Direct air capture systems and methods for operating a direct air capture system

Abstract

The present invention relates to a method for operating a CO2 separation device (100; 200), in particular a direct air capture system (100; 200), comprising a desorption step for desorbing CO2 from a sorbent (2) present in an adsorption or desorption chamber (1), wherein the desorption step comprises applying a negative pressure to the adsorption or desorption chamber (1) and displacing the atmosphere in the adsorption or desorption chamber (1) containing CO2 and H2O, such that a molar ratio of CO2 to H2O is more than 50%.

Description

[0001] The present invention relates to a method for operating a CO2 separation device, in particular a direct air capture system, and to CO2 separation devices, in particular direct air capture systems with lower energy consumption. State of the art

[0002] One greenhouse gas responsible for global warming is carbon dioxide (CO2). There is therefore a worldwide effort to isolate CO2 from the air and then either convert it into other products through synthesis, use it for other purposes (e.g. greenhouses, CO2-containing drinks) or store it permanently in liquid or solid form, for example in underground storage facilities. There are systems with which CO2 can be removed from the air on a large scale and isolated. Such systems are known as direct air capture systems (DAC systems) and comprise an adsorption or desorption chamber (ADC) into which ambient air is introduced, from which CO2 is selectively bound chemically or physically to a liquid or solid sorbent (or adsorbent), thus removing it from the air. When the adsorbed CO2 is needed, it is made available again by heating and, if necessary, by evaporation.The CO2 is desorbed from the sorbent under negative pressure and can then be stored or reused. The desorption of CO2 from the sorbent requires a high energy input and, in particular, high temperatures. Disclosure of the invention

[0003] In contrast, the method according to the invention for operating a CO2 separation device, in particular a DAC system, and the CO2 separation device according to the invention, in particular a DAC system, are characterized by lower energy consumption, since the temperature for desorption of the CO2 from the sorbent can be significantly reduced by about 10 to 50 Kelvin compared to previously published temperatures (100 °C).

[0004] For this purpose, the method according to the invention for operating a CO2 separation device comprises a desorption step for desorbing CO2 from a sorbent present in an adsorption or desorption chamber (hereinafter: ADK) of the CO2 separation device, in which negative pressure is applied to the ADK and the atmosphere in the ADK, which contains CO2 and H2O, is shifted such that a molar ratio of CO2 to H2O is more than 50%.

[0005] Without being bound to theory, it is assumed that the CO2 is bound to the sorbent in the form of hydronium carbamate, which has one molecule of CO2 and one molecule of H2O per adsorption site on the sorbent. Thus, during the desorption of CO2, the atmospheric composition is 50 mol% H2O and 50 mol% CO2. If, during the desorption of CO2 in the ADC, the atmospheric composition changes in favor of CO2 and thus to the detriment of H2O, such that the molar ratio of CO2 to H2O exceeds 50%, the system deviates according to the principle of least constraint, with the hydronium carbamate converting into ammonium carbamate, which contains one molecule of CO2 per two adsorption sites and no water molecule. Thus, for every two binding sites on the sorbent, two molecules of water and one molecule of CO2 are released, even though CO2 is being added to the system.This process takes place without the influence of high temperatures, solely due to the shift in the CO2 partial pressure. Thus, the temperature used for desorption can be significantly reduced, and is therefore particularly below 90 °C, and preferably only in a temperature range of 20 to 80 °C, and in particular from 20 to 60 °C, which is approximately 10 to 50 Kelvin below the otherwise usual desorption temperature of 100 °C. The achieved temperature reduction depends on the negative pressure applied for desorption; this refers to the absolute pressure in the system, which according to the invention is advantageously in a range of 50 to 200 mbar. The remaining required temperature can be achieved by introducing heat into the ADC during the desorption process, for example via a heating element. Any heating device is suitable for this purpose.

