Abstract:
A fuel cell system for converting a first flow and a second flow to electricity, a first spent flow, and a second spent flow. The fuel cell system may include a chamber for combusting the first spent flow and the second spent flow to produce heat and a pathway for the first flow. The pathway may be positioned about the chamber for heat exchange therewith.

Description:
BACKGROUND OF INVENTION  
         [0001]    1. Technical Field  
           [0002]    The present invention relates generally to fuel cell systems and more particularly relates to a fuel cell with an integrated air preheater and tail gas burner.  
           [0003]    2. Background of the Invention  
           [0004]    Fuel cells electrochemically react fuels with oxidants to generate electricity. A fuel cell generally includes a cathode material, an electrolyte material, and an anode material. The electrolyte may be a non-porous material positioned between the cathode and the anode materials. The fuel and the oxidant typically are gases that continually flow about the anode, the cathode, and the electrolyte through separate passageways. A fuel gas may be hydrogen, a short-chain hydrocarbon, or a gas containing a desired chemical species in some form. An oxidant may be an oxygen-containing gas, or quite commonly, air. The fuel and the oxidant typically are pre-heated before being fed to the electrolyte.  
           [0005]    A common fuel cell is a solid oxide fuel cell (“SOFC”). A SOFC uses a solid electrolyte for power generation. The solid electrolyte may be an ion-conducting ceramic or a polymer membrane. For example, the electrolyte may be a non-conductive ceramic, such as a dense yttria-stabilized zirconia (YSZ) membrane. The anode may be a nickel/YSZ cermet and the cathode may be a doped lanthanum manganite.  
           [0006]    The electrochemical conversion occurs at or near the three-phase boundary of each electrode (the cathode and the anode) and the electrolyte. The fuel is electrochemically reacted with the oxidant to produce a direct current electrical output. The anode or the fuel electrode enhances the rate at which the electrochemical reaction occurs on the fuel side. The cathode or the oxidant electrode functions similarly on the oxidant side. The electrochemical reaction between the fuel and the oxidant produces electrical energy, spent fuel, and oxidant exhaust. This conversion of fuel and oxidant to electricity also produces heat, particularly at high current-power densities.  
           [0007]    To achieve higher voltages for a specific application, the individual electrochemical cells may be connected in series to form a fuel cell stack. To achieve higher currents, individual cells may be connected in parallel. The electrical connection between the cells may be achieved by the use of an electrical interconnect between the cathode and the anode of adjacent cells. The electrical interconnect also may provide for passageways for oxygen to flow pass the cathode and fuel to flow pass the anode. Ducts or manifolds generally also are used to conduct the fuel and the oxidant into and out of the stack.  
           [0008]    The heat produced in the reaction generally should be removed from the stack to maintain the fuel cells at an efficient operating temperature. The hot exhaust gas from the stack may be further combusted and/or fed to one or more heat exchangers. For example, the incoming fuel and/or the incoming oxidant may be preheated such that the gases enter the stack at higher, more efficient temperatures. Further, the incoming fuel flow may be processed with air and/or steam before entry into the stack. The exhaust gases also may be used to heat the air or to heat a water stream into steam. The more efficiently the spent gases may be reused in the system may have a significant impact on the efficiency of the system as a whole.  
         SUMMARY OF INVENTION  
         [0009]    The present invention thus provides a fuel cell system for converting a first flow and a second flow to electricity, a first spent flow, and a second spent flow. The fuel cell system may include a chamber for combusting the first spent flow and the second spent flow to produce heat and a pathway for the first flow. The pathway may be positioned about the chamber for heat exchange therewith. The first flow may include a flow of oxidant or fuel and the first spent flow may include a flow of spent oxidant or spent fuel.  
