WO2011124755A1

WO2011124755A1 - A fluidized bed heat exchanger construction for a boiler arrangement

Info

Publication number: WO2011124755A1
Application number: PCT/FI2011/050288
Authority: WO
Inventors: Jouni Miettinen
Original assignee: Foster Wheeler Energia Oy
Priority date: 2010-04-09
Filing date: 2011-04-04
Publication date: 2011-10-13
Also published as: FI20105367A; FI20105367A0

Abstract

The present invention relates to a fluidized bed heat exchanger construction for a boiler arrangement.In accordance with the present invention the solids from the fluidized bed heat exchanger (80)are discharged into the furnace (12) of the boiler by means of a so-called loop-leg that has a bottom (90) at a lower level than the heat exchange chamber bottom (86), and an upright solids feed passage(94) leading to the furnace(12).

Description

A FLUIDIZED BED HEAT EXCHANGER CONSTRUCTION FOR A BOILER ARRANGEMENT

[0001] The present invention relates to a fluidized bed heat exchanger construc- tion for a boiler arrangement. The present invention is especially applicable in connection with the fluidized bed boilers and circulating fluidized bed boilers.

[0002] An ordinary fluidized bed boiler arrangement of prior art comprises a furnace to which the fuel, the bed material and the combustion gas are introduced. When combusting the fuel, heat is generated and both bottom ash and flue gases are formed. The flue gases are taken to a separator which separates solid particles from the gases, and returns the solid particles back to the furnace.

[0003] Structurally, the circulating fluidized bed boiler (CFB) includes generally a furnace having a bottom, side walls and a roof, and at least one particle separator connected in flow communication with the upper part of the furnace. At least some walls of the bottom part of the furnace are normally inclined in such a way that the cross-section of the furnace increases upwardly, i.e. the part of the furnace having the inclined walls may be called a converging bottom part. In practice, all the walls and the roof of the boiler and the separator comprise water or steam tubes to collect heat from the furnace. The walls at the converging bottom part of the furnace are normally covered with refractory material that resists abrasion better than metallic water or steam tube walls. The bottom of the furnace is provided with a grid for introducing combustion or suspending or fluidizing gas, called as primary air, into the furnace, and for removing ash and other debris from the furnace. The side walls of the furnace are provided with means for introducing fuel into the furnace as well as means for introducing secondary air or gas into the furnace. The furnace is also equipped with means for feeding inert bed material that is normally sand into the furnace. Very often the introduction means (for fuel, secondary air, bed material) are positioned in the converging bottom part of the furnace.

[0004] The particle separator separates solid particles from flue gas-solid particles suspension entering the separator from the upper part of the furnace. The flue gases are taken for further treatment from the separator, and separated solids are recycled back to the lower part of the furnace via a recycling conduit including a seal- ing device, like a loop seal, the purpose of which is to prevent gas flow from the furnace to the separator via the recycling conduit. This solids circulation is called external circulation. In addition to vertical upflow of flue gas-solid particles suspension in the furnace entering finally into the separator inlet, there is a vertical downflow of par- tides near and along the furnace walls. This solids circulation is called the internal circulation.

[0005] Very often, in connection with the internal or the external circulation of solid material or both, at least one fluidized bed heat exchange chamber has been arranged to transfer heat from the bed of fluidized particulate solids to a heat transfer medium. Such a fluidized bed heat exchanger is sometimes arranged in the external circulation so that the solids leaving the solids separator are discharged into the heat exchange chamber in their way back to the furnace (see the prior art Fig. 1 for instance). This kind of a fluidized bed heat exchange chamber is hanging from the se- parator at a distance from the furnace wall. The interior of the heat exchange chamber is provided with heat exchange elements for transferring heat from the solid material to the heat transfer medium flowing inside the heat exchange elements.

