JP2018137080A - Fuel cell system and power generation method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a fuel cell system which is superior in the endurance of a fuel cell stack.SOLUTION: A fuel cell system comprises: a first fuel cell stack operable to perform power generation by a fuel gas; separation means for separating at least one of carbon dioxide and water vapor from an anode off-gas containing the un-reacted fuel gas exhausted from the first fuel cell stack; and a second fuel cell stack disposed downstream of the separation means and using the anode off-gas with the at least one of carbon dioxide and water vapor separated therefrom to perform power generation. If the fuel concentration of the anode off-gas when exhausted from the first fuel cell stack is defined as a first fuel concentration, and the fuel concentration of the anode off-gas when exhausted from the second fuel cell stack is defined as a second fuel concentration, the first and second fuel concentrations are each 0.9C or more, where C is a fuel concentration when the first fuel concentration is equal to the second fuel concentration.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a fuel cell system and a power generation method.

  In a fuel cell system, in order to improve power generation efficiency, a technique of providing a plurality of fuel cell stacks and making it a multistage type is known. For example, there is a multistage fuel cell system in which a plurality of fuel cell stacks are provided, and unreacted fuel gas in used fuel gas discharged from the previous fuel cell stack is reused in the subsequent fuel cell stack. It is known (see, for example, Patent Document 1).

JP 2006-31989

  Here, in the multi-stage fuel cell system described in Patent Document 1, no consideration has been given to the fuel concentration in the anode off-gas discharged from the front and rear fuel cell stacks. In a multistage fuel cell system, when the fuel concentration in the anode off-gas discharged from the front and rear fuel cell stacks, that is, the fuel concentration at the outlet of the front and rear fuel cell stacks is low, the fuel cell stack There is a risk that the durability of the fuel cell stack may be reduced by oxidation of the fuel.

  An object of this invention is to provide the fuel cell system excellent in durability of the fuel cell stack, and the electric power generation method using this fuel cell system.

The above problem is solved by, for example, the following means.
<1> At least one of carbon dioxide and water vapor from a first fuel cell stack that generates power using fuel gas, and an anode off-gas containing the unreacted fuel gas discharged from the first fuel cell stack Separating means for separating, and a second fuel cell stack disposed downstream of the separating means and generating power using the anode off-gas from which at least one of carbon dioxide and water vapor is separated, the first fuel When the fuel concentration of the anode off gas at the outlet of the battery cell stack is the first fuel concentration, and the fuel concentration of the anode off gas at the outlet of the second fuel cell stack is the second fuel concentration, the first fuel concentration and the When the value at which the second fuel concentration values are equal is C, the first fuel concentration and the second fuel concentration are respectively Fuel cell system that satisfies 0.9C or higher.

  In the fuel cell system of the present embodiment, at least one of carbon dioxide and water vapor is separated from the anode off-gas discharged from the first fuel cell stack by the separation means, and the anode off-gas is regenerated to increase its fuel concentration. Then, since the anode off-gas whose fuel concentration has been increased is supplied to the second fuel cell stack, the fuel cell system of this embodiment can be operated at a high fuel utilization rate, and high power generation efficiency can be obtained.

  In general, in a multistage fuel cell system, the fuel concentration of the anode gas in the first fuel cell stack is highest at the inlet of the first fuel cell stack, and at the outlet of the first fuel cell stack. The lowest. The same applies to the fuel concentration of the anode gas in the second fuel cell stack. Here, in the first fuel cell stack and the second fuel cell stack, when the fuel concentration of the anode gas in the fuel cell stack decreases, the durability of the fuel cell stack decreases due to oxidation of the fuel cell stack or the like. There is a risk. Therefore, the fuel concentration becomes the lowest and the fuel concentration at the outlet of the first fuel cell stack and the outlet of the second fuel cell stack is increased to suppress oxidation of the fuel cell stack (for example, oxidation of the fuel electrode). It is preferable.

  Therefore, in the fuel cell system of the present embodiment, the anode off gas is regenerated by the separation means to increase the fuel concentration, and the lowest fuel concentration (first fuel concentration) in the first fuel cell stack and the lowest in the second fuel cell stack. By increasing the fuel concentration (second fuel concentration), oxidation or the like of the fuel cell stack is suppressed. More specifically, by making the first fuel concentration and the second fuel concentration equal, both the first fuel concentration and the second fuel concentration can be made high, and oxidation of the fuel cell stack can be reduced. It is possible to provide a fuel cell system that can be suitably suppressed and that is more excellent in durability of the fuel cell stack. Further, when the value when the first fuel concentration and the second fuel concentration are equal is C, the first fuel concentration and the second fuel concentration are 0.9C or more, that is, 90% or more when the respective concentrations are equal. If so, the difference between the first fuel concentration and the second fuel concentration can be suppressed, the risk of oxidation of the fuel cell stack is suppressed, and the fuel cell excellent in durability of the fuel cell stack A system can be provided.

<2> The fuel cell system according to <1>, wherein in the separation unit, one of the carbon dioxide separation rate and the water vapor separation rate is 50% or more and 99% or less.

  In the fuel cell system of the present embodiment, when one of the carbon dioxide separation rate and the water vapor separation rate satisfies the above numerical range, the anode off-gas is regenerated and the fuel concentration suitably increases. Therefore, the fuel cell system of this embodiment can be operated at a higher fuel utilization rate, and higher power generation efficiency can be obtained.

<3> The separation means separates carbon dioxide and water vapor from the anode off gas,
In the separation means, the carbon dioxide separation rate and the water vapor separation rate are both 50% or more and 99% or less, <1>.

  In the fuel cell system of the present embodiment, the carbon dioxide separation rate and the water vapor separation rate satisfy the above numerical ranges, whereby the anode off-gas is regenerated and the fuel concentration is more preferably increased. Therefore, the fuel cell system of this embodiment can be operated at a higher fuel utilization rate, and higher power generation efficiency can be obtained.

<4> When the total effective surface area of the fuel cells provided in the first fuel cell stack is α ₁ , and the total effective surface area of the fuel cells provided in the second fuel cell stack is α ₂ , α ₁ / alpha ₂ is exceeding 2.0 <1> to <3> fuel cell system according to any one of.
<5> When the fuel cell stack included in the first fuel cell stack and the fuel cell stack included in the second fuel cell stack are the same, the number β ₁ of fuel cell stacks in the first fuel cell stack, when the number beta ₂ of the fuel cell stack in the second fuel cell stack, when the _{β 1 / β 2, β 1} / β 2 of 2.0 than <1> - or <4> 1 The fuel cell system described in 1.

In the fuel cell system of the present embodiment, α ₁ / α ₂ or β ₁ / β ₂ is greater than 2.0, so that α ₁ / α ₂ or β ₁ / β ₂ is less than or equal to 2.0. Thus, both the first fuel concentration and the second fuel concentration can be set to high values. Therefore, oxidation of the fuel cell stack can be suitably suppressed, and a fuel cell system that is more excellent in durability of the fuel cell stack can be provided.

<6> In the separation means, when one of carbon dioxide and water vapor is separated, the first fuel concentration and the second fuel concentration are each 10% or more, and any one of <1> to <5> The fuel cell system described.
<7> In the separation means, when carbon dioxide and water vapor are separated, the first fuel concentration and the second fuel concentration are each 11.5% or more, and any one of <1> to <6> The fuel cell system described.

  In the fuel cell system of the present embodiment, by setting the values of the first fuel concentration and the second fuel concentration within the above numerical range, it is possible to suitably suppress the oxidation of the fuel cell stack and the like. A fuel cell system having superior durability can be provided.