           [0010]    A further embodiment of the present invention may provide for a fuel cell system for converting a flow of fuel and a flow of oxidant to electricity, a spent fuel flow, and a spent gas flow. The fuel cell system may include a chamber for combusting the spent fuel flow and the spent oxidant flow to produce heat and a pathway for the flow of oxidant. The pathway may be positioned about the chamber for heat exchange between the heat produced in the chamber and the flow of oxidant in the pathway.  
           [0011]    A further embodiment of the present invention may provide for a fuel cell system for converting a first flow and a second flow to electricity, a first spent flow, and a second spent flow. The system may include an inner chamber for combusting the first spent flow and the second spent flow to produce heated exhaust gases. The inner chamber may include a side wall and an end wall. The side wall may include a number of apertures therein for the heated exhaust gases to flow therethrough. The system also may have a pathway for the first flow. The pathway may be positioned about the inner chamber for heat exchange with the heated exhaust gases. The system also may have an outer chamber to direct the flow of the heated exhaust gases.  
           [0012]    A method of the present invention may provide for heating a flow of oxidant to be used in a fuel cell system producing electricity, a spent fuel flow, and a spent gas flow. The method may include combusting the spent fuel flow and the spent oxidant flow to produce heat, surrounding the combustion with the flow of oxidant, and heating the flow of oxidant.  
           [0013]    These and other features of the present invention will become apparent upon review of the following detailed description when taken in conjunction with the drawings and the appended claims. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0014]    [0014]FIG. 1 is a schematic view of a solid oxide fuel cell system.  
         [0015]    [0015]FIG. 2 is a schematic view of an alternative solid oxide fuel system.  
         [0016]    [0016]FIG. 3 is a cross-sectional view of an integrated recuperator/combustor of the present invention.  
         [0017]    [0017]FIG. 4 is a cross-sectional view of an alternative embodiment of the integrated recuperator/combustor of the present invention.  
         [0018]    [0018]FIG. 5 is a cross-sectional view of an alternative embodiment of the integrated recuperator/combustor of the present invention.  
         [0019]    [0019]FIG. 6 is a cross-sectional view of an alternative embodiment of the integrated recuperator/combustor of the present invention.  
         [0020]    [0020]FIG. 7 is a cross-sectional view of an alternative embodiment of the integrated recuperator/combustor of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0021]    [0021]FIG. 1 shows a schematic view of a fuel cell system  100  for use with the present invention. Numerous variations in the overall fuel cell system  100  may be possible herein. In addition to the hybrid system disclosed below, a single cycle system and other known fuel cell systems may be used herein. The invention also may have applicability beyond and in addition to fuel cell applications.  
         [0022]    The operation of the fuel cell system  100  and the components therein may be set, monitored, and controlled by a microprocessor  105  or a similar type of control device. Various temperature, load, flow, and/or other types of sensors may be used with the microprocessor  105  or otherwise in the fuel cell system  100 .  
         [0023]    The fuel cell system  100  may include a stack assembly  110 . The stack assembly  110  may include solid oxide fuel cells, molten carbonate fuel cells, and other types of fuel cell designs. The stack assembly  110  may include any number of individual fuel cells. As was described above, the fuel and the oxidant may be fed into the stack assembly  110  to produce electricity in the electrochemical reaction. The electrochemical reaction also produces thermal energy in the form of exhaust heat and spent gases.  
         [0024]    Generally described, the fuel cell system  100  may include a fuel cell side  115  with the stack assembly  110  therein and a turbine side  120 . The turbine side  120  components may include a turbine  130  and a compressor  140 . The turbine  130  may be connected to and drive the compressor  140  via a shaft  150 . The turbine  130  and the compressor  140  may be of conventional design. Exhaust gases generated by the stack assembly  110 , as will be described in more detail below, may drive the turbine  130 . The turbine  130  may be in communication with a generator  160 . The mechanical energy of the turbine  130  may be converted to electrical energy in the generator  160 . The generator  160  may be of conventional design. An inverter  165  also may be used to convert the direct current produced by the generator  160  to alternating current.  
