JP2016100136A - Fuel cell system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a fuel cell system which is arranged so that carbon precipitation is suppressed appropriately, and which enables the achievement of a long-term durability in a fuel cell stack, and the achievement of a high power generation efficiency.SOLUTION: A fuel cell system comprises: a first fuel cell stack including a solid oxide electrolyte, a fuel electrode and an air electrode; a second fuel cell stack including a solid oxide electrolyte, a fuel electrode and an air electrode, disposed downstream from the first fuel cell stack, and arranged so that an exhaust gas exhausted from the side of the fuel electrode of the first fuel cell stack is supplied to the fuel electrode thereof; a first reformer disposed upstream of the first fuel cell stack, serving to modify, in quality, a fuel of hydrocarbon, thereby producing a modification gas, and supplying the modification gas to the fuel electrode of the first fuel cell stack; and first fuel-supplying means for supplying the hydrocarbon fuel to the first reformer. The ratio rof a fuel use in second fuel cell stack to a fuel use in the first fuel cell stack satisfies 0.35≤r≤1.0.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a fuel cell system.

  In recent years, attention has been focused on fuel cell power generation systems as new energy sources. There are various types of fuel cells provided in the fuel cell power generation system. Among them, a solid oxide fuel cell (SOFC) using a solid oxide as an electrolyte has the highest power generation efficiency. It is known to be a fuel cell and promising.

As the SOFC, a fuel cell stack including a plurality of single cells each including a solid oxide electrolyte, a fuel electrode, and an air electrode is used. In the power generation method using the SOFC, for example, hydrogen (H ₂ ) and carbon monoxide generated by supplying methane (CH ₄ ) and steam as fuel to a reformer outside the SOFC and performing steam reforming. (CO) is electrochemically reacted at the interface between the electrolyte and the fuel electrode with oxygen ions (O ²⁻ ) generated by dissociating oxygen in the air introduced into the air electrode at the interface with the electrolyte. By this electrochemical reaction, water and carbon dioxide are generated, and electricity is generated by the electrons emitted at that time. In the power generation method using SOFC, the chemical energy of the fuel is directly converted into electric energy, so that energy conversion loss is small and highly efficient power generation is possible.

  For example, Patent Document 1 discloses a highly efficient fuel by cascading fuel gas ventilation methods inside a fuel cell (decreasing multi-stage serialization) and controlling the flow direction of fuel gas between a plurality of cell groups. It is disclosed that a battery can be provided.

  Patent Document 2 discloses a fuel cell system suitable for improving the power generation efficiency of SOFC by the steam reforming method. Specifically, a part of the fuel is supplied to the reformer and the remaining part of the fuel cell system is disclosed. A fuel cell system is disclosed that can reduce the amount of water supplied to the reformer and improve the power generation efficiency by supplying to the second cell stack as compared with the conventional steam reforming system.

JP 2011-210440 A JP2013-258004A

However, in the cascade type fuel cell as described in Patent Document 1, the concentration of the fuel supplied to the fuel cell stack (cell group) on the downstream side is lowered, and as a result, the cells of this fuel cell stack There is a risk that the voltage will drop. When the cell voltage of the fuel cell stack is lowered, the electrode is easily oxidized, and the long-term durability of the fuel cell stack is affected.
In addition, in a fuel cell system including a plurality of fuel cell stacks, the fuel utilization rate in each stack varies depending on the ratio of fuel consumption between the upstream fuel cell stack and the downstream fuel cell stack, resulting in power generation efficiency. affect. Therefore, in order to obtain high power generation efficiency, it is necessary to define the ratio of the fuel consumption.

  Furthermore, in the fuel cell system as described in Patent Document 2, since a part of the fuel is additionally supplied from the bypass pipe to the second cell stack, the fuel is supplied to the second cell stack within the pipe. There is a problem that the carbon activity increases, and the risk of carbon deposition in the piping or in the second cell stack increases.

  The present invention has been made in view of the above problems, and is to provide a fuel cell system in which carbon deposition is suitably suppressed, the fuel cell stack has excellent long-term durability, and high power generation efficiency can be obtained.

The above problem is solved by the following means.
<1> A first fuel cell stack including a solid oxide electrolyte, a fuel electrode, and an air electrode, and a solid oxide electrolyte, a fuel electrode, and an air electrode, which are disposed downstream of the first fuel cell stack. The exhaust gas discharged from the fuel electrode side of the first fuel cell stack is disposed upstream of the second fuel cell stack supplied to the fuel electrode side and the first fuel cell stack. Reforming a hydrocarbon fuel to generate a reformed gas, and supplying the generated reformed gas to the fuel electrode side of the first fuel cell stack; and the first reformer. First fuel supply means for supplying hydrocarbon fuel to the gasifier, the second fuel with respect to the amount of fuel used in the first fuel cell stack (amount of fuel that is electrochemically oxidized) the ratio _{r 1} of the fuel consumption in the cell stack, 0.35 satisfy r ₁ ≦ 1.0, the fuel cell system.

When the ratio r ₁ is 0.35 or more, the value of the cell voltage in the second fuel cell stack is maintained to some extent, and the oxidation of the electrode (fuel electrode) of the second fuel cell stack is suppressed. Therefore, long-term durability of the fuel cell stack can be ensured. Furthermore, in the fuel cell system described above, high power generation efficiency can be obtained.

When the ratio r ₁ is 1.0 or less, the fuel utilization rate in the second fuel cell stack is suppressed from becoming too high. Thereby, while maintaining the long-term stability of a fuel cell stack, high power generation efficiency can be obtained.

  The second fuel cell stack generates power by reacting the exhaust gas discharged from the first fuel cell stack (unreacted gas containing hydrogen and carbon monoxide). The first fuel cell stack No additional hydrocarbon fuel is supplied between the fuel cell stack and the second fuel cell stack. Therefore, an increase in carbon activity is suppressed in the fuel cell system, and carbon deposition is suitably suppressed.

  <2> The number S of steam molecules per unit time supplied to the first reformer and the number C of hydrocarbon fuel carbon atoms per unit time supplied to the first reformer. The fuel cell system according to <1>, wherein a steam carbon ratio S / C that is a ratio is 1.6 to 3.5.

  When S / C is within the above numerical range, the hydrocarbon fuel is efficiently reformed, and a reformed gas containing hydrogen and carbon monoxide is generated.

  <3> The fuel cell system according to <1> or <2>, wherein the total fuel utilization ratio of the first fuel cell stack and the second fuel cell stack is 80% to 95%.

  When the total fuel utilization is 80% to 95%, high power generation efficiency can be obtained in the fuel cell system.

<4> The fuel cell system according to any one of <1> to <3>, wherein the ratio r ₁ satisfies 0.35 ≦ r ₁ ≦ 0.6.

When the ratio r ₁ is not less than 0.35 and not more than 0.6, the high fuel utilization of the entire system without the fuel utilization rates of the first fuel cell stack and the second fuel cell stack being excessively high. Since the rate can be achieved, higher power generation efficiency can be obtained in the fuel cell system.

<5> A solid oxide electrolyte, a fuel electrode, and an air electrode, disposed upstream of the first reformer, and exhaust gas containing water vapor discharged from the fuel electrode side, A third fuel cell stack to be supplied to one reformer, and a ratio r ₂ of a fuel usage in the third fuel cell stack to a fuel usage in the first fuel cell stack is 0 . The fuel cell system according to any one of <1> to <4>, wherein 1 ≦ r ₂ ≦ 0.8 is satisfied.

When the ratio r ₂ is 0.1 or more, high power generation efficiency can be obtained in the fuel cell system.

