FLUIDIZED BED STEAM GENERATOR INCLUDING A SEPARATE RECYCLE BED

Abstract: A fluidized bed steam generator and method of generating steam including a separate recycle bed in which a plurality of vertically stacked fluidized beds are disposed in a furnace enclosure (10) and a heat recovery enclosure (20) is defined adjacent the furnace enclosure (10) for receiving the effluent gases from the fluidized beds (in A, B, C). A fluidized bed is defined in the heat recovery enclosure (20) and one or two separators (22, 24) are provided adjacent the heat recovery enclosure (20) for receiving the effluent gases and separating the entrained solid particles therefrom. The separated solid particles are then injected into the fluidized bed in the heat recovery enclosure (20). Fresh fuel is supplied to the recycle bed and the material inventory and fluidizing velocity in the recycle bed is controlled.

Description: The invention relates to a steam generator, particularly a fluidized bed steam generator with the features of the preamble of claim 1. Fluidized beds are well recognized as attractive heat sources since they enjoy the advantages of an improved heat transfer rate, while permitting a reduction in corrosion, boiler fouling, and sulfur dioxide emission. In a typical fluidized bed arrangement, air is passed upwardly through a mass of particulate material causing the material to expand and take on a suspended or fluidized state. However, there is an inherent limitation on the range of heat input to the water passing in a heat exchange relation to the fluidized bed, largely due to the fact that the quantity of air supplied to the bed must be sufficient to maintain same in a fluidized condition yet must not cause excessive quantities of the particulate material to be blown away. This disadvantage is largely overcome by the heat exchanger disclosed in U.S. Patent No. 3,823,693. In the arrangement disclosed in the latter patent, the furnace section of the heat exchanger is formed by a plurality of vertically stacked chambers, or cells, each containing a fluidized bed. The fluid to be heated is passed upwardly through the fluidized beds in a heat exchange relation thereto to gradually raise the temperature of the fluid. A tube bundle is located in the area above each bed to provide a convection surface for the effluent gases from each bed. The particulate material is separated from the effluent gases exciting from each bed and recycled back into the lowermost bed which functions as a recycle cell to burn off the remaining carbon in the particulate material. However, the volume of space available above each bed to receive the tube bundles is relatively small due to limitations placed on the cross-sectional area of each cell caused by tube spacings, welding accessibility, combustion requirments, etc. As a result, the convection surface defined by the tube bundles is limited to an extent that the mass flow of the effluent gases per area of the convection surface and the resulting heat transfer coefficient above each bed, is less than optimum. Also since the recycle bed is not provided with fresh fuel, undesirable variations in heat input due to fuel variations or steam generator output changes are often encountered. Further, there is no provision to control the inventory and the fluidizing velocity in the recycle cell, which further adds to the problems of controlling the heat in the beds. US-Patent No. 3 863 606 shows a steam generator of the kind mentioned in the preamble of claim 1 to which the invention relates. This steam generator shows besides the furnace section with a plurality of vertically spaced beds means for defining a heat recovery enclosure connected to the fluidized beds through outlets, the recovery enclosure having a separate gas outlet. Further, there is provided in the lower portion of the heat recovery enclosure a separate cell for carbon burnup. This cell has an air gas circuit separate from that of the primary circuit in the form of an inlet through which air passes into a damper for passing through the bed and to excit through a separate gas outlet. As the gas is let through this separate outlet it does not mix with the gas coming through the outlets from the fluidized beds. The mixing of the gas of the outlet of the heat recovery enclosure and the outlet of this separate cell takes place only prior to the entry into a collector. Accordingly, the operation of the known steam generator is limited as the gases from the separate cell pass through outlet with a limited combustion. Also there may be variations in the heat input. It is the problem underlying the invention to provide a steam generator for generating steam which enjoys the advantages of stacked fluidized beds, provide an optimum efficiency, particularly by a convection heat transferred surface of optimum size, and to keep heat input variations to a minimum. This problem is being solved by the features of the characterizing part of claim 1. The mixing of the gases from the vertically spaced fluidized beds with the effluent gases from the fluidized bed in the heat recovery enclosure together with the means for adding additional fuel material to the fluidized bed in the heat recovery enclosure provides a number of advantages. Besides the reduction of the costs for material handling equipment all the effluent gases in chamber 20 have a relatively long residence time since they must travel the full height of the chamber and are maintained at a