Patent Application
20150267968
1. Field of the Invention
The present invention relates to a fluidized bed heat exchanger of a fluidized bed boiler. The invention especially relates to a fluidized bed heat exchanger comprising a front wall adjacent to the fluidized bed boiler, a rear wall opposite to the front wall, and two side walls, the walls being formed as water tube panels, an inlet opening arranged in a first section of an upper portion of the heat exchanger for introducing hot particles from the fluidized bed boiler to the heat exchanger, heat exchange surfaces arranged in the heat exchanger for recovering heat from the particles, and an outlet opening arranged in a second section of the upper portion of the heat exchanger for returning cooled particles as an overflow from the heat exchanger back to the fluidized bed boiler, wherein the heat exchanger comprises a partition wall between the first and second sections of the upper portion of the heat exchanger.
2. Description of Related Art
Usually, the front wall, rear wall, and side walls enclosing the heat exchanger are mainly vertical, and they have a generally known tube-fin-tube construction. The tubes of each wall are then connected to a horizontal lower header arranged below the wall so as to convey heat transfer fluid, usually, water or steam, upwards, to an upper header or another water tube panel. Typically, the water tubes of the enclosure walls function as evaporation surfaces in a drum boiler and water preheating surfaces in a once-through boiler.
Fluidized bed heat exchangers are commonly used in fluidized bed boilers to recover heat from hot solid particles conveyed from the furnace of the boiler to the heat exchanger. After cooling in the heat exchanger, the cooled particles are returned back to the furnace. The fluidized bed heat exchanger may be connected to the hot loop of a circulating fluidized bed boiler or it may receive hot solid particles directly from the furnace. It is also common that hot solid particles are received to the heat exchanger both from the hot loop, i.e., from a particle separator of the fluidized bed boiler, and directly from the furnace.
Basic requirements or desired properties of a heat exchanger include that a sufficient amount of heat can be recovered in the heat exchanger, that the amount of recovered heat can be controlled as desired, that the heat exchanger can be used continuously without problems, and that the operation of the heat exchanger does not harm the processes in the boiler, for example, by increasing the emissions to the environment.
U.S. Pat. No. 7,240,639 discloses a circulating fluidized bed boiler with a fluidized bed heat exchanger that is especially advantageous for efficient control of the recovered heat. It comprises an inlet opening for introducing hot solid particles directly from the furnace and from the particle separator of the boiler to the top of the heat exchanger. A portion of the particles is cooled in heat exchange surfaces and discharged through a lift channel from the bottom of the heat exchanger back to the furnace. Another portion of the particles can, especially at high loads, be discharged without cooling to the furnace as an overflow through another opening and a discharge channel. A disadvantage of this construction is that the existence of a lift channel adjacent to the furnace tends to move the center of gravity of the heat exchanger further away from the furnace, which makes the supporting of the heat exchanger more difficult.
U.S. Pat. No. 6,336,500 shows, in
U.S. Pat. No. 5,533,471 shows, in
In the prior art documents mentioned above, solid particles are conveyed to or from a heat exchanger along a channel that is formed adjacent to a front wall, a top wall or a rear wall of the heat exchanger. The construction of the partition wall forming the channel is not disclosed, but, according to a common art, the partition wall can be made as an uncooled refractory construction or a refractory covered metal plate construction. A cooled partition wall, however, is generally a better solution in terms of endurance or heat balance of the construction. A cooled partition wall close to an adjacent wall can advantageously be constructed by bending a portion of water tubes out from the adjacent wall to run along the partition wall, parallel to the water tubes of the adjacent wall, to be finally bent back to the adjacent wall. An example of forming a discharge channel by bending water tubes of a furnace wall is shown in U.S. Pat. No. 5,526,775. Such a construction may, however, be quite complicated, and may have a disadvantage that, as a result of bending water tubes from the adjacent wall to run along a partition wall, the number of water tubes in the adjacent wall decreases, which may cause harmful thermal stresses to the wall.
A construction in which solid particles are conveyed to or from a heat exchanger along a channel may, especially at high loads, create a situation in which a portion of hot solid particles entering towards the heat exchanger cannot flow through the heat exchanger, but they are let to bypass the heat exchanger and return to the furnace uncooled. Therefore, in order to recover enough energy, the heat exchanger and the heat transfer surfaces have to be made relatively large to increase the residence time of the particles in the heat exchanger so as to cool the particles to a relatively low temperature. A large heat exchanger requires a large space and makes the supporting arrangement more difficult. A relatively low temperature in the heat exchanger may, in some cases, lead to unwanted chemical reactions. Especially, in oxyfuel combustion, in which the partial pressure of CO2 is high, a temperature that is too low may lead to recarbonation of CaO in the heat exchanger.
An object of the present invention is to provide a fluidized bed heat exchanger of a fluidized bed boiler in which at least some of the problems mentioned above are minimized.
