EP2676072B1 - Method for operating a once-through steam generator - Google Patents

Description

Method for operating a continuous steam generator. The invention relates to a method for operating a continuous steam generator with an evaporator, in which a Speisemassenstrom a flow medium with the aid of a feed pump to the evaporator and there is at least partially evaporated, wherein not evaporated flow medium deposited in a separator downstream of the evaporator and Umwälzmassenstrom the deposited flow medium is guided back into the evaporator with the aid of a circulation pump, so that the mass flow of the flow medium flowing through the evaporator, called the evaporator mass flow, is composed of the feed mass flow and the circulation mass flow. Such a method is for example in document DE 32 43 578 A1 oven beard. In a forced flow steam generator, the passage of the usually supplied in the form of feed water flow medium is enforced by the usually provided preheater, the evaporator and the superheater by a correspondingly powerful feed water pump, short feed pump. Thus, the heating of the flow medium to the saturated steam temperature, the evaporation and subsequent overheating takes place continuously in one pass, so that no drum is needed. In contrast to a steam generator, which is designed for a natural circulation operation, a forced once-through steam generator can also be operated in the supercritical range at pressures of 230 bar and more. With forced circulation boilers very large steam outputs can be generated in a relatively small space. Since the amount of flow medium in the system is relatively low, the system has a low inertia and thus allows a fast response to load changes.

Fired forced flow evaporators with spiral around a combustion chamber wound evaporator tubes (so-called spiral tube) are usually designed for a mass flow density of the guided through the evaporator tubes flow medium of about 2000 kg / (sm ² ) at 100% load (full load). According to the previously customary design guidelines, the mass flow density in a vaporizer with smooth tubes at partial load should not fall below a value of about 800 kg / (sm ² ) in order to avoid cooling problems on the tube walls by stratification of the flow. At the aforementioned full load mass flow density of 2000 kg / (sm ² ), this value corresponds to a load value of 40% of the full load. This is then also the load case for which the evaporator minimum mass flow is defined. In start-up and low-load operation, it is ensured by the feedwater control that the evaporator minimum mass flow is always supplied to the evaporator.

Non-evaporated water, which is obtained especially in start-up and low-load operation, is usually separated from the vapor in a downstream of the evaporator water separator (short: separator) and to a water collection vessel (the so-called collection bottle or short bottle), while the steam usually a superheater is supplied. In many cases, a circulating pump is used to recirculate the separated water and before the so-called economizer called feedwater in the feedwater mass flow (short: Speisemassenstrom) integrate, so ultimately return it to the evaporator inlet. In this case, the evaporator mass flow is composed of the feed mass flow and the circulating mass flow, also referred to as recirculation mass flow.

In a hitherto usual mode of operation, the feed mass flow is steadily increased during startup, while the circulation mass flow is regulated down to the same extent. Consequently, in the above example, the circulation pump for a comparatively high circulation mass flow density of approx.

800 kg / (ms ² ) to be designed according to 40% of the full load value of the evaporator mass density, because in zero load operation or just above it almost the entire evaporator mass flow is formed by the Umwälzmassenstrom. This comparatively high design mass flow of the circulation pump causes the circulating pump must be comparatively powerful and large and is therefore associated with high acquisition costs.

The invention is therefore based on the object of specifying a method for operating a continuous steam generator of the type mentioned above, which avoids the disadvantages mentioned, is thus designed with low purchase and operating costs for effective and safe part-load operation with sufficient cooling of the evaporator tubes. Furthermore, a continuous steam generator particularly suitable for carrying out the method should be specified.

With regard to the method, the stated object is achieved according to the invention by the technical features of independent claim 1. The operation in the high load interval is referred to as continuous operation, because in the separator no more water.

The reference to the case of increasing load takes place here only for the purpose of a clear definition; the control characteristic also applies analogously to the case of sinking load. This means, for example, that in the low load interval the feed mass flow is reduced with decreasing load, etc.

The invention is based on the consideration that, although it would be possible in principle to dispense with the Rezirkulationskreisklauf with the circulation pump, thus easily divert the water deposited in the separator when starting and in low load operation and discard (so-called drain operation). However, this would be disadvantageous from a thermodynamic and economic point of view and, moreover, would undesirably increase the thermal load on the superheater heating surfaces downstream of the evaporator because of the lower fluid temperatures at the inlet of the economiser and evaporator and the resulting lower production of cooling steam acting on the heating surfaces Start-up operation.

The present invention is detached from the design guidelines for the recirculation mass flow, which have hitherto been valid and considered to be operationally reliable. It has been surprisingly found that the design mass flow for the circulation pump can be significantly reduced, at least in a low load interval compared to the previous level of knowledge, without having to accept any disadvantages. In particular, in the vicinity of the zero-load state, the evaporator minimum mass flow, which in this case is effected almost exclusively by the circulating-mass flow, can be halved in comparison to the previously established value. The assurance of sufficient cooling of the evaporator tubes under these conditions - even if they are designed as smooth tubes - could be proven by appropriate thermo-hydraulic calculations and simulations. For higher load ranges, the previously customary values for the evaporator minimum mass flow are then predetermined again and achieved by appropriate control of the feed mass flow and the circulation mass flow. The transition between the two control scenarios is preferably continuous, in particular linear.

Advantageously, the feed mass flow is increased linearly with increasing load in the low load interval. When the circulation mass flow rate is kept constant, this means that the total evaporator mass flow - as already mentioned, the sum of the feed mass flow and the circulation mass flow - increases linearly with the load.

Preferably, the feed mass flow is increased linearly with increasing load even in the middle load interval, while the Umwälzmassenstrom is preferably reduced linearly with increasing load. In a particularly preferred embodiment of the Umwälzmassenstrom is thereby reduced to the same extent as the feed mass flow is increased. This means that the sum of the two mass flows, namely the evaporator mass flow, remains constant in the middle load interval.

Conveniently, the low load interval begins at zero load and preferably ends at about 20% of the designed full load. The low load interval is expediently followed immediately by the middle load interval, which preferably ends at approximately 40% of the design full load.

In a particularly preferred embodiment, the circulation mass flow in the low load interval is set to approximately 20% of the full load value of the evaporator mass flow. In this case, in the low load interval, a value of Umwälzmassenstromdichte of about 400 kg / (sm ² ) is particularly advantageous, corresponding to an evaporator mass flow density at full load of about 2000 kg / (sm ² ).

In a further advantageous embodiment, the circulation mass flow and the feed mass flow are adjusted in the middle load interval such that the evaporator mass flow always reaches at least 40% of the full load value in this interval. Particularly preferred is the case that the evaporator mass flow in this load interval by opposite change is kept constant by supply current and circulating current (see above). For carrying out the process according to the invention, a continuous steam generator with an evaporator is necessary, upstream of a feed pump and downstream of a separator for non-evaporated flow medium, the separator being connected to the water-side steam generator inlet via a return line into which a circulation pump is connected. and wherein an electronic control unit for the feed pump and the circulation pump is provided, which performs the method steps of the method described above.

As already indicated, the return line expediently opens downstream of the feed pump and upstream of the feedwater preheater in the feed line. The separator is thus (indirectly) connected to the evaporator inlet via the feedwater preheater.

In the control or regulation unit for the purpose mentioned advantageously a corresponding control or regulation program is implemented in terms of hardware and / or software. The control or regulation unit acts on the feed pump and the circulation pump and controls their delivery rate, ie the respective flow rate of the flow medium (feed water and separated water from the evaporator), by means of suitable manipulators, in accordance with prior operator input (for example startup, shutdown, partial load operation, etc.). , By means of suitable measuring sensors or sensors, the control or regulation unit is expediently supplied with the actual value of relevant operating variables, so that a corresponding readjustment can take place in the event of a deviation from the desired setpoint.

