Abstract:
A solid oxide fuel cell generator ( 10 ) contains stacks of hollow axially elongated fuel cells ( 36 ) having an open top end ( 37 ), an oxidant inlet plenum ( 52 ), a feed fuel plenum ( 11 ), a combustion chamber ( 94 ) for combusting reacted oxidant/spent fuel; and, optionally, a fuel recirculation chamber ( 106 ) below the combustion chamber ( 94 ), where the fuel recirculation chamber ( 94 ) is in part defined by semi-porous fuel cell positioning gasket ( 108 ), all within an outer generator enclosure ( 8 ), wherein the fuel cell gasket ( 108 ) has a laminate structure comprising at least a compliant fibrous mat support layer and a strong, yet flexible woven layer, which may contain catalytic particles facing the combustion chamber, where the catalyst, if used, is effective to further oxidize exhaust fuel and protect the open top end ( 37 ) of the fuel cells.

Description:
GOVERNMENT CONTRACT 
     The Government of the United States of America has rights in this invention pursuant to Contract No. DE-FC26-97FT34139, awarded by the U.S. Department of Energy. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to novel composite positioning gaskets used to separate the combustion chamber of a solid oxide electrolyte fuel cell generator, hereinafter “SOFC generator”, from the other regions of the SOFC generator, such as the recirculation zone or stack region. 
     BACKGROUND OF THE INVENTION 
     High temperature, solid oxide electrolyte fuel cell generator apparatus and fuel cell configurations are well known, and taught, for example, in U.S. Pat. No. 4,395,468 (Isenberg). There, feed fuel, either H 2 +CO, or previously reformed natural gas, is fed into the apparatus at one end and flows parallel to exterior fuel electrode surfaces of elongated tubular fuel cells. Spent fuel is combusted with spent oxidant in a separate combustion chamber and then exits the apparatus. In this patent a power generation chamber extends between an end wall of the generator housing and a porous, stiff positioning/separator board on the other side of which is a combustion chamber containing open ended fuel cells into which air/oxidant feed tubes are passed. The combustion chamber pre-heats incoming air/oxidant passing through the air/oxidant feed tubes and into the interior of tubular closed end fuel cells. Other generator designs utilize an open end fuel cell structure, for example, U.S. Pat. No. 5,200,279 (Draper et al.), or flattened rather than tubular fuel cells, for example, U.S. Pat. No. 4,888,254 (Reichner). 
     In later SOFC embodiments, some spent fuel from the recirculation chamber between the combustion chamber and the power generation chamber, as shown in FIG. 2 of U.S. Pat. No. 4,983,471 (Reichner and Dollard) was recirculated either directly from the recirculation chamber or from openings in the power generation chamber. A similar arrangement is shown in FIG. 4 of U.S. Pat. No. 5,573,867 (Zafred et al.), where some spent fuel captured in a spent fuel recirculation chamber is recirculated to an ejector to mix with fresh feed fuel. The fuel cell positioning board, between a combustion chamber and a spent fuel recirculation chamber is more clearly shown in FIG. 2 of U.S. Pat. No. 6,656,623 B2 (Holmes et al). In some instances, the open end of the fuel cell, located in the combustion chamber, was vulnerable to reduction by spent fuel. Originally, this was solved by Zafred et al., in U.S. Pat. No. 6,221,522 B1, by utilizing open fuel cell end sleeves, but this added to costs and did not completely solve the problem. 
     Originally, upper and lower positioning boards were designed to function as loose separators between the power generation fuel cell stack region, the spent fuel recirculation plenum/chamber and the combustion chamber, and also as open-end cell positioning boards. Typical board material was a low density, vacuum-formed fibrous ceramic board, which was subsequently machined to obtain loose clearance holes for the fuel cells and flow holes through the ligament between adjacent cells. 
     However, inadequate sealing tended to cause some fuel and temperature mal-distribution. Also, differential axial thermal expansion between the cells and the positioning boards, could lead to cells “grabbing” the boards during generator shutdowns and potentially breaking them. Additionally, the use of initially stiff positioning boards required 100% location mapping using a very expensive laser mapping technique which generates a series of XY coordinates for each cell position. These data are inputted into a milling machine which machines the final custom hole pattern to precisely match the irregular cell pattern configuration. 
     What is needed are positioning boards which are both flexurally strong yet soft and compliant and also cost-effective. Furthermore, they should have specific permeation characteristics to allow only a certain fraction of spent fuel to flow through to the combuster chamber while allowing a greater fraction to recirculate through the stack. They also should be initially flexible during insertion into the generator and should also somehow have the capability of protecting the open end of the fuel cell in the combustion chamber from localized burning of hydrogen during transient operating conditions where normal temperatures of 1100° C. can rise for a short time to as high as 1400° C. It is a main object of this invention to provide novel positioning boards, especially those interfacing with the combustion chamber, where the boards meet strong yet compliant requirements and somehow are effective to protect fuel cell open ends during short transient conditions. 
     SUMMARY OF THE INVENTION 
