JP2016181375A - Solid oxide fuel cell system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a solid oxide fuel cell system that secures sufficiently long life period.SOLUTION: A solid oxide fuel cell system comprises a fuel battery cell 16, a reforming part, a fuel supply device 38, a water supply device 28, an evaporation part, an oxidant gas supply device 45, a fuel battery module 2, and a controller. The controller operates the oxidant gas supply device even after power stop, and performs cell protection control, concentration gradient correction control, and purge control. In the cell protection control, a material fuel gas and water are supplied until a temperature is reduced to a temperature band of 350-290°C. In the concentration gradient correction control, an oxidant gas is supplied to make the oxidant gas flow backward to a fuel electrode side and keep the fuel electrode side in an oxidant gas atmosphere. In the purge control, when the temperature is reduced to a temperature band of 200-150°C, the concentration gradient correction control is terminated, and then, the fuel electrode side is purged.SELECTED DRAWING: Figure 1

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 obtained by steam reforming raw fuel gas and an oxidant gas.

  Japanese Patent No. 4906242 (Patent Document 1) describes a method of stopping the operation of a fuel cell. In this method of stopping the operation of the fuel cell, after the power generation by the fuel cell is stopped, the supply of raw fuel gas (hydrocarbon gas) is reduced while continuing to supply air into the fuel cell. A mixed gas of a predetermined amount of raw fuel gas and water vapor is supplied into the fuel cell through the reformer. The supply of the mixed gas after the power generation is stopped is stopped when the temperature of the reforming catalyst falls to the range of the oxidation occurrence temperature of the catalyst ± 150 ° C. Thereafter, air or raw fuel gas is purged as a purge gas through the reformer through the reformer.

  In the invention described in Japanese Patent No. 4906242, the technical problem is that after power generation is stopped, air flows back through the fuel cell to the reformer, and the high-temperature reforming catalyst in the reformer is atmospherically oxidized. In the present invention, the backflow of air is prevented by continuing to supply the mixed gas of raw fuel gas and water vapor through the reformer even after the power generation is stopped. The supply of the mixed gas is stopped until there is no possibility that the catalyst of the reformer is oxidized in the atmosphere, that is, when the temperature falls within the range of the oxidation occurrence temperature of the catalyst ± 150 ° C.

  On the other hand, Japanese Patent No. 5316830 (Patent Document 2) describes a solid oxide fuel cell. In this solid oxide fuel cell, the consumption of fuel that does not contribute to power generation is avoided by stopping the supply of raw fuel gas along with the stop of power generation. Further, in the present invention, temperature drop control for supplying air to the air electrode side of the fuel cell stack is performed in a state where the pressure on the fuel electrode side of the fuel cell stack is high immediately after power generation is stopped. Thereby, the backflow of air is suppressed until the temperature of the fuel electrode of the fuel cell stack is lowered to a temperature at which atmospheric oxidation does not substantially occur, and atmospheric oxidation of the fuel electrode is prevented.

Japanese Patent No. 4906242 Japanese Patent No. 5316830

  However, as in the inventions described in Patent Documents 1 and 2, even if atmospheric oxidation that occurs in a relatively high temperature region is prevented, electrochemical oxidation occurs by a mechanism different from atmospheric oxidation, which adversely affects the fuel cell. The present inventor has found a new technical problem of giving In particular, in the fuel electrode of the fuel cell stack, which is a problem in the invention described in Japanese Patent No. 5316830, a microstructural change occurs due to an electrochemical oxidation reaction. The inventor has found a new problem that damage may occur if the battery system is repeatedly started and stopped many times.

  Therefore, the present invention provides a solid oxide fuel cell that is unlikely to undergo microstructural changes in the fuel electrode even when the fuel cell system is repeatedly started and stopped over many years, and can ensure a sufficiently long service life. The purpose is to provide a system.

  In order to solve the above-described problems, the present invention provides a solid oxide fuel cell system that generates power by reacting hydrogen obtained by steam reforming raw fuel gas and an oxidant gas, from the inside. In order, hydrogen is produced from a fuel cell layer having a nickel-based fuel electrode layer, a solid electrolyte layer, an oxidant gas electrode layer, and raw fuel gas by steam reforming. Of the fuel cell from the lower end, a fuel supply device for supplying raw fuel gas to the reforming portion, a water supply device for supplying water for steam reforming, and the water An evaporation unit that generates water vapor from water supplied from a supply device and supplies the water vapor to a reforming unit, an oxidant gas supply device that supplies an oxidant gas to the oxidant gas electrode side of the fuel cell, and a fuel cell Housed fuel cell module and fuel supply And a controller for controlling the extraction of electric power from the fuel cell module, and the controller also controls the removal of electric power from the fuel cell module. While the operation of the oxidant gas supply device is continued to lower the temperature in the fuel cell module, cell protection control, concentration gradient correction control, and purge control are performed in order to protect the fuel cell after stopping power extraction. In the cell protection control, a smaller amount of raw fuel gas and water are used in the cell protection control than in the power generation operation until the temperature of the fuel cell drops to a temperature range of 350 ° C. to 290 ° C. In the concentration gradient correction control, the fuel supply device and the water supply device are operated so that the fuel supply device and the water supply device are continuously operated. By continuing the operation of the oxidant gas supply device while stopping the operation, the oxidant gas flows backward from the upper end of the fuel cell to the fuel electrode side, and the residual gas on the fuel electrode side is pushed back toward the reforming unit. The fuel electrode side is maintained in the oxidant gas atmosphere, and the purge control is performed through the reforming unit after the concentration gradient correction control is finished when the temperature of the fuel battery cell falls to a temperature range of 200 ° C. to 150 ° C. It is characterized by supplying gas and purging the fuel electrode side of the fuel cell.

  In the conventional fuel cell system, atmospheric oxidation of the fuel electrode after power generation is stopped has been a problem. In particular, when the power generation is stopped (stopping the extraction of power) and the shutdown stop that stops the supply of raw fuel gas, if the oxidant gas flows backward from the oxidant gas electrode side to the fuel electrode side in a high temperature state, The fuel electrode may be oxidized and the fuel cell may be damaged. Such a problem of atmospheric oxidation of the fuel electrode of the fuel battery cell has already been solved by the present inventor as described in Japanese Patent No. 5316830 (Patent Document 2).

  However, the present inventors have found a new technical problem that when the shutdown stop is executed, the fuel electrode can be oxidized by a mechanism completely different from the atmospheric oxidation. That is, after the fuel cell system is shut down and stopped, the oxidant gas on the oxidant gas electrode side of the fuel cell starts to gradually flow back to the fuel electrode side. Since the back flow of this oxidant gas is very little by little, in the temperature range where atmospheric oxidation after shutdown is stopped, the amount of oxidant gas that flows back to the fuel electrode side is very small, and the fuel electrode is substantially in the atmosphere. There is no oxidation. However, the inventors of the present invention have found that the fuel electrode is electrochemically oxidized after a lapse of time, in a state where the temperature is lowered to a zone where atmospheric oxidation of the fuel electrode does not become a problem.

  In other words, in a relatively low temperature range where atmospheric oxidation is not a problem, the oxidant gas electrode layer and the electrolyte layer still maintain catalytic activity, while the catalytic activity of the fuel electrode layer containing nickel as a main component decreased. It becomes a state. In such a state, the oxidant gas electrode layer and the electrolyte layer can permeate oxygen ions, whereas the fuel electrode layer cannot react the permeated oxygen ions with hydrogen ions. It has been found by the present inventors that such oxygen ions that have penetrated to the fuel electrode layer and cannot react with hydrogen ions oxidize the fuel electrode layer electrochemically. Although this electrochemical oxidation is extremely small and does not immediately degrade the performance of the fuel cell, by using the solid oxide fuel cell system for a long period of time and starting and stopping many times, The fuel cell may be damaged.

