JP2005327554A - Solid oxide fuel cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To efficiently utilize heat generated in a fuel cell stack in power generation as a heat source for steam reforming reaction, equalize the temperature distribution of a fuel cell stack by utilizing a cooling action caused by endothermic reaction in reforming, and enhance efficiency of power generation. <P>SOLUTION: The fuel cell stack 1 is constituted by alternately plurally stacking power generation cells 5 and separators 8 having reaction gas passages 11, 12. The fuel cell stack 1 has manifolds 17-19 for introducing reaction gas passing through the stack and arranged in a line in the stacking direction, and the reaction gas is supplied to each power generation cell 5 through gas passages 11, 12 of the separator 8 with which the manifolds are communicated. A reformer 20 filled with a hydrocarbon reforming catalyst is installed in a middle step part excluding at least both end parts of the fuel cell stack 1. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a solid oxide fuel cell having a structure in which power generation cells and separators are alternately stacked, and more particularly, a flat plate stack type in which the temperature distribution in the stacking direction of the fuel cell stack is made uniform to improve power generation efficiency. The present invention relates to a solid oxide fuel cell.

  Solid oxide fuel cells have been developed as third-generation fuel cells for power generation, and three types are currently known: cylindrical, monolithic, and flat plate stack types. Each of these solid oxide fuel cells has a laminated structure in which a solid electrolyte layer made of an oxide ion conductor is sandwiched between an air electrode layer (cathode) and a fuel electrode layer (anode) from both sides. A plurality of power generation cells and separators are alternately stacked to form a stack.

In a solid oxide fuel cell, an oxidant gas (oxygen) is supplied to the air electrode layer side as a reaction gas, and a fuel gas (H ₂ , CO, CH _4, etc.) is supplied to the fuel electrode layer side. The air electrode layer and the fuel electrode layer are both porous layers so that the reaction gas can reach the interface with the solid electrolyte layer.

Here, the power generation reaction of the power generation cell will be described. Oxygen supplied to the air electrode layer side passes through the pores in the air electrode layer and reaches the vicinity of the interface with the solid electrolyte layer. Electrons are received and ionized to oxide ions (O ²⁻ ). 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 such an electrode reaction can be taken out as an electromotive force at an external load of another route.

  By the way, as a fuel gas of the solid oxide fuel cell, a hydrocarbon compound such as city gas (this is called a raw fuel) is usually used. For this reason, in order to supply the power generation cell, it is necessary to reform the raw fuel into a fuel gas mainly containing hydrogen. As a reforming method, when the raw fuel is a hydrocarbon-based gas fuel or liquid fuel, a steam reforming method is usually used.

For example, a reforming reaction using methane gas as a raw fuel is as follows.
The desulfurized methane gas is reformed into hydrogen and carbon monoxide by adding steam in a reformer. This reforming reaction is an endothermic reaction, and a high temperature of about 650 to 800 ° C. is required to perform a stable reforming reaction.
CH ₄ + H ₂ O → 3H ₂ + CO
At this time, the generated carbon monoxide further reacts with water vapor and changes into hydrogen and carbon dioxide.
CO + H ₂ O → H ₂ + CO ₂

  Conventionally, as reforming methods of fuel gas used in solid oxide fuel cells, an external reforming method in which a reformer is installed outside, and an internal reforming method in which a reformer is incorporated inside a high-temperature fuel cell module Is known.

  The external reforming method is a method in which a reformer provided with a hydrocarbon reforming catalyst is installed outside the fuel cell to reform the raw fuel, and this reformed gas is introduced into the fuel cell. Since the quality reaction is an endothermic reaction, it is necessary to supply high heat for the reforming reaction into the external reformer, and wasteful energy is required to obtain this high heat, which reduces the efficiency of the power generation system. There was a problem.

  On the other hand, the internal reforming method is an extremely rational method that uses part of the Joule heat generated by the power generation reaction of the fuel cell as a heat source for the endothermic reaction of the reforming reaction. There is a possibility of realizing the system. In addition, the internal reforming method has attracted attention as a fuel reforming method for solid oxide fuel cells in recent years because it has a cooling effect of absorbing high heat generated during power generation by this endothermic reaction.

