Fluidized bed combustion system having multiple furnace sections

Abstract: A combustion system in which a furnace (10) having a fluidized bed disposed therein is divided into multiple sections (10A,10B) by one or more partitions (70). Openings (70B) are provided in the lower portion of each partition for permitting the fluidized bed material to flow between the furnace sections, and openings (70A) are provided in the upper portion of each partition for equalizing the gas pressure between the furnace sections.

Description: This invention relates to a 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, including a fossil fuel, such as coal, and a sorbent for the oxides of 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. These types of combustion systems are often used in steam generators in which water is passed in a heat exchange relationship to the fluidized bed to generate steam and permit high combustion efficiency and fuel flexibility, high sulphur adsorption and low nitrogen oxides emissions. The most typical fluidized bed utilized in the furnace section of these type systems 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 systems utilize a "circulating" fluidized bed in which the fluidized bed density is below that of a typical bubbling fluidized bed, the fluidizing air velocity is equal to or greater than that of a bubbling bed, and the flue gases passing through the bed entrain a substantial amount of the fine particulate solids to the extent that they are substantially saturated therewith. Circulating fluidized beds are characterized by relatively high internal and external solids recycling which makes them insensitive to fuel heat release patterns, thus minimizing temperature variations and, therefore, stabilizing the sulphur emissions at a low level. The external solids recycling is achieved by disposing a cyclone separator at the furnace section outlet to receive the flue gases, and the solids entrained thereby, from the fluidized bed. The solids are separated from the flue gases in the separator and the flue gases are passed to a heat recovery area while the solids are recycled back to the furnace. This recycling improves the efficiency of the separator, and the resulting increase in the efficient use of sulphur adsorbent and fuel residence time reduces the adsorbent and fuel consumption. U.S. Patent No. 4,609,623 and No. 4,809,625, disclose a fluidized bed reactor in which a dense, or bubbling, bed is maintained in the lower portion of the furnace, while the bed is otherwise operated as a circulating bed. This hybrid arrangement results in several advantages not the least significant of which is the ability to utilize fuel and adsorbent over a relatively large particle size range. In designing a fluidized bed combustion system of the above types, increases in furnace capacity from a given design are usually achieved by increasing the height of the furnace walls. However, this is expensive and there are certain limits to the height of the walls. It has therefore been suggested that the size of the furnace, and therefore its capacity, be increased by increasing the size of the furnace in "plan view" i.e., increasing the width and/or the depth of the furnace. However, this usually requires a common wall, or the like, to be placed in the furnace section to divide the area into two or more fluidized beds which requires separate operating controls, etc. which is expensive. EP-A-444 732 is an example of such an arrangement. Also, the common wall is subjected to lateral loading, especially when the multiple beds operate differently or if one bed is rendered inoperable due to equipment failure. This lateral loading can cause damage to the wall and attendant reduction in operation and efficiency. EP-A-365 723 on which the preamble of Claim 1 is based shows a fluidized bed combustion system where particles entrained form the furnace are separated and returned via a heat exchanger before being passed back to the furnace. EP-A-312 840 shows two different fluidized beds in a common enclosure and separated by a common partition. There are openings through this to allow circulation of the bed material but this does not appear to result in equalisation of the bed heights, possibly because a common air distributor is not used and the beds are operated differently from one another. It is therefore an object of the present invention to provide a fluidized bed combustion system which has an increased capacity without any increase in furnace wall height. According to the invention there is provided a fluidised bed combustion system comprising an enclosure, means for introducing particulate material including fuel to the enclosure, a grate for supporting the particulate material, plenum means for introducing air through the grate and into the particulate material for fluidising the particulate material, separating means disposed adjacent the enclosure for receiving a mixture of flue gases and entrained particulate material and for separating the particulate material from the gases, heat exchange means extending between the separating means and the enclosure for receiving the separated particulate material from the separating means and removing heat from the separated particulate material, and passage means for passing the separated particulate material back to the enclosure, characterised in that partition means are disposed in the enclosure and divide that enclosure into a plurality of sections, the grate is disposed in each of the sections, at least one opening extends through a lower portion of the partition means for permitting the particulate material to flow between the sections to equalise the height of the particulate material in each