GB2042698A - Fluid Bed Furnaces - Google Patents

Abstract

This invention concerns a fluidised-bed furnace bed support 10 comprising loose discrete tile-like parts 14 of non-metallic material having a low coefficient of thermal expansion. Ducts 36 traverse the bed support & comprise the main structural strength of the bed support and the agency by which the support is cooled. Stand pipes 20 pass through holes in between the tiles 14 and ducting 36 to carry air to the bed from a plenum 50. The unapertured heights 42 of the stand pipes ensure a layer 44 of static bed material. <IMAGE>

Description

SPECIFICATION Fluid Bed Furnaces The invention relates to so-called fluid-bed furnaces wherein a bed of incombustible particulate material, such as sand, is fluidised by injection of combustion promoting gas, such as air, in order to burn fuel material added to the incombustible particulate material usually in solid form (e.g. coal), but possibly in liquid form (e.g.

Oil), There are advantages in providing bed supports that permit both gas inlet from below the bed of incombustible particulate material, and also a preheating provision below the bed for initial raising of the bed to ignition temperatures for the fuel material to be burned. We have ourselves made previous proposals for such provisions by way of, inter alia, arched or domed bed support constructions, with fluidising gas release via pipes passing through the bed support.

One of the problems that arises concerns expansion of the bed structure at the high temperatures attainable with luid-bed furnaces, typically as high as possible short of fusion of the bed material and/or fuel waste therein. Expansion is an especially severe problem when using materials such as metals that have a high coefficient of thermal expansion, and is exacerbated by the customary unitary construction of bed supports.

We have considered meeting these problems by using greater depths of uncombustible particulate material and longer lengths of gas pipes to obtain a thick lower static bed part that will act as a heat insulant. However, that increases the volume of bed material, and thus its weight, which leads to greater load carrying requirements, and thus expense, for the bed support structure.

As an alternative, we have also turned our attention to the use of a structure made up of a plurality of discrete, loose plates and support means therefor. This would have the significant advantages of a "knock-down" supply option, simple assembly on site, and ready maintenance.

However, the provision of suitable expansion joints between such plates themselves, and between the plates and gas distribution pipes, has proved to be a very difficult problem to solve using metal plates due to their large coefficient of thermal expansion and the requirement to avoid as much as possible the penetration of sand into and through the joints, especially when the furnace is cold.

We believe that these problems may be mitigated by a bed support made up of loose discrete area parts of non-metallic material having a low coefficient of thermal expansion.

Then, expansion joints between those parts themselves, or between the parts and gas distribution pipes may be of much lower clearance and thus much more readily made substantially impervious to sand or other particulate incombustible bed material.

However, a principal aspect of the invention arises from the form of support that we have devised for such tile-like parts. This takes the form of ducting for combustion promoting gas, which ducting comprises hollow section members of sufficient structural strength to support the furnace bed material in operation and any tile-like parts. Such ducting will serve in delivering the combustion promoting gas to a plenum chamber under the bed for subsequent delivery via stand pipes or the like passing upwards through the bed support. In this way, the combustion promoting gas can traverse the bed support ducting with a cooling action thereon before reaching the plenum chamber which, for preference, also houses burner for preheating of the bed to ignition temperatures.Then, of couse, tile-like parts secured to the underside of the ducting and having good heat insulation properties will serve to protect the ducting, usually of steel, from excessive heating during operation of the ignition burner.

Refractories are suitable for tiles constituting bed support parts, and ceramics may be of particular advantage by way of inherent strength.

Obviously, suitable reinforcements of such materials may be made, including glass-fibres. It may well be that one of the most satisfactory materials will provide to be glass fibre reinforced concrete which is very strong for its weight and readily made to precise shapes using moulds.

An important advantage of the use of such ducting is that the ducting itself will support the bed, most often from mounting of its ends in furnace walling, but possibly also or alternatively or specific other mount walls, posts or pillars therefor. Air or other combustion promoting gas release pipes passing through the ducting, usually between components thereof, and between the tiles and/or through holes in the tiles, serve to communicate air from the plenum chamber into the incombustible particulate bed material. Those pipes preferably also serve in holding the tiles to the ducts by way of abutments secured to the pipes, e.g. integral upper flanges and detachable lower clips, pins or the like. Such abutments may constitute or be parts of expansion interconnection of the tiles.The pipes should be resistant to oxidising by heated gas from action of the ignition burner and also combustion promoting gas alone.

The release pipes will have discharge orifices at a sufficient level above the support structure to provide a static layer of particulate bed material for insulation of the support structure, particularly the ducting from excessive heating from combustion of fuel in the bed.

If desired, a layer of heat resistant material, such as ceramic felt, may be interposed between the tiles and the ducts as an aid to resisting fallthrough of bed material.

