Patent Publication Number: US-2022231316-A1

Title: Fuel cell system

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a continuation application of International Application PCT/JP2020/044501 filed on Nov. 30, 2020 which claims priority from a Japanese Patent Application No. 2019-234467 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 900° C. to 1000° C.) and also the highest power-generating efficiency among currently known classes of fuel cells. 
     In the related art, a solid oxide fuel cell system has been proposed for a solid oxide fuel cell stack (SOFC stack) provided with a plurality of solid oxide fuel cell tubular cells (SOFC tubular cells), the solid oxide fuel cell system being provided with an orifice at the fuel inlet port to restrict the flow rate of the fuel introduced into each of the SOFC tubular cells (for example, see Patent Literature 1). According to this fuel cell system, it is possible to suppress inconsistencies in power generation due to non-uniform fuel supply with respect to each SOFC stack. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Patent Laid-Open No. 2015-185303 
     SUMMARY OF INVENTION 
     Technical Problem 
     By the way, recently, power generation methods using SOFCs have shown promise as a power generation method suited for reducing CO 2 , and there is a demand to increase the capacity of the power output from SOFCs. For example, to achieve higher capacity of the power output from SOFCs, it is conceivable to construct a fuel cell cartridge (SOFC cartridge) by bundling a plurality of SOFC stacks each of which includes a plurality of solid oxide fuel cells (SOFC cells) connected in series, and adopt a fuel cell module provided with a plurality of such SOFC cartridges. Such a fuel cell cartridge includes a fuel supply header that supplies a fuel to the plurality of SOFC stacks, a fuel discharge header that discharges the fuel from the SOFC stacks, an oxidant gas supply header that supplies an oxidant gas, and an oxidant gas discharge header that discharges the oxidant gas from the SOFC stacks. 
     In the case where a fuel cell module is provided with a plurality of SOFC cartridges, achieving a uniform flow rate of the fuel to each of the SOFC cartridges (uniform distribution) is important for preventing degradation of the SOFC stacks (and furthermore the SOFC cells forming the SOFC stacks). In the case of applying the SOFC stack according to Patent Literature 1 to such a fuel cell module, the fuel flow rate is only adjustable for each SOFC stack at the orifice, making it difficult to achieve a uniform flow rate of the fuel to each of the SOFC cartridges (uniform distribution). 
     The present invention has been devised in the light of such circumstances, and one objective thereof is to provide a fuel cell system capable of achieving a uniform flow rate of the fuel to a plurality of fuel cell cartridges. 
     Solution to Problem 
     A fuel cell system according to an aspect of the present invention is a fuel cell system using a plurality of fuel cell stacks each of which includes a plurality of fuel cells that generate electricity through an electrochemical reaction between a fuel gas and an oxidant gas connected in series, the fuel cell system comprising a plurality of fuel cell cartridges each of which supplies the fuel gas and the oxidant gas respectively to the plurality of fuel cell stacks through headers, and also discharges a fuel off-gas and an oxidant off-gas respectively through headers, a fuel gas supply line that supplies the fuel gas to the plurality of fuel cell cartridges, a fuel off-gas discharge line that discharges the fuel off-gas from the plurality of fuel cell cartridges, and a first adjustment member, provided in at least one of the fuel gas supply line or the fuel off-gas discharge line, that adjusts a flow rate of the fuel gas or the fuel off-gas, wherein at least one portion of the first adjustment member includes a flexible pipe. 
     Advantageous Effects of Invention 
     According to the present invention, a uniform flow rate of the fuel to the fuel cell cartridges can be achieved. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a perspective view illustrating an example of a fuel cell module included in a fuel cell system according to the embodiments. 
         FIG. 2  is a plan view illustrating an example of a fuel cell module included in the fuel cell system according to the embodiments. 
         FIG. 3  is a block diagram illustrating a configuration of the fuel cell system according to a first embodiment. 
         FIG. 4  is a block diagram illustrating a configuration of the fuel cell system according to a second embodiment. 
         FIG. 5  is a block diagram illustrating a configuration of the fuel cell system according to a third embodiment. 
         FIG. 6  is a flowchart for describing a method of controlling flow rate adjustment valves in the fuel cell system according to a third embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, a fuel cell module included in a fuel cell system according to the embodiments will be described.  FIG. 1  is a perspective view illustrating an example of a fuel cell module included in a fuel cell system according to the embodiments.  FIG. 2  is a plan view illustrating an example of a fuel cell module included in the fuel cell system according to the embodiments. In  FIG. 2 , a header  30  described later is omitted for convenience, and an inlet pipe  40  of a fuel gas pipe  4  and an inlet pipe  50  of an oxidant gas pipe  5  described later are illustrated. Note that the fuel cell module illustrated below is merely one non-limiting example, and may be modified appropriately. 
     As illustrated in  FIGS. 1 and 2 , a fuel cell module  1  according to the embodiments is configured such that a fuel cell cartridge  3  is disposed inside an airtight container  2 . The airtight container  2  is formed into a bottomed cylindrical shape to cover the fuel cell cartridge  3 . Specifically, the airtight container  2  is provided with a circular bottom wall (not illustrated), a cylindrical side wall  21  rising up from the perimeter of the bottom wall, and a circular top wall  22  that covers an opening above the side wall  21 . The airtight container  2  is formed by a metal material such as stainless steel, for example. 
