JP2022160055A - Solid oxide fuel cell system and method for stopping solid oxide fuel cell system - Google Patents

Abstract

To obtain a highly reliable solid oxide fuel cell system.SOLUTION: A solid oxide fuel cell system includes a reformer 5 that produces reformed gas Ff2, a solid oxide fuel cell stack 2 that outputs DC power through electrochemical reaction between air Fa1 and the reformed gas Ff2, a water vapor generator 9 that generates water vapor Fs2, a recycling circuit 32 that circulates recycling off gas Ff32, and a control unit 4 that coordinated-controls devices in the system. When the system is stopped, the control unit 4 is configured to continue the circulation in a state in which an anode 2a is kept at positive pressure by generating the water vapor Fs2 until the stack temperature T drops to the allowable temperature Tp after the DC power output and the supply of the fuel gas Ff1 are shut off.SELECTED DRAWING: Figure 1

Description

The present application relates to a solid oxide fuel cell system and a method for stopping the solid oxide fuel cell system.

A fuel cell is used in a form called a stack in which many layers of single cells are stacked with an electrolyte sandwiched between a fuel electrode (hereinafter referred to as an anode) and an air electrode (hereinafter referred to as a cathode). . Among fuel cells, a solid oxide fuel cell (SOFC) uses a solid oxide with high ion conductivity such as Yttria Stabilized Zirconia (YSZ) as an electrolyte. Also, the porous ceramic and metal such as nickel (Ni) function as an anode (strictly speaking, the Ni/YSZ interface portion). And it operates at a temperature of 600° C. to 1000° C., which is higher than other fuel cells.

In a solid oxide fuel cell system that uses a hydrocarbon-based fuel, an electrochemical reaction occurs between a hydrogen-containing reformed gas and an oxidant separated by an electrolyte in the stack between the anode and the cathode. to generate electricity. At that time, by providing a recycling circuit that recirculates the gas discharged from the anode (hereinafter referred to as anode off-gas) to the raw fuel side, it becomes possible to reuse the anode off-gas that has not been consumed by the anode, improving the system efficiency. I can expect it.

When such a solid oxide fuel cell system is stopped, the introduction of the reformed gas into the stack is cut off, causing gas to flow backward from the downstream side of the cathode to the anode side, or from the cathode side to the anode side via the electrolyte. The anode may be exposed to an oxidizing atmosphere, such as oxygen diffusion. If this occurs at high temperatures, the anode can oxidize, resulting in stack degradation or even failure. Therefore, a method has been devised to maintain the anode in a reducing atmosphere state by continuing the recycling of the anode off-gas by a recycling circuit after cutting off the raw fuel when the system is stopped (see Reference 1, for example).

Japanese Patent Application Laid-Open No. 2013-243060 (paragraphs 0035 to 0037, FIGS. 1 and 3)

However, as described above, the operating temperature of the SOFC is high, and due to the decrease in temperature until the anode is not affected by oxidation, the inside of the gas system communicating with the anode becomes in a negative pressure state. Therefore, if the anode off-gas is simply circulated, outside air enters the system, and oxygen contained in the outside air oxidizes the anode.

The present application discloses a technique for solving the above problems, and maintains a reducing atmosphere until the temperature of the anode reaches a temperature where it is not affected by oxidation, and realizes a highly reliable solid oxide fuel cell system. with the aim of obtaining

The solid oxide fuel cell system disclosed in the present application includes a reformer that reacts a hydrocarbon-containing fuel with water vapor to produce a hydrogen-containing reformed gas, and an electrochemical reaction between air and the reformed gas. A solid oxide fuel cell stack that generates electrical energy and outputs DC power, a steam generator that generates steam, and a recycling that circulates the anode off-gas discharged from the anode of the stack back to the reformer. A control unit that cooperatively controls a circuit and devices in the system, and the control unit cuts off the output of the DC power and the supply of the fuel when the system is stopped, and then the temperature of the stack drops to an allowable temperature. Until then, the circulation is continued while the positive pressure is maintained at the anode due to the generation of water vapor.

A method for stopping a solid oxide fuel cell system disclosed in the present application includes a reformer that reacts fuel with water vapor to produce a reformed gas, and a solid fuel cell that generates electricity through an electrochemical reaction between air and the reformed gas. A method for stopping a solid oxide fuel cell system comprising a stack of oxide fuel cells and a recycling circuit for circulating the anode off-gas discharged from the anode of the stack back to the reformer, the method comprising: and, during the step of stopping the supply of the fuel and until the temperature of the stack decreases to an allowable temperature, the circulation is continued while the anode is maintained at a positive pressure due to the generation of the water vapor, and the and a cooling step of cooling the stack.

