Thermopile - Thermal Treatment in Situ or On Site Applications
Confronted to classic systems of thermal desorption, thermopile is consuming less energy, producing less emissions and is completely free of noisy, dusty and polluting transport systems for evacuation.
Thermopile is based on thermal desorption consisting in 2 steps: Transfer of pollutants to gaseous phase and gas treatment.
Thermopile is operating through a network of coaxials tubes inserted in the ground. The hot air is introduced in the inner pipe and heat the soil by conduction.
Evaporated pollutants are then sucked within outer pipe and transferred in oxidizing chamber where there are totally destructed and retransferred as hot air in the system.
- In situ treatment without excavation (even under existing buildings)
- On site treatment into specially equipped concrete pools
- Simple system, mounting, maintenance, operation costs reduced
- Time treatment rather short
- No transport of soils
- Gas emissions reduced
- Use of pollutants as energy source for treatment
- Efficiency proved on all type of soils (sand, clay, silt, …)
The Thermopile-technology is an innovating and sustainable application of thermal desorption, developed and patented by Deep Green (ref: EP1604749).
As for any classical thermal desorption technology, the treatment consists of bringing the contaminated materials at relatively high temperature, so that the organic pollutants evaporate and migrate into the gas phase. This gas phase is then collected and oxydized in a special combustion unit, where all organics contaminants are oxydized to CO2 and H2O.
The Thermopile technology represents the next generation of thermal desorption, as it is based on two fundamental principles:
- Optimal use is made of the energy from the organic pollutants contained in the soil to be treated. Those pollutants are used as a secondary heat source in the desorption process.
- The Heat generated by the gas treatment (combustion unit) is used for the heating of the soil (quasi-closed circuit), instead of being wasted at the stack as it is in traditional thermal desorption.
The system consists of coaxial tubes, positioned directly and vertically into the contaminated soil (or material). The tube is heated by circulating hot gasses (never entering the soil) and heats the soil to 250-400°C causing the thermal desorption of the pollutants. The ensuing gasses end up into the external tube via the sidewall perforations (venture effect). They are then sent to the oxidizer for oxidization and destruction. The hot gasses are then re-injected into the system via the internal tubes. This is a quasi-closed system. Once the gasses have been cooled and have possibly passed through a secondary treatment unit (if and when needed), they are released into the atmosphere under permanent monitoring.
The system is particularly fuel efficient (thanks to the recirculation of hot gasses, as well as the optimal use of the pollutants contained in the soil) and easy to implement.
Additionally, soil is heated by conduction, which guarantees an in-depth treatment of the soil, avoiding preferential pathways for conveying heat into the soil. This allows in situ (no excavation) thermal treatment with guaranteed clean soil results!
It can be applied off site (after transportation to a dedicated treatment facility), on site (after excavation – no transportation) or even in situ (no excavation needed, even under existing buildings).
Cleaning the soil
The Thermopile process is based on the same principles as the classical thermal desorption, i.e. a two-phase treatment:
- Evaporate the pollutants contained in the mineral matrix (i.e. the soil) mainly by elevating the temperature.
- Oxydize (transform into C02 and H2O), in a separate phase, the contaminants in gas form.
Those principles, applied for all thermal desorption processes, are applied with Thermopile in a specific manner in order to guarantee clean soil and minimal fuel consumption as well as minimal other nuisances.
The Thermopile process consists of placing into the soil a network of heating pipes, according to a pre-established canvas. Those heating elements are made of two coaxial steel tubes, of which the outer tube is perforated. During the functioning of the process, gasses at high temperature (700-800°C) coming from the combustion chamber, circulate within those heating elements. Thanks to that circulation, the gasses heat the soil and consequently evaporate the volatile pollutants (boiling point < 550°C) contained in the soil.
To the contrary to classical thermal desorption (rotary kiln, afterburner, etc.), where residence time in the kiln is around 20 minutes, heating time for the Thermopile process is a couple of weeks. However, treatment batches can be substantially higher, leading to similar monthly treatment capacities.
