Control method for a fuel cell system, and fuel cell system

Abstract

The present invention provides a control method for a fuel cell system and a fuel cell system that suppresses poisoning of the fuel cell by suppressing the generation of unintended CO-concentrated reformed fuel. [Solution] A control method for a fuel cell system including a reformer 25 for reforming raw fuel into reformed fuel, a fuel cell stack 1 for generating electricity by consuming the reformed fuel, and a heating means (combustor 51, heater (not shown)) for heating the reformer 25, wherein when the output of the fuel cell stack 1 is reduced, the amount of heat supplied (Q1) from the heating means to the reformer 25 is reduced, and the flow rate (m1) of raw fuel to the reformer 25 is gradually reduced.

Description

【Technical Field】 【0001】 The present invention relates to a control method for a fuel cell system and a fuel cell system. 【Background Art】 【0002】 Patent Document 1 discloses a fuel reforming system including an evaporator that generates raw fuel vapor composed of a mixture of a liquid fuel and water, a reformer that generates reformed fuel containing hydrogen using the raw fuel vapor and a gas containing oxygen, and means for controlling the amount of the gas containing oxygen supplied to the reformer. The system further includes means for detecting the amount of the raw fuel vapor, means for detecting the concentration of water vapor in the raw fuel vapor, and means for evaporating water or the raw fuel by heating a hot surface and supplying it to the reformer. By supplying an amount of water or the raw fuel such that the detected concentration of water vapor reaches a predetermined concentration, the CO concentration of the reformed fuel is reduced to an acceptable value level that can be used in a fuel cell. 【Prior Art Documents】 【Patent Documents】 【0003】 【Patent Document 1】 Japanese Patent Application Laid-Open No. 2002-241104 【Summary of the Invention】 【Problems to be Solved by the Invention】 【0004】 However, since Patent Document 1 does not actively control the temperature of the reformer, even if the supply amount of the raw fuel or water is changed, reformed fuel with an unintended CO concentration may be generated, which may cause poisoning of the fuel cell. 【0005】 An object of the present invention is to provide a control method for a fuel cell system that suppresses poisoning of the fuel cell by suppressing the generation of reformed fuel with an unintended CO concentration, and a fuel cell system. 【Means for Solving the Problems】 【0006】 The fuel cell system control method according to the present invention is a control method for a fuel cell system that includes a reformer for reforming raw fuel into reformed fuel, a fuel cell for generating electricity by consuming the reformed fuel, and a heating means for heating the reformer. In this control method, when the output of the fuel cell is reduced, the amount of heat supplied to the reformer by the heating means is reduced, and the flow rate of raw fuel to the reformer is gradually reduced. [Effects of the Invention] 【0007】 According to the present invention, the latent heat and heat absorption are controlled in a way that suppresses a rapid decrease in the latent heat and heat absorption of the raw fuel (and a rapid decrease in the latent heat of the reforming water), thereby enabling active control of the reformer temperature. This suppresses a rapid rise in the temperature of the reformer outlet, i.e., the temperature of the reformed fuel supplied to the fuel cell, thereby suppressing an unintended increase in the CO concentration in the reformed fuel and preventing poisoning and performance degradation of the fuel cell. [Brief explanation of the drawing] 【0008】 [Figure 1] Figure 1 shows the basic configuration of the fuel cell system according to the first embodiment. [Figure 2] Figure 2 is a characteristic curve showing the relationship between the temperature of the reformed fuel and the CO concentration in the reformed fuel, illustrating the change when the ratio of the water flow rate to the raw fuel flow rate is changed. [Figure 3] Figure 3 is a time chart of the fuel cell system of the first embodiment. [Figure 4] Figure 4 is a time chart of the fuel cell system of the second embodiment. [Figure 5] Figure 5 shows the control flow (S01-S05) of the fuel cell system of the third embodiment. [Figure 6] Figure 6 shows the control flow (S06-S11) of the fuel cell system of the third embodiment. [Figure 7] Figure 7 is a time chart of the fuel cell system of the third embodiment. [Modes for carrying out the invention] 【0009】 Embodiments of the present invention will be described below with reference to the attached drawings. 【0010】 [Basic configuration of the fuel cell system of the first embodiment] 【0011】 Figure 1 shows the basic configuration of a fuel cell system according to the first embodiment. The fuel cell system of this embodiment includes a fuel supply line 2 that supplies fuel (anode gas) to a fuel cell stack 1, a steam supply line 3 that supplies steam to the fuel supply line 2, an air supply line 4 that supplies air (cathode gas) to the fuel cell stack 1, an exhaust gas line 5 that exhausts gas discharged from the fuel cell stack 1, a power supply line 6 that extracts power from the fuel cell stack 1, and a control unit 7 that controls the entire system. 【0012】 The fuel supply line 2 includes a fuel tank 21, a pump 22, a desulfurizer 23, a heat exchanger 24, a reformer 25, and a temperature sensor 26. 【0013】 The fuel tank 21 stores raw fuels (e.g., methane (CH4), methanol (CH3OH)) that will be used as raw materials for the anode gas of the fuel cell stack 1. 【0014】 Pump 22 pumps the raw fuel stored in fuel tank 21 to desulfurizer 23. 【0015】 The desulfurizer 23 removes sulfur components from the raw fuel and supplies it to the heat exchanger 24. 