For decades, manufacturers around the world have relied on regenerative thermal oxidizer (RTO) systems to control air emissions. Traditionally used for the destruction of volatile organic compounds (VOCs) and odor reduction, RTOs operate at approximately 1,500°F, delivering high thermal efficiency with minimal maintenance requirements. Their ability to destroy a broad spectrum of air pollutants without generating wastewater has made them a mainstay in industrial air pollution control.
However, RTOs—and combustion-based systems in general—do have limitations. One notable challenge is the treatment of halogenated VOCs. When combusted, these compounds can generate hazardous air pollutants (HAPs), which often require additional treatment. Furthermore, the combination of halogenated VOCs and water vapor produced during combustion can lead to corrosive acid formation, posing a serious threat to system longevity. In cases involving smaller airflow volumes or problematic compounds, alternatives such as wet scrubbers or activated carbon filters are often more appropriate.
The PFAS Challenge
More recently, a new class of air pollutants has come under regulatory scrutiny: per- and polyfluoroalkyl substances (PFAS). Often referred to as “forever chemicals” due to their extreme stability and resistance to degradation, PFAS are used across a wide range of industries. Their persistence in both air and water streams has made effective treatment solutions an urgent priority.
Successfully destroying PFAS in an RTO system requires several critical design modifications. Standard RTOs, which operate at ~1,500°F with a residence time of 1 second, are insufficient for breaking the strong carbon-fluorine bonds that characterize PFAS molecules. Many of these compounds have autoignition temperatures exceeding 1,800°F and slower combustion kinetics due to their long-chain molecular structures.
To achieve high PFAS destruction efficiency, RTOs must therefore be designed to operate at approximately 1,800°F with a minimum residence time of 2 seconds. This effectively doubles the required combustion chamber size for the same air volume when compared to a traditional VOC-targeting RTO.
Engineering for Corrosion and Byproducts
Combusting PFAS generates significant quantities of hydrofluoric acid (HF)—a highly corrosive and hazardous byproduct. As a result, PFAS-capable RTOs must be constructed with specialized metallurgy focused on corrosion resistance. Furthermore, the exhaust stream will require additional treatment to neutralize the acid.
This is typically accomplished with a sodium hydroxide (NaOH) wet scrubber, which effectively removes HF from the exhaust. However, before entering the scrubber, the hot RTO exhaust must be cooled, often using a quench section. Given the high acid concentration, elevated temperature, and humidity, these quenches must also be built from high-performance, corrosion-resistant materials.
Non-Combustion Alternatives
While RTOs offer a viable solution for PFAS destruction, non-combustion options also exist—though with trade-offs. Wet scrubbers can capture PFAS due to their water solubility, but removal efficiency varies based on the specific compounds present. More importantly, wet scrubbing does not destroy PFAS; instead, it transfers them into a wastewater stream, which must then be trucked off-site for disposal—a process that is both logistically challenging and costly.
Activated carbon filtration provides another alternative. These systems adsorb PFAS onto the filter media, which similarly must be disposed of, regenerated, or destroyed off-site. Improper disposal, such as landfilling, can lead to leaching and reintroduction of PFAS into the environment, undermining the treatment effort.
Conclusion
As regulations tighten and awareness grows, effective PFAS treatment has become a critical consideration in industrial air pollution control. While RTOs remain a versatile and proven technology, treating PFAS requires careful attention to design parameters, materials of construction, and byproduct handling systems. In certain scenarios, non-combustion technologies may offer advantages—but all options carry trade-offs in terms of efficiency, cost, and waste management.
By understanding these complexities, facilities can make informed decisions about how best to address PFAS in their emissions—and ensure compliance in a rapidly evolving regulatory landscape.
To learn more, contact The CMM Group.