JPH0824579A - Treatment of low concentration nox containing gas - Google Patents

Abstract

PURPOSE:To efficiently remove a low concn. NOX incorporated in a ventilation gas, etc., from an underground road, etc., by subjecting low concn. NOX- containing gas to corona discharge treatment in the presence of a catalyst to oxidize at least a part of NO in NOX to NO2 and removing formed NOX (NO+NO2) with an adsorbent. CONSTITUTION:At least a part of NO in low concn. NOX-containing gas 1 is oxidized to NO2 by the corona discharge treatment in the presence of an oxidizing catalyst in a discharge reaction vessel 3 and the formed NOX (NO+NO2) is adsorbed by an adsorption tower 4 or 6 and removed. And, corona discharge power is controlled so that a ratio of the NO2 incorporated in the gas after oxidizing NOX may be NO/(NO+NO2)=0.3-0.7 (mol. ratio). Moreover, an air heated with an air heating device 7 is introduced into the adsorption tower 6 and the adsorbed NO and NO2 are removed in a concentrated state. A reducing agent 8 (e.g. ammonia) is added to this gas, then this gas is introduced to a denitrification catalyst tower 9 to reductively purify.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for removing nitrogen oxides (NOx) contained in the atmosphere, and particularly to a low concentration contained in ventilation gas from an automobile tunnel, an underground road or an indoor automobile parking lot. The present invention relates to a method for efficiently removing NOx.

[0002]

2. Description of the Related Art Generally, as a method for detoxifying and removing NOx in exhaust gas, a selective catalytic reduction method in which ammonia is added to NOx-containing exhaust gas and decomposed into harmless N ₂ on a catalyst is mainly applied in a thermal power plant. On the other hand, in a gasoline engine vehicle, a method of reducing and decomposing NOx on a three-way catalyst is applied. All of these methods have a reaction temperature of usually 200 ° C. or higher, and are economical and excellent methods for treating high-temperature exhaust gas, but treat a large amount of gas at room temperature such as tunnel ventilation gas. In some cases it is difficult to adapt. Therefore, the method for treating dilute NOx in the atmosphere is to concentrate NOx in advance with an adsorbent and then add ammonia to a heated high-concentration NOx-containing gas to remove harmless N 2 by the selective catalytic reduction method.
A method of decomposing it into ₂ and releasing it (Japanese Patent Publication No. 5-78369) has been proposed. However, generally 90% or more of the NOx discharged into the atmosphere is NO with poor adsorptivity, and when the process gas contains water, the NO adsorption performance is extremely deteriorated. It is necessary to dehumidify beforehand. Therefore, a dehumidifying agent is required in addition to the NOx adsorbent, which causes an increase in processing cost.

On the other hand, if the form of NOx is NO ₂ , it is relatively easy to remove NOx with an adsorbent or with an alkali even in the presence of water, so that a low concentration of NOx is obtained.
NO in the contained gas is oxidized to NO ₂ by corona discharge,
A method of absorbing and removing the NO ₂ with an alkaline absorbing liquid (Japanese Patent Laid-Open No. 6-99030), or NO ₂ by ozone oxidation
It has been proposed to oxidize and remove NO ₂ by adsorption. These methods have a problem that a large amount of electric power is consumed as energy for oxidizing NO to NO ₂ . Therefore, how to reduce power consumption is a practical point.

[0004]

DISCLOSURE OF THE INVENTION The present invention solves the above-mentioned drawbacks of the prior art, and has a low concentration of N contained in ventilation gas from an automobile tunnel, an underground road or an indoor automobile parking lot.
The purpose is to efficiently remove Ox.

[0005]

SUMMARY OF THE INVENTION A low concentration of N according to the invention.
A method of efficiently removing Ox is to oxidize at least a part of NO in NOx to NO ₂ by performing corona discharge in a low-concentration NOx-containing gas in the presence of a catalyst to generate NO.
x (NO + NO ₂ ) is removed by the adsorbent. Conventionally, it has been known that NO can be oxidized to NO ₂ by corona discharge, but the present inventors have made detailed studies, and by performing corona discharge in the presence of an oxidation catalyst, NO can be further efficiently consumed with low power consumption. It has been discovered that it can be oxidized to NO ₂ .

Still another method of the present invention is to change the ratio of NO ₂ contained in the gas after NO is oxidized to NO /
The corona discharge power is controlled so that (NO + NO ₂ ) (molar ratio) = 0.3 to 0.7. NO for all N
The required power consumption can be reduced to about half as compared with the case of oxidizing to O ₂ .

By using the above two methods in combination, NOx can be removed more efficiently.