The desorbed pollutants (thanks to the progressive temperature rise in the soil) migrate into the heating elements thanks to diffusion and convection mechanism. They enter into the heating elements through perforations into the outer tubes thanks to negative pressure generated by the venture effect. Once inside the heating elements, the desorbed gasses (steam and contaminants) are conveyed into the combustion chamber. Those gasses will then serve as fuel in that chamber, which itself is equipped with a purge system in order to maintain the stoechiometry in the system, as the combustion chamber is also equipped by an auxiliary burner.
Heating the soil
The Thermopile process uses a complete ‘detente/contraction’ cycle of the gasses with outside heat exchange (Figure 1). The components of this cycle are :
- A detente and heating phase of the gasses within a combustion chamber
- A contraction and cooling phase in the soil
At the exit of the combustion chamber, temperature is around 900°C with a pressure slightly above atmospheric pressure. During its circulation into the heating elements (put into the soil), the gasses are cooling down and pressure drops. That pressure drop allows to recuperate all generated contaminated vapours generated into the soil (see below). The return into the combustion chamber closes the cycle.
The soil is heated through conduction. This mechanism is quite unusual in soil treatment technologies, as most heating mechanisms are based upon convection (physically moving a heat-transporting fluid into the contaminated mass). The main advantage of conduction is that it is not sensitive to soil type (thermal conduction varies in a factor 2 to 3 between clay and gravel – whereas permeability to air for example varies in a factor in excess of 100,000 to 1,000,000 between clay and gravel).
As a consequence of this mechanism (conduction), the timing for reaching set temperatures is quite predictable and the cleanliness of the treated material is guaranteed as well, since it responds to the same physical laws (temperature, pressure and residence time).
The heating itself occurs through a network of heating pipes. The tubes are coaxial stainless steel pipes, in which hot gasses circulate. Those gasses remain in the pipes (they enter in the middle pipe, move down to the bottom of the pipe, and exit through the annular zone).
It is important to note that the hot air circulating in the heating elements never enter in contact with the soil. The hot air circulates within the pipes and transfers the heat to the pipes themselves and then by conduction, to the soil. The system is therefore nearly not affected by the heterogeneity of the soil/material to be treated.
The outer pipes are perforated in order to create a negative pressure within the soil pack (venture effect created by the velocity of the circulated gasses) and so attract the desorbed gasses into the annular zone of the pipes and later to the combustion chamber.
Soil temperature raises slowly from the initial 10-12°C to the set temperature needed for complete desorption of all organic pollutants. That temperature depends on the type of pollutants and on the residence time aimed at during the project. Figure 3 presents the typical soil temperature pattern according to heating time.
The Figure 4 shows a temperature evolution at a real in situ project in Liège.
We can clearly distinguish three heating phases:
- First phase during which soil temperature rises to the boiling point of water (100°C).
- Second phase of water evaporation (stable at 100°C).
- Third phase during which soil temperature rises again to the final temperature of the treatment.
During the first phase, the soil and the liquids (water+contaminants) are heated to the boiling point of water. The time needed for this phase depends on the thermal properties of the soil and the quantity of water present. During the second phase, the soil temperature remains around 100°C until the water-steam interface reaches the position of the thermocouple. The speed at which this interface moves depends upon the quantity of water present in the soil, the heating power and the velocity of the steam extracted. When all the water is evaporated, the temperature of the soil rises above 100°C. As the soil temperature increase, the organic components are volatilized and directed towards the openings of the outer tubes (thanks to the venture effect). Depending on the quantity of oxygen present in the soil, oxidative reactions and/or pyrolyses takes place. A large amount of the organic components are thus destroyed in-situ. The remaining quantity is destroyed in the gas circuit and in the thermal oxidizer, where the temperature exceeds 850°C.
Figure 5 represents the evolution of liquid (water and contaminant) saturation during the treatment.
Thanks to the high permeability zone (dry zone formed around tube), the vapors are led easily to the tubes (creating its own permeability to air, even in heavy clay soils).
Figure 6 represents the typical gas and soil temperature fields at the end of treatment (CFD software: Fluent)
Physical and chemical mechanisms during the soil heating
When the soil is heated, the volatile and semi-volatile compounds (VOC’s and SVOCS’s) are vaporised and destroyed through different mechanisms. Those include:
The vaporised components are directed to the heating elements (see below). During their movement into the soil, vapour can be decomposed thanks to oxidations and/or pyrolisis reactions. A substantial part of the contaminants is destroyed into the soil before reaching the heating elements. The main fraction of the contaminant mass which is destroyed into the soil is attributed to the fact that those contaminants are exposed to relatively high temperatures during a long period of time. A couple of days or weeks are sufficient to initiate oxidation or pyrolisis reactions between 300°C and 500°C.
