Improving energy performance in CO2 capture

Abstract

The present disclosure relates to improving the energy performance of CO2 capture. [Solution] A system for capturing carbon dioxide (CO2) from a CO2-containing gas stream, comprising an absorber for contacting the CO2-containing gas stream with an adsorbent, the adsorbent operable to capture CO2 from the CO2-containing gas stream in a first temperature range and release the CO2 in a second temperature range, a desorber for releasing the CO2 from the adsorbent, first and second heat pumps configured to recover heat for the desorber from both a condenser and at least one other point in the system, and the condenser temperature selected to maximize both the heat content of the condensate and the combined coefficient of performance of the first and second heat pumps to minimize the input energy requirements of the heat pump system.

Description

[Technical Field]

[0001] The present disclosure relates to the energy performance of CO2 capture processes. [Background technology]

[0002] Carbon capture and storage is expected to be an important method for reducing the global warming effects of burning fossil fuels.

[0003] Carbon dioxide (CO2) capture can involve techniques to extract CO2 from CO2-containing gases using an absorption medium. Typically, this involves generating a gas flow over an absorption medium under conditions where the absorption medium absorbs CO2 from the gas, and then changing the conditions so that the medium releases the absorbed CO2 for capture and storage. This process can be used to reduce atmospheric CO2 to mitigate anthropogenic emissions related to global warming or climate change. Direct air capture (DAC) is the capture of CO2 from the atmosphere, which involves processing large amounts of air because the atmosphere contains less than 0.05% CO2.

[0004] Heat is used to release the absorbed CO2 in a temperature swing manner, and it has already been proposed that heat pumps can be an effective way to provide that heat in electric heating systems. For example, EP 2 512 628 A1 describes the use of heat pumps to recover low-grade thermal energy from various sources. The proposed low-grade energy sources are condenser and compression systems, as well as solvent cooling systems located within the solvent delivery conduit. However, opportunities remain to increase heat recovery and improve its efficiency, while simultaneously improving the effectiveness of the overall system. Summary of the Invention

[0005] According to a first aspect, there is provided a system for capturing carbon dioxide (CO) from a CO2-containing gas stream, the system comprising an absorber for contacting the CO2-containing gas stream with an adsorbent, the adsorbent operable to capture CO2 from the CO2-containing gas stream at a first temperature range and release CO2 at a second temperature range, the absorber comprising means for moving the CO2-containing gas stream through the absorber from an absorber inlet to an absorber outlet, the system comprising a desorber for releasing CO2 from the adsorbent, the desorber operable to receive a rich adsorbent stream from the absorber and to provide heat to heat the adsorbent using heating means to raise the adsorbent temperature from the first temperature range to a second temperature range, the system comprising an exhaust conduit for supplying an exhaust stream comprising CO2 and steam to a condenser, the system comprising a lean return stream for returning the adsorbent from the desorber to the absorber, the system comprising a desorber for releasing CO2 at a condenser outlet temperature in a third temperature range. a heat pump system comprising first and second heat pumps configured to recover heat from both a condenser operating in a fourth lower temperature range and at least one other point in the system in a fourth lower temperature range, the condenser being fluidly connected to a condensate recovery circuit that delivers recovered condensate to a desorber, the first heat pump being configured to deliver heat to the heating means in 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 one other point in the adsorbent system in the fourth lower temperature range and to deliver that heat to the intermediate circuit at a temperature above the intermediate circuit minimum temperature, the third temperature range being selected 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 the input energy requirements of the heat pump system.

[0006] Preferably, the intermediate circuit 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 requirements of the heat pump system.

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

[0008] In some embodiments, the minimum circuit 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, a second condenser may be present, the lean return stream being connected in heat exchange relationship to the second condenser, and the second heat pump being configured to remove heat from the lean return stream after it has exchanged heat with the second condenser.

[0011] Preferably, the system may include a controller operable to control the heat pump system and a plurality of sensors for measuring the condition of the exhaust air entering the condenser and the temperature of at least one other point in the adsorbent system, the controller being configured to simulate 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 requirements of the heat pump system.

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

[0013] Another aspect is a method of capturing carbon dioxide (CO2) from a CO2-containing gas stream, the method including the steps of providing a first heat pump for recovering heat from an intermediate circuit thermally connected to a condenser in a carbon capture system, and a second heat pump for recovering heat from at least one other point in the system and delivering the heat to the intermediate circuit, wherein the condenser is operable to recover condensate and deliver the condensate to a desorber, and the first heat pump is operable to supply heat to the desorber; and controlling the outlet temperature of the condenser 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 the first and second heat pumps.

[0014] Preferably, in some embodiments, the method also includes controlling the minimum temperature of the intermediate circuit 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 requirements 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 circuit temperature is between 65°C and 69°C.

[0017] As will be appreciated by those skilled in the art, unless mutually exclusive, features described in relation to any one of the above aspects can also be applied to the other aspects, mutatis mutandis. Furthermore, unless mutually exclusive, any feature described herein may be applied to any aspect and / or may be combined with any other feature described herein. [Brief explanation of the drawings]

[0018] Embodiments will now be described, by way of example only, with reference to the figures.

[0019] [Figure 1]1 is a schematic diagram of a prior art carbon capture system. [Figure 2] FIG. 1 is a schematic diagram of a carbon capture system with heat pump heat recovery from an absorber. [Figure 3] FIG. 1 is a schematic diagram of a carbon capture system and desorber condenser with heat pump heat recovery from an absorber recirculation loop. [Figure 4] FIG. 1 is a schematic diagram of a carbon capture system with heat pump heat recovery from an absorber with an additional air source heat exchanger. [Figure 5] FIG. 1 is a schematic diagram of a carbon capture system with heat pump heat recovery from an absorber and heat transfer between the absorber inlet and outlet. [Figure 6] 1 is a chart of adsorbent performance at different temperatures. [Figure 7] 1 is a chart of adsorbent performance at different temperatures. [Figure 8] FIG. 1 is a schematic diagram of a carbon capture system with heat pump heat recovery from the lean return in a cascade system where the heat pump recovers heat from the desorber condenser. [Figure 9] FIG. 1 is a schematic diagram of a carbon capture system with heat pump heat recovery from the hot lean return in a cascade system where the heat pump recovers heat from the desorber condenser. [Figure 10] 10 shows variations and alternative configurations of the system of FIG. 9. [Figure 11] 10 shows variations and alternative configurations of the system of FIG. 9. [Figure 12] 10 shows variations and alternative configurations of the system of FIG. 9. [Figure 13] 10 shows variations and alternative configurations of the system of FIG. 9. [Figure 14] 10 shows variations and alternative configurations of the system of FIG. 9. [Figure 15] 10 shows variations and alternative configurations of the system of FIG. 9. [Figure 16] 1 is a chart of energy usage for an example system at different condenser temperatures. [Figure 17]1 is a series of charts of energy usage for an example system optimized for heat pump performance. [Figure 18] 1 is a chart of energy usage for an example system optimized for heat pump performance at different ambient temperatures. DETAILED DESCRIPTION OF THE INVENTION

