Title of Invention	A FLUIDIZED BED HEAT EXCHANGER FOR A CIRCULATING FLUIDIZED BED BOILER AND A CIRCULATING FLUIDIZED BED BOILER WITH A FLUIDIZED BED HEAT EXCHANGER
Abstract	Abstract A heat exchanger (30) and a circulating fluidized bed boiler (10) with a heat exchanger comprising a first (36) and a second (38) fluidized bed heat 5 exchange chamber arranged in connection with a furnace (12) of the circulating fluidized bed boiler, a first inlet channel (18) for introducing hot solids from a particle separator of the external circulation of the circulating fluidized bed boiler (10) into the first heat exchange chamber (36), a second inlet channel (58) for introducing solids to the second heat exchange chamber (38), first discharge 10 means (54, 56) for removing a first portion of the cooled solids from the first heat exchange chamber (36) to the second inlet channel (58) and second discharge means (61) for removing cooled solids form the second heat exchange chamber (38) to the furnace (12), said heat exchanger comprising inlet means (64) for introducing hot solids directly from the internal circulation of 15 the furnace (12) to the second heat exchange chamber (38). The heat exchanger (30) also preferably comprises third discharge means (72, 74, 76) for removing a second portion of the cooled solids from the first heat exchange chamber (36) directly to the furnace.

Title of Invention

A FLUIDIZED BED HEAT EXCHANGER FOR A CIRCULATING FLUIDIZED BED BOILER AND A CIRCULATING FLUIDIZED BED BOILER WITH A FLUIDIZED BED HEAT EXCHANGER

Abstract

Abstract A heat exchanger (30) and a circulating fluidized bed boiler (10) with a heat exchanger comprising a first (36) and a second (38) fluidized bed heat 5 exchange chamber arranged in connection with a furnace (12) of the circulating fluidized bed boiler, a first inlet channel (18) for introducing hot solids from a particle separator of the external circulation of the circulating fluidized bed boiler (10) into the first heat exchange chamber (36), a second inlet channel (58) for introducing solids to the second heat exchange chamber (38), first discharge 10 means (54, 56) for removing a first portion of the cooled solids from the first heat exchange chamber (36) to the second inlet channel (58) and second discharge means (61) for removing cooled solids form the second heat exchange chamber (38) to the furnace (12), said heat exchanger comprising inlet means (64) for introducing hot solids directly from the internal circulation of 15 the furnace (12) to the second heat exchange chamber (38). The heat exchanger (30) also preferably comprises third discharge means (72, 74, 76) for removing a second portion of the cooled solids from the first heat exchange chamber (36) directly to the furnace.

