Products
BENSON Boilers—
Experience in Nearly 1000 Plants and
Innovative Design Promise
Continuing Success
Manfred Klein, Rudolf Kral, Eberhard Wittchow
In Central Europe, oncethrough boilers have been the
dominant design for more than
30 years, and this boiler design
has also become established in
Japan and South Korea. Growth
countries, too, such as those in
Southeast Asia, are now beginning to switch from subcritical
drum boilers to once-through
steam generators. The BENSON
boiler is the most common implementation of the once-through
design, and it is in use in nearly
1000 plants worldwide. Based
on a patent registered to Mark
Benson, it was developed in the
1920s by Siemens, who initially
manufactured this type of boiler.
Its introduction brought highpressure design to the field of
power plant operation. After
Siemens closed down its boiler
manufacturing operations, it
awarded licenses for the manufacture of BENSON boilers beginning in 1933. Today, the leading
manufacturers of once-through
boilers worldwide are licensees
(Fig. 1).
The activities handled by
Siemens, which cover both the
boiler itself and its incorporation
into power plants, have resulted in
26
Siemens Power Journal 1/96
important innovations in power
plant construction such as oncethrough boilers with variable
evaporation end point, generously dimensioned furnaces, the sin-
Fig. 1
BENSON boilers:
Manufactured by
licensees and
sub-licensees
around the world.
Siemens is licensor for BENSON boilers
Siemens activities include:
Boiler concepts, thermal-hydraulic design,
arrangement of heating surfaces, start-up systems,
control systems, interaction of boiler and turbine,
operating modes, and R&D.
Licensees
Sublicensees
Austria
Austrian Energy &
Environment
(SGP/Waagner-Biró)
Denmark
Vølund Energiteknik
Denmark
Burmeister & Wain Energi
England
Mitsui Babcock Energy
Germany
Deutsche Babcock
Steinmüller
Italy
Ansaldo
Japan
Babcock-Hitachi
Kawasaki Heavy Industries
USA
Babcock & Wilcox
Finnland
Tampella
Mexico
Babcock & Wilcox
de Mexico
Netherlands
Stork Boilers
South Africa
Babcock Africa
South Korea
Halla
Hyundai
Spain
Babcock Espanola
12 pj 1/96 e 04
The unique advantages of the
BENSON boiler are its
suitability for use over a
pressure range of 100 to more
than 350 bar without
modifying the evaporator
system, and its exceptional
suitability for sliding
pressure operation. An
innovative evaporator design
now combines these
operational advantages with
the structural ones of a
vertical-tube combustion
chamber. A low-cost BENSON
boiler with outstanding
operating characteristics can
therefore be realized.
Highest efficiency Use of all
coal grades
Temperature
(545 – 580°C)
Enthalpy
Load
4 – 6 %/min
Modes of
operation
Critical
point
Burners
Pressure (load)
100 to over 350 bar
Load
Constant main
Suitable for
Wide scope in
steam temperature
subcritical and
design, e.g.,
regardless of load,
supercritical
oversized
fuel and degree
pressure without combustion
of fouling.
changing the
chamber or slag
Economical,
evaporator system tap furnace (shown) low-stress start-up.
A variety of different designs
and firing systems make BENSON
boilers exceptionally well-suited
for use in today’s different power
plant environments: In addition to
the trend toward increased efficiency and reduced investment
costs, these boilers also provide
high availability, long times between overhauls, can be used
with a broad range of fuels, and
are characterized by low pollutant
emissions.
Burner
Combustion chamber
Evaporator tubes
Ash hopper
Convection heating
surfaces
Support bars
Start-up system
Recirculation pump
deNOx system
Air heater
Flue gas
Air intake
Coal mills
5
7
9
3
• The single-pass design of the
boiler is structurally straightforward, which among other things
eliminates the need to weld enclosure walls to one another,
which exhibit different temperatures in operation, causing thermal stresses.
• The ash concentration in the
flue-gas flow is uniformly distributed, which minimizes wear on
the tubes of heating surfaces.
Rapid load
changes in
sliding pressure
operation
6
7
8
9
10
11
12
13
6
The current state of the art in
Germany is represented by the
BENSON boiler in the 550-MW
Unit 5 of the Staudinger power
plant (Fig. 3):
• All the superheaters can be
drained, which allows the system
to be started up quickly.
Time
1
2
3
4
5
Current Status of
BENSON Boiler Technology
in Germany
Fig. 2
BENSON boilers:
Well-suited for
efficient, economical and flexible
operation.
12 pj 1/96 e 04
The BENSON boiler has
reached an extraordinarily high
degree of technical refinement
and exhibits excellent operating
characteristics (Fig. 2). In addition
to ongoing optimization, Siemens
has developed an innovative boiler design, and is developing measures to increase plant efficiency.
