Abstract:
A technique includes lowering a temperature of a cathode exhaust flow from an electrochemical cell to produce a second flow and routing the second flow to a contaminant trap.

Description:
[0001]     This application claims the benefit under 35 U.S.C. § 119(e) to U.S. Provisional Application Ser. No. 60/793,762, entitled “IMPROVING PHOSPORIC ACID SCRUBBING EFFICIENCY IN A FUEL CELL SYSTEM,” which was filed on Apr. 21, 2006, and is hereby incorporated by reference in its entirety. 
     
    
     BACKGROUND  
       [0002]     The invention generally relates to recovering a reactant from a fuel cell exhaust flow.  
         [0003]     There are many different types of fuel cells, such as proton exchange membrane (PEM) fuel cells, alkaline fuel cells, molten carbonate fuel cells, direct methanol fuel cells, solid oxide fuel cells and phosphoric acid fuel cells. In its basic function, a fuel cell promotes an electrochemical reaction to produce heat, protons, electrons and water.  
         [0004]     Phosphoric acid is a component of a phosphoric acid fuel cell, and as such, the reactants that flow through the phosphoric acid fuel cell contact the phosphoric acid. Thus, exhaust streams from the fuel cell typically carry some trace amounts of phosphorus in the form of phosphoric acid, P 2 O 4 , etc.  
         [0005]     The exhaust stream from a phosphoric acid fuel cell may contain some concentration of reactants. Therefore, it may be desirable to recover the reactants from the exhaust stream for purposes of improving the fuel cell system&#39;s efficiency. A difficulty with this approach, however, is that the exhaust stream contains trace amounts of phosphorus, which may, as examples, poison catalysts of the fuel cell system, as well as contribute to the corrosion of fuel cell system components, such as heat exchangers, plumbing, etc.  
       SUMMARY  
       [0006]     In an embodiment of the invention, a technique includes lowering a temperature of a cathode exhaust flow from an electrochemical cell to produce a second flow and routing the second flow to a contaminant trap.  
         [0007]     In another embodiment of the invention, a system includes an electrochemical cell to provide a cathode exhaust flow and a mechanism to lower a temperature of the cathode exhaust flow to produce a second flow. A contaminant trap of the system receives the second flow.  
         [0008]     Advantages and other features of the invention will become apparent from the following drawing, description and claims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0009]      FIGS. 1, 3  and  5  are flow diagrams depicting techniques related to recovering a reactant from a fuel cell exhaust flow according to embodiments of the invention.  
         [0010]      FIGS. 2 and 4  are schematic diagrams of fuel cell systems according to embodiments of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0011]     In accordance with embodiments of the invention described herein, for purposes of aiding the entrapment of phosphoric acid from a fuel cell exhaust stream, the exhaust stream is first cooled. Therefore, referring to  FIG. 1 , in accordance with an embodiment of the invention described herein, a technique  10  may be used for purposes of trapping phosphorus from a cathode exhaust flow and recovering a reactant from the stream. The cathode exhaust flow may contain various components, such as hydrogen, methane and carbon monoxide.  
         [0012]     Pursuant to the technique  10 , the temperature of the cathode exhaust flow is lowered (block  14 ) and then the lower temperature cathode exhaust flow is routed (block  18 ) to a phosphoric acid trap. Due to the cooling of the cathode exhaust, the phosphorus is more efficiently removed from the flow. Pursuant to the technique  10 , the exhaust flow from the phosphoric acid trap is routed (block  22 ) to a reformer. The reformer, in turn, recovers hydrogen from the exhaust flow to produce a reformate flow that is furnished back to the fuel cell, pursuant to block  26 .  
         [0013]     As a more specific example,  FIG. 2  depicts an exemplary embodiment  50  of a fuel cell system in accordance with some embodiments of the invention. The fuel cell system  50  includes a fuel cell stack  60 , which may be a stack of phosphoric acid fuel cells, in accordance with some embodiments of the invention. The fuel cell stack  60  has a cathode inlet  62  that receives an incoming oxidant flow from an air blower  66 . The fuel cell stack  60  also includes an anode inlet  70 , which receives a reformate flow, that is provided by a reformer  90 .  
         [0014]     The incoming fuel and oxidant flows to the fuel cell stack  60  are communicated through the respective anode and cathode chambers of the fuel cell stack  60  to promote electrochemical reactions that produce electricity for a load (not shown in  FIG. 2 ) of the system  50 . These flows exit the fuel cell stack  60  to produce corresponding cathode and anode exhaust flows at cathode  74  and anode  64  exhaust outlets, respectively, of the stack  60 . In accordance with some embodiments of the invention, the anode exhaust flow may be communicated at least in part back to the anode inlet  70 , may be vented to ambient or may be communicated to the reformer  90 , depending on the particular embodiment of the invention.  
         [0015]     The cathode exhaust flow is, in general, routed to the reformer  90  for purposes of recovering hydrogen from the flow, which may have migrated from the fuel cell stack&#39;s anode chamber. However, phosphorus compounds are removed from the cathode exhaust flow by communicating the cathode exhaust flow through a phosphoric acid trap  86 . From the phosphoric acid trap  86 , the scrubbed cathode exhaust flow is communicated to a fuel inlet  88  of the reformer  90 .  
