The International Information Center for Geotechnical Engineers

Thermal Desorption - Applicability

Contaminants

Thermal desorption is feasible on a wide variety of materials that are contaminated with many different types of organic contaminants and volatile inorganics. Thermal desorption has been proven effective on soil, sludge, filter cake, and sediments contaminated with petroleum products, volatile and semivolatile organic compounds (VOCs or SVOCs), and pesticides. Generally, any contaminants that are volatile at a temperature at or below 1200 ⁰F can be treated by thermal desorption. Table 1 was generated by the NFESC and illustrates what contaminants have been proven, what contaminants could potentially be removed by thermal desorption, and what contaminants are not expected to be removed from various soils.

 

Table 1: Effectiveness of Thermal Desorption on General Contaminant Groups for Soil, Sludge, Sediments, and Filter Cakes (NFESC, 1998a)

t1

 

Particle Size Distribution

Particle size distribution is important for many remediation technologies, but especially thermal desorption because of the “carry over effect,” i.e., soils that have a high content of fines (particles smaller than 0.075 millimeters or 75 micrometers) are troublesome for thermal desorption because they are small enough to be suspended in the gas stream and can be carried in the exit vapor stream rather than traveling with the bulk of the treated soil. This is problematic because the carry over effect can cause clogging in the treatment system and often impedes the capability of the fines removal system present at the end of the treatment line. Usually, the fines removal system captures fines and returns them to the bulk soil, but if the treatment system is overloaded with fines, some fines might pass through to the atmosphere.

 

Another consideration is that large particles are not desirable for thermal desorption. Generally, particles above 2 inches in diameter are removed before treatment with thermal desorption (NFESC, June 1998). These large particles are removed because they can lead to in insufficient heating due to self-insulation. These large particles have much less surface area per unit mass so there is much less heat transfer between the system and the particle,leading to inefficient volatilization of contaminants. If the particles get larger in size, they might also be a problem for the physical structure of the treatment system (NFESC, 1998b).

 

Composition

It is also important to determine the degree of clay, silt, sand, and gravel of the soil. The ideal composition for treatment via thermal desorption is an unconsolidated sandy soil that may have some gravel. Clays and silts are less ideal because they tend to agglomerate into larger particles which are less susceptible to complete heat transfer and therefore contaminants are not completely volatilized. Agglomeration also occurs in soils with high humic content (FRTR, 2008). Although the high heat volatilizes some organic material, large clumps of soil that have been formed before treatment take much longer to treat than unconsolidated material.

 

To determine the thermal properties of the soil, in situ testing must be used. This can be done by using single and dual heat probes. Single heat probes apply heat continuously at a constant rate, and the temperature of the soil adjacent to the probe is measured. This measures the rate that the heat is conducted by the soil. Dual heat probes uses two prongs, one to transmit the heat and one to receive the response of the soil. The advantage with the dual heat probe is that the distance between the prongs is known, so the thermal diffusivity and volumetric heat capacity can be determined.

 

Plasticity

Plasticity is an indication of how much the soil will agglomerate. Soils that have high plasticity can form clumps and impede heat transfer and volatilization of contaminants. Plasticity also impacts the capability of the soil to stick to other surfaces like the heat transfer of the thermal desorption system. This sticking decreases heat transfer efficiency and also might cause problems with handling.

 

Moisture Content

The moisture content of the soil is very important for thermal desorption. Generally an acceptable range is between 10% and 20% water. Soils that contain less than 10% water may not be heated as efficiently due to the increased heat transfer of the steam. The steam increases thermal efficiency because it can also transfer heat into the contaminated soil.

 

A water content above 20% is detrimental to thermal desorption for two reasons; increased heating requirements for a given soil, and increased material needing treatment by the vapor treatment stream. All the water present in the soil is vaporized because of increased temperatures present in a thermal desorption system and therefore this process takes away some of the heat that could be used to volatilize contaminants. Increased water content may require reducing the feed rate and increasing the temperature therefore increasing cost and treatment time. This steam then increases the strain on the vapor treatment system by increasing volume needing treatment.

 

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