FLUIDIZED BED REACTOR HAVING AN INTEGRATED RECYCLE HEAT EXCHANGER

Abstract: A fluidized bed reactor in which a recycle heat exchanger (40) is located adjacent the furnace (4) of the reactor with each enclosing a fluidized bed and sharing a common wall including a plurality of water tubes. A mixture of flue gases and entrained particulate materials from the fluidized bed in the furnace are separated and the flue gases are passed to a heat recovery area (8) and the separated particulate material to the fluidized bed in the recycle heat exchanger (40). The bed materials in the fluidized bed in the recycle heat exchanger are passed to the fluidized bed in the furnace. Steam is generated, superheated and reheated in the heat exchange surfaces, including water wall tubes, in the furnace, the recycle heat exchanger, and the heat recovery area.

Description: This invention relates to a fluidized bed combustion system and a method of operating same. It is particularly concerned with a fluidized bed reactor in which a recycle heat exchanger is formed integrally with the steam generator. Fluidized bed reactors, such as gasifiers, steam generators, combustors, and the like, are well known. In these arrangements, air is passed through a bed of particulate material, including a fossil fuel such as coal and an adsorbent for the sulphur generated as a result of combustion of the coal, to fluidize the bed and to promote the combustion of the fuel at a relatively low temperature. The entrained particulate solids are separated externally of the bed and recycled back into the bed. The heat produced by the fluidized bed is utilized in various applications such as the generation of steam, which results in an attractive combination of high heat release, high sulphur adsorption, low nitrogen oxides emissions and fuel flexibility. The most typical fluidized bed reactor is commonly referred to as a "bubbling" fluidized bed in which the bed of particulate material has a relatively high density and a well-defined, or discrete, upper surface. Other types of fluidized bed reactors utilize a "circulating" fluidized bed. According to these processes, the fluidized bed density is well below that of a typical bubbling fluidized bed, the air velocity is greater than that of a bubbling bed or the flue gases passing through the bed entrain a substantial amount of particulate solids and are substantially saturated therewith. Also, circulating fluidized beds are characterized by relatively high solids recycling which makes it insensitive to fuel heat release patterns, thus minimizing temperature variations, and therefore, stabilizing the emissions at a low level. The high solids recycling improves the efficiency of the mechanical device used to separate the gas from the solids for solids recycle, and the resulting increase in sulphur adsorbent and fuel residence times reduces the adsorbent and fuel consumption. However, several problems do exist in connection with these types of fluidized bed reactors, and more particularly, those of the circulating type. For example, a sealing device such as a seal pot, a syphon seal, or an "L" valve and a hot expansion joint are required between the low pressure cyclone discharge and the higher pressure furnace section of the reactor, and the transfer of the separated particulate material from the cyclone back to the fluidized bed furnace has to be done by a gravity chute or a pneumatic transport system. Such an arranged is shown in US-A-4 716 856. The addition of these components add to the cost and complexity of the system. Also in these types of reactors the particulate material recycled from the cyclone to the fluidized bed furnace has to be at a fairly precise temperature. This requires an increased furnace height or the installation of wear-prone surfaces in the upper furnace to cool the particulate material before being reinjected into the fluidized bed to the appropriate temperature. This causes the furnace exit flue gases to be cooled to the point where the efficiency of the downstream convection heat exchange surfaces suffer and extra surfaces are required since the heat recovery area requires the installation of all the reheat and superheat surfaces. Further, a hot expansion joint is required between the outlet of the cyclone and the inlet to the fluidized bed furnace which is subjected to positive pressure, a distinct disadvantage. A fluidized bed combustion system to which the present invention relates includes a furnace, at least a portion of the walls of which include tubes for boiler water for supporting a fluidized bed of combustible particulate material disposed in the furnace, a recycle heat exchanger with means for maintaining a fluidized bed of particulate material therein, the heat exchanger being disposed adjacent the furnace and sharing a common wall therewith, separating means for receiving a mixture of flue gases and entrained particulate material from the furnace and separating such particulate material and such flue gases, means for passing separated flue gases to a heat recovery area, and means for passing separated particulate material to the recycle heat exchanger, and is characterised in that a vertical partition is disposed in the recycle heat exchanger for dividing the recycle heat exchanger into a chamber for receiving particulate material from the separating means, and a vertically extending passage located between the common wall and