JPH06198131A - Material for removing nitrogen oxide and method for removing nitrogen oxide - Google Patents

Abstract

PURPOSE:To provide a material and method for removing nitrogen oxides by which nitrogen oxide can effectively be removed even at low temp. from combustion exhaust gas containing nitrogen oxides and a larger amt. of oxygen than the stoichiometric amt. for unburnt component of nitrogen oxide, carbon monoxide, hydrogen, hydrocarbons, etc. CONSTITUTION:This removing material is obtd. by making silver sulfate by 0.2-15wt.% in terms of silver element or 0.2-15wt.% silver and sulfuric acid 1/50-2 times of mols of the silver carry on a porous inorg. oxide. In this catalyst, nitrogen oxides in exhaust gas are reduced at 200-600 deg.C by using hydrocarbon or oxygen-contg. org. compd. as a reducing agent added to the exhaust gas from the outside.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a scavenger capable of effectively removing nitrogen oxides from a combustion exhaust gas containing nitrogen oxides and excess oxygen, and a nitrogen oxides removing method using the same.

[0002]

2. Description of the Related Art Excessive amounts of combustion exhaust gas discharged 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,
It is intended to provide a nitrogen oxide removing material and a removing method capable of effectively removing nitrogen oxide even at a low temperature.

[0009]

As a result of earnest research in view of the above problems, the present inventor has used a removing material in which a specific amount of silver sulfate is supported on a porous inorganic oxide, and is specified as the removing material. The exhaust gas is brought into contact with the exhaust gas at the temperature and contact time of 10%, and the water content is 10% by the hydrocarbon or oxygen-containing organic compound added to the exhaust gas so as to correspond to the amount of nitrogen oxides contained in the exhaust gas.
The present invention was completed by discovering that nitrogen oxides can be effectively removed even with exhaust gas containing a small amount thereof, and that the nitrogen oxide removal performance in a low temperature region is improved.

That is, the nitrogen oxide removing material of the present invention for removing nitrogen oxides from a combustion exhaust gas containing nitrogen oxides and oxygen in an amount larger than the theoretical reaction amount for coexisting unburned components is a porous inorganic oxide. In addition, 0.2 to 15% by weight or 0.2 to 15% by weight of silver sulfate converted to elemental silver and sulfuric acid of 1/50 to 2 times the number of moles of silver are supported. From the hydrocarbon or oxygen-containing organic compound added to the exhaust gas as a reducing agent at 200 to 600 ° C. to reduce nitrogen oxides in the exhaust gas.

Further, the nitrogen oxide removing method of the present invention for removing nitrogen oxides from a combustion exhaust gas containing nitrogen oxides and oxygen in an amount larger than the theoretical reaction amount for coexisting unburned components is the above-mentioned nitrogen oxide removing material. Is installed in the middle of the exhaust gas conduit, a hydrocarbon or an oxygen-containing organic compound is added to the exhaust gas on the upstream side of the removing material, and the exhaust gas is brought into contact with the removing material at 200 to 600 ° C. It is characterized in that the oxygen-containing organic compound is reacted with the nitrogen oxide to remove the nitrogen oxide.

The present invention will be described in detail below. In the present invention, a removing material shown below is used, and exhaust gas is brought into contact with this removing material to remove residual hydrocarbons in the exhaust gas and / or hydrocarbons added to the exhaust gas on the upstream side from the installation site of the removing material or the content of the hydrocarbon. Nitrogen oxides in exhaust gas are reduced and removed using an oxygen organic compound as a reducing agent.

First, the removing material of the present invention comprises silver sulfate or silver and sulfuric acid 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 oxygen-containing organic compound 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 well. 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. The 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 sulfate or silver and sulfuric acid is supported on a porous inorganic oxide. After coating the above-mentioned three-dimensional structure with an inorganic oxide, silver sulfate or silver and sulfuric acid is carried to prepare. 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-shaped porous inorganic oxide loaded with silver sulfate or silver and sulfuric acid.

The amount of silver sulfate 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 sulfate 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). The removing material supporting silver or silver oxide has a low nitrogen oxide removing activity in a low temperature region, whereas the removing material supporting silver sulfate maintains a high removing activity even in a low temperature region.

