Control method for fuel cell system and control device for fuel cell system

Abstract

This control method for a fuel cell system is for controlling a fuel cell system that includes a fuel cell, a reformer, and a combustor, and in the control method for a fuel cell system: oxidant gas thermal energy is estimated; reference fuel gas thermal energy which is thermal energy of the fuel off-gas when a reference fuel gas is used is estimated; reference exhaust gas thermal energy is calculated by adding the oxidant gas thermal energy and the reference fuel gas thermal energy; actual exhaust gas thermal energy, which is the actual total thermal energy of the gas supplied to the combustor, is estimated on the basis of the actual temperature of combustion gas; the difference in the heat quantity between the actual fuel gas, which is the actually used fuel gas, and the reference fuel gas is estimated on the basis of the difference between the reference exhaust gas thermal energy and the actual exhaust gas thermal energy; the hydrogen equivalent amount is calculated using the difference in the heat quantity and the relationship between the heat quantity of a hydrocarbon fuel contained in the fuel gas and the amount of hydrogen produced by reforming; and the flow rate of the fuel gas supplied to the fuel cell is controlled on the basis of the hydrogen equivalent amount.

Description

Fuel cell system control method and fuel cell system control device

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

[0002] The use of city gas as fuel for fuel cells has been proposed for some time. When city gas is used as fuel for fuel cells, it is reformed using a reformer or other device to produce hydrogen. However, although there are regulations regarding the calorific value per unit volume of city gas, the components contained in the gas vary by country and region. Furthermore, methane gas for automobiles contains up to 5% carbon dioxide. As such, different city gas components result in different amounts of hydrogen produced by reforming. Therefore, unless control is performed according to the components of the city gas used, it may not be possible to supply the amount of hydrogen required for the fuel cell's required output.

[0003] To solve this problem, JP2016-111004A describes a control method for estimating the composition of fuel supplied during operation based on the fuel flow rate during operation and the fuel composition that served as the basis for setting a target temperature profile. Specifically, a fuel with a certain composition is set as a reference fuel, and the temperature profile when the reference fuel is used is set as a target temperature profile. The flow rate of the actual fuel is then adjusted so that the temperature profile of the actual fuel matches the target temperature profile, and the calorific value of the actual fuel is calculated under these conditions. The average carbon is then calculated using the calculated calorific value and the relationship between the average carbon number of the fuel and the calorific value, and the actual fuel composition is estimated from the calculated average carbon number. The relationship between the average carbon number and the calorific value is an approximate straight line created by plotting city gas from different countries or regions on a graph with the calorific value on the vertical axis and the average carbon number on the horizontal axis.

[0004] However, methane gas components for automobiles may contain unburned carbon-containing components such as carbon dioxide. Even if the average carbon number is the same, the calorific value differs if carbon dioxide is included. This reduces the accuracy of the estimation method using the approximation line in the above document. This means that the amount of hydrogen required for the fuel cell's required output may not be supplied.

[0005] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a control method that can accurately estimate the amount of hydrogen required to operate a fuel cell and supply an amount of hydrogen that corresponds to the required output of the fuel cell.

[0006] According to one aspect of the present invention, there is provided a fuel cell system control method for controlling a fuel cell system including a fuel cell, a reformer that generates hydrogen from a fuel gas supplied to the fuel cell, and a combustor that generates combustion gas by combusting an oxidant off-gas discharged from a cathode of the fuel cell and a fuel off-gas discharged from an anode of the fuel cell. In this method, an oxidant gas thermal energy, which is the thermal energy of the oxidant off-gas supplied to the combustor, is estimated; a reference fuel gas thermal energy, which is the thermal energy of the fuel off-gas when a reference fuel gas of a known composition is used, is estimated; the oxidant gas thermal energy and the reference fuel gas thermal energy are added together to calculate a reference exhaust gas thermal energy, which is the total thermal energy of the gas supplied to the combustor when the reference fuel gas is used; an actual exhaust gas thermal energy, which is the actual total thermal energy of the gas supplied to the combustor, is estimated based on the actual combustion gas temperature; a difference in heating value between the actual fuel gas, which is the fuel gas actually used, and the reference fuel gas is estimated based on the difference between the reference exhaust gas thermal energy and the actual exhaust gas thermal energy; and a hydrogen equivalent, which is the amount of hydrogen produced when one mole of the actual fuel is reformed, is calculated using the difference in heating value and the relationship between the heating value of the hydrocarbon fuel contained in the fuel gas and the amount of hydrogen produced by reforming, and the flow rate of the fuel gas supplied to the fuel cell is controlled based on the hydrogen equivalent.

