As shown in the section on flares below, concern for flame or hot gas radiation is a design consideration. For this reason and for other reasons such as energy efficiency and cost, thermal energy recovery is important. In a recuperative thermal oxidizer, the combustion chamber is followed by a recuperative plate or tube-type heat exchanger to preheat the incoming waste stream with the flue gas from the combustion chamber. Recuperative oxidizers are generally metallized in structure and temperature limited. The initial cost is lower than for other types, but maintenance costs are high.
Recuperative units are especially applicable to smaller flows and intermittent operations.
They are relatively simple to design and build. In a catalytic oxidizer, waste gas constituents are oxidized on a catalyst surface rather than by direct combustion. Catalysts are usually composed of a rare metal on a metallic or ceramic honeycomb. A heat exchanger can also be used to improve efficiency. While the catalyst is expensive, the combustor is smaller.