JP2005340075A - How to shut down the fuel cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for shutting down a fuel cell capable of rapidly lowering the temperature of a reformer while effectively preventing carbon deposition and catalyst oxidation.
A reformer containing a reforming catalyst and heated and held by a gas discharged from a fuel cell is reformed into a hydrogen-containing fuel gas through a mixed gas of hydrocarbon gas and water vapor, and the fuel A method of stopping the operation of a fuel cell that generates power by supplying gas into a fuel cell in which air is flowing, and supplying the hydrocarbon gas while continuously supplying air into the fuel cell The amount of the mixed gas of the reduced amount of hydrocarbon gas and the water vapor is supplied into the fuel cell through the reformer, and the temperature of the reforming catalyst is changed to an oxidation generation temperature ± 150 ° C. of the catalyst. When the temperature falls to the range, the supply of the mixed gas is stopped, and after the supply of the mixed gas is stopped, air or hydrocarbon gas is purged as a purge gas through the reformer and purged.
[Selection] Figure 5

Description

  TECHNICAL FIELD The present invention relates to a method for stopping operation of a fuel cell. More specifically, the present invention relates to a raw material gas (hydrocarbon gas) in a reformer that is heated and held by gas discharged from the fuel cell as the fuel cell is operated. ) And water vapor to reform the fuel gas rich in hydrogen, and the fuel gas is supplied into the fuel cell to which air is supplied to stop the operation of the fuel cell for generating power.

  In recent years, various fuel cells in which a stack of fuel cells (cells) is housed in a housing have been proposed as next-generation energy. For example, a structure of a solid oxide fuel cell has a structure in which a fuel cell main body in which a plurality of cell stacks in which a plurality of fuel cell units (cells) are stacked is arranged at an appropriate interval is accommodated in a housing. It is operated at a temperature of 1000 ° C.

  As fuel gas for power generation, hydrogen is used, hydrogen gas and oxygen-containing gas (usually air) are supplied into the fuel cell body, and the oxygen-containing gas is brought into contact with the oxygen electrode in the cell. Electric power is generated by bringing hydrogen into contact with the fuel electrode in the cell to cause a predetermined electrode reaction.

  As a method for supplying hydrogen as a fuel gas, a steam reforming method in which a hydrocarbon such as natural gas reacts with steam to generate hydrogen is used, but the reforming reaction between the hydrocarbon and hydrogen is an endothermic reaction. Therefore, in order to stably generate power for the fuel cell, the fuel cell is heated and held at a predetermined operating temperature or higher, and the reformer filled with the reforming catalyst is brought to a temperature at which the reforming reaction is stably performed. It is necessary to keep it heated.

  For this purpose, a reformer is disposed in a heat-insulating housing that contains the cell stack, and heat generated during power generation or combustion heat of gas discharged from the fuel cell is heated by the fuel cell and reformer. It is used for improving thermal efficiency (see, for example, Patent Document 1).

  By the way, in order to stop the operation of the fuel cell, it is necessary to stop the operation of the reformer for reforming the raw material gas (hydrocarbon gas) into a hydrogen-containing fuel gas. However, the operating temperature of the fuel cell is considerably high (usually 700 ° C. or higher), and in the operating fuel cell, the reformer is also heated and held at substantially the same high temperature. Therefore, it takes time to bring the temperature of the fuel cell and the reformer back to room temperature, and it takes a considerable time to stop the operation of the fuel cell. It is required to shorten the time required for the temperature of the mass device to return to room temperature.

As a means to quickly cool the reformer or fuel cell to room temperature while preventing oxidation of the electrodes constituting the fuel cell and the catalyst in the reformer, it is inactive in the reformer and inside the fuel cell. Although it is conceivable to flow a gas, such means requires a supply facility for an inert gas, and causes a significant increase in cost. In addition, there is a problem of an inert gas source in a fuel cell provided for home use. Accordingly, there is a demand for means that does not use an inert gas. For example, Patent Document 2 discloses a method for stopping a reformer attached to a polymer electrolyte fuel cell, and the reformer reforming unit is stopped when the method is stopped. When the temperature of the reforming catalyst layer is lowered, after the reformed gas in the reformer is purged with steam, the temperature of the reforming catalyst layer is equal to or lower than the temperature at which no thermal decomposition of the raw material gas occurs, and the condensation temperature of steam There has been proposed a method for stopping a reformer for a polymer electrolyte fuel cell, characterized by introducing a raw material gas and purging water vapor in the reformer after the above-described decrease.
JP-A-8-287937 JP 2002-151124 A

  The method of Patent Document 2 is modified by flowing steam used for reforming until the temperature of the catalyst layer of the reformer is equal to or lower than the thermal decomposition temperature of the raw material gas (hydrocarbon gas) and equal to or higher than the steam condensation temperature. It is intended to prevent dew condensation by accelerating the temperature drop of the mass vessel and then purging the water vapor with the raw material gas at a temperature equal to or higher than the condensation temperature of the water vapor. Is unnecessary and has an economic advantage. However, in this method, since only water vapor flows in the catalyst layer in a high temperature region, when a base metal catalyst such as a Ni catalyst is used as the reforming catalyst, the water vapor becomes an oxygen source and the catalyst is oxidized. There's a problem. In addition, when the operation of the solid oxide fuel cell is stopped using this method, there is a problem that oxidation of electrodes and the like occurs.

