The pollutants discharged are mainly: paint mist and organic solvents produced by spray paint, and organic solvents produced when drying volatilization. Paint mist mainly comes from the part of solvent coating in air spraying, and its composition is consistent with the coating used. Organic solvents mainly come from the solvents and diluents in the use process of coatings, most of them are volatile emissions, and their main pollutants are xylene, benzene, toluene and so on. Therefore, the main source of the harmful waste gas discharged in the coating is the spray painting room, drying room and drying room.
1. Waste gas treatment method of automobile production line
1.1 Treatment scheme of the organic waste gas in the drying process
The gas discharged from the electrophoresis, medium coating and surface coating drying room belongs to the high temperature and high concentration waste gas, which is suitable for the incineration method. At present, the commonly used waste gas treatment measures in the drying process include: regenerative thermal oxidation technology (RTO), regenerative catalytic combustion technology (RCO), and TNV recovery thermal incineration system
1.1.1 Thermal storage type thermal oxidation technology (RTO)
Thermal oxidator (Regenerative Thermal Oxidizer, RTO) is an energy-saving environmental protection device for treating medium and low concentration volatile organic waste gas. Suitable for high volume, low concentration, suitable for organic waste gas concentration between 100 PPM-20000 PPM. The operation cost is low, when the organic waste gas concentration is above 450 PPM, RTO device does not need to add auxiliary fuel; the purification rate is high, the purification rate of two bed RTO can reach over 98%, the purification rate of three bed RTO can reach over 99%, and no secondary pollution such as NOX; automatic control, simple operation; safety is high.
The regenerative heat oxidation device adopts the thermal oxidation method to treat the medium and low concentration of organic waste gas, and the ceramic heat storage bed heat exchanger is used to recover the heat. It is composed of ceramic heat storage bed, automatic control valve, combustion chamber and control system. The main features are: the automatic control valve at the bottom of the heat storage bed is connected with the intake main pipe and the exhaust main pipe respectively, and the heat storage bed is stored by preheating the organic waste gas coming into the heat storage bed with ceramic heat storage material to absorb and release heat; the organic waste gas preheated to a certain temperature (760℃) is oxidized in the combustion of the combustion chamber to generate carbon dioxide and water, and is purified. The typical two-bed RTO main structure consists of one combustion chamber, two ceramic packing beds and four switching valves. The regenerative ceramic packing bed heat exchanger in the device can maximize the heat recovery of greater than 95%; No or little fuel is used when treating organic waste gas.
Advantages: In dealing with high flow and low concentration of organic waste gas, the operating cost is very low.
Disadvantages: high one-time investment, high combustion temperature, not suitable for the treatment of high concentration of organic waste gas, there are a lot of moving parts, need more maintenance work.
1.1.2 Thermal catalytic combustion technology (RCO)
The regenerative catalytic combustion device (Regenerative Catalytic Oxidizer RCO) is directly applied to medium and high concentration (1000 mg/m3-10000 mg/m3) organic waste gas purification. RCO treatment technology is especially suitable for the high demand for heat recovery rate, but also suitable for the same production line, because of the different products, the waste gas composition often changes or the waste gas concentration fluctuates greatly. It is especially suitable for the need for heat energy recovery of enterprises or drying trunk line waste gas treatment, and the energy recovery can be used for drying trunk line, so as to achieve the purpose of energy saving.
The regenerative catalytic combustion treatment technology is a typical gas-solid phase reaction, which is actually the deep oxidation of reactive oxygen species. In the process of catalytic oxidation, the adsorption of the surface of the catalyst makes the reactant molecules enriched on the surface of the catalyst. The effect of the catalyst in reducing the activation energy accelerates the oxidation reaction and improves the rate of the oxidation reaction. Under the action of specific catalyst, organic matter occurs without eless oxidation combustion at low starting temperature (250~300℃), which is decomposed into carbon dioxide and water, and releases a large amount of heat energy.
The RCO device is mainly composed of the furnace body, the catalytic heat storage body, the combustion system, the automatic control system, the automatic valve and several other systems. In the industrial production process, the discharged organic exhaust gas enters the rotating valve of the equipment through the induced draft fan, and the inlet gas and the outlet gas are completely separated through the rotating valve. The heat energy storage and heat exchange of the gas almost reaches the temperature set by the catalytic oxidation of the catalytic layer; the exhaust gas continues to heat up through the heating area (either by electric heating or natural gas heating) and maintains at the set temperature; it enters the catalytic layer to complete the catalytic oxidation reaction, namely, the reaction generates carbon dioxide and water, and releases a large amount of heat energy to achieve the desired treatment effect. The gas catalyzed by the oxidation enters the ceramic material layer 2, and the heat energy is discharged into the atmosphere through the rotary valve. After purification, the exhaust temperature after purification is only slightly higher than the temperature before the waste gas treatment. The system operates continuously and switches automatically. Through the rotating valve work, all the ceramic filling layers complete the cycle steps of heating, cooling and purification, and the heat energy can be recovered.
Advantages: simple process flow, compact equipment, reliable operation; high purification efficiency, generally over 98%; low combustion temperature; low disposable investment, low operating cost, heat recovery efficiency can generally reach more than 85%; the whole process without wastewater production, purification process does not produce NOX secondary pollution; RCO purification equipment can be used with the drying room, 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 with low boiling point organic components and low ash content, and the waste gas treatment of sticky substances such as oily smoke is not suitable, and the catalyst should be poisoned; the concentration of organic waste gas is below 20%.
1.1.3TNV Recycling type thermal incineration system
Recycling type thermal incineration system (German Thermische Nachverbrennung TNV) is the use of gas or fuel direct combustion heating waste gas containing organic solvent, under the action of high temperature, organic solvent molecules oxidation decomposition into carbon dioxide and water, the high temperature flue gas through supporting multistage heat transfer device heating production process needs air or hot water, full recycling oxidation decomposition of organic waste gas heat energy, reduce the energy consumption of the whole system. Therefore, TNV system is an efficient and ideal way to treat the waste gas containing organic solvents when the production process needs a lot of heat energy. For the new electrophoretic paint coating production line, TNV recovery thermal incineration system is generally adopted.
TNV system consists of three parts: waste gas preheating and incineration system, circulating air heating system and fresh air heat exchange system. The waste gas incineration central heating device in the system is the core part of TNV, which is composed of furnace body, combustion chamber, heat exchanger, burner and main flue regulating valve. Its working process is: with a high pressure head fan will organic waste gas from the drying room, after waste gas incineration central heating device built-in heat exchanger preheating, to the combustion chamber, and then through the burner heating, at high temperature (about 750℃) to the organic waste gas oxidation decomposition, decomposition of organic waste gas into carbon dioxide and water. The generated high temperature flue gas is discharged through the heat exchanger and the main flue gas pipe in the furnace. The discharged flue gas heats the circulating air in the drying room to provide the required heat energy for the drying room. A fresh air heat transfer device is set at the end of the system to recover the waste heat of the system for the final recovery. The fresh air supplemented by the drying room is heated with flue gas and then sent into the drying room. In addition, there is also an electric regulating valve on the main flue gas pipeline, which is used to adjust the flue gas temperature at the outlet of the device, and the final emission of flue gas temperature can be controlled at about 160℃.
The characteristics of waste gas incineration central heating device include: the stay time of organic waste gas in the combustion chamber is 1~2s; the decomposition rate of organic waste gas is more than 99%; the heat recovery rate can reach 76%; and the adjustment ratio of burner output can reach 26 ∶ 1, up to 40 ∶ 1.
Disadvantages: when treating low-concentration organic waste gas, the operation cost is higher; the tubular heat exchanger is only in continuous operation, it has a long life.
1.2 Treatment scheme of organic waste gas in spray paint room and drying room
The gas discharged from the spray paint room and drying room is low concentration, large flow rate and room temperature waste gas, and the main composition of pollutants is aromatic hydrocarbons, alcohol ethers and ester organic solvents. At present, the foreign more mature method is: the first organic waste gas concentration to reduce the total amount of organic waste gas, with the first adsorption method (activated carbon or zeolite as adsorbent) for low concentration of room temperature spray paint exhaust adsorption, with high temperature gas stripping, concentrated exhaust gas using catalytic combustion or regenerative thermal combustion method.
1.2.1 Activated carbon adsorption- -desorption and purification device
Using the honeycomb activated charcoal as the adsorbent, Combined with the principles of adsorption purification, desorption regeneration and concentration of VOC and catalytic combustion, High air volume, low concentration of organic waste gas through honeycomb activated carbon adsorption to achieve the purpose of air purification, When the activated carbon is saturated and then uses hot air to regenerate the activated carbon, Desorbed concentrated organic matter is sent to the catalytic combustion bed for catalytic combustion, Organic matter is oxidized to harmless carbon dioxide and water, The burned hot exhaust gases heat the cold air through a heat exchanger, Some emission of the cooling gas after heat exchange, Part for the desorbitory regeneration of honeycomb activated charcoal, To achieve the purpose of waste heat utilization and energy saving. The whole device is composed of pre-filter, adsorption bed, catalytic combustion bed, flame retardancy, related fan, valve, etc.
Activated carbon adsorption-desorption purification device is designed according to the two basic principles of adsorption and catalytic combustion, using double gas path continuous work, a catalytic combustion chamber, two adsorption bed is used alternately. First organic waste gas with activated carbon adsorption, when fast saturation stop adsorption, and then use hot air flow to remove organic matter from the activated carbon to make the activated carbon regeneration; the organic matter has been concentrated (concentration dozens of times higher than the original) and sent to the catalytic combustion chamber catalytic combustion into carbon dioxide and water vapor discharge. When the concentration of the organic waste gas reaches more than 2000 PPm, the organic waste gas can maintain spontaneous combustion in the catalytic bed without external heating. Part of the combustion exhaust gas is discharged into the atmosphere, and most of it is sent to the adsorption bed for the regeneration of activated carbon. This can meet the combustion and adsorption of the heat energy required, to achieve the purpose of energy saving. The regeneration can enter the next adsorption; in the desorption, the purification operation can be performed by another adsorption bed, suitable for both continuous operation and intermittent operation.
Technical performance and characteristics: stable performance, simple structure, safe and reliable, energy-saving and labor-saving, no secondary pollution. The equipment covers a small area and has a light weight. Very suitable for use in high volume. The activated carbon bed that adsorbs organic waste gas uses the waste gas after catalytic combustion for stripping regeneration, and the stripping gas is sent to the catalytic combustion chamber for purification, without external energy, and the energy saving effect is significant. The disadvantage is that activated carbon is short and its operating cost is high.
1.2.2 Zeolite transfer wheel adsorption- -desorption purification device
The main components of zeolite are: silicon, aluminum, with adsorption capacity, can be used as adsorbent; zeolite runner is to use the characteristics of zeolite specific aperture with adsorption and desorption capacity for organic pollutants, so that the VOC exhaust gas with low concentration and high concentration, can reduce the operation cost of back-end final treatment equipment. Its device characteristics are suitable for the treatment of large flow, low concentration, containing a variety of organic components. The disadvantage is that the early investment is high.
Zeolite runner adsorption-purification device is a gas purification device that can continuously perform adsorption and desorption operation. The two sides of the zeolite wheel are divided into three areas by the special sealing device: adsorption area, desorption (regeneration) area and cooling area. The working process of the system is: the zeolites rotating wheel rotates continuously at a low speed, Circulation through the adsorption area, desorption (regeneration) area and cooling area; When the low concentration and gale volume exhaust gas continuously passes through the adsorption area of the runner, The VOC in the exhaust gas is adsorbed by the zeolite of the rotating wheel, Direct emission after adsorption and purification; The organic solvent adsorbed by the wheel is sent to the desorption (regeneration) zone with the rotation of the wheel, Then with a small air volume heat air continuously through the desorption area, The VOC adsorbed to the wheel is regenerated in the desorption zone, The VOC exhaust gas is discharged together with the hot air; The wheel to the cooling area for cooling cooling can be re-adsorption, With the constant rotation of the rotating wheel, Adsorption, desorption, and cooling cycle is performed, Ensure the continuous and stable operation of the waste gas treatment.
The zeolite runner device is essentially a concentrator, and the exhaust gas containing organic solvent is divided into two parts: clean air that can be discharged directly, and recycled air containing high concentration of organic solvent. Clean air that can be discharged directly and can be recycled in the painted air conditioning ventilation system; the high concentration of VOC gas is about 10 times of the VOC concentration before entering the system. The concentrated gas is treated by high temperature incineration through TNV recovery thermal incineration system (or other equipment). The heat generated by incineration is drying room heating and zeolite stripping heating respectively, and the heat energy is fully utilized to achieve the effect of energy saving and emission reduction.
Technical performance and characteristics: simple structure, easy maintenance, long service life; high absorption and stripping efficiency, convert the original high wind volume and low concentration VOC waste gas into low air volume and high concentration waste gas, reduce the cost of back-end final treatment equipment; extremely low pressure drop, can greatly reduce power energy consumption; overall system preparation and modular design, with minimum space requirements, and provide continuous and unmanned control mode; it can reach the national emission standard; adsorbent uses non-combustible zeolite, the use is safer; the disadvantage is one-time investment with high cost.
Post time: Jan-03-2023
