EP3014178A2

EP3014178A2 - Operating method for an externally heated once-through steam generator

Info

Publication number: EP3014178A2
Application number: EP14758811.5A
Authority: EP
Inventors: Jan BRÜCKNER; Frank Thomas
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 2013-08-28
Filing date: 2014-08-20
Publication date: 2016-05-04
Also published as: WO2015028366A3; WO2015028366A2; IL243949A0; US20160208656A1

Abstract

The invention relates to an operating method for an externally heated once-through steam generator (1), in particular a once-through steam generator that is heated using solar thermal energy and has a steam turbine (2) connected downstream of the once-through steam generator, wherein a pressure regulation device (3), having at least one turbine valve (31) for regulating the pressure, is arranged in the feed water-steam circuit between the once-through steam generator (1) and the steam turbine (2), said pressure regulation device (3) being controlled by a control unit (4') in such a way that, in the event of sudden load reductions, the drop in pressure associated therewith occurs, in a time delayed manner, by means of a throttling of the at least one turbine valve (31).

Description

description

Operating procedure for an externally heated forced-circulation steam generator

The invention relates to an operating method for an externally heated forced once - through steam generator, in particular a so ^¬ larthermically heated forced once - through steam generator, according to the preamble of claim 1.

Solar thermal power plants are an alternative to conventional electricity generation ago ^¬. An already known power plant concept in this area is trough power plant, the so-called parabolic. In this type of power plant thermal oil is typically used as the heat transfer medium, which is the

Parabolic troughs of a solar field flows through and absorbs the heat introduced via the sun as an external heat source and transfers it to a flow medium flowing through the steam generator.

For such a solar thermally heated steam generator provides the once-through principle is the preferred execution ^¬ form. The entering the once-through steam generators and designated at this point as feedwater Strö ^¬ tion medium is warmed up in a single pass, vaporized and overheated. The superheated flow medium is then called

Main steam supplied via a water-steam separator of the steam turbine. The water-steam separator at the outlet of the once-through steam generator is predominantly used during the start-up phase. During normal load ranges ^¬ operating phase contrast superheated Strö ^¬ tion medium at the outlet of the once-through steam generator and therefore also in the water-steam separator must always sufficiently present so that the steam turbine is not supplied with saturated steam. The ^¬ A position of the corresponding fresh steam temperature at the outlet of the once-through steam generator can be set accurately only by the choice of the correct feed-water mass flow, therefore, and are variations of the feed-water mass flow directly linked to fluctuations in the live steam temperature.

To such fluctuations in the main steam temperature in the feed water-steam cycle, in particular during the load ranges ^¬ operating phase solar thermal power plants to counteract a process for pre- diktiven or anticipatory control of Speisewassermas- is senstroms already in WO 2012/110344 Al by a correction value proposed. This type of predictive control of the feed-water mass flow it ^¬ it enables, deviations of the undesirable at the outlet of Zwangdurchlauf- steam generator present specific enthalpy from the target value and the resulting large variations in the steam temperature in all operating states of load operation phase by transient conditions such as, for example, at a load change be kept as low as possible.

It has now been found that where low pressures are present in fast load-cuts in the load phase of operation, and here especially in unte ^¬ load range to unacceptably high in the water steam cycle of the flow medium Tem ^¬ peraturschwankungen of the outlet of the Zwangdurchlauf- Steam generator occurring live steam can come. Decrease for as for example in solar thermal power plants due to reduced absorption of heat by fluctuating sunlight and the mass flow of the Strömungsme ^¬ diums by the once-through steam generator, and takes this in addition at low pressures place at which available from the major differences in the physical properties of water and steam impact ent ^¬ talking serious, these temperature changes can no longer be absorbed by the WO feedwater control concept described 2012/110344 AI may. Since, in the case of a rapid load reduction, the pressures fall too fast and too strongly in accordance with the natural sliding pressure characteristic curve, there is a disproportionately high relative to the quasi-stationary state

Live steam mass flow at the exit of the forced running steam generator before. Decreases at a load reduction in the pressure, so at high load decreases in the lower load range excess steam from the forced flow steam generator expelled, resulting in a relatively large reduction in the steam temperature result. Such temperature fluctuations can then lead depending on the size of an unwanted shutdown of the forced-circulation steam generator downstream steam turbine and thus the entire power plant.

