Control method for fuel cell system and control device for fuel cell system

Abstract

A fuel cell system (1) generates power by reacting a fuel gas that has been reformed from a fuel and supplied to an anode (11) of a fuel cell (10) and a cathode gas that has been supplied to a cathode (13) of the fuel cell (10). According to the present invention, a control method for the fuel cell system involves reduction processing that is performed when the fuel cell system is started and is for passing fuel gas for keeping the anode in a reduced state through an anode gas passage (15) of the anode at a first speed. At the beginning of the reduction processing, there is a replacement period during which a fluid in the anode gas passage is replaced with the fuel gas. The speed at which the fuel gas is passed through the anode gas passage during the replacement period is a second speed that is faster than the first speed. The replacement period is ended when the aggregate amount of fuel gas that has been passed through the anode gas passage at the second speed after the fuel cell system is started has reached a prescribed amount that is at least the volume of the anode gas chamber.

Description

Control Method for Fuel Cell System and Control Device for Fuel Cell System

[0001] The present invention relates to a control method for a fuel cell system and a control device for a fuel cell system.

[0002] Patent Document 1 describes a method for starting a fuel cell. In this method, at the time of starting the fuel cell, a water film (generated water) film is formed on the surface of the cathode catalyst particles, and an oxide layer is formed on the surface of the cathode catalyst particles by an oxidation reaction using the formed water film. This oxide layer acts so that the potential of the cathode gas flow path does not locally increase on the upstream side where fuel is supplied by a power generation reaction using the fuel supplied with the start-up. By this action, at the time of starting the fuel cell, it becomes difficult to form a local battery on the downstream side of the cathode flow path, and deterioration of the cathode catalyst due to local current or reverse current is unlikely to occur.

[0003] International Publication No. 2017 / 150376

[0004] In the method of Patent Document 1, in order to form an oxide layer on the surface of the cathode catalyst particles, a voltage must be applied between the anode and the cathode, and a power source for that is required.

[0005] The present invention has been made in view of the above circumstances, and an object of the present invention is to suppress deterioration of the cathode catalyst without using an external power source during the start-up operation of the fuel cell system.

[0006] One aspect of the present invention, which solves the above-mentioned problems, provides a method for controlling a fuel cell system. In a fuel cell system, electricity is generated by reacting fuel gas, which is reformed from fuel and supplied to the anode electrode of the fuel cell, with cathode gas supplied to the cathode electrode of the fuel cell. In one aspect of the present invention, in the reduction process performed during the startup operation of the fuel cell system, fuel gas is passed through the anode gas flow path of the anode electrode at a first velocity to maintain the anode electrode in a reduced state. At the beginning of the reduction process, a replacement period is provided in which the fluid in the anode gas flow path is replaced with fuel gas. During the replacement period, the speed at which the fuel gas is passed through the anode gas flow path is set to a second velocity, which is faster than the first velocity. When the cumulative flow rate of the fuel gas passed through the anode gas flow path at the second velocity since the startup of the fuel cell system reaches a predetermined flow rate that is greater than or equal to the volume of the anode gas flow path, the replacement period is terminated.

[0007] According to the present invention, degradation of the cathode catalyst during the startup operation of a fuel cell system can be suppressed without using an external power source.

[0008] Figure 1 is a diagram of the fuel cell system according to the embodiment. Figure 2 is a graph showing an example of the potential difference generated between the electrodes of each cell stack in the fuel cell. Figure 3 is a flowchart showing an example of the reduction process. Figure 4 is a graph showing an example of the relationship between the potential difference of the cell stack during the reduction process and the current of the load circuit.

[0009] Embodiments and modifications thereof of the present invention will be described below with reference to the drawings. In the drawings, identical parts are denoted by the same reference numerals and their descriptions are omitted. The present invention can be applied, for example, to a fuel cell system mounted on an electric vehicle. The fuel cell system to which the present invention is applied is not limited to one mounted on an electric vehicle.

[0010] [First Embodiment] As shown in Figure 1, the fuel cell system 1 of the embodiment includes a fuel cell 10, a fuel tank 20, a vaporizer 30, a reformer 40, a first blower 50, a second blower 60, a combustor 70, and a controller 80.

