JP4019924B2 - Fuel cell system - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a fuel cell system including a combustor that combusts fuel gas discharged from a fuel cell stack, and more particularly to reliably ignite the combustor by preventing liquid water from flowing into the combustor. It relates to technology to do.
[0002]
[Prior art]
Fuel cell technology that enables clean exhaust and high energy efficiency is attracting attention as a countermeasure against environmental problems in recent years, particularly air pollution caused by automobile exhaust gas and global warming caused by carbon dioxide.
[0003]
Fuel cell systems supply hydrogen or hydrogen-rich reformed gas and oxygen (air) as fuel to the anode (fuel electrode) and cathode (air electrode) of the fuel cell stack to cause an electrochemical reaction, An energy conversion system that converts energy into electrical energy. At this time, not all of the supplied hydrogen is consumed in the fuel cell stack, and the exhaust gas (anode exhaust gas) from the anode electrode contains a certain level of hydrogen. Therefore, the anode exhaust gas is burned in, for example, a combustor and released into the atmosphere, and an attempt is made to effectively use the heat (see, for example, Patent Document 1).
[0004]
In the invention described in Patent Document 1, off-gas discharged from the fuel cell stack is burned by a catalytic combustor, and the combustion off-gas is introduced into an evaporator to exchange heat with reforming raw fuel and reforming air. Yes. Furthermore, the combustion off gas is introduced into an off gas heater to serve as a heat source for heating the off gas discharged from the fuel cell stack. By heating the off gas from the fuel cell stack with the combustion off gas, water in the off gas can be vaporized and introduced into the catalytic combustor, and generation of condensed water can be prevented.
[0005]
[Patent Document 1]
Japanese Patent Laid-Open No. 2002-198076
[Problems to be solved by the invention]
However, in the technique described in Patent Document 1, the cathode exhaust gas and the anode exhaust gas discharged from the fuel cell stack are heated by the exhaust gas from the combustor, so that it is necessary to always generate heat in the combustor. is there. Therefore, it is necessary to supply unreacted fuel gas before the anode exhaust gas or the fuel cell stack is always supplied to the combustor. In a fuel reforming type fuel cell system that performs fuel reforming, it is necessary to constantly supply heat to the reformer, so there is no problem in always burning hydrogen in the combustor as described above. In the hydrogen fuel cell system, hydrogen, which is a fuel gas, is consumed wastefully.
[0007]
Usually, in a direct hydrogen fuel cell system, a fuel circulation channel for reusing the anode exhaust gas is provided, and the anode exhaust gas in the channel is purged only when the impurity concentration increases. Combusting hydrogen in the combustor at other times leads to wasteful consumption of hydrogen.
[0008]
Therefore, it is conceivable that only the cathode exhaust gas is supplied to the combustor in the normal state, and the anode exhaust gas is burned by the combustor only when the anode exhaust gas is discharged for some reason, for example, only at the time of the purge. .
[0009]
However, in the case of a fuel cell system that normally supplies only the cathode exhaust gas to the combustor, combustion is not performed when only the cathode exhaust gas is supplied, and the temperature of the combustor decreases, and the cathode exhaust gas Due to both factors of condensation of moisture inside the combustor, there arises a problem that ignition is hindered when the anode exhaust gas is supplied to the combustor.
[0010]
The present invention has been proposed for the purpose of overcoming the drawbacks of the prior art, and does not waste fuel gas while preventing combustion of the combustor and generation of condensed water. An object of the present invention is to provide a fuel cell system capable of reliably igniting a combustor.
[0011]
[Means for Solving the Problems]
The fuel cell system of the present invention includes a fuel cell stack including an anode electrode to which a fuel gas is supplied and a cathode electrode to which an oxidant gas is supplied, and power generation is performed by the supply of the fuel gas and the oxidant gas. A combustor for combusting a mixed gas of the anode exhaust gas discharged from the anode electrode and the cathode exhaust gas discharged from the cathode electrode, a cathode exhaust gas bypass passage for allowing the cathode exhaust gas to flow through the combustor, and An anode exhaust gas flow rate control means for controlling the flow rate of anode exhaust gas supplied to the combustor, and a cathode exhaust gas flow rate control means for controlling the flow rate of cathode exhaust gas supplied to the combustor. When the anode exhaust gas is not supplied to the combustor, the cathode exhaust gas bypasses the combustor by the cathode exhaust gas bypass passage.
