TWI636824B - Method for treating exhaust gas containing nitrous oxide and device thereof - Google Patents

Abstract

The present invention provides a treatment method for nitrous oxide-containing exhaust gas which is excellent in removal rate of nitrous oxide and has few by-products and the like, and a treatment apparatus for nitrous oxide-containing waste gas which can prolong life.

Description

Method for treating exhaust gas containing nitrous oxide and device thereof

The present invention relates to an exhaust gas treatment method and apparatus thereof for treating exhaust gas derived from a process of an electronic device such as a semiconductor or a liquid crystal, particularly an exhaust gas containing nitrous oxide (N ₂ O).

A gas of various compounds is used in the process of an electronic device such as a semiconductor or a liquid crystal, wherein the nitrous oxide is a deposition gas used for forming a film for protecting a ruthenium oxide film on a germanium semiconductor substrate.

It is known that nitrous oxide is mostly used as an inhalation anesthetic for medical use, and the exposure limit value is relatively high (TLV value: 50 ppm), although the toxicity to the human body is not a problem, but at the 3rd Global Climate Change Conference (COP) Special mention in 3) that nitrous oxide is equivalent to carbon dioxide, methane and chlorofluorocarbons as strong greenhouse gases (the warming effect is about 300 times that of carbon dioxide), and the half-life in the atmosphere is about 150 years, if directly The release of nitrous oxide into the atmosphere will have a major impact on the global environment.

In the conventional electronic device manufacturing process, the proportion of nitrous oxide is less than that of decane (Si _n H _2n+2 ) or a fluorine compound, so that the exhaust gas containing nitrous oxide and the fluorine-containing compound are contained. The other exhaust gas is removed by an exhaust gas treatment device of various treatment methods such as heat decomposition, and then discharged to the atmosphere.

An exhaust gas treatment device disclosed in Patent Document 1 is known, in which dust or the like contained in harmful exhaust gas is removed by a scrubber of an intake port, and then an exhaust gas treatment tower equipped with an electric heater is used to heat-decompose the exhaust gas. The decomposed gas is then detoxified (ie, removed) by a gas-liquid contact at a wet scrubber.

There is also known an exhaust gas treatment method and apparatus therefor, which are disclosed in the patent document 2, which is used for treating an exhaust gas containing nitrous oxide, and after the exhaust gas containing nitrous oxide is washed with a scrubber of an intake port, the exhaust gas is exhausted. A method of removing a nitrous oxide-containing exhaust gas by adding a reducing gas to prepare a mixed gas and heating the mixed gas to a temperature equal to or higher than a thermal decomposition temperature of nitrous oxide in a reducing atmosphere. The reducing gas disclosed in Patent Document 2 is a gas selected from at least one of hydrogen, ammonia, or hydrocarbon.

[Previous Technical Literature]

[Patent Document 1] Japanese Patent Laid-Open No. 2002-188810

[Patent Document 2] Japanese Patent Laid-Open No. 2005-125285

Generally, when a gas containing nitrous oxide is removed by thermal oxidative decomposition (*1), a high-temperature reaction of about 600 ° C to 1300 ° C is required, so high. In the warm state, in addition to energy consumption, the damage of the treatment tower and the heater is also large, so a treatment method and a device capable of decomposing nitrous oxide at a relatively low temperature are required.

The inventors have found that by introducing a specific ratio of reducing gas and compressed dry air (CDA) at the inlet (bottom) of the treatment column, the reduction reaction (*2) can be used to decompose nitrous oxide at a relatively low temperature. The role of the CDA is to produce a by-product (CO) when the reduction reaction is caused by harmlessness (*3) (the following reaction formula is used to explain the reaction process, and the reaction coefficient is omitted) to obtain an excellent nitrous oxide removal. rate.

*1: Thermal oxidative decomposition reaction..................N ₂ O+O ₂ NO+NO ₂

*2: Reduction reaction (using CH ₄ as an example).........N ₂ O+CH ₄ N ₂ +H ₂ O+CO

*3: CO is harmless...........................CO+O ₂ CO ₂

The inventors have also found that in order to prevent the temperature of the treatment tower from rising too much due to the reaction of nitrous oxide and the combustion of the reducing gas, a system for automatically controlling the addition of nitrogen (N ₂ ) can effectively reduce the treatment tower and The temperature of the heater rises excessively, which in turn increases the life of the processing tower and heater.