     The fuel cell cartridge  3  is constructed by installing a plurality of fuel cell stacks (not illustrated) in parallel (parallel installation), and has a rectangular cuboid shape overall. The fuel cell stacks are constructed by connecting solid oxide fuel cells (SOFC) in series, and is formed into a hollow cylindrical shape that is long in the vertical direction designated the Z direction in  FIG. 1 , for example. The plurality of fuel cell stacks are arranged at a predetermined pitch in the X and Y directions in  FIG. 1 , for example. Each solid oxide fuel cell has a basic configuration in which an electrolyte phase is disposed between an air electrode and a fuel electrode. An SOFC includes 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. 
     In the present embodiment, a single fuel cell cartridge  3  is configured in a rectangular cuboid shape having long rectangular shape in the X direction in a plan view. Also, two fuel cell cartridges  3  are arranged in the transverse direction designated the Y direction inside the airtight container  2 . A first header  30  and a second header  31  for connecting to a fuel gas pipe  4  and an oxidant gas pipe  5  described later are provided on the upper and lower ends of the fuel cell cartridges  3 . The first and second headers  30  and  31  have a generally rectangular cuboid shape. The fuel cell cartridges  3  supply a fuel gas and an oxidant gas to the fuel cell stacks through the first and second headers  30  and  31 , and also discharge fuel off-gas and oxidant off-gas from the fuel cell stacks through the first and second headers  30  and  31 . Note that the configuration and layout of the fuel cell stacks and the fuel cell cartridges  3  are not limited to the above and may be changed appropriately. 
     In addition, the fuel cell module  1  is provided with pipes that form flow channels for supplying the fuel gas or the oxidant gas to the fuel cell cartridges  3  as supply gas. Specifically, the pipes include a fuel gas pipe  4  that forms a fuel gas flow channel and an oxidant gas pipe  5  that forms an oxidant gas flow channel. City gas for example is used as the fuel gas and air for example is used as the oxidant gas. Note that the oxidant gas may also be air mixed with another gas. Moreover, the fuel gas may also be referred to as anode gas, and the oxidant gas may also be referred to as cathode gas. 
     The fuel gas pipe  4  includes an inlet pipe  40  and an outlet pipe  41 . The inlet pipe  40  is disposed on the upper lateral surface of the side wall  21 , and penetrates from the outside into the inside of the airtight container  2 . On the upstream side of the inlet pipe  40 , a fuel gas supply source not illustrated is connected. Also, as illustrated in  FIG. 2 , the inlet pipe  40  branches inside the airtight container  2  for each of the plurality of fuel cell cartridges  3 . Specifically, the inlet pipe  40  includes a first branching part  42  that branches into two channels centrally above the fuel cell cartridges  3 , a pair of first branch pipes  43  extending in the Y direction from the first branching part  42 , second branching parts  44  that branch into two channels at the ends of the first branch pipes  43 , a pair of second branch pipes  45  extending in the X direction from the second branching parts  44 , and connecting pipes  46  that extend inwardly into the airtight container  2  (in the Y direction) from the ends of the second branch pipes  45  and also bend downward to connect to the upper end of each fuel cell cartridge  3 . 
     In addition, the outlet pipe  41  is disposed on the lower end of each fuel cell cartridge  3 . The outlet pipe  41  is disposed on the lower lateral surface of the side wall  21  and projects out from the inside of the airtight container  2  to the outside. The outlet pipe  41  has a branching pattern similar to the inlet pipe  40 , and is configured such that the fuel off-gas (anode off-gas) that has been subjected to a reaction in the fuel cell cartridges  3  flows out from the airtight container  2 . 
     The oxidant gas pipe  5  includes an inlet pipe  50  and an outlet pipe  51 . The upstream side of the inlet pipe  50  is connected to an oxidant gas supply source not illustrated. Also, the inlet pipe  50  branches outside the airtight container  2  for each of the plurality of fuel cell cartridges  3 . Specifically, the inlet pipe  50  includes a first branching part  52  that branches into two channels on the outside of the side wall  21  and a pair of first branch pipes  53  extending horizontally from the first branching part  52  along the outer surface of the side wall  21 . The first branching part  52  is disposed directly above the outlet pipe  41  of the fuel gas pipe  4 . The first branch pipes  53  each wrap around the side wall  21  and are connected internally from the lower lateral surface of the side wall  21  corresponding to the lateral surface in the longitudinal direction of each fuel cell cartridge  3 . 
     As illustrated in  FIG. 2 , each first branch pipe  53  includes a second branching part  54  that branches into two channels inside the airtight container  2  and a pair of second branch pipes  55  extending horizontally from the second branching part  54  along the inner surface of the side wall  21 . The second branch pipes  55  each wrap around the outside of the fuel cell cartridges  3  between the inner surface of the side wall  21  and the lateral surface of the fuel cell cartridges  3 , and are connected to the lateral surface in the transverse direction of each fuel cell cartridge  3 . 
     The outlet pipe  51  includes a pair of third branch pipes  56  projecting out from the upper lateral surface of the side wall  21  corresponding to the lateral surface in the longitudinal direction of each fuel cell cartridge  3 , and a confluent part  57  that combines the pair of third branch pipes  56 . The third branch pipes  56  wrap around the outer surface of the side wall  21  and are connected to the confluent part  57  on the outside of the side wall  21  corresponding to the lateral surface in the transverse direction of the fuel cell cartridges  3 . The confluent part  57  is positioned directly below the inlet pipe  40  of the fuel gas pipe  4 . Note that for convenience, the configuration of the outlet pipe  51  inside the airtight container  2  is omitted. 