  Accordingly, an object of the present invention is to provide a method of shutting down a fuel cell that can quickly lower the temperature of a reformer while effectively preventing oxidation of electrodes and catalysts without using an inert gas. It is in.

According to the present invention, the reformer containing the reforming catalyst and heated and held by the gas discharged from the fuel cell is reformed into a hydrogen-containing fuel gas through a mixed gas of hydrocarbon gas and steam, A method of stopping the operation of a fuel cell that generates power by supplying the fuel gas into a fuel cell in which air is flowing,
While continuously supplying air into the fuel cell, the supply amount of the hydrocarbon gas is reduced, and the mixed gas of the reduced amount of the hydrocarbon gas and the water vapor is supplied into the fuel cell through the reformer. To supply
When the temperature of the reforming catalyst falls in the range of the oxidation occurrence temperature of the catalyst ± 150 ° C., the supply of the mixed gas is stopped,
After stopping the supply of the mixed gas, a method of stopping the operation of the fuel cell is provided, wherein air or hydrocarbon gas is purged as a purge gas through the reformer through the fuel cell.

  In the present invention, the temperature of the reforming catalyst is equal to the temperature of the gas passing through the reforming catalyst layer and is usually higher than the gas temperature on the outlet side of the reforming catalyst layer (generally, higher than the gas temperature on the inlet side). ) Is detected. However, the temperature on the inlet side may become higher due to heat transfer such as reaction heat. In this case, the gas temperature on the inlet side is set as the temperature of the reforming catalyst (reformer temperature). That is, in order to carry out the present invention, it is necessary to always monitor the gas temperature on the outlet side and the inlet side of the reforming catalyst in the reformer. Hereinafter, the temperature of the reforming catalyst may be referred to as the temperature of the reformer.

  Further, the oxidation generation temperature of the catalyst varies depending on the type, and can be determined by confirming the generation of an oxide peak by, for example, XRD. For example, it is 350 degreeC with Ni catalyst.

In the present invention,
(1) The supply of the purge gas is started continuously after the supply of the mixed gas is stopped.
(2) supplying the mixed gas at least until the temperature of the reforming catalyst is lowered to the oxidation generation temperature;
(3) After the supply of the mixed gas is stopped, the steam is supplied to the fuel cell through the reformer, and the steam is supplied at least in a stage where the reforming catalyst is maintained at a temperature higher than the condensation temperature of the steam. Stopping and starting the supply of the purge gas in succession to the stop of the steam,
(4) The supply of the mixed gas is stopped at a stage where the reforming catalyst is maintained at a temperature higher than its oxidation generation temperature.
(5) stopping the supply of the water vapor when the temperature of the reforming catalyst is lowered to 200 to 350 ° C. or when the temperature is lowered to 100 to 200 ° C .;
(6) The reduced supply amount of the hydrocarbon gas is 0.01 to 0.6 times the maximum supply amount during operation,
(7) using a nickel catalyst as the reforming catalyst,
Can do.

In the present invention, no special gas such as an inert gas is used, and only the gas used for power generation of the fuel cell is used for cooling the reformer. Therefore, no special gas source or gas supply facility is required, which is extremely advantageous in terms of cost. In addition, since a reduced amount of raw material gas (hydrocarbon gas) and water vapor are supplied at the initial stage of temperature lowering when the temperature of the reformer is high, the temperature lowering is promoted by reducing the amount of combustion heat and absorbing heat from the steam reforming reaction Furthermore, oxidation of the reforming catalyst is effectively suppressed by hydrogen generated by the steam reforming reaction. Further, when carbon is precipitated by thermal decomposition of the hydrocarbon gas, the precipitated carbon is removed as CO and CH ₄ by a reaction with water vapor supplied in excess as compared with the hydrocarbon gas. Furthermore, since the gas remaining in the reformer is purged by the hydrocarbon gas or air at a temperature higher than the condensation temperature of the steam, the disadvantages caused by the condensation of the steam can be effectively eliminated.

Hereinafter, the present invention will be described in detail based on specific examples shown in the accompanying drawings.
FIG. 1 is a diagram for explaining a gas flow in a fuel cell structure used in the present invention.
FIG. 2 is a diagram showing a schematic structure of a fuel cell structure preferably used in the present invention.
FIG. 3 is a diagram (a cross-sectional view taken along line AA in FIG. 2) showing a schematic structure of a cell stack of the fuel cell main body in the fuel cell structure in FIG.
FIG. 4 is a diagram showing the temperature history of the reformer from the start of operation to the stop of operation of the fuel cell structure of FIG.
FIGS. 5 to 7 are flowcharts showing an example of a method for stopping the operation of the reforming system according to the present invention, together with the temperature history of the reformer.

