JPH08332342A - Method and apparatus for removing nitrogen oxide - Google Patents

Abstract

PURPOSE: To remove nitrogen oxides efficiently with a simple apparatus by arranging an oxygen ion conductive solid electrolyte ceramic between perovskite type oxide ceramic electrodes, and applying voltage between the electrodes. CONSTITUTION: A treatment member 10 has tubular three-layer structure, and a first electrode 11 and a second electrode 12, which are made from LaFeO3 , are set on the inside and outside of a moving layer 13 made from stabilized zirconia. The central part of the treatment member 10 having the electrodes is covered with a cylindrical ceramic heater 14 with a temperature controller. When exhaust gas 16A containing nitrogen oxides is passed through the treatment member 10, the gas is discharged as exhaust gas 16B with nitrogen oxides removed. Nitrogen oxides such as NO and NO2 are decomposed on the first electrode 11 into N2 gas, and oxygen is taken into the moving layer 13 as oxygen ion. The oxygen ion taken into the moving layer 13 reaches the second electrode 12 and releases electrons to be discharged outside as oxygen gas.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a nitrogen oxide removing apparatus and method for removing nitrogen oxides, which are harmful substances, from combustion engine exhaust gas from automobiles, waste incinerator exhaust gas and the like.

[0002]

2. Description of the Related Art Conventionally, various methods for treating nitrogen oxides (NOx) from exhaust gas from automobiles, garbage incinerators and various factories have been studied.

For example, regarding exhaust gas of automobiles, Rh
A method of catalytic decomposition of -Pt-Pd using a three-way catalyst is known. However, in the catalytic decomposition with such a three-way catalyst, the setting range of the processing condition is narrow, for example, the setting range of the condition for efficiently processing NO _x is different depending on the coexisting oxygen concentration, so that a complete effect cannot be expected.

Therefore, as a method for efficiently removing NO _x , a method is known in which a substance such as nitrogen oxide is electrolytically reduced on an electrode by utilizing an electrochemical reaction (Japanese Patent Laid-Open No. 53-51173). No.

[0005]

However, the above-mentioned treatment utilizing the electrochemical reaction is to treat nitrogen oxides and the like with hydrogen generated by the electrolysis of the electrolytic solution held between the electrodes. Therefore, there is a problem that the device configuration becomes complicated, such as having to hold the electrolytic solution between the electrodes.

In view of such circumstances, an object of the present invention is to provide a nitrogen oxide removing apparatus and a nitrogen oxide removing method capable of efficiently removing nitrogen oxides with a simple apparatus.

[0007]

The first aspect of the present invention for achieving the above object is to have an electron conductivity and an ionic conductivity, which is arranged so as to be in contact with a flow of a gas to be treated containing nitrogen oxides. A first electrode layer made of perovskite-type ceramics, an ion transfer layer made of solid electrolyte ceramics provided in contact with the first electrode and having oxygen ion conductivity, and the first transfer layer of the ion transfer layer. A tubular member comprising a second electrode layer made of perovskite ceramics having electronic conductivity and ionic conductivity, which is provided in contact with the side opposite to the electrode; heating for maintaining the tubular member at a predetermined temperature Means;
And a power source for applying a voltage so that the first electrode layer and the second electrode layer of the tubular member serve as a cathode and an anode, respectively.

A second aspect of the present invention is the nitrogen oxide removing apparatus according to the first aspect, characterized in that the treatment member is a tubular member having the first electrode inside.

A third aspect of the present invention is the nitrogen oxide removing apparatus according to the fourth or fifth aspect, characterized in that the solid electrolyte ceramic is pre-stabilized zirconia.

