JP2010123374A - Operation method of fuel cell system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To be capable of appropriately corresponding to the change of an oxidant gas flow rate by simple control and preventing deterioration of a cathode electrode as much as possible. <P>SOLUTION: The operation method of a fuel cell system 10 includes the processes of: setting the target operation temperature T1°C for keeping a constant power generation efficiency during the power generation of a fuel cell stack 28; setting the target fuel utilization factor Uf1 of the fuel cell stack 28; calculating an A/F value to be obtained by dividing the supply amount of oxidant gas to the fuel cell stack 28 by the fuel gas supply amount; measuring the power generation temperature of the fuel cell stack 28; comparing the A/F value with a prescribed value which is previously set; and keeping the power generation temperature of the fuel cell stack 28 to the target operation temperature T1°C by adjusting the supply amount of the oxidant gas when it is determined that the A/F value is equal to or higher than the prescribed value. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a fuel cell stack in which a plurality of fuel cells that generate power by an electrochemical reaction between a fuel gas and an oxidant gas are stacked, a fuel gas supply device that supplies the fuel gas to the fuel cell stack, and the fuel cell stack An oxidant gas supply device for supplying the oxidant gas to the oxidant gas, a control device for controlling a supply amount of the fuel gas from the fuel gas supply device and a supply amount of the oxidant gas from the oxidant gas supply device; It is related with the operating method of a fuel cell system provided with.

  In general, a solid oxide fuel cell (SOFC) uses an oxide ion conductor, for example, stabilized zirconia, as a solid electrolyte, and an electrolyte / electrode in which an anode electrode and a cathode electrode are disposed on both sides of the solid electrolyte. A joined body (hereinafter also referred to as MEA) is sandwiched between separators (bipolar plates). This fuel cell is normally used as a fuel cell stack in which a predetermined number of electrolyte / electrode assemblies and separators are laminated.

  In order to control the operation of this type of fuel cell stack, for example, a fuel cell system disclosed in Patent Document 1 is known. In order to suppress the temperature difference generated in the fuel cell stack, this fuel cell system measures the temperature difference between air and fuel at the inlet of the fuel cell stack and performs feedback control so that the temperature difference becomes small. Yes.

  Further, the solid oxide fuel cell system disclosed in Patent Document 2 is provided with an air content delivery / supply means for distributing and delivering the air flowing toward the air electrode, and the air content delivery / supply system. The means has a central feed flow path section and an end feed flow path section, and when the flow rate of air flowing through the air flow path is increased, the air content delivery feed means is adapted to the total air flow rate flowing through the air flow path. The air is distributed and supplied so that the distribution ratio of the distributed air flow rate distributed and supplied to the end feed flow path portion becomes small.

  Furthermore, in the operation method of the solid oxide fuel cell disclosed in Patent Document 3, even when the load is fluctuated during operation, the occurrence of hunting is prevented, and the temperature inside the stack is kept constant for the power generation reaction. In order to maintain the temperature, the amount of current flowing through the power generation cell in accordance with the power generation reaction is detected, and the supply temperature of the oxidant gas is controlled according to the detected value of the current amount.

JP 2005-071913 A JP 2008-159362 A JP 2004-349214 A

  By the way, during the operation of the SOC, an oxidant gas is supplied to the cathode electrode under the action of the air pump. At this time, if the supply amount of the oxidant gas rapidly decreases due to the stoppage of the air pump or the like, the exhaust gas containing the fuel gas from the anode electrode side may flow backward toward the cathode electrode side. For this reason, the cathode electrode is exposed to a rapid reducing atmosphere, and there is a problem that the constituent material of the cathode electrode is deteriorated or damaged.

  However, in the above Patent Documents 1 to 3, there is a problem that it is not possible to cope with the deterioration of the cathode electrode due to the oxidant gas supply amount being reduced to a predetermined amount or less.

  The present invention solves this type of problem, and can easily cope with fluctuations in the oxidant gas flow rate with simple control, and can prevent deterioration of the cathode electrode as much as possible. It aims at providing the operating method of a battery system.

  The present invention relates to a fuel cell stack in which a plurality of fuel cells that generate power by an electrochemical reaction between a fuel gas and an oxidant gas are stacked, a fuel gas supply device that supplies the fuel gas to the fuel cell stack, and the fuel cell stack An oxidant gas supply device for supplying the oxidant gas to the oxidant gas, a control device for controlling a supply amount of the fuel gas from the fuel gas supply device and a supply amount of the oxidant gas from the oxidant gas supply device; It is related with the operating method of a fuel cell system provided with.

