Systems and methods for capturing carbon dioxide and regenerating a capture solution

Abstract

Techniques according to the present disclosure include contacting carbon dioxide from a dilute gas source with a CO2 capture solution including a first hydroxide (MOH) and a second hydroxide (YOH) to form a carbonate-rich solution including a first carbonate (M2CO3) and a second carbonate (Y2CO3); separating a portion of the M2CO3 and the Y2CO3 from the carbonate-rich solution to form carbonate solids; dissolving solid M2CO3 and solid Y2CO3, and forming a dissolved inorganic carbon species and an electrochemical feed stream; in an EC unit, flowing the EC feed stream to a porous solid electrolyte PSE; and flowing a water stream to at least the alkaline regeneration compartment; applying an electric potential to the EC unit to form at least two EC product streams including a first EC product stream including hydrogen, regenerated MOH and regenerated YOH and a second EC product stream in the PSE; and flowing the first EC product stream including the regenerated MOH and the regenerated YOH to use in the contacting the carbon dioxide.

Description

SYSTEMS AND METHODS FOR CAPTURING CARBON DIOXIDE AND REGENERATING A CAPTURE SOEUTIONTECHNICAL FIELD

[0001] This disclosure relates to systems and methods for capturing carbon dioxide (CO2) and regenerating a CO2 capture solution.BACKGROUND

[0002] Capturing carbon dioxide (CO2) from the atmosphere is one approach to mitigating greenhouse gas emissions and slowing climate change. However, many technologies designed for CO2 capture from point sources, such as flue gas of industrial facilities, are generally ineffective in capturing CO2 from the atmosphere due to the significantly lower CO2 concentrations and large volumes of air required to process. In recent years, progress has been made in finding technologies better suited to capture CO2 directly from the atmosphere.

[0003] Some direct air capture (DAC) systems use a liquid sorbent (sometimes referred to as a solvent or capture solution) to capture CO2 from the atmosphere. An example of such a gas-liquid contact system includes where a fan is used to draw air across a high surface area packing fill that is wetted with a capture solution comprising the liquid sorbent. In some cases, the capture solution can be an aqueous alkaline solution that forms a carbonate-rich solution when reacted with CO2 in the air. The carbonate-rich capture solution is further processed downstream to regenerate a carbonate-lean solution and to release a concentrated carbon stream, for example, CO, CO2 or other carbon products.

[0004] A thermochemical process for regenerating a capture solution and releasing CO2 is the pelletized calcium technology. This process includes precipitating carbonate to form calcium carbonate (CaCOs) solids, calcining the CaCOs solids to recover the capture CO2, thereby producing calcium oxide (CaO) which is hydrated to produce calcium hydroxide (Ca(OH)2). The Ca(OH)2 is then reacted with an alkali carbonate (e.g., potassium carbonate K2CO3 or sodium carbonate Na2COs) to regenerate the capture solution (e.g., potassium hydroxide KOH or sodium hydroxide NaOH). Integrating the thermochemical regeneration process with a DAC system has enabled CO2 capture and recovery at a commercial scale.SUMMARY

[0005] In an example implementation, a method includes contacting carbon dioxide from a dilute gas source with a CO2 capture solution including at least a first hydroxide (MOH) anda second hydroxide (YOH), to form a carbonate-rich solution including at least a first carbonate (M2CO3) and a second carbonate (Y2CO3), M and Y being alkali metals; separating at least a portion of the M2CO3 and the Y2CO3 from the carbonate-rich solution to form carbonate solids, the carbonate solids including solid M2CO3 and solid Y2CO3; dissolving the solid M2CO3 and the solid Y2CO3, and forming a dissolved inorganic carbon (DIC) species and an electrochemical (EC) feed stream; in an EC unit including a porous solid electrolyte (PSE) delimited by a first cation exchange membrane and a second cation exchange membrane, a proton-generating compartment between an anode and the first cation exchange membrane, and an alkaline regeneration compartment between a cathode and the second cation exchange membrane: flowing the EC feed stream to the PSE; and flowing a water stream to at least the alkaline regeneration compartment; applying an electric potential to the EC unit to form at least two EC product streams including a first EC product stream including hydrogen, regenerated MOH and regenerated YOH and a second EC product stream in the PSE; and flowing the first EC product stream including the regenerated MOH and the regenerated YOH to use in the contacting the carbon dioxide.

