JP3253198B2 - Nitrogen oxide reduction equipment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparatus for electrochemically reducing and removing harmful nitrogen oxides released into the atmosphere.

[0002]

BACKGROUND ART nitrogen oxides (NO _X) catalyst method as denitrification technique for removing, adsorption method, a wet absorption method, an electron beam irradiation method,
A plasma method and the like have been considered. The wet absorption method, the electron beam irradiation method, and the plasma method have large cost problems such as requiring a large amount of energy input. On the other hand, the catalyst method and the adsorption method are considered to be inexpensive methods with relatively little input energy. In this catalyst system, an NH ₃ reduction method, a hydrocarbon reduction method, and the like are generally known. The NH ₃ reduction method uses a TiO ₂ —V ₂ O ₃ catalyst and obtains NO
Is reduced. NO + NH ₃ + ／ O ₂ = N ₂ + 3 / 2H ₂ O In this case, the NO removal rate shows a very high value of 95%, but it becomes a large-scale apparatus and is dangerous because it involves handling of NH ₃ . , Equipment and operating costs are higher.

[0003] Next, the hydrocarbon reduction method, Cu- using zeolite catalysts, the presence of hydrocarbons and oxygen, such as _{C 2 H 2, C 3 H} 6, C3H 8, in which the reduction of NO. However, the removal rate is as low as about 25%. In addition, the adsorption method uses Ba 2 _{YCu 3} O ₇ or YSr ₂ Co for selectively adsorbing NO _x.
Materials such as O _X has been attracting attention as a promising adsorbent but, not only at present material development. Actually, there is no practical technology other than the NH ₃ reduction method for large fixed sources, and no useful nitrogen oxide reduction technology for small-scale sources such as household combustors has been established. A solid electrolyte, which is also been proposed a method of removing electrochemically NO _X. For example, this is an example in which a zirconia-based solid electrolyte that is an oxide conductor is used.

[0004]

As described above, a technique for reducing nitrogen oxides that is useful for small-scale sources such as household combustion machines has not been established. In addition, the electrochemically N
In the method of removing O _x , an oxide conductor is used as a solid electrolyte. Therefore, in an atmosphere where oxygen is present, oxygen selectively permeates, making it difficult to remove NO _x . In addition, it is difficult to use at a temperature of 600 ° C. or less in terms of electric conductivity. Therefore,
The present invention has high reliability, and to provide a relatively small, can be removed NO _X in a simple and low-cost device.

[0005]

SUMMARY OF THE INVENTION In view of the above problems, the present invention provides a compound of the formula: BaCe _0.8 M _0.2 O _3-a (where M is Gd, D
y, Tb or Eu), a proton conductive solid electrolyte layer, an anode having hydrogen dissociation ability provided on one surface of the electrolyte layer, and a cathode provided on the other surface of the electrolyte layer. The present invention also provides a nitrogen oxide reducing apparatus characterized in that nitrogen or oxide is reduced at the cathode by supplying hydrogen or steam to the anode to dissociate hydrogen. Where the cathode is Perovs
It is preferred to be composed of a kite type oxide.

In the nitrogen oxide reduction apparatus of the present invention, the solid electrolyte layer is formed in a cylindrical shape, and an anode is provided on an inner surface or an outer surface thereof, and a cathode is provided on the other surface. Further, it has a double structure in which an anode chamber and a cathode chamber are provided inside and outside the solid electrolyte layer.

[0007]

The electrode reaction in the apparatus of the present invention is shown in Chemical formula 1.

[0008]

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However, when excess oxygen is present, oxygen and protons react on the cathode electrode to produce water. It is also possible to use steam as a hydrogen source for the anode, and the anodic reaction in that case is shown in Chemical formula 2.

