JP2015191852A - Method for controlling solid oxide fuel battery system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress the expansion of an anode support owing to carbon precipitation in a solid oxide fuel battery.SOLUTION: In a method for controlling a solid oxide fuel battery system according to an embodiment of the present invention, the control is performed so that the distance from the surface of each Ni particle in an anode support 21 when an oxidation degree of Ni becomes 30 wt.% or larger after stop of a solid oxide fuel battery is 20-100 nm, provided that the oxidation degree of Ni is expressed by the following formula: Oxidation degree of Ni=(Detection weight concentration of Ni oxide)/(all Ni detection weight concentration)×100%.

Description

  The present invention relates to a control method for a solid oxide fuel cell system.

  Solid oxide fuel cells (SOFC) generally have higher power conversion efficiency than solid polymer fuel cells (PEFC). Further, not only hydrogen but also reformed gas obtained by reforming raw fuel such as hydrocarbon, alcohol and dimethyl ether (DME) can be used as the fuel. Furthermore, carbon monoxide, unreformed methane, etc. produced as a by-product during reforming of the raw fuel can also be used as a fuel for power generation. Generally, in one thermal closed system, heat for reforming is supplied by using the amount of heat generated in SOFC or heat obtained by burning surplus gas not used for power generation in the upper part of the cell. The ability to balance heat contributes to the high efficiency of SOFC.

  The operating temperature of SOFC has often been 700 ° C or higher in the past, but it is possible to operate at lower temperatures by using low-temperature (below 700 ° C) electrolyte materials and air electrode materials that have been developed recently. It is becoming. Thereby, the practicality of SOFC can be improved, for example, an inexpensive material with low heat resistance can be adopted. However, on the other hand, if the operating temperature of the cell decreases, the temperature of the reformer that is thermally balanced with the cell also decreases. As a result, fuel gas having a high methane ratio is supplied to the cell due to thermal equilibrium in the reforming reaction. In addition, when using raw fuel containing hydrocarbons having 2 or more carbon atoms (C2 or more hydrocarbons) such as natural gas or LPG with adjusted calorie, C2 or more hydrocarbons are converted to C1 species (carbon number 1) in the reformer. There is a growing concern about the occurrence of slippage of so-called C2 or higher hydrocarbons that are supplied to the cell without being converted into hydrocarbons.

  The problem of the supply of gas with high methane concentration or the occurrence of slippage of C2 or higher hydrocarbons is that the reformer is sufficient at startup and shutdown even for SOFCs operating at normal operating temperatures (700 ° C or higher). It may be caused by some trouble in the system, such as when the heater is not heated, or the reforming water pump stops or the flow rate decreases even during power generation, or the reformer temperature decreases due to the loss of thermal balance between the cell stack and the reformer There is.

  By the way, in an anode (fuel electrode) -supported SOFC, a cermet containing nickel (Ni) having high conductivity is preferably used as an anode support. When the reformed gas supplied from the reformer contains high-concentration methane and C2 or higher hydrocarbons, most of the methane and C2 or higher hydrocarbons are combined with coexisting steam by the catalytic action of Ni contained in the anode support. As the reforming reaction proceeds, the hydrogen-rich gas is used as a fuel for power generation. However, some methane and C2 or higher hydrocarbons may precipitate as carbon on the anode support.

In particular, carbon deposition on the anode support tends to occur under conditions of low S / C (steam / carbon ratio) where the water vapor partial pressure is low and the hydrocarbon partial pressure is high. Examples of the carbon deposition reaction include a direct thermal decomposition reaction of methane (the following formula (1)), a steam reforming reaction (the following formula (2)), or an aqueous transfer reaction (a water gas shift reaction) (the following formula (3)). The disproportionation reaction of CO produced by the above (formula (4) below) is known.

  The carbon deposited on the anode support causes the anode support to expand. As a result, there is a risk of damaging the electrode and electrolyte layer provided on the surface of the anode support. Also, the expansion of the anode support due to carbon deposition and the resulting cracking of the electrolyte layer are irreversible phenomena. For this reason, a case in which the occurrence of cracks in the electrolyte layer due to the expansion of the anode support body has become seriously unusable as a cell (see Non-Patent Documents 1 and 2).

J.H.Koh, Y.S.Yoo, J.W.Park, H.C.Lim, Solid State Ionics, 149 (2002) 157 -166 C. M. Finnery, N.J. Coe, R.H.Cunninghum, R.M.Ormerod, Catal.Today, 46 (1998) 137 -145

  As a means to prevent the problems caused by carbon deposition as described above, the mechanism and threshold are grasped for each phenomenon such as carbon deposition, anode support expansion, and associated electrolyte layer breakage (crack generation). It is extremely important to do so, and a control method of the SOFC system based on these is required.

  However, no effective knowledge has been presented so far on the carbon deposition suppression technology based on the carbon deposition mechanism in the anode support for a solid oxide fuel cell, and there is still room for study.

  The present invention has been made in view of such circumstances, and an object thereof is to provide a technique for suppressing expansion of an anode support due to carbon deposition in a solid oxide fuel cell system.

