HRP20211101A1 - Mobile plant for treatment of polluted soils - Google Patents

Abstract

Plant for the treatment of contaminated soil, characterized by the fact that it consists of a section for pretreatment and supply, a section for thermal desorption and a section for cooling and recovery, separate from each other and interconnected, each of the mentioned sections is placed on a control base. reclamation of contaminated soils, characterized in that it consists of a pre-treatment and feeding compartment, a thermal desorption compartment and a thermal cooling and recovery compartment, separate from each other and interconnected, each of said compartments being arranged on a deck provided with handling means.

Description

The field of technology

The subject invention relates to the area of plants for the treatment of contaminated soil.

Scope of the invention

Techniques for purifying contaminated soil by heating the soil to vaporization of volatile and semi-volatile organic compounds have long been known, and include: removal of contaminated soil fractions, as deep as is considered appropriate, and their subsequent heating in suitable furnaces, to temperatures sufficient to vaporize volatile and semi-volatile organic compounds.

The system in use generally allows furnaces to operate at high temperatures using flames, combustion vapors or pyrolysis, superheating the contaminated soil. These treatment procedures are usually carried out in specific facilities, usually hundreds of kilometers away from the contaminated area: this necessarily implies that the soil to be treated is temporarily stored in predetermined areas, for a very variable period, before being treated in a furnace.

These procedures clearly cause damage to the environment, both because of the very likely loss of harmful components during transport and storage and because of the obvious economic and logistical disadvantages related to ensuring the safe transport of the soil for treatment to the plant, and the subsequent transport aimed at transferring the soil to the previously polluted area.

The complexity of the organization of these operations leads to delays in the reclamation process, with the risk of unwanted spread of pollution; moreover, this procedure leaves room for improper — albeit unintentional — procedures during transport and storage.

Furthermore, the previously known systems involve significant use of thermal energy to obtain properly cleaned soil from that pollution, thereby increasing the pollutants released into the air, with which the gases produced during conventional soil treatment procedures are associated.

The aim of this invention is therefore to provide a mobile plant for the reclamation of polluted soil, i.e. one that can be operated every time close to the pollution area, thus reducing logistical problems and - at least partially - economic, and which can reduce energy costs and limit the risk of pollution and pollution of originally clean area, in the case of proper soil treatment from pollution.

This goal is achieved by a plant for reclamation of contaminated soil, characterized by the fact that it consists of a pretreatment and supply section, a thermal desorption section and a cooling and recovery section, separated from each other and interconnected, each of the mentioned sections is placed on a control base .

A description of the preferred application of the plant according to the invention can be found below with reference to the attached sketches, indicated here only for the purpose of illustration, whereby:

Sketch 1 represents a front view in perspective of the plant according to the invention, where

Sketch 2 rear view;

Figure 3 is a top perspective view of the desorption compartment according to the invention;

Figure 4 is a top perspective view of the connecting part between the desorption section and the cooling and recovery section according to the invention;

Figure 5 is a top perspective view of the cooling and recovery section according to the invention;

Figure 6 is a top perspective view of the pretreatment and supply section of the invention;

Figure 7 is a perspective view of the treated material release part of the plant according to the invention;

Sketch 8 is a schematic representation of the energy sources of the plant according to the invention.

In the description below, similar elements of different parts are identified by the same reference number for clarity and ease of reading.

As can be clearly understood from Sketch 1, the mobile plant consists of a series of parts that make its construction autonomous and fully operational, which is defined by the steps of the procedure.

In particular, these are:

- section for pretreatment and supply 1;

- section for thermal desorption 2; and

- cooling and recovery section 3.

In particular, the pretreatment and supply section 1 consists of a material storage area, a means of collecting the material, a means of screening, mixing and supply to the rest of the plant.

The storage space consists of a space designated for the temporary storage of soil for processing, which is located for practicality, near the hopper 4 for collecting material and for transfer to the next processing procedure.

In the funnel 4 there is also a module for sieving the injected substances so that only substantially more homogeneous material is forwarded to further steps, and more accurate processing is enabled. In the case of coarse material or material of large dimensions, it is possible to install a crushing mill to reduce the size of the material to be processed. The screening and crushing modules, which are already known in the state of the art, are not shown in the sketches, so that the entire plant can be understood more easily.