  The operation method includes a step of setting a target operating temperature for maintaining a constant power generation efficiency during power generation of the fuel cell stack, a step of setting a target fuel utilization rate of the fuel cell stack, and the fuel cell stack. The step of calculating an A / F value obtained by dividing the supply amount of the oxidant gas to the supply amount of fuel gas, the step of measuring the power generation temperature of the fuel cell stack, and the A / F value are preset. A step of comparing with the specified value, and when the A / F value is determined to be greater than or equal to the specified value, the supply amount of the oxidant gas is increased or decreased to set the power generation temperature of the fuel cell stack to the target operation. Maintaining the temperature.

  The operation method further includes a step of comparing the power generation temperature of the fuel cell stack with the target operation temperature, and when it is determined that the power generation temperature is higher than the target operation temperature, the supply amount of the oxidant gas is increased. On the other hand, when it is determined that the power generation temperature is lower than the target operation temperature, it is preferable to reduce the supply amount of the oxidant gas.

  For this reason, the power generation temperature of the fuel cell stack can be maintained at the optimum target operation temperature by simple control of increasing or decreasing the supply amount of the oxidant gas, and efficient operation can be performed.

  Furthermore, this operation method preferably includes a step of comparing the power generation temperature with a corrected target operation temperature lower than the target operation temperature when it is determined that the A / F value is less than the specified value. Accordingly, it is possible to confirm a decrease in the flow rate of the oxidant gas supplied to the fuel cell stack. As a result, by lowering the operating temperature of the fuel cell stack, it becomes possible to prevent the cathode electrode from being deteriorated and to suppress the efficiency from being reduced to a minimum, thereby continuing the operation of the fuel cell stack.

  Furthermore, in this operation method, when it is determined that the power generation temperature is higher than the corrected target operating temperature, the target operating temperature is not less than the corrected target operating temperature and the target operating temperature while maintaining the target fuel utilization rate. It is preferable to have the process of setting to temperature below. Therefore, it is possible to operate the fuel cell stack while suppressing deterioration of the cathode electrode, maintaining a high fuel utilization rate, and minimizing a decrease in efficiency of the fuel cell stack.

  In addition, the operation method may include a step of setting the target fuel utilization rate to be low and setting the target operation temperature to the corrected target operation temperature when it is determined that the power generation temperature is lower than the corrected target operation temperature. preferable. Therefore, it is possible to continuously operate the fuel cell stack while suppressing deterioration of the cathode electrode and minimizing a decrease in efficiency of the fuel cell stack.

  According to the present invention, a desired power generation temperature and fuel utilization rate can be maintained during power generation operation of the fuel cell stack, and highly efficient power generation is reliably performed. In addition, under the conditions that the cathode electrode does not deteriorate, the power generation temperature of the fuel cell stack is optimized by simple control such as increasing / decreasing the supply amount of oxidant gas and adjusting the operating temperature and fuel utilization rate. It is possible to maintain the target operation temperature at a high level, and to perform efficient operation.

  FIG. 1 is a schematic configuration diagram illustrating a mechanical circuit of a fuel cell system 10 to which an operation method according to an embodiment of the present invention is applied, and FIG. 2 is a circuit diagram of the fuel cell system 10.

  The fuel cell system 10 is used for various purposes such as in-vehicle use as well as stationary use. The fuel cell system 10 includes a fuel cell module (SOFC module) 12 that generates power by an electrochemical reaction between a fuel gas (a gas in which methane and carbon monoxide are mixed in hydrogen gas) and an oxidant gas (air), and the fuel cell. A raw fuel supply device (including a fuel gas pump) 16 for supplying raw fuel (for example, city gas) to the module 12 and an oxidant gas supply device (including an air pump) for supplying the oxidant gas to the fuel cell module 12 ) 18, a water supply device (including a water pump) 20 for supplying water to the fuel cell module 12, a power conversion device 22 for converting DC power generated in the fuel cell module 12 into required specification power, And a control device 24 for controlling the power generation amount of the fuel cell module 12.