[0020] Detailed Description Referring to Figure 1, which illustrates a conventional system for carbon capture from CO2-containing gas, an absorber 10 receives a CO2-containing gas stream from an inlet 20 to an outlet 30. An adsorbent stream flows through the absorber, and a lean stream 40 enters the absorber and contacts the CO2-containing gas, becoming a rich stream 50. Adsorbent may be recycled within the absorber as a recycle stream 110, which increases the effective residence time of each portion of the lean stream of adsorbent within the absorber.

[0021] 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 typically uses a heating means 70, which may generate steam to form vapor bubbles into which the desorbed CO2 can diffuse, to heat the rich stream to a temperature at which the CO2 is released from the adsorbent, returning the lean stream of adsorbent to the absorber to repeat the process.

[0022] The vapor and desorbed CO2 exit the desorber at 80, where a condenser 90 is typically used to cool the mixture and condense the vapor, leaving a purer CO2 product stream.

[0023] Turning to Figure 2, the same reference numbers are used for the same or similar features unless otherwise noted. Absorber 10 receives a CO2-containing gas stream from inlet 20 to outlet 30. An adsorbent stream flows through the absorber, with lean stream 40 entering the absorber and contacting the CO2-containing gas to become rich stream 50. Adsorbent may be recycled within the absorber as recycle stream 110, thereby increasing the effective residence time of each portion of adsorbent within the absorber.

[0024] 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. This disclosure provides several embodiments describing heat transfer means, so this heat exchanger may optionally be included, not included, or located at a different location in the circuit. The desorber 60 receives the rich stream and uses heating means 70 to heat the rich stream to a temperature at which the CO2 will be released from the adsorbent.

[0025] The vapor and desorbed CO2 exit the desorber at 80, where a condenser 90 is typically used to cool the mixture and condense the vapor, leaving a purer CO2 product stream.

[0026] The boundary of the absorber system is marked as dotted line 200. The absorber system includes a recycle 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 heating means 70.

[0027] 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 while in contact with the CO2-containing gas. As described below, for any adsorbent, there exists a temperature at which the CO2 absorption rate is maximized for any given system.

[0028] An additional benefit of cooling the absorber is reduced evaporation, thereby helping to maintain the system's fluid balance and reducing water loss, a significant environmental cost for liquid-based DAC plants. Under some atmospheric conditions, it is possible to operate the absorber at a temperature below the ambient air dew point, such that the vapor pressure of the ambient air exceeds the vapor pressure of the air at the absorber temperature. In these conditions, the absorber can obtain water from the ambient air, which helps compensate for water loss due to drift or vapor loss during desorber exhaust. The heat pump system controller can control the absorber temperature based on ambient humidity to maintain liquid balance within the system. The controller can calculate a balance between energy usage, CO2 absorption rate, and water loss rate and adjust the temperature to achieve optimal operating conditions based on the price or environmental cost applicable to each of these parameters. The controller receives environmental cost indicators for CO2 capture, energy usage, and water loss, and can adjust the absorber temperature to minimize overall environmental costs, taking into account ambient environmental conditions such as humidity and temperature, the environmental cost of energy based on the current energy source, as well as local water resources. The controller may estimate the evaporation rate using known equations such as the Penman equation or variations thereof.

[0029] Heat can be recovered from the absorber system from either the gas stream or the adsorbent stream, or both, to maintain the average temperature of the adsorbent within the absorber within a first temperature range that maximizes the CO2 absorption rate.

[0030] The gas and adsorbent streams are in contact within the absorber for at least the residence time of the absorber, and a large contact surface area is preferred to maximize the transfer of CO2 from the gas stream to the adsorbent stream. This can be achieved by the adsorbent being delivered as a thin film coating on packing within the absorber, or by small droplets such as a fine mist. As a result of the contact and transfer of gas to liquid, the two streams rapidly reach thermal equilibrium with each other, so control of heat transfer only requires measurement of the temperature of one stream.

[0031] The offset to the desired temperature range can be based on the difference between either the inlet or outlet temperatures of the streams. This can be calculated by sensors measuring the temperature of each stream during operation. Alternatively, the offset can be calculated by measurement or modeling for a single absorber, and then the calculated offset can be applied to multiple similar units.

[0032] In some cases, there may be multiple absorber units with common lean and rich streams supplied from a central desorber unit and a common recycle stream for all absorbers or groups of absorbers. The temperature of the multiple absorbers can be controlled by cooling the recycle stream, thus supplying cooled or heated absorption liquid to all absorbers.

[0033] In other cases, multiple absorbers may be supplied with gas or air through a common plenum, and the temperature of the absorbers may be controlled by cooling or heating the supplied gas or air in the plenum.

[0034] In Figure 3, features 10-210 are similar to those in Figure 2. Heat is recovered from the absorber system 200 by a heat pump system 210 and delivered to the heating means 70.

[0035] 3, a heat exchanger 320 is shown on the adsorbent feed to the absorber, with the heat exchanger 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, there may be a heat exchanger on either or both of the mixed return stream 40 or the recycle stream 110.

[0036] Heat recovery stream 330 transfers heat from heat exchanger 320 to heat pump system 210. This may be a refrigerant loop or another heat transfer fluid that provides heat transfer between the heat exchanger and the heat pump system.

[0037] There may be multiple absorber systems connected to a single desorber. The multiple absorber systems may share a single recycle stream and heat recovery can be performed by a single heat pump system for all connected absorbers by using heat exchangers.

[0038] In the above embodiment, in some climatic conditions, additional heat input to the desorber may be required to supplement the heat recovered from the absorber. This additional heat may be supplied from an external source, such as steam, waste heat from another process, electrical heating, or any other conventional heat source.

[0039] Also shown in FIG. 3 is a high temperature heat recovery path 340 from the condenser heat exchanger 90, which provides additional heat to the heat pump system.

