Steam Power Stations for Electricity and Heat Generation
4.2 Steam Generators
4.2.4 Operating Regimes and Control Modes
4.2.4.1 Operating Regimes
Power stations can be categorised according to the duty they operate under: peak, intermediate and base loads.
A peak load power station is operated for only a small number of hours per day and only a fraction of the days in the year. Its annual output corresponds to about 2,000 annual full-load hours (equivalent hours of full-load operation per year), where there are 8,760 h in each year. (The actual amount will be greater because of start-up, shut down and partial-load operation). Such a station should reach its rated power within a short time (i.e. start-up time), and it should be possible to shut it down very quickly. The energy losses during start-up and shutdown should be small.
Power stations such as pumped storage power stations or gas turbine power plants are used for peak load.
The base load power station, in contrast, is designed for inexpensive fuels, high efficiency levels and a small number of start-up and shutdowns. The output per year corresponds to about 6,000–8,000 annual full-load hours. It features a relatively small load control range between about 70 and 100% of its rated power, where the load change capability is not a very important criterion because the plant is mostly operated at its rated power. With few outages per year, start-up and shutdown times are of minor importance.
The yearly output of mid-range power stations lies between 2,000 and 6,000 annual full-load hours. Such a plant should be capable of dealing with peak load operation, with daily start-ups and shutdowns, as well as base load operation, with long operating periods and part-load conditions. It features a wide control range of about 30 or 40–100% of its rated power, and a good dynamic transient response and an efficiency as high as possible are sought.
Based on the operating regime of the power plant, the number of start-ups has to be specified in the design phase. Start-ups are classified into cold, warm and hot start-ups:
– Hot start-up: after an outage of maximum 8 h. Such an outage typically occurs overnight. For a hard coal fired power station operated in the mid-range such as the reference power plant, about 3,000–4,500 hot start-ups are scheduled for the lifetime of 40 years.
96 4 Steam Power Stations for Electricity and Heat Generation – Warm start-up: after an outage of 8–72 h. The outage is typically over the week- end. For a medium-range power plant the number of warm start-ups is about 1,000 over the station lifetime.
– Cold start-up: after an outage of more than 72 h. This start-up is quite rare; the total number for the medium-range power plant is about 200 (Zehtner 2009).
4.2.4.2 Primary, Secondary and Tertiary Control
The generation of power within a network such as the UCTE (Union for the Co- ordination of Transmission of Electricity) network in central Europe needs to be controlled and monitored for a secure and high-quality supply of electricity. The goal of the control is to maintain a balance between generation and consumption (demand) of electricity. The key control variable is the frequency of the net, which should be kept stable at 50 Hz, or 60 Hz in the USA or parts of Japan. In case of a drop in the frequency, caused by a higher consumption in comparison to the gen- eration, power plants have to increase their load in order to stabilise the frequency.
The rules of load-frequency control and requirements of power station performance are given in the Transmission Code for the West European UCTE net (UCTE 2004).
The guidelines distinguish between primary, secondary and tertiary control.
Primary Control
The objective of primary control is to rapidly re-establish the balance between gen- eration and consumption within the synchronous area by using turbine speed or turbine governors. By the joint action of all interconnected units, primary control stabilises the system frequency at a stationary value after a disturbance in a time- frame of seconds, but without restoring the reference values of system frequency and power exchanges. Outside periods of correction, the set-point frequency or scheduled frequency value is 50 Hz. Primary control is activated if the frequency deviation exceeds±20 mHz.
All power stations have to be capable of delivering a maximum primary control reserve of 2% of the rated power within 30 s. The maximum reserve has to be acti- vated at a frequency deviation of 200 mHz and has to be maintained over a period of 15 min. At lower frequency deviations, the required increase is correspondingly smaller, though the load change speed of 1% load per 15 s remains the same.
Primary control is supported by the self-regulation of consumption and genera- tion in the network. The self-regulation is assumed to be 1%/Hz, meaning a load decrease of 0.2% occurs in case of a frequency drop of 200 mHz.
