Patent Publication Number: US-9423122-B2

Title: Circulating fluidized bed boiler having two external heat exchangers for hot solids flow

Description:
CLAIM OF PRIORITY 
     Our invention relates to a circulating fluidized bed boiler. In more detail, the circulating fluidized bed boiler of the present invention is preferably a once through utility (OTU) boiler, for example, for power generation or industrial steam production. As the size of boilers increases, the relation of the wall surface area to the volume of the furnace usually becomes disadvantageous, which may cause problems, for example, in positioning of the different devices and conduits related to the furnace, as well as feed the feeding and mixing of different materials. The present invention especially relates to solving problems related to large circulating fluidized bed (CFB) boilers. 
     FIELD OF THE INVENTION 
     Our invention relates to a circulating fluidized bed boiler. In more detail, the circulating fluidized bed boiler of the present invention is preferably a once through utility (OTU) boiler, for example, for power generation or industrial steam production. As the size of boilers increases, the relation of the wall surface area to the volume of the furnace usually becomes disadvantageous, which may cause problems, for example, in positioning of the different devices and conduits related to the furnace, as well as feed and mixing of different materials. The present invention especially relates to solving problems related to large circulating fluidized bed (CFB) boilers. 
     BACKGROUND OF THE INVENTION 
     Generally speaking, a circulating fluidized bed boiler comprises a furnace for combusting fuel, an outlet channel connected to the upper section of the furnace for the discharge of flue gas out of the furnace, a solids separator for receiving the flue gas via the outlet channel from the furnace, and for separating solid particles from the flue gas. The CFB boiler further comprises at the lower portion of the solids separator, a return channel for taking the hot solids separated by means of the solids separator to the lower section of the furnace, and at the upper portion of the solids separator, a flue gas duet for removing cleaned flue gas to the backpass of the boiler, to gas cleaning devices and, further, through the stack to the environment. 
     The outlet channel, solids separator, and the return channel form a so-called external hot circulation, in which the hot solids entrained in the flue gas are first taken out of the furnace, then treated in the separator, and finally, returned to the furnace. Most often, somewhere in the external circulation, in flow communication with the solids return channel, a fluidized bed heat exchanger is arranged. The heat exchanger may be supported to the lower portion of the solids separator such that the return channel takes the solids from the heat exchanger to the lower section of the furnace. Or, the heat exchanger may be supported by a side wall of the furnace, such that the return channel takes the solids from the solids separator to the heat exchange chamber. As to the fluidized bed heat exchangers, they may also be arranged in the internal circulation, i.e., for receiving the solids from the bed material flowing down along the furnace walls. And, naturally, there are also fluidized bed heat exchangers that may receive solids from either the internal or the external circulation, or simultaneously, from both circulations. 
     The lower section of the furnace is provided with feeds for feeding fuel, inert bed material, and possibly sulfur binder to the furnace, and, finally, the bottom of the furnace is provided with feeds for feeding oxide-containing fluidizing gas into the furnace, in other words, a gas inlet channel, a wind box, and nozzles. 
     Published PCT International Application No. WO 2007/128383 A2 discusses a fluidized bed heat exchanger structure for a CFB boiler. The CFB boiler of the PCT publication, or in fact, the fluidized bed heat exchanger, comprises two heat exchange chambers arranged in series in communication with the return channel, such that a first fluidized bed heat exchange chamber supported below the solids separator receives hot solids directly, actually, via a gas seal, from the solids separator, and then, in normal conditions, discharges the cooled solids to a second fluidized bed heat exchange chamber arranged in connection with the wall of the lower section of the furnace. Finally, the cooled solids are returned to the furnace from the second heat exchange chamber. 
     In accordance with the teachings of the PCT publication, the upper heat exchange chamber is also provided with a return for returning cooled solids from the upper heat exchange chamber directly to the furnace. Both heat exchange chambers have internal heat exchange surfaces arranged within the heat exchange chambers for cooling the solids before they are returned to the lower section of the furnace. In other words, the two heat exchange chambers discussed above are connected in series in the external solids circulation of a CFB boiler. 
