Ning Miao1, Shao Xia2, Liu Jie3, Zhang Xin4, Liu Wei* 1
(1. Environmental Planning Institute of the Ministry of Ecology and Environment, 100012; 2. Beijing Institute of Environmental Science, Beijing 100037; 3. China Coatings Industry Association, Beijing 100079; 4. Chinese Academy of Sciences, Institute of Eco-Environmental Research, Beijing 100085)
Abstract:based on a comparative analysis of VOC emissions from solvent-based and water-based coating systems, this document proposes a VOC abatement technology route for industrial coatings in China, focusing on:* Promoting the use of low-VOC coatings, including powder coatings, water-based coatings, high-solids coatings, and UV-curable coatings.* Improving VOC collection efficiency through end-of-pipe treatment, evaluating and selecting VOC-containing spray and drying air treatment technologies from the perspectives of economic cost, emission reduction, and secondary environmental impact.* Determining differentiated, feasible control technologies for different industrial coating industries.Furthermore, this document advocates for strengthening the supervision and regulation of VOC emissions from industrial coatings, establishing a comprehensive emission standard system covering "source, process, and end-of-pipe" management. This includes requirements for enforcement monitoring, self-monitoring, record-keeping, and reporting, as well as information disclosure, to enable the comprehensive and refined management of industrial coating enterprises.
Keywords:Industrial coating; Volatile Organic Compounds (VOCs); Full-process control; Low-VOC coatings; Waste discharge permit.
Traditional industrial coating production processes, due to the use of solvent-based paints, thinners, and cleaning agents, generate a large amount of VOC emissions. According to the 2015 National Inventory of Human-Source VOC Emissions, industrial coating VOC emissions account for more than 20% of the total industrial VOC emissions. Furthermore, monitoring results have shown that the VOC emissions from industrial coating waste contain a high concentration of active and toxic VOCs such as benzene and oxygenated volatile organic compounds (OVOCs). based on this, the "Three-Five" Energy Conservation and Emission Reduction Comprehensive Work Plan issued by the National Development and Reform Commission and the Ministry of Ecology and Environment includes industrial coating as a key area for VOC reduction nationwide during the "Three-Five" period. However, industrial coating involves a wide range of industries, and at present, there is a lack of unified, systematic, and scientific understanding of VOC control in industrial coating across different regions. This article focuses on interpreting the macro thinking of the national VOC emission control of industrial coating from the perspectives of selecting technical routes and building regulatory systems, providing guidance for local areas and industrial coating enterprises to control VOC pollution.
1 Comparison of VOC emissions characteristics of different coating systems
In 2016, China's total paint production reached 18.9978 million tons, a year-on-year increase of 7.2%, with industrial coatings accounting for more than 65% of the total production. Within industrial coatings, solvent-based coatings account for approximately 50%, while the remaining are low-VOC coatings such as water-based, powder, and UV-curable coatings. The proportion of solvent-based coatings is significantly higher than the 20%~30% level in Europe and the United States. To analyze the VOC emission characteristics of industrial coatings, this study collected organized VOC emissions samples from typical industrial coating companies using solvent-based and water-based coatings. GC-MS/FID analysis systems were used to determine various VOC components.
The study selected samples from the VOC emissions of water-based primers and water-based color coatings mixed with solvent-based topcoats, as shown in Figure 1.
Figure 1: Comparison of VOC emissions when mixing water-based primer, water-based color coat, and solvent-based clear coat.
Figure 1 data indicates that the VOC emissions from water-based primers consist of approximately 40% alcohols, 15% ketones, and 20% aromatic hydrocarbons; while for the mixed exhaust of water-based primers and solvent-based topcoats, due to the use of solvent-based topcoat, the VOC emissions contain approximately 70% aromatic hydrocarbons and 17% esters, with alcohols and ketones accounting for less than 10%.
For steel structure manufacturing companies, two companies were selected that use solvent-based and water-based primers, respectively, to analyze VOC emissions.
Figure 2: Analysis of VOC emission components in the coating process of different types of steel structures.