[0006] The now significantly reduced temperature during desorption results in several advantages of the process according to the invention: - The energy costs for operating the DAC system can be effectively reduced. - A lower temperature in the ADC prevents aging of the CO2 sorbent, especially when using amine group-containing or polymer-based sorbents. - By using lower temperatures, the required heating output can be provided from sources other than direct heating. - Heat pumps can also be used as heating elements, which have a higher efficiency the lower the temperature difference to the ambient temperature is.

[0007] The subclaims show preferred developments of the invention.

[0008] The temperature in the ADC during the desorption phase can be set lower the higher the molar ratio of CO2 to water. According to an advantageous development, the atmosphere in the ADC is shifted so that the molar ratio of CO2 to H2O is at least 80% and, in particular, at least 90%.

[0009] In order to ensure that a suitably advantageous atmosphere is established during the desorption phase, the negative pressure in the preferably ADC is regulated or set in a range of greater than or equal to 50 to less than or equal to 200 mbar.

[0010] There are several ways to shift the atmosphere into a desired range, i.e. in favor of CO2, all of which can be used alternatively or additively.

[0011] According to an advantageous development, the atmosphere is shifted by adding CO2 to the ADC. As explained above, this shifts the molar ratio of CO2 to water in favor of CO2, so that more CO2 is released.

[0012] The CO2 can preferably be supplied externally, i.e. from outside the CO2 separation device, if CO2 is available, for example from a storage facility.

[0013] Alternatively or additionally, the CO2 can also be recycled via a gas recirculation line from an ADC outlet to an ADC inlet. However, since the gas exiting the ADC initially contains approximately equal parts of CO2 and water, at least some of the water contained in the ADC exhaust gas must be separated in the gas recirculation line to shift the molar ratio in favor of CO2.

[0014] According to a further advantageous development, the shifting of the atmosphere in favor of CO2 is carried out by reducing a water content in the ADK.

[0015] Reducing the water content in the ADC can advantageously be achieved by contacting the gas mixture from the ADC with a second chamber comprising a drying adsorber, such as, in particular, a moisture-absorbing sorbent, a cold trap, and / or a water separator. Two or more of these water content-reducing devices can also be used in combination.

[0016] Furthermore, a first CO2 separation device or DAC system according to the invention is also described, which comprises an ADC with an air inlet and an air outlet and an exhaust gas outlet at the outlet of the ADC. The CO2 separation device or DAC system according to the invention further comprises a supply line for supplying CO2 to the ADC and / or a return line (also called a recirculation line) for returning exhaust gas from the ADC to an inlet of the ADC, wherein the return line comprises a water separator, a cold trap and / or a dry adsorber in order to separate water from the exhaust gas. For supplying external CO2 to the ADC, a CO2 tank can be provided which is connected to the air inlet of the ADC or directly to a separate inlet of the ADC and can be selectively switched on, for example by actuating a valve, so that CO2 can flow from the CO2 tank into the ADC.

[0017] In the return line, water is separated from the exhaust gas of the ADK by at least one of the above-mentioned devices, so that a high proportion of CO2 can be recovered, which can be used at least partially to shift the atmosphere in favor of CO2 in the ADK.

[0018] Furthermore, a second CO2 separation device or DAC system according to the invention is also described, which also comprises an ADC with an air inlet, an air outlet, and an exhaust gas outlet at the outlet of the ADC. The second CO2 separation device or DAC system according to the invention further comprises a second chamber with a water separator, a cold trap, and / or a drying adsorber, such as, in particular, a moisture-absorbing sorbent, which can be brought into gas-exchanging communication with the ADC during the desorption process of the DAC system, for example by opening corresponding supply lines via valves and the like.

[0019] Both CO2 separation devices or DAC systems according to the invention are designed to shift an atmosphere in the ADC, which comprises CO2 and H2O, such that the molar ratio of CO2 to H2O in this atmosphere is more than 50%. Thus, in both DAC systems, the desorption of CO2 from a sorbent present in the ADC can be carried out at lower temperatures than usual (100 °C). This also results in the following additional advantages for the DAC systems: - The energy costs for operating the DAC systems can be effectively reduced. - A lower temperature in the ADC prevents aging of the CO2 sorbent, especially when using amine group-containing or polymer-based sorbents. - By using lower temperatures, the required heating output can be provided from sources other than direct heating. - Heat pumps can also be used as a heating element, with the advantage that they have a higher efficiency the lower the temperature difference to the ambient temperature is.

