Fuel cell system and its control method

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

Technical Field

[0001] The present invention relates to a fuel cell system and a control method thereof.

Background Art

[0002] A fuel cell that generates electricity using hydrogen as fuel is known. In such a fuel cell, a reforming catalyst may be used to generate hydrogen. In such a fuel cell, a fuel gas containing hydrocarbons is reformed by a reforming catalyst to generate a reformed gas containing hydrogen. Then, power generation is performed using the generated reformed gas.

[0003] Techniques related to fuel cells using a reforming catalyst are described in, for example, Patent Document 1 (JP2005-340075A). Patent Document 1 describes, as a method for stopping the operation of a fuel cell having a specific reformer, while continuously supplying air into the fuel cell, supplying a specific mixed gas into the fuel cell through the reformer, and when the temperature of the reforming catalyst drops within a specific temperature range, stopping the supply of the mixed gas, and after stopping the supply of the mixed gas, flowing air or hydrocarbon gas through the reformer as a purge gas into the fuel cell.

Summary of the Invention

[0004] By the way, when the reforming catalyst is continuously operated, carbon may be deposited so as to cover the reforming catalyst. When the reforming catalyst is covered with carbon, the reaction area decreases and the reforming performance deteriorates. Therefore, it is preferable that the deposited carbon be removed.

[0005] Therefore, an object of the present invention is to provide a technique capable of removing carbon deposited on a reforming catalyst.

[0006] In one aspect, the present invention relates to a control method for a fuel cell system. The fuel cell system comprises a reforming unit having a reforming catalyst, which reforms a fuel gas containing hydrocarbons to produce a reformed gas, and a fuel cell stack configured to generate electricity using the reformed gas as an anode gas. The control method comprises a normal operation step of supplying fuel gas to the reforming unit while causing the fuel cell stack to generate electricity, and a carbon removal step of removing carbon deposited on the reforming catalyst. The carbon removal step includes 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.

[0007] In another aspect, the present invention relates to a fuel cell system. The fuel cell system comprises a reforming unit having a reforming catalyst, which reforms a fuel gas containing hydrocarbons to produce a reformed gas; 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 perform a normal operation step and a carbon removal step. In the normal operation step, the control device supplies fuel gas to the reforming unit while causing the fuel cell stack to generate electricity. In the carbon removal step, the control device supplies 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. [Brief explanation of the drawing]

[0008] [Figure 1] Figure 1 is a schematic block diagram showing a fuel cell system according to the first embodiment. [Figure 2] Figure 2 is a flowchart showing a control method for a fuel cell system according to the first embodiment. [Figure 3] Figure 3 is a flowchart showing the steps for starting the combustor. [Figure 4] Figure 4 is a flowchart showing steps S20 and S30. [Figure 5] Figure 5 is a flowchart showing the POX operation step (step S40). [Figure 6] Figure 6 is a graph showing one modified example of the first embodiment. [Figure 7] Figure 7 is a graph showing other variations of the first embodiment. [Figure 8] Figure 8 is a graph showing further variations of the first embodiment. [Figure 9] Figure 9 is a schematic block diagram showing a fuel cell system according to the second embodiment. [Figure 10] Figure 10 is a schematic block diagram showing a fuel cell according to the third embodiment. [Figure 11] Figure 11 is a graph showing the relationship between operating time and hydrogen generation amount in Experimental Example 1. [Figure 12] Figure 12 is a graph showing the relationship between airflow rate and outlet H2 concentration. [Modes for carrying out the invention]

[0009] Embodiments of the present invention will be described below with reference to the drawings. (1) First Embodiment Figure 1 is a schematic block diagram showing a fuel cell system 1 according to the first embodiment. As shown in Figure 1, the fuel cell system 1 includes a control device 3, a fuel cell stack 4, a reforming unit 5, a fuel tank 6, a heat exchanger 7, a combustor 8, and an air supply unit 9. This fuel cell system 1 is configured to operate steadily at a constant output (rated output) during normal operation.

[0010] The fuel cell stack 4 is the part that realizes the power generation function. Although not shown in the diagram, the fuel cell stack 4 has an anode, an electrolyte, and a cathode. The fuel cell stack 4 generates electricity by receiving anode gas and cathode gas. 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.