[0012]
In the fuel cell system of the present invention, combustion in the combustor is performed only when anode exhaust gas is supplied to the combustor. When the anode exhaust gas is not supplied to the combustor, the cathode exhaust gas flows through the cathode exhaust gas bypass passage and bypasses the combustor.
[0013]
【The invention's effect】
In the fuel cell system of the present invention, combustion in the combustor is performed only when anode exhaust gas is supplied to the combustor, so that fuel gas is not wasted. In addition, when the anode exhaust gas is not supplied to the combustor, the cathode exhaust gas bypasses the combustor through the cathode exhaust gas bypass passage, so that the cathode exhaust gas with high humidity does not flow into the normal temperature combustor. Inconvenience that condensed water is generated inside is also suppressed.
[0014]
Therefore, according to the fuel cell system of the present invention, it is possible to reliably ignite the combustor without wastefully consuming fuel gas and preventing the cooling of the combustor and the generation of condensed water.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of a fuel cell system to which the present invention is applied will be described with reference to the drawings.
[0016]
(First embodiment)
FIG. 1 is a schematic configuration diagram showing the main configuration of the fuel cell system of the present embodiment. This fuel cell system is a power generation system that is mounted on a vehicle and used as a driving power source for the vehicle. The fuel cell system 1 generates power, a fuel supply system that supplies fuel gas to the fuel cell stack 1, a fuel cell system An air supply system for supplying an oxidant gas (air) to the battery stack 1. In particular, this fuel cell system is configured as a direct hydrogen fuel cell system in which hydrogen gas is directly supplied as fuel gas from the fuel supply system to the fuel cell stack 1.
[0017]
The fuel cell stack 1 includes an anode 1a to which hydrogen gas is supplied and a cathode 1b to which an oxidant gas (air) is supplied so as to form an electric power generation cell with an electrolyte / electrode catalyst composite interposed therebetween. The structure has a structure in which a plurality of power generation cells are stacked in multiple stages, and converts chemical energy into electrical energy by an electrochemical reaction. In the anode 1a, hydrogen gas is supplied to dissociate into hydrogen ions and electrons, the hydrogen ions pass through the electrolyte, the electrons pass through an external circuit, generate electric power, and move to the cathode 1b. In the cathode 1b, oxygen in the supplied air reacts with the hydrogen ions and electrons to generate water, which is discharged to the outside.
[0018]
As the electrolyte of the fuel cell stack 1, for example, a solid polymer electrolyte membrane is used in consideration of high energy density, low cost, light weight, and the like. The solid polymer electrolyte membrane is made of an ion (proton) conductive polymer membrane such as a fluororesin ion exchange membrane, and functions as an ion conductive electrolyte when saturated with water.
[0019]
The fuel supply system has a hydrogen supply flow path 2, an ejector 3, an anode exhaust gas exhaust flow path 4, and a hydrogen circulation flow path 5. A hydrogen gas supplied from a hydrogen supply source (not shown) is supplied to the anode electrode 1 a of the fuel cell stack 1 through the hydrogen supply flow path 2 and the ejector 5. The fuel cell stack 1 does not consume all of the supplied hydrogen gas, and the remaining hydrogen gas (hydrogen gas discharged from the anode electrode 1a of the fuel cell stack 1) passes through the hydrogen circulation passage 5 and is ejected. 3 is mixed with the newly supplied hydrogen gas and supplied again to the anode 1 a of the fuel cell stack 1.
