Wall Construction for a Boiler Arrangement

Abstract

A wall construction for a boiler arrangement. The boiler arrangement is formed of at least a furnace and a separator. The furnace has a grid, a bottom part, and an upper part. The separator is arranged by conduits in flow communication with both the upper part and the bottom part of the furnace. The conduits, together with the separator, form an external circulation of bed material. The upper part of the furnace has four vertical walls, and the bottom part of the furnace has a height and four walls extending from the grid up to the vertical walls. The wall construction includes at least one hollow beam being attached to a wall of the bottom part of the furnace and extending substantially over the entire height of the bottom part, and the at least one hollow beam being in flow communication with the external circulation for returning bed material into the furnace.

Description

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates to a wall construction for a boiler arrangement. The present invention is especially applicable in connection with, for example, lower inclined walls of fluidized bed boilers and circulating fluidized bed boilers.

An ordinary fluidized bed boiler arrangement of the prior art comprises a furnace, to which fuel, bed material and combustion air are introduced. When combusting the fuel, heat is generated, and both bottom ash and flue gases are generated. The flue gases are taken to a separator, which separates solid particles from the gases. The solid particles are then returned back to the furnace.

Structurally, a circulating fluidized bed boiler (CFB) generally includes a furnace having a bottom, side walls and a roof, and at least one particle separator connected in flow communication with the upper part of the furnace. At least some walls of the bottom part of the furnace are normally inclined, such that the cross section of the furnace increases upwardly. The part of the furnace having the inclined walls may be called a converging bottom part. In practice, all of the walls and the roof of the boiler and the separator comprise water or steam tubes to collect heat from the furnace. The walls at the converging bottom part of the furnace are normally covered with refractory material that resists abrasion better than metallic, water or steam tube walls. The bottom of the furnace is provided with a grid for introducing combustion, or suspending, or fluidizing gas, called primary air, into the furnace, and for removing ash and other debris from the furnace. The side walls of the furnace are provided with means for introducing fuel and means for introducing secondary air into the furnace, as well as start-up burners. The furnace is also equipped with means for feeding inert bed material, which is normally sand, into the furnace. Very often, the introduction means (for the fuel, the secondary air, and the bed material) are positioned in the converging bottom part of the furnace.

The particle separator separates solid particles from the flue gas and a suspension of the solid particles entering the separator from the upper part of the furnace. The flue gases are taken for further treatment from the separator, and separated solids are recycled back to the lower port of the furnace via a recycling conduit, including a sealing device, such as a loopseal. The purpose of the loopseal is to prevent gas from flowing from the furnace to the separator via the recycling conduit. This solids circulation is called external circulation. In addition to vertical upflow of the flue gas and the suspension of the solid particles in the furnace, entering finally into the separator inlet, there is a vertical downflow of particles near and along the furnace walls. This solids circulation is called internal circulation.

Very often, in connection with the internal or the external circulation of solid material, or both, at least one fluidized bed heat exchange chamber has been arranged to transfer heat from the bed of fluidized particulate solids to a heat transfer medium. Such a fluidized bed heat exchanger is sometimes arranged in the external circulation, so that the solids leaving the solids separator are discharged into the heat exchange chamber on their way back to the furnace (see, for example, FIG. 1 (Prior Art)). This kind of a fluidized bed heat exchange chamber typically hangs from the separator at a distance from the furnace wall. The interior of the heat exchange chamber is provided with heat exchange means for transferring heat from the solid material to the heat transfer medium flowing inside the heat exchange means.

Lately, it has been suggested that a fluidized bed heat exchange chamber for recovering heat from the circulating solids in the external circulation could also be arranged upon the furnace wall, i.e., supported by the vertical furnace wall.

The solids entering the furnace from the external circulation, i.e., directly from the separator, or via the fluidized bed heat exchanger, are normally introduced into the furnace via one or more openings in the lower part of the furnace, i.e., through the inclined walls of the converging bottom part of the furnace. The conduit taking the solids back to the furnace is conventionally an independently suspended channel between the separator and the furnace. This kind of solids return conduit does not run along the outer surface of the furnace, but slopes from the bottom of the separator or of the fluidized bed heat exchange chamber towards the converging bottom part of the furnace, occupying a considerable space between the separator/fluidized bed heat exchanger and the lower part of the furnace. As another alternative, i.e., when the fluidized bed heat exchange chamber is upon the furnace wall, the solids discharged from the chamber are taken down to the grid area of the furnace, outside the furnace and introduced only there into the furnace. This means that the return conduit runs down to the grid area in the vicinity of the furnace wall. This is a controversial feature, as, on the one hand, this kind of a structure leaves the space further outside the furnace wall intact, i.e., makes the structure more compact, but, on the other hand, occupies its space directly from the surface of the furnace wall, preventing any structures from being positioned on or through the furnace wall.

