JP2016054146A - Solid oxide fuel battery system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a solid oxide fuel battery system arranged so that damage to a fuel battery cell can be sufficiently suppressed even in the event of an emergency stop.SOLUTION: A solid oxide fuel battery system comprises: a fuel battery module containing fuel battery cells each having a solid electrolyte, a fuel electrode including a cermet of Ni and ceramic, and an air electrode; a fuel-supply device; a reformer; a water-supply device; an oxygen-supply device; and a controller. After having forcibly stopped the supply of fuel, water and oxygen in the course of a power-generating operation, the size (L) of the fuel electrode when a temperature in the module is 500°C, and the size (L) of the fuel electrode when the temperature is 400°C and an oxygen partial pressure of the air electrode is larger than 10atm satisfy the following condition: (1-L/L)×100≤0.45%.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a solid oxide fuel cell system.

  A solid oxide fuel cell (hereinafter also referred to as “SOFC”) uses an oxide ion conductive solid electrolyte as a solid electrolyte. An air electrode is formed on one side of the solid electrolyte, and oxygen (air, oxygen, etc.) is supplied to the air electrode. A fuel electrode is formed on the other side of the solid electrolyte, and fuel gas is supplied to the fuel electrode.

  In general, a solid oxide fuel cell system performs a power generation operation by supplying fuel gas to a fuel electrode and oxygen to an air electrode under a high temperature condition of, for example, 500 ° C. or more and 1000 ° C. or less. In such a fuel cell system, when an emergency stop such as a power failure occurs, the supply of fuel gas and air into the fuel cell module is stopped in a short time. Then, for example, it is known that the air that has entered from the air electrode or the like contacts the fuel electrode that is in a high temperature state, and the fuel electrode is oxidized, causing expansion or the like in the fuel electrode, which may damage the fuel cell. It has been.

  Japanese Patent Laid-Open No. 2012-190630 (Patent Document 1) discloses that, in an emergency stop, reformed fuel gas is stored in a storage means in order to suppress damage of the fuel cell and performance degradation, and an abnormal stop state is entered. Then, it is described that the oxidation of the fuel electrode is suppressed by adding hydrogen to water vapor before the air reaches the fuel electrode inlet.

JP 2012-190630 A

  In the present invention, at the time of an emergency stop of the solid oxide fuel cell system, the supply of fuel gas or the like is stopped, for example, even when oxygen flows into the fuel electrode from the opening on the outlet side of the fuel gas flow path. An object of the present invention is to provide a fuel cell system capable of suppressing a decrease in output performance due to a problem that a crack occurs in a solid electrolyte or a cell is damaged.

  This problem will be specifically described with reference to FIG. FIG. 8 is a graph showing an example of the temporal change in the oxygen partial pressure of the air electrode and the temperature in the fuel cell module after an emergency stop. FIG. 8 shows changes in the oxygen partial pressure of the air electrode and the temperature of the solid oxide fuel cell module thereafter, assuming that an emergency stop occurs at time t = 0. In FIG. 8, the solid line indicates the oxygen partial pressure of the air electrode, and the broken line indicates the temperature in the solid oxide fuel cell module.

The oxygen partial pressure of the air electrode can be obtained, for example, by the following procedure.
The oxygen partial pressure p _O2ca of the air electrode can be obtained from the Nernst equation (Equation 1). In Formula 1, p _O2ca oxygen partial pressure of the air electrode, p _O2an oxygen partial pressure of the fuel electrode, E is the voltage of the fuel cell module, T is the temperature in the fuel cell module, R represents gas constant, F is Faraday's constant It is. Measured values can be used for the voltage E of the fuel cell module and the temperature T in the fuel cell module, and known values can be used for the gas constant R and the Faraday constant F.
E = _-RT (ln ( _pO2an ) -ln ( _pO2ca )) / 4F _.. Formula 1
When the normal operation of the fuel cell module is stopped, air is supplied to the air electrode and the oxygen partial pressure _pO2ca of the air electrode is controlled to be equal to the atmospheric pressure (0.21 atm). From Equation 1, the oxygen partial pressure _pO2an of the fuel electrode at the temperature at the time of normal operation stop can be calculated.
On the other hand, air cannot be supplied to the air electrode during an emergency stop. Therefore, the oxygen partial pressure _pO2an of the fuel electrode obtained by the above method, the actually measured fuel cell module voltage E, the temperature T in the fuel cell module, the gas constant R, and the Faraday constant F are substituted into Equation 1 to _perform an emergency stop. At this time, the oxygen partial pressure p _{O2ca ′} of the air electrode when no air is supplied to the air electrode can be calculated.

Immediately after the emergency stop, the supply of fuel gas, oxygen and water is stopped, but water and a small amount of fuel gas remain in the reformer. As the water evaporates, a mixed gas of water vapor and a small amount of fuel gas is supplied to the fuel electrode of the solid oxide fuel cell. At this time, since the supply of air to the air electrode is stopped, the pressure of the fuel electrode is higher than that of the air electrode. Therefore, a part of the mixed gas of water vapor and a small amount of fuel gas supplied to the fuel electrode side overflows and is also supplied to the air electrode side, and the oxygen partial pressure _pO2ca on the air electrode side decreases. (Step 1 in FIG. 8)

Thereafter, all the water remaining in the reformer evaporates, and the supply of water vapor and fuel gas to the fuel electrode is stopped. At the same time, the temperature in the fuel cell module gradually decreases. Since the fuel cell module and the plurality of fuel cells accommodated in the fuel cell module have an extremely large heat capacity, the temperature decreases very slowly and stably after an emergency stop. Along with this, the temperature of the fuel electrode of the fuel battery cell and the gas staying in the reformer gradually decreases gradually, and the gas volume starts to shrink. When the gas staying in the fuel cell module contracts, the pressure of the fuel electrode of the fuel cell also decreases gradually and stably over time. Here, after the emergency stop, since the fuel cell module has a large heat capacity, each fuel cell is soaked, so the pressure drop of the fuel electrode is almost the same for all the fuel cells. Occurs stably. Similar to the fuel electrode, the air electrode also takes in outside air as the gas in the fuel cell module contracts, so the oxygen partial pressure p _O2ca of the air electrode increases with time. (Step 2 in FIG. 8)

As the oxygen partial pressure _pO2ca of the air electrode increases due to the intake of outside air and the temperature of each fuel electrode decreases, oxygen is gradually _supplied from the air electrode to the fuel electrode of all the fuel cells little by little at the same timing. Start to flow. This inflow of oxygen causes oxidation of the fuel electrode including the cermet of Ni and ceramics. When the temperature in the fuel cell module is high, the oxygen partial pressure of the air electrode is low, so that the amount of oxygen entering the fuel electrode from the air electrode is small. For this reason, when the temperature inside the fuel cell module is still high after an emergency stop, the oxidation of the fuel electrode is hardly a problem. Actually, it is considered that the oxidation of the fuel electrode occurs when the temperature in the fuel cell module is 500 ° C. or lower. However, when the temperature in the fuel cell module is higher than 400 ° C., as shown in FIG. Since the partial pressure _pO2ca is still low, the oxidation of the fuel electrode hardly proceeds. When the temperature in the fuel cell module is around 400 ° C. and the oxygen partial pressure _pO2ca on the air electrode side is larger than 10 ⁻⁵ atm, the present inventors have found that the oxidation of the fuel electrode is the most serious. I found it. Therefore, after the emergency stop, the cell is controlled by controlling the oxidation state of the fuel electrode when the temperature in the fuel cell module is 400 ° C. and the oxygen partial pressure p _O2ca on the air electrode side is larger than a predetermined value. I found out that I could suppress the damage. (Step 3 in FIG. 8)

Generally, it is known that when the fuel electrode is oxidized, the fuel electrode expands. For example, when coking occurs during startup, the fuel electrode is oxidized and expands. In addition, shortly after the shutdown, after the shutdown to stop the supply of fuel, etc., in the state where the electrolyte is active while the fuel electrode is inactive, the fuel electrode is oxidized by oxygen ions. The inventors have found that the fuel electrode is also oxidized and expands due to mechanical oxidation. Furthermore, at the time of an emergency stop, the fuel electrode is oxidized and expanded by the contact between the air taken in from the outside air and the high temperature fuel electrode. As described above, when a large amount of expansion is accumulated in the fuel electrode, for example, tensile stress is generated in the adjacent solid electrolyte, and a crack is generated in the solid electrolyte.
However, when the fuel electrode includes a cermet of Ni and ceramics, a surprising phenomenon has been found that when the temperature in the fuel cell module is 500 ° C. or lower, the fuel electrode shrinks rather than expands due to oxidation. If the supply of fuel or air is suddenly interrupted due to a power failure during power generation operation, external air is taken into the fuel cell module as the temperature inside the fuel cell module decreases, and the oxygen content on the air electrode side inside the fuel cell module The pressure p _O2ca increases. Then, in the temperature range where the oxidation reaction of the fuel electrode is likely to occur, the oxygen partial pressure _pO2ca increases to a predetermined value or more, for example, 10 ⁻⁵ atm, so that the oxidation of the fuel electrode proceeds. It has been found that the fuel electrode contracts due to this oxidation. By this contraction, expansion due to coking at the time of start-up can be offset, and the dimensional change amount of the fuel electrode from before the start of operation can be reduced. On the other hand, if this contraction is too large and contracts beyond the amount of expansion, compressive stress is generated in the solid electrolyte, and cracks are generated in the solid electrolyte. This phenomenon occurs in the same manner even when the support, not the fuel electrode, includes a cermet of Ni and ceramics, and this support is provided in contact with the fuel electrode. Specifically, as the temperature inside the fuel cell module decreases, outside air is taken into the fuel cell module and the oxygen partial pressure _pO2ca on the air electrode side in the fuel cell module rises to 10 ⁻⁵ atm or higher. Oxidation of the support proceeds in a temperature range where the body oxidation reaction is likely to occur. If the support contracts too much due to this oxidation, a compressive stress is generated in the solid electrolyte, and a crack occurs in the solid electrolyte.