[0010]

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As described above, when nitrogen oxides are electrochemically reduced using an oxide ion conductive solid electrolyte,
Reduction is difficult unless nitrogen oxides are selectively adsorbed on the electrode or oxygen is removed. However, in the present invention, a proton conductive solid electrolyte is used, so that nitrogen oxides can be reduced even in the presence of oxygen. The apparatus of the present invention is a completely new type of nitrogen oxide reduction apparatus that operates at 300 ° C. to 1000 ° C. based on the above reaction principle.
With high conductivity of barium-cerium oxide as a proton conductor, but may be reduced of the NO _X even in a low temperature region of 600 ° C. or less, energy - charged can be reduced, and to lower running costs Can be.

[0012] Further, the proton conductive solid electrolyte layer is formed in a cylindrical shape, and hydrogen or nitrogen oxide is supplied to the inside thereof. Further, a double supply of hydrogen and nitrogen oxide to the inside and outside of the cylindrical solid electrolyte layer is provided. By adopting a tube structure, NO _X can be reduced with high efficiency, and the apparatus can be reduced in size.

[0013]

The present invention will be described below with reference to examples. Example 1 This example is an example in which nitrogen oxide reduction cell composed of a proton conductive solid electrolyte and a pair of electrodes was examined whether actually be what extent reducing the NO _X of the present invention.

FIG. 1 shows a nitrogen oxide reducing apparatus according to this embodiment. The proton conductive solid electrolyte layer 1 has a diameter of 23 mm,
The BaCe0 _.8 Gd _0.2 O _3-a thickness of 0.3mm is used.
An anode 2 is provided by baking Pt on one surface of the electrolyte layer. Further, constituting the cathode 3 of the electrode area 3 cm ² by baking an oxide Ba ₂ FeOx for adsorbing NO _X in the other surface of the electrolyte layer 1 at 900 to 1000 ° C.. 4a, 4
b denotes a pair of Pt leads connected to the anode 2, 5a, 5a
b denotes a pair of Pt leads connected to the cathode 3. One lead of each electrode is used for voltage measurement, and the other is used for current measurement.

On the anode side of the solid electrolyte layer 1, a ceramic tube 6 having a gas outlet 9 is joined via a glass packing 7 to form an anode chamber 8, and a ceramic tube 10 for supplying anode gas is provided. On the other hand, on the cathode side of the solid electrolyte layer 1, a ceramic tube 11 having a gas outlet 14 is joined via a glass packing 12 to form a cathode chamber 13, and a ceramic tube 15 for supplying a cathode gas is provided.

The nitrogen oxide reduction cell constructed as described above is set in an electric furnace 16 as shown in FIG.
Hydrogen gas humidified at room temperature on the hydrogen side (hydrogen dissociation electrode side)
ml / min at the cathode side (actually NO
A gas in which 1% concentration of NO is balanced with N ₂ is supplied at a flow rate of 5 ml / min to the ( _X reducing side) at a flow rate of 5 ml / min.
It was kept at 0 ° C. The anode and cathode were short-circuited by an external circuit, or a voltage was applied to both electrodes to try to reduce NO, and the composition of the outlet gas was examined. Further, assuming an actual cathode gas, the reduction state of NO was examined when oxygen, carbon monoxide, carbon dioxide gas, and sulfurous acid gas were mixed. Table 1 shows the results. The NO conversions were all 90% or more, and were found to be hardly affected by the cathode gas composition. It was also found that NO reduction proceeds even when no voltage is applied. As described above, it has been found that the nitrogen oxide reduction cell composed of the proton conductive solid electrolyte and the pair of electrodes can be effectively reduced with a small amount of input energy.

[0017]

[Table 1]

