Patent Publication Number: US-2023155151-A1

Title: Fuel cell system including fuel exhaust processor and method of operating the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a non-provisional application that claims the benefit of U.S. provisional Application No. 63/278,485, filed on Nov. 12, 2021, the contents of which are herein incorporated by reference in their entirety. 
    
    
     FIELD OF THE INVENTION 
     Aspects of the present invention relate to fuel cell systems and methods, and more particularly, to fuel cell systems including a fuel exhaust processing module configured to generate purified carbon dioxide and hydrogen streams. 
     BACKGROUND OF THE INVENTION 
     Fuel cells, such as solid oxide fuel cells, are electrochemical devices which can convert energy stored in fuels to electrical energy with high efficiencies. High temperature fuel cells include solid oxide and molten carbonate fuel cells. These fuel cells may operate using hydrogen and/or hydrocarbon fuels. There are classes of fuel cells, such as the solid oxide regenerative fuel cells, that also allow reversed operation, such that oxidized fuel can be reduced back to unoxidized fuel using electrical energy as an input. 
     SUMMARY OF THE INVENTION 
     According to various embodiments, provided is a fuel cell system comprising: a hotbox; a fuel cell stack disposed in the hotbox; an anode tail gas oxidizer (ATO) disposed in the hotbox; and a fuel exhaust processor fluidly connected to the hotbox and comprising: a first hydrogen pump configured to extract hydrogen from the anode exhaust received from the fuel cell stack and to output the hydrogen to a first hydrogen stream provided to the fuel cell stack; a second hydrogen pump configured to extract hydrogen from anode exhaust output from the first hydrogen pump and to output the hydrogen to the first hydrogen stream; and a third hydrogen pump configured to extract hydrogen from anode exhaust output from the second hydrogen pump and to output the hydrogen to a second hydrogen stream provided to the ATO. 
     According to various embodiments, provided is a fuel cell system, comprising: a hotbox; a fuel cell stack disposed in the hotbox; an anode tail gas oxidizer (ATO) disposed in the hotbox; and a recycling conduit configured to receive anode exhaust from the fuel cell stack; a splitter fluidly connected to the recycling conduit; a low temperature shift reactor; a hydrogen separator comprising: a first hydrogen pump; a second hydrogen pump that is fluidly connected to the first hydrogen pump; and a third hydrogen pump that is fluidly connected to the second hydrogen pump; a supply conduit that fluidly connects an outlet of the splitter to the hydrogen separator; a first separator conduit fluidly connecting the splitter to the low temperature shift reactor; a second separator conduit fluidly connecting the low temperature shift reactor to the first hydrogen pump; a first return conduit that fluidly connects an outlet of the splitter to the fuel cell stack; a second return conduit that fluidly connects an outlet of the splitter or the separator conduit to the ATO; and a third return conduit that fluidly connects the separator conduit to the second return conduit. 
     According to various embodiments, provided is a fuel cell system comprising: a hotbox; a fuel cell stack disposed in the hotbox; an anode tail gas oxidizer (ATO) disposed in the hotbox; and a fuel exhaust processor fluidly connected to the hotbox and comprising: a first hydrogen pump configured to extract hydrogen from the anode exhaust received from the fuel cell stack; and a second hydrogen pump configured to extract hydrogen from anode exhaust output from the first hydrogen pump; wherein the first hydrogen pump and the second hydrogen pump output hydrogen to one or more components of the hotbox. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated herein and constitute part of this specification, illustrate example embodiments of the invention, and together with the general description given above and the detailed description given below, serve to explain the features of the invention. 
         FIG.  1    is a schematic view of a solid oxide fuel cell (SOFC) system, according to various embodiments of the present disclosure. 
         FIG.  2    is a schematic view of a fuel exhaust processor that may be included in the SOFC system of  FIG.  1   , according to various embodiments of the present disclosure. 
         FIG.  3    is a cross-sectional view of a hydrogen pumping cell that may be included in the fuel exhaust processor of  FIG.  2   , according to various embodiments of the present disclosure. 
         FIG.  4 A  is a perspective view of a shift reactor that may be included in the fuel exhaust processor of  FIG.  2   , according to various embodiments of the present disclosure, and  FIG.  4 B  is a cross-sectional view of the shift reactor of  FIG.  4 A . 
         FIGS.  5 - 8    are schematic views showing fluid flow through the fuel cell system and fuel exhaust processor of  FIGS.  1  and  2   , according to various embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The various embodiments will be described in detail with reference to the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. References made to particular examples and implementations are for illustrative purposes, and are not intended to limit the scope of the invention or the claims. 
     In a solid oxide fuel cell (SOFC) system, a fuel inlet stream may be humidified in order to facilitate fuel reformation reactions such as steam reformation and water-gas shift reactions. In addition, during system startup, shutdown, and power grid interruption events, water may be added to a fuel inlet stream in order to prevent coking of system components such as catalysts. Conventionally, such humidification is performed by vaporizing water in a steam generator containing corrugated tubing. Water flows through the corrugated tubing and is heated by the cathode recuperator heat exchanger exhaust stream which flows around the outside of the tubing. However, utilizing relatively low-temperature cathode recuperator exhaust stream generally requires substantial lengths of corrugated tubing, in order to absorb enough heat to vaporize the water. Further, the steam generator is relative large and bulky, which also adds to the system size, complexity and manufacturing costs. 
     In contrast, embodiments of the present disclosure provide a water injector configured to inject water directly into the anode exhaust recycle stream which provides heat to vaporize the water into steam and/or aerosolize the water into droplets small enough to be entrained in the anode exhaust stream. The anode exhaust recycle stream is recycled into the fuel inlet stream provided into the fuel cell stack, such that humidified fuel is provided to the fuel cells of the fuel cell stack. Thus, the prior art steam generator may be omitted to reduce system size, complexity and cost. In addition, the embodiment system may operate using relatively short, non-corrugated water conduit, which may improve system response times and reduce system size and cost. 
