Abstract:
Combined reformer and fuel cell systems, and their methods of operation, are described in which air is introduced to the system to produce additional water by reacting with hydrogen produced from the reformer during the reformer&#39;s startup partial oxidation mode of operation.

Description:
RELATIONSHIP TO GOVERNMENT CONTRACTS 
       [0001]    This invention was made with Government support under DE-FC26-02NT41246 awarded by DOE. The Government has certain rights in this invention. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    This invention relates to fuel cell systems that include a catalytic reformer to produce reformate as a source of fuel for fuel cell(s) in the system. 
         [0003]    Catalytic reformers are often used in fuel cell systems to provide fuel for the fuel cells. Catalytic reformers are often paired with solid oxide fuel cells (SOFC&#39;s) because SOFC&#39;s can use each of the hydrogen and carbon monoxide reformate components produced by the catalytic reformer as fuel. An SOFC comprises a cathode layer, an electrolyte layer formed of a solid oxide bonded to the cathode layer, and an anode layer bonded to the electrolyte layer on a side opposite from the cathode layer. In use of the cell, air is passed over the surface of the cathode layer, and oxygen from the air migrates through the electrolyte layer and reacts in the anode with hydrogen being passed over the anode surface, forming water and thereby creating an electrical potential between the anode and the cathode of about 1 volt. Typically, each individual fuel cell is arranged as a stage in a stack of fuel cells connected in series to produce a target operating voltage. 
         [0004]    Partial-oxidizing (POX) reformers typically are operated exothermically by using intake air to partially oxidize hydrocarbon fuel as may be represented by the following equation for a hydrocarbon and air: 
         [0000]      C 7 H 12 +3.5(O 2 +3.76N 2 )→6H 2 +7CO+13.16N 2 +heat   (Eq. 1).
 
         [0000]    POX reformers typically are operated slightly fuel-lean of stoichiometric to prevent coking of the anodes from decomposition of non-reformed hydrocarbon within the fuel cell stack. Thus some full combustion of hydrocarbon and reformate occurs within the reformer in addition to, and in competition with, the electrochemical combustion of the fuel cell process. Such full combustion is wasteful of fuel and creates additional heat which must be removed from the reformate and/or stack, typically by passing a superabundance of cooling air through the cathode side of the stack 
         [0005]    It is known to produce a reformate containing hydrogen and carbon monoxide by endothermic steam reforming (SR) of hydrocarbon in the presence of water in the so-called “water gas” process, which may be represented by the following equation: 
         [0000]      C 7 H 12 +7H 2 O+heat.→13H 2 +7CO   (Eq. 2)
 
         [0000]    Many known fuel cell systems use water in the reforming process, either recovered from the fuel cell exhaust or supplied to the system. In the case of recovered water, a large heat exchanger is required to condense the water, adding mass, cost, and parasitic losses to the system. In the case of supplied water, the water must be filtered and deionized, resulting in added cost, complexity, and maintenance requirements. In addition, for vehicular applications, the water must be stored, transported with the reformer, and periodically replenished. 
         [0006]    It is also known to produce a reformate containing hydrogen and carbon monoxide by endothermic reforming of hydrocarbon in the presence of carbon dioxide in the so-called “dry reforming” process, which may be represented by the following equation: 
         [0000]      C 7 H 12 +7CO 2 +heat→6H 2 +14CO   (Eq. 3).
 
         [0007]    U.S. Pat. No. 7,326,482 discloses a highly efficient fuel cell system comprising a reformer and an SOFC stack where a portion of the spent fuel stream (i.e., tail gas) from the fuel cell stack, which contains H 2 O and CO 2 , is introduced to the inlet of the reformer. The patent discloses that at steady state operation, H 2 O and CO 2  in the tail gas provide the oxygen necessary for catalytic reformation of hydrocarbons according to equations (2) and (3) above. A portion of the tail gas is also introduced to a combustor along with exhaust air from the fuel cell stack and combusted to provide heat to the catalytic reformer with combustion exhaust discharged to the atmosphere. The patent further discloses that the added water to the reformer increases protection against anode coking in the fuel cell. However, at startup there is insufficient water and carbon dioxide in the tail gas to provide enough oxygen to reform the fuel, so the patent teaches that air must be provided to the reformer at startup. The patent does not disclose a way of obtaining water&#39;s anti-coking benefits during the start-up phase when the tail gas does not contain high amounts of water. 