An object of the present invention is to provide a method for treating an exhaust gas containing nitrous oxide which is excellent in the removal rate of nitrous oxide and which has few by-products, and the like, and a treatment apparatus for providing an exhaust gas containing nitrous oxide which can prolong the life.

That is, the present invention provides the following invention.

[1] A method of treating an exhaust gas containing nitrous oxide, comprising the steps of: a step of mixing nitrous oxide-containing exhaust gas with a reducing gas and compressed dry air; and heating the arsenic oxide in the exhaust gas by heating the mixed gas in a reducing environment to a temperature at which the nitrous oxide is thermally decomposable a step of removing the mixing ratio of the reducing gas and the compressed dry air in the step of mixing the exhaust gas containing nitrous oxide with the reducing gas and the compressed dry air, Reducing gas: compressed dry air = 7:100~7:130 to mix with nitrous oxide-containing exhaust gas.

[2] The method according to the above [1], wherein the reducing gas is at least one selected from the group consisting of hydrogen, ammonia, and hydrocarbon.

[3] The method according to the above [2], wherein the reducing gas is a hydrocarbon.

[4] The method according to the above [3], wherein the reducing gas is methane.

[5] The method according to any one of the above [1] to [4] wherein, in the step of mixing, nitrogen is further mixed.

[6] An apparatus for treating an exhaust gas containing nitrous oxide according to the method of [1], comprising: an inflow pipe, a reducing gas pipe, a compressed dry air pipe, an exhaust gas treatment tower, a heater, and a discharge Pipe, inflow pipe: one end is connected to a source of exhaust gas containing nitrous oxide, and the other end is connected to an exhaust gas treatment tower; a reducing gas pipe is connected at one end to a supply of reducing gas, and the other end is connected to an inflow pipe; Dry air piping: one end connected to the supply of compressed dry air The other end is connected to the inflow pipe; the exhaust gas treatment tower has one end connected to the inflow pipe and the other end connected to the discharge pipe, and has an exhaust gas decomposition treatment chamber for heating the mixed gas; the heater: located in the exhaust gas treatment tower It is used to heat the exhaust gas decomposition treatment chamber to a temperature above the thermal decomposition temperature of nitrous oxide, and to arrange the end of the inflow pipe at one end; the discharge pipe has one end connected to the exhaust gas treatment tower and the other end connected to the atmospheric environment.

According to the present invention, it is possible to provide a treatment method for nitrous oxide-containing exhaust gas which is excellent in nitrous oxide removal rate and has few by-products and the like, and a treatment apparatus for nitrous oxide-containing waste gas which can prolong life.

1‧‧‧Exhaust gas treatment device

2‧‧‧Exhaust gas treatment tower

20‧‧‧Exhaust gas treatment tower body

21‧‧‧Exhaust gas decomposition treatment room

22‧‧‧heater

3‧‧‧Inflow piping

4‧‧‧ discharge piping

5‧‧‧Reducing gas piping

6‧‧‧Compressed dry air piping

7a, 7b‧‧‧ scrubber

8‧‧‧Additional nitrogen (N ₂ )

P‧‧‧ pump

F‧‧‧Exhaust fan

Fig. 1A is a configuration diagram of an exhaust gas treatment device of the present invention.

Fig. 1B is a schematic view showing temperature measurement points of respective relative positions in the exhaust gas decomposition treatment chamber of the present invention.

[Best form of implementing the invention]

Hereinafter, the present invention will be described based on the drawings. 1A is a structural view of an exhaust gas treatment device (1) of the present invention, which is roughly an exhaust gas treatment tower. (2) An inflow pipe (3), a discharge pipe (4), a reducing gas pipe (5), and a compressed dry air pipe (6).

First, the exhaust gas (G1) containing nitrous oxide is removed from the exhaust gas (G1) in advance by the scrubber (7a) provided in the inflow pipe (3) before the heating reaction, thereby preventing An abnormality caused by dust or water-soluble substances during the heating reaction. Further, a moisture removing means (not shown) may be provided on the way from the scrubber (7a) flowing into the pipe (3) to the bottom of the exhaust gas treatment tower body (20), and the washed exhaust gas (G1) The water removal means is not particularly limited as long as it can remove moisture, and examples thereof include a quenching device and a water absorbing agent.