     Sofc Systems 
       FIG.  1    is a schematic representation of a SOFC system  10 , according to various embodiments of the present disclosure. Referring to  FIG.  1   , the system  10  includes a hotbox  100  and various components disposed therein or adjacent thereto. The hotbox  100  may contain at least one fuel cell stack  102 , such as a solid oxide fuel cell stack containing alternating fuel cells and interconnects. One solid oxide fuel cell of the stack contains a ceramic electrolyte, such as yttria stabilized zirconia (YSZ), scandia stabilized zirconia (SSZ), scandia and ceria stabilized zirconia or scandia, yttria and ceria stabilized zirconia, an anode electrode, such as a nickel-YSZ, a nickel-SSZ or nickel-doped ceria cermet, and a cathode electrode, such as lanthanum strontium manganite (LSM). The interconnects may be metal alloy interconnects, such as chromium-iron alloy interconnects. The stacks  102  may be arranged over each other in a plurality of columns. 
     The hotbox  100  may also contain an anode recuperator heat exchanger  110 , a cathode recuperator heat exchanger  120 , an anode tail gas oxidizer (ATO)  130 , an anode exhaust cooler heat exchanger  140 , a vortex generator  172 , and a water injector  160 . The system  10  may also include a catalytic partial oxidation (CPOx) reactor  200 , a mixer  210 , a CPOx blower  204  (e.g., air blower), a system blower  208  (e.g., main air blower), and an anode recycle blower  212 , which may be disposed outside of the hotbox  100 . However, the present disclosure is not limited to any particular location for each of the components with respect to the hotbox  100 . 
     The CPOx reactor  200  receives a fuel inlet stream from a fuel inlet  300 , through fuel conduit  300 A. The fuel inlet  300  may be a fuel tank or a utility natural gas line including a valve to control an amount of fuel provided to the CPOx reactor  200 . The CPOx blower  204  may provide air to the CPOx reactor  200  during system start-up. The fuel and/or air may be provided to the mixer  210  by fuel conduit  300 B. Fuel flows from the mixer  210  to the anode recuperator  110  through fuel conduit  300 C. The fuel is heated in the anode recuperator  110  by a portion of the fuel exhaust and the fuel then flows from the anode recuperator  110  to the SOFC stack  102  through fuel conduit  300 D. 
     The main air blower  208  may be configured to provide an air stream (e.g., air inlet stream) to the anode exhaust cooler  140  through air conduit  302 A. Air flows from the anode exhaust cooler  140  to the cathode recuperator  120  through air conduit  302 B. The air is heated by the ATO exhaust in the cathode recuperator  120 . The air flows from the cathode recuperator  120  to the SOFC stack  102  through air conduit  302 C. 
     Anode exhaust (e.g., fuel exhaust) generated in the SOFC stack  102  is provided to the anode recuperator  110  through anode exhaust conduit  308 A. The anode exhaust may contain unreacted fuel and may also be referred to herein as fuel exhaust. The anode exhaust may be provided from the anode recuperator  110  to a shift reactor  180 , such as an optional water gas shift (WGS) reactor, by anode exhaust conduit  308 B. In some embodiments, the reactions of WGS reactor may alternatively be reacted in a low-temperature shift (LTS) reactor of the fuel exhaust processor  400 . The water injector  160  may be fluidly connected to the anode exhaust conduit  308 B. The anode exhaust may be provided from the shift reactor  180  to the anode exhaust cooler  140  by anode exhaust conduit  308 C. The anode exhaust heats the air inlet stream in the anode exhaust cooler  140  and may then be provided from the anode exhaust cooler  140  to the fuel exhaust processor  400 . 
     In particular, the anode exhaust may be output from the anode exhaust cooler  140  to the fuel exhaust processor  400  by a first recycling conduit  310 A. In some embodiments, anode exhaust may be provided to the fuel exhaust processor  400  by an optional second recycling conduit  310 B. In particular, the second recycling conduit  310 B may be configured to provide hotter anode exhaust to the fuel exhaust processor  400  than the first recycling conduit  310 A, since anode exhaust is cooled in the anode exhaust cooler  140  prior to entering the first recycling conduit  310 A. In some configurations, anode exhaust can flow through both first recycling conduit  310 A and second recycling conduit  310 B. 
     The shift reactor  180  may be any suitable device that converts components of the fuel exhaust into free hydrogen (H 2 ) and/or water. For example, the shift reactor  180  may comprise a tube or conduit containing a catalyst that converts carbon monoxide (CO) and water vapor in the fuel exhaust stream into carbon dioxide and hydrogen, via the water gas shift reaction (CO+H 2 O ↔CO 2 +H 2 ). Thus, the shift reactor  180  increases the amount of hydrogen and carbon dioxide in the anode exhaust and decreases the amount of carbon monoxide in the anode exhaust. For example, the shift reactor  180  may reduce the amount of carbon monoxide in the anode exhaust to about 5% by volume or less, such as about 4% or less, or about 3% or less. The catalyst may be any suitable catalyst, such as an iron oxide or a chromium-promoted iron oxide catalyst. 
