Patent Publication Number: US-2022223892-A1

Title: Fuel cell system

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a continuation application of International Application PCT/JP2020/044498 filed on Nov. 30, 2020 which claims priority from a Japanese Patent Application No. 2019-234464 filed on Dec. 25, 2019, the contents of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     Technical Field 
     The present invention relates to a fuel cell system. 
     Background Art 
     Recently, the development of solid oxide fuel cells (SOFCs) is progressing. An SOFC is a power generation mechanism in which electrical energy is generated by causing oxide ions generated by an air electrode to pass through an electrolyte and move to a fuel electrode, such that the oxide ions react with hydrogen or carbon monoxide at the fuel electrode. SOFCs have the characteristics of having the highest operating temperatures for power generation (for example, from 600° C. to 1000° C.) and also the highest power-generating efficiency among currently known classes of fuel cells. 
     Patent Literature 1 discloses a fuel cell system provided with a detection means that detects a state in which a fuel is no longer supplied to an SOFC and an emergency stopping means that executes an emergency stop of the SOFC according to a detection result from the detection means. The fuel cell system is further provided with a control means that performs a protection operation of stopping the supply of the fuel and an oxidant on the condition that the detection means no longer detects the fuel, and supplying an inert gas to the SOFC. 
     Patent Literature 2 discloses a power generation system provided with a vent line that branches off from a waste fuel gas line carrying a waste fuel gas from the SOFC, a shutoff valve and an orifice provided in the vent line, and a measurement means that measures and outputs the system differential pressure of the SOFC to a control device. In the power generation system, in the case where a failure occurs in the control device, the systems for supplying and discharging a fuel gas and an oxidant gas are shut off, and the shutoff valve, the orifice, and the like are controlled such that the differential pressure measured by the measurement means reaches a predetermined value. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Patent Laid-Open No. 2006-66244 
     Patent Literature 2: Japanese Patent Laid-Open No. 2016-91644 
     SUMMARY OF INVENTION 
     Technical Problem 
     However, in Patent Literature 1, when the control means stops, the supply of the fuel and the oxidant can no longer be controlled, and moreover the protection operation can no longer be controlled. For this reason, there is a problem in that the fuel electrode can no longer be kept in a reduced state and the fuel electrode is degraded by oxidation. 
     Also, Patent Literature 2 merely maintains the system differential pressure (the differential pressure between the fuel electrode and the air electrode) and does not keep the fuel electrode in a reduced state when a failure occurs in the control device, and consequently there is a problem in that the fuel electrode is degraded by oxidation similarly in Patent Literature 2, too. 
     An object of the present invention, which has been made in the light of such points, is to provide a fuel cell system that can prevent oxidation degradation of the fuel electrode, even in the case where a control unit stops abnormally. 
     Solution to Problem 
     In one aspect, a fuel cell system according to the embodiments comprises a solid oxide fuel cell including an electrolyte interposed between a fuel electrode supplied with a reduction gas and an air electrode supplied with an oxidant gas, the solid oxide fuel cell generating electricity through an electrochemical reaction between the reduction gas and the oxidant gas, a control unit that controls the supply of the reduction gas and the oxidant gas to the solid oxide fuel cell, a detection unit that detects a stopping of a normal signal of the control unit and/or detects an abnormal signal of the control unit transmitted from the control unit, and a maintenance unit that keeps the fuel electrode in a reduced state according to a detection result from the detection unit. 
     Advantageous Effects of Invention 
     According to the present invention, the fuel electrode can be kept in a reduced state by the maintenance unit on the condition that the control unit transmits an abnormal signal or becomes incapable of transmitting a normal signal. With this arrangement, the degradation of the fuel electrode by oxidation at high temperatures can be prevented. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram illustrating a fuel cell system according to a first embodiment. 
         FIG. 2  is a time chart for explaining operations during an abnormal stop of the fuel cell system. 
         FIG. 3  is a block diagram illustrating a fuel cell system according to a second embodiment. 
         FIG. 4  is a block diagram illustrating a fuel cell system according to a third embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     First Embodiment 
       FIG. 1  will be referenced to describe a fuel cell system according to a first embodiment in detail.  FIG. 1  is a block diagram illustrating the fuel cell system according to the first embodiment. 
