Method for treating waste gas through pure oxygen plasma thermal oxidation

Abstract

The invention relates to a method, equipment and system for treating waste gas through plasma thermal oxidation, and the method comprises the following steps: starting an oxygen plasma generator to generate high-temperature oxygen plasma; introducing waste gas to be treated into a plasma reaction device, wherein the waste gas to be treated reacts with the high-temperature oxygen plasma; and the treated waste gas is cooled and discharged. According to the method, the high-temperature oxygen plasma is adopted to react with the waste gas, the treatment effect of special waste gas in the semiconductor industry and the like and the reaction rate of waste gas treatment can be improved, and meanwhile low energy consumption and NOx value are guaranteed.

Description

Technical Field

[0001] The present application relates to the technical field of waste gas treatment, and in particular to plasma thermal oxidation treatment of tail gas with low calorific value and low oxygen content in the fields of semiconductor, photovoltaic and panel in the electronics industry. Background Art

[0002] This application aims to solve the special waste gas system in Chapter 8 of the tail gas treatment equipment (POU) described in GB51401-2019 "Design Standard for Waste Gas Treatment Engineering in Electronic Industry" or the scrubber described at home and abroad, especially the waste gas of special gases generated by the electronics industry such as vacuum etching, chemical vapor deposition (CVD), physical vapor deposition (PVD), etc. These waste gases are generally highly toxic, flammable and explosive, and need to be treated on site and converted into low-toxic, non-flammable and non-explosive safe gases.

[0003] The mainstream processes used by existing POUs or scrubbers to treat waste gas are natural gas combustion water washing and plasma water washing. Among them, in the process of natural gas combustion water washing, since most of the oxygen in the air is consumed by combustion, the oxygen content of the flue gas produced in the combustion step is low, so the reaction speed and reaction temperature are not ideal in the process of oxidizing the waste gas. In addition, the risk of natural gas leakage and explosion fire is high, which is criticized in the semiconductor industry. For the plasma water washing process, it should be pointed out that the plasma referred to in the process is currently an arc (DC arc or AC arc) plasma similar to plasma cutting in the machining industry. In recent years, the plasma water washing process has developed rapidly, and its safety has been greatly improved, gradually replacing the natural gas combustion water washing process. However, in the semiconductor field, nitrogen is currently widely used as the carrier gas for plasma. The high-temperature nitrogen plasma transfers heat to the exhaust gas and oxygen in the air for high-temperature oxidation reaction. In this process, if the reaction temperature is too high, it is easy to generate a large amount of NOx due to oxidation. Therefore, in order to make NOx emissions meet environmental protection requirements, in practice, a large amount of nitrogen is often added to reduce the reaction temperature, which makes the waste gas treatment efficiency not ideal. Compared with the treatment efficiency of 95-99% required in GB51401, the actual treatment efficiency is usually difficult to reach this standard.

[0004] In addition, cost is also an important factor affecting the semiconductor industry's choice of waste gas treatment process. Considering the price of electricity and natural gas as well as process consumption, the cost of using plasma as a heat source in the existing process is higher than that of natural gas. Therefore, the semiconductor and other industries are in urgent need of a waste gas treatment method with lower energy consumption and higher efficiency. Summary of the invention

[0005] The present disclosure is made in view of the above technical problems, and aims to provide a waste gas treatment method for the semiconductor industry, which adopts oxygen plasma for thermal oxidation and is suitable for treating low calorific value and low oxygen content waste gases such as SiH4, H2, PH3, B2H6, BCl3 and purge gas N2 generated in the production process of semiconductor chips, photovoltaics or panels. The method can not only achieve a higher treatment efficiency, but also reduce energy consumption, provide improved waste gas treatment effect and cost control.

[0006] In a first aspect of the present disclosure, a method for treating waste gas by plasma thermal oxidation is provided, comprising the following steps:

[0007] S1: Start the oxygen plasma generator to generate high-temperature oxygen plasma;

[0008] S2: passing the waste gas to be treated into a plasma reaction device, wherein the waste gas to be treated undergoes an oxidation reaction with the high-temperature oxygen plasma;

[0009] S3: Wash the treated waste gas with water, cool it down and discharge it.

[0010] In some embodiments, the oxygen plasma generator uses an oxygen-containing gas as a carrier gas.

[0011] In some embodiments, the oxygen-containing gas is selected from air, oxygen or a combination thereof, preferably oxygen.

[0012] In some embodiments, the oxygen is recycled oxygen derived from a byproduct of an upstream nitrogen production process.

[0013] In some embodiments, the temperature of the jet of high temperature oxygen plasma is 1000-3000K, preferably 1000-2000K.