[0040] The vapor and desorbed CO2 exiting the desorber is at a much higher temperature than the absorber. For example, the desorber may operate at 90°C-120°C, and the absorber may operate at ambient conditions between 0°C-30°C, or may be controlled over a fixed temperature range to improve adsorbent performance as described above. It may be preferable to have a cascade heat pump system to combine the heat recovery from both units.

[0041] A cascade heat pump system may include a first heat pump configured to receive heat from a high temperature heat recovery circuit and a second heat pump configured to remove heat from the absorber and deliver heat to the high temperature heat recovery circuit. The coefficient of performance (COP) of each heat pump is determined in part from the temperature difference between the high temperature circuit, the absorber, and the heating means 70. Thus, through careful design of the systems described herein, the COP can be optimized in conjunction with maximizing available heat.

[0042] In FIG. 4, features 10-210 are similar to those in FIGS. 2 and 3, except that heat pump system 210 is shown as two heat pumps, 210a and 210b. Heat is recovered from absorber system 200 by heat pump 210a and delivered to heating means 70. Heat is recovered from condenser 90 by heat pump 210b. While the two heat pumps are shown connected independently, they can also be arranged as a cascade system, as previously described. Depending on the selection of heat pumps and the refrigerants therein, the overall effective COP of the heat pump system, based on the heat and energy input delivered by both heat pumps, can be maximized by operating heat pump 210a between the absorber temperature and heating means temperature, or between the absorber temperature and the refrigerant temperature. This result also depends on the selection of the condenser heat exchanger off temperature. This allows for maximizing overall heat recovery with a minimum amount of mechanical energy input to the heat pumps that can be powered.

[0043] Also shown is an air heat exchanger 410 connected in heat transfer relationship with the heat pump 210a. This heat transfer relationship is reversible, for example, by a variable refrigerant flow system or other methods known to those skilled in the art. This allows the heat supply to the heating means 70 to be balanced in hot or cold climate conditions while maintaining the absorber temperature within a first operating temperature range where the CO2 absorption rate is maximized.

[0044] In summer conditions, the absorber absorbs heat from the ambient air passing through it, while the recirculation loop 110 is cooled to remove this heat and maintain the absorber within a first temperature range. If the heat recovered from the recirculation loop exceeds the heat required by the heating means 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 prevent freezing. This additional heat can be obtained by operating the air heat exchanger like an air-source heat pump. The air heat exchanger 410 may have its own fan to move air through it, or it may be connected via a plenum to the same air movement means used by the absorber.

[0045] 5, air heat exchangers 510 and 520 are attached to the inlet 20 and outlet 30 of the absorber 10 to exchange heat with the air entering and leaving the desorber. A reversible heat pump 210c is connected to the air heat exchangers 510, 520. This arrangement means that the heat pump 210c can be used to heat or cool the inlet air by transferring heat between the inlet and outlet.

[0046] Heat pump 210c controls the temperature within the absorber system so that the adsorbent is maintained within a preferred temperature range while in contact with the CO2-containing gas. Heat pump 210a may continue to extract heat from recirculation loop 110 to supply heat to heating means 70.

[0047] When operating in cold weather conditions, heat pump 210c cools exhaust air 30 below ambient temperature while warming the inlet air above a desired temperature sufficient to allow recirculation loop cooling to balance the temperature in the absorber at the desired temperature. If outlet heat exchanger 520 begins to accumulate ice, a defrost cycle for heat pump 210c may be required; this process is similar to that used in conventional air-source heat pumps.

[0048] When operating in hot weather conditions, heat pump 210c warms exhaust air 30 above ambient temperature while cooling inlet air sufficiently above the desired temperature to allow recirculation loop cooling to balance the temperature in the absorber at the desired temperature. If inlet heat exchanger 510 begins to accumulate ice, a defrost cycle for heat pump 210c may be required.

[0049] Because air heat exchangers 510 and 520 can extract additional heat from the ambient air, this combination can supply the additional heat demand required by the desorber. If the ambient temperature is already within the desired range, heat pump 210c may be idle.

[0050] If there are multiple absorber units, they may all be connected to a common inlet and outlet plenum, and the air heat exchanger may be located in the common plenum, simplifying the implementation of heat pump 210c.

[0051] Adsorption properties: Sorbents for carbon capture generally have an equilibrium shift between the carbonate form and being in solution with CO2 that is temperature dependent. The sorbent is carried in a solvent, e.g., water, which may contain additional additives that can act as catalysts, modify the physical properties of the solution, reduce degradation or other desirable properties.

[0052] For each adsorbent in solution, an optimal temperature range can be determined for which the rate of absorption of carbon dioxide from the ambient gas in the absorber is high. Given the large number of variables, experimental determination of the optimal temperature range for a particular adsorbent solution under specific absorption conditions may be required.

[0053] Figures 6 and 7 show exemplary plots (601, 701) of the temperature performance of a sorbent solution versus temperature in °C. The temperature performance "R" is the overall rate of CO2 absorption, which may be, for example, Kg CO2 / hr, for a given absorber and sorbent combination, expressed as a percentage of the rate achieved at standard conditions, in this case 20 °C. In Figure 6, lines 602 and 603 represent the lower and upper limits of the operating temperature range desired to achieve at least 100% of the standard rate, in the example, approximately 4 to 21 °C. In Figure 7, lines 702 and 703 represent similar points, but targeting approximately 95% of the standard performance from 4 to 25 °C. The two charts provide exemplary data for two different sorbent solutions, e.g., MEA or amino acid salts. The data in the charts should not be construed as precise; the charts are provided as examples of how the operating range can be determined once the performance of the sorbent solution has been characterized.

[0054] The temperature range can be expressed as the minimum and maximum temperatures of the contact area of ​​the absorber, i.e., the temperature of the adsorbent inside the absorber while it is in contact with the CO2-containing gas. Generally, when the absorber is an air-liquid contactor in a DAC system, the adsorbent temperature and the temperature of the air in the absorber will be very close because the adsorbent is distributed in a thin film or droplets to achieve a high surface area and contact time, and the air flow is relatively high.

[0055] The rate of CO2 absorption from ambient gas can be defined by the change in the number of carbonated sorbent molecules per unit time or per pass through the absorber. For an absorber with constant 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.

[0056] In this application, unless otherwise specified, CO absorption rates are expressed as a percentage of the CO absorption rate at 20°C. Thus, if the adsorbent performs better at other temperatures, the rate may exceed 100%. Expressed in this manner, the rate can be understood whether it applies at laboratory scale or in a large-scale carbon capture plant, even though the actual rate may vary significantly if other parameters are changed.