     It is a specific feature of the second, i.e., the lower heat exchange chamber, of the above-mentioned PCT publication that the heat exchange chambers may receive hot solids, not only from the first heat exchange chamber, but also, from the internal circulation, i.e., the second heat exchange chamber is provided with an inlet arranged in the wall of the lower section of the furnace such that hot solids flowing down along the boiler walls are able to enter the second fluidized bed heat exchange chamber. Further, the heat exchanger arrangement of the PCT publication is provided with an overflow for allowing overflow of solids from the first heat exchange chamber directly to the second heat exchange chamber, in a case that the solids flowing into the first heat exchange chamber is larger than the discharge flowing out of the first heat exchange chamber. In connection with this discussion concerning the heat exchangers, it should be understood that a large CFB boiler is usually provided with several parallel solids separators and heat exchangers connected to their return channel either on one side of the boiler or on both sides thereof, but, for clarity reasons, both above and in the following description of the invention, mainly, only one heat exchange arrangement with one solids separator, is discussed. 
     The starting point in the development of the fluidized bed heat exchanger of the PCT publication discussed above was to be able to construct a heat exchange arrangement that may be used in almost all possible applications due to its versatile controlling possibilities. A problem the construction of the PCT publication solved related to the traditional location of the fluidized bed heat exchange chambers on the outside walls of the lower section of the furnace. While the CFB boilers grew, it was not possible to increase the size of the fluidized bed heat exchange chambers accordingly, as increasing the height of a heat exchange chamber resulted in the increase of pressure losses in the fluidization air, and an increase in the width of the heat exchanger was not possible due to a lack of space. Thus, the growing size of the CFB boilers was taken into account in the PCT publication by arranging the heat exchangers one on top of the other, whereby requirements relating to both the available space and the acceptable pressure Josses were taken into account. And, finally, the adjustability or controllability of the heat exchange arrangement was ensured by providing the arrangement with equipment giving a possibility to run the arrangement in several different ways. 
     When all the above-discussed and other considerations were taken into account in the design of the heat exchange arrangement, however, the construction of the arrangement became less optimal for some specific applications. Such applications are cases when no extensive control is required, or cases when the connection of the heat exchange chambers in series is not desired, for some reason. In other words, the prior art arrangement has a few drawbacks or problems. 
     First, since the upper heat exchange chamber is supposed to discharge the cooled solids to the lower one, the channel between the heat exchange chambers runs between the upper heat exchange chamber and the furnace forcing positioning of the first/upper heat exchange chamber substantially far from the furnace wall. This also means that the solids separator has to be positioned far from the furnace, as the upper heat exchange chamber is normally positioned right below the separator, and supported from the separator. 
     Second, as the lower heat exchange chamber is supposed to be able to receive all the cooled solids from the upper heat exchange chamber, and possibly, also some additional solids from the internal circulation, it is clear that the volume of the lower heat exchange chamber should at least correspond to the one of the upper heat exchange chamber. As already discussed above in connection with the PCT publication (WO 2007/128883 A2), neither the height nor the width (in a direction parallel to the furnace wall) of the lower heat exchanger can be chosen freely, but both the pressure loss in the fluidization, and the space occupied by the heat exchange chamber have to be considered. The above consideration results in that the dimensions of the lower heat exchange chamber are substantially equal with the upper one. Thereby, there is very little room in connection with the lower section of the furnace for the equipment necessary for running the boiler, such as, for example, the start-up burner, a temperature measuring device for measuring the lower furnace temperature, a pressure measurement device for measuring the bed pressure, and feeds for introducing fuel, bed material, secondary air, additives, recirculated flue gas (if in use), etc. 
