Fuel cell system and control method thereof

Abstract

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

Description

Technical Field

[0001] This invention relates to fuel cell systems and their control methods. Background Technology

[0002] Fuel cells that generate electricity using hydrogen as fuel are known. In some cases, modified catalysts are used to generate hydrogen. In these fuel cells, the fuel gas, including hydrocarbons, is modified using a modified catalyst to produce a modified gas containing hydrogen. This modified gas is then used to generate electricity.

[0003] For example, Patent Document 1 (JP2005-340075A) discloses a technology related to fuel cells utilizing modified catalysts. Patent Document 1 describes a method for stopping the operation of a fuel cell equipped with a specific modifier, which involves continuously supplying air into the fuel cell while simultaneously supplying a specific mixed gas through the modifier. When the temperature of the modified catalyst drops to a specific temperature range, the supply of the mixed gas is stopped. After the supply of the mixed gas is stopped, air or hydrocarbon gas is allowed to flow through the modifier as a purified gas within the fuel cell. Summary of the Invention

[0004] However, if the modified catalyst is operated continuously, carbon may sometimes accumulate and cover it. If the modified catalyst is covered by carbon, the reaction area decreases, and the modification performance deteriorates. Therefore, it is preferable to remove the accumulated carbon.

[0005] Therefore, the object of the present invention is to provide a technique for removing carbon deposited on modified catalysts.

[0006] In one embodiment, the present invention relates to a control method for a fuel cell system. The fuel cell system includes: a modification section that modifies a fuel gas containing hydrocarbons to generate a modified gas and has a modification catalyst; and a fuel cell stack configured to generate electricity by using the modified gas as the positive electrode gas. The control method includes the steps of: a normal operation step of supplying fuel gas to the modification section and generating electricity from the fuel cell stack; and a carbon removal step of removing carbon deposited on the modification catalyst. The carbon removal step includes a step of supplying air to the modification section. The flow rate of the air supplied to the modification section in the carbon removal step is greater than the flow rate of hydrocarbons supplied to the modification section in the normal operation step.

[0007] In another embodiment, the present invention relates to a fuel cell system. The fuel cell system includes: a modification section that modifies a fuel gas containing hydrocarbons to generate a modified gas, and has a modification catalyst; a fuel cell stack configured to generate electricity by using the modified gas as the positive electrode 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 modification section and causes the fuel cell stack to generate electricity. In the carbon removal step, the control device supplies air to the modification section. The flow rate of the air supplied to the modification section in the carbon removal step is greater than the flow rate of hydrocarbons supplied to the modification section in the normal operation step. Attached Figure Description

[0008] Figure 1 This is a schematic block diagram illustrating the fuel cell system according to the first embodiment.

[0009] Figure 2 This is a flowchart illustrating the control method of the fuel cell system according to the first embodiment.

[0010] Figure 3 This is a flowchart showing the steps for starting the burner.

[0011] Figure 4 This is a flowchart representing steps S20 and S30.

[0012] Figure 5 This is a flowchart showing the POX operation steps (step S40).

[0013] Figure 6 This is a graph representing a variation of the first embodiment.

[0014] Figure 7 This is a graph representing other variations of the first embodiment.

[0015] Figure 8 This is a graph representing another variation of the first embodiment.

[0016] Figure 9 This is a schematic block diagram illustrating the fuel cell system according to the second embodiment.

[0017] Figure 10 This is a schematic block diagram illustrating the fuel cell according to the third embodiment.

[0018] Figure 11 This is a graph showing the running time and hydrogen production of Experiment Example 1.

[0019] Figure 12 It is a graph showing the relationship between airflow and outlet H2 concentration. Detailed Implementation

[0020] The embodiments of the present invention will now be described with reference to the accompanying drawings.

[0021] (1) First implementation method

[0022] Figure 1 This is a schematic block diagram illustrating the fuel cell system 1 according to the first embodiment. (Example) Figure 1 As shown, the fuel cell system 1 includes a control unit 3, a fuel cell stack 4, a modification unit 5, a fuel tank 6, a heat exchanger 7, a burner 8, and an air supply unit 9. Furthermore, the fuel cell system 1 is configured to operate stably with a constant output (rated output) during normal operation.

[0023] The fuel cell stack 4 is the part that generates electricity. It is not shown, but the fuel cell stack 4 has a positive electrode, an electrolyte, and a negative electrode. The fuel cell stack 4 generates electricity by receiving positive and negative electrode gases. The positive electrode gas is modified gas supplied from the modification section 5. The negative electrode gas is air supplied from the air supply section 9.

[0024] Furthermore, the fuel cell stack 4 is connected to the burner 8 via line 19 downstream of its positive electrode. Additionally, the fuel cell stack 4 is connected to the burner 8 via line 17 downstream of its negative electrode. Thus, both the gas discharged from the positive electrode and the gas discharged from the negative electrode are supplied to the burner 8.