【0016】 The heat exchanger 24 heats the raw fuel (and steam supplied from the steam supply line 3) by exchanging heat with the combustion gas discharged from the combustor 51 (described later) and supplies it to the reformer 25. 【0017】 The reformer 25 reacts the raw fuel with steam to produce reformed fuel (anode gas), which is then supplied to the fuel cell stack 1 (anode). 【0018】 The temperature sensor 26 measures the outlet temperature of the reformer 25 (the internal temperature of the reformer 25), that is, the temperature of the reformed fuel supplied to the fuel cell stack 1. 【0019】 In the steam supply line 3, a water tank 31, a valve 32, a pump 33, and a vaporizer 34 are arranged. The steam supply line 3 merges with the fuel supply line 2 at a position between the heat exchanger 24 of the fuel supply line 2 and the reformer 25. 【0020】 The water tank 31 stores water (reforming water) that serves as a raw material for the steam used in the reformer 25. 【0021】 The valve 32 is opened when the reformer 25 requires steam. 【0022】 The pump 33 pumps the water stored in the water tank 31 to the vaporizer 34. 【0023】 The vaporizer 34 vaporizes water to generate steam and supplies it to the heat exchanger 24 of the fuel supply line 2. 【0024】 In the air supply line 4, a blower 41 and a heat exchanger 42 are arranged. 【0025】 The blower 41 takes in air from the outside and supplies it to the heat exchanger 42. 【0026】 The heat exchanger 42 heats air (cathode gas) by exchanging heat with the combustion gas discharged from a combustor 51 described later and supplies it to the fuel cell stack 1 (cathode). 【0027】 The fuel cell stack 1 has a stack structure obtained by stacking a large number of, for example, proton exchange membrane type battery cells. 【0028】 The fuel cell stack 1 introduces anode gas from the anode inlet of the fuel cell stack 1 and cathode gas from the cathode inlet of the fuel cell stack 1. 【0029】 The fuel cell stack 1 generates electricity through an electrochemical reaction between the anode gas (reformed fuel) supplied to the anode and the cathode gas (oxygen) supplied to the cathode, and outputs the generated electricity to the power supply line 6. 【0030】 Fuel cell stack 1 discharges the anode off-gas used in the electrochemical reaction from the anode outlet of fuel cell stack 1, and discharges the cathode off-gas used in the electrochemical reaction from the cathode outlet of fuel cell stack 1. 【0031】 A combustor 51 is located in the exhaust gas line 5. Upstream of the combustor 51, the exhaust gas line 5 communicates with the anode outlet and cathode outlet of the fuel cell stack 1, while downstream of the combustor 51, it communicates with the heat exchangers 24 and 42 and is open to the outside. 【0032】 The combustor 51 (heating means) burns a mixed gas obtained by mixing the anode-off gas and cathode-off gas discharged from the fuel cell stack 1, and supplies the resulting combustion gas to the heat exchangers 24 and 42. 【0033】 The combustor 51 is formed integrally with the reformer 25 and the fuel cell stack 1, and heats the reformer 25 and the fuel cell stack 1 with the heat generated by the combustion of the mixed gas. The combustor 51 has an ignition device (not shown) for igniting the mixed gas and a catalyst (not shown) for catalytic combustion of the mixed gas. 【0034】 If the combustor 51 is not configured to heat the reformer 25, a configuration in which a heater (main heating means) is attached to the reformer 25 can also be applied. The heater (heating means, for example, a PTC heater) generates heat by receiving power from a battery (power buffer device 62) in accordance with a command from the control unit 7. 【0035】 The power supply line 6 includes a DC / DC converter 61, a power buffer device 62, and an inverter 63. 【0036】 The DC / DC converter 61 boosts the output voltage generated by the fuel cell stack 1 and outputs it to the power buffer device 62. 【0037】 The power buffer device 62 is equipped with a battery that stores the power output from the DC / DC converter 61. Furthermore, when the inverter 63 is driven, the power buffer device 62 outputs a DC voltage to the inverter 63 under control from the control unit 7. 【0038】 The inverter 63 has a switching element driven by a PWM signal, and converts the DC voltage of the power buffer device 62, which receives the PWM signal transmitted from the control unit 7, into a three-phase AC voltage and outputs it to the load (for example, a drive motor). 【0039】 The control unit 7 controls the following: pump 22, valve 32, pump 33, blower 41, combustor 51, DC / DC converter 61, power buffer device 62, and inverter 63. 【0040】 The control unit 7 includes a program for executing the control method of the fuel cell system of this embodiment, and executes control based on the program. 【0041】 When the fuel cell stack 1 is started, the control unit 7 starts the pump 22 to supply raw fuel to the fuel supply line 2 (fuel cell stack 1 (anode), exhaust gas line 5), starts the blower 41 to supply air to the fuel supply line 2 (fuel cell stack 1 (cathode), exhaust gas line 5), and starts the ignition device (not shown) of the combustor 51 to burn the mixed gas of raw fuel and air that flows into the combustor 51, thereby heating the reformer 25 and the fuel cell stack 1 with the heat generated. As described above, the reformer 25 may also be heated by a heater (not shown). 【0042】 The control unit 7 opens the valve 32 and starts the pump 33 to supply water (reforming water) to the reformer 25. 【0043】 The combustion gas generated in the combustor 51 is supplied to the heat exchanger 24 and the heat exchanger 42, where the raw fuel is heated in the heat exchanger 24 and the air is heated in the heat exchanger 42. 【0044】 The raw fuel heated in the heat exchanger 24 heats the reformer 25 and the fuel cell stack 1 (anode), and the air heated in the heat exchanger 42 heats the fuel cell stack 1 (cathode). 