DESORPTION

DESCRIPTION

FIELD OF THE INVENTION

The invention consists of a stand-alone heating system (or unit) equipped with indirect tracer sensors that can be used in the field of soil remediation by thermal desorption using several heating systems. The sensors, coupled with system modelling to anticipate remediation needs, allow for better monitoring of the treatment compared to current systems and thus reduce treatment times and costs. The remote control of the heating system takes into account this additional information and each heating unit is controlled according to the measurements of its direct environment, making it possible to carry out an effective treatment notwithstanding the heterogeneity of the medium to be treated.

In a second aspect, the present invention relates to a method for modelling soil heating. In particular, the invention relates to an approach for modelling polluted soils in thermal desorption treatment. Similarly, the present invention can also be used for modelling soil heating in any type of application (sterilisation of soils contaminated by pathogenic micro organisms, treatment of invasive plants by heating, etc.).

BACKGROUND

Soil contamination is an issue of great importance in a world where the environment and sustainable development are becoming increasingly important. This often- invisible problem can be caused by a wide variety of chemical, biological or even radioactive contaminants and an equally wide range of pollution sources. Left unchecked, contamination can spread and end up in other resources that are essential to the surrounding fauna and flora. It is therefore important, in the interests of environmental protection and public health, to remove these contaminants before they have too great an impact.

Soil remediation technologies are multiple and can be separated into three main categories: thermal, biological and physicochemical. The choice of technique depends on several parameters such as the nature of the contamination, the soil properties, the physical constraints of the site and the total cost of the project. One such technique, thermal desorption, is based on heating the soil to volatilize the contaminants and allow them to be extracted and destroyed or reused after recovery (for example, by simple condensation). Thermal desorption is effective against organic contaminants, cyanides, mercury and any other component that can be volatilised at temperatures below 550°C.

Heating via thermal conduction is one of the techniques used in the field of thermal desorption (W02001078914A8). With this technique, energy from heating tubes is propagated radially through the soil by conduction. This has several advantages over other soil remediation options because thermal conduction allows soil to be heated to temperatures in excess of 350°C (which is not possible with, for example, resistive electrical heating (US5656239A)) and to easily and quickly treat soils contaminated with a wide variety of contaminants, regardless of soil heterogeneity. Indeed, thermal conductivity has the particularity of not fluctuating by large orders of magnitude with soil composition. As a result, thermal conduction is much more efficient than other heat transfer methods in the case of heterogeneous soils.

This technique is applicable ex-situ and in-situ. With ex-situ thermal desorption, the excavated soil is used to form piles or placed in containers that are thermally treated. With in-situ thermal desorption, heating tubes are inserted directly into the polluted soil, thus avoiding excavation and transport of soil. This also allows the treatment of soils in restricted areas and/or with limited access such as remote sites, sites in urban areas, basements of houses, etc. In general, this technique is faster and has a reduced environmental impact.

Current thermal desorption techniques rely exclusively on measuring the temperature of the soil at its coldest point to estimate the progress of the remediation process. Once the contaminant volatilization temperature is reached, if it is maintained for several days and sufficient suction is present in the soil, the contaminants will be removed with near certainty.

However, this conservative approach ignores all the mechanisms that can take place during heating and accelerate remediation. For example, steam stripping occurs when water vapour vaporised at the start of heating carries some of the contaminant out of the soil. A thorough and continuous analysis of the characteristics of the soil and of the vapours generated allows these mechanisms to be taken into account. In this way, heating can be stopped early, resulting in a reduction in fuel consumption, a reduction in total treatment time and ultimately a reduction in treatment cost.

The heating systems that include the heating tubes described above autonomously generate the thermal power transmitted to the medium to be treated. This thermal power set point can be chosen and adapted remotely and independently for each heating point in order to take into account the heterogeneity of the medium to be treated.

In a second aspect, the present invention consists of modelling the thermal treatment of soils using modelling software such as, in a preferred embodiment, ANSYS FLUENT software. Previously, only very basic tools were used to understand the transport phenomena in the soils to be treated, making it impossible to accurately determine the process control parameters, leading to the use of large safety factors to ensure that the remediation objectives were met. These safety margins are a source of high energy and time losses that affect the competitiveness of the technology. Modelling is therefore essential to better understand the phenomena that take place in the ground during a thermal treatment. Modelling downstream of a job site also helps to optimise designs and to provide operational data to the site teams in order to control the treatment in the most optimal way according to the site conditions. Additional data from indirect tracer sensors is used to refine the models, and the output of said models is subsequentially used to remotely control the heating devices via dedicated hardware and software.

DESCRIPTION OF THE FIGURES

Figure 1 is an illustration of the complete heating system, including the control and analysis boxes.

Figure 2 is an illustration of several heating systems used for the treatment of contaminated soil

Figure 3 is an illustration of the installation of different sensors used in the measurement box.

A list of the numbered elements:

1. Heating tube

2. Steam tube

3. Vapour analysis box

4. Soil analysis box

5. Measuring tube

6. Control box

7. Antenna 8. Environment to be treated

9. VTU valve

10. Return valve

11. Cross

12. Flexible

13. Cable gland

14. Acquisition circuit board

15. Vapour Analysis Sensors

16. Waterproof tube

17. Power socket

DETAILED DESCRIPTION OF THE INVENTION

The invention is the addition of a coherent and co-ordinated set of sensors allowing advanced analysis of the process in real time, enabling instantaneous assessment of the progress of the treatment, as well as a new control box that independently pilots each heating system according to a remotely defined set point.

The sensors are used to carry out so-called indirect measurements insofar as they are not a simple immediate measurement of the quantity to be regulated. Indeed, it is rather indirect tracers that are analysed and are not directly linked to the power deployed at a given moment by the heating device.

The sensor data is fed into a model of the whole pollution control process to obtain predictions of the progress of the process. The model used takes into account the prediction errors between the expected results and the reality, in order to refine and improve the quality of the modelling on a permanent basis.

The invention therefore consists of a heating system with real time modulation of power coupled with anticipation of pollution control via a constantly evolving model. In a manner similar to the prior art, a heating tube (1) is inserted into the medium to be treated (8). In a preferred embodiment, this heating tube (1) contains circulating air that can reach a temperature of between 500 and 1900°C, making it possible to generate between 3 and 80 kW of thermal power to be transferred to the medium (8) by thermal conduction.

The vapours generated by the heating are collected by a tube called a steam tube (2), placed next to the heating tube (1). In a preferred embodiment, the vapours are fed from the steam tube (2) to a cross (11) via a hose (12).

To the cross is connected the vapour analysis box (3). This box contains all the electronic equipment necessary to analyse the vapours coming from the ground. In a preferred embodiment, the following non-exhaustive list of quantities is measured: CO concentration, flammable gas concentration, volatile organic compounds concentration, hydrocarbon concentration, moisture content of the vapours. Additional measurement variables can be added depending on the type of contaminant being processed. The unit is capable of wired or wireless communication with the control unit (6). It is also possible to integrate the electronics directly into the control box (6), as the sensors are always in contact with the vapours.

In a preferred application, the cross is connected to two valves: the return valve (10) and the VTU valve (9). In case the contaminant can be used as fuel, the return valve (10) is opened to return the vapours to the heating circuit. If the contaminant is not usable as fuel, the VTU valve (9) is opened so that the vapours are directed to appropriate treatment units. In all cases, the vapours are analysed by the vapour analysis unit (3).

In a preferred embodiment, the soil analysis box (4) comprises all the electronic equipment necessary for the continuous analysis of the properties of the medium to be treated. In a preferred mode of application, the following non-exhaustive list of quantities is measured: resistivity of the medium, pH of the medium, humidity of the medium. It is possible to add other measurement variables depending on the application. The unit is capable of wired or wireless communication with the control unit (6). It is also possible to integrate the electronics directly into the control box (6), as the sensors are always brought into contact with the medium. The sensors can be brought into contact with the soil via a measuring tube (5) inserted into the soil.

In a preferred embodiment, the vapour analysis housing consists of a sealed tube (16) extending through the housing. The vapour analysis sensors (15) are inserted into this tube and can be connected to the acquisition circuit board (14) by removable connectors for ease of assembly and maintenance. Cable glands (13) are used to seal external connections, including the power supply socket (17). In contexts where electrical power is limited, the system can also operate on batteries (rechargeable or not).

The control unit (6) contains all the electronic equipment necessary to collect data from the vapour (3) and soil (4) analysis units. Communication between the different units can be wired or wireless using standard or proprietary communication protocols. It is also possible to concentrate the electronic vapour and soil analysis equipment in the control unit (6). The control box (6) also includes electronic equipment for controlling the heating. In a preferred embodiment, in which the heating tube circulates air heated by the combustion of natural gas, the control box must control, in particular, the combustion air flow rates, the natural gas flow rate and the injection pressure.

