WO1999030080A1

WO1999030080A1 - Fluidized bed combustion plant

Info

Publication number: WO1999030080A1
Application number: PCT/SE1998/002101
Authority: WO
Inventors: Roine Brännström; Tyge Vind
Original assignee: Asea Brown Boveri AB; ABB AB
Current assignee: ABB AB
Priority date: 1997-11-24
Filing date: 1998-11-19
Publication date: 1999-06-17
Anticipated expiration: 2000-05-24
Also published as: SE9704308L; SE9704308D0

Abstract

A combustion plant comprises a combustion chamber (10), which is arranged to contain a fluidised bed (12), formed by a bed material, for the combustion of a fuel. The combustion chamber (10) comprises a bottom structure (15), which is provided with a number of nozzles (16), which are provided in such a way that they extend upwardly from the bottom structure and are arranged to supply an oxygen-containing gas for the combustion of the fuel and the fluidization of the bed material. Said nozzles (16) have an elongated shape and comprise at least one first gas outlet opening (17) in an upper part of the nozzle. Moreover, said nozzles (16) comprise at least one second gas outlet opening (24) in a lower part of the nozzle (16) for supplying a limited amount of said gas to the bed material in order to cool the latter.

Description

Fluidized bed combustion plant.

THE BACKGROUND OF THE INVENTION AND PRIOR ART

The present invention refers to a combustion plant comprising a combustion chamber, which is arranged to contain a fluidised bed, formed by a bed material, for the combustion of a fuel, wherein the combustion chamber comprises a bottom structure, which is provided with at least one nozzle provided in such a way that it extends upwardly from the bottom structure and arranged to supply an oxygen-containing gas for the combustion of the fuel and the fluidization of the bed material, wherein said nozzle has an elongated shape and comprises at least one first gas outlet opening in an upper part of the nozzle.

It is known to use such combustion plants for so-called PFBC-plants (pressurized fluidised bed combustion) , wherein the combustion chamber is enclosed in a pressure vessel and the combustion is usually performed at a pressure of 10 - 25 bars (abs) . The fluidization of the bed material in the combustion chamber takes place by means of the oxygen- containing gas, which is discharged through the first gas outlet openings, i. e. above said gas outlet openings the bed material is fluidised and below the latter the bed material is not fluidised. In order to maintain the combustion in the bed, fuel is supplied, usually crushed coal, often together with an absorbent to take care of sulphur released during the combustion. The smaller particles of ash and absorbent of the bed material and with a diameter less than about 0.5 mm will follow the combustion gases out of the combustion chamber whereas the larger particles of ash and absorbent of the bed material will remain in the bed. Thereby, in order to prevent the bed from growing, which means that the effect increases, and to keep the bed at a desired level, bed material, consequently, has to be discharged from the combustion chamber, which may be performed intermittently or preferably continuously. This spent bed material, which may be called ash for the sake of simplicity and which is discharged from the combustion chamber has, however, still a high temperature, about 800 - 900 °C, and therefore has to be cooled before it may be discharged from the combustion chamber and the pressure vessel, preferably to a temperature of about 300 - 400 °C .

Different ways of cooling the ash are known and one can distinguish between cooling by cooling tubes or cooling by the supply of gas, usually air.

In case of large ash flows, it is common to use cooling by means of an arrangement of cooling tubes, which is provided in a space below the bed and which is arranged to receive water, steam or possibly overheated steam in the cooling tubes. Such an arrangement with cooling tubes may be positioned inside or outside the pressure vessel. By such a cooling of water, it is also possible to cool the bed after a quick stop of the PFBC-plant, a so-called gas turbine trip, wherein the fluidization and the combustion rapidly cease and the bed is brought to such a temperature that sintering is avoided. The drawbacks of such a cooling of water is that the cooling arrangement has to be large and consequently becomes expensive due to the low heat transfer coefficients between the ash and the cooling tubes. Furthermore, unburned carbon particles from the bed may follow the bed mass, whereby the combustion gas produced when these carbon particles burn may cause formation of sinter. In particular, this is often the case when the bed mass is discharged from a so-called column of material, which is described in SE- B- 462 217.

