Process for treating flue gas

Abstract

The present invention relates to a process for treating flue gas. The present invention particularly relates to a process by means of which flue gas can be depleted of iodine and the iodine removed from the flue gas can be collected and recycled or recovered.

Description

[0001] Process for treating flue gas

[0002] The present invention relates to a method for treating flue gas. In particular, the present invention relates to a method in which flue gas can be depleted of iodine, and the iodine removed from the flue gas can thereby be collected and recycled or recovered.

[0003] Chemical processes are known in which iodine is used as a reactant or catalyst. Accordingly, iodine is often found in waste streams. One example is flue gas, which often arises as a waste stream and contains iodine. However, since iodine is a valuable raw material, it is desirable to remove iodine from the waste stream, especially from flue gas, and recycle it.

[0004] DE 25 10 842 A1 discloses a process for the recovery of iodine from waste, in which waste which has been adjusted to an iodine content of 5 wt.% is burned in a combustion chamber and the iodine or the iodine compound is extracted from the resulting combustion or exhaust gas with a basic aqueous solution of sodium thiosulfate or sodium thiosulfite or is washed out of this combustion or exhaust gas.

[0005] CN 1 04 190 211 A and CN 1 03 011 084 also disclose processes for treating iodine-containing gas, in which the gas is introduced into a scrubber and washed with an aqueous scrubbing solution containing a reducing agent.

[0006] However, the state of the art still offers potential for improvement, particularly with regard to flue gas purification, in particular to collect or recover iodine contained in the flue gas more efficiently.

[0007] The object of the present invention is therefore to provide a solution to provide an improved process for smoke purification. In particular, the object of the present invention is to enable improved iodine recovery in a smoke purification process.

[0008] The object is achieved according to the invention at least in part by a method for treating flue gas having the features of claim 1. Preferred embodiments of the invention are described in the subclaims, in the description or the figures, wherein further features described or shown in the subclaims or in the description or the figures may individually or in any combination constitute an object of the invention, unless the context clearly indicates the opposite.

[0009] A process is described for treating iodine-containing flue gas in order to collect iodine contained in the flue gas, the process comprising at least the following process steps: a) providing the iodine-containing flue gas by burning iodine-containing waste having an iodine content of 8 to 40 wt.% with release of the flue gas; b) cooling the flue gas with an aqueous scrubbing solution containing a reducing agent, in particular wherein the aqueous scrubbing solution is partially evaporated; c) scrubbing the flue gas with the aqueous scrubbing solution in a first scrubbing region, wherein the iodine of the flue gas is scrubbed out into the scrubbing solution and wherein the iodine in the scrubbing solution is reduced to iodide by the reducing agent; and d) passing the scrubbing solution into a collecting tank, wherein e) the scrubbing solution is circulated between the collecting tank and the first scrubbing region with concentration of the iodide until a predetermined concentration of iodide has been established in the scrubbing solution in the collecting tank.

[0010] Such a process offers significant advantages over state-of-the-art solutions in the treatment or purification of flue gas, particularly in conjunction with the separation and collection of iodine from the flue gas.

[0011] In particular, it has been shown that, depending on the quality of the wash water used, continued recirculation of the wash solution can lead to what is known as salinization, i.e. the precipitation of salts contained in the original wash water. The iodide concentrations of the wash water cannot therefore be increased indefinitely using conventional processes. It has now surprisingly been found that this salinization occurs to a lesser extent when using iodine-containing waste with an iodide content of 8 to 40 wt.%, i.e. higher than known from the prior art, so that continued recirculation of the wash solution is possible until the wash solution has an iodide concentration in a range of > 15 wt.%. Furthermore, the process according to the invention makes it possible to concentrate the iodide in the wash solution more quickly and thus to recover it more quickly.

[0012] The process described here thus serves to purify iodine-containing flue gas and treat it in such a way that the iodine contained therein can be collected and reused. Collecting iodine within the meaning of the present invention should equally encompass the collection of the reduced form of iodine, i.e., the collection of iodide.

