Fuel cell system

A fuel cell system is disclosed in which the oxidative degradation of an anode of a fuel cell during an operation stop period is restrained.The fuel cell system (39) of the invention comprises a fuel cell (1) configured to generate electric power by use of hydrogen contained in a fuel gas supplied to an anode (1a) and oxygen contained in an oxidizing gas supplied to a cathode (1c); and a combustor (4) configured to combust flammable gas, and is formed such that after stopping the power generation, the flammable gas is introduced into and kept in the cathode (1c) and when discharging the flammable gas from the cathode (1c), the flammable gas is combusted by the combustor (4).

TECHNICAL FIELD

The present invention relates to a fuel cell system and more particularly to a technique for treating flammable gas that has been filled into the cathode of a fuel cell, after stopping power generation.

BACKGROUND ART

As a technique for stopping a fuel cell system, there has heretofore been proposed a method of purging flammable gas from the fuel gas passage of the fuel cell system by use of inert gas such as nitrogen. This purge method using inert gas, however, disadvantageously requires additional provision of a feeding system such as a nitrogen gas cylinder or Ar gas cylinder.

Various shutdown methods without use of inert purge gas have been proposed. A known method uses air for purging the fuel gas passage of a fuel cell system. This method will be outlined below.

As illustrated inFIG. 5, a fuel cell system39has, as chief components, a solid polymer electrolyte membrane type fuel cell1having an anode1aand a cathode1c; a fuel processor2having a reformer (not shown) for generating hydrogen-rich fuel gas by adding water to city gas or natural gas to reform it, which city gas or natural gas serves as a power generation material gas; a water feeder3for supplying water to the reformer of the fuel processor2; a material gas feeder6for supplying the power generation material gas to the reformer of the fuel processor2; a combustor4for combusting remaining fuel gas that has been discharged without being consumed in the anode1aof the fuel cell1; a blower5that serves as an oxidizing gas feeding device for supplying oxidizing gas (air) containing oxygen to the fuel cell1to discharge remaining gas outside from the fuel cell1; and a purge air feeder26for supplying purge air for purge treatment of the inside of the fuel processor2when stopping the power generation of the fuel cell system39.

In the fuel cell system39, a reaction between the hydrogen-rich fuel gas supplied as the fuel gas to the anode1aof the fuel cell1and air supplied as the oxygen-containing oxidizing gas to the cathode1cof the fuel cell1is caused for power generation within the fuel cell1, and at shutdown of the fuel cell system39, the fuel gas passage is finally purged by air. A controller21properly controls the blower5, the material gas feeder6, the water feeder3, the air feeder26and others to perform the above power generation and shutdown operation.

More concretely, when stopping the power generation of the fuel cell, hydrogen-containing fuel gas remaining within the fuel gas passage is removed by vapor which has been generated by supplying water from the water feeder3to the reformer of the fuel processor2. Then, air from the purge air feeder26is allowed to flow into the fuel gas passage, thereby finally performing air purge (see Japanese Patent Document 1).

Compared to the conventional purge treatment process in which when stopping the power generation of the fuel cell system, nitrogen gas is allowed to flow into the fuel processor2and the fuel cell1so that remaining gas (fuel gas etc.) within these members2,1is guided to the combustor4and undergoes treatment within the combustor4, the above fuel cell system39can obviate the need for a storage for storing nitrogen gas so that it can attain cost reduction. The above technique has another advantage that air is supplied to the inside of the fuel cell after the removal of hydrogen gas from the fuel cell by use of vapor, thereby preventing the corrosion of the passages due to water droplets generated from vapor.

Apart from the above shutdown method, there is known another technique (see Patent Document 2) according to which when stopping power generation, air leakage into the anode of the fuel cell is prevented by introducing fuel gas (e.g., hydrogen-rich fuel gas) or power generation material gas (e.g., city gas or natural gas) into the anode and confining it therein, so that the durability of the fuel cell is maintained.Patent Document 1: International Publication No. WO01/97312Patent Document 2: Japanese Laid-Open Patent Application Publication No. 2003-282114

DISCLOSURE OF THE INVENTION

Problems that the Invention is to Solve

Meanwhile, an alloy catalyst comprised of platinum and ruthenium is usually used for the anode of a solid polymer electrolyte fuel cell and if the anode is exposed to air like the case of the fuel cell system disclosed in Patent Document 1, the deterioration of catalytic performance (oxidative degradation) owing to oxidizing atmosphere may occur. Therefore, it is undesirable in view of the service life of the fuel cell system to keep the anode being filled with air when stopping the power generation of the fuel cell system.

Although the fuel cell system shutdown method disclosed in Patent Document 2 seems to prevent the oxidative degradation of the anode, there still remains a possibility that if air (oxygen gas) remains in the cathode after a stop of the power generation of the fuel cell system, the air (oxygen gas) will move to the anode, passing through the porous solid polymer electrolyte membrane with the result that the anode is degraded by oxidation.

The present invention is directed to overcoming the above problems and a primary object of the invention is therefore to provide a fuel cell system capable of restraining the oxidative degradation of the anode of the fuel cell during an operation stop period.

Another object of the invention is to provide a fuel cell system capable of performing proper exhaust gas treatment (e.g., flammable gas combustion treatment) when discharging flammable gas from the cathode of the fuel cell.

Means of Solving the Problems

In accomplishing above objects, there has been provided, in accordance with a first aspect of the present invention, a fuel cell system comprising: a fuel cell configured to generate electric power by use of hydrogen contained in a fuel gas supplied to an anode and oxygen contained in an oxidizing gas supplied to a cathode; and a combustor configured to combust flammable gas,

wherein after stopping the power generation, a flammable gas is introduced into and kept in the cathode and when discharging the flammable gas from the cathode, the flammable gas is combusted by the combustor.

According to a second aspect of the invention, there is provided a fuel cell system comprising: combustion gas feeding device configured to supply a combustion gas to the combustor; and a combustion air feeder configured to supply a combustion air to the combustor;

wherein the combustion air feeder supplies the combustion air in such an amount that an air-fuel ratio within the combustor becomes 1 or more, with respect to flammable gas comprised of at least one of the flammable gas supplied to the combustor and the combustion gas.

According to a third aspect of the invention, there is provided a fuel cell system, wherein the gas discharged from the cathode of the fuel cell is supplied to a passage through which the combustion gas is supplied to the combustor.

According to a fourth aspect of the invention, there is provided a fuel cell system, wherein the gas discharged from the cathode of the fuel cell is supplied to a passage through which the combustion air is supplied to the combustor.

According to a fifth aspect of the invention, there is provided a fuel cell system, wherein, at least during the period of an operation in which the oxidizing gas in the cathode of the fuel cell is replaced with the flammable gas or the flammable gas in the cathode is replaced with the oxidizing gas, the amount of gas supplied to the cathode or the supply amount of the combustion gas is controlled such that the ratio of the flow rate of the flammable gas contained in the combustion gas to the sum of the flow rate of oxygen contained in the gas discharged from the cathode and the flow rate of the flammable gas is below the lower combustible limit of the flammable gas or exceeds the upper combustible limit of the flammable gas based on a mixture of the flammable gas and oxygen.

According to a sixth aspect of the invention, there is provided a fuel cell system, wherein, in cases where air is used as the oxidizing gas, at least during the period of an operation in which air in the cathode of the fuel cell is replaced with the flammable gas or the flammable gas in the cathode is replaced with air, the amount of gas supplied to the cathode or the supply amount of the combustion gas is controlled such that the ratio of the flow rate of the flammable gas contained in the combustion gas to the sum of the flow rate of air discharged from the cathode and the flow rate of the flammable gas is below the lower combustible limit of the flammable gas or exceeds the upper combustible limit of the flammable gas based on a mixture of the flammable gas and air.

According to a seventh aspect of the invention, there is provided a fuel cell system, wherein the flammable gas is hydrogen gas.

According to an eighth aspect of the invention, there is provided a fuel cell system, wherein, at least during the period of an operation in which the oxidizing gas in the cathode of the fuel cell is replaced with the flammable gas or the flammable gas in the cathode is replaced with the oxidizing gas, the amount of gas supplied to the cathode or the supply amount of the combustion gas is controlled such that the ratio of the flow rate of the combustion gas to the sum of the flow rate of oxygen contained in the gas discharged from the cathode and the flow rate of the combustion gas is below the lower combustible limit of the combustion gas or exceeds the upper combustible limit of the combustion gas based on a mixture of the combustion gas and oxygen.

According to a ninth aspect of the invention, there is provided a fuel cell system, wherein, in cases where air is used as the oxidizing gas, at least during the period of an operation in which the air in the cathode of the fuel cell is replaced with the flammable gas or the flammable gas in the cathode is replaced with air, the amount of gas supplied to the cathode or the supply amount of the combustion gas is controlled such that the ratio of the flow rate of the combustion gas to the sum of the flow rate of the air discharged from the cathode and the flow rate of the combustion gas is below the lower combustible limit of the combustion gas or exceeds the upper combustible limit of the combustion gas based on a mixture of the combustion gas and air.

According to a tenth aspect of the invention, there is provided a fuel cell system, wherein, at least during the period of an operation in which the oxidizing gas in the cathode of the fuel cell is replaced with the flammable gas or the flammable gas in the cathode is replaced with the oxidizing gas, the amount of gas supplied to the cathode or the supply amount of the combustion air is controlled such that the ratio of the flow rate of the flammable gas discharged from the cathode to the sum of the flow rate of the flammable gas and the flow rate of the combustion air is below the lower combustible limit of flammable gas or exceeds the upper combustible limit of the flammable gas based on a mixture of flammable gas and air.

According to an eleventh aspect of the invention, there is provided a fuel cell system, wherein, at least during the period of an operation in which the oxidizing gas in the cathode of the fuel cell is replaced with the flammable gas or the flammable gas in the cathode is replaced with the oxidizing gas, the amount of gas supplied to the cathode or the supply amount of the combustion air is controlled such that the ratio of the flow rate of the gas discharged from the cathode to the sum of the flow rate of the gas and the flow rate of the combustion air is below the lower combustible limit of the gas discharged from the cathode or exceeds the upper combustible limit of the gas based on a mixture of the gas and air.

According to a twelfth aspect of the invention, there is provided a fuel cell system, wherein at a start of the power generation, the oxidizing gas is supplied to the cathode, thereby discharging the flammable gas.

