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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:
In utility boilers, it is important to achieve best
possible heat rate to reduce the fuel cost and
hence the operators try to maintain superheat
and reheat steam temperatures at rated value to
the extent possible. In once through boilers, SH
steam temperature is maintained by means of
coordinated feed water flow and spray
attemperation. There are many methods to
control RH steam temperature: like burner tilt,
gas recirculation (GR), divided back pass
dampers (gas biasing), excess air and steam
bypass. Spray, though envisaged as an
emergency control, is not preferred as a means
of RH steam temperature control in constant
pressure operation as it affects plant heat rate.
However, in case of once through boilers which
are generally operated in sliding pressure mode,
quantum of RH spray is expected to be lower. In
this case RH spray attemperation is preferred as
it will result in simpler design and operation of
the boiler and also less maintenance as systems
like burner tilt, GR fans, divided back pass
dampers are eliminated. Above aspects are
discussed in detail in this paper.
Need for steam temperature control:
Superheat and reheat steam temperatures
should not be allowed to increase beyond the
rated value as it will result in metallurgical
problems in superheater and reheater tubes and
also turbine components. On the other hand,
steam temperature lower than rated value will
result in higher cycle heat rate. Typically a
temperature reduction of 10 deg C in large
capacity power plant will result in about 0.3 %
increase in plant heat rate. Hence it is essential
to maintain the superheat and reheat
temperatures within a narrow range around the
rated values.
Steam temperature control methods:
In a coal fired boiler, super heat and reheat pick
up are influenced by many variables like coal
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
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.
Burner tilt:
Tilting burners are provided in corner or
tangential fired boilers. The burners can be tilted
up or down in unison in all the four corners to
move the fire ball inside the furnace either
upward or downward to change the furnace
absorption. When RH temperature is lower than
the rated value, burners are tilted up to reduce
the furnace absorption and increase the furnace
outlet temperature. As more heat is now
available for RH pick up, RH temperature can be
maintained. When RH temperature is more than
the rated value, the burners are tilted down.
Refer Figure-1.
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)
Fig. 1 Burner Tilt
section is located. These two sections are
divided by steam cooled wall or a baffle plate. A
common economiser heat transfer section is
located across both the LTRH and LTSH
sections outlet. The gas mass flow through
LTRH side can be increased or decreased (gas
biasing) by the multi louver dampers positioned
at the outlet of each pass (generally at the outlet
of economizer section in lower gas temperature
region). Refer Figure-2 for a typical arrangement
of dived back pass with control damper. By
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.
Gas recirculation:
Flue gas at economizer outlet or ID fan outlet is
drawn and reintroduced into the furnace by a
Gas Recirculation (GR) fan. Tight shut off
dampers are positioned both upstream and
downstream of the fan. Refer Figure-3 for a
typical arrangement of gas recirculation. As the
quantity of re-circulated gas is changed, the
quantity of heat absorbed in the furnace and the
heat at furnace outlet are changed. When RH
outlet temperature is lower than rated value, GR
quantity is increased to increase the heat
available for RH pick up. In this case power
consumed by the GR fan is additional loss and
will increase the net plant heat rate.
Excess air:
Excess air by itself is not used as a means of
RH steam temperature control as an increase in
excess air will increase the stack loss and
reduces the boiler efficiency. Typically 0.3 to 0.4
% of boiler efficiency will be lost for every 10 %
increase in excess air. In some cases especially
when the control load is very low, in addition to
Fig. 2 Divided Back Pass Damper control
Fig. 3 RH Steam Temp. Control By Gas Recirculation
burner tilt or gas biasing, excess air is also to be
increased to achieve the RH steam temperature.
Effect of RH spray:
The reheat spray is done in a reheat de-
superheater located in the cold reheat piping at
the inlet of low temperature reheat (LTRH)
section or in between stages in a two stage
reheater. Due to the lower operating pressure
for reheat cycle, RH spray is normally taken
from boiler feed pump inter stage. When tapped
at this location, the spray water is not passing
through the HP feed water heaters. Hence this
amount of spray quantity is less regenerative.
Further this spray quantity bypasses HP turbine
and expands only in IP / LP turbines doing less
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.
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.
RH spray as the primary control method in
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
operation. This helps in achieving the rated RH
outlet temperature easily even at part loads.
When designed with 1 to 2% spray at full load,
the rated temperature can be achieved at
control load (70%) with zero spray. Even at
loads below control load the reheat steam
temperature deviation is small compared to
constant pressure operation and hence the
impact on turbine metal temperature and heat
rate are minimal. Figure 4 gives typical reheat
steam temperatures over the load range for
constant pressure operation and variable
pressure operation. Reheat steam temperature
control by spray is a common practice in
Europe for once through boilers. Attached table
(Table 1) gives few supercritical boilers built in
Europe where spray attemperation is the
normal control means for reheat steam
temperature control.
RHO CP – RH outlet temp, constant pressure
RHO SP – RH outlet temp, sliding pressure
RHI CP – RH inlet temp, constant pressure
RHI SP – RH inlet temp, sliding pressure
RHO SP
RHO CP
RHI CP
RHI SP
Rated temp 568 deg c
Table 1
Reference List of European supercritical boilers with reheat temperature control by spray attemperation
Name of plant MaasvlakteWalsum
10
Neurath F&G
(BOA1&2)
Nordjylland-svaerket 3
Iskenderun 1&2
Rostock
Country Netherlands Germany Germany Denmark Turkey Germany
Name of Customer E.ON Evonik RWE VattenfallEvonik/Steag
E.ON
Year of order 2008 2006 2005 1993 2000 1991
Power Output, gross ( MW ) 1100 710 1100 415 660 550
Steam pressure (barg) 284 290 272 290 210 265
Main steam temperature (°C) 600 603 580 582 541 545
RH steam temperature (°C) 620 621 605 580/580 539 562
Boiler typePulverised
coalPulverised
coalPulverised
coalPulverised
coalPulverised
coalPulverised
coal
Type of coal Bituminous Bituminous Lignite Bituminous Bituminous Bituminous
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