JPH0699029A - Treatment of nitrogen oxides and device therefor - Google Patents

Abstract

PURPOSE:To improve the efficiency of NOx treatment in the presence of oxygen by a solid electrolyte by covering both the surfaces of a specified solid electrolyte with electrode materials, applying voltage between electrodes or shortcircuiting between electrodes and feeding oxygen and organic compound contg. gas to be treated to one side and to the other side of the solid electrolyte respectively. CONSTITUTION:Both the surfaces of a solid electrolyte 1 having the property of converting oxygen into oxide ions, the property of allowing oxide ions to permeate and the property of converting oxide ions into active oxygen species are covered with electrode materials and voltage is applied to between electrodes 4 by a power unit 5 or a short circuit is made between electrodes 4. And oxygen is fed to the surface of a cathode 4 on the oxygen feed chamber 2 side of the solid electrolyte 1 and simultaneously NOx and organic compounds contg. gas to be treated is fed to the surface of an anode 4 on the reaction chamber 3 side or to its vicinity. Consequently, the organic compounds are oxidized by active oxygen species generated on the gas to be treated feed side and NOx is reduced by reducing action of the formed oxide.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and apparatus for treating nitrogen oxides using a solid electrolyte. More specifically, the present invention relates to a treatment method and apparatus for indirectly reducing nitrogen oxides through an organic compound coexisting with active oxygen species.

[0002]

2. Description of the Related Art Nitrogen oxide is a causative substance that causes air pollution because it is contained in the gas after combustion such as exhaust gas of automobiles, and the treatment of nitrogen oxide is important from the viewpoint of air purification. Conventionally, a reduction reaction method using a catalyst has been disclosed for the treatment of nitrogen oxides, but more recently, a method of reducing nitrogen oxides using a solid electrolyte has been proposed. That is, electrodes are coated on both sides of a solid electrolyte having a property of transmitting oxygen ions, nitrogen oxide is supplied to one side of the solid electrolyte, and a negative voltage is applied to the electrode (cathode) on the supplied side to generate nitrogen. This is a method in which oxygen in the oxide is converted into nitrogen by drawing it out on the surface of the electrode (Japanese Patent Application Laid-open No. Sho-2004-2006).
50-32092, JP-B-55-3974, JP-A-4-
161223, U.S. Pat. S. DOE Report p403
~ 412, 1990).

However, in the above method, when oxygen gas is contained in the nitrogen oxide-containing gas, the reaction in which the solid electrolyte extracts oxygen ions from oxygen gas is more preferable than the reaction in which oxygen ions are extracted from nitrogen oxide. Is fast,
It is difficult to perform efficient nitrogen oxide reduction treatment, and it is difficult to perform nitrogen oxide treatment in a system containing oxygen such as exhaust gas of a diesel engine.

[0004]

Therefore, it is desired to develop a method capable of efficiently treating nitrogen oxides even in the coexistence of oxygen. An object of the present invention is to provide a method and apparatus for treating nitrogen oxides, which can be treated in the coexistence of oxygen, which has been thought to be difficult in the past, by using a solid electrolyte having a property of allowing oxygen ions to permeate. Especially.

[0005]

Means for Solving the Problems As a result of intensive studies to achieve the above object, the inventors of the present invention utilized the reducing action of an oxide produced by an oxidation reaction using a solid electrolyte,
The inventors have found that the treatment of nitrogen oxides can be efficiently performed by indirectly reducing the nitrogen oxides, and have completed the present invention.

That is, the gist of the present invention is: (1) Both surfaces of a solid electrolyte having the characteristics of converting oxygen into oxygen ions, the characteristics of transmitting oxygen ions and the characteristics of converting oxygen ions into active oxygen species are electrode materials. And applying a voltage between the electrodes or short-circuiting the electrodes to supply oxygen to the electrode surface on one side of the solid electrolyte and to the electrode surface on the other side or in the vicinity thereof, a nitrogen oxide and an organic compound. By supplying a to-be-processed gas containing a gas, the organic compound is oxidized by active oxygen species generated at the to-be-processed gas supply side, and the nitrogen oxide is reduced by a reducing action of an oxide produced. Method for treating nitrogen oxides, and (2) solid-state electrode having properties of converting oxygen into oxygen ions, properties of allowing oxygen ions to permeate, and properties of converting oxygen ions into active oxygen species. A solid electrolyte with electrodes, both surfaces of which are coated with electrode materials, a means for applying a voltage between the electrodes or a short circuit between the electrodes, and supplying oxygen to one electrode surface of the solid electrolyte. In the apparatus for processing a gas to be processed, which comprises a means and a means for supplying a gas to be processed to the electrode surface on the other side or in the vicinity thereof, the gas to be processed contains a nitrogen oxide and an organic compound, The present invention relates to a nitrogen oxide treating apparatus, characterized in that nitrogen oxide is reduced by a reducing action of an oxide produced by oxidizing an organic compound. Hereinafter, the present invention will be described in detail.

