Soil treatment by conventional thermal desorption

Conventional thermal desorption is an ex situ physical technique for treating polluted soils. It involves heating the excavated polluted soil in a rotary kiln, in order to vaporize the pollutants and physically separate them from the matrix. The matrix is then cooled by mixing with water and recycled. The resulting gases containing the pollutants are treated in a plant, where they are oxidized, filtered, neutralized and sometimes even adsorbed before being released into the atmosphere, in compliance with regulations.

The main advantage of the technique lies in its robustness and its application to any type of soil and any type of organic pollutant, including mixtures. Heating ensures that very low or even non-detectable residual levels of the pollutants concerned are reached, enabling the soil to be reused to a very large extent and the pollution to be eliminated once and for all.

The main drawback of this technology, apart from its sometimes high cost due to high energy consumption, is its difficulty of acceptance due to the size of the plants, their continuous operation and the nuisance it can cause during the actual treatment.

Two types of plant are used: fixed plants in industrial environments, where nuisances are well controlled, operating like a waste treatment center, and mobile plants. The latter are set up on industrial sites to be remediated, generally large-scale and far from the population, where the quantities of soil to be treated are significant (several tens or even hundreds of thousands of tonnes) and where the treated soil is returned to the place from which it was excavated.

The main problems covered by conventional thermal desorption are related to coking and gas plants, as well as large-scale chemical sites. The oil industry also makes extensive use of this technique, which was originally developed for refineries, depots and service stations. Today, conventional thermal desorption is mainly applied to oil sites (crude oil), petrochemical sites (refineries), coking and gas plants (coal industry) and large chemical complexes (fertilizers, explosives, pesticides).

The aim of this article is to present the basics of thermal desorption, describe the different types of rotary kilns and gas treatment systems, and outline the limits of use and application for each in relation to the types of soil and pollutants that can be treated.