Method for warming up a fuel cell and fuel cell system

Abstract

To provide a fuel cell warming method that can suppress the structural load applied between the first stack and the second stack during warm-up operation. [Solution] A method for warming up a fuel cell comprising a first stack and a second stack, comprising an oxidizer supply step, a fuel supply step, and a temperature difference control step for controlling the temperature difference between the first stack and the second stack. The temperature difference control step comprises a temperature measurement step of measuring the temperature at the oxidizer inlet portion of the second stack as a first temperature and measuring the temperature at the oxidizer outlet portion of the first stack as a second temperature, and a flow rate control step of controlling the flow rate of fuel or oxidizer supplied to the partial oxidation catalyst so that the difference between the first temperature and the second temperature is within a preset range, based on the measurement results in the temperature measurement step.

Description

【Technical Field】 【0001】 The present invention relates to a method for warming a fuel cell and a fuel cell system. 【Background Art】 【0002】 A fuel cell including a plurality of fuel cell stacks is known. A fuel cell is usually operated in a high-temperature state. 【0003】 On the other hand, there is a limit to the heat resistance of the constituent members of a fuel cell. Therefore, the temperature of the fuel cell needs to be controlled so as not to exceed the heat-resistant temperature. 【0004】 A technique related to temperature control in a fuel cell is described in Patent Document 1 (International Publication No. 2021 / 234426). Patent Document 1 describes a control method for a fuel cell system including a fuel cell having a specific cell stack, a first flow path for supplying a fuel containing hydrocarbons to the cell stack, and a second flow path for supplying an oxidant gas to the cell stack so as to flow opposite to or orthogonal to the fuel, wherein the temperature of the discharged oxidant gas, which is the oxidant gas discharged from the second flow path, is detected, and temperature control of the fuel cell is performed based on the temperature of the discharged oxidant gas. 【Prior Art Documents】 【Patent Documents】 【0005】 【Patent Document 1】 International Publication No. 2021 / 234426 【Summary of the Invention】 【Problems to be Solved by the Invention】 【0006】 By the way, the present inventors have been studying an operation method during warming in a fuel cell including a first stack and a second stack mechanically connected. In particular, they have been studying a method for warming a fuel cell configured such that an oxidant gas flows in the order of the second stack and the first stack, and a fuel flows in the order of the first stack and the second stack. 【0007】 During the investigation, it was discovered that in fuel cells with the above-described configuration, a temperature difference may occur between the first and second stacks during warm-up. This temperature difference can cause thermal stress between the first and second stacks, potentially placing structural loads on the components. From a durability standpoint, it is preferable to avoid such structural loads. 【0008】 Therefore, an object of the present invention is to provide a fuel cell warming method that can suppress the structural load applied between the first stack and the second stack during warm-up operation. [Means for solving the problem] 【0009】 In one embodiment, the present invention provides a method for warming up a fuel cell comprising a mechanically connected first stack and a second stack. This control method comprises an oxidant supply step of supplying an oxidant to the fuel cell so that the heated oxidant flows in the order of the second stack and the first stack; a fuel supply step of supplying fuel to the fuel cell so that the fuel flows in the order of the first stack and the second stack, wherein the fuel is supplied to the first stack via a partial oxidation catalyst located inside the first stack or upstream of the first stack; and a temperature difference control step of controlling the temperature difference between the first stack and the second stack after the commencement of the oxidant supply step and the fuel supply step. The temperature difference control step comprises a temperature measurement step of measuring the temperature at the oxidant inlet portion of the second stack as a first temperature and measuring the temperature at the oxidant outlet portion of the first stack as a second temperature; and a step of controlling the flow rate of fuel or oxidant supplied to the partial oxidation catalyst so that ΔT, the difference between the first temperature and the second temperature, is within a preset range, based on the measurement results of the temperature measurement step. [Effects of the Invention] 【0010】 According to the present invention, a method for warming up a fuel cell is provided that can suppress the structural load applied between the first stack and the second stack. [Brief explanation of the drawing] 【0011】 [Figure 1] Figure 1 is a schematic block diagram showing a fuel cell according to this embodiment. [Figure 2] Figure 2 is a flowchart illustrating the general method for warming up a fuel cell. [Figure 3] Figure 3 is a graph showing an example of the relationship between the mixing ratio of fuel and oxidizer and the heating temperature of the fuel. [Figure 4] Figure 4 is a flowchart showing the warming method related to control example 1. [Figure 5] Figure 5 is a graph showing the relationship between time and temperature when using control example 1. [Figure 6] Figure 6 is a graph showing the relationship between the position on the oxidizer flow path and the temperatures of the first and second stacks. [Figure 7] Figure 7 is a flowchart showing the specific operation method of the temperature difference control step in control example 2. [Figure 8] Figure 8 is a graph showing the relationship between time and temperature when warming up is performed using control example 2. [Figure 9] Figure 9 is a flowchart showing the warming method related to control example 3. [Figure 10] Figure 10 is a graph showing the relationship between time and temperature when warming up is performed using control example 3. [Figure 11] Figure 11 is a flowchart showing the warming method related to control example 4. [Figure 12] Figure 12 is a graph showing the relationship between time and temperature when warming up is performed using control example 4. [Modes for carrying out the invention] 【0012】 Hereinafter, embodiments of the present invention will be described with reference to the drawings. 