     As illustrated in  FIG. 1 , a fuel cell system  1  includes a solid oxide fuel cell (SOFC)  10 . The SOFC  10  includes a cell stack configured as a layering or a collection of a plurality of cells. Each cell has a basic configuration in which an electrolyte is disposed between an air electrode and a fuel electrode (none of which is illustrated), and a separator is interposed between the cells. The cells of the cell stacks are electrically connected in series. The SOFC  10  is a power generation mechanism in which electrical energy is generated by causing oxide ions generated by an air electrode to pass through an electrolyte and move to a fuel electrode, such that the oxide ions react with hydrogen or carbon monoxide at the fuel electrode. 
     The SOFC  10  includes an anode gas flow channel (fuel gas flow channel, reduction gas flow channel)  11  that supplies a fuel gas (reduction gas) to the fuel electrode and a cathode gas flow channel (oxidant gas flow channel)  12  that supplies an oxidant gas to the air electrode. For the fuel gas, a gas containing a hydrocarbon-based fuel, such as city gas (methane gas), natural gas, or biogas such as digestion gas is used. Atmospheric air is one example of the oxidant gas. 
     The fuel cell system  1  is provided with an anode gas supply channel  21  connected to an inlet of the anode gas flow channel  11 , and a cathode gas supply channel  22  connected to an inlet of the cathode gas flow channel  12 . When the SOFC  10  generates electricity, the fuel gas is supplied to the anode gas flow channel  11  through the anode gas supply channel  21 , and the fuel gas flows through the anode gas flow channel  11 . Also, the oxidant gas is supplied to the cathode gas flow channel  12  through the cathode gas supply channel  22 , and the oxidant gas flows through the cathode gas flow channel  12 . By inducing an electrochemical reaction between the fuel gas (reduction gas) supplied to the anode gas flow channel  11  and the oxidant gas supplied to the cathode gas flow channel  12 , a direct current is produced (the SOFC  10  generates electricity). The direct current generated by the SOFC  10  is converted in an alternating current (DC/AC converted) by an inverter (not illustrated). 
     A reaction air blower  24  is provided in the cathode gas supply channel  22 . The cathode gas supply channel  22  supplies atmospheric air brought in by the reaction air blower  24  to the cathode gas flow channel  12  as the oxidant gas. 
     The fuel cell system  1  is provided with an anode gas discharge channel  26  connected to an outlet of the anode gas flow channel  11 , and a cathode gas discharge channel  27  connected to an outlet of the cathode gas flow channel  12 . In addition, the fuel cell system  1  is provided with a combustor  28  connected to the anode gas discharge channel  26  and the cathode gas discharge channel  27 . The anode gas discharge channel  26  discharges an exhaust gas from the outlet of the anode gas flow channel  11  to the combustor  28 , and the cathode gas discharge channel  27  discharges an exhaust gas from the outlet of the cathode gas flow channel  12  to the combustor  28 . The combustor  28  burns the exhaust gases discharged from the SOFC  10  to remove impurities from the exhaust gases, and then exhausts the combusted gas. 
     The fuel cell system  1  is provided with a recirculation channel  31  that branches off from the anode gas discharge channel  26 . The recirculation channel  31  recirculates the exhaust gas from the outlet of the anode gas flow channel  11  to the anode gas supply channel  21  from the anode gas discharge channel  26 . The recirculation channel  31  is provided with a recirculation blower  32  that sends the exhaust gas into the recirculation channel  31 . Here, the recirculation channel  31  and the recirculation blower  32  form a recirculation system  30  that recirculates the exhaust gas to the anode gas supply channel  21 . 
     The fuel cell system  1  includes a control unit  40  that centrally controls the driving of the components of the fuel cell system  1 . More specifically, the control unit  40  is connected to a control valve of the anode gas supply channel  21  not illustrated, the reaction air blower  24 , and the recirculation blower  32 , and executes a driving control, an on/off control, or an open/close control of these components while the SOFC  10  is in operation. Through the control of the above adjustment valve, reaction air blower  24 , and the like by the control unit  40 , the supply of the fuel gas (reduction gas) and the oxidant gas is controlled. For example, the fuel cell system  1  includes a personal computer (PC) or a programmable logic controller (PLC), and the PC or the PLC can be the control unit  40 , i.e., can perform the functions of the control unit  40 . More specifically, the fuel system  1  or the control unit  40  may include a computing device (e.g., a central processing unit (CPU), or a processor) and a memory (storage medium) that stores program instructions. The computing device executes the program instructions to provide the functions of the control unit  40 . 
