Patent Application
20150367284
This document claims priorities to Japanese Patent Application Number 2014-127131 filed Jun. 20, 2014 and Japanese Patent Application Number 2015-050993 filed Mar. 13, 2015, the entire contents of which are hereby incorporated by reference.
In a semiconductor manufacturing process for manufacturing semiconductor devices, liquid crystal panels, LEDs or the like, a process gas is introduced into a process chamber which is being evacuated to perform various processes such as an etching process, a CVD process or the like. Further, the process chamber and exhaust apparatuses connected to the process chamber are cleaned periodically by supplying a cleaning gas thereto. Because exhaust gases such as the process gas, the cleaning gas or the like contain a silane-based gas, a halogen gas, a PFC gas or the like, such exhaust gases have negative effects on the human body and on the global environment such as global warming. Therefore, it is not preferable that these exhaust gases are emitted to the atmosphere as they are.
Accordingly, these exhaust gases are made harmless by the exhaust gas treatment apparatus provided at a downstream side of the vacuum pump, and the harmless exhaust gases are emitted to the atmosphere. As an exhaust gas treatment apparatus, there have been widely used a combustion-type exhaust gas treatment apparatus configured to form flames in a furnace by supplying an oxygen source and a fuel and to combust the exhaust gases by the flames, and a heater-type, plasma-type, catalyst-type, or other-type exhaust gas treatment apparatus configured to oxidatively decompose the exhaust gases by supplying an oxygen source and electric power.
In these exhaust gas treatment apparatuses, when persistent substances such as PFC are treated at a high removal rate, treatment is performed by raising the temperature. Therefore, it is problematic that generation amount of NOx (nitrogen oxide) increases to make the amount of NOx (nitrogen oxide) discharged as a by-product large.
In order to reduce the discharge amount of NOx (nitrogen oxide), air-fuel ratio control and a three-way catalytic process have widely been used for automobiles such as gasoline engines. Specifically, the oxygen concentration in an exhaust gas is measured by an oxygen sensor or the like, and the amount of fuel injection or the like is controlled on the basis of the measured result, thereby controlling the air-fuel ratio to produce a state in which NOx (nitrogen oxide), CO (carbon monoxide), and hydrocarbon coexist in the exhaust gas. In this state, a three-way catalyst is used to cause NOx (nitrogen oxide) to react with CO (carbon monoxide) or hydrocarbon, thus removing NOx (nitrogen oxide). Although the three-way catalytic process is an excellent process capable of simultaneously removing NOx (nitrogen oxide), CO (carbon monoxide), and hydrocarbon, this three-way catalytic process does not function in the coexistence of oxygen. Therefore, in the above exhaust gas treatment apparatus which performs a detoxifying process by oxidatively decomposing the exhaust gas in an oxygen-rich (excess air) state, the three-way catalytic process cannot be employed because a large amount of oxygen remains in the exhaust gas after the detoxifying process (see Japanese Laid-open Patent Publication No. 63-119850).
The present inventors have focused attention on the excellent features of the three-way catalytic process that is capable of simultaneously removing NOx (nitrogen oxide), CO (carbon monoxide), and hydrocarbon, and have made the present invention as a result of a great deal of studies for solving a technical subject matter to be able to use a three-way catalytic process even in an exhaust gas treatment apparatus in which a large amount of oxygen remains in the exhaust gas after the detoxifying process by oxidative decomposition.
According to an embodiment, there is provided an exhaust gas treatment apparatus which can reduce NOx (nitrogen oxide) produced as a by-product at the time of treating an exhaust gas by applying a three-way catalytic process, in the case where a large amount of oxygen remains in the exhaust gas after a detoxifying process by oxidative decomposition.
Embodiments, which will be described below, relate to an exhaust gas treatment apparatus for detoxifying exhaust gases discharged from a manufacturing apparatus or the like for manufacturing semiconductor devices, liquid crystal panels, LEDs or the like.
