Technical field

[0001 ] The invention relates to oxycombustion circulating fluidized bed boilers. The invention particularly relates to a method of operating an oxycombustion circulating fluidized bed boiler according to the preamble of claim 1 .

Background art

[0002] Combustion of solid fuel in a fluidized bed of solid material, such as bubbling (BFB) and circulating fluidized beds (CFB), is known to be advantageous in many respects. Due to the low furnace temperature and the use of hot circulating solids, many beneficial features, such as low NO _x and SO _x emissions, fuel flexibility, and capability of using low grade fuel, are achieved in a CFB- process. Capturing of many substances present in or originating from the com- bustion reactions may be readily accomplished by selecting the used bed material appropriately. For example, sulphur emissions may be considerably reduced by using materials which react with sulphur to facilitate sulphur capturing.

[0003] CFB combustion is well known as air combustion process, an example thereof being a publication CN 1566749 A, which also shows a method of implementing high pressure fluidization blast using steam instead of air in normal operation of a boiler. The fluidization steam source pressure is greater than 60kPa, and the superheating degree is more than 30°C.

[0004] New regulations and other demands limiting the gas emissions related to the so called greenhouse effect have encouraged to develop new tech- nologies aiming at decreasing carbon dioxide emissions from power stations using fossil carbonaceous fuels. While in conventional firing the oxygen required for burning the fuel is fed in the form of air, in oxycombustion the air is replaced with a mixture of substantially pure oxygen and recycled flue gas, which may be called oxidant.

[0005] Oxycombustion is based on combusting carbonaceous fuel with substantially pure oxygen, typically, of at least 95% purity, so as to have carbon dioxide and water as the main components of the exhaust gas discharged from the boiler. Thereby, the carbon dioxide can be captured relatively easily, without having to separate it from a gas stream having nitrogen as its main component, as when combusting the fuel with air.

[0006] One of the mechanisms of sulphur capture in the furnace, when li- mestone is used, is as follows. The limestone calcines in the furnace to form calcium oxide:

CaCO ₃ -> CaO + CO ₂ .

Calcium oxide reacts with SO2 to form calcium sulphate by a reaction: CaO + SO ₂ +½O ₂ -> CaSO ₄ . Another known mechanism is direct sulphatizing with a reaction: CaCO ₃ + ½O ₂ + SO ₂ -> CaSO ₄ + CO ₂ .

CaSO ₄ being solid material, may be removed from the gas by separation. The course of the reactions is naturally dependent on the prevailing temperature and particularly on the partial pressure of CO2. [0007] CaSO ₄ , CaCO ₃ as well as CaO are efficiently mixed into the bed material of the CFB boiler and, thus, they are present also in the external circulation of solid material of the CFB boiler. The formed calcium sulfate can, thus, be removed from the furnace together with the ashes. Therefore, a combustion process using a circulating fluidized bed boiler does not necessarily need addi- tional sulfur reducing equipment in the exhaust gas channel, or the efficiency of such equipment can be relatively low. [0008] In order to maintain the optimum temperature for low emissions and high combustion efficiency within the furnace of a CFB boiler, sufficient heat transfer surface must be provided to remove heat from the combustion products. Heat transfer surface can be provided by arranging a fluidized bed heat ex- changer (FBHE) in the external circulation of solid material, which cools the externally circulated solids before they are returned back to the furnace. This is also the case with oxycombustion CFB boilers.

[0009] US 6,505,567 discloses a circulating fluidized bed steam generator and a method of operating the same. There is also shown a fluidized bed heat exchanger coupled to the external circulation of solid material. The method includes the step of introducing a substantially pure oxygen feed stream into the circulating fluidized bed steam generator and the step of combusting a fuel in the presence of the substantially pure oxygen feed stream to produce a flue gas having carbon dioxide and water vapor as its two largest constituent elements by volume. The method includes the step of separating the flue gas into an end product portion and a recycling portion and directing the recycling portion of the flue gas to the circulating fluidized bed steam generator to contribute to the combustion process therein. The recycling portion of the flue gas is also used for fluidizing gas in the fluidized bed heat exchanger. [0010] It has been recently discovered that in an oxycombustion CFB boiler there may occur disadvantageous behavior in the solids handling system due to the presence of CaO, when the fluidization of solid material is accomplished making use of recycled portion of the flue gas containing high concentration of CO ₂ . This is due to the fact that CaO has a tendency to recarbonating back to CaCO3 at certain circumstances.