  Furthermore, as long as the oxygen ions that do not react in the fuel electrode layer are uniformly generated in each part of the fuel cell, even if the fuel cell system is repeatedly started and stopped many times, there is little problem. . However, in the fuel cell after the shutdown of the fuel cell system, when the oxidant gas starts to flow backward from the oxidant gas electrode side to the fuel electrode side, the hydrogen concentration in the fuel electrode of one fuel cell differs. Thus, it has been found by the present inventor that this concentration gradient promotes electrochemical oxidation of the fuel electrode. That is, if there is a hydrogen concentration gradient on the fuel electrode side, the difference in oxygen partial pressure between the fuel electrode side and the oxidant gas electrode side will be different in each part of the fuel cell. Here, a large electromotive force is generated between the fuel electrode and the oxidant gas electrode in the portion where the oxygen partial pressure difference of the fuel cell is large, while the electromotive force is small in the portion where the oxygen partial pressure difference is small. As described above, the present inventor may cause a “local battery phenomenon” in which a current flows inside one fuel battery cell when a portion having a large electromotive force and a portion having a small electromotive force are present in one fuel battery cell. It was found. When this “local battery phenomenon” occurs, the influence of electrochemical oxidation in the fuel electrode layer is concentrated on a part of one fuel cell, and the start and stop are repeated many times. It may lead to damage.

  In order to avoid such a “local battery phenomenon”, the present inventor supplies raw fuel gas even after the power generation of the fuel cell system is stopped, as in the invention described in Japanese Patent No. 4906242 (Patent Document 1). (The invention described in Patent Document 1 is intended to prevent atmospheric oxidation of the reforming catalyst in the reformer and is intended to avoid electrochemical oxidation of the fuel electrode. The technical idea is completely different from the claimed invention.) As described above, by continuing the supply of the raw fuel gas even after the power generation of the fuel cell system is stopped, the adverse effect of the electrochemical oxidation is reduced, but it is not sufficient. It was confirmed by the present inventors that a microstructural change occurred in the extreme layer.

  That is, in the invention described in Japanese Patent No. 4906242 (Patent Document 1), when the temperature of the reforming catalyst is lowered to about 350 ° C., the supply of the mixed gas of raw fuel gas and water vapor is stopped, and then the air Purging is performed by such means. However, it is possible to avoid the atmospheric oxidation of the reforming catalyst and the fuel electrode layer when such a stopping step is executed, but the “local battery phenomenon” in the fuel cell is promoted on the contrary. Have been found. This is a temperature range of about 350 ° C. at which the gas supplied to the fuel electrode side is switched from the mixed gas to the purge gas is a temperature range in which electrochemical oxidation is likely to occur. When the gas supplied in this zone is switched, A concentration gradient occurs on the fuel electrode side, and a “local cell phenomenon” occurs. Further, if the fuel electrode side of the fuel cell is purged in a temperature range where electrochemical oxidation occurs in order to suppress the generation of the concentration gradient, the concentration gradient is generated during the period during which the fuel electrode side is purged. As a result, a “local battery phenomenon” is induced.

  In order to solve these problems, in the present invention, as cell protection control, a small amount of raw fuel gas and water are supplied until the temperature of the fuel battery cell falls to a temperature range of 350 ° C. to 290 ° C. As gradient correction control, the supply of oxidant gas is stopped while the supply of fuel and water is stopped, so that the oxidant gas flows back to the fuel electrode side, and as purge control, the temperature is lowered to a temperature range of 200 ° C. to 150 ° C. The gas is supplied through the reforming section, and the fuel electrode side of the fuel cell is purged. For this reason, atmospheric oxidation and electrochemical oxidation of the fuel electrode layer of the fuel battery cell are suppressed by the cell protection control. Also, in the temperature range where the risk of atmospheric oxidation has decreased, the concentration gradient correction control causes the oxidant gas to flow backward to the fuel electrode side, and the residual gas on the fuel electrode side is pushed back toward the reforming section to bring the fuel electrode side back. Maintain an oxidant gas atmosphere. As a result, the fuel electrode side is promptly converted to the oxidant gas atmosphere, so that the concentration gradient on the fuel electrode side is suppressed and the occurrence of “local cell phenomenon” is suppressed. Further, in a temperature range where the risk of electrochemical oxidation is reduced, purge control purges the inside of the reforming section and the fuel electrode side of the fuel battery cell, thereby preventing adverse effects due to remaining water vapor.

In the present invention, preferably, in the purge control, the controller operates the fuel supply device to supply only the raw fuel gas to the reforming unit.
According to the present invention configured as described above, since the fuel electrode side of the fuel cell is purged with the raw fuel gas, it is not necessary to provide a device for supplying the oxidant gas to the fuel electrode side, and the solid oxide fuel The cost of the battery system can be reduced.

In the present invention, preferably, the solid electrolyte layer is made of LSGM, and the oxidant gas electrode layer is made of LSCF.
In the present invention configured as described above, the solid electrolyte layer made of LSGM and the oxidant gas electrode layer made of LSCF maintain catalytic activity up to a relatively low temperature, and can transmit oxygen ions. The operating temperature of the solid oxide fuel cell system can be set low, and the power generation efficiency can be improved. On the other hand, since the fuel electrode layer mainly composed of nickel loses its catalytic activity at a temperature higher than that of LSGM and LSCF, electrochemical oxidation becomes a problem as described above. According to the present invention, the cell protection control, the concentration gradient correction control, and the purge control are executed after stopping the extraction of power to effectively suppress the atmospheric oxidation and electrochemical oxidation of the fuel electrode. It is possible to provide a highly efficient solid oxide fuel cell system while suppressing deterioration of the fuel cell.

  According to the solid oxide fuel cell system of the present invention, even if the fuel cell system is repeatedly started and stopped over many years, it is difficult for the microstructure to change in the fuel electrode, and a sufficiently long service life is ensured. Can do.

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) cross-sectional view which show the fuel cell of the fuel cell system by one Embodiment of this 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. 5 is a flowchart showing a processing procedure when shutdown stop is executed in the fuel cell system according to the embodiment of the present invention. 5 is a flowchart showing a processing procedure when shutdown stop is executed in the fuel cell system according to the embodiment of the present invention. 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. In the fuel cell system by one Embodiment of this invention, it is a figure for demonstrating in time series the control in case a stop process is performed, the temperature in a fuel cell module, and the state of the front-end | tip part of a fuel cell. It is front sectional drawing which shows the fuel cell module of the fuel cell system by the deformation | transformation embodiment 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 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 battery cell, an electrolyte, and an air electrode, and a stop process. It is a graph which shows the electromotive force which a single fuel cell produces | generates in the state which supplied hydrogen gas to the fuel electrode side and supplied air to the air electrode side. It is a figure which shows the behavior of a fuel cell when the temperature in a fuel cell module passes an electrochemical oxidation generation | occurrence | production temperature range. It is a figure which shows typically the local battery phenomenon 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 (hydrogen) 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), and the fuel cell stack 14 includes 16 fuel cell cells 16 (see FIG. 4). . Thus, the fuel cell assembly 12 has 160 fuel cells 16, and all of these fuel cells 16 are connected in series.

