JP4654567B2 - Solid oxide fuel cell and method of operating the same - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a solid oxide fuel cell, and more particularly to a preheating structure of a solid oxide fuel cell and a preheating method thereof.
[0002]
[Prior art]
A solid oxide fuel cell having a power generation cell with a laminated structure in which a solid electrolyte layer made of an oxide ion conductor is sandwiched between an air electrode layer and a fuel electrode layer is being developed as a fuel cell for third generation power generation. It is out. In the power generation cell, oxygen (air) as an oxidant gas is supplied to the air electrode side, and fuel gas (H ₂ , CO, etc.) is supplied to the fuel electrode side. The air electrode and the fuel electrode are both porous so that the gas can reach the interface with the solid electrolyte.
[0003]
Oxygen supplied to the air electrode side passes through the pores in the air electrode layer and reaches the vicinity of the interface with the solid electrolyte layer. At this part, it receives electrons from the air electrode and converts them into oxide ions (O ²⁻ ). Ionized. The oxide ions diffuse and move in the solid electrolyte layer toward the fuel electrode. Oxide ions that have reached the vicinity of the interface with the fuel electrode react with the fuel gas at this portion to generate reaction products (H ₂ O, CO _2, etc.), and emit electrons to the fuel electrode.
[0004]
The electrode reaction when hydrogen is used as the fuel is as follows.
Air electrode: 1/2 O ₂ + 2e ⁻ → O ²⁻
Fuel electrode: H ₂ + O ²⁻ → H ₂ O + 2e ⁻
Overall: H ₂ +1/2 O ₂ → H ₂ O
[0005]
The solid electrolyte layer is a moving medium for oxide ions and at the same time functions as a partition for preventing direct contact between the fuel gas and air, and thus has a dense structure that is impermeable to gas. This solid electrolyte layer should have a high oxide ion conductivity, be chemically stable under conditions from the oxidizing atmosphere on the air electrode side to the reducing atmosphere on the fuel electrode side, and be made of a material that is resistant to thermal shock. There is generally used stabilized zirconia (YSZ) to which yttria is added as a material satisfying such requirements.
[0006]
On the other hand, both the air electrode (cathode) layer and the fuel electrode (anode) layer, which are electrodes, must be made of a material having high electron conductivity. Since the air electrode material must be chemically stable in a high-temperature oxidizing atmosphere of at least around 700 ° C., the metal is unsuitable, and a perovskite type oxide material having electron conductivity, specifically LaMnO. ₃ or LaCoO ₃ or a solid solution obtained by substituting a part of these La with Sr, Ca, or the like is generally used. The fuel electrode material is generally a metal such as Ni or Co, or a cermet such as Ni—YSZ or Co—YSZ.
[0007]
Solid oxide fuel cells include a high temperature operation type that operates at a high temperature of about 1000 ° C. and a low temperature operation type that operates at a low temperature of about 700 ° C. A low temperature operation type solid oxide fuel cell can generate power as a fuel cell even at low temperatures by, for example, reducing the thickness of stabilized zirconia (YSZ) to which the electrolyte yttria is added to about 10 μm to reduce the resistance of the electrolyte. Use a solid electrolyte layer modified to:
[0008]
In a high-temperature solid electrolyte fuel cell, for example, ceramics having electronic conductivity such as lanthanum chromite (LaCrO ₃ ) are used as a separator. In a low-temperature operation solid oxide fuel cell, a metal material such as stainless steel is used. Can be used.
[0009]
Also, three types of solid oxide fuel cell structures have been proposed: a cylindrical type, a monolith type, and a flat plate type. Among these structures, since a metal separator can be used for a low temperature operation type solid oxide fuel cell, a flat plate type structure that is easy to give a shape to the metal separator is suitable.
[0010]
A stack of flat plate type solid oxide fuel cells has a structure in which power generation cells, current collectors, and separators are alternately stacked. A pair of separators sandwich the power generation cell from both sides, one being in contact with the air electrode via the air electrode current collector and the other being in contact with the fuel electrode via the fuel electrode current collector. A sponge-like porous body such as a Ni-based alloy can be used for the fuel electrode current collector, and a sponge-like porous body such as an Ag-based alloy can be used for the air electrode current collector. Can do. The sponge-like porous body has a current collecting function, a gas permeation function, a uniform gas diffusion function, a cushion function, a thermal expansion difference absorption function, and the like, and is therefore suitable as a multifunctional current collector material.