         [0016]     To increase the efficiency of the phosphoric acid trap  86 , the incoming flow to the trap  86  is first cooled by combining the cathode exhaust flow from the fuel cell stack  60  with an incoming fuel flow  78  (such as hydrogen) at a junction  76 . The resultant flow, having a lower temperature than the cathode exhaust flow that exits the fuel cell stack  60 , is communicated to an inlet  82  of the acid trap  86 .  
         [0017]     The combination of the cathode exhaust with the incoming fuel flow produces a feed flow for the reformer  90 . The temperature of the incoming fuel flow is relatively low (at ambient temperature, for example) relative to the temperature of the cathode exhaust flow that exits the fuel cell stack  60 . For example, in accordance with some embodiments of the invention, the cathode exhaust flow from the fuel cell stack  60  may have a temperature of approximately 170° C. By combining the cathode exhaust flow that exits the fuel cell stack  60  with the relatively cooler incoming fuel flow, the temperature of the resultant flow that is scrubbed by the phosphoric acid trap  86  is significantly lower than the temperature of the cathode exhaust flow. For example, the temperature of the flow that is received at the inlet  82  of the phosphoric acid trap  86  may be between approximately 150° to 160° C., in accordance with some embodiments of the invention. In general, the scrubbing material of the phosphoric acid trap  86  is more efficient in removing phosphorus compounds from lower temperature flows. Therefore, due to the cooling effect provided by the mixing of the cathode exhaust and incoming fuel flows, the phosphoric acid trap  86  more efficiently traps phosphorus compounds, thereby producing a relatively “cleaner” feed flow to the reformer  90 .  
         [0018]     Referring to  FIG. 3 , to summarize, in accordance with some embodiments of the invention, a technique  100  may be used for purposes of processing a cathode exhaust flow from a fuel cell. Pursuant to the technique  100 , a cathode exhaust from a fuel cell is mixed (block  104 ) with a lower temperature fuel flow. The resultant lower temperature exhaust flow is then routed (block  108 ) to a phosphoric acid trap and further processed in accordance with blocks  22  and  26 , as described above in connection with the technique  10  (see  FIG. 1 ).  
         [0019]      FIG. 4  depicts an exemplary embodiment  150  of a fuel cell system in accordance with another embodiment of the invention. The fuel cell system  150  has a similar design to the fuel cell system  50  (see  FIG. 2 ), with like reference numerals being used to depict similar components. However, unlike the fuel cell system  50 , the fuel cell system  150  cools the cathode exhaust flow using a different technique. In particular, in the fuel cell system  150 , a thermal exchanger  154  transfers thermal energy from the cathode exhaust flow  74  to an incoming coolant inlet flow  158  to the fuel cell stack  60 . Thus, in general, the fuel cell stack  60  has a temperature that is regulated by a coolant subsystem  164 , which circulates a coolant flow through corresponding coolant channels of the fuel cell stack  60 .  
         [0020]     More specifically, the cathode exhaust flow is furnished to the thermal exchanger  154 , which also receives a coolant inlet flow from the coolant subsystem  164 . The coolant inlet flow has a significantly lower temperature for purposes of removing additional thermal energy from the fuel cell stack  60 . Due to the thermal coupling provided by the thermal exchanger  154 , thermal energy of the cathode exhaust flow  74  is lowered before being received at the inlet  82  of the phosphoric acid trap  86 .  
         [0021]     The coolant flow circulates through the thermal exchanger  154  and then flows into the coolant channels of the fuel cell stack  60 , where thermal energy is removed from the stack  60 . The coolant flows from the fuel cell stack  60  back into the coolant subsystem  164 , which removes thermal energy from the coolant. In this embodiment of the invention, an incoming fuel flow may be separately provided to a fuel inlet  174  of the reformer  90  (as depicted in  FIG. 4 ), or alternatively, the flow from the acid trap  86  may be combined with an incoming fuel flow to form a feed flow for the reformer  90  in another embodiment of the invention.  
         [0022]     Referring to  FIG. 5 , to summarize, in accordance with some embodiments of the invention, a technique  200  includes using a coolant inlet flow to a fuel cell to lower the temperature of a cathode exhaust of the fuel cell, pursuant to block  210 . The resultant lower temperature cathode exhaust flow is then processed according to blocks  18 ,  22  and  26 , similar to blocks  18 ,  22  and  26  in a similar manner to the technique s  10  described above.  
         [0023]     Other embodiments are within the scope of the appended claims. For example, in accordance with other embodiments of the invention, the cathode exhaust flow may be communicated in proximity to the coolant inlet flow to the fuel cell stack  60 . Thus, in these embodiments of the invention, an explicit thermal exchanger is not used, as a proximity of the two flows provides sufficient cooling to improve the efficiency of phosphorus capture from the cathode exhaust flow. Therefore, many different variations are possible and are within the scope of the appended claims.  
         [0024]     While the invention has been disclosed with respect to a limited number of embodiments, those skilled in the art, having the benefit of this disclosure, will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of the invention.