[0020] In order to possibly still achieve a temperature higher than room temperature during the desorption phase, the DAC systems according to the present invention may, for example, comprise a heating element or generally any type of heating device.

[0021] The advantages, advantageous effects and further developments described for the method according to the invention also apply to the DAC systems according to the invention. Short description of the drawings

[0022] Embodiments of the invention are described in detail below with reference to the accompanying drawings. In the drawing: Fig. 1 a DAC system according to a first embodiment and Fig. 2 a DAC system according to a second embodiment. Embodiments of the invention

[0023] The figures depict only the essential elements, components, and parts of the DAC systems according to the invention. All other elements, components, and parts are omitted for clarity. Preferably, all identical parts, elements, and / or units are provided with the same reference numerals in all figures.

[0024] Fig. 1 is a schematic representation of a DAC system 100 according to a first embodiment. In detail, the DAC system 100 comprises an ADC 1, which comprises a sorbent 2 configured to reversibly bind (adsorb or absorb) CO2 from the air. The ADC 1 comprises an air inlet 3, a (residual) air outlet 4, and an exhaust gas outlet 5. The respective inlets and outlets can be provided with valves to time the inlet and outlet of the components.

[0025] At the exhaust gas outlet there is a heat exchanger 7 which acts as a water separator, with which water can be removed from the exhaust gas, so that CO2 remains, which can be temporarily stored, for example, in a storage 8.

[0026] During the adsorption process, air is fed via the air supply line 12 by means of the pump 6 to the air inlet 3 of the ADK 1 and introduced into the ADK 1. The exhaust outlet 5 is closed. In the ADK 1, the sorbent 2 removes CO2 and, if necessary, water from the air; and residual air with a reduced CO2 content and, if necessary, water, or even without CO2, leaves the ADK 1 via the air outlet 4.

[0027] During the desorption process, air inlet 3 and air outlet 4 are closed. Negative pressure is applied to the ADC 1 and, if necessary, the temperature is also increased (heating element not shown). The exhaust gas exiting the ADC 1 is then fed to a heat exchanger 7 or, alternatively, a water separator after passing through the exhaust outlet 5, so that water can be condensed from the exhaust gas and removed from the system. The remaining CO2 can pass through the storage tank 8 and is introduced into the ADC 1 via a pump 10 via a return line 9, either directly into another inlet 11 of the ADC 1 or via the air supply line 12, in which case the valve at the air inlet 3 is open.

[0028] Alternatively or additionally, CO2 can also be supplied externally to the ADK 1. The return line 9 can then be omitted. In this case, an additional CO2 storage unit 8a is provided, which can be closed by a valve and is connected via an external CO2 supply line 9a to the additional inlet 11 of the ADK 1 or the air supply line 12, so that CO2 can be introduced into the system from an external source.

[0029] Both alternatives mentioned above, i.e. the external CO2 supply 9a and the supply of CO2 via a return line 9, aim to shift the atmosphere inside the ADC 1, which comprises CO2 and H2O, in favor of CO2 so that a molar ratio of CO2 to H2O is greater than 50%.

[0030] This ensures that, according to the law of least constraint, more CO2 is released by the sorbent than would be expected for the set temperature without any further temperature increase. The temperature required to release a corresponding amount of CO2 from the sorbent 2 by operating the DAC system 100 according to the invention is approximately 10 to 50 Kelvin lower than that required for a DAC system operating at a conventional desorption temperature (100 °C), so that the energy costs for desorption in the DAC system 100 according to the invention are significantly lower, and wear on the sorbent 2 is also reduced.