[0018] [Example 2] This example illustrates the NO _X reduction example of using the steam to hydrogen source to dissociate hydrogen of the nitrogen oxide reduction device composed of a pair of electrodes and the solid electrolyte layer. In the same manner as in Example 1, BaCe _0.8 Dy _0.2 having a diameter of 23 mm and a thickness of 0.3 mm was added to the proton conductive solid electrolyte.
Using O _3-a , Pt was baked on one surface to form an anode, and Pt was also baked on the other surface to an electrode area of 3 cm ² to form a cathode to form a nitrogen oxide reduction cell. This cell is set as shown in FIG.
10 humidified air at 60 ° C on the hydrogen side (hydrogen dissociation electrode side).
ml / min at the cathode side (actually NO
The NO concentration of 1% on the side) to reduce the x balance gas with N ₂ at a flow rate of 5 ml / min, cell temperature 6
It was kept at 00 ° C. In this case, since there is almost no hydrogen concentration gradient between the anode and the cathode, there is almost no voltage difference between the two electrodes. Therefore, when a voltage is applied between both electrodes, NO
_X reduction was performed. When the gas composition in the outlet gas was examined, slight NO was recognized depending on the applied current value,
At 25 mA / cm ² , the NO conversion was 91%, indicating that sufficient reduction was performed.

[Embodiment 3] In this embodiment, in a nitrogen oxide reducing apparatus comprising a pair of electrodes and a solid electrolyte according to the present invention, a chamber for reducing nitrogen and a chamber for dissociating hydrogen are divided. An example using a double tube is shown. With this example,
Nitrogen oxides can be reduced more efficiently, and the apparatus can be downsized. FIG. 2 shows the apparatus of this embodiment. Reference numeral 21 denotes a nitrogen oxide reduction cell formed by baking platinum electrodes on the inner and outer surfaces of _a cylindrical solid electrolyte layer made of BaCe _0.8 Tb _0.2 O _3-a . The three cells are connected by a ceramic joint 22 made of alumina or the like, and a cathode gas inlet tube 23 and a cathode gas outlet tube 24 are connected to both ends, and are installed in a ceramic tube 25. The anode chamber S27 is provided outside the cell. 2
8 is an anode gas inlet, and 29 is an anode gas outlet. Reference numeral 30 denotes a power supply that supplies power to both electrodes through the external circuit 31.

The cell having the above configuration was kept at 600 ° C., and it was examined whether the apparatus operates efficiently. A hydrogen gas is supplied at a flow rate of 5 ml / min as an anode gas, and a simulated gas assuming actual combustion exhaust gas, NO1.5, is used as a cathode gas.
%, O ₂ 6.3%, CO ₂ 8.1%, CO 1.4%, N ₂
A balance gas was flowed at 10 ml / min. Further, a voltage was applied to an external circuit, and reduction was performed at a current density of 25 mA / cm ² . As a result, from the analysis of the outlet cathode gas, N
O was found to be reduced by 99%. As in the present embodiment, a chamber for reducing nitrogen in a nitrogen oxide reduction device,
By dividing the chamber into a chamber for dissociating hydrogen and forming a double-pipe structure, it is possible to continuously reduce NO _x gas, and it is possible to directly introduce the generated combustion exhaust gas. As described above, nitrogen oxides can be reduced with high efficiency, and the apparatus can be simplified and downsized.

[Embodiment 4] This embodiment shows an example in which the solid electrolyte layer of the nitrogen oxide reduction apparatus has a simple cylindrical structure. FIG. 3 shows an example of a cylindrical structure. Cathode 33 made of a tube 32 made of a porous ceramic of platinum on its surface as a support to form a BaCe _0.8 Eu _0.2 O solid electrolyte layer 34 and anode 35 made of platinum consisting _3-a. 3
Reference numeral 6 denotes a lead of the cathode 33.

With such a cylindrical cell configuration, it is possible to configure a reduction device having a double-tube structure as in Embodiment 3 only by using a tube that covers the anode side. Example 3
A hydrogen gas was supplied at a flow rate of 5 ml / min as an anode gas, and a simulated gas, NO1.5, assuming actual combustion exhaust gas was used as a cathode gas.
%, O ₂ 6.3%, CO ₂ 8.1%, CO 1.4%, N ₂
A balance gas was flowed at 10 ml / min. Further, a voltage was applied to an external circuit, and reduction was performed at a current density of 25 mA / cm ² . As a result, NO was 9 at the outlet cathode gas.
9% had been reduced. Thus, NO _X reduction even with cylindrical other reduction cell was found to be efficient Ku performed.
It is clear that the use of the cylindrical cell makes it possible to secure the gas flow path and simplify the gas sealing structure, and also to easily configure the entire apparatus. Further, by making the solid electrolyte layer cylindrical, the mechanical strength can be improved, the reliability can be increased, and the life of the reduction cell can be prolonged.

