JP2006294508A - Fuel cell generator and the shut down method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To make purging by inert gas unnecessary in a shut down. <P>SOLUTION: The shut down method for a fuel cell produces a power generating reaction by supplying fuel gas to a fuel electrode layer side and supplying oxidant gas to an air electrode layer side. When the power generation is stopped, stack temperature is lowered while maintaining the fuel electrode side to a reduced status by supplying water, hydrogen or hydrocarbon system fuel while decreasing the amount of flow. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a fuel cell operation stop method for controlling a supply amount of a reaction gas based on output power of a fuel cell.

A fuel cell with a structure in which a solid electrolyte layer made of an oxide ion conductor is sandwiched between the air electrode layer (cathode) and the fuel electrode layer (anode) from both sides is highly efficient in converting the chemical energy of the fuel directly into electrical energy And it is attracting attention as a clean power generation device. In the solid oxide fuel cell, an oxidant gas (oxygen) is supplied to the air electrode layer side, and a fuel gas (H ₂ , CO, CH _4, etc.) is supplied to the fuel electrode layer side.

Oxygen supplied to the air electrode layer passes through the pores in the air electrode layer and reaches the vicinity of the interface with the solid electrolyte layer, and receives electrons from the air electrode layer at this portion to form oxide ions (O2−). Ionized. The oxide ions diffuse and move in the solid electrolyte layer toward the fuel electrode layer. Oxide ions that have reached the vicinity of the interface with the fuel electrode layer react with the fuel gas at this portion to generate reaction products (H ₂ O, CO _2, etc.), and discharge electrons to the fuel electrode layer. Electrons generated by the electrode reaction can be taken out as an electromotive force at an external load on another route.

By the way, when stopping the operation of the fuel cell, conventionally, the supply of the fuel gas to the fuel electrode layer side and the supply of the oxidant gas to the air electrode layer side are stopped, and then the inert gas is supplied to the fuel cell. A so-called purge is performed in which the inside of the fuel cell is replaced with an inert gas by supplying (for example, nitrogen) (see, for example, Patent Document 1).
This is because the fuel electrode layer is oxidized by oxygen in the oxidant gas remaining inside the fuel cell in a high temperature state after the operation is stopped, and the power generation performance is extremely lowered. It is known that even if the oxidized fuel electrode layer is reduced again by the fuel gas (hydrogen), the power generation performance before oxidation is hardly recovered.

In particular, since the solid oxide fuel cell has an extremely high operating temperature of 600 to 1000 ° C., it is impossible to lower the temperature in a short time in the temperature lowering process after the operation is stopped, and in the outer periphery of the power generation cell. When a sealless structure that does not have a gas leak prevention seal is adopted, the oxygen-containing gas outside (in the fuel cell module) easily enters the inside of the battery due to a decrease in the pressure inside the battery. The amount of inert gas used for purging tended to increase proportionally as the fuel cell size increased.
JP-A-2-244559

  However, when purging with an inert gas, it is necessary to prepare an inert gas for purging in addition to a fuel gas or an oxidant gas as a reaction gas. Generally, a fuel gas is provided with a gas cylinder. A purge inert gas is supplied to the supply path. For this reason, there is a problem that the weight and volume of the entire fuel cell including the gas cylinder and the inert gas supply path increase, and the maintenance work becomes complicated.

  The present invention has been made in view of such a problem, and an object of the present invention is to provide a fuel cell power generation apparatus and an operation stop method that can perform a purge without providing a separate purge gas supply system.

That is, the present invention according to claim 1 is a method for shutting down a fuel cell in which a fuel gas is supplied to the fuel electrode layer side and an oxidant gas is supplied to the air electrode layer side to cause a power generation reaction. When power generation is stopped, the stack temperature is lowered while maintaining the fuel electrode layer side in a reduced state by supplying the fuel cell while reducing the flow rates of water and hydrogen or hydrocarbon fuel. It is said.
In this method, a small amount of water and fuel gas are continuously supplied even after power generation is stopped, steam is generated using the heat capacity of the fuel cell, and a mixed gas of reformed gas and steam is supplied to the fuel electrode. It retains reducibility.