The object of the invention is therefore for power plants, and in particular ^¬ for solar thermal power plants in which, unlike power plants with thermal storage devices, the load change speed is not freely selectable but is dependent on the respective solar radiation to provide an operating method for an externally heated forced once-through steam generator with the help of such inadmissibly high and thus possibly no longer ver ^¬ controllable temperature fluctuations at the outlet of the solar thermal or otherwise externally heated forced once-through steam generator can be avoided even during rapid load decreases in the lower load range.

This is with the operating method for an externally heated, in particular solar thermal heated

Forced-circulation steam generator with the features of claim 1 solved.

The fact that during a caused by an external heat source (such as the sun) fast load reduction of the associated drop in live steam pressure via a throttling of the at least one turbine valve is delayed in time, thus resulting in a modified from the natural sliding pressure deviating Gleitdruckkennlinie, can also for fast load decreases, especially in the lower load range of the load operating phase, the live steam temperature at the outlet of the forced flow steam generator are held in ^{¬ to} allowable limits. As a result, an undesirable shutdown of the forced flow steam generator downstream steam turbine can be avoided and thus the availability of the power plant, in particular the solar thermal ^¬ power plant, permanently ensured.

Thus, the method ^{according to} the invention can additionally stabilize the entire pressure control and, moreover, that in the

Support WO feedwater control concept described 2012/110344 AI towards ^¬ clearly a low-jitter control of the fresh steam temperature, since the pressure during the load reduction and hence on reaching the fixed pressure value has ge ^¬ ringere transients and thus relative to the solid ^¬ pressure value in lower under- and overshoots opens ,

Particularly advantageously, this may be it ^¬ ranges by optimizing the appli ^¬ case of use performance management of the steam turbine. It has been found that a very effective way to reduce the pressure moderate, is be ^¬ is to couple the current power delivery of the steam turbine to the steam generator currently transferred from the heat transfer medium to the heat flow Zwangdurchlauf- functional. Specifically, this means that in relative terms the steam turbine has to reduce its load just as quickly as the heat currently being transferred to the forced-circulation steam generator decreases.

Since the steam production of a solar thermal heated continuous flow steam generator due to its thermal and volumetric storage capacity the current heat supply through the heat transfer medium always follows with a time delay, the demand of equal rates of change of heat release of the heat transfer fluid and Leistungsentbindung the Dampfturbi ^¬ ne by throttling the turbine valves at a load reduction be fulfilled. For this purpose, the pressure control that controls the at least one steam turbine valve to extend suitable. Forming a power setpoint for the steam turbine on Ba ^¬ sis of the currently transmitted to the steam generator heat flow, this is compared with the current power delivery of the steam turbine, so the control deviation formed therefrom can be used directly after appropriate normalization to on the controller used in the pressure control, the steam turbine valves related to the power release of

Steam turbine suitable to control. For this purpose, the current transmitted to the forced flow steam generator heat output of the heat transfer medium must be determined. If this determination is made, for example, according to the feedwater control concept known from WO 2012/110344 A1, then it can be used directly to form the desired power value of the steam turbine without additional effort.

If now such an inventively modified pressure Rege ^¬ development applies, followed by a rapid load reduction only a moderate decrease in pressure. Since this pressure control is self-reinforcing, based on the Laständerungsge ^¬ speed, the pressure drop decreases all the more moderate, the faster the load is lowered. In a subsequent load ^¬ however, did not rise takes the control of the present invention, since the once-through steam generator runs after the heat offered Mestrom the heat transfer medium. Concretely signified ^¬ tet this is that no additional throttling of at least held a turbine valve during a load increase and thus the turbine valves are opened as soon as possible and turbine ensure maximum power delivery of the steam, which is desirable here.

An essential advantage of this modified pressure control is that, in parallel with the known pressure control, the power delivery of the steam turbine can now also be suitably influenced. The control concept according to the invention can thereby be permanently in operation and does not have to be activated and deactivated in the connection for a load increase again ge ^¬ suitable criteria during a load reduction. For the method according to the invention so no additional effort is required because the adjustments in today's usual

Pressure control structures can be introduced and the required measurement and data values are already largely present ^{¬ hand} . The inventive method is independent of the Were ^¬ meträgermedium principle for all externally heated

Forced circulation steam generator applicable. But especially with solar thermally heated once-through steam generators, which are temporally highly dependent on variations in the sunlight, the debonded turbine power and whose time behavior is coupled by the Invention ^¬ method according closer to the performance of the solar panel at a load reduction that the from the perspective of Control of the entire power plant can be beneficial.