[0014] In some embodiments, the carrier gas used in the oxygen plasma generator is introduced through a primary air intake, a secondary air intake, or a tertiary air intake, and all the carrier gases are tangential air intake cyclones in the same direction.

[0015] Among them, the first-stage air intake, the second-stage air intake and the third-stage air intake are all tangential air intakes in the same direction. Unlike the slender plasma jet with the smallest possible diffusion angle required in the plasma cutting process, the plasma jet in the waste gas incineration process needs to quickly exchange heat and mass with the waste gas, so what is needed is a strong swirl plasma jet with a thick and short appearance. This application controls the generated plasma jet to be a strong swirl by adopting the method of tangential air intake in the same direction, so as to achieve efficient heat and mass exchange and improve the efficiency of waste gas treatment.

[0016] In some embodiments, the step S1 further includes:

[0017] Measuring the plasma jet temperature, or calculating the plasma jet temperature based on the measured carrier gas amount and electric power;

[0018] The carrier gas intake mode is set according to the plasma jet temperature.

[0019] In some embodiments, when the oxygen plasma generator is an arc plasma generator, after startup, the cathode current of the oxygen plasma generator is greater than the anode current by more than 0.5%.

[0020] In some embodiments, when the oxygen plasma generator is an arc plasma generator, the cathode material in the oxygen plasma generator is selected from hafnium, zirconium, and silver, preferably metal hafnium.

[0021] In some embodiments, the plasma reaction device includes a reaction chamber, and the length of the reaction chamber is ≤500 mm.

[0022] In some embodiments, the NOx concentration in the treated exhaust gas is less than 30 ppm, preferably less than 10 ppm.

[0023] In some embodiments, the oxygen plasma generator is selected from any one of an arc plasma generator, a high frequency induction plasma generator, a microwave plasma generator, and a radio frequency plasma generator.

[0024] A second aspect of the present invention provides a device for treating waste gas by plasma thermal oxidation, characterized in that the device includes an oxygen plasma generator and a plasma reaction device.

[0025] In some embodiments, it is characterized in that the oxygen plasma generator includes at least one air inlet pipe, and the air inlet pipe is used to introduce a carrier gas, and the carrier gas is selected from air, oxygen or a combination thereof, preferably oxygen.

[0026] In some embodiments, it is characterized in that the cathode material in the oxygen plasma generator is selected from hafnium, zirconium, and silver, preferably metal hafnium.

[0027] In one embodiment, the device further comprises a temperature detection device for measuring the temperature of the plasma jet.

[0028] A third aspect of the present invention provides a system for treating waste gas by plasma thermal oxidation, the system comprising the above-mentioned plasma thermal oxidation waste gas treating equipment and oxygen recovery equipment, wherein the oxygen recovery equipment is used to recover by-product oxygen in the upstream nitrogen production process. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 A schematic diagram of a device for treating waste gas by plasma thermal oxidation according to the present application is shown.

[0030] The reference numerals in the above figure are as follows: 1 is a plasma generator, 2 is a three-way valve, 3 is a reaction chamber, 4 is a water tank, 5 is an air inlet pipe of the reaction chamber, 6 is a secondary air inlet pipe of the plasma generator, and 7 is a primary air inlet pipe of the plasma generator. DETAILED DESCRIPTION

[0031] Hereinafter, the embodiments of the plasma thermal oxidation treatment of waste gas and its equipment disclosed in the present invention will be described in detail with appropriate reference to the drawings. However, there may be cases where unnecessary detailed descriptions are omitted. For example, there are cases where detailed descriptions of well-known matters and repeated descriptions of actually the same structures are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate understanding by those skilled in the art. In addition, the drawings and the following descriptions are provided for those skilled in the art to fully understand the present disclosure and are not intended to limit the subject matter described in the claims.

[0032] "Range" in the present disclosure is defined in the form of lower limit and upper limit, and a given range is defined by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundaries of the particular range. Ranges defined in this way can be inclusive or exclusive of the end values ​​and can be combined arbitrarily, that is, any lower limit can be combined with any upper limit to form a range.

[0033] If not otherwise specified, all embodiments and optional embodiments of the present disclosure may be combined with each other to form a new technical solution.

[0034] If not otherwise specified, all steps of the present disclosure may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), which means that the method may include steps (a) and (b) performed sequentially, or may include steps (b) and (a) performed sequentially. For example, the method may further include step (c), which means that step (c) may be added to the method in any order, for example, the method may include steps (a), (b) and (c), or may include steps (a), (c) and (b), or may include steps (c), (a) and (b), etc.

[0035] Unless otherwise stated, the words “comprises,” “includes,” “has,” “contains” or any other variations thereof mentioned in the present disclosure are intended to cover a non-exclusive inclusion.