One aspect of the present invention is a solid oxide fuel cell having an anode support including particulate Ni metal, Ni oxide that covers at least a part of the surface of the Ni metal, and a porous conductive portion. The system control method, wherein after the solid oxide fuel cell is stopped, the distance at which the Ni oxidation degree on the surface of the Ni particles in the anode support represented by the following formula becomes 30 wt% or more is 20 to 100 nm. Ni oxidation treatment for forming a state is performed.
Ni oxidation degree = (detected concentration of oxidized Ni) / (detected concentration of all Ni) × 100%

  In the control method of the above aspect, the anode support may include nickel cermet. Further, when the solid oxide fuel cell is cooled to a predetermined temperature T1 (200 ° C. <T1 <500 ° C.) when stopped, the supply of fuel gas to the fuel electrode is stopped, and the fuel electrode and / or the air electrode are stopped. The Ni oxidation degree may be controlled by performing air flow to. Further, when the solid oxide fuel cell is cooled to a predetermined temperature T2 (200 ° C. <T2 <T1 <500 ° C.), the air oxidation is stopped to control the Ni oxidation degree. Good.

  According to the present invention, in the solid oxide fuel cell system, the expansion of the anode support due to carbon deposition can be suppressed.

It is a perspective view which shows schematic structure of the principal part of the fuel cell system which concerns on embodiment. It is the schematic which shows typically the structure of the principal part of the fuel cell system which concerns on embodiment. 2 is a horizontal sectional view schematically showing the structure of a fuel cell stack. FIG. It is a schematic diagram which expands and shows a part of anode support. It is a graph which shows the relationship between the heat processing temperature when holding time is 30 minutes, and the oxidation depth when Ni oxidation degree is 30 weight%. It is a graph which shows the linear expansion coefficient 2 hours after hydrocarbon exposure with respect to the oxidation depth of 30 weight% or more of Ni oxidation degree. It is a graph which shows the linear expansion coefficient after 2 hours of hydrocarbon exposure with respect to the Ni oxidation degree in the depth of 30 nm. It is a SEM image of an anode support body when oxidation depth exceeds an upper limit.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In all the drawings, the same reference numerals are given to the same components, and the description will be omitted as appropriate.

  FIG. 1 is a perspective view showing a schematic structure of a main part of a fuel cell system according to an embodiment. FIG. 2 is a schematic diagram schematically showing the structure of the main part of the fuel cell system according to the embodiment. FIG. 1 shows a state where the inside of the module case 2 is seen through. Since FIG. 2 is a schematic diagram, the arrangement, the number of installations, and the like of each part do not necessarily match those in FIG. The fuel cell system 100 includes a hot module 1. The hot module 1 includes a module case 2, a fuel cell stack 10 accommodated in the module case 2, a water vaporizer 50, and a reformer 6.

  The module case 2 is composed of, for example, a substantially rectangular parallelepiped outer frame formed of a heat-resistant metal and a heat insulating material lined on the inner surface of the outer frame. The module case 2 is provided with a supply pipe 3, a supply pipe 4 and a supply pipe 52. The supply pipe 3 is a pipe that supplies raw fuel into the case from the outside of the case. The supply pipe 4 is a pipe for supplying cathode air (oxygen-containing gas) from the outside of the case into the case. The supply pipe 52 is a pipe for supplying reforming water used for the steam reforming reaction from the outside of the case into the case. The module case 2 is provided with an exhaust port 5. As raw fuel, city gas, LPG, methanol, dimethyl ether (DME), kerosene and the like are used. The fuel supply pipe 3, the water supply pipe 52, and the cathode air supply pipe 4 correspond to the fuel, water, and cathode air supply amounts, respectively, so that they can be adjusted. A supply amount control means (pump and / or control valve) (not shown) is provided, and these are appropriately controlled by a control signal from the control unit 200 provided in the fuel cell system 100.

  The water vaporizer 50 and the reformer 6 are arranged in the upper part of the module case 2 and above the fuel cell stack 10. A supply pipe 52 is connected to the water vaporizer 50, and a supply pipe 3 is connected to the reformer 6. The water vaporizer 50 vaporizes the reforming water supplied from the supply pipe 52. Vaporized reforming water (steam) is supplied to the reformer 6. The reformer 6 includes a case formed of a heat-resistant metal and a reforming catalyst that is accommodated in the case and used for reforming raw fuel. The reformer 6 reforms the raw fuel supplied from the supply pipe 3 into a hydrogen-rich fuel gas (reformed gas) by a steam reforming reaction using the steam supplied from the water vaporizer 50.

  One end of a reformed gas supply pipe 7 is connected to the reformed gas outlet 6 a of the reformer 6. The other end of the reformed gas supply pipe 7 is connected to the manifold 8. The manifold 8 is a fuel supply path that is supplied to an anode catalyst layer 23 described later. The manifold 8 is disposed below the fuel cell stack 10 and distributes the reformed gas supplied from the reformer 6 to the plurality of fuel cells 20 included in the fuel cell stack 10. The fuel cell stack 10 is disposed in the lower part of the module case 2, below the reformer 6 and the water vaporizer 50, and fixed on the manifold 8.

