Improvements in energy performance of CO2 capture

Abstract

A system for capturing carbon dioxide (CO2) from a CO2-containing gas stream, the system comprising: an absorber for contacting a CO2-containing gas stream with an adsorbent operable to capture CO2 from the CO2-containing gas stream over a first temperature range and release CO2 over a second temperature range; the desorber is used for releasing CO2 from the adsorbent; and first and second heat pumps configured to recover heat from both the condenser and at least one other point in the system for the desorber, the condenser temperature being selected to maximize both the heat content of the condensate and the combined performance coefficients of the first and second heat pumps in order to minimize the input energy demand of the heat pump system.

Description

Technical Field

[0001] The present disclosure relates to the energetic performance of a CO2 capture process. Background Art

[0002] Carbon capture and storage is expected to be a significant way to reduce the global warming impacts from the burning of fossil fuels.

[0003] Carbon dioxide (CO2) capture can involve techniques that extract CO2 from a CO2-containing gas using an absorbent medium. Typically, this involves creating a gas flowing through the absorbent medium under conditions where the medium will absorb CO2 from the gas, and then changing the conditions so that the medium releases the absorbed CO2, allowing it to be captured and stored. This process can be used to reduce atmospheric CO2 to mitigate anthropogenic emissions associated with global warming or climate change. Direct air capture (DAC) is the capture of CO2 from atmospheric air, involving processing large volumes of air when the atmosphere contains less than 0.05% CO2.

[0004] In variable temperature methods, heat is used to release absorbed CO2, and it has been previously shown that heat pumps can be an effective way to supply this heat in electric heating systems. For example, EP2512628 A1 describes the use of heat pumps to recover low-grade thermal energy from various sources. The proposed low-grade energy sources are the condenser and compression system, as well as a solvent cooling system that is additionally provided in the solvent delivery line. However, there is still opportunity to increase heat recovery and improve its efficiency, while also improving the effectiveness of the entire system. Summary of the Invention

[0005] According to a first aspect, a system for capturing carbon dioxide (CO2) from a CO2-containing gas stream is provided, the system comprising: an absorber for contacting the CO2-containing gas stream with an adsorbent, the adsorbent being operable to capture CO2 from the CO2-containing gas stream within a first temperature range and release CO2 within a second temperature range; the absorber comprising a device for moving the CO2-containing gas stream through the absorber from an absorber inlet to an absorber outlet; a desorber for releasing the CO2 from the adsorbent, the desorber being operable to receive a rich adsorbent stream from the absorber, heating the adsorbent using a heating device to provide heat to increase the adsorbent temperature from the first temperature range to the second temperature range, an exhaust conduit for supplying an exhaust stream comprising CO2 and steam to a condenser; a lean return stream for returning the adsorbent from the desorber to the absorber; a heat pump system comprising a first heat pump and a second heat pump, the first heat pump and the second heat pump Two heat pumps are configured to recover heat from both the condenser operating at a condenser outlet temperature within a third temperature range and from at least one other point in the system at a lower fourth temperature range, the condenser being fluidly connected to a condensate recovery loop that delivers recovered condensate to the desorber, the first heat pump being configured to deliver heat to the heating device at the second temperature range and to receive heat from the condenser and the second heat pump via an intermediate loop operating between the condenser outlet temperature and a minimum temperature of the intermediate loop, the second heat pump being configured to receive heat from one other point in the adsorbent system at the fourth lower temperature range and deliver it to the intermediate loop at a temperature greater than the minimum temperature of the intermediate loop, the third temperature range being selected to maximize both the heat content of the recovered condensate and the combined coefficient of performance of the first and second heat pumps so as to minimize the input energy requirement of the heat pump system.

[0006] Preferably, the intermediate loop minimum temperature is selected based on the condenser outlet temperature to maximize both the heat content of the recovered condensate and the combined coefficient of performance of the first and second heat pumps to minimize the input energy requirement of the heat pump system.

[0007] In some embodiments, the third temperature range is between 68°C and 72°C.

[0008] In some embodiments, the minimum loop temperature is between 65°C and 69°C.

[0009] In some embodiments, the second heat pump has a different refrigerant than the first heat pump.

[0010] In some embodiments, there may be a second condenser, wherein the lean return stream is connected in heat exchange relationship to the second condenser, and wherein the second heat pump is configured to remove heat from the lean return stream after the lean return stream has exchanged heat with the second condenser.

[0011] Advantageously, the system may include a controller operable to control the heat pump system, and a plurality of sensors measuring the condition of the exhaust gas entering the condenser and the temperature of at least one other point in the adsorbent system, the controller being configured to model the coefficient of performance of each heat pump and the heat content of the recovered condensate and to adjust parameters of one or both heat pumps so as to minimize the energy demand of the heat pump system.

[0012] In a preferred embodiment, the CO2-containing gas stream is ambient air.

[0013] On the other hand is a method for capturing carbon dioxide (CO2) from a CO2-containing gas stream, the method comprising: providing a first heat pump and a second heat pump, the first heat pump being used to recover heat from an intermediate loop thermally connected to a condenser in a carbon capture system, the second heat pump being used to recover heat from at least one other point in the system and deliver it to the intermediate loop, the condenser being operable to recover condensate and deliver it to a desorber, the first heat pump being operable to provide heat to the desorber; controlling the temperature of the outlet of the condenser to maximize both the heat content of the recovered condensate and the combined coefficient of performance of the first and second heat pumps so as to minimize the input energy requirements of the first and second heat pumps.

[0014] Advantageously, in some embodiments, the minimum temperature of the intermediate loop is also controlled based on the condenser outlet temperature to maximize both the heat content of the recovered condensate and the combined coefficient of performance of the first and second heat pumps in order to minimize the input energy requirement of the heat pump system.

[0015] In some embodiments, the temperature at the outlet of the condenser is between 68°C and 72°C.

[0016] In some embodiments, the minimum loop temperature is between 65°C and 69°C.

[0017] The skilled person will appreciate that, unless mutually exclusive, the features described with respect to any one of the above aspects may be applied to any other aspects with mutatis mutandis. In addition, unless mutually exclusive, any features described herein may be applied to any aspect and / or combined with any other features described herein. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Embodiments will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0019] Figure 1 is a schematic diagram of a prior art carbon capture system;

[0020] Figure 2 Schematic diagram of a carbon capture system with heat pump heat recovery from the absorber.

[0021] Figure 3 Schematic diagram of a carbon capture system with heat recovery from the absorber recirculation loop and the desorber condenser using a heat pump.

[0022] Figure 4 Schematic diagram of a carbon capture system with heat pump heat recovery from the absorber, with an additional air source heat exchanger.

[0023] Figure 5 Schematic diagram of a carbon capture system with heat pump heat recovery from the absorber and heat transfer between the absorber inlet and outlet.

[0024] Figure 6 and Figure 7 is a graph showing the performance of the adsorbent at different temperatures.

[0025] Figure 8 Schematic diagram of a carbon capture system with heat pump heat recovery from the lean return and heat pump heat recovery from the desorber condenser in a cascade system.