Optionally, the method could if several steam valves, usually is in fact adjacent to the high-pressure turbine valve in systems with intermediate superheating, a low pressure turbine valve provided, on which are applied under different ^¬ union turbine valves. Under these circumstances, the reheater on the heat transfer medium side or the low-pressure turbine on the turbine side could also be included in the pressure control.

The invention will now be explained by way of example with reference to the following figures. 1 shows schematically a known pressure control,

2 shows schematically a development of the invention.

1 shows very schematically a conventional between forced flow steam generator 1 and steam turbine 2 arranged

Pressure control device 3. This pressure control device 3 consists of an adjustable control valve 31 and a pressure measuring device 32 in the steam line 6 between

Forced-circulation steam generator 1 and steam turbine 2. Via the pressure-measuring device 32, the live steam pressure prevailing in the steam line 6 is measured and fed to a control device 4. From the measured steam pressure of the presently applied pressure setpoint 41 übli ^¬ cherweise corresponds to the hard pressure is withdrawn. The Standde control deviation 44 is supplied to a controller 42 after appropriate normalization. This controller 42 may be, for example, a PID, PI, P or a combination of the individual controllers. The controller 42 then controls the controllable valve 31 in accordance with the control deviation via a motor 33 or any other actuator in such a way that the predetermined desired pressure value can be sustainably adjusted. In this case, the tur ^¬ binenventil is usually fully open in normal load operation and the pressure changes in load changes according to the natural Gleitdruckkennlinie. Only when, with a correspondingly large load reduction, does the current live steam pressure fall below the setpoint pressure value does the control begin to intervene and the turbine valve closes in such a way that the preset pressure setpoint (fixed pressure) is established.

As has now been found, in such Regelein ^¬ directions of the pressure mode at load decreases, especially in the lower load range with respect to strong temperature fluctuations at Zwandurchlaufdampferzeugeraustritt a key role. In the case of a rapid load reduction (> 5% / min) in the lower load range, the pressures decrease according to the natural

Sliding pressure characteristic too fast and too strong, so that high pressure transients occur, based on the quasi-stationary state, a disproportionately high fresh steam mass flow at the outlet of the forced flow steam generator before. This is because due to the decrease in pressure in the dining ^¬ water-steam cycle of additional flow medium

Forced-circulation steam generator evaporates. The additional steam requires a comparatively larger volume in the steamer ^¬ zeugerrohren and thus forces due to its expansion more throughput at the exit of the once-through steam generator compared to the water phase in which the steam now also produced still present a short time before. This effect is increasingly exacerbated by decreasing pressures as the density differences between the water and vapor phases increase continuously. As a result, if the pressure drops during a load reduction, then from the mentioned physical see reasons with rapid load decreases in the lower load range excess steam expelled from the forced flow steam generator. This results firstly in a comparison example ^¬ strong lowering of the steam temperature, as is required Liehe additional energy for additional steam production, and secondly in a temporary emptying of the volume of Dampferzeugerheizfläche. Due to the large Heiz ^¬ surface volume of such steam generators, an existing feedwater control, especially in the lower load range, in which the feedwater mass flow is very small, the quasi emptied steam generator tubes fill only delayed again. This results firstly in a strong increase in temperature at the outlet of the once-through steam generator. Due to the temporally upstream flow-side Ausspeicher- effects during the rapid pressure decrease in Lastabsen ^¬ kung and the resulting lower flow medium content, comparatively less steam can now be generated with now higher end temperatures and thus higher final overheating in the further course of time. This negative accompanying effect can be compensated by an injection cooling device 5, so that negative effects on the process are not to be feared. Depending on the strength of the emptying but secondly, a more or less strong decoupling between feedwater mass flow and

Live steam mass flow instead. In concrete terms, this means that the feedwater control can no longer satisfactorily respond to the subsequent temperature fluctuations that result, for example, from load changes. This is exacerbated by the fact that in this decoupling the evaporation end point can move into the forced circulation steam generator very far upstream of the flow medium and, under these circumstances, the steam generator pipe section is greatly extended with superheated flow medium. In consequence, then the heat transfer medium remains wärmeträgermediums- each other in the inlet area of the once-through steam generator on a present compared to the load during the operating phase stationary state we ^¬ sentlich higher temperature. Since here the heat transfer medium also acts as a storage medium, it then takes in the subsequent load operation phase, especially for small feedwater mass flows very long, until this additional stored energy is discharged again. The steam temperature remains at a high temperature level for a very long time under these circumstances. The feedwater control concept is ^¬ built correction controller of main steam temperature during this lengthy period the Speisewassermassen- current increase over charge, since the scheme will try to lichst decrease rapidly compared to the setpoint is raised temperature mög ^¬ so that at some point the steam temperature will suffer inevitably , This temperature decrease is a consequence of the excessive supply and can then not be intercepted by the injection cooling device 5, so that when falling below a minimum steam temperature, the steam turbine has to go out of operation for self-protection. This significantly affects the availability of the entire system. This is where the present invention begins. By the pressure of a modified sliding pressure differing from the natural sliding pressure follows, and so the pressure is moderate reduction during the load change, the time-related ^¬ Lich additional steam production will be lower. An emptying of the forced continuous steam generator with all its