temperature high enough to promote their combustion by the periodic addition of the hot fuel gases entering from the bed cells A, B, and C. Also, any sulfur dioxide entering the chamber is further reacted with the fine adsorbent particles as the gases travel upwardly in the chamber, resulting in a maximum efficiency of sulfur capture and minimum adsorbent requirements control sulfur dioxide emissions. Finally, the extremely tall construction of the heat recovery disclosure above the fluidized bed makes possible an excellent heat exchange. This is essential for an optimum efficiency. The means for adding additional fuel material to said fluidized bed in the heat recovery enclosure are further the basis to make possible a constant heat input. When the fluidizing velocity of the recycle bed is controlled the conditions in the latter bed are maintained steady. Further advantageous developments of claim 1 are given by the teaching of the subclaims. Brief Description of the Drawings The above brief description as well as further objects, features and 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 which is a schematic, vertical sectional, view of the steam generator of the present invention. Description of the Preferred Embodiments The steam generator of the present invention is shown in the drawing, and includes a furnace section formed with three primary fluidized bed cells A, B, and C extending in a chamber 10 defined by a front wall 12, a rear wall 14, a side wall 16, and another side wall not shown. The details of each bed cell A, B, and C will be described later. An additional wall 18 is disposed in a spaced relation to the rear wall 14 to form a chamber 20 adjacent the chamber 10. A pair of cyclone separators 22 & 24 are disposed adjacent the wall 18 and communicate with the chamber 20 via ducts 26 & 28, respectively. Three horizontal, perforated, air distribution plates 30 are disposed in a vertically spaced relation between the walls 12 and 14 and extend within the bed cells A, B, and C, respectively. An air inlet 32 (shown in cross-section) is associated with each bed cell A, B, and C and extends through the side wall 16 into an air plenum chamber 34 extending below each of the plates 30. As a result, air is distributed into each bed cell A, B, and C, with the flow being controlled by dampers, or the like (not shown). Three spreaders 36 are mounted on the front wall 12 at three elevations and communicate with the bed cells A, B, and C, respectively. The spreaders 36 are adapted to receive particulate fuel from an external source, and discharge same into each bed cell in a conventional manner. It is understood that drop pipes, or the like (not shown) may be provided for feeding an adsorbent, such as limestone, into their respective bed cells A, B, and C for adsorbing the sulfur generated as a result of the combustion of the particulate fuel, in a conventional manner. The particulate materials thus form a bed of material in each bed cell A, B, and C which is fluidized by the air passing upwardly through the plates 30 and into each bed. A tube bundle 38 is disposed immediately above the plates 30 and within the fluidized bed formed in each bed cell A, B, and C. Each tube bundle is connected to a system (not shown) for circulating water through the tubes to remove heat from the fluidized beds in a conventional manner. It is understood that appropriate headers, downcomers, and the like (not shown), are provided for circulating water or steam through each tube bundle 38 to transfer heat generated in the bed to the water or steam. Three openings 40 are formed through the wall 14 at three elevations to enable the effluent gases generated in each bed cell A, B, and C to be discharged from the chamber 10 into the chamber 20. A fluidized bed cell D, which is identical to the cells A, B, and C, is disposed in the lower portion of the chamber 20 and has an air inlet 32, an air chamber 34, a spreader 36, and a tube bundle 38 associated therewith, which function in the manner described above in connection with the cells A, B, and C. Also a weir 41 is disposed in the cell D which operates in a conventional manner to control the volume of particulate material in the cell D. The gases entering the chamber 20 from the bed cells A, B, and C, via the openings 40, and the gases from the bed cell D mix in the chamber 20 and rise by natural convection to the upper portion of the latter chamber before exiting through the ducts 26 & 28 and into the cyclone separators 22 and 24, respectively. The cyclone separators 22 and 24 operate in a conventional manner to separate the solid particulate material entrained therein from the gases. The relatively clean gases pass from the separator 22 through an outlet duct 42 to an external heat recovery area (not shown) and the clean gases from the separator 24 pass through an outlet duct 44 to the duct 42. It is understood that the heat recovery area includes a plurality of tube bundles for removing heat from the gases after which the gases pass to a tubular air heater, a baghouse, an induced draft fan and to a stack, all of which are conventional and thus not shown. The separators 22 and 24 each include a hopper portion which collects the fine particles separated from the effluent gases and passes same into injector lines 46 & 48 which inject the particles back to the bed cell D. The particles in the bed cell D combine with the fresh fuel particle bed to the cell by its spreader 36 and the