[0006] In an aspect combinable with the example implementation, flowing the water stream includes flowing a deionized water stream to the alkaline regeneration compartment and to the proton-generating compartment; and applying the electric potential to the EC unit includes: reducing water at the cathode and forming the first EC product stream; and oxidizing water at the anode and forming a third EC product stream including oxygen.

[0007] Another aspect combinable with one, some, or all of the previous aspects includes separating the oxygen from the third EC product stream.

[0008] Another aspect combinable with one, some, or all of the previous aspects includes separating the hydrogen from the first EC product stream before the flowing the first EC product stream.

[0009] In another aspect combinable with one, some, or all of the previous aspects, flowing the water stream includes flowing a deionized water stream only to the alkaline regeneration compartment; and applying the electric potential to the EC unit includes reducing water at the cathode and forming the first EC product stream.

[0010] Another aspect combinable with one, some, or all of the previous aspects includes separating the hydrogen from the first EC product stream and forming a hydrogen feed stream;flowing at least some of the hydrogen feed stream to the proton-generating compartment; and oxidizing the at least some of the hydrogen feed stream at the anode to generate protons for the PSE.

[0011] In another aspect combinable with one, some, or all of the previous aspects, the anode includes a gas diffusion electrode; and flowing the at least some of the hydrogen feed stream includes flowing the at least some of the hydrogen feed stream through the gas diffusion electrode.

[0012] Another aspect combinable with one, some, or all of the previous aspects includes flowing at least part of the second EC product stream for mixing with the DIC species to form the EC feed stream.

[0013] In another aspect combinable with one, some, or all of the previous aspects, the second EC product stream includes carbonic acid, the method including: recovering at least a portion of a carbon dioxide gas stream from the at least part of the second EC product stream and forming a brine stream; wherein flowing the at least part of the second EC product stream for mixing includes flowing the brine stream for mixing with the DIC species to form the EC feed stream.

[0014] In another aspect combinable with one, some, or all of the previous aspects, recovering the at least a portion of the carbon dioxide gas stream from the at least part of the second EC product stream includes: flowing the at least part of the second EC product stream to a vessel; and decreasing a pressure in the vessel to form the at least a portion of the carbon dioxide gas stream.

[0015] In another aspect combinable with one, some, or all of the previous aspects, the second EC product stream includes a proton-shuttling species; and flowing the at least part of the second EC product stream for mixing with the DIC species includes reacting the protonshuttling species with the DIC species to form carbonic acid and the EC feed stream.

[0016] In another aspect combinable with one, some, or all of the previous aspects, the second EC product stream includes carbon dioxide gas and carbonic acid, the method includes: recovering at least a portion of the carbon dioxide gas from the PSE and forming a first carbon dioxide product stream; and recovering a second carbon dioxide product stream from the carbonic acid and forming a brine stream; wherein flowing the at least part of the second ECproduct stream for mixing includes flowing the brine stream for mixing with the DIC species to form the EC feed stream.

[0017] In another aspect combinable with one, some, or all of the previous aspects, the second EC product stream includes carbon dioxide gas, the method including: recovering at least a portion of the carbon dioxide gas from the PSE and forming a carbon dioxide product stream.

[0018] In another aspect combinable with one, some, or all of the previous aspects, M is potassium (K), and Y is sodium (Na).

[0019] In another aspect combinable with one, some, or all of the previous aspects, separating the at least a portion of the M2CO3 and the Y2CO3 includes crystallizing the at least a portion of the M2CO3 and the Y2CO3 to form crystalline M2CO3 and crystalline Y2CO3.

