JP2015191834A - solid oxide fuel cell system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a solid oxide fuel cell system capable of sufficiently suppressing electrochemical oxidation of a fuel electrode while executing shutdown stop.SOLUTION: A solid oxide fuel cell system (1) comprises: a fuel cell module (2); a fuel supply device (38); a reformer (20); a water supply device (28); an oxidant gas supply device (45); and a controller (110) including a shutdown stop circuit (110a). The shutdown stop circuit is programmed so as to execute residual hydrogen discharge control which supplies only an oxidant gas after a standing period has elapsed and before the temperature of the fuel cell module decreases to an oxidation preventive temperature and cooling operation control which supplies the oxidant gas with the temperature of the fuel cell module decreased to the oxidation preventive temperature or lower. The hydrogen outflowing to the oxidant gas electrode side is discharged from the fuel cell module and the hydrogen concentration in the fuel cell module is decreased by the residual hydrogen discharge control.

Description

  The present invention relates to a solid oxide fuel cell system, and more particularly to a solid oxide fuel cell system that generates power by reacting hydrogen with an oxidant gas.

  A solid oxide fuel cell (hereinafter also referred to as “SOFC”) uses an oxide ion conductive solid electrolyte as an electrolyte, attaches electrodes on both sides thereof, and supplies fuel gas on one side, The fuel cell operates at a relatively high temperature by supplying an oxidant gas (air, oxygen, etc.) to the other side.

  Japanese Patent No. 5316830 (Patent Document 1) describes a solid oxide fuel cell. In this fuel cell, when a solid oxide fuel cell in operation is stopped, shutdown stop is executed to simultaneously stop power generation and fuel supply. In the shutdown stop, the supply of fuel is stopped almost simultaneously with the end of power generation, so that fuel that does not contribute to power generation is not supplied, and waste of fuel can be avoided.

  However, if the shutdown stop is executed and the supply of fuel is stopped simultaneously with the stop of power generation, the supply of fuel to the fuel cells is stopped in a high temperature state. Therefore, when the shutdown stop is executed, generally, there is a risk that an oxidant gas such as air entering from the air electrode side of the fuel battery cell contacts the fuel electrode in a high temperature state and oxidizes the fuel electrode. In general, it takes several hours or more for a stack of fuel cells operating at a high temperature to be lowered to a temperature at which there is no risk of oxidation of the fuel electrode after shutdown. If the fuel supply is continued until the temperature of the fuel cell is sufficiently lowered without shutting down in order to avoid the risk of fuel electrode oxidation, the fuel that does not contribute to power generation for several hours or more is used. There is a need to continue to supply, and a great deal of fuel is wasted.

  In the solid oxide fuel cell described in Japanese Patent No. 5316830 (Patent Document 1), each fuel cell and the fuel cell module that accommodates the fuel cell are mechanically structured to prevent the ingress of air. Reduces the risk of polar oxidation. Further, in the above solid oxide fuel cell, temperature drop control is performed to introduce air to the air electrode side in a period when the pressure on the fuel electrode side of each fuel cell immediately after shutdown is high and air does not enter. By doing so, it has succeeded in sufficiently reducing the risk of anode oxidation.

Japanese Patent No. 5316830

  However, by applying the invention described in Japanese Patent No. 5316830, even if the oxidation of the fuel electrode due to air contact with the high temperature fuel electrode can be avoided, the oxidation of the fuel electrode by a different mechanism occurs. Was identified by the inventors. That is, the present inventor has found that the fuel electrode is slightly oxidized even in a lower temperature zone by a mechanism different from the atmospheric oxidation that occurs when air enters the fuel electrode side in a high temperature state.

The mechanism of the fuel electrode oxidation discovered by the present inventors is as follows.
First, the electrolyte of the fuel battery cell can transmit oxygen ions from the air electrode to the fuel electrode in an active state. The active state of the electrolyte has a temperature characteristic, and this temperature characteristic has a lower limit temperature value. That is, the electrolyte becomes active when the temperature is lower than the lower limit temperature, and allows oxygen ions to pass from the air electrode to the fuel electrode. On the other hand, in the active state, the fuel electrode reacts the fuel (hydrogen) with oxygen ions that have permeated through the electrolyte, thereby generating water and releasing electrons. The catalytic activity of the fuel electrode that causes such a reaction also has a temperature characteristic, and this temperature characteristic also has a lower limit temperature value. That is, the fuel electrode is activated when the temperature is lower than the lower limit temperature, and deactivated when the temperature is lower than the lower temperature. In the deactivated state, the fuel electrode cannot act to react with the fuel even if oxygen ions are supplied.

  Generally, there is a temperature difference between the minimum activity temperature of the electrolyte and the minimum activity temperature of the fuel electrode. Therefore, when the lower active temperature limit of the fuel electrode is higher than the lower active temperature limit of the electrolyte, the electrolyte is in an active state while the temperature is decreasing after the shutdown is stopped, while the fuel electrode is in an inactive state. It will pass through the temperature zone. When the air electrode is in an active state, oxygen ions are provided from the electrolyte to the fuel electrode in this temperature range, but the provided oxygen ions cannot react with the fuel.

  Here, the fuel electrode contains a substance (for example, nickel) that can electrochemically react with oxygen ions, and this substance reacts with oxygen ions that cannot react with the fuel to form oxides (for example, nickel oxide). Is generated. Electrons are emitted by this electrochemical oxidation reaction, and an electromotive force is generated.

  As described above, the electrochemical oxidation reaction of the fuel electrode is such that the electrolyte is in the active state and the fuel electrode is in the deactivated state, and the oxygen partial pressure difference between the fuel electrode side and the air electrode (oxidant gas electrode) side. This is caused by the existence of Here, the oxygen partial pressure difference between the fuel electrode side and the air electrode side also occurs in one fuel cell. That is, the oxygen partial pressure difference is small near one end of the fuel cell into which the air that has entered flows in, and the oxygen partial pressure difference increases because no air has yet entered near the other end. As described above, a hydrogen concentration gradient is generated in one of the fuel cells as air enters, so that a large electromotive force is generated in a portion where the oxygen partial pressure difference of the fuel cell is large, whereas a portion where the oxygen partial pressure difference is small. Then, electromotive force is not generated much.

  As a result, an internal battery phenomenon occurs in which electrons move from a large electromotive force portion to a small portion (current flows) inside one fuel cell. When electrons move due to this internal cell phenomenon, the flow of oxygen ions that permeate the electrolyte that has not lost its activity is promoted, and the electrochemical oxidation reaction of the fuel electrode is promoted. In particular, in one fuel battery cell, the flow of oxygen ions is concentrated in a portion where the difference in oxygen partial pressure is large, and many electrochemical oxidation reactions occur in this portion. Although the current due to such an internal battery phenomenon is weak, as described above, the cooling time is several hours, so the time for the fuel cell to pass through the temperature band is long, and the accumulated current amount (ie, , The amount of oxide generated) increases.

  After the stop process, the fuel (hydrogen) is supplied to the fuel electrode when it is restarted next, so that the oxide generated in the fuel electrode by the electrochemical oxidation reaction during the stop process is reduced. Reducing action. As described above, when the fuel cell is stopped / started and the subsequent operation is performed, electrochemical oxidation / reduction reactions are repeatedly generated in the fuel electrode due to the difference in the activation temperature between the electrolyte and the fuel electrode. Here, the fuel electrode undergoes a microstructural change and volume expansion due to an electrochemical oxidation reaction. As a result of repeated stop and start, if changes in the microstructure and volume of the fuel electrode occur repeatedly, not only fatigue is accumulated in the electrolyte, but also microstructural changes and volume expansion are accumulated. Microcracks are generated in the electrolyte. In particular, the oxygen partial pressure difference of the fuel battery cell is large, fatigue is accumulated in a portion where a large amount of electrochemical oxidation reaction has occurred, and microcracks tend to occur in this portion.

  Thus, in the SOFC, the fuel electrode undergoes an electrochemical oxidation reaction due to the difference in the activation temperature between the electrolyte and the fuel electrode in the stop process. When the fuel electrode undergoes an electrochemical oxidation reaction, tensile stress is generated in the electrolyte due to microstructural changes and volume expansion of the fuel electrode, and micro cracks are generated in the electrolyte. The present inventor has ascertained the mechanism of occurrence of damage to the fuel cell due to such microcracks.

  Accordingly, an object of the present invention is to provide a solid oxide fuel cell system capable of sufficiently suppressing electrochemical oxidation of the fuel electrode of the fuel cell while executing shutdown stop.

  In order to solve the above-described problems, the present invention provides a solid oxide fuel cell system that generates electric power by reacting hydrogen with an oxidant gas, the fuel cell module containing the fuel cell, and the fuel cell. A fuel supply device that supplies fuel to the module, a reformer that generates hydrogen by steam reforming the fuel supplied by the fuel supply device, and supplies the generated hydrogen to the fuel electrode side of the fuel cell, A water supply device for supplying water for steam reforming to the reformer, an oxidant gas supply device for supplying an oxidant gas to the oxidant gas electrode side of the fuel cell, a fuel supply device, a water supply device, And a controller that controls the extraction of electric power from the fuel cell module, and the controller supplies the fuel by the fuel supply device and the fuel cell module. A shutdown stop circuit for executing shutdown stop for stopping the extraction of power from the battery, and the shutdown stop circuit after the shutdown stop and after the leaving period when the supply of fuel, water and oxidant gas is stopped, Residual hydrogen discharge control that operates only the oxidant gas supply device for a predetermined period before the temperature in the module drops below the oxidation suppression temperature, and the oxidizer in a state where the temperature in the fuel cell module drops below the oxidation suppression temperature It is programmed to operate the gas supply device and perform cooling operation control to cool the inside of the fuel cell module, and hydrogen flowing out from the fuel electrode side to the oxidant gas electrode side after shutdown is stopped by the residual hydrogen discharge control. It is discharged from the fuel cell module, and the hydrogen concentration in the fuel cell module is reduced It is.

  In the present invention configured as described above, fuel and water are supplied to the reformer by the fuel supply device and the water supply device, respectively, and the reformer steam reforms the fuel. The reformed fuel is supplied to the fuel electrode side of each fuel battery cell. On the other hand, the oxidant gas is supplied to the oxidant gas electrode side of the fuel cell by the oxidant gas supply device. The controller includes a shutdown stop circuit and controls the extraction of electric power from the fuel supply device, the water supply device, the oxidant gas supply device, and the fuel cell module. The shutdown stop circuit executes shutdown stop, executes residual hydrogen discharge control before the temperature in the fuel cell module drops below the oxidation suppression temperature after the leaving period, and suppresses oxidation in the temperature in the fuel cell module Cooling operation control is executed in a state where the temperature drops below the temperature. In the residual hydrogen discharge control, only the oxidant gas supply device is operated for a predetermined period so that the oxidant gas supplied into the fuel cell is not forced to flow in a positive manner. As a result, a large amount of residual hydrogen that has flowed out from the fuel electrode side to the oxidant gas electrode side after shutdown is quickly discharged from the fuel cell module, the hydrogen concentration in the fuel cell module decreases, and the hydrogen concentration in the fuel cell module decreases. And oxygen concentration is optimized. As a result, when the suction starts, residual hydrogen is sucked in, and it is possible to reliably correct the situation where the hydrogen concentration gradient is greatly increased even when the risk of electrochemical oxidation increases. That is, before the risk of electrochemical oxidation increases, the hydrogen concentration gradient can be reliably suppressed and the oxygen partial pressure can be increased in a suitable state throughout the fuel electrode, so that electrochemical oxidation can be reliably suppressed. . Also, by optimizing the hydrogen concentration and oxygen concentration, it is possible to prevent the problem of excessive oxidation due to the oxygen concentration being too high, and to reliably suppress electrochemical oxidation without causing cell damage. be able to. As described above, there is no problem in the state where the cooling operation control is executed because the risk of the occurrence of the hydrogen concentration gradient in the fuel cell can be completely corrected, but the present invention can be applied even during the period until the cooling operation control can be executed. The risk of electrochemical oxidation can be reliably suppressed by executing the residual hydrogen discharge control by using the.

