EP2598789A2

EP2598789A2 - Method for protecting heat exchanger pipes in steam boiler systems, molded body, heat exchanger pipe, and steam boiler system

Info

Publication number: EP2598789A2
Application number: EP11782009.2A
Authority: EP
Inventors: Johannes Martin; Toralf Weber; Thomas Putz; Andreas Kienzle
Original assignee: Martin GmbH fuer Umwelt und Energietechnik; SGL Carbon SE
Current assignee: Martin GmbH fuer Umwelt und Energietechnik; SGL Carbon SE
Priority date: 2010-07-28
Filing date: 2011-07-08
Publication date: 2013-06-05
Also published as: CA2806495A1; JP2013535647A; DE102010032612A1; DE112011102512A5; US20130118421A1; WO2012028127A2; WO2012028127A3

Abstract

In order to protect heat exchanger pipes (1) in steam boiler systems, special casing elements (2) made of fiber-reinforced ceramic are proposed. Said casing elements prevent or reduce the formation of films and corrosion on the heat exchanger pipes and thus enable higher steam parameters of the boiler system and a correspondingly increased thermal efficiency.

Description

Process for protecting heat exchanger tubes in steam boiler plants, shaped bodies, heat exchanger tubes and steam boiler plants

[01] The invention relates to a method for protecting heat exchanger tubes in steam boiler plants and to a mold body for carrying out the method. Furthermore, the invention relates to a heat exchanger tube and a steam boiler system with such a heat exchanger tube.

[02] Incinerators for burning solid fuels such as waste and biomass burning plants have a steam boiler with heat exchanger tubes. These heat exchanger tubes serve in part to vaporize water and partly to overheat evaporated water.

[03] In such systems, there is the problem that the heat exchanger tubes corrode during operation. Numerous studies have shown that this corrosion is induced by adhering deposits of ashes and salts. Gaseous exhaust gas constituents such as HCl and SO2 influence the composition of the deposits, but do not directly lead to corrosion attacks on these components.

In waste and biomass incineration plants can occur in extreme cases, corrosion rates of up to one millimeter per 1,000 hours.

| Confirmation copy | [05] As corrosion protection measures ceramic linings and metallic coatings are used. Ceramic liners are applied either in mortar-like form to the tubes, where they harden by so-called. Dry heating before the actual operation or as fired bricks, which enclose the tube sections, which are exposed to the corrosive attack. The metallic coatings are either job-welded or thermally sprayed on.

[0003] DE 38 23 439 C2 describes a ceramic, finish-sintered protective element made of half-shells toothed together. These shells, preferably made of silicon carbide, have not proven useful in practice, since the material required must be relatively thick and heavy in order to withstand the stress during operation of the boiler system. In addition, the protective element is backfilled with a relatively large amount of mortar. Since the teeth allow no thermal expansion, it comes with the high temperatures present in normal operation to cracking up to the bursting of the shells.

[07] Another ceramic protective cover made of overlapping half shells made of silicon carbide describes the DE 20 2008 006 044 Ul.

[08] Ceramic linings on the walls have proven to work well in the furnace, whereas the use of ceramic shells in superheaters is not practical. In addition to the static load of the steel structure due to the weight of the protective shells subject the heat exchanger tubes in the superheater area mechanical loads during cleaning.

[09] Knock-down devices are widely used, which mechanically act on the pipes in the superheater area in order to remove the deposits. Even with water and steam blowers is trying to remove the deposits, creating additional chemical stress. These loads severely limit the possible uses of ceramic linings for corrosion protection measures in superheater areas.

[10] Build-up welds have proven to be effective corrosion protection in radiation trains. As welding material the material 2.4858 (Inconel 625) has prevailed.

[11] Material temperatures above 400 ° C, as they occur in the superheater area and - at very high operating pressures - in the evaporator tubes, however, significantly limit the corrosion protection of this material. The use of other welding consumables, such as 2.4606 (Inconel 686), experience shows no significant improvement.

