WO2024184064A1

WO2024184064A1 - Improvements to energy performance in co2 capture

Info

Publication number: WO2024184064A1
Application number: PCT/EP2024/054298
Authority: WO
Inventors: Gervas Franceschini; Gregory John SALE; Imogen Francesca PARKER
Original assignee: Equinor Low Carbon Uk Ltd
Current assignee: Equinor Low Carbon Uk Ltd
Priority date: 2023-03-08
Filing date: 2024-02-20
Publication date: 2024-09-12
Anticipated expiration: 2025-09-08
Also published as: GB202303365D0; GB2627940A; AU2024232523A1; CN120835806A

Abstract

A system for the capture of carbon dioxide, with an absorber to contact a CO2 containing gas stream with a sorbent, a desorber to release the CO2 from the sorbent and heat the sorbent to increase the sorbent temperature from a first temperature to a second temperature, a condenser, a lean return stream to return the sorbent from the desorber to the absorber, a flash vessel for reducing the pressure and temperature of the lean return stream and producing a flash vapor stream, a compressor for receiving the flash vapor stream and compressing it to produce a hot flash vapor stream and a controller to control the hot flash vapor stream temperature and inject it into the desorber at a temperature below a threshold.

Description

TITLE

IMPROVEMENTS TO ENERGY PERFORMANCE IN CO2 CAPTURE

Technical Field

The present disclosure concerns improvements to energy performance of a CO2 capture process.

Carbon capture and storage is expected to be a significant way to reduce the effects of global warming from the combustion of fossil fuels.

Capture of carbon dioxide (CO2) involves using technology for extracting CO2 from a CO2 containing gas using an absorbent medium. Typically, this involves creating a gas flow over the absorbent medium under conditions where the medium will absorb CO2 from the gas, and then altering the conditions so that the medium releases the absorbed CO2 allowing it to be captured and stored. This process may be used to reduce atmospheric CO2 to mitigate the anthropogenic emissions that are associated with global warming, or climate change. Direct Air Capture (DAC) is the capture of CO2 from atmospheric air in which, as the atmosphere contains less than 0.05% CO2, involves processing large volumes of air.

Heat is used to release the absorbed CO2 in temperature swing methods, and it has previously been suggested that in an electrically heated system, a heat pump may be an effective way to supply that heat. There remain however, opportunities to increase the heat recovery and improve its efficiency, and to improve the effectiveness of the whole system at the same time.

Summary of Invention

According to a first aspect there is provided a system for the capture of carbon dioxide, CO2, from a CO2 containing gas stream, the system comprising an absorber to contact the CO2 containing gas stream with a sorbent, the sorbent operable to capture CO2 from the CO2 containing gas stream in a first temperature range and release CO2 at a second temperature range; a desorber to release the CO2 from the sorbent, the desorber operable to receive a rich sorbent stream from the absorber, heat the sorbent using heating means to provide heat to increase the sorbent temperature from the first temperature range to the second temperature range, an exhaust conduit to supply an exhaust stream comprising CO2 and vapor to a condenser; a lean return stream to return the sorbent from the desorber to the absorber; a flash vessel for receiving the lean return stream, reducing the pressure and temperature of the lean return stream and producing a flash vapor stream; a compressor for receiving the flash vapor stream and compressing it to produce a hot flash vapor stream; and a controller configured to control the hot flash vapor stream temperature and inject it into the desorber at a temperature below a threshold.

The system may further comprise a mixer or a second heat exchanger, wherein the condenser produces condensate and the hot flash vapor temperature is controllable by either: mixing the hot flash vapor stream with condensate in the mixer, or passing the hot flash vapor stream though the second heat exchanger for exchanging heat with the condensate, and in either case the controller is configured to control the hot flash vapor stream temperature by altering the flow rates of the condensate and hot flash vapor stream.

The system may further comprise a hot flash vapor to desorber heat exchanger, configured to transfer heat from the hot flash vapor to the desorber before the hot flash vapor is injected into the desorber, at a rate of heat transfer that is selected to prevent the temperature within the desorber from exceeding the threshold, and also cooling the hot flash vapor to a temperature below the threshold. The temperature threshold for the hot flash vapor stream is preferably within the second temperature range. The upper end of the second temperature range may be chosen such that the rate of degradation of the sorbent is below a threshold. For example, the upper end of the second temperature range may be less than 120°Celcius.

The sorbent is preferably an amino acid or an amino acid salt.