【0013】 (Fuel cell) First, the configuration of the fuel cell heated by the heating method according to the present embodiment will be described. FIG. 1 is a block diagram schematically showing a fuel cell 1 according to the present embodiment. As shown in FIG. 1, this fuel cell 1 includes a first stack 3-1, a second stack 3-2, an auxiliary structure 4, an oxidant supply device 6-1 (first oxidant supply device), an oxidant supply device 6-2 (second oxidant supply device), a fuel supply device 7, a partial oxidation catalyst 8, a first temperature sensor 9-1, a second temperature sensor 9-2, an oxidant flow path 10-1, and a fuel flow path 10-2. 【0014】 The first stack 3-1 and the second stack 3-2 are parts that realize the function of a fuel cell, that is, the power generation function. Although not shown, each stack is provided with a fuel electrode layer, an air electrode layer, and an electrolyte layer. During operation, fuel is supplied to the fuel electrode layer and an oxidant is supplied to the air electrode layer. 【0015】 In FIG. 1, for clarity, the first stack 3-1 and the second stack 3-2 are drawn separately, but they are mechanically connected. In the present embodiment, the first stack 3-1 and the second stack 3-2 are mechanically connected via the auxiliary structure 4. 【0016】 The auxiliary structure 4 incorporates peripheral devices such as a combustor 5. 【0017】 The oxidizer supply device 6-1 is configured to supply an oxidizer to the fuel cell 1. For example, air is used as the oxidizer. Specifically, the oxidizer supply device 6-1 supplies the oxidizer to the fuel cell 1 via the oxidizer flow path 10-1. The oxidizer flow path 10-1 connects the components so that the oxidizer flows in the following order: combustor 5, second stack 3-2, first stack 3-1, and combustor 5. Specifically, the oxidizer supplied by the oxidizer supply device 6-1 is first heated by heat exchange in the combustor 5. The heated oxidizer is supplied to the air electrode of the second stack 3-2. Next, the oxidizer is sent from the second stack 3-2 to the air electrode of the first stack 3-1. Next, the oxidizer is sent from the first stack 3-1 to the combustor 5 and used for the combustion of fuel in the combustor 5. In this specification, some components within the first stack 3-1 and the second stack 3-2 can also be considered as part of the oxidizer flow path 10-1. 【0018】 The fuel supply device 7 is configured to supply fuel to the fuel cell 1. For example, a hydrocarbon fuel such as methane is used as the fuel. Specifically, the fuel supply device 7 supplies fuel to the fuel cell 1 via the fuel flow path 10-2. The fuel flow path 10-2 connects the components so that the fuel flows in the order of the first stack 3-1, the second stack 3-2, and the combustor 5. Specifically, the fuel is first sent to the fuel electrode layer of the first stack 3-1. Then, the fuel is sent from the first stack 3-1 to the fuel electrode layer of the second stack 3-2. Then, the fuel is sent from the second stack 3-2 to the combustor 5 and burned. In this specification, some components within the first stack 3-1 and the second stack 3-2 can also be considered to constitute part of the fuel flow path 10-2. 【0019】 The partial oxidation catalyst 8 is provided for fuel reforming and other purposes. The partial oxidation catalyst 8 is located on the fuel flow path 10-2. In the example shown in Figure 1, the partial oxidation catalyst 8 is located inside the first stack 3-1. Specifically, the partial oxidation catalyst 8 is located in the fuel electrode layer of the first stack 3-1. However, the partial oxidation catalyst 8 may also be located outside the first stack 3-1. That is, the partial oxidation catalyst 8 may be located upstream of the first stack 3-1, on the fuel flow path 10-2. Furthermore, the partial oxidation catalyst 8 may be provided together with other internal reforming catalysts, etc. 【0020】 The oxidizer supply device 6-2 is configured to add an oxidizer to the fuel supplied to the partial oxidation catalyst 8. In the example shown in Figure 1, the oxidizer supply device 6-2 is configured to add an oxidizer to the fuel flow path 10-2 upstream of the partial oxidation catalyst 8 (upstream of the first stack 3-1). When fuel and oxidizer are supplied to the partial oxidation catalyst 8, an exothermic reaction proceeds on the partial oxidation catalyst 8. The heat generated by this exothermic reaction is used for temperature control in the warm-up method described later. 【0021】 Exothermic reactions include partial oxidation (sometimes referred to as POX reaction) and complete combustion. For example, when the fuel is methane, the partial oxidation reaction is represented as CH4 + 0.5O2 → CO + H2. The complete combustion reaction is represented as CH4 + 2O2 → CO2 + 2H2O. Whether a partial oxidation reaction or a complete combustion reaction proceeds depends on the flow rates of the fuel and oxidizer, etc. 【0022】 The first temperature sensor 9-1 is configured to measure the temperature at the oxidizer inlet portion of the second stack 3-2 as the first temperature T1. The second temperature sensor 9-2 is configured to measure the temperature at the oxidizer outlet portion of the first stack 3-1 as the second temperature T2. 