There are basically four types of thermal oxidizer. Direct fired thermal oxidizer, recuperative thermal oxidizer, regenerative thermal oxidizer (RTO), and flameless thermal oxidizer (FTO). All of these thermal oxidizers operate on the same principle of thermally promoting an oxidation reaction between the pollutant compound in the exhaust gas and the available oxygen.
The flameless thermal oxidizer (FTO) is typically built as a reactor vessel in which external heat is applied, as in a furnace-style design, through either the combustion of supplemental fuel or electrical coil heating. The typical design temperature for a FTO reactor is between 1,200 and 2,000 degrees Fahrenheit, which is selected based on contaminant loading rate, residence time, and the thermodynamic properties of the chemical stream (EPA, 2006).
Conversely, a catalytic oxidizer utilizes a specific chemical catalyst screen, which is designed to react with the excited particles of gas moving across it. As a result of the chemical reaction enhancing the oxidation of the contaminants, reactor temperatures are typically lower, in the range of 500 to 900 degrees Fahrenheit. However, this feature also limits the variety of chemicals that may be treated by the unit, once constructed. Catalysts are chosen specifically for one type of reaction chemistry or family of contaminants. If additional chemicals are detected in the SVE off-gas stream, adjustments or a complete re-design of the system may be required.
The ideal design of an oxidizing unit involves a residence time for SVE off-gas flow of approximately 2 seconds, as it flows through the reactor vessel, and makes use of both the exothermic heat of oxidation, as well as the surplus heat of the furnace to pre-heat the incoming air stream (Cooper, 1994).
The most significant limitation of this technology is, again, the use of supplemental fuel, and it’s availability and cost. Similarly significant are the metallurgy requirements, due to the formation of hydrochloric acid as a result of the oxidation of chlorinated compounds.
Thermal oxidation (or incineration) can be an effective remediation technology under the correct site and operating conditions. Generally, thermal and catalytic oxidation are employed for incineration / destruction of petroleum hydrocarbon and compounds with attributable amounts of British Thermal Units (BTU). When considering oxidation for petroleum hydrocarbon applications, the selection of either thermal or catalytic depends on the type and concentration in parts per million by volume of constituents in the vapor stream or alternatively the BTU value of the target vapors. Thermal oxidation is more useful for higher concentration (~4,000 ppmV) influent vapor streams (which require less natural gas usage) than catalytic oxidation at ~2,000 ppmV.