The exhaust gases in the painting workshop mainly consist of organic solvents contained in the coatings and decomposition products formed during spraying and baking, collectively referred to as volatile organic compounds (VOCs). The primary components are toluene and xylene. These substances are harmful to human health and the living environment and emit a pungent odor; prolonged exposure to low concentrations can Organic waste gas It can trigger chronic respiratory diseases such as cough, chest tightness, wheezing, and even emphysema, and is currently recognized as a potent carcinogen.

1. Preface

Organic waste gases play a crucial role in the formation of photochemical smog and acid rain. To reduce VOCs in coatings, waterborne and powder coatings have been developed; however, waterborne coatings still contain a certain proportion of organic solvents. In response, countries worldwide have enacted relevant regulations to limit emissions of such gases. China’s GB 16297 “Integrated Emission Standard for Air Pollutants,” promulgated and implemented in 1997, sets emission limits for 33 pollutants, including volatile organic solvents such as benzene, toluene, and xylene. In recent years, with growing public environmental awareness, continuous improvement of environmental protection laws and regulations, and stricter enforcement, automobile manufacturers are required to equip newly built painting lines with Waste gas treatment Equipment: Old coating lines are also being gradually equipped with exhaust gas treatment systems, and exhaust gases can only be discharged after they have been treated to meet regulatory standards. For different Painting exhaust gases Different manufacturers have adopted different approaches; below is a preliminary analysis and discussion of exhaust gas treatment technologies for automotive painting.

According to automotive painting production processes, coating exhaust gases primarily originate from the spraying and drying stages. The main pollutants emitted include paint mist and organic solvents generated during spraying, as well as organic solvents volatilized during the drying process. Paint mist mainly arises from the overspray of solvent-based coatings during air spraying operations, with its composition consistent with that of the coatings used. Organic solvents primarily stem from solvents and thinners employed in the coating application process; the vast majority are volatile emissions, with major constituents including xylene, benzene, and toluene. Therefore, the primary sources of harmful exhaust gases in the painting process are the spray booths, flash-off rooms, and baking ovens.

2. Exhaust Gas Treatment Methods for Automotive Production Lines

2.1 Treatment Plan for Organic Waste Gases in the Drying Process

The exhaust gases emitted from electrophoresis, mid-coat, and topcoat baking ovens are high-temperature, high-concentration waste gases, making incineration an appropriate treatment method. Currently, the commonly used waste-gas treatment technologies for the baking process include regenerative thermal oxidation (RTO), regenerative catalytic oxidation (RCO), and TNV heat recovery–based thermal incineration systems.

2.1.1 Regenerative Thermal Oxidation (RTO) Technology

A Regenerative Thermal Oxidizer (RTO) is an energy-efficient, environmentally friendly device designed for the treatment of low- to medium-concentration volatile organic waste gases. It is particularly suitable for applications involving large air volumes and low concentrations, with optimal performance when the concentration of organic waste gases ranges from 100 ppm to 20,000 ppm. The RTO features low operating costs: when the concentration of organic waste gases exceeds 450 ppm, no auxiliary fuel is required. It also delivers high purification efficiency—over 98% for two-bed RTOs and over 99% for three-bed RTOs—while producing no secondary pollutants such as NOx. Additionally, the system offers fully automated control, simple operation, and enhanced safety.

Regenerative thermal oxidizers employ thermal oxidation to treat medium- and low-concentration organic waste gases, utilizing a ceramic heat-storage bed heat exchanger to recover heat. The system comprises a ceramic heat-storage bed, automatic control valves, a combustion chamber, and a control system. Its key features include: automatic control valves at the bottom of the heat-storage bed are connected respectively to the inlet manifold and the outlet manifold; the heat-storage bed alternates between heat absorption and heat release via a reversing valve, capturing the high-temperature heat from the combustion chamber exhaust and preheating the incoming organic waste gas; the heat-storage bed is constructed from ceramic heat-storage materials that absorb and release heat; the preheated organic waste gas—once it reaches a specified temperature (≥760°C)—is combusted in the combustion chamber, undergoing an oxidation reaction to produce carbon dioxide and water, thereby achieving purification. A typical two-bed RTO consists of one combustion chamber, two ceramic packing beds, and four switching valves (see the figure below). The regenerative ceramic-packed-bed heat exchanger in this system enables maximum heat recovery, with a heat recovery rate exceeding 95%; moreover, during the treatment of organic waste gases, little or no auxiliary fuel is required.