  The fuel cell module 12 includes a solid oxide fuel cell stack 28 in which a plurality of solid oxide fuel cells 26 are stacked in the vertical direction. The fuel cell 26 includes, for example, a plate-like electrolyte / electrode assembly (MEA) in which a cathode electrode and an anode electrode are provided on both surfaces of an electrolyte made of an oxide ion conductor such as stabilized zirconia.

  As shown in FIG. 2, a fuel gas supply communication hole 30 is provided at the center of the fuel cell stack 28 so as to extend in the stacking direction (the direction of arrow A), and each fuel cell is extended from the fuel gas supply communication hole 30. Fuel gas is supplied to the 26 anode electrodes.

  A plurality of oxidant gas supply communication holes 32 are provided concentrically around the fuel gas supply communication hole 30 at the center edge of the fuel cell stack 28, and each fuel cell 26 is connected through the oxidant gas supply communication hole 32. Air is supplied to the cathode electrode. The oxidant gas supply communication hole 32 also serves as the exhaust gas communication hole 34 for discharging the fuel gas used in the anode electrode and the air used in the cathode electrode.

  As shown in FIG. 1, on the upper end side (or lower end side in the stacking direction) of the fuel cell stack 28, a heat exchanger 36 for heating before supplying the oxidant gas to the fuel cell stack 28, and the raw fuel In order to generate a mixed fuel of water and water vapor, an evaporator 38 for evaporating water and a reformer 40 for reforming the mixed fuel to generate a reformed gas are provided.

  On the lower end side (or upper end side in the stacking direction) of the fuel cell stack 28, a load is applied to apply a tightening load along the stacking direction (arrow A direction) to the fuel cells 26 constituting the fuel cell stack 28. A mechanism 42 is disposed (see FIG. 2).

The reformer 40 removes higher hydrocarbons (C ₂₊ ) such as ethane (C ₂ H ₆ ), propane (C ₃ H ₈ ) and butane (C ₄ H ₁₀ ) contained in city gas (raw fuel). , A pre-reformer for steam reforming to a fuel gas mainly containing methane (CH ₄ ), hydrogen, and CO, and is set to an operating temperature of several hundred degrees Celsius.

  The fuel cell 26 has an operating temperature as high as several hundred degrees Celsius. In the electrolyte / electrode assembly, methane in the fuel gas is reformed to obtain hydrogen and CO, and this hydrogen and CO are supplied to the anode electrode. The

  The heat exchanger 36 includes an exhaust gas passage 44 for flowing used reaction gas (hereinafter also referred to as exhaust gas) discharged from the fuel cell stack 28, and air for flowing air, which is a fluid to be heated, in a counterflow with the exhaust gas. And a passage 46. The upstream side of the air passage 46 communicates with the air supply pipe 48, and the downstream side of the air passage 46 communicates with the oxidant gas supply communication hole 32 of the fuel cell stack 28.

  The evaporator 38 is provided with a raw fuel passage 50 and a water passage 52. The raw fuel passage 50 is connected to the raw fuel supply device 16, and the reformer 40 communicates with the fuel gas supply communication hole 30. The water passage 52 is connected to the water supply device 20, while the oxidant gas supply device 18 is connected to the air supply pipe 48.

  The raw fuel supply device 16, the oxidant gas supply device 18, and the water supply device 20 are controlled by a control device 24, and a detector 56 that detects fuel gas is electrically connected to the control device 24. . For example, a commercial power source 58 (or a load, a secondary battery, or the like) is connected to the power conversion device 22 (see FIG. 2).

  As shown in FIGS. 1 and 2, the fuel cell system 10 includes a temperature sensor 60 that detects the temperature of the fuel cell stack 28, and the flow rate of raw fuel (fuel gas) supplied from the raw fuel supply device 16 to the evaporator 38. And a second flow rate sensor 62b for detecting the flow rate of air (oxidant gas) supplied from the oxidant gas supply device 18 to the heat exchanger 36.

  The temperature sensor 60, the first flow sensor 62a, and the second flow sensor 62b are connected to the control device 24. The control device 24 also has a function of controlling the amount of fuel gas supplied from the raw fuel supply device 16 and the amount of air supplied from the oxidant gas supply device 18.

  The operation of the fuel cell system 10 configured as described above will be described below.

