[0001] The present invention relates to a method of and an arrangement for controlling the operation of a fluidized bed boiler. The present invention is especially applicable in controlling the operation of a fluidized bed heat exchanger in connection with circulating fluidized bed boilers.

[0002] An ordinary fluidized bed boiler arrangement of prior art comprises a fur- nace to which fuel, bed material and combustion air are introduced. When combusting the fuel, heat is generated and both bottom ash and flue gases are 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 or combustion chamber 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 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 or solids 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 sol- ids are recycled back to the lower part of the furnace via a recycling conduit including a sealing 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 solid particles or solids 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 solids or both, at least one fluidized bed heat exchange chamber has been arranged to transfer heat from the bed of fluidized 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 exchanger is hanging from the separator at a distance from the furnace wall. The heat exchanger includes a heat exchange chamber, the interior of which 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 exchange chamber, more generally a fluidized bed heat exchanger, could be arranged in communication with the furnace i.e. forming a part of the internal circulation. Advantageously such a fluidized bed heat exchanger is arranged in the lower part of the boiler arrangement i.e. in communication with and outside of the inclined walls of the furnace for recovering heat from the solids circulating in the internal circulation. Figure 2 illustrates an exemplary fluidized bed heat exchanger that has been positioned at the bottom or lower part of the boiler arrangement. This kind of a fluidized bed heat exchanger receives solids flowing down along the internal wall of the furnace into its heat exchange chamber, 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 fluidized bed heat exchanger receives solids from the internal circulation via one or more openings arranged in the wall of the furnace. Often, but not necessarily, the opening/s is/are arranged in the converging bottom area of the furnace i.e. through the inclined wall of the furnace. The solids from the internal circulation enter the fluidized bed heat exchange chamber from above and fill the chamber such that the heat exchange surfaces arranged in the heat exchange chamber are surrounded by the solids i.e. the circulating bed material. A fluidized bed heat exchanger has a bottom surface having nozzles or openings for the fluidizing gas. Below the bottom surface there is at least one wind box for introducing fluidizing gas in the nozzles. Normally there are two wind boxes, the first one below the heat exchange elements of the heat exchange chamber, and the second one/s below the solids discharge channel/s arranged to at least one side of the heat exchange chamber.

[0007] Now that this kind of fluidized bed heat exchangers has been used for a while, it has been learned that there is no possibility of controlling and regulating the internal circulation of solids from the furnace, i.e. from the combustion chamber to the heat exchanger. It means, in practice, that the heat duty of the fluidized bed heat exchanger cannot be influenced. The reason for the above problem is the fact that the gas used for fluidizing the solids in the fluidized bed heat exchange chamber flows from the heat exchange chamber into the furnace via the same opening/s the solids are introduced into the heat exchange chamber from the furnace. In other words, the counter current gas flow prevents some solids from entering the heat exchange chamber. Keeping the above operation principle in mind it is easy to understand that the fluidized bed heat exchanger reaches soon after the start-up of the boiler a state where the fluidization in the heat exchange chamber is appropriate for ensuring an optimal operation of the fluidized bed heat exchanger. The optimal operation requires a certain constant fluidization gas flow, which has a certain constant effect on the solids in the internal circulation, more closely in the solids introduction opening. In other words, the fluidized bed heat exchanger receives a constant amount of solids, a con- stant amount of gas is needed for fluidization, and a constant amount of heat is recovered/collected by the heat exchange surfaces in the heat exchange chamber.

[0008] However, there are some operating conditions where either an additional solids flow into the fluidized bed heat exchanger could be utilized or the solids flow into the fluidized bed heat exchanger could be reduced. For example in dynamic load changes it could be useful to be able to adjust the flow in fluidized bed heat exchanger to minimize the furnace side bed pressure change during the load change.

[0009] An object of the present invention is to find at least one solution to the above discussed problem. [0010] Another object of the present invention is to improve the construction of the fluidized bed heat exchange arrangement such that its operation can be controlled more easily, reliably and comprehensively than before in all running conditions of the boiler arrangement.

[001 1] Yet another object of the present invention is to design a novel discharge flow path for the fluidization gas out of the fluidized bed heat exchanger that gives wider control possibilities than the prior art structures.

[0012] A further object of the present invention is to make it possible to change and regulate the internal circulation by proper configuration of gas flow in the area of a fluidized bed heat exchanger. The internal flow within a fluidized bed heat exchanger can be regulated by adjusting the direction and velocity of the gas flow in the openings of the fluidized bed heat exchanger.

[0013] The above and other objects of the present invention are met with the method and arrangement in accordance with independent claims 1 and 9. [0014] Other features of the method and arrangement of the present invention can be seen in the dependent claims.

[0015] By means of the method and the arrangement of controlling the operation of a fluidized bed boiler 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,

• More accurate control of the heat duty of the fluidized bed heat exchanger.

· Possibility to store the material in the fluidized bed heat exchanger chamber if needed during the dynamic load changes.

[0016] In the following, the method and the arrangement of controlling the operation of a fluidized bed boiler of the present invention will be explained in more detail with reference to the following drawings, of which Fig. 1 is a schematic cross sectional representation of a circulating fluidized bed boiler arrangement of prior art,

Fig. 2 is a schematic partial vertical cross sectional representation of a prior art furnace having a fluidized bed heat exchanger arranged on the outside wall of the bot- torn part of the furnace,

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

Fig. 4 is a partial perspective view of the arrangement of the first preferred embodiment of the present invention.

[0017] 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 or lower part having four side walls of which two are normally, but not necessarily, 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 recirculation 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 air, 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 fur- nace. 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 fluidized 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.

[0018] It is known from prior art, like illustrated in a very schematic manner in Figure 2, that fluidized bed heat exchangers 38 have also been arranged in flow communication with the internal circulation, and advantageously been positioned at the bottom part of the boiler arrangement. These fluidized bed heat exchangers 38 receive a part of the solids flowing down along the internal wall 34 of the furnace 12 i.e. from the internal circulation via a solids introduction opening 40 arranged in the wall 34 of the furnace. 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 fluidized bed heat exchange chamber illustrated in Fig. 2 has a top wall, side walls, and a bottom surface 42 having nozzles or openings for the fluidizing gas. Below the bottom surface 42 there are, in this example, two wind boxes 44 and 48, the first one 44 below the heat exchange elements in the fluidized bed heat exchange 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 via which the solids that have been cooled on the heat exchange surfaces are able to escape to the lift leg. The first wind box 44 is used for providing fluidizing air into the fluidized bed heat exchange chamber 46 for maintaining the bed of solids in the fluidized bed heat exchanger 38 in a fluidized state, whereas the second wind box 48 is needed for providing air for feeding the solids out of the fluidized bed heat exchange chamber 46 along the upright lift leg, i.e. the solids discharge channel 50. Sometimes the number of wind boxes is only one. In such a case the adjustability of the discharge of the solids from the heat exchange chamber is clearly weaker than in the above example having two wind boxes. Sometimes the fluidized bed heat exchanger has been provided with two lift legs arranged in connection with opposite walls of the fluidized bed heat exchange chamber where- by, possibly three wind boxes are needed below the bottom of the fluidized bed heat exchanger.

[0019] The problem leading to the present invention is easy to understand when studying the prior art Fig. 2, and the prior art fluidized bed heat exchanger in more detail. Fig. 2 shows that the heat exchange chamber 46 has one inlet for the solids i.e. the solids introduction opening 40, one inlet (wind box 44) for the fluidization gas, and one outlet (lift leg 50) for the solids, which has its own wind box 48, the purpose of which is partially to prevent the gas flow from the heat exchange chamber 46 to the lift leg 50. Thus, though prior art has not paid any attention to what happens to the fluidization gas introduced into the heat exchange chamber 46 after the gas has done its task and fluidized the solids and by doing that entered above the fluidized bed of solids in the heat exchange chamber 46, it is clear that the gas cannot but escape to the furnace 12 via the same opening 40 the solids are supposed to enter the heat ex- change chamber 46. It is obvious that the counter current gas flow prevents some solids from entering the heat exchange chamber, and thus, in a way, reduces the efficiency of the heat exchange chamber. Naturally the effect of the counter current flow of fluidization gas on the solids depends on the size of the introduction opening 40 and the volume flow of fluidization gas in the fluidized bed heat exchange chamber, just to name a few factors.

[0020] As discussed already earlier the operation of the fluidized bed heat exchanger, soon after starting the boiler, reaches a balance such that a certain amount of solids enter the heat exchange chamber, a certain amount of air is needed for the fluidization the solids in the heat exchange chamber, and a certain amount of heat is recovered from the solids. The described construction of the heat exchange chamber does not allow any adjustment during the operation of the boiler.

[0021] To improve the adjustability and controllability of a fluidized bed boiler arrangement, and more closely that of the fluidized bed heat exchanger the present invention takes into use specific control means arranged between and in communication with the upper part of the fluidized bed heat exchange chamber and the furnace. This control means makes it possible to change and regulate the internal circulation to the fluidized bed heat exchange chamber by proper configuration of the fluidization gas flow in the heat exchanger area. The control means makes it also possible to re- gulate the internal flow in the heat exchanger by adjusting the direction and velocity of the gas in the heat exchange chamber openings. The above objects are met by arranging proper venting in the upper part of the fluidized bed heat exchange chamber.

