Control method and control device for fuel cell systems

Abstract

This system achieves both the suppression of fuel cell stack degradation and the maintenance of the reforming reaction temperature when the electrical load increases. [Solution] A control method for a fuel cell system comprising a heat exchanger for heating raw fuel, a reformer for reforming the heated raw fuel into fuel gas, a fuel cell stack for generating electricity using fuel gas and air, a combustor for mixing and burning cathode exhaust gas and anode exhaust gas discharged from the fuel cell stack, and a heat supply passage for supplying heat generated in the combustor to the heat exchanger and the reformer, characterized in that the amount of raw fuel is obtained, which is the amount of raw fuel contained in the input gas fed into the reformer, the reformer temperature is obtained, the input gas temperature is obtained, and the proportion of raw fuel in the input gas is controlled based on the amount of raw fuel, the reformer temperature and the input gas temperature.

Description

Technical Field

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

Background Art

[0002] A fuel cell system is known that includes a reformer and generates electricity using a fuel gas obtained by reforming a raw fuel such as methanol. As a reforming method, steam reforming is generally used, in which the raw fuel and steam are reacted through a reforming catalyst in a high-temperature environment to produce hydrogen as a fuel gas. In the case of this steam reforming, since the reforming reaction is an endothermic reaction, it is necessary to maintain the reaction temperature by supplying heat from the outside in order to maintain the reforming reaction. For example, a burner (combustor) that burns the anode off-gas discharged from the fuel cell is provided, and the heat generated by the burner is used to maintain the reaction temperature.

[0003] By the way, when the electrical load increases from the state where the fuel cell is operating steadily, the consumption amount of the fuel gas inside the fuel cell increases by the increased amount of the electrical load. Therefore, the fuel supply amount to the burner decreases, and the calorific value generated by the burner decreases. In addition, since the reformer has a large heat capacity, there is a delay until the temperature of the reformer rises after increasing the calorific value generated by the burner. Due to these factors, when the electrical load increases from steady operation, the temperature of the reformer may decrease, and it may become impossible to maintain the reaction temperature.

[0004] Control for solving this problem is disclosed in Patent Document 1. Patent Document 1 discloses a fuel cell system including a vaporizer that generates raw fuel vapor, a reformer, and a burner, and supplies the heat generated by the burner to the vaporizer and the reformer. In the control described in the above document, when the electrical load increases, the outlet temperature of the vaporizer is detected, and based on the detected temperature, the fuel supply amount supplied to the burner is controlled, thereby controlling the energy supplied to the vaporizer and the reformer. Specifically, by increasing the amount of heat supplied to the vaporizer, the temperature of the gas (that is, raw fuel vapor) introduced into the reformer is increased.

Prior Art Documents

[0005] [Patent Document 1] Japanese Patent Publication No. 63-48773 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] However, with the control described in the above-mentioned literature, the temperature of the raw fuel vapor increases as the amount of heat required to sustain the reforming reaction increases. And, the higher the temperature of the raw fuel vapor, the higher the carbon monoxide concentration in the reformed gas. Since carbon monoxide is a factor that degrades the fuel cell stack, the control described in the above-mentioned literature may accelerate the degradation of the fuel cell stack.

[0007] Therefore, the present invention aims to provide a control method that can suppress the deterioration of the fuel cell stack and maintain the reaction temperature of the reforming reaction when the electrical load increases. [Means for solving the problem]

[0008] According to one aspect of the present invention, a control method for a fuel cell system is provided, comprising: a heat exchanger for heating raw fuel; a reformer for reforming the heated raw fuel into fuel gas; a fuel cell stack for generating electricity using the fuel gas and air; a combustor for mixing and burning cathode exhaust gas and anode exhaust gas discharged from the fuel cell stack; and a heat supply passage for supplying heat generated in the combustor to the heat exchanger and the reformer. This method includes obtaining the raw fuel amount, which is the amount of raw fuel contained in the input gas fed into the reformer; obtaining the reformer temperature, which is the temperature of the reformer; obtaining the input gas temperature, which is the temperature of the input gas; and controlling the proportion of raw fuel in the input gas based on the raw fuel amount, the reformer temperature, and the input gas temperature. [Effects of the Invention]