     Cathode exhaust generated in the SOFC stack  102  flows to the ATO  130  through cathode exhaust conduit  304 A. The vortex generator  172  may be disposed in the cathode exhaust conduit  304 A and may be configured to swirl the cathode exhaust. The ATO fuel conduit  304 A may be fluidly connected to the vortex generator  172  or to the cathode exhaust conduit  304 A or the ATO  130  downstream of the vortex generator  172 . The swirled cathode exhaust may mix with hydrogen output from the fuel exhaust processor  400  at the ATO  130  (e.g., at an ATO injector space). The mixture may be oxidized in the ATO  130  to generate ATO exhaust. The ATO exhaust flows from the ATO  130  to the cathode recuperator  120  through the cathode exhaust conduit  304 B. Exhaust flows from the cathode recuperator  120  and out of the hotbox  100  through cathode exhaust conduit  304 C. 
     Water flows from a water source  206 , such as a water tank or a water pipe, to the water injector  160  through the water conduit  306 . The water injector  160  injects water directly into first portion of the anode exhaust provided in the anode exhaust conduit  308 C. Heat from the first portion of the anode exhaust (also referred to as a recycled anode exhaust stream) provided in the anode exhaust conduit  308 C vaporizes the water to generate steam. The steam mixes with the anode exhaust, and the resultant mixture is provided to the anode exhaust cooler  140 . The mixture is then routed through the fuel exhaust processor  400  and provided to the mixer  210 . The mixer  210  is configured to mix natural gas (or CPOx effluent if the CPOx is lit) and anode recycle from the stream of the anode recycle blower  212 . This humidified fuel mixture may then be heated in the anode recuperator  110  by the anode exhaust, before being provided to the SOFC stack  102 . The system  10  may also include one or more fuel reforming catalysts  112 ,  114 , and  116  located inside and/or downstream of the anode recuperator  110 . The reforming catalyst(s) partially (e.g., 15%, 20%, etc.) reform the humidified fuel mixture before it is provided to the SOFC stack  102 . 
     The system  10  may further include a system controller  225  configured to control various elements of the system  10 . The system controller  225  may include a central processing unit configured to execute stored instructions. For example, the system controller  225  may be configured to control fuel and/or air flow through the system  10 , according to fuel composition data. 
     Fuel Exhaust Processors 
       FIG.  2    is a schematic view showing components of the fuel exhaust processor  400 , according to various embodiments of the present disclosure. Referring to  FIGS.  1  and  2   , the fuel exhaust processor  400  may include a hydrogen separator  410 , a system controller  425 , a splitter  440 , a low temperature shift reactor  450 , and a heat exchanger  444 . The system controller  425  may be a central processing unit configured to execute stored instructions. For example, the system controller  425  may be configured to control anode exhaust, hydrogen and/or carbon dioxide flow through the fuel exhaust processor  400 . In some embodiments, the system controller  425  may be operatively connected to the system controller  225  of the SOFC system  10 , such that the system controller  425  may control the fuel exhaust processor based on operating conditions of the SOFC system  10 . 
     The splitter  440  may be configured to receive anode exhaust from the first recycling conduit  310 A. The splitter  440  may be fluidly connected to the hotbox  100  and the hydrogen separator  410 . For example, a first return conduit  406 A may fluidly connect an outlet of the splitter  440  to the hotbox  100 , and a first separator conduit  401 A and a second separator conduit  401 B may fluidly connect an outlet of the splitter  440  to the hydrogen separator  410 . In particular, a first portion of the anode exhaust may be output from the splitter  440  and provided to the shift reactor  450  via the first separator conduit  401 A, and anode exhaust output form the shift reactor  450  may be supplied to the hydrogen separator  410  by the second separator conduit  401 B. A second portion of the anode exhaust may be output from an outlet of the splitter  440  to the first return conduit  406 A. Anode exhaust output from the fuel exhaust processor  400  may be move through the first return conduit  406 A to the mixer  210  of the SOFC system  10 , by the anode recycle blower  212 . However, the anode recycle blower  212  may be disposed in any other suitable location. 
     The shift reactor  450  may be a WGS reactor similar to the shift reactor  180 , but may configured to operate at a lower temperature than the shift reactor  180 . Accordingly, the shift reactor  180  may be referred to as a high temperature shift reactor, and the shift reactor  450  may be referred to as a low temperature shift reactor. The shift reactor  450  may be configured to further reduce the carbon monoxide content of the anode exhaust provided to the fuel exhaust processor  400 . For example, the shift reactor  450  may be configured to reduce the carbon monoxide content of the anode exhaust to less than about 0.3% by volume, such as less than about 0.2%, or less than about 0.1%. 
     Purified anode exhaust (e.g., low carbon monoxide content anode exhaust) output from the shift reactor  450  may be provided to the hydrogen separator  410  by a second separator conduit  401 B. The heat exchanger  444  may be operatively connected to the second separator conduit  401 B and may be configured to cool anode exhaust passing there through. For example, the heat exchanger  444  may include fans and/or cooling fins configured to transfer heat to air supplied thereto. Accordingly, the heat exchanger  444  may be configured to cool the anode exhaust, in order to prevent overheating and/or damage to the hydrogen separator  410 . In some embodiments, the heat exchanger  444  may be omitted. For example, if the shift reactor  450  includes an internal cooling system, as disclosed below with respect to  FIGS.  4 A and  4 B , the heat exchanger  444  may optionally be omitted. 
     In various embodiments, the fuel exhaust processor  400  may be fluidly connected to multiple SOFC systems  10 . For example, the fuel exhaust processor  400  may be configured to process anode exhaust output from two or more fuel cell systems, and may be configured to return hydrogen rich fuel streams to both fuel cell systems. 
     The hydrogen separator  410  may include one or more hydrogen pumps, which may each include electrochemical hydrogen pumping cells  420 . For example, as shown in  FIG.  2   , the hydrogen separator  410  may include a first hydrogen pump  414 A, a second hydrogen pump  414 B, and a third hydrogen pump  414 C, that each comprise stacked hydrogen pumping cells  420 . However, the present disclosure is not limited to any particular number of hydrogen pumps. For example, in various embodiments, the first hydrogen pump  414 A and the second hydrogen pump  414 B may be combined into a single stack of hydrogen pumping cells  420 . In other embodiments, the first, second, and third hydrogen pumps  414 A,  414 B,  414 C may be combined into a single stack of hydrogen pumping cells  420 . 