In the present invention, NO removed by the adsorbent
x is characterized in that the adsorbent is desorbed as a concentrated gas by heating and regenerating the adsorbent, and after reducing agent is added, it is reductively decomposed into N ₂ on the catalyst, but this method is not always necessary. That is, a method of washing and regenerating the adsorbent is also possible.

The oxidation catalyst used in the present invention is not particularly limited as long as it is a catalyst having NO oxidation performance, but Pt, Pd, R
A catalyst in which a noble metal such as h or Ru is supported on a carrier such as alumina, titania or silica, a transition metal oxide such as Co, Mn, Ni or Fe, or a composite oxide catalyst thereof can be used. Also, a complex oxide such as barium titanate having a high dielectric constant is a good catalyst. Further, zeolite or activated carbon, or a catalyst in which a transition metal oxide such as Co, Mn, Ni or Fe or a noble metal such as Pt, Pd, Rh or Ru is supported on these is also preferable. Normally, these catalysts do not show NO oxidizing ability at high temperatures, but in a corona discharge field, they accelerate NO oxidation even at room temperature.

The catalyst may be used in the form of particles, honeycombs, etc. in the corona discharge reactor,
It is also possible to apply these catalysts to the inner wall of the reactor.

The corona power consumption per unit NO of corona discharge is preferably in the range of 0.01 to 0.2 Wh / m ³ · ppm. Below this, the oxidation rate of NO is too low to be effective, and above this, power consumption is too high and it is not economical.

[0012]

In the method for removing low concentration NOx according to the present invention,
By performing the corona discharge in the presence of the NO oxidation catalyst, it is possible to significantly reduce the power consumption as compared with the corona discharge alone. Normally, the NO oxidation catalyst shows no activity unless it is at a temperature of 300 ° C. or higher, but it shows the NO oxidation ability even at room temperature in the corona discharge field. The reason for this is not always clear at present, but one of the reasons is that the reaction between the NO activated by adsorption on the catalyst and the active oxygen species generated by the corona discharge easily proceeds on the catalyst. Conceivable. Since this reaction is faster than the reaction between NO and active oxygen in the gas phase,
It is thought to proceed efficiently. Therefore, it was found that in the presence of a catalyst, similar NO oxidation efficiency can be obtained even if the power consumption is reduced to about half that of corona discharge alone.

According to another method of the present invention, NOx is used.
The ratio of NO ₂ contained in the gas after oxidizing the NO
_{/ (NO + NO 2) =} 0.3~0.7 and controlling the corona discharge power so. According to detailed experiments by the present inventors,
All NO not expedient to the NO _2, the ratio of NO ₂ contained in the gas after oxidizing the NO NO / (NO +
NO ₂ ) (molar ratio) = 0.3 to 0.7, NO
It was found that the NOx adsorption performance was sufficiently exhibited even in the presence of water, and the adsorption capacity was double or more as compared with the case of only. That is, in the conventional adsorbents, but towards the form of NOx is oxidized to NO ₂ is advantageous over adsorption, NO / NO ₂ (molar ratio) as a composition of NOx = 1/1 in the same amounts and around its long if, without necessarily oxidized to nO _2, inferior no NOx adsorption performance could be obtained in the nO ₂ gas. The reason for this is not clear at present, but the reaction of NO + NO ₂ = N ₂ O ₃ occurs on the adsorbent,
It is considered that the adsorption is promoted. In this case, N
Power consumption is about 1 compared to the case of oxidizing all O into NO _2.
It can be less than / 2.

Further, the composition of NOx in the desorbed gas is NO / N.
If the O ₂ (molar ratio) is in the vicinity of 1/1, the reaction rate in the ammonia reduction denitration method is remarkably higher than that in the case of the gas of NO or NO ₂ alone, so that there is also an advantage that the reactor can be made compact.

Further, in this way, the composition of NOx is NO /
When (NO + NO ₂ ) = 0.3 to 0.7 is controlled, in the process of decomposing the desorbed gas into N ₂ by the ammonia reduction method and rendering it harmless,
It turned out to be very advantageous. That is, it is generally said that the catalytic reduction reaction of NO and NO ₂ with ammonia proceeds according to the (formula 1) and (formula 2), but in the case of an equal mixture of NO and NO ₂ , it reacts according to the formula (formula 3). Moreover, it proceeds at a lower temperature and a higher speed than the formulas (1) and (2). Therefore, if the composition of NOx in the process gas can be controlled to NO / (NO + NO ₂ ) = 0.3 to 0.7, which is close to an equimolar mixture of NO and NO ₂ , the ammonia catalytic reduction reaction step will be performed.
Compared with the case of treating exhaust gas with only O or NO _2, there is a great advantage that it can be carried out economically.