[0025] The modification section 5 is configured to modify the fuel gas supplied from the fuel tank 6 to generate a modified gas containing hydrogen. The modification section 5 generates the modified gas by modifying water vapor. The water vapor can be supplied from the fuel tank 6 together with the fuel gas, or it can be supplied to the modification section 5 via a different path (not shown).

[0026] Modification section 5 includes a modified catalyst. Examples of modified catalysts include Pt / CeO2, Ni / CeO2, and Rh / CeO2.

[0027] The modification section 5 is connected to the positive electrode of the fuel cell stack 4 via line 18. The modified gas generated in the modification section 5 is supplied to the positive electrode of the fuel cell stack 4 via line 18 in the manner described above.

[0028] Fuel tank 6 is the part that stores fuel gas. Fuel tank 6 is connected to modification unit 5 via line 13. Valve 15 is provided on line 13. In addition, fuel tank 6 is also connected to burner 8 via line 14. Valve 16 is provided on line 14.

[0029] The fuel gas contains hydrocarbons. Methane is preferably an example of a hydrocarbon. When using methane, a methane steam modification reaction can be carried out in the modification section 5. Methane is a non-alcohol fuel, so the modification catalyst in the modification section 5 is less prone to degradation due to acid sites.

[0030] Air supply unit 9 is provided to supply air to the negative electrode of fuel cell stack 4 as negative electrode gas. Air supply unit 9 is implemented, for example, by a blower. Air supply unit 9 is connected to the negative electrode of fuel cell stack 4 via air supply line 10.

[0031] The air supply unit 9 can also supply air to the modification unit 5. Specifically, the air supply line 10 is connected to the modification unit 5 via the purification line 11. A valve 12 is provided in the purification line 11. By operating the valve 12, air can be supplied from the air supply unit 9 to the modification unit 5.

[0032] A heat exchanger 7 is installed in the air supply line 10. The heat exchanger 7 is thermally connected to the burner 8. Thus, the air flowing in the air supply line 10 is heated by heat from the burner 8.

[0033] Furthermore, the heat exchanger 7 is positioned further downstream than the connection between the purification line 11 and the air supply line 10. Therefore, the air supplied from the air supply unit 9 to the modification unit 5 via the purification line 11 is not heated by the heat exchanger 7.

[0034] As described above, the burner 8 is provided for heating air via the heat exchanger 7. Fuel gas from the fuel tank 6, exhaust gas from the negative electrode of the fuel cell stack 4, and exhaust gas from the positive electrode of the fuel cell stack 4 are supplied to the burner 8. These gases are combusted in the burner 8. The combustion gas generated in the burner 8 is discharged to the outside as exhaust gas.

[0035] In addition, the burner 8 is connected to the modification section 5 so that the combustion gas is discharged as exhaust gas after heat exchange with the modification section 5.

[0036] The control device 3 is provided for controlling the operation of the entire fuel cell system 1. The control device 3 is implemented, for example, by a computer having a processing unit such as a CPU and a storage device such as RAM and ROM. That is, the control device 3 performs its functions by executing the control program stored in the storage device through the processing unit.

[0037] The above is a general structural description of fuel cell system 1. The operation method of fuel cell system 1 will now be explained.

[0038] First, the operation of the fuel cell system 1 during normal operation will be explained. During normal operation, the fuel cell system 1 operates at a predetermined constant output (rated output).

[0039] Specifically, control device 3 closes valve 12 and opens valves 15 and 16. This supplies fuel gas from fuel tank 6 to modification section 5 and burner 8. Furthermore, to achieve a constant output, fuel gas is supplied to modification section 5 at a constant flow rate. Modified gas is generated in modification section 5 by water vapor modification. The generated modified gas is then transported to the positive electrode of fuel cell stack 4 via line 18. After being used for power generation, the modified gas supplied to the positive electrode is discharged as positive electrode exhaust gas and transported to burner 8 via line 19.

[0040] Air is supplied from the air supply unit 9 to the negative electrode of the fuel cell stack 4 via the air supply line 10 and the heat exchanger 7. To achieve a constant output, air is supplied to the negative electrode at a constant flow rate. After being used for power generation, the air supplied to the negative electrode is discharged as negative electrode exhaust gas and transported to the burner 8 via line 17.

[0041] The gases supplied from lines 14, 17, and 19 are burned in burner 8. This heats the air via heat exchanger 7. In other words, heated air is supplied to fuel cell stack 4. Fuel cells typically function at high temperatures. The temperature required for power generation in fuel cell stack 4 is maintained by supplying heated air. Furthermore, the combustion gases generated in burner 8 are discharged as exhaust gases. Additionally, a portion of the combustion gases is discharged after heat exchange with modification section 5. Therefore, the temperature of modification section 5 is also maintained at the temperature required for the modification reaction.

[0042] The above describes the normal operating procedures.

[0043] Here, as a result of the operation, carbon sometimes precipitates on the modified catalyst at modification section 5. This can be attributed to the heterogeneous reaction of CO as a byproduct of the modification reaction (2CO→C+CO2). If carbon covers the surface of the modified catalyst, the efficiency of the modification reaction decreases.