【0023】 Note that the first temperature T1 and the second temperature T2 do not necessarily need to be measured directly. For example, the temperature of the oxidizer before it is supplied to the second stack 3-2 reflects the temperature of the oxidizer inlet portion of the second stack 3-2. Therefore, the first temperature sensor 9-1 may be configured to measure the temperature of the oxidizer before it is supplied to the second stack 3-2. Similarly, the temperature of the oxidizer after it has been discharged from the first stack 3-1 reflects the temperature of the oxidizer outlet portion of the first stack 3-1. Therefore, the second temperature sensor 9-2 may be configured to measure the temperature of the oxidizer downstream of the first stack 3-1. 【0024】 The above describes the configuration of the fuel cell 1 that is warmed up in this embodiment. 【0025】 (Method for warming up a fuel cell) Next, a method for warming up the fuel cell according to this embodiment will be described. The main entity of each operation included in the warming method described below is not particularly limited. However, in this embodiment, the case where the main entity of each operation is the control device 2 will be described. That is, as shown in Figure 1, the control device 2 is connected to the fuel cell 1. The control device 2 is a computer. The control device 2 realizes its function by executing a control program, which is stored in a storage device such as ROM, by an arithmetic unit such as a CPU. In this specification, the fuel cell 1 and the control device 2 together may be referred to as the fuel cell system. 【0026】 The fuel cell warming method according to this embodiment is a method performed to raise the temperature of the fuel cell 1 to a desired temperature when the fuel cell 1 is started up. 【0027】 Figure 2 is a flowchart illustrating the fuel cell warming method. As shown in Figure 2, this warming method includes an oxidizer supply step (S1), a fuel supply step (S2), and a temperature difference control step (S3). The temperature difference control step (S3) includes a temperature measurement step (S3-1) and a flow rate control step (S3-2). Each step will be described below. 【0028】 (Step S1) Oxidizing agent supply When fuel cell 1 is started, an oxidizer is first supplied to it. The oxidizer is supplied to fuel cell 1 from the oxidizer supply device 6-1 via the oxidizer flow path 10-1. That is, the oxidizer is first heated via the combustor 5. The heated oxidizer flows in the following order: second stack 3-2, first stack 3-1, and then combustor 5. 【0029】 (Step S2) Fuel supply During startup, fuel is also supplied to the fuel cell 1. The fuel is supplied from the fuel supply device 7 to the fuel cell 1 via the fuel flow path 10-2. The fuel flows in the order of the first stack 3-1, the second stack 3-2, and the combustor 5. 【0030】 (Step S3) Temperature difference control After the start of steps S1 and S2, the temperatures of the first stack 3-1 and the second stack 3-2 rise. The heat source in this process is the heated oxidizer. That is, the heated oxidizer flows from the second stack 3-2 to the first stack 3-1, causing the temperature of each stack (3-1 and 3-2) to rise. Here, the second stack 3-2, located upstream in the flow of the oxidizer, is heated more easily than the first stack 3-1, located downstream. Therefore, over time, the second stack 3-2 becomes hotter than the first stack 3-1. Over time, the temperature difference increases. When a large temperature difference occurs, a large thermal stress may be applied between the first stack 3-1 and the second stack 3-2. This thermal stress can cause a large structural load on the fuel cell 1, potentially leading to the destruction of its components. 【0031】 Therefore, in this embodiment, the temperature difference between the first stack 3-1 and the second stack 3-2 is controlled so that a large temperature difference does not occur. 【0032】 To control the temperature difference, the exothermic reaction on the partial oxidation catalyst 8 is utilized. That is, when an oxidizer is supplied to the partial oxidation catalyst 8 in addition to the fuel, an exothermic reaction proceeds on the partial oxidation catalyst 8, heating the fuel. When the heated fuel is supplied to the first stack 3-1, the rate of temperature rise of the first stack 3-1 increases. Since the heat from the fuel is also transferred to the second stack 3-2, the rate of temperature rise of the second stack 3-2 also increases. However, the amount of increase is greater for the first stack 3-1, which is located upstream with respect to the fuel flow. Therefore, by increasing the rate of temperature rise of the first stack 3-1 to the extent that the rates of temperature rise reverse, the temperature difference between the first temperature T1 and the second temperature T2 can be reduced. 【0033】 Furthermore, by controlling the flow rates of fuel and oxidizer supplied to the partial oxidation catalyst 8, the amount of heat generated in the partial oxidation catalyst 8 can be controlled, and thus the rate of temperature rise of the first stack 3-1 can be controlled. Figure 3 is a graph showing an example of the relationship between the mixing ratio of fuel and oxidizer and the heat generation temperature (calorific value) of the fuel. Figure 3 shows a graph when methane is used as the fuel and air (oxygen) is supplied as the oxidizer. The amount of heat generated is greatest when the mixing ratio of fuel and oxidizer is the stoichiometric mixing ratio λ, and decreases as it deviates from the stoichiometric mixing ratio. Therefore, by controlling this mixing ratio, that is, by controlling the flow rate of fuel or oxidizer, the rate of temperature rise of the first stack 3-1 can be controlled. This allows the temperature difference between the first stack 3-1 and the second stack 3-2 to be controlled to stay within a predetermined range. For example, if the temperature of the first stack 3-1 rises too high, the flow rate of fuel or oxidizer can be controlled to reduce the amount of heat generated. This reduces the rate of temperature rise of the first stack 3-1, and the temperature difference ΔT can be kept within a desired range. On the other hand, if the temperature of the first stack 3-1 is too low compared to the temperature of the second stack 3-2, the flow rate of fuel or oxidizer is controlled to increase the heat output. This increases the rate at which the temperature of the first stack 3-1 rises, allowing the temperature difference ΔT to be kept within the desired range. 