[0057] Thus, the optimum temperature range for the adsorbent is expressed as a minimum and maximum temperature between which the rate of CO2 absorption exceeds a threshold, for example, the rate exceeds 95%.

[0058] A complete model for operating a CO2 capture system may need to consider factors other than absorption efficiency, such as energy efficiency, sorbent degradation rate, sorbent loss, maintenance costs, etc. For any CO2 capture system, an absorption rate threshold can be set taking into account all other factors to give the best overall operation of the system.

[0059] For some adsorbent chemistries, the optimum range is narrow and it is desirable to operate the thermal management closely to keep it within the desired range. For other adsorbent chemistries, the optimum range is much wider and the thermal management is really only needed on very cold and very hot days.

[0060] The adsorbent may include an alkaline absorbent such as a hydroxide or an organic adsorbent.

[0061] Alkaline adsorbents may include potassium hydroxide or calcium hydroxide.

[0062] The organic adsorbent may include an amine, an amino acid, etc. The amine may include an ethanolamine (2-aminoethanol, monoethanolamine, ETA, or MEA).

[0063] Preferred adsorbents include amino acids or alkaline salt solutions of amino acids. The amino acids may be selected from the group consisting of alanine, arginine, asparagine, aspartic acid, cysteine, glutamic acid, glutamine, glycine, histidine, isoleucine, leucine, lysine, methionine, ornithine, phenylalanine, proline, sarcosine, selenocysteine, serine, taurine, threonine, tryptophan, tyrosine, or valine. The amino acids may be amino acid-derived compounds such as methylamine or diethylamine.

[0064] The preferred alkaline component of the amino acid salt is potassium or sodium. Examples of the amino acid salt include sodium glycinate and potassium lysinate.

[0065] Amino acids are preferred because they require less heat for desorption, decompose less than amines, and are understood to be less hazardous in use than many alternatives. However, amino acids generally have a smaller temperature range for absorption to achieve a desired CO2 absorption rate. The invention described herein is particularly advantageous when applied to CO2 capture systems that use amino acids and their salts and compounds, because maintaining the absorption medium within an optimal temperature range improves carbon capture performance.

[0066] Combining heat recovery using a heat pump with control of the absorber temperature provides a dual benefit to the carbon capture system that both increases the amount of CO2 captured per unit of energy consumed: each mole of sorbent absorbs more CO2 per pass through the absorber, while the energy used to desorb the CO2 from the sorbent is reduced by the heat pump.

[0067] When combined with other heat recovery means described herein, the entire heat requirement of the desorber can be provided using only electrical or mechanical energy input to the heat pump, requiring less primary energy than would otherwise be the case.

[0068] High temperature side placement Referring now to Figure 8, only the hot side of the system is shown. The absorber and associated heat recovery options may be in any of the configurations shown in Figures 1-5, connected to the lean line 40 and rich line 50.

[0069] The heat pump system of Figure 9 comprises heat pump 210b, which recovers heat from the condenser 90, and heat pump 210d, which recovers heat from the lean line 40. Heat pump 210d recovers heat from the lean return at a temperature intermediate between the main heat exchanger 100 and the absorber, and cools the lean return before returning to the absorber. The heat is then fed to intermediate circuit 810, where it is combined with heat from the condenser. Heat pump 210b can operate with a high coefficient of performance due to the small temperature difference between the condenser and heating means 70.

[0070] The heating means may require heat of 120-130°C, but the condenser can be sized to cool exhaust stream 80, for example, to just below the dew point of the exhaust mixture, recovering most of the latent heat from the exhaust. Condensate line 820 returns the hot condensate, which is mostly water, to desorber 60. By keeping the condensate temperature high, less sensible heat is required to reheat the condensate to the desorber temperature, while the heat pump's COP is high, reducing the overall thermal demand of the system.

[0071] Additional moisture present in the exhaust stream after the condenser can be removed later at lower temperatures, 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 water balance.

[0072] Because heat pump 210d only raises the temperature of heat from the lean return temperature to the condenser temperature, not to the temperature required for the heating means, a wider selection of heat pumps is available, as heat pumps capable of delivering heat above 100°C tend to be complex and require specialized refrigerants. This means that a dedicated heat pump with a high coefficient of performance between the condenser temperature and the reboiler can be run for the entire required amount of heat, and heat can be supplied to the intermediate-temperature reservoir 810 from either the condenser or another heat source (including another low-temperature heat pump) so that the heat extracted from the intermediate reservoir matches the heat supplied to the intermediate reservoir. External heat can also be added from the absorber side. This can be from a separate air-source heat pump, ground-source heat pump, water-source heat pump, or waste heat from an external industrial process or district. External heat can also come from a combination of sources.

[0073] Figure 9 shows another option for hot-side heat recovery. All of the features of Figure 8 are present as previously described. Additionally, a second condenser 920 is shown that recovers latent and sensible heat from the exhaust gas 80 after it has passed through the first heat exchanger of the condenser 90. The lean return line 910, after being cooled by the rich line 50 in the main heat exchanger, passes through the second condenser 920, which recovers some of the remaining enthalpy of the exhaust. This additional heat is then recovered by the heat pump 210d, which now has an improved COP and more available heat to recover due to the higher temperature of the lean return 910.

[0074] FIG. 10 shows similar features to FIG. 9 , except that a hot condensate 820 may be recovered after the first condenser heat exchanger 90. The second condenser is cooled by a cold rich line 50 returning from the absorber at an absorber temperature that may be near ambient temperature, and is preheated in a condenser 920 before receiving additional heat in the main heat exchanger 100 from a lean line that is at a temperature near the desorber temperature. In this example, the main heat exchanger may be smaller than typically used, or less heat may be removed from the lean line 910, meaning that the lean line has more heat available, i.e., is hotter, for recovery by the heat exchanger 1110 and the heat pump system 210d. The result of the lean return 910 exiting the main heat exchanger 100 at a higher temperature is an improvement in the COP of the heat pump 210d, making more heat available for recovery. Because the rich line 50 is near or below ambient temperature, when the features of Figures 2-5 are combined with those shown, the heat recovered from the second condenser 920 provides preheat to the rich line with little additional energy requirement. The resulting CO2 emissions are also drier, reducing the amount of post-treatment required before storage or further use. Heat pump 210d may deliver heat directly to heating means 70 as shown, or may deliver heat to the input of heat pump 210d to form a cascade heat pump system as described elsewhere, for example, in Figure 8.