[0026] Figure 9 Schematic diagram of a carbon capture system with heat pump heat recovery from the heat lean return and heat pump heat recovery from the desorber condenser in a cascade system.

[0027] Figure 10-15 Shown Figure 9 Variations and alternative arrangements of the system,

[0028] Figure 16 A graph showing the energy usage of an example system at different condenser temperatures,

[0029] Figure 17 A series of graphs showing energy usage for an example system optimized for heat pump performance,

[0030] Figure 18 A graph showing energy usage for an example of a system optimized for heat pump performance at different ambient temperatures. DETAILED DESCRIPTION

[0031] refer to Figure 1, which shows a conventional system for carbon capture from a CO2-containing gas. An absorber 10 receives a CO2-containing gas stream from an inlet 20 to an outlet 30. A sorbent stream flows through the absorber, with a lean stream 40 entering the absorber, contacting the CO2-containing gas, and becoming a rich stream 50. The sorbent can be recycled within the absorber as a recycle stream 110, which increases the effective residence time of each portion of the lean sorbent stream in the absorber.

[0032] The rich stream is typically passed through a heat exchanger 100 to recover some heat from the lean stream returning from the desorber 60. The desorber 60 receives the rich stream and heats it to a temperature where the CO2 will be released from the adsorbent, typically using a heating device 70, which also generates steam to form vapor bubbles into which the desorbed CO2 can diffuse, leaving a lean adsorbent stream to return to the absorber to repeat the process.

[0033] The vapor and desorbed CO 2 exit the desorber at 80 , where a condenser 90 is typically used to cool the mixture, causing the vapor to condense, leaving a purer CO 2 product stream.

[0034] Go to Figure 2 Except where noted, the same reference numerals are used for the same or similar features. An absorber 10 receives a CO2-containing gas stream from an inlet 20 to an outlet 30. A sorbent stream flows through the absorber, with a lean stream 40 entering the absorber, contacting the CO2-containing gas, and becoming a rich stream 50. The sorbent can be recycled within the absorber as a recycle stream 110, which increases the effective residence time of each portion of the sorbent in the absorber.

[0035] The rich stream is optionally passed through a main heat exchanger 100 to recover some heat from the lean stream returning from the desorber 60. Since the present disclosure provides a variety of embodiments describing a heat transfer device, such a heat exchanger may or may not be included or may be located at different locations in the circuit. The desorber 60 receives the rich stream and heats it using a heating device 70 to a temperature at which CO2 will be released from the adsorbent.

[0036] The vapor and desorbed CO 2 exit the desorber at 80 , where a condenser 90 is typically used to cool the mixture, causing the vapor to condense, leaving a purer CO 2 product stream.

[0037] The boundary of the absorber system is marked by dashed line 200. The absorber system comprises a recirculating stream 110 and an inlet 20 and an outlet 30. Heat is recovered from the absorber system by a heat pump system 210 and delivered to a heating device 70.

[0038] The heat pump system has a controller that controls the temperature within the absorber system so that the adsorbent is maintained within a preferred temperature range when in contact with the CO2-containing gas. As described below, for any adsorbent, there is a temperature where the CO2 absorption rate is maximized for any given system.

[0039] An additional benefit of cooling the absorber is reduced evaporation, helping to maintain the system's fluid balance and reducing water loss, a significant environmental cost for liquid-based DAC devices. Under certain atmospheric conditions, the absorber can be operated at a temperature below the dew point of the ambient air, causing the vapor pressure of the ambient air to exceed the vapor pressure of air at the absorber temperature. Under these conditions, the absorber can obtain water from the ambient air, which helps to compensate for water lost through drift or vapor loss in the exhaust from the desorber. The heat pump system controller can control the absorber temperature based on ambient humidity to maintain fluid balance within the system. The controller can calculate the balance between energy use, CO2 absorption rate, and water loss rate and adjust the temperature to achieve optimal operating conditions based on the price or environmental cost applied to each of these parameters. The controller can receive environmental cost indicators for CO2 capture, energy use, and water loss and adjust the absorber temperature to minimize overall environmental costs, taking into account ambient conditions such as humidity and temperature, the environmental cost of energy based on current energy sources, and the environmental cost of local water resources. The controller can estimate the evaporation rate using known formulas such as the Penman equation or variations thereof.

[0040] Heat from the gas stream or the sorbent stream, or both, may be recovered from the absorber system, allowing the average temperature of the sorbent within the absorber to be maintained within a first temperature range in which the CO2 absorption rate is maximized.

[0041] The gas stream and the adsorbent stream are in contact within the absorber for at least the absorber's residence time, and to maximize the transfer of CO2 from the gas stream to the adsorbent stream, the contact surface area is preferably large. This can be achieved by delivering the adsorbent as a thin film coating onto the filler within the absorber, or via small droplets such as a fine mist. As a result of the contact and gas-to-liquid transfer, the two streams will quickly reach thermal equilibrium with each other, and thus controlling heat transfer requires only measuring the temperature of one stream.

[0042] The required temperature range can be compensated for based on differences in the inlet or outlet temperatures of the streams. This can be calculated by using sensors to measure the temperature of each stream during operation. Alternatively, the compensation can be calculated by measuring or modeling a single absorber and then applying the calculated compensation to multiple similar units.

[0043] In some cases, there may be multiple absorber units, supplied with common lean and rich streams from a central desorber unit, and all absorbers or absorber groups have a common recycle stream. The temperature of the multiple absorbers can be controlled by cooling the recycle stream, thereby supplying cooled or heated absorbent solution to all absorbers.

[0044] In other cases, multiple absorbers may be supplied with gas or air via a common plenum, and the temperature of the absorbers may be controlled by cooling or heating the supplied gas or air in the plenum.

[0045] exist Figure 3 In the case of features 10 to 210, Figure 2 Heat is recovered from the absorber system 200 by the heat pump system 210 and delivered to the heating device 70.

[0046] Figure 3 , a heat exchanger 320 is shown on the adsorbent fed to the absorber, where it is in thermal contact with both the lean return stream 40 from the desorber and the recycle stream 110. In this example, the two streams are mixed before being delivered to the absorber. In other examples, a heat exchanger may be present on one or both of the mixed return stream 40 or recycle stream 110.

[0047] The heat recovery stream 330 transfers heat from the heat exchanger 320 to the heat pump system 210. This may be a refrigerant loop, or it may be another heat transfer fluid that provides heat transfer between the heat exchanger and the heat pump system.

[0048] There may be multiple absorber systems connected to a single desorber. The multiple absorber systems may share a single recycle stream and heat recovery may be performed by a single heat pump system using heat exchangers for all connected absorbers.

[0049] In the embodiments described above, under certain climate conditions, it may be necessary to input additional heat to the desorber to supplement the heat recovered from the absorber. This additional heat can be supplied by an external heat source, such as steam, waste heat from other processes, electrical heating, or any other conventional heat source.

[0050] Figure 3 Also shown is a high temperature heat recovery path 340 from the condenser heat exchanger 90, which provides additional heat to the heat pump system.