[0011] The fuel cell 10 has a cell stack 14. The cell stack 14 is constructed by sequentially stacking an anode electrode 11, an electrolyte layer 12, and a cathode electrode 13. The cell stack 14 generates electricity by reacting the fuel gas supplied to the anode electrode 11 with the cathode gas supplied to the cathode electrode 13. In Figure 1, only one cell stack 14 of the fuel cell 10 is schematically shown, but the fuel cell 10 has multiple cell stacks 14. The multiple cell stacks 14 are arranged in a line along the flow of fuel gas and cathode gas supplied to the anode electrode 11 and cathode electrode 13 from the inlet to the outlet of the fuel cell 10. Figure 2 shows the case where the fuel cell 10 has 11 cell stacks 14, labeled CS1 to CS11. The cell stack 14 of CS1 is located on the inlet side of the fuel cell 10 in the flow of fuel gas and cathode gas, and the cell stack 14 of CS11 is located on the outlet side of the fuel cell 10 in the flow of fuel gas and cathode gas. The number of cell stacks 14 in the fuel cell 10 is not limited to the number shown in Figure 2, and may be 10 or less, or 12 or more. When the switch 92 of the load circuit 91 connecting the anode electrode 11 and cathode electrode 13 in Figure 1 to the load 90 is turned on, the electricity generated in each cell stack 14 of the fuel cell 10 is supplied to the load 90. The fuel gas can be, for example, hydrogen (H2), and the cathode gas can be, for example, air. The anode off gas discharged from the anode gas flow path 15 is supplied to the combustor 70 via the flow control valve 17. The cathode gas that has passed through the cathode gas flow path 16 is supplied to the combustor 70.

[0012] The fuel tank 20 stores the fuel to be reformed into fuel gas in the reformer 40. For example, methane (CH4) can be used as the fuel. The fuel may also be a hydrocarbon other than methane. The fuel from the fuel tank 20 is supplied to the vaporizer 30 by the power of the pump 21. The amount of fuel supplied to the vaporizer 30 per unit time can be adjusted by the flow control valve 22. The vaporizer 30 vaporizes the fuel supplied from the fuel tank 20 when it is superheated above the vaporization temperature. The vaporized fuel is supplied to the reformer 40. The reformer 40 has a reforming catalyst that causes the fuel to undergo a reforming reaction into fuel gas. For example, nickel, platinum, etc., can be used as the reforming catalyst. The reforming catalyst does not contribute to reforming at temperatures below the reformization temperature, but begins to contribute to reforming when heated above the reformization temperature. The fuel gas reformed from the fuel in the reformer 40 is supplied to the anode gas flow path 15 of the anode electrode 11. The fuel gas passes through the anode gas flow path 15 at a rate corresponding to the amount of fuel supplied to the vaporizer 30 per unit time. The anode electrode 11 is reduced by the fuel gas passing through the anode gas flow path 15. The cathode electrode 13 is oxidized by the cathode gas passing through the cathode gas flow path 16. The fuel cell 10 generates electricity from the potential difference between the reduced anode electrode 11 and the oxidized cathode electrode 13. The anode off gas discharged from the anode gas flow path 15 is supplied to the combustor 70. The cross-sectional area of ​​the supply path that supplies the anode off gas from the anode gas flow path 15 to the combustor 70 can be increased or decreased by the valve opening of the flow control valve 17 on the supply path. The first blower 50 supplies air. The destination of the air supply from the first blower 50 can be switched between the fuel cell 10 and the combustor 70, which will be described later, by a three-way valve 51. When the three-way valve 51 is switched to the fuel cell 10 side, air from the first blower 50 is supplied as cathode gas to the cathode gas passage 16 of the cathode electrode 13. The cathode-off gas discharged from the cathode gas passage 16 is supplied to the combustor 70. When the three-way valve 51 is switched to the combustor 70 side, air from the first blower 50 is supplied to the combustor 70, bypassing the fuel cell 10. The three-way valve 51 may be replaced with two two-way valves.The combustor 70 burns the fuel gas contained in the anode off gas that has passed through the anode gas flow path 15 with the cathode off gas that has passed through the cathode gas flow path 16 or with oxygen from the air from the first blower 50 that bypasses the fuel cell 10. By burning the fuel gas, the combustor 70 generates combustion heat used to heat the vaporizer 30 and the reformer 40.

