Patent Publication Number: US-2022223888-A1

Title: Fuel cell system and operating method

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a continuation application of International Application PCT/JP2020/044499 filed on Nov. 30, 2020 which claims priority from a Japanese Patent Application No. 2019-234465 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 and an operating method. 
     Background Art 
     In the invention described in Patent Literature 1, when a solid oxide fuel cell stops, steam is generated by heating a water vaporizer with a ceramic heater to reform a fuel gas. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Patent Laid-Open No. 2011-119055 
     SUMMARY OF INVENTION 
     Technical Problem 
     However, in the invention described in Patent Literature 1, it takes time for the heat from the heater to raise the temperature of the water vaporizer enough for the water vaporizer to reach a temperature at which steam can be generated. For this reason, the steam is generated after a delay from the stopping of the solid oxide fuel cell. Consequently, after the solid oxide fuel cell stops, there is time in which the steam is not supplied, and during this time, the fuel gas is still supplied to the fuel cell stack. According to this configuration, the steam to carbon ratio (S/C) is lowered, carbon is deposited on the catalyst in the reformer and the fuel cell stack, and the catalyst is degraded in a phenomenon also referred to as coking. 
     An object of the present invention, which has been made in the light of such problems, is to provide a fuel cell system and an operating method capable of generating steam immediately after the solid oxide fuel cell stops. 
     Solution to Problem 
     A fuel cell system according to one aspect of the present invention comprises an anode gas flow channel, a cathode gas flow channel, a solid oxide fuel cell which is supplied with a fuel gas from the anode gas flow channel and air from the cathode gas flow channel to generate electricity through an electrochemical reaction, and a steam generator that generates steam to be mixed with the fuel gas when the solid oxide fuel cell stops, wherein the steam generator is disposed such that heat is exchangeable with a gas flowing through the anode gas flow channel or the cathode gas flow channel. 
     An operating method of a fuel cell system according to another aspect of the present invention is an operating method of a fuel cell system that mixes steam with a fuel gas when a solid oxide fuel cell, which is supplied with the fuel gas from an anode gas flow channel and air from a cathode gas flow channel to generate electricity through an electrochemical reaction, stops, the operating method comprising disposing a steam generator such that heat is exchangeable with a gas flowing through the anode gas flow channel or the cathode gas flow channel, and maintaining the steam generator at a temperature sufficient for generating steam through heat exchange with the gas while the solid oxide fuel cell is generating electricity, and causing the steam generator to generate the steam when the solid oxide fuel cell stops generating electricity. 
     Advantageous Effects of Invention 
     According to the present invention, steam can be generated immediately after the solid oxide fuel cell stops. Consequently, it is possible to reduce the time in which steam is not supplied after the solid oxide fuel cell stops, and thereby prevent degradation of the catalyst in the reformer and the fuel cell stack. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a conceptual diagram of a fuel cell system according to a first embodiment of the present invention. 
         FIG. 2  is a perspective view of a steam generator according to the present embodiment. 
         FIG. 3  is a schematic cross section illustrating the steam generator and a gas flow channel. 
         FIG. 4  illustrates a temperature profile from power generation to stopping in the solid oxide fuel cell in a comparative example in which the steam generator does not contact the gas flow channel. 
         FIG. 5  illustrates a temperature profile from startup to power generation and stopping in the solid oxide fuel cell in the present embodiment in which the steam generator contacts the gas flow channel. 
         FIG. 6  is a graph illustrating an example of an operating method when the solid oxide fuel cell stops in a fuel cell system according to the present embodiment. 
         FIG. 7  is a conceptual diagram of a fuel cell system according to a second embodiment. 
         FIG. 8  is a conceptual diagram of a fuel cell system according to a third embodiment. 
         FIG. 9  is a conceptual diagram of a fuel cell system according to a fourth embodiment. 
         FIG. 10  illustrates a cross section of a gas flow channel having a steam generation function. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments of the present invention will be described in detail. However, the present invention is not limited to the following embodiments, and may also be modified in various ways while remaining within the scope of the present invention. 
     First Embodiment 
       FIG. 1  is a conceptual diagram of a fuel cell system according to a first embodiment of the present invention. As illustrated in  FIG. 1 , a fuel cell system  1  includes a solid oxide fuel cell (SOFC)  2 , a steam generator  3 , an anode gas flow channel  4 , and a cathode gas flow channel  5 . Note that the anode gas flow channel  4  and the cathode gas flow channel  5  may be referred to as the “gas flow channel(s)” when not being distinguished individually. 
     The solid oxide fuel cell  2  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, and a separator is interposed between the cells. The cells of the cell stack are electrically connected in series. The solid oxide fuel cell 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 anode gas flow channel  4  includes an anode gas inlet channel L 1  on the inlet side from the perspective of the solid oxide fuel cell  2  and an anode gas outlet channel L 2  on the outlet side from the perspective of the solid oxide fuel cell  2 . 
     The anode gas inlet channel L 1  functions as a fuel gas supply channel that supplies a fuel gas to the solid oxide fuel cell  2 . The flow rate of the fuel gas is adjusted by a fuel supply blower not illustrated. The anode gas outlet channel L 2  functions as an exhaust channel that releases an anode exhaust gas. Also, the anode gas outlet channel L 2  is provided with a recirculation channel L 3  that branches off partway through and recirculates the anode exhaust gas to the anode gas inlet channel L 1 . As illustrated in  FIG. 1 , a recirculation blower  6  is provided in the recirculation channel L 3  to adjust the flow rate of the recirculated anode exhaust gas. 
     In the first embodiment illustrated in  FIG. 1 , the steam generator  3  is disposed so as to allow heat exchange with the fuel gas flowing through the anode gas inlet channel L 1 . The steam generator  3  is disposed on the portion of the anode gas inlet channel L 1  between the solid oxide fuel cell  2  and the recirculation channel L 3 , for example. As illustrated in  FIG. 1 , a water supply channel L 5  is provided on the inlet side of the steam generator  3 . Also, a steam supply channel L 6  is provided on the outlet side of the steam generator  3 , and steam generated by the steam generator  3  passes through the steam supply channel L 6  and is mixed with the fuel gas flowing through the anode gas inlet channel L 1 . 
     As illustrated in  FIG. 1 , the cathode gas flow channel  5  includes a cathode gas inlet channel L 7  on the inlet side from the perspective of the solid oxide fuel cell  2  and a cathode gas outlet channel L 8  on the outlet side from the perspective of the solid oxide fuel cell  2 . 
     Air is supplied to the solid oxide fuel cell  2  from the cathode gas inlet channel L 7  by an air blower  7 . A regenerative heat exchanger  8  is provided in the cathode gas inlet channel L 7 . 
     As illustrated in  FIG. 1 , the cathode gas outlet channel L 8  that acts as an exhaust channel for the cathode exhaust gas is connected to the regenerative heat exchanger  8  to form a flow channel that recirculates the cathode exhaust gas. In the regenerative heat exchanger  8 , the air flowing through the cathode gas inlet channel L 7  exchanges heat with the cathode exhaust gas, and the temperature rises. 
     The steam generator  3  will be described. As illustrated in  FIGS. 2 and 3 , the steam generator  3  includes a housing  10 , a tubular part  11  provided on the front surface (the surface facing the inlet side) of the housing  10 , a steam release pipe  12  provided on a side surface of the housing  10 , a heater  13  disposed on the underside of the housing  10 , and a fixture  14  for affixing the steam generator  3  to a predetermined location in the fuel cell system  1 . The arrangement of the tubular part  11  and the steam release pipe  12  may also be different from  FIG. 2 . 
     The tubular part  11  and the steam release pipe  12  lead into the housing  10 . The tubular part  11  is connected to the water supply channel L 5  illustrated in  FIG. 1 . The steam release pipe  12  forms all or part of the steam supply channel L 6  illustrated in  FIG. 1 . In the case where the steam release pipe  12  forms all of the steam supply channel L 6 , the steam release pipe  12  is connected directly to the anode gas inlet channel L 1 . 
     As illustrated in  FIG. 3 , the steam generator  3  contacts the anode gas inlet channel L 1 . For this reason, the steam generator  3  is capable of exchanging heat with the fuel gas flowing through the anode gas inlet channel L 1 , and is kept in a high-temperature state (at or above 300° C., for example). Note that the temperature of the steam generator  3  is measured by a temperature measuring instrument  3   a  (see  FIG. 1 ). 
     Consequently, when water is supplied to the steam generator  3  through the water supply channel L 5 , steam can be generated immediately, and the steam can be supplied from the steam release pipe  12  to the fuel gas flowing through the anode gas inlet channel L 1 . 
     As illustrated in  FIG. 3 , the heater  13  is disposed out of contact with the anode gas inlet channel L 1 . If the heater  13  is made to contact the anode gas inlet channel L 1  directly, thermal shock is imparted due to sudden gas temperature changes and the like, which leads to damage to the heater  13 . Consequently, the heater  13  preferably is disposed so as not to contact the anode gas inlet channel L 1 , and may also be disposed somewhere other than the underside of the housing  10 . 
     The heater  13  has a role of providing assistive heating to keep the steam generator  3  at a high temperature. 
     Hereinafter,  FIGS. 4 and 5  will be used to describe temperature profiles from power generation to stopping in the solid oxide fuel cell according to a comparative example and the present embodiment. 
       FIG. 4  is the temperature profile of the comparative example. In the comparative example, unlike the present embodiment, the steam generator  3  does not contact the anode gas inlet channel L 1 . 
     As illustrated in  FIG. 4 , while the solid oxide fuel cell  2  is generating electricity, the steam generator  3  is not exchanging heat with the fuel gas flowing through the anode gas inlet channel L 1  and remains at a normal temperature. As illustrated in  FIG. 4 , when the solid oxide fuel cell  2  stops generating electricity, the heater  13  of the steam generator  3  is activated to raise the temperature of the steam generator  3 . The temperature of the steam generator  3  is ultimately raised to approximately 300° C. As illustrated in  FIG. 4 , water is supplied to the steam generator  3 , and if the temperature of the steam generator  3  is at or above 100° C. at this time, steam begins to form. However, as illustrated in  FIG. 4 , the generation of the steam is delayed by a time t from when the solid oxide fuel cell  2  stopped. 
     On the other hand,  FIG. 5  is the temperature profile of the present embodiment. In the present embodiment, as illustrated in  FIGS. 1 and 3 , the steam generator  3  is made to contact the anode gas inlet channel L 1 . Note that  FIG. 5  is used to describe a temperature profile from startup to power generation and stopping in the solid oxide fuel cell  2 . 
     As illustrated in  FIG. 5 , from the startup of the solid oxide fuel cell  2  until a time (1), the temperature of the steam generator  3  rises due to the transfer of heat from the fuel gas. During the period between the time (1) and a time (2), the heater  13  provided in the steam generator  3  is activated to further raise the temperature of the steam generator  3 . In this way, the temperature of the steam generator is raised to approximately 300° C. by the transfer of heat from the fuel gas and by heating provided by the heater. 
     As illustrated in  FIG. 5 , when the time (2) is reached, steam is generated and mixed with the fuel gas. With this arrangement, steam reforming of the fuel gas can be performed. 
     While the solid oxide fuel cell  2  is generating electricity (from a time (3) to a time (4) illustrated in  FIG. 5 ), the supply of steam is stopped to achieve water self-reliance. As illustrated in  FIG. 5 , while the solid oxide fuel cell  2  is generating electricity, the steam generator  3  can be kept at approximately 300° C. (hot standby) through the transfer of heat from the fuel gas. 
     At the time (4), the solid oxide fuel cell  2  stops generating electricity, and at the same time, water is supplied to the steam generator  3 . At this time, because the steam generator  3  is maintained at a temperature of approximately 300° C., steam can be generated immediately after the water is supplied. 
     As illustrated in  FIG. 5 , during the period from the time (4) to a time (5), the temperature of the steam generator  3  falls briefly due to the generation of steam, but by activating the heater  13 , the steam generator  3  can be brought back and kept to a temperature of approximately 300° C. through heating provided by the heater. 
     As illustrated in  FIG. 5 , the gas temperature continues to fall from the time (4) when the solid oxide fuel cell  2  stops generating electricity. In the period from the time (5) to a time (6), due to the falling of the gas temperature, steam is generated by heating the steam generator  3  mainly with heating provided by the heater. 
     As illustrated in the temperature profile according to the present embodiment illustrated in  FIG. 5 , unlike the comparative example in  FIG. 4 , steam can be generated once the solid oxide fuel cell  2  stops. As a result, the degradation of the catalyst in the reformer and the fuel cell stack can be suppressed after the solid oxide fuel cell  2  stops, and coking can be prevented effectively. 
       FIG. 6  is a graph illustrating an example of an operating method when a stop occurs in the fuel cell system according to the present embodiment. 
     In step ST 1 , the solid oxide fuel cell  2  stops generating electricity (time (4) in  FIG. 5 ). Next, in step ST 2 , water is supplied to the steam generator  3 . At this time, the steam generator  3  is being maintained at a temperature sufficient for generating steam, and therefore steam can be generated by the steam generator  3  immediately by supplying the water. 
     In step ST 3 , the temperature of the steam generator  3  is measured by the temperature measuring instrument  3   a  (see  FIG. 1 ), and when the temperature of the steam generator  3  falls below 280° C. as illustrated in the period from the time (4) to the time (5) in  FIG. 5 , for example, the flow proceeds to step ST 4 . Additionally, the heater  13  attached to the steam generator  3  is activated. With this arrangement, the temperature of the steam generator  3  can be raised back up to 300° C. 
     As above, the steam generator  3  is maintained at a temperature sufficient for generating steam, and therefore the steam generator  3  can generate steam immediately after the solid oxide fuel cell  2  stops generating electricity. When a certain time elapses from the stopping of the solid oxide fuel cell  2 , the temperature of the steam generator  3  begins to fall. Consequently, heating provided by the heater  13  is used to keep the steam generator  3  at a predetermined temperature, thereby making it possible to continue generating steam for a certain time for clearing up coking immediately after the solid oxide fuel cell  2  stops. 
     In the first embodiment illustrated in  FIG. 1 , the steam generator  3  is disposed on the anode gas inlet channel L 1  of the anode gas flow channel  4 . With this arrangement, the steam supply channel L 6  can be shortened, the stream can be mixed with the fuel gas immediately after the solid oxide fuel cell  2  stops, and coking can be prevented effectively. 
     In this way, in the present embodiment, the steam generator  3  preferably is disposed on the anode gas inlet channel L 1  of the anode gas flow channel  4 , but the steam generator  3  is not limited thereto and may also be disposed at another location in a gas flow channel. Hereinafter, examples of disposing the steam generator  3  at a different location from  FIG. 1  will be described. 
     OTHER EMBODIMENTS 
       FIG. 7  is a conceptual diagram of a fuel cell system according to a second embodiment,  FIG. 8  is a conceptual diagram of a fuel cell system according to a third embodiment, and  FIG. 9  is a conceptual diagram of a fuel cell system according to a fourth embodiment. 
     In the embodiments in  FIGS. 7 to 9 , signs that are the same as in  FIG. 1  denote the same portions. In the second embodiment illustrated in  FIG. 7 , the steam generator  3  is disposed on the anode gas outlet channel L 2  on the outlet side of the anode gas flow channel  4 . By causing the steam generator  3  to contact the anode gas outlet channel L 2 , similarly to  FIG. 3 , heat can be exchanged effectively with the exhaust gas flowing through the anode gas outlet channel L 2 . Note that the steam generator  3  may also be disposed in contact with the recirculation channel L 3 . 
     In the third embodiment illustrated in  FIG. 8 , the steam generator  3  is disposed on the cathode gas outlet channel L 8  on the outlet side of the cathode gas flow channel  5 . By causing the steam generator  3  to contact the cathode gas outlet channel L 8 , similarly to  FIG. 3 , heat can be exchanged effectively with the exhaust gas flowing through the cathode gas outlet channel L 8 . Preferably, the steam generator  3  is disposed in contact with the recirculation channel of the cathode gas outlet channel L 8 . 
     In the fourth embodiment illustrated in  FIG. 9 , the steam generator  3  is disposed on the cathode gas inlet channel L 7  on the inlet side of the cathode gas flow channel  5 . By causing the steam generator  3  to contact the cathode gas inlet channel L 7 , similarly to  FIG. 3 , heat can be exchanged effectively with the oxidant gas flowing through the cathode gas inlet channel L 7 . 
     Additionally, in the embodiments in  FIGS. 7 to 9 , when the solid oxide fuel cell  2  stops, steam can be generated by supplying water to the steam generator  3 . By passing the steam through the steam supply channel L 6  to mix with the fuel gas flowing through the anode gas inlet channel L 1 , steam reforming of the fuel gas can be performed immediately after the solid oxide fuel cell  2  stops. With this arrangement, degradation of the catalyst in the reformer and the fuel cell stack can be suppressed, and coking can be prevented effectively. 
     Also, as illustrated in  FIG. 10 , the steam generator according to the embodiments may also be integrated with a portion of a gas flow channel. In  FIG. 10 , the gas flow channel has a double-walled pipe structure with a heater layer  21  provided on the outer circumference of a pipe  20 . A space allowing the passage of water from the water supply channel L 5  is provided between the heater layer  21  and the pipe  20 . With this arrangement, steam can be generated by heat exchange with a gas flowing inside the pipe  20 . The space between the heater layer  21  and the pipe  20  leads to the steam supply channel L 6  at a location different from the water supply channel L 5 . In addition, through the steam supply channel L 6 , the steam is mixed with the fuel gas flowing through the anode gas inlet channel L 1 . In this way, by configuring the gas flow channel as a double-walled pipe structure, the gas flow channel itself can be given a steam generation function with a high heat exchange ratio, making it possible to supply steam efficiently. Moreover, it is possible to provide a stable supply of steam even with a heater of low capacity. 
     Note that although embodiments of the present invention have been described, the above embodiments and modifications thereof 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. 
     For example, the embodiments may also have a structure in which the heater  13  is not provided in the steam generator  3 . In this case, when the temperature of the steam generator  3  falls as illustrated during the period between the time (4) and the time (5) in  FIG. 5 , steam can be generated for a longer time by controlling factors such as reducing the quantity of steam to be supplied. However, by providing the heater  13  as an external power source in the steam generator  3 , when the temperature of the steam generator  3  falls, heating can be provided by the heater  13  to keep the temperature of the steam generator  3  at a certain value, making it possible to supply a fixed quantity of steam continually. With this arrangement, a high S/C can be maintained and the risk of fuel cell degradation can be reduced. 
     Also, in the above embodiments, the steam generator  3  is made to contact a gas flow channel, but the steam generator  3  does not have to contact the gas flow channel insofar heat exchange is possible with the gas flowing through the gas flow channel. For example, an intermediate layer may exist between the steam generator  3  and the gas flow channel, or alternatively, some space may be provided between the steam generator  3  and the gas flow channel. 
     This application is based on Japanese Patent Application No. 2019-234465 filed on Dec. 25, 2019, the content of which is hereby incorporated in entirety.