[0020]
A hydrogen purge valve 6 serving as an anode exhaust gas flow rate control means is provided on the outlet side of the fuel cell stack 1, that is, on the anode exhaust gas exhaust passage 4. The hydrogen purge valve 6 is for discharging the hydrogen gas as necessary in order to suppress a reduction in efficiency of the fuel cell stack 1 caused by circulating the hydrogen gas. That is, when hydrogen gas is circulated, impurities, nitrogen, and the like accumulate in the hydrogen circulation flow path 5, leading to a decrease in hydrogen partial pressure, which may reduce the efficiency of the fuel cell stack 1. In such a case, by opening the hydrogen purge valve 6 and purging the gas in the hydrogen circulation channel 5, impurities, nitrogen, and the like can be removed from the hydrogen circulation channel 5.
[0021]
The air supply system has an air supply flow path 7 and a cathode exhaust gas exhaust flow path 8. For example, air compressed by a compressor is supplied to the cathode electrode 1 b of the fuel cell stack 1 through the air supply flow path 7. It has become so. Further, oxygen and other components (cathode exhaust gas) not consumed in the fuel cell stack 1 are discharged from the fuel cell stack 1 through the cathode exhaust gas exhaust pipe 8.
[0022]
In the fuel cell system of this embodiment, a combustor 9 for treating the exhaust gas of the fuel cell stack 1 is installed, and the anode exhaust gas exhaust passage 4 of the fuel supply system and the cathode exhaust gas exhaust passage 8 of the air supply system. Are connected to the combustor 9. The anode exhaust gas discharged from the anode electrode 1a of the fuel cell stack 1 and the cathode exhaust gas discharged from the cathode electrode 1b are mixed and burned by the combustor 9. As the combustor 9, for example, a catalytic combustor having a combustion catalyst therein is used.
[0023]
In particular, in the fuel cell system of the present embodiment, a cathode exhaust gas bypass passage 10 that bypasses the combustor 9 is branched in the cathode exhaust gas exhaust passage 8 so that the cathode exhaust gas from the cathode electrode 1b can be used as needed. The lever is configured to flow through the cathode exhaust gas bypass passage 10. The cathode exhaust gas exhaust passage 8 and the cathode exhaust gas bypass passage 10 are provided with an exhaust air control valve 11 and a bypass control valve 12, respectively. The exhaust air control valve 11 and the bypass control valve 12 are for controlling the flow rate of the cathode exhaust gas supplied to the combustor 9 (cathode exhaust gas flow rate control means), and are configured to operate in conjunction with each other exclusively. ing. The opening / closing operation of the exhaust air control valve 11 and the bypass control valve 12 and the opening / closing operation of the hydrogen purge valve 6 are controlled by the controller 13 based on the operation information from the fuel cell stack 1.
[0024]
In the fuel cell system having the above configuration, when the fuel cell stack 1 is caused to generate electric power, air as an oxidant is supplied to the fuel cell stack 1 through the air supply flow path 7. Further, hydrogen gas as a fuel gas is supplied to the fuel cell stack 1 through a hydrogen supply channel 2 from a hydrogen supply source (not shown). Hydrogen gas that has not contributed to power generation is supplied again to the fuel cell stack 1 via the hydrogen circulation passage 5 and the ejector 4.
[0025]
At this time, normally, the exhaust air control valve 11 is closed and the bypass control valve 12 is opened, and the cathode exhaust gas from the fuel cell stack 1 is released into the atmosphere through the cathode exhaust gas bypass passage 10. The electric power generated by the fuel cell stack 1 is consumed by a load (not shown) such as a motor for driving the vehicle.
[0026]
On the other hand, for some reason, for example, the hydrogen concentration on the anode electrode 1a side decreases due to, for example, a nitrogen crossover inside the fuel cell stack 1, and the hydrogen gas inside the fuel cell stack 1 and the hydrogen circulation channel 5 is released to the atmosphere. When the need arises, the controller 13 performs the following operation.
[0027]
That is, when the controller 13 determines that the hydrogen concentration is in a lowered state, the controller 13 opens the hydrogen purge valve 6 and supplies hydrogen (anode exhaust gas) discharged from the fuel cell stack 1 to the combustor 9. Further, the exhaust air control valve 11 is opened and the bypass control valve 12 is closed to supply the cathode exhaust gas to the combustor 9.
[0028]
FIG. 2 shows a functional configuration of the controller 13. The controller 13 includes a purge determination unit 13a, a hydrogen purge valve control unit 13b, a time constant setting unit 13c, and a cathode exhaust gas control unit 13d.
[0029]
The controller 13 is connected to an operation state detection unit (not shown) of the fuel cell stack 1, and an output from the operation state detection unit is given to the purge determination unit 13a and the time constant setting unit 13c. The purge determination unit 13a determines the necessity of the purge operation from the operation state of the fuel cell stack 1, and outputs the command to the hydrogen purge valve control unit 13b and the cathode exhaust gas control unit 13d. The hydrogen purge valve control unit 13b performs opening / closing control of the hydrogen purge valve 6 in accordance with a command from the purge determination unit 13a.
[0030]
Since the time from the opening of the hydrogen purge valve 6 to the arrival of the anode exhaust gas at the combustor 9 varies depending on the operating pressure of the fuel cell stack 1 and the like, the time constant setting unit 13c receives the input operating state of the fuel cell stack 1 Therefore, a time constant τ used for opening / closing control of the exhaust air control valve 11 and the bypass control valve 12 is set.
[0031]
In the cathode exhaust gas control unit 13d, as shown in FIG. 3, based on the time constant τ given by the time constant setting unit 13c and the command from the purge determination unit 13a, the following equations (1) and (2) are used. The open / close control of the exhaust air control valve 11 and the bypass control valve 12 is performed.
[0032]
u1 = 1 / (1 + τ · s) (1)
u2 = 1−u1 (2)
Here, s is a Laplace operator, u1 is an opening command value of the exhaust air control valve 11, and u2 is an opening command value of the bypass control valve 12.
[0033]
FIG. 4 shows a simulation result of the ratio of the flow rate of exhaust air (cathode exhaust gas) and exhaust hydrogen (anode exhaust gas) flowing into the combustor 9 in the fuel cell system of the present embodiment in comparison with the conventional fuel cell system. . In addition, the graph shown with a continuous line in FIG. 4 is a thing of this embodiment, and the graph shown with a broken line is a thing of a prior art example.
[0034]
By controlling the exhaust air control valve 11 and the bypass control valve 12 as in this embodiment, even if the combustor 9 is in a state where the temperature is low and the ignitability is low, the exhaust hydrogen and exhaust gas at the beginning of combustion start. The ratio of air can be made appropriate, and ignition can be prevented from becoming poor. Further, the ratio of exhaust hydrogen and exhaust air can be made appropriate even in a steady state, and the combustor 9 can be reacted stably.
[0035]
In other words, in the fuel cell system of the present embodiment, the ratio of the flow rate of the anode exhaust gas supplied to the combustor 9 and the flow rate of the cathode exhaust gas is set to an optimum desired value according to the state of the combustor 9. In addition, opening / closing control of the exhaust air control valve 11 and the bypass control valve 12 is performed. Here, the ratio between the flow rate of the anode exhaust gas supplied to the combustor 9 and the flow rate of the cathode exhaust gas is such that the combustion temperature of the combustor 9 is the same as that of the combustor 9 while the anode exhaust gas is being stably supplied to the combustor 9. The range is determined by the characteristics, and immediately after the supply of the cathode exhaust gas and the anode exhaust gas to the combustor is started, the range is set to promote the ignition of the combustor 9.
[0036]
In the fuel cell system of the present embodiment, while the anode exhaust gas is not supplied to the combustor 9, the cathode exhaust gas bypasses the combustor 9, so that the combustor 9 is cooled or the moisture of the cathode exhaust gas It is possible to ensure ignition performance when the anode exhaust gas is supplied without condensing the exhaust gas.
[0037]
Moreover, the combustion state in the combustor 9 can be controlled by supplying the cathode exhaust gas to the combustor 9 using the state quantity relating to the flow rate of the anode exhaust gas supplied to the combustor 9. At this time, if the time delay is set so that the cathode exhaust gas and the anode exhaust gas are started to be supplied to the combustor substantially simultaneously, abnormal combustion or the like can be prevented from occurring in the combustor 9 at the initial stage of combustion, and the durability of the combustor 9 can be prevented. Will not be damaged. By using the time delay until the anode exhaust gas is supplied to the combustor 9 as the state quantity related to the flow rate of the anode exhaust gas, the present invention can be implemented without adding a special sensor or the like.
[0038]
Furthermore, the flow rate of the cathode exhaust gas is controlled so that the ratio of the flow rate of the anode exhaust gas flowing into the combustor 9 and the flow rate of the cathode exhaust gas becomes a desired value, thereby improving the ignition performance and ensuring the control performance of the combustion temperature. Can do. In particular, while the anode exhaust gas is stably supplied to the combustor 9, and when the anode exhaust gas and the cathode exhaust gas start to be supplied to the combustor 9, a first predetermined ratio suitable for the ratio of the anode exhaust gas and the cathode exhaust gas respectively. By controlling to the range and the second predetermined range, the steady combustion temperature can be controlled to a range determined by the characteristics of the combustor 9, and anode exhaust gas and cathode exhaust gas are supplied at a special ratio to promote ignition. Therefore, it is possible to improve the ignition performance while preventing the combustion temperature from becoming too high and impairing the durability of the combustor 9, and preventing the combustion temperature from being lowered too much to cause a misfire.
[0039]
In this embodiment, flow control valves are used for the exhaust air control valve 11 and the bypass control valve 12, but for example, on / off valves are used for these, and the exhaust air control valve 11 and bypass control are performed in the time constant setting unit 13c. A similar effect can be obtained by setting a dead time for opening and closing the valve 12.
[0040]
(Second Embodiment)
Next, a second embodiment of the present invention will be described. The principal part structure of the fuel cell system of this embodiment is shown in FIG. The fuel cell system of this embodiment differs from the fuel cell system of the first embodiment described above in that a pressure sensor 14 is installed between the hydrogen purge valve 6 and the combustor 9 and internal processing in the controller 13. Therefore, in the following, these characteristic parts will be mainly described, and the description of the parts common to the first embodiment will be omitted.
[0041]
In the combustion cell system of the present embodiment, the pressure sensor 14 measures the pressure of the anode exhaust gas (exhaust hydrogen) immediately before the combustor 9. The cathode exhaust gas control unit 13d of the controller 13 calculates the target opening degree U of the exhaust air control valve 11 and the bypass control valve 12 using the output of the pressure sensor 14 and a predetermined gain K. .
[0042]
FIG. 6 shows a functional configuration diagram of the controller 13 in the fuel cell system of the present embodiment, and FIG. 7 shows a state of calculation processing in the cathode exhaust gas control unit 13d. In the fuel cell system of the present embodiment, the controller 13 includes a purge determination unit 13a, a hydrogen purge valve control unit 13b, and a cathode exhaust gas control unit 13d. The purge determination unit 13a determines the necessity of the purge operation from the operation state of the fuel cell stack 1, and outputs the command to the hydrogen purge valve control unit 13b and the cathode exhaust gas control unit 13d. The cathode exhaust gas control unit 13d controls the target opening of the exhaust air control valve 11 and the bypass control valve 12 from the calculated target opening based on a command from the purge determination unit 13a.
[0043]
In this embodiment, since the anode exhaust gas pressure immediately before the combustor 9 is used as the state quantity related to the flow rate of the anode exhaust gas, a relatively inexpensive pressure sensor 14 can be used, and the control of the present invention can be performed at a lower cost. it can.
[0044]
In the present embodiment, flow control valves are used for the exhaust air control valve 11 and the bypass control valve 12. For example, an on / off valve is used for these, and the time constant setting unit 13 c uses the exhaust air control valve 11 and the bypass control valve. Similar to the case of the first embodiment, the same effect can be obtained even when the dead time for opening and closing the control valve 12 is set.
[0045]
Further, in the present embodiment, the opening control of the exhaust air control valve 11 and the bypass control valve 12 is performed using the pressure immediately before the combustor 9, but it is also performed using the flow rate of exhaust hydrogen immediately before the combustor 9. Is possible. If the flow rate of the anode exhaust gas is directly used as the state quantity relating to the flow rate of the anode exhaust gas, the air-fuel ratio can be controlled more directly.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram showing a main configuration of a fuel cell system according to a first embodiment.
FIG. 2 is a functional block diagram showing a functional configuration of a controller in the fuel cell system according to the first embodiment.
FIG. 3 is a diagram showing a state of calculation processing in a cathode exhaust gas control unit of the controller.
FIG. 4 is a diagram showing the ratio of the flow rate of exhaust air and exhaust hydrogen supplied to a combustor in the fuel cell system of the first embodiment in comparison with the conventional example.
FIG. 5 is a schematic configuration diagram showing a main configuration of a fuel cell system according to a second embodiment.
FIG. 6 is a functional block diagram showing a functional configuration of a controller in the fuel cell system according to the second embodiment.
FIG. 7 is a diagram showing a state of calculation processing in a cathode exhaust gas control unit of the controller.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Fuel cell stack 1a Anode electrode 1b Cathode electrode 5 Hydrogen circulation flow path 6 Hydrogen purge valve 7 Air supply flow path 8 Cathode exhaust gas exhaust flow path 9 Combustor 10 Cathode exhaust gas bypass flow path 11 Exhaust air control valve 12 Bypass control valve 13 Controller 14 Pressure sensor

Claims (10)

A fuel cell stack including an anode electrode to which fuel gas is supplied and a cathode electrode to which oxidant gas is supplied, and power generation is performed by supplying these fuel gas and oxidant gas;
A combustor that burns a mixed gas of anode exhaust gas discharged from the anode electrode and cathode exhaust gas discharged from the cathode electrode;
A cathode exhaust gas bypass passage for circulating the cathode exhaust gas by bypassing the combustor;
Anode exhaust gas flow rate control means for controlling the flow rate of anode exhaust gas supplied to the combustor;
Cathode exhaust gas flow rate control means for controlling the flow rate of the cathode exhaust gas supplied to the combustor,
The fuel cell system, wherein when the anode exhaust gas is not supplied to the combustor, the cathode exhaust gas bypasses the combustor by the cathode exhaust gas bypass passage. 2. The fuel cell system according to claim 1, wherein the flow rate of the cathode exhaust gas supplied to the combustor is controlled based on a change in a state quantity related to the flow rate of the anode exhaust gas supplied to the combustor. The fuel cell system according to claim 2, wherein the flow rate of the cathode exhaust gas is controlled so that the cathode exhaust gas and the anode exhaust gas are supplied to the combustor almost simultaneously. The supply of the cathode exhaust gas to the combustor is started based on a time delay from the operation of the anode exhaust gas flow rate control means to the supply of the anode exhaust gas to the fuel device. Item 4. The fuel cell system according to Item 3. Comprising a flow rate detection means for detecting the flow rate of the anode exhaust gas supplied to the combustor immediately before the combustor,
The fuel cell system according to claim 3, wherein the flow rate of the cathode exhaust gas to the combustor is controlled based on the output of the flow rate detection means. Pressure detecting means for detecting the pressure of the anode exhaust gas supplied to the combustor immediately before the combustor;
4. The fuel cell system according to claim 3, wherein the flow rate of the cathode exhaust gas to the combustor is controlled based on the output of the pressure detection means. The flow rate of the cathode exhaust gas to the combustor is controlled so that a ratio between the flow rate of the anode exhaust gas supplied to the combustor and the flow rate of the cathode exhaust gas becomes a desired value. Item 7. The fuel cell system according to any one of Items 2 to 6. The ratio is in a range in which the combustion temperature of the combustor is determined by the characteristics of the combustor while the anode exhaust gas is stably supplied to the combustor.
The fuel cell system according to claim 7, wherein immediately after the cathode exhaust gas and the anode exhaust gas are supplied to the combustor, the ignition of the combustor is promoted. The fuel cell system according to claim 1, further comprising a fuel circulation channel for reusing the fuel gas. 10. The fuel cell system according to claim 1, wherein the fuel cell system is mounted on a vehicle and used as a driving power source for the vehicle.

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