The structures of a fluidized bed boiler not only need to withstand the load caused by the combustion, i.e., heat, which is, by nature, a continuous, normally non-changing load, but also, both sub- and super-atmospheric pressures that tend to bend or to flex the planar boiler walls to curved ones. Since the boiler walls are made of welded water or steam tubes provided with fins or membrane plates between the tubes, the wall structure is very weak against bending or flexing, and needs specific reinforcement structures. Therefore, the walls are provided with both vertical and horizontal reinforcements or stiffeners, called buckstays. Normally, vertical buckstays are welded on the boiler wall so that their thermal expansion is the same as that of the boiler wall. Horizontal buckstays are arranged outside the vertical ones, and are arranged in slidable connection with the vertical buckstays, such that their thermal expansion may be different from (that is, less than) the boiler wall and the vertical buckstays.

Reinforcing the walls of the boiler furnace with horizontal and vertical buckstays does not, in general, pose a problem. There is plenty of room for all reinforcements used for stiffening the vertical walls in the upper part of the furnace, as there are not that many other pieces of equipment arranged on or through the furnace wall. However, the lower part of the furnace and, specifically, the converging bottom part of the furnace, is provided with so many conduits and accessories that positioning both the conduits and accessories, as well as the reinforcements, in an optimal manner is difficult, and sometimes, in practice, impossible. The bottom part of the furnace has to be provided with at least the following conduits: fuel feed, bed material feed, primary air feed (in addition to the grid), secondary air feed, returning bed material feed (both from the separator, from the fluidized bed heat exchange chamber in connection with the separator, and from the fluidized bed heat exchange upper chamber on the wall of the furnace), and a connection for at least one, and, more often, several, start-up burners. Additionally, there may be one or more fluidized bed heat exchangers positioned outside the converging walls of the bottom part of the furnace. And, further, the converging bottom part of the furnace is also provided with service access doors. And, finally, since the number of the above-listed conduits, heat exchangers and openings is normally more than one, it is easy to see that finding an optimal location for each component is such a challenging task that most often several compromises have to be made.

One way to optimize the structures available on the furnace wall is (as is already known from the prior art, and illustrated in a very schematic manner in FIG. 2) to attach some parts of the conduit, returning bed material to the bottom part of the furnace, to the wall of the bottom part of the furnace. The prior art return conduit receives circulating bed material from an ordinary separator arranged at a distance from the boiler. The return channel originating from the separator is connected to the top of the return conduit by means of a bellows arrangement that allows some movement due, for instance, to changing temperatures. The return conduit is formed of water/steam tube panels, such that it has two side walls and a back wall. One long vertical edge of a side wall is welded to the back wall, and the other opposite edge to the outer surface of the furnace wall. The return conduit has, at its lower end, a bottom wall that slopes from the lower end of the back wall towards an opening in the lower part of the inclined furnace wall for allowing the returning bed material to flow from the return conduit into the furnace. The side walls have extensions extending from the bottom wall down to the level of the grid, whereby, the side walls extend the full height of the inclined furnace wall. The water/steam tube panels forming the side walls are welded on both the vertical and inclined walls of the furnace, such that the upper end of the panels extends at a distance above the transition between the inclined wall and the vertical wall. The back wall of the return conduit, on its part, extends from the upper ends of the side wall panels in parallel with the vertical furnace wall, downwards to the side of the inclined wall, such that the back wall of the return conduit terminates at a distance of, e.g., about one third of the height of the inclined wall below the transition.