In an embodiment, there is provided an exhaust gas treatment apparatus having an oxidative decomposition unit configured to oxidatively decompose an exhaust gas and an exhaust gas cleaning unit configured to clean the exhaust gas after oxidative decomposition, comprising: a nitrogen oxide removing unit configured to remove nitrogen oxide contained in the exhaust gas, the nitrogen oxide removing unit being disposed at a stage subsequent to the oxidative decomposition unit; wherein the nitrogen oxide removing unit is configured to supply at least one of hydrocarbon and carbon monoxide into the exhaust gas discharged from the oxidative decomposition unit to cause the at least one of hydrocarbon and carbon monoxide to react with oxygen remaining in the exhaust gas, thereby removing oxygen from the exhaust gas, and thereafter to cause the nitrogen oxide in the exhaust gas to react with the at least one of hydrocarbon and carbon monoxide.
In an embodiment, a supply amount of the at least one of hydrocarbon and carbon monoxide is an amount corresponding to an air-fuel ratio of less than 1 where the at least one of hydrocarbon and carbon monoxide becomes incomplete combustion.
In an embodiment, the nitrogen oxide removing unit comprises a hydrocarbon and carbon monoxide supply section configured to supply at least one of hydrocarbon and carbon monoxide into the exhaust gas discharged from the oxidative decomposition unit, an exothermic reaction section configured to cause the at least one of hydrocarbon and carbon monoxide to react with oxygen remaining in the exhaust gas in the presence of a catalyst, and a denitration reaction section configured to cause the nitrogen oxide in the exhaust gas to react with the at least one of hydrocarbon and carbon monoxide in the exhaust gas in the presence of a catalyst.
In an embodiment, the nitrogen oxide removing unit is configured to cause the nitrogen oxide in the exhaust gas to react with the at least one of hydrocarbon and carbon monoxide in the exhaust gas, and thereafter to supply air or oxygen into the exhaust gas to cause oxygen in the supplied air or the supplied oxygen to react with the at least one of hydrocarbon and carbon monoxide remaining in the exhaust gas.
In an embodiment, the nitrogen oxide removing unit comprises a supply section configured to supply air or oxygen into the exhaust gas, the supply section being disposed at a stage subsequent to the exothermic reaction section and the denitration reaction section; and a CO oxidation reaction section configured to cause oxygen in the supplied air or the supplied oxygen to react with the at least one of hydrocarbon and carbon monoxide remaining in the exhaust gas in the presence of a catalyst.
In an embodiment, the catalysts used in the respective reaction sections comprise a carrier of silica (SiO2) and/or alumina (Al2O3), and one or more of platinum (Pt), palladium (Pd), rhodium (Rh), copper oxide, and manganese oxide which are carried by the carrier.
In an embodiment, the nitrogen oxide removing unit is disposed in the exhaust gas cleaning unit or at a stage subsequent to the exhaust gas cleaning unit.
In an embodiment, the exhaust gas treatment apparatus further comprises a cooler configured to cool the exhaust gas, the cooler being disposed between the oxidative decomposition unit and the exhaust gas cleaning unit; wherein the nitrogen oxide removing unit is disposed in the cooler or at a stage subsequent to the cooler.
In an embodiment, the oxidative decomposition unit comprises one or more of a combustion system configured to oxidatively decompose the exhaust gas by heat of a combustion reaction between a fuel and oxygen, a plasma system configured to decompose the exhaust gas by plasma and to oxidatively decompose the exhaust gas by a reaction between the decomposed gas and oxygen, a heater system configured to heat the exhaust gas by a heater and to oxidatively decompose the exhaust gas by causing the exhaust gas to react with oxygen, and a catalyst system configured to oxidatively decompose the exhaust gas by bringing the exhaust gas and oxygen into contact with an oxidative catalyst.
According to the above-described embodiments, in the exhaust gas treatment apparatus in which a large amount of oxygen remains in the exhaust gas after a detoxifying process, NOx (nitrogen oxide) produced as a by-product at the time of treating the exhaust gas can be remarkably reduced by applying a three-way catalytic process, and thus environmental burdens can be reduced.
An exhaust gas treatment apparatus according to embodiments will be described below with reference to
Fuel and oxygen are mixed in a premixer 16 in advance to form mixed fuel, and this mixed fuel is supplied to the burner 11. Further, air as an oxygen source for combusting (oxidizing) the exhaust gas is supplied to the burner 11. The burner 11 combusts the mixed fuel to form swirling flames in the combustion chamber 12, and the exhaust gas is combusted by the swirling flames. A UV sensor (not shown) is disposed inside the burner 11 and it is monitored by the UV sensor whether the swirling flames are formed normally. Air and nitrogen are supplied around the UV sensor as purge gas (not shown). Water W1 is supplied to the upper part of the combustion chamber 12. This water W1 flows down along the inner surface of the combustion chamber 12 and a water film is formed on the inner surface of the combustion chamber 12. The combustion chamber 12 is protected from heat of the swirling flames and corrosive gas by the water film. Further, a cooling water passage (not shown) through which cooling water W2 for cooling the burner 11 flows is provided between the burner 11 and the combustion chamber 12.
The exhaust gas introduced into the combustion chamber 12 through the burner 11 is combusted by the swirling flames. Thus, combustible gases and persistent substances such as silane, disilane, PFC and the like contained in the exhaust gas are combusted (oxidized). At this time, silica (SiO2) is produced as powdery product. This silica exists in the exhaust gas as fine dust.
A part of such powdery product is accumulated on the burner 11 or the inner surface of the combustion chamber 12. Therefore, the combustion unit 10 is configured to operate a scraper (not shown) periodically so that the powdery product accumulated on the burner 11 or the inner surface of the combustion chamber 12 is scraped off. A circulating water tank 20 is disposed below the combustion chamber 12. A weir 21 is provided inside the circulating water tank 20, and the circulating water tank 20 is partitioned by the weir 21 into a first tank 20A at an upstream side and a second tank 20B at a downstream side. The powdery product scraped off by the scraper falls in the first tank 20A of the circulating water tank 20 through the combustion unit connecting pipe 13 and is accumulated on the bottom of the first tank 20A. Further, the water film which have flowed down along the inner surface of the combustion chamber 12 flows into the first tank 20A. Water in the first tank 20A flows over the weir 21 and flows into the second tank 20B.
The combustion chamber 12 communicates with the exhaust gas cleaning unit 30 through the cooling unit 25. This cooling unit 25 has a piping 26 extending toward the combustion unit connecting pipe 13, and spray nozzles 27, 27 arranged in the piping 26 and at the outlet of the piping 26. The spray nozzle 27 sprays water countercurrently into the exhaust gas flowing in the piping 26. Therefore, the exhaust gas treated by the combustion unit 10 is cooled by water sprayed from the spray nozzle 27. The ejected water is recovered to the circulating water tank 20 through the piping 26.
The exhaust gas cooled in the cooling unit 25 is then introduced into the exhaust gas cleaning unit 30. This exhaust gas cleaning unit 30 is an apparatus for cleaning the exhaust gas with water and removing water-soluble harmful components and fine dust contained in the exhaust gas. This dust is mainly composed of powdery product produced by combustion (oxidization) in the combustion unit 10.
The exhaust gas cleaning unit 30 comprises a wall member 31 for forming a gas passage 32, and a first mist nozzle 33A, a first water film nozzle 33B, a second mist nozzle 34A and a second water film nozzle 34B disposed in the gas passage 32. These mist nozzles 33A and 34A and water film nozzles 33B and 34B are located at the central portion of the gas passage 32. The first mist nozzle 33A and the first water film nozzle 33B constitute a first nozzle unit, and the second mist nozzle 34A and the second water film nozzle 34B constitute a second nozzle unit. Therefore, in this embodiment, two sets of nozzle units are provided. One set of nozzle units or three or more sets of nozzle units may be provided.
The first mist nozzle 33A is disposed further upstream in a flowing direction of an exhaust gas than the first water film nozzle 33B. Similarly, the second mist nozzle 34A is disposed further upstream than the second water film nozzle 34B. Specifically, the mist nozzle and the water film nozzle are alternately disposed. The mist nozzles 33A and 34A, the water film nozzles 33B and 34A, and the wall member 31 are composed of corrosion-resistant resin (e.g., PVC: polyvinyl chloride). In the illustrated example, four nozzles are shown. However, the number of nozzles may be properly changed, or the number of mist nozzles and the number of water film nozzles may be properly changed.
As shown in
Fine dust having a diameter of less than 1 μm contained in the exhaust gas easily adheres to water particles forming mist by diffusion action (Brownian movement), and thus the fine dust is trapped by the mist. Dust having a diameter of not less than 1 μm is mostly trapped by the water particles in the same manner. Since a diameter of the water particles is approximately 100 μm, the size (diameter) of the dust adhering to these water particles becomes large apparently. Therefore, the water particles containing dust easily hit the water film at the downstream side due to inertial impaction, and the dust is thus removed from the exhaust gas together with the water particles. Dust having a relatively large diameter which has not been trapped by the mist is also trapped by the water film in the same manner and is removed.
