JP4797352B2 - Solid oxide fuel cell - Google Patents

Description

  The present invention relates to an internal reforming fuel cell, and more particularly to a high-efficiency solid oxide fuel cell that effectively uses exhaust heat of a fuel cell stack.

  Solid oxide fuel cells are being developed as third-generation fuel cells for power generation. At present, three types of solid oxide fuel cells have been proposed: a cylindrical type, a monolith type, and a flat plate type, all of which include a solid electrolyte composed of an oxide ion conductor formed by an air electrode layer and a fuel electrode layer. It has a laminated structure sandwiched between them. A fuel cell stack is configured by alternately laminating power generation cells and separators made of this laminate.

The power generation cell is supplied with oxygen (air) as an oxidant gas on the air electrode side and fuel gas (H ₂ , CO, CH _4, etc.) on 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. 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.

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

  By the way, since the fuel gas used in the fuel cell is a hydrocarbon compound (referred to as raw fuel) such as natural gas (methane gas), the raw fuel is actually reformed into a fuel gas mainly composed of hydrogen. Need to use. 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 added with water vapor in the reformer to become hydrogen and carbon monoxide. This reforming reaction is an endothermic reaction, and the temperature is as high as 650 to 800 ° C.
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

Conventional fuel cell reforming methods include an external reforming method in which a reformer is installed outside the fuel cell and a direct internal reforming method in which a direct fuel reforming mechanism is incorporated inside a high-temperature fuel cell stack. Are known. Since the steam reforming reaction is an endothermic reaction, the external reforming method that requires the supply of heat for the reforming reaction is not suitable for the fuel reforming mechanism of the fuel cell due to its poor power generation efficiency. An efficient internal reforming method that can utilize part of the heat generated from the fuel cell for the endothermic reaction of the reforming reaction has attracted attention.

  As described above, the reforming reaction is an endothermic reaction, and the reforming catalyst needs to be heated to at least 640 ° C. or more, preferably 700 ° C. or more in order to perform a sufficient reforming reaction. In the fuel cell of the type, high-temperature exhaust heat from the fuel cell stack is used as thermal energy for reforming.

Patent Document 1 discloses a technique for recovering exhaust heat from a fuel cell stack and effectively using it for preheating reaction gas (fuel gas, air), reforming reaction, or the like.
Japanese Patent Laid-Open No. 62-283570

  By the way, taking a solid oxide fuel cell as an example, a high-temperature solid oxide fuel cell having an operating temperature of around 1000 ° C. has a large amount of discharged thermal energy, and thus recovers the thermal energy required for reforming. However, in the case of a low-temperature operation type solid oxide fuel cell with an operating temperature of around 700 ° C., the amount of heat energy discharged is smaller than that of the previous high-temperature type, and the thermal space is relaxed. Therefore, the reforming reaction may become insufficient unless efficient heat recovery is performed. If the reforming is insufficient, the battery performance will drop sharply due to carbon deposition from methane (unreformed gas), or if methane is introduced into the power generation cell, the fuel electrode will peel off due to thermal stress due to endothermic reaction. This causes the negative effect of shortening the service life.

  Therefore, in order to obtain the stable power generation performance without the above-mentioned adverse effects, it is a big issue how to efficiently recover the surplus energy (exhaust heat) discharged from the fuel cell and effectively use it for the power generation reaction. Yes.

  The present invention has been made in view of such conventional problems, and an object of the present invention is to provide a solid oxide fuel cell with high thermal efficiency that efficiently uses exhaust heat from the fuel cell stack.

That is, according to the first aspect of the present invention, a fuel cell stack is configured by alternately stacking power generation cells and separators, accommodated in a power generation reaction chamber, and a reaction gas is supplied into the fuel cell stack during operation. In the internal reforming solid oxide fuel cell that generates a power generation reaction, the fuel cell stack is disposed near the center of the power generation reaction chamber, and the fuel is modified on one side of the fuel cell stack. the quality unit arranged, and, on the other side to face the fuel reformer is characterized in that place the least steam generator and an air heat exchanger.

  According to a second aspect of the present invention, in the solid oxide fuel cell according to the first aspect, the fuel reformer is disposed in the vicinity of the fuel cell stack.

