JPH0618611B2 - Exhaust gas purification method - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for purifying exhaust gas, and more particularly to a method for purifying exhaust gas containing nitrogen oxides (nitric oxide and nitrogen dioxide), carbon-containing powder, and dilute sulfur oxides contaminated by automobile exhaust gases and the like.

BACKGROUND ART Conventionally, methods for removing nitrogen oxides contained in gases have been known, such as a method using a catalyst, an ultraviolet method, an absorption method, an adsorption method, and an electron beam irradiation method. However, no effective method has yet been found that can efficiently, economically, and safely remove nitrogen oxides and the like from exhaust gases contaminated with automobile exhaust gases and the like, which contain low concentrations of nitric oxide and nitrogen dioxide and carbon-containing powder.

For example, the method of reducing nitrogen oxides to nitrogen gas using a catalyst is not suitable as a method of removing nitrogen oxides from gas at room temperature because the catalytic effect is not exhibited unless the reaction temperature is high, such as 300°C or higher.

The ultraviolet method involves adding ammonia and converting nitrogen oxides into _N2.
This method can separate nitrogen oxides into _O2 and also converts nitrogen oxides into ammonium nitrate, but the reaction time is slow and it cannot be applied to flowing gases. Furthermore, a treatment method that combines ozone generated by ultraviolet irradiation with nitrogen monoxide to generate nitrogen dioxide through a reaction between the ozone and nitrogen monoxide, and then adsorbs this nitrogen dioxide onto an adsorbent, is not economical because the current adsorbents have low nitrogen dioxide removal capacity and require frequent replacement. The aforementioned method of generating ozone using ultraviolet light is energy inefficient and uneconomical. Similarly, the method of using a conventional ozone generator also has energy efficiency issues and economical problems with nitrogen dioxide treatment, making it an ineffective method.

The absorption method is a wet method that uses various absorption chemicals.
This method uses a chemical reaction to remove nitrogen monoxide, but at present no effective chemicals have been found that react with nitrogen monoxide at room temperature, making it unsuitable for gases with high nitrogen monoxide content such as those contaminated by automobile exhaust fumes.

The adsorption method is a method of physically or chemically removing nitrogen oxides (mainly nitrogen dioxide) using an adsorbent.
While this method is currently the most widely used method for purifying air containing low concentrations of nitrogen dioxide, it is not an effective method for treating the gases containing a certain amount of nitrogen oxides (primarily nitric oxide) that we are attempting to treat. The atmospheric concentration of nitrogen dioxide is typically less than several tens of parts per billion (ppb), a fraction of the concentration we are attempting to treat. While adsorption is effective for gases containing such low concentrations of nitrogen dioxide, for gases containing a high proportion of nitric oxide (50% or more), even at levels on the order of several ppm, the adsorbent's removal capacity is short and it cannot withstand long-term use. In our testing, the nitrogen dioxide removal capacity of adsorbents such as zeolite, activated carbon, and alkali-impregnated carbon, even with the best impregnated carbon, fell to less than 50% within several tens of hours at a linear velocity (LV) of 0.5 m/sec. At the very least, for practical use, the removal capacity must be maintained for at least one month, and the cost of maintaining this capacity is also an important factor in selecting an effective nitrogen oxide removal method.

Next, the electron beam irradiation method is a technology developed by the present applicant, in which highly reactive radicals such as O, OH, and _HO2 are generated by electron beam irradiation, which converts SOx and NOx into sulfuric acid and nitric acid, which are then reacted with ammonia that has been added in advance to recover ammonium sulfate and ammonium nitrate.
This method has been put to practical use as a method for treating combustion gases. Furthermore, the present applicant has previously filed a patent application and published it as Japanese Patent Application Laid-Open No. 63-291626 as a method for treating gases containing nitrogen monoxide and nitrogen dioxide contaminated by automobile exhaust gases, etc. However, this method involves irradiating exhaust gas to which no ammonia has been added with an electron beam, and then treating the gas with an activated carbon adsorbent or a zeolite adsorbent. Therefore, the activated carbon adsorbent and the zeolite adsorbent have low removal capacity for nitrate ions and nitrogen dioxide, and in order to maintain this removal capacity,
There was an economic problem in that the adsorbent had to be replaced quite frequently.

As mentioned above, various methods have been known for removing nitrogen oxides, but there has been no practical and effective method for purifying gases polluted by automobile exhaust gases and the like.

Therefore, the present invention aims to efficiently and economically remove harmful substances from exhaust gases containing nitrogen oxides (NOx), carbon-containing powders, and diluted sulfur oxides contaminated by automobile exhaust gases, etc.
Moreover, the object is to provide a purification method that safely removes the pollutants.

DISCLOSURE OF THE INVENTION The above object can be achieved by adding ammonia to exhaust gas containing harmful substances originating from automobile exhaust gases, etc., irradiating the gas with ionizing radiation, and then passing the gas through a wet electrostatic precipitator.

Specifically, the present invention relates to a method for purifying exhaust gas containing nitrogen oxides, carbon-containing powder, and dilute sulfur oxides, which is generated from automobile exhaust gases and the like, by adding ammonia to the gas, irradiating the gas with ionizing radiation, and then removing the resulting dust and/or mist and carbon with a wet electrostatic precipitator. The present invention also relates to a method for purifying exhaust gas which, after treatment with the wet electrostatic precipitator, further passes the gas through an activated carbon filler layer.

BRIEF DESCRIPTION OF THE DRAWINGS Figures 1 and 2 are process diagrams showing one embodiment of the present invention;
FIG. 3 is a graph showing the relationship between absorbed dose, denitration rate and ozone concentration, and FIG. 4 is a conceptual diagram of the method of the present invention when installed in an automobile tunnel.

BEST MODE FOR CARRYING OUT THE INVENTION First, the principle of the present invention will be explained. When a gas such as air is irradiated with ionizing radiation, highly reactive radicals such as N, O, OH, and _HO2 are generated from nitrogen, oxygen, and moisture, which are components of the gas such as air (Reaction Formula 1).

N₂,O₂,H₂O^e- N, O, OH, HO₂(Reaction Scheme 1) These radicals react with the nitrogen oxides to be removed.
The reaction produces nitrogen gas and nitric acid as shown in Reactions 2 to 4.
It will proceed in the direction of completion.

NO + N → _N2 + 1/ _2O2 (Reaction 2) NO + _HO2 → _HNO3 (Reaction 3) _NO2 + OH → _HNO3 (Reaction 4) The nitrogen gas produced in reaction 2 is released as a harmless gas, and the nitric acid produced in reaction 3 or 4 becomes ammonium nitrate in the presence of ammonia, as shown in reaction 5.

HNO ₃ + NH ₃ → NH ₄ NO ₃ (Reaction Formula 5) Reactions 1 to 5 proceed instantaneously within one second. Dilute sulfur oxides react in the same way to produce a small amount of ammonium nitrate.

The ammonium nitrate and ammonium sulfate obtained in the above reaction are recovered using a wet electrostatic precipitator.

When flue gas is treated by an ionizing radiation method, ammonium sulfate is usually present and is recovered together with ammonium nitrate. Furthermore, the amount of unburned carbon is usually small relative to the amount of ammonium nitrate. Therefore, when treating flue gas by an ionizing radiation method, there is no problem in recovering the by-product in a dry state.

However, when treating exhaust gases from automobile exhausts using the ionizing radiation method, the amount of sulfur oxides is small, and the majority of the by-product that can be recovered is ammonium nitrate. Furthermore, the proportion of unburned carbon is significantly higher than in the by-products from treating flue gas. Therefore, the mixture of ammonium nitrate and unburned carbon creates the risk of heat generation or explosion.
Therefore, when treating automobile exhaust gases and the like using the ionizing radiation method, it is necessary to use a wet recovery method as a safe by-product recovery method.

On the other hand, the wet recovery method also includes the scrubber method, but in this case, when treating large volumes of air like in this case, the pressure loss is high and there is also the problem of mist leakage, so it is not suitable in this case.

Therefore, the method of recovering by-products using a wet electrostatic precipitator is a method that can safely recover by-products mainly composed of ammonium nitrate that are generated when automobile exhaust gases, etc. are treated by the ionizing radiation method, without causing problems such as pressure loss or mist streaks.

Furthermore, the wet electrostatic precipitator absorbs any remaining unreacted ammonia through the effects of its spray water and weeping walls, thereby reducing leaked ammonia.

Furthermore, by repeatedly recycling the water used in the wet electrostatic precipitator and increasing the concentration of ammonium nitrate, the water can be recovered and used as fertilizer, which also improves economic efficiency.

Furthermore, by using activated carbon fillers at the final location of the equipment where ionizing radiation irradiation and wet electrostatic precipitators are installed, ozone generated by ionizing radiation irradiation and charging of the wet electrostatic precipitator can be decomposed. However, in this case, the activated carbon fillers
The purpose of this packing is not to remove Ox. The ozone decomposition capacity of activated carbon packing is such that it can decompose ozone in excess of its own weight, making its use economical. Increasing the dose of ionizing radiation is effective in increasing the nitrogen oxide removal efficiency, but ozone also tends to increase with the dose of ionizing radiation. Because ozone is harmful to plants, animals, and humans when released into the atmosphere, it has been an obstacle to increasing nitrogen oxide removal capacity. However, the use of activated carbon packing, which can economically decompose ozone, allows the dose of ionizing radiation to be increased safely, making it effective for improving nitrogen oxide removal capacity.

It was also found that by using a positive electrode as the discharge electrode of a wet electrostatic precipitator, the amount of ozone generated by charging is less than when the electrode is a negative electrode, as is commonly used.

Activated carbon fillers are fibrous, granular, or aggregate materials containing activated carbon. These materials are used to transport gases that have been treated with ionizing radiation and a wet electrostatic precipitator at a linear velocity (LV),
A flow rate of approximately 0.01 to 1.0 m/sec allows contact with minimal pressure loss. Granular activated carbon is preferably mixed with zeolite or silica gel, as this eliminates the risk of combustion that can occur when high-concentration ozone is decomposed by activated carbon and improves ozone decomposition performance. Impregnated activated carbon, in which an agent such as an alkali metal carbonate is added to activated carbon, can also be used.

As described above, the activated carbon filler that can be used in the present invention is
Activated carbon, which is normally used as an adsorbent,
Anything can be used.

Activated carbon fillers have almost no ability to remove nitrogen monoxide, and their ability to remove nitrogen dioxide deteriorates rapidly within several tens of hours at concentrations of a few ppm, so they must be replaced quite frequently and are not economical to use for that purpose, as mentioned in the prior art. However, it has been confirmed that activated carbon fillers have the ability to reduce harmful nitrogen dioxide to the less toxic nitric oxide, in addition to their ability to decompose ozone.

Furthermore, by using activated carbon packing as a moving layer, dust adhering to the surface of the activated carbon packing is removed,
This eliminates unused surfaces, and also makes it easier to replace materials that have lost their ozone decomposition or nitrogen dioxide reduction capabilities with new activated carbon fillers.

The ionizing radiation to be irradiated to the nitrogen oxide-containing gas of the present invention may be electron beams, α-rays, β-rays, γ-rays, X-rays, neutron beams, etc., but electron beams are more convenient. The irradiation dose is sufficient if it is within the range of 0.001 Mrad to 0.5 Mrad. Furthermore, the irradiation can be carried out at room temperature. Furthermore, the applied voltage of the wet electrostatic precipitator is about 10 to 10
A range of 0 kV is preferred.

According to the present invention, low-order oxides of nitrogen contained in the gas,
In particular, nitric oxide is generated from nitrogen, oxygen, and moisture in the gas by irradiation with ionizing radiation, and is a compound of N, O, OH, and HO.
It reacts with radicals such as ₂ , oxidizing it to harmless nitrogen gas or the higher oxide nitric acid, which then reacts with ammonia to form ammonium nitrate. The ammonium nitrate is safely recovered in the circulating water of the wet electrostatic precipitator, and harmful ozone is also decomposed by the activated carbon filler placed in the downstream stage. The activated carbon filler also reduces nitrogen dioxide, which cannot be treated by these methods, to relatively less toxic nitric oxide.

The gas to be treated by this invention is a gas containing nitrogen oxides at a level of about several ppm, and is particularly effective for purifying exhaust gases originating from automobile exhaust gases such as exhaust gases in automobile tunnels and exhaust gases in underground parking lots. Exhaust gases in automobile tunnels contain about 0.1 ppm of NOx.
It is known that the air contains about 0.05 to 1.0 mg/ ^m³ of carbon dioxide, about 10 to 80% of the dust, and a trace amount of _SO₂ . Figure 4 shows a conceptual diagram of the application of the method of the present invention to an automobile tunnel. In Figure 4, (10) is the automobile tunnel, (11) is the exhaust duct, and the air from the exhaust duct is treated by the method of the present invention.

[Examples] The present invention will be described in more detail below with reference to examples.
The present invention is not limited to these examples.

Example 1 Figure 1 is a process diagram showing an example of the present invention. In Figure 1, (1) is an ammonia addition section, (2) is an ionizing radiation irradiation device, (3) is a reactor, (4) is a wet electrostatic precipitator, (5) is a water-soluble polymer.
indicates a blower. After ammonia is added to the treated gas (1), it enters the reactor (3) and is irradiated with ionizing radiation.
The wastewater enters the wet electrostatic precipitator (4). In the wet electrostatic precipitator (4), water from which dust has been removed in the circulating water dust removal device (6) is sprayed from a pipe (8), where ammonium nitrate and carbon from the reactor (3) are collected, and the purified gas is discharged from a blower (5). Meanwhile, the concentrated ammonium nitrate is extracted as fertilizer from (7). Next, a specific example using this FIG. 1 will be explained.

NOx 1.8-2.7 ppm (NO content 80%-90% other N
_O2 ) Dust: approx. 0.1-0.7mg/ ^m3 (solid carbon: 50% of the dust
%) and a trace amount of _SO2 at room temperature.
Nm ³ /h into the reactor (3), where an electron beam generator
The electron beam from (2) was irradiated in the range of 0.004 Mrad to 0.12 Mrad. At this time, ammonia equivalent to 0.65 to 1.1 equivalents based on the NOx concentration was added from (1) to the gas before electron beam irradiation.

The NOx concentration in the gas after electron beam irradiation was measured using a chemiluminescence NOx meter, and the results are shown in Figure 3 as the removal rate (denitrification rate) relative to the inlet NOx concentration. Figure 3 is a graph showing the relationship between absorbed dose, denitrification rate (ηNOx), and ozone concentration. Most of the NOx in the outlet gas was detected in the form of nitrogen dioxide. The denitrification rate also improved with increasing absorbed dose, and a denitrification rate of over 80% was obtained at an irradiation dose of around 0.05 Mrad. Even at an irradiation dose of around 0.005 Mrad, a denitrification rate of approximately 50% was obtained. Furthermore, for the same irradiation dose, the denitrification rate (ηNOx) varied depending on the NOx concentration, with a tendency for ηNOx to be higher at lower NOx concentrations. That is, at 0.004 Mrad, NOx was 2.70p
At pm(c), ηNOx was 31%, at 2.30 ppm(b), 37%, and at 2.0
At 5 ppm (a), the figure is 47%. The tendency is similar for other irradiation amounts. Therefore, the irradiation amount to the exhaust gas is
The dose varies depending on the NOx concentration and the required denitrification rate. However, considering the NOx concentration range in the target gas in this case (approximately 0.1 to 5.0 ppm), an irradiation dose of 0.001 to 0.5 Mrad is considered sufficient. During this test, the ozone concentration was also measured, and it was found that the ozone concentration increased with the increase in absorbed dose, reaching 0.05 Mrad.
Ozone concentrations of 2.0 ppm were measured in nearby irradiation.
However, the ozone concentration was below 0.1 ppm at irradiation doses around 0.005 Mrad.

Example 2 Figure 2 is a process diagram showing another example of the present invention. Figure 2 is the same as Figure 1 except that an activated carbon filler layer (9) is provided after the wet electrostatic precipitator (4) in Figure 1.

The test was carried out in the same manner as in Example 1 using this Figure 2.
The electron beam irradiation was carried out mainly at an absorbed dose of about 0.05 Mrad to about 0.1 Mrad. The packed bed (9) was packed with crushed activated carbon, and a part of the gas after the electron beam irradiation was introduced into the packed bed (9) having a layer thickness of 30 mm at a linear velocity of 0.8 m/sec.
The x concentration and ozone concentration were measured. As a result, the denitrification ability of the crushed activated carbon was lost after several tens of hours of testing.
The NOx concentration in the outlet gas was almost the same as in Example 1, but more than 60% of it was detected as nitrogen monoxide. In addition, no ozone was detected in the gas that passed through the crushed activated carbon layer. As can be seen from the results of these examples, the present invention was able to efficiently remove dilute NOx on the ppm order.

INDUSTRIAL APPLICABILITY As described above, the present invention has the following advantages.

1) It is possible to efficiently, economically and safely remove nitrogen oxides from pollutant gases such as air, which contain nitrogen oxides at the ppm level, including a high proportion of nitric oxide, which is difficult to remove by other methods.

2) By irradiating the waste with ionizing radiation, lower nitrogen oxides are converted into harmless nitrogen gas or higher oxides (nitric acid), which are then combined with ammonia that has been added beforehand to form ammonium nitrate. By passing the ammonium nitrate through a wet electrostatic precipitator, it can be safely recovered as an aqueous ammonium nitrate solution and used as fertilizer.

3) By using activated carbon fillers, ozone generated by exposure to ionizing radiation or by the charging of a wet electrostatic precipitator can be decomposed, and nitrogen dioxide, which cannot be treated by these methods, can be reduced to relatively less toxic nitric oxide.

4) It becomes possible to remove nitrogen oxides, especially nitric oxide, from polluted air at room temperature and pressure efficiently, safely, and economically.

──────────────────────────────────────────────────── Continued from the front page (72) Inventor: Toru Kamisatoda 11-1 Haneda Asahi-cho, Ota-ku, Tokyo Ebara Corporation (72) Inventor: Kyoichi Okamoto 11-1 Haneda Asahi-cho, Ota-ku, Tokyo Ebara Corporation (56) References: JP 52-140499 (JP, A) JP 63-291626 (JP, A) JP 51-151258 (JP, A)

Claims (7)

[Claims] Claim 1: A method for purifying exhaust gas containing nitrogen oxides (NOx), carbon-containing powder, and diluted sulfur oxides (SOx), comprising adding an amount of ammonia not more than approximately equivalent to the NOx and SOx contained in the exhaust gas, and adjusting the amount of ammonia to 0.001 to 0.5 Mr depending on the required denitration rate and inlet NOx concentration.
After irradiating with ionizing radiation within the range of 10 to 10
A method for purifying exhaust gas, characterized in that the dust particles are removed using a wet electrostatic precipitator under an applied voltage of 0 kV. 2. The method according to claim 1, wherein the exhaust gas containing the dust and/or mist and carbon is treated with a wet electrostatic precipitator and then passed through a layer of activated carbon filler at a linear velocity of 0.01 to 1.0 m/sec. 3. The method according to claim 1, wherein the wet electrostatic precipitator has a discharge electrode as a positive electrode. 4. The activated carbon filler according to claim 2, wherein the activated carbon filler is in the form of fibers, particles, or lumps containing activated carbon.
Or the method described in 3. 5. The method according to claim 2, wherein the activated carbon filler layer is a fixed bed or a moving bed. Claim 6: Ionizing radiation includes electron beams, α rays, β rays,
6. The method according to claim 1, wherein one or more types of radiation selected from the group consisting of rays, γ rays, X-rays, and neutron rays are used. 7. The method according to any one of claims 1 to 6, wherein the water used in the wet electrostatic precipitator is recycled and recovered as fertilizer.
10. The method according to any one of claims 1 to 9.