First, the nitrogen oxide processing apparatus of the present invention will be described. The solid electrolyte that can be used in the present invention is not particularly limited, but is preferably a metal oxide composed of one kind or two or more kinds, and a crystal structure having a fluorite structure. In the fluorite type structure of the metal oxide, the arrangement of oxygen ions is far from the closest packing, and there is an empty site with eight coordinations of oxygen ions. It is considered that a large amount of lattice defects are likely to occur due to such a loose structure. In other words, it is said that the fluorite structure retains its basic crystal structure even if some heteroatoms enter due to its looseness and various defects occur (Tetsuichi Kudo and Kazuo Fueki, "Solid Eye"). Onyx] Kodansha Scientific 1
Issued the first printing on May 20, 986). By utilizing the lattice defects of oxygen ions in such a fluorite structure, a property of transmitting oxygen ions is generated.

Examples of the metal oxide having the fluorite structure include cerium oxide, thorium oxide, zirconium oxide, hafnium oxide, bismuth oxide, and magnesium oxide, calcium oxide, scandium oxide, and oxides of these metal oxides. Examples include solid solutions of yttrium, lanthanum oxide, niobium oxide, tungsten oxide, neodymium oxide, samarium oxide, gadnium oxide, erbium oxide, and ytterbium oxide. Among these, zirconium oxide in which yttrium oxide is dissolved in solid solution, zirconium oxide in which calcium oxide is dissolved in solid solution, and cerium oxide in which scandium oxide is dissolved in solid solution are preferably used from the viewpoints of wide application temperature range and easy availability.

The shape of the solid electrolyte is not particularly limited, but may be any shape having a substantially constant thickness such as a plate shape or a tube shape. Further, as long as it has a substantially constant thickness, it is also preferable that the structure is not flat but has an uneven structure and a large surface area so that a reaction field can be efficiently provided. Here, the thickness of the solid electrolyte is usually 1 μm to
It is 5 mm, preferably 10 μm to 2 mm. 1 μm
Thinner ones are difficult to manufacture and are likely to have many holes through which molecular oxygen can pass. Further, there is a problem in strength because it is easily cracked due to a sudden temperature change. On the other hand, if the thickness is greater than 5 mm, good oxygen ion permeability will not be seen. However, the thinner the thickness of the solid electrolyte, the larger the applied voltage per unit thickness with respect to the same applied voltage, thereby increasing the permeation amount of oxygen ions, which is suitable for the treatment method of the present invention. Can be done. Since the mechanical strength decreases as the thickness of the solid electrolyte decreases (for example, 0.1 mm or less), a porous material with low diffusion resistance of oxygen molecules (for example, porous alumina, porous zirconia, etc.)
A solid electrolyte may be attached to the surface of a support consisting of.

Examples of the method for producing the solid electrolyte include, but are not limited to, firing method, plasma spraying, sol-gel method, vacuum deposition and sputtering. The solid electrolyte as described above has the property of converting oxygen into oxygen ions, the property of transmitting oxygen ions, and the property of converting oxygen ions into active oxygen species. It can be expressed even if oxygen is present on the generating side.

In the present invention, both surfaces of the solid electrolyte are coated with electrode materials. This electrode efficiently donates electrons to convert oxygen supplied to one side into oxygen ions and accepts electrons to convert oxygen ions permeating the solid electrolyte into active oxygen species on the other side. It is provided for. The electrode material may be one having sufficient conductivity, for example, metals such as gold, platinum, silver, copper, iron, aluminum, nickel, zinc, tin, alloys composed of two or more metals, or carbon. Etc. Such an electrode is formed by a method of applying an electrode material in a paste form, a method of coating the solid electrolyte surface by a sputtering method or a vacuum deposition method, a method of adhering a metal mesh to the solid electrolyte surface, and the like. It is not limited.

The treatment apparatus of the present invention comprises such a solid electrolyte with electrodes, a means for applying a voltage between the electrodes or a short circuit between the electrodes, and oxygen on one electrode surface of the solid electrolyte. A means for supplying and an active oxygen species generated on the other side of the solid electrolyte oxidize the added organic compound to generate an oxide, and nitrogen oxide is reduced by the reducing action of the oxide, A means for supplying a gas to be treated containing nitrogen oxides and an organic compound (hereinafter, may be simply referred to as “gas to be treated”) to the electrode surface of the solid electrolyte or its vicinity is provided.

In the present invention, in order to permeate oxygen ions in the solid electrolyte, it is necessary to generate at least one of a potential difference, a pressure gradient of oxygen and a concentration gradient of oxygen between both electrodes. You may combine two or more. In the case of a potential difference, means for applying a voltage between the electrodes is required, and there is no particular limitation as long as a direct current voltage can be applied. For example, a stabilizing power supply, a dry battery, a lead storage battery or the like can be used.

When only a pressure gradient or a concentration gradient of oxygen is used, the electrodes may be short-circuited, and the means thereof include a method of connecting both electrodes with a material having sufficient conductivity. Examples of means for generating a pressure gradient include means for pressurizing the oxygen supply side with a pressurizing pump, means for depressurizing the target gas supply side with a vacuum pump, and the like. As a means for generating a concentration gradient, a means for supplying an oxygen-containing gas having a high oxygen concentration to the oxygen supply side, a means for increasing the pressure of the oxygen-containing gas by a pressure pump to supply a gas having a high oxygen concentration, a treatment target Inert gas (nitrogen,
Examples include a means for supplying a gas to be processed mixed with helium or the like.

As means for supplying oxygen, for example, a method of supplying oxygen or a mixture of oxygen such as air with a high-pressure gas cylinder, a method of supplying with an air pump or an air compressor, a method of exposing to the atmosphere, and the like can be mentioned. However, it is not particularly limited. Oxygen is supplied to the electrode surface on one side of the solid electrolyte by this oxygen supply means, and oxygen is converted into oxygen ions.

Examples of means for supplying the gas to be treated include a method of supplying it with a pump or the like, a method of diluting the gas to be treated with an inert gas (helium, nitrogen, etc.) in a pressurized state and supplying it. To be The gas to be treated is preferably supplied to the electrode surface or the vicinity thereof. When the organic compound in the gas to be treated is liquid or solid, it may be vaporized by an evaporator or the like and supplied.

In the nitrogen oxide treating apparatus of the present invention, it is possible to use a reactor as shown in FIG. 1 when supplying the oxygen and the gas to be treated.
That is, it is possible to adopt a method of supplying the respective substances to the oxygen supply chamber and the reaction chamber by using a reactor partitioned by the solid electrolyte with electrode into the oxygen supply chamber and the reaction chamber. Besides such a reactor, for example, a reactor in which the inside or outside of the solid electrolyte with a tubular electrode is an oxygen supply chamber or a reaction chamber,
It is possible to use various types of reactors such as a reactor in which one side of the solid electrolyte with electrode is exposed to the atmosphere and is an open system. However, in the present invention, it is not always necessary to use a reactor having a shape having such a closed system.

Next, the method for treating nitrogen oxides of the present invention will be described. The nitrogen oxide treatment method of the present invention can be suitably carried out using the above-mentioned treatment apparatus. The feature of the treatment method of the present invention is that the active oxygen species generated on the supply side of the treated gas oxidize the organic compound in the treated gas, and the nitrogen oxide is reduced by the reducing action of the generated oxide.

The oxygen used may be a pure component oxygen molecule gas, air, or a gas obtained by mixing oxygen and another gas (for example, nitrogen, helium, etc.), and such a substance is supplied by the above-mentioned supply means. To be done. The gas to be treated used contains nitrogen oxides and organic compounds as described above, but in the present invention, even if it contains oxygen, treatment can be performed efficiently.
The nitrogen oxide to be reduced may be nitrogen monoxide or nitrogen dioxide, which is a pure component gas thereof, or a nitrogen oxide-containing gas such as exhaust gas generated from a diesel engine or the like.

As the organic compound, many compounds are possible and are not particularly limited, but saturated and unsaturated hydrocarbons having 1 to 10 carbon atoms, saturated hydrocarbons having 1 to 10 carbon atoms, Unsaturated alcohols with 1 to 10 carbon atoms
Saturated or unsaturated amines, saturated or unsaturated amides having 1 to 10 carbon atoms, aromatic amines having 6 to 20 carbon atoms, or aromatic amines having 1 to 10 carbon atoms Examples include saturated and unsaturated compounds represented by RCOONH ₄ , amino acids and the like. For example, as hydrocarbons, methane,
Ethane, propane, ethylene, propylene, octane,
Octene and the like, alcohols such as methanol, ethanol and butanol, amines such as monomethylamine, monoethylamine and monooctylamine, and amides such as acetamide, urea and oxamide.
The aromatic amines include aniline and benzylamine, the compounds represented by RCOONH ₄ include ammonium formate and ammonium acetate, and the amino acids include glutamine and asparagine. The above organic compounds can be used alone or in combination of two or more. The amount of the organic compound to be added is usually selected to be equal to or higher than that of the nitrogen oxide, preferably 1.5 times or higher. If it is less than the same molar concentration, the reduction reaction of nitrogen oxide becomes insufficient.