【0034】 In the temperature difference control step S3, the temperature difference is controlled using the principle described above. Specifically, as shown in Figure 2, first, in step S3-1, temperature measurement is performed. That is, the first temperature T1 is measured by the first temperature sensor 9-1. In addition, the second temperature T2 is measured by the second temperature sensor 9-2. 【0035】 Next, in step S3-2, the flow rate of fuel or oxidizer supplied to the partial oxidation catalyst 8 is controlled based on the first temperature T1 and the second temperature T2. These flow rates are controlled so that ΔT, the difference between the first temperature T1 and the second temperature T2, is within a preset range. For example, if ΔT exceeds a preset range when no oxidizer is supplied to the partial oxidation catalyst 8 (the exothermic reaction has not started), the supply of oxidizer to the partial oxidation catalyst 8 (addition of oxidizer to the fuel) is started to cause an exothermic reaction, and the flow rate of the fuel or oxidizer is further controlled. If ΔT falls outside the preset range while oxidizer is already supplied, the flow rate is controlled to increase or decrease the amount of heat generated on the partial oxidation catalyst 8 so that ΔT falls within the predetermined range. 【0036】 The above describes the heating method according to this embodiment. As described above, according to this embodiment, the temperature difference ΔT between the first stack 3-1 and the second stack 3-2 is controlled to be within a predetermined range, so that structural load due to thermal stress can be suppressed. 【0037】 Next, the processing in the temperature difference control step (S3) will be explained in more detail with reference to a control example. However, the processing content in the temperature difference control step (S3) according to this embodiment is not limited to the following control example. 【0038】 (Control example 1) Figure 4 is a flowchart showing the warming method related to Control Example 1, and is a flowchart showing the operation method of the temperature difference control step (S3). Figure 5 is a graph showing the relationship between time and temperature when this control example is adopted. As a reference example, Figure 5 also shows a graph when the temperature difference control step is not performed. As shown in Figure 4, in this control example, the temperature difference control step S3 includes the processing of steps S10 to S15. The processing in each step will be explained below. 【0039】 (Step S10) After steps S1 and S2 (supply of oxidizer and fuel) are initiated, temperature measurements are performed as shown in step S10 of Figure 4. Specifically, the first temperature T and the second temperature T2 are measured. In the control example shown in Figure 5, steps S1 and S2 are initiated at time t0. Therefore, temperature measurements are performed after time t0. 【0040】 (Step S11) Next, in step S11, it is determined whether the temperature difference ΔT is greater than or equal to a predetermined value. As shown in Figure 5, the first temperature T1 and the second temperature T2 increase from time t0 onward. In this case, the first temperature T1 increases faster than the second temperature T2. Therefore, a temperature difference ΔT is created. The temperature difference ΔT widens over time. In this step, it is determined whether this ΔT has reached a predetermined value. 【0041】 The predetermined "specified value" referred to here is a value determined from the perspective of whether or not the constituent members of each stack (3-1 and 3-2) will be destroyed by structural loads resulting from thermal stress. Specifically, if the coefficient of linear expansion of the constituent members of each stack (3-1 and 3-2) is α, the thermal strain occurring between the stacks is expressed by the following equation 1. 【0042】 【number】 【0043】 Furthermore, the thermal stress generated between the stacks is expressed by the following formula, using the elastic modulus E of the constituent material of each stack. 【number】 【0044】 Furthermore, if the yield point and thermal stress of each stack component (stack structural material) are in the following relationship, the component will not fail. 【number】 【0045】 Therefore, the "predetermined value" referenced in step S11 is set in advance within a range of temperature difference ΔT such that the relationship described in equation 3 above holds. 【0046】 (Step S12) If ΔT exceeds a predetermined value, an exothermic reaction is initiated in step S12. Specifically, an oxidizer is added to the fuel supplied to the partial oxidation catalyst 8, and an exothermic reaction is initiated on the partial oxidation catalyst 8. In the example shown in Figure 5, ΔT reaches a predetermined value at time t1. As a result, an exothermic reaction (POX reaction in the example shown in Figure 5) is carried out from time t1 onward. 【0047】 As a result of the exothermic reaction, the second temperature T2 approaches the first temperature T1, and ΔT decreases. 【0048】 (Steps S13-14) Even after the exothermic reaction has started, the measurement of the first temperature T1 and the second temperature T2 continues (step S13). Then, it is determined whether the second temperature T2 has become equal to or greater than the first temperature T1 (step S14). 