Disclosure of the invention

[001 1 ] An object of the invention is to provide a method of operating an oxycombustion CFB boiler, which solves the above mentioned and other prob- lems of the prior art. A particular object of the invention is to provide a method of operating an oxycombustion CFB boiler, in which recirculating flue gas is used as fluidizing gas and sulphur reducing agent comprising CaCO ₃ is used for sulphur capture, and which method provides a reliable operation of an external solid material handling system of the oxycombustion CFB boiler and which minimizes the disadvantageous behavior in the external solid material handling system due to the presence of CaO in the solid material, when the fluidization is accomplished making use of gas containing high concentration of CO ₂ .

[0012] Objects of the invention are met by a method comprising steps of arranging a circulating fluidized bed in the oxycombustion CFB boiler; introducing an oxidant gas into the CFB boiler through the grid as fluidizing gas, the fluidizing gas comprising recirculating flue gas; introducing fuel material into the circulating fluidized bed; introducing a sulphur reducing agent comprising CaCO ₃ into the circulating fluidized bed; circulating solid material out of the furnace and providing an external circulation of solid material via the external solid material handling system; fluidizing the solid material in the solid material handling system by introducing fluidizing medium comprising recirculating flue gas into the external solid material handling system.

[0013] It is characteristic to the invention that a predetermined amount of steam is introduced into the external solid material handling system as a component of the fluidization medium.

[0014] This way, the control and operation of the external solid material handling system is reliable since the recarbonization of CaO at temperatures below the equilibrium calcination temperature is minimized and the disadvanta- geous phenomena caused by excessive formation of CaCO3 in system is substantially avoided or at least minimized. Thus, potential bed agglomeration/sintering by formation of CaCO3 is minimized. Additionally, since the reaction of recarbonization of CaO consumes CO ₂ and the reaction is substantially avoided, a loss of fluidizing gas is accordingly substantially avoided. [0015] According to an embodiment of the invention, the steam is mixed into the fluidizing medium prior to its introduction into the external solid material handling system as fluidizing medium.

[0016] According to another embodiment of the invention, the steam is dis- tributed evenly into the external solid material handling system as fluidizing medium separately from the other components of the fluidizing medium introduced into the external solid material handling system.

[0017] According to yet another embodiment of the invention, the steam is mixed into the fluidizing medium during its introduction into to the external solid material handling system as fluidizing medium.

[0018] According to a further embodiment of the invention, the steam is generated as expansion steam from water originally at a first pressure and at a first temperature by depressurizing the water to a second pressure being lower than the first pressure prior to the introduction of the steam into the external solid material handling system as fluidizing medium.

[0019] According to yet another embodiment of the invention, the steam is generated as expansion steam from water originally at a first pressure and at a first temperature by depressurizing the water to a second pressure being lower than the first pressure during the introduction of the steam into the external solid material handling system as fluidizing medium.

[0020] The predetermined amount of steam is mixed into the fluidizing medium so as to replace CO2 in the fluidizing medium. High concentration of CO2 in the fluidizing medium is, thus, reduced by replacing CO2 with steam.

