WO2000076682A1 - Method and device for cleaning soil - Google Patents

Abstract

This cleaning takes place in two steps. During the first step (2), the soil is heated to a relatively low temperature. This temperature is such that the water which is present therein is substantially completely evaporated. On the other hand, the temperature is sufficiently low to ensure that hydrocarbons which are present do not evaporate. During the second step (3), the temperature is increased further, so that the hydrocarbons do now evaporate. The result is a useful gas which can be used to heat the soil. The water vapor which is obtained is condensed and returned to the end product.

Description

Method and device for cleaning soil

The present invention relates to a method for continuously cleaning contaminated soil, in which the soil is subjected to heating and the gases which are liberated from this contamination during the operation are burnt, the heating taking place in two steps, a first step during which this soil is heated to 80-200°C in order to substantially remove the water which is present therein, and a second, subsequent step in which the soil is heated to above 250°C in order to remove non-aqueous liquids. A method of this type is known from EP-A1 -0,891,799. It has been found at filling stations, factories and other industrial complexes, for example, that there is considerable contamination to the ground. This contamination often consists of organic substances such as oil, PAHs, cyanide and non- volatile and volatile hydrocarbons (Eox/Vox). Of course, the ground also contains water, usually in the form of rainwater. To clean soil of this nature, hitherto this soil has been dug up and transported to a cleaning installation by lorry. A cleaning installation of this type is a large installation and has a rotating drum into which the contaminated soil is introduced. This drum, together with the material contained therein, is heated. This heating is to above 400°C and will generally be to approximately 600°C. In the process, the hydrocarbons decompose and the vapours present in the soil are extracted. These gases are cleaned and after-burnt. Dust filters are present. To make sure that certain harmful substances cannot escape, there is generally an afterburner, in which the combustion gases are heated to 800-1200°C with the addition of further air. The soil which has been cleaned in this way is then loaded back into lorries or other means of transport and returned to the site of contamination. Although clean soil can be obtained in this way, the energy consumption is considerable. Firstly, relatively large amounts of fuel are required to transport the soil from the contaminated site to the processing installation. Secondly, large amounts of energy are required in the processing installation to heat the soil and to afterburn the gases released. Since the cleaning of soil is an environmental measure, it will generally be desirable for this environmental measure also to be carried out in as environmentally friendly way as possible, i.e. using as little energy as possible. After all, hitherto approximately 3300 tonnes of propane for combustion were required to treat 100,000 tonne of soil, for example, without even considering the energy required for transportation.

US-A-5,248,092 describes a batchwise distillation process. Under a vacuum, soil is subjected to successive distillation steps. In this case, the soil is heated again for each load. The result is a discontinuous process, making the quality of the flue gases difficult to control and limiting the throughput which can be achieved. Moreover, a method of this type is expensive.

EP-A1-0,891,799, which comprises the measures of the preamble of Claim 1, discloses a method and device for cleaning soil. The preheating step is carried out using electrical means and the gases released during the heating in the second step are extracted and, if appropriate, burnt elsewhere. The heat which is released in the process can be used to provide heating in the second step. Since in this step the temperature is relatively high, the flue gases will be held at this higher temperature for a prolonged period. Consequently, there is a risk of NO_x formation. Moreover, external energy is still required for the preheating step in which water is removed.

The object of the present invention is to avoid these drawbacks.

In the method described above, this object is achieved in that the heat which is released during this burning of the said gases is fed to the said first step.

According to the invention, the heating of the soil is carried out in two steps. During a first, less hot step, substantially evaporation of water occurs. This requires heating at from 80-200°C and above, in particular approximately 150°C. Standard organic substances will not escape at this temperature. This steam can be cleaned in a relatively simple way (if appropriate after condensation), and there is certainly no need for afterburning. The water which is released can then be fed to the cleaned soil, i.e. does not have to be discharged to the air, and can be returned to the place of origin with the cleaned soil.