In the present invention, specific conditions for permeating oxygen ions through the solid electrolyte may be that at least one of a potential difference, a pressure gradient and a concentration gradient is generated through the solid electrolyte. Alternatively, a plurality of combinations may be used. When oxygen ion permeation due to the potential difference is not performed, the electrodes are short-circuited. To generate a potential difference, a voltage is applied between the electrodes. The applied voltage per 1 mm thickness of the solid electrolyte is usually 0.1 V / mm or more, preferably 0.5 V / mm or more.

In order to generate a pressure gradient, the method of pressurizing the oxygen supply side and the method of depressurizing the gas to be treated as mentioned above may be mentioned. At this time, the differential pressure for causing the permeation of oxygen ions is usually 10 mmH as the oxygen partial pressure.
g or more, preferably 50 mmHg or more. When a concentration gradient is to be generated, a method of supplying a gas with a high oxygen concentration to the oxygen supply side as described above, a method of pressurizing the oxygen-containing gas to increase the oxygen concentration or supplying a gas with a high oxygen concentration to the oxygen supply side, or The gas to be processed mixed with the active gas may be supplied to the gas supply side to be processed. At this time, the difference in oxygen concentration for causing the permeation of oxygen ions is usually 5 mol% or more, preferably 10 mol% or more.

The reaction temperature is 50 to 800 ° C. It is preferably 100 to 600 ° C. At a temperature lower than 50 ° C., the permeation rate of oxygen ions in the solid electrolyte is extremely low, and at a temperature higher than 800 ° C., the electrode material and the like are limited. The pressure on the gas-to-be-treated side may be normal pressure, but it may be reduced pressure when a pressure gradient is generated through the solid electrolyte as described above. In this case, the decompression condition is 500 mmHg or less, preferably 200 m
It is preferably mHg or less. The pressure on the oxygen supply side may be normal pressure, but may be increased when a pressure gradient is generated through the solid electrolyte as described above. The pressurizing condition in this case is 0.1 kgf / cm ² or more, preferably 1 kgf
/ Cm ² or more.

As described above, in the present invention, the active oxygen species obtained by the present invention are used to oxidize the added organic compound, and the nitrogen oxide is indirectly and efficiently produced by the reducing action of the produced oxide. It can be reduced.
Therefore, it is possible to treat and remove nitrogen oxides contained in the exhaust gas in various combustion furnaces such as incinerators in general and internal combustion engines.

The mechanism of the method for treating nitrogen oxide of the present invention is as follows.
It can be considered as follows. A solid electrolyte composed of one or more kinds of metal oxides having a fluorite type crystal structure has a property of transmitting oxygen ions through lattice defects of oxygen ions. When both sides of this solid electrolyte are covered with electrodes and oxygen is brought into contact with one side of the solid electrolyte, the surface of the electrodes is halved.
The reaction of O ₂ + 2e ⁻ → O ^{2 −} occurs and oxygen ions enter into the lattice defects in the solid electrolyte. Oxygen ions in the lattice defects are transported to the other side due to the potential difference due to the voltage applied between the electrodes, the oxygen concentration gradient, or the oxygen pressure gradient. On the surface of the electrode on the other side, a reaction of O ^2- → O ^* + 2e ⁻ occurs and active oxygen species are generated. The active oxygen species oxidize the organic compounds existing in the gas to be treated together with the nitrogen oxides to generate oxides, and the reducing action of the oxides reduces the nitrogen oxides and converts them into nitrogen.

[0026]

The present invention will be described in more detail with reference to the following examples, but the present invention is not limited to these examples.