section, and at least one opening extends through an upper portion of the partition means for equalising the gas pressure between the sections. In a fluidized bed combustion system according to the invention the common wall is vented to equalise the gas pressure across the wall and to enable the fluidized beds to maintain substantially the same height. The invention will be described by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a schematic representation depicting the system of the present invention; Fig. 2 is an enlarged cross-sectional view taken along the line 2-2 of Fig. 1; Fig. 3 is a cross-sectional view taken along the line 3-3 of Fig. 2; and Figs. 4 and 5 are views similar to Fig. 2 but depicting alternate embodiments of the system of the present invention. The drawings depicts the fluidized bed combustion system of the present invention used for the generation of steam. The system includes an upright water-cooled furnace, referred to in general by the reference numeral 10, having a front wall 12, a rear wall 14 and two sidewalls 16a and 16b (Fig. 2). The upper portion of the furnace 10 is enclosed by a roof 18 and the lower portion includes a floor 20. A perforated plate, or grate, 22 extends across the lower portion of the furnace 10 and extends parallel to the floor 20 to define an air plenum 24 which is adapted to receive air from an external source (not shown) and selectively distribute the air through the perforations in the plate 22, as will be described. Two feeder systems, 25a (Fig. 1) and 25b (shown partially in Fig. 2) are provided adjacent the front wall 12 for introducing particulate fuel material into the furnace 10. Adsorbent, such as limestone, in particle form can also be introduced into the furnace 10 by a ducts connected to the feeder systems 25a and 25b respectively, or by a separate feeder system. The particulate fuel and adsorbent material are fluidized by the air from the plenum 24 as it passes upwardly through the plate 22. This air promotes the combustion of the fuel which generates combustion gases, and the resulting mixture of the combustion gases and the air (hereinafter collectively termed "flue gases") rises in the furnace 10 by convection and entrains a portion of the particulate material as will be described. Two cyclone separators 26a and 26b (Fig. 2) extend adjacent the furnace 10. A duct 28 extends from an outlet opening provided in the rear wall 14 of the furnace 10 to an inlet opening provided through the wall of the separator 26a. It is understood that another duct (not shown) connects the rear wall 14 to the separator 26b. The separators 26a and 26b receive the flue gases and the entrained particle material from the furnace 10 in a manner to be described and operate in a conventional manner to disengage the particulate material from the flue gases due to the centrifugal forces created in the separators. The separated flue gases in the separator 26a, which are substantially free of solids, pass, via a duct 30 located immediately above the separator 26a, into a heat recovery section 32, via an inlet 32a provided through a wall thereof. It is understood that the separated flue gases from the separator 26b also pass, via another duct (not shown), into the heat recovery section 32. The heat recovery section 32 includes an enclosure 34 divided by a vertical partition 36 into a first passage which houses a reheater 38, and a second passage which houses a primary superheater 40. An economizer is provided and has an upper section 42a located in the above-mentioned second passage and a lower section 42b in the lower portion of the heat recovery section 32. An opening 36a is provided in the upper portion of the partition 36 to permit a portion of the gases to flow into the passage containing the superheater 40 and the economizer sections 42a and 42b. The reheater 38, the superheater 40 and the economizer sections 42a and 42b are all formed by a plurality of heat exchange tubes extending in the path of the gases as they pass through the enclosure 34. After passing across the reheater 38, the superheater 40 and the economizer sections 42a and 42b in the two parallel passes, the gases exit the enclosure 34 through an outlet 34a. As shown in Fig. 1, the floor 20 and the plate 22 extend past the rear wall 14, and a vertically-extending partition 50 extends upwardly from the floor 20 and parallel to the rear wall 14. A roof 52 extends from the partition 50 to the rear wall 14 and, as shown in Fig. 2, the lower portions of the sidewalls 16a and 16b are also extended past the rear wall 14 to complete an enclosure which defines a recycle heat exchange section 56. The heat exchange section 56 communicates with the furnace section through openings 14b formed through the lower portion of the rear wall 14, for reasons to be described. A bank of heat exchange tubes 58 are disposed in the heat exchange section 56 and extend between an inlet feeder 59a and an outlet feeder 59b. The plennum 24 extends underneath the heat exchange section 56 for introducing air to the latter section in a manner to be described. The lower portion of the separator 26a is conically shaped and is connected to a dip leg 60a which, in turn, is connected to a J-valve 62a. An inlet conduit 64a connects the outlet of the J-valve 62a to the heat exchange section 56 to transfer the separated solids from the separator 26a to the latter section. The J-valve 62a functions in a conventional manner to prevent back-flow of solids from the furnace section 54 and the heat exchange section 56 to the separator 26a. As shown in Fig. 2, a dip leg 60b, a J-valve 62b and an inlet conduit 64b are all associated with the separator 26b and are identical to the dip