  This will be described in more detail below. Conventionally, when performing a shutdown stop in a solid oxide fuel cell system, an oxidant gas such as air enters the fuel electrode side from the oxidant gas electrode side at a high temperature after the fuel supply is stopped. As a result, the atmospheric oxidation of the fuel electrode was a problem. The present inventor has conducted intensive research and has taken various measures to sufficiently prevent the oxidant gas from entering the fuel electrode until the temperature drops to a temperature at which the fuel electrode does not oxidize after shutdown. Was successful in doing.

  However, it has been found by the present inventor that oxidation of the fuel electrode has occurred even in a low temperature range where it has been conventionally considered that oxidation of the fuel electrode does not occur. The present inventor has conducted research and found that the oxidation of the fuel electrode that occurs in this low temperature range is generated by a mechanism different from the atmospheric oxidation of the fuel electrode, which has been regarded as a problem in the past. Oxidation by this newly discovered mechanism occurs in a temperature range where the electrolyte layer of the fuel cell is in an active state while the fuel electrode has lost its activity. That is, when the electrolyte layer is in an active state, oxygen ions on the oxidant gas electrode side can permeate the electrolyte layer, but the permeated oxygen ions are fuel (hydrogen) by the fuel electrode that has lost its activity. ) Can not react. For this reason, oxygen ions that cannot be processed by the fuel electrode are accumulated on the fuel electrode side and react with the material constituting the fuel electrode, whereby the fuel electrode is electrochemically oxidized.

  This electrochemical oxidation occurs based on the difference in oxygen partial pressure between the fuel electrode side and the oxidant gas electrode side of the fuel cell. Therefore, if there is a hydrogen concentration gradient in one fuel battery cell and there are parts where the oxygen partial pressure difference is large and small, the amount of oxygen ions that permeate the electrolyte layer will be different. Oxygen ions permeate and oxidize the material constituting the fuel electrode. The electric charge generated by this electrochemical oxidation reaction moves to a portion where the permeation amount of oxygen ions is relatively small, and a local battery phenomenon occurs in which the electric charge moves within one fuel cell. When this local cell phenomenon occurs, electrochemical oxidation is promoted in a portion where the oxygen partial pressure difference is large, and electrochemical oxidation is concentrated in that portion. Such electrochemical oxidation is extremely small compared to atmospheric oxidation that occurs at high temperatures. However, since oxidation is concentrated on a part of each fuel cell, the fuel cell is repeatedly executed by shutting down. May lead to damage.

  As described above, electrochemical oxidation occurs based on the difference in oxygen partial pressure between the fuel electrode side and the oxidant gas electrode side, in other words, based on the hydrogen concentration gradient in the fuel cell. The oxidant gas such as air is supplied to the fuel electrode side when the fuel electrode activity is still high while the atmospheric oxidation risk is reduced, and the hydrogen concentration gradient is already present when the fuel electrode activity decreases. It is thought that this can be prevented by reducing the oxygen partial pressure difference so as to be eliminated. However, the temperature range in which the atmospheric oxidation of the fuel electrode occurs and the temperature range in which electrochemical oxidation occurs partially overlap. For this reason, if an oxidant gas is supplied to the fuel electrode side in order to prevent electrochemical oxidation due to incorrect timing or high concentration of oxygen, the fuel electrode is excessively oxidized in the atmosphere, thus preventing electrochemical oxidation. Even though the means to do it was clear, it was very difficult to achieve it.

  Therefore, in the present invention, when the leaving period has elapsed after the shutdown stop, the residual hydrogen discharge control is executed. When the residual hydrogen discharge control is executed, the oxidant gas is supplied to the oxidant gas electrode side of the fuel cell module, and the oxidant gas electrode of the fuel cell is exhausted by exhausting the residual hydrogen remaining in the fuel cell module. The oxygen concentration on the side is increased to an appropriate value, and the hydrogen concentration is decreased to an appropriate value. It is necessary to control the supply amount of the oxidant gas supplied into the fuel cell module so that the oxidant gas is not forcibly supplied to the fuel electrode side of the fuel cell.

On the other hand, after the shutdown is stopped, 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 drops very slowly and stably after shutdown. Along with this, the temperature of the fuel electrode side of the fuel battery cell and the gas staying in the reformer gradually decreases gradually and the volume starts to shrink. As the staying gas contracts, the pressure on the fuel electrode side of the fuel cell also decreases gradually and stably over time. Here, since the fuel cell module has a large heat capacity after shutdown, each fuel cell is soaked, so the pressure drop on the fuel electrode side is almost the same for all fuel cells. Stable to the extent. As a result, as the temperature of each fuel electrode decreases, the optimal concentration of oxidant gas in a state where high-concentration hydrogen is gradually discharged from the oxidant gas electrode side to the fuel electrode side of all fuel cells at the same timing. The same amount begins to invade stably. The intrusion of the oxidant gas to the fuel electrode side occurs stably at the same timing very slowly from the time when the temperature of the fuel electrode is appropriately lowered. In addition, the gas entering from the oxidant gas electrode side is appropriately controlled in oxygen concentration by the residual hydrogen discharge control, so that the fuel electrode is not excessively oxidized by the invading gas.
Increase the partial pressure of oxygen in the entire fuel cell to ensure that excess hydrogen concentration gradients are corrected by air that has entered in a temperature range where electrochemical oxidation occurs without excessive oxidation at high temperatures However, according to the present invention, the solid oxide fuel cell has a very large heat capacity. Therefore, if this characteristic is used, it is ensured that all cells are surely transmitted at the same temperature. It is possible to use the characteristic that the suction action is generated in the fuel cell, and by using this, the residual hydrogen sprayed into the fuel cell module is appropriately discharged and the oxygen concentration is optimized to be very simple. It was possible to easily correct the concentration gradient of hydrogen in the entire cell, which was thought to be difficult. That is, only by executing the residual hydrogen discharge control, the gas whose oxygen concentration is appropriately controlled can be made to enter the fuel electrode side of all the fuel cells at an appropriate temperature almost simultaneously. The hydrogen concentration gradient was reliably corrected before the risk of electrochemical oxidation increased, avoiding the risk of excessive oxidation.

  In the present invention, the shutdown stop circuit preferably operates the oxidant gas supply device to lower the temperature in the fuel cell module during a predetermined period in which the pressure on the fuel electrode side of the fuel cell is sufficiently high immediately after the shutdown is stopped. The temperature drop control is programmed to be executed, and the residual hydrogen discharge control is executed after the leaving period has elapsed after the temperature drop control is completed.

  According to the present invention configured as described above, the fuel electrode side and the oxidant gas electrode side can be obtained by executing the temperature drop control in a state where the pressure on the fuel electrode side is high and no oxidant gas enters. The temperature is made uniform. Thereby, when the temperature on the fuel electrode side decreases, the gas on the fuel electrode side contracts, and a large amount of oxidant gas can be prevented from suddenly entering the fuel electrode side. Further, by making the temperature uniform by temperature drop control, the temperature state in the fuel cell module after shutdown is set to a specified state. As a result, the state inside the fuel cell module at the time when the residual hydrogen discharge control is started after the leaving period has elapsed is appropriately controlled, and the timing at which the fuel cell module gradually enters the fuel electrode side as the temperature decreases and the oxygen concentration contained in the gas are reduced. It can set appropriately, and can suppress atmosphere oxidation and electrochemical oxidation of a fuel electrode more reliably.

  In the present invention, preferably, the residual hydrogen discharge control is executed by intermittently operating the oxidant gas supply device a plurality of times after the temperature drop control is finished and before the cooling operation control is started.

  According to the present invention configured as described above, as the residual hydrogen discharge control, the oxidant gas supply device is intermittently operated a plurality of times, so that the amount of oxidant gas introduced into the oxidant gas electrode side at one time. Can be reduced. For this reason, by supplying the oxidant gas before the temperature inside the fuel cell module falls below the oxidation suppression temperature, the oxidant gas is forced to enter the fuel electrode side, and the fuel electrode is excessively oxidized at a high temperature. It is easy to avoid the risk of getting stuck. At the same time, it becomes easy to optimize the hydrogen concentration and oxygen concentration on the oxidant gas electrode side by exhausting appropriate amounts of residual hydrogen in the fuel cell module before the temperature drops below the oxidation suppression temperature. Therefore, the hydrogen and oxygen concentration contained in the gas that gradually enters the fuel electrode side as the temperature decreases can be optimized, so the risk of electrochemical oxidation due to temperature decrease while avoiding the risk of atmospheric oxidation. When the value becomes high, the oxygen partial pressure on the fuel electrode side can be sufficiently increased, and the risk of electrochemical oxidation can be sufficiently suppressed.

  In the present invention, preferably, the flow rate of the oxidant gas supplied by the residual hydrogen discharge control is set to be smaller than the flow rate of the oxidant gas supplied by the temperature drop control.

  Since the temperature drop control is executed in a state where the pressure on the fuel electrode side is sufficiently high, the risk of oxidant gas entry is extremely low. On the other hand, when the residual hydrogen discharge control is executed, the pressure on the fuel electrode side is reduced, so that the risk of the oxidant gas entering the fuel electrode side is high. According to the present invention configured as described above, since the flow rate of the oxidant gas in the residual hydrogen discharge control is set to be smaller than that in the temperature drop control, the oxidant gas is sufficiently suppressed while suppressing the risk of entry. Can be supplied to the oxidant gas electrode side.

  In the present invention, the flow path resistance from the oxidant gas electrode side of the fuel battery cell to the fuel electrode side is preferably larger than the flow path resistance from the oxidant gas electrode side of the fuel battery cell to the outside of the fuel cell module. The shutdown stop circuit is set, and controls the oxidant gas supply device so that the flow rate of the oxidant gas supplied to the oxidant gas electrode side in the residual hydrogen discharge control is gradually increased.

  In the present invention configured as described above, the flow path resistance to the fuel electrode side is larger than the flow path resistance to the outside of the fuel cell module, so that the oxidant gas is introduced to the oxidant gas electrode side. The gas staying inside the fuel cell module is likely to be discharged outside without entering the fuel electrode side. Further, in the residual hydrogen discharge control, the flow rate of the oxidant gas is gradually increased. Therefore, the flow rate is increased after the gas flow from the inside of the fuel cell module to the outside is generated with a small amount of supply, and the flow to the fuel electrode side is increased. Intrusion is less likely to occur. According to the present invention configured as described above, the risk of oxidant gas entry in residual hydrogen discharge control can be greatly reduced.

  In the present invention, it is preferable that the shutdown stop circuit operates the oxidant gas supply device for a predetermined time or supplies an oxidant gas electrode so that a predetermined amount of oxidant gas is supplied in the residual hydrogen discharge control. The oxidant gas supply device is controlled by feeding back the amount of oxidant gas supplied to the side.

  According to the present invention configured as described above, the oxidant gas supply device is operated for a predetermined time or the amount of oxidant gas is fed back. Oxidant gas can be introduced. Thereby, the oxygen partial pressure on the oxidant gas electrode side can be set to a desired value while reducing the hydrogen concentration to an optimum amount, and the oxygen concentration to enter the fuel electrode side can be set appropriately.

  In the present invention, preferably, the intermittent operation of the oxidant gas supply device in the residual hydrogen discharge control is performed with a lower frequency as the temperature in the fuel cell module decreases, or the temperature in the fuel cell module decreases. It is executed so that the oxidant gas supply amount in one intermittent operation decreases as the number of operations increases.