  1 and 2, the fuel cell structure used in the present invention has a structure in which a fuel cell 1 and a reformer 2 are provided in a predetermined heat insulating housing (not shown). A combustion zone 3 is formed in the upper part of the fuel cell 1, and a reformer 2 is arranged in the upper part of the combustion zone 3. That is, the reformer 2 is heated by the combustion heat in the combustion zone 3.

That is, a hydrocarbon gas such as city gas (usually CH ₄ gas) is supplied to the reformer 2 via the desulfurizer 5 (not shown in FIG. 2), reformed into a hydrogen-rich fuel gas, and reformed. The fuel gas thus supplied is supplied into the fuel cell 1. On the other hand, air is supplied into the fuel cell 1 through another route, and power is generated by supplying these gases. The gas after power generation is discharged to the upper part of the fuel cell 1 and diffused and burned in the combustion zone 3 by ignition by an ignition burner or the like, and combustion waste gas (exhaust gas) is released to the outside.

  As shown in FIG. 2, the fuel cell 1 has a structure in which a plurality of cell stacks 10 connected to a plurality of cell units are arranged at appropriate intervals. A manifold 11 is provided. That is, the reformed gas (fuel gas) obtained through the reformer 2 is supplied into the cell unit of each cell stack 10 via the manifold 11 and discharged from the upper part of the cell stack 10 to the combustion zone 3. It has become so.

  On the other hand, a power generation gas supply pipe 13 extends downward from above between the cell stacks 10, and power generation air is supplied through the supply pipe 13. That is, air for power generation is supplied between the cells of the cell stack 10 from the lower end of the supply pipe 13 and is discharged to the combustion zone 3 from the upper part between the cell stacks 10. Combustion by diffusive combustion is performed by supplying a sufficient amount of oxygen.

  Referring to FIG. 3 showing the structure of the cell stack 10, the cell stack 10 provided on the manifold 11 has a plate-like and column-like upright cell unit 20 that is elongated in the vertical direction. In the space between the plurality of cell stacks 10, a power supply gas supply pipe 13 extends downward from above.

  In each cell unit 20, a fuel electrode layer 23, a solid electrolyte layer 25, and an oxygen electrode layer 27 are laminated in this order on one surface of the electrode support substrate 21, and a fuel electrode layer 23 is stacked on the other surface of the electrode support substrate 21. The interconnector 29 is laminated so as to face the polar layer 23. The solid electrolyte layer 25 is provided so as to completely cover the fuel electrode layer 23, and the fuel electrode layer 23 and the solid electrolyte layer 25 wrap around to the other surface of the electrode support plate 21. 29 is joined to both ends. A plurality of gas holes 21a communicating with the manifold 11 are formed inside the electrode support plate 21, and the fuel gas (reformed gas) supplied into the manifold 11 passes through the gas holes 21a. It passes through and is discharged into the upper combustion zone 3. That is, power is generated by supplying fuel gas (reformed gas) to the gas holes 21 a in the cell unit 20 and supplying an oxygen-containing gas for power generation from the gas supply pipe 13 between the cell stacks 10. It has a structure that is called.

  Adjacent cell units 20 are connected by a current collecting member 31, and the interconnector 31 of one cell unit 20 and the oxygen electrode layer 27 of the other cell unit 20 are connected by the current collecting member 31. ing. Although not shown, the current collecting members 31 located at both ends of the cell stack 10 are connected to a circuit to which power extraction means are connected, and the generated current is extracted.

  In the cell unit 20 described above, the electrode support substrate 21 needs to be gas permeable in order to allow the fuel gas to permeate to the fuel electrode layer 23, and to conduct electricity to collect current via the interconnector 29. It can be formed from a porous conductive ceramic (or cermet) that satisfies this requirement.

  The electrode support substrate 21 is preferably composed of an iron group metal component such as Ni and a specific rare earth oxide in order to be produced by simultaneous firing with the fuel electrode layer 23 and the solid electrolyte layer 25. In order to provide gas permeability, the open porosity is preferably 30% or more, particularly 35 to 50%, and the conductivity is preferably 300 S / cm or more, particularly 440 C / cm or more.

The fuel electrode layer 23 can be formed of a porous conductive ceramic, for example, ZrO ₂ (referred to as stabilized zirconia) in which a rare earth element is dissolved, and Ni and / or NiO.

The solid electrolyte layer 25 has a function as an electrolyte for bridging electrons between the electrodes, and at the same time needs to have a gas barrier property in order to prevent leakage between fuel gas and air. In general, it is formed from ZrO ₂ in which ₃ to 15 mol% of a rare earth element is dissolved.

The oxygen electrode layer 27 can be formed of a conductive ceramic made of a so-called ABO ₃ type perovskite oxide. The oxygen electrode layer 27 is required to have gas permeability, and preferably has an open porosity of 20% or more, particularly 30 to 50%.

The interconnector 29 can be formed from a conductive ceramic. However, since it is in contact with hydrogen gas (fuel gas) and oxygen-containing gas (air), the interconnector 29 must have reduction resistance and oxidation resistance. Therefore, lanthanum chromite-based perovskite oxides (LaCrO ₃ -based oxides) are preferably used. The interconnect 29 must be dense to prevent leakage of fuel gas passing through the gas holes 21a formed in the electrode support substrate 21 and air flowing outside the electrode support substrate 21, and is 93% or more In particular, it is desirable to have a relative density of 95% or more.