     The above needs/objects are met and problems solved by providing a solid oxide fuel cell generator comprising: stacks of hollow, axially elongated fuel cells having at least one open top above an electrochemically active portion, which can react and operate on feed oxidant and feed fuel to generate electricity; an oxidant inlet plenum supporting oxidant feed tubes; a feed fuel inlet plenum; and a combustion chamber for receiving reacted oxidant/spent fuel, located above the active portion of the fuel cells and below the oxidant inlet plenum and containing the at least one open top of the fuel cells where the combustion chamber is defined by the oxidant inlet plenum and a semi-porous fuel cell positioning gasket, which gasket allows permeation of spent fuel into the combustion chamber; all surrounded by insulation and all within an outer generator enclosure, and where the fuel cell positioning gasket has a laminate structure comprising at least a compliant fibrous mat sealing layer and a flexible, woven layer, the laminate structure closely gasketing the fuel cells so that spent fuel permeation into the combustion chamber is controlled. This “control” can be through use of measured porosity within the gasket to provide just a slightly lower pressure in the combustion chamber vs. the chamber opposite the combustion chamber. Tubular fuel cells are much preferred but elongated/flattened hollow cells and other types are possible. 
     This invention also resides in a solid oxide fuel cell generator comprising: stacks of hollow tubular, axially elongated fuel cells having at least one open top above an electrochemically active portion, which can operate on feed oxidant and feed fuel to generate electricity; an oxidant inlet plenum supporting oxidant feed tubes; a feed fuel inlet plenum; a combustion chamber for receiving reacted oxidant/spent fuel, located above the active portion of the fuel cells and below the oxidant inlet plenum and containing the at least one open top of the fuel cells where the combustion chamber is defined by the oxidant inlet plenum and a fuel cell positioning gasket, which gasket allows permeation of spent fuel into the combustion chamber; and a fuel recirculation chamber, below the combustion chamber, said fuel recirculation chamber defined at least by the fuel cell positioning gasket; all surrounded by insulation and all within an outer generator enclosure, where the oxidant feed tubes have an open end in the oxidant inlet plenum and which oxidant feed tubes pass through the combustion chamber and into the at least one open fuel cell top within the combustion chamber, and where the fuel cell positioning gasket seals around the fuel cells to control gas flow into the combustion chamber and has a laminate structure comprising at least a compliant fibrous mat layer and a flexible woven layer facing the combustion chamber. The woven layer can contain catalytic nickel. 
     The fibrous mat can also be laminated to a flexible, woven, bottom non-catalyzed layer to form a three layer composite gasket sheet. The flexible woven layer contains high purity alumina fibers, which matrix, when facing the combustion chamber, may be impregnated with nickel slurry to form a catalytic mat, where hydrogen fuel combustion would preferably occur, thus protecting the open ends of the cells from hot spots in the combustion chamber. The composite assembly is very flexible, even in the initial state, allowing ease of installation over bundle rows with realistic tolerances in cell pitch without resorting to expensive mapping and custom manufacturing. This compliance effectively seals against the fuel cells as a gasket which in most instances prevents spent fuel getting through gaps between the gasket and fuel cells and burning near the open fuel cell ends. If effective, nickel catalyst is not needed. 
     Another embodiment of the invention resides in a plurality of fuel cell stacks in a fuel cell generator, the stacks containing a plurality of axially elongated fuel cells all supported at least in part by a semi-porous gasket, where the fuel cells are tightly disposed through perforations in the gasket and at least one end of the fuel cells is disposed in a combustion chamber adjacent the gasket, said gaskets having flexible initial form but made of material that may stiffen at temperatures over 1000° C., said gasket comprising at least a compliant fibrous mat support layer, and a flexible woven layer. 
     The compliant gasket, even when later baked/cured/sintered effectively absorbs mechanical and thermal loads induced by differential thermal expansion of the SOFC cells and the adjacent structures during generator operation, and maintains the important range of allowable pressure drop which has now been found in this environment to be between about 0.25-2.0 inches of water, to control spent fuel permeation through the gasket. Additionally, the simple fabrication of the fuel cell positioning gasket makes it ideal for low cost production. A lower positioning board if used, can be of the same construction or standard stiff board construction. The fuel cells can be closed or open ended, tubular or of flattened design. Thus, this provides a non seal-less design. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other advantages of this invention will be more apparent from the following description in view of the drawings, where: 
         FIG. 1 , which best shows the invention, is a side view in section of one embodiment of the fuel cell generator of the invention, utilizing closed end fuel cells, showing fuel cell gasket and optional lower fuel cell positioning boards as well as the oxidant inlet plenum; 
