Steam Turbines & Power Plants
BENSON BOILER
Best Choice
Excellent performance features make the Benson boiler the most widely used type
of once-through boiler
by JOACHIM FRANKE and RUDOLF KRAL
Power for generations
Steam Turbines & Power Plants
BENSON BOILER
Best Choice
High capacity and excellent
operating characteristics are main
features of the Benson boiler,
making it the most widely used
type of once-through boiler
by Dr. JOACHIM FRANKE,
expert on Benson boiler technology, in charge of
licensing and continued development of this
steam generator type, Siemens Power Generation
and RUDOLF KRAL,
expert for steam generator technology in the
engineering sector and technical consultant to
Benson licensees, Siemens Power Generation
With a total of more than 1,000 units
having established a proven track record
over many years in the power generation
industry, the Benson boiler is the most
commonly used type of once-through
boiler. It operates at power levels of up to
1300 MW, steam pressures of up to 350 bar
and steam temperatures of up to more
than 600°C. About a quarter of the units
operate at supercritical pressures.
Given its performance features and operating characteristics, it is no surprise
that Benson technology has been successful. Up to now, its use has been most widespread in central Europe, Japan and South
Africa, but numerous countries are showing growing interest in this technology.
Convincing Technology
Steam generators using the Benson design—one pass through the evaporator
section and variable endpoints of evaporation—incorporate features which are critical to economic success in today’s competitive power markets. These features include
• a highly efficient steam/water cycle as a
result of supercritical pressures and high
steam temperatures,
• ability to undergo rapid load changes
due to a design that allows unconstrained
thermal expansion and a variable-pressure operating mode in the part-load
range,
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• insensitivity of steam output and superheated steam
temperature to fluctuating
fuel properties.
Benson boilers are equally
well suited for both subcritical
and supercritical pressures;
steam pressure is limited almost solely by the properties
of the materials used. Due to a
design that allows unconstrained thermal expansion—
no drum, low storage masses—Benson boilers are ideal
for use in the mid- and peakload operating ranges. The
variable-pressure
operating
mode prevents stressing of the
steam turbine materials due
to changes in temperature.
Environmental protection
also benefits due to the low
specific exhaust gas emissions
achieved as the result of high
efficiencies, as well as through
the use of low-NOx combustion methods and proven,
powerful flue-gas cleaning
processes.
Performance Features
Highest Steam Pressures and
Temperatures
Supercritical steam pressures were already being used
in central Europe in the 1960s,
while superheated steam temperatures remained in the
540°C/560°C range until the
mid-1990s. The 550-MW Unit 5
Furnace of a 930-MW lignitefired unit under construction
Steam Turbines & Power Plants
pact combustion chamber which, along
with the two separate flue-gas paths, provides advantages in terms of installation
options. Moreover, the separate cross sections of the second pass and combustion
chamber make it possible to optimize the
configuration of the convective heat-exchange surfaces, and thus also their size. A
typical example of the double-pass design
is the hard-coal-fired steam generator at
the Hemweg power plant in The Netherlands.
at the Staudinger power plant in Germany
is representative of this level of technology with supercritical steam pressure at
full load and outlet temperatures of 545°C
(with HP superheater) or 562°C (with reheater). Other features of this unit include
variable-pressure operation and a net efficiency of 43%.
Nordjyllandsvaerket plant
410 MWel; imported coal
270 kg/s main steam
310 bar, 582°C/580°C
Denmark
Staudinger plant, Unit 5
550 MWel; hard-coal-fired
417 kg/s main steam
285 bar, 545°C/562°C
Germany
At these steam temperatures, wellproven and cost-effective ferritic materials are adequate for the end stages of the
high-pressure and reheater heat-exchange surfaces. At higher temperatures,
more advanced ferritic chromium steels
are required for the thick-walled components, while austenitic materials are required for the final superheater heat-exchange surfaces. The value of such materials has been proven over many years of
service in the Benson boilers of the Danish
Nordjyllandsvaerket and Skaerbekvaerket
power plants, for example, at steam temperatures of 580°C. Operating experience
at 600°C has to date been gained only
from boilers in Japan.
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Today, state-of-the-art developments in
materials permit steam temperatures of
up to 600°C (with HP superheater) or
620°C (with reheater).
Variable Designs
Depending on plant-specific requirements, Benson boilers are of single- or
double-pass design.
The single-pass steam generator has
clear operational advantages when coals
with a high ash content or severely corrosive ash are used, since there is no reversal
of the flue gases and thus no additional
ash concentration in the vicinity of the
heat-exchange surfaces. All modern European lignite-fired steam generators are
therefore of the single-pass design. Other
advantages of this type of steam generator include single-wall design with its favorable unconstrained thermal expansion
characteristic.
The double-pass steam generator is
characterized by its low height and a com-
Hemweg plant
660 MWel; imported coal
530 kg/s main steam
261 bar, 540°C/540°C
The Netherlands
Flexible Use of Fuel
Since Benson boilers can be designed to
accommodate a wide range of coal types,
plant operators are able to use the lowestpriced coal available. The combustion
chamber size and shape, the firing system
and the thermodynamics of the boiler are
then matched to the combustion properties of the range of fuels in question. Due
to the variable endpoint of evaporation,
the combustion chamber can be dimen-
Steam Turbines & Power Plants
sioned solely in accordance with the corresponding firing characteristics.