As shown in
A mist trap 35 is provided above the water film nozzle 34B. This mist trap 35 has a plurality of baffle plates or filling materials therein and serves to trap the mist. In this manner, the exhaust gas from which the mist has been removed is supplied to the subsequent stage.
As shown in
In the nitrogen oxide removing unit 40 having the above structure, the case where hydrocarbon (CnHm) is supplied from the hydrocarbon and carbon monoxide supply section 44 into the exhaust gas will be described below. If carbon monoxide is supplied from the hydrocarbon and carbon monoxide supply section 44, then the substance which reacts with oxygen is replaced from hydrocarbon to carbon monoxide, and no CO (carbon monoxide) is generated from a reaction between hydrocarbon and oxygen. Only this point is different from the following description.
The exhaust gas that contains hydrocarbon flows into the exothermic reaction section 41 in which the hydrocarbon reacts with oxygen that remains in a large amount in the exhaust gas in the presence of an oxidative catalyst, thus producing heat, CO (carbon monoxide), and CO2. The exothermic reaction section 41 has been heated by the heater 46 to a temperature capable of initiating an exothermic reaction, before hydrocarbon starts to be supplied. Once the exothermic reaction is started, the heater 46 is turned off because the temperature of the reaction section is maintained by the generated heat. The amount of hydrocarbon supplied from the hydrocarbon and carbon monoxide supply section 44 is an amount corresponding to an air-fuel ratio of less than 1 so that hydrocarbon is larger in amount than oxygen and becomes incomplete combustion. Thus, after oxygen has been removed from the exhaust gas, the exhaust gas flows, together with hydrocarbon that has not yet been combusted and remains and CO (carbon monoxide) that has been generated in the exothermic reaction section 41, into the denitration reaction section 42. Therefore, NOx (nitrogen oxide), CO (carbon monoxide), and hydrocarbon coexist in the exhaust gas that flows into the denitration reaction section 42. In the denitration reaction section 42, NOx (nitrogen oxide) is allowed to react with hydrocarbon and CO (carbon monoxide), and thus NOx (nitrogen oxide) in the exhaust gas is removed (or the amount of NOx (nitrogen oxide) in the exhaust gas is reduced). After NOx (nitrogen oxide) has been removed (or the amount of NOx (nitrogen oxide) has been reduced) by the reaction in the denitration reaction section 42, the exhaust gas contains CO (carbon monoxide) and hydrocarbon which have become excessive due to the reaction with the NOx (nitrogen oxide). The exhaust gas is then supplied with air from the air supply section 45. The exhaust gas that has become to contain oxygen by air supply flows into the CO oxidation reaction section 43. In the CO oxidation reaction section 43, CO (carbon monoxide) and hydrocarbon that remain in the exhaust gas react with oxygen, thus turning into CO2 and H2O. The exhaust gas from which NOx (nitrogen oxide) has been removed (the amount of NOx (nitrogen oxide) has been reduced) is discharged from the nitrogen oxide removing unit 40. The exhaust gas discharged from the nitrogen oxide removing unit 40 is cooled, and is then emitted to the atmosphere through an exhaust duct.
The respective reactions that occur in the presence of the catalysts in the nitrogen oxide removing unit 40 will be described below.
If methane (CH4) is added as a hydrocarbon source, the following reaction occurs at an air-fuel ratio of less than 1 where hydrocarbon is larger in amount than oxygen in the exothermic reaction section 41:
CH4+O2→CO2+CO+H2O
Heat is generated by the oxidative reaction of CH4, thus heating the catalyst and consuming O2 that obstructs a denitration reaction. Then, a reductive reaction by CH4 whose reaction rate is slower than that of the oxidative reaction of CH4 and a reductive reaction by CO (carbon monoxide) generated from the oxidative reaction of CH4 occur in the denitration reaction section 42.