The above-described structure utilizes the side walls of the return conduit as vertical buckstays, i.e., they reinforce the inclined bottom wall of the furnace against pressure fluctuations in the furnace. However, the use of a water/steam tube panel as a reinforcing structure in the manner discussed above has a few disadvantages. First, since, for the most part of the height of the inclined walls, the side walls alone carry the load, i.e., the load carrying members are planar, practically, two-dimensional, and at right angles to the surface they are supposed to reinforce, the construction of the side walls requires special attention due to the local tendency of buckling of a planar reinforcement. Second, to be able to carry the load, the planar walls have to extend rather far from the inclined wall occupying some space, which, at least, could be used more effectively. Third, for a substantial part of the side wall height, i.e., for the height below the bottom wall of the return conduit, the side walls are not in contact with hot returning bed material, whereby, the medium circulating in the wall panels can only dissipate the heat that it has been able to recover from the hot bed material in the upper portion of the conduit.

SUMMARY OF THE INVENTION

An object of the present invention is to find at least one solution to at least one of the problems discussed above.

Another object of the present invention is to reconsider the use of water/steam tube panels as reinforcing structures in view of the risk of local buckling of planar stiffeners.

Yet another object of the present invention is to reconsider the use of load carrying structures as heat transfer surfaces in view of improving their overall efficiency.

And a still further object of the present invention is to optimize the use of space in the vicinity of the inclined wall of the boiler bottom part.

The above, and other objects of the present invention, are met with the wall construction for a boiler arrangement of this invention, the boiler arrangement being formed of at least a furnace and a separator, the furnace having a grid, a bottom part, and an upper part, the separator being arranged by means of conduits in flow communication with both the upper part and the bottom part of the furnace, the conduits together with the separator forming an external circulation of bed material, the upper part of the furnace having four vertical walls, and the bottom part of the furnace having a height, the wall construction comprising at least one hollow beam extending substantially over the entire height of the bottom part, the at least one hollow beam being attached to a wall of the bottom part, and the at least one hollow beam being in flow communication with the external circulation for returning bed material into the furnace.

Other features of the wall construction of the present invention can be determined from the appended claims.

By means of the wall construction of the present invention, at least the following advantages over the prior art have been achieved:

• • The bed material return conduit can be used as a beam reinforcing the lower inclined walls of a boiler furnace.
  • The heat recovery surface used as a reinforcing structure is also used for recovering heat from the returning bed material far more efficiently than before.
  • The extension of both the return conduit and the reinforcing structures is substantially reduced when compared to prior art structures.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the wall construction of the present invention will be explained in more detail with reference to the following drawings.

FIG. 1 is a schematic representation of a circulating fluidized bed boiler arrangement of the prior art.

FIG. 2 is a schematic vertical cross-sectional representation of a prior art furnace having a bed material return conduit attached on the outside wall of the furnace.

FIG. 3 is a schematic vertical cross-sectional representation of a first preferred embodiment of the present invention.

FIG. 4 is a more detailed cross-sectional view of the first preferred embodiment of the present invention.

FIG. 5 is a schematic representation of a second preferred embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 schematically illustrates a circulating fluidized bed boiler 10 of the prior art. The boiler 10 comprises a furnace 12 with an upper part having four substantially vertical side walls 32, a bottom part having four side walls, of which two are normally inwardly inclined side walls 34, a discharge conduit 14 in the upper part or upper end of the furnace 12 for taking the flue gas and solid particles suspended thereby to a solids separator 16, a passage 18 arranged in the upper end of the solids separator 16 for the removal of cleaned exhaust gas from the solids separator 16, a recirculation conduit 20 at the lower end of the solids separator 16 for returning at least a portion the separated solids, i.e., mostly circulating bed material, back to the bottom part of the furnace 12, a fuel feed 22 arranged at a lower side wall 34 of the furnace 12, and means 24 and 26 for introducing primary air and secondary air, respectively, arranged at the bottom part of the furnace 12. The fuel feed 22 may include a screw feeder, a drop leg, or a pneumatic feeder, just to name a few alternatives. The primary air 24 is the primary combustion gas that is also used to fluidize the bed material, and is thus fed into the furnace 12 through the grid 36 arranged at the bottom of the furnace 12. The secondary air 26 is introduced into the furnace 12 through the lower side wall 34 thereof slightly above the grid 36.