【0049】 (Step S15) When the second temperature T2 becomes equal to or greater than the first temperature T1, that is, when the second temperature T2 reaches the first temperature T1, a calorific value reduction operation is performed (step S15). That is, the flow rate of fuel or oxidizer supplied to the partial oxidation catalyst 8 is controlled so that the calorific value is reduced. Alternatively, the flow rate of fuel or oxidizer may be controlled so that the exothermic reaction itself is stopped. This controls the second temperature T2 so that it does not exceed the first temperature T1. In the example shown in Figure 5, at time t2, the second temperature T2 is the same as the first temperature T1. Therefore, a calorific value reduction operation is performed from time t2 onward. 【0050】 As described above, the second temperature T2 is controlled so as not to exceed the first temperature T1, thereby more reliably preventing the failure of components due to thermal stress. For details, please refer to Figure 6. Figure 6 is a graph showing the relationship between the position on the oxidizer flow path and the temperatures of the first and second stacks. When an exothermic reaction occurs on the partial oxidation catalyst 8, the area where the partial oxidation catalyst 8 is located is where the temperature is most likely to rise. If the partial oxidation catalyst 8 is located inside the first stack 3-1, then the area where the temperature is most likely to rise will be inside the first stack 3-1. As a result, the internal temperature of the first stack 3-1 may be higher than the second temperature T2 (temperature at the oxidizer outlet). In other words, a temperature difference greater than the temperature difference between the second temperature T2 and the first temperature T1 may occur between the first stack 3-1 and the second stack 3-2. As a result, unacceptable thermal stress may occur. In contrast, according to this control example, the second temperature T2 is controlled so as not to exceed the first temperature T1, which further prevents the generation of unacceptable thermal stress and more reliably prevents the failure of structural members. 【0051】 (Control example 2) Next, we will explain Control Example 2. Figure 7 is a flowchart showing the specific operation method of temperature difference control step 3 in Control Example 2. Figure 8 is a graph showing the relationship between time and temperature when warming is performed using this control example. As a reference example, Figure 8 also shows a graph of the case when the flow rate control related to Control Example 2 is not implemented. 【0052】 In this control example, as in control example 1, first, after the start of steps S1 and S2, the temperature is measured and it is determined whether ΔT is above a predetermined value (steps S20-21). Then, if ΔT reaches the predetermined value (time t1 in Figure 8), the flow rate of the fuel or oxidizer supplied to the partial oxidation catalyst 8 is controlled so that the exothermic reaction is started (step S22). 【0053】 Temperature measurements are performed from step S22 onward (step S23). Then, in step S24, the first temperature T1 and the second temperature T2 are compared with a threshold for heat generation control. The heat generation suppression threshold is set from the perspective of whether or not the temperature of each stack exceeds the heat resistance temperature, and is set to a temperature slightly lower than the heat resistance temperature of each stack. 【0054】 If at least one of the first temperature T1 and the second temperature T2 exceeds the heat generation suppression threshold, a heat generation reduction operation is performed in step S25. In the example shown in Figure 8, the first temperature T1 reaches the heat generation suppression threshold at time t2. Therefore, a heat generation reduction operation is performed from time t2 onward. The specific details of the heat generation reduction operation can be the same as in control example 1. That is, the flow rate of fuel or oxidizer supplied to the partial oxidation catalyst 8 is controlled so that the amount of heat generated on the partial oxidation catalyst 8 decreases, or so that the exothermic reaction stops. 【0055】 According to this control example, the failure of the fuel cell 1 can be prevented more reliably. When the exothermic reaction proceeds on the partial oxidation catalyst 8, the temperature of the entire fuel cell 1, especially around the first stack 3-1, tends to rise more easily. Therefore, if no control is performed in this regard, the temperature may exceed the heat resistance temperature, as shown in the reference example in Figure 8. In contrast, according to this embodiment, since the heat generation reduction operation is performed when the temperature exceeds the heat generation suppression threshold, the temperature of the fuel cell 1 can be prevented from reaching the heat resistance temperature in advance. This prevents the failure of the fuel cell 1. 【0056】 (Control example 3) Next, we will explain control example 3. Figure 9 is a flowchart showing the warming method related to control example 3, and is a flowchart showing the specific operation method of temperature difference control step S3. Figure 10 is a graph showing the relationship between time and temperature when warming is performed using this control example. 【0057】 In this control example, as in control example 1, after the start of steps S1 and S2, temperature measurement is performed and it is determined whether ΔT is greater than or equal to a predetermined value (steps S30-S31). In the example shown in Figure 10, at time t1, ΔT has reached the predetermined value. 【0058】 When ΔT reaches a predetermined value, the first temperature T1 is compared with the oxidation temperature threshold (step S31). Specifically, it is determined whether the first temperature T1 is below the oxidation temperature threshold. The oxidation temperature threshold is a preset value, set from the perspective of whether or not there is a risk of oxidation of the components of each stack. 