JPH0649133B2 - Method for removing nitrogen oxides in exhaust gas - Google Patents

Description

TECHNICAL FIELD The present invention mainly relates to a refuse incinerator, a pulverized coal burning combustion furnace,
The present invention relates to a method for effectively removing nitrogen oxides (NOx) in combustion exhaust gas discharged from a gas fired boiler, a kerosene fired boiler, and the like.

Conventional technology and its problems Conventionally, nitrogen oxides in this kind of exhaust gas have been removed by treating the exhaust gas by a non-catalytic denitration method, a catalytic denitration method, an exhaust gas circulation method or the like. By the way, when, for example, the waste incineration exhaust gas is treated by these methods, the dischargeable concentration of nitrogen oxides is 50 to 8 in the non-catalytic denitration method.
0 ppm, 40 to 60 ppm in the catalytic denitration method and 60 to 80 ppm in the exhaust gas circulation method (both oxygen concentration 12
% Conversion value). Therefore, when it is necessary to remove nitrogen oxides to a low concentration, the catalytic denitration method will be adopted, but this method requires an expensive denitration catalyst, so the initial cost and running required for its operation Both costs were a heavy burden.

Under such circumstances, the present inventors have previously applied for a patent for a method for removing nitrogen oxides and mercury in exhaust gas at the same time by wet-cleaning the exhaust gas with a liquid containing an oxidant (Japanese Patent Application No. SHO 6-96).
No. 1-246623, Japanese Patent Application No. 62-83538), and as a more efficient method for removing nitrogen oxides, a patent application has been filed for a method using an absorbent containing bromine ions together with an oxidizing agent (Japanese Patent Application No. 63-24146). ).

This invention is an extension of the invention of the above patent application, and as a result of repeated studies on the development of a more efficient nitrogen oxide removal method, a solution containing ammonium ions together with an oxidizing agent, and an oxidizing agent It has been completed by finding that the absorbing solution containing ammonium ion and bromine ion shows a remarkable effect.

Means for Solving Problems A method for removing nitrogen oxides in exhaust gas according to the present invention is a method in which an exhaust gas is wet-cleaned with an oxidizing agent-containing liquid, and at the same time, the same liquid is used as an absorbing liquid for absorbing nitrogen oxides in the exhaust gas. A liquid containing ammonium ion and bromine ion together with an oxidizing agent is used. The pH of the absorbent is
It is preferably 4 to 6.

Typical examples of the exhaust gas to which the method according to the present invention can be applied are the exhaust gas of a refuse incinerator, the exhaust gas of a pulverized coal burning furnace, the exhaust gas of a gas burning furnace or the exhaust gas of a kerosene burning furnace.

The absorption liquid containing ammonium ions is generally prepared by adding ammonia water or an ammonium salt such as ammonium chloride, but it can also be prepared from wastewater containing ammonia such as clinker wastewater of a refuse incinerator, and It can also be prepared by absorbing ammonia gas, ammonium chloride, etc. in the exhaust gas into the absorbing liquid. In the method of preparing the absorption liquid from the ammonia gas in the exhaust gas, ammonia generated when the suppression combustion by the exhaust gas circulation method is performed, or ammonia that leaks when the reduction denitration by the non-catalytic denitration method is performed as described later Will be used. In the non-catalytic denitration method, reduction denitration is performed by injecting or spraying an ammonia gas or a liquid containing an ammonia generating substance into the exhaust gas to be treated.

Representative examples of the ammonia-containing substance-containing liquid include aqueous ammonia and aqueous urea. These substances are
When sprayed into hot exhaust gas, it produces ammonia gas.

The temperature of the exhaust gas is usually 800 to 950 ° C, but this is not limiting.

The non-catalytic reduction denitration reaction is represented by the following reaction formula.