During a second step of the method according to the invention, heating takes place to a higher temperature, i.e. higher than 250°C, and preferably in the range from 450-600°C. In the process, the organic substances escape, possibly decomposing in vapour form. Since there is no steam present, these gases are very easy to burn once they have been treated if required. Consequently, it is possible for the burners with which the soil is heated in the second step to operate to a large extent or even completely thereon. This means that the energy supplied to the burners in the second step can be reduced considerably or even eliminated altogether. This is because the absence of steam in the gases which are released during the heating in the second step means that the calorific value of the gas obtained in this way is considerably higher. According to the invention, the relatively hot flue gases which are formed during combustion in the second step are (partly) used for the heating in the first step. The residual heat content of the flue gases is then used to preheat the air which is used in the burners of the second step.

According to the invention, the hot flue gases are cooled considerably by heat exchange with the first step, so that the formation of hazardous substances can be limited. Since all this in addition takes place in the same environment, it is possible to accurately control the residence time of the flue gases, thus guaranteeing, on the one hand, that certain toxic substances, such as dioxines, are broken down completely and, on the other hand, that there are no new toxic substances, such as nitrogen oxide, formed. According to an advantageous embodiment of the invention, the soil is turned during the conveying. In this way, firstly heat transfer to all soil particles is ensured and, secondly, it is made easier for the related vapours to escape.

According to a further advantageous embodiment of the invention, the flue gases which are formed during the combustion of the volatile components which emerge from the contaminated soil are cooled in steps. Initial cooling takes place during the heat exchange with the second step, and further cooling takes place during the heating in the first step. Then, the flue gases, optionally after further heat-exchange cleaning and the like, are discharged to atmosphere.

According to a further preferred embodiment of the invention, the flue gases which form are at a temperature of at least 1000°C and preferably approximately 1250°C. During heat exchange with the second step, the temperature falls to at least 700°C and preferably approximately 850°C. The third temperature, i.e. that reached during heat exchange with the first step, is preferably at least 300°C, and more particularly approximately 450°C. It is important for the residence time of the flue gases at the second temperature to be accurately controlled in order, on the one hand, to sufficiently break down specific toxic substances and also to prevent the formation of other toxic substances. In particular, such a residence time is at least one second and more particularly about two seconds.

To avoid the risk of explosions and the like while the soil is being heated, it is preferable to apply an inert atmosphere around this soil during the heating. An inert atmosphere of this nature can be achieved by introducing nitrogen.

The invention also relates to a device for cleaning contaminated soil under atmospheric pressure, comprising means for conveying soil, provided with heating means, and discharge means for the gases which are released during the heating, and burning means for the said gases, in which device there are at least two heating chambers for soil, which adjoin one another, a first chamber for heating to a first, relatively low temperature, and a second chamber for heating to a relatively high temperature, the said second chamber being provided with the said discharge for gases and the said first chamber being provided with a discharge for water vapor, in which device the said means for conveying soil comprise continuously operating conveyors, and said heating means for the said first chamber comprise the flue gas discharge from the burner into the said second chamber.

This device has shown to be of particularly compact design. It is possible to accommodate the first and second chambers in a standard (40-foot) container. Consequently, it is possible to position the device at the location of contamination and to carry out the operations at that location. In addition to the above-described considerable limitation of energy costs as a result of the reduced consumption of propane or the like, a further considerable energy saving is obtained since it is no longer necessary for lorries to drive to and from between an installation which is at a great distance from the site of contamination. Furthermore, traffic can be limited in this way.

According to an advantageous embodiment of the device described above, the conveyor is designed as a lifting screw provided with partitions.

The invention will be explained in more detail below with reference to a preferred embodiment which is illustrated in the drawing, in which: Fig. 1 diagrammatically depicts, in cross section, the device according to the invention as a mobile arrangement, and

Fig. 2 diagrammatically depicts a partially cut-away view of part of a screw conveyor. The device according to the invention is denoted overall by 1. It comprises a first part 2 and a second part 3. The first part 2, like the second part, is designed as a 40-foot container. The first part is situated in container 4, while the second part is situated in container 17. These containers can be transported by road in the usual way and, near to the place of contamination, can either be put down or remain on a trailer.