  An object of the present invention is to provide a solid oxide fuel cell system that can prevent damage to fuel cells even when an emergency stop occurs.

In order to solve the above-described problem, a solid oxide fuel cell system that generates power by reacting hydrogen and oxygen, the solid oxide fuel cell system includes a cermet of a solid electrolyte, Ni, and ceramics A fuel cell module containing a fuel cell having a fuel electrode and an air electrode, a fuel supply device for supplying fuel to the fuel cell module, and steam reforming the fuel supplied by the fuel supply device to hydrogen A reformer that supplies the generated hydrogen to the fuel electrode, a water supply device that supplies water for steam reforming to the reformer, an oxygen supply device that supplies oxygen to the fuel cell module, A controller that controls the fuel supply device, the water supply device, and the oxygen supply device, and that controls the extraction of electric power from the fuel cell module. After the fuel supply device, water supply device, and oxygen supply device are forcibly stopped during the power generation operation of the fuel cell system, the dimensions of the fuel electrode when the temperature inside the fuel cell module is 500 ° C. (L ₅₀₀ ) And the fuel electrode dimensions (L ₄₀₀ ) when the temperature inside the fuel cell module is 400 ° C. and the oxygen partial pressure of the air electrode is greater than 10 ⁻⁵ atm is (1−L ₄₀₀ / L ₅₀₀ ) × 100 ≦ 0.45%.
A solid oxide fuel cell system that generates electricity by reacting hydrogen and oxygen, the solid oxide fuel cell system being provided in contact with a fuel electrode, a solid electrolyte, a fuel electrode, an air electrode, and Ni A fuel cell module containing a fuel cell having a support including a cermet with ceramics, a fuel supply device for supplying fuel to the fuel cell module, and steam reforming the fuel supplied by the fuel supply device A reformer that supplies hydrogen to the fuel electrode, a water supply device that supplies water for steam reforming to the reformer, and an oxygen supply that supplies oxygen to the fuel cell module And a controller that controls the fuel supply device, the water supply device, and the oxygen supply device, and controls the extraction of electric power from the fuel cell module. After the fuel supply device, the water supply device, and the oxygen supply device are forcibly stopped during the power generation operation of the fuel cell system, the dimensions of the support when the temperature inside the fuel cell module is 500 ° C. (L ₅₀₀ ) And the dimension (L ₄₀₀ ) of the support when the temperature inside the fuel cell module is 400 ° C. and the oxygen partial pressure of the air electrode is greater than 10 ⁻⁵ atm is (1−L ₄₀₀ / L ₅₀₀ ) × 100 ≦ 0.45%.

In the present invention, after the fuel supply device, the water supply device, and the oxygen supply device are forcibly stopped during the power generation operation of the fuel cell system (hereinafter referred to as an emergency stop), the temperature in the fuel cell module is 400. When the oxygen partial pressure is greater than 10 ⁻⁵ atm, the dimension L _{400 of the} fuel electrode and / or the support including the cermet of Ni and ceramics is It shrinks with respect to the dimensions, and the amount of shrinkage is reduced within a predetermined range. Therefore, it is possible to suppress the occurrence of excessive compressive stress in the solid electrolyte, and it is possible to suppress the occurrence of cracks and cell breakage in the solid electrolyte.

  According to the solid oxide fuel cell system of the present invention, it is possible to sufficiently suppress the fuel cell from being damaged even if an emergency stop is performed.

1 is an overall configuration diagram showing a fuel cell system according to an embodiment of the present invention. It is a front sectional view showing a fuel cell module of a fuel cell system by one embodiment of the present invention. FIG. 3 is a cross-sectional view taken along line III-III in FIG. 2. It is (a) partial sectional view and (b) transverse sectional view showing a fuel cell unit of a fuel cell system by one embodiment of the present invention. It is a figure which shows the relationship between the temperature in the fuel cell module by one Embodiment of this invention, and the dimension of a fuel cell. 1 is a perspective view showing a fuel cell stack of a fuel cell system according to an embodiment of the present invention. 1 is a block diagram showing a fuel cell system according to an embodiment of the present invention. It is a figure which shows the time change of the oxygen partial pressure of the air electrode after an emergency stop, and the temperature of a fuel cell module. No. 2 is a scanning electron microscope image of a support of a solid oxide fuel cell obtained in 2. FIG. 10 is a diagram obtained by performing image processing on FIG. 9.

Definition In the present invention, “emergency stop” means that during power generation operation of a solid oxide fuel cell system, for example, power is taken out from the fuel cell module due to a power failure or a water pump failure, and the fuel to the fuel cell module is This refers to a stopped state in which the supply of gas, water, and air is suddenly stopped. That is, at the time of emergency stop, in the high temperature state (for example, 500 ° C. or more and 1000 ° C. or less) during the power generation operation, the fuel cell module is forcibly stopped without being supplied with fuel gas, water, and air.

Solid Oxide Fuel Cell System The solid oxide fuel cell system of the present invention will be described with reference to FIGS.
FIG. 1 is an overall configuration diagram showing a solid acid fuel cell (SOFC) system according to an embodiment of the present invention. FIG. 2 is a side sectional view showing a solid oxide fuel cell (SOFC) fuel cell module according to an embodiment of the present invention, and FIG. 3 is a sectional view taken along line III-III in FIG. . FIG. 4A is a partial cross-sectional view showing a fuel cell unit of a solid oxide fuel cell (SOFC) system according to an embodiment of the present invention. FIG. 4B is a partial cross-sectional view of the fuel cell unit.

The solid oxide fuel cell system 1 according to the present invention generates power by reacting hydrogen and oxygen. As shown in FIG. 1, the solid oxide fuel cell system 1 includes a fuel cell module 2, a fuel supply device 38, a reformer 20, a water supply device 28, oxygen supply devices 44 and 45, and a controller. 110. The fuel cell module 2 accommodates fuel cells 12, 14, 16 having an inner electrode layer 90, a solid electrolyte 94, and an outer electrode layer 92. The inner electrode layer 90 is, for example, a fuel electrode, and the outer electrode layer 92 is, for example, an air electrode. Hereinafter, the fuel cell module 2 in which the inner electrode layer 90 is a fuel electrode and the outer electrode layer 92 is an air electrode will be described as an example. The fuel electrode includes a cermet of Ni and ceramics.
The fuel supply device 38 supplies fuel to the fuel cell module 2. The reformer 20 steam reforms the fuel supplied from the fuel supply device 38 to generate hydrogen, and supplies the generated hydrogen to the fuel electrode 90. The water supply device 28 supplies water for steam reforming to the reformer 20. The oxygen supply devices 44 and 45 supply oxygen to the fuel cell module 2. The controller 110 controls the fuel supply device 38, the water supply device 28, and the oxygen supply devices 44 and 45, and also controls the extraction of electric power from the fuel cell module 2. Alternatively, the fuel cells 12, 14, and 16 may have a support, and the support and the fuel electrode may be provided in contact with each other so that the support includes a cermet of Ni and ceramics.
In the present invention, after the fuel supply device 38, the water supply device 28, and the oxygen supply devices 44 and 45 are forcibly stopped during the power generation operation of the fuel cell system 1, The dimensions (L ₅₀₀ ) of the fuel electrode 90 and / or the support when the temperature T is 500 ° C., the temperature T in the fuel cell module 2 is 400 ° C., and the oxygen partial pressure p _O2ca of the air electrode 92 is 10 ⁻ The dimension (L ₄₀₀ ) of the fuel electrode 90 and / or the support when it is greater than ⁵ atm is (1−L ₄₀₀ / L ₅₀₀ ) × 100 ≦ 0.45%.

Here, (1-L ₄₀₀ / L ₅₀₀ ) × 100 is the oxidation contraction rate Lr of the fuel electrode 90 and / or the support. By reducing the oxidation shrinkage ratio Lr of the fuel electrode 90 to 0.45% or less, the amount of shrinkage with respect to the dimensions of the fuel electrode 90 during power generation operation can be reduced to a predetermined range or less, and cracks and cells in the solid electrolyte 94 Breakage can be prevented. More preferably, the oxidation shrinkage ratio Lr (1-L ₄₀₀ / L ₅₀₀ ) × 100 of the fuel electrode 90 is 0.35% or less.

With reference to FIGS. 5A and 5B, the temperature T change in the fuel cell module 2 after the emergency stop and the change in the dimension L of the fuel electrode 90 and / or the support will be described. Here, a change in the dimensions of the fuel electrode 90 will be described as an example.
FIGS. 5A and 5B are diagrams showing the relationship between the temperature T in the fuel cell module 2 and the dimension L of the fuel cell according to one embodiment of the present invention. FIG. 5A is a schematic diagram showing a change in the temperature T in the fuel cell module 2 with respect to time t. FIG. 5B is a schematic diagram showing a change in the dimension L of the fuel cell with respect to time t. For convenience, the size of the fuel cell is regarded as the size L of the fuel electrode 90.
In the fuel cell system 1, for example, the power generation operation is performed when the temperature T in the fuel cell module 2 is 500 ° C. or higher and 1000 ° C. or lower. FIG. 5A shows a case where the power generation operation is performed at a temperature T of 500 ° C. as an example.

