Method for heating a heat exchange medium in a fluidized bed boiler, fluidized bed boiler, and loop seal heat exchanger

Abstract

A method for heating a heat exchange medium in a fluidized bed boiler (100), the method comprising the steps of: burning a first fuel (165) in a first furnace (162) of the fluidized bed boiler (100) to produce a first flue gas (163); recovering heat from the first flue gas (163) to a heat exchange medium using a first heat exchanger (310); and transporting the heat exchange medium from the first heat exchanger (310) to a second heat exchanger (320), wherein the second heat exchanger (320) The method includes the steps of: disposing at least a portion of a second fuel (175) in contact with the fluidized bed of the fluidized bed boiler (100); burning a second fuel (175) in a second furnace (172) of the fluidized bed boiler (100) to generate a second flue gas (173); transporting a heat exchange medium from the second heat exchanger (320) to a third heat exchanger (330); and recovering heat from the second flue gas (173) to the heat exchange medium using the third heat exchanger (330). A fluidized bed boiler (100) for carrying out this method is provided. The loop-seal heat exchanger (400), when installed in the loop seal of a circulating fluidized bed boiler, is configured to combust a second fuel (175) in a second furnace (172) of the loop-seal heat exchanger (400) to produce a second flue gas (173), transport a heat exchange medium from the second heat exchanger (320) to a third heat exchanger (330), and recover heat from the second flue gas (173) to the heat exchange medium using the third heat exchanger (330).

Description

[Technical Field]

[0001] The present invention relates to fluidized bed boilers. The present invention relates to fluidized bed boilers of the circulating bed type. The present invention relates to loop seal heat exchangers for circulating fluidized bed boilers. The present invention relates to the production of steam by boiling water. The present invention relates to the production of steam at a temperature high enough to be used in steam turbines for power generation. The present invention relates to a method for reducing corrosion of heat transfer surfaces. The present invention relates to the production of steam by burning low quality fuels. The present invention relates to operating a fluidized bed boiler at low loads. [Background technology]

[0002] Superheated steam is required for the efficient generation of mechanical energy from heat, e.g., electricity generation. Saturated steam can be produced by boiling water, and the steam can be further heated (i.e., superheated) in a superheater (i.e., first heat exchanger) that recovers heat from the flue gas. However, when burning low-quality fuels, the flue gas contains large amounts of alkalis and / or halogens, which corrode heat transfer surfaces at certain temperatures. For example, gaseous alkali halides condense on heat transfer surfaces that are cooler than the flue gas. This thus sets an upper temperature limit for superheated steam. For efficiency reasons, higher temperatures of steam are required.

[0003] In a fluidized bed boiler, this limit can be exceeded by applying another superheater (i.e., a second heat exchanger) downstream of the first heat exchanger, which is placed in a fluidized bed of solid particle material. Within the fluidized bed, the flue gas contains much less corrosive components, and heat transfer from the fluidized bed to the second heat exchanger is much better. This reduces corrosion because fewer compounds condense on the heat transfer surfaces and the surface temperatures of the heat transfer surfaces are higher.

[0004] With this design, under normal operating conditions, high temperature steam can be produced by subsequently heating the heat transfer medium in the first and second heat exchangers.

[0005] However, when the load on the fluidized bed boiler decreases, less fuel is burned. As a result, the temperature of the bed material and / or the amount of bed material circulated may become so low that the second heat exchanger may not be able to superheat the steam sufficiently for the steam turbine's purposes. In such a case, the steam turbine must be shut down to prevent turbine failure. Thus, even if that is the purpose of a power plant containing a fluidized bed boiler, it cannot generate electricity. Operating the boiler using excess fuel at low loads would significantly reduce efficiency, even if it were possible. Summary of the Invention

[0006] It has been found that steam coming from the second superheater, i.e., the second heat exchanger, can be further heated in a third heat exchanger. The heat required for the third heat exchanger can be provided by burning a second fuel. Preferably, the second fuel is of high quality to avoid the corrosion problems detailed in the background art of the first heat exchanger. This method is more specifically disclosed in claim 1. The fluidized bed boiler is more specifically disclosed in claim 11. The fluidized bed boiler can be part of a power plant, as disclosed in claim 15. The second fuel can be burned in a loop-seal heat exchanger. The loop-seal heat exchanger is more specifically disclosed in claim 16. The loop-seal heat exchanger is suitable for use in a circulating fluidized bed boiler. The dependent claims specifically disclose several preferred embodiments. The detailed description and drawings disclose these and other embodiments. [Brief explanation of the drawings]

[0007] [Figure 1a] 1 shows a circulating fluidized bed boiler for carrying out a method for heating a heat exchange medium in a fluidized bed boiler; [Figure 1b] 1 shows a bubbling fluidized bed boiler for carrying out a method for heating a heat exchange medium in a fluidized bed boiler. [Figure 2a] FIG. 1 is a diagram illustrating operation of a fluidized bed boiler at low load. [Figure 2b]FIG. 1 is a diagram illustrating operation of a fluidized bed boiler at high load. [Figure 2c] FIG. 10 is a diagram illustrating the operation of a damper. [Figure 3] FIG. 1b is a cross-sectional view of the loop-seal heat exchanger shown in FIG. 1a taken along line III-III. [Figure 4] FIG. 4 is a cross-sectional view of the loop-seal heat exchanger shown in FIG. 3 taken along line IV-IV. [Figure 5] FIG. 4 is a VV cross-sectional view of the loop-seal heat exchanger shown in FIG. 3. [Figure 6] 6 is a cross-sectional view of the loop-seal heat exchanger shown in FIG. 3 taken along the line VI-VI. [Figure 7] FIG. 7 is a cross-sectional view of the loop-seal heat exchanger shown in FIG. 3 taken along line VII-VII. [Figure 8a] FIG. 1 illustrates a method for circulating a second flue gas used as a fluidizing gas. [Figure 8b] FIG. 1 illustrates a method for circulating a second flue gas used as a fluidizing gas. [Figure 9a] FIG. 1 is a cross-sectional view of a loop-seal heat exchanger. [Figure 9b] 9b is a cross-sectional view of the loop-seal heat exchanger shown in FIG. 9a taken along line IXb-IXb. [Figure 10a] FIG. 1 is a cross-sectional view of a loop-seal heat exchanger. [Figure 10b] 10b is a cross-sectional view of the loop-seal heat exchanger shown in FIG. 10b taken along the line Xb-Xb. [Figure 10c] 10b is a cross-sectional view of the loop-seal heat exchanger of FIG. 10b, with a portion of the wall formed by a damper, the damper being in a first position. [Figure 10d] FIG. 10c is a cross-sectional view of the loop-seal heat exchanger of FIG. 10c, with the damper in a second position. [Figure 10e] FIG. 10 illustrates the recovery of heat from excess secondary flue gas to heat fluidizing gas in a chamber containing a second heat exchanger. [Figure 10f] FIG. 10 illustrates the use of excess secondary flue gas as a fluidizing gas in the first furnace. [Figure 11]FIG. 4 is a horizontal cross-sectional view of a loop-seal heat exchanger having fewer chambers than the loop-seal heat exchanger of FIG. 3. [Figure 12] FIG. 9c is a vertical cross-sectional view of a loop-seal heat exchanger with fewer chambers than the loop-seal heat exchanger of FIG. 9b. [Figure 13] FIG. 10c is a vertical cross-sectional view of a loop seal heat exchanger with fewer chambers than the loop seal heat exchanger of FIG. 10b. [Figure 14] FIG. 1 is a cross-sectional view of a loop-seal heat exchanger. [Figure 15] 1 shows the integrated header of the second and third heat exchangers. In the figure, Sx, Sy, and Sz indicate three mutually orthogonal directions oriented according to the right-hand rule. That is, the vector product (i.e., cross product) of Sx and Sy, in that order, is equal to Sz. In use, the direction Sz is vertically upwards, i.e., against the direction of gravity. DETAILED DESCRIPTION OF THE INVENTION

[0008] Figures 1a and 1b show an embodiment of a fluidized bed boiler 100. The fluidized bed boiler in Figure 1a is a circulating fluidized bed boiler 100. The fluidized bed boiler in Figure 1b is a bubbling fluidized bed boiler 100.

