EP2257696B1 - Method for starting a continuous steam generator - Google Patents

Description

The invention relates to a method for starting a continuous steam generator with a number of burners having combustion chamber whose evaporator tubes downstream of a water-vapor separator downstream of the flow medium side.

In a power plant with a steam generator, the energy content of a fuel used for evaporation of a flow medium in the steam generator. The steam generator has evaporator tubes for the evaporation of the flow medium, the heating of which leads to an evaporation of the flow medium guided therein. The steam provided by the steam generator can in turn be provided, for example, for a connected external process or else for the drive of a steam turbine. If the steam drives a steam turbine, usually a generator or a working machine is operated via the turbine shaft of the steam turbine. In the case of a generator, the power generated by the generator may be provided for feeding into a composite and / or island grid.

The steam generator can be designed as a continuous steam generator. A continuous steam generator is from the essay " Evaporator Concepts for BENSON Steam Generators "by J. Franke, W. Köhler and E. Wittchow, published in VGB-Kraftwerkstechnik 73 (1993), No. 4, pp. 352-360 known. In a continuous steam generator, the heating of steam generator tubes provided as evaporator tubes leads to an evaporation of the flow medium in the steam generator tubes in a single pass.

In order to achieve a particularly high efficiency of the continuous steam generator, the evaporator tubes are downstream of the flow medium side superheater tubes, which are the enthalpy further increase the exiting steam. The superheater pipes are designed to pass steam and can be damaged when water enters. Therefore, they are usually upstream of the flow medium side, a water-vapor separator, which may include, for example, water vapor separator and a water bottle, the so-called water collection vessel, or combinations of separators and water bottle. The water-vapor separator does not completely separate evaporated water from the steam, collects it first and releases it via an outlet valve. The separated water can either be discarded or re-circulated for re-evaporation.

In the water-vapor separator comparatively little or no water flows in the continuous operation state of the continuous steam generator, since the water pumped in the evaporator tubes practically completely evaporated. A much larger amount of water, however, flows during the startup process in the water-vapor separator. When starting a continuous steam generator, namely usually the first evaporator tubes for reasons of sufficient tube cooling flows through a minimum evaporator mass flow and ignited the burner with a partial load. Before the onset of evaporation while the entire water flow of the water-vapor separator is supplied. At the onset of evaporation, the consequent sudden increase in volume ejects a portion of the water content between the start of vaporization and the water vapor separator. In order to largely avoid unwanted transmission of unvaporised flow medium into the downstream superheater tubes despite this water discharge, usually a correspondingly large dimensioning of all components of the water-vapor separator and the downstream water discharge device (such as expander, condenser, drain line, etc.) required, which is associated with high material and cost.

A method for starting a continuous steam generator with a number of burners having combustion chamber whose evaporator tubes flow medium side has a water-vapor separator with a downstream level measuring device, are for example in the US-A-3,690,303 . US-A-3,478,726 and in the GB-A-1,207,366 disclosed.

A method for starting a continuous steam generator, with which the water discharge can be avoided or kept low, is from the DE 19528438 known. In this method, the ratio of firing capacity and feedwater flow is adjusted such that the water pumped into the evaporator tubes completely evaporated even in the partial load range and thus no or almost no water enters the water-vapor separator or the superheater tubes. The water output is thus minimized by a correspondingly low supply of feed water.

However, in continuous steam generators, as in the DE 195 28 438 described, for safe cooling of the evaporator tubes, even with a minimum installed fire power a minimum mass flow density and thus a minimum feed water mass flow necessary. A reduction of the feedwater mass flow to avoid water discharge is therefore not possible.

The invention is therefore based on the object of specifying an alternative method for starting up a continuous steam generator, in which the amount of water flowing in during the starting process into the water-steam separator and the water discharge device is kept low so that a smaller dimensioning of the water-steam generator is achieved. Separator and / or water discharge device is possible, at the same time a sufficient cooling of the evaporator tubes should be guaranteed. This should be achieved with simple means for a suitable for carrying out the process continuous steam generator.

