EP2567151B1 - Method for operating a steam generator - Google Patents

Description

The invention relates to a method for operating a steam generator having a combustion chamber with a plurality of evaporator heating surfaces connected in parallel on the flow medium side. It further relates to such a steam generator. A steam generator is a closed, heated vessel or piping system designed to produce high pressure, high temperature steam for heating and service purposes (eg, for operation of a steam turbine). At particularly high steam outputs and pressures such as in power generation in power plants water tube boilers are used, in which the flow medium - usually water - is located in steam generator tubes. Also in the solid combustion water tube boilers are used, since the combustion chamber, in which the heat is generated by combustion of the respective raw material, can be arbitrarily designed by the arrangement of pipe walls.

Such a steam generator in the design of a water tube boiler thus comprises a combustion chamber, the surrounding wall is at least partially formed of tube walls, ie gas-tight welded steam generator tubes. On the flow medium side, these steam generator tubes form, as evaporator heating surfaces, first an evaporator, into which the unevaporated medium is introduced and evaporated. The evaporator is usually arranged in the hottest region of the combustion chamber. He is downstream of the flow medium side, where appropriate, a device for separating water and steam and a superheater, in which the steam is further heated above its evaporation temperature to in a subsequent heat engine such. B. at the Enspannung in a steam turbine to achieve high efficiency. The evaporator can be upstream of the flow medium side, a pre-heater (so-called economizer), the feed water under Exploitation of waste or residual heat preheats and so also increases the efficiency of the entire system.

The US 5 293 842 A describes a method for operating a steam generating plant in which steam is generated from water by indirect heat exchange with hot flue gas, wherein condensed water is first preheated and then the preheated water is evaporated under high pressure. The EP 0 359 735 A1 describes a heat recovery steam generator with a feedwater tank, which performs the function of a steam drum.

Depending on the design and geometry of the steam generator further steam generator tubes can be arranged within the combustion chamber, which are connected on the flow medium side parallel to the steam generator tubes forming the enclosure walls. These can be summarized or welded to an inner wall, for example. Depending on the desired arrangement of evaporator heating surfaces or inner walls within the combustion chamber, it may be necessary to interconnect interior walls on the flow medium side in succession and to connect their steam generator tubes via an intermediate collector.

This is the case, for example, with the so-called "pant-leg" design for steam generators with fluidized bed firing. Here, two in the combustion chamber symmetrically arranged, at least partially formed from further steam generator tubes inner walls upstream of an intermediate collector flow medium side. In the intermediate collector, the medium flow from the upstream inner wall combines and it serves as an inlet collector for a downstream inner wall. In the pant-leg design, a better mixing of the fuel mixture and thus lower possible distribution problems on the firing side is achieved.

In certain operating conditions, however, it may already come in the intermediate collector to a vapor content greater than zero. With Such a vapor content, a uniform distribution of the medium on the downstream inner wall with a simple collector is not possible, so that water-steam segregation can occur. Individual pipes of the downstream inner wall can thus already have such high vapor contents or enthalpies at their inlet that overheating of these pipes is very likely. Such overheating can lead to pipe damage during prolonged operation.

The object of the invention is therefore to provide a method for operating a steam generator of the type mentioned above and a steam generator, which allow a particularly long service life and a particularly low repair susceptibility of the steam generator.

This object is achieved by a method having the features of claim 1.

The invention is based on the consideration that a particularly long service life and a particularly low repair susceptibility of an evaporator in a steam generator would be achievable by avoiding overheating of the steam generator tubes by excessively high vapor contents or enthalpies. In this case, these high levels of steam occur in particular by the fact that in intermediate collectors already teilverdampftes flow medium is distributed unevenly to the downstream steam generator tubes. This unequal distribution should therefore be prevented by avoiding a two-phase mixture of water and steam in the intermediate collector. This would be achievable by keeping the inner walls upstream of the intermediate collector untouched, so that the medium undercooled and enters the intermediate collector without further preheating. However, this solution has constructive disadvantages. Therefore, rather, the temperature of the flow medium should be reduced at the entrance to the steam generator.