[0007] FIG. 1 is a schematic diagram of a fuel cell system. FIG. 2 is a diagram showing the relationship between the calorific value and carbon number of fuel gas. FIG. 3 is a diagram showing the relationship between the calorific value and hydrogen equivalent of fuel gas. FIG. 4 is a schematic diagram of the periphery of a combustor when a reference fuel gas is flowed. FIG. 5 is a schematic diagram of the periphery of a combustor when an actual fuel gas is flowed. FIG. 6 is a flowchart showing a control routine for fuel gas flow rate control.

[0008] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0009] 1 is a schematic diagram of a fuel cell system 100 to which control according to this embodiment is applied. The fuel cell system 100 includes a fuel cell (also referred to as a fuel cell stack) 1, a fuel supply mechanism 2 that supplies a mixed gas (hereinafter also referred to as a fuel gas) serving as fuel to the anode of the fuel cell stack 1, a blower 5 that supplies air to the cathode of the fuel cell stack 1, and a control unit 11 that controls the entire system. The fuel cell system 100 also includes a combustor 7 that generates combustion gas for heating air serving as an oxidant gas supplied to the fuel cell stack 1 at startup, a heat exchanger 8 that exchanges heat between the combustion gas generated in the combustor 7 and the air supplied from the blower 5, and an evaporator 20 that generates water vapor using the combustion gas generated in the combustor 7.

[0010] The combustor 7 is connected to a first flow path 17 to which the oxidant off-gas discharged from the fuel cell 1 is supplied, and a second flow path 18 to which the fuel off-gas is supplied. A first temperature sensor 12 and a first flow rate sensor 13 are provided in the first flow path 17. A second temperature sensor 14 and a second flow rate sensor 15 are provided in the second flow path 18.

[0011] The combustor 7 and the heat exchanger 8 are connected by a third flow path 19 , and a third temperature sensor 16 is provided in the third flow path 19 .

[0012] The steam generated in the evaporator 20 is subjected to so-called steam reforming.

[0013] The fuel cell stack 1 is a solid oxide fuel cell (SOFC) that has a catalyst on the anode side that reforms fuel gas and can generate reformed gas (anode gas) containing hydrogen by reforming the fuel gas. The fuel cell stack 1 generates electricity through an electrochemical reaction between the anode gas (hydrogen) and the cathode gas (oxygen) via an electrolyte. The reformer may be separate from the SOFC.

[0014] The fuel supply mechanism 2 includes a fuel tank 3, a main injector 4 that supplies fuel to the fuel cell stack 1, and a startup injector 6 that supplies fuel to a combustor 7. Note that a pressure reducing valve and a pressure sensor (not shown) are installed in the gas flow paths between the fuel tank 3 and the main injector 4 and startup injector 6.

[0015] The fuel tank 3 stores fuel gas at a predetermined pressure. City gas is used as the fuel gas.

[0016] The main injector 4 receives a signal from the control unit 11 and adjusts the amount of fuel gas supplied.

[0017] The flow path of the air supplied from the blower 5 includes a flow path that passes through the heat exchanger 8 and flows into the fuel cell stack 1, as well as a bypass flow path 10 that bypasses the heat exchanger 8 and flows into the fuel cell stack 1. A bypass valve 9 is installed in the bypass flow path 10 to adjust the flow rate of air flowing through that flow path.

[0018] The control unit 11 controls the operation of the fuel cell stack 1. The control unit 11 is composed of a microcomputer equipped with a central processing unit (CPU), a read-only memory (ROM), a random access memory (RAM), and an input / output interface (I / O interface). The control unit 11 can also be composed of multiple microcomputers.

[0019] City gas used as fuel gas may contain multiple types of saturated chain hydrocarbons, but its composition is not specified; rather, it is specified by the calorific value per unit volume. Therefore, when the fuel cell system 100 is installed in a vehicle, the composition of the fuel gas in the fuel tank 3 may change before and after filling due to fuel gas being filled in different regions. The amount of hydrogen produced by the reforming reaction varies depending on the fuel composition. In other words, even if the fuel flow rate is constant, if the fuel composition changes, the amount of hydrogen supplied to the fuel cell stack 1 also changes, resulting in fluctuations in the output of the fuel cell stack 1. Furthermore, if the amount of hydrogen changes in the direction of a shortage, this may lead to deterioration of the fuel cell stack 1.

[0020] Therefore, in order to supply an amount of hydrogen corresponding to the required output to the fuel cell stack 1, it is necessary to accurately estimate the amount of hydrogen to be generated from the fuel gas in the fuel tank 3.

