EP3827200B1 - Feed water control for forced throughput by-product steam generator - Google Patents

Description

The invention relates to a method for operating a once-through steam generator designed as a heat recovery steam generator. It also relates to a once-through steam generator for carrying out the process.

The feedwater control concept for Benson evaporators is essentially based on the calculation of a pilot control signal for the feedwater mass flow using measured process variables. Such a pre-control signal is typically calculated from known reference values or disturbance variables of the control loop or changes thereto and finally corrected multiplicatively with the output signal of the controller. It anticipates the reaction of the controller to a setpoint change or a disturbance variable and increases the dynamics of the controller so that the desired overheating at the evaporator outlet (setpoint) is set as well as possible in all conceivable phases of the process. When a Benson evaporator was used for the first time in a heat recovery steam generator of vertical design, it has now been shown that, due to the design, said controller intervention must be significantly stronger than in the known horizontal design. However, this also increases the ability of the control loop to oscillate. As a result, insufficient positioning accuracy of the feedwater control valves (e.g. due to poor hardware quality) is becoming even more important. In extreme cases, undesired residual process fluctuations of a significant magnitude can be observed in otherwise stationary plant operation.

A feed water control for Benson heat recovery steam generators is for example in EP 2 212 618 B1 disclosed. There it is assumed that a sufficiently reliable predictive mass flow control that can also be used for steam generators switched as waste heat boilers should be largely adapted to the special features of the waste heat boiler. In particular, it should It must be taken into account that, unlike with fired boilers, in this case the firing capacity is not a suitable parameter that allows a sufficiently reliable conclusion to be drawn about the heat flow balance on which it is based. In particular, it should be taken into account that with a value that is equivalent for waste heat boilers, namely the current gas turbine output or parameters correlating with this, other gas turbine-internal parameters can also occur, so that no acceptable conclusions can be drawn about the enthalpy conditions when the hot gas enters the flue gas duct of the steam generator is. The heat flow balance used to determine the required feedwater flow should therefore be based on other, particularly suitable parameters, such as the hot gas temperature when it enters the evaporator and the mass flow of the hot gas.

the EP 2 297 518 B1 further discloses that characteristic correction values are taken into account for the time derivative of the enthalpy at the inlet of one or more of the evaporator heating surfaces.

For use in the solar thermal power plant reveals the DE 10 2010 040 210 A1 also a method in which a correction value characteristic of the time derivative of the enthalpy, the temperature or the density of the flow medium at the inlet of one or more of the heating surfaces is taken into account for the creation of the set value for the feedwater mass flow.

the U.S. 2014/034044 A1 claims not only a solar thermal steam generator itself, but also a method for operating this solar thermal steam generator, in which the setting of the feed water mass flow is controlled predictively. For this purpose, a correction value is also used here, by means of which thermal storage effects of stored or withdrawn thermal energy are corrected.

Finally also reveals the DE 10 2011 004 263 A1 a method for operating a solar-heated heat recovery steam generator, in which a device for adjusting the feedwater mass flow is supplied with a target value for the feedwater mass flow, a characteristic correction value being taken into account, by means of which thermal storage effects of thermal energy stored or withdrawn in one or more of the heating surfaces are corrected .

Since the present problem occurred during the initial application of a Benson evaporator in a vertical heat recovery steam generator, there are no further approaches to solving the problem. The solution to the problem chosen in this specific case was to slightly reduce the gain of the controller again. However, with this approach, depending on the given boundary conditions, poorer and, in extreme cases, undesirable operating behavior of the system must be accepted.

The object of the invention is therefore to provide a method for operating a once-through steam generator designed as a waste-heat steam generator, in which improved feedwater control leads to stable operating behavior of the plant. Furthermore, a once-through steam generator that is particularly suitable for carrying out the method is to be specified.

The invention is defined by the features of claim 1 and solves the task of a method by providing that in a once-through steam generator designed as a heat recovery steam generator with a preheater, comprising a number of preheater heating surfaces, and with an evaporator, comprising a number of the Preheater heating surfaces downstream from the evaporator heating surfaces on the flow medium side, in which a setpoint value for the feedwater mass flow is fed to a device for adjusting a feedwater mass flow, with a waste heat flow transferred to a fluid in the evaporator heating surfaces being determined when the setpoint for the feedwater mass flow is generated as well as mass storage and energy storage in the fluid in the evaporator heating surfaces during transient system operation, a temporal behavior of the mass storage in the evaporator is coupled to a temporal behavior of a mass storage in the preheater, with scaling using a ratio of the density changes in the evaporator and in the preheater.

