VOCs emission characteristics and pollution control technology of automobile coating industry in Jiangsu Province

China Coatings Online News Information: With the
demand for automobiles, Jiangsu Province's auto industry will continue to maintain a high growth rate. Volatile organic matter produced during surface coating in automobile manufacturing industry is an important preludes to stratospheric ozone (O3) and secondary organic aerosols (SOA) and a major factor in increasing atmospheric oxidation. At the same time, the industry emissions of benzene and other toxic and harmful VOCs to serious damage to human health.
the automobile manufacturing industry paint consumption is large, VOCs composition is complex, emission strength is large, ISOS pollution control focus. Beijing, Shanghai and the Pearl River Delta region have carried out a typical automobile coating enterprise VOCs emissions test, obtained some source spectrum information. The United States, Europe and other developed countries, as well as Taiwan, Hong Kong, Beijing, Shanghai, Guangzhou and other places according to the characteristics of emissions have developed a series of VOCs pollution prevention and control measures, including VOCs limits in coatings, automotive coating VOCs emission standards, pollution control technical guidelines.
, there is less research on pollution control measures based on VOCs emission characteristics in the automotive coating industry. This study will analyze the VOCs emission characteristics of the automotive coating industry, identify the key components of VOCs, summarize the best applicable technologies for pollution control, and provide scientific and reliable technical support for the development of VOCs control strategies for the automotive coating industry based on improving air quality.
1, research methods Select the typical enterprises of different models (cars and buses) in Jiangsu Province to carry out the investigation of THE FULL-process emission characteristics of VOCs, screen the key emission links, and carry out the research on the source emission characteristics and the best applicable technology.
1.1 Unit Coating Area VOCs Emissions Estimation
Using material balancing method to calculate VOCs emissions per vehicle unit coating area, understand the total VOCs emission characteristics of different models per unit area, and calculate the total voCs emissions per unit of coating area in formula (1):
(1)
, total VOCs emissions per unit of coating area, g/m2; The amount of VOCs in thinners, sealants and cleaning solvents, kg/month, O1 is the amount of VOCs recovered per month (reusable or waste disposal), kg/month, O2 is the amount of VOCs destroyed by monthly pollution control equipment, kg/month, A is the area painted at the end of each month, refers to the total area of the underbody of the vehicle, the body area of the computer-aided design system, m2/month.
1.2 source emission
test is used to understand the voCs emission concentration level and species composition of enterprise exhaust cylinders. Enterprise categories, exhaust gas treatment facilities and sampling conditions as shown in Table 1, including the use of different exhaust gas treatment measures of the two car manufacturers, as well as a light passenger car production enterprises, focusing on large emissions of paint rooms and drying room sampling testing, sampling frequency 3 times a day, a total of 2 days, samples collected through the chimney. The determination of the source emission sample is made using solid adsorption/thermal de-attachment-gas chromatography, which uses a filling poly2,6-dibenzene-based benzene ether (Tenax) sampling tube to collect volatile organic matter in ambient or indoor air at room temperature, and for the higher temperature exhaust tube sampling tube is connected to a thermal desorption meter, the adsorption component is imported into the gas phase chromatography detector with hydrogen flame ionization detector (FID) after heating.
Table 1VOCs sampling enterprise situation
2, results analysis and discussion 2.1 coating process and sewage links
Jiangsu Province car production enterprises mostly use electrophoresis primer, medium coating, paint traditional 3-layer coating system The primer uses water-based epoxy cathode electrophoresis primer, basically free of organic solvents, medium and finish mainly use urethane paint, acrylic paint and polyester paint and other coatings, organic solvent content is higher, the use of more than 50%. In order to reduce VOCs emissions, two enterprises have taken the lead in adopting a water-free mid-coating process, which reduces VOCs emissions per coating area from 50-90g/m2 to less than 25g/m2, which has a significant effect on reducing VOCs emissions. The surface coating of passenger cars and micro- and light goods vehicles has relatively low requirements for rust protection, stone-resistant, weather resistance and UV isolation, so the medium coating is omitted by the 2C2B coating system. At present, the province's passenger cars and micro, light goods vehicle production enterprises in the primer spraying basically realized electrophoresis spraying, and paint spraying mainly in accordance with customer needs, the use of artificial spraying methods. According to the results of the typical enterprise survey, the VOCs in the coating mainly include methyl ethyl ketone, inter- xylene, ethyl benzene, toluene, isopropyl alcohol, ethyl acetate, acetone, phthalates, methyl isoclode, 1,2,4-tritobenzene, 1,3,5-tritobenzene, benzene, styrene and butyl alcohol, isodyl alcohol and butyl steril.
the process of coating the car VOCs mainly arise from: medium and paint spraying and drying process and plastic parts processing of the painting process. Due to the relatively low VOCs content in PVC and protective wax, the body seal and spray protection wax are not the main drainage links. The organic exhaust gas produced by body coating has the characteristics of high wind volume and low and medium concentration. (1) Paint room: the main harmful components in the exhaust gas discharge chamber are organic solvents volatile during the painting process. The exhaust volume of the paint chamber is very large and the total concentration of organic matter in the exhaust gas is very low, usually below 100mg/m3. In addition, the exhaust of the paint chamber often contains a small amount of unprocessed complete paint mist. In particular, dry paint fog catch spray room, exhaust paint fog more. (2) Drying room: the composition of drying exhaust gas is more complex, in addition to the organic solvent itself, but also contains thermal decomposition of the generator and reaction production. Although electrophoresis coating belongs to water-based paint, it still contains more organic components in drying gas. In addition to the electrophoresis coating itself contains a small amount of alcohol ether organic matter, but also contains the drying process of thermal decomposition of the results (such as aldehyde small molecules). The concentration of total organic matter in electrophoresis drying exhaust gas is generally 500-1000mg/m3, which is lower than that of solvent-based coatings. (3) Solid waste storage place: waste paint barrels and paint mist collection after the production of paint slag if not timely treatment, often produce organic exhaust gas.
2.2 total emissions per unit coating area
the total emission results per unit area VOCs calculated by the material scale algorithm as shown in Table 2, the unit coating area VOCs emissions of different models have a large difference, the car due to the coating workshop closed facilities, paint The amount is relatively low, the VOCs emission per coating area is low, between 40-60g/m2, and the passenger truck, especially the bus, uses artificial spraying, workshop closure and other pollution control measures are weak, the emissions reached more than 300g/m2. Table
2 The total VOC emissions per unit coating area of typical automobile coating enterprises in Jiangsu Province
2.3VOCs emission concentration and component characteristics
the main species composition of VOCs emissions of 3 typical enterprises such as Table 3, benzene is an important component of VOCs emissions of automobile coating enterprises, the highest proportion of emissions of enterprises in the range of 33.2%-64.6%. The most important species of benzene is xylene, which accounts for 10.3%-22.9% of spraying workshops. Butyl acetate, isopropyl alcohol, butanol and other alcohol-like substances accounted for a relatively high proportion, the highest proportion of emissions of enterprises in the 29.6%-61.2%. With the gradual increase of pollution control in the solvent industry, the emission composition has changed significantly. Substances such as esters and alcohols have been used as substitutes for benzene solvents in recent years, and their use has increased significantly, especially in some thinners and cleaning agents. Overall, there are similarities in the emission species categories of each enterprise, but there are still large differences in the composition and concentration contribution of specific species, one is due to the use of different coating composition, dilution ratio is not the same, and the other is due to the use of different end treatment technology on the emission components of exhaust emissions, it is difficult to use a unified source spectrum to represent the entire automotive spraying industry emission characteristics.
Table 3 typical enterprise characteristics VOCs part concentration
2.4 best applicable technology
According to the results of the results of the pollution emission characteristics study, the automotive coating industry VOCs emissions concentrated in the spraying and drying links, mainly from the use of coatings, emissions of the parts are basically consistent with the use of coatings in the parts. Pollution control measures can be divided into two categories: source control and end control.
