Feedwater control for a forced-flow waste-heat steam generator

Abstract

A method for operating a forced-flow steam generator constructed as a waste-heat steam generator having a pre-heater, including pre-heater heating surfaces, and having an evaporator including evaporator heating surfaces connected downstream on the flow medium side of the pre-heater heating surfaces. A device for adjusting a feed water mass flow has a set point for the feed water mass flow. During the creation of the set point for the feed water mass flow, a waste-heat flow transferred to a fluid in the evaporator heating surfaces is determined, and mass storage and energy storage in the fluid in the evaporator heating surfaces is detected during non-steady-state plant operation. A behaviour over time of a mass storage in the evaporator is coupled with a behaviour over time of a mass storage in the pre-heater, wherein scaling is carried out with a ratio of the density changes in the evaporator and pre-heater.

Claims

1. A method for operating a once-through steam generator designed as a waste-heat steam generator, with a pre-heater, comprising a number of pre-heater heating surfaces, and with an evaporator, comprising a number of evaporator heating surfaces connected downstream on the flow medium side of the pre-heater heating surfaces, the method comprising: determining a setpoint value for a feedwater mass flow based on a waste heat flow transferred to a fluid in the evaporator heating surfaces, and detecting mass storage and energy storage in the fluid in the evaporator heating surfaces during non-steady-state plant operation, wherein a behavior over time of the mass storage in the evaporator is coupled to a behavior over time of a mass storage in the pre-heater, and wherein scaling is carried out with a ratio of the changes in density in the evaporator and in the pre-heater.

2. The method as claimed in claim 1, wherein storage terms for mass storage and energy storage are determined from current measured values.

3. The method as claimed in claim 2, wherein the current measured values are pressures and temperatures at the pre-heater input, at the pre-heater output or at the evaporator input and at the evaporator output.

4. The method as claimed in claim 1, wherein a specific enthalpy of the fluid in the evaporator required for the estimation of the energy storage is approximated by the arithmetic mean value of the boiling enthalpy and saturation enthalpy.

5. The method as claimed in 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 or at the evaporator output.

6. The method as claimed in claim 5, wherein temporal derivatives of the boiling and saturation enthalpies in the evaporator and also a density of the flow medium in the pre-heater are evaluated.

7. The method as claimed in claim 6, wherein the temporal derivatives are determined by way of first and second differential elements.

8. The method as claimed in claim 7, wherein the first differential element, describing the variation over time of the change in density in the pre-heater 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.

9. The method as claimed in claim 7, wherein the first differential element is subjected to a time constant corresponding to substantially half the transit time of the flow medium through the evaporator.

10. The method as claimed in claim 7, wherein the second differential element for the estimation of the energy storage is subjected to a time constant that lies between 5 s and 40 s.

11. A forced-flow waste-heat steam generator, comprising: a number of evaporator heating surfaces, a number of pre-heater heating surfaces connected upstream on the flow medium side, and a device for setting the 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 as claimed in claim 1.

12. The method as claimed in claim 1, wherein during steady-state plant operation, the temperatures and pressures measured at a specific location in the evaporator at different times are the same, such that the temporal derivatives describing the process become zero.

13. The method as claimed in claim 1, wherein during non-steady-state plant operation, changes in the temperatures and pressures measured at a specific location in the evaporator at different times are taken into account.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention is explained more specifically by way of example on the basis of the schematic drawings, in which:

(2) FIG. 1 shows a diagram of the algorithm for calculating the feedwater mass flow and

(3) FIG. 2 shows a representation of the measured variables and the approximations derived therefrom for the changes in the algorithm for calculating the setpoint value of the feedwater mass flow, as they are to be implemented in automation of the power plant.

DETAILED DESCRIPTION OF INVENTION

(4) FIG. 1 schematically shows the change in the algorithm resulting from the invention for calculating the setpoint value for the feedwater mass flow {dot over (M)}.sub.FW. In this case, the component of the algorithm that is relevant to the invention is shown inside the surrounding border indicated by dashed lines and the prior art is shown outside.

(5) The setpoint value for the feedwater mass flow {dot over (M)}.sub.FW is accordingly made up of the feedwater mass flow for the evaporator {dot over (M)}.sub.Ev,in and the mass flow {dot over (M)}.sub.S,E stored in the pre-heater or withdrawn from it, corrected by a factor f.sub.Ctrl.

(6) The feedwater mass flow for the evaporator {dot over (M)}.sub.Ev,in is obtained according to the prior art as the quotient of the heat flow {dot over (Q)}.sub.Ev,fl transferred from the waste gas to the fluid in the evaporator and the setpoint value for the change in enthalpy in the evaporator Δh.sub.Ev,set. The heat flow {dot over (Q)}.sub.Ev,fl transferred to the fluid in the evaporator is obtained once again from the heat flow in the waste gas {dot over (Q)}.sub.EG minus the heat storage in the material of the wall of the heating surface tube {dot over (Q)}.sub.S,W.

(7) 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.

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

(9) The mass storage effect is represented in the heat flows of FIG. 1 by the product of

(10) $\frac{{\mathrm{dM}}_{\mathrm{Ev}}}{\mathrm{dt}}$
(mass storage) and h.sub.Ev,out,set (enthalpy at the outlet of the evaporator)

(11) $\frac{{\mathrm{dU}}_{\mathrm{Ev}}}{\mathrm{dt}}$
stands for the energy storage effect.

(12) These values are suitably approximated according to the invention, so that they can be determined from measured process variables.

