Improvement of energy performance in CO2 capture

Abstract

A system for capturing carbon dioxide, comprising: an absorber for bringing a CO2-containing gas stream into contact with an absorbent; a desorber for releasing CO2 from the absorbent and heating the absorbent to raise its temperature from a first temperature to a second temperature; a condenser; a lean return stream for returning the absorbent from the desorber to the absorber; a flash vessel for reducing the pressure and temperature of the lean return stream and generating a flash vapor stream; a compressor for receiving the flash vapor stream and compressing it to generate a high-temperature flash vapor stream; and a controller for controlling the temperature of the high-temperature flash vapor stream and injecting the high-temperature flash vapor stream into the desorber at a temperature below a threshold.

Description

Technical Field

[0001] The present disclosure relates to improving the energy performance of a CO2 capture process.

Background Art

[0002] Carbon capture and storage is expected to be an important method for reducing the impact of global warming caused by the combustion of fossil fuels.

[0003] The capture of carbon dioxide (CO2) involves using technologies to extract CO2 from a CO2-containing gas using an absorbent medium. Typically, this involves generating a gas stream over an absorbent medium under conditions where the medium absorbs CO2 from the gas, and then changing the conditions so that the medium releases the absorbed CO2, thereby enabling the CO2 to be captured and stored. This process can be used to reduce atmospheric CO2 and mitigate anthropogenic emissions associated with global warming or climate change. Direct air capture technology (DAC) is the capture of CO2 from the air in the atmosphere. In DAC, since the atmosphere contains less than 0.05% CO2, it is necessary to process a large amount of air.

[0004] In the temperature swing process, heat has conventionally been suggested as an effective way for a heat pump to supply the heat in a system that is heated electrically in order to release the absorbed CO2. However, opportunities remain to increase heat recovery, improve its efficiency, and at the same time improve the effectiveness of the overall system.

Summary of the Invention

[0005] According to a first embodiment, a system is provided for capturing carbon dioxide (CO2) from a CO2-containing gas stream, the system comprising: an absorber for bringing the CO2-containing gas stream into contact with an absorbent, wherein the absorbent is operable to capture CO2 from the CO2-containing gas stream in a first temperature range and release CO2 in a second temperature range; and a desorber for releasing CO2 from the absorbent, wherein the desorber receives a rich absorbent stream from the absorber and heats the absorbent using heating means for providing heat to raise the absorbent temperature from the first temperature range to the second temperature range. The system comprises an operable desorber, an exhaust conduit for supplying an exhaust flow containing CO2 and steam to a condenser, a lean return flow for returning an absorbent from the desorber to the absorber, a flash vessel for receiving the lean return flow and reducing its pressure and temperature to generate a flash steam flow, a compressor for receiving the flash steam flow and compressing it to generate a high-temperature flash steam flow, and a controller configured to control the temperature of the high-temperature flash steam flow and inject the high-temperature flash steam flow into the desorber at a temperature below a threshold.

[0006] The system may further include a mixer or a second heat exchanger, in which the condenser generates condensate, and the high-temperature flash vapor temperature can be controlled by either mixing the high-temperature flash vapor stream with the condensate in the mixer, or by passing the high-temperature flash vapor stream through a second heat exchanger to exchange heat with the condensate, in either case the controller is configured to control the temperature of the high-temperature flash vapor stream by changing the flow rates of the condensate and the high-temperature flash vapor stream.

[0007] The system may further include a high-temperature flash vapor-to-desorber heat exchanger configured to transfer heat from the high-temperature flash vapor to the desorber at a heat transfer rate selected to prevent the temperature inside the desorber from exceeding a threshold before the high-temperature flash vapor is injected into the desorber, and to cool the high-temperature flash vapor to a temperature below the threshold. The temperature threshold of the high-temperature flash vapor flow is preferably within a second temperature range. The upper limit of the second temperature range may be selected so that the rate of degradation of the absorbent is below the threshold. For example, the upper limit of the second temperature range may be less than 120°C.

[0008] The absorbent is preferably an amino acid or an amino acid salt.

[0009] The CO2-containing gas stream may be ambient air.

