METHOD AND DEVICE FOR THE EFFICIENT REDUCTION OF CARBON DIOXIDE EMISSIONS

Description

[0001] The invention relates to a method for safely binding carbon dioxide with old building materials, for example cement stone, and thus ensuring safe long-term storage.

[0002] It is becoming increasingly necessary to conserve natural resources and utilize recycled materials. Carbon dioxide emissions are also a critical cause of global warming. Therefore, there is a growing focus on capturing carbon dioxide from exhaust gases and storing or utilizing it permanently. One possible method is injecting liquefied carbon dioxide into the ground. However, this method is not without controversy, as its long-term retention is not guaranteed, and any release would exacerbate the greenhouse effect. Furthermore, capturing and storing carbon dioxide requires additional energy, potentially producing more carbon dioxide in the process.

[0003] One of the most carbon-intensive industries is the cement industry. Firstly, the process requires a great deal of energy, which, when using conventional fossil fuels, leads to carbon dioxide emissions. Secondly, carbon dioxide is released from the raw material, such as limestone, as a result of the process itself.

[0004] On the other hand, large quantities of old concrete are generated when concrete structures are demolished. Therefore, the recycling of concrete, for example to produce new cement, is currently being discussed. However, a problem arises because components such as sand and hardened cement are often difficult to separate and are inextricably mixed and bonded together. The sand-free or at least low-sand component of old concrete is also referred to as recycled cement. It is known that concrete can absorb carbon dioxide during its service life, but only a fraction of the carbon dioxide released from the limestone during its production. After a long period, for example in very old buildings, this value can be around 20% relative to the calcium content of the concrete; thus, approximately one-fifth of the originally released carbon dioxide is reabsorbed very slowly and over long periods.

[0005] From WO 2020 / 058 247 A1 a process and a plant for processing material containing cement stone are known.

[0006] The use of carbon dioxide from and for cement is known from EP 3 656 750 A2.

[0007] From the subsequently published DE 10 2022 132 073 a method and a device for the efficient reduction of carbon dioxide emissions are known.

[0008] From the subsequently published DE 10 2023 113 943 a method and a device for the efficient reduction of carbon dioxide emissions are known.

[0009] The activation of old concrete using a plowshare mixer and a mill is known from the subsequently published DE 10 2023 123 525.

[0010] From AU 2020 281 669 A1, an improved process and a device for the carbonation of old concrete and / or the sequestration of CO2 are known.

[0011] From RALF HABERMANN: "Investigations into the movement behavior of bulk materials in horizontal plowshare mixers", CHEMIE INGENIEUR TECHNIK, WILEY VCH. VERLAG, WEINHEIM; DE, Vol. 89, No. 4, March 9, 2017 (2017-03-09), pages 475-479, XP071039970, ISSN: 0009-286X, DOI: 10.1002 / CITE.201600127 shows investigations into the movement behavior of bulk materials in horizontal plowshare mixers.

[0012] The purpose of the invention is to optimize the carbonation of old concrete or old cement brick.

[0013] This problem is solved by the method with the features specified in claim 1. Advantageous further developments are described in the dependent claims, the following description, and the drawing.

[0014] The process according to the invention serves for the carbonation of waste concrete or waste concrete products as the starting material. Waste concrete products are materials manufactured from waste concrete, mostly for the purpose of increasing the proportion of recycled cement and reducing the SiO₂ component. 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 these components. The advantage is that this ensures safe and long-term storage of the carbon dioxide. In a first step, the starting material is crushed. Even if it is usually already coarsely crushed, for example during the demolition of a building, the usual storage outdoors leads to the hardening of still-reactive components due to moisture, resulting in re-clumping. In addition, this coarse crushing is usually insufficient for further processing anyway.However, the initial coarse crushing process can lead to a depletion of the quartz component, resulting in a product that is essentially old concrete. Regardless of the initial coarse crushing, further crushing is necessary because the required fineness is typically not achieved in the first stage. Therefore, further crushing, if necessary, is crucial to ensure the largest possible surface area and good pore accessibility, allowing for the most efficient reaction between the solid and gas phases. In a second step, the material is moistened and treated with a carbon dioxide-containing gas. This binds the carbon dioxide within the solid, removes it from the gas, and ensures its long-term safe storage.