[0011] Furthermore, the fuel cell stack 4 is connected to the combustor 8 via line 19 downstream of its anode. Also, the fuel cell stack 4 is connected to the combustor 8 via line 17 downstream of its cathode. As a result, exhaust gases from both the anode and cathode are sent to the combustor 8.

[0012] The reforming unit 5 is configured to reform the fuel gas supplied from the fuel tank 6 to produce a reformed gas containing hydrogen. The reforming unit 5 produces the reformed gas by steam reforming. The steam may be supplied from the fuel tank 6 together with the fuel gas, or it may be supplied to the reforming unit 5 through a separate route (not shown) from the fuel gas.

[0013] The reforming section 5 contains a reforming catalyst. Examples of reforming catalysts that can be used include Pt / CeO2, Ni / CeO2, and Rh / CeO2.

[0014] The reforming unit 5 is connected to the anode of the fuel cell stack 4 via line 18. The reformed gas produced in the reforming unit 5 is supplied to the anode of the fuel cell stack 4 via line 18, as described above.

[0015] The fuel tank 6 is the part that stores fuel gas. The fuel tank 6 is connected to the reforming unit 5 via line 13. A valve 15 is provided in line 13. The fuel tank 6 is also connected to the combustor 8 via line 14. A valve 16 is provided in line 14.

[0016] The fuel gas contains hydrocarbons. Methane is a preferred hydrocarbon. 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.

[0017] The air supply unit 9 is provided to supply air as the cathode gas to the fuel cell stack 4. The air supply unit 9 is realized, for example, by a blower. The air supply unit 9 is connected to the cathode of the fuel cell stack 4 via an air supply line 10.

[0018] 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 in 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.

[0019] The heat exchanger 7 is provided in the air supply line 10. The heat exchanger 7 is thermally connected to the combustor 8. Thus, the air flowing through the air supply line 10 is heated by the heat from the combustor 8.

[0020] Note that the heat exchanger 7 is provided on the downstream side of the connection portion 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.

[0021] As described above, the combustor 8 is provided to heat 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. In the combustor 8, these gases burn. The combustion gas generated in the combustor 8 is discharged to the outside as exhaust gas.

[0022] Also, the combustor 8 is connected to the reforming unit 5 so that the combustion gas is discharged as exhaust gas after heat-exchanging with the reforming unit 5.

[0023] The control device 3 is provided to control the operation of the entire fuel cell system 1. The control device 3 is implemented by a computer having, for example, an arithmetic unit such as a CPU and a memory device such as RAM and ROM. In other words, the control device 3 realizes its function by having the arithmetic unit execute a control program stored in the memory device.

[0024] The above is a general overview of the fuel cell system 1. Next, the operation method of the fuel cell system 1 will be explained.

[0025] First, let's explain the operation of the fuel cell system 1 during normal operation. During normal operation, the fuel cell system 1 operates at a predetermined constant output (rated output).

[0026] Specifically, the control device 3 closes valve 12 and opens valves 15 and 16. This supplies fuel gas from the fuel tank 6 to the reforming unit 5 and the combustor 8. The reforming unit 5 is supplied with fuel gas at a constant flow rate to obtain a constant output. In the reforming unit 5, the fuel gas is reformed by steam reforming, and reformed gas is generated. The generated reformed gas is sent to the anode of the fuel cell stack 4 via line 18. After being used for power generation, the reformed gas supplied to the anode is sent to the combustor 8 via line 19 as anode exhaust gas.

[0027] Air is supplied 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 supplied to the cathode at a constant flow rate. After being used for power generation, the air supplied to the cathode is sent to the combustor 8 via line 17 as cathode exhaust gas.

[0028] In the combustor 8, gases supplied from lines 14, 17, and 19 are burned. This heats the air via the heat exchanger 7. In other words, heated air is supplied to the fuel cell stack 4. Fuel cells typically function at high temperatures. The supply of heated air maintains the temperature necessary for power generation 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 unit 5. Therefore, the temperature of the reforming unit 5 is also maintained at the temperature necessary for the reforming reaction.

[0029] The above describes the operation during normal operation.

[0030] During operation, carbon deposits may form on the reforming catalyst in reforming section 5. This is thought to be due to the disproportionation reaction of CO (2CO → C + CO2), a byproduct of the reforming reaction. When the surface of the reforming catalyst is coated with carbon, the efficiency of the reforming reaction decreases.

