JP2014188479A - Oxygen permeable membrane, oxygen separation method, and fuel cell system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an oxygen permeable membrane capable of highly holding oxygen permeation performance for a long time even if being in contact with a CO-containing gas and to provide a fuel cell system using the oxygen permeable membrane.SOLUTION: In an oxygen permeable membrane consisting of a BaO-SrO-CoO-FeO-based complex oxide, the BaO-SrO-CoO-FeO-based complex oxide contains at least one of Zr and Ti by 3-20 mol% to the total amount of Co and Fe in the case of Zr and 10-50 mol% to the total amount of Co and Fe in the case of Ti. In a fuel cell system, a cathode off-gas from which oxygen is separated by using the oxygen permeable membrane is added to an anode off-gas and then a high concentration CO-containing anode off-gas is obtained.

Description

The present invention relates to an oxygen permeable membrane, and more particularly to a BaO—SrO—CoO—Fe ₂ O ₃ -based oxygen permeable membrane. The present invention also relates to an oxygen separation method and a fuel cell system using the oxygen permeable membrane.

The anode offgas is burned by oxygen separated from the cathode offgas (air) of the solid oxide fuel cell (SOFC), and the heat generated during combustion is used again as heat for reforming, and CO ₂ is recovered at a high concentration. A system is described in US Pat. Patent Document 1 discloses a Ba _0.5 Sr _0.5 Co _0.8 Fe _0.2 O ₃ film and a La _0.7 Sr _0.3 Ga _0.6 as oxygen permeable films for separating oxygen from air. Examples are Fe _0.4 O ₃ films and Pr _0.7 Sr _0.3 Fe _0.8 Al _0.2 O ₃ films.

Patent Document 2 discloses a composite oxide film of Sr: Ba: Co: Fe = x: (1-x) :( 1-y): y as a BaO—SrO—CoO—Fe ₂ O ₃ -based oxygen permeable film. Is described (0.1 ≦ x ≦ 0.9, 0.2 ≦ y ≦ 0.7).

JP 2012-164423 A JP 2005-15320 A

When the BaO—SrO—CoO—Fe ₂ O ₃ based oxygen permeable membrane separates oxygen from the fuel cell cathode offgas, when the BaO—SrO—CoO—Fe ₂ O ₃ based membrane comes into contact with the anode offgas, BaO—SrO— The CoO—Fe ₂ O ₃ based film reacts with CO ₂ contained in the anode off gas, and a carbonate is formed on the BaO—SrO—CoO—Fe ₂ O ₃ based oxygen permeable film, or before the carbonate is formed. It has been found by the present inventor that the oxygen permeation performance is abruptly lowered by the adsorption of CO ₂ on the membrane surface.

An object of the present invention is to provide an oxygen permeable membrane that maintains high oxygen permeation performance for a long time even when it comes into contact with a CO ₂ -containing gas, and an oxygen separation method and a fuel cell system using the oxygen permeable membrane. And

The oxygen permeable membrane of the present invention is an oxygen permeable membrane made of a BaO—SrO—CoO—Fe ₂ O ₃ composite oxide, wherein the BaO—SrO—CoO—Fe ₂ O ₃ composite oxide is at least of Zr and Ti. One of them is characterized by containing 3 to 20 mol% with respect to the total amount of Co and Fe in the case of Zr, and 10 to 50 mol% with respect to the total amount of Co and Fe in the case of Ti.

The matrix composition other than ZrO ₂ and TiO _{2 of} this BaO—SrO—CoO—Fe ₂ O ₃ composite oxide is Sr _x Ba _1-x Co _1-y Fe _y O _{3− (x + y)} (however, 0.1 ≦ x ≦ 0.9, 0.2 ≦ y ≦ 0.7).

  The oxygen separation method of the present invention separates oxygen from the oxygen-containing gas by the oxygen permeable membrane of the present invention.

  The fuel cell system of the present invention includes a solid oxide fuel cell, reformed gas supply means for supplying a reformed gas to the anode side of the fuel cell, and an anode off-gas flow for discharging the anode off-gas from the fuel cell. A path member, oxygen-containing gas supply means for supplying an oxygen-containing gas to the cathode side of the fuel cell, means for discharging a cathode off-gas in contact with the cathode side of the fuel cell, and oxygen in the cathode off-gas is separated. In the fuel cell system having an oxygen permeable membrane introduced into the anode off-gas flow path, the oxygen permeable membrane is the oxygen permeable membrane of the present invention.