As shown in FIG. 5B, when the temperature T in the fuel cell module 2 rises after startup, the dimension L of the fuel electrode 90 increases. For example, if L ₀ is the dimension of the fuel electrode at the start of the fuel cell module operation, and L ₁ is the dimension of the fuel electrode 90 being activated, then L ₁ > L ₀ . That is, the fuel electrode 90 extends more than before the start of operation. In addition, the size of the fuel electrode also increases due to the occurrence of coking at startup. Thereafter, during the power generation operation at a substantially constant temperature, the dimensional change of the fuel electrode 90 hardly occurs.
On the other hand, when the emergency stop is performed, the temperature T in the fuel cell module 2 decreases, and the dimension L of the fuel electrode 90 decreases. Ideally, if the dimension L _S of the fuel electrode 90 when the temperature T in the fuel cell module 2 drops to the temperature before the start of operation is substantially the same as the dimension L ₀ of the fuel electrode at the start of operation, Cell degradation does not occur. However, since the fuel electrode is actually oxidized by coking or the like, the dimension L _S after the operation is stopped is larger than the dimension L ₀ at the start of the operation. On the other hand, since the fuel electrode undergoes oxidative shrinkage due to the ingress of air after an emergency stop, the expansion due to coking is offset and further contracted. For example, the dimension L _S of the fuel electrode 90 is smaller than the dimension L ₀ before the start of operation, and a compressive stress is generated in the solid electrolyte, and as a result, the cell may be damaged.

The fuel electrode 90 _undergoes oxidation when the temperature T in the fuel cell module 2 is higher than a predetermined value and the oxygen partial pressure _pO2ca of the air electrode 92 is higher than a predetermined value. At the time of emergency stop, for example, when the temperature T in the fuel cell module 2 is still high, the contraction amount of the gas in the fuel cell module 2 is small, and the oxygen partial pressure _pO2ca of the air electrode 92 is not so high. The oxidation of the fuel electrode 90 is not a problem. When the temperature T in the fuel cell module 2 decreases to about 400 ° C. and the oxygen partial pressure _pO2ca of the air electrode 92 becomes higher than 10 ⁻⁵ atm, the oxidation of the fuel electrode 90 becomes a serious problem.

As described above, it is known that when the fuel electrode 90 is oxidized, the fuel electrode 90 expands. However, when the fuel electrode contains a cermet of Ni and ceramics, a surprising phenomenon has been found that when the temperature in the fuel cell module is 500 ° C. or lower, the fuel electrode contracts rather than expands due to oxidation. By reducing the amount of contraction within a predetermined range, a crack can be generated in the solid electrolyte 94 as the fuel electrode 90 contracts. Specifically, in the present invention, when the temperature T in the fuel cell module 2 is 400 ° C. and the oxygen partial pressure p _O2ca of the air electrode 92 is 10 ⁻⁵ atm or higher / larger, The dimension L ₄₀₀ and the dimension L ₅₀₀ of the fuel electrode when the temperature T in the fuel cell module 2 is 500 ° C. are (1−L ₄₀₀ / L ₅₀₀ ) × 100 ≦ 0.45%. That is, by reducing the contraction of the fuel electrode at 400 ° C. to some extent, the dimensional change ΔL (ΔL = L ₀ −L _S ) of the fuel electrode can be reduced even in the case of an emergency stop, and the solid electrolyte 94 It is possible to suppress cracks from occurring.

  The temperature T in the fuel cell module 2 is the temperature of the fuel cell stack 14. For example, the temperature is estimated by a power generation chamber temperature sensor 142 described later, and can be regarded as the temperature of the air electrode, the fuel electrode, or the support of the fuel cell.

The oxygen partial pressure p _O2ca at the air electrode refers to the oxygen partial pressure at the air electrode / electrolyte interface. Specifically, the oxygen partial pressure p _O2ca of the air electrode can be obtained from the Nernst equation (Equation 1), for example, as described above. In Formula 1, p _O2ca oxygen partial pressure of the air electrode, p _O2an oxygen partial pressure of the fuel electrode, E is the voltage of the fuel cell module, T is the temperature in the fuel cell module, R represents gas constant, F is Faraday's constant It is. Measured values can be used for the voltage E of the fuel cell module and the temperature T in the fuel cell module, and known values can be used for the gas constant R and the Faraday constant F.
E = _-RT (ln ( _pO2an ) -ln ( _pO2ca )) / 4F _.. Formula 1

When the fuel cell module 2 is stopped normally, air is supplied to the air electrode 92 and the oxygen partial pressure _pO2ca of the air electrode 92 is controlled to be equal to the atmospheric pressure (0.21 atm). The oxygen partial pressure p _O2an of the fuel electrode 90 at the temperature at the time of normal operation stop is calculated from Equation 1. _Substitute the oxygen partial pressure p _O2an of the fuel electrode 90 obtained by the above method, the actually measured fuel cell module voltage E, the temperature T in the fuel cell module, the gas constant R, and the Faraday constant F into Equation 1 and _perform an emergency stop. At this time, the oxygen partial pressure _{pO2ca ′} of the air electrode 92 when no air is supplied to the air electrode 92 is calculated.

As the dimension L of the fuel electrode 90 of the present invention, any dimensions such as the length, thickness, and volume of the fuel electrode 90 can be used. Here, the length of the fuel electrode 90 is, for example, a direction from the fuel cells 12, 14, 16 toward the reformer 20 (air heat exchanger 22) in the fuel cell module (for example, the Z-axis direction in FIG. 3). ) Along the length of the fuel electrode 90. The thickness of the fuel electrode 90 is, for example, the length of the fuel electrode 202 between the support 201 and the solid electrolyte 203 shown in FIG. Alternatively, the dimensional change of the fuel electrode may be inferred from the residual stress of the solid electrolyte 94.
As a method for estimating the dimensional change of the fuel electrode 90 from the residual stress of the solid electrolyte 94, for example, the following method can be used.

For example, the residual stress of a solid electrolyte can be measured by the sin ² ψ method of X-ray diffraction (XRD). Specifically, the sample before and after the operation was cut into an appropriate size and measured under the following conditions. For example, X'Pert PRO MPD manufactured by PANalytical can be used to measure the residual stress. For example, the measurement area is 2 mm × 2 mm, the X-ray output is 40 mA, 45 kV, the measurement range is 94.0-95.7, the angle ψ between the sample surface normal and the crystal surface normal is changed, and the diffraction angle 2θ is obtained. The stress σ can be obtained from the following equation using constants obtained from the Young's modulus, Poisson's ratio, and reflection angle θ in the unstrained state.

From the stress value of the solid electrolyte determined above, for example, a change in the strain of the electrolyte can be calculated and used as a change in the dimensions of the fuel electrode.

The oxidation shrinkage ratio Lr of the fuel electrode 90 can be obtained by the following method, for example.
The reduced solid oxide fuel cell is cut to a length of 15 mm and polished so that the cut surfaces are parallel to prepare a sample. About this sample, oxidation shrinkage ratio Lr is calculated | required using the thermomechanical analyzer TMA (for example, TMA-60 by Shimadzu Corporation). The load during measurement is 10 g.
The sample is heated from room temperature at 6 ° C./min in hydrogen to, for example, 650 ° C. assuming that the solid oxide fuel cell system is in operation. Thereafter, the temperature is lowered to 400 ° C. at 6 ° C./min in hydrogen. Stabilize at 400 ° C. for 10 minutes, then hold in nitrogen for 5 minutes. Thereafter, assuming that the operation of the solid oxide fuel cell system is stopped, the solid oxide fuel cell system is maintained for 18 hours in a mixed gas of air and nitrogen so that the oxygen partial pressure is 0.001 atm. The oxidation shrinkage ratio Lr (%) is obtained from the dimension at 500 ° C. (L ₅₀₀ ) and the dimension at the time of holding at 400 ° C. under an oxygen partial pressure of 0.001 atm (L ₄₀₀ ).
Oxidation shrinkage ratio Lr (%) = (1−L ₄₀₀ / L ₅₀₀ ) × 100

The reason why the fuel electrode 90 undergoes oxidative contraction rather than oxidative expansion in a predetermined temperature range is not clear, but is considered as follows. Here, the case where the fuel electrode includes cermet of Ni and ceramics will be described as an example.
When the Ni particles contained in the fuel electrode 90 are exposed to an oxidizing atmosphere, the Ni particles react as follows to oxidize the Ni particles to NiO.
2Ni (s) + O ₂ (g) → 2NiO (s)
This oxidation reaction is faster at higher temperatures. After the emergency stop, since the oxygen partial pressure of the air electrode is still low at high temperatures, the progress of oxidation is slow. When the temperature of the fuel cell module is 400 ° C., the oxygen partial pressure p _O2ca of the air electrode _increases to 10 ⁻⁵ atm or more, and oxidation proceeds. This oxidation reaction proceeds from the surface of the Ni particles. When the Ni particles are oxidized, the surface energy changes, so that the contact area between the Ni particles and the ceramic particles increases. The fuel electrode has a porous structure, and in a cermet of Ni and ceramics having a porous structure, a microstructural change is caused so that the Ni particles and the ceramic particles approach each other. This change is considered to cause contraction. In addition, at a temperature of 500 ° C. or higher, when an oxidation reaction occurs, oxidation expansion occurs instead of oxidation shrinkage. The reason for this is thought to be that NiO precipitates on the surface of Ni particles in large quantities and expands because the oxidation of Ni particles proceeds vigorously as the temperature rises. On the other hand, since the oxidation rate of Ni particles is very slow below 200 ° C., no shrinkage is observed within the module stop time.

In the fuel electrode 90, it is preferred interface length per unit area of the Ni and the ceramic at the beginning operation of the fuel cell system is 2 [mu] m / [mu] m ² or more 10 [mu] m / [mu] m ² or less. As a result, the contraction amount of the fuel electrode can be reduced within a predetermined range, so that it is possible to prevent electrolyte cracks and cell breakage. More preferably, it is 3.5 μm / μm ² or more and 10 μm / μm ² or less. Here, the start of operation of the fuel cell system refers to the time when the fuel cell system is started for the first time.