  The pressure on the fuel electrode side decreases as the temperature decreases after the shutdown is stopped. According to the present invention configured as described above, since the supply amount of the oxidant gas is reduced along with the pressure drop on the fuel electrode side, it is possible to reliably prevent the oxidant gas from entering the fuel electrode side.

  In the present invention, preferably, the controller further includes a voltage monitoring circuit for monitoring an open circuit output voltage from the fuel cell module, and the shutdown stop circuit is configured to detect residual hydrogen when the open circuit output voltage is excessively reduced. Stop supply of oxidant gas by emission control.

  According to the present invention configured as described above, since the open circuit output voltage is monitored by the voltage monitoring circuit, if excessive entry of the oxidant gas to the fuel electrode side occurs, the residual hydrogen is immediately Emission control can be stopped, and atmospheric oxidation of the fuel electrode can be reliably prevented.

  In the present invention, preferably, the fuel electrode of the fuel cell includes a nickel component, and the residual hydrogen discharge control is executed such that oxidation of the fuel electrode occurs at a temperature of 400 ° C. or lower and oxidation shrinkage occurs.

  The present inventor newly found out that the nickel component contained in the fuel electrode contracts when atmospheric oxidation starts at a temperature of 400 ° C. or lower. According to the present invention configured as described above, high-concentration oxygen diffuses to the fuel electrode side by residual hydrogen discharge control, and the fuel electrode is appropriately oxidized and contracted at a temperature of 400 ° C. or lower. The Thereby, expansion of the fuel electrode generated by electrochemical oxidation can be offset, and adverse effects on the fuel cells due to electrochemical oxidation can be reduced.

  In the present invention, preferably, the shutdown stop circuit is programmed to execute pre-stop control for supplying a predetermined amount of fuel and water to the reformer for a predetermined period immediately before executing the shutdown stop. .

  According to the present invention configured as described above, the state of the fuel cell module at the time of shutdown stop can be set to a specified state by the pre-stop control, so that the fuel electrode side to the fuel electrode side based on the residual hydrogen discharge control can be set. The amount of oxidant gas entering, the oxygen concentration, and the timing of entry can be made more reliable. Thereby, atmospheric oxidation and electrochemical oxidation of the fuel electrode can be reliably prevented.

  The present invention also relates to a solid oxide fuel cell system that generates electric power by reacting hydrogen with an oxidant gas, a fuel cell module containing fuel cells, and a fuel that supplies fuel to the fuel cell module The reformer for supplying hydrogen to the fuel electrode side containing the nickel component of the fuel battery cell by generating steam by reforming the fuel supplied from the fuel supply device by steam reforming, and the reformer A water supply device for supplying water for steam reforming to the gasifier, an oxidant gas supply device for supplying an oxidant gas to the oxidant gas electrode side of the fuel cell, a fuel supply device, a water supply device, and an oxidation And a controller for controlling extraction of electric power from the fuel cell module, and the controller supplies fuel by the fuel supply device and the fuel cell module. And a shutdown circuit for performing shutdown stop to stop taking out the power of the fuel cell module. After the shutdown period, the fuel cell module is stopped after the standing period in which the supply of fuel, water and oxidant gas is stopped. Residual hydrogen discharge control in which only the oxidant gas supply device is operated for a predetermined period before the temperature in the fuel cell module drops below the oxidation suppression temperature, and the oxidant gas in a state where the temperature in the fuel cell module has dropped below the oxidation suppression temperature. It is programmed to execute the cooling operation control to operate the supply device and cool the inside of the fuel cell module, and the hydrogen that flows out from the fuel electrode side to the oxidant gas electrode side after the shutdown is stopped by the residual hydrogen discharge control. As the hydrogen concentration in the fuel cell module is lowered from the battery module, 400 Is the fuel electrode oxidation at a temperature, is characterized in that oxidation shrinkage occurs.

  According to the solid oxide fuel cell system of the present invention, it is possible to sufficiently suppress the electrochemical oxidation of the fuel electrode of the fuel cell while executing shutdown stop.

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. 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 time chart which shows an example of each supply_amount | feed_rate of fuel etc. in the starting process of the fuel cell system by one Embodiment of this invention, and the temperature of each part. 4 is a time chart schematically showing an example of a stop behavior in a fuel cell system according to an embodiment of the present invention in time series. 6 is an enlarged time chart showing a state immediately after the shutdown is stopped in the stop behavior of the fuel cell system according to the embodiment of the present invention. In the fuel cell system by one Embodiment of this invention, it is a figure which shows typically the supply aspect of the electric power for electric power generation in residual hydrogen discharge | emission control. The figure for demonstrating in time series the control in the case of a stop process being performed in the fuel cell system by one Embodiment of this invention, the temperature in a fuel cell module, a pressure, and the state of the front-end | tip part of a fuel cell unit. It is. 5 is a graph showing a linear expansion coefficient when the temperature of the fuel electrode material is raised in an air atmosphere in the fuel cell system according to the embodiment of the present invention. It is a figure which shows typically the supply aspect of the electric power for electric power generation in residual hydrogen discharge control in the fuel cell system by the modification of this invention. In the conventional solid oxide fuel cell system, it is a figure for demonstrating in time series the temperature in the fuel cell module, the pressure, and the state of the front-end | tip part of a fuel cell unit when shutdown stop is performed. It is explanatory drawing which shows typically the relationship between the active temperature of the fuel electrode which comprises a fuel cell unit, an electrolyte, and an air electrode, and shutdown control. It is a figure which shows the behavior of a fuel cell when the temperature in a fuel cell module passes the temperature range which can generate an abnormal current. It is a figure which shows typically the abnormal current in a fuel cell.

Next, a solid oxide fuel cell (SOFC) system according to an embodiment of the present invention will be described with reference to the accompanying drawings.
FIG. 1 is an overall configuration diagram showing a solid oxide fuel cell (SOFC) system according to an embodiment of the present invention. 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 oxidant gas (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 an oxidant gas supplied from an air supply source 40, a reforming air flow rate adjusting unit 44 that adjusts the flow rate of air, and a power generation air flow rate adjustment. A 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 first heater for heating the power generating air supplied to the power generation chamber 2 heaters 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.

Next, the internal structure of the fuel cell module of the solid oxide fuel cell (SOFC) system according to the embodiment of the present invention will be described with reference to FIGS. 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. .
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 to the upper end of 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.

Next, the fuel cell unit 16 will be described with reference to 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.
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 extending in the vertical direction, and includes a cylindrical inner electrode layer 90 that forms a fuel gas flow path 88 therein, a cylindrical outer electrode layer 92, an inner electrode layer 90, and an outer side. An electrolyte layer 94 is provided between the electrode layer 92 and the electrode layer 92. The inner electrode layer 90 is a fuel electrode through which fuel gas passes and becomes a (−) electrode, while the outer electrode layer 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 portion 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 electrolyte layer 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.

  The inner electrode layer 90 includes, for example, a mixture of Ni and zirconia doped with at least one selected from rare earth elements such as Ca, Y, and Sc, and Ni and ceria doped with at least one selected from rare earth elements. The mixture is formed of at least one of Ni and a mixture of lanthanum garade doped with at least one selected from Sr, Mg, Co, Fe, and Cu.

  The electrolyte layer 94 is, for example, 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, lanthanum gallate doped with at least one selected from Sr and Mg, Formed from at least one of the following.

  The outer electrode layer 92 includes, for example, 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. It is formed from at least one of lanthanum cobaltite doped with at least one selected from the group consisting of silver and silver.

Next, the structure of the fuel cell 84 will be described in detail with reference to FIG.
As shown in FIG. 4B, the inner electrode layer 90 is composed of a first fuel electrode 90d and a second fuel electrode 90e, and further, a fuel electrode protection layer 90f is formed as the first fuel electrode 90d and the second fuel electrode. It is included between 90e. The outer electrode layer 92 includes an air electrode 92a and a current collecting layer 92b.

In the present embodiment, the first fuel electrode 90d is made of Ni / YSZ, and is formed by firing a mixture of Ni and YSZ that is Y-doped zirconia into a cylindrical shape. The fuel electrode protective layer 90f is a thin film made of a mixture of strontium zirconate and nickel (Ni / SrZrO ₃ ) and having a thickness smaller than that of the first fuel electrode 90d and the second fuel electrode 90e. The second fuel electrode 90e is Ni / GDC, and is formed by forming a mixture of Ni and GDC, which is ceria doped with Gd, on the outside of the fuel electrode protective layer 90f.

  In the present embodiment, the electrolyte layer 94 is formed by laminating LSGM, which is lanthanum gallate doped with Sr and Mg, on the outside of the second fuel electrode 90e. A fired body was formed by firing the formed body.

  In the present embodiment, the air electrode 92a is formed by depositing LSCF, which is lanthanum cobaltite doped with Sr and Fe, on the outside of the fired body. The current collecting layer 92b is configured by forming an Ag layer outside the air electrode 92a.

Next, the fuel cell stack 14 will be described with reference to FIG. FIG. 5 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. 5, 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 ceramic, and at the upper end, four fuel cell units 16 at both end portions 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 includes a fuel electrode connection portion 102a that is electrically connected to an inner electrode terminal 86 attached to the inner electrode layer 90 that is a fuel electrode, and an outer peripheral surface of the outer electrode layer 92 that is an air electrode. It is integrally formed so as to connect the air electrode connecting portion 102b that is electrically connected. In addition, a silver thin film is formed on the entire outer surface of the outer electrode layer 92 (air electrode) of each fuel cell unit 16 as an electrode on the air electrode side. 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.

Next, 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. 6 is a block diagram illustrating a solid oxide fuel cell (SOFC) system according to an embodiment of the present invention.
As shown in FIG. 6, 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 disposed 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 from the upper end of the fuel cell unit 16 through the many holes provided on the upper surface thereof, through the fuel electrode side of the fuel cell unit 16, that is, through the inside of the fuel cell unit 16. . 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 that is the oxidant gas 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 a power generation oxidant gas supply device. 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 comes into contact with the outer surface of each fuel cell unit 16 on the air electrode side (oxidant gas electrode side) of the fuel cell stack 14, and a part of oxygen in the air is in contact with it. Used for power generation. 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.

Next, control in the starting process of the solid oxide fuel cell system 1 will be described with reference to FIG.
FIG. 7 is a time chart showing an example of each supply amount of fuel and the temperature of each part in the starting process. In addition, the scale of the vertical axis | shaft of FIG. 7 has shown temperature, and each supply amount of fuel etc. has shown those increase / decrease roughly.

In the starting process shown in FIG. 7, the temperature of the fuel cell stack 14 at room temperature is raised to a temperature at which power generation is possible.
First, at time t0 in FIG. 7, supply of power generation air and reforming air is started. Specifically, the control unit 110 that is a controller sends a signal to the power generation air flow rate adjustment unit 45 that is a power generation oxidant gas supply device to operate it. As described above, the power generation air is introduced into the fuel cell module 2 through the power generation air introduction pipe 74 and flows into the power generation chamber 10 through the air heat exchanger 22 and the power generation air supply path 77. . In addition, the control unit 110 sends a signal to the reforming air flow rate adjustment unit 44 which is a reforming oxidant gas supply device to operate it. The reforming air introduced into the fuel cell module 2 flows into the interior of each fuel cell unit 16 through the reformer 20 and the manifold 66, and flows out from the upper end thereof. Note that at time t0, no reforming reaction occurs in the reformer 20 because fuel has not yet been supplied. In the present embodiment, the supply amount of power generation air started at time t0 in FIG. 7 is about 100 L / min, and the supply amount of reforming air is about 10.0 L / min.

