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Suzhou Huawen|Use of low concentration VOCs waste gas treatment skills:

Exhaust gas treatment

With the development of the economy and the improvement of people's living standards, the problem of volatile organic compounds (VOCs) in the air has received increasing attention. VOCs refer to volatile organic substances that have a vapor pressure greater than 70.9 lPa and a boiling point of 260 ° C or less at room temperature, and are considered to be the second largest class of air pollutants after dust. In 1990, the US Clean Air Act amended 189 kinds of toxic and hazardous substances, most of which were VOCs. From 1993 to 2003, China issued the "Insulation Rules for Inorganic Pollutants", "Specifications for Emissions of Odor Pollutants" and "Indoor Air Quality Specifications". Therefore, it is imperative to develop applicable VOCs management skills.

Because the VOCs in the exhaust gas tend to have low concentrations (<3000mg/m3), the gas volume is large and the pollution is wide. Traditional processing techniques such as thermal incineration, catalytic combustion, condensation, absorption and adsorption are often not applicable, and the treatment effect is not At the request, the capital or operating costs are too high, forcing people to seek and develop new applicable skills.

In recent years, new skills such as low temperature plasma, photocatalytic oxidation and biological treatment have shown their technical advantages and good market prospects in dealing with low concentration VOCs. This paper will introduce these new skills and exhaust gas treatment equipment.

1 low temperature plasma skills

1.1 Principle

A plasma is a fourth form of a substance containing many electrons, ions, molecules, neutral atoms, excited atoms, photons, and radicals. The total positive and negative charge numbers are flat and microscopically electrically neutral, but have electrical and electromagnetic properties and exhibit high chemical activity. Plasmas are generally classified into high temperature plasmas and low temperature plasmas (including thermal plasmas and cold plasmas) depending on system energy state, temperature, and ion density. The ionization of high-temperature plasma is close, the temperature of various particles is almost the same, and the system is in thermodynamic equilibrium. It is mainly used in the study of controlled thermonuclear reaction. The low temperature plasma is in a thermodynamic non-equilibrium state, and the temperature of various particles is not the same.

Low-temperature plasma can be obtained by high-voltage pulse discharge with steep front edge and narrow pulse width (nanosecond order) at normal temperature and pressure. The high-energy electrons and active particles such as O ? and OH 可 can be combined with various pollutants such as CO and HC. The seizure effects such as NOX, SOX, H2S, and RSH are converted into harmless or low-harm substances such as CO2, H2O, N2, S, and SO2, thereby purifying the exhaust gas. It can promote some chemical reactions that are difficult to carry out under normal conditions, even in a very short time, so it is a cutting-edge skill in the management of low-concentration VOCs.

1.2 Discussion

Low-temperature plasma is mainly caused by gas discharge, and it is closely related to modern industry and is widely used. According to the discharge method, it can be divided into glow discharge, corona discharge, dielectric resist discharge, radio frequency discharge and microwave discharge. Pulse corona is a new type of plasma technology. It belongs to cold plasma, can work under normal pressure and low temperature, and has moderate electron energy. Therefore, it is generally used to treat harmful gases such as VOCs. In the mid-1980s, Masuda and Mizuno first proposed it, and now it has been studied in countries such as China, Japan, Russia, and Canada [1, 2].

FutamuraS et al. [3] studied the catalytic activity of metal oxides in low temperature plasma chemical treatment of harmful atmospheric pollutants (HAP). In the absence of MnO2 as catalyst, the molar conversion of benzene was 30%, while in the presence of MnO2 When used as a catalyst, the conversion of benzene can be as high as 94%. KangM et al [4] used a plasma TiO2 catalytic system to remove toluene waste gas with an initial concentration of 1000 mg/m3 under normal pressure. The toluene removal rate was 40% only in the absence of TiO2 catalyst in O2 plasma; under TiO2/O2 plasma The removal rate reached 70%; in the O2 plasma, when TiO2 was supported on γ-Al2O3, the removal rate of toluene reached 80%. These studies have shown that the synergistic effect of plasma and catalytic response is used to improve the organic gas purification rate and reduce energy consumption.

In recent years, domestic scholars have also deepened their research on low-temperature plasma. Yu Yong et al [5] used a dielectric shield to degrade CF3Br, and the degradation rate reached 55%. Li et al [6] studied the differentiation characteristics of chlorobenzene and toluene by introducing bipolar pulse high pressure into the medium resisting reverberator. Feng Chunyang [7], Yan Naiqiang [8] and Huang Liwei [9] and others conducted a pulse corona removal of a variety of organic waste gas, the initial concentration of 76.8mg / m3, benzene removal rate reached 61.4%, and The removal rate of toluene in the two types of reverberators, line-and-tube and plate-type, was compared. When Mn, Fe, etc. were used as catalysts, the removal rate was improved. The order of catalyst activity was Mn>Fe>Co>Ti>Ni. >Pd>Cu>V, formaldehyde is most easily removed in removing various organic waste gases. Methylene chloride is the most difficult, and toluene, ethanol and acetone are in between. Zhou Yuanxiang [10] also used low temperature plasma technology to deal with dioxin in dust, with a removal rate of 81%.

