Research Progress on Exhaust Gas Treatment in 4S Store Painting Processes
As people's demands increase, cars are gradually becoming more popular in households, and the pollution generated by car 4S stores during the painting process has also drawn increasing attention. This article focuses on analyzing and comparing the methods for disposing of exhaust gas pollution during the automotive painting process, such as adsorption, absorption, condensation, oxidation, low-temperature plasma, and biological methods. Based on this analysis, a new treatment measure is introduced: soil microbial method. The research results of this article can provide a reference for the environmental treatment research of painting exhaust gas in 4S stores.
When cars are repaired in 4S stores, they often need to undergo baking and painting operations. During these operations, the 4S stores generate a large amount of strong irritating gas. These gases contain a large amount of benzene, toluene, xylene, and other volatile organic compounds (VOCs). If these gases are directly discharged into the environment without treatment, they will cause serious harm to people exposed to them for a long time.
The painting exhaust gas from 4S stores seriously pollutes the environment and the treatment of painting exhaust gas is very important. This article reviews various treatment technologies for the main pollutants in painting exhaust gas and compares the recent domestic and international developments in the painting exhaust gas industry. It proposes some possible trends in the treatment of painting exhaust gas.
1. Existing treatment technologies for painting exhaust gas
In recent years, many scholars have conducted research on the treatment technologies for painting exhaust gas in 4S stores and exhaust gases containing benzene compounds. Some of the earlier traditional technologies include adsorption, absorption, and condensation. New technologies that have emerged in recent years include direct oxidation, photocatalytic oxidation, low-temperature plasma, and microbial treatment.
1.1 Adsorption method
The adsorption method for organic exhaust gas mainly includes chemical adsorption and physical adsorption.
Cui Qinghua et al. selected sodium citrate, dried mint extract, etc. as five absorbents and optimized the experimental conditions. The results showed that, compared with others, the absorption efficiency of BDO was better and it had the advantages of low treatment cost and high efficiency in engineering applications. Tao Dongdong et al. conducted experiments to remove xylene exhaust gas. They studied the effects of adding inorganic salts, etc. on the removal of xylene by sodium citrate absorbents. The experimental results showed that when 8% sodium citrate absorbents were added with 1.5% Tween-80, 1.5% Span-80, 0.5% sodium silicate, and 0.5% sodium phosphate, the removal efficiency of xylene was significantly improved.
The physical absorption method has low cost and is easy to operate and is widely used in simple treatment. Activated carbon is one of the commonly used substances in the adsorption method. Granular activated carbon has good affinity and uniform pores. Due to the longer distance through which gas is processed, the removal rate is relatively low. Fibrous activated carbon has small pore diameters and is directly outward, and the adsorption process is faster. The activated carbon adsorption method for 4S store exhaust gas can achieve an efficiency of 90% to 95%, and it is inexpensive, but it has the following disadvantages: the adsorption efficiency decreases with time; the usage time is limited; it requires regular replacement; the regeneration treatment is troublesome; and the waste as hazardous materials needs to be processed at a cost. To improve the benzene adsorption capacity of activated carbon, Yao et al. modified activated carbon with alkaline solution, and after modification, the adsorption capacity for benzene and surface area reached 1210 In/g and 423 mg/g. Tham et al. modified activated carbon with H3PO4 solution, prepared a batch of activated carbon (DSAC), and investigated the removal effect of different activated carbon on different concentrations of xylene exhaust gas. The results showed that the maximum BET specific surface area of activated carbon reached 1404 m/g, and the highest xylene removal efficiency was 30% at PO.
Activated carbon fibers are a third-generation activated carbon product made by carbonizing ultra-fine fibers such as viscose fibers and then activating them with water vapor. The micropores of activated carbon fibers are directly exposed to the outer surface, and the diffusion distance of adsorption molecules is shorter, which is 1 to 10 times higher than that of ordinary activated carbon.
1.2 Absorption method
Absorption method utilizes the principle of similarity and compatibility, using substances with similar physical and chemical properties to absorb the waste gas containing benzene series compounds. The absorption method has the advantages of simple equipment structure, easy maintenance, simple process, easy operation, and low cost. However, it also has the following constraints: the selection of absorbent, the recovery, regeneration, and treatment of secondary pollution. These factors limit its application. Chen Wei et al. designed a circulating spray device made of used engine oil and iron filings (see Figure 1), using the principle of similarity and compatibility to absorb the waste gas. The experimental results showed that the removal efficiency of benzene, toluene, and xylene was above 90%. This device has the advantages of small land occupation and waste treatment. However, it also has the disadvantage of organic gas volatilization during the absorption process.
Zhu Yongjun et al. used sodium citrate aqueous solution, Tween-80 aqueous solution, new mineral oil, and new mineral oil-water as research objects. They experimentally studied the absorption effects of several absorbents on xylene waste gas. The experimental results showed that the sodium citrate aqueous solution had limited absorption capacity for xylene waste gas. Adding salts could slightly enhance the absorption effect of the aqueous solution on xylene waste gas. Among them, the Tween-80 aqueous solution had a higher absorption rate for xylene waste gas than the sodium citrate aqueous solution; the absorption rate of the new mineral oil-water mixed absorbent per unit volume was 46% higher than that of the new mineral oil absorbent, and it also had the advantages of regeneration and low cost.
1.3 Condensation Method
The principle of the condensation method is to utilize the different vapor pressures of the waste gas containing benzene series compounds, through physical properties to cause pollutants to condense, so that the useful components in the waste gas can be recovered and the waste gas can be purified. The secondary condensation process for treating benzene series waste gas conducted by Li Hongxia showed that it could meet the standard for discharge. The research on the treatment of condensation method mainly focuses on the design of the condenser, achieving condensation and recovery of useful substances through changes in the structure and heat dissipation of the condenser. However, due to the complex pollution components in automotive coating waste gas and the excessive toxic substances, this method is prone to secondary pollution, and thus is rarely used after strict environmental supervision.
1.4 Oxidation Method
The commonly used measures for treating automotive spraying waste gas through oxidation include: regenerative thermal oxidation technology (RTO) [L18] (see Figure 2), catalytic oxidation, etc. The regenerative thermal oxidizer is a device for treating medium and low concentration organic waste gas. It is suitable for waste gas with an organic content between 1.0×10 and 2.0×10. Photocatalytic oxidation technology has been widely used in the treatment of wastewater, and using it to treat VOCs waste gas is a new technology. In the light environment, semiconductor materials such as TiO will generate active substances with strong oxidation ability that directly convert benzene series compounds into harmless inorganic substances. This method not only has a fast reaction speed and good treatment effect, but also has the advantages of easy recovery and being unaffected by solvents.
Yao Zhenhua et al. used manganese dioxide/graphene ozone catalytic oxidation of toluene. The experimental results showed that MnO was uniformly and firmly anchored on the surface of graphene, and the deposition of MnO prevented the composite from being combined in multiple layers, thereby making the composite have a larger specific surface area. Under the same conditions, MnO/graphene had better ozone catalytic oxidation performance of toluene compared to pure MnO or graphene. Lu SY et al. used tungsten and vanadium pentoxide as carriers to catalyze nano-sized titanium dioxide, improving the degradation efficiency of benzene-containing waste gas.
1.5 Low-Temperature Plasma Method
Low-temperature plasma technology is achieved through discharge, generating a large number of high-energy electrons and superoxide particles, hydroxyl radical particles, etc. Active particles with high reactivity, converting benzene series compounds in the waste gas into CO, HO, etc., harmless or low-harmful substances.
Shi Yao et al. used a nano-TiO: /SMF electrode for plasma catalytic degradation of paint waste gas. The research results showed that the nano-TiO/SMF electrode achieved a higher degradation efficiency than the untreated SMF, with the degradation efficiency of xylene reaching up to 92.1%. The nitrogen in the carrier gas could significantly enhance the catalytic activity of the nano-TiO: /SMF catalytic electrode. Liang Wenjun et al. used low-temperature plasma in combination with vanadium-titanium catalyst to degrade toluene. The results showed that the filling of vanadium-titanium catalyst had a significant promoting effect on the degradation of toluene, with a decrease in energy density and ozone generation, and a significant improvement in energy efficiency. The degradation efficiency of toluene decreased slowly with the extension of the reaction time, with the degradation rate dropping from 87.5% to 83.6% within the first 100 minutes, and reaching an average efficiency of 83% within 900 minutes. Jiang N et al. used silver-sodium double-metal plasma to catalytically degrade benzene-containing waste gas, achieving a degradation efficiency of 96.2%.
