Supercritical Boiler with Low Mass Flux, Vertical Wall Design
J. Franke, Siemens AG, Erlangen, Germany
T. Balakrishnan, V. Balarathinam, CETHAR Vessels Limited, Tiruchirapalli, India
Abstract
Supercritical Boiler technology is gaining acceptance worldwide as Clean Coal Technology due to
its significant advantages like higher overall plant efficiency, reduced coal consumption, reduced
gaseous emissions like SOx, NOx and CO2 and particulate emission. Large capacity units of size
600 to 1000 MW are now built up with supercritical parameters. In India, the trend is clearly towards
large capacity Mega / Ultra Mega Power Projects based on supercritical technology. Evaporators of
once through supercritical boilers are designed with either “high mass flux spiral/vertical wall” or
“low mass flux vertical wall”. This paper deals with low mass flux vertical wall design developed by
Siemens AG, Germany. The authors of this paper feel that the low mass flux vertical wall design
has many distinct advantages over high mass flux spiral/vertical wall design.
Introduction
The major difference between a drum type
boiler and once through supercritical boiler is
the furnace wall design. An once through
furnace wall design needs to take care of the
temperature difference between tube-to-tube at
furnace wall outlet due to the variation in
furnace heat absorption. Also the occurrence
of boiling crisis like DNB and Dryout and
associated metal temperatures need to be
critically analyzed and taken care in the furnace
wall design. There are basically two types of
furnace wall designs used namely helically
wound spiral wall and vertical wall. The design
aspects of these two types of furnace wall are
discussed in this paper.
Furnace wall design requirements
Furnace sizing is decided by the heat input,
coal and ash characteristics like fouling &
slagging
tendency,
initial
deformation
temperature, and furnace heat loadings which
are derived from the experience of the boiler
manufacturer in firing different types of coal.
In short, furnace sizing philosophy is same for
both drum type and once through boilers.
However, the furnace wall design (evaporator
system) is different in case of once through
boilers.
A drum type boiler operates with circulation
ratio of normally around four to six (4 - 6).
This results in a steam quality of about 1725% (% of steam by weight) of the two phase
flow at the outlet of the furnace walls. With
this high water content of the two phase flow
the tube walls remain wetted at all loads. The
water and steam mixture leaves the furnace
wall at saturation temperature and there is no
temperature difference between adjacent
furnace wall tubes.
In once through boiler, the fluid passes
through the furnace walls only once and
enters superheater for further heating. When
the boiler operates at supercritical pressure at
higher loads (typically above 70 %), the
furnace walls have single phase fluid. The
supercritical fluid from economizer outlet
enters the furnace walls and its temperature
continuously increases along the furnace
height. When the boiler operates at lower
loads (below 70%), it will operate at sub
critical pressure. Hence at lower loads, single
phase water from economizer outlet enters the
furnace walls and upon addition of latent heat
of evaporation, gets converted into a two
phase mixture (water and steam). At certain
height along the furnace wall, all the water will
be evaporated into saturated steam. Further
addition of heat will increase the temperature
of steam such that it becomes superheated at
furnace wall outlet.
In other words, the fluid temperature at furnace
wall outlet in an once through boiler is not a
fixed temperature and there is tube-to-tube
temperature variation. The heat absorption of
furnace wall tubes varies due to their location
within the furnace enclosure, difference in
length and hence difference in hydraulic
resistance, variation in burner heat release
pattern and furnace cleanliness. For example,
the corner tubes receive less heat than the
middle tubes. Due to this variation in heat
absorption, the temperature of the medium at
furnace wall outlet is different from tube-totube.
A good furnace wall design of an once through
boiler should take care of the following:
1. The furnace heat absorption variation and the
temperature difference between tubes at
furnace wall outlet
2. Boiling crisis like DNB and Dryout are
properly addressed and associated peak tube
metal temperatures are kept within allowable
limits for the material selected.
contact between the water and the tube wall
which results in deterioration of heat transfer
and rise in tube metal temperature. Refer Fig.1.
