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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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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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Benson is a registered trademark of Siemens AG.
Copyright 2011 by Babcock & Wilcox Power Generaon Group, Inc.
a Babcock & Wilcox company
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