[12] In addition, thermal spraying processes are being used more and more often as a corrosion protection measure. Experiments with a wide variety of material compositions as anticorrosive coating on the boiler components showed that such protective coatings can be seen in a short time in advance. can say. Long-term corrosion protection therefore can not be guaranteed with such methods.

The corrosion protection of boiler tubes on the one hand has an impact on the efficiency of the steam generator, since the applied deposits can affect the heat transfer. On the other hand, most waste and biomass combustion plants are operated only with steam temperatures of up to 400 ° C at a maximum of 40 bar steam pressure to keep the corrosion within manageable limits. An increase in the steam parameters is associated with significantly increasing corrosion rates on the pressure element and thus a reduction in the availability of the system. The known corrosion protection measures could not offer satisfactory improvements here.

The invention is therefore based on the object to reduce the corrosion of heat exchanger tubes in steam boiler plants while minimizing the disadvantages described.

This object is achieved with a method for the protection of heat exchanger tubes in steam boiler plants, are surrounded in the heat exchanger tubes of the boiler plant at least partially with fiber-reinforced ceramic. The invention is based on the finding that corrosion from heat exchanger tubes in steam boiler systems is induced by the adhering coatings. Keeping away the deposits containing a mixture of salmon zen and ashes, from the surface of the pipe experience has shown that it leads to a significant reduction or even to a standstill of the corrosion processes.

[17] The coverings can be kept away from the heat exchanger tubes of the steam boiler systems by at least partially surrounding the heat exchanger tubes with fiber-reinforced ceramic.

It has been found that even with the high temperatures in the superheater area and the strong mechanical loads of the cleaning systems fiber-reinforced ceramic can be used to reduce the formation of deposits on the heat exchanger tubes. Fiber-reinforced ceramics can withstand high temperatures without damage and it has good resistance to water vapor-containing atmospheres. In addition, the material has a good thermal conductivity and a low Wärmeaus stretch. The use of fiber-reinforced ceramic to protect the heat exchanger tubes allows operating the boiler system at much higher temperatures, whereby the thermal efficiency of the system can be significantly improved.

In order to avoid stresses between the ceramic shell and the steel of a heat exchanger tube, it is proposed that the ceramic is slidably disposed relative to the tube. These ceramic tubes or sleeves before mounting the heat exchanger tubes on the pipes are attached. As a result, the ceramic is arranged in the form of a plurality of mutually adjacent enveloping elements.

[21] In particular, when the ceramic is to be applied to mounted heat exchanger, threading ceramic rings or sleeves onto the heat exchanger tube without damaging it is no longer possible. Therefore, it is proposed that the enveloping elements are formed from circular segment shells. For example, two circular segment shells can be assembled into a sleeve. Such a sleeve can be retrofitted to a pipe by applying the sleeve halves to the pipe from opposite sides.

The sleeve halves can then be connected to each other or snap into each other. It is advantageous if the circular segment shells are axially and / or radially positively connected to each other. For example, by undercuts or steps, a Z-joint can be formed. Two opposing semicircular shells can interlock with each other or be connected to each other so that even at the junction of a particle is prevented from entering the heat exchanger tube.

[23] However, enveloping elements that lie against each other axially may also have interdependent undercuts or steps in order, for example, to restrict the access of particles between two enveloping elements to the heat exchanger tube by means of a Z-joint. [24] In this case, the sheath elements can be fixed by brackets, pipe bends and / or by welding points on the heat exchanger tubes in their position.

[25] The fiber-reinforced ceramic can have a wide variety of additives to improve its stability and surface properties. It is advantageous if the ceramic has carbon fibers. Carbon fibers are hardly combustible and allow a special stability of the ceramic, which is very important in particular with regard to the mechanical knock cleaning methods. [26] In order to keep the cost of corrosion protection low and to influence the heat transfer as little as possible, it is proposed that the ceramic has a thickness between inner diameter and outer diameter of less than 10 millimeters and preferably less than 5 millimeters. The fiber-reinforced ceramic can also be applied as a coating directly on the tubes in order to keep the thickness of the material as low as possible and to allow expansion of the ceramic material together with the tubes. If the ceramic material is firmly connected to the tube, even cracks in the ceramic see material can be accepted, since they affect the function of the lining only insignificantly. [28] The tubes can also be surrounded by fibrous materials such as fibrous ceramic mats. The ceramic can be created prior to application to the pipe, after application to the pipe in an oven or even when heating the material after commissioning of the boiler in the combustion aungsge.

[29] The boiler tubes can be wrapped or surrounded by the material. For this purpose, a material in the form of mats, fabric or in a kind of chain mail is suitable. These materials either already have fiber-reinforced ceramic or the ceramic is formed only after the application to the tube by sintering, curing or similar processes-

[30] Experiments have shown that it is thus possible that the ceramic is exposed to temperatures of over 400 ° C.

[31] Accordingly, the heat exchanger tubes can be exposed on their inner side to a pressure of over 40 bar.

[32] Metal tube and ceramic can also be firmly connected to each other, for example, by producing a ceramic compound tube.

[33] In order that the enveloping elements are suitable for use on heat exchanger tubes of industrial incinerators, it is proposed that the ceramic has an internal diameter of more than 30 mm, preferably about 40 to 60 mm. The invention also relates to a molded article with a fiber-reinforced ceramic for carrying out the method, which is adapted to envelop a heat exchanger tube. Furthermore, the subject of the invention is a heat exchanger tube, which is surrounded with such a Fonnkörper. Between the molded body and the heat exchanger tube, a preferably annular gap may be arranged. Finally, the invention relates to a steam boiler system with such a heat exchanger tube.

[35] In the following, the invention will be explained in more detail by means of an exemplary embodiment. The single figure shows a view of a heat exchanger tube with a

Envelope element.

[36] The heat exchanger tube 1 shown in FIG. 1 is a tube of many heat exchanger tubes of a heat exchanger (not shown) of a steam boiler system (not shown). This heat exchanger tube 1 is surrounded by a plurality of enveloping elements 2. Of these Hüllelementen 2 only the circular segment shell 3 of a Hüllelementes is shown. This circular segment shell 3 has an inner side 4, which rests against the outer side 5 of the heat exchanger tube 1.

[37] At a distance of, for example, 5 millimeters from the inner side 4, the circular segment shell 3 has an outer side 6, which is particularly smooth in order to avoid deposits. [38] On the outside 6, a structure for influencing the flow, such as a wave structure or flow s may be provided to improve the heat transfer by turbulence or solely by the surface enlargement. By means of a suitable structure, the deposition behavior on the surface of the enveloping elements can also be positively influenced. While the microscopic structure of the outer side 6 of the circular segment shell 3 should be as smooth as possible in order to avoid deposits, the macroscopic structure on a smooth surface may, for example, have corrugations. [39] One embodiment therefore envisages that, for example, a very smooth coating of the ceramic surface is achieved by nanoparticles in order to minimize the caking of particles such as dusts from the flue gas.

[40] The circular segment shell 3 has peg-shaped protruding elements 7, 8, which cooperate with corresponding recesses in an opposing circular segment shell to allow a positive and possibly also positive, fitting connection between two radially opposite circular segment shells.

[41] The circular segment shell 3 has on its other end face 9 two blind holes 10, 1 1, which can cooperate with pins of an opposite circular segment shell (not shown). Pins and holes can be mounted at an angle of, for example, about 45 ° be. This leads to a positioning of the shells relative to each other and to a sufficient attachment of the shells to each other.

[42] A symmetrical design of the circular segment shells makes it possible to use these shaped parts for two opposite, form-fitting connectable circular segment shells.

The formation of the circular segment shell 3 also allows a positive connection between two axially abutting circular segment shells.

