To promote pollution control in typical industries in Foshan City, guide enterprises to develop and use advanced pollution control technologies, and help enterprises improve their environmental management level, the Foshan Municipal Bureau of Ecology and Environment has launched a column on the "Foshan Ecological Environment" WeChat official account entitled "[Pollution Control Technologies in Typical Industries]" to publicize and showcase standardized process management, advanced technologies, and typical cases in typical industries for relevant enterprises to learn from.
In this issue, we'll take you through the coating process in the machinery manufacturing industry and how companies can reduce VOCs generation and emissions at each stage of production.
Machinery manufacturing industry
The mechanical coating process in the machinery manufacturing industry generates and emits VOCs. Mechanical coating has the following characteristics:
1. Mechanical coating mainly involves paint spraying and powder coating, and the types of coatings that can be used are relatively limited.
2. VOCs are generated in processes such as paint mixing, spraying, leveling, drying, and cleaning. VOCs originate from raw and auxiliary materials containing VOCs, such as paints, thinners, cleaning agents, curing agents, adhesives, and sealants.
3. The operating environment of the machinery is relatively complex, requiring high-quality coatings. Therefore, the quality requirements for raw and auxiliary materials are high, necessitating multi-layer spraying or multiple coatings, and some require special treatments, such as special anti-corrosion and waterproofing, which also increases the consumption of raw and auxiliary materials.
4. Due to the large size of the machinery, a large area is required for coating production; due to the large differences in the shape of the machinery, the complexity of coating production is increased; some large-volume machinery can only be coated in a semi-open or open environment, making it difficult to collect VOCs and resulting in a large amount of fugitive VOC emissions.
Production process
I. Basic Process
Mechanical coating processes can be categorized based on the raw materials used into paint spraying and powder coating. Paint spraying can be further divided into water-based paint spraying and oil-based paint spraying. Mechanical coating processes can also be categorized based on the production environment into closed production, semi-closed production, and open production. Furthermore, mechanical coating processes can be categorized based on the tools used into automated production line spraying, robotic arm spraying, and manual spraying.
Differences in production processes can affect both the total amount of VOCs emissions (for example, the total VOCs emissions of powder coating and oil-based paint differ significantly) and the amount of fugitive emissions (for example, even when using the same raw and auxiliary materials, the fugitive emissions of closed production and open production differ significantly).
VOCs are generated in the processes and places of mechanical coating production, including spraying primer, leveling, drying, and mixing.
II. Raw and auxiliary materials
01 Raw materials for paint spraying
The raw materials used in spray painting include oil-based paint and water-based paint. When using oil-based paint, hardeners and thinners are also required. VOC emissions come not only from the oil-based paint itself, but also from the auxiliary materials used.
① When using oil-based paint, not only is the oil-based paint itself high in VOCs, but the additives used also have high VOCs content.
② When using water-based paint, not only is the VOC content of the water-based paint much lower than that of oil-based paint, but the VOC content of the additives used is also low.
02 Powder Coating Raw Materials
Analysis of the raw material composition shows that the powder does not contain VOCs. However, during the production process, especially during high-temperature curing and drying, a small amount of VOCs may be released.
III. VOCs Collection and Treatment
01 Collection
In mechanical coating, VOCs collection is a crucial aspect. On one hand, the machinery to be coated is often large, requiring ample space for coating operations; the larger the space, the more difficult it is to collect VOCs. On the other hand, a significant portion of mechanical coating production takes place in semi-open or open environments, limiting VOCs collection.
02 Preprocessing
In mechanical coating, VOCs-containing waste gas should be collected and pretreated according to the different characteristics of the waste gas collected in each production process.
For example, the exhaust gas collected in the painting process is at a low temperature, but has a high moisture content due to water film treatment. Pretreatment does not require cooling, but water removal. The waste collected in the drying process is at a higher temperature and has a lower moisture content, requiring cooling but not dehydration.
03 Treatment Technology
When oil-based paints are used for mechanical coating, the concentration of VOCs in the exhaust gas is relatively high, and the air volume is also large. Therefore, adsorption-catalytic combustion or adsorption-regenerative combustion should be the primary treatment process. When water-based paints or powder coatings are used for mechanical coating, the air volume of the exhaust gas is large, but the VOCs concentration is low. In this case, activated carbon adsorption can be selected as the treatment process.
Clean production methods for mechanical coating
I. Green Product Design and Production
01. Properly combine primer, intermediate coat, and topcoat.
Properly combining primer, intermediate coat, and topcoat can reduce paint consumption, thereby reducing VOC emissions. This involves reducing the amount of each coat used and using paints with lower VOC content.
02. Rationally determine product quality standards
Raising product quality standards inevitably increases the consumption of raw and auxiliary materials, leading to increased VOC emissions. Therefore, it is crucial to rationally determine product quality standards, ensuring they are neither excessively high nor insufficient to meet the requirements of machinery use. Strictly controlling quality standards allows for strict control of raw and auxiliary material consumption, ultimately achieving strict control of VOC emissions.
II. Improvement and Selection of Raw and Auxiliary Materials
01 Use water-based paint instead of oil-based paint
In mechanical coating, the most practical and effective method for reducing VOC emissions is the use of water-based paints. Water-based paint technology is maturing and widely used in many machinery manufacturing applications, such as fans, presses, slitting machines, cross-cutting machines, pumps, extruders, and electrical cabinets. Water-based paints can be used in virtually any product. The use of water-based paints includes the following situations:
① Water-based primer + low-VOC oil-based topcoat;
② Water-based primer + low-VOC oil-based intermediate coat + low-VOC oil-based topcoat;
③ Water-based primer + water-based intermediate coat + low VOC content oil-based topcoat.
④The primer, intermediate coat and topcoat are all water-based paints.
Companies can adopt different methods depending on the characteristics of their products and the quality of water-based paints.
02 Powder coatings as a substitute for oil-based paints
Powder coatings are virtually VOC-free, and replacing oil-based paints with powder coatings can reduce VOC emissions. In some aspects, powder coatings also offer superior quality compared to water-based paints. Therefore, powder coating is an effective way to reduce VOC emissions compared to oil-based paint. However, machinery is often bulky, and some practical technical challenges need to be addressed when using powder coating. For smaller, lighter mechanical assemblies, powder coating can replace paint coating.
03 Choose raw and auxiliary materials with lower VOC content
Raw and auxiliary materials containing VOCs have varying VOC emissions due to differences in the types and amounts of VOCs they contain. Therefore, raw and auxiliary materials with lower VOC content should be selected during the production process.
III. Improve production processes and equipment
01 Improve the accuracy of coating amount
In paint spraying, improving the precision of the spray volume means reducing the amount of paint and powder used, which not only reduces VOC emissions but also lowers production costs. Improving the precision of the spray volume requires accurate calculations of consumption, as well as precise control over airflow and spraying speed.
02 Improve the spraying method
With technological advancements, various spraying methods have emerged, including compact coating, electrostatic spraying, high-pressure airless spraying, air-assisted airless spraying, and thermal spraying. Companies can choose different spraying methods based on product characteristics and the production environment to improve production efficiency and reduce the release of raw materials and auxiliary materials during spraying.
03 Improve the automation level of spraying
Increasing the level of automation in spray painting can not only improve production efficiency and reduce the labor intensity of employees, but also reduce VOC emissions. For example, robotic arm spraying can improve spraying accuracy. Due to the large size and heavy weight of machinery, automated spray painting production lines are not suitable; robotic arm spraying is a more appropriate method.
04 Strictly control process parameters
Strictly controlling the process parameters of VOCs generation processes is beneficial to reducing VOCs emissions. For example, strictly controlling the painting speed, the drying temperature, and the leveling temperature and time are all helpful in reducing VOCs release.
