EP0054601B2 - Forced-circulation steam boiler - Google Patents

Description

The invention relates to a forced-flow steam generator system with the features of the preamble of claim 1

Such a system is known from FR-PS 1 574 394. In this system, the water separator is switched on between the wall-forming evaporator heating surface and the wall-forming first superheater heating surface connected to it. The water exiting this separator is connected on the one hand via a circulation pump to the inlet of the evaporator heating surface and on the other hand is provided with a drain pipe in which a valve controlled by the level in the separator is arranged. The level-dependent control signal also acts on the feed pump At the outlet of the wall-forming superheater heating surface, another separator is connected, the water outlet of which is connected to the feed water line and the steam outlet of which is connected to a bulkhead superheater heating surface, which is suspended in the combustion chamber near its outlet. The further water separator, which can be bridged on the steam side by means of a bypass line, has the purpose of taking up the excess water that is supplied to the evaporator when starting up and not separated by the first water separator and returning it to the feed water line. The water not separated by the first water separator therefore also flows through the wall-forming superheater heating surface before it reaches the further separator.

Such an occurrence of water in the wall-forming superheater heating surface also takes place at partial load, and the amount of water decreases with an increasing load.

As a result of sudden load changes or malfunctions in the fuel supply, it can happen that, even with a higher load of the steam generator, water gushes from the first separator into the wall-forming superheater heating surface, which is disadvantageous for this heating surface.

A further disadvantage of the steam generator, which operates above the limit load of 50% of full load with forced passage and below this limit load with forced circulation, is that a switchover must take place when the limit load is passed, by switching on the level control signal of the first separator as the load increases and the circulation pump must be switched off, whereas with decreasing load the level control signal must be switched off while the circulation pump is switched on. In addition, the second separator is taken out of operation when the load increases by opening the bypass line. Due to the fact that water flows through the wall-forming superheater heating surface at partial load, the efficiency is reduced, and more so the lower the load. Since the drain line is shut off during forced circulation operation of the known steam generator, this mode of operation cannot be maintained for a long time because salt deposits occur in the evaporator.

The known steam generator, which is mainly intended for base load, is therefore not suitable for continuous operation at part load or for rapid load changes.

The invention has for its object to improve a forced-flow steam generator system of the type mentioned so that it enables safe continuous operation at part load and, moreover, rapid load changes and that the design effort is reduced.

This object is achieved by the features of the characterizing part of claim 1. The connection of the water outlet of the water separator with the flow of working fluid between the treatment plant and the high-pressure feed pump makes it unnecessary to arrange a circulation pump and a second water separator. This simplifies the construction of the steam generator system, to the extent that devices for switching the level control signal and the circulation pump on and off are eliminated. This makes it easier to operate the system, as no switchover operations are necessary when the load changes over the limit load of 50%. The provision of the final evaporator between the wall-forming evaporator and the water separator prevents water from the wall-forming evaporator from reaching the wall-forming superheater heating surface at low partial load. This heating surface is thus protected against temperature shocks, and the efficiency of the steam generator system is also improved. By attaching the final evaporator in the flue gas stream immediately upstream of the economizer, a mixing section is created in the connecting line from the wall-forming evaporator to the final evaporator, which ensures that the working fluid emerging from the wall-forming evaporator, which is still partially undevaporated, has been heated differently when this evaporator flows through is present as a homogeneous mixture at the inlet into the final evaporator. The final evaporator thus compensates for the disadvantage of uneven heating, which is caused by the vertical arrangement of the tubes of the wall-forming evaporator. Another advantage of attaching the final evaporator is that the wall-forming evaporator can be designed so that there is always a certain amount of water in the flow of working fluid at its outlet. This ensures good cooling of this evaporator. Finally, the feature of the internal grooves allows the combustion chamber walls to be subjected to higher thermal loads. With these walls, the side of the tubes exposed to flame radiation is heated to a greater extent, so that Film evaporation can occur on the inside facing this heating, which leads to impermissible pipe wall temperatures. The helically arranged grooves on the inside of the working medium are forced to rotate due to its longitudinal flow, as a result of which the heavier, liquid phase of the working medium is centrifuged against the wall. It is thus possible to increase the thermal strength of the pipes beyond what is to be expected from the increase in surface area.

Decades ago, when today's water treatment technology was not yet known, salt deposits appeared on the water side in the end parts of the evaporator tubes bordering the combustion chamber, which insulated the tubes from the cooling working fluid, causing them to overheat and burst. In order to avoid this phenomenon, the end section of the evaporator was moved to a weakly heated area of the steam generator (DE-AS 1000 828). This achieved two things: Because of the lower temperature difference between the flue gas and the working fluid, the final evaporation extended to a longer pipe section, which meant that the build-up of the salt deposits proceeded slowly. The salt deposits were then periodically removed by rinsing. Because of the lower thermal load, even salt layers that were several times thicker did not yet result in inadmissible overheating of the pipes. With the advent of modern water treatment techniques, which led to the high purity values specified in claim 1, the attachment of the final evaporator in the weakly heated area was abandoned.

The development of the invention according to claim 2 results in a reduction in the weight and material requirements of the pressure-carrying parts of the steam generator.

The invention will now be explained in more detail using an exemplary embodiment shown schematically in the drawing:

The system contains a condenser 1, in which steam from a turbine group 2 is condensed. An additional water line 3 with an additional water pump 4 and an additional water treatment system 5 is connected to the condenser 1. From the bottom of the condenser, a condensate line 6 leads via a condensate pump 7, a condensate treatment system 8 and two condensate preheaters 9 and 10 to the inlet of a degasser 12 seated on a feed water vessel 13.

A feed water line 15 with a feed pump 16 and two high-pressure preheaters 17 and 19 leads from the water area of the feed water container 13 to the input of an economizer 20 of a once-through steam generator 22.

The outlet of the economizer 20 is connected via a connecting line 23 to the distributor 25 of an evaporator heating surface 26. This consists of tubes 27 which are tightly welded to one another and form a funnel-like base 29 and four flat walls 29 of a combustion chamber 30 of the steam generator 22. In the walls 29, the tubes 27 run vertically; in section A they are provided with helical internal grooves. The combustion chamber 30 has a furnace 32.

The wall-forming tubes 27 are alternately bent outwards from the walls 29 at the height of one and the other of two horizontal planes E and F and led to collectors 35. These collectors 35 are connected via a line 36 to a final evaporator 40, which consists of a system of finned tubes 41 and is arranged in a flue gas duct 60 starting from the combustion chamber 30 directly below the economizer 20. The outlet of the final evaporator 40 is connected via a line 42 to the inlet of a water separator 44, from the bottom of which a line 45 with a level-controlled valve 46 leads back to the feed water vessel 13.

At the steam outlet of the separator 44, a connecting pipe 50 is connected, which opens into a ring distributor 51, from which wall pipes 53 lead to a ring collector 55. The wall tubes 53 alternately enter the combustion chamber walls 29 in the horizontal planes E and F. They are tightly welded to one another and to the tubes 27, so that the flue gas duct 60 connects seamlessly to the combustion chamber 30. The train 60 is bounded in its uppermost part by uncooled sheet metal walls 62 and a ceiling 63, to which a chimney 65 connects.

A second superheater 72 and a final superheater 75 are connected in series to the collector 55 of the wall pipes 53 forming a first superheater, and a live steam line 77 leads from the outlet of the final superheater 75 to a high-pressure turbine 78, the outlet of which is via a feed line 90 connected to an intermediate superheater 82, which is arranged in the flue gas duct 60 between the two superheaters 72 and 75. From the outlet of the intermediate superheater 82, a return line 84 leads to a low-pressure turbine 86 which, together with the high-pressure turbine 78 and a generator 88, sits on a common shaft, forms the turbine group 2.

The condensate treatment system 8 is designed such that the treated condensate has practically no salt, which corresponds to a conductivity of 0.2 μ Siemens, and that the silicon content is below 0.02 ppm. Salt deposits in the evaporator are therefore negligible.

The additional water treatment system 5 serves to relieve the load on the condensate treatment system 8 and also to protect the condenser 1.

The system is particularly suitable for sliding pressure operation, with full load operation before there may be supercritical pressure. In the following description of the operation of the system, it is initially assumed that the feed pump delivers subcritical pressure, since this condition also occurs at part load in systems operated under sliding pressure, which are operated at full load with supercritical pressure.

In normal operation, the condensate accumulating in the condenser 1 is practically completely desalinated together with the make-up water flowing in via line 3 in the condensate treatment system 8, which preferably contains a cation exchanger, a C0 ₂ Riesler, an anion exchanger and a mixed bed filter. It is then heated by the two preheaters 9 and 10, which are connected to the two lowermost withdrawals 11 of the low-pressure turbine 86 in a manner not shown, and fed into the degasser 12, from which it flows into the feed vessel 13. The working medium is now no longer called condensate, but called feed water - is now brought to a pressure dependent on the load of the system, possibly supercritical pressure at full load, in the two high-pressure preheaters 17 and 18, which come from two tapping points 19 of the low-pressure turbine 86 with the feed pump Tap steam are fed, the feed water is heated. A further heating, in the assumed operation with subcritical pressure close to the evaporation temperature, takes place in the economizer 20. Subsequently, the water is divided as evenly as possible onto the tubes 27. Adjustable throttling elements are installed in the mouths of the tubes 27 because the heating of the individual pipes are not exactly the same among themselves, the working fluid flows of the individual pipes absorb an uneven amount of heat and accordingly an unevenly large amount of water evaporates in the different pipes.