The entire pretreatment and supply section 1 is placed on a base 5 having wheels 6 and telescopic fixing legs 7 to ensure easier disassembly both at the plant site and for transport to other sites.

Downstream of the first section 1 is a second thermal desorption section 2 connected to the first conveyor belt 8 for conveying the processing material. This conveyor belt 8 is inclined relative to the road surface, and its surface vibrates, so that the soil to be treated enters the desorption area with the appropriate moisture level. In this way, by combining these two possibilities, the inclination of the moving body and the vibration of the floor, cause the rest of the water in the soil to drip: a mechanical drying step is therefore implemented, which benefits the desorber and the treatment process because it ensures less moisture in the following steps.

The desorption section 2, in which the material is conveyed by a conveyor belt 8, consists mainly of a housing 9 designed to thermally insulate the rotating desorption drum, or desorber. The contaminated and sieved soil is introduced into the drum, whose inner surface has blades arranged according to a spiral geometry: this ensures — in use — the use of the correct path and consequently the soil inside. The desorption section also consists of a heat generator, usually an air burner, designed to maintain the temperature in the drum at around 900°C.

Like the pretreatment and supply section and the desorption section, it is mounted on a base 5 with wheels 6 and telescopic fixing legs 7 to ensure easier disassembly both at the plant site and for transport to other locations.

The desorbed soil passes the last screening in the tower 12 to compensate separately the sand and powder for the subsequent purpose, retaining the 300 µm sieve. To enable adequate management of aggregates, in this step there is also a humidifier 10, which reduces the risk of dust and enables the soil to be easy to handle and transport.

The steam that expands in the desorber is rich in pollution, and must be subjected to subsequent treatment, in order to further reduce the pollution load. For this purpose, the steam passes through the tube 10 to the post-combustion chamber 11, in which the contaminating substances that have not yet spread inside the desorber are burned. For this purpose, the temperature in the afterburner chamber 11 is maintained at about 750°C. The oxidation of the oxidized contaminant releases energy in the form of thermal energy, which is transferred within the lines 10, so that it can be fed to the micro-mesh bag filter 13 for dust collection, and then transferred to the chimney 14.

Inside the pipe 10 there is a bundle of pipes — not shown, of conventional construction — for countercurrent heating of the environment, and therefore clean, air is introduced through the fan 15 into the pipe 10 to move towards the interior of the desorber 9.

Like the sections already described, the thermal cooling and energy recovery section is mounted on a base 5 with wheels 6 and telescopic fixing legs 7 to ensure easier disassembly both at the plant site and for transport to other locations. Furthermore, in order to comply with traffic rules, the use of a telescopic chimney is foreseen, the height of which varies depending on the type and temperature of emissions at the end of processing, as will be clearer from the description of the procedure that follows.

In the plant, in addition to the already mentioned fan, there are also other fans for gas control and management, in order to enable proper operation and movement of gases, and to reduce the deposition of particles in the pipes.

Moreover, preferably, the drum in the desorber is placed with a slight depression to prevent steam from escaping from the drum itself.

The entire plant is controlled from the control unit 16 and driven by the power plant 17, which in this example is upstream of the hopper 1, and placed for transport on the bed 5 of the pretreatment and supply section 1.

The process in the plant described here seems quite simple: after soil decortication, operators, after peeling the soil, feed it into hopper 1, where the first screening takes place. The sieve is adjusted, according to the size and number of meshes, as a function of the specific chemical and physical conditions of the soil.

If the screened product is suitable, it is forwarded for recovery; otherwise, the material is transferred to the crushing module, so that the oversize material can be processed and screened again.

Once the desired size of the material to be processed is reached, it is guided by a conveyor belt to the desorber. In the transfer from the hopper to the desorber, the soil is mechanically dehumidified to allow a faster and more efficient spread of pollutants.

When the soil enters the desorber, the rotating drum - by means of blades placed on the inner surface - lifts it and causes a gravitational fall, while the superheated air coming from the heat generators connected to the desorber and enriched with hot air from the heat exchanger invests the soil to be treated countercurrently, thus enabling the first separation of different contaminated material from the soil.

At the end of processing, steam rich in water contaminants comes to the afterburner for further processing, while the solid matter is sent to the screen, and later distributed to the next specific activities to obtain the appropriate type of product.