A gas lock 28 has been arranged in the return conduit 20 for preventing gas from flowing from the furnace 12 via the return conduit 21 into the solids separator 16. Here, the return conduit 20 is further provided with a fluidized bed heat exchange chamber 30 for collecting heat from the recirculating solids to a heat transfer medium. The path of the recirculating solids/bed material is called the external circulation, and includes the separator 16 and all conduits and equipment between the upper part of the furnace 12 and the bottom part of the furnace 12 used for returning the bed material back to the furnace 12. The upper and lower side walls, 32 and 34, respectively, of the boiler 10, as well as the ones of the solids separator 16 usually comprise water or steam tubes, or are made of water/steam tube panels, so that the water or steam acts as the heat transfer medium. The fluidized bed heat exchange chamber may, in accordance with recent suggestions, be arranged on the outside wall of the furnace 12, too, whereby the recirculation conduit 20 or return leg would be running down to the grid area closer to the furnace wall than in prior art arrangements.

It is known from the prior art, for example, as illustrated in a very schematic manner in FIG. 2, to attach some parts of the conduit 60 returning bed material to the bottom part of the furnace 12 to the wall of the bottom part of the furnace 12. The prior art conduit 60 receives circulating bed material from an ordinary separator 16 arranged at a distance from the boiler 10. The return channel originating from the separator 16 is connected to the top of the return conduit 60 by means of a bellows arrangement that allows some movement due to, for instance, changing temperatures. The return conduit 60 is formed of water/steam tube panels, such that it has two side walls 62 and a back wall 64. One longitudinal vertical edge of a side wall 62 is welded to the back wall 64, and the other opposite longitudinal edge to the outer surface of the furnace wall 32/34. The return conduit 60 has further a bottom wall 66 that slopes from the lower end of the back wall 64 towards opening 68 in the lower part of the inclined furnace wall 34. The side walls 62 of the conduit 60 have extensions 62′ extending from the bottom wall 66 down to the level of the grid 36, whereby the side walls 62, 62′ extend the full height of the inclined furnace wall 34, i.e., the full height of the bottom part of the furnace 12. The water/steam tube panels forming the side walls 62, 62′ are welded on both the vertical 32 and inclined 34 walls of the furnace 12, such that the upper end of the panels extends at a distance above the transition between the inclined wall 34 and the vertical wall 32. The back wall 64, on its part, extends from the upper ends of the side wall panels in parallel to the vertical furnace wall 32, downwards to the side of the inclined wall 34, such that the back wall 64 of the return conduit 60 terminates at a distance of, e.g., about one third of the height of the inclined wall 34 below the transition. The side walls 62, 62′ of the return conduit 60 are utilized as vertical buckstays, i.e., they reinforce the inclined bottom wall 34 of the furnace 12 against sub- and super-atmospheric pressures in the furnace 12.

However, the use of a water/steam tube panel as a reinforcing structure in the manner discussed above has a few disadvantages. First, since, for the most part of the height of the inclined wall, or that of the bottom part of the furnace 12, the side walls alone carry the load, i.e., the load carrying members are planar, practically, two-dimensional, and at right angles to the surface that they are supposed to reinforce, the construction of the side walls requires special attention due to the local buckling tendency of planar reinforcements. Second, to be able to carry the load, i.e., to be strong enough, the planar walls have to extend rather far from the inclined wall. This both adds to the risk of buckling, and occupies some space that at least could be used more effectively. Third, for a substantial part of the side wall height, i.e., for the height below the bottom wall of the return conduit, the side walls are not in contact with hot returning bed material, whereby the medium circulating in the wall panels can only dissipate heat that it has been able to recover from the hot bed material in the upper portion of the conduit.

FIG. 3 illustrates, as a first preferred embodiment of the present invention, means to overcome at least some of the disadvantages of the prior art, which have been discussed above. In connection with FIG. 3, a novel structure of arranging a return conduit 70 in connection with the inclined wall 34 of the bottom part of a furnace 12 is discussed. When compared to the prior art return conduit 60, it is easy to see that the return conduit 70 of the present invention solves at least some problems of the prior art. The return conduit 70 forms a three-dimensional beam that extends, in this embodiment of the present invention, over the entire height of, and along the inclined lower wall 34 of the furnace 12, and thus, forms a vertical buckstay reinforcing the inclined wall 34. The beam/return conduit 70 is advantageously formed of two side walls 72 and a back wall 74, all made of water/steam tube panels. The side walls 72 have two opposite longitudinal edges each, and so does the back wall 74. Each side wall is attached, preferably, by welding, by means of its longitudinal edge to the longitudinal edge of the back wall to form a U-shaped beam (the cross section thereof being shown in more detail in FIG. 4). The beam is welded to the inclined wall 34 of the bottom part of the furnace along the other longitudinal edges of the side walls 72, such that a box-like hollow rectangular beam 70 is formed, the back wall 74 of the beam being parallel with the inclined wall 34 of the furnace. In the vicinity of its lower end, the beam 70 is provided with a sloping bottom wall 76 that guides the returning bed material through an opening 68 in the inclined wall 34 to the grid area of the furnace 12. In this embodiment of the present invention, the bottom wall has been arranged within the walls of the beam 70 such that the water/steam tube walls of the beam 70 extend substantially to the level of the grid 36, to which the front wall of the beam 70, i.e., the inclined wall, is attached.