said chamber for receiving particulate material from the chamber, the common wall having an opening registering with the lower end of the passage for feeding particulate material therein to the fluidized bed in the furnace, and the height of the passage being such that the quantity of particulate material maintained therein is sufficient to seal against backflow of flue gases from the furnace through the recycle heat exchanger to the separating means. Also a method of operating a fluidized bed combustion system to which the invention relates comprises the steps of fluidizing a bed of combustible particulate material in a furnace, at least a portion of the walls of which include tubes for boiler water, combusting the particulate material to thereby discharge a mixture of flue gases and entrained particulate material, separating the particulate material and flue gases in the mixture, passing the separated flue gases to a heat recovery area and passing the separated particulate material into a receiving chamber of a recycle heat exchanger, the recycle heat exchanger sharing a common wall with the furnace, and fluidizing the particulate material in said chamber, and is characterised in that particulate material in the chamber of the recycle heat exchanger is passed to a passage which is located between the common wall and the chamber and which is divided from the chamber by a vertical partition, the depth of said separated particulate material being maintained sufficient to seal against backflow of flue gases from the furnace through the recycle heat exchanger to the separating means, and from the passage to the fluidized bed in the furnace. By adopting the present invention there is no need for pneumatic transport devices between the separator and the furnace section of the reactor. Also the height of the furnace section of the reactor can be reduced and the need for wear-prone surfaces in the upper furnace section eliminated. Further radiant superheater and/or reheater surfaces in the upper portion of the furnace is eliminated. Finally the efficiency of the heat exchange surfaces is increased, and optimum bed temperatures can be achieved. The invention will now be illustrated by the following detailed description of the presently preferred but nonetheless illustrative embodiment taken in conjunction with the accompanying drawing which is a schematic representation depicting the system of the present invention. Referring specifically to the drawing, the reference numeral 2 refers, in general, to a fluidized bed reactor which includes a furnace section 4, a separating section 6, and a heat recovery area 8. The furnace sect on 4 includes an upright enclosure 10 and an air plenum 12 disposed at the lower end portion of the enclosure for receiving air from an external source. An air distributor 14 is provided at the interface between the lower end of the enclosure 10 and the air plenum 12 for allowing the pressurized air from the plenum to pass upwardly through the enclosure 10. A bed 15 of particulate material is supported on the air distributor 14 and one or more inlets 16 are provided through the front wall of the enclosure 10 for introducing a particulate material onto the bed, and a drain pipe 17 registers with an opening in the air distributor 14 for discharging spent particulate material from the bed 15. The particulate material can include coal and relatively fine particles of an adsorbent material, such as limestone, for adsorbing the sulphur generated during the combustion of the coal, in a known manner. The air from the plenum 12 fluidizes the particulate material in the bed 15. It is understood that the walls of the enclosure 10 include a plurality of water tubes disposed in a vertically extending relationship and that flow circuitry (not shown) is provided to pass water through the tubes to convert the water to steam. Since the construction of the walls of the enclosure 10 is conventional, the walls will not be described in any further detail. The separating section 6 includes one or more cyclone separators 18 provided adjacent the enclosure 10 and connection thereto by ducts 20 which extend from openings formed in the upper portion of the rear wall of the enclosure 10 to inlet openings formed in the upper portion of the separators 18. The separators 18 receive the flue gases and entrained particulate material from the fluidized bed 15 in the enclosure 10 and operate in a conventional manner to disengage the particulate material from the flue gases due to the centrifugal forces created in the separator. The separated flue gases pass, via ducts 22, into and through the heat recovery area 8. The heat recovery area 8 includes an enclosure 24 housing a superheater 26, a reheater 28 and an economizer 30, all of which are formed by a plurality of heat exchange tubes 34 extending in the path of the gases that pass through the enclosure 24. The superheater 26, the reheater 28 and the economizer 30 all are connected to fluid flow circuitry (not shown) extending from the tubes forming the walls of the furnace section 10 to receive heated water or vapour for further heating. It is understood that the tubes 34 are formed in bundles, in a conventional manner. After passing through the superheater 26, the reheater 28 and the economizer 30, the gases exit the enclosure 24 through an outlet 38 formed in the rear wall thereof. The separated solids from the separator 18 pass