         [0025]    The compressor  140  may compress incoming ambient air. The air may be pressurized to about four (4) atmospheres, although any pressure may be used. The incoming air then may be preheated in a recuperator  170 . The recuperator  170  may be in communication with the incoming air stream and the flow of exhaust gases leaving the turbine  130 . The recuperator  170  largely acts as a heat exchanger such that the exhaust gases from the turbine  130  may heat the incoming air stream. A fuel cell air preheater  190  then may further heat the incoming air. The fuel cell air preheater  190  may be in communication with the incoming air stream and a flow of exhaust gases from the fuel cell side  115 . The fuel cell air preheater  190  also acts largely as a heat exchanger such that the exhaust gases from the fuel cell side  115  may heat the incoming air stream. Heating the air may make the electrochemical reaction in the fuel stack  110  more efficient. After being heated in the air preheater  190 , the air may be fed into the stack assembly  110  on the fuel cell side  115 .  
         [0026]    Due to the high temperature operations in the fuel cell system  110 , these heat exchangers may be made of relatively expensive metals. For example, the recuperator  170  and the fuel cell air preheater  190  may be made out of stainless steel, Inconel alloys, or similar types of materials. Inconel alloys are generally nickel-chromium-iron alloys sold by Special Metals of New Hartford, Conn. Other heat exchange devices in the system  100  as a whole also may use similar materials.  
         [0027]    Fuel, such as natural gas or similar fuels, may be provided to the stack assembly  110  via a compressor  200 . The compressor  200  may be a standard compressor, a fan, or similar type of device. The fuel may be pressurized to about two (2) atmospheres, although any pressure may be used.  
         [0028]    The compressor  200  may compress the fuel and forward it onto a gas preheater/steam generator  210 . The fuel may be heated within the gas preheater/steam generator  210  via the exhaust gases from the recuperator  170  on the turbine side  120 . Other sources of heat also may be used. Heating the fuel also may make the electrochemical reaction in the fuel stack  110  more efficient. The gas preheater/steam generator  210  also may receive a flow of water from a water pump  220 . The water may be heated by the gases and turned into steam. The gas preheater/steam generator  210  generally includes at least two (2) separate heat exchangers, one to heat the fuel and one to produce the steam. For example, the gas preheater/steam generator  210  may be made out of stainless steel or similar types of materials.  
         [0029]    The fuel then may be fed to a reformer  230 . The reformer  230  uses the steam generated within the gas preheater and steam generator  210  for a steam reforming or an autothermal (air and steam) process to convert partially the fuel into a gas containing H 2  and CO. Other types of fuel processing methods and devices may be used.  
         [0030]    The reformed fuel stream is then supplied to the stack assembly  110  where the H 2  and CO are electrochemically reacted with oxygen in the incoming air stream to produce electrical power as is described above. An inverter  235  also may be used with the stack assembly  110 . The electricity produced by the generator  160  and the stack assembly  110  may be provided to an electrical grid  245  or applied to any type of load.  
         [0031]    Any fuel remaining after the electrochemical process then may be oxidized in a stack combustor  240  with the spent airflow. The exhaust heat from the stack combustor  240  may be used to heat the reformer  230 . As was described above, the exhaust gases also may be supplied to the fuel cell preheater  190  so as to heat the incoming air stream from the recuperator  170  on its way to the stack assembly  110 . The exhaust gases then also may be supplied to the turbine  130 .  
         [0032]    It is again important to note that the fuel cell system  100  as described above is for purposes of example only. Any type of fuel cell system may be used with the present invention as is described in more detail below.  
