JPH0275164A - Molten carbonate type fuel cell electricity generating device and operating method for the same - Google Patents

Abstract

PURPOSE:To decrease fuel consumption and enhance the power generating efficiency by sensing the output of a fuel cell of molten carbonate type, and controlling so that the oxygen concentration of cathode gas is greater and the carbon dioxide gas concentration is smaller with with larger. output. CONSTITUTION:An anode 2 is supplied with a fuel in hydrocarbon series such as LNG which is previously converted into hydrogen by a modifier 4. Air and part of the fuel having passed through an air control valve 7 and a fuel control valve 6, respectively, are sent to a catalyst burner 5 and combusted to be turned into a mixture gas of produced carbon dioxide gas and air as residue and forwarded to a cathode 1. The output of battery and the gas composition supplied to this cathode 1 are sensed by respective sensors 8A, 8B to serve for control of valves 6, 7 by a controller 8. That is, either the valve 6 is throttled or the one 7 opened when the output is increased, and thereby the proportion of oxygen and carbon dioxide gas is varied, which should enhance the power generating efficiency.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a molten carbonate fuel cell using molten carbonate as an electrolyte, and in particular improves the performance of the molten carbonate fuel cell and improves power generation efficiency. The present invention relates to a molten carbonate fuel cell power generation device that is a good enemy to the above, and a method of operating the power generation device.

[Conventional technology]

Generally, the cathode gas for molten carbonate fuel cells is
A mixture of air and carbon dioxide gas is used. In other words, carbon dioxide gas is produced by mixing carbon dioxide gas filled in a cylinder with air, or by burning hydrocarbon fuel.
A mixed gas of air and carbon dioxide obtained by mixing carbon dioxide and air is used.

Incidentally, the electromotive reaction of a molten carbonate fuel cell is expressed by the following equation.

Anode: H2+C03”←H20+CO2+ 2
e --- (1) Cathode: 1/2 o, +co, +
2 e−←CO3”−・・・= (2) Overall:
H2+1/20□←11□0 ・・・・・・(3
) At the cathode, 172 moles of oxygen, 1 mole of carbon dioxide, and electrons from the anode react electrochemically to produce carbonate ions. Therefore, the gas composition supplied to the cathode is adjusted so that the ratio of oxygen to carbon dioxide gas is 1:2 by volume.

In reality, it is difficult to make the volume ratio 1:2 due to combustion reactions, but efforts are being made to make it as close to 1:2 as possible.

[Problem to be solved by the invention]

However, as mentioned above, fixing the ratio of oxygen and carbon dioxide gas as cathode gas to 1:2 or a value close to this means that, for example, when cathode gas cooling is adopted, it is necessary to circulate a large amount of carbon dioxide gas. Because there is
There is a problem in that the amount of fuel burned as a source of carbon dioxide becomes enormous, the amount of fuel wasted increases, and the power generation efficiency with respect to the amount of fuel consumed decreases.

An object of the present invention is to provide a molten carbonate fuel cell power generation device and a method for operating the power generation device, which can reduce the amount of fuel wasted and improve power generation efficiency relative to the amount of consumed fuel.

[Means to solve the problem]

In order to achieve the above object, a first aspect of the power generation device of the present invention is to supply an anode gas containing hydrogen to the anode of a molten carbonate fuel cell, and supply a cathode gas containing oxygen and carbon dioxide to the cathode. In a molten carbonate fuel cell power generation device that supplies power to the molten carbonate fuel cell, the molten carbonate fuel cell has output means for detecting the output of the molten carbonate fuel cell, and a cell output of the molten carbonate fuel cell based on a signal from the output detection means. Detected,
means for increasing the oxygen concentration and decreasing the carbon dioxide concentration of the cathode gas as the battery output increases;
It is equipped with the following.

The second aspect of the power generation device of the present invention is to supply a hydrocarbon fuel to the anode of a molten carbonate fuel cell, convert the hydrocarbon fuel into hydrogen, and supply a cathode containing oxygen and carbon dioxide gas to the cathode. In an internal reforming type molten carbonate fuel cell power generation device that supplies gas to generate electricity, the molten carbonate fuel cell includes an output detection means for detecting the output of the molten carbonate fuel cell; The apparatus is equipped with a control means that detects the cell output of the carbonate fuel cell, and controls the oxygen concentration of the cathode gas to be increased and the carbon dioxide gas concentration to be decreased as the cell output increases.