4NO + 4NH ₃ + O ₂ → 4N ₂ + 6H ₂ O In this reaction, the relationship between the NH ₃ / HOx molar ratio and the NOx removal rate is as shown in FIG. 3 in the case of an application example to a waste incinerator exhaust gas. According to this, a significant part of the amount of NH ₃ supplied does not contribute to the reaction,
It can be seen that NH ₃ is left as it is or is reacted with hydrogen chloride gas in the exhaust gas to generate ammonium chloride (NH ₄ C1), which leaks from the reaction system. As a reference, FIG. 4 shows the relationship between the NH ₃ / NOx molar ratio and the leak NH ₃ in the case of application to a waste incinerator exhaust gas.

Next, when the exhaust gas is washed with a liquid containing an oxidant or a liquid containing bromine ions together with the oxidant, about 60% of the leak ammonia (NH ₃ or NH ₄ C1) is absorbed and removed in the cleaning liquid. The ammonia (NH ₃ or NH ₄ C1) absorbed in the cleaning liquid in this manner effectively acts on the absorption and removal of NOx remaining in the exhaust gas.

As the oxidizing agent of the absorbing solution, sodium salt, potassium salt or calcium salt of hypochlorous acid or chlorous acid, ozone and the like are often used. The hypochlorite may be produced by electrolyzing an alkali chloride-containing liquid that has absorbed hydrogen chloride gas in the exhaust gas.

The concentration of the oxidant in the absorption liquid is preferably several times as available chlorine.
When the concentration of nitrogen oxides in the exhaust gas is high, the hypochlorite is also used at a high concentration in the range of ppm to several hundreds ppm.

On the other hand, an absorbing solution containing bromide ions is generally prepared by adding a bromide such as alkali bromide. Further, this absorbing solution can be prepared from seawater, and further, it can also be prepared from smoke washing wastewater discharged from a refuse incineration plant.

The bromine ion concentration in the absorption liquid is preferably Br ₂ / C.
a 3 / 100-1 / 10 l ₂ molar ratio.

The reaction between nitrogen oxides and hypochlorite, which is an oxidant, is estimated as follows.

ClO ⁻ + NO → NO ₂ + Cl ^{− In} this reaction, the action mechanism of bromine ion and ammonia ion coexisting in the absorbing solution is not clear, but they act as a catalyst for the oxidation reaction of nitrogen oxides, and ammonium ion is the liquid. It is considered that it also acts as a neutralizing agent for NO ₂ absorbed therein.

As a result of the exhaust gas treatment by the wet cleaning of the present invention, the nitrogen oxides are oxidized and absorbed and removed by the action of the oxidizing agent, ammonium ions and bromine ions into a form that is easily absorbed by water.

EFFECTS OF THE INVENTION According to the nitrogen oxide removing method of the present invention, a liquid containing ammonium ions and bromine ions together with an oxidant is used as the absorbing liquid, so that nitrogen oxides in combustion exhaust gas can be effectively removed. . In particular, the effect is remarkable when the latter absorbing liquid is used, without requiring a large initial cost and running cost due to an expensive denitration catalyst like the catalytic denitration method described at the beginning of this document. An extremely high denitration rate equal to or higher than that of the catalytic denitration method can be obtained.

Further, in the wet cleaning treatment of the present invention, in addition to nitrogen oxides, hydrogen chloride gas, sulfur oxides and mercury in exhaust gas can be removed extremely effectively at the same time.

Further, when the wet cleaning treatment of the present invention is used in combination with the non-catalytic denitration method, NH ₃ supplied by the denitration method does not contribute to the reaction in a considerable proportion, and NH ₃ remains as it is or in the exhaust gas. Leaked from the reaction system as a reaction product with hydrogen chloride gas, ammonium chloride (NH ₄ Cl), this leaked ammonia (NH ₃ or NH ₄ Cl) is absorbed and removed in the cleaning liquid in the wet cleaning treatment of the present invention, Since it effectively contributes to the absorption and removal of NOx remaining in the exhaust gas, the supply amount of the chemicals used in each step is set by setting each denitration rate of the non-catalytic denitration method and the wet cleaning treatment of the present invention to an appropriate value. Can be lean. Therefore, it is particularly effective to carry out the wet cleaning treatment of the present invention in combination with the non-catalytic denitration method.