leg 60a, the J-valve 62a and the conduit 64a associated with the separator 26a. As shown in Figs. 2 and 3, a partition 70 is provided in the furnace 10 and extends between the walls 12 and 14 and parallel to, and midway between, the sidewalls 16a and 16b for the height of the furnace to divide the furnace into two sections 10a and 10b. A plurality of openings 70a are provided in the upper portion of the partition 70 and a plurality of openings 70b are provided in the lower portion of the partition 70, for reasons to be described. It is understood that the outlet openings in the upper portion of the rear wall 14 connect both furnace sections 10a and 10b with the duct 28 extending to the separator 26a and with the duct (not shown) extending to the separator 26b, respectively. As better shown in Fig. 2 the front wall 12, the rear wall 14, the sidewalls 16a and 16b, as well as the walls defining the heat recovery enclosure 34 all are formed by a plurality of spaced tubes 72 having continuous fins 74 extending from diametrically opposed portions thereof to form a gas-tight membrane in a conventional manner. As shown in Fig. 3, a plurality of rows of air distributors, or nozzles, 76 extend through the perforations in the plate 22 in the furnace sections 10a and 10b for distributing air from the plenum 24 upwardly into the furnace sections 10a and 10b. It is understood that a plurality of similar nozzles (not shown) extend through the perforations in the plate 22 in the heat exchange section 56. The outlet conduit of the feeder systems 25a and 26b extend through openings in the front wall 12 and into the furnace section 10a and 10b, respectively. It is understood that drain pipes, or the like, may be associated with the plate 22 as needed for discharging the particulate material from the furnace sections 10a and 10b and the heat exchange section 56 as needed. A steam drum 80 (Fig. 1) is located above the enclosure 10 and, although not shown in the drawings, it is understood that a plurality of headers are disposed at the ends of the various water-tube walls described above. As shown in general by the reference numeral 82, a plurality of downcomers, pipes, etc. are utilized to establish a steam and water flow circuit through these headers, the steam drum 80, the tubes forming the aforementioned water tube walls, and the tubes 58 in the heat exchange section 56, with connecting feeders, risers, headers being provided as necessary. The boundary walls of the cyclone separators 26a and 26b and the tubes forming the reheater 38 and the superheater 40 are thus steam cooled while the economizer portions 42a and 42b receive feed water and discharge it to the steam drum 80. Thus, water is passed, in a predetermined sequence through this flow circuitry, including the downcomers and pipes 82, to convert the water to steam and heat the steam by the heat generated by combustion of the particulate fuel material in the furnace 10. In operation, particulate fuel material and particulate sorbent material are introduced into the furnace sections 10a and 10b through the feeder systems 25a and 25b, respectively. Alternately, sorbent may also be introduced independently through openings formed through one or more of the furnace walls 12, 14, 16a and 16b. Air from an external source is introduced at a sufficient pressure into that portion of the plenum 24 extending below the furnace sections 10a and 10b, and the air passes through the nozzles 76 at a sufficient quantity and velocity to fluidize the solids in the latter sections. A lightoff burner (not shown), or the like, is provided to ignite the fuel material, and thereafter the fuel material is self-combusted by the heat in the furnace sections 10a and 10b. A homogeneous mixture of the fuel particles and the adsorbent particules, in various stages of combustion and reaction, is thus formed in the furnace sections 10a and 10b, which mixture is hereinafter referred to as "particulate material". The openings 70b in the partition 70 are sized to permit adequate flow of the particulate material between the furnace sections 10a and 10b so that the respective heights of the solids in each section are substantially the same. The flue gases pass upwardly through the furnace sections 10a and 10b and entrain, or elutriate, a portion of the particulate material. The quantity of particulate material introduced into the furnace sections 10a and 10b and the quantity of air introduced, via the air plenum 24, through the nozzles 76 and into the interior of the furnace sections is established in accordance with the size of the particulate material so that a dense bed is formed in the lower portions of the furnace sections 10a and 10b and a circulating fluidized bed is formed in the upper portions thereof, i.e. the particulate material is fluidized to an extent that substantial entrainment or elutriation thereof is achieved. Thus the density of the particulate material is relatively high in the lower portion of the furnace sections 10a and 10b, decreases with height throughout the length of the furnace sections and is substantially constant and relatively low in the upper portions of the furnace sections. The flue gases passing into the upper portion of the furnace sections 10a and 10b are substantially saturated with the particulate material and pass, via the outlet openings in the upper portion of the rear wall 14, into the cyclone separators 26a and 26b. The openings 70a in the upper portion of the partition 70 equalize the gas pressure in each furnace section 10a and 10b and thus eliminate any pressure drop across the partition 70. In the separators 26a and 26b, the particulate material is separated from the flue gases and the former passes