According to a thirteenth aspect of the invention, there is provided a fuel cell system comprising a fuel processor having a reformer for generating the fuel gas containing hydrogen from a power generation material,

wherein the combustor is a fuel processing burner for heating the reformer.

According to a fourteenth aspect of the invention, there is provided a fuel cell system comprising a hydrogen feeder capable of supplying hydrogen gas as the fuel gas for the fuel cell.

According to a fifteenth aspect of the invention, there is provided a fuel cell system, wherein the combustion gas is the fuel gas discharged from the fuel processor or remaining fuel gas discharged from the fuel cell.

According to a sixteenth aspect of the invention, there is provided a fuel cell system, wherein the combustion gas is the hydrogen gas supplied from the hydrogen feeder or remaining hydrogen gas discharged from the fuel cell.

According to a seventeenth aspect of the invention, there is provided a fuel cell system, wherein during the period of the operation in which the oxidizing gas in the cathode of the fuel cell is replaced with the flammable gas or the flammable gas in the cathode is replaced with the oxidizing gas, the combustion air feeder supplies air in such an amount that an air-fuel ratio within said combustor as a fuel processing burner becomes 1 or more, with respect to flammable gas and the combustion gas in the fuel processing burner.

According to an eighteenth aspect of the invention, there is provided a fuel cell system, wherein before the operation in which the oxidizing gas in the cathode of the fuel cell is replaced with the flammable gas or the flammable gas in the cathode is replaced with the oxidizing gas, the temperature of the reformer is controlled so as to be lower than a specified target temperature for normal operation.

These objects as well as other objects, features and advantages of the invention will become apparent to those skilled in the art from the following detailed description of preferred embodiments with reference to the accompanying drawings.

Effects of the Invention

According to the invention, operation is stopped by filling the cathode of the fuel cell with flammable gas such as city gas, whereby the oxidative degradation of the anode owing to air in the cathode of the fuel cell in an operation stop period can be prevented and proper exhaust gas treatment (e.g., flammable gas combustion treatment) can be performed when discharging the flammable gas from the cathode of the fuel cell.

DESCRIPTION OF REFERENCE NUMERALS

BEST MODE FOR CARRYING OUT THE INVENTION

Referring now to the accompanying drawings, preferred embodiments of the invention will be described below.

First Embodiment

FIG. 1is a block diagram illustrating a rough outline of the structure of a fuel cell system according to a first embodiment.

A fuel cell system39comprises, as chief components, a material gas feeder6for supplying a power generation material gas to a reformer (not shown) provided in a fuel processor2through a material feed pipe23, the power generation material gas containing at least a flammable organic compound comprised of carbon and hydrogen (e.g., city gas and natural gas); a solid polymer electrolyte fuel cell1for generating electric power by use of a hydrogen-containing fuel gas and an oxygen-containing oxidizing gas (air); a fuel processor2having the reformer for generating a hydrogen-rich fuel gas by reforming the power generation material gas through water addition; a water feeder3for supplying water to the reformer of the fuel processor2; a combustor4serving as a fuel processing burner for burning remaining fuel gas to heat the reformer of the fuel processor2which remaining fuel gas has been sent from an anode1aof the fuel cell1without being consumed therein; a combustion fan18serving as the combustion air feeding device (air feeder) of the invention for supplying combustion air to the combustor4through a combustion air feed passage50; and a blower5that serves as an oxidizing gas feeder for supplying an oxidizing gas to a cathode1cof the fuel cell1and purging remaining oxidizing gas from the cathode1c.

The gas pipe system of the fuel cell system39includes: a cathode feed pipe7that serves as an oxidizing gas flow path for guiding air from the blower5to the cathode1cof the fuel cell1; a cathode exhaust pipe8that serves as an oxidizing gas flow path for discharging remaining air from the cathode1cof the fuel cell1to the atmosphere; a cathode shut-up device12constituted by a first outlet-side opening/closing valve12a(first oxidizing gas flow path valve) for opening and closing the outlet of the cathode1cof the fuel cell1and a second inlet-side opening/closing valve12b(second oxidizing gas flow path valve) for opening and closing the inlet of the cathode1cof the fuel cell1; a material feed pipe23for guiding the power generation material gas from the material gas feeder6to the fuel processor2; an anode feed pipe24for guiding the fuel gas sent from the fuel processor2to the anode1aof the fuel cell1through a flow path switching device14; a fuel gas back flow pipe25that serves as the combustion gas feeding means of the invention for supplying the combustor4with the fuel gas discharged from the fuel processor2or the remaining fuel gas discharged from the anode1aof the fuel cell1(these fuel gases are the combustion gas of the invention); a back flow pipe valve15disposed in the fuel gas back flow pipe25, for opening and closing the fuel gas back flow pipe25; an anode bypass pipe13for guiding the fuel gas sent from the fuel processor2to the fuel gas back flow pipe25on the downstream side of the back flow pipe valve15by means of the flow path switching device14; a material cathode feed pipe22for connecting the material feed pipe23to the cathode feed pipe7on the downstream side of the second inlet-side opening/closing valve12b; and a material cathode feeder11(flammable gas feeding device) disposed in a material cathode feed pipe22, for guiding the power generation material gas to the cathode1c.

Further, there are provided a first cathode combustion pipe16for connecting the outlet of the cathode1con the upstream side of the first outlet-side opening/closing valve12ato the combustion air feed passage50; and a first combustion pipe opening/closing valve17for switching the first cathode combustion pipe16between open and closed states. With such an arrangement, the oxidizing gas flowing in the cathode line (i.e., the first cathode combustion pipe16) can be introduced into the combustor4without coming into contact with the fuel gas flowing in the anode line (i.e., the fuel gas back flow pipe25), so that a mixture of the fuel gas and oxidizing gas is not produced in the flammable gas passage that extends to the combustor4. This is desirable in the light of the control of the combustion properties of the combustor4.

Herein, the flow path switching device14is configured with of, for example, a three-way valve and the material cathode feeder11is configured with, for example, a flow rate regulating valve or pump.

A controller21controls the blower5, the material gas feeder6, the water feeder3, the material cathode feeder11, the combustion fan18and the valves12a,12b,14,15,17to control the operation of the gas feeding system of the fuel cell system39. In the drawings, the objects that the controller controls are indicated by dashed line. Although not shown in the drawings, the controller21receives detection signals from various sensors (such as temperature sensors and flow meters) and properly controls the operation of the fuel cell system39based on these detection signals.

Reference is made toFIG. 1for hereinafter describing the operations of the fuel cell system39during the power generation period of the fuel cell system39. Specifically, the operation for stopping the power generation and the operation for starting the power generation (start-up of the system39) will be separately described.

During the power generation period of the fuel cell system39, while the temperature of the reformer of the fuel processor2being kept at about 700° C., the hydrogen-rich fuel gas is generated by causing, under control of the controller21, a reforming reaction between the power generation material gas supplied from the material gas feeder6and water supplied from the water feeder3within the reformer of the fuel processor2. Then, the fuel gas sent from the fuel processor2is sent to the anode1aof the fuel cell1after passing through the flow path switching device14disposed in the anode feed pipe24(the flow path switching device14is controlled by the controller21such that the anode feed pipe24is communicated with the anode1a). The air supplied from the blower5passes through the second inlet-side opening/closing valve12bin its open state by way of the cathode feed pipe7and is then sent to the cathode1cof the fuel cell1. In this way, hydrogen contained in the fuel gas and oxygen contained in the air are consumed thereby to generate electric power within the fuel cell1.

The fuel gas, which has remained without being consumed in the power generation of the fuel cell1, is sent to the combustor4after passing through the back flow pipe valve15in its open state by way of the fuel gas back flow pipe25and then burnt within the combustor4to be utilized as a heat source for heating the reformer of the fuel processor2. The air, which has remained without being consumed by the power generation of the fuel cell1, passes through the first outlet-side opening/closing valve12ain its open state by way of the cathode exhaust pipe8and is then discharged to the atmosphere.

When stopping the power generation of the fuel cell system39, the controller21stops the operation of the blower5, thereby stopping the supply of air from the blower5to the cathode1c, while closing the second inlet-side opening/closing valve12bas well as the first outlet-side opening/closing valve12aand opening the first combustion pipe opening/closing valve17.

The controller21controls the flow path switching device14so as to form a bypass flow path (a passage by which the anode feed pipe24is communicated with the anode bypass pipe13) and closes the valve15. Thus, the fuel gas (hydrogen-rich gas) staying in the anode1aof the fuel cell1can be sealed within the anode1aand in this condition; the supply of the fuel gas from the fuel processor2to the anode1ais stopped.

At this point, the material gas feeder6continues the supply of the power generation material gas to continue the combustion in the combustor4, while the controller21operates the material cathode feeder11to guide the power generation material gas (flammable gas) to the cathode feed pipe7located on the downstream side of the second inlet-side opening/closing valve12bthrough the material cathode feed pipe22and then, the power generation material gas is supplied to the cathode1cof the fuel cell1through the cathode feed pipe7. The amount of power generation material gas supplied to the cathode1cof the fuel cell1by the material cathode feeder11is set by the controller21to a value that is about two or three times the inner volume of the cathode1c, so that the air within the cathode1ccan be thoroughly replaced with the power generation material gas, that is, a flammable gas. At that time, the power generation material gas exceeding the inner volume of the cathode1cis supplied to the combustion air feed passage50by way of the first cathode combustion pipe16, so that the power generation material gas is mixed with the combustion air and the mixture is then introduced into and burnt in the combustor4.

Herein, the pressure of the power generation material gas within an area of the material feed pipe23which area is close to the outlet of the material gas feeder6is raised by about 2 kPa. Therefore, the power generation material gas can be allowed to flow into the cathode1cfrom the cathode feed pipe7located on the downstream side of the second inlet-side opening/closing valve12bwith the use of the inner pressure of the power generation material gas, by opening the flow rate regulating valve, which serves as the material cathode feeder11and is disposed in the material cathode feed pipe22, in a condition where one end of the material cathode feed pipe22is connected to the area of the material feed pipe23close to the outlet of material gas feeder6whereas the other end is connected to the cathode feed pipe7located on the downstream side of the second inlet-side opening/closing valve12b. If the supply pressure used for supplying the power generation material gas is insufficient, a feed pump may be used as the material cathode feeder11to forcibly send the power generation material gas into the cathode1cby pumping.