When the ratio r ₂ is 0.8 or less, an increase in the fuel utilization rate in the third fuel cell stack is suppressed, and the amount of fuel consumed before being supplied to the first fuel cell stack is suppressed. can do. As a result, an increase in the fuel utilization rate of the third fuel cell stack and a decrease in the closed circuit voltage (OCVAi) at the inlet of the first fuel cell stack can be suppressed at the same time, and the fuel cell with excellent long-term durability of the fuel cell stack A system can be provided.

  <6> Arranged upstream of the third fuel cell stack, reforming hydrocarbon fuel to generate reformed gas, and generating the reformed gas on the fuel electrode side of the third fuel cell stack The fuel cell system according to <5>, further comprising: a second reformer to be supplied; and second fuel supply means for supplying hydrocarbon fuel to the second reformer.

  According to the above configuration, the reformed gas containing hydrogen and carbon monoxide is generated by reforming the hydrocarbon fuel in the second reformer, and the reformed gas is supplied to the third fuel cell stack. Then, power generation is performed.

  <7> A number S of water vapor molecules per unit time supplied to the second reformer and a number C of hydrocarbon fuel carbon atoms per unit time supplied to the second reformer. The fuel cell system according to <6>, wherein the steam carbon ratio S / C, which is the ratio, is 1.6 to 3.5.

  When S / C is within the above numerical range, the hydrocarbon fuel is efficiently reformed, and a reformed gas containing hydrogen and carbon monoxide is generated.

<8> The fuel cell system according to any one of <5> to <7>, wherein the ratio r ₂ satisfies 0.35 ≦ r ₂ ≦ 0.8.

When the ratio r ₂ is 0.4 or more, it is possible to provide a fuel cell system with high power generation efficiency. Further, when the ratio r ₂ is 0.8 or less, it is possible to provide a fuel cell system with excellent long-term durability of the fuel cell stack.

<9> The ratio r ₂ satisfies 0.4 ≦ r ₂ ≦ 0.7, and the flow rate of the hydrocarbon fuel supplied to the first reformer and the second reformer are supplied. The fuel cell system according to <6> or <7>, wherein a ratio r _{3 to} a flow rate of the hydrocarbon fuel to be satisfied satisfies 0.6 ≦ r ₃ ≦ 0.9.
<10> The ratio r ₂ satisfies 0.5 ≦ r ₂ ≦ 0.6, and the flow rate of the hydrocarbon fuel supplied to the first reformer and the second reformer are supplied. The fuel cell system according to <6> or <7>, wherein a ratio r _{3 to} a flow rate of the hydrocarbon fuel to be satisfied satisfies 0.5 ≦ r ₃ ≦ 1.2.

  According to these configurations, it is possible to provide a fuel cell system with excellent long-term durability of the fuel cell stack and high power generation efficiency.

  According to the present invention, it is possible to provide a fuel cell system in which carbon deposition is suitably suppressed, the fuel cell stack has excellent long-term durability, and high power generation efficiency can be obtained.

FIG. 1 is a schematic configuration diagram showing a fuel cell system according to an embodiment of the present invention, (a) is a diagram showing an overall configuration of the fuel cell system, and (b) is a fuel cell stack in a fuel cell system. It is a figure which shows having been laminated | stacked. 1 is a schematic exploded perspective view showing a fuel cell stack included in a fuel cell system according to an embodiment of the present invention. In the present embodiment, it is a graph showing the power generation efficiency of the fuel cell system when changing the r _1. In the present embodiment, (a) is a graph showing a fuel utilization rate of the first fuel cell stack and the second fuel cell stack at the time of changing the r _1, (b) is changed r ₁ It is a graph which shows the cell voltage of the 1st fuel cell stack and the 2nd fuel cell stack when it was made to do. FIG. 4 is a schematic configuration diagram showing a fuel cell system according to another embodiment of the present invention, (a) is a diagram showing an overall configuration of the fuel cell system, and (b) is a fuel cell stack in the fuel cell system. It is a figure which shows that is laminated | stacked.

  In the present 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.

  Hereinafter, an exemplary embodiment 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 this embodiment, (a) is a diagram showing an overall configuration of the fuel cell system, and (b) is a fuel cell stack in the fuel cell system. It is a figure which shows that is laminated | stacked.

[Fuel cell system 100]
A fuel cell system 100 according to an embodiment of the present invention includes a first fuel cell stack 5 including a solid oxide electrolyte, a fuel electrode, and an air electrode; a solid oxide electrolyte, a fuel electrode, and an air electrode; The second fuel cell stack 7 is disposed downstream of the first fuel cell stack 5 and exhaust gas discharged from the fuel electrode side of the first fuel cell stack 5 is supplied to the fuel electrode side. , Disposed upstream of the first fuel cell stack 5, reforming the hydrocarbon fuel to generate a reformed gas, and supplying the generated reformed gas to the fuel electrode side of the first fuel cell stack 5. The first reformer 3 and first fuel supply means for supplying hydrocarbon fuel to the first reformer 3, and the amount of fuel used in the first fuel cell stack 5 (the fuel is electric Second fuel cell stack 7 against the amount of chemically oxidized) The ratio _{r 1} of the fuel consumption satisfies 0.35 ≦ _{r 1} ≦ 1.0.

As shown in FIG. 1B, the fuel cell system 100 is a two-stage fuel cell system in which a first fuel cell stack 5 and a second fuel cell stack 7 are stacked and electrically connected in series. . By electrically connecting in series, the structure of the fuel cell system 100 is simplified, and the cost of the system can be reduced. The ratio r ₁ of the fuel usage amount in the second fuel cell stack 7 to the fuel usage amount in the first fuel cell stack 5 satisfies 0.35 ≦ r ₁ ≦ 1.0.

When the ratio r ₁ is 0.35 or more, the value of the cell voltage in the second fuel cell stack 7 is maintained to some extent, and the oxidation of the electrode (fuel electrode) of the second fuel cell stack 7 is suppressed. . Therefore, long-term durability of the fuel cell stack can be ensured. Furthermore, in the fuel cell system 100 described above, high power generation efficiency can be obtained.

When the ratio r ₁ is 1.0 or less, it is possible to suppress the fuel utilization rate in the second fuel cell stack 7 from becoming too high. Thereby, the long-term stability of the fuel cell stack can be maintained.

  Exhaust gas (unreacted gas containing hydrogen and carbon monoxide) discharged from the fuel electrode side of the first fuel cell stack 5 passes through the exhaust gas supply pipe 6 to the fuel electrode side of the second fuel cell stack 7. To be supplied. Electric power is generated in the second fuel cell stack 7 by reacting the supplied gas, and additional hydrocarbon fuel is added between the first fuel cell stack 5 and the second fuel cell stack 7. We do not supply. Therefore, an increase in carbon activity is suppressed in the fuel cell system 100 (for example, inside the exhaust gas supply pipe 6 or inside the second fuel cell stack 7), and carbon deposition is suitably suppressed.

In order for the ratio r ₁ of the fuel usage in the second fuel cell stack 7 to the fuel usage in the first fuel cell stack 5 to satisfy 0.35 ≦ r ₁ ≦ 1.0, simply When the current density per battery (cell) electrode area is equal between the first fuel cell stack 5 and the second fuel cell stack 7, the second fuel cell stack 7 with respect to the effective electrode area of the first fuel cell stack 5. The ratio of the effective electrode area or the ratio of the number of stacked cells of the second fuel cell stack 7 to the number of stacked cells of the first fuel cell stack 5 corresponds to r _1. What is necessary is just to adjust to 1.0 or less.
Here, the ratio of the effective electrode area of the second fuel cell stack 7 to the effective electrode area of the first fuel cell stack 5 is the area when the cell electrode areas of the stacked cells of each fuel cell stack are summed. It means ratio. The ratio of the number of stacked cells in the second fuel cell stack 7 to the number of stacked cells in the first fuel cell stack 5 is based on the assumption that the cell areas of each fuel cell stack are equal.