Full Text	A FLUIDIZED BED HEAT EXCHANGER FOR A CIRCULATING FLUIDIZED BED BOILER AND A CIRCULATING FLUIDIZED BED BOILER WITH A FLUIDIZED BED HEAT EXCHANGER The present invention relates to a fluidized bed heat exchanger for a circulating fluidized bed boiler (CFB boiler), in accordance with the preamble of claim 1, and to a circulating fluidized bed boiler with such a heat exchanger. More specifically, the invention relates to the external hot circulation of the CFB boiler, in other words to an efficient heat exchanger arranged in the return channel for the solids which have been separated from the exhaust gas of the CFB boiler by a particle separator and which are to be returned to the furnace. Especially, the invention relates to arranging an efficient heat exchanger in a supercritical once-through utility boiler, which is provided with reheating. In CFB boilers, generation of hot steam from feed water takes place in several stages; for example, by means of heat exchangers arranged in the backpass of the boiler, by means of water tube panels of the furnace and backpass walls, and in heat exchange chambers arranged in the external hot circulation. When larger and larger CFB boilers are developed which are more and more efficient, heat exchange chambers in the external hot circulation become increasingly important. Therefore, we have to find ways of advantageously providing the boilers with heat exchange chambers which are capable of producing a sufficiently high heat transfer power, still operating flexibly in various operating conditions. Once-through utility boilers (OTU boilers) have the advantage of not needing the density difference between water and steam to provide the driving force for water circulation, to cool the evaporator tubes of the furnace walls. Instead of the density difference the feed water pump of the boiler acts as the driving force for the water circulation. Therefore, in OTU boilers it is possible to heat the steam to high temperatures at pressures above the critical point of water (220 bar), which improves the efficiency of the water vapour generation process of the boiler. In suspension-fired boilers in operation and having capacities of about 1000 MWe, in which the temperature of the flue gas exiting the furnace may be about 1300°C, the achieved end temperature of steam at 300 bar pressure has been 610°C. In CFB boilers where the furnace temperature is typically 850 to 900°C, achieving corresponding steam values and especially a high reheat temperature, e.g. 620°C, calls for new solutions in the designing of boiler heat exchangers. A heat exchanger has a high efficiency when a large amount of solids having a high inlet temperature and low outlet temperature flows through it. In general, it is possible to raise the efficiency of the heat exchanger by increasing its heat exchange surface, which requires that the volume of the fluidized bed in the heat exchange chamber is large enough. Increasing the height of the fluidized bed increases the pressure loss of the fluidizing gas, and growing its width and depth may lead to disadvantageous solutions in view of the structure or space consumption. To avoid these problems, it is advantageous to use at least two separate heat exchange chambers instead of one large heat exchange chamber. US patent No. 5,275,788 discloses a heat exchanger of a CFB boiler, comprising two heat exchange chambers arranged in association with the furnace wall, one on top of the other, but in parallel in view of the particles flow. Desirable portions of the solids separated from the boiler exhaust gas by means of a particle separator may be introduced into these heat exchange chambers. With this kind of a heat exchanger, the solids to be introduced into both heat exchange chambers have the same temperature, and the end temperature of the solids may remain high. Thus, the heat exchange efficiency of the heat exchanger and the adjustability of the heat exchange efficiency may be inadequate, especially at low loads. US patent No. 5,537,941 discloses a heat exchanger with two stacked sections, an upper and a lower section, connected with each other in series, both sections having two heat exchange chambers connected in parallel. Both the upper section and the lower one also comprise a bypass channel through which a portion of the solids entering each section may be passed in a non-cooled condition past the heat exchange chambers into the solids exiting the section. The adjustability of this kind of a heat exchanger is quite good, but even here the efficiency and flexibility of the heat exchanger are not necessarily sufficient in all operational conditions of the boiler. An object of the present invention is to provide a heat exchanger to be arranged in the external hot circulation of a circulating fluidized bed boiler, for reducing the above-mentioned drawbacks of prior art heat exchangers of the circulating fluidized bed boiler. Another object of the invention is especially to provide a heat exchanger to be arranged in the external hot circulation of the circulating fluidized bed boiler, which heat exchanger is applicable to high efficiency once-through utility circulating fluidized bed boilers provided with reheating. A further object of the invention is also to provide a circulating fluidized bed boiler with a heat exchanger as described hereinabove. To solve the above problems involved in prior art, a heat exchanger and a circulating fluidized bed boiler are provided, the characterizing features whereof are disclosed in the characterizing part of the independent apparatus claims. Hence, it is a characterizing feature of the heat exchanger in accordance with the present invention that it comprises a first and a second heat exchange chamber arranged in association with a furnace of a circulating fluidized bed boiler, a first inlet channel for introducing hot solids from the particle separator of the external circulation of the circulating fluidized bed boiler into the first heat exchange chamber which is provided with first means for fluidizing solids, a second inlet channel for introducing solids into the second heat exchange chamber, which is provided with second means for fluidizing solids, first discharge means for removing a first portion of the cooled solids from the first heat exchange chamber into the second inlet channel and second discharge means for removing the cooled solids from the second heat exchange chamber into the furnace, and inlet means for introducing hot solids from the internal circulation of the furnace directly into the second heat exchange chamber. Thus, the present invention offers a new solution for providing an efficient heat exchanger, according to which solution the heat exchanger comprises two heat exchange chambers connected in series in the external hot circulation of a CFB boiler, and means for introducing hot solids from the internal circulation of the furnace directly into the latter heat exchange chamber. In this kind of a heat exchanger, it is possible to obtain a sufficiently high flow of solids, a sufficiently high inlet temperature of the solids and, at the same time, a relatively low outlet temperature of the solids in both heat exchange chambers. in accordance with a preferred embodiment of the present invention, the heat exchanger also comprises third discharge means for removing a second portion of the cooled solids from the first heat exchange chamber directly into the furnace. Preferably, these third discharge means and the first discharge means referred to hereinabove comprise controlling means for controlling the amounts of the first and the second portions of the cooled solids. Thus, it is possible, e.g. to prevent the solids cooled in the first heat exchange chamber from flowing into the second heat exchange chamber, if necessary, in which case solids flow to the second heat exchange chamber directly from the furnace, only. Thus, the inlet temperature of the solids in the second heat exchange chamber is the highest possible, and in the second heat exchange chamber, for example, reheating of the steam returning from the high-pressure turbine to a sufficiently high temperature can be accomplished. A heat exchanger arrangement similar to the one described in the above embodiment is also applicable to a circulating fluidized bed boiler driven by different combustion modes, in which one combustion mode requires efficient cooling of solids in two heat exchanger chambers connected in series, and another combustion mode requires cooling of the solids in one heat exchange chamber, only. In the latter case, it is possible to return the solids to the furnace directly from the first heat exchange chamber, the second heat exchange chamber being not used at all. Of these two combustion modes, the first may correspond, e.g. to combustion in which the oxidizing gas is air enriched with oxygen, or even pure oxygen, whereas the latter corresponds to combustion with ordinary air. The two heat exchange chambers of the heat exchanger in accordance with the present invention, which are connected in series, may be arranged adjacently in connection with the furnace wall of the CFB boiler, but in accordance with an especially advantageous embodiment of the invention, the first heat exchange chamber of the heat exchanger is arranged above the second heat exchange chamber. This embodiment is especially advantageous in large CFB boilers which comprise several efficient and relatively small particle separators, in which case two superposed and separate heat exchange chambers may be arranged in the space remaining below them. In two superposed heat exchange chambers, the pressure loss of the fluidizing gas in the fluidized bed of the solids remains lower than in a corresponding undivided high chamber. When the heat exchanger in accordance with the invention is connected to a supercritical OTU boiler, the end temperature required for superheating may be considerably high, for example 610°C, and the end temperature required for reheating may be even higher, for example 620°C. In this case, when the temperature in the CFB boiler furnace is for example 850 to QOO'C, the heat exchanger containing the last heat exchange surfaces of the boiler steam cycle must be arranged in a very effective manner, in order to gain the desired superheating temperatures. It is possible to advantageously implement this kind of a heat exchanger, in accordance with the present invention, so that the last superheater of the steam cycle is arranged in the first heat exchange chamber of the heat exchanger, and the last reheater of the boiler steam cycle is arranged in the second heat exchange chamber which is most preferably arranged below the first heat exchange chamber. When, according to the invention, hot solids are fed from the furnace directly to one of the heat exchange chambers connected in series, it is possible to achieve a sufficient reheating temperature in this heat exchange chamber, at all loads of the boiler. This results, above all, from the surprising discovery that in large once-through utility circulating fluidized bed boilers, the temperature in the lower section of the boiler is usually higher than the temperature in the upper section of the boiler because of the considerable height of the boiler and the heat exchange surfaces being arranged in the furnace. Therefore, solids fed directly from the internal circulation of the furnace, the temperature of which is close to the temperature of the lower section of the boiler, are hotter than the solids separated from the exhaust gases of the furnace, the temperature of which corresponds to the temperature in the upper section of the boiler. Especially, it has been found that the temperature difference between the upper and lower sections of the CFB boiler furnace becomes more prominent at low loads, when reaching a sufficient reheat temperature with normal modes is especially difficult. Achieving a sufficient superheating temperature, on the other hand, is no problem, because at all loads, the efficiency of the boiler is raised so high that the desired superheating temperature will be reached. The invention is described 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 in accordance with a preferred embodiment of the present invention. Fig. 2 is a schematic vertical cross section of a heat exchanger in accordance with a second preferred embodiment of the present invention. Fig. 3 is a schematic horizontal cross section of a heat exchanger in accordance with a third preferred embodiment of the present invention. Fig. 1 illustrates a CFB boiler 10 in accordance with a preferred embodiment of the present invention, which boiler comprises a furnace 12, an outlet channel 14 connected with the upper section of the furnace, a particle separator 16 for the external hot circulation, connected with channel 14, the lower portion of said particle separator being joined with a return channel 18 which returns the solids separated with particle separator 16 to the lower section of the furnace 12, and the upper portion of said particle separator being joined with a flue gas duct 20 for removing cleaned flue gas to the backpass of the boiler, gas cleaning devices and further through the stack to the environment. (As the last mentioned devices are known from prior art and as they are not specifically part of the present invention, they are not shown in Fig. 1.) The CFB boiler 10, for example, may be of a natural circulation type or a supercritical OTU boiler. The lower section of the furnace 12 is provided with means 22 for feeding fuel, inert bed material and possible sulphur binder to the furnace, and the bottom of the furnace is provided with means for feeding oxide-containing fluidizing gas, in other words a gas inlet channel 24, wind box 26 and nozzles 28. In operation of the boiler, oxide-containing fluidizing gas, for example air, fed through nozzles 28 at an adequate velocity, makes the fuel burn in a fluidized bed, typically at a temperature of about 850 to 900°C, in which case flue gas and entrained solids, primarily ashes, inert bed material and unburned fuel, exit the upper section of the boiler through the outlet channel 14 and enter the particle separator 16. The particle separator 16 separates hot solids from the flue gas, which hot solids are passed through the return channel 18 to the heat exchanger 30, where heat exchange surfaces 32, 34 arranged in said heat exchanger cool the solids before they are returned to the lower section of the furnace 12. A large CFB boiler is usually provided with several parallel particle separators and heat exchangers connected to their return channel, but for clarity reasons. Fig. 1 illustrates an arrangement related to one particle separator, only. Normally, the walls of the furnace 12 are made of water tube panels serving as so-called evaporating 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. The steam temperature is further raised in superheaters, the last stage of said superheaters being normally 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 reaheters, for reheating. Advantageously, the last stage of the reaheaters may be arranged also 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. Steam generation exploiting reheat is known per se, and it is therefore unnecessary to describe it here in further detail. In a preferred embodiment of the present Invention, illustrated in Fig. 1, the heat exchanger 30 comprises a first heat exchange chamber 36 and a second heat exchange chamber 38 arranged below the first heat exchange chamber 36, each heat exchange chamber being provided with a 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, wind box 44, 46 and nozzles 48, 50 for fluidizing the bed of solids being formed in the heat exchange chambers. In accordance with a preferred embodiment shown in Fig. 1, 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 preferably connected with a lifting channel 54, the lower section of said lifting channel being provided with nozzles 56 which make the solids flow at a desired velocity through the heat exchange chamber 36 and to be further discharged through the upper part of the lifting channel 54 into an inlet channel 58 of the second heat exchange chamber. The upper section of the heat exchange chamber 36 is preferably arranged with an overflow channel 60 wherethrough excess solids are discharged if the amount of solids to be discharged through the lifting channel 54 is smaller than the amount of solids entering the heat exchange chamber 36 through the separator 16. The amount of solids passing through the heat exchange chamber is preferably adjustable by means of the lifting channel 54 and overflow channel 60. In some cases, also other known arrangements for the heat exchange chamber may be used, for example, such as is disclosed in US patent No. 5,537,941. 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 the flow of particles entering the heat exchange chamber is passed from the upper part of the lifting channel 54 of the upper 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 heat exchange chamber 38. 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. In accordance with a preferred embodiment of the present invention, shown in Fig. 1, the upper section of the lower heat exchange chamber 38, preferably the inlet channel 58, comprises inlet openings 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 are preferably arranged on the oblique surfaces 66 in the lower section of the furnace, 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. In accordance with a preferred embodiment of the invention, the heat exchange surface 32 of the upper heat exchange chamber 36 is the last superheater of the steam cycle of the boiler 10 and the heat exchange surface 34 of the lower heat exchange chamber 38 is the last reheater of the steam cycle. As the temperature of the furnace in a large once-through utility circulating fluidized bed boiler is at its highest in the lower section of the boiler, especially at low loads, this arrangement is capable of providing a sufficiently high reheat temperature also at low loads. The heat exchange surfaces 32, 34 of the heat exchange chambers 36, 38 may be also other heat exchange surfaces; for example, both of them can be either superheaters or reheaters. Fig. 2 illustrates a heat exchanger 68 in accordance with a second preferred embodiment of the invention. The heat exchanger 68 differs from the heat exchanger 30 shown in Fig. 1 only in that the upper heat exchange chamber 70 of it is provided with a second lifting channel 72 by the side of the lifting channel leading to the lower heat exchange chamber, which second lifting channel passes solids being discharged from the heat exchange chamber 68 along the discharge channel 74 directly to the furnace 12. The lower part of the lifting channel 72 is provided with separate fluidizing gas nozzles 76, so that by passing fluidizing gas either to the first nozzles 56 in accordance with Fig. 1 or to the second nozzles 76 in accordance with Fig. 2, it is possible to introduce solids cooled in the heat exchange chamber 68 either to the lower heat exchange chamber 78 or directly to the furnace 12. The material cooled in the upper heat exchange chamber 70 is preferably passed to the lower heat exchange chamber 78 when it is desirable to recover as much energy as possible from the solids separated by a particle separator. Correspondingly, the material cooled in the upper heat exchange chamber 70 is preferably passed directly to the furnace 12, when it is desirable that the temperature of the solids entering the lower heat exchange chamber is as high as possible, In doing like this, only non-cooled solids enter the lower heat exchange chamber 78, either directly from the internal circulation of the furnace exclusively, through inlet openings 80, or possibly also from the external hot circulation through the overflow channel 82. Fig. 3 is a schematic illustration of a horizontal cross section of a heat exchanger 84 in accordance with a third preferred embodiment of the invention. This embodiment deviates from the embodiments described hereinabove specifically in that it comprises a first heat exchange chamber 86 and a second heat exchange chamber 88, which are connected in series in view of the solids flow and arranged in parallel in association with the wall of the furnace 12. The solids discharged from the particle separator of the external hot circulation of the circulating fluidized bed boiler are passed along the return channel to the first heat exchange chamber 86, from the lower section whereof it is possible to return it to the furnace 12 through a lifting channel 90. If there is not enough fluidizing gas introduced into the lifting channel 90 through fluidizing gas nozzles 92 arranged in the lower part of the channel, the solids entering the heat exchange chamber, or a portion of it, may end up in the furnace 12 through an overflow channel 94 attached to the upper section of the chamber 86. A specific feature of the first heat exchange chamber 86 illustrated in Fig. 3 Is a second lifting channel 96 attached to the lower section thereof, by means of which lifting channel it is possible to pass solids from the lower section of the heat exchange chamber 86 to the upper section of the second heat exchange chamber 88. The lower part of the lifting channel 96 is provided with separate fluidizing gas nozzles 98, so by feeding fluidizing gas in a suitable proportion through the nozzles 92, 98 of the lifting channels 90, 96, it is possible to pass a desirable portion of the material cooled in the first heat exchange chamber 86 to the second heat exchange chamber 88. Preferably, the first and the second heat exchange chambers 86, 88 comprise inlet means corresponding to those illustrated e.g. in Fig. 1, through which inlet means it is possible to feed hot solids also directly from the internal circulation of the furnace 12. Fig. 3 illustrates two heat exchange chambers 86, 88 connected in series. In accordance with another preferred embodiment, the heat exchanger comprises three heat exchange chambers arranged in parallel, the utmost two of said heat exchange chambers serving as first heat exchange chambers, with hot solids directly from the particle separator being introduced into both of them. A third chamber is preferably arranged between the first two chambers so that it is possible to feed cooled solids, if desired, from either of the first chambers or from both of them, to this midmost chamber. Fig. 3 shows, for clarity reasons, that there is only one lifting channel of each type 90, 96, but for structural and operational reasons, it is often better to divide the lifting channel into two or more parallel channels. Thus, for example an overflow channel 94 may preferably be arranged between two parallel lifting channels. Naturally, the number of chambers connected in different manners may also be higher than what has been described above. It is also possible that part of the chambers are connected in parallel, as shown in Fig. 3, and another part in a superposed manner, as shown in Figs. 1 and 2. The invention has been described above with reference to a few exemplary arrangements. These arrangements are not intended to limit the scope of the invention, but the invention is limited only by the accompanying claims and the definitions therein. CLAIMS 1. A heat exchanger (30, 68, 84) of a circulating fluidized bed boiler (10), comprising - a first (36, 70, 86) and a second (38, 78, 88) heat exchange chamber arranged in connection with a furnace (12) of the circulating fluidized bed boiler, - a first inlet channel (18) for introducing hot solids from a particle separator (16) of the external circulation of the circulating fluidized bed boiler (10) into the first heat exchange chamber (36, 70, 86) which is provided with first means (40, 44, 48) for fluidizing solids, - a second inlet channel (58, 96) for introducing solids into the second heat exchange chamber (38, 78, 88) which is provided with second means (42, 46, 50) for fluidizing solids, - first discharge means (54, 56, 98) for removing a first portion of the cooled solids from the first heat exchange chamber (36, 70, 86) to a second inlet channel (58, 96) and - second discharge means (61) for removing cooled solids from the second heat exchange chamber (38, 78, 88) to the furnace (12), characterized in that the heat exchanger comprises inlet means (64, 80) for introducing hot solids from the internal circulation of the furnace (12) directly to the second heat exchange chamber (38, 78, 88). 2. A heat exchanger (30) in accordance with claim 1, characterized in that the heat exchanger comprises third discharge means (72, 74, 76, 90, 92) for removing a second portion of the cooled solids from the first heat exchange chamber (70, 86) directly to the furnace (12). 3. A heat exchanger (30) in accordance with claim 1, characterized in that the first discharge means (54, 56, 98) and the third discharge means (72, 74, 76, 90, 92) comprise controlling means (56, 76, 92, 98) for controlling the amounts of the first and second portions of the cooled solids. 4. A heat exchanger (30) in accordance with any of the above claims, characterized in that the first heat exchange chamber (36, 70) is arranged above the second heat exchange chamber (38, 78). 5. A heat exchanger (30) in accordance with any of the above claims, characterized in that the first heat exchange chamber (36, 70, 86) comprises the last superheater of the boiler steam cycle and that the second heat exchange chamber (38, 78, 88) comprises the last reheater (48) of the boiler steam cycle. 6. A circulating fluidized bed boiler (10), which comprises a furnace (12), a particle separator (16) for the external hot circulation and a heat exchanger (30, 68, 84) arranged in the return channel (18) of the external hot circulation, characterized in that the heat exchanger (30) is in accordance with any of the above claims 1 to 5. 7. A circulating fluidized bed boiler (10), characterized in that the circulating fluidized bed boiler is a supercritical once-through utility boiler.

