Fluidized bed combustion system having a recycle heat exchanger with a non-mechanical solids control system

Abstract: A fluidized bed combustion system and method in which a recycle heat exchange section is located within an enclosure housing the furnace section of the combustion system. The flue gases and entrained solids from a fluidized bed in the furnace section are separated and the flue gases are passed to a heat recovery section and the separated particulate material to the heat exchange section. The heat exchange section includes a bypass chamber for permitting the separated solids to pass directly from the separator to the furnace section. A heat exchange chamber is provided in the recycle heat exchange section which receives the separated materials from the bypass chamber and transfers heat from the separated material to a fluid flow circuit. The separated material in the heat exchange chamber is then passed back to the furnace section.

Description: This invention relates to a fluidized bed combustion system and, more particularly, to such a system in which a recycle heat exchanger is formed adjacent to the furnace section of the 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 sulfur 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 times reduces the adsorbent and fuel consumption. In the operation of these types of fluidized beds, and, more particularly, those of the circulating type, there are several important considerations. For example, the flue gases and entrained solids must be maintained in the furnace section at a particular temperature (usually approximately 871°C) consistent with proper sulphur capture by the adsorbent. As a result, the maximum heat capacity (head) of the flue gases passed to the heat recovery area and the maximum heat capacity of the separated solids recycled through the cyclone and to the furnace section are limited by this temperature. In a cycle requiring only superheat duty and no reheat duty, the heat content of the flue gases at the furnace section outlet is usually sufficient to provide the necessary heat for use in the heat recovery area of the steam generator downstream of the separator. Therefore, the heat content of the recycled solids is not needed. However, in a steam generator using a circulating fluidized bed with sulphur capture and a cycle that requires reheat duty as well as superheater duty, the existing heat available in the flue gases at the furnace section outlet is not sufficient. At the same time, heat in the furnace cyclone recycle loop is in excess of the steam generator duty requirements. For such a cycle, the design must be such that the heat in the recycled solids is utilized before the solids are reintroduced to the furnace section. To provide this extra heat capacity, a recycle heat exchanger is sometimes located between the separator solids outlet and the fluidized bed of the furnace section. The recycle heat exchanger includes heat exchange surfaces and receives the separated solids from the separator and functions to transfer heat from the solids to the heat exchange surfaces at relatively high heat transfer rates before the solids are reintroduced to the furnace section. The heat from the heat exchange surfaces is then transferred to cooling circuits to supply reheat and/or superheat duty. US 4 813 479, upon which the precharacterising part of claim 1 is based, shows such an arrangement although it does not specify the use to which the heat removed from the particles is put. The recycle heat exchanger usually includes a bypass channel for permitting direct flow of the recycled solids from the recycle heat exchanger inlet to the furnace section in order to avoid contacting the solids with the heat exchange surfaces in the heat exchanger during start-up or low load conditions. However, this type of arrangement usually requires mechanical valves, or the like, for selectively controlling the flow of the solids from the inlet, through the bypass channel and to the furnace section; or from the inlet, through an area containing the heat exchange surfaces and to the furnace section. These mechanical valves are large, expensive and require periodic replacement which adds to the cost of the system. US 4 716 856 shows an arrangement where there is normally a direct return of separated particles to the furnace. However, a mechanical valve controls the return of said particles via a heat exchange chamber positioned adjacent the furnace section. According to the invention there is provided a fluidised bed combustion system comprising a furnace section, means forming a fluidised bed in the furnace section, a separating section for receiving a mixture of flue gases and entrained particulate material and separating the flue gases from the particulate material, and a recycle heat exchange section disposed adjacent to the furnace section for receiving the separated particulate material, that recycle heat exchange section having a bypass for receiving the separated solids and heat exchange means, means for connecting the bypass to the furnace section, a plurality of air nozzles for selectively controlling the flow of solids from the bypass to the furnace section and from the bypass to the heat exchange means, characterised in that a partition is disposed in the recycle heat exchange section for defining a bypass compartment and a heat exchange compartment for the heat exchange means, an opening is formed in that partition to connect the bypass compartment to the heat exchange compartment, and the plurality of air nozzles are disposed in the bypass compartment and in the heat exchange compartment. In a system according to the present invention the recycle heat exchange section includes