JP3371127B2 - Exhaust gas purification method - Google Patents

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 from combustion exhaust gas containing nitrogen oxides and excess oxygen, which is capable of effectively removing nitrogen oxides .

[0002]

2. Description of the Related Art Excessive amounts of combustion exhaust gas emitted from internal combustion engines such as automobile engines, combustion equipment installed in factories, household fan heaters, etc. Nitrogen oxides such as nitric oxide and nitrogen dioxide are contained together with oxygen. Here, "containing excess oxygen" means containing more oxygen than the theoretical oxygen amount necessary to burn unburned components such as carbon monoxide, hydrogen, and hydrocarbons contained in the exhaust gas. To do. Moreover, the nitrogen oxide in the following refers to nitric oxide and / or nitrogen dioxide.

This nitrogen oxide is considered to be one of the causes of acid rain and is a serious environmental problem. Therefore, various methods for removing nitrogen oxides in exhaust gas discharged from various combustion devices have been studied.

As a method for removing nitrogen oxides from combustion exhaust gas containing excess oxygen, a selective catalytic reduction method using ammonia is used, particularly for large-scale fixed combustion devices (large combustors such as factories). It has been put to practical use.

However, in this method, ammonia used as a reducing agent for nitrogen oxides is expensive, and ammonia is toxic, so that unreacted ammonia is discharged so that nitrogen oxides in exhaust gas are not discharged. There are problems that the amount of ammonia injection must be controlled while measuring the concentration and that the apparatus is generally large.

Therefore, there has been proposed a method for removing nitrogen oxides by adding a reducing agent in an amount equal to or less than a theoretical reaction amount with oxygen in exhaust gas by using zeolite or a catalyst supporting a transition metal thereon (for example, a special method). Kaisho 63-100919, 63-28372
No. 7, JP-A-1-130735, and 59th Annual Meeting of the Chemical Society of Japan
(1990) 2A526, 60th Autumn Meeting (1990) 3L420, 3L42
2, 3L423, "Catalyst" vol.33 No.2, page 59, 1991 etc.). In addition, a method using a catalyst supporting silver has been proposed (JP-A-4-218844).

However, although these methods can remove nitrogen oxides with high efficiency from simulated exhaust gas that does not contain water, the actual exhaust gas contains about 10% of water, so It was found that the oxide removal rate was significantly reduced. Also, with these methods,
There is also the disadvantage that the optimum temperature for the reduction reaction of nitrogen oxides is as high as 400 to 600 ° C. Further, a catalyst supporting silver or silver oxide has a drawback that the catalytic activity is lowered when exposed to light and the removal rate of nitrogen oxides is lowered.

Therefore, an object of the present invention is to remove nitrogen oxides, carbon monoxide, hydrogen, hydrocarbons, etc., such as combustion exhaust gas from a fixed combustion apparatus and a gasoline engine, a diesel engine, etc. that burn under an excess oxygen condition. From the combustion exhaust gas that contains more than the theoretical reaction amount of oxygen for unburned components,
An object of the present invention is to provide a method for removing nitrogen oxides, which is optically stable and can effectively remove nitrogen oxides .

[0009]

As a result of earnest research in view of the above problems, the present inventor has found that a removal material formed by supporting a specific amount of silver chloride on a porous inorganic oxide is used to remove nitrogen contained in exhaust gas.
Hydrocarbons or alcohols corresponding to the amount of elementary oxides
The exhaust gas added with is contacted with the above-mentioned removal material at a specific temperature and
Moisture content of approx. 10%
Nitrogen oxides are optically stable against the exhaust gas
The present invention has been completed by discovering that it can be effectively removed .

[0010]

That is, the nitrogen oxide removing method of the present invention for removing nitrogen oxides from a combustion exhaust gas containing nitrogen oxides and oxygen in a larger amount than the theoretical reaction amount for coexisting unburned components is a porous inorganic oxide. Silver chloride to 100% by weight
The nitrogen oxide removing material supported by 0.2 to 15% by weight is installed in the middle of the exhaust gas conduit, and hydrocarbons or alcohols are added to the exhaust gas on the upstream side of the removing material,
The nitrogen oxide is removed by bringing the exhaust gas into contact with the removing material at 200 to 600 ° C. and reacting the hydrocarbon or alcohol with the nitrogen oxide.

The present invention will be described in detail below. In the present invention, using a removal agent shown below, by contacting the exhaust gas to the removal member, hydrocarbons or alcohols upstream of the installation site of the residual hydrocarbons及beauty dividing Sazai in the exhaust gas is added to the exhaust gas It s the reduction removal of nitrogen oxides in the exhaust gas as a reducing agent.

First, the removing material of the present invention comprises silver chloride supported on a porous inorganic oxide. As the porous inorganic oxide, porous alumina, titania, zirconia, and their composite oxides can be used, but γ-alumina or alumina-based composite oxide is preferably used.
By using γ-alumina or an alumina-based composite oxide, the reaction between the added hydrocarbon or alcohol and the nitrogen oxide in the exhaust gas occurs efficiently.

The specific surface area of the porous inorganic oxide is 10 m ²
/ G or more is preferable. Specific surface area of 10 m ² / g
If it is less than the above range, the dispersion of the silver component (silver active species) in the inorganic oxide becomes poor, and the nitrogen oxide cannot be removed satisfactorily. The specific surface area of the porous inorganic oxide is preferably 30 m ² / g or more.

The removing material of the present invention can be used in the form of pellets, powder, honeycomb, foam, plate or the like. A preferred form of the removing material of the present invention is to coat the surface of a three-dimensional structure such as a ceramic or metal honeycomb-like or foam-like one with a catalyst in which silver chloride is supported on a porous inorganic oxide, or the above three-dimensional structure. It is prepared by coating the body with an inorganic oxide and then supporting silver chloride. Examples of the ceramic material include cordierite and mullite, which have excellent heat resistance. The inorganic oxide is coated on the three-dimensional structure by a known wash coating method or the like. Another preferred form of the removing material of the present invention is a pellet-like porous inorganic oxide loaded with silver chloride.

The amount of silver chloride supported on the above-mentioned inorganic oxide such as γ-alumina is 0.2 to 15% by weight (in terms of silver element), with 100% by weight of the inorganic oxide.
And When the content of silver chloride is less than 0.2% by weight (converted to silver element), the removal rate of nitrogen oxides on the low temperature side decreases. Further, when the amount of silver that exceeds 15% by weight (converted to silver element) is carried, the hydrocarbon itself is easily burned, and the nitrogen oxide removal rate is rather lowered. Preferably, the supported amount of silver exceeds 15% by weight with respect to 100% by weight of the inorganic oxide.
Weight% or less (converted to silver element). When the remover supporting silver or silver oxide is placed in a bright place, silver is reduced and the activity decreases, whereas the remover supporting silver chloride is hard to undergo photoreduction and is active even in a bright place. Does not decrease.

As a method for supporting the silver component on the inorganic oxide such as γ-alumina, a known impregnation method, a kneading method or the like can be used. Adjustment of the removing material after loading is 50 to 15
After drying at about 0 ° C., it is preferable to raise the temperature stepwise at 100 to 600 ° C. and bake. The firing is preferably performed in air or under nitrogen flow, or under hydrogen gas flow, or while evacuating. It is preferable that the removal material fired under the flow of nitrogen gas or hydrogen gas is finally subjected to an oxidation treatment.

Next, the method of the present invention will be described. First, the above-mentioned removal material is installed in the middle of the exhaust gas conduit.

The exhaust gas contains acetylene, methane, ethane, propylene and the like as residual hydrocarbons, but when the residual hydrocarbons are not contained in an amount sufficient to reduce NOx in the exhaust gas, Hydrocarbon or Arco from the outside
The Lumpur compounds introduced into the exhaust gas. Hydrocarbon or alcohol
The introduction position of the class is upstream of the position where the removing material is installed.

As the hydrocarbon introduced from the outside, in addition to the gaseous hydrocarbon in the standard state such as propylene, acetylene, propane, etc., a hydrocarbon in the liquid state in the standard state can be used. As a liquid hydrocarbon in the standard state,
Specific examples include light oil, cetane, heptane, and kerosene. Examples of alcohols include ethanol.
You can These additives can be introduced into the exhaust gas by a method such as spraying.

Hydrocarbon or alcohol introduced from the outside
The amount of the class is preferably 5 times or less the weight of nitrogen oxides in the exhaust gas. If the addition amount exceeds 5 times, the fuel efficiency is deteriorated. It is more preferably 0.2 to 4 times.

In the present invention, the time during which the exhaust gas containing hydrocarbons or alcohols is in contact with the above-mentioned removing material is adjusted so that the reaction between the additive and the nitrogen oxides proceeds efficiently. From a practical standpoint, the contact time between the exhaust gas containing hydrocarbons or alcohols and the removal material is 0.006 g · sec / ml.
That is all. The preferable contact time is 0.007 g · sec / ml or more.

Further, in the present invention, hydrocarbon or alcohol
Keeping the temperature of the exhaust gas to 200 to 600 ° C. Le acids and nitrogen oxides in the removal member installation site is a site to be reacted. If the temperature of the exhaust gas is less than 200 ° C., the reduction reaction of nitrogen oxides does not proceed, and it is not possible to remove nitrogen oxides satisfactorily. On the other hand, hydrocarbon or Al and a temperature in excess of 600 ° C.
Combustion of coal products itself begins and nitrogen oxides cannot be reduced and removed. The preferred exhaust gas temperature is 300-550 ° C.

[0024]

The present invention will be described in more detail by the following specific examples. Example 1 Commercially available γ-alumina molded body (diameter 1.5 mm, length about 6 mm,
After immersing 10 g of a specific surface area of 260 m ² / g) in an aqueous solution of silver nitrate (a solution of 0.67 g of silver nitrate in 20 ml of water) for 20 minutes, 2 hours at 80 ° C in air and 2 hours at 180 ° C under a dry nitrogen stream. After drying for an hour, silver nitrate was supported on the molded body. Next, after cooling to room temperature under a stream of dry nitrogen,
An aqueous solution of ammonium chloride (a solution prepared by dissolving 0.5 g of ammonium chloride in 20 ml of water) was added to the γ-alumina molded body.
The silver nitrate was converted to silver chloride by immersing in silver chloride for 12 hours. Then, the γ-alumina molded body was taken out of the ammonium chloride solution, dried in air at 80 ° C. for 2 hours, and then oxygen 10
% Nitrogen at a rate of 2.5 ° C./min to 550 ° C., followed by firing at 550 ° C. for 5 hours to obtain 2.1% by weight (elemental conversion value) of silver with respect to the γ-alumina compact. Was supported in the form of silver chloride.

Example 2 Alumina sol (Cataloid AS- manufactured by Catalyst Kasei Co., Ltd.)
3) 0.4 g of silver chloride powder was suspended in 200 g, and water was evaporated on a water bath while stirring. Stop heating in a semi-dried state and mold into pellets, then
Dry for 2 hours at ℃, under a nitrogen stream containing 10% oxygen, 20
2 hours at 0 ° C, 2 hours at 300 ° C, 2 hours at 400 ° C,
It was calcined at 550 ° C. for 5 hours, and 2.1% by weight (elemental conversion value) of silver was supported on alumina in the form of silver chloride. The size of the pellet catalyst after calcination was 1-2 mmφ × 2-3 mm.

Examples 3 and 4 The removing materials prepared in Examples 1 and 2 were exposed to sunlight for 1 hour and then placed in a reaction tube, and the gas having the composition shown in Table 1 (nitrogen monoxide, carbon dioxide, oxygen, A total of 100% by volume of dry components consisting of propylene and nitrogen, to which 10% by volume of water was added, was flowed at a flow rate of 2 liters per minute (standard state) (contact time: 0.3 g · sec / ml, Space velocity 6
400 hr ^-1 ), the temperature of the exhaust gas in the reaction tube is 300 to 55
Propylene was reacted with nitrogen oxides while maintaining the range of 0 ° C.

The concentration of nitrogen oxides (total amount of nitric oxide and nitrogen dioxide) of the gas after passing through the reaction tube was measured by a chemiluminescence type nitrogen oxide analyzer to determine the removal rate of nitrogen oxides. The results are shown in Table 2.

Table 1 Component Concentration Nitric oxide 500 ppm Carbon dioxide 10% by volume Oxygen 10% by volume Propylene 500 ppm Nitrogen balance Moisture 10% by volume based on the amount of gas composed of the above components

Comparative Example 1 The same method as in Example 1 was carried out by carrying 2.0 wt% (in terms of silver element) of silver nitrate on the same γ-alumina pellets used in Example 1 using an aqueous solution of silver nitrate. After preparing
A test for removing nitrogen oxides was performed under the same conditions as in Example 3 except that one obtained by photoreducing the carried silver was used as a removing material after being exposed to sunlight for 1 hour. The results are shown in Table 2.

Table 2 Nitrogen oxide removal rate (%) Temperature (° C.) Example 3 Example 4 Comparative Example 1 300 0.0 0.0 0.0 0.0 350 350 5.8 5.0 2.0 2.0 400 27. 5 25.8 10.4 450 63.3 60.5 43.2 500 60.5 57.7 45.0 550 27.6 22.9 10.5

As can be seen from Table 2, in the examples using silver chloride, good removal of nitrogen oxides was observed at the exhaust gas temperature of 400 to 550 ° C. On the other hand, in Comparative Example 1 of the silver catalyst subjected to photoreduction, the nitrogen oxide removal rate was decreased over the entire temperature range.

[0032]

As described above in detail, according to the method of the present invention, nitrogen oxides in exhaust gas containing excess oxygen, optical
Can be removed stably and efficiently. Further, according to the method of the present invention, nitrogen oxides can be efficiently removed even when the exhaust gas contains about 10% of water.

The nitrogen oxide removing method of the present invention can be widely used for removing nitrogen oxides contained in exhaust gas from various combustors, automobiles and the like.

─────────────────────────────────────────────────── ─── Continuation of the front page (58) Fields surveyed (Int.Cl. ⁷ , DB name) B01D 53/86, 53/94 B01J 21/00-38/74

Claims (3)

(57) [Claims] 1. A method for removing nitrogen oxides from a combustion exhaust gas containing nitrogen oxides and oxygen in a larger amount than the theoretical reaction amount for coexisting unburned components, wherein a porous inorganic oxide is used.
0.2 to 15% by weight of silver chloride converted to 100% by weight
%, The nitrogen oxide removing material supported is installed in the middle of the exhaust gas conduit, hydrocarbons or alcohols are added to the exhaust gas on the upstream side of the removing material, and the exhaust gas is removed at 200 to 600 ° C. Contact with the hydrocarbon or alcohol
A method for removing nitrogen oxide, which comprises removing the nitrogen oxide by reacting a substance with the nitrogen oxide. 2. The method for removing nitrogen oxides according to claim 1, wherein the porous inorganic oxide is alumina or an alumina-based composite oxide.
Way . 3. Removal of nitrogen oxides according to claim 1 or 2.
In the method , the nitrogen oxide removing method is characterized in that the removing material further contains a ceramic or metal three-dimensional structure, and the porous inorganic oxide is coated on the three-dimensional structure.