[0013] The second blower 60 is located in the middle of the recirculation system 61. The recirculation system 61 is located upstream of the flow control valve 17 in the anode off-gas flow and connects the outlet of the anode gas flow path 15 to the inlet of the vaporizer 30. While the second blower 60 is operating, it recirculates the anode off-gas discharged from the anode gas flow path 15 to the vaporizer 30 via the recirculation system 61. The anode off-gas recirculated to the vaporizer 30 by the second blower 60 merges with the fuel supplied from the fuel tank 20 and vaporized by the vaporizer 30, and is then supplied to the reformer 40. If the recirculation rate of anode off-gas to the vaporizer 30 per unit time exceeds the amount of fuel supplied to the vaporizer 30 per unit time, the fuel vaporized in the vaporizer 30 is supplied to the reformer 40 by riding on the anode off-gas flow. In this embodiment, while the second blower 60 is stopped, the fuel gas reformed by the reformer 40 passes through the anode gas passage 15 at a first velocity, corresponding to the amount of fuel supplied to the vaporizer 30 by the pump 21 per unit time. When the second blower 60 is operated to recirculate the anode off gas to the vaporizer 30, the fuel gas and anode off gas pass through the anode gas passage 15 at a second velocity, which is faster than the first velocity. The anode off gas discharged from the anode gas passage 15 while the second blower 60 is operating splits into two. One portion of the anode off gas flows to the combustor 70 via the flow control valve 17, and the other portion flows to the recirculation system 61. Increasing the valve opening of the flow control valve 17 increases the cross-sectional area of ​​the path through which the anode off gas flowing to the combustor 70 passes, reducing the flow resistance and increasing the volumetric flow rate of the anode off gas passing through the path, making it easier to increase the gas passage velocity in the anode gas passage 15. If the pressure at the inlet of the vaporizer 30 rises above a predetermined pressure due to the anode off gas recirculated into the vaporizer 30, the relief valve 24 in the relief path 23 connected to the inlet of the vaporizer 30 opens. The relief path 23 is connected to the outlet side of the anode gas flow path 15, downstream of the flow rate control valve 17 in the anode off gas flow. The opened relief valve 24 releases the pressure exceeding the predetermined pressure at the inlet of the vaporizer 30 to the outlet side of the anode gas flow path 15, bypassing the anode electrode 11, and reduces the pressure at the inlet of the vaporizer 30 to the predetermined pressure.

[0014] The control method for the fuel cell system 1 can be performed by the controller 80 shown in Figure 1, which functions as a control device for the fuel cell system 1. The controller 80 has a general-purpose microcontroller equipped with an arithmetic unit and an input / output unit. The arithmetic unit includes a CPU (Central Processing Unit) and memory. The memory includes a read-only ROM (Read Only Memory) and a read and writeable RAM (Random Access Memory). The arithmetic unit of the controller 80 can virtually construct multiple information processing circuits by having the CPU execute a program stored in the ROM. The information processing circuits constructed in the arithmetic unit of the controller 80 can, for example, constitute the voltage measurement unit 81, current extraction unit 82, reduction operation unit 83, and speed adjustment unit 84 of the controller 80. The information processing circuits of the controller 80 may also be configured with dedicated hardware. Dedicated hardware includes devices such as application-specific integrated circuits (ASICs) arranged to perform the necessary information processing functions, and conventional circuit components.

[0015] The voltage measurement unit 81 measures the potential difference between the anode electrode 11 and the cathode electrode 13 of each cell stack 14 of the fuel cell 10. The current extraction unit 82 forms a circuit to extract the current generated by the fuel cell 10 to the load 90. The current extraction unit 82 can form a circuit to extract the current from the fuel cell 10 to the load 90 by turning on the switch 92 of the load circuit 91. The reduction operation unit 83 performs a reduction process on the anode electrode 11 during the startup operation that starts the fuel cell system 1 before entering steady-state operation in which the fuel cell 10 generates power to supply power to the load 90. During the reduction process, the fuel cell 10 is idled. During idling, fuel gas is passed through the anode gas flow path 15 at a first velocity to keep the anode electrode 11 in a reduced state. If the fuel cell system 1 stops operating while air is present in the anode gas flow path 15, the anode electrode 11 is oxidized by oxygen in the air during the shutdown. If steady-state operation of the fuel cell system 1 is started with the anode electrode 11 in an oxidized state, the reduction of the anode electrode 11 will not start immediately even if fuel gas is supplied to the anode gas flow path 15, and it will take time for the fuel cell 10 to start generating electricity. The anode electrode 11, which was oxidized while the fuel cell system 1 was stopped, returns to a reduced state during the start-up operation of the fuel cell system 1 through idling. The speed adjustment unit 84 adjusts the speed of the fuel gas passing through the anode gas flow path 15 during the reduction process.

[0016] If air is present in the anode gas flow path 15 while the fuel cell system 10 is shut down, the air in the anode gas flow path 15 is replaced with fuel gas by idling. The replacement of the anode gas flow path 15 proceeds sequentially from the cell stack 14 (CS1) on the inlet side of the fuel cell 10 to the cell stack 14 (CS11) on the outlet side, following the flow of fuel gas in the fuel cell 10. In the cell stack 14 on the inlet side of the fuel cell 10 after replacement is complete, the anode electrode 11 is reduced by the fuel gas in the anode gas flow path 15, and the cathode electrode 13 is oxidized by the cathode gas in the cathode gas flow path 16 or oxygen in the air. At the same time, in the cell stack 14 on the outlet side of the fuel cell 10 where replacement is not yet complete, the anode electrode 11 is oxidized by oxygen in the air in the anode gas flow path 15, and a local galvanic reaction occurs at the cathode electrode 13. Localized galvanic reactions occur at the cathode electrode 13 of the outlet cell stack 14 to generate electrons that are consumed in the oxidation of the cathode electrode 13 in the inlet cell stack 14. In localized galvanic reactions, the cathode catalyst of the cathode electrode 13 is consumed and degraded by CO (carbon monoxide) poisoning in order to reduce the cathode electrode 13. In the cell stack 14 where a localized galvanic reaction has occurred, the power generation efficiency deteriorates due to the consumption of the cathode catalyst. The speed adjustment unit 84 adjusts the fluid passage speed in the anode gas passage 15 to a second speed, which is faster than the first speed during idling, during the replacement period at the beginning of the reduction process when the fluid in the anode gas passage 15 is replaced with fuel gas. By adjusting the fluid passage speed in the anode gas passage 15 to the second speed, the situation in which a localized galvanic reaction occurs in the outlet cell stack 14 of the fuel cell 10 can be resolved more quickly. The replacement period is defined as the period until the cumulative flow rate of the fuel gas passed through the anode gas flow path 15 at the second velocity reaches a predetermined flow rate equal to or greater than the volume of the anode gas flow path 15. The cumulative flow rate of the fuel gas can be calculated based on the elapsed time from the point in time when the fluid flow velocity through the anode gas flow path 15 is adjusted to the second velocity.