The exhaust gas treatment tower (2) heat-decomposes the nitrous oxide-containing exhaust gas (G1) to form a sealed gas (G2), and the exhaust gas treatment tower (2) is a cylindrical exhaust gas treatment tower body ( 20) An exhaust gas decomposition processing chamber (21) provided inside the main body (20) and one or more heaters (22) vertically disposed in the exhaust gas decomposition processing chamber (21).

The exhaust gas treatment tower body (20) has a cylindrical outer casing jacket made of steel, and the inner portion of the inner portion that is in contact with the exhaust gas decomposition treatment chamber (21) can be made of a high temperature resistant metal material or a lining formed of a fireproof material. The member allows the exhaust gas (G1) to be in direct contact with the inner metal material or the lining member.

The bottom of the exhaust gas treatment tower main body (20) is made of a metal pipe excellent in heat resistance and corrosion resistance, and is connected to an end portion of the inflow pipe (3), and a heater is disposed around the metal pipe so as to be surrounded ( twenty two). Exhaust gas The treatment tower body (20) is located near the bottom end of the exhaust gas decomposition treatment chamber (21) surrounded by the heater (22) and the inside of the exhaust gas treatment tower body (20), and has an end connected to the discharge pipe (4). At the office.

The heater (22) heats the exhaust gas decomposition treatment chamber (21) to a thermal decomposition temperature of nitrous oxide (specifically, about 600 ° C to 1300 ° C) to thermally decompose the exhaust gas (G1), and the heater (22) It is preferable to use an electrothermal heater formed of a solid or hollow rod-shaped heating element composed of tantalum carbide. In the present invention, the bottom end portion of the heater (22) is provided in such a manner as to surround the metal pipe, and the gas system to be treated such as exhaust gas (G1) passes through the space enclosed by the heater (22) and is bottomed up. The ground flows, and after the gas reaches the tip end portion of the heater (22), it flows through the space between the heater (22) and the exhaust gas decomposition processing chamber (21), and then flows to the discharge pipe (4).

The inflow pipe (3) is connected to the exhaust gas treatment tower body (20) at one end (as described above), and the other end is connected to a generation source of the exhaust gas (G1) of the semiconductor manufacturing device or the like to introduce the exhaust gas (G1) into the exhaust gas decomposition treatment. Piping of the chamber (21).

The discharge pipe (4) discharges the treated gas (G2) after the treatment in the exhaust gas decomposition treatment chamber (21) to the atmosphere, and an exhaust fan (F) can be installed in the middle of the discharge pipe (4). The treated gas (G2) in the exhaust gas decomposition processing chamber (21) is sucked and discharged to the atmosphere. Further, a scrubber (7b) may be provided in the middle of discharging the pipe (4) to remove dust or the like in the treated gas (G2).

The reducing gas pipe (5) is used for reducing gas (R) such as hydrogen, ammonia, or hydrocarbons, and exhaust gas (G1) flowing through the inflow pipe (3). The mixing pipe is connected, and the reducing gas pipe (5) is connected to one end of the reducing gas supply (one device/not shown), and the other end is connected (inserted) to the inflow pipe (3).

The compressed dry air pipe (6) is a pipe for mixing compressed dry air (CDA) with reducing gas (R) and exhaust gas (G1), and the compressed dry air pipe (6) is connected at one end to compressed dry air (CDA). The supply (a device/not shown) is connected (inserted) to the inflow pipe (3).

In addition to the above configuration, the exhaust gas treatment device (1) may be connected to another wiring or measuring device (for example, a flow meter, a flow rate regulator, etc.) or the like, and stored in the processing device.

Further, the flow portion (inflow pipe (3)) of the exhaust gas (G1) other than the exhaust gas treatment tower (2), the flow portion (the discharge pipe (4)) of the treated gas (G2), and the reducing gas pipe (5) ), etc., because it contains exhaust gas (G1) or acid gas generated by decomposing the exhaust gas (G1), in order to prevent corrosion of these parts, the gas circulation part is applied to polyvinyl chloride or polyethylene. Corrosion-resistant lining or coating film such as (polyethylene), unsaturated polyester resin, or fluororesin.

Next, the operation of the exhaust gas treatment device (1) of the present invention will be described.

The exhaust gas generating source such as a semiconductor manufacturing device is connected to the inflow pipe (3), and after the power of the exhaust gas treating device (1) is turned on, the heater (22) starts heating the exhaust gas treating tower (2). available A pipe (not shown) connected to the tip end of the exhaust gas treatment tower body (20) supplies an inert gas into the exhaust gas treatment tower (2).

