Method and device for the efficient reduction of carbon dioxide emissions

Abstract

The present invention relates to a process for the carbonation of old concrete or old concrete products as starting product, wherein the starting product is crushed in a first step and moistened in a second step and exposed to a carbon dioxide-containing gas, characterized in that the second step is carried out in a mixer 20 a mechanical fluidized bed with a tool Froude number between 3 and 11.

Description

The invention relates to a method for securely binding carbon dioxide with old construction materials, for example cement brick, and thus ensuring a secure long-term storage.It is increasingly necessary to conserve natural resources and also to rely on recycled material. Likewise, the emissions of carbon dioxide are critical as the cause of global warming. Therefore, there is an increasing emphasis on the separation of carbon dioxide from the exhaust gas and its permanent storage or use. One possible form is the introduction as liquefied carbon dioxide into the soil. However, this method is not indispensable, since permanent retention is not necessarily guaranteed and, if it escapes, the greenhouse effect would be reinforced again, especially since further energy is required for separation and storage and carbon dioxide is thus potentially produced again.One of the carbon dioxide intensive industries is the cement industry. On the one hand, a large amount of energy is required for the process, which leads to carbon dioxide emission in the case of conventional fossil fuels. On the other hand, carbon dioxide is released from the raw material, for example limestone, as a result of the process.On the other hand, when concrete structures are torn off, large amounts of used concrete are obtained. Thus, it is currently discussed to refurbish concrete to make new cement, for example. However, it is problematic here that, for example, sand and the set cement are mixed and connected to one another in a difficult-to-separate manner. The sandy or at least low-sand component of the used concrete is also referred to as used cement brick. It is known that concrete can absorb carbon dioxide during the lifetime, but only a fraction of the carbon dioxide released from the limestone during production. After a long time, for example in very old buildings, this value can be about 20% based on the calcium content of the concrete, and it is therefore very slow and thus restored to about 1 / 5 of the originally released carbon dioxide over long periods of time.WO 2020 / 058 247 A1 discloses a method and a plant for preparing material containing cement brick.EP 3 656 750 A2 discloses the use of carbon dioxide for cement.From post-published DE 10 2022 132 073 a method and a device for the efficient reduction of carbon dioxide emissions is known.From post-published DE 10 2023 113 943 a method and a device for the efficient reduction of carbon dioxide emissions is known.The post-published DE 10 2023 123 525 discloses the activation of used concrete by means of a ploughshare mixer and a mill.It is the object of the invention to optimize the carbonation of used concrete or used cement brick.This object is achieved by the method having the features specified in claim 1. Advantageous further developments are evident from the dependent claims, the following description and the drawing.The method according to the invention serves for the carbonation of used concrete or used concrete products as the starting product. Old concrete products are materials produced from old concrete, mostly with the purpose of enriching the proportion of old cement brick and reducing the SiO 2- Coumponte. This is because only components such as CaO, MgO and FeO are capable of binding carbon dioxide. Carbon dioxide, for example from exhaust gases, is to be bound to this. An advantage is that this provides a safe and long-term storage of the carbon dioxide. The starting product is comminuted in a first step. Even if this is usually already roughly comminuted, for example, when a building is torn off, the usual outdoor storage leads to the setting of still reactive constituents and thus to renewed agglomeration due to wetting. In addition, this coarse comminution is usually not sufficient for further processing in any case. However, during the coarse comminution, a first depletion of the quartz component can occur, that is to say a used concrete product. Regardless of the first coarse comminution, further comminution is necessary, since the necessary finenesses cannot usually be achieved in the first coarse comminution. Therefore, optionally renewed comminution is important in order to ensure the largest possible surface area and good pore accessibility, so that a reaction between solid and gas phase can take place as efficiently as possible. In a second step, the material is humidified and acted upon with a carbon dioxide-containing gas. As a result, the carbon dioxide is bound in the solid, removed from the gas and stored in a long-term secure manner.According to the invention, the second step is carried out in a mixer with a mechanical fluidized bed having a tool Froude number between 3 and 11. An exemplary and preferred mixer with which such a mechanical fluidized bed is simple to implement is the ploughshare mixer. Ploughshare mixers are known from the prior art, but the particular use for carbonation is not. EP 0 500 561 B1 discloses an apparatus for mixing and thermally treating solid particles with a substantially horizontally arranged container. DE 1 051 250 discloses a method and a device for mixing pulverulent or fine-grained compositions with liquids. DE 27 29 477 C2 discloses a ploughshare-like mixing tool for such apparatuses. A similar mixing tool for such devices is also known from DE 197 06 364 C2. Corresponding mixing devices are offered by Gebrüder Lödige Machinbau GmbH under the name Ploughshare mixer and produce a mechanical fluidized bed in their interior.In another embodiment of the invention, a ploughshare mixer with cutter heads is used. A launcher mixer, to which the ploughshare mixer belongs, can optionally be equipped with so-called cutter heads. These cutter heads are fast-running and therefore well suited for the destruction of agglomerates. At this point, the cutter heads are particularly advantageous since moist powder tends to agglomerate