  In this fuel cell system, it is preferable that the oxygen permeable membrane is provided so as to constitute at least a part of the anode off-gas flow path member.

In the oxygen-permeable membrane made of a _{BaO-SrO-CoO-Fe 2} O 3 composite oxide, by including at least one of ZrO ₂ and TiO ₂ to _{BaO-SrO-CoO-Fe 2} O 3 composite oxide, It has been found that carbonate formation and CO ₂ adsorption are suppressed when the oxygen permeable membrane is contacted with a CO ₂ containing gas. Even when the oxygen permeable membrane of the present invention is in contact with a CO ₂ -containing gas, the oxygen permeable performance becomes high over a long period of time.

The fuel cell system of the present invention uses this oxygen permeable membrane to separate oxygen from the cathode offgas, supply this oxygen to the anode offgas, oxidize unreacted hydrogen, CO in the anode offgas, H ₂ O and and CO _2. By condensing and separating this H ₂ O, a CO ₂ -containing gas containing CO ₂ at a high concentration can be obtained. This CO ₂ -containing gas is useful as a raw material for producing liquefied carbonic acid, dry ice and the like.

It is typical sectional drawing of the fuel cell part of the fuel cell system which concerns on embodiment. It is a perspective view of a fuel cell unit. It is a fragmentary sectional view of a fuel cell unit. It is a graph showing the result of an Example and a comparative example. It is a graph showing the result of an Example and a comparative example. It is a graph showing the result of an Example and a comparative example. It is a graph showing the result of an Example and a comparative example.

  Hereinafter, the present invention will be described in more detail.

The BaO—SrO—CoO—Fe ₂ O ₃ composite oxide constituting the oxygen permeable membrane of the present invention is Sr _x Ba _1-x Co _1-y Fe _y O _{3-(x + y)} (provided that 0.1 ≦ x ≦ 0.9
, 0.2 ≦ y ≦ 0.7) (hereinafter sometimes referred to as “mother composition”). In this Sr _x Ba _1-x Co _1-y Fe _y O _3-z , Sr and Ba are located at the A site of the general formula ABO ₃ showing a perovskite structure, and Co and Fe are located at the B site. It has a structure. The substitution amount x of Ba with Sr at the A site is in the range of 0.1 ≦ x ≦ 0.9. Further, the substitution amount of Co with Fe is in the range of 0.2 ≦ y ≦ 0.7. By setting it as such a composition, the rapid oxygen absorption-and-release at the time of raising / lowering temperature and the sudden volume change accompanying this become difficult, and it is prevented that the film | membrane cracks at the time of raising / lowering temperature. In particular, in the case of 0.4 ≦ x ≦ 0.8, the rapid oxygen absorption / release during the temperature rise and fall and the sudden volume change associated therewith are further suppressed, and oxygen equal to or higher than that in the case of x = 0. Transmission performance is obtained.

  Note that when x> 0.9, the oxygen permeation performance decreases, and when x <0.1, oxygen absorption / release during the temperature rise / fall occurs abruptly. Changes tend to cause cracks in the film. In addition, when y <0.2 and 0.7 <y, the movement of oxygen ions or oxygen vacancies is difficult to occur, and the oxygen permeability is lowered.

The composite oxide constituting the oxygen permeable membrane of the present invention has a composition in which at least one of ZrO ₂ and TiO ₂ is added to the mother composition. The Zr content in the entire composite oxide is 3 to 20 mol%, particularly 3 to 10 mol%, based on the total amount of Co and Fe in the composite oxide, and the Ti content is the total amount of Co and Fe in the composite oxide. It is preferable that it is 10-50 mol% with respect to 10-30 mol% especially. In this way, by containing at least one of ZrO ₂ and TiO ₂ in the BaO—SrO—CoO—Fe ₂ O ₃ composite oxide, carbonate formation and CO in the case where the oxygen permeable membrane comes into contact with the CO ₂ containing gas. ₂ Adsorption is suppressed, and the oxygen permeation performance becomes high over a long period of time.