【0059】 If the first temperature T1 is below the oxidation temperature threshold, fuel and oxidizer are supplied to the partial oxidation catalyst 8 at a flow rate equal to or greater than the stoichiometric mixing ratio, and an exothermic reaction is carried out (step S33). For the stoichiometric mixing ratio, please refer to Figure 3. "Equal to the stoichiometric mixing ratio" means a flow rate ratio in which oxygen is in excess of the fuel. In the example shown in Figure 10, an exothermic reaction is carried out at a flow rate equal to or greater than the stoichiometric mixing ratio from time t1 onward. 【0060】 When fuel and oxidizer are supplied at a flow rate above the stoichiometric ratio, the Reynolds number in the fuel flow path 10-2 increases. As a result, the heat exchange rate in the first stack 3-1 increases. This allows the temperature of the first stack 3-1 to approach the temperature of the second stack 3-2 more quickly, and the temperature difference ΔT can be eliminated more rapidly. 【0061】 Furthermore, temperature measurement continues thereafter (step S34), and the process from step S32 onward is repeated. If the first temperature T1 exceeds the oxidation temperature threshold, fuel and oxidizer are supplied to the partial oxidation catalyst 8 at a flow rate less than the stoichiometric mixing ratio (step S35). In the example shown in Figure 10, at time t2, the first temperature T1 exceeds the oxidation temperature threshold. Therefore, from time t2 onward, exothermic operation is performed at a flow rate less than the stoichiometric mixing ratio. 【0062】 As mentioned above, if the fuel-oxidant mixture ratio is greater than or equal to the stoichiometric mixture ratio, the second temperature T2 tends to rise. However, this results in excess oxidant (oxygen) on the partial oxidation catalyst 8, causing a large amount of oxidant to flow into the fuel passage 10-2. As a result, the components of each stack may oxidize. In contrast, according to this control example, when the first temperature T1 exceeds the oxidation temperature threshold, the mixture ratio is controlled to be less than the stoichiometric mixture ratio. This reduces the flow rate of oxidant through the fuel passage 10-2, thereby preventing oxidation. 【0063】 (Control example 4) Next, we will explain control example 4. Figure 11 is a flowchart showing the warming method related to control example 4, and is a flowchart showing the specific operation method of temperature difference control step S3. Figure 12 is a graph showing the relationship between time and temperature when warming is performed using this control example. 【0064】 In this control example, after the start of steps S1 and S2, in step S40, the second temperature T2 is compared with a preset POX-enabled temperature. The POX-enabled temperature is the temperature at which a partial oxidation reaction (POX reaction) becomes possible in the partial oxidation catalyst 8. 【0065】 If the second temperature T2 is above the POX-capable temperature, in step S41, fuel and oxidizer are supplied to the partial oxidation catalyst 8 at a flow rate that initiates the partial oxidation reaction. In the example shown in Figure 12, the second temperature T2 reaches the POX-capable temperature at time t1. Therefore, the partial oxidation (POX) reaction is carried out from time t1 onward. From time t1 onward, the fuel is heated on the partial oxidation catalyst 8 by the partial oxidation reaction, so the rate of temperature rise of the first stack 3-1 increases. If the rate of temperature rise of the first stack 3-1 increases to the point where the rate of temperature rise reverses, the temperature difference ΔT will decrease. 【0066】 According to this control example, when the second temperature T2 reaches the POX-enabled temperature, the partial oxidation reaction is initiated before ΔT reaches a predetermined value. Therefore, the increase in ΔT can be prevented more reliably, and fracture due to thermal stress can be prevented more reliably. 【0067】 Furthermore, the processing from step S41 onward can be the same as, for example, the embodiments and control examples described above. That is, the flow rate of fuel or oxidizer supplied to the partial oxidation catalyst 8 is controlled so that ΔT is within a predetermined range. 【0068】 The present invention has been described above using embodiments and control examples 1 to 4. These embodiments and their control examples 1 to 4 are not independent of each other, and can be combined and adopted within a non-contradictory range. 【0069】 (Note) The configurations and effects of the present invention are summarized below as an appendix. 【0070】 (Note 1) A method for warming up a fuel cell 1 comprising a mechanically connected first stack 3-1 and second stack 3-2, comprising: an oxidant supply step (S1) of supplying an oxidant to the fuel cell 1 such that the heated oxidant flows in the order of the second stack 3-2 and the first stack 3-1; and a fuel supply step (S2) of supplying fuel to the fuel cell such that the fuel flows in the order of the first stack 3-1 and the second stack 3-2, wherein the fuel is supplied to the first stack 3-1 via a partial oxidation catalyst 8 located inside the first stack 3-1 or upstream of the first stack; and the oxidant supply step and fuel supply step A method for warming up a fuel cell, comprising: a temperature difference control step (S3) for controlling the temperature difference between a first stack and a second stack after the start of the warming process, wherein the temperature difference control step (S3) includes a temperature measurement step (S3-1) for measuring the temperature at the oxidizer inlet portion of the second stack 3-2 as a first temperature and measuring the temperature at the oxidizer outlet portion of the first stack 3-1 as a second temperature, and a flow rate control step (S3-2) for controlling the flow rate of fuel or oxidizer supplied to the partial oxidation catalyst 8 so that the difference ΔT between the first temperature and the second temperature is within a preset range, based on the measurement results in the temperature measurement step. 