[0006] Lately, it has been suggested that a corresponding fluidized bed heat ex- change chamber recovering heat from the solids circulating in the internal circulation could be arranged in the lower part of the furnace i.e. in communication with and outside of the inclined walls of the furnace. Figures 2a and 2b illustrate two types of such fluidized bed heat exchangers that have been positioned at the bottom part of the boiler arrangement. Both fluidized bed heat exchangers receive solids flowing down along the internal wall of the furnace, though it would, naturally, be possible to accept solids from another source i.e. from the separator or, more generally, from the external circulation, too. The difference between the two fluidized bed heat exchanger types can be seen in the bottom part thereof. The type shown in Fig. 2a (discussed for instance in EP-B1 -0 785 821 ) has a flat bottom surface having nozzles or open- ings for the fluidizing gas in the entire bottom area. Below the bottom surface there are two wind boxes, the first one below the heat exchange elements, and the second one below the solids discharge channel. The heat exchange chamber is separated from the solids discharge channel by means of a partition wall that is positioned above the wall separating the two wind boxes. The partition wall leaves a gap be- tween its lower edge and the bottom surface. The first wind box is used for providing fluidizing gas into the fluidized bed heat exchange chamber for maintaining the solids in a fluidized state in the fluidized bed heat exchanger, whereas the second wind box is needed for providing gas for feeding the solids out of the fluidized bed heat exchange chamber along the upright discharge channel, a so called lift leg. However, it has been learned when using this type of boilers or fluidized bed heat exchangers that the control of the fluidization between the fluidized bed heat exchange chamber and the discharge channel requires in some running conditions a lot of attention. In other words, the two fluidization areas, though they should have a totally different function, may be mixed in such a way that in certain running conditions the fluidiza- tion in the heat exchange chamber assists in lifting solids along the lift leg, whereby the residence time of the solids in the fluidized bed heat exchange chamber remains too short. Also, sometimes the fluidization at the discharge side participates in the heat exchange chamber fluidization and slows down the solids flow into the lift leg area, whereby the residence time of the solids in the fluidized bed heat exchange chamber remains too long. In accordance with our studies, a factor influencing the above described mixing of the fluidization areas is the furnace pressure. If, for some reason, the furnace pressure decreases, the counter pressure in the lift leg is decreased, too, giving the gas current creating the fluidization in the heat exchange chamber a possibility to escape to the lift leg.

[0007] The fluidized bed heat exchanger type shown in Fig. 2b (discussed in EP-B1 -0 390 776) has a bottom with two separate bottom surfaces provided with nozzles and/or openings for fluidizing gas and two wind boxes below the bottom surfaces. Now, the bottom surfaces have been arranged vertically at different levels in such a way that the first bottom surface situated below the fluidized bed heat exchange chamber is clearly upper than the second bottom surface. The second bottom surface is situated directly below the partition wall between the fluidized bed heat exchange chamber and the furnace. The first bottom surface does not, thus, extend over all the width of the fluidized bed heat exchange chamber but leaves a flow area between the side wall and the edge of the first bottom surface. In this fluidized bed heat exchanger version, the discharge of the solids from the fluidized bed heat exchange chamber takes place laterally, i.e. along a channel extending in horizontal direction from the fluidized bed heat exchange chamber towards the bed area of the furnace. The purpose of the first wind box below the first bottom surface is exactly the same as that of the first fluidized bed heat exchanger version, but the purpose of the second wind box and the gas blown therefrom is not to raise the solids anywhere but to control the flow of solids in the channel. The actual purpose of controlling the discharge flow in the channel is to control the solids flow into the fluidized bed heat exchange chamber. In accordance with EP-B1 -0 390 776, the flow of solid material into the heat exchange chamber can either be increased or even caused to stop by means of the control gas blown in the channel. In other words, the disclosure of EP- B1 -0 390 776 is not concerned about the discharge of solids from the fluidized bed heat exchange chamber but the control of solids flow therein. Thus, this solids discharge structure is even more difficult to control than the one discussed in connection with Fig. 2a. For instance, if the counter pressure in the furnace is not sufficient, the fluidization gas in the heat exchange chamber is sufficient for making the solids flow out of the heat exchange chamber towards the furnace.

[0008] In other words, the problems the present invention is solving relate to the operation both prior art fluidized bed heat exchanger structures. In the structure of EP-B1 -0 785 821 the flow resistances in the fluidized bed heat exchange chamber and in the lift leg are equal in equilibrium state, but in a situation when there is heavy solids flow in the fluidized bed heat exchange chamber the flow resistance is smaller in the lift leg whereby part of the fluidization gas escapes from the fluidized bed heat exchange chamber via the lift leg, which, at least in some cases, weakens the adjustability of the operation. In the structure of EP-B1 -0 390 776 the fluidization gas of the heat exchange chamber has a tendency to escape via the lateral channel from the heat exchange chamber, as the flow resistance is the weakest therein. Therefore the control does not function properly but is more like an on/off control.