[0021 ] This way, the control and operation of the loop seal is reliable, since the recarbonization of CaO at temperatures below the equilibrium for calcination temperature is minimized and the phenomena caused by excessive formation of CaCO3 in the loop seal, namely potential bed agglomeration/sintering and loss of fluidizing gas, is accordingly substantially avoided. [0022] According to an embodiment of the invention, solid material is transported with the flue gases to a solid material separator and solid material is separated from the flue gases in the solid material separator and the separated solid material is recycled from the separator back to the furnace through the ex- ternal solid material handling system. The external solid material handling system comprises a loop seal, in which the solid material is controllably fluidized to control the flow of the solid material back to the furnace. A predetermined amount of steam is mixed into the fluidizing medium prior to or during its introduction into the loop seal and fluidizing the solid material. [0023] According to an embodiment of the invention, the external solid material handling system comprises a fluidized bed heat exchanger and the solid material is circulated out of the furnace of the CFB boiler into a fluidized bed heat exchanger and in the method a predetermined amount of steam is mixed into the fluidizing medium prior to or during its introduction into the fluidized bed heat exchanger for fluidizing the solid material therein. This way, the control and the operation of the fluidized bed heat exchanger is reliable since the recarboni- zation of CaO is minimized and the phenomena caused by excessive amount of CaCO3 are minimized. Such disadvantageous phenomena are inter alia potential bed agglomeration/sintering, loss of fluidizing gas and carburization of heat surface tubes in high CO2 environment, is substantially avoided or at least minimized.

[0024] The fluidizing medium comprises recirculating flue gas and the predetermined amount of steam is controlled so that that the CO2 partial pressure in the fluidizing medium is maintained below the equilibrium pressure for recar- bonation of CaO at the prevailing temperature in the external solid material handling system.

[0025] According to an embodiment of the invention, the fluidizing medium comprises a first portion of the recirculating flue gas, a second portion of oxygen and a third portion of steam; and the method comprises a step of determining the flow rate or amount of sulphur reducing agent comprising CaCO3 introduced into the circulating fluidized bed; and a step of defining the relative shares of the first portion, the second portion and the third portion based on the determined flow rate or amount of introduced sulphur reducing agent; and introducing the fluidizing medium comprising the defined relative shares of the first portion, the second portion and the third portion of the recirculating flue gas, the oxygen and the steam, respectively.

[0026] Further, according to an embodiment of the invention, CO ₂ content of the recirculating flue gas is determined, which is taken into account when the CO ₂ partial pressure in the fluidizing medium is determined.

[0027] According to an embodiment of the invention, the temperature of the solid material in the external solid material handling system is measured directly or indirectly and the step of defining the relative shares of the first portion, the second portion and the third portion comprises utilization of the actual solid material temperature information.

[0028] The steam used, i.e. introduced into the external solid material handling system, in method of the invention is preferably superheated and pressure is substantially at the prevailing pressure level in the target environment. For ex- ample, the pressure of the introduced steam is preferably 0,5 - 5 bar above the atmospheric pressure and the temperature is 140 -200°C.

[0029] According to an embodiment of the invention, the steam is generated utilizing the heat produced in the oxycombustion CFB boiler.

[0030] According to another embodiment of the invention, the steam is generated as expansion steam.

Brief Description of Drawings

[0031 ] In the following, the invention will be described with the reference to the accompanying schematic drawings, in which • Figure 1 illustrates an oxycombustion CFB boiler in which the method of operating an oxycombustion CFB boiler according to an embodiment of the invention may be advantageously practiced,

• Figure 2 illustrates an oxycombustion CFB boiler in which the method of operating an oxycombustion CFB boiler according to another embodiment of the invention may be advantageously practiced,

• Figure 3 illustrates an exemplary diagram of the equilibrium for calcium carbonate and calcium oxide as function of CO2 partial pressure,

• Figure 4 illustrates an oxycombustion CFB boiler in which the method of operating an oxycombustion CFB boiler according to yet another embodiment of the invention may be advantageously practiced, and

• Figure 5 illustrates an oxycombustion CFB boiler in which the method of operating an oxycombustion CFB boiler according to yet another embodiment of the invention may be advantageously practiced.

Best mode for carrying out the invention

[0032] Figure 1 schematically discloses an oxycombustion CFB boiler 10, which may also be called in the following a CFB boiler. The CFB boiler 10 comprises a furnace 12 with an upper part having four substantially vertical side walls, a bottom part having four side walls of which two are normally inwardly inclined side walls, 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 solid material separator 16, a flue gas passage 18 arranged in the upper end of the solid material separator 16 for removing the exhaust gas from the solid ma- terial separator 16.