[0036] Further, in the following specification, reference will be made to a number of expressions, which are defined as having the following meanings.

[0037] The term "plasma generator" refers to a device used to generate plasma and may also be referred to as a "plasma torch".

[0038] The term "high temperature oxygen plasma" refers to a high temperature plasma formed by ionizing oxygen, including positive ions, negative ions, O + , O 2+ , O - , O 2- , O 3- , ground state particles, excited state particles and other particles.

[0039] Figure 1 A schematic diagram of a pure oxygen plasma thermal oxidation waste gas treatment device is provided as an example. The device includes a plasma generator 1, a three-way valve 2, a reaction chamber 3, a water tank 4 and a first-stage air inlet pipe 7.

[0040] The plasma generator 1 is loaded with direct current, and the carrier gas introduced through the primary air inlet pipe 7 pulls out the plasma arc to form a plasma jet. The carrier gas is selected from air, oxygen or a combination thereof, preferably oxygen. Optionally, the device also includes a secondary air inlet pipe 6 for introducing the carrier gas, that is, the carrier gas can be introduced in a primary air inlet or a secondary air inlet.

[0041] The outlet of the plasma generator 1 is passed into the reaction chamber 3, and the exhaust gas is passed into the reaction chamber 3 through the three-way valve 2. In the reaction chamber 3, the exhaust gas reacts with the oxygen plasma, and after the reaction is completed, it is washed and cooled by the water tank 4 and then discharged. The present disclosure does not limit the subsequent washing tower and other devices, and any commonly used water washing and cooling equipment can be used to further treat the exhaust gas. In some cases, the water tank 4 is optionally provided.

[0042] Optionally, the device further includes a reaction chamber air inlet pipe 5. When the plasma flame temperature is relatively low and it is not easy to ensure the reaction, it is necessary to increase the amount of oxygen from the reaction chamber air inlet pipe 5, and reduce the air intake of the secondary air intake to increase the position of the flame center. That is, the air intake method can also be a three-stage air intake. Optionally, the gas introduced into the reaction chamber through the reaction chamber air inlet pipe 5 can be pure oxygen, or compressed air or a combination of compressed air and oxygen. Unlike traditional VOC (Volatile Organic Compounds) waste gas and other classic waste gases, the special waste gas generated in the semiconductor processing process usually has the characteristics of low calorific value and no oxygen. Therefore, in the case of the prior art using nitrogen plasma as a heat source, a large amount of nitrogen is used as a carrier gas and a purge gas during plasma combustion. The gas obtained by mixing nitrogen and part of the special waste gas has very few combustible components and almost no oxygen. Even if air is further used as a combustion-supporting gas, the oxygen content in the air is only 20.8%, and the entire thermal oxidation process of waste gas treatment is in a state of very low oxygen content, which is a huge difference from the thermal oxidation characteristics of most other industries. In addition, the low oxygen content in the working gas also makes the waste gas treatment efficiency in this industry much worse than the classic waste gas treatment efficiency in other industries.

[0043] The above problems are common working conditions in the semiconductor industry, and are even worse in the photovoltaic and panel industries, where insufficient heat and low oxygen content are more serious. Especially when dealing with waste gas with a high hydrogen content, since the waste gas cannot react completely in the reaction chamber, it is easy to explode at the tail of the scrubber, causing factory accidents.

[0044] In order to improve the problem caused by insufficient oxygen content in the exhaust gas, the prior art attempts to increase the amount of combustion-supporting gas introduced to increase the amount of oxygen involved in the reaction. However, introducing more combustion-supporting gas will reduce the overall combustion temperature, resulting in reduced reaction efficiency. Therefore, the prior art has the following paradox in improving the POU or Scrubber: increasing the oxygen content of the mixed exhaust gas by adding combustion-supporting gas (such as compressed air or oxygen) from the reaction chamber will reduce the reaction efficiency, while not adding combustion-supporting air will cause the thermal oxidation reaction to be in a particularly low-oxygen state, making it difficult to oxidize completely. Both cannot achieve an ideal exhaust gas treatment effect.

[0045] To solve the above problems, an exemplary embodiment of the present disclosure provides a method for treating waste gas by plasma thermal oxidation, the core of which is to replace the heat source used to treat the waste gas from nitrogen plasma commonly used in the prior art with oxygen plasma.

[0046] Specifically, the method includes: S1: starting an oxygen plasma generator to generate high-temperature oxygen plasma; S2: passing the waste gas to be treated into a plasma reaction device, and the waste gas to be treated reacts with the high-temperature oxygen plasma; S3: washing the treated waste gas with water, cooling it, and discharging it.