A combustion chamber 18 as a combustion portion 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, residual fuel (off-gas) remaining without being used for the power generation reaction. And the remaining oxidant (air) are 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 raw fuel gas 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. doing. 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). The auxiliary unit 4 includes a gas cutoff valve 32 that shuts off the raw fuel gas supplied from the fuel supply source 30 such as city gas, a desulfurizer 36 for removing sulfur from the raw fuel gas, and a raw fuel gas. And a valve 39 for shutting off the raw fuel gas flowing out from the fuel flow rate adjusting unit 38 when the 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 air flow rate, 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, the fuel cell module 2 is connected with a combustion catalyst 49 containing a combustion catalyst for purifying the exhaust gas, and the exhaust gas purified by the combustion catalyst 49 is supplied to the hot water production apparatus 50. It has become so. The combustion catalyst 49 has a built-in catalyst heater 49a for heating the catalyst to the activation temperature. 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, raw fuel gas to be reformed, and reforming air on the end side surface of 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 raw fuel gas and pure water are supplied to both ends of the T-shaped tube 62a extending in the substantially horizontal direction. Pipes for supply 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 evaporator 20a is configured to heat and evaporate the pure water introduced through the reformer introduction pipe 62. Further, the evaporator 20a is provided with a heater 81 (FIG. 2) for assisting the heating of the evaporator 20a. The heater 81 is energized when water vapor is generated in a state where the temperature in the fuel cell module 2 is lowered, such as after power generation is stopped, and is configured to evaporate the supplied pure water.

  Further, the water vapor generated in the evaporation unit 20a is mixed with the raw fuel gas in the mixing unit 20b and flows into the reforming unit 20c. The steam and raw fuel gas introduced into the reforming unit 20c are reformed by the reforming catalyst filled in the reforming unit 20c. In the present embodiment, a reforming catalyst having ruthenium added to the surface of an alumina sphere is used. As the reforming catalyst, a noble metal-based reforming catalyst such as a rhodium-provided alumina sphere surface can be used. Note that the noble metal-based reforming catalyst generally has an oxidation temperature of about 700 ° C. or higher and is very high temperature, even if air enters the reformer 20 after the solid oxide fuel cell system 1 is stopped. The reforming catalyst is not oxidized in the atmosphere.

  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 is supplied into the fuel cell 16. Is done.

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 battery cell 16 will be described with reference to FIG. FIG. 4A is a partial cross-sectional view showing a fuel cell 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 battery cell.
As shown in FIG. 4A, the fuel cell 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. A solid 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 (hydrogen) passes and becomes a (−) electrode, while the outer electrode layer 92 is an air electrode in contact with air and becomes a (+) electrode. Yes.

  Since the inner electrode terminal 86 attached to the lower end side, which is the first end portion of the fuel cell 84, and the upper end side, which is the second end portion, has the same structure, the inner side attached to the upper end side here. The electrode terminal 86 will be specifically described. The upper part 90 a of the inner electrode layer 90 includes an outer peripheral surface 90 b and an upper end surface 90 c exposed to the solid electrolyte 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 channel narrow tube 98 is an elongated throttle portion 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 solid electrolyte layer 94 includes, 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, and lanthanum gallate doped with at least one selected from Sr and Mg. , And at least one kind.

  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 that constitute the fuel electrode layer, and the fuel electrode protection layer 90f is further formed as the first fuel electrode. It is included between 90d and the second fuel electrode 90e. The outer electrode layer 92 includes an air electrode 92a that is an oxidant gas electrode layer 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. Thus, in the present embodiment, the fuel electrode layer is mainly composed of nickel.

  In the present embodiment, the solid 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 cells 16, and these fuel cells 16 are arranged in two rows of 8 each. Each fuel cell 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 cells 16 at both ends are supported on two approximately square pieces. It is supported by the 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.

  Further, a current collector 102 and an external terminal 104 are attached to the fuel cell 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 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 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 16 at the end of the adjacent fuel cell stack 14, and as described above, all 160 fuel cells 16 are connected in series. It is like that.

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 placed outdoors. Further, a sensor for measuring the humidity or the like of the outside air may be provided.

  Signals from these sensors are sent to the control unit 110, which controls the water flow rate adjustment unit 28, the fuel flow rate adjustment unit 38, the ignition air flow rate adjustment unit 44, the oxidation based on the data based on these signals. A control signal is sent to the power generation air flow rate adjustment unit 45, which is an agent gas supply device, 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 raw fuel gas is introduced into the evaporator 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, the T-shaped pipe 62a, and the reformer introduction pipe 62 are introduced into the evaporator 20a. Accordingly, the supplied raw fuel gas 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 raw fuel gas are mixed in the mixing unit 20 b and flow into the reforming unit 20 c of the reformer 20. The raw fuel gas 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. 16 communicates with the inside. The fuel introduced into the manifold 66 flows out from the upper end of the fuel cell 16 through the many holes provided on the upper surface thereof, through the fuel electrode side of the fuel cell 16, that is, through the inside of the fuel cell 16. Further, when hydrogen gas as a fuel passes through the inside of the fuel cell 16, it reacts with oxygen in the air passing outside the fuel cell 16 as an air electrode (oxidant gas electrode) to generate a charge. . The remaining fuel that is not used for power generation flows out from the upper end of each fuel cell 16 and is combusted in a combustion chamber 18 provided above the fuel cell stack 14.

  On the other hand, the power generation air as 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 as the 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 16 on the air electrode side (oxidant gas electrode side) of the fuel cell stack 14, and a part of oxygen in the air generates power. Used for 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 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. Further, the exhaust gas discharged from the fuel cell module 2 flows into the combustion catalyst 49, where harmful components such as carbon monoxide are removed and sent to the hot water production apparatus 50.

Next, with reference to FIG. 7, the control in the starting process of the solid oxide fuel cell system 1 will be described.
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 start-up process shown in FIGS. 7 and 8, 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, ignition air, and water is started (pre-purge process). 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. . Further, the control unit 110 sends a signal to the ignition air flow rate adjustment unit 44 which is an oxidant gas supply device for ignition to operate it. The ignition air introduced into the fuel cell module 2 flows into the interior of each fuel cell 16 via the reformer 20 and the manifold 66, and flows out from the upper end thereof. Further, the control unit 110 sends a signal to the water flow rate adjusting unit 28 to operate it. The pure water pumped from the water flow rate adjusting unit 28 reaches the T-shaped tube 62a through the water supply pipe 63a. At least during a predetermined time (water filling period: t0 to t1) from time t0, the water supply pipe 63a from the water flow rate adjustment unit 28 to the T-shaped pipe 62a is filled with water, and a small amount of water is It arrange | positions in the evaporation part 20a. Note that at time t0, no reforming reaction occurs in the reformer 20 because fuel has not yet been supplied. In this embodiment, the supply amount of power generation air started at time t0 in FIG. 7 is about 50 L / min, the supply amount of ignition air is about 4.8 L / min, and the supply amount of water is 2.5 cc / min.

Next, at time t1 after a predetermined time from time t0 in FIG. 7, fuel supply is started, and an ignition process for the supplied fuel is started. 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 4.0 L / min.
The fuel introduced into the fuel cell module 2 flows into the inside of each fuel cell 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.

In the ignition process, the control unit 110 sends a signal to an ignition device 83 (FIG. 2) as ignition means, and ignites the fuel flowing out from the upper end of each fuel cell 16. The ignition device 83 repeatedly generates a spark near the upper end of the fuel cell stack 14 and ignites the fuel flowing out from the upper end of each fuel cell 16.
Further, at time t1, the water supply pipe 63a is filled with water and a small amount of water is disposed in the evaporation unit 20a, so the control unit 110 reduces the supply amount of water to 2.06 cc / min. . Thereby, a small amount of water starts to be supplied to the reformer 20 during the ignition process.