Furthermore, when silver and sulfuric acid are supported on the above-mentioned inorganic oxide such as γ-alumina, the amount of the silver component supported is 0.2 to 15% by weight (silver) with 100% by weight of the inorganic oxide. Element conversion value). If the silver component is less than 0.2% by weight (converted to silver element), the nitrogen oxide removal rate on the low temperature side is lowered. 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 is 5% by weight with respect to 100% by weight of the inorganic oxide.
To 15% by weight or less (silver element conversion value). The removing material supporting silver or silver oxide has a low nitrogen oxide removing activity in a low temperature region, whereas the removing material supporting silver and sulfuric acid maintains a high removing activity even in a low temperature region. The amount of sulfuric acid carried need not be the same number of moles as the amount of silver carried. The amount of sulfuric acid carried is preferably 1/50 to 2 times (molar ratio) the amount of silver carried.

As a method for supporting silver sulfate or the like on an inorganic oxide such as γ-alumina, a known impregnation method, a kneading method or the like can be used. Adjustment of the removal material after loading is 50-
It is preferable that after drying at about 150 ° C., the temperature is raised stepwise at 100 to 600 ° C. for firing. 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, A hydrocarbon or an oxygen-containing organic compound is introduced into the exhaust gas from the outside. The introduction position of the hydrocarbon or the oxygen-containing organic compound 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. As the oxygen-containing organic compound, alcohols such as ethanol are preferable. These additives can be introduced into the exhaust gas by a method such as spraying.

The amount of hydrocarbon or oxygen-containing organic compound introduced from the outside is preferably not more than 5 times the weight of nitrogen oxide 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 the hydrocarbon or the oxygen-containing organic compound 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 oxygen-containing organic compounds and the removal material is 0.006 g.
・ Seconds / ml or more. Preferred contact time is 0.007g
・ Seconds / ml or more.

Further, in the present invention, the temperature of the exhaust gas at the site where the removing material is installed, which is the site where the hydrocarbon or the oxygen-containing organic compound reacts with the nitrogen oxide, is maintained at 200 to 600 ° C. If the temperature of the exhaust gas is less than 200 ° C., the reduction reaction of nitrogen oxides does not proceed, and good removal of nitrogen oxides cannot be performed. On the other hand, if the temperature exceeds 600 ° C., the combustion of the hydrocarbon or the oxygen-containing organic compound itself starts, and the nitrogen oxide cannot be reduced and removed. Preferred exhaust gas temperature is 30
The temperature is 0 to 550 ° C.

[0025]

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,
10 g of a specific surface area of 260 m ² / g) was added to an aqueous solution of silver sulfate (2.
It was immersed in a solution of 0.61 g of silver sulfate in 20 ml of 5 M nitric acid) for 20 minutes, and then in air at 80 ° C. for 2 hours. Next, under a nitrogen stream containing 10% oxygen, 2.5 per minute
After the temperature was raised to 550 ° C. at 5 ° C., it was baked at 550 ° C. for 5 hours, and 2.0% by weight (elemental conversion value) of silver was supported in the form of silver sulfate on the γ-alumina formed body.

Example 2 Commercially available γ-alumina molded body (diameter 1.5 mm, length about 6 mm,
10 g of a specific surface area of 260 m ² / g) was immersed in 20 ml of a 97 mM aqueous sulfuric acid solution for 20 minutes, and then dried in air at 80 ° C. for 2 hours. Next, under a nitrogen stream containing 10% oxygen,
After heating to 550 ° C. at 2.5 ° C./min, firing was performed at 550 ° C. for 5 hours and cooling to room temperature. Then, the above γ-alumina was immersed in a silver nitrate solution (solution in which 0.67 g of silver nitrate was dissolved in 20 ml of water) for 20 minutes, and then, in air, at 80
After drying at ℃ for 2 hours, under a nitrogen stream containing 10% oxygen, the temperature was raised to 550 ℃ at 2.5 ℃ / min, and baked at 550 ℃ for 5 hours, 2 wt% ( Silver (elemental conversion value) and 0.09% by weight of sulfuric acid were supported.