When the temperature in the exhaust gas treatment tower (2) is sufficiently raised to the state of the thermally decomposable exhaust gas (G1), the exhaust fan (F) is activated to cause the exhaust gas (G1) to enter the inflow pipe (3), at this time, in order to make the exhaust gas ( G1) Dilution or uniform flow, usually a small amount of inert gas is introduced into the exhaust gas (G1), and the reducing gas (R) and compressed dry air (CDA) are respectively reduced from the bottom of the exhaust gas treatment tower body (20). The gas pipe (5) and the compressed dry air pipe (6) are introduced into the inflow pipe (3) to be a mixed gas of the exhaust gas (G1), the reducing gas (R), and the compressed dry air (CDA). ). The mixed gas (M) enters the exhaust gas decomposition treatment chamber (21) from the metal pipe at the bottom of the exhaust gas treatment tower body (20), and the exhaust gas decomposition treatment chamber (21) is maintained at a specified high temperature of about 600 ° C to 1300 ° C. Next, the mixed gas (M) is thermally decomposed in the exhaust gas decomposition treatment chamber (21) to be converted into the treated gas (G2).

At this time, if the reducing gas (R) is hydrogen (H ₂ ), the conversion reaction of the nitrous oxide-containing exhaust gas (G1) will be N ₂ O+H ₂ N ₂ +H ₂ O; if the reducing gas (R) is ammonia (NH ₃ ), the conversion reaction will be 3N ₂ O+2NH ₃ 4N ₂ +3H ₂ O; if the reducing gas (R) is a hydrocarbon, in the case of methane (CH ₄ ), the conversion reaction will be 4N ₂ O+CH ₄ 4N ₂ + 2H ₂ O + CO ₂ .

The decomposition reaction of the nitrous oxide (N ₂ O) and the reducing gas (R) is a reduction reaction, that is, an exothermic reaction. With the heat of reaction between nitrous oxide (N ₂ O) and the reducing gas (R), other decomposition reactions are promoted in a chain, and decomposition of the nitrous oxide (N ₂ O) and the reducing gas (R) is accelerated. Therefore, it is possible to decompose nitrous oxide (N ₂ O) in the exhaust gas (G1) at the same time, and once the decomposition reaction starts, the other decomposition reaction can be promoted in a chain by using the generated reaction heat, so that the decomposition can be reduced. Energy for heating to achieve energy savings.

Among the reducing gas (R), hydrocarbons are preferably used from the viewpoint of storage stability, availability, and the like. Among the hydrocarbons, for example, methane, ethane, ethylene, propylene, etc., preferably methane (CH ₄ ) is used.

The present inventors have found that when the reducing gas (R) and compressed dry air (CDA) are mixed in a specific ratio, an excellent nitrous oxide removal rate (N ₂ O DRE (%)) can be achieved, wherein methane (CH) is used. ₄ ) When the reducing gas (R) is used, the mixing ratio of methane to compressed dry air (flow ratio / slm) is CH ₄ : CDA = 7: 100 to 7: 130, that is, CH ₄ : CDA = 1:14.3 When it is ~1:18.6, more than 95% of N ₂ O DRE can be obtained, and carbon monoxide (CO) is an excellent removal effect of ≦5 (ND). In addition, the nitrous oxide removal rate (N ₂ O DRE (%)) refers to the removal rate of nitrous oxide before and after removal (ie, (G2)) of the exhaust gas (G1), and The closer N ₂ O DRE (%) is to 100%, the better.