     In some embodiments, the first hydrogen pump  414 A may include a larger number of hydrogen pumping cells  420  than the second and/or third hydrogen pumps  414 B,  414 C. For example, the first hydrogen pump  414 A may include twice the number of hydrogen pumping cells  420  as the second hydrogen pump  414 B and/or the third hydrogen pump  414 C. 
     In still other embodiments, the fuel exhaust processor  400  may output only a single hydrogen stream. For example, the third hydrogen pump  414 C may be omitted. In particular, heat generated by exothermic reactions in the ATO  130  may be used to offset heat losses due to endothermic fuel reformation reactions occurring in the anode recuperator  110 , by using the ATO exhaust to heat air provided to the fuel cell stack  102  in the cathode recuperator  120 . When there is no H 2  fuel fed to the ATO  130 , then there is no exothermic reaction in the ATO  130 . Accordingly, when the SOFC stacks are operated at steady state conditions where heat from the ATO  130  is not needed, then the fuel stream to the ATO  130  can be omitted. 
     The second separator conduit  401 B may provide anode exhaust to an anode inlet of the first hydrogen pump  414 A. An anode outlet of the first hydrogen pump  414 A may be fluidly connected to an anode inlet of the second hydrogen pump  414 B by a first exhaust conduit  402 A. An anode outlet of the second hydrogen pump  414 B may be fluidly connected to an anode inlet of the third hydrogen pump  414 C, by a second exhaust conduit  402 B. An anode outlet of the third hydrogen pump  414 C may be fluidly connected to the carbon dioxide storage device  50 , by a carbon dioxide storage conduit  52 . 
     The carbon dioxide storage device  50  may include a carbon dioxide processor and a carbon dioxide storage tank. The processor may operate to compress and/or cool a carbon dioxide stream received from the fuel exhaust processor  400 . The processor may be a condenser and/or dryer configured to remove water from the carbon dioxide stream. The carbon dioxide stream may be provided to the carbon dioxide storage device  50  in the form of a vapor, liquid, solid or supercritical carbon dioxide. The carbon dioxide storage device  50  also may condense the carbon dioxide into a liquid phase after compression. 
     A first hydrogen conduit  404 A may be fluidly connected to a cathode outlet of the first hydrogen pump  414 A, a second hydrogen conduit  404 B may be fluidly connected to a cathode outlet of the second hydrogen pump  414 B, and a third hydrogen conduit  404 C may be fluidly connected to a cathode outlet of the third hydrogen pump  414 C. The first hydrogen conduit  404 A may be fluidly connected to a first return conduit  406 A, and the second hydrogen conduit  404 B may be fluidly connected to the first hydrogen conduit  404 A. In particular, the first return conduit  406 A may be configured to provide hydrogen extracted from the anode exhaust by the first hydrogen pump  414 A, the second hydrogen pump  414 B, and or the third hydrogen pump  414 C to the mixer  210 , such that the hydrogen may be recycled to the SOFC stack  102 . 
     The third hydrogen conduit  404 C may be fluidly connected to the SOFC system  10  by a second return conduit  406 B. In particular, the second return conduit  406 B may be configured to provide hydrogen extracted from the anode exhaust by the third stack  414 C to the second return conduit  406 B, which may provide the hydrogen to the ATO  130 . 
     In some embodiments, an optional fourth hydrogen conduit  404 D may fluidly connect the third hydrogen conduit  404 C to the first hydrogen conduit  404 A. An optional fifth hydrogen conduit  404 E may fluidly connect the second hydrogen conduit  404 B to the third hydrogen conduit  404 C. An optional hydrogen storage conduit  56  may fluidly connect the first hydrogen conduit  404 A to the hydrogen storage device  54 . 
     The hydrogen storage device  54  may include, for example, a condenser and a hydrogen storage tank. The condenser may be an air-cooled or water-enhanced, air-cooled condenser and/or heat exchanger configured to cool a hydrogen stream received from the fuel exhaust processor  400 , to a temperature sufficient to condense water vapor in the hydrogen stream. The hydrogen storage device  54  may further include compression to the desired storage pressure. A wide range of pressure above ambient is feasible, but is likely set to a value that corresponds with allowed piping standards (e.g., 150 psig, 300 psig, 600 psig, 1500 psig, etc.) 
     The first return conduit  406 A may fluidly connect the splitter  440  to the mixer  210  of the SOFC system  10 . The second return conduit  406 B may fluidly connect the first separator conduit  401 A to the ATO  130 , and may also be fluidly connected to the third hydrogen conduit  404 C. In other embodiments, the second return conduit  406 B may be fluidly connected to an outlet of the splitter  440 . A third return conduit  406 C may fluidly connect the second separator conduit  401 B to the second return conduit  406 B. 
     In various embodiments, the fuel exhaust processor  400  may include various valves to control fluid flow. For example, a first separator conduit valve  401 V 1  and a second separator conduit valve  401 V 2  may be respectively configured to control anode exhaust flow through the first and second separator conduits  401 A,  401 B. A first hydrogen conduit valve  404 V 1 , a second hydrogen conduit valve  404 V 2 , a third hydrogen conduit valve  404 V 3 , a fourth hydrogen conduit valve  404 V 4 , and a fifth hydrogen conduit valve  404 V 5  may be configured to respectively control hydrogen flow through the first, second, third, fourth, and fifth hydrogen conduits  404 A,  404 B,  404 C,  404 D,  404 E. A hydrogen storage valve  56 V, such as a two way valve, may be configured to control hydrogen flow from the first hydrogen conduit  404 A into the hydrogen storage conduit  56 . A second return conduit valve  406 V 2  and a third return conduit valve  406 V 3 , may be configured to respectively control anode exhaust flow through the second and third return conduits  406 B,  406 C. 