[0011]
The separator has a function of electrically connecting the power generation cells and supplying gas to the power generation cells. The fuel is introduced from the outer peripheral surface of the separator and discharged from the surface facing the separator fuel electrode layer. Each has a gas passage and an oxidant gas passage which introduces air as an oxidant gas from the outer peripheral surface of the separator and discharges it from the surface facing the air electrode layer of the separator.
[0012]
By the way, when operating the above-described solid oxide fuel cell, it is necessary to start the operation after preheating the power generation cell to the above-described power generation reaction temperature (see Patent Document 1). A method of raising the temperature with a heater arranged on the outer periphery or a method of introducing a gas heated from the outside around the fuel cell stack is employed. These are all heating the power generation cell from the outer periphery.
[0013]
[Patent Literature]
Japanese Patent Laid-Open No. 6-124721
[Problems to be solved by the invention]
However, when preheating the power generation cell, if the temperature is not increased while maintaining the uniform temperature of the entire fuel cell stack, a temperature distribution is generated in the power generation cell and thermal stress is generated, which may lead to damage of the power generation cell. For this reason, in the method of heating the power generation cell from the outer peripheral portion, it is difficult to maintain the thermal uniformity, so it is necessary to gradually raise the temperature over a very long time, and the standby time until the start of operation becomes longer. There's a problem. Also, in the temperature lowering operation when the operation is stopped, if the temperature is lowered from the operating temperature at once, problems such as breakage of the power generation cell occur as in the case of the temperature increase. It has passed.
[0015]
In addition, conventionally, Ni is mainly used as a material for the fuel electrode layer. Therefore, the temperature rising operation at the time of start-up and the temperature lowering operation at the time of operation stop are repeatedly performed. There is a problem that oxidization causes peeling at the interface with the electrolyte layer due to sintering shrinkage due to reduction during power generation. Such a peeling phenomenon of the fuel electrode layer loses electrode activity, thereby deteriorating the power generation characteristics and significantly lowering the durability. For this reason, conventionally, in order to prevent oxidation of Ni in the fuel electrode layer during preheating, N ₂ or the like is introduced from an inert gas cylinder and the temperature is raised while maintaining the fuel electrode side in a reducing atmosphere. Has been taken.
[0016]
The present invention has been made in view of the above problems, and is a suitable solid oxide fuel that can efficiently perform preheating in a short time while preventing damage to the power generation cell and separation due to oxidation of the fuel electrode. It aims at providing a battery and its operating method.
[0017]
[Means for Solving the Problems]
That is, according to the present invention, the fuel cell stack configured by alternately stacking the power generation cells and the separators is housed in the power generation reaction chamber, and the fuel gas supply pipe and the oxidation are put into the fuel cell stack during operation. A solid oxide fuel cell in which a fuel gas and an oxidant gas are respectively supplied from an agent gas supply pipe to cause a power generation reaction, and a gas preheating chamber having a burner is provided at a lower portion of the power generation reaction chamber, and the gas In the preheating chamber, a preheating fuel gas pipe to which water vapor and methane gas or hydrocarbon gas are supplied at the time of temperature rise and / or temperature drop and a preheating oxidant gas pipe to which air is supplied are arranged, and the preheating fuel is provided. with arranging the steam reforming catalyst in the gas tube, connecting the preheating fuel gas pipe to the fuel gas supply pipe, and connecting the preheating oxidizer gas pipe to the oxidant gas supply pipe It is characterized in that was.
[0018]
In the above configuration, the hydrocarbon gas introduced into the preheating fuel gas pipe and the steam are brought into contact with the reforming catalyst in the pipe to perform the reforming reaction of the hydrocarbon gas, and the reformed gas (H ₂ , CO, CO ₂ ) is obtained. This reforming reaction is an endothermic reaction, and the heat required for the reforming reaction can be the combustion heat of the burner.