In case of smaller ash flows, it is also known to cool the ash by means of air supply in a conical combustion chamber bottom. Thereby, air or possibly some other gas is supplied into the non-fluidised bed material, which is located below the nozzles of fluidization. An advantage of such an air- cooling is that it does not require any additional space below the bed but one may use the existing bed bottom structure. A further advantage of such an air cooling is that the ash may be cooled to a desired handling temperature for the transportation of the ashes, preferably 85 °c or lower, i.e. the ash may be cooled to substantially the same temperature as the entering cooling air. However, a serious drawback of this cooling method is that its cooling capacity is limited. If the ash flow increases, the supplied amount of cooling air also has to be increased to a corresponding degree. However, the air flow may not, by the methods available today, be increased to any amount, since then the ash will be fluidised. In a pressurized fluidised bed combustion plant available today and having a bed bottom which is defined by so-called "sparge tubes", i.e. air distribution channels which extend in the combustion chamber, the flow of cooling air may not be larger than about 1 - 2 % of the total air flow through the bed. If the flow of cooling air becomes larger than this value, the ash between the "sparge tubes" will fluidise and thereby these will not any longer receive necessary cooling. Such a fluidization results in that the ash will be mixed and the heat transfer to the air distribution channels increases. Moreover, this heat transfer is not evenly distributed. Thereby, one has been able to observe a bending of the air distribution channels. Moreover, it is necessary in connection with air cooling to replace the air with some inert gas, usually nitrogen, in the cases the combustion in the bed has to be stopped quickly, i.e. in connection with so-called gas turbine trip.

Such a known application of air cooling is to release a part of the air, which is supplied to the air distribution channels through apertures in a lower part of these, see US- A- 4 805 405. Thereby, this air will pass upwardly between the air distribution channels and cool the ash located there between. However, as mentioned above, one may only discharge a small amount of cooling air through these openings, due to the fact that the ash otherwise will fluidise. Furthermore, US- A- 4 805 405 describes the supply of cooling air further down in the space below the air distribution channels.

US-A-4 227 488 discloses a combustion plant with a fluidised bed, which is supported by an air distribution plate. Below the plate, a chamber is provided which via a channel, which extends through the plate, is connected to the bed. The ash is discharged through said channel to the chamber. In order to cool the ash, cooling tubes are provided in the chamber and furthermore air can be fed into the chamber from below in such a way that air passes through the bed material.

SE-A-450 163 discloses a further ash-cooling device in which the air, which is supplied to the air distribution channels and the fluidization nozzles, firstly passes a cooling tubes system, which is provided in the space below the air distribution channels.

Furthermore, the applicant has, with regard to fluidised beds with an open bottom, i.e. with substantially horizontal or parallel air distribution channels, worked with different ideas to improve heat transfer between the ash and these channels. In Fig 1 - 6, different embodiments to achieve such an improvement are disclosed. Common for these embodiments are that the air distribution channels have a rhombic cross-sectional shape and are relatively elongated, i.e. have a relatively large extension in the direction of height. By this shape of the air distribution channels, it has been shown that heat transfer between the ash and the air distribution channels is better than for air distribution channels with a conventional rectangular cross- section shape. Furthermore, such air distribution channels are easier to produce. Fig 1 schematically discloses a section through a combustion chamber 1 with a fluidised bed