The continuous steam generator is preferably fired directly by a number of burners. He preferably has one Combustion chamber or a throttle cable, the surrounding wall is formed of a plurality of gas-tight welded together evaporator tubes, wherein at least a portion of the enclosure wall forms the actual evaporator (next to possibly other areas that form the feedwater or the superheater). The throttle cable is preferably designed as a vertical gas train and has at least in the evaporator section a spiral tube, that is spirally or helically within the enclosure wall about the longitudinal axis of the gas draft convoluted evaporator tubes on. The evaporator tubes are preferably smooth tubes; but there are also conceivable provided with a Innenberippung pipes.

When using internally ribbed tubes in volute evaporators, the minimum mass flow density at the highest load in recirculation mode can be reduced from the typical smooth tube value of 800 kg / (sm ² ) to about 500 kg / (sm ² ). Therefore, an evaporator with internally finned tubes can be run in continuous operation at loads above 25% of full load when the full load mass flow density of the evaporator is 2000 kg / (sm ² ). Even with the use of innenberippten pipes in a spiral evaporator, the circulation pump according to the invention can be dimensioned particularly compact. In a spiral evaporator with internally tipped tubes, the transition from recirculation to continuous operation is about 25% load rather than 40% load. The previous and following descriptions, which are numerically designed for a smooth-tube evaporator, can be transferred to an evaporator with internally-tipped tubes, taking into account this constraint.

The advantages achieved by the invention are, in particular, that an operation of a forced once-through steam generator with return of the deposited on or after the evaporator liquid flow medium (water) in the feedwater is made possible by the deliberate departure from previously relevant design principles (so-called Forced-circulation mixing system), in which despite a comparatively low selected Umwälzmassenstrom in the vicinity of the zero-load range, a high operational safety and sufficient pipe cooling is guaranteed. The circulation pump can be dimensioned particularly compact in this case and be correspondingly inexpensive to purchase.

FIG 1

ein Blockschaltbild eines Durchlaufdampferzeugers,

FIG 2

ein Diagramm, in dem verschiedene für den Durchfluss von Strömungsmedium durch entsprechende Komponenten des Durchlaufdampferzeugers charakteristische und für seine bisherige Betriebssteuerung maßgebliche Kennlinien als Funktion der Last aufgetragen sind, und

FIG 3

ein weiteres derartiges Diagramm, wobei der Kennlinienverlauf einer neuartigen, erfindungsgemäß verbesserten Betriebssteuerung entspricht.

An embodiment of the invention will be explained in more detail with reference to drawings. In each case show in a highly simplified and schematic representation:

FIG. 1

a block diagram of a continuous steam generator,

FIG. 2

a diagram in which various characteristic for the flow of fluid through corresponding components of the continuous steam generator and relevant for its previous operation control characteristics are plotted as a function of the load, and

FIG. 3

another such diagram, wherein the characteristic curve of a novel, inventively improved operation control corresponds.

The in FIG. 1 illustrated flow steam generator 2 comprises an evaporator 4 for the evaporation of a flow medium M, which is preceded by a feedwater heater 6 also referred to as economizer flow side. The evaporator 4 comprises a plurality of fluidly connected in parallel, gas-tight welded together and designed as smooth tubes steam generator tubes which form a region of a peripheral wall of a combustion chamber in the manner of a spiral tube, which is heated via a number of burners (not shown in detail here). The evaporator 4 is followed by a superheater 8 with a number of Überhitzerheizflächen flow medium side. During operation of the continuous steam generator 2, the feedwater preheater 6 via the feed line 10 by means of a feed pump 12, the flow medium M supplied in the form of feed water S, preheated in the feedwater pre-heater 6, then passed through the evaporator inlet 14 into the evaporator 4 and evaporated there. The vapor D leaving the evaporator 4 via the evaporator outlet 16 is finally superheated in the superheater 8 and then supplied to its intended use, for example in a steam turbine.