Example 3 Commercially available γ-alumina molded body (diameter 1.5 mm, length about 6 mm,
10 g of specific surface area 260 m ² / g) in silver nitrate solution (water 20
It was immersed in a solution of 0.67 g of silver nitrate in 20 ml) for 20 minutes and then dried in air at 80 ° C. for 2 hours. next,
550 ° C at 2.5 ° C / min under nitrogen stream containing 10% oxygen
After heating to 50 ° C., the mixture was baked at 550 ° C. for 5 hours and cooled to room temperature. Then, the above γ-alumina was immersed in 20 ml of 97 mM sulfuric acid aqueous solution for 20 minutes, and then, in air,
After drying at ℃ for 2 hours, under nitrogen stream containing 10% oxygen, the temperature was raised to 550 ℃ at 2.5 ℃ per minute, and baked at 550 ℃ for 5 hours, and 2 wt% ( Silver (elemental conversion value) and 0.09% by weight of sulfuric acid were supported.

Examples 4 to 6 The removing materials prepared in Examples 1 to 3 were placed in a reaction tube,
A gas having the composition shown in Table 1 (a total of 10 dry components consisting of nitric oxide, carbon dioxide, oxygen, propylene, and nitrogen).
Using 0% by volume and 10% by volume of water added), the mixture was allowed to flow at a flow rate of 2 liters per minute (standard state) (contact time: 0.3 g · sec / ml, space velocity: 6400 hr).
^-1 ), the exhaust gas temperature of the reaction tube was kept in the range of 300 to 550 ° C., propylene was reacted with nitrogen oxide, and the nitrogen oxide removal rate was obtained. The results are shown in Table 2.

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 Prepared by carrying out 2.0% by weight (in terms of silver element) of silver nitrate on the same γ-alumina pellets as used in Example 1 using an aqueous solution of silver nitrate, drying and baking to 600 ° C. A nitrogen oxide removal test was conducted under the same conditions as in Example 4 except that the above removing material was used. The results are shown in Table 2.

Table 2 Nitrogen oxide removal rate (%) Temperature (° C.) Example 4 Example 5 Example 6 Comparative Example 1 300 11.8 11.0 10.5 0.0 350 33.9 30.5 28.0 2.0 400 59.5 45.8 40.5 17.2 450 67.8 60.5 63.3 57.3 500 57.7 56.7 60.5 56.7 550 22.9 30 .2 27.6 37.8

As can be seen from Table 2, in the embodiment, 40
Removal of nitrogen oxide was observed even at 0 ° C or lower. On the other hand, in Comparative Example 1, the nitrogen oxide removal rate decreased at the low temperature side.

[0034]

As described in detail above, according to the method of the present invention, nitrogen oxides in exhaust gas containing excess oxygen can be efficiently removed. 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 method for removing nitrogen oxides 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 (72) Inventor Kiyohide Yoshida 4-1-1 Suehiro, Kumagaya-shi, Saitama Stock company Riken Kumagaya Works

Claims (4)

[Claims] 1. A removal material for removing nitrogen oxides from a combustion exhaust gas containing nitrogen oxides and oxygen in an amount larger than the theoretical reaction amount for coexisting unburned components, wherein silver sulfate is silver element as a porous inorganic oxide. 0.2 to 15% by weight or 0.2 to 15% by weight of silver and 1/50 to 2 of the number of moles of silver
It is characterized by supporting twice the amount of sulfuric acid and reducing the nitrogen oxides in the exhaust gas at 200 to 600 ° C. by using a hydrocarbon or an oxygen-containing organic compound externally added to the exhaust gas as a reducing agent. Nitrogen oxide removing material. 2. The nitrogen oxide removing material according to claim 1, wherein the porous inorganic oxide is alumina or an alumina-based composite oxide. 3. The nitrogen oxide removing material according to claim 1 or 2, wherein the removing material further contains a ceramic or metal three-dimensional structure, and the porous inorganic oxide is the three-dimensional structure. Nitrogen oxide removing material characterized by being coated on. 4. 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, the nitrogen oxidation according to claim 1. Substance removal material is installed in the middle of the exhaust gas conduit, hydrocarbons or oxygen-containing organic compounds are added to the exhaust gas on the upstream side of the removal material, and the exhaust gas is contacted with the removal material at 200 to 600 ° C., A method for removing nitrogen oxide, which comprises reacting a hydrocarbon or an oxygen-containing organic compound with the nitrogen oxide to remove the nitrogen oxide.