<8> A reformer that reforms the raw material gas to generate a fuel gas is further provided, and at least one of carbon dioxide and water vapor separated by the separation unit is used for the reforming of the raw material gas <1> to <7 > The fuel cell system according to any one of

  In the fuel cell system of this embodiment, it is not necessary to supply at least one of carbon dioxide and steam used for reforming the raw material gas from the outside to the reformer, or carbon dioxide and steam supplied from the outside to the reformer. The amount of at least one of the above can be reduced.

<9> A power generation method using the fuel cell system according to any one of <1> to <8>.
<10> The power generation method according to <9>, wherein power generation is performed by operating the first fuel cell stack and the second fuel cell stack at an equal current density.

  In the power generation method of this embodiment, since power generation is performed using the above-described fuel cell system, the fuel cell stack is excellent in durability and stable power generation is possible over the long term.

<11> The sum of the product of the effective surface area of the fuel cell provided in the first fuel cell stack and the current density of the fuel cell when the first fuel cell stack is operated is γ ₁ , when the sum of the product of the current density of the fuel cell when operated the effective surface area of the fuel cell provided in the second fuel cell stack a second fuel cell stack was γ _2, γ _{1 /} γ The power generation method according to <9> or <10>, wherein power generation is performed by adjusting ₂ to greater than 2.0.

In the power generation method of the present embodiment, since γ ₁ / γ ₂ is greater than 2.0, the first fuel concentration and the second fuel concentration are compared with the case where γ ₁ / γ ₂ is 2.0 or less. Both values can be high. Therefore, the oxidation of the fuel cell stack can be suitably suppressed, the fuel cell stack is more excellent in durability, and more stable power generation is possible over a long period.

  ADVANTAGE OF THE INVENTION According to this invention, the fuel cell system excellent in durability of the fuel cell stack and the electric power generation method using this fuel cell system can be provided.

1 is a schematic configuration diagram showing a fuel cell system according to a first embodiment. It is a schematic block diagram which shows the fuel cell system which concerns on 2nd Embodiment. When the removal rate of H 2 _O and CO ₂ are both 95%, and the fuel utilization rate of the first fuel cell stack is a graph showing the relationship between the first fuel concentration and a second fuel concentration. When H 2 _O removal rate is 95%, and the fuel utilization rate of the first fuel cell stack is a graph showing the relationship between the first fuel concentration and a second fuel concentration. It is a graph showing the relationship between removal rate and the stack ratio of H _{2 O} and CO ₂ in the anode off-gas. It is a graph showing the relationship between the H _{2 O} removal ratio in the anode off gas and the stack ratios.

In this specification, a numerical range expressed using “to” means a range including numerical values described before and after “to” as a lower limit value and an upper limit value.
In the numerical ranges described stepwise in this specification, the upper limit value or the lower limit value described in one numerical range may be replaced with the upper limit value or the lower limit value of another numerical range. Good. Further, in the numerical ranges described in the present specification, the upper limit value or the lower limit value of the numerical range may be replaced with a value shown in an example of the embodiment.

[First Embodiment]
Hereinafter, an embodiment of a fuel cell system of the present invention will be described with reference to FIG. FIG. 1 is a schematic configuration diagram showing a fuel cell system according to the first embodiment. A fuel cell system 10 according to the first embodiment includes a first fuel cell stack 11 that generates power using fuel gas, and an anode off-gas containing the unreacted fuel gas discharged from the first fuel cell stack 11. From the separation membrane 16 that separates at least one of carbon dioxide and water vapor, and the second fuel battery cell that is arranged downstream of the separation membrane 16 and that generates power using the anode off-gas from which at least one of carbon dioxide and water vapor is separated A fuel concentration of the anode off-gas at the outlet of the first fuel cell stack 11, and a fuel concentration of the anode off-gas at the outlet of the second fuel cell stack 12 as the second fuel concentration. When the value when the first fuel concentration and the second fuel concentration are equal is C, the first fuel concentration and the second fuel concentration Concentration satisfies the above 0.9C, respectively.
Furthermore, the fuel cell system 10 according to the present embodiment includes a reformer 14 having a reforming unit 19 that reforms a raw material gas to generate a fuel gas and a combustion unit 18 that heats the reforming unit 19 by a combustion reaction. I have.

  In the fuel cell system 10 of the present embodiment, at least one of carbon dioxide and water vapor is separated from the anode off gas discharged from the first fuel cell stack 11 by the separation membrane 16, and the anode off gas is regenerated and its fuel concentration is reduced. To rise. Then, since the anode off-gas having an increased fuel concentration is supplied to the second fuel cell stack 12, the fuel cell system 10 can be operated at a high fuel utilization rate, and high power generation efficiency can be obtained.

  In general, in a multistage fuel cell system, the fuel concentration of the anode gas in the first fuel cell stack is highest at the inlet of the first fuel cell stack, and at the outlet of the first fuel cell stack. The lowest. The same applies to the fuel concentration of the anode gas in the second fuel cell stack. Here, in the first fuel cell stack and the second fuel cell stack, when the fuel concentration of the anode gas in the fuel cell stack decreases, the durability of the fuel cell stack decreases due to oxidation of the fuel cell stack or the like. There is a risk. Therefore, it is preferable to suppress the oxidation of the fuel cell stack by increasing the fuel concentration at the outlet of the first fuel cell stack and the outlet of the second fuel cell stack where the fuel concentration is the lowest.

  Therefore, in the fuel cell system 10, the anode offgas is regenerated by the separation membrane 16 which is a kind of separation means to increase the fuel concentration, and the minimum fuel concentration (first fuel concentration) and the second fuel concentration in the first fuel cell stack 11. By increasing the minimum fuel concentration (second fuel concentration) in the fuel cell stack 12, oxidation or the like of the fuel cell stack is suppressed. More specifically, by making the first fuel concentration and the second fuel concentration equal, both the first fuel concentration and the second fuel concentration can be made high, and oxidation of the fuel cell stack can be reduced. It can suppress suitably, and can provide a system superior in durability of the fuel cell stack. Further, when the value when the first fuel concentration and the second fuel concentration are equal is C, the first fuel concentration and the second fuel concentration are 0.9C or more, that is, 90% or more when the respective concentrations are equal. If this is the case, the difference between the first fuel concentration and the second fuel concentration can be suppressed, the risk of oxidation of the fuel cell stack can be suppressed, and a system with excellent durability of the fuel cell stack can be achieved. Can be provided.

The fuel cell system 10 according to the present embodiment has, for example, the effective surface area of the fuel cell included in the second fuel cell stack with respect to the total effective surface area (α ₁ ) of the fuel cell included in the first fuel cell stack 11. The ratio (α ₁ / α ₂ ) of the total (α ₂ ) is adjusted, or the fuel cell stack included in the second fuel cell stack with respect to the number (β ₁ ) of fuel cell stacks included in the first fuel cell stack 11 By adjusting the ratio (β ₁ / β ₂ ) of the number (β ₂ ) or adjusting the degree of separation of carbon dioxide and water vapor in the separation means, the first fuel concentration and the second fuel concentration are set to 0. It can be adjusted to 9C or more.

  Hereinafter, each configuration of the fuel cell system 10 according to the present embodiment will be described.