[0075] 11 shows a system 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 a heat exchanger 1110 on the lean line 40 after the condenser, thus recovering heat from both the condenser and the lean line return using only a single additional heat exchanger and heat pump. By raising the lean return temperature, condenser enthalpy is captured and the single heat pump has a high coefficient of performance, but it also captures residual heat in the lean line before it is lost in the absorber.

[0076] 12 is a variation of the system shown in FIG. 11, but in this case the rich line returning from the absorber at an absorber temperature that may be close to ambient temperature is preheated in condenser 90 before receiving additional heat from the lean line in main heat exchanger 100. In this example, the main heat exchanger may be smaller or may remove less heat from lean line 910, meaning that the lean line has more heat available for recovery by heat exchanger 1110 and heat pump system 210.

[0077] In both Figures 11 and 12, additional heat may be recovered from an absorber or other low temperature source to provide additional heat to the heating means 70 using an additional heat pump either feeding the heating means directly or in a cascade arrangement with heat recovery from the high temperature lean line.

[0078] FIG. 13 shows another variation of the high-temperature lean-line system of FIGS. 11 and 12. As in FIG. 9, a second condenser 920 is used to further cool the exhaust from the desorber 80, allowing the first condenser 90 to be optimally sized to maximize latent heat recovery from the exhaust stream and return the high-temperature condensate 820 to the desorber. Here, the second condenser is cooled by the low-temperature lean line 910, which then enters the first condenser 90. The high-temperature lean line then passes through a heat exchanger 1110, where heat is collected by the heat pump system 210 and supplied to the heating means 70. The advantage of this configuration is that the temperature of the condensate 820 can be set by sizing the two condensers and balancing the liquid by returning the high-temperature condensate to the desorber and the low-temperature condensate from the second heat exchanger elsewhere in the system. At the same time, only a single heat pump needs to be used to manage the condensate and lean-line heat recovery.

[0079] Figure 14 is a similar configuration to Figure 13, except that a low temperature rich return 50 is used to cool both condensers. As in Figure 12, the rich line 50 returning from the absorber at absorber temperature, which may be near ambient temperature, is preheated in condensers 920 and 90 before receiving more heat from the lean line in main heat exchanger 100. In this example, the main heat exchanger may be smaller or may have less heat removed from lean line 910, meaning that the lean line has more heat available for recovery by heat exchanger 1110 and heat pump system 210.

[0080] Figure 15 is a variation of the systems of Figures 13 and 14, in which the cold rich line 50 returning from the absorber is used to cool the second condenser 920 before being further heated by the main heat exchanger 100. The lean line is cooled in the main heat exchanger 100 and then reheated in the first condenser 90.

[0081] Because the rich line 50 may be very cold, especially when the absorber is operated in cold climates, additional heat may be recovered by a second condenser 920 while still providing the advantages of Figures 13 and 14 for heat recovery from a single heat pump and a first condenser 90 for recovering heat from the condenser and lean return.

[0082] One aspect of the present disclosure is the recovery of heat from the absorber itself, as described with reference to Figures 2-7. As previously mentioned, heat recovery from the absorber can also be used to control the absorber temperature by cooling either the air or liquid within the absorber. This has the advantage of maintaining the CO2 capture rate by the absorbent, which in most cases decreases in high ambient conditions. Also, water loss from the absorber through evaporation can be mitigated, or water can even be recovered from the ambient air in some conditions.

[0083] Another aspect of the present disclosure is the use of a lean line return to transfer heat to the heat pump after it has been recovered from the condenser. This allows for the simplification of the heat pump system by combining multiple heat sources into a single heat source at a constant temperature. Examples of this are shown in Figures 9-15.

[0084] Another aspect of the present disclosure is the control of the condenser temperature within a range. There is a trade-off between maximizing the enthalpy recovered in the exhaust stream from the desorber, which is primarily gaseous HO and carbon dioxide, the efficiency of the heat pump system, whose coefficient of performance varies with temperature difference, and the heat demand imposed on the desorber by reinjecting the hot condensate. Because liquid water and CO combine to form an acidic solution that can damage plant and piping, it is desirable to remove HO from the exhaust stream before further processing or transport. Water must also be recovered to maintain liquid balance within the desorber (maintaining desorber pressure) and to maintain liquid balance in the carbon capture system as a whole.

[0085] Control of the temperature of the condenser 90 can be achieved using various systems described herein. In Figures 8, 9, and 10, the temperature of the condenser 90 is set by a heat pump 210b, which can be selected to provide a desired condenser temperature at desired operating conditions. Alternatively, the heat pump may include controls to vary the flow rate of refrigerant through the condenser, either by controlling the heat pump compressor or by diverting a portion of the refrigerant to another heat source. Other methods of temperature control are known to those skilled in the art of heat pumps and refrigeration.

[0086] 11-15, control of the condenser temperature is achieved by the flow of sorbent through the condenser. The condenser may be sized to achieve the desired temperature under expected operating conditions, or the control system may vary the flow to maintain the desired temperature. For example, a bypass valve may operate to divert the sorbent flow around the condenser.

[0087] In any of Figures 8-15, the control system may include one or more sensors that measure values ​​indicative of the condenser temperature or condensate temperature and control the condenser temperature based on the sensor readings as described above.

[0088] As the condenser outlet temperature increases, the enthalpy available for heat recovery decreases because more heat is lost in the exhaust stream exiting 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 means, increases. Additionally, when the condensate is injected into the desorber to maintain liquid balance, the condensate requires sensible heat to raise the condensate to the desorber temperature. The resulting total heat demand has been calculated for several exemplary systems and is shown in Figure 16. In principle, this model applies, with minor modifications within the skill of the art, to any desorber in a carbon capture system when a heat pump is used to recover heat from the absorber. Because absolute temperatures depend on at least the choice of adsorbent, the ambient operating conditions of the absorber, and the pressure in the desorber, the horizontal chart in Figure 16 is scaled from 0 to 1, 0 representing ambient conditions, and 1 representing the maximum temperature of the desorber exhaust stream when no heat is recovered. There is always a balance between sensible and latent heat to be determined, which depends on the selection of the operating pressure of the desorption system and usually results in the dew point of the exhaust stream being a relatively fixed percentage of the maximum temperature.