During partial load operation, in particular during startup or shutdown of the continuous steam generator 2, the flow medium M is not completely evaporated in the evaporator 4, but it remains at the evaporator outlet 16, a proportion of non-evaporated, liquid flow medium M, namely water W. This water content is in a flow medium side between the evaporator 4 and the superheater 8 connected separator 18 from the vapor portion, which is forwarded to the superheater 8, separated and separated. The separated water W is collected in a collecting vessel 20 connected to the separator 18, and from there, depending on the operating state, is guided to varying degrees via a return line 22 to the inlet of the feedwater pre-heater 6. For this purpose, a circulation pump 24 is connected in the return line 22, and the return line 22 is connected to the feed line 10 downstream of the feed pump 12 and upstream of the feedwater pre-heater 6. Excess water W is discharged from the collecting vessel 20 via a discharge line 26.

The mass flow of the evaporator 4 flowing through the flow medium M, namely the evaporator mass flow VM, is thus additively from the mass flow of supplied feedwater S, namely the feed mass flow SM, and the mass flow of previously separated water W, namely recirculated by means of the circulation pump 24 the Umwälzmassenstrom UM, together. Instead of the term mass flow colloquially also the term flow is used.

A on the feed pump 12 and the circulation pump 24 and optionally not shown here adjusting or control valves in the line system of the flow medium M acting electronic control or regulating unit 28 is used for operating state-dependent control or regulation of these mass flows, especially during start-up or low load operation. For detecting the operational actual state, a number of sensors connected to the control or regulation unit 28 are furthermore provided (not shown here).

FIG. 2 shows the course of relevant characteristics according to a conventional control scheme. Plotted as a function of the load L here are Umwälzmassenstrom UM, the feed mass flow SM and the evaporator mass flow VM. The load values on the abscissa are each expressed as a percentage value of the maximum load, and similarly, the flow rate and mass flow values are indicated on the ordinate as the percentage values of the designed maximum evaporator mass flow VM at full load. As can be seen, the Umwälzmassenstrom UM increases steadily and in particular linearly to the value 0% (corresponding to 40% load) with increasing load from the output value of 40% (corresponding to 0% load), while the value of the feed mass flow SM in the corresponding load interval linearly from 0 % rises to 40%. The sum of the feed mass flow SM and the circulation mass flow UM, which represents the evaporator mass flow VM, therefore has the constant value 40% in this load interval. For even larger loads, the circulation mass flow UM remains at the value 0%, while the feed mass flow SM and thus the evaporator mass flow VM increase 100% up to the full load value (not shown in the diagram). The circulation pump 24 must therefore be designed for a comparatively high mass flow value of 40% of the evaporator mass flow VM at full load.

In contrast, shows FIG. 3 a with respect to the requirements of the circulation pump 24 improved control scheme in a too FIG. 2 analogue diagrammatic representation. Similar to the through FIG. 2 is represented control variant the feed mass flow SM increased in the load interval between 0% and 40% load linearly from the value 0% to the value 40%. Notwithstanding the previous variant, the Umwälzmassenstrom UM is now in a first load interval between 0% and 20% load, here referred to as low load interval I, on a opposite FIG. 2 reduced value of 20% kept constant. Only in the subsequent middle load interval II between 20% load and 40% load is the circulation mass flow reduced linearly to the value 0%. Accordingly, the evaporator flow in the low load interval I increases from the value of 20% linearly to the value of 40% and is maintained at 40% in the middle load interval II. In the right next high load interval beyond 40% load (not shown) then increases as in the previously discussed case of the feed mass flow SM and thus the evaporator mass flow VM to full load value 100%.

By reducing the design mass flow for the recirculation pump 24 to one opposite FIG. 2 halved value of 20% of the maximum evaporator mass flow VM, the requirements for the circulation pump 24 are significantly reduced, without jeopardizing the sufficient cooling of the evaporator tubes of the evaporator 4 in the low load range.

