JPH0654674B2 - Fuel cell power generator - Google Patents

Description

TECHNICAL FIELD OF THE INVENTION The present invention relates to a fuel cell power generator capable of preventing deterioration of an electrode due to an electrochemical reaction and prolonging its life.

[Technical background of the invention and its problems]

Conventionally, a fuel cell is a device that directly converts the chemical energy of fuel into electrical energy. In this fuel cell, a pair of porous electrodes are usually placed across the electrolyte, and a fuel gas such as hydrogen is brought into contact with the back surface of one electrode and an oxidant gas such as oxygen is brought into contact with the back surface of the other electrode. The electric energy generated by the electrochemical reaction that occurs at this time is taken out from the pair of electrodes. In this case, the electrolyte may be a molten salt, an alkaline solution, an acidic solution, or the like. Here, the principle of the phosphoric acid fuel cell using phosphoric acid as an electrolyte as a typical fuel cell will be described as an example.

FIG. 1 shows the basic configuration of a fuel cell of this type. In the figure, the electrolyte layer 1 is a fibrous sheet or mineral powder impregnated with phosphoric acid. Reference numerals 2 and 3 denote a pair of porous (carbonaceous) electrodes, an anode and a cathode, which are arranged so as to sandwich the electrolyte layer 1. Colloidal platinum is supported on the carbonaceous powder between itself and the electrolyte layer 1. The catalyst layers 4 and 5 formed by coating the respective materials are arranged. Further, 6 is a chamber in which a fuel gas such as hydrogen flows, and 7 is a chamber in which an oxidant gas such as oxygen (usually air) flows.

In such a fuel cell, hydrogen flowing into the chamber 6 diffuses in the void space of the anode electrode 2 and reaches the catalyst layer 4. Here, the hydrogen gas is dissociated into hydrogen ions and electrons by the action of the catalyst. The reaction formula is H ₂ → 2H ⁺ + 2e (1). Then, the hydrogen ions enter the electrolyte layer 1 and migrate toward the cathode electrode 3 by the action of the electromotive force and the concentration diffusion. On the other hand, the electrons separated by the dissociation of hydrogen gas flow into the anode electrode 2 and the electrode 2 is negatively charged. In the cathode electrode 3, hydrogen ions that have migrated from the anode electrode 2 side, oxygen that has been supplied to the room 7 as an oxidant and has diffused in the voids of the cathode electrode 3, and the anode electrode 2 that passes through an external power load. The three electrons that have returned to the cathode 3 of the battery that has worked as a work cause the following reaction on the surface of the catalyst layer 5.

4H ⁺ 4e + O ₂ → 2H ₂ O (2) Electromotive force and heat are generated in the process of reduction and oxidation at the electrodes 2 and 3, and the total is equal to the energy when hydrogen is oxidized. If all this energy is converted to electric energy, theoretically a voltage of about 1.23 (V) is generated, but the amount of electric energy actually extracted to the outside depends on the internal resistance of the battery. The voltage drop is subtracted. This loss is caused by activation polarization dominated by the activity of the catalyst, concentration polarization determined by the hydrogen concentration and oxygen concentration near the reaction point of the electrode, voltage drop when ions flow in the electrolyte 1, electrode and contact part. The total voltage drop due to the resistance in the isoelectron flow path is the loss inside the battery, that is, the voltage drop. In this case, it is difficult to directly measure the voltage between the catalyst layers 4 and 5 which are arranged with the electrolyte 1 in between, but it is necessary to add the voltage drop between the electrodes 2 and 3 to the voltage measured externally. It is considered to be almost equal.

By the way, when the voltage between the catalyst layers 4 and 5 becomes high, there is a phenomenon that the electrochemical action causes the dissolution of platinum which is an electrode constituent member of the fuel cell and the electrochemical oxidation of carbon particles of the platinum carrier. It is known that the electrodes 2 and 3 are rapidly deteriorated due to the occurrence.

FIG. 2 is a diagram for explaining that deterioration of the electrodes 2 and 3 occurs depending on operating conditions. The horizontal axis represents cumulative operating time, the vertical axis represents temperature 180 (° C.), current density 200 (mA / cm ² ). , And shows the voltage per battery when operating under atmospheric pressure. In the figure, (a) is the current density of 200 (mA / cm ² ) and the voltage is 0.65 (V) when continuously operated, and ( ^b ) is the current density of 2
00 (mA / cm ² ), voltage 0.9 (V) in continuous operation, current density is 200 (mA / cm ² ) every few hours, showing the progress of voltage respectively. . The difference between the curves a and b is considered to be due to the deterioration due to the electrochemical reaction.