In another embodiment a further or alternative layer of ceramic tiles may be disposed above the air feed ducting with or without a ceramic felt or similar layer over the ducting or the tiles themselves. It is possible that washers of heat resistant material such as ceramic felt could be used at abutments of the pipes, possibly only at upper abutments.

One particular embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a section through part of a bed support of the invention; Figure 2 is a section through a preferred fluidised bed furnace; and Figure 3 is a section on line A-A of Figure 2.

Referring first to Figure 1, a bed support 10 is shown to comprise a layer 12 of refractory tiles 14 all a loose fit edge-to-edge to accommodate whatever small thermal expansion may occur.

Clearly, lipping of the tile edges by rebating from one side of the tiles would allow a patchwork assembly with adjacent tiles relatively inverted, which may assist in preventing drop-through of any sand or other incombustible particulate bed material reaching that layer 12, but would not allow for such ready disassembly as will be particularly described.

Passing through holes in or between the tiles 14 are stand pipes 20 with sufficient clearances 22 to allow for expansion of those pipes which should resist oxidising and will usually be of metal for example stainless steel. The clearances 22 could be much smaller, even substantially nonexistent, if the stand pipes 20 or at least lower parts thereof were of a material having much the same coefficient of thermal expansion as the tiles themselves.

A layer 30 of low expansion heat-resistant material for example of ceramic felting is shown overlying the tiles 14 as a protection against penetration of the tile layer 1 2 by bed material.

This layer 30 may be of discrete areas, but preferably not corresponding and registering with the tiles 14, and no clearance is shown about the stand pipes 20. As will become clear, this layer 30 may be dispensed with in certain circumstances.

Above the refractory tile layer 12, and any layer 30, the stand pipes pass between ducts 36 that traverse the bed support 10 from end to end, or side to side, and comprise both the main structural strength of the bed support as a whole and also the agency by which that support is cooled. These ducts 36 will usually be of metal when clearances 38 between them and the stand pipes will be required and may be greater than the clearances 22 to tolerate any thermal expansion that may take place, though it is to be understood that cooling of the bed by passage of air through the ducts 36 will keep temperature rise thereof to a satisfactorily low level.

The stand pipes 20 extend significantly above the ducting 36 and terminate in apertured air distribution heads 40 that, in use, will be within the sand or other incombustible particulate material of a furnace bed. The unapertured heights 42 of the stand pipes above the ducting will ensure a layer 44 of static bed material to protect the ducting 36 from excessive and damaging temperatures generated by a fluid bed furnace both in operation and also when shut off at which time the fluidised bed material will slump. To cover the possibility that the static layer 44 is of insufficient depth to insulate the ducting 36 in slump conditions, provision may be made to continue air circulation through the ducting 36 but not into a plenum chamber 50 under the tile layer 12 as is the normal manner of bed fluidisation by air feed to and through the stand pipes 20.

As shown, the ducting 36 serves as the main support means of the bed support as a whole.

This is achieved by having abutments 54 secured to the stand pipes 20 above the ducting 36 and more than covering the clearances 38, and further abutments 56 releasably secured to the stand pipes 20 below the tile layer 12. These abutments 56 may be pins through the stand pipes 20, or ships about the stand pipes 20, or nuts on threaded ends of the stand pipes 20, or, indeed any other arrangement and can be arranged to give a clamping action between the tile layer 12 and the ducting 36.

As an alternative or addition to the layer 30, washer-like components of a similar material could be inserted between the upper stand pipe abutments 54 and the upper sides of the ducting 36. In both cases good sealing against penetration by bed material, at least to the tile layer 12 is provided together with cushioning that assists in the clamping action between stand pipe abutments 54 and 56.

The ducting 36 will not only traverse the bed support proper but will usually protrude at each end and then provide satisfactory means for mounting the bed on walls of a furnace, see 60, 62, 64 of Figure 2. It will be appreciated that the tiles 14 will, at least if not of interengaging edge lipped form, permit removal and replacement of individual tiles from below at any time by removal of appropriate ones of the lower stand pipe abutments 56.

From the point of view of even better overall structural integrity and protection of the ducting 36, we further propose a structure in which a further layer of refractory tiles is interposed between the upper stand pipes abutments 54 and the ducting 36 as indicated in dashed lines at 58 in Figure 1. This may utilise an additional upper layer of felting or the like between the upper tiles and the ducting 36, or simply rely upon washers, or exclude both if the resulting structure will accommodate any small thermal expansion of the tiles without such cushioning.

A preferred furnace is shown in Figures 2 and 3 where the above reference numerals are used when appropriate. A combustion chamber 66 has a sealed solid fuel feed 68 from hopper 70 into one end of that chamber and onto a bed of incombustible particulate material 72 shown on a bed support 10 with a static lower layer 44 and a fluidised upper layer 76 in which the fuel is burnt. An upper exhaust flue 78 is shown at the other end of the furnace chamber 66 above a sloping shelf 80 from which deposited parts of initially entrained material can drop back into the bed proper.