【0045】 The control unit 7 extinguishes the ignition device when the temperature of the combustor 51 reaches a temperature at which catalytic combustion is possible. 【0046】 When the temperature of the reformer 25 reaches a temperature at which the raw fuel can be converted into reformed fuel (anode gas) and the temperature of the fuel cell stack 1 reaches a temperature at which power generation can be started, the control unit 7 controls the DC / DC converter 61 to extract a predetermined amount of current and start power generation. 【0047】 The control unit 7 increases the flow rate of raw fuel when the load demand increases. This increases the heat output in the combustor 51 and raises the temperature of the fuel cell stack 1, thereby improving the IV characteristics of the fuel cell stack 1 and increasing the output of the fuel cell stack 1. 【0048】 The control unit 7 controls the output of the pump 33 so that the ratio of the flow rate of water (reforming water) to the flow rate of raw fuel maintains a predetermined flow rate ratio. 【0049】 The control unit 7 reduces the flow rate of raw fuel (and water) when the load requirement decreases. 【0050】 At this time, the flow rate of anode-off gas supplied to the combustor 51 also decreases, but the amount of heat supplied to the reformer 25 by the residual heat of the combustor 51 decreases more slowly than when the flow rate of the raw fuel is reduced. Therefore, the temperature of the reformer 25 does not decrease immediately from the moment the raw fuel is reduced, but decreases after a predetermined delay. 【0051】 On the one hand, since the reforming reaction occurring in the reformer 25 is an endothermic reaction, when the flow rate of the raw fuel (and water) decreases, the amount of heat absorbed decreases. Also, when the flow rates of the raw fuel and water decrease, the latent heat of the raw fuel and water also decreases. As a result, the temperature of the raw fuel (and water) temporarily rises rapidly, the concentration of carbon monoxide (CO) in the reformed fuel increases, and when reformed fuel with a concentration above a predetermined upper limit concentration (for example, 5%) is supplied to the fuel cell stack 1, the fuel cell stack 1 (the fuel cells) is poisoned and the performance of the fuel cell stack 1 deteriorates. 【0052】 Therefore, in the present embodiment, when reducing the flow rate of the raw fuel to lower the output of the fuel cell stack 1, the temporary temperature rise of the raw fuel (reformed fuel) is suppressed and the concentration of carbon monoxide in the reformed fuel is suppressed, thereby suppressing the poisoning of the fuel cell stack 1 and suppressing the deterioration of the performance of the fuel cell stack 1. 【0053】 [Relationship between the temperature of the reformed fuel and the CO concentration in the reformed fuel] FIG. 2 is a characteristic curve showing the relationship between the temperature of the reformed fuel and the CO concentration in the reformed fuel, and shows the changes when the flow rate ratio of water to the flow rate of the raw fuel is changed. As shown in FIG. 2, the CO concentration in the reformed fuel has a characteristic curve that monotonically increases as the temperature of the reformed fuel rises. 【0054】 The characteristic curves shown in FIG. 2 show the cases where the flow rate ratio of water to the flow rate of the raw fuel (SC) is 1.5, 2.0, and 2.5. The characteristic curves (CO concentration) have the relationship SC = 2.5 < SC = 2.0 < SC = 1.5, and the slopes of the characteristic curves also have the relationship SC = 2.5 < SC = 2.0 < SC = 1.5. 【0055】 Therefore, the CO concentration in the reformed fuel monotonically decreases as the flow rate ratio (SC) increases, and the slope of the characteristic curve also monotonically decreases. 【0056】 When the flow rate of the raw fuel (and the flow rate of water) is increased, the amount of heat absorbed and the amount of latent heat increase, so the temperature of the reformed fuel decreases and the CO concentration also decreases. Conversely, when the flow rate of the raw fuel (and the flow rate of water) is decreased, the amount of heat absorbed and the amount of latent heat decrease, so the temperature of the reformed fuel rises and the CO concentration also increases. 【0057】 Increasing the flow ratio (SC) shifts the chemical equilibrium between the reformed fuel (hydrogen) and carbon monoxide towards the reformed fuel (hydrogen) side, thus decreasing the CO concentration. Conversely, decreasing the flow ratio (SC) shifts the chemical equilibrium towards the carbon monoxide side, thus increasing the CO concentration. 【0058】 When the control unit 7 reduces the flow rate of raw fuel to reduce the output of the fuel cell stack 1, it reduces the amount of heat supplied from the heating means (combustor 51, heater) to the reformer 25 and gradually reduces the flow rate of raw fuel (and water) to the reformer 25 so that the CO concentration in the reformed fuel does not exceed a set concentration (e.g., 3%) which is lower than the upper limit concentration (e.g., 5%) that can efficiently reduce poisoning. 【0059】 When the heating means is a combustor 51, the control unit 7 increases the output of the blower 41 from its rated value to increase the airflow rate and cool the combustor 51, thereby lowering the temperature of the reformer 25. When the heating means is a heater, the control unit 7 lowers the temperature of the reformer 25 by lowering the output of the heater from its rated value. However, the temperature of the reformer 25 is lowered within a range where the temperature of the reformer 25 after the temperature reduction is above the lower limit of the temperature at which the raw fuel can be reformed. 【0060】 In this embodiment, when reducing the output of the fuel cell stack 1, the flow rate of the raw fuel is gradually reduced from the first flow rate (the flow rate before the reduction in the output of the fuel cell stack 1) to a second flow rate smaller than the first flow rate. At this time, the control unit 7 gradually reduces the output of the pump 22. 