However, a reduction in the inlet temperature of the flow medium leads to a lower efficiency of the steam process. This is not desirable, moreover, such reduction in less heated steam generator tubes or pipe walls without intermediate collector - especially in the Umfassungswänden the steam generator - not necessary. Therefore, in these steam generator tubes to improve the efficiency no reduction of the inlet temperature should take place. This can be achieved by evaporator heating with downstream intermediate collector -. B. the inner walls of the pant-leg design - flow medium is supplied at a lower temperature than other Verdampferheizflächen.

To improve the efficiency or to optimize the Heizflächenanordnung the entrances of the perimeter walls and the inner walls of a steam generator is preceded by a preheater. This uses waste heat to preheat the flow medium. Due to the lower exhaust gas temperature generated by the use of waste heat so a higher overall efficiency of the steam generator is achieved. A particularly simple construction of a steam generator is therefore possible by the different temperature at the inner wall and the peripheral wall of the steam generator is achieved by structural measures on the preheater, d. H. by providing media with different degrees of preheating. For this purpose, a first part of the flow medium is conducted past the preheater. This can be done by means of a bridging line. Thus, a bypass of the preheater of the preheater is achieved in a structurally simple manner and achieved a lower heat input into the bridged part of the flow medium. This can then be supplied to the entry of the first evaporator heating surface at a lower temperature.

In order to achieve a not excessively reduced temperature in the acted upon with cooler fluid flow evaporator heating surfaces, the first part of the flow medium with a second fluid flow side after the Preheater diverted part to be mixed. Thus, a particularly adapted reduction of the temperature of the first evaporator heating surfaces supplied flow medium is achieved.

Advantageously, the mass flow of the second partial flow is limited upwards. This limitation can take place via a manual control or control valve for setting a flow rate limitation of the second control flow. Furthermore, a directional boundary should be provided by a check valve to prevent the main flow of the preheater exit stream, from which the second partial flow is diverted, not to cool unintentionally.

In order to achieve a particularly simple adaptation of the temperature of the flow medium supplied to the first evaporator heating surface, the mass flow rate of the first substream should advantageously be regulated on the basis of thermodynamic parameters at a measuring point downstream of the inlet of the first evaporator heating surface. For this purpose, a control valve can be arranged in the bypass line of the preheater. If the system is operated at supercritical pressures, where at no temperature water and steam can occur simultaneously and thus no phase separation is possible, there is no danger of segregation described above and the part of the flow medium passed by the preheater can be reduced to zero. If the steam generator operated with subcritical pressures in the evaporator, such. B. partial load operation of a modern Gleitdruckkessels, it must be adhered to avoid segregation of the two media a certain supercooling, which is determined by means of thermodynamic parameters at a measuring point behind the first evaporator heating.

In order to achieve a particularly targeted consideration of the thermodynamic states in the intermediate collector of the inner wall in the previously described steam generators in the pant-leg design, where the problem of separation of steam and water content leads to uneven distribution to the subsequent tubes, the measuring point should advantageously in one of the first evaporator heating surface downstream intermediate collector can be arranged.

The consideration of the thermodynamic parameters is carried out in an advantageous embodiment such that pressure and temperature are used as thermodynamic parameters, wherein the saturated steam temperature is determined from the measured pressure and the actual value of the subcooling is determined on the basis of the measured temperature. Thus, directly undercooling is determined as the decisive size for the problems explained.

For particularly simple control while a setpoint value for the subcooling is advantageously set and controlled the mass flow of the first partial flow based on the deviation of the actual and setpoint of subcooling. Advantageously, the mass flow rate of the first partial flow is increased at a lower actual value than the nominal value of the subcooling. Thus, if the subcooling is too low, the control valve in the partial flow withdrawn upstream of the preheater is opened further, so that the temperature of the flow medium supplied to the inlets is reduced and thus the subcooling is increased. If too much hypothermia, the control valve is closed.

With decreasing or increasing load of the steam generator is fed to the evaporator more or less flow medium through the main feedwater control loop. The proportions of the flow medium mass flow supplied to the various parallel evaporator heating surfaces remain nearly constant across the load. Thus, a nominal value for the mass flow for the first evaporator heating surface can be calculated via design calculations. In order to achieve a particularly accurate mass flow control for acting with colder flow medium Verdampferheizflächen, the mass flow of the second partial flow is advantageously controlled by the mass flow of the first evaporator heating supplied flow medium.

Further regulation of the mass flow rate of the flow medium supplied to the first evaporator heating surface can take place taking into account a water-steam separation device arranged downstream of the evaporator heating surfaces. In an advantageous embodiment, the current of the first evaporator heating surface fed medium regulated by the exit enthalpy of the evaporator.