[0019]
Further, the invention of claim 2 is introduced in the solid oxide fuel cell according to claim 1, the power generation reaction chamber combustion gas of the burner in the partition wall between the gas preheating chamber and the power generation reaction chamber It is characterized by having an air passage for doing this.
In the above-described configuration, hot air (combustion gas) generated in the burner combustion operation is introduced into the power generation reaction chamber through the air passage when the temperature rises, and the fuel cell stack is heated from the outside in the process of rising in the power generation reaction chamber.
[0020]
Further, the invention according to claim 3, in a solid oxide fuel cell according to claim 2 is characterized in that arranging a plurality of said air passage around the fuel cell stack.
In the above configuration, the fuel cell stack is uniformly heated over the entire area by the combustion gas introduced from the air passage.
[0021]
Further, the present invention according to claim 4 is the operation method of the solid oxide fuel cell according to any one of claims 1 to 3 , wherein the combustion operation of the burner is performed during preheating. The preheating air in the preheating oxidant gas pipe is heated and supplied to the oxidant electrode layer of the power generation cell through the separator, and a mixed gas of water vapor and hydrocarbon gas is supplied to the preheating fuel gas pipe. Introducing, generating a gas containing hydrogen below the lower limit of explosion by a reforming reaction, supplying the fuel electrode layer of the power generation cell through the separator, heating the power generation cell from the inside, and burning the burner The power generation cell is heated from the outside with gas.
In the above operation method, since the power generation cell can be heated in parallel from the outside and the inside during the temperature increase, the temperature increase of the power generation cell is suppressed while keeping the temperature difference between the outer peripheral portion and the inside of the fuel cell stack small. Can be promoted. Therefore, it is possible to efficiently raise the temperature of the power generation cell in a short time while preventing cracking of the power generation cell. Further, during heating, since the reducing atmosphere is maintained by hydrogen supplied to the fuel electrode layer side, oxidation of Ni in the fuel electrode layer can be prevented.
[0022]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to FIGS.
FIG. 1 shows an internal configuration of a solid oxide fuel cell according to the present invention, FIG. 2 shows a configuration of heat exchange piping, and FIG. 3 shows a schematic configuration of a main part of a fuel cell stack.
[0023]
In FIG. 1, reference numeral 1 is a solid oxide fuel cell, reference numeral 2 is a housing in which a heat insulating material 26 is attached, and reference numeral 3 is a fuel cell stack disposed inside the housing 2 with the stacking direction being vertical. . As shown in FIG. 3, the fuel cell stack 3 includes a power generation cell 7 in which a fuel electrode layer 5 and an air electrode layer (oxidant electrode layer) 6 are disposed on both surfaces of a solid electrolyte layer 4, and an outer side of the fuel electrode layer 5. A structure in which a fuel electrode current collector 8, an air electrode current collector (oxidant electrode current collector) 9 outside the air electrode layer 6, and a separator 10 outside each current collector 8, 9 are laminated in order. Have.
[0024]
Here, the solid electrolyte layer 4 is composed of stabilized zirconia (YSZ) or the like to which yttria is added, and the fuel electrode layer 5 is composed of a metal such as Ni or Co or a cermet such as Ni—YSZ or Co—YSZ, and air. The electrode layer 6 is made of LaMnO ₃ , LaCoO ₃ or the like, the fuel electrode current collector 8 is made of a sponge-like porous sintered metal plate such as a Ni-based alloy, and the air electrode current collector 9 is made of an Ag-based alloy or the like. The separator 10 is made of stainless steel or the like.
[0025]
Further, on the side of the fuel cell stack 3, a fuel manifold 13 that supplies fuel gas to the fuel gas passages 17 (see FIG. 3) of each separator 10 through the connection pipe 11, and an oxidant gas passage 18 of each separator 10. An oxidant manifold 14 for supplying air as an oxidant gas through the connecting pipe 12 is provided extending in the stacking direction of the power generation cells 7 (see FIG. 3).
[0026]
Further, a fuel gas supply pipe 15 for supplying an external fuel gas necessary for a power generation reaction during operation is provided at a substantially central portion of the fuel manifold 13, and a power generation reaction is provided at a substantially central portion of the oxidant manifold 14. An oxidant gas supply pipe 16 for supplying external oxidant gas (air) necessary for the operation is connected.