into a hopper 18a connected to the lower end of the separator and then into a dipleg 39 connected to the outlet of the hopper. The dipleg 39 extends into a relatively small enclosure 40 disposed adjacent the lower rear wall portion of the enclosure 10 for receiving particulate material from the dipleg. An air distributor 42 is disposed at the lower end portion of the enclosure 40 and defines an air plenum 44 to introduce air received from an external source into and through the air distributor 42 and into the interior of the enclosure. A partition 46 extends between rear wall of the enclosure 10 and the air distributor 44 to define a passage 48 which registers with an opening 50 formed in the latter rear wall to allow the particulate material from the vessel 40 to overflow and pass into the interior of the enclosure 10 and into the bed 15. A drain pipe 52 discharges the spent particulate material from the enclosure and a bundle of heat exchange tubes 54 are disposed in the enclosure 40 for circulating a cooling fluid, such as water through the interior of the enclosure 40 to cool the bed of particulate material on the air distributor 42. According to a feature of the present invention, the lower rear wall portion of the enclosure 10 serves as a common wall for the enclosure 40 and, as such, forms the front wall of the latter enclosure. It is understood that the remaining walls of the enclosure 40 can include water tubes in the manner described in connection with the walls of the enclosure 10. In operation, particulate fuel material from the inlet 16 is introduced into the enclosure 10 and adsorbent material can also be introduced in a similar manner, as needed. Pressurized air from an external source passes into and through the air plenum 12, through the air distributor 14 and into the bed 15 of particulate material in the enclosure 10 to fluidize the material. A lightoff burner (not shown), or the like, is disposed in the enclosure 10 and is fired to ignite the particulate fuel material. When the temperature of the material reaches a relatively high level, additional fuel from the inlet 16 is discharged into the enclosure 10. The material in the enclosure 10 is self-combusted by the heat in the furnace section 10 and the mixture of air and gaseous products of combustion (hereinafter referred to as "flue gases") passes upwardly through the enclosure 10 and entrain, or elutriate, the relatively fine particulate material in the enclosure. The velocity of the air introduced, via the air plenum 12, through the air distributor 14 and into the interior of the enclosure 10 is established in accordance with the size of the particulate material in the enclosure 10 so that a circulating fluidized bed is formed, i.e. the particulate material is fluidized to an extent that substantial entrainment or elutriation of the particulate material in the bed is achieved. Thus the flue gases passing into the upper portion of the enclosure 10 are substantially saturated with the particulate material. The saturated flue gases pass to the upper portion of the enclosure 10 and exit through the ducts 20 and pass into the cyclone separators 18. In the separators 18, the solid particulate material is separated from the flue gases and the former passes through the hoppers 18a and is injected, via the diplegs 39, into the enclosure 40. The cleaned flue gases from the separators 18 exit, via the duct 22, to the heat recovery area 8 for passage through the enclosure 24 and across the superheater 26, the reheater 28 and the economizer 30, before exiting through the outlet 38 to external equipment. In the enclosure 40, the temperature of the separated solids accumulating on the air distributor 44 is controlled by the fluid circulating through the tubes 54. These solids overflow the enclosure 40 and pass, via the passage 48, through the opening 50 in the rear wall of the enclosure 10 and into the fluidized bed 15 where they mix with the other solids in the bed. Air is injected, via the plenum 44 and the air distributor 42 to fluidize the particulate material in the enclosure 40 and seal against a backflow of flue gases from the enclosure 10 through the passage 48 and the dipleg 39 and into the separator 18 in a direction opposite from the normal system flow described above. Water is passed through the economizer 30, to the steam drum 32, then through the walls of the furnace section 10 to exchange heat with the fluidized bed 15 and generate steam. The steam then passes through fluid flow circuitry (not shown) to the bundles of tubes 34 forming the superheater 26, the reheater 28 and the economizer 30 in the heat recovery area 8. The steam thus picks up additional heat from the hot gases passing through the heat recovery area 8 before the steam is discharged to external equipment such as a steam turbine. It is thus apparent that several advantages result from the foregoing. The use of sealing devices, and pneumatic transport devices between the cyclone separator solids outlet and the furnace section of the reactor are eliminated. Also, the height of the furnace section of the reactor is reduced and the need for wear-prone surfaces in the upper furnace section is eliminated. Further, the radiant superheater and/or reheater surface in the upper portion of the furnace is eliminated and the efficiency of the downstream heat exchange surfaces is increased. Still further, optimum bed temperatures are achieved.

