Operating_Experience.fh8 09.04.2001 8:17 Uhr Seite 1
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COMMUNICATING POWER TECHNOLOGY WORLDWIDE
Coal Fired Plants:
Horizontal Boilers Make 70 0°C Steam Economic
Reprint from
ModernPowerSystems
Author:
David Smith
Power for Generations Siemens Power Generation
Probedruck
Horizontal boilers make
700°C steam economic
A novel design of Benson boiler with horizontal furnace and internally rifled vertical tubes has been developed by
Siemens. In association with the EU-funded Thermie advanced 700°C PF power plant programme it has been
designed for steam conditions of 350 bar/700°C/720°C. The reduced height of the boiler minimises the amount of
expensive high nickel alloys required for the steam lines to a point where the economics are competitive. Mounting
the steam turbine at the level of the boiler steam outlet headers results in further cost reductions.
David Smith
I
n a world of rapidly advancing deregulation, it has become increasingly difficult for coal fired power generation to
compete. Now the pendulum may be
beginning to swing back.
It is not just the forecast of a doubling in
natural gas prices in the next three years that
is driving this trend. Under the influence of
stringent political regulatory regimes, unstable gas prices, the prospect of heavy emissions
trading costs, carbon taxes and the response
to Kyoto, interest in advanced coal fired generation technology is enjoying a resurgence.
Often dismissed by many senior pundits as
having little prospect of economic viability in
the foreseeable future, ultra supercritical coal
fired utility boilers to generate steam at over
700°C and 350 bar is beginning to be more
vigorously pursued, particularly in Europe,
and in Germany and Denmark in particular.
The main hurdle in the past has been the
horrendously high cost of the high nickel alloys needed to withstand these temperatures
and pressures. However, new designs of boiler using a horizontal furnace configuration
have now been developed by Benson licensor
Siemens that greatly reduce the impact of the
cost of high nickel alloys (see Figure 1). The
concept has been developed in the context of
a study of future advanced coal fired plants
with ultracritical steam conditions.
Development work has been underway on
horizontal furnace boiler technology for some
years. In the medium term, a demonstration
plant incorporating the new boiler, working
at current state of the art conditions, could
prove the viability of the design. A fully developed ultra supercritical version could start
operation in 2010.
Figure 1. Layout of the
new horizontal furnace
boiler, with vertical tubes.
Note the vortex burners
located on the front of the
furnace
Figure 2. Internally
rifled tube
In the 1980s Sulzer developed a concept
that used internally ribbed tubing which has
been used in the 2 x 700 MWe gas fired power
plant at the Kawagoe site and the Matsuura
700 MWe coal fired plant in Japan.
Full load mass flow density in the tubes for
this design of boiler is in the range 1600–2000
kg/m2s.
Horizontal vertical tube boilers
There is nothing new about vertical tube
Benson boilers – from as early as 1930 up to
the mid 1960s the use of vertical tubes with
refractory linings was the popular design approach. Since then the furnace tubing has generally been configured in a spiral
configuration with the tubes welded together to form membrane walls.
Various concepts for using vertical tubes in
the form of membrane walls were developed
in the USA in the late 1950s. The main problem with this arrangement was achieving adequate cooling of all of the tubes under a wide
range of load conditions. The use of high mass
flow densities was the generally adopted solution.
.
Mass flux m (kg/m2s)
1000
980
960
940
920
900
880
860
840
820
800
∑ qn/qo = 1
Relative heat input (qn/qo)
q2/qo
q3/qo
.
m q1/qo
Corner tube
Middle tube
(without burner)
Middle tube
(with burner)
0.40
0.38
0.36
0.34
0.32
0.30
0.28
0.26
0.24
0.22
0.20
Figure 3. Typical measurements on a supercritical
boiler with vertical rifled-tube water walls, showing
the relatiuonship between heat input and mass flux.
To accommodate temperature
variations that would occur at the
evaporator outlet due to differences in heat input conditions,
tubes expected to be subject to
insufficient heat input are fitted
with flow restrictors. But rather
than assume complete evaporation in the furnace, a convection
evaporator section is added for
completion of the evaporation process.
Siemens has been conducting experimental research into heat transfer and flow conditions in such tubes for some years in a high
pressure test loop in Erlangen, and many reports have been published by Joachim Franke,
Rudolf Kral and his colleagues over the last
decade. They have produced an extensive
database which shows that the heat transfer
is highly sensitive to changes in internal rifling
rib geometry.
