The hottest issue on VOCs production and emission

2022-08-05
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Calculation and analysis of VOCs production and emission from coating in the automotive industry

calculation and analysis of VOCs production and emission from coating in the automotive industry

September 27, 2017

[China coating information]

China is the world's largest automobile consumer, and the pollution caused by automobile production is becoming more and more serious. A large number of coatings and organic solvents are used in the automobile surface coating process, which are important VOCs emission sources. These organic solvents mainly include low-level toluene, xylene, acetone, etc. According to the guidance on promoting joint prevention and control of air pollutants to improve regional air quality, VOCs has been included in the key pollutants of joint prevention and control of air pollutants. However, the current formulation and implementation of VOCs emission standards for the automotive industry in China lags behind last year, and there is a lack of relevant national standards for VOCs emission from automotive surface coating operations. The calculation of VOCs production and emission in the automotive industry will certainly become an important work for the enterprise's pollutant compliance management

1 coating process flow and pollution production link in the traditional automobile industry

production process and VOCs Pollution production link in the coating workshop of the automobile industry: body in white → degreasing → surface conditioning → phosphating → cathodic electrophoresis (unorganized emission) → electrophoresis drying (exhaust funnel emission) → strong cooling → rough sealing (unorganized emission) → floor protection → fine sealing (unorganized emission) → PVC drying (exhaust funnel emission) → strong cooling → electrophoretic polishing → intermediate coating manual wiping → ostrich hair automatic wiping machine wiping → manual spraying of intermediate coating (inner cavity) → automatic coating machine spraying of intermediate coating (partial exhaust funnel discharge, partial unorganized discharge) → drying → intermediate coating drying (incineration discharge) → intermediate coating strong cooling → intermediate coating grinding → top coating wiping → ostrich hair automatic wiping machine wiping → manual spraying of color paint (inner cavity) (exhaust funnel discharge) → drying → automatic coating machine spraying (some exhaust pipes discharge and some unorganized discharge) → the air station automatically sprays metallic paint (some exhaust pipes discharge and some unorganized discharge) → air drying → the automatic coating machine sprays finishing varnish (some exhaust pipes discharge and some unorganized discharge) → check and repair paint → drying of finish paint (incineration discharge) → strong cooling of finish paint → decoration → inspection → decoration → general assembly

2 data required for calculation of VOCs generation and emission of coating in the automotive industry

the enterprise operator shall record the VOCs emission per unit coating area of the automotive manufacturing coating production line and the data in the accounting of VOCs generation every month. Specific data include: 1) name of raw materials containing volatile organic compounds; 2) Monthly consumption of raw materials; 3) Content of volatile organic compounds in raw materials; 4) Monthly recovery of raw materials containing volatile organic compounds; 5) The amount of volatile organic compounds removed by pollution control equipment; 6) Monthly total vehicle coating area; 7) Annual or regular environmental protection monitoring data of the enterprise

3 calculation of VOCs production and emission by total material balance method

verify the production of VOCs according to the collection of raw material consumption, organic solvent content, vehicle coating area, production, working conditions and other data provided by each coating workshop. According to the national cleaner production coating industry standard, the calculation formula is: organic solvent production = organic solvent volatilization (g/a)/total coating production area (m2/a)

the actual volatile amount of organic solvent can be calculated from the solid content and organic solvent content of paint in the coating workshop, including electrophoretic paint, glue, wax solvent, paint solvent, diluent, etc; Coating area = coating area of each vehicle type × Model output. The coating area of each vehicle type can be given according to the relevant parameters of vehicle type R & D, or the primer area of approximate vehicle type =2 × Net mass of steel plate (kg)/(original thickness of steel plate (m) × Steel plate density (kg/m3)). The calculated amount of volatile organic compounds will contain toluene and xylene

in the process and VOCs generation process, VOCs produced by drying the intermediate coating are discharged into the atmosphere through the chimney. In the unorganized emission, it is mainly evaporation and a small amount of VOCs volatilized from waste water and paint slag. However, the calculation of this part of VOCs in the unorganized emission is too difficult and small. At the same time, the VOCs discharged unorganized in the coating room are finally discharged to the atmosphere through the ventilation equipment in the workshop. Therefore, when calculating the actual emission of VOCs 14. Displacement resolution: 0.001mm, there is no distinction between organized and unorganized. The following formula can be used to calculate the emission:

vocs emission = production - treatment capacity of intermediate coating and finishing coating - waste solvent recovery

in the actual calculation, the VOCs in the waste solvent recovery can be directly subtracted from the total VOCs production to obtain the VOCs production in the solvent recovery

treatment capacity of intermediate coat and finish coat = incineration treatment efficiency (99%) × VOCs entering incineration treatment equipment (30%) × (content of organic solvent in intermediate paint + content of organic solvent in finish paint)/total coating area

4 calculate the production and emission of VOCs by combining the material balance algorithm with the analogy method

in the process of calculating the production and emission of VOCs by using the material balance algorithm of toxic and harmful elements, it is also necessary to collect the consumption, composition and content of paint and solvent in the coating workshop. According to the analogy of the same domestic process and the characteristics of fugitive volatilization, the generation and emission of VOCs in each link are calculated. This calculation can be simplified into two sections, i.e. intermediate coating section, finishing coating section and pretreatment, electrophoresis and PVC section, for analysis and calculation respectively

(view all pictures of Chinese coatings)

(view all pictures of Chinese coatings)

figures 1~2 show the actual calculation by the author using this method. See Figure 1 for the material balance diagram of VOCs generation and emission in the intermediate coating and finishing coat drying section. See Figure 2 for VOCs generation and emission balance diagram of pretreatment, electrophoresis and PVC sections

the paint for intermediate coating and surface coating is divided into two parts: solid component and organic solvent. The solid component is mainly attached to the vehicle body and the paint mist (purification measures). The organic solvent mainly contains non methane total hydrocarbon and xylene. VOCs are mainly produced in paint spraying room, drying room and drying room. About 50% of VOCs will be generated in the paint booth, and finally exist in the paint slag and chimney discharge, and about 9% of volatile organic compounds will enter the aqueous phase. The drying room will also produce 30%~40% VOCs, and the VOCs in the drying room will be incinerated with a treatment efficiency of more than 98%. Carbon dioxide and water will be generated after incineration. Fugitive emissions of about 2% of the total amount will be generated in the drying room and spray booth

pretreatment, electrophoresis and PVC sections are basically the same as those in the intermediate coating and finishing coating sections, but generally there is no tail gas incineration treatment facility in the PVC drying section. If there is a tail gas incineration treatment facility, the removal efficiency should be calculated

the actual VOCs emission per unit coating area can be obtained by dividing the calculated emission by the total coating area

5 emissions and emissions calculated by actual measurement and backstepping method

the above two methods may produce large errors due to the inconsistency between data or analogy assumptions and actual production. In order to verify the accuracy of VOCs generation and emissions calculated by the above methods, the organized and unorganized emissions of VOCs in each process can be actually monitored, and the consistency of various data can be compared under the verification of actual working conditions and actual conditions, Analyze the differences, find out the causes, and further improve the calculation accuracy. For example, according to the monitoring data of the environmental protection department entrusted by a company in 2014, the total emission of non methane total hydrocarbon in the exhaust funnel of PVC, electrophoresis, intermediate coating and finishing coating is 26.082kg/h. According to 250D, three shifts and 24h production, vocs=g/a is calculated. Taking the annual average coating area of the coating workshop in 2014 as m2, vocs=11g/m2 is calculated. The calculated total volatile organic compounds and the volatile organic compounds in each section of the process are compared with those calculated by the above two methods. Local adjustments are made according to the actual situation to verify the consistency of the values calculated by the three methods. Because there are inaccurate factors in the coating area, working hours, working conditions and monitoring data, and VOCs are highly volatile, VOCs will be dispersed in the unorganized evaporation and waste water during the production process. However, this part of VOCs is not monitored in this monitoring, so the VOCs emission calculated according to the measured data will be less than the actual amount

6 conclusion

through mutual verification and comparison of several methods, analysis of differences and their causes, and repeated demonstration, more accurate VOCs production and emission data can be obtained

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