At time t2 in FIG. 7, the ignition process shifts to the combustion process. In the combustion process, the temperature of the reformer 20 is increased by the off-gas combustion heat.
When the ignition process ends, the control unit 110 stops the supply of ignition air at time t2 (supply amount 0.0 L / min). Therefore, the ignition air flows through the reformer 20 before ignition, but does not flow through the reformer 20 after ignition, and the reforming reaction (POX) in the reformer 20 does not flow. It is not intended to be supplied for (reaction). Therefore, as another embodiment, the ignition air may be supplied to the fuel cell 16 through another pipe without passing through the reformer 20.

  Further, at time t2, the controller 110 reduces the fuel supply amount to 3.14 L / min and increases the power generation air supply amount to about 70 L / min. The supply amount of water is maintained at 2.06 cc / min, and is maintained at a minimum value during the start-up process at least immediately after ignition and during the combustion process.

  In the combustion process, the supplied fuel flows out as off-gas from the upper end of each fuel cell 16 and is burned here. This combustion heat heats the reformer 20 disposed above the fuel cell stack 14. Here, an evaporating chamber heat insulator 23 is disposed above the reformer 20 (above the case 8), so that immediately after the start of fuel combustion, the temperature of the reformer 20 suddenly increases from room temperature. To rise. Since the outside air is introduced into the air heat exchanger 22 disposed on the heat insulating material 23 for the evaporation chamber, the air heat exchanger 22 has a low temperature, particularly immediately after the start of combustion, and serves as a cooling source. Cheap. In the present embodiment, the evaporation chamber heat insulating material 23 is disposed between the upper surface of the case 8 and the bottom surface of the air heat exchanger 22, so that the reformer 20 disposed in the upper portion of the case 8 The movement of heat to the air heat exchanger 22 is suppressed, and heat is easily trapped in the vicinity of the reformer 20 in the case 8. In addition, the space above the rectifying plate 21 above the evaporation unit 20a is configured as a gas retention space 21c (FIG. 2) in which the flow of combustion gas is slow, so that the vicinity of the evaporation unit 20a is double insulated. And the temperature rises more rapidly.

  As described above, the temperature of the evaporation unit 20a rapidly increases, so that water vapor can be generated in a short time after the start of off-gas combustion. Further, since the reforming water is supplied to the evaporation unit 20a little by little (minimum amount during the start-up process), a little heat is required compared to the case where a large amount of water is stored in the evaporation unit 20a. Water can be heated to the boiling point, and the supply of water vapor can be started immediately. Furthermore, since water flows in immediately after the start of the operation of the water flow rate adjustment unit 28, it is possible to avoid an excessive increase in temperature of the evaporation unit 20a and a delay in supply of water vapor due to a delay in supply of water.

In this way, the temperature of the reformer 20 rises during the combustion process, and at time t3, the combustion process shifts to the first steam reforming reaction process (SR1 process). In the SR1 process, a steam reforming reaction is caused by the fuel and steam that have flowed into the reforming unit 20c through the evaporation unit 20a. This steam reforming reaction is an endothermic reaction represented by the following formula (1).
C _m H _n + xH ₂ O → aCO ₂ + bCO + cH ₂ (1)

Further, when the first steam reforming reaction step (SR1 step) is started at time t3, the amount of water consumption increases, so the control unit 110 maintains the supply amount of fuel and power generation air. Increase the water supply only to 3.82cc / min.
In the time chart shown in FIG. 7, the reformer temperature at time t3 is about 250.degree.

  Next, when the temperature detected by the reformer temperature sensor 148 reaches about 450 ° C., the first steam reforming reaction process (SR1 process) to the second steam reforming reaction process (SR2) at time t4 in FIG. (Process) At time t4, the power generation air supply rate is reduced to 59 L / min, and the water supply rate is further increased to 5.0 cc / min. In addition, the previous fuel supply amount is maintained. Thereby, in the SR2 process, the molar ratio S / C of water vapor and carbon is increased as compared with the SR1 process.

  Furthermore, when the temperature detected by the power generation chamber temperature sensor 142 reaches about 550 ° C. at time t5 in FIG. 7, the second steam reforming reaction step (SR2 step) to the third steam reforming reaction step (SR3 step). It is transferred to. Accordingly, the fuel supply amount is reduced to 2.02 L / min, the power generation air supply amount is further reduced to 47 L / min, and the water supply amount is reduced to 4.75 cc / min. Thereby, in the SR3 step, the molar ratio S / C of steam and carbon is further increased as compared with the SR2 step, and is set to a value suitable for the steam reforming reaction.

  Further, after the SR3 step is executed for a predetermined time, when the temperature detected by the power generation chamber temperature sensor 142 reaches about 600 ° C., the process proceeds to the power generation step. 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. 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, a shutdown stop process in the conventional solid oxide fuel cell system will be described with reference to FIGS. 12 to 15b.

  First, when the shutdown stop is performed, the supply of raw fuel gas, 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 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) that existed on the air electrode side of each fuel battery cell is based on the pressure difference between the pressure on the air electrode side (pressure in the power generation chamber) and atmospheric pressure. It is 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 naturally decreases.

  However, by providing a throttle part, which is a narrow tube having a large flow path resistance part, at the upper end of each fuel cell, the pressure drop on the fuel electrode side after the fuel supply and power generation is stopped It is configured so that the fuel remains 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 pressure on the oxidation side 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, such an oxidation suppression temperature is considered to be about 350 ° C. to 400 ° C., and the oxidation lower limit temperature is considered to be about 250 ° C. to 300 ° C.

  FIG. 12 is a diagram for explaining the operation after shutdown of the conventional solid oxide fuel cell system. The upper part of FIG. 12 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 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 600 to 700 ° C. Further, as shown in the lower part (1) of FIG. 12, the fuel gas remaining without being used for power generation is ejected from the upper end of the fuel cell, and this ejected fuel gas is combusted at the upper end. Next, when the supply of the raw fuel gas, the reforming water, and the power generation air is stopped due to the shutdown stop, the flow rate of the ejected fuel gas decreases, as shown in the lower part (2) of FIG. The flame at the tip of the fuel cell disappears.

  As shown in the lower part (3) of FIG. 12, the pressure inside the fuel cell (fuel electrode side) is higher than the outside (air electrode side) even after the flame disappears after the shutdown is stopped. The ejection of fuel gas from the battery cell is continued. Also, as shown in the upper part of FIG. 12, 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 decreases as the pressure difference between the fuel electrode side and the air electrode side decreases (lower stage (4) (5) in FIG. 12).

  When about 5 to 6 hours have passed after shutdown, and when the temperature in the fuel cell module drops to the oxidation suppression temperature, both the fuel electrode side and the air electrode side of each fuel cell drop to almost atmospheric pressure, The pole side air begins to diffuse to the fuel pole side (lower stage (6) and (7) in FIG. 12). 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. Further, as shown in the lower part (8) of FIG. 12, 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 is filled with air. Never happen. 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 when the atmospheric oxidation of the fuel electrode is prevented, the present inventors have found a phenomenon that the fuel electrode is electrochemically oxidized by a mechanism different from the atmospheric oxidation.

Next, the electrochemical oxidation phenomenon of the fuel electrode will be described with reference to FIG.
FIG. 13 is an explanatory view schematically showing the relationship between the activation temperature of the fuel electrode, the electrolyte, and the air electrode constituting the fuel cell 16 and the stop 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 solid electrolyte layer 94 is formed of LSGM, and the air electrode 92a is formed of LSCF. ing. 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, the risk that the nickel contained in the first fuel electrode 90d and the second fuel electrode 90e is oxidized in the atmosphere increases. The risk of this atmospheric oxidation decreases as the temperature decreases. When the temperature in the fuel cell module 2 decreases to the nickel oxidation suppression temperature Te (about 350 ° C.), the risk decreases sufficiently. Disappear. In the present embodiment, the control is performed by setting the atmospheric oxidation safety temperature Ts to about 300 ° C.

