Abstract:
A top-supported circulating fluidized bed boiler system includes a furnace, having sidewalls of a tube wall construction, for combusting fuel and producing combustion products, a particle separator, connected to the furnace, for separating particles from the combustion products from the furnace, an external, preferably non-cooled, heat exchange chamber connected to the particle separator for removing heat from the combustion products, a return duct, connected to the heat exchange chamber, for returning particles separated by the separator to the furnace, a rigid support construction for supporting elements of the system, and a suspension arrangement for suspending the heat exchange chamber from the rigid support construction. The suspension arrangement includes, for preferably 60% or more of its length, at least one of steam tubes and water tubes at a temperature of about 300 to about 550° C.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a circulating fluidized bed combustion system and a heat exchange chamber utilized therein, and, more particularly, to a system in which the heat exchange chamber is provided between a separating section and a furnace section of the circulating fluidized bed combustion system. 
     Fluidized bed combustion systems are well known and include a furnace section in which air is passed through a bed of particulate material to fluidize the bed and to promote combustion of fuel in the bed at a relatively low temperature. The bed may include fossil fuel, such as coal, sand and a sorbent for the sulfur oxides generated as a result of the combustion of the coal. These types of combustion systems are often used in steam generators in which water is passed in a heat exchange relation with the fluidized bed to generate steam and permit high combustion efficiency and fuel flexibility, high sulfur adsorption and low nitrogen emissions. 
     In circulating fluidized bed systems, the fluidizing air velocity is such that the gases passing through the bed entrain a substantial amount of the fine particulate solids. External solids recycling is achieved by disposing a particle separator, usually a cyclone separator, at the furnace outlet to receive the flue gases, and the solids entrained therewith, from the fluidized bed. The solids are separated from the flue gases and the flue gases are passed to a heat recovery section while the solids are recycled back to the furnace. This recycling extends the fuel retention and improves the efficiency of utilization of a sulfur adsorbent, thus reducing consumption of both the adsorbent and fuel. 
     Circulating fluidized beds are characterized by relatively intensive internal and external solids recycling, which makes them insensitive to fuel heat release patterns, thus minimizing temperature variations and stabilizing sulfur emissions at a low level. When fluidized bed systems are used to generate steam, the heat released in the exothermal reactions taking place in the furnace may be recovered by heat exchange surfaces disposed in several locations in the system. The walls of the furnace section are usually so-called tube walls, made by welding tubes together with fins. A heat transferring fluid, usually water or steam, is led through the tube walls in order to cool the furnace walls, and to transfer heat therefrom. Other heat exchange surfaces may be located within the system, such as in the walls of a cooled cyclone, in the heat recovery section downstream of the cyclone or in a separate heat exchange chamber, which may be in flow connection with the internal or external recycling of the solids. 
     The furnace section and the cyclone separator may be bottom-supported, the structure being rigidly supported at its bottom, and the main thermal expansion taking place upwards from the bottom. When designing a large bottom-supported unit, the mechanical loads on the tube walls have to be well considered as the whole weight of the furnace section is transferred through the walls to the lower parts of the boiler, with the tube walls in compressive stress. A significant share of the load may need to be carried from the top steel structure via constant load springs, which may increase the costs significantly. 
     Therefore, it is, especially in large units, conventional to construct a top-supported furnace and cyclone, i.e., to support them on a steel structure constructed on and above the system, with the main thermal expansion taking place downwards. A top-supported unit is generally easier to assemble than a bottom-supported unit. In top-supported systems, the furnace walls do not have to be stiffened due to the weight of the boiler, because the tube walls can easily endure the tensile stress caused by the load. 
     The most typical way of manufacturing a heat exchange chamber is to make it of steel plates, which are thermally isolated and protected against wear by a relatively thick layer of refractory material. Such enclosures are cost-effective to construct but, due to different thermal expansions, difficult to join to other units of the system constructed of tube walls. To solve this problem, one has to use flexible joints, such as metal or fabric baffles to accommodate the relative motions between the different parts of the system. Such baffles, however, are expensive and prone to wear. 