Heat transfer in a rifled (also called ribbed)
tube (Figure 2) is exceptionally good, especially during evaporation. This is because centrifugal force transports the water fraction of
the wet steam to the tube wall. The resulting
May 2000 Modern Power Systems
37
Figure 4. Cottam, UK, uses a horizontal HRSG
wall wetting causes excellent heat transfer
from the wall to the fluid. This has the following advantages over smooth tubes:
● No deterioration of heat transfer even in
the range of high steam quality
● Very good heat transfer even at low mass
flux
● Only slight increase in wall temperature in
case of film boiling near critical pressure
(interval from about 200 bar to critical
pressure)
● Potential for increased heat transfer by optimisation of rifling geometry.
The low mass flux design not only enables
downward extension of the output limits for
vertical tubes to 300 or 200 MW and use of
large-diameter tubes, but in particular it also
changes the flow characteristic of a oncethrough system: with increased heating of an
individual tube, the throughput of that tube
increases instead of decreasing.
In a rifled tube, the boiling crisis does not
take place until steam quality is less than 0.9
shortly before the end of evaporation due to
the swirl flow generated by the spiral ribs inside the tubes. Differences in centrifugal force
separate the water from the steam fraction and
force the water towards the tube wall. This
maintains wetting up to high steam quality levels, resulting in high flow velocities even at
the boiling crisis location.
The main advantages of vertical internally
rifled tubes in a Benson boiler can be summarised as:
● Reduced mass flow, from 2000 to 1000
kg/s, with flow characteristics as in drum
boilers, ie increased heat input to an indi
vidual tube increases throughput in that
tube (as shown in Figure 3).
● Cost effective fabrication and assembly
● Minimum Benson output can be as low as
20 per cent.
● Simple start up system for 20 per cent
evaporator throughput
● Reduced slagging on combustion chamber
walls.
Siemens,
Babcock
Lentjes
Kraftwerkstechnik and Steinmüller carried
out large scale testing in a rig installed in
PreussenElektra’s supercritical 320 MWe
Farge coal fired power plant in 1993 which
served to verify the experimental results and
yield input to design codes for a new vertical
tube Benson boiler concept.
The theoretical conclusions for this concept regarding pressure drop and thus for flow
distribution with non-uniform heating were
tested in practice in the Farge plant. It was important to achieve the physical height which
Figure 5. Mass flow distribution, steam quality and temperatures in the HRSG for the Cottam combined cycle plant
is significant for a natural circulation characteristic but which cannot be attained in laboratory operation. A furnace heat exchange
surface with the low mass flux design was in
trouble-free operation at the Farge plant for
more than 10 000 hours. This confirmed the
calculation fundamentals and at the end of
trial operation the tubes were still practically
as good as new, the rib profile not smoothed
by deposits.
Heat transfer measurements were not performed in Farge, as important factors such as
heating and the thickness of the insulating ash
layer on the tubes vary constantly, preventing
reliable, reproducible measurement results.
But the low mass flux design with its optimised internally rifles tubes has even greater
benefits when applied to the low profile horizontal furnace boiler configuration.
91m
63m
The thermohydraulic principles of low
mass flux design have already been proven in
commercial operation in the horizontal
Benson heat recovery steam generator used in
Siemens’ most advanced V94.3A gas turbine
combined cycle field development plant, at
Cottam, UK (see MPS, September 1999, pp
40-43), shown in Figure 4.
The parallel tubes of the evaporator for the
HP and IP stages arranged sequentially in the
exhaust flow path are characterised by extremely different heat uptakes. In the selected concept, mass flows automatically adjust
to the heat input ie all parallel tubes of the HP
evaporator show saturation temperature at
first pass outlet and low temperature differences between the rows of the second pass
(Figure 5). The thermoelastic construction of
the Benson boiler significantly increases flexibility of the combined-cycle power
plant over that of a
Figure 6. Size comparison
drum boiler, espeof coal fired boilers for
cially during start550 MW output
up.
One third of
the height
Figures 1 and 6
show the schematic layout of the horizontal coal fired
boiler for which
very considerable
cost reductions are
claimed, particular-
31m
Tower
Two pass
Horizontal
Figure 7. The modular design of the boiler lends itself to variations in output rating
May 2000 Modern Power Systems
39
Relative power plant
investment costs (%)
160
140
120
∑ 120%
∑ 100%
∑ 107%
100
Steam
piping
80
Boiler
60
Turboset
40
Other
20
0
540ºC
Conventional
design
side of the combustion chamber.
The modular design of the boiler lends itself to variations in output rating – a 700
MWe version can be put together with
dual furnace sections to make a 700 MWe
unit with twice the width but the same
height as a 350 MWe unit (see Figure 7).
Whereas with more conventional steam
conditions the superheated steam lines account for some 3 per cent of the power plant
costs (see Figure 8), this number increases to
about 15 per cent in plants with steam temperatures of 700 °C (using Ni base alloys and
state of the art power block design). But with
the compact horizontal furnace boiler the figure decreases once again to around 3 per cent.
This is because with the horizontal furnace
boiler the length of pipes is reduced to 20 per
cent of that in the conventional design.
