Exhaust air purification and energy recovery

In companies in the coating industry, e.g. in the powder coatings sector, powder coatings are applied to various materials and then dried at temperatures of approx. 180 to 220°C. This process is known as powder coating. During the drying of the coated components, organic compounds are released and imitated into the ambient air via exhaust air lines. Exhaust air concentrations for total carbon content are in the range of 220mg/m³ at an exhaust air temperature of max. 180°C.

Components can be coated with different materials continuously in process plants or discontinuously in paint booths. The continuous process plants are operated in recirculation mode. In this process, the exhaust air in the coating lines is replaced only proportionally by fresh air, so that the VOC concentration in the process plants increases. Typical VOC concentrations in the exhaust air are around 4000 mg/m³. The vapors are thermally post-treated or incinerated in a subsequent process step.

Discontinuously operated paint shops consist of paint booths. These paint booths are equipped with a separator and a fan, so that the paint exhaust air is discharged directly into the exhaust air duct and thus into the ambient air. Typical VOC concentrations in the exhaust air are around 500mg/m³, which is currently cleaned with scrubbers and/or activated carbon filters. The disadvantage of activated carbon is that the saturation of the activated carbon with exhaust air constituents is difficult to determine.

Different materials such as wood, metals or plastics are coated. The coatings contain either high solvent contents or, in the case of water-based coating agents, low solvent contents.

During coating or in the subsequent drying process, aliphatic solvent components such as butyl glycol or dimethyl formamide or aromatic solvents such as benzyl alcohol or condensation products from polymer formation such as formaldehyde are released into the exhaust air. In addition to VOCs, however, dusts can also be produced that are detectable in the exhaust air. These emissions are evaluated for their VOC content or with odor units.

The pollutants released in the coating industry and in paint stores can be neutralized with UV ozone technology and with a combination process consisting of plasma and UV-C.

UV ozone technology can be used to break down particularly polar organic mono-compounds or complex mixtures of different organic compounds. The oxidative degradation of the organic compounds is based on a radical reaction cascade.

With the combined technology of plasma and UV-C, non-polar organic compounds such as long-chain ester compounds are also degraded. In the first stage, the plasma initiates the radical process. The subsequent UV-C process supports it and increases its effectiveness. As an additional module, a water diffuser is used between the plasma technology and the UV-C technology to optimize reactivity in the downstream UV-C unit. Tests show that VOC contents above 1g/m³ can be degraded in this way.

The heat in the exhaust air can be recovered and then reintroduced into the process.

A prerequisite for the use of these technologies is knowledge of the exhaust air constituents, their content in the exhaust air, and the physical parameters such as relative humidity and exhaust air temperature. Based on these values, highly energy-efficient exhaust air purification systems can be set up and operated.

• The TA Air is complied with
• Energy consumption is relatively low
• Heat from the exhaust air can be recovered and reintroduced into the process.