     In some embodiments, the fuel exhaust processor  400  may be fluidly connected to multiple hotboxes  100 . For example, the splitter  440  may receive anode exhaust from multiple recycling conduits  310 A/ 310 B, and may be fluidly connected to multiple return conduits  406 A,  406 B. For example, the recycling conduits  310 A/ 310 B and the first and second return conduits  406 A,  406 B may be branched and connected to different hotboxes  100 . 
     Hydrogen Pumping Cells 
       FIG.  3    is a cross-sectional view of a hydrogen pumping cell  420  that may be included in the fuel exhaust processor  400 , according to various embodiments of the present disclosure. Referring to  FIG.  4   , the hydrogen pumping cell  420  may be polymer electrolyte (PEM) cells that include an anode gas diffusion layer (GDL)  422 , a cathode GDL  424 , and a membrane electrode assembly (MEA) disposed there between. The MEA may include the GDL, an anode  426 , a cathode  428 , and a polymer membrane  430  disposed there between. The hydrogen pumping cell  420  may be disposed between bipolar plates  432 . The bipolar plates  432  may include channels to deliver reactants to the hydrogen pumping cells  420  disposed thereon. The bipolar plates  432  may be formed of material such as graphite. 
     The GDLs  422 ,  424  may be formed of a porous medium configured to distribute or remove the reactants received from adjacent bipolar plates  432 . The GDLs may comprise, for example, carbon paper treated with a hydrophobic material, such as polytetrafluoroethylene (PTFE), to reduce water accumulation. The anode  426  may be configured to oxidize hydrogen. The cathode  428  may be configured to evolve hydrogen (i.e., convert 2H +  into H 2 ). For example, the anode  426  and the cathode  428  may include a Pt/C catalyst. In some embodiments, the cathode  428  may have a higher or lower catalyst loading than the anode  426 , for example. The catalyst type (e.g., CO tolerant catalyst on the cathode and/or anode side) and loading may vary at anode  426  and/or cathode  428 . The membrane  430  may be configured to transport ions. For example, the membrane  430  may include an ionomer such as Nafion. When an electrical potential is applied between the anode  426  and the cathode  428 , hydrogen ions are generated at the anode  426 , the hydrogen ions are driven through the membrane  430  by the applied electrical potential, and the hydrogen ions are recombined to evolve hydrogen gas at the cathode  428 . In some embodiments, the anodes  426  may be carbon monoxide tolerant anodes, as disclosed in Indian Provisional Application number 2021-11016645 filed Mar. 8, 2021 and U.S. application Ser. No. 17/715,353, each of which is incorporated herein by reference in its entirety. 
     Shift Reactor With Active Cooling 
       FIG.  4 A  is a perspective view of a low-temperature shift reactor  450  that may be included in the fuel exhaust processor  400  of  FIG.  2   , according to various embodiments of the present disclosure, and  FIG.  4 B  is a cross-sectional view of the shift reactor  450 . Referring to  FIGS.  4 A and  4 B , the shift reactor  450  may include a cover  451 , an inlet  452 , an outlet  454 , a first chamber  456 , a second chamber  458 , cooling conduits  460 , a fan housing  462 , at least one fan  464 , a first screen  466 , a second screen  468 , a first catalyst bed  470 , a second catalyst bed  472 , and a control unit  474 . 
     The cover  451  may cover the first chamber  456 , and the first chamber  456  may be disposed over the second chamber  458 . The cooling conduits  460  may be disposed between the first and second chambers  456 ,  458 . The fan housing  462  and the fans  464  may be connected to the cooling conduits  460 . The first screen  466  may be disposed adjacent the bottom of the first chamber  456 , and the second screen  468  may be disposed adjacent to the bottom of the second chamber  458 . The first catalyst bed  470  may be disposed in the first chamber  456  and on the first screen  466 , and the second catalyst bed  472  may be disposed in the second chamber  458  and on the second screen  468 . 
     The first and second catalyst beds  470 ,  472  may include any suitable WGS reaction catalysts, such as an iron oxide or a chromium-promoted iron oxide catalyst. Depending on the temperature range, zinc-copper or ferrochromium alloy catalysts or other known catalysts may be used. The catalyst may have an acceptable catalytically active at temperature ranging from about 200° C. to about 300° C. Alternatively, the catalyst can continue to be active to as low as 150° C. in some instances. In some embodiments, the first and second catalyst beds  470 ,  472  may include the same catalyst or different catalysts. In various embodiments, the shift reactor  450  may be configured to reduce the carbon monoxide content of the anode exhaust to about 0.3% or less. 
     In various embodiments, the first catalyst bed  470  may not completely fill the first chamber  456 . In particular, a first space S1 may be formed between the upper surface of the first catalyst bed  470  and a lower surface of the cover  451 . The first screen  466  may separate the first catalyst bed  470  and the cooling conduits  460 , and the top surface of the second catalyst bed  472  may be spaced apart from the cooling conduits  460 , such that a second space S2 is formed around the cooling conduits  460 , between the first and second chambers  456 ,  458 . Similarly, the second screen  468  may separate the second catalyst bed  472  from the bottom of the second chamber  458 , such that a third space S3 is formed under the second screen  468 . 