It is important to understand that in the present invention an observer is not connected to a fluid particle in a figurative sense and flows with it through the evaporator, but that the observer regards the evaporator as a balance space into which fluid flows in and out. During normal operation of the system, a fluid particle will always absorb energy on the way from the evaporator inlet to the evaporator outlet, regardless of whether the system operation is stationary or transient. It is different when considering the system according to the invention, where during stationary operation of the system (the evaporator) the same temperatures and pressures are measured at a specific location in the evaporator at different times and thus the time derivatives of the corresponding terms in the formulas describing the process become zero. The changes in these parameters over time during transient operation of the evaporator are now taken into account by the inventive method. Of course, this can lead to energy or mass injections as well as energy or mass withdrawals.

With this method, in which the algorithm for calculating the pre-control signal, which in the simplest state of the art only takes into account the heat flow Q̇ _Ev , _fl transferred to the fluid in the evaporator, which results from the heat flow in the exhaust gas Q̇ _EG minus the heat storage in the wall material of the heating surface tubes Q̇ _s , _w is expanded by the influence of the fluid-side mass and energy storage effects in the evaporator, the quality of the pre-control signal is particularly high for the described application of the vertical Heat recovery steam generator further improved, thus minimizing the necessary correction by the controller. The potential consequence of this is that the controller can then be parameterized more weakly, so that the problem described above does not occur, but at the same time the operating behavior of the system is not negatively influenced.

The storage terms for mass storage and energy storage are advantageously determined from current measured values. This enables a particularly reliable evaluation of the heat flow balance and thus the determination of a particularly precisely precalculated desired feed water value.

The current measured values are expediently pressures and temperatures at the preheater inlet, at the preheater outlet or evaporator inlet and at the evaporator outlet.

It is advantageous if a specific enthalpy of the fluid in the evaporator required for estimating the energy storage is approximated by the arithmetic mean of the boiling and saturation enthalpy.

It is expedient here if the boiling enthalpy and saturation enthalpy are determined via at least one pressure measurement at the evaporator inlet or at the evaporator outlet.

The correction values for mass storage and energy storage for determining the setpoint for the feedwater mass flow are advantageously determined taking into account the time derivatives of the boiling and saturation enthalpies in the evaporator and a density of the flow medium in the preheater. With regard to the density, an average fluid density in the preheater can be defined and calculated in particular by suitable measurements of temperature and pressure at the inlet and outlet of the respective preheater, a linear density profile being expediently used as a basis. This can be used to compensate for mass memory effects that occur with transient processes. If, for example, the heat supply to the evaporator heating surfaces decreases due to a load change, fluid is temporarily stored there. If the flow rate of the feedwater pump were constant, the mass flow would drop when the heating surface exited. This can now be compensated for by temporarily increasing the feedwater mass flow.

In practice, these temporally variable processes or temporal derivatives are advantageously determined via a first and a second differentiating element, preferably DT1 elements, to which parameters such as temperature and pressure are fed at suitable measuring points on the input side.

It is advantageous if the first differentiating element, which describes the change in density over time in the preheater for estimating the mass storage, is subjected to an amplification factor that corresponds to the total volume of the flow medium in the evaporator heating surfaces. The correction signals generated with the invention for the feedwater mass flow can represent the effects of mass and energy storage in a particularly advantageous manner if suitable amplifications and time constants are selected for the respective DT-1 element.

In particular, it is advantageous if the first differentiating element is subjected to a time constant corresponding essentially to half the flow time of the flow medium through the evaporator.

It is also advantageous if the second differentiating element for estimating the energy storage is subjected to a time constant of between 5 s and 40 s. With regard to the once-through steam generator, the stated object is achieved by a once-through steam generator according to claim 11 with a number of evaporator heating surfaces and a number of preheater heating surfaces connected upstream on the flow medium side and with a device for adjusting the feedwater mass flow, which can be guided using a target value for the feedwater mass flow, the target value being designed using the inventive method.