The flow rate of the combustion air supplied by the combustion fan18and the flow rate of the power generation material gas supplied by the material cathode feeder11are set by the controller21such that the concentration of the flammable gas contained in the mixture of the combustion air and the power generation material gas is out of the combustible range and more preferably lower than the lower combustible limit, so that a back fire does not occur in the combustion air feed passage50.

For example, the controller21may control the amount of power generation material gas to be supplied to the cathode1cor the amount of combustion air supplied from the combustion fan18, such that, in a power generation stop period of the fuel cell system39during which the air existing in the cathode1cof the fuel cell1is replaced with the power generation material gas, the ratio of the flow rate of the power generation material gas discharged from the cathode1cto the sum of the flow rate of the power generation material gas and the flow rate of the combustion air is out of the combustible range of the power generation material gas and, more preferably, lower than its lower combustible limit based on a mixture of the power generation material gas and air.

The above control is performed based on such a concept that the combustion of the flammable gas can be more easily controlled by adjusting the flow rate of the power generation material gas to a value lower than the lower combustible limit to make the flammable gas concentration of the mixed gas be out of the combustible range when feeding the power generation material gas to the combustion air feed passage50filled with air. The reason for this is that if the flow rate of the power generation material gas is adjusted to a value exceeding the upper combustible limit, the flammable gas concentration of the mixed gas in the combustion air feed passage50will temporarily fall in the combustible range before it becomes greater than the upper combustible limit.

Suppose that city gas13A used in large cities is employed as the power generation material gas. Since the city gas13A has a combustible range of about 5 to 15% when mixed with air, the flow rate of the power generation material gas supplied from the material gas cathode feeder11is adjusted by the controller21to a value less than one twentieth of the flow rate of the combustion air supplied from the combustion fan18.

The above flow rate of the power generation material gas may be derived from the flow rate of the power generation material gas contained in the gas discharged from the cathode1c. For more reliable safety, it may be equal to the flow rate of cathode off gas discharged from the cathode1con assumption that all of the gas discharged from the cathode1cis the power generation material gas.

The amount of air sent to the combustor4by the combustion fan18should be such an amount that at least the mixture of the power generation material gas discharged from the first cathode combustion pipe16and the fuel gas sent from the fuel gas back flow pipe25can be perfectly combusted. In other words, it is necessary to send air to the mixture of the power generation material gas discharged from the first cathode combustion pipe16and the fuel gas sent from the fuel gas back flow pipe25in an amount that makes the air-fuel ratio within the combustor4be 1 or more. Accordingly, the combustion fan18is controlled by the controller21so as to send air to the combustor4in an amount that at least enables perfect combustion of the mixture of the power generation material gas and the fuel gas which is sent to the combustor4(an amount that makes the air-fuel ratio within the combustor4be 1 or more). It should be noted that the air-fuel ratio is the ratio (A/A0) of the actual supply amount of air A to the theoretical amount of air (the minimum amount of air necessary for perfect fuel combustion) A0and that if the air-fuel ratio is less than 1, imperfect fuel combustion is likely to occur.

In the above discussion, the amount of power generation material gas supplied to the combustion air feed passage50through the first cathode combustion pipe16is adjusted to a value less than one twentieth of the flow rate of the combustion air supplied by the combustion fan18by controlling the opening of the flow rate regulating valve of the material cathode feeder11. Instead, it may be adjusted to a value less than one twentieth of the flow rate of the combustion air, by controlling the output of the combustion fan18.

In the latter case, the combustion fan18is controlled by the controller21so as to send air to the combustor4in such an amount that the mixture of the power generation material gas and the fuel gas sent to the combustor4is perfectly combusted and the power generation material gas concentration of the mixture of combustion air and the power generation material gas within the combustion air feed passage50becomes less than the lower combustible limit.

In the above-described power generation stop operation, during the replacement of the air within the cathode1cof the fuel cell1with the power generation material gas, the power generation material gas is supplied to the combustor4in addition to the fuel gas supplied to the combustor4in the normal operation, so that the calorie of combustion heat increases. As a result, the temperature of the fuel processor2and more particularly the reformer increases so that it may become higher than the upper limit (e.g., 750° C.) of the temperature range that ensures the heat resistance of the reforming catalyst. Therefore, it is desirable to control the output of the combustion fan18by the controller21such that air is sent to the combustor4in an amount more than the amount of air required for perfect combustion of the mixture of the power generation material gas and fuel gas sent to the combustor4or in an amount more than the amount of air necessary for making the power generation material gas concentration of the mixture of combustion air and the power generation material gas within the combustion air feed passage50lower than the lower combustible limit. For instance, the supply amount of air, which makes the air-fuel ratio within the combustor4exceed 1, is desirable. With this arrangement, the increase in the temperature of the reformer can be restrained by the air cooling effect of the combustion air supplied from the combustion fan18. It should be noted that a large amount of air such as described above may be supplied in a continuous manner in the course of the replacement of the air within the cathode1cof the fuel cell1with the power generation material gas. Alternatively, air may be supplied by increasing the output of the combustion fan18according to rises in the temperature of the reformer so that the increase of the temperature of the reformer is restrained.

It is desirable in the light of energy efficiency that the controller21perform control instead of the above operation during the power generation stop period such that: prior to the replacement of the air within the cathode1cof the fuel cell1with the power generation material gas, the flow rate of the power generation material gas from the material gas feeder6drops to a value lower than the flow rate for the normal operation in order to lower the temperature of the reformer to a value (e.g., 620° C.) below a specified target temperature (e.g., 650° C.) for the normal operation, and during the replacement of the air within the cathode1cwith the power generation material gas, the temperature of the reformer does not exceed the upper limit (e.g., 750° C.) of the temperature range that ensures the heat resistance of the reforming catalyst.

At the time when the amount of power generation material gas supplied to the cathode1cof the fuel cell1by the material cathode feeder11has reached a value that is about two or three times the inner volume of the cathode1c, the controller21controls the material gas feeder6and the material cathode feeder11so as to stop the supply of the power generation material gas, and closes the first combustion pipe opening/closing valve17, stopping the combustion fan18.

After stopping the power generation through the above procedure, the anode1acan be filled with the hydrogen-rich fuel gas, while the cathode1ccan be filled with the power generation material gas, which is a flammable gas, so that the oxidative degradation of the anode1acan be prevented.

When starting the power generation of the fuel cell system39(a start-up of the system39), the power generation material gas is supplied from the material gas feeder6to the fuel processor2through the material feed pipe23in a condition where a bypass flow path has been formed by controlling the flow path switching device14with the controller21. The gas, which has passed through the fuel processor2, is sent to the flow path switching device14and then to the combustor4by way of the anode bypass pipe13and the fuel gas back flow pipe25. In the combustor4, the gas is combusted. Meanwhile, the controller21controls the water feeder3to supply water to the fuel processor2. Then, the temperature of the reformer of the fuel processor2is raised to about 700° C. by utilizing the combustion heat of the combustor4, and the reformer is kept in a temperature condition where the hydrogen-rich fuel gas can be generated from the power generation material gas and vapor.

At the time when the temperature of a carbon monoxide removing section (not shown) of the fuel processor2is allowed to reach a reaction stabilization temperature, thereby reducing the carbon monoxide concentration of the fuel gas to such a degree (about 20 ppm) that the anode electrode of the fuel cell1does not degrade, the controller21opens the back flow pipe valve15disposed in the fuel gas back flow pipe25and switches the flow path switching device14from the side of the anode bypass pipe13, thereby forming the feed flow path for the anode1a. In this condition (in which the anode feed pipe24is communicated with the anode1a), the fuel gas, which has been sent from the fuel processor2, is guided to the anode1aof the fuel cell1through the flow path switching device14, and the remaining fuel gas which has not been consumed in the anode1ais allowed to flow back to the combustor4through the fuel gas back flow pipe25and the back flow pipe valve15to combust the remaining fuel gas within the combustor4, so that a supply of gas to the anode1aof the fuel cell1is resumed to enable power generation.

At the same time, the second inlet-side opening/closing valve12bof the cathode shut-up device12and the first combustion pipe opening/closing valve17are opened by the controller21to start air blasting by the blower5.

At that time, the flow rate of air supplied to the cathode1cof the fuel cell1by the blower5becomes equal to the flow rate of the power generation material gas forced out from the first cathode combustion pipe16toward the combustor4by the air supplied to the cathode1c. Therefore, the flow rate of air, which is supplied by the blower5such that the flammable gas concentration of the mixed gas within the combustion air feed passage50becomes lower than the lower combustible limit similarly to the case discussed earlier, is adjusted by the controller21to a value less than one twentieth of the flow rate of combustion air supplied by the combustion fan18.

For example, the controller21may control the amount of air to be supplied to the cathode1cor the amount of combustion air supplied from the combustion fan18, such that, in the power generation stop period of the fuel cell system39during which the power generation material gas existing in the cathode1cof the fuel cell1is replaced with air, the ratio of the flow rate of the power generation material gas discharged from the cathode1cto the sum of the flow rate of the power generation material gas and the flow rate of the combustion air is out of the combustible range of the power generation material gas and, more preferably, lower than its lower combustible limit based on a mixture of the power generation material gas and air.

In the above discussion, the amount of power generation material gas supplied to the combustion air feed passage50through the first cathode combustion pipe16is adjusted to a value less than one twentieth of the flow rate of the combustion air supplied by the combustion fan18by controlling the opening of the flow rate regulating valve of the material cathode feeder11. Instead, it may be adjusted to a value less than one twentieth of the flow rate of the combustion air by controlling the output of the combustion fan18.

At that time, the combustion fan18is controlled by the controller21so as to send air to the combustor4in such an amount that the mixture of the power generation material gas and the fuel gas sent to the combustor4is perfectly combusted and the power generation material gas concentration of the mixture of combustion air and the power generation material gas within the combustion air feed passage50becomes less than the lower combustible limit.

The above flow rate of the power generation material gas may be derived from the flow rate of the power generation material gas contained in the gas discharged from the cathode1c. For more reliable safety, it may be equal to the flow rate of cathode off gas discharged from the cathode1con assumption that all of the gas discharged from the cathode1cis the power generation material gas.

The amount of air sent to the combustor4should be such an amount that at least the mixture of the power generation material gas discharged from the first cathode combustion pipe16and the fuel gas sent from the fuel gas back flow pipe25can be perfectly combusted (i.e., the air amount with which the air-fuel ratio within the combustor4is 1 or more). Specifically, the combustion fan18is controlled by the controller21so as to send air to the combustor4in such an amount that at least perfect combustion of the power generation material gas and fuel gas sent to the combustor4becomes possible.