The gaseous part is fed inside the afterburner for the following oxidation: inside the afterburner - due to the oxidation of pollutants, which are therefore rendered harmless for discharge into the environment - excess heat is generated which is reused in the plant. This excess heat, which cannot even be released into the environment without causing harmful effects to the ecosystem, is specially fed into the pipe bundle for heat exchange, thanks to the counter-current transfer of the surrounding air directed towards the desorber. In turn, the vapors coming from the afterburner reduce their temperature and are cleaned by a particulate filter, which may still contain contaminants and which would certainly reduce the efficiency of the chimney draft, before reaching the open air - through the chimney itself - at a temperature low enough not to be harmful, but high enough to prevent the formation of condensation inside the chimney itself.

The energy balance of the system therefore enables the temperature inside the desorber to be around 900°C, while the temperature inside the afterburner reaches 750°C. The surrounding air, which has been superheated by means of heat exchangers with vapors created during heat treatment of the soil, reaches a temperature of around 450°C at the end of its journey, that is, when it enters the desorber. Therefore, the temperature of the steam fed into the bag filter is about 294°C. At the end of processing, the gas going into the chimney has a temperature of around 200°C.

As is better understood by analyzing sketch no. 8, the described plant provides a series of control elements for the appropriate operating temperature. In particular, in the described preferred example, the drum contains burners 18, preferably on methane or LPG, with a control module, with fixed air, filled at a pressure of 300 mbar; the supply voltage of the panel is 400V-50Hz 3 phase.

Power control in the furnace burner is performed by the thermal controller 19 based on the discharge temperature of the solids detected by the infrared probe 19a, so that variations in the gas supply correspond to variations in temperature with respect to the adjusted value.

In order to achieve the appropriate temperature inside the afterburner, the thermocouple 20 detects the temperature of the steam at the outlet and sends it to the thermal regulator 20a: if the temperature exceeds the adjustable warning value (e.g. 160°C), the flame in the burner 21 located in the afterburner 8 is extinguished . At the same time, the probe 22 detects the temperature in the afterburner by means of the thermocouple 22a, and substantially ensures that the temperature in the afterburner is constant.

To reduce energy consumption, the rotating drum is ignited by a thermal group consisting of a special burner with a refractory head, working with fixed air, preheated by a heat exchanger 10.

The following table 1 shows the technical data of the plant according to the invention, with the aim of obtaining the appropriate dimensions.

Table 1

The main features of the heat treatment plant

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1 (depending on humidity and contaminant concentration)

Economic evaluation of plant operation

To correctly estimate the cost of managing the plant, let's take a soil moisture of 13%. Based on these data, table 2 shows the expected costs per ton of processed waste, which are as follows:

Table 2

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Since about 50% of the transferred material due to removal from the polluted soil consists of gravel and pebbles, and therefore is not amenable to processing, the economic and energy balance must be halved. The cost per ton of processing is therefore around €2/t.

The calculation is made taking into account that the transferred material contains 15% moisture that is eliminated, and therefore a weight of about 150 kg is expected for each ton of material to be processed.

Since the energy required to evaporate the water is 600 kcal/l, about 90,000 kcal/t must be used just to evaporate the water in a ton of material.

Taking into account the total power of energy in the desorber, the theoretical power of the plant reaches about 50 t/hr.

Based on the data shown above, i.e. at maximum capacity with 15% moisture is equal to 50 t/hr, with a favorable flow rate of 30 t/hr (working capacity per hour in full operation), the desorber works up to 60% of its capacity.

From the above, it is understood that the plant was designed by assessing the opportunity for complete processing of the contaminating substance into thermal energy during processing in the afterburner. This makes it possible, in an obvious and extremely satisfactory way, to reduce the need for external energy, i.e. to minimize the contribution of the burner in the drum for the desorption step, since the contaminant is successfully recycled into heat energy.

As can be understood, then, the plant therefore enables a direct link between soil contamination for processing and the amount of power that must be externally supplied for processing: the higher the pollution, the lower the external energy contribution. This makes the entire facility highly environmentally friendly.

This plant is particularly profitable when it is necessary to process the soil from hydrocarbons: in this case, energy is also produced by pyrolysis, which takes place directly in the drum, especially if the pollution is very high. Such a reaction helps raise the temperature in the rotating drum, causing a reduction in methane gas consumption. The greater the pollution, at the same humidity, the lower the consumption.