     As illustrated in  FIG. 1 , tubular heat-insulating covers  6  and  7  are provided to cover the outer circumference of the inlet pipe  40  and the outlet pipe  41  forming the fuel gas pipe  4 . In addition, tubular heat-insulating covers  8  and  9  are provided on the inlet pipe  50  and the outlet pipe  51  forming the oxidant gas pipe  5 . These heat-insulating covers are formed by a metal material such as stainless steel like the airtight container  2 , and are formed having a predetermined gap with respect to the outer circumferential surface of each pipe. For example, by disposing a high-temperature heat-insulating material (not illustrated) such as glass wool between the heat-insulating covers and the pipes, the diffusion of heat from the pipes to the outside can be prevented. Note that a heat-insulating material may also be provided on the outer circumferential side of the heat-insulating covers. Also, the heat-insulating material may be affixed by winding a metal wire of a certain wire gauge. 
     In the fuel cell module  1  configured in this way, a fuel gas from the fuel gas supply source is supplied to the fuel cell cartridges  3  through the fuel gas pipe  4 . On the other hand, an oxidant gas from the oxidant gas supply source is supplied to the fuel cell cartridges  3  through the oxidant gas pipe  5 . By inducing a chemical reaction between the fuel gas and the oxidant gas inside the fuel cell cartridges  3 , electrical energy (direct-current power) is generated. The generated direct-current power is converted into alternating-current power by an inverter not illustrated, for example. The fuel gas and the oxidant gas after the reaction are discharged to the outside of the fuel cell module  1  through respective pipes. 
     Incidentally, in the case where the fuel cell module  1  is provided with a plurality of fuel cell cartridges  3 , achieving a uniform flow rate of the fuel to each of the fuel cell cartridges  3  (uniform distribution) is important for preventing degradation of the fuel cell stacks (and furthermore the fuel cells forming the fuel cell stacks). Preferably, a uniform flow rate of the fuel to the fuel cell cartridges  3  is achieved without increasing the overall bulk or the manufacturing costs of the fuel cell module  1 . In particular, in the case of applying the present invention to a solid oxide fuel cell (SOFC) or a molten carbonate fuel cell (MCFC) for example, the fuel cell stacks contain ceramic, which makes it difficult to achieve uniform dimensions after firing the ceramic. For this reason, there is a limit to achieving uniform dimensions by design, and inconsistencies in the fuel flow rate occur among the fuel cell cartridges (the same also applies to the oxidant gas). This problem is especially pronounced for a solid oxide fuel cell (SOFC) in which the highest operating point is approximately 1000° C. and the firing temperature of the cell stack exceeds 1500° C. 
     The inventions focused on how in the fuel cell module  1 , non-uniform flow rates of the fuel gas flowing through the plurality of fuel cell cartridges  3  affects the uniformity of the fuel flow rate. Furthermore, the inventors discovered that matching the flow rates of the fuel gas among the fuel cell cartridges contributes to achieving a uniform flow rate with respect to the fuel cell cartridges  3 , and thereby conceived of the present invention. 
     In other words, the gist of the fuel cell system according to the present invention is to match the flow rates of the fuel gas among the fuel cell cartridges by installing a flexible pipe as a part of an adjustment member that adjusts the flow rate of the fuel gas or the fuel off-gas in at least one of the fuel gas supply line that supplies the fuel gas to the plurality of fuel cell cartridges  3  or the fuel off-gas discharge line that discharges the fuel off-gas from the plurality of fuel cell cartridges. 
     According to the fuel cell system according to the present invention, because a flexible pipe is installed as a part of an adjustment member that adjusts the flow rate of the fuel gas or the fuel off-gas in at least one of the fuel gas supply line or the fuel off-gas discharge line, it is possible to match the flow rates of the fuel gas among the fuel cell cartridges, thereby making it possible to achieve a uniform flow rate of the fuel with respect to the plurality of fuel cell cartridges. 
     Hereinafter, configurations of the fuel cell system according to embodiments of the present invention will be described. 
     First Embodiment 
       FIG. 3  is a block diagram illustrating a configuration of a fuel cell system  100  according to a first embodiment. For convenience, only the components related to the present invention are illustrated in  FIG. 3 . Note that in  FIG. 3 , components shared in common with  FIG. 1  are denoted with the same signs and further description of such components is omitted. In  FIG. 3 , the flow channels of fluids such as the fuel gas and the oxidant gas are illustrated by solid lines. Note that the flow channels of fluids inside the SOFC cartridges  3  are illustrated by chain lines for convenience. 
     As illustrated in  FIG. 3 , the fuel cell system  100  includes the fuel cell module  1 . The fuel cell module  1  is provided with a pair of fuel cell cartridges (hereinafter referred to as the “SOFC cartridges”)  3  ( 3   a ,  3   b ). Note that because the SOFC cartridges  3   a  and  3   b  share a common configuration, the SOFC cartridge  3   a  will be described as a representative example. The SOFC cartridge  3   a  includes an oxidant gas flow channel (cathode gas flow channel)  32  and a fuel gas flow channel (anode gas flow channel)  34 . 
     The oxidant gas (air) and other gases brought in by a reaction air blower (oxidant gas supplier) B 10  are supplied to an inlet  32   a  of the oxidant gas flow channel  32 , and oxidant off-gas is discharged from an outlet  32   b  of the oxidant gas flow channel  32 . The oxidant gas (air) is supplied to the inlet  32   a  of the oxidant gas flow channel  32  through an oxidant gas supply line P 10  that connects an outlet B 11  of the reaction air blower B 10  to the inlet  32   a  of the oxidant gas flow channel  32 . Additionally, the oxidant off-gas is discharged from the outlet  32   b  of the oxidant gas flow channel  32  through an oxidant gas discharge line P 11  connected to the outlet  32   b  of the oxidant gas flow channel  32 . 