Claims: A fluidized bed combustion system including a furnace (4), at least a portion of the walls of which include tubes for boiler water for supporting a fluidized bed (15) of combustible particulate material disposed in the furnace, a recycle heat exchanger (40) with means for maintaining a fluidized bed of particulate material therein, the heat exchanger being disposed adjacent the furnace and sharing a common wall therewith, separating means (18) for receiving a mixture of flue gases and entrained particulate material from the furnace (4) and separating such particulate material and such flue gases, means for passing separated flue gases to a heat recovery area (8), and means for passing separated particulate material to the recycle heat exchanger (40), CHARACTERISED IN THAT a vertical partition (46) is disposed in the recycle heat exchanger (40) for dividing the recycle heat exchanger into a chamber for receiving particulate material from the separating means (18), and a vertically extending passage (48) located between the common wall and said chamber for receiving particulate material from the chamber, the common wall having an opening registering with the lower end of the passage (48) for feeding particulate material therein to the fluidized bed in the furnace (4), and the height of the passage being such that the quantity of particulate material maintained therein is sufficient to seal against backflow of flue gases from the furnace (4) through the recycle heat exchanger (40) to the separating means (18). A system according to Claim 1 CHARACTERISED IN THAT the separating means (18) comprises an outlet pipe (39) extending into the bed of particulate material in said chamber of the recycle heat exchanger. A system according to Claim 1 or Claim 2 including internal heat exchange means (54) disposed in the recycle heat exchanger (40) for passing a fluid in a heat exchange relation to the fluidized bed therein to control the temperature of separated particulate material passed from the recycle heat exchanger (40) to the furnace (4). A method of operating a fluidized bed combustion system comprising the steps of fluidizing a bed (15) of combustible particulate material in a furnace (4), at least a portion of the walls of which include tubes for boiler water, combusting the particulate material to thereby discharge a mixture of flue gases and entrained particulate material, separating the particulate material and flue gases in the mixture, passing the separated flue gases to a heat recovery area (8) and passing the separated particulate material into a receiving chamber of a recycle heat exchanger (40), the recycle heat exchanger sharing a common wall with the furnace, and fluidizing the particulate material in said chamber, CHARACTERISED IN THAT particulate material in the chamber of the recycle heat exchanger is passed to a passage (48) which is located between the common wall and the chamber and which is divided from the chamber by a vertical partition (46), the depth of said separated particulate material being maintained sufficient to seal against backflow of flue gases from the furnace (4) through the recycle heat exchanger (40) to the separating means (18), and from the passage (48) to the fluidized bed in the furnace (4). A method according to Claim 4 including the step of passing a fluid in a heat exchange relation through the fluidized bed of particulate material in said chamber of the recycle heat exchanger (40). A method according to Claim 4 or Claim 5 including the step of controlling the temperature of the separated particulate material passed from the recycle heat exchanger to the furnace (4).

Inventors: ALLISTON, MICHAEL GERARD, CAMPBELL, WALTER ROBERT, JR., SISSON, BENJAMIN HAWES
Applicants: FOSTER WHEELER ENERGY CORPORATION
Application year: 1988
Main classification: F22B31
IPC codes: B01J8, F22B31