  In this embodiment, the active state of the catalyst of the first fuel electrode 90d and the second fuel electrode 90e starts to decrease from about 450 ° C., and the lower limit temperature Ta of the fuel electrode, which is the lower limit temperature of the active state, is about 290 ° C. It is. The fuel electrode receives oxygen ions from the solid electrolyte layer 94 and reacts the received oxygen ions with fuel (hydrogen) to generate water and release electrons, but this catalytic action decreases with a decrease in temperature. When the temperature of the fuel electrode decreases to the activation lower limit temperature Ta, the ability is lost, and even if oxygen ions are received, they cannot react with the fuel and cause a power generation reaction.

On the other hand, the active state in which the solid electrolyte layer 94 can permeate oxygen ions from the air electrode to the fuel electrode starts to decrease from about 350 ° C., and its lower limit temperature Tb is about 150 ° C. The activity lower limit temperature Tb of the electrolyte is lower than the activity lower limit temperature Ta of the fuel electrode. The solid electrolyte layer 94 can permeate 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 solid electrolyte layer 94 becomes 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.
Further, the air electrode is in an active state in which oxygen in the air can be converted into oxygen ions above the lower limit activation temperature Tb of the solid electrolyte layer 94.

  In general, in order to enable power generation at a lower temperature, it is preferable that the activation temperatures of the fuel electrode, the solid electrolyte layer, and the air electrode of the fuel cell are low. Lowering 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. As the activation temperature is lowered as described above, the activation lower limit temperatures of the fuel electrode, the solid electrolyte layer, and the air electrode can be set lower than the oxidation inhibition temperature Te of nickel. Moreover, the activity minimum temperature of a solid electrolyte layer and an air electrode becomes lower than the activity minimum temperature of a fuel electrode.

  FIG. 13 shows the relationship between the lower limit temperature Ta of the fuel electrode and the lower limit temperature Tb of the electrolyte. As described above, the active state of the fuel electrode starts to decrease from about 450 ° C., and becomes inactive at the activation lower limit temperature Ta (about 290 ° C.). On the other hand, the active state of the solid electrolyte layer starts to decrease from about 350 ° C., and becomes inactive at the activity lower limit temperature Tb (about 150 ° C.). Therefore, from the power generation operating temperature to the lower limit temperature Tb of the solid electrolyte layer, the solid electrolyte layer 94 is in a state where oxygen ions can permeate the fuel electrode, but the active state of the fuel electrode starts to decrease from about 450 ° C. Yes. Therefore, in the temperature zone Tc (electrochemical oxidation generation temperature zone) from about 450 ° C. to the lower limit temperature Tb of the solid electrolyte layer, the solid electrolyte layer 94 transmits oxygen ions, but the oxygen ions that have passed through the fuel electrode. The ability to react with fuel (hydrogen) has been reduced.

FIG. 14 is a graph showing an electromotive force generated by a single fuel cell 16 in a state where hydrogen gas is supplied to the fuel electrode side and air is supplied to the air electrode side.
In the graph of FIG. 14, the horizontal axis indicates the temperature of the fuel cell 16 and the vertical axis indicates the electromotive force generated. The broken line in FIG. 14 indicates the electromotive force obtained by theoretical calculation assuming that the fuel electrode layer, the solid electrolyte layer, and the air electrode layer are all in an active state, and the solid line indicates the electromotive force measured by experiment. Indicates power. As is apparent from this graph, in the temperature range of about 450 ° C. or higher, the electromotive force calculated theoretically and the electromotive force obtained by the experiment agree relatively well. On the other hand, it can be seen that the electromotive force obtained by the experiment is lower than the theoretically calculated electromotive force as the temperature decreases. This is considered to be because the activity of the fuel electrode layer having the highest activation temperature among the fuel electrode layer, the solid electrolyte layer, and the air electrode layer is reduced and the ability to generate a power generation reaction is reduced.

Next, with reference to FIG. 15a and FIG. 15b, the behavior of the fuel cell when the temperature in the fuel cell module passes the electrochemical oxidation generation temperature range will be described.
As described above, in the temperature zone Tc, the solid electrolyte layer 94 and the air electrode 92a are in an active state, and oxygen ions can pass through the air electrode 92a and the solid electrolyte layer 94 to the fuel electrode, but the first fuel electrode 90d. The second fuel electrode 90e is in a state where the catalytic activity is lowered. Therefore, the ability of the fuel electrode to generate a power generation reaction that causes the supplied oxygen ions to react with the fuel (hydrogen) is reduced.

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. In nickel, nickel is in a state in which it hardly reacts with oxygen in the air.
However, even in a temperature range lower than the oxidation suppression temperature Te, when oxygen ions that have passed through the solid electrolyte layer 94 are supplied to the fuel electrode, the nickel in the fuel electrode becomes oxygen ions by an electrochemical oxidation reaction. Can be combined with nickel oxide. 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 active state of the fuel electrode is not lowered, 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 substantially occur.

  On the other hand, in the stopping process, since the extraction of electric power from the fuel cell module 2 is stopped, it has been considered that even if electric charges are generated by the electrochemical oxidation reaction, substantially no current flows. 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 the air electrode side becomes large. Oxygen ions easily flow. However, when the oxygen partial pressure difference is uniform in each part of the single fuel battery cell 16, the fuel electrode into which oxygen ions have flowed through the solid electrolyte layer 94 has the same potential. Almost no current flows.

  However, as described with reference to FIG. 12, when the time after the shutdown stops, air begins to gradually diffuse from the air electrode side to the fuel electrode side, so that the concentration of hydrogen remaining in the vicinity of the upper end of the fuel cell 16 is increased. descend. As a result, as shown in FIG. 15b, in one fuel battery cell 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 the oxygen partial pressure in the vicinity of the lower end portion is small. The difference is large.

  In this way, a large amount of electric charge is generated near the lower end of the fuel cell 16 due to the occurrence of many electrochemical oxidation reactions, and relatively less electric charge is generated near the upper end. As a result, a potential difference is generated in the fuel electrode of one fuel battery cell 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 proceeds significantly. In particular, in the vicinity of the lower end portion of each fuel battery cell 16 where the difference in oxygen partial pressure between the fuel electrode side and the air electrode side is large, a large amount of oxygen ions flow into the fuel electrode through the solid electrolyte layer 94, and a lot of electrochemical ions are generated in this portion. Oxidation reaction occurs.

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 solid 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 16, finally a minute crack is generated in the solid 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 found that the microstructure and volume of the fuel electrode changed, and tensile stress was applied to the solid electrolyte layer, and finally a minute crack was generated in the solid electrolyte layer, thereby damaging the fuel cell.

Next, the stop process in the solid oxide fuel cell system 1 according to the embodiment of the present invention will be described. This stopping step is configured for the purpose of sufficiently suppressing the deterioration of the fuel cell due to the above atmospheric oxidation and electrochemical oxidation.
8a and 8b are flowcharts showing a processing procedure when a normal stop process is executed in the solid oxide fuel cell system 1 according to the embodiment of the present invention. FIG. 9 is a time chart schematically showing an example of the stop behavior when the shutdown stop is executed. FIG. 10 is a diagram for explaining the control, the temperature in the fuel cell module, and the state of the tip of the fuel cell in time series when the stop process is executed.