     In the fuel cell system  1 , it is necessary to anticipate the case where the control unit  40  stops abnormally because the supply of power to the control unit  40  is cut off or the control unit  40  itself malfunctions due to unforeseen circumstances during power generation. In this case, in the SOFC  10  in a high-temperature state, the oxide ions generated at the air electrode and passing through the electrolyte cause the fuel electrode to oxidize and become degraded. Accordingly, the fuel cell system  1  according to the present embodiment is provided with the configuration described hereinafter to keep the fuel electrode in a reduced state and suppress oxidation degradation, even in the case where the control unit  40  stops abnormally. 
     The fuel cell system  1  according to the present embodiment includes a signal transmission unit  41  provided in the control unit  40 , a detection unit  45  that detects a signal transmitted by the signal transmission unit  41 , and a maintenance unit  50  that operates according to a detection result from the detection unit  45 . Similar to the control unit  40 , the CPU can execute instructions stored in the memory to perform the functions of the detection unit  45  and the maintenance unit  50 . Additionally, the fuel cell system  1  includes a solenoid valve (valve)  46  provided on the downstream side of the branch point of the recirculation channel  31  in the anode gas discharge channel  26 . Here, the control unit  40 , the detection unit  45 , the maintenance unit  50 , and the solenoid valve  46  are supplied with power through an uninterruptible power supply (UPS) not illustrated to ensure operations for a certain time even in the case where the supply of power to the fuel cell system  1  as a whole is stopped. 
     The signal transmission unit  41  is provided with a function that switches the transmission of a signal to the detection unit  45  between a case where an abnormality has occurred in which the components described above cannot be controlled normally due to the control unit  40  itself or an external factor such as a cutoff of the supplied power, and a normal case where the above abnormality has not occurred. For example, a configuration is adopted such that the signal transmission unit  41  transmits a normal signal to the detection unit  45  intermittently or continuously only in the normal case, and transmits an abnormal signal to the detection unit  45  only in the case where an abnormality has occurred. 
     The detection unit  45  is provided with a function of receiving the normal signal or the abnormal signal transmitted from the signal transmission unit  41 . In addition, the detection unit  45  is provided with a function of detecting the state in which the transmission of the normal signal has stopped or detecting the transmission of the abnormal signal, transmitting an actuation signal that actuates the maintenance unit  50  and the solenoid valve  46  on the condition of the above detection, or cutting off an energizing current to the maintenance unit  50  and the solenoid valve  46 . 
     The maintenance unit  50  is provided with a hydrogen supply system  51  that supplies hydrogen gas to the anode gas supply channel  21  as a reduction gas. Examples of the source that supplies the hydrogen gas in the hydrogen supply system  51  include a hydrogen gas cylinder filled with hydrogen gas or a hydrogen supply system in a facility or the like where the fuel cell system  1  is installed. The hydrogen supply system  51  is provided with a solenoid valve for allowing or stopping the supply of the hydrogen gas in the hydrogen gas supply channel. For example, the solenoid valve closes and stops the supply of the hydrogen gas when in an energized state, and opens and allows the supply of the hydrogen gas when in a non-energized state. Consequently, the supply of the hydrogen gas to the anode gas supply channel  21  can be initiated by transmitting an actuation signal from the detection unit  45  to cut off the energizing current or by cutting off the energizing current from the detection unit  45 . 
     The maintenance unit  50  is further provided with an inert gas supply system  52 . In the present embodiment, nitrogen gas is adopted as the inert gas, but a gas such as carbon dioxide or steam may also be adopted as the inert gas. Examples of the source that supplies the nitrogen gas in the inert gas supply system  52  include a nitrogen gas cylinder filled with nitrogen gas or a nitrogen supply system in a facility or the like where the fuel cell system  1  is installed. The nitrogen gas supply channel in the inert gas supply system  52  may converge with the hydrogen gas supply channel or may be connected to the anode gas supply channel  21  independently from the hydrogen gas supply channel. The inert gas supply system  52  is provided with a solenoid valve for allowing or stopping the supply of the nitrogen gas in the nitrogen gas supply channel. For example, the solenoid valve closes and stops the supply of the nitrogen gas when in an energized state, and opens and allows the supply of the nitrogen gas when in a non-energized state. Consequently, the supply of the nitrogen gas to the anode gas supply channel  21  can be initiated by transmitting an actuation signal from the detection unit  45  to cut off the energizing current or by cutting off the energizing current from the detection unit  45 . 