[0047] Different from the prior art which uses a nitrogen plasma generator, this embodiment uses an oxygen plasma generator to generate high-temperature oxygen plasma to treat the waste gas. Specifically, this embodiment uses oxygen-containing gas as a carrier gas to generate a high-temperature oxygen plasma with strong oxidizing properties to react with the waste gas.

[0048] Nitrogen has become a common carrier gas for plasma generators due to its stability, protection and low cost. As a gas with low cost and stable chemical properties, nitrogen plasma is widely used in traditional waste gas treatment and material surface treatment. However, for low calorific value and low oxygen content waste gas generated by the semiconductor industry, the treatment effect of nitrogen plasma as a heat source is not ideal. For example, the carrier gas delivery volume of a typical 10kw plasma is 100 liters / minute. If the amount of waste gas to be treated is 600 liters / minute, the amount of combustion air delivered is 200 liters / minute, and the total waste gas volume after mixing is 900 liters / minute. According to practice, for the waste gas generated by semiconductor processing, the oxygen content of the above mixed waste gas is only about 4.67% when it is not reacted. This kind of combustion in a low-oxygen environment is very rare in common thermal oxidation occasions. For example, the oxygen content at the outlet of the common incinerator in traditional industries is still 6%.

[0049] Faced with the huge difference in gas composition between traditional waste gas and special waste gas from industries such as semiconductors, the inventors creatively proposed using oxygen-containing gas as the carrier gas of the plasma generator in the special waste gas treatment process, and using oxygen plasma as a heat source to react with the special waste gas.

[0050] Oxygen plasma has a very high oxidation capacity and can fully oxidize special waste gas with low calorific value and low oxygen content, improve the waste gas treatment effect and improve the waste gas treatment efficiency. Schematically, taking silane as an example, the combustion rate of silane in oxygen is 10.7 times that in air. If the oxygen content in the combustion air is increased by 1 percentage point, the flame temperature can be increased by 70°C, and the ignition energy is rapidly reduced. The flame length in pure oxygen is only 1 / 4 of that in air.

[0051] At the same time, the inventor surprisingly found that the cathode current of the oxygen plasma generator is significantly greater than the anode current. For example, when the cathode current of the plasma is 50 amperes, the anode current is 49 amperes. What is meant here is that the cathode current is more than 0.5% greater than the anode current. This breaks the traditional understanding that the plasma theory holds that the plasma is electrically neutral as a whole. Contrary to the above findings, the cathode and cathode currents are equal when nitrogen and argon are used as plasma carrier gases, or the difference between the cathode and cathode currents is not macroscopically perceptible.

[0052] Inferentially, the significant difference between the cathode and anode currents in the oxygen plasma generator is due to the presence of a large number of negative oxygen ions in the oxygen plasma. The oxidation activity of negative oxygen ions is very high. After reacting with the combustible components in the exhaust gas, the negative oxygen ions are not consumed all at once. After the negative oxygen ions oxidize a waste gas molecule, the electrons are in a suspended state. Without hitting the wall, the electrons will continue to adsorb on the nearby oxygen molecules to form the next negative oxygen ion, because electrons are very easy to adsorb on the surface of polar oxygen molecules but not on the surface of non-polar nitrogen molecules. It can be reasonably inferred that when oxygen is used as a carrier gas, the combustion of exhaust gas is closer to ion reaction, and it has higher efficiency in treating exhaust gas.

[0053] It can be seen that using pure oxygen instead of nitrogen as a plasma carrier gas in a POU or a scrubber can effectively increase the speed and processing efficiency of the chemical reaction. Furthermore, the increase in reaction speed and processing efficiency enables the method according to the present application to achieve an ideal waste gas treatment effect with lower energy consumption and cost.

[0054] Optionally, the carrier gas used in this embodiment can be air, oxygen, or a combination of air and oxygen, preferably oxygen.

[0055] When compressed air is used as a plasma carrier gas, although it cannot achieve the oxidation effect of pure oxygen on exhaust gas, it can also provide an effect that is better than the use of nitrogen in the prior art, so the use of compressed air as a plasma carrier gas should also be regarded as a type of the present disclosure.

[0056] Optionally, the oxygen used in this embodiment is recycled oxygen, which is derived from a by-product of an upstream nitrogen production process.