  The manifold 8 includes a box-shaped main body having an opening and an upper lid that closes the opening of the box-shaped main body. The upper lid is fixed to the box-shaped main body by, for example, an inorganic glass composition. The upper lid functions as a holding member for the fuel cell stack 10. The upper lid is provided with a plurality of holes for fixing the anode support 21 of each fuel cell 20, and the lower end portion of each anode support 21 is inserted into each opening, and a fixing material such as an adhesive is provided. It is fixed by. Examples of the adhesive used as the fixing material include silica-alumina based inorganic adhesives. In this case, the anode support 21 is fixed to the upper lid by solidifying the inorganic adhesive by firing in a state where the lower end portion of each anode support 21 is held in each opening of the upper lid via the inorganic adhesive. can do. Further, an adhesive layer for fixing the anode support 21 and the upper lid is covered with a sealing material. Thereby, the junction part of the anode support body 21 and an upper cover is sealed gas tight (airtight).

  The fuel cell stack 10 is an assembly of a plurality of fuel cells 20 (cells). When, for example, a cylindrical flat plate type fuel cell is used as the fuel cell 20, a plurality of (e.g., five are shown for simplicity) vertically long fuel cells 20 are arranged in a row in the horizontal direction. A current collecting member 30 is interposed between the side surfaces of the adjacent fuel cells 20. A plurality of fuel cells 20 in a row are arranged on a plane, so that a large number of fuel cells 20 are arranged in a matrix (see FIG. 3). The outermost current collecting member 30 in each row is electrically connected to the conductive member 40.

  Each of the fuel cells 20 is provided with a plurality of gas flow paths 22 extending from the lower end to the upper end of the fuel cell 20. Each gas channel 22 communicates with the manifold 8 at its lower end. The upper end portion of the gas flow path 22 is opened to a space sandwiched between the reformer 6 and the water vaporizer 50 and the fuel cell stack 10. The space constitutes a combustion portion for fuel gas discharged from the upper end portion of the gas flow path 22. Most of the fuel gas flowing from the manifold 8 into the gas flow path 22 is supplied to the fuel cell 20. Excess fuel gas that has not been used for electrode reaction (power generation) is supplied from the upper end of the gas flow path 22 to the combustion section.

  Next, the structure of the fuel cell stack 10 will be described in more detail. FIG. 3 is a horizontal sectional view schematically showing the structure of the fuel cell stack. As shown in FIG. 3, in the present embodiment, a cylindrical flat plate fuel cell 20 is illustrated. The fuel cell 20 is an anode-supported solid oxide fuel cell, and includes an anode support 21, a gas flow path 22, an anode catalyst layer 23 (fuel electrode layer), and an electrolyte layer 24 (solid oxide electrolyte layer). A cathode catalyst layer 25 (air electrode layer), and an interconnector 26.

  The anode support 21 is a porous body containing at least one of Ni metal and Ni oxide. The anode support 21 is a plate-like piece having a flat oval horizontal cross-sectional shape and extending in the vertical direction (vertical direction) (see FIG. 2). The anode support 21 has a lower surface located on the manifold 8 side, an upper surface located on the reformer 6 side, two flat side surfaces (flat side surfaces) facing each other, and two semi-cylindrical surface shapes facing each other. And side surfaces (two curved side surfaces arranged in a direction orthogonal to the direction in which the two flat side surfaces are arranged).

  The anode support 21 has a plurality of gas flow paths 22 at the center thereof. The gas flow path 22 is provided along the longitudinal direction of the anode support 21. One end (lower end) of the gas passage 22 is located on the lower surface of the anode support 21, and the other end (upper end) of the gas passage 22 is located on the upper surface of the anode support 21. In the present embodiment, four gas flow paths 22 are provided in each anode support 21. The anode support 21 is fixed to the manifold 8, and one end of the gas flow path 22 is communicated with the manifold 8. The reformed gas generated in the reformer 6 is distributed to the gas flow path 22 of each fuel cell 20 by the manifold 8 and flows from one end of the gas flow path 22 to the other end side.

  The interconnector 26 is provided on one flat side surface of the anode support 21 (on the left flat side surface in the fuel cell stack 10-1 in the first row in FIG. 3). The interconnector 26 is a conductive member for collecting current from the anode of each fuel cell 20. The interconnector 26 can be made of, for example, a conductive ceramic.

  The anode catalyst layer 23 is provided on the surface of the anode support 21. In the present embodiment, the anode catalyst layer 23 is laminated at least on the other flat side surface of the anode support 21 (on the right flat side surface in the fuel cell stack 10-1 in the first row in FIG. 3).

  The electrolyte layer 24 is provided on the surface of the anode catalyst layer 23 opposite to the anode support 21. In the present embodiment, the electrolyte layer 24 covers the entire surface of the anode catalyst layer 23. The cathode catalyst layer 25 is provided on the surface of the electrolyte layer 24 opposite to the anode catalyst layer 23. In the present embodiment, the cathode catalyst layer 25 is laminated on the main surface of the electrolyte layer 24 and is disposed on the opposite side of the interconnector 26 with the anode support 21 interposed therebetween. That is, in the fuel cell 20, the anode catalyst layer 23, the electrolyte layer 24, and the cathode catalyst layer 25 are laminated in this order on one surface of the anode support 21 having the gas flow path 22, and the other surface of the anode support 21. And an interconnector 26 is formed. The constituent materials of the anode support 21, the anode catalyst layer 23, the electrolyte layer 24, and the cathode catalyst layer 25 will be described in detail later.