【0071】 According to the method described above, the temperature difference between the first stack 3-1 and the second stack 3-2 is controlled to remain within a certain range, thereby suppressing structural loads caused by thermal stress. 【0072】 (Note 2) A method for warming up a fuel cell as described in Appendix 1, wherein the flow rate control step (S3-2) further comprises a step (S14-S15) of controlling the flow rate of fuel or oxidizer supplied to the partial oxidation catalyst so that the second temperature does not exceed the first temperature, based on the measurement results of the temperature measurement step. 【0073】 According to the method described above, the second temperature is controlled so as not to exceed the first temperature. Therefore, when the partial oxidation catalyst 8 is placed in the first stack 3-1, it is possible to prevent an unacceptable temperature difference from occurring between the first stack 3-1 and the second stack 3-2. As a result, the occurrence of structural load due to thermal stress can be suppressed more reliably. 【0074】 (Note 3) A method for warming up a fuel cell as described in Appendix 1 or 2, wherein the temperature difference control step (S3) further includes a heat generation reduction step (S25) in which the flow rate of fuel or oxidizer supplied to the partial oxidation catalyst 8 is changed so as to reduce the amount of heat generated in the partial oxidation catalyst 8 when at least one of the first temperature and the second temperature exceeds the heat generation suppression threshold. 【0075】 According to this method, the amount of heat generated in the partial oxidation catalyst 8 is reduced in advance, before the heat resistance temperature of the components of each stack is exceeded. Therefore, it is possible to prevent the temperature of each stack from exceeding its heat resistance temperature. 【0076】 (Note 4) A method for warming up a fuel cell as described in any of Appendix 1 to 3, wherein the temperature difference control step (S3) further includes a step (S32 to S33) of comparing a first temperature with a preset oxidation temperature threshold, and supplying fuel and oxidizer to the partial oxidation catalyst at a flow rate equal to or greater than the stoichiometric mixing ratio when the first temperature is less than or equal to the oxidation temperature threshold. 【0077】 This method increases the Reynolds number during fuel supply, which increases the heat exchange rate between the fuel and the first stack, causing the first stack 3-1 to heat up more rapidly. Therefore, if the temperature of the first stack 3-1 is lower than that of the second stack 3-2, the temperature difference can be quickly eliminated. 【0078】 (Note 5) The method for warming up a fuel cell as described in Appendix 4, A method for warming up a fuel cell, wherein the temperature difference control step (S3) further includes a step (35) of supplying fuel and oxidant to a partial oxidation catalyst at a flow rate less than the stoichiometric mixing ratio when the first temperature exceeds the oxidation temperature threshold. 【0079】 This method reduces the amount of oxidizer in the fuel supplied to each stack, thus preventing oxidation of the components of each stack even when the temperature of each stack exceeds its oxidation temperature. 【0080】 (Note 6) A method for warming up a fuel cell as described in any of Appendix 1 to 5, wherein the temperature difference control step (S3) further includes a step (S41) of comparing a second temperature with a preset POX-capable temperature, and supplying fuel and oxidant to a partial oxidation catalyst at a flow rate such that a partial oxidation reaction is initiated when the second temperature becomes equal to or greater than the POX-capable temperature. 【0081】 With this method, the POX reaction begins as soon as POX becomes possible, so ΔT is less likely to become large. Structural loads due to thermal stress are less likely to occur, and the failure of each stack component is more reliably prevented. 【0082】 (Note 7) A fuel cell 1 comprising a mechanically connected first stack and a second stack, and a control device 2 for controlling the operation of the fuel cell, wherein the fuel cell 1 includes a first oxidant supply device 6-1 that supplies oxidant to the fuel cell 1 so that the heated oxidant flows in the order of the second stack 3-2 and the first stack 3-1, and a fuel supply device 7 that supplies fuel to the fuel cell 1 so that the fuel flows in the order of the first stack 3-1 and the second stack 3-2, wherein the fuel is supplied to the first stack 3-1 via a partial oxidation catalyst 8 located inside the first stack 3-1 or upstream of the first stack 3-1, and a second oxidant supply device 6-2 that adds oxidant to the fuel supplied to the partial oxidation catalyst 8, and at the oxidant inlet portion in the second stack 3-2 A fuel cell system comprising a first temperature sensor 9-1 that measures the temperature as the first temperature, and a second temperature sensor 9-2 that measures the temperature at the oxidizer outlet portion of the first stack 3-1 as the second temperature, wherein the control device 2 supplies oxidizer to the fuel cell 1 via a first oxidizer supply device 6-1 and supplies fuel to the fuel cell via a fuel supply device 7, measures the first and second temperatures via the first temperature sensor 9-1 and the second temperature sensor 9-2 after the start of supplying oxidizer and fuel, and controls the flow rate of fuel or oxidizer supplied to the partial oxidation catalyst 8 by the fuel supply device 7 or the second oxidizer supply device 6-2 so that the difference ΔT between the first and second temperatures is within a preset range based on the measurement results of the first and second temperatures. 【0083】 According to the above configuration, the temperature difference between the first stack 3-1 and the second stack 3-2 is controlled to remain within a certain range, thereby suppressing structural loads caused by thermal stress. [Explanation of Symbols] 【0084】 1. Fuel cell, 2. Control device, 3-1. First stack, 3-2. Second stack, 4. Auxiliary structure, 5. Combustor, 6-1. Oxidizer supply device, 6-2. Oxidizer supply device, 7. Fuel supply device, 8. Partial oxidation catalyst, 9-1. First temperature sensor, 9-2. Second temperature sensor, 10-1. Oxidizer flow path, 10-2. Fuel flow path

