EP2655811B1 - Method for regulating a brief increase in power of a steam turbine - Google Patents

Description

The invention relates to a method for controlling a short-term increase in output of a steam turbine with an upstream fossil-fueled continuous steam generator with a number of a flow path forming, flowed through by a flow medium economizer, evaporator and superheater heating, and for example from the US 6301895 B1 known.

A fossil-fueled steam generator produces superheated steam using the heat generated by burning fossil fuels. Fossil fueled steam generators are mostly used in steam power plants, which are mainly used for power generation. The generated steam is fed to a steam turbine.

Analogous to the various pressure stages of a steam turbine, the fossil-fueled steam generator also comprises a plurality of pressure stages with different thermal states of the respectively contained water-steam mixture. In the first (high) pressure stage, the flow medium first passes through economizers on its flow path, using residual heat to preheat the flow medium, and then various stages of evaporator and superheater heating surfaces. In the evaporator, the flow medium is evaporated, then separated any residual moisture in a separator and further heated the remaining steam in the superheater. Thereafter, the superheated steam flows into the high-pressure part of the steam turbine, where it is expanded and fed to the following pressure stage of the steam generator. There it is again superheated (reheater) and fed to the next pressure part of the steam turbine.

Due to a wide variety of external influences, the heat output transferred to the superheaters can fluctuate greatly. Therefore It is often necessary to regulate the overheating temperature. Usually, this is usually achieved by an injection of feedwater before or after individual Überhitzerheizflächen for cooling, ie, an overflow branches off from the main stream of the flow medium and leads to there correspondingly arranged injection coolers. In this case, the injection is usually regulated by means of valves via a characteristic value characteristic of the temperature deviations from a predetermined temperature setpoint at the outlet of the superheater.

Modern power plants not only require high levels of efficiency but also the most flexible mode of operation possible. Apart from short start-up times and high load change speeds, this also includes the possibility of compensating for frequency disturbances in the power grid. To meet these requirements, the power plant must be able to provide more power of, for example, 5% and more in relation to full load within a few seconds.

Such power changes of a power plant block in the second range are possible only by a coordinated interaction of steam generator and steam turbine. The contribution that the fossil-fueled steam generator can make is the use of its storage, d. H. of the steam but also of the fuel storage, as well as rapid changes of the control variables feedwater, injection water, fuel and air.

This can be done, for example, by opening partially throttled turbine valves of the steam turbine or a so-called step valve, whereby the vapor pressure is lowered in front of the steam turbine. Steam from the steam storage of the upstream fossil-fueled steam generator is thereby stored and fed to the steam turbine. With this measure, an increase in performance is achieved within a few seconds.

This additional power can be released in a relatively short time, so that the delayed power increase can be at least partially compensated by the increase in the firing capacity. The entire block makes by this measure immediately a jump in performance and can also permanently maintain or exceed this level of performance by a subsequent increase in the firing capacity, provided that the system was at the time of additionally requested power reserves in the partial load range.

However, a permanent throttling of the turbine valves to provide a reserve always leads to a loss of efficiency, so that for an economic driving the degree of throttling should be kept as low as absolutely necessary. In addition, some types of fossil-fueled steam generators, such. B. forced flow steam generator may have a significantly smaller storage volume than z. B. natural circulation steam generator. The difference in the size of the memory in the method described above has an influence on the behavior of power plant block power changes. In addition, especially in the upper load range by throttling the design pressure in the entire steam generator must not be exceeded, so that this measure in the upper load range only limited or can not be applied.

It is therefore an object of the invention to provide a method for controlling a short-term power increase of a steam turbine, which is particularly suitable to allow a short-term increase in output of a downstream steam turbine, without affecting the efficiency of the steam process is unduly affected.

This object is achieved according to the invention by increasing the flow of the flow medium through the fossil-fired steam generator for short-term increase in output of the steam turbine.

The invention is based on the consideration that the introduced heat output into the steam generator is determined by the firing capacity and only has a comparatively slow effect in the event of a sudden change. An additional benefit payment in the steam turbine should therefore be made by using the heat energy stored in the heating surfaces of the steam generator. The withdrawal of this heat requires a lowering of the average material temperature. This should be done by increasing the flow, i. H. the amount of flow medium flowing through per unit of time can be achieved. Due to the higher flow with comparatively lower medium temperatures, the average material temperature of all heating surfaces is lowered by this measure and as a result thermal energy is released from all these heating surfaces and released in the steam turbine in the form of additional power.

In an advantageous embodiment, the enthalpy desired value at the outlet of an evaporator heating surface is reduced for a short-term increase in output of the steam turbine. The setpoint for the specific enthalpy is used in the control system of the steam generator as a control variable for determining the setpoint for the flow of the flow medium. This switching action has two effects: First, the basic setpoint for the evaporator flow rate calculated in the feedwater setpoint determination increases. Secondly, the enthalpy correction controller - in particular if the reduction takes place particularly quickly (abruptly) - increases its output signal by means of a now greater control deviation in order to reduce the enthalpy at the evaporator outlet as quickly as possible. As a result, the feedwater quantity at the beginning of this measure even increases disproportionately and it is a particularly rapid withdrawal of heat from the heating surfaces with the associated Leistungsentbindung in the steam turbine possible.