  The plurality of fuel cells 20 are arranged in a row so that one cathode catalyst layer 25 and the other interconnector 26 in two adjacent fuel cells 20 face each other, and are joined to each other via a current collecting member 30. The That is, as shown in FIG. 3, the interconnector 26 located on the left side of each fuel cell 20 is joined to the cathode catalyst layer 25 located on the right side of the adjacent fuel cell 20 via the current collecting member 30. As a result, a plurality of fuel cells 20 arranged in a row are connected in series to form the fuel cell stack 10. In the present embodiment, the fuel cell stack 10 includes a first row of fuel cell stacks 10-1 and a second row of fuel cell stacks 10-2. The current collecting member 30 can be composed of a member having a predetermined shape formed from a metal or alloy having elasticity, or a member obtained by performing a predetermined surface treatment on a felt made of metal fiber or alloy fiber.

  In the hot module 1 having the above-described configuration, raw fuel for hydrogen production is supplied from the supply pipe 3 to the reformer 6, and reforming water is supplied from the supply pipe 52 to the water vaporizer 50. The reformer 6 reforms the raw fuel by a steam reforming reaction to generate a hydrogen-rich reformed gas used for power generation of the fuel cell 20. The generated reformed gas is supplied to the manifold 8 via the reformed gas supply pipe 7. The reformed gas supplied to the manifold 8 is distributed to each fuel cell 20 and rises in the gas flow path 22 of each fuel cell 20. In this process, hydrogen in the reformed gas permeates through the anode support 21 and reaches the anode catalyst layer 23. On the other hand, cathode air is introduced into the module case 2 from the supply pipe 4. The cathode air introduced into the module case 2 is supplied to each fuel cell 20 through an air supply passage (not shown) disposed in the module case 2, and oxygen in the cathode air reaches the cathode catalyst layer 25.

In the cathode catalyst layer 25 located on the outer peripheral side of each fuel cell 20, an electrode reaction represented by the following formula (5) occurs. O ²⁻ produced in the cathode catalyst layer 25 permeates the electrolyte layer 24 and reaches the anode catalyst layer 23. Further, an electrode reaction represented by the following formula (6) occurs in the anode catalyst layer 23 located on the center side of the fuel cell 20. Thereby, power generation is performed in the fuel cell 20.
1 / 2O ₂ + 2e ⁻ → O ²⁻ (5)
O ²⁻ + H ₂ → H ₂ O + 2e ⁻ (6)

  Of the reformed gas flowing through the gas flow path 22, surplus reformed gas that has not been used for the electrode reaction is released into the module case 2 from the upper end of the anode support 21. This reformed gas is burned in the combustion section located between the reformer 6 and the water vaporizer 50 and the fuel cell stack 10. Predetermined ignition means (not shown) is provided in the module case 2, and when the reformed gas starts to be released to the combustion section, the ignition means is activated and combustion of the reformed gas is started. Of the cathode air introduced into the module case 2, surplus air that has not been used for the electrode reaction is used for combustion of the reformed gas. The inside of the module case 2 becomes a high temperature of about 600 ° C. to 1000 ° C., for example, due to heat generated by the power generation of each fuel cell 20 and heat generated by combustion of the surplus reformed gas. The water vaporizer 50 and the reformer 6 perform the vaporization and reforming reaction of the reforming water using the heat generated from the fuel cell 20 and the heat generated by the combustion of the reformed gas. The exhaust gas generated by the combustion of the reformed gas in the module case 2 is discharged out of the module case 2 through the exhaust port 5.

  Next, the composition of each part constituting the fuel cell 20 will be described in detail. FIG. 4 is an enlarged schematic view showing a part of the anode support. The anode support 21 is required to have gas permeability in order to permeate fuel from the gas flow path 22 to the anode catalyst layer 23. Further, the anode support 21 is required to have conductivity in order to transmit electrons generated in the anode catalyst layer 23 to the interconnector 26.

  Therefore, the anode support 21 according to the present embodiment has a base 210 that is porous. By making the base 210 porous, the reformed gas can be transmitted from the gas flow path 22 to the anode catalyst layer 23. Further, the base 210 includes at least one of Ni metal and Ni oxide. Ni contained in the base 210 takes the state of Ni oxide during the manufacture of the anode support 21 and the fuel cell 20, and becomes substantially Ni metal after a predetermined reduction process. Further, before driving the fuel cell system 100, a mixed state of oxidized Ni / Ni metal in which a part of the particle surface is oxidized is taken, and reducing reformed gas is circulated while the fuel cell system 100 is driven. In the meantime, most of the metal is reduced and can be substantially in the state of Ni metal. In FIG. 4, Ni metal and Ni oxide are not distinguished from each other and are illustrated as a nickel portion 212. Since the base 210 has the nickel portion 212, conductivity can be imparted to the anode support 21.