from the separators 26a and 26b into the heat exchange section 56 as described above. The cleaned flue gases from the separators 26a and 26b pass to the heat recovery section 32 for passage through the enclosure 34 and across the reheater 38, the superheater 40, and the economizer sections 42a and 42b, before exiting through the outlet 34a to external equipment. With reference to Figs. 2 and 3, the separated particulate material from each of the separators 26a and 26b enters the heat exchange section 56 and builds up on that portion of the plate 22 in the latter section. Assuming normal operation, fluidizing air is introduced, via the plenum 24, to the nozzles 76 in the heat exchange section 56 to fluidize the particulate material therein. The particulate material gives up heat to the water/steam in the tubes 58 in the heat exchange section 56 before passing through the openings in the lower portion of the wall 14 and back to the furnace sections 54a and 54b. Feedwater is introduced to and circulated through the flow circuit described above including the water wall tubes, the tubes 58 in the heat exchange section 56 and the steam drum 80, in a predetermined sequence to convert the feed water to steam and to reheat and superheat the steam. To this end, the heat removed from the solids by the tubes 58 in the heat exchange section 56 can be used to provide reheat and/or full or partial superheat. It is understood that the heat exchange section 56 can be provided with a bypass compartment, or compartment that does not contain any heat exchange tubes, so that, during initial start up and low load conditions the solids can be routed through this bypass compartment and directly back to the furnace 10 without encountering any tubes. In this manner, start up and low load operation can be achieved without exposing the banks of tubes 58 to the hot recirculating solids. The system and method of the present invention have several advantages. For example, the use of two adjacent furnace sections 10a and 10b defined by the partition 70 enables the size of the furnace 10, and therefore the load capacity, to be increased without increasing the height of the furnace. Moreover, the provision of openings 70b provided in the lower portion of the partition 70 equalizes the height of the respective beds in the furnace sections 10a and 10b, thus correcting for imbalances in the fuel feed from the feeder systems 25a and 25b, or the like. Further, since the adjacent furnace sections 10a and 10b are the same, a single control scheme can be utilized which operates both sections. Moreover, the provision of the openings 70a in the upper portion of partition 70 enables the respective gas pressures in the furnace sections 10a and 10b to be equalized, thus minimizing or eliminating any lateral loading across the partition 70 and possible damage. Also, the openings 70a enable a predetermined gas pressure drop to be set across the furnace 10 and enables the entrainment and circulation to be identical in each furnace section 10a and 10b. Also, the provision of the openings 70a enables identical combustion environments to be established above the dense bed in both furnace sections 10a and 10b. Although the preferred embodiment described above utilizes a single partition 70 to divide the furnace section into two identical furnace sections 10a and 10b, it is understood that multiple partitions can be used in a similar manner to divide the enclosure into multiple sections. As examples of this, Figs. 4 and 5 depict alternate embodiments of the present invention which include structure identical to that of the embodiment of Figs. 2 and 3 which structure is referred to by the same reference numerals. According to the embodiment of Fig. 4 the widths of the walls 12, 14 and 50 are increased and a partition 84 is provided in the furnace 10 which extends from the wall 12 to the wall 14, and which is disposed between, and parallel to, the partition 70 and the sidewall 16b to define another furnace section 10c. A feeder system 25c (shown partially in Fig. 4) is added for supplying coal and sorbent into the furnace section 10c. Also, another separator 26c is added, along with a dip leg 60c, a J-valve 62c and an inlet conduit 64c which is connected to the heat exchange section 56 and is in alignment with the furnace section 10c. The partition 84 has openings (not shown) provided in its upper portion and its lower portion similar to the openings 70a and 70b of the partition 70, and the embodiment of Fig. 4 functions in the same manner as described above in connection with the embodiment of Figs. 1-3 while enjoying the added capacity and flexibility of the additional furnace section 10c. According to the embodiment of Fig. 5 the widths of the walls 12, 14 and 50 are increased even more and another partition 86 is provided in the furnace 10 which extends from the wall 12 to the wall 14 and which is disposed between, and parallel to, the partition 84 and the sidewall 16b to define another furnace section 10d. A feeder system 25d (shown partially in Fig. 5) is added for supplying coal and sorbent into the furnace section 10d. Also, another separator 26d is added, along with a dip leg 60d, a J-valve 62d and an inlet conduit 64d which is connected to the heat exchange section 56 and is in alignment with the furnace section 10d. The partition 86 has openings (not shown) provided in its upper portion and its lower portion similar to the openings 70a and 70b of the partition 70. The embodiment of Fig. 5 functions in the same manner as described above in connection with the embodiment of Figs. 1-3 and the embodiment of Fig. 4 and enjoys the added capacity and flexibility of the additional furnace section 10d.