In the above-described power generation stop operation, during the replacement of the power generation material gas within the cathode1cof the fuel cell1with air, the power generation material gas is supplied to the combustor4in addition to the fuel gas supplied to the combustor4in the normal operation, so that the calorie of combustion heat increases. As a result, the temperature of the fuel processor2and more particularly the reformer is likely to increase so that it may become higher than the upper limit (e.g., 750° C.) of the temperature range that ensures the heat resistance of the reforming catalyst. Therefore, it is desirable to control the output of the combustion fan18by the controller21such that air is sent to the combustor4in an amount more than the amount of air required for perfect combustion of the mixture of the power generation material gas and fuel gas sent to the combustor4or in an amount more than the amount of air necessary for making the power generation material gas concentration of the mixture of combustion air and the power generation material gas within the combustion air feed passage50lower than the lower combustible limit. For instance, the supply amount of air, which makes the air-fuel ratio within the combustor4exceed 1, is desirable. With this arrangement, the increase in the temperature of the reformer can be restrained by the air cooling effect of the combustion air supplied from the combustion fan18. It should be noted that a large amount of air such as described above may be supplied in a continuous manner in the course of the replacement of the power generation material gas within the cathode1cof the fuel cell1with air. Alternatively, air may be supplied by increasing the output of the combustion fan18according to rises in the temperature of the reformer, so that the increase of the temperature of the reformer is restrained.

It is desirable in the light of energy efficiency that the controller21perform control instead of the above operation during the power generation start period such that: prior to the replacement of the air within the cathode1cof the fuel cell1with the power generation material gas, the flow rate of the power generation material gas from the material gas feeder6drops to a value lower than the flow rate for the normal operation in order to lower the temperature of the reformer to a value (e.g., 620° C.) below a specified target temperature (e.g., 650° C.) for the normal operation, and during the replacement of the power generation material gas within the cathode1cwith air, the temperature of the reformer does not exceed the upper limit (e.g., 750° C.) of the temperature range that ensures the heat resistance of the reforming catalyst. After completion of the replacement of the power generation material gas in the cathode1cwith air, the flow rate of the power generation material gas is increased to the value of the flow rate for the normal operation.

After the replacement of the power generation material gas in the cathode1cof the fuel cell1with air, the controller21opens the first outlet-side opening/closing valve12aof the cathode shut-up device12and closes the first combustion pipe opening/closing valve17, whereby the amount of air supplied by the blower5is set to a value required for the power generation of the fuel cell1, and then, the power generation of the fuel cell1starts.

Thus, the power generation material gas discharged from the cathode1cof the fuel cell1at a start or stop of the power generation in the fuel cell system39is sent from the first cathode combustion pipe16to the combustion air feed passage50to be mixed with combustion air and this mixed gas is sent to the combustor4. Thereby, the power generation material gas discharged from the cathode1ccan be completely combusted and discharged from the fuel cell system39.

In addition, the flow rate of power generation material gas sent from the first cathode combustion pipe16to the combustion air feed passage50is adjusted by the controller21to a value less than one twentieth of the flow rate of combustion air supplied from the combustion fan18, whereby it becomes possible to perform proper operation free from the risk of a back fire that occurs from the combustor4toward the combustion air feed passage50.

Further, since the condition where the cathode1cof the fuel cell1is filled with the power generation material gas can be maintained during the power generation stop period of the fuel cell system39, not only the flammable gas (fuel gas) can be sealed in the anode1abut also the cause of the oxidation of the catalyst of the anode1aof the fuel cell1can be thoroughly eliminated, so that the durability of the anode1aof the fuel cell system39can be prevented from decreasing.

Second Embodiment

FIG. 2is a block diagram illustrating a rough outline of the structure of a fuel cell system according to a second embodiment. The second embodiment is formed by modifying the configuration of the first cathode combustion pipe16of the first embodiment that serves as a cathode bypass passage. In the second embodiment, the parts thereof corresponding to those ofFIG. 1are identified by the same reference numerals as inFIG. 1and a detailed description thereof is omitted herein.

As seen fromFIG. 2, the second embodiment differs from the first embodiment in the following points. The first cathode combustion pipe16for connecting the outlet of the cathode1cto the combustion air feed passage50in order to send the power generation material gas discharged from the cathode1cof the fuel cell1to the combustor4after mixed with combustion air is replaced with a second cathode combustion pipe19for connecting the outlet of the cathode1clocated on the upstream side of the first outlet-side opening/closing valve12ato the fuel gas back flow pipe25located on the downstream side of the back flow pipe valve15in order to send the power generation material gas discharged from the cathode1cof the fuel cell1to the combustor4after mixed with the fuel gas. The first combustion pipe opening/closing valve17disposed on the first cathode combustion pipe16is replaced with a second combustion pipe opening/closing valve20disposed on the second cathode combustion pipe19.

During the power generation period of the fuel cell system39, while the reformer of the fuel processor2being kept at a temperature of about 700° C., a reforming reaction within the reformer is caused between the power generation material gas supplied from the material gas feeder6and water supplied from the water feeder3, these feeders being controlled by the controller21, so that hydrogen-rich fuel gas is generated. The fuel gas coming out from the fuel processor2passes through the flow path switching device14disposed in the anode feed pipe24(the flow path switching device14is controlled by the controller21such that the anode feed pipe24is communicated with the anode1a) and is then introduced into the anode1aof the fuel cell1. The air coming out from the blower5passes through the second inlet-side opening/closing valve12bin its open state through the cathode feed pipe7and is then introduced into the cathode1cof the fuel cell1. In the fuel cell1, hydrogen contained in the fuel gas and oxygen contained in the air are thus consumed, thereby generating electric power. The remaining fuel gas which has not been consumed in the power generation of the fuel cell1is sent to the combustor4after passing through the back flow pipe valve15by way of the fuel gas back flow pipe25. Then, the remaining fuel gas is combusted within the combustor4to generate heat that is utilized as a heat source for heating the reformer of the fuel processor2. The remaining air, which has not been consumed in the power generation of the fuel cell1, is discharged to the atmosphere after passing through the first outlet-side opening/closing valve12ain its open state by way of the cathode exhaust pipe8.

At a stop of the power generation of the fuel cell system39, the controller21stops the operation of the blower5so that the supply of air from the blower5to the cathode1cis stopped, while closing the second inlet-side opening/closing valve12band the first outlet-side opening/closing valve12aand opening the second combustion pipe opening/closing valve20.

The controller21controls the flow path switching device14so as to form a bypass flow path (the passage for communicating the anode feed pipe24with the anode bypass pipe13) and close the valve15. In this way, the fuel gas (hydrogen-rich gas) staying in the anode1aof the fuel cell1can be sealed in the anode1a. While maintaining this condition, the supply of the fuel gas from the fuel processor2to the anode1ais stopped.

At that time, the material gas feeder6continues the supply of the power generation material gas to continue the combustion in the combustor4, while the controller21operates the material cathode feeder11to guide the power generation material gas (flammable gas) to the cathode feed pipe7located on the downstream side of the second inlet-side opening/closing valve12bby way of the material cathode feed pipe22. The power generation material gas is then supplied to the cathode1cof the fuel cell1through the cathode feed pipe7.

Since the amount of air staying in the cathode1ccan be grasped beforehand, the amount of power generation material gas to be supplied to the cathode1cof the fuel cell1by the material cathode feeder11can be set to a value (that is normally two or three times the inner volume) equal to or greater than the amount of the air by the controller21.

Herein, the pressure of the power generation material gas within the area of the material feed pipe23which area is close to the outlet of the material gas feeder6is raised by about 2 kPa. Therefore, the power generation material gas can be allowed to flow from the cathode feed pipe7located on the downstream side of the second inlet-side opening/closing valve12binto the cathode1cwith the use of the inner pressure of the power generation material gas, by opening the flow rate regulating valve disposed in the material cathode feed pipe22as the material cathode feeder11, in a condition where one end of the material cathode feed pipe22is connected to the area of the material feed pipe23close to the outlet of the material gas feeder6whereas the other end is connected to the cathode feed pipe7located on the downstream side of the second inlet-side opening/closing valve12b. If the supply pressure used for supplying the power generation material is insufficient, a feed pump may be used as the material cathode feeder11to forcibly send the power generation material gas into the cathode1cby pumping.

The fuel gas supplied from the fuel gas back flow pipe25to the combustor4can be joined with the power generation material gas that flows in the fuel gas back flow pipe25from the cathode1cof the fuel cell1by way of the second cathode combustion pipe19, by supplying the power generation material gas to the cathode1cof the fuel cell1with the material cathode feeder11. In short, the power generation material gas discharged from the cathode1cis sent onto the fuel gas back flow pipe25by way of the second cathode combustion pipe19, so that the power generation material gas is mixed with the fuel gas and transferred to the combustor4for combustion.

The flow rate of the air supplied to the combustor4after flowing in the fuel gas back flow pipe25is set by the controller21such that the flammable gas concentration of the mixed gas comprised of the flammable gas (that is hydrogen gas contained in the fuel gas flowing in the fuel gas back flow pipe25) and air in the fuel gas back flow pipe25is out of the combustible range and more preferably greater than the upper combustible limit in order to prevent a back fire from occurring in the fuel gas back flow pipe25.

For example, the controller21controls the amount of power generation material gas to be supplied to the cathode1cor the amount of fuel gas flowing in the fuel gas back flow pipe25, such that, in the power generation stop period of the fuel cell system39during which the air existing in the cathode1cof the fuel cell1is replaced with the power generation material gas, the ratio of the flow rate of the flammable gas contained in the fuel gas to the sum of the flow rate of the air discharged from the cathode1cand the flow rate of the flammable gas is out of the combustible range of the flammable gas and, more preferably, greater than its upper combustible limit based on a mixture of the flammable gas and air.

The above control is performed based on such a concept that when air flows from the second cathode combustion pipe19into the fuel gas back flow pipe25filled with the fuel gas (more particularly, just after the replacement of the gas in the cathode1cwith the power generation material gas), the combustion of the flammable gas can be more easily controlled by adjusting the flow rate of the air (i.e., the flow rate of the power generation material gas used in the air replacement in the cathode1c) such that the flammable gas concentration of the mixed gas becomes greater than the upper combustible limit, thereby making the flammable gas concentration of the mixed gas become out of the combustible range. The reason for this is that if the flow rate of the air (i.e., the flow rate of the power generation material gas) is adjusted to a value below the lower combustible limit, the flammable gas concentration of the mixed gas in the fuel gas back flow pipe25will temporarily fall in the combustible range before it becomes equal to the lower combustible limit.