The CO2 containing gas stream may be ambient air. In another aspect there is provided a method for the capture of carbon dioxide, CO2, from a CO2 containing gas stream, the method comprising contacting the CO2 containing gas stream with a sorbent in an absorber, the sorbent operable to capture CO2 from the CO2 containing gas stream in a first temperature range and release CO2 at a second temperature range; releasing the CO2 from the sorbent in a desorber, the desorber operable to receive a rich sorbent stream from the absorber, heating the sorbent in the desorber using heating means to provide heat to increase the sorbent temperature from the first temperature range to the second temperature range; supplying an exhaust stream comprising CO2 and vapor from the desorber to a condenser; returning the sorbent from the desorber to the absorber as a lean return stream; flashing the lean return stream in a flash vessel, reducing the pressure and temperature of the lean return stream and producing a flash vapor stream; compressing the flash vapor stream in a compressor to produce a hot flash vapor stream; and controlling the hot flash vapor stream temperature below a threshold and injecting it into the desorber.

The condenser produces condensate, and the method may include controlling the hot flash vapor temperature by one or both of: mixing the hot flash vapor stream with the condensate in a mixer, or passing the hot flash vapor stream though a second heat exchanger for exchanging heat with the condensate, and altering the flow rates of the condensate and hot flash vapor stream to provide a stream for injection into the desorber below the threshold temperature.

Additionally, the method may include transferring heat from the hot flash vapor to the desorber before the hot flash vapor is injected into the desorber, using a hot flash vapor to desorber heat exchanger, the hot flash vapor to desorber heat exchanger having a rate of heat transfer selected to prevent the temperature within the desorber from exceeding the threshold, and also cooling the hot flash vapor to a temperature below the threshold.

A third aspect of the invention is a controller, configured to implement the method described herein. The skilled person will appreciate that except where mutually exclusive, a feature described in relation to any one of the above aspects may be applied mutatis mutandis to any other aspect. Furthermore except where mutually exclusive any feature described herein may be applied to any aspect and/or combined with any other feature described herein.

Brief Description of Drawings

Embodiments will now be described by way of example only, with reference to the Figures, in which:

Fig. 1 is a schematic of a prior art carbon capture system;

Fig. 2 is a schematic of a carbon capture system with a lean flash vapor compression subsystem.

Fig. 3 is a schematic of a carbon capture system with a condensate mixer to control the temperature of the compressed lean flash vapor.

Fig. 4 is a schematic of a carbon capture system with a condensate heat exchanger to control the temperature of the compressed lean flash vapor.

Fig. 5 is a schematic of a carbon capture system with a condensate heat exchanger to control the temperature of the compressed lean flash vapor and a further heat exchanger to reduce the compressed lean flash vapor.

Fig. 6 is a schematic of a control system for a carbon capture system of any of figures 2 to 5.

Detailed

With reference to Fig. 1 , which shows a conventional system for carbon capture from a CO2 containing gas. Absorber 10 receives a CO2 containing gas stream flowing from the inlet 20 to outlet 30. The absorber could also be reversible, with the input and outlet combined. A sorbent stream flows through the absorber, a lean stream 40 enters the absorber, contacts the CO2 containing gas and becomes a rich stream 50. Sorbent may be recirculated within the absorber as a recirculation stream 110, which increases the effective residence time of each portion of the lean stream of sorbent in the absorber. While the sorbent is in contact with the CO2 containing gas, particles within the sorbent may sorb CO2 until an equilibrium is reached, or leave the absorber before this point.

The rich stream is typically passed through main heat exchanger 100 to recover some heat from the lean stream returning from the desorber 60 and preheat the rich stream, although other heat optimisations may be used instead of heat exchanger 100. Desorber 60 receives the rich stream and heats it up to a temperature where the CO2 will be released form the sorbent, typically using heating means 70 which may also generate steam to form vapour bubbles into which the desorbed CO2 can diffuse, leaving a lean stream of sorbent to return to the absorber to repeat the process.

The vapour and desorbed CO2 exit the desorber as exhaust stream 80, where a condenser 90 is used to cool the mixture causing the vapour to condense leaving a purer CO2 product stream. Depending on the temperature set in the condenser, more or less heat may be present in the condensate stream 120. The heat recovered at the condenser may be recovered by e.g. a heat pump and used elsewhere in the system, e.g. to supply heat to heating means 70. The condenser may be a two stage condenser, with the first stage removing heat at a high temperature to improve the co-efficient of performance of a heat pump, and the second stage operating at a lower temperature producing cooler condensate 120. Heat is recovered using a main heat exchanger 100 that transfers heat from the hot lean absorbent stream that has just exited the desorber column to the cold rich absorbent stream that is about to enter the desorber column. This is only able to recover a limited amount of heat so additional heat sources are still required in the desorber reboiler.