For this purpose, a gradation 14, 15 and 16, 17 is provided on axially opposite end faces 12, 13, which makes it possible to insert the axially projecting element 16, 17 into the recess 14, 15 in the next adjacent circular segment shell.

[45] The form shown is only one embodiment, which illustrates the basic structure of a Hüllelementes. It will be readily apparent to those skilled in the art that there are various other possibilities for designing enveloping elements within the scope of the invention which preferably cooperate with one another in a form-fitting, radial and possibly also axial manner. As a result, good protection of the heat exchanger tube 1 is achieved.

[46] The fit between the heat exchanger tube 1 and the cladding element 2 is chosen such that the expansion of the heat exchanger tube 1 relative to the cladding element 2 does not lead to the destruction of the cladding element 2 and, on the other hand, the distance between the inner surface 4 of the cladding element 2 ment 3 and the outer surface 5 of the heat exchanger tube 1 is minimally selected. This causes the heat exchanger tube at operating temperature firmly against the fiber-reinforced ceramic, but without exerting too high pressure on this. [47] In the gap, which remains between the inner surface 4 of the Hüllelementes 3 and the outer surface 5 of the heat exchanger tube, a material can be introduced, which positively influences the heat transfer.

[48] The gap can also be dimensioned so that the fiber-reinforced ceramic can simply be pulled over the heat exchanger tube and the inner surface of the ceramic is coated so that it foams on reaching a specific temperature to fill the gap. For this purpose, special under the influence of heat foaming materials are known. [49] When installing the enveloping elements, it is also possible to apply a material to the boiler tubes that disappears (for example, evaporates) during initial start-up, thus releasing the gap for thermal expansion.

[50] In order to allow an expansion of the Hüllelementes in expanding heat exchanger tube 1, the Hüllelement 2 may also consist of several radially assembled and axially divided Hüllelementen. [51] In particular, a stepped end of circular segment shells of a Hüllelementes allows some radial expansion of a Hüllelements at a thermal expansion of Wärmetau scherrohres without particles find a direct access to the heat exchanger tube. [52] By special undercuts Hüllelementteile, such as circular segment shells, are hung radially into each other and / or hung axially together, so that without screwing alone by plugging a heat exchanger tube can be surrounded with enveloping elements. [53] It is understood that in areas in which a heat exchanger tube is designed as a pipe bend, the Hüllelemente must be designed to match adjusted.

[54] A variant for producing a cladding tube of the method according to the invention is then explained by way of example. [55] In a first step, fiber bundles are produced that do not react in the subsequent silicization process. Carbon fiber strands comprising 50,000 nearly parallel single filaments are impregnated with a phenolic resin to form a prepreg having a mass-based resin content of 35% and a basis weight of 320 g / m ² . This prepreg is continuously kneaded at a speed of 1 m / min at a pressure of 1 MPa on a belt press at a temperature of 180 ° C. to form a fabric web having a thickness of 200 μm. seals and at the same time cured so far that a dimensionally stable fabric web is obtained. The gauze web is subsequently separated into individual bands with a width of 50 mm each. These are cut as described above to segments 9.4 mm long and 1 mm wide. 2400 g of the fiber bundles are transferred to a tumble mixer and coated with 600 g of powdered resin and mixed together for 5 minutes.

[56] The pressing tool is filled with the molding compound. In order to achieve a preferably tangential alignment of the fiber bundles, a filling grid is used which comprises a plurality of concentric rings whose spacing is less than or equal to the length of the fiber bundles. During filling, the molding compound with the fiber bundles falls through the spaces between the concentric rings of the filling grid, and the fiber bundles assume a substantially tangential arrangement. The filled press tool is exposed to a pressure of 4.0 N / mm ² and a temperature of 160 ° C for 30 minutes on a hot extrusion press and then demoulded. During the pressing process, the phenolic resin cures. It is obtained a near net shape green body in the form of an annular disc whose inner diameter corresponds to that of the later to be protected pipe. Of these slices 10 pieces are coated by means of a phenolic resin adhesive containing SiC powder, clamped in a clamping device so that the individual slices lie exactly above each other and the joint gap is less than 0.5 mm. The clamped disks are transferred with a tensioning device into a drying cabinet and cured at 180 ° C. for 30 minutes. tet. Subsequently, the resulting cylinder, called green body, removed from the jig and carbonized.