       SUMMARY OF THE INVENTION 
       [0008]    A method of operating a fuel cell system is provided for a fuel system that comprises a catalytic reformer having an inlet and an outlet, and a fuel cell assembly that comprises a plurality of fuel cells having cathodes and anodes, an air passage in contact with the fuel cell cathodes and having an air inlet and exhaust outlet, and a reformate passage in contact with the fuel cell anodes and having a reformate inlet and a tail gas outlet. The method comprises:
       (a) introducing fuel and air to the catalytic reformer inlet;   (b) operating the catalytic reformer to produce a reformate stream comprising hydrogen and carbon monoxide from the reformer outlet, the operation of the catalytic reformer performed under partial oxidation conditions during a start-up mode and under endothermic conditions during a steady state mode;   (c) introducing reformate from the reformate stream to the fuel cell assembly reformate inlet, and introducing air to the fuel cell assembly air inlet to produce electricity, an air exhaust stream from the fuel cell assembly air exhaust outlet, and a tail gas stream from the fuel cell assembly tail gas outlet;   (d) introducing the tail gas stream to the reformer inlet;   (e) during the start-up mode: introducing oxygen to the reformate stream downstream of the reformer outlet and upstream of the fuel cell assembly reformate inlet, or introducing oxygen to the tail gas stream downstream of the fuel cell assembly tail gas outlet and upstream of the reformer inlet, or introducing oxygen to the reformate stream downstream of the reformer outlet and upstream of the fuel cell assembly reformate inlet and introducing oxygen to the tail gas stream downstream of the fuel cell assembly tail gas outlet and upstream of the reformer inlet; and   (f) reacting the oxygen introduced in (e) with hydrogen and carbon monoxide in the reformate stream, the tail gas stream, or both the reformate and tail gas streams to produce water in the reformate stream, the tail gas stream, or both the reformate and tail gas streams.       
 
         [0015]    Also provided is a fuel cell system comprising:
       (a) a catalytic reformer having an inlet and an outlet;   (b) a fuel cell assembly that comprises a plurality of fuel cells having cathodes and anodes, an air passage in contact with the fuel cell cathodes and having an air inlet and exhaust outlet, and a reformate passage in contact with the fuel cell anodes and having a reformate inlet and a tail gas outlet, the reformate inlet being in fluid communication with the reformer outlet; and   (c) a combustor having a tail gas inlet in fluid communication with the fuel cell assembly tail gas outlet, an air inlet, and an outlet in fluid communication with the reformer inlet.       
 
         [0019]    Also provided is a fuel cell system comprising:
       (a) a catalytic reformer having an inlet and an outlet;   (b) a fuel cell assembly that comprises a plurality of fuel cells having cathodes and anodes, an air passage in contact with the fuel cell cathodes and having an air inlet and exhaust outlet, and a reformate passage in contact with the fuel cell anodes and having a reformate inlet and a tail gas outlet, the reformate inlet being in fluid communication with the reformer outlet; and   (c) an air inlet in the reformate stream between the reformer outlet and the fuel cell assembly reformate inlet.       