The base 210 includes at least one of Ni metal and Ni oxide (that is, the nickel portion 212), and at least one oxide 214 selected from the group consisting of Y ₂ O ₃ , a zirconia-based composite oxide, and a ceria-based composite oxide. It is preferable that the composite is That is, the base 210 is preferably a nickel cermet composed of Ni metal and / or Ni oxide and a porous conductive ceramic. Examples of the zirconia-based composite oxide include YSZ, ScSZ (Scandia Stabilized Zirconia) and the like. Examples of the ceria-based composite oxide include SDC (Samaria-Doped Ceria), YDC (Yttria-Doped Ceria), LDC (La ₂ O ₃ -doped Ceria), GDC (Gadolinia-doped Ceria), and the like.

  The anode support 21 preferably has an open porosity of 20% or more, more preferably 25% to 50%. The anode support 21 preferably has a conductivity of 100 S / cm or higher, more preferably 200 S / cm or higher. In order to satisfy these requirements, when the anode support 21 is manufactured, the base 210 is preferably 40% by mass or more, more preferably 40% by mass or more, in terms of Ni oxide, with respect to the total mass of the anode support 21. It contains 80 mass%, more preferably 40 mass% to 75 mass% Ni atoms. By setting the content of Ni atoms in the base 210 to 40 mass% or more in terms of Ni oxide, desired conductivity can be more reliably imparted to the anode support 21. Moreover, a desired open porosity can be more reliably imparted to the anode support 21 by setting the content of Ni atoms in the base 210 to 80% by mass or less in terms of Ni oxide. As for the dimension of the anode support 21, the height (length from the upper end to the lower end) is, for example, 7 cm to 20 cm.

  As a manufacturing method of the base 210, a composite composition containing nickel oxide and oxide 214 is processed into a support shape by means of extrusion molding or the like, and subjected to appropriate firing treatment to obtain predetermined dimensions and porosity. A preferred example is a method of forming a cermet having the same. The cermet is subjected to a reduction treatment before it is used as a fuel cell after all cell constituent layers are finally formed. This reduction treatment is usually performed by heating the fuel cell 20 or the fuel cell stack 10 to a predetermined temperature in a reducing gas stream such as hydrogen gas or hydrogen gas diluted with nitrogen. By this reduction treatment, almost all of the nickel oxide contained in the base portion 210 is reduced to Ni metal.

  Conventionally, the operating temperature of SOFC is often a high temperature of 700 ° C. or higher. On the other hand, in recent years, improvements in electrolyte materials and air electrode materials have progressed, and SOFCs capable of operating at a low temperature of about 600 ° C. or lower are being used. However, when the operating temperature of the fuel cell 20 decreases, the temperature of the reformer 6 also decreases in the hot module 1 that performs the reforming reaction in the reformer 6 using the heat generated by the fuel cell 20. In this case, a reformed gas having a high methane ratio may be generated due to thermal equilibrium.

  Further, when a raw fuel containing C2 or higher hydrocarbons is used, the reformer 6 may cause the C2 or higher hydrocarbons to be supplied to the fuel cell 20 without being converted into C1 chemical species (C2 or higher hydrocarbon slips). Occurrence). Further, the fuel cell system 100 may be stopped due to various reasons such as a user instruction, a purpose of improving the energy saving effect, and a trouble of the device. In the system stop process and the subsequent restart process, the reformer 6 may not be sufficiently heated even with a conventional SOFC operating at a high temperature. Also in this case, the above-described methane concentration increase or a situation where the reformed gas mixed with C2 or more hydrocarbons is supplied to the fuel cell 20 may occur.

  In particular, in the system stop process, after the power generation of the fuel cell 20 is stopped, the fuel cell 20 may be gradually cooled while the reformed gas is continuously supplied to the fuel cell 20. The reformed gas is supplied for the purpose of preventing air from flowing into the anode support 21 and the anode catalyst layer 23 and re-oxidation of the anode support 21. In this case, the reformed gas is supplied to the fuel cell 20 for several hours even after the power generation of the fuel cell 20 is stopped. On the other hand, as the temperature of the fuel cell 20 decreases, the temperature of the reformer 6 also decreases. For this reason, there is a high possibility that high-concentration methane or unreformed C2 or higher hydrocarbon compound is supplied from the reformer 6 to the fuel cell 20.

  When high-concentration methane or C2 or higher hydrocarbon compounds are supplied to the fuel cell 20, the fuel cell system having the conventional anode support may cause various forms of carbon deposition in various places. Such carbon deposition is likely to occur, for example, in the manifold, the upstream portion of the anode support in the fuel gas flow direction, in the pores of the anode support, or in places where the potential changes due to the external current collecting structure in the hot module. .