Claims

[Claim 1] A method for warming up a fuel cell comprising a mechanically connected first stack and a second stack, An oxidant supply step, in which the heated oxidant is supplied to the fuel cell so that it flows through the second stack and then the first stack in that order, A fuel supply step of supplying fuel to the fuel cell such that the fuel flows in the order of the first stack and the second stack, wherein the fuel is supplied to the first stack via a partial oxidation catalyst located inside the first stack or upstream of the first stack. After the commencement of the oxidizer supply step and the fuel supply step, a temperature difference control step is performed to control the temperature difference between the first stack and the second stack. Equipped with, The aforementioned temperature difference control step is: A temperature measurement step in which the temperature at the oxidizer inlet portion of the second stack is measured as the first temperature, and the temperature at the oxidizer outlet portion of the first stack is measured as the second temperature, The system includes a flow rate control step, which controls the flow rate of fuel or oxidizer supplied to the partial oxidation catalyst based on the measurement results in the temperature measurement step, such that ΔT, the difference between the first temperature and the second temperature, falls within a preset range. How to warm up a fuel cell. [Claim 2] A method for warming up a fuel cell according to claim 1, The flow rate control step further includes a step of controlling the flow rate of fuel or oxidizer supplied to the partial oxidation catalyst so that the second temperature does not exceed the first temperature, based on the measurement results of the temperature measurement step. How to warm up a fuel cell. [Claim 3] A method for warming up a fuel cell according to claim 1 or 2, The temperature difference control step further includes a heat generation reduction step, in which the first temperature and the second temperature are compared with a preset heat generation suppression threshold, and if at least one of the first temperature and the second temperature exceeds the heat generation suppression threshold, the flow rate of the fuel or oxidizer supplied to the partial oxidation catalyst is changed so as to reduce the amount of heat generated in the partial oxidation catalyst. How to warm up a fuel cell. [Claim 4] A method for warming up a fuel cell according to claim 1 or 2, The temperature difference control step further includes comparing the first temperature with a preset oxidation temperature threshold, and, if the first temperature is below the oxidation temperature threshold, supplying fuel and oxidizer to the partial oxidation catalyst at a flow rate equal to or greater than the stoichiometric mixing ratio. How to warm up a fuel cell. [Claim 5] A method for warming up a fuel cell according to claim 4, The temperature difference control step further includes supplying fuel and oxidizer to the partial oxidation catalyst at a flow rate less than the stoichiometric mixing ratio when the first temperature exceeds the oxidation temperature threshold. How to warm up a fuel cell. [Claim 6] A method for warming up a fuel cell according to claim 1 or 2, The temperature difference control step further includes comparing the second temperature with a preset POX-enabled temperature, and supplying fuel and oxidant to the partial oxidation catalyst at a flow rate such that the partial oxidation reaction starts when the second temperature becomes equal to or greater than the POX-enabled temperature. How to warm up a fuel cell. [Claim 7] A fuel cell comprising a mechanically connected first stack and a second stack, A control device for controlling the operation of the fuel cell, Equipped with, The aforementioned fuel cell, A first oxidant supply device supplies oxidant to the fuel cell such that the heated oxidant flows through the second stack and then the first stack in that order, A fuel supply device that supplies fuel to the fuel cell such that the fuel flows in the order of the first stack and the second stack, wherein the fuel is supplied to the first stack via a partial oxidation catalyst located inside the first stack or upstream of the first stack. A second oxidizer supply device adds an oxidizer to the fuel supplied to the partial oxidation catalyst, A first temperature sensor measures the temperature at the oxidizer inlet portion of the second stack as the first temperature, A second temperature sensor measures the temperature at the oxidizer outlet portion of the first stack as the second temperature, Equipped with, The control device is The oxidizer is supplied to the fuel cell via the first oxidizer channel. Fuel is supplied to the fuel cell via the fuel passage, After the supply of the oxidizer and the fuel is started, the first temperature and the second temperature are measured via the first temperature sensor and the second temperature sensor. Based on the measurement results of the first and second temperatures, the flow rate of fuel or oxidizer supplied to the partial oxidation catalyst by the fuel supply device or the second oxidizer supply device is controlled so that the difference ΔT between the first and second temperatures falls within a preset range. Fuel cell system.

Images