Therefore, since the chief component of the fuel gas is hydrogen and hydrogen has a combustible range of about 4 to 75% when mixed with air, the flow rate of the air supplied to the combustor4after flowing in the fuel gas back flow pipe25, in other words, the flow rate of the power generation material gas supplied from the material cathode feeder11is adjusted by the controller21to a value less than one fourth of the flow rate of the fuel gas supplied from the fuel gas back flow pipe25to the combustor4.

For any of the flammable gas components contained in the fuel gas flowing in the fuel gas back flow pipe25, the flow rate control for preventing a back fire as described above is performed so as to satisfy the above conditions. In this embodiment, most of the flammable gas contained in the fuel gas flowing in the fuel gas back flow pipe25is hydrogen and the combustible range of hydrogen when mixed with air is 4 to 75 vol %. Since the lower and upper combustible limits of hydrogen are both strict compared to those of the unreformed power generation material (city gas13A) that is another flammable gas component of the fuel gas, the flow rate of hydrogen gas contained in the fuel gas is employed as the flammable gas flow rate of the fuel gas. For more reliable safety, the flow rate of the fuel gas may be used in place of the flow rate of hydrogen gas contained in the fuel gas.

Similarly to the first embodiment, the combustion fan18is controlled, at that time, by the controller21so as to send air to the combustor4in such an amount that at least a mixture of the power generation material gas and the fuel gas sent to the combustor4is perfectly combusted (i.e., an amount that makes the air to fuel ratio within the combustor4be 1 or more).

During the replacement of the air within the cathode1cof the fuel cell1with the power generation material gas in the power generation stop operation described above, it is preferable to control the output of the combustion fan18by the controller21similarly to the first embodiment such that air is sent to the combustor4in an amount more than that required to perfectly combust the mixture of the power generation material gas and the fuel gas which mixture is sent to the combustor4. Thereby, the temperature rise of the reformer can be restrained by the air cooling effect of the combustion air supplied from the combustion fan18. It should be noted that a large amount of air such as described above may be supplied in a continuous manner in the course of the replacement of the air within the cathode1cof the fuel cell1with the power generation material gas. Alternatively, air may be supplied by increasing the output of the combustion fan18according to rises in the temperature of the reformer, so that the increase of the temperature of the reformer is restrained.

It is desirable in the light of energy efficiency that, similarly to the first embodiment, the controller21perform control instead of the above operation during the power generation stop period such that: prior to the replacement of the air within the cathode1cof the fuel cell1with the power generation material gas, the flow rate of the power generation material gas from the material gas feeder6drops to a value lower than the flow rate for the normal operation in order to lower the temperature of the reformer to a value (e.g., 620° C.) below a specified target temperature (e.g., 650° C.) for the normal operation, and during the replacement of the air within the cathode1cwith the power generation material gas, the temperature of the reformer does not exceed the upper limit (e.g., 750° C.) of the temperature range that ensures the heat resistance of the reforming catalyst.

At the time when the amount of power generation material gas supplied to the cathode1cof the fuel cell1by the material cathode feeder11has reached a value equal to or more than the inner volume of the cathode1c(this value is usually two or three times the inner volume), the controller21stops the supply of the power generation material gas by the material gas feeder6and the material cathode feeder11, closes the second combustion pipe opening/closing valve20and stops the combustion fan18.

After stopping the power generation through the above procedure, the hydrogen-rich fuel gas is introduced into the anode1aand kept in this condition, while the power generation material gas, which is flammable gas, being kept in the cathode1c, so that the oxidative degradation of the anode1acan be prevented.

Next, at a start of the power generation of the fuel cell system39(start-up of the system39), the power generation material gas is supplied from the material gas feeder6into the reformer of the fuel processor2through the material feed pipe23, while the flow path switching device14being controlled by the controller21so as to form the bypass passage. Then, the gas, which has come out from the fuel processor2after passing therethrough, goes to the anode bypass pipe13and the fuel gas back flow pipe25by way of the flow path switching device14and then to the combustor4where it is combusted. Meanwhile, the reformer within the fuel processor2is supplied with water by the water feeder3controlled by the controller21. Thereafter, the temperature of the reformer of the fuel processor2is raised to about 700° C. by the combustion heat of the combustor4so that the reformer can be kept in a temperature condition in which hydrogen-rich fuel gas can be generated from the power generation material gas and vapor.

At the time when the temperature of the carbon monoxide removing section (not shown) housed in the fuel processor2has reached the reaction stabilization temperature, thereby reducing the carbon monoxide concentration of the fuel gas to such a degree (about 20 ppm) that the anode electrode of the fuel cell1does not degrade, the controller21opens the back flow pipe valve15placed in the fuel gas back flow pipe25and switches the flow path switching device14from the side of the anode bypass pipe13, thereby forming the feed flow path for the anode1a. In this condition (where the anode feed pipe24is communicated with the anode1a), the fuel gas coming out from the fuel processor2is guided to the anode1aof the fuel cell1through the flow path switching device14and the remaining gas which has not been consumed in the anode1ais allowed to flow back to the combustor4through the fuel gas back flow pipe25and the back flow pipe valve15and then combusted within the combustor4. Thereby, a supply of gas to the anode1aof the fuel cell1is resumed to enable power generation.

At the same time, the controller21opens the second inlet-side opening/closing valve12bof the cathode shut-up device12and the second combustion pipe opening/closing valve20to start air blasting by the blower5.

At that time, the flow rate of the air sent from the cathode1cof the fuel cell1to the combustor4through the second cathode combustion pipe19by the blower5is adjusted by the controller21to a value less than one fourth of the flow rate of the fuel gas supplied from the fuel gas back flow pipe25to the combustor4, so that the flammable gas concentration of the mixture of the fuel gas and air within the fuel gas back flow pipe25becomes greater than the upper combustible limit, like the case described earlier.

For example, the controller21controls the amount of air supplied to the cathode1cor the amount of fuel gas flowing in the fuel gas back flow pipe25, such that, in the power generation start period of the fuel cell system39during which the power generation material gas existing in the cathode1cof the fuel cell1is replaced with air, the ratio of the flow rate of the flammable gas contained in the fuel gas to the sum of the flow rate of the air discharged from the cathode1cand the flow rate of the flammable gas is out of the combustible range of the flammable gas and, more preferably, greater than its upper combustible limit based on a mixture of the flammable gas and air.

For any of the flammable gas components contained in the fuel gas flowing in the fuel gas back flow pipe25, the flow rate control for preventing a back fire as described above is performed so as to satisfy the above conditions. In this embodiment, most of the flammable gas contained in the fuel gas flowing in the fuel gas back flow pipe25is hydrogen and the combustible range of hydrogen when mixed with air is 4 to 75 vol %. Since the lower and upper combustible limits of hydrogen are both strict compared to those of the unreformed power generation material (city gas13A) that is another flammable gas component of the fuel gas, the flow rate of hydrogen gas contained in the fuel gas is employed as the flow rate of flammable gas of the fuel gas. For more reliable safety, the flow rate of the fuel gas may be used in place of the flow rate of hydrogen gas contained in the fuel gas.

Further, the amount of air sent to the combustor4by the combustion fan18is kept by the cathode1csimilarly to the first embodiment during the power generation stop period. When the blower5starts to send air to the cathode1c, the amount of air, which at least enables perfect combustion of the power generation material gas discharged from the second cathode combustion pipe19from the beginning (i.e., just after the replacement of the gas within the cathode1cwith air) and the fuel gas contained in the mixed gas sent from the fuel gas back flow pipe25, becomes necessary. That is, the combustion fan18is controlled by the controller21so as to send air to the combustor4in such an amount that the power generation material gas and fuel gas sent to the combustor4can be perfectly combusted.

During the replacement of the power generation material gas within the cathode1cof the fuel cell1with air in the power generation start operation described above, it is preferable to control the output of the combustion fan18by the controller21similarly to the first embodiment such that air is sent to the combustor4in an amount more than that required to perfectly combust the mixture of the power generation material gas and the fuel gas which mixture is sent to the combustor4. Thereby, the temperature rise of the reformer can be restrained by the air cooling effect of the combustion air supplied from the combustion fan18. It should be noted that a large amount of air such as described above may be supplied in a continuous manner in the course of the replacement of the air within the cathode1cof the fuel cell1with the power generation material gas. Alternatively, air may be supplied by increasing the output of the combustion fan18according to rises in the temperature of the reformer so that the increase of the temperature of the reformer is restrained.

It is desirable in the light of energy efficiency that the controller21perform control instead of the above operation during the power generation stop period such that: prior to the replacement of the air within the cathode1cof the fuel cell1with the power generation material gas, the flow rate of the power generation material gas from the material gas feeder6drops to a value lower than the flow rate for the normal operation in order to lower the temperature of the reformer to a value (e.g., 620° C.) below a specified target temperature (e.g., 650° C.) for the normal operation, and during the replacement of the power generation material gas within the cathode1cwith air, the temperature of the reformer does not exceed the upper limit (e.g., 750° C.) of the temperature range that ensures the heat resistance of the reforming catalyst. After completion of the replacement of the power generation material gas in the cathode1cwith air, the flow rate of the power generation material gas is increased to the value of the flow rate for the normal operation.

After the replacement of the power generation material gas in the cathode1cof the fuel cell1with air, the controller21opens the first outlet-side opening/closing valve12aof the cathode shut-up device12and closes the second combustion pipe opening/closing valve20; the air supply amount of the blower5is set to a value required for the power generation of the fuel cell1; and then, the power generation of the fuel cell1is started.

Thus, the power generation material gas discharged from the cathode1cof the fuel cell1at the time of a start or stop of the power generation of the fuel cell system39is sent from the second cathode combustion pipe19to the combustor4by way of the fuel gas back flow pipe25, whereby the power generation material gas discharged from the cathode1ccan be completely combusted and discharged from the fuel cell system39.

The flow rate of the air sent from the second cathode combustion pipe19to the combustor4by way of the fuel gas back flow pipe25is adjusted by the controller21to a value less than one fourth of the flow rate of the fuel gas supplied from the fuel gas back flow pipe25to the combustor4, thereby enabling proper operation free from the risk of a back fire that occurs from the combustor4toward the fuel gas back flow pipe25.