The data results in Figure 2 show that steel structure manufacturing companies using water-based primers emit approximately 65% of VOCs as ketone esters, 28% as aromatic hydrocarbons, and approximately 15% as alkanes and alkenes. Conversely, companies using solvent-based primers emit approximately 50% of VOCs as aromatic hydrocarbons, less than 10% as ketone esters, and 12% as alkanes and alkenes.
based on typical industrial coating enterprises' VOC emission monitoring results, the main VOC substances emitted by industrial coating enterprises are summarized in Figure 3.
Figure 3: Analysis of the photochemical reactivity of main VOC emissions from industrial painting.
based on typical industrial coating VOC emission detection results, it is evident that in industrial coating applications using solvent-based coatings, the detection rates of VOCs such as benzene, toluene, ethylbenzene, xylene, and trimethylbenzene are high. These VOCs have high photochemical reactivity, meaning they have a high ozone generation potential and secondary organic aerosol generation potential per unit emission, thus contributing significantly to composite air pollution. Conversely, the VOC emission concentrations from industrial coating applications using water-based coatings are relatively low, and the emissions are rich in VOCs with low reactivity, such as alcohols and esters.
2. Selection of VOC Pollution Control Technology Routes in Industrial Painting
2.1 Strengthening Source and Process Control
By comparing the VOC emission characteristics of different coating systems, it is clear that the most effective ways to reduce industrial coating VOC emissions and their impact on the atmosphere are to implement source and process controls. This means using low-VOC coatings at the source of production. Specifically, Article 46 of China's "Air Pollution Prevention and Control Law" states that "Industrial coating enterprises shall use coatings with low volatile organic matter (VOC) content." Low-VOC coatings include various types such as powder coatings, water-based coatings, and high-solids coatings, and should not be simply equated with water-based coatings. Powder coatings have a solid content close to 100%, and the coating process virtually eliminates VOC emissions. Replacing solvent-based coatings with powder coatings can reduce VOC emissions by more than 95%.
Typical paint composition testing results for automotive and steel structure manufacturing companies are shown in Figure 4 and Figure 5. Replacing solvent-based paints with water-based paints with lower solid content can reduce VOC emissions by approximately 50%. Since the "Air Pollution Prevention and Control Law" does not explicitly define paints with low VOC content, local authorities should develop different industry directories for low-VOC content paints, based on local air quality improvement needs and the development trends of paint coating technologies in each industry, to clearly specify the types and VOC content limits for applicable low-VOC content paints.
Figure 4 shows the composition of different coating materials for automotive applications.
Figure 5: Composition of different types of primer for steel structures.
Regarding the control of the coating process, the goal is to establish a closed-loop production system, with large-scale spraying using component disassembly and segmented spraying methods to lay the groundwork for subsequent VOC collection and management. Furthermore, it is necessary to promote the application of automated, continuous spraying lines, utilizing high-efficiency coating technologies to improve coating utilization and reduce coating consumption. Currently, most industrial coating enterprises in China typically use traditional manual air spraying guns with low coating transfer efficiency, with transfer efficiency usually ranging from 25% to 40%. This leads to over-spraying, increasing coating consumption and VOC emissions, as shown in Table 1. Therefore, industrial coating enterprises should optimize product design, implement component coating, and develop coating processes based on the material and shape of each component, selecting the coating technology and equipment with the optimal transfer efficiency, such as air-assisted spraying, airless spraying, and electrostatic spraying, to reduce coating usage and VOC emissions.
