WO2024218981A1 - Fuel cell system and control method for same - Google Patents

Abstract

This control method for a fuel cell system comprises: a normal operation step for, while supplying fuel gas to a modification part, causing a fuel cell stack to generate power; and a carbon removal step for removing carbon deposited on a modification catalyst. The carbon removal step includes a step for supplying air to the modification part. The flow rate of the air supplied to the modification part in the carbon removal step is greater than the flow rate of hydrocarbon supplied to the modification part in the normal operation step.

Description

Fuel cell system and control method thereof

The present invention relates to a fuel cell system and a control method thereof.

Fuel cells that use hydrogen as fuel to generate electricity are known. In such fuel cells, a reforming catalyst may be used to generate hydrogen. In such fuel cells, fuel gas containing hydrocarbons is reformed by the reforming catalyst to generate reformed gas containing hydrogen. The generated reformed gas is then used to generate electricity.

Technology relating to fuel cells using reforming catalysts is described, for example, in Patent Document 1 (JP2005-340075A). Patent Document 1 describes a method for stopping the operation of a fuel cell having a specific reformer, in which a specific mixed gas is supplied into the fuel cell through the reformer while continuing to supply air into the fuel cell, and when the temperature of the reforming catalyst falls to within a specific temperature range, the supply of the mixed gas is stopped, and after the supply of the mixed gas is stopped, air or hydrocarbon gas is passed through the reformer as a purge gas into the fuel cell.

However, when a reforming catalyst is operated continuously, carbon may accumulate and cover the reforming catalyst. When the reforming catalyst is covered with carbon, the reaction area decreases and the reforming performance decreases. Therefore, it is preferable to remove the accumulated carbon.

The object of the present invention is to provide a technology that can remove carbon that has accumulated on a reforming catalyst.

In one aspect, the present invention relates to a control method for a fuel cell system. The fuel cell system includes a reforming unit that reforms a fuel gas containing hydrocarbons to generate reformed gas, the reforming unit having a reforming catalyst, and a fuel cell stack configured to generate electricity using the reformed gas as an anode gas. The control method includes a normal operation step of causing the fuel cell stack to generate electricity while supplying fuel gas to the reforming unit, and a carbon removal step of removing carbon deposited on the reforming catalyst. The carbon removal step includes a step of supplying air to the reforming unit. The flow rate of air supplied to the reforming unit in the carbon removal step is greater than the flow rate of hydrocarbons supplied to the reforming unit in the normal operation step.

In another aspect, the present invention relates to a fuel cell system. The fuel cell system includes a reforming section that reforms a fuel gas containing hydrocarbons to generate reformed gas, the reforming section having a reforming catalyst; a fuel cell stack configured to generate electricity using the reformed gas as an anode gas; and a control device. The control device is configured to execute a normal operation step and a carbon removal step. In the normal operation step, the control device causes the fuel cell stack to generate electricity while supplying fuel gas to the reforming section. In the carbon removal step, the control device supplies air to the reforming section. The flow rate of the air supplied to the reforming section in the carbon removal step is greater than the flow rate of the hydrocarbons supplied to the reforming section in the normal operation step.

FIG. 1 is a schematic block diagram showing a fuel cell system according to a first embodiment. FIG. 2 is a flowchart showing a control method for the fuel cell system according to the first embodiment. FIG. 3 is a flow chart illustrating the steps for starting up a combustor. FIG. 4 is a flowchart showing steps S20 and S30. FIG. 5 is a flow chart showing the POX operation step (step S40). FIG. 6 is a graph showing a modification of the first embodiment. FIG. 7 is a graph showing another modified example of the first embodiment. FIG. 8 is a graph showing yet another modified example of the first embodiment. FIG. 9 is a schematic block diagram showing a fuel cell system according to the second embodiment. FIG. 10 is a schematic block diagram showing a fuel cell according to the third embodiment. FIG. 11 is a graph showing the relationship between the operation time and the amount of hydrogen generated in Experimental Example 1. FIG. 12 is a graph showing the relationship between air flow rate and outlet _H2 concentration.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
(1) First embodiment Fig. 1 is a schematic block diagram showing a fuel cell system 1 according to a first embodiment. As shown in Fig. 1, the fuel cell system 1 has a control device 3, a fuel cell stack 4, a reformer 5, a fuel tank 6, a heat exchanger 7, a combustor 8, and an air supply unit 9. Note that this fuel cell system 1 is configured to steadily operate at a constant output (rated output) during normal operation.

The fuel cell stack 4 is the part that realizes the power generation function. Although not shown, the fuel cell stack 4 has an anode, an electrolyte, and a cathode. The fuel cell stack 4 receives anode gas and cathode gas to generate power. The reformed gas supplied from the reforming unit 5 is used as the anode gas. The air supplied from the air supply unit 9 is used as the cathode gas.

Fuel cell stack 4 is also connected to combustor 8 via line 19 downstream of its anode. Fuel cell stack 4 is also connected to combustor 8 via line 17 downstream of its cathode. This allows both exhaust from the anode and exhaust from the cathode to be sent to combustor 8.