[0044] Therefore, in this embodiment, a carbon removal step is performed to remove the precipitated carbon. In the carbon removal step, air is supplied to the modification section 5. Specifically, the control device 3 is used to control valve 12 (see reference 12). Figure 1 The valve is set to the open state, and air is supplied to the modification section 5 via the purification line 11. Meanwhile, the valve 15 is set to the closed state, meaning that fuel gas is not supplied to the modification section 5.

[0045] In the carbon removal step, the air supplied to the modification section 5 is at a high flow rate. Specifically, air is supplied to the modification section 5 at a flow rate greater than or equal to the flow rate of hydrocarbons supplied to the modification section 5 during normal operation. By supplying air to the modification section 5 at this amount, carbon deposited on the modified catalyst can be removed, and the function of the modified catalyst can be restored. Furthermore, the phrase "flow rate of hydrocarbons supplied to the modification section during normal operation" refers to the flow rate of hydrocarbons, not the flow rate of all fuel gas. For example, if the hydrocarbons contained in the fuel gas are methane, the flow rate of methane is "the flow rate of hydrocarbons supplied to the modification section 5".

[0046] Preferably, the air flow rate supplied to the modification section 5 in the carbon removal step is more than four times the flow rate of hydrocarbons supplied to the modification section 5 during normal operation. Supplying air at this flow rate allows for more reliable carbon removal.

[0047] Alternatively, if based on molar number, it is preferable that, in the carbon removal step, the molar flow rate (O2) of the oxygen-based air supplied to the modification section 5 is 0.84 or more relative to the molar flow rate (C) of the carbon-based fuel gas supplied to the modification section 5 during normal operation. Supplying air at this flow rate allows for more reliable carbon removal.

[0048] The above is a general overview of the structure and operation of this embodiment. As described above, according to this embodiment, by supplying air to the modification section 5 at a high flow rate, the precipitated carbon can be removed. Therefore, the modification function can be restored.

[0049] Furthermore, 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, carbon removal and the start-up of the fuel cell system 1 can be performed simultaneously, resulting in higher efficiency. The control method for the fuel cell system 1 according to this embodiment will be specifically described below using the case where the carbon removal step is performed when the fuel cell system 1 is started as an example. Unless otherwise stated, the control method described below is performed by the control device 3.

[0050] Figure 2This is a flowchart illustrating the control method of the fuel cell system 1 according to this embodiment. The control method of this fuel cell system generally includes a start-up step (S10-S40) and a normal operation step (S50). The start-up step (S10-S40) includes a step of starting the burner 8 (S10), a step of determining whether carbon removal is needed (S20), a carbon removal step (S30), and a step of implementing POX operation (S40). Furthermore, after the start-up step (S10) of the burner 8, heated air is supplied to the negative electrode of the fuel cell stack 4 via the air supply line 10. Therefore, the fuel cell stack 4 is heated. That is, the fuel cell stack 4 is warmed up. Therefore, it can be said that in the warm-up step of the fuel cell stack 4, steps S20-S40, which are performed after the burner 8 is started (after step S10), are implemented.

[0051] The following is a detailed description of each step.

[0052] (Step S10: Start-up of the burner)

[0053] First, start burner 8. Figure 3 This is a flowchart showing the steps for starting burner 8.

[0054] First, the air supply unit 9 is activated (step S11). Air then flows from the air supply unit 9 through the air supply line 10, in the order of heat exchanger 7, the negative electrode of the fuel cell stack 4, and burner 8. Furthermore, valve 12 is set to the closed state. That is, air is not supplied to the modification unit 5.

[0055] Additionally, the heater (not shown) of burner 8 is activated (step S12).

[0056] Next, it is determined whether the temperature of the burner 8 is greater than or equal to a predetermined temperature (step S13). Specifically, the control device 3 measures the temperature of the burner 8 via a temperature sensor (not shown) or the like. Furthermore, the control device 3 compares the measured result with a pre-stored predetermined temperature. The predetermined temperature referred to here is a temperature set from the viewpoint that a combustion reaction can occur in the burner 8. For example, if the burner 8 contains a combustion catalyst, the quenching temperature of the combustion catalyst (e.g., 350°C) is set as the predetermined temperature.

[0057] If the temperature of burner 8 is greater than or equal to a specified temperature, fuel gas is introduced into burner 8 (step S14). That is, valve 16 is opened, and fuel gas is supplied from fuel tank 6 to burner 8 via line 14. Thus, a combustion reaction takes place in burner 8, and burner 8 starts.

[0058] As described above, after the burner 8 is started, air heated by the heat exchanger 7 is supplied to the negative electrode of the fuel cell stack 4. That is, as described above, the fuel cell stack 4 is warmed up. In addition, the modification section 5 is also heated by the heat of the combustion gas supplied from the burner 8.

[0059] (Step S20: Determining whether carbon removal is necessary)

[0060] Figure 4 This is a flowchart illustrating step S20 and the subsequent carbon removal step S30. After the burner 8 is started, it is first determined whether a carbon removal step is required (step S20).