Therefore, the feasibility and conditional tests for the use of low-temperature plasma skills have been abundant, and there are many theoretical foundations; the new skills of this process are simple, applicable, short-flow, low-energy, easy to operate and automated. It has laid a solid foundation.

2 photocatalysis skills

2.1 Mechanism of action

In recent years, the use of photocatalytic skills to treat gaseous pollutants has also received increasing attention from countries around the world. The research indicates that this skill can differentiate organic matter in exhaust gas into CO2, H2O and other inorganic substances under normal temperature and normal pressure conditions, and has great potential use value. Since the discovery of TiO2 single crystal electrode differentiation water by Fujishima and HondaL in Japan in 1972, it has marked the beginning of a new era of nano-phase heterogeneous photocatalysis. In the semiconductor catalyst used in multi-phase photocatalytic reverberation, TiO2 powder is generally used as a photocatalyst in foreign countries. Degradation of benzene series [11], but

TiO2 has a wide forbidden band, and solar energy can only account for 3% of total solar energy. In order to improve the utilization rate of solar energy, scholars around the world have surrounded the preparation of high-activity nano-TiO2, multi-phase photocatalytic mechanism and the photocatalytic power of TiO2. A lot of exploration work.

Nano-TiO2 is an n-type semiconductor with three different crystal phase structures: anatase, brookite, and rutile. In the meantime, anatase TiO2 has high photocatalytic oxidation ability, and its forbidden band width is Eg=3.2eV, which is equivalent to the energy of light with a wavelength of 387 nm, and is in the ultraviolet region. Under the action of ultraviolet light, the electrons (e-) in its valence band can be excited to transition to the conduction band, and the corresponding holes (h+) occur on the valence band, and then h+ and e- are adsorbed on the surface of TiO2. The action of H2O, O2, etc., produces high-activity groups such as ?OH, ?O2-, and of course the holes and electrons of the attack may be complex. The mechanism is as follows:

Photooxidation equipment

Regarding pure TiO2, photogenerated electrons (e-) and photogenerated holes (h+) occur when irradiated with ultraviolet light having a wavelength of λ = 387.5 nm. E- and h+ can also be recombined to convert light energy into heat energy; when there is a proper catcher or surface vacancy, the complex of e- and h+ will be inhibited, and the redox reaction will occur. The capture agent of photogenerated electrons is primarily O2 adsorbed on the surface of TiO2. O2 absorbs electrons and is capable of generating H2O2 and a series of free radicals. OH ? is the primary free radical in the photocatalytic system. The free radical has a strong oxidation effect, and its oxidation is almost non-selective, and it can oxidize various organic substances including compounds that are difficult to biodegrade. Photogenerated electrons can also react with substances such as O2 and H2O to generate a series of free radicals, which in turn oxidize organic matter, thereby achieving the purpose of eliminating pollutants.

2.2 Discussion and development

TiO2 photocatalytic technology has strong processing ability for industrial wastewater, and it has been widely used. The use of TiO2 as a photocatalyst to purify the air has gradually become more sophisticated in foreign countries, but the domestic research is in the ascendant.

Most of the organic pollutants in the air can be removed by photocatalytic oxidation of TiO2. The photocatalytic degradation of gaseous organic compounds such as olefins, alcohols, ketones, aldehydes, aromatic compounds, organic acids, amines, organic compounds and trichloroethylene has been reported in the literature. Its quantum power is more than 10 times that of degrading the same organic matter in an aqueous solution. Vorontsov et al. [12] found that the primary gas phase products contained (C2H2)2S2, CH3CHO, CH3CH2OH, C2H4 and trace amounts of CH3COOH, C2H5S(CO)CH3 and SO2 in the gas phase photocatalytic degradation of TiO2 (C2H5)2S.

Domestic TiO2 photocatalytic use in exhaust gas treatment is still relatively rare. In recent years, preliminary research and kinetics of indoor air and low concentration benzene series have been made. See Table 1 for details.

Photooxidation is used in the spray booth

It has also been noted that magnetic photocatalysis can use magnetic fields to make TiO2 easy to recover. Therefore, the function and development of magnetic TiO2 photocatalysts are explored. The TiO2 photocatalytic suspension system has a large specific surface area to produce new high-efficiency light. Catalytic reverberator.

3 biological purification

3.1 purification mechanism

Soil is the base of microbial organisms. As early as 1957, the United States invented the patent for the deodorization of H2S by soil filtration. The skill at that time was only composed of covering the soil on the gas pipe. In the 1980s, a considerable amount of exhaust gas biological treatment equipment was put into operation in Europe. In the 1980s and 1990s, it was the golden age of biological waste treatment in Europe [17-18]. For example, the proportion of biological treatment processes used in Germany in 1994 has reached 78%. Because it has the advantages of good function, stability, low operating cost, no secondary pollution, it has become the preferred technology for sophisticated technology and treatment of VOCs-containing waste gas in developed countries; its superiority is increasingly recognized in China, and Get more and more widely used.