Claims (12)

1. Method for operating a once-through steam generator (2) with an evaporator (4), in which a feeding mass flow (SM) of a flow medium (M) is fed with the aid of a feed pump (12) to the evaporator (4) and at least partly evaporated there, wherein non-evaporated flow medium (W) is separated in a separator (18) connected downstream from the evaporator (4) and is fed back to a circulating mass flow (UM) of the separated flow medium (W) with the aid of a circulating pump (24) into the evaporator (4), so that the mass flow referred to as the evaporator mass flow (VM) of the flow medium (M) flowing through the evaporator (4) is composed additively of the feeding mass flow (SM) and the circulating mass flow (UM), characterised in that

   - in a low-load interval (I) the feeding mass flow (SM) is increased as the load (L) rises, while the circulating mass flow (UM) is kept substantially constant,

   - in a moderate-load interval (II) the feeding mass flow is increased further with increasing load (L) and the circulating mass flow (UM) is reduced to zero, and

   - if necessary in a high load interval the feeding mass flow (SM) is increased further with increasing load (L) and the circulating mass flow (UM) is kept at zero.

   and wherein, for the moderate-load interval (II) a minimum value is defined for the evaporator mass flow (VM) and, for the low-load interval (I), the circulating mass flow (UM) is substantially kept constant at the halved minimum value.
2. Method according to claim 1, in which in the low-load interval (I) the feeding mass flow (SM) is increased linearly with increasing load (L).
3. Method according to claim 1 or 2, in which in the moderate-load interval (II) the feeding mass flow (SM) is increased linearly with increasing load (L).
4. Method according to one of claims 1 to 3, in which in the moderate-load interval (II) the circulating mass flow (UM) is reduced linearly with increasing load (L).
5. Method according to claim 1 or 2, in which in the moderate-load interval (II) the feeding mass flow (SM) is increased linearly with increasing load (L) and the circulating mass flow (UM) is reduced linearly to the same extent with increasing load (L).
6. Method according to one of claims 1 to 5, in which the low-load interval (I) begins at no load.
7. Method according to claim 6, in which the low-load interval (I), when smooth tubes are used, ends at approximately 20%, when inner-ribbed tubes are used, ends at approximately 12.5% of the full load provided for by the design.
8. Method according to one of claims 1 to 7, in which the moderate-load interval (II) follows directly on from the low-load interval (I).
9. Method according to claim 8, in which the moderate-load interval (II), when smooth tubes are used, ends at approximately 40%, when inner-ribbed tubes are used, ends at approximately 25% of the full load provided for by the design.
10. Method according to one of claims 1 to 9, in which the circulating mass flow (UM) in the low-load interval (I), when smooth tubes are used, is set at approximately 20%, when inner-ribbed tubes are used is set at approximately 12.5% of the full load value of the evaporator mass flow (VM).
11. Method according to one of claims 1 to 10, in which in the low-load interval (I), when smooth tubes are used, a circulating mass flow density of approximately 400 kg/(sm²), when inner-ribbed tubes are used a circulating mass flow density of approximately 250 kg/(sm²) is set.
12. Method for operating a once-through steam generator (2) with an evaporator (4), in which a feeding mass flow (SM) of a flow medium (M) is fed with the aid of a feed pump (12) to the evaporator (4) and at least partly evaporated there, wherein non-evaporated flow medium (W) is separated in a separator (18) connected downstream from the evaporator (4) and is fed back to a circulating mass flow (UM) of the separated flow medium (W) with the aid of a circulating pump (24) into the evaporator (4), so that the mass flow referred to as the evaporator mass flow (VM) of the flow medium (M) flowing through the evaporator (4) is composed additively of the feeding mass flow (SM) and the circulating mass flow (UM), characterised in that

    - in a moderate-load interval (II) the feeding mass flow (SM) is decreased with decreasing load (L) and the circulating mass flow (UM) is increased, starting from zero, and

    - in a low-load interval (I) the feeding mass flow (SM is decreased further with decreasing load (L), while the circulating mass flow (UM) is kept substantially constant.

    and wherein, for the moderate-load interval (II) a minimum value is defined for the evaporator mass flow (VM) and, for the low-load interval (I), the circulating mass flow (UM) is substantially kept constant at the halved minimum value.

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