ONCE-THROUGH STEAM GENERATOR FOR USING AT STEAM TEMPERATURES OF ABOVE 650°C

Abstract

The invention relates to a forced-flow steam generator for using steam temperatures of above 650° C. Said forced-flow steam generator (1) comprises a combustion chamber (2) and a waste gas flue (3) connected to the upper end thereof, and peripheral walls (4) surrounding said flue. Said walls (4) are formed from tubular walls (5), the tubes thereof guiding the water/steam working medium. The combustion chamber (2) comprises at least one burner (6), and downstream heating surfaces (7) are arranged in the waste gas flue (3). Part of the peripheral walls (4) is covered in the region of the combustion chamber (2) by at least one bulk-head heating surface (8), the size of which on the surface side being determined such that the heat absorption of the peripheral walls (4) and therefore the temperature thereof are reduced to a value enabling the formation of the peripheral walls (4) from modified, heat-resistant 2.25-2.5% chrome steel that does not require any heat aftertreatment following the welding treatment thereof.

Description

The invention relates to a once-through steam generator for the use of steam temperatures of above 650° C., the once-through steam generator having a combustion chamber, a flue gas pass which adjoins the upper end of said combustion chamber, and enclosure walls which enclose them, the enclosure walls being formed from tube walls, the tubes of which conduct the working medium water/steam, the combustion chamber having at least one burner, and heating surfaces being arranged in the flue gas pass.

Once-through steam generators are known from the publication “Kraftwerkstechnik” [Power plant technology], Springer-Verlag, 2nd edition 1994, chapter 4.4.2.4—Forced flow (pages 171 to 174), Prof. Dr.-Ing. Karl Strauβ, and are used in power plants to generate electric energy by burning, for example, fossil fuels. In a once-through steam generator, the heating of the tube walls or enclosure walls which form the combustion chamber or the gas flue leads, in contrast to a natural circulation or forced circulation steam generator with only partial evaporation of the water/steam mixture conducted in the circulation, to an evaporation of the flow or working medium in the tubes of the tube walls or enclosure walls in a single throughflow.

The desire for once-through steam generators with higher degrees of efficiency, which forced-flow steam exchangers help, inter alia, to reduce the specific CO₂ emission into the atmosphere, leads, inter alia, to the increase in the steam parameters of the once-through steam generator. Achieving or realizing higher steam parameters, that is to say higher pressures and temperatures of the working medium steam at the outlet of the once-through steam generator (fresh steam outlet), makes high requirements of the material concept of the once-through steam generator. Once-through steam generators with steam parameters of approximately 280 bar/600° C. (fresh steam parameters) represent the current prior art. Here, the highly loaded parts (oblique winding and perpendicular bore) of the enclosure walls which are configured as tube walls are made with the special materials T23 (a material which is approved by the ASME (American Society of Mechanical Engineers)), T24 (7CrMoVTiB10-10) or other materials with a similar chemical composition which all belong to the category of modified, heat-resistant 2.25-2.5% chromium steels. The material T23 is listed, for example, in the VdTÜV material sheet 511/2, edition 06.2001 and the material T24 is listed, for example, in the standard specification sheet DIN EN 10216-2, edition October 2007. These materials have the advantage that they are especially suitable for the abovementioned steam parameters and that they can be welded without thermal post-treatment, and therefore the production of the enclosure walls or tube walls and their assembly on the construction site can be carried out simply.

If, in the course of a further improvement of the degree of efficiency, once-through steam generators with even higher steam parameters are desired, such as with 350 bar/700° C. (fresh steam parameters), the steam temperature and therefore the associated design temperature in the enclosure walls of the steam generator rise further. At said increased enclosure wall temperatures, however, the abovementioned materials T23, T24 or other materials with a similar chemical composition are no longer sufficient with regard to their strength parameters. Possible materials for said increased enclosure wall temperatures are martensitic 9-12% chromium steels such as T91 (X10CrMoVNb9-1), T92 (X10CrWMoVNb9-2) and VM12-SHC (company designation of the Vallourec-Mannesmann company) or Ni-based alloys such as Alloy 617 (NiCr23Co12Mo) or Alloy 617mod (NiCr23Co12Mo mod). The abovementioned materials are specified for the greatest part in material sheets, for example the material T91 in the VdTÜV material sheet 511/2, edition 06.2009; the material T92 in the VdTÜV material sheet 552/2, edition 03.2008; the material Alloy 617 in the VdTÜV material sheet 485, edition 09/2001 and the material VM12 in the VdTÜV material sheet 560/2, edition 03/2009.