a direct bypass for routing the separated solids directly to the furnace section without passing over any heat exchange surfaces during start-up, shut-down, unit trip, and low load conditions. Also, a non-mechanical control system is provided for selectively passing the separated solids through the bypass or over the heat exchange surfaces. The invention will be described with reference to the following detailed description taken in conjunction with the accompanying drawings, in which: Fig. 1 is a schematic representation depicting the system of the present invention; Fig. 2 is a partial cross-section, partial schematic 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; Fig. 4 is a partial, enlarged perspective view of a portion of a wall of the enclosure of the system of Fig. 1; Fig. 5 is an enlarged sectional view taken along the line 4-4 of Fig. 2; and Fig. 6 and 7 are views similar to Figs. 2 and 3, respectively, but depicting an alternate embodiment of the present invention. Fig. 1 of 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 enclosure, referred to in general by the reference numeral 10, having a front wall 12, a rear wall 14 and two sidewalls one of which is shown by the reference number 16a. The upper portion of the enclosure 10 is enclosed by a roof 18 and the lower portion includes a floor 20. A plate 22 extends across the lower portion of the enclosure 10 and is spaced from the floor 18 to define an air plenum 24 which is adapted to receive air from an external source (not shown) and selectively distribute the air through perforations in the plate 22 and to nozzles (not shown in Fig. 1) mounted on the plate as will be described. A coal feeder system, shown in general by the reference numeral 25, is provided adjacent the front wall 12 for introducing particulate material containing fuel into the enclosure 10. The particulate material is fluidized by the air from the plenum 24 as it passes upwardly through the plate 22. This air promotes the combustion of the fuel and the resulting mixture of combustion gases and the air (hereinafter termed "flue gases") rises in the enclosure by forced convection and entrains a portion of the solids to form a column of decreasing solids density in the upright enclosure 10 to a given elevation, above which the density remains substantially constant. It is understood that an absorbent, such as limestone, in particle form can also be introduced into the enclosure by a separate feeder system or by a duct connected to the feeder system 25. A cyclone separator 26 extends adjacent the enclosure 10 and is connected thereto via a duct 28 extending from an outlet 14a provided in the rear wall 14 of the enclosure 10 to an inlet 26a provided through the separator wall. Although reference is made to one separator 26, it is understood that one or more additional separators (not shown) can be disposed behind the separator 26. The separator 26 receives the flue gases and the entrained particle material from the enclosure 10 in a manner to be described and operates 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, which are substantially free of solids, pass, via a duct 30 located immediately above the separator 26, into a heat recovery section 32, via an inlet 32a provided through a wall thereof. 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 36, 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 the plate 22 and the sidewalls 16a and 16b extend past the rear wall 14, and a vertically-extending partition 50 extends upwardly from the floor 18 and parallel to the rear wall 14. A roof 52 extends from the partition 50 to the rear wall 14. The front wall 12 and the rear wall 14 define a furnace section 54, and the rear wall 14 and the partition 50 define a recycle heat exchange section 56. The floor 20, the plate 22, and therefore the plennum 24 extend 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 26 includes a hopper 26a which is connected to a dip leg 60 connected to an inlet "J" valve, shown in general by the reference numeral 62. An inlet conduit 64 connects the outlet of the J-valve 62 to the heat exchange section 56 to transfer the separated solids from the separator 26 to the latter section. The J-valve 62 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 26. Figs. 2 and 3 depict the other sidewall 16b of the enclosure 10 as well as a pair of transverse spaced partitions 70 and 72 extending between the rear wall 14 and the partition 50. As shown in Fig. 3, the partitions 70 and 72 extend for a height less than the walls forming the heat exchange section 56. The front wall 12, the rear wall 14, the sidewalls 16a and 16b and the partitions 50 and 70, as well as the walls defining the heat recovery enclosure 34 all are formed in a manner depicted in Fig. 4. As shown, each wall is formed by a plurality of spaced tubes 74 having continuous fins 74a extending from diametrically opposed portion thereof to form a gas-tight membrane. Referring to Figs. 2 and 3, the partitions 70 and 72 divide the lower portion of the heat exchange section into three compartments 56a, 56b and 56c. The inlet conduit 64 registers with an opening in the partition 50 communicating with the compartment 56b. A plurality of rows of air distributors, or nozzles, 76 extend through the plate 22 in the furnace section 54 for distributing air from the plenum 24 upwardly into the furnace section. A plurality of rows of nozzles 78 extend through the perforations in the plate 22 in the heat exchange section 56. Each nozzle 78 consists of a central portion extending through the perforation and a horizontal discharge portion registering with the vertical portion. As shown in Figs. 2 and 3, the nozzles 78 in the