JP2007080761A - Fuel cell and its starting method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fuel cell and its starting method capable of quickly starting the fuel cell. <P>SOLUTION: The fuel cell 1 includes a steam generator 20, a fuel reformer 30 for steam-reforming hydrocarbon fuel, and a fuel cell stack 3 for causing power generation reaction with reformed gas supplied from the fuel reformer 30 and supplied oxidant gas. The steam generator 20 includes a heat exchanger 21 using exhaust gas from the fuel cell stack 3 as a heat source, and a combustion means for burning unburned gas in the exhaust gas in starting is installed in an exhaust gas passage of the heat exchanger 21. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a fuel cell that generates power by supplying a fuel gas obtained by steam reforming raw fuel gas and an oxidant gas, and a starting method thereof.

  In recent years, fuel cells that directly convert chemical energy of fuel into electrical energy have attracted attention as highly efficient and clean power generators. In particular, the solid oxide fuel cell has a number of advantages such as high power generation efficiency and high operating temperature compared to other fuel cells, so that exhaust heat can be effectively used. Research and development is progressing as a fuel cell for power generation.

This solid oxide fuel cell 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. As described above, an oxidant gas (oxygen) is supplied to the air electrode layer side, and a fuel gas (H ₂ , CO, 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.

In the power generation cell, 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. It is ionized to (O ²⁻ ). This oxide ion diffuses and moves in the solid electrolyte layer toward the fuel electrode layer and reaches the vicinity of the interface with the fuel electrode layer, where it reacts with the fuel gas to produce reaction products (H ₂ O, produce CO _2, etc.), releasing 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 starting the operation (power generation) of the fuel cell, it is necessary to raise the temperature of the fuel cell stack to an operating temperature (for example, 650 or more in the case of a solid oxide fuel cell) in advance by some heating device at the time of startup. is there. This is for activating the power generation reaction of the power generation cell, and usually an electric heater, a burner, or the like is used as a heating device for preheating.
For example, Patent Literature 1 is disclosed as a technology related to preheating and temperature rise at the start of the fuel cell.
JP 2002-124288 A

  By the way, when the temperature of the fuel cell stack is raised at the time of startup, conventionally, a heating device such as the above-described electric heater or burner is disposed around the fuel cell stack, and the stack surface is externally exposed by radiant heat from these heating devices. A method of heating was performed. However, such a preheating method using radiant heat has a problem that it takes time to raise the temperature, and it takes a long time (for example, 5 hours or more) until the rated power generation is possible.

In particular, in a fuel cell with a sealless structure that discharges exhaust gas from the outer periphery of the power generation cell, fuel gas is supplied to the power generation cell at the same time as startup, and exhaust gas discharged from the stack, that is, not used for power generation reaction. Combustion gas is burned around the stack, and by using the combustion heat of the unburned gas together with the radiant heat from the heating device to raise the temperature of the fuel cell stack, the startup time is also shortened. When a hydrocarbon-based fuel such as city gas is used as the fuel gas, carbon deposition occurs in an atmosphere of 300 ° C. or higher during exhaust gas combustion, so a large amount of fuel gas cannot be supplied without darkness. Needed to be gradually supplied. For this reason, the temperature rising effect due to the combustion heat of the exhaust gas cannot be expected so much, which has been a major obstacle to shortening the startup time.
In order to prevent carbon deposition during combustion, it is known that water vapor may be mixed with a hydrocarbon fuel. Normally, a fuel cell includes a water vapor generator using exhaust gas as a heat source, but it is difficult to stably generate water vapor in the water vapor generator under an atmosphere of 300 ° C. or lower where carbon deposition does not occur.

  The present invention has been made in view of such problems at the time of start-up. At the time of start-up, the steam generator is heated by the combustion heat of the exhaust gas (unburned gas), and steam is generated by the steam generator immediately after start-up. Accordingly, an object of the present invention is to provide a fuel cell and a starting method thereof that can solve the above-described problem of carbon deposition and enable rapid start-up.

  That is, the present invention described in claim 1 supplies a steam generator, a fuel reformer for steam reforming a hydrocarbon-based fuel, and a reformed gas and an oxidant gas from the fuel reformer. And a fuel cell stack that generates a power generation reaction, wherein the steam generator includes a heat exchanger that uses exhaust gas from the fuel cell stack as a heat source, and an exhaust gas flow path of the heat exchanger. Further, it is characterized in that a combustion means for burning unburned gas in the exhaust gas at the time of startup is provided.

  The present invention described in claim 2 is characterized in that, in the fuel cell described in claim 1, a combustion catalyst is used as the combustion means.

  According to a third aspect of the present invention, there is provided the fuel cell according to the first or second aspect, wherein the fuel cell is fixed with a sealless structure that discharges exhaust gas from the outer periphery of the power generation cell. It is an oxide fuel cell.

  According to a fourth aspect of the present invention, there is provided the fuel cell starting method according to any one of the first to third aspects, wherein the fuel cell stack is heated when the temperature rises at the time of starting. In addition to heating in the fuel cell stack, hydrocarbon fuel and oxidant gas are supplied into the fuel cell stack, the unburned gas is burned by the combustion means to heat the heat exchanger, and generated in the heat exchanger The steam is introduced into the fuel reformer together with the hydrocarbon fuel.

  According to the present invention, the fuel means is provided in the heat exchanger of the steam generator, the exhaust gas is burned at the time of start-up, the heat exchanger is heated, and steam is generated. Sufficient water vapor can be supplied together with the hydrocarbon-based fuel, and this water vapor prevents carbon deposition when the exhaust gas (unburned gas) burns outside the fuel cell stack, and is also generated by reforming. As the unburned gas is burned more quickly by hydrogen, the temperature of the fuel cell stack can be raised in a short time, and thus rapid start-up becomes possible.

Hereinafter, embodiments of the present invention will be described with reference to FIGS.
FIG. 1 shows an internal schematic configuration of a solid oxide fuel cell to which the present invention is applied, FIG. 2 shows an internal structure of a steam generator, and FIG. 3 shows a main configuration of the fuel cell stack.

  In FIG. 1, reference numeral 1 is a solid oxide fuel cell (fuel cell module), reference numeral 2 is a housing (can body) provided with a heat insulating material (not shown) on the inner wall, and reference numeral 3 is a stacking direction vertical. It is the fuel cell stack installed inside the housing 2 in a state.

  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 are disposed on both surfaces of a solid electrolyte layer 4, and a fuel electrode current collector outside the fuel electrode layer 5. 8, an air electrode current collector 9 outside the air electrode layer 6, and a large number of separators 10 outside the current collectors 8 and 9 are stacked in order.

The solid electrolyte layer 4 is composed of stabilized zirconia (YSZ) or the like to which yttria is added, the fuel electrode layer 5 is composed of a metal such as Ni or a cermet such as Ni—YSZ, and the air electrode layer 6 is composed of LaMnO ₃ , LaCoO _3. The fuel electrode current collector 8 is composed of a sponge-like porous sintered metal plate such as Ni, and the air electrode current collector 9 is composed of a sponge-like porous sintered metal plate such as Ag. The separator 10 is made of stainless steel or the like.

  The separator 10 has a function of electrically connecting the power generation cells 7 and supplying a reaction gas to the power generation cells 7. The fuel gas supplied from the fuel gas manifold 13 is introduced from the outer peripheral surface of the separator 10. Then, an oxidant gas supplied from an oxidant gas manifold 14 and an oxidant gas manifold 14 discharged from a substantially central portion of the separator 10 facing the fuel electrode current collector 8 is introduced from the outer peripheral surface of the separator 10. An oxidant gas passage 12 that discharges from substantially the center of the surface facing the air electrode current collector 9 is provided.

  This solid oxide fuel cell 1 has a sealless structure in which no gas leakage prevention seal is provided on the outer periphery of the power generation cell 7, and during operation, as shown in FIG. 3, the fuel gas passage 11 and the oxidant gas The fuel electrode layer 5 and the air electrode layer 6 are diffused in the outer circumferential direction of the power generation cell 7 while the fuel gas and the oxidant gas (air) discharged toward the power generation cell 7 from the substantially central portion of the separator 10 through the passage 12. The power generation reaction is caused to spread over the entire surface with a good distribution, and the remaining gas (exhaust gas) that has not been consumed by the power generation reaction is freely released from the outer periphery of the power generation cell 7 to the outside. The emitted gas is burned around the fuel cell stack.