2 with a plurality of such rhombic air distribution channels

3 according to a first embodiment. Fig 3 more closely shows how the nozzles 4, for the combustion and the fluidization air, are provided in an upper part of each air distribution channel. According to this embodiment each nozzle comprises only an opening 4 through the wall of the air distribution channel 3. Fig 2 schematically shows a section through a combustion chamber 1 with a fluidised bed 2 with air distribution channels 3 according to a second embodiment. In this case, air outlet members 5 are provided also in a lower part of each air distribution channel 3 for discharge of cooling air for cooling the ash between the air distribution channels 3. Fig 4 discloses a third embodiment, in which the nozzles 4 are formed by means of a plate 6, which is provided above the air distribution channels and in this way a gap 4 is formed between the plate 6 and an upper side edge of each of the side walls of each air distribution channel 3. By a suitable dimension of the distance a, it is possible to prevent particles from passing backwards into the air distribution channels 3. In Fig 5, a fourth embodiment is shown in which the gap shown in Fig 4 has been formed as a nozzle, which at least partly is of a Laval type and which enables a more precise control of the air flow, gives lower flow resistance, gives better combustion and a lower pressure difference along the wall of the combustion chamber. Fig 6 discloses a fifth embodiment in which the embodiment shown in Fig 5 has been provided with possibilities to regulate the size of the gap 4 by means of the plate 6 being displaceable upwardly and downwardly, for example via a regulating bar 7. To the left in Fig 6 an open position is shown and to the right a closed position is shown. It is of course also possible to position the plate 6 in intermediate positions and in this way control the level of opening of the nozzle and thereby adapt the air flow to the local instantaneous carbon flow. Furthermore, it is possible by completely close the carbon flow, to stop the combustion and much later, i.e. after several hours, restart the combustion without the necessity of cold or warm blowoff .

SUMMARY OF THE INVENTION

The object of the present invention is to achieve a combustion plant of the type mentioned above and with an improved cooling of the bed material, which is discharged from the fluidised bed.

This object is obtained by the combustion plant initially defined and characterized in that said nozzle also comprises at least one second gas outlet opening in a lower part of the nozzle for supplying a limited amount of said gas to the bed material in order to cool the latter. By such a second gas outlet opening it is possible to discharge a large amount of gas, which however is smaller than the gas amount which is discharged through the first upper gas outlet openings, without the bed material between the nozzles being fluidised, i.e. it is possible to obtain an effective cooling of the bed material. The total structure height for cooling may also be small since space-demanding cooling tubes system below the bed bottom may to a large extent be dispensed with. For the gas which is discharged through these second gas outlet openings a large portion of the bed surface is available. In particular, this is the case in comparison with the previously known method of discharging cooling air below the air distribution channels, where only a smaller portion of the bed surface between these channels are available. Advantageously, said nozzle has a relatively large length in comparison with previously known nozzles, preferably larger than 200 mm, especially larger than 400 mm.

According to an embodiment of the invention, said nozzle comprises a flange which extends outwardly from the nozzle and is provided in a lower part of the nozzle above said second gas outlet opening. By such a flange, it is guaranteed that the gas may penetrate the material and not move upwardly along the wall of the nozzle. Furthermore, the risk of the second gas outlet opening getting filled up is reduced.

According to a further embodiment of the invention, said first gas outlet opening is arranged to supply a substantially larger amount of gas to the bed material than said second gas outlet opening.

According to a further embodiment of the invention, said bottom structure comprises a number of gas distribution channels, which extend in the combustion chamber and are provided at a distance from each other. By the nozzles according to the invention a large amount of air will pass through the air distribution channels, so-called "sparge tubes" and the wall of these may contribute to cool the bed material, which passes between the air distribution channels. Advantageously, said air distribution channels extend substantially in parallel to each other and substantially horizontally. Each such a gas distribution channel may comprise a plurality of said gas nozzles. According to another embodiment of the invention, said bottom structure comprises a bottom plate which forms a lower limitation of the combustion chamber. Such a "whole" bed bottom may be produced in a simple and inexpensive manner. To further improve the cooling, the bottom plate may comprise cooling tubes, which extend in parallel thereto. Advantageously, the bottom plate extends in a substantially horizontal plane.