[0031] Therefore, in this embodiment, a carbon removal step is performed to remove the precipitated carbon. During the carbon removal step, air is supplied to the reforming unit 5. Specifically, the control device 3 opens valve 12 (see Figure 1), and air is supplied to the reforming unit 5 via the purge line 11. Valve 15 is kept closed. In other words, fuel gas is not supplied to the reforming unit 5.

[0032] In the carbon removal step, the air supplied to the reforming unit 5 is at a high flow rate. Specifically, the air supplied to the reforming unit 5 is at a flow rate greater than the flow rate of hydrocarbons supplied to the reforming unit 5 during normal operation. By supplying air to the reforming unit 5 in such quantities, carbon deposited on the reforming catalyst can be removed, and the function of the reforming catalyst can be restored. It should be noted that the "flow rate of hydrocarbons supplied to the reforming unit during normal operation" referred to here is the flow rate of hydrocarbons, not the flow rate of the entire fuel gas. For example, if the hydrocarbon contained in the fuel gas is methane, then the flow rate of methane is the "flow rate of hydrocarbons supplied to the reforming unit 5".

[0033] Preferably, the flow rate of air supplied to the reforming unit 5 in the carbon removal step is four times or more the flow rate of hydrocarbons supplied to the reforming unit 5 during normal operation. When air is supplied at such a flow rate, carbon can be removed more reliably.

[0034] Alternatively, using moles as a reference, it is preferable that the oxygen-based molar flow rate (O2) of the air supplied to the reforming unit 5 in the carbon removal step be 0.84 or higher than the carbon-based molar flow rate (C) of the fuel gas supplied to the reforming unit 5 during normal operation. When air is supplied at such a flow rate, carbon can be removed more reliably.

[0035] The above describes the general configuration and operation of this embodiment. As described above, according to this embodiment, precipitated carbon can be removed by supplying air to the modification section 5 at a large flow rate. This restores the modification function.

[0036] The timing of the carbon removal step is not particularly limited. However, performing the carbon removal step when the fuel cell system 1 is started up is efficient because it allows for simultaneous carbon removal and startup of the fuel cell system 1. Below, the control method of the fuel cell system 1 according to this embodiment will be specifically described using the case where the carbon removal step is performed when the fuel cell system 1 is started up as an example. Unless otherwise specified, the following control method is performed by the control device 3.

[0037] Figure 2 is a flowchart showing the control method for the fuel cell system 1 according to this embodiment. The control method for this fuel cell system generally includes a startup step (S10-S40) and a normal operation step (S50). The startup step (S10-S40) includes a step of starting the combustor 8 (S10), a step of determining whether carbon removal is necessary (S20), a carbon removal step (S30), and a step of performing POX operation (S40). After the combustor 8 startup step (S10), heated air is supplied to the cathode of the fuel cell stack 4 via the air supply line 10. Therefore, the fuel cell stack 4 is heated; in other words, the fuel cell stack 4 is warmed up. Thus, steps S20-S40, performed after the combustor 8 is started (from step S10 onwards), can be considered to be performed during the warm-up step of the fuel cell stack 4.

[0038] The following details each step. (Step S10: Starting the combustor) First, start the combustor 8. Figure 3 is a flowchart showing the steps for starting the combustor 8.

[0039] First, the air supply unit 9 is activated (step S11). This causes air to flow 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 then to the combustor 8 in that order. The valve 12 is kept closed; that is, no air is supplied to the reforming unit 5.

[0040] Also, the heater (not shown) of the combustor 8 is activated (step S12).

[0041] Next, it is determined whether the temperature of the combustor 8 has reached a predetermined temperature or higher (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 that has been stored in advance. The predetermined temperature here is a temperature set in relation to whether or not a combustion reaction is possible in the combustor 8. For example, if the combustor 8 contains a combustion catalyst, the light-off temperature of the combustion catalyst (e.g., 350°C) is set as the predetermined temperature.

[0042] When the temperature of the combustor 8 reaches a predetermined temperature or higher, fuel gas is introduced into the combustor 8 (step S14). That is, valve 16 is opened, and fuel gas is supplied from fuel tank 6 to combustor 8 via line 14. As a result, the combustion reaction proceeds in the combustor 8, and the combustor 8 starts up.

