A brief description of VOCs comprehensive emission reduction in coating production!

Introduction:
coating provides decorative and protective functions for materials such as metals, and coating is one of the core production processes in the manufacturing industry. Traditional coatings use large amounts of organic solvents in production and construction, and solvents eventually evaporate into the atmosphere. As China's air pollution control work continues to advance, NOx, SOx, PM and other pollutant concentrations continue to decrease, ozone has become the most important pollutants in Shanghai and other

. VOC (volatile organic matter) is a key preludes for ozone generation, and SOA (secondary organic aerosols), SOA will also increase the concentration of microparticles such as PM2.5, with increasingly stringent CONTROL of VOC emissions represented by organic solvents.
With the promulgation of relevant regulations and regulations such as the Ministry of Ecological Environment's Program for comprehensive management of volatile organic matter in key industries, provinces and cities have also formulated regulations or measures for the management of volatile organic matter, of which industrial coating is the key rectification industry. In order to achieve environmental protection production, China China Car Company has carried out a lot of research and practice on the reduction of coating VOC emission reduction, and implemented the three-year action plan for the prevention and control of ecological pollution prevention and control of mid-week vehicles, with remarkable results.
current VOC emission reduction is mainly divided into source reduction, process control, end-of-control three directions. For the field of industrial coating, three types of measures need to be considered comprehensively and applied flexibly, and it is difficult to achieve satisfactory results in terms of economic and environmental benefits to achieve emission standards from a certain aspect alone, which is not conducive to the long-term competitiveness and green development of enterprises.
1, source reduction,
source reduction is currently the most concerned direction, is considered to be one of the most effective means. Source reduction strict limits on VOC content in coatings, and the use of less polluting organic solvents, many provinces and cities have introduced policies to promote source emission reduction. VOC content reduction can directly reduce the total emission volume and emission concentration, but if the single pursuit of very small VOC content, will lead to a significant increase in coating costs, construction difficulties and the risk of overall performance decline of coating, and ultimately not conducive to the gradual promotion of low VOC products. VoC limits in coatings should take full account of current technical levels and cost tolerance. The low VOC content coatings used in source reduction mainly include: water-based coatings, powder coatings, high solids (solvent-free) coatings and radiation curing coatings.
1.1 water-based coatings
water-based coatings using water as a dispersed medium, replacing organic solvents, its coating construction equipment and solvent-based coating basic general use, construction skills requirements are basically the same, product appearance is close, V0c drop significantly, is currently the most concerned about the source of reduction programs. High-quality water-based coatings used in general industry have reduced VOCs by 80% or more compared to conventional solvent-based products. Organic solvents in water-based coatings are mainly used as solvents to improve coating film-forming and construction properties, and their reaction activity to ozone is less than that of traditional solvents such as xylene, and the photochemical reaction level is low.
for the centralized continuous production workshop such as rail transit vehicles, the direct emission of exhaust gas from water-based coating is more difficult, but the post-treatment requirements are greatly reduced. At present, zhongcheng Puzhen city rail vehicle coating has all used water-based paint, many years of operation practice shows that: water-based paint construction is stable, reliable performance. Water-based coatings differ significantly from solvent-based coatings in some details, and need to focus on surface contaminants, temperature and humidity during construction and drying, coating maladies such as hanging and tweezers, ambient temperature when transporting and storing coatings, and repairing surface treatment requirements.
1.2 powder coating
powder coating is a solid powder state construction to the surface of the coating, the whole process does not require solvents as a dispersion medium, coating process close to 0VOC emissions. The film-forming process of the thermostatized powder coating with more application mainly includes spray construction, heating melting, leveling and curing. Powder coating coatings are excellent in appearance, performance and green.
powder coatings are developing rapidly in the field of small parts coating, and some large simple structural parts are also used. In the body coating of rail vehicles, powder coating is still difficult. Powder coating curing temperature is generally as high as 160 to 200 degrees C, while the subway and other body coating up to 3 to 4 layers, resulting in powder curing process requires a lot of energy and larger baking workshop investment. If you can improve the surface quality of the car body, a small amount of putty patching before coating can achieve surface flatness requirements, and eventually achieve two layers of powder or even multiple layers of powder baked into film at once, which can greatly reduce energy consumption costs, improve efficiency. Powder coatings are constructed using electrostectric spraying, and the ferrada cage shielding effect causes the local complex structure to become film-free inside, which also limits its popularization and application.
, powder coating is an environmentally friendly and efficient product, currently in the body of some metal plates and interior parts have mature applications. With the maturity of low temperature curing and 2C1B technology, the application range of powder coatings will be further expanded.
1.3 high solid (or solvent-free) coatings
high solid coatings generally refer to solids with a solid score of up to 80% or higher, while solvent-free coatings may have solids of more than 96%. Compared with VOC emissions per unit coating area, some high solid sub-coating emissions are even lower than water-based coatings. Because of its coating dispersion system and formulation composition close to traditional solvent-based products, high solid sub-coatings have significant advantages in coating cost, construction performance, coating physicophysication index and so on.
At present, high solid sub-paint products overall high viscosity, do not add diluent construction, its coating appearance and traditional solvent-based products and water-based coatings still have a large gap, the appearance of products have higher requirements for rail transit equipment and other products are not applicable. In the future, with the development of high-solidity low-stick technology and the use of low-pollution-exempt solvents, high solid sub-coatings will also become an important component of green coatings for rail transportation equipment.
1.4 Radiation curing (UV) coating
radiation curing paint generally refers to the use of ultraviolet light or electron beam to achieve coating curing products, wet film contains a large number of thinners, but most of the active monosome is not volatile, its curing becomes an important part of the coating film, so the construction process VOC volatilization is very small.
radiation curing coating environmental protection, high efficiency, energy saving, in wood, furniture, flooring, electronics and other industries widely used. Germany has reported the use of UV paint vehicle coating, but the domestic is still in the exploration stage, for rail transport equipment has the following difficulties:
1) coating thickness. Light curing coatings need to be exposed to ultraviolet light, etc. in order to complete the coating reaction, deep curing and colored coating curing has certain restrictions. The proportion of pigment fillers in subway paint is high, the penetration ability of light limits the thickness of single-layer coating film, and there is a large gap between the current requirements of 50 to 60um thickness of single-layer film for rail vehicles.
2) Radiation intensity. The radiation intensity of ultraviolet light is closely related to the distance and angle of the coated work piece, and the radiation intensity determines the curing time and effect of the product.
2 process control
whether water-based paint or solvent-based coatings, the construction of VOC release, process control is extremely important. The process control of broad significance includes two aspects, on the one hand, controlling VOC escape, on the other hand, reducing the total amount of VOC emissions.
coatings continue to volatile organic solvents into the atmosphere from the time they are mixed and stirred, and most unit exhaust gas treatment is currently limited to spraying and baking workshops. The evaporation of the process of mixing and mixing paint, transfer of paint after mixing, repair of open area, cleaning of construction tools, waste residue VOC, etc. needs to be incorporated into the relevant management norms. More importantly, part of the coating workshop to consider cleanliness and other reasons, spray room and baking room for micro-positive pressure design, that is, the air supply is slightly greater than the air discharge. When the chamber seal is poor, the fan continuously has gas containing mechanical solvents escaping into the surrounding air during operation. Strengthen VOC monitoring at all locations in the production-related areas, continuously improve and improve collection measures, can effectively reduce the actual emissions of organic solvents.
Through the design of reasonable coating matching, the selection of more efficient transmission of spray equipment, more scientific construction process, more environmentally friendly oil removal system, reduce the deployment and washing of guns in the process of paint and solvent waste, reduce the use of paint due to rework such as film abuse, and ultimately will reduce the subsequent emission pressure, but also reduce production costs.
3 end treatment
coating industry exhaust gas treatment technology routes, including activated carbon adsorption, molecular sieve adsorption, zeolite adsorption and other adsorption method, direct combustion (TO), catalyst combustion (CO), heat storage catalytic combustion (RCO), heat storage thermal oxidation combustion (RTO) and other combustion programs, as well as photocatalytic degradation, plasma treatment. Each treatment method has significant differences in exhaust gas concentration requirements, energy consumption, treatment efficiency, etc., and needs to be considered according to the actual emission requirements, exhaust gas characteristics, total amount of treatment and cost investment budget.
: the method of adsorption and separation of organic solvents by using porous solid materials such as activated carbon, zeolite, molecular sieve and alumina. When adsorption reaches a certain concentration, desorption treatment obtains a high concentration of exhaust gas, exhaust gas through combustion and other means to achieve the standard emissions, desorption of porous materials can be reused. The initial investment cost of this method is relatively low, but the applicability of the coating workshop with large VOC treatment is poor, the regeneration of adsorption material needs to be combined with other treatment methods, and the operating maintenance cost is high.
: including thermal combustion and catalytic combustion, due to VOC removal efficiency, good stability, etc. , is widely recommended for application.
TO direct combustion: refers to the use of fuel to generate high heat, organic solvents directly burned and degradation of the treatment method, the reaction temperature is generally more than 900 degrees C.
CO catalytic combustion: exhaust gases are heated to a decomposable temperature, and the catalytic bed is decomposed into CO2 and H2O at a lower temperature than the TO scheme.
RTO heat storage thermal oxidation combustion: the exhaust gas containing VOC is heated to more than 750 degrees C, the organic solvent is broken down into CO2 and H2O, while the ceramic heat storage components are recovered and decomposed during the process of heat release, reducing energy consumption. Its decomposition efficiency is more than 99%. Generally suitable for handling high wind volume, VOC concentration range of 2 to 8g/m3 organic exhaust gas. In use with devices such as zeolite wheel concentration, the lower concentration of exhaust gases can be processed.
the RCO heat storage catalytic combustion: compared to RTO, a catalyst device has been added to handle lower concentrations of organic solvent exhaust gas. The equipment first increases the VOC concentration by adsorption, the catalyst also reduces the reaction activity energy, improves the reaction rate, its reaction temperature can be reduced to less than 400 degrees C, greatly reduces the energy consumption during the operation of the equipment, and when the exhaust gas concentration is high enough, the equipment does not even need additional fuel.
catalysis method: the surface of the catalyst adsorption of organic solvents, under the action of ultraviolet light, broken down into CO2 and H2O, the treatment efficiency of such methods and VOC types and environmental conditions are more associated. For industrial organic exhaust gases with high emission speed and high VOC content, photo catalytic removal efficiency is low. Ozone is also included in some post-catalytic end products.
plasma treatment: the use of radio frequency discharge to produce high-energy treatment of organic solvents, organic solvents are broken down into atoms, non-toxic molecules or freelance, and eventually all into harmless products of a treatment method.
4 Conclusion
The current VOC emission reduction is still one of the important contents of China's atmospheric environmental management, emission reduction pressure is still severe, in the process of governance, source reduction, process control, end-of-term governance is indispensable. It is an inevitable requirement to achieve sustainable development and green development to formulate strategies according to the actual emission characteristics and emission reduction targets of each plant. VOC governance can also achieve effective and economic reductions only through systematic consideration, process-wide analysis, and careful implementation.
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