BE1028861B1 - Combination of thermal desorption and in situ chemical treatment - Google Patents

Abstract

The present invention aims to extend the field of application of treatment by thermal desorption, in particular, to the treatment of the saturated zone of the soil and to the treatment of underground water. It consists of a coupling: thermal desorption and chemical treatment (in-situ and ex-situ). This coupling appears to be a promising means of reducing the costs of thermal desorption treatment, its negative effects on the environment and the duration of conventional chemical treatment. The coupling is carried out thanks to a system (or an installation) which adapts as well to an in-situ treatment as to an ex-situ treatment. The installation is made up of two circuits: a heating circuit and a thermoactive substance injection circuit. The thermoactive chemical substance can be an oxidant or a reducing agent, which reacts with the pollutant to give less harmful or biodegradable species. The injection of the thermoactive substance can be carried out at the start of the heating or during the cooling phase of the ground (therefore after treatment by thermal desorption). In ex-situ mode (or thermal battery), the thermoactive substance can be mixed in solid or liquid form with the contaminated land before assembly of the battery or injected, during the heating phase, through perforated tubes placed above stack and/or through horizontal tubes.

Description

1 a Combination thermal desorption and in situ chemical treatment BE2020/0128

DESCRIPTION FIELD OF THE INVENTION The present invention aims to extend the field of application of treatment by thermal desorption, in particular, to the treatment of the saturated zone of the soil and to the treatment of groundwater. It consists of a coupling: thermal desorption and chemical treatment (in-situ and ex-situ). This coupling is a promising way to reduce the costs of thermal desorption treatment, its negative effects on the environment and the duration of a conventional chemical treatment. I BACKGROUND OF THE INVENTION The environment is one of the sectors on which technology focuses the most nowadays to ensure human well-being because its alteration by toxic substances gives rise to a good number of respiratory pathologies. , cardiovascular or allergies and cancers. Many human activities (agricultural, industrial, urban, etc.) are the cause of this pollution. The fight against environmental pollution is therefore a major challenge. This pollution exists in various forms and can thus degrade our environment in different ways. Various soil and groundwater (or groundwater) depollution techniques have been developed nowadays to combat environmental pollution. There are 4 main families of processes, namely chemical (by the use of reagents; chemical to reduce/eliminate the toxicity of pollutants), physical (by the use of fluids to transport pollutants), thermal (by the use of heat to vaporize, incinerate or vitrify pollutants) or biological (using micro-organisms to reduce/eliminate pollutants). A 5th family which is phytoremediation (using plants to fix pollutants) is beginning to emerge. The choice of technique depends on several parameters such as the nature of the contamination, the characteristics of the soil, the physical — constraints of the site and the total cost of the project. Although finding real success in “many applications and following their modes of application, they are very often expensive, energy-intensive, long in treatment, and difficult to implement depending on access to . site or the composition of the soil. Moreover, before beginning any depollution actions on a site, it is essential to have knowledge of the contaminants involved and their — main characteristics because the behavior of a pollutant in a soil depends so much on its physico- chemicals than those of the soil. Among the major pollutants, polycyclic aromatic hydrocarbons (PAH), BTEX (Benzene, Toluene, Ethyibenzene and Xylene) and petroleum hydrocarbons are priority compounds. They are the main pollutants of soils and groundwater. None of the techniques cited above 40 allows simultaneous treatment of the saturated zone (or groundwater) and the unsaturated zone (or vadose zone) of the soil. It is therefore with a view to meeting this new challenge that this invention is part of: coupling of a treatment by thermal desorption and a treatment by oxidation or chemical reduction. | Thermal desorption is one of the thermal techniques used to treat polluted soil. It is based on heating the ground to volatilize the contaminants and allow their extraction and destruction/reuse after condensation. Thermal desorption is effective in the unsaturated zone of polluted soil, against organic contaminants, cyanides, mercury and any other component that can be volatilized at temperatures below 550°C. Heating via thermal conduction is one of the techniques used in the field of thermal desorption. With this technique, the energy coming from heating tubes propagates radially in the ground by conduction regardless of the heterogeneity of the ground. This technique is applicable in ex situ mode and in situ mode. In ex-situ thermal desorption, excavated soil is used to form piles called “thermal piles” or placed in containers that are heat treated. With in-situ thermal desorption, heating elements (generally metal tubes) are directly inserted into the polluted soil, thus avoiding excavation and transport of soil.