[0033] The CFB boiler also comprises a fuel feeding means 22 arranged at a side wall of the furnace, and means for introducing a sulphur reducing agent comprising CaCO3 into the circulating fluidized bed 24 also arranged at a side wall of the furnace. The oxycombustion CFB boiler 10 comprises a grid 26 at the bottom section of the furnace 12 and a wind box 28 arranged in connection with the grid. Fluidizing medium is introduced through the wind box and the grid into the furnace in the operation of the CFB boiler. There are conduits with control valves arranged for introducing at least recycling gas and oxygen into the wind box 28.

[0034] The flue gas passage 18 extending from the solid material separator 16 is provided with a flue gas treatment system 30 which advantageously comprises heat exchangers 32 for recovering heat from the flue gases. The passage 18 is also provided with a branch 36 which provides an outlet con- nected to a flue gas recycling conduit 38 through which the recycling portion of the flue gas is made available to the furnace and/or its auxiliary equipment. The recycling conduit 38 is provided with a recycling blower 40 for raising the pressure of the recycling flue gas to an adequate level. In an oxycombustion CFB boiler, the recycling flue gas is used as fluidizing gas which recycling flue gas contains considerably great amounts of CO2.

[0035] A first solid material returning system is also arranged at the lower end of the solid material separator 16 for circulating solid material, thus providing an external circulation of solid material. This may be considered to be the first external solid material handling system 20 in the CFB boiler of Figure 1 . The first solid material handling system 20 may be used for controllably returning at least part of the separated solid material, i.e. bed material, back to the bottom part of the furnace 12. In this context, the external solid material handling system means an arrangement in which the bed material of the CFB boiler is handled outside the furnace, the handling comprising a step of fluidizing the solid material. The solid material separator 16 preferably comprises one or more cyclone separators.

[0036] There is also a second external solid material handling system 21 in the CFB boiler 10 of Figure 1 comprising a fluidized bed heat exchanger 45 arranged next to the furnace 12 having a common partition wall 50 with the fur- nace 12. The common partition wall has at least one inlet opening 52 provided connecting the furnace 12 with the fluidized bed heat exchanger 45. The at least one inlet opening is arranged to allow passing solid material from the furnace to the heat exchanger and thus circulating solid material out of the furnace providing an external circulation of solid material by the second solid material handling system. There is also at least one outlet opening 54 in the partition wall. The outlet opening is in connection with a solid material discharge system 56 and it opens into the lower part of the furnace above the grid 26 for controllably recycling at least part of the solid material from the fluidized bed heat exchanger back to the bottom part of the furnace 12. The fluidized bed heat exchanger 45 is also provided with a wind box 58 having a separate fluidization section 59 for the solid material discharge system 56. The solid material discharge system is here a so called lifting chamber, the operation of which is controlled by controlling the introduction of fluidizing gas through the section 59 in the wind box.

[0037] The oxycombustion CFB boiler 10 is in connection with a source of oxygen 42 and a source of steam 44 by means of oxygen conduit 46 and steam conduit 48, respectively, the purpose of which is explained in the following.

[0038] The recycling conduit 38 is in flow communication with wind box 28 of CFB reactor in order to feed the recycling flue gas back into the furnace as fluidizing gas. Additionally, the oxygen conduit 46 is connected to the wind box 28 and during the operation of the oxycombustion CFB reactor oxidant gas comprising a mixture of recycling flue gas and the oxygen is fed into the furnace at least in steady state condition.

[0039] In the oxycombustion CFB boiler, there is arranged a circulating fluidized bed by introducing an oxidant gas into the CFB boiler through the grid 26. The oxidant gas fed through the grid 26 operates as fluidizing gas. The oxidant gas is fed into the furnace so that a sufficient gas velocity is maintained for creating a fluidized bed therein so that at least part of the solid material (bed material) is conveyed along with the gas into the solid material separator 16. Fuel material is also fed into the circulating fluidized bed by the fuel feeding means 22. The fuel material is combusted in the furnace with the aid of the oxygen in the oxidant gas.

