JP2010113867A - Fuel cell system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fuel cell system not always requiring a condenser, an evaporator, or a water tank, improving response to a variation of an external load, and elongating its life time. <P>SOLUTION: The fuel cell system includes a plurality of solid oxide fuel cell stacks generating power by separately flowing fuel gas and air of flow rates according to a required output of an external load, and gas returning passages returning fuel offgas containing moisture flowed through and exhausted from one of the solid oxide fuel cell stacks to other solid oxide fuel cell stack. In each of the gas returning passages, flow rate adjusters for increasing or decreasing the flow rates of the fuel offgas flowing through the gas returning passages, and there are provided a moisture volume calculating means C3 calculating the flow rates of moisture corresponding to the required output of the external load, and a fuel offgas supplying means C4 increasing and decreasing supplying flow rates of the fuel offgas through the flow rate adjusters 40 to 42 so as to return the moisture of the flow rate calculated to each of the solid oxide fuel cell stacks. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a fuel cell system having a plurality of solid oxide fuel stacks.

Conventionally, there exists a thing of the structure disclosed by patent document 1 as this kind of fuel cell system.
In the fuel cell system described in Patent Document 1, a fuel cell unit having an oxide-permeable electrolyte and a fuel reforming unit that supplies a reformed gas obtained by reforming fuel to the fuel cell unit are used as unit units. A plurality of unit units, a gas piping section that guides exhaust gas and fuel from the fuel cell section of this unit unit to a fuel reforming section of another unit unit, and water separation that separates water or water vapor from the exhaust gas It has a mechanism and a water piping part which guides this separated water or water vapor to the fuel reforming part of the unit unit.
JP 2004-63341 A

  However, in the thing of patent document 1, since the water is collect | recovered only in the last stage, the quantity of water accumulates between the stacks of each stage, and a fuel electrode may be oxidized. Moreover, the control regarding only the amount of required water is not carried out at the time of load fluctuation.

  Therefore, the present invention has an object to provide a fuel cell system that can ensure necessary water vapor in accordance with load fluctuations without necessarily requiring a condenser, an evaporator, or a water tank.

  In order to achieve the above object, the present invention has a plurality of solid oxide fuel cell stacks that generate power by separating and circulating fuel gas and air at a flow rate corresponding to the required output of an external load. And a gas return path for returning the fuel off-gas containing water discharged through one of the solid oxide fuel cell stacks to the other solid oxide fuel cell stack, and A fuel cell system in which each gas return path is provided with a flow regulator for increasing or decreasing the flow rate of the fuel off-gas flowing therethrough, and the moisture amount for calculating the flow rate of water corresponding to the required output of the external load Calculation means and fuel off-gas supply for increasing / decreasing the supply flow of fuel off-gas via a flow regulator so that the calculated water flow is returned to each solid oxide fuel cell stack It is characterized by having a stage.

  According to the present invention, it is possible to improve the water supply responsiveness to fluctuations in the external load and increase the life without necessarily requiring a condenser, an evaporator or a water tank.

  The best mode for carrying out the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram illustrating a configuration of a fuel cell system according to an embodiment of the present invention, and FIG. 2 is a block diagram illustrating functions of an arithmetic / control unit that forms part of the fuel cell system.

  A fuel cell system A1 according to an embodiment of the present invention includes a fuel tank 10 that stores hydrocarbon fuel, three fuel cells B1 to B3, a compressor, a fuel pump (all not shown), a reformer 20, and In addition to the calculation / control unit C, the fuel cell system includes an oxygen partial pressure gauge 31, an ammeter 32, a thermometer 33, a flow meter 34, and flow regulators 40 to 45 provided for each of the fuel cells B1 to B3. .

The compressor is for supplying air to the fuel cells B1 to B3, and the fuel pump is for supplying hydrocarbon fuel stored in the fuel tank 10 to the fuel cells B1 to B3. belongs to.
These compressors and fuel pumps are controlled under the control of a calculation / control unit C having a function to be described in detail later.

The oxygen partial pressure gauge 31 measures the oxygen partial pressure of the fuel off gas returned to the fuel cells B1 to B3, and is disposed in each of the fuel cells B1 to B3.
The oxygen partial pressure value measured by the oxygen partial pressure gauge 31 is wired so as to be input to the calculation / control unit C.

  The ammeter 32 measures the output current of a solid oxide fuel cell stack (hereinafter simply referred to as “stack”) described later for each of the fuel cells B1 to B3. A voltmeter may be employed instead of the ammeter.