【0061】 When gradually reducing the flow rate of water (reforming water), the control unit 7 gradually reduces the output of pump 33. When reducing the output of fuel cell stack 1 to zero, the second flow rate is set to zero, and the outputs of pumps 22 and 33 are also set to zero. 【0062】 The time constant when gradually reducing the raw fuel (and water) from the first flow rate to the second flow rate can minimize the time (time from t1 to t11 in Figure 3) from the time the raw fuel flow rate is switched from the first flow rate to the second flow rate until the outlet temperature of the reformer 25 (temperature of the reformed fuel) reaches its peak, for example, when the amount of heat supplied to the reformer 25 by the heating means is reduced and the flow rate of the raw fuel is switched from the first flow rate to the second flow rate in a step function manner. 【0063】 Furthermore, the maximum value of the time constant can be the sum of the minimum value and the time constant from the time after the peak until the outlet temperature of the reformer 25 (temperature of the reformed fuel) changes gradually (the time from t1 to t12 in Figure 3). 【0064】 As shown in Figure 2, for example, when the temperature of the reformer 25 (outlet temperature) before reducing the output of the fuel cell stack 1 is 225°C, if the flow rate of the raw fuel (in this case, for example, SC=2.0) is reduced in a step function from the first flow rate to the second flow rate (first flow rate > second flow rate), as in the conventional method, the temperature of the reformer 25 will rise to 500°C, and the CO concentration will rise from 0.5% to 5.5%, potentially poisoning the fuel cell stack 1. On the other hand, in this embodiment, by gradually reducing the flow rate of the raw fuel (in this case, for example, SC=2.0) from the first flow rate to the second flow rate (first flow rate > second flow rate), the temperature of the reformer 25 will rise to 300°C, for example, and the CO concentration will rise only from 0.5% to 1.5%, thus reducing the poisoning of the fuel cell stack 1. 【0065】 [Time chart of the fuel cell system of the first embodiment] Figure 3 is a time chart of the fuel cell system of the first embodiment. Figure 3 shows the time chart of the fuel cell system of the first embodiment (solid line) and the time chart of the fuel cell system of the comparative example (dashed line). Figure 3 shows the time changes of the raw fuel flow rate (m), the amount of heat supplied from the heating means to the reformer 25 (Q1), the amount of heat absorbed by the mixed fuel of raw fuel and water (Q2), the latent heat of the mixed fuel (Q3), the reformer inlet temperature, and the reformer outlet temperature. 【0066】 In the comparative example and the first embodiment, the heat supply amount (Q1) is stepwise decreased from Q11 to Q12 (Q11 > Q12) at time t1. Further, when the heating means of the reformer 25 is the combustor 51, the output of the blower 41 is increased from its rated value at time t1, and when the flow rate (m) of the raw fuel reaches the second flow rate (m2) at time t2, the output of the blower 41 is returned to its rated value. 【0067】 In the comparative example, the flow rate (m1) of the raw fuel is switched from m11 (the first flow rate) to m12 (the second flow rate) (m11 > m12) at time t1. As a result, the heat absorption amount (Q2) and the latent heat amount (Q3) (heat capacity) also stepwise decrease at time t1. 【0068】 The inlet temperature of the reformer 25 in the comparative example monotonically decreases after time t1 by decreasing the heat supply amount (Q1) at time t1, but since the latent heat amount (Q3) also decreases, the decrease amount (gradient) of the inlet temperature is also gentler than in the case of this embodiment. 【0069】 At time t1 in the comparative example, the heat absorption amount (Q2) stepwise decreases from Q21 to Q22 (Q21 > Q22), and the latent heat amount (Q3) stepwise decreases from Q31 to Q32 (Q31 > Q32). As a result, the outlet temperature of the reformer 25 in the comparative example (the temperature of the reformed fuel supplied to the fuel cell stack 1) rapidly increases due to the non-uniform distribution of the flow rate of the mixed fuel of the raw fuel and water in the reformer 25 and reaches a peak at time t11 (t1 < t11 < t2). Thereafter, at time t12 (t11 < t12 < t2), as the distribution of the flow rate of the mixed fuel in the reformer 25 stabilizes, the outlet temperature of the reformer 25 decreases gently and monotonically. 【0070】 Before and after the peak at time t2, the outlet temperature of the reformer is a temperature at which there is a risk that the CO concentration in the reformed fuel will be above the upper limit concentration. Therefore, reformed fuel with a high CO concentration is supplied to the fuel cell stack 1, and the fuel cell stack 1 is poisoned. 【0071】 On the other hand, in this embodiment, the flow rate of the raw fuel (m1) (and similarly the flow rate of water (m2)) is gradually reduced from time t1 to m11 and then to m12. As a result, the heat absorbed (Q2) and latent heat (Q3) (heat capacity) of the mixed fuel of the raw fuel and water also gradually decrease from time t1. 【0072】 In this embodiment, the inlet temperature of the reformer 25 decreases monotonically after time t1 by reducing the heat supply amount (Q1). However, since the decrease in the latent heat amount (Q3) is smaller than in the comparative example, the decrease (slope) of the inlet temperature is also slightly steeper than in the comparative example. 【0073】 As described above, in this embodiment, the absorbed heat (Q2) and latent heat (Q3) gradually decrease from time t1. As a result, the outlet temperature of the reformer 25 in this embodiment (temperature of the reformed fuel supplied to the fuel cell stack 1) does not rise sharply to the set concentration (upper limit concentration) as in the comparative example, because the uniformity of the flow rate distribution of the mixed fuel of raw fuel and water in the reformer 25 is relatively maintained, but rises slightly and then decreases monotonically. 