JP2002313402A - How to improve power generation efficiency of fuel cells - Google Patents

Abstract

(57) [Summary] [PROBLEMS] To increase the fuel utilization rate and improve the power generation efficiency in a fuel cell. In a fuel cell, unused fuel gas discharged without being used for power generation at the time of operation of the fuel cell is converted into CO 2 therein.
_{2. A} method for improving the power generation efficiency of a fuel cell, wherein the fuel cell ₂ is absorbed and removed by an absorbent and then reused as power generation fuel.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to relates to a power generation efficiency method of the fuel cell, by removing the carbon dioxide in the used fuel (carbon dioxide = CO ₂₎ in more detail in the fuel cell, the fuel cell The present invention relates to a method for increasing the fuel utilization rate in a plant and improving power generation efficiency.

[0002]

2. Description of the Related Art In a fuel cell such as a solid oxide fuel cell (SOFC) and a molten carbonate fuel cell (MCFC), when a hydrocarbon such as methane is used as a fuel, hydrogen obtained by reforming the hydrocarbon is used. And CO contribute to the electrochemical reaction. In these fuel cells, 80 to 90% of the supplied fuel is usually used to increase the fuel utilization rate, and the remaining 10 to 20% is discharged outside the cell stack without being used.

There are a cylindrical type (cylindrical type) and a flat type (flat type) as SOFCs, and FIG. 1 is a view schematically showing a configuration example of a cylindrical type SOFC. As shown on the left side of FIG.
A plurality of cylindrical cells are supported and arranged in the casing 1. The casing 1 is accommodated in a casing 2, and the casing 2 also forms a header or the like at an upper portion thereof. Each cylindrical cell consists of a double tube, and the outer tube is made of an electrolyte material. The inner tube is an air supply tube which is spaced apart in the outer tube.

[0004] Air is introduced into the air supply header,
From here, it passes through each inner pipe, turns back at the lower part, and flows upward between the inner pipe and the outer pipe. On the other hand, the fuel is introduced from the lower part of the casing 1 and flows upward outside each outer tube (that is, each cylindrical cell), and air (the side in which the fuel flows) of each cell on the fuel side (the side where the fuel flows). Electric power is generated by an electrochemical reaction with oxygen.

[0005] As described above, in an SOFC, hydrogen and CO in a fuel, that is, reformed gas, cannot all be used for power generation. Therefore, the unused fuel in the used fuel is burned by the unused air and discharged as exhaust gas. In the example of FIG. 1, unused fuel is burned by unused air and discharged as exhaust gas in the upper part in the casing 1. The figure on the right side in FIG.
FIG. 4 is an enlarged view of a part of an upper part of the casing 1, showing a combustion state of hydrogen and CO contained in fuel not used in the fuel cell, that is, used fuel in the fuel cell. In addition,
The used fuel in the fuel cell includes not only unused hydrogen and CO, but also other gases such as water vapor, but in the present specification, the used fuel is also referred to as “unused fuel” or the like as appropriate. .

When the hydrocarbon is methane, the hydrogen and CO in the reformed gas have a 3: 1 composition, but (CH ₄ + H ₂ O →
3H ₂ + CO), hydrogen used for power generation is water (steam)
Then, CO becomes CO ₂ . If the reformed fuel (mainly hydrogen and CO in the reformed gas) can be used at 100%, the power generation efficiency can be increased. However, when the fuel utilization is high, CO ₂ increases in the downstream side of the fuel flow. Accordingly, oxygen increases due to an equilibrium reaction (CO ₂ → CO + / O ₂ ). For this reason, the electromotive force of the cell is reduced, and the current cannot be sufficiently taken out. For this reason, the fuel utilization cannot be increased. FIG. 2 shows an example of the configuration of a flat-plate SOFC. The above points are the same in the case of a flat-type or integrally-stacked SOFC as shown in FIG.

[0007] There are two types of reformed gas generation: an internal reforming method in which reforming is performed inside the cell, and an external reforming method in which reforming is performed in a separately provided reformer and then introduced into the cell. Internal reforming S
In the OFC, it is necessary to simultaneously supply a certain amount of water in accordance with the amount of methane in order to cause a reforming reaction of a hydrocarbon, for example, methane directly inside the cell. It is possible to separately install a water supply system, but it is also considered to use water (steam) contained in spent fuel.
FIG. 3 is a diagram schematically showing an SOFC of a type for recycling spent fuel. As shown in FIG. 3, part of the spent fuel exiting the cell is recycled, and the water contained therein is used for reforming a fresh fuel that supplies the cell, ie, hydrocarbons such as methane.