The controller 110, which is a controller, executes the processes of the flowcharts shown in FIGS. 8a and 8b by using a built-in microprocessor, memory, and a program for operating them. In addition, a microprocessor, a memory, and the like that are operated by a built-in program function as a cell protection circuit 110a (FIG. 6) and a purge control circuit 110b (FIG. 6) that execute cell protection control.
First, in step S1 of FIG. 8a, it is determined whether or not the solid oxide fuel cell system 1 is in a power generation operation. If the power generation operation is being performed, the process proceeds to step S2. If the power generation operation is not being performed, the one-time processing of the flowcharts shown in FIGS. 8a and 8b is terminated.
Next, in step S2, it is determined whether the stop process to be executed is a normal stop. In the case of an abnormal stop, the one-time process of the flowchart shown in FIGS. 8a and 8b is terminated. In the case of a normal stop, the processing after step S3 in FIG. 8a is executed. In the present embodiment, “normal stop” refers to a stop based on a stop operation by the user of the solid oxide fuel cell system 1 or a periodic stop set in advance by a program built in the control unit 110. Corresponds to this.

  In step S3, the extraction of electric power from the fuel cell module 2 is stopped. In the example of the time chart shown in FIG. 9, the stop switch is operated by the user at the time t301 (the upper stage (1) in FIG. 10), and the power output from the fuel cell module 2 is zero.

  Next, in step S4, restart of the solid oxide fuel cell system 1 is prohibited. That is, if the solid oxide fuel cell system 1 is started again after the stop process is started in step S3 and before the process is terminated, the temperature state inside the fuel cell module 2 becomes inappropriate, and the fuel cell Since there is a possibility of deteriorating the cell 16, restart is prohibited.

  Further, in step S 5, the fuel supply amount and the reforming water supply amount are set to the minimum supply amount that can be supplied by the fuel flow rate adjustment unit 38 and the water flow rate adjustment unit 28. That is, during the stop process, the cell protection circuit 110a built in the control unit 110 executes cell protection control for supplying a smaller amount of raw fuel gas and water than during power generation operation. The supply amounts of the raw fuel gas and water are always maintained at constant values during the cell protection control. Further, the S / C, which is the ratio of the amount of steam supplied to the fuel supplied at this time, is about 3, which is about the same as the S / C during the power generation operation. Note that the S / C can be further increased during the stopping step. On the other hand, the power generation air (however, the power generation is completely stopped) is supplied at the maximum supply amount of the power generation air flow rate adjustment unit 45 as the cooling control after the stop operation at time t301 in FIG. . As a result, the air in the fuel cell module 2 (air electrode side of the fuel cell stack 14), the combustion gas of the remaining fuel, and the fuel that flows out from the fuel electrode side of the fuel cell stack 14 during the stop process (in FIG. 10) The upper stage (2)) is discharged from the fuel cell module 2. Further, the combustion of the fuel gas flowing out from the upper end of each fuel cell 16 gradually disappears when the supply amount of the raw fuel gas is minimized.

  Since the minimum amount of fuel supply is continued even after the stop operation at time t301 in FIG. 9, even if the power generation air is supplied, the air does not enter the fuel electrode side. Thereby, in the high temperature state during the stop process, the backflow of air to the fuel electrode side of each fuel cell 16 is suppressed, and atmospheric oxidation of the fuel electrode layer of each fuel cell 16 is prevented. In addition, even after the stop operation at time t301, the reforming unit 20c is still at a high temperature, so that a steam reforming reaction is generated by the supplied raw fuel gas and steam, and hydrogen gas is generated. Such a reforming reaction after the stop operation does not gradually occur as the temperature of the reforming portion 20c decreases, and unreformed raw fuel gas and water vapor are supplied to the fuel electrode side of each fuel cell 16. (Lower part of FIG. 10).

  Next, in step S6 of FIG. 8a, it is determined whether or not the temperature of the fuel cell stack 14 in the fuel cell module 2 has decreased to 350 ° C. When it falls to 350 degrees C or less, it progresses to step S7, and when not falling, the control state to step S5 is maintained. Here, when the temperature of the fuel cell stack 14 is in a temperature range of about 450 ° C. to about 150 ° C., the fuel electrode layer may be subjected to electrochemical oxidation (FIG. 13). However, in this embodiment, even after the stop operation at time t301 in FIG. 9, the supply of a constant amount of fuel continues, so the atmosphere on the fuel electrode side of each fuel cell 16 is constant, and the fuel concentration gradient Is suppressed. By suppressing the concentration gradient in each part on the fuel electrode side, the occurrence of a local battery phenomenon in each fuel cell 16 is suppressed. As a result, electrochemical oxidation of the fuel electrode layer of each fuel battery cell 16 is prevented (lower stage in FIG. 10). In addition, since the supply of water vapor is continued along with the supply of fuel, carbon deposition on the fuel electrode layer of the fuel cell 16 and the reforming catalyst in the reforming unit 20c is also prevented.

  In the present embodiment, the temperature detected by the thermocouple, which is a temperature sensor arranged in the vicinity of the fuel cell stack 14, is referred to as the temperature of the fuel cell stack 14. By referring to the temperature of the fuel cell stack 14, atmospheric oxidation and electrochemical oxidation of the fuel electrode layer of each fuel cell 16 can be reliably suppressed. On the other hand, the temperature of the reforming portion 20c changes greatly due to the steam reforming reaction, which is an endothermic reaction that occurs inside, so it is difficult to grasp the temperature of the fuel electrode layer, and the atmospheric oxidation and electrochemical oxidation of the fuel electrode layer are difficult to grasp. It is not suitable for suppression.

  In step S7, the cell protection circuit 110a sends a signal to the fuel flow rate adjustment unit 38 and the water flow rate adjustment unit 28, stops the supply of the raw fuel gas and water, and ends the cell protection control (time t302 in FIG. 9). In the present embodiment, the cell protection control is finished when the temperature of the fuel cell stack 14 drops to 350 ° C., but the temperature at which the cell protection control is finished depends on the structure of the fuel cell stack 14 and the like. Accordingly, a temperature range between about 350 ° C. and about 290 ° C. can be set. On the other hand, the supply of air to the air electrode side by the power generation air flow rate adjustment unit 45 is continued as concentration gradient correction control. That is, in the temperature range near 350 ° C. where the concentration gradient correction control is started, the risk of atmospheric oxidation of the fuel electrode layer of each fuel cell 16 is reduced, while the risk of electrochemical oxidation is increased. Yes. In such a state, when all the supply of raw fuel gas, water and air to the air electrode side is stopped, the raw fuel gas and water vapor remaining on the fuel electrode side of each fuel cell 16 are discharged from the upper end. While flowing out, air on the air electrode side gradually enters the fuel electrode side. In this state, the oxygen concentration of each part is different on the fuel electrode side of one fuel cell 16 (the oxygen concentration near the upper end of the fuel cell 16 is increased). A concentration gradient occurs inside. If this state is left unattended, a local cell phenomenon based on the concentration gradient occurs, resulting in the fuel electrode layer undergoing electrochemical oxidation.

  In the present embodiment, as the concentration gradient correction control, the supply of the raw fuel gas and water is stopped, and air is supplied to the air electrode side of the fuel cell 16. Thus, air is actively sent from the air electrode side of each fuel cell 16 to the fuel electrode side through the upper end of the fuel cell 16. In this way, when air is actively sent to the fuel electrode side, the raw fuel gas and water vapor remaining on the fuel electrode side are pushed toward the reformer 20 toward the upstream side, and each fuel is supplied. The fuel electrode side of the battery cell 16 becomes an air atmosphere (upper stage (3) → (4) in FIG. 10). Thereby, the oxygen concentration gradient on the fuel electrode side of each fuel cell 16 is corrected, and electrochemical oxidation of the fuel electrode layer is suppressed.

  However, when air is sent from the upper end of the fuel cell 16, the pressure in the fuel supply flow path from the reformer 20 to the fuel electrode side of the fuel cell 16 increases. For this reason, the air sent from the upper end of the fuel battery cell 16 makes the fuel electrode side of each fuel battery cell 16 an air atmosphere. This air passes through the manifold 66 and the fuel gas supply pipe 64 (FIG. 2). The reverse flow does not reach the reformer 20. Therefore, even during the concentration gradient correction control, the interior of the reformer 20 is maintained in a state where the raw fuel gas and water vapor are filled.