In container 4 there is a worm conveyor 5 for soil. This conveyor is driven with a motor 6. Soil is delivered with a conveyor 14, which may be a moveable conveyor which, if appropriate, is fed direct from the contamination site. Moreover, the container 4 contains a feedline 9 for air, which is connected to a flexible line 18 which, in turn, is connected to inlet 19 which opens out in container 17. On the other side, line 9 is connected to an air pump 7 which sucks in air from the atmosphere and conveys it through line 9 to air inlet 19. This line 9 is accommodated in a flue-gas duct 10 which on one side is connected to a flue gas box 11 in which the worm conveyor 5 is situated and on the other side opens out to atmosphere. If appropriate, cleaning installations, such as an active carbon filter and the like, are present in the vicinity of the outlet 15 from flue-gas duct 10, in order to remove contaminants in the flue gas. At least part of the walls of container 4 is provided with heat-insulating refractory material 12. In addition, there is a discharge 13 for steam. The flue-gas box 11 is connected to the gas-discharge line 29 of container

17 via a flexible connection 20. Worm conveyor 5 extends to above worm conveyor 22 to which the material is transferred. Worm conveyor 22 extends through container 17 and is driven by motor 16. In the vicinity of the end of this worm conveyor 22 there is a lock wheel 30, via which soil is discharged from the container 17 onto the removal conveyor 31. Around the worm conveyor 22 there is a heat-resistant, gas-tight plate 24 which extends over the entire width of the container 17 and thus separates the latter into a separate top part and bottom part. The insulation for container 17 is denoted by 23.

In a manner which is not illustrated, the chamber in which conveyor 5 is situated and/or the chamber in which conveyor 22 is situated can be filled with an inert gas, such as nitrogen, in order to limit, inter alia, the risk of explosion.

The top part of container 17 is provided with a gas outlet 25 which is connected to line 28. If appropriate, cleaning and processing devices may be present. These devices can be used to filter out toxic substances and, for example, retain small particles and return them to the contaminated soil which is to be processed. The feedline 28 for gas is connected to the burners 21 via line 26. Moreover, there is a gas source 27, which preferably contains propane. If appropriate, a further substance may be fed to the burners, for example ammonia may be fed in order to prevent or limit toxic emissions during combustion. Air derived from air inlet 19 and gas derived from line 26 are premixed in a manner not shown in more detail and are fed to the burners 21. In the process, the plate 24 is heated, as is the chamber above it in which the worm conveyor 22 is arranged. Gas derived from the burners is discharged via lines 20 and 29. Moreover, the containers may be provided with a lining which as far as possible prevents attack from, for example, acidic water or steam.

As can be seen from Fig. 2, each or only one of the conveyors may be provided with partitions 40 which may extend over part or all of the radius of the screw. This prevents the soil from being displaced through the worm conveyor in the form of a string and ensures optimum heat transfer to the underside of the worm conveyor. This underside may be provided with heat-transfer ribs. The turning also ensures a homogeneous treatment and promotes the escape of vapours.

The device described above functions as follows. Contaminated soil derived from conveyor 14 is fed to the worm conveyor 5. This worm conveyor can rotate at any speed which is known in the prior art, for example at between 5 and 10 revolutions per minute. The flue gases derived from burners 21 heat the space around worm conveyor 5 which is present in flue box 11 to a temperature of approximately 150°C. In the process, water disappears through outlet 13. This water vapor can be condensed and fed to line 33 and can be introduced into the soil product which is discharged onto discharge conveyor 31 with the aid of injectors 32. To monitor the combustion from burners 21, it is possible to determine the oxygen content of the gases present in line 20. According to an advantageous design, there must be at least 6% oxygen, so that complete combustion of the gases is always ensured.

The flue gases in flue box 11 will be cooled by heat transfer to the soil in worm conveyor 5. The remaining heat which is present in the flue gases can be extracted when the flue gases move to the environment via flue-gas duct 10. In the process, the inlet air is heated. A test set-up has shown that the final temperature of the flue gases at outlet 15 is approximately 230°C. The residence time of the flue gases in the section in which they heat conveyor 22 through to further cooling at conveyor 5 is important. This is because during this period the flue gases are at a relatively high temperature, for example 850°C, at which temperature, on the one hand, toxic substances such as dioxines can be broken down but, on the other hand, there is a risk of new, undesirable substances, such as nitrogen oxides, being formed. According to an advantageous embodiment of the invention, this residence time is controlled in such a way that it is approximately 2 seconds. It will be understood that the design of the device, and more particularly the flue box 11, can be altered in such a manner that the residence time can be optimized as a function of the device.