(Raw gas supply route)
The fuel cell system 10 according to the present embodiment is disposed on the permeation side 16B of the separation membrane 16 and feeds at least one of carbon dioxide and water vapor separated by the separation membrane 16 and the feed gas to the reformer 14. A supply path 24 is provided. In addition, a blower 25 for sending at least one of carbon dioxide and water vapor separated by the separation membrane 16 and the raw material gas to the reformer 14 is installed in the raw material gas supply path 24.

  The source gas flowing through the source gas supply path 24 is not particularly limited as long as it is a reformable gas, and includes hydrocarbon gas. Examples of the hydrocarbon gas include natural gas, LP gas (liquefied petroleum gas), coal reformed gas, and lower hydrocarbon gas. Examples of the lower hydrocarbon gas include lower hydrocarbons having 4 or less carbon atoms such as methane, ethane, ethylene, propane, butane, and methane is particularly preferable. The hydrocarbon gas may be a mixture of two or more of the above-described lower hydrocarbon gases, and may be a gas obtained by mixing the above-described lower hydrocarbon gas with a gas such as natural gas, city gas, or LP gas. There may be.

(Reformer)
The fuel cell system 10 according to the present embodiment includes a reformer 14 that reforms a raw material gas to generate a reformed gas. The reformer 14 includes, for example, a combustion unit 18 in which a burner and / or a combustion catalyst is disposed, and a reforming unit 19 including a reforming catalyst.

  The reformer 19 is connected to the raw material gas supply path 24 on the upstream side, and is connected to the fuel gas supply path 42 on the downstream side. Therefore, a raw material gas such as methane is supplied to the reforming unit 19 through the raw material gas supply path 24, and the reformed gas generated after the reforming unit 19 reforms the raw material gas through the fuel gas supply path 42. One fuel cell stack 11 is supplied.

  The combustion unit 18 is connected to the air supply path 44 and the off-gas path 46 on the upstream side, and is connected to the exhaust path 48 on the downstream side. The combustion unit 18 burns a mixed gas of oxygen-containing gas supplied through the air supply path 44 and off-gas supplied through the off-gas path 46, and heats the reforming catalyst in the reforming unit 19. Exhaust gas from the combustion unit 18 is discharged through an exhaust path 48.

  A heat exchanger 41 is installed in the exhaust path 48 and the air supply path 44, and the heat exchanger 41 causes the exhaust gas flowing in the exhaust path 48, the gas containing oxygen flowing in the air supply path 44, and Heat exchange between the two. As a result, the exhaust gas flowing through the exhaust passage 48 is cooled and discharged, and the gas containing oxygen flowing through the air supply passage 44 is heated to a temperature suitable for the operating temperature of the first fuel cell stack 11. After that, it is supplied to the cathode of the first fuel cell stack 11.

  Since the reforming in the reforming unit 19 involves a large endotherm, it is necessary to supply heat from the outside for the progress of the reaction. For this reason, the reforming unit 19 is heated by the combustion heat generated in the combustion unit 18. It is preferable. Alternatively, the reforming unit 19 may be heated using heat released from each fuel cell without installing the combustion unit 18, or may be heated by both combustion heat and heat released from each fuel cell.

Examples of the reforming of the raw material gas include steam reforming, carbon dioxide reforming, partial oxidation reforming, shift reaction reforming and the like, and the reforming unit 19 includes at least carbon dioxide reforming and steam reforming of the raw material gas. The structure which performs one side may be sufficient, the structure which produces | generates fuel gas by partial oxidation reforming may be sufficient, and a shift reaction may be accompanied at the time of reforming. In the present embodiment, a configuration in which a hydrocarbon gas C _n H _m (n and m are both positive integers) is used as a raw material gas, and methane is steam reformed in the reforming unit 19 to generate a fuel gas is described below. explain.

When a hydrocarbon gas represented by C _n H _m (n and m are both positive integers) is steam reformed as a raw material gas, the reforming unit 19 performs monoxide oxidation by the reaction of the following formula (a). Carbon and hydrogen are produced.
C _n H _m + nH ₂ O → nCO + [(m / 2) + n] H ₂ ... (A)

When methane, which is an example of the raw material gas, is steam reformed, carbon monoxide and hydrogen are generated in the reforming unit 19 by the reaction of the following formula (b).
CH ₄ + H ₂ O → CO + 3H ₂ ... (B)

  The reforming catalyst installed in the reforming unit 19 is not particularly limited as long as it becomes a catalyst for the reforming reaction, but Ni, Rh, Ru, Ir, Pd, Pt, Re, Co, Fe and A reforming catalyst containing at least one of Mo as a catalyst metal is preferable.

  The number S of steam molecules per unit time supplied to the reforming unit 19 of the reformer 14 and the number of carbon atoms C of the raw material gas per unit time supplied to the reforming unit 19 of the reformer 14 The steam carbon ratio S / C, which is the ratio, is preferably 1.5 to 3.5, more preferably 2.0 to 3.0, and further preferably 2.0 to 2.5. preferable. When the steam carbon ratio S / C is in this range, the raw material gas is efficiently steam reformed, and a fuel gas containing hydrogen and carbon monoxide is generated. Furthermore, carbon deposition in the fuel cell system 10 can be suppressed, and the reliability of the fuel cell system 10 can be improved.

  Moreover, it is preferable that the combustion part 18 heats the modification part 19 to 600 to 800 degreeC from a viewpoint which performs steam reforming efficiently, and it is more preferable to heat to 600 to 700 degreeC.

  In the fuel cell system according to this embodiment (particularly, a fuel cell system including a high-temperature fuel cell), the reformer does not need to be attached to the outside of the first fuel cell stack, and the first fuel cell. At least one of raw material gas, water vapor, and carbon dioxide is directly supplied to the stack, reforming (internal reforming) is performed inside the first fuel cell stack, and the generated fuel gas is supplied to the first fuel cell stack. The structure used for electric power generation may be sufficient. Particularly when the first fuel cell stack is a high-temperature fuel cell, the internal reaction temperature is as high as 600 ° C. to 800 ° C., so that reforming can be performed in the first fuel cell stack. is there.

  When performing carbon dioxide reforming, the ratio (A: B) of the number of carbon atoms (A) of the source gas (preferably methane) supplied to the reforming section and the number of molecules of carbon dioxide (B) is: From the viewpoint of efficiently performing carbon dioxide reforming, for example, 700 ° C. is preferably 1: 2.7 to 5.0, and more preferably 1: 2.7 to 4.0. Furthermore, from the viewpoint of suppressing carbon deposition, a part of water vapor may be added.