With regard to the method, this object is achieved according to the invention by adjusting the firing capacity of at least one of the burners as a function of a fill level characteristic value for the water-vapor separator.

The invention is based on the consideration that sufficient cooling of the evaporator tubes then ensured remains when the supplied feed water quantity is sufficiently large. Avoiding the discharge of water by simply reducing the amount of feed water is therefore not expedient. Nevertheless, a comparatively smaller dimensioning of the water-vapor separator and the water discharge device should be achieved, since this would mean the saving of significant material and manufacturing costs in the design of the water-vapor separator and the water discharge device. Therefore, the water discharge occurring during the startup process should be reduced by other means than by influencing the feedwater quantity. This can be achieved by distributing the water output over a longer period of time. For this purpose, the incipient evaporation of the water should be slowed down during the start-up process, since the water discharge is caused by the sudden onset of evaporation in the evaporator tubes and the resulting volume increase. This can be achieved by a corresponding influence on the heat input into the evaporator tubes. This is in turn determined by the firing capacity and should thus be controlled taking into account the onset of evaporation. In order to determine the time of the incoming evaporation, the water discharge caused by the evaporation can be used as an indicator. Since the water discharge is indicated in particular by an increase in the influence of water in the water-vapor separator, this can be done by measuring a level characteristic of the water-vapor separator.

For the determination of the incipient discharge of water, the evaluation of different characteristics characteristic of the level in the water-vapor separation device is conceivable. For example, a flow measurement at the inlet of the water-vapor separator could be made, can be concluded indirectly from the level. A particularly reliable implementation can be achieved by providing in a particularly advantageous embodiment, a direct measurement of the level of the water-vapor separator is. An increase in the level in the water-vapor separator shows a beginning of water discharge particularly reliable and can be measured by simple means.

In a further advantageous embodiment of the method, the rate of change of the measured fill level characteristic value can additionally be taken into account, since a particularly rapid rise provides a further indicator for incipient water discharge and the level of water discharge.

To counteract the discharge of water sufficiently, the heat supply to the evaporator tubes should be influenced and in particular throttled. During a typical in the start-up phase of increasing the firing capacity, this can be achieved by suspending the increase in firing capacity at the time of onset of evaporation. As a result, the evaporation process is slowed down and prevents over-feeding of the water-vapor separator with water. Since the incipient discharge of water is indicated in particular by a relatively high increase of the filling level in the water-vapor separator, this reduction can advantageously take place when a limit value of the measured filling level characteristic value of the water-vapor separating device is reached. This allows a technically particularly easy to implement circuit.

In a further advantageous embodiment, when a limit value of the measured fill level characteristic value is reached, the firing output of the burners can not only be kept constant, but even reduced. This causes an even greater reduction of the heat input into the evaporator tubes and thus an even stronger slowing down of the evaporation process. This allows even more effective reduction of water discharge and limitation of water entry into the water-vapor separator.

In a further advantageous embodiment, however, it is taken into account here that a minimum stationary start-up firing power, which, depending on the design of the continuous steam generator with regard to the stability of the combustion, can for example be between 2% and 5% of the maximum firing capacity (corresponds to a firing capacity at 100% load), should not be fallen below. For this purpose, the reduction of the firing rate when reaching the limit value is advantageously 1% to 5% of the maximum firing rate.

A particularly effective plant operation can be achieved by the continuous steam generator as soon as possible and immediately after the expelled after the onset of evaporation water is brought into its desired operating condition. For this purpose, the firing rate is expediently increased again after a waiting period. In order to ensure a complete outflow of the ejected water from the evaporator tubes, it should be advantageously maintained a waiting time of 1 to 3 minutes.

In a further advantageous embodiment of the invention, in order to ensure a temporally even better coordinated with the end of the water output increase the firing performance, these are increased again when reaching a lower limit of the level characteristic for the water-vapor separator. This allows a comparatively even more effective and time-saving starting process.