However, as described above, in a SOFC of a type that recycles a part of spent fuel, CO ₂ generated during power generation is circulated again in addition to water (steam), unused hydrogen and CO. Will be. For this reason,
The oxygen concentration in the fuel increases due to the equilibrium reaction (CO ₂ → CO + / O ₂ ). As a result, the electromotive force in the fuel cell decreases, and the power generation efficiency decreases. The above points are MCF
The same applies to the case of C.

[0009]

SUMMARY OF THE INVENTION The present invention is to solve the above-mentioned problems in conventional fuel cells such as a solid oxide fuel cell (SOFC) and a molten carbonate fuel cell (MCFC). It is an object of the present invention to provide a method for removing or possibly removing CO _{2 in} spent fuel, thereby increasing a fuel utilization rate in a fuel cell and improving power generation efficiency.

[0010]

According to the present invention, there is provided (1) a fuel cell comprising: an unused fuel gas discharged without being used for power generation during operation of the fuel cell after absorbing and removing CO ₂ therein; Another object of the present invention is to provide a method for improving power generation efficiency of a fuel cell, wherein the method is reused as power generation fuel.

According to the present invention, (2) in a fuel cell, cells are divided into a first cell and a second cell, and the fuel cell is discharged without being used for power generation in the first cell during operation. that the unused fuel gas, after absorbing removed by absorber of CO ₂ therein, to provide power efficiency method for a fuel cell, characterized in that the reused as the power generation fuel in the second cell.

According to the present invention, (3) in a fuel cell, a cell is divided into two parts, a first cell and a second cell, and a pair of absorbents is arranged between the first cell and the second cell. By alternately switching through the valve, the absorption and removal of CO ₂ in the unused fuel gas discharged without being used for power generation in the first cell by one absorbent and the regeneration of the other absorbent are alternately performed. And supplying the unused fuel gas from which the CO ₂ has been absorbed and removed to the second cell and reusing it as the fuel for power generation in the second cell. .

The present invention provides (4) a fuel cell for recycling used fuel from a cell, wherein CO _{2 in} unused fuel gas discharged without being used for power generation during its operation.
Using an absorbent to mix the unused fuel gas from which the CO ₂ has been absorbed and removed into fresh fuel to be supplied to the fuel cell and reusing it as fuel for power generation. Provide a way to improve efficiency.

According to the present invention, (5) in a fuel cell for recycling spent fuel from a cell, a pair of absorbents are arranged, and when the fuel cell is operated, the fuel cells are alternately switched via a pipe and a switching valve to use one of the absorbents. The CO ₂ in the unused fuel gas discharged without being used for power generation is alternately removed and the other absorbent is regenerated, and the unused fuel gas from which the CO ₂ has been absorbed and removed is supplied to the fuel cell. Provided is a method for improving power generation efficiency of a fuel cell, wherein the method is mixed with fresh fuel and reused as power generation fuel.

[0015]

DETAILED DESCRIPTION OF THE INVENTION In the present invention (1), in the fuel cell, after the unused fuel gas exhausted without being used for power generation during the operation, were absorbed and removed by the absorbent of CO ₂ therein, Reuse as fuel for power generation. In the present invention (2), in the fuel cell, the cells are divided into two parts, a first cell and a second cell. Then, the unused fuel gas discharged without being used for power generation in the first cell during the operation is removed by absorbing CO ₂ therein by an absorbent, and then reused as power generation fuel for the second cell. I do.

According to the present invention (3), in the fuel cell, the cells are divided into two parts, a first cell and a second cell, and a pair of absorbents are arranged between them. Switch alternately via And during that driving,
The absorption and removal of CO ₂ in the unused fuel gas discharged without being used for power generation in the first cell by one of the absorbents and the regeneration of the other absorbent were performed alternately, and the CO ₂ was absorbed and removed. Unused fuel gas is supplied to the second cell and is reused as fuel for power generation in the second cell.

According to the present invention (4), in a fuel cell for recycling used fuel from a cell, C is contained in unused fuel gas discharged without being used for power generation during operation.
O ₂ is removed using an absorbent, and the unused fuel gas from which the CO ₂ has been absorbed and removed is mixed with fresh fuel supplied to the fuel cell and reused as power generation fuel.

