Painted VOCs waste gas treatment solution

The waste gas in the paint working place is mainly the organic solvent contained in the paint and the decomposition of the coating film during spraying and drying, they are collectively referred to as volatile organic compounds (VOCs), and their components are mainly toluene and xylene. These ingredients are harmful to people's health and living environment, and have stench. If people inhale low concentrations of organic waste gas for a long time, they will cause chronic respiratory diseases such as cough, chest tightness, wheezing and emphysema, it is currently recognized as a strong carcinogen.

1. Foreword

Organic waste gas plays a very important role in the formation of photochemical smog and acid rain. In order to reduce VOCs in coatings, aqueous coatings and powder coatings have been developed, but aqueous coatings still contain a certain proportion of organic solvents. To this end, many countries have issued corresponding decrees to limit the emission of such gases. In 1997, China has promulgated and implemented the GB16297 <Integrated Emission Standard for Air Pollution>, limiting the emission limits of 33 kinds of pollutants, including volatile organic solvents such as benzene, toluene and xylene. In recent years, with the improvement of people's awareness of environmental protection, the continuous improvement of environmental protection regulations and the constant improvement of law enforcement, automobile production plants need to deploy exhaust gas treatment equipment in the new coating line, and the old coating line is also gradually supplementing the exhaust gas treatment device, the exhaust gas can only be discharged after it has been treated. Different manufacturers have adopted different methods for coating exhaust gases, the following is a preliminary analysis of automotive coating exhaust gas treatment technology.

2. Automobile production line exhaust gas treatment method

The gas discharged from the electrophoresis, intermediate coating and surface coating drying chambers is a high-temperature, high-concentration exhaust gas, and is suitable for treatment by incineration. At present, the exhaust gas treatment measures commonly used in the drying process are: regenerative catalytic technology (RCO), and TNV recycling thermal incineration system.

2.1.1 Regenerative thermal oxidation technology (RTO)

Regenerative Thermal Oxidizer (RTO) is an energy-saving environmental protection device for treating low and medium concentrations of volatile organic waste gas. Suitable for high air volume and low concentration, suitable for organic waste gas concentration between 100PPM and 20000PPM. The operation cost is low, when the organic waste gas concentration is above 450 PPM, the RTO device does not need to add auxiliary fuel; the purification rate is high, the two-bed RTO purification rate can reach 98% or more, and the three-bed RTO purification rate can reach 99% or more, and does not produce secondary pollution such as NOX; fully automatic control, simple operation; high safety.

The regenerative thermal oxidizer applies a thermal oxidation process to treat low-concentration organic waste gases, and a ceramic regenerator heat exchanger to recover heat. It consists of ceramic regenerator bed, automatic control valve, combustion chamber and control system. The main feature is that the automatic control valve at the bottom of the regenerator bed is connected to the intake manifold and the exhaust manifold respectively, and the regenerator bed is replaced by the reversing valve, and the heat of the high temperature gas from the combustion chamber is accumulated, and preheating organic waste gas entering the regenerator bed, the regenerator bed uses ceramic heat storage material to absorb and release heat; the organic waste gas preheated to a certain temperature (≥760 ° C) is oxidized in the combustion chamber to generate carbon dioxide and water to be purified. A typical two-bed RTO body consists of a combustion chamber, two ceramic packed beds and four switching valves (see figure below). The regenerative ceramic packed bed heat exchanger in the device allows for maximum recovery of thermal energy with a heat recovery of greater than 95%; no or little fuel is used to treat organic waste gases.

Advantages: the operating cost is very low when dealing with large flows of low concentrations of organic waste gas.

Disadvantages: higher one-time investment, higher combustion temperature, not suitable for handling high concentration of organic waste gas, there are many moving parts, and needs more maintenance work.

2.1.2 Regenerative catalytic oxidizer (RCO)

The Regenerative Catalytic Oxidizer (RCO) is directly applied to the purification of organic waste gas with a medium to high concentration (1000 mg/m³ to 10000 mg/m³). The RCO treatment technology is especially suitable for occasions where the heat recovery rate is high, and it is also suitable for the same production line where the exhaust gas composition often changes or the exhaust gas concentration fluctuates greatly due to different products. It is especially suitable for enterprises that need heat recovery or drying line exhaust gas treatment. The energy recovery can be used for drying lines to save energy.

Regenerative catalytic combustion control technology is a typical gas-solid phase reaction, which is essentially the deep oxidation of active oxygen. In the catalytic oxidation process, the adsorption of the catalyst surface enriches the reactant molecules on the surface of the catalyst, and the action of reducing the activation energy of the catalyst accelerates the progress of the oxidation reaction and increases the rate of the oxidation reaction. Under the action of a specific catalyst, the organic matter undergoes flameless oxidative combustion at a lower light-off temperature (250-300 ° C), and oxidatively decomposes into CO₂ and water. And releases a lot of heat.

The RCO device is mainly composed of several systems such as a furnace body, a catalytic heat storage body, a combustion system, an automatic control system, and an automatic valve. In the industrial production process, the discharged organic tail gas enters the rotary valve of the equipment through the induced draft fan, and the inlet gas and the outlet gas are completely separated by the rotary valve. The gas is first preheated by the ceramic material layer 1 to generate heat reserve and heat exchange, and the temperature thereof is almost the temperature set by the catalytic layer for catalytic oxidation, at which time some of the pollutants are oxidatively decomposed; the exhaust gas continues to pass through the heating zone (Electric heating or natural gas heating method) heats up and maintains at the set temperature; it further enters the catalytic layer to complete the catalytic oxidation reaction, that is, the reaction generates CO₂ and H₂O, and releases a large amount of heat to achieve the desired treatment effect. The catalytically oxidized gas enters the ceramic material layer 2, and the heat energy is recovered and discharged to the atmosphere through a rotary valve, the exhaust gas temperature after purification is only slightly higher than the temperature before the exhaust gas treatment. The system runs continuously and automatically switches. Working through the rotary valve, all ceramic filling layers complete the heating, cooling and purification cycle steps, and the heat is recovered.

Advantages: simple process, compact equipment, reliable operation; high purification efficiency, generally more than 98%; low combustion temperature compared with RTO; low one-time investment, low operating cost, its heat recovery efficiency can generally reach more than 85%; no waste water produced in the whole process, and the purification process does not produce secondary pollution such as NOX; RCO purification equipment can be used together with the drying room, and the purified gas can be directly reused in the drying room to achieve the purpose of energy saving and emission reduction;

Disadvantages: the catalytic combustion device is only suitable for the treatment of organic waste gas containing low-boiling organic components and low ash content, it is not suitable for the treatment of exhaust gas containing oleaginous materials such as oily smoke, and the catalyst should be poisoned; the concentration of treated organic waste gas is below 20%.

2.1.3 Thermische Nachverbrennung

(German ThermischeNachverbrennung, referred to as TNV) is the direct combustion of gas or fuel to heat the exhaust gas containing organic solvent. under the action of high temperature, the organic solvent molecules are oxidized and decomposed into CO₂ and water, and the generated high-temperature flue gas heats the air or hot water required for the production process through the matched multi-stage heat exchange device, fully recovers the heat energy generated by the oxidative decomposition of the organic waste gas, and reduces the energy consumption of the entire system. Therefore, the TNV system is an efficient and ideal treatment method for treating organic solvent-containing exhaust gas when the production process requires a large amount of heat, for the new coating production line, the TNV recovery type thermal incineration system is generally used.

The TNV system consists of three major components: exhaust gas preheating and incineration systems, circulating air heating systems and fresh air heat exchange systems. The exhaust gas incineration central heating device in the system is the core part of TNV, which is composed of a furnace body, a combustion chamber, a heat exchanger, a burner and a main flue regulating valve. The working process is: using a high-pressure head fan to extract organic waste gas from the drying chamber, and preheating through the built-in heat exchanger of the exhaust gas incineration central heating device, reaching the combustion chamber, and then heated by the combustion machine, at a high temperature (about 750 ° C), the organic waste gas is oxidatively decomposed to CO₂ and water. The generated high-temperature flue gas is discharged through the heat exchanger in the furnace and the main flue gas duct, and the exhausted flue gas heats the circulating air of the drying chamber to provide the required heat for the drying chamber. A fresh air heat exchange device is arranged at the end of the system, and the residual heat of the system is finally recovered, and the fresh air supplemented by the drying chamber is heated by the flue gas and sent to the drying room. In addition, an electric regulating valve is arranged on the main flue gas duct for adjusting the flue gas temperature at the outlet of the device, and the temperature of the flue gas finally discharged can be controlled at about 160 °C.

The characteristics of the waste gas incineration central heating device include: the residence time of the organic waste gas in the combustion chamber is 1 to 2 s; the decomposition rate of the organic waste gas is greater than 99%; the heat recovery rate is up to 76%; the adjustment ratio of the burner output can reach 26:1 up to 40:1.

Disadvantages: When dealing with low-concentration organic waste gas, the operating cost is high; the tubular heat exchanger only has a long life when it is continuously operated.

2.2 Treatment plan for organic waste gas in spray booth and drying room

The gas discharged from the spray booth and the drying chamber is a low-concentration, high-flow, ambient-temperature exhaust gas, the main components of the pollutants are aromatic hydrocarbons, alcohol ethers and ester organic solvents. At present, the more mature method in foreign countries is: firstly concentrate the organic waste gas to reduce the total amount of organic waste gas to be treated, adopt adsorption method (activated carbon or zeolite as adsorbent) to adsorb low-concentration normal temperature paint exhaust gas, and take off with high-temperature gas,the concentrated exhaust gas is treated by catalytic combustion or regenerative thermal combustion.

2.2.1 Activated carbon adsorption-desorption purification device

Using the honeycomb activated carbon as adsorbent, combined with adsorption purification, desorption regeneration and concentration of VOCs, and catalytic combustion, that is, the high-volume, low-concentration organic waste gas is adsorbed by honeycomb activated carbon to achieve the purpose of purifying air, when the activated carbon is saturated and then desorbed with hot air, the activated carbon is regenerated, and the concentrated organic matter is sent to the catalytic combustion bed for catalytic combustion, and the organic matter is oxidized into harmless CO₂ and H₂0, and the burned hot exhaust gas heats the cold air through the heat exchanger, the gas that is cooled after the heat exchange is partially discharged, and a part of gas is used for the desorption regeneration of the honeycomb activated carbon to achieve the purpose of waste heat utilization and energy conservation. The whole device consists of pre-filter, adsorption bed, catalytic combustion bed, flame retardant, related fans and valves.

Activated carbon adsorption-desorption purification device is designed according to two basic principles of adsorption and catalytic combustion, it uses two gas paths for continuous operation, one catalytic combustion chamber, and two adsorption beds are used alternately. The organic waste gas is first adsorbed by activated carbon, and when it reaches saturation, the adsorption is stopped, and then the organic matter is desorbed from the activated carbon by a hot gas stream to regenerate the activated carbon; the desorbed organic matter has been concentrated (concentration is increased several tens times) and it is sent to the catalytic combustion chamber for catalytic combustion into carbon dioxide and water vapor. When the concentration of the organic waste gas reaches 2000 ppm or more, the organic waste gas can maintain self-ignition in the catalytic bed without external heating. Part of the exhaust gas after combustion is discharged into the atmosphere, and most of it is sent to the adsorption bed for regeneration of activated carbon. This can meet the heat energy required for combustion and adsorption, and achieve energy saving. After regeneration, the next adsorption can be entered; at the time of desorption, the purification operation can be carried out by another adsorption bed, which is suitable for both continuous operation and intermittent operation.

Technical performance and characteristics: stable performance, simple structure, safe and reliable, energy saving, no secondary pollution. The equipment has a small footprint and light weight. Extremely suitable for use under high air volume. The activated carbon bed adsorbing the organic waste gas is desorbed and regenerated by the exhaust gas after the catalytic combustion, and the desorbed gas is sent to the catalytic combustion chamber for purification, and no external energy is needed, and the energy saving effect is remarkable. The disadvantage is that the activated carbon has a short service life and high operating cost.

2.2.2 Zeolite runner adsorption-desorption purification device

The main components of zeolite are: silicon and aluminum, which have adsorption capacity and can be used as adsorbents. Zeolite runners are characterized by the specific pore size of zeolites for adsorption and desorption of organic pollutants, which makes the original low concentration and high air volume, the VOC exhaust gas is converted into a small air volume and a high concentration gas by concentration of the zeolite runner, which can reduce the operating cost of the final end processing equipment. Its device characteristics are suitable for treating large-flow, low-concentration, exhaust gas containing a variety of organic components. The disadvantage is that the upfront investment is high.

The zeolite runner adsorption-purification device is a gas purification device that can continuously perform adsorption and desorption operations. The sides of the zeolite runner are divided into three zones by a special sealing device: adsorption zone, desorption (regeneration) zone and cooling zone. The working process of the system is: the zeolite runner continuously rotates at a lower speed, and circulates through the adsorption zone and the desorption (regeneration) zone and the cooling zone; when the low-concentration, high-volume exhaust gas continuously passes through the adsorption zone of the runner, the VOC in the exhaust gas is adsorbed by the zeolite of the runner, and the adsorbed and purified gas is directly discharged; the organic solvent adsorbed by the wheel is sent to the desorption (regeneration) zone along with the rotation of the runner, and then continuously passes through the desorption zone with a small amount of hot air, the VOCs adsorbed on the runner are regenerated by desorption in the desorption zone, and the VOC exhaust gas is discharged together with the hot air; the runner can be re-adsorbed after being cooled to the cooling zone for cooling and cooling. As the runner rotates continuously, the adsorption, desorption and cooling cycles are carried out to ensure continuous and stable operation of the exhaust gas treatment.

The zeolite runner device is essentially a concentrator, and the organic solvent-containing exhaust gas after the rotor treatment is divided into two parts: clean air that can be directly discharged and regeneration air containing a high concentration of organic solvent. The clean air that can be directly discharged can be recycled into the paint air conditioning ventilation system; the high concentration of VOCs gas is about 10 times the concentration of VOCs before entering the system, and the concentrated gas passes through the TNV recovery type thermal incineration system ( Or other equipment) high-temperature incineration treatment, the heat generated by incineration is the heating of the drying chamber and the desorption of the zeolite runner, and the heat is fully utilized to achieve the effect of energy saving and emission reduction.

Technical performance and characteristics: simple structure, convenient maintenance, long service life; high suction and desorption efficiency, so that the original high-volume, low-concentration VOCs waste gas is converted into low-air volume, high-concentration exhaust gas, reducing the end-end processing equipment cost; the pressure drop generated by zeolite runners adsorbing VOCs is extremely low, which can greatly reduce the power consumption; the overall system adopts pre-group and modular design, which has the minimum space requirement and provides continuous and unmanned control modal; the exhaust gas after the concentration of the runner can reach the national emission standard; the adsorbent uses non-combustible hydrophobic zeolite, which is safer to use; the disadvantage is that the one-time investment is higher.
3. Conclusion
Reducing the pollution of painting is not only the development direction of coating technology, but also the responsibility of the coating industry. As the domestic demand for environmental quality continues to increase, the management of volatile organic waste gas is gradually being carried out. According to different characteristics, the treatment effect and treatment cost of different treatment methods should be analyzed, adopt effective technical methods to meet emission standards, protect the global environment and create a harmonious society.