The initial state of a continuous steam generator is very different for hot and cold start: The temperature of the various components has a direct influence on the parameters of the startup process. Advantageously, therefore, different limits are given for hot and cold start of the continuous steam generator. If the water-vapor separation device comprises different exhaust valves for hot and cold start, then in the warm start, in which the pressure in the water-vapor separator usually in is above the lock-up pressure for the cold-start drain valve, the upper limit, for example, be the uppermost value of the control range for the warm start valve. On the other hand, in the cold start, where the pressure in the water-vapor separator is below the lock-up pressure for the cold-start drain valve, the upper limit may be, for example, the uppermost value of the cold-start drain valve level control range. Thus, a corresponding optimization of the startup is possible.

With regard to the continuous-flow steam generator with a combustion chamber having a number of burners, the evaporator tubes downstream of which is a water-steam separator, the object is achieved by providing a control unit for setting the firing capacity on the data input side with a sensor for measuring a fill-level characteristic of the water-steam trap. Separating device is connected.

Advantageously, the sensor directly measures the level of the water-vapor separator. The level of the water-vapor separator offers a particularly easy-to-process size for the control of the firing capacity.

The advantages achieved by the invention are in particular that by the measurement or observation of the amount of water in the water-vapor separator an early detection of incipient water discharge during the start-up phase, ie in the first 20 minutes after the ignition of the burner and below 15% the maximum firing capacity is possible and can be mitigated by a needs-based control of the firing capacity, in particular a reduction of the firing capacity. Thus, the amount of water introduced into the water-vapor separator and the water-vapor separator and water discharge device can be made smaller overall, so that significant material and manufacturing costs can be saved.

FIG 1

schematisch einen Durchlaufdampferzeuger mit einer Wasser-Dampf-Abscheideeinrichtung, hier beispielhaft mit Umwälzpumpe, und einer Steuereinrichtung für die Feuerungsleistung, und

FIG 2

eine grafische Darstellung des Anfahrvorgangs eines Durchlaufdampferzeugers.

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

FIG. 1

schematically a continuous steam generator with a water-vapor separator, here for example with circulation pump, and a control device for the firing, and

FIG. 2

a graphic representation of the startup process of a continuous steam generator.

The continuous steam generator 1 according to FIG. 1 is designed in a vertical design. The amount of fuel B introduced through the fuel inlet 2 is influenced by a control valve 4 which is adjusted by a control device 6. The control device 6 thus directly controls the firing output of the burners 7. The hot gas produced by the combustion process flows through the combustion chamber 8 and enters a gas flue 9. The throttle cable 9 can not be shown other components such. B. be followed by an economizer.

Flow medium side enters water W through a water inlet 10 first in the evaporator tubes 12, the discharge side open into the water-vapor separator 14. Unevaporated water is collected in the water-vapor separator 14 and, because it is under pressure, either completely removed by a drain valve 15 from the system, or it is in an evaporator system with circulation appropriately proportionately a division of the total flow mass flow from the Water-vapor separator on a circulation pump 20 (with downstream circulation control valve 21) and a drain valve 15 instead. Thus, the discharged water can either be discarded or re-fed via the water inlet 10 into the system. Instead of the individual drain valve 15 shown here, it is also possible to provide different drain valves for hot and cold start, which in their design correspond to the different initial states the continuous steam generator 1 are adapted for hot and cold start.

The generated steam D exits the water-steam separator 14 into the superheater tubes 16 where it is further overheated, and is then fed through the steam outlet 18 for further use. Typically, the steam is supplied for power generation of a steam turbine, not shown here.

The control device 6 for the firing power is designed such that an excessive water discharge is prevented by the sudden onset of evaporation during startup by timely influence, in particular temporary reduction of the firing capacity. For this purpose, the water-vapor separation device 14 is equipped with various sensors for measuring level characteristics: These include one or more level sensors 30, which are connected via a data line 36 to the control device 6. The level characteristic values of the water-vapor separator are thus read out by the control device 6 and thus make it possible to detect a sudden increase in the fill level in the water-steam separator 14. This level change is a consequence of the water output from the evaporator tubes 12, which in turn is triggered by the incipient evaporation. Via the level sensors 30, the control device 6 thus receives reliable data on the incipient evaporation in the evaporator tubes 12 and is designed for a timely intervention in the burner control in order to limit further evaporation and thus the entry of water into the water-vapor separator.