  The present invention described in claim 3 is the solid oxide fuel cell according to claim 1 or 2, wherein the water vapor generator is connected to the air heat exchanger with respect to the fuel cell stack. It is characterized by being arranged more rearward.

  Moreover, the present invention described in claim 4 is the solid oxide fuel cell according to any one of claims 1 to 3, wherein the steam generator and the air heat exchanger are provided with fins, The steam generator is filled with alumina beads.

  The present invention described in claim 5 is the solid oxide fuel cell according to any one of claims 1 to 4, wherein the power generation reaction chamber has an airtight structure with an internal can body. It is a feature.

  According to a sixth aspect of the present invention, in the solid oxide fuel cell according to any one of the first to fifth aspects, the high-temperature exhaust gas from the fuel cell stack is supplied to the outside of the fuel cell module as a heat source. A water exhaust gas heat exchanger is provided, and heat exchange water from the water exhaust gas heat exchanger is supplied to the steam generator.

  Further, according to a seventh aspect of the present invention, in the solid oxide fuel cell according to the sixth aspect, a combustion catalyst is disposed near the exhaust port of the fuel cell module, and the combustion gas generated by the combustion catalyst is reduced. It is used as a heat source for the water exhaust gas heat exchanger.

In the power generation reaction chamber, the fuel reformer needs a high temperature for the reforming reaction (endothermic reaction), while the steam generator needs a larger amount of heat than a high temperature like the reforming reaction. .
Therefore, in the present invention, a fuel reformer that requires high temperature is disposed in the vicinity of the periphery of the fuel cell stack that can directly receive the radiant heat from the fuel cell stack, and the above-described steam generator, air heat exchanger, etc. These heat exchange devices are placed in opposite positions across the fuel cell stack so that the endothermic action of the fuel does not affect the endothermic reaction of the fuel reformer. It was set as the structure isolate | separated into.

In addition, among the heat exchange devices, in particular, the steam generator requires a large amount of heat rather than a high temperature. Therefore, in order to minimize the above-described thermal influence, air separated from the fuel cell stack is used. It arrange | positioned in the site | part outside the heat exchanger.
As a result, the fuel reformer is in the vicinity of the fuel cell stack and can efficiently receive the radiant heat to perform sufficient reforming, and the steam generator has a high temperature atmosphere in the power generation reaction chamber. Underneath it can absorb a large amount of heat to generate high temperature steam and supply it to the fuel reformer. As a result, a highly efficient power generation system that effectively uses exhaust heat from the fuel cell can be realized.

  Further, as described in claims 6 and 7, a water exhaust gas heat exchanger is provided outside the housing, and the water supplied to the steam generator is heated in advance, which is necessary for water vaporization in the steam generator. The amount of heat can be reduced, and accordingly, the steam generator can be disposed on the outer side as far as possible from the fuel cell stack, and the thermal influence of the water vaporization of the steam generator on the fuel reformer can be reduced.

As described above, according to the present invention, the fuel reformer can directly and efficiently receive the radiant heat from the fuel cell stack, and can perform sufficient reforming. The high-temperature steam can be efficiently generated by absorbing a large amount of heat in a high-temperature atmosphere by the high-temperature exhaust gas in the power generation reaction chamber.
In addition, a fuel reformer, an air preheater, and a steam generator are arranged opposite to each other with the fuel cell stack interposed therebetween, and the fuel reformer and these heat exchange devices are thermally separated, so that the steam generator and the air heat The endothermic effect of the exchanger can prevent thermal effects on the endothermic reaction of the fuel reformer. In addition, due to the airtight structure of the power generation reaction chamber, high-temperature exhaust gas is discharged wastefully from the gaps in the heat insulating material. Therefore, a stable high-temperature atmosphere state in which a sufficient reforming reaction can be performed around the fuel reformer can be secured.
As a result, a highly efficient power generation system that effectively uses exhaust heat from the fuel cell can be realized.

  Hereinafter, embodiments of the present invention will be described with reference to FIGS.

  1 and 2 show the internal schematic configuration of a solid oxide fuel cell to which the present invention is applied, and FIG. 3 shows the gas flow during operation in the fuel cell stack.

