Patent Publication Number: US-2023155144-A1

Title: Fuel cell system including anode exhaust diversion and method of operating the same

Description:
FIELD 
     Aspects of the present invention relate to fuel cell systems and methods, and more particularly, to fuel cell systems including anode exhaust diversion components. 
     BACKGROUND 
     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 
     According to various embodiments, a fuel cell system includes a stack of fuel cells, an anode tail gas oxidizer (ATO) configured to oxidize anode exhaust output from the stack, an exhaust conduit configured to output the anode exhaust from the fuel cell system, a bypass conduit configured to divert anode exhaust output from the stack to the exhaust conduit, a bypass valve configured to control anode exhaust flow through the bypass conduit, and an ATO conduit configured to provide anode exhaust to the ATO. 
     According to various embodiments, a method of operating a fuel cell system includes providing fuel and air to a stack of fuel cells located in a hotbox, operating the stack to generate an anode exhaust and a cathode exhaust, in a startup mode, providing a first amount of the anode exhaust and the cathode exhaust to an anode tail gas oxidizer (ATO) located in the hotbox to oxidize the anode exhaust and to generate heat which is provided to the stack, and in a steady-state mode, stopping providing the anode exhaust to the ATO or providing to the ATO a second amount of the anode exhaust which is smaller than the first amount, and providing the anode exhaust and the cathode exhaust outside the hotbox. 
     According to various embodiments, a method of operating a fuel cell system includes providing fuel and air to a stack of fuel cells located in a hotbox, operating the stack to generate an anode exhaust and a cathode exhaust, in a low current draw steady-state mode in which insufficient current is drawn from the stack to sustain a predetermined steady-state stack operating temperature, providing a first amount of the anode exhaust and the cathode exhaust to an anode tail gas oxidizer (ATO) located in the hotbox to oxidize the anode exhaust and to generate heat which is provided to the stack, and in a regular steady-state mode in which sufficient current is drawn from the stack to sustain the predetermined steady-state stack operating temperature, stopping providing the anode exhaust to the ATO or providing to the ATO a second amount of the anode exhaust which is smaller than the first amount, and providing the anode exhaust and the cathode exhaust outside 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, together with the general description given above and the detailed description given below. 
         FIG.  1    is a schematic of a solid oxide fuel cell (SOFC) system, according to a first embodiment of the present disclosure. 
         FIG.  2    is a schematic of a SOFC system, according to a second embodiment of the present disclosure. 
         FIGS.  3    is a schematic view of a SOFC system, according to a third embodiment of the present disclosure. 
         FIG.  4    is a cross-sectional view of a portion of a central column  101  that may be included in the SOFC systems of  FIGS.  1 ,  2   , and/or  3 . 
         FIG.  5    is a schematic view of a combined heat and power (CHP) system connected to a fuel cell system, according to various embodiments of the present disclosure. 
         FIG.  6    is a flow diagram illustrating a method of using a SOFC system, according to various embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     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 relatively 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. 
       FIG.  1    is a schematic representation of a SOFC system  10 , according to a first embodiment 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 , an optional splitter  170 , 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., 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  202  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 the fuel exhaust and the fuel then flows from the anode recuperator  110  to the stack  102  through fuel conduit  300 D. 
     The system 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 stack  102  through air conduit  302 C. 
     An anode exhaust (e.g., fuel exhaust stream) generated in the stack  102  is provided to the anode recuperator  110  through an anode exhaust conduit  308 . 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 the mixer  210  by a recycling conduit  310 , which may include a first recycling conduit  310 A and a second recycling conduit  310 B. In particular, the first recycling conduit  310 A may fluidly connect an outlet of the anode recuperator  110  to an inlet of the anode exhaust cooler  140 . The second recycling conduit  310 B may fluidly connect an outlet of the anode exhaust cooler  140  to an inlet of the mixer  210 . 