     A fuel gas (fuel) and other gases are supplied to an inlet  34   a  of the fuel gas flow channel  34  from a fuel gas supplier (not illustrated). Fuel off-gas is discharged from an outlet  34   b  of the fuel gas flow channel  34 . The fuel gas (fuel) is supplied to the inlet  34   a  of the fuel gas flow channel  34  through a fuel gas supply line P 12  that connects a valve V 10  to the inlet  34   a  of the fuel gas flow channel  34 . Additionally, the fuel off-gas is discharged from the outlet  34   b  of the fuel gas flow channel  34  through a fuel gas discharge line P 13  connected to the outlet  34   b  of the fuel gas flow channel  34 . 
     In the fuel cell system  100 , a heat exchanger H 10  is connected to the oxidant gas supply line P 10  and the oxidant gas discharge line P 11 . The heat exchanger H 10  transfers heat from the oxidant off-gas flowing through the oxidant gas discharge line P 11  to the oxidant gas flowing through the oxidant gas supply line P 10 . With this arrangement, the oxidant gas (air) brought in by the reaction air blower B 10  is heated by the heat exchanger H 10  and supplied to the inlet  32   a  of the oxidant gas flow channel  32 . 
     Also, on the outside of the fuel cell module  1 , a fuel gas recirculation line P 14  is connected to the fuel gas discharge line P 13  and the fuel gas supply line P 12 . The fuel gas recirculation line P 14  is provided with a blower B 12  that recirculates the fuel off-gas. A portion of the fuel off-gas discharged from the SOFC cartridges  3   a  and  3   b  to the fuel gas discharge line P 13  is introduced into the fuel gas recirculation line P 14  by the blower B 12  and sent to the fuel gas supply line P 12 . With this arrangement, the fuel gas (fuel) from the fuel gas supplier (not illustrated) is heated by being mixed with the fuel off-gas, and is supplied to the inlet  34   a  of the fuel gas flow channel  34 . Also, moisture generated at the fuel electrode in association with the recirculation of the fuel off-gas is usable as reforming water for the fuel gas, and consequently a configuration for supplying reforming steam from an external source while the fuel cell module  1  is in operation can be omitted. As a result, a more compact fuel cell system can be achieved and the manufacturing costs can be lowered. 
     In the fuel cell system  100  illustrated in  FIG. 3 , the oxidant gas supply line P 10  includes the inlet pipe  50  of the oxidant gas pipe  5  while the oxidant gas discharge line P 11  includes the outlet pipe  51  of the oxidant gas pipe  5 , for example (see  FIG. 1 ). Similarly, the fuel gas supply line P 12  includes the inlet pipe  40  of the fuel gas pipe  4  while the fuel gas discharge line P 13  includes the outlet pipe  41  of the fuel gas pipe  4 . 
     In the fuel cell system  100 , of the inlet pipe  40  of the fuel gas pipe  4 , the first branch pipes  43  connected to the SOFC cartridge  3   a  are provided with a flow rate adjustment member (hereinafter simply referred to as the “adjustment member”) AD 10  (see  FIG. 3 ). The adjustment member AD 10  includes a member that adjusts the flow rate of the fuel gas flowing through the inlet pipe  40  (first branch pipes  43 ) of the fuel gas pipe  4  toward the SOFC cartridge  3   a . The adjustment member AD 10  constitutes one example of a first adjustment member. Note that the adjustment member AD 10  may also be referred to as a resistive element with respect to the fuel gas flowing through the first branch pipes  43 . The same applies to other adjustment members. 
     Any member can be selected as the adjustment member AD 10  on the condition that the flow rate of the fuel gas flowing through the inlet pipe  40  (first branch pipes  43 ) of the fuel gas pipe  4  is adjusted. For example, the adjustment member AD 10  includes a flexible pipe, an orifice, a control valve, or a combination of the above. In the present embodiment, flexible pipes are used as the first branch pipes  43 , and in addition, the adjustment member AD 10  includes an orifice  43   a  disposed between the first branch pipes  43  and the second branching parts  44  (see  FIG. 2 ). 
     By using flexible pipes to construct the adjustment member AD 10 , the flow rate of the fuel gas in the inlet pipe  40  can be adjusted while absorbing the thermal expansion of the pipes associated with the operation of the fuel cell module  1 . Note that the flow rate of the fuel gas can be adjusted on the basis of measured data obtained while the fuel cell module  1  is in operation. For example, in the case of using flexible pipes to construct the adjustment member AD 10 , the flow rate of the fuel gas can be adjusted by selecting the length and degree of bend in the flexible pipes on the basis of the measured data. 
     For example, in the case where the flow rate in the first branch pipe  43  connected to the SOFC cartridge  3   a  is lower than the flow rate in the first branch pipe  43  connected to the SOFC cartridge  3   b , the resistance to the fluid flowing through the first branch pipe  43  is increased to lower the flow rate. For example, in the case of using flexible pipes to construct the adjustment member AD 10 , the resistance to the fluid flowing through the first branch pipe  43  is increased by extending the length or bending the shape of the flexible pipe. 
     Conversely, in the case where the flow rate in the first branch pipe  43  connected to the SOFC cartridge  3   a  is higher than the flow rate in the first branch pipe  43  connected to the SOFC cartridge  3   b , the resistance to the fluid flowing through the first branch pipe  43  is decreased to lower the flow rate. For example, in the case of using flexible pipes to construct the adjustment member AD 10 , the resistance to the fluid flowing through the first branch pipe  43  is decreased by straightening the shape of the flexible pipe. 
