In the coating process, VOCs are mainly produced in the spraying and drying process of the middle coat and top coat over plastic parts finishing(sealing and spraying due to relatively small content of VOCs). The organic waste gas produced by painting has the characteristics of large air volume and medium to low concentration.
1)Paint spray booth: The main harmful component from paint booth exhausting is the organic solvent volatilized during the painting process. The spray booth exhaust volume is large but the total concentration of organic matter is low, generally below 100mg/m3. In addition, the exhaust from paint spray booths often contains a small amount of paint mist that has not been fully treated. Especially in the dry paint filter spray booth, there are more paint mist exhausted.
2)Drying chamber: In addition to the composition of the organic solvent itself, the composition of dry waste gas is relatively complex, as it contains both thermal decomposition and reaction products. Although electrophoretic coatings are water-based, their drying gases contain more organic components. In addition to the electrophoretic coating itself containing a small amount of alcohol ether organic matter, there’s also thermal decomposition products (such as small molecules of aldehydes and ketones) during the drying process. The total concentration of organic matter in the electrophoretic drying exhaust gas is generally 500-1000mg/m3, and the drying exhaust gas is lower than solvent-based coatings.
3)Solid waste storage: If the paint residue generated after the collection of waste paint barrels and paint mist cannot be treated in time, organic waste gas will often be generated.
According to the research of pollution emission characteristics of automobile painting industry, VOCs emissions are mainly concentrated in the spraying and drying stations, mainly from the coating being used. The emission composition is basically the same as the coating used.
Pollution control measures can be divided into two categories: source control and terminal control.
Reduce VOCs emissions with powder coatings, waterborne coatings and high-solids coatings instead of solvent coatings. Most automotive coatings use cathodic electrophoresis + solvent-based intermediate coating + solvent-based pigment + one-component varnish process system. VOCs emissions are much higher than those of environmentally friendly (low) VOCs coatings. At present, a small number of large automobile enterprises adopt waterborne coating technology in their new production lines, which greatly reduces the emission level of VOCs of automotive surface coatings and very close to or meets with the emission standards.
The environmentally friendly paint spray booth adopts advanced coating technology, which can reduce the amount of paint, and the use of low-VOCs content coatings can greatly reduce VOCs emissions. At present, the advanced coating process includes 3 C1B technologies and water-based no-middle coating process. 3C1B cancelled the 3C2B process for baking and grinding requirement, waiting for the three coating layers to dry and cure together (medium coating, color paint and top paint), which can greatly save the use of paint. Waterborne neutral coating technology simplified the commercial vehicles coating from a 3-coat system to a 2-coat system, and the 3-coat system C2B simplifies the body painting process compared to the painting process, reducing VOCs emissions by 20%.
The final treatment measures of the automotive spraying industry include paint collection for paint rooms, exhaust gas collection and treatment measures for VOCs. Paint mist collection in painting booth is an important measure for exhaust gas pretreatment in the automotive coating industry, plays a key role of VOCs terminal governance. The paint mist treatment methods mainly include filtration method, condensation method and liquid absorption method, among which filtration method (dry) and liquid absorption method (wet) are widely used. Venturi hydrocyclone (vortex) paint mist separation technology is one of the widely used wet treatment methods. Because wet (washed) paint mist separation technology will produce hazardous wastes such as paint residue, its restricted in some countries and gradually adopt dry paint mist separation technology which can effectively reduce energy consumption, and basically do not produce chemical condensate.
The spray paint exhaust gas and leveling and drying waste gas after paint mist treatment mainly contain VOCs organic waste gas, and are generally degraded by adsorption, combustion and some combinations. When using the direct combustion method to treat the waste gas, in order to increase the exhaust gas temperature and reduce the consumption of fuel, usually we use heat exchange method before anf after combustion of exhaust gas. According to the heat exchange and waste heat utilization form, the common form is RTO (regenerative thermal combustion system) and TAR (regenerative thermal combustion system), automobile enterprises mainly use RTO for VOCs exhaust gas end treatment. For the newly built automobile painting production line, TAR is used at the end of the drying chamber, and the treatment efficiency can reach more than 99%.
Adsorption-desorption regeneration technology can also achieve more than 90% treatment efficiency. Due to the large air volume and low concentration of the paint spray room, combustion can not be directly used. Instead, runner concentrated adsorption-regenerative incineration technology is one of the best treatment technologies, the use of adsorption-desorption-concentrated incineration three continuous procedures, the low concentration of waste gas adsorption concentration, and then desorption by incineration technology treatment, treatment efficiency is more than 99%.