In order to gain an insight into the substances that are spread at the end of conventional tillage according to the plant of the present invention, a practical example is shown in Table 3.

Table 3

Emission and efficiency characteristics of the plant

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From the above description and from the practical numerical example presented, the advantages provided by the plant according to the present invention can be summarized as follows:

- the same general advantages of known plants, since it is able not to create dioxins and furans, the polluted soil is still able to retain most of its organic and chemical properties, etc.;

- highly adaptable system from a structural point of view, with the possibility of offering additional modules to meet the most diverse needs for processing and energy sources;

- very limited processing cost, both due to the reduction of power supply to the processing system and due to minor logistical problems associated with ex situ processing

- limited utilization of energy sources.

Furthermore, the mobile contaminated soil treatment plant according to the invention provides a reduction of organic pollutants (from C4 to C40) up to 99% compared to the concentrations of the incoming pollutants. The use of a thermal oxidation system also enables the removal of organic substances in exhaust gases, with an efficiency of 99 to 99.9% and at a processing speed of 32-54 t/hr.

Therefore, it is completely clear that the intended goals have been achieved, with the realization of a plant with reduced energy and impact on the environment and limited logistical impact.

Compared to thermal-destructive treatments, moreover, the operating conditions can only continue with the evaporation of the pollutant, without oxidation or direct destruction, avoiding the risk of dioxin or furan formation.

It is understood that the above description refers to a particular preferred example, without limiting the technical scope as defined in the accompanying claims.

Claims (11)

1. Plant for reclamation of contaminated soil, characterized by the fact that it consists of a section for pretreatment and supply, a section for thermal desorption and a section for cooling and recovery, separated from each other and interconnected, each of the mentioned sections is placed on a control base. 2. The plant according to claim 1, characterized in that the pretreatment and supply section consists of a hopper for material collection and transfer to the next processing steps, the hopper has a module for checking added ingredients and separating large bodies, and is connected by a conveyor belt to the thermal section desorption, which tape is tilted and its surface vibrates. 3. Plant according to claim 1 or 2, characterized in that the desorption section consists of a housing designed to accommodate a heat-insulated rotating desorption drum, from the inner wall of which concentric vanes are placed according to spiral geometry. 4. Plant according to claim 3, characterized in that the heat generators consist of burners for air veins which have a refractory head and are connected to the drum. 5. A plant according to any of the preceding claims, characterized in that the cooling and recovery section consists of a combustion chamber placed downstream of the desorber, collects and oxidizes vapors rich in pollutants that come from the desorber through the connecting channel. 6. Plant according to claim 5, characterized in that the heat exchanger consists of a plate tube that is fed by steam coming out of said afterburner and a flow of countercurrent ambient air supplied by a fan extending from that afterburner chamber. 7. Plant based on claim 6, characterized in that the air coming from the supply of superheated ambient air is brought to the desorption chamber and that the vapors coming from the combustion chamber are brought to the bag filter and chimney. 8. Plant according to claim 7, characterized in that said chimney is a telescopic chimney. 9. A plant according to any of the preceding claims, characterized in that it contains a control unit powered by a power plant. 10. Plant based on any of the preceding claims, characterized in that the desorbed and regenerated soil is fed to a sieve for separating the powder from larger parts. 11. The melioration procedure in the plant based on requirements 1 to 10, characterized by the fact that it consists of the following steps: - bringing the soil into the funnel, screening according to the specific chemical and physical properties of the soil, and conveying it by a conveyor belt with mechanical means for dehumidification to the desorption chamber; - handling of the soil by raising the blades and falling by gravity in the presence of a superheated air current at a temperature of 800°C to 900°C in the direction opposite to the handling of the soil, and separating pollution from the soil itself; - separation and inspection of the soil that came out of the desorber; - sending steam rich in contaminants through a channel to the afterburning chamber, and oxidizing the contaminants at around 750°C; - supplying steam without pollutants into the bundle of heat exchanger tubes, into which ambient air directed towards the desorber flows countercurrently; - passing overheated ambient air to the desorber; - sending cooled steam to bag filters and separating particles; - transfer of steam at 200°C to the chimney.