[0021] FIG. 2 is a diagram showing the relationship between the calorific value and the carbon number of a fuel gas, which is used in the method for specifying the composition of a fuel gas disclosed in the above-mentioned JP2016-111004A. ｎ H （２ｎ+２） Assuming that the calorific value is proportional to the value of n (average carbon number), an approximate straight line can be drawn as shown in the figure. In the above document, this relationship is used to estimate the average carbon number of the fuel gas from the calorific value of the actual fuel gas, and the composition of the fuel gas is estimated from the average carbon number.

[0022] However, as mentioned above, methane gas components for automobiles contain carbon dioxide (CO ２ ) may contain unburned components containing carbon. ４ is 100%, and the approximate line is drawn based on this assumption. ２ With 5% of CH ４ When the ratio reaches 95%, the amount of heat generated decreases due to the inclusion of unburned components, and the value deviates from the approximate straight line in the figure.

[0023] In other words, estimation based on the relationship between the calorific value and the average carbon number shown in Figure 2 cannot accurately estimate the fuel composition, and there is a problem in that it is not possible to supply an amount of hydrogen corresponding to the required output for the fuel cell stack 1.

[0024] The applicants conducted extensive research into the above-mentioned problem and found that there is a relationship between the calorific value of fuel gas and the amount of hydrogen generated by reforming, even when the fuel contains unburned components. Therefore, the amount of hydrogen generated when 1 mol of fuel is reformed is defined as the hydrogen equivalent, and the hydrogen equivalent is estimated from the calorific value to set the fuel gas flow rate. Below, we will explain how to estimate the hydrogen equivalent and how to set the fuel gas flow rate.

[0025] [Hydrogen Equivalent] Figure 3 is a diagram showing the relationship between the calorific value of fuel gas and hydrogen equivalent. In Figure 3, the vertical axis represents the hydrogen equivalent and the horizontal axis represents the calorific value. In this embodiment, the higher calorific value is used as the calorific value, but a similar approximation formula can be created for the lower calorific value.

[0026] The hydrogen equivalent is the amount of hydrogen generated when 1 mol of fuel is reformed as described above. For example, if the hydrocarbon fuel contained in the fuel gas is methane (CH ４ ), and when the reforming method is steam reforming, the ratio of hydrogen (H ２ ) is 4, so the hydrogen equivalent is 4.

[0027]

[0028] The hydrogen equivalent and the calorific value when each fuel gas is burned in the combustor 7 were calculated for various fuel gases, including not only fuels specified by the standards but also those containing carbon dioxide and hydrogen, and the calculation results were plotted on the graph in Figure 3. As a result, it was found that each fuel gas was aligned on a single approximate straight line, as shown in the figure.

[0029] In this embodiment, the relationship between the calorific value of the fuel gas and the hydrogen equivalent amount shown in FIG. 3 is determined in advance and stored in the control unit 11, and is used to calculate the hydrogen equivalent amount, which will be described below.

[0030] [Calculation of Hydrogen Equivalent Amount] In this embodiment, a fuel with a known composition is set as a reference fuel gas, and the reference exhaust gas thermal energy, which is the total thermal energy of the gas supplied to the combustor 7 when the reference fuel gas is flowed at a predetermined flow rate, is calculated and this is also stored in the control unit 11.

[0031] Then, based on the combustion gas temperature when actual fuel gas, which is the fuel gas actually used, is flowed, the actual exhaust gas thermal energy, which is the actual total thermal energy of the gas supplied to the combustor 7, is estimated, and the calorific value difference, which is the difference in calorific value between the reference fuel gas and the actual fuel gas, is estimated from the difference between the reference exhaust gas thermal energy and the actual exhaust gas thermal energy.

[0032] If the difference in calorific value is known, the calorific value of the actual fuel gas can be determined from the above-mentioned approximation line, and the hydrogen equivalent can be calculated from the calorific value.

[0033] 4 and 5 are diagrams schematically illustrating the configuration of the combustor 7 and its surroundings in the fuel cell system 100 shown in Fig. 1. Fig. 4 illustrates the case where a reference fuel gas is flowed, and Fig. 5 illustrates the case where an actual fuel gas is flowed. In this embodiment, the reference fuel gas is 13A as defined by the standard, and the actual fuel gas is G20 as defined by the standard.

[0034] If the gas temperature of the reference fuel gas is 25° C. and the combustion gas temperature is 800° C., the combustion equation of the reference fuel gas is expressed by equation (2).