According to the solid oxide fuel cell system disclosed in the present application or its shutdown method, positive pressure is maintained until the temperature of the anode reaches a temperature at which it is not affected by oxidation, thereby preventing outside air from entering and maintaining a reducing atmosphere. Therefore, a highly reliable solid oxide fuel cell system can be obtained.

1 is a flowchart showing the configuration of a solid oxide fuel cell system according to Embodiment 1; FIG. 4 is a flowchart showing control operations in the solid oxide fuel cell system according to Embodiment 1; BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram showing a hardware configuration of a controller or an arithmetic execution part for executing control of a solid oxide fuel cell system according to Embodiment 1 and subsequent embodiments; FIG. 6 is a flow diagram showing the configuration of a solid oxide fuel cell system according to Embodiment 2; FIG. 10 is a flowchart showing the configuration of a solid oxide fuel cell system according to Embodiment 3;

Hereinafter, with reference to the accompanying drawings, a detailed description of each embodiment of the solid oxide fuel cell system disclosed by the present application will be given. In addition, the embodiment shown below is an example, and this application is not limited by these embodiments.

Embodiment 1.
1 to 3 are for explaining the configuration of the solid oxide fuel cell system according to Embodiment 1 and the control operation including the stop method thereof. FIG. 1 shows the solid oxide fuel cell system. FIG. 2 is a flow chart for explaining the control operation in stopping, that is, the method for stopping the solid oxide fuel cell system. In addition, FIG. 3 shows the hardware configuration of the control unit for controlling the solid oxide fuel cell system according to the first embodiment and each subsequent embodiment, or the arithmetic execution unit for executing the control. It is a block diagram.

Before describing the configuration characteristic of the solid oxide fuel cell system of the present application, using a diagram showing the configuration of the solid oxide fuel cell system according to Embodiment 1, general solid oxide fuel The configuration and operation common to the battery system will be described. As shown in FIG. 1, the solid oxide fuel cell system 1 includes, as main components, a reformer 5 that reforms the fuel gas Ff1, and an electrochemical reaction between the reformed gas Ff2 and air Fa1 that is an oxidant. and includes a stack 2 that generates power for use by a load 99 . It also has a control unit 4 for cooperatively controlling equipment for supplying or processing raw materials such as fuel gas Ff1, air Fa1, water Fs1, etc. necessary for operating the main constituent equipment, and for each equipment in the system. ing.

The stack 2 is formed by stacking single cells each having an electrolyte 2e sandwiched between a cathode 2c and an anode 2a and using battery members such as flow paths and separators (not shown). The reformed gas Ff2 and air Fa1 supplied to the stack 2 are discharged from the stack 2 as an anode off-gas Ff3 and a cathode off-gas Fa2 after hydrogen, oxygen, etc. are consumed in an electrochemical reaction as will be described later. A part of the anode offgas Ff3 passes through the anode offgas combustion circuit 31 to the combustor 6, and the rest passes through the recycle circuit 32 to the upstream side of the reformer 5 for recirculation. Cathode offgas Fa<b>2 is supplied to combustor 6 .

The reformer 5 is equipped with a catalyst and the like inside, and performs a reforming reaction using the fuel gas Ff1 and water vapor Fs2 that have passed through the desulfurizer 7 and the recycle offgas Ff32 branched from the anode offgas Ff3 as raw materials. A reformed gas Ff2 containing a large amount of hydrogen is generated. The catalyst of the reformer 5 is heated by the heat generated by the combustor 6 .

The combustor 6 combusts the combustion offgas Ff31 branched from the anode offgas Ff3 and supplied through the anode offgas combustion circuit 31 having the blower 12 and the cathode offgas Fa2 to generate combustion gas. The combustion gas raises the temperature of the catalyst in the reformer 5, exchanges heat with the air Fa1 in the heat exchanger 8, and is then discharged outside as exhaust gas Fx. Ignition is also performed by an ignition transformer located within the combustor 6 .

The blower 12 supplies the combustion off-gas Ff31 to the combustor 6 . Further, when differential pressure control of the stack 2 is required, the flow rate ratio of the combustion off-gas Ff31 and the recycling off-gas Ff32 is changed by controlling the operation of the blower 12 to control the flow rate of the reformed gas Ff2 flowing through the anode 2a. to control the differential pressure between the anode 2a and the cathode 2c. If differential pressure control of the stack 2 is unnecessary, the blower 12 may be omitted and a valve may be installed. The blower 13 has a function of circulating the recycle off-gas Ff32 upstream of the reformer 5, and controls the amount of recycle. The fuel gas cutoff valve 15 cuts off the supply of the fuel gas Ff1. A water vapor cutoff valve 16 cuts off the supply of water vapor.