Example 1 FIG. 1 shows an example of a processing apparatus suitable for carrying out the processing of nitrogen oxides of the present invention. 1 is a zirconium oxide disc (manufactured by Nippon Fine Ceramics Co., Ltd., thickness 0.1 mm, diameter 80 m) in which yttrium oxide is solid-solved at a concentration of 8 mol% with a solid electrolyte having a property of transmitting oxygen ions.
m). Reference numeral 2 is an oxygen supply chamber that supplies oxygen into the solid electrolyte. Reference numeral 3 is a reaction chamber in which a gas to be treated containing a nitrogen oxide and an organic compound is supplied to react the active oxygen species with the organic compound and the nitrogen oxide with the oxide of the organic compound. 4 is paste gold (manufactured by Nippon Kin Liquid Co., Ltd.)
Was applied with a brush to prepare gold electrodes on both sides of the solid electrolyte. Reference numeral 5 is a stabilized DC power supply for applying a voltage between the solid electrolytes. 6 is an ammeter for measuring the current flowing in the system when a voltage is applied. Reference numeral 7 is a changeover switch for selecting application and short-circuiting of voltage between the electrodes.

Example 2 Treatment of nitrogen oxide was carried out using the apparatus shown in FIG. The processing method and the result are shown below. Air was supplied to the second oxygen supply chamber at a flow rate of 100 ml / min. On the other hand, 500 m of dry air containing 985 ppm of nitric oxide and 1522 ppm of methane was supplied to the third reaction chamber.
It was distributed at 1 / min. The reaction temperature was heated to 400 ° C. with a heater, and the switch 7 was connected to the voltage application side under normal pressure. The driving force for oxygen ion permeation was a potential difference, and a voltage of 5 V / mm was applied to the 4 electrodes on the anode side by the power supply device of 5 to carry out the reaction. The quantification of the reaction product was performed by measuring nitric oxide, oxygen and methane in the inlet and outlet gases of the reaction chamber. Nitric oxide and oxygen are NOx-O ₂
A measuring device (manufactured by Shimadzu Corporation) and methane were measured by gas chromatography (GC-8A manufactured by Shimadzu Corporation). As the analysis conditions, WG-100 (manufactured by GL Sciences) was used as a column, column temperature: 50 ° C., carrier gas: helium, carrier pressure: 0.8 kg / cm ² , and detector: TCD. Table 1 shows the measured values of each substance at the inlet and outlet of the reaction chamber.

[0029]

[Table 1]

From Table 1, it was confirmed that the present invention enables efficient reduction treatment of nitrogen oxides even in the presence of oxygen.

Comparative Example 1 In the reaction method shown in Example 2, nitrogen oxide was treated under the same conditions except that methane was not supplied. The results are shown in Table 2.

[0032]

[Table 2]

As shown in Table 2, the nitrogen oxide reduction treatment did not proceed in the system containing no organic compound.

[0034]

According to the present invention, it has become possible to efficiently carry out the treatment of nitrogen oxides in the coexistence of oxygen with a solid electrolyte, which has hitherto been considered difficult to carry out with efficient treatment. This has enabled a wide range of applications of the invention.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram showing an example of a processing apparatus suitable for carrying out the processing of nitrogen oxides of the present invention.

[Explanation of symbols]

 1 Solid Electrolyte 2 Oxygen Supply Chamber 3 Reaction Chamber 4 Electrode 5 Power Supply Device 6 Ammeter 7 Changeover Switch 8 Reactor

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Keiichi Deng 4-1-1 Sekido, Wakayama City, Wakayama Prefecture (72) Inventor Masaki 2-31-24 Koenji Minami, Suginami-ku, Tokyo

Claims (2)

[Claims] 1. A solid electrolyte having properties of converting oxygen into oxygen ions, properties of allowing oxygen ions to permeate, and properties of converting oxygen ions into active oxygen species is coated with electrode materials, and a voltage is applied between the electrodes. By applying or short-circuiting the electrodes, oxygen is supplied to the electrode surface on one side of the solid electrolyte and a gas to be treated containing a nitrogen oxide and an organic compound is supplied to the electrode surface on the other side or in the vicinity thereof. Thus, the organic compound is oxidized by the active oxygen species generated on the supply side of the gas to be treated, and the nitrogen oxide is reduced by the reducing action of the generated oxide. 2. A solid electrolyte with electrodes in which both surfaces of a solid electrolyte having properties of converting oxygen into oxygen ions, properties of allowing oxygen ions to permeate, and properties of converting oxygen ions into active oxygen species are coated with electrode materials. A means for applying a voltage between the electrodes or a short circuit between the electrodes, a means for supplying oxygen to the electrode surface on one side of the solid electrolyte, and a gas to be treated on or near the electrode surface on the other side And a means for treating the gas to be treated, wherein the gas to be treated contains nitrogen oxides and an organic compound, and the organic compound is oxidized to reduce nitrogen oxides by a reducing action of an oxide produced. An apparatus for treating nitrogen oxide, characterized in that the substance is configured to be reduced.