and is associated with a reduction in the free surface. The cutter heads destroy the agglomerates formed, so that the specific surface area remains large.Habermann, Investigations on the movement behavior of bulk materials in horizontal ploughshare mixers, Chem. Ing. Tech., 2017, 89, No. 4, 475-479, Wiley-VCH, it is known that the determination of the tool Froude number in mixers is familiar to the skilled person.In a further embodiment of the invention, the comminution in the first step is carried out in an agitator ball mill. It has been found that the stirred ball mill enables better accessibility of the metal oxide component and thus easier and faster carbonation.In a further embodiment of the invention, a degree of carbonation to be achieved is predetermined. In concrete itself, this value can naturally increase to up to 20% after a long time, which thus represents the upper limit of the input value. Although a degree of carbonation of 100% would be desirable, the reaction rate decreases sharply at the end, so that for economic reasons the predefined degree of carbonation to be achieved is between 80% and 95%. After the second step, the material is examined for the current degree of carbonation. The actual degree of carbonation and the degree of carbonation to be achieved are compared. It is thus determined whether or not the material has already bound sufficient carbon dioxide as desired. Accordingly, the residence time of the material in the mixer is adjusted as a function of the degree of carbonation already achieved. If the degree of carbonation is too low, the residence time is increased, if the degree of carbonation is too high, the residence time can be decreased in order to increase the throughput or the absorption rate.In a further embodiment of the invention, the carbon dioxide content of the carbon dioxide-containing gas fed to the mixer is determined and the carbon dioxide content of the gas coming from the mixer is also determined. The residence time of the material in the mixer is adjusted as a function of the two concentrations. This is particularly preferred if the mixer is operated discontinuously. The further the reaction proceeds, the lower is the proportion of carbon dioxide removed from the gas stream. If this proportion becomes too low, this is an indication of a carbon dioxide-saturated material. It is then expedient to end the batch run (and thus the residence time) in order to be able to achieve a higher absorption again with fresh material.In a further embodiment of the invention, a moisture content of between 5 and 25 wt %, preferably between 10 and 15 wt %, is set in the mixer. For example, the material can be moistened, for example supplied with 25 wt % moisture, and then dried out in the mixer during the flow. However, a water supply for moistening can also be provided in the mixer in order to keep the moisture constant. Moisture is understood to mean unbound water, i.e. which is removable up to 100° C., i.e. for example and in particular no water of crystallization. Since this bound water is not freely available, it is also not available for the actual reaction and is therefore not relevant.In a further embodiment of the invention, a filling degree of 30 to 60% is set in the mixer (based on the volume).In a further embodiment of the invention, intermediate storage takes place between the first step and the second step. This is, for example and in particular, the case in a batch operating mode in order to simplify and speed up the filling and emptying of the mixer.In a further embodiment of the invention, drying of the material is carried out before the first step. The carbon dioxide-containing gas is particularly preferably used for the drying. The carbon dioxide-containing gas is often an exhaust gas which has a certain elevated temperature. On the one hand, the heat can be utilized in this way, and on the other hand, the gas containing the carbon dioxide is cooled further in order not to introduce unnecessary (additional) heat into the downstream mixer, since this would promote drying out, for example. In addition, the actual reaction in the subsequent mixer is exothermic, so that a further supply of energy is therefore already counterproductive. Furthermore, a preceding drying of the material is advantageous for the comminution process, since a formation of scabs is thus prevented.In a further embodiment of the invention, drying of the material is carried out after the second step. The humidified and cooled carbon dioxide-containing gas is then fed to the mixer.In a further embodiment of the invention, the carbon dioxide-containing gas is used in the first step. This applies in particular if the starting material has a higher moisture content than the required 25% by weight.The device according to the invention is explained in more detail below with reference to an exemplary embodiment shown in the drawing.FIG. 1 shows an example of a deviceIn Fig. 1, the apparatus is shown highly schematically. From a raw material storage 30, the material is fed to a mill 10 and comminuted there. The comminuted material is transferred into the mixer 20, in particular a horizontal ploughshare mixer. At the same time, water is supplied to the mixer 20 via the water supply 22 in order to set the moisture content of, for example, 10% by weight. The product enriched with carbon dioxide is transferred from the mixer 20 into a dryer 40 and the degree of carbonation is subsequently determined in an analysis 50.The carbon dioxide-containing gas, for example an off-gas, is first fed to the dryer via the CO 2- feed in order to dry the product and in the process to be cooled and humidified itself. This gas stream is then passed into the mixer 20 where parts of the carbon dioxide are bound to the used concrete. The depleted gas is then discharged as exhaust gas 70.Reference numerals denote reference numerals10 Mill 20 Mixer 22 Water feed 30 Raw material store 40 Drying device 50 Analytics 60 CO 2- feed 70 Waste gasReferences included in the specificationThis list of documents cited by the applicant has been produced in an automated manner and is only included for the better information of the reader. The list is not part of the German patent application or utility model application. The DPMA does not take any adhesion for any faults or omissions.Patent Literature citedWO 2020 / 058 247 A1