[0017] The CPU of the controller 80 executes a program stored in memory during the startup operation of the fuel cell system 1, causing the fuel cell system 1 to perform the reduction process shown in Figure 3. During the reduction process, the three-way valve 51 is switched to the combustor 70 side, and air from the first blower 50 is supplied to the combustor 70, bypassing the fuel cell 10. At the start of the reduction process, the supply of fuel to the vaporizer 30 has not yet begun. The reduction operation unit 83 of the controller 80 starts heating the vaporizer 30 and the reformer 40 with the combustion heat of the combustor 70 during the reduction process (step S11). The combustion heat of the combustor 70 may be used to heat the fuel cell 10 in addition to heating the vaporizer 30 and the reformer 40. The vaporizer 30 does not vaporize fuel at temperatures below its vaporization temperature, and the reformer 40 does not reform fuel into fuel gas at temperatures below its reforming temperature. The CPU checks whether the vaporizer 30 has been heated to the vaporization temperature (step S13). The vaporization temperature is lower than the operating temperature, which is the lower limit of the temperature range in which the fuel cell 10 can generate electricity. Even if the fuel cell 10 is heated to the vaporization temperature together with the vaporizer 30, the reaction rate of the electrochemical reaction that occurs in the fuel cell 10 is slower than the reaction rate of the electrochemical reaction that occurs in the fuel cell 10 when it is heated to the operating temperature. If the vaporizer 30 has not been heated to the vaporization temperature (NO in step S13), step S13 is repeated. If the vaporizer 30 has been heated to the vaporization temperature (YES in step S13), the CPU checks whether the reformer 40 has been heated to the reforming temperature (step S15). The reforming temperature is higher than the vaporization temperature and lower than the operating temperature of the fuel cell 10. Even if the fuel cell 10 is heated to a reformable temperature together with the reformer 40, the reaction rate of the electrochemical reaction generated in the fuel cell 10 is slower than the reaction rate of the electrochemical reaction generated in the fuel cell 10 when heated to its operating temperature. The processes in steps S13 and S15 may be carried out using the temperatures of the vaporizer 30 and reformer 40 measured by, for example, a temperature sensor (not shown). If the reformer 40 is not heated to a reformable temperature (NO in step S15), step S15 is repeated.When the reformer 40 is heated to a reformable temperature (YES in step S15), the speed control unit 84 starts the recirculation of anode off-gas from the anode gas flow path 15 to the vaporizer 30 by the second blower 60 (step S17). Upon starting the recirculation of anode off-gas, the fluid passage speed in the anode gas flow path 15 becomes the second speed. Once the recirculation of anode off-gas is started, the recirculated anode off-gas passes through the vaporizer 30, reformer 40 and anode gas flow path 15 and is supplied to the combustor 70. When the reformer 40 starts supplying the reformed fuel gas to the anode electrode 11, the anode off-gas containing the fuel gas is supplied from the anode gas flow path 15 to the combustor 70, and the mixed gas with air supplied from the first blower 50 is burned in the combustor 70. The CPU sets the amount of air supplied per unit time from the first blower 50 to the combustor 70, bypassing the fuel cell 10, to an amount exceeding a predetermined amount λ1, in preparation for the case where the mixed gas is burned in the combustor 70 (step S19). The predetermined amount λ1 represents the amount of air supplied from the first blower 50 to the combustor 70 necessary to completely burn off the combustion gas in the anode off gas in the combustor 70. The predetermined amount λ1 may be determined, for example, based on the amount of combustion gas supplied per unit time to the combustor 70 when the fluid passage velocity in the anode gas flow path 15 is at a second velocity. Alternatively, the predetermined amount λ1 may be determined based on the amount of combustion gas supplied per unit time to the combustor 70 when the fluid passage velocity in the anode gas flow path 15 is at a first velocity. The CPU opens the flow control valve 17 to its fullest extent (step S21), and the reduction operation unit 83 starts supplying fuel to the vaporizer 30 by the pump 21 (step S23). When fuel supply is started in step S23, the fuel is vaporized by the vaporizer 30, which is heated to a vaporization temperature, and reformed into fuel gas by the reformer 40, which is heated to a reforming temperature. When the reformed fuel gas is supplied to the anode gas flow path 15, the idling of the fuel cell 10 starts and the replacement period begins. When idling starts, the anode electrode 11 is reduced by the fuel gas, and the cathode electrode 13 is oxidized by oxygen in the air from the three-way valve 51, creating a potential difference between the anode electrode 11 and the cathode electrode 13.The CPU checks whether the potential difference between the electrodes of the fuel cell 10 or one cell stack 14, as measured by the voltage measurement unit 81, has risen from 0V. If the potential difference between the electrodes has risen, as shown in the cell voltage graph in Figure 4, the CPU begins preparations to extract the current generated by the potential difference to the load 90, as shown in Figure 3 (step S25). Specifically, the current extraction unit 82 turns on the switch 92 to form a load circuit 91, which is a circuit that extracts the current generated by the potential difference between the electrodes from the fuel cell 10 to the load 90. Once the load circuit 91 is formed, a current corresponding to the potential difference between the electrodes flows through the load circuit 91, as shown from time t1 in Figure 4 onward. The load circuit 91 becomes a path for moving electrons from the anode electrode 11 to the cathode electrode 13, and the movement of electrons in the load circuit 91 suppresses the occurrence of localized galvanic reactions at the cathode electrode 13 of the cell stack 14, where the replacement of the anode gas flow path 15 has not been completed. When current flows through the load circuit 91, the CPU sets the control mode of the fuel cell system 1 to a mode in which the power generated by the fuel cell 10 is controlled to a control voltage Vcont by voltage control.