If martensitic 9-12% chromium steels or Ni-based alloys are used for the enclosure walls, complicated manufacturing and assembly operations are necessary. The martensitic 9-12% chromium steels have to be thermally treated after the welding in the workshop and during the assembly. To this end, special tempering furnaces are required in the workshop and special annealing boxes are required on the installation site. Excessively great shrinking processes have to be coped with during the manufacture and/or assembly of Ni-based alloys. In addition, there is the fact in the case of the Ni-based alloys that their procurement costs are considerably higher than those of steels. Higher costs are therefore to be expected for both of these solutions, as far as both the costs of the materials and the manufacturing and assembly costs are concerned.

As has already been mentioned above, natural circulation and forced circulation steam generators are also known in addition to the once-through steam generators. In these known recirculation or circulation steam generators with subcritical steam parameters, only a defined quantity of heat can be absorbed by the evaporator of the steam generator. Said quantity of heat is defined by the operating pressure of the plant. As a result of the properties of the fuel (ash composition, fusion properties of the ash), the quantity of heat to be transferred from the combustion in the firing chamber or in the combustion chamber can be higher than that which can be absorbed by the evaporator. Platen heating surfaces have then been used in circulation steam generators, in order to limit the quantity of heat to be transferred to the evaporator and/or to emit the excess quantity of heat to the platen heating surface. In contrast to the natural circulation and forced circulation steam generators, the heat absorption is not limited in a once-through steam generator in its evaporator, since the medium temperature at the evaporator outlet is already superheated during forced-flow operation and the level of the superheating can be fixed variably. The associated temperature level of the steam or the corresponding design temperature in the enclosure walls is controlled by a suitable material selection with respect to the enclosure walls.

It is then an object of the invention to provide a once-through steam generator for the use of steam temperatures of above 650° C., in which once-through steam generator the abovementioned disadvantages are avoided or, with respect to the enclosure walls or the tube walls of the once-through steam generator, simple manufacturing and assembly processes are to be carried out rather than complicated ones which are difficult to control.

The abovementioned object is achieved by the entirety of the features of patent claim 1.

Advantageous refinements of the invention can be gathered from the subclaims.

The solution according to the invention provides a once-through steam generator for the use of steam temperatures of above 650° C., which once-through steam generator has the following advantages:

• • avoidance of a complicated manufacturing and assembly concept for the enclosure walls of the once-through steam generator;
  • reduction of material costs and manufacturing and assembly costs for the enclosure walls of the once-through steam generator;
  • savings on the use of annealing furnaces and boxes and their operating costs.

One advantageous embodiment provides that the platen heating surface which covers part of the enclosure walls in the region of the combustion chamber is arranged between the upper edge of the uppermost burner and the lower edge of the lowermost downstream heating surface. As a result of this measure, a defined region of the combustion chamber is covered by way of a platen heating surface, at which region a large part of the heat would otherwise pass out of the combustion chamber to the enclosure walls and would increase the medium temperature in the enclosure wall and the wall temperature in such a way that higher quality materials would have to be used.

In one advantageous refinement of the invention, at least part of the enclosure walls is formed from one of the materials T23, T24 or another material with a similar chemical composition. Here, at least that part of the enclosure walls is configured with the abovementioned materials which is thermally loaded highly or more highly than the remaining part of the enclosure walls. The materials T23, T24 or another material with a similar chemical composition are high-quality materials which are commercially available and satisfy the desired requirements or on which no thermal post-treatment has to be carried out after they have been welded.

One advantageous embodiment of the invention provides for the platen heating surface to be formed or produced from martensitic materials with a 9-12% chromium proportion, austenitic materials or nickel-based alloys. This ensures that, with regard to the temperatures, the requirements made of the platen heating surface which lies exposed in the combustion chamber are satisfied.

It is advantageous that the platen heating surface is configured as a superheater heating surface or a reheater heating surface. The platen heating surface is therefore incorporated efficiently into the water/steam circuit of the once-through steam generator or into the water/steam circuit of a power plant which comprises a once-through steam generator of this type.

One advantageous embodiment provides that the platen heating surface is arranged parallel to the enclosure wall. This achieves a situation where the platen heating surface is arranged vertically just like the enclosure wall and affords as small as possible an action surface for ash or cinder from the combustion chamber.

One expedient embodiment provides that the platen heating surface runs such that it bears against the enclosure wall. This ensures that the enclosure wall is covered as satisfactorily as possible by the platen heating surface and the smallest possible quantity of heat passes to the enclosure wall.