In the present invention, the reason why the amount of contraction of the fuel electrode can be made smaller than a predetermined value when the interface length per unit area between Ni and ceramics of the fuel electrode is in a specific range is considered as follows. However, the present invention is not limited to this. Since the surface energy of Ni changes when oxidized, the contact area between Ni and ceramics increases. That is, in the cermet of porous Ni and ceramics, the microstructure changes so that Ni and ceramics approach each other. This change is considered to cause contraction. It is considered that the shrinkage amount of the entire cermet can be reduced within the predetermined range by setting the interface length per unit area of Ni and ceramics within the predetermined range. Specifically, when the interface length is 2 μm / μm ² or more and 10 μm / μm ² or less, the microstructure change can be suppressed and the shrinkage can be suppressed within a predetermined range. Therefore, it is thought that cracking of the solid electrolyte and cell damage can be prevented.

The inventors discovered that the shrinkage due to oxidation of Ni and ceramics cermet occurs at 200 ° C. or more and 500 ° C. or less and becomes the largest around 400 ° C.
When the temperature is higher than 500 ° C., the reason why the oxidative expansion occurs instead of the oxidative shrinkage is considered to be because, as described above, the Ni particles are oxidized at a high temperature and a large amount of NiO is deposited on the surface of the Ni particles. . Note that after the emergency stop, the oxygen partial pressure of the air electrode at a high temperature is low, so that the oxidation itself does not occur so much. On the other hand, since the oxidation rate of Ni particles is very low at 200 ° C. or lower, no contraction of the fuel electrode is observed within the module stop time.
As described above, in the fuel cell system of the present invention, after the emergency stop, when the temperature in the fuel cell module is 400 ° C. and the oxygen partial pressure p _O2ca is greater than 10 ⁻⁵ atm, the oxidation contraction of the fuel electrode is within a predetermined range. It is thought that it can be inside.

  The interface length per unit area between Ni and ceramics can be determined as follows. The solid electrolyte fuel cell before power generation is observed with an electron microscope. A sample for observation with an electron microscope can be prepared by polishing the fuel electrode of the solid oxide fuel cell after the reduction treatment and then performing ion milling treatment with IM4000 manufactured by Hitachi, Ltd. This sample is observed with a scanning electron microscope (S-4100, manufactured by Hitachi, Ltd.) at an acceleration voltage of 5 kV, a secondary electron image and a reflected electron image, and a magnification of 10,000 times to obtain an electron microscope image (reflected electron image). In this electron microscope image, Ni, ceramics, and pores are distinguished by the difference in brightness. The interface between Ni and ceramics is determined visually. The length of this interface indicates the interface length between Ni and ceramics. The total value of the interface lengths of Ni and ceramics is obtained by adding all the interface lengths shown in the electron microscope image. A value obtained by dividing the total value by the visual field area of the electron microscope image is defined as an interface length per unit area between Ni and ceramics.

  In addition, the method of discriminating Ni, ceramics, and pores is not limited to the method using the light / dark difference in the electron microscope image. For example, element mapping can be obtained by SEM-EDS in the same visual field, and ternarization can be performed by superimposing Ni and ceramic mapping images.

If the fuel electrode comprises a cermet of Ni and YSZ, the interface length per unit area of Ni and YSZ in the fuel electrode is preferably 2 [mu] m / [mu] m ² or more 6 [mu] m / [mu] m ² or less. More preferably, it is preferably 3.5 [mu] m / [mu] m ² or more 5.5 [mu] m / [mu] m ² or less.

When the fuel electrode includes a cermet of Ni and GDC, the interface length per unit area between Ni and GDC of the fuel electrode is preferably 6 μm / μm ² or more and 10 μm / μm ² or less. More preferably, it is 7 μm / μm ² or more and 10 μm / μm ² or less.

Referring again to the attached drawings, a solid oxide fuel cell (SOFC) system according to an embodiment of the present invention will be described.
As shown in FIG. 1, a solid oxide fuel cell (SOFC) system 1 according to an embodiment of the present invention includes a fuel cell module 2 and an auxiliary unit 4.

  The fuel cell module 2 includes a housing 6, and a metal case 8 is built in the housing 6 via a heat insulating material 7. A fuel cell assembly 12 that performs a power generation reaction with fuel and oxygen (air) is disposed in a power generation chamber 10 that is a lower portion of the case 8 that is a sealed space. The fuel cell assembly 12 includes ten fuel cell stacks 14 (see FIG. 5). The fuel cell stack 14 includes 16 fuel cell units 16 (see FIG. 4), which are fuel cells. ). Thus, the fuel cell assembly 12 has 160 fuel cell units 16, and all of these fuel cell units 16 are connected in series.

A combustion chamber 18 that is a combustion section is formed above the above-described power generation chamber 10 of the case 8 of the fuel cell module 2. In this combustion chamber 18, the remaining fuel and the remaining oxidant that have not been used for the power generation reaction. (Air) is combusted to generate exhaust gas. Further, the case 8 is covered with a heat insulating material 7 to suppress the heat inside the fuel cell module 2 from being diffused to the outside air.
Further, a reformer 20 for reforming the fuel is disposed above the combustion chamber 18, and the reformer 20 is heated to a temperature at which a reforming reaction can be performed by the combustion heat of the residual gas. Yes. Further, an air heat exchanger 22, which is a heat exchanger for heating the power generation air with the remaining combustion gas and preheating the power generation air, is disposed above the reformer 20.

  Next, the auxiliary unit 4 stores pure water tank 26 that stores water condensed from moisture contained in the exhaust from the fuel cell module 2 and makes it pure water with a filter, and water supplied from the water storage tank. Is provided with a water flow rate adjusting unit 28 (such as a “water pump” driven by a motor). In addition, the auxiliary unit 4 adjusts the flow rate of the fuel gas, the gas shutoff valve 32 for shutting off the fuel supplied from the fuel supply source 30 such as city gas, the desulfurizer 36 for removing sulfur from the fuel gas, A fuel flow adjustment unit 38 (such as a “fuel pump” driven by a motor) and a valve 39 that shuts off fuel gas flowing out from the fuel flow adjustment unit 38 when power is lost. Further, the auxiliary unit 4 includes an electromagnetic valve 42 that shuts off air that is oxygen supplied from the air supply source 40, a reforming air flow rate adjustment unit 44 that adjusts the flow rate of air, and a power generation air flow rate adjustment unit 45. (Such as an “air blower” driven by a motor), a first heater 46 for heating the reforming air supplied to the reformer 20, and a second heater for heating the power generating air supplied to the power generation chamber 48. The first heater 46 and the second heater 48 are provided in order to efficiently raise the temperature at startup, but may be omitted.

Next, a hot water production apparatus 50 to which exhaust gas is supplied is connected to the fuel cell module 2. The hot water production apparatus 50 is supplied with tap water from the water supply source 24, and the tap water is heated by the heat of the exhaust gas and supplied to a hot water storage tank of an external hot water heater (not shown).
The fuel cell module 2 is provided with a control box 52 for controlling the amount of fuel gas supplied and the like.
Furthermore, the fuel cell module 2 is connected to an inverter 54 that is a power extraction unit (power conversion unit) for supplying the power generated by the fuel cell module to the outside.

Fuel Cell Module Referring to FIGS. 2 and 3 again, the internal structure of a fuel cell module of a solid oxide fuel cell (SOFC) system according to an embodiment of the present invention will be described.
As shown in FIGS. 2 and 3, the case 8 in the housing 6 of the fuel cell module 2 has a fuel cell assembly 12, a reformer 20, and an air heat exchanger in order from the bottom as described above. 22 is arranged.

The reformer 20 is provided with a reformer introduction pipe 62 for introducing pure water, fuel gas to be reformed, and reforming air on the end side surface on the upstream end side.
The reformer introduction pipe 62 is a circular pipe extending from the side wall surface at one end of the reformer 20, is bent by 90 ° and extends in a substantially vertical direction, and penetrates the upper end surface of the case 8. The reformer introduction pipe 62 functions as a water introduction pipe for introducing water into the reformer 20. Further, a T-shaped tube 62a is connected on the reformer introduction tube 62, and fuel gas and pure water are supplied to both ends of the T-shaped tube 62a extending in a substantially horizontal direction. Pipes for connecting are connected to each other. The water supply pipe 63a extends obliquely upward from one side end of the T-shaped pipe 62a. The fuel gas supply pipe 63b extends in the horizontal direction from the other side end of the T-shaped pipe 62a, then bends in a U shape, and extends substantially horizontally in the same direction as the water supply pipe 63a.

  On the other hand, an evaporation unit 20a, a mixing unit 20b, and a reforming unit 20c are formed in the reformer 20 sequentially from the upstream side, and the reforming unit 20c is filled with a reforming catalyst. The fuel gas and air mixed with the steam (pure water) introduced into the reformer 20 are reformed by the reforming catalyst filled in the reformer 20. As the reforming catalyst, a catalyst obtained by imparting nickel to the alumina sphere surface or a catalyst obtained by imparting ruthenium to the alumina sphere surface is appropriately used.

  A fuel gas supply pipe 64 is connected to the downstream end side of the reformer 20, and the fuel gas supply pipe 64 extends downward and further in an manifold 66 formed below the fuel cell assembly 12. It extends horizontally. A plurality of fuel supply holes 64 b are formed in the lower surface of the horizontal portion 64 a of the fuel gas supply pipe 64, and the reformed fuel gas is supplied into the manifold 66 from the fuel supply holes 64 b. Further, in the middle of the vertical portion of the fuel gas supply pipe 64, a pressure fluctuation suppressing flow path resistance portion 64c having a narrow flow path is provided, and the flow path resistance of the fuel gas supply flow path is adjusted. The adjustment of the channel resistance will be described later.