compartments 56a and 56c are disposed in parallel rows with their discharge portions 78 facing towards the sidewalls 16a and 16b, respectively. Two parallel rows of nozzles 78 are provided in the compartment 56b with their discharge portions facing towards the partitions 70 and 72, respectively. A single row of nozzles 80 are also located in the compartment 56b and extend between the two rows of nozzles 78. The nozzles 80 are longer than the nozzles 78 for reasons to be explained. A manifold 82 is located in the plenum 24 and is connected to the nozzles 80 for supplying air to the nozzles independently of the flow of air from the plenum 24, through the plate 22 and to the nozzles 76 and 78. As shown in Figs. 3 and 5, a bank of heat exchange tubes 84 are disposed in each of the compartments 56a and 56c. The tubes 84 extend between headers 86a and 86b (Fig. 5) for circulating fluid through the tubes. Three horizontally spaced elongated openings 14a, 14b and 14c (Fig. 3) are provided through a portion of those portions of the wall 14 defining the compartments 56a, 56b and 56c, respectively. The opening 14b extends at an elevation higher than the openings 14a and 14c for reasons to be described. The openings are shown schematically in Fig. 3 for the convenience of presentation, it being understood that they actually are formed by cutting away the fins 74a, or bending the tubes 74 out of the plane of the wall 14, in a conventional manner. Also, a plurality of openings 70a and 72a (Fig. 3) are formed in the lower portions of the partitions 70 and 72, respectively, for reasons to be described. A steam drum 90 (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 walls described above. As shown in general by the reference numeral 92, a plurality of downcomers, pipes, etc. are utilized to establish a steam and water flow circuit through these headers, the steam drum 90 and the tubes 74 forming the aforementioned water tube walls, with connecting feeders, risers, headers being provided as necessary. The boundary walls of the cyclone separator 26, the heat exchanger tubes 84 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 82. Thus, water is passed, in a predetermined sequence through this flow circuitry, including the downcomers and pipes 92, to convert the water to steam and heat the steam by the heat generated by combustion of the particulate fuel material in the furnace section 54. In operation, particulate fuel material and a sorbent material (hereinafter referred to as "solids") are introduced into the furnace section 54 through the feeder system 25. Alternately, sorbent may also be introduced independently through openings 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 section 54 and the air passes through the nozzles 76 disposed in the furnace section 54 at a sufficient quantity and velocity to fluidize the solids in the latter section. A lightoff burner (not shown), or the like, is provided to ignite the fuel material in the solids, and thereafter the fuel material is self-combusted by the heat in the furnace section 54. The mixture of air and gaseous products of combustion (hereinafter referred to as "flue gases") passes upwardly through the furnace section 54 and entrains, or elutriates, a majority of the solids. The quantity of the air introduced, via the air plenum 24, through the nozzles 76 and into the interior of the furnace section 54 is established in accordance with the size of the solids so that a circulating fluidized bed is formed, i.e. the solids are fluidized to an extent that substantial entrainment or elutriation thereof is achieved. Thus the flue gases passing into the upper portion of the furnace section 54 are substantially saturated with the solids and the arrangement is such that the density of the bed is relatively high in the lower portion of the furnace section 54, decreases with height throughout the length of this furnace section and is substantially constant and relatively low in the upper portion of the furnace section. The saturated flue gases in the upper portion of the furnace section 54 exit into the duct 28 and pass into the cyclone separator 26. In the separator 26, the solids are separated from the flue gases and the former passes from the separators through the dipleg 60 and are injected, via the J-valve 62 and the conduit 64 into the heat exchange section 56. The cleaned flue gases from the separator 26 exit, via the duct 30, and 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 solids from the conduit 64 enter the compartment 56b of the heat exchange section 56. Assuming normal operation, fluidizing air is introduced, via the plenum 24, to the nozzles 78 in the compartments 56a, 56b and 56c of the heat exchange section 56, while the air flow to the manifold 82, and therefore to the nozzles 80, is turned off. Since the two rows of nozzles 78 in the compartment 56b are directed towards the walls 70 and 72, respectively, the solids pass from the compartment 56b into the compartments 56a and 56c, respectively. The solids mix and build up in the compartments 56a and 56c and thus give up heat to the water/steam in the tubes 84 in the latter compartments. The cooled solids then pass through the openings 14a and 14c in the wall 14 and back to the furnace section 54. Feed water is introduced to and circulated through the flow circuit described above including the water wall tubes 74 and the steam drum 90, 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 in the heat exchange section 56 can be used to provide reheat and/or full or partial superheat. For example, the banks of tubes 84 in the compartments 56a and 56c, respectively, can function to provide different stages of heating such as primary, intermediate