  For example, carbon deposition includes a large amount of hydrocarbons containing C2 or more and low S / C reformed gas is supplied to the fuel cell, or high concentration of methane is included and low S / C reformed gas is supplied. Occurs when it is done. When carbon is deposited in the pores of the anode support 21 or on the Ni particles, this causes dimensional expansion of the anode support. When the anode support is expanded, there is a possibility that stress deformation and cracks may occur. In addition, stress deformation occurs in the sealing material covering the joint between the anode support and the manifold, which may lead to cracking or peeling. In particular, when the raw fuel contains a hydrocarbon compound having 2 or more carbon atoms in an amount of 5 mol% or more based on the total molar amount of the raw fuel, the above-described carbon deposition tends to occur.

  The anode catalyst layer 23 is a composite composed of at least one of Ni metal and Ni oxide and at least one composite oxide selected from the group consisting of zirconia composite oxide, ceria composite oxide, and LSGM. Preferably there is. Specifically, the anode catalyst layer 23 is made of Ni / YSZ, Ni / ScSZ, Ni / SDC, Ni / YDC, Ni / LDC, Ni / GDC, Ni / LSGM (La—Sr—Ga—Mg composite oxide). Preferably, at least one cermet selected from the group consisting of: The layer thickness of the anode catalyst layer 23 is, for example, 0.1 μm to 50 μm.

  The electrolyte layer 24 functions as an electrolyte that bridges electron conduction and ion conduction between the electrodes, and also functions as a gas barrier layer for preventing leakage of fuel gas and air. The electrolyte layer 24 preferably includes at least one composite oxide selected from the group consisting of a zirconia composite oxide, a ceria composite oxide, and LSGM. Specifically, a solid electrolyte selected from the group consisting of YSZ, ScSZ, SDC, YDC, LDC, GDC, and LSGM is preferably used for the electrolyte layer 24. The layer thickness of the electrolyte layer 24 is, for example, 0.2 μm to 200 μm.

The cathode catalyst layer 25 can be formed using a conductive ceramic composed of a so-called ABO ₃ type perovskite oxide. The cathode catalyst layer 25 is required to have gas permeability. Therefore, the open porosity of the cathode catalyst layer 25 is preferably 20% or more, and more preferably 30% to 50%. Therefore, the cathode catalyst layer 25 preferably contains a composite oxide such as LSM (La Sr Mn), LSC (La Sr Co), or LSCF (La Sr Co Fe). The layer thickness of the cathode catalyst layer 25 is, for example, 2 μm to 200 μm. However, this does not apply when the cathode catalyst layer 25 has a function of electrically connecting to the current collector 30 or when the cathode catalyst layer 25 itself has a function of collecting current. Depending on the materials of the anode catalyst layer 23, the electrolyte layer 24 and / or the cathode catalyst layer 25 described above, the operation of the fuel cell 20 at a temperature of about 600 ° C. or lower can be realized.

  The anode catalyst layer 23, the electrolyte layer 24, and the cathode catalyst layer 25 are laminated on the anode support 21 in this order. Further, the interconnector 26 is sintered to the anode support 21. Thereby, the fuel cell 20 is formed. In the process of forming the anode support 21 described above, by sintering the composite composition to be the anode support 21 and simultaneously sintering the anode catalyst layer 23 and the electrolyte layer 24 to the composite composition, residual stress can be reduced. A structure including the anode support 21, the anode catalyst layer 23, and the electrolyte layer 24 can be formed in a small amount.

(Knowledge about cell expansion due to hydrocarbon exposure)
With respect to the nickel cermet used for the anode support 21, the present inventors have found that the carbon accumulation behavior on the anode support 21 under the condition where the low S / C hydrocarbon-containing gas is exposed, the structure change of the support and the accompanying change. As a result of earnestly examining the macroscopic dimensional expansion behavior of the support itself, as to the mechanism that causes carbon deposition, the state / physical property change of the anode support 21 when carbon deposition occurs, and the critical conditions that cause dimensional expansion, the following Obtained knowledge.

(1) Low S / C, specifically, a mixed gas of hydrocarbons and steam with S / C <1 in which hydrocarbon reforming of hydrocarbons occurs without complete steam reforming in thermodynamic equilibrium with nickel cermet When exposed to a containing anode support at elevated temperatures, carbon deposits on the nickel particles in the anode support.

(2) Carbon deposition proceeds sequentially from the surface of the anode support on the side in contact with the fuel gas, and gradually diffuses into the inside through the pores of the nickel cermet, so that the carbon deposition site gradually expands inside. To do.

(3) When carbon deposition proceeds, at least the surface exposed to the mixed gas expands two-dimensionally, and if there are a plurality of exposed surfaces depending on the shape of the anode support, the entire anode support expands three-dimensionally. Then, dimensional expansion is observed by a displacement meter (Dilatometer). At this time, the depth from the surface of the Ni particles of the anode support (hereinafter sometimes referred to as oxidation depth) in which the Ni oxidation degree defined by the following formula (1) is 30% by weight or more is within a predetermined range. In this case, the dimensional expansion of the anode support is suppressed.
Ni oxidation degree = (detected weight concentration of oxidized Ni) / (detected weight concentration of all Ni) × 100% (1)

  The analysis results related to the findings (3) are described below.