  Next, the fuel supply is started at time t1 after a predetermined time from time t0 in FIG. Specifically, the control unit 110 sends a signal to the fuel flow rate adjustment unit 38 which is a fuel supply device to operate it. In the present embodiment, the fuel supply amount started at time t1 is about 5.0 L / min. The fuel introduced into the fuel cell module 2 flows into the interior of each fuel cell unit 16 through the reformer 20 and the manifold 66, and flows out from the upper end thereof. At time t1, the reformer reaction is not generated in the reformer 20 because the temperature of the reformer is still low.

  Next, at time t2 when a predetermined time has elapsed from time t1 in FIG. 7, an ignition process for the supplied fuel is started. Specifically, in the ignition process, the control unit 110 sends a signal to an ignition device 83 (FIG. 2) that is an ignition means, and ignites the fuel flowing out from the upper end of each fuel cell unit 16. The ignition device 83 repeatedly generates a spark in the vicinity of the upper end of the fuel cell stack 14 and ignites the fuel flowing out from the upper end of each fuel cell unit 16.

When ignition is completed at time t3 in FIG. 7, supply of reforming water is started. Specifically, the control unit 110 sends a signal to the water flow rate adjustment unit 28 (FIG. 6), which is a water supply device, to activate it. In the present embodiment, the supply amount of water started at time t3 is 2.0 cc / min. At time t3, the fuel supply amount is maintained at about 5.0 L / min. Further, the supply amounts of the power generation air and the reforming air are also maintained at the previous values. At this time t3, the ratio O ₂ / C of oxygen O ₂ in the reforming air and carbon C in the fuel becomes about 0.32.

  After being ignited at time t3 in FIG. 7, the supplied fuel flows out from the upper end of each fuel cell unit 16 as off-gas and is burned here. This combustion heat heats the reformer 20 disposed above the fuel cell stack 14.

In this way, at time t4 when the temperature of the reformer 20 rises, the fuel and the reforming air that have flowed into the reforming unit 20b through the evaporation unit 20a undergo the partial oxidation reforming reaction represented by the equation (1). Get up.
_{C m H n + xO 2 →} aCO 2 + bCO + cH 2 (1)
Since this partial oxidation reforming reaction is an exothermic reaction, when the partial oxidation reforming reaction occurs in the reforming part 20b, the ambient temperature rapidly rises locally.

On the other hand, in the present embodiment, the supply of the reforming water is started from time t3 immediately after the ignition is confirmed, and the temperature of the evaporation unit 20a is rapidly increased. At time t4, water vapor has already been generated in the evaporator 20a and supplied to the reformer 20b. That is, after the off-gas is ignited, the supply of water is started a predetermined time before the temperature of the reforming unit 20b reaches the temperature at which the partial oxidation reforming reaction occurs, and reaches the temperature at which the partial oxidation reforming reaction occurs. At that time, a predetermined amount of water is stored in the evaporation unit 20a, and water vapor is generated. For this reason, when the temperature rapidly rises due to the occurrence of the partial oxidation reforming reaction, a steam reforming reaction occurs in which the reforming steam supplied to the reforming unit 20b reacts with the fuel. This steam reforming reaction is an endothermic reaction shown in Formula (2), and occurs at a higher temperature than the partial oxidation reforming reaction.
C _m H _n + xH ₂ O → aCO ₂ + bCO + cH ₂ (2)

As described above, when the time t4 in FIG. 7 is reached, the partial oxidation reforming reaction occurs in the reforming unit 20b, and the steam reforming is caused by the temperature rise caused by the partial oxidation reforming reaction. Reaction will also occur at the same time. Therefore, the reforming reaction that occurs in the reforming unit 20b after time t4 is an autothermal reforming reaction (ATR) shown in Formula (3) in which the partial oxidation reforming reaction and the steam reforming reaction are mixed. That is, the ATR1 process is started at time t4.
_{C m H n + xO 2 +} yH 2 O → aCO 2 + bCO + cH 2 (3)

Next, when the temperature detected by the reformer temperature sensor 148 reaches about 500 ° C. or higher, the process moves from the ATR1 process to the ATR2 process at time t5 in FIG. At time t5, the water supply amount is changed from 2.0 cc / min to 3.0 cc / min. Further, the fuel supply amount, the reforming air supply amount, and the power generation air supply amount are maintained at the previous values. As a result, the steam / carbon ratio S / C in the ATR2 step is increased to 0.64, while the reforming air / carbon ratio O ₂ / C is maintained at 0.32. Thus, by maintaining the ratio of reforming air and carbon O ₂ / C constant, the ratio S / C of steam and carbon is increased, so that the amount of carbon that can be partially oxidized and reformed is not reduced. In addition, the amount of carbon that can be steam reformed is increased. This makes it possible to increase the amount of carbon subjected to steam reforming as the temperature of the reforming unit 20b increases while reliably avoiding the risk of carbon deposition in the reforming unit 20b.

Furthermore, in the present embodiment at time t6 in FIG. 7, when the temperature detected by the power generation chamber temperature sensor 142 reaches about 400 ° C. or more, the process shifts from the ATR2 process to the ATR3 process. Accordingly, the fuel supply amount is changed from 5.0 L / min to 4.0 L / min, and the reforming air supply amount is changed from 9.0 L / min to 6.5 L / min. Also, the previous values are maintained for the water supply amount and the power generation air supply amount. This increases the steam / carbon ratio S / C in the ATR3 step to 0.80, while the reforming air to carbon ratio O ₂ / C is reduced to 0.29.

  Furthermore, when the temperature detected by the power generation chamber temperature sensor 142 reaches about 550 ° C. or higher at time t7 in FIG. 7, the process proceeds to the SR1 process. Along with this, the fuel supply amount is changed from 4.0 L / min to 3.0 L / min, and the water supply amount is changed from 3.0 cc / min to 7.0 cc / min. Further, the supply of the reforming air is stopped, and the power supply air supply amount is maintained at the previous value. Thus, in the SR1 process, steam reforming occurs exclusively in the reforming unit 20b, and the steam / carbon ratio S / C is 2 which is appropriate for steam reforming the entire amount of supplied fuel. .49. At time t7 in FIG. 7, since the temperature of both the reformer 20 and the fuel cell stack 14 is sufficiently increased, the steam reforming is performed even if the partial oxidation reforming reaction has not occurred in the reforming unit 20b. The reaction can be generated stably.

  Next, when the temperature detected by the power generation chamber temperature sensor 142 reaches about 600 ° C. or higher at time t8 in FIG. 7, the process proceeds to the SR2 step. Accordingly, the fuel supply amount is changed from 3.0 L / min to 2.5 L / min, and the water supply amount is changed from 7.0 cc / min to 6.0 cc / min. In addition, the previous value of the power supply air supply amount is maintained. Thereby, in the SR2 step, the ratio S / C of water vapor to carbon is set to 2.56.

  Further, after executing the SR2 process for a predetermined time, the process proceeds to the power generation process. In the power generation process, power is extracted from the fuel cell stack 14 to the inverter 54 (FIG. 6), and power generation is started. In the power generation process, the reforming unit 20b reforms the fuel exclusively by steam reforming. Further, in the power generation process, the fuel supply amount, the power generation air supply amount, and the water supply amount are changed corresponding to the output power required for the fuel cell module 2.

  Next, stop processing in the conventional solid oxide fuel cell system will be described with reference to FIGS.

  First, when the shutdown is stopped, the supply of fuel, the supply of water, and the supply of power generation air are stopped in a short time. Also, the extraction of power from the fuel cell module is stopped (output current = 0). After the shutdown stop, the fuel cell module is left in this state. For this reason, the fuel which existed on the fuel electrode side inside each fuel cell unit is ejected to the air electrode side based on the pressure difference from the external air electrode side. In addition, the air (and the fuel ejected from the fuel electrode side) existing on the air electrode side of each fuel cell unit is based on the pressure difference between the pressure on the air electrode side (pressure in the power generation chamber) and the atmospheric pressure. And discharged outside the fuel cell module. Therefore, after the shutdown is stopped, the pressure on the fuel electrode side and the air electrode side of each fuel cell unit naturally decreases.

  However, by providing a thin tube with a large flow path resistance at the upper end of each fuel cell unit, the pressure drop on the fuel electrode side after fuel supply and power generation is stopped is slower than the pressure drop on the air electrode side. The fuel is configured to remain for a long time after the shutdown is stopped. As described above, even in the conventional solid oxide fuel cell system, after being left to stand after being shut down, the pressure on the fuel electrode side is kept below the oxidation suppression temperature while maintaining the pressure higher than the pressure on the air electrode side. The risk that the fuel electrode is oxidized in the atmosphere is suppressed.

  In the present specification, the oxidation suppression temperature means a temperature at which the risk that the fuel electrode is oxidized in the atmosphere is sufficiently reduced. The risk of the anode being oxidized gradually decreases with decreasing temperature and eventually becomes zero. For this reason, even if the oxidation suppression temperature is in a temperature range slightly higher than the oxidation lower limit temperature, which is the lowest temperature at which oxidation of the fuel electrode can occur, the risk of fuel electrode oxidation can be sufficiently reduced. In a general fuel cell unit, such an oxidation suppression temperature is considered to be about 350 ° C. to 400 ° C., and an oxidation lower limit temperature is considered to be about 250 ° C. to 300 ° C.

  FIG. 14 is a diagram illustrating an operation after shutdown of a conventional solid oxide fuel cell system. The upper part of FIG. 14 shows a graph schematically showing the pressure change on the fuel electrode side and the air electrode side, the middle part shows the control operation and the temperature in the fuel cell module in time series, and the lower part shows the fuel at each time point. The state of the upper end part of the battery cell unit is shown.

  First, a normal power generation operation is performed before the shutdown stop in the middle stage of FIG. In this state, the temperature in the fuel cell module is about 700 ° C. Further, as shown in the lower part (1) of FIG. 14, the fuel gas remaining without being used for power generation is ejected from the upper end of the fuel cell unit, and this ejected fuel gas is combusted at the upper end. Next, when the supply of fuel gas, reforming water, and power generation air is stopped due to shutdown stop, the flow rate of the jetted fuel gas decreases, and as shown in the lower part (2) of FIG. The flame at the tip of the battery cell unit disappears.

  As shown in the lower part (3) of FIG. 14, the pressure inside the fuel cell unit (fuel electrode side) is higher than the outside (air electrode side) even after the flame has disappeared after the shutdown is stopped. The ejection of fuel gas from the fuel cell unit is continued. Further, as shown in the upper part of FIG. 14, immediately after the shutdown is stopped, the pressure on the fuel electrode side is higher than the pressure on the air electrode side, and each pressure decreases while maintaining this relationship. The pressure difference between the fuel electrode side and the air electrode side decreases with the ejection of the fuel gas after the shutdown stop. The amount of fuel gas ejected from the fuel cell unit decreases as the pressure difference between the fuel electrode side and the air electrode side decreases (lower stage (4) (5) in FIG. 14).

  When about 5 to 6 hours have passed after the shutdown, and when the temperature in the fuel cell module decreases to the oxidation suppression temperature, both the fuel electrode side and the air electrode side of each fuel cell unit decrease to almost atmospheric pressure, Air on the air electrode side begins to diffuse toward the fuel electrode side (lower stage (6) and (7) in FIG. 14). In the vicinity of the oxidation suppression temperature, the temperature of the fuel electrode is low, and even when the fuel electrode is in contact with air, little oxidation occurs, and the adverse effects caused by oxidation can be substantially ignored. Furthermore, as shown in the lower part (8) of FIG. 14, after the temperature of the fuel electrode falls below the oxidation lower limit temperature, the fuel electrode is atmospherically oxidized even if the fuel electrode side of each fuel cell unit is filled with air. It will never be done. As described above, in the conventional solid oxide fuel cell system, the mechanical structure prevents the air that has entered the fuel electrode above the oxidation suppression temperature from coming into contact and preventing the fuel electrode from being oxidized in the atmosphere.