According to the present invention (5), in a fuel cell for recycling spent fuel exiting from a cell, a pair of absorbents is arranged and switched alternately via a pipe and a switching valve. During the operation, absorption and removal of CO ₂ in the unused fuel gas discharged without being used for power generation by one absorbent and regeneration of the other absorbent are performed alternately, and the CO ₂ is absorbed and removed. The unused fuel gas is mixed with fresh fuel supplied to the fuel cell and reused as power generation fuel. In the present invention, these make it possible to use a fuel cell,
It is possible to increase the fuel utilization rate of the OFC or the MCFC and improve the power generation efficiency.

The CO ₂ absorbent used in the present invention includes:
Any substance that can absorb CO ₂ in the spent fuel is used. As a phenomenon in which a gas such as CO ₂ is absorbed (absorbed) by a solid or a liquid, in addition to so-called absorption, there are adsorption and sorption accompanied by reaction or dissolution in adsorption. The substance that absorbs CO ₂ in the spent fuel due to these phenomena is referred to as a CO ₂ absorbent or simply an absorbent. The absorbent can be used in an appropriate mode, such as filling the container in the form of granules or granules, or supporting it on a honeycomb-shaped heat-resistant base material and disposing it in the container.

A preferred example of the CO ₂ absorbent used in the present invention is lithiated zirconia (Li ₂ ZrO ₃ or Li ₂ ZrO _3).
4 ZrO ₄ ). Li ₂ ZrO ₃ absorbs CO ₂ by the reaction of the following formula (1). This reaction is
It is a reversible reaction, which proceeds to the right at low temperatures and to the left at high temperatures, for example, at around 700 ° C. (depending on pressure conditions etc.). Moreover, the reaction rate is sufficiently fast in the temperature range, the per 600 ° C. CO ₂ as that 520 times the lithium zirconia is absorbed by volume. In the present invention, CO _{2 in} spent fuel can be removed in such a temperature range by using such an absorbent.

[0021]

[Formula 1]

For example, spent fuel at about 600 ° C. in an SOFC is passed through this absorbent made of lithiated zirconia to absorb CO _2, and after the absorption is saturated (or immediately before saturation), this time, for example, 700 ° C. more air extent to release CO ₂ through a lithium zirconia. By repeating this operation, CO ₂ in the spent fuel can be removed, and the absorbent can be regenerated to repeatedly remove the CO ₂ in the spent fuel.

In an SOFC, for example, an operating temperature of 10
When the operation is performed at about 00 ° C., the fuel utilization rate is about 85%, and the cell electromotive force near the fuel outlet drops to about 0.78 V. On the other hand, when the used fuel discharged from the fuel outlet is subjected to the CO ₂ removal treatment according to the present invention and then supplied again as fuel, the battery electromotive force recovers to about 0.8V.

This value is substantially equal to the battery electromotive force near the fuel outlet when the battery is operated at a fuel utilization of 80%. The fuel which has been subjected to the CO ₂ removal treatment is supplied again to the cell,
When the current is taken out until the electromotive force near the fuel outlet becomes about 0.78 V, the fuel utilization rate can be finally improved to about 90%. As described above, according to the present invention, by removing CO ₂ in the spent fuel, the cell electromotive force can be increased even in the downstream region of the fuel flow.

[0025]

EXAMPLES Hereinafter, the present invention will be described in more detail with reference to Examples, but it goes without saying that the present invention is not limited to these Examples.

<Embodiment 1> In this embodiment, a fuel cell (SOF
In C), the spent fuel, that is, the unused fuel discharged without being used for power generation during the operation,
This is an example in which O ₂ is removed using an absorbent and then reused as a fuel for power generation. As shown in FIG. 4, the cell is divided into two parts, A and B, and the spent fuel conduit and the air conduit from the cell A are arranged between the cells A and B. Then, a CO ₂ absorbent layer is arranged in the spent fuel conduit. The CO ₂ absorbent layer can be arranged in a usual manner, for example, by filling or disposing a granular or honeycomb-shaped CO ₂ absorbent in a container. This is the same in the following examples.

During operation of the fuel cell, that is, during power generation, the spent fuel from the cell A passes through the CO ₂ absorbent layer, where CO ₂ is absorbed and removed. Since the spent fuel (fuel after treatment) introduced into the cell B has CO ₂ removed,
Since the equilibrium reaction (CO ₂ → CO + / O ₂ ) does not occur without removing CO ₂ as in the conventional case,
Since there is no increase in oxygen, a decrease in electromotive force of the cell can be prevented, and the fuel utilization can be increased. As described above, since the absorbent such as lithiated zirconia has a suitable temperature for absorbing CO _2, an appropriate or optimal absorbent is selected as the CO ₂ absorbent depending on the temperature of the spent fuel from the cell A. However, if necessary, a heat exchanger or the like for adjusting the used fuel to the appropriate temperature may be provided. Further, the absorbent eventually becomes a state where CO ₂ is saturated (or nearly saturated). In this case, the absorbent may be regenerated by heating or the like, or may be replaced with a new absorbent. These points are the same in the following embodiments.