[0020] Another aspect combinable with one, some, or all of the previous aspects includes removing ion species from the EC feed stream.

[0021] In another aspect combinable with one, some, or all of the previous aspects, flowing the water stream includes flowing a hydroxide stream to at least the alkaline regeneration compartment.

[0022] Another aspect combinable with one, some, or all of the previous aspects includes: separating the hydrogen from the first EC product stream; and subsequently, flowing the hydroxide stream from some of the first EC product stream to at least the alkaline regeneration compartment.

[0023] In another example implementation, an electrochemical (EC) unit for regenerating a CO2 capture solution includes: a proton-generating compartment between an anode and a first cation exchange membrane, the proton-generating compartment includes a proton feed inlet configured to receive a hydrogen-containing feed stream; an alkaline regeneration compartment between a cathode and a second cation exchange membrane, the alkaline regeneration compartment including: a water feed inlet configured to receive a water feed stream; and an alkaline regeneration compartment outlet; a porous solid electrolyte (PSE) delimited by the first cation exchange membrane and the second cation exchange membrane, the PSE including: a PSE inlet configured to receive an EC feed stream including at least a first salt of M and a second salt of Y, M and Y being alkali metals; and a PSE outlet; and a circuit extending between the anode and the cathode and configured to carry an electric currentto decompose at least the water feed stream and the EC feed stream and form: a first EC product stream in the alkaline regeneration compartment being flowable through the alkaline regeneration compartment outlet, the first EC product stream including a regenerated first hydroxide (MOH) and a regenerated second hydroxide (YOH); and a second EC product stream in the PSE being flowable through the PSE outlet.

[0024] In an aspect combinable with the example implementation, the proton-generating compartment includes a gaseous products outlet.

[0025] In another aspect combinable with one, some, or all of the previous aspects, the proton-generating compartment is in fluid communication with the alkaline regeneration compartment, the hydrogen-containing feed stream including hydrogen gas from the first EC product stream.

[0026] In another aspect combinable with one, some, or all of the previous aspects, the anode includes a gas diffusion electrode.

[0027] In another example implementation, a system for capturing carbon dioxide from a dilute gas source includes: a CO2 capture subsystem configured to generate a carbonate-rich solution including at least a first carbonate (M2CO3) and a second carbonate (Y2CO3), M and Y being alkali metals; a carbonate separation subsystem fluidly coupled to the CO2 capture subsystem and operable to receive the carbonate-rich solution, the carbonate separation subsystem configured to separate carbonate solids from the carbonate-rich solution, the carbonate solids including solid M2CO3 and solid Y2CO3; and a regeneration subsystem fluidly coupled to the carbonate separation subsystem, the regeneration subsystem including: a dissolving tank fluidly coupled to the carbonate separation subsystem, the dissolving tank configured to dissolve at least a portion of the solid M2CO3 and of the solid Y2CO3, and form an electrochemical (EC) feed stream including at least a first salt of M and a second salt of Y; and at least one EC unit fluidly coupled to the dissolving tank, the at least one EC unit including: a proton-generating compartment between an anode and a first cation exchange membrane, the proton-generating compartment including a proton feed inlet configured to receive a hydrogen-containing feed stream; an alkaline regeneration compartment between a cathode and a second cation exchange membrane, the alkaline regeneration compartment including: a water feed inlet configured to receive a water feed stream; and an alkaline regeneration compartment outlet; a porous solid electrolyte (PSE) delimited by the first cationexchange membrane and the second cation exchange membrane, the PSE including: a PSE inlet configured to receive the EC feed stream; a PSE outlet; and a circuit extending between the anode and the cathode and configured to carry an electric current to decompose at least the water feed stream and the EC feed stream and form: a first EC product stream in the alkaline regeneration compartment being flowable through the alkaline regeneration compartment outlet, the first EC product stream including a regenerated first hydroxide (MOH), a regenerated second hydroxide (YOH) and hydrogen; and a second EC product stream in the PSE being flowable through the PSE outlet.

