Description of Direct Thermal Desorption Mobile Plant (DTD)
Mobile unit for direct thermal desorption (DTD)
Whereas indirect thermal desorption (IDT) can be applied to a large range of pollutants in soil, direct thermal desorption (DTD) is especially used for the treatment of soils contaminated with hydrocarbons and/or other pollutants containing volatile compounds.
Hydrocarbons concentration (TPH) in soil should not exceed 10% and moisture 15%, exceeding such limits could lead to considerably low productivity.
Operation, stages, results:
- soil conditioning in order to be introduced in thermal desorption plant – coarse screening for foreign matters separation (stones, roots, wooden pieces etc.), followed by drying until moisture decreases below 15%;
- introducing contaminated soil in the desorber, on a conveyor belt after a fine screening up to a granulation of max. 3 mm;
- in the first stage of the process, in desorber, contaminated soil comes directly into contact with combustion gases, at controlled temperature between 370 and 540oC, water vapors and vapors of contaminated toxic substances from soil being released;
- in the second stage, in oxidator, resulted gaseous phase is heated and oxidized at temperatures of 850 – 870oC, until getting carbon dioxide or other non-hazardous oxides;
- in the third stage, gaseous phase, which drove also solid particles, is cooled up to abt. 77 – 120oC by the help of a water spraying system; by cooling in washing scrubber and then passing through the powercyclone, this phase is separated into a non-hazardous gaseous component which is released in the air and a solid component which is recycled in the oxidator;
- the water used in spraying system is entrapped, cooled and recycled in order to retain the dust and condesable volatile substances removed from contaminated soil;
- the soil decontaminated in the first stage is cooled, checked by specific laboratory analyses and stored for being reused.
The major advantage of direct thermal desorption is a bigger productivity compared with indirect thermal desorption. This advantage is due to both the fact that the soil contaminated comes into direct contact with the combustion gases in the desorber, as well as the fact that the petroleum products from soil maintain combustion, improving the unit efficiency.
The mobile unit, including also its electricity generator, is mounted on a trailer, which enables easy assembly in areas close to contaminated site and implicitly a considerable reduction of costs for contaminated soil transport.
Mobile unit for indirect thermal desorption is equipped with all measure and control devices needed for monitoring and adjusting in real time of flow parameters.