According to a further embodiment of the invention the bottom plate comprises at least a recess which is arranged to receive substantially spent bed material for the discharge of the latter from the combustion chamber. In this way, the ash formed may be discharged in a simple way and in order to improve the cooling further said recess may comprise a gas supply member, which is provided in a lower part of the recess for the supply of gas to said substantially spent bed material. Thereby, a stirring of the bed material is achieved which contributes to a substantial improvement of the heat transfer coefficient. Such a gas supply member may be arranged to enable intermittent supply of gas .

To further improve the cooling of the formed ash, said recess may comprise cooling tubes, which are arranged to cool said substantially spent bed material. These may carry water and extend in a wall member, which delimits said recess and/or in said recess in such a way that said cooling tubes are surrounded by the substantially spent bed material.

According to a further embodiment of the invention the device comprises a discharge channel which is arranged to discharge the substantially spent material to a collecting member via a tube portion of the discharge channel, which portion is arranged to form a column of the substantially spent bed material. By such a column of material it is possible to discharge the ash in an elegant and simple way. However, it is necessary that the ash is sufficiently cooled for avoiding sintering in the column of material . Such sufficient cooling may be achieved by the cooling according to the invention. Thereby, the combustion chamber may be provided in a pressure vessel and said tube portion may extend through a wall of the pressure vessel, wherein the column of material is arranged to reduce the pressure from a first pressure level in the pressure vessel to substantially a second pressure level in the collecting member. In such a way, one may avoid so-called "lock hopper" systems which are previously often used and which are expensive and complicated.

According to a further embodiment of the invention the discharge channel is connected to a material outlet in a lower part of said recess. Thereby, a gas flow may be permitted to pass through said tube portion and the column of material from said recess to the collecting chamber. Furthermore, said gas supply member may be arranged to supply a gas in such a way that at least a part of the gas passes through said tube portion and the column of material to the collecting member.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is now to be described more closely by means of different embodiments and with reference to the drawings attached.

Fig 1 discloses schematically a section through a combustion chamber for a fluidised bed with air distribution channels according to a first embodiment. ig 2 discloses schematically a section through a combustion chamber for a fluidised bed with air distribution channels according to a second embodiment . ig 3 discloses more closely two of the air distribution channels in Fig 1. Fig 4 discloses air distribution channels according to a third embodiment . Fig 5 discloses air distribution channels according to a fourth embodiment.

Fig 6 discloses air distribution channels according to a fifth embodiment. Fig 7 discloses schematically a section through a combustion plant according to a first embodiment of the invention.

Fig 8 discloses more closely the air inlet nozzles of the combustion plant in Fig 7. Fig 9 discloses schematically a section through a combustion plant according to a second embodiment of the invention.

Fig 10 discloses more closely an ash-discharging ditch of the combustion plant in Fig 9. Fig 11 discloses a view from above of a bed bottom of the combustion plant in Fig 9. Fig 12 discloses a section along the line XII - XII in

Fig 11.

DETAILED DESCRIPTION OF DIFFERENT EMBODIMENTS OF THE INVENTION

Fig 7 discloses a first embodiment of the invention and more precisely a part of a combustion plant, which comprises a combustion chamber 10, which is enclosed in a pressure vessel 11 and arranged to contain a fluidised bed 12. The pressure vessel 11 and the combustion chamber 10 is pressurized by means of a compressor 13, which via a schematically shown channel supplies gas containing oxygen, in the embodiment shown air, to the interior of the pressure vessel 11. The compressor 13 is driven by a turbine, not shown, which in turn is driven by the combustion gases, which are produced in connection with the combustion in the combustion chamber. The air supplied to the pressure vessel is directed via an inlet channel 14 to a number of air distribution channels 15, often called "sparge tubes". In connection with each air distribution channel 15 a number of nozzles 16 are provided in such a way that they extend upwardly from the air distribution channels 15. The nozzles 16 are arranged to supply air via a number of first upper outlet openings 17 to the bed in order to fluidise the bed material and to supply combustion air for the combustion of the fuel in the bed. The nozzles 16, shown more closely in Fig 8, have an elongated shape and are longer than such nozzles previously known. For example the nozzles 16 may have a length, which exceeds 200 mm, in particular exceeds 400 mm.