     For example, the solenoid valve  46  opens and allows the discharge of the fuel gas from the anode gas discharge channel  26  to the combustor  28  when in an energized state, and closes and stops the discharge of the fuel gas when in a non-energized state. Consequently, the fuel gas can be confined to the anode gas discharge channel  26  and the recirculation channel  31  by transmitting an actuation signal from the detection unit  45  to cut off the energizing current or by cutting off the energizing current from the detection unit  45 . In addition, the solenoid valve  46  is provided with a timer or the like for switching from the closed state to the open state after a certain time elapses since the energizing current was cut off, or is provided with a function of switching from the closed state to the open state according to the residual pressure of the hydrogen gas described later. 
       FIG. 2  is a time chart for explaining operations during an abnormal stop of the fuel cell system according to the first embodiment. Hereinafter,  FIGS. 1 and 2  will be referenced to describe the operations during an abnormal stop of the fuel cell system  1  in detail. 
     Here, the case where the supply of power to the fuel cell system  1  as a whole is stopped and the supply of power to the control unit  40  is also stopped due to unforeseen circumstances will be described as the abnormal stop. As illustrated in  FIG. 2 , a first line and a second line exist as the supply systems of the maintenance unit  50 , and in the present embodiment, the first line is taken to be the hydrogen supply system  51 , and the second line is taken to be the inert gas supply system  52 . 
     During operations before the abnormal stop, that is, during normal operations, the SOFC  10  is set to a high operating temperature from 600° C. to 1000° C. for example. If the supply of power to the control unit  40  stops in this state, the temperature of the SOFC  10  will fall gradually but still remain in a high-temperature state for some time. 
     Also, if the supply of power to the control unit  40  stops, the stopping of the transmission of the normal signal, or the transmission of the abnormal signal, from the signal transmission unit  41  is detected by the detection unit  45 . On the condition of the above detection, the detection unit  45  transmits an actuation signal to the maintenance unit  50  and the solenoid valve  46 , or cuts off the energizing current to the maintenance unit  50  and the solenoid valve  46 . With this arrangement, in the maintenance unit  50 , the supply of the hydrogen gas in the hydrogen supply system  51  acting as the first line and the supply of the nitrogen gas in the inert gas supply system  52  acting as the second line are started, and the solenoid valve  46  switches from the open state to the closed state. 
     By supplying hydrogen gas from the hydrogen supply system  51  (first line) and nitrogen gas from the inert gas supply system  52  (second line) through the anode gas supply channel  21 , a hydrogen gas of predetermined concentration (reduction gas) is supplied to the anode gas flow channel  11  of the SOFC  10 . With this arrangement, the fuel electrode (anode) in the SOFC  10  can be kept in a reduced state, and degradation caused by the oxidation reaction of the fuel electrode can be prevented. 
     Also, the closing of the solenoid valve  46  makes it possible to regulate the discharge from the anode gas discharge channel  26  of the hydrogen gas of a predetermined concentration supplied from the hydrogen supply system  51  and the inert gas supply system  52 , and thereby also contribute to maintaining the reduced state of the fuel electrode. Furthermore, as the temperature falls, the gas contracts in the anode gas flow channel  11 , but the closing of the solenoid valve  46  makes it possible to regulate the inflow of air and the like from outside the system into the anode gas flow channel  11  through the anode gas discharge channel  26 , and thereby also prevent oxidation degradation of the fuel electrode. 
     The closing of the solenoid valve  46  causes the hydrogen gas that has passed through the anode gas flow channel  11  to flow into the recirculation channel  31 . In other words, the recirculation channel  31  functions as a buffer that stores the hydrogen gas. Additionally, the recirculation channel  31  is also maintained in a high-temperature state during the abnormal stop, and therefore can be used as an evaporation heat source for turning water produced in the SOFC  10  into reforming water. 
     When the temperature of the SOFC  10  falls to a temperature T 1  (from 300° C. to 500° C., such as 400° C. for example) at which the oxidation reaction no longer occurs at the fuel electrode, the supply of the hydrogen gas from the hydrogen supply system  51  (first line) is stopped. The timing of the stop can be set by pre-calculating the cooling time for the SOFC  10  to cool down to the temperature T 1  and pre-adjusting the hydrogen gas capacity in the supply source of the hydrogen supply system  51  and the opening degree of a valve for adjusting the quantity of hydrogen gas to be supplied during the cooling period. 