Patent Documents 1 to 3 and the like are disclosed as such an internal reforming type flat plate laminated fuel cell.
Patent Document 1 discloses a fuel cell in which a reforming catalyst is provided in a fuel passage of a separator, and Patent Document 2 discloses a fuel cell in which a reforming catalyst is provided in a fuel electrode current collector. These effectively perform the reforming reaction using the heat of the battery reaction. Patent Document 3 discloses a fuel cell in which power generation efficiency is improved by combining a heat generating stack and a heat absorbing stack.
Japanese Patent Application Laid-Open No. 06-243881 Japanese Patent Laid-Open No. 07-045293 JP 2003-272645 A

By the way, it is known that the temperature distribution in the stacking direction extremely decreases in the vicinity of both ends of the stack in the flat plate stack type fuel cell stack. This is because the Joule heat generated by the power generation reaction of the power generation cell is more easily diffused at the end of the fuel cell stack than at the middle portion. In the portion where the temperature is low, the power generation reaction is not activated as compared with the high temperature portion, and the power generation performance is reduced.
In particular, in a fuel cell stack configured by connecting a plurality of power generation cells in series, when such a temperature distribution with both shoulders in the stacking direction of the fuel cell stack occurs, the power generation performance of the entire fuel cell is reduced to the low temperature part. Since it is limited by the power generation performance, there is a problem that efficient power generation cannot be performed.

  However, as described above, since the reforming reaction is an endothermic reaction, it is considered that such nonuniformity of the temperature distribution in the stack can be improved by effectively using the cooling action by the endothermic reaction. It is done.

  The present invention has been made based on such an idea, and efficiently uses the high heat discharged from the fuel cell stack during power generation as a heat source for the steam reforming reaction, and also absorbs heat during reforming. An object of the present invention is to provide a solid oxide fuel cell in which the temperature distribution of the fuel cell stack is made uniform by utilizing a cooling action by reaction to improve the efficiency of power generation.

That is, according to the present invention, the fuel cell stack is configured by alternately stacking a plurality of separators each having a power generation cell and a reaction gas passage, and the fuel cell stack penetrates the stack direction. A solid oxide fuel cell comprising a manifold for introduction of a reaction gas arranged in a row and configured to supply a reaction gas to each power generation cell through a gas passage of a separator that communicates with the manifold. Is characterized in that a reformer filled with a hydrocarbon reforming catalyst is disposed in a middle portion excluding at least both ends of the catalyst.
In this configuration, the heat generated from the power generation cells located in the middle part of the fuel cell stack that becomes hot can be efficiently absorbed and used for the reforming reaction, and the fuel cell stack can be used for the endothermic reaction of the reforming reaction. The middle part can be cooled. Thereby, the temperature distribution in the stacking direction can be made uniform, and efficient power generation becomes possible.

Further, according to a second aspect of the present invention, in the solid oxide fuel cell according to the first aspect, the reformer is disposed at a plurality of locations in the stacking direction.
In this configuration, since the cooling can be performed uniformly over a wide range in the stacking direction, the temperature distribution in the stacking direction can be further smoothed.

The present invention described in claim 3 is the solid oxide fuel cell according to claim 1, wherein the reformer introduces an unreformed fuel gas. And a gas discharge port for discharging the reformed fuel gas, the gas introduction port communicates with the unreformed fuel gas manifold, the discharge port communicates with the fuel gas manifold, and the fuel The fuel gas reformed through the gas manifold is supplied to each power generation cell.
In this configuration, the fuel gas reformed by each reformer is temporarily joined to the fuel gas manifold and distributed to each power generation cell while being mixed in the process of flowing through the manifold. Uniform flow distribution to the cells is possible, and a stable power generation reaction without variation is obtained in each power generation cell.

According to a fourth aspect of the present invention, in the solid oxide fuel cell according to any one of the first to third aspects, the reformer is a recess for filling a hydrocarbon reforming catalyst. And the gas inlet is provided at the center of the recess, the gas discharge port is provided at the edge of the recess, and the cavity through which the reformed fuel gas flows is formed at the periphery of the recess. It is characterized by providing.
In this configuration, the fuel gas discharged from the peripheral portion of the reforming catalyst layer can be merged and mixed in the hollow portion and smoothly introduced into the fuel gas manifold through the gas discharge port.

