Abstract:
A circulating fluidized bed (CFB) boiler has one or more bubbling fluidized bed enclosures containing heating surfaces and located within a lower portion of the CFB boiler to provide a compact, efficient design with a reduced footprint area. The heating surfaces are provided within the bubbling fluidized bed located above a CFB grid and/or in a moving packed bed below the CFB grid inside the lower portion of the CFB boiler. Solids in the bubbling fluidized bed are maintained in a slow bubbling fluidized bed state by separately controlled fluidization gas supplies. Separately controlled fluidization gas is used to control bed level in the bubbling fluidized beds or to control the throughput of solids through the bubbling fluidized beds. Solids ejected from the bubbling fluidized beds can be returned directly into the surrounding CFB environment of the CFB boiler, or purged from the system for disposal or recycle back into the CFB. Solids which are recycled back to the CFB have less heat and can be used to control the temperature of the fast moving bed in the CFB.

Description:
FIELD AND BACKGROUND OF THE INVENTION 
     The present invention relates generally to the field of circulating fluidized bed (CFB) reactors or boilers such as those used in electric power generation facilities and, in particular, to a new and useful CFB reactor arrangement which permits temperature control within the CFB reaction chamber and/or of the effluent solids. The CFB reactor arrangement according to the invention contains and supports not only the CFB but also one or more bubbling fluidized bed(s) (BFB&#39;s) in a lower portion of the CFB reactor enclosure; i.e., one or more slow bubbling bed region(s) are maintained and located within a fast CFB region. An arrangement of heating surface is located within the bubbling fluidized bed(s) (BFB&#39;s). Heat transfer to the heating surface is controlled by providing separately controlled fluidizing gas to the bubbling fluidized bed(s) (BFB&#39;s) to either maintain a desired bed level or control a throughput of solids through the bubbling fluidized bed(s) (BFB&#39;s). 
     Most prior arts bubbling bed heat exchangers known to the inventors are located outside of the CFB reaction chamber and occupy at least one of the enclosure walls. 
     For example, U.S. Pat. Nos. 5,526,775 and 5,533,471 to Hyppänen each disclose a CFB having an adjacent bubbling fluidized bed with an integral heat exchanger. U.S. Pat. No. 5,533,471 teaches placing the slow bubbling fluidized bed below and to the side of the bottom of the faster moving CFB chamber. In U.S. Pat. No. 5,526,775, the slow bubbling bed is above and to the side of the fast CFB. Each of the slow beds is controlled by permitting particles to escape back into the main CFB chamber from an opening in the side of the slow bed chamber. These heat exchangers further require a different gas distribution grid level for each bed, which substantially complicates the structure of the CFB systems. The plan area of the CFB can be increased as a result. 
     Other patents disclose heat exchanger elements located above the grid of a CFB furnace, but not within a slow bubbling bed region of a fast CFB. U.S. Pat. No. 5,190,451 to Goldbach, for example, illustrates a CFB chamber having a heat exchanger immersed within a fluidized bed at the lower end of the chamber. The bed has only one air injector for controlling the circulation rate for the entire bed. 
     U.S. Pat. No. 5,299,532 to Dietz discloses a CFB having a recycle chamber immediately adjacent the main CFB chamber. The recycle chamber receives partially combusted particulate from a cyclone separator connected between the recycle chamber and the upper exhaust of the main CFB chamber. A heat exchanger is provided inside the recycle chamber, and the recycle chamber is separated from the main CFB chamber by water walls and occupies part of the lower portion of the furnace enclosure; the recycle chamber does not extend outwardly from the furnace enclosure. 
     U.S. Pat. No. 5,184,671 to Alliston et al. teaches a heat exchanger having multiple fluidized bed regions. One region has heat exchange surfaces, while the other regions are used to control the rate of heat transfer between the fluidized bed material and the heat exchanger surfaces. 
     None of these prior art bubbling beds is incorporated in a manner which simplifies the overall construction of the CFB reactor and permits easy access to enclosure walls for feeding reagents, maintenance and inspections. 
