Reheat steam temperature control concept in Once-through boilers - A Review
Dr. Joachim Franke, SIEMENS AG, Germany
Ponnusami K Gounder, CETHAR VESSELS LTD
V.Balarathinam, CETHAR VESSELS LTD
SYNOPSIS
In once through boilers, superheated steam temperature is controlled by means of coordinated feed
water flow and spray attemperation. For reheat (RH) steam temperature control, many methods are
being adopted namely burner tilt, gas recirculation, divided back pass dampers, excess air and steam
bypass as primary control and feed water attemperation is envisaged as emergency control. When the
boiler is operated in sliding pressure mode the cold reheat steam temperature is higher compared to
constant pressure operation. The adjustment required for maintaining constant reheat outlet temperature
is larger in constant pressure operation mode. In general spray is not used for RH steam temperature
control for boilers designed for constant pressure operation since the spray quantity required will be large
and its impact on plant heat rate. In Europe utility boilers are operated under sliding pressure mode and
hence RH steam temperature control by spray is a common practice especially for once-through boilers.
This paper deals with the benefits and losses of using spray for RH steam temperature control in lieu of
other control mechanisms.
Introduction:
pressure operation as it affects plant heat rate.
In utility boilers, it is important to achieve best
However, in case of once through boilers which
possible heat rate to reduce the fuel cost and
are generally operated in sliding pressure mode,
hence the operators try to maintain superheat
quantum of RH spray is expected to be lower. In
and reheat steam temperatures at rated value to
this case RH spray attemperation is preferred as
the extent possible. In once through boilers, SH
it will result in simpler design and operation of
steam temperature is maintained by means of
the boiler and also less maintenance as systems
coordinated
spray
like burner tilt, GR fans, divided back pass
attemperation. There are many methods to
dampers are eliminated. Above aspects are
control RH steam temperature: like burner tilt,
discussed in detail in this paper.
feed
water
flow
and
gas recirculation (GR), divided back pass
dampers (gas biasing), excess air and steam
Need for steam temperature control:
bypass.
an
Superheat and reheat steam temperatures
emergency control, is not preferred as a means
should not be allowed to increase beyond the
of RH steam temperature control in constant
rated value as it will result in metallurgical
Spray,
though
envisaged
as
problems in superheater and reheater tubes and
Burner tilt:
also turbine components. On the other hand,
Tilting burners are provided in corner or
steam temperature lower than rated value will
tangential fired boilers. The burners can be tilted
result in higher cycle heat rate. Typically a
up or down in unison in all the four corners to
temperature reduction of 10 deg C in large
move the fire ball inside the furnace either
capacity power plant will result in about 0.3 %
upward or downward to change the furnace
increase in plant heat rate. Hence it is essential
absorption. When RH temperature is lower than
to
reheat
the rated value, burners are tilted up to reduce
temperatures within a narrow range around the
the furnace absorption and increase the furnace
rated values.
outlet temperature. As more heat is now
maintain
the
superheat
and
available for RH pick up, RH temperature can be
Steam temperature control methods:
maintained. When RH temperature is more than
In a coal fired boiler, super heat and reheat pick
the rated value, the burners are tilted down.
up are influenced by many variables like coal
Refer Figure-1.
quality,
cleanliness / dirtiness of the furnace,
fouling of heat transfer sections, etc,. When the
furnace is cleaner compared to the design
condition, the furnace absorption is more
resulting in lower furnace outlet temperature
(FOT)
and
hence
lower
SH
and
RH
temperatures. On the other hand, when slagging
/ fouling occurs due to deterioration in coal
quality, furnace absorption will be lower resulting
in higher furnace outlet temperature and hence
higher SH and RH outlet temperatures. Normally
superheat steam temperature is maintained over
the load range by means of coordinated feed
Fig. 1 Burner Tilt
water flow and spray attemperation. Various
methods are employed to maintain the reheat
steam temperature at rated value over the
control load range. The after effect of reheat
temperature control on superheat temperature
increase or decrease is regulated by feed water
attemperation.
The
methods
employed
for
reheat temperature control are discussed below.
Divided back pass dampers:
The divided back pass arrangement is used in
wall fired boilers with fixed burners. In wall fired
boilers, the convective back pass is divided into
two gas passes. On one side, Low Temperature
Reheat (LTRH) section is located and on the
other side Low Temperature Superheat (LTSH)
section is located. These two sections are
dampers are positioned both upstream and
divided by steam cooled wall or a baffle plate. A
downstream of the fan. Refer Figure-3 for a
common economiser heat transfer section is
typical arrangement of gas recirculation. As the
located across both the LTRH and LTSH
quantity of re-circulated gas is changed, the
sections outlet. The gas mass flow through
quantity of heat absorbed in the furnace and the
LTRH side can be increased or decreased (gas
heat at furnace outlet are changed. When RH
biasing) by the multi louver dampers positioned
outlet temperature is lower than rated value, GR
at the outlet of each pass (generally at the outlet
quantity is increased to increase the heat
of economizer section in lower gas temperature
available for RH pick up. In this case power
region). Refer Figure-2 for a typical arrangement
consumed by the GR fan is additional loss and
of dived back pass with control damper. By
will increase the net plant heat rate.
opening the dampers on LTRH side, the heat
transfer in LTRH section which is predominantly
convective is increased due to the increase in
gas mass flow thereby increasing the RH steam
temperature. In this type of control, draft loss
through the dampers will increase the power
consumed by induced draft fans.
Fig. 3 RH Steam Temp. Control
By Gas Recirculation
Excess air:
Excess air by itself is not used as a means of
Fig. 2 Divided Back Pass Damper control
RH steam temperature control as an increase in
excess air will increase the stack loss and
Gas recirculation:
reduces the boiler efficiency. Typically 0.3 to 0.4
Flue gas at economizer outlet or ID fan outlet is
% of boiler efficiency will be lost for every 10 %
drawn and reintroduced into the furnace by a
increase in excess air. In some cases especially
Gas Recirculation (GR) fan. Tight shut off
when the control load is very low, in addition to
burner tilt or gas biasing, excess air is also to be
the rated temperature can be achieved at
increased to achieve the RH steam temperature.
control load (70%) with zero spray. Even at
loads below control load the reheat steam
Effect of RH spray:
temperature deviation is small compared to
The reheat spray is done in a reheat de-
constant pressure operation and hence the
superheater located in the cold reheat piping at
impact on turbine metal temperature and heat
the inlet of low temperature reheat (LTRH)
rate are minimal. Figure 4 gives typical reheat
section or in between stages in a two stage
steam temperatures over the load range for
reheater. Due to the lower operating pressure
constant
for reheat cycle, RH spray is normally taken
pressure operation. Reheat steam temperature
from boiler feed pump inter stage. When tapped
control by spray is a common practice in
at this location, the spray water is not passing
Europe for once through boilers. Attached table
through the HP feed water heaters. Hence this
(Table 1) gives few supercritical boilers built in
amount of spray quantity is less regenerative.
Europe where spray attemperation is the
Further this spray quantity bypasses HP turbine
normal
and expands only in IP / LP turbines doing less
temperature control.
pressure
control
operation
means
for
and
variable
reheat
steam
work. Because of these reasons, the cycle
efficiency reduces and heat rate increases. The
reduction in efficiency (or increase in heat rate)
is a function of the quantity of spray water used.
RHO CP
Rated temp 568 deg c
RHO SP
Since the spray water required under sliding
pressure operating mode is less, its impact on
heat rate is minimal. Typically the cycle
efficiency decreases by about 0.08 % for every
1% RH spray.
RHI SP
RH spray as the primary control method in
RHI CP
once through boilers:
In once through boilers which are generally
operated
in
sliding
pressure
mode,
the
temperature of steam entering RH at all loads is
higher than it would be in constant pressure
RHO CP – RH outlet temp, constant pressure
operation. This helps in achieving the rated RH
RHO SP – RH outlet temp, sliding pressure
outlet temperature easily even at part loads.
RHI CP – RH inlet temp, constant pressure
When designed with 1 to 2% spray at full load,
RHI SP – RH inlet temp, sliding pressure
Table 1
Reference List of European supercritical boilers
with reheat temperature control by spray attemperation
Name of plant
Maasvlakte
Walsum
10
Neurath
F&G
(BOA1&2)
Nordjyllandsvaerket 3
Iskenderun
1&2
Country
Netherlands
Germany
Germany
Denmark
Germany
Name of Customer
E.ON
Evonik
RWE
Vattenfall
Year of order
Power Output, gross ( MW )
Steam pressure (barg)
Main steam temperature (°C)
RH steam temperature (°C)
2008
1100
284
600
620
2006
710
290
603
621
2005
1100
272
580
605
1993
415
290
582
580/580
Turkey
Evonik/Ste
ag
2000
660
210
541
539
Boiler type
Pulverised
coal
Pulverised
coal
Pulverised
coal
Pulverised
coal
Pulverised
coal
Pulverised
coal
Type of coal
Bituminous
Bituminous
Lignite
Bituminous
Bituminous
Bituminous
Rostock
E.ON
1991
550
265
545
562
Design simplification:
Generally for coal fired boilers either "Burner Tilt" or "Divided Backpass" is used for reheat steam
temperature control. The control response is slow due to the large inertia involved with these control
mechanisms. So emergency spray is provided in addition to either of these control methods. In many
operating plants, the operators resort to spray for control flexibility though other control mechanism is
provided in the design. As a result the real heat rate advantage is not realized in day to day operation.
The design can be simplified if "Burner Tilt" or "Divided Backpass" is not considered for control.
Elimination of the control mechanisms with their associated Controls & Instrumentation will also result in
reduced capital cost and operating cost.
Conclusion:
In view of the marginal effect on heat rate, elimination of maintenance prone control mechanisms, design
simplification, RH steam temperature control by spray attemperation in once through boilers is good for
both boiler suppliers and plant owners. The consultants and owners who are in the process of making the
specification for once through boilers should seriously consider adopting spray as the normal control
means for reheat steam temperature control.
Reference:
1.
Power plant Engineering – Black and Veatch
2.
J.C. Peeraer , "Gegenüberstellung
unterschiedlicher Zwischenüberhitzer temperaturregelungen",
Diploma Thesis Delft University of Technology, Netherlands, 1996