Commonly Used Technologies
for Gas and Vapor Control from SVE Operations (8/22/02)
Control of gas and vapor pollutants from SVE
operations can be carried out in several ways.
Commonly applied technologies for treating gases and vapor include the
following: granular activated carbon (adsorption), thermal oxidation and catalytic oxidation. Each of these treatment techniques are
typically economic over a certain range of concentrations, and some are better
suited to treat some pollutants than others.
Granular activated carbon is a material used to filter harmful chemicals from polluted air or
water. This type of technology is
called adsorption. As polluted water or air flows through an
activated carbon filter, chemicals adsorb
or stick to the surface and within the pores of the carbon. Because activated carbon is very porous and has a
large surface area it is an ideal adsorbent material. Most tap water
filters and fish tank filters at home contain activated carbon and work the
same way. When the available surface of the activated
carbon fills up with chemicals, the carbon is said to be spent and needs to be either replaced or regenerated. Since pollutants are only trapped,
and not destroyed by adsorption, spent adsorbent may need to be managed as a
hazardous waste. If replaced, the spent carbon with the adsorbed chemicals is
disposed at an approved landfill or incinerator. If regenerated, the carbon is normally heated off-site to release
the adsorbed contaminants that are then further treated. GAC is best
suited to sites where the vapor has a consistent flow rate and low
concentration.
Click here for more technical
information on GAC, including a report on its performance at a hazardous waste
site.
Thermal
oxidation is a process that converts organic compounds to
principally carbon dioxide (CO2) and water (H2O). Thermal oxidation is used for the
destruction of a wide variety of organic vapors. In this process, the vapors are heated to high temperatures
(usually in the 1,200 to 1,800 °F range) to oxidize (burn) the organic
compounds. Thermal oxidation is
normally used when the amount and concentration of organic vapors is high
enough so that little or no supplemental fuel is required to maintain the
desired temperature.
The
term "thermal oxidizer"
generally refers to units that do not use a catalyst. Contaminants in the gases or vapors are heated to a temperature
sufficiently high to quickly react with oxygen to form less harmful end
products, e.g., carbon dioxide and water vapor. In the case of treating vapor containing chlorinated solvents, hydrochloric
acid gas is produced which is typically controlled using a scrubber. Thermal oxidation is commonly applied where
it is important to destroy the pollutant molecules, e.g., toxic compounds such
as chlorinated solvents. Design and operating considerations are important
to ensure complete destruction of contaminants, and in the case of treating
chlorinated organics, to prevent the formation of unwanted byproducts such as
dioxins.
Click here for more technical
information on thermal oxidation.
Catalytic
oxidation is a process where a catalyst promotes the oxidation
reaction at lower temperatures.
Different types of catalysts can be used and they are affected
differently by the type and concentration of the contaminant(s) being
treated. Catalytic oxidation units
generally operate between 600 and 950 °F. Catalytic oxidation is normally used
when the amount and concentration of contaminants is fairly low. A small burner operated with natural gas (methane) or propane gas
is commonly used to heat the catalytic units.
Catalytic
oxidizer units modify the thermal oxidizer concept by adding a
catalyst to promote the oxidation reaction, providing a faster reaction rate
and/or reduced reaction temperature.
This allows for a more cost-effective operation at low contaminant
concentrations. A faster reaction
requires a smaller reaction chamber, thus reducing capital costs; and low
operating temperatures generally reduce auxiliary fuel requirements, thus
reducing operating costs. Design and
operating considerations are critical because the catalyst may be adversely
affected by high temperatures and high concentrations of organics. Levels of particulate matter, certain
metals, and halogenated (e.g. chlorine-containing) organics can also impact
performance of a catalyst over time..
Click
here for more technical
information on catalytic oxidation.
For additional information on vapor control
technologies, including biofiltration, click
here.