{ Chemical treatment is a technique used mainly for the treatment of the saturated zone of a soil (underground water or water table). This technique is done by injecting a chemical reagent directly In-situ to stabilize the pollutants or to transform them into . 25 less harmful or biodegradable species. The combination of two techniques: thermal desorption and chemical treatment, requires the use of a thermoactive substance, i.e. a substance which, once heated to a temperature above ambient temperature, becomes active and leads to the degradation of the pollutant. As an example of a thermoactive substance, persulfate (5.037) in its three forms: sodium persulfate, potassium persulfate and ammonium persulfate. Sodium persulfate is the salt commonly used for the chemical treatment of soils and groundwater. The persulfate salt is a solid compound in the form of white crystals that is very soluble in water (545 g/L at 20°C), which has a huge advantage when used as a base product for the oxidation of the pollutants present in soils and groundwater. It dissociates in water to produce persulfate anions (S.027), which have a strong oxidizing power (E° = 2.01 V). It is very effective for the chemical oxidation of organic pollutants. Its action is all the better as the degradation takes place by radical means. For this, its SO7' radicals are activated, which have an oxidizing power (E° = 2.6 V) greater than 5.02. The best persulfate activator is still heat. The activation of the 40 persulfate by heat takes place relatively well and increases with temperature.

. S BE2020/0128 Its half-life time allows us to deduce an optimal temperature range between 60° and 70°C. Indeed past 70°C, it only has 1 to 2 hours as a half-life and thus becomes difficult to set up in a field because it risks being completely consumed before. to reach the target area. And below 60°C it is quite the opposite, it has much too long half-lives which will have an impact on the processing time of the sites.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is an illustration of the complete system with the floor heating unit comprising the heating tube, the burner, the dual purpose tube (vapor collection and injection of the product), the injection pump under pressure of the product (thermoactive substance), the filter medium (gravel or sand), the bentonite plug and the layer of concrete. Figure 2 is an illustration of a possible implementation of the system in in-situ mode. Figure 3 is an illustration of an implementation of the system in groundwater for pollution plume treatment. , Figure 4 is an illustration of a possible implementation of the system in ex-situ mode (thermal stack).

LEGEND OF FIGURES . ;

1. Heating tube

2. Dual purpose tube (vapour collection/thermoactive substance injection)

3. Holes. | | 25 4. Thermoactive substance injection pump

5. Wells

6. Filter media (gravel/sand)

7. Burner

8. Impermeable media (bentonite)

9. Concrete |

10. Groundwater (or water table)

11. Direction of water flow | ;

12. Thermal stack

13. Thermoactive substance injection tube | 35 14. Product supply circuit

15. Contaminated soil DETAILED DESCRIPTION OF THE INVENTION The invention is a new method for decontaminating soil and/or groundwater via coupling thermal desorption and chemical treatment. Said method was developed with the primary objective of reducing the energy consumption (and therefore reducing the cost) of a conventional heat treatment, but also and above all the reduction of its negative effects on the environment. This treatment method also widens the field of application of thermal desorption, it allows for example the treatment of the source zone of pollution and its plume in groundwater. ; To do this, the coupling is carried out thanks to a system (or an installation) which adapts as well to an in-situ treatment as to an ex-situ treatment.

The installation is made up of two circuits: a heating circuit and a thermoactive substance injection circuit. The two circuits are independent. The heating circuit includes heating elements (Figure 1). Each heating element is made up of a metal heating tube (1) and a burner (7) or an electric heating element. In in-situ mode of use, each tube is placed in a well (5) dug to the depth of the pollution and containing, in addition to the heating tube, a filter medium (6) and a stopper (8). The burner (7) uses a gaseous or liquid fuel. It makes it possible to heat the metal tube (1) to a temperature between 50°C and 400°C. The injection circuit comprises metal tubes (2 and 13) perforated on their lower parts (3). Each perforated tube (2 and 13) is connected, at its end, to a distribution network (14). The distribution of the thermoactive substance is carried out thanks to one or more pumps (4). In a preferred embodiment, additional perforated tubes (2) are used and are placed in independent wells. The distribution of wells, in the area to be treated, can be triangular (figure 2) or square with interdistances between 1 and 20m.