The reforming unit 5 is configured to reform the fuel gas supplied from the fuel tank 6 to generate a reformed gas containing hydrogen. The reforming unit 5 generates the reformed gas by steam reforming. The steam may be supplied from the fuel tank 6 together with the fuel gas, or may be supplied to the reforming unit 5 via a route (not shown) separate from the fuel gas.

The reforming section 5 includes a reforming catalyst, such as Pt/ _CeO2 , Ni/ _CeO2 , or Rh/ _CeO2 .

The reforming section 5 is connected to the anode of the fuel cell stack 4 via line 18. The reformed gas generated in the reforming section 5 is supplied to the anode of the fuel cell stack 4 via line 18 as described above.

The fuel tank 6 is a section for storing fuel gas. The fuel tank 6 is connected to the reforming section 5 via a line 13. A valve 15 is provided on the line 13. The fuel tank 6 is also connected to the combustor 8 via a line 14. A valve 16 is provided on the line 14.

The fuel gas contains a hydrocarbon. A preferred example of the hydrocarbon is methane. When methane is used, a methane steam reforming reaction can be carried out in the reforming section 5. Since methane is a non-alcohol fuel, the reforming catalyst in the reforming section 5 is less likely to deteriorate due to acid sites.

The air supply unit 9 is provided to supply air as a cathode gas to the fuel cell stack 4. The air supply unit 9 is realized by, for example, a blower. The air supply unit 9 is connected to the cathode of the fuel cell stack 4 via an air supply line 10.

The air supply unit 9 can also send air to the reforming unit 5. Specifically, the air supply line 10 is connected to the reforming unit 5 via a purge line 11. A valve 12 is provided on the purge line 11. By operating the valve 12, it is possible to supply air from the air supply unit 9 to the reforming unit 5.

The heat exchanger 7 is provided in the air supply line 10. The heat exchanger 7 is thermally connected to the combustor 8. This allows the air flowing through the air supply line 10 to be heated by the heat from the combustor 8.

The heat exchanger 7 is located downstream of the connection between the purge line 11 and the air supply line 10. Therefore, the air sent from the air supply unit 9 to the reforming unit 5 via the purge line 11 is not heated by the heat exchanger 7.

As described above, the combustor 8 is provided to heat the air via the heat exchanger 7. The combustor 8 is supplied with fuel gas from the fuel tank 6, exhaust gas from the cathode of the fuel cell stack 4, and exhaust gas from the anode of the fuel cell stack 4. These gases are combusted in the combustor 8. The combustion gas generated in the combustor 8 is discharged to the outside as exhaust gas.

The combustor 8 is also connected to the reforming section 5 so that the combustion gas is discharged as exhaust gas after heat exchange with the reforming section 5.

The control device 3 is provided to control the operation of the entire fuel cell system 1. The control device 3 is realized, for example, by a computer having an arithmetic unit such as a CPU and storage devices such as RAM and ROM. In other words, the control device 3 realizes its functions by the arithmetic unit executing a control program stored in the storage device.

The above is a general configuration of fuel cell system 1. Next, we will explain how fuel cell system 1 operates.

First, the operation of the fuel cell system 1 during normal operation will be described. During normal operation, the fuel cell system 1 operates at a predetermined constant output (rated output).

Specifically, the control device 3 closes valve 12 and opens valves 15 and 16. This allows fuel gas to be supplied from the fuel tank 6 to the reformer 5 and combustor 8. To obtain a constant output, fuel gas is supplied to the reformer 5 at a constant flow rate. In the reformer 5, the fuel gas is reformed by steam reforming to generate reformed gas. The generated reformed gas is sent to the anode of the fuel cell stack 4 via line 18. The reformed gas supplied to the anode is used for power generation and then sent to the combustor 8 via line 19 as anode exhaust gas.

Air is sent from the air supply unit 9 to the cathode of the fuel cell stack 4 via the air supply line 10 and the heat exchanger 7. To obtain a constant output, air is sent to the cathode at a constant flow rate. After being used to generate electricity, the air supplied to the cathode is sent to the combustor 8 via line 17 as cathode exhaust gas.

In the combustor 8, the gas sent from lines 14, 17, and 19 is burned. This heats the air via the heat exchanger 7. That is, heated air is supplied to the fuel cell stack 4. Fuel cells normally function at high temperatures. By supplying heated air, the temperature required for power generation is maintained in the fuel cell stack 4. The combustion gas generated in the combustor 8 is discharged as exhaust gas. In addition, a portion of the combustion gas is discharged after heat exchange with the reforming section 5. Therefore, the temperature of the reforming section 5 is also maintained at the temperature required for the reforming reaction.

The above is how it works during normal operation.

As a result of operation, carbon may be deposited on the reforming catalyst in the reformer 5. This is believed to be due to the disproportionation reaction (2CO→C+ _CO2 ) of CO, a by-product of the reforming reaction. When the surface of the reforming catalyst is covered with carbon, the efficiency of the reforming reaction decreases.