[0061] For example, the need for a carbon removal step can be determined based on the cumulative operating time. For instance, control device 3 is configured to store the cumulative operating time since the last carbon removal step as data. Control device 3 compares this cumulative operating time with a preset time. Furthermore, if the cumulative operating time exceeds the preset time, it is determined that a carbon removal step is required.

[0062] Alternatively, the need for a carbon removal step can be determined based on the modification performance during the previous operation. For example, the modification performance can also be determined based on the outlet temperature of the modification section 5. The modification reaction in the modification section 5 is an endothermic reaction. Therefore, if the modification performance decreases, the outlet temperature of the modification section 5 increases. That is, it can be said that the outlet temperature of the modification section 5 reflects the modification performance of the modification section 5. Therefore, the need for a carbon removal step can also be determined based on the outlet temperature of the modification section 5 during the previous operation. For example, the control device can compare the outlet temperature of the modification section 5 during the previous operation with the outlet temperature of the modification section 5 in a preset initial state, and determine whether a carbon removal step is needed if the difference is greater than or equal to a specified value (e.g., 10% of the value in the initial state).

[0063] As a result of the determination, if no carbon removal step is required, it is determined at the next startup whether a carbon removal step is needed, and the processing after step S40 is implemented. Therefore, unnecessary carbon removal steps are avoided, improving energy efficiency.

[0064] On the other hand, if it is determined that a carbon removal step is required, the next carbon removal step S30 is performed.

[0065] (Step S30: Carbon removal step)

[0066] In the carbon removal step, as described above, valve 12 (refer to...) Figure 1 The air supply unit 9 is turned on (step S31). Air is then supplied to the modification unit 5 from the air supply unit 9 via the purification line 11. As described above, air is supplied to the modification unit 5 at a predetermined high flow rate at this time.

[0067] The carbon released from the modified section 5 is removed by supplying air. The removed carbon is then transported to the burner 8 via the positive electrode of the fuel cell stack 4 and line 19. It is then burned in the burner 8 and discharged to the outside after being rendered harmless.

[0068] Furthermore, air is supplied to the modification section 5 from the air supply section 9 without heating. If air is supplied to the modification section 5 at high temperatures, the modification catalyst may sometimes oxidize and deteriorate. However, according to this embodiment, air is supplied without heating, thus suppressing the oxidative deterioration of the modification catalyst. In addition, the oxidative deterioration of the electrode material at the positive electrode of the fuel cell stack 4 located downstream of the modification section 5 is also suppressed.

[0069] During the air supply, the modification section 5 is also heated by heat from the burner 8, thus its temperature continuously increases. Therefore, the carbon removal step is performed until the temperature of the modification section 5 reaches a predetermined temperature. Specifically, the control device 3 determines whether the temperature of the modification section 5 has reached a preset first temperature during the air supply (step S32). For example, the control device 3 uses a sensor (not shown) to measure the temperature of the modification section 5. The temperature of the modification section 5 can be indirectly determined by measuring the outlet temperature of the modification section 5.

[0070] If, in step S32, it is determined that the temperature of the modification section 5 has reached the first temperature, the control device 3 closes the valve 12. This stops the air supply to the modification section 5 (step S33). That is, the carbon removal step ends.

[0071] The first temperature is set based on the viewpoint of the deterioration of the substances contained in the modified section 5. If air is supplied to the modified section 5 at a high temperature, the modified catalyst may deteriorate due to sintering and oxidation. Furthermore, the positive electrode of the downstream fuel cell stack 4 is also prone to oxidation and deterioration. In contrast, stopping the air supply at the first temperature stage prevents the oxidation and deterioration of the modified catalyst and the positive electrode. The first temperature is, for example, 300°C.

[0072] In addition, the carbon removal step is performed for at least 15 seconds, preferably at least 30 seconds, and more preferably at least 1 minute.

[0073] (Step S40: POX operation)

[0074] After the carbon removal step (S30) is completed, the POX operation step (S40) is carried out. Figure 5 This is a flowchart illustrating the POX operation steps (step S40). POX operation is implemented to facilitate the warm-up of the fuel cell stack 4.

[0075] Specifically, after the carbon removal step is completed, the modification section 5 is also heated by combustion gas from the burner 8. Therefore, in step S41, the control device 3 determines whether the temperature of the modification section 5 is greater than or equal to a preset second temperature. The second temperature is determined based on whether a POX reaction (partial oxidation reaction: CH4 + 1 / 2O2 → CO + 2H2) is carried out, and is a temperature higher than the first temperature, for example, 400°C.

[0076] If it is determined in step S41 that the temperature of the modified part 5 has reached the second temperature, then valve 12 opens, and air flows through purification line 11 (see reference). Figure 1 And supplied to the modification section 5 (step S42). Additionally, valve 15 (see...) Figure 1 The fuel tank 6 is opened, and fuel gas is supplied to the modification section 5 (step S43). Thus, a POX reaction takes place in the modification section 5. Furthermore, the amount of air supplied to the modification section 5 in step S42 only needs to be sufficient to induce a POX reaction, and its amount is sufficiently small compared to the amount in the carbon removal step (S30).

[0077] Then, it is determined whether stable power generation can be achieved (e.g., whether the temperature of the modified section 5 has reached the desired temperature) (step S44). If it is determined that stable power generation can be achieved, normal operation is implemented. Furthermore, during normal operation, valve 12 is closed as described above, and air is not supplied to the modified section 5.

[0078] 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 modification section 5 at a large flow rate, so that the carbon precipitated from the modified catalyst can be removed and the modification performance can be restored.

[0079] Furthermore, by performing a carbon removal step (S30) during the startup process, the startup of the fuel cell system 1 and carbon removal can be carried out simultaneously. This allows for efficient carbon removal.

[0080] Furthermore, in this embodiment, a carbon removal step (S30) is performed after the burner 8 is started. Therefore, the carbon removed from the modification section 5 can be burned in the burner 8. As a result, the removed carbon can be discharged to the outside while achieving harmlessness.

[0081] Furthermore, the amount of air supplied to the modification section 5 in the carbon removal step (S30) can be constant or variable. The following describes a modified example to illustrate the air supply amount.

[0082] Figure 6This is a graph illustrating a variation of this embodiment, showing the relationship between the flow rate and time of the air supplied to the modification section 5 in the carbon removal step (S30). In this example, air is supplied at a constant flow rate after the start of the carbon removal step (S30). It is possible to supply air to the modification section 5 at a constant flow rate, as in this variation.

[0083] on the other hand, Figure 7 The graph shown is a variation of the graph, illustrating the relationship between the flow rate and time of the air supplied to the modification section 5 in the carbon removal step (S30). In this example, the air is supplied intermittently. Furthermore, the flow rate during air supply (refer to "A" in the attached figure) is greater than the flow rate of hydrocarbons during normal operation.

[0084] according to Figure 7 In the modified example shown, by intermittently supplying air, the contact time between the modified catalyst and air can be reduced. Therefore, oxidative degradation of the modified catalyst can be prevented. Furthermore, for the same reason, oxidative degradation of the positive electrode can also be suppressed in the fuel cell stack 4 located downstream of the modification section 5.

[0085] Figure 8 This is a graph illustrating yet another variation, showing the relationship between the flow rate and time of the air supplied to the modification section 5 in the carbon removal step (S30). In this example, the air supply increases in a stepwise manner. Moreover, the final air flow rate (refer to "A" in the attached figure) is greater than the hydrocarbon flow rate during normal operation.

[0086] according to Figure 8 In the modified example shown, the air flow rate is increased in a stepwise manner, thus facilitating the treatment of the entire surface of the modified catalyst with air. This allows for more efficient carbon removal.

[0087] The application of the fuel cell system 1 in this embodiment is not particularly limited, but it can be used, for example, in a vehicle. Regarding the fuel cell system 1 for vehicle use, miniaturization is required. As a result, it is used under stringent conditions (e.g., high SV, low S / C). Therefore, carbon is easily deposited on the modified catalyst. This embodiment can solve the problem related to this carbon deposition, and is therefore particularly useful in fuel cell systems for vehicle use.

[0088] In this embodiment, the S / C ratio (molar ratio of water vapor to carbon) during normal operation is, for example, less than or equal to 5, preferably 1 to 5, and more preferably 1 to 3. Regarding fuel cells used with such a low S / C ratio, as mentioned above, carbon is easily deposited on the modified catalyst. This embodiment solves the problems related to carbon deposition and is therefore particularly valuable for fuel cells operating with this S / C ratio.

[0089] In this embodiment, during normal operation, the space velocity (GHSV) of the gas supplied to the modification unit 5 is, for example, greater than or equal to 30,000 h⁻¹. -1 Preferably 40,000~100,000h -1 Regarding fuel cells used under such GHSV conditions, which are essentially fuel cells operating under harsh conditions, as mentioned above, carbon is prone to precipitation on the modified catalyst. This embodiment solves the problems associated with carbon precipitation and is therefore particularly valuable for fuel cell systems used under such GHSV conditions.

[0090] (2) Second implementation method

[0091] Next, the second embodiment will be described. Figure 9 This is a schematic block diagram illustrating the fuel cell system 1 according to the second embodiment. Furthermore, detailed explanations regarding the ability to employ the same structure as the first embodiment are omitted.

[0092] like Figure 9 As shown, in this embodiment, the modification section 5 is disposed at the positive electrode of the fuel cell stack 4. That is, the positive electrode of the fuel cell stack 4 contains a modified catalyst. Therefore, the purification line 11 is connected to the positive electrode of the fuel cell stack 4. The line 13 for supplying fuel gas is also connected to the positive electrode of the fuel cell stack 4.

[0093] 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 burner inlet temperature reflects the modification performance of the modification section 5. Therefore, in step S20 (refer to...) Figure 4 In step S32, it is possible to determine whether a carbon removal step is needed based on the burner inlet temperature during the previous operation. Furthermore, the positive electrode outlet temperature reflects the temperature of the modification section 5. Therefore, in step S32 (refer to...), Figure 4 ) and step S41 (refer to Figure 5 In this process, the temperature of the modified part 5 can be determined based on the outlet temperature of the positive electrode to determine whether the temperature has reached the specified temperature.

[0094] In this embodiment, similar to the first embodiment, the carbon removal step removes the deposited carbon, restoring the modified properties. Furthermore, according to this embodiment, the modified catalyst is disposed inside the fuel cell stack 4, thus reducing the volume of the fuel cell system. This leads to an increase in power generation efficiency. Additionally, a cost reduction due to the reduced number of components is also expected.

[0095] (3) Third implementation method

[0096] Next, the third embodiment will be described. Figure 10This is a schematic block diagram illustrating the fuel cell system according to the third embodiment. Furthermore, detailed explanations regarding the ability to employ the same structure as in the embodiments described above are omitted.

[0097] 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 a positive electrode, an electrolyte layer (not shown), and a negative electrode.

[0098] Modification section 5 is disposed at the positive electrode of the first pile 4-1. Therefore, purification line 11 is connected to the positive electrode of the first pile 4-1. Line 13 for supplying fuel gas is also connected to the positive electrode of the first pile 4-1. The positive electrode of the first pile 4-1 is connected to the positive electrode of the second pile 4-2 on its downstream side. That is, the first pile 4-1 and the second pile 4-2 are connected so that gas flows from the positive electrode of the first pile 4-1 toward the positive electrode of the second pile 4-2. Furthermore, the positive electrode of the second pile 4-2 is connected to the burner 8 on its downstream side via line 19.

[0099] On the other hand, air supply line 10 is connected to the negative terminal of the second stack 4-2. The negative terminal of the second stack 4-2 is connected downstream to the negative terminal of the first stack 4-1. That is, the first stack 4-1 and the second stack 4-2 are connected so that gas flows from the negative terminal of the second stack 4-2 toward the negative terminal of the first stack 4-1. Furthermore, the negative terminal of the first stack 4-1 is connected downstream to the burner 8 via line 17.

[0100] The control method for the fuel cell system in this embodiment is the same as that in the embodiments described above. That is, as follows: Figure 2 As shown, first, the burner 8 is started (step S10). Then, during the warm-up of the fuel cell stack 4, a carbon removal step is performed (S30).

[0101] In this embodiment, during the warm-up of the fuel cell stack 4 (steps S20-S40), the heated air flows in the order from the negative electrode of the second stack 4-2 to the negative electrode of the first stack 4-1. Therefore, the second stack 4-2 is heated before the first stack 4-1. In other words, during the warm-up, the temperature of the first stack 4-1 is unlikely to be higher than that of the second stack 4-2.

[0102] On the other hand, in the carbon removal step (S30), air is supplied from the purification line 11 to the modification section 5 located in the first pile 4-1. That is, air is supplied to the modification section 5 of the first pile 4-1, which has a lower temperature. Since air is not supplied to the modified catalyst at a high temperature, the oxidative degradation of the modified catalyst can be suppressed more reliably.

[0103] The embodiments of the present invention have been described above, but the above embodiments only illustrate a part of the application examples of the present invention, and their purpose is not to limit the technical scope of the present invention to the specific structures of the above embodiments.

[0104] The following is a summary of representative relationships regarding the structure and effects of embodiments of the present invention.

[0105] In one embodiment, this method relates to a control method for a fuel cell system 1. The fuel cell system 1 includes: a modification section 5 that modifies fuel gas, including hydrocarbons, to generate modified gas and has a modification catalyst; and a fuel cell stack 4 configured to generate electricity using the modified gas as a cathode gas. The control method includes the following steps: a normal operation step (S50) in which fuel gas is supplied to the modification section 5 and the fuel cell stack 4 generates electricity; and a carbon removal step (S30) in which carbon deposited on the modified catalyst is removed. The carbon removal step (S30) includes a step of supplying air to the modification section 5. The flow rate of the air supplied to the modification section 5 in the carbon removal step (S30) is greater than the flow rate of hydrocarbons supplied to the modification section 5 in the normal operation step (S50). According to this method, by supplying air to the modification section 5 at a specific flow rate, the deposited carbon can be removed.

[0106] Preferably, the flow rate of air supplied to the modification section 5 in the carbon removal step (S30) is more than four times the flow rate of hydrocarbons supplied to the modification section 5 in the normal operation step (S50). According to this method, the precipitated carbon can be removed more reliably.

[0107] Preferably, the fuel gas contains methane. According to this method, a methane steam modification reaction can be carried out in the modification section 5. Since methane is a non-alcohol fuel, the modifying catalyst in the modification section 5 becomes less susceptible to degradation due to acidic sites.

[0108] In a preferred embodiment, the control method includes a step (S20) of deciding whether to implement a carbon removal step based on the implementation time of the normal operating steps. This avoids unnecessary implementation of the carbon removal step, thereby improving energy efficiency.