[0027]
Further, the solid oxide fuel cell 1 has a sealless structure in which a gas leakage prevention seal is not provided on the outer peripheral portion of the power generation cell 7, and during operation, as shown in FIG. The fuel electrode layer 5 and the air electrode layer are diffused while the fuel gas and the oxidant gas (air) discharged from the substantially central portion of the separator 10 through the agent gas passage 18 toward the power generation cell 7 are diffused in the outer peripheral direction of the power generation cell 7. The power generation reaction is caused to spread over the entire surface of 6 with a good distribution, and the remaining gas that has not been consumed in the power generation reaction is freely released from the outer periphery of the power generation cell 7 to the outside. Further, the housing 2 is provided with an exhaust pipe (exhaust hole) 19 for discharging excess gas discharged into the power generation reaction chamber 20 formed inside the housing 2 to the outside of the housing 2.
[0028]
By the way, in the solid oxide fuel cell 1 of the present embodiment, the temperature rise at the start of operation or the operation stop is performed via the partition wall 25 formed by attaching the heat insulating material 26 to the lower part of the power generation reaction chamber 20 described above. A gas preheating chamber 30 for efficiently preheating the fuel cell stack 3 at the time of temperature drop is disposed, and a burner 21 is installed inside the heat insulating material 26.
The burner 21 is configured so that fuel gas is supplied from the outside of the gas preheating chamber 30 through the burner combustion gas supply pipe 22 piped to the burner 21 when the temperature is raised. The combustion operation in the burner 21 is performed together with the air supplied from the plurality of intake holes 23 provided.
[0029]
Further, during the combustion operation, a preheating fuel gas pipe 27 and a preheating oxidant gas pipe 28 which are heat exchange pipes for preheating are disposed in the vicinity of the flame formed by the burner 21. As shown in FIG. 2, each of the preheating fuel gas pipe 27 and the preheating oxidant gas pipe 28 is formed of a metal pipe formed in a spiral shape, and these pipes are formed on a plane. A heat exchanging piping structure 31 having a high heat exchanging efficiency combined with a nested state is configured, and a steam reforming catalyst (not shown) (for example, not shown) is disposed inside the inner preheating fuel gas pipe 27 of the pipe. , Ni, Ru, Pt, Rh, Ce, Ir, etc.).
[0030]
The preheating fuel gas pipe 27 has an outer end piped to the outside of the housing 2, and a central end inserted through a vent 25 a provided in the thickness direction of the upper partition wall 25 to generate a power generation reaction chamber. The end of the power generation reaction chamber 20 is connected to the fuel gas supply pipe 15. On the other hand, the preheating oxidant gas pipe 28 has an outer end piped to the outside of the housing 2, and a central end inserted through the vent 25 a of the upper partition wall 25 into the power generation reaction chamber 20. Inducted and connected to the oxidant gas supply pipe 16 in the power generation reaction chamber 20.
A side window of the gas preheating chamber 30 is provided with a see-through viewing window 29 so that the combustion state of the burner 21, that is, the state of flame can be monitored from the outside.
In the configuration of the gas preheating chamber 30 described above, the number of burners 21 is not limited to one, and a plurality of burners 21 may be installed. A plurality of fuel cell stacks 3 are arranged in a ring shape so as to surround the fuel cell stack 3 so as to be efficiently guided to the side 20.
[0031]
Next, the preheating operation at the start of operation by the preheating mechanism having the above configuration will be described.
[0032]
During preheating at the start of operation, first, fuel gas such as external methane gas or hydrocarbon gas is supplied to the burner 21 from the burner combustion gas supply pipe 22 of the gas preheating chamber 30. The burner 21 starts a combustion operation with a mixed gas of the combustion gas and combustion air supplied from the intake hole 23, and a spiral preheating fuel gas pipe 27 disposed in the vicinity of the upper portion of the burner 21 by the flame and The heat exchange piping structure 31 by the preheating oxidant gas pipe 28 is heated. Incidentally, in the preheating operation of the embodiment, while monitoring the flame formed by the burner 21 from the observation window 29, the preheating is gradually heated by the melting fire, and then the heating power is increased to a predetermined temperature. To do.