Now that the hollow beam 70 extends the entire height of the bottom part of the furnace 12, or of the inclined wall 34, the beam 70, including its back wall 74, is able to carry the pressure load of the furnace 12. Due to the back wall 75 carrying a substantial part of the pressure load, the horizontal extension (i.e., the extension perpendicular to the inclined side wall 34 of the furnace 12) of the side walls 72 may be made significantly smaller than the side walls 62, 62′ in the prior art. This saves some space in the vicinity of the furnace 12. Also, partially due to the smaller horizontal extension, a significantly larger part of the inner surfaces of the side walls 72 is in heat transfer communication with the returning bed material, whereby, the heat recovery properties of the side walls 72 are significantly better than those of the side walls 62, 62′ of the prior art. Further, due to the box-like structure of the reinforcements, the risk of local buckling of the side walls has, in practice, disappeared.

In accordance with the embodiment illustrated in FIG. 3, the hollow beam 70 extends not only along the lower inclined wall 34 of the furnace 12, but also, along the upper vertical wall 32 of the furnace 12. A prerequisite for this structure is that the vertical upper wall 32 is not provided with horizontal buckstays at its lower part, but the horizontal supporting has been performed some other way. It is also possible, however, in accordance with another preferred embodiment of the present invention (illustrated later on in FIG. 5), that the beam 70 runs along the inclined lower wall 34 up to the upper end of the inclined wall, and then, bends outwardly to pass the lowermost horizontal buckstay on the vertical furnace wall 32 or a secondary air header, if such is arranged to replace the horizontal buckstay.

FIG. 4 illustrates a horizontal cross section of the inclined wall 34 of the bottom part of the furnace 12, and two hollow beams 70 attached thereon. As shown in FIG. 4, the beam is formed of a back wall 74 and two side walls 72. The side walls 72 are welded on the outside surface of the inclined wall 34, so that the furnace wall 34 forms the fourth, i.e., front wall of the beam 70. Naturally, the cross section of the beam/return conduit 70 need not be always rectangular, but also, other shapes are applicable. In view of the present invention, it is important that both side walls 72 and the back wall 74 of the beam 70 participate in carrying the load subjected to the inclined wall 34 of the furnace 12. As another alternative, the beam 70 may also be prefabricated of four water/steam tube panels, to a full box-like structure, such that when installing and attaching the beam on the inclined wall, one of the beam walls (the front wall) is positioned against the furnace wall, whereby, there are two walls between the return conduit and the furnace cavity. In this alternative, naturally, the additional front wall of the beam carries some load, too.

FIG. 5 schematically illustrates the beams/return conduits 70 of an inclined lower wall 34 of the furnace 12 in accordance with a second preferred embodiment of the present invention. Here, it is assumed that on the upper portion of the vertical furnace wall 32, there are three fluidized bed heat exchange chambers, from which the returning bed material is taken down to the bed area of the furnace, by means of six return conduits (two for each fluidized heat exchanger) that have been positioned substantially evenly along the horizontal width of the inclined lower wall 32 of the furnace 12. The overall construction of the beams is similar to the one already discussed above. However, the lower ends of the beam are somewhat different from the one shown in FIG. 3, as here, the lower ends of the side walls terminate at the bottom wall of the return conduit/beam, the bottom wall sloping towards the grid, as has been discussed earlier. This kind of beam construction is possible, as the forces subjected by the inclined wall to the beam are very limited in the vicinity of the grid, as the inclined wall is supported at its lower end by the grid, and is thus not able to bend as much farther away above the grid level. Thus, it could be said that the full three-dimensional beam having preferably, but not necessarily, fixed width side walls, should extend over at least 80%, preferably, at least 90% of the inclined wall height. For the rest of the beam length, the beam cross-sectional area, or the width of the side walls, could be decreasing towards the lower end of the inclined wall either continuously or discontinuously.