In order to be able to be controlled remotely, the control unit (6) is equipped with a wired or wireless communication protocol.

In a preferred embodiment, the analysis units (3, 4) as well as the control unit (6) communicate via a local network such as Bluetooth, Wi-Fi or LoRa. Other devices on the same network allow the operators on site to have direct access to the data and allow fine, real-time monitoring of the treatment, even in the event of an Internet connection failure (which can frequently happen in the isolated locations in which depollution sites sometimes take place). A gateway is used to connect the local network to the Internet and centralise the data to a dedicated server for analysis. Once the data has been examined and interpreted (by specialised algorithms or by human intervention), heating instructions are generated and transmitted back to the control units. This process of data export, analysis, setpoint and then regulation can be carried out entirely autonomously.

This communication with the control system can be carried out via a local area network (WLAN) and possibly connected to the Internet via a gateway.

In a preferred embodiment, the control (6), vapour analysis (3) and soil (4) boxes are designed to be deployable outdoors. This implies that they are resistant to moisture and dust. For example, an IP56 rating against solids and liquids (international standard as defined by European standard EN 60529) is recommended, meaning effective protection against dust, microscopic residues and strong jets of water from all directions. Thermal insulation is also required to ensure the proper functioning of the electronics in the vicinity of the heating systems. In order to increase the communication range, it may be necessary to add an antenna (7) outside the enclosure. This antenna is systematically necessary if the enclosure is made of metallic material.

In an embodiment, the modelling system uses a method for the modelling of heat transfer wherein the evaporation of water present in a porous medium is solved by a fixed term of the energy equation. By using a model which takes the evaporation of water into account, a better anticipation of the requirements regarding the treatment are obtained. This allows for a better management or follow-up of the treatment. By using a model which takes the evaporation of water into account, a better, more accurate and more optimal estimation of the required heat can be anticipated, and thus improving the treatment method.

The invention presents a method for modelling the heating of soils, in particular soils being cleaned up by thermal desorption.

In a preferred embodiment, the modelling takes into account soil properties due to the presence of moisture and pollutants in the soil and includes and quantifies physico-chemical phenomena such as evaporation and pyrolysis of pollutants that occur during treatment.

In another preferred embodiment, the modelling is directed towards treatment plants by simulating the combustion that occurs in the burners of the heating tubes and by simulating the phenomena that occur in a polluted vapour treatment unit.

The invention concerns a method for modelling soil heating that takes into account certain properties of a soil to be treated, such as its initial water content, its concentration of pollutants, etc.

Simulation software, such as ANSYS FLUENT, allows soil properties to be considered that vary with temperature, that vary with time or that are a constant value. However, the thermal properties of the soil vary with moisture. Most simulation software is able to take this factor into account but requires the resolution of several model equations integrating the phase change of water, requiring tedious calculations.

The present invention concerns the formula developed, written in the C programming language, for these properties which takes into account the variation of humidity over time under the effect of the heat created by the heating elements. This formula initially considers a porous medium consisting of a type of soil initially containing a certain percentage of water. It also numerically considers the energy consumed by the evaporation of this amount of water. With this proprietary formula, using the "used defined function (UDF)" option in the modelling software, the model solves the following energy equation:

The robustness of the formula developed in the present invention has been verified by comparing the results obtained by modelling with on-site measurements of soil temperature. Temperature is a parameter that can be measured on site over time and is a major parameter in thermal technologies. In the simulation, a heat treatment zone was drawn in 2D and the position of the thermocouples was recorded in the software so that the site layout and the simulation were identical. By juxtaposing the temperature curves measured on site over time with those obtained numerically by the simulation, several observations were made.

Firstly, the formula developed in the present invention is functional: the general behaviour of the curve is similar to that observed on the construction site. Secondly, the heterogeneity of the soil is difficult to exploit by simulation but could highlight problems on site such as a poorly exploited water table. Finally, the simulation revealed the influence of the other two surrounding heating tubes on this central point, the so-called cold spot.

The present invention has also provided an understanding of heat transfer in the ground. When the temperature profile between a hot point (temperature of the wall tubes) and the corresponding cold point (furthest from the heating tubes) is known, it is possible to carry out a thermography at a given moment. Measurements taken at the site have shown that the temperature curve tends towards a parabolic profile. The present invention is able to provide the equation of the profile between the different hot and cold points, and thus generate a thermography as close as possible to reality.

In one embodiment, a 2D model is used. In another preferred embodiment, 3D models are used.