As shown in FIGS. 1 and 2, under the driving action of the raw fuel supply device 16, for example, city gas (CH ₄ , C ₂ H ₆ , C ₃ H ₈ , C ₄ H ₁₀₎ is provided in the raw fuel passage 50. And other raw fuel is supplied. On the other hand, under the driving action of the water supply device 20, water is supplied to the water passage 52, and oxidant gas is supplied to the air supply pipe 48 via the oxidant gas supply device 18. Is supplied.

In the evaporator 38, steam is mixed with the raw fuel to obtain a mixed fuel, and this mixed fuel is supplied to the reformer 40. The mixed fuel is steam reformed in the reformer 40, and C ₂₊ hydrocarbons are removed (reformed) to obtain a reformed gas mainly composed of methane. This reformed gas is supplied to the fuel gas supply communication hole 30 of the fuel cell stack 28.

  On the other hand, when the air supplied from the air supply pipe 48 to the heat exchanger 36 moves along the air passage 46 of the heat exchanger 36, the air is heated between the exhaust gas that moves along the exhaust gas passage 44, which will be described later. Exchange is performed and preheated to the desired temperature. The air heated by the heat exchanger 36 is supplied to the oxidant gas supply communication hole 32 of the fuel cell stack 28.

  Therefore, fuel gas is supplied to the anode electrode of each fuel cell 26 and air is supplied to the cathode electrode, and power generation is performed by a chemical reaction. The exhaust gas containing the fuel gas and air used for the reaction is discharged from the fuel cell stack 28 through the exhaust gas passage 44 as an off gas.

  Next, the operation method according to the present embodiment will be described along the flowchart shown in FIG.

  First, the control device 24 sets a target operating temperature T1 ° C. for the fuel cell system 10 (step S1). The temperature of the fuel cell stack 28 and the power generation efficiency have the relationship shown in FIG. 4, and a desired power generation efficiency can be obtained at the stack temperature T1 ° C., while the decrease in efficiency is relatively low until the stack temperature T2 ° C. If the stack temperature is less than T2 ° C., the efficiency is significantly reduced. Here, the stack temperature T1 ° C. is, for example, 750 ° C. to 800 ° C., and the stack temperature T2 ° C. is, for example, 700 ° C.

  Next, it progresses to step S2 and the target fuel utilization factor Uf1 is set. The fuel utilization rate Uf is calculated from the output current / theoretical current of the supplied fuel, and the fuel utilization rate Uf and the power generation efficiency have the relationship shown in FIG. The target fuel utilization rate Uf1 is set to a value that can maintain high efficiency under the condition that no redox reaction occurs in the anode electrode.

  In the fuel cell module 12, raw fuel is supplied from the raw fuel supply device 16 and air is supplied from the oxidant gas supply device 18. The supply amount of raw fuel supplied to the fuel cell module 12 is detected via the first flow rate sensor 62a (step S3), and the supply amount of air supplied to the fuel cell module 12 is the second flow rate. It is detected via the sensor 62b (step S4).

  The control device 24 calculates A / F (air supply amount / raw fuel supply amount) using the detected raw fuel supply amount and air supply amount (step S5), and the power generation temperature of the fuel cell stack 28 is calculated. It is detected via the temperature sensor 60 (step S6).

  The detected A / F is compared with a specified value (step S7). The specified value of A / F is set to a value that does not deteriorate due to exposure of the cathode electrode to a reducing atmosphere due to a decrease in the air supply amount. When it is determined that the detected A / F is equal to or greater than the specified value (YES in step S7), the process proceeds to step S8.

  In step S8, the detected power generation temperature is compared with the target operating temperature T1 ° C. If it is determined that the detected power generation temperature is higher than the target operating temperature T1 ° C. (YES in step S8), the process proceeds to step S9, and the supply amount of air is increased. For this reason, the fuel cell stack 28 is cooled by the increase in the air flow rate, and the power generation temperature is lowered. And it progresses to step S10 and said process is performed until a driving | operation is complete | finished.

  If it is determined in step S8 that the power generation temperature is lower than the target operating temperature T1 ° C. (NO in step S8), the process proceeds to step S11, and the supply amount of air is reduced. For this reason, in the fuel cell stack 28, the power generation temperature rises due to a decrease in the air supply amount having a cooling function, and this power generation temperature is maintained at the target operating temperature T1 ° C. or higher.