Further, since the condition where the cathode1cof the fuel cell1is filled with the power generation material gas can be maintained during the power generation stop period of the fuel cell system39, not only the flammable gas (fuel gas) can be sealed in the anode1abut also the cause of the oxidation of the catalyst of the anode1aof the fuel cell1can be thoroughly eliminated, so that the durability of the anode1aof the fuel cell system39can be prevented from decreasing.

In the first embodiment (FIG. 1) and the second embodiment (FIG. 2) described earlier, the power generation material gas supplied from the material gas feeder6to the anode1ais used as one example of the flammable gas with which the cathode1cis filled during the power generation stop period of the fuel cell system39.

Either the power generation material gas or the fuel gas may be arbitrarily selected as the flammable gas according to changes in the arrangement of specified pipes. It is thought to be desirable in view of the durability of the platinum catalyst to fill the cathode1cwith a gas having the highest possible hydrogen gas concentration. However, hydrogen gas should be more carefully treated when discharged to the atmosphere, because the combustible range of hydrogen gas is wider than those of other flammable gases.

Although air is used as the oxidizing gas in the foregoing embodiments, the oxidizing gas is not necessarily limited to air and other gases may be used. In the latter case, during the period in which the oxidizing gas existing in the cathode1cof the fuel cell1is replaced with the power generation material gas or the power generation material gas in the cathode1cis replaced with the oxidizing gas (i.e., the power generation stop period or the power generation start period), the controller21controls the amount of gas to be supplied to the cathode1cor the supply amount of fuel gas flowing in the fuel gas back flow pipe25such that the ratio of the flow rate of the flammable gas contained in the fuel gas to the sum of the flow rate of oxygen contained in the gas discharged from the cathode1cand the flow rate of the flammable gas is out of the combustible range of the flammable gas and, more preferably, greater than its upper combustible limit based on a mixture of the flammable gas and oxygen.

Third Embodiment

FIG. 3is a block diagram illustrating a rough outline of the structure of a fuel cell system according to a third embodiment. The fuel cell system139of the third embodiment is formed by modifying the gas supply system of the fuel cell1shown in the first embodiment (FIG. 1). In the third embodiment, a detailed description of parts corresponding to those ofFIG. 1is omitted. (It should be noted that the parts corresponding toFIG. 1are indicated with the same reference numerals as in the first embodiment with addition of the number 100).

As seen fromFIGS. 1 and 3, in the fuel cell system139of the third embodiment, a hydrogen feeder102is disposed in place of the fuel processor2, the material gas feeder6and the water feeder3which have been described earlier in the first embodiment (FIG. 1). The hydrogen feeder102is capable of storing a fixed quantity of hydrogen gas serving as the fuel gas and sending the hydrogen gas to an anode101aof a fuel cell101. In addition, a combustor104is disposed in place of the combustor4for heating the fuel processor (reformer)2described in the first embodiment (FIG. 1), the dedicated combustor104being used for processing the hydrogen gas discharged from a cathode101cof the fuel cell101(described later) at a stop or start-up of the fuel cell system139. It should be noted that the hydrogen cathode feeder111(that corresponds to the material cathode feeder11of the first embodiment) shown inFIG. 3functions to supply hydrogen gas to the cathode101c.

During the power generation period of the fuel cell system139, the hydrogen gas coming out from the hydrogen feeder102passes through a flow path switching device114disposed in an anode feed pipe124(the flow path switching device114is controlled by a controller121so as to communicate the anode feed pipe124with the anode101a) and is then sent into the anode101aof the fuel cell101.

On the other hand, the air sent from the blower105passes through a second inlet-side opening/closing valve112bin its open state by way of a cathode feed pipe107and is then sent into the cathode101cof the fuel cell101.

Thus, hydrogen and oxygen contained in the air are consumed, thereby executing power generation within the fuel cell101. The remaining hydrogen gas (combustion gas), which has not been consumed in the power generation of the fuel cell101, is sent to the combustor104after passing through a back flow pipe valve115in its open state by way of a hydrogen gas back flow pipe125and is then combusted within the combustor104. Alternatively, the hydrogen gas (combustion gas) coming out from the hydrogen feeder102may be sent from the hydrogen gas back flow pipe125to the combustor104by the switching operation (for communicating the anode feed pipe124with an anode bypass pipe113) of the flow path switching device114.

The combustion heat (exhaust heat) generated in the combustor104may be recovered and used as a heat source for an appropriate exhaust heat utilization system (e.g., hot water supply system).

The remaining air, which has not been consumed in the power generation of the fuel cell system101, is discharged to the atmosphere after passing through the first outlet-side opening/closing valve112ain its open state by way of a cathode exhaust pipe108.

When stopping the power generation of the fuel cell system139, the controller121controls a blower105to stop its operation, thereby stopping the supply of air from the blower105to the cathode101c, and closes the second inlet-side opening/closing valve112band the first outlet-side opening/closing valve112a, while opening a first combustion pipe opening/closing valve117.

The controller121controls the flow path switching device114to form a bypass flow path (a passage for communicating the anode feed pipe124with the anode bypass pipe113) and closes the valve115. In this way, the hydrogen gas staying in the anode101aof the fuel cell101is sealed within the anode101aand in this condition; the supply of hydrogen gas from the hydrogen feeder102to the anode101ais stopped.

At that time, the hydrogen feeder102continues the supply of hydrogen gas thereby continuing the combustion in the combustor104, while the controller121operates a hydrogen cathode feeder111such that hydrogen gas (i.e., flammable gas) is guided into the cathode feed pipe107located on the downstream side of the second inlet-side opening/closing valve112bthrough a hydrogen cathode feed pipe122and supplied to the cathode101cof the fuel cell101through the cathode feed pipe107.

The amount of hydrogen gas supplied to the cathode101cof the fuel cell101by the hydrogen cathode feeder111is set by the controller121to a value that is about two or three times the inner volume of the cathode101c, and the air in the cathode101cis thoroughly replaced with the hydrogen gas that is a flammable gas. At that time, the hydrogen gas exceeding the inner volume of the cathode101cis supplied onto a combustion air feed passage150by way of a first cathode combustion pipe116, whereby the hydrogen gas and combustion air are mixed with each other and transferred to the combustor104where they are combusted.

Herein, the pressure of the hydrogen gas within the area of the anode feed pipe124which area is close to the outlet of the hydrogen feeder102is raised by about 2 kPa. Therefore, the hydrogen gas can be allowed to flow into the cathode101cfrom the cathode feed pipe107located on the downstream side of the second inlet-side opening/closing valve112bwith the use of the inner pressure of the hydrogen gas, by opening the flow rate regulating valve, which serves as the hydrogen cathode feeder111and is disposed in the hydrogen cathode feed pipe122, in a condition where one end of the hydrogen cathode feed pipe122is connected to the area of the anode feed pipe124close to the outlet of the hydrogen gas feeder102whereas the other end is connected to the cathode feed pipe107located on the downstream side of the second inlet-side opening/closing valve112b. If the supply pressure used for supplying the hydrogen gas is insufficient, a feed pump may be used as the hydrogen cathode feeder111to forcibly send the hydrogen gas into the cathode101cby pumping.

The flow rate of the combustion air supplied by a combustion fan118and the flow rate of the hydrogen gas supplied to the hydrogen cathode feeder111are set by the controller121to such values that the flammable gas concentration of the mixture of them is out of the combustible range and more preferably lower than the lower combustible limit.

For example, the controller121may control the amount of hydrogen gas supplied to the cathode101cor the amount of combustion air supplied from the combustion fan118, such that, in a power generation stop period of the fuel cell system139during which the air existing in the cathode101cof the fuel cell101is replaced with the hydrogen gas, the ratio of the flow rate of the hydrogen gas discharged from the cathode101cto the sum of the flow rate of the hydrogen gas and the flow rate of the combustion air is out of the combustible range of hydrogen gas and, more preferably, lower than its lower combustible limit based on a mixture of hydrogen gas and air.

The above control is performed based on such a concept that the combustion of the flammable gas can be more easily controlled by adjusting the flow rate of the hydrogen gas to a value lower than the lower combustible limit to make the flammable gas concentration of the mixed gas be out of the combustible range when feeding the hydrogen gas to the combustion air feed passage150filled with air. The reason for this is that if the flow rate of the hydrogen gas is adjusted to a value exceeding the upper combustible limit, the flammable gas concentration of the mixed gas in the combustion air feed passage150will temporarily fall in the combustible range before it becomes greater than the upper combustible limit.

Since hydrogen gas has a combustible range of about 4 to 75% when mixed with air, the flow rate of the hydrogen gas supplied from the hydrogen cathode feeder111controlled by the controller121is preferably adjusted to a value less than one twenty-fifth of the flow rate of the combustion air supplied from the combustion fan118.

The above flow rate of the hydrogen gas may be derived from the flow rate of the hydrogen gas contained in the gas discharged from the cathode101c. For more reliable safety, it may be equal to the flow rate of cathode off gas discharged from the cathode101con assumption that all of the gas discharged from the cathode101cis hydrogen gas.

The amount of air sent to the combustor104by the combustion fan118should be such an amount that at least all of the hydrogen gas discharged from the first cathode combustion pipe116and the hydrogen gas sent from the hydrogen gas back flow pipe125can be perfectly combusted. In other words, it is necessary to send air in an amount that makes the air-fuel ratio within the combustor104with respect to a total amount of hydrogen (i.e., the sum of the hydrogen gas discharged from the first cathode combustion pipe116and the hydrogen gas sent from the hydrogen gas back flow pipe125) be 1 or more. Accordingly, the combustion fan118is controlled by the controller121so as to send air to the combustor104in an amount that at least enables perfect combustion of the total hydrogen sent to the combustor4(an amount that makes the air-fuel ratio within the combustor104be 1 or more). It should be noted that the air-fuel ratio is the ratio (A/A0) of the actual supply amount of air to the theoretical amount of air (the minimum amount of air necessary for perfect fuel combustion) A0and that if the air-fuel ratio is less than 1, imperfect fuel combustion is likely to occur.

In the above description, the amount of hydrogen gas supplied to the combustion air feed passage150through the first cathode combustion pipe116is adjusted to a value less than one twenty-fifth of the flow rate of the combustion air supplied by the combustion fan118by controlling the opening of the flow rate regulating valve of the hydrogen cathode feeder111. Instead, it may be adjusted to a value less than one twenty-fifth of the flow rate of the combustion air, by controlling the output of the combustion fan118.