<Embodiment 2> This embodiment is the same as Embodiment 1 in that the cell is divided into A and B, but a pair (two) of CO ₂ absorbent layers are arranged and switched alternately. This is an example in which CO ₂ is absorbed and removed by the absorbent and the absorbent is regenerated. As Figure 5, a pair of CO ₂ absorbent layer 1 and the CO ₂ absorbent layer 2 disposed as CO ₂ absorbent layer, placing a piping and switching valve 1-4 to this. 5 (a) is carried out absorption removal of CO ₂ from the used fuel in the cell A in a CO ₂ absorbent layer 1, when reproducing the CO ₂ absorbent layer 2, FIG. 5 (b) CO ₂ absorbent This is the case where CO ₂ is absorbed and removed from the spent fuel in the layer 2 to regenerate the CO ₂ absorbent layer 1.

The changeover valve 1 is a valve for switching the used fuel from the cell A in any of the CO ₂ absorbent layer 1 and the CO ₂ absorbent layer 2, FIGS. 5 (a) the CO ₂ absorbent layer It is set to distribute to 1. The switching valve 2 is a CO ₂
A valve for switching the fuel after the absorption treatment with either the absorbent layer 1 or the CO ₂ absorbent layer 2 (the processed fuel = the spent fuel after absorbing and removing the CO ₂ ) to the cell B. In FIG. 5A, it is set so that the processed fuel from the CO ₂ absorbent layer 1 is supplied to the cell B. The switching valve 3 converts the used air from the cell A into the CO ₂ absorbent layer 1.
And a valve to switch to any of the CO ₂ absorbent layer 2, is set so as to be circulated to FIGS. 5 (a) the CO ₂ absorbent layer 2. Switching valve 4, the used air that has passed through one of the CO ₂ absorbent layer 1 and the CO ₂ absorbent layer 2 (C
The O ₂ air after subjected to regeneration of the absorbent layer 1 and the CO ₂ absorbent layer 2) with a valve for switching manner to supply to the cell B, FIGS. 5 (a) from CO ₂ absorbent layer 2, It is set to supply used air to the cell B.

At the stage of FIG. 5A, the spent fuel from the cell A passes through the CO ₂ absorbent layer 1, where CO ₂ is absorbed and removed, and is introduced into the cell B. CO ₂ has been removed from the spent fuel introduced into the cell B. Therefore,
As is conventional, the equilibrium reaction CO ₂ is occurring remains not removed _{(CO 2 → CO + 1 /} 2O 2) no increase in oxygen because does not occur, thereby preventing an electromotive force reduction of the cell, the fuel utilization ratio Can be raised. On the other hand, the used air from the cell A passes through the CO ₂ absorbent layer 2 where
After the absorbent in the O ₂ absorbent layer 2 is regenerated, it is supplied to the cell B and used for power generation in the cell B.

The CO ₂ absorbent layer 1 is in a state where CO ₂ is saturated (or nearly saturated). At this time, the switching valve 1
４4 is switched. FIG. 5B shows this stage. In the stage of FIG. 5B, the spent fuel from the cell A passes through the CO ₂ absorbent layer 2, where CO ₂ is absorbed and removed, and is introduced into the cell B. The spent fuel has the CO ₂ removed, and thus, as before, the equilibrium reaction (CO ₂ → CO +) that occurs without the CO ₂ being removed.
Since 1/2 O ₂ ) does not occur, there is no increase in oxygen, thereby preventing a decrease in the electromotive force of the cell and increasing the fuel utilization. On the other hand, the used air from the cell A is CO ₂
After passing through the absorbent layer 1 and regenerating the absorbent in the CO ₂ absorbent layer 1 here, it is supplied to the cell B and used for power generation in the cell B.

Embodiment 3 As shown in FIG. 3 described above, in an SOFC of a type in which part of the spent fuel is recycled, part of the spent fuel exiting the cell is recycled, and the water (steam) contained therein is recycled. ) Is used for methane reforming.
This embodiment is an example in which the present invention is applied to this type of SOFC. As shown in FIG. 6, a CO ₂ absorbent layer is placed in a recycle conduit branched from a spent fuel conduit from the cell.