[0043] As previously described, after the combustor 8 is started, heated air is supplied to the cathode of the fuel cell stack 4 via the heat exchanger 7. In other words, as previously described, the fuel cell stack 4 is warmed up. The reforming unit 5 is also heated by the heat of the combustion gas sent from the combustor 8. (Step S20: Determining whether carbon removal is necessary) Figure 4 is a flowchart showing step S20 and the subsequent carbon removal step S30. After the combustor 8 is started, it is first determined whether the carbon removal step is necessary (step S20).

[0044] The necessity of the carbon removal step is determined, for example, by the cumulative operating time. For instance, the control device 3 is configured to store the cumulative operating time since the last carbon removal step as data. The control device 3 compares this cumulative operating time with a predetermined time. If the cumulative operating time exceeds the predetermined time, it determines that the carbon removal step is necessary.

[0045] Alternatively, the necessity of the carbon removal step may be determined based on the reforming performance during the previous operation. The reforming performance can be determined, for example, based on the outlet temperature of the reforming section 5. The reforming reaction in the reforming section 5 is an endothermic reaction. Therefore, if the reforming function deteriorates, the outlet temperature of the reforming section 5 rises. In other words, the outlet temperature of the reforming section 5 can be said to reflect the reforming performance in the reforming section 5. Accordingly, 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 greater than or equal to a predetermined value (for example, 10% of the initial state value).

[0046] If the determination determines that the carbon removal step is unnecessary, the necessity of the carbon removal step will be determined at the next startup, and the processing from step S40 onward will be carried out. This prevents the carbon removal step from being performed unnecessarily, thereby improving energy efficiency.

[0047] On the other hand, if it is determined that a carbon removal step is necessary, the next carbon removal step S30 is performed. (Step S30: Carbon removal step) In the carbon removal step, as previously described, valve 12 (see Figure 1) is opened (step S31). This allows air to be supplied from the air supply unit 9 to the reforming unit 5 via the purge line 11. As previously described, at this time, air is supplied to the reforming unit 5 at a predetermined large flow rate.

[0048] The supply of air removes the carbon precipitated from the reforming section 5. The removed carbon is sent to the combustor 8 via the anode and line 19 of the fuel cell stack 4. There, it is burned and rendered harmless before being discharged to the outside.

[0049] Furthermore, 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 undergo oxidative degradation. However, according to this embodiment, since the air is supplied without being heated, oxidative degradation of the reforming catalyst can be suppressed. Similarly, oxidative degradation of the electrode material is also suppressed at the anode of the fuel cell stack 4 located downstream of the reforming unit 5.

[0050] Even while air is being supplied, the reforming unit 5 is heated by the heat from the combustor 8, so its temperature increases. Therefore, the carbon removal step is performed until the temperature of the reforming unit 5 reaches a predetermined temperature. Specifically, the control device 3 determines whether the temperature of the reforming unit 5 has reached a preset first temperature while air is being supplied (step S32). For example, the control device 3 measures the temperature of the reforming unit 5 using a sensor (not shown). The temperature of the reforming unit 5 may also be determined indirectly by measuring the outlet temperature of the reforming unit 5.

[0051] If the control device 3 determines in step S32 that the temperature of the modification unit 5 has reached the first temperature, it closes the valve 12. This stops the supply of air to the modification unit 5 (step S33). In other words, the carbon removal step is completed.

[0052] The first temperature mentioned above is set from the perspective of preventing deterioration of the substances contained in the reforming section 5. If air is supplied to the reforming section 5 when the temperature is high, the reforming catalyst may deteriorate due to sintering and oxidation. Furthermore, the anode in the downstream fuel cell stack 4 is also more susceptible to oxidative deterioration. In contrast, by stopping the supply of air when the first temperature is reached, oxidative deterioration of the reforming catalyst and anode can be prevented. The first temperature is, for example, 300°C.

[0053] Furthermore, the carbon removal step is preferably performed 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 performed. Figure 5 is a flowchart of the POX operation step (step S40). The POX operation is performed to promote the warming up of the fuel cell stack 4.

[0054] Specifically, even after the carbon removal step is completed, the reforming unit 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 unit 5 has reached a preset second temperature or higher. The second temperature is determined by whether or not the POX reaction (partial oxidation reaction: CH4 + 1 / 2O2 → CO + 2H2) proceeds, and is a higher temperature than the first temperature, for example, 400°C.