According to an aspect, the present invention provides a fluidized bed heat exchanger of a fluidized bed boiler, the heat exchanger comprising a front wall adjacent to the fluidized bed boiler, a rear wall opposite to the front wall, and two side walls, the walls being formed as water tube panels, an inlet opening arranged in a first section of an upper portion of the heat exchanger for introducing hot particles from the fluidized bed boiler to the heat exchanger, heat exchange surfaces arranged in the heat exchanger for recovering heat from the particles, and an outlet opening arranged in a second section of the upper portion of the heat exchanger for returning cooled particles as an overflow from the heat exchanger back to the fluidized bed boiler, wherein the heat exchanger comprises a partition wall between the first and second sections of the upper portion of the heat exchanger, and wherein the partition wall extends from the front wall to a center section of the heat exchanger, and the partition wall is formed by bending water tubes from at least one side wall of the heat exchanger.
By the expression that “the partition wall extends to a center section of the heat exchanger” is herein denoted that the partition wall extends to a portion of the heat exchanger that is approximately at an equal distance from the front wall and from the rear wall. Therefore, the partition wall is not arranged adjacent to a front wall or a rear wall, but it is directed from the front wall to a center section of the heat exchanger. Preferably, the partition wall extends from the front wall up to a portion of the heat exchanger that is spaced apart from the front wall and the rear wall by about 30% to about 70%, even more preferably, about 40% to about 60%, of the total depth of the heat exchanger, i.e., from the distance between the rear wall and the front wall. An advantage of the present invention is that, because the partition wall extends from the front wall only to a center section of the heat exchanger, and not to the vicinity of the rear wall, the incoming particles have a relatively wide cross-sectional area for their flow. Thereby, there is no constriction in the flow path of the particles that would limit the particle flow to the heat exchanger.
By the expression of “returning cooled particles from the heat exchanger as an overflow back to the boiler” is herein denoted that the particles are discharged from the heat exchanger directly from the top of the main fluidized bed, i.e., of the fluidized bed in which the heat exchange surfaces are arranged. Thus, the outlet of the heat exchanger does not comprise a lifting channel in which particles are discharged upwards from the bottom of the heat exchanger. Advantageously, the fluidized bed of the heat exchanger is undivided. Thus, there is no partition wall, for example, a vertical partition wall, dividing an essential portion of the portion of the fluidized bed in which the heat exchange surfaces are arranged into two sections.
The expression of “the partition wall being formed by bending water tubes from at least one side wall of the heat exchanger” herein refers to the partition wall comprising water tubes that are a continuation of corresponding water tubes of at least one side wall of the heat exchanger.
Because the particle flow through the heat exchanger can be, by utilizing the features of the present construction described above, easily designed such as to render possible a relatively high particle flow, in practice, any desired heat recovery rate can be obtained without cooling the particles to a very low temperature in the heat exchanger. Therefore, the heat exchanger can be made relatively small. Moreover, due to the low level of cooling, the risk of unwanted chemical reactions, especially, in oxyfuel combustion, is minimized.
By making the partition wall to extend to a center section of the heat exchanger, it becomes more difficult to arrange cooling for the partition wall than for a partition wall adjacent to the rear wall or front wall of the heat exchanger. One possibility would be to arrange a transverse header at the lower end of the partition wall to provide water, to water tubes, flowing along the partition wall to the front wall. In such a construction, however, it is relatively difficult to continue the water tubes further from the front wall. Therefore, the present invention provides a solution, in which the partition wall is formed by bending water tubes from at least one side wall of the heat exchanger.
According to a preferred embodiment of the present invention, the partition wall is formed by bending tubes from at least one side wall as a first horizontal section extending towards a center section of the partition wall and as a second horizontal section from the center section of the partition wall back to the side wall. Preferably, the partition wall is formed by bending tubes from each of the two side walls as a first horizontal section extending towards a center section of the partition wall and from the center section as a second horizontal section back to the side wall. Advantageously, the first horizontal section is arranged below the second horizontal section.
Preferably, the partition wall is, as seen from the front wall, slanted downwards, so as to guide particles introduced into the heat exchanger closer to the rear wall, further away from the furnace. Thereby, particles introduced towards the heat exchanger at any location at the top surface of the heat exchanger are guided to the rear portion of the heat exchanger. The particles are discharged from the heat exchanger through an outlet opening in the front wall, and, therefore, they proceed in the fluidized bed of the heat exchanger horizontally towards the front wall. By changing the fluidizing velocity of the fluidized bed, it is possible to efficiently control movement of particles around the heat exchange surfaces, and the rate of heat recovery in the heat exchanger. Thus, the present construction provides an efficient means for controlling, for example, the temperature of superheated or reheated steam.
According to a preferred embodiment of the present invention, the heat exchange surfaces are arranged below the vertical level of the lower edge of the outlet opening. Because the lower edge of the outlet opening defines the vertical level of the top surface of the fluidized bed forming in the heat exchanger, the construction provides the advantage that the heat exchange surfaces are always in use embedded in the fluidized bed. Preferably, the lower end of the partition wall is approximately at the same vertical level as, even more preferably, at a slightly lower vertical level than, the lower edge of the outlet opening. This is advantageous, because, if the lower end of the partition wall would be at a higher level than the lower edge of the outlet opening, there would be a possibility that particles could float on the surface of the fluidized bed directly from the inlet to the outlet without cooling in the heat exchange surfaces. Preferably, the lower end of the partition wall is at most 0.2 m, even more preferably, at most 0.1 m, below the vertical level of the lower edge of the outlet opening.