In particular, it has become possible to advance the technology relating to fuel cells and to increase the pressure of the supply gas, and the rise in the generated voltage has resulted in the acceleration of the above deterioration due to overvoltage at light load.

Therefore, in order to prevent such overvoltage, measures such as connecting a load resistor at the time of light load and consuming electric power to this, or restricting the supply amount of the oxidant gas to suppress the voltage have been considered. . However, the former method wastes power in the former method, and the latter method causes an imbalance in the amount of oxidant gas supplied to each battery in a practical battery in which a large number of batteries are stacked and used. There is a problem in that the voltage generated in each battery causes a large variation, and it is not possible to suppress all the batteries to a voltage within a necessary limit, so that the purpose cannot be achieved.

[Object of the Invention]

The present invention has been made in consideration of the above circumstances,
An object of the present invention is to provide a fuel cell power generation device capable of preventing the electrode from being deteriorated due to an electrochemical reaction and maintaining a long life while suppressing the device from becoming complicated and wasting energy.

[Summary of the Invention] In order to achieve the above-mentioned object, a fuel cell power generator of the present invention has a pair of porous electrodes of an anode and a cathode arranged with an electrolyte in between, and allows a fuel gas to pass through the back surface of the anode electrode. A fuel cell that allows an oxidant gas to pass through the back surface of the cathode electrode and takes out electrical energy generated by an electrochemical reaction at this time from a pair of electrodes, and is driven by being supplied with exhaust gas from the cathode electrode side of the fuel cell,
The turbine compressor that supplies the oxidant gas to the cathode electrode side of the fuel cell, and the exhaust gas discharged from the turbine after the exhaust gas from the cathode electrode side of the fuel cell drives the turbine, The exhaust gas supply pipeline for supplying to the intake oxidant gas supply pipeline, the valve body for controlling the flow rate provided in the middle of the exhaust gas supply pipeline, the fuel cell has a low output, and the voltage of the fuel cell is predetermined. The load was reduced by providing a controller that controls to open the valve opening of the valve element when the voltage exceeds the voltage that causes the electrode components of the fuel cell to deteriorate due to an electrochemical reaction. At the time of operation, the exhaust gas is mixed with the oxidant gas supplied to the fuel cell at a predetermined ratio to reduce the oxygen concentration in the exhaust gas and suppress the occurrence of overvoltage. It is characterized in.

Example of Invention

An embodiment of the present invention shown in the drawings will be described below.
FIG. 3 shows an example of the configuration of a fuel cell power generator equipped with a gas supply device to which the present invention is applied. Note that the fuel gas of the fuel cell is hydrogen, but in the case of fuel cells for general electric power use, there are many systems that supply reformed natural gas, naphtha, methanol, etc., so here natural gas is the fuel gas. The case will be described.

In the figure, 8 is a reformer consisting of a catalyst tube 9 and a combustion chamber 10. A fuel gas A is introduced into the catalyst tube 9 through a fuel gas control valve 11 together with an appropriate amount of water vapor B, from which an anode of a fuel cell 12 is introduced. The gas is supplied to the gas distribution chamber 6 of the electrode 2. Then, the anode exhaust gas containing the unreacted fuel gas from the gas distribution chamber 6 is supplied to the combustion chamber 10 of the reformer 8 together with an oxidant gas C described later and burned.

On the other hand, 13 is a turbine that rotates with the energy of the exhaust gas D from the mixer, which will be described later.
Is driven to compress the intake air E and supply it as the oxidant gas C to the gas flow chamber 7 of the cathode electrode 3 of the fuel cell 12 through the oxidant gas control valve 15. Further, 16 is a mixer for mixing the exhaust gas from the combustion chamber 10 of the reformer 8 and the cathode exhaust gas from the gas distribution chamber 7 obtained via the pressure regulating valve 17, and the mixed gas is used as the exhaust gas D in the turbine 13 Supply to. Further, 18 is the fuel gas A obtained through the control valve 19 and the control valve 2
In the auxiliary combustor for introducing and burning the oxidant gas C obtained through 0, the combustion exhaust gas is additionally supplied to the mixer 16. That is, this auxiliary combustor 18
When the energy for driving the turbine 13 is insufficient only with the exhaust gas of the combustion chamber 10 and the cathode exhaust gas,
The operation is performed by operating the control valves 19 and 20. Furthermore, the intake air E to the compressor 14
A damper 21 as a valve is provided in the supply pipeline of the turbine 13, and a damper 22 is provided between the supply pipeline and the exhaust pipeline of the exhaust gas F from the turbine 13 so that the exhaust gas F of the turbine 13 is sucked in. It can be mixed with air E. Reference numeral 23 is a controller for controlling the dampers 21 and 22, which compares the output voltage of the fuel cell 12 with a reference voltage,
Damper 2 when output voltage is higher (light load operation)
2 is controlled to open, and conversely, when the output voltage is lower, the damper 22 is controlled to be closed, and the opening degree of the damper 21 is controlled as necessary. Therefore, a suitable reference voltage is 0.8 (V) to 0.9 (V) per cell. If it is set to 0.9 (V), efficient power generation can be performed, but since it is close to the voltage that causes deterioration due to an electrochemical reaction, 0.8 (V) is preferable if the life is important.