The desulfurizer 7 removes sulfur components in the fuel gas Ff1 when the fuel gas Ff1 contains sulfur components such as city gas. This may be omitted if fuel gas Ff1 containing no sulfur component such as methane is used. Further, as the fuel gas Ff1, not only methane but also other hydrocarbon-based gases can be applied as long as they can be reformed by the reformer 5. FIG.

The heat exchanger 8 exchanges heat between the air Fa1 before being supplied to the cathode 2c and the exhaust gas Fx discharged from the reformer 5 to increase the temperature of the air Fa1 to be supplied to the cathode 2c. The steam generator 9 performs heat exchange between the anode offgas Ff3 and the water Fs1 to generate steam Fs2 from the water Fs1. Conversely, since the anode off-gas Ff3 is deprived of heat, the temperature of the anode off-gas Ff3 decreases.

The heat exchanger 10 further cools the anode off-gas Ff3 and adjusts the moisture contained in the anode off-gas Ff3. A part of the water vapor contained in the anode off-gas Ff3 is condensed by the heat exchanger 10, and the condensed water is returned to the water supply device 19 via a gas-liquid separator (not shown), and converted into the water Fs1 supplied to the water vapor generator 9. used. Part of the water vapor remaining in the anode offgas Ff3 passes through the recycling circuit 32 via the blower 13 and is used as the raw material of the reformer 5 (recycled offgas Ff32).

Since the temperature of the anode off-gas Ff3 is lowered by the heat exchanger 10 in this manner, the blowers 12 and 13 can be operated below the heat-resistant temperature of general blowers (for example, 80° C.). For example, the circulation blower used in Patent Literature 1 must withstand the high temperature of the anode off-gas, cannot use a general blower, and requires a hard-to-find high-temperature fuel gas blower.

Next, the operation of the solid oxide fuel cell system during power generation will be described. During system power generation, the fuel gas cutoff valve 15 and the water vapor cutoff valve 16 are open. As a result, the fuel gas Ff1 and steam Fs2 are supplied to the reformer 5, and the reformed gas Ff2 containing a large amount of hydrogen is supplied to the anode 2a. Air Fa1 passes through heat exchanger 8 and is supplied to cathode 2c. As a result, the oxygen contained in the air Fa1 and the hydrogen in the reformed gas Ff2 are consumed by the electrode reactions represented by the formulas (1) and (2) at the cathode 2c and the anode 2a, respectively. Electron transfer (DC power) due to this reaction is supplied to the load 99 .

1/2 O ₂ +2e ⁻ → O ²⁻ (1)
H ₂ +O ²⁻ → H ₂ O+2e ⁻ (2)

At the anode 2a, hydrogen corresponding to the amount of electron transfer is consumed by the electrode reaction, and the hydrogen partial pressure decreases toward the anode outlet, but the same amount of water is produced and the water vapor partial pressure increases. On the other hand, at the cathode 2c, the oxygen corresponding to the electron transfer is consumed by the electrode reaction, and the gas flow rate and the oxygen partial pressure decrease toward the cathode outlet. In the case of the SOFC, internal reforming is possible in which the electrode reaction and the reforming reaction proceed simultaneously at the anode 2a. can be made

After passing through the anode 2a, the reformed gas Ff2 becomes the anode off-gas Ff3 having the composition described above, and heat is removed by the steam generator 9 and the heat exchanger 10. FIG. Then, part of it is led to the combustor 6 as combustion off-gas Ff31 by the blower 12 and used for combustion. The remainder is sent to the inlet side of the reformer 5 as the recycle off-gas Ff32 by the blower 13 and reused as the raw material of the reformed gas Ff2. The blower 12 is used for differential pressure adjustment of the stack as required.

Based on the configuration and basic operation described above, the solid oxide fuel cell system 1 of the present application will be described. In the solid oxide fuel cell system 1 according to Embodiment 1 of the present application, which is common to the solid oxide fuel cell systems 1 according to the following embodiments, the atmosphere of the gas system on the anode 2a side at the time of stopping is characterized by a configuration having a function of controlling the

The operation of such a system when it is stopped, that is, the method of stopping the solid oxide fuel cell system 1 executed by the control unit 4 will be described with reference to the flowchart of FIG. When the system is stopped, the current output to the load 99 is shut off (step S100), and the fuel gas shutoff valve 15 is closed to shut off the fuel gas Ff1 (step S110). Then, the blower 12 is stopped to stop the supply of the combustion off-gas Ff31 to the combustor 6 (step S120). After that, while continuing to supply the water vapor Fs2 to maintain the positive pressure of the anode 2a with respect to the outside air, the operation of the blower 13 is continued, and the recycling by the recycling circuit 32 is continued (step S130).