[0089] The power requirement of the desorber, typically expressed in electrical kilowatts, is the energy input to the heat pump to generate the required heat, taking into account the COP, plus any additional heat input required, assumed to be direct electrical heating, if the demand cannot be met by the heat pump. The desorber requires energy at a rate high enough to break the bonds between the sorbent and CO2, raise the sorbent temperature from that in the absorber to a temperature below the equilibrium carbonation of the sorbent molecules in the absorber, and generate vapor-like bubbles in the sorbent solution through which the CO2 diffuses to facilitate its escape from the solution. This rate depends on several conditions, including the flow of sorbent through the desorber, which can be fixed-flow. The power supply can be grid power or can be provided by local low-carbon energy sources such as wind, solar, or nuclear power. On the vertical axis of the chart, this is expressed as the percentage of heat required if the condenser were cooled to ambient temperature and all available enthalpy were recovered.

[0090] If a second heat pump is used to recover heat from a second heat source, the electrical input to the second heat pump is included in the desorber's energy usage. Carbon capture systems may have other energy demands, such as fan and pump power, which may be assumed to be constant for purposes of illustrating the effects of this disclosure, and therefore these are not included in the exemplary calculations.

[0091] By modeling the enthalpy flow rate in the exhaust stream, the mass flow rate and temperature of the condensate, and the coefficient of performance of the heat pump system (including one or more heat pumps), a curve of the net energy input required to heat the adsorbent in the desorber can be obtained. A simulation system can be configured to calculate the energy usage of each portion of the heat pump system and generate desired temperature ranges for controlling the temperatures 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.

[0092] In Figure 16, four exemplary traces are plotted showing the total energy usage of the absorber system.

[0093] Trace 1610 (dotted line) shows the energy (E) requirements of the absorber when heat is supplied by a single heat pump recovering heat from a single condenser at temperature (T). As previously mentioned, the horizontal axis, T, is scaled as a relative temperature range, from an absorber temperature of 0 to a maximum condenser temperature of 1. In one 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 an energy usage of 0.54. At low values ​​of T, maximum latent and sensible heat is recovered from the exhaust stream, but because the condenser heat pump 210b has a low COP, more work is provided by the heat pump as heat. As the temperature reaches 50% of the range shown, there is less heat to recover and the HP adds less heat as work. Above this temperature, additional heat input is required by the absorber, which for purposes of illustration in this trace is assumed to be direct electrical heating. Thus, total energy use increases with temperature above 0.5 until, above T=0.8, the net energy use exceeds that which would have been required at T=0. In one example, T=0.5 corresponds to 70°C, and the energy requirement is equivalent to 1.67 GJ of electricity per tonne of CO2 emitted for an MEA-based carbon capture system.

[0094] Trace 1620 (dashed and dotted lines) shows the absorber energy (E) requirements when heat is supplied by two independent heat pumps, except the heat pumps are connected in parallel rather than in cascade, with one heat pump recovering heat from a condenser at temperature (T) and the other recovering heat from the lean return reheated by the second condenser, as shown in FIG. 9, for example. That is, the high temperature output of heat pump 210d is the temperature of heating means 70, not the temperature of intermediate circuit 810. As previously mentioned, the horizontal axis T is scaled from an absorber temperature of 0 to a maximum condenser temperature of 1. In one example, the ambient temperature is 25°C and the maximum condenser temperature is 115°C. Trace 1620 shows a minimum at T=0.72, with an energy usage of 0.48 compared to a fully cooled condenser. Compared to trace 1610, the additional heat in the lean line, including the residual heat from the main heat exchanger 100 and the additional heat obtained from the second condenser 920, is recovered at a lower COP than the condenser heat pump, but uses much less input power than direct electrical heating. At T = 0.5 to 0.8, when the backup heater is the second heat pump, the backup heater provides the additional heat needed without increasing the power input. Power usage is reduced between T = 0.5 and 0.72, then rises slowly, only exceeding that of the single heat pump example of trace 1610 for T > 0.83 (the range between the absorber and desorber temperatures). Thus, the system can operate efficiently with a condenser controlled or configured to operate in the relative temperature range of T = 0.5 to 0.83, preferably T = 0.6 to T = 0.8, and most preferably T = 0.7 to T = 0.75. These ranges have actual temperatures of 70-99°C, 79-97°C, and 88-92°C for examples using MEA adsorbents, or other adsorbents that can be used to adsorb at 0-30°C and desorb above 90°C.

[0095] Trace 1630 (dashed line) shows the energy (E) requirements of the absorber when heat is supplied by two heat pumps in a cascade arrangement, such as the arrangement of FIG. 9. One heat pump 210b recovers heat from a condenser at temperature (T), and the other heat pump 210d recovers heat from the lean return that has been reheated by the second condenser 920, as shown in FIG. 9. In this case, the heat pump recovering heat from the lean return supplies heat to the intermediate circuit 810, where it is combined with heat from the condenser and then transferred to a higher temperature by the condenser heat pump. Trace 1630 shows slightly higher energy usage than trace 1620 over the same operating temperature range of the condenser. However, this difference is small and can be improved by optimizing the selection of the 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 up to the condenser temperature setpoint, for example 70-99°C, 79-97°C, or most preferably 88-92°C. This means that this second heat pump is simpler and can use more widely available refrigerants than a heat pump that needs to deliver heat to about 120°C directly to the heating means.

[0096] The performance of the heat pump 210d may be improved by operating a lean-line heat pump from a counter-flow heat exchanger to increase the heat pump's heat source temperature, thereby increasing the COP.

[0097] Trace 1640 shows the energy performance of a system in which a rich line is used to cool the second condenser 920. As a result, the low-temperature rich adsorbent stream returning from the absorber is preheated before entering the main heat exchanger 100, resulting in a higher-temperature lean line return due to less heat transfer in the main heat exchanger. This configuration is shown in FIG. 10 and described above. Trace 1640 shows similar energy performance to trace 1620 up to a relative temperature T of around 0.8, but the performance range is extended at higher condenser temperatures. Energy usage remains below the energy requirements of the single heat pump system of trace 1610, with a minimum between T=0.5 and T=0.86. The minimum is at T=0.72, which is equivalent to 90°C for the MEA adsorbent example, and the energy requirement is 0.48 of the energy requirement for a fully cooled condenser with a single heat pump. From T=0.64 to T=0.80, energy usage is less than half that of the low-temperature single heat pump example. In absolute terms, best performance is achieved at about 85°C-95°C, with an energy usage of less than 1.49 GJ per tonne of CO2 emitted, for the example MEA adsorbent.