     SUMMARY OF THE INVENTION 
     The present invention seeks to overcome the limitations of the prior art CFB slow bed heat exchangers by providing a CFB boiler or reactor having an internal heat exchanger in a slow bubbling bed, and without increasing the plan area of the CFB. 
     Accordingly, one aspect of the present invention is drawn to a circulating fluidized bed (CFB) boiler, comprising: a CFB reaction chamber having side walls and a grid defining a floor at a lower end of the CFB reaction chamber for providing fluidizing gas into the CFB reaction chamber. Means are provided for supplying an amount of fluidizing gas to a first portion of the grid sufficient to produce a fast moving bed of fluidized solids in a first zone of the CFB reaction chamber, and for providing an amount of fluidizing gas to a second portion of the grid sufficient to produce a bubbling fluidized bed of fluidized solids in a second zone of the CFB reaction chamber. The amount of fluidizing gas provided to one zone is controllable independently of the amount of fluidizing gas provided to the other zone. Finally, means are provided for removing solids from the first and second zones for purging the solids from or recycling the solids to the CFB boiler to control the fast moving bed. 
     Thus, the CFB boiler is partitioned into two portions: a first portion or zone which is operated as a fast moving circulating fluidized bed, and a second region or zone which is operated as a slow bubbling fluidized bed. 
     The slow bubbling bed height is controlled within the range corresponding to the height of its enclosure walls. Mechanisms for controlling the slow bed height include outlets through the top of the enclosure and a valved outlet through the bottom side edges of the enclosure. 
     In an alternate embodiment, a portion of the floor-level grid has openings sufficient to allow particles to fall through. A heat exchanger is located directly below the main CFB chamber. A secondary fluidizing gas supply is provided in the region of the grid above the heat exchanger. The amount of particles falling through into the area below the grid with the slow bubbling bed can be controlled by controlling their purge or recycle rate. 
     In a further embodiment, the above-grid enclosure for one heat exchanger is combined with the below-grid position of a second heat exchanger. 
     The improved CFB design of the invention permits a reduced footprint size of the CFB and allows the enclosure walls to be straightened. The design is simpler in construction and provides easier access to the enclosure walls for feeding reagents. 
     The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which a preferred embodiment of the invention is illustrated. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1 is a sectional side elevational view of a CFB boiler according to a first embodiment of the invention, illustrating a bubbling fluidized bed (BFB) enclosure within the CFB boiler; 
     FIG. 2 is a sectional plan view of the CFB boiler of FIG. 1, viewed in the direction of arrows  2 — 2 ; 
     FIG. 3 is a partial sectional side elevational view of a CFB boiler according to a second embodiment of the invention illustrating removal of solids from the bubbling fluidized bed (BFB) enclosure via one or more internal conduits; 
     FIG. 4 is a partial sectional side elevational view of a CFB boiler according to a third embodiment of the invention illustrating removal of solids from the bubbling fluidized bed (BFB) enclosure via one or more non-mechanical valves; 
     FIG. 5 is a partial sectional side elevational view of a CFB boiler according to a fourth embodiment of the invention illustrating placement of heating surface below an arrangement of air supply tubes located below an upper surface of a grid level of the CFB boiler; 
     FIG. 6 is a partial sectional side elevational view of a CFB boiler according to a fifth embodiment of the invention illustrating placement of heating surface within an arrangement of air supply tubes located below an upper surface of a grid level of the CFB boiler; 
     FIG. 7 is a partial sectional side elevational view of a CFB boiler according to a sixth embodiment of the invention illustrating placement of heating surface both within and below an arrangement of air supply tubes located below an upper surface of a grid level of the CFB boiler; 
     FIG. 8 is a partial sectional side elevational view of a CFB boiler illustrating the application of several principles of the invention; 
     FIGS. 9-14 are top plan views of alternate locations or positions inside the CFB boiler of the bubbling fluidized bed (BFB) enclosures which contain the heating surfaces according to the invention; 
     FIG. 15 is a perspective view of a lower portion of the CFB boiler illustrating one form of the construction of the bubbling fluidized bed (BFB) enclosure; and 
     FIG. 16 is another perspective view of a lower portion of the CFB boiler illustrating another form of the construction of the bubbling fluidized bed (BFB) enclosure. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As used herein, the term CFB boiler will be used to refer to CFB reactors or combustors wherein a combustion process takes place. While the present invention is directed particularly to boilers or steam generators which employ CFB combustors as the means by which the heat is produced, it is understood that the present invention can readily be employed in a different kind of CFB reactor. For example, the invention could be applied in a reactor that is employed for chemical reactions other than a combustion process, or where a gas/solids mixture from a combustion process occurring elsewhere is provided to the reactor for further processing, or where the reactor merely provides an enclosure wherein particles or solids are entrained in a gas that is not necessarily a byproduct of a combustion process. 