Advantageously, the outlet enthalpy is determined on the basis of the temperature of the flow medium at the last evaporator heating surface downstream of the first evaporator heating surface and the pressure in the water-steam separator. Favorable here is a control of the outlet enthalpy to the mean fluid enthalpy in the separator. The set point of the evaporator outlet enthalpy should be stored dependent on the load in the main control loop. In any case, the outlet temperature of the fluid se should be limited so that the maximum permissible material temperature is not exceeded.

The advantages achieved by the invention are in particular that the problem of water-steam segregation in the intermediate collector is reliably avoided by the use of two media with different degrees of supercooling for feeding the various evaporator parts (enclosing walls and inner walls). In contrast to a solution with reduced enthalpy of entry for all evaporator parts, the evaporator does not have to be increased or only slightly enlarged in order to ensure a sufficiently high outlet enthalpy at the evaporator.

A design of the steam generator as a once-through boiler has several advantages: forced-circulation steam generators can be used for both subcritical and supercritical pressure without changing the process technology. Only the wall thickness of the pipes and collectors must be dimensioned according to the intended pressure. Thus, the continuous flow principle meets the globally recognizable trend towards increasing efficiencies by increasing the steam conditions.

Furthermore, an operation of the entire system in sliding pressure is possible. In sliding pressure mode, the temperatures remain in the high pressure part the turbine in the entire load range constant. Due to the large dimensions in terms of diameter and wall thickness of the component, the turbine is much more heavily loaded than the boiler components. This results in Gleitdruckbetrieb advantages in terms of load change speeds, number of load changes and starts.

FIG 1

schematisch den unteren Teil der Brennkammer eines Zwangdurchlaufdampferzeugers mit Wirbelschichtfeuerung mit teilweise überbrückter Vorwärmeinrichtung,

FIG 2

den Durchlaufdampferzeuger aus FIG 1 mit Regelung des Durchflusses zu den Innenwänden,

FIG 3

den Durchlaufdampferzeuger aus FIG 1 mit Regelung der Austrittsenthalpie der Innenwände, und

FIG 4

einen Graphen, der spezifische Enthalpie und Druck des Strömungsmediums in verschiedenen Bereichen des Durchlaufdampferzeugers bei verschiedenen Lastfällen zeigt.

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

FIG. 1

schematically the lower part of the combustion chamber of a forced once-through steam generator with fluidized bed combustion with partially bridged preheating device,

FIG. 2

the continuous steam generator FIG. 1 with regulation of the flow to the inner walls,

FIG. 3

the continuous steam generator FIG. 1 with control of the exit enthalpy of the interior walls, and

FIG. 4

a graph showing specific enthalpy and pressure of the flow medium in different areas of the continuous steam generator in different load cases.

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

The steam generator 1 in a schematic representation according to the FIG. 1 is designed as a forced flow steam generator. It comprises a plurality of tube walls formed from steam generator tubes and flowed through from bottom to top, namely an enclosing wall 2 and symmetrically arranged, inclined aligned interior walls 4, to which an additional interior wall 8 is connected downstream via an intermediate collector 6 on the flow medium side. The continuous steam generator 1 is thus designed in the so-called "pant-leg" design.

Through each of the enclosure wall 2 and the inner walls 4 associated entries 10, 12 occurs flow medium in the pipe walls. In the interior 14, a solid fuel is burned in the manner of fluidized bed combustion and thus reaches a heat input into the tube walls, which causes heating and evaporation of the flow medium. If the medium now enters all tube walls with the same enthalpy, a high vapor content can already be created in the intermediate collector 6 such that an uneven distribution takes place on the tubes of the inner wall 8 and the tubes with high steam content overheat here.

In order to avoid the consequent disadvantages such as a lower life or a higher repair susceptibility, the intermediate walls 6 upstream inner walls 4 flow medium at a lower temperature is supplied as the Umfassungswand 2. In the steam generator 1, first modifications of the preheater 16 are provided, the different heat inputs in ensure the different medium flows.

The preheater 16 after the FIG. 1 For this purpose, a branch point 18 is upstream of the flow medium side. A portion of the flow medium is thus passed around the preheater 16 in a bypass line 20. In the flow-medium-side direction, the preheater 16 is first followed by a further branching point 22, from which a line is led to the inlets 10 of the surrounding wall 2. A part of the preheated flow medium is thus supplied to the enclosure wall 2. Another part of the preheated flow medium is guided in a line 24, which meets in a mixing point 26 with the bypass line 20. Here, a medium of lower temperature is achieved by the mixing of the medium streams, which is then fed to the inlets 12 of the inner walls 4.

In line 24, a check valve 30 is arranged, which is an unwanted cooling by reflux in the branching point 22 prevented. Furthermore, a manual flow control valve 32 is provided, which limits the diverted mass flow preheated medium upwards. By an automatic flow control valve 28 in the bypass line 20 then the amount of bridged flow medium and thus the temperature of the inner walls 4 supplied flow medium can be easily controlled.

The pressure p and the temperature T in the intermediate collector 6 serve as input variables for the automatic regulation in the flow control valve 28. From the pressure determined, the saturated steam temperature is initially determined whose difference to the determined temperature T results in the actual undercooling. In order to prevent segregation of water and steam in the intermediate collector 6, a target subcooling in the intermediate collector 6 is specified. If the actual subcooling exceeds the desired subcooling, the automatic flow control valve 28 is closed further, so that the temperature at the inlets 12 increases. In the opposite case, the flow control valve 28 is opened further. If pressure and temperature are above the critical point of the flow medium, the flow control valve 28 is completely closed, since at supercritical pressures at no temperature water and steam can occur simultaneously and thus no segregation in the intermediate collector 6 can occur more.

An alternative embodiment of the invention shows FIG. 2 , The steam generator 1 is here except for the flow control valve 32 for FIG. 1 identical. The flow control valve 32 is here as the control valve 28 automated. This makes it possible to regulate the amount of the inner walls 4 supplied medium. The total flow F to the inlets 12, which is determined at a measuring point 34, serves as the input variable for the control. In this case, the total flow F is guided by means of a setpoint determined by design calculations.

A further embodiment of the invention is in FIG. 3 shown. Here is the steam generator 1 to FIG. 2 identical, it However, further components are shown, namely the outlet 36 of the inner wall 8 and the outlet 38 of the enclosure wall 2. The media streams from the outlets 36, 38 are brought together and fed into a water-steam separator 40. Here is also the main control loop is shown, which controls the total amount of supplied flow medium in the steam generator 1 by means of a flow control valve 42. In this case, pressure p and temperature T at the outlet on the steam side of the water-steam separator 40 serve as input variables for the regulation of the total medium flow.

In FIG. 3 the amount of flow medium supplied to the inner walls 4 via the inlets 12 is regulated as a function of the exit enthalpy of the inner wall 8. This is determined on the basis of the temperature T at the outlet 36 of the inner wall 8 and the pressure p in the water-steam separator 40. In this case, the average fluid enthalpy in the water-steam separator 40 is provided as a setpoint for the outlet enthalpy of the inner wall 8. In addition, the outlet temperature at the outlet 40 is limited beyond the maximum permissible material temperature.

FIG. 4 finally shows a state diagram for water / steam, in which the states of the flow medium are located in different areas of the steam generator. The diagram plots the specific enthalpy h in kJ / kg against the pressure p in bar. In this case, first lines of the same temperature T, ie isotherms 44 are shown, whose respective temperature values are indicated on the right-hand axis of the graph in degrees Celsius. The bump-shaped structure 46 on the left graphite side provides information about the vapor content of the water / steam mixture. Outside the structure 46, the medium is single-phase, ie, there is only medium in an aggregate state. The tip of the structure 46 at about 2100 kJ / kg and 221 bar here marks the critical point 48. If the pressure rises above 221 bar, water and steam do not occur at any temperature.

Within structure 46 is a water-steam mixture. The proportion of water and steam is shown with curves 50 in 10-percent intervals, from 0% vapor content at characteristic 52% to 100% vapor content at characteristic 54. The curves 50, 52, 54 converge at the critical point 48. Within the structure 46, the isotherms 44 are perpendicular to the pressure axis, so are also isobars. An energy input into the medium at constant pressure thus causes no higher temperature, but rather a shift of the water-steam content to more steam out.

Depending on the load condition of the steam generator 1, the steam process within the steam generator 1 runs on different load characteristic curves 56, 58, 60, which are not isobars, since the pressure losses of the heating surfaces are represented. The load essentially determines the pressure within the overall system. Load curve 56 represents the steam process at 100% load, load curve 58 at 70% load and load curve 60 at 40% load. The points A, B, C, D in each case represent the state of the flow medium at different points of the steam generator 1, and first without the separate regulation according to the invention of the temperature at the inlets 12 of the inner walls 4: point A the state at the entrance of the Preheater 16, point B the state at the inlet 12 of the inner walls 4, point C the state in the intermediate collector 6 and point D the state at the outlet of the evaporator.

As FIG. 4 shows, the steam generator is fully operated at 100% load in the supercritical range. At no point A, B, C, D on the load curve 56 is a distinction of water and steam possible, so that no segregation can occur. At 70% load, the subcritical range is already reached, but only a small part of the load characteristic 58 is within the structure 46. The points A, B, C of the load characteristic 58 are still below the structure 46, here is single-phase water. Again, it can not come to segregation in the intermediate collector 6.

At 40% load, however, a considerable part of the load characteristic 60 lies within the structure 46. The points A and B on the load characteristic 60 are still below the structure 46, so that one-phase water is still present here. However, the point C of the load characteristic 60 is within the structure 46 at a vapor content of 10%. Here it can thus come to the described segregation in the intermediate collector 6. However, if a portion of the flow medium past the preheater 16, which is achieved in pressure ranges below the load curve 62 by opening the flow control valve 28, the temperature and thus the energy content of the flow medium is selectively reduced. In this case, on the load characteristic 60, point E shows the state of the flow medium at the entrance 12 of the inner walls 4 at a reduced temperature. As a result, the energy content in the intermediate collector 6 is reduced, represented by point F on the load curve 60. This point F is now outside the structure 46, so that here single-phase water is present and segregations are reliably prevented.

Claims (11)

1. Method for operating a steam generator (1) with a combustion chamber having a plurality of evaporator heating surfaces (2, 4, 8) which are connected in a parallel manner on the flow medium side, wherein flow medium is supplied to an inlet (12) of a first evaporator heating surface (4) at a lower temperature than to an inlet (10) of a second evaporator heating surface (2)
   characterised in that
   a preheater (16) is connected upstream of the inlets (10, 12) on the flow medium side and wherein a first part of the flow medium is conducted past the preheater (16) and wherein the first part of the flow medium is mixed with a second part that is branched downstream of the preheater (16) on the flow medium side.
2. Method according to claim 1, wherein the mass throughflow of the second part-flow has an upper limit.
3. Method according to one of claims 1 or 2, wherein the mass throughflow of the first part-flow is regulated based on thermodynamic characteristics at a measurement point downstream of the inlet (12) of the first evaporator heating surface (4).
4. Method according to claim 3, wherein the measurement point is disposed in an intermediate collector (6) connected downstream of the first evaporator heating surface.
5. Method according to claim 3 or 4, wherein pressure (p) and temperature (T) are used as thermodynamic characteristics, with the saturated steam temperature being determined from the measured pressure (p) and the actual subcooling value being determined based on the measured temperature (T).
6. Method according to claim 5, wherein a setpoint value is predefined for subcooling and wherein the mass throughflow of the first part-flow is regulated based on the deviation between the actual and setpoint values for subcooling.
7. Method according to claim 6, wherein if the actual value for subcooling is lower than the setpoint value, the mass throughflow of the first part-flow is increased.
8. Method according to one of the preceding claims, wherein the mass throughflow of the second part-flow is regulated based on the mass throughflow of the flow medium supplied to the first evaporator heating surface (4).
9. Method according to one of the preceding claims, wherein the flow of the medium supplied to the first evaporator heating surface (4) is regulated based on the outlet enthalpy of the last evaporator heating surface (8) connected downstream of the first evaporator heating surface (4) on the flow medium side.
10. Method according to claim 9, wherein the outlet enthalpy of the evaporator heating surface (8) is determined based on the temperature at the outlet (36) of the flow medium at the last evaporator heating surface (8) connected downstream of the first evaporator heating surface (4) on the flow medium side and the pressure in a water/steam separator (40) connected downstream of the evaporator heating surfaces (2, 4, 8) on the flow medium side.
11. Steam generator (1) having means for executing the method according to one of claims 1 to 10.

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