<Preparation of anode support>
NiO (trade name NiO-FP, manufactured by Sumitomo Metal Mining Co., Ltd.) and YSZ (trade name TZ8YS, manufactured by Tosoh Corporation) are wet-mixed at a volume ratio of 40:60, and crosslinked polymethyl methacrylate fine particles (as a pore-forming material ( Sekisui Chemical Co., Ltd.) was mixed at a mass ratio of 30% to obtain a support material paste. After processing this paste into a support by extrusion molding, the mixture was dried at room temperature for 12 hours, then pre-fired in air at 800 ° C. for 1 hour, and then heated to 1500 ° C. at 10 ° C./min. Firing was carried out at that temperature for 8 hours to produce a support molded product.

  For measurement with a thermal dilatometer, a pellet of approximately 2 × 2 × 10 mm rectangular parallelepiped shape was cut out from this support molded product and subjected to reduction treatment at 800 ° C. for 5 hours in hydrogen to provide a support made of Ni / YSZ. A body sample (unoxidized cell) was obtained. The diameter of the Ni particles in the produced Ni / YSZ was 1 to 2 μm.

<Evaluation of oxidation state of cell>
A plurality of the support samples were prepared, and the heat treatment time was appropriately changed to obtain a sample in which the oxidation depth with a Ni oxidation degree of 30% by weight or more was changed. Using X-ray photoelectron spectroscopy (XPS), the oxidation state of each sample was evaluated. Specifically, the surface of the anode support was fixed to a holder so as to be measured, and after sufficiently reducing the pressure, the sample was irradiated with X-rays, and the generated photoelectrons were measured with a detector. When evaluating the depth direction from the support sample surface, sputtering was performed with Ar ions, and the sputtering depth in terms of SiO ₂ was calculated from the irradiation time of sputtering. As for the oxidation state of Ni, waveform analysis was performed on the Ni2p spectrum (850 to 860 eV) of XPS, and the ratios of Ni, Ni ²⁺ , and Ni ³⁺ were obtained from the respective peak areas, respectively, and total Ni (Ni, Ni ²⁺ , The ratio of Ni ^{2+ to} the sum of Ni ³⁺ was defined as the Ni oxidation degree. FIG. 5 is a graph showing the relationship between the heat treatment temperature when the holding time is 30 minutes and the oxidation depth when the Ni oxidation degree is 30 wt% or more. As shown in FIG. 5, by increasing the heat treatment temperature, it is possible to increase the oxidation depth of the Ni oxidation degree of 30% by weight or more, and there is a predetermined correspondence between the two. Using this correspondence, the oxidation depth of the anode support can be set to a desired value.

<Measurement of deformation caused by exposure to hydrocarbon fuel>
Deformation evaluation of the support sample due to exposure to hydrocarbon-based fuel was performed using an expansion measurement device (Dilameter, manufactured by Netzsch). First, each support was set in an expansion measuring device so as to measure a change in length in the long side direction of the support sample. The atmosphere of each support sample was a nitrogen atmosphere, the temperature was raised at 20 ° C./min, and tests were performed in two types of holding states of 250 ° C. and 300 ° C. After confirming that the length change due to thermal expansion of each support sample converged and its shape stabilized, supply of hydrocarbon fuel to each support sample was started. For hydrocarbon exposure, propane having a concentration of 2% diluted with nitrogen as a hydrocarbon fuel was supplied at a flow rate of 200 cc / min for 2 hours. Then, for each support sample, the change in the length of the hydrocarbon exposure measured by the expansion measurement device to calculate the linear expansion rate _{(ΔL / L 0) (ppm} ). ΔL is the amount of change in the length of the support sample after 2 hours due to hydrocarbon exposure, and L ₀ is the length of the support sample before the hydrocarbon exposure treatment. After completion of the hydrocarbon exposure for a predetermined time, dry nitrogen was supplied to the support sample and purged and allowed to cool.

  FIG. 6 is a graph showing the linear expansion coefficient after 2 hours of hydrocarbon exposure with respect to an oxidation depth of 30% by weight or more of Ni oxidation degree. As shown in FIG. 6, it was confirmed that the expansion of the anode support due to exposure to hydrocarbons can be suppressed when the oxidation depth with a Ni oxidation degree of 30% by weight or more is 20 nm or more.

  FIG. 7 is a graph showing the linear expansion coefficient after 2 hours of hydrocarbon exposure with respect to the Ni oxidation degree at a depth of 30 nm. As shown in FIG. 7, it was confirmed that the expansion of the anode support due to hydrocarbon exposure can be suppressed when the Ni oxidation degree is in the range of 30% by weight or more.

  6 and 7, it was confirmed that the expansion suppression effect of the anode support becomes more prominent when exposed to hydrocarbons at 300 ° C.

(4) Since the above-described dimensional expansion due to exposure to hydrocarbons selectively occurs at the support Ni cermet site and does not occur at the adjacent electrolyte layer or the like, stress (extension stress with respect to the electrolyte) occurs in the support / electrolyte. When this stress exceeds a certain critical point, the electrolyte layer breaks (crack).

(Control method of solid oxide fuel cell)
A control method of the solid oxide fuel cell according to the embodiment devised based on the above knowledge will be described below.