Further, the present invention according to claim 5 is the solid oxide fuel cell according to any one of claims 1 to 4, wherein the reformer has the same shape and the same size as the separator, It is characterized by being closely and closely arranged between the separators.
In this configuration, by closely contacting the reformer and the separator, the heat generated by the battery reaction can be efficiently absorbed into the reformer via the separator, and the reformer is configured in the same shape as the separator. The fuel cell stack itself can be gathered compactly.

As described above, according to the present invention, since the reformer is disposed in the middle stage excluding at least both ends of the fuel cell stack, the middle stage part of the fuel cell stack where the temperature becomes high due to the endothermic effect of the reforming reaction. Only the temperature can be cooled to make the temperature distribution in the stacking direction uniform. As a result, the efficiency of power generation can be improved.
The reformer has the same shape and the same size as the separator, and the heat generated by the cell reaction can be absorbed efficiently through the upper and lower separators by placing the reformer in close contact between the separator and the fuel cell stack itself. Can be summarized in a compact.

Hereinafter, an embodiment of the present invention will be described with reference to FIGS.
FIG. 1 shows the configuration of a flat plate type solid oxide fuel cell stack, FIG. 2 shows the structure of a separator used in the fuel cell stack, and FIG. 3 shows the internal structure of the reformer disposed in the fuel cell stack. Show.

  As shown in FIG. 1, the fuel cell stack 1 includes a power generation cell 5 configured by disposing a fuel electrode layer 3 and an air electrode layer 4 on both surfaces of a solid electrolyte layer 2, and a fuel disposed outside the fuel electrode layer 3. A single cell 10 is constituted by an electrode current collector 6, an air electrode current collector 7 disposed outside the air electrode layer 4, and a separator 8 disposed outside each current collector 6, 7. 10 is laminated.

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

  As shown in FIG. 2, the separator 8 is formed of a hexagonal stainless steel plate having a thickness of about several millimeters, and includes a fuel gas passage 11 through which fuel gas flows and an oxidant gas (generally air). ) Through which an oxidant gas passage 12 flows.

One end (upstream side) of the fuel gas passage 11 communicates with the fuel gas introduction hole 13 provided at the corner of the separator 8, and the other end (downstream side) faces the fuel electrode current collector 6. 8 communicates with the central gas discharge hole 11a. One end (upstream side) of the oxidant gas passage 12 communicates with an oxidant gas introduction hole 14 provided at another corner of the separator 8, and the other end (downstream side) is the air electrode current collector 7. It communicates with the gas discharge hole 12a in the central part of the separator facing the.
An unreformed fuel gas introduction hole 15 to be described later is formed in another corner of the separator 8 in addition to the fuel gas introduction hole 13 and the oxidant gas introduction hole 14 described above. The quality fuel gas introduction hole 15 does not affect the gas flow into the separator 8.

  Incidentally, in the present invention, as shown in FIG. 1, a plate-like reformer 20 is disposed near the middle stage of the fuel cell stack 1. The reformer 20 has the same hexagonal shape as the separator 8 and is formed in the same size. The reformer 20 is disposed at several places in the stacking direction in close contact with each other between the separator 8 and the separator 8. ing. In addition, arrangement | positioning of the reformer 20 shown in FIG. 1 is an example, Comprising: An arrangement | positioning position can be arbitrarily set according to thermal design.

The reformer 20 is made of a stainless steel plate having excellent thermal conductivity like the separator 8, and the reforming catalyst layer 22 is arranged in a circular recess 21 provided in the center of the interior as shown in FIG. It is installed.
The reforming catalyst layer 22 has a thin disk shape in which hydrocarbon reforming catalyst particles are supported on a porous metal body (for example, Ni foam metal), and its outer diameter is somewhat smaller than the diameter of the recess 21. Thus, when the reforming catalyst layer 22 is disposed at the center of the recess 21, a ring-shaped cavity 23 surrounding the reforming catalyst layer 22 is formed.
The reforming catalyst layer 22 may be composed of a hydrocarbon reforming catalyst supported on ceramic pieces (for example, pellets, spheres, flakes).