In this embodiment, therefore, a carbon removal step is carried out to remove the deposited carbon. In the carbon removal step, air is supplied to the reforming section 5. Specifically, the control device 3 opens the valve 12 (see FIG. 1), and air is supplied to the reforming section 5 via the purge line 11. The valve 15 is closed. In other words, fuel gas is not supplied to the reforming section 5.

The air supplied to the reforming section 5 in the carbon removal step is at a high flow rate. Specifically, air is supplied to the reforming section 5 at a flow rate equal to or greater than the flow rate of the hydrocarbons supplied to the reforming section 5 during normal operation. By supplying air to the reforming section 5 at this rate, carbon deposited on the reforming catalyst can be removed, and the function of the reforming catalyst can be restored. Note that the "flow rate of the hydrocarbons supplied to the reforming section during normal operation" referred to here does not refer to the flow rate of the fuel gas as a whole, but rather to the flow rate of the hydrocarbons. For example, if the hydrocarbon contained in the fuel gas is methane, the flow rate of the methane is the "flow rate of the hydrocarbons supplied to the reforming section 5."

Preferably, the flow rate of air supplied to the reforming section 5 in the carbon removal step is at least four times the flow rate of the hydrocarbons supplied to the reforming section 5 during normal operation. If air is supplied at such a flow rate, carbon can be removed more reliably.

Alternatively, when the number of moles is used as a standard, the oxygen-based molar flow rate ( _O2 ) of the air supplied to the reforming unit 5 in the carbon removal step is preferably 0.84 or more relative to the carbon-based molar flow rate (C) of the fuel gas supplied to the reforming unit 5 during normal operation. If air is supplied at such a flow rate, carbon can be removed more reliably.

The above is a general description of the configuration and operation of this embodiment. As described above, according to this embodiment, by supplying air to the reforming section 5 at a large flow rate, the precipitated carbon can be removed. This makes it possible to restore the reforming function.

The timing of the carbon removal step is not particularly limited. However, if the carbon removal step is performed when the fuel cell system 1 is started up, carbon removal and startup of the fuel cell system 1 can be performed simultaneously, which is efficient. Below, a specific description will be given of the control method of the fuel cell system 1 according to this embodiment, using as an example a case where the carbon removal step is performed when the fuel cell system 1 is started up. Unless otherwise specified, the following control method is performed by the control device 3.

FIG. 2 is a flowchart showing a control method for the fuel cell system 1 according to this embodiment. This control method for the fuel cell system generally includes start-up steps (S10 to S40) and a normal operation step (S50). The start-up steps (S10 to S40) include a step (S10) of starting the combustor 8, a step (S20) of determining whether carbon removal is required, a carbon removal step (S30), and a step (S40) of performing POX operation. After the start-up step (S10) of the combustor 8, heated air is supplied to the cathode of the fuel cell stack 4 via the air supply line 10. This heats up the fuel cell stack 4. In other words, the fuel cell stack 4 is warmed up. Therefore, steps S20 to S40, which are performed after the start-up of the combustor 8 (after step S10), can be said to be performed in the warm-up step of the fuel cell stack 4.

Each step is described in detail below.
(Step S10: Start-up of combustor)
First, start up the combustor 8. Figure 3 is a flow chart showing the steps of starting up the combustor 8.

First, the air supply unit 9 is started (step S11). As a result, air flows from the air supply unit 9 through the air supply line 10 to the heat exchanger 7, the cathode of the fuel cell stack 4, and the combustor 8 in that order. Note that the valve 12 is closed. In other words, air is not supplied to the reformer 5.

The heater (not shown) of the combustor 8 is also operated (step S12).

Then, it is determined whether the temperature of the combustor 8 has reached or exceeded a predetermined temperature (step S13). Specifically, the control device 3 measures the temperature of the combustor 8 via a temperature sensor (not shown) or the like. The control device 3 then compares the measurement result with a predetermined temperature stored in advance. The predetermined temperature here is a temperature that is set based on whether or not a combustion reaction is possible in the combustor 8. For example, if the combustor 8 includes a combustion catalyst, the light-off temperature of the combustion catalyst (e.g., 350°C) is set as the predetermined temperature.

When the temperature of the combustor 8 reaches or exceeds a predetermined temperature, fuel gas is introduced into the combustor 8 (step S14). That is, the valve 16 is opened, and fuel gas is supplied from the fuel tank 6 to the combustor 8 via the line 14. This causes a combustion reaction to proceed in the combustor 8, and the combustor 8 starts up.

As described above, after the combustor 8 is started up, heated air is supplied to the cathode of the fuel cell stack 4 via the heat exchanger 7. That is, as described above, the fuel cell stack 4 is warmed up. The reformer 5 is also heated by the heat of the combustion gas sent from the combustor 8.
(Step S20: Determining whether carbon removal is necessary)
4 is a flow chart showing step S20 and the subsequent carbon removal step S30. After the start-up of the combustor 8, first, it is determined whether or not a carbon removal step is required (step S20).