[0010] In another embodiment, a method is provided for capturing carbon dioxide (CO2) from a CO2-containing gas stream, the method comprising bringing the CO2-containing gas stream into contact with an absorbent in an absorber, wherein the absorbent is operable to capture CO2 from the CO2-containing gas stream in a first temperature range and release CO2 in a second temperature range, and releasing CO2 from the absorbent in a desorber, wherein the desorber is operable to receive a rich absorbent stream from the absorber, and providing heat to raise the absorbent temperature from the first temperature range to the second temperature range. The method includes heating the absorbent in the desorber using a heating means; supplying an exhaust flow containing CO2 and steam from the desorber to the condenser; returning the absorbent from the desorber to the absorber as a lean return flow; flushing the lean return flow in a flash vessel to reduce the pressure and temperature of the lean return flow and generate a flash vapor flow; compressing the flash vapor flow in a compressor to generate a high-temperature flash vapor flow; controlling the temperature of the high-temperature flash vapor flow to below a threshold and injecting the high-temperature flash vapor flow into the desorber.

[0011] The condenser generates condensate, and the method may include controlling the high-temperature flash vapor temperature by either or both of the following: mixing a high-temperature flash vapor stream with the condensate in a mixer, or passing the high-temperature flash vapor stream through a second heat exchanger to exchange heat with the condensate, and changing the flow rates of the condensate and the high-temperature flash vapor stream to provide a flow for injection into a desorber below a threshold temperature.

[0012] In addition, the method may include transferring heat from the high-temperature flash steam to the desorber using a high-temperature flash steam-to-desorber heat exchanger before the high-temperature flash steam is injected into the desorber, wherein the high-temperature flash steam-to-desorber heat exchanger has a heat transfer rate selected to prevent the temperature inside the desorber from exceeding a threshold, and cooling the high-temperature flash steam to a temperature below the threshold.

[0013] A third aspect of the present invention is a controller configured to carry out the method described herein.

[0014] Those skilled in the art will understand that, except where mutually exclusive, features described in relation to any one of the above embodiments may be applied mutatis mutandis to other embodiments. Furthermore, except where mutually exclusive, any feature described herein may be applied to any embodiment and / or combined with any other feature described herein. [Brief explanation of the drawing]

[0015] Herein, the embodiments will be described below with reference to the figures, merely as examples.

[0016] [Figure 1] This is a schematic diagram of a conventional carbon capture system. [Figure 2] This is a schematic diagram of a carbon capture system equipped with a lean-flash vapor compression subsystem. [Figure 3]This is a schematic diagram of a carbon capture system having a condensate mixer for controlling the temperature of compressed lean flash steam. [Figure 4] This is a schematic diagram of a carbon capture system having a condensate heat exchanger for controlling the temperature of compressed lean flash steam. [Figure 5] This is a schematic diagram of a carbon capture system having a condensate heat exchanger for controlling the temperature of compressed lean flash steam and an additional heat exchanger for reducing the compressed lean flash steam. [Figure 6] Figures 2 through 5 show schematic diagrams of the control system for one of the carbon capture systems. [Modes for carrying out the invention]

[0017] Detailed explanation Refer to Figure 1, which shows a conventional system for carbon capture from CO2-containing gas. The absorber 10 receives the CO2-containing gas flow from the inlet 20 to the outlet 30. The absorber may also be reversible with a combined inlet and outlet. The absorbent flow flows through the absorber, and the lean flow 40 enters the absorber, comes into contact with the CO2-containing gas, and becomes a rich flow 50. The absorbent may be recirculated within the absorber as a recirculation flow 110, thereby increasing the effective residence time of each portion of the lean flow of absorbent in the absorber. While the absorbent is in contact with the CO2-containing gas, the particles in the absorbent may absorb CO2 until equilibrium is reached, or may leave the absorber before this point.

[0018] Other thermal optimizations may be used instead of the heat exchanger 100, but typically, a rich flow flows through the main heat exchanger 100, recovering some heat from the lean flow returning from the desorber 60 and preheating the rich flow. The desorber 60 receives the rich flow and heats it to a temperature at which the CO2 is released from the absorbent, using a heating means 70 which may also generate steam to form vapor bubbles from which the desorbed CO2 can diffuse, thereby returning to the absorber to repeat the process, leaving a lean flow of the absorbent.