By stopping the blower 12, the supply of the combustion off-gas Ff31 to the combustor 6 is interrupted and cut off. On the other hand, in order to cool the stack 2, the supply of air Fa1 to the cathode 2c is continued (step S140). At this time, it is desirable to supply, for example, the maximum amount of air Fa1 for cooling, and if a blower (not shown) is provided as an air source, its output is maximized. On the other hand, if equipment such as compressed air piping is provided at the system installation site as an air source, a flow control valve (not shown) is provided to maximize the opening of the valve. Of course, it is not always necessary to maximize the output, and it may be controlled to an appropriate ratio as necessary. In either case, it is controlled by the controller 4 .

Also, part of the water vapor in the anode off-gas Ff3 is condensed in the heat exchanger 10 and discharged to the outside of the closed circuit. However, new steam Fs2 is supplied from the steam generator 9 to the inlet of the reformer 5, and the steam Fs2 circulates as a reducing gas together with the recycle offgas Ff32, thereby always keeping the circuit of the recycle offgas Ff32 at a positive pressure. becomes possible. In this state, the temperature of the stack 2 (stack temperature T) is monitored until the anode 2a is cooled below the permissible temperature Tp (for example, about 300° C.) at which the anode 2a is considered not to be affected by oxidation (in step S200 "No"), the reducing gas supply to the anode 2a is maintained while maintaining positive pressure.

When the stack temperature T drops below the permissible temperature Tp ("yes" in step S200), the blower 12 is temporarily activated to guide the anode offgas Ff3 to, for example, the combustor 6 and release it to the outside as the exhaust gas Fx. (step S210). Then, the water vapor cutoff valve 16 in the water vapor supply circuit 33 is closed to stop the supply of the water vapor Fs2 (step S220). Thereafter, the recycling circuit 32 (including the anode offgas combustion circuit 31) through which the anode offgas Ff3 flows is replaced with non-flammable gas such as air or inert gas (step S230).

As described above, when the supply of the water vapor Fs2 is stopped after the anode off-gas Ff3 is discharged, the flow path leading to the anode 2a including the recycling circuit 32 is almost filled with the water vapor Fs2, and there is no combustible gas. Become. However, since the pressure becomes negative as the temperature drops, if, for example, a path leading to the outside air is opened, the air will naturally replace the air without using any special power. If an inert gas source is connected to that path, it will be replaced with an inert gas.

This makes it possible to maintain a stable stopped state. After that, the blower 13 is stopped and the recycling by the recycling circuit 32 is stopped (step S240).

With the configuration and control as described above, when the system is stopped, the supply of the fuel gas Ff1 is cut off, and the positive pressure is maintained to maintain the reducing atmosphere of the anode 2a. 2 can be cooled and the system can be stopped. In addition, no combustible gas remains in the recycle circuit 32, the anode 2a of the stack 2, and the anode off-gas Ff3 after the system is stopped, which facilitates handling during storage. For example, the stack 2 can be taken out of the system and inspected without special processing.

That is, by introducing water vapor Fs2 into the recycle circuit 32 when the system is stopped, the reducing atmosphere is maintained while maintaining positive pressure and suppressing outside air from entering the anode 2a until the stack temperature T drops to the allowable temperature Tp. It became possible. As a result, it is possible to obtain the solid oxide fuel cell system 1 in which the temperature of the stack 2 can be lowered and stopped without deterioration or damage.

Furthermore, before stopping recycling after the stack temperature T drops below the allowable temperature Tp, the blower 12 that supplies the combustion off-gas Ff31 in the recycling circuit 32 to the combustor 6 is operated to to release combustible components. At this time, since the recycle circuit 32 is maintained at a positive pressure by replenishment with the water vapor Fs2, the partial pressure of the combustible gas is lower than in the case of replenishing the combustible gas, and the labor for treating the combustible components is reduced. As a result, no combustible gas remains in the recycling circuit 32 while it is stopped, and handling during storage is facilitated. In addition, the blower 12 that supplies the combustion off-gas Ff31 to the combustor 6 can also be used to maintain the differential pressure of the stack 2 during system operation, making it possible to improve the reliability of stack operation.