Further, the present invention according to claim 2 is the method of stopping the operation of the fuel cell according to claim 1, wherein the cell voltage of the fuel cell is 0.5 V or more when the stack temperature is 300 ° C. It is characterized by reducing the supply amount of hydrogen or hydrocarbon fuel.
If the fuel gas supply is stopped when the stack temperature is 300 ° C. or higher, the fuel electrode layer side may be oxidized and deteriorated. However, the cell voltage should be 0.5 V until the stack temperature drops to about 300 ° C. By gradually reducing the fuel gas so as not to fall below, the fuel electrode layer side can be kept in a reduced state.

Further, according to a third aspect of the present invention, in the fuel cell operation stop method according to the first or second aspect, when the supply of water to the fuel cell is stopped, the water vapor temperature by the water is reduced. It is characterized by reducing the amount of water supplied so that it is 200 ° C. or higher.
If the water vapor temperature is 200 ° C. or lower when the supply of water is stopped, the temperature of the water vapor drops to 100 ° C. all at once, making it difficult to continuously generate water vapor. As a result, liquid water is supplied into the cell, and the cell may be deteriorated or cracked. For this reason, the water vapor temperature at the time of stopping the supply of water needs to be 200 ° C. or higher.

  According to a fourth aspect of the present invention, there is provided a fuel cell that outputs electric power in accordance with a fuel gas supply amount and an oxidant gas supply amount, a fuel supply system that supplies fuel gas to the fuel cell, and an oxidant gas. An oxidant gas supply system for supplying water, a water supply system for supplying water, and a control unit that controls these systems to perform the operation stop control according to any one of claims 1 to 3. It is characterized by that.

  According to a fifth aspect of the present invention, there is provided the fuel cell power generator according to the fourth aspect, wherein the fuel cell releases a residual gas that has not been used in a power generation reaction from an outer peripheral portion of the power generation cell. It is a solid oxide fuel cell having a structure.

According to the present invention, when the power generation is stopped, the reducing property of the fuel electrode layer can be maintained by supplying the fuel cell with a reduced flow rate of water and hydrogen or hydrocarbon fuel. In addition, the oxidation of the power generation cell and the accompanying performance deterioration of the power generation cell in the heating / cooling cycle can be prevented, and the life can be extended.
In addition, since purging with a conventional inert gas is unnecessary, there is no need to provide a purge gas supply system including an inert gas cylinder (for example, a nitrogen cylinder), which simplifies maintenance work and reduces the size of the apparatus itself. Can be

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 shows a schematic configuration of a fuel cell power generator to which the present invention is applied, FIG. 2 shows a configuration of a fuel cell stack used in the fuel cell power generator, and FIG. 3 shows an operation stop control mode of the present invention. Yes.

  As shown in FIG. 1, the fuel cell power generator of this embodiment includes a solid oxide fuel cell 1 (fuel cell stack 1) that generates a direct current output in accordance with a fuel gas supply amount and an air supply amount, a fuel gas ( For example, a fuel cell module 10 configured by housing a fuel reformer 15 or the like that reforms a mixed gas of methane gas, city gas) and water vapor into a hydrogen rich gas and supplies the fuel cell stack 1 in a heat insulating housing, And a fuel supply system 40 that is disposed around the fuel cell module 10 and that includes a fuel gas blower 21, a desulfurizer 23, each fuel gas supply pipe, and the like, and introduces fuel gas into the fuel reformer 15, and an air blower Or an air supply pipe or the like, and an air supply system 30 for supplying an oxidant gas (air) to the fuel cell stack 1, a water supply pump 25, and a water supply pipe. (Note that this water is converted into water vapor by a water vapor generator (not shown) in the fuel cell module), the DC output from the fuel cell stack 1 is converted into AC output, and AC power Pa is externally loaded. It comprises an inverter 24 supplied to (not shown), a control unit 20 for controlling the flow rate of each of the air supply system 30, the fuel supply system 40, and the water supply system 50 described above.

  Further, the control unit 20 sends cell voltage information V, stack temperature information T1, water vapor temperature information T2, and outputs sent from detectors (not shown) disposed at appropriate positions in the fuel cell power generator. Various detection information such as power information Pa is input.

  Here, as shown in FIG. 2, the fuel cell stack 1 is disposed outside the fuel electrode layer 3, and a power generation cell 5 in which the fuel electrode layer 3 and the air electrode layer 4 are disposed on both surfaces of the solid electrolyte layer 2. A single cell 9 composed of a fuel electrode current collector 6, an air electrode current collector 7 arranged outside the air electrode layer 4, and a separator 8 arranged outside each current collector 6, 7 is vertically arranged. Many are stacked to form a stack.

