JPS585699B2 - How to remove nitrogen oxides - Google Patents

Description

[Detailed Description of the Invention] The problem of air pollution caused by nitrogen oxides (NOx) is considered to be a serious problem due to the toxicity of nitrogen oxides themselves at the environmental concentration level and the fact that they are the cause of photochemical smog. ing.

Since the current level of pollution is much higher than the environmental standards, measures are being taken to address the sources, such as exhaust gas regulations for power-driven vehicles and NOx emission standards for stationary sources.

However, even if these measures are successful and, for example, the environmental standard level is achieved for the atmosphere as a whole, the problem of localized high concentration pollution during the transient period or near the source remains a serious problem. be.

The present invention mainly aims to purify nitrogen oxides in the atmosphere by using a dry method that is easy to handle, and provides a method that can treat nitrogen oxides in the atmosphere with high performance and is stable in terms of life. It is.

Conventionally well-known methods for removing nitrogen oxides from the atmosphere are mostly large-scale stationary source technologies, and when applied as a dry method, selective reduction methods using reducing agents such as ammonia are used. etc. are promising, but these cannot be used to treat nitrogen oxides at low environmental concentration levels.

Adsorption methods, absorption methods, etc. are considered to be effective for treating nitrogen oxides at such levels.

In the case of adsorption methods, activated carbon is the most effective technology.

In the case of activated carbon, nitrogen dioxide (NO2) among nitrogen oxides
It has the property of reducing nitrogen dioxide, although it can be removed to some extent, and its removal ability improves as the water vapor (relative humidity) increases, but its lifespan is short.

Its removal capacity is approximately 2 to 3 m9N02/1ml activated carbon.

Another method for treating nitrogen oxides at environmental concentration levels is the wet absorption method.

Since N02 is absorbed by an alkaline solution, this technology mainly uses a porous carrier, such as Japanese paper or non-woven fabric, to absorb the alkaline solution and carry it along with a moisturizing agent such as glycerin. be.

This technology was developed to remove other air pollutants, mainly SO2, rather than NO2.
The absorption efficiency (removal rate) of 02 is low, and since the amount of alkali supported is small, the reaction life is also short.

However, since it is originally a wet method, it is not possible to increase the space velocity during operation due to considerations for droplet scattering, so the operation must be performed at a space velocity of about 1000 to 2000 (h-1). It is inevitable that the volume of the removing agent portion will be enormous.

On the other hand, the method of the present invention is particularly excellent in removing NO2 among nitrogen oxides in the atmosphere, but it is easy to handle, has high reactivity, and is superior to activated carbon adsorption methods. It has a much longer lifespan.

Moreover, looking at the current state of nitrogen oxide pollution, most of the NOx at the source is nitrogen dioxide (No), while this NO is gradually oxidized in the atmosphere and changes to NO2, but recently According to air pollution data released by the Pollution Monitoring Center of Japan, most of the air contains NO and NO.
The ratio with 2 is around 1/1.

This is because the oxidation rate of NO slows down as the concentration decreases, but it may be that the rate of NO oxidation is balanced at around 1/1.

The present invention is evaluated as a practical technique because it exhibits extremely excellent effects in treating NOx with a composition of NO/NO2 of around 1/1.

The present invention involves kneading an alkali, a curing agent, and powdered activated carbon with water, molding the particles to an appropriate particle size, and curing the particles, and then bringing nitrogen oxide-containing gas into contact with the particles to absorb and remove nitrogen oxides from the gas. It is characterized by:

The hardening materials used here are hydraulic cement materials such as alumina cement and Bortland cement, calcined gypsum (calcium sulfate), bentonite, diatomaceous earth, or calcium hydroxide.

The present invention will be explained below with reference to Examples.

The reaction between an aqueous alkaline solution and nitrogen oxides is well known, and when potassium carbonate K2CO3 is used as the alkali, the reaction is as follows.

(1) Does not react to NO.

(2) Reaction with NO2 2NO2+K2CO3→KNOs+KNO2+CO2(
3) Reaction with N203 N20s + K2COs → 2KNO2 + C02 As alkalis, NaOH, KOH, Na2C03tCa (OH
)2, hydroxides or carbonates of alkali metals or alkaline earth metals such as Mg(OH)2, and other NH40H
, NH4co3, etc. are known to undergo similar reactions.

These are naturally liquid phase reactions, and chemical absorption reactions centered on ionic reactions in the presence of moisture.

In the case of wet absorption, as shown above, the space velocity (SV) cannot be increased in order to prevent water droplets from scattering, resulting in poor operability.

On the other hand, those made by granulation molding with alkali and hardening agent have NO/
Various tests were carried out by sending a gas of about 1/1 concentration in NO2, but as shown in the table below, these methods hardly remove NOx.

This is probably due to the N02 absorption rate on the reactant surface, N20
3. This is thought to be due to the slow absorption rate.

Since the reaction rate of dry absorption in such a system is slow for N02, it is thought that it is necessary to consider further increasing the absorption rate for N203.

Therefore, the present inventors added various substances expected to have a catalytic effect to a dry absorption composition consisting of an alkali and a hardening agent, and tested it. It is a headline that has an effect.

The following table compares the NOx removal rates for various compositions.

The sample used was prepared by adding 20 to 30% by weight of water to a mixture of various raw materials, kneading it, drying it at 100°C for about an hour to harden it, then crushing it and classifying it into a particle size of 4 to 8 mesh. there was.

Note that activated carbon of 80 mesh was used.

The removal rate of NOx is determined by using air containing 100 ppm of NOx with a composition of NO/NO2 of around 1/1 at a temperature of 30°C and a relative humidity of 50% at a space velocity of 25,000 (h-1).
), the sample particles were passed through a packed bed, and the outlet gas concentration was measured, and the average value is shown for 30 minutes from the initial stage.

Although not shown in the table, K2CO3tNa2s20
3tNaOHtNa2SO3,CO(NH2)2,Ca
(HPO4)3, KOH, and other hardening materials such as alumina cement, bentonite, and baked stone were similarly prepared and tested, and the NOx removal rate was low in all cases, indicating that they were the best. However, the removal rate was only about 5%.

Combining powdered activated carbon with alkali and hardening agent reduces NOx
The removal rate is excellent, and the removal rate of N02 is excellent compared to those using activated alumina or zeolite instead of activated carbon.

In particular, a method using K2CO3 as the alkali and calcium hydroxide Ca(OH)s as the hardening agent was effective in combination with activated carbon.

This is a reaction product with NOx, Ca(NO2)2,
This is thought to be due to the fact that the reaction proceeds effectively due to the low solubility of Ca(NO3)2 and the like.

As shown in the table, K2C03 is used as the alkali, and Ca is used as the hardening agent.
(OR)2 and calcium sulfate CaSO4・1/2H20
In those using K2CO3/Ca(OH)2
/CaSO4・1/2H20/Activated carbon-3/3/2/2
The composition was the best.

This composition has a removal capacity of 48 ■N per ml of its volume.
It showed O2.

This value is about 20 times that of the same amount of activated carbon alone, and it is clear that this is an extremely excellent method for removing nitrogen oxides from the atmosphere.

Also, in a packed bed of particles with the same composition, NO2 - 1.4 ppm,
As a result of testing at a temperature of 30°C and a relative temperature of 60% through air containing NO-0.2ppm, SV = 25,00.
At 0 (h-1)t, NO2 removal rate is 100%, NO removal rate is 10%, SV = 50,000 (h-1), NO2 removal rate is 95%, No removal rate is 5%, SV = 100,000 ( h
-1), the NO2 removal rate was 90% and the NO removal rate was 0.

Thus, the composition used in the present invention exhibits extremely good performance in removing NOx, especially NO2, at low concentration levels in the atmosphere.

The reason for this is thought to be as follows.

In other words, in the absorption reaction of nitrogen oxides into alkali, as shown in the reaction formula above, reactions with N02 and N203 can occur, but the reaction rate with NO2 is slow. The nitrogen oxide state N203, which is known to have a high reaction rate with N203, exists as an intermediate product but does not exist as a stable compound in the atmosphere.

On the other hand, the adsorption of nitrogen oxides by activated carbon is considered to be in the state of N203.

If activated carbon itself passes NO2 in the atmosphere, NO2
It has the property of reducing NO to NO.

The adsorption capacity of activated carbon itself for N203 is 2■NO2/1m4
If the coal is old, its lifespan will soon reach saturation.

Therefore, in the present invention, by applying these properties of activated carbon, N
A cycle of adsorbing NOx and absorbing the adsorbed N203 with an alkali is performed, and activated carbon is used as a type of catalyst to increase the reaction rate between NOx and the alkali.

Since the absorption reaction between NOx and alkali is the reaction that is actually progressing, the absorption capacity (removal capacity) of NOx is approximately equal to the level of the equivalent absorption reaction between NOx and alkali. , 50~N O 2/1ml
It is thought that a removal capacity comparable to that of the sample was obtained.

Therefore, activated carbon is considered to act as a catalyst, rather than as an adsorbent, as described above.

The drawing shows K2COa/Ca(OH)2/CaSO4・1
2 shows the change in NO2 removal rate when the amount of activated carbon added is changed for the composition /2H20/-3/3/2.

The test conditions are the same as in the table above.

Addition of activated carbon is effective from about 5% by weight, and if it exceeds 30% by weight, there is a disadvantage that the strength decreases.

Therefore, a range of 5 to 30% by weight is preferred.

As mentioned above, the coexistence of alkali such as K2CO3 and Ca(OH)2 and powdered activated carbon is a necessary condition for high NOx removal performance, especially NO2 removal performance, but as a hardening agent, it is not directly related to the reaction. Since it is used for granulating and curing into a porous and stable form, a relatively wide range of hardening materials can be used.

Among these, hydraulic cement materials and those that can be hardened by drying or the like after being kneaded with water are advantageous because they can uniformly disperse alkali.

Taking moldability into consideration, hydraulic cement, bentonite, diatomite, calcined gypsum (calcium sulfate), and calcium hydroxide are practically effective, and especially when calcined gypsum is used, it has the best NO2 removal performance. is obtained.

As described above, according to the present invention, nitrogen oxides in the atmosphere can be removed by an extremely simple dry method.

[Brief explanation of drawings]

The drawing shows K2CO3-Ca(OH)2-CaSO4・1/
The relationship between the amount of activated carbon added to the 2H20 system and the NO2 removal rate is shown.

Claims (1)

[Scope of Claims] 1 Nitrogen is added to an aggregate of particles such as a molded product of a hardening material selected from the group consisting of hydraulic cement material, calcined gypsum, bentonite, diatomaceous earth, and calcium hydroxide, alkali, and powdered activated carbon. A method for removing nitrogen oxides, which comprises contacting with an oxide-containing gas to absorb and remove nitrogen oxides in the gas. 2. The method for removing nitrogen oxides according to claim 1, wherein the alkali is selected from the group of hydroxides and carbonates of alkali metals or alkali metals. 3. The method for removing nitrogen oxides according to claim 2, wherein the alkali is potassium carbonate and the curing agent is calcium hydroxide. 4. The method for removing nitrogen oxides according to claim 1, wherein the activated carbon content of the particles is 5 to 30% by weight.