[0009] An object of the present invention is to find at least one solution to at least one of the above discussed problems.

[0010] Another object of the present invention is to improve the construction of the fluidized bed heat exchange chamber such that its operation can be controlled easily and reliably in all running conditions of the furnace.

[001 1] Yet another object of the present invention is to design a novel discharge passage structure for a fluidized bed heat exchanger that gives wider control possibil- ities than the prior art structures. [0012] The above and other objects of the present invention are met with the fluidized bed heat exchanger construction in accordance with claim 1 . [0013] Other features of the construction of the present invention can be seen in the appended claims.

[0014] By means of the fluidized bed heat exchanger construction of the present invention at least the following advantages over the prior art have been achieved,

· Easy control of the operation of the fluidized bed heat exchange chamber in all its running conditions.

[0015] In the following, the fluidized bed heat exchanger construction of the present invention will be explained in more detail with reference to the following draw- ings, of which

Fig. 1 is a schematic cross sectional representation of a circulating fluidized bed boiler arrangement of prior art,

Fig. 2a is a schematic vertical cross sectional representation of a prior art furnace having a fluidized bed heat exchanger and a bed material return conduit attached on the outside wall of the furnace discussed in more detail in EP-B1 -0 785 821 ,

Fig. 2b is a schematic vertical cross sectional representation of another prior art furnace having a fluidized bed heat exchanger and a bed material return conduit attached on the outside wall of the furnace discussed in more detail in EP-B1 -0 390 776, and

