s
Innovative Boiler Design to Reduce Capitel Cost and
Construction Time
Presented originally at
Power-Gen 2000
Authors: Joachim Franke
Rudolf Kral
Power for Generations Siemens Power Generation
Content
Introduction – 3
Power Plant Design – 3
Once-Through Operation – 6
Vertical BENSON Evaporator Design – 7
Horizontal Furnace – 10
Studies for the New Concept
Concentrate on Four Aspects – 10
Summary – 12
2
Innovative Boiler Design to Reduce Capital Cost and
Construction Time
Joachim Franke, Rudolf Kral
Siemens Power Generation
Introduction
The requirements to be met by the next generation of power plants are determined on the
basis of various criteria. The most important factors on the assessment scale are efficiency and
environmental protection, operational flexibility and power generation costs. The steam generator is of major significance as no other power plant component is more cost-intensive or has
such a major impact on availability.
The economic efficiency of steam power plants is enhanced by elevating steam parameters to
the supercritical level, making the use of once-through boilers necessary. Among the various
types of once-through boilers, the BENSON boiler for which Siemens is licensor exhibits clear
advantages based on its suitability for flexible operating modes with rapid load changes and
brief start-up times.
The market demand for a reduction in manufacturing and installation costs led to the development of a new BENSON boiler concept incorporating a furnace with vertical tubing.
Power Plant Design
In recent years, the requirements for coal-fired power plants in countries outside of Europe
have also shifted from base load towards intermediate peaking and peak load duty. Frequent
startups and shutdowns as well as rapid load changes necessarily lead to a change in operating
mode from constant to variable pressure. The reason for this is that under such conditions,
large temperature changes and hence high material loads on the HP turbine can be prevented
only with variable-pressure operation (Fig. 1). In addition, omission of the complex turbine control stage is only possible with variable-pressure operation.
Figure 1
Comparison of different operation modes
3
The transition to variable pressure has also had a decisive influence on boiler design as the
main steam pressure, which must be increased to over 200 bar in order to enable achievement
of higher efficiencies (Fig. 2). Different requirements thus also lead to different power plant
concepts (Fig. 3). While all available types of boilers and turbines can be implemented for baseload operation, for intermediate peaking and peak load duty only once-through boilers suitable
for variable-pressure operation remain. Various typical power plant types have established themselves based on the differing requirements in force in the various regions (Fig. 4).
Previously, only base-load plants of intermediate efficiency were required in the USA as well as
in Southeast Asia. Drum-boiler technology is correspondingly widespread in these areas.
Figure 2
Efficiency potential of advanced coal-fired power plants
Figure 3
Power plant design is defined by the requirements
4
A start was made along the road to higher pressures and thus to higher efficiencies in the USA
with a boiler technology requiring supercritical pressure in the evaporator over the entire load
range. For intermediate peaking duty this method has the disadvantage that part-load operation
either entails a significant reduction in plant efficiency – with throttling downstream of the evaporator, turbine in variable-pressure operation – or that the number or rates of load changes must
be significantly reduced – no throttling downstream of evaporator, turbine with control stage in
fixed-pressure operation.
In Europe and Japan, the combined requirement of higher efficiency and more operating flexibility in intermediate peaking and peak load duty lead to the early implementation of once-through
boilers with supercritical steam parameters and suitability for variable-pressure operation.
The availability of such plants with supercritical pressures is on the same level as that of subcritical plants (Fig. 5), and the large number of reference plants also demonstrates the maturity
of this technology.
Figure 4
Typical power plant design
Figure 5
Energy unavailability not postponable (EU) of German power plants
5
Once-Through Operation
With approx. 1,000 units constructed, the BENSON boiler is the once-through boiler with the
greatest representation worldwide. The various types of once-through boilers differ primarily in
their evaporator systems (Fig. 6).
Since the 60s, the evaporators in BENSON boilers have been equipped with tubes welded to
form membrane walls in a spiral configuration around the furnace. This method, which has
since been adopted by most boiler manufacturers worldwide, enables parallel upward flow at
high mass flux through all evaporator tubes, and can thus be operated at both subcritical and
supercritical pressures.
In contrast, the Universal Pressure (UP) or Combined Circulation systems introduced in the USA
in the 50s with vertical-tubed evaporators and higher mass velocity are suitable only for operation with supercritical pressure in the evaporator. Variable-pressure operation is possible only by
throttling downstream of the evaporator and is thus associated with efficiency losses.