In the first embodiment and the following embodiments, when software using a microcomputer constitutes the execution part of the arithmetic processing, the control unit 4 includes a processor 401 and a storage device 402 as shown in FIG. It is also conceivable to configure a single microcomputer 400 having The storage device 402 includes a volatile storage device such as a random access memory and a non-volatile auxiliary storage device such as a flash memory (not shown).

Also, an auxiliary storage device such as a hard disk may be provided instead of the flash memory. Processor 401 executes a program input from storage device 402 . In this case, the program is input from the auxiliary storage device to the processor 401 via the volatile storage device. Further, the processor 401 may output data such as calculation results to the volatile storage device of the storage device 402, or may store the data in an auxiliary storage device via the volatile storage device.

Embodiment 2.
In the first embodiment, an example has been described in which cooling of the anode off-gas and adjustment of the water content are performed by heat exchange with cooling water. In the second embodiment, an example in which air, which is an oxidizing agent, is used instead of cooling water will be described. Other configurations are the same as those of the first embodiment. FIG. 4 is a schematic flow diagram for explaining the configuration of the solid oxide fuel cell system according to the second embodiment. The configuration and operation other than the heat exchange between the air and the anode off-gas are the same as those described in the first embodiment. FIG. 3 is used.

As shown in FIG. 4, the solid oxide fuel cell system 1 according to the second embodiment has a heat exchanger for heat exchange between the air Fa1 and the anode offgas Ff3 instead of the heat exchanger 10 described in the first embodiment. An exchanger 11 is provided. That is, the anode offgas Ff3 is cooled and moisture content is adjusted by exchanging heat with the air Fa1, and other configurations are the same as those described in the first embodiment.

The operation when the solid oxide fuel cell system 1 according to the second embodiment is stopped, that is, the method of stopping the solid oxide fuel cell system 1 executed by the control unit 4 will be described with reference to the flowchart of FIG. do. When the system is stopped, the current output to the load 99 is shut off (step S100), and the fuel gas shutoff valve 15 is closed to shut off the fuel gas Ff1 (step S110). Then, the blower 12 is stopped to stop the supply of the combustion off-gas Ff31 to the combustor 6 (step S120). After that, while continuing to supply the water vapor Fs2 to maintain the positive pressure of the anode 2a with respect to the outside air, the operation of the blower 13 is continued, and the recycling by the recycling circuit 32 is continued (step S130).

By stopping the blower 12, the supply of the combustion off-gas Ff31 to the combustor 6 is interrupted and cut off. On the other hand, the supply of air Fa1 to the cathode 2c is continued in order to cool the stack 2, cool the anode offgas Ff3, and adjust the water content (step S140). At this time, at least for cooling the stack 2, it is desirable to supply the maximum amount of air Fa1.

Here, in the reforming reaction, it is necessary to keep the ratio of carbon and moisture in the gas within a certain range. is an important control item. However, during power generation of the stack 2, unless the air utilization rate and fuel utilization rate are changed, the flow rate ratio of the air Fa1 and the fuel gas Ff1 is constant regardless of the power supplied to the load 99, and the heat exchanger 11 The flow rate ratio between the air Fa1 and the anode off-gas Ff3 at is also constant. That is, unless the temperature of the air Fa1 changes significantly compared to the temperature difference from the anode offgas Ff3, the moisture content of the recycle offgas Ff32 is kept substantially constant.

On the other hand, if only the flow rate of the air Fa1 is increased in step S140, the flow rate of the air Fa1 with respect to the anode offgas Ff3 becomes excessive, and the moisture content of the recycle offgas Ff32 becomes lower than normal.

However, as described in Embodiment 1, new steam Fs2 is supplied from the steam generator 9 to the inlet of the reformer 5, and the steam Fs2 is circulated as reducing gas together with the recycle offgas Ff32. Therefore, the circuit of the recycle offgas Ff32 can always be kept at a positive pressure, and an excessive decrease in the water content can be suppressed. In this state, the stack temperature T is monitored, and the supply of the reducing gas to the anode 2a is maintained while maintaining the positive pressure until the stack temperature T drops to the allowable temperature Tp ("No" in step S200).

When the stack temperature T drops below the permissible temperature Tp ("yes" in step S200), the blower 12 is temporarily activated to guide the anode offgas Ff3 to, for example, the combustor 6 and release it to the outside as the exhaust gas Fx. (step S210). Then, the water vapor cutoff valve 16 in the water vapor supply circuit 33 is closed to stop the supply of the water vapor Fs2 (step S220).