     Third, due to the various running alternatives, i.e., control options in the prior art boiler, there are conduits and channels for each alternative. For instance, the upper heat exchange chamber has one inlet from the separator, and several outlet channels and lift channels. One lift channel and outlet channel leading to the lower heat exchanger, another lift channel and outlet channel leading to the furnace, and overflow channels leading to both the lower heat exchange chamber and to the furnace. In addition to the channels, rather complicated fluidization means and controllers for adjusting the fluidizations are also required at the bottom of the upper heat exchange chamber. If, and when, the various channels and conduits require bellows to separate components in different temperatures, the bellows, again, occupy space, and also increase the costs of the heat exchanger arrangement together with the already numerous channels, conduits, fluidization equipment, and control systems that have been discussed above. And, still further, all of the channels and conduits need to be either made of water/steam tube walls and connected to the rest of the steam/water system, or made of a refractory material. Irrespective of the manufacture, this adds to the expenses as constructing the channels of water/steam tube walls or refractory material is a complicated and time-consuming task. 
     For the above reasons, it has been found necessary to improve the construction of a CFB boiler and its heat exchanger arrangement. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a circulating fluidized bed boiler, in which problems and drawbacks of the prior art discussed above are minimized. 
     A further object of the present invention is to provide a simpler heat exchanger arrangement as compared to the prior art. 
     Yet another, further object of the present invention is to provide a heat exchanger arrangement that offers the boiler designer more alternatives in positioning various components of the boiler system in the lower section of the furnace. 
     In order to solve the above-mentioned problems of the prior art, our invention provides a circulating fluidized bed boiler (CFB) with a novel heat exchanger arrangement. The CFB boiler comprises a furnace for combusting solid carbonaceous fuel in a fast fluidized bed, the furnace having walls made of water/steam tube panels and used for evaporating the water fed therein, a solids separator arranged adjacent to a sidewall of the furnace for separating solids entrained with exhaust gas discharged via an outlet channel from an upper portion of the furnace, a gas seal for conveying at least a portion of the separated solids to a first fluidized bed heat exchange chamber arranged downstream of the gas seal and having internal heat exchange surfaces, a first lift channel, having a lower end connected to a bottom portion of the first fluidized bed heat exchange chamber and an upper end connected to an upper end of a first return channel for discharging solids from the first fluidized bed heat exchange chamber and taking the cooled solids to a lower portion of the furnace, a second fluidized bed heat exchange chamber arranged adjacent to a lower sidewall of the furnace and having internal heat exchange surfaces, an inlet channel arranged between the second fluidized bed heat exchange chamber and the furnace for introducing hot solids from the furnace to the second heat exchange chamber, a second lift channel having a lower end connected to a bottom portion of the second fluidized bed heat exchange chamber and an upper end connected to discharge the solids to the lower portion of the furnace, the first fluidized bed heat exchange chamber being positioned above the second fluidized bed heat exchange chamber, wherein the first heat exchange chamber has two first lift channels and two first return channels arranged at the lateral sides thereof such that the second heat exchange chamber is situated between the lower ends of the two first return channels. 
     Other features of the present invention will be discussed in more detail as follows. 