First, the structure of a solid oxide fuel cell will be described with reference to FIGS.
1 and 2, reference numeral 1 denotes a solid oxide fuel cell 1 (fuel cell module 1), reference numeral 2 denotes a cylindrical module can body as a housing, and reference numeral 28 denotes a cylindrical inner can body. A heat insulating material 27 is interposed between the can body 2 and the inner can body 28. The internal can body 28 holds the inside of the can (power generation reaction chamber 21) in an airtight state. The fuel cell stack 3 is arranged in the center of the power generation reaction chamber 21 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 6 (oxidant electrode layer) are disposed on both surfaces of a solid electrolyte layer 4, and an outer side of the fuel electrode layer 5. The fuel electrode current collector 8, the air electrode current collector 9 outside the air electrode layer 6 (oxidant electrode current collector), and the separator 10 outside each current collector 8, 9 are laminated in order. It is a cylindrical body which has.

Here, for example, 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. The air 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 base. The separator 10 is made of stainless steel or the like, and is made of a sponge-like porous sintered metal plate such as an alloy.

  Further, on the side of the fuel cell stack 3, a fuel manifold 13 for supplying fuel gas to the fuel passage 26 of each separator 10 through the connection pipe 11, and an oxidant through the connection pipe 12 to the oxidant passage 25 of each separator 10. An oxidant manifold 14 for supplying air as gas is provided extending in the stacking direction of the power generation cells 7.

Further, the fuel cell module 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. During operation, the fuel cell module 1 passes through the fuel passage 26 and the oxidant passage 25 as shown in FIG. The fuel gas and the oxidant gas (air) supplied from the substantially central portion of the separator 10 toward the power generation cell 7 are diffused in the outer peripheral direction of the power generation cell 7 and are excellent on the entire surface of the fuel electrode layer 5 and the air electrode layer 6. The power generation reaction is caused to spread with a wide distribution, and the remaining high-temperature gas that has not been consumed in the power generation reaction is freely discharged from the outer periphery of the power generation cell 7 into the power generation reaction chamber 21.
Further, the module can body 2 is provided with exhaust pipes 22 a and 22 b for discharging surplus gas released into the power generation reaction chamber 21 to the outside of the fuel cell module 1.

  By the way, in the fuel cell module 1 of this embodiment, as shown in FIGS. 1 and 2, in the power generation reaction chamber 21, there is a hydrocarbon in the vicinity of the fuel cell stack 3 that enables the radiant heat transfer of the fuel cell stack 3. A fuel reformer 30 filled with a catalyst is disposed so as to cover one side of the fuel cell stack 3 so that the heat radiation from the fuel cell stack 3 can be received efficiently.

  The gas inlet 39 of the reformer 30 is connected to one end of the fuel heat exchanger 15 by piping, and the outlet of the reformer 30 is connected to the fuel manifold 13 by piping (FIG. 1). reference). The fuel heat exchanger 15 is for fuel gas preheating.

  The reformer 30 may have a fuel reforming structure in which a gas catalyst is filled with a pellet catalyst (for example, a Ni-based or Ru-based hydrocarbon reforming catalyst dispersed and adhered), Alternatively, a honeycomb (honeycomb) catalyst may be used. Further, a gas flow path may be formed by providing a partition plate in the box.

In any case, the size of the gas flow path and the catalyst can be secured so that the SV (SV: reactive gas volume flow rate / catalyst volume) in the fuel reformer 30 can be secured to 500 to 5000 h ⁻¹ at which a good reforming reaction can be obtained. The amount is set.

  On the other hand, an air heat exchanger 16 and a steam generator 34 are disposed at a portion opposite to the power generation reaction chamber 21 facing the fuel reformer 30 with the fuel cell stack 3 interposed therebetween.

The air heat exchanger 16 is a heat exchanger for preheating the air supplied to the oxidant manifold 14 and requires a relatively high temperature for preheating. Therefore, the air heat exchanger 16 emits radiation from the fuel cell stack 3. The fuel cell stack 3 that can directly receive heat is disposed in the vicinity of the periphery of the fuel cell stack 3 so as to cover one side of the stack 3.
The air heat exchanger 16 is connected to an oxidant gas supply pipe 18 for introducing an oxidant gas (air) from the outside.