[0005] EP 3 656 750 A2

[0006] DE 10 2022 132 073

[0007] DE 10 2023 113 943

[0008] DE 10 2023 123 525

[0009] EP 0 500 561 B1

[0013] DE 1 051 250

[0013] DE 27 29 477 C2

[0013] DE 197 06 364 C2

[0013] Cited Non-Patent LiteratureHabermann, Investigations on the movement behavior of bulk materials in horizontal ploughshare mixers, Chem. Ing. Tech., 2017, 89, No. 4, 475-479, Wiley-VCH

[0015]

Claims

Method for the carbonation of used concrete or used concrete products as the starting product, wherein the starting product is comminuted in a first step and moistened in a second step and is acted upon with a carbon dioxide-containing gas, characterized in that the second step is carried out in a mixer (20) having a mechanical fluidized bed at a tool Froude number between 3 and 11.Method according to claim 1, characterised in that the comminution in the first step is carried out in an agitator ball mill.Method according to one of the preceding claims, characterized in that the second step is carried out in a ploughshare mixer.Method according to one of the preceding claims, characterized in that a degree of carbonation to be achieved is predetermined, wherein the material after the second step is examined for the current degree of carbonation, wherein the current degree of carbonation and the degree of carbonation to be achieved are compared, wherein the residence time of the material in the mixer (20) is adapted as a function of the comparison.Method according to one of the preceding claims, characterized in that the carbon dioxide content of the carbon dioxide-containing gas fed to the mixer (20) is determined and the carbon dioxide content of the gas coming from the mixer (20) is determined, wherein the residence time of the material in the mixer (20) is adjusted as a function of the two concentrations.Method according to one of the preceding claims, characterized in that a moisture content of between 5 and 25 wt.%, preferably between 10 and 15 wt.%, is set in the mixer (20).Method according to one of the preceding claims, characterized in that a filling degree of 30 to 60% is set in the mixer (20).Method according to one of the preceding claims, characterized in that intermediate storage takes place between the first step and the second step.Method according to one of the preceding claims, characterized in that drying is carried out before the first step.Method according to one of the preceding claims, characterized in that drying is carried out after the second step.Method according to claim 10, characterised in that the carbon dioxide-containing gas is used for the drying.Process according to one of the preceding claims, characterized in that the carbon dioxide-containing gas is used in the first step.

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