  A lower support plate 68 having a through hole for supporting the fuel cell stack 14 described above is attached above the manifold 66, and the fuel gas in the manifold 66 flows into the fuel cell unit 16. Supplied.

On the other hand, an air heat exchanger 22 is provided above the reformer 20.
Further, as shown in FIG. 2, an ignition device 83 for starting combustion of fuel gas and air is provided in the combustion chamber 18.

Fuel Cell Unit Next, the fuel cell unit 16 will be described with reference again to FIG.
As shown in FIG. 4A, the fuel cell unit 16 includes a fuel cell 84 and inner electrode terminals 86 that are caps respectively connected to both ends of the fuel cell 84.
The fuel cell 84 is a tubular structure that extends in the vertical direction, and includes a cylindrical inner electrode 90 that forms a fuel gas flow path 88 therein, a cylindrical outer electrode 92, an inner electrode 90, and an outer electrode 92. And a solid electrolyte 94 between them. The inner electrode 90 is a fuel electrode through which fuel gas passes and becomes a (−) electrode, while the outer electrode 92 is an air electrode in contact with air and becomes a (+) electrode.

  Since the inner electrode terminal 86 attached to the upper end side and the lower end side of the fuel cell 84 has the same structure, the inner electrode terminal 86 attached to the upper end side will be specifically described here. The upper part 90 a of the inner electrode layer 90 includes an outer peripheral surface 90 b and an upper end surface 90 c exposed to the solid electrolyte 94 and the outer electrode layer 92. The inner electrode terminal 86 is connected to the outer peripheral surface 90b of the inner electrode layer 90 through a conductive sealing material 96, and is further in direct contact with the upper end surface 90c of the inner electrode layer 90, thereby Electrically connected. At the center of the inner electrode terminal 86, a fuel gas channel capillary 98 that communicates with the fuel gas channel 88 of the inner electrode layer 90 is formed.

  The fuel gas passage narrow tube 98 is an elongated thin tube provided so as to extend in the axial direction of the fuel cell 84 from the center of the inner electrode terminal 86. For this reason, a predetermined pressure loss occurs in the flow of the fuel gas flowing from the manifold 66 (FIG. 2) into the fuel gas passage 88 through the fuel gas passage narrow tube 98 of the lower inner electrode terminal 86. . Accordingly, the fuel gas flow passage narrow tube 98 of the lower inner electrode terminal 86 acts as an inflow side flow passage resistance portion, and the flow passage resistance is set to a predetermined value. Further, a predetermined pressure loss also occurs in the flow of the fuel gas flowing out from the fuel gas flow path 88 to the combustion chamber 18 (FIG. 2) through the fuel gas flow path narrow tube 98 of the upper inner electrode terminal 86. Therefore, the fuel gas flow passage narrow tube 98 of the upper inner electrode terminal 86 acts as an outflow side flow passage resistance portion, and the flow passage resistance is set to a predetermined value.

With reference to FIG. 4B, the structure of the fuel cell 84 will be described in detail.
The solid oxide fuel cell 210 according to an embodiment of the present invention includes, for example, a support 201, a (first / second) fuel electrode 202, a (first / second) solid electrolyte 203, and a (first / second). 2) It is composed of an air electrode 204 and a current collecting layer 205. The fuel electrode 202 includes, for example, a first fuel electrode layer and a second fuel electrode layer. The first fuel electrode layer is provided between the second fuel electrode layer and the support 201. The first fuel electrode layer is, for example, a fuel electrode layer. The second fuel electrode layer is, for example, a fuel electrode catalyst layer. The solid electrolyte 203 includes, for example, a first solid electrolyte layer and a second solid electrolyte layer. The first solid electrolyte layer is provided between the second solid electrolyte layer and the fuel electrode 202. The first solid electrolyte layer is, for example, a reaction suppression layer. The second solid electrolyte layer is, for example, a solid electrolyte layer. The air electrode 204 includes, for example, a first air electrode layer and a second air electrode layer. The first air electrode layer is provided between the second air electrode layer and the solid electrolyte 203. The first air electrode layer is, for example, an air electrode catalyst layer. The second air electrode layer is, for example, an air electrode layer. In the solid oxide fuel cell of the present invention, the preferred thickness of each layer is 0.5 to 2 mm for the support, 10 to 200 μm for the fuel electrode layer (first fuel electrode layer), and the fuel electrode catalyst layer (second electrode). The fuel electrode layer) is 0-30 μm, the reaction suppression layer (first solid electrolyte layer) is 0-20 μm, the solid electrolyte layer (second solid electrolyte layer) is 5-60 μm, and the air electrode catalyst layer (first air electrode layer). Is 0 to 30 μm, and the air electrode layer (second air electrode layer) is 10 to 200 μm.

  FIG. 4B shows an example in which the inner electrode is a fuel electrode in the fuel cell, but the inner electrode may be a fuel electrode or an air electrode. The inner electrode may be configured as a support or may be configured as a separate body. When the inner electrode and the support are configured separately, the inner electrode may be a fuel electrode, and the fuel electrode may be provided in contact with the support. In a fuel cell (such as a cylindrical cell or a hollow plate cell) having a gas flow path inside the support, the inner electrode is preferably a fuel electrode. When the inner electrode is a fuel electrode, the outer electrode is an air electrode. In this case, a gas flow path is provided inside the fuel electrode, and a solid electrolyte, an air electrode, and a current collecting layer are formed on the surface of the fuel electrode.

As the fuel electrode 202, for example, one made of cermet of Ni and ceramics can be used. As the ceramic, at least one selected from zirconia doped with at least one selected from rare earth elements such as Ca, Y, and Sc, ceria doped with at least one selected from rare earth elements, Sr, Mg, Co, Fe, and Cu. A doped lantern garade can be mentioned. Preferred examples include zirconia (YSZ) doped with yttria (Y ₂ O ₃ ) and ceria (GDC) doped with gadria (Gd ₂ O ₃ ).

  In the fuel electrode 202, the mixing ratio of Ni and ceramic is preferably 30:70 or more and 70:30 or less by weight. More preferably, it is 50:50 or more and 70:30 or less. Thereby, high electronic conductivity and high strength can be realized.

  In the present invention, when the support and the fuel electrode are provided separately, even when a support made of cermet of Ni and ceramics is provided in contact with the fuel electrode, the same effect can be exhibited. In this case, a well-known fuel electrode can be adopted.

Examples of the solid electrolyte 203 include zirconia doped with at least one selected from rare earth elements such as Y and Sc, ceria doped with at least one selected from rare earth elements, and lanthanum gallate doped with at least one selected from Sr and Mg. And at least one of them. Preferably, lanthanum gallate or YSZ doped with Sr and Mg can be used. More preferably the general formula _{La 1-a Sr a Ga 1} -bc Mg b Co c O 3 ( where, 0.05 ≦ a ≦ 0.3,0 <b <0.3,0 ≦ c ≦ 0.15) The lanthanum gallate (LSGM) represented by these can be used.

Examples of the air electrode 204 include lanthanum manganite doped with at least one selected from Sr and Ca, lanthanum ferrite doped with at least one selected from Sr, Co, Ni and Cu, Sr, Fe, Ni and Cu. At least one of lanthanum cobaltite doped with at least one selected from the group consisting of silver and silver can be used. The air electrode 204 may be a single layer or a multilayer. As an example of the case where the outer electrode is a multilayer air electrode, for example, a layer containing L _0.6 Sr _0.4 Co _0.2 Fe _0.8 O ₃ on the electrolyte side (for example, an air electrode catalyst layer, a first air electrode layer) is used. A layer containing L _0.6 Sr _0.4 Co _0.8 Fe _0.2 O ₃ (for example, an air electrode layer or a second air electrode layer) can be used as the outermost layer.

  From the viewpoint of improving the power generation performance, a fuel electrode catalyst layer (second fuel electrode layer) can be provided between the fuel electrode 202 and the solid electrolyte 203. Examples of the fuel electrode catalyst layer include Ni / cerium-containing oxides and Ni / lanthanum gallate oxides from the viewpoint of excellent electronic conductivity and oxygen ion conductivity.

  In order to further improve the power generation performance, a reaction suppression layer (first solid electrolyte layer) can be provided between the fuel electrode 202 or the fuel electrode catalyst layer and the solid electrolyte 203. Examples of the reaction suppressing layer include cerium-containing oxides from the viewpoint of excellent oxygen ion conductivity.

  The method for producing each layer of the solid oxide fuel cell in the present invention can be preferably carried out by a slurry coating method for coating a slurry liquid, a tape casting method, a doctor blade method, a transfer method or the like. A printing method can also be used, and a screen printing method, an inkjet method, or the like can be used.

  The air electrode, solid electrolyte, and fuel electrode are prepared by adding a molding aid such as a solvent (water, alcohol, etc.), a dispersant, a binder, etc. to each raw material powder, coating it, and drying it. It can obtain by baking (1100 degreeC or more and less than 1400 degreeC).

  Firing may be performed each time the electrodes and the solid electrolyte layer are formed, but “co-firing” in which a plurality of layers are fired at once is also possible. Further, the firing is preferably performed in an oxidizing atmosphere so that the solid electrolyte is not denatured by diffusion of the dopant. More preferably, firing is performed in an atmosphere having an oxygen concentration of 20% by mass to 30% by mass using a mixed gas of air and oxygen.

