JP2000202236A - METHOD FOR DENITRATING WASTE GAS WITH NOx CONCENTRATION VARIABLE - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the amt. of catalyst to be packed and to curtail running costs by bringing a waste gas into contact with water to absorb and liberate NOx to level the NOx concn. and then removing the NOx in the waste gas in a denitration stage. SOLUTION: A waste gas contg. NOx is supplied from an inlet pipe 10 into a tower 1 for absorbing and liberating the NOx, water is supplied from a water circulating pipe 16, the waste gas is brought into countercurrent contact with the water in a packed bed 6, hence the NOx in the waste gas are absorbed, and the NOx in the water are liberated. The water descending in the packed bed 6 of the tower 1 is retained in a reservoir at the bottom and then circulated above the top of the packed bed 6 by a circulating pump 8. The waste gas with the NOx concn. leveled in the tower 1 is then supplied to a wet denitration tower 2 from a connecting pipe 11, a liq. absorbent is suppied from an absorbent circulating pipe 13, the waste gas is brought into countercurrent contact with the absorbent in a packed bed 7, and the NOx in the waste gas are absorbed and removed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a denitration method for removing nitrogen oxides from exhaust gas containing nitrogen oxides,
More specifically, the present invention relates to a denitration method for leveling nitrogen oxide concentration in exhaust gas in which nitrogen oxide concentration fluctuates and performing denitration treatment.

[0002]

2. Description of the Related Art Relatively high concentrations of nitrogen oxide-containing gas are emitted from semiconductor manufacturing plants, petrochemical plants, steelworks, and the like.
A wet denitration method of absorbing and removing nitrogen oxides with an absorbing solution and a dry denitration method of decomposing and removing nitrogen oxides using a catalyst are used.

In the wet denitration method, an alkali absorption method using an aqueous alkali solution containing only an alkali component such as alkali hydroxide or ammonia as an absorbing solution, or an alkali solution in which an oxidizing agent such as permanganate or chlorite is dissolved. An oxidation absorption method using an aqueous solution as an absorbing solution, a reduction absorption method using an alkaline aqueous solution in which a reducing agent such as thiosulfate or sulfide is dissolved as an absorbing solution, or a hydroxide of an alkali metal containing hydrogen peroxide and And / or aqueous carbonate solutions are known.

In the dry denitration method, a reducing substance such as ammonia is added to exhaust gas, and the exhaust gas is brought into contact with a catalyst such as a titanium oxide catalyst, an iron oxide catalyst or activated carbon to decompose nitrogen oxides into nitrogen. The removal method is often used.

[0005]

The concentration of nitrogen oxides in the exhaust gas containing nitrogen oxides generally fluctuates largely due to reasons such as repetition of batch operations in a short time. In particular, gases discharged from semiconductor cleaning processes in semiconductor manufacturing plants and coke ovens in steel mills alternately emit relatively high-concentration nitrogen oxide-containing gas and almost no nitrogen oxide-containing gas. Has been exhausted. Incidentally, the concentration of nitrogen oxides in the exhaust gas discharged from the semiconductor manufacturing plant varies between a maximum of 1000 ppm and a minimum of 0 ppm and is discharged alternately.

Further, a denitration apparatus for removing nitrogen oxides in exhaust gas containing nitrogen oxides is based on the maximum concentration of nitrogen oxides in exhaust gas in order to surely perform denitration treatment.
When the size of the equipment and operating conditions are determined and the nitrogen oxide concentration fluctuates, the size of the equipment such as a denitration tower, the absorption in the wet denitration method, Since operating conditions such as a liquid circulation amount or a catalyst filling amount in the dry denitration method are excessive, there has been a problem that equipment costs and operating costs of the apparatus increase.

Accordingly, the present invention provides a method for supplying a nitrogen oxide-containing gas having a relatively high concentration and a gas containing almost no nitrogen oxide to an exhaust gas having a variable nitrogen oxide concentration, such as an alternate discharge, to a denitration apparatus. Leveling the nitrogen oxide concentration in the exhaust gas, it is possible to increase the SV (superficial tower speed), reduce the amount of circulating absorbent in the wet denitration method, and reduce the catalyst loading in the dry denitration method. Another object of the present invention is to provide a denitration method capable of reducing equipment costs and operation costs by reducing the size of a device such as a denitration tower.

[0008]

The gist of the present invention to achieve the above object is to provide a method for removing nitrogen oxides in an exhaust gas containing nitrogen oxides according to the present invention. A concentration control step of leveling the nitrogen oxide concentration by contacting the exhaust gas with water to absorb and diffuse nitrogen oxides, and a denitration step of removing nitrogen oxides in the exhaust gas after the concentration control step. A method for denitration of exhaust gas in which the concentration of nitrogen oxide fluctuates.