The thermometer 33 measures the temperature of each of the stacks 21 to 23 and is wired so that the measured temperature data is input to the arithmetic / control unit C.
The flow meter 34 measures the flow rate of the fuel gas supplied to each of the fuel cells B <b> 1 to B <b> 3, and the measured flow rate of the fuel gas is wired so as to be input to the calculation / control unit C. .

  The fuel cells B <b> 1 to B <b> 3 shown in the present embodiment have a configuration in which stacks 21 to 23 formed by stacking a plurality of cell units (not shown) with gaps are accommodated in a case 24.

These fuel cells B1 to B3, and therefore the three stacks 21 to 23, are arranged in series along the fuel off-gas discharge path. By arranging in this way, a simple structure can be obtained.
In the present embodiment, the fuel cells B1 to B3 are described as examples having the same structure, but fuel cells having different structures can be combined.

In the stacks 21 to 23, fuel gas and air in amounts corresponding to the required output of an external load such as a motor are distributed separately from each other, thereby generating power.
The “gap” is for allowing the fuel gas to flow between other adjacent cell units.

The case 24 has an airtight cylindrical shape surrounding the peripheral wall 24c over the entire circumference of the bottom wall 24a and the upper wall 24b that are circular in plan view.
In FIG. 1, only the fuel cell B1 is provided with reference numerals corresponding to the bottom wall 24a, the upper wall 24b, and the peripheral wall 24c, and the reference numerals are omitted for the other fuel cells B2 and B3.

  The peripheral wall 24 c has a first gas inlet 25 for introducing fuel gas into the case 24, a second gas inlet 26 for introducing fuel off-gas to be described later into the case 24, and the case 24. A gas discharge port 27 for discharging the fuel off-gas from the opening is formed.

  Between the adjacent fuel cells B1, B2, B2, B3, and between the second gas inlet 26 and the gas outlet 27 of the fuel cell B3 and the fuel cell B1, return pipes 50 to 52 as gas return paths are provided. The return pipes 50 to 52 are connected with flow rate adjusters 40 to 42, respectively.

In the present embodiment, the fuel off-gas discharged from the gas discharge port 27 of the fuel cell B1 is supplied to the fuel cell B2, and the fuel off-gas discharged from the gas discharge port 27 of the fuel cell B2 is supplied to the fuel cell B3. Further, the return pipes 50 to 52 are arranged so that the fuel off-gas discharged from the gas discharge port 27 of the fuel cell B3 is returned to the fuel cell B1.
That is, the return pipe 52 is disposed between the stacks 21 and 23 arranged on the most upstream side and the most downstream side of the discharge path.

The flow regulators 40 to 42 are so-called directional control valves, and have functions of increasing or decreasing the flow rate of the fuel off gas flowing through the return pipes 50 to 52 and discharging a part of the fuel off gas outside the flow path. Yes, it is driven and controlled by the arithmetic / control unit C.
That is, the flow rate adjusters 40 to 42 have a function as a gas discharger for discharging a part of the fuel off gas flowing through the gas return pipes 50 to 52 out of the flow path.

Reference numeral 60 denotes a gas supply path for supplying fuel gas toward each of the fuel cells B1 to B3, one end of which is connected to the reformer 20 and the other end is connected to the fuel cell B3. Has been.
61 and 62 are branch passages for diverting the fuel gas from the gas supply passage 50 toward the fuel cells B1 and B2, and the flow rate adjusting portions 43 and 44 are disposed in the branch passages 61 and 62, respectively. In addition, a flow rate adjusting unit 45 is provided at each end of the gas supply path 60.
The flow rate adjusting units 43 to 45 control the increase and decrease of the flow rate of the fuel gas supplied to the fuel cells B1 to B3, and are wired so as to be driven and controlled under the calculation / control unit C.

As shown in FIG. 2, the arithmetic / control unit C includes a controller 70 including a CPU (Central Processing Unit), an interface circuit, and the like, and a storage unit 71 including a hard disk, a semiconductor memory, and the like.
By executing the program used in the fuel cell system stored in the storage unit 71, the following functions are exhibited.