[0013] Iodine-containing flue gas is generated in many processes, particularly waste disposal. For example, waste streams are often thermally converted or incinerated. The resulting flue gas often contains iodine. Since iodine must be removed from a waste stream for environmental reasons and is also valuable as a raw material in many processes, the iodine should be removed from the flue gas and collected for potential further use or recycling.

[0014] For this purpose, the described method comprises at least the following method steps.

[0015] According to process step a), the iodine-containing flue gas is provided. For example, the flue gas can be generated by burning iodine-containing waste or an iodine-containing waste stream, releasing the flue gas. Iodine-containing waste, in particular, is often burned, thereby generating iodine-containing flue gas. Furthermore, such gas streams are particularly preferred for the process described here, as they have a high iodine concentration. This iodine should then be effectively removed from the flue gas.

[0016] The starting material for providing the iodine-containing flue gas in process step a) is a material, in particular an iodine-containing waste or an iodine-containing waste stream with an iodine content of 8 to 40 wt.%, preferably with an iodine content of 10 to 25 wt.%, particularly preferably with an iodine content of 13 to 18 wt.%.

[0017] As described above, the flue gas usually originates from the thermal treatment or incineration of a waste stream or waste, so the flue gas usually has a high temperature. This can be in the range of > 100°C, approximately > 250°C, for example > 500°C, approximately > 1000°C.

[0018] According to process step b), the flue gas is cooled with an aqueous scrubbing solution containing a reducing agent. This process step can also be referred to as quenching and serves primarily to rapidly cool the flue gas, which usually still has very high temperatures. For example, the flue gas can be cooled to a temperature in the range of below 100 °C, typically around 80 to 85 °C.

[0019] Furthermore, according to process step c), the flue gas is scrubbed with the aqueous scrubbing solution in a first scrubbing zone, whereby the iodine from the flue gas is scrubbed into the scrubbing solution, and the iodine in the scrubbing solution is reduced to iodide by the reducing agent. This step therefore serves in particular to remove the iodine from the flue gas. For this purpose, the iodine is scrubbed out of the flue gas by the scrubbing solution, so that it collects in the scrubbing solution. Furthermore, the iodine is directly reduced to iodide by the reducing agent in the scrubbing solution, and thus collects in ionic form in the scrubbing solution. This allows a comparatively harmless form of iodine to be collected, which can also be easily maintained in an aqueous solution.

[0020] In a particularly advantageous embodiment, the reducing solution is added via the scrubber nozzles in order to obtain the largest possible surface area for the reduction reaction.

[0021] In principle, process steps b) and c) can be performed in the same washing area, or they can be performed in different areas. Furthermore, according to process step d), the washing solution is directed into a collection tank. This collection tank serves to collect the iodide-containing washing solution and prepare it for later separation.

[0022] According to process step e), it is further provided that the washing solution is circulated between the collection tank and the first washing area, concentrating the iodide, until a predetermined iodide concentration is established in the washing solution in the collection tank. For example, the washing solution can be circulated to the area in which process step b) is carried out and to the collection tank.

[0023] This process thus takes advantage of the fact that if the scrubbing solution is continuously circulated, the same scrubbing solution continuously scrubs flue gas, or in particular, removes iodine from the flue gas and reduces it to iodide. This allows the scrubbing solution to be constantly concentrated in iodide. This cycle can be maintained until a predetermined iodide concentration is reached in the scrubbing solution.

[0024] The concentration of iodine or iodide in the scrubbing solution can be further increased by taking advantage of the fact that the flue gas has not yet cooled below 100 °C during the iodine scrubbing process or before process step b), and in particular has a temperature such that water can be evaporated by the flue gas. Accordingly, it can be exploited that water is always evaporating during quenching, i.e. in process step b), and thus the amount of scrubbing water is reduced, while at the same time the iodine or iodide content increases as described. It is of course possible to add new water, but care must be taken to ensure that the amount of water added is not too high as to reduce the iodide concentration again too drastically. For example, a constant amount of scrubbing water can be assumed.