[0051] The vapor and desorbed CO2 leaving the desorber will be at a much higher temperature than the absorber. For example, the desorber can be operated at 90 to 120 degrees Celsius, while the absorber can be operated at 0 to 30 degrees Celsius under ambient conditions, or controlled within a fixed temperature range to improve adsorbent performance as described above. In order to combine heat recovery from both units, it may be preferable to have a cascade heat pump system.

[0052] The cascade heat pump system can include a first heat pump configured to receive heat from a high-temperature heat recovery loop and a second heat pump configured to remove heat from the absorber and deliver heat to the high-temperature heat recovery loop. The coefficient of performance (COP) of each heat pump will be determined in part by the temperature difference between the high-temperature loop, the absorber, and the heating device 70. Therefore, by carefully designing the system as described herein, the COP can be optimized along with maximizing the available heat.

[0053] Figure 4 In the case of features 10 to 210, Figure 2 and Figure 3 Those features are similar, except that the heat pump system 210 is shown as two heat pumps 210a and 210b. Heat is recovered from the absorber system 200 by the heat pump 210a and is delivered to the heating device 70. Heat is recovered from the condenser 90 by the heat pump 210b. The two heat pumps are shown as being connected independently, but they can also be arranged as a cascade system as described above. Depending on the selection of the heat pump and the refrigerant therein, by operating the heat pump 210a between the absorber temperature and the heating device temperature or between the absorber temperature, the overall effective COP of the heat pump system based on the heat and energy input delivered by the two heat pumps can be maximized. This result will also depend on the selection of the condenser heat exchanger shut-off temperature (offtemperature). This makes it possible to maximize total heat recovery with a minimum amount of mechanical energy input to the heat pump (which can be electric).

[0054] Also shown is an air heat exchanger 410, which is also connected to the heat pump 210a in a heat transfer relationship. This heat transfer relationship can be reversible, for example in a variable refrigerant flow system or other means known to those skilled in the art. This makes it possible to maintain a balance in the heat supply to the heating device 70 in hot or cold climates while maintaining the absorber temperature within the first operating temperature range in which the CO2 absorption rate is maximized.

[0055] In summer conditions, the absorber will absorb heat from the ambient air passing through it, while the recirculation loop 110 is cooled to remove this heat and keep the absorber within the first temperature range. If the heat recovered from the recirculation loop exceeds the heat required by the heating device 70, the excess heat can be discharged through the air heat exchanger 410. In winter conditions, it may be necessary to add heat to the absorber to maintain the minimum temperature within the first temperature range and also to prevent ice formation. This additional heat can be obtained by operating the air heat exchanger in the manner of an air source heat pump. The air heat exchanger 410 can have its own fan to move air through it, or it can be connected via an air supply to the same air moving device used by the absorber.

[0056] exist Figure 5 In the embodiment, air heat exchangers 510 and 520 are fixed to the inlet 20 and outlet 30 of the absorber 10 so that they exchange heat with the air entering and leaving the desorber. Air heat exchangers 510 and 520 are connected to a reversible heat pump 210c. This arrangement means that the inlet air can be heated or cooled by transferring heat between the inlet and outlet using the heat pump 210c.

[0057] Heat pump 210c controls the temperature within the absorber system so that the adsorbent is maintained within a preferred temperature range when in contact with the CO 2 -containing gas. Heat pump 210a can continuously extract heat from recirculation loop 110 to supply heat to heating device 70.

[0058] When operating in cold weather conditions, the heat pump 210c cools the exhaust air 30 to below ambient temperature while warming the inlet air to above a preferred temperature sufficient to allow the recirculation loop to cool to balance the temperature within the absorber at the desired temperature. If the outlet heat exchanger 520 begins to ice up, a defrost cycle may be required on the heat pump 210c, a process similar to that used in conventional air source heat pumps.

[0059] When operating in hot climates, the heat pump 210c warms the exhaust air 30 to above ambient temperature while cooling the inlet air to above a preferred temperature sufficient to allow the recirculation loop to cool to balance the temperature within the absorber at the desired temperature. If the inlet heat exchanger 510 begins to ice up, a defrost cycle may be required on the heat pump 210c.

[0060] This combination can supply the additional heat demand required by the desorber since the air heat exchangers 510 and 520 are able to extract additional heat from the ambient air.When the ambient temperature is already within the desired range, the heat pump 210c can be idle.

[0061] When there are multiple absorber units, they may all be connected to a common inlet and outlet charge, and the air heat exchanger may be placed in the common charge, simplifying the arrangement of the heat pump 210c.

[0062] Adsorbent characteristics:

[0063] Adsorbents used for carbon capture typically have a varying equilibrium between the carbonate form and being in solution with CO2, depending on temperature. The adsorbent is supported in a solvent, such as water, which may contain additional additives that can act as catalysts, change the solution's physical properties, reduce degradation, or other desired properties.

[0064] For each adsorbent in solution, an optimal temperature range can be determined within which the rate of absorption of carbon dioxide from the surrounding gas in the absorber is high. Given the large number of variables, experimental measurements of the optimal temperature range for a specific adsorbent solution under specific absorption conditions may be required.

[0065] Figure 6 and Figure 7 Exemplary curves (601, 701) each show the temperature performance of an adsorbent solution versus temperature (in ° C.) The temperature performance "R" is the total CO2 absorption rate, which can be expressed in kg CO2 / hour for a given absorbent and adsorbent combination, expressed as a percentage of the rate achieved under standard conditions (in this case, 20° C.). Figure 6 In FIG. 6 , lines 602 and 603 are the lower and upper limits of the operating temperature range required to achieve at least 100% standard rate, which in this example is about 4 to 21° C. Figure 7 , 702 and 703 are similar points, but the goal is to achieve approximately 95% of the standard performance between 4 and 25°C. These two graphs are illustrative data for two different adsorbent solutions (e.g., MEA or amino acid salts). The data in the graphs should not be considered precise; these graphs are provided as examples of how the operating range can be determined after characterizing the performance of the adsorbent solution.

[0066] The temperature range can be expressed as the minimum and maximum temperatures of the absorber contact area, i.e., the temperature inside the absorber when the adsorbent contacts the CO₂-containing gas. Typically, when the absorber is an air-to-liquid contactor in a DAC system, the temperature of the adsorbent and the temperature of the air inside the absorber will be very close, as the adsorbent is distributed as a thin film or droplets to achieve high surface area and contact time, and the air flow rate is relatively high.

[0067] The rate of CO2 absorption from the surrounding gas can be defined by the change in the number of carbonized sorbent molecules per unit time or per passage through the absorber. For an absorber with fixed gas and liquid flow rates and a difference in CO2 saturation between the rich and lean streams, the rate can be expressed as the rate of CO2 removal from the gas stream per unit time.

[0068] Throughout this application, unless otherwise stated, the CO2 absorption rate will be expressed as a percentage of the CO2 absorption rate at 20°C. Therefore, the rate can exceed 100% when the adsorbent performs better at other temperatures. Expressed in this manner, the rate can be understood whether it is applied at a laboratory scale or in a large-scale carbon capture facility, even though the actual rate can vary significantly when other parameters are varied.

[0069] Therefore, the optimal temperature range of the adsorbent is represented by the lowest temperature and the highest temperature at which the rate of CO 2 absorption exceeds a threshold value (eg, a rate greater than 95%).