[0019] The steam and desorbed CO2 exit the desorber as exhaust flow 80, where a condenser 90 is used to cool the mixture, thereby condensing the steam and leaving a purer CO2 product flow. Depending on the temperature set in the condenser, more or less heat may be present in the condensed flow 120. The heat recovered in the condenser may be recovered by, for example, a heat pump and used elsewhere in the system, for example, to supply heat to a heating means 70. The condenser may be a two-stage condenser, where the first stage removes heat at a high temperature to improve the performance coefficient of the heat pump, and the second stage, operating at a lower temperature, produces a lower-temperature condensate 120. Heat is recovered using a main heat exchanger 100, which transfers heat from the high-temperature lean absorbent flow just leaving the desorber column to the lower-temperature rich absorbent flow about to enter the desorber column. Because only a limited amount of heat can be recovered, an additional heat source is still required in the desorber reboiler.

[0020] Unless otherwise stated, the same reference numbers are used throughout this document for the same or similar features.

[0021] Moving to Figure 2, the heating means 70 may be a reboiler that uses steam or recovered heat (e.g., from a heat pump) to heat the absorbent mixture in the desorber 60. To maintain the temperature in the desorber at an optimal temperature for desorption, a portion of the lean absorbent is heated and returned to the desorber, while a portion of the lean absorbent is separated to be returned to the absorber, preferably via the main heat exchanger 100. At this point, the absorbent is typically at a temperature of about 120°C or less and a pressure of about 2 bar. The drawing shows this high-temperature lean flow 210 leaving the reboiler, although this flow may branch off before the reboiler.

[0022] The high-temperature lean stream 210 enters the flash vessel 220, where the pressure is reduced to atmospheric pressure, i.e., an absolute pressure of about 1 bar (bara). As a result, a portion of the solvent carrying the absorbent, e.g., water, flashes into vapor in the flash stream 230, while the now cooler liquid phase continues to flow from the bottom of the flash vessel 220 to the main heat exchanger 100 (or other heat recovery system). The flash stream 230 is introduced into the steam compressor 240, where the flash stream 230 is compressed and the temperature of the steam increases. Then, by injecting this high-temperature compressed flash steam into the desorber 60, additional heat is provided, thereby reducing the load on the reboiler 70. However, the compressed flash steam may become too hot and may cause degradation of the absorbent when it is injected. In the system of FIG. 2, this can be controlled by using a controller to reduce the speed of the steam compressor 240, where the controller varies the compressor speed based on the temperature of the high-temperature flash stream or, using known compressor performance, varies the compressor speed based on the temperature and pressure of the high-temperature lean stream and the calculated temperature of the high-temperature flash stream based on thermodynamic calculations of the expected high-temperature flash stream.

[0023] Here, FIG. 3 shows an addition to the system shown in FIG. 3. Mixer 310 mixes the high-temperature flash vapor from compressor 240 with the condensate stream 120 to produce a mixed stream for injection into the stripper. The temperature of the mixed stream can be controlled to provide a maximum temperature for injection that is below the temperature range that degrades the absorbent. Control of the temperature of the mixed stream can be achieved by diverting a portion of the condensate stream from mixer 310. Preferably, the temperature is controlled by controlling the temperature of the condensate stream by controlling the rate of cooling of condenser 90. The control of the compressor speed described with respect to FIG. 2 can also be used to control the high-temperature flash vapor temperature. When the stripper is operating at, for example, 2 bara, flashing the lean return stream to 1 bara and then compressing the vapor to 2 bara for reinjection can cover approximately 25% of the stripper heat requirement with the compressed flash vapor. However, this usually results in compressed vapor that is hotter than the desired stripper temperature. As described below, mixing the stream with the condensate or cooling the stream with the condensate allows for a safe temperature for injection with maximum energy recovery.