Among the constituent elements of the unit cell 9, the solid electrolyte layer 2 is made of stabilized zirconia (YSZ) or the like to which yttria is added, and the fuel electrode layer 3 is made of a metal such as Ni or Co or Ni-YSZ or Co-YSZ. It is composed of cermet, the air electrode layer 4 is composed of LaMnO ₃ , LaCoO ₃ or the like, and the fuel electrode current collector 6 is composed of a sponge-like porous sintered metal plate such as a Ni-based alloy. 7 is composed of a sponge-like porous sintered metal plate such as an Ag-based alloy, and the separator 8 is composed of stainless steel or the like.

  The separator 8 has a function of electrically connecting the power generation cells 5 and supplying a reaction gas to the power generation cells 5. The fuel of the separator 8 is introduced by introducing fuel gas from the outer peripheral surface of the separator 8. The fuel gas passage 11 discharged from the substantially central portion 11a of the surface facing the electrode current collector 6 and the surface of the surface of the separator 8 facing the air electrode current collector 7 by introducing oxidant gas from the outer peripheral surface of the separator 8 An oxidant gas passage 12 that is discharged from the substantially central portion 12a is provided.

  A fuel gas manifold 17 and an oxidant gas manifold 18 extending in the stacking direction are provided in the fuel cell stack 1, and the reformed fuel gas flows through the manifold 17. Air supplied from the outside flows, and each gas is introduced into each gas passage 11 and 12 of each separator 8 from the manifolds 17 and 18 and discharged from each gas discharge port 11a and 12a to each electrode of each power generation cell. Distributed and supplied. A pair of end plates 8a, 8b made of stainless steel or the like are disposed at both ends of the fuel cell stack 1, and the generated power of the fuel cell stack 1 can be taken out through the end plates 8a, 8b. It has become.

  In addition, the fuel cell stack 1 employs a sealless structure in which a gas leakage prevention seal is not provided on the outer periphery of the power generation cell 5, and surplus gas (high temperature exhaust gas) that has not been consumed in the power generation reaction during operation is used. It is discharged freely from the outer periphery of the power generation cell 5 into the housing. Note that the high-temperature exhaust gas discharged into the internal space of the housing is discharged out of the module through the upper exhaust hole.

  Next, operation stop control of the fuel cell power generation device having the above configuration will be described with reference to FIG. The operation stop control is performed by the control unit 20 based on various detection information (V, Pa, T1, T2, etc.) input from the various detectors described above.

  The operation stop control shown in FIG. 3 is performed in a state where the flow rate of air supplied to the fuel cell stack 1 is maintained at a constant flow rate. In FIG. 3, the left vertical axis indicates the supply amount of fuel gas (methane) and the supply amount of water serving as a water vapor source, the right vertical axis indicates the stack temperature and the battery output, and the horizontal axis indicates the elapsed time.

As shown in FIG. 3, in the rated power generation period (output 1 kW, stack temperature 750 ° C.), when the operation stop operation is performed, the respective flow rates of methane and water supplied to the fuel cell module 10 are reduced, and about The battery output is reduced from 1 kW to 0 W in 4 hours (output reduction period), and then the stack temperature is reduced from about 700 ° C. to 300 ° C. or less in about 15 hours (temperature reduction period). The present invention is a purge process for avoiding an oxidation phenomenon of the fuel electrode layer by maintaining the fuel electrode layer side in a reduced state in a high temperature atmosphere during the output reduction period to the temperature falling period after the operation stop operation.
That is, the operation stop control of the present embodiment continues to supply a small amount of methane and water to the fuel cell module 10 even when power generation is stopped, generates steam using the heat capacity of the fuel cell module 10, and The reducibility of the fuel electrode layer is maintained by generating hydrogen by a quality reaction and supplying a gas mixture with water vapor to the fuel electrode layer side.

  In the operation stop control, the flow rate of methane or water is changed by controlling the operation of the fuel gas blower 21 (which may be a control valve) or the water supply pump 25 (which may be a control valve) by the control unit 20. be able to.