With the present invention, the correction of the pilot control signal by the controller can be noticeably reduced and the controller can be parameterized with a lower gain. The problem described above of undesired residual process fluctuations of a significant magnitude can thus be eliminated. The operating behavior of the system is not negatively influenced.

Correction factors found empirically for the pilot control signal (or even entire parameter fields) are also conceivable. Finding them, however, takes a lot of effort. In contrast to this, the invention described is based on physical approaches and does not have to be further parameterized.

Figur 1

eine Skizze des Algorithmus zur Berechnung des Speisewassermassenstroms und

Figur 2

eine Darstellung der Messgrößen und der daraus abgeleiteten Approximationen für die Änderungen im Algorithmus zur Berechnung des Sollwerts des Speisewassermassenstroms, wie sie in der Kraftwerksautomatisierung zu implementieren sind.

The invention is explained in more detail by way of example with reference to the drawings. They show schematically:

figure 1

a sketch of the algorithm for calculating the feedwater mass flow and

figure 2

a representation of the measured variables and the approximations derived from them for the changes in the algorithm for calculating the target value of the feedwater mass flow, as they are to be implemented in the power plant automation.

the figure 1 shows schematically the change in the algorithm resulting from the invention for calculating the desired value for the feedwater mass flow Ṁ _FW . The part of the algorithm relevant to the invention is shown within the dashed border and the prior art outside.

The setpoint for the feedwater mass flow Ṁ _FW is therefore made up of the feedwater mass flow for the evaporator Ṁ _Ev,in and the mass flow Ṁ _S , _E stored or withdrawn in the preheater, corrected with a factor f _Ctrl .

According to the state of the art, the feedwater mass flow for the evaporator Ṁ _Ev , _in results from the quotient of the heat flow Q̇ _Ev , _fl transferred from the exhaust gas to the fluid in the evaporator and the setpoint for the enthalpy change in the evaporator Δh Ev, set. The heat flow Q̇ _Ev,fl transferred to the fluid in the evaporator results from the heat flow in the exhaust gas Q̇ _EG minus the heat storage in the wall material of the heating surface tubes Q̇ _S , _W .

According to the invention, the term for the heat flow transferred to the fluid in the evaporator is supplemented and corrected by two further terms.

The first correction concerns the mass storage effect in the evaporator, the second correction concerns the energy storage effect in the evaporator.

(Massenspeicherung) und h_Ev, _out, _set (Enthalpie am Austritt des Verdampfers) dargestellt. $\frac{{\mathit{dU}}_{\mathit{Ev}}}{\mathit{dt}}$

steht für den Energiespeichereffekt.The mass storage effect is in the heat flows figure 1 through the product $\frac{{\mathit{dm}}_{\mathit{possibly}}}{\mathit{German}}$

(mass storage) and h _Ev , _out , _set (enthalpy at the outlet of the evaporator). $\frac{{\mathit{you}}_{\mathit{possibly}}}{\mathit{German}}$

stands for the energy storage effect.

According to the invention, these values are suitably approximated so that they can be determined from measured process variables.

figure 2 shows these measured variables or the measuring points in the once-through heat recovery steam generator and their processing.

The once-through heat recovery steam generator according to figure 2 includes a preheater 1, also known as an economizer, for as Feed water provided for the flow medium, with a number of preheater heating surfaces 2, and an evaporator 3 with a number of evaporator heating surfaces 4 downstream of the preheater heating surfaces 2 on the flow medium side. The evaporator 3 is followed by a superheater 12 with corresponding superheater heating surfaces 13. The heating surfaces are located in a gas flue, not shown in detail , Which is acted upon by the exhaust gas of an associated gas turbine system.

As already explained, the once-through steam generator is designed for a controlled application of feed water. For this purpose, a feedwater pump 31 is followed by a throttle valve 33 controlled by a servomotor 32, so that the feedwater quantity conveyed by the feedwater pump 31 in the direction of the preheater 1 or the feedwater mass flow can be adjusted by suitably controlling the throttle valve 33. A measuring device 34 for determining the feedwater mass flow through the feedwater line 35 is connected downstream of the throttle valve 33 in order to determine a current characteristic value for the feedwater mass flow supplied. The servomotor 32 is actuated via a control element 36 which is acted upon on the input side by a setpoint value for the feedwater mass flow Ṁ _FW supplied via a data line 37 and by the current actual value of the feedwater mass flow determined by the measuring device 34 . A correction requirement is transmitted to the controller 36 by forming the difference between these two signals, so that if the actual value deviates from the desired value, the throttle valve 33 is corrected accordingly via the activation of the motor 32 .