         FIG. 2  is a three dimensional view of a flexible bendable fuel cell positioning gasket laminate; 
         FIG. 3  is a three dimensional view of a still flexible/baked fuel cell positioning gasket laminate; and 
         FIG. 4  is a three dimensional view of a “sewn pillow type” configuration embodiment of a fuel cell positioning gasket. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to  FIG. 1 , solid oxide fuel cell generator  10  is shown. It contains a plurality of fuel cell stacks, generally designated as number  20 , of fuel cells  36 . Each fuel cell is in hollow, axially elongated form, preferably tubular as shown, having an open top end  37  and, as shown in this embodiment, a closed bottom end  38 . Use of dual open ended fuel cells is also an option. The fuel cells contain interior self supporting air electrodes  40 , usually of a doped LaMnO 3  ceramic, exterior fuel electrodes  42 , usually of a nickel zirconia cermet, with solid electrolyte  44 , usually comprising yttria stabilized zirconia, therebetween, as shown by the one fuel cell shown. The fuel cells operate on a feed oxidant  50 , usually pre-heated air, and reformed fuel  12 , usually internally or externally reformed to provide H 2 +CO anode gas. The fuel passes from a bottom fuel inlet plenum  11 , through fuel entry boards  90  having holes therethrough, at the bottom  8  of the fuel cell generator. These fuel entry boards can also help support the fuel cell bottoms  38  as at point  92 . 
     An oxidant inlet plenum  52  can be formed by enclosing insulation  76 ,  76 ′ and  76 ″ and includes an oxidant feed tube support board  77 , having holes for oxidant feed tubes  51  therethrough, which member  77  is located at the top portion  9  of the fuel cell generator. Below the oxidant feed tube support board  77  and above the fuel cells  36  is a reacted or spent oxidant/reacted or spent fuel exhaust combustion chamber  94  useful to heat incoming feed oxidant in the oxidant feed tubes  51 . In operation, oxidant flows down the oxidant feed tubes  51  into the open end  37  of fuel cells  36 , to the bottom  38  of the fuel cells where it reverse flows, contacting the inner air electrode  40 , reacts to provide depleted oxidant  104  which exits the open end  37  of the fuel cells into oxidant/fuel exhaust combustion chamber  94 . The reformed fuel  12  passes outside of the fuel cells,  36 , contacting exterior fuel  14  some of which exits into fuel recirculation chamber  106  and depleted fuel  16  which exits into reacted oxidant/fuel exhaust combustion chamber  94 . 
     The fuel cells  36  are partly supported by fuel cell positioning gasket  108  and optional lower fuel cell positioning board  110 , which can define the fuel recirculation chamber  106 , as shown in this embodiment. The fuel cell positioning gasket  108  is usually spaced by vertical supports, not shown. The oxidant feed tubes  51  pass through combustion chamber  94  into the open center of the fuel cells. Insulation  76 , most of it in bulk ceramic fiber form, about 70% porous by volume, surrounds all these components which are enclosed by a metallic, usually, steel canister, providing the pressure boundary of the system. Exhaust outlet is shown as  116 , with exhaust flow shown as  120 . 
     The fuel cell positioning gasket  108  is a composite/laminate of at least two layers, top layer  122  and base layer  124 . Optional bottom layer  126  is also shown in  FIG. 1 . The top layer  122  is preferably a woven textile matrix comprised of high purity alumina fibers which, optionally, can be impregnated with nickel particles or nickel oxide slurry to form a catalytic mat close to spent fuel combustion in the generator chamber  94 . 
     The composite fuel cell positioning gasket  108  is designed to facilitate the assembly of the generator stack, lower the cost of the fuel cell generator, and ultimately offer a viable solution towards producing highly efficient SOFC systems. The positioning gasket  108  is designed to function as a semi-porous partition barrier between, for example, a recirculation chamber  106  and a combustion chamber  94  of the SOFC fuel cell generator  10 . The compliant layer  124  is designed to seal against the fuel cells effectively. 
     The impregnated top woven mat  122  can be designed to help combust the unreacted fuel in the combustion chamber  94 , limit the peak combustor and exhaust exit temperature to acceptable values. The essential requirements met by the high temperature ceramic composite fuel cell positioning gasket  108  include: utilization of stable, continuous alumina fibers; a semi-porous matrix, either about 90 vol. % to 99 vol. % porous or having higher density material with flow holes distributed through the gasket in any suitable configuration, to allow controlled spent/depleted fuel gas permeation/diffusion; compliance to accommodate variable cell bundle pitch; flexibility to allow for ease of installation; resistance to both oxidizing and reducing atmosphere; long term stability under severe operating conditions as high as 1400° C.; mechanical strength under thermal loads and pressure transients; and cost effectiveness. 