In addition to the fuel cell stack 3 described above, a heater 31 (electric heater), a fuel reformer 30 and the like are disposed in the housing 2 as a heating device for temperature. The fuel reformer 30 is filled with a hydrocarbon catalyst, and reforms a hydrocarbon-based fuel introduced from the outside into a hydrogen-based fuel gas.
A fuel gas supply pipe 15 from the outside is connected to the inlet side of the fuel reformer 30 and the outlet side is connected to a fuel gas manifold 13 disposed inside the fuel cell stack. An oxidant gas supply pipe 16 from the outside is connected to the oxidant gas manifold 14 inside the fuel cell stack (see FIG. 3).

  At startup, hydrocarbon fuel (for example, city gas, other LPG, kerosene, etc. can be used) is introduced into the fuel gas supply pipe 15 and air is introduced into the oxidant gas supply pipe 16. It is like that.

  On the other hand, at the bottom of the housing 2, a water vapor generator 20 is disposed that uses high-temperature exhaust gas discharged from the upper fuel cell stack 3 as a heat source.

The water vapor generator 20 is composed of a heat exchanger 21 accommodated in a space surrounded by a heat insulating material (not shown), and externally supplied water is supplied to the heat exchanger 21 through a water supply pipe 17. In the process of being introduced from the lower part and flowing upward, the heat exchanger 21 exchanges heat with the high-temperature exhaust gas in the housing 2 introduced from the upper exhaust gas inlet 26 to form high-temperature steam, and the fuel cell through the pipe 27 at the upper end part. The gas is guided into the module 1, and is merged and mixed with the city gas from the fuel gas supply pipe 15 in the module to be mixed into the fuel reformer 30.
Further, the exhaust gas introduced from the upper exhaust gas inlet 26 finishes heat exchange with the supply water in the process of flowing downward, becomes low temperature gas and is discharged to the outside from the lower exhaust pipe 19. Yes.

By the way, in this embodiment, as the above-mentioned heat exchanger 21, the water from the exhaust pipe 23 with the fin 24 through which the exhaust gas from the fuel cell stack 3 circulates and the water from the water supply pipe 17 circulate as shown in FIG. Plate fin type heat exchangers (or corrugated fin heat exchangers) in which water flow paths 22 with fins 24 (supply water is immediately vaporized and water vapor flows) are arranged in a crossed or opposed manner in units of plates. And a combustion catalyst 25 for combusting unburned gas in the exhaust gas flowing through the exhaust gas passage 23 is disposed on the fin 24 in the exhaust gas passage 23.
As the combustion catalyst 25, for example, Pt (platinum), Pd (palladium), or the like can be used, and these are applied in layers on the surface of the fin 24 described above.

  As described above, the combustion catalyst is disposed in the heat exchanger 21 (in the exhaust gas flow path 23), and the unburned gas is quickly ignited and burned, so that the conduction heat from the high temperature exhaust gas and the combustion of the unburned gas are performed. The heat increases the heat capacity of the heat exchanger, and the heat exchange performance can be remarkably improved. Thereby, externally supplied water can be efficiently heated and vaporized to stably obtain a large amount of water vapor in a short time.

In the solid oxide fuel cell 1 having the above-described configuration, the heater 31 is operated simultaneously with the start-up, the fuel cell stack 3 is directly heated and heated by heat conduction, and the hydrocarbon fuel (city) is supplied from the outside through the fuel gas supply pipe 15. Gas) is introduced into the fuel cell stack through the oxidant gas supply pipe 16. Then, these reaction gases introduced into the fuel cell stack are directly discharged from the outer periphery of the stack as exhaust gas (unburned gas), and since the power generation reaction cannot occur immediately after startup, the supplied fuel gas Part of the gas is burned around the stack, and the fuel cell stack 3 is heated and heated from the outer peripheral portion by the combustion heat of the exhaust gas together with the radiant heat from the heater 31.
In this case, a combustion catalyst 32 may be disposed in the vicinity of the fuel cell stack 3 as shown in FIG. As the combustion catalyst 32, for example, a thin plate-shaped honeycomb catalyst using Pt, Pd or the like supported on an alumina carrier is used, and this is disposed along the stacking direction.