 
         [0023]    In some embodiments, heat generated by the reaction of oxygen with hydrogen and carbon monoxide from the reformate stream and/or tail gas stream is removed. In some embodiments, sufficient heat is removed to maintain the reformate stream temperature at or below 900° C., more specifically less than 850° C., and even more specifically less than 750° C. Heat can be removed actively with heat exchangers or passively by heat flow to the surrounding thermal mass in the system. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]    The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
           [0025]      FIG. 1  is a schematic drawing of a fuel cell system in which fuel cell tail gas is combusted with air and fed to a catalytic reformer; 
           [0026]      FIG. 2  is a schematic drawing of a fuel cell system in which air is added to a reformate stream; and 
           [0027]      FIG. 3  is a schematic drawing of a fuel cell system in which fuel cell tail gas is combusted with air and fed to a catalytic reformer and in which air is added to a reformate stream 
       
    
    
     DETAILED DESCRIPTION 
       [0028]    Referring now to the Figures, the invention will be described with reference to specific embodiments, without limiting same. The figures are not intended to represent comprehensive diagrams of all components, but only those components necessary for illustrating the concepts and principles described herein. Missing components, including but not limited to pumps, sensors, control valves, etc., will be readily inferred by those of ordinary skill in the art. Where practical, reference numbers for like components are commonly used among multiple figures. 
         [0029]    Referring to  FIG. 1 , fuel cell system  10  is shown with fuel feed  12   a  and air feed  14   a  introduced to a fuel delivery unit  16 , which in turn delivers a fuel air mixture to catalytic reformer  18  through conduit  20 . The catalytic reformer  18  produces reformate that contains hydrogen and carbon monoxide, and delivers the reformate through conduit  22  to equalizer heat exchanger  24  where the temperatures of the reformate and cathode air feed for the fuel cell are equalized. Reformate exiting from the heat exchanger  24  is delivered through conduit  26  to fuel desulfurizer  28 , from where it is delivered through conduit  30  to the anode inlet of fuel cell stack  32 . The air feed  14   b  for the cathode side of the fuel cell stack  32  is introduced to recycle cooler heat exchanger  34  where it is heated by hot tail gas discharged from the fuel cell stack  32 . Air is delivered from the recycle cooler heat exchanger  34  through conduit  36  to cathode air pre-heat heat exchanger  38  where the air is further heated by exhaust from combustor/heat exchanger  40 , from where it is delivered through conduit  42  to equalizer heat exchanger  24  and then through conduit  44  to the cathode inlet of fuel cell stack  32 . Exhaust air from the fuel cell stack  32  is delivered through conduit  46  to combustor/heat exchanger  40 , where it is combusted with fuel feed  12   b  and/or tail gas delivered through conduit  48 . The combustion exhaust from combustor/heat exchanger  40  is delivered through conduit  49  to cathode air preheat heat exchanger  38  where it pre-heats cathode air feed for the fuel cell and then is discharged through exhaust  51 . Fuel cell tail gas containing CO 2  and water along with unspent fuel exits the fuel cell stack  32  through conduit  50 , and is distributed through control valve  52  through conduit  48  as fuel for combustor/heat exchanger  40  and/or through conduit  54  to be recycled back to the catalytic reformer  18 . The tail gas recycle stream in conduit  54  is introduced to combustor  56  where it can be combusted during operation of the catalytic reformer  18  in exothermic partial oxidation mode during startup of the system. The outlet of combustor  56  delivers the recycle stream through conduit  58  to the recycle cooler heat exchanger  34 , from where it flows through conduit  60  (optionally with the aid of a pump (not shown)) to fuel delivery unit  16 . 
         [0030]    During operation of the  FIG. 1  system at startup, heat is initially supplied to the catalytic reformer  18  to bring the catalyst up to operating temperatures (e.g., 350° C. to 600° C.). This heat can be supplied by combusting fuel  12   b  in heat exchanger combustor  40  to provide heat to the catalytic reformer, and/or fuel delivery unit  16  can include a heating functionality to pre-heat the air/fuel mixture, and/or the catalyst can be heated by electrical resistance heating. After the catalyst reaches operating temperature, the reformer will be operating in exothermic partial oxidation mode with oxygen for the reforming reaction coming primarily from atmospheric oxygen. During this stage of operation, water can be introduced to the system shown in  FIG. 1  by combusting tail gas in the combustor  56  to produce water, and recycling the combustion reaction products into catalytic reformer  18 . In some embodiments, all of the tail gas stream is combusted in the combustor  56 . Any excess heat from the combustion not utilized to pre-heat the cathode air in the cathode air pre-heat heat exchanger  34  can be discharged outside of the system, pumped elsewhere in the system to enhance overall thermal efficiency, or stored for later use in the system (e.g., to provide heat for the endothermic reforming stage that will follow the startup exothermic reforming stage). 