Thereafter, the recycling circuit 32 (including the anode offgas combustion circuit 31) through which the anode offgas Ff3 flows is replaced with non-flammable gas such as air or inert gas (step S230). This makes it possible to maintain a stopped state in which there is no need to worry about combustible gases. Then, the blower 13 is stopped and the recycling by the recycling circuit 32 is stopped (step S240).

By using the heat exchanger 11 that exchanges heat between the air Fa1 and the anode offgas Ff3, the system of the cooling water Fc (the cooling water device 18 and the heat exchanger 10) can be omitted. Furthermore, as in the first embodiment, when the stack 2 is stopped, the positive pressure is maintained while the supply of the fuel gas Ff1 is cut off, and the reducing atmosphere of the anode 2a is maintained. The temperature of the stack 2 can be lowered as soon as possible, and the system can be stopped in a state that facilitates storage. In addition, no combustible gas remains in the recycling circuit 32 after the stack is stopped, and handling during storage is also good.

Embodiment 3.
In Embodiments 1 and 2 above, examples were described in which an additional heat exchanger was used in addition to the steam generator for cooling the anode off-gas and adjusting the water content. In the third embodiment, an example in which the anode off-gas is cooled and the water content is adjusted only by the water vapor generator will be described. Other configurations are the same as those of the first and second embodiments. FIG. 5 is a schematic flow diagram showing the configuration of a solid oxide fuel cell system according to Embodiment 3. FIG. The configuration and operation other than the heat exchange of the anode off-gas are the same as those described in Embodiments 1 and 2, and the description of the same parts is omitted, and FIGS. 3.

The solid oxide fuel cell system 1 according to Embodiment 3, as shown in FIG. It is designed to cool Ff3 and adjust the water content, and other configurations are the same as those described in the first and second embodiments.

The operation when the solid oxide fuel cell system 1 according to Embodiment 3 is stopped, that is, the method of stopping the solid oxide fuel cell system 1 executed by the control unit 4 will be described with reference to the flowchart of FIG. . When the system is stopped, the current output to the load 99 is shut off (step S100), and the fuel gas shutoff valve 15 is closed to shut off the fuel gas Ff1 (step S110). Then, the blower 12 is stopped to stop the supply of the combustion off-gas Ff31 to the combustor 6 (step S120). After that, while continuing to supply the water vapor Fs2 to maintain the positive pressure of the anode 2a with respect to the outside air, the operation of the blower 13 is continued, and the recycling by the recycling circuit 32 is continued (step S130).

By stopping the blower 12, the supply of the combustion off-gas Ff31 to the combustor 6 is interrupted and cut off. On the other hand, in order to cool the stack 2, the supply of air Fa1 to the cathode 2c is continued (step S140). At this time, it is desirable to supply the maximum amount of air Fa1 for cooling the stack 2 .

Also, part of the water vapor in the anode off-gas Ff3 is condensed in the water vapor generator 9, which is a heat exchanger, and discharged to the outside of the closed circuit. However, new steam Fs2 is supplied from the steam generator 9 to the inlet of the reformer 5, and the steam Fs2 circulates as a reducing gas together with the recycle offgas Ff32, thereby always keeping the circuit of the recycle offgas Ff32 at a positive pressure. becomes possible. Also, by heat exchange with the anode off-gas Ff3 having the same temperature as the stack temperature T, high-pressure water vapor Fs2 is obtained.

In this state, the stack temperature T is monitored, and the supply of the reducing gas to the anode 2a is maintained while maintaining the positive pressure until the stack temperature T drops to the allowable temperature Tp ("No" in step S200). When the stack temperature T drops below the permissible temperature Tp ("yes" in step S200), the blower 12 is temporarily activated to guide the anode offgas Ff3 to, for example, the combustor 6 and release it to the outside as the exhaust gas Fx. (step S210). Then, the water vapor cutoff valve 16 in the water vapor supply circuit 33 is closed to stop the supply of the water vapor Fs2 (step S220).

Thereafter, the recycling circuit 32 (including the anode offgas combustion circuit 31) through which the anode offgas Ff3 flows is replaced with non-flammable gas such as air or inert gas (step S230). As a result, it is possible to maintain a stopped state in which storage can be easily carried out without worrying about combustible gas. Then, the blower 13 is stopped and the recycling by the recycling circuit 32 is stopped (step S240).

With the configuration and control as described above, cooling and water content adjustment of the anode offgas Ff3 are performed by the water vapor generator 9 instead of the heat exchangers 10 and 11, which will be described in the first and second embodiments. additional cooling system can be omitted. Furthermore, as in the first embodiment, when the stack 2 is stopped, the positive pressure is maintained while the supply of the fuel gas Ff1 is cut off, and the reducing atmosphere of the anode 2a is maintained. The temperature of the stack 2 can be lowered in a short period of time, and the system can be stopped without special handling. In addition, no combustible gas remains in the recycle circuit 32 after the stack is stopped, so handling during storage is improved.