Placement of the heating elements
The heating elements are placed at interdistances varying from 1 to 2.5 meters. The placement occurs through various drilling techniques, depending on the local circumstances and the depth of the pipes.
The Figure 7 : Drilling units presents a drilling unit used for shallow heating elements. It works thanks to a hammerhead and pushes the head into the soil by means of compressed air.
Disposition – Network
The network of heating elements (Figure 8) is put in place under an equilateral triangular grid. This configuration allows to have a regular heating and treatment time for each heating element, and consequently for the whole zone.
Desorbing the polluants
Soil heating time
The soil heating time depend on:
- The heating power (temperature and gas velocity).
- Distance between tubes.
- The final soil temperature at coldest point (triangle center).
- Soil characteristics.
- Water and contaminant concentrations.
The Figure 9 presents the relation between the heating time and the distance between the heating elements for sand with 12% (wt.) water and 1% TPH. The heating power is 2000W/m of tubes. Durée de chauffe en fonction de la distance entre les tubes
Hydrocarbons are molecules composed mainly out of Hydrogen (H) and Carbon (C). Evaporation temperatures of hydrocarbons are mostly proportionate to the number of Carbon atoms composing the hydrocarbon. As a rule of tomb, one can consider that the more carbon atoms are composing the hydrocarbon (“heavy hydrocarbons”), the higher the boiling point will be. Hydrocarbons present in contaminated soils will have different behaviours in function of their molecular weight. The higher that molecular weight will be, the higher the temperature or the residence time will be for complete desorption (evaporation) of those hydrocarbons.
Among the hydrocarbons, one considers often the aliphatic hydrocarbons, the mono-aromatic (BTEX) and the poly-aromatic (PAH) ones. Many other types of hydrocarbons are also present (see below). The first and second type of hydrocarbons mentioned (aliphatics and BTEX) are often associated with oil derivates, whereas the PAH are more associated with coal derivates.
In all cases, hydrocarbons are volatilized during a thermal desorption process (incl the Thermopile process) and, as gasses, they are conveyed to a combustion chamber where they are oxidized to CO2 and H2O, with a generation of energy.
In some case, hydrocarbons may contain other elements, like sulphur or chlorine, which are not oxidized to CO2 and H2O and need additional gas treatment (see secondary gas treatment below) in order not to exceed the emission standards set for the release of clean gasses into the atmosphere.
Table 1 presents the various boiling temperatures (at atmospheric pressure) of the most common pollutants found in soils and treated by thermal desorption (incl Thermopile).
Other pollutants can be treated by Thermopile in the same way as they are treated by classical thermal desorption, i.e. any pollutant with a boiling point lower than approx. 550°C. Their behaviour during the Thermopile process, however, can differ, depending on the type of pollutant.
The pure organic contaminants (only hydrogen and carbon) will not only be desorbed, but also be completely oxidized in the combustion chamber. There will be no other release into the atmosphere than CO2 and H2O.
For halogenated hydrocarbons (chlorinated solvents, PCB, dioxins, furans, pesticides, etc.) the principle of desorption remains the same, i.e. that they will evaporate and be collected in the gas phase (their boiling points are all well below 550°C). Once oxidized, the halogens will remain in the gas phase and therefore a secondary gas treatment is required before release into the atmosphere (see below).
Regarding the treatable inorganics, such as cyanides and mercury, their behaviour is very specific. Whereas cyanides are desorbing easily and are oxidized to CO2 et NOx, requiring a specific secondary treatment for the NOx release, the mercury impacted materials require special attention and a dedicated unit. Mercury will desorb (nearly all form of mercury have boiling points well below 550°C) and it will be collected in the gas circuit. Mercury cannot be oxidized and therefore a special system for that kind of contaminant has been designed.
Recuperating the polluants
Once the pollutants have been transferred into the gas phase, they migrate into the heating elements. This occurs through preferred channels, thanks to a negative pressure. The negative pressure is generated by the high velocity of the gasses circulating into the tubes (Venturi effect).