Also, in the horizontal furnace boiler, the
convection section with the horizontal and
vertical passes is located downstream of the
horizontal furnace, and is largely identical
with proven two-pass boilers (see Figures 1
and 6).
●
700ºC
700ºC
Conventional Conventional design
design
with HF boiler
Figure 8. Reductions in costs gained by horizontal
furnace (HF) boiler design for the highest steam
temperatures
ly for the most highly supercritical power
plants. Such boilers will have a height of little
more than 30 m.
Typical turbine plinth levels today are
around 16 m high. But with the new horizontal furnace there is the possibility of raising the turbine floor level to the boiler main
steam outlet pipe level, ie to about about 30
m above datum, to minimise superheated
steam pipe length and complexity. Power
plant designs being developed with the horizontal furnace boiler combined with
Table 1.
Materials for steam generators
with high steam temperatures
Components
Material
Temperature for
105h creep at 100
N/mm2 (°C)
Membrane wall
13CrMo44
7CrMoVTiB910
HCM12
NF12/SAVE12
515
580
600
640
Superheater tubes X3CrNiMoN1713
Esshete 1250
TP347HFG
Alloy 617
Alloy 625
630
640
655
~690
~740
Headers
590
615
640
655
~700
P91
E911/NF616
NF12
TP347HFG
Modified 617
gression of high temperature steam system
alloy applications to date.
Materials
Increasingly purposeful negotiations have recently been held between nickel alloy supplier Inco and the 40 strong members of the
Thermie 700 project, which include all of the
boiler makers in Europe, many utilities and
Figure 9. Siemens has designed
a new four-stage steam turbine
generator set for highly
supercritical power plants
Four-stage turbine
As already mentioned, the turbine envisaged
for use with the horizontal furnace boiler has
four stages: HP1, HP2, IP and LP (see Figure
9). The design takes into account the high
Old design
Siemens’ new four-stage turbine have this configuration.
The main advantages of the low-profile boiler configuration are obvious:
● Reduced structural steelwork costs.
● Simplified installation.
● Installation time is reduced as the furnace,
lateral pass and vertical pass can be installed in parallel, which also reduces interest during construction.
● The steam lines between the boiler and
turbine are shorter and more direct.
● All of the burners are mounted on one
IP
Rotor – nickel based alloy
Casing G17CrMoV5-10
LP
LP
HP1 HP2
LP
LP
IP
manufacturing
concerns.
Significantly, other German
utilities and the power plant
operators association VGB
are expecting to join the
HP1
HP2
group before long.
It seems that the projected
cost of the high nickel alloys needed to hancosts of nickel-based alloys and the need to redle the 700 °C steam is now down to around
strict component weights. Accordingly, the
10 x the cost of present P91 and P92 materiHP cylinder is split into separate HP1 and HP2
als instead of the 40 x figure recently mooted.
cylinders. The HP1 cylinder includes parts
The new boiler layout clearly reduces the
made of nickel-based alloys and is designed to
amount of High Nickel Alloy 617 in the sube very compact. The HP1 exhaust steam
perheated steam pipes, but thick castings in
flows directly into the HP2 cylinder, which
this material are still needed for the high prescan be designed for moderate steam condisure turbine casings. Table 1 indicates the protions and thus manufactured from conventional materials, eg cost effective 9 ... 12 per
cent chromium steels. Reheat steam enters the
HP1
IP cylinder, in which the hot areas will be manufactured from nickel-based materials, while
Inner casing – nickel based alloy
conventional materials will be used for the
colder areas (Figure 10).
Rotor – nickel based alloy
Inlet casing 9...12% Cr steel
Outlet casing 9...12% Cr steel
Inner casing –
nickel based alloy
HP
IP
New design
Figure 10. Materials for the IP and HP2 stages of
the 700°C steam turbine, 400 MWe single reheat
power plant
Reference
J. Franke and R. Kral, Advanced boiler design for
high efficiency power plants, to be presented at
Parsons 2000, Cambridge, 3-7 July 2000. These authors acknowledge participants in the advanced
700°C PF power plant project carried out under the
EU funded Thermie programme and the financial
contributions from the European Commission and
the Swiss government.
May 2000 Modern Power Systems
41
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This article appeared in:
Modern Power Systems
May 2000, page 37-41
Published by and copyright 2000
Siemens AG
Power Generation Group (KWU)
Freyeslebenstraße 1
91058 Erlangen, Germany
Phone: +49 9131 18-37 87
E-mail: contact@erl11.siemens.de
http:www.siemens.de/kwu
Siemens Westinghouse
Power Corporation
The Quadrangle
4400 Alafaya Trail
Orlando, FL 3 28 26-23 99 (USA)
Phone: +001 4 07 7 36-20 00
http://www.siemenswestinghouse.com
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