A fourth aspect of the present invention is to provide a first electrode layer made of a perovskite type ceramic having electron conductivity and ionic conductivity, and a layer provided in contact with the first electrode and having oxygen ion conductivity. And an ion transfer layer made of a solid electrolyte ceramics, which is provided on the opposite side of the ion transfer layer from the first electrode, and made of a perovskite type ceramics having electron conductivity and ion conductivity. While maintaining a temperature of the processing member including the electrode layer of (1) to a predetermined temperature, and applying a voltage so that the first electrode layer and the second electrode layer serve as a cathode and an anode, respectively. A method for removing nitrogen oxide is characterized in that a gas to be treated containing nitrogen oxide is flowed so as to come into contact with the electrode layer.

A fifth aspect of the present invention is the method for removing nitrogen oxides according to the fourth aspect, characterized in that the treatment member is heated to 400 ° C to 1000 ° C. A sixth aspect of the present invention is the method for removing nitrogen oxides according to the fourth or fifth aspect, characterized in that the solid electrolyte ceramic is pre-stabilized zirconia.

Here, the perovskite type ceramics having electron conductivity and ionic conductivity used for the first and second electrode layers of the present invention is represented by the general formula ABO ₃ (A: rare earth element, B: metal element, O: It is a ceramic having a structure represented by (oxygen) and exhibits decomposition catalytic activity for various substances depending on its composition. In the present invention, it is preferable to use one having a chemical composition that selectively has a high catalytic activity with respect to nitrogen oxides, and specifically, LaMO ₃ (M =
Mn, Fe, Ni, Rh), La _1-x A _x MnO ₃ (A =
Ca, Sr, Ba, Hf), La _1-x A _x Mn _{1 -y} Ru _y O
_It is preferable to use ₃ (A = K, Pb), La ₂ Cu _1-y Zr _y O ₄ ) or the like.

The solid electrolyte ceramics having oxygen ion conductivity used in the present invention include yttria (Y ₂ O ₃ ), calcia (Ca ₂ O ₃ ), Sc ₂ O ₃ and Yb ₂
Zirconia (ZrO ₂ ), ceria (CeO ₂ ), and thoria (Th) containing 7 to 20% by weight of O ₃ , Gd ₂ O _{3 and the} like.
O ₂ ) and the like can be mentioned. Typically, there is stabilized zirconia which is yttria-containing zirconia, and this stabilized zirconia can be preferably used.

In the present invention, the removal rate of nitrogen oxides is greatly affected by the migration rate of oxygen ions (O ²⁻ ) generated by the decomposition of nitrogen oxides in the solid electrolyte,
Almost match. Here, using the formula of the current flowing through the conductor shown below, the electron charge is 1.6 × 10 ⁻¹⁹ , the Avogadro's number is 6.02 × 10 ²³ , and the moving speed of the ion (value of n × v) ) Is shown in Table 1 below.

[0015]

## EQU1 ## I = nvze I: current [A] or [C / cm ² ] n: density of charge [cm ⁻³ ] v: velocity of charge [cm / sec] z: valence of charged particles e: electron Charge [C]

[0016]

[Table 1]

As is clear from the numerical values shown in Table 1,
It can be seen that the moving speed (n × v) of oxygen ions in the solid electrolyte is proportional to the current. On the other hand, in general, the concentration of nitrogen oxides in exhaust gas is 1600 ppm in natural gas combustion,
Since it is 1200ppm in automobile exhaust gas, 50
It is necessary to pass a current of mA or more between the electrodes sandwiching the solid electrolyte.

Further, for example, the electric resistance value of the stabilized zirconia ceramics greatly depends on its temperature as shown in FIG. Current, voltage and resistance are Ohm's law V = I
According to R, at a temperature of 200 ° C. or lower, a voltage of about 1000 V is required to obtain a current of about 50 mA, which is not realistic. Therefore, it is desirable to heat the solid electrolyte to 400 ° C. to 1000 ° C. in consideration of the configuration of the entire device.

In the present invention, the power source means a DC power source,
10 to 100 mA, preferably 30 to 6 for the solid electrolyte
A constant current power source that supplies a current of about 0 mA, or 1 to 10
A constant voltage power source for applying a voltage of 0 V, preferably 3 to 60 V may be used.

[0020]

The perovskite type ceramic electrode has a catalytic activity for decomposing nitrogen oxides and decomposes nitrogen oxides such as NO and NO ₂ into nitrogen and oxygen. The mechanism of such decomposition is considered to be (1) adsorption of NO, (2) dissociation of NO, and (3) desorption of N ₂ and O ₂ . Further, in the present invention, oxygen ion conductive solid electrolyte ceramics are arranged between the perovskite oxide ceramics electrodes and a voltage is applied between the electrodes, so that in addition to the catalytic activity of the perovskite oxide ceramics electrode Since the oxygen reduction action is added, the decomposition of nitrogen oxides on the electrode surface is further promoted.

In the present invention, NO in the exhaust gas, first, is adsorbed on the surface of the first electrode (cathode) by its affinity with the perovskite type oxide. Next, the NO molecules are forcibly dissociated into nitrogen radicals (N.) and oxygen ions (O ^2- ) by the electrons supplied from the electrodes. The stability of the nitrogen radical is very small, and it immediately reacts with the same type of radical in the vicinity to become N ₂ and is desorbed from the electrode. On the other hand, oxygen ions move to the second electrode (anode) in the solid electrolyte according to the potential gradient between the electrodes. The oxygen ions reaching the anode again release electrons at the interface between the anode and the electrolyte to become O ₂ gas, which is released outside the electrode. Oxygen ions generated by the decomposition of nitrogen oxides at the first electrode are pulled to the second electrode side through the solid electrolyte, so that the decomposition of nitrogen oxides is further promoted.

As described above, when the exhaust gas containing nitrogen oxides is caused to flow from one end of the tubular member to the first electrode side, the nitrogen oxides in the exhaust gas are electrochemically decomposed into nitrogen gas. It is discharged from the other end.

[0023]

EXAMPLES The present invention will be described in detail below with reference to examples.

FIG. 1 shows an outline of an exhaust gas treatment apparatus according to one embodiment. As shown in FIG. 1, the treatment member 10 has a three-layered tubular shape, and a first electrode 11 made of LaFeO _{3 is formed on} both inner and outer surfaces of a moving layer 13 made of stabilized zirconia.
And the second electrode 12 is provided. Specifically, the processing member 10 is a stabilized zirconia tube (ZrO ₂ -1) having an inner diameter of 7 mm, an outer diameter of 10 mm, and a length of 300 mm.
0% Y ₂ O ₃ ) LaF on both inner and outer sides of the central part 100 mm
The electrode layer of eO ₃ is formed by a firing method.

The central portion of the processing member 10 having the electrodes is covered with a cylindrical ceramic heater (inner diameter 15 mm) 14 with a temperature controller. The heater 14 is not particularly limited in its method as long as it is a known heating means capable of heating the treatment member 10 to, for example, 400 ° C. or higher, and an ordinary electric heater or the like may be used.

A DC power source 15 composed of a constant current power source (model 237 manufactured by Kessley Co., Ltd.), a first electrode 11 is a cathode, and a second electrode 12 is a first electrode 11 and a second electrode 12.
Is connected through a platinum wire so that it becomes the anode.

When the exhaust gas 16A containing nitrogen oxides is flown into the processing member 10, it is discharged as exhaust gas 16B from which nitrogen oxides have been removed. NO in the exhaust gas 16A,
Nitrogen oxides such as NO ₂ are decomposed on the first electrode 11 to become N ₂ gas, and oxygen is used as oxygen ions to form the moving layer 1.
Taken in 3. Similarly, the oxygen gas contained in the exhaust gas 16A is also reduced and converted into oxygen ions in the moving layer 13A.
Taken in. Then, the oxygen ions thus taken into the moving layer 13 reach the second electrode 12, release electrons, and are discharged to the outside as oxygen gas.