[0009] 1a and 1b, the fluidized-bed boiler 100 includes a first furnace 162 for burning a first fuel 165 to generate a first flue gas 163. The first fuel may be low-quality. Typically, the first fuel 165 includes a solid material, such as biomass and / or residue-derived fuel. To recover heat from the first flue gas 163, the fluidized-bed boiler 100 includes a first heat exchanger 310 for recovering heat from the first flue gas 163 to a heat exchange medium. The first heat exchanger 310 may be a superheater, i.e., a heat exchanger configured to receive and heat steam. The heat exchange medium includes at least one of water and steam. For example, saturated steam is composed of both gaseous and liquid H2O, i.e., steam and water. However, superheated steam does not contain liquid water. As described in detail in the Background section, the purpose of a fluidized-bed boiler is to generate superheated steam from water.

[0010] For the reasons mentioned in the background section, the steam coming from the first heat exchanger 310 needs to be further heated. Thus, the fluidized bed boiler 100 includes a second heat exchanger 320 and a first pipeline 312 for transporting the heat exchange medium from the first heat exchanger 310 to the second heat exchanger 320. Preferably, the first pipeline 312 does not include a heat exchanger configured to heat or cool the heat exchange medium between the first heat exchanger 310 and the second heat exchanger 320. Thus, preferably, in the direction of steam flow within the steam circuit of the fluidized bed boiler 100, the first heat exchanger 310 is the last heat exchanger, such that no other heat exchanger arranged to contact the first flue gas 163 is arranged downstream of the first heat exchanger 310. Thus, in such a case, the first heat exchanger 310 is also the last heat exchanger (see FIGS. 1a and 1b) that is not positioned to contact only the first flue gas 163, a heat exchanger whose steam, in use, is at least as hot as the first heat exchanger 310. Of course, downstream of the steam turbine 152, the steam may be condensed and recycled to the economizer 122 in contact with the first flue gas 163. Furthermore, the second heat exchanger 320 is not positioned to contact only the first flue gas, but is in contact with the bed material. Furthermore, as will be described in more detail below, the third heat exchanger 330 preferably does not contact the first flue gas 163 at all.

[0011] The second heat exchanger 320 is positioned such that, in use, a fluidized bed of bed material is configured to contact the second heat exchanger 320. The bed material comprises a solid, refractory granular material. The bed material contacting the second heat exchanger 320 can thus be fluidized by injecting a sufficient amount of fluidizing gas therein. The bed material is refractory so as not to combust within the first furnace 162. The advantages of contacting the second heat exchanger with a fluidized bed are discussed in the Background section.

[0012] For example, the second heat exchanger 320 may be located within the first chamber (412, 162) of the fluidized bed boiler 100, or within a wall of the first chamber (412, 162), within which the fluidized bed is formed during use. In this sense, the first furnace 162 may be considered a chamber.

[0013] 1a, in a circulating fluidized bed boiler, the first chamber 412 may be the chamber of the loop-seal heat exchanger 400. Heat exchange surfaces, such as heat exchange pipes, of the second heat exchanger 320 may be disposed within the first chamber 412. Additionally or alternatively, heat exchange surfaces, such as heat exchange pipes, of the second heat exchanger 320 may be disposed on the walls of the first chamber 412.

[0014] 1b, in a bubbling fluidized bed boiler, the first furnace 162 can function as a first chamber. A heat exchange surface, such as a heat exchange pipe of the second heat exchanger 320, can be disposed within the lower portion of the first furnace 162 such that a bubbling fluidized bed is formed in the lower portion of the first furnace 162.

[0015] 1a and 1b, to further heat the heat exchange medium, the fluidized bed boiler 100 includes a second furnace 172 for combusting a second fuel 175 to generate a second flue gas 173, and a third heat exchanger 330 for recovering heat from the second flue gas 173 to the heat exchange medium received from the second heat exchanger 320. FIG. 2a shows in more detail the second furnace 172, the second fuel 175, the second flue gas 173, the third heat exchanger 330 disposed within the second chamber 422, and a passage 178 that conveys the second flue gas 173 to the second chamber 422. As shown in FIG. 2a, a burner 176 is disposed within the second furnace 172. The burner 176 is configured to combust the second fuel 175. This applies to at least one of the fluidized bed boiler 100 and the loop-seal heat exchanger for a circulating fluidized bed boiler 400. In this manner, the fluidized bed boiler 100 or the loop-seal heat exchanger 400 includes a burner 176 disposed within the second furnace 172. The burner 176 may be configured to combust a gaseous or liquid second fuel 175. Even if not explicitly shown, the burner 176 may be similarly disposed in the second furnace 172 of the other figures.

[0016] 1a, the second and third heat exchangers 320, 330 may be positioned close to each other, such that the heat exchange medium can flow directly from the second heat exchanger 320 to the third heat exchanger 330, for example, via a short pipe, which may be considered part of one of the heat exchangers 320, 330 or part of the second pipeline 322.

[0017] 1b, the fluidized bed boiler 100 may include a second pipeline 322 for transporting the heat exchange medium from the second heat exchanger 320 to the third heat exchanger 330. Preferably, the second pipeline 322 does not include a heat exchanger configured to heat or cool the heat exchange medium between the second heat exchanger 320 and the third heat exchanger 330.

[0018] 2a and 2b, the loop-seal heat exchanger 400 for the fluidized bed boiler 100 and / or circulating fluidized bed boiler can operate in two modes. Referring to FIG. 2a, in the first mode (i.e., the first period), the second furnace 172 is used to combust the second fuel 175, thus further heating the heat exchange medium through the second flue gas 173. In the first mode, the second heat exchanger 320 does not need to heat the heat exchange medium. Referring to FIG. 2b, in the second mode (i.e., the second period), the second furnace 172 is used without use (i.e., the second fuel 175 is not combusted), or a much smaller amount of the second fuel 175 is combusted. Thus, in effect, the third heat exchanger 330 is not used.

[0019] 1a and 1b, in a first mode and a second mode, a method of heating a heat exchange medium in a fluidized bed boiler 100 includes combusting a first fuel 165 in a first furnace 162 of the fluidized bed boiler 100 to generate a first flue gas 163 and recovering heat from the first flue gas 163 to a heat exchange medium using a first heat exchanger 310. From the first heat exchanger 310, the heat exchange medium is conveyed to a second heat exchanger 320. As described in detail above, at least a portion of the second heat exchanger 320 is positioned in contact with the fluidized bed of the fluidized bed boiler 100. More preferably, all heat transfer surfaces of the second heat exchanger 320 are positioned in contact with the fluidized bed of the fluidized bed boiler 100. The fluidized bed may be positioned in a loop-seal heat exchanger 400 (FIG. 1a) or the first furnace 162 (FIG. 1b). The method further includes conveying the heat exchange medium through the second heat exchanger 320 and from the second heat exchanger 320 to a third heat exchanger 330 .

[0020] Referring to FIG. 2a, in a first mode, i.e., a first time period, the method includes combusting a second fuel 175 in a second furnace 172 of the fluidized bed boiler 100 to generate a second flue gas 173 and recovering heat from the second flue gas 173 to a heat exchange medium using a third heat exchanger 330. The first mode may correspond to an operating condition in which the fluidized bed boiler 100 is under a low load. Thus, in one embodiment, during the first time period, the load of the fluidized bed 100 is below a threshold. Also, because the fluidized bed in which at least a portion of the second heat exchanger 320 is disposed does not need to be hot during the first time period, there is no need to heat the heat exchange medium in the second heat exchanger 320, although the heat exchange medium is heated in the third heat exchanger 330. Furthermore, the fluidized bed is typically slightly hotter than the heat exchange medium flowing through the second heat exchanger 320. Thus, preferably, the method also includes, during the first period, recovering heat from the fluidized bed of the fluidized bed boiler 100 to a heat exchange medium using a second heat exchanger 320, at least a portion of which (the second heat exchanger 320) is positioned in contact with the fluidized bed of the fluidized bed boiler 100.

[0021] 1a and 1b for disposing at least a portion of the second heat exchanger 320 in contact with the fluidized bed of the fluidized bed boiler 100. In the case of the circulating fluidized bed boiler 100 (FIG. 1a), the circulating fluidized bed is disposed within the first furnace 162. Heating the heat exchange medium within the circulating fluidized bed boiler includes circulating the bed material from the first furnace 162 to the cyclone 132, from the cyclone 132 to the loop seal 140, and from the loop seal 140 to the first furnace 162, for example, via the return channel 136. However, the loop seal 140 may also be disposed in contact with the wall of the first furnace 162, thereby shortening the return channel 136 to, for example, an opening in the wall. In use, another fluidized bed is disposed within the loop seal 140 of the circulating fluidized bed boiler 100 to facilitate circulation of the bed material through the loop seal. In the embodiment of Figure 1a, the second heat exchanger 320 is located at the loop seal 140 of the fluidized bed boiler 100. This is also the preferred location for the second heat exchanger 320 for several reasons: - Less corrosion than the first furnace due to lower particle velocity, - less corrosive than the first furnace, since the cyclone 132 separates the corrosive first flue gas 163 into the flue gas channel 120; - Easier integration with the second furnace 172 and the third heat exchanger 330.