Table 1: Coating Transfer Efficiency and Application Range of Different Coating Technologies
2.2 Significantly improve the efficiency of end-of-pipe collection and treatment.
Currently, end-of-pipe treatment remains one of the most effective methods for reducing VOC emissions in China's industrial coating industry. However, VOC treatment technologies in China still face several challenges. Due to the different VOC emission concentrations, types, volumes, and continuity in various industries, as well as the uneven quality of available treatment technologies, there is a significant gap between the expected and actual VOC removal efficiency. To improve the overall VOC removal efficiency of end-of-pipe treatment facilities, it is crucial to first enhance VOC collection efficiency, increase the amount of VOC entering the treatment facility, and reduce uncontrolled VOC emissions. Specifically, VOC-containing materials such as coatings, solvents, and cleaning agents should be stored in a closed manner, and if possible, centralized supply systems should be used. When centralized supply systems are not available, these materials should be transported in closed containers to minimize the transportation distance. All processes, including coating mixing, spraying, drying (air drying, natural drying), and equipment cleaning, should be carried out in independent, enclosed spaces or equipment. Areas with VOC emissions should operate under negative pressure and have pressure monitoring instruments. Enclosure systems should be constructed to increase the overall VOC collection efficiency of the entire industrial coating process to 80% or higher.
To identify the most suitable end-of-pipe treatment technologies for industrial coating applications, a set of evaluation criteria should be established, encompassing 15 criteria: "removal rate, emission control level, operating stability, complexity of operation and maintenance, technical compatibility, safety, maturity, initial investment, energy consumption, consumables, labor costs, resource recovery, energy recovery, environmental benefits, secondary pollution." These criteria should be evaluated and selected using the AHP analysis method. based on the different characteristics of common waste gas emissions, such as spray painting and drying (air drying) waste gases, and drying waste gases, different treatment technologies should be recommended. The results show that for spray painting and drying (air drying) waste gases, due to the presence of paint fumes and large volumes of low-concentration air, a combination of technologies should be used. It is recommended to adopt advanced fume removal devices such as dry filter for high-efficiency paint fume removal, "wet curtain + multi-stage filtration and dehumidification" combined device, and electrostatic paint fume collection. When using solvent-based coatings, a "sorption and combustion" treatment facility should be constructed. If the waste gas volume and concentration from spray painting are small and low, and the surrounding environment is not sensitive, the combination of "activated carbon adsorption + low-temperature plasma" technologies can be considered, considering the initial investment and operating costs. Low-temperature plasma and photocatalytic dry oxidation technologies can also be combined with absorption technologies. For drying processes of solvent-based coatings, which generate organic waste gas with high concentration and temperature, heat-based combustion devices or catalytic combustion devices should be used for independent treatment. based on safety and equipment conditions, heat recovery combustion devices can be considered, where the heat generated can be used as a heat source for drying equipment, fully utilizing the calorific value of VOCs, which has good economic benefits and high removal efficiency, and can meet national and local emission standards.
2.3 Implement differentiated control based on different industries.
Due to the broad application of industrial coating in various industries, differentiated control should be implemented based on the specific characteristics of each industry. Overall, the technical routes for VOC emissions control in industrial coating can be divided into 3 categories. The first category consists of industries where replacement technologies with low (or zero) VOC content coatings are relatively mature, such as automotive, wood furniture, and container manufacturing. These industries should focus on source control and can implement the widespread use of low-VOC coatings across the industry to promote green transformation. In the container manufacturing industry, in addition to the initial sanding process, the sanding of the entire container, the coating of the inside and outside of the container, and the coating of the frame and wooden floor should all use water-based coatings. For the sanding process using solvent-based coatings, VOC collection and treatment facilities should be built to match. In automotive manufacturing, the primer and color coats should use water-based coatings, or the primer, color coat, and clear coat should be fully replaced with high-solid coatings, accompanied by compact processes such as "three coats and one bake" and "no primer". VOC emissions from spraying and leveling, and drying of solvent-based coatings should be classified and treated separately. In wood furniture production, water-based and UV-curable coatings are used to replace traditional coatings, and powder coatings are promoted for the production of particleboard furniture. Water-based adhesives are used comprehensively.
The second category involves end-of-pipe treatment technologies and models, which are relatively mature in industries such as roll material manufacturing. These industries typically use high-boiling-point solvents in their coatings, and VOC emissions are mainly concentrated during the drying process. To treat these emissions, combustion devices are used to convert the heat into energy, and activated carbon adsorption devices are used to treat the spray waste gas. For these industries, the focus should be on ensuring that all companies in the industry implement end-of-pipe treatment, and to strengthen the monitoring and operation of end-of-pipe treatment facilities, thereby increasing the operational rate.
Third, for industries such as shipbuilding, steel structures, and construction machinery, the technical routes for VOC reduction are not clearly defined. Comprehensive measures should be taken to promote VOC reduction. In the shipbuilding industry, high-solids content coatings should be used comprehensively. Water-based coatings should be promoted for use in engine rooms and interior spaces. Coating processes should be optimized, and the proportion of organized emission coating should be increased. By the end of 2020, 60% of coating operations in shipbuilding enterprises should be carried out in closed spraying. VOC emissions from spraying workshops should be collected and treated. In the construction machinery industry, high-solids content, ultra-high-solids content, and powder coatings should be promoted. Water-based coatings should be piloted in some processes. Spray workshops should be strengthened. In the steel structure manufacturing industry, high-solids content coatings should be used comprehensively. Water-based coatings should be piloted in areas with severe composite atmospheric pollution. Open-air spraying of steel structures should be prohibited, and the work should be carried out in workshops. Spraying waste gas collection and treatment facilities should be constructed.
3. Recommendations for Establishing a Robust VOC Regulatory System for Industrial Coatings
3.1 Develop an industrial coating emission standard system covering "source – process – end"
Currently, there are no national-level VOC emission standards for industrial coatings, which poses challenges for environmental regulation. Both national and local authorities should accelerate the establishment of a VOC emission standard system for industrial coatings, as illustrated in Figure 6.
Figure 6: VOC Emission Standard System Architecture for Industrial Painting
The design of the VOC emission standards system for industrial coatings should focus on industries with high VOC emissions, concentrated emission points, and significant impact on air quality, such as automotive, furniture, container, and shipbuilding industries. Combining industry-specific VOC pollution control technology routes, develop separate and targeted industry-specific emission standards. For other industrial coating processes, implement unified comprehensive emission standards.In selecting control indicators, considering that VOC emissions from industrial coatings are significant and often unorganized, the management approach should shift from controlling the concentration of emissions from end-of-pipe outlets to controlling emissions throughout the entire production process. This should be reflected in the standard by including VOC content limits for different industry coatings. Additionally, establish monitoring points in coating workstations or coating workshops to increase the requirements for unorganized emissions. This will encourage companies to improve their processes and achieve enclosed production.Furthermore, to reflect the principle of whole-process control, develop VOC emission total control indicators for the entire coating process, such as VOC emission limits per unit product or per coating area. This encourages companies to adopt various combined measures to reduce VOC emissions.The setting of standard limits should balance feasibility and advanced technology, with regional differences. Areas with composite and severe atmospheric pollution should implement special emission limits, or develop more stringent local standards.
3.2 Accelerate the establishment of industrial coating VOC monitoring and surveillance system
Implement comprehensive monitoring of VOC emissions from industrial coating enterprises through both supervisory monitoring and self-monitoring. In supervisory monitoring, strengthen the equipment and capabilities of first-level local and district environmental enforcement personnel, explore opening up the supervisory monitoring market to third-party testing agencies, standardize VOC enforcement monitoring work for industrial coating enterprises, and strengthen daily supervision of VOC emissions from industrial coating enterprises. In self-monitoring, revise and update the list of key polluting units at the provincial level, including coating enterprises with high single VOC emissions in the list, install VOC automatic monitoring equipment at major emission points, and connect with local and provincial environmental departments. In addition, VOC emission sources should also be equipped with environmental monitoring facilities at the factory boundary. For enterprises not included in the list of key polluting units, require them to conduct VOC waste gas treatment facility inlet and outlet monitoring and boundary unorganized monitoring concentration monitoring at least once a year, with monitoring indicators including the main characteristic pollutants and non-methane total hydrocarbons in raw materials, and calculate VOC treatment efficiency based on the parameters of the waste gas treatment facility inlet and outlet monitoring. Strengthen the environmental accounting management of industrial coatings. Industrial coating enterprises should gradually complete the organic waste gas monitoring accounting, organic waste gas treatment facility operating accounting, and accounting of VOC organic raw materials consumption, including usage, waste, destination, and VOC content, and accounting of organic waste gas treatment materials (absorbents, catalysts, etc.) usage, replacement, and disposal. The retention period for the accounting shall not be less than 3 years. Establish a management system for abnormal operating conditions, including reporting and filing to local environmental departments in cases of project shutdown, waste gas treatment facility shutdown, or sudden environmental accidents.
4. Conclusion
1. During the "13th Five-Year" period, comprehensive control of VOC emissions from industrial coatings will be implemented nationwide. The automotive and furniture manufacturing industries should complete the control of VOC emissions in industrial coating processes by 2019, while other industrial coating industries should complete the control by the end of 2020. Industrial coating industries located in key regions such as the Beijing-Tianjin-Hebei and surrounding areas, the Yangtze River Delta, and the Pearl River Delta should complete VOC control by the end of 2018, and have the ability to meet national and local emission standards, laying the foundation for the implementation of a permit system.2. Implementation of fine-grained management based on a permit systemIn accordance with the national unified framework, industrial coating enterprises that use solvent-based coatings and diluents in excess of 10t per year will be subject to special management under a permit system. By 2019, furniture manufacturing and automotive manufacturing industries will fully implement the permit system. By 2020, other transportation equipment manufacturing and electronic information manufacturing industries will fully implement it. Key regions such as the Beijing-Tianjin-Hebei region, the Yangtze River Delta region, and the Pearl River Delta region will accelerate the issuance of permits.The permit system is the sole administrative permit for a company's emissions behavior, clearly defining the environmental management requirements and legal responsibilities that the company must comply with. By using the permit system, ensure that industrial coating enterprises use raw materials with low VOC content; promote the implementation of enclosed production by enterprises through unorganized emission permits, and improve VOC collection efficiency; and through organized emission permits, encourage enterprises to build efficient VOC control facilities, and comprehensively achieve full compliance with annual total permit emission requirements. At the same time, implement enterprise self-monitoring, record-keeping, regular reporting, and information disclosure systems to improve the fine-grained level of enterprise environmental management. Industrial coating enterprises should obtain permits in accordance with the law and comply with the permit requirements; local and municipal environmental protection departments will issue permits based on the legal commitments made during the application for a permit, and strengthen mid-term and post-implementation supervision, and severely punish illegal emissions. The entire process of applying for, issuing, and supervising permits will be open to public supervision.
2、Industrial coating VOC emissions control should prioritize source control, with strong promotion of powder, water-based, high solid content, and UV-curable coatings with low VOC content. Simultaneously, end-of-pipe treatment should be implemented, including the construction of comprehensive VOC collection systems to achieve VOC collection efficiency of 80% or higher. VOC exhaust (including drying exhaust) should be classified and treated separately. The selection of end-of-pipe treatment technologies should consider economic costs, emission reduction benefits, and secondary environmental impacts.
3. During the "13th Five-Year" period, different industrial coating industries have adopted various approaches to VOC emission control. Industries such as automotive, wooden furniture, and container manufacturing are mandated to use low-VOC coatings, driving the overall industry's green transformation and upgrading. Industries such as roll materials, which primarily emit VOCs during drying, have mature end-of-pipe emission control solutions, and all companies in the industry should implement comprehensive measures. Industries such as shipbuilding, steel structures, and engineering machinery should adopt a multi-pronged approach, implementing "source-process-end" measures to reduce VOC emissions.
4. Accelerate the establishment and improvement of industrial coating VOC emission regulatory systems. Strengthen the supervisory monitoring and self-monitoring of VOC emissions from industrial coating processes, and gradually improve the VOC monitoring records, VOC treatment facility operation records, VOC-containing organic raw material consumption records, VOC treatment material replacement and disposal records, and establish a non-normal operating condition reporting and management system. Establish VOC emission regular reporting and information disclosure systems, and implement all management requirements of the discharge permit system.
The article was published in the 12th issue of "Coatings Industry" in 2017.