         [0031]    During operation of the  FIG. 1  system at steady state, the catalytic reformer  18  operates endothermically, utilizing oxygen supplied by water and CO 2  in the tail gas recycle for reforming according to equations (2) and/or (3) above, with little or no added air. Heat is supplied for the endothermic reaction by the combustion of a portion of the tail gas in combustor/heat exchanger  40 . During this steady state endothermic stage of operation, combustor  56  is inactive, with the tail gas recycle either flowing through the combustor with no air feed  14   c  added and no combustion, or the tail gas recycle can be routed around the combustor  56  to cathode air pre-heat heat exchanger  34 . 
         [0032]    Turning now to  FIG. 2 , fuel feed  12   a  and air feed  14   a  are introduced to a fuel delivery unit  16 , which in turn delivers a fuel air mixture to catalytic reformer  18  through conduit  20 . The catalytic reformer  18  produces reformate that contains hydrogen and carbon monoxide, and delivers the reformate through conduit  22  to equalizer heat exchanger  24  where the temperatures of the reformate and cathode air feed for the fuel cell are equalized. Conduit  22  also includes air inlet  14   d . Reformate exiting from the heat exchanger  24  is delivered through conduit  26  to fuel desulfurizer  28 , from where it is delivered through conduit  30  to the anode inlet of fuel cell stack  32 . Conduit  26  also includes air inlet  14   e , fuel desulfurizer  28  includes an air inlet  14   f , and conduit  30  includes air inlet  14   g . The air feed  14   b  for the cathode side of the fuel cell stack  32  is introduced to recycle cooler heat exchanger  34  where it is heated by hot tail gas discharged from the fuel cell stack  32 . Air is delivered from the recycle cooler heat exchanger  34  through conduit  36  to cathode air pre-heat heat exchanger  38  where the air is further heated by exhaust from combustor/heat exchanger  40 , from where it is delivered through conduit  42  to equalizer heat exchanger  24  and then through conduit  44  to the cathode inlet of fuel cell stack  32 . Exhaust air from the fuel cell stack  32  is delivered through conduit  46  to combustor/heat exchanger  40 , where it is combusted with fuel feed  12   b  and/or tail gas delivered through conduit  48 . The combustion exhaust from combustor/heat exchanger  40  is delivered through conduit  49  to cathode air preheat heat exchanger  38  where it pre-heats cathode air feed for the fuel cell and then is discharged through exhaust  51 . Fuel cell tail gas containing CO 2  and water along with unspent fuel exits the fuel cell stack  32  through conduit  50 , and is distributed through control valve  52  through conduit  48  as fuel for combustor/heat exchanger  40  and/or through conduit  54  to be recycled back to the catalytic reformer  18 . The tail gas recycle stream in conduit  54  is introduced to the recycle cooler heat exchanger  34 , from where it flows through conduit  60  (optionally with the aid of a pump (not shown)) to fuel delivery unit  16 . 