In order to determine a particularly needs-based setpoint for the feedwater mass flow Ṁ _FW in the manner of a predictive, anticipatory setting of the feedwater mass flow or an adjustment based on future or current needs, the data line 37 is connected on the input side to a feedwater flow control 38 designed to specify the setpoint for the feedwater mass flow Ṁ _FW . This is for it designed to determine the target value for the feedwater mass flow Ṁ _FW based on a heat flow balance in the evaporator heating surfaces 4, the target value for the feedwater mass flow Ṁ _FW being determined in that a waste heat flow transferred to a fluid in the evaporator heating surfaces 4 is determined and also mass storage and energy storage in the fluid in the evaporator heating surfaces 4 are taken into account. At the expense of completeness, but in favor of clarity, the figure 2 in the feedwater flow control 38 only the elements that are relevant for the correction of the feedwater mass flow setpoint Ṁ _FW according to the invention. The part known from the prior art is not shown.

The measured values for determining a setpoint for the feedwater mass flow Ṁ _FW are pressure and temperature values and the measuring points are in the areas of preheater inlet 5, preheater outlet 6 or evaporator inlet 7 and evaporator outlet 8.

The measured values determined are processed in function elements 14, 15, 16, 17 and 18. Using the first, second and third function elements 14, 15 and 16, the density of the fluid at various locations on the heating surfaces of preheater 1 and evaporator 3 is determined from the measured values for pressure and temperature. The fourth and fifth function elements 17 and 18 deliver the boiling and saturation enthalpy from measured pressure values.

wird approximiert, indem aus den ermittelten Dichten am Vorwärmereingang 5 und am Vorwärmerausgang 6 zuerst über ein erstes Addierglied 19 und ein erstes Multiplikationsglied 20 ein Mittelwert gebildet wird, der anschließend im ersten Differenzierglied 9 mit einer entsprechend gewählten Zeitkonstanten weiter verarbeitet und mit einem dem Gesamtvolumen V_Ev des Strömungsmediums in den Verdampferheizflächen 4 entsprechenden Verstärkungsfaktor im zweiten Multiplikationsglied 21 beaufschlagt wird.The storage term for mass storage $\frac{{\mathit{dm}}_{\mathit{possibly}}}{\mathit{German}}$

is approximated by first forming an average value from the densities determined at the preheater inlet 5 and at the preheater outlet 6 via a first adder element 19 and a first multiplier element 20, which is then further processed in the first differentiator element 9 with a correspondingly selected time constant and with a value corresponding to the total volume V _Ev of the flow medium in the evaporator heating surfaces 4 corresponding Amplification factor in the second multiplier 21 is applied.

Further scaling takes place in a subsequent third multiplication element 22 with a ratio of the changes in density of the fluid in the evaporator 3 and in the preheater 1, which is determined by means of the first and second subtraction elements 23 and 24 and the first dividing element 25 in the manner shown in FIG figure 2 shown.

wird approximiert, indem aus den ermittelten Enthalpien ein Mittelwert mit Hilfe des zweiten Addierglieds 26 und des vierten Multiplikationsglieds 27 gebildet wird. Dieser Mittelwert stellt eine gute Annahme für die spezifische Enthalpie des Fluids im Verdampfer 3 dar.The storage term for energy storage $\frac{{\mathit{you}}_{\mathit{possibly}}}{\mathit{German}}$

is approximated in that an average value is formed from the determined enthalpies using the second adder 26 and the fourth multiplier 27 . This average represents a good assumption for the specific enthalpy of the fluid in evaporator 3.

