Steam generating system utilizing separate fluid flow circuitry between the furnace section and the separating section

Abstract: A fluidized bed steam generating system in which a duct (16) is disposed between a furnace outlet and a separator inlet. The duct is formed by first and second wall panels (22a, 22b) which are comprised of a plurality of parallel tubes (18) extending perpendicular to the direction of the flow of the combustion gases. The tubes comprising the wall panels are bent and welded to a fin (20) extending from corresponding portions of adjacent tubes to form a gas tight structure. The ends of the tubes are connected in fluid flow communication with first and second headers which enables cooling fluid to flow through the tubes for recovering heat from the combustion gases as they flow from the furnace into the separator.

Description: This invention relates to a fluidized bed steam generating system and, more particularly, to such a system which includes a separate fluid flow circuitry between the furnace section and the separating section. Fluidized bed combustion systems in connection with separators are well known. In these arrangements, air is passed through a bed of particulate fuel, possibly coal, wood or dehydrated sewage sludge, to fluidized the bed, and thereby, effectuate high combustion efficiency at a relatively low temperature. This process, however, results in flue gases which retain a large amount of fine particulates. The gas stream is therefore passed into a separator which separates the particulates from the gas and recycles them back into the bed. In conventional steam generating systems, the passage between the furnace and the separator is usually defined by a relatively expensive, high temperature, refractory-lined duct due to the extreme temperature of the flue gases. This duct is either left relatively thin due to the expense and weight of the refractory material which results in excessive heat losses to the environment, thereby reducing the system's efficiency, or it is made relatively thick which adds to the bulk, weight and cost of the separator. Even when the duct is thick, all the heat losses cannot be prevented since perfect insulation would raise the duct's temperature to an unacceptable degree. A further problem associated with the use of a refractory-lined duct is the lengthy time required to warm the walls before putting the system on line to eliminate premature cracking of the refractory material. This lengthy delay is inconvenient and adds to the cost of the process. For relatively small steam generating systems, these problems can be prevented by forming the duct directly out of the walls of the furnace and separator. This is accomplished by bending a plurality of cooling tubes of each device out of their planes to form both an outlet and inlet. Such an arrangement is shown in Swedish Patent Specification No. 437 124. This process is not feasible in larger systems due to the engineering requirement that the duct leading into the separator be several feet in length in order to maintain an acceptable separator collection efficiency. Further, this process is complex and expensive due to the elaborate bending patterns required. According to the invention there is provided a steam generating system, comprising a furnace having a gas outlet and formed at least in part by a plurality of water tubes, a separator having a gas inlet and a duct for directing gases from the gas outlet of the furnace to the separator inlet, the duct comprising a plurality of tubes bent and arranged to form the duct characterised in that means connect adjacent tubes to form a gas-tight structure, and a fluid flow circuit circulated fluid through the tubes independently of any fluid flow through the water tubes of the furnace to recover heat from the gases as they pass through the duct. In a steam generating system according to the invention heat losses are reduced to increase the efficiency of the system and expensive, high temperature, refractory-lined ductwork is minimized. Therefore, the system can be put into use relatively quickly without any significant warm up period. Also the efficiency of the steam generating system is increased by transferring heat from a duct connecting the furnace and the separator to a power generating system. Further, the duct between the furnace and separator can be maintained at the same temperature as the furnace and separator to reduce the thermal stresses in the system. The above brief description as well as advantages of the present invention will be more fully appreciated by reference to the following detailed description of presently preferred but nonetheless illustrative embodiments in accordance with the present invention when taken in conjunction with the accompanying drawings wherein: FIG. 1 is a schematic view of the steam generating system of the present invention; and FIG. 2 is an enlarged perspective view of a duct in the system of FIG. 1. Referring to FIG. 1 of the drawings, the reference numeral 10 refers in general to a steam generating system which includes a furnace 12 and a separator 14. A duct 16 connects the rear wall 12a of the furnace 12 to the front wall 14a of the separator 14, and the walls of the duct 16 are formed by a group of spaced, parallel, hollow tubes 18. As shown in FIG. 2, a fin 20 is welded to, and extends from, diametrically opposed wall portions of each tube 18 and between the adjacent walls of each adjacent pair of tubes 18. Each fin 20 extends for the entire length of each pair of tubes 18 thus forming two air-tight finned tube panels 22a and 22b. As shown in FIG. 1, the walls of both the furnace 12 and the separator 14 are formed by finned tube panels in a like manner to the walls of the duct 16. The tubes 18 forming the panel 22a are connected at their upper ends to an upper header 24, extend downwardly vertically from the upper header 24, are bent 90° in their mid-sections and are connected at their other ends to a lower header 26, so that the panel 22a creates the bottom and one side of the duct 16. The tubes 18 forming the panel 22b extend horizontally from the upper header 24, are bent 90° downwardly in their mid-sections and are connected to the lower header 26, so that the panel 22b forms the top and the other side of the duct 16. The respective ends of all the tubes 18 are thus connected to the headers 24 and 26 so that fluid can flow from the upper header 24 through the tubes 18 and into the lower header 26. A pipe, or pipes 28a, shown schematically in FIG. 1, extends upwardly from the upper header 24, and a pipe, or pipes 28b, extends downwardly from the lower header 26. The pipes 28a are connected to a vessel 30, which may be in the form of a steam drum or a header, and the pipes 28b are connected to a header 14b disposed at the lower end of the separator 14. It is understood that the vessel 30 can be a source of cooling fluid, such as water, steam or a mixture of both, which passes from the pipes 28a into the upper header 24, through the tubes 18, and into the lower header 26 before being discharged, via the pipes 28b, into the header 14b. From the header 14b, the fluid passes upwardly through the length of the tubes forming the walls of the separator 14, before discharging into a header 14c disposed at the upper end of the separator 14. A pipe, or pipes 31, connects the header 14c to a vessel 32, which may be in the form of a steam drum or a header. Referring to FIG. 2, the finned tube rear wall 12a of the furnace 12 contains a gas outlet 12b in the upper portion of the furnace 12 for directing furnace gases out of the furnace 12. This furnace outlet 12b is formed in a conventional manner by bending a portion of the tubes of the wall 12a 90° out of the plane of the furnace wall 12a, then outwardly, downwardly and around to define the outlet 12b, and finally back into the plane of the wall 12a. Although not depicted in the drawings in detail, it is understood that the separator 14 contains a gas inlet formed by bending a portion of the tubes comprising the finned tube front wall 14a of the separator 14 out of the plane of the separator wall to form an opening in a similar manner. The duct 16, as formed by the finned tube panels 22a and 22b, is connected to the rear wall 12a of the furnace 12 by welding a fin edge 20a of the duct 16 to a fin edge 12c of the furnace outlet 12b as depicted schematically in FIG. 2. Similarly, the duct 16 is connected to the front wall 14a of the separator 14 by welding a fin edge 20b of the duct 16 to the inlet (not shown) formed in the wall of the separator 14. In operation, fuels are combusted in the furnace 12 and the mixture of air and gaseous products of combustion (referred to generally as "the flue gases") passes upwardly in the furnace 12 by natural convection, through the outlet 12b in the upper portion of the furnace 12, and through the duct 16 into the inlet of the separator 14. Simultaneously and continuously, a cooling fluid flows from the vessel 30 into the upper header 24 via the pipes 28a. The cooling fluid then flows into and through the plurality of tubes 18 of both finned tube panels 22a and 22b forming the duct 16. While flowing through the tubes 18, heat from the flue gases passing from the furnace 12 to the separator 14 is transferred into the cooling fluid via the tubes 18, thus warming the cooling fluid. The cooling fluid continues on to the lower header 26 where it then enters the pipes 28b and is passed through the separator 14 and, via the pipes 31, to the vessel 32. Several advantages result from the foregoing arrangement. For example, the duct 16 of the present invention reduces heat losses and minimizes the requirement for internal refractory insulation. The heat is instead transferred via a cooling fluid through the tubes 18 to increase the efficiency of the steam generating system. Also, the duct 16 can be maintained at the same temperature as the furnace 12 and the separator 14, thereby reducing thermal stresses in the system. It is understood that several variations may be made in the foregoing without departing from the scope of the present invention. For example, the direction of fluid flow described above can be reversed such that the flow originates from the vessel 32 and continues downward through the separator 14 then upward through the walls of the duct 16 and on to the vessel 30. Furthermore, the fluid flow passing through the tubes 18 of the duct 16 need not flow through the tubes of the separator 14, but can instead pass solely from a header, through the tubes 18 of the duct 16, and back to the originating, or on to a secondary, header.

