Quest For Clean Air: Thermal or Catalytic?

Regenerative thermal oxidizers and catalytic oxidizers both destroy volatile organic compounds and hazardous air pollutants, but which one is right for you?

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Concern over the effects of air pollution on the environment is not new. However, increased public awareness of U.S. Environmental Protection Agency (EPA) regulations regarding harmful air emissions such as volatile organic compounds (VOCs) and/or hazardous air pollutants (HAPs), along with an industry-wide long-term commitment to comply, have sharpened the focus on control-technology developments. To meet stringent local, state and federal clean air requirements, companies in a wide variety of industries are installing new emission control systems or replacing existing technology with more modern and efficient equipment. With more than 25 years of proven success, both catalytic and thermal oxidation technologies represent effective methods for controlling VOC and/or HAP emissions.

Catalytic and thermal oxidation systems destroy the harmful VOCs and HAPs emitted by processing operations by collecting the emissions and destroying them using high temperatures and/or an industrial-grade catalyst. Both catalytic and thermal oxidation ensure thorough VOC and/or HAP destruction. Today's modern and efficient systems use high-efficiency heat exchangers, natural-gas-fired burners, industrial-grade blowers, electric or pneumatic actuators and programmable logic controllers (PLCs) to ensure safe, efficient operation.

Regenerative Thermal Oxidizers

Regenerative thermal oxidizers (RTOs) destroy VOCs/HAPs found in process exhaust fumes. They typically can achieve a destruction efficiency of greater than 98 percent. They can achieve a more than 99 percent destruction efficiency through the use of an integrated valve-switch purge containment chamber.

Thermal oxidization promotes a chemical reaction of the VOCs/HAPs with oxygen at elevated temperatures. This reaction destroys the pollutants in the air stream by converting them into carbon dioxide (CO2), water and heat. Three interdependent and critical factors -- time, temperature and turbulence -- control the rate of reaction.

Process exhaust fumes are forced into the RTO inlet manifold (with a high-pressure supply fan) and directed into one of the energy-recovery canisters via inlet control (switching) valves. The VOC/HAP-laden air passes from the valve assembly vertically upward through the first of the two heat-exchanger canisters, where it adsorbs heat from the ceramic media, eventually cooling the media.

This preheated air then enters the combustion chamber, typically at a temperature very close to that required for oxidation. It is mixed thoroughly for temperature uniformity and held in the combustion chamber at elevated temperatures from 1,500°F to 1,800°F for a residence time between 0.3 second (sec.) and 1.0 sec.

VOC/HAP destruction takes place within the combustion chamber; auxiliary fuel is introduced if necessary.

After passing through the combustion chamber, the hot, clean air is routed vertically downward through the second energy-recovery canister, where the ceramic media adsorb the heat generated during thermal oxidation and preheat the media for the next cycle. The cooled, clean air is routed to the atmosphere through outlet control (switching) valves, the exhaust manifold and, ultimately, through the exhaust stack. To maximize the heat exchange, the switching valves alternate the airflow path between the canisters to regenerate continuously the heat stored within the ceramic media. Thermal efficiency ranges from 85 percent to 95 percent. To maintain low external shell temperatures and minimize radiant heat loss, the combustion chamber is insulated with long-life ceramic fiber modules.

Figure 1. Bring on the Heat

RTOs use very high temperatures to promote a chemical reaction of the VOCs/HAPs with oxygen, offering a destruction efficiency of more than 98 percent.

Catalytic oxidizers

Catalytic oxidizers also are designed to destroy VOCs/HAPs from process exhaust fumes. The oxidizers typically can achieve a greater than 98 percent destruction efficiency. A more than 99 percent destruction efficiency is possible with additional catalyst material.

Catalytic oxidation uses an industrial-grade catalyst to promote the chemical reaction at lower temperatures than those required for thermal oxidation. The VOCs/HAPs still are mixed with oxygen and heated to an elevated temperature, destroying the pollutants in the air stream by converting them to CO2, water and heat. The reaction rate is controlled by the temperature in the catalyst chamber and the amount of time the pollutants spend within the catalyst. Catalytic oxidizers usually require less energy to operate than do thermal oxidizers because the technology operates at lower temperatures.

Process exhaust fumes are forced into the catalytic oxidizer inlet plenum (using a high-pressure supply fan) and directed through the "cold" side of a high-efficiency counter-flow plate-type heat exchanger. The VOC/HAP-laden air then enters the combustion chamber, typically at a temperature very close to that required for oxidation. It then is mixed thoroughly for temperature uniformity.

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