Overall solution for coating VOCs exhaust gas
First, the source of exhaust gas
The exhaust gas from the workshop mainly comes from the spraying and drying processes. The pollutants emitted are: paint mist and organic solvents generated during spraying, and organic solvents generated during drying and volatilization. The paint mist mainly comes from the scattered part of the solvent-based paint in the air spraying operation, and its composition is consistent with the paint used. Organic solvents mainly come from solvents and thinners during the use of coatings. Most of them are volatile emissions. The main pollutants are xylene, benzene and toluene. These ingredients are harmful to human health and living environment, and have a foul odor. If people inhale low concentrations of organic waste gas for a long time, they can cause chronic respiratory diseases such as cough, chest tightness, asthma and even emphysema. They are currently recognized as strong carcinogens.
2.1 Economic aspects
Due to the small scale of the enterprise, the unstable benefits and the average economic capacity, it cannot afford the high cost of environmental protection equipment.
2.2 Process facilities
The printing and spraying processes of most enterprises are simple and backward, and they are mainly based on open labor. Most of the inks and coatings they choose are solvent-based products. A few enterprises are equipped with dedicated operation areas and exhaust gas treatment facilities, but the operation areas are not standardized and closed, the treatment facilities are simple and backward, and the functions are single. They cannot meet the current environmental protection requirements, and the site lacks management, and the scattered pollution is more serious, and some are dangerous. Products are not properly placed, there are certain safety risks.
2.3 Operating conditions
Due to the peculiarities of the machinery industry, most enterprises are unable to form assembly line operations, and are mostly staged spraying operations. Through communication with a number of enterprises, rough operating conditions data was collected, as shown in Table 1.
Third, the overall solution
The overall solution of VOCs exhaust gas mainly includes four aspects: exhaust emission reduction, exhaust gas collection, exhaust gas treatment, and management monitoring.
3.1 Exhaust emission reduction
3.1.1 Use clean and environmentally friendly materials
Depending on the process, it is recommended to use environmentally-friendly raw materials with low VOCs content instead of the original solvent-based raw materials. For example, the printing and packaging industry uses water-based, hot-melt, solvent-free, UV-curable, high-solid-content, and biodegradable adhesives, and inks are water-based inks, UV inks, gravure inks, radiation-curable inks, etc. ; The coating industry chooses green coating products such as water-based coatings, powder coatings, high solids coatings, solvent-free coatings, and UV coatings.
Due to the prominent environmental problems in recent years, the state has strongly promoted the application of environmentally friendly coatings. At present, the environmental assessment of new production lines requires the use of environmentally friendly coatings. Due to the strong applicability and reasonable price of water-based coatings, it has gradually spread to various coating industries. Its main component is water and the solvent content is 10% -25%, which is far lower than solvent-based coatings. The use of water-based coatings can reduce VOCs emissions in exhaust gas from the root cause. The reference values of VOCs content of various coatings ("Interim Measures for the Calculation of Volatile Organic Compound Emissions in Key Industries in Jiangsu Province") are listed in Table 2.
3.1.2 Process improvement and optimization to increase raw material utilization
Encourage enterprises to implement continuous and automated technological transformation, promote the use of high-efficiency coating processes such as electrostatic spraying, shower coating, roll coating, dip coating, etc., promote the use of advanced process technologies, and optimize processes and equipment.
In the coating process, the coating is often used in the form of spraying. During the spraying process, more paint mist is generated. The paint mist is the source of particulate matter in the exhaust gas of the spray paint. Therefore, increasing the utilization rate of the paint can greatly reduce the particulate matter in the exhaust gas and reduce the difficulty of exhaust gas treatment. The coating utilization rates of different spraying methods are listed in Table 3.
It can be seen from Table 3 that only a large-capacity low-pressure air spray gun or electrostatic spraying can satisfy at least 65% of the coating utilization rate. In addition, the use of robotic spraying can also improve the utilization rate of coatings. At present, the domestic coating industry is promoting the use of internal spraying robots, that is, the entire coating line is sprayed to achieve unmanned use. All robots are sprayed, which greatly improves the use of coatings. rate.
3.2 Exhaust gas collection
At present, most companies use open-air spraying operations without any exhaust gas collection facilities. Organic exhaust gas is directly emitted into the atmosphere. A small number of companies use exhaust gas hoods to collect exhaust gas. The general exhaust gas collection efficiency is only 30-50%. Uncollected gas still exists in the atmosphere in an unorganized form, which is far from meeting the requirements for exhaust gas collection. Only the collected exhaust gas can be treated. Therefore, enterprises are encouraged to implement a closed production process, set up a closed and independent operating area, and be equipped with an efficient organic gas collection device to form a closed negative pressure collection state. The organic waste gas emitted from the workshop needs to be concentrated by the exhaust system Exhaust and run synchronously with the production process to eliminate open and open spraying operations. The workshop should be equipped with good ventilation equipment, and there is no obvious odor in the space outside the workshop.
3.3 Exhaust gas treatment
3.3.1 analysis of exhaust gas treatment technology
At present, the treatment technologies recognized in the waste gas treatment industry can be divided into recovery technology and destruction technology. The specific breakdown of the overall VOCs pollution prevention technology is shown in Figure 1.