  On the other hand, if it is determined in step S7 that the calculated A / F is less than the specified value (NO in step S7), the process proceeds to step S12, where the power generation temperature is compared with the stack temperature T2 ° C. As shown in FIG. 4, the stack temperature T2 ° C. is set to a temperature at which the decrease in efficiency is relatively small.

  Accordingly, when it is determined that the power generation temperature is equal to or higher than the stack temperature T2 ° C. (YES in step S12), the process proceeds to step S13, and the power generation temperature higher than the stack temperature T2 ° C. (T2 ° C. ≦ power generation temperature <T1 ° C.). ) Is set to the corrected target operating temperature. At that time, the target fuel utilization rate Uf1 is maintained. The fuel cell stack 28 is operated based on the corrected target operation temperature.

  If it is determined in step S12 that the power generation temperature is lower than the stack temperature T2 ° C. (NO in step S12), the process proceeds to step S14. In step S14, a low target fuel utilization rate Uf2 (see FIG. 5) is set. By reducing the fuel utilization rate, the concentration of the fuel gas discharged to the exhaust gas passage 44 increases, and the unreacted gas burns, so the exhaust gas temperature increases and the stack temperature rises. Therefore, the temperature of the fuel cell stack 28 can be kept high.

  Further, in step S14, a corrected target operating temperature T2 ° C. is set. This is to maintain the minimum efficiency reduction. Then, the fuel cell stack 28 is operated based on the target fuel utilization rate Uf2 and the corrected target operation temperature T2 ° C.

  In this case, in the present embodiment, during the power generation of the fuel cell stack 28, a step (step S1) of setting a target operating temperature T1 ° C. for maintaining a constant power generation efficiency, and the target fuel utilization of the fuel cell stack 28 A step of setting the rate Uf1 (step S2) and an A / F (air supply amount / fuel gas supply amount) value obtained by dividing the supply amount of oxidant gas to the fuel cell stack 28 by the supply amount of fuel gas is calculated. A step (step S5) of measuring, a step of measuring the power generation temperature of the fuel cell stack 28 (step S6), a step of comparing the A / F value with a preset specified value (step S7), When it is determined that the A / F value is equal to or greater than the specified value (YES in step S7), the supply amount of the oxidant gas is increased / decreased (steps S9 and S11). The power generation temperature and a step of maintaining said target operating temperature T1 ° C..

  Therefore, during the power generation operation of the fuel cell stack 28, the desired power generation temperature (T1 ° C.), fuel utilization rate (Uf1), and A / F value can be maintained, and highly efficient power generation is reliably performed. . Moreover, under conditions where the cathode electrode does not deteriorate, the power generation temperature of the fuel cell stack 28 can be controlled by simple control such as increasing or decreasing the supply amount of the oxidant gas or adjusting the operating temperature or the fuel utilization rate. The optimum target operating temperature (T1 ° C.) can be maintained, and an effect that efficient operation can be performed is obtained (see the first region in FIG. 6).

  In addition, this operation method includes a step (step S8) of comparing the power generation temperature of the fuel cell stack 28 with the target operation temperature (T1 ° C), and when the power generation temperature is higher than the target operation temperature (T1 ° C). When the determination is made (YES in step S8), the supply amount of the oxidant gas is increased (step S9), while when the power generation temperature is determined to be lower than the target operating temperature (T1 ° C.), the oxidant The gas supply amount is decreased (step S11).

  Therefore, the power generation temperature of the fuel cell stack 28 can be maintained at the optimum target operating temperature (T1 ° C.) by simple control of increasing / decreasing the supply amount of the oxidant gas, and efficient operation can be performed. Become.

  Further, in this operation method, when it is determined that the A / F value is less than the specified value (NO in step S7), the power generation temperature is set to a corrected target operation temperature (T2 ° C.) lower than the target operation temperature. It has the process of comparing (step S12). For this reason, it is possible to confirm the decrease in the flow rate of the oxidant gas supplied to the fuel cell stack 28 and to lower the operating temperature of the fuel cell stack 28. As a result, it is possible to prevent the cathode electrode from deteriorating and to suppress the minimum efficiency reduction, and to continue the operation of the fuel cell stack 28.

  Furthermore, in this operation method, when it is determined that the power generation temperature is higher than the corrected target operation temperature (T2 ° C.) (YES in step S12), the target operation temperature is maintained while maintaining the target fuel utilization rate (Uf1). Is set to a temperature equal to or higher than the corrected target operating temperature (T2 ° C.) and lower than the target operating temperature (T1 ° C.) (step S13).