[0098] Trace 1650 (dash-dotted line) shows the combined condensate temperature in degrees Celsius plotted against the right-hand axis. In any example described herein, if there are two condensers, the condensate 820 may be recovered after the first or second condenser 920, or both. An optional external heat source may be used. The temperature of the first condenser may be set higher to allow the heat pump 210b to operate at a high, consistent COP to primarily recover latent heat with little temperature drop across the condenser 920. The second condenser 920 may be sized 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 requirements of the heat pump system.

[0099] If the first condenser is operated to recover approximately 88% to 71% of the available water vapor, the second condenser can be operated at a much lower temperature to recover the remaining water, for example, by using a subambient rich return line to cool the second condenser. Because the mass flow rate from the second condenser is only 12% to 29% of the available water, the combined temperature of the combined stream remains high. In one example, if the condenser outlet temperature of the first condenser is 85 to 100°C and the second condenser is cooled to 25°C or less, the condensate temperature of the combined stream can exceed 78°C. This minimizes the heat requirements for reheating the returned condensate in the desorber. This temperature range also overlaps with the temperature range over which two heat pump systems can operate, minimizing the system's energy requirements.

[0100] In all traces in Figure 16, performance data is derived from an MEA adsorbent-based system with the desorption reboiler operating at approximately 120 °C and the absorber operating at ambient conditions of 25 °C. However, the scale has been normalized to 0 to 1 between these two figures because heat pump performance and condenser performance are expected to 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 different adsorbents than those shown.

[0101] Figures 17 and 18 illustrate the benefits of the aforementioned condenser-heat pump combination, particularly the surprising improvement in energy efficiency that can be achieved by deliberately controlling temperatures at two points in the system. For each of the above system variations using one or two condensers, the system can be generalized as including an intermediate circuit, such as a water loop directly cooling the first condenser, a low-temperature heat pump extracting heat from another location, e.g., the second condenser, directly or indirectly using one of the other fluid circuits, and a heat pump extracting heat from the water loop and supplying it to the desorber heating means. While some heat can always be recovered from both locations, optimizing the heat recovery and maximizing the heat pump's coefficient of performance requires specific control points to be implemented in the system. The optimal temperatures of the first condenser and water loop also vary with ambient temperature. This configuration identifies an optimal operating temperature range that can be used in conjunction with several of the above embodiments, particularly the illustrated embodiment.

[0102] FIG. 17 shows contour plots of energy usage for four different system architectures. Numbered lines 34-48 are contours of the desorber's energy performance expressed as a percentage relative to the desorber without the heat pump. Thus, contour "40" indicates an operating regime in which the desorber system would use 40% of the required energy compared to a directly heated desorber. That is, as discussed above with respect to FIG. 16, the energy input to the heat pump to generate the required heat, taking into account the COP, plus the required additional heat input, assumed to be direct electric heating, if the demand is not met by the heat pump, expressed as a percentage of the heat demand without the heat pump.

[0103] The horizontal axis T CX is the outlet temperature of the first condenser (°C). The high-temperature heat pump extracts heat from the first condenser, which receives exhaust from the desorber outlet, to reduce the temperature of the exhaust stream and recover water and sorbent from the captured CO2. A water loop or other heat transfer medium connects the high-temperature heat pump to the first condenser. In both of the example heat pump systems shown in Figures 17 and 18, there is a second heat pump that removes heat from the second condenser heat exchanger directly or indirectly through 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 minimum temperature of the water loop. As those skilled in the art will appreciate, the actual flow and return temperatures of the heat pumps on either side will actually be separated by a temperature depending on the heat exchanger configuration and flow rate. The minimum temperature of the water loop is plotted on the vertical axis and is in WLMT (degrees Celsius).

[0104] Similar to the contour lines, dashed boundary lines 1710 are shown, which represent limits beyond which additional heat may not be recovered because the approach temperature of the heat exchanger is too low, i.e., the fluid temperature in the heat transfer loop is too close to the temperature in the condenser or the temperature of the second heat exchanger.

[0105] Also shown are two or three dashed best-fit lines based on quadratic equations derived for the minimum water loop temperature for each desired condenser temperature, TCX. The lines do not need to be read from the chart, as the equations are provided in Tables 1 and 2 below. For the example chart in FIG. 17 in #2, 1720 is the best-fit line for achieving approximately 34% of the default energy usage, 1730 is the best-fit line at 36%, and 1740 is the best-fit line at 38%. The plant will use the least energy when operating in conditions between the 34% line and boundary line 1710, although practical considerations, fluctuating ambient conditions, and limited availability of precisely sized heat pumps and heat exchangers may require some flexibility. Thus, the plant can be designed to operate in the control region indicated by the 38% line, preferably the 36% line, and even better above the 34% line.

[0106] The controller may be configured to control the temperature of the WLMT so that the plant operates at better than 38% of the default energy usage based on the condenser temperature, TCX. The controller can control the condenser temperature, TCX, to minimize the heat requirements for reheating the returned condensate in the desorber, as described above with reference to FIG. 16. The energy performance shown in the contour plots of FIGS. 17 and 18 includes both of these optimizations. Note that TCX is cooler than the condensate temperature described for FIG. 16 because it is the outlet temperature of the condenser heat exchanger, not the condensate itself.

[0107] The four DAC system architectures whose performance is shown in Figure 17 are:

[0108] Architecture (#1): The first condenser is cooled by a first refrigerant loop, and the second condenser CX2 is cooled by a low-temperature heat pump either directly or via a second refrigerant loop. The high-temperature heat pump transfers heat from the first refrigerant loop to the heating means of the desorber.

[0109] Architecture (#2): The lean return is cooled by a low temperature heat pump 210d which heats a refrigerant loop 810 which also cools the condenser 90 as shown in FIG.

[0110] Architecture (#3): A second condenser CX2 preheats the low temperature rich line 50; the lean line return 40 supplies the low temperature heat pump 210d as shown in FIG. 10, except that the low temperature heat pump 210d delivers heat to a water loop 810 connected to the first condenser rather than directly to the desorber heating means 70.

[0111] Architecture (#4): Second condenser CX2 heats the lean line; the lean line return feeds the low temperature heat pump as shown in Figure 9, 810 is provided by the water loop.