     Referring now to the drawings, wherein like reference numerals designate the same or functionally similar elements throughout the several drawings, and to FIG. 1 in particular, there is illustrated a circulating fluidized bed (CFB) reactor or boiler, generally referred to as CFB boiler  10 . The CFB boiler  10  has a reactor or reaction chamber or furnace enclosure  12  containing a circulating fluidized bed  14 . As is known to those skilled in the art, the furnace enclosure  12  is typically rectangular in cross-section and comprises fluid cooled membrane tube enclosure walls  16  typically comprised of water and/or steam conveying tubes separated from one another by a steel membrane to achieve a gas-tight reactor enclosure  12 . 
     Air  18 , fuel  20  and sorbent  22  are provided into a lower portion of the furnace  12  and react in a combustion process to produce hot flue gas and entrained particles  24  which pass up through the furnace  12  reactor. The hot flue gases and entrained particles  24  are then conveyed through several cleaning and heat removal stages,  28 ,  30 , respectively, before the hot flue gases are conveyed to an exhaust flue  32  as shown. Collected particles  26  are returned to the lower portion of the furnace where further combustion or reaction can occur. 
     The lower portion of the furnace  12  is provided with a fluidization gas distribution grid  34  (advantageously a perforated plate or the like provided with a multiplicity of bubble caps (not shown)) up through which fluidizing gas (typically air) is provided under pressure to fluidize the bed of fuel  20 , sorbent  22 , collected solids particles  26 , and recycled solids particles  40  (described infra) which had been purged from the system. Any additional air needed for complete combustion of the fuel  20  is advantageously provided through the enclosure walls  16  as shown at  18 . The fast moving CFB  14  is thus created above the distribution grid  34 , with solids particles moving rapidly within and through the flue gases resulting from the combustion process. 
     Although the CFB  14  features a vigorous circulation of entrained solids, some of these solids cannot be supported by the upward gas flow from grid  34  and thus fall back toward the grid  34 , while others continue upward through the furnace  12  as described earlier. Some solids particles are removed from the lower portion of the furnace  12  via bed drains  36  and may be purged from the system as shown at  38 , or recycled as shown at  40 . The flow of solids removed via the bed drains  36  may be controlled in any known manner, such as with mechanical rotary valves or screws, or air-assisted conveyors or valves, or combinations thereof. In any event, it will be appreciated that the lower portion of the furnace  12  is exposed to an intensive downfall of solids particles. 
     According to the present invention, in its simplest form, a bubbling fluidized bed (BFB) enclosure  42  having enclosure walls  44  is provided above the grid  34  within the furnace  12  in the lower portion thereof, and contains a bubbling fluidized bed (BFB)  46  during operation of the CFB boiler  10 . The enclosure walls  44  separate the bubbling fluidized bed (BFB)  46  from the CFB  14 . The bubbling fluidized bed (BFB)  46  is created by separately supplying and controlling fluidizing gas to it up through the grid  34 ; that is, separate from that portion of the fluidizing gas provided up through the grid  34  which establishes the CFB  14 . The CFB boiler  10  is thus partitioned into two general types of regions or zones above the grid, wherein the zones are created by providing and controlling different amounts of fluidizing gas through the grid into each zone. The first zone, of course, is the main circulating fluidized bed (CFB) zone, while the second zone is a bubbling fluidized bed (BFB) region or zone  46  which is contained within the CFB zone  14 . 