     The advantages gained by the construction and the design of the CFB boiler of the present invention are as follows;
         a smaller-sized lower heat exchange chamber;   the lower heat exchange chamber has a lighter construction;   the lower heat exchange chamber is easier to support from the furnace wall;   the lower heat exchange chamber leaves room for other equipment;   there is no return channel from the upper fluidized bed heat exchange chamber to the lower one;   a simple heat exchanger arrangement construction;   a separator and an upper heat exchange chamber that are closer to the furnace;   a possibility to position equipment necessary for the working of the CFB boiler to the sides of the lower fluidized bed heat exchange chamber;   the ability to provide different temperatures of the solids entering the upper and lower heat exchange chambers;   no need to use bellows in connection with the lower heat exchange chamber;   mixing of fuel in the solids discharged from the upper heat exchange chamber;   mixing of the fuel and the solids discharged from the lower heat exchange chamber in the bed area of the furnace;   separately supported upper and lower heat exchange chambers, the weight of the chambers divided between the solids separator and the wall of the lower section of the furnace; and   returning solids from the upper heat exchange chamber at a higher temperature to the lower part of the furnace, as the solids pass only one heat exchange chamber.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention is described in more detail in the following with reference to the attached drawings, of which 
         FIG. 1  is a schematic vertical cross section of a circulating fluidized bed boiler provided with a heat exchanger arrangement in accordance with the prior art; 
         FIG. 2  is a schematic vertical cross section of a heat exchanger arrangement in accordance with a preferred embodiment of the present invention; and 
         FIG. 3  is a schematic back view of a heat exchanger arrangement in accordance with the preferred embodiment of the present invention shown in  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  illustrates a prior art circulating fluidized bed (CFB) boiler  10  comprising a furnace  12  for combusting fuel, an outlet channel  14  connected to the upper section of the furnace  12  for the discharge of flue gas out of the furnace  12 , a solids separator  16  for receiving the flue gas via the outlet channel  14  from the furnace  12 , and for separating solid particles from the flue gas. The CFB boiler  10  further comprises, at the lower portion of the solids separator  16 , a return channel  18  for taking the hot solids separated by the solids separator  16  out of the separator  16  towards the lower section of the furnace  12 , and, at the upper portion of the solids separator  16 , a flue gas duct  20  for removing cleaned flue gas to the backpass of the boiler  10 , gas cleaning devices, and further, through the stack to the environment. The outlet channel  14 , the solids separator  16 , and the return channel  18  form a so-called external hot circulation, where the hot solids entrained in the fine gas are first taken out of the furnace  12 , then treated in the separator  16 , and finally, returned to the furnace  12 . The lower section of the furnace  12  is provided with a feed  22  for feeding fuel, inert bed material, secondary air, and possibly, sulfur binder to the furnace. Finally, the bottom of the furnace  12  is provided with a feed for feeding oxide-containing fluidizing gas into the furnace  12 , in other words, the feed comprises a gas inlet channel  24 , a wind box  26 , and nozzles  28 . 
     Most often, somewhere in the external circulation, a fluidized bed heat exchanger ( 36 ) is arranged. The fluidized bed heat exchanger may be supported to the lower portion of the solids separator  16  such that the return channel  18  takes the solids from the heat exchanger to the lower section of the furnace  12 . Or, the fluidized bed heat exchanger may be supported by the side wall of the furnace  12  such that the return channel  18  takes the solids from the solids separator  16  to the heat exchange chamber. The prior art also shows fluidized bed heat exchange chambers arranged outside the furnace wall in the internal circulation, which means that the fluidized bed heat exchange chamber receives solids flowing down along the furnace walls, cools the solids, and returns them back to the furnace. 
       FIG. 1  illustrates a further developed construction where the fluidized bed heat exchanger between the solids separator  16  comprises two heat exchange chambers, a first or upper heat exchange chamber  36  and a second or lower heat exchange chamber  38  arranged below the first heat exchange chamber  36 , each heat exchange chamber being provided with an internal heat exchange surface  32 ,  34 . The bottoms of the first and second heat exchange chambers  36 ,  38  are provided with a gas inlet duct  40 ,  42 , a wind box  44 ,  46 , and nozzles  48 ,  50  for fluidizing the bed of solids being formed in the heat exchange chambers  36 ,  38 . 