[0112] Each of these system architectures #1-#4 has one or two condensers and two heat pumps in the heat pump system, as described in some embodiments above. Optionally, a third heat pump can be added to the third condenser to obtain further efficiency improvements. A balance must be 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. The temperature of the absorbent at various sampling locations varies due to ambient temperature fluctuations. For example, the rich return from the absorber is close to the ambient wet-bulb temperature, except in systems where the absorber temperature is controlled. This has a knock-on effect on the temperature of the lean stream to the absorber as it passes through the main heat exchanger. The rate of heat transfer in each condenser can be controlled, as previously described, by varying the fluid flow within the heat exchanger or by bypassing the condenser. Therefore, the temperature at the outlet of the first condenser can be controlled, as can the minimum 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. The lowest temperature in the water loop is at the outlet from the heat exchanger that supplies heat to the high temperature heat pump (e.g., 210b in Figures 8, 9, and 10).

[0113] A controller configured to control the first condenser outlet temperature and the water loop minimum temperature can be provided to provide the maximum effective coefficient of performance (COP) of the overall heat pump system as ambient conditions vary. Alternatively, the condenser and heat exchanger can be sized to provide the best average effective COP over typical ambient conditions for the plant location.

[0114] The best performance of these systems is the architecture (#3) described in Figure 10, i.e., the second condenser CX2 preheats the low temperature rich line 50 and the lean line return 40 supplies the low temperature heat pump 210d as shown in Figure 10, except that the low temperature heat pump 210d delivers heat to the water loop 810 connected to the first condenser rather than directly to the desorber heating means 70.

[0115] Figure 18 shows the performance of System #3 at three different ambient conditions with ambient air temperatures of 5, 25, and 40° C. As can be seen, the desorber energy usage can be lower when the ambient temperature is high due to the additional heat available to recover from the rich line.

[0116] The best available performance at ambient T = 25°C, without a heat pump, is TCX = 72°C and WLMT = 69°C for #3, providing a minimum electrical duty of 32.7% of the requirement. Therefore, a good operating regime is TCX between 68°C and 72°C and WLMT between 65°C and 69°C. This allows a direct air capture plant to operate using less than one-third of the energy required to directly heat the desorber. This may allow direct air capture to operate using electricity more cheaply than plants that use fossil fuels to heat the desorber. Using low-carbon electricity, such as renewable energy or nuclear power, makes direct air capture a smart option for reducing global warming.

[0117] The TCX of 68°C to 76°C and WLMT of 65°C to 69°C also provide good performance relative to other architectures. To avoid crossing the boundary, a safer WLMT range is 61-67°C, and the TCX is 68-72°C, which is optimal for most operating conditions.

[0118] However, as mentioned above, design constraints may mean that the optimum is not achievable. 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 in a 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.

[0119] [Table 1]

[0120] The refrigerants used in the heat pumps can be the same or different. Only a few refrigerants can operate in the saturated region up to 130°C or higher (for simplicity, we will ignore more complex cycles that use supercritical properties, such as transcritical cycles). These include, for example, R717 (ammonia), R718 (not suitable for low-temperature operation, but steam / water), R600 (butane, flammable), and R1233zd (1-chloro-3,3,3-trifluoroprop-1-ene, developed as a low global warming potential refrigerant for future air conditioning products). Of note, high temperature refrigerants 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), and R1234ze(E) (1,3,3,3-tetrafluoropropene, GWP 6), while 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).

[0121] It will be understood that the present invention is not limited to the foregoing embodiments, and that various modifications and improvements can be made without departing from the concepts described herein. Except where mutually exclusive, any of the features may be used separately 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. Carbon dioxide (CO ２ ) to CO ２ 1. A system for capturing from a containing gas stream, comprising: The system includes: ２ an absorber for contacting the CO 2 -containing gas stream with an adsorbent, the adsorbent being adapted to adsorb the CO 2 -containing gas stream at a first temperature range; ２ CO from the containing gas stream ２ and CO in a second temperature range. ２ and The absorber ２ means for moving a contained gas stream through said absorber from an absorber inlet to an absorber outlet; The system removes the CO from the adsorbent. ２ a desorber for releasing a rich adsorbent stream from the absorber, the desorber being operable to receive the rich adsorbent stream from the absorber and to supply heat to heat the adsorbent using heating means to raise the adsorbent temperature from the first temperature range to the second temperature range, the system comprising: ２ and an exhaust conduit for supplying the steam-containing exhaust stream to the condenser; the system includes a lean return stream for returning the adsorbent from the desorber to the absorber; the system comprises a heat pump system comprising first and second heat pumps configured to recover heat from both the condenser operating at a condenser outlet temperature in a third temperature range and at least one other point in the system; the condenser is fluidly connected to a condensate recovery circuit for delivering recovered condensate to the desorber; the first heat pump is configured to deliver heat to the heating means in 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; a second heat pump configured to receive heat from another point in the adsorbent system in a fourth lower temperature range and deliver that heat to the intermediate circuit at a temperature above the intermediate circuit minimum temperature; the third temperature range is selected 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 the heat pump system. system.

2. 2. The system of claim 1, wherein the intermediate circuit 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 of claim 2 , wherein the third temperature range is from 68° C. to 72° C.

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

5. 5. The system of claim 1, wherein the second heat pump has a different refrigerant than the first heat pump.

6. 6. The system of claim 1, further comprising a second condenser, the lean return stream being connected in heat exchange relationship to the second condenser, and the second heat pump being configured to remove heat from the lean return stream after it has exchanged heat with the second condenser.

7. 7. The system of claim 1, further comprising a controller operable to control the heat pump system and a plurality of sensors for measuring the condition of the exhaust entering the condenser and the temperature of the at least one other point in the adsorbent system, the controller configured to simulate 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 requirements of the heat pump system.

8. The CO ２ 8. The system of claim 1, wherein the containing gas stream is ambient air.

9. Carbon dioxide (CO ２ ) to CO ２ 1. A method for capturing from a containing gas stream, comprising: providing a first heat pump for recovering heat from an intermediate circuit thermally connected to a condenser in a carbon capture system, and a second heat pump for recovering heat from at least one other point in the system and delivering the heat to the intermediate circuit, the condenser operable to recover condensate and deliver the condensate to a desorber, and the first heat pump operable to supply heat to the desorber; controlling the temperature at the outlet of the condenser within a third temperature range 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 the input energy requirements of the first and second heat pumps; A method comprising:

10. 10. The method of claim 9, further comprising controlling the minimum temperature of the intermediate circuit 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.

11. 11. The method of claim 10, wherein the third temperature range is from 68°C to 72°C.

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

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

14. 14. The method of claim 9, further comprising the steps of connecting a lean return stream from the desorber to a second condenser and cooling the lean return stream using the second heat pump after the lean return stream has exchanged heat with the second condenser.

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