At that time, the combustion fan118is controlled by the controller121so as to send air to the combustor104in such a proper amount that the total hydrogen gas sent to the combustor104is completely combusted and the hydrogen gas concentration of the mixture of combustion air and hydrogen gas within the combustion air feed passage150becomes less than the lower combustible limit.

At the time when the amount of hydrogen gas supplied to the cathode101cof the fuel cell101by the hydrogen cathode feeder111has reached a value that is about two or three times the inner volume of the cathode101, the controller121controls the hydrogen feeder102and the hydrogen cathode feeder111so as to stop the supply of hydrogen gas and closes the first combustion pipe opening/closing valve117, so that the combustion fan118stops.

After stopping the power generation through the above procedure, hydrogen gas, which serves as the power generation gas, can be kept staying in the anode101a, while hydrogen gas, which is a flammable gas, is kept staying in the cathode101c, so that the oxidative degradation of the anode101acan be prevented.

When starting the power generation of the fuel cell system139(start-up of the system139), the controller121opens the back flow pipe valve115disposed in the hydrogen gas back flow pipe125and switches the flow path switching device114from the side of the anode bypass tube113to form a feed flow path for the anode101a. In this condition (in which the anode feed pipe124is communicated with the anode101a), the hydrogen gas, which has been sent from the hydrogen feeder102, is guided to the anode101aof the fuel cell101through the flow path switching device114and the remaining hydrogen gas which has not been consumed in the anode101ais allowed to flow back to the combustor104through the hydrogen gas back flow pipe125and the back flow pipe valve115to combust the remaining fuel gas within the combustor4, so that a supply of gas to the anode1aof the fuel cell1is resumed to enable power generation.

At the same time, the second inlet-side opening/closing valve112bof the cathode shut-up device112and the first combustion pipe opening/closing valve117are opened by the controller121to start air blasting by the blower105.

At that time, the flow rate of the air supplied to the cathode101cof the fuel cell101by the blower105becomes equal to the flow rate of the hydrogen gas forced out from the first cathode combustion pipe116toward the combustor104by the air supplied to the cathode101c. Therefore, the flow rate of the air, which is supplied by the blower105such that the flammable gas concentration of the mixed gas within the combustion air feed passage150becomes lower than the lower combustible limit similarly to the above case, is adjusted by the controller121to a value less than one twenty-fifth of the flow rate of the combustion air supplied by the combustion fan118.

For example, the controller121may control the amount of air to be supplied to the cathode101cor the amount of combustion air supplied from the combustion fan118, such that, in the power generation start period of the fuel cell system139during which the hydrogen gas existing in the cathode101cof the fuel cell101is replaced with air, the ratio of the flow rate of the hydrogen gas discharged from the cathode101cto the sum of the flow rate of the hydrogen gas and the flow rate of the combustion air is out of the combustible range of hydrogen gas and, more preferably, lower than its lower combustible limit based on a mixture of hydrogen gas and air.

The above flow rate of the hydrogen gas may be derived from the flow rate of the hydrogen gas contained in the gas discharged from the cathode101c. For more reliable safety, it may be equal to the flow rate of cathode off gas discharged from the cathode101con assumption that all of the gas discharged from the cathode101cis hydrogen gas.

The amount of air sent to the combustor104should be such an amount that at least all the hydrogen gas discharged from the first cathode combustion pipe116and the hydrogen gas sent from the hydrogen gas back flow pipe125can be perfectly combusted (i.e., the air amount with which the air-fuel ratio within the combustor104is 1 or more). Specifically, the combustion fan118is controlled by the controller121so as to send air to the combustor104in such an amount that at least perfect combustion of all of the hydrogen gas sent to the combustor104becomes possible.

In the above description, the amount of hydrogen gas supplied to the combustion air feed passage150through the first cathode combustion pipe116is adjusted to a value less than one twenty-fifth of the flow rate of the combustion air supplied by the combustion fan118by controlling the opening of the flow rate regulating valve of the hydrogen cathode feeder111. Instead, it may be adjusted to a value less than one twenty-fifth of the flow rate of the combustion air, by controlling the output of the combustion fan118.

At that time, the combustion fan118is controlled by the controller121so as to send air to the combustor104in such a proper amount that all the hydrogen gas sent to the combustor104is perfectly combusted and the hydrogen gas concentration of the mixture of combustion air and hydrogen gas within the combustion air feed passage150becomes less than the lower combustible limit.

After the hydrogen gas which has been introduced into the cathode101cof the fuel cell101is replaced with air, the controller121opens the first outlet-side opening/closing valve112aof the cathode shut-up device112, closes the first combustion pipe opening/closing valve117, sets the amount of air supplied by the blower105to a value necessary for the power generation of the fuel cell101, and starts the power generation of the fuel cell101.

As described above, at a start or stop of the power generation of the fuel cell system139, the hydrogen gas discharged from the cathode101cof the fuel cell101is sent from the first cathode combustion pipe116to the combustion air feed passage150and mixed with combustion air. This mixed gas is sent to the combustor104, so that the hydrogen gas discharged from the cathode101ccan be completely combusted and discharged from the fuel cell system139.

In addition, the flow rate of the hydrogen gas sent from the first cathode combustion pipe116to the combustion air feed passage150is adjusted by the controller121to a value less than one twenty-fifth of the flow rate of combustion air supplied by the combustion fan118, so that proper operation free from the risk of a back fire that occurs from the combustor104toward the combustion air feed passage150becomes possible.

Further, since the condition in which the cathode101cof the fuel cell101is filled with hydrogen gas can be maintained during the power generation stop period of the fuel cell system139, not only the flammable gas (hydrogen gas) can be sealed in the anode101abut also the cause of the oxidation of the catalyst of the anode101aof the fuel cell101can be thoroughly eliminated, so that the durability of the anode101aof the fuel cell system139can be prevented from decreasing.

Fourth Embodiment

FIG. 4is a block diagram illustrating a rough outline of the structure of a fuel cell system according to a fourth embodiment. The fuel cell system139of the fourth embodiment is formed by modifying the gas supply system of the fuel cell1shown in the second embodiment (FIG. 2). In the fourth embodiment, a detailed description of parts corresponding to those ofFIG. 2is omitted. (It should be noted that the parts corresponding toFIG. 2are indicated with the same reference numerals as in the second embodiment with addition of the number 100).

As seen fromFIGS. 2 and 4, in the fuel cell system139of the fourth embodiment, a hydrogen feeder102is disposed in place of the fuel processor2, the material gas feeder6and the water feeder3which are described in the second embodiment (FIG. 2). The hydrogen feeder102is capable of storing a fixed quantity of hydrogen gas serving as the fuel gas and sending the hydrogen gas to an anode101aof a fuel cell101. In addition, a combustor104is disposed in place of the combustor4for heating the fuel processor (reformer)2described in the second embodiment (FIG. 2), the dedicated combustor104being used for processing hydrogen gas discharged from a cathode101cof the fuel cell101(described later) at a stop or start-up of the fuel cell system139. It should be noted that the hydrogen cathode feeder111(that corresponds to the material cathode feeder11of the second embodiment) shown inFIG. 4functions to supply hydrogen gas to the cathode101c.

During the power generation period of the fuel cell system139, the hydrogen gas coming out from the hydrogen feeder102passes through a flow path switching device114disposed in an anode feed pipe124(the flow path switching device114is controlled by a controller121so as to communicate the anode feed pipe124with the anode101a) and is then sent into the anode101aof the fuel cell101.

On the other hand, the air sent from the blower105passes through a second inlet-side opening/closing valve112bin its open state by way of a cathode feed pipe107and is then sent into the cathode101cof the fuel cell101.

Thus, hydrogen and oxygen contained in the air are consumed, thereby executing power generation within the fuel cell101. The remaining hydrogen gas, which has not been consumed in the power generation of the fuel cell101, is sent to the combustor104after passing through a back flow pipe valve115in its open state by way of a hydrogen gas back flow pipe125and is then combusted within the combustor104. The combustion heat (exhaust heat) generated in the combustor104may be recovered and used as a heat source for an appropriate exhaust heat utilization system (e.g., hot water supply system).

The remaining air, which has not been consumed in the power generation of the fuel cell system101, is discharged to the atmosphere after passing through the first outlet-side opening/closing valve112ain its open state by way of a cathode exhaust pipe108.

When stopping the power generation of the fuel cell system139, the controller121controls a blower105to stop its operation, thereby stopping the supply of air from the blower105to the cathode101c, and closes the second inlet-side opening/closing valve112band the first outlet-side opening/closing valve112awhile opening a first combustion pipe opening/closing valve117.

The controller121controls the flow path switching device114to form a bypass flow path (a passage for communicating the anode feed pipe124with the anode bypass pipe113) and closes the valve115. In this way, the hydrogen gas staying in the anode101aof the fuel cell101is sealed within the anode101aand in this condition; the supply of hydrogen gas from the hydrogen feeder102to the anode101ais stopped.

At that time, the hydrogen feeder102continues the supply of hydrogen gas thereby continuing the combustion in the combustor104, while the controller121operates a hydrogen cathode feeder111to guide hydrogen gas (i.e., flammable gas) into the cathode feed pipe107located on the downstream side of the second inlet-side opening/closing valve112bby way of a hydrogen cathode feed pipe122and supply the hydrogen gas to the cathode101cof the fuel cell101through the cathode feed pipe107.

The amount of hydrogen gas supplied to the cathode101cof the fuel cell101by the hydrogen cathode feeder111is set by the controller121to a value that is about two or three times the inner volume of the cathode101c, and the air in the cathode101cis thoroughly replaced with the hydrogen gas that is a flammable gas.

Herein, the pressure of the hydrogen gas within the area of the anode feed pipe124which area is close to the outlet of the hydrogen feeder102is raised by about 2 kPa. Therefore, the hydrogen gas can be allowed to flow into the cathode101cfrom the cathode feed pipe107located on the downstream side of the second inlet-side opening/closing valve112bwith the use of the inner pressure of the hydrogen gas, by opening the flow rate regulating valve, which serves as the hydrogen cathode feeder111and is disposed in the hydrogen cathode feed pipe122, in a condition where one end of the hydrogen cathode feed pipe122is connected to the area of the anode feed pipe124close to the outlet of the hydrogen gas feeder102whereas the other end is connected to the cathode feed pipe107located on the downstream side of the second inlet-side opening/closing valve112b. If the supply pressure used for supplying the hydrogen gas is insufficient, a feed pump may be used as the hydrogen cathode feeder111to forcibly send the hydrogen gas into the cathode101cby pumping.