     The inlet  452  may be configured to provide anode exhaust to the first chamber  456 . In particular, the anode exhaust may be provided to the first space S1, before entering the first catalyst bed  470 . Accordingly, the anode exhaust may be dispersed in the first space S1 and uniformly distributed in the first catalyst bed  470 . The anode exhaust may then pass through the first screen  466  and into the second space S2, where the anode exhaust may be dispersed around the cooling conduits  460 , before entering the second catalyst bed  472 . The cathode exhaust may then flow through the second catalyst bed  472 , the second screen  470 , and into the third space S3, before entering the outlet  454 . 
     The fans  464  may be variable speed fans configured to force air into the fan housing  462  and through the cooling conduits  460 . The fan housing  462  may be triangular in cross-section and may be configured to channel the air output from the relatively large diameter fans  464  into the relatively small diameter the cooling conduits  460 . Air flowing through the cooling conduits  460  may reduce the temperature of the cooling conduits  460 , which may be heated by interaction with the anode exhaust. As such, the cooling conduits  460  may be configured to reduce the temperature of the anode exhaust, by transferring heat to the air flowing there through. In some embodiments, an optional air filter (not shown) may be disposed within the fan housing  462 . 
     In various embodiments, the control unit  474  may be configured to control the speed of the fans  464 , based on the temperature of the anode exhaust flowing through the shift reactor  450  and/or a temperature of the catalyst beds  470 ,  472 . In particular, exothermic oxidation reactions occurring during operation of the shift reactor  450  may increase the temperature of the anode exhaust flowing there through. During steady state operation, the control unit  474  may be configured to operate the fan at a speed sufficient maintain the temperature of the anode exhaust within a desired temperature range, such as a temperature ranging from about 200° C. to about 250° C., such as a temperature ranging from about 210° C. to about 240° C. 
     In particular, the speed of the fans  464  may be controlled based on an anode exhaust flow rate through the shift reactor  450 , with higher fan speeds being utilized at higher anode exhaust flow rates. In other embodiments, the fan speed may be controlled based on a temperature of the shift reactor  450  and/or a temperature of anode exhaust flowing past the cooling conduits  460  and/or output from the shift reactor  450 . For example, the control unit  474  may include, or be operatively connected to, a temperature sensor, such as a thermocouple, configured to detect the anode exhaust temperature. For example, the temperature sensor may be configured to detect the temperature of the anode exhaust adjacent to the cooling conduits  460  and/or adjacent to the top of the second catalyst bed  472 . In some embodiments, the shift reactor  450  may include multiple temperature sensors, in order to provide temperature detection redundancy, for example. 
     In some embodiments, the fans  464  may not be operated if relatively cool anode exhaust is provided to the shift reactor  450 , such as during system startup. However, in some embodiments, the fans  464  may be operated during system startup, in order to prevent excessive heat accumulation due exothermic reactions that may occur due to adsorption of gas species to the catalyst material. In some embodiments, the shift reactor  450  may optionally include a heating element (not shown), such as heating tape disposed on an outer surface of the shift reactor  450 . The heating element may be used, for example, during system startup, in order to heat the shift reactor  450  (e.g., to heat the first and/or second catalyst beds  470 ,  472 ) to a desired operating temperature. 
     In some embodiments, the first screen  466  and/or the cover  451  may be removable, so as to facilitate catalyst loading. For example, the cover  451  and the first screen  466  may be removed, a catalyst material may be filled between the cooling conduits  460  to form the second catalyst bed  472  in the second chamber  458 . The first screen  466  may be installed and secured via tack welding or mechanical fasteners, and then a catalyst material may be filled into the first chamber  456  to form the first catalyst bed  470 . The cover  451  may then be attached. 
     The cooling conduits  460  may be separated by a gap ranging from about 2 to about 7 mm, such as from about 3 to about 5 mm, in order to permit loading of a catalyst material into the second chamber  458 . The cooling conduits  460  may have a triangular pitch or a rectangular pitch, in some embodiments. In various embodiments, the cooling conduits  460  may include heat transfer structures, such as external fins or the like, to increase heat transfer. In various embodiments, the shift reactor  450  may include a single row or cooling conduits  460 , as shown in  FIGS.  4 A and  4 B . In other embodiments, the shift reactor  450  may include multiple rows of cooling conduits  460  in the second space S2. In other embodiments, the shift reactor  450  may include more than two catalyst beds, and at least one row of cooling conduits disposed between each pair of catalyst beds. For example, the shift reactor  450  may include three catalyst beds and two rows of cooling conduits. 
     Methods of SOFC System Operation Including Fuel Exhaust Processing 
       FIG.  5    is a schematic view showing fuel flow through of the SOFC system  10  during a startup mode, according to various embodiments of the present disclosure, wherein fuel flow is shown by dashed lines. Referring to  FIGS.  1  and  5   , during startup, various components of the SOFC system  10  may heated from ambient temperatures to operating temperatures. Accordingly, the SOFC system may be configured to heat components as quickly as possible to operating temperatures. 
     In particular, anode exhaust may be output from the hotbox  100  to the splitter  440  of the fuel exhaust processor  400 , via the second recycling conduit  310 B. However, in some embodiments, the first recycling conduit  310 A may be used to provide the anode exhaust. The splitter  440  may output a first portion of the anode exhaust back to the hotbox  100 , via the first return conduit  406 A. In particular, the first portion of the anode exhaust may be used to maintain a carbon to oxygen ratio in fuel provided to the SOFC stack  102 . 
     The splitter  440  may output a second portion of the anode exhaust to the hotbox  100 , via the second return conduit  406 B. The first portion of the anode exhaust may be provided to the SOFC stack  102 , and the second portion of the anode exhaust may be provided to the ATO  130 . 
     In particular, the system controller  425  may close the first separator conduit valve  401 V 1  and the third return conduit valve  406 V 3 , and may open the second return conduit valve  406 V 2 , in order to direct the second portion of the anode exhaust away from the hydrogen separator  410  and back to the ATO  130 . In other words, the hydrogen separator  410  may not be operated during system startup. 