NOx+CH4→N2+CO2+H2O
NOx+CO→N2+CO2
Excessive CH4 and CO (carbon monoxide) that have not been used in the reductive reactions in the preceding-stage denitration reaction section 42 can be detoxified by adding oxygen or air to cause the following reaction in the CO oxidation reaction section 43:
CH4+CO+O2→CO2+H2O
Further, if CO (carbon monoxide) is supplied from the hydrocarbon and carbon monoxide supply section 44, the following reaction occurs at an air-fuel ratio of less than 1 where CO (carbon monoxide) is larger in amount than oxygen in the exothermic reaction section 41:
CO+O2→CO2
Heat is generated by the oxidative reaction of CO (carbon monoxide), thus heating the catalyst and consuming O2 that obstructs a denitration reaction. Then, a reductive reaction by CO (carbon monoxide) occurs in the denitration reaction section 42.
NOx+CO→N2+CO2
Excessive CO (carbon monoxide) that has not been used in the reductive reactions in the preceding-stage denitration reaction section 42 can be detoxified by adding oxygen or air to cause the following reaction in the CO oxidation reaction section 43:
CO+O2→CO2
The exothermic reaction section 41, the denitration reaction section 42, and the CO oxidation reaction section 43 may have a continuous structure or a discrete structure.
As catalysts used in the respective reaction sections, silica (SiO2) and/or alumina (Al2O3) is used as a carrier, and one or more of platinum (Pt), palladium (Pd), rhodium (Rh), copper oxide, and manganese oxide that are carried by the carrier are used. As a promoter for preventing the catalysts from being deteriorated, ceria (CeO2), lantana (La2O3), or zirconia (ZrO2) may be contained. In the respective reaction sections, catalysts of the same type or different types may be used.
The temperatures of the respective reaction sections are in the range of 300° C. to 600° C., more preferably in the range of 350° C. to 500° C.
The test results of the exhaust gas treatment conducted by using the combustion-type exhaust gas treatment apparatus 1 shown in
The tests were carried out under the conditions that the exhaust gas discharged from an etching apparatus was introduced into the combustion-type exhaust gas treatment apparatus 1 shown in
In Runs 1 to 3, exhaust gases having respective different NOx concentrations were treated. Specifically, the NOx concentration in the exhaust gas at the inlet of the nitrogen oxide removing unit 40 was 500 ppm in the test of Run 1, 1500 ppm in the test of Run 2, and 3000 ppm in the test of Run 3. The CO concentrations in the exhaust gases at the inlet of the nitrogen oxide removing unit 40 were lower than 10 ppm in Run 1, Run 2, and Run 3.
As is clear from Tables 1 to 3, even though the NOx concentration was progressively higher from 500 ppm (Run 1) to 1500 ppm (Run 2) then to 3000 ppm (Run 3), NOx (nitrogen oxide) was reduced in the nitration reaction section 42, and the NOx concentration at the outlet of the denitration reaction section 42 was lower than 10 ppm in any of Run 1, Run 2, and Run 3.
On the other hand, the CO concentration at the inlet of the nitrogen oxide removing unit 40 was lower than 10 ppm in any of Run 1, Run 2, and Run 3. However, CO (carbon monoxide) generated by the oxidative reaction of CH4 supplied from the hydrocarbon and carbon monoxide supply section 44 was supplied to the denitration reaction section, and thus the concentration of CO (carbon monoxide) that has not been used in the reductive reaction of NOx (nitrogen oxide), i.e., the CO concentration at the outlet of the denitration reaction section 42 was in the range of 4000 to 5000 ppm and thus became high. Thereafter, air (oxygen-containing gas) was added to the exhaust gas discharged from the denitration reaction section 42, and CH4 and CO (carbon monoxide) remaining in the exhaust gas are oxidized in the CO oxidation reaction section 43, and thus the CO concentration at the outlet of the CO oxidation reaction section 43 was lower than 10 ppm.
The test results from Run 1 to Run 3 are put together in Table 4.
As is clear from Table 4, when the exhaust gas treatment was carried out by using the exhaust gas treatment apparatus shown in
Further, even though the CO concentration at the outlet of the denitration reaction section was in the range of 4000 to 5000 ppm, the CO concentration at the outlet of the CO oxidation reaction section could be lower than 10 ppm.
Although the embodiments of the present invention have been described herein, the present invention is not intended to be limited to these embodiments. Therefore, it should be noted that the present invention may be applied to other various embodiments within a scope of the technical concept of the present invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2014-127131 | Jun 2014 | JP | national |
| 2015-050993 | Mar 2015 | JP | national |