     Note that the adjustment member AD 10  may also be constructed by changing the pattern of the inlet pipe  40  in a corresponding location. For example, the adjustment member AD 10  may be constructed by changing the pipe diameter or the pipe length in a location corresponding to the adjustment member AD 10 , or by performing bending work in a location corresponding to the adjustment member AD 10 . By constructing the adjustment member AD 10  by changing the pattern of the corresponding location in this way, increases in the costs for manufacturing the fuel gas pipe  4  can be reduced. 
     In this way, in the fuel cell system  100  according to the first embodiment, the fuel gas pipe  4  connected to the SOFC cartridge  3   a  is provided with the adjustment member AD 10  that adjusts the flow rate of the fuel gas. With this arrangement, the flow rates of the fuel gas flowing through the inlet pipe  40  of the fuel gas pipe  4  connected to the SOFC cartridges  3   a  and  3   b  can be matched, and therefore a uniform flow rate of the fuel with respect to the SOFC cartridges  3   a  and  3   b  can be achieved. As a result, degradation of the SOFC stacks (and furthermore the SOFC cells forming the SOFC stacks) due to non-uniform fuel flow rates with respect to the SOFC cartridges  3  can be prevented, thereby making it possible to achieve stable, high-capacity power generation. 
     In particular, the adjustment member AD 10  is provided in the fuel gas pipe  4  connected to the SOFC cartridges  3  and the flow rate of the fuel gas flowing through the fuel gas pipe  4  is adjusted, thereby making it possible to suppress increases in the costs associated with manufacturing the fuel gas pipe  4  compared to the case of adjusting the flow rate of the fuel gas with respect to the SOFC stacks forming the SOFC cartridges  3 , or moreover the SOFC cells forming the SOFC stacks. As a result, a uniform flow rate of the fuel with respect to the SOFC cartridges  3  can be achieved while also keeping manufacturing costs down. 
     Additionally, in the fuel cell system  100 , of the outlet pipe  51  of the oxidant gas pipe  5 , the third branch pipes  56  connected to the SOFC cartridge  3   b  are provided with an adjustment member AD 20 . The adjustment member AD 20  is configured using a member similar to the adjustment member AD 10 , and includes a member that adjusts the flow rate of the oxidant gas flowing through the outlet pipe  51  (third branch pipes  56 ) of the oxidant gas pipe  5 . The adjustment member AD 20  constitutes one example of a second adjustment member. 
     By adjusting the flow rate of the oxidant gas flowing the third branch pipes  56  with the adjustment member AD 20 , the flow rates of the oxidant off-gas flowing through both of the third branch pipes  56  connected to the SOFC cartridges  3   a  and  3   b  can be matched. Consequently, a uniform flow rate of the oxidant off-gas discharged from the SOFC cartridges  3   a  and  3   b  can be achieved. As a result, a situation in which the temperature of one of the SOFC cartridges  3  rises because of non-uniform flow rates of the oxidant off-gas from the SOFC cartridges  3  can be avoided, and damage or the like to the SOFC cartridges  3  can be prevented. 
     Second Embodiment 
     A fuel cell system according to a second embodiment differs from the fuel cell system  100  according to the first embodiment in the number of flow rate adjustment members disposed in the inlet pipe  40  of the fuel gas pipe  4  and the number of flow rate adjustment members disposed in the outlet pipe  51  of the oxidant gas pipe  5 . 
     Hereinafter, the configuration of the fuel cell system according to the second embodiment will be described while mainly focusing on the points that differ from the fuel cell system  100  according to the first embodiment.  FIG. 4  is a block diagram illustrating a configuration of a fuel cell system  200  according to the second embodiment. Note that in  FIG. 4 , components shared in common with  FIG. 3  are denoted with the same signs and further description of such components is omitted. 
     As illustrated in  FIG. 4 , in the fuel cell system  200 , an adjustment member AD 11  is provided in addition to the adjustment member AD 10  in the first branch pipes  43  of the inlet pipe  40  of the fuel gas pipe  4 . Like the adjustment member AD 10 , the adjustment member AD 11  includes a member that adjusts the flow rate of the fuel gas flowing through the inlet pipe  40  (first branch pipes  43 ) of the fuel gas pipe  4  toward the SOFC cartridge  3   b . Note that the adjustment members AD 10  and AD 11  may be configured using the same member or different members. 
     In the fuel cell system  200  according to the second embodiment, the flow rate of the fuel gas flowing through the first branch pipes  43  is adjusted by both the adjustment member AD 10  and the adjustment member AD 11 . Consequently, the flow rate in the inlet pipe  40  overall can be adjusted more effectively compared to the case of adjusting the flow rate in the inlet pipe  40  with the adjustment member AD 10  alone. With this arrangement, a uniform flow rate of the fuel gas with respect to the SOFC cartridges  3   a  and  3   b  can be achieved with high precision. 
     Also, in the fuel cell system  200 , of the outlet pipe  51  of the oxidant gas pipe  5 , the third branch pipes  56  are provided with an adjustment member AD 21  in addition to the adjustment member AD 20 . Like the adjustment member AD 20 , the adjustment member AD 21  includes a member that adjusts the flow rate of the oxidant gas flowing through the outlet pipe  51  (third branch pipes  56 ) of the oxidant gas pipe  5 . Note that the adjustment members AD 20  and AD 21  may be configured using the same member or different members. 
     In the fuel cell system  200  according to the second embodiment, the flow rate of the oxidant gas flowing through the third branch pipes  56  is adjusted by both the adjustment member AD 20  and the adjustment member AD 21 . Consequently, the flow rate in the outlet pipe  51  overall can be adjusted more effectively compared to the case of adjusting the flow rate in the outlet pipe  51  with the adjustment member AD 20  alone. With this arrangement, a uniform flow rate of the oxidant off-gas discharged from the SOFC cartridges  3   a  and  3   b  can be achieved with high precision. 
     Third Embodiment 
     A fuel cell system according to a third embodiment differs from the fuel cell system  200  according to the second embodiment in that an adjustment valve is included in the flow rate adjustment members disposed in the inlet pipe  40  of the fuel gas pipe  4  and the outlet pipe  51  of the oxidant gas pipe  5 , and the adjustment valves are controlled on the basis of the state of the fuel cell module  1 . Additionally, the fuel cell system according to the third embodiment differs from the fuel cell system  200  according to the second embodiment in that an adjustment valve is disposed externally to the fuel cell module  1  to ensure the operation of the adjustment valves as flow rate adjustment members. Due to the arrangement of the adjustment valve external to the fuel cell module  1 , the paths of the inlet pipe  40  of the fuel gas pipe  4  and the outlet pipe  51  of the oxidant gas pipe  5  are partially changed. 
     Hereinafter, the configuration of the fuel cell system according to the third embodiment will be described while mainly focusing on the points that differ from the fuel cell system  200  according to the second embodiment.  FIG. 5  is a block diagram illustrating a configuration of a fuel cell system  300  according to the third embodiment. Note that in  FIG. 5 , components shared in common with  FIG. 4  are denoted with the same signs and further description of such components is omitted. Also, in  FIG. 5 , the flow channels of fluids such as the fuel gas and the oxidant gas are illustrated by solid lines, and signal lines of control signals in the fuel cell system  300  are illustrated by dashed lines. 
     As illustrated in  FIG. 5 , in the fuel cell system  300 , an adjustment valve AD 12  is provided instead of the adjustment member AD 10  in the first branch pipe  43  connected to the SOFC cartridge  3   a  of the inlet pipe  40  of the fuel gas pipe  4 . In the fuel cell system  300 , because the adjustment valve AD 12  is installed in the first branch pipe  43 , unlike the fuel cell system  200  according to the second embodiment, a portion of the first branch pipe  43  is configured to be exposed to the outside of the fuel cell module  1 . Under control by a control unit  301  described later, the adjustment valve AD 12  adjusts the flow rate of the fuel gas flowing through the inlet pipe  40  (first branch pipe  43 ) of the fuel gas pipe  4  toward the SOFC cartridge  3   a.    
     Additionally, in the fuel cell system  300 , of the outlet pipe  51  of the oxidant gas pipe  5  the third branch pipe  56  connected to the SOFC cartridge  3   b  is provided with an adjustment valve AD 22  instead of the adjustment member AD 21 . Like the adjustment valve AD 12 , the adjustment valve AD 22  adjusts the flow rate of the oxidant gas flowing through the outlet pipe  51  (third branch pipe  56 ) of the oxidant gas pipe  5  under control by the control unit  301  described later. 
     The fuel cell system  300  is provided with a temperature sensor T that detects the internal temperature of the SOFC cartridges  3   a  and  3   b  and a voltage sensor V that detects the voltage of the SOFC cartridges  3   a  and  3   b . In addition, a concentration sensor (first concentration detection unit) S 1  that detects the oxygen concentration is provided in the oxidant gas supply line P 10  leading to the SOFC cartridges  3   a  and  3   b . Furthermore, a concentration sensor (second concentration detection unit) S 2  that detects the fuel off-gas concentration is provided in the fuel gas discharge line P 13  from the SOFC cartridges  3   a  and  3   b . The temperature sensor T, the voltage sensor V, and the concentration sensors S 1  and S 2  output detection results to the control unit  301  described later. 
     Also, the fuel cell system  300  is provided with the control unit  301  that controls the adjustment valves AD 12  and AD 22 . The control unit  301  controls the adjustment valve AD 12  and/or the adjustment valve AD 22  on the basis of the various detection results received from the temperature sensor T, the voltage sensor V, and the concentration sensors S 1  and S 2 . For example, the control unit  301  controls the adjustment valve AD 22  on the basis of the detection result from the temperature sensor T and/or the concentration sensor S 1 . With this arrangement, as a result of adjusting the flow rate in the outlet pipe  51  of the oxidant gas pipe  5 , the flow rate of the air (oxidant gas) from the SOFC cartridges  3   a  and  3   b  is adjusted. In addition, the control unit  301  controls the adjustment valve AD 12  on the basis of the detection result from the voltage sensor V and/or the concentration sensor S 2 . With this arrangement, as a result of adjusting the flow rate in the inlet pipe  40  of the fuel gas pipe  4 , the flow rate of the fuel gas to the SOFC cartridges  3   a  and  3   b  is adjusted. 
     Here, the operations of controlling the adjustment valves AD 12  and AD 22  in the fuel cell system  300  will be described with reference to  FIG. 6 .  FIG. 6  is a flowchart for describing the control of the adjustment valves AD 12  and AD 22  in the fuel cell system  300  according to the third embodiment. Note that in  FIG. 6 , the case of controlling the adjustment valves AD 12  and AD 22  on the basis of the detection results from the voltage sensor V and the temperature sensor T is described for convenience. 
     In the fuel cell system  300 , when power generation by the fuel cell module  1  is started, the control unit  301  determines the possibility of degradation in the SOFC cartridges  3   a  and  3   b . At this point, the control unit  301  acquires voltage values V 1  and V 2  from the voltage sensor V connected to the SOFC cartridges  3   a  and  3   b  (step (hereinafter designated “ST”)  601 ). Additionally, the control unit  301  determines whether the absolute value of the difference between the voltage values V 1  and V 2  is greater than a predetermined voltage value V T  (ST 602 ). 
     In the case where the absolute value of the difference between the voltage values V 1  and V 2  is greater than the voltage value V T  (ST 602 : Yes), the control unit  301  determines that there is a possibility of degradation in the SOFC cartridges  3   a  and  3   b . The determination is made in consideration of the property that the voltage values V 1  and V 2  in the SOFC cartridges  3   a  and  3   b  rise according to the concentration of the supplied fuel gas. If one of the voltage values V 1  and V 2  in the SOFC cartridges  3   a  and  3   b  is low, the possibility that the SOFC cartridge  3  with the low voltage value has degraded or is degrading is inferred. Consequently, the control unit  301  uses the adjustment valve AD 12  to adjust the flow rate in the inlet pipe  40  and thereby adjust the flow rate of the fuel supplied to the SOFC cartridges  3   a  and  3   b  (ST 603 ). 
     Here, by adjusting the flow rate of the fuel supplied to the SOFC cartridges  3   a  and  3   b , a uniform flow rate of the fuel supplied to the SOFC cartridges  3   a  and  3   b  is achieved. This arrangement makes it possible to avoid a situation in which a reduced quantity of the fuel is supplied to the SOFC cartridge  3   a  or  3   b  recognized as having a low voltage value according to the voltage sensor V, and inhibit the progression of degradation in the affected SOFC cartridge  3 . 
     After adjusting the flow rate of the fuel supplied to the SOFC cartridges  3   a  and  3   b  in ST 603 , or in the case where the absolute value of the difference between the voltage values V 1  and V 2  is the voltage value V T  or less (ST 602 : No), the control unit  301  determines the possibility of damage to the SOFC cartridges  3   a  and  3   b . At this point, the control unit  301  acquires temperatures T 1  and T 2  from the temperature sensor T connected to the SOFC cartridges  3   a  and  3   b  (ST 604 ). Additionally, the control unit  301  determines whether the absolute value of the difference between the temperatures T 1  and T 2  is greater than a predetermined temperature T T  (ST 605 ). 
     In the case where the absolute value of the difference between the temperatures T 1  and T 2  is greater than the temperature T T  (ST 605 : Yes), the control unit  301  determines that there is a possibility of damage to the SOFC cartridges  3   a  and  3   b . The determination is made in consideration of how the SOFC cartridges  3   a  and  3   b  may be damaged if the temperatures T 1  and T 2  rise to an extreme degree. If one of the temperatures T 1  and T 2  in the SOFC cartridges  3   a  and  3   b  is low, the possibility of damage to the SOFC cartridge  3  with the high temperature is inferred. Consequently, the control unit  301  uses the adjustment valve AD 22  to adjust the flow rate in the outlet pipe  51  and thereby adjust the flow rate of the air (oxidant off-gas) discharged from the SOFC cartridges  3   a  and  3   b  (ST 606 ). 
     Here, by adjusting the flow rate of the air (oxidant off-gas) discharged from the SOFC cartridges  3   a  and  3   b , a uniform flow rate of the air supplied to the SOFC cartridges  3   a  and  3   b  is achieved. This arrangement makes it possible to avoid a situation in which the temperature rises to an extreme degree in the SOFC cartridge  3   a  or  3   b  recognized as having a high temperature according to the temperature sensor T, and deter damage to the affected SOFC cartridge  3 . 
     On the other hand, in the case where the absolute value of the difference between the temperatures T 1  and T 2  is the temperature T T  or less (ST 605 : No), the control unit  301  returns the process to ST 601  and repeats the process from ST 601  to ST 606 . In other words, the control unit  301  repeats the processes for determining the possibility of degradation in the SOFC cartridges  3   a  and  3   b  and the possibility of damage to the SOFC cartridges  3   a  and  3   b . After adjusting the flow rate of the air discharged from the SOFC cartridges  3   a  and  3   b  in ST 606 , the control unit  301  ends the series of operations. Thereafter, after the operations end, the control illustrated in  FIG. 6  is executed again after a certain time elapses, for example. 
     In this way, in the fuel cell system  300  according to the third embodiment, the flow rate of the fuel gas flowing through the inlet pipe  40  (first branch pipes  43 ) of the fuel gas pipe  4  is adjusted on the basis of the voltage values of the SOFC cartridges  3   a  and  3   b . With this arrangement, the flow rate of the fuel gas can be adjusted flexibly according to the voltage conditions in the SOFC cartridges  3   a  and  3   b , and a uniform flow rate of the fuel gas with respect to the SOFC cartridges  3   a  and  3   b  can be achieved with high precision. 
     Moreover, in the fuel cell system  300  according to the third embodiment, the flow rate of the oxidant off-gas flowing through the outlet pipe  51  (third branch pipes  56 ) of the oxidant gas pipe  5  is adjusted on the basis of the temperatures of the SOFC cartridges  3   a  and  3   b . With this arrangement, the flow rate of the oxidant off-gas can be adjusted flexibly according to the temperature conditions in the SOFC cartridges  3   a  and  3   b , and a uniform flow rate of the oxidant off-gas discharged from the SOFC cartridges  3   a  and  3   b  can be achieved with high precision. 