       FIG.  6    is a schematic view showing fuel flow through of the SOFC system  10  during a low-current steady-state mode, according to various embodiments of the present disclosure, wherein fuel flow is show by dashed lines. Referring to  FIGS.  1  and  6   , if a relatively low current load, for example a current load of less than about 25 amps, is applied to the hotbox  100  and/or stacks  102  included therein, the amount of heat generated in the hotbox may be insufficient for stable power generation, without additional heat being generated by the ATO  130 . Accordingly, the system controller  425  may be configured to provide anode exhaust from the fuel exhaust processor  400  to the ATO  130 . 
     For example, anode exhaust may be output from the hotbox  100  to the splitter  440 , via the first recycling conduit  310 A. A first portion of the anode exhaust may be output from the splitter  440  and returned to the hotbox  100 , via the first return conduit  406 A. 
     A second portion of the anode exhaust is output from the splitter  440  to the shift reactor  450 , via the first separator conduit  401 A. The anode exhaust output from the shift reactor  450  may be provided to the optional heat exchanger  444 , via the second separator conduit  401 B, before being provided to the third return conduit  406 C. In the alternative, the anode exhaust may be provided from the second separator conduit  401 B directly to the third return conduit  406 C. The anode exhaust may then be provided to the hotbox  100  for use in the ATO, via the second return conduit  406 B. 
     Accordingly, during the low-current mode, the hydrogen separator  410  is not provided with anode exhaust and is not operated. In particular, the system controller  425  may close the second separator conduit valve  401 V 2 , the second return conduit valve  406 V 2 , and the first hydrogen conduit valve  404 V 1 , to isolate the hydrogen separator  410 , and may open the first separator conduit valve  401 V 1  and the third return conduit valve  406 V 3 , to provide anode exhaust to the ATO  130 . 
       FIG.  7    is a schematic view showing fuel flow through of the SOFC system  10  during a high-current steady-state mode, according to various embodiments of the present disclosure, wherein fuel flow is show by dashed lines. Referring to  FIGS.  1 ,  2 , and  7   , if a relatively high current load, for example a current load of at least 25 amps, is applied to the hotbox  100  and/or stacks  102  included therein, an amount of heat may be generated in the hotbox  100  may be sufficient or nearly sufficient for stable power generation. As such, the heat output and fuel consumption of the ATO  130  may be minimized. 
     For example, anode exhaust may be output from the hotbox  100  to the splitter  440 , via the first recycling conduit  310 A. A first portion of the anode exhaust may be output from the splitter  440  and returned to the hotbox  100 , via the first return conduit  406 A. 
     A second portion of the anode exhaust is output from the splitter  440  to the shift reactor  450 , via the first separator conduit  401 A. The shift reactor  450  may reduce the carbon monoxide content of the anode exhaust from about 5% by volume to less than about 0.3% by volume. In some embodiments, this further reduction in carbon monoxide content may reduce and/or prevent deactivation of anode catalysts of the hydrogen separator  410 . 
     For example, the shift reactor  450  may be actively cooled during operation, such that the anode exhaust is output from the shift reactor  450  at a temperature of less than about 240° C., such as less than about 220° C., or less than about 200° C., which may be sufficient to prevent damage to the membranes of the hydrogen pumping cells  420 . Accordingly, when the actively cooled shift reactor  450  is used, the heat exchanger  444  may be omitted in some embodiments. 
     However, in other embodiments, the heat exchanger  444  may be used to further reduce the temperature of the anode exhaust output from the shift reactor  450 . For example, the heat exchanger  444  may reduce the temperature of the anode exhaust to about 100° C. or less, such as to a temperature of from about 80° C. to about 50° C., from about 75° C. to about 55° C., or to about 65° C. The heat exchanger  444  may cool the anode exhaust by transferring heat to ambient air. 
     The anode exhaust may be output from the shift reactor  450  and/or the heat exchanger  444  to the anode inlet of the first hydrogen pump  414 A, via the second separator conduit  401 B. In particular, the system controller  425  may close the second return conduit valve  406 V 2  and the third return conduit valve  406 V 3 , and may open the first and second separator conduit valves  401 V 1 ,  401 V 2 , such that the second portion of the anode exhaust is provided only to the fuel exhaust processor  400 . 
     The anode exhaust may be distributed to the anodes of each hydrogen pumping cell  420  in the first hydrogen pump  414 A. Power may be provided to the hydrogen pumping cells  420  to separate hydrogen from the anode exhaust. The evolved hydrogen may be output from the cathode outlet of the first hydrogen pump  414 A to the first return conduit  406 A, via the first hydrogen conduit  404 A. The remaining anode exhaust may be output from the anode outlet of the first hydrogen pump  414 A to the anode inlet of the second hydrogen pump  414 B, via the first exhaust conduit  402 A. 
     Power may be applied to the hydrogen pumping cells  420  of the second hydrogen pump  414 B to separate hydrogen from the anode exhaust flowing therethrough. The separated hydrogen may be output from the cathode outlet of the second hydrogen pump  414 B to the first hydrogen conduit  404 A, via the second hydrogen conduit  404 B. The remaining anode exhaust may be output from the anode outlet of the second hydrogen pump  414 B to the anode inlet of the third hydrogen pump  414 C, via the second exhaust conduit  402 B. 