Full Text

A FLUIDIZED BED HEAT EXCHANGER FOR A CIRCULATING FLUIDIZED BED BOILER
AND A CIRCULATING FLUIDIZED BED BOILER WITH A FLUIDIZED BED HEAT EXCHANGER
The present invention relates to a fluidized bed heat exchanger for a circulating fluidized bed boiler (CFB boiler), in accordance with the preamble of claim 1, and to a circulating fluidized bed boiler with such a heat exchanger. More specifically, the invention relates to the external hot circulation of the CFB boiler, in other words to an efficient heat exchanger arranged in the return channel for the solids which have been separated from the exhaust gas of the CFB boiler by a particle separator and which are to be returned to the furnace. Especially, the invention relates to arranging an efficient heat exchanger in a supercritical once-through utility boiler, which is provided with reheating.
In CFB boilers, generation of hot steam from feed water takes place in several stages; for example, by means of heat exchangers arranged in the backpass of the boiler, by means of water tube panels of the furnace and backpass walls, and in heat exchange chambers arranged in the external hot circulation. When larger and larger CFB boilers are developed which are more and more efficient, heat exchange chambers in the external hot circulation become increasingly important. Therefore, we have to find ways of advantageously providing the boilers with heat exchange chambers which are capable of producing a sufficiently high heat transfer power, still operating flexibly in various operating conditions.
Once-through utility boilers (OTU boilers) have the advantage of not needing the density difference between water and steam to provide the driving force for water circulation, to cool the evaporator tubes of the furnace walls. Instead of the density difference the feed water pump of the boiler acts as the driving force for the water circulation. Therefore, in OTU boilers it is possible to heat the steam to high temperatures at pressures above the critical point of water (220 bar), which improves the efficiency of the water vapour generation process of

the boiler. In suspension-fired boilers in operation and having capacities of about 1000 MWe, in which the temperature of the flue gas exiting the furnace may be about 1300°C, the achieved end temperature of steam at 300 bar pressure has been 610°C. In CFB boilers where the furnace temperature is typically 850 to 900°C, achieving corresponding steam values and especially a high reheat temperature, e.g. 620°C, calls for new solutions in the designing of boiler heat exchangers.
A heat exchanger has a high efficiency when a large amount of solids having a high inlet temperature and low outlet temperature flows through it. In general, it is possible to raise the efficiency of the heat exchanger by increasing its heat exchange surface, which requires that the volume of the fluidized bed in the heat exchange chamber is large enough. Increasing the height of the fluidized bed increases the pressure loss of the fluidizing gas, and growing its width and depth may lead to disadvantageous solutions in view of the structure or space consumption. To avoid these problems, it is advantageous to use at least two separate heat exchange chambers instead of one large heat exchange chamber.
US patent No. 5,275,788 discloses a heat exchanger of a CFB boiler, comprising two heat exchange chambers arranged in association with the furnace wall, one on top of the other, but in parallel in view of the particles flow. Desirable portions of the solids separated from the boiler exhaust gas by means of a particle separator may be introduced into these heat exchange chambers. With this kind of a heat exchanger, the solids to be introduced into both heat exchange chambers have the same temperature, and the end temperature of the solids may remain high. Thus, the heat exchange efficiency of the heat exchanger and the adjustability of the heat exchange efficiency may be inadequate, especially at low loads.
US patent No. 5,537,941 discloses a heat exchanger with two stacked sections, an upper and a lower section, connected with each other in series, both sections having two heat exchange chambers connected in parallel. Both the

upper section and the lower one also comprise a bypass channel through which a portion of the solids entering each section may be passed in a non-cooled condition past the heat exchange chambers into the solids exiting the section. The adjustability of this kind of a heat exchanger is quite good, but even here the efficiency and flexibility of the heat exchanger are not necessarily sufficient in all operational conditions of the boiler.
An object of the present invention is to provide a heat exchanger to be arranged in the external hot circulation of a circulating fluidized bed boiler, for reducing the above-mentioned drawbacks of prior art heat exchangers of the circulating fluidized bed boiler.
Another object of the invention is especially to provide a heat exchanger to be arranged in the external hot circulation of the circulating fluidized bed boiler, which heat exchanger is applicable to high efficiency once-through utility circulating fluidized bed boilers provided with reheating.
A further object of the invention is also to provide a circulating fluidized bed boiler with a heat exchanger as described hereinabove.
To solve the above problems involved in prior art, a heat exchanger and a circulating fluidized bed boiler are provided, the characterizing features whereof are disclosed in the characterizing part of the independent apparatus claims.
Hence, it is a characterizing feature of the heat exchanger in accordance with the present invention that it comprises a first and a second heat exchange chamber arranged in association with a furnace of a circulating fluidized bed boiler, a first inlet channel for introducing hot solids from the particle separator of the external circulation of the circulating fluidized bed boiler into the first heat exchange chamber which is provided with first means for fluidizing solids, a second inlet channel for introducing solids into the second heat exchange chamber, which is provided with second means for fluidizing solids, first discharge means for removing a first portion of the cooled solids from the first