【0074】 In this embodiment, when gradually reducing the raw fuel flow rate (m1) from the first flow rate (m11) to the second flow rate (m12), the temperature of the reformer 25 may be reduced by lowering the heat supply amount (Q1), and control may be started to gradually reduce the raw fuel flow rate (m1) after the temperature converges to a predetermined temperature (a temperature above the lower limit of the temperature at which the reformer 25 can reform the raw fuel). This lowers the temperature of the mixed fuel of raw fuel and water (reformed fuel) and reduces the CO concentration in the reformed fuel. 【0075】 [Second Embodiment] Figure 4 is a time chart of the fuel cell system of the second embodiment. The configuration of the fuel cell system of the second embodiment is the same as that of the first embodiment, but a control is added that increases the ratio of the water flow rate to the fuel flow rate when the flow rate of the raw fuel (m1) is reduced. 【0076】 Before time t1, the control unit 7 controls the flow rate ratio (SC) to SC1 (for example, SC = 1.5). At this time, the water flow rate (m2) is m2 = m1 × SC1. 【0077】 At time t1, the control unit 7 switches the flow rate ratio (SC) from SC1 to SC2 (SC = 2.0, SC1 < SC2). At this time, the water flow rate (m2) is m2 = m1 × SC2. 【0078】 At time t3, when the raw fuel flow rate (m1) reaches m12, the control unit 7 switches the flow rate ratio (SC) from SC2 to SC1. At this time, the water flow rate (m2) is m2 = m1 × SC1. 【0079】 In the second embodiment, the change in the heat absorption amount (Q2) is the same as that in the first embodiment. 【0080】 The latent heat amount (Q3) is Q31 before time t1, the same as in the first embodiment, but increases to Q311 (Q311 > Q31) at time t1, and then monotonically decreases to the value Q312 (Q311 > Q312) immediately before reaching time t2. 【0081】 At time t2, the latent heat amount (Q3) decreases from Q312 to Q32. Therefore, the latent heat amount (Q3) (solid line) from time t1 to time t2 is larger than the latent heat amount (Q3) (dashed line) of the first embodiment. 【0082】 As described above, due to the increase in the latent heat amount (Q3) at time t1, the inlet temperature and outlet temperature (solid line) of the reformer 25 after time t1 are lower than those in the case of the first embodiment (dashed line), so the CO concentration in the reformed fuel is lower than that in the first embodiment. Furthermore, since the flow rate ratio of the second embodiment from time t1 to time t2 is also lower than that of the first embodiment, the CO concentration of the reformed fuel is even lower than that of the first embodiment. 【0083】 [Third Embodiment] Figure 5 shows the control flow (S01-S05) of the fuel cell system of the third embodiment. Figure 6 shows the control flow (S06-S11) of the fuel cell system of the third embodiment. Figure 7 is a time chart of the fuel cell system of the third embodiment. 【0084】 In the third embodiment, a target flow rate is set when reducing the raw fuel flow rate from a first flow rate (m1) to a second flow rate (m2). The target flow rate is updated by decreasing it by a predetermined flow rate at each cycle of the control loop, thereby reducing the target flow rate from the first flow rate to the second flow rate. This predetermined flow rate is set to a step size obtained by dividing the difference obtained by subtracting the second flow rate from the first flow rate into equal parts (e.g., 100 equal parts). This allows the target flow rate to be set in a stepped manner between time t1 and time t2. Alternatively, the magnitude of the predetermined flow rate may be changed each time the target flow rate is updated so that the raw fuel flow rate (m1) that changes due to the update of the target flow rate between time t1 and time t2 changes in a stepped manner along the curve of the raw fuel flow rate (m1) shown in Figure 3. 【0085】 In step S01, the control unit 7 sets the target flow rate of the raw fuel to an initial value (first flow rate (m1)) when reducing the output of the fuel cell stack 1. 【0086】 In step S02, the control unit 7 reduces the amount of heat supplied (Q1) to the heating means (combustor 51, heater). 【0087】 In step S03, the control unit 7 updates the target flow rate (updated target flow rate = pre-update target flow rate - predetermined flow rate) and controls the pump 22 so that the flow rate of raw materials (m) becomes the updated target flow rate. It also controls the output of the pump 33 (water flow rate) so that the ratio of the water flow rate to the flow rate of raw materials (m) remains constant before and after updating the target flow rate. 【0088】 In step S04, the control unit 7 measures the temperature of the reformer 25 using the temperature sensor 26. Alternatively, the control unit 7 estimates the outlet temperature of the reformer 25 using, for example, C(Q1 / (m1+m2))(C: arbitrary constant), based on the amount of heat supplied to the reformer 25 (Q1), the flow rate of the raw fuel (m1), and the flow rate of water (m2) (flow rate of raw fuel (m1) × flow rate ratio (SC)). 【0089】 When the heating means for the reformer 25 is a combustor 51, the supplied heat quantity (Q1) can be simply estimated by using the supplied heat quantity (Q10) when the raw fuel flow rate (m1) is set to the first flow rate (m11) and the blower 41 is outputting at its rated value as a known value, and calculating Q1 = Q10 × (rated value / (current output of blower 41)) × (current flow rate of raw fuel (m1) / m11). When the heating means for the reformer 25 is a heater, the supplied heat quantity (Q1) can be simply estimated by using the supplied heat quantity (Q10) when the heater is outputting at its rated value as a known value, and calculating Q1 = Q10 × (current output of heater / rated value). 