[0022] The bed material may be conveyed from the bottom of the cyclone 132 via a dipleg channel 134 to a loop seal heat exchanger 400 located within the loop seal 140. The term dipleg refers to a channel configured to allow the bed material to flow primarily downward. From the loop seal heat exchanger 400, the bed material is configured to return to the first furnace 162 via a return channel 136.

[0023] The fluidized bed boiler 100 (either circulating or bubbling) and / or the loop-seal heat exchanger 400 for the circulating fluidized bed boiler 100 may include a damper, i.e., a third damper 475 as shown in Figure 2c. When the third damper 475 is used, in a first mode, i.e., during a first time period, the third damper 475 is in an open position to allow circulation of the second flue gas 173.

[0024] Referring to FIG. 2b, in a second mode, i.e., a second time period, the method includes recovering heat by a second heat exchanger 320 from a fluidized bed to a heat exchange medium disposed in contact with at least a portion of the second heat exchanger 320.

[0025] Preferably, the fluidized bed boiler 100 is designed so that it is not necessary to control the circulation of the heat exchange medium when switching from the first mode to the second mode. Thus, in one embodiment, even in the second mode, the method includes the steps of conveying the heat exchange medium from the second heat exchanger 320 to the third heat exchanger 330 and conveying the heat exchange medium through the third heat exchanger 330. Thus, the heat exchange medium can be circulated in the second mode in a manner similar to the first mode. This may not be necessary to heat the heat transfer medium, which reduces the investment cost of the fluidized bed boiler 100 and improves its robustness.

[0026] However, in the second mode, the heat exchange medium is conveyed through the third heat exchanger 330 without burning the second fuel 175 in the second furnace 172 during the second time period. This can be done to reduce use of the second fuel 175. Alternatively, a smaller amount of the second fuel 175 (in terms of average mass per hour) can be burned during the second time period than during the first time period. For example, the consumption of the second fuel 175 (in terms of average mass per hour) during the second time period may be less than half, less than a quarter, or less than a tenth of the consumption of the second fuel 175 (in terms of average mass per hour) during the first time period.

[0027] Because the heat exchange medium circulates through the third heat exchanger 330 even in the second mode, i.e., during the second time period, the fluidized bed boiler 100 and / or the loop-seal heat exchanger 400 for the fluidized bed boiler 100 may include a third damper 475. The purpose of the third damper is to prevent air circulation through the third heat exchanger 330 in the second mode. Thus, during the second time period, the third damper 475 may be in a closed position, as shown in FIG. 2c. In this manner, heat loss to air circulation is minimized during the second time period. However, depending on the design details, the third damper 475 may not be required because natural convection of air through the third heat exchanger 330 is very small. The third damper 475 may be slidable, for example. The third damper 475 may be pivotable, for example, about an axis 476 (see FIG. 2c). Additional or other dampers may be used for purposes described in more detail below. 2c, the third damper 475 is disposed downstream of the third heat exchanger 330 (downstream in the flow direction of the second flue gas 173 during the first period). Although not shown, the third damper 475 may be disposed upstream of the burner 176. Also, although not shown, the third damper 475 may be disposed downstream of the burner 176 and upstream of the third heat exchanger 330. During the second period, even if a damper is used, the steam may be slightly cooled while propagating through the third heat exchanger 330.

[0028] The second mode can accommodate operating conditions where the load on the fluidized boiler 100 is high. Thus, in one embodiment, during the second time period, the load on the fluidized boiler 100 is greater than during the first time period. For example, during the second time period, the load on the fluidized boiler 100 can be at least equal to a threshold value. See the load threshold discussed in connection with the first mode. While the exact value of the threshold value will vary depending on the specifics of the case, for a typical circulating fluidized bed boiler, the threshold value can be, for example, 30% to 70% of the fluidized bed boiler's maximum load, such as 50% of the fluidized bed boiler's maximum load.

[0029] In a preferred embodiment, the third heat exchanger 330 is not in contact with the first flue gas 163. This has the advantage that corrosive compounds in the first flue gas 163 will not corrode the third heat exchanger 330, even if the first flue gas 163 is produced using a low-quality fuel as the first fuel 165.

[0030] In a preferred embodiment, the third heat exchanger 330 is not in contact with the fluidized bed of the fluidized bed boiler 100. In particular, in one embodiment, the third heat exchanger 330 is not in contact with the same fluidized bed that is in contact with the second heat exchanger 320. Preferably, the third heat exchanger 330 is not in contact with the fluidized bed composed of fluidized solid, refractory, granular material. This has the advantage that the heat of the second fuel 175 can be directly utilized in the third heat exchanger 330. Thus, the heat of the second flue gas 173 is not consumed to heat the bed material of the fluidized bed. Furthermore, since the bed material does not need to be heated when switching from the second mode to the first mode, the process can be controlled more quickly. Also, the bed material does not need to be cooled when switching from the first mode to the second mode. Furthermore, problems associated with agglomeration and / or sintering of bed material are avoided, at least near the third heat exchanger 330, when the third heat exchanger 330 is not in contact with the fluidized bed of the fluidized bed boiler 100. Furthermore, because the heat exchanger 330 is not in contact with the fluidized bed of the fluidized bed boiler 100, the surfaces of the third heat exchanger 330 are avoided from problems associated with erosion of the heat exchanger surface by the bed material.

[0031] The method and fluidized bed boiler allow for the use of the fluidized bed boiler 100 at low loads (by operating in the first mode) and also allows for the use of the boiler 100 when a lower quality fuel is used as the first fuel 165. As used herein, the term fuel quality refers to at least the total alkali and halogen content of the fuel (as it applies to the first fuel 165 and the second fuel 175). The term alkali refers to elements in Group 1 of the IUPAC Periodic Table of Elements, excluding hydrogen, and the term halogen refers to elements in Group 17 of the IUPAC Periodic Table of Elements. The alkali and halogen are typically included in the compound of the fuel. Upon combustion, at least a portion of the alkali and halogen are ultimately included in the flue gas (this applies to the first flue gas 163 and the second flue gas 173, although the second fuel may be substantially free of alkali and halogen). In the flue gas, these elements typically form alkali halides, i.e., compounds containing alkali and halogen elements, examples of which include NaCl, NaF, NaBr, KCl, KF, and KBr. Some of the alkali and halogen may remain in the ash. Corrosion problems associated with low-quality fuels are avoided by using high-quality fuels as the second flue gas 175.

[0032] Thus, in one embodiment, the second flue gas 173 is lower in alkali and halogen than the first flue gas 163. The alkali and / or halogen may be comprised by alkali halides in the flue gases (163, 173). These compounds are due to the quality of the fuel. Thus, in the same or another embodiment, the second fuel 175 is lower in alkali and halogen than the first fuel 165. More specifically, the content of compounds containing alkali and / or halogen in the second fuel 175 is lower than the content of compounds containing alkali and / or halogen in the first fuel 173.

[0033] In a preferred embodiment, the second fuel 175 contains less than 500 ppm or less than 100 ppm by weight of alkali and halogen atoms. Of course, the atoms are not free but are part of the chemical composition of the second fuel 175. Furthermore, in one embodiment, the second flue gas 173 contains less than 500 ppm, preferably less than 100 ppm by weight of alkali halides.

[0034] Typically, the high-quality fuel is a gas or liquid, such as natural gas or diesel. Thus, in one embodiment, the second fuel 175 is a liquid or gas, such as a gas containing natural gas, or a liquid containing oil, such as diesel, at a temperature of 20° C. and a pressure of 1 atmosphere. More preferably, to ensure smooth delivery of the second fuel 175, the second fuel is free of solid particles at the aforementioned temperature and pressure.

[0035] However, as indicated above, first fuel 165 does not need to be of high quality. Furthermore, to save on operating costs, first fuel 164 is preferably of low quality. Thus, in one embodiment, first fuel 165 comprises solid material such as biomass and / or residue-derived fuel at a temperature of 20° C. and a pressure of 1 atmosphere. Even more preferably, at the aforementioned temperatures and pressures, [A] second fuel 175 is gaseous or liquid, and [B] first fuel 165 comprises solid material.