Secondary Control
Secondary control restores primary control reserves and maintains a balance between generation and consumption of electricity within each control area in a timeframe of seconds to, typically, 15 min. Accordingly, load variations of differing magni- tudes must be corrected in the control area within this timeframe. Secondary control
4.2 Steam Generators 97 is based on secondary control reserves which are under automatic control by the operator of the network area. Secondary control is accomplished by increasing the fuel input of a power plant and thus puts requirements on the dynamic behaviour of power plants. The recommendations of the Deutsche Verbundgesellschaft (the association of German transmission system operators), located in Heidelberg, in 1991 fixed a required load change rate of coal-fired power plants of between 4 and 8%/min, referring to the rated load, within a load range of 40–100% (Verbundge- sellschaft 1991). With the expanded European interconnected network system, these strict limits are no longer valid. It is now the responsibility of the operator of the network area to cater for a sufficient secondary control reserve (Verbundgesellschaft 1996; VDN 2007).
Tertiary Control
Tertiary control reserve is required to restore the secondary control reserves. Tertiary control reserve is usually activated manually after activation of secondary control and frees secondary reserve. Tertiary control is achieved by re-scheduling power generation of operating plants or start-up of additional plants. Tertiary control thus corresponds to the operation planning of all power plants within a network area.
4.2.4.3 Constant-Pressure and Sliding-Pressure Operation
The output of a condensation power station is set by means of the live steam mass flow ˙mLS(Doleˇzal 1990). The mechanical power, Pm, of the turbine shaft depends on the live steam pressure pLS, the cross-section of the opening A, or the lifting of the turbine intake valves, and the live steam temperature, TLS, according to the following relation:
Pm≈m˙LS≈A pLS
√TLS
(4.1) The live steam temperature should remain constant throughout the whole load control range, so that a high efficiency rate is also achieved during part load and to avoid stress on the turbine caused by temperature changes. The turbine output and the live steam mass flow to the turbine are set during steady-state conditions, either when the live steam pressure is at a constant cross-section of the turbine intake valves (sliding or variable pressure) or when the intake cross-section is at a constant steam pressure (constant or fixed pressure).
Constant-Pressure Operation
The control in constant-pressure operation is subdivided into throttle control and governing control. In constant-pressure governing control, the first turbine stage is designed as a control wheel and is preceded by sets of nozzle valves (see Fig. 4.20).
As the load increases, the nozzle valves are sequentially opened.
98 4 Steam Power Stations for Electricity and Heat Generation Fig. 4.20 Turbine with
nozzle set and control wheel (Traupel 2001)
Under any load condition, only one of the valves is partially open, so loss through throttling only occurs there. The other valves are already fully opened or still closed.
Because only a partial flow is subject to throttling losses, the part-load efficiency of the turbine is high. In such a case, the first stage of the turbine, the control stage, is charged by a high pressure pBonly for part of the circumference, where this pressure is slightly lower than the live steam pressure (i.e. constant pressure). The control stage cuts the pressure back to the wheel chamber pressure pWand homogenises the steam distribution over the blading of the circumference of the following turbine stage (Traupel 1982).
In constant-pressure operation with throttle control, the total live steam mass flow is controlled by throttling the steam pressure through all the live steam valves at the same time. The turbine does not need a control stage, since the first turbine stage is charged uniformly and with lower pressure than the nozzle set governed stage.
The pressure losses in throttling have a disadvantageous effect on the heat rate in part-load operation. In full-load operation, the heat rate may be somewhat better than in constant-pressure operation with the nozzle set governing, because there is no efficiency-decreasing impact of the control stage.
In the balanced steady-state conditions of a power station unit, the steam pro- duced and the steam consumed by the turbine are equal. Fuel flow and steam gen- eration correspond. The steam production is controlled by the fuel mass flow, the changes of which, however, have a delayed effect due to the thermal inertia of the steam generator. In constant-pressure operation, the steam energy stored in the boiler is used to control rapid load changes. By further opening the intake cross-section of the turbine control valve, additional steam is extracted from the steam generator and used to bridge the interval until the conditions are balanced out by the fuel supply.