     In operation, the heat exchanger of  FIG. 1 , functions such that the hot solids flowing from separator  16  are passed along the return channel  18  through a gas seal  52  into the upper part of the fluidized bed of particles in the first heat exchange chamber  36 . The lower section of the heat exchange chamber is provided with a lifting channel  54 , the lower section of the lifting channel having nozzles  56 , which make the solids flow at a desired velocity through the heat exchange chamber  36  to be further discharged through the upper part of the lifting channel  54  into an inlet channel  58  of the second heat exchange chamber  38 . The upper section of the first heat exchange chamber  36  is preferably arranged with an overflow channel  60  via which excess solids are discharged either to the second heat exchange chamber  38  or back to the furnace  12 , if the amount of solids to be discharged through the lifting channel  54  is less than the amount of solids entering the heat exchange chamber  36  through the separator  16 . The amount of solids passing through the first heat exchange chamber  36  is preferably adjustable by means of the lifting channel  54  and the overflow channel  60 . 
     In the arrangement of  FIG. 1 , the lower heat exchange chamber  38  is equal to the upper heat exchange chamber  36 , except that, in the lower heat exchange chamber  38 , the flow of particles entering the heat exchange chamber is received from the upper part of the lifting channel  54  of the upper, i.e., the first heat exchange chamber  36 , and from the overflow channel  60  along the inlet channel  58  into the upper part of the fluidized bed of particles in the lower, i.e., the second heat exchange chamber  38 . In the manner of the first heat exchange chamber  36 , the second heat exchange chamber  38 , too, has a lifting channel  61  for discharging cooled solids from the chamber  38 , and an overflow channel  62  in case the amount of solids entering the heat exchange chamber  38  is larger than what the lifting channel  61  is able to discharge. Furthermore, the solids to be discharged from the upper part of the lifting channel  61  of the lower heat exchange chamber  38  and from the overflow channel  62  are passed into the furnace  12 . 
     Further,  FIG. 1  also shows how the upper section of the lower heat exchange chamber  38 , preferably, the inlet channel  58 , comprises inlet opening(s)  64  for passing solids into the heat exchange chamber  38  directly from the internal circulation of the solids in the furnace  12 . The inlet openings  64  are preferably arranged in the oblique surfaces  66  in the lower section of the furnace  12 , in which case, hot solids flow through openings  64  into the heat exchange chamber  38 , also, at small loads of the boiler  10 , in which case, the fluidizing velocity of the solids in the furnace  12  is relatively low. 
     Normally, the walls of the furnace  12 , as well as the walls of the solids separator  16 , of the fluidized bed heat exchange chambers  36 ,  38 , and also, of some conduits and channels, are made of water tube panels (sometimes called membrane walls) serving as so-called evaporating surfaces or as water heating surfaces, in which water tube panels, the high-pressure feed water of the boiler steam cycle, heated in an economizer (not shown in  FIG. 1 ) arranged in the boiler backpass, is converted to steam, or feed water is further heated. The steam temperature is further, after the evaporating surfaces, raised in superheaters, the last stage of the superheaters normally being arranged in the heat exchanger  30  of the external hot circulation. The superheated steam is passed into a high pressure steam turbine, having a generator connected therewith, for generating electricity. In high-efficiency boilers, the steam leaving the high-pressure turbine at a lower pressure is passed to reheaters, for reheating. Advantageously, the last stage of the reheaters may also be arranged in the heat exchanger  30  of the external hot circulation. The hot steam generated thereby is further passed to a lower-pressure steam turbine, in order to increase the quantity of produced electricity and the total efficiency of the plant. 
     As has already been explained above, however, the heat exchanger arrangement of  FIG. 1  has a number of drawbacks and problems related thereto. 
     First, since the upper heat exchange chamber  36  is supposed to discharge the cooled solids to the lower one  38 , the channel between the heat exchangers runs between the upper heat exchanger  36  and the furnace  12  forcing positioning of the first heat exchanger  36  substantially far from the furnace  12 . This also means that the solids separator  16  has to be positioned far from the furnace  12 , as the heat exchange chamber  36  is normally positioned right below the solids separator  16 , and is supported by the separator  16 . 