[0035]

[0036] Qair is the thermal energy [kW] of air as an oxidant gas, Ffuel is the fuel flow rate [mol / s], H13A (25°C) is the calorific value of a reference fuel gas at 25°C, Fexh is the combustion gas flow rate [mol / s], and Hexh (x°C) is the combustion gas energy [kJ / mol] at x°C. In this embodiment, since it is assumed that all combustion gases are gaseous, the lower heating value is used as the calorific value of the fuel gas, but the higher heating value may also be used.

[0037] Qair can be calculated from the air flow rate and temperature. For example, if the air flow rate is Fair [mol] and the air temperature is Tair [K], the air enthalpy Hair [kJ / mol] can be expressed as a function f(Tair) of the air temperature. Since the air enthalpy can be obtained from a thermodynamic database, Qair can be calculated by multiplying the air enthalpy Hair by the air flow rate Fair.

[0038] The second term on the left side is the thermal energy of the fuel gas, and like the thermal energy of air, it can be determined by obtaining the enthalpy of the components contained in the fuel gas from a thermodynamic database. The same applies to the term on the right side (thermal energy of combustion gas). Since the components of the reference fuel gas are known, the specific energy (heat value) can be determined using the above method. For simplicity of explanation, the heat value of the reference fuel gas is assumed to be X [kW].

[0039] On the other hand, equation (3) holds true for the actual fuel gas. Here, it is assumed that the actual fuel gas is G20, and the combustion gas temperature when the actual fuel gas is flowed is 700° C. The fuel flow rate and air flow rate are the same as those for the reference fuel gas.

[0040]

[0041] HG20(25°C) in equation (3) is the calorific value of the actual fuel gas at 25°C.

[0042] When the fuel cell system 100 is actually operated, it is not known that the actual fuel is G20, so the calorific value of the actual fuel gas cannot be calculated using thermodynamic data. However, since most of the components of air are nitrogen and oxygen, if the air flow rate and fuel flow rate are the same as those for the reference fuel gas, the impact of differences in the fuel gas components on the composition of the combustion gas is small. Therefore, in this embodiment, the left side of equation (3) is calculated using the composition of the exhaust gas when the reference fuel gas is burned. The calorific value obtained as a result of the calculation is defined as Y [kW].

[0043] Subtracting equation (3) from equation (2) gives the difference in calorific value caused by the difference in fuel gas as X-Y [kW]. If the difference between the calorific value of the reference fuel gas and the above calorific value is known, the hydrogen equivalent of the actual fuel gas can be calculated from the approximation line in Figure 3. If the hydrogen equivalent is known, it becomes possible to estimate the amount of hydrogen that can be generated when the actual fuel gas is supplied to the fuel cell system 100.

[0044] [Fuel Gas Flow Rate] Once the hydrogen equivalent is known, the fuel flow rate required for power generation can be set from the preset fuel utilization rate and hydrogen equivalent rate. The fuel utilization rate should be set within a range that does not cause deterioration of the fuel cell 1, and generally, a range of 70% or more is considered preferable.

[0045] This makes it possible to avoid fuel shortage during power generation and suppress deterioration of the fuel cell 1. It also makes it possible to avoid excessive supply of fuel gas and suppress a decrease in system efficiency.

[0046] Incidentally, when operating the fuel cell system 100, it is also necessary to control the temperature of the combustion gas. If the temperature of the combustion gas is excessively high, deterioration of the combustor 7 will be accelerated, and if the temperature of the fuel gas is excessively low, there will be adverse effects such as a decrease in the amount of steam generated in the evaporator 20 that uses the combustion gas. As can be seen from equation (3), the temperature of the combustion gas can be controlled by the fuel flow rate. Therefore, the fuel flow rate is controlled as follows:

[0047] If the difference in calorific value is negative, that is, if the calorific value of the actual fuel gas is higher than that of the reference fuel gas, the temperature of the combustion gas will be higher than when the reference fuel gas is used. Therefore, in this case, the fuel flow rate is reduced from the fuel flow rate set on the assumption that the reference fuel gas will be used. On the other hand, if the difference in calorific value is positive, that is, if the calorific value of the actual fuel gas is lower than that of the reference fuel gas, the temperature of the combustion gas will be lower than when the reference fuel gas is used. Therefore, in this case, the fuel flow rate is increased from the fuel flow rate set on the assumption that the reference fuel gas will be used. Specifically, this can be determined from the following equation (4):

[0048]

[0049] In equation (4), Ttarget is the target temperature of the combustion gas (e.g., 800°C), Tin is the temperature of the fuel gas at the inlet of the combustor 7 (e.g., 25°C), and Δheat value is the difference in heat value between the reference fuel gas and the actual fuel gas. By controlling the fuel gas flow rate to the value calculated by this equation, the fuel gas flow rate for the reference fuel gas is corrected as described above. However, in either case, the temperature of the combustion gas is limited to a temperature below which deterioration of the combustor 7 is not accelerated.