Flexible
operating mode
12
10
2
11
1
4
8
13
12 pj 1/96 e 04
gle-pass design, sliding-pressure
operation and spiral-wound tubing.
Economical,
low-stress operation
Fig. 3
Unit 5 of the coalfired Staudinger
power plant: The
100-meter-high,
550-MW singlepass BENSON
boiler is equipped
with an opposed
firing system.
Siemens Power Journal 1/96
27
Fig. 5
The Deutsche
Babcock whirlstage burner is
characterized by
exceptionally
low NOx emissions with levels
down to 130 ppm
(measured downstream of the
steam generator).
Opposed
Corner
Slag tap
Dry vertical
930 MW
Lignite
930 MW
Hard coal
400 MW
Hard coal
770 MW
Anthracite
350 MW
Anthracite
Burner
IR flame monitor
Ignition burner
Secondary air
Tertiary air
• The airflow and flue-gas passages are designed as single
28
Siemens Power Journal 1/96
Swirl plates
Ignition lance
Core air
Pulverized coal
UV flame monitor
• At full load, the boiler operates
at supercritical pressure and steam
conditions of 260 bar/545/562 °C.
At these temperatures, it is still
possible to use low-cost martensitic steels for the final stages of
the high-pressure and reheater
heating surfaces. The optimum
feedwater temperature was determined to be 276 °C. Net unit
efficiency was measured at 43
percent.
12 pj 1/96 e 04
Tangential
Stabilizer
Swirl insert
trains. This design, which has
also proven effective in several
700-MW units, has been extended at the Staudinger plant to include the scrubber in the flue-gas
desulfurization system as well.
This design simplifies operation
while maintaining the same level
of availability.
One interesting solution was
implemented in the 770-MW unit
at the Ibbenbüren plant. The slagtap firing system used in that
unit burns anthracite and is the
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Fig. 4
The unrestricted
design of the combustion chamber
in the BENSON
boiler with its water/steam section
is particularly
well matched to
the broad range
of firing system
requirements. The
BENSON boiler
can therefore be
used with a number of different
firing systems.
largest of its kind in the world.
The unit can be operated under
part-load conditions at down to
40 percent of full-load capacity
without backup firing, and with
only six percent volatile matter
(water- and ash-free).The BENSON
boiler also operates at full load
with supercritical pressure in the
evaporator.
A Variety of Different Firing
Systems Are Possible
One of the advantages of the
BENSON boiler is the unrestrict-
tured thus far, NOx values measured downstream of the steam
generator have ranged from
400 mg/m3 (200 ppm) down to
260 mg/m3 (130 ppm), depending
on the composition of the fuel.
The new burner design has a
greater influence than previous
models on NOx formation directly
at the point of origin. It features
special inserts in the pulverizedcoal pipe and a swirl flow of combustion air on the secondary and
tertiary sides. In terms of combustion, the burner promotes
Design features implemented
since the end of the 1970s to
reduce NOx emissions on the
primary side have brought
about fundamental changes in
Drum boiler
Subcritical operation
Vertical tubing
BENSON boiler
Subcritical and supercritical operation
Spiral tubing
to superheater
Recirculation
pump
Siemens has further developed
the vertical-tube design for the
BENSON boiler (Fig. 6) not only in
order to reduce investment costs,
but also to facilitate the introduction of boilers operating at supercritical pressure in countries in
BENSON boiler
Subcritical and supercritical operation
Spiral tubing
to superheater
to superheater
Start-up
system
in the combustion chamber walls
(Fig. 6). While this design has
reached an exceptionally high
level of technical refinement,
higher manufacturing and installation costs put it at a disadvantage as compared to a verticaltube design.
Start-up
system
to flash
tank
to
economizer
(Diagram showing two-pass design)
the design of pulverization and
firing systems, including finer
pulverization and graded airflow
with supplementary turbulence
and over-fire air supply, to name
but a few. The whirl-stage burner
designed by Deutsche Babcock
(Fig. 5) is representative of the
high level of technical refinement
which has been achieved. This
burner design meets the pertinent requirements in terms of
flame stability, coal burnout and
minimum NOx emissions. In the
more than 200 burners manufac-
intensive and early coal pyrolysis
in a low-oxygen zone. This also
leads to very stable ignition, and
has the advantage of allowing
the use of a wide variety of fuel
grades without having to change
the burner settings.