     Also, at this timing, the supply of the hydrogen gas stops, the residual pressure of the hydrogen gas drops, and the solenoid valve  46  switches from the closed state to the open state according to the operation of the timer in the solenoid valve  46 . Furthermore, at the same timing, the supply of the nitrogen gas from the inert gas supply system  52  (second line) is still ongoing. In other words, after the supply of the hydrogen gas from the hydrogen supply system  51  stops, the inert gas supply system  52  can supply the nitrogen gas as an inert gas to the fuel electrode and thereby use the inert gas to purge the hydrogen gas from the fuel electrode. Through the inert gas purge, the hydrogen gas can be discharged outside the system through the open solenoid valve  46  and the anode gas discharge channel  26 , safety can be ensured, and safety-related standards can be upheld. 
     Thereafter, at the timing when the temperature of the SOFC  10  falls to reach a predetermined temperature T 2  and the inert gas purge at the fuel electrode is completed, the supply of the nitrogen gas from the inert gas supply system  52  (second line) is stopped. The timing of the stop can be set by pre-calculating the time it takes to complete the inert gas purge and pre-adjusting the nitrogen gas capacity in the supply source of the inert gas supply system  52  and the opening degree of a valve for adjusting the quantity of nitrogen gas to be supplied during this time. With the above, the operations after an abnormal stop of the control unit  40  are completed. 
     Note that although the above describes the case where the control unit  40  stops abnormally, similar operations preferably are also performed in the case where the uninterruptible power supply described above stops abnormally. With this arrangement, the oxidation degradation of the fuel electrode in the SOFC  10  can also be prevented not only when the control unit  40  fails, but also when the uninterruptible power supply or the like fails. 
     As above, in the above fuel cell system  1  according to the first embodiment, the hydrogen gas can be supplied to the SOFC  10  as a reduction gas by the hydrogen supply system  51  of the maintenance unit  50 , even in the case where the control unit  40  stops abnormally. With this arrangement, the fuel electrode in the SOFC  10  can be kept in a reduced state, and oxidation degradation of the hot fuel electrode can be prevented. 
     Next, other embodiments of the present invention will be described. Note that the following description uses the same signs to refer to configuration portions which are the same or similar to the embodiment(s) described before the embodiment being described, and a description of such portions will be omitted or simplified. 
     Second Embodiment 
     Next, a second embodiment of the present invention will be described with reference to  FIG. 3 .  FIG. 3  is a block diagram illustrating a fuel cell system according to the second embodiment. As illustrated in  FIG. 3 , in the second embodiment, the configuration of a maintenance unit  60  is changed with respect to the first embodiment. 
     The maintenance unit  60  according to the second embodiment is provided with a fuel supply system  61  that supplies a fuel gas (hydrocarbon-based fuel) to the anode gas supply channel  21 . For example, the fuel supply system  61  may include a gas cylinder filled with a fuel gas such as methane gas as a supply source. The fuel supply system  61  is provided with a solenoid valve for allowing or stopping the supply of the fuel gas in the supply channel, and the solenoid valve works similarly to the solenoid valve in the hydrogen supply system  51  described above. 
     The maintenance unit  60  is provided with a water supply system  63  that supplies water to an evaporator  62  provided in the anode gas supply channel  21 . For example, the water supply system  63  may include a tank storing pure water as a water supply source. The water supply system  63  likewise is provided with a solenoid valve for allowing or stopping the supply of the water in the supply channel, and the solenoid valve works similarly to the solenoid valve in the hydrogen supply system  51  described above. 
     The maintenance unit  60  is additionally provided with a reforming unit  64 . The reforming unit  64  has a function of using steam generated by the evaporator  62  to reform the fuel gas supplied from the fuel supply system  61  into a reduction gas. The reforming unit  64  supplies the reduction gas to the fuel electrode through the anode gas flow channel  11 . Although the diagram illustrates the case where the reforming unit  64  is provided in the anode gas supply channel  21  downstream from the evaporator  62 , the reforming unit  64  may also be provided inside the SOFC  10 . 
     The maintenance unit  60  is additionally provided with the inert gas supply system  52  similar to the first embodiment. 
     Next,  FIGS. 2 and 3  will be referenced to describe the operations during an abnormal stop of the fuel cell system  1  according to the second embodiment. In the following description, the first line in  FIG. 2  is taken to be the fuel supply system  61  and the water supply system  63 , and the second line is taken to be the inert gas supply system  52 . The operations and function of the solenoid valve  46  are similar to the first embodiment, and therefore a description is omitted. 