[0033]
During preheating, preheating air is supplied to the preheating oxidant gas pipe 28 from the outside. The preheating air is heat-exchanged in the process of passing through the spiral gas pipe 28, and the heated air is It is guided to the power generation reaction chamber 20 by the preheating oxidant gas pipe 28 and is introduced from the oxidant gas supply pipe 16 to each separator 10 through the oxidant manifold 14, the numerous connection pipes 12, and the like. The air is further discharged to the air electrode side through the oxidant gas passage 18 inside, and diffuses and moves in the air electrode current collector 9 to reach the air electrode layer 6. Such a gas flow passage is as shown in FIG. 3, and the heated air exchanges heat with the metallic separator 10 in the process of passing through the oxidant gas passage 18 of the separator 10 to heat the separator 10 from the inside.
[0034]
In parallel with this, at the time of preheating, the preheating fuel gas pipe 27 is supplied with a mixed gas of water vapor and methane gas or hydrocarbon gas (fuel gas for fuel cells is diverted). A steam reforming reaction of hydrocarbon gas or methane gas occurs in contact with the reforming catalyst in a spiral tube, and reformed gas (H ₂ , CO, CO ₂ ) is generated. This reforming reaction is an endothermic reaction, and the heat necessary for the reforming reaction can use the combustion heat of the burner 21.
Here, the hydrogen obtained by the reforming reaction is less than the explosion limit for safety and the like, and the amount of hydrogen in the reformed gas is about 3% or less. Therefore, the amount of fuel gas supplied to the preheating fuel gas pipe 27 during preheating is extremely small.
[0035]
The reformed gas generated by this reforming reaction is guided to the power generation reaction chamber 20 through the preheating fuel gas pipe 27 and is supplied from the fuel gas supply pipe 15 to each separator 10 through the fuel manifold 13, a large number of connection pipes 11, and the like. It is introduced and discharged from the separator side portion through the internal fuel gas passage 17 to the fuel electrode side, diffuses and moves in the fuel electrode current collector 8 and reaches the fuel electrode layer 5. The heated reformed gas exchanges heat with the metallic separator 10 in the process of passing through the fuel gas passage 17 and heats the separator 10 from the inside in the process of passing through the fuel gas passage 17 as well as the above-described effect of the heated air. By introducing hydrogen to the side, the fuel electrode layer 5 can be made a reducing atmosphere during the temperature rise, and as a result, oxidation of Ni which is the material of the fuel electrode layer 5 is prevented. Therefore, the hydrogen obtained by the reforming reaction in the preheating fuel gas pipe 27 is not used as a fuel gas for power generation unlike the conventionally known internal reforming.
[0036]
Further, as described above, while the preheating from the inside of the power generation cell is performed, the combustion gas generated from the burner 21 enters the power generation reaction chamber 20 through the upper vent 25a, and the reaction chamber is filled with the combustion gas. In the process of flowing upward, the outer periphery of the power generation cell is contacted, and the power generation cell 7 is also heated from the outer periphery. As described above, since the plurality of vent holes 25a are arranged around the fuel cell stack 3, the entire outer periphery of the fuel cell stack 3 can be uniformly heated from below to above. The combustion gas is sucked upward by a blower (not shown) and exhausted to the outside through the upper exhaust hole 19.
[0037]
When the temperature of the fuel cell stack 3 is raised to the temperature at which power generation reaction is possible, the supply of the fuel gas is cut off, the combustion operation of the burner 21 is stopped, and the preheating fuel gas pipe 27 and the preheating oxidant gas pipe 28 are stopped. The supply of the heating gas and air is also stopped. After the temperature rise, the fuel gas is supplied from the fuel gas supply pipe 15 and the air is supplied from the oxidant gas supply pipe 16 to start the operation (power generation) of the fuel cell.