Evaporator Design with
Vertical-Tube Combustion
Chamber
A typical feature of BENSON
boilers has been the spiral configuration of the evaporator tubes
12 pj 1/96 e 04
ed design of the combustion
chamber, the walls of which are
designed as evaporator heating
surfaces, allowing the water/
steam section to be particularly
well matched to the requirements
of the firing system. Fig. 4 shows
the firing system designs which
have to date been used in highcapacity BENSON boilers.
Fig. 6
The innovative concept of the verticaltube combustion
chamber with rifled
evaporator tubes
combines the advantages of the
BENSON boiler with
evaporator tubes in
spiral configuration
and those of the
drum boiler. Hence,
a low-cost BENSON
boiler with outstanding operating
characteristics can
be realized.
which the drum boiler (Fig. 6) has
been the dominant design. By using rifled tubes with high heat
transfer capability, it was possible
to reduce the mass velocity in the
tubes from a value of 2000 to
2500 kg/m2 s, which is typical of
the spiral evaporator tube configuration, to less than 1200 kg/m2 s
in the case of the vertical-tube
design. Siemens has made more
than 80,000 measurements during this development initiative.
The resulting advantages in plant
operation are:
Siemens Power Journal 1/96
29
• The evaporator system exhibits
a flow characteristic similar to
that of a natural-circulation boiler
(drum boiler). In this design, an
increase in the heat input to individual tubes increases the flow
rate through them; thus the outlet
temperature is only marginally
affected. This in turn tends to
51
%
50
350 bar
700/
720°C
48
315 bar
300 bar 620/
620°C
270 bar 600/
620°C
585/
0,4
250 bar 600°C
0,7
540/
1,1
560°C
47
46
45
44
43
42
167 bar
538/
538°C
2,1
1,5
Enhanced Efficiency through
New Materials
Fig. 7
Higher steam conditions and higher
efficiencies are
made possible
through the use of
new materials.
F 12
P 91
NF 616 Austenitic Inconel
steel
Material
12 pj 1/96 e 04
41
F 12
• Early changeover to oncethrough operation mode keeps
start-up losses so low that recirculation pumps are not required.
The vertical-tube design, therefore, combines the structural advantages of the drum boiler and
the operating advantages of the
BENSON boiler with evaporator
tubes in a spiral configuration.
This vertical-tube design can be
used for capacities of around 300
MW up to more than 1000 MW,
and it has proven to be a successful, innovative step toward a lowcost, structurally simple boiler
design with superior operating
characteristics.
Net efficiency
49
shutdowns. This is of particular
advantage in the case of hightemperature systems with their
thick-walled components, which
can only be started up and shut
down slowly.
make the outlet temperatures of
the tubes uniform despite varying heat input. An evaporator
heating surface based on this design was successfully tested for a
period of more than 10,000 hours
by Siemens together with Deutsche
Babcock and Steinmüller in the
320-MW Farge unit.
• The minimum load in oncethrough operation can be reduced from the previous value of
35 to 20 percent without reduced
main steam temperatures, thus
avoiding frequent start-ups and
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Siemens Power Journal 1/96
The potential increase in efficiency through the use of new
materials and hence the scope
for attaining higher steam conditions is shown in Fig. 7.
Over the last several years, it
has been possible to develop
non-austenitic materials with increased creep rupture strength.
In power plants currently under
construction, thick-walled components are now able to withstand main-steam temperatures
of 585°C and reheat temperatures of 600 °C, and a further
increase to 630°C is expected.
Given these new developments,
it makes little sense to continue
to use austenitic steel, with its unfavorable behavior, for headers,
piping, and turbine shafts and
casings, since the potential in-
crease in temperature to 650°C
would result in only a marginal increase in efficiency.The European
power plant industry is therefore
looking at the possibility of
achieving steam temperatures of
700 °C, and thus net efficiencies
of more than 50 percent, through
the use of nickel-base alloys. Manfred Klein
received a degree
(Dipl.-Ing.) in mechanical engineering. Since 1970
he has been employed by Babcock
Lentjes Kraftwerkstechnik GmbH,
where he is involved in the design of boiler systems
for power plants, and since 1993 has
served in the capacity of manager for
bid generation with responsibility
for the design and sales & marketing
of large-capacity boilers.
Rudolf Kral
joined KWU in
1970 after receiving a degree
(Dipl.-Ing.) in mechanical engineering. Since then he
has worked in the
field of BENSON
boiler technology,
and is currently responsible for advising BENSON boiler licensees on technical issues, and for continued BENSON
boiler development.
Eberhard Wittchow
joined Siemens in
1958 after receiving a degree
(Dipl.-Ing.) in energy economics.
He has previously
worked in the
fields of design
and commissioning of steam generators and power
plants. At present, he is responsible for
the areas of BENSON boiler licensing
and special steam power plant issues.