In the above-described operation stop control, regarding the flow rate of methane, it is necessary to decrease the supply amount of methane so that the cell voltage V is maintained at 0.5 V or higher when the stack temperature T1 is 300 ° C.
This is because if the supply of methane is stopped when the stack temperature T1 is 300 ° C. or higher, the fuel electrode layer containing Ni as a main component may be oxidized by the heat and NiO may be generated. The redox reaction in the fuel electrode layer significantly reduces the performance of the power generation cell. Therefore, until the stack temperature T1 is lowered to about 300 ° C., it is necessary to gradually decrease the flow rate of methane so that the cell voltage V does not fall below 0.5V, as shown in FIG. Thereby, the fuel electrode layer side can be kept in a reduced state. In the control of the methane flow rate, the stack voltage may be monitored instead of the cell voltage V.

  On the other hand, with respect to the flow rate of water, it is necessary to reduce the amount of water supplied so that the temperature of water vapor is maintained at 200 ° C. or higher when water supply is stopped. This is because when the water vapor temperature is 200 ° C. or lower, the temperature of the water vapor drops to 100 ° C. at a stretch, making it difficult to continuously generate water vapor. As a result, liquid water is supplied into the cell instead of water vapor. This is because there is a possibility that the cell is deteriorated or cracked.

As described above, in the present invention, when power generation is stopped, the fuel cell is supplied while reducing the flow rates of water and fuel gas, so that the reducing property on the fuel electrode layer side in a high-temperature atmosphere after power generation is stopped. As a result, it is possible to prevent oxidation of the power generation cell in the temperature increasing / decreasing cycle and deterioration of the performance of the power generation cell associated therewith, thereby extending the life.
In addition, this operation stop method does not require purging with a conventional inert gas, so there is no need to provide a purge gas supply system including an inert gas cylinder (for example, a nitrogen cylinder), and the maintenance work is simplified. In addition, the device itself can be downsized.

  In particular, in the solid oxide fuel cell, since the operating temperature is as high as 600 to 1000 ° C., the heat capacity of the module such as metal or ceramic is large during the cooling period, and it is impossible to lower the stack temperature in a short time. In a sealless structure, the oxygen-containing gas outside (in the fuel cell module) tends to enter the battery due to a decrease in the pressure inside the battery. The operation stop method of the present invention, which requires a purge process but does not require a purge with an inert gas, is extremely effective for such a high temperature operation type fuel cell.

The figure which shows schematic structure of the fuel cell power generator to which this invention was applied. The figure which shows the structure of the solid oxide fuel cell stack used for the fuel cell power generation device of FIG. The figure which shows the driving | operation stop control form by this invention.

Explanation of symbols

1 Fuel cell (fuel cell stack)
3 Fuel electrode layer 4 Air electrode layer 20 Control unit 30 Oxidant gas supply system 40 Fuel supply system 50 Water supply system

Claims (5)

A fuel cell operation stopping method for supplying a fuel gas to the fuel electrode layer side and supplying an oxidant gas to the air electrode layer side to cause a power generation reaction,
When power generation is stopped, the stack temperature is lowered while maintaining the fuel electrode layer side in a reduced state by supplying the fuel cell while reducing the flow rates of water and hydrogen or hydrocarbon fuel. To stop the operation of the fuel cell. 2. The fuel cell according to claim 1, wherein a supply amount of the hydrogen or hydrocarbon fuel is decreased so that a cell voltage of the fuel cell becomes 0.5 V or more when a stack temperature is 300 ° C. 3. How to stop operation. 3. The fuel according to claim 1, wherein when the supply of water to the fuel cell is stopped, the supply amount of water is reduced so that a water vapor temperature of the water is 200 ° C. or more. How to stop battery operation. A fuel cell that outputs electric power according to a fuel gas supply amount and an oxidant gas supply amount; a fuel supply system that supplies fuel gas to the fuel cell; an oxidant gas supply system that supplies oxidant gas; and water. A fuel cell power generator comprising: a water supply system to be supplied; and a control unit that controls each of these systems to perform operation stop control according to any one of claims 1 to 3. 5. The fuel cell power generation according to claim 4, wherein the fuel cell is a solid oxide fuel cell having a sealless structure that discharges residual gas that has not been used in a power generation reaction from an outer peripheral portion of the power generation cell. apparatus.

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