[0015] 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 can be easily realized is the plowshare mixer. Plowshare mixers are known from the prior art, but their specific use for carbonation is not. EP 0 500 561 B1 discloses a device 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 powdered or fine-grained masses with liquids. DE 27 29 477 C2 discloses a plowshare-like mixing tool for such devices. A similar mixing tool for such devices is also known from DE 197 06 364 C2.The company Gebrüder Lödige Maschinenbau GmbH offers corresponding mixing devices under the name ploughshare mixer, which create a mechanical fluidized bed inside.

[0016] Selecting a tool Froude number between 3 and 11 results in a particularly favorable material distribution for the carbonation of old concrete or concrete products. Since this is a reaction between the solid and the gas, close contact and efficient use of space within the mixer are crucial for effective and efficient processing. A tool Froude number between 3 and 11 ensures the highly efficient formation of a mechanically generated fluidized bed, meaning the solid is distributed throughout virtually the entire volume of the mixer. Lower tool Froude numbers, on the other hand, would lead to, for example, better mixing within the solid bed, which may be advantageous for the mixer's basic function but not for carbonation.Excessively high tool Froude numbers lead to an area in the center that is not filled with solids due to centrifugal force, thus resulting in lower efficiency. Therefore, a tool Froude number between 3 and 11 is technically advantageous for carbonation.

[0017] In a further embodiment of the invention, a plowshare mixer with cutter heads is used. A throwing mixer, such as the plowshare mixer, can optionally be equipped with so-called cutter heads. These cutter heads rotate at high speed and are therefore well suited for breaking up agglomerates. The cutter heads are particularly advantageous here because moist powder tends to agglomerate, thus reducing the free surface area. The cutter heads break up the resulting agglomerates, so that the specific surface area remains large.

[0018] From 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 those skilled in the art.

[0019] In a further embodiment of the invention, the comminution in the first step is carried out in a stirred ball mill. It has been found that the stirred ball mill allows better accessibility of the metal oxide component and thus easier and faster carbonation.

[0020] In a further embodiment of the invention, a target degree of carbonation is specified. In the concrete itself, this value can naturally increase to up to 20% over a long period, which thus represents the upper limit of the input value. While a carbonation level of 100% would be desirable, the reaction rate decreases significantly towards the end, so for economic reasons, the target carbonation level is set between 80% and 95%. After the second step, the material is examined for its current degree of carbonation. The current degree of carbonation and the target degree of carbonation are compared. This determines whether the material has already bound a sufficient amount of carbon dioxide as desired. Accordingly, the residence time of the material in the mixer is adjusted depending on 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 reduced to increase the throughput or absorption rate.

[0021] In a further embodiment of the invention, the carbon dioxide content of the carbon dioxide-containing gas supplied to the mixer is determined, as is the carbon dioxide content of the gas exiting the mixer. The residence time of the material in the mixer is adjusted depending on these two concentrations. This is particularly advantageous when the mixer is operated discontinuously. The further the reaction progresses, the lower the proportion of carbon dioxide removed from the gas stream. If this proportion becomes too low, it indicates that the material is saturated with carbon dioxide. In this case, it is advisable to terminate the batch process (and thus reduce the residence time) in order to achieve a higher absorption rate with fresh material.