220.127.116.11 The absorption method uses low-volatile or non-volatile organic solvents to absorb VOCs. Due to the addition of organic solvents in the waste gas treatment process, secondary pollution such as mixed solvent waste water is generated, which does not meet the concept of clean production. Machinery companies do not have wastewater treatment capabilities, so they are not suitable for the coating industry.
18.104.22.168 Adsorption method is to use solid adsorbents (activated carbon, molecular sieve, etc.) to adsorb and purify pollutants. Although the adsorption process is mature and has a certain treatment effect, and the equipment investment is moderate, the adsorbent is a consumable product. Although the adsorbent regeneration function can be configured, the performance of the adsorbent after regeneration is greatly reduced and the life is short. Agents are hazardous waste and require additional treatment costs. In addition, the general equipment is relatively large and occupies a large amount of land. Most machinery companies have a tight internal land and cannot provide too much space for exhaust gas treatment equipment, so they are not suitable for painting. industry.
22.214.171.124 The condensing method is to reduce the temperature to change the VOCs exhaust gas from a gaseous state to a liquid state. Although it can play a recovery role, the recovery rate is not high, generally only 40-60%, and it is only suitable for high-concentration exhaust gas pre-treatment. Processing, so it is not suitable for the coating industry.
126.96.36.199 Membrane separation method is to use the ability of the membrane to selectively penetrate different substances for the treatment of VOCs. The equipment investment is high, and the membrane's selective ability has certain limitations, so it is not suitable for the coating industry.
188.8.131.52 The combustion method and the activated activated carbon adsorption / zeolite runner adsorption + RTO / RCO are currently recognized as the most thorough treatment of VOCs, but the equipment investment is high, usually ranging from 1 to 5 million, and the equipment occupies a relatively large area. Large, far beyond the economic scope of SMEs, so it is not suitable for SMEs.
184.108.40.206 Biological method is to degrade VOCs by microbial metabolic activities. The process of converting it into harmless small-molecule substances, because the biological bacteria have a strong specificity for the digestion of organic matter, are sensitive to environmental conditions, and generate a large amount of excreta during the degradation process, causing solid waste pollution The overall equipment occupies a large area, so it is not suitable for small, medium and micro enterprises.
220.127.116.11 Low temperature plasma technology is to use the plasma with extremely high temperature generated by dielectric discharge to repeatedly bombard gas molecules in the exhaust gas at extremely fast speed, deactivate, ionize, and crack various components in the exhaust gas. A series of complex chemical reactions have transformed complex macromolecular pollutants into safe substances with small molecules (such as carbon dioxide and water), or transformed toxic and harmful substances into non-toxic and harmless or low-toxic and low-hazard substances. Although the VOCs removal efficiency of this technology is high, its plasma temperature is high, and there are great safety risks when handling flammable and explosive volatile organic compounds. Therefore, the Tianjin Safety Supervision Bureau issued the "Special Safety on Environmental Protection and Governance Facilities to Learn Lessons from Accidents" In the "Notice of Inspection", the government officially halted low-temperature plasma technology for the first time.
18.104.22.168 UV photolysis technology is to use high-energy UV light to crack organic gas molecules and break chemical bonds to form free atoms or groups (C *, H *, O *, etc.). At the same time, the oxygen in the mixed gas is replaced by Ultraviolet light cracks to form free oxygen atoms and combines to generate ozone [UVO2 → OO * (active oxygen) O * O2 → O3 (ozone)]; water vapor in the mixed gas is decomposed by ultraviolet light to generate hydroxyl groups [UVH2O → HOH- (hydroxyl )], And these generated ozone and hydroxyl groups are extremely oxidizing, which can oxidize the atoms and groups generated by the cracking of exhaust gas molecules into non-polluting low-molecular compounds such as H2O and CO2.
3.3.2 Overall Solution Development
Existing complete sets of equipment for collecting and treating exhaust gas, the exhaust gas collection efficiency of the entire set of equipment is more than 90%, the efficiency of exhaust gas treatment is more than 95%, and the exhaust emissions meet the current emission standards. Fully consider the actual situation of small and medium-sized enterprises, the equipment is economical and reasonable, high security, small footprint, strong adaptability, low operating cost, simple and convenient daily maintenance.
The whole device consists of ALF-PF closed paint spray booth, ALF-AH dust removal filter, ALF-UV photolysis, ALF-AC activated carbon adsorption, pipeline, and fan.
3.4 Management monitoring
In order to ensure the long-term effective operation of the exhaust gas treatment device, the enterprise should compile standardized management regulations, and at the same time strictly follow the equipment instructions to do maintenance work, and actively respond to the relevant requirements of the "Guide to Key Industries in Jiangsu Province Volatile Organic Compound Pollution Control Guide". Exhaust gas sampling facilities are included in the planning scope, and management and monitoring measures are used to ensure that the company's exhaust emissions meet environmental protection requirements.
Reducing the emission of volatile organic compounds is not only the direction of enterprise technology development, but also the responsibility of enterprises. Enterprises should choose appropriate environmental protection coatings according to their product characteristics and operating conditions, purchase reliable and economical exhaust gas collection and processing equipment, adopt standardized and effective management monitoring to ensure the long-term stable operation of the exhaust gas treatment device, and ensure that exhaust gas emissions are met. National and local emission standards protect the global environment and create a harmonious society.