SYSTEM AND METHOD FOR PYROLYSING ORGANIC WASTE

Abstract

The invention provides a system for pyrolysing organic waste. The system comprises a conical housing (4) configured to temporarily, substantially hermetically, enclose the waste and a mixing device provided with a drive shaft rotatably mounted relative to the housing and a conical mixing body (25) configured inside the housing to fluidise the waste, which mixing body fixedly attached substantially does not touch the housing. The system further comprises heating means (24) for heating the side wall of the housing. This system makes it possible to carry out the processing of organic waste in a batch process. The mixing body prevents a portion of the waste from sticking together by fluidising the waste and keeping it fluidised, whereby the heat generated by the heating means can gradually spread through the waste inside the housing.

Claims

1. System for pyrolysing organic waste into pyrolysed material and gasified material, said system comprising: a frame; a conical housing mounted on the frame and configured to temporarily, substantially hermetically, enclose said waste, which housing has a longitudinal direction, a side wall, an upper side and an underside and is provided with: an inlet configured to communicate with a supply device to introduce said waste into the housing; an outlet configured to communicate with a discharge device for discharging said pyrolysed material from the housing, which outlet is provided on the underside of the housing; and a gas outlet configured to communicate with a gas discharge device for discharging said gasified material from the housing, a cross-section through the conical housing having a surface which substantially perpendicular to said longitudinal direction decreases downwards; a mixing device mounted on the frame and provided with: a drive shaft rotatably mounted relative to the housing, said drive shaft extending through the upper side of the housing along said longitudinal direction with a first portion located inside the housing and a second portion located outside the housing; and a conical mixing body inside the housing and configured to fluidise said waste, which mixing body is fixedly attached to the first portion of the drive shaft and substantially does not touch the housing; and heating means for heating the side wall of the housing.

2. System according to claim 1, wherein the second portion of the drive shaft is rotatably mounted on said frame by means of at least two bearings.

3. System according to claim 1, wherein the mixing body has a free bottom side that is located at substantially the same height as the underside of the housing.

4. System according to claim 1, wherein a shortest distance between the side wall of the housing and the mixing body measured in a cross-section through the conical housing which is substantially perpendicular to said longitudinal direction amounts to at most 5% and in particular at most 3% of the maximum internal diameter of the housing.

5. System according to claim 4, wherein said shortest distance is substantially independent of the height at which the cross-section is taken.

6. System according to claim 1, wherein the mixing body is attached to said shaft with one or more cross connections such that, in use, the mixing body is configured to transport said waste upwards along the side wall of the housing.

7. System according to claim 1, wherein the heating means comprise electrical heating elements mounted on, preferably integrated into, the outside of the side wall of the housing, the electrical heating elements being configured to generate a temperature within the housing of at least at least 200° C., in particular at least 300° C., more in particular at least 400° C. and most in particular at least 500° C.

8. System according to claim 1, wherein the system further comprises an inertisation device with at least one storage tank in which nitrogen gas is stored, the inertisation device comprising a plurality of conduits configured for supplying nitrogen gas to near each aperture in the housing.

9. System according to claim 1, wherein the mixing device further comprises a drive, in particular an electric motor, configured for rotating the drive shaft, which drive is located outside said housing.

10. System according to claim 1, wherein the inlet of the housing is closed by a first sluice valve device and wherein the outlet of the housing is closed by a second sluice valve device.

11. System according to claim 1, wherein no ceramic and/or metal balls are present within the housing.

12. System according to claim 1, wherein said waste is radioactive and in particular comprises radioactive resin or radioactive sludge.

13. Method for drying and pyrolysing organic waste into pyrolysed material and gasified material, said method comprising the following steps: a) introducing said waste into a housing; b) after step a), substantially hermetically sealing the housing; c) fluidising said waste within the housing by rotating a mixing body; d) heating the housing for heating said waste to successively dry and pyrolyse the waste; e) extracting said gasified material from the housing; f) collecting said pyrolysed material at the bottom of the housing until said waste is completely pyrolysed; and g) unlocking the housing to remove the collected pyrolysed material.