(13) FIG. 2 shows these measured variables and the measuring points in the forced-flow waste-heat steam generator and their processing.

(14) The forced-flow waste-heat steam generator according to FIG. 2 comprises a pre-heater 1, also referred to as an economizer, for feedwater provided as a flow medium, with a number of pre-heater heating surfaces 2, and an evaporator 3, with a number of evaporator heating surfaces 4 connected downstream on the flow medium side of the pre-heater heating surfaces 2. The evaporator 3 is followed by a superheater 12 with corresponding superheater heating surfaces 13. The heating surfaces are located in a gas exhaust, which is not shown any more specifically and to which the waste gas of an assigned gas turbine plant is admitted.

(15) As already stated, the forced-flow steam generator is designed for controlled admission of feedwater. For this purpose, a throttle valve 33 activated by a servomotor 32 is arranged downstream of a feedwater pump 31, so that, by way of suitable activation of the throttle valve 33, the amount of feedwater delivered by the feedwater pump 31 in the direction of the pre-heater 1 or the feedwater mass flow can be set. For determining a current characteristic value for the fed feedwater mass flow, arranged downstream of the throttle valve 33 is a measuring device 34 for determining the feedwater mass flow through the feedwater line 35. The servomotor 32 is activated by way of a control element 36, which is subjected on the input side to a setpoint value for the feedwater mass flow {dot over (M)}.sub.FW, fed via a data line 37, and the current actual value of the feedwater mass flow, determined by way of the measuring device 34. By forming the difference between these two signals, an adjustment requirement is transmitted to the controller 36, so that, if there is a deviation of the actual value from the setpoint value, a corresponding adjustment of the throttle valve 33 is performed by way of the activation of the motor 32.

(16) For determining a setpoint value for the feedwater mass flow {dot over (M)}.sub.FW that is particularly appropriate for the requirement, in the manner of a setting of the feedwater mass flow that is predictive, forward-looking or based on the future or current requirement, the data line 37 is connected on the input side to a feedwater flow control 38 designed for selecting the setpoint value for the feedwater mass flow {dot over (M)}.sub.FW. This is designed to determine the setpoint value for the feedwater mass flow {dot over (M)}.sub.FW on the basis of an enthalpy balance in the evaporator heating surfaces 4, wherein the setpoint value for the feedwater mass flow {dot over (M)}.sub.FW is determined by providing that a waste heat flow transferred to a fluid in the evaporator heating surfaces 4 is determined and furthermore mass storage and energy storage in the fluid in the evaporator heating surfaces 4 are taken into account. At the expense of completeness, but to the benefit of overall clarity, FIG. 2 only shows in the feedwater flow control 38 the elements that are relevant to the correction according to the invention of the feedwater mass flow setpoint value {dot over (M)}.sub.FW. The part known from the prior art is not shown.

(17) The measured values for determining a setpoint value for the feedwater mass flow {dot over (M)}.sub.FW are pressure and temperature values and the measuring points lie in the regions of the pre-heater input 5, pre-heater output 6 or evaporator input 7 and evaporator output 8.

(18) The measured values determined are processed in functional elements 14, 15, 16, 17 and 18. By means of the first, second and third functional elements 14, 15 and 16, the density of the fluid at various locations of the heating surfaces of the pre-heater 1 and evaporator 3 are determined from the measured values for pressure and temperature. The fourth and fifth functional elements 17 and 18 provide the boiling enthalpy and saturation enthalpy from measured pressure values.

(19) The storage term for the mass storage

(20) $\frac{{\mathrm{dM}}_{\mathrm{Ev}}}{\mathrm{dt}}$
is approximated, in that first a mean value is formed from the determined densities at the pre-heater input 5 and at the pre-heater output 6, by way of a first adding element 19 and a first multiplying element 20, the mean value is subsequently processed further with a correspondingly chosen time constant in the first differential element 9 and subjected to a gain factor corresponding to the total volume V.sub.Ev of the flow medium in the evaporator heating surfaces 4 in the second multiplying element 21.

(21) Further scaling takes place in a following third multiplying element 22 with a ratio of the changes in density of the fluid in the evaporator 3 and in the pre-heater 1, which is determined by means of the first and second subtracting elements 23 and 24 and the first dividing elements 25 in the way shown in FIG. 2.

(22) The storage term for the energy storage

(23) $\frac{{\mathrm{dU}}_{\mathrm{Ev}}}{\mathrm{dt}}$
is approximated, in that a mean value is formed from the determined enthalpies with the aid of the second adding element 26 and the fourth multiplying element 27. This mean value represents a good assumption for the specific enthalpy of the fluid in the evaporator 3.

(24) The storage term for the energy storage is

(25) $\frac{{\mathrm{dU}}_{\mathrm{Ev}}}{\mathrm{dt}}$
then determined by the sum of two terms. The first term is determined by the specific enthalpy of the fluid in the evaporator 3 being processed further with a correspondingly chosen time constant in the second differentiating element 10 and subjected to a mean value of the fluid masses M.sub.Ev in the evaporator under maximum and minimum load in the fifth multiplying element 28. For the sake of simplicity, this mean value is regarded as a time-constant value. The second term is determined in that the specific enthalpy of the fluid in the evaporator 3 is multiplied by the storage term for the mass storage

(26) $\frac{{\mathrm{dM}}_{\mathrm{Ev}}}{\mathrm{dt}}.$
This takes place in the sixth multiplying element 29.

(27) In the third adding element 30, the two terms are brought together.

(28) The corresponding algorithm is to be implemented in the functional plans of the feedwater control, and consequently in the automation of the power plant.