[0024] FIG. 4 shows an alternative configuration where a flash stream heat exchanger 410 exchanges heat between the condensate stream 120 and the high-temperature flash stream exiting compressor 240 instead of the mixer as shown in FIG. 3. This configuration also allows the temperature for injection of the high-temperature flash vapor to be controlled, and the same control mechanisms used for the system shown in FIG. 3 may also be used in this configuration, for example, by diverting a portion of the condensate around the flash heat exchanger or by controlling the rate of cooling of condenser 90 to control the temperature of the condensate, thereby achieving the desired temperature for injection. The condensate heated by the flash heat exchanger is cooler than the injection stream and can be injected into the stripper at different positions corresponding to the temperature profile within the stripper.

[0025] The remaining condensate may be injected directly into the desorber, bypassing contact with the flash vapor, or it may be used to replenish the fluid at another point in the system. A control valve may be used to divert the flow of condensate to achieve this.

[0026] Figure 5 shows another embodiment of the system, which is a variation of the system shown in Figure 4 and has an additional heat exchanger 510, which lowers the temperature of the compressed flash vapor flow by transferring heat from the compressed flash vapor flow to the desorber, so that the compressed flash vapor flow is injected into the desorber at a temperature close to the temperature of the desorber itself. This heat exchanger can be used in conjunction with any of the systems shown in Figures 2, 3, or 4. The heat exchanger exchanges heat with the desorber over a wide area, which means that the heat transfer rate does not raise the temperature of the absorbent in the desorber above a desired threshold temperature to prevent the degradation of the absorbent. This second heat exchanger 510 with a desorber vessel should be employed if the mass flow rate of the vapor is too high or the vapor temperature is too high after compression so that the condensation flow cannot adequately cool the vapor and still maintain the liquid condensate. The second heat exchanger supplies heat to the vessel in such a manner that there are no hot spots in the desorber column that exceed the thermal degradation temperature that limits the lifetime of the absorbent. One exemplary technique is a coil wrapped around a container, as shown in Figure 5.

[0027] The heat exchanger 510 may be integrated with the desorber tower, and one way to implement such a heat exchanger is by using an external coil wound around the tower (this coil may be placed in a thermally conductive sleeve such as aluminum and wound externally with additional insulation). An alternative to integrating the condensate supply is to use only the desorber outer periphery heat exchanger without mixing with or cooling the condensate. This can be achieved by designing the heat exchanger 510 such that the desorber is not overheated by the heating means 70 and the heat exchanger 510 gradually transfers heat to the desorber without raising the temperature of the absorbent above a threshold.

[0028] Referring to Figures 2 to 5, the system described above may include a control system for controlling the temperature of the compressed flash vapor injected into the desorber. The control system may receive values ​​indicating one or more of the following: compressor speed, compressed flash vapor temperature, condenser temperature, desorber temperature at one or more locations within the desorber, and condenser temperature. The controller may have adjustable operating values ​​including one or more of the following: maximum injection temperature to maintain absorbent degradation below a threshold, required condensate temperature, heating means, and energy cost for power.

[0029] Figure 6 shows one embodiment of a controller 610 having a plurality of optional sensors 620 and control points. In some embodiments, the controller controls the speed of the compressor 650 to maintain the compressed flash vapor temperature below a safe temperature for injection into the desorber, or to achieve a desired compressed flash vapor temperature such that the compressed flash vapor temperature is at an appropriate temperature after mixing with the condensate, after heat exchange with the condensate, or after heat exchange with the absorbent.

[0030] In some embodiments, the controller controls one or more fluid control valves 640 that regulate the flow of high-temperature flash steam and / or condensate. Depending on the relative fluid flow between the compressed flash steam, condensate, and the flow rates through the desorber, the controller may control only the flow of condensate to the mixer in Figure 2 or Figure 3, or only the flash steam heat exchanger in Figure 4, to provide the necessary temperature for the compressed flash steam for injection.