     It is a common practice to construct an external heat exchange chamber as a bottom-supported structure. If the furnace section and the cyclone separator of the system are bottom-supported as well, the relative motions between the different units may be relatively small and the joints therebetween do not have to accommodate large motions. As the heat exchange chamber is typically located near the ground, it is also common, in larger units, to construct the heat exchange chamber as being bottom-supported, while the furnace section and the cyclone separator are top-supported. In such a construction, the relative thermal motions may be very large, and special expansion joints are required to accommodate the motions between the cyclone and the heat exchange chamber and between the heat exchange chamber and the furnace. Typically, these expansion joints are very expensive metal joints. 
     Another method of constructing a heat exchange chamber is to make its enclosure as a cooled tube wall structure. U.S. Pat. No. 5,911,201 describes a suspending unit comprising a cooled heat exchange chamber integrated with a cyclone separator. U.S. Pat. No. 5,425,412 discloses a method of making a furnace, a cyclone and a heat exchange chamber of tube walls and to integrate them all closely together. In such a system, the temperatures of these units are very close to each other, and thus, due to similar materials and constructions, their thermal expansions are very much alike, and no flexible joints are needed between the units. A drawback in such cooled heat exchange chambers, however, is that the construction, especially if it includes complicated structures and cooled inlet and outlet connections, requires a lot of manual bending and welding of the tubes, and is thus time-consuming and expensive to manufacture. Also, in some applications, the heat exchange chambers tightly integrated with the furnace may take too much space around the lower part of the furnace. This is especially the case in large units, where very high total heat exchange capacity, and, e.g., many fuel feeding ducts, as well, are required in the lower part of the furnace. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a fluidized bed combustion system and a heat exchange chamber utilized therein in which the above-mentioned problems are minimized or overcome. 
     It is a more specific object of the present invention to provide a fluidized bed combustion system, and a heat exchange chamber utilized therein, which is cost-effective to construct. 
     Especially, it is an object of the present invention to provide a fluidized bed combustion system and a heat exchange chamber utilized therein in which the costs of the flexible joints in the connections to the heat exchange chamber are minimized. 
     It is a still further object of the present invention to provide a compact fluidized bed combustion system and a heat exchange chamber utilized therein in which a lot of free space is provided around the lower part of the combustion chamber to be used, e.g., for feeding in various materials. 
     Towards the fulfillment of these and other objects, the present invention provides a top-supported fluidized bed boiler system comprising a furnace, having sidewalls of a tube wall construction, for combusting fuel and producing combustion products, a particle separator, connected to the furnace, for separating particles from the combustion products from the furnace, an external heat exchange chamber connected to the particle separator for removing heat from the combustion products, a return duct, connected to the heat exchange chamber, for returning particles separated by the separator to the furnace, a rigid support construction for supporting elements of said system, and suspension means, comprising at least one of steam tubes and water tubes, for suspending said heat exchange chamber from said rigid support construction. 
     The heat exchange chamber may be a simple chamber or a unit which includes several chambers, valves, etc. The supporting hot steam or water tubes, which, when the boiler is in operation, contain water or steam near or above the boiling temperature of water at high pressure, are thus at a temperature of about 300 to about 550° C. Therefore, the hot steam or water tubes have a similar thermal expansion to that of the furnace. Suspending the heat exchange unit by suspension means comprising hot steam or water tubes, instead of supporting it on the ground or hanging it by rigid, cool hanger rods, significantly reduces the relative thermal motions between the furnace and the thermal exchange unit. 
     A large fluidized bed boiler may be several tens of meters high, and thus, the thermal motions may be on the order of a tenth of a meter. As an example, a 30 m long steel wall, steel having a thermal expansion coefficient of 12×10 −6 /° C., lengthens in a temperature change of 300° C. by about 11 cm. Thus, if the upper parts of a furnace separator and a heat exchange chamber located 30 m lower are fixed, the duct from the heat exchange chamber to the lower part of the furnace needs a flexible joint which is able to lengthen vertically by more than 11 cm. 