[0050] Next, the amount of steam generated for reforming will be described.

[0051] In this embodiment, hydrogen is produced from hydrocarbon fuel by steam reforming as described above. At this time, an insufficient amount of steam will cause carbon to deposit, resulting in deterioration such as coking, so it is necessary to produce an appropriate amount of steam. While this depends on the type of reforming catalyst, it is generally desirable for the ratio of steam (Steam) to carbon (C) to be 2 or greater.

[0052] The amount of water vapor can be calculated using an approximation line determined from the relationship between the calorific value and the number of carbon atoms. The approximation line used here is the same as that shown in Figure 2. However, as mentioned above, methane gas for automobiles contains unburned components, which can cause the calorific value to deviate by up to 5%, so an approximation line corrected to take this deviation into account is used.

[0053] Since the calorific value of the fuel gas can be calculated as described above, the average carbon number can be calculated from the approximate line. The required amount of water vapor can then be calculated as the average carbon number and the flow rate of the fuel gas.

[0054] Next, a specific example of the above-mentioned fuel gas flow rate control will be described with reference to FIG.

[0055] 5 is a flowchart showing a control routine for fuel gas flow rate control executed by the first to sixth calculation units and the control unit 11 as a flow rate control unit. This control routine is preprogrammed in the control unit 11.

[0056] In step S100, the control unit 11 detects the combustion gas temperature by reading the value detected by the third temperature sensor 16.

[0057] In step S110, the control unit 11 estimates the difference in the amount of heat generated by the method described above.

[0058] In step S120, the control unit 11 determines whether the difference in the amount of heat generated is greater than zero, and if it is positive, the process of step S130 is executed, and if it is equal to or less than zero, the process of step S140 is executed.

[0059] In steps S130 and S140, the control unit 11 sets the fuel gas flow rate required for reforming. Specifically, in step S130, the control unit 11 sets the fuel gas amount reduced from the fuel gas flow rate for the reference fuel gas in accordance with the difference in calorific value, and in step S140, the control unit 11 sets the fuel gas amount increased from the fuel gas flow rate for the reference fuel gas in accordance with the difference in calorific value. The fuel gas flow rate set in step S130 or step S140 is set as the gas flow rate (i).

[0060] In step S150, the control unit 11 calculates the hydrogen equivalent of the actual fuel gas using the difference in the calorific value.

[0061] In step S160, the control unit 11 calculates the fuel gas flow rate required for power generation from the hydrogen equivalent amount and the fuel utilization rate, and the calculation result is set as the gas flow rate (ii).

[0062] In step S170, the control unit 11 determines whether the gas flow rate (i) is greater than the gas flow rate (ii), and if so, executes the process of step S180, otherwise executes the process of step S190.

[0063] In step S180, the control unit 11 sets the gas flow rate (i) as the fuel gas flow rate and supplies the fuel gas.

[0064] By performing the processes in steps S170 to S190, it is possible to satisfy both the fuel gas flow rate required for reforming and the fuel gas flow rate required for power generation.

[0065] As described above, in this embodiment, a fuel cell system control method is provided for controlling a fuel cell system including a fuel cell 1, a reformer that generates hydrogen from fuel gas supplied to the fuel cell 1, and a combustor 7 that generates combustion gas by combusting oxidant off-gas discharged from the cathode of the fuel cell 1 and fuel off-gas discharged from the anode of the fuel cell. In this method, the oxidant gas thermal energy, which is the thermal energy of the oxidant off-gas supplied to the combustor 7, is estimated; the reference fuel gas thermal energy, which is the thermal energy of the fuel off-gas when a reference fuel gas with a known composition is used, is estimated; the oxidant gas thermal energy and the reference fuel gas thermal energy are added to calculate the reference exhaust gas thermal energy, which is the total thermal energy of the gas supplied to the combustor 7 when the reference fuel gas is used; the actual exhaust gas thermal energy, which is the actual total thermal energy of the gas supplied to the combustor 7, is estimated based on the actual combustion gas temperature; the difference in heating value between the actual fuel gas (the fuel gas actually used) and the reference fuel gas is estimated based on the difference between the reference exhaust gas thermal energy and the actual exhaust gas thermal energy; the hydrogen equivalent, which is the amount of hydrogen produced when one mole of the actual fuel is reformed, is calculated using the difference in heating value, the heating value of the hydrocarbon fuel contained in the fuel gas, and the amount of hydrogen produced by reforming; and the flow rate of the fuel gas supplied to the fuel cell is controlled based on the hydrogen equivalent. This allows for accurate estimation of the amount of hydrogen produced when the actual fuel gas is supplied to the fuel cell 1. Since the fuel gas flow rate is controlled based on the amount of hydrogen, it is possible to supply the amount of fuel gas necessary for power generation, thereby suppressing deterioration of the fuel cell 1 due to a lack of fuel and a decrease in system efficiency due to an oversupply of fuel gas.