IV. Clean Production Environment
01 Reduce the volatilization of raw and auxiliary materials
Keep containers holding VOCs-containing raw and auxiliary materials sealed, strictly control and appropriately reduce the temperature of storing VOCs-containing raw materials, and avoid exposure to air as much as possible when weighing, batching and transporting VOCs-containing raw and auxiliary materials.
02 Improve the weighing, mixing and conveying of raw and auxiliary materials.
Because mechanical coating involves large areas and uses a large quantity of raw and auxiliary materials, manual weighing and mixing are time-consuming. The weighing, mixing, and transporting of raw and auxiliary materials cause liquid disturbance and evaporation. To reduce VOCs release during weighing, mixing, and transport, automated equipment should be used as much as possible. Automated equipment offers advantages such as high speed, good sealing, accuracy, and reduced possibility of error.
03 Reduce leakage during the production process
Mechanical coating production should be carried out in a production workshop, and open-air operations are prohibited. When coating production is carried out in a production workshop, processes such as spraying should be conducted in a closed space. If a fully enclosed space is not possible, a movable semi-enclosed isolation space should be constructed and equipped with an exhaust system to collect fugitive VOC emissions. If open-air production is the only option, the production workshop must be enclosed, and a gas collection device must be installed to collect VOCs.
V. Optimize collection and treatment processes
01 Strengthen VOCs collection
① Strengthen the control of fugitive VOC emissions. To reduce fugitive VOC emissions, it is necessary to enhance the sealing of production workshops and improve the gas collection system.
② Improve the airtightness of the production environment. Painting, leveling, and drying/baking processes must be carried out under closed conditions. Enterprises are encouraged to adopt automated closed collection systems for small workpieces and to use track conveying combined with closed spraying for large workpieces, ensuring the collection of exhaust gases from paint mixing rooms and spraying operations.
③ Where conditions permit, ensure proper collection and pretreatment of waste gas by category. For example, if there are paint booths, leveling tunnels, or drying rooms, ensure proper collection and pretreatment of waste gas by category.
02 Optimize the combination of processing techniques
Choosing the right combination of waste gas treatment processes and adjusting the order of these processes are essential for achieving stable emissions compliance.
03 Strictly control the treatment process parameters
Besides selecting a suitable waste gas treatment process, it is essential to adjust the treatment process parameters in a timely manner according to the actual production situation and to strictly control these parameters. The main waste gas treatment process parameters include air volume, air velocity, waste gas temperature, combustion temperature, and activated carbon saturation, among others.
Typical Case
This company is a large-scale wind turbine manufacturer in China, and it prioritizes energy conservation, emission reduction, environmental protection, and carbon reduction. The solar photovoltaic power generation system it has installed is sufficient to meet all of its electricity needs.
With the continuous maturation of water-based paint technology, and after a long period of research and continuous adjustments, the company has been able to use water-based paints for conventional products such as fans, pumps, and electrical cabinets, instead of oil-based paints. Water-based paints account for over 80% of the company's total product output. However, specialized equipment used in nuclear power plants and hydropower plants still uses oil-based paints due to limitations in operating conditions.
Based on the characteristics of its products, the company creates a closed production environment. The paint mixing room is connected to the spray booth, with access only through doors to the corresponding spray booth. The spray booth uses a rail conveyor and is equipped with a small crane to facilitate the collection of exhaust gases during spraying operations. The water-based paint spraying production line and the oil-based paint spraying production line operate independently, with separate collection and treatment.
After extensive research and exploration, the company decided to adopt a classified collection and treatment approach for different types of waste gas. The waste gas from the water-based paint production line is treated using a water jet mixing tower + activated carbon adsorption process. The waste gas from the oil-based paint production line is treated using a water jet mixing tower + three-stage filter + cylindrical rotary adsorption + regenerative catalytic combustion process.
Source: Foshan Municipal Bureau of Ecology and Environment
