Method for starting a continuous steam generator

Abstract

A method for starting a continuous steam generator is provided. The steam generator includes a combustion chamber provided with a plurality of burners, a water-steam separation device that is mounted downstream of evaporator tubes of the water-steam separation device on a flow-medium side. The amount of water flowing into the water-steam separation device during a starting process is kept to a minimum. The firing power of at least one of the burners is adjusted in accordance with a filling level characteristic value of the water-steam separation device.

Claims

1. A method of starting a continuous steam generator, comprising: providing a combustion chamber comprising a plurality of burners, a water-steam separation device being arranged downstream of a plurality of evaporator tubes of the combustion chamber on a flow medium side, and a plurality of superheater tubes arranged downstream of both the plurality of evaporator tubes and the water-steam separation device; measuring by a sensor a value of a fill level of water-steam separation device; adjusting a firing power of at least one of the burners by a control unit as a function of a value of the fill level of water for the water-steam separation device to minimize the water quantity flowing into the water-steam separation device during a starting process, wherein the firing power is adjusted as a function of a change in speed of the value of the fill level such that the firing power is adjusted when the value of the fill level is increasing, and wherein the water is a result of water discharge due to incipient evaporation.

2. The method as claimed in claim 1, wherein the firing power is not increased when an upper limit value of the value of the fill level is reached.

3. The method as claimed in claim 1, wherein the firing power is reduced when an upper limit value of the value of the fill level is reached.

4. The method as claimed in claim 3, wherein the firing power is reduced by 1% to 5% of a maximum firing power.

5. The method as claimed in claim 2, wherein the firing power is subsequently increased after a holding time for enabling a complete outflow of discharged water from the evaporator tubes expires.

6. The method as claimed in claim 3, wherein the firing power is subsequently increased after a holding time for enabling a complete outflow of discharged water from the evaporator tubes expires.

7. The method as claimed in claim 4, wherein the firing power is subsequently increased after a holding time for enabling a complete outflow of discharged water from the evaporator tubes expires.

8. The method as claimed in claim 5, wherein the holding time is 1 minute to 3 minutes and begins after the upper limit value is reached.

9. The method as claimed in claim 6, wherein the holding time is 1 minute to 3 minutes and begins after the upper limit value is reached.

10. The method as claimed in claim 7, wherein the holding time is 1 minute to 3 minutes and begins after the upper limit value is reached.

11. The method as claimed in claim 2, wherein the firing power is increased again when a lower limit value of the value of the fill level is achieved.

12. The method as claimed in claim 3, wherein the firing power is increased again when a lower limit value of the value of the fill level is achieved.

13. The method as claimed in claim 2, wherein different limit values are predetermined for warm and cold start processes of the continuous steam generator.

14. The method as claimed in claim 3, wherein different limit values are predetermined for warm and cold start processes of the continuous steam generator.

15. A continuous steam generator, comprising: a combustion chamber with a plurality of burners and evaporator tubes; a water-steam separation device being arranged downstream of the evaporator tubes on a flow medium side and upstream of a plurality of superheater tubes; a sensor for measuring a value of a fill level of the water-steam separation device wherein water in the water-steam separation device is a result of water discharge due to incipient evaporation; and a control unit for adjusting a firing power, wherein the control unit is connected on a data input side to the sensor for measuring a value of the fill level of the water-steam separation device wherein the control unit adjusts the firing power as a function of the value to minimize the water quantity flowing into the water-steam separation device during a starting process wherein the water quantity provides sufficient cooling of the evaporator tubes, and wherein the control unit adjusts the firing power as a function of a change in speed of a value of the fill level such that the firing power is adjusted when the value of the fill level is increasing.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) An exemplary embodiment of the invention is described in more detail with the aid of a drawing, in which;

(2) FIG. 1 shows a schematic continuous steam generator with a water-steam separation device, here for instance with a circulating pump, and a control device for the firing power and

(3) FIG. 2 shows a graphic representation of the starting process of a continuous steam generator.