         [0033]    [0033]FIG. 2 shows an alternative to the fuel cell system  100 , in this case, a fuel cell system  250 . In the fuel cell system  250 , an integrated recuperator/combustor  260  may be used in place of the fuel cell air preheater  190  and the stack combustor  240 . This embodiment also may incorporate the recuperator  170  on the turbine side  120 . The present invention may use any combination or orientation of the integrated recuperator/combustor  260 , the fuel cell air preheater  190 , and the recuperator  170 . The incoming air stream thus may travel from the compressor  130  to the recuperator  170  and then to the integrated recuperator/combustor  260 . Alternatively, the air may be fed directly from the compressor  130  to the integrated recuperator/combustor  260 . The incoming air stream also may travel from the integrated recuperator/combustor  260  and then to the fuel cell air preheater  190 . Further, either or both the fuel cell air preheater  190  and/or the recuperator  170  may be eliminated if the integrated recuperator/combustor  260  is used.  
         [0034]    The integrated recuperator/combustor  260  also may be used in combination with any other heat exchange device or devices in the fuel cell system  100  as a whole such as the gas preheater/steam generator  210 , the reformer  230 , or otherwise. Either or both the incoming air or fuel stream may be heated.  
         [0035]    [0035]FIG. 3 shows one embodiment of the integrated recuperator/combustor  260 . The integrated recuperator/combustor  260  may include an outer shell  270  defining a combustion chamber  280 . The outer shell  270  may be made out of stainless steel, Inconel alloys, or similar types of materials. The combustion chamber  280  may include a cathode flow exhaust inlet  290 , an anode flow exhaust inlet  300 , and an exhaust gas outlet  310 . The combustion chamber  280  also may include an igniter  320 .  
         [0036]    As was described above, spent air from the stack assembly  110  enters via the cathode flow exhaust inlet  290  while spent fuel from the stack assembly  110  enters via the anode flow exhaust inlet  300 . The air and the fuel are ignited by the igniter  320  and the exhaust gases exit via the exhaust gas outlet  310 . The combustion chamber  280  also may include a turbulator  330  surrounding the inner wall of the combustion chamber  280  so as to insure proper turbulence in the air and fuel flow and to promote combustion.  
         [0037]    The integrated recuperator/combustor  260  also may include a recuperator  340 . The recuperator  340  may include a compressor input  350  and a stack output  360 . The compressor input  350  and the stack output  360  may be separated by a recuperator tube  370 . The recuperator tube  370  may form one or more spiral paths about the outer shell  270 . Any other type or number of pathways also may be used. The recuperator tube  370  may be made out of stainless steel, Inconel alloys, or similar types of materials with good heat transfer characteristics. As is shown in FIG. 3, the outer shell  270  may define an exterior wall  380  and an interior wall  390 . The recuperator tube  370  may be positioned on or within the exterior wall  380 . The outer shell  270  may define a channel  375  therein for the positioning of the recuperator tube  370 . Specifically, the recuperator tube  370  may be welded or brazed to the exterior wall  380 . Similar types of attachment means also may be used.  
         [0038]    [0038]FIGS. 4-6 show various alternative embodiments of the recuperator  340 . In FIG. 4, the recuperator tube  370  may be positioned on the exterior wall  380  of the outer shell  270 . The recuperator tube  370  may be attached by welding, brazing, or similar methods. The configurations shown in FIGS. 3 and 4 use both conduction and radiation modes of heat transfer. The configuration of FIG. 3 may provide a more efficient path for conduction given the positioning of the recuperator tube  370  within the channel  375  of the outer shell  270 .  
         [0039]    [0039]FIG. 5 shows the recuperator tube  370  positioned within the outer shell  270  along the interior wall  390 . The recuperator tube  370  may be attached by welding, brazing, or similar methods. The configuration of FIG. 5 thus uses three modes of heat transfer, namely conduction, convection, and radiation. In this configuration, the input and output  350 ,  360  should be tightly sealed to avoid any leaks from the combustion chamber  280 .  
         [0040]    [0040]FIG. 6 shows the recuperation tube  370  positioned within the combustion chamber  280  of the outer shell  270  but not in contact with the interior wall  390 . The configuration provides effective convective heat transfer but no conduction. There also is no need for welding or brazing in this configuration.  