Such as is shown in Fig 7 the air distribution channels 15 are provided at a distance from each other so that a space is formed therebetween through which space the bed material may pass. Consequently, when the plant is in operation, the part of the bed material, which is located on a level of and above the outlet openings 17, will be fluidised and in this fluidised bed, a combustion of fuel, present in the bed, for example crushed coal, will take place. Spent bed material, which includes ash and spent absorbent and which henceforth is called ash, will move downwardly in the bed and pass between the air distribution channels 15 down into collecting cones 18. In Fig 7, two such cones are shown but of course the combustion plant may comprise fewer or more such cones. These cones do not have to have a conical shape but is preferably only downwardly tapered. From these cones, the ash is discharged through a discharge channel, formed of a tube system 19, which comprises a tube portion 20, in which the ash forms a standing column of material. Downstream the column of material, there is a feeding member 21, preferably a so-called cell feeder, which feeds the ash to a collecting container 22. Because the pressure in the pressure vessel 11 and the combustion chamber 10 is considerably higher, about 10 - 25 bars (abs) , than the atmospheric pressure in the collecting container 22, the pressure has to been reduced in the tube system 19. This can be achieved by means of the formed standing column of material, which gives a flow resistance to the gas, that will flow therethrough, such as the pressure may be reduced to the atmospheric pressure. To enable the gas to flow through the standing column of material, a by-pass conduit 23 is provided past the feeding member 21. Consequently, no substantial pressure difference is existing over the feeding member 21. It is to be noted that the collecting chamber 22 is only schematically shown. In a real plant, a conveyance device may be provided below the pressure vessel 11 in connection with the extension of the tube portion 20, since it is a shortage of space directly below the pressure vessel 11. Such a conveyance device may comprise a conveying band or a tube channel for pneumatic transport and may convey the ash to a collecting silo for the storage of the ash.

However, to enable such a discharge of the ash by means of a standing column of material, it is necessary to cool the ash for preventing sintering in the column of the material, since the ash may contain small amounts of combustible material. Such a cooling may, according to the present invention, be achieved by means of second air outlet openings 24 of the nozzles 16. Since the nozzles 16, seen from above, cover a relatively small part of the surface of the bed, a large part of the surface is available to the air, which flows out from the lower outlet openings 24. Consequently, it is possible to discharge a relative large amount of air through these outlet openings 24 without the bed material between the nozzles 16 being fluidised, in particular in comparison with such cooling air being discharged in the lower part of the air distribution channels 15 or therebelow, since then all air has to pass through the space between the air distribution channels 15, which constitutes a relatively small part of the total surface of the bed seen from above. Consequently, it is possible by the air outlet openings 24, according to the invention, to achieve an effective cooling of the bed material, which exists between the nozzles 16 and which moves downwardly towards the collecting cones 18. Thereby, the bed material will not only be cooled by the air discharged through the outlet openings 24 but also by its direct heat transferring contact with the walls of the air distribution channels 15.

In order to guide the air from the outlet openings 24 to the bed material and in order to prevent penetration of bed material through the outlet openings 24, a flange or a collar 25 is provided above the outlet openings 24 in such a way that it extends outwardly from the nozzles 16 and around the latter. Such as is shown in Fig 8, the flange 25 extends substantially radially outwardly from the nozzle in question. However, this flange may be designed in many different ways, for example as a cone which incline downwardly.

In Figs 9 and 10, a second embodiment of the invention is shown. It is to be noted that components, which have substantially the same function have been provided with the same reference signs in the shown embodiments . The second embodiment differs from the first embodiment since it does not show any air distribution channels but a so-called whole bed bottom, i.e. a bottom plate 30, which forms a lower limitation of the combustion chamber 10. The nozzles 16, for the supply of fluidised and combustion air to the bed 12 via the first air outlet openings 17, extend through the bottom plate 30. Consequently, the supplied pressurized air, supplied via the compressor 13, to the pressure vessel 11 may pass to the combustion chamber 10 via said nozzles 16. Some part of the pressurized air also passes through the lower outlet openings 24 for cooling the bed material in the same way as in the first embodiment.