  This composite oxide can be basically prepared by a method including heat treatment. For example, an oxide or a compound containing each metal atom of strontium, barium, cobalt, iron, which can be converted into an oxide in a heat treatment process such as calcination or sintering, for example, strontium oxide, barium oxide, cobalt oxide, oxidation Oxides such as iron, zirconium oxide and titanium oxide, inorganic acid salts such as nitrates, carbonates, sulfates and phosphates, organic acid salts such as acetates and oxalates, chlorides and bromides, iodine A halide such as a halide, a hydroxide, an oxyhalide, and the like are mixed in a desired ratio, and heat treatment is performed.

  Alternatively, a desired composite oxide may be obtained by subjecting a precipitate obtained by a so-called coprecipitation method, in which a mixed aqueous solution of metal salt is hydrolyzed with an alkaline aqueous solution such as aqueous ammonia, to obtain a desired composite oxide. Furthermore, each metal mixture or alloy may be oxidized by heat treatment.

  Preferable conditions for calcination of this raw material mixture are conditions of holding at 850 ° C. to 1050 ° C. for 6 to 24 hours in the air. The obtained calcined powder is pulverized and adjusted to a predetermined particle size.

  The obtained calcined fine powder is pressed into a plate or the like by a uniaxial press molding method or a cold isostatic pressing method, or a self-supporting film is formed by a doctor blade method, or by an extrusion molding method or an injection molding method. A composite oxide film is manufactured by forming a tube-shaped formed body, or forming a coating film on the surface of the porous support by a screen printing method or a slurry coating method, and firing the formed body. Firing is preferably carried out in the air at 1000 ° C. to 1200 ° C. for 1 to 10 hours. For example, most of them can be made into cubic crystals by holding at 1100 ° C. for 6 hours. At the time of firing such a molded body, it is preferable that the temperature raising rate and the temperature lowering rate are 50 ° C./hour or less, respectively. This is because if the heating rate and the cooling rate are 50 ° C./hour, oxygen is suddenly absorbed and discharged from the fired body, and cracks are likely to occur in the fired body.

  When a sintered body is produced by firing a press-molded body, an oxygen permeable film having a desired thickness can be produced by performing a slicing process, a grinding process, or a polishing process as necessary.

  When forming a membrane on the surface of a porous support, the porous support has the same composition as the membrane to be formed, or does not react with the formed membrane to form a composite oxide such as spinel, and thermal expansion. A porous support made of a material having substantially the same coefficient is preferably used. Such a porous support is produced by controlling its thickness, porosity, pore diameter and the like so as to obtain an appropriate mechanical strength and gas diffusion coefficient.

  Other methods for producing the oxygen permeable membrane of the present invention include sputtering and thermal spraying. In the sputtering method, a sintered body obtained by molding and firing calcined powder by press molding or the like can be used as a target. In the thermal spraying method, the calcined powder may be used directly as a spraying raw material, or a sintered body obtained by firing the calcined powder by press molding or the like may be reground and used.

In order to separate oxygen from an oxygen-containing gas such as air using the oxygen permeable membrane made of the ZrO ₂ and / or TiO ₂ -containing BaO—SrO—CoO—Fe ₂ O ₃ film thus produced, A chamber is provided on both sides of the oxygen permeable membrane, an oxygen-containing gas such as air is supplied to one chamber, and the pressure conditions in both chambers are set so that the oxygen partial pressure in the other chamber is lower than the oxygen partial pressure. . For example, a chamber to which air is supplied (hereinafter referred to as an “air chamber”) is set to a normal pressure or pressurized state, the other chamber is set to a reduced pressure, or the other chamber is replaced with nitrogen gas or the like with the air chamber set to a normal pressure. Reduce pressure. Thus, oxygen moves from the air chamber to the other chamber on the low oxygen partial pressure side through the oxygen permeable membrane.