The development of new once-through evaporator concepts with vertical tubes began in the
80s. A concept developed by MHI is characterized by a mass velocity in the evaporator tubes
which is reduced but is still more than double that in natural-circulation systems, and by an
additional evaporator in the convection section of the boiler.
In contrast, the Siemens development of vertical evaporator tubes is characterized by low mass
velocities comparable with those in natural-circulation boilers and the simplest possible structural design.
Figure 6
6
Once-through steam generators
Vertical BENSON Evaporator Design
Furnaces with spiral-wound tubes can look back on more than 30 years of development, and
operating experience with several hundred boilers. With their high availability, they represent
the current state of the art. The only criticism in comparison with vertical tubing is the higher
cost of manufacturing and installation due to welding of the support straps and the numerous
field welds required. Global efforts therefore target replacement of the spiral-wound tube configuration with vertical tubes.
The vertically-tubed furnace with Low Mass Flux design was not feasible until Siemens performed further development of rifled tubes to give improved heat transfer. Fig. 7 shows why
heat transfer in a rifled tube is so good, especially during evaporation: Centrifugal force transports the water fraction of the wet steam to the tube wall. The resulting wall wetting causes
excellent heat transfer from the wall to the fluid. This has the following advantages over smooth
tubes:
Steam quality
Rifled
tube
Fluid
1.0
Smooth
tube
0.8
0.6
Pressure: 150 bar
Mass flux: 500 kg/m2s
0.4
Rifled
tube
Figure 7
Smooth
tube
Heat flux: 300 kW/m2
100
200
300
400
500
600
Inside wall temperature (°C)
Wall temperatures and boiling crisis of 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 optimization of rifling geometry.
Changes in rifling geometry permit significant improvements in heat transfer to be achieved
(Fig. 8). A Siemens high-pressure test rig – the largest in the world, with an electrical heater
capacity of more than 2,000 kW – was used to generate more than 150,000 data points in an
investigation of standard commercial rifled tubes and of tubes with modified rifling geometry.
For unchanged inner wall temperatures it proved possible to reduce the mass velocity by
approximately 25 % compared to the use of standard commercial rifled tubes.
7
The Low Mass Flux design mentioned in the foregoing 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 once-through system: With increased heating of an individual tube, the throughput of that tube increases instead of decreasing.
Figure 8
Optimized rifled tubes reduce wall temperatures or allow mass flux reduction
This flow behavior – well known from drum boilers – is called a natural circulation or positive
flow characteristic.
The standard flow characteristic of once through boilers, where excess heating impedes the flow
in individual tubes, is transformed into a natural circulation characteristic when the full-load mass
flux is reduced to the value known from drum boilers.For an evaporator tube with 25 % higher
heat input than an average tube this transition is plotted in Fig. 9 as a function of mass velocity.
The example is valid for a supercritical 600 MW BENSON boiler with vertically tubed furnace. A
natural circulation characteristic is established below a mass flux of roughly 1,050 kg/m2s. Below
this value the increase in outlet temperature is more or less compensated by a corresponding
increase in mass flux in the individual tube. At values above 1050 kg/m2s the throughput decreases with increasing heat input and the outlet temperature increases disproportionately.
The theoretical conclusions for this concept regarding flow distribution with non-uniform
heating were tested in practice in the supercritical 320-MW FARGE plant.
A 47.00 m high furnace heat exchange surface of 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, with
the rib profile not smoothed by deposits.
In addition, the thermohydraulic principles of Low Mass Flux design have already been proven
in commercial operation in the horizontal heat recovery steam generator at the COTTAM GUD®
combined-cycle power plant. The parallel tubes of the BENSON evaporator for the HP and IP
stages arranged sequentially in the exhaust flow path are characterized by extremely different
heat uptakes to which the mass flow automatically adjust (Fig. 10).
8
Figure 9
Mass flow characteristic of vertically tubed BENSON boilers
The characteristics of the vertically tubed furnace can be summarized as follows:
Mass flux reduction from 2000 to 1000 kg/m2s flow characteristic as in drum boilers:
increased heat input to an individual tube increases throughput in that tube
Cost-effective fabrication and assembly
Minimum BENSON output: 20 %
Simple startup system for 20 % evaporator throughput
Reduced slagging on furnace walls
Figure 10
Heat and mass flow distribution in the HP evaporator of a once-through HRSG
9
Horizontal Furnace
The scientifically founded design basis, the results of the practically oriented large-scale test
and the operating experience with the BENSON heat recovery steam generator at the Cottam
CCPP have enabled the establishment of a solid knowledge base for the next step of innovation, the horizontal furnace boiler (HF-boiler).