Whether or not a carbon removal step is required is determined, for example, based on the accumulated operating time. For example, the control device 3 is configured to store the accumulated operating time since the previous carbon removal step was performed as data. The control device 3 compares this accumulated operating time with a preset specified time. Then, if the accumulated operating time exceeds the specified time, it determines that a carbon removal step is required.

Alternatively, the necessity of the carbon removal step may be determined based on the reforming performance during the previous operation. The reforming performance can also be determined based on, for example, the outlet temperature of the reforming section 5. The reforming reaction in the reforming section 5 is an endothermic reaction. Therefore, when the reforming function decreases, the outlet temperature of the reforming section 5 increases. In other words, it can be said that the outlet temperature of the reforming section 5 reflects the reforming performance in the reforming section 5. Therefore, the necessity of the carbon removal step can also be determined based on the outlet temperature of the reforming section 5 during the previous operation. For example, the control device can compare the outlet temperature of the reforming section 5 during the previous operation with the outlet temperature of the reforming section 5 in a preset initial state, and determine the necessity of the carbon removal step if the difference is equal to or greater than a predetermined value (for example, 10% of the value in the initial state).

If the result of the determination is that the carbon removal step is not necessary, the necessity of the carbon removal step will be determined the next time the device is started, and the processing from step S40 onwards will be carried out. This prevents the carbon removal step from being carried out unnecessarily, thereby improving energy efficiency.

On the other hand, if it is determined that a carbon removal step is necessary, the next carbon removal step S30 is carried out.
(Step S30: Carbon Removal Step)
In the carbon removal step, as described above, the valve 12 (see FIG. 1) is opened (step S31). This causes air to be supplied from the air supply unit 9 to the reforming unit 5 via the purge line 11. As described above, at this time, the air is supplied to the reforming unit 5 at a predetermined large flow rate.

The supply of air removes the carbon that has precipitated from the reforming section 5. The removed carbon is sent to the combustor 8 via the anode of the fuel cell stack 4 and line 19. It is then burned in the combustor 8, rendered harmless, and then discharged to the outside.

The air is supplied from the air supply unit 9 to the reforming unit 5 without being heated. If air is supplied to the reforming unit 5 at a high temperature, the reforming catalyst may be oxidized and deteriorated. However, according to this embodiment, the air is supplied without being heated, so that the oxidative deterioration of the reforming catalyst can be suppressed. Similarly, in the anode of the fuel cell stack 4 located downstream of the reforming unit 5, the oxidative deterioration of the electrode material is suppressed.

Even while air is being supplied, the reforming section 5 is heated by heat from the combustor 8, and so its temperature increases. Therefore, the carbon removal step is performed until the temperature of the reforming section 5 reaches a predetermined temperature. Specifically, while air is being supplied, the control device 3 determines whether the temperature of the reforming section 5 has reached a preset first temperature (step S32). For example, the control device 3 measures the temperature of the reforming section 5 using a sensor not shown. The temperature of the reforming section 5 may be indirectly determined by measuring the outlet temperature of the reforming section 5.

If it is determined in step S32 that the temperature of the reforming unit 5 has reached the first temperature, the control device 3 closes the valve 12. This stops the supply of air to the reforming unit 5 (step S33). In other words, the carbon removal step is completed.

The above-mentioned first temperature is set from the viewpoint of deterioration of the substances contained in the reforming section 5. If air is supplied to the reforming section 5 when the temperature of the reforming section 5 is high, the reforming catalyst may deteriorate due to sintering or oxidation. Furthermore, the anode in the downstream fuel cell stack 4 is also prone to oxidation deterioration. In response to this, by stopping the supply of air when the first temperature is reached, it is possible to prevent oxidation deterioration of the reforming catalyst and anode. The first temperature is, for example, 300°C.

The carbon removal step is preferably carried out for, for example, 15 seconds or more, preferably 30 seconds or more, and more preferably 1 minute or more.
(Step S40: POX operation)
After the carbon removal step (S30) is completed, the POX operation step (S40) is carried out. Fig. 5 is a flow chart showing the POX operation step (S40). The POX operation is carried out to promote warming up of the fuel cell stack 4.

Specifically, even after the carbon removal step is completed, the reforming section 5 is heated by the combustion gas from the combustor 8. Therefore, in step S41, the control device 3 determines whether the temperature of the reforming section 5 has reached a preset second temperature or higher. The second temperature is determined based on whether a POX reaction (partial oxidation reaction: _CH4 + 1/ _2O2 → CO + _2H2 ) is progressing, and is a temperature higher than the first temperature, e.g., 400°C.

If it is determined in step S41 that the temperature of the reforming section 5 has reached the second temperature, the valve 12 is opened and air is supplied to the reforming section 5 via the purge line 11 (see FIG. 1) (step S42). In addition, the valve 15 (see FIG. 1) is opened and fuel gas is supplied from the fuel tank 6 to the reforming section 5 (step S43). This causes the POX reaction to proceed in the reforming section 5. Note that the amount of air supplied to the reforming section 5 in step S42 only needs to be an amount sufficient to cause the POX reaction to proceed, and this amount is sufficiently smaller than the amount in the carbon removal step (S30).