Sliding-Pressure Control
In sliding-pressure operation, the turbine output and the steam flow are adjusted by the pressure at the outlet of the boiler. In natural sliding-pressure operation, the live steam valves of the turbine are completely opened, and the cross-section of the turbine intake is constant throughout the whole load range.
4.2 Steam Generators 99 An output change using this control type can only be carried out by changing the fuel flow, a consequence of which is a long delay control characteristic of a change in the steam generator. Given that, in sliding-pressure operation, the pressure rises with increasing output, it is necessary that an increased steam flow is produced by the boiler before the output of the turbine increases.
In industrial practice, in order to diminish the disadvantages of the delayed con- trol characteristic of natural sliding-pressure operation, modified sliding-pressure control is used. The live steam valves in this operation are opened about 95% dur- ing steady-state conditions, so that in the case of a power demand similar to that of constant-pressure operation, the valves open and thus increase the steam flow to the turbine. By this slight throttling of the turbine intake valves, a limited loss is chosen in favour of better control dynamics (Baehr 1985).
Advantages of sliding-pressure control are a load-independent temperature dis- tribution in the turbine, a lower pressure stress on the steam generator and a lower power demand of the boiler feed water pump in part-load operation. Disadvan- tages are the changes of the boiling temperature in the evaporator, due to the pres- sure changes. The advantage of the decreasing power requirement for boiler feed pumping is stronger when the live steam pressure becomes higher. The general outcome in applying natural sliding pressure is a heat rate (including that of the boiler feed pumping power) which is slightly better than with nozzle-governed constant-pressure operation; with modified sliding pressure the heat rate is higher (Adrian et al. 1986; Baehr 1985). See also Sect. 4.4.4.
4.2.4.4 Impacts on the Turbine by Sliding-Pressure or Constant-Pressure Operation
The comparison of the different control modes in Fig. 4.21 shows that, in constant- pressure operation with the nozzle set controlling, the pressure pBafter the turbine inlet valves and before the blading remains almost constant over the load range. In sliding-pressure control, in contrast, and also in constant-pressure operation with throttle control, the pressure shows a linear rise with the output.
Both in sliding-pressure and in constant-pressure operation with throttle control, the stage pressures change to the same degree depending on the output, so that the stage temperatures are constant. In constant pressure operating with the nozzle set controlling, the pressure drop over the control stage pB−pWbecomes steeper with a decreasing output, so that the stage temperatures of the stages drop as well (Strauß 2006).
The influence of the control mode on the temperature in the high-pressure sec- tion is shown in Fig. 4.22. Load changes in constant-pressure operation cause con- siderable changes in temperature in the area of the first stage of the high-pressure turbine. Thermal stress arising in the process therefore limits the load change rate, in particular in the case of high-capacity turbines. In sliding-pressure operation, the temperature conditions in the turbine remain almost constant, so load changes are possible even with large turbines, even abruptly. This means that in sliding-pressure operation, the steam generator determines the dynamic performance of the power
100 4 Steam Power Stations for Electricity and Heat Generation
Fig. 4.21 Influence of the control mode on the pressure pattern at the turbine intake (not to scale) (Baehr 1985)
station unit, with the turbine having much higher allowable load change rates than in constant-pressure operation.
4.2.4.5 Impacts on Circulation or Once-Through Steam Generators by Sliding-Pressure or Constant-Pressure Operation
Circulation or once-through steam generators can in principle be operated with slid- ing or constant pressure. Circulation steam generators, however, are not operated with sliding pressure because it would involve considerable restrictions on load changes. Circulation systems are almost exclusively operated with constant-pressure
4.2 Steam Generators 101 Fig. 4.22 Temperatures in the
high-pressure section of the turbine with different control modes (Wittchow 1982)
control, while once-through systems mostly use sliding pressure, though in some cases constant pressure as well. For steam generators operated with constant pres- sure only in the evaporator, sliding-pressure operation does have the well-known operating advantage, though not the economic advantage, of the disproportionately decreasing power consumption of the boiler feed pump in part-load operation.