[0025] Furthermore, it may be preferred if reducing agent is added to the scrubbing water constantly or in specific batches to ensure that the reducing effect of the scrubbing water is always maintained. Accordingly, it can generally be advantageous for the scrubbing water to be continuously checked with regard to at least one of the iodide content and the reducing agent content. Such a check can, for example, be carried out according to a predetermined pattern, for example according to a time pattern, so that a corresponding check takes place after a predefined period of time. Alternatively, a corresponding check can also take place when a predefined amount of flue gas is scrubbed or treated with the scrubbing solution or when a certain amount of iodide has accumulated in the scrubbing water.Such a determination or verification can be performed, for example, by radiometric analysis. As the iodide concentration in the wash solution increases, other loads in the system also increase. This can also be monitored by radiometric density measurement. At 17 wt% iodide content, the density should be approximately 1.3 g / cm. 3 This can therefore be a preferred embodiment for continuously and easily monitoring the process, particularly with regard to progress.

[0026] This design ensures that the iodine is always removed from the flue gas to the desired extent and is also reliably reduced to iodide in the solution, since a sufficient amount of reducing agent is still present in the scrubbing solution. Following the above, it may be particularly preferred that, regardless of the specific reducing agent used, it be continuously added to the scrubbing solution. Furthermore, this ensures that iodide accumulates in the scrubbing solution at the desired concentration.

[0027] A previously described process offers the particular advantage of producing a concentrated iodide solution directly during the flue gas treatment process. This is particularly advantageous because concentrated solutions are particularly advantageous for further use or recycling of the iodide solution, or for recycling the iodine. This eliminates the need for subsequent concentration.

[0028] This advantage can be achieved in a simple and highly defined manner according to the invention and can also be implemented in existing systems. A further advantage can be seen in the fact that further recycling is simplified by the comparatively highly concentrated iodide solution, since transporting a concentrated solution is more efficient than, for example, a solution with only a low concentration. In principle, the washing solution can be drained from the collection container for further use.

[0029] Further use of the washing solution or the iodide it contains can in principle be applied wherever iodine or iodide is needed.

[0030] After draining the iodide-concentrated wash solution, new wash solution can be added and a new cycle can start in which the iodide-concentrated wash solution is again added.

[0031] The pH of the aqueous solution or the washing solution can preferably be adjusted to a value in a range from 5 to 8. In particular, the washing solution can thus be slightly basic, which is easily possible, for example, by adding sodium hydroxide or other bases, either as a solution or in solid form. It can also be preferred if the pH is also subjected to continuous monitoring, as described above, for example, for the iodide concentration and the reducing agent concentration. The advantage of this embodiment can be seen, for example, in the fact that the reducing agent remains stable because the solution is not too acidic, and furthermore, no unwanted base-catalyzed reactions with iodine occur.

[0032] It may also be advantageous to carry out process step e) until the wash solution has an iodide concentration in a range of > 15 wt.%. For example, process step e) can be carried out until the wash water has an iodide concentration in a range of > 17 wt.%. It has been shown that such iodide concentrations in the wash water can be particularly advantageous for enabling efficient further use of the iodide.

[0033] It may also be advantageous for the flue gas to be further scrubbed with an aqueous solution in a second scrubbing area, in particular one that is spatially separate from the first scrubbing area. The second scrubbing area may be arranged approximately downstream of the first scrubbing area and configured such that the scrubbing solution used in the second scrubbing area does not participate in the circuit of the first scrubbing area. However, it is also possible for the solution from the second scrubbing area to be at least partially transferred to the circuit of the first scrubbing area, and vice versa. Furthermore, it may be advantageous for the scrubbing water from the second scrubbing area or the second scrubber stage to also contain a reducing agent and / or have a pH value as described above. Furthermore, the scrubbing water from the second scrubbing area can also be tested for the concentration of reducing agent and / or iodide.