  Next, in step S8 of FIG. 8a, it is determined whether or not the temperature of the fuel cell stack 14 has decreased to 150.degree. If the temperature has decreased to 150 ° C. or lower, the process proceeds to step S9 (FIG. 8b) to end the concentration gradient correction control. If not, the control state up to step S7 is maintained. In the example shown in FIG. 9, the temperature of the fuel cell stack 14 has decreased to 150 ° C. at time t303. In the present embodiment, the concentration gradient correction control is terminated when the temperature of the fuel cell stack 14 is reduced to 150 ° C., but the temperature at which the concentration gradient correction control is terminated is the structure of the fuel cell stack 14. Depending on the temperature range, it can be set to a temperature range between about 200 ° C. and about 150 ° C.

  In step S9, the purge control circuit 110b of the control unit 110 resumes the supply of the raw fuel gas by the fuel flow rate adjustment unit 38 as the purge control. The fuel supply amount in the purge control is set to the minimum supply amount that can be supplied by the fuel flow rate adjustment unit 38. On the other hand, the supply of water by the water flow rate adjustment unit 28 is maintained in a stopped state, and the supply of air by the power generation air flow rate adjustment unit 45 is continued with the previous supply amount (time t303 in FIG. 9). As described above, the purge control circuit 110b built in the control unit 110 stops the supply of water to stop the generation of water vapor, and restarts the supply of the raw fuel gas, whereby the fuel of each fuel cell 16 is recovered. Water vapor remaining on the pole side is purged by the raw fuel gas (upper stage (5) in FIG. 10). This prevents water vapor remaining on the fuel electrode side of each fuel battery cell 16 from solidifying and adversely affecting the fuel electrode layer. When the temperature of the fuel cell 16 is in a temperature range between about 200 ° C. and about 150 ° C., the concentration gradient correction control is terminated and the purge control is executed, thereby generating electrochemical oxidation and dew condensation. Both prevention can be achieved. Further, in the state where the temperature of the fuel cell stack 14 is lowered to 150 ° C., carbon deposition does not occur even if only the raw fuel gas is supplied to each fuel cell 16.

Next, in step S10, it is determined whether or not 10 minutes have elapsed after the supply of the raw fuel gas is resumed. If 10 minutes have passed, the process proceeds to step S11. If not, the control state up to step S9 is maintained.
Next, in step S11, the supply of the raw fuel gas by the fuel flow rate adjustment unit 38 is stopped. On the other hand, the supply of air by the power generation air flow rate adjustment unit 45 is continued with the previous supply amount (time t304 in FIG. 9). As a result, the raw fuel gas remaining on the fuel electrode side of each fuel battery cell 16 gradually flows out to the air electrode side, and eventually all the raw fuel gas is replaced with air (the upper stage of FIG. 10). (6) to (7)). In this way, by stopping the supply of the raw fuel gas and continuing the supply of air, the raw fuel gas flowing out from the fuel electrode side of each fuel cell 16 to the air electrode side is supplied to the air electrode side. Purge the fuel cell module 2 together with the air.

Next, in step S12, it is determined whether or not 10 minutes have elapsed after the supply of the raw fuel gas is stopped. If 10 minutes have passed, the process proceeds to step S13, and if not, the control state up to step S11 is maintained.
Further, in step S13, it is determined whether or not the temperature of the fuel cell stack 14 has decreased to 90 ° C. When the temperature of the fuel cell stack 14 has decreased to 90 ° C., the process proceeds to step S14, and when it has not elapsed, the control state up to step S13 is maintained.

Next, in step S14, the supply of air by the power generation air flow rate adjustment unit 45 is stopped, and the supply of raw fuel gas, water, and power generation air is all stopped (time t305 in FIG. 9). As described above, in the state where the temperature of the fuel cell stack 14 is lowered to 90 ° C. or lower, the risk of the atmospheric oxidation and electrochemical oxidation of the fuel electrode layer is completely eliminated, and the solid oxide fuel cell is obtained. Even if the system 1 is restarted, each fuel cell 16 is not adversely affected.
Further, in step S15, the restart of the solid oxide fuel cell system 1 prohibited in step S4 of FIG. 8a is permitted, and one process of the flowcharts shown in FIGS. 8a and 8b is ended. Thereby, a stop process is completed.

  According to the solid oxide fuel cell system 1 of the embodiment of the present invention, as the cell protection control, a small amount of raw fuel gas and water are supplied until the temperature of the fuel cell 16 is lowered to 350 ° C. (FIG. 8a, steps S5 and S6, times t301 to t302 in FIG. 9). Further, as concentration gradient correction control, the air is caused to flow backward to the fuel electrode side by stopping the supply of fuel and water (steps S7 and S8 in FIG. 8a, times t302 to t303 in FIG. 9). As the purge control, after the temperature is lowered to 150 ° C., the raw fuel gas is supplied through the reforming unit 20a (steps S9 and S10 in FIG. 8a, times t303 to t304 in FIG. 9), and the fuel electrode of the fuel cell 16 Purging side. For this reason, atmosphere oxidation and electrochemical oxidation of the fuel electrode layer in the high temperature zone are suppressed by the cell protection control. Further, in a temperature range where the risk of atmospheric oxidation has decreased, the concentration gradient correction control causes air to flow backward to the fuel electrode side, and the residual gas on the fuel electrode side is pushed back toward the reforming portion 20c to air the fuel electrode side. Maintain the atmosphere. As a result, the fuel electrode side is promptly converted to an air atmosphere, so that the concentration gradient on the fuel electrode side is suppressed and the occurrence of “local cell phenomenon” is suppressed. Further, in a temperature range where the risk of electrochemical oxidation is reduced, purge control purges the reformer 20c and the fuel electrode side of the fuel cell, thereby preventing adverse effects of residual water vapor.

  Further, according to the solid oxide fuel cell system 1 of the present embodiment, the fuel electrode side of the fuel cell 16 is purged with the raw fuel gas (steps S9 and S10 in FIG. 8a, times t303 to t304 in FIG. 9). Therefore, it is not necessary to provide a device for supplying air to the fuel electrode side, and the cost of the solid oxide fuel cell system 1 can be reduced.

  Furthermore, in the solid oxide fuel cell system 1 of the present embodiment, the solid electrolyte layer made of LSGM and the air electrode layer made of LSCF maintain catalytic activity up to a relatively low temperature and allow oxygen ions to pass therethrough. Therefore, the operating temperature of the solid oxide fuel cell system 1 can be set low, and the power generation efficiency can be improved. On the other hand, since the fuel electrode layer mainly composed of nickel loses its catalytic activity at a temperature higher than that of LSGM or LSCF (FIG. 13), electrochemical oxidation becomes a problem as described above. According to the solid oxide fuel cell system 1 of this embodiment, after stopping the extraction of power (time t301 in FIG. 9), cell protection control (time t301 to t302), concentration gradient correction control (time t302 to t303). And purge control (time t303 to t304) are executed to effectively suppress the atmospheric oxidation and electrochemical oxidation of the fuel electrode, so that the highly efficient solid oxide form while suppressing the deterioration of the fuel cell 16 The fuel cell system 1 can be provided.

  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 reformer 20 is disposed inside the fuel cell module 2, and an evaporator that generates steam for steam reforming is provided inside the reformer 20, The present invention can be applied to a solid oxide fuel cell system including fuel cell modules having different configurations.