Consequences can be counteracted effectively by the additional DampfProdukti ^¬ on being caused by the decrease in pressure lengthened and thus be maintained feed water and live steam mass flow rather in equilibrium.

According to the invention, therefore, as shown in FIG 2, a mo ^¬ modified control device 4 ^λ provided. This includes in addition to the embodiment shown in FIG 1 loop control circuit 41 to 44, a further loop control circuit with which the refreshes ^¬ elle power delivery of the steam turbine according to a pre-given ^¬ power setpoint can be controlled. For this purpose, the power setpoint 46 in the additional control loop is which can be determined in the form of a mathematical function on the basis of the introduced into the steam generator heat output, subtracted from a means of a corresponding measuring device 21 currently measured steam turbine power. The resulting control deviation is 47 after appropriate normalization 48 is also supplied to the controller 42 so as then according einzu the required performance of the steam turbine set ^¬. As should become ^¬ additionally responds to deviations from a predetermined pressure desired value in addition to the power control in the present embodiment in FIG 2 is to always use a Min selection 45 the afflicted with a negative sign largest deviation which provides for a corresponding closing of the control valves , Depending on the Re ^¬ gelabweichung the individual control loops so either the power control of the steam turbine or the pressure control takes the lead for the throttling behavior of the shown in FIG 2 a control valve 31. For the simplest case in which only the fixed pressure is set as a constant value as the pressure setpoint, takes over at a load lowering the control loop loop 46-48 (power regulation of the steam turbine) so long ^¬ the lead until the actual turbine inlet pressure falls below the predetermined fixed pressure level. From this time ^¬ point takes over the loop loop 41 to 44, the guide and ensures that the fixed pressure setpoint can be ensured by further throttling the control valves.

Specifically, the moderate reduction of pressure is to couple the power release of the steam turbine to the heat output of the heat transfer medium to the forced flow steam generator. Normally, during a load reduction and at the same time fully opened turbine valves, the turbine output temporarily remains at a higher level compared to the heat consumption of the once-through steam generator due to the above-mentioned withdrawal effects of the once-through steam generator (after completion of the load change, a balance between absorbed heat and discharged again ^¬ ner turbine power on). Now with the same Lastabsen- kung the decoupled from the steam turbine power gedros ^¬ rare, so that this heat power to the recorded

Forced continuous steam generator fits (this is achieved by teilteil ^¬ ges closing the turbine valves), so temporarily more fluid remains in the forced flow steam generator. As a result the pressure in the once-through steam generator decreases comparatively slowly and a dreaded emptying of the once-through steam generator does not occur or only abge ^¬ weakens. In this way, the live steam temperature at the outlet of a solar thermal heated forced-circulation steam generator remains controllable even with rapid load decreases in the lower load range by the feedwater control described in WO 2012/110344 AI.

Claims

claims

1. Operating method for an externally heated forced once-through steam generator (1), in particular a solar thermal heated forced once-through steam generator with a

Zwangdurchlaufdampferzeuger downstream steam turbine (2), wherein in the feedwater-steam cycle between

Forced flow steam generator (1) and steam turbine (2) a pressure control device (3) with at least one turbine valve (31) is provided for regulating the pressure,

d a d u r c h e s e n c i n e s, d a s s

the pressure control device (3) is controlled by a control device (4 ^λ ) in such a way that, during rapid load decreases, the concomitant drop in pressure over one

Throttling the at least one turbine valve (31) takes place delayed ^¬ time.

2. Operating method according to claim 1,

d a d u r c h e s e n c i n e s, d a s s

the modified Gleitdruckkennlinie is designed for optimized power management of the steam turbine.

3. Operating method according to claim 2,

d a d u r c h e s e n c i n e s, d a s s

forming a power setpoint for the steam turbine for the optimized power command of the steam turbine, on Ba ^¬ sis of a currently transmitted to the once-through steam generator heat flow and this is compared with a current power delivery of the steam turbine.