[0036] As described in detail above, preferably, the second furnace 172 is disposed in the loop seal 142 of the circulating fluidized bed boiler. More preferably, the second furnace 172 and the third heat exchanger 330 are disposed as part of the loop seal heat exchanger 400. Figures 3 to 7 show the details of the loop seal heat exchanger 400. When the loop seal heat exchanger 400 is installed in the loop seal of the circulating fluidized bed boiler 100, - burning a second fuel 175 in a second furnace 172 of a loop seal heat exchanger 400 to produce a second flue gas 173; - conveying the heat exchange medium from the second heat exchanger 320 to the third heat exchanger 330 of the loop seal heat exchanger 400; and - configured to recover heat from the second flue gas 173 to a heat exchange medium using a third heat exchanger 330.

[0037] Other components of the fluidized bed boiler 100 shown in FIG. 1a for purposes of understanding the embodiment include a feeder 164 configured to supply a first fuel 165 into the first furnace 162 and air channels 104 for supplying combustion air 106 to the first furnace 162. The combustion air 106 serves as an oxygen source for combustion and as (at least a portion of) the fluidizing gas. As described in more detail below, the term "combustion air" may refer to a mixture of air and some other gas, particularly a mixture of air and a secondary flue gas 173. As described in more detail below, other gases, particularly the secondary flue gas, may also be used to further fluidize the material in the first furnace 162.

[0038] Between the air channels 104, ash channels 112 are provided for removing bottom ash from the first furnace 162. A bottom ash cooler 114 is configured to receive the hot bottom ash and recover heat therefrom. A first heat exchanger 310 is arranged in the flue gas duct 120. The fluidized bed boiler may include further heat exchangers, such as an economizer 122, configured to recover heat from the first flue gas 163. Generally, an economizer is a heat exchanger that receives a liquid heat exchange medium, in particular water. Typically, water is heated but does not boil in the economizer. In the direction of flow of the heat exchange medium, the economizer 122 is arranged upstream of the first heat exchanger 310. A drum 124 may be provided between the first heat exchanger 310 and the economizer 122 to separate the liquid portion of the heat exchange medium (e.g., water) from the gas portion of the heat exchange medium (e.g., steam). Additionally, another heat exchanger (not shown) may be connected to the drum 124 to boil water to produce saturated steam.

[0039] The heated heat exchange medium is preferably used to generate mechanical energy in a steam turbine 152. Thus, in one embodiment of the method, the heat exchange medium includes steam, and the method includes conveying the steam from a third heat exchanger 330 to the steam turbine 152. Preferably, the steam is conveyed from the third heat exchanger 330 to the steam turbine 152 such that there is no heat exchanger between the third heat exchanger 330 and the steam turbine 152. Thus, preferably, the third heat exchanger 330 is the last heat exchanger before the steam turbine 152. Similarly, the power plant includes a fluidized bed boiler 100 and a steam turbine 152, as shown in FIGS. 1a and 1b. The power plant further includes a third pipeline 332 configured to convey the heat exchange medium from the third heat exchanger 330 to the steam turbine 152. Preferably, the third pipeline 332 does not include a heat exchanger configured to heat or cool a heat exchange medium between the third heat exchanger 330 and the steam turbine 152 .

[0040] Preferably, the method further includes using the steam turbine 152 to operate a generator 155, thus generating electricity. The corresponding power plant includes the generator 155 arranged in mechanical connection with the steam turbine 152. For example, a shaft may be configured to be rotated by the steam turbine 152 to operate the generator, i.e., rotate components of the generator 155.

[0041] 3 shows a cross-sectional view of an embodiment of a loop-seal heat exchanger 400 from above (see FIG. 1a for section line III-III). In use, the loop-seal heat exchanger 400 is placed in the loop seal 140 of the circulating fluidized bed boiler 100. Thus, the loop-seal heat exchanger 400 is suitable for such purposes even though it is not part of the boiler. The first chamber 412 of the fluidized bed boiler 100 is, in this embodiment, the first chamber 412 of the loop-seal heat exchanger 400. Furthermore, in this embodiment, the third heat exchanger 330 is placed in the second chamber 422 of the loop-seal heat exchanger 400.

[0042] The walls of the loop-seal heat exchanger 400 define a first chamber 412, a second chamber 422, and a second furnace 172 (i.e., the loop-seal heat exchanger 400 includes the first chamber 412, the second chamber 422, and the second furnace 172), the function of which has been described in detail above. The second heat exchanger 320 is disposed in the first chamber 412, the third heat exchanger 330 is disposed in the second chamber 422, and a second pipeline 322 connects the second heat exchanger 320 and the third heat exchanger 330. The second pipeline 322 may run only within the chambers 412, 422 as shown in FIG. 3, or a portion of the second pipeline may run outside the chambers 412, 422 as shown in FIG. 9b.

[0043] The loop-seal heat exchanger includes an inlet chamber 431 (see FIG. 1a) into which circulating bed material enters from dipleg channels 134. From inlet chamber 431, the bed material flows to at least one of an inlet chamber 433 and a bypass chamber 432 (see FIG. 3).

[0044] The bypass chamber 432 has two functions. First, heat recovery by the second heat exchanger 320 can be controlled by controlling the amount of bed material flowing through the bypass chamber 432. The bypass chamber 432 does not have a heat exchanger surface for heating the heat exchange medium. Thus, by directing the flow of bed material solely or primarily through the bypass chamber rather than through the first chamber 412, the heat exchange medium is heated to a lesser extent in the second heat exchanger 320. Second, the bypass chamber 432 functions as a gas lock. The bypass chamber 432 is an upleg; that is, bed material flows primarily upward within the bypass chamber 432. This, in conjunction with the dipleg channel 134, provides a gas lock that prevents bed material from flowing in the wrong direction, i.e., from the first furnace 162 to the loop-seal heat exchanger 400.

[0045] The inlet chamber 433 is also designed as an upleg. Thus, in combination with the dipleg channel 134, the inlet chamber 433 primarily functions as a gas lock. Furthermore, another function of the inlet chamber 433 is to supply the bed material to the first chamber 412. From the first chamber 412, which is formed as a dipleg, the bed material flows into the outlet chamber 435, which is designed as an upleg.

[0046] In the embodiment of Figure 3, the second heat exchanger 320 is disposed inside the first chamber 412 of the loop-seal heat exchanger 400, and the third heat exchanger 330 is disposed inside the second chamber 422 of the loop-seal heat exchanger 400. Locating the heat exchanger within a chamber is preferred because it increases the heat transfer surface area and facilitates maintenance. However, the second heat exchanger 320 could be disposed within the wall of the first chamber 412. Similarly, the third heat exchanger 330 could be disposed within the wall of the second chamber 422.

[0047] From the outlet chamber 435 and / or the bypass chamber 432, the bed material flows into the return channel 136. The direction of bed material flow is indicated by arrows 451, 452, 453, 454, 455, and 457 in FIG. 3. The geometric details of the loop-seal heat exchanger 400 can be varied. For example, as shown in FIG. 14, a portion of the return channel 136 can be disposed between parallel walls of the bypass chamber 432 and the outlet chamber 435, with the bed material configured to flow through openings in these parallel walls into the return channel 136. Other types of loop-seal heat exchangers are described in more detail below.

[0048] As such, the present invention also relates to a new type of fluidized-bed heat exchanger 400. As described in detail above and shown in FIGS. 3-10b and 14, one embodiment of the fluidized-bed heat exchanger 400 includes a first chamber 412 and a second chamber 422. The second heat exchanger 320 is disposed within (i.e., in) the first chamber 412 or within (i.e., as part of) a wall that bounds the first chamber 412. The third heat exchanger 330 is disposed within (i.e., in) the second chamber 422 or within (i.e., as part of) a wall that bounds the second chamber 422. The loop-seal heat exchanger 400 includes a second pipeline 322 for conveying a heat exchange medium from the second heat exchanger 320 to the third heat exchanger 330, and may include a first nozzle 462 configured to fluidize the bed material in the first chamber 412. To further heat the heat exchange medium already in the loop-seal heat exchanger 400, the loop-seal heat exchanger 400 includes a second furnace 172 that combusts a second fuel 175 to produce a second flue gas 173, and a passage 178 for conveying the second flue gas 173 to a second chamber 422.

[0049] In Figures 3, 11, and 14, the second chamber 422 is located next to the first chamber 412. Additionally, in Figures 3, 11, and 14, the second furnace 172 is located either next to the inlet chamber 433 (Figures 3 and 14) or next to the inlet chamber 431 (Figure 11). Both of these features, alone or in combination, are beneficial from an energy recovery perspective. With this arrangement, the second furnace 172 is heated through the first wall 441. Thus, the second furnace 172 is heated not only by the combustion of the second fuel but also by the heated bed material located in the adjacent chamber. Similarly, the second chamber 422 is heated through the first wall 441. Thus, the second chamber is heated not only by the combustion of the second fuel but also by the heated bed material. Thus, in one embodiment, a first side of the wall of the loop-seal heat exchanger 400 (i.e., a first side of the first wall 441) defines the first chamber 412, and a second side opposite the wall of the loop-seal heat exchanger 400 (i.e., the first wall) defines the second chamber 422. Furthermore, the first wall 441 preferably includes a heat transfer tube for transferring heat to a heat exchange medium. This applies to both the circulating fluidized bed boiler 100 and the loop-seal heat exchanger 400. Furthermore, because the second heat exchanger 320 and the third heat exchanger 330 are located on opposite sides of the first wall 441 and the normal N (see FIG. 1 ) of the first wall 441 penetrates both the second heat exchanger 320 and the third heat exchanger 330, the second heat exchanger 320 and the third heat exchanger 330 can be easily integrated. This means that the connecting second pipeline 322 can be shortened. This simplifies the structure of the loop-seal heat exchanger and reduces manufacturing costs.

[0050] Furthermore, the second chamber 422 and the first chamber 412 share a common wall. Referring to Figure 3, the second chamber 422 and the first chamber 412 typically share a second wall 443 of the loop-seal heat exchanger 400. Referring to Figure 3, the second chamber 422 and the first chamber 412 typically share a third wall 445 of the loop-seal heat exchanger 400. Furthermore, preferably, the second wall 443 includes a heat transfer tube for recovering heat to a heat exchange medium. Furthermore, preferably, the third wall 445 includes a heat transfer tube for recovering heat to a heat exchange medium.

[0051] The term "wall" as used herein refers to a planar object that defines a chamber. Thus, first wall 441 defines chambers 433, 172, 422, and 412 (see FIG. 3). As shown in FIG. 3, second wall 443 defines chambers 422, 412, and 435, and third wall 445 defines all of the chambers in the loop-seal heat exchanger of FIG. 3.

[0052] Thus, in one embodiment, a first side of the loop-seal heat exchanger wall (i.e., second wall 443) confines the first chamber 412, and a first side of the loop-seal heat exchanger wall (i.e., second wall 443) confines the second chamber 422. This is also beneficial from the standpoint of keeping the profile of the loop-seal heat exchanger 400 easy to install.

[0053] The location of the third wall 445 is not critical, but for manufacturing reasons it is preferred that it confines both the first chamber 412 and the second chamber 422. Thus, in Figure 3, a first side of the loop-seal heat exchanger wall (i.e., the third wall 445) confines the first chamber 412, and a first side of the loop-seal heat exchanger wall (i.e., the third wall 445) confines the second chamber 422. As shown in the figure, the second and third walls (443, 445) are spaced apart from each other, i.e., they are not different portions of the same wall.

[0054] In the above-described structure, the second pipeline 322 can be shortened or omitted. For example, referring to Fig. 15, the second heat exchanger 320 can include an inlet header 320a and an outlet header 320b, such that steam from the first superheater 310 can be distributed to the tubes of the second superheater 320 via the inlet header, and the steam that has flowed through the tubes of the second superheater 320 can be collected in the outlet header 320b of the second heat exchanger 320. Similarly, the third heat exchanger 330 can include an inlet header 330a for distributing steam to the tubes of the third heat exchanger 330 and an outlet header 330b for collecting the steam that has flowed through the tubes of the third heat exchanger 330.

[0055] Referring to FIG. 15 , the outlet header 320b of the second heat exchanger 320 and the inlet header 330a of the third heat exchanger 330 can be arranged to form part of a steam chamber 335. A portion of the steam chamber 335 can be seen to form the second pipeline 322. Having the steam chamber 335 function as both the outlet header 320b of the second heat exchanger 320 and the inlet header 330a of the third heat exchanger 330 further simplifies the structure of the loop-seal heat exchanger 400, thereby reducing manufacturing costs. Furthermore, such a structure is mechanically robust. This can be easily achieved when the second and third heat exchangers 320, 330 are arranged in adjacent chambers 412, 422, as shown in the figure. This also relates to the issue that the normal N of the first wall 441 of the loop-seal superheater passes through both the second heat exchanger 320 and the third heat exchanger 330. This issue will be discussed in more detail below.

[0056] 15, the inlet header 320a of the second heat exchanger 320 may be mechanically connected to the outlet header 330b of the third heat exchanger 330. However, in such a case, for example, if the connection between the headers 320a and 330b is a pipeline, a plug 325 may be used to prevent steam from flowing directly from the inlet header 320a of the second heat exchanger 320 to the outlet header 330b of the third heat exchanger 330. The inlet header 320a of the second heat exchanger 320 and the outlet header 330b of the third heat exchanger 330 may be part of the same tubular structure with the plug 325.

[0057] With regard to the circulation of bed material within the loop-seal heat exchanger 400 and through the first chamber 412, a first opening 451′ is provided at the bottom of the inlet chamber 431 to allow the bed material to flow into the inlet chamber 433, as indicated by arrow 451 (see FIGS. 3 and 4). A second opening 453′ is provided at the top of the inlet chamber 433 to allow the bed material to flow into the first chamber 412, as indicated by arrow 453 (see FIGS. 4 and 7). A third opening 455′ is provided at the bottom of the first chamber 412 to allow the bed material to flow into the outlet chamber 435, as indicated by arrow 455 (see FIG. 7). Finally, a fourth opening 457′ is provided at the top of the outlet chamber 435 to allow the bed material to exit the loop-seal heat exchanger 400, as indicated by arrow 457.

[0058] With regard to the circulation of bed material within the loop-seal heat exchanger 400 and through the bypass chamber 432, a fifth opening 452' is provided at the bottom of the inlet chamber 431 to allow the bed material to flow into the bypass chamber 432, as indicated by arrow 452 (see FIGS. 3 and 4). A sixth opening 454' is provided at the top of the bypass chamber 432 to allow the bed material to exit the loop-seal heat exchanger 400, as indicated by arrow 454.

[0059] The bed material is fluidized in the chambers of the loop-seal heat exchanger 400 except for those chambers without bed material, i.e., at least the first furnace 172 and the second chamber 422. However, the bed material does not need to be fluidized in all chambers simultaneously. For example, fluidization of the material in the bypass chamber 432 can be stopped to control the flow through the other chambers. For fluidization purposes, the loop-seal heat exchanger 400 includes a nozzle 460 (see FIGS. 4 and 5) located at the bottom of the chamber 400 where the material is to be fluidized. Regarding the more detailed function of the nozzle 460, the loop-seal heat exchanger 400 includes a first nozzle 462 (see FIGS. 5 and 7) configured to fluidize the bed material in the first chamber 412. In this way, a fluidized bed is formed in the first chamber 412 so that the second heat exchanger 320 contacts the fluidized bed of the fluidized bed boiler.

[0060] The loop-seal heat exchanger 400 includes a second nozzle 464 (see FIG. 4) configured to fluidize the bed material in the bypass chamber 432. The loop-seal heat exchanger 400 includes a third nozzle 466 (see FIG. 4) configured to fluidize the bed material in the inlet chamber 433. By controlling the amount of fluidizing air in the bypass chamber 432 and the inlet chamber 433, it is possible to control how much bed material is sent to the bypass chamber 432 and how much bed material is sent to the inlet chamber 433. Typically, the more fluidizing gas is supplied, the easier the bed material will flow, thus increasing the flow of bed material, or vice versa.

[0061] Since the purpose of the third heat exchanger 330 is to superheat the steam to the final temperature during the first period (see FIG. 2a), the temperature of the heat exchange medium in the third heat exchanger 330 is highest during the first period. Thus, the second flue gas 173 is still at a very high temperature downstream of the third heat exchanger 330 and can be used in at least the following ways: - heating the heat exchange medium upstream of the third heat exchanger 330, for example using the second heat exchanger 320 and / or the first heat exchanger 310; and / or Heating a heat exchange medium as or as part of the fluidizing gas of the fluidized bed boiler 100.