[0109] In a preferred embodiment, the control method includes a step (S20) of determining whether to perform a carbon removal step based on the outlet temperature of the modification section. This avoids unnecessary carbon removal and improves energy efficiency.

[0110] In a preferred embodiment, the control method includes a startup step that starts the fuel cell stack 4 before the normal operation step (S50). A carbon removal step (S30) is performed during the startup step. According to this method, startup of the fuel cell stack and carbon removal can be performed simultaneously, thus enabling the fuel cell system 1 to operate efficiently.

[0111] In a preferred embodiment, the fuel cell system 1 further comprises: an air supply line 10 that supplies air to the negative electrode of the fuel cell stack 4; and a burner 8 thermally connected to the air supply line 10 for heating the air. The start-up step includes a burner start-up step that starts the burner to a temperature above a predetermined temperature. A carbon removal step (S30) is performed after the burner start-up step. The carbon removal step (S30) includes the step of conveying the removed carbon to the burner 8. According to this method, the carbon removed in the carbon removal step can be burned using the burner 8. Thus, the carbon can be rendered harmless and then discharged to the outside.

[0112] In a preferred embodiment, the fuel cell stack 4 has a first stack 4-1 and a second stack 4-2. A modification section 5 is disposed at the positive electrode of the first stack 4-1. An air supply line 10 is connected to the negative electrode 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 positive electrode of the first stack 4-1 to the positive electrode of the second stack 4-2, and gas flows from the negative electrode of the second stack 4-2 to the negative electrode of the first stack 4-1. The start-up step includes a warm-up step that heats the fuel cell stack 4 by supplying air to the negative electrode of the second stack 4-2 via the air supply line 10 after the burner start-up step. A carbon removal step (S30) is performed in the warm-up step. According to this method, in the carbon removal step, air is supplied to the modification section 5 of the first stack 4-1, which has a lower temperature. This avoids treating the modified catalyst with oxygen at high temperatures, thus suppressing the oxidative degradation of the modified catalyst.

[0113] In a preferred embodiment, the carbon removal step (S30) includes stopping the air supply to the modification section 5 when the temperature of the modification section reaches a first temperature. According to this method, supplying air to the modification section 5 at a high temperature is avoided. This suppresses the oxidative degradation of the modified catalyst.

[0114] In a preferred embodiment, the carbon removal step (S30) includes supplying air from the air supply unit 9 to the modification unit 5 without heating. According to this method, the modification catalyst is protected from treatment with high-temperature oxygen. Therefore, oxidative degradation of the modified catalyst can be suppressed.

[0115] In a preferred embodiment, the carbon removal step (S30) includes intermittently supplying air to the modification section 5. According to this method, the contact time between the modified catalyst and oxygen can be reduced. As a result, oxidative degradation of the modified catalyst can be suppressed.

[0116] In a preferred embodiment, the carbon removal step (S30) includes supplying air to the modification section 5 in a stepwise increasing manner. According to this method, treating the entire surface of the modified catalyst with air enables more efficient carbon removal.

[0117] In a preferred embodiment, the modified catalyst 5 is disposed within the fuel cell stack 4. According to this method, the modified catalyst is disposed inside the fuel cell stack 4, thus reducing the volume of the fuel cell system. This can lead to an increase in power generation efficiency. Furthermore, it can be expected that the cost will be reduced due to the decrease in the number of components.

[0118] In one embodiment, the fuel cell system 1 includes: a modification section 5 that modifies fuel gas, including hydrocarbons, to generate modified gas and has a modification catalyst; a fuel cell stack 4 configured to generate electricity by using the modified gas as the positive electrode 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 modification section 5 and causes the fuel cell stack to generate electricity. In the carbon removal step, the control device supplies air to the modification section 5. The flow rate of the air supplied to the modification section 5 in the carbon removal step is greater than the flow rate of hydrocarbons supplied to the modification section 5 in the normal operation step. According to this fuel cell system 1, by supplying air to the modification section 5 at a specific high flow rate, the released carbon can be removed.

[0119] Example

[0120] The following description, by the inventors of the present invention, illustrates embodiments to provide a more detailed explanation of the invention. However, the invention should not be limited to these embodiments.

[0121] (Experimental Example 1)

[0122] Utilizing in the third embodiment (see Figure 10 The fuel cell system 1 described in [reference to a document] underwent normal operation for 350 hours. The S / C ratio was set to 2. A mixture of methane, water vapor, and N2 was used as the fuel gas. The GHSV was set to 65,000 h. -1 Furthermore, the flow rate of methane (i.e., the flow rate of hydrocarbons) supplied to the modification section 5 is 0.35 NL / min. The operating temperature of the modification section is set to 650 °C. A mixture containing Pt and Ni is used as the modification catalyst.

[0123] After 350 hours of normal operation, a carbon removal step was performed. Specifically, air was supplied to modification section 5 at a flow rate four times the normal methane flow rate (1.4 NL / min) for 5 minutes. Normal operation was then performed again after the carbon removal step. The hydrogen concentration at the outlet of modification section 5 was measured before and after the carbon removal step.