It should be noted that although the present application has described exemplary embodiments, the various features, aspects, and functions described in the embodiments are not limited to application of particular embodiments, but alone. , or in various combinations. Accordingly, numerous variations not illustrated are envisioned within the scope of the technology disclosed herein. For example, modifications, additions, or omissions of at least one component shall be included.

For example, an example in which the blower 12 is used as a flow controller for controlling the supply/shutoff of the combustion off-gas Ff31 is shown, but the present invention is not limited to this. good too. In particular, by providing a shut-off valve, it becomes unnecessary to use a blower with high shut-off performance.

As described above, according to the solid oxide fuel cell system 1 of the present application, the reformer 5 that reacts the hydrocarbon-containing fuel (fuel gas Ff1) with the water vapor Fs2 to produce the hydrogen-containing reformed gas Ff2 , a solid oxide fuel cell stack 2 that generates electrical energy through an electrochemical reaction between the air Fa1 and the reformed gas Ff2 and outputs DC power, a water vapor generator 9 that generates water vapor Fs2, and an anode 2a of the stack 2. A recycling circuit 32 (including a blower 13) that circulates the anode offgas Ff3 (strictly speaking, a recycle offgas Ff32) discharged from the reformer 5 back to the reformer 5, and a control unit 4 that cooperatively controls devices in the system. When the system is stopped, the controller 4 cuts off the output of DC power and the supply of fuel (fuel gas Ff1) until the temperature of the stack 2 (stack temperature T) drops to the allowable temperature Tp. is generated, the circulation is continued while the anode 2a is maintained at a positive pressure (relative to the outside air). By maintaining the positive pressure until the anode 2a reaches a temperature (permissible temperature Tp) at which the anode 2a is not oxidized, the inside of the anode 2a can be kept in a reducing atmosphere by preventing the intrusion of outside air. A physical fuel cell system 1 can be obtained.

If the controller 4 is configured to increase the flow rate of the air Fa1 supplied to the cathode 2c until the temperature of the stack 2 (stack temperature T) drops to the allowable temperature Tp, the time required for cooling can be shortened. can.

If the water vapor generator 9 is configured to generate water vapor Fs2 by heat exchange with the anode offgas Ff3, the thermal efficiency will be increased and heating for generating the water vapor Fs2 will not be required during the cooling period.

A combustor 6 that burns a part of the anode offgas Ff3 (combustion offgas Ff31) to heat the reformer 5 is provided, and the controller 4 controls the temperature of the stack (stack temperature T) until the temperature of the stack (stack temperature T) drops to the allowable temperature Tp. During this period, the flow of the anode offgas Ff3 (combustion offgas Ff31) to the combustor 6 is stopped, and after the temperature drops to the permissible temperature Tp, the gas circulating in the recycling circuit 32 via the combustor 6 is discharged outside the system. If the circulating gas is replaced with air or an inert gas, no combustible gas remains, which facilitates handling during storage.

The control unit 4 controls the flow rate of the anode offgas Ff3 (combustion offgas Ff31) to the combustor 6 so that the differential pressure between the anode 2a and the cathode 2c (in the stack 2) falls within a predetermined range during system operation. If controlled, damage due to differential pressure can be prevented.

Since the heat exchangers 10 and 11 (and the water vapor generator 9) for adjusting the temperature and moisture content of the anode offgas Ff3 are provided, thermal efficiency is increased.

In this case, if the heat exchanger 11 that exchanges heat between the anode offgas Ff3 and the air Fa1 is used as the heat exchanger, the flow rate ratio between the anode offgas Ff3 and the air Fa1 is kept constant, so special control is required. Even without it, the temperature and water content of the anode off-gas Ff3 can be maintained at constant levels.

As described above, according to the method of stopping the solid oxide fuel cell system 1 of the present application, the reformer 5 that causes the fuel (fuel gas Ff1) to react with the water vapor Fs2 to generate the reformed gas Ff2, the air Fa1 and the reformed gas Ff2, and the anode offgas Ff3 (of which, the recycle offgas Ff32) discharged from the anode 2a of the solid oxide fuel cell stack 2 that generates power through an electrochemical reaction with the reformer 5 A method of stopping the solid oxide fuel cell system 1 including the recycling circuit 32 (and the blower 13) that circulates back to S110), and until the temperature of the stack 2 (stack temperature T) drops to the permissible temperature Tp, the generation of water vapor Fs2 keeps the anode 2a at a positive pressure (relative to the outside air) while continuing circulation. and cooling the stack 2 (steps S130 to S200). By maintaining the positive pressure until the anode 2a reaches a temperature (permissible temperature Tp) at which the anode 2a is not oxidized, the inside of the anode 2a can be kept in a reducing atmosphere by preventing outside air from entering. A physical fuel cell system 1 can be obtained.