In the above, industrial water, tap water, pure water or seawater can be used as the water in the nitrogen oxide concentration control step. However, industrial water is used because of its availability and cost. Preferably, as a contact device between the exhaust gas and water in the concentration control step, a packed tower, an absorption and diffusion tower such as a plate tower or a spray tower is used. A packed tower using a cylindrical packing material is preferred. However, the invention is not limited to those structures.

In the nitrogen oxide concentration adjusting step, when the nitrogen oxide concentration in the exhaust gas is relatively high,
The amount of absorption of nitrogen oxides by water also increases, and the nitrogen oxide concentration in the exhaust gas supplied to the denitration process is supplied with a gas that is lower than the initial concentration by a certain amount. Or, if not contained, the nitrogen oxides absorbed at the time of the high concentration is dissipated, and the nitrogen oxide concentration in the exhaust gas supplied to the denitration step is supplied with a gas that has increased by a certain amount from the initial concentration. Therefore, even if the nitrogen oxide concentration in the exhaust gas fluctuates greatly, the nitrogen oxide concentration supplied to the denitration step is absorbed and diffused, and supplied evenly.

In the denitration step, either a wet denitration method or a dry denitration method may be used. Generally, nitrogen oxides in exhaust gas have a low concentration of nitrogen monoxide and a high concentration of nitrogen dioxide. Since the wet denitration method has a higher denitration efficiency, it is preferable to use a wet denitration method.In the case of using a wet denitration method, a wet denitration apparatus such as a packed tower, a plate tower or a spray tower is used. A packed tower using a cylindrical filler made of a synthetic resin is preferred from the viewpoint of absorption efficiency and ease of handling. However, the invention is not limited to those structures.

In the case where a wet denitration apparatus is used in the denitration step, the absorbing solution contains an alkaline aqueous solution containing only an alkaline component, an alkaline aqueous solution containing an oxidizing agent, an alkaline aqueous solution containing a reducing agent, or hydrogen peroxide. An absorbent solution used in conventional wet denitration equipment such as an alkaline aqueous solution can be used.However, if an alkaline solution containing hydrogen peroxide is used, wastewater discharged from a semiconductor manufacturing plant can be effectively used, and wastewater treatment can be performed. It is extremely economical because it reduces the cost of chemicals for use and the cost of chemicals for absorbing liquid, and also has a high absorption efficiency and a low processing load in the processing of the absorbing liquid discharged after the absorption processing. This is preferable because it is possible to reduce the chemical cost of the chemical treatment in the wastewater treatment device and to prevent the growth of microorganisms in the biological treatment from being inhibited.

In the case of using a dry denitration apparatus, a reducing substance such as ammonia is added to exhaust gas,
A method of contacting with a catalyst to decompose and remove nitrogen oxides into nitrogen is used. Further, as the catalyst, it is possible to use a general catalyst formed into a particle shape such as titanium oxide, iron oxide or activated carbon, or into a cylindrical shape, a cylindrical shape, a honeycomb shape, a saddle shape, or the like. It is preferable to use a honeycomb-shaped titanium oxide catalyst from the viewpoints of reactivity and reactivity.

[0014]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a system diagram of one embodiment of the present invention, FIG. 2 is a graph showing a temporal change of the nitrogen oxide concentration in the exhaust gas before the nitrogen oxide concentration adjusting step, and FIG. 3 is an exhaust gas after the nitrogen oxide concentration adjusting step. Graph of nitrogen oxide concentration over time in FIG. 4, FIG. 4 is a graph of nitrogen oxide concentration over time in the processing gas after the denitration step, and FIG. 5 is a graph showing the relationship between the nitrogen oxide concentration in the exhaust gas supplied to the denitration step and SV. It is a graph shown.

Reference numeral 1 denotes a nitrogen oxide absorption / dissipation tower in a concentration control step of bringing a nitrogen oxide-containing exhaust gas into contact with water to absorb and disperse the nitrogen oxides and level the nitrogen oxide concentration. A filling layer 6 is provided by filling a cylindrical, columnar, saddle-shaped filler or the like made of a synthetic resin, ceramic, or the like therein, and a liquid pool in which a certain amount of the absorbing liquid is retained at the bottom. A water circulation pipe 16 is provided below the packed bed 6 for introducing an exhaust gas containing nitrogen oxide and a water circulating pump 8 for communicating with the liquid reservoir and extracting water with a water circulation pump 8. , A water supply pipe 17 for supplying new water such as industrial water, and a water discharge pipe 18 are connected.
Further, a connection pipe 11 and a water circulation pipe 16 for supplying the exhaust gas having the nitrogen oxide concentration leveled to the subsequent denitration step are connected to the upper part.