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

[0016] Fig. 1 schematically illustrates a circulating fluidized bed boiler 10 of prior art. The boiler 10 comprises a furnace 12 with an upper part having four substantially vertical side walls 32, a bottom part having four side walls of which two are normally inwardly inclined side walls 34, a discharge conduit 14 in the upper part or upper end of the furnace 12 for taking the flue gas and solid particles suspended thereby to a solids separator 16, a passage 18 arranged in the upper end of the solids separator 16 for the removal of the cleaned exhaust gas from the solids separator 16, a recircu- lation conduit 20 at the lower end of the solids separator 16 for returning at least part of the separated solids, i.e. mostly circulating bed material, back to the bottom part of the furnace 12, fuel feed means 22 arranged at a lower side wall 34 of the furnace, and means 24 and 26 for introducing primary and secondary gas, respectively, arranged at the bottom part of the furnace 12. The fuel feed means may include a screw feeder, a drop leg, or a pneumatic feeder, just to name a few alternatives. The primary air 24 is the primary combustion gas that is also used to fluidize the bed material, and is, thus, fed into the furnace 12 through the grid 36 arranged at the bottom of the furnace 12. The secondary gas 26 is introduced into the furnace 12 through the lower side wall 34 thereof slightly above the grid 36. A gas lock 28 has been arranged in the return conduit 20 for preventing the gas from flowing from the furnace 12 via the return conduit 20 into the solids separator 16. Here, the return conduit 20 is further provided with a fluidized bed heat exchange chamber 30 for collecting heat from the recirculating solids to a heat transfer medium. The path of the recirculating solids/bed material is called the external circulation, and includes the separator and all conduits and equipment between the upper part of the furnace and the bottom part of the furnace used for returning the bed material back to the furnace. The upper and lower side walls, 32 and 34 respectively, of the boiler 10 as well as the ones of the solids separator usually comprise water or steam tubes, or are made of water/steam tube panels, so that the water or steam acts as the heat transfer medium. The flui- dized bed heat exchange chamber may, in accordance with recent suggestions, be arranged on the outside wall of the furnace, too, whereby the return conduit or return leg would be running down to the grid area closer to the furnace wall than in prior art arrangements. [0017] It is known from prior art, like illustrated in a very schematic manner in Figures 2a and 2b, that there are at least two types of such fluidized bed heat exchangers that have been positioned at the bottom part of the boiler arrangement. Both fluidized bed heat exchangers receive solids flowing down along the internal wall of the furnace i.e. from the internal circulation, though it would, naturally, be possible to accept solids from another source i.e. from the separator or, more generally, from the external circulation, too. The difference between the two fluidized bed heat exchanger types can be seen in the bottom part thereof. The type shown in Fig. 2a has a flat bottom surface 42 having nozzles or openings for the fluidizing gas in the entire bottom area. Below the bottom surface 42 there are two wind boxes 44 and 48, the first one 44 below the heat exchange elements in the fluidized bed heat ex- change chamber 46, and the second one 48 below the solids discharge channel 50, i.e. below the so called lift leg. The heat exchange chamber 46 is separated from the solids discharge channel 50 by means of a partition wall 52 that is positioned above the wall separating the two wind boxes 44 and 48. The partition wall 52 leaves a gap between its lower edge and the bottom surface 42. The first wind box 44 is used for providing fluidizing gas into the fluidized bed heat exchange chamber 46 for maintaining the solids in the fluidized bed heat exchanger 40 in a fluidized state, whereas the second wind box 48 is needed for providing gas for feeding the solids out of the fluidized bed heat exchange chamber 46 along the upright lift leg, i.e. the solids dis- charge channel 50. In particular running conditions, i.e. for instance when the furnace pressure has started to fluctuate, it has been learned that the balance between the gas currents from the wind boxes starts fluctuating, too, resulting in the problems discussed already above. In other words, when the counter pressure in the lift leg decreases part of the fluidization gas introduced into the heat exchange chamber flows to the lift leg and aids in lifting solids along the lift leg to the furnace. And when the furnace pressure increases resulting in the increase in the counter pressure the gas current from the second wind box turns partially towards the heat exchange chamber decreasing the solids discharge from the chamber. [0018] The fluidized bed heat exchanger type shown in Fig. 2b has a bottom with two bottom surfaces 62 and 64 provided with nozzles and/or openings for fluidizing gas and two wind boxes 66 and 68 below the bottom surfaces 62 and 64, respectively. Now, the bottom surfaces 62 and 64 have been arranged vertically on different levels such that the first bottom surface 62 situated below the fluidized bed heat ex- change chamber 70 is clearly upper than the second bottom surface 64. The second bottom surface 64 is situated directly below the side wall 72 between the fluidized bed heat exchange chamber 70 and the furnace 12. The first bottom surface 62 does not extend over the entire width of the fluidized bed heat exchange chamber 70 but leaves a flow area between the side wall 72 and the closest edge of the first bottom surface 62. In this fluidized bed heat exchanger version the discharge of the solids into the furnace takes place substantially laterally i.e. along a channel 74 extending in substantially horizontal direction towards the bed area of the furnace 12. The purpose of the first wind box 66 below the first bottom surface 62 is exactly the same as of that in the first fluidized bed heat exchanger version of Fig. 2a, but the purpose of the second wind box 68 and the gas blown therefrom is not to raise the solids anywhere but to control the flow of solids in the channel 74. The actual purpose of controlling the discharge flow in channel 74 is to control the solids flow into the fluidized bed heat exchange chamber. In accordance with EP-B1 -0 390 776, the flow of solid material into the heat exchange chamber can either be increased or even caused to stop by means of the control gas blown in the channel 74. However, this kind of control is very problematic, as there is a great risk of discharging solids from the fluidized bed heat exchange chamber too quickly. This would mean that the heat exchange surfaces are only partially covered by the solids, whereby the heat recovery is not as efficient as it should be. The reason is that as soon as the gas flow through the second bottom surface is sufficient to create fluidization in the solids layer above the bottom, the solids flow from the fluidized bed heat exchange chamber is uncontrollable as there is nothing that would slow down the discharge of the fluidized solids from the fluidized bed heat exchange chamber to the furnace. Thus, in order to ensure a steady solids flow from the fluidized bed heat exchange chamber to the furnace very accurate adjustment of the fluidizing gas through the second bottom surface would be required. However, such an accuracy is, in practice, impossible, and the result is that the adjustment of the fluidizing gas flow fluctuates, and results in short-term blockages of the channel 74 (as the gas flow is too low), as well as short-term rapid discharges (as the gas flow is too high) of the solids from the fluidized bed heat ex- change chamber.

[0019] Figure 3 illustrates a first preferred embodiment of the present invention. In this embodiment of the present invention the fluidized bed heat exchanger 80 has been arranged outside the lower wall 34 of the furnace 12. Solids flowing down along the wall 34 of the furnace 12 enter the fluidized bed heat exchanger from above as shown by arrows S, i.e. from the internal circulation. However, it has to be understood that the solids may enter the fluidized bed heat exchanger also from the separator, or from both the separator and from the flow down along the inside wall of the furnace. The fluidized bed heat exchanger 80 is formed of a fluidized bed heat exchange chamber 81 having a top 82 forming an inlet for the solids flow, side walls, of which only the back wall 83 has been shown, a partition wall means 84 facing the furnace 12 and having a lower edge 85, and a bottom 86. The fluidized bed heat exchange chamber 81 houses a plurality of heat exchange surfaces 87 therein. The bottom 86 of the fluidized bed heat exchange chamber 81 is provided with openings or nozzles for introducing fluidizing gas into the heat exchange chamber 81 from the wind box 88 beneath the bottom 86.

[0020] Above, the fluidized bed heat exchange chamber has been shown to be positioned just as close to the furnace as possible. However, it is possible that in some applications the heat exchange chamber is farther away from the furnace such that, for instance, the partition wall means may be formed, not only of a single preferably water-tube wall, but of a thicker wall or even of two, preferably water-tube, walls having an air, or some other cavity therebetween. Also, it has to be understood, that the solids when entering the heat exchange chamber from a separator, or, more generally, from the external circulation, may be freely introduced into the chamber, not necessarily at the top thereof (though such is a viable option, too) but at the bottom of the chamber. A further option, in case the solids are coming from the separator, is to arrange the gas seal, which is normally at the bottom of the separator, in connection with the heat exchange chamber, preferably at the bottom level thereof. A practical solution is that the solids from the separator are discharged to the bottom level of the heat exchange chamber from where they pass laterally sideways underneath a partition wall means into the actual fluidized bed heat exchange chamber. [0021] At the side of the bottom 86 facing the partition wall means 84 there is an outlet port 89 for the solids flow, the outlet port 89 being formed of an open space left between the bottom 86 and the partition wall means 84. The outlet port 89 may also be called the entrance port for allowing the solids to enter a loop-leg. The loop-leg is a solids discharge passage leading from the fluidized bed heat exchange chamber 81 to the furnace 12. The loop-leg is situated above the loop-leg bottom 90, and has three parts: the outlet/entrance port 89 left between the heat exchange chamber bottom 86 and the partition wall means 84, a conduit 92 left between the loop-leg bottom 90 and the lower edge 85 of the partition wall means 84, and an upright solids feed passage 94. The upright solids feed passage 94 has an upper end such that the loop- leg terminates at its upper end in an opening 96 in the wall 34 of the furnace 12. The loop-leg bottom 90 has openings or nozzles for fluidizing gas that is introduced into the loop-leg from the wind box 98 below the loop-leg bottom 90.

[0022] Here also, a few options need to be discussed. In connection with flui- dized bed heat exchange chambers, it is customary practice that if the solids enter the chamber from the top thereof the discharge is from the bottom and vice versa. In other words, in case the solids enter the chamber at the bottom thereof, for instance from the external circulation, the discharge of the solids is arranged from the upper part of the heat exchange chamber. Thus, in accordance with a second preferred embodiment of the present invention, the loop-leg is formed of an upright solids discharge passage running down from at least the level of the upper ends of the heat exchange surfaces of the heat exchange chamber either along the wall of the heat exchange chamber or at a distance thereof. In other words, there is at least a water- tube wall between the heat exchange chamber and the upright solids discharge pas- sage, but it is as well possible that there is a thicker wall or that the wall is formed of two separate parts having a cavity therebetween, whereby a substantially horizontal (preferably, but not necessarily) passage is required to take the solids from the heat exchange chamber to the upright solids discharge passage. This upright solids discharge passage takes the solids to the bottom of the loop-leg, from whereon the loop- leg structure is as elsewhere discussed in this description.