     Water flows from a water source  206 , such as a water tank or a water pipe, to the water injector  160  through a water conduit  306 . The water injector  160  may be configured to inject water into anode exhaust flowing through the first recycling conduit  310 A. Heat from the anode exhaust (also referred to as a recycled anode exhaust stream) vaporizes the water to generate steam which humidifies the anode exhaust. The humidified anode exhaust is provided to the anode exhaust cooler  140 . Heat from the anode exhaust provided to the anode exhaust cooler  140  may be transferred to the air inlet stream provided from the system blower  208  to the cathode recuperator  120 . The cooled humidified anode exhaust may then be provided from the anode exhaust cooler  140  to the mixer  210  via the second recycling conduit  310 B. The anode recycle blower  212  may be configured to move the anode exhaust through the second recycling conduit  310 B. 
     The mixer  210  is configured to mix the humidified anode exhaust with fresh fuel (i.e., fuel inlet stream). This humidified fuel mixture may then be heated in the anode recuperator  110  by the anode exhaust, before being provided to the 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) reform the humidified fuel mixture before it is provided to the stack  102 . 
     The splitter  170  may be operatively connected to the first recycling conduit  310 A and may be configured to divert a portion of the anode exhaust to the ATO  130  via an ATO conduit  312 A. The ATO conduit  312 A may be fluidly connected to the cathode exhaust conduit  304 A or the ATO  130 . 
     Cathode exhaust generated in the stack  102  is provided to the ATO  130  by cathode exhaust conduit  304 A. The cathode exhaust may be mixed with the anode exhaust before or after being provided to the ATO  130 . The mixture of the anode exhaust and the cathode exhaust may be oxidized in the ATO  130  to generate an ATO exhaust. The ATO exhaust flows from the ATO  130  to the cathode recuperator  120 , through cathode exhaust conduit  304 B. Exhaust flows from the cathode recuperator  120  and out of the hotbox  100  through cathode exhaust conduit  304 C. 
     The system  10  may further include a system controller  225  configured to control various elements of the system  10 . The controller  225  may include a central processing unit configured to execute stored instructions. For example, the controller  225  may be configured to control fuel and/or air flow through the system  10 , according to fuel composition data. 
     The present inventors have determined that during steady-state operations when a sufficient electrical load is applied to a fuel cell stack, ATO heat generation may not be required to maintain the desired SOFC stack  102  steady-stage operating temperature (e.g., a temperature above 700° C., such as 750 to 900° C.). However, in conventional systems, the ATO may still be operated during steady-state mode in order to oxidize carbon monoxide present in the anode exhaust provided thereto. However, this oxidation may result in the release of a significant amount of heat that may not be necessary for steady-state operations. 
     Conventionally, the flow rate of the system air flow (i.e., the air inlet stream) provided by the system blower  208  may be increased in order to compensate for such ATO heat release. However, as the air flow rate is increased, the temperature range of the fuel cells in the stack may also increase. For example, higher air flow rates may result in certain fuel cells operating at temperatures that are less than optimal, which may reduce overall power production. In addition, higher air flow rates may also result in a higher pressure drop along the air flow path. Both a higher air flow rate and a higher pressure drop may lead to higher overall system balance of plant power consumption and lower overall system efficiency. 
     In view of the above and/or other problems of conventional systems, in some embodiments of the present disclosure, at least during steady-state operation of the system, the anode exhaust may be mixed with the cathode exhaust leaving the hotbox  100 , or with an alternate fresh air stream, instead of being provided to the ATO. In some embodiments, a catalyst is utilized to support oxidation of the carbon monoxide, hydrocarbon fuel and/or hydrogen which remains in the anode exhaust. In alternative embodiment systems which exclusively use hydrogen fuel, the anode exhaust may be vented directly from the hotbox  100  if desired. For combined heat and power (CHP) configurations, then anode exhaust may be vented into a cathode exhaust conduit, upstream of a CHP heat exchanger, to create heat for recovery in the CHP heat exchanger. 