     If the supply of power to the control unit  40  stops, the supply of the fuel gas and the water by the fuel supply system  61  and the water supply system  63  acting as the first line is initiated in the maintenance unit  60  through the detection unit  45 . The supply causes reforming into a reduction gas in the reforming unit  64  as described above, and the supply of the nitrogen gas in the inert gas supply system  52  acting as the second line is also initiated, and consequently a reduction gas of a predetermined concentration is supplied to the anode gas flow channel  11 . With this arrangement, the fuel electrode (anode) in the SOFC  10  can be kept in a reduced state, and degradation caused by the oxidation reaction of the fuel electrode can be prevented. 
     When the SOFC  10  falls to the temperature T 1 , the supply of the fuel gas and the water from the fuel supply system  61  and the water supply system  63  (first line) is stopped. At this timing, the supply of the nitrogen gas from the inert gas supply system  52  (second line) is still ongoing, and the nitrogen gas can be used to perform an inert gas purge of the reduction gas at the fuel electrode. Thereafter, at the timing when the temperature of the SOFC  10  falls to reach a predetermined temperature T 2  and the inert gas purge at the fuel electrode is completed, the supply of the nitrogen gas from the inert gas supply system  52  is stopped. With the above, the operations after an abnormal stop of the control unit  40  are completed. 
     In the second embodiment, operations different from the operations during the abnormal stop described above can be performed. In the different operations, the first line in  FIG. 2  is taken to be the fuel supply system  61  and the second line is taken to be the water supply system  63 . 
     If the supply of power to the control unit  40  stops, the supply of the fuel gas by the fuel supply system  61  acting as the first line and the supply of the water by the water supply system  63  acting as the second line are initiated in the maintenance unit  50 . Through the supply, steam is generated in the evaporator  62 , the fuel gas is reformed into a reduction gas in the reforming unit  64 , and a reduction gas of a predetermined concentration is supplied to the anode gas flow channel  11 . With this arrangement, the fuel electrode in the SOFC  10  is kept in a reduced state. 
     When the SOFC  10  falls to the temperature T 1 , the supply of the fuel gas from the fuel supply system  61  (first line) is stopped. At this timing, the supply of the water (steam) from the water supply system  63  (second line) is still ongoing, and the steam can be used to perform a steam purge at the fuel electrode. Thereafter, at the timing when the temperature of the SOFC  10  falls to reach the predetermined temperature T 2  and the steam purge at the fuel electrode is completed, the supply of the water from the water supply system  63  is stopped, and the operations after an abnormal stop of the control unit  40  are completed. Additionally, the present embodiment may also include the inert gas supply system  52 , and if the supply of power to the control unit  40  stops, the supply of an inert gas from the inert gas supply system  52  may be initiated and remain ongoing even after the completion of the steam purge, and after an inert gas purge is completed, the supply of the inert gas may be stopped and the operations after an abnormal stop of the control unit  40  may be completed. 
     As above, in the above fuel cell system  1  according to the second embodiment, the fuel electrode in the SOFC  10  can be kept in a reduced state to prevent oxidation degradation of the fuel electrode, similarly to the first embodiment. In addition, it is not necessary to provide components such as a hydrogen gas cylinder for supplying the hydrogen gas, and consequently the equipment costs can be reduced. 
     Furthermore, in the case where the reforming unit  64  is provided inside the SOFC  10 , the SOFC  10  can be cooled by the endothermic reaction of reforming the fuel gas from the fuel supply system  61 , and thereby also prevent oxidation degradation of the fuel electrode. 
     Third Embodiment 
     Next, a second embodiment of the present invention will be described with reference to  FIG. 4 .  FIG. 4  is a block diagram illustrating a fuel cell system according to the third embodiment. As illustrated in  FIG. 4 , in the third embodiment, the configuration of a maintenance unit  70  is changed with respect to the first embodiment. 
     The maintenance unit  70  according to the third embodiment is provided with an ammonia supply system  71  that supplies aqueous ammonia to the anode gas supply channel  21 . For example, the ammonia supply system  71  may include a tank storing aqueous ammonia as a supply source. The ammonia supply system  71  is provided with a solenoid valve for allowing or stopping the supply of the fuel gas in the supply channel, and the solenoid valve works similarly to the solenoid valve in the hydrogen supply system  51  described above. Also, the ammonia supply system  71  includes an aqueous ammonia evaporation unit (not illustrated) that vaporizes the ammonia in the aqueous ammonia while also evaporating the water for reforming. 
     The maintenance unit  70  is additionally provided with a reforming unit  74  provided in the anode gas supply channel  21 . The reforming unit  74  has a function of reforming the aqueous ammonia and steam supplied from the ammonia supply system  71  into hydrogen gas (reduction gas) and nitrogen gas (inert gas). The reforming unit  74  supplies the hydrogen gas and the nitrogen gas to the fuel electrode through the anode gas flow channel  11 . Although the diagram illustrates the case where the reforming unit  74  is provided in the anode gas supply channel  21 , the reforming unit  74  may also be provided inside the SOFC  10 . 
     The maintenance unit  70  is additionally provided with the inert gas supply system  52  similar to the first embodiment. 
     Next,  FIGS. 2 and 4  will be referenced to describe the operations during an abnormal stop of the fuel cell system  1  according to the third embodiment. In the following description, the first line in  FIG. 2  is taken to be the ammonia supply system  71  and the second line is taken to be the inert gas supply system  52 . The operations and function of the solenoid valve  46  are similar to the first embodiment, and therefore a description is omitted. 
     If the supply of power to the control unit  40  stops, the supply of the aqueous ammonia and the steam by the ammonia supply system  71  acting as the first line is initiated in the maintenance unit  70  through the detection unit  45 . The supply causes reforming into the hydrogen gas (reduction gas) and the nitrogen gas (inert gas) in the reforming unit  74  as described above, and the supply of the nitrogen gas in the inert gas supply system  52  acting as the second line is also initiated, and consequently a hydrogen gas of a predetermined concentration is supplied to the anode gas flow channel  11 . With this arrangement, the fuel electrode (anode) in the SOFC  10  can be kept in a reduced state, and degradation caused by the oxidation reaction of the fuel electrode can be prevented. 
     When the SOFC  10  falls to the temperature T 1 , the supply of the aqueous ammonia and the steam from the ammonia supply system  71  (first line) is stopped. At this timing, the supply of the nitrogen gas from the inert gas supply system  52  (second line) is still ongoing, and the nitrogen gas can be used to perform an inert gas purge at the fuel electrode. Thereafter, at the timing when the temperature of the SOFC  10  falls to reach a predetermined temperature T 2  and the inert gas purge at the fuel electrode is completed, the supply of the nitrogen gas from the inert gas supply system  52  is stopped. With the above, the operations after an abnormal stop of the control unit  40  are completed. 
     Note that the third embodiment may also be configured such that the inert gas supply system  52  (second line) is omitted and the nitrogen gas is not supplied as part of the above operations when an abnormal stop occurs. 
     As above, in the above fuel cell system  1  according to the third embodiment, the fuel electrode in the SOFC  10  can be kept in a reduced state to prevent oxidation degradation of the fuel electrode, similarly to the first embodiment. In addition, it is not necessary to provide components such as a gas cylinder for supplying the hydrogen gas or the fuel gas, and consequently a space savings can be attained with the equipment. 
     Although the above embodiments are provided with the recirculation channel  31 , the recirculation channel  31  may also be omitted, and the exhaust gas from the anode gas discharge channel  26  may also be discharged to the combustor  28 . Also, although the recirculation channel  31  is described as acting like a heat source, a high-temperature part in a location different from the recirculation channel  31  inside the fuel cell system  1  may also be used as a heat source. 
     In addition, embodiments of the present invention have been described, but the above embodiments may also be combined in full or in part and treated as another embodiment of the present invention. 
     Also, embodiments of the present invention are not limited to the embodiments described above, and various modifications, substitutions, and alterations are possible without departing from the scope of the technical idea according to the present invention. Further, if the technical idea according to the present invention can be achieved according to another method through the advancement of the technology or another derivative technology, the technical idea may be implemented using the method. Consequently, the claims cover all embodiments which may be included in the scope of the technical idea according to the present invention. 
     INDUSTRIAL APPLICABILITY 
     The fuel cell system according to the present invention is suitable for application to fuel cell systems for domestic use, commercial use, and all other industrial fields. 
     This application is based on Japanese Patent Application No. 2019-234464 filed on Dec. 25, 2019, the content of which is hereby incorporated in entirety.