[0009] According to the above embodiment, it is possible to suppress the deterioration of the fuel cell stack while maintaining the reaction temperature of the reforming reaction even when the electrical load increases. [Brief explanation of the drawing]

[0010] [Figure 1] Figure 1 is a schematic block diagram of a fuel cell. [Figure 2] Figure 2 shows the relationship between the ratio of raw fuel to water and the input gas temperature when the amount of input gas is increased in response to an increase in electrical load. [Figure 3] Figure 3 shows an example of a time chart for a transition from steady-state operation to steady-state operation after an increase in electrical load, followed by transient operation. [Figure 4] Figure 4 shows the relationship between the ratio of raw fuel, water, and recycled gas and the input gas temperature when the amount of input gas is increased in response to an increase in the electrical load. [Modes for carrying out the invention]

[0011] Embodiments of the present invention will be described below with reference to the drawings.

[0012] [First Embodiment] [Fuel cell] The configuration of the fuel cell controlled by the control method according to this embodiment will now be described. Figure 1 is a schematic block diagram showing the fuel cell 1 according to this embodiment. The fuel cell 1 comprises a reformer 9 that reforms the raw fuel into a gas containing hydrogen (fuel gas), a fuel cell stack 3 that generates electricity using the fuel gas and air, and a combustor 13 that burns the off-gases discharged from the anode and cathode of the fuel cell stack 3, respectively.

[0013] The fuel cell stack 3 comprises an anode, a cathode, and an electrolyte layer (not shown). During operation, fuel gas is supplied to the anode and an oxidizer is supplied to the cathode. In this embodiment, air is used as the oxidizer, and a hydrocarbon fuel such as methane is used as the raw fuel.

[0014] The raw fuel is supplied from the raw fuel tank 4 to the desulfurizer 6 via the booster 5. After the sulfur content is removed in the desulfurizer 6, it is supplied to the first heat exchanger 8 along with water. The water is supplied from the water supply unit 7, which includes a water tank, valves, and a water pump. The first heat exchanger 8 uses the heat supplied from the combustor 13 to raise the temperature of the raw fuel and water to produce raw fuel steam containing water vapor (hereinafter also referred to as input gas). The reformer 9 uses a reforming catalyst to react the raw fuel and water vapor in the input gas to produce fuel gas containing hydrogen, which is then supplied to the anode of the fuel cell stack 3.

[0015] Meanwhile, the air supplied to the cathode of the fuel cell stack 3 is taken in from the atmosphere via the first blower 10, heated in the second heat exchanger 11, and then supplied to the cathode.

[0016] The fuel cell stack 3 generates electricity by reacting fuel gas with air. The generated electricity is supplied to electrical equipment via the inverter 12. The electrical equipment includes, for example, a drive motor when the fuel cell 1 is installed in an electric vehicle.

[0017] The combustor 13 burns the cathode-off gas and anode-off gas discharged from the fuel cell stack 3. The gas whose temperature has risen due to combustion (combusted gas) is supplied to the first heat exchanger 8 via the heat supply passage 16, and after passing through the first heat exchanger 8, it is supplied to the second heat exchanger 11, and then discharged into the atmosphere. The heat supply passage 16 is also arranged to exchange heat with the reformer 9. In other words, the first heat exchanger 8, the reformer 9, and the second heat exchanger 11 are heated by the heat of the combusted gas.

[0018] The fuel cell 1 is equipped with an off-gas recycling passage 17 that recirculates a portion of the anode off-gas to the upstream side of the first heat exchanger 8. The off-gas recycling passage 17 branches off from between the anode and the combustor 13 of the fuel cell stack 3 and merges into the flow path between the desulfurizer 6 and the first heat exchanger 8. A flow control valve 14 and a second blower 15 are interposed in the off-gas recycling passage 17.