[0057] In the production process of semiconductors, photovoltaic cells and panels, a large amount of nitrogen is usually used as a carrier gas, purge gas or protective gas. For example, in the steps of CVD (Chemical Vapor Deposition), PVD (Physical Vapor Deposition), etching, doping, ion implantation, packaging, etc., a large amount of nitrogen is usually used, and the inert atmosphere of nitrogen is used to control the atmosphere to prevent unnecessary oxidation. In addition, since semiconductor chip factories use a large amount of toxic, corrosive, flammable and explosive special gases as process gases, nitrogen is also commonly used as a protective gas in the process pipelines of semiconductor chip factories. Generally, the existing general factory technology in factories of a certain scale adopts cryogenic air separation or pressure swing adsorption technology to produce nitrogen. The above method usually uses air as a raw material, liquefies the air through heat exchange, and then uses the difference in boiling points between the main components of liquid oxygen and liquid nitrogen in liquid air to separate the two to obtain nitrogen. Among them, the by-product oxygen is often directly discharged into the factory atmosphere. In the method provided in this embodiment, the carrier gas required for use in the plasma generator is precisely the by-product oxygen produced in the upstream production process. Therefore, in one possible implementation, the by-product oxygen that should have been discharged in the nitrogen production link can be recycled and reused as the carrier gas required for subsequent waste gas treatment to generate oxygen plasma to participate in the step of thermal oxidation of the waste gas.

[0058] The inventors would like to emphasize that the practice of directly discharging oxygen, a byproduct of nitrogen production, in the prior art not only causes a waste of resources, but also greatly increases the risk of fire in the field. In addition, a high concentration of oxygen environment can also have an impact on human health. Therefore, the method provided in this embodiment can reduce the oxygen concentration in the factory while achieving efficient waste gas treatment. And this method does not bring additional processing costs, but reduces overall energy consumption through waste utilization, thereby improving the economic benefits of overall production.

[0059] Currently, the top three energy consumption items of POU or Scrubber are electricity, compressed air and nitrogen. The main energy consumption statistics are shown in Table 1.

[0060] Table 1 Energy consumption of existing photovoltaic 3000L / min scrubber (POU)

[0061]

[0062] 229.44 yuan / day

[0063] 82598.4 yuan / year

[0064] Different from the prior art, the present application uses an oxygen plasma generator to produce oxygen plasma as a heat source. This change can improve the processing efficiency while reducing the process cost. Specifically, the main energy consumption statistics of the waste gas by oxygen plasma oxidation heat treatment are shown in Table 2.

[0065] Table 2 Energy consumption of pure oxygen photovoltaic 3000L / min Scrubber (POU)

[0066]

[0067] 86.88 yuan / day

[0068] 31711.2 yuan / year

[0069] Comparing Table 1 and Table 2, it can be seen that in the photovoltaic process, for the same 3000 liters / minute air intake, the annual energy consumption cost of the existing scrubber (POU) is 82598.4 yuan / year, while the annual energy consumption cost of the scrubber (POU) according to this embodiment is 31711.2 yuan / year, which is a significant reduction in cost. In general, the cost reduction mainly involves two factors: one is the lower carrier gas cost; the other is that when treating the same amount of waste gas, the whole machine power under the method of this embodiment is lower. The specific description is as follows.

[0070] As for the first factor, as mentioned above, the oxygen used in this embodiment comes from the byproduct oxygen produced in the upstream production process, while the nitrogen carrier gas used in the prior art needs to be obtained through deep cold air separation, and the operation of the air compressor will incur corresponding costs. Therefore, although the amount of oxygen used when using pure oxygen plasma is greater than the amount of nitrogen used in the existing waste gas treatment process due to the violent reaction of oxygen plasma and the low reaction temperature required, according to this embodiment, the cost of using oxygen in obtaining the carrier gas is lower.

[0071] It can be seen that using the byproduct oxygen as a carrier gas can not only improve processing efficiency, but also save costs and reduce energy consumption. For companies currently engaged in the production of semiconductor chips, display panels and photovoltaic cells, the pure oxygen plasma process has obvious advantages.

[0072] As for the second factor, according to Table 1 and Table 2, when treating 600 liters / minute of waste gas, if nitrogen is used as carrier gas, the carrier gas volume of plasma is 100 liters / minute; to ensure the treatment effect, combustion-supporting air needs to be introduced into the reaction chamber, and the amount of air introduced is about 200 liters / minute. The total amount of waste gas after mixing is 900 liters / minute, and the corresponding whole machine power is 10kw. Since the oxygen content of the mixed waste gas is only 4.67% when it is not reacted, after the carrier gas is replaced with pure oxygen, for the same treatment of 600 liters / minute of waste gas, the carrier pure oxygen volume of plasma is 100 liters / minute, no air is introduced, and the total amount of waste gas after mixing is 700 liters / minute, and the corresponding whole machine power is 7kw. At this time, the oxygen content of the mixed waste gas rises to 14.3% when it is not reacted.