14. Method according to claim 13, wherein step d) comprises gradually increasing the temperature of the housing from an ambient temperature to at least 200° C., in particular at least 300° C., more in particular at least 400° C., and most in particular at least 500° C.

15. Method according to claim 14, wherein step d) comprises adjusting the temperature based on the waste processing process.

16. Method according to claim 13, wherein step c) comprises transporting said waste upwards along the side wall of the housing.

17. Method according to claim 13, wherein step d) further comprises heating the housing for heating said waste to successively evaporate, dry and pyrolyse the waste.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0066] The invention will hereafter be explained in further detail by way of the following description and the accompanying drawings.

[0067] FIG. 1 shows a perspective view of a system for processing organic resin according to the present invention.

[0068] FIG. 2 shows a perspective view of the pyrolysis portion of the system of FIG. 1.

[0069] FIG. 3 shows a front view of FIG. 2.

[0070] FIG. 4 shows a schematic representation of the system of FIG. 1.

[0071] FIG. 5 shows a perspective view of the mixing device.

[0072] FIG. 6 shows a perspective view of the waste gas processing portion of the system of FIG. 1.

[0073] FIG. 7 shows a section through the oxidation portion of the system of FIG. 1.

EMBODIMENTS OF THE INVENTION

[0074] Although the present invention will hereinafter be described with respect to particular embodiments and with reference to certain drawings, the invention is not limited thereto and is only defined by the claims. The drawings shown here are merely schematic representations and are not limiting. In the drawings, the dimensions of some of the elements may be exaggerated and thus not drawn to scale for illustrative purposes. The dimensions and the relative dimensions do not necessarily correspond to actual reductions to practice of the invention.

[0075] In addition, terms such as ‘first’, ‘second’, ‘third’, and the like are used in the description and in the claims in order to make a distinction between similar elements and not necessarily in order to indicate a sequential or chronological order. The terms in question are interchangeable under appropriate circumstances and the embodiments of the invention can operate in other sequences than described or illustrated herein.

[0076] Moreover, terms such as ‘top’, ‘bottom’, ‘above’, ‘under’ and the like in the description and the claims are used for descriptive purposes. The terms thus used are interchangeable under appropriate circumstances and the embodiments of the invention can operate in other orientations than described or illustrated herein.

[0077] The term ‘comprising’, or its derivatives, as used in the claims, should not be interpreted as being restricted to the means listed thereafter; the term does not preclude other elements or steps. The term should be interpreted as specifying the stated features, integers, steps or components referred to, but does not preclude the presence or addition of one or more other features, integers, steps or components, or groups thereof. Thus, the scope of an expression such as ‘a device comprising means A and B’ is not solely limited to devices consisting only of components A and B. In contrast, what is meant is that, with respect to the present invention, the only relevant components of the device are A and B.

[0078] As used herein, the term ‘inert atmosphere’ means an atmosphere in which less than 2% oxygen is present.

[0079] The present invention comprises a method and a system for decomposing organic waste, so that the volume and mass of the waste to be removed is considerably reduced relative to the initial volume and the original mass. The present invention also relates to the rendering harmless of those components of the processed waste that are released (e.g. exhaust gases) before they end up in the environment.

[0080] The present method will be described in particular with regard to radioactive waste, and in particular with regard to radioactive ion exchange resin, but other types of organic waste may be processed in accordance with the following process and with the components of the system. The organic wastes that can be processed according to the present invention therefore comprise not only ion exchange resins, but also, among other things, cleaning solutions for steam generators, solvents, oils, decontamination solutions, antifreeze, dirt, sludge, nitrates, phosphates and contaminated water.

