US20060213405A1

US20060213405A1 - Grate covering

Info

Publication number: US20060213405A1
Application number: US11/207,215
Authority: US
Inventors: Bernhard Zimmermann
Original assignee: Alstom Technology AG
Current assignee: GE Vernova GmbH
Priority date: 2005-03-22
Filing date: 2005-08-19
Publication date: 2006-09-28
Also published as: EP1705425A1; JP2006266676A

Abstract

A grate covering for an incineration grate of an incineration furnace having a combustion chamber, includes a plurality of metal grate bars, each having a surface facing the combustion chamber that includes a layer made of a temperature-resistant, corrosion-resistant and abrasion-resistant non-metallic material, wherein a thickness of the layer is minimally sufficient to bear pressure and thrust loading. The thickness of the layer is about 10 mm to 15 mm. For a forward feed grate, the layer on each metal grate bar is present only in a zone of contact with an adjacent metal grate bar, and for a roller grate, and wherein the layer is present on the entire surface facing the combustion chamber.

Description

FIELD OF THE INVENTION

The invention relates to the field of incineration technology. It pertains to a grate covering having a protective layer made of a temperature-resistant, corrosion-resistant and abrasion-resistant non-metallic material that can be used especially for a grate in an incineration furnace for burning waste.

DESCRIPTION OF RELATED ART

It is known from the state of the art to burn waste on an incineration grate, for example, a forward feed grate or roller grate, whose grate covering made up of air-cooled or water-cooled grate bars transports the waste through the combustion chamber by means of a defined movement.
In the case of air-cooled grates, primary air flows through the grate, where it is first used to cool the grate bars and subsequently as a requisite reactant for the combustion. This system entails the following drawbacks:
The marked fluctuation in the quality of the waste in terms of gross calorific value, density, flow resistance, moisture, etc. leads to enormous temperature fluctuations, and often to excessively high temperatures of the air-cooled cast grate bars, which cannot be tolerated.
The movement of the bars relative to each other and relative to the waste leads to a high wear and tear due to abrasion, a phenomenon that occurs at an accelerated rate at high temperatures.
The frequent temperature changes with large temperature gradients and temperature transients lead to the thermomechanical failure of individual grate bars. The resultant changed flow conditions for the primary air ultimately damage larges sections of the grate.
An attempt is made to overcome these drawbacks by using water-cooled grate bars. Although the expensive cast alloys that have to be used for air-cooled grates can be replaced in water-cooled grates by less expensive sheet-metal constructions, water-cooled grates entail a number of disadvantages. On the one hand, they are technically much more complex than air-cooled grates since they require a separate cooling-water circulation system, in addition to which they are very damage-prone. If leaks occur, they have to be shut down. On the other hand, a detrimentally large amount of heat energy is withdrawn from the system at a low level, which reduces the thermal efficiency of the waste incineration plant.
In actual practice, in addition to the above-mentioned problems, so-called weld build-ups are encountered with the described grate bars. This phenomenon has to do with metal mixtures that form from the product being incinerated, that are molten and that cool when they come into contact with the grate covering, thus creating a tight bond with the grate covering, similar to a welded joint. The weld build-up can be very small, but it can also be elevated by as much as 10 mm or more. The frequency and extent of the weld build-ups depend on the composition of the waste.
Such weld build-ups on forward feed grates are especially detrimental when they occur in the zones where two grate bars come into contact with each other. This leads to greater wear and tear of the contact areas and, in the case of water-cooled grate bars, can cause premature failure when leaks occur due to excessive material abrasion especially in this area.
In addition, the operation of the installation can be impaired if the weld build-up hinders or blocks the movement of the rows of grate bars. In this case, the transport of the product being incinerated is reduced or stops altogether. The consequence of this can be malfunctions in the course of the incineration, installation standstills and operational failures.
In the case of roller grates, weld build-ups are detrimental if they are elevated to such an extent that they come into contact with the stripping plate. This can result in greater wear and tear of the stripping plate or in a shifting of the stripping block and/or of the stripping plate itself. This can lead to a deterioration of the primary air distribution and cause more material to fall through the grate. In addition, the operation of the installation can be severely impaired if the rotation of the rollers is hindered or blocked by the weld build-up. In this case, the transport of the product being incinerated is impeded or even comes to a halt. The consequence can be installation standstills and operational failures.
European Patent Application EP 0 382 045 A2 discloses a grate bar comprising a load-bearing lower element made of steel or cast iron and an upper plate-like element made of ceramic material, whereby the ceramic element constitutes the bed for the waste being incinerated. Thanks to the ceramic material employed, the upper element is highly temperature-resistant and durable, so that the grate can be air-cooled. The upper ceramic element and the lower element of each grate bar are joined in a form-fitting manner by means of bolts and elastic elements such as, for example, springs. A drawback of this state of the art is the fact that, in case of impact stress, the upper ceramic element configured as a plate easily breaks since the ceramic material is very brittle and highly susceptible to impact while exhibiting low elongation at break. Moreover, it is not possible to sufficiently compensate for the large temperature differences and the different thermal expansion behavior of the two elements, with the result that undesired strains and warpage occur, thus shortening the service life of the grate. Furthermore, a high manufacturing tolerance of the joints is needed so that the assembly procedure itself does not already cause damage. This translates into high production costs.
A similar solution is known from German Patent DE 33 14 098 C2 in which the grate block or grate bar, as a component of a grate covering for a waste incineration furnace, is configured in such a way that the grate block or the front part of the grate bar that is exposed to the incineration consists of a fine-ceramic material while the block or rod holder with a bearing part (grate block or back part of the grate bar) is made of steel or cast steel. Here, too, the two parts are held together by means of screwed connections. In addition to the disadvantages listed above, this version also has the drawback that it is very expensive since almost the entire grate block or grate bar is made of expensive ceramic material.
German Patent DE 197 14 573 C1 discloses a furnace grate or individual elements for such a grate made completely of a high temperature-resistant, abrasion-resistant and corrosion-resistant ceramic material. This technical solution is likewise very expensive.
German Patent Application DE 198 57 416 A1 describes a high temperature-resistant grate bar for the grate of a waste incineration furnace that has a layer consisting of numerous ceramic elements on its surface facing the waste bed, said elements being embedded in the interstices of a grid-like metal frame structure. This grate bar is characterized by low wear and tear and by a good absorption of impact stress. However, the above-mentioned weld build-ups with their attendant drawbacks can form in the area of the metal frame structure.