FIG. 11 is a front sectional view of a fuel cell module provided in a solid oxide fuel cell system according to a modification of the present invention.
As shown in FIG. 11, the fuel cell module 200 in this modification example distributes fuel gas to a module case 208, a plurality of fuel cells 216 accommodated in the module case 208, and these fuel cells 216. It has a manifold 266, a reformer 220 that is a reforming unit disposed above the fuel cell 216, and an air heat exchanger 222 that is a heat exchanger disposed above the module case 208. Further, the fuel cell module 200 includes a combustion catalyst 249 disposed above the air heat exchanger 222, an evaporator 224 that is an evaporation section disposed above the combustion catalyst 249, and a combustion catalyst 249. And a heater 250 arranged between the evaporator 224 and the heater 250 for heating them. Moreover, the heat insulating material 207 is arrange | positioned so that the above structure may be surrounded, The heat dissipation in the module case 208 is suppressed. Further, a heat insulating material that suppresses the movement of heat between the module case 208 and the air heat exchanger 222 and between the combustion catalyst 249 and the air heat exchanger 222 is also disposed.

  In this modification, the remaining fuel that is not used for power generation is combusted at the upper end of each fuel battery cell 216 and heats the reformer 220 disposed above. The high-temperature exhaust gas discharged from the module case 208 flows into the air heat exchanger 222 disposed outside the module case 208, and heats the supplied power generation air. The exhaust gas that has heated the power generation air flows into the combustion catalyst 249 and heats the combustion catalyst. Further, the exhaust gas that has heated the combustion catalyst flows into the evaporator 224 and heats the supplied water to generate water vapor.

  The generated water vapor is mixed with the raw fuel gas and flows into the reformer 220. The raw fuel gas is steam reformed in the reformer 220 and flows into the manifold 266. The fuel gas that has flowed into the manifold 266 flows into the fuel electrode inside each fuel cell 216 and flows upward in each fuel cell 216. On the other hand, the power generation air heated by the air heat exchanger 222 flows downward through an air passage (not shown) provided outside the module case 208 and is provided at the lower portion of the side wall surface of the module case 208. Injected from a plurality of outlets. The power generation air injected from the air outlet flows into the air electrode side (outside each fuel cell 216) in the module case 208, and a power generation reaction occurs between the fuel in each fuel cell 216. The remaining fuel that is not used for power generation is burned at the upper end of each fuel cell 216.

  The heater 250 is energized in the start-up process of the solid oxide fuel cell system, and heats the evaporator 224 and the combustion catalyst 249 in a low temperature state at the initial start-up. Thereby, since the temperature of the evaporator 224 and the combustion catalyst 249 can be raised from the initial stage of startup, the evaporator 224 can generate water vapor at an early stage, and the exhaust gas can be purified at an early stage by the combustion catalyst. Is possible. Further, the heater 250 is energized even in the stop process of the solid oxide fuel cell system, and after the power generation is stopped, the evaporator 224 and the combustion catalyst 249 whose temperature has decreased are heated. As a result, even after a lapse of time from the stop of power generation, steam can be generated in the evaporator 224 and exhaust gas can be purified by the combustion catalyst 249. In addition, when the temperature of the evaporator 224 and the combustion catalyst 249 sufficiently rises during the start-up process or the power generation operation, the energization to the heater 250 is stopped, and the evaporator 224 and the combustion catalyst 249 are only activated by the heat of the exhaust. Heated. The start-up of the solid oxide fuel cell system according to this modification is the same as that of the above-described embodiment of the present invention except for the above points.

On the other hand, the processing after the power generation stop in the present modification is also the same as that of the above-described embodiment of the present invention. However, when the temperature decreases after the power generation stops, the water in the evaporator 224 is surely evaporated. The heater 250 may be energized.
Further, in the above-described embodiment, as the fuel cell 16, one cell has one set of the fuel electrode and the air electrode. The present invention is applied to a fuel cell system using an arbitrary fuel cell such as a cylindrical horizontal stripe type fuel cell in which a plurality of air electrodes are formed and single cells formed by electrodes of each set are connected in series. can do.

DESCRIPTION OF SYMBOLS 1 Solid oxide fuel cell system 2 Fuel cell module 4 Auxiliary machine unit 7 Heat insulating material 8 Case 8a Communication opening 8b Falling wall 10 Power generation chamber 12 Fuel cell assembly 14 Fuel cell stack 16 Fuel cell 18 Combustion chamber (combustion) Part)
20 Reformer 20a Evaporating section (evaporating chamber)
20b Mixing part (pressure fluctuation absorbing means)
20c reforming part 21 current 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 adjustment unit for ignition 45 Air flow adjustment unit for power generation (oxidant gas supply device)
46 1st heater 48 2nd heater 49 Combustion catalyst 49a Catalyst 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 81 Heater 82 Exhaust gas discharge pipe 83 Ignition device 84 Fuel cell 85 Exhaust valve 86 Inner electrode terminal (cap)
90d First fuel electrode (fuel electrode layer)
90e Second fuel electrode (fuel electrode layer)
92a Air electrode (oxidant gas electrode layer)
94 Solid electrolyte layer 98 Fuel gas flow channel (throttle part)
110 Controller (Controller)
110a Cell protection circuit 110b Purge control 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 Air Temperature Sensor 200 Fuel Cell Module 207 Insulating Material 208 Module Case 216 Fuel Cell 220 Reformer (Reformer)
222 Heat exchanger for air (heat exchanger)
224 Evaporator (evaporator)
249 Combustion Catalyst 250 Heater 266 Manifold

Claims (3)

A solid oxide fuel cell system that generates electricity by reacting hydrogen obtained by steam reforming raw fuel gas and an oxidant gas,
In order from the inside, a fuel electrode layer mainly composed of nickel, a solid electrolyte layer, a cylindrical fuel battery cell provided with an oxidant gas electrode layer,
A reforming unit that generates hydrogen from raw fuel gas by steam reforming, and supplies the generated hydrogen to the fuel electrode side of the fuel cell from the lower end;
A fuel supply device for supplying raw fuel gas to the reforming section;
A water supply device for supplying water for steam reforming;
An evaporation unit that generates water vapor from the water supplied from the water supply device and supplies the water vapor to the reforming unit;
An oxidant gas supply device for supplying an oxidant gas to the oxidant gas electrode side of the fuel cell;
A fuel cell module containing the fuel cell, and
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 continues the operation of the oxidant gas supply device even after stopping the extraction of electric power from the fuel cell module, and lowers the temperature in the fuel cell module. On the other hand, after stopping the extraction of electric power, the controller Are programmed to perform cell protection control, concentration gradient correction control, and purge control in this order,
In the cell protection control, the fuel supply device and the fuel supply device are configured so that a smaller amount of raw fuel gas and water are supplied than in the power generation operation until the temperature of the fuel cell is lowered to a temperature range of 350 ° C. to 290 ° C. Continue the operation of the water supply device,
In the concentration gradient correction control, the operation of the oxidant gas supply device is continued while stopping the operation of the fuel supply device and the water supply device, so that the oxidant gas is supplied from the upper end of the fuel cell. Back to the pole side, the residual gas on the fuel electrode side is pushed back toward the reforming part to maintain the fuel electrode side in the oxidant gas atmosphere,
In the purge control, when the temperature of the fuel battery cell is lowered to a temperature range of 200 ° C. to 150 ° C., after the concentration gradient correction control is finished, gas is supplied through the reforming unit, and the fuel battery cell Purging the fuel electrode side of the solid oxide fuel cell system   2. The solid oxide fuel cell system according to claim 1, wherein in the purge control, the controller operates the fuel supply device to supply only the raw fuel gas to the reforming unit.   The solid oxide fuel cell system according to claim 2, wherein the solid electrolyte layer is made of LSGM, and the oxidant gas electrode layer is made of LSCF.

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