The fuel cell stack 14 will be described with reference to FIG. FIG. 6 is a perspective view showing a fuel cell stack of a solid oxide fuel cell (SOFC) system according to an embodiment of the present invention.
As shown in FIG. 6, the fuel cell stack 14 includes 16 fuel cell units 16, and these fuel cell units 16 are arranged in two rows of 8 each. Each fuel cell unit 16 is supported at its lower end by a rectangular lower support plate 68 (FIG. 2) made of ceramics, and at the upper end, four fuel cell units 16 at both ends are provided, each having a generally square shape. It is supported by the upper support plate 100. The lower support plate 68 and the upper support plate 100 are formed with through holes through which the inner electrode terminal 86 can pass.

  Furthermore, a current collector 102 and an external terminal 104 are attached to the fuel cell unit 16. The current collector 102 is electrically connected to the fuel electrode connection portion 102a electrically connected to the inner electrode terminal 86 attached to the inner electrode 90 which is a fuel electrode, and the outer peripheral surface of the outer electrode 92 which is an air electrode. Are integrally formed so as to be connected to the air electrode connecting portion 102b connected to the air electrode. In addition, a silver thin film is formed on the entire outer surface of the outer electrode 92 (air electrode) of each fuel cell unit 16 as an electrode of the air electrode. When the air electrode connecting portion 102b contacts the surface of the thin film, the current collector 102 is electrically connected to the entire air electrode.

  Furthermore, two external terminals 104 are connected to the air electrode 86 of the fuel cell unit 16 located at the end of the fuel cell stack 14 (the far left side in FIG. 5). These external terminals 104 are connected to the inner electrode terminal 86 of the fuel cell unit 16 at the end of the adjacent fuel cell stack 14, and as described above, all 160 fuel cell units 16 are connected in series. It has come to be.

The sensors and the like attached to the solid oxide fuel cell (SOFC) system according to the present embodiment will be described with reference to FIG. FIG. 7 is a block diagram illustrating a solid oxide fuel cell (SOFC) system according to an embodiment of the present invention.
As shown in FIG. 7, the solid oxide fuel cell system 1 includes a control unit 110 that is a controller. The control unit 110 includes operations such as “ON” and “OFF” for operation by the user. An operation device 112 having a button, a display device 114 for displaying various data such as a power generation output value (wattage), and a notification device 116 for issuing a warning (warning) in an abnormal state are connected. Yes. The notification device 116 may be connected to a remote management center and notify the management center of an abnormal state. In addition, the control unit 110 incorporates a microprocessor, a memory, and a program (not shown) for operating these components, and thereby, based on input signals from the respective sensors, the auxiliary unit 4, The inverter 54 and the like are controlled.

Next, signals from various sensors described below are input to the control unit 110.
First, the combustible gas detection sensor 120 is for detecting a gas leak, and is attached to the fuel cell module 2 and the auxiliary unit 4. The CO detection sensor 122 detects whether or not CO in the exhaust gas originally discharged to the outside through the exhaust gas passage 80 or the like leaks to an external housing (not shown) that covers the fuel cell module 2 and the auxiliary unit 4. Is to do. The hot water storage state detection sensor 124 is for detecting the temperature and amount of hot water in a water heater (not shown).

  The power state detection sensor 126 is for detecting the current and voltage of the inverter 54 and the distribution board (not shown). The power generation air flow rate detection sensor 128 is for detecting the flow rate of power generation air supplied to the power generation chamber 10. The reforming air flow sensor 130 is for detecting the flow rate of the reforming air supplied to the reformer 20. The fuel flow sensor 132 is for detecting the flow rate of the fuel gas supplied to the reformer 20.

  The water flow rate sensor 134 is for detecting the flow rate of pure water supplied to the reformer 20. The water level sensor 136 is for detecting the water level of the pure water tank 26. The pressure sensor 138 is for detecting the pressure on the upstream side outside the reformer 20. The exhaust temperature sensor 140 is for detecting the temperature of the exhaust gas flowing into the hot water production apparatus 50.

  As shown in FIG. 3, the power generation chamber temperature sensor 142 is provided on the front side and the back side in the vicinity of the fuel cell assembly 12, and detects the temperature in the vicinity of the fuel cell stack 14 to thereby detect the fuel cell stack. 14 (ie, the fuel cell 84 itself) is estimated. The combustion chamber temperature sensor 144 is for detecting the temperature of the combustion chamber 18. The exhaust gas chamber temperature sensor 146 is for detecting the temperature of the exhaust gas in the exhaust gas chamber 78. The reformer temperature sensor 148 is for detecting the temperature of the reformer 20, and calculates the temperature of the reformer 20 from the inlet temperature and the outlet temperature of the reformer 20. The outside air temperature sensor 150 is for detecting the temperature of the outside air when the solid oxide fuel cell (SOFC) system is placed outdoors. Further, a sensor for measuring the humidity or the like of the outside air may be provided.

  Signals from these sensors are sent to the control unit 110, and the control unit 110, based on data based on these signals, the water flow rate adjustment unit 28, the fuel flow rate adjustment unit 38, the reforming air flow rate adjustment unit 44, A control signal is sent to the power generation air flow rate adjusting unit 45 to control each flow rate in these units.

  As shown in FIGS. 2 and 3, the air heat exchanger 22 includes a plurality of combustion gas pipes 70 and a power generation air flow path 72. As shown in FIG. 2, an exhaust gas collecting chamber 78 is provided at one end of the plurality of combustion gas pipes 70, and the exhaust gas collecting chamber 78 is communicated with each combustion gas pipe 70. ing. Further, an exhaust gas discharge pipe 82 is connected to the exhaust gas collecting chamber 78. Further, the other end of each combustion gas pipe 70 is open, and this open end communicates with the combustion chamber 18 in the case 8 via a communication opening 8 a formed on the upper surface of the case 8. Has been.

  The combustion gas pipe 70 is a plurality of metal circular pipes oriented in the horizontal direction, and the circular pipes are arranged in parallel. On the other hand, the power generation air flow path 72 is constituted by a space outside each combustion gas pipe 70. Further, a power generation air introduction pipe 74 is connected to an end of the power generation air flow path 72 on the exhaust gas discharge pipe 82 side, and the air outside the fuel cell module 2 is connected to the power generation air introduction pipe 74. And is introduced into the power generation air flow path 72. The power generation air introduction pipe 74 protrudes in the horizontal direction from the air heat exchanger 22 in parallel with the exhaust gas discharge pipe 82. Further, a pair of communication flow paths 76 (FIG. 3) are connected to both side surfaces of the other end of the power generation air flow path 72, and each of the power generation air flow path 72 and each communication flow path 76 is connected to each other. The communication is made through the outlet port 76a.

  As shown in FIG. 3, a power generation air supply passage 77 is provided on each side surface of the case 8. Each communication channel 76 provided on both side surfaces of the air heat exchanger 22 communicates with an upper portion of a power generation air supply channel 77 provided on both side surfaces of the case 8. In addition, a large number of air outlets 77 a are arranged in the horizontal direction at the lower portion of each power generation air supply passage 77. The power generation air supplied through each power generation air supply path 77 is injected toward the lower side surface of the fuel cell stack 14 in the fuel cell module 2 from a large number of air outlets 77a.

A rectifying plate 21 that is a partition wall is attached to the ceiling surface inside the case 8, and the rectifying plate 21 has an opening 21 a.
The rectifying plate 21 is a plate member disposed horizontally between the ceiling surface of the case 8 and the reformer 20. The rectifying plate 21 is configured to adjust the flow of the gas flowing upward from the combustion chamber 18 and to guide it to the inlet of the air heat exchanger 22 (communication opening 8a in FIG. 2). The power generation air and the combustion gas traveling upward from the combustion chamber 18 flow into the upper side of the rectifying plate 21 through the opening 21 a provided in the center of the rectifying plate 21, and the upper surface of the rectifying plate 21 and the ceiling surface of the case 8. 2 flows to the left in FIG. 2 and is led to the inlet of the heat exchanger 22 for air. The opening 21a is provided above the reforming unit 20c of the reformer 20, and the gas rising through the opening 21a is exhausted on the left side in FIG. 2 on the side opposite to the evaporation unit 20a. It flows into the passage 21b. For this reason, the space above the evaporation unit 20a (the right side in FIG. 2) acts as a gas retention space 21c in which the exhaust flow is slower than the space above the reforming unit 20c and the exhaust flow is stagnant.

  Further, a vertical wall 21d is provided at the edge of the opening 21a of the rectifying plate 21 over the entire circumference, and the vertical wall 21d allows the exhaust above the rectifying plate 21 from the space below the rectifying plate 21. The flow path flowing into the passage 21b is narrowed. Further, a falling wall 8b (FIG. 2) is provided over the entire circumference at the edge of the communication opening 8a that allows the exhaust passage 21b and the air heat exchanger 22 to communicate with each other. The flow path flowing into the air heat exchanger 22 is narrowed. By providing the vertical wall 21d and the falling wall 8b, the flow resistance in the exhaust passage from the combustion chamber 18 to the outside of the fuel cell module 2 through the air heat exchanger 22 is adjusted. The adjustment of the channel resistance will be described later.

Next, the flow of fuel, power generation air, and exhaust gas during the power generation operation of the solid oxide fuel cell system 1 will be described.
First, the fuel is introduced into the evaporation section 20a of the reformer 20 through the fuel gas supply pipe 63b, the T-shaped pipe 62a, and the reformer introduction pipe 62, and the pure water is supplied to the water supply pipe 63a, It is introduced into the evaporator 20a through the T-shaped tube 62a and the reformer introducing tube 62. Accordingly, the supplied fuel and water are merged in the T-shaped tube 62 a and introduced into the evaporation unit 20 a through the reformer introduction tube 62. During the power generation operation, since the evaporation unit 20a is heated to a high temperature, the pure water introduced into the evaporation unit 20a is evaporated relatively quickly to become water vapor. The evaporated water vapor and fuel are mixed in the mixing unit 20 b and flow into the reforming unit 20 c of the reformer 20. The fuel introduced into the reforming unit 20c together with the steam is steam reformed here and reformed into a fuel gas rich in hydrogen. The fuel reformed in the reforming unit 20c goes down through the fuel gas supply pipe 64 and flows into the manifold 66 which is a dispersion chamber.