In the cooling step (steps S130 to S200), the flow of the anode offgas Ff3 (combustion offgas Ff31) to the combustor 6 that heats the reformer 5 is stopped (step S120), and after the temperature drops to the allowable temperature Tp, combustion A step of discharging the gas circulating in the recycling circuit 32 through the vessel 6 to the outside of the system and replacing the circulating gas with air or an inert gas (steps S210 to S230). does not remain, making it easy to handle during storage.

1: Solid oxide fuel cell system 2: Stack 2a: Anode 2c: Cathode 2e: Electrolyte 4: Control unit 5: Reformer 6: Combustor 7: Desulfurizer 8: Heat exchanger, 9: steam generator, 10: heat exchanger, 11: heat exchanger, 12: blower (combustion off-gas flow controller), 13: blower, 15: fuel gas shutoff valve, 16: steam shutoff valve, 18: Cooling water device 19: Water supply device 31: Anode offgas combustion circuit 32: Recycle circuit 33: Water vapor supply circuit 99: Load Fa1: Air Fa2: Cathode offgas Ff1: Fuel gas Ff2: reformed gas Ff3: anode off-gas Ff31: combustion off-gas Ff32: recycle off-gas Fx: exhaust gas T: stack temperature Tp: allowable temperature.

Claims (9)

a reformer that reacts a hydrocarbon-containing fuel with water vapor to produce a hydrogen-containing reformed gas;
A solid oxide fuel cell stack that generates electrical energy through an electrochemical reaction between air and the reformed gas and outputs DC power;
a steam generator for generating the steam;
a recycling circuit that circulates the anode off-gas discharged from the anode of the stack back to the reformer;
After the output of the DC power and the supply of the fuel are cut off when the system is stopped, the control unit keeps the anode at a positive pressure by generating the water vapor until the temperature of the stack drops to an allowable temperature. A solid oxide fuel cell system characterized in that the circulation is continued while maintaining the solid oxide fuel cell system. 2. The solid oxide fuel cell system according to claim 1, wherein the controller increases the flow rate of the air supplied to the cathode until the temperature of the stack drops to the allowable temperature. 3. The solid oxide fuel cell system according to claim 1, wherein said steam generator generates said steam by heat exchange with said anode off-gas. a combustor that burns a portion of the anode off-gas to heat the reformer;
The control unit stops the flow of the anode off-gas to the combustor until the temperature of the stack drops to the allowable temperature, and after the temperature drops to the allowable temperature, the recycle circuit through the combustor. 4. The solid oxide fuel cell system according to any one of claims 1 to 3, wherein the circulating gas is discharged outside the system, and the circulating gas is replaced with air or an inert gas. 5. The solid state fuel according to claim 4, wherein the controller controls the flow rate of the anode off-gas to the combustor so that the differential pressure between the anode and the cathode falls within a predetermined range during system operation. Oxide fuel cell system. 6. The solid oxide fuel cell system according to any one of claims 1 to 5, further comprising a heat exchanger for adjusting the temperature and moisture content of said anode off-gas. 7. The solid oxide fuel cell system according to claim 6, wherein a heat exchanger for exchanging heat between said anode off-gas and said air is used as said heat exchanger. A reformer for reacting a fuel with water vapor to produce a reformed gas, a stack of solid oxide fuel cells for generating electricity through an electrochemical reaction between air and the reformed gas, and an anode of the stack that discharges A method for stopping a solid oxide fuel cell system comprising a recycling circuit for returning anode off-gas to the reformer for circulation, comprising:
During the step of stopping the power generation and the supply of the fuel, and until the temperature of the stack drops to an allowable temperature, the steam is generated to continue the circulation while maintaining the anode at a positive pressure. , a cooling step for cooling the stack;
A method for stopping a solid oxide fuel cell system, comprising: In the cooling step, the flow of the anode off-gas to the combustor that heats the reformer is stopped;
After the temperature has decreased to the permissible temperature, a step of discharging the gas circulating in the recycling circuit through the combustor to the outside of the system and replacing the circulating gas with air or an inert gas;
9. The method of stopping a solid oxide fuel cell system according to claim 8, comprising:

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