Pyrex glass tubes (not shown) are connected to both ends of the processing member 10 of the exhaust gas treatment device with an instant adhesive (trade name: Aron Ceramic), and an NO concentration meter (made by Best Sokki Co., Ltd.) not shown is provided at the outlet end. Connected. Then, the heater 14 is heated to 500 ° C., and N ₂ gas is used as a balance gas from the inlet end and NO is 500 to 1200 ppm.
The simulated exhaust gas having the concentration of 100 ml / min was passed, and the current shown below was passed between the electrodes 11 and 12 to measure the NO concentration at the outlet end, and the results shown in Table 2 below were obtained.

[0029]

[Table 2]

The treatment apparatus described above is capable of effectively and continuously removing nitrogen oxides, and is particularly suitable for use in automobile exhaust gas treatment, garbage incinerator exhaust gas treatment and the like. is there. Further, for example, when it is used for exhaust gas treatment of an automobile, it can be used together with a treatment device using a three-way catalyst so as to effectively treat carbon dioxide and sulfur oxides.

Such a treatment apparatus is for treating nitrogen oxides electrochemically, and is a kind of exhaust gas to be treated.
The nitrogen oxide can be effectively treated without being largely affected by the treatment conditions and the like.

Although the treatment member is tubular in the above embodiment, it is not particularly limited as long as it can effectively contact the gas to be treated with the first electrode.
The gas to be processed may be caused to flow in the flow path that comes into contact with the electrode.

[0033]

As described above, according to the present invention,
The nitrogen oxides in the exhaust gas can be treated effectively and continuously.

[Brief description of drawings]

FIG. 1 is a cross-sectional view schematically showing an exhaust gas treatment device according to an embodiment of the present invention.

FIG. 2 is a graph showing the relationship between the electric resistance value of zirconia ceramics and temperature.

[Explanation of symbols]

 10 Processing Member 11 First Electrode 12 Second Electrode 13 Moving Layer 14 Heater 15 DC Power Supply 16A, 16B Exhaust Gas

Claims (6)

[Claims] 1. A first perovskite ceramic which is arranged so as to come into contact with a flow of a gas to be treated containing nitrogen oxides and which has electron conductivity and ion conductivity.
Electrode layer, an ion transfer layer which is provided in contact with the first electrode and is made of solid electrolyte ceramics having oxygen ion conductivity, and the first transfer layer of the ion transfer layer.
A treatment member comprising a second electrode layer made of perovskite-type ceramics, which is provided in contact with the other side of the electrode and has electron conductivity and ionic conductivity; and this treatment member is kept at a predetermined temperature. A heating means; and a power supply for applying a voltage so that the first electrode layer and the second electrode layer of the processing member serve as a cathode and an anode, respectively; . 2. The nitrogen oxide removing apparatus according to claim 1, wherein the processing member is a tubular member having the first electrode inside. 3. The nitrogen oxide removing apparatus according to claim 1 or 2, wherein the solid electrolyte ceramic is pre-stabilized zirconia. 4. A first electrode layer made of perovskite type ceramics having electronic conductivity and ionic conductivity,
The ion transfer layer is provided in contact with the first electrode and is made of a solid electrolyte ceramic having oxygen ion conductivity, and is provided in contact with the ion transfer layer opposite to the first electrode. Further, a treatment member including a second electrode layer made of perovskite-type ceramics having electron conductivity and ion conductivity is maintained at a predetermined temperature, and the first electrode layer and the second electrode layer respectively serve as a cathode and a cathode. A method for removing nitrogen oxides, wherein a voltage is applied so as to serve as an anode, and a gas to be treated containing nitrogen oxides is caused to flow so as to come into contact with the first electrode layer of the treatment member. 5. The treatment member according to claim 4, wherein
A method for removing nitrogen oxides, which comprises heating to 0 ° C to 1000 ° C. 6. The method for removing nitrogen oxide according to claim 4, wherein the solid electrolyte ceramic is pre-stabilized zirconia.