The bottom wall of the return conduit/beam is preferably made of a water/steam tube panel. In other words, as a preferred option, the bottom wall could be manufactured of the same panel as that of the back wall, just by bending the panel sufficiently. Another thing worth mentioning, in view of the embodiment shown in FIG. 5, is that the upper parts of the beams have been shown to bend outwardly, so that they leave a space between the furnace wall and the beam. In this variation of the invention, the lower end of the vertical furnace wall has been provided with a horizontal buckstay or a secondary air channel acting as a buckstay, which, on the one hand, is used as the upper attachment point for the vertical beams, but which, on the other hand, the beams have to pass to be able to act as return conduits for the circulating bed material. As to the vertical wall of the furnace and its stiffening, it is possible to use the return conduits or return legs as the vertical buckstays, but the vertical wall may as well have vertical buckstays of its own, as there is space enough for both buckstays and return legs.

The return conduits or beams of the present invention may be arranged in flow communication, in addition to a fluidized bed heat exchange chamber arranged on the upper wall of the boiler 10, with solids separator 16, with a fluidized bed heat exchange chamber hanging from the solids separator 16, and with a fluidized bed heat exchange chamber supported separate from the boiler 10 and the solids separator 16.

In view of the description above, it has to be understood that only a few most preferred embodiments of the present invention have been discussed. Thus, it is obvious that the invention is not limited to the above disclosed embodiments only, but that it can be modified in many ways within the scope of the appended claims. It has to be understood, too, that features of a specific embodiment of the invention may be applied in connection with features of other embodiments within the basic idea of the present invention, or that the features from different embodiments may be combined, as long as they result in a working and technically feasible construction.

Additionally, it is clear, as already discussed above, that one or more beams/return conduits may be used to reinforce the wall of the bottom part of the furnace. The beams may form the sole vertical reinforcing means of the bottom part of the furnace, or they may be used together with ordinary buckstays, such that at least one buckstay and at least one return conduit/beam form the vertical reinforcement. In a similar manner, the return conduits/beams may be used, in the above-described manner, to reinforce not only one inclined wall of the bottom part of the furnace, but also, the opposite inclined wall, and optionally, also at least one vertical wall of the furnace bottom part. And, finally, the invention is applicable in reinforcing one or more side walls of the bottom part of the furnace irrespective of the inclination of the wall. That is, the wall/walls may be vertical or inclined in any direction.

Claims

1. A wall construction for a boiler arrangement, the boiler arrangement being formed of at least a furnace and a separator, the furnace having a grid, a bottom part, and an upper part, the separator being arranged by conduits in flow communication with both the upper part and the bottom part of the furnace, the conduits together with the separator forming an external circulation of bed material, the upper part of the furnace having four vertical walls, and the bottom part of the furnace having a height and four walls extending from the grid up to the vertical walls, the wall construction comprising:

at least one hollow beam being attached to a wall of the bottom part of the furnace and extending substantially over the entire height of the bottom part, and the at least one hollow beam being in flow communication with the external circulation for returning bed material into the furnace.

2. The wall construction according to claim 1, further comprising an opening in the wall of the bottom part of the furnace for receiving bed material from the external circulation, the opening being arranged in flow communication with the at least one hollow beam.

3. The wall construction according to claim 1, wherein the at least one hollow beam extends over at least 80% of the height of the wall of the bottom part of the furnace.

4. The wall construction according to claim 1, wherein the at least one hollow beam is attached, at its lower part, to the grid of the furnace.

5. The wall construction according to claim 1, further comprising a lowermost horizontal reinforcement at the lower end of the vertical wall of the upper part of the furnace, wherein the at least one hollow beam has an upper part, and the beam is attached, at its upper part, to the lowermost horizontal reinforcement.

6. The wall construction according to claim 1, wherein the at least one hollow beam is arranged in flow communication with one of (i) a fluidized bed heat exchange chamber arranged on the vertical wall of the upper part of the boiler, (ii) a fluidized bed heat exchange chamber hanging from the separator, (iii) a fluidized bed heat exchange chamber supported separate from the boiler and the separator, and (iv) a separator.

7. The wall construction according to claim 1, wherein the at least one hollow beam is formed of water/steam tube panels forming at least two side walls and one back wall, the panels being attached to each other and to the wall of the bottom part of the furnace for forming a box-like structure.