Then, it is determined whether steady-state power generation is possible (for example, whether the temperature of the reforming unit 5 has reached the desired temperature) (step S44), and if it is determined that steady-state power generation is possible, normal operation is performed. During normal operation, as described above, the valve 12 is closed and no air is supplied to the reforming unit 5.

The first embodiment has been described above. As described above, according to this embodiment, in the carbon removal step (S30), air is supplied to the reforming section 5 at a large flow rate, so that carbon deposited on the reforming catalyst can be removed and the reforming performance can be restored.

In addition, since the carbon removal step (S30) is performed during the startup step, startup of the fuel cell system 1 and carbon removal can be performed simultaneously. This allows carbon to be removed efficiently.

In addition, in this embodiment, the carbon removal step (S30) is performed after the combustor 8 is started. Therefore, the carbon removed from the reformer 5 can be burned in the combustor 8. This makes it possible to render the removed carbon harmless and then discharge it to the outside.

The amount of air supplied to the reforming unit 5 in the carbon removal step (S30) may be constant, but it does not have to be. Below, the amount of air supplied will be explained while listing modified examples.

FIG. 6 is a graph showing a variation of this embodiment, which shows the relationship between the flow rate of air supplied to the reforming section 5 in the carbon removal step (S30) and time. In this example, air is supplied at a constant flow rate after the start of the carbon removal step (S30). As in this variation, air can be supplied to the reforming section 5 at a constant flow rate, for example.

On the other hand, FIG. 7 is a graph showing another modified example, which shows the relationship between the flow rate of air supplied to the reforming section 5 in the carbon removal step (S30) and time. In this example, air is supplied intermittently. The flow rate when air is supplied (see "A" in the figure) is greater than the flow rate of hydrocarbons during normal operation.

In the modified example shown in FIG. 7, the contact time between the reforming catalyst and air can be reduced by supplying air intermittently. This prevents oxidation degradation of the reforming catalyst. For the same reason, oxidation degradation of the anode electrode can also be suppressed in the fuel cell stack 4 located downstream of the reforming section 5.

Figure 8 is a graph showing yet another modified example, which shows the relationship between the flow rate of air supplied to the reforming section 5 in the carbon removal step (S30) and time. In this example, the amount of air supplied is increased in stages. The final air flow rate (see "A" in the figure) is greater than the flow rate of hydrocarbons during normal operation.

In the modified example shown in Figure 8, the air flow rate increases stepwise, making it easier for the entire surface of the reforming catalyst to be treated with air. This allows for more efficient carbon removal.

The use of the fuel cell system 1 in this embodiment is not particularly limited, but for example, it is for use on a vehicle. The fuel cell system 1 for use on a vehicle is required to be compact. As a result, it is used under severe conditions (for example, high SV, low S/C). This makes it easy for carbon to precipitate on the reforming catalyst. This embodiment can solve the problem of carbon precipitation, and is therefore particularly useful for use in a fuel cell system for use on a vehicle.

In this embodiment, the S/C ratio (molar ratio of water vapor to carbon) during normal operation is, for example, 5 or less, preferably 1 to 5, and more preferably 1 to 3. In fuel cells used at such low S/C ratios, as described above, carbon is prone to deposit on the reforming catalyst. This embodiment is particularly valuable in fuel cells operated at such S/C ratios, as it can solve the problem of carbon deposition.

In this embodiment, the space velocity GHSV of the gas supplied to the reforming unit 5 during normal operation is, for example, 30,000 h ^-1 or more, and preferably 40,000 to 100,000 h ^-1 . Fuel cells used in such GHSVs are used under severe conditions, and as described above, carbon is likely to deposit on the reforming catalyst. This embodiment is particularly valuable in fuel cell systems used in such GHSVs, since it can solve the problem of carbon deposition.
(2) Second embodiment Next, a second embodiment will be described. Fig. 9 is a schematic block diagram showing a fuel cell system 1 according to a second embodiment. Note that detailed description will be omitted for the points where the same configuration as the first embodiment can be adopted.

As shown in FIG. 9, in this embodiment, the reforming section 5 is provided at the anode of the fuel cell stack 4. That is, the anode of the fuel cell stack 4 contains a reforming catalyst. Therefore, the purge line 11 is connected to the anode of the fuel cell stack 4. A line 13 for supplying fuel gas is also connected to the anode of the fuel cell stack 4.

The control method of the fuel cell system in this embodiment is basically the same as that in the first embodiment. However, in this embodiment, the combustor inlet temperature reflects the reforming performance in the reformer 5. Therefore, in step S20 (see FIG. 4), it is possible to determine whether or not a carbon removal step is required based on the combustor inlet temperature during the previous operation. In addition, the temperature of the reformer 5 is reflected in the anode outlet temperature. Therefore, in steps S32 (see FIG. 4) and S41 (see FIG. 5), it may be determined whether or not the temperature of the reformer 5 has reached a predetermined temperature based on the anode outlet temperature.