[0019] Also, a first temperature sensor 18 as an input gas temperature acquisition unit for detecting the temperature of the input gas is provided in the flow path between the first heat exchanger 8 and the reformer 9. Further, a second temperature sensor 19 as a reformer temperature acquisition unit for detecting the temperature of the reformer 9 is provided in the reformer 9. Detection signals of these first temperature sensor 18 and second temperature sensor 19 are read into the control device 2. The control device 2 controls the fuel cell 1 based on these temperatures and various other signals. In particular, the control device 2 includes a raw fuel amount acquisition unit for acquiring the amount of raw fuel, which is the amount of raw fuel contained in the input gas introduced into the reformer 9, and a raw fuel amount control unit for controlling the ratio of the raw fuel in the input gas based on the raw fuel amount, the reformer temperature, and the input gas temperature. Note that since the control device 2 itself determines the raw fuel amount according to the electric load, the value can be acquired by reading it.

[0020] Note that the control device 2 is a computer. The control device 2 realizes its functions by a control program stored in a storage device such as a ROM in advance being executed by an arithmetic device such as a CPU. In this specification, the fuel cell 1 and the control device 2 may be collectively referred to as a fuel cell system. [Input Gas Control]

[0021] Next, the input gas control executed by the control device 2 will be described. Here, the case where the fuel cell system is used as a power source of an electric vehicle will be described.

[0022] The control device 2 controls the supply of raw fuel and water, as well as the supply of air, according to the electrical load such as the drive motor, to cause the fuel cell 1 to generate electricity. In other words, if the electrical load increases, the supply of raw fuel is also increased accordingly. However, as mentioned above, when the electrical load increases, it is necessary to suppress the decrease in the reaction temperature inside the reformer 9. Furthermore, if the temperature of the input gas becomes excessively high, the carbon monoxide concentration in the fuel gas increases, leading to deterioration of the fuel cell stack 3. In addition, if the temperature of the reformer 9 becomes excessively high due to the heat from the combustor 13 directly raising the temperature of the reformer 9, or by introducing high-temperature fuel gas, it becomes difficult to design the reformer 9 to be heat-resistant.

[0023] To solve these problems, this embodiment implements input gas control that suppresses the temperature rise of the fuel gas while supplying the thermal energy necessary for the reforming reaction to the reformer 9. Specifically, it is as follows.

[0024] The input gas is the gas fed into the reformer 9 and contains raw fuel and water. The control device 2 controls the supply amount of raw fuel and the supply amount of water so that the ratio of raw fuel to water is predetermined according to the magnitude of the electrical load. In this embodiment, the predetermined ratio is expressed as S / C, which is the ratio of moles of water vapor to moles of carbon contained in the raw fuel, and when the fuel cell 1 is operating at a constant power generation rate (also called steady-state operation), for example, S / C = 1.5.

[0025] In contrast, during transient operation when the electrical load increases and the amount of power generated increases accordingly, the control device 2 increases the amount of raw fuel supplied and the amount of water supplied in accordance with the increase in the amount of power generated, and increases the S / C ratio by making the rate of increase of the amount of steam greater than the rate of increase of the raw fuel supplied. In other words, the proportion of raw fuel is reduced compared to steady-state operation, for example, to S / C = 2.0.

[0026] Figure 2 shows the relationship between S / C and input gas temperature when the input gas amount is increased in response to an increase in electrical load. The horizontal axis of the figure represents the ratio of the amount of substance, and the vertical axis represents the input gas temperature. That is, the area of ​​the hatched portion in the figure can be considered to be the thermal energy of the input gas, which is the sum of the thermal energy originally contained in the raw fuel and water, and the thermal energy supplied from the combustor 13.

[0027] When S / C = 1.5, the same as during steady-state operation, the input gas temperature rises to Tsg2, whereas when S / C = 2.0, the input gas temperature is kept lower at Tsg1, which is lower than Tsg2. This is because increasing the amount of water for the same amount of raw fuel increases the S / C ratio, which increases the heat capacity of the input gas. In the case of S / C = 2.0, the increased water absorbs the thermal energy equivalent to the difference between input gas temperatures Tsg1 and Tsg2 in the case of S / C = 1.5 (the area enclosed by the dashed line on the left side of the figure).