The volatilized pollutants follow any preferential path, all of them leading to the heating elements. Even in heavy clay, preferential paths are created because of the drying of the soil, which generates cracks, leading to preferential paths and sufficient permeability to convey the desorbed contaminants.
Additionally, the vaporization of water and contaminants creates locally a high pressure area, which itself generates movement of those fluids toward to low pressure area (the pipes).
After heating at maximum 400°C (measured at the coldest points) the soil is cooled with the fresh air (later with water). Air at 30°C (maximum temperature of fresh air) is circulated in the tubes thanks to the oxidizer fan. The aim of this study is to evaluate the airflow needed for fast soil cooling (few days). Calculation is carried out with Fluent (CFD software).
System: Air, tube and soil around (D=1.5m).
Model: 86000 meshes, 2d/axisymmetric, unsteady, turbulent flow/k-? (see drawing below).
As Figure 11 shows soil temperature drops below 145°C after 20 days (Uair = 25m/s at 30°C),
Figure 12 indicates air temperature in the outlet tube over time.
The following figure gives the soil temperature profiles at various times.
Cleaning the gasses
Primary gas treatment
The primary gas treatment consists in oxidizing all organic components which have been desorbed and not oxidized in situ or in the piping system.
As those gasses are collected and conveyed into the combustion chamber, they are oxidized completely and transformed into CO2 and H20 for the hydrocarbon part.
Within the process, and before the gaseous components reach the oxidizer chamber, other types of reactions occur such as hydrolysis and pyrolysis. All those reactions can be summarized as follows:
The complete oxidation of the gasses (ultimate and main process) takes place into the combustion chamber. In order to reach 100% of destruction efficiency, one needs to take into account the following two main parameters:
- Residence time
Those two parameters are linked to each other by the Arrhenius Kinetics Law :
- C/Co : Residual concentration
- t : Residence time
- T: Temperature
- R: Perfect Gas Constant
- E : Activation Energy
- A : Constant dependent upon the nature of the pollutant
Residence time and temperature are therefore depending on the type of pollutant. Equipment and project design is always based on the heaviest pollutant.
Key parameters and control
The key parameters for the primary treatment are temperature, residence time and oxygen concentration. Complete combustion control is monitored through a CO monitoring, which indicated total oxidation of all organic compounds (i.e. successful completion of the primary treatment).
Secondary gas treatment
Depending on the type of pollution treated, a secondary gas treatment may be needed. This occurs when elements such as S, Cl, Br, Hg or NOx are present in the soil and/or in the treated gasses exiting the combustion chamber.
If those elements exceed the maximum concentrations allowed for the release into the atmosphere, the purge gasses need a secondary gas treatment. It is important to notice that this secondary treatment only applies to the purge gasses, and not to the complete treatment gasses (which circulate in quasi closed loop). The order of magnitude of the gas flow for secondary treatment ranges from 300 to 1,500 Nm³/h depending on the type of unit, which are very low flow rates.
The secondary gas treatment devices are of various kinds and will depend on the contaminants to be treated. In any case, they are well-known technologies, applying gas treatment systems for dust-free gasses at relatively high temperatures with low flow rates. Activated carbon unit, catalytic oxidation, acid gas scrubbing and condensation steps are the most commonly used for this type of treatment.
Implementing a soil treatment technology can be done in various ways. As a general terminology, one uses often the terms in situ , off site, on situ, ex-situ, etc. In order to avoid misunderstandings, we have adopted the definitions as follows:
- In situ: All treatments requiring no excavation of the soil
- Ex-situ: All treatments requiring excavation of the soil
- On site: Treatment without transportation of the soil outside of its area or origination
- Off site: Treatment of the soil on another location than where the pollution occurred originally.
The Thermopile in-situ technology uses vertically placed heating elements which are put in place directly in the soil (Figure 14). Those elements are then connected to a combustion unit (T1 or T2) which is positioned on the surface, preferably as close as possible to the area to treat.