[0081] An ion exchange resin is made from organic materials, usually styrene to which amino groups are grafted to make anion resins or to which sulfone groups are grafted to form cation resins. Since these resins are used to purify cooling water in a nuclear reactor, they accumulate up to about 7% iron, calcium, silica and small amounts of other metals and cations.

[0082] The method is based on pyrolysis in a closed reactor. The solid residue from the processing of the waste, namely an inorganic grain with a high metal oxide content, is packaged for subsequent storage for a period of 200 years to 300 years. The method can further utilise a conventional exhaust gas treatment (e.g. post-combustion), but also an oxidation of the waste gas as further described. Pyrolysis is the destruction of organic material with the aid of heat in the absence of a stoichiometric amount of oxygen, i.e. in an inert atmosphere. Hence, a reactor for use in the present invention should be substantially hermetically sealable.

[0083] In the present method, the organic components of the resin are destructively distilled by heating. Upon heating, the weak chemical compounds of the polymer resins break into compounds with lower carbon numbers, including carbon, metal oxides, metal sulphides, and pyrolysis gases, which in turn comprise carbon dioxide, carbon monoxide, water, nitrogen, and hydrocarbon gases. The small volume of solid residue that remains after pyrolysis contains the vast majority of the radionuclides.

[0084] Although pyrolysis can take place over a wide range of temperatures, the present method is a pyrolysis at low temperatures, generally about 300° C. to 600° C., to prevent radioactive metals from volatilising in the ion exchange resins. These metals are therefore retained in the residue of the reactor. As a result, the low-active synthetic pyrolysis gases can then be converted at higher temperatures into carbon dioxide and water without concern for volatile radioactive metals such as caesium.

[0085] Oxidation of the waste gas, e.g. the pyrolysis gas comprising hydrocarbons, is the destruction of an organic gas at high temperatures, where a minimum amount of oxygen gas must be present. Typically, the hydrocarbon vapours are converted to carbon dioxide and water at a temperature of at least 850° C. with a residence time of the gases of at least 2 seconds and an oxygen content of at least 6 vol %. This does not constitute combustion of the waste gas.

[0086] The system for processing the ion exchange resin comprises a frame 1 on which a collecting chamber 2 is mounted, in which the resin is transported by means of transport water, in particular via supply line (not shown) and inlet 3, which inlet can be closed via closing valve 9. Optionally, the transport water can be filtered from the collecting tank 1 and discharged via a discharge line (not shown). The separation of the transport water results in a dryer resin, which lowers the residence time in the pyrolysis chamber.

[0087] From the collecting chamber 2, the resin (including or excluding the transport water) is transported to the conical pyrolysis chamber 4 via line 5 (shown in FIG. 2) and enter the housing 4 via inlet 6. In the pyrolysis chamber 4, the transport water is evaporated (if necessary) and the resin is dried and pyrolysed as described below. The waste gases generated by evaporation, drying and pyrolysis are discharged via gas outlet 7 connected to line 8. Due to gravity, the pyrolysed material falls down inside the housing 4 and leave it via outlet 12, which opens into a collection tray 10. In the embodiment shown, a sluice valve device 11 (shown in FIG. 3) is also provided between the outlet 12 and the collection tray 10. Such a sluice valve is then closed during use so that non-pyrolysed material cannot yet end up in the collection tray 10. A similar sluice valve device 13 (also shown in FIG. 3) is provided between the collection tray 2 and the inlet 6 of the pyrolysis chamber 4.

[0088] In order for the pyrolysis chamber 4 to be hermetically sealed, i.e. to ensure that substantially no oxygen is present inside the pyrolysis chamber 4, nitrogen gas is led from storage tanks 19 to each opening of the pyrolysis chamber 4. This nitrogen gas prevents oxygen gas from getting through one of the openings in the pyrolysis chamber 4 and thus disrupting the state of an inert atmosphere, causing an oxidation or combustion reaction which could lead to such high temperatures that the housing 4 could be damaged and/or an unsafe condition arises. It will be appreciated that the storage tanks 19 can also be replaced by another nitrogen supply, e.g. a nitrogen supply network.