Reference numeral 2 denotes a wet denitration tower in a denitration step for removing nitrogen oxides from the exhaust gas passing through the nitrogen oxide absorption and diffusion tower 1, and has a cylindrical or columnar shape made of synthetic resin, ceramic or the like. , A filling material such as a saddle is filled, a filling layer 7 is formed, a liquid pool portion in which a certain amount of the absorbing liquid is retained is formed at the bottom, and a nitrogen reservoir is formed below the filling layer 7. An absorbent circulating pipe 1 which communicates with a connecting pipe 11 for the exhaust gas discharged from the concentration control step of the oxide concentration and a liquid reservoir, extracts an absorbent by an absorbent circulating pump 4 and circulates it to a packed bed 7.
3. An absorption liquid supply pipe 14 for supplying new absorption liquid from the absorption liquid tank 3 via the absorption liquid supply pump 5, an absorption liquid discharge pipe 15, and the like are connected. Further, an absorption liquid circulation pipe 13 and a processing gas discharge pipe 12 for discharging the processing gas into the atmosphere are connected to the upper part.

Hereinafter, a wet denitration method for absorbing and removing nitrogen oxides from an exhaust gas containing nitrogen oxides by the denitration apparatus having the above configuration will be described. In the nitrogen oxide concentration adjusting step, the exhaust gas containing nitrogen oxide is supplied from the introduction pipe 10 to the nitrogen oxide absorption / dissipation tower 1, flows through the packed bed 6 in an upward flow, and the water is circulated in the water. The exhaust gas is supplied from the pipe 16 and flows through the packed bed 6 in a downward flow, so that the exhaust gas and the water come into countercurrent contact with each other.
The nitrogen oxides in the exhaust gas are absorbed and the nitrogen oxides in the water are released.

That is, when the concentration of nitrogen oxides in the exhaust gas is relatively high, the amount of nitrogen oxides absorbed by water in the packed bed 6 also increases, and the concentration of nitrogen oxides in the exhaust gas supplied to the denitration step is increased. Is supplied with a gas that is lower than the initial concentration by a certain amount. Also, the nitrogen oxide concentration is low, or
When not contained, the nitrogen oxides absorbed at the time of the high concentration are diffused, and the nitrogen oxide concentration in the exhaust gas supplied to the denitration step is supplied with a gas which is increased by a certain amount from the initial concentration. become. Therefore, even if the nitrogen oxide concentration in the exhaust gas fluctuates greatly, the nitrogen oxide concentration supplied to the denitration step is absorbed and diffused, and is supplied evenly.

The water descending down the packed bed 6 of the absorption and diffusion tower 1
After staying in the liquid reservoir provided at the bottom, the water is sucked and extracted by the circulation pump 8 provided in the water circulation pipe 16, and circulated and supplied from the water circulation pipe 16 to the upper part of the packed bed 6.

Further, a part of the water is withdrawn from the liquid reservoir through the drain pipe 18 to the outside of the absorption and diffusion tower 1, treated by a wastewater treatment device (not shown), and then discharged to a river or the like. Is controlled by the degree of water degradation (content of impurities, etc.). In addition, new water such as industrial water is supplied from the water supply pipe 17 to the reduced water.

In the denitration step, the exhaust gas whose nitrogen oxide concentration has been leveled through the absorption and diffusion tower 1 is supplied from the connection pipe 11 to the wet denitration tower 2, flows through the packed bed 7 in an upward flow, and The absorbing liquid is supplied from the absorbing liquid circulation pipe 13 and
The exhaust gas and the absorbing liquid are brought into countercurrent contact with each other, whereby nitrogen oxides in the exhaust gas are absorbed and removed.

The absorbent is retained in a liquid reservoir provided at the bottom of the wet type denitration tower 2 and is then drawn out by a circulating pump 4 provided in the absorbent circulating pipe 13 to be withdrawn. 13 circulates and supplies the upper part of the packed bed 7. The circulated absorption liquid is supplied to the liquid storage part with the absorption liquid extracted from the absorption liquid tank 3 by the absorption liquid supply pump 5 to be adjusted in concentration.