The exhaust gas treatment tower body (20) composed of a cylindrical outer casing jacket made of steel on the outside and a high-temperature resistant metal material on the inside has manufacturing cost advantages, but is combined with an exhaust gas decomposition treatment chamber (21) and a heater. (22) The high temperature contact metal material is deliberate to cause damage due to temperature rise (ie, the life is shortened). Figure 1B shows the exhaust gas decomposition treatment room (21) A schematic diagram of the internal temperature of each relative position in the position, the TC1 point is located at a position surrounded by the heater (22) and is about 170 mm from the top of the exhaust gas decomposition treatment chamber (21); the TC2 point is located at the exhaust gas The top of the decomposition processing chamber (21) is calculated to be about 10 mm; the TC3 point is located inside the heater (22) and the exhaust gas treatment tower body (20) and is 170 mm from the top of the exhaust gas decomposition treatment chamber (21). The left and right positions; the TC4 point is located at the inner side of the heater (22) and the exhaust gas treatment tower body (20) and is about 310 mm from the top of the exhaust gas decomposition treatment chamber (21); the TC5 point is located at the heater (22) A position surrounded by the inside of the exhaust gas treatment tower main body (20) and at a position of about 620 mm from the top end of the exhaust gas decomposition treatment chamber (21). In general, the temperature at the TC2 point is the highest in the exhaust gas decomposition treatment chamber (21), but the upper limit temperature of the metal material used is usually 1050 ° C, so it is necessary to remove the nitrous oxide from the exhaust gas decomposition treatment chamber (21). The internal temperature is controlled below the upper limit temperature of the metal material, preferably below 960 ° C, and more preferably below 900 ° C.

The present invention further finds that nitrogen (N ₂ ) is passed through an automatic control system and added (added) to the exhaust gas (G1) in a specific amount while the reducing gas (R) and the compressed dry air (CDA) are mixed in a specific ratio. The specific amount of nitrogen added is calculated as the flow ratio (slm) as CH ₄ : CDA: N ₂ = 7: 100: 150 to 7: 130: 180, that is, CH ₄ : CDA = 1:14.3: When 21.4 to 1:18.6: 25.7, the temperature in the exhaust gas decomposition treatment chamber (21) can be lowered while maintaining a high N ₂ O DRE.

Further, when the mixed gas (M) is introduced into the exhaust gas decomposition treatment chamber (21), the mixed gas (M) can be passed through a honeycomb (not shown) to finely fluidize the gas stream, and can be sufficiently diffused to The whole of the exhaust gas decomposition treatment chamber (21).

Finally, the nitrous oxide is the removed treated gas (G2), and is discharged to the atmosphere through the discharge pipe (4) by suction of the exhaust fan (F).

[Examples]

Hereinafter, the present invention will be described in more detail by way of examples, but the invention should not be construed as limited.

Regarding the relationship between the mixing ratio of the reducing gas (R) and the compressed dry air (CDA), the removal efficiency (DRE), and the CO emission concentration when the nitrous oxide is removed, methane (CH ₄ ) is exemplified as follows.

The exhaust gas (G1) having a flow rate of nitrous oxide of 6, 12, and 18 L/min (slm) flows to the inflow pipe (3), and at this time, the flow rate of nitrogen (N ₂ ) added to the exhaust gas (G1) is flown. When the value is set to a fixed value of 180 L/min, the flow rate of methane (CH ₄ ) introduced into the inflow pipe (3) through the reducing gas pipe (5) and mixed with the exhaust gas (G1) is set to a fixed value of 7 L/min. The flow rate of compressed dry air (CDA) introduced into the inflow pipe (3) through the compressed dry air pipe (6) and mixed with the exhaust gas (G1) is set to A group: 70 L/min, Group B: 100 L/min, Group C: 130L/min and Group D: 150L/min, after obtaining the mixed gas (M), the mixed gas (M) is passed through the space surrounded by the heater (22) at a temperature of 850 °C, and the mixed gas ( M) After reaching the top end of the heater (22), the space between the heater (22) and the exhaust gas decomposition processing chamber (21) flows, and then flows to the discharge pipe (4) and is discharged to the atmosphere. The above various treatment conditions, nitrous oxide removal rate (N ₂ O DRE (%)), CO concentration, and the like are shown in Table 1.

From the results shown in Table 1, it can be seen that the flow rate of compressed dry air (CDA) is nitrous oxide removal rate (N ₂ O DRE (%)) of Group A (70 L/min) is 90% or less, and CO The highest concentration reached ≧2000; the flow rate of compressed dry air (CDA) was B group (100L/min), and the nitrous oxide removal rate (N ₂ O DRE (%)) was 95% or more, and the CO concentration was ≦5. (ND); the flow rate of compressed dry air (CDA) is ₅ % of the nitrous oxide removal rate (N ₂ O DRE (%)) of Group C (130 L/min), and the CO concentration is ≦5 (ND). The flow rate of compressed dry air (CDA) is ≦5 (ND) for the D group (150L/min), but the nitrous oxide removal rate when the flow rate of N ₂ O is 6slm (N ₂ O DRE) (%)) is 90% or less. Therefore, it can be known that when the mixing ratio of methane to compressed dry air is CH ₄ : CDA = 7: 100 to 7: 130, that is, when CH ₄ : CDA = 1:14.3 to 1:18.6, excellent oxidation oxide is obtained. The nitrogen removal rate and carbon monoxide (CO) are excellent effects of 未5 which is not found.

