Direct Fired Thermal Desorption Systems
Basic Thermal Desorber Design
Direct fired thermal desorbers consist of an inclined rotating cylinder with a burner injecting an open flame from one end. Materials are fed into the desorber at the inclined end and as the desorber rotates, the materials travel to the discharge point at the lower end. Solids are typically discharged at temperatures of 500 to 800°F. An exhaust fan maintains a negative draft on the desorber, removing vaporized hydrocarbons, combustion gasses, and entrained dust. Contaminants are typically destroyed in downstream organic vapor control devices such as a thermal oxidizer.
Thermal Desorber Operation
Processed materials are discharged at a defined temperature and re-moisturized for reuse. Production rates for direct fired thermal desorbers range from 5 to 120 tons per hour and is dependent upon equipment size, configuration, operating temperature, and moisture content of the feed material.
Thermal desorption History and Status
Direct fired rotary desorbers have been used extensively over the years for petroleum contaminated soils and soils contaminated with RCRA hazardous wastes as defined by the USEPA. A 1992 paper on treating petroleum contaminated soils estimated that between 20 and 30 contractors have 40 to 60 rotary dryer systems available. Today, it is probably closer to 6 to 10 contractors with 15 to 20 portable systems commercially available. The majority of these systems utilize a secondary combustion chamber (afterburner) or catalytic oxidizer to thermally destroy the volatilized organics. A few of these systems also have a quench and scrubber after the oxidizer which allows them to treat soils containing chlorinated organics such as solvents and pesticides. The desorbing cylinder for full scale transportable systems is typically four to ten feet in diameter with heated lengths ranging from twenty to fifty feet. The maximum practical solids temperature for these systems is around 750 to 900°F depending on the material of construction of the cylinder. Total residence time in this type of desorber normally ranges from 3 to 15 minutes.