     Second, as the lower heat exchange chamber  38  is supposed to be able to receive all of the cooled solids from the upper heat exchange chamber  36 , and possibly, also some additional solids from the internal circulation, it is clear that the volume of the lower heat exchange chamber  38  should be at least that of the upper heat exchange chamber. As has already been discussed above in the PCT publication (WO 2007/128883 A2), neither the height nor the width of the lower heat exchanger  38  can be chosen freely, but both the pressure loss in the fluidization, and the space occupied by the heat exchanger have to be optimized. This results in that the dimensions of the lower heat exchange chamber  38  are substantially equal to the upper one  36 . Thereby, there is very little room in connection with the lower section of the furnace  12  for the equipment necessary for running the boiler  10 , such as, for example, the start-up burner, a measuring device for measuring the lower furnace temperature, a measuring device for measuring the bed pressure, and feeds for introducing fuel, bed material, secondary air, additives, recirculated flue gas (if applicable), etc. 
     Third, due to the various running alternatives, conduits and channels are necessary for each alternative. For instance, the upper heat exchange chamber  36  has one inlet from the separator  16 , and several outlet channels and lift channels. One lift channel and outlet channel leading to the lower heat exchanger  38 , another lift channel and outlet channel leading to the furnace  12 , and an overflow channel leading to the lower heat exchange chamber  38 . In addition to the channels, rather complicated fluidization means and controller for adjusting the fluidization also are required at the bottom of the upper heat exchange chamber  36 . If, and when, the various channels and conduits require bellows to separate components in different temperatures, the bellows, again, occupy space, and also increase the costs of the heat exchanger arrangement together with the numerous channels, conduits, fluidization equipment, and control systems that have already been discussed above. 
     A solution to at least some of the above-mentioned drawbacks and problems is illustrated in  FIGS. 2 and 3 , which show a novel heat exchanger  70  arrangement for a CFB boiler  10 . The heat exchanger arrangement  70  comprises two heat exchanger chambers  72  and  74 . The upper heat exchange chamber  72  is in flow communication with the solids separator  16  via a gas seal  52 . Preferably, the upper heat exchange chamber  72  is supported from the separator  16 , but since the upper heat exchange chamber  72  is very close to the furnace wall, the heat exchange chamber  72  may also be supported by the furnace wall and its reinforcement structures. The heat exchange chamber  72  is also provided with internal heat exchange surfaces  76 , and nozzles  78  at the bottom of the chamber  72 . Below the nozzles  78  is a wind box  80  for blowing fluidization air  82  into the fluidized bed heat exchange chamber  72  for fluidizing the solids entering the chamber from the separator  16 . In this preferred embodiment of the invention, the upper fluidized bed heat exchange chamber  72  is provided with two lift channels  84  on both lateral sides of the chamber  72 , and, naturally, also, two return channels  86  for taking the cooled solids back to the furnace  12 . In accordance with an additional embodiment of the present invention, the return channel  86  is provided with a feed  88  for introducing fuel into the solids flow. 
     A lower fluidized bed heat exchange chamber  74  is arranged below the upper fluidized bed heat exchange chamber  72 , and, preferably, in connection with the wall of the furnace lower section. Further, the lower heat exchange chamber  74  is situated between the return channels  86  of the upper heat exchange chamber  72 , in fact, between the lower ends of the return channels  86 . The lower heat exchange chamber  74  is provided with an inlet channel  90  for receiving hot solids directly from the furnace  12  via an opening  92  in the, preferably oblique, furnace wall  94 . The lower heat exchange chamber  74  further has internal heat exchange surfaces  96 , bottom nozzles  98 , and a wind box  100  below the bottom from where fluidization air  102  is blown into the fluidized bed heat exchange chamber  74 . The lower fluidized bed heat exchanger  74  further has a lift channel  104  along which solids from the chamber  74  are discharged into the lower section of the furnace  12 . The lift channel  104  needs its own nozzles, wind box, and air feed to be able to lift the solids into the lift channel  104 . 