[0062] With regard to the former, the second flue gas 173 may be mixed with the first flue gas 163, for example, upstream of the first heat exchanger 310. However, from a heat recovery perspective, it may be beneficial to supply the second flue gas 173 further upstream of the first heat exchanger, where the second flue gas 173 may be used, for example, as a fluidizing gas.

[0063] With regard to the latter, because the temperature of the second flue gas 173 is so high, using the second flue gas 173 as or as part of the fluidizing gas means that the remaining thermal energy of the second flue gas 173 is transferred to the bed material of the fluidized bed and is thus available for use in the process and can be recovered by conventional means. Examples of fluidized beds that may utilize the second flue gas 173 as a fluidizing gas include the first furnace 162 (see Figures 1a and 1b) and the first chamber 412 (see Figure 1a).

[0064] The fluidizing gas may include gases other than the second flue gas 173. It is not necessary to use all of the second flue gas as at least a portion of the fluidizing gas. Thus, a preferred embodiment includes using a fluidizing gas including at least a portion of the second flue gas 173 to fluidize the fluidized bed in the fluidized bed boiler 100. For reasons of process simplification, preferably, all of the second flue gas 173 is used in the same manner. Thus, a more preferred embodiment includes using at least 75% (by volume) of the second flue gas 173 to fluidize one or more fluidized beds of the fluidized bed boiler 100. Here, the fluidized bed may be the fluidized bed of the first furnace 162 or the fluidized bed of the first chamber 412. The second flue gas 173 may be used as at least a portion of the fluidizing gas in both fluidized beds.

[0065] Correspondingly, the fluidized bed boiler 100 preferably includes a channel 179 for conveying at least a portion of the second flue gas 173 to such a chamber of the fluidized bed boiler, where a fluidized bed is formed during use. As an example, the fluidized bed boiler 100 may include a second flue gas channel 179 for conveying at least a portion of the second flue gas 173 to such a portion of the first furnace 162, where a fluidized bed is formed. With reference to FIG. 1a, during use of the circulating fluidized bed boiler, substantially the entire material in the first furnace 162 is fluidized. With reference to FIG. 1b, during use of the bubbling fluidized bed boiler, the material in the lower part of the first furnace 162 is fluidized.

[0066] For example, in the context of the embodiments of FIGS. 1a and 3-7, as shown in FIG. 8a, the second flue gas 173 can be conveyed through the second flue gas channel 179 to the return channel 136. At some point within the second flue gas channel 179, the second flue gas 173 mixes with the bed material of the return channel 136. Then, through the combined second flue gas channel 179 and return channel 136, both the bed material and the second flue gas 173 are conveyed into the first furnace 162. Therein, the second flue gas 173 is mixed with other fluidizing gas (i.e., combustion air) and thus used as part of the fluidizing gas for the first furnace. Thus, the second flue gas 173 does not need to be fed through the combustion air passage 104 of the fluidized bed boiler. However, as shown in Figure 8b, the second flue gas 173 can be mixed with the combustion air supplied to the first furnace through the combustion air channel 104. If the combustion air is preheated, the second flue gas is typically mixed with the preheated combustion air (i.e., downstream of the combustion air preheater) to improve the efficiency of the preheating. In this way, a portion of the combustion air piping forms part of the second flue gas channel 179.

[0067] As discussed above, the third heat exchanger 330 may be disposed side-by-side (in the horizontal direction Sx) with respect to the second heat exchanger 320. In other words, the first wall 441 may be a vertical wall.

[0068] 9a and 9b, the third heat exchanger 330 may be disposed above the second heat exchanger 320. In such a case, as shown in detail in FIG. 9b, the first wall 441 is substantially horizontal. As discussed above, the second heat exchanger 320 is disposed on a first side of the first wall 441, and the third heat exchanger 330 is disposed on a second side opposite the first wall 441. Furthermore, the first wall 441 defines a first chamber 412 in which the second heat exchanger 320 is disposed, and the first wall 441 defines a second chamber 422 in which the third heat exchanger 330 is disposed. Finally, the second heat exchanger 320 and the third heat exchanger 330 are disposed such that the normal N of the first wall 441 passes through both the second heat exchanger 320 and the third heat exchanger 330. See above for technical effects. The outlet header 320b of the second heat exchanger 320 may be connected to the inlet header of the third heat exchanger 330. See Figure 15. However, in such a case, the vapor chamber 335 may be vertical.

[0069] A second wall 443 and a third wall 445 are shown in FIG. 9a. As discussed above, the second heat exchanger 320 is disposed on a first side of the second wall 443, and the third heat exchanger 330 is disposed on the first side of the second wall 443. Furthermore, the second wall 443 confines a first chamber 412 in which the second heat exchanger 320 is disposed, and the second wall 443 confines a second chamber 422 in which the third heat exchanger 330 is disposed. As discussed above, the second heat exchanger 320 is disposed on a first side of the third wall 445, and the third heat exchanger 330 is disposed on a first side of the third wall 445. Furthermore, the third wall 445 confines the first chamber 412 in which the second heat exchanger 320 is disposed, and the third wall 445 confines the second chamber 422 in which the third heat exchanger 330 is disposed.

[0070] In the embodiment of Figures 9a and 9b, the second flue gas 173 can be used as a fluidizing gas in the first furnace 162, for example, by providing a flue gas passage in the top of the outlet chamber 435. As can be seen from Figures 3-7, 9a, and 9b, the loop-seal heat exchanger 400 of Figures 3-7, 9a, and 9b can be similar with respect to the chambers through which the bed material is configured to flow, i.e., chambers 431, 433, 412, 435, and 432. Thus, in the embodiment of Figures 9a and 9b, the second flue gas can flow through a flue gas passage in the roof of the outlet chamber 435, through the outlet chamber 435 to the return channel 136, and finally through the return channel 136 to the first furnace 162. The outlet chamber 435 thus forms part of a channel 179 for conveying at least a portion of the second flue gas 173 to such chamber of a fluidized bed boiler, where, during use, a fluidized bed is configured to form (see Figure 9b for the other parts of the channel 179).

[0071] 9a and 9b apply to both the loop seal heat exchanger 400 and the fluidized bed boiler 100.

[0072] 10a-10f, the third heat exchanger 330 may be disposed below the second heat exchanger 320. In such a case, as detailed in FIG. 10b, the first wall 441 is substantially horizontal. As discussed above, the second heat exchanger 320 is disposed on a first side of the first wall 441, and the third heat exchanger 330 is disposed on a second side opposite the first wall 441. Furthermore, the first wall 441 defines a second chamber 422 in which the third heat exchanger 330 is disposed. Finally, the second heat exchanger 320 and the third heat exchanger 330 are disposed such that the normal N of the first wall passes through both the second heat exchanger 320 and the third heat exchanger 330. See above for technical effects. The outlet header 320b of the second heat exchanger 320 may be connected to the inlet header of the third heat exchanger 330. See FIG. 15. However, in such cases, the vapor chamber 335 may be vertical.

[0073] A second wall 443 and a third wall 445 are shown in FIG. 10a. As discussed above, the second heat exchanger 320 is disposed on a first side of the second wall 443, and the third heat exchanger 330 is disposed on the first side of the second wall 443. Furthermore, the second wall 443 confines a first chamber 412 in which the second heat exchanger 320 is disposed, and the second wall 443 confines a second chamber 422 in which the third heat exchanger 330 is disposed. As discussed above, the second heat exchanger 320 is disposed on a first side of the third wall 445, and the third heat exchanger 330 is disposed on a first side of the third wall 445. Furthermore, the third wall 445 confines the first chamber 412 in which the second heat exchanger 320 is disposed, and the third wall 445 confines the second chamber 422 in which the third heat exchanger 330 is disposed.

[0074] In the embodiment of Figures 10a-10d, the second flue gas 173 can be used as a fluidizing gas in the first chamber 412. The principle is shown in Figure 10b, where the second flue gas 173 flows through the heat exchanger pipes of the third heat exchanger 330 and from there through the first nozzle 462 into the first chamber 412. In this embodiment, air ("Air" in Figure 10b) is used as an oxidizing medium for the second fuel 175 (i.e., combustion air in the second furnace) and as a fluidizing gas in the first chamber 412. Thus, during the second operating mode, i.e., the second time period (see Figure 2b), the air can cool the heat exchange medium flowing through the third heat exchanger 330.