The same reference numbers will be used for the same or similar features throughout this document except where mentioned.

Moving on to Fig. 2, heating means 70 may be a reboiler using steam or recovered heat (e.g. from a heat pump) to heat the sorbent mixture in the desorber 60. Some of the lean sorbent is heated and returned to the desorber to maintain the temperature within the desorber at the optimum temperature for desorption, while some of the lean sorbent leaves to be returned to the absorber, preferably via main heat exchanger 100. The sorbent at this point is typically at a temperature around or below 120 Celcius and pressure about 2 Bar. Although the drawing shows this hot lean stream 210 leaving the reboiler, this stream may branch off before the reboiler.

Hot lean stream 210 enters flash vessel 220, where the pressure is reduced to atmospheric pressure, approximately 1 bar absolute (bare), resulting in some of the solvent, e.g. water, that is carrying the sorbent to flash to steam in flash stream 230, while the now cooler liquid phase continues from the bottom of the flash vessel 220 to main heat exchanger 100 (or other heat recovery system). The flash stream 230 passes into vapor compressor 240 where it is compressed raising the temperature of the vapor. This hot, compressed flash vapor is then injected into the desorber 60 providing additional heat and thus reducing the load on the reboiler 70. However, there is a risk that the compressed flash vapor may be too hot, causing degradation of the sorbent when it is injected. In the system of Figure 2 this may be controlled by reducing the speed of the vapor compressor 240 using a controller, the controller varying the compressor speed based on the temperature of the hot flash stream, or a calculated temperature of the hot flash stream based on the hot lean stream temperature and pressure and thermodynamic calculations of the expected hot flash stream using the known compressor performance.

Figure 3 now shows an addition to the system shown in Figure 3. Mixer 310 combines the hot flash vapor from the compressor 240 with the condensate stream 120 to produce a mixed stream for injection into the desorber. The temperature of the mixed stream can be controlled to provide a maximum temperature for injection that is below the range of temperatures that would degrade the sorbent. Control of the mixed stream temperature may be achieved by diverting a portion of the condensate stream away from the mixer 310. Preferably the temperature is controlled by controlling the temperature of the condensate stream by controlling the rate of cooling the condenser 90. The control of compressor speed described with respect to figure two may also be used to control the hot flash vapor temperature. When the desorber is operating at e.g. 2 bara, flashing the lean return stream to 1 bara then compressing the vapor to 2 bara for reinjection, around 25% of the desorber heat requirements can be met by the compressed flash vapor. However this normally results in a compressed vapor that is hotter than the desired desorber temperature. Mixing the stream with condensate or cooling it with condensate as described below allows a safe temperature for injection with maximum energy recovery.

Figure 4 shows an alternative arrangement where instead of a mixer as shown in Figure 3, a flash stream heat exchanger 410 exchanges heat between the condensate stream 120 and the hot flash stream leaving compressor 240. This arrangement also allows the temperature for injection of the hot flash vapor to be controlled, and the same control mechanisms as may be used for the system shown in figure 3 may also be used here, for example diverting a portion of the condensate around the flash heat exchanger, or controlling the temperature of the condensate by controlling the rate of cooling the condenser 90, so as to achieve the desired temperature for injection. The condensate that has been heated by the flash heat exchanger will be cooler than the injection stream, and may be injected into the desorber at a different position commensurate with the temperature profile within the desorber.

The remaining condensate may bypass the contact with the flash vapor and be injected directly into the desorber, or may be used to make up the fluid at another point in the system. A control valve may divert the flow of condensate to achieve this.

Figure 5 shows another embodiment of the system, which as shown is a modification of the system shown in figure 4 and has an additional heat exchanger 510 to transfer heat from the compressed flash vapor stream to the desorber, reducing the temperature of the compressed flash vapor stream so that it is injected into the desorber at a temperature close to the temperature of the desorber itself. This heat exchanger could be used with any of the systems shown in figure 2, 3 or 4. The heat exchanger exchanges heat with the desorber over an extended area, meaning that the heat transfer rate does not raise the temperature of sorbent within the desorber above the desired threshold temperature to prevent sorbent degradation. If the mass flow rate of steam is too high or its temperature is too high after compression for the condensate stream to cool it sufficiently and still maintain liquid condensate then this second heat exchanger 510 with the desorber vessel is to be employed. The second heat exchanger provides heat to the vessel in such a way that there are no hot spots in the desorber column above the life-limiting thermal degradation temperature of the absorbent. One example approach is a coil wrapped around the vessel as shown in Figure 5.