Further, in the reformer 20, in addition to the concave portion 21, reforming in which an unreformed gas passage 24 through which unreformed fuel gas flows and a reformed gas (that is, fuel gas) flows. A gas passage 25 is formed.
One end (upstream side) of the unreformed gas passage 24 communicates with an unreformed fuel gas introduction hole 15 provided at a corner of the reformer 20, and the other end (downstream side) is connected to the reforming catalyst layer. 22 communicates with a gas inlet 24a provided at a central position. Further, one end (upstream side) of the reformed gas passage 25 communicates with a gas discharge port 25 a provided at the edge of the recess 21, and the other end (downstream side) is provided at another corner of the reformer 20. The fuel gas introduction hole 13 communicates.

In addition to the unreformed fuel gas introduction hole 15 and the fuel gas introduction hole 13, an oxidant gas introduction hole 14 is formed at another corner of the reformer 20. 14 does not contribute to the gas flow into the reformer 20.
The positions of the gas introduction holes 13 to 15 of the reformer 20 coincide with the positions of the gas introduction holes 13 to 15 of the separator 8.

  The fuel gas introduction hole 13, the oxidant gas introduction hole 14, and the unreformed fuel gas introduction hole 15 described above pass through each separator 8 and each reformer 20 except for the upper and lower ends in the plate thickness direction. Then, an insulating manifold ring 16 is arranged in each of the upper and lower openings of the gas introduction holes 13 to 15 and a large number of single cells 10 are stacked, so that these manifold rings 16 are connected to the gas introduction holes 13 and 14 of the separator 8. , 15 are connected in a straight line in the vertical direction (stacking direction), and each of the fuel gas manifold 17, the oxidant gas manifold 18, and the unreformed fuel gas manifold 19 is formed in a tubular shape inside the stack. A system manifold is constructed.

  In the fuel cell stack 1 configured as described above, unreformed fuel gas (for example, a mixed gas of methane and water vapor) and oxidant gas (air) are supplied as reaction gases from the outside of the stack. Unreformed fuel gas is introduced into the unreformed fuel gas manifold 19 through a pipe (not shown), and air is introduced into the oxidant gas manifold 18 through a pipe (not shown).

  The unreformed fuel gas introduced into the unreformed fuel gas manifold 19 is introduced into the reformer 20 from the unreformed fuel gas introduction holes 15 at a plurality of locations in the middle part of the fuel cell stack 1. It is discharged through the unreformed gas passage 24 to the central portion of the reforming catalyst layer 22 facing from the gas inlet 24a at the end of the passage.

The unreformed fuel gas is brought into contact with the hydrocarbon catalyst in the process of radially diffusing and moving in the reforming catalyst layer 22 from the central portion toward the peripheral portion, and a reforming reaction is performed. The unreformed fuel gas is reformed into a hydrogen-rich fuel gas, is discharged from the peripheral portion of the reforming catalyst layer 22, flows through the peripheral cavity 23, and passes through the reformed gas passage 25 from the gas discharge port 25a. It is introduced into the fuel gas manifold 17.
In this way, the fuel gas ejected from the peripheral portion of the reforming catalyst layer 22 is merged in the cavity 23 and circulated in the cavity, thereby uniformizing the radial flow of the fuel gas in the reforming catalyst layer. The gas can be sufficiently mixed, and the introduction of the gas into the fuel manifold 17 can be facilitated.

  The fuel gas and air introduced into the fuel gas manifold 17 and the oxidant gas manifold 18 circulate in the manifolds 17 and 18 extending in the longitudinal direction, respectively, and the gas introduction holes 13 in each layer (single cell). , 14 and supplied to the electrode portions of the power generation cells 5 through the gas passages 11 and 12 of the separator 8 respectively.

  That is, the fuel gas in the fuel gas manifold 17 is introduced into the fuel gas passage 11 from the fuel gas introduction hole 13 of each separator 8 and discharged from the gas discharge hole 11a at the end of the passage to face the fuel electrode current collector 6. , Passes through here while diffusing, and reaches the fuel electrode layer 3 of the power generation cell 5. On the other hand, the air in the oxidant gas manifold 18 is introduced into the oxidant gas passage 12 from the oxidant gas introduction hole 14 of each separator 8, and is discharged from the gas discharge hole 12a at the end of the passage to face the air electrode current collector. It is supplied to the body 7, passes through here while diffusing, and reaches the air electrode layer 4 of the power generation cell 5. Hereinafter, the power generation reaction in each power generation cell 5 is as described above.