The r ₁ may satisfy 0.35 ≦ r ₁ ≦ 1.0, but preferably satisfies 0.35 ≦ r ₁ ≦ 0.6, and satisfies 0.4 ≦ r ₁ ≦ 0.6. It is more preferable. By the ratio r ₁ is 0.35 or more and 0.6 or less, without the respective fuel utilization too high of the first fuel cell stack 5 and the second fuel cell stack 7, a high fuel of the entire system Since the utilization rate can be achieved, higher power generation efficiency can be obtained in the fuel cell system.

  The hydrocarbon fuel used in the fuel cell system 100 according to the present embodiment is a gas that is reformed by the first reformer 3. Examples of the hydrocarbon fuel include natural gas, LPG (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, and butane, and methane is preferable. The hydrocarbon fuel may be a mixture of the above-described lower hydrocarbon gas, or a gas such as natural gas, city gas, or LP gas containing the above-described lower hydrocarbon gas.

When reforming hydrocarbon fuel with a reformer (first reformer, second reformer), reforming methods include steam reforming, CO ₂ reforming (dry reforming), and the like. Can be mentioned. In the following fuel cell system 100, a case where steam reforming is adopted as a reforming method will be described, but the present invention is not limited to this.

  The hydrocarbon fuel is supplied from the first fuel supply source (not shown) to the first reformer 3 through the hydrocarbon fuel supply pipe 2. Further, the reforming water used for the steam reforming of the hydrocarbon fuel is supplied from the reforming water supply source (not shown) to the first reformer 3 through the reforming water supply pipe 1. The first fuel supply means includes a hydrocarbon fuel supply pipe 2 and a first fuel supply source, and the hydrocarbon fuel supply pipe 2 constitutes a part of the first fuel supply means.

  The first reformer 3 is for generating reformed gas containing carbon monoxide and hydrogen by steam reforming the supplied hydrocarbon fuel. The first reformer 3 is disposed upstream of the first fuel cell stack 5 and is connected to the reformed water supply pipe 1 and the hydrocarbon fuel supply pipe 2.

  The first reformer 3 includes, for example, a combustion section in which a burner or a combustion catalyst is disposed and a reforming section that includes a reforming catalyst. The reforming section is provided with a reforming catalyst in which a metal such as Ni or Ru is supported on a carrier such as alumina.

  The steam reforming that occurs in the reforming part involves a large endotherm, so that it is necessary to supply heat from the outside for the progress of the reaction, and therefore, the reforming part can be heated by the combustion heat generated in the combustion part. preferable. Or you may heat using the heat discharge | released from each fuel cell stack.

  When methane, which is an example of a hydrocarbon fuel, is steam reformed, carbon monoxide and hydrogen are generated in the first reformer 3 by the reaction of the following formula (a), and the following formula (b ) To generate carbon dioxide and hydrogen.

CH ₄ + H ₂ O → CO + 3H ₂ ... (A)
CO + H ₂ O → CO ₂ + H ₂ ... (B)

  It is a ratio between the number S of water vapor molecules per unit time supplied to the first reformer 3 and the number C of carbon atoms of hydrocarbon fuel per unit time supplied to the first reformer 3. The steam carbon ratio S / C is preferably 1.6 to 3.5, more preferably 2.0 to 3.5, and still more preferably 2.5 to 3.0. When the steam carbon ratio S / C is within this range, the hydrocarbon fuel is efficiently steam reformed, and a reformed gas containing hydrogen and carbon monoxide is generated. Furthermore, carbon deposition in the fuel cell system 100 can be suppressed.

  The reformed gas generated by the first reformer 3 is supplied to the first fuel cell via the reformed gas supply pipe 4 that communicates the first reformer 3 and the first fuel cell stack 5. It is supplied to the fuel electrode side of the stack 5.

  The first fuel cell stack 5 includes a solid oxide electrolyte, a fuel electrode, and an air electrode, and is connected to the reformed gas supply pipe 4 and the exhaust gas supply pipe 6 on the fuel electrode side. The first fuel cell stack 5 electrochemically reacts the reformed gas supplied from the reformed gas supply pipe 4 to the fuel electrode side and oxygen ions derived from oxygen supplied to the air electrode side. To generate electricity. The reformed gas after the reaction and the unreacted reformed gas are discharged to the exhaust gas supply pipe 6 as exhaust gas. The specific configuration of the fuel cell stack, the solid oxide electrolyte, the fuel electrode, and the air electrode will be described later.

  The exhaust gas is discharged from the fuel electrode side of the first fuel cell stack 5 and supplied to the fuel electrode side of the second fuel cell stack 7 through the exhaust gas supply pipe 6.

  The second fuel cell stack 7 includes a solid oxide electrolyte, a fuel electrode, and an air electrode, and is disposed downstream of the first fuel cell stack 5. Connected. The second fuel cell stack 7 generates electricity by electrochemically reacting the exhaust gas supplied from the exhaust gas supply pipe 6 to the fuel electrode side and the oxygen ions derived from oxygen supplied to the air electrode side. To do. The exhaust gas after the reaction may be exhausted outside the second fuel cell stack 7.

  Hereinafter, the structure of the first fuel cell stack 5 will be described with reference to FIG. FIG. 2 is a schematic exploded perspective view showing the first fuel cell stack 5 provided in the fuel cell system 100 according to the present embodiment. The first fuel cell stack 5 includes a plurality of unit cells 208 each having a solid oxide electrolyte 202, a fuel electrode 204, and an air electrode 206. In FIG. 2, the first fuel cell stack 5 includes two unit cells, but is not limited to this configuration. That is, the fuel cell stack used in the present invention may include a plurality (n; n is an integer satisfying n ≧ 2). Note that the structure of the second fuel cell stack 7 is the same as that of the first fuel cell stack 5, and therefore the description thereof is omitted.

  The first fuel cell stack 5 has a stack body having a plurality of unit cells 208 and the fuel electrode 204 and the reformed gas supply pipe 4 (not shown) of each unit cell 208 in the first fuel cell stack 5. Are connected, and the fuel electrode 204 of each unit cell 208 and the exhaust gas supply pipe 6 (not shown) are connected. Further, in the first fuel cell stack 5, the air electrode of each unit cell 208 and an oxidant gas supply pipe (not shown) for supplying oxidant gas are connected, and the exhaust gas on the air electrode side is connected. Is provided with a discharge port (not shown).

  As shown in FIG. 2, the unit cell 208 of the first fuel cell stack 5 includes a layered solid oxide electrolyte 202 and a layered fuel electrode 204 joined to one surface of the layered solid oxide electrolyte 202. , And a layered air electrode 206 joined to one surface of the layered solid oxide electrolyte 202. The plurality of single cells 208 are stacked via the interconnector 210. That is, each of the plurality of single cells 208 has a structure sandwiched between a pair of interconnectors 210. In addition, although not shown in figure, each cell 208 and the interconnector 210 are in the state which clamped the gas seal body in the outer periphery part.