According to a preferred embodiment of the present invention, a windbox is arranged below the heat exchanger, which windbox is divided into a first section, which is mainly below the partition wall, and a second section, which is mainly not below the partition wall. The portions of the fluidized bed above the first section and the second section of the windbox can thus be called an outlet side and an inlet side of the fluidized bed, respectively. A divided windbox renders it possible to have a different fluidizing velocity on the inlet side and the outlet side of the fluidized bed. Preferably, a slightly higher fluidizing velocity is used on the outlet side of the fluidizing bed than on the inlet side of the fluidized bed, in order to enhance the circulation of particles in the fluidized bed. In oxyfuel combustion, it is possible to have different oxygen contents of the fluidizing gases introduced through the first and second windbox sections. It is also possible that, especially in a large fluidized bed heat exchanger, at least one of the first and second wind box sections is further divided into two, or even more than two, sections.
The above brief description, as well as further objects, features, and advantages of the present invention will be more fully appreciated by reference to the following detailed description of the currently preferred, but nonetheless illustrative, embodiments in accordance with the present invention, when taken in conjunction with the accompanying drawings, wherein:
A bed of particles 28 fluidized by fluidizing gas introduced to the heat exchanger 10 through a windbox 30 and nozzles 32 is formed in the lower portion of the heat exchanger 10. Heat exchange surfaces 34 are arranged in the lower portion of the heat exchanger 10 for recovering heat from the particles. After cooling on the heat exchange surfaces 34, the particles are discharged from the heat exchanger 10 back to the furnace 12 as an overflow through an outlet opening 36 in the front wall 16.
Because the height of the fluidized bed 28 is determined by the vertical level of the lower edge of the outlet opening 36, the outlet opening 36 is located in the upper portion of the heat exchanger 10. Thus, the partition wall 26 divides the upper portion of the heat exchanger 10 into a first section 38 comprising the inlet opening 24, and a second section 40 comprising the outlet opening 36.
In order to prevent attrition of the heat exchange surfaces 34 by particles falling on top of the fluidized bed 28, the heat exchange surfaces 34 are advantageously arranged below the vertical level of the lower edge 42 of the outlet opening 36. According to a preferred embodiment of the present invention, the partition wall 26 has a lower end 44 approximately at the same vertical level as the lower edge 42 of the outlet opening 36. Preferably, the lower end 44 of the partition wall 26 is at a vertical level within 0.2 m, even more preferably, within 0.1 m, from that of the lower edge 42 of the outlet opening 36. In order to prevent direct floating of particles on the surface of the particle bed 28 from the rear side of the particle bed to the outlet opening 36, the lower end 44 of the partition wall 26 is advantageously at a vertical level slightly below the vertical level of the lower edge 42 of the outlet opening 36.
The partition wall 26 advantageously extends from the front wall 16 downwards in an angle of about forty-five degrees, preferably, between thirty and sixty degrees, even more preferably, between forty and fifty degrees, from horizontal, to a center section of the heat exchanger 10. The lower end 44 of the partition wall 26 is advantageously approximately as far from the front wall 16 and the rear wall 18. Preferably, the lower end 44 of the partition wall 26 is in a horizontal cross section spaced apart from the front wall 16 by about 30% to about 70%, even more preferably, by about 40% to about 60%, of the distance between the rear wall 18 and the front wall 16.
The front wall 16 of the heat exchanger 10 is preferably a common wall with the furnace 12. The front wall 16 is in
The side walls of the heat exchanger 10 are preferably formed as a water tube panel, i.e., as a tube-fin-tube construction, of mainly vertical water tubes leading water from a lower header 52 to an upper header (not shown in the Figures). The partition wall 26 is preferably formed of water tubes 54 covered by a layer of refractory 56. According to a preferred embodiment of the present invention, the partition wall 26 is formed by bending water tubes from at least one side wall, preferably, from two side walls, of the heat exchanger 10. Thereby, the forming of the partition wall does not weaken or complicate the arrangement of water tubes in the front wall 16.
According to a preferred embodiment of the present invention, the windbox 30 below the heat exchanger 10 is divided into a first section 30 that is mainly not below the partition wall 26 and a second section 30′ that is mainly below the partition wall 26.
An exemplary way of forming of the partition wall 26 by bending water tubes from the sidewalls of the heat exchanger 10 is schematically shown in
While the invention has been described herein by way of examples in connection with what are at present considered to be the most preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but is intended to cover various combinations or modifications of its features and several other applications included within the scope of the invention as defined in the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 20126064 | Oct 2012 | FI | national |
This application is a U.S. national stage application of PCT International Application No. PCT/FI2013/050975, filed Oct. 9, 2013, published as International Publication No. WO 2014/057173 A1, and which claims priority from Finnish patent application number 20126064, filed Oct. 11, 2012.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/FI2013/050975 | 10/9/2013 | WO | 00 |