and finishing superheating. Since, during the above operation, there is no air introduced into the nozzles 80 in the compartment 56b very little, if any, flow of solids occurs through the latter passage. During initial start up and low load conditions the fluidizing air flow to the plenum 24 is turned off and the air flow to the manifold 82, and therefore to the nozzles 80, is turned on. As a result, the volume of solids in the compartments 56a and 56c slump and therefore seal each volume from further flow. Thus, the solids from the conduit 64 pass directly through the compartment 56b and, after building up to the level of the opening 14b, pass through the latter opening into the furnace section 54. Since the compartment 56b does not contain heat exchanger tubes, start up and low load operation can be achieved without exposing the banks of tubes 84 to the hot recirculating solids. It is understood that a drain pipe, or the like, may be provided on the plate 22 as needed for discharging spent solids from the furnace section 54 and the heat exchange section 56 as needed. The system of the present invention has several advantages. For example, heat is removed from the separated solids exiting from the separator 26 before they are reintroduced to the furnace section 54 without reducing the temperature of the separated flue gases. Also, the separated gases are at a sufficient temperature to provide significant heating of the system fluid while the recycle heat exchanger can function to provide additional heating. Further, the recycled solids can be passed directly from the J-valve 62 to the furnace section 54 during start-up or low load conditions prior to establishing adequate cooling steam flow to the heat exchange tubes 84. Also, the heat exchanger section 56 is formed integrally with the furnace section 54 and operates at the same saturation temperature of the cooling fluid, thus permitting the all welded boundary wall construction as shown in Fig. 4. Also, the flow of separated solids back to the furnace section 54 can be achieved precisely and quickly by controlling the flow of fluidizing air from the plenum 24. Further, a relatively large space is provided in the compartments 56a and 56c for accommodating the heat exchange tubes. The embodiment of Figs. 6 and 7 is similar to the previous embodiment and includes identical components which are given the same reference numerals. According to the embodiment of Figs. 6 and 7, a single transverse partition 100 is provided in the heat exchange section 56 to divide it into compartments 56d and 56e. An opening 100a (Fig. 7) is provided through the lower portion of the partition 100 to permit the separated solids to flow from the compartment 56e to the compartment 56d, as will be described. A plurality of rows of nozzles 78 are provided in the compartment 56d all of which face towards the sidewall 16a. Two rows of nozzles 78 are provided in the compartment 56e which face towards the partition 100 and the sidewall 16b, respectively. A row of nozzles 80 extend between the two rows of nozzles 78 in the compartment 56e, and the nozzles 80 are connected to the manifold 82 (Fig. 7) disposed in the plenum 24. A plurality of heat exchange tubes 84 are provided in the compartment 56d and the inlet conduit 64 extends through an opening in the wall 50 and registers with the compartment 56e. An opening 14d is formed through the wall 14 which connects the compartment 56d to the furnace section 54. An opening 14e is formed through the wall 14 which connects the compartment 56e to the furnace section 54 and which is located at an higher elevation than the opening 14d. The embodiment of Figs. 6 and 7 as otherwise identical to that of the embodiment of Figs. 1-5. The operation of the embodiment of Figs. 6 and 7 is similar to that of the embodiment of Figs. 1-5. Thus in normal operations, air flow to the nozzles 78 in the compartments 56d and 56e is turned on, while the air flow to the nozzles 80 in the compartment 56e is turned off. The furnace section 54, the separator 26 and the heat recovery section 32 operate as described above. Thus, separated solids from the separator 26 are directed, via the conduit 64, into the compartment 56e. The row of nozzles 78 located adjacent the partition 100 direct the solids towards and through the opening 100a in the partition 100, into the compartment 56d and across the heat exchange tubes 84 for removing heat from the solids. As the level of cooled solids in the compartment 56d rises, the solids pass into the furnace section 54, via the opening 14c. During start-up and low load conditions, the nozzles 78 are turned off and the nozzles 80 are turned on. As a result, very little, if any, flow of solids occurs from the compartment 56e to the compartment 56d. The solids thus build up in the compartment 56e and pass into the furnace section 54, via the opening 14d. It is understood that several variations may be made in both of foregoing embodiments without departing from the scope of the present invention. For example, the heat removed from the solids in the heat exchange section 56 can be used for heating the system fluid in the furnace section or the economizer, etc. and other types of beds may be utilized in the furnace, such as a circulating transport mode bed with constant density through its entire height. Further, a series heat recovery arrangement can be provided with superheat, reheat and/or economizer surface, or any combination thereto. Also, the number and/or location of the bypass channels in the recycle heat exchanger can be varied and the number and size of separators used can be varied in accordance with the capacity of the steam generator and economic considerations.