The solid oxide electrolyte 202 is made of, for example, a dense oxide oxide conductive solid oxide. Examples of the solid oxide include stabilized zirconia and partially stabilized zirconia. Specific examples of the stabilized zirconia include yttria stabilized zirconia (YSZ), scandia stabilized zirconia (ScSZ), and the like. Specific examples of the partially stabilized zirconia include yttria partially stabilized zirconia (YSZ), scandia partially stabilized zirconia (ScSZ), and the like. In addition, as the solid oxide, for example, ceria-based oxide doped with Sm, Gd, etc .; La _0.8 Sr ₀ in which LaGaO ₃ is used as a base and a part of La and Ga are respectively replaced by Sr and Mg. _.2 Perovskite oxides such as Ga _0.8 Mg _0.2 O _(3-δ) ;

  The fuel electrode 204 is an anode. In the fuel electrode 204, oxygen ions and the reformed gas fuel react to emit electrons. The fuel electrode 204 is preferably, for example, porous, has high ion conductivity, and does not easily cause a solid-solid reaction with the solid oxide electrolyte 202 or the like at a high temperature. The fuel electrode 204 is preferably made of, for example, Ni, yttria stabilized zirconia (YSZ) / nickel metal porous cermet, scandia stabilized zirconia (ScSZ) / nickel metal porous cermet, and the like. The fuel electrode 204 may be made of a mixed material obtained by mixing two or more of the above materials.

The air electrode 206 is a cathode. At the air electrode 206, oxygen in the oxidant gas takes in electrons and oxygen ions are formed. The air electrode 206 is preferably, for example, porous, has high electron conductivity, and does not easily cause a solid-solid reaction with the solid oxide electrolyte 202 or the like at a high temperature. The air electrode 206 is preferably composed of, for example, a PrCoO ₃ oxide, a LaFeO ₃ oxide, a LaCoO ₃ oxide, a LaMnO ₃ oxide, or the like. Specific examples of LaMnO ₃ -based oxides include La _0.8 Sr _0.2 MnO ₃ (LSM), La _0.6 Ca _0.4 MnO ₃ (LCM), and the like. Examples of the LaCoO ₃ oxide include La _0.6 Sr _0.4 Fe _0.8 Co _0.2 O ₃ (LSCF). The air electrode 206 may be composed of a mixed material obtained by mixing two or more of the above materials.

  The interconnector 210 is composed of an electron conductive member in order to exchange electrons with the fuel electrode 204 or the air electrode 206. The interconnector 210 supplies the reformed gas flow path forming groove 210A for supplying the reformed gas to the surface facing the fuel electrode 204 and the oxidant gas to the surface facing the air electrode 206. An oxidant gas flow path forming groove 210B is formed. The reformed gas flow path forming groove 210A and the oxidant gas flow path forming groove 210B are formed, for example, along directions intersecting each other. The reformed gas channel forming groove 210 </ b> A functions as a reformed gas channel for supplying the reformed gas to the fuel electrode 204 when the interconnector 210 is disposed in close contact with the fuel electrode 204. On the other hand, the oxidant gas flow path forming groove 210 </ b> B functions as an oxidant gas flow path for supplying an oxidant gas to the air electrode 206 when the interconnector 210 is disposed in close contact with the air electrode 206.

The constituent material of the interconnector 210 is not particularly limited. The interconnector 210 generally uses an alloy. For example, SUS310S and ZMG232 (made by Hitachi Metals, Ltd.) are mentioned. The constituent material of the interconnector 210 may be an oxide having electronic conductivity, specifically, a dense body of LaCrO ₃ oxide.

  Next, an electrochemical reaction that occurs in the unit cell 208 will be described. First, an oxygen-containing oxidant gas is supplied to the air electrode 206 of the unit cell 208 to cause a reaction represented by the following formula (c). At this time, oxygen ions are exchanged inside the solid oxide electrolyte 202. To move.

O ₂ + 4e ⁻ → 2O ²⁻ (c)

  Further, the reformed gas containing carbon monoxide and hydrogen is supplied to the fuel electrode 204, and electrons are transferred from the oxygen ions moving inside the solid oxide electrolyte 202 at the interface between the fuel electrode 204 and the solid oxide electrolyte 202. Is received, the reaction shown in the following formula (d) occurs.

CO + 3H ₂ + 4O ²⁻ → CO ₂ + 3H ₂ O + 8e ⁻ (d)

  By the electrochemical reaction of the reformed gas in the unit cell as shown in the above formula (d), water vapor and carbon dioxide are mainly generated, and electric power is generated.

  Carbon monoxide and hydrogen that are not used in the first fuel cell stack 5 are discharged from the fuel electrode side of the first fuel cell stack 5, and the second fuel cell stack is connected via the exhaust gas supply pipe 6. 7 is supplied directly to the fuel electrode side and used for power generation. As described above, since no additional hydrocarbon fuel is supplied between the first fuel cell stack 5 and the second fuel cell stack 7, the carbon activity in the fuel cell system 100 is increased. Is suppressed, and carbon deposition is suitably suppressed.

  Next, it will be described below that the fuel cell system 100 having a two-stage stack has excellent long-term stability and high power generation efficiency. First, the performance and conditions of the fuel cell stack (single stage stack) used in the fuel cell system 100 are set as follows.

[Fuel cell stack performance and conditions]
When the fuel utilization rate of a fuel cell stack (single stage stack) is 75%, the system efficiency (power generation efficiency) of the fuel cell stack is 55% LHV (low heating value), and the loss due to efficiency conversion from the stack to the system is reduced. Assume 10%. From this assumption, the stack efficiency is calculated to be 61.1%. When the current density is 0.2 A / cm ² , this performance is obtained if the internal resistance (ASR) of the fuel cell stack is 0.3517 ohm (Ω) · cm ² when the fuel utilization rate is low.
It is assumed that the steam carbon ratio S / C is 3.0 for the hydrocarbon fuel and steam supplied to the reformer provided upstream of the fuel cell stack. Further, the oxygen partial pressure P _O2 (in) at the fuel electrode side inlet of the fuel cell stack is 3.77 × 10 ⁻²¹ (atm), and the oxygen partial pressure P _O2 (out) at the fuel electrode side outlet of the fuel cell stack is 8 915 × 10 ⁻¹⁹ (atm), the reaction temperature inside the stack is 750 ° C., and the supplied hydrocarbon fuel is methane.
At this time, the oxygen potential at the fuel electrode side inlet of the stack is calculated as -0.989V in terms of air potential, and the oxygen potential at the fuel electrode side outlet of the stack is -0.877V.

[Two-stage fuel cell system]
Using the single-stage stack having the system efficiency of 55% LHV as described above, the fuel cell stack (the first fuel cell stack 5 and the second fuel cell stack 7) having a two-stage structure as shown in FIG. 1 is provided. The fuel cell system 100 is prepared. The overall fuel usage rate of the fuel cell system 100 (Uf (total), unit%), the fuel usage rate of the first fuel cell stack 5 (Uf (A), unit%), and the second fuel cell stack 7 The relationship with the fuel utilization rate (Uf (B), unit%) will be described below.

  First, the overall fuel utilization rate (Uf (total)) was used in the first fuel cell stack 5 and the second fuel cell stack 7 with respect to the amount of fuel supplied to the first fuel cell stack 5. Calculated as a percentage of the total amount of fuel. Therefore, the supply amount to the first fuel cell stack 5 is S, the fuel use amount in the first fuel cell stack 5 is Con (A), and the fuel use amount in the second fuel cell stack 7 is Con (B ), The following relational expression (1) is established.