On the other hand, the exhaust gas in the housing 2 is introduced into the heat exchanger 21 through the exhaust gas inlet 26, and ignited and burned by the combustion catalyst 25 in the exhaust gas passage 23, and steam is generated in the heat exchanger 21 in just a few minutes. The vaporization temperature to be obtained, for example, 300 ° C. or higher, is heated and heated, whereby the water vapor generator 20 can stably generate a large amount of high temperature water vapor.
Incidentally, the temperature in the housing at this time (temperature in the module) is about 200 ° C.

The water vapor is guided into the housing 2 through the pipe 27, and is mixed and mixed with the city gas from the fuel gas supply pipe 15 in the housing 2 to be mixed into the fuel reformer 30.
That is, only the city gas (no steam is introduced) is introduced into the fuel reformer 30 at the time of start-up, but a large amount of high-temperature steam is introduced immediately after that. In addition, since it is possible to prevent carbon deposition when the exhaust gas (unburned gas) burns outside the fuel cell stack, a large amount of city gas can be supplied at one time to promote combustion outside the stack of exhaust gas, In the fuel reformer 30, hydrogen is partially generated by the reforming reaction due to the introduction of the water vapor, so that the unburned gas is burned more quickly by the hydrogen generated by the reforming, and the fuel The battery stack 3 can be heated in a short time.
In addition, unrated power generation is performed by hydrogen generated by the reforming reaction, and the fuel cell stack 3 is also heated from the inside by surreal heat at that time.

Incidentally, although it has conventionally required 5 hours or more from the time of start-up to the start of power generation operation, in the present invention, it can be shortened to 2-3 hours, thereby enabling rapid start-up. In addition, if rapid start-up can be realized, purge at start-up, which was conventionally required, can be made unnecessary.
Purging means reducing the inside by supplying an inert gas (for example, nitrogen) to the fuel cell in order to prevent the fuel electrode layer from being oxidized by oxygen remaining inside the fuel cell when the temperature is raised. A process that maintains the atmosphere.

  In this embodiment, the combustion catalyst 25 is used as an ignition / combustion means for the exhaust gas in the steam generator 20, but it is of course possible to use an electric heater (ignition heater), an igniter, or the like.

The figure which shows the internal schematic structure of the solid oxide fuel cell to which this invention was applied. The principal part sectional drawing which shows the internal structure of the water vapor generator which concerns on this invention. It is a principal part schematic block diagram of the fuel cell stack which concerns on this invention, and shows the flow of the gas at the time of driving | operation.

Explanation of symbols

1 Fuel cell (solid oxide fuel cell)
3 Fuel cell stack 7 Power generation cell 20 Steam generator 21 Heat exchanger 23 Exhaust gas passage 25 Combustion means (combustion catalyst)
30 Fuel reformer

Claims (4)

A steam generator, a fuel reformer for steam reforming a hydrocarbon-based fuel, and a fuel cell stack for supplying a reformed gas and an oxidant gas from the fuel reformer to generate a power generation reaction In a fuel cell,
The steam generator includes a heat exchanger that uses exhaust gas from the fuel cell stack as a heat source, and combustion means for burning unburned gas in the exhaust gas at the time of startup in the exhaust gas passage of the heat exchanger A fuel cell comprising: The fuel cell according to claim 1, wherein a combustion catalyst is used as the combustion means. 3. The fuel cell according to claim 1, wherein the fuel cell is a fixed oxide fuel cell having a sealless structure that discharges exhaust gas from an outer peripheral portion of the power generation cell. 4. A method for starting a fuel cell according to any one of claims 1 to 3, comprising:
When the temperature rises at the time of startup, the fuel cell stack is heated by a heating device, and hydrocarbon fuel and oxidant gas are supplied into the fuel cell stack,
A fuel cell characterized in that the unburned gas is burned by the combustion means to heat the heat exchanger, and water vapor generated in the heat exchanger is introduced into the fuel reformer together with a hydrocarbon fuel. How to start.

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