【0090】 In step S05, the control unit 7 estimates the CO concentration in the reformed fuel based on the flow rate ratio (SC) and the temperature of the reformer 25. At this time, the control unit 7 estimates the CO concentration in the reformed fuel by inputting the flow rate ratio (SC) information and the temperature of the reformer 25 information into the characteristic curve (map) shown in Figure 2. 【0091】 In step S06, the control unit 7 determines whether the CO concentration in the reformed fuel is less than the set concentration (for example, 3%). If it is YES, it proceeds to step S09; otherwise, it proceeds to step S07. 【0092】 In step S07, the control unit 7 adds a correction flow rate to the target flow rate, or increases the flow rate ratio (SC) by a predetermined flow rate ratio. Here, the correction flow rate is a flow rate with an absolute value smaller than the predetermined flow rate. The control unit 7 also controls the output of the blower 41 so that the flow rate ratio (SC) remains constant before and after adding the correction flow rate to the target flow rate. The predetermined flow rate ratio is set to, for example, about 5%. 【0093】 In step S08, the control unit 7 calculates the ratio of the water flow rate (SC) to the flow rate of the raw fuel, and then proceeds to step S04. 【0094】 In step S09, the control unit 7 calculates the upper limit temperature of the reformer 25 (the upper limit temperature at which the CO concentration reaches the set concentration) based on the flow rate ratio (SC) and the set concentration. 【0095】 In step S10, the control unit 7 determines whether the temperature of the reformer 25 measured or estimated in step S04 is below the upper limit temperature calculated in step S09. If it is YES, it proceeds to step S11; otherwise, it proceeds to step S03. In step S10, the control unit 7 determines NO if there is a risk that the temperature of the reformer 25 will exceed the upper limit temperature in the future, causing the CO concentration in the reformed fuel to reach an upper limit concentration (e.g., 5%) higher than the set concentration (e.g., 3%). 【0096】 In step S11, the control unit 7 determines that the temperature of the reformer 25 will not reach the upper limit temperature in the future, nor will the CO concentration in the reformed fuel reach the set concentration (upper limit concentration), and sets the target flow rate of the raw fuel to the second flow rate (m12). 【0097】 In Figure 13, the flow rate of the raw fuel (m1) is shown with a dashed line for the first embodiment and with a solid line for the third embodiment. Figure 13 also shows the case where the flow rate ratio (SC) is increased in step S07. 【0098】 At time t1, the target flow rate (flow rate of raw fuel (m1)) gradually decreases from the first flow rate (m11) each time the target flow rate is updated, but the outlet temperature of the reformer 25 gradually increases from time t1, similar to the first embodiment. 【0099】 As described above, the target flow rate gradually decreases after time t1, and consequently the flow rate ratio gradually increases, so the upper temperature limit gradually increases. 【0100】 After time t1, at time t13 (t1 < t13 < t2), since the outlet temperature of the reformer 25 becomes lower than the upper and lower temperatures, it is determined that the temperature of the reformer 25 will not reach the upper limit temperature in the future and the set concentration (upper limit concentration) of CO in the reformed fuel will not be reached, and the target flow rate of the raw fuel is set to the second flow rate (m12). 【0101】 With the above control, since the flow rate (m1) of the raw fuel from time t13 to time t2 can be set lower than in the case of the first embodiment, the consumption of the raw fuel can be suppressed accordingly. 【0102】 In addition, when adding the correction flow rate to the target flow rate in step S07, there is almost no change in the above upper limit temperature. On the other hand, the decrease rate of the raw fuel becomes lower, and accordingly, the decrease amounts of the latent heat amount and the heat absorption amount also become smaller, and the temperature of the reformer 25 decreases faster. Therefore, when the temperature of the reformer 25 becomes lower than the upper limit temperature, the target flow rate (the flow rate (m1) of the raw fuel) can be set to the second flow rate (m12). 【0103】 [Effect of this embodiment] The control method of the fuel cell system of this embodiment is a control method of a fuel cell system including a reformer 25 that reforms raw fuel into reformed fuel, a fuel cell (fuel cell stack 1) that consumes the reformed fuel to generate electricity, and heating means (a combustor 51, a heater (not shown)) that heats the reformer 25. When reducing the output of the fuel cell (fuel cell stack 1), the heat supply amount (Q1) to the reformer 25 of the heating means (the combustor 51, the heater (not shown)) is reduced and the flow rate (m1) of the raw fuel to the reformer 25 is gradually reduced. 【0104】 With the above method, since the latent heat amount and the heat absorption amount are controlled in a form that suppresses a rapid decrease in the latent heat amount and the heat absorption amount of the raw fuel (and a rapid decrease in the latent heat amount of the reforming water), the temperature of the reformer 25 can be actively controlled. Thereby, it becomes possible to suppress an unexpected increase in the CO concentration in the reformed fuel by suppressing a rapid temperature rise of the reformed fuel supplied to the fuel cell (fuel cell stack 1), and poisoning and performance degradation of the fuel cell (fuel cell stack 1) can be suppressed. 