Vapor-Phase Oxidation
(Thermal, Catalytic,
and Ultra-Violet)
Description
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.
Applicability
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.
Web
Links
http://www.frtr.gov/matrix2/section4/4-59.html
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.
See
the technology descriptions of the Flameless Thermal Oxidizer
and Ultra-Violet
Oxidation.