The arithmetic / control unit C includes a CPU (Central Processing Unit), an interface circuit, and the like, and exhibits the following functions by executing a required program.
(1) A function of calculating the fuel gas supply flow rate corresponding to the required output of the external load. This is referred to as “fuel supply amount calculation means C1”.
Specifically, the supply flow rate of the fuel gas to be supplied to each of the fuel cells B1 to B3 is calculated corresponding to the required output of the external load that increases or decreases.
The amount of water returned to the stack is adjusted to increase or decrease by measuring the value of the oxygen partial pressure PO ₂ of the stack.
That is, the value of the oxygen partial pressure PO ₂ continues as it operated in the case of a predetermined value, in the case of PO ₂ values> predetermined value reduces the refunded flow water (refunded flow rate of the fuel gas), the value of the PO ₂ < In the case of a predetermined value, the water return flow rate is increased. At this time, if the output value becomes excessive or insufficient, the fuel gas supply flow rate is recalculated to increase or decrease the fuel supply flow rate.

(2) A function of feeding hydrocarbon fuels stored in the fuel tank 10 to the fuel cells B1 to B3 via the flow rate regulators 43 to 45 based on the calculated fuel gas feed flow rate. This is referred to as “fuel supply means C2”.
That is, the flow rate of the fuel gas supplied to the fuel cells B1 to B3 is increased or decreased by the flow rate regulators 43 to 45.

(3) A function of calculating the flow rate of moisture corresponding to the required output based on the required output of the external load. This function is referred to as “moisture amount calculation means C3”.
In other words, the supply flow rate of the fuel off gas to be returned and supplied to each of the stacks 21 to 23 is calculated.
Specifically, when the required output of the external load increases, the flow rate of the fuel gas supplied to each of the fuel cells B1 to B3 increases accordingly, and the carbon deposition on the fuel electrode is caused by the increase in the flow rate of the fuel gas. The flow rate of moisture that does not occur is calculated.
The relationship between the flow rate of the fuel gas and the flow rate of water, that is, the relationship between the flow rate of the fuel gas and the flow rate of the fuel off-gas to be returned is stored in the storage unit 71 as a lookup table (not shown), With reference to the look-up table, the supply flow rate of the fuel off gas to be returned toward each of the stacks 21 to 23 is calculated.
In the present embodiment, “moisture” is a concept including both water and water vapor. The calculated water flow rate varies depending on the type of hydrocarbon fuel.

(4) A function of increasing / decreasing the supply flow rate of the fuel off gas so that the calculated water flow rate is returned to each stack via the flow rate regulators 40 to 42. This is referred to as “fuel off-gas feeding means C4”.

(5) A function of discharging the remaining fuel off-gas to be returned to the other fuel cells B1 to B3 via the flow rate regulators 40 to 42 as gas exhausters. This is referred to as “residual gas discharging means C5”.

(6) A function of selecting one of the stacks according to increase / decrease in request output. This is referred to as “stack selection means C6”.
For example, a plurality of stacks are selected in combination so that an output corresponding to an increase in the required output is obtained. Specifically, considering the outputs of the fuel cells B1 to B3, one or a combination of two or more of the fuel cells B1 to B3 is selected so as to satisfy the required output.
In this case, each output of the fuel cells B1 to B3 and a correspondence relationship obtained by combining these outputs are stored in advance in the storage unit 71 as a lookup table.

The order of selection may be determined in advance. For example, the stack may be sequentially selected from the most upstream stack (fuel cell) along the discharge path to the downstream one.
Note that all stacks may be driven at the same time regardless of increase / decrease in request output.

(7) A function for predicting carbon deposition in any stack. This is referred to as “deposition prediction means C7”.
That is, when there is a stack with a bad temperature rise at the time of startup, carbon deposition is predicted in the stack, so the flow rate of moisture returned to the stack, that is, the return flow rate of the fuel off gas is increased.
In general, when the temperature of the stack is low, carbon deposition tends to occur easily. Therefore, the possibility of carbon deposition can be predicted by detecting the stack temperature and the amount of moisture.

Specifically, the temperature rise of the stack at startup is measured by the thermometer 33, and the rate of increase is compared with a predetermined rate of increase set in advance.
In this case, the “predetermined rate of increase” is stored in advance in the storage unit 71 described above.
In addition, in order to increase the flow rate of returning water, the flow rate of fuel off-gas from the stack upstream of the stack discharge path where the temperature rises poor may be increased, and the discharge of fuel off-gas from all stacks may be increased. The flow rate may be increased.