One advantage of Benson boiler operation is that the required steam output and
main steam temperature are always
achieved, even if the properties of the coal
are highly variable. A prime example is
the Benson boiler at the Staudinger power
plant, which is capable of firing a wide
range of coals—from highly volatile hard
coals up to and including anthracite—in
its amply dimensioned combustion chambers with opposed firing.
Benson boilers can be outfitted with
various types of firing systems. In addition to conventional pulverized-coal firing, systems with liquid-ash separation,
with circulating fluidized beds, for example, as well as systems for firing biomass
can be used. Examples are the boiler with
liquid-ash removal at the 660-MW Ibbenbüren power plant that has two slag-tap
furnaces with a common gas radiation
chamber, as well as the boiler with circulating fluidized-bed firing at the 100-MW
Ibbenbüren plant
750 MWel; anthracite
600 kg/s main steam
220 bar, 530°C/530°C
Germany
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Moabit power plant. At the Moabit plant
the fluidized-bed coolers, in addition to
the walls of the combustion chambers
and the ash cyclone, are also utilized as
evaporators.
In Demand Worldwide
In Germany, hard-coal-fired Benson
boilers in the 800-MW to 1000-MW output range have been operating successfully and at high availability levels for several
years. The two 930-MW units at the Lippendorf power plant achieved this same
output using lignite, a much more problematic fuel. A steam generator of the
same capacity is also installed at the Boxdorf power plant. These Benson boilers are
among the largest in the world, with a
combustion chamber cross section of
24×24 m and a boiler height of more than
160 m.
As early as the 1950s and 1960s, a number of power plants built in the U.S. were
operating at supercritical steam pressure.
Since the boilers used at that time were
Lippendorf plant
930 MWel; lignite
672 kg/s main steam
285 bar, 554°C/583°C
Germany
only marginally suitable for variable-pressure operation, American manufacturers
today are also setting their sights on modern Benson boiler technology. Although
the American market has seen almost no
new plants built since the mid-1980s, this
technology is being used successfully for
export.
In Japan, the development trend in recent years has been toward very high
steam pressures and temperatures. Since
1980, once-through boilers have been essentially the only type used in power
plants in Japan, about half of them Benson boilers operating at supercritical pressures. Steam generators operating superheated steam temperatures of 600°C and
with capacities of up to 1000 MW have already been in operation for many years. In
Japan, traditional preference has been given to the double-pass design, such as that
used at the Hekinan power plant. Due to
the low ash content of the coals used, the
operational advantages of the single-pass
design with regard to wear and tear of the
heat-exchange surfaces would not be significant.
Hekinan plant
700 MWel; imported coal
639 kg/s main steam
255 bar, 543°C/569°C
Japan
Steam Turbines & Power Plants
At the beginning of the 1970s, South
Africa recognized the advantages of Benson boiler technology. Since that time,
twenty-four large units with ratings between 500 and 730 MW have been outfitted with Benson boilers. The single-pass
design is preferred due to the high ash
content of the coal in that region.
In China, both drum-type and oncethrough boilers are used for unit ratings of
up to 300 MW. In the future, plants in the
700/900-MW class will be designed only
for supercritical steam pressures and temperatures. A supercritical, hard-coal-fired,
double-pass Benson boiler is currently being designed for the 700-MW Taishan
plant.
Leading manufacturers around the world
are marketing the
advantages of Benson boiler technology through licensing
agreements. At
Siemens, activities
related to the Benson boiler include
boiler design, thermodynamic and hydraulic design, startup systems, control
concepts and operating principles.
Licensees for Benson Boiler Manufacture
Austria
Austrian Energy & Environment
Denmark
FLS miljØ (Burmeister & Wain)
England
Mitsui Babcock Energy
Germany
Babcock Borsig Power
(Deutsche Babcock, Steinmüller)
India
BHEL
Italy
Ansaldo
Japan
Babcock-Hitachi
Kawasaki Heavy Industries
USA
Babcock & Wilcox
Foster Wheeler
Outlook on Further Development
Worldwide use of the Benson boiler is
in no small measure the result of ongoing
efforts at Siemens Power Generation to
further develop this technology. The expanded knowledge base obtained
through detailed studies, particularly of
the heat transfer mechanisms within the
combustion chamber tubes, has made an
important contribution to this effort.
New evaporator designs with vertical
tubing will continue to improve operating
behavior and make manufacture of the
boilers more cost-effective. An initial reference boiler that employs this new evap-
Hekinan
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Hemweg
Siemens power journal online, October 2001
orator design is already under development for refurbishment of a coal-fired
300-MW steam generator in China.
The Benson boiler is also ideally suited
for even higher steam pressures and temperatures (designs for temperatures of
700°C with plant efficiencies of more than
50% are being studied as part of the EU‘s
THERMIE project) due to its variable, freely
expandable construction and high operating flexibility.
■
Nordjyllandsvaerket Staudinger
Ibbenbüren
Lippendorf