     The composite fuel cell positioning gasket  108  has perforations therethrough shown in  FIGS. 2-4 , having inner surface/circumference against which the outer surface/circumference of the fuel cells fit tightly. The gasket may be comprised of sinterable material such as a pure alumina fiber, a matrix of discontinuous fiber, 95-97 wt. % alumina, 3-5 wt. % silica and an optional pure alumina bond phase to bond/cement the layers together. Bonding the facings  122  and  126  to the core material  124  is important in the overall integrity of the sandwich gasket panel  108 . High temperature thread can also be used around the perforation and/or along the edges of a pillow type gasket, to effect bonding of the layers. The overall composition of the composite gasket positioning board is about 60 wt. % to 98 wt. % alumina and 2 wt. % to 40 wt. % silica. Less stringent requirements on material purity may be adopted to use up to 40 wt. % silica, providing that no cell voltage degradation is detected as a result of long-term running in a fuel cell environment. Usually, the limit on silica is up to about 10 to 30 wt. % silica content. 
     The flexible ceramic gasket  108  is configured to accommodate a range of SOFC design conditions requiring pressure drops between the recirculation chamber  106  and combustion zone  94  of 0.25 inches of water to 2.0 inches of water. The gasket  108  can be tailored to meet this by utilizing materials with typical porosities of 90-99 vol. % and allowing the gas to diffuse evenly through the board, or by using higher density materials and flow holes in the range of 0.070 inches to 0.100 inches (0.178 cm-0.254 cm). In either case, the flexible ceramic gasket will include holes through which the fuel cells  36 , of typical diameter 0.87 inches (2.21 cm), protrude. These holes are undersized to ensure the gasket seals effectively against the fuel cells. The gasket is constructed of flexible ceramic material that effectively seals against the fuel cells while controlling the flow rate, flow distribution, and pressure drop between the recirculation and combustion chambers. 
     During normal operation the composite gasket fuel cell positioning board  108  is exposed to temperatures up to 1100° C. in the combustion atmosphere of chamber  94  containing spent fuel which typically contains 50% water vapor, 14% hydrogen and 28% carbon dioxide. However, transient operating conditions may significantly increase the temperature to 1300° C.-1400° C. as a result of hydrogen oxidation in the combustion zone, 94. To protect the open ends of the cells from localized burning of hydrogen, the woven top layer  122  is or can be impregnated with a slurry, preferably nickel oxide which would adhere to the woven matrix after curing. The nickel impregnation process applied to a woven matrix very importantly promotes flameless catalytic combustion of spent fuel in combustion chamber  94 . 
     A preferred configuration of the composite assembly positioning gasket  108  includes a composite single strip spanning a 3-bundle row assembly. This design includes a central perforated section for installation over the bundles, holding/supporting the fuel cells and stack bundles, where the fuel cells fit tightly through the perforations  128  of  FIGS. 2 ,  3  and  4 . Typical dimensions of the composite gasket positioning board are: Perforated width 3.25 in. (8.25 cm); overall length 29.75 in. (75.6 cm- to accommodate 3 groups of 3×8 hole patterns); thickness ⅛ in. to ½ in. (0.32 cm to 1.27 cm). 
     The design can be a stitched, “pillow” style, shown in one embodiment in  FIG. 4 , with central perforated section, with perforations  128 , where any appropriate configuration of stitching can be used so long as it integrates the composite, for example stitching  134 , is shown at only one location for the sake of simplicity. Also, a plain peripheral double layer fabric strip  130  is used to retain it between adjacent boards, or an outer rigid frame with two offset slots designed to capture and retain the woven inserts is used. Here, no flow holes are needed, as gas permeates through the fabric  130 . The outer frame structure (not shown) is made of high purity alumina fibrous insulation material. A flexible, positioning board is shown in  FIG. 2 , with perforations  128 , and gas flow holes  132 , whose dimensions were detailed earlier. The gasket can be used with or without these gas flow holes  132 , naturally it is advantageous and less expensive to rely solely on the gasket material to control flow. This can easily be positioned within the fuel cell bundles. A baked/cured product of  FIG. 2  is shown as  FIG. 3  in cured form having a rigid top and compliant core. 
     Two of the most original aspects of the flexible ceramic gasket  108  are its capabilities to seal against the cells to ensure no unwanted leakage occurs, while concurrently controlling the flow rate of the spent fuel electrochemically consumed. The first feature is accomplished by utilizing the strength and flexibility of the woven fabric material to securely retain the alumina fiber inner mat, which is cut smaller than the cell diameters to ensure adequate sealing. The second feature is controlled by varying the thickness and or type of alumina fiber inner mat and/or the weave of the fabric, to tailor/control the flow rate or pressure drop across the gasket. The accompanying Table 1 illustrates this behavior, where for a desired flow rate, variations in the fabric and inner mat provide a wide range of pressure drop, compared to conventional top or bottom rigid boards currently in use in SOFC generators, which require precision machining of numerous flow holes (typically 268 flow holes through the ligament between adjacent cells, 0.070 to 0.096 inches in diameter; 0.178 to 0.243 cm), and where gas leakage between the board and fuel cell have always been the accepted norm. The appropriate pressure drop for efficient operation of this gasket positioned next to the combustion chamber is 0.25-2.0 inches of water, preferably 0.35 to 1.75 inches of water, so construction must be managed to be within that range. 
     