Embodiment Next, an embodiment of the present invention will be described with reference to the accompanying drawings.

Example 1 A nitrogen oxide removal experiment was conducted in the experimental apparatus shown in FIG. In this experiment, O ₂ 14% and CO ₂ 5%
N ₂ -based simulated exhaust gas containing
Then, one of them was directly led to a NOx analyzer (33) to measure the nitrogen oxide concentration in the gas before treatment. The other is a reaction bottle
(31) and the C1 ₂ absorption bottle (32) were sequentially passed, and then introduced into a NOx analyzer (34) to measure the nitrogen oxide concentration in the gas after the treatment.

The absorption solutions of Experiment Nos. 1 to 7 shown in Table 1 were sequentially placed in the reaction bottle (31), and 1N NaOH aqueous solution was placed in the Cl ₂ absorption bottle (32) to collect the vaporized chlorine gas. . Also, N
The gas flow rate through the Ox analyzer (34) was adjusted to 2 / min. Measured values by NOx analyzers (33) (34)
Recorded by (35). Further, in Experiment No. 5 in Table 1, ammonia gas was injected into the supplied simulated gas.

In this way, an experiment was conducted for each absorbent.
Table 1 shows the measured NOx concentration recorded on the recorder (35) and the NOx removal rate derived from this value.

Embodiment 2 FIG. 2 shows an example in which the method for removing nitrogen oxides according to the present invention is applied to the treatment of exhaust gas from a refuse incinerator in combination with a non-catalytic denitration method. In the figure, garbage incinerator (1)
Ammonia gas or ammonia water or urea water is sprayed on the top part (1a) of the above, and a noncatalytic reduction denitration reaction is performed at an exhaust gas temperature of 800 to 950 ° C. The NH ₃ / NOx molar ratio is 0.
It is appropriately set in the range of 5 to 2.0.

Exhaust gas emitted from the refuse incinerator (1) is passed through an electrostatic precipitator (2) to remove dust.

Then, the exhaust gas from which dust was removed was sent to the wet washing tower (3), and a liquid containing an alkali agent and sodium hypochlorite as an oxidizing agent and potassium bromide as a bromine ion was used as an absorbing liquid.
Disperse in a smoke-washing tower (3) at a temperature of 60 to 70 ° C.

As a result of treating the exhaust gas containing nitrogen oxides in this way, nitrogen oxides were removed at a high removal rate of 70 to 80%.

[Brief description of drawings]

FIG. 1 is the flow sheet of Example 1, FIG. 2 is the flow sheet of Example 2, and FIG. 3 is the NH ₃ / NOx molar ratio and NOx.
FIG. 4 is a graph showing the relationship between the removal rates, and FIG. 4 is a graph showing the relationship between the NH ₃ / NOx molar ratio and the leak NH ₃ .

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Tadao Murakawa 1-6-14 Edobori, Nishi-ku, Osaka City, Osaka Prefecture Hitachi Shipbuilding Co., Ltd. (72) Inventor Akio Hirojo 1-6-14 Edobori, Nishi-ku, Osaka Hitachi Shipbuilding Co., Ltd. (72) Inventor Toshiharu Kobayashi 8-7 Tomogaoka, Suma-ku, Kobe City, Hyogo Prefecture (56) References JP-A-50-152991 (JP, A) JP-A-52-32871 (JP, A) Special Kai-49-67869 (JP, A) JP-A-51-100966 (JP, A)

Claims (1)

[Claims] 1. A wet cleaning treatment of an exhaust gas with an oxidizing agent-containing liquid, and at the same time absorbing nitrogen oxides in the exhaust gas into the same, a liquid containing ammonium ions and bromine ions together with an oxidizing agent is used as an absorbing liquid. A method for removing nitrogen oxides in exhaust gas, which is characterized by being used.