(Thermal, Catalytic, and Ultra-Violet)
Oxidation equipment is used for destroying contaminants in the exhaust gas from air strippers and soil vapor extraction (SVE) systems. There are three primary types of oxidation technologies used: thermal oxidation, catalytic oxidation, and ultra-violet (UV) oxidation. There is also a Flameless Thermal Oxidizer, which is described as a separate technology.
1. Thermal Oxidation. Thermal oxidation units are typically single chambered, equipped with a propane or natural gas burner and a stack. Air containing the organic vapors is heated to a temperature that oxidizes the compounds.
2. Catalytic Oxidation. Catalytic destruction devices are similar to the pollution control device on automobiles. They use a metal catalyst, commonly platinum or palladium, to oxidize the contaminants at lower temperatures. Lower combustion temperatures decrease the amount of nitrous oxides (NOx) that are produced. NOx is a major component of air pollution. The addition of a catalyst to the basic thermal oxidation configuration accelerates the rate of oxidation by sorbing the oxygen from the air stream and the contaminant vapor onto the catalyst surface where they react to form carbon dioxide, water, and hydrochloric gas.
3. UV Oxidation. UV oxidation, or photo-oxidation, destroys organic contaminants in vapors by exposing them to high intensity UV light, breaking their chemical bonds.
Organic contaminants in air can also be used as fuel and burned in an internal combustion engine. When the concentration of organics is too low, auxiliary fuel is added to enhance the oxidation.
Limitations and Concerns
For both catalytic and thermal oxidation, if chlorinated compounds are in the contaminant mix, there is a concern that either incomplete combustion or other chemical processes will lead to the formation of dioxins and furans. These substances may be toxic in the parts per trillion range. Therefore, a treatability study should be performed prior to implementation of the technology. Additionally, continuous emission monitoring is desirable.
With catalytic oxidation, the catalyst can be poisoned (i.e., deactivated) by emissions containing sulfur, halogenated compounds or some metals, such as lead. Destruction of halogenated compounds requires special catalysts and the addition of a flue-gas scrubber to reduce acid gas emissions.
To avoid the danger of explosion when using catalytic or thermal oxidation, gas concentrations must be less than 25% of the lower explosive limit.
UV oxidation systems often use hydrogen peroxide. Storing large quantities of this substance presents a safety risk.
The target contaminant groups for thermal and catalytic oxidation are non-chlorinated volatile organic compounds (VOCs), semi- volatile organic compounds (SVOCs), and fuel hydrocarbon vapors in off-gases. UV oxidation handles these, as well as other chlorinated compounds.
Technology Development Status
Thermal and catalytic oxidation technologies are commercially available. Catalysts have been developed that are reportedly capable of destroying chlorinated hydrocarbons. Complete conversion of organic contaminants to carbon dioxide, water and possibly chlorine has been difficult to achieve using the UV process. Hence, UV oxidation is still being field tested.
Other Resources and Demonstrations
See http://www.clu-in.org/download/remed/EPA542R05028.pdf, ÒOff-Gas Treatment Technologies for Soil Vapor Extraction Systems: State of the Practice,Ó EPA, 2006.