[0018] As shown in Figure 3, the CPU checks whether the replacement of air with fuel gas in the anode gas flow path 15 is complete (step S27). When the replacement of air with fuel gas in the anode gas flow path 15 is complete, the potential difference between the two electrodes reaches an electromotive force of a magnitude corresponding to the specifications of the cell stack 14 of the fuel cell 10. When the potential difference between the two electrodes reaches an electromotive force, it can be determined that the replacement of air with fuel gas in the anode gas flow path 15 is complete. In Figure 2, a potential difference is generated between the electrodes of the cell stacks 14 of CS1 to CS9 on the inlet side of the anode gas flow path 15, and in the cell stacks 14 of CS1 to CS6 on the inlet side, the potential difference generated between the electrodes has reached the electromotive force of the cell stack 14. If an electromotive force is detected between the electrodes of the cell stack 14 of CS11, which is located on the outlet side of the anode gas flow path 15, it may be determined that the replacement of the anode gas flow path 15 is complete for the entire fuel cell 10. If electromotive force is detected between the electrodes of all cell stacks 14 (CS1 to CS11) of the fuel cell 10, it may be determined that the replacement of the anode gas flow path 15 has been completed throughout the entire fuel cell 10. If the replacement of the anode gas flow path 15 has been completed in some of the cell stacks 14 on the inlet side, the hydrogen concentration in the anode off-gas will decrease and oxygen will be generated in the cathode off-gas in the cell stacks 14 on the outlet side where the replacement has not been completed due to the formation of a local cell. When the replacement of the anode gas flow path 15 has been completed in all cell stacks 14 of the fuel cell 10, the hydrogen concentration in the off-gas discharged from the fuel cell 10 will increase and oxygen will no longer be present. The CPU may estimate that the replacement of the anode gas flow path 15 has been completed in all cell stacks 14 of the fuel cell 10 by monitoring the change in the gas composition of the off-gas of the fuel cell 10 detected by the gas sensor 93.

[0019] If there are any cell stacks 14 in which the replacement of the anode gas flow path 15 has not been completed (NO in step S27), step S27 is repeated. If the replacement of the anode gas flow path 15 has been completed for all cell stacks 14 of the fuel cell 10 (YES in step S27), the CPU terminates the replacement period. Once the replacement period is complete, the speed control unit 84 changes the fluid flow rate through the anode gas flow path 15 from the second speed to the first speed (step S29). Specifically, the speed control unit 84 terminates the recirculation of anode off-gas by the second blower 60. When the fluid flow rate through the anode gas flow path 15 is reduced to the first speed, the fuel cell system 1 enters a mode in which the power generated by the fuel cell 10 is controlled by current. In conjunction with reducing the fluid flow rate through the anode gas flow path 15 to the first speed, the CPU adjusts the valve opening of the flow control valve 17 from fully open to the normal opening (step S31). The flow resistance of the path through which the anode off-gas flows to the combustor 70 does not hinder the fluid passage velocity in the anode gas flow path 15 to the first velocity when the flow control valve 17 is normally open. The CPU switches the three-way valve 51 to the fuel cell 10 side and starts supplying air from the first blower 50 as cathode gas to the cathode gas flow path 16 (step S33). Upon starting the supply of cathode gas, the potential difference generated between the electrodes of each cell stack 14 of the fuel cell 10 reaches the electromotive force corresponding to the specifications of the cell stack 14, and the fuel cell 10 starts generating electricity (step S35). When the fuel cell 10 starts generating electricity, the CPU switches the fuel cell 10 to a mode in which the power generated by the fuel cell 10 is controlled by current, as shown from time t2 in Figure 4 onwards. When the CPU switches the fuel cell 10 to the current control mode, it transitions the fuel cell system 1 from startup operation to steady-state operation (step S37) and ends the replacement process.

