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