[0055] 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 Figure 1) (step S42). Also, the valve 15 (see Figure 1) is opened and fuel gas is supplied from the fuel tank 6 to the reforming section 5 (step S43). As a result, the POX reaction proceeds in the reforming section 5. The amount of air supplied to the reforming section 5 in step S42 only needs to be enough for the POX reaction to proceed, and this amount is sufficiently small compared to the amount in the carbon removal step (S30).

[0056] Subsequently, 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). 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.

[0057] 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.

[0058] Furthermore, since the carbon removal step (S30) is performed during the startup step, the startup of the fuel cell system 1 and carbon removal can be performed simultaneously. This allows for efficient carbon removal.

[0059] Furthermore, in this embodiment, the carbon removal step (S30) is performed after the combustor 8 is started. Therefore, the carbon removed from the reforming unit 5 can be burned in the combustor 8. This allows the removed carbon to be rendered harmless before being discharged to the outside.

[0060] In addition, the amount of air supplied to the modification unit 5 in the carbon removal step (S30) may be constant, or it may not be constant. The amount of air supplied will be explained below with reference to variations.

[0061] Figure 6 is a graph showing a modified example of this embodiment, illustrating the relationship between the flow rate of air supplied to the modification unit 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 shown in this modified example, air can be supplied to the modification unit 5 at a constant flow rate, for example.

[0062] On the other hand, Figure 7 is a graph showing another modified example, illustrating the relationship between the flow rate of air supplied to the reforming unit 5 in the carbon removal step (S30) and time. In this example, air is supplied intermittently. The flow rate during air supply (see "A" in the figure) is greater than the hydrocarbon flow rate during normal operation.

[0063] As shown in the modified configuration in Figure 7, the intermittent supply of air reduces the contact time between the reforming catalyst and the air. Therefore, oxidative degradation of the reforming catalyst can be prevented. Similarly, for the same reason, oxidative degradation of the anode electrode can also be suppressed in the fuel cell stack 4 located downstream of the reforming section 5.

[0064] Figure 8 is a graph illustrating further variations, showing the relationship between the airflow rate supplied to the reforming section 5 in the carbon removal step (S30) and time. In this example, the air supply is increased in stages, and the final airflow rate (see "A" in the figure) is greater than the hydrocarbon flow rate during normal operation.

[0065] As shown in the modified version in Figure 8, the airflow rate is increased in stages, making it easier for the entire surface of the reforming catalyst to be treated with air. This allows for more efficient removal of carbon.

[0066] The application of the fuel cell system 1 in this embodiment is not particularly limited, but it is used, for example, in vehicles. In vehicle fuel cell systems 1, miniaturization is required. As a result, they are used under harsh conditions (e.g., high SV, low S / C). Therefore, carbon tends to precipitate on the reforming catalyst. This embodiment can solve the problem of such carbon deposition and is therefore particularly useful in vehicle fuel cell systems.

[0067] 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 with such a low S / C ratio, carbon tends to precipitate on the reforming catalyst, as described above. This embodiment is particularly valuable in fuel cells operated at such an S / C ratio because it can solve the problem of carbon deposition.

[0068] 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 The above applies, preferably 40,000 to 100,000 hours. -1 Therefore, fuel cells used in such GHSVs are used under harsh conditions, and as mentioned above, carbon tends to precipitate on the reforming catalyst. This embodiment is particularly valuable in fuel cell systems used in such GHSVs because it can solve the problem of carbon deposition. (2) Second embodiment Next, a second embodiment will be described. Figure 9 is a schematic block diagram showing the fuel cell system 1 according to the second embodiment. Detailed explanations of aspects that can be adopted in the same way as in the first embodiment will be omitted.

[0069] As shown in Figure 9, in this embodiment, the reforming unit 5 is located at the anode of the fuel cell stack 4. That is, the reforming catalyst is located at the anode of the fuel cell stack 4. Therefore, the purge line 11 is connected to the anode of the fuel cell stack 4. The line 13 for supplying fuel gas is also connected to the anode of the fuel cell stack 4.