At the time of power generation, part of the spent fuel from the cell passes through the CO ₂ absorbent layer, where CO ₂ is absorbed and removed, and mixed with fresh fuel (hydrocarbon such as methane) supplied to the cell. You. Since CO ₂ has been removed from the spent fuel, the shift reaction in the cell shifts in the direction of generating hydrogen (CO + H ₂ O → CO ₂ + H ₂ ), the equilibrium oxygen partial pressure also decreases, and the electromotive force decreases. Can be enhanced. In this regard, as in the conventional case (FIG. 3), in a SOFC that recycles a part of the spent fuel, water, unused hydrogen,
In addition to CO, CO ₂ generated during power generation will be circulated again. Therefore, conventionally, the shift reaction shifts in a direction of consuming hydrogen (CO ₂ + H ₂ → CO + H).
₂ O) Although the equilibrium oxygen partial pressure also increases, according to the present invention, the shift reaction is shifted in the direction in which hydrogen is generated, whereby the electromotive force can be increased and the fuel utilization can be increased.

[0034] <Example 4> this example, but in terms of applying the present invention to the type of the SOFC to recycle the used fuel exiting the cell is the same as in Example 3, a pair of CO ₂ absorbent layer (2 In this example, CO ₂ is absorbed and removed and regenerated by switching alternately. As Figure 7, a pair of CO ₂ absorbent layer 1 and the CO ₂ absorbent layer 2 disposed as CO ₂ absorbent layer, placing a piping and switching valve 1-4 to this. FIG.
(A) shows the used fuel recycled from the cell as C
Through O ₂ absorber layer 1 perform absorption removal of CO _2, CO ₂
When playing the absorbent layer 2 of absorbent, FIG. 7 (b), performs the absorption removal of CO ₂ through the used fuel is recycled from the cell to the CO ₂ absorbent layer 2, CO ₂ absorbent layer In this case, the absorbent of No. 1 is regenerated.

The switching valve 1 is a switching valve provided in a recycle conduit branched from a conduit for spent fuel from the cell.
In (a), the fuel is set to flow to the CO ₂ absorbent layer 1. Switching valve 2 is switched recycled fuel after the absorption treatment with either CO ₂ absorbent layer 1 and the CO ₂ absorbent layer 2 (the used fuel after the CO ₂ has been absorbed and removed) to supply to the cell In FIG. 7A, the valve is set so that the recycled fuel after CO ₂ absorption and removal in the CO ₂ absorbent layer 1 is mixed with the fuel supplied to the cell. Switching valve 3 is a valve for switching the used air from the cell to one of the CO ₂ absorbent layer 1 and the CO ₂ absorbent layer 2, FIGS. 7 (a) the CO ₂ absorbent layer 2
It is set to be distributed to. The switching valve 4 is a CO
The used air that has passed through one of the ₂ absorbent layer 1 and the CO ₂ absorbent layer 2 (the used air after subjected to regeneration of CO ₂ absorbent layer 1 or CO ₂ absorbent layer 2 of the absorbent) In FIG. 7 (a), the valve is set to supply the used air from the CO ₂ absorbent layer 2 to the used fuel not to be recycled. I have.

At the stage shown in FIG. 7A, the spent fuel recycled among the spent fuel from the cell passes through the CO ₂ absorbent layer 1 where CO ₂ is absorbed and removed. Since CO ₂ has been removed from this recycled fuel, the shift reaction in the cell shifts in the direction of generating hydrogen (CO + H ₂ O → CO ₂ + H ₂ ), and the electromotive force of the cell can be increased. At this stage, the use has been air from the cell through the CO ₂ absorbent layer 2, wherein after playing absorber CO ₂ absorbent layer 2, thereby burning the used fuel is not the used fuel i.e. recycling of residual .

At the stage shown in FIG. 7B, the spent fuel recycled among the spent fuel from the cell passes through the CO ₂ absorbent layer 2 where CO ₂ is absorbed and removed. The CO ₂ has been removed from the recycled fuel, so that the shift reaction in the cell is shifted in a direction to generate hydrogen, and thus the electromotive force of the cell can be increased. At this stage, the use has been air from the cell through the CO ₂ absorbent layer 1, wherein after playing CO ₂ absorbent layer 1 of absorbent to burn the used fuel is not the used fuel i.e. recycling of residual . As described above, since the absorbent such as lithiated zirconia has a suitable temperature for absorbing and regenerating CO ₂ , the temperature of the spent fuel from the cell A and the temperature of the used air from the cell A may be different. The type of the absorbent is selected according to the conditions, but if necessary, a heat exchanger for cooling or heating to an appropriate temperature can be appropriately disposed.