  The manifold 66 is a rectangular parallelepiped space having a relatively large volume, which is disposed on the lower side of the fuel cell stack 14, and a plurality of holes provided on the upper surface thereof constitute each fuel cell constituting the fuel cell stack 14. It communicates with the inside of the unit 16. The fuel introduced into the manifold 66 flows out of the upper end of the fuel cell unit 16 through the fuel electrode of the fuel cell unit 16, that is, through the inside of the fuel cell unit 16 through a number of holes provided on the upper surface thereof. Further, when hydrogen gas as a fuel passes through the inside of the fuel cell unit 16, it reacts with oxygen in the air passing outside the fuel cell unit 16 as an air electrode (oxidant gas electrode) to generate a charge. Is done. The remaining fuel that is not used for power generation flows out from the upper end of each fuel cell unit 16 and is combusted in a combustion chamber 18 provided above the fuel cell stack 14.

  On the other hand, the power generation air, which is oxygen, is sent into the fuel cell module 2 through the power generation air introduction pipe 74 by the power generation air flow rate adjustment unit 45 that is an oxygen supply device for power generation. The air sent into the fuel cell module 2 is introduced into the power generation air passage 72 of the air heat exchanger 22 via the power generation air introduction pipe 74 and preheated. The preheated air flows out to each communication channel 76 via each outlet port 76a (FIG. 3). The power generation air flowing into each communication channel 76 flows downward through the power generation air supply passages 77 provided on both side surfaces of the fuel cell module 2, and the fuel cell stack 14 from a number of outlets 77a. Toward the power generation chamber 10.

  The air injected into the power generation chamber 10 contacts the outer surface of each fuel cell unit 16 that is the air electrode (oxygen electrode side) of the fuel cell stack 14, and a part of oxygen in the air is used for power generation. Is done. Moreover, the air injected to the lower part of the power generation chamber 10 through the blower outlet 77a rises in the power generation chamber 10 while being used for power generation. The air rising in the power generation chamber 10 burns the fuel flowing out from the upper end of each fuel cell unit 16. The combustion heat generated by this combustion heats the evaporation section 20a, the mixing section 20b, and the reforming section 20c of the reformer 20 disposed above the fuel cell stack 14. The combustion gas generated by burning the fuel heats the upper reformer 20 and then flows into the upper side of the rectifying plate 21 through the opening 21 a above the reformer 20. The combustion gas that has flowed into the upper side of the rectifying plate 21 is guided to the communication opening 8 a that is the inlet of the air heat exchanger 22 through the exhaust passage 21 b formed by the rectifying plate 21. The combustion gas that has flowed into the air heat exchanger 22 from the communication opening 8 a flows into the open end of each combustion gas pipe 70 and flows through the power generation air flow path 72 outside each combustion gas pipe 70. Are exchanged with each other and collected in an exhaust gas collecting chamber 78. The exhaust gas collected in the exhaust gas collection chamber 78 is discharged to the outside of the fuel cell module 2 through the exhaust gas discharge pipe 82. As a result, the evaporation of water in the evaporation unit 20a and the steam reforming reaction, which is an endothermic reaction in the reforming unit 20c, are promoted, and the power generation air in the air heat exchanger 22 is preheated.

  In this example, a cylindrical solid oxide fuel cell having both ends of a type using a fuel electrode as a support was opened. Details of the method for producing the cylindrical solid oxide fuel cell will be described below. The present invention is not limited to these examples.

1. Fabrication of a cell using a fuel electrode as a support A cylindrical solid oxide fuel cell including a solid electrolyte layer 203 formed on the outside of the fuel electrode support 201 and an air electrode 204 formed on the outside of the solid electrolyte 203 A cell was produced. In this example, the fuel electrode is configured as a support. A fuel electrode reaction catalyst layer was provided between the fuel electrode support 201 and the solid electrolyte layer 203. A reaction preventing layer was provided between the solid electrolyte layer 203 and the fuel electrode reaction catalyst layer. The production procedure will be described in detail below.

No. 1, 5, 6
NiO powder and (ZrO ₂ ) _0.90 (Y ₂ O ₃ ) _0.10 (hereinafter abbreviated as YSZ) powder were mixed so that the weight ratio of Ni: YSZ was 59:41. A cylindrical molded body was produced by extruding the powder. The formed body was heat treated at 900 ° C. to obtain a calcined body. On the surface of the calcined body, a fuel electrode reaction catalyst layer, a reaction prevention layer, and a solid electrolyte were formed in this order by a slurry coating method. These laminated molded bodies were co-fired at 1300 ° C. with the support. At this time, the thickness of the reaction preventing layer was adjusted to be between 2 and 10 μm. The thickness of the solid electrolyte layer was adjusted to be between 30 and 60 μm. Next, the cell was masked so that the area of the air electrode was 32.7 cm ² , and the air electrode was formed on the surface of the solid electrolyte. This molded body was fired at 1100 ° C., and the solid oxide fuel cell, Sample No. 1, 5, 6 were obtained. The fuel electrode support had dimensions after co-firing, an outer diameter of 10 mm, a wall thickness of 1 mm, and an effective cell length of 110 mm. The details of the slurry preparation method for each layer used in the slurry coating method are shown in (1-a) to (1-d) below. Sample No. A plurality of cells 1, 5, and 6 were prepared, a test piece was prepared using a part of each cell, and the oxidation shrinkage ratio Lr and the interface length were measured. In addition, a start / stop test was performed on the remaining cells.

(1-a) Preparation of fuel electrode reaction catalyst layer slurry A mixture of NiO powder and GDC powder was prepared by a coprecipitation method, followed by heat treatment to obtain a fuel electrode reaction catalyst layer powder. The mixing ratio of NiO powder and GDC powder was 50:50 by weight. The average particle size was adjusted to 0.5 μm. 40 parts by weight of the powder is mixed with 100 parts by weight of a solvent (ethanol), 2 parts by weight of a binder (ethyl cellulose), 1 part by weight of a dispersant (polyoxyethylene alkyl phosphate ester) and 1 part by weight of an antifoaming agent (sorbitan sesquiolate). Then, the slurry was prepared by sufficiently stirring.

(1-b) Preparation of Slurry for Reaction Prevention Layer (Ce _0.6 La _0.4 ) O ₂ (hereinafter abbreviated as LDC) powder was used as the material for the reaction prevention layer. As a sintering aid, 0.2 part by weight of Ga ₂ O ₃ powder is mixed, 100 parts by weight of solvent (ethanol), 2 parts by weight of binder (ethyl cellulose), 1 part by weight of dispersant (polyoxyethylene alkyl phosphate ester), After mixing with 1 part by weight of an antifoaming agent (sorbitan sesquiolate), the slurry was prepared by sufficiently stirring.

(1-c) Slurry Preparation of Solid Electrolyte Layer As the material for the second layer, LSGM powder having a composition of La _0.8 Sr _0.2 Ga _0.8 Mg _0.2 O ₃ was used. 40 parts by weight of LSGM powder, 100 parts by weight of solvent (ethanol), 2 parts by weight of binder (ethyl cellulose), 1 part by weight of dispersant (polyoxyethylene alkyl phosphate ester), 1 part by weight of antifoaming agent (sorbitan sesquiolate) After mixing, the slurry was prepared by sufficiently stirring.

(1-d) Preparation of Air Electrode Slurry A powder having a composition of La _0.6 Sr _0.4 Co _0.8 Fe _0.2 O ₃ was used as the air electrode material. 40 parts by weight of the powder is 100 parts by weight of a solvent (ethanol), 2 parts by weight of a binder (ethyl cellulose), 1 part by weight of a dispersant (polyoxyethylene alkyl phosphate ester) and 1 part by weight of an antifoaming agent (sorbitan sesquiolate). After mixing, the slurry was prepared by sufficiently stirring.

No. 2-4
A mixture of NiO powder and Ce _0.9 Gd _0.1 O _1.95 (hereinafter abbreviated as GDC) powder was prepared by a wet mixing method, followed by heat treatment and pulverization to obtain a fuel electrode support raw material powder. The mixing ratio of the NiO powder and the GDC powder was the composition shown in Table 1 in terms of the Ni: GDC weight ratio. Other than this, no. 1 was performed, and a plurality of solid oxide fuel cells were obtained.

  The prepared sample No. Each evaluation was performed about the 1-6 solid oxide fuel cell. The results are shown in Table 1.

(Measurement of oxidation shrinkage of fuel electrode)
First, sample no. 1 to 6 solid oxide fuel cells were subjected to reduction treatment, and then each sample was subjected to oxidation treatment, and the oxidation shrinkage ratio Lr after the oxidation treatment was measured.
First, sample no. About 1-6, the reduction process of the fuel electrode was performed. Specifically, sample no. Nitrogen 200 cc / min was supplied to the fuel electrode of the solid oxide fuel cells 1 to 6 and air 2000 cc / min was supplied to the air electrode, and the temperature was raised from room temperature to 700 ° C. over 1 hour and a half. Thereafter, hydrogen was maintained at a temperature of 700 ° C., 90 cc / min of hydrogen was supplied to the fuel electrode, 40 cc / min of nitrogen, and 2000 cc / min of air was supplied to the air side and held for 8 hours. Thereafter, hydrogen 30 cc / min and nitrogen 200 cc / min were supplied to the fuel electrode side, and air 2000 cc / min was supplied to the air electrode side, and the temperature was lowered to room temperature in 3 hours.