In this embodiment, as in the first embodiment, by carrying out the carbon removal step, the deposited carbon can be removed and the reforming performance can be restored. In addition, according to this embodiment, since the reforming catalyst is disposed inside the fuel cell stack 4, the volume of the fuel cell system can be reduced. This is expected to improve the power generation efficiency. Also, a reduction in costs due to a reduction in the number of parts can be expected.
(3) Third embodiment Next, a third embodiment will be described. Fig. 10 is a schematic block diagram showing a fuel cell system according to a third embodiment. Note that detailed description will be omitted for the points where the same configuration as the above-mentioned embodiments can be adopted.

In this embodiment, the fuel cell stack 4 has a first stack 4-1 and a second stack 4-2. Each stack (4-1, 4-2) has an anode, an electrolyte layer (not shown), and a cathode.

The reforming section 5 is disposed at the anode of the first stack 4-1. Therefore, the purge line 11 is connected to the anode of the first stack 4-1. A line 13 for supplying fuel gas is also connected to the anode of the first stack 4-1. The anode of the first stack 4-1 is connected to the anode of the second stack 4-2 on its downstream side. In other words, the first stack 4-1 and the second stack 4-2 are connected so that gas flows from the anode of the first stack 4-1 to the anode of the second stack 4-2. The anode of the second stack 4-2 is connected to the combustor 8 via a line 19 on its downstream side.

On the other hand, the air supply line 10 is connected to the cathode of the second stack 4-2. The cathode of the second stack 4-2 is connected downstream to the cathode of the first stack 4-1. In other words, the first stack 4-1 and the second stack 4-2 are connected so that gas flows from the cathode of the second stack 4-2 to the cathode of the first stack 4-1. The cathode of the first stack 4-1 is connected downstream to the combustor 8 via a line 17.

The control method of the fuel cell system in this embodiment is the same as that of the previously described embodiment. That is, as shown in FIG. 2, first, the combustor 8 is started (step S10). Then, while the fuel cell stack 4 is warming up, a carbon removal step (S30) is performed.

In this embodiment, during warm-up of the fuel cell stack 4 (steps S20 to S40), heated air flows from the cathode of the second stack 4-2 to the cathode of the first stack 4-1 in that order. Therefore, the second stack 4-2 is heated before the first stack 4-1. In other words, during warm-up, the temperature of the first stack 4-1 is less likely to rise than that of the second stack 4-2.

On the other hand, in the carbon removal step (S30), air is supplied from the purge line 11 to the reforming section 5 provided in the first stack 4-1. That is, air is supplied to the reforming section 5 of the first stack 4-1, which is at a relatively low temperature. Since air is not supplied to the reforming catalyst, which is at a high temperature, oxidation deterioration of the reforming catalyst can be more reliably suppressed.

　Although the embodiments of the present invention have been described above, the above embodiments merely show some of the application examples of the present invention, and are not intended to limit the technical scope of the present invention to the specific configurations of the above embodiments.

Below is a summary of the most representative relationships between the configurations and effects of embodiments of the present invention.

In one aspect, the present embodiment relates to a control method for a fuel cell system 1. The fuel cell system 1 includes a reforming unit 5 that reforms a fuel gas containing hydrocarbons to generate a reformed gas, the reforming unit 5 having a reforming catalyst, and a fuel cell stack 4 configured to generate electricity using the reformed gas as an anode gas. The control method includes a normal operation step (S50) in which the fuel cell stack 4 generates electricity while supplying fuel gas to the reforming unit 5, and a carbon removal step (S30) in which carbon deposited on the reforming catalyst is removed. The carbon removal step (S30) includes a step of supplying air to the reforming unit 5. The flow rate of the air supplied to the reforming unit 5 in the carbon removal step (S30) is greater than the flow rate of the hydrocarbons supplied to the reforming unit 5 in the normal operation step (S50). According to this method, the deposited carbon can be removed by supplying air to the reforming unit 5 at a specific flow rate.

Preferably, the flow rate of air supplied to the reforming unit 5 in the carbon removal step (S30) is at least four times the flow rate of hydrocarbons supplied to the reforming unit 5 in the normal operation step (S50). This method makes it possible to more reliably remove precipitated carbon.

Preferably, the fuel gas contains methane. With this method, a methane steam reforming reaction can be carried out in the reforming section 5. Since methane is a non-alcohol fuel, the reforming catalyst in the reforming section 5 is less likely to deteriorate due to acid sites.

In a preferred embodiment, the control method includes a step (S20) of determining whether or not to perform a carbon removal step based on the execution time of the normal operation step. This prevents the carbon removal step from being executed unnecessarily, thereby improving energy efficiency.

In a preferred embodiment, the control method includes a step (S20) of determining whether or not to perform a carbon removal step based on the outlet temperature of the reforming section. This prevents the carbon removal step from being performed unnecessarily, thereby improving energy efficiency.

In a preferred embodiment, the control method includes a startup step of starting up the fuel cell stack 4 before the normal operation step (S50). The carbon removal step (S30) is performed in the startup step. According to this method, the startup of the fuel cell stack and the carbon removal can be performed simultaneously, so that the fuel cell system 1 can be operated efficiently.

In a preferred embodiment, the fuel cell system 1 further includes an air supply line 10 that supplies air to the cathode of the fuel cell stack 4, and a combustor 8 that is thermally connected to the air supply line 10 and heats the air. The start-up step includes a combustor start-up step of starting the combustor so that the temperature is equal to or higher than a predetermined temperature. The carbon removal step (S30) is performed after the combustor start-up step. The carbon removal step (S30) includes a step of sending the removed carbon to the combustor 8. According to this method, the carbon removed in the carbon removal step can be burned by the combustor 8. This makes it possible to render the carbon harmless and then discharge it to the outside.

In a preferred embodiment, the fuel cell stack 4 has a first stack 4-1 and a second stack 4-2. The reforming section 5 is disposed at the anode in the first stack 4-1. The air supply line 10 is connected to the cathode of the second stack 4-2. The first stack 4-1 and the second stack 4-2 are connected so that gas flows from the anode of the first stack 4-1 to the anode of the second stack 4-2, and gas flows from the cathode of the second stack 4-2 to the cathode of the first stack 4-1. The start-up step includes a warm-up step in which the fuel cell stack 4 is heated by supplying air to the cathode of the second stack 4-2 via the air supply line 10 after the combustor start-up step. The carbon removal step (S30) is performed during the warm-up step. According to this method, in the carbon removal step, air is supplied to the reforming section 5 of the first stack 4-1, which is at a relatively low temperature. This prevents the reforming catalyst from being treated with oxygen at high temperatures, suppressing oxidative deterioration of the reforming catalyst.

In a preferred embodiment, the carbon removal step (S30) includes a step of stopping the supply of air to the reforming section 5 when the reforming section temperature reaches the first temperature. This method prevents air from being supplied to the reforming section 5 at a high temperature. This makes it possible to suppress oxidation deterioration of the reforming catalyst.

In a preferred embodiment, the carbon removal step (S30) includes a step of supplying air from the air supply unit 9 to the reforming unit 5 without heating the air. This method prevents the reforming catalyst from being treated with high-temperature oxygen. This makes it possible to suppress oxidative deterioration of the reforming catalyst.

In a preferred embodiment, the carbon removal step (S30) includes a step of intermittently supplying air to the reforming section 5. This method can reduce the contact time between the reforming catalyst and oxygen. As a result, oxidation deterioration of the reforming catalyst can be suppressed.

In a preferred embodiment, the carbon removal step (S30) includes a step of supplying air to the reforming section 5 at a flow rate that increases stepwise. With this method, the entire surface of the reforming catalyst is treated with air, allowing carbon to be removed more efficiently.

In a preferred embodiment, the reforming section 5 is disposed within the fuel cell stack 4. With this method, the reforming catalyst is disposed inside the fuel cell stack 4, so the volume of the fuel cell system can be reduced. This is expected to improve power generation efficiency. In addition, costs can be reduced by reducing the number of parts.

In one aspect, the fuel cell system 1 includes a reforming unit 5 having a reforming catalyst that reforms a fuel gas containing hydrocarbons to generate a reformed gas, a fuel cell stack 4 configured to generate electricity using the reformed gas as an anode gas, and a control device 3. The control device 3 is configured to execute a normal operation step and a carbon removal step. In the normal operation step, the control device 3 causes the fuel cell stack to generate electricity while supplying fuel gas to the reforming unit 5. In the carbon removal step, the control device supplies air to the reforming unit 5. The flow rate of the air supplied to the reforming unit 5 in the carbon removal step is greater than the flow rate of the hydrocarbons supplied to the reforming unit 5 in the normal operation step. According to this fuel cell system 1, the precipitated carbon can be removed by supplying air to the reforming unit 5 at a specific high flow rate.

In the following, examples carried out by the present inventors will be described in more detail in order to explain the present invention in more detail. However, the present invention should not be construed as being limited to the following examples.
(Experimental Example 1)
Using the fuel cell system 1 described in the third embodiment (see FIG. 10), normal operation was performed for 350 hours. The S/C ratio was set to 2. A mixed gas containing methane, water vapor, and _N2 was used as the fuel gas. The GHSV was set to 65000 h ^-1 . The flow rate of methane (i.e., the flow rate of hydrocarbons) supplied to the reforming unit 5 was 0.35 NL/min. The operating temperature of the reforming unit was set to 650°C. A mixture containing Pt and Ni was used as the reforming catalyst.

After 350 hours of normal operation, a carbon removal step was carried out. Specifically, air was supplied to the reforming section 5 for 5 minutes at a flow rate (1.4 NL/min) four times the flow rate of methane during normal operation. After the carbon removal step, normal operation was carried out again. The hydrogen concentration at the outlet of the reforming section 5 was measured before and after the carbon removal step.