[0028] As described above, when increasing the amount of raw fuel in response to an increase in electrical load, even if the increase in raw fuel and the thermal energy supplied from the combustor 13 are the same, a larger S / C ratio results in a lower input gas temperature than a smaller S / C ratio. In other words, as in this embodiment, by increasing the S / C ratio during transient operation, the temperature of the reformer 9 can be increased while suppressing the input gas temperature, thereby maintaining the temperature necessary for the reforming reaction.

[0029] In the above explanation, the S / C ratio during transient operation was assumed to be 2.0, but this is just an example. In practice, the amount of steam (water supply) is determined based on the temperature of the reformer 9, the input gas temperature, and the amount of raw fuel, so that the input gas temperature is above the reforming reaction temperature and below the heat resistance temperature of the reformer 9. That is, the more steam you have, the lower the input gas temperature can be, but if it is too low, the reforming reaction will not occur easily, so an upper limit is determined for the amount of steam to ensure that the reforming reaction occurs. On the other hand, if the amount of steam is too low, the effect of suppressing the input gas temperature is small and there is a risk of exceeding the heat resistance temperature of the reformer 9, so a lower limit is determined for the amount of steam to prevent exceeding the heat resistance temperature of the reformer 9. The method for determining the amount of steam may be feedback control using detected values ​​of the input gas temperature and the reformer temperature, or it may be a method using a map that has been created in advance.

[0030] Figure 3 is an example of a time chart for the transition from steady-state operation to steady-state operation after an increase in electrical load, followed by transient operation. The dashed line in the figure indicates the reformer temperature. The solid line in the figure shows the supply gas temperature when the control according to this embodiment (S / C=2.0) is executed to achieve the above reformer temperature, and the dashed line shows the supply gas temperature when the control of the comparative example is executed to achieve the above reformer temperature. During steady-state operation, S / C=1.5 is used in both this embodiment and the comparative example. The control of the comparative example remains at S / C=1.5 even during transient operation.

[0031] If the electrical load increases at timing T1 during steady-state operation, transient operation control, which is a control for transient operation, is started. As the amount of raw fuel increases in response to the increase in electrical load, the input gas temperature rises. At this time, according to the control of this embodiment, the input gas temperature is kept below the heat resistance temperature Tup of the reformer 9, but in the comparative example, it exceeds the heat resistance temperature Tup.

[0032] The input gas temperature initially rises, then begins to decrease due to the increased heat absorption associated with the increase in the reforming reaction. Transient control is terminated at timing T2 when the reformer temperature rises to a predetermined temperature. The predetermined temperature is, for example, the temperature at which the input gas temperature falls below the heat resistance temperature Tup when S / C = 1.5, and can be determined by mapping the relationship between the reformer temperature 9, the input gas temperature, and the amount of raw fuel.

[0033] When transient control is terminated at timing T2, both the control of this embodiment and the S / C ratio of the input gas become 1.5. However, as described above, even with S / C = 1.5 at timing T2, the input gas temperature remains below the heat resistance temperature Tup. After the transient operation control is terminated, the input gas temperature decreases, similar to the control in the comparative example.

[0034] As described above, this embodiment provides a control method for a fuel cell system comprising: a first heat exchanger 8 for raising the temperature of raw fuel; a reformer 9 for reforming the heated raw fuel into fuel gas; a fuel cell stack 3 for generating electricity using fuel gas and air; a combustor 13 for mixing and burning cathode exhaust gas and anode exhaust gas discharged from the fuel cell stack 3; and a heat supply passage 16 for supplying the heat generated in the combustor 13 to the first heat exchanger 8 and the reformer 9. This method includes obtaining the amount of raw fuel, which is the amount of raw fuel contained in the input gas fed into the reformer 9; obtaining the reformer temperature, which is the temperature of the reformer 9; obtaining the input gas temperature, which is the temperature of the input gas; and controlling the proportion of raw fuel in the input gas based on the amount of raw fuel, the reformer temperature, and the input gas temperature. This makes it possible to secure the thermal energy necessary for the reforming reaction and control the input gas temperature at the same time. In other words, it is possible to suppress the deterioration of the fuel cell stack and maintain the reaction temperature of the reforming reaction when the electrical load increases at the same time.