[0031] The controller may control the condenser temperature to change both the volumetric flow rate and temperature of the condensate produced. The condenser 90 removes water vapor from the exhaust flow from the desorber 60. Ultimately, in carbon capture applications, it is usually desirable to remove as much vapor as possible from the exhaust. However, to minimize the plant's thermal requirements, some of the vapor may be condensed in a first condensation stage at a set temperature, while more vapor is removed in later stages. The controller may vary the condenser temperature to minimize the energy input required for the desorber. The condenser may be cooled by one or more heat pumps; for example, the condenser may be divided into one or more stages, and the heat pump may be selected to recover waste heat from the first stage of the condenser at a higher temperature (as shown in the figure) and use the heat pump output to supply heat to the heating means at a high-performance coefficient. The condenser may have a second stage that operates at a lower temperature and is cooled by another heat pump that supplies heat to the first heat pump via a cascade configuration. The controller may control the operation of the heat pump using an algorithm that determines the minimum energy requirements of the desorber, taking into account the energy available from the high-temperature flash vapor compression mixed with the condensate and the heat available from the heat pump, taking into account a performance coefficient under the conditions in the condenser. Alternatively, the condenser may be cooled using a coolant, and the controller may control the condenser temperature by altering the flow of the coolant.

[0032] Absorbent properties: Absorbents for carbon capture generally exhibit a shift in equilibrium between a carbonate or carbamate form and a state in a CO2-containing solution that is partially dependent on temperature. The absorbent is transported in a solvent, such as water, and the solvent may further contain additional additives that act as catalysts, modifying the physical properties of the solution and suppressing degradation or other desirable properties.

[0033] In a complete model of how a CO2 capture system operates, factors other than absorption efficiency may need to be considered, such as energy efficiency, absorbent degradation rate, absorbent loss, and maintenance costs. For any CO2 capture system, the absorption rate threshold may be set considering all other factors to achieve the best overall operation of the system.

[0034] The absorbent may include alkaline absorbents such as hydroxides or organic absorbents.

[0035] The alkaline absorbent may contain potassium hydroxide or calcium hydroxide.

[0036] Organic absorbents may include amines and amino acids. The amines may include ethanolamines (2-aminoethanol, monoethanolamine, ETA, or MEA).

[0037] Preferred absorbents include amines, amino acids, or alkaline salt solutions of amino acids. Amino acids may be derived from the group consisting of alanine, arginine, asparagine, aspartic acid, cysteine, glutamic acid, glutamine, glycine, histidine, isoleucine, leucine, lysine, methionine, ornithine, phenylalanine, proline, sarcosine, selenocysteine, serine, taurine, threonine, tryptophan, tyrosine, or valine. The amino acids may be compounds of amino acids such as methylamine or diethylamine.

[0038] The preferred alkaline component of amino acid salts is potassium or sodium. Examples of amino acid salts include sodium glycinate and potassium ricinate.

[0039] Amino acids are preferred because they have lower thermal requirements for elimination, degrade less than amines, and are understood to be less hazardous in use than many alternatives. However, amino acids generally have a narrower temperature range for absorption to achieve the desired CO2 absorption rate.

[0040] Degradation of absorbents reduces the effectiveness of carbon capture systems and increases maintenance requirements. Both amino acids and amines degrade rapidly when exposed to high temperatures. Therefore, the desorber temperature must be carefully controlled to achieve a sufficient rate of CO2 desorption while avoiding accelerated degradation. The invention described herein is particularly advantageous when applied to CO2 capture systems using amino acids and their salts and compounds, since the degradation of amino acids increases rapidly when exposed to temperatures above approximately 120°C. For any absorbent, the temperature-dependent degradation rate can be determined experimentally, and a threshold temperature for controlling the compressed flash vapor injection temperature can be set based on an economic or technical model of system operation. Depending on the relative availability of replacement absorbent or heating energy, this threshold level will vary according to the plant's operating model.

[0041] The present invention is not limited to the embodiments described above, and it will be understood that various modifications and improvements can be made without departing from the concepts described herein. Any of the features may be employed separately or in combination with any other features, except where mutually exclusive, and this disclosure extends to and includes all combinations and partial combinations of one or more features described herein.