mixture is fluidized and combusted in a manner similar to the particulate coal in the fluidized bed cells A, B, and C, as described above. It is understood that the walls 12, 14, and 16 are each formed by a plurality of vertically extending tubes connected in a conventional manner to form part of a natural circulation flow circuit which includes a steam drum 50, a plurality of headers such as shown by the reference numeral 52 at the ends of the above walls, and a plurality of pipes two of which are shown by reference numeral 52. Since this type of arrangement is conventional it will not be described in any further detail. In operation, air is passed into each fluidized bed disposed in the bed cells A, B, and C to fluidize each bed, it being understood that the velocity and rate of flow of the air is regulated so that it is high enough to fluidized the particulate fuel and to obtain economical burning, or heat release rates, per unit area of bed, yet is low enough to avoid the loss of too many fine fuel particles from the bed and to allow sufficient residence time of gases for good sulfur removal by the adsorbent added to the bed. The heated air, after passing through each fluidized bed, combines with the combustion products from the bed and the resulting mixture, or gas (hereinafter referred to as the effluent gases) exits through the openings 40 in the wall 14 and flows into the heat recovery chamber 20. The effluent gases from the bed cell D, along with the gases from the cells A, B, and C, rise by natural convection in chamber 20, exit from the chamber through the ducts 26 & 28, respectively, and flow into the separators 22 & 24, respectively. The solid fuel and adsorbent particles entrained in the effluent gases are separated therefrom in the separators 22 & 24, with the gases exiting through the ducts 40 & 42 and into the heat recovery area. The separated particles, which include flyash and unreacted fuel and adsorbent are injected to the fluidized bed in the cell D, where they mix with the fresh fuel supplied by the spreader 36 associated with the latter cell. The velocity of the air from the inlets 32 to each bed cell A, B, C, and D is regulated, and the amount of material in the cell D is carefully controlled by the weir 41 in the cell D to prevent any increases in inventory in the latter cell once an inventory sufficient to maintain steady conditions in the latter cell is attained.

Claims: A steam generator comprising a furnace section, means (A,B,C) in said furnace section for defining a plurality of vertically spaced beds of particulate material including fuel, means (30,32,34) for introducing air into said beds to fluidize said beds, one boundary wall (14) of said furnace section having openings (40) therein for permitting the discharge of effluent gases from said fluidized beds, means (20, 14) including said one boundary wall (14) for defining a heat recovery enclosure (20) adjacent said furnace section for receiving said effluent gases, means disposed adjacent said heat recovery enclosure (20) for receiving the effluent gases from the heat recovery enclosure, and separating the entrained solid particles from said gases, means (D) defining a bed of said material in said heat recovery enclosure (20), means (30,32,34) for introducing air into said bed in said heat recovery enclosure (20) to fluidize said material in said latter enclosure, and means for injecting the separated solid particles into the fluidized bed in said heat recovery enclosure (20) characterized by means (36) for adding additional fuel material to said fluidized bed (D) in said heat recovery enclosure (20), the fluidized bed (D) in said heat recovery enclosure (20) being located in the lower portion of said enclosure so that the effluent gases from said latter fluidized bed pass upwardly through the entire length of said heat recovery enclosure (20) before exiting same and that the effluent gases from said vertically spaced fluidized beds (in A,B,C) combine with the effluent gases from said fluidized bed (in D) in said heat recovery enclosure (20). The steam generator of claim 1, further comprising means for varying the fluidized velocity of the air introduced into said fluidized bed in said heat recovery enclosure (20). The steam generator of claim 2, further comprising means for controlling the amount of material in said fluidized bed in said heat recovery enclosure (20). The steam generator of claim 1, wherein two walls of said furnace section and said heat recovery enclosure (20) are formed by two continuous walls (14,18) spanning the width of said generator. The steam generator of claim 1 further comprising means in each of said vertically spaced fluidized beds removing heat from said latter beds. The steam generator of claim 1 further comprising means in said fluidized bed in said heat recovery enclosure (20) for removing heat from said latter fluidized bed. The steam generator of claim 6 wherein said heat removal means comprises tube means, and means for passing water through said tube means. The steam generator of claim 1 wherein said separating means comprises a multi-cyclone type device (22, 24) disposed adjacent said heat recovery enclosure (20) for receiving the effluent gases from all of said beds (in A,B,C and in D).

Inventors: ALLISTON, MICHAEL GERARD, GARCIA-MALLOL, JUAN ANTONIO
Applicants: FOSTER WHEELER ENERGY CORPORATION
Application year: 1987
Main classification: F22B31
IPC codes: F23C10, F22B1, F22B31, F23C9