  The current collecting member 31 can be composed of a member having an appropriate shape formed from a metal or alloy having elasticity, or a member obtained by adding a required surface treatment to a felt made of metal fiber or alloy fiber.

That is, in a state where the cell unit 20 as described above is heated to a predetermined operating temperature (about 700 to 1000 ° C.), an oxygen-containing gas (air) for power generation is caused to flow from the supply pipe 13, and the fuel gas is supplied to the gas hole 21a. When (hydrogen) is flowed, in the oxygen electrode layer 27,
1 / 2O ₂ + 2e ⁻ → O ²⁻ (solid electrolyte)
The electrode reaction occurs, and in the fuel electrode layer 23,
O ²⁻ (solid electrolyte) + H ₂ → H ₂ O + 2e ⁻
As a result, electric power is generated.

  The structure of the cell unit 20 is not limited to the above-described example. For example, the electrode support plate 21 can be a fuel electrode, and the positional relationship between the fuel electrode layer 23 and the oxygen electrode layer 27 can be determined. Conversely, it is also possible to adopt a structure in which oxygen-containing gas (air) for power generation is supplied to the gas holes 21 a and fuel gas (reformed gas) is supplied between the cell stacks 10.

  By the way, in order to generate power using the fuel cell structure as described above, as described above, a hydrocarbon gas typified by city gas is used as a raw fuel as a fuel source. Is reformed into a hydrogen-rich fuel gas by a reforming reaction through the reformer 2 and supplied to the fuel cell body 1. This reforming reaction is classified into a partial oxidation reforming reaction, an autothermal reforming reaction, and a steam reforming reaction according to the type of gas supplied to the reformer. That is, if a hydrocarbon gas and an oxygen-containing gas (that is, air) are supplied to the reformer 2, a reforming reaction is performed by a partial oxidation reaction, and the hydrocarbon gas, the oxygen-containing gas, and water vapor are supplied to the reformer 2. If a hydrocarbon gas and an oxygen-containing gas are supplied to the reformer 2, a steam reforming reaction is obtained. In order to cause such a reforming reaction, it is necessary to heat the reformer 2 (reforming catalyst) above the reforming reaction start temperature according to the type of the reforming catalyst. In particular, since the steam reforming reaction is an endothermic reaction, in order to continue this, it is necessary to continue heating so as not to cause a temperature drop. It is also necessary to heat the cell 20 to the operating temperature. Therefore, in order to perform such heating without using a special heating source, a combustion zone 3 is provided in the upper part of the fuel cell main body 1, and the combustion zone 3 is provided between the reformer 2 and the fuel ionization main body 1. By arranging, combustion in the combustion zone 3 is utilized. The reformer 2 is supplied with reforming steam together with the raw material gas (hydrocarbon gas). The steam is generated using a separate heating source and supplied to the reformer 2. However, the reformer 2 can be provided with a vaporization chamber for generating steam, and steam can be generated by the combustion heat in the combustion zone 3 to supply the reformer 2 with steam, thereby increasing thermal efficiency. , Energy costs can be reduced. The reformer 2 having such a structure is shown in FIG.

  As shown in FIG. 2, such a reformer 2 is disposed so as to be positioned over the entire upper portion of the fuel cell main body 1 (cell stack 10) with the combustion zone 3 interposed therebetween. That is, the entire reformer 2 is heated by the combustion in the combustion zone 3.

  The reformer 2 is a cylindrical body having a ring-shaped cross section (of course, the cross section may be rectangular), and has a catalyst storage chamber 40 in the upper portion and a vaporization chamber 41 in the lower portion. An inlet chamber (gas mixing chamber) 43 is formed at one end (gas supply side) of the catalyst storage chamber 40, and an outlet chamber 44 is formed at the other end (gas discharge side). . A gas supply pipe 45 for supplying raw fuel gas to be reformed is connected to the inlet chamber 43, and an exhaust pipe 47 for discharging the gas reformed in the catalyst storage chamber 40 is connected to the outlet chamber 44. The exhaust pipe 47 communicates with the manifold 11 of the fuel cell main body 1.

  A reforming catalyst 49 is accommodated in the catalyst accommodating chamber 40, and the catalyst accommodating chamber 40 is provided with an inlet chamber 43 and an outlet chamber by a partition wall 50 such as a mesh so that the accommodated reforming catalyst 49 does not leak. 44. The inlet chamber 43 and the gas supply side portion of the vaporization chamber 41 are also partitioned by a partition wall 51 such as a mesh so that water vapor flows.

  A water supply pipe 53 is connected to the gas discharge side portion of the vaporizing chamber 41 so that water is supplied into the vaporizing chamber 41. That is, when water is supplied from the water supply pipe 53, the water is vaporized in the vaporization chamber 41 due to the heating by the combustion heat from the combustion zone 3 described above, and the generated water vapor is supplied into the catalyst storage chamber 40 through the inlet chamber 43. The Rukoto. In the reformer 2 having such a structure, excessive heating of the catalyst storage chamber 40 and the catalyst inlet temperature in the storage chamber 40 (temperature of the inlet chamber 43) is suppressed by the heat of vaporization of water, and carbon deposition occurs. Very suitable for prevention.