  The steam generator 34 is a heat exchanger for obtaining high-temperature steam for reforming, and requires a large amount of heat rather than high temperature. Therefore, the steam generator 34 is separated from the fuel cell stack 3 and air The heat exchanger 16 is disposed outside the heat exchanger 16.

  The steam generator 34 is composed of a plurality of finned heating towers extending in the height direction and disposed at predetermined intervals so as to surround the central fuel cell stack 3. And the lower end part of each heating tower is connected by the pipe member which is not illustrated, The water for water vapor generation supplied from the water exhaust gas heat exchanger 40 mentioned below is introduced into each heating tower through warm water supply pipe 41, and is heated. Heat is exchanged in the tower to generate high-temperature steam. The high-temperature steam is introduced into the fuel reformer 30 from a pipe member (not shown) from the upper end of each heating tower.

  The heating tower 37 is filled with alumina beads (not shown) having high thermal conductivity, and the heat exchange efficiency of the steam generator 34 is greatly increased by the heat conduction between the fins on the outer surface and the alumina beads. It has improved.

  Fuel reforming requires a high temperature (650-800 ° C.) for the reforming reaction, while a high temperature like the reforming reaction is not necessary to obtain high-temperature steam for reforming. Requires a large amount of heat. Incidentally, if the ambient temperature of the steam generator 34 can be secured at about 300 ° C., it is sufficient for the generation of steam.

  In this configuration, the fuel reformer 30 can directly and efficiently receive the radiant heat from the fuel cell stack 3 in the power generation reaction chamber 21, perform sufficient reforming, and be connected to the steam generator 34. As a result, a large amount of heat can be absorbed in a high temperature atmosphere in the power generation reaction chamber 21 to efficiently generate high temperature steam.

Further, as in the present embodiment, the fuel reformer 30, the air preheater 16, and the water vapor generator 34 are disposed in opposed positions across the fuel cell stack 3, and the fuel reformer 30 and these heat exchange devices are heated. Therefore, the endothermic action by the steam generator 34, the air heat exchanger 16 and the like can be prevented from affecting the endothermic reaction of the fuel reformer 30 in addition to the internal heat. Due to the airtight structure of the power generation reaction chamber 21 by the can body 28, it is possible to prevent the high-temperature exhaust gas in the power generation reaction chamber 21 from being discharged from the gap between the surrounding heat insulating materials 27 to the outside, and around the fuel reformer 30. It is possible to secure a stable high-temperature atmosphere that can perform a sufficient reforming reaction.
As a result, a highly efficient power generation system that effectively uses exhaust heat from the fuel cell can be realized.

  By the way, as shown in FIG. 2, a water exhaust gas heat exchanger 40 is disposed at the lower part of the fuel cell module 1.

This water exhaust gas heat exchanger 40 includes a heat exchanging portion 45 into which externally supplied water (water, nitrogen) flows into a space surrounded by a heat insulating material 44, and inside the heat exchanger 40, This water exchanges heat with the high-temperature exhaust gas introduced from the power generation reaction chamber 21 through the exhaust pipe 22 b, and guides the hot water to the steam generator 34 through the hot water supply pipe 41.
A combustion catalyst 43 is disposed around an exhaust port 42 that communicates with the exhaust pipe 22 b, and the combustion gas generated by the combustion catalyst 43 is used as a heat source in the heat exchange unit 45. Pt, Rh, Ce, Os or the like is used as the combustion catalyst 43 and dispersed around the exhaust port 42 on the water exhaust gas heat exchanger 40 side in a state of being supported on alumina, or wash coated and arranged in layers. .
Thereby, the thermal energy of exhaust gas can be used effectively and the efficiency of the power generation system can be further enhanced.

  Thus, by preheating the water supplied to the steam generator 34 (partly being steam), the amount of heat required for water vaporization in the steam generator 34 can be reduced. It is possible to arrange the generator 34 on the outer side of the inner can body 28 as far as possible from the fuel cell stack 3, and thereby the thermal effect of the water vaporization action of the water vapor generator 34 on the fuel reformer 30. Can be less.