  The control unit 200 controls the oxidation depth from the surface at which the Ni oxidation degree of the Ni particles in the anode support becomes 30% by weight or more to 20 to 100 nm after the solid oxide fuel cell is stopped. By setting the oxidation depth at which the Ni oxidation degree from the surface of the Ni particles in the anode support is 30% by weight or more to 20 nm or more, as described in the above knowledge, the expansion of the anode support due to hydrocarbon exposure is suppressed. An effect can be obtained. On the other hand, when the oxidation depth at which the Ni oxidation degree is 30% by weight or more exceeds 100 nm, the oxidation degree of the anode support 21 is increased, and redox is repeated between this state and the reduced state during operation. This is not preferred because there is a risk of problems such as expansion / contraction and loss of conductivity. FIG. 8 shows an SEM image of the anode support when the oxidation depth at which the Ni oxidation degree is 30% by weight or more is 150 nm. As shown in FIG. 8, when the oxidation depth exceeded the upper limit, cracks occurred in the anode support. Such cracks are not observed at all when the oxidation depth at which the Ni oxidation degree is 30% by weight or more is 100 nm or less and begin to be observed at 105 nm or more. It is necessary to control the following. Furthermore, in consideration of the expansion suppressing effect and the occurrence of cracks, it is desirable to control to 25 to 75 nm, and most preferably 30 to 50 nm.

  As a method of controlling the oxidation depth at which the Ni oxidation degree is 30% by weight or more to be 20 to 100 nm, the solid oxide fuel cell is stopped and a predetermined temperature T1 (200 ° C. <T1 <500 ° C.) As shown in FIG. 5, the supply of fuel gas to the anode (fuel electrode) is stopped and the air flow to the anode and / or cathode (air electrode) is performed. The method of adjusting the temperature conditions required for the above is mentioned. When the temperature condition necessary for Ni oxidation is satisfied, for example, when the solid oxide fuel cell is cooled to a predetermined temperature T2 (200 ° C. <T2 <T1 <500 ° C.), the air flow is stopped. May be. The temperature of the solid oxide fuel cell may be detected by a temperature sensor (not shown) installed in the vicinity of the solid oxide fuel cell and input to the control unit 200 as needed. The supply of fuel gas to the anode, and the air flow to the anode and / or cathode, for example, supply the cathode air in a state where the supply of fuel gas by the fuel / water supply pipe 3 shown in FIG. 4 to achieve. At this time, since the anode side has a negative pressure, cathode air supplied to the cathode flows in.

  In order to obtain the temperature condition necessary for Ni oxidation, a heating means such as a heater is provided in the vicinity of the hot module or the solid oxide fuel cell, and is heated for a certain period of time so that the vicinity of the anode support becomes a desired temperature. May be warm. By using these heating means, it is possible not only to easily control the Ni oxidation degree, but also to perform the oxidation treatment before starting the solid oxide fuel cell.

  When the solid oxide fuel cell system is restarted, the effect of suppressing the expansion of the anode support due to hydrocarbon exposure can be obtained when the temperature of the fuel gas (hydrocarbon) supplied to the cell is less than 350 ° C. (See FIGS. 6 and 7).

  According to the solid oxide fuel cell system described above, the knowledge about the carbon deposition mechanism can be utilized to suppress the expansion of the anode support due to the carbon deposition, thereby effectively suppressing the cell destruction.

  The present invention is not limited to the above-described embodiments, and various modifications such as design changes can be added based on the knowledge of those skilled in the art. Embodiments to which such modifications are added Can also be included in the scope of the present invention.

2 module case, 100 fuel cell system, 1 hot module, 50 water vaporizer, 6 reformer, 3 supply pipe, 4 supply pipe, 50 water vaporizer, 52 supply pipe

Claims (4)

Ni oxide in particulate form and Ni oxide covering at least a part of the surface of the Ni metal;
A porous conductive portion;
A method for controlling a solid oxide fuel cell having an anode support comprising:
After stopping the solid oxide fuel cell,
A solid oxide that performs a Ni oxidation treatment to form a state in which the distance at which the Ni oxidation degree on the surface of the Ni particles in the anode support represented by the following formula is 30 wt% or more is 20 to 100 nm Control method for a fuel cell system.
Ni oxidation degree = (detected concentration of oxidized Ni) / (detected concentration of all Ni) × 100%   The method for controlling a solid oxide fuel cell system according to claim 1, wherein the anode support includes nickel cermet. When the solid oxide fuel cell is cooled to a predetermined temperature T1 (200 ° C. <T1 <500 ° C.) when stopped,
Stopping supply of fuel gas to the anode,
By providing air flow to the fuel electrode and / or air electrode,
The method for controlling a solid oxide fuel cell system according to claim 1 or 2, wherein the Ni oxidation degree is controlled. When the solid oxide fuel cell is cooled to a predetermined temperature T2 (200 ° C. <T2 <T1 <500 ° C.),
The method for controlling a solid oxide fuel cell system according to claim 3, wherein the Ni oxidation degree is controlled by stopping the air flow.

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