Brief Introduction to the Exhaust Gas Treatment Technology for VOCs Arising from Automotive Coating
In recent years, environmental problems have become major problems affecting China's development. It is clearly pointed out on the 19th CPC National Congress that environmental problems are critical to the satisfaction of people's growing requirements for the living standards, the comprehensive building-up of a well-off society, the high-quality development of the economy, and the construction of beautiful China. At present, China's emissions of VOCs have reached 2500 tons based on rough estimations, making China the country with the world's largest emissions of VOCs. In order to accelerate the improvement of the ambient air quality, the State Council has issued "Three-year Action Plan for Winning the Blue Sky Protection Campaign", in which the industrial coating industry is designated as the key industry for VOCs emission.
The characteristics of and treatment technology for the exhaust gas arising from automotive coating
A large quantity of VOCs exhaust gases will be generated during the production process of the coating workshop. The characteristics of such exhaust gases include low concentration and high blowing rate. Using conventional incineration method to treat the exhaust gases will give rise to high operating costs. The zeolite roller and activated carbon are both devices used to concentrate low-concentration and high-blowing-rate VOCs gases into high-concentration and low-blowing-rate ones. The working conditions and principles of the zeolite roller and activated carbon are similar, but the performance of the zeolite roller is superior to activated carbon in many respects.
Therefore, at present, the exhaust gas treatment process generally adopted by coating workshops is the zeolite roller combined with the incineration method. The RTO in the incineration method has advantages including higher thermal efficiency (≥95%) and low operating cost compared with conventional catalytic-combustion and direct-fired thermal oxidizer (TO). The combination of "zeolite roller + RTO" has become the mainstream treatment technology among large-scale automotive coating factories.
Introduction to the treatment facilities and processes
- Pretreatment System
The exhaust gas must be filtered within the pretreatment system before it enters the zeolite roller. Such filtration aims at removing particulate matters from the exhaust gas and preventing the service life of the zeolite roller from being reduced due to the plugging by particulate matters of the zeolite roller. In addition, the wet filtration method is commonly used by automobile coating plants to treat the paint residue in the end-of-pipe exhaust gas, resulting in exhaust gases with high relative humidity. When the relative humidity exceeds 85%, zeolite roller's treatment capacity of the exhaust gases will be reduced, so dehumidification devices should be added in the pretreatment system in order to reduce the relative humidity of exhaust gases under 70%.
- Zeolite Roller System
Zeolite roller is a kind of gas purifying device which can continuously carry out the adsorption and desorption. Its two sides are divided by sealing devices into three zones: adsorption zone desorption zone, and cooling zone.
The zeolite roller rotates continuously at a low speed. When the low-concentration and high-blowing-rate VOC exhaust gas continuously passes through the adsorption zone of the roller, the VOC component in the exhaust gas is adsorbed by the zeolite of the roller, and the gas after adsorption and purification is directly discharged if it is up to standard; The organic solvent adsorbed by the roller is sent to the desorption area with the rotation of the roller, and the heated mixed gas output from the RTO at a temperature of 180-220℃ passes through the desorption zone, and the VOC exhaust gas is discharged to RTO. After the roller is rotated to the cooling zone and cooled, it can be used again for the adsorption (the cooling gas changes to a temperature of about 120℃). With the continuous rotation of the roller, the adsorption, desorption, and cooling are carried out cyclically to ensure the continuous and stable treatment of exhaust gas.
- RTO System
The principle of the Regenerative Thermal Oxidizer (abbreviated as RTO) is to oxidize the organic compounds (VOCs) in the exhaust gas to corresponding carbon dioxide and water at high temperature(>760℃), so as to purify the exhaust gas and recycle the heat released during the decomposition of the exhaust gas. There are mainly three forms of RTO: two-compartment RTO, three-compartment RTO, and rotating RTO. Each of the three forms has its own advantages and disadvantages. In the following, we take the three-compartment RTO as an example for explanation.
The high-concentration exhaust gas generated by desorption from the zeolite roller is brought into RTO by the RTO draft fan to carry out the incineration, and the VOCs in the exhaust gas are decomposed into CO2 and H2O at high temperature.
Each regenerator of RTO goes through the process of "heat storage-heat release-purging" in turn, cyclically, and continuously; After the "heat release" of the regenerator, the clean gas of the oxidation chamber is used to purge the regenerator, and the unreacted exhaust gas in the heat storage bed is sent back to the inlet of the main draft fan, and the "heat storage" procedure is started after the purging is completed.
The gas to be processed receives heat from the regenerator of the first can in the process of passing through the inlet, and then it is oxidized in the oxidation chamber. After oxidation, the treated high-temperature gas releases heat into the regenerator of the third chamber in the process of passing through the outlet. After the heat is released, the gas that has been treated is discharged.
In Procedure 1, the regenerator of the third chamber is heated and thus has stored the thermal energy. The valve switches the gas flow, and the third chamber becomes the "inlet". The untreated gas from the first chamber will be purged and returned into the gas to be processed, in order to avoid the discharge of untreated gas upon switching.
After purifying the untreated gas, the first chamber becomes the outlet. In order to maintain the heat balance of the thermal medium, the valve periodically repeats the switching cycle. Through the action of the switching valve in the lower chambers, each regenerator of the upper chambers is alternately kept in the state of gas intake & heat release, purging & purification, and gas exhausting & heat storage; The oxidation reaction of VOC is completed at high temperature and generates CO2and H2O and releases heat.
Roller + RTO exhaust gas treatment system of a large domestic coating factory
Roller + RTO exhaust gas treatment system of a large domestic coating factory
At present, the State has launched a VOCs regulation enforcement campaign to severely crack down on illegal pollutant discharge, and at the same time, regulations on the online VOCs monitoring have been enforced in many places and according to such regulations enterprises are required to install online monitoring equipment for VOCs, which will be connected to the local environmental protection bureaus. The environmental protection bureaus monitor the emissions of enterprises in real time and enterprises that have discharged excessive emission will be held accountable in accordance with the law, in order to ensure that the total VOCs emissions in 2020 will be reduced by more than 10% compared with 2015. Using the combination of the zeolite roller and RTO as post-treatment equipment for the automotive coating workshop can effectively reduce the emission of VOCs. Through the reasonable selection of the model and design according to the actual working conditions on the site, the final VOCs emissions will be far less than the national and local standards. Therefore, it can not only reduce the sewage charges to be paid by enterprises and help enterprises comply with the laws and regulations regarding its production, but can also help enterprises assume the corresponding social responsibilities.
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