         [0041]    In use, air from the compressor  130  may be sent to the integrated recuperator/combustor  260 . The air enters via the compressor input  350  into the recuperator tube  370 . While in the recuperator tube  370 , the air is heated via conduction, convention, and/or radiation depending upon the configuration of the recuperation tube  370 . The heated air then exists via the stack output  360  and travels to the fuel cell stack  110 . The electrochemical reaction then takes place within the fuel cell stack  110 . The spent fuel and air exits the fuel cell stack  110  and enters the integrated recuperator/combustor  260  via the cathode flow exhaust inlet  290  and the anode flow exhaust inlet  300 . The air and the fuel are ignited by the igniter  320  so as to heat the incoming air within the recuperator tube  370 . The exhaust gas exits the integrated recuperator combustor  260  via the exhaust gas outlet  310  and travels to the reformer  230  or elsewhere. As described above, the present invention also may use any combination or orientation of the integrated recuperator/combustor  260 , the fuel cell air preheater  190 , and the recuperator  170 , and/or any other heat exchange structure within the fuel cell system  100  as a whole.  
         [0042]    The present invention thus provides improved reliability, maintainability, and lower costs in that two separate fuel cell system components may be combined and improved. The present invention thus improves the efficiency of the air preheating process in specific and the efficiency of the fuel cell system  100  as a whole.  
         [0043]    [0043]FIG. 7 shows a further embodiment of an integrated recuperator/combustor  400 . The integrated recuperator/combustor  400  may include the outer shell  270  defining the combustion chamber  280 . The outer shell  270  maybe made out of stainless steel, Inconel alloys, or similar types of materials. The combustion chamber  280  may include the cathode flow exhaust inlet  290 , the anode flow exhaust inlet  300 , and the exhaust gas outlet  310 . The combustion chamber  280  also may include the igniter  320 .  
         [0044]    Positioned within the outer shell  270  of the integrated recuperator/combustor  400  may be an inner shell  410 . The inner shell  410  may be made out of stainless steel, Inconel alloys, or similar types of materials. The inner shell  410  may include an elongated, substantially tubular side wall  420  that ends in a solid end wall  430 . The side wall  420  may have a number of apertures or perforations  440  positioned therein. The inner shell  410  may be spaced about one (1) to about ten (10) centimeters from the outer shell  270 , although any spacing may be used.  
         [0045]    The integrated recuperator/combustor  400  also may include the recuperator  340 . The recuperator  340  may include the compressor outlet  350  and the stack outlet  360 . The compressor input  350  and the stack output  360  may be separated by the recuperator tube  370 . The recuperator tube  370  may be made out of stainless steel, Inconel alloys, or similar types of materials with good heat transfer characteristics.  
         [0046]    The recuperator tube  370  may be placed inside the side wall  420  of the inner shell  410 , although the recuperator tube  370  also may be placed within or outside the side wall and/or otherwise about the inner shell  410 . The recuperator tube  370  may form one or more spiral paths about the inner shell  410 . Any other type or number of pathways also may be used. The recuperator tube  370  may be within the inner shell  410  or the tube  370  may be welded or brazed to the inner shell  410 . Similar types of attachment means also may be used.  
         [0047]    In use, the cathode flow exhaust and the anode flow exhaust are ignited within the combustion chamber  280  via the igniter  320  as described above. Because of the solid end wall  430 , the gases are forced to flow radially over the recuperator tube  370  and exit the inner shell  410  via the perforations  440  within the side wall  420 . This forced flow path provides for good heat transfer with the recuperator tube  370 . The exhaust gases then exit via the exhaust gas outlet  310 . The gases are collected within the outer shell  270  and flow towards the exhaust gas outlet  310 . The exhaust gases then may be used in further heat exchangers.  
         [0048]    It should be apparent that the foregoing relates only to the preferred embodiments of the present invention and that numerous changes and modifications may be made herein without departing from the spirit and scope of the invention as defined by the following claims and the equivalents thereof.