Claims: A fluidised bed combustion system comprising an enclosure (10), means (25a, 25b) for introducing particulate material including fuel to the enclosure, a grate (22) for supporting the particulate material, plenum means (24) for introducing air through the grate (22) and into the particulate material for fluidising the particulate material, separating means (26a, 26b) disposed adjacent the enclosure (10) for receiving a mixture of flue gases and entrained particulate material and for separating the particulate material from the gases, heat exchange means (58) extending between the separating means (26a, 26b) and the enclosure (10) for receiving the separated particulate material from the separating means and removing heat from the separated particulate material, and passage means (14b) for passing the separated particulate material back to the enclosure (10), characterised in that partition means (70, 84, 86) are disposed in the enclosure (10) and divide that enclosure (10) into a plurality of sections (10a, 10b, 10c, 10d), the grate (22) is disposed in each of the sections (10a, 10b, 10c, 10d), at least one opening (70b) extends through a lower portion of the partition means (70, 84, 86) for permitting the particulate material to flow between the sections to equalise the height of the particulate material in each section (10a, 10b, 10c, 10d), and at least one opening (70a) extends through an upper portion of the partition means (70, 80, 82) for equalising the gas pressure between the sections (10a, 10b, 10c, 10d). A system as claimed in Claim 1 in which the heat exchange means (58) comprise a housing (56) disposed adjacent to, and sharing a common wall (14) with the enclosure (10) and the passage means (14b) comprising an opening extending through the common wall. A system as claimed in Claim 2 in which the heat exchange means further comprise a plurality of tubes (58) disposed in the housing (56) for passing a fluid in a heat exchange relation to the separated particulate material in the housing (56) to heat the fluid and control the temperature of the separated particulate material. A system as claimed in any preceding claim in which the plenum means comprise an air plenum (24) extending below the grate means (22) for receiving fluidising air, and a plurality of nozzles (76) supported by the grate means (22) for receiving air from plenum (24) and introducing that air into the sections (10a, 10b, 10c, 10d). A system as claimed in any preceding claim in which the separating means comprise a plurality of separators (26a, 26b) and the plurality of sections (10a, 10b, 10c, 10d) correspond in number to the separators (26a, 26b). A system as claimed in Claim 5 in which the housing (56) extends between the plurality of separators (26a, 25b) and the plurality of enclosure sections (10a, 10b, 10c, 10d). A system as claimed in any preceding claim in which the partition means comprise one partition (70) disposed in the enclosure (10) dividing that enclosure into two sections (10a, 10b). A system as claimed in any one of claims 1 to 6 in which the partition means comprise two partitions (70, 84) disposed in the enclosure (10) dividing that enclosure into three sections (10a, 10b, 10c). A system as claimed in any one of claims 1 to 6 in which the partition means comprise three partitions (70, 84, 86) disposed in the enclosure (10) dividing that enclosure into four sections (10a, 10b, 10c, 10d).

Inventors: DIETZ DAVID HAROLD, DIETZ, DAVID HAROLD
Applicants: FOSTER WHEELER ENERGY CORP, FOSTER WHEELER ENERGY CORPORATION
Application year: 1992
Main classification: F23C11
IPC codes: F22B31, F23C10, F23C6