Further, a part of the absorbing liquid is withdrawn from the liquid reservoir through the absorbing liquid extracting pipe 15 to the outside of the wet denitration tower 2, treated by a wastewater treatment device (not shown), and then discharged to a river or the like. The withdrawal amount of the absorbing liquid is controlled by the composition of the absorbing liquid. The processing gas from which the nitrogen oxides have been removed is discharged into the atmosphere from the processing gas discharge pipe 12.

An example in which nitrogen oxides in exhaust gas discharged from a semiconductor manufacturing process are removed by the denitration method will be described in detail below. The exhaust gas discharged from the semiconductor manufacturing process has a nitrogen oxide concentration ranging from 0 ppm to 1000 ppm as shown in the graph of the nitrogen oxide concentration change with time in FIG.
The exhaust gas fluctuates intermittently, and the exhaust gas is supplied to a nitrogen oxide absorption / dissipation tower, which is a nitrogen oxide concentration adjusting step, and is processed. As shown by, it is leveled to about 500 ppm.

The exhaust gas whose nitrogen oxide concentration has been leveled in the concentration control step is supplied to a wet denitration tower which is a denitration step to be subjected to denitration treatment. In addition, most nitrogen oxides can be stably absorbed and removed.

The SV (superficial velocity) in the above-mentioned wet absorption tower is such that the nitrogen oxide concentration at the inlet of the absorption tower is about 1000 p.
pm and an exhaust gas of about 500 ppm were subjected to a denitration treatment by a reduction absorption method using a thiosulfate solution, and the SV for maintaining the nitrogen oxide concentration at the outlet of the absorption tower at 100 ppm was obtained and graphed. As is clear from the graph showing the relationship between the concentration and the SV, it depends on the nitrogen oxide concentration in the supplied exhaust gas, and the SV value is about 1000 ppm when the nitrogen oxide concentration in the supplied exhaust gas is 500 ppm.
About twice as large as Therefore, by leveling the nitrogen oxide concentration in the exhaust gas at a low value, the amount of exhaust gas treatment can be increased, so that a device such as a wet denitration tower can be miniaturized,
The circulating amount of the absorbing liquid can be reduced.

[0027]

According to the present invention, there is provided a method for denitrifying an exhaust gas having a variable nitrogen oxide concentration, such as an exhaust gas in which a relatively high concentration of nitrogen oxide-containing gas and a gas containing almost no nitrogen oxide are alternately discharged. By leveling the nitrogen oxides in the nitrogen oxide-containing gas supplied to the denitration device, the SV
Since it is possible to increase the (superficial tower speed) and decrease the amount of the circulating absorbent, the size of the apparatus such as the denitration tower can be suppressed, and the operating cost can be reduced.

[Brief description of the drawings]

FIG. 1 is a system diagram of an embodiment of the present invention.

FIG. 2 is a graph showing the change over time of the nitrogen oxide concentration in the exhaust gas before the nitrogen oxide concentration adjusting step.

FIG. 3 is a graph showing the change over time of the nitrogen oxide concentration in the exhaust gas after the nitrogen oxide concentration adjusting step.

FIG. 4 is a graph showing a change with time of a nitrogen oxide concentration in a processing gas after a denitration process.

FIG. 5 is a graph showing a relationship between a nitrogen oxide concentration in an exhaust gas supplied to a denitration process and SV.

[Explanation of symbols]

 1: Absorption and diffusion tower 2: Wet denitration tower 3: Absorption liquid tank 4: Absorption liquid circulation pump 5: Absorption liquid supply pump 6: Absorbent layer 7: Packed layer 8: Water circulation pump 10: Intake pipe for exhaust gas 11: Connection pipe 12: Process gas discharge pipe 13: Absorbent liquid circulation pipe 14: Absorbent liquid supply pipe 15: Absorbent liquid discharge pipe 16: Water circulation pipe 17: Water supply pipe 18: Water discharge pipe

Continued on front page F-term (reference) 4D002 AA12 AC03 AC10 BA02 BA03 BA05 BA06 CA01 CA07 DA01 DA07 DA11 DA12 DA14 DA21 DA22 DA35 DA41 DA52 EA01 EA02 GA02 GA03 GB01 GB02 HA03

Claims (1)

[Claims] 1. A denitration method for removing nitrogen oxides in an exhaust gas containing nitrogen oxides, wherein the exhaust gas is brought into contact with water to absorb and disperse the nitrogen oxides, thereby leveling the nitrogen oxide concentration. An exhaust gas denitration method in which the nitrogen oxide concentration fluctuates, comprising a control step and a denitration step of removing nitrogen oxides in the exhaust gas after the concentration control step.