  The temperature at the time of oxygen separation by the oxygen permeable membrane is usually 400 ° C to 1200 ° C, preferably 500 to 1000 ° C. A catalyst for promoting oxygen separation may be applied to the surface of the oxygen permeable membrane. Examples of the catalyst include metals or metal oxides such as platinum, palladium, gold, silver, bismuth, barium, vanadium, molybdenum, cerium, ruthenium, manganese, cobalt, rhodium, and praseodymium.

  Next, a fuel cell system using this oxygen permeable membrane will be described.

  FIG. 1 is a longitudinal sectional view schematically showing a configuration of a fuel cell portion of the fuel cell system, FIG. 2 is a perspective view of the fuel cell unit, and FIG. 3 is a schematic enlarged longitudinal sectional view of the fuel cell unit.

  An air supply manifold 2 is installed in the lower part of the housing 1 constituting the outer shell of the fuel cell, an exhaust manifold 3 is installed in the upper part, and the fuel cell unit 4 is installed between the manifolds 2 and 3. As shown in FIGS. 2 and 3, the fuel cell unit 4 includes a support 5 made of porous ceramic having air permeability, and fuel cells (hereinafter abbreviated as cells) 6 provided on the outer surface of the support 5. . The support 5 is provided with a through hole 5h that penetrates from the lower end surface to the upper end surface. The fuel cell unit 4 is installed such that the lower end of the through hole 5 h faces the air supply manifold 2 and the upper end of the through hole 5 h faces the exhaust manifold 3.

  The cell 6 is a SOFC type cell having a solid electrolyte layer 6b, and has a three-layer structure in which a solid electrolyte layer 6b is interposed between an anode 6a and a cathode 6c. The anode 6a is disposed in contact with the support 5, and the cathode 6c is exposed to the air. In this embodiment, the support 5 has a flat plate shape, and a plurality of holes 5h are provided in parallel. A plurality of cells 6 are provided on both side surfaces of the support 5, and the cells 6 on each side surface are connected in series so that a high voltage output can be obtained. However, the configuration of the fuel cell unit 4 is not limited to this.

  A horizontal partition plate 7 is installed on the upper surface of the exhaust manifold 3, and a fuel reformer 10 is installed in a space 8 above the partition plate 7. The exhaust manifold 3 is located below the partition plate 7. At least part of the wall surface and bottom surface of this exhaust manifold 3 (part of the wall surface in this embodiment) is constituted by the oxygen permeable membrane 9 of the present invention. Through this oxygen permeable membrane 9, oxygen in the cathode off gas can diffuse into the exhaust manifold 3.

  In order to configure at least a part of the wall surface and bottom surface of the exhaust manifold 3 with the oxygen permeable membrane 9, it is preferable to provide an opening in the exhaust manifold 3 and attach the oxygen permeable membrane 9 to the opening, but the present invention is not limited to this. . For example, the exhaust manifold may be configured by using a frame constituting the skeleton of the exhaust manifold and attaching the plate-like oxygen permeable membrane 9 to the frame.

  A fuel reformer 10 is installed in the space 8 so as to be in contact with the partition plate 7. The fuel reformer 10 is heated by the heat of the anode off gas transmitted through the partition plate 7.

The fuel reformer 10 is filled with a reforming catalyst, a shift catalyst, and a CO selective oxidation catalyst, and contains a hydrocarbon-containing gas whose main component is a hydrocarbon such as methane (CH ₄ ) and water vapor (H ₂ O). ) Are introduced through the supply pipes 11 and 12, and air for shift reaction and CO selective oxidation reaction is introduced through the air supply pipe (not shown), and the reforming reaction, shift reaction and CO selective oxidation are introduced. The reaction is performed, and a reformed gas containing H ₂ , CO, CO ₂ and a small amount of unburned CH ₄ is generated. Heat transmitted through the partition plate 7 as heat necessary for the reforming reaction (endothermic reaction), shift reaction heat in the reformer 10, and CO selective oxidation reaction heat are used. The reformed gas is introduced into the supply manifold 2 via the pipe 13 and is distributed and supplied to the through holes 5 h of the fuel cell units 4. Hydrogen in the reformed gas reaches the anode 6 a of the cell 6 through the pores of the porous support 5. The anode off gas that has passed through the through hole 5h is sent out from the exhaust manifold 3 through the exhaust port.