DETAILED DESCRIPTION OF INVENTION

(4) The continuous steam generator 1 according to FIG. 1 is embodied in a vertical structure. The quantity of fuel B introduced by 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 power of the burner 7. The hot gas generated by the combustion process flows through the combustion chamber 8 and enters a gas pass 9. Further components (not shown) like for instance an economizer can be arranged downstream of the gas pass 9.

(5) Water W initially enters the evaporator tubes 12 on the flow medium side through a water inlet 10, said evaporator tubes opening into the water steam separation device 14 on the outlet side. Non-evaporated water is collected in the water-steam separation device 14 and is, as it is pressurized, either completely removed from the system by a discharge valve 15 or in the case of an evaporator system with a recirculation circuit a proportional division of the whole discharge mass flow from the water-steam separation device between a circulating pump 20 (with downstream circulating control valve 21) and a discharge valve 15 takes place proportionately. The discharged water can thus either be rejected or fed back into the system by way of the water inlet 10. Instead of the discharge valve 15 shown here, different discharge valves can also be provided for the warm and cold start, which are adjusted in terms of their design to the different initial states of the continuous steam generator 1 during the hot and cold start.

(6) The generated steam D escapes from the water-steam separation device 14 into the superheater tubes 16, where it is superheated again and subsequently supplied for further use by means of the steam outlet 18. The steam is typically supplied to generate power in a steam turbine (not shown here).

(7) The control device 6 for the firing power is configured such that an excessive water discharge as a result of the sudden onset of evaporation during the starting process is prevented by a prompt influence, in particular temporary reduction in the firing power. To this end, the water-steam separation device 14 is equipped with different sensors for measuring the fill level characteristic values. This includes one or more fill level sensors 30, which are connected to the control device 6 by way of a data line 36. The fill level characteristic values of the water-steam separation device are thus read out by the control device 6 and thus enable a prompt increase in the fill level in the water-steam separation device 14. This fill level change is a result of the water discharge from the evaporator tubes 12, which is triggered for its part by the incipient evaporation. The control device 6 thus receives reliable data relating to the incipient evaporation in the evaporator tubes 12 by way of the fill level sensors 30 and is configured for a prompt intervention in the burner control in order to restrict a further evaporation and thus the ingress of water into the water-steam separation device.

(8) The temporal course of a starting process of the continuous steam generator is shown with the aid of the relevant parameters or data in the diagram according to FIG. 2. The process data of a typical starting process which is determined with a simulation program is plotted against the time in FIG. 2. Here line L1 shows the firing power of the burner 7 in terms of percentage of the maximum firing power, controlled by the control device 6. Line L2 shows the intake mass flow in the water-steam separation device 14, line L3 shows the discharge mass flow of the water quantity through the discharge valve 15. Line L4 shows the data of the fill level sensor 30 and thus the fill level of the water-steam separation device 14.

(9) In region I, the burners 7 are initially brought up to a firing power of 5% of the maximum firing power. After approximately 75 seconds, the evaporation starts in the evaporator tubes 12, which initiates a water discharge which can be identified by the sudden increase in the intake mass flow into the water-steam separation device. After approximately 90 seconds, the discharge mass flow achieves the maximum throughput capacity of the discharge valve 15 and the water level of the water-steam separation device 14 rises.

(10) When the limit value of 1.2 m for the fill level in the water steam separation device 14 is reached, a reduction in the firing power by 2.5% of the maximum firing power is triggered in region II. Other measured variables could also be used here as indicators, for instance the first derivative, i.e. the change in speed of the fill level could be used as an indicator.

(11) By reducing the firing power, the heat input into the evaporator tubes is lowered and the evaporation process is thus slowed down. By slowing down the volume increase determined by the evaporation process, the water discharge is reduced and the further increase in the fill level in the water steam separation device 14 can be limited to approximately 2.9 m. This enables a corresponding cost-effective smaller dimensioning of all components of the water-steam separation device and the water discharge device.

(12) After a holding time of approximately 60 seconds, the firing power in region III is increased by the previously reduced 2.5% of the maximum firing power. Furthermore, the firing power is further increased and the permanent operating state of the continuous steam generator is thus established.

(13) The method thus effectively restricts the maximum fill level of the water-steam separation device 14 by prompt intervention in the firing power of the burner 7 and thus reliably prevents water ingress into the superheater tubes 16.