[0066] In this embodiment, the temperature and flow rate of the oxidant gas are obtained by a first temperature sensor 12 and a first flow rate sensor 13 provided in a first flow path 17 that connects the fuel cell 1 and the combustor 7 and through which the oxidant gas flows, and the oxidant gas thermal energy is estimated using the temperature and flow rate of the oxidant gas. This makes it possible to directly obtain the temperature and flow rate of the oxidant gas.

[0067] In this embodiment, the temperature and flow rate of the fuel gas are obtained by the second temperature sensor 14 and the second flow rate sensor 15 provided in the second flow path 18, which connects the fuel cell 1 and the combustor 7 and through which the fuel gas flows, and the reference fuel gas thermal energy is estimated using the temperature and flow rate of the fuel gas and the calorific value of the reference fuel gas. This makes it possible to directly obtain the temperature and flow rate of the fuel gas.

[0068] In this embodiment, the flow rate of the oxidant gas is obtained by a first flow sensor 13 provided in a first flow path 17 that connects the fuel cell 1 and the combustor 7 and through which the oxidant gas flows, the flow rate of the fuel gas is obtained by a second flow sensor 15 provided in a second flow path 18 that connects the fuel cell 1 and the combustor 7 and through which the fuel gas flows, and the temperature of the combustion gas is obtained by a third temperature sensor 16 provided in a combustion gas flow path 19. The actual exhaust gas thermal energy is estimated using the flow rates of the oxidant gas, the fuel gas, and the combustion gas temperature. This allows the calorific value of the fuel gas to be estimated with high accuracy.

[0069] In this embodiment, an approximation formula that approximates the relationship between the calorific value of the fuel and the hydrogen equivalent is created in advance based on the relationship between the calorific value of the hydrocarbon fuel contained in the fuel gas and the amount of hydrogen produced by reforming, and the hydrogen equivalent of the calorific value of the actual fuel gas estimated from the calorific value and the difference between the calorific values ​​of the reference fuel is calculated. This makes it possible to estimate the amount of hydrogen produced, regardless of whether carbon dioxide or other contaminants are present.

[0070] In this embodiment, if the calculated hydrogen equivalent amount of the actual fuel gas is lower than the hydrogen equivalent amount of the reference fuel, the flow rate of the fuel is increased, and if the calculated hydrogen equivalent amount of the actual fuel gas is higher than the hydrogen equivalent amount of the reference fuel, the flow rate of the fuel is decreased, thereby making it possible to prevent a shortage of fuel gas and an excess supply of fuel gas during power generation.

[0071] In this embodiment, the required fuel flow rate, which is the fuel flow rate required to generate power in the fuel cell 1, is calculated using a preset fuel utilization rate and hydrogen equivalent amount, thereby making it possible to more accurately set the amount of fuel gas required for power generation.

[0072] In this embodiment, when the difference in calorific value is calculated by subtracting the calorific value of the actual fuel gas from the calorific value of the reference fuel gas, if the difference in calorific value is negative, the fuel flow rate is increased relative to the initial fuel flow rate set based on the hydrogen equivalent of the reference fuel gas, and if the difference in calorific value is positive, the fuel flow rate is decreased relative to the initial fuel flow rate, thereby making it possible to set an appropriate combustion gas temperature.

[0073] In this embodiment, by limiting the fuel flow rate, the temperature of the combustion gas is limited to a temperature lower than the temperature at which the combustor deteriorates, thereby suppressing deterioration of the combustor.

[0074] In this embodiment, the fuel cell system 100 further includes an evaporator 20 that generates steam to be supplied to the reformer, and an approximation equation that approximates the relationship between the calorific value of the fuel gas and the average number of carbon atoms contained in the fuel gas is created in advance. The average number of carbon atoms in the actual fuel gas is calculated from the approximation equation and the calorific value of the actual fuel gas, and the amount of steam required for reforming is calculated based on the average carbon number. This makes it possible to supply the amount of steam required for reforming to the evaporator 20, thereby preventing deterioration of the reformer.