Furthermore, the bottom plate 30 comprises a number of recesses 31, which extend downwardly from the combustion chamber 10 and which act as ash discharging ditches, compare also Fig 11. Consequently, the ash will be fed down in these recesses 31 during the operation of the plant. In the lower part of the recesses 31, there is a material outlet 32, which is connected to a tube system 19, which in the same way as in the first embodiment comprises a tube portion 20, a feeding member 21, a collecting container 22 and a by-pass conduit 23 for discharging the ash via a standing column of material. The bottom plate 30 and the limiting walls of the recesses 31 comprise cooling tubes 33 for water, steam or overheated steam to further cool the ash. Furthermore, such cooling tubes 34 may also be provided in such a way that they extend through a central part of each recess, see Fig 10. Furthermore, a gas supply member 35 may be provided in a lower part of each recess 31 and arranged to discharge gas, preferably air, intermittent or continuously to the ash, that is present in the recess 31 via one or several outlet openings 36. Consequently, by means of this air, the ash may be cooled and the air may also provide a stirring of the ash which improves the heat transfer between the ash and the cooling tubes 33 and 34. The object of a part of this supplied air is to supply air to the standing column of material to prevent flue gas from penetrating down in the latter. The flue gas contains sulphur dioxide and sulphur trioxide, which at the cooling condensate and form sulphuric acid and sulphurous acid, respectively. The air to the gas supply members 35 is supplied via a supply conduit 37, which in turn is arranged to be supplied with air from the pressure vessel 11 via a conduit 38. The conduit 38 comprises a regulating valve 39, by which the air supply to the gas supply member 35 may be controlled. Furthermore, it is possible, although not necessary, to provide a booster compressor 40 in connection with the conduit 38 in order to increase the pressure of the air, that is supplied the gas supply member 35 to a level above the pressure level in the pressure vessel 11. It is also possible to supply the air to the gas supply member 35 directly from the pressure vessel 11, i.e. the gas supply members 35 are open towards the pressure vessel 11.

In Fig 11 a bottom plate 30 is shown in a view from above, from which it can be seen that the bottom plate 30 may comprise a plurality of recesses 31. Furthermore, in Fig 11 a possible positioning of a set of fuel inlet nozzles 41 is shown. From Fig 12, it is disclosed that the bottom plate 30, which extends in a substantially horizontal plane, may be folded, i.e. the bottom plate 30 has a wave like shape, wherein the recesses 31 are provided in a respective valley of the waves. In this way the discharging of sintering agglomeration is facilitated which thereby may slide or roll down the inclined planes towards a recess 31. A plurality of nozzles are provided between each recess 31 which are shown schematically in Fig 12.

The present invention is not limited to the embodiments shown above but can be varied and modified within the scope of the following claims. Though, the invention has been explained in connection with a pressurized fluidised bed it is to be noted that it is also applicable in connection with fluidised beds, which operate at atmospheric pressure and in connection with bubble beds as well as in connection with circulating beds .

Claims

1. A combustion plant comprising a combustion chamber (10), which is arranged to contain a fluidised bed (12), formed by a bed material, for the combustion of a fuel, wherein the combustion chamber (10) comprises a bottom structure (15, 30) , which is provided with at least one nozzle (16) provided in such a way that it extends upwardly from the bottom structure and arranged to supply an oxygen- containing gas for the combustion of the fuel and the fluidization of the bed material, wherein said nozzle (16) has an elongated shape and comprises at least one first gas outlet opening (17) in an upper part of the nozzle, characterized in that said nozzle (16) also comprises at least one second gas outlet opening (24) in a lower part of the nozzle (16) for supplying a limited amount of said gas to the bed material in order to cool the latter.