[0070] The control method for the fuel cell system in this embodiment is basically the same as in the first embodiment. However, in this embodiment, the combustor inlet temperature reflects the reforming performance in the reforming unit 5. Therefore, in step S20 (see Figure 4), the necessity of the carbon removal step can be determined based on the combustor inlet temperature during the previous operation. Also, the temperature of the reforming unit 5 is reflected in the anode outlet temperature. Therefore, in steps S32 (see Figure 4) and S41 (see Figure 5), it may be determined whether the temperature of the reforming unit 5 has reached a predetermined temperature based on the anode outlet temperature.

[0071] In this embodiment, as in the first embodiment, the precipitated carbon can be removed by performing a carbon removal step, thereby restoring the reforming performance. In addition, according to this embodiment, since the reforming catalyst is located inside the fuel cell stack 4, the volume of the fuel cell system can be reduced. This is expected to improve power generation efficiency. Furthermore, a reduction in cost can be expected due to a reduction in the number of parts. (3) Third Embodiment Next, a third embodiment will be described. Figure 10 is a schematic block diagram showing a fuel cell system according to the third embodiment. Detailed explanations will be omitted for aspects where the same configuration as in the previously described embodiments can be adopted.

[0072] In this embodiment, the fuel cell stack 4 comprises 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.

[0073] The reforming unit 5 is located 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. The 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 downstream to the anode of the second stack 4-2. In other words, the first stack 4-1 and the second stack 4-2 are connected such 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 downstream to the combustor 8 via line 19.

[0074] Meanwhile, 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 such 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 line 17.

[0075] The control method for the fuel cell system in this embodiment is the same as in the previously described embodiment. That is, as shown in Figure 2, first the combustor 8 is started (step S10). Then, while the fuel cell stack 4 is warming up, the carbon removal step (S30) is performed.

[0076] In this embodiment, during the 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. 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 does not rise as easily as that of the second stack 4-2.

[0077] On the other hand, in the carbon removal step (S30), air is supplied from the purge line 11 to the reforming section 5 located in the first stack 4-1. In other words, 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 at a high temperature, oxidative degradation of the reforming catalyst can be suppressed more reliably.

[0078] Although embodiments of the present invention have been described above, these embodiments only represent a part 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.

[0079] The following summarizes representative examples of the relationship between the configuration and effects of embodiments of the present invention.

[0080] In one embodiment, this embodiment relates to a control method for a fuel cell system 1. The fuel cell system 1 is a reforming unit 5 that reforms a fuel gas containing hydrocarbons to produce reformed gas, and comprises a 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 comprises a normal operation step (S50) in which the fuel gas is supplied to the reforming unit 5 and the fuel cell stack 4 is made to generate electricity, 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 air supplied to the reforming unit 5 in the carbon removal step (S30) is greater than the flow rate of hydrocarbons supplied to the reforming unit 5 in the normal operation step (S50). With this method, the deposited carbon can be removed by supplying air to the reforming unit 5 at a specific flow rate.

[0081] Preferably, the flow rate of air supplied to the reforming unit 5 in the carbon removal step (S30) is four times or more the flow rate of hydrocarbons supplied to the reforming unit 5 in the normal operation step (S50). This method makes it possible to remove precipitated carbon more reliably.

[0082] Preferably, the fuel gas contains methane. This method allows the methane steam reforming reaction to 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.

[0083] In a preferred embodiment, the control method includes a step (S20) of determining whether or not to perform the carbon removal step based on the duration of the normal operation step. This prevents the carbon removal step from being performed unnecessarily, thereby improving energy efficiency.

[0084] 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.

[0085] In a preferred embodiment, the control method includes a startup step for starting the fuel cell stack 4 before the normal operation step (S50). The carbon removal step (S30) is performed in the startup step. This method allows the fuel cell stack to be started and carbon to be removed simultaneously, thus enabling efficient operation of the fuel cell system 1.

[0086] In a preferred embodiment, the fuel cell system 1 further includes an air supply line 10 for supplying air to the cathode of the fuel cell stack 4, and a combustor 8 thermally connected to the air supply line 10 for heating the air. The startup step includes a combustor startup step for starting the combustor to a predetermined temperature or higher. A carbon removal step (S30) is performed after the combustor startup step. The carbon removal step (S30) includes sending the removed carbon to the combustor 8. With this method, the carbon removed in the carbon removal step can be burned by the combustor 8. This makes it possible to detoxify the carbon and then discharge it to the outside.