Sample No. subjected to reduction treatment 1 to 6 solid oxide fuel cells were cut to a length of 15 mm and polished so that the cut surfaces were parallel to obtain samples 1 to 6 for measuring the oxidation shrinkage rate. Assuming a situation in which the temperature inside the fuel cell module drops to a predetermined temperature after the solid oxide fuel cell system is stopped for these samples 1 to 6, a thermomechanical analyzer TMA (TMA-60 manufactured by Shimadzu Corporation) is assumed. ) Was used to carry out the following test. The load during measurement was 10 g.
First, samples 1 to 6 were heated from room temperature to 650 ° C. at 6 ° C./min in hydrogen. This temperature is a temperature that is assumed during operation of the solid oxide fuel cell system. Thereafter, the temperature was lowered to 400 ° C. at 6 ° C./min in hydrogen. Stabilized at 400 ° C. for 10 minutes and then held in nitrogen for 5 minutes. Thereafter, assuming that the operation of the solid oxide fuel cell system was stopped, the solid oxide fuel cell system was maintained in a mixed gas of air and nitrogen so that the oxygen partial pressure was 0.001 atm for 18 hours. From the dimension at 500 ° C. (L ₅₀₀ ) and the minimum dimension (L ₄₀₀ ) when held at 400 ° C. under an oxygen partial pressure of 0.001 atm, the oxidation shrinkage ratio Lr of each fuel electrode of Samples 1 to 6 is obtained by the following formula. Asked.
Oxidation shrinkage ratio Lr (%) = (1−L ₄₀₀ / L ₅₀₀ ) × 100
Here, the length of the fuel electrode in the Z-axis direction was used as the dimension L of the fuel electrode.

(Calculation of interface length)
First, the produced sample No. 1 to 6 were observed with an electron microscope. Samples for electron microscope observation include sample no. Test pieces 1 to 6 in which 1 to 6 were cut out to an appropriate size were used. After the cut porous supports of the test pieces 1 to 6 were polished, ion milling processing was performed using IM4000 manufactured by Hitachi, Ltd. These test pieces 1 to 6 were observed with a scanning electron microscope (S-4100, manufactured by Hitachi, Ltd.) at an acceleration voltage of 5 kV, a secondary electron image and a reflected electron image, and a magnification of 10,000 times. )
FIG. It is an electron microscope image of 2. FIG. In the electron microscope image (reflected electron image) shown in the figure, Ni, GDC, and pores are individually displayed due to the difference in brightness. In FIG. 9, Ni is displayed as “gray”, GDC as “white”, and pores as “black”.

(Electron microscope image processing)
FIG. 10 is a diagram obtained by performing image processing on the electron microscope image shown in FIG. 9 using image analysis software (WinROOF manufactured by Mitani Corporation). In FIG. 10, the interface between Ni and GDC is shown in white. This interface distinguishes Ni, GDC, and pores from the contrast of the electron microscope image (reflected electron image), and visually determines the interface between Ni and GDC. The length of this interface is the difference between Ni and GDC. Indicates the interface length.
The average value of the interface length per unit area between Ni and GDC was determined by the following method.
The total interface length of Ni and GDC was determined by adding all the interface lengths shown in FIG. A value obtained by dividing the total value by the visual field area of the electron microscope image was defined as the interface length per unit area between Ni and GDC per unit area. In addition, in order to reduce the dispersion | variation by the visual field of an electron microscope image, image processing was performed about the electron microscope image of 10 visual fields per sample, and the average value of the interface length per unit area was computed.

(Start / stop test)
In addition, the obtained sample No. 1 to 6 solid oxide fuel cells were incorporated into a solid oxide fuel cell system and a power generation test was conducted. As power generation conditions, the fuel was a mixed gas of city gas 13A, water and air. The oxidizing agent was air. The steady state power generation temperature was set to 650 ° C., and after operation at a current density of 0.2 A / cm 2, the solid oxide fuel cell system was stopped by the following method.
Gas supply, water supply, and air supply on the fuel electrode side are stopped in a short time. At the same time, the supply of air on the air electrode side is also stopped. Also, the extraction of power from the fuel cell module is stopped (output current = 0). After the stop, the fuel cell module was left in this state. The above start / stop test was repeated 5 cycles, and the presence or absence of cracks in the solid oxide fuel cell after the test was confirmed using an optical microscope (Keyence digital microscope VHX-1000).

1 Solid Oxide Fuel Cell System 2 Fuel Cell Module 4 Auxiliary Unit 7 Heat Insulation Material (Heat Storage Material)
8 Case 8a Communication opening 8b Falling wall 10 Power generation chamber 12 Fuel cell assembly 14 Fuel cell stack 16 Fuel cell unit (fuel cell)
18 Combustion chamber (combustion section)
20 Reformer 20a Evaporating section (evaporating chamber)
20b Mixing part (pressure fluctuation absorbing means)
20c reforming part 20d evaporation / mixing part partition 20e partition opening 20f mixing / reforming part partition (pressure fluctuation absorbing means)
20g communication hole (narrow channel)
21 Rectifier plate (partition wall)
21a Opening 21b Exhaust passage 21c Gas residence space 21d Vertical wall 22 Air heat exchanger (heat exchanger)
23 Heat insulating material for evaporation chamber (internal heat insulating material)
24 Water supply source 26 Pure water tank 28 Water flow rate adjustment unit (water supply device)
30 Fuel supply source 38 Fuel flow rate adjustment unit (fuel supply device)
39 Valve 40 Air supply source 44 Reforming air flow rate adjusting unit (reforming oxygen supply device)
45 Air flow adjustment unit for power generation (oxygen supply device for power generation)
46 1st heater 48 2nd heater 50 Hot water production apparatus (heat exchanger for exhaust heat recovery)
52 Control box 54 Inverter 62 Reformer introduction pipe (water introduction pipe, preheating part, condensation part)
62a T-shaped tube (condensation part)
63a Water supply pipe 63b Fuel gas supply pipe 64 Fuel gas supply pipe 64c Pressure fluctuation suppressing flow path resistance 66 Manifold (dispersion chamber)
76 Air introduction pipe 76a Air outlet 82 Exhaust gas discharge pipe 83 Ignition device 84 Fuel cell 85 Exhaust valve 86 Inner electrode terminal (cap)
98 Fuel gas passage narrow tube 110 Controller (controller)
110a Shutdown stop circuit 110b Voltage monitoring circuit 112 Operation device 114 Display device 116 Alarm device 126 Power state detection sensor 132 Fuel flow rate sensor (fuel supply amount detection sensor)
138 Pressure sensor (reformer pressure sensor)
142 Power generation chamber temperature sensor (temperature detection means)
148 Reformer Temperature Sensor 150 Outside Temperature Sensor

Claims (6)

A solid oxide fuel cell system that generates electricity by reacting hydrogen and oxygen,
A fuel cell module containing a fuel cell comprising a fuel electrode and a cathode containing a cermet of solid electrolyte, Ni and ceramics; and
A fuel supply device for supplying fuel to the fuel cell module;
A reformer for generating hydrogen by steam reforming the fuel supplied by the fuel supply device, and supplying the generated hydrogen to the fuel electrode;
A water supply device for supplying water for steam reforming to the reformer;
An oxygen supply device for supplying oxygen to the fuel cell module;
A controller for controlling the fuel supply device, the water supply device, and the oxygen supply device, and for controlling the extraction of electric power from the fuel cell module,
The fuel electrode when the temperature in the fuel cell module is 500 ° C. after the fuel supply device, the water supply device, and the oxygen supply device are forcibly stopped during the power generation operation of the fuel cell system. the dimensions (L ₅₀₀₎ of a temperature of 400 ° C. in the fuel cell module, when the oxygen partial pressure is greater than 10 -5 ^atm of the air electrode, the dimension of the fuel electrode and the (L _400), but (1-L ₄₀₀ / L ₅₀₀ ) × 100 ≦ 0.45%, solid oxide fuel cell system A solid oxide fuel cell system that generates electricity by reacting hydrogen and oxygen,
A fuel cell module containing a fuel cell comprising: a solid electrolyte; a fuel electrode; an air electrode; and a support including a cermet of Ni and ceramics provided in contact with the fuel electrode;
A fuel supply device for supplying fuel to the fuel cell module;
A reformer for generating hydrogen by steam reforming the fuel supplied by the fuel supply device, and supplying the generated hydrogen to the fuel electrode;
A water supply device for supplying water for steam reforming to the reformer;
An oxygen supply device for supplying oxygen to the fuel cell module;
A controller for controlling the fuel supply device, the water supply device, and the oxygen supply device, and for controlling the extraction of electric power from the fuel cell module,
After at least one of the fuel supply device, the water supply device, and the oxygen supply device is forcibly stopped during the power generation operation of the fuel cell system, the temperature in the fuel cell module is 500 ° C. The size of the support (L ₅₀₀ ) and the temperature of the support when the temperature in the fuel cell module is 400 ° C. and the oxygen partial pressure of the air electrode is greater than 10 ⁻⁵ atm. ₄₀₀ ) and (1−L ₄₀₀ / L ₅₀₀ ) × 100 ≦ 0.45%, a solid oxide fuel cell system. ^2. The solid oxide fuel cell system according to claim 1, wherein in the cermet, an interface length per unit area between the Ni and the ceramic is 2 μm / μm ² or more and 10 μm / μm ² or less.   4. The solid oxide fuel cell system according to claim 1, wherein the fuel electrode includes at least one of Ni and YSZ cermets or Ni and GDC cermets. 5. 3. The solid oxide fuel cell system according to claim 2, wherein in the cermet, an interface length per unit area between the Ni and the ceramic is 2 μm / μm ² or more and 10 μm / μm ² or less.   The solid oxide fuel cell system according to claim 2 or 5, wherein the support includes at least one of Ni and YSZ cermet or Ni and GDC cermet.