[0022] Fig. 3 is a schematic cross section of fluidized bed boiler in accordance with a preferred embodiment of the present invention. The fluidized bed boiler, as well as the fluidized bed heat exchanger 38 arranged in connection therewith, is, in principle, similar to that shown in Figs. 1 and 2. The only exceptions can be seen above the fluidized bed heat exchanger 38. The top wall 56 supporting the heat exchanger 38 from the inclined wall 34 of the lower portion of the furnace 12 is in accordance with the present invention provided with an opening 58, whereas the corresponding wall in the prior art structures in Figs. 1 and 2 is closed. The opening 58 leads to a substantially vertically extending chamber 60, which is at its upper end provided with a first end of a further upwardly extending passage 62. The passage 62 is provided with a bend 64 where the passage 62 turns towards the furnace 12. Finally the passage 62 is at its second end fastened to the upright wall 32 of the furnace 12, and the furnace wall 12 has been provided with an outlet opening 66 that opens a flow communication between the furnace 12 and the fluidized bed heat exchange chamber via the passage 62, and the chamber 60. The flow path for fluidization gas extending from opening 58 in the top wall 56 of the heat exchange chamber through chamber 60 and passage 62 to opening 66 in the vertical furnace wall 32 allows the fluidization gas introduced into the heat exchange chamber to escape to the furnace 12 without interfering the introduction of solids into the heat exchanger 38. And fur- ther, the passage 62 is at the bend 64 or close to it provided with means 68 for controlling, on the one hand, the flow of the fluidization gas along the flow path to the furnace 12, and on the other hand, for affecting the flow of solids into the fluidized bed heat exchange chamber. [0023] The above described arrangement may be utilized in several different manners.

[0024] Firstly, it is possible to increase solids flow to the fluidized bed heat exchange chamber from the internal circulation. This may be done by allowing the flui- dization gas enter the above discussed flow path and be taken into the furnace via the flow path, whereby the fluidization gas does not flow countercurrent to the solids entering the heat exchanger. The flow path i.e. the vertically extending chamber 60 and the passage 62 have two preferred, but not completely necessary, features. Firstly, the flow path should have a large cross sectional area to keep the pressure loss at its minimum, and secondly, the opening 66 in the vertical furnace wall should be positioned high up in the furnace wall 32 where the furnace pressure is lower than that at the level of the solids introduction opening 40 at the bottom part of the furnace 12. By taking the above discussed preferred features into use the fluidization gas enters the inventive flow path on its own due to lower counter pressure whereby there is no counter current gas flow in the introduction opening 40, and more solids are able to enter the heat exchanger.

[0025] Secondly, by means of the arrangement of the present invention it is also possible to decrease the solids flow to the fluidized bed heat exchanger. Such a function may be achieved by either reducing the fluidization gas flow along the flow path, or by preventing such totally. Natural results of the above operations are that the fluidization gas will either partially or totally flow counter current to the solids flow, and thus prevent some solids from entering the heat exchange chamber. The fluidization gas escaping the heat exchange chamber has a substantially high velocity in the solids introduction opening 40 that affects the solids flow into the chamber. To achieve the desired control of solids flow into the heat exchange chamber the passage 62 is provided with means 68 for controlling the flow of the fluidization gas along the flow path to the furnace 12. There are various options for the control means 68. A preferred option is a nozzle 68 for introducing gas counter current to the upstream direction of flow of the fluidization gas. By blowing gas downwards from the nozzle 68 the pressure both in the upright chamber 60 and in the heat exchanger above the bed of solids increase, whereby the fluidizing gas is not able to enter the upright chamber 60 as easily as before the gas blow from the nozzle 68 resulting in at least part of the fluidization gas exiting the heat exchanger via the solids introduction opening 40 into the furnace 12.

[0026] By means of the above control means i.e. the gas nozzle and the gas blown therethrough it is not only possible to adjust the fluidization gas flow through the solids introduction opening 40 between 0 and 100% of the fluidization gas used for fluidization of the solids bed in the fluidized bed heat exchange chamber, but also adjust the downward gas flow from the nozzle such that some gas blown through the nozzle 68 will be introduced to the furnace via the same opening 40, whereby the amount of solids entering the fluidized bed heat exchanger 38 may be still reduced. By doing that the resulting counter current gas flow through said solids introduction opening 40 exceeds the fluidization gas flow into the fluidized bed heat exchanger 38. In other words, by means of the present invention it is possible to both increase the solids flow, and decrease the solids flow from that known from prior art fluidized bed heat exchangers.

[0027] Fig. 4 illustrates as a partial perspective view of the arrangement in ac- cordance with the present invention. Actually, Fig. 4 only shows that the substantially vertical chamber 60 collecting the fluidization air from a fluidized bed heat exchanger may be continued by a number of passages 62 such that each passage has its own gas discharge opening 66 in the wall 32 of the furnace. Naturally it is also possible that each substantially vertical chamber extends as a single passage up to the gas discharge opening 66, or that there are two passages 62 for each chamber 60 etc. Also it should be understood that each fluidized bed boiler arrangement may comprise one or more fluidized bed heat exchangers, part or all of which may be position in flow communication with the internal circulation, and, again, part of all of which may be provided with the above discussed inventive fluidization gas flow path.

[0028] 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.