【0105】 In this embodiment, water for reforming is supplied to the reformer 25, and the flow rate ratio (SC) of the water for reforming to the flow rate (m1) of the raw fuel is set to a predetermined first flow rate ratio (SC1). While the flow rate (m1) of the raw fuel is gradually reduced from a first flow rate (m11) to a second flow rate (m12) lower than the first flow rate (m11), the flow rate ratio (SC) is set to a second flow rate ratio (SC2) higher than the first flow rate ratio (SC1). When the flow rate (m1) of the raw fuel reaches the second flow rate (m12), the flow rate ratio (SC) is set to the first flow rate ratio (SC1). 【0106】 By increasing the flow rate ratio (SC) while keeping the raw fuel flow rate (m1) approximately constant, the latent heat increases. This reduces the outlet temperature of the reformer 25, i.e., the temperature of the reformed fuel supplied to the fuel cell stack 1, thereby lowering the CO concentration in the reformed fuel. Increasing the flow rate ratio (SC) also shifts the chemical equilibrium between the reformed fuel (hydrogen) and carbon monoxide towards the reformed fuel (hydrogen) side, further reducing the CO concentration. Therefore, poisoning and performance degradation of the fuel cell stack 1 can be efficiently suppressed. 【0107】 In this embodiment, water for reforming is supplied to the reformer 25, and the flow rate ratio (SC) of the water for reforming is set to a predetermined flow rate ratio with respect to the flow rate (m1) of the raw fuel. When a characteristic curve (Figure 2), which is represented in a coordinate space with the temperature of the reformer 25 (outlet temperature or internal temperature of the reformer 25) and the concentration of carbon monoxide in the reformed fuel as axes, changes such that the concentration of carbon monoxide (CO concentration) increases monotonically with an increase in the temperature of the reformer 25, and the concentration of carbon monoxide (CO concentration) decreases monotonically with an increase in the flow rate ratio (SC), the flow rate ratio (SC) is controlled so that the carbon monoxide (CO concentration) represented by the operating point on the characteristic curve, which is set based on the temperature of the reformer 25 and the flow rate ratio (SC), does not exceed a predetermined upper limit concentration (e.g., 5%). 【0108】 By using the above method, when gradually reducing the raw fuel flow rate (m1) from the first flow rate (m11) to the second flow rate (m12), the flow rate (m1) of the raw fuel is controlled precisely according to the temperature of the reformer 25, thereby shortening the time it takes for the raw fuel flow rate (m1) to change from the first flow rate to the second flow rate, and suppressing the increase in CO concentration in the reformed fuel. 【0109】 In this embodiment, the upper limit concentration is set to 5%. 【0110】 The above method can efficiently reduce the poisoning of fuel cell stack 1. 【0111】 In this embodiment, reforming water (water) is supplied to the reformer 25, and the flow rate ratio (SC) of the reforming water (water) to the flow rate (m1) of the raw fuel is set to a predetermined flow rate ratio (e.g., SC1). When the output of the fuel cell (fuel cell stack 1) is reduced, the amount of heat supplied (Q1) from the heating means (combustor 51, heater (not shown)) to the reformer 25 is reduced, and the flow rate (m1) of the raw fuel to the reformer 25 is gradually reduced from a first flow rate (m11) to a second flow rate (m12) which is lower than the first flow rate (m11). A target flow rate (raw fuel flow rate (m1)) is set, which gradually decreases from the first flow rate (m12) towards the second flow rate (m12). The temperature of the reformer 25 (outlet temperature, or internal temperature of the reformer 25) is measured or estimated. Based on the temperature of the reformer 25 and the flow rate ratio (SC), the concentration of carbon monoxide (CO concentration) in the reformed fuel is estimated. If the concentration of carbon monoxide (CO concentration) is lower than the upper limit concentration (e.g., 5%) and equal to or greater than the set concentration (e.g., 3%), a predetermined correction flow rate is added to the target flow rate (raw fuel flow rate (m1)) or the flow rate ratio (SC) is increased. 【0112】 By using the above method, when gradually reducing the flow rate (m1) of the raw fuel from the first flow rate (m11) to the second flow rate (m12), the flow rate (m1) of the raw fuel can be controlled without excess or deficiency according to the temperature of the reformer 25, and an unexpected rise in the temperature of the reformer 25 can be suppressed. 【0113】 In this embodiment, the upper limit temperature of the reformer 25 is calculated from the flow ratio (SC) after adding a correction flow rate to the target flow rate (flow rate of raw fuel (m1)) or after increasing the flow ratio (SC), and the set concentration. When the temperature of the reformer 25 falls below the upper limit temperature, the target flow rate is set to the second flow rate (m12). 【0114】 By using the method described above, the target flow rate can be set to the second flow rate while the target flow rate is gradually decreasing from the first flow rate to the second flow rate, thereby reducing the consumption of raw materials and fuels. 【0115】 In this embodiment, the heat supply amount (Q1) is reduced to lower the temperature of the reformer 25, and then the flow rate (m1) of raw fuel to the reformer 25 is gradually reduced. 【0116】 By using the method described above, the flow rate (m1) of raw fuel to the reformer 25 is gradually reduced after the overall temperature of the reformer 25 has decreased. Therefore, even if the temperature of the reformer 25 (outlet temperature) rises in some way, it is possible to reduce the likelihood of the CO concentration reaching the upper limit, thereby reducing the poisoning of the fuel cell stack 1. 