In a preferred embodiment, the installation in the water table (or in the groundwater) “(10) is carried out upstream of the source zone of pollution and its plume and according to the direction of flow (11) of the waters (Figure 3). In the latter case, the distance between wells is reduced to less than 5m to better distribute the heat and the thermoactive substance.

‚30 In a preferred embodiment, the perforated tubes are made of stainless steel or PTFE (Teflon), cylindrical or rectangular in shape and whose diameter varies between 1/4” and 2”. In one - preferred embodiment, the perforations on the injection tube are circular or rectangular in size between 0.1 and 15 mm and whose distribution along the tube can be uniform or limited to a part of the tube. In a preferred embodiment, certain perforated tubes can be used during heating, periodically or continuously, to collect the vapors produced in the ground (15).

In a preferred embodiment, the filter medium (6) is sand or gravel whose particle size varies between 3 and 35mm. In a preferred embodiment, the well plug (8) is made of bentonite, refractory concrete or any other heat-resistant material, the thickness of which varies between 15 and 50 cm. In a preferred embodiment, a layer of concrete (9) 5 to 30 cm thick is used to cover the area to be treated. | |

In a preferred embodiment, the thermoactive substance distribution network is made of plastic material.

It is equipped with one or more product storage tanks (thermoactive substance) and one or more well supply pumps (4).

In a preferred embodiment, the filling of the wells is done under pressure (between 0.1 and 10 bars) or by the simple effect of gravity.

In a preferred embodiment, injection of the thermoactive substance begins when the ground in the area to be treated reaches the target temperature; minimum activation temperature of the thermoactive substance.

In a preferred embodiment, the thermoactive chemical substance can be an oxidant or a reducing agent, which reacts with the pollutant to give less harmful or biodegradable species.

In a preferred embodiment, the inner tube and the burner (7) are replaced by an electrical resistance heater.

In a preferred embodiment method in ex-situ mode (or thermal battery), the thermoactive substance, in solid or liquid form, is mixed with the contaminated earth (15) before assembly of the battery.

In a preferred embodiment in thermal stack mode, the injection of the thermoactive substance takes place through perforated tubes (13) placed above the stack and/or through the horizontal tubes (2) (FIG. 4 ). In a preferred embodiment, a layer of concrete (9) 5 to 30 cm thick is used to cover the thermal stack.

Claims (9)

CLAIMS 7 1. A system used in the depollution of soil and groundwater (or groundwater) comprising: - Heating elements distributed in the area to be treated at the level of the unsaturated zone and/or also in the groundwater according to a well-designed grid that allows you to evenly heat the floor to the desired objective temperature. - Perforated tubes distributed in the area to be treated at the level of the unsaturated zone and/or also in the groundwater according to a well-designed grid and which makes it possible to distribute the chemical substance effectively. - A filter medium is used in the wells containing the tubes, with a bentonite plug and a layer of concrete on top. - A circuit dedicated to the injection of the chemical substance comprising a network of tubes in stainless steel or any other material resistant to the substance and pumps. - A thermoactive chemical substance: when the substance heats up to a predetermined temperature, it reacts with the pollutant. The reaction products are less harmful species than the reactants or biodegradable. - A thermoactive substance which can be an oxidizing or reducing chemical compound. - A chemical substance that heats thanks to the energy provided by the heating elements. . 2. An installation as in claim 1, where the injection of the thermoactive substance takes place under pressure using pumps or by the simple effect of gravity. NOT 3. An installation as in claims 1 to 2, in which the perforated tubes are placed close to the heating elements or at a distance "d" from the latter. 4. An installation as in claims 1 to 3, wherein the thermoactive substance is injected continuously or intermittently. 5. An installation as in claims 1 to 4, in which the thermoactive substance is injected at the start or at the end of a thermal desorption treatment. 6. An installation as in claims 1 to 5, which can be used for in-situ or ex-situ treatment (thermal battery). 7. An installation as in claims 1 to 6, which can be used in situ mode in the groundwater upstream of the source zone of pollution or its plume depending on the direction of the groundwater. 8. An installation as in claims 1 to 7, which can be used in ex-situ mode (or thermal cell), where the thermoactive substance is mixed in solid or liquid form with the contaminated earth before assembly of the cell. 7; BE2020/0128 9. An installation as in claims 1 to 8, which can be used in ex-situ mode (or thermal pile), where the injection of the thermoactive substance is done through perforated tubes placed above the pile and /or through the horizontal tubes (figure 4).