[0035] In this embodiment, when an increase in the amount of raw fuel is detected due to an increase in the electrical load, the proportion of raw fuel in the input gas is reduced based on the amount of raw fuel, the reformer temperature, and the input gas temperature. This allows the input gas temperature to be kept low while securing the thermal energy necessary for the reforming reaction by increasing the amount of gas other than raw fuel during transient operation when the amount of power generation is increased in response to an increase in the electrical load. As a result, the reformer 9 can be kept at an appropriate temperature, thereby suppressing the generation of hydrocarbons. In addition, as the input gas temperature decreases, the temperature difference with the heat supplied from the combustor 13 increases, and the heat exchange efficiency increases, allowing the first heat exchanger 8 to be miniaturized. Furthermore, as the input gas temperature decreases, the heat resistance performance required of the reformer 9 decreases, thus reducing the cost of the reformer 9.

[0036] In this embodiment, the fuel cell system further includes a water supply unit 7 that supplies water to the raw fuel or raw fuel vapor, and the proportion of raw fuel is reduced by increasing the amount of water supplied from the water supply unit 7. As a result, the S / C ratio increases, making it less likely for carbon to precipitate.

[0037] In this embodiment, the input gas temperature is maintained above the reforming reaction temperature of the raw fuel by setting an upper limit on the water supply amount. This allows the reforming reaction to continue even during transient operation.

[0038] In this embodiment, the reformer 9 is kept below its heat resistance temperature by setting a lower limit on the water supply. This helps to suppress deterioration of the reformer 9 and improve its reliability.

[0039] [Second Embodiment] Next, a second embodiment will be described. The configuration of the fuel cell system is the same in this embodiment and the first embodiment, but the input gas control, particularly the transient operation control, differs.

[0040] In the transient operation control of the first embodiment, the proportion of raw fuel in the input gas is increased compared to normal operation by increasing the water supply amount. However, in the transient operation control of this embodiment, as shown in Figure 4, the S / C ratio remains the same as in normal operation, while the amount of recycled gas introduced is increased. The upper and lower limits of the recycled gas introduced amount are the same as the amount of water vapor in the first embodiment, and are determined so that the input gas temperature is above the reforming temperature and below the heat resistance temperature of the reformer 9.

[0041] Recycled gas is the gas emitted from the fuel cell stack 3, whose temperature has risen due to the exothermic reaction of power generation, and therefore its temperature is higher than that of water. For this reason, the method of this embodiment, which increases the amount of recycled gas introduced, can suppress heat loss compared to the method of the first embodiment, which increases the amount of water supplied.

[0042] Furthermore, as can be seen from the fact that the stack-off gas used for combustion in the combustor 13 is discharged after passing through the first heat exchanger 8 and the second heat exchanger 11, there is ample capacity for the stack-off gas. Therefore, even if the flow rate of the off-gas recycling passage is increased for control purposes in this embodiment, the effect on the heat output in the combustor 13 is negligible.

[0043] As described above, according to this embodiment, the fuel cell system is further equipped with an off-gas recycling passage 17 that supplies a portion of the anode exhaust gas to the raw fuel steam, and the proportion of raw fuel is reduced by increasing the amount of anode exhaust gas supplied from the off-gas recycling passage 17. This makes it possible to secure the thermal energy necessary for the reforming reaction and control the input gas temperature at the same time as in the first embodiment. Furthermore, since high-temperature anode exhaust gas supplied from the combustor 13 is used, heat loss can be reduced compared to when water is used.

[0044] In this embodiment, the input gas temperature is maintained above the reforming reaction temperature of the raw fuel by setting an upper limit on the anode exhaust gas supply amount. This allows the reforming reaction to continue even during transient operation.