In addition, the controller 23 may be configured to compare the output current of the fuel cell 12 with the reference current and open the damper 22 when the output current is smaller than the reference current.

In addition to the above elements, the fuel cell power generation system includes a cell cooling / heating device, an electric output regulator, a heat exchanger for preheating and heat recovery of fuel gas and oxidant gas, and a steam generator. Since these are not directly related to the present invention, the illustration and description thereof will be omitted here. Further, in the above, the catalyst pipe 9 of the reformer 8 uses a nickel / alumina system as its catalyst, and converts natural gas such as methane as the fuel gas A into hydrogen, carbon dioxide and carbon monoxide.

Next, the operation of the fuel cell power generator of the present invention based on such a configuration will be described.

First, as described above, the theoretical value of the terminal voltage of the fuel cell, that is, the voltage between electrodes is 1.23 (V), but it is a practical operating condition due to the voltage drop due to activation polarization, concentration polarization, and electric resistance. 0.7 at current density near (mA / cm ² ).
The voltage will be around (V). In this case, the concentration polarization is directly related to the present invention. The higher the hydrogen concentration near the reaction point of the anode electrode and the higher the oxygen concentration near the reaction point of the cathode electrode, the more active the reaction becomes. Generate high voltage. Therefore, the highest voltage is obtained when pure hydrogen and pure oxygen are supplied to the fuel cell, respectively, and conversely, the inert gas and carbon dioxide are mixed and the reaction component becomes thinner, the lower the generated voltage becomes. Further, in this case, it has been found that the voltage drop is more remarkable when the concentration of oxygen is lower than that of hydrogen, and the generated voltage can be lowered by lowering the concentration.

In the present invention, paying attention to the above-mentioned phenomenon, at the time of operation when the load at which the output voltage of the fuel cell becomes excessively small is reduced, the oxidant gas supplied to the fuel cell is mixed with the exhaust gas at a predetermined ratio to perform the operation. This is specifically described below with reference to FIG.

First, in FIG. 3, the intake air E supplied through the damper 21 is compressed by the compressor 14 driven by the turbine 13, and the combustion of the reformer 8 is performed as the oxidant gas C through the oxidant gas control valve 15. While being branched into the chamber 10, the gas is supplied to the gas flow chamber 7 of the cathode electrode 3 of the fuel cell 12, and the cathode exhaust gas is introduced into the mixer 16 from this.

On the other hand, the fuel gas A is introduced into the catalyst tube 9 of the reformer 8 through the fuel gas control valve 11 together with an appropriate amount of water vapor B to be hydrogenated, and is supplied to the gas distribution chamber 6 of the anode electrode 2 of the fuel cell 12. . Then, most of this hydrogen is consumed in the fuel cell 12, and the unreacted fuel gas is introduced into the combustion chamber 10 of the reformer 8 where it is combusted and heats the catalyst tube 9 to allow the above-mentioned mixing. It is introduced into the container 16. As a result, the mixer 16 mixes the exhaust gas from the combustion chamber 10 with the cathode exhaust gas, supplies the mixed exhaust gas to the turbine 13, and rotates it by its energy. If the exhaust gas from the combustion chamber 10 and the cathode exhaust gas do not have sufficient energy to drive the turbine 13, the control valve 1
The fuel gas A and the oxidant gas C are introduced into the auxiliary combustor 18 by 9, 20 and the burned exhaust gas is additionally introduced into the mixer 16 to obtain a predetermined energy.

Further, in the fuel cell 12, due to the above-described electrochemical reaction between hydrogen supplied to the anode electrode 2 and air supplied to the cathode electrode 3, a voltage of a predetermined magnitude is generated between the electrodes 2 and 3. Occurs, and this is supplied to a load (not shown).