[0040] During the operation of the oxycombustion CFB boiler, a sulphur reducing agent comprising CaCO3, such as limestone, is introduced into the fur- nace and, thus, into the circulating fluidized bed. The limestone calcines in the furnace to form calcium oxide (CaCO3 -> CaO + CO ₂ ), which reacts with SO ₂ to form calcium sulphate (CaO + SO ₂ +½O ₂ -> CaSO ₄ ).

[0041 ] Calcium sulphate, being solid material, may be removed from the gas by separation. CaSO ₄ as well as CaO are efficiently mixed into the bed ma- terial of the CFB boiler and, thus, they are present also in the external circulation of solid material of the CFB boiler.

[0042] In the CFB boiler shown in Figure 1 , the main path of external circulation of solid material runs from the furnace 12 to the particle separator 16 of the CFB boiler 10. The separated solid particles are led further from the particle separator 16 at least partly back to the furnace 12 through the external solid material handling system 20. In the external solid material handling system 20, the admission of solid particles back to the CFB boiler is controlled by a loop seal 46 arranged in the system 20. The operation of the loop seal is controlled by fluidiz- ing medium, i.e., gas introduction into the loop seal. [0043] The fluidization medium, which is introduced into the loop seal 46, is a mixture of gaseous fluidizing medium and a predetermined amount of steam therein.

[0044] Now, according to a preferred embodiment of the invention, a predetermined amount of steam from the source of steam 44 is mixed into the fluidiz- ing medium prior to its introduction into to the external solid material handling system as fluidizing medium. This way, the recarbonization of CaO, originating from the sulphur reducing process being present in the external circulation of solid material in the first solid material handling system 20, particularly in the loop seal 46, is minimized while being fluidized by the fluidizing medium. This way, the control and the operation of the loop seal is reliable, since the recar- bonization of CaO is minimized and the phenomena caused by excessive amount of CaCO3 in the loop seal, namely potential bed agglomeration/sintering and loss of fluidizing medium (CO ₂ ) in the recarbonization reaction, is accord- ingly substantially avoided.

[0045] The loop seal 46 is provided with fluidization system 60, into which a gas conduit 62 is connected through which the fluidization medium is introduced into the fluidization system 60. The gas conduit is provided with a mixing device 64 by means of which the mixture of the fluidizing medium is controlled. The source of steam 44 is in flow connection with the mixing device 64 by means of a first inlet conduit 66 having a control valve 68. Also, the source of oxygen 42 is in flow connection with the mixing device 64 by means of a second inlet conduit 70 having a control valve 72. Further, the recycling conduit 38 is in flow connection with the mixing device 64 by means of a third inlet conduit 74 having a con- trol valve 76 arranged in the third inlet conduit. The mixing device 64 makes it possible to control the ratio of the components of the fluidizing gas which is available through the first, the second and the third inlet conduits. It should be understood that contrary to the presentation of Figure 1 , the mixing device may be integrated into the fluidization system 60 or the loop seal 46. [0046] In the CFB boiler shown in Figure 1 , there is another path of external circulation of solid material by the second solid material handling system 21 . This comprises an external fluidized bed heat exchanger 45 next to the furnace 12 and finally a path back to the furnace. The fluidized bed heat exchanger 45 is provided with a wind box 58 with a separate section 59 for the solid material dis- charge system 56. The fluidization medium which is introduced into the wind box 58 is a mixture of gaseous fluidizing medium and a predetermined amount of steam mixed prior to the introduction into the fluidized bed heat exchanger.