[0023] The fluidized bed heat exchanger 80 functions such that when the boiler is running solids, i.e. mainly circulating bed material, flow into the heat exchange chamber 81 either from the internal or the external circulation or both. The fluidizing gas blown from the wind box 88 through the bottom 86 prevents the accumulation of the solids on the bottom 86 of the chamber 81 by suspending the solids in the upward gas current. However, due to the continuous flow of solids into the heat exchange chamber a part of the solids enter the discharge area in the heat exchange chamber i.e., in the embodiment of Fig. 3, the lowermost left hand corner of the chamber 81 that is closest to the furnace. However, it has to be understood that the discharge area of the heat exchange chamber may also be elsewhere, for instance, at the top part of the chamber, as discussed already above. Yet another option is to arrange the discharge of the solids from the heat exchange chamber not at a corner thereof, but somewhere along the bottom 86 thereof. In such a case, the discharged solids need to be transferred laterally along a more or less horizontal solids transfer passage farther away from the heat exchange chamber to the lower end of the upright solids feed passage 94. Such a transfer requires, in practice, fluidization, whereby the bottom of the loop-leg with its wind box (either a separate wind box for the lateral transfer passage or an extension of the wind box situated directly below the upright solids feed passage) can be considered to extend, in this option, literally below the bottom of the heat exchange chamber, i.e. at least substantially to the entire length of the lateral solids transfer, or solids discharge passage. However, in each embodiment or option, a part of the solids in the fluidized bed heat exchange chamber fall through the outlet port (shown by reference numeral 89 in Fig. 3) down above the loop-leg bottom 90. The gas current via the nozzles or openings in the loop-leg bottom 90 prevents the accumulation of the solids on the loop-leg bottom 90, and allows the solids entering the loop-leg push the solids fluidized by the gas current towards, and finally into the upright solids feed passage 94. The solids present in the upright solids feed passage 94 are taken to the furnace 12 by means of the gas current from wind box 98.

[0024] Now that both the loop-leg bottom 90 has been brought to a lower level than the bottom 86 of the fluidized bed heat exchange chamber 81 , and the lower edge 85 of the partition wall means 84 has also been brought below the bottom 86 of the fluidized bed heat exchange chamber 81 , it is easier to control both the flow of solids to the loop-leg and the flow of solids along the loop-leg to the furnace 12, as the effect of the gas current through the bottom 86 of the heat exchange chamber 81 on the solids discharge is minimized. Another feature assisting in controlling the operation of the heat exchange chamber is the height of the upright solids feed passage 94 i.e. the distance from the loop-leg bottom 90 to the discharge opening 96 in the wall 34 of the furnace 12. Both features add the flow resistance of the solids discharge passage decreasing the effects of the furnace pressure on the heat exchange chamber 81 . As to dimensioning of the loop-leg, there are a few features that ensure the proper operation of the loop-leg. Firstly, the lower edge 85 of the partition wall means 84 should be located either at the same level with the fluidized bed heat exchange chamber bottom 86 or lower. Secondly, as a starting point, the cross- sectional flow area of the channels/passages/openings for discharging solids from the heat exchange chamber all the way to the upright solids feed passage should be maintained the same over the entire length of the solids path. In other words, in the embodiment of Fig. 3, the depth of the loop-leg bottom 90 measured from the fluidized bed heat exchange chamber bottom 86 should be at least of the order of the distance between the partition wall means 84 and the edge of the fluidized bed heat exchange chamber bottom 86 facing the partition wall means 84. And thirdly, the height of the upright solids feed passage 94 needed for improving the adjustability or controllability of the solids discharge from the fluidized bed heat exchange chamber should, in principle, be such that the discharge opening 96 of the upright solids feed passage 94 is at the same level as the upper ends of the heat exchange elements 87. This would be just for ensuring that the solids in the fluidized bed heat exchange chamber cover the heat exchange surfaces of the elements 87. However, it has to be understood that if the flow resistance at the loop-leg bottom, i.e. mainly between the loop-leg bottom 90 and the lower edge 85 of the partition wall means 84, or somewhere along the length of the solids discharge passage, either the upright one leading from the top of the heat exchange chamber down to the loop-leg bottom or the lateral one extending below the heat exchange chamber bottom, is increased by for instance decreasing the cross-sectional flow area, the height of the upright solids feed passage 94 may be reduced. But to maintain the controllability of the solids discharge at a sufficient level the discharge opening 96 of the upright solids feed passage 94 should be positioned at least to the level of half the height of the heat exchange elements 87. Thus, the height of the upright solids feed passage is at least the sum of half the height of the heat exchange elements 87, the distance between the lower end of the heat exchange elements 87 and the fluidized bed heat exchange chamber bottom 86, and the depth of the loop-leg bottom 90 measured from the fluidized bed heat exchange chamber bottom 86. In accordance with a preferred embodiment of the present invention the height of the upright solids feed passage 94 is from about the same as the height of the heat exchange elements 87 up to about 2- fold the height of the heat exchange elements 87, naturally depending on the depth of the loop-leg bottom 90.

