Thermal Desorber Off Gas Systems
Regardless of the type of thermal desorber used, its purpose is to separate organic compounds from some media. Once the compounds are vaporized, they are removed from the desorber via off gas handling equipment. There are only three basic options available: discharge to atmosphere, collection, and destruction. In addition to managing the organics, particulate solids (dust) that exit the desorber must also be removed from the off gas.
Collection systems are typically used with indirect fired thermal desorption units. When a collection system is used, the off gas from the desorber is cooled to somewhere between 120 and 40°F to condense the bulk of the volatilized water and organic contaminants into a liquid. Even at 40°F, there may be measurable amounts of non-condensed organics remaining in the gas and require further treatment by carbon adsorption, or thermal oxidation. The condensed liquid from cooling the off gas 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, or recycled as a supplemental fuel
Destruction systems are typically used with direct fired thermal desorption units, but can also be found on some indirect fired equipment. Desorbers using off gas destruction systems use combustion to thermally destroy the volatilized organics components forming CO, CO2, NOx, and SOx. For contaminants containing chlorine, the destruction unit will cause the formation of HCL and require the addition of a quench and acid gas scrubber. Destruction units may be referred to as 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 from the soil or waste.