8. The wall construction according to claim 7, wherein the back wall of the at least one hollow beam is parallel with the wall of the bottom part of the furnace.

9. The wall construction according to claim 7, wherein the at least one hollow beam has a lower part and a bottom wall is arranged to the lower part of the beam.

10. The wall construction according to claim 9, wherein the bottom wall is arranged between the side walls of the at least one hollow beam.

11. The wall construction according to claim 9, wherein the bottom wall is formed by bending the back wall of the at least one hollow beam between the side walls.

12. The wall construction according to claim 9, wherein the bottom wall of the at least one hollow beam is arranged to slope towards the opening.

13. The wall construction according to claim 1, wherein the at least one hollow beam is formed of two side walls and a back wall, each having two longitudinal edges, the side walls having a longitudinal edge and being attached by their one longitudinal edge to the wall of the bottom part of the furnace and by an opposite longitudinal edge to the longitudinal edges of the back wall, to form a box-like structure.

14. The wall construction according to claim 13, wherein the box-like structure forms a hollow beam.

15. The wall construction according to claim 1, wherein the wall of the bottom part of the furnace has one or more hollow beams acting as substantially vertical reinforcements or buckstays.

16. A boiler arrangement comprising:

a furnace having a grid, a bottom part, and an upper part, the upper part of the furnace having four vertical walls, and the bottom part of the furnace having a height and four walls extending from the grid up to the vertical walls;

a separator arranged by conduits in flow communication with both the upper part and the bottom part of the furnace, the conduits together with the separator forming an external circulation of bed material; and

a wall construction comprising at least one hollow beam being attached to a wall of the bottom part of the furnace and extending substantially over the entire height of the bottom part, and the at least one hollow beam being in flow communication with the external circulation for returning bed material into the furnace.

17. The boiler arrangement according to claim 16, further comprising an opening in the wall of the bottom part of the furnace for receiving bed material from the external circulation, the opening being arranged in flow communication with the at least one hollow beam.

18. The boiler arrangement according to claim 16, wherein the at least one hollow beam extends over at least 80% of the height of the wall of the bottom part of the furnace.

19. The boiler arrangement according to claim 16, wherein the at least one hollow beam is attached, at its lower part, to the grid.

20. The boiler arrangement according to claim 16, further comprising a lowermost horizontal reinforcement at the lower end of the vertical wall of the upper part of the furnace, wherein the at least one hollow beam has an upper part, and the beam is attached, at its upper part, to the lowermost horizontal reinforcement.

21. The boiler arrangement according to claim 16, wherein the at least one hollow beam is arranged in flow communication with one of (i) a fluidized bed heat exchange chamber arranged on the vertical wall of the upper part of the boiler, (ii) a fluidized bed heat exchange chamber hanging from the separator, (iii) a fluidized bed heat exchange chamber supported separate from the boiler and the separator, and (iv) a separator.

22. The boiler arrangement according to claim 16, wherein the at least one hollow beam is formed of water/steam tube panels forming at least two side walls and one back wall, the panels being attached to each other and to the wall of the bottom part of the furnace for forming a box-like structure.

23. The boiler arrangement according to claim 22, wherein the back wall of the at least one hollow beam is parallel with the wall of the bottom part of the furnace.

24. The boiler arrangement according to claim 22, wherein the at least one hollow beam has a lower part and a bottom wall is arranged to the lower part of the at least one hollow beam.

25. The boiler arrangement according to claim 24, wherein the bottom wall is arranged between the side walls of the at least one hollow beam.

26. The boiler arrangement according to claim 24, wherein the bottom wall is formed by bending the back wall of the at least one hollow beam between the side walls.

27. The boiler arrangement according to claim 24, wherein the bottom wall of the at least one hollow beam is arranged to slope towards the opening.

28. The boiler arrangement according to claim 16, wherein the at least one hollow beam is formed of two side walls and a back wall, each having two longitudinal edges, the side walls having a longitudinal edge and being attached by their one longitudinal edge to the wall of the bottom part of the furnace and by an opposite longitudinal edge to the longitudinal edges of the back wall, to form a box-like structure.

29. The boiler arrangement according to claim 28, wherein the box-like structure forms a hollow beam.

30. The wall construction as recited in claim 16, wherein the wall of the bottom part of the furnace has one or more hollow beams acting as substantially vertical reinforcements or buckstays.