By appropriate adjustment of the throttling members, an attempt is made to adjust the working fluid flows in the tubes of the evaporator 26 so that at the end of each evaporator tube 27 an equal amount of water remains undevaporated. Since the heating of the individual pipes changes due to changes in the flame position or due to varying contamination of the pipes on the flue gas side, the evaporator 26 is dimensioned so small that it is very likely that even under partial load operation, even in the outlet cross section of the pipe 27 with the most unfavorable conditions, a small proportion of unevaporated Water flows In this way it is avoided that individual pipes assume an excessive temperature.

The steam-water mixture of different water content now flowing into the collector 35 is mixed on its way through the line 36 and - with possibly still considerable differences in the water content - distributed into the parallel pipes 41 of the final evaporator 40. Since the final evaporator 40 is arranged in a weakly heated area of the flue gas stream, that is to say in an area where the flue gas temperature is not much higher than the temperature of the evaporating water, its flue gas-side surface can, even if the working medium is distributed very unevenly on the pipes, do not assume dangerously high temperatures.

When designing the final evaporator 40, an optimum can be striven for with regard to the expenditure for a good distribution of the steam / water mixture at the inlet of the parallel tubes of the final evaporator 40 or with regard to the heating surface size of the final evaporator 40.

The working medium flows from the final evaporator 40, preferably slightly overheated at full load, into the separator 44. After any water that may still be there has been separated off, the now dry steam flows through the wall tubes 53 forming the first superheater at high speed and guaranteeing good heat transfer and homogeneous temperature.

The temperature difference between the welded tubes 27 of the evaporator 26 and the tubes 53 of the first superheater is mainly determined by the position of the final evaporator 40 in the flue gas stream. This position is chosen so that the temperature difference mentioned does not lead to inadmissibly high thermal stresses. To limit the temperature difference, means for influencing the flue gas-side heat supply to the final evaporator can be provided, which can be brought about, for example, by flue gas circulation or through a shunt channel through which flue gases can be directed past the final evaporator. On the working medium side, the temperature difference can also be checked by a bypass line to the final evaporator 40 or, for example, by a temperature-controlled injection element in the area of the line 42.

The superheated steam flows out of the ring chamber 55 through the second superheater 72, in which further heating takes place, and then via an injection element 74 in the line 73 through the final superheater 75. A temperature measuring element, not shown, is provided on the connecting live steam line 77, the one not shown Control means acts on the injection member 74.

After a first expansion in the high-pressure turbine 78 associated with a drop in temperature, the steam in the reheater 82 is reheated and fed to the low-pressure turbine 86, in which it is expanded to the vacuum generated in the condenser 1.

While in normal operation the amount of feed water depends, for example, on the outlet temperature of the final evaporator 40 is influenced, the delivery rate of the feed pump 16 is preferably kept constant for starting and in a load range below a certain limit load. At the outlet of the final evaporator 40, there is a load-dependent water content. The water is separated out in the separator 44 and returned to the feed water vessel 13 via the valve 46, which is controlled by the level in the separator 44.

In systems for supercritical operation, it may be expedient to arrange a bypass line with throttle element parallel to the final evaporator 40, so that a partial flow of the working medium can be bypassed the final evaporator in the event of operation with high load. The temperature difference between the tubes 27 and 53 in the region where they are welded to one another can thus be reduced, as a result of which the thermal stresses are reduced.

Thermal stresses in the areas of levels E and F can also be reduced by directly welding the tubes 27 and 53 to each other only over short lengths and the sealing being achieved by means of a skin construction.

Claims (1)

1. A fossil-fuel fired forced-flow steam-raising plant in which the following components are arranged in series in relation to the work medium flow:

- A feed water desalination facility (8) being designed for a feed water conductivity of less than 0.2 microsiemens/cm and a reduction of the feed water silicon content below 0.02 ppm ;

- A high-pressure feed pump (16) ;

- An economiser (20) ;

- An evaporator (26) embodied by sealingly welded vertical tubes (27) and forming combustion- chamber walls (29) of the steam raiser ;

- A water separator (44) ;

- A number of superheater heating surfaces (53, 75, 72) ; wall-forming tubes (53) of the first superheater heating surface being connected to the vertical evaporator tubes (27), the watt-forming tubes (53) being sealingly welded to one another and to those of the evaporator (26) and being connected at the junctions (51) by way of steam outlet lines (50) to the water separator (44), the forced-flow steam-raising plant being designed to operate on a one-through flow of the work medium through the evaporator (26) in the load range above 50% of full load, characterized in that the water outlet of the water separator (44) communicates by way of a return line (45) with the work medium flow between the facility (8) and the high-pressure feed pump (16), that a final evaporator (40) is provided in the work medium flow between the evaporator (26) and the water separator (44) and is disposed in the waste gas flue (60) between the economiser (20) and the final superheater heating surface- (72) flowed round on all sides by the flue gas and that the heating surface forming the final evaporator (40) and embodied by tubes (41) is enlarged on the gas side by ribs which extend preferably in the peripheral direction of the tubes.

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