Claims: A fluidised bed combustion system comprising a furnace section (54), means forming a fluidised bed in the furnace section (54), a separating section (26) for receiving a mixture of flue gases and entrained particulate material and separating the flue gases from the particulate material, a heat recovery section for receiving said separated flue gases, and a recycle heat exchange section (56) disposed adjacent to the furnace section (54) for receiving the separated particulate material, that recycle heat exchange section (56) having a bypass (56b,56e) for receiving the separated solids and heat exchange means (84), means (14b, 14e) for connecting the bypass (56b,56e) to the furnace section (54), a plurality of air nozzles (78,80) for selectively controlling the flow of solids from the bypass (56b,56e) to the furnace section (54) and from the bypass (56b,56e) to the heat exchange means (84), characterised in that a partition (70,72,100) is disposed in the recycle heat exchange section (56) for defining a bypass compartment (56b,56e) and a heat exchange compartment (56a,56c,56d) for the heat exchange means (84), an opening (70a,72a,100a) is formed in that partition (70,72,100) to connect the bypass compartment (56b,56e) to the heat exchange compartment (56a,56c,56d), and the plurality of air nozzles (78,80) are disposed in the bypass compartment (56b,56e) and in the heat exchange compartment (56a,56c,56d). A system as claimed in Claim 1 further comprising means (14a, 14c, 14d) for connecting the heat exchange compartment (56a, 56c, 56d) to the furnace section (54) to permit the flow of solids from the heat exchange compartment (56a, 56c, 56d) to the furnace section (54). A system as claimed in Claim 2 in which the means (14a, 14c, 14d) connecting the heat exchange compartment (56a, 56c, 56d) to the furnace section (54) comprise openings (14a, 14c, 14d) formed in the rear wall (14) of the furnace section (54). A system as claimed in any preceding claim in which the means (14b, 14e) connecting the bypass compartment (56b, 56e) to the furnace section (54) comprises openings (14b, 14e) formed in the rear wall (14) of the furnace sections (54). A system as claimed in any preceding claim further comprising a vertical wall (50) which, together with the rear wall (14) and sidewalls (15a, 16b) of the furnace section (54) define the recycle heat exchange section (56). A system as claimed in any preceding claim comprising means for introducing air to the air nozzles (78) an air plenum (24) for receiving fluidised air for fluidising the particles in the recycle heat exchange section (56) and an air distributor (22) extending above the air plenum (24) for supporting the air nozzles (78) and the separated particulate material. A system as claimed in any preceding claim in which air nozzles (78, 80) in the bypass compartment (56b, 56e) are positioned at a first height to pass the separated material from the bypass compartment (56b, 56e) to the heat exchange compartment (56a, 56c, 56d) and at a second height to pass separated material from the bypass chamber (56b, 56e) to the furnace section (84), means (24, 82) being provided to selectively pass air to the nozzles (78, 80). A system as claimed in any preceding claim in which said exchange means comprises water tubes (84) for passing a fluid in a heat exchange relation to the separated particulate material to heat the fluid and control the temperature of the separated particulate material. A system as claimed in Claim 8 further comprising a fluid flow circuit including the water tubes (84) and further water tubes (74) forming at least a portion of the walls (12, 14, 16a, 16b) of the furnace section (54).

Inventors: DIETZ DAVID H, DIETZ, DAVID H.
Applicants: FOSTER WHEELER ENERGY CORP, FOSTER WHEELER ENERGY CORPORATION
Application year: 1991
Main classification: F22B31
IPC codes: F23C10, F22B31