In the embodiment, BaCe _0.8 Gd _0.2 O _3-a and BaCe _0.8 Dy _0.2 are used as the proton conductive solid electrolyte.
O _3-a or the like may be used. In addition to the platinum, a material having a hydrogen dissociation ability at the operating temperature of the cell, such as a metal or an alloy mainly composed of nickel or iron, or an oxide thereof is used for the anode. The cathode is made of platinum or Ba ₂
Perovskite oxide represented by FeO _X is used, in particular the latter is a preferred material from the place of selectively adsorbing nitrogen oxides.

The synthesis and production methods of these electrolytes and electrodes include coating, vapor deposition, sputtering and C.I. V. A known method such as the D method can be used. The operating temperature of the cell may be any temperature at which protons of the solid electrolyte are conducted. For example, the conductivity of the perovskite oxide used in the examples is 300
℃ at ^{10 -5 S / cm, 1000 2} × 10 -1 S / cm at ℃
It is about.

The cell may have any shape as long as the anode gas and the cathode gas continuously flow through the divided chambers. In the case of the cylindrical structure, the cell may be a cylinder or a square tube. In other words, any cylindrical shape having a hollow structure may be used.

[0026]

As described above, the present invention has high reliability,
An object of the present invention is to provide a simple and low-cost apparatus for electrochemically reducing and removing nitrogen oxides. In addition, the reduction device can be downsized by having a cylindrical structure.
INDUSTRIAL APPLICABILITY The nitrogen oxide reduction device of the present invention is applicable not only to small-sized combustion equipment but also to removal of nitrogen oxides in gas discharged from large-scale combustion equipment.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view of a nitrogen oxide reduction device according to an embodiment of the present invention.

FIG. 2 is a longitudinal sectional view of a nitrogen oxide reduction device according to another embodiment of the present invention.

FIG. 3 is a perspective view of a nitrogen oxide reduction cell according to another embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Solid electrolyte layer 2 Anode 3 Cathode 4a, 4b Electrode lead 5a, 5b Electrode lead 6, 11 Ceramic tube 7, 12 Glass packing 8 Anode room 9, 14 Gas outlet 10, 15 Ceramic tube 13 Cathode room 16 Electric furnace 21 Reduction cell 22 Joint 23 Cathode gas inlet tube 24 Cathode gas outlet tube 25 Ceramic tube 26 Cathode room 27 Anode room 28 Anode gas inlet 29 Anode gas outlet 30 Power supply 31 External circuit

Continuation of front page (58) Field surveyed (Int.Cl. ⁷ , DB name) B01D 53/56 B01D 53/32

Claims (4)

(57) [Claims] 1. The formula: BaCe _0.8 M _0.2 O _3-a (wherein M
Is a proton conductive solid electrolyte layer represented by Gd, Dy, Tb or Eu), an anode having hydrogen dissociation ability provided on one surface of the electrolyte layer, and a cathode provided on the other surface of the electrolyte layer And supplying hydrogen or steam to the anode to dissociate the hydrogen, thereby reducing the nitrogen oxide at the cathode. 2. The method according to claim 1, wherein the cathode is a perovskite oxide.
Nitrogen oxide reduction device according to claim 1, wherein you characterized in that it consists of. Wherein a said proton conductive solid electrolyte layer is cylindrical, the anode is provided on the inner or outer surface, the nitrogen of claim 1, wherein in that a said cathode on the other surface Oxide reduction device. Wherein the anode compartment or cathode compartment to the inside of the proton conductive solid electrolyte layer, this having a double pipe structure in which each of the cathode compartment or the anode compartment to the outside
The nitrogen oxide reduction device according to claim 1, wherein: