style with

horizontal tube banks) and parallel gas path biasing in the

downpass. This arrangement is not new to the industry. Foran A-USC boiler using Indian coal, the modied tower with

gas recirculation (GR) is used in a series back pass arrange-

ment because the gas velocity limits are very low due to

the very high ash content in the Indian coal and the heating

surface area becomes larger and less effective without GR.

(See Figure 4.) To achieve a wider range of reheat (RH)

temperature control turndown, gas bias with a parallel pass

and GR might also be included. RH control priority in this

design would be to position the biasing dampers and then

complement with gas recirculation ow.

Table 1Materials Selection for Steam Generator Components

Alloy Composition

(Nominal) Application

210C, 106C Carbon steel Econ, piping, headers

T12 1Cr-0.5Mo Water walls

T22 2.25Cr-1Mo Water walls, RH

T232.25Cr-1.6W-V-

NbWater walls, RH

T91 9Cr-1Mo-V Water walls, RH

T92 9Cr-2W Water walls, RH, piping

347 HFG 18Cr-10Ni-Nb SH, RH

310 HCbN 25Cr-20Ni-Nb-N SH, RH

Super 304H18Cr-9Ni-3Cu-

Nb-NSH, RH, piping, headers

61755Ni-22Cr-9Mo-

12Co-Al-TiSH, RH, piping, headers

23057Ni-22Cr-14W-

2Mo-LaSH, RH, piping, headers

740H50Ni-25Cr-20Co-

2Ti-2Nb-V-AlSH, RH, piping, headers

28258Ni-10Cr-8.5Mo-

2.1Ti-1.5AlPiping, headers

Fig. 2 Expected material ASME I Code allowable stress.

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Design of a steam generator using indian coal

Indian coals that are widely used for new power projects

in India are generally low sulfur content and have a reduced

likelihood of reside corrosion problems. U.S. western coals

from the Powder River Basin (PRB) are similarly low in

sulfur and have fared much better regarding coal ash corro-

sion testing. Low sulfur U.S. coals are preferred in the fuels

selection for A-USC applications by providing lower risk to

reside corrosion. Low sulfur Indian coals are expected to

have this same lower risk for reside corrosion.

However, the ash content of Indian coals is very high

and the silica/quartz content is very high, thus very erosive,

requiring much lower gas velocities passing through theconvection tube banks, about 50% less than a higher grade

U.S. coal. Special erosion protection provisions are also re-

quired on the pulverizers and boiler components. The impact

to the design arrangement and cost is signicant. The size

of the gas ow area increases about 50% and the amount of

heating surface increases due to lower heat transfer rates.

Compared to a boiler using U.S. eastern bituminous coal,

the furnace of a boiler using Indian coal is about 78% larger

in volume and about 50% taller. Lower furnace exit gas

temperatures are specied. The furnace width is about 38%

more, impacting the length of the nickel alloy superheater/

reheater outlet headers. Furnace wall average absorption

rates are lower while the peak rates will be expected to be

nearly the same. Staged ring for nitrogen oxides (NOx)

reduction may be required at some plants. The lower furnace

walls may be fabricated starting with lower chrome steel,

T12, and T22 for the middle water walls. The A-USC upper

water walls of the furnace will operate at about 55C (100F)

higher temperature than current practice and thereby require

different material. At this higher temperature T92 tubing

is preferred for wall construction and brings new welding

procedures to the furnace erection requirements. B&W PGG

has been performing R&D on T92 panel fabrication, erection

and repair procedures [6].

Convection heating surface is arranged sequentially as

follows: 1) from the furnace exit plane with the primary

superheater platen, 2) three superheater banks in parallel gas

ow, 3) reheat outlet banks in parallel/counter ow, 4) over

the pendant crossover with the pendant reheat inlet bank,

5) primary superheater banks interlaced with the horizontal

reheat banks, and 6) economizer banks.A vertical or spiral tube furnace enclosure may be used

based on the steam ow to perimeter ratio. With Indian

coal and its larger furnace perimeter requirement, a spiral

design is used. The heating surface arrangement and steam

temperature control method will need to result in component

operating temperatures that change very little versus load,

refer to reference [6]. It is desirable not to have large magni-

tude changes in the material temperature of thick components

like the superheater and reheater outlet headers. Rapid cyclic

temperature changes will cause fatigue damage and reduce

component life. The vertical steam separator (VS) is a thick

wall component that must be located in the steam generator

ow sequence considering the cyclic temperature changes

of start up and load changing. The location will also impact

the Benson point load where the steam generator will begin

to operate in once-through mode.

Fig. 3 Conceptual design of a two-pass (Carolina) A-USC

boiler using U.S. coal.

Fig. 4 Conceptual design of an 840 MW modied tower

A-USC.

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A-USC steam generator control andoperation

The B&W PGG A-USC plant design operates at full load

above critical pressure 22.1 MPa (3208 psia) and on a vari-

able pressure ramp at lower load so it is capable of permitting

appropriately located dryout to occur when the furnace is in

the subcritical pressure two-phase region.

Control must handle the transition from the minimumcirculation ow recirculating mode for initial ring using the

boiler circulation pump to the once-through mode where all

the water entering the economizer leaves from the superheat-

er outlet. Control of the equipment must achieve cold startup,

warm restarts, hot restarts, load cycling and shutdown. The

load where the vertical steam separator runs dry is called the

Benson point. For A-USC, this is estimated to be at about

45% load. At this dry separator point, the boiler circulation

pump is shut off and the boiler feed pump is controlled so

the feedwater ow will meet the demanded furnace enthalpy

pickup function (from the economizer outlet to the primary

superheater inlet) in once-through operation. Final steam

temperature control range meets set point from about 50 to100% load. Reheat steam temperature control range meets

set point from about 60 to 100% load.

Steam temperature is controlled by multiple stages of

spray attemperation. The steam temperature control for

faster transients must account for the time delay of the wa-

ter entering the economizer to leave the superheater outlet,

which takes about 15 minutes at minimum circulation ow

load, and about 3 minutes at maximum continuous rating

(MCR) load.

A-USC turbine conditions

The OCDO/DOE study conditions at the boiler terminalsare: 792 MW gross; 34.6 MPa (5015 psia) throttle and 36.2

MPa (5250 psia) superheater; 735.6C (1356F) / 761C (1402);

333C (631F) feedwater; 508.4 kg/s (4,035,000 lb/hr) main

steam; and 389.2 kg/s (3,089,000 lb/hr) reheat. HP cooling

steam is 7.6 kg/s (60,318 lb/hr) at 566C (1050F) from the

primary superheater outlet. Since this study began (in 2002),

the throttle pressure has not been changed, although a lower

pressure is now considered very likely. Studies in the 1980s

and early 1990s used 44.8 MPa (6500 psi) [10]. The avail-

able energy with a given steam temperature reaches a at

gradual optimum as a function of pressure so that the expense

of high design pressure will increase cost more rapidly than

the benet to thermal efciency.

In the current B&W PGG and Toshiba design study using

an Indian coal specication, the steam conditions are: 30 MPa

(4350 psia), 700C (1292F throttle / 730C (1346F) reheat,

330C (626F) feedwater, to produce 840 MW gross generation.

Turbine rotor welding development

The rotor is one of the largest components of the steam

turbine system. (See Figure 5.) The weight of a high pres-

sure turbine rotor or intermediate pressure turbine rotor isover 20 tons.

The materials and design for rotors are unique to each

manufacturer. Rotors for Toshibas A-USC design involve

welding nickel-based alloy and ferrite steel to minimize

use of expensive nickel-based alloy, and due to difculties

in producing a large ingot for mono-block nickel-based

alloy rotors. As shown in Figure 5, the middle of the rotor

is nickel-based alloy, and the ends are ferrite steel. Actual

size weld trials are being conducted that test welding nickel-

based alloy TOS1X-II to ferrite steel, and welding TOS1X-II

to TOS1X-II. The welds will be evaluated for mechanical

properties later this year.

Turbine materials

Table 2 shows candidate materials for application in

high temperature turbine components designed by Toshiba.

For current A-USC turbine design studies, it is necessary

to apply nickel-based alloys for rotor forging materials.

Nickel-based alloys for rotors are required for high creep

strength at elevated temperatures. (See Table 3.) The ability

to forge and weld are also important issues for large rotor

production. The castings of a steam turbine are large struc-

tures with complex shapes that must provide the pressure

containment for the steam turbine. The major requirement

for casing materials is the ability to cast them into the

required size and shape through the air casting process.

Weldability is also an important issue for pipe connecting

Fig. 5 Steam turbine welded rotor.

Table 2Candidate Materials for High Temperature Turbine

Components

Components Properties Candidate Materials

Rotor

High creep strength

Good forge-ability

in the large size

Good weld-ability

TOS1X

TOS1X-II

Casing

High creep strengthGood cast-ability

in the large size

Good weld-ability

Alloy625

TOS3X

etc.

Valve chest

High creep strength

Good cast-ability

in the large size

Good weld-ability

Alloy625

TOS3X

etc.

Blade and boltHigh creep strength

Machine-ability

Alloys used in gas

turbines (U520,

IN738LC, etc.)

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and repair welding. Blades and bolts will also be made of

nickel-based alloys.

Figure 6 shows the creep rupture strength of conventional

steels and nickel-based alloy for turbine rotor candidates.

TOS1X and TOS1X-II have higher creep strength than al-

loy 617. These materials have demonstrated good forging

and welding characteristics. For turbine casing and valve

chest, TOS3X provides potentially better creep strength

than alloy 625.

Steam turbine confguration

Figure 7 shows a conceptual drawing of Toshibas single

reheat steam turbine system of a 840 MW power plant with

30 MPa (4350 psi), 700C (1292F) for main steam, and 6

MPa (870 psi), 730C (1346F) for reheat steam. It consists

of a single ow HP turbine, a double ow IP turbine, and a

double ow LP turbine with 48 in. last stage blade length.

For the HP and IP turbines, nickel-based alloys are ap-

plied to those parts directly in contact with high temperature

steam. These include the inner casing, high temperature

rotor, nozzle box (rst stage nozzle), and higher tempera-

ture nozzles and blades. The other parts are constructed of

conventional ferrite steel. Rotor designs require dissimilarwelding nickel-based alloys (TOS1X-II) and ferrite steels to

minimize the weight of expensive nickel-based alloys, and

due to difculties in producing a large mono-block nickel-

based alloy ingot. Conventional cast steel can be used for

the outer casing because high temperature steam is isolated

by cooling steam.

Materials and conguration for the LP turbine are similar

to that for the 600C class USC turbine.

Performance

A comparison of the technical operating parameters be-

tween 600C USC and 700C A-USC is summarized in Table

4. Thermal efciency is improved by 6% with 700C A-USC

steam conditions.

Steam turbine generator consists of one single ow HP

turbine, one double ow IP turbine, one double ow LP tur-

bine, and one generator in a tandem arrangement. (See Figure

8.) The overall length of the turbine-generator is 42 m. The

LP turbine is downward exhaust. The turbine is rated at 840

MW gross with steam inlet conditions of 30 MPa and 700C,

reheat to 730C. The rated speed is 3000 rpm.

Main steam from the boiler ows through the four main

stop valves and four control valves and enters the HP turbine.

It expands through the HP turbine and exhausts as cold reheat

to the boiler. Hot reheat steam from the boiler ows through

the four reheat stop valves and four intercept valves and

enter the IP turbine. It expands through the IP turbine and

then enters the crossover piping, which transports the steam

to the LP turbine. The steam expands through the LP turbine

and exhausts into the condenser. The steam turbine is oper-

Fig. 6 105hour creep rupture strength of turbine rotor alloys.

Table 3Chemical Composition of Nickel-Based Alloys for Turbine Rotors

Ni C Cr Al Ti Mo Co Ta Nb

Alloy 617 Bal.0.05 ~

0.15

20.0 ~

24.00.8 ~ 1.5

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ated in throttle governing, sliding pressure. Sliding pressure

improves the efciency of partial load operation.

The turbine provides for nine feedwater extraction points.

Final feedwater temperature at full load is 330C with de-

superheaters. The steam turbine exhaust pressure at design

conditions was selected based on typical Indian conditions at

the condenser. The electrical generator is rated at 1050 MVA,

50Hz with a power factor of 0.85.

Table 5 lists the turbine and major auxiliary equipmentdesign parameters.

A-USC steam generator and steamturbine cost allowance with U.S. coal

In 2003 as part of the DOE/OCDO Boiler Materials De-

velopment project, the current U.S. two-pass steam generator

design arrangement was evaluated on the basis of economic

viability.[2, 3] It was determined that with the improved heat

rate for the 750 MW net A-USC plant, the breakeven cost of

electricity was attainable when the capital cost was within

13% above the cost of a conventional subcritical plant. The

higher plant efciency allowed cost reductions because of

the lower fuel cost per MW and smaller size of the equipment

for the steam generator and the boiler balance of plant (fuel

handling, emissions systems, fans and auxiliary power, etc.).

The steam generator cost would need to stay within 40%above the cost of the steam generator of a subcritical plant.

The DOE/OCDO A-USC steam generator had 7% more

suspended weight than the conventional supercritical unit

while it was 20% narrower. The narrower arrangement

reduces the cost of the alloy headers and piping. There is a

13% weight increase of the overall tubing due to the lower

temperature difference of the ue gas to steam resulting in

lower heat transfer rates. The resulting cost estimate for

the A-USC steam generator was 28% above the subcritical

boiler and within the 40% allowance.

Nickel alloy tubing is estimated to cost 46 times the cost

of a T22 tubing. A current estimate is being developed to

adjust to a newer assessment of nickel alloy cost for thesteam leads between the boiler and steam turbine. The al-

lowance for capital cost of the steam generator and steam

turbine is expected to require less than a 25% increase over

a subcritical plant.

Conclusion

A primary need in A-USC development is to conrm the

capability of suppliers to support the new materials required

and to meet the schedule demands so plant projects may be

initiated. Suppliers will need to make investments based on

increased certainty of the timing when the A-USC market

demand will form. First generation demonstration plants are

needed to establish a working understanding of the necessary

relationships and put into practice the procurement standards

for A-USC components.

The value of owning an A-USC power plant will be

determined by the balance of lifecycle cost saving of the

impact to resource demands and infrastructure requirements

with the increased capital cost of using nickel-based alloys.

Table 4Operating Parameters for 600C USC and 700C A-USC Turbine Arrangement

600C USC 700C A-USC

General output 840 MW 840 MW

Main steam (pressure and temp.) 24.1 MPa, 600C 30 MPa, 700C

Reheat steam (pressure and temp.) 4.3 MPa, 600C 6.0 MPa, 730C

Condenser pressure 683 mm Hg vac. 683 mm Hg vac.Boiler feedwater temp. 292C 330C

Thermal efficiency Base 6% improvement

Table 5Turbine and Major Auxiliary Equipment

Steam turbine Tandem-compound(three casings)

High pressure section Single flow

Intermediate pressure section Double flow

Low pressure section Double flow

Rated speed 3000 rpm

MSV/CV 4 valves

CRV 4 valves

Overload valve 1 valve

Feedwater pumps Electrically driven

Heater

De-superheater 1 or 2

HP heater 4 heaters

Deaerator 1 deaerator

LP heater 4 heaters

Generator

Number of poles 2

Power factor 0.85

Rated output 1005000 kVA

Cooling Water

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5. Duffy, T., et.al., Advanced High Performance Steam

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CE/40746-TI (1987)

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Supercritical Power Plants 700 to 760C, ASME

Power 2011, Denver, CO (2011).

7. Rao, K.R., (ed.), Energy and Power Generation Hand-

book, ASME, New York, 2011.

8. Kitto, JB, Stultz, SC., Steam/its generation and use,

Edition 41, The Babcock & Wilcox Company, Bar-

berton, OH 2005.

9. Silvestri, G.J., Eddystone Station, 325 MW Gener-ating Unit 1-A Brief History, ASME, March 2003.

10. Silvestri, G.J., et.al., Optimization of Advanced

Steam Condition Power Plants, Diaz-Tous, I.A., (ed.),

Steam Turbines in Power Generation PWR-Vol. 3,

Book No. H00442, ASME, 1992.

Benson is a registered trademark of Siemens AG.

Copyright 2011 by Babcock & Wilcox Power Generaon Group, Inc.

a Babcock & Wilcox company

All rights reserved.

No part of this work may be published, translated or reproduced in any form or by any means, or incorporated

into any informaon retrieval system, without the wrien permission of the copyright holder. Permission re -

quests should be addressed to: Markeng Communicaons, Babcock & Wilcox Power Generaon Group, Inc.,

P.O. Box 351, Barberton, Ohio, U.S.A. 44203-0351. Or, contact us from our website at www.babcock.com.

Disclaimer

Although the informaon presented in this work is believed to be reliable, this work is published with the

understanding that Babcock & Wilcox Power Generaon Group, Inc. (B&W PGG) and the authors are supplying

general informaon and are not aempng to render or provide engineering or professional services. Neither

B&W PGG nor any of its employees make any warranty, guarantee, or representaon, whether expressed or

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