(First fuel cell stack)
The fuel cell system 10 according to the present embodiment includes a first fuel cell stack 11 that generates power using the fuel gas supplied from the reforming unit 19 of the reformer 14 through the fuel gas supply path 42. As the first fuel cell stack 11, for example, a fuel cell stack including at least one fuel cell stack in which a plurality of fuel cells each including an air electrode (cathode), an electrolyte, and a fuel electrode (anode) are stacked or connected is used. It is. As the first fuel cell stack, a high-temperature fuel cell that operates at about 600 ° C. to 800 ° C., for example, a solid oxide fuel cell that operates at about 600 ° C. to 800 ° C., about 600 ° C. to 700 ° C. A molten carbonate fuel cell operating at

When the first fuel cell stack 11 is a solid oxide fuel cell, air is supplied to the cathode (not shown) of the first fuel cell stack 11 through the air supply path 44. By supplying air to the cathode, the reaction shown in the following formula (c) occurs, and oxygen ions move inside the solid oxide electrolyte (not shown).
O ₂ + 4e ⁻ → 2O ²⁻ (c)

When the first fuel cell stack 11 is a solid oxide fuel cell, a fuel gas containing hydrogen and carbon monoxide is supplied to the anode (not shown) of the first fuel cell stack 11 through the fuel gas supply path 42. Supplied. When hydrogen and carbon monoxide react with oxygen ions moving inside the solid oxide electrolyte at the interface between the anode and the solid oxide electrolyte, reactions shown in the following formulas (d) and (e) occur. .
H ₂ + O ²⁻ → H ₂ O + 2e ⁻ (d)
CO + O ²⁻ → CO ₂ + 2e ⁻ (e)

When the first fuel cell stack 11 is a molten carbonate fuel cell, a gas containing oxygen and carbon dioxide is supplied to the cathode (not shown) of the first fuel cell stack 11 through the air supply path 44. . When a gas containing oxygen and carbon dioxide is supplied to the cathode, the reaction shown in the following formula (f) occurs, and at this time, carbonate ions move inside the electrolyte (not shown).
O ₂ + 2CO ₂ + 4e ⁻ → 2CO ₃ ² −... (F)

When the first fuel cell stack 11 is a molten carbonate fuel cell, a fuel gas containing hydrogen and carbon monoxide is supplied to the anode (not shown) of the first fuel cell stack 11 through the fuel gas supply path 42. Supplied. When hydrogen and carbon monoxide react with carbonate ions moving inside the electrolyte at the interface between the anode and the electrolyte, reactions shown in the following formulas (g) and (h) occur.
H ₂ + CO ₃ ²⁻ → H ₂ O + CO ₂ + 2e ⁻ (g)
CO + CO ₃ ²⁻ → 2CO ₂ + 2e ⁻ (h)

  As shown in the above formula (d), formula (e), formula (g), and formula (h), the solid oxide fuel cell is produced by the electrochemical reaction of the fuel gas in the first fuel cell stack 11. In the molten carbonate fuel cell, water vapor and carbon dioxide are mainly produced. Further, the electrons generated at the anode move to the cathode through an external circuit. In this way, the electrons move from the anode to the cathode, so that power generation is performed in the first fuel cell stack 11.

  The cathode off gas discharged from the cathode is supplied to the cathode (not shown) of the second fuel cell stack 12 through the downstream air supply path 44.

  On the other hand, the anode off gas containing unreacted fuel gas discharged from the anode is supplied to the supply side 16 </ b> A of the separation membrane 16 through the off gas path 52. Here, the anode off gas containing unreacted fuel gas is a mixed gas containing hydrogen, carbon monoxide, carbon dioxide, water vapor and the like.

  The heat exchanger 21 is installed in the off-gas path 52 and the off-gas path 54, and at least one of the anode off-gas flowing in the off-gas path 52 and the carbon dioxide and water vapor flowing in the off-gas path 54 by the heat exchanger 21. Is exchanged with the anode off-gas separated. As a result, the anode offgas flowing in the offgas passage 52 is cooled to a preferable temperature when separating at least one of carbon dioxide and water vapor by the separation membrane 16, and at least one of carbon dioxide and water vapor flowing in the offgas passage 54. The anode off gas from which is separated is heated to a temperature suitable for the operating temperature of the second fuel cell stack 12. Therefore, the power generation efficiency and thermal efficiency of the entire system are further improved.

(Separation membrane)
The fuel cell system 10 according to the present embodiment includes a separation membrane 16 that separates at least one of carbon dioxide and water vapor from an anode off gas containing unreacted fuel gas discharged from the first fuel cell stack 11. . The separation membrane 16 is a kind of separation means.

  The anode offgas flowing in the offgas passage 52 is supplied to the supply side 16A of the separation membrane 16, and at least one of carbon dioxide and water vapor in the anode offgas flows from the supply side 16A to the permeation side 16B in the direction of the arrow A. Pass through. The anode off-gas after separating at least one of carbon dioxide and water vapor flows through the off-gas path 54 from the supply side 16 </ b> A and is supplied to the second fuel cell stack 12. On the other hand, at least one of the separated carbon dioxide and water vapor is mixed with the raw material gas flowing in the raw material gas supply path 24 and flows in the raw material gas supply path 24 from the permeate side 16B. To the unit 19. Therefore, at least one of the separated carbon dioxide and water vapor is used for reforming the raw material gas. Therefore, in the fuel cell system 10 according to the present embodiment, it is not necessary to supply at least one of carbon dioxide and steam used for reforming the raw material gas to the reforming unit 19 of the reformer 14 from the outside, or from the outside. The amount of at least one of carbon dioxide and steam supplied to the reforming unit 19 of the reformer 14 can be reduced.

  Furthermore, in the fuel cell system 10 according to the present embodiment, the raw material gas supply path 24 is disposed on the permeation side 16B of the separation membrane 16 so that the raw material gas functions as at least one of the separated carbon dioxide and water vapor sweep gas. Yes. Therefore, it is not necessary to separately provide a path for supplying a sweep gas such as air and an air blower or a decompression pump to the permeation side 16B of the separation membrane, and the system is simplified.

  In the fuel cell system 10, since at least one of carbon dioxide and water vapor is separated by the separation membrane 16, at least one of the water vapor concentration and the carbon dioxide concentration in the anode off-gas supplied to the second fuel cell stack 12 is lowered. The power generation efficiency of the fuel cell system 10 can be increased.

  The separation membrane is not particularly limited as long as it is a membrane that transmits at least one of carbon dioxide and water vapor, and examples thereof include an organic polymer membrane, an inorganic material membrane, an organic polymer-inorganic material composite membrane, and a liquid membrane. The separation membrane is more preferably a glassy polymer membrane, a rubbery polymer membrane, an ion exchange resin membrane, an alumina membrane, a silica membrane, a carbon membrane, a zeolite membrane, a ceramic membrane, an amine aqueous solution membrane or an ionic liquid membrane. preferable.

  The thickness of the separation membrane is not particularly limited, but from the viewpoint of mechanical strength, it is usually preferably in the range of 10 μm to 3000 μm, more preferably in the range of 10 μm to 500 μm, and still more preferably in the range of 15 μm to 150 μm.

  Further, the separation membrane is preferably a membrane satisfying one of the carbon dioxide separation rate and the water vapor separation rate of 50% or more and 99% or less from the viewpoint of suitably increasing the fuel concentration by regenerating the anode off-gas, A film satisfying 60% to 99% is more preferable, a film satisfying 70% to 99% is further preferable, and a film satisfying 80% to 99% is particularly preferable. By regenerating the anode off gas and suitably increasing its fuel concentration, the fuel cell system can be operated at a higher fuel utilization rate, and higher power generation efficiency can be obtained.

  Further, the separation membrane is a membrane satisfying a water vapor separation rate of 50% or more and 99% or less from the viewpoint of suitably suppressing the carbon deposition while regenerating the anode off gas and suitably increasing its fuel concentration. A film satisfying 60% to 99% is more preferable, a film satisfying 70% to 99% is more preferable, and a film satisfying 80% to 99% is particularly preferable.

The separation membrane may be supported by a porous support. Examples of the material for the support include paper, cellulose, polyester, polyolefin, polyamide, polyimide, polysulfone, polycarbonate, metal, glass, and ceramic. When a support is provided, the thickness of the separation membrane is preferably in the range of 100 nm to 100 μm, more preferably in the range of 100 nm to 50 μm, from the viewpoint of suitably ensuring carbon dioxide permeability and water vapor permeability.
The thickness of the support (preferably larger than the thickness of the separation membrane) is preferably in the range of 10 μm to 500 μm, more preferably in the range of 30 μm to 300 μm.