[0087] In a preferred embodiment, the fuel cell stack 4 comprises a first stack 4-1 and a second stack 4-2. The reforming unit 5 is located at the anode of the first stack 4-1. An 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 such 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 startup step includes a warm-up step, which, after the combustor startup step, heats the fuel cell stack 4 by supplying air to the cathode of the second stack 4-2 via the air supply line 10. A 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 unit 5 of the first stack 4-1, which is at a relatively low temperature. Since the reforming catalyst is not treated with oxygen at high temperatures, oxidative degradation of the reforming catalyst can be suppressed.

[0088] In a preferred embodiment, the carbon removal step (S30) includes stopping the supply of air to the reforming section 5 when the reforming section temperature reaches a first temperature. This method avoids supplying air to the reforming section 5 when it is at a high temperature. This suppresses oxidative degradation of the reforming catalyst.

[0089] In a preferred embodiment, the carbon removal step (S30) includes supplying air from the air supply unit 9 to the reforming unit 5 without heating. This method avoids treating the reforming catalyst with high-temperature oxygen, thereby suppressing oxidative degradation of the reforming catalyst.

[0090] In a preferred embodiment, the carbon removal step (S30) includes a step of intermittently supplying air to the reforming section 5. This method reduces the contact time between the reforming catalyst and oxygen. As a result, oxidative degradation of the reforming catalyst can be suppressed.

[0091] In a preferred embodiment, the carbon removal step (S30) includes supplying air to the reforming section 5 at a progressively increasing flow rate. This method ensures that the entire surface of the reforming catalyst is treated with air, thereby enabling more efficient carbon removal.

[0092] In one preferred embodiment, the reforming unit 5 is located within the fuel cell stack 4. This method allows for a reduction in the volume of the fuel cell system because the reforming catalyst is located inside the fuel cell stack 4. This is expected to improve power generation efficiency and reduce costs due to a reduction in the number of components.

[0093] In one embodiment, the fuel cell system 1 comprises a reforming unit 5 that reforms a fuel gas containing hydrocarbons to produce reformed gas, the reforming unit 5 having a reforming catalyst, 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 perform a normal operation step and a carbon removal step. In the normal operation step, the control device 3 supplies fuel gas to the reforming unit 5 while causing the fuel cell stack to generate electricity. In the carbon removal step, the control device supplies air to the reforming unit 5. The flow rate of air supplied to the reforming unit 5 in the carbon removal step is greater than the flow rate of hydrocarbons supplied to the reforming unit 5 in the normal operation step. According to this fuel cell system 1, precipitated carbon can be removed by supplying air to the reforming unit 5 at a specific large flow rate. [Examples]

[0094] Examples of the present invention conducted by the inventors are described below to illustrate the present invention in more detail. However, the present invention should not be interpreted as being limited to the following examples. (Experimental Example 1) In the third embodiment (see Figure 10), the fuel cell system 1 described above was subjected to 350 hours of normal operation. The S / C ratio was set to 2. A mixed gas containing methane, water vapor, and N2 was used as the fuel gas. GHSV was 65,000 h -1 The flow rate of methane (i.e., hydrocarbon flow rate) supplied to the reforming section 5 was 0.35 NL / min. The operating temperature of the reforming section was 650°C. A mixture containing Pt and Ni was used as the reforming catalyst.

[0095] After 350 hours of normal operation, a carbon removal step was performed. Specifically, air was supplied to the reforming unit 5 at a flow rate four times that of methane during normal operation (1.4 NL / min) for 5 minutes. After the carbon removal step, normal operation was performed again. The hydrogen concentration at the outlet of the reforming unit 5 was measured before and after the carbon removal step.

[0096] The results are shown in Figure 11. Figure 11 shows a graph of the relationship between operating time and hydrogen generation. In Figure 11, region A represents the timing when the carbon removal step was performed. As shown in Figure 11, hydrogen generation decreased with normal operation. In contrast, it was confirmed that hydrogen generation increased and reforming performance was restored by performing the carbon removal step. (Experimental Example 2) The experiment was conducted in the same manner as in Experimental Example 1, by changing the amount of air supplied to the reforming section 5 during the carbon removal step. Specifically, air was supplied to the reforming section 5 at flow rates 1, 3, 4, and 10 times the methane flow rate during normal operation, and the carbon removal step was performed. After the carbon removal step, the hydrogen concentration at the outlet of the reforming section 5 was measured.