Advantages: When treating high-flow, low-concentration organic waste gases, the operating costs are very low.

Disadvantages: relatively high initial capital investment, high combustion temperatures, unsuitability for treating high-concentration organic waste gases, numerous moving parts, and substantial maintenance requirements.

2.1.2 Regenerative Catalytic Oxidation (RCO) Technology

Regenerative catalytic combustion unit ( The Regenerative Catalytic Oxidizer (RCO) is directly applied to the purification of organic waste gases at medium to high concentrations (1,000–10,000 mg/m³). RCO technology is particularly well suited for applications with high heat recovery requirements, as well as for production lines where the composition of waste gases frequently changes or the concentration fluctuates significantly due to product variations. It is especially ideal for enterprises or drying lines that require heat recovery, enabling the recovered thermal energy to be reused in the drying process and thereby achieving energy savings.

Regenerative catalytic combustion treatment technology is a typical gas - Solid-phase reactions are, in essence, deep oxidation processes involving active oxygen. During catalytic oxidation, adsorption on the catalyst surface enriches reactant molecules at the catalyst interface, while the catalyst’s ability to lower the activation energy accelerates the oxidation reaction and increases its rate. Under the action of a specific catalyst, organic compounds undergo flameless oxidative combustion at relatively low ignition temperatures (250–300°C), oxidizing and decomposing into CO2 and water, with substantial heat being released in the process.

The RCO unit is primarily composed of several subsystems, including the furnace body, catalytic heat-storage media, combustion system, automatic control system, and automated valves. During industrial production, the emitted organic exhaust gases are drawn into the unit by an induced-draft fan and directed into a rotary valve, which completely separates the inlet and outlet gas streams. The gas first passes through ceramic bed layer 1, where it is preheated, storing thermal energy and undergoing heat exchange until its temperature nearly reaches the set operating temperature for catalytic oxidation in the catalyst bed; at this point, a portion of the pollutants undergoes oxidative decomposition. The exhaust gas then continues through a heating zone—either electrically or via natural-gas-fired heating—to reach and maintain the set temperature, before entering the catalyst bed to complete the catalytic oxidation reaction, producing CO2 and H2O while releasing substantial amounts of heat, thereby achieving the desired treatment performance. After catalytic oxidation, the gas flows into ceramic bed layer 2, where residual heat is recovered before being discharged to the atmosphere via the rotary valve; the temperature of the treated effluent is only slightly higher than that of the raw exhaust gas prior to treatment. The system operates continuously with automatic switching. Through the action of the rotary valve, all ceramic packing beds sequentially undergo heating, cooling, and purification cycles, enabling efficient heat recovery.

Advantages: simple process flow, compact equipment, and reliable operation; high purification efficiency, typically reaching More than 98%; lower combustion temperatures compared with RTOs; lower initial capital investment and operating costs, with typical heat recovery efficiencies exceeding 85%; no wastewater generated throughout the process, and the purification process does not produce secondary pollutants such as NOx; RCO purification equipment can be integrated with drying ovens, allowing the purified exhaust gas to be directly recirculated back into the oven for reuse, thereby achieving energy conservation and emission reduction.

Disadvantages: Catalytic combustion units are only suitable for treating organic exhaust gases containing low-boiling-point organic components and with low ash content; they are not appropriate for treating exhaust gases containing viscous substances such as cooking fumes, as the catalyst is prone to poisoning. The concentration of organic exhaust gases to be treated should be in Less than 20%.

2.1.3 TNV Recirculating Thermal Incineration System

Regenerative thermal oxidation system (German Thermal post-combustion, abbreviated as TNV, involves the direct combustion of gas or oil to heat exhaust gases containing organic solvents. Under high-temperature conditions, the organic solvent molecules are oxidized and decomposed into CO2 and water. The resulting high-temperature flue gas then passes through a multi-stage heat-exchange system to preheat the air or hot water required for the production process, thereby fully recovering and utilizing the thermal energy generated during the oxidative decomposition of the organic exhaust gases and reducing the overall energy consumption of the system. Consequently, the TNV system is an efficient and ideal solution for treating exhaust gases containing organic solvents when the production process demands substantial amounts of heat; for newly built coating production lines, a TNV-based heat-recovery thermal oxidation system is typically employed.

The TNV system comprises three main subsystems: an exhaust-gas preheating and incineration system, a recirculating-air heating system, and a fresh-air heat-exchange system. At the heart of the system is the centralized exhaust-gas incineration and heat-generation unit, which consists of a furnace body, a combustion chamber, a heat exchanger, a burner, and a main flue-gas regulating valve. The operating process is as follows: a high-pressure fan draws organic exhaust gases from the drying chamber; the gases are first preheated by the internal heat exchanger of the centralized exhaust-gas incineration and heat-generation unit before entering the combustion chamber. Inside the combustion chamber, the gases are further heated by the burner and undergo oxidative decomposition at a high temperature of approximately 750°C, converting the organic compounds into carbon dioxide and water. The resulting high-temperature flue gases pass through the furnace’s heat exchanger and the main flue-gas duct before being discharged, where they heat the recirculating air in the drying chamber, thereby providing the necessary thermal energy. At the system’s outlet, a fresh-air heat-exchange unit is installed to recover any remaining waste heat, using the flue gases to preheat the fresh air supplied to the drying chamber before it is introduced. In addition, an electrically actuated regulating valve is fitted on the main flue-gas duct to control the flue-gas temperature at the unit’s outlet, ensuring that the final flue-gas discharge temperature is maintained at around 160°C.

The characteristics of the waste gas incineration-based centralized heating system include: the residence time of organic waste gases in the combustion chamber is 1–2 seconds; organic waste gas decomposition rate greater than 99%; heat recovery efficiency up to 76%; burner output modulation ratio up to 26:1, with a maximum of 40:1.

Disadvantages: Operating costs are relatively high when treating low-concentration organic waste gases; tubular heat exchangers only achieve a long service life under continuous operation.

2.2 Treatment Plan for Organic Waste Gases in the Paint Spraying Room and Drying Room

The exhaust gases from paint spray booths and drying rooms are low-concentration, high-flow, ambient-temperature waste gases, with aromatic hydrocarbons, alcohols, ethers, and esters as the primary organic solvent components. Currently, a well-established approach internationally is to first concentrate the organic waste gases to reduce the total volume of organic emissions requiring treatment. This involves using adsorption—employing activated carbon or zeolites as adsorbents—to capture the low-concentration, ambient-temperature paint-spray exhaust, followed by desorption with high-temperature gas. The concentrated waste gases are then treated via catalytic combustion or regenerative thermal oxidation.

2.2.1 Activated Carbon Adsorption–Desorption Purification Unit

Employing honeycomb-shaped activated carbon as the adsorbent, this process integrates adsorption-based purification with desorption-based regeneration and concentration. The principle of VOCs treatment via catalytic combustion involves passing a large volume of low-concentration organic waste gas through honeycomb-shaped activated carbon for adsorption, thereby purifying the air. Once the activated carbon becomes saturated, it is desorbed using hot air to regenerate the carbon; the concentrated organic compounds released during desorption are then sent to a catalytic combustion bed for catalytic oxidation, where they are converted into harmless CO2 and H2O. The resulting hot exhaust gas is subsequently passed through a heat exchanger to preheat cold air; part of the cooled gas is discharged, while the remainder is recirculated to desorb and regenerate the honeycomb activated carbon, thus achieving waste-heat recovery and energy conservation. The complete system comprises a pre-filter, an adsorption bed, a catalytic combustion bed, a flame arrester, as well as associated fans, valves, and other components.

Activated carbon adsorption —The desorption-and-purification unit is designed based on the two fundamental principles of adsorption and catalytic combustion, employing a dual-air-path continuous operation with one catalytic combustion chamber and two adsorption beds that operate in alternating fashion. First, the organic waste gas is adsorbed by activated carbon; when the adsorption bed approaches saturation, adsorption is halted, and a hot gas stream is used to desorb the organic compounds from the activated carbon, thereby regenerating the bed. The desorbed organic compounds are concentrated—increasing in concentration by several dozen times—and then sent to the catalytic combustion chamber for catalytic oxidation into carbon dioxide and water vapor, which are subsequently discharged. When the concentration of the organic waste gas reaches 2,000 ppm or higher, the gas can sustain self-ignition in the catalytic bed without external heating. After combustion, a portion of the exhaust gas is released into the atmosphere, while the majority is routed back to the adsorption bed for activated-carbon regeneration. This approach provides the thermal energy required for both combustion and adsorption, achieving energy savings. Once regenerated, the activated carbon can be reused for the next adsorption cycle; during desorption, purification operations can be carried out using the other adsorption bed, making the system suitable for both continuous and batch operations.

Technical performance and features: stable performance, simple structure, safe and reliable operation, energy efficiency and labor savings, and no secondary pollution. The equipment occupies a small footprint and is lightweight, making it particularly suitable for applications with large air volumes. An activated carbon bed used to adsorb organic waste gases is desorbed and regenerated using the exhaust gas from catalytic combustion; the desorbed gas is then fed back into the catalytic combustion chamber for further purification, eliminating the need for external energy and delivering significant energy-saving benefits. However, the activated carbon has a short service life, resulting in high operating costs.

2.2.2 Zeolite Rotary Adsorption–Desorption Purification Unit

The main components of zeolite are silicon and aluminum, and it possesses adsorption capacity, making it suitable for use as an adsorbent. The zeolite rotary wheel leverages the characteristic of zeolite’s specific pore size to adsorb and desorb organic pollutants, thereby treating streams that originally have low concentrations but large air volumes. VOC exhaust gases are concentrated and converted by a zeolite rotary wheel into a small-volume, high-concentration stream, which can reduce the operating costs of downstream end-of-pipe treatment equipment. The system is particularly well suited for treating large-flow, low-concentration exhaust streams containing multiple organic components. Its main drawback is the high initial capital investment.

Zeolite Rotary Adsorption - A purification unit is a gas purification system capable of continuous adsorption and desorption operations. The zeolite rotor is divided into three zones by specially designed sealing devices on both sides: the adsorption zone, the desorption (regeneration) zone, and the cooling zone. The operating process of this system is as follows: the zeolite rotor rotates continuously at a low speed, cycling through the adsorption zone, the desorption (regeneration) zone, and the cooling zone. As low-concentration, high-volume waste gas continuously passes through the adsorption zone of the rotor, the VOCs in the gas are adsorbed by the zeolite on the rotor, and the purified gas is then directly discharged. The organic solvents adsorbed by the rotor are carried to the desorption (regeneration) zone as the rotor turns; a small flow of heated air is then continuously passed through the desorption zone, causing the VOCs adsorbed on the rotor to be desorbed and regenerated due to the heat, with the VOC-containing exhaust gas being discharged along with the heated air. After the rotor moves into the cooling zone for cooling and temperature reduction, it is ready to resume adsorption. As the rotor continues to rotate, the cycles of adsorption, desorption, and cooling repeat continuously, ensuring the sustained and stable operation of the waste gas treatment process.

The zeolite rotary concentrator is essentially a concentration unit. After treatment by the rotor, the organic-solvent-containing exhaust gas is separated into two streams: clean air that can be directly discharged, and regeneration air with a high concentration of organic solvents. The clean air that can be directly discharged can be recirculated within the paint-spraying HVAC system; the high-concentration The VOC gas concentration is approximately ten times the original inlet concentration. The concentrated gas is then fed into a TNV regenerative thermal oxidation system (or other suitable equipment) for high-temperature incineration. The heat generated during incineration is used to supply thermal energy to the drying chamber and to desorb the adsorbent in the zeolite rotor, ensuring efficient heat recovery and achieving significant energy savings and emission reductions.

Technical performance and features: simple structure, easy maintenance, long service life; high adsorption and desorption efficiency. This process converts VOC exhaust gases with high air volume and low concentration into those with low air volume and high concentration, thereby reducing the cost of downstream end-of-pipe treatment equipment. The pressure drop generated by zeolite rotary adsorption of VOCs is extremely low, significantly lowering electricity consumption. The entire system features pre-assembly and modular design, minimizing space requirements while enabling continuous, unattended operation. After concentration by the rotary wheel, the exhaust gas can meet national emission standards. The adsorbent used is a non-combustible, hydrophobic zeolite, enhancing safety. The main drawback is the relatively high initial capital investment.

3. Conclusion

Reducing coating-related environmental pollution is both a key direction for the development of coating technologies and a fundamental responsibility of the coating industry. As domestic requirements for environmental quality continue to rise, efforts to control volatile organic compound emissions are being progressively implemented. With regard to exhaust gases from automotive painting, it is essential to analyze the treatment effectiveness and costs of various methods in light of their specific characteristics, adopt practical and effective technological solutions, ensure compliance with emission standards, protect the global environment, and foster a harmonious society.