     Referring again to  FIG.  1   , the SOFC system  10  may include a bypass conduit  316 , a bypass valve  320 , and an exhaust oxidizer  330 . The bypass conduit  316  fluidly connects the second recycling conduit  310 B to the cathode exhaust conduit  304 C. In some embodiments, the bypass conduit  316  may be connected to the second recycling conduit  310 B downstream of the anode recycle blower  212 , with respect to an anode exhaust flow direction through the second recycling conduit  310 B, in order to provide additional exhaust flow pressure to the cathode exhaust conduit  304 C. However, in other embodiments, the bypass conduit  316  may be fluidly connected to the second recycling conduit  310 B, upstream of the anode recycle blower  212 . 
     The exhaust oxidizer  330  may be configured to oxidize ATO exhaust output from the ATO  130  and/or anode exhaust output from the bypass conduit  316 . For example, the exhaust oxidizer  330  may comprise a tube or conduit containing a catalyst that promotes oxidization of carbon monoxide and/or hydrogen into carbon dioxide and/or water, respectively. In one embodiment, the exhaust oxidizer  330  is located outside the hotbox  100 . Thus, the heat generated by the exhaust oxidizer  330  is not used to heat the SOFC stack  102 . 
     The ATO conduit  312 A provides anode exhaust (e.g., an ATO fuel stream) output from the splitter  170  to the ATO  130 . Anode exhaust flow from the splitter  170  through the ATO conduit  312 A may be controlled by controlling the speed of the anode recycle blower  212 . For example, higher anode recycle blower  212  speeds may result in lower anode exhaust flow to the ATO  130 , while lower anode recycle blower  212  speeds may result in higher anode exhaust flow to the ATO  130 . In some embodiments, the speed of the anode recycle blower  212  may be limited, in order to prevent backflow of cathode exhaust through the ATO conduit  312 A and into the splitter  170 . 
     The bypass valve  320  may be located outside of the hotbox  100 , in order to prevent damage to the bypass valve  320  due to exposure to high temperatures inside of the hotbox  100 . The bypass valve  320  may be configured to control the anode exhaust flow through the bypass conduit  316 , from the anode exhaust conduit  310 . In particular, during system  10  startup, the bypass valve  320  may be closed such that the anode exhaust is not provided to the exhaust oxidizer  330  and the cathode exhaust conduit  304 C, and the—anode recycle blower  212  may be operated at a speed that does not pull all the anode exhaust out of the hotbox  100 , such that the ATO fuel stream is provided to the ATO  130 . Thus, the ATO  130  operates during the startup mode of the system  10  to oxidize the anode exhaust using the cathode exhaust, and to generate heat. The ATO heat is used to increase the temperature of the stack  102  during the startup mode before the stack  102  reaches it steady-state operating temperature (e.g., a temperature above 700° C., such as 750 to 900° C.). 
     In contrast, during the steady-state operation after the stack  102  reaches it steady-state operating temperature (e.g., a temperature above 700° C., such as 750 to 900° C.), the bypass valve  320  may be open, such that at least a portion of anode exhaust is provided to cathode exhaust conduit  304 C and the exhaust oxidizer  330  via the bypass conduit  316 , and the anode recycle blower  212  speed/flow rate may be increased, to minimize the amount of anode exhaust provided to the ATO  130  via conduit  312 A, while preventing the backflow of cathode exhaust. 
     Therefore, if sufficient current is drawn from the stack  102  (i.e., if the stack outputs electrical power above a threshold value specific to the stack  102 ), then the stack  102  generates sufficient heat during the steady-state mode to sustain a desired steady-state operating temperature, and the stack  102  does not require ATO heat. In some embodiments, the anode exhaust is provided to the ATO  130  during the start-up mode and/or during a low current draw steady-state mode when insufficient current is drawn from the stack  102  to sustain the desired steady-state operating temperature. 
     For example, if the system controller  225  detects that the current drawn from the stack  102  falls below a predetermined threshold current value required to maintain the desired steady-state operating temperature (i.e., the stack operating temperature value above a threshold temperature value (e.g., a temperature above 700° C., such as 750 to 900° C.)), then a portion of the anode exhaust is provided to the ATO  130  to generate heat in the ATO  130 . The ATO heat is provided to the stack  102  to maintain the stack above the threshold temperature value. The threshold current value depends on the stack size, fuel cell composition, fuel composition provided to the stack, cumulative level of stack degradation, etc. In one embodiment, the threshold current value may comprise 10 to 30 Amps, such as 20 to 25 Amps. The anode exhaust may or may not be provided to the exhaust oxidizer  330  during the low current draw steady-state mode. 
     Once the system controller  225  detects that sufficient current is drawn on the stack  102  is equal to or above the threshold current value needed to maintain the stack above the threshold temperature value, then the system  10  exits the low current draw steady-state mode and enters a regular steady-state mode. In the regular steady-state mode, the flow rate of anode exhaust through the anode recycle blower  212  is increased and a minimal amount of anode exhaust is provided to the ATO  130 . For example, the anode recycle blower  212  may be operated at a relatively high speed, such that a majority of the anode exhaust is pulled from the hotbox  100  and provided to the exhaust oxidizer  330  and none of or only a small minority of (e.g., less than 20 volume percent, such as 1 to 10 volume percent) the anode exhaust is provided to the ATO  130 , during the regular steady-state mode. 
     In one embodiment, the bypass valve  320  may be a proportionate valve configured to control the anode exhaust flow rate through the bypass conduit  316 . In some embodiments, the system controller  225  may be configured to gradually open the bypass valve  320  and gradually increase the speed of the anode recycle blower  212 , during transition from startup to steady-state operation, and/or during steady-state operation. In addition, the system controller  225  may be configured to gradually reduce the speed of the system blower  208 , in order to compensate for a reduction in the heat output of the ATO  130 . 
     The exhaust oxidizer  330  may be disposed outside of the hotbox  100 , such that heat generated by the oxidation reactions thereby does not unnecessarily heat system components. As a result, total system airflow may be reduced. In particular, the system air blower  208  may be operated at a lower speed, as compared to if the anode exhaust was provided to the ATO  130  during steady-state operation. In other words, the steady-state power consumption of the system blower  208  may be significantly reduced. In addition, cell-to-cell temperature variations may be reduced, which may increase cell voltage and efficiency. 
     In some embodiments, the system  10  may optionally include a cabinet air blower  209  configured to provide cabinet air to cathode exhaust conduit  304 C or the system exhaust conduit  332 . In particular, the system  10  may be disposed in a cabinet, and the cabinet air blower  209  may be configured to provide cabinet air to cool the exhaust output from the system  10 , in embodiments where cooler system exhaust is needed. 
     In various other embodiments, the system  10  may include a system exhaust conduit  332  configured to provide exhaust output from the exhaust oxidizer  330  to a combined heat and power (CHP) system  400 , as discussed below with regard to  FIG.  5   . In one embodiment shown in  FIG.  1   , additional air may be provided by the cabinet air blower  209  to the cathode exhaust conduit  304 C in order to reduce exhaust temperatures. 
     In some embodiments, some anode exhaust may be provided to the ATO  130  during startup and steady-state operating modes. In other embodiments shown in  FIGS.  2  and  3   , the splitter  170  and the ATO conduit  312 A may be omitted. In these embodiments, no anode exhaust is directly provided to the ATO  130  from within the hotbox  100 , during any operating mode. 
       FIG.  2    is a schematic representation of a SOFC system  12 , according to a second embodiment of the present disclosure. The SOFC system  12  is similar to the SOFC system  10  of  FIG.  1   . Accordingly, only the differences there between will be discussed in detail. 