[0022] In a further embodiment of the invention, a moisture content between 5 and 25 wt.%, preferably between 10 and 15 wt.%, is maintained in the mixer. For example, the material can be moistened, for instance, by being supplied with 25 wt.% moisture, and then allowed to dry out in the mixer during flow. Alternatively, a water supply can be provided in the mixer for moistening in order to maintain a constant moisture content. Moisture refers to unbound water, i.e., water that can be removed at temperatures up to 100 °C, and therefore, for example, and in particular, not water of crystallization. Since this bound water is not freely available, it is not available for the actual reaction and is therefore irrelevant.

[0023] In a further embodiment of the invention, a fill level of 30 to 60% (based on volume) is set in the mixer.

[0024] In a further embodiment of the invention, intermediate storage takes place between the first and second steps. This is the case, for example, and particularly so, in batch operation, in order to simplify and accelerate the filling and emptying of the mixer.

[0025] In a further embodiment of the invention, the material is dried before the first step. Carbon dioxide-containing gas is particularly preferred for this drying process. This carbon dioxide-containing gas is often exhaust gas, which has a certain elevated temperature. This allows the heat to be utilized and further cools the carbon dioxide-containing gas, preventing unnecessary (additional) heat from being introduced into the downstream mixer, as this would, for example, promote drying. Furthermore, the actual reaction in the subsequent mixer is exothermic, so any further energy input is counterproductive. Additionally, prior drying of the material is advantageous for the comminution process, as it prevents the formation of scale.

[0026] In a further embodiment of the invention, the material is dried after the second step. Subsequently, the moistened and cooled carbon dioxide-containing gas is fed to the mixer.

[0027] 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.

[0028] The device according to the invention is explained in more detail below with reference to an embodiment shown in the drawing. Fig. 1 Exemplary device

[0029] In Fig. 1The device is shown in a highly schematic form. Material from a raw material storage area 30 is fed to a mill 10 and crushed there. The crushed material is transferred to the mixer 20, in particular a horizontal plowshare mixer. Simultaneously, water is supplied to the mixer 20 via the water supply 22 to adjust the moisture content to, for example, 10 wt.%. The carbon dioxide-enriched product is transferred from the mixer 20 to a dryer 40, and the degree of carbonation is subsequently determined in an analytical instrument 50.

[0030] The carbon dioxide-containing gas, for example exhaust gas, is first fed to the dryer via the CO₂ supply to dry the product and, in the process, is itself cooled and moistened. This gas stream is then directed to the mixer 20, where some of the carbon dioxide binds to the old concrete. Subsequently, the depleted gas is released as exhaust gas 70. Reference sign

[0031] 10 Mill 20 Mixer 22 Water supply 30 Raw material storage 40 Drying device 50 Analysis 60 CO2 supply 70 Exhaust gas

Claims

1. Method for carbonating old concrete or old concrete products as a starting product, wherein the starting product is crushed in a first step and moistened and exposed to a carbon dioxide-containing gas in a second step, characterized in that , the second step is carried out in a mixer (20) with a mechanical fluidized bed at a tool Froude number between 3 and 11.

2. Method according to claim 1, characterized in that the comminution in the first step is carried out in an agitator ball mill.

3. Method according to one of the preceding claims, characterized in that the second step is carried out in a plowshare mixer.

4. Method according to one of the preceding claims, characterized in that a degree of carbonation to be achieved is specified, wherein the material is examined after the second step 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 adjusted depending on the comparison.

5. 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 out of the mixer (20) is determined, whereby the residence time of the material in the mixer (20) is adjusted depending on the two concentrations.

6. 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).

7. Method according to one of the preceding claims, characterized in that a filling level of 30 to 60% is set in the mixer (20).

8. Method according to one of the preceding claims, characterized in that intermediate storage takes place between the first step and the second step.

9. Method according to one of the preceding claims, characterized in that drying is carried out before the first step.

10. Method according to one of the preceding claims, characterized in that drying is carried out after the second step.

11. Method according to claim 10, characterized in that the carbon dioxide-containing gas is used for drying.

12. Method according to one of the preceding claims, characterized in that the carbon dioxide-containing gas is used in the first step.