Claims: A steam generating system, comprising a furnace (12) having a gas outlet (12b) and formed at least in part by a plurality of water tubes, a separator (14) having a gas inlet (14a) and a duct (16) for directing gases from the gas outlet (12b) of the furnace to the separator inlet (14a), the duct (16) comprising a plurality of tubes (18) bent and arranged to form the duct (16), characterised in that means (20) connect adjacent tubes (18) to form a gas-tight structure, and a fluid flow circuit circulates fluid through the tubes (18) independently of any fluid flow through the water tubes of the furnace (12) to recover heat from the gases as they pass through the duct (16). A system as claimed in Claim 1 in which fins (20) extending from corresponding portions of adjacent tubes (18) connect the tube (18) to form a gas-tight structure. A system as claimed in Claim 1 or Claim 2 in which the tubes (18) extend perpendicular to the direction of the flow of the gases through the duct (16). A system as claimed in any preceding claim in which the fluid flow circuit comprises first and second vessels (30,32), first and second headers (24, 26) in fluid flow communication with the respective ends of each of the tubes (18), first piping (28a) connecting the first vessel (30) in fluid flow communication with the first header (24), and second piping (28b) connecting the second vessel (32) in fluid flow communication with the second header (26). A system as claimed in Claim 4 in which the first vessel (30) is a steam drum and the second vessel (32) is a header. A system as claimed in any preceding claim in which the tubes (18) are divided into first and second panels (22a, 22b), the tubes (18) forming the first panel (22a) being connected at one of their ends to a first header (24), extend downwardly from the first header, are bent 90°, and are connected at their other ends to a second header (26), and the tubes (18) forming the second panel (22b) are connected at one of their ends to the first header (24), extend horizontally from the first header, and are bent 90° downwardly and are connected at their other ends to the second header (26). A system as claimed in any preceding claim in which the fluid flow circuit for the tubes (18) is independent of any fluid flow through the separator (14).

Inventors: GARKAWE MICHAEL, SOLTYS ROGER A, GARKAWE, MICHAEL, SOLTYS, ROGER A
Applicants: FOSTER WHEELER ENERGY CORP, FOSTER WHEELER ENERGY CORPORATION
Application year: 1992
Main classification: F22B31
IPC codes: F22B1, F22B31