         [0033]    During operation of the  FIG. 2  system at startup, heat is initially supplied to the catalytic reformer  18  to bring the catalyst up to operating temperatures (e.g., 700° C. to 1000° C.). This heat can be supplied by combusting fuel  12   b  in heat exchanger combustor  40  to provide heat to the catalytic reformer, and/or fuel delivery unit  16  can include a heating functionality to pre-heat the air/fuel mixture, and/or the catalyst can be heated by electrical resistance heating. After the catalyst reaches operating temperature, the reformer will be operating in exothermic partial oxidation mode with oxygen for the reforming reaction coming primarily from atmospheric oxygen. During this stage of operation, water can be introduced to the system shown in  FIG. 2  by adding oxygen to the reformate stream downstream of the catalytic reformer  18  and upstream of the fuel cell stack  32 . The oxygen will react with hydrogen in the reformate stream to produce water. As this reaction is highly exothermic, care must be taken to control the rate at which air is added to the reformate stream. Adding air at too high a rate can produce heat sufficient to drive temperatures in the reformate stream, and adding all of the air required to produce the desired amount of water in the reformate stream at a single location can produce detrimental temperature levels. However, during the startup mode, the temperature of surrounding materials and components of the system is typically lower than it is at steady state, thereby creating a higher temperature gradient so that sufficient heat can be transferred to the surrounding thermal mass of the system materials and components if the air is added to the reformate stream at a plurality of locations. These locations should be sufficiently spaced apart so that sufficient heat can be transferred away from the reformate stream before additional air is added. In some embodiments, the locations are sufficiently spaced apart to maintain any surrounding steel (including stainless, austenitic 300 series, and ferritic 400 series steels) at or below 800° C. (more specifically at or below 750° C.), and/or any surrounding nickel-based components (including Inconel and similar alloys) at or below 950° C. (more specifically below 850° C.) 
         [0034]    In some exemplary embodiments, air is added at two or more locations of the reformate stream. In some exemplary embodiments, air is added at three or more locations of the reformate stream. In a more specific exemplary embodiment, the system includes a heat exchanger having one side disposed in the reformate stream between the reformate outlet and the fuel cell assembly reformate inlet and one side disposed in an air flow feed stream connected to the fuel cell assembly air inlet, a desulfurizer disposed in the reformate stream between the heat exchanger and the fuel cell assembly reformate inlet, a first air inlet in the reformate stream at three locations selected from the group consisting of: a first location between the reformer outlet and the heat exchanger, a second air inlet at a second location between the heat exchanger and the desulfurizer, a third air inlet at a third location inside the desulfurizer, and a fourth air inlet at a fourth location between the desulfurizer and the fuel cell assembly reformate inlet. 
         [0035]    During operation of the  FIG. 2  system at steady state, the catalytic reformer  18  operates endothermically, utilizing oxygen supplied by water and CO 2  in the tail gas recycle for reforming according to equations (2) and/or (3) above, with little or no added air. Heat is supplied for the endothermic reaction by the combustion of a portion of the tail gas in combustor/heat exchanger  40 . During this steady state endothermic stage of operation, the air feeds  14   d ,  14   e , and  14   f  are inactive. 
         [0036]    Turning now to  FIG. 3 , fuel feed  12   a  and air feed  14   a  are introduced to a fuel delivery unit  16 , which in turn delivers a fuel air mixture to catalytic reformer  18  through conduit  20 . The catalytic reformer  18  produces reformate that contains hydrogen and carbon monoxide, and delivers the reformate through conduit  22  to equalizer heat exchanger  24  where the temperatures of the reformate and cathode air feed for the fuel cell are equalized. Conduit  22  also includes air inlet  14   d . Reformate exiting from the heat exchanger  24  is delivered through conduit  26  to fuel desulfurizer  28 , from where it is delivered through conduit  30  to the anode inlet of fuel cell stack  32 . Conduit  26  also includes air inlet  14   e , fuel desulfurizer  28  includes an air inlet  14   f , and conduit  30  includes air inlet  14   g . The air feed  14   b  for the cathode side of the fuel cell stack  32  is introduced to recycle cooler heat exchanger  34  where it is heated by hot tail gas discharged from the fuel cell stack  32 . Air is delivered from the recycle cooler heat exchanger  34  through conduit  36  to cathode air pre-heat heat exchanger  38  where the air is further heated by exhaust from combustor/heat exchanger  40 , from where it is delivered through conduit  42  to equalizer heat exchanger  24  and then through conduit  44  to the cathode inlet of fuel cell stack  32 . Exhaust air from the fuel cell stack  32  is delivered through conduit  46  to combustor/heat exchanger  40 , where it is combusted with fuel feed  12   b  and/or tail gas delivered through conduit  48 . The combustion exhaust from combustor/heat exchanger  40  is delivered through conduit  49  to cathode air preheat heat exchanger  38  where it pre-heats cathode air feed for the fuel cell and then is discharged through exhaust  51 . Fuel cell tail gas containing CO 2  and water along with unspent fuel exits the fuel cell stack  32  through conduit  50 , and is distributed through control valve  52  through conduit  48  as fuel for combustor/heat exchanger  40  and/or through conduit  54  to be recycled back to the catalytic reformer  18 . The tail gas recycle stream in conduit  54  is introduced to combustor  56  where it can be combusted during operation of the catalytic reformer  18  in exothermic partial oxidation mode during startup of the system. The outlet of combustor  56  delivers the recycle stream through conduit  58  to the recycle cooler heat exchanger  34 , from where it flows through conduit  60  (optionally with the aid of a pump (not shown)) to fuel delivery unit  16 . 