The soil from which water has been removed is then fed to container 17. There, further heating to 450-600°C takes place. This is where the burners act directly on the worm conveyor 22. In the process, hydrocarbon vapours and other vapours containing combustible gases escape via line 25. After harmful constituents or non-combustible components have been removed, these gases are fed to the burners 21 via lines 28 and 26. To start up the process and, if appropriate, add additional heat, gas can be withdrawn from the propane store 27.

It has been found that the device shown here makes it possible to reduce the propane consumption or other energy consumption considerably while, when the process operates continuously, it is even possible to supply no energy at all, apart from the energy required for the various drive motors, although in an extreme case, it is even possible to allow generators to run on the combustion gases. Since, moreover, this installation can be used in mobile form, the energy costs involved in transport (and of course the labour costs) are eliminated. Consequently, it is possible to clean soil on site in a particularly efficient way. Using two standard 40-foot containers, it is possible to completely clean 15-25 tonnes of soil with an average contamination level per hour. Although the invention is described above with reference to a preferred embodiment, it will be understood that numerous modifications may be made without departing from the scope of the present application, for example, it is possible for the device not to be of mobile design or to be of more compact design. Modifications of this nature all lie within the scope of the person skilled in the art. Moreover, it is possible to carry out treatments in order to process or clean the substances formed during the treatment further. It is also possible to omit certain steps as described above. Modifications of this nature are all deemed to lie within the scope of the appended claims.

Claims

1. Method for continuously cleaning contaminated soil, in which the soil is subjected to heating and the gases which are liberated from this contamination during the operation are burnt, the heating taking place in two steps, a first step during which this soil is heated to 80-200°C in order to substantially remove the water which is present therein, and a second, subsequent step in which the soil is heated to above 250°C in order to remove non-aqueous liquids, characterized in that the heat which is released during this burning of the said gases is fed to the said first step.

2. Method according to Claim 1, in which the heat released during this burning of the said gases is fed to at least the said second step.

3. Method according to Claim 1 or 2, in which the flue gases which are formed as part of the burning during the heating in the second step are fed for heating to the first step.

4. Method according to Claim 3, in which the flue gases which are formed during this burning and are at a first temperature are firstly fed to the said second step and, in the process, cool to a second temperature, and then are fed to the said first step and, in the process, cool to a third temperature, after which they are discharged to atmosphere.

5. Method according to Claim 4, in which the said first temperature is at least

1000°C, the said second temperature is at least 700°C, and the said third temperature is at least 300°C.

6. Method according to one of Claims 4 or 5, in which the said flue gases remain at the said second temperature for at least one second.

7. Method according to one of the preceding claims, in which an inert atmosphere is established around said soil which is to be heated.

8. Method according to one of the preceding claims, in which said soil is turned while it is being moved.

9. Method according to one of the preceding claims, in which the water obtained in the first step is fed to the soil product obtained at the end of the second step.

10. Device (1) for cleaning contaminated soil under atmospheric pressure, comprising means (5, 22) for conveying soil, provided with heating means (11, 21), and discharge means (13, 25) for the gases which are released during the heating, and burning means (21) for the said gases, in which device there are at least two heating chambers (2, 3) for soil, which adjoin one another, a first chamber for heating to a first, relatively low temperature, and a second chamber for heating to a relatively high temperature, the said second chamber being provided with the said discharge for gases and said first chamber being provided with a discharge (13) for water vapor, characterized in that said means (5, 22) for conveying soil comprise continuously operating conveyors, and in that the said heating means for the said first chamber comprise the flue gas discharge (11) from the burner into the said second chamber.

11. Device according to Claim 10, in which the said first and second chambers are arranged in a container (4, 17).

12. Device according to Claim 10 or 11, in which the said second chamber is provided with a burner (21) having a gas supply (26, 28) comprising the said discharge for gases.

13. Device according to one of Claims 10-12, in which the said means for turning the soil comprise a screw conveyor (5, 22).

14. Device according to one of Claims 10-13, in which the said conveyors comprise turning means (40).