Heat exchanger 510 may be integrated with the desorber column; one method of implementing such a heat exchanger by using an external coil wrapped around the column (this coil may be set into a thermally conductive sleeve such as aluminium and wrapped externally by additional insulation). An alternative approach instead of integrating the condensate feed is to use the desorber wraparound heat exchanger only without mixing or cooling with the condensate. This can be achieved by heating the desorber less with heating means 70, and designing the heat exchanger 510 so that it transfers heat gradually to the desorber without raising the temperature of the sorbent above the threshold.

In the systems described above with reference to figures 2 to 5, a control system may be included which controls the temperature of the compressed flash vapor injection into the desorber. The control system may receive values indicative of one or more of the compressor speed, the compressed flash vapor temperature, the condenser temperature, the desorber temperature at one or more positions within the desorber, the condenser temperature. The controller may have adjustable operating values including one or more of maximum injection temperature to keeps sorbent degradation below a threshold, required condensate temperature, energy price for heating means and power.

Figure 6 shows an embodiment of the controller 610 with a number of optional sensors 620 and control points. In some embodiments, the controller controls the speed of the compressor 650 to achieve a desired compressed flash vapor temperature, either to keep it below the safe temperature for injection into the desorber, or so that it will have the correct temperature after mixing with condensate, exchanging heat with condensate or exchanging heat with sorbent.

In some embodiments, the controller controls one or more fluid control valves 640 that adjust the flow of hot flash vapor and/or condensate. Depending on the relative fluid flows between compressed flash vapor, condensate and the rate of flow though the desorber, the controller may control just the condensate flow into the mixer of figures 2 or 3, or just the flash vapor heat exchanger of figure 4, in order to provide the required temperature of compressed flash vapor for injection.

The controller may control the temperature of the condenser to alter both the volumetric rate and temperature of condensate produced. The condenser 90 removes water vapor from the exhaust stream from the desorber 60. Ultimately in a carbon capture application it is usually desirable to remove as much vapor as possible from the exhaust. However to minimise heat requirements of the plant, some of the vapor may be condensed in a first condensing stage at a set temperature, while more vapor is removed at later stages. The controller may vary the temperature of the condenser to minimise the energy input required to the desorber. The condenser may be cooled by one or more heat pumps, e.g. the condenser may be split into one or more stages, and a heat pump may be selected to recover waste heat ( as shown on the figures) from the first stage of the condenser at a higher temperature, and use the heat pump output to supply heat to the heating means at a high coefficient of performance. The condenser may have a second stage that is cooled by another heat pump operating at a lower temperature and delivering heat via a cascade arrangement to the first heat pump. The controller may control the operation of the heat pumps using an algorithm that determines the minimum energy requirement of the desorber taking into account the energy available from the hot flash vapor compression combined with the condensate, and the heat available from the heat pump taking into account the coefficient of performance under the conditions in the condenser. Alternatively the condenser may be cooled using a coolant and the controller may control the condenser temperature by varying the flow of coolant.

Sorbent

Sorbents for carbon capture generally have a changing equilibrium between the carbonate or carbamate form and being in solution with CO2 that depends in part on temperature. Sorbents are carried in a solvent, for example water, which may contain further additives that can act as catalysts, modify the solution physical properties, reduce degradation or other desirable properties. In a complete model of operating a CO2 capture system system, other factors than absorbing efficiency may need to be taken into account, such as energy efficiency, sorbent degradation rate, loss of sorbent, maintenance costs etc. For any CO2 capture system the threshold for rate of absorbing may be set taking into account all the other factors, to give the best overall operation of the system.

Sorbents may include alkaline absorbents such as hydroxides or organic sorbents. Alkaline sorbents may include potassium hydroxide or calcium hydroxide.

Organic sorbents may include amines, amino acids. Amines may include Ethanolamine (2-aminoethanol, monoethanolamine, ETA, or MEA).

Preferred sorbents include amines, amino acids or alkali salt solutions of amino acids. The 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 acid may be a compound of an amino acid, such as a methyl amine or diethyl amine.

Preferred alkali component of the amino acid salts is potassium or sodium. Examples of amino acid salts include, sodium glycinate, potassium lysinate.

Amino acids are preferred because they are understood to have lower heat requirements for desorption, have less degradation than amines, and are less hazardous in use than many of the alternatives. They do however generally have a smaller range of temperatures for absorption to achieve a desired CO2 absorption rate.