Further, in the first operating method of the present invention, an anode gas containing hydrogen is supplied to the anode of the molten carbonate fuel cell, and a cathode gas containing oxygen and carbon dioxide is supplied to the cathode, thereby causing an electrochemical reaction. In the method for operating a molten carbonate fuel cell power generation device that generates electricity using a molten carbonate fuel cell, when the molten carbonate fuel cell is operated at a low current density, the ratio of oxygen to carbon dioxide gas in the cathode gas is approximately 1:1. 2, when operating at a high current density, the proportion of oxygen in the cathode gas is increased and the proportion of carbon dioxide gas is decreased.

In addition, the second operating method of the present invention is to supply an anode gas containing hydrogen to the anode of the molten carbonate fuel cell, and to supply a cathode gas containing oxygen and carbon dioxide to the cathode, thereby causing an electrochemical reaction. A method of operating a molten carbonate fuel cell power generation device that uses a molten carbonate fuel cell to generate electricity is to control the output of a molten carbonate fuel cell by changing the proportion of oxygen in the cathode gas.

[Effect]

According to the present invention, when operating at a high current density, the proportion of oxygen is increased and the proportion of carbon dioxide is decreased. This means that the amount of oxygen dissolved in the molten carbonate is approximately 10-'moQ.
/'drn'-atm, whereas carbon dioxide is about 1
0-2moR/dm・atm, which is 10" times larger. In other words, carbon dioxide is by far the most dissolved gas present in the electrochemical reaction field, so when the current value is increased, The elementary reactions at the cathode shown in the following formulas (4) and (5) become rate-determining and increase cathode polarization, so in order to prevent this, the ratio of carbon dioxide gas is reduced.

1/202+2e-←02-・・・・・・・・・・・・
・・・・・・・・・・・・(4) o2−+, co, ←C○
3'=・・・・・・・・・・・・・・・Old・・Mountain(5)
FIG. 2 shows changes in cathode units with respect to each current density when the oxygen concentration and carbon dioxide concentration in the gas supplied to the cathode are changed. The dotted line in the figure shows what happens when the ratio of oxygen to carbon dioxide is 1:2. As you can see from the figure, when no current is drawn, that is, when the circuit is open,
The cathode potential is most critical at the part indicated by the dotted line where oxygen/carbon dioxide gas = 1/2, where polarization is small and performance is high. However, when a large amount of current is extracted, the polarization is small and the performance is improved when the oxygen concentration is high and the carbon dioxide concentration is low. For example, at a current density of 150 mA/cJ, the polarization is lowest when oxygen/carbon dioxide = 2/1, which is the opposite of the case when the circuit is open. It is thought that this phenomenon occurs because there is a difference in the amounts of oxygen and carbon dioxide dissolved in the molten carbonate. That is, reaction formula (4), (
The elementary reaction at the cathode shown in 5) is that the above gas first dissolves into the molten carbonate, then reaches the cathode surface, and (
This is thought to be due to the reaction shown in equations 4,) and (5) occurring.

〔Example〕

An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 shows the overall configuration of a molten carbonate fuel cell power generation device according to the present invention. As shown in the figure, a molten carbonate fuel cell has a cathode 1. It consists of an anode 2 and an electrolyte 3 consisting of molten carbonate. A hydrocarbon fuel such as LNG or methane is converted into hydrogen by a reformer 4 and supplied to the anode 2 .

On the other hand, a part of the fuel and air are sent to the cathode 1 through a fuel control valve 6 and an air control valve 7, respectively, to a catalytic burner 5, where carbon dioxide gas generated by burning the fuel and the remaining air are sent to the cathode 1. It is supplied as a mixed gas. Further, a controller 8 is provided which detects the output of the fuel cell and the gas composition supplied to the cathode 1 with detectors 8A and 813, respectively, and controls the fuel control valve 6 and the air control valve 7. When the output of the battery increases, by throttling the fuel control valve 6 or further opening the air control valve 7, the ratio of oxygen and carbon dioxide gas is changed to an arbitrary suitable ratio to maintain high battery performance. We are trying to increase power generation efficiency.