         [0037]    During operation of the  FIG. 3  system at startup, heat is initially supplied to the catalytic reformer  18  to bring the catalyst up to operating temperatures (e.g., 300° C. to 600° C.). This heat can be supplied by combusting fuel  12   b  in heat exchanger combustor  40  to provide heat to the catalytic reformer, and/or fuel delivery unit  16  can include a heating functionality to pre-heat the air/fuel mixture, and/or the catalyst can be heated by electrical resistance heating. After the catalyst reaches operating temperature, the reformer will be operating in exothermic partial oxidation mode with oxygen for the reforming reaction coming primarily from atmospheric oxygen. During this stage of the reaction, water can be introduced to the system shown in  FIG. 3  by adding oxygen to the reformate stream downstream of the catalytic reformer  18  and upstream of the fuel cell stack  32 . The oxygen will react with hydrogen in the reformate stream to produce. As this reaction is highly exothermic, care must be taken to control the rate at which air is added to the reformate stream. Adding air at too high a rate can produce heat sufficient to drive temperatures in the reformate stream, and adding all of the air required to produce the desired amount of water in the reformate stream at a single location can produce detrimental temperature levels. However, water can also be introduced to the system shown in  FIG. 1  by combusting tail gas in the combustor  56  to produce water, and recycling the combustion reaction products into catalytic reformer  18 , so adding some oxygen to a single location in the reformate stream can be combined with adding oxygen to the combustor  56 . In some exemplary embodiments, all of the tail gas is combusted in combustor  56 . In some exemplary embodiments, air is added at two or more locations of the reformate stream. In some exemplary embodiments, air is added at three or more locations of the reformate stream. In a more specific exemplary embodiment, the system includes a heat exchanger having one side disposed in the reformate stream between the reformate outlet and the fuel cell assembly reformate inlet and one side disposed in an air flow feed stream connected to the fuel cell assembly air inlet, a desulfurizer disposed in the reformate stream between the heat exchanger and the fuel cell assembly reformate inlet, a first air inlet in the reformate stream at three locations selected from the group consisting of: a first location between the reformer outlet and the heat exchanger, a second air inlet at a second location between the heat exchanger and the desulfurizer, a third air inlet at a third location inside the desulfurizer, and a fourth air inlet at a fourth location between the desulfurizer and the fuel cell assembly reformate inlet. 
         [0038]    During operation of the  FIG. 3  system at steady state, the catalytic reformer  18  operates endothermically, utilizing oxygen supplied by water and CO 2  in the tail gas recycle for reforming according to equations (2) and/or (3) above, with little or no added air. Heat is supplied for the endothermic reaction by the combustion of a portion of the tail gas in combustor/heat exchanger  40 . During this steady state endothermic stage of operation, the air feeds  14   d ,  14   e ,  14   f , and  14   g  are inactive, and combustor  56  is inactive, with the tail gas recycle either flowing through the combustor with no air feed  14   c  added and no combustion. Alternatively, the tail gas recycle can be routed around the combustor  56  to cathode air pre-heat heat exchanger  34 . 
         [0039]    While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description.