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Babcock & Wilcox Company Supercritical (Once Through) Boiler Technology Babcock & Wilcox
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J.W. Smith
Babcock & Wilcox
Barberton, Ohio, U.S.A.
Published: May 1998
Babcock & Wilcox Company
Supercritical (Once Through) Boiler Technology
BR-1658
Abstract
This paper provides an overview of the Babcock & Wilcox
(B&W) Once Through Boiler Technology. This review includes
the history of the boiler development beginning with the worlds
first ultra-supercritical steam system, which began operation at
the American Electric Power (AEP) Philo Station in 1957,
through the development of the worlds largest boilers, the pul-
verized coal fired 1300 MW class. The most recent of the 1300
MW units began operation in 1990 at the Zimmer Power Sta-
tion jointly owned by Cincinnati Gas & Electric, Dayton Power
& Light and American Electric Power. The design features of
this boiler style, which is designed for base load and load cy-
cling operation, are discussed.
Early History
Once through boilers have long been the vision of boiler de-
sign engineers. In the United States, patents for once through
boiler concepts date from as early as 1824. These early inven-
tors were undoubtedly motivated by the desire to improve the
product safety because of the notoriety of pressure vessel fail-
ures associated with the early fire tube and water tube boilers.
While advances in the boiler industry in the late 1800s such as
the developments by The Babcock & Wilcox Company (founded
in 1867) significantly improved product safety, interest contin-
ued in the development of once through boilers both as a way to
eliminate the need for the steam drum and with the hope that
the design would better cope with impurities contained in the
water. B&Ws research in once through boilers dates from 1916
when boiler research was begun at the companys Bayonne, New
Jersey Laboratory. In keeping with the technology of the time,
this early research unit was operated at a pressure of 4 MPa.
The first significant commercial application of once through
boilers was made by Mark Benson, a Czechoslovakian inven-
tor, when he in 1923 provided 4 ton/hr unit for English Electric
Co., Ltd. at Rugby, England. This unit was designed to operate
at critical pressure with the belief that operating at this pres-
sure, where there is not density difference between steam and
water, would avoid boiler tube overheating and solids deposi-
tion. Mark Benson continued his development work which in-
cluded the installation in 1930 of a 113 ton/hr unit in Belgium.
Like the unit for English Electric, this unit was intended to op-
erate at critical pressure. The hoped elimination of problems by
operating at critical pressure, however, was not fulfilled and it
was necessary to reduce the boiler operating pressure to over-
come problems with tube failures. In this case, the boiler
inventors vision outreached the technology available at this time
for both tube materials and water chemistry control. Nonethe-
less, these early units were successful in operation and served
as the foundation for the boiler development work that set the
direction for European boiler development. Mark Bensons con-
cepts were ultimately acquired by Siemens and it is from these
concepts that the Benson Boiler Technology now licensed world-
wide by Siemens was developed.
B&W also continued with their experimental work on once
through boilers in the 1920s as boiler and power plant engi-
neers envisioned the efficiency gains that could be achieved by
the use of ultra supercritical pressure cycles. B&W in 1928 be-
gan experimental work at its research center in a test facility
capable of operation at 34.5 MPa and 520C. This test facility 2
Babcock & Wilcox
was used to examine thermal-hydraulic and heat transfer effects
in the pressure range from 10 MPa to the maximum operating
pressure of the unit. The test rig was then transferred to Purdue
University where research continued in collaboration with B&W.
Much of this work was reported in Technical Papers in the early
1930s by authors such as Kerr and Potter.
Through the 1930s and 1940s power plant operating condi-
tions were limited to the subcritical regime because of limita-
tions of metallurgy and water chemistry control technology. In
Europe, boiler technology followed the once through philoso-
phy. This at least in part was driven by material availability
constraints and took advantage of the fact that the once through
boiler generally used smaller diameter and thinner walled tubes
then did the natural circulation boiler. In addition, the once
through boiler eliminated the need for thick steel plate for the
steam drum. In the United States where material was more
readily available, the technology continued to rely on the natu-
ral circulation boiler design. In both Europe and the United
States, the steam cycles used had similar steam conditions re-
sulting in similar power generation efficiencies. For example,
as early as 1941 B&W had supplied boilers to American Elec-
tric Power for operation at 16 MPa.
B&W Once Through Boiler Development
The era following the second World War brought on rapid
economic development in the United States. The rapid economic
development increased the desire for more efficient power plant
operation. This, coupled with the improvements in both boiler
tube metallurgy and water chemistry technology, brought a re-
newed interest in the supercritical cycle. B&W increased its
research work and in 1951 established another 34.5 MPa heat
transfer test facility at its Alliance (Ohio) Research Center. In
addition, to assimilate the European once through boiler tech-
nology, B&W established a working relation with the then Si-
emens-Schuckertwerke Company, the holder of the Benson tech-
nology, and the Durrwerke Company, at the time the builder of
the more boilers than any other Benson licensee, both of Ger-
many. While the European experience was all for subcritical
cycles, this technology transfer was invaluable in accelerating
B&Ws development of the supercritical application.
The vision of the supercritical power plant was also held by
American Electric Power and General Electric (for the steam
turbine). American Electric Power entered into contract with
both B&W and General Electric to build the worlds first ultra-
supercritical power plant. This 125 MW installation at the Philo
Plant operated at main steam condition of 31 MPa and 621C
with two stages of reheating first to 565C and then to 538C.
The decision to proceed with this plant was made in 1953 and
operation was begun in 1957. While the intent of the plant was
to demonstrate the feasibility of the supercritical pressure cycle,
this unit was commercially operated until 1979.
The boiler used B&Ws cyclone firing technology and was
equipped with three cyclone furnaces. The boiler arrangement,
shown in Figure 1, is based on horizontal gas flow over the
majority of the convection heat transfer surfaces. This arrange-
ment is quite different than the typical boiler arrangement of
today, but was very similar to the natural circulation boiler ar-
rangements of the time, such as the 90 MW, 10 MPa boiler shown
in Figure 2. The furnace tube arrangement was quite different
Figure 1 125 MW AEP Philo boiler (UP-1). Babcock & Wilcox
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from that used in boilers of the Benson technology design. The
Benson technology boilers, because they were designed for op-
eration in the subcritical regime, made use of the meandering
tube arrangement where the tubes passed completely around the
furnace enclosure as a means of obtaining more uniform heat
absorption from tube to tube. This construction was necessary
for the subcritical design as a means to minimize the tempera-
ture difference between tubes. The meandering tube design used
the refractory and skin casing construction.
Figure 2 90 MW drum boiler, 1956 design.
Since the Philo unit operated in the supercritical regime, the
concern for differential heat absorption and the resulting im-
pact on tube temperature difference was not nearly as great.
Therefore, this unit was designed with vertical tubes following
the construction techniques already employed in natural circu-
lation boilers. Following the practice of United States boiler
construction, the design made use of partial membraning to
minimize the amount of refractory and skin casing. In order to
obtain the necessary mass flux within the tubes to provide ad-
equate heat transfer and tube cooling, a multiple pass arrange-
ment was adopted. Because the uni