  However, even if atmospheric oxidation of the fuel electrode is prevented, the present inventors have found a phenomenon that the fuel electrode is electrochemically oxidized in a temperature range below the oxidation suppression temperature.

Next, the electrochemical oxidation phenomenon of the fuel electrode will be described with reference to FIG.
FIG. 15 is an explanatory view schematically showing the relationship between the activation temperature of the fuel electrode, electrolyte, and air electrode constituting the fuel cell unit 16 and the shutdown control.
In the present embodiment, the first fuel electrode 90d is formed of Ni / YSZ, the second fuel electrode 90e is formed of Ni / GDC, the electrolyte layer 94 is formed of LSGM, and the air electrode 92a is formed of LSCF. Yes. A fuel electrode protective layer 90f made of Ni / SrZrO ₃ is formed between the first fuel electrode 90d and the second fuel electrode 90e.

  In the stop process in the present embodiment, the first fuel electrode 90d and the second fuel electrode 90e are kept until the temperature in the fuel cell module 2 is lowered to a temperature lower than at least the oxidation inhibition temperature Te (about 350 ° C.) of nickel. There is a risk that the nickel contained will be oxidized in the atmosphere.

  In the present embodiment, the lower limit temperature Ta of the fuel electrode, which is the lower limit temperature at which the catalyst of the first fuel electrode 90d and the second fuel electrode 90e is activated, is lower than the oxidation suppression temperature Te and the temperature at which air cooling is started. A low of about 290 ° C. The fuel electrode receives oxygen ions from the electrolyte layer 94 and reacts the received oxygen ions with fuel (hydrogen) until the temperature decreases from the operating temperature of the fuel cell module 2 to the activation lower limit temperature Ta, thereby generating water. At the same time, electrons can be emitted. However, when the temperature of the fuel electrode is lower than the activation lower limit temperature Ta, even if oxygen ions are received, the fuel electrode cannot react with the fuel and cause a power generation reaction.

On the other hand, the minimum active temperature Tb of the electrolyte in which the electrolyte layer 94 is in an active state in which oxygen ions can permeate from the air electrode to the fuel electrode is about 200 ° C., which is lower than the minimum active temperature Ta of the fuel electrode. The electrolyte layer 94 can transmit oxygen ions received from the air electrode to the fuel electrode until the operating temperature of the fuel cell module 2 decreases to the activation lower limit temperature Tb. However, when the temperature of the electrolyte layer 94 is lower than the lower limit activation temperature Tb, oxygen ions cannot be transmitted to the fuel electrode even if oxygen ions are received from the air electrode.
Note that the air electrode is in an active state in which oxygen in the air can be converted into oxygen ions at the activation lower limit temperature Tb or higher of the electrolyte layer 94.

  In general, in order to enable power generation at a lower temperature, the activation temperatures of the fuel electrode, the electrolyte layer, and the air electrode of the fuel cell are preferably low. The lowering of the activation temperature is achieved not only by selecting the material itself but also by tuning the particle size, layer thickness, manufacturing process parameters, etc. of the selected material. When the activation temperature is lowered as described above, the activation lower limit temperatures of the fuel electrode, the electrolyte layer, and the air electrode can be configured to be lower than the oxidation inhibition temperature Te of nickel. Moreover, the activity minimum temperature of an electrolyte layer and an air electrode becomes lower than the activity minimum temperature of a fuel electrode.

FIG. 15 shows the relationship between the lower limit temperature Ta of the fuel electrode and the lower limit temperature Tb of the electrolyte. In the temperature range (temperature range in which an abnormal current can be generated) Tc between these activation lower limit temperatures Ta and Tb, the electrolyte layer 94 is in an active state capable of transmitting oxygen ions to the fuel electrode, but the fuel electrode has catalytic activity. In a lost state of inactivity.
Therefore, after the shutdown is stopped, when the inside of the fuel cell module 2 is in this temperature band Tc, oxygen ions are supplied to the fuel electrode through the electrolyte layer.

Next, with reference to FIG. 16A, the behavior of the fuel cell when the temperature in the fuel cell module passes through the temperature range where the abnormal current can be generated will be described.
In the temperature zone Tc, the electrolyte layer 94 and the air electrode 92a are in an active state, and oxygen ions can permeate to the fuel electrode through the air electrode 92a and the electrolyte layer 94, but the first fuel electrode 90d and the second fuel electrode 90e The catalytic activity is in a deactivated state. Therefore, the fuel electrode cannot cause a normal power generation reaction that causes the supplied oxygen ions to react with the fuel (hydrogen).

The fuel electrode contains nickel, and nickel reacts with oxygen in the air and tends to be nickel oxide in a high temperature atmosphere higher than the oxidation suppression temperature Te (about 350 ° C.), but the temperature range is lower than the oxidation suppression temperature Te. At Tc, nickel is in a state where it is difficult to react with oxygen in the air.
However, when oxygen ions that have passed through the electrolyte layer 94 are supplied to the fuel electrode in the temperature zone Tc, nickel in the fuel electrode can be combined with oxygen ions to become nickel oxide by an electrochemical oxidation reaction. When this reaction occurs, electrons are emitted. Therefore, when the above electrochemical oxidation reaction occurs, a charge is generated by a mechanism different from that of a normal power generation reaction.
When the fuel electrode is in an active state, it is considered that only a normal power generation reaction in which oxygen ions are combined with fuel occurs, and an abnormal reaction in which oxygen ions are combined with nickel does not occur substantially.

  On the other hand, after the shutdown is stopped, the extraction of electric power from the fuel cell module 2 is stopped, so that it is considered that no current flows even if charges are generated by the electrochemical oxidation reaction. . However, the present inventor has found that even if the extraction of electric power is stopped and the fuel electrode and the air electrode are electrically non-conductive (open circuit), a weak current can flow locally. I found it.

  That is, the electrochemical oxidation reaction occurs based on the difference in oxygen partial pressure between the fuel electrode side and the air electrode side. Therefore, in the state where a large amount of hydrogen remains on the fuel electrode side, the oxygen partial pressure on the fuel electrode side is low, and the difference in oxygen partial pressure with respect to the air electrode side is large, so that oxygen ions flow through the electrolyte layer 94. It becomes easy. Here, as described with reference to FIG. 14, when the time after the shutdown stops, the air gradually diffuses from the air electrode side to the fuel electrode side, so that the remaining hydrogen in the vicinity of the upper end of the fuel cell unit 16 is increased. The concentration decreases. That is, as shown in FIG. 16B, in one fuel cell unit 16, the difference in oxygen partial pressure between the fuel electrode side and the air electrode side is small in the vicinity of the upper end portion, and oxygen in the vicinity of the lower end portion. The difference in partial pressure is large.

  Thus, a large amount of charge is generated by the occurrence of many electrochemical oxidation reactions near the lower end of the fuel cell unit 16, and relatively less charge is generated near the upper end. As a result, a potential difference is generated in the fuel electrode of one fuel cell unit 16, and local charge movement (weak current) occurs. When a weak current due to such a local battery phenomenon flows, the electrochemical oxidation reaction of nickel further proceeds. In particular, the flow of oxygen ions concentrates near the lower end of each fuel cell unit 16 where the difference in oxygen partial pressure between the fuel electrode side and the air electrode side is large, and many electrochemical oxidation reactions occur in this portion.

Nickel oxide generated by the electrochemical oxidation reaction is reduced by the fuel (hydrogen) supplied to the fuel electrode during the power generation operation. As described above, when the nickel contained in the fuel electrode undergoes an electrochemical oxidation / reduction reaction by stopping and starting and subsequent operation, the fuel electrode changes in the microstructure and volume.
Furthermore, in addition to the frequent stopping and starting of the fuel cell module, the electrochemical oxidation reaction of nickel occurs for a relatively long time in the stopping and starting process, particularly in the stopping process that requires a long time.

Thus, when a change in the microstructure and volume of the fuel electrode occurs, tensile stress is applied to the electrolyte layer. Each fuel cell where the flow of oxygen ions due to local battery phenomenon concentrates as the cumulative amount of nickel that undergoes electrochemical oxidation / reduction reactions increases many times due to long-term use. In the vicinity of the lower end of the unit 16, a minute crack is finally generated in the electrolyte layer.
Thus, the present inventors are in a state where nickel, which is a specific substance contained in the fuel electrode, is subjected to electrochemical oxidation in the temperature zone Tc, and is electrochemically oxidized and reduced during the subsequent operation. Upon receipt, it was discovered that the microstructure and volume of the fuel electrode changed, gave tensile stress to the electrolyte layer, and finally caused minute cracks in the electrolyte layer to damage the fuel cell.

Next, the shutdown stop in the solid oxide fuel cell system 1 according to the embodiment of the present invention will be described. This shutdown stop is configured to sufficiently suppress the deterioration of the fuel cell due to the above atmospheric oxidation and electrochemical oxidation.
FIG. 8 is a time chart schematically showing an example of the stop behavior when the shutdown stop is executed in the solid oxide fuel cell system 1 according to the embodiment of the present invention. FIG. 9 is an enlarged time chart showing a state immediately after the shutdown stop in FIG. FIG. 10 is a diagram for explaining the control, the temperature and pressure in the fuel cell module, and the state of the tip of the fuel cell unit in the time series when the shutdown stop is executed.

  In the example of the time chart shown in FIG. 9, at time t301, the stop switch is operated by the user, and the pre-stop control is started. In the pre-stop control, first, output to the outside by the fuel cell module 2 is stopped. As a result, as indicated by a thin one-dot chain line in FIG. 9, the current and power extracted from the fuel cell module 2 rapidly decrease. In the pre-stop control, the current output from the fuel cell module 2 to the outside is stopped, but a certain weak current (about 1 A) for operating the auxiliary unit 4 of the solid oxide fuel cell system 1 is stopped. ) Is continued for a predetermined period. For this reason, a weak current is taken out from the fuel cell module 2 during the pre-stop control even after the generated current significantly decreases at time t301. Further, as indicated by a broken line in FIG. 9, the output voltage of the fuel cell module 2 increases with a decrease in the extracted current. In this way, during the pre-stop control, by limiting the amount of power to be extracted and continuing the power generation with a predetermined power while extracting a weak current, a part of the supplied fuel is used for power generation. Thus, a significant increase in surplus fuel that remains unused is avoided, and the temperature in the fuel cell module 2 is lowered.

  Further, in the pre-stop control, after time t301, the fuel supply amount indicated by the dotted line in FIG. 9 and the supply amount of reforming water indicated by the thin solid line are linearly reduced. On the other hand, the air supply amount for power generation indicated by a thick dashed line is increased linearly. Therefore, during the pre-stop control, more air is supplied than the amount corresponding to the electric power extracted from the fuel cell module 2. In this way, by increasing the air supply amount, heat is taken from the reformer 20 and the temperature rise in the fuel cell module 2 is suppressed. Subsequently, in the example shown in FIG. 9, at time t302 about 20 seconds after time t301, the fuel supply amount and the water supply amount are reduced to the supply amount corresponding to the weak current extracted from the fuel cell module 2. And then the reduced supply is maintained. As described above, as the pre-stop control, by reducing the fuel supply amount and the water supply amount, the air flow in the fuel cell module 2 caused by the rapid stop of the large flow rate of fuel when the fuel supply is completely stopped. It prevents turbulence and a large amount of fuel remaining in the reformer 20 and the manifold 66 after the fuel supply is completely stopped. Note that, after the time t301, the temperature of air on the air electrode side in the fuel cell module 2 indicated by a thick solid line in FIG. 9 is lowered by decreasing the fuel supply amount and increasing the air supply amount. However, a large amount of heat is still accumulated in the heat insulating material 7 and the like surrounding the fuel cell module 2. In addition, during the pre-stop control, the output of current to the outside is stopped, but the fuel and water supply continues. Therefore, even if the supply of air for power generation is continued, each fuel cell unit No air enters the fuel electrode 16 inside. Therefore, the supply of air can be continued safely.