[0075] A solution to this problem is shown in Figures 10c and 10d. The loop-seal heat exchanger of Figures 10c and 10d includes a first damper 471 and, optionally, a first pivot 472. The first damper 471 is pivotable about the first pivot 472. However, the damper 471 can also be implemented without the pivot, using two slidable dampers (one horizontal and one vertical corresponding to the position of the first damper 471). Referring to Figure 10c, during a first period, the first damper 471 is positioned to guide the second flue gas 173 to the third heat exchanger 330. Referring to Figure 10d, during a second period, the first damper 471 is positioned to guide the fluidizing air or the second flue gas 173 to, possibly, bypass the third heat exchanger 330. In this embodiment, there is no need to combust even a small amount of the second fuel 175 during the second time period. Thus, typically in this embodiment, no second flue gas is produced, and only air is directed by the first damper to [A] the third heat exchanger 330 and [B] the first nozzle 462.

[0076] To further ensure that air does not cool the third heat exchanger 330 during the second time period, the loop-seal heat exchanger 400 may include a second damper 473 and, optionally, a second pivot 474, as shown in Figures 10c and 10d. Figures 10c and 10d show two second dampers 473 and a second pivot 474 for each second damper 473.

[0077] The second damper 473 is pivotable about a second pivot 474. As discussed in detail above, a slidable damper may alternatively or additionally be used. Referring to FIG. 10c, during a first time period, the second damper 473 is disposed in a position configured to direct the second flue gas 173 from the third heat exchanger 330 to the first nozzle 462. Referring to FIG. 10d, during a second time period, the second damper 473 is disposed in a position configured to prevent the fluidizing air or, as the case may be, the second flue gas 173 from flowing into the third heat exchanger 330. In combination, the first and second dampers 471, 473 serve the same purpose as the third damper 475 discussed in connection with FIG. 2c.

[0078] Thus, an embodiment of a loop seal heat exchanger for a fluidized bed boiler 100 or a circulating fluidized bed boiler includes a damper device including at least one damper (471, 473, 475), the damper device comprising: - allowing the circulation of the second flue gas 173 gas from the second furnace 172 to the third heat exchanger 330; and - configured to prevent air circulation through the third heat exchanger 330;

[0079] The former applies to a first period (see Figures 2a, 2c, and 10c), and the latter applies to a second period (see Figures 2b, 2c, and 10d). With regard to the latter, the air therein may contain secondary flue gas, provided that a small amount of secondary flue gas is also burned during the second period.

[0080] Furthermore, it has been found that in the first mode, i.e., first time period, more combustion air is typically required to combust the second fuel 175 than is required to fluidize the bed material in the first chamber 412. It has been found that the excess heat can be recovered in the combustion air used for combustion in the second furnace 172.

[0081] Referring to FIG. 10e, in an embodiment, the loop-seal heat exchanger 400 includes a fourth heat exchanger 340. The fourth heat exchanger is configured to recover heat from the second flue gas 173 to air, specifically the combustion air used in the second furnace 172. This embodiment also includes a fourth pipeline 342 configured to transport the heated air from the fourth heat exchanger 340 to the second furnace 172, where it is used as an oxidizing medium. This embodiment also includes a valve 344 configured to restrict the flow of the second flue gas through the fourth heat exchanger 340. In this manner, the valve 344 can be used to control how much of the second flue gas is used as a fluidizing gas in the loop-seal heat exchanger. The second flue gas can also be used to fluidize bed material in a chamber separate (or other) than the first chamber 412 of the loop-seal heat exchanger.

[0082] To recover heat from the excess secondary flue gas 173, it is also possible to convey a portion of the secondary flue gas 173 to the first furnace 162 and use only a portion of the secondary flue gas 173 as a fluidizing medium in the loop-seal heat exchanger 400. An example of such a solution is shown in FIG. 10f, in which a portion of the secondary flue gas 173 is taken from a location upstream of the fluidizing nozzle 460 and conveyed through a portion of the channel 179 to the return channel 136. The secondary flue gas 173 flows through the return channel 136 to the first furnace 162, for example, as described in detail above in connection with FIG. 8a. A valve (not shown) can be applied to control the division of the secondary flue gas 173 into the portion conveyed to the nozzle 460 and the portion conveyed to the first furnace 162. Although not shown, the excess secondary flue gas 173 can be mixed with the combustion air of the first furnace 162, as described in detail in FIG. 8b.

[0083] The heat of the portion of the second flue gas 173 used to fluidize the bed material in the second chamber 412 and thus conveyed through the nozzle 460 can be recovered by using that gas as part of the fluidizing gas in the first furnace 162 or in other ways as detailed above. See Figures 8a, 8b, 9a, 9b. For example, an opening can be provided in the top of the exit chamber 435 to allow the used fluidizing gas to flow into the return channel 136, and yet another portion of the second flue gas used, as shown in Figure 10e or 10f.

[0084] Although not shown, the excess second flue gas 173, i.e., the portion not conveyed through the nozzle 460, can be mixed with the first flue gas 163 downstream of the fluidized bed arranged in the first furnace 162, possibly downstream of the cyclone 132 of a circulating fluidized bed boiler, for example. In this case, it is advantageous to mix the flue gases 173, 163 upstream of the first heat exchanger 310. However, from the viewpoint of energy recovery, it is more advantageous to use the excess second flue gas or all of the second flue gas as part of the fluidizing gas for the first furnace 162.

[0085] 10a-10f apply to both the loop seal heat exchanger 400 and the fluidized bed boiler 100.

[0086] On the other hand, such chambers (431, 432, 433, 412, 435) through which the bed material is configured to pass in the loop-seal heat exchanger of Figures 3 to 10f are arranged similarly to each other, but the second furnace 172 and the third heat exchanger 330 can also be provided in a simpler loop-seal heat exchanger 400 as shown in Figures 11 to 13.

[0087] In the loop-seal heat exchanger of FIGS. 11-13, the bed material enters the inlet chamber 431 through the dipleg channel 134, as in FIG. 3. However, at least a portion of the bed material passes directly from the inlet chamber 431 into the first chamber 412, as indicated by arrow 451. For this purpose, an opening can be provided in the bottom of the inlet chamber 431. The first chamber 412 is provided with the second heat exchanger 320. In the first chamber 412, the flowing material flows upward and also in the negative Sx direction in FIG. 11. The bed material exits the first chamber 412 into the return channel 136, as indicated by arrow 457. For this purpose, an opening can be provided in the top of the first chamber 412. The loop-seal heat exchanger 400 of FIG. 11 also includes a bypass chamber 432. At least a portion of the bed material can bypass the second heat exchanger 320 by flowing through the bypass chamber 432, as indicated by arrows 452 and 454. The same statements made regarding the bed material circulating through the bypass chamber 432 in the embodiment of FIG. 3 apply here as well.

[0088] The loop-seal heat exchanger 400 in Figure 11 also includes a second chamber 422 in which the third heat exchanger 330 is disposed, and a second furnace 172 equipped with a burner 176. Although not shown in Figure 11, the second flue gas is configured to flow from the second furnace 172 to the second chamber 422 in which the third heat exchanger 330 is disposed. For reference, see Figure 6. In Figure 11, the second chamber 422 is also disposed in the positive Sy direction from the second furnace 172.

[0089] The loop-seal heat exchanger 400 of FIG. 11 includes a first wall 441. What was stated about the first wall 441 and its normal N in connection with FIGS. 3-7 applies. The loop-seal heat exchanger 400 of FIG. 11 includes a second wall 443. What was stated about the second wall 443 in connection with FIGS. 3-7 applies. The loop-seal heat exchanger 400 of FIG. 11 includes a third wall 445. What was stated about the third wall 443 in connection with FIGS. 3-7 applies. As shown in FIG. 15, the outlet header 320b of the second heat exchanger 320 may be connected to the inlet header 330a of the third heat exchanger 330.

[0090] Although not shown, the loop-seal heat exchanger 400 is provided with a nozzle 460 (see nozzle 462 above) for blowing fluidizing gas into the first chamber 412 and a nozzle 460 (see nozzle 464 above) for blowing fluidizing gas into the bypass chamber 432, thereby fluidizing the bed material therein.