[0031] Fig. Figure 2 shows a schematic view of a DAC system 200 according to a second embodiment. In this embodiment, neither a recirculation line 9 nor an external CO2 supply line 9a is provided. However, these can be added.

[0032] The DAC system 200 comprises a second chamber 13, which includes a water separator, a cold trap, and / or a drying adsorber, such as, in particular, a moisture-absorbing sorbent 14. During the desorption process, the air inlet 3 and the air outlet 4 are closed via the valves. For this purpose, the supply line 15 into the second chamber 13 and the discharge line 16 from the second chamber 13 into the ADC 1 are opened, so that the activation of the second chamber 13 results in an additional volume from which water is removed.As a result, the atmosphere in the overall system ADK 1 and second chamber 13 is shifted in favor of CO2, so that the advantages as for the DAC system 100 also arise here: the temperature which is necessary to release a corresponding amount of CO2 from the sorbent 2 by operating the DAC system 200 according to the invention is about 10 to 50 Kelvin lower compared to a DAC system which is operated at a conventional desorption temperature (100 °C), so that the energy costs for the desorption in the DAC system 200 according to the invention are significantly lower and, in addition, the wear of the sorbent 2 is reduced.

Claims

[1] Method for operating a CO2 separation device (100, 200), in particular a direct air capture system (100; 200), comprising a desorption step for desorbing CO2 from a sorbent (2) present in an adsorption or desorption chamber (1), the desorption step comprising: - Applying a negative pressure to the adsorption or desorption chamber (1), - Shifting the atmosphere in the adsorption or desorption chamber (1) containing CO2 and H2O so that a molar ratio of CO2 to H2O is more than 50%. [2] Method according to claim 1, wherein the atmosphere in the adsorption or desorption chamber (1) is shifted such that the molar ratio of CO2 to H2O is at least 80% and in particular at least 90%. [3] Method according to claim 1 or 2, wherein the negative pressure is or is set in a range of greater than or equal to 50 to less than or equal to 200 mbar. [4] Method according to one of the preceding claims, wherein the displacement of the atmosphere is carried out by supplying CO2 to the adsorption or desorption chamber (1). [5] The method according to claim 4, wherein the CO2 is supplied externally. [6] Method according to claim 4 or 5, wherein the CO2 is returned via a return line (9) from an exhaust gas outlet (5) of the adsorption or desorption chamber (1) to an inlet (3, 11) of the adsorption or desorption chamber (1), wherein in the return line (9) at least part of the water contained is separated from the exhaust gas of the adsorption or desorption chamber (1). [7] Method according to one of claims 1 to 3, wherein the shifting of the atmosphere is carried out by reducing a water content in the adsorption or desorption chamber (1). [8] Method according to claim 7, wherein the reduction of the water content is carried out by bringing the gas mixture from the adsorption or desorption chamber (1) into contact with a second chamber (13) comprising a dry adsorber, a cold trap and / or a water separator (14). [9] CO2 separation device (100), in particular a direct air capture system (100) comprising an adsorption or desorption chamber (1) with an air inlet (3), an air outlet (4) and an exhaust gas outlet (5) at the outlet of the adsorption or desorption chamber (1), the CO2 separation device (100) further comprising: - a supply line (11) for supplying CO2 into the adsorption or desorption chamber (1) or - a return line (9) for returning exhaust gas from the adsorption or desorption chamber (1) to an inlet (11) of the adsorption or desorption chamber (1), wherein the return line (9) comprises a water separator, a cold trap and / or a dry adsorber (7). [10] CO2 separation device (200), in particular a direct air capture system (200) comprising an adsorption or desorption chamber (1) with an air inlet (3), an air outlet (4) and an exhaust gas outlet (5) at the outlet of the adsorption or desorption chamber (1), the CO2 separation device (200) further comprising: - a second chamber (13) with a water separator, a cold trap and / or a dry adsorber (14), which can be brought into gas-exchanging communication with the adsorption or desorption chamber (1) during the desorption process of the CO2 separation device (200).

Images