[0016]

[Formula 1] NO + NH ₃ + 1 / 4O ₂ = N ₂ + 3 / 2H ₂ O (Formula 1)

[0017]

Embedded image 6NO ₂ + 8NH ₃ = 7N ₂ + 12H ₂ O (Formula 2)

[0018]

Embedded image NO + NO ₂ + 2NH ₃ = 2N ₂ + 3H ₂ O (Chemical Formula ₃ ) The catalyst is not particularly limited in the decomposition of NOx by the ammonia reduction method, but a titanium oxide-based or zeolite-based catalyst is preferable.

As the method for treating NOx after concentration, a reduction method using an oxygen-containing hydrocarbon compound such as urea, various hydrocarbons, alcohols, etc. can be used in addition to the ammonia reduction method.

[0020]

【Example】

(Example 1) An example of the NOx removal process to which the present invention is applied will be described with reference to the overall flow of FIG. The low concentration NOx-containing exhaust gas 1 is introduced into the discharge reaction device 3 by the fan 2. In the discharge reactor 3, a part of NO in the exhaust gas is oxidized to NO ₂ by the corona discharge and the oxidation catalyst. The exhaust gas after the oxidation treatment is introduced into the adsorption tower 4, where NO and NO ₂ are adsorbed and removed, and then discharged as the purified gas 5.

The adsorption tower 4 is paired with the adsorption tower 6, which are switched at regular intervals to adsorb the adsorbent and purify concentrated NO and NO ₂ by the following method. The air heated by the air heating device 7 is introduced into the adsorption tower 6, and the adsorbed NO and NO ₂ are desorbed in a concentrated state. After adding the reducing agent 8 to this gas, it is introduced into the denitration catalyst tower 9 for reduction purification. For example, a system using ammonia as the reducing agent 8 and a titanium oxide-based denitration catalyst as the denitration catalyst is possible. The purified gas is circulated through the adsorption tower 4 again by the fan 10, and then released as purified gas 5 into the atmosphere.

(Example 2) An example of a corona discharge reactor to which the method of the present invention is applied will be described with reference to the sectional view of the discharge reaction tube shown in FIG. The reaction tube has a cylindrical shape, and it is possible to configure a reaction device in which the required number of reaction tubes are arranged in parallel according to the amount of exhaust gas to be introduced and the required purification performance.

The exhaust gas 11 containing lean NO is introduced into a ceramic discharge reaction tube 12. A center electrode 13 is installed inside the reaction tube, an outer electrode 14 is installed on the outer wall, and an oxidation catalyst 15 is applied to the inner wall of the discharge reaction tube. 13 and 1
Corona discharge is performed by applying electric power during 4 and the oxidation reaction of NO to NO ₂ proceeds in the discharge field 16. At the same time, due to the action of the oxidation catalyst 15, the oxidation reaction proceeds more efficiently. The exhaust gas 17 after the oxidation treatment is NO and N
Both O ₂ are contained, and the treatment with the adsorbent and the denitration catalyst installed downstream can be efficiently performed.

Example 3 The NO oxidation reaction using the discharge reaction tube shown in Example 2 was carried out. A cylindrical tube of mullite porcelain having an outer diameter of 41 mm, an inner diameter of 35 mm and a length of 400 mm was used as the discharge reaction tube, and an oxidation catalyst was applied to the inner surface. A platinum zeolite catalyst obtained by wet-kneading dinitrodiamineplatinic acid and ZSM-5 zeolite and then calcining in air at 500 ° C. for 2 hours was used as the oxidation catalyst. The platinum content of this catalyst was 0.5% by weight. The oxidation catalyst was applied to the inner surface of the cylindrical tube by the following method. Platinum zeolite catalyst powder and water were thoroughly mixed at a weight ratio of 15:85 to prepare a slurry, which was poured into the inner surface of the cylindrical tube and wash-coated so as to be adhered substantially uniformly. After that, after removing the extra slurry attached to the cylindrical tube, it was dried in air and further calcined in air at 500 ° C. for 2 hours. This coating and firing operation was repeated several times to form an oxidation catalyst layer.

A stainless steel threaded rod having an outer diameter of 20 mm was used for the center electrode. Aluminum foil is used for the external electrodes,
The cylindrical tube was wrapped so that it completely covered the outer wall. The length of the discharge part was about 200 mm.

Simulated exhaust gas having the composition shown in Table 1 was passed through the reaction tube at a flow rate of 0.12 m ³ / hr, and one hour after that, corona discharge was started at a discharge power of 0.01 Wh. Immediately before and 1 hour after the start of discharge, NO in the discharge reaction tube outlet gas
Table 2 shows the results of measuring the NO ₂ and NO ₂ concentrations with a chemiluminescent nitrogen oxide analyzer.

[0027]

[Table 1]

[0028]

[Table 2]

Corona discharge causes about 50% of NO to be NO ₂
It was found that the reaction tube outlet gas contained both NO and NO ₂ after being oxidized.

Comparative Example 1 The same experiment as in Example 3 was conducted using a discharge reaction tube not coated with an oxidation catalyst.
The amount of gas flowing through the reaction tube, gas composition, operating conditions, etc. were the same as in Example 3. After the start of the corona discharge, the corona discharge power was adjusted at any time so that approximately 1/2 of the NO introduced into the reaction tube was oxidized to NO ₂ . As a result of measuring the discharge power 1 hour after the start of discharge, the power required was 0.054 Wh.

This result shows that higher electric power is required as compared with the case where the discharge reaction tube coated with the oxidation catalyst of Example 3 is used, and the oxidation catalyst should be installed near the discharge field. It was found that NO oxidation is promoted and the discharge power can be reduced.

(Example 4) The same experiment as in Example 3 was repeated except that 0.5% by weight of palladium (Pd) was used as an oxidation catalyst in Al ₂ O _3.
Was carried out using the catalyst supported on. The catalyst was obtained by wet kneading a predetermined amount of palladium nitrate with Al ₂ O ₃ powder together with water,
It was obtained by firing in air at 500 ° C. for 2 hours. The amount of gas flowing through the reaction tube, gas composition, discharge power, and operating conditions were the same as in Example 3.

Table 3 shows the results of measurement of NO and NO ₂ concentrations at the outlet of the discharge reaction tube immediately before and 1 hour after the start of discharge.

[0034]

[Table 3]

About 60% of NO is NO ₂ due to corona discharge
It was found that the reaction tube outlet gas contained both NO and NO ₂ after being oxidized.

(Embodiment 5) A part of NO is converted to N in the discharge reaction tube.
The adsorption characteristics of the exhaust gas oxidized to O ₂ by the adsorbent were examined. As the adsorbent, γ-Al ₂ O ₃ having a particle size of 0.85 to 1.7 mm is used.
The 6 cm ³ used, and filled in the quartz cylinder tube having an inner diameter of 20mm and baked 1 hour at previously 500 ° C. in air, which was used as the adsorption tower.

Exhaust gas having a composition shown in Table 4, Example gas composition 5 in an adsorption tower kept at a temperature of 25 ° C. was 0.12 m ³ / h.
flow at a flow rate of r
The NO ₂ concentration was measured, and the NOx removal rate was measured by the following equation (1). Here, NOx is a general term for NO and NO ₂ .

[0038]

[Table 4]

[0039]

[Equation 1]

The NOx removal rate shows almost 100% at the start of NOx adsorption, and gradually decreases with time. When the NOx adsorption reaches saturation, the removal rate becomes almost 0%. The NOx removal rate is continuously measured from the start of adsorption until the removal rate drops to 80%, that is, 8
Table 5 shows the time when it was 0% or more.

[0041]

[Table 5]

(Examples 6 and 7) Example gas composition 6 in Table 4
Using the exhaust gases of Nos. 7 and 7, the adsorption performance test was conducted in the same manner as in Example 5. The results are shown in Table 5.

Comparative Examples 2 and 3 Comparative Example Gas Composition 2 in Table 4
Using the exhaust gases of Nos. 3 and 3, the adsorption performance test was conducted in the same manner as in Example 5. The results are shown in Table 5.

In the case of Examples 5 to 7 in which the exhaust gas contains both NO and NO ₂ , it is possible to maintain a high NOx removal rate for a significantly longer time than in Comparative Example 2 in which the exhaust gas contains only NO. found. This result shows that by oxidizing a part of NO in the exhaust gas to NO ₂ , the NOx adsorption performance of the adsorbent is sufficiently exhibited and the adsorption capacity is significantly improved. Comparative Example 3 is a case where all NO in the exhaust gas is oxidized to NO ₂ , but the adsorption performance is not much different from that of Examples 5 to 7, and high NOx adsorption performance can be obtained without necessarily oxidizing all NO to NO _2. It has been found.

(Embodiment 8) FIG. 3 shows the N to which the present invention is applied.
It is an example of an Ox removal process. The low concentration NOx-containing exhaust gas 1 is introduced into the discharge reaction device 3 by the fan 2. In the discharge reactor 3, a part of NO in exhaust gas is oxidized to NO ₂ by corona discharge and an oxidation catalyst. The exhaust gas after the oxidation treatment is introduced into the adsorption tower 4, where NO and NO ₂ are adsorbed and removed, and then discharged as the purified gas 5.