[0020] In this embodiment, during the replacement process, the fluid flow rate through the anode gas flow path 15 is changed to a second speed, which is faster than the first speed, until the replacement period is completed, when the air in the anode gas flow path 15 of all cell stacks 14 has been replaced with fuel gas. During the replacement period, in the cell stack 14 on the outlet side of the fuel cell 10 where the replacement of the anode gas flow path 15 has not been completed, the cathode catalyst deteriorates due to local galvanic reactions occurring at the cathode electrode 13, resulting in a decrease in power generation efficiency. By passing fuel gas through the anode gas flow path 15 at the second speed during the replacement period, the situation in which the cathode catalyst deteriorates due to local galvanic reactions can be quickly resolved, and the deterioration of the cathode catalyst can be suppressed without using an external power source.

[0021] When an electromotive force corresponding to the specifications of the fuel cell 10 is detected between the anode electrode 11 and the cathode electrode 13 of the fuel cell 10, it is considered that the replacement of the anode gas flow path 15 is complete and the anode electrode 11 is being reduced by the fuel gas in the anode gas flow path 15. Once the electromotive force between the electrodes of the fuel cell 10 is detected, the replacement period is terminated and the fluid flow rate through the anode gas flow path 15 is reduced to a first velocity. This reduces the amount of fuel gas consumed to maintain the anode electrode 11 in a reduced state after the anode gas flow path 15 has been replaced with fuel gas.

[0022] During the replacement process, if an electromotive force corresponding to the specifications of the cell stack 14 is detected between the electrodes of the cell stack 14 at the outermost outlet of the anode gas flow path 15, then an electromotive force should be detected between the electrodes of all the cell stacks 14 in the fuel cell 10. If an electromotive force is detected between the electrodes of all the cell stacks 14, then the replacement of all the anode gas flow paths 15 in the fuel cell 10 should be complete. By detecting the electromotive force for the cell stack 14 at the outermost outlet of the fuel cell 10, it is possible to detect that the air in the anode gas flow path 15 has been completely replaced with fuel gas up to the cell stack 14 at the outermost outlet of the fuel cell 10.

[0023] During the replacement process, if the replacement of the anode gas flow path 15 is not completed in at least some of the cell stacks 14, the gas composition of the off-gas discharged from the fuel cell 10 will be the same as when a local galvanic reaction occurs at the cathode electrode 13. If the replacement of the anode gas flow path 15 is completed in all of the cell stacks 14, the gas composition of the off-gas from the fuel cell 10 will be the same as when a local galvanic reaction does not occur at the cathode electrode 13. For example, the off-gas when a local galvanic reaction occurs has a higher concentration of fuel gas than the off-gas when a local galvanic reaction does not occur. For example, the off-gas when a local galvanic reaction occurs contains oxygen, while the off-gas when a local galvanic reaction does not contain oxygen. By detecting the gas composition of the off-gas from the fuel cell 10 and monitoring the change in gas composition, it is possible to determine the end of the replacement period in which the fuel gas is passed through the anode gas flow path 15 at a second rate.

[0024] In this embodiment, during the substitution process, a load circuit 91 is formed when a potential difference smaller than the electromotive force corresponding to the specifications is detected between the anode electrode 11 and the cathode electrode 13 in one of the fuel cell 10 or the cell stack 14 of the fuel cell 10 during the substitution period. The load circuit 91 serves as a path for electron movement from the anode electrode 11 to the cathode electrode 13, and this electron movement suppresses the occurrence of local galvanic reactions at the cathode electrode 13 of the cell stack 14 where the substitution of the anode gas flow path 15 has not been completed. By forming the load circuit 91, the formation of local galvanic reactions in the cell stack 14 where the substitution of the anode gas flow path 15 has not been completed can be suppressed, thereby suppressing a decrease in the power generation efficiency of the fuel cell 10.