wird nun durch die Summe zweier Terme bestimmt. Der erste Term wird dadurch ermittelt, dass die spezifische Enthalpie des Fluids im Verdampfer 3 im zweiten Differenzierglied 10 mit einer entsprechend gewählten Zeitkonstanten weiter verarbeitet und mit einem Mittelwert der Fluidmassen M _Ev im Verdampfer bei maximaler und minimaler Last im fünften Multiplikationsglied 28 beaufschlagt wird. Dieser Mittelwert wird der Einfachheit halber als zeitlich konstanter Wert angesehen. Der zweite Term wird ermittelt, indem die spezifische Enthalpie des Fluids im Verdampfer 3 mit dem Speicherterm für die Massenspeicherung $\frac{{\mathit{dM}}_{\mathit{Ev}}}{\mathit{dt}}$

multipliziert wird. Dies erfolgt im sechsten Multiplikationsglied 29.The storage term for energy storage $\frac{{\mathit{you}}_{\mathit{possibly}}}{\mathit{German}}$

is now determined by the sum of two terms. The first term is determined by further processing the specific enthalpy of the fluid in the evaporator 3 in the second differentiating element 10 with a correspondingly selected time constant and with a mean value of the fluid masses M _Ev in the evaporator at maximum and minimum load in the fifth multiplier 28 is applied. For the sake of simplicity, this mean value is regarded as a value which is constant over time. The second term is determined by comparing the specific enthalpy of the fluid in the evaporator 3 with the storage term for mass storage $\frac{{\mathit{dm}}_{\mathit{possibly}}}{\mathit{German}}$

is multiplied. This takes place in the sixth multiplication element 29.

In the third adder 30, the two terms are combined.

The corresponding algorithm is to be implemented in the function plans of the feedwater control and thus in the power plant automation.

Claims (11)

1. Method for operating a once-through steam generator designed as a waste-heat steam generator, with a pre-heater (1), comprising a number of pre-heater heating surfaces (2), and with an evaporator (3), comprising a number of evaporator heating surfaces (4) connected downstream on the flow medium side of the pre-heater heating surfaces (2), in which a device for setting a feedwater mass flow is fed a setpoint value for the feedwater mass flow, wherein a waste heat flow transferred to a fluid in the evaporator heating surfaces (4) is determined in the creation of the setpoint value for the feedwater mass flow and furthermore mass storage and energy storage in the fluid in the evaporator heating surfaces (4) are detected during non-steady-state plant operation,
   characterized in that a behavior over time of the mass storage in the evaporator (3) is coupled to a behavior over time of a mass storage in the pre-heater (1), wherein scaling is carried out with a ratio of the changes in density in the evaporator (3) and in the pre-heater (1).
2. Method according to Claim 1, wherein storage terms for mass storage and energy storage are determined from current measured values.
3. Method according to Claim 2, wherein the current measured values are pressures and temperatures at the pre-heater input (5), at the pre-heater output (6) or at the evaporator input (7) and at the evaporator output (8).
4. Method according to one of the preceding claims, wherein a specific enthalpy of the fluid in the evaporator (3) required for the estimation of the energy storage is approximated by the arithmetic mean value of the boiling enthalpy and saturation enthalpy.
5. Method according to Claim 4, wherein the boiling enthalpy and the saturation enthalpy are determined by way of at least one pressure measurement either at the evaporator input (7) or at the evaporator output (8).
6. Method according to Claim 5, wherein temporal derivatives of the boiling and saturation enthalpies in the evaporator (3) and also a density of the flow medium in the pre-heater (1) are evaluated.
7. Method according to Claim 6, wherein the temporal derivatives are determined by way of first and second differential elements (9, 10).
8. Method according to Claim 7, in which the first differential element (9), describing the variation over time of the change in density in the pre-heater (1) for the estimation of the mass storage, is subjected to a gain factor corresponding to the total volume of the flow medium in the evaporator heating surfaces (4).
9. Method according to either of Claims 7 and 8, wherein the first differential element (9) is subjected to a time constant corresponding to substantially half the transit time of the flow medium through the evaporator (3).
10. Method according to Claim 7, wherein the second differential element (10) for the estimation of the energy storage is subjected to a time constant that lies between 5 s and 40 s.
11. Forced-flow waste-heat steam generator (11) with a number of evaporator heating surfaces (4) and a number of pre-heater heating surfaces (2) connected upstream on the flow medium side, characterized in that the forced-flow waste-heat steam generator comprises a device for setting a feedwater mass flow, which can be guided on the basis of a setpoint value for the feedwater mass flow, wherein the setpoint value is designed on the basis of the method according to one of Claims 1 to 10.

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