  In addition, the size of the catalyst storage chamber 40 is not particularly limited, but considering the case where the fuel cell structure is used particularly for home use, the size of the catalyst storage chamber 40 is compact and the gas passing through the reformer and the catalyst are mixed. From the standpoint of ensuring a contact time during which sufficient reforming can be performed, the ratio L / D of the length L to the opening diameter D is preferably set to about 5 to 15.

  In addition, as the reforming catalyst 49 accommodated in the catalyst accommodating chamber 40, a known catalyst is used per se, such as a base metal catalyst such as a Ni catalyst (oxidation generation temperature: 350 ° C.), Ru, Pt or the like. A noble metal catalyst is used. Base metal catalysts such as Ni catalysts have low oxidation activity, and the reforming reaction start temperature is as high as about 350 ° C., which is equal to the oxidation generation temperature, while noble metal catalysts usually have high oxidation activity when hydrocarbon gas is passed through. The reforming reaction start temperature is about 250 to 300 ° C.

  The vaporization chamber 41 may have any structure as long as it easily vaporizes water and smoothly supplies water vapor to the catalyst storage chamber 40 via the inlet chamber 43. For example, the vaporization chamber 41 can be filled with heat-resistant ceramic particles having a high heat capacity.

  In the reformer 2 having the above-described structure, a temperature sensor (not shown) such as a thermocouple is provided on the partition wall 50 that partitions the inlet chamber 43 and the outlet chamber 44 before and after the catalyst housing chamber 40. The following operation is performed while monitoring the temperature of the gas introduced into the reforming catalyst 49 filled in the catalyst housing chamber 40 or the temperature of the gas discharged from the reforming catalyst 49.

(Operation of fuel cell)
In order to generate electricity by operating the fuel cell structure described above, a fuel cell is produced by reforming a raw material gas (hydrocarbon gas) using a reformer 2 while supplying air for power generation to the fuel cell 1. The reforming system supplied to 1 is operated. The temperature history of the reformer 2 during this operation is as shown in FIG. The temperature history of the fuel cell 1 (cell unit 20) is substantially the same.

Referring to FIG. 4, when the fuel cell structure 1 is operated, a mixed gas of hydrocarbon gas and water vapor is supplied to the fuel cell 1 through the reformer 2 and at the same time, power generation air is fueled by another path. The battery 1 is supplied to start activation. At the start-up, since the reforming catalyst 49 in the reformer 2 has not reached the predetermined reaction start temperature, the reforming is not performed, and the mixed gas of hydrocarbon gas and steam remains as it is. Supplied to the fuel cell 1. In this case, it is also possible to start by supplying air (O ₂ ) together with hydrocarbon gas instead of water vapor. In addition, when supplying water vapor, it is necessary to generate water vapor from a separate heat source, but when using air (hereinafter, air supplied instead of water vapor may be referred to as starting air). Does not require such a separate heat source.

  As described above, the mixed gas supplied into the fuel cell and the air for power generation supplied through another path are discharged to the combustion zone 3 above the fuel cell 1 (cell stack 10). Combustion occurs by ignition in the combustion zone 3, and the temperature of the reformer 2 and the fuel cell 1 rises due to the heat of combustion.

  When the temperature of the reformer 2 (temperature of the reforming catalyst 49) rises and reaches a predetermined reforming reaction start temperature, the reforming of the hydrocarbon gas is performed, and a hydrogen-rich fuel gas is generated. A fuel gas containing hydrogen gas is supplied to the fuel cell 1.

In the above reforming reaction, for example, when CH ₄ gas is used as the hydrocarbon gas, when steam is supplied together with the hydrocarbon gas, the following formula:
CH ₄ + H ₂ O → 3H ₂ + CO (endothermic reaction)
When air (oxygen) is supplied, the following formula:
CH ₄ + 1 / 2O ₂ → 2H ₂ + CO (exothermic reaction)
It is represented by That is, when steam is supplied, although the hydrogen generation efficiency is high, it is an endothermic reaction, so it takes time to perform the reforming reaction stably (that is, the startup time is long). On the other hand, when starting by supplying air, since the reforming reaction is an exothermic reaction, the temperature reaches a temperature at which the reforming reaction occurs stably in a relatively short time. There is a drawback that it is complete combustion and carbon is likely to precipitate. For this reason, at the time of start-up, air is supplied together with the hydrocarbon gas, and when the temperature reaches a temperature at which the reforming reaction is stably performed, the reforming is performed by switching the air to steam. The reforming reaction start temperature varies depending on the oxidation activity of the reforming catalyst 49 and, as described above, is about 350 ° C. when a base metal catalyst such as a Ni catalyst is used, and when a noble metal catalyst is used. It is about 250-300 degreeC. Accordingly, the timing for switching from air to oxygen is preferably when the gas temperature monitored in the outlet chamber 44 of the reformer 2 is about 50 ° C. higher than the reforming reaction start temperature.