can effectively reduce VOCs emissions by replacing solvent-based coatings with powder coatings, water-based coatings and high-solid-component coatings. The vast majority of domestic car coating using cathode electrophoresis and solvent-based medium coating and solvent-based paint and single-part hood varnish process system. Car primers have achieved a comprehensive replacement, but the medium coating, paint or solvent-based, VOCs emissions higher than developed countries (regions) emission levels, from environmental protection (low VOCs, water-based) coating application development is still a certain distance. At present, a small number of large domestic automotive enterprises new production lines using water-based coating technology, greatly reducing the level of automotive surface coating VOCs emissions, close to or meet the European automotive coating VOCs emission standards.
table 4 different body coating supporting system VOCs emission level
using advanced coating technology, can reduce the use of coatings, combined with the use of low VOCs content of coatings, can significantly reduce VOCs emissions. At present, advanced coating technology includes 3C1B technology and water-free medium coating process. 3C1B coating process eliminated the 3C2B process of medium coating baking and grinding process, to be applied, color paint, hood paint three layers of coating together for drying and curing treatment, can save the use of paint. The water-free coating process simplifies the economy car and commercial body from a 3 coating system to a 2 coating system, simplifying the body coating process and reducing VOCs emissions by 20% compared to the 3C2B coating process.
of the automotive spraying industry include paint chamber paint mist collection measures, VOCs exhaust gas collection and control measures. Paint mist collection in paint rooms is an important waste gas pre-treatment measure in the automotive coating industry and play a key role in the end control effect of VOCs. Paint mist treatment method mainly includes filtration method, condensation method and liquid absorption method, among which the filtration method (dry) and liquid absorption method (wet type) have a wide applicability, the most widely used Venturi-type water spin (vortex) paint mist separation technology is a wet treatment method. Because wet (washed) paint mist separation technology produces hazardous waste such as paint residue, the United States and Europe have restricted its use, and gradually adopted dry paint mist separation technology, effectively reduce energy consumption, and basically do not produce chemical condensate.
the spray exhaust gas and level, drying exhaust gas mainly contains VOCs organic exhaust gas, generally by adsorption, combustion and some combination of degradation treatment. When using direct combustion method to treat exhaust gas, in order to improve the temperature of exhaust gas treatment and reduce fuel consumption, the exhaust gas after combustion and the exhaust gas before combustion are usually exchanged heat, according to the different forms of heat exchange and waste heat utilization, the common forms of direct combustion are RTO (heat storage thermal combustion system) and TAR (recycled thermal combustion system), large domestic automobile enterprises coating more use OFO for VOCs exhaust end treatment. For the new automobile coating production line, European and American automobile enterprises preferred TAR to dry the end of the oven VOCs exhaust gas treatment, processing efficiency can reach more than 99%. The use of adsorption method - desorption regeneration technology, such as equipment maintenance length can also achieve more than 90% processing efficiency. Due to the large air volume and low concentration of the paint chamber, often can not be directly used incineration treatment, wheel concentrated adsorption -heat storage incineration technology is currently one of the best spray exhaust gas treatment effect technology, the use of adsorption-de-attachment-concentrated incineration and other three continuous procedures, low concentration of exhaust gas adsorption concentration, and then de-attachment using incineration technology treatment, treatment efficiency of more than 99%.
3, the conclusion(1) VOCs emissions from the automotive coating industry mainly come from the coating and paint spraying and drying process, and the emissions are mainly affected by the use of coatings, coating processes, exhaust gas collection and treatment measures.
(2) Benzene is an important component of automobile coating enterprises, the highest proportion of emissions of typical enterprises in the 33.2%-64.6%, of which the most important component is xylene, accounting for 10.3%-22.9% of the spraying workshop. In recent years, butyl acetate, isopropyl alcohol, butanol and other alcohol esters are widely used to replace benzene solvents, typical enterprise emissions accounted for the highest proportion between 29.6% and 61.2%.
(3) bus paint is generally used manual spraying, workshop closure and other pollution control measures are weak, unit coating area VOCs emissions reached more than 300g/m2, while cars in 40-60g/m2 rooms.
(4) using 3C1B technology, water-free medium coating and other advanced coating technology, and the use of powder coatings, water-based coatings and high solid composition coatings instead of solvent-based coatings, can effectively control VOCs emissions from the source. The removal rate of VOCs can reach more than 99% by using advanced technologies such as dry paint mist separation technology and wheel concentrated adsorption-heat storage incineration technology.
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