[0070] In a complete model of the operational CO2 capture system, other factors besides absorption efficiency may need to be considered, such as energy efficiency, adsorbent degradation rate, adsorbent loss, maintenance costs, etc. For any CO2 capture system, an absorption rate threshold can be set to give the best overall operation of the system taking into account all other factors.

[0071] For some adsorbent chemistries, the optimal range is narrow, and it is desirable to operate the thermal management function closely to stay within the required range. For other adsorbent chemistries, the optimal range is quite broad, and thermal management functions are only really needed on very cold and very hot days.

[0072] Adsorbents may include alkaline absorbents such as hydroxides or organic adsorbents.

[0073] The alkaline adsorbent may include potassium hydroxide or calcium hydroxide.

[0074] The organic adsorbent may include amines, amino acids, or ethanolamines (2-aminoethanol, monoethanolamine, ETA, or MEA).

[0075] Preferred adsorbents include amino acids or alkaline salt solutions of amino acids. The amino acids can 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 can be amino acid compounds such as methylamine or diethylamine.

[0076] The preferred basic component of the amino acid salt is potassium or sodium. Examples of the amino acid salt include sodium glycinate and potassium lysine.

[0077] Amino acids are preferred because they are understood to have lower heat requirements for desorption, have lower degradation than amines, and are less hazardous to use than many alternatives. However, they generally have a smaller absorption temperature range to achieve the desired CO2 absorption rate. The invention described herein is particularly advantageous when applied to CO2 capture systems using amino acids and their salts and compounds because by maintaining the absorption medium within the optimal temperature range, carbon capture performance is enhanced.

[0078] By combining heat recovery using a heat pump with absorber temperature control, the carbon capture system experiences a dual benefit, which together increases the amount of CO2 captured per unit of energy consumed. Each mole of sorbent absorbs more CO2 per passage through the absorber, while the energy used to desorb the CO2 from the sorbent is reduced by the heat pump.

[0079] When combined with the other heat recovery options described herein, using only electrical or mechanical energy input to the heat pump can provide the entire heat requirement of the desorber, requiring less primary energy than would otherwise be the case.

[0080] Hot side layout

[0081] Now go to Figure 8 , only the hot side of the system is shown. The absorber and associated heat recovery options can be Figures 1 to 5 Any arrangement shown connects to a lean line 40 and a rich line 50 .

[0082] Figure 9 The heat pump system in FIG. 1 includes heat pump 210b, which recovers heat from condenser 90, and heat pump 210d, which recovers heat from lean line 40. Heat pump 210d recovers heat from the lean return at a temperature intermediate between that of the main heat exchanger 100 and the absorber, cools the lean return, and then returns it to the absorber. The heat is then supplied to intermediate loop 810, where it is combined with heat from the condenser. Due to the small temperature difference between the condenser and heating device 70, heat pump 210b can operate with a high coefficient of performance.

[0083] The heating device may require heat at 120-130°C, and the condenser size can be varied to cool the exhaust gas stream 80 to, for example, slightly below the dew point of the exhaust gas mixture, recovering most of the latent heat from the exhaust gas. The condensate line 820 returns the hot condensate, which is primarily water, to the desorber 60. By keeping the condensate temperature high, the sensible heat required to reheat the condensate to the desorber temperature is low, and the COP of the heat pump is high, reducing the overall heat demand of the system.

[0084] Additional moisture present in the exhaust stream after the condenser can be removed later at a lower temperature, or even during the compression cycle if the CO2 is compressed for further processing. This additional condensate can be injected back into the system elsewhere to help maintain the water balance.

[0085] Since the heat pump 210d only raises the temperature of the heat from the lean return temperature to the condenser temperature, rather than up to the temperature required by the heating device, a greater selection of heat pumps is available because heat pumps capable of supplying heat above 100°C tend to be complex and require specialized refrigerants. This means that for all the heat required, we can run a dedicated heat pump with a high performance coefficient between the condenser temperature and the reboiler, and feed the intermediate temperature reservoir 810 with heat from the condenser or other sources (including from other lower temperature heat pumps) so that the heat extracted from the intermediate reservoir matches the heat supplied to the intermediate reservoir. Optionally, external heat can be added from the absorber side. It can come from a separate air source heat pump, a ground source heat pump, a water source heat pump, an external industrial process, or waste heat from the local area. It can come from a combination of sources.

[0086] Figure 9 Another option for hot side heat recovery is shown. Figure 8 All features of the exhaust gas 90 are present as described above. Additionally, a second condenser 920 is shown, which recovers latent and sensible heat from the exhaust gas 80 after it has passed through the first heat exchanger of condenser 90. Lean return line 910 has been cooled by rich line 50 in the main heat exchanger and then passed through second condenser 920, where it recovers some of the remaining enthalpy in the exhaust gas. This additional heat is then recovered by heat pump 210d, which now has an improved COP and more usable heat recovered due to the higher temperature of lean return 910.

[0087] Figure 10 Shown with Figure 9In a similar manner, hot condensate 820 can be recovered after the first condenser heat exchanger 90. The second condenser is cooled by a cold rich line 50 return from the absorber at absorber temperature, which may be close to ambient temperature, and preheated in the condenser 920 which then receives additional heat in the main heat exchanger 100 from the lean line, which is at a temperature close to the desorber temperature. In this example, the main heat exchanger can be smaller than is typically used, or remove less heat from the lean line 910, which means that the lean line has more available heat, i.e. it is hotter, for recovery via the heat exchanger 1110 and the heat pump system 210d. The result of the lean return 910 leaving the main heat exchanger 100 at a higher temperature is an improvement in the COP of the heat pump 210d and makes more heat available for recovery. Since the rich line 50 will be close to ambient temperature, or if Figures 2 to 5 The characteristics of the combined with those shown are even below ambient temperature, so the heat recovered from the second condenser 920 provides preheating for the rich line with almost no additional energy requirements. The resulting CO2 exhaust is also drier, which reduces the post-processing required before storage or further use. The heat pump 210d can deliver heat directly to the heating device 70 as shown, or alternatively, it can deliver heat to the input of the heat pump 210d to form a cascade heat pump system, such as for example Figure 8 As otherwise described in.

[0088] Figure 11 Now, a system is shown in which the lean line 910 is reheated by the condenser 90 after exchanging heat with the rich line 50 in the main heat exchanger 100. In this example, the heat pump system 210 recovers heat from the heat exchanger 1110 on the lean line 40 after the condenser, thereby using only a single additional heat exchanger and heat pump to recover heat from both the condenser and the lean line return. By making the lean return hot, the enthalpy of the condenser is captured, and the single heat pump will have a high coefficient of performance while also capturing any residual heat in the lean line before it is lost in the absorber.

[0089] Figure 12 yes Figure 11 A variation of the system shown, but in this case, the rich line returning from the absorber is preheated in condenser 90 at absorber temperature (which may be close to ambient temperature) before it receives additional heat from the lean line in main heat exchanger 100. In this example, the main heat exchanger may be smaller or remove less heat from the lean line 910, meaning that the lean line has more heat available for recovery via heat exchanger 1110 and heat pump system 210.