In addition, in this embodiment, the detectors 8A and 8B serve as detection means.
The fuel control valve 6, the air control valve 7, and the controller 8 each constitute a control means.

Furthermore, according to this embodiment, it is also clear that the battery output can be controlled by changing the gas composition supplied to the cathode.

In addition to the output of the battery, the amount of fuel supplied to the reformer 4, the amount of gas supplied at the inlet of the anode 2, changes in battery temperature, etc. are detected, and the ratio of carbon dioxide and oxygen is changed according to the changes. It is also possible to control the composition to an optimum composition.

Furthermore, in so-called internal reforming fuel cells, which generate electricity by reforming hydrocarbon fuels such as LNG and methane into hydrogen on the anode side inside the molten carbonate fuel cell,
Similarly, by detecting battery output, cathode inlet gas composition, fuel supply amount, and battery temperature changes, the supply amount of oxygen and carbon dioxide can be controlled, improving battery performance and increasing power generation efficiency. .

According to the study results of this example, it was found that in a no-load state, that is, in an open circuit with zero current, a ratio of 1:2 between oxygen and carbon dioxide is suitable, and a ratio of 4:1 is suitable when the current density is around 250 mA/cJ. Also. A ratio of 2:1 is suitable at a current density of around 15-11=Om A/aJ, which is generally employed in molten carbonate fuel cells.

Note that it is also obvious that the current density or output during power generation may be determined in advance, and the ratio of oxygen and carbon dioxide gas supplied to the cathode may be set in advance to an optimum value for that value.

Next, an experimental example (1) to explain an experimental example related to the present invention
) The results shown in FIG. 2 were investigated only for the cathode, and it was also investigated whether the method according to the present invention is effective for actual batteries. A known molten carbonate fuel cell was assembled using a mixed carbonate consisting of 62 moles of lithium carbonate and 38 moles of potassium carbonate as the electrolyte, a nickel porous plate for the anode, and a nickel oxide porous plate for the cathode. 72% hydrogen, 18% carbon dioxide, 1 water vapor
An anode gas consisting of 0% was supplied, and a cathode gas with varying concentrations of air and carbon dioxide was supplied to the cathode.

The carbon dioxide gas used at this time was in a cylinder.

FIG. 3 shows the relationship between the cell voltage when operating at a current density of 1.50 mA/aJ and the concentration of air and carbon dioxide gas supplied to the cathode.

From this result, it is clear that the cell voltage becomes higher when the carbon dioxide concentration is smaller than what has been thought so far, and it becomes clear that the present invention is effective.

Experimental Example (2) As in Example (1), a power generation system having the configuration shown in FIG. 1 was created using a known molten carbonate fuel cell. LNG was used as a fuel, and gas that was reformed into hydrogen by the reformer 2 was supplied to the anode. In addition, a part of LNG is guided to the cathode to the catalytic burner 5, and this L N
A mixed gas of gas containing carbon dioxide produced by burning G and the remaining air used for LNG combustion was supplied. The cathode gas composition during no-load power generation was oxygen, nitrogen, carbon dioxide, and water vapor, and the ratio of oxygen to carbon dioxide was 1:6. Increase current density to 1.50 mA,/c
When the voltage was set to i, the cell voltage decreased from 1.07V at no load to 0.77V. Next, when the fuel supplied to the catalytic burner was reduced to a small amount using the fuel control valve 6, and the ratio of carbon dioxide to carbon dioxide was set to 2:1, the cell voltage increased to 0.82V. Further, when the current density was set to 250 mA/aK, the cell voltage was highest when the ratio of oxygen to carbon dioxide was 4:1.

〔Effect of the invention〕

As described above, according to the present invention, by reducing unnecessary carbon dioxide gas, the amount of fuel consumed can be reduced, and the power generation efficiency of the fuel cell can be improved.

