Solid oxide fuel cell with selective anode tail gas circulation

Abstract

A solid oxide fuel cell system includes a solid oxide fuel cell stack, a fuel supply inlet conduit adapted to provide a hydrocarbon fuel inlet stream into the solid oxide fuel cell stack and a fuel exhaust outlet conduit adapted to remove a fuel exhaust stream from the solid oxide fuel cell stack. The system also includes a water vapor and enthalpy recovery unit operatively connected to the fuel supply inlet conduit and to the fuel exhaust outlet conduit. The water vapor and enthalpy recovery unit is adapted to selectively transfer at least a portion of water vapor and heat from the fuel exhaust stream to the hydrocarbon fuel inlet stream. The system also includes a hydrogen recovery unit connected to the fuel supply inlet conduit and to the fuel exhaust outlet conduit. The hydrogen recovery unit is adapted to selectively transfer at least a portion of hydrogen from the fuel exhaust stream to the hydrocarbon fuel inlet stream.

1 - 5 . (canceled)  
     
     
         6 . A solid oxide fuel cell system, comprising: 
 a solid oxide fuel cell stack;    a fuel supply inlet conduit adapted to provide a hydrocarbon fuel inlet stream into the solid oxide fuel cell stack;    a fuel exhaust outlet conduit adapted to remove a fuel exhaust stream from the solid oxide fuel cell stack; and    a fuel exhaust processing subsystem operatively connected to the fuel supply inlet conduit and to the fuel exhaust outlet conduit, wherein the fuel exhaust processing subsystem comprises:    a carbon dioxide scrubbing unit, wherein the carbon dioxide scrubbing unit is adapted to remove at least a portion of carbon dioxide present in the fuel exhaust stream to form a scrubbed fuel exhaust stream;    a water vapor recovery unit, wherein the water vapor recovery unit is adapted to transfer at least a portion of water vapor from the scrubbed fuel exhaust stream to the hydrocarbon fuel inlet stream; and    a hydrogen recovery unit, wherein the hydrogen recovery unit is adapted to transfer at least a portion of hydrogen from the scrubbed fuel exhaust stream to the hydrocarbon fuel inlet stream.    
     
     
         7 . The system of  claim 6 , wherein the fuel exhaust processing subsystem comprises an absorption-adsorption gas recovery and separation system.  
     
     
         8 . The system of  claim 6 , wherein the fuel exhaust processing subsystem comprises a molecular sieve.  
     
     
         9 . The system of  claim 6 , further comprising: 
 a hydrogen delivery conduit connecting the fuel exhaust processing subsystem and the fuel supply inlet conduit; and    a water vapor delivery conduit connecting the fuel exhaust processing subsystem and the fuel supply inlet conduit, wherein the water vapor delivery conduit is located further upstream from the hydrogen delivery conduit relative to the solid oxide fuel cell stack.    
     
     
         10 . The system of  claim 6 , further comprising: 
 a heat exchanger connected to the fuel supply inlet conduit and to the fuel exhaust outlet conduit, wherein the heat exchanger is adapted to transfer heat from the fuel exhaust stream to the hydrocarbon fuel inlet stream; and    a methane fuel storage vessel connected to the fuel supply inlet conduit.    
     
     
         11 . A method of operating a solid oxide fuel cell system, comprising: 
 providing a hydrocarbon fuel inlet stream into a solid oxide fuel cell stack;    removing fuel exhaust stream from the solid oxide fuel cell stack;    recovering at least a portion of water vapor and heat from the fuel exhaust stream;    providing the at least a portion of the recovered water vapor and heat into the hydrocarbon fuel inlet stream being provided into the solid oxide fuel cell stack;    recovering at least a portion of hydrogen from the fuel exhaust stream; and    separately providing the at least a portion of the recovered hydrogen into the hydrocarbon fuel inlet stream being provided into the solid oxide fuel cell stack.    
     
     
         12 . The method of  claim 11 , wherein the water vapor and heat are recovered using an enthalpy wheel without using aspirators or blowers.  
     
     
         13 . The method of  claim 11 , wherein: 
 the hydrogen is recovered using a pressure swing absorption hydrogen recovery device or an electrochemical pump without using aspirators or blowers; and    the step of providing the at least a portion of hydrogen comprises providing hydrogen without providing carbon dioxide present in the fuel exhaust stream.    
     
     
         14 . The method of  claim 11 , wherein hydrogen is provided into the hydrocarbon fuel inlet stream further upstream from where the water vapor is provided into the hydrocarbon fuel inlet stream, relative to the solid oxide fuel cell stack.  
     
     
         15 . The method of  claim 14 , wherein the hydrocarbon fuel comprises methane.  
     
     
         16 . The method of  claim 11 , further comprising independently controlling an amount of water vapor and hydrogen provided into hydrocarbon fuel inlet stream.  
     
     
         17 . The method of  claim 16 , wherein: 
 a hydrogen flow rate into the hydrocarbon fuel inlet stream is higher than a water vapor flow rate into the hydrocarbon fuel inlet stream; and    only a portion of the water vapor present in the fuel exhaust stream is provided into the hydrocarbon fuel inlet stream.    
     
     
         18 . The method of  claim 17 , further comprising controlling an amount of water vapor provided into the hydrocarbon fuel inlet stream to decrease or prevent reforming of the hydrocarbon fuel and water vapor in a fuel inlet conduit upstream from the solid oxide fuel cell stack.  
     
     
         19 . The method of  claim 18 , wherein the step of controlling the amount of water vapor comprises providing water vapor into the hydrocarbon fuel inlet stream such that the water vapor and hydrocarbon fuel mixture contains less than two water molecules for each carbon atom.  
     
     
         20 . The method of  claim 18 , further comprising: 
 heating the hydrocarbon fuel inlet stream to about the solid oxide fuel cell stack operating temperature before the hydrocarbon fuel inlet stream enters the solid oxide fuel cell stack and after the hydrogen and the water vapor are provided into the hydrocarbon fuel inlet stream; and    reforming the hydrocarbon fuel and the water vapor in the solid oxide fuel cell stack in parallel with an oxidation reaction proceeding in the solid oxide fuel cell stack.    
     
     
         21 . A method of operating a solid oxide fuel cell system, comprising: 
 providing a hydrocarbon fuel inlet stream into a solid oxide fuel cell stack;    removing a fuel exhaust stream from the solid oxide fuel cell stack;    removing at least a portion of carbon dioxide present in the fuel exhaust stream to form a scrubbed fuel exhaust stream; and    providing at least a portion of the scrubbed fuel exhaust stream into the hydrocarbon fuel inlet stream.    
     
     
         22 . The method of  claim 21 , wherein only a portion of the scrubbed fuel exhaust stream is provided into the hydrocarbon fuel inlet stream.  
     
     
         23 . The method of  claim 22 , wherein about 45 to about 55% of the scrubbed fuel exhaust stream is provided into the hydrocarbon fuel inlet stream to obtain a fuel utilization of about 70 to about 80%.  
     
     
         24 . The method of  claim 23 , further comprising recovering at least a portion of hydrogen and water vapor from the scrubbed fuel exhaust stream.  
     
     
         25 . The method of  claim 24 , wherein the step of providing at least a portion of the scrubbed fuel exhaust stream into the hydrocarbon fuel inlet stream comprises providing a portion of the recovered hydrogen and water vapor into the hydrocarbon fuel inlet stream.  
     
     
         26 - 32 . (canceled)