When air is supplied instead of water vapor, the start-up air and the hydrocarbon gas have a mixed molar ratio (O ₂ / C) of oxygen (O ₂ ) to hydrocarbon (C) of 0.5 to 1. It is preferably adjusted so as to be in the range of 0.0. For example, when the supply amount of starting air is less than the above range, when the temperature of the reforming catalyst 49 becomes equal to or higher than the reforming reaction start temperature, the amount of oxygen that reacts with hydrocarbons on the reforming catalyst 49 is Due to the shortage, hydrocarbons may be thermally decomposed and carbon (soot) may be generated. When the supply amount of air is larger than the above range, when the temperature of the reforming catalyst 49 becomes equal to or higher than the reforming reaction start temperature, complete combustion occurs, and the temperature of the reformer 2 rises all at once. Then, the reformer 2 may be damaged. Moreover, since it is complete combustion, there is a risk of backfire that burns in the pipe from the inside of the reformer 2.

  In addition, when air is supplied together with the hydrocarbon gas at the start-up, and then the air is switched to steam, it is particularly preferable to use the reformer 2 having the structure shown in FIG. That is, when switching to steam, in the reformer 2 of this structure, the vaporization chamber 41 is sufficiently heated, and water vapor is generated in the vaporization chamber 41 by the supply of water from the water supply pipe 53. It is because it is not necessary to use.

  As described above, a mixed gas of hydrocarbon gas and water vapor is supplied to the reforming catalyst 49 in the reformer 2, and the temperature of the fuel cell 1 (cell unit 20) is set at a predetermined power generation temperature (700 to 1000 ° C.). In this state, the electrode reaction described above occurs and power generation is started. That is, in a stable operation state in which power generation is stably continued, the temperature of the reformer 2 is maintained at a temperature of 500 ° C. or higher by combustion in the combustion zone 3.

In the stable operation as described above, the mixing molar ratio (H ₂ O / C) of water vapor and hydrocarbon gas is preferably adjusted to be in the range of 1.5 to 4.0.

(Operation stopped)
In a fuel cell structure that is operating stably, when the circuit to which the fuel cell 1 is connected is interrupted, the flow of current stops and power generation is stopped, the supply of gas to the reformer 2 is stopped, and the reforming catalyst. 49, the operation is stopped in the present invention, as shown in FIG. The flowchart in FIG. 5 shows an example in which a Ni catalyst having an oxidation generation temperature of 350 ° C. is used as the reforming catalyst.

  That is, referring to FIG. 5, when the fuel cell structure is in operation, the fuel cell 1 is heated and held at the operating temperature (usually 700 ° C. or higher), and the combustion heat of the gas discharged from the fuel cell 1 The reformer 2 is heated and held. Since this reformer is housed in a heat insulating housing together with the fuel cell and is disposed adjacent to the gas combustion zone, it is heated and maintained at a high temperature of 500 ° C. or higher during such operation. A reforming reaction is carried out. In the present invention, in order to stop the operation using the reformer 2 as described above, first, the power generation of the fuel cell 1 is stopped. Along with this power generation stop, generation of reaction heat and Joule heat due to power generation reaction. Therefore, the temperature drop of the fuel cell 1 and the reformer 2 starts. At this time, the supply of air to the fuel cell 1 is continued. That is, the air supplied to the fuel cell 1 for power generation functions as a cooling gas when the power generation is stopped, and the supply of air to the fuel cell 1 is continued until the temperature of the fuel cell 1 drops to about room temperature. You can continue.

  In the following description, the temperature of the reformer 2 has already been described, but corresponds to the temperature of the reforming catalyst 49, and usually the gas on the reforming catalyst outlet side measured in the outlet chamber 44. When the temperature on the inlet side is higher than the temperature on the outlet side, the gas temperature on the reforming catalyst inlet side is measured in the inlet chamber 43.

  In the present invention, the supply amount of the raw material gas (hydrocarbon gas) supplied to the reformer 2 is reduced along with the start of temperature drop due to the stop of power generation, and the reduced amount of raw material gas and water vapor are still kept at a high temperature. The fuel is supplied into the fuel cell 1 through the reformer 2. That is, at this stage, since the temperature is higher than the spontaneous ignition temperature of hydrogen (about 550 ° C.), the combustion of the gas discharged from the fuel cell is not stopped. Since the steam reforming reaction, which is a reaction, continues, the temperature drop of the reformer 2 and the fuel cell 1 is promoted. Further, since the hydrocarbon gas coexists with an excessive amount of water vapor, the carbon deposition due to the thermal decomposition of the hydrocarbon is effectively suppressed, and further, the oxidation of the reforming catalyst 49 is performed by the hydrogen generated by the reforming reaction. Alternatively, oxidation of metal components (for example, Ni) constituting the electrodes of the cell unit 20 can be effectively prevented.