Uf (total) / 100 = (Con (A) + Con (B)) / S (1)

By using the ratio r ₁ of the fuel consumption in the second fuel cell stack 7 to the fuel consumption at the first fuel cell stack 5, the fuel used in the first fuel cell stack and the second fuel cell stack The following equation (2) for the quantity is obtained:

Con (B) = r ₁ Con (A) (2)

  The following equation (3) is obtained from the relationship between the fuel utilization rate and the fuel usage amount in the first fuel cell stack 5, and the relationship between the fuel utilization rate and the fuel usage amount in the second fuel cell stack 7 is obtained. Thus, the following formula (4) is obtained.

Uf (A) / 100 = Con (A) / S (3)
Uf (B) / 100 = Con (B) / S (1-Uf (A) / 100) (4)

  From the above formulas (1) to (4), the following formulas (5) and (6) are established for Uf (A) and Uf (B).

Uf (A) = Uf (total) / (1 + r ₁ ) (5)
Uf (B) = r ₁ Uf (total) / (1 + r ₁ −Uf (total) / 100) (6)

FIG. 4A is a graph showing the fuel utilization rates of the first fuel cell stack 5 and the second fuel cell stack 7 when r ₁ is changed. This graph is obtained by setting Uf (total) = 85 (%) in the above formulas (5) and (6). As shown in the figure, when r ₁ is decreased, the fuel utilization rate (Uf (A)) of the first fuel cell stack 5 is increased and the fuel utilization rate (Uf (B)) of the second fuel cell stack 7 is increased. ) Becomes smaller. On the other hand, when r ₁ is increased, the fuel utilization rate (Uf (A)) of the first fuel cell stack 5 is decreased and the fuel utilization rate (Uf (B)) of the second fuel cell stack 7 is increased. .

If Uf (A) and Uf (B) greatly exceed 75%, the long-term durability is reduced. Therefore, Uf (A) and Uf (B) are preferably 80% or less, and more preferably 75% or less. Therefore, the range of _{r 1,} preferably Uf (A) and Uf (B) is 0.063 ≦ _{r 1} ≦ 2.4 which satisfies 80% or less, Uf (A) and Uf (B) is It is more preferable that 0.134 ≦ r ₁ ≦ 1.125 satisfying 75% or less. Furthermore, to maintain over a long stability of the first fuel cell stack 5 and the second fuel cell stack 7, the range of r _1, preferably from 0.2 ≦ r ₁ ≦ 1.0, More preferably, 0.35 ≦ r ₁ ≦ 1.0.

Next, FIG. 4B is a graph showing the cell voltages of the first fuel cell stack 5 and the second fuel cell stack 7 when r ₁ is changed. In the figure, V (A) is the cell voltage of the first fuel cell stack 5, V (B) is the cell voltage of the second fuel cell stack 7, and V (mean) is the first fuel cell. The weighted average of V (A) and V (B) taking into account the ratio of the fuel usage of the stack and the second fuel cell stack is shown. These are cell voltages obtained by using methane as a hydrocarbon fuel and setting Uf (total) = 85%. As can be seen, as r ₁ increases, V (A) and V (B) also increase. Here, the range of _{r 1,} preferably from 0.35 ≦ _{r 1,} and more preferably 0.4 ≦ _{r 1.} When r ₁ satisfies this range, when an electrode containing Ni or the like is used, electrode oxidation (for example, Ni oxidation) caused by a low cell voltage (less than 0.80 V in the figure) can be suppressed. In particular, it is preferable because the oxidation of the electrode of the second fuel cell stack 7 can be suppressed.

FIG. 3 is a graph showing the power generation efficiency of the fuel cell system 100 when r ₁ is changed in the present embodiment. That is, this graph shows a change in the power generation efficiency of the fuel cell system 100 having a two-stage configuration including the first fuel cell stack 5 and the second fuel cell stack 7 when r ₁ is changed. Note that it is assumed that Uf (total) = 85% and the stack has an internal resistance (ASR) of 0.3517 Ω · cm ² when operating at a current density of 0.20 A / cm ² .
Here, the power generation efficiency of the fuel cell system refers to the ratio of the actual power generation amount to the theoretical power generation amount obtained from the fuel supplied to the fuel cell system (%: low calorific value standard).

FIG. 3 shows that the power generation efficiency is the maximum value (about 62%) when r ₁ is in the vicinity of 0.4 to 0.5. Therefore, the range of _{r 1,} preferably from 0.35 ≦ _{r 1} ≦ 0.6, it is preferable that 0.4 ≦ _{r 1} ≦ 0.6.

  The overall fuel utilization rate (Uf (total), unit%) of the first fuel cell stack 5 and the second fuel cell stack 7 is preferably 80% to 95%. Thereby, high power generation efficiency can be obtained.

  Here, when Uf (A) and Uf (B) match, the following equation (7) is established from equations (5) and (6).

r ₁ = (100−Uf (total)) ^1/2 / 10... (7)

As r ₁ , Uf (A) and Uf (B) match (in the vicinity of r ₁ = 0.4 as shown in FIG. 4A), or Uf (A) and Uf (B) Are preferably close to each other, and assuming that Uf (total) is 80% to 95%, 0.25 ≦ r ₁ ≦ 0.6 is preferable, and 0.35 ≦ r ₁ ≦ 0. .6 is more preferable. As the distance from the range in which Uf (A) and Uf (B) coincide with each other, either Uf (A) or Uf (B) becomes a higher value as shown in FIG. For this reason, it is desirable that Uf (A) and Uf (B) have the same value or close values.

Considering the graph shown in FIGS. 3 and 4 and the range of r ₁ when Uf (A) and Uf (B) are equal or close to each other, as r ₁ , 0.35 ≦ r ₁ ≦ 1 0.0 is preferable, and 0.35 ≦ r ₁ ≦ 0.6 is more preferable. As described above, the ratio r ₁ of the fuel usage is the ratio of the effective electrode area of the second fuel cell stack 7 to the effective electrode area of the first fuel cell stack 5 or the cells of the first fuel cell stack 5. Since this corresponds to the ratio of the number of stacked cells of the second fuel cell stack 7 to the number of stacked layers, it is necessary to satisfy 0.35 ≦ r ₁ ≦ 1.0, preferably 0.35 ≦ r ₁ ≦ 0.6. The effective electrode area or the number of stacked cells in the first fuel cell stack 5 and the second fuel cell stack 7 may be set. The effective electrode area or the number of stacked cells in the fuel cell stacks 5 and 7 can be set to a desired range in the design stage of the fuel cell system 100 or the like. Thereby, the fuel cell system 100 having excellent long-term stability and high power generation efficiency can be obtained.

[Fuel cell system 200]
Next, a fuel cell system 200 according to another embodiment of the present invention will be described with reference to FIG. FIG. 5 is a schematic configuration diagram showing a fuel cell system according to another embodiment, (a) is a diagram showing an overall configuration of the fuel cell system, and (b) is a fuel cell in the fuel cell system. It is a figure which shows that a stack is laminated | stacked.
In addition, about the structure of the same name as each structure of the fuel cell system 100 mentioned above, the description in the fuel cell system 100 shall be used, and the detailed description is abbreviate | omitted.

Similar to the fuel cell system 100, the fuel cell system 200 according to another embodiment of the present invention includes a first fuel cell stack 15, a second fuel cell stack 17, and a first reformer 13. ing. Furthermore, the fuel cell system 200 includes a solid oxide electrolyte, a fuel electrode, and an air electrode, is disposed upstream of the first reformer 13, and is exhausted from the fuel electrode side, including water vapor. A third fuel cell stack 25 that supplies gas to the first reformer 13 is provided, and the ratio of the fuel usage in the third fuel cell stack 25 to the fuel usage in the first fuel cell stack 15 r ₂ satisfies 0.1 ≦ r ₂ ≦ 0.8.