In the housing 1, air is supplied from a lower air supply port 1a by a blower (not shown). This air comes into contact with the cathode 6 c of the cell 6. Oxygen in the air diffuses and permeates through the solid electrolyte layer 6b and reaches the anode 6a to generate power. The cathode off gas is sent out from the exhaust port 1 b at the top of the housing 1. In the housing 1, part of oxygen in the cathode off-gas passes through the oxygen permeable membrane 9 and flows into the exhaust manifold 3, reacts with CO, H ₂ , unburned CH _4, etc. in the anode off-gas, and CO _2. , H ₂ O.

The anode off gas discharged from the exhaust port 14 may be directly cooled by a heat exchanger to separate condensed water and used as a high CO ₂ concentration gas. Further, when the anode off gas contains a relatively large amount of H ₂ and CO as unburned components, a _second solid oxide fuel cell that receives the anode off gas is installed, and the second solid oxide fuel cell It may be used for power generation. In this way, an anode off gas having a high CO ₂ concentration can be obtained from the second solid oxide fuel cell.

<Preparation of oxygen permeable membrane sample>
BaCO ₃ , SrCO ₃ , CoO, Fe ₂ O ₃ , ZrO ₂ , and TiO ₂ (purity are all 99.9%) as raw materials are weighed to the following target composition, and mixed for 6 h using a planetary ball mill did. The obtained mixed powder was compacted by a hydraulic uniaxial hand press at 35 MPa for 1 min to form a pellet having a diameter of 27 mm. Subsequently, it was calcined at 1150 to 1200 ° C. in the atmosphere for 5 hours to obtain a powder sample having a target composition. This powder sample was pulverized in an alumina mortar and pulverized for 24 hours using a planetary ball mill to obtain a fine powder. After mixing this fine powder and a binder, it was pressed for 1 min at 35 MPa by a hydraulic uniaxial hand press to form a pellet having a diameter of 27 mm. Thereafter, the pellets were isotropically compressed at 250 MPa by a cold isostatic press (CIP) for 1 min and then sintered in the atmosphere at 1150 to 1200 ° C. for 10 hours to obtain a sintered body having a diameter of 20 mm. It was. This sintered body was coarsely polished with a diamond lap polishing machine, and both surfaces were mirror-polished using a diamond solution to obtain oxygen-permeable sample films A to D.
Sample film A (comparative example): Ba _0.5 Sr _0.5 Co _0.8 Fe _0.2 O _3-δ
Sample film B (present invention 1): Ba _0.5 Sr _0.5 (Co _0.8 Fe _0.2 ) _1-0.03 Zr _0.03 O _3-δ
Sample film C (Invention 2): Ba _0.5 Sr _0.5 (Co _0.8 Fe _0.2 ) _1-0.05 Zr _0.05 O _3-δ
Sample film D (Invention 3): Ba _0.5 Sr _0.5 (Co _0.8 Fe _0.2 ) _1-0.1 Ti _0.1 O _3-δ

The thickness of each film A to D and the ratio (mol%) to the total amount of Co and Fe in the composite oxide of Zr or Ti are as follows.
Sample film A: 0.67 mm (Zr, Ti = 0 mol%)
Sample film B: 0.66 mm (Zr = 3 mol%)
Sample film C: 0.58 mm (Zr = 5 mol%)
Sample film D: 0.67 mm (Ti = 10 mol%)

<Measurement of oxygen transmission rate into CO ₂ gas>
The oxygen permeable membrane was sandwiched between an alumina support tube having an outer diameter of 20 mm and an inner diameter of 15 mm and an alumina spacer having the same diameter, and the sample and the support tube were brought into close contact by using an alumina jig with a spring. Pyrex glass was sandwiched between the sample and the support tube as a sealing material and heated to about 900 ° C. to melt and seal. In a state heated to 800 ° C. or 850 ° C. by an electric heater, air was circulated on one side of the sample film, and CO ₂ -containing Ar gas having a CO ₂ concentration of 0.13 to 20% was circulated on the other side. The gas flow rate was constant at 50 sccm for air and 250 sccm for Ar gas containing CO ₂ .