[Brief explanation of the drawing]

Fig. 1 is an overall configuration diagram of a molten carbonate fuel cell power generation device according to the present invention, Fig. 2 is a graph showing the relationship between the ratio of oxygen and carbon dioxide supplied to the cathode and cathode performance, and Fig. 3 is a graph showing the relationship between the cathode performance and the ratio of oxygen and carbon dioxide gas supplied to the cathode. It is a graph showing the relationship between gas composition and cell voltage. DESCRIPTION OF SYMBOLS 1... Cathode, 2... Anode, 3... Electrolyte, 4... Reformer, 5... Catalyst burner, 6... Fuel control valve, 7... Air control valve, 8...・・Controller, 8
A, 8B...Detector.

Claims (1)

[Claims] 1. Molten carbonate fuel cell power generation that generates electricity by supplying an anode gas containing hydrogen to the anode of the molten carbonate fuel cell and supplying cathode gas containing oxygen and carbon dioxide to the cathode. In the apparatus, an output detection means for detecting the output of the molten carbonate fuel cell, and a cell output of the molten carbonate fuel cell is detected based on a signal from the output detection means, and as the cell output becomes larger, A molten carbonate fuel cell power generation device comprising: control means for controlling the oxygen concentration of the cathode gas to be high and the carbon dioxide concentration to be low. 2. The power generating apparatus according to claim 1, wherein the control means includes a combustion section that burns hydrocarbon fuel, and a control valve that controls the amount of the hydrocarbon fuel and air supplied to the combustion section. , a molten carbonate fuel cell power generation device. 3. The power generating apparatus according to claim 1, wherein the control means includes a combustion section that burns hydrocarbon fuel, and is provided downstream of the combustion section, and controls carbon dioxide gas from the combustion section according to the battery output. A molten carbonate fuel cell power generation device is provided with an absorption section that absorbs. 4. Internal reforming that supplies hydrocarbon fuel to the anode of a molten carbonate fuel cell, converts the hydrocarbon fuel into hydrogen, and supplies cathode gas containing oxygen and carbon dioxide to the cathode to generate electricity. type of molten carbonate fuel cell power generation device, comprising: an output detection means for detecting the output of the molten carbonate fuel cell; and a cell output of the molten carbonate fuel cell based on a signal from the output detection means. 1. A molten carbonate fuel cell power generation device, comprising: control means that increases the oxygen concentration of the cathode gas and controls the carbon dioxide concentration to decrease as the battery output increases. 5. A molten carbonate fuel cell that supplies an anode gas containing hydrogen to the anode of the molten carbonate fuel cell and supplies a cathode gas containing oxygen and carbon dioxide to the cathode to generate electricity using an electrochemical reaction. In the method for operating a fuel cell power generation device, when the molten carbonate fuel cell is operated at a low current density, the ratio of oxygen to carbon dioxide gas in the cathode gas is approximately 1:2, and when the molten carbonate fuel cell is operated at a high current density. teeth,
A method of operating a molten carbonate fuel cell power generation apparatus in which the ratio of oxygen in the cathode gas is increased and the ratio of carbon dioxide gas is lowered. 6. The operating method according to claim 5, in which the cathode gas containing oxygen and carbon dioxide gas is produced by supplying excess air and combusting the hydrocarbon fuel;
A method of operating a molten carbonate fuel cell power plant that reduces the amount of said hydrocarbon fuel when operating at high current densities. 7. The operating method according to claim 5, in which the cathode gas containing oxygen and carbon dioxide gas is produced by supplying excess air and combusting the hydrocarbon fuel;
A method of operating a molten carbonate fuel cell power plant in which the amount of air is increased when operating at high current densities. 8. The operating method according to claim 5, 6 or 7, wherein the ratio of oxygen to carbon dioxide in the cathode gas is set within the range of 1:2 to 4:1. Method. 9. A molten carbonate fuel cell that generates electricity by using an electrochemical reaction by supplying an anode gas containing hydrogen to the anode of the fuel cell and supplying a cathode gas containing oxygen and carbon dioxide to the cathode. A method of operating a molten carbonate fuel cell power generation device, wherein the output of the molten carbonate fuel cell is controlled by changing the ratio of oxygen and carbon dioxide in the cathode gas.