[0090] exist Figure 11 and Figure 12In both, additional heat may be recovered from the absorber or other low temperature source to provide additional heat to the heating device 70 using an additional heat pump directly to feed the heating device or in a cascade arrangement with heat recovery from the heat lean line.

[0091] exist Figure 13 In the Figure 11 and Figure 12 Another variation of the hot lean line system. Figure 9 In this arrangement, the second condenser 920 is used to further cool the exhaust gas from the desorber 80, allowing the first condenser 90 to be optimally sized to maximize latent heat recovery from the exhaust gas stream and return the hot condensate 820 to the desorber. The second condenser here is cooled by the cold lean line 910, which is then transferred to the first condenser 90. The hot lean line now passes through the heat exchanger 1110, where the heat is collected by the heat pump system 210 to supply the heating device 70. The advantage of this arrangement is that the temperature of the condensate 820 can be set by varying the size of the two condensers, returning the hot condensate to the desorber, and returning the cooler condensate from the second heat exchanger to other parts of the system to replenish the liquid balance. At the same time, only a single heat pump is required to manage the condensate and lean line heat recovery.

[0092] Figure 14 is with Figure 13 Similar arrangement except that the cold rich return 50 is used to cool both condensers. Figure 12 In the example, the rich line 50 returning from the absorber at absorber temperature (which may be close to ambient temperature) is preheated in condensers 920 and 90 before it receives additional heat from the lean line in the main heat exchanger 100. In this example, the main heat exchanger may be smaller or remove less heat from the lean line 910, which means that the lean line has more heat available for recovery via heat exchanger 1110 and heat pump system 210.

[0093] Figure 15 yes Figure 13 and Figure 14 In this case, the cold rich line 50 returning from the absorber is used to cool the second condenser 920 and then further heated by the main heat exchanger 100. The lean line is cooled by the main heat exchanger 100, but then reheated by the first condenser 90.

[0094] Since the rich line 50 can be quite cold, especially when the absorber is operating in cold climates, the second condenser 920 can recover additional heat while still providing a single heat pump with heat recovery from the first condenser 90 and heat recovery from the condenser and lean return. Figure 13 and Figure 14 benefits.

[0095] One aspect of the present disclosure is to recover heat from the absorber itself, as described in reference Figures 2 to 7 As described above, recovering heat from the absorber can also be used to control the absorber temperature by cooling the air or liquid within the absorber. This has the advantage of maintaining the CO2 capture rate of the adsorbent, which in most cases decreases under warmer ambient conditions. It can also mitigate water loss from the absorber through evaporation, or even recover water from the ambient air under some conditions.

[0096] Another aspect of the present disclosure is to use a lean line return to transfer heat to a heat pump after recovering heat from the condenser. This allows for simplification of the heat pump system by combining multiple heat sources into a single heat source at a constant temperature. Figures 9 to 15 This example is shown.

[0097] Another aspect of the present disclosure is to control the condenser temperature within a certain range. There is a trade-off between maximizing the enthalpy recovered in the exhaust stream from the desorber (primarily gaseous HO and CO2), the efficiency of the heat pump system (where the coefficient of performance varies with the temperature difference), and the need to place heat in the desorber by reinjecting the hot condensate. It is desirable to remove as much HO as possible from the exhaust stream before further processing or transportation, because liquid water and CO2 combine to form an acidic solution, which can be destructive to equipment and piping systems. Water recovery is also required to maintain liquid balance in the desorber (to maintain the desorber pressure) and the overall carbon capture system.

[0098] Controlling the condenser 90 temperature can be accomplished using various systems as described herein. Figure 8 、 Figure 9 and Figure 10 In FIG. 1 , the condenser 90 temperature is set by heat pump 210 b, which can be selected to provide the desired condenser temperature under the desired operating conditions. Alternatively, the heat pump can include controls that vary the flow rate of the refrigerant through the condenser by controlling the heat pump compressor or by routing a portion of the refrigerant to another heat source. Other temperature control methods are known to those skilled in the art of heat pumps and refrigeration.

[0099] exist Figures 11 to 15 In a condenser, the condenser temperature is controlled by the flow of adsorbent through the condenser. The condenser dimensions can be changed to achieve the desired temperature under the expected operating conditions, or the control system can vary the flow to maintain the desired temperature. For example, a bypass valve can be operated to divert the flow of adsorbent around the condenser.

[0100] exist Figures 8 to 15In either, the control system may include one or more sensors that measure a value indicative of the condenser temperature or condensate temperature and control the temperature of the condenser based on the sensor readings as described above.

[0101] As the condenser outlet temperature increases, the available enthalpy for heat recovery decreases because more heat is lost in the exhaust stream leaving the condenser. At the same time, the coefficient of performance of the heat pump, which extracts heat from the condenser and supplies it to the heating device, increases. In addition, when the condensate is injected into the desorber to maintain liquid equilibrium, the condensate requires sensible heating to raise it to the desorber temperature. The resulting total heat requirements have been calculated for some exemplary systems and are shown in Figure 16 In principle, this model is applicable with minor variations to any desorber in a carbon capture system when a heat pump is used to recover heat from the absorber. The absolute temperature will depend at least on the choice of adsorbent, the ambient operating conditions of the absorber, and the pressure in the desorber, and thus Figure 16 The horizontal axis of the graph in FIG has been plotted on a scale of 0-1, with 0 representing ambient conditions and 1 representing the maximum temperature of the desorber exhaust stream when no heat is recovered. While there will always be a balance to be determined between sensible and latent heat, this will depend on the choice of operating pressure for the desorber system and will generally result in a dew point in the exhaust stream that is a relatively fixed percentage of the maximum temperature.

[0102] The power requirement of the desorber, usually expressed in kW electricity, is the energy input to the heat pump to generate the required heat (taking into account COP) plus any additional heat input required (if the heat pump does not meet the demand, direct electric heating is assumed). The desorber requires energy at a rate high enough to break the bonds between the adsorbent and the CO2, raise the temperature of the adsorbent from the temperature in the absorber to a temperature where the equilibrium carbonization of the adsorbent molecules is lower than the state in the absorber, and also generate bubbles (such as steam) in the adsorbent solution for the CO2 to diffuse into to promote the CO2 leaving the solution. This rate will depend on a variety of conditions, including the flow rate of adsorbent through the desorber, which can be a fixed flow rate. The electricity supply can be grid electricity, or it can be supplied by a local low-carbon energy source (such as wind, solar or nuclear power). On the vertical axis of the graph, this is expressed as the fraction of the heat required if the condenser were cooled to ambient temperature and all available enthalpy was recovered.

[0103] When a second heat pump is used to recover heat from a second source, the electrical input to the second heat pump is included in the desorber energy usage. The carbon capture system may have other energy requirements, such as fan power and pump power, which, for the purposes of illustrating the effects of the present disclosure, may be assumed to be constant and are therefore not included in the exemplary calculations.

[0104] By modeling the enthalpy flow in the exhaust stream, the mass flow and temperature of the condensate, and the coefficient of performance of a heat pump system (including one or more heat pumps), a curve can be obtained that shows the net energy input required to heat the adsorbent in the desorber. The simulation system can be configured to calculate the energy usage of various components of the heat pump system and generate a desired temperature range for controlling the temperature to minimize energy usage. In particular, the temperature of the condenser can be controlled to minimize the energy usage of the desorber in the system.

[0105] exist Figure 16 In , four exemplary traces are plotted showing the total energy usage of the absorber system.

[0106] Trace 1610 (dotted line) shows the energy (E) requirement of the absorber when heat is supplied by a single heat pump that recovers heat from a single condenser at temperature (T). As explained above, the horizontal axis T is plotted on a scale from absorber temperature at 0 to a maximum condenser temperature of 1 (a relative temperature range). In the example, the ambient temperature is 25°C and the maximum condenser temperature is 115°C. Trace 1610 shows a minimum at T = 0.5, with energy usage of 0.54. At low values of T, maximum latent and sensible heat is recovered from the exhaust stream, but the condenser heat pump 210b has a low COP, so more work provided by the heat pump is supplied as heat. Once the temperature reaches 50% of the range shown, there is less heat recovered and the HP also adds less heat as work. Above this temperature, the absorber requires additional heat input, and for illustrative purposes, it is assumed to be direct electric heating in this trace. Thus, total energy use increases as temperature rises above 0.5, until, at a point above T = 0.8, the net energy use exceeds the energy use required at T = 0. In this example, T = 0.5 corresponds to 70°C, and in an MEA-based carbon capture system, the energy demand is equivalent to 1.67 GJ of electricity per ton of CO2 released.

[0107] Trace 1620 (dashed line) shows the energy (E) requirement of the absorber when heat is supplied by two independent heat pumps, one heat pump recovering heat from the condenser at temperature (T), and the lean return having been reheated by the second condenser (e.g., Figure 9), except that the heat pumps are connected in parallel rather than in cascade. That is, the heat output of heat pump 210d is at the temperature of heating device 70 rather than at the temperature of intermediate loop 810. As explained above, the horizontal axis T is plotted on a scale from absorber temperature at 0 to maximum condenser temperature 1. In the example, the ambient temperature is 25°C and the maximum condenser temperature is 115°C. Trace 1620 shows a minimum at T = 0.72, and the energy usage is 0.48 relative to a fully cooled condenser. Compared to trace 1610, at a lower COP than that of the condenser heat pump, but using much less input power than direct electric heating, additional heat is recovered in the lean line, which includes residual heat from main heat exchanger 100 plus additional heat obtained from secondary condenser 920. Between T = 0.5 and 0.8, when the backup heater is the second heat pump, it supplies the required additional heat without increasing the power input. Between T = 0.5 and 0.72, power usage decreases and then slowly rises, exceeding the power usage of the single heat pump example of trace 1610 only when T>0.83 (of the range between the absorber temperature and the desorber temperature). Thus, the system can operate efficiently with the condenser controlled or configured to operate within a relative temperature range between T = 0.5 and 0.83, preferably between T = 0.6 and T = 0.8, and most preferably between T = 0.7 and T = 0.75. In the example using MEA adsorbent or other adsorbents that can be used for absorption at 0 to 30°C and desorption above 90°C, these ranges have actual temperatures of 70 to 99°C, 79 to 97°C, and 88 to 92°C.

[0108] Trace 1630 (dashed line) shows the heat transfer when heat is transferred from two heat pumps arranged in cascade, such as Figure 9 One heat pump 210b recovers heat from the condenser at temperature (T), and another 210d recovers heat from the lean return, which has been reheated by the second condenser 920, as shown in FIG. Figure 9 As shown. In this case, the heat pump that recovers heat from the lean return supplies heat to the intermediate loop 810 so that it is combined with the heat from the condenser and then moved to a higher temperature by the condenser heat pump. Over the same operating temperature range of the condenser, trace 1630 shows slightly higher energy usage than trace 1620. However, this difference is small and can be improved by optimizing the choice of heat pump. The advantage of a cascade arrangement over a stand-alone heat pump is that the lean line heat pump only needs to operate to the temperature set point of the condenser, for example 70 to 99°C, 79 to 97°C, and most preferably 88 to 92°C. This means that this second heat pump can be simpler and use a wider range of available refrigerants than a heat pump that needs to deliver approximately 120°C of heat directly to the heating device.

[0109] The performance of heat pump 210d can be improved by running the lean line heat pump from a counter-flow heat exchanger to increase the source temperature of the heat pump, thereby improving the COP.

[0110] Trace 1640 shows the energy performance of a system in which the rich line is used to cool the second condenser 920 so that the cold rich adsorbent stream returning from the absorber is preheated before entering the main heat exchanger 100, resulting in a hotter lean line return due to less heat transfer occurring in the main heat exchanger. This arrangement is shown in Figure 10 and described above. Trace 1640 shows similar energy performance to trace 1620 up to a relative temperature of T of about 0.8, but the performance range extends at higher condenser temperatures. Energy usage remains below the energy requirement of the single heat pump system of trace 1610 and has a minimum between T=0.5 and T=0.86. The minimum is at T=0.72, which in the MEA adsorbent example is equivalent to 90°C, where the energy requirement is 0.48 of the energy requirement of a fully cooled condenser with a single heat pump. Between T=0.64 and T=0.80, the energy usage is less than half of that of the cold single heat pump example. In absolute terms, optimal performance is achieved at about 85°C to 95°C, with energy usage of less than 1.49 GJ / ton of CO2 released in the MEA adsorbent example.

[0111] Trace 1650 (dash-dotted line) shows the mixed condensate temperature (in degrees Celsius), which is plotted against the right-hand axis. When there are two condensers, in any of the examples described herein, the condensate 820 can be recovered after the first or second condenser 920 or both. An optional external heat source can be used. The temperature of the first condenser can be set higher to allow the heat pump 210b to operate with a high and consistent COP so that it will mainly recover latent heat with almost no temperature drop on the condenser 90. The size of the second condenser 920 can be changed to maximize the heat content of the recovered condensate and the combined performance coefficient of the first and second heat pumps in order to minimize the input energy demand of the heat pump system.

[0112] If the first condenser is operated so that approximately 88% to 71% of the available water vapor is recovered, the second condenser can be operated at a much colder temperature to recover the remaining water, for example, by using a rich return line at or below ambient temperature to cool the second condenser. Since the mass flow from the second condenser is only between 12% and 29% of the available water, the mixed temperature of the combined stream is still high. In an example, when the condenser outlet temperature of the first condenser is between 85 and 100°C and the second condenser is cooled to 25°C or below, the condensate temperature of the mixed stream can be higher than 78°C. This minimizes the heat requirement for reheating the returning condensate in the desorber. This temperature range also overlaps with the temperature range in which the dual heat pump system can minimize the energy demand of the system.

[0113] exist Figure 16 In all traces, the performance data is derived from a system based on MEA adsorbent, where the desorber reboiler is operated at approximately 120°C and the absorber is operated at ambient conditions of 25°C. However, the scale has been normalized to 0-1 between the two figures because it is expected that heat pump performance and condenser performance will behave similarly in other systems with different end points of the operating range. Therefore, the preferred relative condenser temperature ranges can be applied to other systems using adsorbents different from those shown.

[0114] Figure 17 and Figure 18 The advantages of the condenser and heat pump combination described above are illustrated, and in particular the surprising energy efficiency improvements that can be achieved by deliberately controlling the temperatures at two points in the system. For each variation of the above system using one or two condensers, the system can be summarized as an intermediate circuit such as a water loop that directly cools the first condenser, a low-temperature heat pump that extracts heat from elsewhere, such as a second condenser that uses one of the other fluid loops directly or indirectly, and a heat pump that extracts heat from the water loop and supplies it to the heating device of the desorber. Although some heat can always be recovered from two locations, optimizing heat recovery and maximizing the coefficient of performance of the heat pump require implementing specific control points in the system. The optimal temperatures of the first condenser and the water loop also vary with the ambient temperature. In this configuration, an optimal operating temperature range has been determined that can be used in combination with some of the above embodiments (particularly those shown).

[0115] exist Figure 17 , a contour plot of energy usage for four different system architectures is shown. Numbered lines 34 to 48 are contour plots of the energy performance of the desorber, shown as a percentage relative to a desorber without a heat pump. Thus, contour line "40" shows an operating scenario where the desorber system would use 40% of the required energy compared to a directly heated desorber. That is, as discussed above with respect to Figure 16, the energy input to the heat pump to generate the required heat (taking into account COP) plus any additional heat input required (assuming direct electric heating if the heat pump does not meet the demand), expressed as a percentage of the heat demand without the heat pump.

[0116] The horizontal axis TCX is the outlet temperature of the first condenser in degrees Celsius. A high temperature heat pump extracts heat from the first condenser, which receives the exhaust gas from the desorber outlet and reduces the temperature of the exhaust gas stream to recover water and adsorbent from the captured CO2. A water loop or other heat transfer medium connects the high temperature heat pump to the first condenser. Figure 17 and Figure 18 In all of the dual heat pump system examples shown, there is a second heat pump that removes heat from the second condenser heat exchanger directly or indirectly via one of the other fluid systems and delivers heat to the water loop. Therefore, the second heat pump can be described as operating between the temperature of the second condenser and the lowest temperature of the water loop. It will be understood by those skilled in the art that the actual flow and return temperature of the heat pumps on either side will actually be separated in temperature depending on the configuration and flow rate of the heat exchanger. The lowest temperature of the water loop is plotted on the vertical axis, with WLMT in degrees Celsius.

[0117] In addition to the contour lines, a dashed boundary line 1710 is shown, which represents the limit beyond which additional heat cannot be recovered because the approach temperature of the heat exchanger becomes too small (i.e., the fluid temperature in the heat transfer loop is too close to the temperature in the condenser or the second heat exchanger temperature).

[0118] Also shown are two or three dashed best fit lines based on quadratic equations derived for the respective minimum water loop temperatures for the desired condenser temperature T CX. There is no need to read these lines from the graph as the equations are provided in Tables 1 and 2 below. As an example, in Figure 17 , for graph #2, 1720 is the best-fit line achieving approximately 34% of the default energy usage, 1730 is the best-fit line achieving 36% of the default energy usage, and 1740 is the best-fit line achieving 38% of the default energy usage. When operating under conditions between the 34% line and boundary line 1710, the device will use the lowest energy, but practical considerations, variations in ambient conditions, and the limited availability of appropriately sized heat pumps and heat exchangers may necessitate some flexibility. Therefore, the device can be designed to operate within the control region indicated by the 38% line, preferably the 36% line, and even better, above the 34% line.

[0119] The controller may be configured to control the temperature of the WLMT to operate the unit at 38% better than the default energy usage based on the condenser temperature T CX. The controller may control the condenser temperature T CX to minimize the heat requirement for reheating the condensate returning from the desorber, as described above with reference to Figure 16 described. Figure 17 and Figure 18 The energy performance shown in the contour plot includes both optimizations. It should be noted that TCX Figure 16 The condensate temperature described in is lower because it is the outlet temperature of the condenser heat exchanger, not the condensate itself.

[0120] Figure 17 The four DAC system architectures that illustrate performance are as follows:

[0121] Architecture (#1): The first condenser is cooled by the first coolant loop, and the second condenser CX2 is cooled by a low-temperature heat pump directly or via a second coolant loop. The high-temperature heat pump transfers heat from the first coolant loop to the heating device of the desorber.

[0122] Architecture (#2): Lean return is cooled by a low temperature heat pump 210d which heats the coolant loop 810 which also cools the condenser 90 as shown. Figure 8 shown.

[0123] Architecture (#3): Second condenser CX2 preheats the cold rich line 50; the lean line returns 40 to feed the low temperature heat pump 210d, such as Figure 10 As shown, except that the low temperature heat pump 210 d delivers heat to a water loop 810 connected to the first condenser, rather than directly to the desorber heating device 70 .

[0124] Architecture (#4): The second condenser CX2 heats the lean line; the lean line returns to feed the low-temperature heat pump, such as Figure 9 As shown, 810 is provided by a water loop.

[0125] Each of these system architectures #1 to #4 has one or two condensers and two heat pumps in a heat pump system, as described in some of the embodiments above. As an option, a third condenser can be added with a third heat pump to obtain further improvements in efficiency. There is a balance achieved between the coefficient of performance of each heat pump, the temperature of the condensate returned to the desorber, and the delivery of dry CO2. There are changes in the adsorbent temperature at different sampling locations caused by changes in ambient temperature. For example, the rich return from the absorber will be close to the ambient wet bulb temperature, except in the system where the absorber temperature is controlled. When the lean stream is passed through the main heat exchanger, this has a knockon effect on the temperature of the lean stream to the absorber. As mentioned above, the ratio of heat transfer to each condenser can be controlled by changing the fluid flow in the heat exchanger or by bypassing the condenser. Therefore, the outlet temperature of the first condenser can be controlled, and the lowest temperature in the water loop connecting the outlet of the second heat pump to the inlet of the high-temperature heat pump that supplies heat to the desorber can also be controlled. The lowest temperature in the water loop is at the point where the water is fed to the high-temperature heat pump (e.g., Figure 8 、 Figure 9 and Figure 10 210b) at the outlet of the heat exchanger that supplies heat.

[0126] A controller may be provided that is configured to control the first condenser outlet temperature and the water loop minimum temperature to provide a maximum effective coefficient of performance (COP) for the entire heat pump system as ambient conditions vary. Alternatively, the condenser and heat exchanger may be sized to provide the best average effective COP under ambient conditions typical for the location of the device.

[0127] The best performing of these systems is Figure 10 The described architecture (#3): the second condenser CX2 preheats the cold rich line 50; the lean line returns 40 to feed the low temperature heat pump 210d, as shown Figure 10 As shown, except that the low temperature heat pump 210 d delivers heat to a water loop 810 connected to the first condenser, rather than directly to the desorber heating device 70 .

[0128] Figure 18 The performance of system #3 is shown under three different ambient conditions, with ambient air temperatures of 5, 25, and 40° C. It can be seen that the desorber energy usage can be lower when the ambient temperature is higher due to the additional heat available for recovery from the rich line.

[0129] The best performance achievable at ambient T = 25°C gives a minimum power load = 32.7% of the demand without a heat pump for #3 when TCX = 72°C and WLMT = 69°C. Therefore, a good operating regime would be with TCX between 68 and 72°C and WLMT between 65 and 69°C. This enables a direct air capture plant to operate using less than one-third of the energy required to directly heat the desorber. This can enable direct air capture to operate using cheaper electricity than plants using fossil fuels to heat the desorber. Using low-carbon electricity, such as renewable or nuclear power, makes direct air capture a smart option for reducing global warming.

[0130] For other architectures, a TCX between 68 and 76°C and a WLMT between 65 and 69°C will also provide good performance. To avoid crossing the boundary line, a safer WLMT range of 61 to 67°C with a TCX of 68 to 72°C is optimal under most operating conditions.

[0131] However, as mentioned above, design limitations may mean that optimal conditions cannot be achieved. Compromise operating conditions can be obtained for any of the described architectures by selecting a range of operating points on or above the 38% performance line (or a best fit to the 38% line) and designing or controlling the system to operate within the control region bounded by the 38% line or the 38% best fit line to form a quadrant-like shape with its apex at TCX=72°C and WLMT=69°C.

[0132] Table 1 - Best fit water loop temperatures required to achieve 38% less energy use than the base case for different ambient conditions.

[0133]

[0134]

[0135] The refrigerants used in heat pumps can be the same or different. There are only a few refrigerants that can operate in the saturated regime up to 130°C or higher (for simplicity, more complex cycles that use supercritical properties, such as transcritical cycles, are ignored). These include, for example, R717 (ammonia), R718 (steam / water, although this is not suitable for low-temperature operation), R600 (butane, flammable), and R1233zd (1-chloro-3,3,3-trifluoroprop-1-ene, developed as a lower global warming potential refrigerant for future air conditioning products). As a note, high temperature refrigerants for temperatures below 120°C include R245fa (1,1,1,3,3-pentafluoropropane, GWP 858), R717 (ammonia, GWP 0), R744 (carbon dioxide, GWP 1), R134a (1,1,1,2-tetrafluoroethane, GWP 1300), R1234ze(E) (1,3,3,3-tetrafluoropropylene, GWP 6); and those being developed for applications above 120°C include R1336mzz(Z) (cis-1,1,1,4,4,4-hexafluoro-2-butene, GWP 2), R718 (water / steam), R245fa, R1234ze(E), R600 (butane, GWP 4-6.5) and R601 (pentane, GWP 4±2).

[0136] It should be understood that the present invention is not limited to the above-described embodiments and that various modifications and improvements can be made without departing from the concepts described herein. Unless mutually exclusive, any feature can be used alone or in combination with any other feature, and the present disclosure extends to and includes all combinations and subcombinations of one or more features described herein.

Claims

1. A system for capturing carbon dioxide (CO2) from a CO2-containing gas stream, the system comprising: an absorber for contacting the CO 2 -containing gas stream with an adsorbent operable to capture CO 2 from the CO 2 -containing gas stream within a first temperature range and release CO 2 within a second temperature range; The absorber includes means for moving the CO2-containing gas stream through the absorber from an absorber inlet to an absorber outlet; a desorber for releasing the CO 2 from the adsorbent, the desorber being operable to receive a rich adsorbent stream from the absorber, heating the adsorbent using a heating device to provide heat to increase the adsorbent temperature from the first temperature range to the second temperature range, an exhaust conduit for supplying an exhaust stream comprising CO 2 and steam to a condenser; a lean return stream for returning adsorbent from the desorber to the absorber; a heat pump system comprising a first heat pump and a second heat pump configured to recover heat from both the condenser and at least one other point in the system operating at a condenser outlet temperature within a third temperature range, The condenser is fluidly connected to a condensate recovery circuit for delivering recovered condensate to the desorber, the first heat pump being configured to deliver heat to the heating device at the second temperature range and to receive heat from the condenser and the second heat pump via an intermediate circuit operating between the condenser outlet temperature and an intermediate circuit minimum temperature, the second heat pump being configured to receive heat from another point in the adsorbent system at a fourth lower temperature range and deliver it to the intermediate loop at a temperature greater than a minimum temperature of the intermediate loop, The third temperature range is selected to maximize both the heat content of the recovered condensate and the combined coefficient of performance of the first and second heat pumps in order to minimize the input energy requirements of the heat pump system.

2. The system according to claim 1, wherein: The intermediate loop minimum temperature is selected based on the condenser outlet temperature to maximize both the heat content of the recovered condensate and the combined coefficient of performance of the first and second heat pumps to minimize input energy requirements of the heat pump system.

3. The system according to claim 2, wherein: The third temperature range is between 68°C and 72°C.

4. The system according to claim 3, wherein: The minimum temperature of the intermediate circuit is between 65°C and 69°C.

5. A system according to any preceding claim, wherein: The second heat pump has a different refrigerant than the first heat pump.

6. A system according to any preceding claim, further comprising a second condenser, wherein The lean return stream is connected in heat exchange relationship to the second condenser, and wherein the second heat pump is configured to remove heat from the lean return stream after the lean return stream has exchanged heat with the second condenser.

7. The system of any preceding claim, further comprising a controller operable to control the heat pump system, and a plurality of sensors measuring the condition of the exhaust gas entering the condenser and the temperature of at least one other point in the adsorbent system, the controller being configured to model the coefficient of performance of each heat pump and the heat content of the recovered condensate and to adjust parameters of one or both heat pumps to minimize the energy demand of the heat pump system.

8. A system according to any preceding claim, wherein: The CO2-containing gas stream is ambient air.

9. A method for capturing carbon dioxide (CO2) from a CO2-containing gas stream, the method comprising: providing a first heat pump and a second heat pump, the first heat pump being operable to recover heat from an intermediate loop thermally connected to a condenser in the carbon capture system, the second heat pump being operable to recover heat from at least one other point in the system and deliver it to the intermediate loop, the condenser being operable to recover condensate and deliver it to a desorber, the first heat pump being operable to provide heat to the desorber; The temperature of the outlet of the condenser is controlled within a third temperature range to maximize both the heat content of the recovered condensate and the combined coefficient of performance of the first and second heat pumps to minimize input energy requirements of the first and second heat pumps.

10. The method of claim 9, further comprising controlling a minimum temperature of the intermediate loop based on the condenser outlet temperature to maximize both the heat content of the recovered condensate and a combined coefficient of performance of the first and second heat pumps to minimize input energy requirements of a heat pump system.

11. The method according to claim 10, wherein: The third temperature range is between 68°C and 72°C.

12. The method according to claim 11, wherein The minimum temperature of the intermediate circuit is between 65°C and 69°C.

13. The method according to any one of claims 9 to 12, wherein: The second heat pump has a different refrigerant than the first heat pump.

14. The method of any one of claims 9 to 13, further comprising connecting a lean return stream from the desorber to a second condenser, and using the second heat pump to cool the lean return stream after the lean return stream has exchanged heat with the second condenser.

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