  In such a stage, the amount of reduction of hydrocarbon gas is usually set to be 0.01 to 0.6 times, particularly 0.05 to 0.3 times the maximum amount of hydrocarbon gas during stable operation. Is preferred. By reducing the hydrocarbon gas within this range, it is possible to generate a steam reforming reaction moderately, effectively promote the temperature drop of the reformer 2, and at the same time most reliably prevent carbon deposition due to steam shortage. it can. In addition, the hydrogen flow generated by the reforming reaction can effectively prevent reoxidation of the reforming catalyst and oxidation of metal components such as Ni used for the electrodes in the fuel cell 1.

  The temperature continues to decrease as described above, and the temperature of the reformer 2 is near the oxidation generation temperature of the reforming catalyst (shown in FIG. 5 which is 350 ° C., which is the oxidation generation temperature of the Ni catalyst). When the temperature reaches the temperature range of ± 150 ° C., the supply of the mixed gas of hydrocarbon gas and steam to the reformer 2 is stopped. That is, this temperature range is lower than the spontaneous ignition temperature range of hydrogen, and by stopping the supply of the mixed gas, combustion of the gas discharged from the fuel cell can be stopped quickly, and the reformer 2 and the fuel cell can be stopped. The temperature drop of 1 can be further promoted.

  After the supply of the mixed gas is stopped as described above, the purge gas is supplied to the reformer 2 and flows to the fuel cell 1, whereby the water vapor present in the reformer 2, the fuel cell 1, or the piping is removed. By discharging, it is possible to prevent re-oxidation of a reforming catalyst or the like using steam as an oxygen source, and subsequent condensation of steam due to a temperature drop.

  In the present invention, as the purge gas, an inert gas is not used, but a raw material gas (hydrocarbon gas) or air is used. That is, when an inert gas is used, it is necessary to provide a gas supply mechanism separately from the gas supply mechanism for power generation, and an inert gas supply source must be installed, which increases costs. Or, the equipment becomes complicated. However, since the source gas and air are used for power generation, no special supply mechanism is required, and an increase in cost or the like can be effectively avoided. For example, in general, the raw material gas is used as the purge gas, but when air is used for the above-described activation, the air can be used as the purge gas because the air supply pipe is connected to the reformer. . In particular, when air is used as the purge gas, it is advantageous in that wasteful consumption of the raw material gas can be avoided and the running cost can be reduced.

  The supply of the purge gas after the supply of the mixed gas is stopped may be performed at a temperature where the temperature of the reformer 2 is higher than the condensation temperature of steam (usually 100 ° C.), but in general, the supply of the mixed gas is stopped. Subsequent to this, it is preferable to continuously supply the purge gas without leaving a time lag. That is, it is advantageous to continue the flow of gas through the reformer 2 in order to promote the temperature drop. Further, it is preferable to continue the supply of the mixed gas up to at least the oxidation generation temperature of the reforming catalyst and start the supply of the purge gas following the stop of the supply of the mixed gas in order to prevent reoxidation of the reforming catalyst. In particular, when air is used as the purge gas, it is most preferable to start supplying the purge gas at an oxidation generation temperature or lower in order to prevent reoxidation of the reforming catalyst.

  It is sufficient that the purge gas is supplied to such an extent that the water vapor existing in the reformer 2, the fuel cell 1, or the piping is exhausted. Thus, it is desirable that the reformer 2 be heated until the temperature of the reformer 2 falls to 100 ° C. or lower, particularly near room temperature.

  As described above, the temperature of the reformer 2 and the fuel cell 1 is lowered, the temperature of the fuel cell 1 is lowered to near room temperature, and the supply of air into the fuel cell 1 in another path is stopped, The operation stop operation according to the present invention ends.

In the example of FIG. 5 described above, the purge gas is supplied after the supply of the mixed gas is stopped. However, in the present invention, after the supply of the mixed gas is stopped, the steam is once supplied (that is, the hydrocarbon gas is supplied). Only the supply is stopped and the supply of water vapor is continued), and the purge gas can be supplied. That is, even if carbon deposition occurs in the mixed gas supply stage or stable operation described above, the deposited carbon is converted into the following reaction (steaming) by supplying water vapor:
C + H ₂ O → CO + H ₂
It is because it can be removed by.

  Thus, the flowchart of the aspect which supplies water vapor | steam (steam) prior to supply of purge gas was shown in FIG.6 and FIG.7.

  In the example of FIGS. 6 and 7, the supply stop temperature X of the mixed gas of the reduced hydrocarbon gas and steam is set to a temperature higher than the oxidation generation temperature of the reforming catalyst, preferably from the oxidation generation temperature to the oxidation generation temperature + 150. It is preferable to set the temperature in the range of ° C. and continuously supply water vapor following the supply of the mixed gas. That is, in the temperature range from the oxidation generation temperature to the oxidation generation temperature + 150 ° C., it is possible to effectively remove the precipitated carbon by the above steaming reaction, and at this temperature range, the oxidation rate of the catalyst is slow, This is because it is possible to suppress the reoxidation of the reforming catalyst using NO as an oxygen source to a level that can be ignored.