[0047] Now, correspondingly to the behavior in the loop seal, as described above, a predetermined amount of steam from the source of steam 44 is mixed into the fluidizing medium prior to its introduction into the external solid material handling system, i.e., into the wind box 58 as fluidizing medium. This way, the recarbonization of CaO, originating from the sulphur reducing process being present in the external fluidized bed heat exchanger 45 is minimized while being fluidized by the fluidizing medium. This way, the control and the operation of the fluidized bed heat exchanger 45 is reliable, since the recarbonization of CaO is min imized and the phenomena caused by excessive formation of CaCO3 _, namely potential bed agglomeration/sintering, loss of fluidizing gas and carburi- zation of heat surface tubes in high CO2 environment, is accordingly substantially avoided. [0048] The external fluidized bed heat exchanger 45 is provided with fluidi- zation system, i.e. the wind box 58, 59 into which a gas conduit 80 is connected, through which the fluidization medium is introduced into the wind box 58 and its separate section 59. The gas conduit 80 is provided with a mixing device 64, by means of which the mixture of the fluidization medium is controlled. The source of steam 44 is in flow connection with the mixing device 64 by means of a first inlet conduit 66 having a control valve 68. Also the source of oxygen 42 is in flow connection with the mixing device 64 by means of a second inlet conduit 70 having a control valve 72. Further, the recycling conduit 38 is in flow connection with the mixing device 64 by means of a third inlet conduit 74 having a control valve 76 arranged in the third inlet conduit. The mixing device 64 makes it possible to control the ratio of the components of the fluidizing gas available through the first, the second and the third inlet conduits.

[0049] According to an embodiment of the invention, the temperature of the solid material in the external solid material handling system is measured directly or indirectly by a temperature measurement system 82 and the step of defining the relative shares of the first portion, the second portion and the third portion comprises utilization of the actual solid material temperature information. More specifically, the predetermined amount of steam is controlled so that the CO2 partial pressure in the fluidizing medium is maintained below the equilibrium pressure for recarbonation of CaO at the measured temperature of the solid material. [0050] Figure 1 also shows that each of several solid material handling systems or parts thereof where fluidization is required, is provided with a mixing device 64 of their own.

[0051 ] Figure 2 schematically describes an oxycombustion CFB boiler in which the method of operating an oxycombustion CFB boiler according to an embodiment of the invention may be advantageously practiced. To a large extent, it is similar to that of Figure 1 and, thus, corresponding reference numerals are used for the corresponding elements. The embodiment of Figure 2 differs from the one in the Figure 1 by the fact that the solid material returning system at the lower end of the solid material separator 16 for circulating solid material and the fluidized bed heat exchanger 45 arranged next to the furnace 1 2 are connected with each other. Thus, it is clear that there is a loop seal in the fluidized bed heat exchanger, too. This means that the solid material separated by the separator 16 is directed to the fluidized bed heat exchanger. Even if it is not shown here, the solid material may be directed directly back to the furnace or the separated material is cooled in the heat exchanger prior to feeding back to the furnace.

[0052] In Figure 2 there is also shown only one mixing device 64 which illustrates that according to an embodiment of the invention, the CFB boiler is provided with only one mixing device 64 in which a predetermined amount of steam 44 is mixed into the fluidizing medium from which mixing device the fluid- izing medium is directed to each of several solid material handling systems or parts thereof where fluidization is required.

[0053] Figure 3 shows an exemplary diagram of equilibrium for calcium carbonate and calcium oxide as function of CO2 partial pressure and temperature. It can be seen that, for example, at the temperature of 800°C the CO2 partial pressure should be less than about 0,2 atm in order to avoid recarbonization of CaO. Thus, according to an embodiment of the invention, the predetermined amount of steam mixed into the fluidizing medium is controlled so that the CO2 partial pressure in the fluidizing medium is maintained below the equilibrium pressure for recarbonation of CaO at the prevailing temperature of the fluidizing medium and/or solid material to be fluidized.

[0054] Figure 4 illustrates yet another embodiment of the invention. Mainly it is similar to that of Figure 1 and, thus, corresponding reference numbers are used in connection with corresponding features and elements. It also operates to a large extent similarly. The most significant difference in the oxycombustion CFB boiler 10 of Figure 4 relates to the concept of external solid material handling system, and particularly to the fluidizing medium.