[0089] As shown in FIG. 4, the conical housing 4 on the side wall is provided with heating means 24, in particular electric heating means, for heating the side wall. In an advantageous embodiment, these heating means 24 are incorporated in ceramic elements which are directly attached to the conical housing 4 and are suitable for generating a temperature within the housing 4 of at least 200° C., in particular at least 300° C., more in particular at least 400° C. and most in particular at least 500° C. The temperature that needs to be generated depends on the type of waste that needs to be processed and also on the phase the processing is in. For example, a temperature of 120° C. may suffice during the evaporation of the transport water, while a temperature of approximately 300° C. (depending on the type of resin) is required during the pyrolysis.

[0090] Inside the housing 4 a conical mixing body 25 is provided which is attached via cross connections 41 to a drive shaft 26 which extends through the upper side of the housing along said longitudinal direction with a first portion which is inside the housing 4 and a second portion which is outside the housing 4. The conical mixing body 25 is configured to fluidise waste inside the housing 4 by transporting it upwards along the side wall of the housing 4 by rotating the mixing body 25.

[0091] As shown schematically in FIG. 4, the mixing body 25 does not touch the housing 4 and the mixing body 25 has a free bottom side at the underside of the housing 24, as described above, to avoid accumulation of waste at the bottom of the housing 4. Such a construction is possible because the second portion of the drive shaft 26 is bearing-mounted on the frame 1 and therefore no mounting is required inside the housing 24. In particular, a double bearing is used when attaching the drive shaft 26 to the frame 1. As shown in FIG. 4, the drive shaft 26 extends in the longitudinal direction 27 of the housing 4. The drive shaft 26 is driven by an electric motor 28 shown in FIG. 1.

[0092] In an embodiment, the shortest distance between the mixing body 25 and the side wall of the housing 4 is at most 5% and in particular at most 3%, as described above, in order to avoid build-up of residue on the side wall.

[0093] The waste gas from the evaporation, drying and pyrolysis is supplied via line 8 to a supply line 14, which discharges into an inlet 33 of an oxidising device 15 (also known as an oxidiser). The supply line 14 is provided for supplying superheated oxygen gas, in particular superheated air (such as ambient air), which air was heated by means of a superheater 20 (shown schematically in FIG. 4). The oxygen gas can be provided in a tank 21 and supplied via pump 22 as shown in FIG. 4. The tank 21 is preferably the chamber in which the oxidiser 15 is located and the supplied air is hence ambient air. The pump 22 also determines how much oxygen gas is supplied and can also be used to control the speed (for example between 1 and 5 m/s) at which the mixture of oxygen gas and waste gas is supplied.

[0094] As shown in FIGS. 4 and 6, the oxidising device 15 comprises an outer chamber 16 and an inner chamber 17. Electric heating means 23 (shown in FIG. 7) are provided in the outer chamber 16 and heat the inner chamber 17, through which the mixture of waste gas and superheated air flows. This mixture is oxidised by the temperature so that an oxidised gas is discharged from the oxidising device 15 via outlet 32 and discharge line 18. The temperature of the superheated air (in particular at least 850° C., preferably at least 900° C. and more preferably almost 1000° C.) already ensures an initial heating of the waste gas such that the oxidation reaction already starts at the beginning of the inner chamber 17.

[0095] The outer chamber 16 is provided with a front wall, a rear wall, a left wall, a right wall, an upper wall and a lower wall, each of which are provided on their inner side with electric heating means 23 schematically shown in FIG. 7, a depth of the outer chamber 16 being defined as a shortest distance between its front wall and its rear wall, a width of the outer chamber 16 being defined as a shortest distance between its left wall and its right wall and a height of its outer chamber 16 being defined as a shortest distance between its lower wall and its upper wall. The outer chamber 16 is further provided with a first opening 29 and a second opening 30, through which the supply line 14 and the discharge line 18 extend, respectively.