[0025] In the above discussed loop-leg structure of the present invention the flow resistances are, in equilibrium state, equal at the loop-leg bottom but as the feed of the fluidization gas into the fluidized bed heat exchange chamber takes place from above the loop-leg bottom, and the lateral channel 92, the fluidization gas is, in its entirety, conveyed through the heat exchange chamber, and the control of the bed is performed fully by the loop-leg fluidization gas only. Thus the controllability of the flui- dized bed heat exchanger structure is by far better than using the prior art fluidized bed heat exchanger structures.

[0026] A few possible additional embodiments of the invention relates to the loop-leg bottom. Firstly, the loop-leg bottom may have different properties of fluidizing the solids in the loop-leg. This function may be achieved by arranging different open- ings or nozzles in different parts of the loop-leg bottom 90. For instance, the bottom part situated, in Fig. 3, to the right of the partition wall means 84 may have smaller openings or nozzles than the bottom part to the left i.e. below the upright solids feed passage 94, such that the gas current entering the right-hand side of the loop-leg from the wind box 98 is weaker than that entering the opposite side of the partition wall means 84. The aim of this construction is to ensure, on the one hand, that the incoming gas current does not interfere the functioning of the fluidized bed heat exchange chamber, and on the other hand, that the gas current in the upright solids feed passage 94 is strong enough to lift the solids easily into the furnace.

[0027] Another alternative embodiment is to arrange two or more different wind boxes, or wind box compartments below the loop-leg bottom 90. Thus, by adjusting the volume flows from the boxes or compartments, a similar function as described above may be achieved.

[0028] Yet another alternative embodiment is to arrange at least part of the gas nozzles/openings at the right-hand end of the loop-leg to blow gas in lateral direction, i.e. for feeding solids from the vicinity of the fluidized bed heat exchange chamber 81 towards the upright solids feed passage 94. This could be arranged either by provid- ing the loop-leg bottom with directional nozzles or by arranging the substantially vertical right-hand end wall (in Fig. 3) of the loop-leg with horizontally oriented nozzles.

[0029] It should also be understood that the above alternative embodiments, i.e. the different openings or nozzles in the loop-leg bottom, several wind boxes along the length of the loop-leg bottom, and the directional nozzles in the loop-leg bottom, are also applicable in connection with the embodiment where the solids are discharged from the heat exchange chamber to a laterally extending solids discharge passage, or where the solids discharge passage is an upright one. Further, the words 'lateral' and 'upright' should be understood in the above description such that when a pas- sage is lateral the moving of solids therein requires fluidization whereas when a passage is upright the solids move downwards by means of mere gravity. In other words, lateral direction is mainly horizontal, and upright direction is mainly vertical. And finally it should be understood that the present invention applies to any combustion mode available, i.e. to both air and oxy combustion. [0030] In view of the above description, it has to be understood that only a few most preferred embodiments of the present invention has been discussed. Thus, it is obvious that the invention is not limited to the above disclosed embodiments only, but it can be modified in many ways within the scope of the appended claims. It has to be understood, too, that features of a specific embodiment of the invention may be applied in connection with features of other embodiments within the basic idea of the present invention or that features from different embodiments may be combined as long as they result in a working and technically feasible construction.

Claims

We claim:

1 . A fluidized bed heat exchanger construction for a boiler arrangement, the boiler arrangement being formed of at least a furnace (12) and at least one fluidized bed heat exchanger (80) arranged in flow communication therewith; the furnace (12) having a grid (36), and a plurality of side walls (34), the at least one fluidized bed heat exchanger (80) being arranged in connection with one of said side walls (34); the at least one fluidized bed heat exchanger (80) being formed of a fluidized bed heat exchange chamber (81 ) having side walls (83), a top, a solid material inlet (82) for intro- ducing solids into the heat exchange chamber, a bottom (86), heat exchange elements (87) between the top and the bottom (86), a discharge outlet (89) for discharging solids from the heat exchange chamber (81 ), and an upright solids feed passage separate from the heat exchange chamber (81 ) for taking the solids to the furnace (12), the upright solids feed passage (94) having an upper end terminating in a dis- charge opening (96) in said one of said furnace side walls (34), the fluidized bed heat exchanger construction being characterized in a loop-leg for discharging solids from the fluidized bed heat exchanger (80) to the furnace (12), the loop-leg having