[0034] This design allows for further scrubbing of iodine remaining in the flue gas after the first scrubbing stage. This allows the scrubbed flue gas to be cleaned or depleted of iodine particularly effectively. Furthermore, the iodine can also be collected particularly effectively and as completely as possible, enabling particularly high added value.

[0035] It may also be advantageous for the flue gas to be further purified after process step e) or after the iodine has been washed out by the scrubbing solution. In this embodiment, and thus through further purification, additional waste materials, and in particular other environmentally hazardous substances, can be removed from the flue gas, thus preferably producing a clean gas that can be released into the environment either directly or after further conventional purification steps.

[0036] The further purification steps can be selected, in particular, depending on the substances contained in the flue gas to be removed. For example, the purification steps can include adding an absorbent followed by filtration. This can remove, for example, dioxins or halogen residues from the flue gas. However, the further purification steps should not be fundamentally limited to this.

[0037] It can also be advantageous for the process to be operated under reduced pressure. In this embodiment, the flue gas can be drawn from a feeder or from a furnace, which is the location where the flue gas is generated, through the plant, for example by means of an induced draft fan, to a corresponding outlet where, for example, clean gas can leave the plant. Accordingly, the flue gas only has a limited residence time in the plant, and in the scrubber stages, for example. By setting a reduced pressure, for example using a suction fan, very defined conditions can be set with regard to the residence time of the flue gas and thus the treatment. Accordingly, in this embodiment, the collection of iodide-containing solution is possible in a particularly defined manner. Furthermore, this solution can be easily implemented in terms of equipment.

[0038] Advantageously, the reducing agent may further comprise thiosulfate. Thiosulfate has been shown to be stable under the given conditions and is also electrochemically suitable for reducing iodine to iodide. Thus, in this embodiment in particular, it can be ensured that the iodine is safely removed from the flue gas and collected as iodide.

[0039] For further advantages or technical features of the method, reference is made to the description of the device, to the figures and to the description of the figures.

[0040] Also described is an apparatus for treating iodine-containing flue gas to collect iodine contained in the flue gas, the apparatus comprising at least i) a flue gas line for conducting flue gas; ii) a scrubber having a first scrubbing region for scrubbing the flue gas, the scrubber being connected to a feed line for supplying aqueous scrubbing solution into the scrubber and to the flue gas line; iii) a collection vessel connected to the scrubber by a fluid line; and iv) a circuit for circulating scrubbing solution from the collection vessel to the scrubber.

[0041] The device serves, in particular, to carry out a method described above. Accordingly, the advantages described in detail with regard to the method can also be achieved.

[0042] To carry out the process, the device thus has a flue gas line for conveying flue gas. The flue gas line can, for example, be routed from a furnace for incinerating iodine-containing waste. The furnace, which can be configured as a rotary kiln with an afterburner chamber, can be part of the device described here.

[0043] The device further comprises a scrubber with a first scrubbing zone for scrubbing the flue gas, for example in the flue gas duct, wherein the scrubber is connected to a supply line for supplying aqueous scrubbing solution to the scrubber and to the flue gas duct. Accordingly, the flue gas can be treated with the scrubbing solution in the scrubbing zone, thus scrubbing out the iodine. If a reducing agent, such as sodium thiosulfate, is present in the scrubbing solution, the iodine in the scrubbing solution is immediately reduced to iodide. Furthermore, the flue gas duct can have a quench zone in a transition from the furnace to the scrubbing zone, in which the flue gas can be rapidly cooled, for example by scrubbing solution.

[0044] Furthermore, a collection container connected to the scrubber by a fluid line is provided as part of the device, in which the iodine- or iodide-enriched scrubbing solution can be collected. The collection container can further have an outlet from which the iodine solution can be discharged.

[0045] In order to enrich the scrubbing solution in iodide, as described in detail with regard to the process, the device further comprises a circulation system for circulating scrubbing solution from the collection container to the scrubber. The scrubbing solution can be discharged directly into the scrubbing area or partially into the quenching area described above. Accordingly, the device can preferably comprise a pump with which the scrubbing solution can be conveyed in the circuit.

[0046] The device may further comprise a feed for supplying a base or a reducing agent. This allows the conditions of the washing solution to be kept constant.

[0047] Furthermore, for improved reaction control, the device can have a detector for detecting at least one of the pH value and the reducing agent content in the scrubbing water. To reliably remove all iodine even in the event of fluctuating iodine concentrations in the flue gas, the device can also have a second scrubbing area, which can be separate from the first scrubbing area. This second scrubbing area can, for example, be connected to the first scrubbing area, so that the above-described circulation of the scrubbing solution can also include the second scrubbing area.

[0048] For further advantages or technical features of the device, reference is made to the description of the method, the figures and the description of the figures.

[0049] The invention will be explained below by way of example with reference to the accompanying drawings using preferred embodiments, wherein the features presented below can represent an aspect of the invention both individually and in combination. It shows:

[0050] Fig. 1. a schematic representation of an apparatus for carrying out a method for treating iodine-containing flue gas in order to collect iodine contained in the flue gas.

[0051] Figure 1 schematically shows an apparatus 10 for carrying out a method for treating iodine-containing flue gas in order to collect iodine contained in the flue gas.

[0052] The device 10 comprises a flue gas line 12, through which flue gas from an incinerator 14, which may be a suitable furnace with an afterburner chamber, can be conducted. Iodine-containing waste can be burned in the incinerator 14, so that the flue gas conducted in the flue gas line 12 contains iodine.

[0053] The flue gas line 12 leads into a scrubber 16, in which the flue gas can be treated with scrubbing solution. For this purpose, the scrubber 16 initially comprises a quench zone 18, in which the flue gas can be quenched or rapidly cooled by the scrubbing water. The flue gas then flows into a first scrubbing zone 20, in which the iodine contained in the flue gas can be scrubbed out of the flue gas in a first stage. For this purpose, appropriate spray heads 24 can be provided, which spray the scrubbing water into the flue gas. The flue gas then flows through a second scrubbing zone 22, in which further iodine can be removed from the flue gas. Accordingly, spray heads 26 are again provided for discharging the scrubbing solution. The spray heads 24, 26 are supplied with aqueous scrubbing solution via a corresponding supply line 28.

[0054] The device 10 further comprises a collection container 30, which is connected to the scrubber 16 by a fluid line 32. The collection container 30 serves to collect the scrubbing solution. In particular, the collection container 30 can have an outlet 38 through which scrubbing solution can be removed from the device 10. This allows the scrubbing solution to be used for further processing, for example, or to be analyzed for its constituents.

[0055] To enrich the scrubbing solution in iodide, the device 10 further comprises a circulation line 34 for circulating scrubbing solution from the collection tank 30 to the scrubber 16. For example, the circulation line 34 can lead from the collection tank 30 into the feed line 28, so that the scrubbing solution is sprayed again into the flue gas in the first scrubbing area 20, the second scrubbing area 22, and / or the quenching area 18. For this purpose, the scrubbing solution can be taken from the first scrubbing area 20 at a collection tank 40 or from the bottom 42 of the second scrubbing area 22.

[0056] In order to adjust the pH of the washing solution, in particular to a value in a range of 7-8 and / or to allow reducing conditions in the washing solution, an inlet 36 may further be provided through which a base, such as sodium bicarbonate, and / or a reducing agent, such as sodium thiosulfate, can be introduced into the washing solution.

[0057] In order to further purify the flue gas downstream of the scrubber 16, the device 10 may comprise further purification units, as shown below by way of example.

[0058] According to the design shown in Figure 1, a condenser 44 is provided downstream of the scrubber 16, which can be used to condense any iodine still present in the flue gas after the scrubber 16. Furthermore, according to Figure 1, a droplet separator 46 is provided downstream of the condenser 44, which can be supplied with fresh water or production water through an inlet 48. The droplet separator 46 can reduce the moisture content of the flue gas.

[0059] Furthermore, a heating chamber 50 is provided, which can be fed with natural gas via an inlet 52 to heat the flue gas. The thus heated flue gas can be brought into contact with, in particular, a solid adsorbent through an inlet 54 to effect additional flue gas purification.

[0060] The correspondingly loaded adsorbent can then be separated in a filter unit 56, for example, comprising a fabric filter. Downstream of the filter unit 56, the gas can now be purified and can be conveyed by a suction fan 58 into a chimney 60, through which the gas can now leave the device.

[0061] It should be noted that, since the combustion process and flue gas scrubbing, and thus also iodine recycling, are directly related, a variety of parameters throughout the process must be monitored and adjusted. This allows the corresponding boundary conditions to be at least partially controlled. Such boundary conditions include, for example, the negative pressure, which is preferably also present in the incineration plant or combustion chamber, the amount of waste fed in, the temperatures in the combustion chamber, the flow and pressure in the scrubber nozzles and the tangential lines of the quench, the pH values ​​of the scrubber stages, the excess of the reducing agent, in particular sodium thiosulfate, and any fluctuations in the composition of the waste materials fed in, in particular the contents of iodine, chlorine, sulfur, etc. However, this is readily possible for a specialist familiar with the respective plant.

[0062] 10 Device

[0063] 12 Flue gas line 14 Incineration plant

[0064] 16 washers

[0065] 18 Quench area

[0066] 20 first washing area

[0067] 22 second washing area 24 spray head

[0068] 26 spray head

[0069] 28 supply line

[0070] 30 collection containers

[0071] 32 Fluid line 34 Circulation

[0072] 36 Entrance

[0073] 38 Outlet

[0074] 40 collection containers

[0075] 42 Floor 44 Capacitor

[0076] 46 drop separators

[0077] 48 Entrance

[0078] 50 Heating chamber

[0079] 52 Inflow 54 Inlet

[0080] 56 filter unit

[0081] 58 induced draft

[0082] 60 fireplace

Claims

1. A process for treating iodine-containing flue gas in order to collect iodine contained in the flue gas, the process comprising at least the following process steps: a) providing the iodine-containing flue gas by burning iodine-containing wastes having an iodine content of 8 to 40 wt.% with release of the flue gas; b) cooling the flue gas with an aqueous scrubbing solution containing a reducing agent; c) scrubbing the flue gas with the aqueous scrubbing solution in a first scrubbing region (20), wherein the iodine of the flue gas is scrubbed out into the scrubbing solution and wherein the iodine in the scrubbing solution is reduced to iodide by the reducing agent; and d) passing the scrubbing solution into a collecting container (30), wherein e) the scrubbing solution is circulated between the collecting container (30) and the first scrubbing region (20) with concentration of the iodide until a predetermined concentration of iodide has been established in the scrubbing solution in the collecting container (30).

2. Process according to claim 1, characterized in that the pH of the washing solution is adjusted to a range of 5 to 8.

3. Process according to one of claims 1 or 2, characterized in that process step e) is carried out until the washing solution has an iodide concentration in a range of > 15 wt.%.

4. A process according to any one of claims 1 to 3, characterized in that the flue gas is produced by burning iodine-containing waste with an iodine content of 10 to 25 wt.%.

5. A process according to any one of claims 1 to 4, characterized in that the flue gas is produced by burning iodine-containing waste having an iodine content of 13 to 18 wt.%.

6. Process according to one of claims 1 to 5, characterized in that the flue gas before process step b) has a temperature in a range of > 100 °C.

7. Process according to one of claims 1 to 6, characterized in that the flue gas is further purified after process step e).

8. Process according to one of claims 1 to 7, characterized in that the reducing agent comprises thiosulfate.

9. Method according to one of claims 1 to 8, characterized in that the method is carried out under a negative pressure.

10. Process according to one of claims 1 to 9, characterized in that reducing agent is continuously added to the washing solution.

11. A process according to any one of claims 1 to 10, characterized in that the washing solution is continuously checked for at least one of the iodide content and the reducing agent content.

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