That is, the fuel supply amount calculation means C1 calculates the supply flow rate of the fuel gas to be supplied to another stack so as to supplement the output of the stack where carbon deposition is predicted.
The fuel off-gas feeding means C4 increases the return flow rate of the fuel off-gas corresponding to the flow rate of the fuel gas fed to the other stack calculated so as to supplement the output of the stack where carbon deposition is predicted.

(8) A function for detecting a decrease in output of each stack. This is referred to as “output reduction detection means C8”.
Specifically, the output decrease of the stack is detected by measuring the output currents of the fuel cells B1 to B3 by the ammeter 32 which is an output detector.
When the output reduction of the stack is detected by the output reduction detection means C8, the fuel supply means C2 supplies the fuel gas and the fuel off-gas to the other stack so as to compensate for the output reduction of the stack. Increase each.

  The operation of the fuel cell system having the above configuration will be described with reference to FIGS. FIG. 3 is a flowchart showing a case where the outputs of all fuel cells are increased with respect to load fluctuations, and FIG. 4 is a flowchart showing a case where the output of one fuel cell is increased with respect to load fluctuations.

<When increasing the output of all fuel cells>
Step 1 (abbreviated as “S1” in the figure. The same applies hereinafter): When the required output increases, the flow rate of the fuel gas supplied to each of the fuel cells B1 to B3 is increased correspondingly. Thereby, the output of each fuel cell B1-B3 increases.

Step 2: The temperature of the stacks 21 to 23 of the fuel cells B1 to B3 is detected, and the value of S / C is calculated.
If the stack temperature, fuel amount, and water flow rate are known, the S / C value (ratio of H2O amount / hydrocarbon amount) to prevent carbon deposition in the fuel cell is known from experience. Can be calculated.
Step 3: It is determined whether or not there is an excess or deficiency in the flow rate of water. If it is determined that the flow rate is sufficient, the process proceeds to Step 4;
Step 4: Continue normal operation.
Step 5: Increase / decrease the return flow of fuel off gas to all stacks.

<When increasing the output of one fuel cell>
Step 1 (abbreviated as “Sa1” in the figure. The same applies hereinafter): When the required output increases, for example, the flow rate of the fuel gas supplied to the fuel cell B1 is increased. As a result, the output of the fuel cell B1 increases.

Step 2: The temperature of the stacks 21 to 23 of the fuel cells B1 to B3 is detected, and the value of S / C is calculated.
If the stack temperature, fuel amount, and water flow rate are known, the S / C value (ratio of H2O amount / hydrocarbon amount) to prevent carbon deposition in the fuel cell is known from experience, so The flow rate can be calculated.
Step 3: It is determined whether or not there is an excess or deficiency in the flow rate of water. If it is determined that the flow rate is sufficient, the process proceeds to Step 4;
Step 4: Continue normal operation.
Step 5: Increase / decrease the return flow rate of the fuel off gas to the other stack.

  Next, a case where the temperature of one stack does not rise and a case where carbon deposition is detected in one stack will be described with reference to FIGS. FIG. 5 is a flowchart showing a case where the temperature of one stack does not rise, and FIG. 6 is a flowchart showing a case where carbon deposition is detected in one stack.

<When the temperature of one stack does not rise>
Step 1 (abbreviated as “Sb1” in the figure. The same applies hereinafter): Based on the temperature measured by the thermometer 33 provided in each of the fuel cells B1 to B3, the temperature of one of the stacks becomes a predetermined temperature. It is determined whether or not the temperature has reached, and when it is determined that the temperature does not reach the predetermined temperature, the process proceeds to Step 2. This step is so-called stack temperature determination means.
The “predetermined temperature” is, for example, 700 ° C., which is the starting temperature, and if the water flow rate is increased when this temperature is not reached, no carbon deposition occurs. Such a reference temperature is stored in advance in the storage unit 71 in accordance with the type of hydrocarbon fuel.

Step 2: Calculate the temperature of the stacks 21 to 23 of each of the fuel cells B1 to B3, the type of fuel, the flow rate of fuel gas, and the value of S / C.
Step 3: Based on the temperature of the stacks 21 to 23 of the fuel cells B1 to B3, the type of fuel, the flow rate of the fuel gas, and the value of S / C, the possibility of carbon deposition is determined. If yes, go to Step 4; otherwise, go to Step 5.

Step 4: Increase the return flow rate of the fuel off gas to the stack upstream of the stack, and go to Step 6.
Step 5: Continue normal operation.

Step 6: It is determined whether or not the temperature of the stack has reached a predetermined temperature. If it is determined that the temperature of the stack has reached a predetermined temperature, the process proceeds to Step 7; otherwise, the process returns to Step 2.
Step 7: Return the return flow rate of the fuel off gas to the stack upstream of the stack.

<When carbon deposition is detected in one stack>
Step 1 (abbreviated as “Sc1” in the figure. The same applies hereinafter): It is determined whether or not carbon deposition (carbon deposition) is detected in any stack. Proceed to 2.

Step 2: The flow rate of returning the fuel off gas to the stack upstream of the stack is increased, and the process proceeds to Step 3.
Step 3: The flow rate of returning the fuel off gas to the stack upstream of the stack is increased, and the process proceeds to Step 4.
Step 4: It is determined whether or not the output of the stack has increased. If it is determined that the output of the stack has increased, the process proceeds to Step 5; otherwise, the process returns to Step 2.
Step 5: Return the fuel gas supply flow rate to the upstream stack and the fuel off-gas return supply flow rate to the original values.

According to the fuel cell system according to the embodiment described in detail above, the following effects can be obtained.
-A condenser, an evaporator, and a water tank are not necessarily required, and the responsiveness to fluctuations in the external load can be improved and the life can be extended.
・ Each gas return passage is provided with a gas discharger for discharging a part of the fuel off-gas flowing through these gas return passages to the outside of the flow passage, and through the gas discharger, another solid oxide fuel Since the remaining fuel off-gas that is returned to the battery stack is discharged, the following effects can be obtained.

(A) That is, the fuel off-gas contains a large amount of moisture such as water vapor. If a part of the fuel off-gas is discharged out of the path, the stack and the return pipe are damaged, and oxidation of the fuel electrode due to moisture can be prevented.
In other words, if not discharged, the pressure in the stack may increase rapidly due to gas accumulation, and the stack, pipes, etc. may be damaged, and if the concentration of moisture such as water vapor increases, the fuel electrode of the fuel cell (Ni is often used). Is easily oxidized by the moisture.
(B) Furthermore, as the fuel off-gas is returned and circulated, the water in the stack accumulates more and more. Therefore, when the fuel electrode enters the oxygen partial pressure region where the fuel electrode is oxidized, the work of reducing the amount of fuel off-gas circulation is reduced. You need it, but you don't have to do it.

(C) Controllability and followability can be further improved by selecting one of the solid oxide fuel cell stacks according to the increase or decrease of the required output.
(D) Since the flow rate of the fuel off-gas returned to the solid oxide fuel cell stack where carbon deposition is predicted is increased, it is possible to prevent the output of the solid oxide fuel cell stack from decreasing.
(E) The supply flow rate of the fuel off-gas returned to the other solid oxide fuel cell stack is increased so as to supplement the output of the solid oxide fuel cell stack in which the carbon deposition is predicted. It can cope with the increase in output.
(F) When a decrease in the output of any one of the solid oxide fuel cell stacks is detected, the fuel gas and the fuel off-gas to other solid oxide fuel cell stacks are compensated to compensate for the decrease in the output of the solid oxide fuel cell stack. Since the supply flow rate of each is increased, it is possible to cope with an increase in required output.

The present invention is not limited to the above-described embodiments, and the following modifications can be made.
In the above-described embodiment, three fuel cells have been described as an example, but it is needless to say that two or four or more fuel cells may be used.

In the embodiment described above, an example in which three fuel cells are arranged in series along the fuel off-gas discharge path has been described. However, two or more return paths on the discharge side of one solid oxide fuel cell stack are provided. And a solid oxide fuel cell stack may be arranged on each branch path.
Further, the fuel off-gas discharged through one solid oxide fuel cell stack may be arranged so as to be returned to the other plurality of solid oxide fuel cell stacks.

In the above-described embodiment, for the three fuel cells, a stack formed by stacking a plurality of cell units with a gap between each other has been described as an example. It may be configured to accommodate a plurality of stacks.

In the above-described embodiment, the example in which each function described above is realized by a single arithmetic / control unit has been described. However, it is distributed by a plurality of controller units and therefore by a plurality of computers that control the fuel cell. You may make it process.
-In above-mentioned embodiment, although the gas exhaust device and the flow volume regulator were demonstrated as an example, it is needless to say that a separate thing may be adopted.