[0038]
According to the above operation method, the fuel cell stack 3 is heated from the outer periphery by the combustion gas of the burner 21 introduced into the power generation reaction chamber 20 at the time of temperature rise, and the preheating fuel gas pipe 27 and the preheating oxidant gas are used. Since the fuel cell stack 3 is also heated from the inside by the heated gas (reformed gas and air) introduced from the pipe 28, the temperature difference between the outer peripheral portion and the inside of the fuel cell stack 3 is kept small, and the power generation cell 7. The temperature rise can be promoted. Thereby, it is possible to efficiently raise the temperature of the power generation cell 7 while preventing the power generation cell 7 from cracking. Of course, the preheating operation can be applied not only when the temperature is increased at the start of operation but also when the temperature is decreased when operation is stopped.
[0039]
Further, by supplying a gas containing hydrogen generated by the reforming reaction in the preheating fuel gas pipe 27 to the fuel electrode layer 5, the fuel electrode layer 5 can be made a reducing atmosphere, and the fuel cell is repeatedly performed. In the temperature rising / falling operation, the peeling phenomenon of the fuel electrode layer 5 due to oxidation of Ni which is the material of the fuel electrode layer 5 can be prevented, and stable power generation characteristics can be maintained.
[0040]
【The invention's effect】
As described above, according to the present invention, when the temperature is raised, the power generation cell is heated from the outside and the inside at the same time. Therefore, while the temperature difference between the outer peripheral portion and the inside of the fuel cell stack is kept small, Can promote temperature. Accordingly, it is possible to efficiently raise the temperature of the power generation cell while preventing the power generation cell from cracking.
In addition, during the temperature rise, hydrogen below the explosion limit is supplied to the fuel electrode side, so that the peeling phenomenon of the fuel electrode layer due to oxidation of Ni which is the material of the fuel electrode layer is prevented, and the durability is improved. Stable power generation characteristics can be maintained even during repeated heating and cooling operations of the fuel cell.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view showing an internal configuration of a solid oxide fuel cell according to the present invention.
FIG. 2 is a schematic configuration diagram of a main part of a fuel cell stack used for explaining an embodiment of the present invention, and shows a gas flow during operation.
FIG. 3 is a diagram showing a configuration of a heat exchange piping structure according to the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Solid oxide fuel cell 3 Fuel cell stack 5 Fuel electrode layer 7 Power generation cell 10 Separator 20 Power generation reaction chamber 21 Burner 25 Partition 25a Vent (vent)
27 Preheating fuel gas pipe 28 Preheating oxidant gas pipe 30 Gas preheating chamber

Claims (4)

A fuel cell stack configured by alternately stacking power generation cells and separators is housed in a power generation reaction chamber, and fuel gas and oxidant gas are respectively supplied from the fuel gas supply pipe and the oxidant gas supply pipe into the fuel cell stack during operation. A solid oxide fuel cell that generates a power generation reaction by supplying
A gas preheating chamber provided with a burner is provided in the lower part of the power generation reaction chamber, and a preheating fuel gas pipe and air to which water vapor and methane gas or hydrocarbon gas are supplied when the temperature is raised and / or lowered. A preheating oxidant gas pipe is provided, a steam reforming catalyst is disposed in the preheating fuel gas pipe , the preheating fuel gas pipe is connected to the fuel gas supply pipe, and the preheating is performed. A solid oxide fuel cell comprising an oxidant gas pipe connected to the oxidant gas supply pipe.   2. The solid oxide fuel cell according to claim 1, further comprising an air passage for introducing combustion gas of the burner into the power generation reaction chamber in a partition wall between the power generation reaction chamber and the gas preheating chamber. 3. The solid oxide fuel cell according to claim 2, wherein a plurality of the air passages are disposed around the fuel cell stack.   A method for operating a solid oxide fuel cell according to any one of claims 1 to 3, wherein, during preheating, the burner is started for combustion operation to preheat an oxidant gas pipe for preheating. The air is heated and supplied to the oxidant electrode layer of the power generation cell through the separator, and a mixed gas of water vapor and hydrocarbon gas is introduced into the preheating fuel gas pipe, and the lower limit of explosion is reached by a reforming reaction A gas containing hydrogen is generated and supplied to the fuel electrode layer of the power generation cell through the separator, whereby the power generation cell is heated from the inside, and the power generation cell is heated from the outside by the combustion gas of the burner. A method for operating a solid oxide fuel cell.

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