     According to the present invention, the suspension means of the heat exchanger unit mainly comprises hot steam or water tubes, and thus, the required elasticity of the ducts leading to the heat exchange chamber is clearly less than that in the previous example. According to a preferred embodiment of the present invention, the heat exchange unit is suspended from a steel structure above the boiler system, and more than 60%, more preferably even more than 80%, of the length of the suspension means of the heat exchange unit includes hot steam or water tubes. 
     The particle recycling section of a fluidized bed boiler typically comprises a separator section having a cylindrical upper part, a conical lower part and a return duct connected to a heat exchange chamber. The separator section, or at least the upper part of it, can be made as a cooled tube wall construction. Typically, the horizontal cross section of the heat exchange chamber is about as large as that of the upper part of the particle separator. In such a system, the heat exchange chamber may, according to a preferred embodiment of the present invention, be arranged below the separator section in such a way that the suspension means of the heat exchange chamber includes hanger means which is connected to a cooled upper part of the particle separator. 
     According to another preferred embodiment of the present invention, the suspension means of a heat exchange unit includes hanger means, which comprises hot water or steam tubes and short rigid hanger rods. Such cooled hanger means is preferably arranged between the heat exchange unit and the upper part of a particle separator. According to a preferred embodiment, at least 50%, and even more preferably at least 70%, of the length of the hanger means between the upper part of the particle separator and the heat exchange unit is made of hot water or steam tubes. The hot water or steam tubes between the upper part of the particle separator and the heat exchange unit may be, e.g., steam or water supply lines or extensions of the cooling tubes in the upper part of the particle separator. 
     According to an advanced construction, described in, e.g., U.S. Pat. No. 5,281,398, the particle separator may have a rectangular upper part and a non-symmetrical lower part, where the sidewall of the separator closest to the furnace section extends nearly vertically all the way down to the lower part of the return duct. The manufacturing and maintenance of such a separator is very cost-effective, and it can be connected to the furnace in a compact way. In a preferred embodiment of the present invention, which is especially applicable to non-symmetrical particle separators, as described above, a heat exchange chamber is suspended by hanger means, a part of which is connected to the return duct or to the lower part of the particle separator and another part to the upper section of the particle separator. 
     Preferably, in the above-mentioned embodiment, the part of the hanger means connected to the upper part of the separator comprises hot water or steam tubes and short rigid hanger rods. Correspondingly, the part of the hanger means connected to the return duct or to the lower part of the particle separator preferably comprises short rigid hanger rods connected to an extended horizontal inlet header feeding hot water or steam to vertical tubes of a cooled return duct or of the lower part of the particle separator. 
     Particles are usually conducted from the heat exchange unit back to the lower part of the furnace via a duct having a flexible joint. Because the heat exchange unit, suspended according to the present invention, more or less follows the thermal motions of the furnace, the flexible joint in the duct between the heat exchange unit and the furnace also does not have to endure very large motions, and a joint with a moderate flexibility is sufficient. 
     Compared to the heat exchange unit disclosed in U.S. Pat. No. 5,425,412, the present construction also provides a compact solution, but does not require as much space at the lower part of the furnace. Thus, there is a lot of room for various connections for feeding, e.g., fuel, bed material, sorbent and secondary air to the bed. 