  In the example shown in FIG. 9, the fuel supply amount and the reforming water supply amount are made zero at time t303, approximately two minutes after the time t301 when the pre-stop control is started, and the current taken out from the fuel cell module 2 It is also zeroed and shutdown is stopped.

  In the example shown in FIG. 9, the supply of power generation air (however, the power generation is completely stopped) is continued as the temperature drop control even after the shutdown stop at time t303. As a result, the air in the fuel cell module 2 (air electrode side of the fuel cell stack 14), the remaining fuel combustion gas, and the fuel flowing out from the fuel electrode side of the fuel cell stack 14 after shutdown is discharged. The temperature drop control functions as an exhaust process.

  In the present embodiment, after the fuel supply is completely stopped at time t303, a large amount of power generation air is continuously supplied for a predetermined period until time t304. In the present embodiment, the air supply amount for power generation is increased to 80 L / min, which is the maximum air supply amount during the pre-stop control, and this air supply amount is maintained thereafter. During the period in which the temperature drop control is performed, fuel remains in the fuel electrode side of the fuel cell stack 14 and the reformer 20, and the fuel electrode side (inside each fuel cell unit 16) The pressure is still high enough. Therefore, in the predetermined period in which the temperature drop control is performed, even if the power generation air is supplied in a state where the fuel supply is stopped, the air does not enter the fuel electrode side.

As shown in FIG. 8, after the supply of power generation air is stopped at time t304, it is left undisturbed for a predetermined leaving period. After the supply of power generation air is stopped, the shutdown stop circuit 110a starts the residual hydrogen discharge control at time t305 when the temperature in the fuel cell module 2 decreases to about 550 ° C.
In the present embodiment, as the residual hydrogen discharge control, the power generation air flow rate adjustment unit 45 that is a power generation oxidant gas supply device is intermittently operated a plurality of times. That is, in the residual hydrogen discharge control, every time the inside of the fuel cell module 2 is lowered to a predetermined temperature, a small flow of power generation air is supplied for a short time (however, power generation is completely stopped). In the present embodiment, the time t305 when the temperature in the fuel cell module 2 decreases to about 550 ° C., the time t306 when it decreases to about 500 ° C., the time t307 when it decreases to about 450 ° C., and the time t308 when it decreases to about 350 ° C. In addition, power generation air is supplied for a short time.

  By operating the power generation air flow rate adjustment unit 45, air is supplied to the air electrode side of the fuel cell stack 14 in the fuel cell module 2. The air supplied into the fuel cell module 2 cools the fuel cell stack 14 (each fuel cell unit 16) accommodated therein. In addition, when air is supplied, the air remaining in the fuel cell module 2 and the fuel flowing out from the fuel electrode side of the fuel cell stack 14 pass through the air heat exchanger 22 and the fuel cell module 2. Is discharged outside. In the present embodiment, the volume of air introduced into the fuel cell module 2 by one operation of the power generation air flow rate adjustment unit 45 is smaller than the internal volume of the fuel cell module 2. For this reason, the gas (air and outflowed fuel) staying in the fuel cell module 2 is not completely discharged by one operation of the power generation air flow rate adjusting unit 45.

FIG. 10 is a diagram schematically illustrating a supply mode of power generation air in the residual hydrogen discharge control.
As shown in FIG. 10, in each operation of the power generation air flow rate adjustment unit 45 by residual hydrogen discharge control, the air flow rate is gradually increased over about 10 seconds, and this flow rate is reached when the flow rate reaches 20 L / min. Is maintained for about 1 second, and then the power generation air flow rate adjustment unit 45 is stopped. Thus, in the residual hydrogen discharge control, the power generation air flow rate adjustment unit 45 is operated for a predetermined time.

  Here, a fuel gas channel narrow tube 98 is provided at the upper end of each fuel cell unit 16, and the channel resistance of the fuel gas channel narrow tube 98 is the heat exchange for air from the inside of the fuel cell module 2. The flow path resistance of the flow path leading to the outside air via the vessel 22 is configured to be larger. For this reason, even when air is introduced into the fuel cell module 2 in a state where the fuel supply is stopped, the fuel cell stack 14 enters the fuel electrode side (inside each fuel cell unit 16) from the air electrode side. The amount of air to be discharged is small, and most of the air is discharged out of the fuel cell module 2. In addition, as shown in FIG. 10, since the flow rate of air introduced into the fuel cell module 2 is gradually increased and the introduction of air is stopped in a short time, the amount of air entering the fuel electrode side can be ignored. It is suppressed to the extent.

  Further, the control unit 110 includes a voltage monitoring circuit 110b (FIG. 6). The voltage monitoring circuit 110b is opened from the fuel cell module 2 detected by the power state detection sensor 126 (FIG. 6) after the shutdown is stopped. The circuit output voltage is monitored. That is, when excessive entry of air from the air electrode side to the fuel electrode side of the fuel cell unit 16 occurs, the difference in oxygen partial pressure between the air electrode side and the fuel electrode side rapidly decreases, so that the open circuit output voltage Decreases. For this reason, air intrusion can be detected by monitoring the open circuit output voltage. When the open circuit output voltage decreases excessively and falls below a predetermined voltage, the voltage monitoring circuit 110b stops the residual hydrogen discharge control by the shutdown stop circuit 110a and stops the supply of air. As a result, when excessive air enters the fuel electrode side, the supply of air is immediately stopped, so that atmospheric oxidation of the fuel electrode can be prevented.

  Further, in the present embodiment, the operation of the power generation air flow rate adjustment unit 45 by the residual hydrogen discharge control causes the temperature in the fuel cell module 2 to drop to about 550 ° C., about 500 ° C., about 450 ° C., about 350 ° C. Each time it is executed. As described above, the intermittent operation of the power generation air flow rate adjustment unit 45 is performed with a lower frequency as the temperature in the fuel cell module 2 decreases.

  Next, at time t309 when the temperature in the fuel cell module 2 is reduced to about 300 ° C., the shutdown stop circuit 110a starts the cooling operation control. In the cooling operation control, the power generation air flow rate adjustment unit 45 is continuously operated at a large flow rate. That is, in the present embodiment, if the temperature of the fuel cell unit 16 is reduced to about 300 ° C., there is no risk that the fuel electrode is oxidized even if air enters the fuel electrode side. For this reason, the fuel cell stack 14 is rapidly cooled by sending a large amount of air into the fuel cell module 2 when the temperature drops to about 300 ° C. By cooling the fuel cell stack 14, the temperature of the electrolyte layer 94 of each fuel cell unit 16 is lowered. When the electrolyte layer 94 is cooled to a deactivated state, oxygen ions do not permeate the electrolyte layer 94 and the risk of electrochemical oxidation of the fuel electrode is eliminated. In addition, when air is continuously fed to the air electrode side of the fuel cell unit 16 at a large flow rate, air enters the fuel electrode side. As a result, the oxygen partial pressures on the air electrode side and the fuel electrode side become substantially equal, so the risk of electrochemical oxidation of the fuel electrode is greatly reduced. In the present embodiment, air is introduced into the fuel cell module 2 at a flow rate of 70 L / min in the cooling operation control.

  Further, at time t310 when the temperature in the fuel cell module 2 is lowered to about 150 ° C., the shutdown stop circuit 110a operates the reforming air flow rate adjustment unit 44. By operating the reforming air flow rate adjusting unit 44, air is directly supplied to the fuel electrode side (inside each fuel cell unit 16) of the fuel cell stack 14 via the reformer 20 and the manifold 66. Is done. Further, the supply of air by the power generation air flow rate adjustment unit 45 is continued as it is. By operating the reforming air flow rate adjustment unit 44 and supplying air directly to the fuel electrode side, the fuel, water vapor, etc. remaining inside the reformer 20, the manifold 66, each fuel cell unit 16, etc. Is discharged. As a result, the oxygen partial pressures on the air electrode side and the fuel electrode side are completely equal, so that the risk of electrochemical oxidation of the fuel electrode is substantially zero. The supply of air to the fuel electrode side by the reforming air flow rate adjustment unit 44 may start at a temperature of about 300 ° C. in the fuel cell module 2.

  FIG. 11 is a diagram for explaining the operation of the stopping process, and the upper stage shows a graph schematically showing the pressure change on the fuel electrode side and the air electrode side, and the middle stage shows the control operation by the control unit 110 and the fuel cell module 2. The temperature inside is shown in time series, and the lower part shows the state of the upper end portion of the fuel cell unit 16 at each time point.

  First, the power generation operation is performed before the stop switch operation in the middle stage of FIG. 11, and the pre-stop control is executed after the stop switch operation. In the pre-stop control, the supply amount of the fuel gas is reduced, so that the flame at the upper end of each fuel cell unit 16 that was large during the power generation operation, as shown in the lower part (1) of FIG. It becomes small as shown in. Thus, since the supply amount of fuel gas and the power generation amount are reduced, the temperature in the fuel cell module 2 is lowered than during the power generation operation. After the pre-stop control for about 2 minutes, shutdown stop is performed (fuel supply stop and power generation stop). After the shutdown is stopped, power generation air is supplied for 2 minutes by the power generation air flow rate adjustment unit 45 as temperature drop control. After the temperature drop control, the power generation air flow rate adjustment unit 45 is stopped and then left naturally for a predetermined leaving period.

  As described above, since the pressure on the fuel electrode side of each fuel cell unit 16 is higher than the pressure on the air electrode side at the time of shutdown stop, the fuel gas on the fuel electrode side after the fuel supply is stopped. Is ejected from the upper end of each fuel cell unit 16. The flame in which the fuel gas is burned disappears when the shutdown is stopped. After the shutdown is stopped, the fuel gas ejected from the upper end of each fuel cell unit 16 is the largest immediately after the shutdown is stopped, and then gradually decreases. A large amount of fuel gas ejected immediately after the shutdown is stopped is discharged out of the fuel cell module 2 by power generation air supplied by temperature drop control. Even after the temperature drop control is finished, the fuel gas is ejected from the upper end of each fuel cell unit 16, but the amount of the fuel gas is relatively small.

  For this reason, hydrogen, which is the fuel gas ejected after the end of the temperature drop control, stays in the upper part of the fuel cell module 2 (above the fuel cell stack 14), but the ejected fuel gas remains in each fuel cell. The air electrode of the cell unit 16 does not substantially contact. Therefore, the fuel gas is reduced by contacting the hot air electrode, and the air electrode is not deteriorated. In the pre-stop control before the shutdown stop, an appropriate amount of water in a predetermined range is supplied, and a part of this water remains in the evaporation unit 20a. For this reason, after the shutdown is stopped, the pressure on the fuel electrode side of each fuel cell unit 16 is increased by evaporating water in the evaporation section 20a.

  Next, after the temperature drop control is completed, the fuel cell module 2 is left as a standing period until the temperature in the fuel cell module 2 drops to about 550 ° C. When the temperature in the fuel cell module 2 drops to about 550 ° C., residual hydrogen discharge control is started, and the power generation air flow rate adjustment unit 45 is intermittently operated a plurality of times.