     Referring to  FIG.  2   , SOFC system  12  may include a bypass conduit  316 A, an ATO conduit  312 B, and an ATO valve  324 . The bypass conduit  316 A may fluidly connect the second recycling conduit  310 B to the cathode exhaust conduit  304 C upstream of the anode recycle blower  212 . However, in other embodiments, the bypass conduit  316 A may be connected to the second recycling conduit  310 B downstream of the anode recycle blower  212 , if higher a higher anode exhaust flow rate is needed. The ATO conduit  312 B may fluidly connect the bypass conduit  316  to the ATO  130 . 
     The bypass valve  320  may be configured to control the anode exhaust flow through the bypass conduit  316 A, and the ATO valve  324  may be configured to control the anode exhaust (e.g., an ATO fuel stream) flow through the ATO conduit  312 B to the ATO  130 . In some embodiments, the valves  320  and  324  may be proportionate valves configured to provide various flow rates through their respective conduits  316 A and  312 B. In particular, during system startup, the bypass valve  320  may be closed and the ATO valve  324  may be opened by the system controller  225 , such that anode exhaust is provided to the ATO  130  and no anode exhaust is provided to the cathode exhaust conduit  304 C and the exhaust oxidizer  330 . 
     During steady-state operation, the bypass valve  320  may be opened and the ATO valve  324  may be closed by the system controller  225 , such that some of the anode exhaust in the second recycling conduit  310 B is diverted into the cathode exhaust conduit  304 C and provided to the exhaust oxidizer  330 , and the anode exhaust is not provided to the ATO  130  via the ATO conduit  312 B. 
     In some embodiments, the system controller  225  may be configured to gradually open the bypass valve  320  and gradually close the ATO valve  324 , during transition from startup to steady-state operation, and/or during steady-state operation. In addition, the system controller  225  may be configured to gradually reduce the speed of the system blower  208 , in order to compensate for a reduction in the heat output of the ATO  130 , as the ATO fuel stream is reduced and/or after the ATO fuel stream is stopped. 
       FIG.  3    is a schematic representation of a SOFC system  14 , according to a third embodiment of the present disclosure. The SOFC system  14  is similar to the SOFC system  12  of  FIG.  2   . Accordingly, only the differences there between will be discussed in detail. 
     Referring to  FIG.  3   , the SOFC system  14  may include a bypass conduit  316 B that fluidly connects the first recycling conduit  310 A to the cathode exhaust conduit  304 C. In other words, the bypass conduit  316 B may be configured to the divert anode exhaust flowing from the anode recuperator  110  to the anode exhaust cooler  140 , to the cathode exhaust conduit  304 C, such that a portion of the anode exhaust may be diverted to the exhaust oxidizer  330  upstream of the anode exhaust cooler  140 . The optional ATO conduit  312 B fluidly connects the bypass conduit  316 B to the ATO  130 . 
     The bypass valve  320  may be configured to control anode exhaust flow through the bypass conduit  316 B, and the ATO valve  324  may be configured to control anode exhaust flow through the ATO conduit  312 B to the ATO  130 . In particular, during system startup, the bypass valve  320  may be closed and the ATO valve  324  may be opened by the system controller  225 , such that anode exhaust is provided to the ATO  130  and anode exhaust is not provided to the cathode exhaust conduit  304 C and the exhaust oxidizer  330 . During steady-state operation, the bypass valve  320  may be opened and the ATO valve  324  may be closed by the system controller  225 , such that anode exhaust is provided to the exhaust oxidizer  330  and anode exhaust is not provided to the ATO  130 . 
     In some embodiments, the system controller  225  may be configured to gradually open the bypass valve  320  and gradually close the ATO valve  324 , during transition from startup to steady-state operation, and/or during steady-state operation. In addition, the system controller  225  may be configured to gradually reduce the speed of the system blower  208 , in order to compensate for a reduction in the heat output of the ATO  130 , as the ATO fuel stream is reduced and/or after the ATO fuel stream is stopped. 
       FIG.  4    is a cross-sectional view of a portion of a central column  101  that may be included in the SOFC systems  10 ,  12 , and/or  14 . Referring to  FIGS.  1 - 4   , the central column  101  may include the anode recuperator  110 , the ATO  130 , and the anode exhaust cooler  140 . The anode recuperator  110  is located in the core of the central column  101 . The ATO  130  may comprise a toroidal manifold containing an ATO catalyst which surrounds the anode recuperator  110 . The anode exhaust cooler  140  may be located above anode recuperator  110  and the ATO  130 . The fuel cell stacks  102  may surround the ATO  130 , and the cathode recuperator  120  may surround the fuel cell stacks  102 . 
     An ATO conduit  312 , which may be any of the ATO conduits  312 A,  312 B,  312 C described above, may be divided into two or more column conduits (e.g., two to six pipes of manifolds)  314  that extend from the top of the central column  101  to an ATO injector  172 . The ATO injector  172  may comprise an annular space having an inner radial sidewall  174  connected to the outlets of the column conduits  314 , and an outer sidewall  176  which protrudes into the top of the ATO  130 . In one embodiment, the outer sidewall  176  may have a curved vertical profile with a middle section protruding radially outward from the tapered top and bottom sections. The outer sidewall  176  contains one or more openings  178 , such as slits located in the middle section. The openings  178  fluidly connect the interior space of ATO injector  172  to the interior space of the ATO  130 . In particular, the column conduits  314  and may be configured to provide multiple anode exhaust streams from the ATO conduit  312  to the ATO  130  through the openings  178  in the ATO injector  172 , as shown by the dashed arrows in  FIG.  4   . The radial separation of the anode exhaust stream into multiple anode exhaust streams in the ATO injector improves radial flow and mixing of the anode exhaust in the ATO  130 . 
     Referring again to  FIGS.  1 - 3   , according to the first through third embodiments, the exhaust oxidizer  330  may be omitted from the SOFC systems  10 ,  12 ,  14 , if the systems are operated using hydrogen as a fuel source. In one embodiment shown in  FIG.  1   , additional air may be provided by the cabinet air blower  209  to the cathode exhaust conduit  304 C in order to reduce exhaust temperatures if the exhaust oxidizer  330  is omitted. 
     In various embodiments, the exhaust oxidizer  330  may include an oxidation catalyst as described in U.S. Provisional Application No. 63/220,659, filed on Jul. 12, 2021, and which is incorporated herein by reference in its entirety. For example, the oxidation catalyst may include a D-block metal, such as gold (Au) and/or one or more platinum group metals such as, platinum (Pt), palladium (Pd), rhodium (Rh), iridium (Jr), osmium (Os), ruthenium (Ru), or a combination thereof. In some embodiments, Au, Pt, Pd, and Rh may exhibit the highest catalytic activity. In some embodiments, the oxidation catalyst may include Au and/or a platinum group metal stabilized with another metal, such as manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), and/or copper (Cu). 
     In various embodiments, the anode exhaust may be diverted from an anode exhaust stream flowing from the anode recuperator  110  to the anode exhaust cooler  140 , or from the anode exhaust stream output from the anode exhaust cooler  140 , and provided to the exhaust oxidizer  330  disposed outside of the hotbox  100 . Anode exhaust flow to the ATO  130  may be reduced and/or cut off during steady-state operations to reduce ATO  130  heat generation, and system air flow may be correspondingly reduced. As such, overall system temperature uniformity and performance may be increased. 
     In some embodiments, the SOFC systems  10 ,  12 ,  14  may be operated (e.g., in the steady-state mode) under low current load conditions or in response to a transient in the current load conditions, such that additional heat may be needed to maintain stack operating temperature. In such conditions, the anode exhaust may be periodically supplied to the ATO  130  to periodically increase the temperature in the hotbox  100 . For example, the anode exhaust may be supplied to the ATO on a schedule based on an amount of additional heating needed to maintain the stack operating temperature, such as for 5-20 seconds per minute, 30 seconds to three minutes every ten minutes, five to fifteen minutes an hour, etc. In these embodiments, valve  320  may be periodically closed and/or valve  324  (if present) may be periodically opened during steady-state mode to provide a portion of the anode exhaust to the ATO  130 . After the anode exhaust has been provided to the ATO via conduit  312 A or  312 B, the valve  320  may be opened and/or valve  324  (if present) may be closed. These steps of opening and closing the valve(s)  320  and/or  324  may be repeated periodically. Alternatively or in addition, the speed/flow rate of the anode recycle blower  212  may be varied periodically to control the amount of anode exhaust provided to the ATO  130 . For example, the speed may be decreased for a first time period to provide or increase the amount of the anode exhaust provided to the ATO  130 . The speed may then be increased for a second time period to stop or decrease the amount of the anode exhaust flowing to the ATO  130 . The above steps may be repeated periodically in response to low output current or current transient. The low output current or current transient may be detected by measuring the current output by the stack and/or by measuring the temperature of the stack using a thermocouple or another suitable temperature sensor. 
       FIG.  5    is a schematic diagram of a CHP system  400 , according to various embodiments of the present disclosure. Referring to  FIG.  5   , the CHP system  400  may include a heat exchanger  410 , a boiler  420 , a steam turbine  422 , and a generator  424 . The boiler  420  provides steam to operate the turbine  422 , and the turbine  422  spins the generator  424  to generate electricity. 
     The heat exchanger  410  may be configured to receive water from a water inlet conduit  412  and a hot exhaust stream output from an exhaust oxidizer  330  via the conduit  332  of the SOFC system  10 ,  12  or  14 , as described above. In particular, the heat exchanger  410  may be configured to extract heat from the exhaust stream, in order to heat the water provided from the water inlet conduit  412  to generate steam and/or hot water. The heated water (or a mixture of heated water and steam) may be provided to the boiler  420 . Alternatively, a completely vaporized (and possibly superheated) steam stream may be provided to the turbine  422 . The hot exhaust stream is cooled in the heat exchanger  410  and is exhausted from the heat exchanger  410  through an outlet conduit  414 . Accordingly, the heat exchanger  410  may reduce the fuel consumption of the boiler  420  for a given amount of power output from the generator  424 , by utilizing heat output from an SOFC system. 
     Additional water and fuel may be provided to the boiler  420  to boil the water in the boiler and to provide steam from the boiler  420  to the steam turbine  422 . The steam and/or hot water may be provided from the steam turbine  422  to a cooling or heating system  430  of a structure  432  (e.g., a building or facility, such as a factory). The electricity generated by the generator  424  is provided to the structure  432  and/or to the power grid  434 . In some embodiments, a steam superheater  421  may be fluidly connected between the boiler  420  and steam turbine  422 . The steam superheater  421  may be configured to superheat the steam output from the boiler  420 , by extracting heat from combustion exhaust generated by combusting the boiler fuel. 
       FIG.  6    is a flow chart illustrating steps of a method of operating a fuel cell system, according to various embodiments of the present disclosure. The method is described with respect to a generic SOFC system, which may include components as described with respect to any of the fuel cell system  10 ,  12 ,  14  disclosed herein. 
     Referring to  FIGS.  1 - 6   , in step  502 , the SOFC system is operated in a startup mode. In particular, fuel may be provided to the stack  102  from the fuel inlet  300 , air may be provided to the stack by the system air blower  208 . In addition, anode and cathode exhaust generated by the stack  102  may be provided to the ATO  130  for oxidation, which may provide heat to increase the temperature of the stack  102 . 
     For example, the speed of the anode recycle blower  212  may be decreased and/or the ATO valve  324  may be opened to provide anode exhaust to the ATO  130  via the respective ATO conduit  312 A or  312 B. In addition, the bypass valve  320  may be closed, such that anode exhaust does not flow through the bypass conduit  316 . In some embodiments, the CPOx reactor  200  may be operated to partially reform the fuel during the startup mode. 