Advantageously, the enthalpy target value is reduced to a predetermined minimum enthalpy value. As a result, on the one hand in all load conditions, a maximum performance deduction while maintaining operational safety guaranteed.

In a particularly advantageous embodiment, the Mindestenthalpiewert is dimensioned such that in all load conditions of the fossil-fired steam generator complete evaporation of the flow medium is achieved in the evaporator heating. In particular, in subcritical operation, namely, should be ensured that the enthalpy at the evaporator outlet is not lowered too far and consequently a seizure of residual water in a downstream separator can be safely avoided. Thus, a maximum increase in additional feed water and thus additional performance relief should be achieved with the safest possible driving.

It should be emphasized that the higher the actual enthalpy at the evaporator outlet in stationary operation is selected, ie the greater the distance to the fixed minimum enthalpy, the more thermal energy can also be stored, ie the more steam turbine power can be generated in the short term. Accordingly, in the case of a boiler design tailored to this measure, the greatest possible distance from the minimum enthalpy in stationary operation or in frequency support mode should be sought. It should be noted, however, that under the circumstances mentioned inadmissible high temperature imbalances at the evaporator outlet are to be avoided only by a suitable boiler design. In addition, the occurring transient loads in the design or for the existing steam generator design are to be considered, which can lead to a corresponding material fatigue depending on the size and frequency. It should be mentioned, however, that especially in supercritical steam generator operation, in which the greatest possible reduction of the evaporator outlet enthalpy can be realized, due to the water-steam properties of the flow medium only moderate temperature reductions at the evaporator outlet are to be expected, and thus the material load of the evaporator is kept within limits.

Advantageously, the parameters of the measures taken are matched to the required power release in the steam turbine and optimized. For this purpose, the amount and / or duration of the reduction of the enthalpy target value are determined on the basis of the required power increase.

In an alternative or additional advantageous embodiment, flow medium removed in the flow path in the region of a superheater heating surface of the steam generator is injected for short-term increase in output of the steam turbine. Namely, such injections can make a further contribution to the short-term rapid power change. By this additional injection in the superheater namely the steam mass flow can be temporarily increased. Here, the stored thermal energy is also used for a temporary increase in power of the steam turbine. This results in the additional advantage that a suitable co-ordination of all measures available a particularly high power surplus can be maintained quickly and for as long as possible at a constant level. By staggering the individual measures, the material load can also be positively influenced.

In a further advantageous embodiment, the heat input is increased in the fossil-fired steam generator, that is, increases the firing capacity of the burner. Thus, a temperature reduction at the evaporator outlet can be favorably influenced or even completely avoided by the described method, since the measure acts as a Vorhaltsignal on the feed water. Thus, the method not only allows a short-term increase in performance, but is also used for faster adjustment of a longer-term performance increase.

In an advantageous embodiment, a control system for a fossil-fired steam generator with a number of flow-forming, flowed through by a flow medium economizer, evaporator and superheater heating means comprises means for carrying out the method. In a further advantageous embodiment, a fossil-fired steam generator for a steam power plant comprises such a control system and a steam power plant such a fossil-fired steam generator.

The advantages achieved by the invention are, in particular, that by the short-term increase in the amount of feed water, a particularly fast power release in the steam turbine downstream steam turbine is made possible by using the heat energy stored in all heating surfaces. In addition, this measure can only be implemented without invasive structural measures with minimal adjustments to the feedwater control concept, so that no additional costs are incurred despite considerably increased system flexibility.

In addition, compared to the use of the injections as a power-increasing measure, the stored thermal energy of the economizer, the evaporator and the first superheater heating surfaces, which are located on the flow medium side before the first injection, can be used as additional energy source. Thus, a much larger reservoir of stored thermal energy is available for the additionally requested power. As a result, either a greater increase in power (peak) can be generated, or an additionally released power can be maintained longer at a lower level.

In particular, in the upper load range, in which, for example, a throttling of the turbine valves must be limited to a certain extent in order not to exceed the maximum design pressure in the high pressure part, if necessary By the method described a high power surplus can be ensured. And especially in the upper load range, the advantages of this measure to advantage, since here move the temperature changes at the evaporator outlet due to the water-steam properties of the flow medium within tolerable limits.