1.6 Biotechnology
Biological treatment processes can be classified into biological filter tower systems, biological scrubber systems, and biological drip filter tower systems (BTF) based on the operation conditions of the system and the form of microorganisms present. Biotechnology has low secondary pollution and low operating costs, and is suitable for the treatment of medium and low concentration organic waste gas. However, the gas must be water-soluble and biodegradable, and the reactor startup and the waste gas treatment process require a long time. It also needs to provide the necessary conditions for the survival of microorganisms.
The principle of treating benzene-containing waste gas by biological methods is to control environmental conditions and domesticate specific microorganisms. The microorganisms use benzene-containing waste gas as a carbon source and energy source to maintain life activities, and convert it into inorganic substances such as CO: and H.0, thereby achieving purification. Biotechnology for treating waste gas is already quite mature abroad and has gradually been used in the research of treating benzene-containing waste gas.
Zhou Min et al. used a biological drip filter bed to treat toluene waste gas, and the experiment showed that the treatment effect was obvious, achieving a removal rate of 93%. Sun Lishi et al. used a biological drip filter bed to treat benzene-containing waste gas, and the results showed that the removal efficiency could be maintained at around 90%. Xu Mengmeng et al. used a filter bag-type membrane bioreactor to purify xylene, and the experimental results showed that under the condition of 4-9°C, the removal capacity of xylene increased with the increase of residence time and inlet load, and the maximum removal capacity could reach 74.3g/(m˙h), with a benzene removal efficiency of 90%, comparable to the operating performance of a general biological drip filter tower. In this method, microorganisms mainly attach to the filter bag surface and grow well. R.A. Pandey et al. used a biological filter to treat benzene-containing waste gas, achieving a treatment rate of 97%.
2 Soil Microbial Treatment Method
Soil microorganisms have good degradation ability for pollutants. In the process of treating waste gas by soil method, the organic components in the waste gas are adsorbed while being degraded by microorganisms. As early as 1996, the Korean scientist Sung Ji-tae reported that the Korean used soil microorganisms to treat toxic gases produced in enterprises. By using various microorganisms with the ability to decompose waste gas, heavy metals and the foul smell of coal smoke in the emissions from chemical factories and food factories were removed, with an odor removal rate of 99.9%. The design principle is: pressurize the waste gas and pass it through the soil layer, allowing the original components to be adsorbed by particles or dissolved by soil water, and then be oxidized and decomposed by soil microorganisms. The specific treatment process: pressurize the waste gas and pass it through four soil layers: sand, sand-milled soil mixed layer, general sand layer, and sand-milled soil layer for cultivating microorganisms. Remove heavy metals and toxic substances with foul gas and odor from the waste gas, turning it into clean gas and releasing it into the atmosphere. Using this soil microbial waste gas treatment equipment to treat toxic gas only requires replacing the microorganisms and soil once every seven years, and it can be used continuously, which is a semi-permanent equipment.
Based on this equipment, improvements can be made, such as changing the soil composition or the layout of each layer of fillers, and cultivating different types of microorganisms to treat special or mixed waste gases. This research direction is currently in its infancy.
3 Conclusion
With the acceleration of social development, people's requirements for living and working environments are gradually increasing. The high hazard of VOCs gases generated in coating waste gas has also received increasing attention. China's governance and management of VOCs is relatively weak and insufficient, but as air pollution becomes increasingly severe, the country's efforts to manage waste gas emissions and disposal have become increasingly strict. The Ministry of Ecology and Environment published the "Technical Guidelines for VOCs Control in Key Industries" in 2014, and the requirements for end-of-pipe treatment have gradually increased. This paper reviews the research on the treatment of waste gas containing benzene compounds, which can provide a reference for the treatment of waste gas containing benzene compounds.