2. A plant according to claim 1, characterized in that said nozzle (16) comprises a flange (25) which extends outwardly from the nozzle and is provided in a lower part of the nozzle above said second gas outlet opening (24) .

3. A plant according to claim 2, characterized in that said second gas outlet opening (24) is provided between the flange (25) and the bottom structure (15, 30) immediately below the flange (25) in such a way that the bed material is prevented from penetrating said second gas outlet opening (24) .

4. A plant according to any one of claims 1-3, characterized in that said first gas outlet opening (17) is arranged to supply a substantially larger amount of gas to the bed material than said second gas outlet opening (24) .

5. A plant according to any one of the preceding claims, characterized in that the bottom structure comprises a number of gas distribution channels (15) which extend in the combustion chamber (10) and are provided at a distance from each other.

6. A plant according to claim 5, characterized in that said gas distribution channels (15) extend substantially in parallel to each other and substantially horizontally.

7. A plant according to any one of claims 5 and 6, characterized in that each gas distribution channel (15) comprises a plurality of said nozzles (16) .

8. A plant according to any one of claims 1 and 4, characterized in that the bottom structure comprises a bottom plate (30) which forms a lower limitation of the combustion chamber (10) .

9. A plant according to claim 8, characterized in that the bottom plate (30) comprises cooling tubes (33) which extend in parallel thereto.

10. A plant according to any one of claims 8 and 9, characterized in that the bottom plate (30) extends in a substantially horizontal plane.

11. A plant according to any one of claims 8 - 10, characterized in that the bottom plate (30) comprises at least one recess (31) which is arranged to receive substantially spent bed material for the discharge thereof from the combustion chamber (10) .

12. A plant according to claim 11, characterized in that said recess (31) comprises a gas supply member (35) which is provided in a lower part of the recess (31) for the supply of gas to said substantially spent bed material.

13. A plant according to claim 12, characterized in that said gas supply member (35) is arranged to enable intermittent supply of gas.

14. A plant according to any one of claims 12 and 13, characterized in that the gas supply member (35) via a supply conduit (37, 38) is connected to a compressor (40) which is arranged to supply an oxygen-containing gas at an increased pressure level.

15. A plant according to any one of claims 11 - 14, characterized in that said recess (31) comprises cooling tubes (33, 34) which are arranged to cool said substantially spent bed material .

16. A plant according to claim 15, characterized in that said cooling tubes (33) extend in a wall member which delimits said recess (31) .

17. A plant according to any one of claims 15 and 16, characterized in that said cooling tubes (34) extend in said recess (31) in such a way that said cooling tubes (34) are surrounded by the substantially spent bed material.

18. A plant according to any one of the preceding claims, characterized by a discharge channel (19) which is arranged to discharge the substantially spent material to a collecting member (22) via a tube portion (20) of the discharge channel (19) , which portion is arranged to form a column of the substantially spent bed material.

19. A plant according to claim 18, characterized in that the combustion chamber (10) is provided in a pressure vessel (11) and that said tube portion (20) extends through a wall of the pressure vessel (11) , wherein the column of material is arranged to reduce the pressure from a first pressure level in the pressure vessel (11) to substantially a second pressure level in the collecting member (22) .

20. A plant according to claims 10 and 19, characterized in that the discharge channel (19) is connected to a material outlet (32) in a lower part of said recess (31) .

21. A plant according to claim 20, characterized in that a gas flow is permitted to pass through said tube portion (20) and the column of material from said recess (31) to the collecting chamber (22) .

22. A plant according to claims 12 and 21, characterized in that said gas supply member (35) is arranged to supply a gas in such a way that at least a part of the gas passes through said tube portion (20) and the column of material to the collecting member (22) .

23. A plant according to any one of claims 21 and 22, characterized by a feed member (21) , which is provided downstream of the tube portion (20) and arranged to discharge the substantially spent material from the tube portion (20) , and a by-pass conduit (23) which is provided to permit said gas flow to pass the feed member (21) .