  In addition, the supply of water vapor should be stopped at a temperature higher than at least the condensation temperature, but the specific stop temperature Y varies depending on the water vapor generating means. That is, when steam is generated using the combustion heat in the fuel cell structure as shown in FIG. 2 as described above, as shown in FIG. 6, at least up to the oxidation generation temperature of the reforming catalyst. It is preferable to supply the water vapor and to set the supply stop temperature Y to a relatively high region, for example, 200 to 350 ° C., in order to stably supply the water vapor. In addition, when water vapor is generated by providing a heat source completely separate from the fuel cell structure, water vapor can be stably supplied. Therefore, as shown in FIG. For example, it is preferable to set the temperature in the range of 100 to May 28, 2004. If the supply is stopped in this range, the steam can be completely removed by the purge gas until the temperature of the reformer 2 is lowered to the steam condensing temperature (usually 100 ° C.). is there.

  In the present invention, when a source gas (hydrocarbon gas) is used as the purge gas, in any of the above-described embodiments shown in FIGS. 5 to 7, the temperature is lower than the oxidation generation temperature of the reforming catalyst 49 and is carbonized. In order to reliably prevent carbon deposition, it is preferable to start purging at a temperature lower than the thermal decomposition temperature of hydrogen (usually about 300 to 400 ° C.). Depending on the type of reforming catalyst, even if it is below the oxidation generation temperature, it may be above the thermal decomposition temperature of the hydrocarbon. In this case, the supply of the raw material gas that is the purge gas is started below the thermal decomposition temperature. Is good.

  In the present invention, by reducing the temperature of the reformer as described above, the temperature of the reformer is lowered to room temperature more quickly without causing condensation of water vapor, and carbon deposition and oxidation of the reforming catalyst are also effective. In addition, the fuel cell 1 can be effectively avoided in the same manner from the precipitation of carbon and the oxidation of the metal constituting the cell. In particular, when a Ni catalyst is used as the reforming catalyst, the same antioxidant effect can be obtained for the fuel cell. That is, the fuel cell 1 uses Ni as the electrode support and fuel electrode material, and exhibits the same oxidation behavior as the Ni catalyst.

The figure for demonstrating the flow of the gas in the fuel cell used for this invention. The figure which shows schematic structure of the fuel cell structure used for this invention. The figure which shows the schematic structure of the cell stack of the fuel cell main body in the fuel cell structure of FIG. 2 (AA sectional drawing of FIG. 2). The figure which shows the thermal history of the reformer from an operation to a stop. The figure which shows the flowchart of an example of the operation stop method of this invention with the temperature history of a reformer. The figure which shows the flowchart of the other example of the operation stop method of this invention with the temperature history of a reformer. The figure which shows the flowchart of the further another example of the operation stop method of this invention with the temperature history of a reformer.

Explanation of symbols

1: Fuel cell 2: Reformer 3: Combustion zone 40: Catalyst storage chamber 41: Vaporization chamber 43: Inlet chamber 44: Outlet chamber 49: Reforming catalyst 53: Water pipe

Claims (9)

A reformer containing the reforming catalyst and heated and held by the gas discharged from the fuel cell is reformed into a hydrogen-containing fuel gas through a mixed gas of hydrocarbon gas and water vapor. A method of stopping the operation of a fuel cell that is supplied into a flowing fuel cell to generate power,
While continuously supplying air into the fuel cell, the supply amount of the hydrocarbon gas is reduced, and the mixed gas of the reduced amount of the hydrocarbon gas and the water vapor is supplied into the fuel cell through the reformer. To supply
When the temperature of the reforming catalyst falls in the range of the oxidation occurrence temperature of the catalyst ± 150 ° C., the supply of the mixed gas is stopped,
A method of stopping the operation of a fuel cell, wherein after the supply of the mixed gas is stopped, air or hydrocarbon gas is purged as a purge gas through the reformer through the reformer.   The fuel cell operation stopping method according to claim 1, wherein the supply of the purge gas is started continuously after the supply of the mixed gas is stopped.   The operation stop method according to claim 2, wherein the mixed gas is supplied at least until the temperature of the reforming catalyst falls below its oxidation generation temperature.   After the supply of the mixed gas is stopped, water vapor is allowed to flow through the reformer to the fuel cell, and the supply of the water vapor is stopped at a stage where at least the reforming catalyst is maintained at a temperature higher than the condensation temperature of the water vapor. The method for stopping the operation of the fuel cell according to claim 1, wherein the supply of the purge gas is started continuously after the stop of water vapor.   The operation stop method according to claim 4, wherein the supply of the mixed gas is stopped at a stage where the reforming catalyst is maintained at a temperature higher than its oxidation generation temperature.   The operation stop method according to claim 5, wherein the supply of the water vapor is stopped when the temperature of the reforming catalyst is lowered to 200 to 350 ° C.   The operation stop method according to claim 5, wherein the supply of the water vapor is stopped when the temperature of the reforming catalyst falls to 100 to 200 ° C.   The operation stop method according to any one of claims 1 to 7, wherein a reduced supply amount of the hydrocarbon gas is 0.01 to 0.6 times a maximum supply amount during operation.   The operation stop method according to any one of claims 1 to 8, wherein a nickel catalyst is used as the reforming catalyst.

Images