SUMMARY OF THE INVENTION

The invention endeavors to avoid the disadvantages of the known state of the art. It is based on the objective of developing a grate covering comprising a plurality of grate bars for an incineration grate, especially for a forward feed grate or roller grate, wherein the formation of weld build-ups on the grate bars during operation is prevented.
According to the invention, this is achieved with a grate covering in accordance with the generic part of claim 1 in that said layer is so thin that it merely has to bear pressure and thrust loading and in that, in the case of a forward feed grate, said layer on the grate bar is only applied in the contact zone with the adjacent grate bar, whereas in the case of a roller grate, said layer is applied onto the entire surface of the grate bar facing the combustion chamber.
The advantages of the invention lie in the fact that, on the one hand, only the parts of the grate bar that are exposed to the corrosive and oxidizing atmospheres at high temperatures are provided with a high temperature-resistant, abrasion-resistant and corrosion-resistant non-metallic layer, so that the grate bar is relatively inexpensive and, on the other hand, weld build-ups on the grate bars are prevented in the critical areas as a result of which malfunctions in the course of the incineration, installation standstills and operational failures are all avoided.
It is advantageous for the layer to be applied or cemented in by means of an adhesive bond since this can be done quickly and inexpensively.
It is likewise advantageous for the layer to be applied to the surface of the grate bar by means of a mechanical joint, especially a countersunk screwed connection. Then it can be detached from the grate covering and replaced relatively easily.
It is also advantageous for the layer to be made of a ceramic layer since this material definitely has the requisite combination of properties (high temperature-resistance and, at the same time, high corrosion and abrasion-resistance).
Moreover, it is advantageous if a metal web is formed at the edge of the metal grate bar in order to delimit the circumference of the layer, said web having either a rectangular contour or advantageously a slanted contour that widens in the direction of the surface. In the latter case, this serves to provide an additional anchoring of the layer.
Finally, it is advantageous for the layer to be applied in several segments. This serves, among other things, to compensate for and reduce the influence of different thermal expansions between the layer and the grate bar.