  Here, immediately after the end of the temperature drop control, the temperature of the fuel cell stack 14 is high. Therefore, when air enters from the air electrode side to the fuel electrode side, the risk that the fuel electrode is oxidized in the atmosphere is extremely high. From this state, the temperature of the fuel cell stack 14 gradually decreases after a standing period, so that the risk that the entering air contacts the fuel electrode and the fuel electrode is oxidized in the atmosphere gradually decreases. On the other hand, immediately after the end of the temperature drop control, the temperature of the fuel cell stack 14 is high, and therefore the electrolyte layer 94 and the fuel electrode of each fuel cell unit 16 are in an active state. Therefore, oxygen ions that have passed through the electrolyte layer 94 can react with hydrogen remaining in the fuel electrode, and no electrochemical oxidation reaction occurs. When the temperature of the fuel cell stack 14 is lowered from this state, the activity of the fuel electrode starts to gradually decrease with the temperature drop. Therefore, oxygen ions that have passed through the electrolyte layer 94 are less likely to react with hydrogen at the fuel electrode as the temperature decreases, and the risk of an electrochemical oxidation reaction occurring at the fuel electrode is gradually increased.

Thus, the risk that the fuel electrode is oxidized in the atmosphere decreases as the temperature of the fuel cell stack 14 decreases, while the risk that the fuel electrode is electrochemically oxidized increases as the temperature decreases.
As described with reference to FIG. 15, the temperature range in which the fuel electrode is easily oxidized in the atmosphere is a temperature range of about 350 ° C. or higher, which is higher than the oxidation suppression temperature, and the temperature at which the fuel electrode is easily oxidized electrochemically. The band is a temperature band Tc (approximately 200 to 290 ° C.) where an abnormal current can be generated. However, even in the temperature range of about 290 ° C. or more, which is the lower limit temperature Ta of the fuel electrode, the activity of the fuel electrode decreases with a decrease in temperature, so that the fuel electrode can be electrochemically generated. Similarly, atmospheric oxidation of the fuel electrode can occur even in a temperature range below about 350 ° C., which is the oxidation suppression temperature. In addition, since the temperature of each fuel cell unit 16 constituting the fuel cell stack 14 is not completely uniform, for example, even if the temperature detected by the power generation chamber temperature sensor 142 is less than 350 ° C. The temperature of the fuel cell unit 16 may exceed 350 ° C. Therefore, in practice, a part of the temperature range where the atmospheric oxidation of the fuel electrode can occur and the temperature range where the electrochemical oxidation can occur overlap. In the present embodiment, the risk of atmospheric oxidation and the risk of electrochemical oxidation partially overlapping in this way are suppressed by residual hydrogen discharge control.

  Here, when the residual hydrogen discharge control is started after the standing period is over, air is introduced into the fuel cell module 2 and flows out from the fuel electrode side to the air electrode side and stays in the fuel cell module 2. A part of the fuel (hydrogen) was discharged. As a result, the hydrogen concentration on the air electrode side of the fuel cell unit 16 decreases. For this reason, by executing the residual hydrogen discharge control, the concentration of hydrogen contained in the gas gradually diffusing from the air electrode side to the fuel electrode side through the upper end of each fuel cell unit 16 decreases (see FIG. 11). Lower row (4)). That is, by executing the residual hydrogen discharge control, the concentration of oxygen contained in the gas diffusing to the fuel electrode side of each fuel cell unit 16 increases. However, gas diffusion proceeds slowly with a decrease in the temperature of the fuel electrode, so that atmospheric oxidation that has an adverse effect on the fuel electrode does not occur.

  In this way, the residual hydrogen discharge control is executed, and the oxygen partial pressure on the fuel electrode side of the fuel cell unit 16 is increased by increasing the concentration of oxygen contained in the gas diffusing to the fuel electrode side of the fuel cell unit 16. Rises very slowly. That is, as the temperature of the fuel cell unit 16 decreases over a long period of time and the risk of atmospheric oxidation of the fuel electrode decreases, the oxygen partial pressure of the gas present on the fuel electrode side gradually increases. Further, when the temperature of the fuel cell unit 16 decreases and the risk of electrochemical oxidation of the fuel electrode increases, the oxygen partial pressure on the fuel electrode side increases, and the difference in oxygen partial pressure between the fuel electrode side and the air electrode side increases. Is getting smaller. Thereby, the electrochemical oxidation reaction of the fuel electrode in a state where the temperature of the fuel cell unit 16 is lowered is suppressed.

  On the other hand, when the residual hydrogen discharge control is not executed as in the case of the conventional shutdown stop, the hydrogen flowing out from the fuel electrode side to the air electrode side stays in the fuel cell module 2, and this is the fuel from the air electrode side. It diffuses again to the pole side. As described above, when a gas having a high hydrogen concentration diffuses from the air electrode side to the fuel electrode side, even when the risk of electrochemical oxidation is increased due to a decrease in the temperature of the fuel cell stack 14, the fuel electrode side and the air still remain. The difference in oxygen partial pressure on the pole side is maintained large. In particular, if a hydrogen concentration gradient occurs within one fuel cell unit 16 (on the fuel electrode side), a local cell phenomenon occurs as described with reference to FIG. Promotes oxidation.

  In the present embodiment, after the standing period, the residual hydrogen on the air electrode side of the fuel cell unit 16 is discharged by the residual hydrogen discharge control to indirectly increase the oxygen partial pressure on the fuel electrode side. . This increase in the oxygen partial pressure on the fuel electrode side occurs very slowly as the temperature of the fuel cell unit 16 decreases. Therefore, both high-risk atmospheric oxidation at high temperatures and high-risk electrochemical oxidation at low temperatures are ensured. Can be avoided.

Next, the behavior when the fuel electrode is oxidized will be described with reference to FIG.
FIG. 12 is a graph showing temperature (° C.) on the horizontal axis and linear expansion coefficient (%) on the vertical axis. A thick line I in the graph indicates a linear expansion coefficient when the reductant of the material used for the fuel electrode is gradually heated in an air atmosphere. As described above, the fuel electrode is made of a nickel-containing composite material made of a mixture of nickel and zirconia doped with yttrium (Ni / YSZ). A broken line II in the graph indicates a linear expansion coefficient when the Ni / YSZ oxidant is similarly heated. The thin line III in the graph indicates the linear expansion coefficient when the temperature of the nickel reductant is similarly raised. Both the fuel electrode material and nickel were heated from room temperature (25 ° C.) to 700 ° C. at a rate of 0.5 ° C./min. The linear expansion coefficient of the anode material and nickel was measured using a thermomechanical analysis (TMA) apparatus based on the volume at room temperature.

The fuel electrode material used for the measurement is a reductant of a nickel-containing composite material made of a mixture of nickel and zirconia doped with yttrium (NiYSZ). The mixing ratio of Ni and YSZ oxide is 50:50.
Since the fuel electrode material is supplied with hydrogen during operation of the fuel cell, it is a reductant immediately after the fuel cell is stopped.

  First, paying attention to the broken line II in the graph, the length of the oxidant of the composite material monotonously increases at a substantially constant rate as the temperature increases. Since the oxidant of the composite material has already been oxidized, this volume increase is considered to be due to thermal expansion and is not affected by oxidation.

  Next, paying attention to the thick line I in the graph, the increase in volume of the reductant composite material in the region from room temperature to around 350 ° C. and in the region higher than about 480 ° C. is that the thick line I is almost parallel to the broken line II. Therefore, it is thought that it is due to thermal expansion. On the other hand, the length of the reductant composite material decreases slightly from around 350 ° C. to around 400 ° C., then becomes minimal at around 380 ° C., and then suddenly increases from around 400 ° C. to around 480 ° C. It has increased. As described above, the reductant composite material has conventionally been thought to only expand due to oxidation, but in fact, the present inventors have found the fact that it shrinks in the temperature range from about 350 ° C. to about 400 ° C. It was done.

  Furthermore, paying attention to the thin line III in the graph, it can be seen that the volume of nickel alone also decreases from 350 ° C. to 480 ° C. as the temperature increases. From this, the peculiar volume change of the reductant composite material from about 350 ° C. to about 480 ° C. shown in the thick line I is not the already oxidized YSZ oxide among the components of the reductant composite material, but nickel It is thought to be caused by

  Here, as described with reference to FIG. 11, the residual hydrogen discharge control by the shutdown stop circuit 110 a is started when the temperature in the fuel cell module 2 is lowered to 550 ° C. When the residual hydrogen discharge control is started, air is introduced into the fuel cell module 2, so that a part of the hydrogen staying on the air electrode side of the fuel cell unit 16 is discharged and oxygen on the air electrode side is discharged. The concentration becomes high. The gas on the air electrode side gradually diffuses to the fuel electrode side through the fuel gas channel narrow tube 98 at the upper end of each fuel cell unit 16. For this reason, when the residual hydrogen discharge control is executed, the fuel electrode is easily oxidized in the atmosphere, although the amount is slightly smaller than when the residual hydrogen discharge control is not executed.

  However, after the residual hydrogen discharge control is started, the air gradually begins to diffuse from the air electrode side to the fuel electrode side, and the amount of diffused air becomes a certain amount because the temperature of the fuel cell stack 14 is When the temperature drops to about 400 ° C. As described above, when the fuel electrode is atmospherically oxidized in this temperature range, the material of the fuel electrode slightly oxidizes and shrinks. Thus, the fuel electrode is oxidized and contracted by slightly oxidizing the fuel electrode in the atmosphere in a temperature range of about 400 ° C. or lower. Next, when the temperature of the fuel cell stack 14 decreases and the fuel electrode is subjected to electrochemical oxidation, the fuel electrode is oxidized and expanded, but the fuel electrode is oxidized and contracted by the above atmospheric oxidation. The effect of this oxidative expansion is offset. That is, the stress generated by atmospheric oxidation of the fuel electrode and the stress generated by electrochemical oxidation are offset, and adverse effects on each fuel cell unit 16 due to oxidation are suppressed. In this embodiment, by starting the residual hydrogen discharge control from about 550 ° C., slight atmospheric oxidation of the fuel electrode is generated in a temperature range of about 400 ° C. or less, thereby suppressing adverse effects due to oxidation.

  According to the solid oxide fuel cell system 1 of the embodiment of the present invention, the residual hydrogen discharge control is executed before the temperature in the fuel cell module 2 falls below the oxidation suppression temperature (time t308 in FIG. 8) ( At time t305 in FIG. 8, only the power generation air flow rate adjustment unit 45 is operated. As a result, hydrogen that has flowed out from the fuel electrode side to the oxidant gas electrode side after the shutdown stop (time t303 in FIG. 8) is discharged from the fuel cell module 2, and the hydrogen concentration in the fuel cell module 2 is reduced. By executing the residual hydrogen discharge control and discharging the hydrogen on the oxidant gas electrode side, the oxygen concentration of the gas diffused to the fuel electrode side becomes high.

  In the present embodiment, the oxygen concentration on the air side is increased by residual hydrogen discharge control from a state where there is a high risk of atmospheric oxidation before the inside of the fuel cell module 2 falls to 350 ° C. or less, which is the oxidation suppression temperature. Therefore, the oxygen concentration of the gas diffusing from the air electrode side to the fuel electrode side can be increased very little by little, and when the temperature decreases with time and the risk of electrochemical oxidation increases, the fuel The oxygen partial pressure on the pole side increases to some extent. Thereby, the oxygen partial pressure on the fuel electrode side can be gradually increased while avoiding the atmospheric oxidation of the fuel electrode, and at the time of reaching the temperature band (Tc in FIG. 15) where the risk of electrochemical oxidation increases. The difference in oxygen partial pressure between the fuel electrode side and the air electrode side is reduced, and the occurrence of electrochemical oxidation can be effectively suppressed. Further, in a state where the temperature in the fuel cell module 2 is lowered below the oxidation suppression temperature and the risk of atmospheric oxidation of the fuel electrode is sufficiently reduced, cooling operation control for cooling the inside of the fuel cell module 2 is executed (FIG. 8). Time t309). Thereby, the temperature of the electrolyte layer of the fuel cell unit 16 is lowered and its activity is lost, so that the cause of electrochemical oxidation is fundamentally removed. According to this embodiment, it is possible to sufficiently suppress both atmospheric oxidation and electrochemical oxidation of the fuel electrode while executing shutdown stop.