     As illustrated in FIG. 1, the fluidizing gas provided to the bubbling fluidized bed (BFB)  46  is designated  48 , and controlled by valve or control means schematically indicated at  50 . The fluidizing gas provided to establish the CFB  14  is designated  52 , and is controlled by valve or control means schematically indicated at  54 . 
     Located within the bubbling fluidized bed (BFB) enclosure  42  is an arrangement of heating surface  56  which absorbs heat from the bubbling fluidized bed (BFB)  46 . The heating surface  56  may advantageously be superheater, reheater, economizer, evaporative (boiler), or combinations of such types of heating surface which are known to those skilled in the art. The heating surface  56  is typically a serpentine arrangement of tubes which convey a heat transfer medium therethrough, such as water, a two-phase mixture of water and steam, or steam. While the overall furnace  12  operates in a CFB mode, the bubbling fluidized bed (BFB)  46  is operated and controlled as such by separately controlling, as at  50 , the amount of fluidizing gas  48  provided up through that portion of the grid  34  beneath the bubbling fluidized bed (BFB) enclosure  42 . Downfalling solids particles  24  from the CFB  14  within the lower portion of the furnace  12  feed the bubbling fluidized bed (BFB)  46 . 
     The enclosure walls  44  of the bubbling fluidized bed (BFB) enclosure  42  may all be the same height or different, and vertical, sloped or a combination thereof. The top of the bubbling fluidized bed (BFB) enclosure  42  may be inclined or substantially horizontal and, if necessary, may be partially covered. However, it will be appreciated that the maximum level or height of the bubbling fluidized bed (BFB)  46  within the enclosure  42  is limited by the height of the shortest enclosure wall  44  of the enclosure  42 . As illustrated in FIG. 2, one preferred location of the bubbling fluidized bed (BFB) enclosure  42  is in a central portion of the furnace  12 . However, as illustrated in FIGS. 9-14, infra, other locations for the bubbling fluidized bed (BFB) enclosure  42  within a lower portion of the furnace  12  are also acceptable. 
     An important aspect of the present invention is that the bubbling fluidized bed (BFB)  46  may be controlled to control the heat transfer to the heating surface  56  located within the bubbling fluidized bed (BFB)  46 . This can be accomplished by either controlling the level of the solids within the bubbling fluidized bed (BFB)  46 , or by controlling the throughput of solids across the heating surface  56  located within the bubbling fluidized bed (BFB)  46 . 
     FIG. 3 illustrates one optional means for controlling the heat transfer within the bubbling fluidized bed (BFB)  46 , which comprises provision of one or more conduits  58  extending from a lower part of the bed  46  just above the grid  34  to an upper level at or above the lowest portion of the walls  44 , and the conduit(s)  58  may have any general configuration which satisfies this criteria. Below each of the conduit(s)  58  there is provided a gas conduit  57  and separate fluidizing means which introduces fluidizing gas  60  controlled via valve means  62 . By fluidizing the solids particles in the conduit(s)  58  located directly above the gas conduit  57 , their upward movement through the conduit(s)  58  is promoted, causing the solids particles to be discharged from the bubbling fluidized bed (BFB)  46  into the surrounding CFB  14 . When the fluidizing gas  60  rate is increased, or additional conduits  58  are put into operation, the overall solids discharge from the bubbling fluidized bed (BFB)  46  will eventually exceed the solids influx into the bed  46  from the CFB  14 , causing the bed level to decrease. The more the solids discharge from the bed  46  exceeds the solids influx from the CFB  14 , the lower the bed level will become. 