[0055] Firstly, the loop seal 46 is provided with fluidization system 60 into which a gas conduit 62 is connected through which the fluidizing medium is introduced into the fluidization system 60. The gas conduit is provided with a mixing device 64 by means of which the mixture of the fluidization medium is formed and controlled. It should be understood that the mixing device may be integrated into the fluidization system 60. In this embodiment, the source of steam 44 is in flow connection directly with the fluidization system 60. The arrangement is provided with a first inlet conduit 66 having a control valve 68 for introducing the steam. The source of oxygen 42 is in flow connection with the mixing device 64 by means of a second inlet conduit 70 having a control valve 72. Further, the recycling conduit 38 is in flow connection with the mixing device 64 by means of a third inlet conduit 74 having a control valve 76 arranged in the third inlet conduit. This way, the oxygen is preferably mixed with the recycled flue gas prior to its introduction into the fluidization system 60, i.e. the loop seal in this case.

[0056] According to another embodiment of the invention, the steam is in- traduced as water at a first pressure and at a first temperature and the water is depressurized to a second pressure being lower than the first pressure in such a manner that expansion steam is generated during the introduction of the steam as fluidizing medium. This is depicted in the Figure 4 by an expansion unit 1 10 in the wind box 58 or immediately prior to it. [0057] It is also illustrated in the Figure 4 that the source of steam 44 is in direct flow connection with the wind box 28 of the boiler 10. Thereby, the steam is evenly distributed into the area, through which the fluidization medium is introduced into the boiler. [0058] The fluidizing medium of the fluidized bed heat exchanger 45 comprises a mixture of gaseous fluidization medium and a predetermined amount of steam mixed prior to the introduction into the fluidized bed heat exchanger. In the embodiment of Figure 4, the source of steam 44 is in flow connection directly with the wind box 58. The gas conduit 80 connected to the wind box 58 is pro- vided with a mixing device 64 by means of which the mixture of the fluidization medium is partially controlled. The source of steam 44 is now in separate flow connection with the wind box 58. However, the source of oxygen 42 is in flow connection with the mixing device 64 by means of a second inlet conduit 70 having a control valve 72. Further, the recycling conduit 38 is in flow connection with the mixing device 64 by means of a third inlet conduit 74 having a control valve 76 arranged in the third inlet conduit. The mixing device 64 makes it possible to control the ratio of the components of the fluidizing gas fed through the conduits. The mixing device does not have to be separate from the wind box, but it may be as well integrated thereto. [0059] Figure 4 also shows an embodiment of steam production unit 100 which serves as the source of steam. The steam production unit 100 is arranged to receive heat from the flue gas flowing the flue gas passage 18. The steam production unit comprises a water pump 102 which increases the pressure of the water to desired level above the atmospheric pressure. Pressurized water is led to a heat exchanger 104 connected to the flue gas passage 18 in which the water is evaporated and superheated.

[0060] Figure 5 illustrates yet another embodiment of the invention. Mainly, it is similar to that of Figure 4 and, thus, corresponding reference numbers are used in connection with corresponding features and elements. Figure 5 depicts another embodiment of steam production unit 100, which serves as the source of steam 44. The steam production unit 100 is also arranged to receive heat from the flue gas flowing in the flue gas passage 18. There is a circuit comprising a flash tank 106 in which expansion steam is produced, which circuit is used as the source of steam 44. The lower part of the flash tank 106 is connected to the heat exchanger 104 and a water pump 102' is arranged into the connection conduit. The pump 102' increases the pressure of the water to a desired level above the atmospheric pressure. Pressurized water is led to a heat exchanger 104 connected to the flue gas passage 18, in which the water is heated. Subsequently, the pressurized and heated water is introduced in the flash tank to a lower pressure thus producing expansion steam. There is an inlet for make-up water 108 in the circuit to compensate the produced and used expansion steam. This has the advantage that the water used for producing expansion steam does not need any or needs only minor treatment, which makes the arrangement e.g. straightforward to operate. [0061 ] It is to be noted that only a few most advantageous embodiments of the invention have been described in the above. Thus, it is clear that the invention is not limited to the above-described embodiments, but may be applied in many ways within the scope of the appended claims. Thus, it is clear that the source of steam may practically any available low pressure steam source, like steam turbine extraction etc. The features disclosed in connection with various embodiments can also be used in connection with other embodiments within the inventive scope and/or different assemblies can be combined from the disclosed features, should it be desired and should it be technically feasible.