【0117】 The fuel cell system of this embodiment includes a reformer 25 that reforms raw fuel into reformed fuel, a fuel cell (fuel cell stack 1) that generates electricity by consuming the reformed fuel, and a heating means (combustor 51, heater (not shown)) that heats the reformer 25, wherein when the output of the fuel cell (fuel cell stack 1) is reduced, the amount of heat supplied (Q1) from the heating means (combustor 51, heater (not shown)) to the reformer 25 is reduced, and the flow rate (m1) of raw fuel to the reformer 25 is gradually reduced. 【0118】 With the above configuration, the latent heat and heat absorption amounts are controlled in a way that suppresses a rapid decrease in the latent heat and heat absorption amounts of the raw fuel (and a rapid decrease in the latent heat of the reforming water), thereby enabling active control of the temperature of the reformer 25. This suppresses a rapid rise in the temperature of the outlet temperature of the reformer 25, i.e., the temperature of the reformed fuel supplied to the fuel cell (fuel cell stack 1), thereby suppressing an unintended increase in the CO concentration in the reformed fuel and preventing poisoning and performance degradation of the fuel cell (fuel cell stack 1). 【0119】 Although embodiments of the present invention have been described above, these embodiments represent only a part of the application examples of the present invention, and are not intended to limit the technical scope of the present invention to the specific configurations of the above embodiments. Furthermore, the above embodiments can be combined as appropriate. [Explanation of Symbols] 【0120】 1 fuel cell stack, 25 reformers, 51 combustors

Claims

[Claim 1] A control method for a fuel cell system comprising a reformer for reforming raw fuel into reformed fuel, a fuel cell for generating electricity by consuming the reformed fuel, and a heating means for heating the reformer, A control method for a fuel cell system, which reduces the amount of heat supplied by the heating means to the reformer and gradually reduces the flow rate of the raw fuel to the reformer when reducing the output of the fuel cell. [Claim 2] Water for reforming is supplied to the reformer. The ratio of the flow rate of the reforming water to the flow rate of the raw fuel is set to a predetermined first flow rate ratio. While gradually reducing the flow rate of the raw fuel from a first flow rate to a second flow rate lower than the first flow rate, the flow rate ratio is set to a second flow rate ratio higher than the first flow rate ratio. A control method for a fuel cell system according to claim 1, wherein the flow rate ratio is set to the first flow rate ratio when the flow rate of the raw fuel reaches the second flow rate. [Claim 3] Water for reforming is supplied to the reformer. The ratio of the flow rate of the reforming water to the flow rate of the raw fuel is set to a predetermined flow rate ratio. In a coordinate space with the temperature of the reformer and the concentration of carbon monoxide in the reformed fuel as axes, where the characteristic curve showing a monotonically increasing carbon monoxide concentration with respect to an increase in the temperature of the reformer changes such that the carbon monoxide concentration monotonically decreases with respect to an increase in the flow rate ratio, A control method for a fuel cell system according to claim 1, wherein the flow rate ratio is controlled at an operating point on the characteristic curve, which is set based on the temperature of the reformer and the flow rate ratio, so that the concentration of carbon monoxide represented by the operating point does not exceed a predetermined upper limit concentration. [Claim 4] The control method for a fuel cell system according to claim 3, wherein the upper limit concentration is set to 5%. [Claim 5] Water for reforming is supplied to the reformer. The ratio of the flow rate of the reforming water to the flow rate of the raw fuel is set to a predetermined flow rate ratio. When reducing the output of the fuel cell, the amount of heat supplied by the heating means to the reformer is reduced, and the flow rate of the raw fuel to the reformer is gradually reduced from a first flow rate to a second flow rate lower than the first flow rate, while setting a target flow rate that gradually decreases from the first flow rate to the second flow rate, The temperature of the reformer is measured or estimated, Based on the temperature of the reformer and the flow rate ratio, the concentration of carbon monoxide in the reformed fuel is estimated. A control method for a fuel cell system according to claim 1, wherein a predetermined correction flow rate is added to the target flow rate or the flow rate ratio is increased when the concentration of carbon monoxide is equal to or greater than a set concentration. [Claim 6] The upper limit temperature of the reformer is calculated from the flow rate ratio after adding the correction flow rate to the target flow rate or after increasing the flow rate ratio, and the set concentration. A control method for a fuel cell system according to claim 5, wherein the target flow rate is set to the second flow rate when the temperature of the reformer falls below the upper limit temperature. [Claim 7] A method for controlling a fuel cell system according to any one of claims 1 to 6, wherein the amount of heat supplied is reduced to lower the temperature of the reformer, and then the flow rate of the raw fuel to the reformer is gradually reduced. [Claim 8] A fuel cell system comprising a reformer for reforming raw fuel into reformed fuel, a fuel cell for generating electricity by consuming the reformed fuel, and a heating means for heating the reformer, A fuel cell system that reduces the amount of heat supplied by the heating means to the reformer and gradually reduces the flow rate of the raw fuel to the reformer when reducing the output of the fuel cell.

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