     FIG. 4 illustrates another means for controlling the heat transfer within the bubbling fluidized bed (BFB)  46  which involves provision of one or more non-mechanical valve(s)  64  each with its own controlled gas supply  66  controlled via gas conduit  57  and valve means  68 . Gas flow to the vicinity of the valve(s)  64  promotes solids discharge from the lower part of the bubbling fluidized bed (BFB)  46  into the CFB  14 . Again, by controlling the gas flow rate and/or the number of valve(s)  64  in operation, the bubbling fluidized bed (BFB) level can be controlled in a manner similar to that described above. 
     When the overall solids discharge is lower than the solids influx, the bed  46  level is constant, being determined by the height of the lowest enclosure wall  44 . In this situation, increasing the solids discharge from the lower part of the bed  46  (via either of the approaches of FIGS. 3 or  4 ) will cause an increased supply of “fresh” influx solids from the upper portion of the bed  46  to the heating surface  56 . This will intensify the heat transfer between the bed  46  and the heating surface  56 . If the discharge rate from the bed  46  is increased further, the bed level will decrease, thereby reducing the area of heating surface  56  immersed in the bed  46  solids. Since the heat transfer rate for non-immersed portions of heating surface is significantly lower than for immersed portions, the overall heat transfer rate to the heating surface, and its heat transfer medium being conveyed therethrough, will decrease. This provides an operator of the CFB boiler  10  with increased operational flexibility, since overall heat transfer can be controlled in different modes-with a constant or variable bed  46  level-as dictated by operational requirements or convenience. 
     When heat is transferred from the solids to the heating surface  56 , the solids temperature in the bubbling fluidized bed (BFB)  46  will differ from that in the CFB  14 . When a solids purge from the lower part of the CFB boiler  10  is required, it may be beneficial to discharge these solids from the bubbling fluidized bed (BFB)  46 , since purging cooled bottom ash from a CFB furnace  12  reduces the sensible heat loss that would otherwise occur if hotter solids were purged. 
     FIG. 5 illustrates another way of implementing the invention. In this embodiment, the lower portion of the CFB furnace  12  again has a fluidization grid  34  with its own fluidizing gas supply  52 . However, one or more portions  70  of the grid  34  is provided with its own, separately controlled gas supply  72 . Portion  70  of the grid has an arrangement of air supply tubes  76  provided with bubble caps  78  spaced from one another to provide openings sufficient for bed solids particles to fall downwardly through the grid. In one aspect of the present invention, these particles fall across a heating surface  74  located in the vicinity of the grid  34  but below the upper surface of the grid  34  level. In this configuration, the heating surface  74  is well suited to the task of cooling the discharged solids prior to purging (as described above) or recycling them back into the CFB boiler  10 . 
     Solids particles traveling downwardly will pass across the heating surface  74  resulting in heat transfer between the solids particles and the heating surface  74 . Again, the overall heat transfer can be controlled by controlling solids flow rate across the heating surface  74 ; solids can then be purged or recycled back to the CFB  14  as before. Such purge and recycle flows can be handled by known means such as mechanical devices, e.g., a rotary valve or a screw, or non-mechanical devices, e.g., an air-assisted conveyor or valve, or a combination of mechanical and non-mechanical devices. FIGS. 6 and 7 illustrate other variations in the placement of the heating surface  74  below the grid level. In FIG. 6, heating surface  80  is located interspersed inbetween the air supply tubes of portion  70 , while in FIG. 7, the heating surface  74  is located below the air supply tubes of portion  70  while an additional heating surface  80  is located interspersed inbetween the air supply tubes of portion  70 . 
     By developing a way to place the bubbling fluidized bed (BFB) enclosure  42  with the heating surface  74 ,  80  within the CFB chamber  12 , as opposed to being offset to the sides outside of the CFB boiler  10 , the overall footprint, or plan area of the CFB boiler  10  is reduced. Further, the CFB chamber  12  may have straight side walls  16 , which reduces maintenance and erosion, while providing easier access to the enclosure walls  16  for feeding reagents to the combustion process, installing additional structure and performing maintenance. Straight furnace enclosure walls  16  can be used when the total area of the grid  34  occupied by the bubbling fluidized bed (BFB) enclosure  42  and the balance of the CFB grid  34  is selected to be equal to the plan area of the upper part of the CFB chamber  12 . The required upward gas velocity can still be achieved in the lower part in such case. 