The results are shown in Figure 11. Figure 11 shows a graph of operation time and amount of hydrogen generation. In Figure 11, region A is the timing when the carbon removal step was performed. As shown in Figure 11, the amount of hydrogen generation decreased as normal operation was performed. In contrast, it was confirmed that the amount of hydrogen generation increased and reforming performance was restored by performing the carbon removal step.
(Experimental Example 2)
The amount of air supplied to the reforming unit 5 in the carbon removal step was changed, and an experiment was performed in the same manner as in Experimental Example 1. Specifically, air was supplied to the reforming unit 5 at flow rates 1, 3, 4, and 10 times the flow rate of methane during normal operation, and the carbon removal step was performed. After the carbon removal step, the hydrogen concentration at the outlet of the reforming unit 5 was measured.

The results, along with the initial performance, are shown in Figure 12. As shown in Figure 12, when air was supplied at four times the flow rate, there was a significant increase in the outlet _H2 concentration compared to the lower flow rates.
(Experimental Example 3)
In Experimental Example 1, the air supply time in the carbon removal step was changed to 1 minute. The other conditions were the same as those in Experimental Example 1. Then, when the reforming performance was similarly confirmed before and after the carbon removal step, it was confirmed that the reforming performance was restored.
(Experimental Example 4)
In Experimental Example 1, the reforming catalyst was changed to one containing Rh. The other conditions were the same as those in Experimental Example 1. Then, when the reforming performance was similarly confirmed before and after the carbon removal step, it was confirmed that the reforming performance was restored.

Claims (14)

A method for controlling a fuel cell system, comprising:
The fuel cell system includes:
a reforming unit that reforms a fuel gas containing a hydrocarbon to generate a reformed gas, the reforming unit having a reforming catalyst;
a fuel cell stack configured to generate electricity using the reformed gas as an anode gas;
The control method includes:
a normal operation step of causing the fuel cell stack to generate power while supplying the fuel gas to the reformer;
A carbon removal step of removing carbon deposited on the reforming catalyst,
The carbon removal step includes a step of supplying air to the reforming section,
The flow rate of the air supplied to the reforming unit in the carbon removal step is greater than the flow rate of the hydrocarbons supplied to the reforming unit in the normal operation step.
A method for controlling a fuel cell system. 2. The control method according to claim 1,
A flow rate of the air supplied to the reforming unit in the carbon removal step is four times or more than a flow rate of the hydrocarbons supplied to the reforming unit in the normal operation step.
Control methods. 3. The control method according to claim 1 or 2,
The method of claim 1, wherein the fuel gas comprises methane. 3. The control method according to claim 1 or 2,
Furthermore,
A step of determining whether to perform the carbon removal step based on an implementation time of the normal operation step.
Control methods. 3. The control method according to claim 1 or 2,
Furthermore,
A step of determining whether or not to perform the carbon removal step based on an outlet temperature of the reforming section.
Control methods. 3. The control method according to claim 1 or 2,
Furthermore,
a start-up step of starting up the fuel cell stack before the normal operation step;
The decarbonization step is carried out during the start-up step.
Control methods. 7. The control method according to claim 6,
The fuel cell system further comprises:
an air supply line for supplying the air to the cathode of the fuel cell stack;
a combustor thermally connected to the air supply line for heating the air;
The starting step includes a combustor starting step of starting the combustor so that the temperature of the combustor becomes equal to or higher than a predetermined temperature,
The decoking step is performed after the combustor start-up step;
The carbon removal step includes sending the removed carbon to the combustor.
Control methods. 8. The control method according to claim 7,
The fuel cell stack comprises:
A first stack;
a second stack,
the reformer is disposed on the anode of the first stack,
the air supply line is connected to the cathode of the second stack;
the first stack and the second stack are connected such that a gas flows from the anode of the first stack to the anode of the second stack and from the cathode of the second stack to the cathode of the first stack;
the startup step includes a warm-up step of heating the fuel cell stack by supplying the air to the cathode of the second stack through the air supply line after the combustor startup step;
The decoking step is performed during the warm-up step.
Control methods. 3. The control method according to claim 1 or 2,
The carbon removal step includes a step of stopping the supply of the air to the reforming unit when the temperature of the reforming unit reaches a first temperature.
Control methods. 3. The control method according to claim 1 or 2,
The carbon removal step includes a step of supplying the air from an air supply unit to the reforming unit without heating the air.
Control methods. 3. The control method according to claim 1 or 2,
The carbon removal step includes a step of intermittently supplying the air to the reforming unit.
Control methods. The control method according to claim 1 or 2,
The carbon removal step includes a step of supplying the air to the reforming unit so that the flow rate increases stepwise.
Control methods. 3. The control method according to claim 1 or 2,
The reformer is disposed in the fuel cell stack.
Control methods. a reforming unit that reforms a fuel gas containing a hydrocarbon to generate a reformed gas, the reforming unit having a reforming catalyst;
a fuel cell stack configured to generate electricity using the reformed gas as an anode gas;
A control device;
Equipped with
The control device is configured to perform a normal operation step and a carbon removal step;
In the normal operation step, the control device controls the fuel cell stack to generate power while supplying the fuel gas to the reforming unit,
In the carbon removal step, the control device supplies air to the reforming section,
The flow rate of the air supplied to the reforming unit in the carbon removal step is greater than the flow rate of the hydrocarbons supplied to the reforming unit in the normal operation step.
Fuel cell system.

Images