As the separation membrane for separating at least one of carbon dioxide and water vapor, for example, a membrane described in “Journal of Membrane Science Vol. 320 (2008) 390-400, The upper bound revisited” may be used. Further, as a separation membrane for separating at least one of carbon dioxide and water vapor, particularly as a separation membrane for separating carbon dioxide, the polymer membrane described in Japanese Patent No. 5329207 and the CO ₂ facilitated transport described in Japanese Patent No. 496528 A membrane, a separation membrane described in Japanese Patent No. 5743639, a permeable membrane described in Japanese Patent No. 5738704, or the like may be used.

  The anode off-gas after separating at least one of carbon dioxide and water vapor flows through the off-gas path 54 from the supply side 16 </ b> A and is supplied to the second fuel cell stack 12. At this time, as described above, the anode offgas from which at least one of carbon dioxide and water vapor flowing through the offgas passage 54 is separated by the heat exchanger 21 installed in the offgas passage 52 and the offgas passage 54 is the second fuel. The battery cell stack 12 is heated to a temperature suitable for the operating temperature.

(Second fuel cell stack)
The fuel cell system 10 according to the present embodiment is disposed downstream of the supply side 16A of the separation membrane 16 and generates power using an anode offgas from which at least one of carbon dioxide and water vapor is separated. It has. As the second fuel cell stack 12, for example, a fuel cell stack including at least one fuel cell stack in which a plurality of fuel cells each including an air electrode (cathode), an electrolyte, and a fuel electrode (anode) are stacked or connected is used. It is. In addition, since the electrochemical reaction in the 2nd fuel cell stack 12 is the same as that of the above-mentioned 1st fuel cell stack 11, the description is abbreviate | omitted.

  In the fuel cell system 10, the second fuel cell stack 12 generates power using an anode off gas from which at least one of carbon dioxide and water vapor is separated. Therefore, in the second fuel cell stack 12, the theoretical voltage due to the oxygen partial pressure difference between the electrodes is improved, and the concentration overvoltage is increased due to the separation of at least one of carbon dioxide and water vapor in the anode off-gas. Reduced, and can exhibit high performance, particularly at high current densities. Therefore, the fuel cell system 10 can obtain high power generation efficiency.

<Preferable conditions for the first fuel cell stack and the second fuel cell stack>
In the fuel cell system 10 according to the present embodiment, when the value when the first fuel concentration and the second fuel concentration are equal is C, the first fuel concentration and the second fuel concentration are 0.9C or more, respectively. The first fuel concentration and the second fuel concentration may each satisfy 0.95C or more. The upper limits of the first fuel concentration and the second fuel concentration are not particularly limited and may be, for example, 1.1 C or less.

In the fuel cell system 10 according to the present embodiment, the total effective surface area of the fuel cells provided in the first fuel cell stack 11 is α ₁ , and the total effective surface area of the fuel cells provided in the second fuel cell stack 12 is determined. When α ₂ , α ₁ / α ₂ is preferably more than 2.0. As a result, both the values of the first fuel concentration and the second fuel concentration can be made higher than when α ₁ / α ₂ is 2.0 or less. Can be suitably suppressed, and a fuel cell system that is more excellent in durability of the fuel cell stack can be provided. In addition, when the effective surface area of a fuel cell increases, the consumption of fuel gas also tends to increase.
α ₁ / α ₂ is more preferably 2.5 or more.

α ₁ / α ₂ is preferably 20.5 or less from the viewpoint of suppressing a decrease in the values of the first fuel concentration and the second fuel concentration, in particular, from the viewpoint of suppressing a decrease in the value of the first fuel concentration, It is more preferably 15 or less, still more preferably 10 or less, particularly preferably 8.0 or less, and even more preferably 6.0 or less.

When the separation means separates carbon dioxide and water vapor, α ₁ / α ₂ is preferably more than 2.0 and less than 15 and more preferably more than 2.0 and less than 10 and more than 2.05 and 6 Is more preferably 0.0 or less, and particularly preferably 2.05 or more and 5.0 or less.

When the fuel cell stacks included in the first fuel cell stack 11 and the second fuel cell stack 12 are the same, the number of fuel cell stacks in the first fuel cell stack 11 β ₁ When the number of fuel cell stacks β ₂ in the second fuel cell stack 12 is β ₂ , β ₁ / β ₂ is preferably more than 2.0. A preferable numerical range of β ₁ / β ₂ is the same as α ₁ / α ₂ described above.

  In addition, when power generation is performed using the fuel cell system 10 according to the present embodiment, it is preferable to generate power by operating the first fuel cell stack 11 and the second fuel cell stack 12 with equal current density. Power generation is performed by operating the first fuel cell stack 11 and the second fuel cell stack 12 at equal current densities while satisfying the above-described <preferred conditions for the first fuel cell stack and the second fuel cell stack>. As a result, the durability of the fuel cell stack is excellent and stable power generation is possible over the long term.

Further, the sum of the product of the effective surface area of the fuel cell provided in the first fuel cell stack 11 and the current density of the fuel cell when the first fuel cell stack 11 is operated is γ ₁ , the second fuel. When the total product of the effective surface area of the fuel cells provided in the battery cell stack 12 and the current density of the fuel cell when the second fuel cell stack 12 is operated is γ ₂ , γ ₁ / γ ₂ It is also preferable to adjust the power to more than 2.0 and perform power generation. Thereby, compared with the case where (gamma) ₁ / (gamma) ₂ is 2.0 or less, since both the value of a 1st fuel concentration and a 2nd fuel concentration can be made into a high value, oxidation of a fuel cell stack, etc. Can be suitably suppressed, and the durability of the fuel cell stack is excellent, and stable power generation is possible over a long period of time. A preferable numerical range of γ ₁ / γ ₂ is the same as that of α ₁ / α ₂ described above.
In addition, the product of the effective surface area of the specific fuel cell provided in the first fuel cell stack 11 and the current density of the fuel cell is obtained, and another fuel cell provided in the first fuel cell stack 11 Similarly, the sum of these products when the product is obtained corresponds to γ ₁ , and the same applies to γ ₂ .
Further, γ ₁ and γ ₂ are proportional to the fuel gas consumption in the first fuel cell stack 11 and the second fuel cell stack 12, respectively.

  In the separation means, when one of carbon dioxide and water vapor is separated, it is preferable that the first fuel concentration and the second fuel concentration are each 10% or more from the viewpoint of suitably suppressing the oxidation of the fuel cell stack. It is more preferably 12% or more, further preferably 14% or more, particularly preferably 16% or more, and further preferably 17% or more.

  In the separation means, when carbon dioxide and water vapor are separated, it is preferable that the first fuel concentration and the second fuel concentration are each 11.5% or more from the viewpoint of suitably suppressing the oxidation of the fuel cell stack. It is more preferably 14% or more, further preferably 17% or more, particularly preferably 19% or more, and further preferably 21% or more.

  In the fuel cell system 10 according to the present embodiment, the fuel utilization rate is preferably 80% or more, more preferably 85% or more, and further preferably 85% or more and 95% or less. In the fuel cell system 10 according to the present embodiment, by providing the separation means, the fuel utilization rate is increased and the first fuel concentration and the second fuel concentration are adjusted so as to satisfy the above conditions. Therefore, the fuel cell system according to the present embodiment achieves both improvement in power generation efficiency due to a high fuel utilization rate and improvement in durability of the fuel cell stack.

  The cathode off gas discharged from the cathode of the second fuel cell stack 12 is supplied to the combustion unit 18 of the reformer 14 through the air supply path 44. Similarly, the anode off gas discharged from the anode of the second fuel cell stack 12 is supplied to the combustion unit 18 of the reformer 14 through the off gas path 46.

[Second Embodiment]
Hereinafter, a second embodiment of the fuel cell system of the present invention will be described with reference to FIG. FIG. 2 is a schematic configuration diagram showing a fuel cell system according to the second embodiment. In the fuel cell system 20 according to the second embodiment, instead of providing the separation membrane 16 between the first fuel cell stack 11 and the second fuel cell stack 12, a water tank 36 which is a kind of separation means is provided. In addition, the condensed water stored in the water tank 32 is supplied to the reforming unit 19 of the reformer 14 and used for the steam reforming of the raw material gas, mainly with the fuel cell system 10 according to the first embodiment. Is different. In the present embodiment, the same components as those in the first embodiment are denoted by the same reference numerals, and the description thereof is omitted.

  The source gas supply path 24 is connected to a later-described steam supply path 37, and steam flowing through the steam supply path 37 is supplied to the source gas supply path 24. Then, the steam supplied from the steam supply path 37 is supplied to the reforming unit 19 of the reformer 14 together with the raw material gas, and is used for the steam reforming of the raw material gas. The raw material gas supply path 24 is not connected to the steam supply path 37 and is directly supplied to the reforming unit 19 of the reformer 14 through the steam supply path 37 from the viewpoint of preventing condensation of the steam in the path. It may be a configuration.

  The heat exchanger 21 exchanges heat between the anode off-gas flowing through the off-gas passage 52 and the anode off-gas from which the water vapor flowing through the off-gas passage 54 has been recovered, and then the inside of the off-gas passage 52 The anode off gas flowing through the water tank 36 is supplied to the water tank 36. At this time, the anode offgas flowing in the offgas passage 52 is cooled, approaches the temperature at which the water vapor is condensed in the water tank 36, and the off gas from which the water vapor supplied to the second fuel cell stack 12 is recovered is Heated to a temperature suitable for power generation.

  The water tank 36 is a container for collecting water vapor contained in the anode off gas by storing condensed water obtained by condensing water vapor contained in the anode off gas flowing in the off gas passage 52.

  The water tank 36 is connected to the water tank 32 via the condensed water supply path 35. For example, the condensed water stored in the water tank 36 is supplied to the water tank 32 via the condensed water supply path 35 when the condensed water stored in the water tank 36 reaches a predetermined amount. May be. Further, instead of the water tank 36, a separation membrane for separating water vapor, an adsorbent for adsorbing water vapor, or the like may be provided. A preferable range of the water vapor separation rate in the water tank is the same as the numerical range described in the above-described separation membrane. For example, the water vapor separation rate can be adjusted as appropriate by adjusting the temperature of the water tank.

  The fuel cell system 20 according to this embodiment includes an anode off-gas discharged from the first fuel cell stack 11 as a kind of separation means between the first fuel cell stack 11 and the second fuel cell stack 12. You may provide the carbon dioxide removal part which removes a carbon dioxide from. For example, the carbon dioxide removal unit is disposed at a position B1 or B2 in FIG. The carbon dioxide removing unit is not particularly limited as long as it can separate carbon dioxide contained in the anode off gas, and examples thereof include a separation membrane, an absorbent, and an adsorbent. A preferable range of the carbon dioxide separation rate in the carbon dioxide removal unit is the same as the numerical range described in the above-described separation membrane.

  In the fuel cell system 20, the exhaust gas flowing in the exhaust path 48 exchanges heat with the reformed water flowing in the reformed water supply path 33 in the heat exchanger 23 having the role of a vaporizer. Thus, the exhaust gas flowing through the exhaust passage 48 is cooled and then supplied to the water tank 32 as a condenser, and the reformed water flowing through the reformed water supply passage 33 is vaporized and then passed through the water vapor supply passage 37. It is supplied to the source gas supply path 24.

  The water tank 32 is a container for collecting the water vapor contained in the exhaust gas by storing the condensed water obtained by condensing the water vapor contained in the exhaust gas flowing through the exhaust passage 48. In the water tank 32, exhaust gas other than water vapor is discharged to the outside, and when a predetermined amount or more of water is stored, drainage is performed by overflow, for example.

  The water tank 32 is connected to the reforming water supply path 33, and the reforming water supply path 33 is provided with a reforming water pump 34. The water stored in the water tank 32 by the reforming water pump 34 is supplied as reforming water to the heat exchanger 23 through the reforming water supply path 33.

  The structure for separating the water vapor from the exhaust gas flowing in the exhaust path 48 is not limited to the water tank 32. For example, the water vapor and the gas other than the water vapor may be separated by a separation membrane, Gas for steam reforming may be separated by adsorbing gas other than steam.

  Further, instead of the heat exchanger 23 that performs heat exchange between the exhaust gas flowing in the exhaust path 48 and the reformed water flowing in the reformed water supply path 33, the combustion unit 18, the first fuel cell unit. A vaporizer that vaporizes the reformed water using heat released from at least one of the stack 11 and the second fuel cell stack 12 may be provided.

  In the fuel cell system 20, the exhaust gas heat-exchanged by the heat exchanger 23 exchanges heat with the oxygen-containing gas flowing in the air supply path 44 by the heat exchanger 22. As a result, the exhaust gas is cooled and then supplied to the water tank 32 (condenser), and the gas containing oxygen is heated and then supplied to the cathode of the first fuel cell stack 11 and used for power generation.

(Modification)
In the first embodiment and the second embodiment, since the air supply path 44 is in series, after the air is supplied to the first fuel cell stack 11, the first fuel cell is supplied to the second fuel cell stack 12. The cathode off gas discharged from the stack 11 is supplied, but the air supply paths 44 may be parallel. That is, the air supply path 44 through which air flows may be branched to supply air to the cathodes of the first fuel cell stack 11 and the second fuel cell stack 12.

Although 1st Embodiment demonstrated the structure which provides one separation membrane which isolate | separates at least one of a carbon dioxide and water vapor | steam, this invention is not limited to this, The structure which arrange | positions a some separation membrane may be sufficient. . For example, a carbon dioxide separation membrane that separates carbon dioxide and a water vapor separation membrane that separates water vapor may be separately disposed as separation membranes. There is no need to strictly distinguish between a carbon dioxide separation membrane and a water vapor separation membrane, and the carbon dioxide separation membrane may be a separation membrane that transmits water vapor together with carbon dioxide. It may be. The material for the water vapor separation membrane and the carbon dioxide separation membrane may be the same as the material for the separation membrane described above. Further, as the carbon dioxide separation membrane, a CO ₂ facilitated transport membrane may be used.

  In the first embodiment, when a water vapor separation membrane for separating water vapor is disposed as the separation membrane 16, the fuel cell system 10 has an anode downstream of the first fuel cell stack 11 and upstream of the second fuel cell stack 12. You may further provide the carbon dioxide removal part which removes a carbon dioxide from off gas. The carbon dioxide removal unit is the same as that used in the second embodiment.

  In the first embodiment, when a carbon dioxide separation membrane that separates carbon dioxide is disposed as the separation membrane 16, the fuel cell system 10 is disposed downstream of the first fuel cell stack 11 and upstream of the second fuel cell stack 12. A water vapor removing unit that removes water vapor from the anode off gas may be further provided. Thereby, since the 2nd fuel cell stack 12 performs electric power generation using anode off gas from which water vapor and carbon dioxide were separated, electric power generation efficiency can be raised more.

  The water vapor removing unit is for removing water vapor from the anode off gas, and may be a separation membrane that separates water vapor, an adsorbent that adsorbs water vapor, a condenser that condenses water vapor, or the like.

  Moreover, in 1st Embodiment, it is not limited to the structure which uses at least one of the water vapor | steam and carbon dioxide isolate | separated by arrange | positioning the raw material gas supply path 24 in the permeation | transmission side of the separation membrane 16, and reforming of raw material gas. For example, a configuration in which a supply path for supplying a sweep gas such as air is provided on the permeate side of the separation membrane 16, a configuration in which an air supply path 44 through which cathode gas or cathode off gas flows is provided, and a configuration in which an exhaust path 48 through which exhaust flows are provided. Or a configuration in which the permeation side of the separation membrane 16 is depressurized.

  Further, in the present invention, the reformer is not an essential configuration, and the first fuel gas containing at least one of hydrogen and carbon monoxide is disposed without arranging the reformer upstream of the first fuel cell stack. The configuration may be such that power is generated by supplying the fuel cell stack.

  The present invention is not limited to the first embodiment, the second embodiment, and the modifications described above, and is implemented by a person skilled in the art in combination with the above-described embodiments within the technical idea of the present invention. Further, the installation location and combination of heat exchangers are not limited to these embodiments. Furthermore, the present invention also includes a power generation method using the fuel cell system described in the first embodiment, the second embodiment, and the modification.

[Example of the present invention]
Hereinafter, an example of the fuel cell system of the present invention will be considered. The fuel cell system includes a reformer that reforms hydrocarbon gas upstream of the first fuel cell stack in addition to the first fuel cell stack, the separating unit, and the second fuel cell stack. investigated. The conditions for the study are as follows.
<Conditions>
1st fuel cell stack and 2nd fuel cell stack: Solid oxide fuel cell Hydrocarbon gas: Methane Reform: Steam reforming Steam carbon ratio (S / C): 2.5
Cell load current: Same for the first fuel cell stack and the second fuel cell stack Overall fuel utilization: 90 (%)
Relationship between fuel utilization rates: U _{f (total)} = [U _{f (first)} + (1-U _{f (first)} ) · U _{f (second)} ] (where U _{f (total)} is the total fuel utilization rate) U _{f (first)} indicates the fuel utilization rate of the first fuel cell stack and U _{f (second)} indicates the fuel utilization rate of the second fuel cell stack.)
Fuel concentration C ₀ at the inlet of the first fuel cell stack: 72.73 (%) [(H ₂ gas amount + CO gas amount) / total gas amount] × 100 (%)
<Calculation formula for each parameter>
Fuel utilization rate of the second fuel cell stack: U _{f (second)} = (U _{f (total)} −U _{f (first)} ) / (1−U _{f (first)} )
Ratio of the number of stacks of the first fuel cell stack to the number of stacks of the second fuel cell stack (the fuel cell stack and current density constituting the fuel cell stack are all the same, corresponding to β ₁ / β ₂ ): U _{f (first)} / (U _{f (total)} −U _{f (first)} )
Anode off-gas concentration at the outlet of the first fuel cell stack (first fuel concentration C ₁ ): C ₀ × (1−U _{f (first)} )
Fuel concentration at the inlet of the second fuel cell stack (third fuel concentration C ₃ ): C ₁ / {1−removal rate (1−C ₁ )}
Anode off-gas concentration at the outlet of the second fuel cell stack (second fuel concentration C ₂ ): C ₃ × (1-U _{f (second)} )

(Examination 1)
A case where both H ₂ O and CO ₂ are removed by the separation means will be described. Hereinafter, a fuel cell system in the case where 95% of both H ₂ O and CO ₂ is removed by the separation means (when the “removal rate” in the above calculation formula is 0.95) will be considered.

Here, when the removal rates of H ₂ O and CO ₂ are both 95%, the relationship between the fuel utilization rate of the first fuel cell stack and the first and second fuel concentrations is shown in FIG. . If the overall fuel utilization in the fuel cell system is constant, as shown in FIG. 3, an increase in the fuel utilization ratio of the first fuel cell stack, a first fuel concentration C ₁ decreases. On the other hand, when the fuel usage rate of the first fuel cell stack increases under the condition that the overall fuel usage rate of the fuel cell system is constant, the fuel usage rate of the second fuel cell stack is greatly reduced. as shown in FIG. 3, the second fuel concentration C ₂ is increased. Then, as shown in FIG. 3, when the first fuel concentration C ₁ and the second fuel concentration C ₂ is equal, the minimum value of the first fuel concentration C ₁ and the second fuel concentration C ₂ is maximized, the fuel concentration It is possible to reduce the risk of cell stack oxidation or the like due to a decrease in the cell.

When 95% of both H ₂ O and CO ₂ are removed by the separation means, when the first fuel concentration C ₁ and the second fuel concentration C ₂ are equal, the following values are calculated based on the above-described calculation formula. The
Fuel usage rate U _{f (first) of} the first fuel cell stack: 65.40%
Fuel utilization rate U _{f (second) of} the second fuel cell stack: 71.10%
Value when the first fuel concentration _{C 1} and the second fuel concentration _{C 2} is equal (maximum concentration): 25.16%
Ratio of stack number of first fuel cell stack to stack number of second fuel cell stack (stack ratio): 2.659

Next, the first fuel concentration C ₁ and the second fuel concentration C ₂ are 90% or more when the minimum values of the first fuel concentration C ₁ and the second fuel concentration C ₂ are maximized, that is, 25.16%. 90% or more, the first fuel concentration C ₁ and the second fuel concentration C ₂ can both be high, and the fuel concentration decreases in the first fuel cell stack and the second fuel cell stack. It is possible to reduce the risk of cell stack oxidation due to. Accordingly, the 22.65% when calculating the first lower limit value of the fuel concentration _{C 1} and the second fuel concentration _{C 2.}

When the first fuel concentration C ₁ is 22.65%, based on the calculation formula described above, the following values are calculated.
Fuel usage rate U _{f (first) of} the first fuel cell stack: 68.86%
Fuel utilization rate of second fuel cell stack U _{f (second)} : 67.88%
Ratio of stack number of first fuel cell stack to stack number of second fuel cell stack (stack ratio): 3.258

Also, if the second fuel concentration C ₂ is 22.65%, based on the calculation formula described above, the following values are calculated.
Fuel utilization rate of the first fuel cell stack U _{f (first)} : 60.75%
Fuel utilization rate U _{f (second) of second} fuel cell stack: 74.52%
Ratio of stack number of first fuel cell stack to stack number of second fuel cell stack (stack ratio): 2.077

Therefore, when 95% of H ₂ O and CO ₂ are both removed by the separation means while satisfying the above conditions, the first fuel concentration C ₁ and the second fuel concentration are reduced from the viewpoint of reducing the risk of cell stack oxidation and the like. C ₂ is preferably 22.65% or more, and the stack ratio is preferably 2.077 or more and 3.258 or less.

Next, in Study 1, the maximum concentration, U _{f (first)} , U _{f (second)} , stack ratio, and 90% when H ₂ O and CO ₂ are both removed by 50% to 90% by the separation means. The concentration (90% of the maximum concentration) was calculated respectively.
The results are shown in Table 1 and FIG. 5 together with the case where 95% of both H ₂ O and CO ₂ are removed by the separation means. The stack ratio preferably takes a range between the maximum value and the minimum value in FIG.

(Examination 2)
A case where H ₂ O is removed without removing CO ₂ by the separation means will be described. Hereinafter, a fuel cell system in which 95% of H ₂ O is removed by the separation means (“H ₂ O removal rate” is 0.95) will be examined. In this case, the parameters of the fuel concentration (third fuel concentration C ₃ ) at the inlet of the second fuel cell stack are calculated in the same manner as in Study 1.

Here, when the H ₂ O removal rate is 95%, the relationship between the fuel utilization rate of the first fuel cell stack and the first fuel concentration and the second fuel concentration is shown in FIG. Similar to FIG. 3, also in FIG. 4, when the first fuel concentration C ₁ and the second fuel concentration C ₂ is equal, the minimum value of the first fuel concentration C ₁ and the second fuel concentration C ₂ is maximized, the fuel Risks such as cell stack oxidation due to a decrease in concentration can be reduced.

When 95% of H ₂ O is removed by the separation means, the following values are calculated based on the above-described calculation formula when the first fuel concentration C ₁ and the second fuel concentration C ₂ are equal.
Fuel utilization rate of first fuel cell stack U _{f (first)} : 72.59%
Fuel utilization rate of the second fuel cell stack U _{f (second)} : 63.52%
Value when the first fuel concentration _{C 1} and the second fuel concentration _{C 2} is equal (maximum concentration): 19.94%
Ratio of stack number of first fuel cell stack to stack number of second fuel cell stack (stack ratio): 4.168

Next, the first fuel concentration _{C 1} and the second fuel concentration _{C 2} is more than 90% when the minimum value of the first fuel concentration _{C 1} and the second fuel concentration _{C 2} is maximum, i.e., 19.94% 90% or more, the first fuel concentration C ₁ and the second fuel concentration C ₂ can both be high, and the fuel concentration decreases in the first fuel cell stack and the second fuel cell stack. It is possible to reduce the risk of cell stack oxidation due to. Accordingly, the 17.94% when calculating the first lower limit value of the fuel concentration _{C 1} and the second fuel concentration _{C 2.}

When the first fuel concentration C ₁ is 17.94%, based on the calculation formula described above, the following values are calculated.
Fuel utilization rate U _{f (first) of} the first fuel cell stack: 75.33%
Fuel utilization rate U _{f (second) of second} fuel cell stack: 59.47%
Ratio of stack number of first fuel cell stack to stack number of second fuel cell stack (stack ratio): 5.134

Also, if the second fuel concentration C ₂ is 17.94%, based on the calculation formula described above, the following values are calculated.
Fuel utilization rate U _{f (first) of} the first fuel cell stack: 64.76%
Fuel utilization rate of the second fuel cell stack U _{f (second)} : 71.62%
Ratio of stack number of first fuel cell stack to stack number of second fuel cell stack (stack ratio): 2.566

Therefore, when 95% of H ₂ O is removed by the separation means while satisfying the above conditions, both the first fuel concentration C ₁ and the second fuel concentration C ₂ are reduced in order to reduce the risk of cell stack oxidation and the like. The stack ratio is preferably set to 17.94% or more and the stack ratio to 2.566 or more and 5.134 or less.

Next, in Study 2, the maximum concentration, U _{f (first)} , U _{f (second)} , stack ratio, and 90% concentration (maximum concentration ₎ when 50% to 90% of H ₂ O is removed by the separation means. Of 90%) was calculated.
Results with the case of removing the H _{2 O} 95% in the separation unit are shown in Table 2 and FIG. 6 a. The stack ratio preferably takes a range between the maximum value and the minimum value in FIG.

  DESCRIPTION OF SYMBOLS 10, 20 ... Fuel cell system, 11 ... 1st fuel cell stack, 12 ... 2nd fuel cell stack, 14 ... Reformer, 16 ... Separation membrane, 16A ... Supply side, 16B ... Permeation side, 18 ... Combustion , 19 ... reforming section, 21, 22, 23, 41 ... heat exchanger, 24 ... raw material gas supply path, 25, 26 ... blower, 32, 36 ... water tank, 33 ... reforming water supply path, 34 ... Reformed water pump, 35 ... condensed water supply path, 37 ... steam supply path, 42 ... fuel gas supply path, 44 ... air supply path, 46, 52, 54 ... off-gas path, 48 ... exhaust path

Claims (11)

A first fuel cell stack that generates power using fuel gas;
Separation means for separating at least one of carbon dioxide and water vapor from the anode offgas containing the unreacted fuel gas discharged from the first fuel cell stack;
A second fuel cell stack disposed downstream of the separation means and generating power using the anode offgas from which at least one of carbon dioxide and water vapor is separated;
With
When the fuel concentration of the anode off gas at the outlet of the first fuel cell stack is a first fuel concentration and the fuel concentration of the anode off gas at the outlet of the second fuel cell stack is a second fuel concentration, A fuel cell system in which the first fuel concentration and the second fuel concentration satisfy 0.9 C or more, respectively, where C is a value when the fuel concentration and the second fuel concentration are equal.   2. The fuel cell system according to claim 1, wherein in the separation means, one of the carbon dioxide separation rate and the water vapor separation rate is 50% or more and 99% or less. The separation means separates carbon dioxide and water vapor from the anode off-gas;
2. The fuel cell system according to claim 1, wherein in the separation means, both the carbon dioxide separation rate and the water vapor separation rate are 50% or more and 99% or less. When the total effective surface area of the fuel cells provided in the first fuel cell stack is α ₁ and the total effective surface area of the fuel cells provided in the second fuel cell stack is α ₂ , α ₁ / α ₂ The fuel cell system according to any one of claims 1 to 3, wherein is more than 2.0. When the fuel cell stack included in the first fuel cell stack and the fuel cell stack included in the second fuel cell stack are the same, the number of fuel cell stacks β ₁ in the _first fuel cell stack, the second fuel when the number beta ₂ of the fuel cell stack in the battery cell stack, when the beta _{1 /} beta _2, according to any one of claims 1 to 4 beta _{1 /} beta ₂ is exceeding 2.0 Fuel cell system.   The fuel according to any one of claims 1 to 5, wherein in the separation means, when one of carbon dioxide and water vapor is separated, the first fuel concentration and the second fuel concentration are each 10% or more. Battery system.   The fuel according to any one of claims 1 to 6, wherein in the separation means, when the carbon dioxide and the water vapor are separated, the first fuel concentration and the second fuel concentration are each 11.5% or more. Battery system. It further comprises a reformer that reforms the raw material gas to generate fuel gas,
The fuel cell system according to any one of claims 1 to 7, wherein at least one of carbon dioxide and water vapor separated by the separation means is used for reforming the raw material gas.   A power generation method using the fuel cell system according to any one of claims 1 to 8.   The power generation method according to claim 9, wherein power generation is performed by operating the first fuel cell stack and the second fuel cell stack at an equal current density. The sum of the product of the effective surface area of the fuel cell provided in the first fuel cell stack and the current density of the fuel cell when the first fuel cell stack is operated is γ ₁ , and the second fuel cell When the total product of the effective surface area of the fuel cell provided in the cell stack and the current density of the fuel cell when the second fuel cell stack is operated is γ ₂ , γ ₁ / γ ₂ is 2 The power generation method according to claim 9 or 10, wherein power generation is performed by adjusting to more than 0.0.