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 sprayattemperation. 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) 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 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
Table 1Reference List of European supercritical boilers
with reheat temperature control by spray attemperation
Name of plant / Maasvlakte / Walsum 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 / Vattenfall / Evonik/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 type / Pulverised
coal / Pulverised
coal / Pulverised
coal / Pulverised
coal / Pulverised
coal / Pulverised
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:
- Power plant Engineering – Black and Veatch
- J.C. Peeraer , "Gegenüberstellung
unterschiedlicher Zwischenüberhitzer temperaturregelungen",
Diploma Thesis Delft University of Technology, Netherlands, 1996