BRIEF DESCRIPTION OF THE DRAWING

The drawing shows several embodiments of the invention. The following is shown:
FIG. 1 a longitudinal section through a schematically depicted grate covering according to the invention, in a first embodiment, for a forward feed grate;
FIG. 2 a longitudinal section through a schematically depicted grate covering according to the invention, in a second embodiment, for a roller grate;
FIG. 3 a section through two adjacent grate rollers and the stripping device of a roller grate and
FIG. 4 a cross section through a grate bar according to the invention in the area of the coating.
The only elements shown are those that are essential for understanding the invention. The same elements are each provided with the same reference numerals.

WAYS TO EXECUTE THE INVENTION

Below, the invention will be explained with reference to embodiments and to FIGS. 1 to 4.
FIG. 1 shows a longitudinal section through a schematically depicted grate covering, in a first embodiment of the invention. The grate covering consists of a plurality of metal grate bars 1 and is intended for an air-cooled forward feed grate. Such forward feed grates have several rows of grate bars 2 a, 2 b, etc. that are arranged like roof tiles above each other and that are alternately movable and stationary, whereby several grate bars 1, in turn, are arranged next to each other within the rows of grate bars 2 a, 2 b, etc. The grate bars 1 are preferably cast bars. Of course, the invention can also be used for water-cooled grate bars.
The grate is arranged in an incineration furnace (not shown here) in which waste, for example, household garbage, is incinerated. By means of a defined movement of the grate bars 1, the waste is transported through the combustion chamber 3 of the incinerating furnace. Primary air flows through the grate, first cooling the grate bars 1 and then being used to incinerate the waste. Since the surface 4 of the grate bar 1 facing the combustion chamber 3 is exposed to temperatures of more than 700° C. [1292° F.] and also to oxidizing and corrosive atmospheres, the surface 4 facing the combustion chamber 3 is at least partially provided with a layer 5 made of temperature-resistant, corrosion-resistant and abrasion-resistant non-metallic material. This is the known state of the art.
According to the present invention, this layer 5 is configured to be so thin that it merely has to bear pressure and thrust loading. The layer thickness is, for example, about 10 mm to 15 mm.
In the case of a forward feed grate of the type described in the first embodiment, the layer 5 on the grate bar 1 is applied only in the contact zone 6 with the adjacent grate bar 1. In FIG. 1, this area is indicated by the hatched depiction of the position of the upper grate bar 1. The material properties of the layer 5 are selected in such a way that the grate bar 1 and liquid material from the product being incinerated cannot fuse in this area and so that no weld build-ups can be formed that would impair the movement of the grate bar and so that little or no wear and tear occurs. Advantageously, ceramic material is used as the material for the layer 5 because of its good combination of properties.
Since the layer 5 is very thin, it can advantageously be applied or cemented in by means of an adhesive bond. However, application by means of a mechanical joint, for example, a countersunk screwed connection, is likewise possible. Then it can be detached from the grate covering and replaced relatively easily.
FIGS. 2 and 3 show a second embodiment. FIG. 2 shows a longitudinal section through a schematically depicted grate covering according to the invention for a roller grate. FIG. 3 shows a section through two adjacent grate rollers and the stripping device of a roller grate.
The roller grate consists of a support construction (not shown in FIGS. 3 and 4) that is inclined with respect to the horizontal in the direction of the waste discharge. Several consecutive cylindrical grate rollers 7 are suspended in this support construction and they serve to transport the waste through the furnace as a result of a rotating movement. The grate rollers 7 are provided with a plurality of grate rods 1, usually made of gray cast iron. Strippers 8 are arranged behind each grate roller 7 and they strip off the waste lying on the grate roller 7 and feed it to the next grate roller 7. The strippers 8 have replaceable stripping plates 9 that are positioned close to the grate rollers 7 and are arranged on the stripper block 10 (see FIG. 3).
According to the invention, with this embodiment, that is to say, in the case of a roller grate, the layer 5 made of a temperature-resistant, corrosion-resistant and abrasion-resistant non-metallic material is applied onto the entire surface 4 of the grate bar 1 facing the combustion chamber 3. As already described above, it can be bonded, cemented in or applied by means of a mechanical joint.
In this manner, the formation of elevated weld build-ups and a resultant contact with the stripping plates 9 are avoided and so is a shifting of the stripper block 10 and/or of the stripping plate 9 itself, which would lead to a deterioration of the primary air distribution and cause more material to fall through the grate. Of course, this also prevents a hindrance or blocking of the grate rollers 7 caused by weld build-ups.
FIG. 4 shows a cross section through a grate bar 1 according to the invention in the area of the coating. At the edge of the cast metal grate bar 1, there is a web 11, likewise made of cast material, so that the edge of the layer is completely surrounded by cast material. This delineation of the circumference of the layer 5 can be configured either so that the edge secures the layer 5 by means of a right-angled flank of the web 11, that is to say, the web 11 has a right-angled contour (see, for instance, FIG. 2) or else the web 11 has a contour that widens in the direction of the surface 4 of the grate bar 1 facing the combustion chamber 3. Thus, the non-metallic layer 5 has a slightly smaller cross section directly at the surface 4 than at a certain distance away from the surface 4. This has the advantage that the layer material is held more securely and the layer 5 is effectively prevented from tearing loose or being detached during the incineration operation.
In another embodiment, the non-metallic layer 5 can also be applied in several segments. This serves, among other things, to compensate for and reduce the influence of different thermal expansions between the layer 5 and the grate bar 1.
Naturally, the invention is not limited to the embodiments described.