[0124] Figure 11 The results are shown in the figure. Figure 11 The graph shows the operating time and hydrogen production. Figure 11In the diagram, region A represents the timing of the carbon removal step. For example... Figure 11 As shown, hydrogen production decreases with normal operation. In contrast, an increase in hydrogen production and restoration of modified properties were confirmed by implementing a carbon removal step.

[0125] (Experimental Example 2)

[0126] The experiment was conducted in the same manner as in Experimental Example 1, with variations in the air supply to the modification section 5 during the carbon removal step. Specifically, the carbon removal step was performed by supplying air to the modification section 5 at 1, 3, 4, and 10 times the flow rate of methane during normal operation. After the carbon removal step, the hydrogen concentration at the outlet of the modification section 5 was measured.

[0127] Figure 12 The results are shown together with the initial performance. Figure 12 As shown, when the air supply is four times the normal flow rate, the outlet H2 concentration is significantly higher compared to the case where the flow rate is less than the normal flow rate.

[0128] (Experimental Example 3)

[0129] The air supply time for the carbon removal step in Experimental Example 1 was changed to 1 minute. All other conditions were set to the same as in Experimental Example 1. Furthermore, the modification performance was checked before and after the carbon removal step, and it was confirmed that the modification performance had recovered.

[0130] (Experimental Example 4)

[0131] The modified catalyst in Experimental Example 1 was changed to a substance containing Rh. All other conditions were set to be the same as in Experimental Example 1. Furthermore, the modification performance was checked before and after the carbon removal step, and it was confirmed that the modified performance had been restored.

Claims

1. A control method for a fuel cell system, wherein, The fuel cell system has the following features: A modification section that modifies fuel gases, including hydrocarbons, to generate modified gases, and includes a modification catalyst; and A fuel cell stack is configured to generate electricity by using the modified gas as the positive electrode gas. The control method comprises the following steps: The normal operating steps for supplying the modified section with the fuel gas and enabling the fuel cell stack to generate electricity; and The carbon removal step involves removing the carbon deposited on the modified catalyst. The carbon removal step includes supplying air to the modified section. The flow rate of air supplied to the modification section in the carbon removal step is greater than the flow rate of hydrocarbon supplied to the modification section in the normal operation step.

2. The control method according to claim 1, wherein, The air flow rate supplied to the modification section in the carbon removal step is more than four times the flow rate of the hydrocarbon supplied to the modification section in the normal operation step.

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

4. The control method according to claim 1 or 2, wherein, The control method further includes the step of deciding whether to implement the carbon removal step based on the implementation time of the normal operation step.

5. The control method according to claim 1 or 2, wherein, The control method further includes the step of determining whether to implement the carbon removal step based on the outlet temperature of the modification section.

6. The control method according to claim 1 or 2, wherein, The control method also includes a startup step that starts the fuel cell stack before the normal operating steps. The carbon removal step is performed during the startup step.

7. The control method according to claim 6, wherein, The fuel cell system also has: An air supply line that supplies air to the negative electrode of the fuel cell stack; and A burner, thermally connected to the air supply line, heats the air. The startup step includes a burner startup step that starts the burner to bring it to a temperature above a specified temperature. The carbon removal step is performed after the burner start-up step. The carbon removal step includes the following steps: conveying the removed carbon to the burner.

8. The control method according to claim 7, wherein, The fuel cell stack has a first stack and a second stack. The modified part is disposed at the positive electrode of the first pile. The air supply line is connected to the negative terminal of the second pile. The first pile and the second pile are connected such that gas flows from the positive electrode of the first pile to the positive electrode of the second pile, and gas flows from the negative electrode of the second pile to the negative electrode of the first pile. The startup step includes a warm-up step, which involves supplying air to the negative electrode of the second stack via the air supply line to heat the fuel cell stack after the burner startup step. The carbon removal step is performed during the warm-up process.

9. The control method according to claim 1 or 2, wherein, The carbon removal step includes the following step: when the temperature of the modification section reaches a first temperature, the air supply to the modification section is stopped.

10. The control method according to claim 1 or 2, wherein, The carbon removal step includes the following steps: supplying air from the air supply unit to the modification unit without heating.

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

12. The control method according to claim 1 or 2, wherein, The carbon removal step includes the following steps: supplying air to the modification section in a stepwise manner.

13. The control method according to claim 1 or 2, wherein, The modified part is disposed within the fuel cell stack.

14. A fuel cell system, wherein, The fuel cell system has the following features: The modification section modifies fuel gases, including hydrocarbons, to generate modified gases and includes a modification catalyst. A fuel cell stack configured to generate electricity by using the modified gas as the positive electrode gas; and Control device, The control device is configured to perform normal operating steps and carbon removal steps. During the normal operating procedure, the control device supplies the fuel gas to the modification section and causes the fuel cell stack to generate electricity. In the carbon removal step, the control device supplies air to the modification section. The flow rate of air supplied to the modification section in the carbon removal step is greater than the flow rate of hydrocarbon supplied to the modification section in the normal operation step.

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