FIG 1

ein Diagramm mit Simulationsergebnissen zur Verbesserung der Sofortreserve eines fossil befeuerten Durchlaufdampferzeugers durch Erhöhung der Speisewassermenge zusammen mit Einspritzung von Hochdruck-Dampf, Zwischenüberhitzungs-Dampf und jeweils in beiden Drucksystemen in einem oberen Lastbereich, und

FIG 2

ein Diagramm mit Simulationsergebnissen zur Verbesserung der Sofortreserve eines fossil befeuerten Durchlaufdampferzeugers durch Erhöhung der Speisewassermenge zusammen mit Einspritzung von Hochdruck-Dampf, Zwischenüberhitzungs-Dampf und jeweils in beiden Drucksystemen in einem unteren Lastbereich.

An embodiment of the invention will be explained in more detail with reference to a drawing. Show:

FIG. 1

a graph of simulation results to improve the immediate reserve of a fossil-fired continuous steam generator by increasing the amount of feedwater together with injection of high-pressure steam, reheat steam and in each case in an upper load range in both pressure systems, and

FIG. 2

a graph with simulation results to improve the immediate reserve of a fossil-fired continuous steam generator by increasing the amount of feed water together with injection of high-pressure steam, reheat steam and in each case in a lower load range in both pressure systems.

Identical parts are provided with the same reference numerals in all figures.

FIG. 1 shows a diagram with simulation results using the control method in a fossil-fired steam generator, ie a sudden reduction of enthalpy target value at the evaporator outlet to increase the feedwater quantity at constant held firing performance. Plotted is the percent additional power in terms of full load 1 versus time 2 in seconds after a sudden reduction in the setpoint of evaporator specific enthalpy by 100 kJ / kg at 95% load. This reduction provides in the control concept for an increase in the feedwater flow rate. Curve 4 shows the result without additional use of injections, while curves 6 and 8 represent the results for additional use of injections in the high-pressure stage or in the high-pressure and medium-pressure stage. For comparison, more curves 10, 12, 14 are shown, which show the results without increasing the amount of feedwater, but by using only the injections in high-pressure stage (curve 10), medium pressure stage (curve 12) and two pressure levels (curve 14). In each case, the injection is achieved by reducing the setpoint for live steam temperature and, if appropriate, reheaction temperature by 20 K.

In FIG. 1 It can be seen that the maxima of the curves 4, 6 and 8 are arranged higher than those of the curves 10, 12 and 14. The additional unbound power is thus higher. In particular, a combination of the measures relating to feed water and injections shows a significant increase in power (curves 6, 8). Curve 4 already shows that at the high load in FIG. 1 the increase in the feedwater flow shows the highest power yield of all individual measures (comparison of the curves 10, 12, 14). However, the use of injections ensures even faster delivery of additional power, as can be seen from the peaks of the corresponding curves lying farther to the left in the graph.

FIG. 2 is opposite FIG. 1 only slightly modified and shows the simulated curves 4, 6, 8, 10, 12, 14 for 40% load, all other parameters coincide FIG. 1 the same applies to the curves 4, 6, 8, 10, 12, 14. Here, in particular, the curves 4, 6, 10 show a substantially flatter course than in FIG. 1 that is, there is a slower increase in power at a lower level. Also, the power surplus due to the feedwater flow increase is less pronounced, albeit still significant.

Only the modification of the reheat, shown in curve 12 shows a comparatively high power increase about 60 seconds after the change of the setpoint, which then rapidly drops again to go to the maximum of the flat course. This increase in performance is also reflected in a modification of both pressure levels according to curves 8 and 14. In all cases, however, shows that the increase in power with an increase in the feed water quantity allows the highest power output at a higher duration, this effect especially in the high load range pronounced is.

Claims (6)

1. Method for regulating a brief increase in power of a steam turbine that has an upstream fossil-fired once-through steam generator featuring a number of economiser, evaporator and superheater heating surfaces which form a flow path and through which a flow medium flows, wherein the flow of the flow medium through the fossil-fired once-through steam generator is increased in order to achieve the brief increase in power of the steam turbine, wherein a desired enthalpy value at the outlet of an evaporator heating surface is used as a control variable when determining the desired value for the flow of the flow medium through the fossil-fired once-through steam generator, and the desired enthalpy value is reduced in order to achieve the brief increase in power of the steam turbine.
2. Method according to claim 1, wherein the desired enthalpy value is reduced to a predetermined minimum enthalpy value.
3. Method according to claim 2, wherein the minimum enthalpy value is dimensioned in such a way that complete evaporation of the flow medium in the evaporator heating surfaces is achieved under all load conditions of the fossil-fired once-through steam generator.
4. Method according to one of claims 1 to 3, wherein the magnitude and/or duration of the reduction in the desired enthalpy value is determined with reference to the required increase in power.
5. Method according to one of the preceding claims, wherein in order to achieve the brief increase in power of the steam turbine, flow medium taken from the flow path is injected in the region of a superheater heating surface of the fossil-fired once-through steam generator.
6. Method according to one of the preceding claims, wherein the heat supply to the fossil-fired once-through steam generator is increased.

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