       
         
               
               
               
             
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Pressure Drop 
                 Flow Rate 
               
               
                 Configuration 
                 [inches of water] 
                 [liters/min] 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Conventional Positioning Board with 
                 0.25 
                 34 
               
               
                 0.070″ and 0.096″ flow holes 
               
               
                 2 layers Woven Fabric #1 + Alumina 
                 0.01 
                 34 
               
               
                 Fiber Mat A 
               
               
                 2 layers Woven Fabric #1 + Alumina 
                 0.7 
                 34 
               
               
                 Fiber Mat B 
               
               
                 2 layers Woven Fabric #2 + Alumina 
                 2 
                 34 
               
               
                 Fiber Mat C 
               
               
                   
               
             
          
         
       
     
     Compliant and complete sealing of the flexible ceramic gasket against the exterior of the fuel cell is a very important feature of this invention, because it eliminates the undesirable effect of radial gap size and cell eccentricity within the clearance hole. Eccentricity between the cell outside diameter and inside diameter of the clearance hole introduces a wide variation of flow through former solid type positioning boards causing flow mal-distributions which play a big role in stack temperature, all solved by the intimate contact of the gaskets with the outside of the fuel cells as described herein. 
     Referring back to  FIG. 1 , power leads  32 , are shown, for the sake of simplicity on the side where oxidant inlet plenum  52  and exhaust outlet  116  are located. They are, however, electrically connected to each parallel bus bars  17  and to the fuel cells  36 , through flexible power take-off connectors  35 , which contact each fuel cell. The power leads  32  are transverse to the axis  36 ′ of the fuel cells and are embedded in insulation material. As shown, external ducting  81  is kept to a minimum by integral entrance and exit conducts  82 . 
     EXAMPLE 
     Prototypes of two potential gasket positioning board styles with square-pitched 3×8 hole arrays were made. The first was a pillow type gasket, with undersized holes for the cells punched out, then sewn with high-temperature ceramic thread to consolidate the gasket composite and prevent unraveling. The second was a quilted gasket, with cell holes cut using a laser. Both would be useful and accomplish all objectives of the invention, especially when the top would be impregnated with catalytic nickel oxide. To control pressure drop through the gasket, a number of parameters can be varied to optimize the consistency and the porosity of the matrix. Gas permeability can be controlled by using low-density ceramic fiber blankets or medium density papers or felts. The preferred method of fabrication consists in stitching a 3×8 hole pattern by the continuous intertwining of thin ceramic thread through the gasket thickness, relying on the uniformity of the gasket permeability rather than manufactured flow holes to promote uniform burning above the gasket. 
     An alternative method used a filament winding process, which simultaneously deposited both continuous and discontinuous fibers on a fixture consisting of a 3×8 array of mandrels of the same diameter as the fuel cell. This method eliminates the need to perform a secondary hole punching operation that may cause fiber thinning/breakage in the perforated gasket ligaments. One major advantage of this method was that the gasket was not machined but it was integrally woven to a final net shape. Additional advantages of this woven construction include ease of assembly, adaptability to irregular cell pitch, commonality between parts, better serviceability and gasket quick-replacement capability. 
     While specific embodiments of the invention have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular arrangements disclosed are meant to be illustrative only and not limiting as to the scope of invention which is to be given the full breadth of the claims appended and any and all equivalent thereof.