[0097] Figure 12 shows the results along with the initial performance. As shown in Figure 12, when air was supplied at four times the flow rate, the outlet H2 concentration increased significantly compared to when the flow rate was lower. (Experimental Example 3) In Experimental Example 1, the air supply time in the carbon removal step was changed to 1 minute. All other conditions were the same as in Experimental Example 1. When the modification performance was checked before and after the carbon removal step, it was confirmed that the modification performance had recovered. (Experimental Example 4) In Experimental Example 1, the reforming catalyst was changed to one containing Rh. All other conditions were the same as in Experimental Example 1. When the reforming performance was checked before and after the carbon removal step, it was confirmed that the reforming performance had recovered.

Claims

1. A method for controlling a fuel cell system, The aforementioned fuel cell system A reforming unit that produces a reformed gas by reforming a fuel gas containing hydrocarbons, comprising a reforming unit having a reforming catalyst, The fuel cell stack is configured to generate electricity using the reformed gas as the anode gas, The control method described above is A normal operation step involves supplying the fuel gas to the reforming unit while generating electricity with the fuel cell stack, The system comprises a carbon removal step for removing carbon deposited on the reforming catalyst, The carbon removal step includes supplying air to the modification section, In the carbon removal step, the flow rate of air supplied to the reforming unit is greater than the flow rate of hydrocarbons supplied to the reforming unit in the normal operation step. A method for controlling a fuel cell system.

2. A control method according to claim 1, In the carbon removal step, the flow rate of air supplied to the reforming unit is four times or more the flow rate of hydrocarbons supplied to the reforming unit in the normal operation step. Control method.

3. A control method according to claim 1 or 2, The aforementioned fuel gas contains methane Control method.

4. A control method according to claim 1 or 2, Furthermore, The system includes a step of determining whether or not to perform the carbon removal step based on the time taken to perform the normal operation step. Control method.

5. A control method according to claim 1 or 2, Furthermore, The system includes a step of determining whether or not to perform the carbon removal step based on the outlet temperature of the modification section. Control method.

6. A control method according to claim 1 or 2, Furthermore, Prior to the normal operation step, the system includes a startup step for starting the fuel cell stack. The carbon removal step is performed in the startup step. Control method.

7. A control method according to claim 6, The aforementioned fuel cell system further, An air supply line that supplies the air to the cathode of the fuel cell stack, The system includes a combustor that is thermally connected to the air supply line and heats the air, The startup step includes a combustor startup step that starts the combustor to a predetermined temperature or higher, The carbon removal step is performed after the combustor startup step, The carbon removal step includes sending the removed carbon to the combustor. Control method.

8. A control method according to claim 7, The aforementioned fuel cell stack is The first stack and, It has a second stack, The modification unit is located at the anode in 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 gas flows from the anode of the first stack to the anode of the second stack, and gas flows from the cathode of the second stack to the cathode of the first stack. The startup step includes a warm-up step, after the combustor startup step, in which the fuel cell stack is heated by supplying the air to the cathode of the second stack via the air supply line. The carbon removal step is performed during the warm-up step. Control method.

9. A control method according to claim 1 or 2, The carbon removal step includes stopping the supply of air to the modifying section when the temperature of the modifying section reaches a first temperature. Control method.

10. A control method according to claim 1 or 2, The carbon removal step includes supplying the air from the air supply unit to the modification unit without heating it. Control method.

11. A control method according to claim 1 or 2, The carbon removal step includes the step of intermittently supplying the air to the modification section. Control method.

12. The control method according to claim 1 or 2, The carbon removal step includes supplying the air to the modification section in such a stepwise manner that the flow rate increases. Control method.

13. A control method according to claim 1 or 2, The reforming unit is located within the fuel cell stack. Control method.

14. A reforming unit that produces a reformed gas by reforming a fuel gas containing hydrocarbons, comprising a reforming unit having a reforming catalyst, A fuel cell stack configured to generate electricity using the reformed gas as the anode gas, Control device and 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 supplies the fuel gas to the reforming unit while causing the fuel cell stack to generate electricity. In the carbon removal step, the control device supplies air to the modification section. In the carbon removal step, the flow rate of air supplied to the reforming unit is greater than the flow rate of hydrocarbons supplied to the reforming unit in the normal operation step. Fuel cell system.

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