  Therefore, by keeping the A / F constant, the deterioration of the cathode electrode is suppressed, the fuel utilization rate is maintained high, and the decrease in the efficiency of the fuel cell stack 28 is suppressed to the minimum. The vehicle can be operated (see the second region in FIG. 6).

  Further, in this operation method, when it is determined that the power generation temperature is lower than the corrected target operation temperature (T2 ° C.) (NO in step S12), the target fuel utilization rate (Uf2) is set low and the target operation temperature is set to be lower. And a step of setting the corrected target operating temperature (step S14).

  For this reason, the exhaust gas temperature is raised by the low target fuel utilization rate (Uf2) to raise the temperature of the fuel cell stack 28, and the power generation temperature of the fuel cell stack 28 can be maintained at the corrected target operating temperature (T2 ° C.). . Accordingly, the deterioration of the cathode electrode can be suppressed by keeping A / F constant, and the decrease in the efficiency of the fuel cell stack 28 can be suppressed to the minimum, and the fuel cell stack 28 can be continuously operated. This becomes possible (see the third region in FIG. 6).

1 is a schematic configuration explanatory diagram showing a mechanical circuit of a fuel cell system to which an operation method according to an embodiment of the present invention is applied. It is a circuit diagram of the fuel cell system. It is a flowchart explaining the said driving | running method. It is a relation explanatory view of stack temperature and power generation efficiency. It is an explanatory view of the relationship between the fuel utilization rate and the power generation efficiency. It is an explanatory view of a relation between an output and an A / F value.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... Fuel cell system 12 ... Fuel cell module 16 ... Raw fuel supply device 18 ... Oxidant gas supply device 20 ... Water supply device 22 ... Power converter 24 ... Control device 26 ... Fuel cell 28 ... Fuel cell stack 36 ... Heat exchange Unit 38 ... Evaporator 40 ... Reformer 60 ... Temperature sensor 62a, 62b ... Flow rate sensor

Claims (5)

A fuel cell stack in which a plurality of fuel cells that generate electricity by an electrochemical reaction between a fuel gas and an oxidant gas are stacked;
A fuel gas supply device for supplying the fuel gas to the fuel cell stack;
An oxidant gas supply device for supplying the oxidant gas to the fuel cell stack;
A control device for controlling the supply amount of the fuel gas from the fuel gas supply device and the supply amount of the oxidant gas from the oxidant gas supply device;
A method for operating a fuel cell system comprising:
Setting a target operating temperature for maintaining a constant power generation efficiency during power generation of the fuel cell stack;
Setting a target fuel utilization rate of the fuel cell stack;
Calculating an A / F value obtained by dividing the supply amount of the oxidant gas to the fuel cell stack by the supply amount of the fuel gas;
Measuring the power generation temperature of the fuel cell stack;
Comparing the A / F value with a preset specified value;
Maintaining the power generation temperature of the fuel cell stack at the target operating temperature by increasing or decreasing the supply amount of the oxidant gas when it is determined that the A / F value is equal to or greater than the specified value;
A method for operating a fuel cell system, comprising: The operation method according to claim 1, further comprising a step of comparing the power generation temperature of the fuel cell stack with the target operation temperature.
When it is determined that the power generation temperature is higher than the target operating temperature, the supply amount of the oxidant gas is increased,
An operation method of a fuel cell system, wherein when the power generation temperature is determined to be lower than the target operation temperature, the supply amount of the oxidant gas is decreased.   The operation method according to claim 1 or 2, wherein when the A / F value is determined to be less than the specified value, the power generation temperature is compared with a corrected target operation temperature lower than the target operation temperature. A method for operating a fuel cell system, comprising:   4. The operation method according to claim 3, wherein when the power generation temperature is determined to be higher than the corrected target operation temperature, the target operation temperature is set to be equal to or higher than the corrected target operation temperature while maintaining the target fuel utilization rate. And a method of operating the fuel cell system, comprising a step of setting the temperature to be lower than the target operating temperature.   4. The operation method according to claim 3, wherein when it is determined that the power generation temperature is lower than the corrected target operating temperature, the target fuel utilization rate is set low, and the target operating temperature is set to the corrected target operating temperature. A method of operating a fuel cell system, comprising the step of:

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