Different system characteristics determine different degrees of suitability of drum boilers and once-through boilers for rapid load changes. While the thick-walled drums of circulation steam generators limit the allowable rate of load change, the stress of a once-through boiler is lower at the same pressure rating due to the thinner walls of the separators. However, with higher pressures and temperatures involved, thick-walled construction parts of once-through steam generators, such as separa- tors, do limit the allowable load change rates.
In the case of a short-term increased power output demand of about 5%, the output can be increased by opening the turbine valves, which is possible both using modified sliding pressure and at constant-pressure control. Steam released in the first 20 s comes essentially from the live steam pipe and the superheater. Only afterwards does the evaporator add to the extra steam supply. The greater storage capacity of the drum boiler is an advantage in this case compared to once-through boilers. Delays in steam production if a step load change occurs can be bridged for a longer period until the compensation by the firing rate takes effect (Wittchow 1982).
In both boiler systems, greater output changes are always initiated by increasing the firing rate. Drum and once-through boilers differ in controlling the feed water.
In drum boilers, the feed water is designed to be controlled by the drum water level.
The feed water control is coupled with the fuel control via the evaporator and the circulation system. Changes in the feed water flow do not immediately influence the flow through the superheater. When the firing rate is increased, delayed steam generation in the circulation system, due to the large storage capacity of the evap- orator, may result in insufficient superheater cooling. With rapid load changes, the spray attemperators often do not suffice to control the live steam temperatures, so this circumstance places another limit on the load change rate in drum boilers.
102 4 Steam Power Stations for Electricity and Heat Generation The once-through boiler, compared to the drum boiler, has less steam storage capacity. In addition, in sliding-pressure operation, a large load change involves the boiler being more highly pressurised. For the once-through boiler, the enthalpy after the evaporator is used as the controlling variable for the feed water control. By means of a short increase in the feed water flow, the pressurising can be accelerated and the cooling of the superheater ensured. The limits of the once-through boiler thus result from delays in steam production in consequence to fuel flow changes.
So it can be said that different control modes and operation of once-through steam generators determine both the dynamic behaviour of the unit and the load-dependent heat rate. Once-through boilers are capable of coping with load change rates of 5–8% per minute, which is higher than the rates of 2–3% per minute that drum boilers can deal with (Wittchow 1982). The influence of the different control modes on the heat rate is described in Sect. 4.4.4.
4.2.4.6 Start-Up
The operation of a power station unit in the lower intermediate load range and peak load range also involves frequent start-ups and shutdowns. Start-up losses should be kept at a minimum in order not to impair the economic efficiency of power generation.
These losses are smaller with shorter start-up times, and the earlier the electrical unit output reaches the minimum output that allows the shutting of steam bypasses to the turbine.
After ignition, fuel flow and electric power consumption rise very quickly, but they cannot be used for power generation until the turbine generator is connected to the electrical grid. After connection to the grid, the start-up losses decrease as turbine bypasses are closed.
Once-through and circulation steam generators today are usually started up with water – steam separation behind a filled evaporator, which ensures that only steam is fed to the superheater. In all steam-generating systems, sufficient cooling of all heating surfaces must be guaranteed in the start-up process. Additional restrictions may arise due to thick-walled parts (Adrian et al. 1986; Wittchow 1982).
During start-up, a natural-circulation steam generator can only slowly increase its firing rate, because sufficient cooling of the heated risers becomes effective only when the circulating flow starts, that is, after evaporation has set in. It is also because steam must be available for the cooling of the superheater. In once-through or cir- culation systems, the evaporator and each tube already have a defined flow before ignition of the burners, both in the initial water phase and in the following water – steam phase. Due to the small storage capacity of water/steam in the system, steam generation can quickly be increased.
The reliable cooling of all superheater surfaces is a prerequisite for a rapid increase in the firing rate. It is ensured by an adequate turbine bypass system (see Fig. 4.23) (Adrian et al. 1986). Separated bypass systems for the high-pressure section (HPS), and the intermediate- and the low-pressure sections (IPS, LPS) of the turbine allow independent charging of the turbine parts while maintaining the