The oxygen transmission rate jo ₂ (sccm · cm ⁻² ) was calculated by the following formula after quantitatively analyzing the air on the oxygen supply side using a gas chromatograph. Here, [O ₂ ] (%) is the oxygen concentration in the air analyzed by gas chromatography, f (sccm) is the flow rate of the air circulated to the oxygen supply side, and S (cm ² ) is the permeation cross-sectional area of the sample. .
jo ₂ = (20.9476- [O ₂ ]) · (1/100) · f · (1 / S)

  The results are shown in FIGS.

<Discussion>
FIG. 4 shows the results at 800 ° C. using the sample films A, B, and C. As shown in FIG. 4, the sample membrane C has a CO ₂ concentration of 6% or more and a higher oxygen transmission rate than the sample membrane A, and the sample membrane B has a CO ₂ concentration of 7.5% or more and a higher oxygen transmission rate than the sample membrane A. . Note that the sample membrane A has an oxygen transmission rate of almost zero when CO _{2 is} 20%.

FIG. 5 shows the results at 850 ° C. using the sample films A, B, and C. As shown in FIG. 5, the sample membranes B and C both have a CO ₂ concentration of 15% or more and a higher oxygen transmission rate than the sample membrane A.

FIG. 6 shows the results at 800 ° C. using the sample films A and D. As shown in FIG. 6, the sample film D has a CO ₂ concentration of 8% or more and a higher oxygen transmission rate than the sample film A.

FIG. 7 shows the results at 850 ° C. using the sample films A and D. As shown in FIG. 7, it is estimated that the sample membrane D has a higher oxygen permeation rate than the sample membrane A at a CO ₂ concentration of 20% or more.

From the above Examples and Comparative Examples, it was suggested that the reaction rate with CO ₂ is reduced by substitution of (Co, Fe) -sites with Zr or Ti. This is considered to be the reason why the reduction of the oxygen transmission rate in the CO ₂ gas atmosphere was suppressed with the increase of the Zr and Ti substitution amounts. As the amount of substitution of Zr and Ti increases, the reaction rate between the BaO—SrO—CoO—Fe ₂ O ₃ -based oxygen permeable membrane and CO ₂ decreases, and the adsorption of CO ₂ onto the sample surface brings about a decrease in oxygen transmission rate. It can be said that the formation of carbonate due to the reaction with CO ₂ hardly occurs.

DESCRIPTION OF SYMBOLS 1 Housing 2 Air supply manifold 3 Exhaust manifold 4 Fuel cell unit 5 Support body 5h Through-hole 6 Cell 9 Oxygen permeable membrane 10 Fuel reformer

Claims (5)

In the oxygen-permeable membrane made of a _{BaO-SrO-CoO-Fe 2} O 3 composite oxide, the _{BaO-SrO-CoO-Fe 2} O 3 composite oxide of at least one of Zr and Ti, when the Zr Co, An oxygen permeable membrane comprising 3 to 20 mol% with respect to the total amount of Fe, and 10 to 50 mol% with respect to the total amount of Co and Fe in the case of Ti. In Claim 1, the mother composition other than ZrO ₂ and TiO ₂ of the BaO—SrO—CoO—Fe ₂ O ₃ composite oxide is Sr _x Ba _1-x Co _1-y Fe _y O _{3− (x + y)} 0.1 ≦ x ≦ 0.9, 0.2 ≦ y ≦ 0.7).   An oxygen separation method for separating oxygen from an oxygen-containing gas using the oxygen permeable membrane according to claim 1. A solid oxide fuel cell;
A reformed gas supply means for supplying a reformed gas to the anode side of the fuel cell;
An anode off-gas flow path member for discharging anode off-gas from the fuel cell;
Oxygen-containing gas supply means for supplying an oxygen-containing gas to the cathode side of the fuel cell;
Means for discharging the cathode off gas in contact with the cathode side of the fuel cell;
A fuel cell system having an oxygen permeable membrane that separates oxygen in the cathode offgas and introduces it into the anode offgas flow path;
A fuel cell system, wherein the oxygen permeable membrane is the oxygen permeable membrane according to claim 1 or 2.   5. The fuel cell system according to claim 4, wherein the oxygen permeable membrane is provided in the anode off-gas flow path member.

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