In the HF-boiler, the convection section with the horizontal and vertical passes is located in the
gas path downstream of the horizontal furnace, and is largely identical with proven two-pass
boilers (Fig. 11). The vortex burners are shown here in a front configuration. HF-boiler evaporator
tubes are vertical. This concept is characterized by a very low building height. The potential reduction in structural steelwork, connecting lines to the turbine as well as installation costs and
time in comparison with traditional single and two-pass designs is clearly evident.
Figure 11
Steam generator with horizontal furnace for 350 MW
Studies for the New Concept Concentrate on Four Aspects:
Firing: NOX formation and unburned carbon must be evaluated. In addition, the horizontal
flame orientation may effect heat flux distribution and thus have an impact on the flow
design of the furnace walls.
Furnace walls: The evaporator tubes can only be vertical. The influence of the large heat
input differences of 3:1 and more over the length of furnace on the outlet temperature of
the evaporator tubes connected in parallel must still be investigated.
Transition from furnace to horizontal section: Large temperature differences occur here
between adjacent tubes and tubes which are welded together.
Reinforcement of surrounding walls needs attention because of relatively large spans.
The problem of the water/steam side design of the furnace walls is characterized by the fact
that an evaporator tube in the main combustion zone has a heat input roughly 3 times as great
as at the furnace outlet. Despite this unfavorable situation, the outlet temperatures from the individual parallel tubes must under no circumstances exceed either allowable maximum values
10
or set tolerance limits from other tubes. Once again, as already seen in case of HRSG Cottam
the Low Mass Flux design provides the solution to this problem (Fig. 12). The natural circulation
characteristic described earlier results in a higher mass flux and thus greater throughput in a
tube with higher heat input than in a tube with lower heat input. This behavior is further supported by different dimensioning of the tubes and fins.
Fig. 12 also illustrates the effect of this design on the tube outlet temperature: The temperature
difference between any two tubes is less than 50 K, with that between adjacent tubes even
less, below 30 K. Tube throughput also automatically adjusts to new heating conditions, such as
a change in heat input due to fouling or soot blowing.
Figure 12
Mass flow distribution adjusts automatically to heat flux distribution
A boiler with a horizontal furnace is only about 30 m high, which allows simple and fast installation. The heights of comparable conventional steam generators are between 60 m and 90 m
(Fig. 13).
The advantages of this horizontal, low-profile design are obvious:
Reduced cost of structural steelwork
Straightforward installation
Short installation time due to parallel installation of the furnace, lateral pass and
vertical pass
Shorter steam lines between boiler and turbine.
The modular design of the boiler with its furnace, lateral pass and vertical pass also makes
adaptation to other power ratings and other fuels a relatively simple matter. A 700-MW plant
with dual furnace, for example, has only twice the width but otherwise the same dimensions
as a 350-MW boiler.
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Summary
The requirements for modern power plants with regard to efficiency and operating behavior
necessarily lead to the implementation of once-through boilers. On the one hand, this boiler
type enables supercritical steam conditions, while on the other it is especially suitable for
variable-pressure operation.
The further development by Siemens of the BENSON once-through boiler with vertical-tube
furnace walls constitutes a step in the direction of simpler and thus more cost-effective design
with improved operating behavior.
The development is based on extensive fundamental research in heat transfer and pressure
drop in rifled tubes. Optimization of the rifled tubes enables a significant reduction in mass flux
and hence a furnace wall flow design yielding increased throughput in tubes with higher heat
input. This behavior is known from natural circulation evaporators. In addition to the cost reduction, the reduction of the minimum BENSON output to 20 % is especially attractive, as this
makes night and weekend shutdowns unnecessary in many cases.
Tower
Figure 13
Two Pass
Horizontal
Size comparison of coal-fired steam generators with horizontal furnace for 550 MW output
The development of the HF boiler focused on reducing investment costs by incorporating the
operational advantages of vertical tubes. Investigations of possible problem areas were initiated
and solutions found. The HF boiler will constitute a milestone in boiler construction for manufacturers and operators alike. The cost advantages continue to increase with increasing steam conditions.
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This paper is based on a lecture
held at Power Gen 2000
Published by and copyright 2000
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