     FIG. 8 is a partial sectional side elevational view of a CFB boiler illustrating the application of several principles of the invention. As shown, heating surface  56 , located above the grid  34 , and heating surface  74  located below the air supply tubes  76  may be provided. Heating surface  80 , as before, could also be included if desired. In this embodiment, means for controlling the heat transfer within the bubbling fluidized bed (BFB)  46  involves provision of the one or more non-mechanical valve(s)  64  each with its own controlled gas supply  66  (not shown) controlled via gas conduit  57  and valve means  68  (not shown). 
     While to this point each of the embodiments has illustrated the bubbling fluidized bed (BFB) enclosure  42  as being substantially in the center of the CFB chamber  12 , the one or more bubbling fluidized bed (BFB) enclosure(s)  42  may be located in different positions within the CFB boiler, as illustrated in FIGS. 9-14. FIGS. 9-14 each illustrate different locations in the CFB boiler  10  where one or more bubbling fluidized bed (BFB) enclosures  42  can be located. As seen in each case, the enclosure  42  is located entirely within the furnace enclosure walls  16  of the CFB chamber  12 , thereby providing a reduced plan area of the CFB boiler  10 . Regardless of the particular location within the CFB boiler  10 , the bubbling fluidized bed (BFB) enclosures  42  can be used as described above to control the operation of the CFB  10  in an effective manner while reducing the footprint space needed for the CFB boiler  10 . 
     The enclosure walls  44  forming the bubbling fluidized bed (BFB) enclosure  42  may be constructed in several ways. Preferably, the enclosure walls  44  would be comprised of fluid cooled tubes covered with erosion resistant material such as brick or refractory to prevent erosion of the tubes during operation. FIG. 15 is a perspective view of a lower portion of the CFB chamber  12  illustrating one form of the construction of the bubbling fluidized bed (BFB) enclosure  42 , and which is particularly suited for an enclosure  42  which is not adjacent to any of the furnace enclosure walls  16 . The walls  44  are made of fluid cooled tubes  82  covered with brick or refractory  84 . Inlet or outlet headers may be provided as required to provide or collect the fluid conveyed through the tubes  82  in known fashion. In FIG. 15, for example, an inlet header  86  may be provided underneath the grid  34 , and which supplies the tubes  82 . After encircling the bubbling fluidized bed (BFB) enclosure  42 , the tubes  82  then form a division wall  90  which could extend throughout the entire height (not shown in FIG. 15) of the CFB furnace  12 , terminating at an upper outlet header (also not shown) above a roof of the furnace  12 . 
     Another design option may be used when a bubbling fluidized bed (BFB) enclosure  42  is adjacent to at least one furnace enclosure wall  16 . FIG. 16 is another perspective view of a lower portion of the CFB chamber  12  illustrating such a construction of the bubbling fluidized bed (BFB) enclosure  42 . Again, the enclosure walls  44  are made of refractory covered tubes  82 ; in this case, they penetrate through the furnace enclosure walls  16 , and are provided with inlet header  86  and outlet header  88 . 
     While specific embodiments of the invention have been shown and described in detail to illustrate the application of the principles of the invention, those skilled in the art will appreciate that changes may be made in the form of the invention covered by the following claims without departing from such principles. For example, the present invention may be applied to new construction involving circulating fluidized bed reactors or combustors, or to the replacement, repair or modification of existing circulating fluidized bed reactors or combustors. In some embodiments of the invention, certain features of the invention may sometimes be used to advantage without a corresponding use of the other features. Accordingly, all such changes and embodiments properly fall within the scope of the following claims.