[0025] In this embodiment, fuel supply to the vaporizer 30 is started after the vaporizer 30 has been heated to a vaporization temperature. The fuel supply to the vaporizer 30 is started after the reformer 40 has been heated to a reformation temperature. When fuel is supplied to the vaporizer 30, the supply of the reformed fuel gas from the reformer 40 to the fuel cell 10 begins, but the rate of local reactions occurring in the cell stack 14, where the replacement of the anode gas flow path 15 has not been completed, is slower than when the fuel cell 10 is at its operating temperature. By starting fuel vaporization before the fuel cell 10 is heated to its operating temperature and starting the supply of the reformed fuel gas to the anode electrode 11, the rate of deterioration of the cathode catalyst in the cell stack 14 during the replacement of the anode gas flow path 15 can be suppressed.

[0026] In this embodiment, the second blower 60 of the recirculation system 61, which recirculates the anode off gas back to the vaporizer 30, is started before the vaporization of the fuel by the vaporizer 30 begins. When the vaporization of the fuel by the vaporizer 30 begins, the fuel gas reformed from the vaporized fuel is supplied to the anode gas flow path 15, and the anode off gas is discharged from the anode gas flow path 15. If the second blower 60 of the recirculation system 61 is started after the vaporization of the fuel by the vaporizer 30 has begun, the anode off gas is supplied to the vaporizer 30, and the fuel gas in the anode off gas is reused for power generation by the fuel cell 10. Before the vaporization of the fuel by the vaporizer 30 begins, reforming from the vaporized fuel to fuel gas does not occur, and the reformed fuel gas is not supplied to the fuel cell 10, so the anode off gas does not contain fuel gas. Even if the second blower 60 of the recirculation system 61 is started, if the anode off gas does not contain fuel gas, reuse of the fuel gas by the fuel cell 10 cannot be expected. However, when the second blower 60 of the recirculation system 61 is started, the anode off gas recirculated from the anode gas passage 15 to the vaporizer 30 accelerates the fluid flow from the vaporizer 30 to the anode gas passage 15. When passing the fuel gas to be replaced with air through the anode gas passage 15, starting the second blower 60 of the recirculation system 61 before the vaporization of the fuel by the vaporizer 30 begins allows the speed at which the fuel gas passes through the anode gas passage 15 to be easily set to the second speed from the initial stage.

[0027] In this embodiment, when the pressure of the fuel vaporized in the vaporizer 30 reaches a predetermined pressure due to the anode off gas recirculated into the vaporizer 30, any pressure exceeding the predetermined pressure on the inlet side of the vaporizer 30 is released to the outlet side of the anode gas flow path 15, bypassing the anode electrode 11. By recirculating the anode off gas to the inlet side of the vaporizer 30 via the recirculation system 61, if the pressure of the fuel vaporized by the vaporizer 30 rises, any pressure exceeding the predetermined pressure is released to the outlet side of the anode gas flow path 15, bypassing the anode electrode 11. For example, by setting the predetermined pressure based on the fuel pressure at which the ignition point of the fuel drops to the ambient temperature of the location where the fuel cell 10 is located during operation, it is possible to suppress the pressure of the fuel vaporized by the vaporizer 30 from rising to the pressure at which it ignites and burns at the ambient temperature of the fuel cell 10.

[0028] In this embodiment, during steady-state operation of the fuel cell system 1 in which the fuel cell 10 generates electricity, air is supplied to the cathode electrode 13 as cathode gas. During startup operation before the fuel cell system 1 switches to steady-state operation, cathode gas is supplied to the combustor 70, which receives anode off gas discharged from the anode gas flow path 15, bypassing the cathode electrode 13. During startup operation, a mixture of anode off gas and cathode gas is burned in the combustor 70, and the amount of cathode gas supplied to the combustor 70 is controlled to exceed the amount necessary to completely burn the fuel gas in the anode off gas in the combustor 70. By controlling the flow rate of cathode gas supplied to the combustor 70 so that the fuel gas in the anode off gas supplied to the combustor 70 does not remain unburned due to a shortage of cathode gas, purification by combustion of the fuel can be appropriately performed during startup operation.

[0029] In this embodiment, the cross-sectional area of ​​the supply path that supplies anode off gas from the anode gas flow path 15 to the combustor 70 can be increased or decreased by the valve opening of the flow control valve 17. During the replacement period of the anode gas flow path 15, the cross-sectional area of ​​the supply path becomes larger than after the replacement period ends, and the pressure experienced by the fuel gas passing through the supply path decreases. Since the pressure experienced by the fuel gas passing through the anode gas flow path 15 connected to the supply path is lower during the replacement period than after the replacement period ends, the velocity of the fuel gas passing through the anode gas flow path 15 during the replacement period can be easily set to a second velocity that is faster than the first velocity after the replacement period ends.

[0030] The embodiments described above are merely examples of the present invention. Therefore, the present invention is not limited to the embodiments described above, and various modifications are possible in forms other than those described above, as long as they do not depart from the technical spirit of the present invention, depending on the design and other factors.