[0045] In this embodiment, the reformer 9 is kept below its heat resistance temperature by setting a lower limit on the anode exhaust gas supply amount. This helps to suppress deterioration of the reformer 9 and improve its reliability.

[0046] Although embodiments of the present invention have been described above, these embodiments only represent a part of the application examples of the present invention, and are not intended to limit the technical scope of the present invention to the specific configurations of the above embodiments. [Explanation of symbols]

[0047] 1: Fuel cell, 2: Control device, 3: Fuel cell stack, 9: Reformer, 13: Combustor, 18: First temperature sensor, 19: Second temperature sensor

Claims

1. A heat exchanger that raises the temperature of the raw fuel, A reformer for converting the heated raw fuel into fuel gas, A fuel cell stack that generates electricity using the aforementioned fuel gas and air, A combustor that mixes and burns the cathode exhaust gas and anode exhaust gas discharged from the fuel cell stack, A heat supply passage that supplies the heat generated in the combustor to the heat exchanger and the reformer, In a control method for a fuel cell system comprising, The amount of raw fuel, which is the amount of raw fuel contained in the input gas fed into the reformer, is obtained. The reformer temperature, which is the temperature of the reformer, is obtained. The input gas temperature, which is the temperature of the input gas, is obtained. A control method for a fuel cell system, characterized by controlling the proportion of the raw fuel in the input gas based on the amount of raw fuel, the reformer temperature, and the input gas temperature.

2. In the control method for a fuel cell system according to claim 1, A control method for a fuel cell system, which, when an increase in the amount of raw fuel is detected due to an increase in the electrical load, reduces the proportion of the raw fuel in the input gas based on the amount of raw fuel, the reformer temperature, and the input gas temperature.

3. In the control method for a fuel cell system according to claim 2, The fuel cell system further includes a water supply unit that supplies water to the raw fuel or raw fuel vapor, A control method for a fuel cell system, which reduces the proportion of raw fuel by increasing the amount of water supplied from the water supply unit.

4. In the control method for a fuel cell system according to claim 3, A control method for a fuel cell system, wherein the input gas temperature is maintained above the reforming reaction temperature of the raw fuel by setting an upper limit on the amount of water supplied.

5. In the control method for a fuel cell system according to claim 3, A control method for a fuel cell system, wherein a lower limit is set on the amount of water supplied, thereby maintaining the reformer below its heat resistance temperature.

6. In the control method for a fuel cell system according to claim 2, The fuel cell system further includes an off-gas recycling passage that supplies a portion of the anode exhaust gas to the raw fuel steam. A control method for a fuel cell system, which reduces the proportion of raw fuel by increasing the amount of anode exhaust gas supplied from the off-gas recycling passage.

7. In the control method for a fuel cell system according to claim 6, A control method for a fuel cell system, wherein the input gas temperature is maintained above the reforming reaction temperature of the raw fuel by setting an upper limit on the anode exhaust gas supply amount.

8. In the control method for a fuel cell system according to claim 6, A control method for a fuel cell system, wherein a lower limit is set on the anode exhaust gas supply amount to maintain the reformer below its heat resistance temperature.

9. A heat exchanger that raises the temperature of the raw fuel, A reformer for converting the heated raw fuel into fuel gas, A fuel cell stack that generates electricity using the aforementioned fuel gas and air, A combustor that mixes and burns the cathode exhaust gas and anode exhaust gas discharged from the fuel cell stack, A heat supply passage that supplies the heat generated in the combustor to the heat exchanger and the reformer, In a control device for a fuel cell system, A raw fuel quantity acquisition unit acquires the raw fuel quantity, which is the amount of raw fuel contained in the input gas fed into the reformer. A reformer temperature acquisition unit acquires the reformer temperature, which is the temperature of the reformer, An input gas temperature acquisition unit acquires the input gas temperature, which is the temperature of the input gas, A fuel quantity control unit controls the proportion of the fuel in the input gas based on the amount of raw fuel, the reformer temperature, and the input gas temperature. A control method for a fuel cell system, characterized by comprising:

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