Next, the operation of this embodiment will be described.
During operation of the fuel cell, a mixed gas of hydrocarbon gas supplied from the outside through the fuel gas supply pipe 17 and high-temperature steam from the steam generator 34 is supplied from the gas inlet 39 of the fuel reformer 30 to the reformer. 30. The fuel gas is preheated by the fuel heat exchanger 15 shown in FIG. 1 after being merged and mixed with high-temperature steam generated by the steam generator 34 through a pipe (not shown).

This mixed gas is guided to the reformer 30 and in contact with the hydrocarbon catalyst in the course of flowing through the reformer, the reforming reaction of the hydrocarbon gas by the steam reforming method is performed. This reforming reaction is an endothermic reaction, and high heat (for example, 650 to 800 ° C.) necessary for the reforming reaction is obtained by receiving radiant heat from the fuel cell stack 3.
Since the fuel reformer 30 is disposed at a suitable position capable of radiant heat transfer in the circumferential direction of the fuel cell stack 3, it can directly receive a sufficiently high heat required for the endothermic reaction of the reforming reaction.

The fuel gas reformed by the reformer 30 is guided to the fuel manifold 13 shown in FIG. 1 through a pipe, and is introduced from this to the side of each separator 10 through each connection pipe 11.
As shown in FIG. 3, the reformed gas is further discharged from the side surface of the separator 10 to the fuel electrode side through the fuel passage 26, diffused and moved in the fuel electrode current collector 8, and reaches the fuel electrode layer 5. Done.

  On the other hand, the oxidant gas (air) supplied from the outside through the oxidant gas supply pipe 18 is preheated by the air heat exchanger 16, and this preheated air is guided to the oxidant manifold 14 and each connection pipe. 12 is introduced into the side of each separator 10. As shown in FIG. 3, air is further discharged from the side surface of the separator 10 to the air electrode side through the oxidant passage 25, diffused and moved in the air electrode current collector 9, and reaches the air electrode layer 6.

The top view which shows the internal schematic structure of the fuel cell stack which concerns on this invention. The side view which shows the internal schematic structure of a fuel cell stack. The gas flow at the time of operation in a fuel cell stack is shown.

Explanation of symbols

1 Solid oxide fuel cell (fuel cell module)
3 Fuel cell stack 7 Power generation cell 10 Separator 16 Air heat exchanger 21 Power generation reaction chamber 28 Internal can body 30 Fuel reformer 34 Steam generator 40 Water exhaust gas heat exchanger

Claims (7)

A fuel cell stack is configured by alternately stacking power generation cells and separators, and is housed in a power generation reaction chamber, and an internal reforming type that generates a power generation reaction by supplying a reaction gas into the fuel cell stack during operation. In a solid oxide fuel cell,
The fuel cell stack is disposed near the center of the power generation reaction chamber, a fuel reformer is disposed on one side across the fuel cell stack, and on the other side facing the fuel reformer, solid oxide fuel cell characterized by being placed at least steam generator and an air heat exchanger. 2. The solid oxide fuel cell according to claim 1, wherein the fuel reformer is disposed in the vicinity of the fuel cell stack. 3. The solid oxide fuel cell according to claim 1, wherein the water vapor generator is disposed behind the air heat exchanger with respect to the fuel cell stack. The solid oxide form according to any one of claims 1 to 3, wherein fins are provided in the steam generator and the air heat exchanger, and alumina beads are filled in the steam generator. Fuel cell. The solid oxide fuel cell according to any one of claims 1 to 4, wherein the power generation reaction chamber has an airtight structure with an internal can body. A water exhaust gas heat exchanger using a high temperature exhaust gas from the fuel cell stack as a heat source is provided outside the fuel cell module, and heat exchange water from the water exhaust gas heat exchanger is supplied to the steam generator. The solid oxide fuel cell according to any one of claims 1 to 5. The solid oxide form according to claim 6, wherein a combustion catalyst is disposed in the vicinity of an exhaust port of the fuel cell module, and the combustion gas generated by the combustion catalyst is used as a heat source of the water exhaust gas heat exchanger. Fuel cell.

Images