Degradation of the sorbent reduces the effectiveness of the carbon capture system and leads to increased maintenance requirements. Both amino acids and amines degrade more rapidly when exposed to high temperatures. Therefore the desorber temperature must be carefully controlled to achieve a sufficient rate of desorption of CO2, while avoiding an acceleration of degradation. The invention described herein is particularly advantageous when applied to a CO2 capture system using amino acids and their salts and compounds, as the degradation of amino acids increase rapidly if exposed to temperatures above around 120 Celcius. For any sorbent, the degradation rate variation with temperature can be determined experimentally, and a threshold temperature for controlling the compressed flash vapor injection temperature can be set based on economic or technical models of the system operation. Depending on the relative availability of replacement sorbent or heating energy, the level of this threshold will be varied according to the operating model of the plant.

It will be understood that the invention is not limited to the embodiments above- described and various modifications and improvements can be made without departing from the concepts described herein. Except where mutually exclusive, any of the features may be employed separately or in combination with any other features and the disclosure extends to and includes all combinations and subcombinations of one or more features described herein.

Claims

1. A system for the capture of carbon dioxide, CO2, from a CO2 containing gas stream, the system comprising: an absorber to contact the CO2 containing gas stream with a sorbent, the sorbent operable to capture CO2 from the CO2 containing gas stream in a first temperature range and release CO2 at a second temperature range; a desorber to release the CO2 from the sorbent, the desorber operable to receive a rich sorbent stream from the absorber, heat the sorbent using heating means to provide heat to increase the sorbent temperature from the first temperature range to the second temperature range, an exhaust conduit to supply an exhaust stream comprising CO2 and vapor to a condenser; a lean return stream to return the sorbent from the desorber to the absorber; a flash vessel for receiving the lean return stream, reducing the pressure and temperature of the lean return stream and producing a flash vapor stream; a compressor for receiving the flash vapor stream and compressing it to produce a hot flash vapor stream; a controller configured to control the hot flash vapor stream temperature and inject it into the desorber at a temperature below a threshold.

2. The system of claim 1 , further comprising a mixer or a second heat exchanger, wherein the condenser produces condensate and the hot flash vapor temperature is controllable by one of: mixing the hot flash vapor stream with condensate in the mixer, or passing the hot flash vapor stream though the second heat exchanger for exchanging heat with the condensate, and in either case the controller is configured to control the hot flash vapor stream temperature by altering the flow rates of the condensate and hot flash vapor stream.

3. The system of claim 1 or 2, further comprising a hot flash vapor to desorber heat exchanger, configured to transfer heat from the hot flash vapor to the desorber before the hot flash vapor is injected into the desorber, at a rate of heat transfer that is selected to prevent the temperature within the desorber from exceeding the threshold, and also cooling the hot flash vapor to a temperature below the threshold.

4. The system of any previous claim, wherein the temperature threshold for the hot flash vapor stream is within the second temperature range.

5. The system of claim 4, wherein the upper end of the second temperature range is chosen such that the rate of degradation of the sorbent is below a threshold.

6. The system of claim 5, wherein the upper end of the second temperature range is less than 120°Celcius.

7. The system of any previous claim, wherein the sorbent is an amino acid or an amino acid salt.

8. The system of any previous claim, wherein the CO2 containing gas stream is ambient air.

9. A method for the capture of carbon dioxide, CO2, from a CO2 containing gas stream, the method comprising: contacting the CO2 containing gas stream with a sorbent in an absorber, the sorbent operable to capture CO2 from the CO2 containing gas stream in a first temperature range and release CO2 at a second temperature range; releasing the CO2 from the sorbent in a desorber, the desorber operable to receive a rich sorbent stream from the absorber, heating the sorbent in the desorber using heating means to provide heat to increase the sorbent temperature from the first temperature range to the second temperature range, supplying an exhaust stream comprising CO2 and vapor from the desorber to a condenser; returning the sorbent from the desorber to the absorber as a lean return stream; flashing the lean return stream in a flash vessel, reducing the pressure and temperature of the lean return stream and producing a flash vapor stream; compressing the flash vapor stream in a compressor to produce a hot flash vapor stream; and controlling the hot flash vapor stream temperature below a threshold and injecting it into the desorber.

10. The method of claim 9, wherein the condenser produces condensate, further comprising: controlling the hot flash vapor temperature by either mixing the hot flash vapor stream with the condensate in a mixer, or passing the hot flash vapor stream though a second heat exchanger for exchanging heat with the condensate, and altering the flow rates of the condensate and hot flash vapor stream to provide a stream for injection into the desorber below the threshold temperature.

11 . The method of claim 9 or 10, further comprising: transferring heat from the hot flash vapor to the desorber before the hot flash vapor is injected into the desorber, using a hot flash vapor to desorber heat exchanger, the hot flash vapor to desorber heat exchanger having a rate of heat transfer selected to prevent the temperature within the desorber from exceeding the threshold, and also cooling the hot flash vapor to a temperature below the threshold.