Ex-situ (on site or off site)
Thermopile can also treat excavated soils/materials. In such a configuration, the materials to treat are first put into a basin by traditional transportation means (Figure 15). The size and the number of basins depend on the total quantity of soil/material to treat as well as the type of unit used (T1 or T2). When the materials are put in place, the heating elements are placed according to the pre-established grid. They are then connected through flexible stainless steel tubes to the horizontal collector network. Then, the collectors are connected to the treatment unit.
Treatment capacities depend on the type of treatment unit (T1 or T2) but also on the total quantity of materials to treat. Depending on the design and the number of basins, large treatment capacities can be achieved.
The main parameter measured continuously during the Thermopile process are temperatures. Both in the gas flow (hot and cold gasses) and in the soil (at the coldest point in the soil), temperature is measured continuously. Pressure drops and other parameters are also measured continuously in the process. The Figure 20 presents the classical Process control measures and location of sampling/testing.
Within the soil pile, the main parameter followed during the process is the temperature, which is measured by Thermocouples placed at different depths, at the coldest points (equidistant from 3 heating elements). This parameter is the determining one for defining when the treatment is fulfilled. The Figure 21 presents the thermocouple location for the process follow-up.
Gas analysis - atmospheric emissions
The exhaust gases (released into the atmosphere) are analyzed continuously. The analyzer is located within the Thermopile installation. The analyser can measure the concentrations of the following gases: O2, CO2, CO, H2O, NO, NO2, HCl, SO2, HCT, HF.
Thermopile T2 system characteristics
- Thermal capacity oxidizer : 3 MW
- Combustion temperature : >850°C
- Residence time in oxidizer : > 2sec.
- Oxygen concentration in purge gasses :min 6% - max 11%
- Purge gas flow rate : 1500Nm³/h (@ 20°C)
- Total number heating elements per batch (approx. 2500m³) : 288
- Total length heating elements : 1152 m
- Average fuel consumption : 80 to 175 l/h
- Final temperature in soil (TPS or equivalent) : 200 to 250°C (coldest point)
- Final temperature in soil (PAH or equivalent) : 350 to 400°C (coldest point)
Release of environmental responsibilities
The application of Thermopile in-situ is an innovative approach in the field of remediation. Even in the most constraint circumstances, the use of Thermopile allows, without excavation, for the complete volatilisation of organics pollutants in the soil, as a result of a heating process. The gathered gases are thereupon completely oxidised in a combustion chamber. A difference with the majority of other treatment methods is that desorption releases the customer from all environmental liabilities for the cleaned soil (even if target values or legislation on contaminated soil change in the future).
As treatment goes relatively fast (a couple of weeks per batch), the duration and related costs and nuisances of a site operation are minimised.
Where the classical way of thermal treatment is characterised by high energy consumption, Deep Green felt driven in the development of Thermopile to strongly reduce energy consumption. The recuperated contaminants (after volatilisation) are re-used in the heating process of the soil and serve as fuel! The polluted soil and other material are treated and recycled on the site, and consequently avoid the nuisances and risks of transport. Pre-eminently being a solution of recycling, the thermal desorption fits perfectly in a sustainable development perspective.
Compared to traditional thermal treatment, atmospheric emissions are reduced with a factor 2 to 10 when the Thermopile process is applied.
Respecting the neighbourhood
The nuisances of noise and smell are minimal. There are no blocked roads, there is no traffic of trucks for transport of soil.
Deep Green guarantees to achieve the clean-up targets.
Thermal desorption is a technical solution that has the flexibility to manage and control eventual surprises that could occur while excavating as well as regarding the concentrations of pollutants. There are only minimal uncertainties with regards to the effectiveness of the treatment.
Respecting treatment time
The parameters of treatment are easily mastered. As the development and progress of the Thermopile process is highly predictable, Deep Green can guarantee timing for the remediation process as well.
Type of contaminants
Thermal desorption can only be applied on organic pollutants and cyanides, but has no impact on the concentration of heavy metals (with the exception of mercury), as the boiling point of heavy metals is over 600 °C. On the other hand, the presence of heavy metals in the soil does not have any impact on the feasibility of a thermal desorption treatment.
Thermopile cannot be applied in the groundwater zone yet, as currently energy consumption is too high. Therefore, in order to use Thermopile when groundwater is present, it must first be lowered with traditional pump& treat methods before Thermopile can be used.