Fig.1
The occurrence of both boiling crisis (DNB and
Dryout) must be considered in the evaporator
design. In both cases the steam cooling must
be sufficient to ensure a reliable cooling.
Evaporator system
There are mainly two types of evaporator
system suitable for sliding pressure operation
being used today namely spiral wall design and
vertical wall design. Refer Fig.2.
Departure from Nucleate Boiling (DNB)
Under certain conditions of heat flux, mass flux,
tube geometry, steam quality and pressure, the
steam bubbles formed inside the furnace walls
do not collapse but coalesce to form a film of
superheated steam over the inner wall of the
tube. This condition is known as film boiling.
The point at which film boiling occurs is known
as departure from nucleate boiling (DNB).
Refer fig 1. When DNB occurs, the tube metal
temperature shoots up. For high heat flux, DNB
may occur at low steam quality.
Fig.2
Dryout
Spiral wall high mass flux design
As the water and steam mixture flows along the
furnace wall, at higher steam quality boiling
crisis known as “Dryout” occurs. During the
occurrence of “Dryout” there is a sudden loss of
In an once through boiler, the feed water flow
matches the superheated steam output and
hence the flow per tube is less compared to a
drum type circulation boiler. In order to increase
the flow per tube or mass flux to ensure
sufficient cooling of the tubes throughout the
boiler operating regime, a spiral wall evaporator
system with the following design concept is
adopted.
1. Smaller OD tubes.
2. Helically wound spiral wall construction in
which the tubes are inclined (15 to 25 deg.)
and furnace tubes pass through the
circumference of the furnace more than one
time and connected to a transition header
above the burner zone. Above the transition
header the furnace enclosure is made up of
vertical water wall tubes. The spiral wall
concept reduces the number of parallel
tubes and hence increases the mass flux
through the tubes. As all tubes pass
through all the furnace walls, any variation
in heat absorption is applicable to all these
tubes and hence the temperature difference
between these tubes is minimized.
3. To achieve reliable cooling the mass flux
generally adopted is around 2000 kg/(m2s)
at full load. It may be chosen higher for
other reasons, e.g. to lower the minimum
load for once through operation.
4. Smooth tubes are adequate, as the mass
flux is high.
In a spiral wall design with high mass flux, a
tube which receives more heat draws less flow
due to higher frictional loss. The fluid flow
response with respect to heat absorption
variation is illustrated in Fig.3.
3. A mass flux of around 1000 kg/(m2s) is
used and this is known as low mass
flux design.
4. Rifled tubes with optimized tube
geometry are used in the lower
furnace and smooth tubes are used in
the upper furnace.
The vertical wall low mass flux design is based
on an important flow characteristic known from
natural circulation boilers and therefore called
“Natural flow characteristic” which is illustrated
in figure.3
In a parallel tube water / steam flow circuit, the
system hydraulic resistance i.e. pressure drop
is the same in all tubes which come from the
same inlet header and have the same outlet
header. However, there is variation in heat
absorption among these tubes as explained
earlier. A tube which receives more heat
produces more steam. But since the mass flux
is already low by design, the increase in
frictional pressure loss (dynamic head) is low
compared to the decrease in static head and
therefore draws more flow to maintain the
system pressure drop in the circuit. Similarly, a
tube having less absorption will receive lower
flow. Thus a natural flow characteristic is
established with the low mass flux design. This
ensures that the temperature difference
between adjacent tubes at furnace wall outlet is
kept to a minimum. Refer Fig.3
Behaviour of Individual Tubes with Higher Heat Input
Vertical wall low mass flux design
In vertical wall evaporator system the following
design concept is adopted.
1. Smaller OD tubes
2. Furnace wall construction is made up
of vertical single pass upward flow
tubes.
Fig.3
Use of Rifled tubes in the lower furnace
As the heat release rate is high in the lower
furnace particularly in the burner region, it is
essential to ensure sufficient cooling of the
tubes in order to protect them at the location of
boiling crisis and associated peak metal
temperatures.
lowest for OMLR tubes compared to smooth
tubes and standard rifled tubes with the same
mass flux. For identical wall temperatures the
mass flux in an OMLR tube can be halved
compared to a smooth tube.
To reduce the peak metal temperatures,
BENSON low mass flux vertical wall design
uses specially developed “Optimized Multi Lead
Rifled” (OMLR) tubes in the lower furnace
region. Fig.4 illustrates that an OMLR vertical
tube ensures wetting of the inner wall such that
the Dryout occurs - compared to a smooth tube
- at a safer elevation above the burner zone
and at higher steam quality with higher steam
velocities.
Fig.5
Vertical wall, high mass flux design:
Fig.4
The advantages of using OMLR tubes are
illustrated in Fig.5. It can be observed from the
figure that the peak metal temperature is the
Vertical wall vs. spiral wall design
There are some designs with vertical single
pass upward flow tubes with higher mass
fluxes. This design does not exhibit the natural
circulation characteristics as in low mass flux
design. Standard rifled tubes with orifices are
used in this design to take care of the tube-totube
temperature
differences.
Following table gives a comparison of the different evaporator designs.
Parameter
Spiral wall
High mass flux,
Vertical wall
Low mass flux
Vertical wall
Mass flux
High
High
(around 2000 kg/(m2-s) at full load or
1800 to 2000 kg/(m2-s) at
above)
at full load
Low
(around 1000 kg/(m2-s) at full load)
Furnace wall tube
Smooth tubes
Rifled tubes & Orificing
Optimized Multi Lead Rifled Tubes
(OMLR)
Pressure drop in
furnace wall
Higher
Higher
Lower ; Good savings in BFP power
consumption
Furnace
wall attachments &
buckstays
Complex
Simple,
Typical increase in weight for a 660 MW Self supporting
boiler is approx. 400 MT.
Simple
Self supporting
Manufacturing and
construction
Difficult compared to vertical design Easier
Easier
Experience of vertical low mass flux design
Yaomeng power station in China had 2 x 300MW subcritical units. The first unit was refurbished by Doosan
Babcock into an once through vertical wall low mass flux design based on BENSON boiler technology and
commissioned in year 2002. The operating experience has proved successful and thus established the
vertical low mass flux design concept. Based on the success in unit-1, order for refurbishment of unit-2 also
has been placed on Doosan Babcock in 2007.
Lagisza, 1 x 460MW CFB Boiler has been built with vertical wall low mass flux design by Foster Wheeler
Finland. The unit is commissioned in 2009.
Baima, 1 x 600MW CFB Boiler by Dongfang China is under construction.
Foster Wheeler is constructing a pulverized coal fired Boiler of 750 MW capacity using vertical low mas
flux design for Longview project, USA. The unit is planned for commissioning by 2011.
In addition, many Anthracite fired once through supercritical boilers are in various stages of execution in
China.
Conclusions
Once through boiler with supercritical parameters improve the overall plant cycle efficiency and heat rate
which results in reduced coal consumption, reduced gaseous emissions and pollutants. The vertical wa
low mass flux design developed based on the test results of vast number of experiments conducted b
Siemens AG, Germany offers significant advantages like natural flow characteristic, reduced feed wate
pump power consumption, a furnace wall design simpler to fabricate, erect, operate and maintain ensuring
the reliability and availability on par with drum type units. The concept is also validated by the experience
gained from some of the operating units.
References
1. BENSON training manual by Siemens AG, Germany
2. “The world’s first supercritical boiler FW-Benson vertical PC Boiler – The Long view power project” b
Stephen J. Goidich, Richard J. Docherty, Kenneth P.Melzer – Powergen Europe, Colonge, Germany
May 26-28, 2009
3. “Seven Years On The world’s first commercial low mass flux vertical tube Benson boiler” by P J Bel
C H Chen & I R Torkington, Doosan Babcock Energy - Electric Power 09.
4. “Introduction of supercritical / Ultra supercritical technology in India” by T.Balakrishnan &
V.Balarathinam – Energex 2008, Tiruchirapalli, India.