     The advantages of the present invention may be seen in both  FIGS. 2 and 3 . It has been shown that the separator  16  and the upper fluidized bed heat exchange chamber  72  are located much closer to the furnace  12  than in the prior art construction of  FIG. 1 . The cause for this improvement is the fact shown in  FIG. 3  that the lift channels  84  and return channels  86  have been arranged to the lateral sides of the fluidized bed heat exchange chamber  72 , and not between the chamber and the furnace wall, as in the prior art. A further option would be to arrange the lift channel  84  and the return channel  86  such that they both have a common wall with the chamber  72  such that, in the illustration as in  FIG. 3 , the channels would not be side by side (as in  FIG. 3 ), but one after the other, whereby the usage of space would be very effective, and would make it possible to bring the adjacent heat exchange chamber (and the separator) even closer to each other. 
       FIG. 3  clearly shows how the lower fluidized bed heat exchange chamber  74  may be built narrower than the upper heat exchange chamber  72 , as the lower heat exchange chamber  74  receives high temperature solids from the furnace  12  only, and thereby the size, i.e., the width of the chamber  74 , may be reduced. Thus, this construction offers room for other equipment at the sides of the lower heat exchange chamber  74 . Here, such has been exemplified by openings  106  in the wall  94  of the furnace  12 . The openings  106  may be provided with feeds for introducing fuel, bed material, secondary air, etc., into the furnace  12  or with a start-up burner. 
     As to the heat exchange surfaces of the fluidized bed heat exchange chambers  72 ,  74 , it is normal practice that the internal surfaces  76  and  96  ( FIGS. 2 and 3 ) are used in the steam cycle. A viable option is to use the heat exchange surfaces  76  of the upper heat exchanger  72  as the last superheater stage before the steam is introduced into the high pressure turbines. A similarly viable option is to use the heat exchange surfaces  96  of the lower heat exchanger  74  for reheating the steam entering from the high pressure turbines before being introduced into low pressure turbines. The utilization of the membrane walls or the fluidized bed heat exchange chambers is, however, not that self-evident. 
     One alternative to utilize the wall surfaces of the heat exchange chambers  72 ,  74  is to arrange such in the water circulation, i.e., for preheating the water to be fed into the steam cycle of the furnace  12 . For instance, one option is to feed water via an economizer in the flue gas conduit to the walls of the lower fluidized bed heat exchange chamber  74 , and then, to introduce the preheated water to the evaporator tubes in the furnace walls. A further option is to take the feed water after the lower heat exchange chamber  74  to the walls of the upper heat exchange chamber  72 , and only thereafter introduce the preheated water to the evaporator panels of the furnace  12 . A yet further option is to take the feed water after the lower heat exchange chamber  74  to the walls of the discharge conduit that leads from the upper heat exchange chamber  72  to the furnace  12 , and, thereafter, to the walls of the upper heat exchange chamber  72 . This way, the feed water path from the feed water pump to the evaporator tubes in the furnace walls is as follows: feed water pump—economizer—lower heat exchange chamber walls—return channel walls  86 ′ —upper heat exchange chamber walls—water/steam tube panels of the furnace  12 . The feed water path may also be provided with water cooled hanger tubes between the economizer and the lower heat exchange chamber  74  walls. As a further option, it is also possible that the walls of the upper heat exchange chamber  72  may be steam cooled, and, optionally, integrated with the steam cooled separator. 
     The invention has been described above in connection with exemplary arrangements, but the invention also comprises various combinations or modifications of the disclosed embodiments. Especially, the number of separators and heat exchangers may vary from what is disclosed in  FIGS. 2 and 3 . Thus, it is obvious that the exemplary embodiments disclosed herein are not intended to limit the scope of the invention, but several other embodiments are also included in the invention, the embodiments being limited only by the appended claims and the definitions therein.