By the way, the reforming reaction in the reformer 20 is an endothermic reaction, and the heat (for example, 650 to 800 ° C.) necessary for the reforming reaction uses the power generation reaction heat of the power generation cell 5.
As described above, the reformer 20 is disposed in several places in the middle stage of the stack in a state where the reformer 20 is in close contact between the separator 8 and the separator 8. Heat from the power generation cell 5 positioned can be efficiently absorbed through the upper and lower separators 8, 8 having excellent thermal conductivity and used for the reforming reaction, and the fuel cell stack is obtained by the endothermic reaction at the time of reforming. One middle stage portion can be uniformly cooled over a wide range in the stacking direction. Thereby, the temperature distribution in the stacking direction is made uniform, and efficient power generation becomes possible.

That is, as shown in FIG. 4, in the conventional fuel cell stack 1 in which the reformer is not provided, the temperature distribution in the stacking direction is high in the middle portion as shown by the broken line, but has a characteristic of both shoulders falling. In the case of the fuel cell stack 1 of the present invention in which the reformers 20 are arranged at a plurality of locations in the middle stage of the stack, the temperature near the placement of the reformer 20 decreases as shown by the solid line due to the cooling effect of the endothermic reaction. Therefore, the temperature difference between the high temperature portion and the low temperature portion is eliminated, and a substantially uniform temperature distribution can be obtained over the entire region in the stacking direction.
In addition, by reducing the temperature of the high temperature portion, damage to the power generation cell 5 such as peeling of the fuel electrode layer 3 due to thermal stress can be prevented, and the durability (thermal cycle characteristics) of the fuel cell stack is also improved.

  In addition, in the conventional internal reforming mechanism in which the reforming catalyst is supported in the gas passage of the separator or the current collector, the influence of the carbon deposition on the power generation cell and the fuel gas distributed from the manifold are collected in the gas passage and the collector of the separator. In the present invention, the fuel gas reformed in each reformer 20 is temporarily converted into a fuel gas manifold. 17, and is distributed to each power generation cell 5 while being mixed in the process of flowing through the manifold, so that it is possible to distribute to each power generation cell 5, and stable power generation in each power generation cell 5. A reaction is obtained.

The figure which shows the structure of the fuel cell stack which concerns on this invention. The top view which shows the structure of a separator. The cross-sectional view which shows the internal structure of a reformer. The figure which shows the temperature distribution of the lamination direction of a fuel cell stack.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Fuel cell stack 5 Power generation cell 8 Separator 17 Manifold 18 for fuel gas Manifold 19 for oxidant gas 19 Manifold 20 for unreformed fuel gas Reformer 21 Recess 23 Cavity 24a Gas inlet 25a Gas outlet

Claims (5)

A plurality of separators having power generation cells and reaction gas passages are alternately stacked to form a fuel cell stack, and a reaction gas introduction manifold arranged in the stacking direction through the fuel cell stack is provided. A solid oxide fuel cell configured to supply a reaction gas to each power generation cell through a gas passage of a separator that communicates with the manifold;
A solid oxide fuel cell, wherein a reformer filled with a hydrocarbon reforming catalyst is disposed in a middle portion excluding at least both ends of the fuel cell stack. The solid oxide fuel cell according to claim 1, wherein the reformer is disposed at a plurality of locations in the stacking direction. The reformer includes a gas inlet for introducing unreformed fuel gas and a gas outlet for discharging the reformed fuel gas, and the gas inlet communicates with a manifold for unreformed fuel gas. The discharge port communicates with a fuel gas manifold and supplies the reformed fuel gas to each power generation cell via the fuel gas manifold. A solid oxide fuel cell according to claim 1. The reformer has a recess for filling the hydrocarbon reforming catalyst;
In addition, the gas introduction port is provided at the center of the recess, the gas discharge port is provided at the edge of the recess, and the cavity through which the reformed fuel gas flows is provided at the periphery of the recess. The solid oxide fuel cell according to any one of claims 1 to 3. 5. The solid according to claim 1, wherein the reformer has the same shape and the same size as the separator, and is integrally and closely disposed between the separator and the separator. Oxide fuel cell.

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