text: There are only three basic options for offgas treatment available. The volatilized contaminants in the offgas can either be discharged to atmosphere, collected or destroyed. In some cases, both a collection and destruction system are employed. In addition to managing the volatilized components, the particulate solids (dust) that exit the desorber must also be removed from the offgas.
  When a collection system is used, the offgas must be cooled to condense the bulk of the volatilized components into a liquid. The offgas will exit most desorbers in the 350–900 °F range. The offgas is then typically cooled to somewhere between 120 and 40 °F to condense the bulk of the volatilized water and organic contaminants. Even at 40 °F, there may be measurable amounts of non-condensed organics. For this reason, after the condensation step, further treatment of the offgas is usually required. The cooled offgas may be treated by carbon adsorption, or thermal oxidation. Thermal oxidation can be accomplished using a catalytic oxidizer, an afterburner or by routing the offgas to the combustion heat source for the desorber. The volume of gas requiring treatment for indirect fired desorbers is a fraction of that required for a direct fired desorber. This requires smaller air pollution control trains for the gaseous process vent emissions. Some thermal desorption systems recycle the carrier gas, thereby further reducing the volume of gaseous emissions.
  The condensed liquid from cooling the offgas is separated into organic and aqueous fractions.  The water is either disposed of or used to cool the treated solids and prevent dusting. The condensed liquid organic is removed from the site.  Depending on its composition, the liquid is either recycled as a supplemental fuel or destroyed in a fixed base incinerator.  A thermal desorber removing 500 mg/kg of organic contaminants from 20,000 tons of soil will produce less than  of liquid organic.  In essence 20,000 tons of contaminated soil could be reduced to less than one tank truck of extracted liquid residue for off-site disposal.
  Desorbers using offgas destruction systems use combustion to thermally destroy the volatilized organics components forming CO, CO, NOx, SOx and HCl. The destruction unit may be called an afterburner, secondary combustion chamber, or thermal oxidizer. Catalytic oxidizers may also be used if the organic halide content of the contaminated media is low enough. Regardless of the name, the destruction unit is used to thermally destroy the hazardous organic constituents that were removed (volatilized) from the soil or waste.