     The main idea of the present invention is that the suspension of the heat exchange unit is not at a constant temperature, but instead, mainly consists of hot water or steam tubes, which approximately follow the temperature of the tube walls of the boiler system. This construction significantly reduces the relative motions between the heat exchange unit and the rest of the boiler system. Thus, large-motion expansion joints are not needed. The reduced motions will also reduce the costs of the expansion joints, and allow the use of fabric baffles rather than very expensive metal baffles. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     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 presently preferred, but nonetheless illustrative, embodiments in accordance with the present invention, when taken in conjunction with the accompanying drawings wherein: 
     FIG. 1 is a schematic elevational view of a fluidized bed combustion system according to a first exemplary embodiment of the present invention; 
     FIG. 2 is another schematic elevational view of a fluidized bed combustion system according to the first embodiment of the present invention; 
     FIG. 3 is a schematic elevational view of a second embodiment of the present invention; and 
     FIG. 4 is a schematic elevational view of a third embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1 and 2 depict a fluidized bed combustion system  10  according to a preferred embodiment of the present invention. The combustion system  10  is used for the generation of steam and includes a furnace section  12 , a separating section  14  (such as a cyclone separator) and a heat exchange chamber  16 . The furnace section  12  includes an upright water-cooled enclosure, having a front wall  18 , a rear wall  20 , two sidewalls  22  and  24 , a floor  26  and a roof  28 . 
     A conduit  30  is provided in the upper portion of the furnace section  12  for permitting combustion flue gases produced in the furnace section  12  to pass from the furnace section  12  into the separating section  14 . It is understood that proper ducting (not shown) is provided to permit the separated gases to pass from the top of the separating section  14  to a heat recovery section, dust separator and stack (not shown). 
     The walls  18 ,  20 ,  22  and  24  of the furnace section  12 , as well as the walls  74 ,  76 ,  80  and  82  of the separating section  14 , are formed by a plurality of heat exchange tubes formed in a parallel, gas-tight manner to carry fluid to be heated, such as water or steam. It is also understood that a plurality of headers, of which only header  72  is shown, is disposed at both ends of each of the tube walls which, along with additional tubes and associated flow circuitry, would function to route the water through the water tubes of the reactor in a conventional manner. 
     An air distributor system including a plurality of air distributor nozzles (not shown) are mounted in corresponding openings formed in a tube panel  32  extending across the lower portion of the enclosure  12 . The tube panel  32  is spaced from the floor  26  to define an air plenum  34 , which is adapted to receive air from an external source (not shown) and to distribute the air through the nozzles into the furnace section  12 . 
     The separating section  14  comprises a straight upper part  36 , a hopper-like lower part  38  and a return duct  40 . The separated particulate material passes from the separating section  14  through the return duct  40  into the heat exchange chamber  16 . The heat exchange chamber  16  is made cost-effectively of metal plates covered by a relatively thick layer of insulation to prevent both erosion and heat loss from the chamber. Thus, the outer walls of the chamber  16  are not cooled. Naturally, the interior of the heat exchange chamber  16  comprises heat exchange surfaces (not shown) to recover heat from the recirculating particulate material into a fluid, such as water or steam, flowing through the interior of the heat exchange surfaces in the heat exchange chamber  16 . 
     From the heat exchange chamber  16 , the recirculating material is conducted, via a conduit  44 , back to the furnace section  12  of the combustion system  10 . Into the conduit  44  may be connected a fuel feeder  46 , by which particulate material containing fuel may be introduced into the furnace section  12 . Additional feeders  48  for fuel, as well as for inert bed material, a sulfur adsorbing agent, etc., may be located in the lower portion of the furnace section  12 . Secondary air is introduced into the furnace section  12  by inlets  50 . 
     A plurality of vertically extending steel support columns  52  extends from the ground  54  to a plurality of spaced horizontally extending beams  56 . A plurality of hanger rods  58  extends downwardly from the beams  56  for supporting the furnace section  12  and the separating section  14 . 
     According to the present invention, the heat exchange chamber  16  is supported by a plurality of short hanger rods  60  and  62 , which are supported by hot water or steam tubes. In the embodiment shown in FIGS. 1 and 2, the hanger rods  60  are supported by the horizontal inlet header  72 , which feeds hot water or steam to a planar wall  74  of the separating section  14 . As seen in FIG. 2, even if the return duct  40  is downwardly tapered, the wall  74  maintains its full width all the way down to the header  72 , allowing the hanger rods  60  to be connected on both sides of the return duct  40 . 