• a loop-leg bottom (90) arranged to a lower level than the bottom (86) of the heat exchange chamber (81 ), and receiving solids from the heat exchange chamber (81 );

• a substantially lateral conduit (92) arranged below the level of the heat exchange chamber bottom (86) and above the loop-leg bottom (90) for transferring the solids laterally farther away from the heat exchange chamber to the lower end of the upright solids feed passage (94); and

· the discharge opening (96) of the upright solids feed passage (94) positioned a least at a level of half the height of the heat exchange elements (87).

2. The fluidized bed heat exchanger construction as recited in claim 1 , characterized in that the discharge outlet is arranged in the bottom (86) of the heat ex- change chamber (81 ).

3. The fluidized bed heat exchanger construction as recited in claim 1 , characterized in that the discharge outlet is arranged in the top end of the heat exchange chamber (81 ).

4. The fluidized bed heat exchanger construction as recited in claim 3, characterized in that the discharge outlet in the top end of the heat exchange chamber is in flow communication with the bottom of the loop-leg by means of an upright solids discharge passage.

5. The fluidized bed heat exchanger construction as recited in any one of the preceding claims, characterized in that the upright solids feed passage (94) for introducing solids from the bottom of the loop-leg to the furnace (12) is separated from the heat exchange chamber (81 ) by means of partition wall means (84).

6. The fluidized bed heat exchanger construction as recited in claim 5, characterized in that the partition wall means (84) is one of a water-tube partition wall, a thicker wall, and two walls with a cavity therebetween.

7. The fluidized bed heat exchanger construction as recited in claim 5, characterized in that the discharge outlet is arranged in the bottom (86) of the heat exchange chamber (81 ) at a distance from the partition wall means (84).

8. The fluidized bed heat exchanger construction as recited in claim 6, characterized in that the discharge outlet (89) is arranged in the bottom (86) of the heat exchange chamber (81 ) originating from the partition wall means (84).

9. The fluidized bed heat exchanger construction as recited in any one of the preceding claims, characterized in that the loop-leg bottom (90) has a depth dimensioned such that the cross-sectional flow area between the partition wall means (84) and the loop-leg bottom (90) corresponds to the area of the discharge outlet (89).

10. The fluidized bed heat exchanger construction as recited in any one of claims 6 - 9, characterized in that the lower edge (85) of the partition wall means (84) is at the same level or lower than the bottom (86) of the fluidized bed heat exchange chamber.

1 1 . The fluidized bed heat exchanger construction as recited in any one of the preceding claims, characterized in that the lateral conduit runs at least partially below the bottom (86) of the heat exchange chamber (81 ).

12. The fluidized bed heat exchanger construction as recited in claim 1 1 , characterized in that the loop-leg bottom extends over the substantial length of the lateral conduit.

13. The fluidized bed heat exchanger construction as recited in any one of claims 6 - 12, characterized in that the lateral conduit (92) runs between a lower edge (85) of the partition wall means (84) and the bottom (90) of the loop-leg.

14. The fluidized bed heat exchanger construction as recited in any one of the preceding claims, characterized in that the loop-leg bottom (90) has openings or nozzles for gas, and that the openings or nozzles are directional.

15. The fluidized bed heat exchanger construction as recited in any one of the preceding claims, characterized in that the height of the upright solids feed passage (94) measured from the loop-leg bottom (90) is from 1 ,0 to 2,0-fold compared to the height of the heat exchange elements in the fluidized bed heat exchange chamber.

16. The fluidized bed heat exchanger construction as recited in any one of the preceding claims, characterized in that the upright solids feed passage (94) has a discharge opening (96) at least at a level of the full height of the heat exchange elements (87).

17. The fluidized bed heat exchanger construction as recited in any one of the preceding claims, characterized in a wind box (98) below the loop-leg bottom (90), the wind box (98) being divided into at least two different compartments.