[0075] Next, modifications of the above embodiment will be described. The following modifications also fall within the scope of the present invention, just like the above embodiment.

[0076] [Modification] In the above embodiment, the fuel gas is reformed by steam reforming. However, reforming using an oxidant gas, so-called POX reforming, may also be used.

[0077] The POX reforming is an exothermic reaction, and the reaction formula is, for example, as shown in formula (5).

[0078]

[0079] That is, the amount of hydrogen generated when 1 mol of CH4 is reformed is 4 in steam reforming, but 2 in POX reforming. Despite this difference, the fuel gas flow rate can be set so as not to cause carbon deposition due to reforming, based on the same concept as in the above embodiment. Note that steam reforming and POX reforming may be used in combination.

[0080] Furthermore, in the above embodiment and modified examples, the fuel used is city gas, but the fuel is not limited to this and may be any fuel whose main component is saturated chain hydrocarbons (e.g., ethanol).

[0081] Although the embodiments of the present invention have been described above, the above embodiments merely illustrate some of the application examples of the present invention, and it is not intended that the technical scope of the present invention be limited to the specific configurations of the above embodiments.

Claims

1. A method for controlling a fuel cell system comprising: a fuel cell; a reformer which generates hydrogen from a fuel gas supplied to the fuel cell; and a combustor which generates a combustion gas by combusting an oxidant off-gas discharged from a cathode of the fuel cell and a fuel off-gas discharged from an anode of the fuel cell, the method comprising the steps of: estimating an oxidant gas thermal energy which is the thermal energy of the oxidant off-gas supplied to the combustor; estimating a reference fuel gas thermal energy which is the thermal energy of the fuel off-gas when a reference fuel gas having a known composition is used; calculating a reference exhaust gas thermal energy which is the total thermal energy of the gas supplied to the combustor when the reference fuel gas is used by adding the oxidant gas thermal energy and the reference fuel gas thermal energy; estimating an actual exhaust gas thermal energy which is the actual total thermal energy of the gas supplied to the combustor based on an actual temperature of the combustion gas; estimating a difference in heat value between the actual fuel gas which is the fuel gas actually used and the reference fuel gas based on the difference between the reference exhaust gas thermal energy and the actual exhaust gas thermal energy; a hydrogen equivalent amount, which is the amount of hydrogen produced when one mole of the actual fuel gas is reformed, is calculated using the difference in heating value and a relationship between the heating value of a hydrocarbon fuel contained in the fuel gas and the amount of hydrogen produced by reforming, and a flow rate of the fuel gas supplied to the fuel cell is controlled based on the hydrogen equivalent amount.

2. A fuel cell system control method as described in claim 1, comprising the steps of: acquiring the temperature and flow rate of the oxidant gas using a first temperature sensor and a first flow rate sensor provided in a first flow path that connects the fuel cell and the combustor and through which the oxidant gas flows; and estimating the oxidant gas thermal energy using the temperature and flow rate of the oxidant gas.

3. A fuel cell system control method as described in claim 1, comprising the steps of: acquiring the temperature and flow rate of the fuel gas using a second temperature sensor and a second flow rate sensor provided in a second flow path that connects the fuel cell and the combustor and through which the fuel gas flows; and estimating the reference fuel gas thermal energy using the temperature and flow rate of the fuel gas and the heat value of the reference fuel gas.

4. A fuel cell system control method as described in claim 1, comprising: obtaining the flow rate of the oxidant gas by a first flow sensor provided in a first flow path that connects the fuel cell and the combustor and through which the oxidant gas flows; obtaining the flow rate of the fuel gas by a second flow sensor provided in a second flow path that connects the fuel cell and the combustor and through which the fuel gas flows; obtaining the temperature of the combustion gas by a third temperature sensor provided in the combustion gas flow path; and estimating the actual exhaust gas thermal energy using the flow rate of the oxidant gas, the flow rate of the fuel gas, and the temperature of the combustion gas.

5. A fuel cell system control method as claimed in claim 1, comprising the steps of: creating in advance an approximation equation that approximates the relationship between the heat value of the fuel gas and the hydrogen equivalent, based on the relationship between the heat value of the hydrocarbon fuel contained in the fuel gas and the amount of hydrogen produced by reforming; and calculating the hydrogen equivalent in the heat value of the actual fuel gas estimated from the difference between the heat value of the reference fuel gas and the heat value.