heat exchange chamber into the second inlet channel and second discharge means for removing the cooled solids from the second heat exchange chamber into the furnace, and inlet means for introducing hot solids from the internal circulation of the furnace directly into the second heat exchange chamber.
Thus, the present invention offers a new solution for providing an efficient heat exchanger, according to which solution the heat exchanger comprises two heat exchange chambers connected in series in the external hot circulation of a CFB boiler, and means for introducing hot solids from the internal circulation of the furnace directly into the latter heat exchange chamber. In this kind of a heat exchanger, it is possible to obtain a sufficiently high flow of solids, a sufficiently high inlet temperature of the solids and, at the same time, a relatively low outlet temperature of the solids in both heat exchange chambers.
in accordance with a preferred embodiment of the present invention, the heat exchanger also comprises third discharge means for removing a second portion of the cooled solids from the first heat exchange chamber directly into the furnace. Preferably, these third discharge means and the first discharge means referred to hereinabove comprise controlling means for controlling the amounts of the first and the second portions of the cooled solids. Thus, it is possible, e.g. to prevent the solids cooled in the first heat exchange chamber from flowing into the second heat exchange chamber, if necessary, in which case solids flow to the second heat exchange chamber directly from the furnace, only. Thus, the inlet temperature of the solids in the second heat exchange chamber is the highest possible, and in the second heat exchange chamber, for example, reheating of the steam returning from the high-pressure turbine to a sufficiently high temperature can be accomplished.
A heat exchanger arrangement similar to the one described in the above embodiment is also applicable to a circulating fluidized bed boiler driven by different combustion modes, in which one combustion mode requires efficient cooling of solids in two heat exchanger chambers connected in series, and another combustion mode requires cooling of the solids in one heat exchange

chamber, only. In the latter case, it is possible to return the solids to the furnace directly from the first heat exchange chamber, the second heat exchange chamber being not used at all. Of these two combustion modes, the first may correspond, e.g. to combustion in which the oxidizing gas is air enriched with oxygen, or even pure oxygen, whereas the latter corresponds to combustion with ordinary air.
The two heat exchange chambers of the heat exchanger in accordance with the present invention, which are connected in series, may be arranged adjacently in connection with the furnace wall of the CFB boiler, but in accordance with an especially advantageous embodiment of the invention, the first heat exchange chamber of the heat exchanger is arranged above the second heat exchange chamber. This embodiment is especially advantageous in large CFB boilers which comprise several efficient and relatively small particle separators, in which case two superposed and separate heat exchange chambers may be arranged in the space remaining below them. In two superposed heat exchange chambers, the pressure loss of the fluidizing gas in the fluidized bed of the solids remains lower than in a corresponding undivided high chamber.
When the heat exchanger in accordance with the invention is connected to a supercritical OTU boiler, the end temperature required for superheating may be considerably high, for example 610°C, and the end temperature required for reheating may be even higher, for example 620°C. In this case, when the temperature in the CFB boiler furnace is for example 850 to QOO'C, the heat exchanger containing the last heat exchange surfaces of the boiler steam cycle must be arranged in a very effective manner, in order to gain the desired superheating temperatures. It is possible to advantageously implement this kind of a heat exchanger, in accordance with the present invention, so that the last superheater of the steam cycle is arranged in the first heat exchange chamber of the heat exchanger, and the last reheater of the boiler steam cycle is arranged in the second heat exchange chamber which is most preferably arranged below the first heat exchange chamber.

When, according to the invention, hot solids are fed from the furnace directly to one of the heat exchange chambers connected in series, it is possible to achieve a sufficient reheating temperature in this heat exchange chamber, at all loads of the boiler. This results, above all, from the surprising discovery that in large once-through utility circulating fluidized bed boilers, the temperature in the lower section of the boiler is usually higher than the temperature in the upper section of the boiler because of the considerable height of the boiler and the heat exchange surfaces being arranged in the furnace. Therefore, solids fed directly from the internal circulation of the furnace, the temperature of which is close to the temperature of the lower section of the boiler, are hotter than the solids separated from the exhaust gases of the furnace, the temperature of which corresponds to the temperature in the upper section of the boiler. Especially, it has been found that the temperature difference between the upper and lower sections of the CFB boiler furnace becomes more prominent at low loads, when reaching a sufficient reheat temperature with normal modes is especially difficult. Achieving a sufficient superheating temperature, on the other hand, is no problem, because at all loads, the efficiency of the boiler is raised so high that the desired superheating temperature will be reached.
The invention is described 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 in accordance with a preferred embodiment of the present invention.
Fig. 2 is a schematic vertical cross section of a heat exchanger in
accordance with a second preferred embodiment of the present invention.
Fig. 3 is a schematic horizontal cross section of a heat exchanger in
accordance with a third preferred embodiment of the present
invention.

Fig. 1 illustrates a CFB boiler 10 in accordance with a preferred embodiment of the present invention, which boiler comprises a furnace 12, an outlet channel 14 connected with the upper section of the furnace, a particle separator 16 for the external hot circulation, connected with channel 14, the lower portion of said particle separator being joined with a return channel 18 which returns the solids separated with particle separator 16 to the lower section of the furnace 12, and the upper portion of said particle separator being joined with a flue gas duct 20 for removing cleaned flue gas to the backpass of the boiler, gas cleaning devices and further through the stack to the environment. (As the last mentioned devices are known from prior art and as they are not specifically part of the present invention, they are not shown in Fig. 1.) The CFB boiler 10, for example, may be of a natural circulation type or a supercritical OTU boiler. The lower section of the furnace 12 is provided with means 22 for feeding fuel, inert bed material and possible sulphur binder to the furnace, and the bottom of the furnace is provided with means for feeding oxide-containing fluidizing gas, in other words a gas inlet channel 24, wind box 26 and nozzles 28.
In operation of the boiler, oxide-containing fluidizing gas, for example air, fed through nozzles 28 at an adequate velocity, makes the fuel burn in a fluidized bed, typically at a temperature of about 850 to 900°C, in which case flue gas and entrained solids, primarily ashes, inert bed material and unburned fuel, exit the upper section of the boiler through the outlet channel 14 and enter the particle separator 16. The particle separator 16 separates hot solids from the flue gas, which hot solids are passed through the return channel 18 to the heat exchanger 30, where heat exchange surfaces 32, 34 arranged in said heat exchanger cool the solids before they are returned to the lower section of the furnace 12. A large CFB boiler is usually provided with several parallel particle separators and heat exchangers connected to their return channel, but for clarity reasons. Fig. 1 illustrates an arrangement related to one particle separator, only.

Normally, the walls of the furnace 12 are made of water tube panels serving as so-called evaporating 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. The steam temperature is further raised in superheaters, the last stage of said superheaters being normally 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 reaheters, for reheating. Advantageously, the last stage of the reaheaters may be arranged also 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. Steam generation exploiting reheat is known per se, and it is therefore unnecessary to describe it here in further detail.
In a preferred embodiment of the present Invention, illustrated in Fig. 1, the heat exchanger 30 comprises a first heat exchange chamber 36 and a second heat exchange chamber 38 arranged below the first heat exchange chamber 36, each heat exchange chamber being provided with a 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, wind box 44, 46 and nozzles 48, 50 for fluidizing the bed of solids being formed in the heat exchange chambers.
In accordance with a preferred embodiment shown in Fig. 1, 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 preferably connected with a lifting channel 54, the lower section of said lifting channel being provided with nozzles 56 which make the solids flow at a desired velocity through the heat exchange chamber 36 and to be further discharged through the upper part of the lifting channel 54 into an inlet channel 58 of the second heat exchange chamber. The upper section of the heat exchange

chamber 36 is preferably arranged with an overflow channel 60 wherethrough excess solids are discharged if the amount of solids to be discharged through the lifting channel 54 is smaller than the amount of solids entering the heat exchange chamber 36 through the separator 16. The amount of solids passing through the heat exchange chamber is preferably adjustable by means of the lifting channel 54 and overflow channel 60. In some cases, also other known arrangements for the heat exchange chamber may be used, for example, such as is disclosed in US patent No. 5,537,941.
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 the flow of particles entering the heat exchange chamber is passed from the upper part of the lifting channel 54 of the upper 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 heat exchange chamber 38. 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.
In accordance with a preferred embodiment of the present invention, shown in Fig. 1, the upper section of the lower heat exchange chamber 38, preferably the inlet channel 58, comprises inlet openings 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 are preferably arranged on the oblique surfaces 66 in the lower section of the furnace, 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.
In accordance with a preferred embodiment of the invention, the heat exchange surface 32 of the upper heat exchange chamber 36 is the last superheater of the steam cycle of the boiler 10 and the heat exchange surface 34 of the lower heat exchange chamber 38 is the last reheater of the steam cycle. As the

temperature of the furnace in a large once-through utility circulating fluidized bed boiler is at its highest in the lower section of the boiler, especially at low loads, this arrangement is capable of providing a sufficiently high reheat temperature also at low loads. The heat exchange surfaces 32, 34 of the heat exchange chambers 36, 38 may be also other heat exchange surfaces; for example, both of them can be either superheaters or reheaters.
Fig. 2 illustrates a heat exchanger 68 in accordance with a second preferred embodiment of the invention. The heat exchanger 68 differs from the heat exchanger 30 shown in Fig. 1 only in that the upper heat exchange chamber 70 of it is provided with a second lifting channel 72 by the side of the lifting channel leading to the lower heat exchange chamber, which second lifting channel passes solids being discharged from the heat exchange chamber 68 along the discharge channel 74 directly to the furnace 12. The lower part of the lifting channel 72 is provided with separate fluidizing gas nozzles 76, so that by passing fluidizing gas either to the first nozzles 56 in accordance with Fig. 1 or to the second nozzles 76 in accordance with Fig. 2, it is possible to introduce solids cooled in the heat exchange chamber 68 either to the lower heat exchange chamber 78 or directly to the furnace 12.
The material cooled in the upper heat exchange chamber 70 is preferably passed to the lower heat exchange chamber 78 when it is desirable to recover as much energy as possible from the solids separated by a particle separator. Correspondingly, the material cooled in the upper heat exchange chamber 70 is preferably passed directly to the furnace 12, when it is desirable that the temperature of the solids entering the lower heat exchange chamber is as high as possible, In doing like this, only non-cooled solids enter the lower heat exchange chamber 78, either directly from the internal circulation of the furnace exclusively, through inlet openings 80, or possibly also from the external hot circulation through the overflow channel 82.
Fig. 3 is a schematic illustration of a horizontal cross section of a heat exchanger 84 in accordance with a third preferred embodiment of the invention.

This embodiment deviates from the embodiments described hereinabove specifically in that it comprises a first heat exchange chamber 86 and a second heat exchange chamber 88, which are connected in series in view of the solids flow and arranged in parallel in association with the wall of the furnace 12. The solids discharged from the particle separator of the external hot circulation of the circulating fluidized bed boiler are passed along the return channel to the first heat exchange chamber 86, from the lower section whereof it is possible to return it to the furnace 12 through a lifting channel 90. If there is not enough fluidizing gas introduced into the lifting channel 90 through fluidizing gas nozzles 92 arranged in the lower part of the channel, the solids entering the heat exchange chamber, or a portion of it, may end up in the furnace 12 through an overflow channel 94 attached to the upper section of the chamber 86.
A specific feature of the first heat exchange chamber 86 illustrated in Fig. 3 Is a second lifting channel 96 attached to the lower section thereof, by means of which lifting channel it is possible to pass solids from the lower section of the heat exchange chamber 86 to the upper section of the second heat exchange chamber 88. The lower part of the lifting channel 96 is provided with separate fluidizing gas nozzles 98, so by feeding fluidizing gas in a suitable proportion through the nozzles 92, 98 of the lifting channels 90, 96, it is possible to pass a desirable portion of the material cooled in the first heat exchange chamber 86 to the second heat exchange chamber 88. Preferably, the first and the second heat exchange chambers 86, 88 comprise inlet means corresponding to those illustrated e.g. in Fig. 1, through which inlet means it is possible to feed hot solids also directly from the internal circulation of the furnace 12.
Fig. 3 illustrates two heat exchange chambers 86, 88 connected in series. In accordance with another preferred embodiment, the heat exchanger comprises three heat exchange chambers arranged in parallel, the utmost two of said heat exchange chambers serving as first heat exchange chambers, with hot solids directly from the particle separator being introduced into both of them. A third chamber is preferably arranged between the first two chambers so that it is

possible to feed cooled solids, if desired, from either of the first chambers or from both of them, to this midmost chamber.
Fig. 3 shows, for clarity reasons, that there is only one lifting channel of each type 90, 96, but for structural and operational reasons, it is often better to divide the lifting channel into two or more parallel channels. Thus, for example an overflow channel 94 may preferably be arranged between two parallel lifting channels. Naturally, the number of chambers connected in different manners may also be higher than what has been described above. It is also possible that part of the chambers are connected in parallel, as shown in Fig. 3, and another part in a superposed manner, as shown in Figs. 1 and 2.
The invention has been described above with reference to a few exemplary arrangements. These arrangements are not intended to limit the scope of the invention, but the invention is limited only by the accompanying claims and the definitions therein.

CLAIMS
1. A heat exchanger (30, 68, 84) of a circulating fluidized bed boiler (10), comprising
- a first (36, 70, 86) and a second (38, 78, 88) heat exchange chamber arranged in connection with a furnace (12) of the circulating fluidized bed boiler,
- a first inlet channel (18) for introducing hot solids from a particle separator (16) of the external circulation of the circulating fluidized bed boiler (10) into the first heat exchange chamber (36, 70, 86) which is provided with first means (40, 44, 48) for fluidizing solids,

- a second inlet channel (58, 96) for introducing solids into the second heat exchange chamber (38, 78, 88) which is provided with second means (42, 46, 50) for fluidizing solids,
- first discharge means (54, 56, 98) for removing a first portion of the cooled solids from the first heat exchange chamber (36, 70, 86) to a second inlet channel (58, 96) and
- second discharge means (61) for removing cooled solids from the second heat exchange chamber (38, 78, 88) to the furnace (12),
characterized in that the heat exchanger comprises inlet means (64, 80) for introducing hot solids from the internal circulation of the furnace (12) directly to the second heat exchange chamber (38, 78, 88).
2. A heat exchanger (30) in accordance with claim 1, characterized in that the heat exchanger comprises third discharge means (72, 74, 76, 90, 92) for removing a second portion of the cooled solids from the first heat exchange chamber (70, 86) directly to the furnace (12).
3. A heat exchanger (30) in accordance with claim 1, characterized in that the first discharge means (54, 56, 98) and the third discharge means (72, 74, 76, 90, 92) comprise controlling means (56, 76, 92, 98) for controlling the amounts of the first and second portions of the cooled solids.

4. A heat exchanger (30) in accordance with any of the above claims,
characterized in that the first heat exchange chamber (36, 70) is arranged
above the second heat exchange chamber (38, 78).
5. A heat exchanger (30) in accordance with any of the above claims,
characterized in that the first heat exchange chamber (36, 70, 86) comprises
the last superheater of the boiler steam cycle and that the second heat
exchange chamber (38, 78, 88) comprises the last reheater (48) of the boiler
steam cycle.
6. A circulating fluidized bed boiler (10), which comprises a furnace (12), a
particle separator (16) for the external hot circulation and a heat exchanger (30,
68, 84) arranged in the return channel (18) of the external hot circulation,
characterized in that the heat exchanger (30) is in accordance with any of the
above claims 1 to 5.
7. A circulating fluidized bed boiler (10), characterized in that the circulating
fluidized bed boiler is a supercritical once-through utility boiler.

Documents:

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Patent Number

269952

Indian Patent Application Number

6097/CHENP/2008

PG Journal Number

47/2015

Publication Date

20-Nov-2015

Grant Date

19-Nov-2015

Date of Filing

10-Nov-2008

Name of Patentee

Amec Foster Wheeler Energia Oy

Applicant Address

Metsänneidonkuja 8, FI-02130 Espoo

Inventors:

#	Inventor's Name	Inventor's Address
1	KAUPPINEN, KARI	TUULASTIE 4, FI-78250 VARKAUS

PCT International Classification Number

F22B31/00

PCT International Application Number

PCT/FI07/50264

PCT International Filing date

2007-05-09

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	20065308	2006-05-10	Finland