  Further, according to the solid oxide fuel cell system 1 of the present embodiment, by performing temperature drop control (time t303 to t304 in FIG. 8) in a state where no air enters the fuel electrode side. The temperatures on the fuel electrode side and the air electrode side can be made uniform, and the ingress of air based on the contraction of the gas accompanying the temperature decrease can be suppressed. Further, by making the temperature uniform by the temperature drop control, the temperature state in the fuel cell module 2 after the shutdown is stopped can be set to a specified state, and the leaving period has elapsed (time t304 to t305 in FIG. 8). Thereafter, residual hydrogen discharge control (time t305 to time t308 in FIG. 8) can be executed at an appropriate timing.

  Furthermore, according to the solid oxide fuel cell system 1 of the present embodiment, the power generation air flow rate adjustment unit 45 is intermittently operated a plurality of times as residual hydrogen discharge control (time t305, t306, t307 in FIG. 8). , T308), it is possible to reduce the amount of air introduced into the air electrode at a time. For this reason, it is possible to reliably prevent air from entering the fuel electrode.

  Further, according to the solid oxide fuel cell system 1 of the present embodiment, the flow rate of the air supplied by the residual hydrogen discharge control (time t305 to t308 in FIG. 8) is the temperature drop control (time t303 to time t303 in FIG. 8). It is set to be smaller than the flow rate of air supplied by t304). Since the temperature drop control is executed in a state where the pressure on the fuel electrode side is sufficiently high, the risk of air entry is extremely low. On the other hand, when the residual hydrogen discharge control is executed (time t305 in FIG. 8), the pressure on the fuel electrode side is low, so the risk of entry is high. According to this embodiment, since the air flow rate in the residual hydrogen discharge control is set to be smaller than that in the temperature drop control, air can be supplied to the air electrode side while sufficiently suppressing the risk of entry. .

  Further, according to the solid oxide fuel cell system 1 of the present embodiment, the shutdown stop circuit 110a adjusts the power generation air flow rate so that the flow rate of the air supplied to the air electrode side is gradually increased in the residual hydrogen discharge control. The unit 45 is controlled (FIG. 10). In this embodiment, the flow path resistance from the air electrode side to the fuel electrode side is the flow path resistance of the flow path from the air electrode side to the outside of the fuel cell module 2 through the air heat exchanger 22 or the like. When the air is introduced to the air electrode side, the gas staying inside the fuel cell module 2 is easily discharged outside without entering the fuel electrode side. Further, in the residual hydrogen discharge control, the flow rate of air is gradually increased. Therefore, the flow rate increases after a gas flow from the inside of the fuel cell module 2 to the outside is generated with a small amount of supply, and the flow toward the fuel electrode is increased. Is less likely to occur. According to this embodiment, the risk of air ingress in the residual hydrogen discharge control can be greatly reduced.

  Further, according to the solid oxide fuel cell system 1 of the present embodiment, the power generation air flow rate adjustment unit 45 is operated for a predetermined time (FIG. 10), so that it is accurately managed in the residual hydrogen discharge control. A large amount of air can be introduced. Thereby, the oxygen partial pressure on the air electrode side can be set to a desired value, and the amount of oxygen diffused to the fuel electrode side can be set appropriately.

  Further, according to the solid oxide fuel cell system 1 of the present embodiment, the intermittent operation of the power generation air flow rate adjustment unit 45 in the residual hydrogen discharge control is less frequent as the temperature in the fuel cell module 2 decreases. (FIG. 8). The pressure on the fuel electrode side decreases with a decrease in temperature after the shutdown is stopped (time t303 in FIG. 8). According to the present embodiment, the air supply amount is reduced along with the pressure drop on the fuel electrode side, so that air can be reliably prevented from entering the fuel electrode side.

  Further, according to the solid oxide fuel cell system 1 of the present embodiment, the open circuit output voltage is monitored by the voltage monitoring circuit 110b, so that when air enters the fuel electrode side, it immediately occurs. In addition, the residual hydrogen discharge control can be stopped and the atmospheric oxidation of the fuel electrode can be reliably prevented.

  Furthermore, according to the solid oxide fuel cell system 1 of the present embodiment, the state of the fuel cell module 2 at the time of shutdown stop (time t303 in FIG. 9) is controlled by the pre-stop control (time t301 to t303 in FIG. 9). Therefore, the air can be diffused to the fuel electrode side based on the residual hydrogen discharge control at an appropriate timing. Thereby, atmospheric oxidation and electrochemical oxidation of the fuel electrode can be reliably prevented.

  As mentioned above, although preferable embodiment of this invention was described, a various change can be added to embodiment mentioned above. In particular, in the above-described embodiment, the power generation air having the same flow rate is supplied a plurality of times in the residual hydrogen discharge control. However, as a modification, the flow rate of each time can be changed.

  In the modification shown in FIG. 13, when the temperature in the fuel cell module 2 decreases to about 500 ° C., decreases to about 450 ° C., and decreases to about 400 ° C., the flow rate and supply time of air to be supplied respectively. It has changed. That is, in the modification shown in FIG. 13, the lower the temperature in the fuel cell module 2, the smaller the air supply flow rate and the shorter the supply time. Thereby, the supply amount of air decreases as the temperature in the fuel cell module 2 decreases.

  In the above-described embodiment, in the residual hydrogen discharge control, the power generation air is supplied for a preset time. However, as a modification, the actual air supply amount can be controlled. That is, a sensor (not shown) for measuring the actual flow rate of the power generation air is provided, and a predetermined amount of air is supplied into the fuel cell module 2 by feeding back the flow rate measured by this sensor. The present invention can also be configured as described above.

DESCRIPTION OF SYMBOLS 1 Solid oxide fuel cell system 2 Fuel cell module 4 Auxiliary machine 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 Air flow rate adjustment unit for reforming (oxidizing gas supply device for reforming)
45 Air flow adjustment unit for power generation (oxidizer gas 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 (11)

A solid oxide fuel cell system that generates electricity by reacting hydrogen with an oxidant gas,
A fuel cell module containing fuel cells, 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 side of the fuel cell;
A water supply device for supplying water for steam reforming to the reformer;
An oxidant gas supply device for supplying an oxidant gas to the oxidant gas electrode side of the fuel cell;
A controller for controlling the fuel supply device, the water supply device, and the oxidant gas supply device, and for controlling the extraction of electric power from the fuel cell module,
The controller includes a shutdown stop circuit for executing a shutdown stop for stopping the supply of fuel by the fuel supply device and the extraction of electric power from the fuel cell module,
The shutdown stop circuit is configured such that after the shutdown period, the fuel, water, and oxidant gas supply is stopped and the oxidant is cooled before the temperature in the fuel cell module drops below the oxidation suppression temperature. Residual hydrogen discharge control in which only the gas supply device is operated for a predetermined period, and the oxidant gas supply device is operated in a state where the temperature in the fuel cell module is lowered below the oxidation suppression temperature to cool the inside of the fuel cell module. Programmed to perform cooling operation control,
According to the residual hydrogen discharge control, hydrogen that flows out from the fuel electrode side to the oxidant gas electrode side after shutdown is discharged from the fuel cell module, and a hydrogen concentration in the fuel cell module is reduced. Solid oxide fuel cell system.   The shutdown stop circuit operates the oxidant gas supply device during a predetermined period in which the pressure on the fuel electrode side of the fuel cell is sufficiently high immediately after the shutdown is stopped, and a temperature drop that lowers the temperature in the fuel cell module. 2. The solid oxide fuel cell system according to claim 1, programmed to execute control, wherein the residual hydrogen discharge control is executed after the standing period has elapsed after completion of the temperature drop control.   The residual hydrogen discharge control is executed by intermittently operating the oxidant gas supply device a plurality of times during a period after the temperature drop control is completed and before the cooling operation control is started. Solid oxide fuel cell system.   The solid oxide fuel cell system according to claim 3, wherein a flow rate of the oxidant gas supplied by the residual hydrogen discharge control is set to be smaller than a flow rate of the oxidant gas supplied by the temperature drop control.   The flow resistance from the oxidant gas electrode side to the fuel electrode side of the fuel cell is set to be larger than the flow resistance from the oxidant gas electrode side of the fuel cell to the outside of the fuel cell module. 4. The solid oxide according to claim 3, wherein the shutdown stop circuit controls the oxidant gas supply device so that the flow rate of the oxidant gas supplied to the oxidant gas electrode side in the residual hydrogen discharge control is gradually increased. Type fuel cell system.   The shutdown stop circuit operates the oxidant gas supply device for a predetermined time or is supplied to the oxidant gas electrode side so that a predetermined amount of oxidant gas is supplied in the residual hydrogen discharge control. 4. The solid oxide fuel cell system according to claim 3, wherein the oxidant gas supply device is controlled by feeding back the amount of the oxidant gas.   The intermittent operation of the oxidant gas supply device in the residual hydrogen discharge control is performed less frequently as the temperature in the fuel cell module decreases, or 1 as the temperature in the fuel cell module decreases. The solid oxide fuel cell system according to claim 6, which is executed so that the supply amount of the oxidant gas in the intermittent operation is reduced.   The controller further includes a voltage monitoring circuit for monitoring an open circuit output voltage from the fuel cell module, and the shutdown stop circuit is configured to control the residual hydrogen discharge when the open circuit output voltage is excessively decreased. 4. The solid oxide fuel cell system according to claim 3, wherein the supply of the oxidant gas is stopped.   The fuel electrode of the fuel cell includes a nickel component, and the residual hydrogen discharge control is executed such that oxidation of the fuel electrode occurs and oxidation shrinkage occurs at a temperature of 400 ° C. or lower. Solid oxide fuel cell system.   4. The shutdown stop circuit according to claim 3, wherein the shutdown stop circuit is programmed to execute pre-stop control for supplying a predetermined amount of fuel and water to the reformer for a predetermined period immediately before executing the shutdown stop. Solid oxide fuel cell system. A solid oxide fuel cell system that generates electricity by reacting hydrogen with an oxidant gas,
A fuel cell module containing fuel cells, and
A fuel supply device for supplying fuel to the fuel cell module;
A reformer for steam-reforming the fuel supplied by the fuel supply device to generate hydrogen, and supplying the generated hydrogen to the fuel electrode side containing the nickel component of the fuel cell;
A water supply device for supplying water for steam reforming to the reformer;
An oxidant gas supply device for supplying an oxidant gas to the oxidant gas electrode side of the fuel cell;
A controller for controlling the fuel supply device, the water supply device, and the oxidant gas supply device, and for controlling the extraction of electric power from the fuel cell module,
The controller includes a shutdown stop circuit for executing a shutdown stop for stopping the supply of fuel by the fuel supply device and the extraction of electric power from the fuel cell module,
The shutdown stop circuit is configured such that after the shutdown period, the fuel, water, and oxidant gas supply is stopped and the oxidant is cooled before the temperature in the fuel cell module drops below the oxidation suppression temperature. Residual hydrogen discharge control in which only the gas supply device is operated for a predetermined period, and the oxidant gas supply device is operated in a state where the temperature in the fuel cell module is lowered below the oxidation suppression temperature to cool the inside of the fuel cell module. Programmed to perform cooling operation control,
By the residual hydrogen discharge control, hydrogen that flows out from the fuel electrode side to the oxidant gas electrode side after the shutdown is stopped is discharged from the fuel cell module, and the hydrogen concentration in the fuel cell module is lowered and 400 ° C. A solid oxide fuel cell system characterized in that the fuel electrode is oxidized at the following temperature to cause oxidation shrinkage.

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