The biological treatment process of the exhaust gas is essentially that the microorganisms attached to the biological filler medium use the pollutants in the exhaust gas as a carbon source and energy under appropriate environmental conditions, maintain their life activities, and differentiate them into CO2, H2O, etc. The process of harming inorganic matter. The pollutants in the exhaust gas mainly undergo mass transfer from the gas phase to the solid/liquid phase, and then are degraded by the microorganisms in the solid/liquid phase. There are three main types of reverberators: biological scrubbers, biofilters, and bio-tricks. In addition, the types of reverbs that have recently entered the field of study have bio-drums.

3.2 Discussion

The mass transfer process in the biological treatment process of exhaust gas is mainly explained by two theories. One is the "absorption-biofilm" theory proposed by Dutch scholar Ottengraf according to the double membrane theory of absorption operation. One is Sun Peishi et al. The proposed adsorption-biofilm theory. In recent years, the research of biological treatment process has focused on repairing and improving the above theory. For example, Zarook, Delhomenietomski et al [19~21] proposed the theory of axial dispersion of gaseous pollutants, thinking that the size and surface area of the filler are affecting its gaseous pollutants. The primary factor of degradation, and the use of mass and energy balance theory to verify the moisture change in the packed bed, and discuss the mechanism of water change.

Both the biological filter bed and the biological trickling filter can handle mixed waste gas such as odor and VOCs [22], acetone, a mixture of toluene and trichloroethylene [17], toluene, ethanol and butanol [23], and Achieve good processing. LeCloirec et al [24] described the removal of VOCs, commented on the development of different processes of biological filter bed, biological trickling filter and biological scrubbing bed, and thought that the mass transfer of hydrophobic VOCs in biological trickling filter is a constraint factor. Deshusses et al. [25] also modeled VOCs for biological filter beds and bio-trick bed treatments, optimized the planning and treatment of the reverberators, and considered the appropriate pH, salinity, and concentration of metabolites in the biotrick filter bed. Make up for nutrient solution. Cho et al. [26] studied novel thermophilic microorganisms, including higher temperature VOCs.

Domestic biological methods have also been rapidly developed in low-concentration VOC waste gas treatment in recent years. Sun Shi et al [27,28] used biological methods to treat low-concentration reclaimed rubber industrial waste gas, and achieved good results. Together with gaseous pollution such as toluene, styrene, formaldehyde, CS2, SO2, H2S, NOX, etc. commonly found in industrial waste gas. The substance was subjected to a purification test. Sun Yumei [29] studied biological filtration to remove ethyl acetate and ammonia-containing waste gas, Chen Jianmeng et al. [30] selected Pseudomonas sp. GD11 strain to inoculate a biotrickling filter membrane to purify the concentration of 0.709mg/m3. The methyl chloride waste gas has an EBRT of 11.8 s, a removal rate of 97.6%, an optimum pH of 7.0 ± 0.5, and a temperature of 28.5 ± 2 °C.

4 develop a vision

In summary, low-temperature plasma, photocatalytic skills and biological skills are feasible in dealing with low-concentration VOCs, especially their safety, high efficiency, low energy consumption, no secondary pollution, and their use prospects are very broad; However, at present, they are not still in the stage of trial and discussion, that is, they have not yet become sophisticated skills, and there are still many homework to be done to achieve industrial use. In view of this, the author proposes that the prospects for the development of three new skills are shown in Table 2.

Exhaust gas treatment equipment

5 Conclusions and claims

Three new skills, low temperature plasma, photocatalysis and biological treatment, can effectively solve the problem that traditional skills have no suitable skills for dealing with low concentrations of atmospheric exhaust gas in the near future. With the rapid development of China's economy, the pollution caused by the emission of many VOCs from industrial enterprises is still very serious every year. However, China is developing our country and needs to use limited pollution management funds to be effective. These three new skills have the characteristics of low capital contribution, low operating cost, short exhaust time, high efficiency, stability, thorough response and no secondary pollution. They can overcome the shortcomings of traditional methods such as high operating cost and huge reverberant. The disadvantage of secondary pollution. It will play a huge role in the field of low concentration VOCs pollution management.

The development and use of new skills and new processes must be invested in the funds and strengths of the meeting, and deepen theoretical and technological research; on the other hand, because of the wide variety of VOCs, the process of exhaust emissions is complicated, and the appropriate skills are selected according to the specific situation. And crafts will be one of our top priorities.

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Application of Waste Gas Treatment Skills for Low Concentration VOCs in Suzhou Huaene

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Application of Waste Gas Treatment Skills for Low Concentration VOCs in Suzhou Huaene

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