Now, in such a state, for example, when the load decreases to a light load state, the active polarization, the concentration polarization, and the resistance polarization that cause the voltage drop of the fuel cell are reduced as the load decreases. As a result, the output voltage of the fuel cell 12 rises and approaches the theoretical voltage value between the electrodes of the fuel cell. The theoretical voltage between the electrodes of the fuel cell is usually higher than a voltage that causes a deterioration reaction due to an electrochemical reaction of electrode constituent members, that is, a reference voltage. Therefore, as the load is reduced, the reference voltage becomes equal to or higher than a voltage at which the electrochemical reaction of the electrode constituent member that is predetermined as the reference voltage rapidly progresses.

Here, the predetermined reference voltage is set as the maximum voltage of the fuel cell, and the controller 23 constantly causes the fuel cell voltage and the reference voltage to be compared and monitored. When the controller 23 detects that the fuel cell voltage exceeds the reference voltage due to the reduction of the load, the controller 23 opens the damper 22 so that the fuel cell voltage becomes the reference value or lower. As a result, the exhaust gas F of the turbine / compressor 13 which has consumed oxygen in advance in the fuel cell and used to drive the turbine / compressor 13 or which has a low oxygen concentration or does not contain oxygen is supplied to the fuel cell as an oxidant gas. Is mixed in at the inlet of the turbine 13. This reduces the concentration of the oxidant gas supplied to the fuel cell,
The fuel cell voltage decreases due to an increase in active polarization and concentration polarization.

By the way, the electric output of the fuel cell is represented by electric output = voltage of the fuel cell × current. Therefore, under a low load such that the fuel cell voltage exceeds the reference voltage, the voltage of the fuel cell shown in the above formula is lowered to the oxygen concentration in the oxidant gas and fixed to the reference voltage, so that only the increase / decrease of the current is achieved. The output of the fuel cell can be controlled with.

That is, the controller 23 constantly monitors the difference between the reference voltage and the fuel cell voltage, which increases or decreases depending on the load of the fuel cell, that is, the current, and determines the oxygen concentration in the oxidant gas supplied to the fuel cell from the damper 22. The fuel cell voltage is controlled to be equal to or smaller than the reference voltage by opening and closing.
In this case, in this example, since the intake air E mixes with the exhaust gas F, the oxygen concentration of the intake air E is reduced and the flow rate (pressure) is not extremely decreased. Therefore, even in a battery stack in which a large number of batteries are stacked, each battery is The voltage can be reduced substantially uniformly.

In the above, when the oxygen concentration above the predetermined value cannot be obtained even when the damper 22 is fully opened, the damper 2
By narrowing the opening degree of the damper 21 while keeping 2 fully open, it is possible to increase the mixing ratio of the exhaust gas F into the intake air E and achieve the above effect. Further, since the exhaust gas from the turbine 13 is supplied to the intake air supply pipeline at the inlet of the turbine 13 / compressor 14 to reduce the oxygen concentration of the air that is the oxidant gas at the primary pressure, The pressure fluctuation caused by circulating the exhaust gas does not significantly affect the pressure of the cathode electrode 3, which is the secondary pressure, and therefore the inter-electrode differential pressure hardly occurs.

FIG. 4 shows the relationship between the output current density and the generated voltage in the fuel cell 12. In the figure, c indicates that the operating temperature of the battery is 195 (℃) on average, the supply pressure of fuel gas and air is 3.5 (Kg / cm ² ), the damper 22 is fully closed, and the damper 2
No. 1 is fully open, and only the intake air E is supplied to the cathode electrode 3 side. Further, the oxygen utilization rate in the fuel cell 12 (the ratio of oxygen actually consumed in the cell 12 to the amount of oxygen in the air E supplied to the cell 12) is 60.
Similarly, the hydrogen utilization rate is 70%. Under this operating condition, when the load current becomes smaller than 80 (mA / cm ² ) in terms of current density, the voltage per cell becomes higher than 0.8 (V), which is in the dangerous range of electrochemical deterioration.

On the other hand, D is the case where the damper 22 is also opened and the exhaust gas F is mixed in the intake air E, and the operating conditions are that the exhaust gas mixture ratio of the cathode supply gas is 49 to 50 (%) and the oxygen concentration is 11 to 12 ( %) And the oxygen utilization rate is 40 to 45 (%). Although not shown, if the exhaust gas mixing ratio is further increased, it will fall below the curve d,
It can be easily inferred that it is suitable for operation under light load.