It will be noted that the sloping shelf 80 is one exterior of an air supply chamber 82 fed from a fan 84 housed against the furnace proper. This chamber 82 acts as a plenum supply chamber to the ducting 36 which discharges at its other end into another chamber 86 as a supply to the stand pipe feed plenum chamber 50 under the bed, at least partially via auxiliary or preignition heating burner(s) 88 used to bring the fluidised bed up to fuel ignition temperature.

A second route for air to the plenum chamber 50 is also shown at 90 directly from the chamber 82 for use in supplying air to both ends of the under bed plenum chamber 50 after preheating is complete and furnace ignition is achieved. A damper arrangement 92 is shown for controlling this possibility.

The possibility of venting to atmosphere at the chamber 86 is, also envisaged for air cooling of the bed support structure even in slumped bed conditions, should that be desired.

Various access doors are indicated at 94, 96, 98 for clearing and maintenance purposes.

In general, embodiments of this invention may be viewed as bed support structures in the form of an air or other combustion promoting gas distributor with the air or other gas serving to cool the support structure to keep to temperatures at which thermal expansion and oxidation problems are negligible or at least readily tolerated and accommodated. Metal support ducting of such a distributor can thus be held to fairly low expansion differentials relative to refractory tilelike insulation or cladding, even on ignition preheating from below. Also, of course heating of the air or other gas as it traverses the bed support ducting will enhance combustion potential both of preheating fuel and main in-bed fuel.

Claims (25)

Claims

1. A fluid-bed furnace bed support comprising loose discrete tile-like parts of non-metallic material having a low coefficient of thermal expansion.

2. A bed support as claimed in claim 1 wherein the tile-like parts are made of refractory material.

3. A bed support as claimed in claim 2 wherein the tile-like parts are made of ceramic material.

4. A bed support as claimed in claim 2 or 3 wherein said tile-like part material is reinforced.

5. A bed support as claimed in claim 4 wherein said tile-like part material is reinforced with glassfibres.

6. A bed support as claimed in claim 1 wherein the tile-like parts are made of glass fibre reinforced concrete.

7. A fluid-bed furnace having a bed support as claimed in any one of claims 1 to 6 which bed support is itself supported by ducting for combustion promoting gas, wherein the ducting comprises hollow section members of sufficient strength to support the furnace bed material in operation and any tile-like parts.

8. A fluid-bed furnace as claimed in claim 7 wherein the ducting serves to deliver the combustion promoting gas to a plenum chamber under the bed for subsequent delivery to the bed through the bed support.

9. A fluid-bed furnace as claimed in claim 8 wherein the plenum chamber houses a burner for preheating of the bed to ignition temperatures.

10. A fluid-bed furnace as claimed in claim 7, 8 or 9 wherein the ducting has its ends mounted in furnace walling.

11. A fluid-bed furnace as claimed in any one of claims 7 to 10 wherein mounting of the ducts is on specific mount walls, posts or pillars therefor.

12. A fluid-bed furnace as claimed in any one of claims 7 to 11 wherein air or other combustion promoting gas release pipes pass through the ducting and between the tiles and/or through holes in the tiles.

13. A fluid-bed furnace as claimed in claim 12 wherein the release pipes pass between components of the ducting.

14. A fluid-bed furnace as claimed in claim 12 or 1 3 wherein the release pipes also serve in holding the tiles to the ducting by way of abutments secured to the pipes.

1 5. A fluid-bed furnace as claimed in claim 14 wherein the abutments comprise .integral upper flanges and detachable lower clips, pins or the like.

16. A fluid-bed furnace as claimed in claim 14 or 1 5 wherein the abutments constitute or are parts of expansion interconnection of the tiles.

17. A fluid-bed furnace as claimed in any one of claims 12 to 1 6 wherein the release pipes have discharge orifices at a sufficient level above the support structure to provide a static layer of particulate bed material for insulation of the support structure from excessive heating from combustion of fuel in the bed.

1 8. A fluid-bed furnace as claimed in any one of claims 7 to 1 7 wherein a layer of heat resistant material is interposed between the tiles and the ducts.

19. A fluid-bed furnace as claimed in any one of claims 7 to 18 wherein a further layer of ceramic tiles is disposed above the air feed ducting.

20. A fluid-bed furnace as claimed in claim 1 9 wherein a layer of heat resistant material is placed over the ducting or the tiles.

21. A fluid-bed furnace as claimed in any one of claims 14 to 20 wherein heat resistant washers are used at upper abutments of the pipes.

22. A fluid-bed furnace as claimed in claim 21 wherein heat resistant washers are also used at lower abutments of the pipes.

23. A fluid-bed furnace as claimed in any one of claims 1 8 to 22 wherein the heat resistant material is a ceramic felt.

24. A fluid-bed furnace bed support substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

25. A fluid-bed furnace substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.