It is a block diagram which shows the structure of the fuel cell system which concerns on one Embodiment of this invention. It is a block diagram which shows the function of the arithmetic / control unit which makes a part of fuel cell system same as the above. It is a flowchart which shows the case where the output of all the fuel cells is raised with respect to load fluctuation. It is a flowchart which shows the case where the output of one fuel cell is raised with respect to load fluctuation. It is a flowchart which shows the case where the temperature of one stack does not rise. It is a flowchart which shows the case where carbon deposition is detected in one stack.

Explanation of symbols

21-23 Solid oxide fuel cell stacks 40-42 Flow regulators 40-42 Flow regulators 50-52 as gas dischargers Gas return path C2 Fuel supply means C3 Moisture amount calculation means C4 Fuel off-gas supply means C5 Residue Gas discharge means C6 Stack selection means C7 Precipitation prediction means C8 Output drop detection means

Claims (12)

It has a plurality of solid oxide fuel cell stacks for generating power by separating and circulating fuel gas and air at a flow rate according to the required output of the external load, and one of them is a solid electrolyte type A gas return path for returning the fuel off-gas containing water discharged through the fuel cell stack to the other solid oxide fuel cell stack is provided, and these gas return paths are circulated through each gas return path. A fuel cell system provided with a flow rate regulator for increasing or decreasing the flow rate of fuel off gas,
A moisture amount calculating means for calculating a flow rate of moisture corresponding to the required output of the external load;
A fuel cell system comprising fuel off gas feeding means for increasing / decreasing a fuel off gas feeding flow rate via a flow rate regulator so as to return the calculated water flow rate to each solid oxide fuel cell stack. .   Multiple solid oxide fuel cell stacks are arranged in series along the fuel off-gas discharge path, and the fuel off-gas discharged from the solid oxide fuel cell stack located upstream of the discharge path is relatively discharged. 2. A fuel cell system according to claim 1, further comprising a gas return path for feeding to a solid oxide fuel cell stack located on the downstream side of the fuel cell stack.   A gas return path is provided to return the fuel off-gas discharged from the solid oxide fuel cell stack arranged on the most downstream side of the discharge path to the solid oxide fuel cell stack arranged on the most upstream side. The fuel cell system according to claim 2, wherein:   The gas return path for returning the fuel off-gas toward another solid oxide fuel cell stack is provided for each solid oxide fuel cell stack. 2. The fuel cell system according to item 1. Each gas return path is provided with a gas discharger for discharging a part of the fuel off gas flowing through these gas return paths out of the distribution path,
5. A residual gas discharge means for discharging the remaining fuel off-gas to be returned and fed toward another solid oxide fuel cell stack through a gas discharger is provided. The fuel cell system according to claim 1.   The fuel cell system according to any one of claims 1 to 5, wherein the moisture amount calculating means calculates a flow rate of moisture to be returned and sent to each solid oxide fuel cell stack.   The fuel cell system according to any one of claims 1 to 6, further comprising a stack selection unit that selects any one of the solid oxide fuel cell stacks according to an increase or decrease in a required output.   8. The fuel cell system according to claim 7, wherein the stack selection means selects a combination of solid oxide fuel cell stacks so as to obtain an output corresponding to an increase in the required output. Having a deposition prediction means for predicting carbon deposition in any of the solid oxide fuel cell stacks;
9. The fuel off-gas feeding unit increases the flow rate of the fuel off-gas fed back to the solid oxide fuel cell stack predicted by the deposition predicting unit. 10. Fuel cell system.   The fuel off-gas supply means increases the flow rate of the fuel off-gas to be returned to another solid oxide fuel cell stack so as to compensate for the output of the solid oxide fuel cell stack in which carbon deposition is predicted. The fuel cell system according to claim 1, wherein: An output detector for detecting the output of each solid oxide fuel cell stack is provided, and through the output detector, there is an output decrease detecting means for detecting an output decrease of each solid oxide fuel cell stack. And
When the output decrease of the solid oxide fuel cell stack is detected by the output decrease detection means, the fuel gas to the other solid oxide fuel cell stack is compensated for to compensate for the output decrease of the solid oxide fuel cell stack. The fuel cell system according to any one of claims 1 to 10, wherein the supply flow rates of the fuel off gas and the fuel off gas are increased.   The output decrease detection means is characterized by increasing the flow rates of fuel gas and fuel off-gas to other solid oxide fuel cell stacks arranged upstream of the solid oxide fuel cell stack in which the output decrease is detected. The fuel cell system according to claim 11.

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