[0073] By comparison, it can be seen from the above data that after using oxygen as a plasma carrier gas, the oxygen content in the waste gas to be treated increases from 4.67% to 14.3%, and the oxygen content increases significantly. At the same time, since the method according to this embodiment does not require the introduction of combustion-supporting air, the total gas volume of the waste gas to be treated can be reduced from 900 liters / minute to 700 liters / minute, with a significant decrease, and the required whole machine power is reduced accordingly. Therefore, when achieving similar waste gas treatment effects, the energy consumption can be significantly reduced when using oxygen plasma to treat waste gas. Therefore, using oxygen-containing gas, especially oxygen, instead of nitrogen as a plasma carrier gas has obvious advantages for treating such waste gas with low calorific value and low oxygen content.

[0074] Furthermore, the inventors were surprised to find that the use of pure oxygen plasma greatly reduced the accumulation of dust on the combustion chamber walls. In industries such as semiconductors, slagging of the combustion chamber walls caused by the treatment of special exhaust gases is the largest workload for inspection and maintenance in the exhaust gas treatment process. The method provided in this embodiment can significantly reduce the slagging phenomenon. In theory, this is because ash and slag are easy to form on the combustion chamber walls in a reducing atmosphere, but not easy to form slagging in an oxidizing atmosphere. On the other hand, when oxygen plasma exchanges heat and mass and reacts chemically with exhaust gas, the diffusion direction of oxygen is completely opposite to that of oxygen during conventional combustion or the use of nitrogen plasma, which may also lead to less slagging in the present application.

[0075] Optionally, the plasma water cooling process and the plasma air cooling process are both common tail gas treatment processes in the semiconductor, photovoltaic or panel industries. The application of pure oxygen plasma process in the plasma air cooling process should also be regarded as one of the present applications.

[0076] As for the specific method of generating plasma, similar to the common arc plasma technology, high frequency induction plasma technology and microwave plasma technology under atmospheric pressure are also common plasma technologies. Therefore, arc plasma, high frequency induction plasma and microwave plasma technology using pure oxygen should all be regarded as one of the present application.

[0077] There is a technical prejudice against the application of oxygen plasma in traditional industries, and it is believed that there are three main defects in using oxygen as a carrier gas: first, high NOx emission concentration will be caused when oxygen plasma treats nitrogen-containing waste gas; second, the electrode life of pure oxygen plasma is short; third, the energy consumption of pure oxygen preparation is high, or the cost of purchasing pure oxygen is high. The method of the embodiment of the present application can overcome the above defects, as described in detail below.

[0078] Nitrogen oxides (NOx) are a class of pollutants including nitric oxide (NO) and nitrogen dioxide (NO2). Since NOx is one of the main pollutants that form acid rain and photochemical smog, controlling NOx emissions has always been a focus of factory environmental protection work. Since the waste gas generated in industries such as semiconductors often contains nitrogen, under the treatment conditions commonly used in the prior art, if more oxygen is introduced to participate in waste gas treatment, the nitrogen in the waste gas is easily oxidized to generate nitrogen oxides, making it difficult to control NOx emissions to a compliant value. In this embodiment, when oxygen plasma is used to react with waste gas, in a possible implementation, the plasma jet temperature is controlled to be 1000-3000K, preferably 1000-2000K.

[0079] Traditional nitrogen plasma has no reaction activity. When treating exhaust gas, heat must be transferred to the exhaust gas and oxygen in the combustion air before reacting, so a high-temperature plasma jet of 3000-6000K is usually used. In contrast, this embodiment uses oxygen plasma with strong oxidation ability. When oxygen plasma encounters combustible components in the exhaust gas, it can immediately undergo oxidation reaction without the need for intermediate heat transfer. Therefore, the plasma jet temperature only needs to be 1000-3000K to complete the reaction with the exhaust gas. The generation of NOx is closely related to the temperature during the combustion process. This embodiment uses a lower reaction temperature, especially controlling the temperature below the generation temperature of NOx, which can effectively achieve the goal of compliant emissions while ensuring the exhaust gas treatment effect.

[0080] Optionally, step S1 of the method of this embodiment further includes:

[0081] Measuring the plasma jet temperature, or calculating the plasma jet temperature based on the measured plasma carrier gas volume and electric power;

[0082] Set the carrier gas intake method according to the plasma jet temperature.

[0083] The temperature is adaptively controlled to control the outlet temperature of the plasma generator to a temperature that will not oxidize a large number of nitrogen molecules into NO. In general, the oxygen consumption is about 2 to 3 times the amount of nitrogen plasma carrier gas. By controlling the plasma temperature at a lower level, the NO value produced by the method of this embodiment is also significantly lower than the concentration of traditional nitrogen plasma, and is significantly lower than the NOx value produced by common natural gas combustion and existing nitrogen plasma treatment of waste gas. This embodiment can control NO emissions to below 30ppm, or even below 10ppm, while ensuring the efficiency of waste gas treatment. This is mainly subject to the nitrogen content in pure oxygen. During normal idling and normal incineration of waste gas, NO is less than 10ppm, and the stability is high.

[0084] The carrier gas intake mode can be one-stage intake, two-stage intake or three-stage intake. In this embodiment, the one-stage intake means that oxygen-containing gas is introduced into the plasma generator only through a single intake pipe to generate high-temperature plasma to ensure normal stable operation of the plasma; the two-stage intake means that on the basis of the one-stage intake, another intake pipe is introduced to introduce the carrier gas, and the two-stage intake mode can adjust and optimize the reaction conditions, such as the reaction temperature, by introducing additional carrier gas into the plasma jet zone; the three-stage intake means that on the basis of the two-stage intake, the reaction chamber intake pipe is introduced, and the temperature of the plasma flame outlet is adjusted to 1000-3000K before and after the arc falling point of the plasma anode.

[0085] Optionally, this embodiment uses water washing to cool down in the cooling step. Even if the high temperature plasma in this embodiment contains oxygen plasma, the ratio of NO and NO2 in the treated waste gas formed by the reaction is different from the 95:5 of the conventional nitrogen carrier gas, and the ratio of NO2 is higher. However, since NO2 will be removed by water washing, even if oxygen-containing gas is used as the carrier gas, compared with the prior art when nitrogen is used as the carrier gas, the final discharged treated waste gas contains mainly NO gas, which contains almost no NO2.

[0086] To ensure the life of the electrode, this embodiment selects a metal with good corrosion resistance and arc burn resistance and good conductivity as the cathode material. In a possible implementation, the cathode material in the oxygen plasma generator is selected from metal hafnium, zirconium, and silver, preferably metal hafnium. The structure of the hafnium electrode is generally to embed a hafnium wire with a diameter of 3 to 4 mm into a copper or silver base. In addition, when metal hafnium is used as the cathode, oxygen-free copper can be selected as the anode.

[0087] In addition, since the plasma jet temperature used in the present application is relatively low, the electrodes in the plasma generator do not need to withstand the intense ablation and consumption of the plasma flame of the conventional nitrogen plasma at 3000-6000K.

[0088] Regarding oxygen cost, as described above, when the present disclosure is applied to the semiconductor and photovoltaic manufacturing industries, it is possible to recycle and reuse the byproduct oxygen used to prepare nitrogen, and energy consumption can be reduced by replacing the traditional nitrogen carrier gas with the byproduct oxygen.

[0089] In addition, the method disclosed in the present invention can further improve the safety of the waste gas treatment process. Since the prior art is low-oxygen combustion, that is, the plasma flame is required to transfer heat to the waste gas first, and then the waste gas and air are mixed to burn. The defect caused by this combustion method is that it cannot guarantee the stability of the flame and the completeness of the combustion. When the combustion is incomplete in the reaction chamber, the high-temperature gas is prone to explosion and re-combustion accidents at the tail. In comparison, the present application uses oxygen plasma, so there is no intermediate process of heat transfer. The high-temperature oxygen plasma burns immediately after encountering the combustible components in the waste gas, and the flame stability is strong and the combustion is complete. The present application can eliminate the phenomenon in the prior art that the dust accumulated in the tail exhaust pipe after the incineration of the waste gas can be re-combusted after meeting the air and the heat source. The fully oxidized waste gas is not easy to explode when entering the scrubbing tower, which can greatly reduce the explosion and re-combustion accidents at the tail, and improve the safety of the waste gas treatment process.

[0090] Furthermore, in a possible implementation, the length of the reaction chamber in the plasma reaction device is ≤500mm. In the prior art, the length of the reaction chamber of the Scrubber and POU is usually >600mm, which is due to the aforementioned reaction speed and the low oxygen content of the exhaust gas. After applying the method provided by the present disclosure, the length between the upper cover of the reaction chamber and the water tank can be greatly shortened. Schematically, the length of the reaction chamber can be ≤500mm. According to practice, a reaction chamber with a length of about 400mm is usually sufficient. Setting a reduced reaction chamber can reduce the size of the equipment, miniaturize the equipment, occupy less space, and thus reduce costs and simplify maintenance.

[0091] It can be seen that the present embodiment adopts oxygen plasma to treat the waste gas, the speed of the pure oxygen chemical reaction of the waste gas is increased compared with the prior art, the ion reaction is more obvious, the waste gas treatment efficiency is greatly increased, and the risk factor of the waste gas after treatment is reduced; due to the decrease in the power of the plasma whole machine, nitrogen and compressed air consumption, the energy consumption is greatly reduced, and the energy saving rate of the present embodiment is more than 50%; due to the low temperature of the plasma jet in the present embodiment, the concentration of NOx generated during the reaction is only about 10ppm.

[0092] In summary, the use of oxygen plasma can greatly improve the treatment efficiency of special waste gases in industries such as semiconductors, reduce energy consumption, and at the same time reduce NOx emission concentration and reduce the risk of tail reburning explosion.

[0093] The present disclosure also provides a system for treating waste gas with plasma thermal oxidation. The system includes: Figure 1 The waste gas treatment equipment shown, and the oxygen recovery equipment for recovering the by-product oxygen in the upstream nitrogen production process. As mentioned above, industries such as semiconductors, photovoltaic cells, and panels prepare and use a large amount of nitrogen in their production processes, which will produce by-product oxygen accordingly. In practice, oxygen is often directly discharged, resulting in waste of resources and hidden dangers of oxygen enrichment in the factory. According to the oxygen recovery equipment included in the system provided in this embodiment, the by-product oxygen can be recovered and reused in the waste gas treatment equipment. While improving the efficiency of resource utilization, it can also reduce energy consumption and costs in the waste gas treatment process and improve the economic benefits of the enterprise.

[0094] It should be noted that the present disclosure is not limited to the above-mentioned embodiments. The above-mentioned embodiments are merely examples, and embodiments having substantially the same structure as the technical idea and exerting the same effects within the scope of the technical solution of the present disclosure are all included in the technical scope of the present disclosure. In addition, within the scope of the present disclosure, various modifications that can be thought of by those skilled in the art to the embodiments and other methods of combining some of the constituent elements in the embodiments are also included in the scope of the present disclosure.

Claims

1. A method for treating waste gas by plasma thermal oxidation, characterized in that: The following steps are involved: S1: Start the oxygen plasma generator to generate high-temperature oxygen plasma; S2: passing the waste gas to be treated into a plasma reaction device, where the waste gas to be treated reacts with the high-temperature plasma; S3: The treated waste gas is washed with water, cooled and then discharged.

2. The method according to claim 1, characterized in that The oxygen plasma generator uses oxygen-containing gas as carrier gas.

3. The method according to claim 2, characterized in that The oxygen-containing gas is selected from air, oxygen or a combination thereof, preferably oxygen.

4. The method according to claim 3, characterized in that The oxygen is recycled oxygen, which comes from a by-product of an upstream production process.

5. The method according to claim 1, characterized in that The temperature of the jet of high temperature oxygen plasma is 1000-3000K, preferably 1000-2000K.

6. The method according to claim 1, characterized in that The carrier gas used in the oxygen plasma generator is introduced through a primary air intake, a secondary air intake or a tertiary air intake, and all the carrier gases are tangential air intake cyclones in the same direction.

7. The method according to claim 5, characterized in that The step S1 further comprises: Measuring the plasma jet temperature, or calculating the plasma jet temperature based on the measured plasma carrier gas volume and electric power; The carrier gas intake mode is set according to the plasma jet temperature.

8. The method according to claim 1, characterized in that When the oxygen plasma generator is an arc plasma generator, after starting, the cathode current of the oxygen plasma generator is greater than the anode current by more than 0.5%.

9. The method according to claim 1, characterized in that: When the oxygen plasma generator is an arc plasma generator, the cathode material in the oxygen plasma generator is selected from hafnium, zirconium, and silver, preferably metal hafnium.

10. The method according to claim 1, characterized in that The plasma reaction device comprises a reaction chamber, and the length of the reaction chamber is ≤500 mm.

11. The method according to claim 1, characterized in that: The NOx concentration in the treated exhaust gas is less than 30 ppm, preferably less than 10 ppm.

12. The method according to claim 1, characterized in that The oxygen plasma generator is selected from any one of an arc plasma generator, a high-frequency induction plasma generator, a microwave plasma generator, and a radio frequency plasma generator.

13. A device for treating waste gas by plasma thermal oxidation, characterized in that: The equipment comprises an oxygen plasma generator and a plasma reaction device.

14. The device according to claim 13, characterized in that The oxygen plasma generator comprises at least one air inlet pipe, and the air inlet pipe is used to introduce a carrier gas, and the carrier gas is selected from air, oxygen or a combination thereof, preferably oxygen.

15. The device according to claim 13, characterized in that The cathode material in the oxygen plasma generator is selected from hafnium, zirconium, and silver, preferably metal hafnium.

16. A system for treating waste gas by plasma thermal oxidation, characterized in that: The system comprises the device according to any one of claims 13 to 15 and an oxygen recovery device, wherein the oxygen recovery device is used to recover by-product oxygen in an upstream production process.

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