Patent ID: 12224076

EMBODIMENTS OF THE INVENTION

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.

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.

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.

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.

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

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.

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.

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.

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.

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.

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.

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.

The system for processing the ion exchange resin comprises a frame1on which a collecting chamber2is mounted, in which the resin is transported by means of transport water, in particular via supply line (not shown) and inlet3, which inlet can be closed via closing valve9. Optionally, the transport water can be filtered from the collecting chamber2and 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.

From the collecting chamber2, the resin (including or excluding the transport water) is transported to the conical pyrolysis chamber4via line5(shown inFIG.2) and enter the housing4via inlet6. In the pyrolysis chamber4, 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 outlet7connected to line8. Due to gravity, the pyrolysed material falls down inside the housing4and leave it via outlet12, which opens into a collection tray10. In the embodiment shown, a sluice valve device11(shown inFIG.3) is also provided between the outlet12and the collection tray10. Such a sluice valve is then closed during use so that non-pyrolysed material cannot yet end up in the collection tray10. A similar sluice valve device13(also shown inFIG.3) is provided between the collection tray2and the inlet6of the pyrolysis chamber4.

In order for the pyrolysis chamber4to be hermetically sealed, i.e. to ensure that substantially no oxygen is present inside the pyrolysis chamber4, nitrogen gas is led from storage tanks19to each opening of the pyrolysis chamber4. This nitrogen gas prevents oxygen gas from getting through one of the openings in the pyrolysis chamber4and thus disrupting the state of an inert atmosphere, causing an oxidation or combustion reaction which could lead to such high temperatures that the housing4could be damaged and/or an unsafe condition arises. It will be appreciated that the storage tanks19can also be replaced by another nitrogen supply, e.g. a nitrogen supply network.

As shown inFIG.4, the conical housing4on the side wall is provided with heating means24, in particular electric heating means, for heating the side wall. In an advantageous embodiment, these heating means24are incorporated in ceramic elements which are directly attached to the conical housing4and are suitable for generating a temperature within the housing4of 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.

Inside the housing4a conical mixing body25is provided which is attached via cross connections41to a drive shaft26which extends through the upper side of the housing along said longitudinal direction with a first portion which is inside the housing4and a second portion which is outside the housing4. The conical mixing body25is configured to fluidise waste inside the housing4by transporting it upwards along the side wall of the housing4by rotating the mixing body25.

As shown schematically inFIG.4, the mixing body25does not touch the housing4and the mixing body25has a free bottom side at the underside of the housing24, as described above, to avoid accumulation of waste at the bottom of the housing4. Such a construction is possible because the second portion of the drive shaft26is bearing-mounted on the frame1and therefore no mounting is required inside the housing24. In particular, a double bearing is used when attaching the drive shaft26to the frame1. As shown inFIG.4, the drive shaft26extends in the longitudinal direction27of the housing4. The drive shaft26is driven by an electric motor28shown inFIG.1.

In an embodiment, the shortest distance between the mixing body25and the side wall of the housing4is at most 5% and in particular at most 3%, as described above, in order to avoid build-up of residue on the side wall.

The waste gas from the evaporation, drying and pyrolysis is supplied via line8to a supply line14, which discharges into an inlet33of an oxidising device15(also known as an oxidiser). The supply line14is provided for supplying superheated oxygen gas, in particular superheated air (such as ambient air), which air was heated by means of a superheater20(shown schematically inFIG.4). The oxygen gas can be provided in a tank21and supplied via pump22as shown inFIG.4. The tank21is preferably the chamber in which the oxidiser15is located and the supplied air is hence ambient air. The pump22also 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.

As shown inFIGS.4and6, the oxidising device15comprises an outer chamber16and an inner chamber17. Electric heating means23(shown inFIG.7) are provided in the outer chamber16and heat the inner chamber17, 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 device15via outlet32and discharge line18. 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 chamber17.

The outer chamber16is 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 means23schematically shown inFIG.7, a depth of the outer chamber16being defined as a shortest distance between its front wall and its rear wall, a width of the outer chamber16being defined as a shortest distance between its left wall and its right wall and a height of its outer chamber16being defined as a shortest distance between its lower wall and its upper wall. The outer chamber16is further provided with a first opening29and a second opening30, through which the supply line14and the discharge line18extend, respectively.

The inner chamber17is completely surrounded by the outer chamber16as shown inFIG.6and has dimensions that are as close as possible to the dimensions of the outer chamber16. 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 chamber16. It has been found that such dimensions allow the inner chamber16to 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 oxidiser15. As shown inFIG.7, the inner chamber17is supported on the outer chamber16by fastening means31so that direct contact between the two chambers16,17is avoided. This avoids damage to the heating means23.

It is clear that the atmosphere within inner chamber17is substantially completely sealed off from the atmosphere around the inner chamber17in the outer chamber16. In this way the gas to be oxidised is prevented from coming into contact with the outer chamber17, in particular with the heating means23.

In an embodiment, the heating means23are designed as ceramic elements which are provided on their side facing the outer chamber16with heat-resistant insulation.

In order to limit the dimensions of the oxidiser15, the inner chamber17is provided with mutually substantially parallel partitions35which increase the residence time inside the inner chamber17relative to an identically dimensioned chamber without partitions. Due to these partitions35, the mixture must flow through a series of substantially U-shaped loops36which stimulate the mixing of the gases.

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 oxidiser15.

In the embodiment shown, the length of each partition35is approximately 85% of the height of the inner chamber17, 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 chamber17.

The number of partitions35and the dimensions of the inner chamber17are selected based on the desired residence time of the mixture. This residence time is preferably at least 2 seconds. The pump22can also be used to adjust the flow rate so that, given a certain total distance, the residence time is sufficient.

To prevent damage to the partitions35, 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 chamber17is also insulated.

In the embodiment shown, use is made of an odd number of partitions35so that the openings29,30can be provided in the same wall of the outer chamber16, on which optionally no heating means23are then provided, but preferably insulation is provided.

In the embodiment shown, a third opening38is also provided in the outer chamber16to which a urea supply device37is connected via gas inlet40positioned within the opening38. 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 device37is connected to the nearest U-shaped loop36such that the supplied urea has a sufficient residence time.

In an embodiment, the outer chamber16is provided with a door34that forms a wall thereof. This is advantageous since it allows the inner chamber17to be replaced or cleaned in its entirety. This also makes it possible to perform maintenance on the heating means23.

In the embodiment shown, a control device39is 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 pump22so 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 chamber17.

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.