[57] The green body is heated to a temperature of 900 ° C in a protective gas oven under a nitrogen atmosphere at a rate of 1 K / min. The phenolic resins are decomposed to a residue consisting essentially of carbon. This temperature is maintained for one hour. Thereafter, the carbonized molded body is cooled to room temperature. Subsequently, the resulting porous CFC cylinder is transferred to a crucible made of graphite and spilled with silicon and heated in an oven under vacuum to temperatures of 1700 ° C. In this case, liquid silicon enters the porous preform from a temperature of 1420 ° C. and converts the matrix carbon into silicon carbide. Subsequently, the formed C / SiC pipe in the outer and inner regions is ground to the desired final geometry. [58] The C / SiC moldings produced in this way have a strength of 50 - 300 MPa and a thermal conductivity of 50 - 150 W / mK. Depending on the manufacturing process, the material composition of the moldings may be given as follows: 2 - 30% carbon, 50 - 70% silicon carbide and 5 - 15% silicon. The porosity of the material is very low at <2%.

Claims

claims:

1. A method for protecting heat exchanger tubes in steam boiler systems, characterized in that heat exchanger tubes of the steam boiler system are at least partially surrounded by ceramic.

2. The method according to claim 1, characterized in that the ceramic is fiber-reinforced.

3. The method according to claim 1 or 2, characterized in that the ceramic is at least partially formed of silicon carbide.

4. The method according to claim 3, characterized in that the ceramic is at least partially formed by siliconizing a graphite or carbon film, in particular a sheet of expanded graphite.

5. The method according to any one of the preceding claims, characterized in that the ceramic relative to the heat exchanger tube (1) is arranged displaceably.

6. The method according to any one of the preceding claims, characterized in that the ceramic is arranged in the form of a plurality of adjacent cladding elements.

7. The method according to any one of the preceding claims, characterized in that the enveloping elements are formed from circular segment shells.

8. The method according to claim 4, characterized in that the circular segment shells are axially and / or radially positively connected with each other.

9. The method according to any one of the preceding claims, characterized in that the ceramic comprises carbon fibers.

10. The method according to any one of the preceding claims, character- ized in that the ceramic is exposed to temperatures of about 400 ° C.

11. The method according to any one of the preceding claims, characterized in that the heat exchanger tubes are exposed on their inside a pressure of about 40 bar.

12. The method according to any one of the preceding claims, characterized in that the ceramic has a thickness between inner diameter and outer diameter of less than 10 millimeters and preferably less than 5 millimeters.

13. The method according to any one of the preceding claims, character- ized in that the ceramic has an inner diameter of more than 30 mm.

14. The method according to any one of the preceding claims, characterized in that the surface of the ceramic structures for influencing the flow and for influencing the deposition behavior of particles.

15. The method according to any one of the preceding claims, characterized in that the enveloping elements are fixed by brackets or welding points in their position.

16. molded body with a fiber-reinforced ceramic for performing a method according to any one of the preceding claims, which is adapted to envelop a heat exchanger tube.

17. Shaped body according to one of the preceding claims, whose surface is coated to prevent deposits or caking with nanoparticles.

18. Heat exchanger tube, which is surrounded by a shaped body according to claim 9.

19. Heat exchanger tube according to claim 18, characterized in that a preferably annular gap is arranged between the shaped body and the heat exchanger tube.

20. Heat exchanger tube which is surrounded by fiber materials such as fiber ceramic mats. Steam boiler plant comprising heat exchanger pipes according to one of the preceding claims.