The adsorption tower 4 has a movable adsorbent unit 18
~ 21 are loaded. One of these units on the exhaust gas inlet side is taken out and loaded into the adsorption tower 6, and after the adsorbed NO and NO ₂ are purified, the adsorption tower 4 can be loaded again.

Adsorbent unit 22 loaded in adsorption tower 6
Purifies the adsorbed NO and NO ₂ by the following method.
The air heated by the air heating device 7 is introduced into the adsorption tower 6 to desorb NO and NO ₂ adsorbed in the adsorbent unit 22 in a concentrated state. After adding the reducing agent 8 to this gas, it is introduced into the denitration catalyst tower 9 for reduction purification. The purified gas is circulated through the adsorption tower 4 again by the fan 10, and then released as purified gas 5 into the atmosphere.

The adsorbent unit 22 for which the purification process has been completed
Is loaded inside the adsorption tower 4 downstream of the adsorbent unit 21, and a new unit 18 is loaded into the adsorption tower 6. After that, the adsorbent units 19 to 22 inside the adsorption tower 4 are moved to the exhaust gas inlet side, and the state shown in FIG. 4 is obtained.

By successively taking out the adsorbent unit and performing the purifying process in this manner, the amount of the adsorbent used can be reduced and the NOx can be purified more efficiently.

[0050]

According to the present invention, low concentration NOx in the atmosphere is obtained.
Since it can be efficiently removed, it is effective in purifying NOx contained in ventilation gas from an automobile tunnel or an underground parking lot.

[Brief description of drawings]

FIG. 1 is a block diagram of an exhaust gas purification system according to an embodiment of a method of the present invention.

FIG. 2 is an explanatory view showing an example of a reaction apparatus used in the method of the present invention.

FIG. 3 is a block diagram of an exhaust purification system according to a second embodiment of the method of the present invention.

FIG. 4 is a block diagram of an exhaust gas purification system according to a third embodiment of the method of the present invention.

[Explanation of symbols]

1 ... Exhaust gas containing low-concentration NOx, 2 ... Fan, 3 ... Discharge reaction device, 4, 6 ... Adsorption tower, 5 ... Purification gas, 7 ... Air heating device, 8 ... Reducing agent, 9 ... Denitration catalyst tower, 10 ... Fan .

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical display location B01D 53/34 ZAB 53/56 53/74 53/86 ZAB B01D 53/34 129 C 53/36 ZAB (72) Inventor Hiroshi Miyadera 1-1-1, Omika-cho, Hitachi-shi, Ibaraki Prefecture Hitachi Ltd. Hitachi Research Laboratory

Claims (6)

[Claims] 1. At least part of NO in a gas containing low concentration NOx is converted to N by corona discharge in the presence of an oxidation catalyst.
A method for treating a low-concentration NOx-containing gas, which comprises oxidizing to O ₂ . 2. At least a part of NO in a low concentration NOx-containing gas is subjected to corona discharge in the presence of an oxidation catalyst.
_A method for treating a low-concentration NOx-containing gas, which comprises oxidizing to ₂ and removing generated NOx (NO + NO ₂ ) by adsorption or absorption. 3. The ratio of NO ₂ contained in the gas after oxidizing NOx according to claim 1, is NO / (NO + N
A method for treating a low-concentration NOx-containing gas, which controls the corona discharge power so that O ₂ ) = 0.3 to 0.7 (molar ratio). 4. The low-concentration NOx desorbed as NOx removed by the adsorbent as a concentrated gas by regenerating the adsorbent as a concentrated gas by addition of a reducing agent, and then reducing and decomposing into N ₂ on the denitration catalyst. Treatment method of contained gas. 5. The method for treating a low-concentration NOx-containing gas according to claim 1, 2, 3 or 4, wherein the low-concentration NOx-containing gas is an exhaust gas from an automobile tunnel. 6. NO in a gas containing low concentrations of NOx is oxidized to at least part of NO ₂ by corona discharge, and the NOx is discharged.
In a method of condensing (NO + NO ₂ ) by adsorption or absorption and then releasing it again to purify it on a catalyst in the presence of a reducing agent, NO contained in the gas after oxidizing NOx
_A method for treating a low-concentration NOx-containing gas, which comprises controlling the corona discharge power so that the ratio of ₂ becomes NO / (NO + NO ₂ ) = 0.3 to 0.7 (molar ratio).