The timing of a start-up process of the continuous steam generator is based on the relevant parameters or data in the diagram after FIG. 2 shown. In this case, the process data of a typical starting process determined in a simulation program are in FIG. 2 applied against time. there Line L1 shows the firing rate of the burners 7 as a percentage of the maximum firing rate, controlled by the controller 6. The line L2 shows the inlet mass flow into the water vapor separator 14, the line L3 shows the outlet mass flow rate of the water through the drain valve 15. The line L4 shows the data of the level sensor 30 and thus the level of the water-vapor separator 14th

In area I, the burners 7 are first raised to a firing rate of 5% of the maximum firing rate. After about 75 seconds, evaporation begins in the evaporator tubes 12, which initiates water ejection, evidenced by the sudden increase in the inlet mass flow into the water-vapor separator. After about 90 seconds, the exit mass flow reaches the maximum throughput capacity of the drain valve 15 and the water level of the water-steam separator 14 increases.

Upon reaching the limit of 1.2 m for the level in the water-vapor separator 14, a reduction of the firing capacity is triggered by 2.5% of the maximum firing capacity in area II. It could also be a different measure used as an indicator, such as the first derivative, d. H. the rate of change of the level could serve as an indicator.

By reducing the firing capacity of the heat input is throttled into the evaporator tubes and thus slows the evaporation process. By slowing down the increase in volume caused by the evaporation process, the water output is reduced and the further increase in the level in the water-vapor separator 14 can be limited to approximately 2.9 m. This allows a correspondingly inexpensive smaller dimensions of all components of the water-vapor separator and the water discharge device.

After a waiting period of about 60 seconds, the firing capacity in area III is increased by the previously reduced 2.5% of the maximum firing capacity. Furthermore, the firing capacity is further increased, thus producing the continuous operating state of the continuous steam generator.

The method thus effectively limits the maximum level of the water-steam separator 14 by timely intervention in the firing capacity of the burner 7 and thus reliably prevents water from entering the superheater 16.

Claims (12)

1. Method for starting a continuous steam generator (1) having a combustion chamber (8) comprising a number of burners (7), a water-steam separation device (14) being arranged downstream of the evaporator tubes (12) thereof on the flow medium side, with the firing power of at least one of the burners (7) being adjusted as a function of a fill level characteristic value for the water-steam separation device (14).
2. Method according to claim 1, in which the firing power is adjusted as a function of the fill level of the water-steam separation device (14).
3. Method according to claim 1 or 2, in which the firing power is also adjusted as a function of the change in speed of the fill level characteristic value.
4. Method according to one of claims 1 to 3, in which the firing power is not increased further when an upper limit value of the fill level characteristic value is reached.
5. Method according to one of claims 1 to 4, in which the firing power is reduced when an upper limit value of the fill level characteristic value is reached.
6. Method according to claim 5, in which the reduction amounts to 1% to 5% of the maximum firing power.
7. Method according to one of claims 4 to 6, in which the firing power is increased further after a holding time.
8. Method according to claim 7, in which a holding time of 1 to 3 minutes is maintained after reaching the limit value.
9. Method according to one of claims 4 to 8, in which the firing power is increased again when a lower limit value of the fill level characteristic value is reached.
10. Method according to one of claims 4 to 9, in which different limit values are predetermined for warm and cold start processes of the continuous steam generator (1).
11. Continuous steam generator (1) having a combustion chamber (8) comprising a number of burners (7), a water-steam separation device (14) being arranged downstream of the evaporator tubes (12) thereof on the flow medium side, with a control unit provided for adjusting the firing power being connected on the data input side to a sensor for measuring a fill level characteristic value of the water-steam separation device (14).
12. Continuous steam generator (1) according to claim 11, in which the sensor (30) measures the fill level of the water-steam separation device (14).

Images