Claims

1. CO2 ２ CO ２ A system for collecting contained gases from a gas stream, The aforementioned CO ２ An absorber for bringing a contained gas flow into contact with an absorbent, wherein the absorbent contains the CO in a first temperature range. ２ CO from the gas stream ２ Collect CO in the second temperature range. ２ An absorber that is operable to release, From the absorbent to the CO ２ A desorber for releasing a rich absorbent flow from an absorber, wherein the desorber is operable to receive a rich absorbent flow from the absorber and to heat the absorbent using heating means for providing heat to raise the absorbent temperature from a first temperature range to a second temperature range, CO ２ and an exhaust conduit for supplying the exhaust flow containing steam to the condenser, A lean return flow for returning the absorbent from the desorber back to the absorber, A flash vessel for receiving the lean return flow, reducing the pressure and temperature of the lean return flow, and generating a flash vapor flow, A compressor for receiving the flash vapor flow and compressing the flash vapor flow in order to generate a high-temperature flash vapor flow, A controller configured to control the temperature of the high-temperature flash vapor flow and inject the high-temperature flash vapor flow into the desorber at a temperature below a threshold, A system that includes this.

2. Further comprising a mixer or a second heat exchanger, The system according to claim 1, wherein the condenser generates condensate, and the high-temperature flash vapor temperature is controllable by either mixing the high-temperature flash vapor flow with the condensate in the mixer, or passing the high-temperature flash vapor flow through the second heat exchanger to exchange heat with the condensate, and in either case, the controller is configured to control the temperature of the high-temperature flash vapor flow by changing the flow rates of the condensate and the high-temperature flash vapor flow.

3. The system according to claim 1 or 2, further comprising a high-temperature flash steam to desorber heat exchanger configured to transfer heat from the high-temperature flash steam to the desorber at a heat transfer rate selected to prevent the temperature inside the desorber from exceeding the threshold before the high-temperature flash steam is injected into the desorber, and to cool the high-temperature flash steam to a temperature below the threshold.

4. The system according to any one of claims 1 to 3, wherein the temperature threshold of the high-temperature flash vapor flow is within the second temperature range.

5. The system according to claim 4, wherein the upper limit of the second temperature range is selected such that the rate of degradation of the absorbent is below a threshold.

6. The system according to claim 5, wherein the upper limit of the second temperature range is less than 120°C.

7. The system according to any one of claims 1 to 6, wherein the absorbent is an amino acid or an amino acid salt.

8. said CO ２ The system according to any one of claims 1 to 7, wherein the gas stream containing is ambient air.

9. CO2 ２ CO ２ A method of collecting from a gaseous stream containing the gas, The aforementioned CO ２ A step of bringing a contained gas flow into contact with an absorbent in an absorber, wherein the absorbent is in a first temperature range and the CO ２ CO from the gas stream ２ Collect CO in the second temperature range. ２ A process that is operable to release, From the absorbent in the desorber to the CO ２ A step of releasing a rich absorbent stream, wherein the desorber is operable to receive a rich absorbent stream from the absorber. A step of heating the absorbent in the desorber using heating means for providing heat to raise the temperature of the absorbent from the first temperature range to the second temperature range, CO ２ and a step of supplying the exhaust flow containing steam from the desorber to the condenser, A step of returning the absorbent from the desorber to the absorber as a lean return flow, The process involves flushing the lean return flow in a flash container to reduce the pressure and temperature of the lean return flow and generate a flash vapor flow. A step of compressing the flash vapor flow in a compressor in order to generate a high-temperature flash vapor flow, The steps include controlling the temperature of the high-temperature flash vapor flow to below a threshold and injecting the high-temperature flash vapor flow into the desorber. Methods that include...

10. The aforementioned condenser generates condensate, A step of controlling the high-temperature flash vapor temperature by either a step of mixing the high-temperature flash vapor flow with the condensate in a mixer, or a step of passing the high-temperature flash vapor flow through a second heat exchanger to exchange heat with the condensate, A step of changing the flow rates of the condensate and the high-temperature flash vapor flow in order to provide a flow for injection into the desorber below the threshold temperature. The system method according to claim 9, further comprising:

11. A step of transferring heat from the high-temperature flash steam to the desorber using a high-temperature flash steam-to-desorber heat exchanger before the high-temperature flash steam is injected into the desorber, wherein the high-temperature flash steam-to-desorber heat exchanger has a heat transfer rate selected to prevent the temperature inside the desorber from exceeding a threshold; and a step of cooling the high-temperature flash steam to a temperature below the threshold. The method according to claim 9 or 10, further comprising:

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