Further, regarding the removal of nitrous oxide, the relationship between the flow rate of nitrogen (N ₂ ) and the internal temperature of the exhaust gas treatment column (2) is as follows. As shown in FIG. 1B, the exhaust gas (G1) having a flow rate of nitrous oxide of 20 L/min (slm) is circulated to the inflow pipe (3), and at this time, methane (CH ₄ ) added to the exhaust gas (G1) is added. The flow rate is set to a fixed value of 7 L/min, and the flow rate of the compressed dry air (CDA) added to the exhaust gas (G1) is set to a fixed value of 100 L/min, and nitrogen gas (N ₂ ) added to the exhaust gas (G1) is added. The flow rate is set to 120, 150, and 180 L/min, respectively, and after the respective mixed gas (M) is obtained, the mixed gas (M) is passed through a space surrounded by a heater (22) having a temperature of 850 ° C, and the mixed gas ( M) After reaching the top end of the heater (22), the space between the heater (22) and the exhaust gas decomposition processing chamber (21) flows, and then flows to the discharge pipe (4) and is discharged to the atmosphere. At this time, the temperatures of the TC1 point, the TC2 point, the TC3 point, the TC4 point, and the TC5 point in the exhaust gas decomposition processing chamber (21) were measured, and the measured temperatures and the like of the respective points are shown in Table 2.

It can be seen from the results shown in Table 2 that when the flow rate of nitrogen (N ₂ ) is set to 120 L/min, the temperature of the highest temperature TC2 point in the exhaust gas treatment tower (2) rises to 992 ° C, when nitrogen (N ₂ ) When the flow rate is set to 150 L/min, the temperature at the TC2 point rises to 958 °C. When the flow rate of nitrogen (N ₂ ) is set to 180 L/min, the temperature at the TC2 point is less than 900 °C. When the upper limit temperature of the metal material to be used is 1050 ° C, the effect of lowering the temperature in the exhaust gas decomposition treatment chamber (21) can be obtained when the flow rate of nitrogen (N ₂ ) is set to 180 L/min.

Claims (2)

A method for treating an exhaust gas containing nitrous oxide, comprising the steps of: mixing a waste gas containing nitrous oxide with a reducing gas, compressed dry air, and nitrogen; and heating the mixed gas in a reducing environment to a step of removing nitrous oxide in the exhaust gas at a temperature at which the nitrous oxide is thermally decomposable at 600 ° C or higher, and characterized in that the nitrous oxide-containing exhaust gas and the reducing gas, the compressed dry air, and In the step of mixing nitrogen gas, the mixing ratio of the reducing gas, the compressed dry air, and the nitrogen gas is calculated as a reducing gas by a flow ratio: compressed dry air: nitrogen=7:100:150~7:130:180 The exhaust gas containing nitrous oxide is mixed, and the reducing gas is methane. An apparatus for treating a waste gas containing nitrous oxide according to the method of claim 1, comprising: an inflow pipe, a reducing gas pipe, a compressed dry air pipe, an exhaust gas treatment tower, a heater, and a discharge pipe, and an inflow pipe One end is connected to the source of the exhaust gas containing nitrous oxide, the other end is connected to the exhaust gas treatment tower, and nitrogen is added to the inflow pipe; the reducing gas pipe is connected at one end to the supply of the reducing gas, and the other end is connected to the inflow. Inside the piping; compressed dry air piping: one end is connected to the supply of compressed dry air, and the other end is connected to the inflow piping; Exhaust gas treatment tower: one end is connected to the inflow pipe, the other end is connected to the discharge pipe, and has an exhaust gas decomposition treatment chamber for heating the mixed gas; the heater is located in the exhaust gas treatment tower for heating the exhaust gas decomposition treatment chamber to oxidation The nitrous has a thermal decomposition temperature or higher and is disposed so as to surround the end of the inflow pipe at one end; the discharge pipe has one end connected to the exhaust gas treatment tower and the other end connected to the atmospheric environment.