LIST OF REFERENCE NUMERALS

1 grate bar
2 a, 2 b, etc. grate bar row
3 combustion chamber
4 surface of the grate bar
5 layer
6 contact zone
7 grate roller
8 stripper
9 stripping plate
10 stripping block
11 web

Claims

1-10. (canceled)

11. A grate covering for an incineration grate of an incineration furnace having a combustion chamber, the grate covering comprising:

a plurality of metal grate bars, each having a surface facing the combustion chamber that includes a layer made of a temperature-resistant, corrosion-resistant and abrasion-resistant non-metallic material, wherein a thickness of the layer is minimally sufficient to bear pressure and thrust loading.

12. The grate covering as recited in claim 11, wherein the thickness of the layer is about 10 mm to 15 mm.

13. The grate covering as recited in claim 11, wherein the incineration grate is a forward feed grate, wherein the plurality of metal grate bars includes adjacent metal grate bars in moveable contact with one another, and wherein the layer on each metal grate bar is present only in a zone of contact with an adjacent metal grate bar.

14. The grate covering as recited in claim 11, wherein the incineration grate is a roller grate, and wherein the layer is present on the entire surface facing the combustion chamber.

15. The grate covering as recited in claim 11, wherein the layer is attached to each of the metal grate bars by means of an adhesive bond.

16. The grate covering as recited in claim 11, wherein the layer is cemented into each of the metal grate bars.

17. The grate covering as recited in claim 11, wherein the layer is connected to each of the metal grate bars by means of a mechanical joint.

18. The grate covering as recited in claim 17, wherein the mechanical joint is a countersunk screwed connection.

19. The grate covering as recited in claim 11, wherein the non-metallic material is a ceramic material.

20. The grate covering as recited in claim 11, wherein a circumference of the layer is delimited by a metal web formed at an edge of each metal grate bar.

21. The grate covering as recited in claim 20, wherein the web has a rectangular contour.

22. The grate covering as recited in claim 20, wherein the web has a slanted contour that widens in a direction of the surface.

23. The grate covering as recited in claim 11, wherein the layer includes several segments.