     In the embodiment of FIGS. 1 and 2, it is possible to fix the hanger rods directly to the header  72  of the tubes of wall  74  because the return duct of the cyclone separator of the separating section  14  is located non-symmetrically, as a continuation of the wall  74 . On the opposite, “outboard” side, the corresponding sidewall  76  of the separating section  14  does not extend down as low as on the “inboard” side, and thus, a different supporting system has to be used. If a rigid connecting rod extended all the way from the heat exchange chamber  16  to the upper part  36  of the cyclone separator of separating section  14 , the relative thermal motions between the inboard and outboard sides would be large, and a special arrangement would be required to compensate for the difference. 
     According to a further embodiment of the present invention, when a heat exchange chamber  16  is to be supported by the upper part of the cyclone separator of separating section  14 , vertical sections  68  of water or steam supply lines  66  are used as a part of the supporting system. The main function of the lines  66  is to supply water or steam to the tube walls of the separating section  14  or some other part of the boiler system of the combustion system  10 . In the embodiment shown in FIGS. 1 and 2, the lower part of the vertical section  68  of the supply line  66  is connected to the heat exchange section  16  by a short hanger rod  62 . Correspondingly, the upper part of the vertical section  68  of the supply line  66  is connected to the upper part of the cyclone separator  14  by a short hanger rod  64 . 
     Because the thermal expansion of the hanger means at the “inboard” and “outboard” sides of the heat exchange chamber  16  can, according to the disclosed constructions, be made very much alike, no special arrangements are needed to compensate for their difference. Also, the thermal expansion of the hanger means is close to that of the return duct  40  and the lower part  38  of the separating section  14 , and thus, a relatively short baffle  70  suffices to compensate for their relative thermal motions. 
     The suspension system of the heat exchange chamber  16  closely follows the thermal motion of the rest of the top-supported fluidized bed reactor system  10 . 
     Therefore, the connection between the heat exchange chamber  16  and the lower part of the furnace section  12  also can be made simply, by using a mainly slant tube  44 , which includes a vertical portion with a short baffle  78 . The disclosed construction is compact in the sense that the heat exchange chamber  16  is located close to the separating section  14  and the furnace section  12 . However, the heat exchange chamber  16  does not take up any space near the lower part of the furnace section  12  or near the ground  54 . Therefore, a lot of room remains to arrange other possible conduits and reservoirs near the lower part of the furnace section  12 . 
     FIG. 3 schematically shows the suspension system of a heat exchange chamber  16  according to another embodiment of the present invention. In fact, FIG. 3 shows a modification of a portion of FIG. 1, where hot steam or water is fed to the wall tubes of sidewall  80 , and of sidewall  82  (which is not shown in this figure), of the separating section  14  via horizontal inlet headers  84 . The heat exchange chamber  16  is suspended by rigid hanger rods  86  fixed to the inlet headers  84 . FIG. 3 shows three hanger rods, but naturally, their number can vary in practical applications. One can also combine the types of suspension means shown in FIGS. 1 and 3, if required. It is also possible to extend a portion, e.g., every fifth tube, of the wall tubes from wall  76  in FIG. 1 down, e.g., to the level of the inlet header  84 , and to utilize these tubes as a part of the suspension system of the heat exchange chamber  16 . 
     FIG. 4 schematically shows a suspension system of a heat exchange chamber  16  in connection with a symmetrical separating section  14 , according to a third embodiment of the present invention. In FIG. 4, all the hanger means of the heat exchange chamber  16  include vertical sections  68  of hot water or steam tubes  66 . These vertical sections  68  are connected to the heat exchange chamber  16  and to the lower edge of the cylindrical upper part  36  of the separating section  14  by short rigid hanger rods  62  and  64 , respectively. Thus, the thermal expansion of the hanger means nearly corresponds to that of the lower part  38  of the separating section  14  and the return duct  40 , and a short baffle  70  suffices to compensate for their relative thermal motions. 
     While the invention has been herein described 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 their features and several other applications included within the scope of the invention as defined in the appended claims.