Exemplary embodiments of the invention are described in greater detail below using the description and the drawing, in which:

FIG. 1 diagrammatically shows a longitudinal section through a once-through steam generator according to the invention, and

FIG. 2 shows the same as FIG. 1, but in an alternative embodiment.

FIG. 1 diagrammatically shows a once-through steam generator 1 (designation means the same, namely the generation of the steam within the steam generator in one pass) of tower design, that is to say the tube walls 5 (as enclosure walls 4) and all the heating surfaces 7 are accommodated on or in a single vertical gas flue. The vertical gas flue which is formed or delimited by gastight enclosure walls 4 contains, in its lower region, the combustion chamber 2 and the flue gas pass 3 which adjoins above said combustion chamber 2. The combustion chamber 2 terminates to the bottom as a rule with a combustion chamber hopper and reaches upward as far as the lowermost heating surface 7. One or more burners 6 for burning a fossil fuel are arranged in the lower region of the combustion chamber 2. The burners 6 can be arranged either in the corners (corner burners) or in the walls (wall burners) of the combustion chamber 2. The various heating surfaces 7 are arranged as convection heating surfaces in the flue gas pass 3. Said heating surfaces 7 are as a rule economizer heating surfaces, superheater heating surfaces and reheater heating surfaces. The flue gas pass 3 terminates at the top with a ceiling and has a flue gas outlet 9 laterally at its upper end.

According to the invention, the once-through steam generator 1 has at least one platen heating surface 8 which covers part of the enclosure walls 4 in the region of the combustion chamber 2 and the surface-side size of which is defined such that the heat absorption of the enclosure walls 4 and, as a consequence, their temperature are reduced to a value which permits the configuration of the enclosure wall 4 from modified, heat-resistant 2.25-2.5% chromium steels which do not require any thermal post-treatment after they have been processed using welding technology. In other words, the platen heating surface 8, which covers the enclosure wall 4 in the region of the combustion chamber 2 with a predefined surface-side size, absorbs so much heat from the combustion chamber 2 that the heat absorption of the enclosure wall 4 is reduced as a consequence of the covering, in such a way that the maximum medium temperature at the enclosure wall 4 remains below a value which allows the use of modified, heat-resistant 2.25-2.5% chromium steels which do not require any thermal post-treatment after they have been processed using welding technology. They can be, for example, the materials T23 (a material which is approved by the ASME (American Society of Mechanical Engineers)), T24 (7CrMoVTiB10-10) or another material with a similar chemical composition which can cover steam temperatures up to approximately 500-510° C. and which are listed, for example, in the brochure “The T23/T24 Book, New Grades for Waterwalls and Superheaters from Vallourec & Mannesmann Tubes” (brochure about modified, heat-resistant 2.25-2.5% chromium steels). As a result of the reduction in the medium temperature in the enclosure wall 4 by means of the platen heating surface 8 which is arranged according to the invention, the use of high temperature-resistant materials such as martensitic, 9-12% chromium-containing steels or nickel base alloys for the enclosure wall 4 can be dispensed with, which, after they have been processed using welding technology, would have to undergo complex thermal post-treatment (martensitic, 9-12% chromium-containing steels) or would have to undergo complex processing because of their high contraction properties (nickel-base alloys).

The high-quality materials which are now used and do not require any thermal post-treatment after they have been processed using welding technology or which do not require complex processing can either be used everywhere on the enclosure wall 4 or, according to one commercially more advantageous variant, at least at those parts of the enclosure walls 4 where the high thermal loading makes it necessary. They are, for example, the regions at the burners and directly above the burners 6 within the combustion chamber 2. In order to reduce the investment costs in comparison with the abovementioned high-quality materials, lower-quality materials, such as 16Mo3 or 13CrMo45, are used at those parts of the enclosure walls 4 where the thermal loading is lower, such as in the lower part of the combustion chamber 2 (below the burners 6 including combustion chamber hopper) with medium temperatures of approximately ≦400-460° C. in the tube walls. Said materials likewise do not require any thermal post-treatment after they have been processed using welding technology or do not require further complex processing.

The enclosure walls 4 which are configured as tube walls 5 are produced as a rule from a welded tube-web-tube combination, the tubes of the tube walls 5 conducting the working medium water/steam, and it being possible for them to be formed within the enclosure walls 4 either helically or vertically or from a combination of helically and vertically. The tubes which are arranged in the enclosure walls 4 are used in the lower and middle part of the combustion chamber 2 as evaporator tubes, that is to say the water which is fed in and pre-heated is evaporated in said evaporator tubes. In the upper part of the combustion chamber 2 which has perpendicular tubes as a rule, the tubes which are arranged in the enclosure wall 4 can already be connected as a superheater heating surface.