The hydrogen gas supplied from the hydrogen gas back flow pipe125to the combustor104can be joined with the hydrogen gas that flows in the hydrogen gas back flow pipe125from the cathode101cof the fuel cell101by way of a second cathode combustion pipe119, by supplying the hydrogen gas to the cathode101cof the fuel cell101with the hydrogen cathode feeder111. In short, the hydrogen gas discharged from the cathode101cis sent onto the hydrogen gas back flow pipe125by way of the second cathode combustion pipe119, so that the hydrogen gases from the two systems are mixed with each other and transferred to the combustor104for combustion.

The flow rate of the air supplied to the combustor104after flowing in the hydrogen gas back flow pipe125is set by the controller121such that the flammable gas concentration of the mixed gas comprised of air and flammable gas (that is the hydrogen gas contained in the fuel gas flowing in the hydrogen gas back flow pipe125) in the hydrogen gas back flow pipe125is out of the combustible range and more preferably greater than the upper combustible limit in order to prevent a back fire from occurring in the hydrogen gas back flow pipe125.

For example, the controller121may control the amount of hydrogen gas supplied to the cathode101cor the amount of fuel gas flowing in the hydrogen gas back flow pipe125, such that, in the power generation stop period of the fuel cell system139during which the air existing in the cathode101cof the fuel cell101is replaced with hydrogen gas, the ratio of the flow rate of the flammable gas contained in the fuel gas to the sum of the flow rate of the air discharged from the cathode101cand the flow rate of the flammable gas is out of the combustible range of the flammable gas and, more preferably, greater than the upper combustible limit based on a mixture of the flammable gas and air.

The above control is performed based on such a concept that the combustion of the flammable gas can be more easily controlled by adjusting the flow rate of the air (i.e., the flow rate of the hydrogen gas to be used for the air replacement in the cathode101c) such that the flammable gas concentration of the mixed gas becomes greater than the upper combustible limit to make the flammable gas concentration be out of the combustible range, when air flows from the second cathode combustion pipe119into the hydrogen gas back flow pipe125filled with the flammable gas (more specifically, just after the replacement of the air in the cathode101cwith hydrogen gas). The reason for this is that if the flow rate of the air (i.e., the flow rate of the hydrogen gas) is adjusted to a value lower than the lower combustible limit, the flammable gas concentration of the mixed gas in the hydrogen gas back flow pipe125will temporarily fall in the combustible range before it becomes equal to the lower combustible limit.

Since hydrogen gas has a combustible range of about 4 to 75% when mixed with air, the flow rate of the air that flows in the hydrogen gas back flow pipe125and is to be supplied to the combustor4, that is, the flow rate of the hydrogen gas supplied by the hydrogen cathode feeder111is adjusted by the controller121to a value less than one fourth of the flow rate of the hydrogen gas supplied from the hydrogen gas back flow pipe125to the combustor104.

For any of the flammable gas components contained in the hydrogen gas flowing in the hydrogen gas back flow pipe125, the flow rate control for preventing a back fire as described above is performed so as to satisfy the above conditions. In this embodiment, most of the flammable gas contained in the fuel gas flowing in the hydrogen gas back flow pipe125is hydrogen and the combustible range of hydrogen when mixed with air is 4 to 75 vol %. Since the lower and upper combustible limits of hydrogen are both strict compared to those of the unreformed power generation material (city gas13A) that is another flammable gas component of the fuel gas, the flow rate of hydrogen gas contained in the fuel gas is employed as the flow rate of flammable gas of the fuel gas. For more reliable security, the flow rate of the fuel gas may be used in place of the flow rate of hydrogen gas contained in the fuel gas.

Similarly to the third embodiment described earlier, the amount of air sent to the combustor104by the combustion fan118is adjusted by the controller121to such a value that all of the hydrogen gas discharged from the second cathode combustion pipe119and the hydrogen gas discharged from the hydrogen gas back flow pipe125can be perfectly combusted (i.e., the amount of air with which the air-fuel ratio in the combustor104becomes 1 or more).

In this way, at the time when the amount of hydrogen gas supplied to the cathode101cof the fuel cell101by the hydrogen cathode feeder111has reached a value equal to or more than the inner volume of the cathode101c(this value is usually two or three times the inner volume), the controller121stops the supply of hydrogen gas by the hydrogen feeder102and the hydrogen cathode feeder111; closes a second combustion pipe opening/closing valve120; and stops the combustion fan118.

After stopping the power generation through the above procedure, hydrogen gas, which serves as the power generation gas, is introduced into the anode101aand kept in this condition, while hydrogen gas, which serves as the flammable gas, being kept in the cathode101c, so that the oxidative degradation of the anode101acan be properly prevented.

When starting the power generation of the fuel cell system139(start-up of the system139), the controller121opens the back flow pipe valve115disposed in the hydrogen gas back flow pipe125and switches the flow path switching device114from the side of the anode bypass pipe113to form a feed flow path for the anode101a. In this condition (where the anode feed pipe124is communicated with the anode101a), the hydrogen gas coming out from the hydrogen gas feeder102is introduced into the anode101aof the fuel cell101through the flow path switching device114and the remaining hydrogen gas which has not been consumed in the anode101ais allowed to flow back to the combustor104through the hydrogen gas back flow pipe125and the back flow pipe valve115and then, combusted within the combustor104. Thereby, a gas supply to the anode101aof the fuel cell101is resumed to enable power generation.

At the same time, the controller121opens the second inlet-side opening/closing valve112bof the cathode shut-up device112and the first combustion pipe opening/closing valve117to start air blasting by the blower105.

At that time, the flow rate of the air sent from the cathode101cof the fuel cell101to the combustor104through the second cathode combustion pipe119by the blower105is adjusted by the controller121to a value less than one fourth of the flow rate of the hydrogen gas supplied from the hydrogen gas back flow pipe125to the combustor104, so that the flammable gas concentration of the mixture of hydrogen gas and air within the hydrogen gas back flow pipe125becomes greater than the upper combustible limit similarly to the above case.

For example, the controller121controls the amount of air supplied to the cathode101cor the amount of fuel gas flowing in the hydrogen gas back flow pipe125, such that, in the power generation start period of the fuel cell system139during which the hydrogen gas existing in the cathode101cof the fuel cell101is replaced with air, the ratio of the flow rate of the flammable gas contained in the fuel gas to the sum of the flow rate of the air discharged from the cathode101cand the flow rate of the flammable gas is out of the combustible range of the flammable gas and, more preferably, greater than the upper combustible limit based on a mixture of the flammable gas and air.

For any of the flammable gas components contained in the fuel gas flowing in the hydrogen gas back flow pipe125, the flow rate control for preventing a back fire as described above is performed so as to satisfy the above conditions. In this embodiment, most of the flammable gas contained in the fuel gas flowing in the hydrogen gas back flow pipe125is hydrogen and the combustible range of hydrogen when mixed with air is 4 to 75 vol %. Since the lower and upper combustible limits of hydrogen are both strict compared to those of the unreformed power generation material (city gas13A) that is another flammable gas component of the fuel gas, the flow rate of hydrogen gas contained in the fuel gas is employed as the flow rate of flammable gas of the fuel gas. For more reliable safety, the flow rate of the fuel gas may be used in place of the flow rate of hydrogen gas contained in the fuel gas.

Further, the amount of air sent to the combustor104by the combustion fan118is maintained by the cathode101csimilarly to the third embodiment during the power generation stop period. When the blower105starts to send air to the cathode101c, the amount of air, which at least enables perfect combustion of all of the hydrogen gas initially discharged from the second cathode combustion pipe119(just after the replacement of the gas within the cathode101cwith air) and the hydrogen gas contained in the mixed gas sent from the hydrogen gas back flow pipe125, becomes necessary. That is, the combustion fan118is controlled by the controller121so as to send air to the combustor104in such an amount that all of the hydrogen gas sent to the combustor104can be completely combusted.

After the hydrogen gas in the cathode101cof the fuel cell101is thus replaced with air, the controller121opens the first outlet-side opening/closing valve112aof the cathode shut-up device112; closes the second combustion pipe opening/closing valve120; and sets the amount of air supplied from the blower105to a value necessary for the power generation of the fuel cell101. Then, the power generation of the fuel cell101starts.

Thus, the hydrogen gas discharged from the cathode101cof the fuel cell101at a start or stop of the power generation of the fuel cell system139is sent from the second cathode combustion pipe119to the combustor104by way of the hydrogen gas back flow pipe125, so that the hydrogen gas discharged from the cathode101ccan be perfectly combusted and discharged from the fuel cell system139.

Further, the flow rate of air sent from the second cathode combustion pipe119to the combustor104by way of the hydrogen gas back flow pipe125is adjusted by the controller121to a value less than one fourth of the flow rate of the hydrogen gas supplied from the hydrogen gas back flow pipe125to the combustor104, whereby proper operation free from the risk of a back fire that occurs from the combustor104toward the hydrogen gas back flow pipe125becomes possible.

In addition, since hydrogen gas can be kept staying in the cathode101cof the fuel cell101during the power generation stop period of the fuel cell system139, not only the flammable gas (hydrogen gas) can be sealed in the anode101a, but also the cause of the oxidation of the catalyst in the anode101aof the fuel cell101can be thoroughly eliminated so that the durability of the anode101aof the fuel cell system139can be prevented.

Although air is used as the oxidizing gas in the foregoing embodiments, the oxidizing gas is not necessarily limited to air. In this case, during the period in which the hydrogen gas existing in the cathode101cof the fuel cell101is replaced with the oxidizing gas or the oxidizing gas existing in the cathode101cis replaced with the hydrogen gas (i.e., the power generation stop period or the power generation start period), the controller121controls the amount of gas to be supplied to the cathode101cor the supply amount of fuel gas flowing in the hydrogen gas back flow pipe125such that the ratio of the flow rate of the flammable gas contained in the fuel gas to the sum of the flow rate of oxygen contained in the gas discharged from the cathode101cand the flow rate of the flammable gas is lower than the lower combustible limit of the flammable gas or greater than its upper combustible limit based on a mixture of the flammable gas and oxygen.

INDUSTRIAL APPLICABILITY

The fuel cell system of the invention is useful as, for instance, a fuel cell system for household use since it has the effect of providing increased durability by preventing the oxidative degradation of the anode of the fuel cell and properly exhausting the flammable gas kept in the cathode.