     The flowchart illustrated in  FIG. 6  is used to describe the case of controlling the adjustment valves AD 12  and AD 22  on the basis of the detection results from the voltage sensor V and the temperature sensor T. However, the detection results from the sensors used when controlling the adjustment valves AD 12  and AD 22  are not limited to the above and may be changed appropriately. For example, the control unit  301  may also control the adjustment valve AD 22  on the basis of the detection result from the concentration sensor S 1  and control the adjustment valve AD 12  on the basis of the detection result from the concentration sensor S 2 . Even in the case of controlling the adjustment valves AD 12  and AD 22  by using the detection results from the concentration sensors S 1  and S 2  in this way, effects similar to the above embodiment can be obtained. 
     Note that the present invention is not limited to the embodiments described above, and various modifications are possible. In the embodiments described above, properties such as the sizes, shapes, and functions of the components illustrated in the accompanying drawings are not limited to what is illustrated, and such properties may be modified appropriately insofar as the effects of the present invention are still achieved. Otherwise, other appropriate modifications are possible without departing from the scope of the present invention. 
     For example, in the fuel cell system  300  according to the third embodiment above, a case is described in which the adjustment valve AD 22  is disposed in the outlet pipe  51  (third branch pipes  56 ) of the oxidant gas pipe  5  and the flow rate of the oxidant off-gas flowing through the outlet pipe  51  is adjusted. However, the placement of the adjustment valve AD 22  is not limited to the above and may be changed appropriately. 
     For example, the adjustment valve AD 22  may also be provided in a portion of the oxidant gas supply line P 10  (inlet pipe  50  of the oxidant gas pipe  5 ). In this case, the adjustment valve AD 22  may be disposed inside the fuel cell module  1  or outside the fuel cell module  1 . In the former case, the adjustment valve AD 22  is provided in the second branch pipes  55  of the inlet pipe  50 , and in the latter case, the adjustment valve AD 22  is provided in the first branch pipes  53  of the inlet pipe  50 . In the case where the adjustment valve AD 22  is provided outside the fuel cell module  1  (in the first branch pipes  53  of the inlet pipe  50 ), it is not necessary to make a space for disposing the adjustment valve AD 22  in the fuel cell module  1 , and consequently the dimensions of the fuel cell module  1  can be reduced. 
     Note that although the above examples describe a solid oxide fuel cell (SOFC), the present invention is not limited thereto, and obviously the present invention is applicable to any fuel cell having headers for respectively supplying or discharging a fuel gas and an oxidant gas to a plurality of fuel cell stacks. Such fuel cells include a polymer electrolyte fuel cell (PEFC), a phosphoric acid fuel cell (PAFC), and a molten carbonate fuel cell (MCFC), for example. 
     Features of the above embodiments are summarized below. The fuel cell system described in the above embodiments is a fuel cell system using a plurality of fuel cell stacks each of which includes a plurality of fuel cells that generate electricity through an electrochemical reaction between a fuel gas and an oxidant gas connected in series, the fuel cell system comprising a plurality of fuel cell cartridges in which the fuel cell stacks are connected in parallel and provided with headers so as to respectively supply the fuel gas and the oxidant gas to the plurality of fuel cell stacks through the headers and also respectively discharge a fuel off-gas and an oxidant off-gas through the headers, a fuel gas supply line that supplies the fuel gas to the plurality of fuel cell cartridges, a fuel off-gas discharge line that discharges the fuel off-gas from the plurality of fuel cell cartridges, and a first adjustment member, provided in at least one of the fuel gas supply line or the fuel off-gas discharge line, that adjusts a flow rate of the fuel gas or the fuel off-gas, wherein at least one portion of the first adjustment member includes a flexible pipe. 
     Also, the fuel cell system described in the above embodiments further comprises an oxidant gas supply line that supplies the oxidant gas to the fuel cell cartridges, an oxidant gas discharge line that discharges the oxidant off-gas from the fuel cell cartridges, and a second adjustment member, provided in at least one of the oxidant gas supply line or the oxidant gas discharge line, that adjusts a flow rate of the oxidant gas or the oxidant off-gas, wherein at least one portion of the second adjustment member includes a flexible pipe. 
     Also, the fuel cell system described in the above embodiments further comprises an adjustment valve provided in at least portion of the first adjustment member or the second adjustment member. 
     Also, the fuel cell system described in the above embodiments further comprises a control unit that controls the adjustment valve. 
     Also, the fuel cell system described in the above embodiments further comprises a temperature detection unit that detects a temperature of the fuel cell cartridges, wherein the control unit controls the adjustment valve according to a detection result from the temperature detection unit. 
     Also, the fuel cell system described in the above embodiments further comprises a voltage detection unit that detects a voltage of the fuel cell cartridges, wherein the control unit controls the adjustment valve according to a detection result from the voltage detection unit. 
     Also, the fuel cell system described in the above embodiments further comprises a first concentration detection unit that detects a concentration of the oxidant gas discharged from the fuel cell cartridges, wherein the control unit controls the adjustment valve according to a detection result from the first concentration detection unit. 
     Also, the fuel cell system described in the above embodiments further comprises a second concentration detection unit that detects a concentration of the fuel gas supplied to the fuel cell cartridges, wherein the control unit controls the adjustment valve according to a detection result from the second concentration detection unit. 
     Also, in the fuel cell system described in the above embodiments, solid oxide fuel cells are included as the fuel cells. 
     INDUSTRIAL APPLICABILITY 
     As described above, the present invention is effective at achieving a uniform flow rate of a fuel with respect to fuel cell cartridges, and is particularly useful in a fuel cell system provided with a solid oxide fuel cell module. 
     This application is based on Japanese Patent Application No. 2019-234467 filed on Dec. 25, 2019, the content of which is hereby incorporated in entirety.