[0038]

According to the present invention, in a fuel cell such as a solid oxide fuel cell or a molten carbonate fuel cell, the fuel utilization can be increased and the power generation efficiency can be improved. Further, according to the present invention, an equilibrium reaction (CO ₂ → CO + 1 / 2O) that occurs without removing CO ₂ as in the prior art.
_{Since 2} ) does not occur, there is no increase in oxygen, which prevents a decrease in the electromotive force of the cell and increases the fuel utilization. Furthermore, according to the present invention, in a fuel cell of a type that recycles a part of spent fuel, the fuel utilization is improved by shifting the shift reaction in a direction in which hydrogen is generated (CO + H ₂ O → CO ₂ + H ₂ ). Various useful effects can be obtained, such as an increase in the induced electromotive force.

[Brief description of the drawings]

FIG. 1 is a diagram schematically illustrating a configuration example of a cylindrical SOFC.

FIG. 2 is a diagram schematically illustrating a configuration example of a flat panel SOFC.

FIG. 3 is a diagram schematically showing an SOFC of a type for recycling spent fuel in a cell.

FIG. 4 is a diagram showing a first embodiment of the present invention.

FIG. 5 is a diagram showing a second embodiment of the present invention.

FIG. 6 is a diagram showing a third embodiment of the present invention.

FIG. 7 is a diagram showing a fourth embodiment of the present invention.

Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat II (reference) H01M 8/04 H01M 8/04 J N 8/12 8/12 F term (reference) 4D020 AA03 BA01 BA06 BA08 BB01 BC01 BC04 CA03 CA05 CC01 CC12 CC20 CC21 4G040 FA04 FB04 FC02 FD07 FE01 4G066 AA13B AA23B BA07 CA35 DA04 DA05 EA20 GA07 GA08 GA37 5H026 AA05 AA06 5H027 AA05 AA06 BA16 BA19

Claims (7)

[Claims] In a fuel cell, an unused fuel gas discharged without being used for power generation during operation of the fuel cell is converted into CO 2 therein.
_{2. A} method for improving the power generation efficiency of a fuel cell, wherein the fuel cell ₂ is absorbed and removed by an absorbent and then reused as power generation fuel. 2. In a fuel cell, a cell is divided into a first cell and a second cell, and an unused fuel discharged without being used for power generation in the first cell during operation. A method for improving the power generation efficiency of a fuel cell, wherein the gas is reused as power generation fuel for a second cell after absorbing and removing CO ₂ therein by an absorbent. 3. In a fuel cell, a cell is divided into a first cell and a second cell, and a pair of absorbents is disposed between the first cell and the second cell. By alternately switching, the absorption and removal of CO ₂ in the unused fuel gas discharged without being used for power generation in the first cell by one of the absorbents and the regeneration of the other absorbent are performed alternately. _{2. A} method for improving the power generation efficiency of a fuel cell, wherein the unused fuel gas from which ₂ has been absorbed and removed is supplied to the second cell and reused as power generation fuel for the second cell. 4. In a fuel cell for recycling used fuel from a cell, CO ₂ in unused fuel gas discharged without being used for power generation during operation is removed using an absorbent, and the CO _{2 is} removed. A method for improving the power generation efficiency of a fuel cell, comprising mixing unused fuel gas from which fuel has been absorbed and removed into fresh fuel supplied to the fuel cell and reusing it as fuel for power generation. 5. In a fuel cell for recycling spent fuel from a cell, a pair of absorbents is disposed, and during operation thereof, the pair is alternately switched via a pipe and a switching valve to be used for power generation by one of the absorbents. The CO ₂ in the unused fuel gas discharged without being removed is alternately removed and the other absorbent is regenerated, and the CO ₂ absorbed and removed unused fuel gas is converted into fresh fuel to be supplied to the fuel cell. A method for improving power generation efficiency of a fuel cell, wherein the fuel cell is mixed and reused as fuel for power generation. 6. The method for improving power generation efficiency of a fuel cell according to claim 1, wherein said absorbent is lithiated zirconia. 7. The method according to claim 1, wherein the fuel cell is a solid oxide fuel cell or a molten carbonate fuel cell.