The present invention is not limited to the above embodiment, but can be carried out as follows.

(a) In the above embodiment, as shown in FIG. 3, the oxidant gas C from the compressor 14 is supplied to the combustion chamber 10 of the reformer 8.
5 is not directly introduced into the reformer 8, but a heat exchanger 24 is provided in the middle thereof so that the high-temperature reformed gas discharged from the reformer 8 and the mixed air to be supplied are heat-exchanged with each other. C may be preheated and supplied to the combustion chamber 10. With this configuration, there is no fear that the oxidant gas C in which the exhaust gas is mixed and the oxygen concentration is lowered does not perform stable combustion and causes misfire.

(b) Further, as shown in FIG. 6, even if the mixed air is introduced into the combustion chamber 10 by exchanging heat with the combustion exhaust gas from the reformer 8 by the heat exchanger 25, the same as in (a). Not only the effect can be obtained, but also the fuel gas is preheated, so that the overall thermal efficiency can be improved.

[Effects of the Invention] As described above, according to the present invention, a pair of porous electrodes of an anode and a cathode are arranged with an electrolyte in between, a fuel gas is allowed to pass through the back surface of the anode electrode, and oxidation is performed on the back surface of the cathode electrode. A fuel cell that allows an agent gas to pass through and takes out electrical energy generated by an electrochemical reaction at this time from a pair of electrodes, and is driven by supplying exhaust gas from the cathode electrode side of the fuel cell to the cathode electrode side of the fuel cell. Intake oxidant gas supply pipe at the compressor inlet of the turbine / compressor for exhaust gas discharged from the turbine / compressor that supplies the oxidant gas to the Exhaust gas supply pipe to be supplied to the passage, a valve body for flow rate control provided in the middle of the exhaust gas supply pipe, and a fuel cell Is low output, and when the voltage of the fuel cell is above a predetermined voltage that causes deterioration of the electrode component members of the fuel cell by an electrochemical reaction, the valve opening of the valve body is opened. When the load is reduced, the oxidant gas supplied to the fuel cell is mixed with the exhaust gas at a predetermined ratio to reduce the oxygen concentration during operation when the load is reduced. Since the voltage is controlled so that it does not occur, the deterioration of the electrode due to the electrochemical reaction is prevented, the life of the fuel cell is increased and the power generation efficiency of the fuel cell is improved A device can be provided.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing the principle configuration of a fuel cell, FIG. 2 is a characteristic diagram for explaining conventional problems, FIG. 3 is a configuration diagram showing an embodiment of the present invention, and FIG. FIG. 5 and FIG. 6 are characteristic diagrams for explaining the operation of the invention, and are diagrams showing another embodiment of the invention. 1 ... Electrolyte, 2 ... Anode electrode, 3 ... Cathode electrode, 4,5 ... Catalyst layer, 6, 7 ... Gas distribution chamber, 8 ...
… Reformer, 9 …… Catalyst tube, 10 …… Combustion chamber, 11
...... Fuel gas control valve, 12 ...... Fuel cell, 13 ...... Turbine, 14 ...... Compressor, 15 ...... Oxidant gas control valve, 16 …… Mixer, 17 …… Pressure control valve, 18 …… Auxiliary combustor , 19, 20 ... Control valve 21, 22 ... Damper, 23 ... Controller, 24, 25 ... Heat exchanger, A ...
Fuel gas, B ... Steam, C ... Oxidizer gas, D, F ...
… Exhaust gas, E… Intake air.

Claims (1)

[Claims] 1. A pair of porous electrodes, an anode and a cathode, are arranged with an electrolyte in between, and a fuel gas is passed through the back surface of the anode electrode and an oxidant gas is passed through the back surface of the cathode electrode. Fuel cell for extracting electric energy generated by a static reaction from the pair of electrodes, and driven by being supplied with exhaust gas from the cathode electrode side of the fuel cell, and supplying the oxidant gas to the cathode electrode side of the fuel cell The exhaust gas discharged from the turbine / compressor and the exhaust gas from the cathode electrode side of the fuel cell after driving the turbine is supplied to the intake oxidant gas supply pipeline at the compressor inlet of the turbine / compressor. An exhaust gas supply pipeline, a valve body for controlling a flow rate provided in the exhaust gas supply pipeline, the fuel cell When the pond is low-powered and the voltage of the fuel cell is higher than a predetermined voltage that causes deterioration of the electrode constituent members of the fuel cell by an electrochemical reaction, the valve opening of the valve body is opened. A fuel cell power generation device comprising: