Talking about the Method of Organic Waste Gas Treatment in Automobile Painting

Automotive coating production line is the most "three wastes" in the automobile manufacturing process, of which coating exhaust gas is the main part of "three wastes" painting. Exhaust gas from the paint shop mainly consists of organic solvents contained in paint and decomposition products of paint films when sprayed and dried. They are collectively referred to as volatile organic compounds (VOCs) and mainly consist of toluene and xylene. These ingredients are harmful to people's health and living environment and have bad smell. People who breathe low concentrations of organic waste gas for a long time can cause chronic respiratory diseases such as cough, chest tightness, asthma and even emphysema, and are currently recognized as strong carcinogens. In addition, organic waste gas plays a very important role in the formation of photochemical smog and acid rain. To reduce VOCs in coatings, waterborne coatings and powder coatings have been developed, but aqueous coatings still contain a proportion of organic solvents. To this end, various countries have enacted corresponding decrees to limit the emission of such gases. The GB16297 Integrated Air Pollution Emission Standard promulgated and implemented by China in 1997 has limited the emission limits of 33 pollutants, including benzene, toluene , Xylene and other volatile organic solvents. In recent years, as people's awareness of environmental protection, environmental laws and regulations continue to improve and continue to improve law enforcement, automobile manufacturing plant in the new coating line to be configured exhaust gas treatment equipment, the old coating line is also gradually replenish the exhaust gas treatment device, exhaust gas After handling the standard before discharge. Different coatings for the exhaust gas, different manufacturers have adopted different methods, the following car exhaust gas treatment technology for the shallow analysis.

According to the car painting production process, painting exhaust mainly from the spraying, drying process. Emissions of pollutants are mainly: Paint spray generated when the fog and organic solvents, volatile volatile organic solvents. Paint mist mainly comes from the part of the solvent-based paint which is sprayed in the air spraying operation, and its composition is consistent with the paint used. Organic solvents mainly come from the solvents and diluents used in coatings. Most of them are volatile emissions. The main pollutants are xylene, benzene and toluene. Therefore, the main source of emission of harmful exhaust paint painting room, drying room, drying room.

1, automobile production line exhaust gas treatment methods

1.1 drying process of organic waste gas treatment program

Electrophoresis, coating, surface coating drying gas is the high temperature exhaust gas, suitable for incineration method for processing. Currently used in the drying process exhaust treatment measures are: regenerative thermal oxidation (RTO), regenerative catalytic combustion (RCO), TNV recovery heat incineration system

1.1.1 Regenerative Thermal Oxidation Technology (RTO)

Regenerative Thermal Oxidizer (RegenerativeThermalOxidizer, referred to as RTO) is an energy-saving environmental protection device for the treatment of low-concentration volatile organic waste gas. Suitable for wind volume, low concentration, suitable for organic waste gas concentration between 100PPM-20000PPM. The operating cost is low. When the organic waste gas concentration is above 450PPM, the RTO device does not need to be supplemented with auxiliary fuel. The purification rate is high, the purification rate of two-bed RTO can reach over 98%, the three-bed RTO purification rate can reach over 99% No secondary pollution such as NOx; Fully automatic control, easy to operate; safe.

Regenerative thermal oxidizer using thermal oxidation of low concentrations of organic waste gas treatment, ceramic regenerator bed heat exchanger to recover heat. By the ceramic heat storage bed, automatic control valve, combustion chamber and control system and other components. The utility model is characterized in that the automatic control valve at the bottom of the heat storage bed is respectively connected with an air intake manifold and an exhaust manifold, the heat storage bed is alternately reversed by a reversing valve, the high-temperature gas heat exiting from the combustion chamber is stored and preheated into the storage Hot bed organic waste gas, heat storage bed using ceramic heat storage material to absorb and release heat; preheated to a certain temperature (≥760 ℃) of organic waste gas in the combustion chamber oxidation reaction occurs, generate carbon dioxide and water, to be purified. A typical two-bed RTO main structure consists of a combustion chamber, two ceramic packed beds and four switching valves (see below). The regenerative ceramic-packed bed heat exchanger in this unit maximizes thermal energy recovery with greater than 95% heat recovery and eliminates or uses very little fuel when treating organic waste gases.

Advantages: Operating costs are very low when dealing with low levels of organic waste gas at high flow rates.

Disadvantages: a one-time investment, high combustion temperature, not suitable for the treatment of high concentrations of organic waste gas, there are many moving parts, requiring more maintenance work.

1.1.2 regenerative catalytic combustion technology (RCO)

Regenerative Catalytic Oxidizer (RCO) is applied to the purification of organic waste gas of medium to high concentration (1000mg / m3 to 10000mg / m3). RCO treatment technology is particularly suitable for high heat recovery needs of the occasion, but also applies to the same production line, due to different products, the composition of the exhaust gas often changes or exhaust gas concentrations fluctuate larger occasions. It is especially suitable for the waste gas treatment of enterprises or drying lines that need heat recovery, which can recover the energy for the drying line so as to save energy.

Regenerative catalytic combustion control technology is a typical gas-solid reaction, the essence of reactive oxygen species involved in the depth of oxidation. In the catalytic oxidation process, the adsorption on the surface of the catalyst enriches the reactant molecules on the surface of the catalyst. The effect of the catalyst on reducing the activation energy accelerates the oxidation reaction and increases the oxidation reaction rate. Under the action of a specific catalyst, the organic material is flamelessly oxidized and oxidized to CO 2 and water at a lower light-off temperature (250-300 ° C). And release a lot of heat.

RCO device mainly consists of furnace, catalytic regenerator, combustion system, automatic control system, automatic valve and several other systems. In the industrial process, the discharged organic exhaust 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 selector valve. The gas first undergoes a reserve of heat and heat exchange after the preheating of the ceramic material layer 1 at a temperature almost reaching the temperature set by the catalytic oxidation of the catalytic layer, at which point some of the pollutants oxidize and decompose; the exhaust gas continues to pass through the heating zone Electric heating or natural gas heating) and maintain the temperature at the set temperature; it re-enter the catalytic layer to complete the catalytic oxidation reaction, that is, the reaction generates CO2 and H2O, and release a lot of heat to achieve the desired treatment effect. After the catalytic oxidation of gas into the ceramic layer 2, after the recovery of heat through the rotary valve discharge to the atmosphere, the exhaust temperature after purification is only slightly higher than the exhaust gas before treatment temperature. System continuous operation, automatic switching. Through the rotary valve work, all the ceramic filled layer are completed heating, cooling, cleaning cycle, the heat can be recovered.

Advantages: simple process, compact equipment, reliable operation; high purification efficiency, generally up to 98% or more; low combustion temperature compared with RTO; low investment, low operating costs, the heat recovery efficiency are generally up to 85 % Or more; no waste water is generated in the whole process, no secondary pollution such as NOX is generated in the purification process; the RCO purifying equipment can be used in combination with a drying house to directly reuse the cleaned gas for drying house to achieve energy saving and emission reduction purposes;

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, which is not suitable for the exhaust gas treatment of viscous substances such as fume, and the catalyst should be poisoned; the concentration of organic waste gas treated is below 20%.

1.1.3TNV recovery thermal incineration system

Recycling thermal incineration system (German ThermischeNachverbrennung TNV for short) is the use of gas or fuel direct combustion heating organic waste gas containing organic solvent molecules under high temperature oxidation and decomposition of CO2 and water, the resulting high-temperature flue gas through the supporting more Level heat exchanger heats the air or hot water required in the production process to fully recover the thermal energy generated by the oxidative decomposition of organic waste gas and reduce the energy consumption of the entire system. Therefore, the TNV system is an efficient and ideal way to deal with organic solvent exhaust gas when a large amount of heat is needed in the production process. For a new coating production line, a TNV recovery thermal incineration system is generally adopted.

TNV system consists of three parts: exhaust gas preheating and incineration systems, circulating air heating system, fresh air heat exchange system. The exhaust gas incineration central heating system in this system is the core part of TNV. It consists of furnace body, combustion chamber, heat exchanger, combustion machine and main flue regulating valve. The working process is as follows: a high-pressure head fan is used to extract the organic waste gas from the drying room, after the waste heat incineration central heating unit is preheated by the built-in heat exchanger and reaches the combustion chamber, and then heated by the combustion machine, (About 750 ℃) to oxidize and decompose the organic waste gas, and the decomposed organic waste gas becomes CO2 and water. The generated high-temperature flue gas is discharged through the heat exchanger in the furnace and the main flue gas pipe, and the exhausted flue gas heats the circulating air in the drying chamber to provide the required heat for the drying chamber. In the end of the system set fresh air heat exchanger, the system for the final recovery of waste heat, the drying room to add fresh air heated with flue gas into the drying room. In addition, the main flue gas pipe is also equipped with electric control valve, used to regulate the exit flue gas temperature, the final emission of flue gas temperature can be controlled at about 160 ℃.

The characteristics of exhaust gas incineration centralized heating unit include: the residence 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%; the burner output regulation ratio can reach 26: , Up to 40: 1.

Disadvantages: High cost of operation when dealing with low concentrations of organic waste gas; tube heat exchangers have a longer life expectancy only during continuous operation.