[0096] The inner chamber 17 is completely surrounded by the outer chamber 16 as shown in FIG. 6 and has dimensions that are as close as possible to the dimensions of the outer chamber 16. In particular, the height, depth and width are each at most 15%, in particular at most 10%, less than a respective one of the depth, width and height of the outer chamber 16. It has been found that such dimensions allow the inner chamber 16 to expand due to the high temperature, but also that the total volume of the outer chamber is as small as possible to form a compact oxidiser 15. As shown in FIG. 7, the inner chamber 17 is supported on the outer chamber 16 by fastening means 31 so that direct contact between the two chambers 16, 17 is avoided. This avoids damage to the heating means 23.

[0097] It is clear that the atmosphere within inner chamber 17 is substantially completely sealed off from the atmosphere around the inner chamber 17 in the outer chamber 16. In this way the gas to be oxidised is prevented from coming into contact with the outer chamber 17, in particular with the heating means 23.

[0098] In an embodiment, the heating means 23 are designed as ceramic elements which are provided on their side facing the outer chamber 16 with heat-resistant insulation.

[0099] In order to limit the dimensions of the oxidiser 15, the inner chamber 17 is provided with mutually substantially parallel partitions 35 which increase the residence time inside the inner chamber 17 relative to an identically dimensioned chamber without partitions. Due to these partitions 35, the mixture must flow through a series of substantially U-shaped loops 36 which stimulate the mixing of the gases.

[0100] Preferably, the corners of the U-shaped loops are rounded to avoid a vortex flow that could temporarily retain a portion of the waste gas, thereby reducing the efficiency of the oxidiser 15.

[0101] In the embodiment shown, the length of each partition 35 is approximately 85% of the height of the inner chamber 17, but other lengths are also possible. In general, this length is preferably between 60% and 95% of the height (or of the width or length if the partitions are oriented according to the width or length direction). It has been found that this allows sufficient flow and also maximises the total distance that the mixture has to travel based on the ideally as low as possible dimensions of the inner chamber 17.

[0102] The number of partitions 35 and the dimensions of the inner chamber 17 are selected based on the desired residence time of the mixture. This residence time is preferably at least 2 seconds. The pump 22 can also be used to adjust the flow rate so that, given a certain total distance, the residence time is sufficient.

[0103] To prevent damage to the partitions 35, these can also be provided with insulation (not shown). This is especially advantageous with the first partition since this is the hottest zone due to the supply of the superheated air. In addition, this wall of the inner chamber 17 is also insulated.

[0104] In the embodiment shown, use is made of an odd number of partitions 35 so that the openings 29, 30 can be provided in the same wall of the outer chamber 16, on which optionally no heating means 23 are then provided, but preferably insulation is provided.

[0105] In the embodiment shown, a third opening 38 is also provided in the outer chamber 16 to which a urea supply device 37 is connected via gas inlet 40 positioned within the opening 38. As described above, the supply of urea leads to the avoidance of the formation of nitrogen oxides during the oxidation. As shown, the urea supply device 37 is connected to the nearest U-shaped loop 36 such that the supplied urea has a sufficient residence time.

[0106] In an embodiment, the outer chamber 16 is provided with a door 34 that forms a wall thereof. This is advantageous since it allows the inner chamber 17 to be replaced or cleaned in its entirety. This also makes it possible to perform maintenance on the heating means 23.

[0107] In the embodiment shown, a control device 39 is also provided which checks what volume percentage of oxygen gas is present in the oxidised gas. If this is too low, a control mechanism is activated that adjusts pump 22 so that more oxygen gas is supplied. In this way, the desired oxygen gas percentage (e.g. 6 vol %) can be obtained and maintained throughout the entire inner chamber 17.

[0108] Although certain aspects of the present invention have been described with respect to specific embodiments, it is clear that these aspects may be implemented in other forms within the scope of protection as defined by the claims.