     Once the stack  102  reaches a set steady-state operating temperature, the SOFC system may transition to the steady-state mode. In particular, in step  504  the anode exhaust flow to the ATO  130  may be stopped or reduced. For example, the speed of the anode recycle blower  212  may be increased and/or the ATO valve  324  may be closed to stop anode exhaust flow through the respective ATO conduit  312 A or  312 B to the ATO  130 . 
     In step  506 , a portion of the anode exhaust generated by the stack  102  may be diverted to the exhaust oxidizer  330 , where a mixture of anode exhaust and cathode exhaust is reacted to oxidize carbon monoxide and/or hydrogen. For example, the bypass valve  320  may be opened to divert a portion of the anode exhaust to the exhaust oxidizer  330 . For example, the anode exhaust may be diverted from either conduit  310 A or  310 B upstream or downstream of the anode exhaust cooler  140 . 
     Anode exhaust may be diverted to the exhaust oxidizer  330  by closing the ATO valve  324 , and a portion of the anode exhaust generated by the stack  102  may be diverted to the exhaust oxidizer  330 , in order to oxidize the cathode exhaust. For example, the bypass valve  320  may be opened, such that a portion of the anode exhaust is diverted from conduit  310 A or  310 B to the exhaust oxidizer  330  via the bypass conduit  316 ,  316 A, or  316 B. In various embodiments, a flow rate of the anode exhaust to the exhaust oxidizer  330  during steady-state operation may be less than an anode exhaust flow rate to the ATO  130 , during startup operation. 
     In step  508 , the air flow rate of the system may be optionally adjusted based on a change in the temperature of the SOFC system. For example, the speed of the system air blower  208  may be adjusted, based on the temperature of the stack  102 . In particular, if the temperature of the stack  102  is reduced due to a reduction in the heat output of the ATO  130 , then the air flow rate of the system air blower  208  may be correspondingly reduced. 
     In some embodiments, in step  510 , the method may optionally include providing exhaust output from the exhaust oxidizer  330  to the combined heat and power (CHP) system  400 . For example, the exhaust may be provided to the heat exchanger  410  to generate steam and/or hot water. 
     In some embodiments, the method may include operating the SOFC system in a low current draw steady-state mode. For example, if a current load applied to the stack is insufficient to sustain a predetermined steady-state stack operating temperature, step  504  may include periodically providing anode exhaust to the ATO  130  to generate heat that is supplied to the stack  102 , such that the temperature of the stack  102  may be maintained within a selected operating temperature range. For example, the anode exhaust may be periodically provided to the ATO, where it is mixed with the cathode exhaust, which may be continuously provided to the ATO  130 . For example, the anode exhaust may be provided according to schedule, which may be based on an amount of heating needed to maintain the stack  102  at a particular operating temperature or temperature range. For example, the anode exhaust may be provided for X minutes during a time period of Y, wherein X may range from about 10 seconds to about five minutes, such as from about 30 seconds to about 3 minutes, and Y may range from about 5 minutes to about one hour, such as from about 10 minutes to about 30 minutes. 
     Thus, in the start-up mode and/or in the low current draw steady-state mode in which insufficient current is drawn from the stack to sustain a predetermined steady-state stack operating temperature, a first amount (e.g., a first volume or flow rate) of the anode exhaust and the cathode exhaust are provided to the ATO  130  located in the hotbox  100  to oxidize the anode exhaust and to generate heat which is provided to the stack  120 . In contrast, in a regular steady-state mode in which sufficient current is drawn from the stack to sustain the predetermined steady-state stack operating temperature, the anode exhaust flow to the ATO is stopped, or a second amount (e.g., a second volume or flow rate) of the anode exhaust which is smaller than the first amount is provided to the ATO  130 , while another portion (i.e., amount) of the anode exhaust and the cathode exhaust are provided to the exhaust oxidizer  330  located outside the hotbox  100 . 
     Fuel cell systems of the embodiments of the present disclosure are designed to reduce greenhouse gas emissions and have a positive impact on the climate. 
     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.