     Power may be applied to the hydrogen pumping cells  420  of the third hydrogen pump  414 C to separate hydrogen from the anode exhaust flowing there through. The separated hydrogen may be output from the cathode outlet of the third hydrogen pump  414 C to the second return conduit  406 B, via the third hydrogen conduit  404 C. The remaining anode exhaust may be output from the anode outlet of the third hydrogen pump  414 C to the third hydrogen conduit  404 C. In some embodiments, the remaining anode exhaust may comprise at least 95%, such as at least 97% or at least 98% by volume carbon dioxide. Accordingly, a purified carbon dioxide stream may be output from the fuel exhaust processor  400  and stored in the carbon dioxide storage device  50 . The stored carbon dioxide may be provided to carbon dioxide consumers, such as the beverage industry, in order to recycle the carbon dioxide and provide supplemental income. 
     A first hydrogen stream (e.g., hydrogen-enriched fuel stream), including the hydrogen output from the first hydrogen pump  414 A and the second hydrogen pump  414 B, may be output to the hotbox  100  via the first hydrogen conduit  404 A, the second hydrogen conduit  404 B, and the first return conduit  406 A. In particular, the first hydrogen stream may be provided to the mixer  210  and recycled for use in to the fuel cell stack  102 . 
     In the alternative, all or a portion of the first hydrogen stream may be provided to the hydrogen storage via the hydrogen storage conduit  56  and stored in the hydrogen storage device  54 . In some embodiments, the hydrogen storage valve  56 V may be used to control how much of the first hydrogen stream is stored in the hydrogen storage device  54  and how much is provided to the SOFC system  10 . 
     A second hydrogen stream (e.g., hydrogen-enriched fuel stream), including the hydrogen output from the third hydrogen pump  414 C via the third hydrogen conduit  404 C, may be provided to the hotbox  100  via the third hydrogen conduit  404 C and the second return conduit  406 B. In particular, the second hydrogen stream may be provided to the ATO  130  for oxidation by the second return conduit  406 B. 
     A high-purity carbon dioxide stream may be output from the hydrogen separator  410  to the carbon dioxide storage device  50 , via the carbon dioxide storage conduit  52 . For example, the carbon dioxide stream may be at least 98 volume percent carbon dioxide. In some instances, the purity of the carbon dioxide stream may be less than 98 volume percent if that is compatible with the downstream use, such as greater than 95 volume percent. 
       FIG.  8    is a schematic view showing fuel flow through of the SOFC system  10  during an alternative steady-state mode, according to various embodiments of the present disclosure, wherein fuel flow is show by dashed lines. The alternative high-current steady-state mode may be similar to the high-current steady-state mode of  FIG.  7   . As such, only the difference there between will be discussed in detail. 
     Referring to  FIGS.  1 ,  2 , and  8   , it has been determined that by providing hydrogen rich recycled fuel to the hotbox  100 , fuel reformation reactions may be correspondingly reduced, thereby reducing heat loss within the SOFC system  10 . As a result, fuel cell stack operating temperatures may be maintained during steady-state operation, without providing fuel to the ATO  130 . 
     Accordingly, the system controller  425  may be configured to close the third hydrogen conduit valve  404 V 3  and open the fourth hydrogen conduit valve  404 V 4 , such that hydrogen extracted by the third hydrogen pump  414 C is diverted into the fourth hydrogen conduit  404 D and provided to the first return conduit  406 A, via the first hydrogen conduit  404 A. In other words, the hydrogen output from the third hydrogen pump  414 C may be added to the first hydrogen stream provided to the fuel cell stack  102 . In other words, a second hydrogen stream may not be provided to the ATO  130 . 
     In some embodiments, the SOFC system  10  may be operated in a hydrogen generation mode to maximize hydrogen extraction and/or storage. In particular, the system controller  225  may be configured to increase hydrogen extraction by the fuel exhaust processor  400 , by decreasing the fuel utilization rate of the SOFC stack  102  (e.g., a ratio of current drawn from the SOFC stack  102  to a flow rate of fresh fuel supplied to the SOFC stack  102  from the fuel inlet  300 ). 
     For example, the fuel utilization rate may be decreased by decreasing the current drawn from the SOFC stack  102  and/or by increasing the flow rate of fresh fuel to SOFC stack  102 , such that the anode exhaust output to the fuel exhaust processor  400  has a higher hydrogen content. The higher hydrogen content of the anode exhaust may increase the amount of hydrogen extracted by the hydrogen separator  410 . Thus, the hydrogen generation mode may include reducing a fuel utilization rate to increase hydrogen extraction. 
     In some embodiments, the hydrogen generation mode may utilize the hydrogen flow configuration shown in  FIG.  7   . In particular, hydrogen generated by the third hydrogen pump  414 C may be provided to the ATO. Since the amount of hydrogen extracted by the third hydrogen pump  414 C may be relatively low, as compared to the amounts of hydrogen extracted by the first and second hydrogen pumps  414 A,  414 B, the rate of hydrogen generation and storage may not be significantly decreased. 
     In the various embodiments, three hydrogen pumps  414 A,  414 B,  414 C are shown, but the embodiments are not so limited. In some embodiments, varying numbers of hydrogen pumps may be used. For example, two hydrogen pumps may be used in some configurations. In the two hydrogen pump configuration, the first hydrogen pump and the second hydrogen pump output hydrogen to one or more components of the hotbox as fuel. Any of the hydrogen pumps may be configured to send the hydrogen stream to ATO  130  or back to hotbox  100  as fuel. Independent of the number of hydrogen pumps used, it is preferred to provide the output of the hydrogen pumps back to hotbox  100  as fuel, as this is thermally stable under most conditions. If thermal stability (e.g. due to low current SOFC operation) is not achievable, then the hydrogen from any of the hydrogen pumps may be output to ATO  130  either steadily, or for periodic timed durations (e.g. 1 minute out of 5, etc). 
     The preceding description of the disclosed aspects is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these aspects will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other aspects without departing from the scope of the invention. Thus, the present invention is not intended to be limited to the aspects shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.