6. A fuel cell system control method as claimed in claim 1, further comprising the steps of: increasing the flow rate of the fuel gas when the calculated hydrogen equivalent amount of the actual fuel gas is lower than the hydrogen equivalent amount of the reference fuel gas; and decreasing the flow rate of the fuel gas when the calculated hydrogen equivalent amount of the actual fuel gas is higher than the hydrogen equivalent amount of the reference fuel gas.

7. A method for controlling a fuel cell system as claimed in claim 6, wherein a required fuel flow rate, which is the fuel flow rate required when generating electricity in the fuel cell, is calculated using a preset fuel utilization rate and the hydrogen equivalent amount.

8. A fuel cell system control method as described in claim 1, wherein, when the difference in heat generation amount is a value obtained by subtracting the heat generation amount of the actual fuel gas from the heat generation amount of the reference fuel gas, if the difference in heat generation amount is negative, the fuel flow rate is increased relative to an initial fuel flow rate set based on the hydrogen equivalent of the reference fuel gas, and if the difference in heat generation amount is positive, the fuel flow rate is decreased relative to the initial fuel flow rate.

9. A method for controlling a fuel cell system according to claim 8, further comprising the step of limiting the temperature of the combustion gas to a temperature lower than the temperature at which the combustor deteriorates by restricting the fuel flow rate.

10. A fuel cell system control method according to claim 1, wherein the fuel cell system further comprises an evaporator which generates water vapor to be supplied to the reformer, an approximation equation which approximates the relationship between the heat value of the fuel gas and the average number of carbon atoms contained in the fuel gas is prepared in advance, the average carbon number of the actual fuel gas is calculated from the approximation equation and the heat value of the actual fuel gas, and the amount of water vapor required for reforming is calculated based on the average carbon number.

11. A fuel cell system control method as claimed in claim 1, wherein the fuel cell system further comprises an oxidant gas supply device which supplies an oxidant gas to the reformer, an approximation equation which approximates the relationship between the heat value of the fuel gas and the average number of carbon atoms contained in the fuel gas is prepared in advance, the average number of carbon atoms of the actual fuel gas is calculated from the approximation equation and the heat value of the actual fuel gas, and the amount of the oxidant gas required for reforming is calculated based on the average number of carbon atoms.

12. A fuel cell system control method as claimed in claim 1, wherein the fuel cell system further comprises an evaporator which generates water vapor to be supplied to the reformer, and an oxidant gas supply device which supplies oxidant gas to the reformer, the method comprising: creating in advance an approximation equation which approximates the relationship between the heating value of the fuel gas and the average number of carbon atoms contained in the fuel gas; and calculating the amount of water vapor and the amount of oxidant gas required for reforming based on the average number of carbon atoms.

13. A method for controlling a fuel cell system according to claim 1, wherein the fuel gas contains a main component of saturated chain hydrocarbons.

14. A fuel cell system control device for controlling a fuel cell system including: a fuel cell; a reformer for generating hydrogen from a fuel gas supplied to the fuel cell; and a combustor for generating a combustion gas by combusting an oxidant off-gas discharged from a cathode electrode of the fuel cell and a fuel off-gas discharged from an anode electrode of the fuel cell, the control device comprising: a first calculation unit for estimating an oxidant gas thermal energy which is the thermal energy of the oxidant off-gas supplied to the combustor; a second calculation unit for estimating a reference fuel gas thermal energy which is the thermal energy of the fuel off-gas when a reference fuel gas having a known composition is used; a third calculation unit for calculating a reference exhaust gas thermal energy which is the total thermal energy of the gas supplied to the combustor when the reference fuel gas is used by adding the oxidant gas thermal energy and the reference fuel gas thermal energy; and a fourth calculation unit for estimating an actual exhaust gas thermal energy which is the actual total thermal energy of the gas supplied to the combustor based on the actual temperature of the combustion gas. a fifth calculation unit that estimates a difference in heat value between actual fuel gas, which is the fuel gas actually used, and the reference fuel gas based on the difference between the reference exhaust gas thermal energy and the actual exhaust gas thermal energy; a sixth calculation unit that calculates a hydrogen equivalent amount, which is the amount of hydrogen produced when one mole of the actual fuel gas is reformed, using the difference in heat value and a relationship between the heat value of a hydrocarbon fuel contained in the fuel gas and the amount of hydrogen produced by reforming; and a flow rate control unit that controls a flow rate of the fuel gas supplied to the fuel cell based on the hydrogen equivalent amount.

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