The platen heating surface 8 itself, which then absorbs part of the heat from the combustion chamber 2, is formed using suitable materials according to the temperature requirements. Since very high temperatures have to be handled, martensitic 9-12% chromium-containing steels, austenitic steels or nickel-based alloys have proven to be suitable for this purpose. They can be, for example, the martensitic materials T91 (X10CrMoVNb9-1), T92 (X10CrWMoVNb9-2) or VM12-SHC, the austenitic steels SUPER 304H, HR3c, DMV304HCu, DMV3101N or nickel-based alloys such as Alloy 617 (NiCr23Co12Mo) or Alloy 617mod (NiCr23Co12Mo mod). The platen heating surface 8 can consist of individual tubes, which are arranged close to one another and in parallel, or of a tube-web-tube construction. The tubes of the platen heating surface 8 run as a rule horizontally within the heating surface, but can also run vertically.

The platen heating surface 8 is preferably arranged parallel to the enclosure wall 4 and more preferably so as to bear against the latter. This arrangement ensures that the enclosure wall 4 is covered very efficiently by the platen heating surface 8, and the transmission of heat to the enclosure wall 4 is therefore suppressed as far as possible. FIG. 2 shows one advantageous variant of the platen heating surface 8 according to the invention. Here, the enclosure wall 4 and tube wall 5, which as a rule contain the front and rear walls and two side walls of the once-through steam generator, are covered partially by one or more platen heating surfaces 8 in the region of the combustion chamber 2, to be precise between the upper edge of the uppermost burner 6 and the lower edge of the lowermost heating surface 7 (the region is marked or denoted by “S” in FIG. 2), one platen heating surface 8 being arranged on each individual tube wall, that is to say a total of four, according to FIG. 2. As a result of the targeted arrangement of the platen heating surface 8 specifically in this region of the combustion chamber 2, that region of the enclosure wall 4 or tube wall 5 within the combustion chamber 2 which is hottest as a rule can be covered in a very targeted manner. The platen heating surface 8 can advantageously be used as a superheater heating surface within the once-through steam generator 1. However, the use as a reheater heating surface is also possible.

LIST OF DESIGNATIONS

• 1 Once-through steam generator
• 2 Combustion chamber
• 3 Flue gas pass
• 4 Enclosure wall
• 5 Tube wall
• 6 Burner
• 7 Heating surface
• 8 Platen heating surface
• 9 Flue gas outlet

Claims

1. A once-through steam generator for the use of steam temperatures of above 650° C., the once-through steam generator (1) having a combustion chamber (2), a flue gas pass (3) which adjoins the upper end of said combustion chamber (2), and enclosure walls (4) which enclose them, the enclosure walls (4) being formed from tube walls (5), the tubes of which conduct the working medium water/steam, the combustion chamber (2) having at least one burner (6), and downstream heating surfaces (7) being arranged in the flue gas pass (3), part of the enclosure walls (4) being covered in the region of the combustion chamber (2) by at least one platen heating surface (8), the surface-side size of which is defined such that the heat absorption of the enclosure walls (4) and, as a consequence, their temperature are reduced to a value which permits the configuration of the enclosure walls (4) from modified, heat-resistant 2.25-2.5% chromium steels which do not require any thermal post-treatment after they have been processed using welding technology.

2. The once-through steam generator as claimed in claim 1, characterized in that the platen heating surface (8) which covers part of the enclosure walls (4) in the region of the combustion chamber (2) is arranged between the upper edge of the uppermost burner (6) and the lower edge of the lowermost heating surface (7).

3. The once-through steam generator as claimed in claim 1, characterized in that at least part of the enclosure walls (4) is formed from one of the materials T23, T24 or another material with a similar chemical composition.

4. The once-through steam generator as claimed in claim 1, characterized in that the platen heating surface (8) is formed from martensitic steels with a 9-12% chromium proportion, austenitic steels or nickel-based alloys.

5. The once-through steam generator as claimed in claim 1, characterized in that the platen heating surface (8) is configured as a superheater heating surface.

6. The once-through steam generator as claimed in claim 1, characterized in that the platen heating surface (8) is configured as a reheater heating surface.

7. The once-through steam generator as claimed in claim 1, characterized in that the platen heating surface (8) is arranged parallel to the enclosure wall (4).

8. The once-through steam generator as claimed in claim 1, characterized in that the platen heating surface (8) extends such that it bears against the enclosure wall (4).