As shown in FIG. 5, the fuel cell system 200 is a three-stage fuel cell system in which a first fuel cell stack 15, a second fuel cell stack 17, and a third fuel cell stack 25 are stacked. The ratio r ₂ of the fuel usage amount in the third fuel cell stack 25 to the fuel usage amount in the first fuel cell stack 15 satisfies 0.1 ≦ r ₂ ≦ 0.8.

When the ratio r ₂ is 0.1 or more, high power generation efficiency can be obtained in the fuel cell system 200.

When the ratio r ₂ is 0.8 or less, an increase in the fuel utilization rate in the third fuel cell stack 25 is suppressed, and the amount of fuel consumed before being supplied to the first fuel cell stack 15. Can be suppressed. As a result, an increase in the fuel utilization rate of the third fuel cell stack 25 and a decrease in the closed circuit voltage (OCVAi) at the inlet of the first fuel cell stack can be suppressed at the same time.

  The third fuel cell stack 25 includes a solid oxide electrolyte, a fuel electrode, and an air electrode, and is disposed upstream of the first reformer 13, and the reformed gas supply pipe 24 is disposed on the fuel electrode side. The exhaust gas supply pipe 11 is connected. The third fuel cell stack 25 electrochemically reacts the reformed gas supplied from the reformed gas supply pipe 24 to the fuel electrode side and oxygen ions derived from oxygen supplied to the air electrode side. To generate electricity. Then, the water (water vapor) generated by the electrochemical reaction during the power generation (reaction shown in the formula (d)) is discharged from the third fuel cell stack 25 and is supplied to the first through the exhaust gas supply pipe 11. The reformer 13 is used for steam reforming of hydrocarbon fuel. The structure of the third fuel cell stack 25 is the same as that of the first fuel cell stack 5 described above, and a detailed description thereof will be omitted.

  The fuel cell system 200 is disposed upstream of the third fuel cell stack 25, steam reforms the hydrocarbon fuel to generate a reformed gas, and the generated reformed gas is supplied to the third fuel cell stack 25. A second reformer (other reformer) 23 to be supplied, and a second fuel supply means for supplying hydrocarbon fuel to the second reformer 23 are further provided.

  The second reformer 23 is for generating a reformed gas containing carbon monoxide and hydrogen by steam reforming the supplied hydrocarbon fuel. The second reformer 23 is disposed upstream of the third fuel cell stack 25 and is connected to the reformed water supply pipe 21 and the hydrocarbon fuel supply pipe 22.

  The hydrocarbon fuel is supplied from the second fuel supply source (not shown) to the second reformer 23 via the hydrocarbon fuel supply pipe 22. Further, the reforming water used for the steam reforming of the hydrocarbon fuel is supplied from the reforming water supply source (not shown) to the second reformer 23 via the reforming water supply pipe 21. The second fuel supply means includes a hydrocarbon fuel supply pipe 22 and a second fuel supply source, and the hydrocarbon fuel supply pipe 22 constitutes a part of the second fuel supply means.

  Here, the hydrocarbon fuel supplied to the second reformer 23 and the hydrocarbon fuel supplied to the first reformer 13 are preferably the same. At this time, the first reformer 13 and the second reformer 23 have a common hydrocarbon fuel supply source, that is, the first fuel supply source and the second fuel supply source are the same. It is preferable from the viewpoint of reducing the equipment cost. Here, the hydrocarbon fuel supplied to the second reformer 23 is preferably methane.

  The second reformer 23 has the same configuration as the first reformer 3 described above. A reformed gas containing carbon monoxide and hydrogen is generated by steam reforming the hydrocarbon fuel in the second reformer 23. The reformed gas is supplied to the fuel electrode side of the third fuel cell stack 25 via the reformed gas supply pipe 24.

  As shown in FIG. 5A, the fuel cell system 200 includes a first fuel cell stack 15 and a fuel electrode side of the first fuel cell stack 15 that is disposed downstream of the first fuel cell stack 15. The exhaust gas discharged from the fuel cell is disposed upstream of the second fuel cell stack 17 and the first fuel cell stack 15 that are directly supplied to the fuel electrode side, and reforms and reforms the hydrocarbon fuel. A first reformer 13 that generates gas and supplies the generated reformed gas to the fuel electrode side of the first fuel cell stack 15, and is disposed upstream of the first reformer 13 and fuel A third fuel cell stack 25 that supplies exhaust gas containing water vapor discharged from the pole side to the first reformer 13 and an upstream side of the third fuel cell stack 25 are arranged to improve the hydrocarbon fuel. To produce reformed gas, and the produced reformed gas is And a second reformer 23 is supplied to the fuel electrode side of the fuel cell stack 25.

That is, the fuel cell system 200 has a configuration in which the second reformer 23 and the third fuel cell stack 25 are added to the fuel cell system 100 described above. As a result, the exhaust gas of the third fuel cell stack 25 is reformed by the first reformer 13 provided in the uppermost stream of the fuel cell system 100, so that the ratio r ₁ of the fuel usage is appropriately set. Therefore, the driving conditions of the fuel cell system 100 set to be highly efficient and maintained are reliably maintained, and the fuel cell system 100 can be expanded (addition of functions and configurations) flexibly and easily.

  This is a ratio between the number S of water vapor molecules per unit time supplied to the second reformer 23 and the number C of carbon atoms of the hydrocarbon fuel per unit time supplied to the second reformer 23. The steam carbon ratio S / C is preferably 1.6 to 3.5, more preferably 2.0 to 3.5, and still more preferably 2.5 to 3.0. When the steam carbon ratio S / C is within this range, the steam reforming of the hydrocarbon fuel can be performed efficiently, and the carbon deposition in the fuel cell system 200 can be suppressed.

  In the fuel cell system 200, the reformer does not have to be attached to the outside of the first fuel cell stack 15 or the second fuel cell stack 17, and the steam reforming (internally) inside each fuel cell stack. It may be configured to perform (modification). Since the reaction temperature inside the fuel cell stack is as high as 700 ° C. to 1000 ° C., steam reforming can be performed in the fuel cell stack by the catalytic action of nickel constituting the fuel electrode.

[3-stage fuel cell system]
Using a single-stage stack having a system efficiency of 55% LHV as described above, a fuel cell stack having a three-stage configuration as shown in FIG. 5 (first fuel cell stack 15, second fuel cell stack 17 and third A fuel cell system including a fuel cell stack 25) is prepared. The overall fuel utilization rate of the fuel cell system 200 (Uf (total), unit%), the fuel utilization rate of the first fuel cell stack 15 (Uf (A), unit%), and the second fuel cell stack 17 The relationship between the fuel usage rate (Uf (B), unit%) and the fuel usage rate (Uf (C), unit%) of the third fuel cell stack 25 will be described below.

  Note that the overall fuel usage rate (Uf (total), unit%) of the fuel cell system 200 is the total fuel usage rate of the first fuel cell stack 15 and the second fuel cell stack 17 as described above. (The third fuel cell stack 25 is not included). Therefore, also in the fuel cell system shown in FIG. 5, Uf (A) and Uf (B) satisfy the above formulas (5) and (6).

Next, for the fuel cell system 200 having a three-stage configuration, the ratio r ₂ of the fuel usage amount in the third fuel cell stack 25 to the fuel usage amount in the first fuel cell stack 15, and the first reformer The ratio (mass flow rate ratio) r ₃ (flow rate) of the flow rate (flow rate 1) of the hydrocarbon fuel supplied to 13 and the flow rate (flow rate 2) of the hydrocarbon fuel additionally supplied to the second reformer 23 2 / closed circuit voltage (OCVAi) at the inlet of the first fuel cell stack, carbon activity (ActCAi) at the inlet of the first fuel cell stack, and fuel utilization rate of the third fuel cell stack when the flow rate 1) is changed The long-term durability is evaluated by obtaining a numerical value as an index of long-term stability. In addition, the power generation efficiency of the fuel cell system 200 is also obtained.

As shown in FIG. 3, when the overall fuel utilization rate satisfies 85%, the ratio r ₁ of the fuel usage in the second fuel cell stack to the fuel usage in the first fuel cell stack is 0. When it is 4 to 0.5, the power generation efficiency of the two-stage fuel cell stack is maximized. Accordingly, the overall fuel utilization rate Uf (total) is fixed to 85% and r ₁ is fixed to 0.5, and a numerical value serving as the above-mentioned long-term stability index is obtained. At this time, from the above formulas (5) and (6), Uf (A) is 56.7% and Uf (B) is 65.4%. Note that methane is assumed as the hydrocarbon fuel.

  In the two-stage fuel cell system 100 as shown in FIG. 1, the open circuit voltage at the first fuel cell stack inlet is 0.99V. Further, in the three-stage fuel cell system 200 as shown in FIG. 5, if the open circuit voltage at the first fuel cell stack inlet is 0.99 V or more, the cell voltage of the second fuel cell stack 17 is Since it becomes more than the cell voltage of the 2nd fuel cell stack 7 in a 2 step | paragraph structure and the oxidation of an electrode (fuel electrode) is suppressed in the 2nd fuel cell stack 17, it is preferable from the point of long-term stability. Therefore, the open circuit voltage (OCVAi) at the inlet of the first fuel cell stack is based on 0.99 V or more.

  When the carbon activity (ActCAi) at the inlet of the first fuel cell stack is 1.0 or more, the risk of carbon deposition and damage to the stack becomes extremely high. Therefore, considering the safety factor, the carbon activity at the inlet of the first fuel cell stack is based on 0.2 or less.

Here, in order to lower the carbon activity at the inlet of the first fuel cell stack, it is necessary to increase r ₃ to some extent and increase the amount of steam supplied to the first reformer 13. However, if too large _{r 3,} a first fuel cell stack inlet of OCVAi decreases.

Then, a large r _2, a first fuel cell stack inlet of OCVAi fuel consumption amount consumed by the third fuel cell stack 25 is increased is reduced. Therefore, when r ₂ and r ₃ are increased, the OCVAi at the inlet of the first fuel cell stack is lowered and is easily below the reference value. Therefore, there is a suitable range for r ₂ and r ₃ .

  If Uf (C) greatly exceeds 75%, which is the premise of the single-stage stack performance, it causes a decrease in long-term durability. Therefore, Uf (C) is based on 80% or less.

  In the results of long-term durability evaluation shown below, when OCVAi, ActCAi, and Uf (C) all meet the standards, the stability is A (good stability), and when one does not meet the standards, it is stable. Property B (stability failure).

First, the ratio r ₂ of the amount of fuel used in the third fuel cell stack 25 to the amount of fuel used in the first fuel cell stack 15 is fixed to 0.1 and supplied to the first reformer 13. and the flow rate of the hydrocarbon fuel, Table 1 the results of the long-term durability of the indicator and the power generation efficiency when changing the ratio r ₃ between the flow rate of the hydrocarbon fuel supplied to the second reformer 23 to 20 Shown in The hydrocarbon fuel supplied to the first reformer 13 and the second reformer 23 is methane regardless of whether r ₂ is 0.1 to 0.9. The value of r ₃ is adjusted by changing the flow rate of methane supplied to the second reformer 23 with reference to the flow rate of methane supplied to the reactor 13.

First, as shown in Table 1, the fuel cell system 200 using three single-stage stacks having a system efficiency (power generation efficiency) of 55% LHV has an overall power generation efficiency of over 62.5%. It shows higher power generation efficiency.
Further, as shown in Table 1, when r ₃ is 1, ActCAi is more than 0.2, and when r ₃ is 20, OCVAi is less than 0.99V. Therefore, when r ₃ is 1 or less, or 20 or more, it is determined that the stability is poor, it is determined that the stability is good otherwise.

Next, Table 2 shows long-term durability indicators and results of power generation efficiency when r ₂ is fixed to 0.2 and r ₃ is changed to 0.5 to 7.

As shown in Table 2, when r ₃ is 0.5, ActCAi is more than 0.2, and when r ₃ is 7, OCVAi is less than 0.99V. Therefore, when r ₃ is 0.5 or less or 7 or more, it is determined that the stability is poor, otherwise determines that stability is good.

Next, Table 3 shows long-term durability indicators and results of power generation efficiency when r ₂ is fixed to 0.3 and r ₃ is changed to 0.5 to 4.

As shown in Table 3, when r ₃ is 0.5, ActCAi is more than 0.2, and when r ₃ is 4, OCVAi is less than 0.99V. Therefore, when r ₃ is 0.5 or less or four or more, it is determined that the stability is poor, it is determined that the stability is good otherwise.

Next, Table 4 shows long-term durability indices and results of power generation efficiency when r ₂ is fixed to 0.4 and r ₃ is changed to 0.5 to 3.

As shown in Table 4, when _{r 3} is 0.5, ActCAi 0.2 ultra next, when _{r 3} is 3, OCVAi has become less than 0.99 V. Therefore, when r ₃ is 0.5 or less or 3 or more, it is determined that the stability is poor, but otherwise, it is determined that the stability is good.
Furthermore, when r ₂ is 0.4, if r ₃ satisfies 0.6 to 1, the power generation efficiency satisfies 65% or more, and high power generation efficiency is obtained.

Next, Table 5 shows long-term durability indices and results of power generation efficiency when r ₂ is fixed to 0.5 and r ₃ is changed to 0.4 to 1.7.

As shown in Table 5, when _{r 3} is 0.4, ActCAi 0.2 ultra next, when _{r 3} is 1.7, OCVAi has become less than 0.99 V. Therefore, when r ₃ is 0.4 or less or 1.7 or more, it is determined that the stability is poor, but otherwise, it is determined that the stability is good.

Next, Table 6 shows long-term durability indicators and results of power generation efficiency when r ₂ is fixed to 0.6 and r ₃ is changed to 0.4 to 1.3.

As shown in Table 6, when _{r 3} is 0.4, when Uf (C) is 80% ultra next, _{r 3} is 1.3, OCVAi has become less than 0.99 V. Therefore, when r ₃ is 0.4 or less or 1.3 or more, it is determined that the stability is poor, it is determined that the stability is good otherwise.

Next, Table 7 shows long-term durability indicators and results of power generation efficiency when r ₂ is fixed to 0.7 and r ₃ is changed to 0.5 to 1.

As shown in Table 7, when r ₃ is 0.5, Uf (C) exceeds 80%, and when r ₃ is 1, OCVAi is less than 0.99V. Therefore, when r ₃ is 0.5 or less or 1 or more, it determines that the stability is poor, it is determined that the stability is good otherwise.

Next, Table 8 shows long-term durability indices and results of power generation efficiency when r ₂ is fixed to 0.8 and r ₃ is changed to 0.6 to 0.8.

As shown in Table 8, when _{r 3} is 0.6, when Uf (C) is 80% ultra next, _{r 3} is 0.8, OCVAi has become less than 0.99 V. Therefore, when r ₃ is 0.6 or less or 0.8 or more, it is determined that the stability is poor, it is determined that the stability is good otherwise.

Next, Table 9 shows long-term durability indices and results of power generation efficiency when r ₂ is fixed to 0.9 and r ₃ is changed to 0.7 to 0.9.

As shown in Table 9, when r ₃ is 0.7, Uf (C) exceeds 80% and OCVAi is less than 0.99 V, and when r ₃ is 0.8 or 0.9, OCVAi is 0. It is less than 99V. Therefore, when r ₂ was set to 0.9, it was determined that the stability was poor regardless of the numerical range of r ₃ .

As described above, as shown in Tables 1 to 9, in order to ensure the long-term stability of the fuel cell system 200, the fuel in the third fuel cell stack 25 with respect to the amount of fuel used in the first fuel cell stack 15. It is preferable that the ratio r _{2 of the} amount used satisfies 0.1 ≦ r ₂ ≦ 0.8. Here, in order to satisfy this numerical range, when the current density per unit cell (cell) electrode area is equal between the first fuel cell stack 5 and the second fuel cell stack 7, the first fuel cell stack 15 The ratio of the effective electrode area of the third fuel cell stack 25 to the effective electrode area of the third fuel cell stack 25 or the ratio of the cell stack number of the third fuel cell stack 25 to the number of cell stacks of the first fuel cell stack 15 is r _2. Therefore, this value may be adjusted to 0.1 or more and 0.8 or less.
Here, the ratio of the effective electrode area of the third fuel cell stack 25 to the effective electrode area of the first fuel cell stack 15 is the area when the cell electrode areas of the stacked cells of each fuel cell stack are summed. It means ratio. In addition, regarding the ratio of the cell stack number of the third fuel cell stack 25 to the cell stack number of the first fuel cell stack 15, it is assumed that the cell areas of each fuel cell stack are equal.

The r ₂ may satisfy 0.1 ≦ r ₂ ≦ 0.8, but preferably satisfies 0.35 ≦ r ₂ ≦ 0.8, and satisfies 0.4 ≦ r ₂ ≦ 0.8. It is more preferable. Thereby, it is possible to obtain a power generation efficiency equal to or higher than that of the state-of-the-art large-scale concentrated thermal power generation, and it is possible to provide a distributed fuel cell system with higher power generation efficiency.

Further, the ratio r ₂ satisfies 0.4 ≦ r ₂ ≦ 0.7, and the flow rate of the hydrocarbon fuel supplied to the first reformer 13 and the second reformer 23 are supplied. The ratio r _{3 to} the flow rate of the hydrocarbon fuel preferably satisfies 0.6 ≦ r ₃ ≦ 0.9. In addition, the ratio r ₂ satisfies 0.5 ≦ r ₂ ≦ 0.6, and the flow rate of the hydrocarbon fuel supplied to the first reformer 13 and the second reformer 23 The ratio r _{3 to} the flow rate of the supplied hydrocarbon fuel preferably satisfies 0.5 ≦ r ₃ ≦ 1.2. According to these configurations, it is possible to provide a fuel cell system with excellent long-term stability of the fuel cell stack and high power generation efficiency.

  The fuel cell system according to the present invention is not limited to a fuel cell system having a two-stage stack and a fuel cell system having a three-stage stack. Therefore, a fuel cell system having a two-stage stack may be provided as a fuel cell system having a three-stage stack or more by providing another fuel cell stack downstream of the second fuel cell stack. Regarding the fuel cell system having the stack having the configuration, another fuel cell stack may be provided on the downstream side of the second fuel cell stack to provide a fuel cell system having a stack of four or more stages.

100, 200 Fuel cell system 1, 21 Reformed water supply pipe 2, 12, 22 Hydrocarbon fuel supply pipe 3, 13 First reformer 4, 14, 24 Reformed gas supply pipe 5, 15 First fuel Battery stack 6, 11, 16 Exhaust gas supply pipe 7, 17 Second fuel cell stack 23 Second reformer 25 Third fuel cell stack 202 Solid oxide electrolyte 204 Fuel electrode 206 Air electrode 208 Single cell 210 Inter Connector 210A Reformed gas flow path forming groove 210B Oxidant gas flow path forming groove

Claims (10)

A first fuel cell stack comprising a solid oxide electrolyte, a fuel electrode, and an air electrode;
A solid oxide electrolyte, a fuel electrode, and an air electrode are disposed downstream of the first fuel cell stack, and an exhaust gas discharged from the fuel electrode side of the first fuel cell stack is a fuel. A second fuel cell stack supplied to the pole side;
A first fuel cell is disposed upstream of the first fuel cell stack, reforms the hydrocarbon fuel to generate a reformed gas, and supplies the generated reformed gas to the fuel electrode side of the first fuel cell stack. 1 reformer,
First fuel supply means for supplying hydrocarbon fuel to the first reformer;
With
The ratio r ₁ of the fuel usage amount in the second fuel cell stack to the fuel usage amount in the _first fuel cell stack satisfies 0.35 ≦ r ₁ ≦ 1.0.   It is a ratio between the number S of steam molecules per unit time supplied to the first reformer and the number of carbon atoms C of hydrocarbon fuel per unit time supplied to the first reformer. The fuel cell system according to claim 1, wherein the steam carbon ratio S / C is 1.6 to 3.5.   The fuel cell system according to claim 1 or 2, wherein a total fuel utilization ratio of the first fuel cell stack and the second fuel cell stack is 80% to 95%. 4. The fuel cell system according to claim ₁ , wherein the ratio r ₁ satisfies 0.35 ≦ r ₁ ≦ 0.6. 5. An exhaust gas including water vapor, which includes a solid oxide electrolyte, a fuel electrode, and an air electrode, is disposed upstream of the first reformer, and is exhausted from the fuel electrode side. A third fuel cell stack for supplying to the mass device;
The ratio r ₂ of the fuel usage amount in the third fuel cell stack to the fuel usage amount in the first fuel cell stack satisfies 0.1 ≦ r ₂ ≦ 0.8. 5. The fuel cell system according to any one of 4 above. A first fuel cell is disposed upstream of the third fuel cell stack, reforms hydrocarbon fuel to generate reformed gas, and supplies the generated reformed gas to the fuel electrode side of the third fuel cell stack. Two reformers;
Second fuel supply means for supplying hydrocarbon fuel to the second reformer;
The fuel cell system according to claim 5, further comprising:   The ratio between the number S of water vapor molecules per unit time supplied to the second reformer and the number of carbon atoms C of hydrocarbon fuel per unit time supplied to the second reformer. The fuel cell system according to claim 6, wherein the steam carbon ratio S / C is 1.6 to 3.5. The fuel cell system according to claim 5, wherein the ratio r ₂ satisfies 0.35 ≦ r ₂ ≦ 0.8. The ratio r ₂ satisfies 0.4 ≦ r ₂ ≦ 0.7, and the flow rate of the hydrocarbon fuel supplied to the first reformer and the carbonization supplied to the second reformer. 8. The fuel cell system according to claim 6, wherein a ratio r _{3 to} a flow rate of the hydrogen fuel satisfies 0.6 ≦ r ₃ ≦ 0.9. The ratio r ₂ satisfies 0.5 ≦ r ₂ ≦ 0.6, and the flow rate of the hydrocarbon fuel supplied to the first reformer and the carbonization supplied to the second reformer. 8. The fuel cell system according to claim 6, wherein a ratio r _{3 to} a flow rate of hydrogen fuel satisfies 0.5 ≦ r ₃ ≦ 1.2.