[0031] 1 Fuel cell system 10 Fuel cell 11 Anode electrode 13 Cathode electrode 14 Cell stack 15 Anode gas flow path 30 Vaporizer 61 Recirculation system 70 Combustor 80 Controller (Fuel cell system control device) 83 Recirculation operation unit 84 Speed ​​control unit 90 Load 91 Load circuit

Claims

1. A control method for a fuel cell system that generates electricity by reacting a fuel gas supplied to the anode electrode of a fuel cell, which is reformed from fuel, with a cathode gas supplied to the cathode electrode of the fuel cell, the method comprising: in a reduction process performed during the startup operation of the fuel cell system, the fuel gas is passed through the anode gas flow path of the anode electrode at a first velocity to maintain the anode electrode in a reduced state; a replacement period is provided at the beginning of the reduction process in which the fluid in the anode gas flow path is replaced with the fuel gas; the speed at which the fuel gas is passed through the anode gas flow path during the replacement period is set to a second velocity which is faster than the first velocity; and when the cumulative flow rate of the fuel gas passed through the anode gas flow path at the second velocity since the startup of the fuel cell system reaches a predetermined flow rate which is greater than or equal to the volume of the anode gas flow path, the replacement period is terminated.

2. A control method for a fuel cell system according to claim 1, wherein the replacement period is terminated when an electromotive force of a magnitude corresponding to the specifications of the fuel cell is detected between the anode electrode and the cathode electrode.

3. A control method for a fuel cell system according to claim 2, which involves detecting the electromotive force of the cell stack located furthest from the outlet side of the anode gas flow path among a plurality of cell stacks of the fuel cell.

4. A control method for a fuel cell system according to claim 1, which involves detecting the gas composition of the off-gas discharged from the fuel cell and determining the end of the replacement period based on the detected gas composition.

5. A control method for a fuel cell system according to any one of claims 1 to 4, wherein, during the replacement period, a circuit is formed to extract a current from the fuel cell to a load when a potential difference smaller than the electromotive force corresponding to the specifications of the fuel cell is detected between the anode and cathode electrodes of one of the plurality of cell stacks of the fuel cell or the fuel cell itself.

6. A control method for a fuel cell system according to any one of claims 1 to 5, wherein heating of a vaporizer that vaporizes the fuel to be reformed into fuel gas is started by starting the fuel cell system, and when the vaporizer is heated to a vaporization temperature of the fuel that is lower than the lower limit of the operating temperature range in which the fuel cell can generate electricity, vaporization of the fuel by the vaporizer is started.

7. A control method for a fuel cell system according to any one of claims 1 to 6, wherein a recirculation system is started before the start of vaporization of the fuel by the vaporizer, to recirculate the anode off gas discharged from the anode gas flow path to the vaporizer that vaporizes the fuel to be reformed into the fuel gas.

8. A control method for a fuel cell system according to claim 7, wherein when the pressure of the fuel vaporized by the vaporizer reaches a predetermined pressure, the pressure exceeding the predetermined pressure on the inlet side of the vaporizer is released to the outlet side of the anode gas flow path, bypassing the anode electrode.

9. A control method for a fuel cell system according to any one of claims 1 to 7, wherein the cathode gas supplied to the cathode electrode during steady-state operation of the fuel cell system in which the fuel cell generates electricity is supplied to a combustor that is supplied with anode off gas discharged from the anode gas flow path, bypassing the cathode electrode, during the startup operation of the fuel cell system before switching to steady-state operation, the mixture of the anode off gas and the cathode gas is burned in the combustor, and the amount of cathode gas supplied to the combustor is controlled to exceed the amount necessary to completely burn off the fuel gas in the anode off gas in the mixture gas.

10. A control method for a fuel cell system according to any one of claims 1 to 8, wherein the cross-sectional area of ​​the supply path that supplies the anode off gas discharged from the anode gas flow path to the combustor is made larger during the replacement period than after the end of the replacement period.

11. A control device for a fuel cell system that generates electricity by reacting a fuel gas supplied to the anode electrode of a fuel cell, which is reformed from fuel, with a cathode gas supplied to the cathode electrode of the fuel cell, comprising: a reduction operation unit that, in the startup operation of the fuel cell system, causes a reduction process to be performed in which the fuel gas is passed through the anode gas flow path of the anode electrode at a first velocity in order to maintain the anode electrode in a reduced state; and a speed adjustment unit that, during the replacement period at the beginning of the reduction process in which the fluid in the anode gas flow path is replaced with the fuel gas, sets the speed at which the fuel gas is passed through the anode gas flow path to a second velocity faster than the first velocity, wherein the speed adjustment unit terminates the replacement period when the cumulative flow rate of the fuel gas passed through the anode gas flow path at the second velocity since the startup of the fuel cell system reaches a predetermined flow rate that is greater than or equal to the volume of the anode gas flow path.

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