[0091] In FIG. 11 , the first chamber 412 and the second chamber 422 are arranged side by side in the horizontal direction (Sx). However, as shown in FIG. 12 , the first and second chambers may be arranged such that the third heat exchanger 330 is disposed above the second heat exchanger. In the embodiment of FIG. 12 , the second flue gas 173 downstream of the third heat exchanger 330 is conveyed through the first chamber 412 to the return channel 136 and can be used as a flow medium in both the first chamber 412 and the first furnace 162. The loop-seal heat exchanger 400 of FIG. 12 includes a first wall 441. What was said about the first wall 441 and its normal N in connection with FIGS. 9 a and 9 b applies. Although not shown in FIG. 12 , the loop-seal heat exchanger 400 of FIG. 12 includes second and third walls 443 and 445 for the same purposes as the walls 443 and 445 disclosed in connection with FIGS. 9 a and 9 b.

[0092] Furthermore, as shown in FIG. 13, the first chamber 412 and the second chamber 422 may be arranged such that the third heat exchanger 330 is located below the second heat exchanger 320. In this case, the second flue gas 173 can be used to fluidize the bed material in the first chamber 412. If there is an excess of available second flue gas, some of the flue gas need not be conveyed through the nozzles 460 (particularly 462). The excess second flue gas can be used in one of the ways described above in connection with FIGS. 10e-10f. Cooling of the third heat exchanger 330 by the combustion air for the second fuel 175 can be prevented by the solution described above in connection with FIGS. 10c-10d.

[0093] The loop-seal heat exchanger 400 of Figure 13 includes a first wall 441. What was said about the first wall 441 and its normal N in relation to Figures 10a to 10f applies. Although not shown in Figure 13, the loop-seal heat exchanger 400 of Figure 13 includes second and third walls 443, 445 that serve the same purpose as the walls 443 and 445 disclosed in relation to Figures 10a to 10f.

Claims

1. A method for heating a heat exchange medium in a fluidized bed boiler (100), the method comprising: In the first period, Combusting a first fuel (165) in a first furnace (162) of the fluidized bed boiler (100) to produce a first flue gas (163); recovering heat from the first flue gas (163) to a heat exchange medium using a first heat exchanger (310); conveying the heat exchange medium from the first heat exchanger (310) to a second heat exchanger (320), at least a portion of which is positioned in contact with the fluidized bed of the fluidized bed boiler (100); burning a second fuel (175) in a second furnace (172) of the fluidized bed boiler (100) to produce a second flue gas (173); conveying the heat exchange medium from the second heat exchanger (320) to a third heat exchanger (330); recovering heat from the second flue gas (173) to the heat exchange medium using the third heat exchanger (330); circulating the fluidized bed from the first furnace (162) to a cyclone (132), from the cyclone (132) to a loop seal (140), and from the loop seal (140) to the first furnace (162); a first side of the walls (441, 443, 445) of the loop-seal heat exchanger (400) bounding a first chamber (412); the first side or the opposite second side of the wall (441, 443, 445) of the loop-seal heat exchanger (400) bounds a second chamber (422); the second heat exchanger (320) is disposed within the first chamber (412) or within a wall that bounds the first chamber (412); the third heat exchanger (330) is disposed within the second chamber (422) or within a wall that bounds the second chamber (422); The third heat exchanger (330) is not in contact with the fluidized bed of the fluidized bed boiler (100); the second flue gas (173) is lower in alkalis and halogens than the first flue gas (163); and / or the second fuel (175) is lower in alkali and halogen than the first fuel (165); method.

2. The method of claim 1, wherein the third heat exchanger (330) is not in contact with the first flue gas (163).

3. In the second period, recovering heat from the fluidized bed of the fluidized bed boiler (100) to the heat exchange medium using the second heat exchanger (320); conveying the heat exchange medium from the second heat exchanger (320) to the third heat exchanger (330); [A] not combusting the second fuel (175) in the second furnace (172) during the second period; or [B] burning less second fuel (175) in the second furnace (172) during the second time period than during the first time period; Preferably, the method further comprises, during the second period: conveying the heat exchange medium through the third heat exchanger (330); 3. The method according to claim 1 or 2.

4. During the second period, the load of the fluidized bed boiler (100) is greater than the load of the fluidized bed boiler (100) during the first period, for example: During the first period, the load of the fluidized bed boiler (100) is below a threshold value; The method of claim 3, wherein during the second period of time, the load of the fluidized bed boiler (100) is at least equal to the threshold value.

5. The method of any one of claims 1 to 4, wherein the second fuel (175) is a liquid or a gas.

6. The method of claim 5, wherein the second fuel (175) comprises natural gas or a liquid containing oil.

7. A method described in any one of claims 1 to 6, wherein the first fuel (165) comprises a solid material.

8. The method of claim 7, wherein the first fuel (165) comprises biomass and / or residual-derived fuel.

9. 9. The method of claim 1, further comprising fluidizing a fluidized bed in the fluidized bed boiler using a fluidizing gas comprising at least a portion of the second flue gas.

10. A fluidized bed boiler (100), comprising: a first furnace (162) for combusting a first fuel (165) comprising a solid material to produce a first flue gas (163); a first heat exchanger (310) for recovering heat from the first flue gas (163) to a heat exchange medium; a second heat exchanger (320) disposed at a location in the fluidized bed boiler (100) such that, in use, the fluidized bed is configured to contact the second heat exchanger (320); a first pipeline (312) for conveying the heat exchange medium from the first heat exchanger (310) to the second heat exchanger (320); a second furnace (172) for combusting a second fuel (175) to produce a second flue gas (173); a burner (176) disposed within the second furnace (172); a third heat exchanger (330) for recovering heat from the second flue gas (173) to the heat exchange medium received from the second heat exchanger (320); Cyclone (132) and a loop seal (140); The fluidized bed boiler (100) includes a loop seal heat exchanger (400) disposed within the loop seal (140), The loop-seal heat exchanger (400) includes a first chamber (412) and a second chamber (422); the second heat exchanger (320) is disposed within the first chamber (412) or within a wall that bounds the first chamber (412); the third heat exchanger (330) is disposed within the second chamber (422) or within a wall that bounds the second chamber (422); a first side of the walls (441, 443, 445) bounding the first chamber (412); the first side of the wall (443, 445) or the opposite second side of the wall (441) bounds the second chamber (422); The third heat exchanger (330) is configured not to come into contact with the fluidized bed of the fluidized bed boiler (100); The burner (176) is configured to combust a second fuel (175), which may be liquid or gaseous. A fluidized bed boiler (100).

11. 11. The fluidized bed boiler (100) of claim 10, comprising a channel (179) for conveying at least a portion of the second flue gas (173) to a portion of the fluidized bed boiler (100) configured to form a fluidized bed during use.

12. A power plant, the power plant comprising: A fluidized bed boiler (100) according to claim 10 or 11; a steam turbine (152); a pipeline (332) configured to convey a heat exchange medium from the third heat exchanger (330) to the steam turbine (152); a generator (155) arranged to be mechanically connected to the steam turbine (152), Power plant.

13. A loop seal heat exchanger (400), comprising: a first chamber (412) and a second chamber (422); a second heat exchanger (320) disposed within the first chamber (412) or within a wall bounding the first chamber (412); a third heat exchanger (330) disposed within the second chamber (422) or within a wall limiting the second chamber (422); a first nozzle (462) for fluidizing the fluidized bed in the first chamber (412); a second furnace (172) for combusting a second fuel (175) to produce a second flue gas (173); a burner (176) disposed within the second furnace (172); a passageway (178) for conveying the second flue gas (173) to the second chamber (422); the third heat exchanger (330) is configured so as not to contact the fluidized bed; The burner (176) is configured to combust a second fuel (175), which may be liquid or gaseous. A loop seal heat exchanger (400).

14. a first wall (441); the second heat exchanger (320) is disposed on a first side of the first wall (441), and the third heat exchanger (330) is disposed on a second side of the first wall (441); 14. The loop-seal heat exchanger (400) of claim 13, wherein a normal (N) to the first wall (441) passes through both the second heat exchanger (320) and the third heat exchanger (330).

15. a second wall (443); a first side of the second wall (443) bounding the first chamber (412); the first side of the second wall (443) bounding the second chamber (422); Preferably, the loop-seal heat exchanger (300) further comprises a third wall (445); a first side of the third wall (445) bounding the first chamber (412); 15. The loop-seal heat exchanger (400) of claim 13 or 14, wherein the first side of the third wall (445) confines the second chamber (422).

16. a damper device including at least one damper (471, 473, 475); The damper device comprises: allowing circulation of second flue gas (173) from said second furnace (172) to said third heat exchanger (330); The loop-seal heat exchanger (400) of any one of claims 13 to 15, configured to prevent circulation of air through the third heat exchanger (330).

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