[0028] In an aspect combinable with the example implementation, the proton-generating compartment includes a proton compartment outlet; the hydrogen-containing feed stream is configured to include a proton compartment water feed stream; and the circuit is configured to carry the electric current to decompose the proton compartment water feed stream and form oxygen.

[0029] Another aspect combinable with one, some, or all of the previous aspects includes an oxygen degassing vessel in fluid communication with the proton compartment outlet.

[0030] In another aspect combinable with one, some, or all of the previous aspects, the CO2 capture subsystem is in fluid communication with the alkaline regeneration compartment outlet.

[0031] Another aspect combinable with one, some, or all of the previous aspects includes a hydrogen degassing vessel downstream of the alkaline regeneration compartment outlet and upstream of the CO2 capture subsystem, the hydrogen degassing vessel configured to separate the hydrogen from the first EC product stream.

[0032] In another aspect combinable with one, some, or all of the previous aspects, the alkaline regeneration compartment outlet is in fluid communication with the proton-generating compartment; and the hydrogen-containing feed stream is configured to include at least some of the hydrogen of the first EC product stream.

[0033] Another aspect combinable with one, some, or all of the previous aspects includes a hydrogen degassing vessel downstream of the alkaline regeneration compartment outlet and upstream of the proton-generating compartment, the hydrogen degassing vessel configured to separate the hydrogen from the first EC product stream.

[0034] In another aspect combinable with one, some, or all of the previous aspects, In another aspect combinable with one, some, or all of the previous aspects, the dissolving tank is in fluid communication with the PSE outlet.

[0035] Another aspect combinable with one, some, or all of the previous aspects includes a CO2 degassing vessel downstream of the PSE outlet and upstream of the dissolving tank, wherein: the circuit is configured to carry the electric current to decompose the EC feed stream and form the second EC product stream including carbonic acid; and the CO2 degassing vessel is configured to separate carbon dioxide gas from the second EC product stream and form a brine stream.

[0036] In another aspect combinable with one, some, or all of the previous aspects, the dissolving tank is configured to react the brine stream with the dissolved at least a portion of the solid M2CO3 and of the solid Y2CO3 and form the EC feed stream.

[0037] In another aspect combinable with one, some, or all of the previous aspects, the circuit is configured to carry the electric current to decompose the EC feed stream and form the second EC product stream including a proton-shuttling species; and the dissolving tank is configured to: react the proton-shuttling species with the dissolved at least a portion of the solid M2CO3 and of the solid Y2CO3 and form carbonic acid and the EC feed stream; and separate carbon dioxide gas from the carbonic acid.

[0038] Another aspect combinable with one, some, or all of the previous aspects includes a CO2 degassing vessel downstream of the PSE outlet and upstream of the dissolving tank, wherein: the circuit is configured to carry the electric current to decompose the EC feed stream and form the second EC product stream including carbon dioxide gas and carbonic acid; the PSE is configured to flow at least a portion of the carbon dioxide gas from the PSE outlet as a first carbon dioxide gas stream; the CO2 degassing vessel is configured to separate a second carbon dioxide gas stream from the carbonic acid of the second EC product stream and form a brine stream; and the dissolving tank is configured to react the brine stream with the dissolved at least a portion of the solid M2CO3 and of the solid Y2CO3 and form the EC feed stream.

[0039] In another aspect combinable with one, some, or all of the previous aspects, the circuit is configured to carry the electric current to decompose the EC feed stream and form the second EC product stream including carbon dioxide gas; and the PSE is configured to flowat least a portion of the carbon dioxide gas from the PSE outlet as a carbon dioxide product stream.

[0040] In another aspect combinable with one, some, or all of the previous aspects, M is potassium (K), and Y is sodium (Na).

[0041] In another aspect combinable with one, some, or all of the previous aspects, the carbonate separation subsystem includes a crystallizer operable to crystallize the carbonate- rich solution and form a crystalline M2CO3 and a crystalline Y2CO3; and the dissolving tank is configured to receive at least some of the crystalline M2CO3 and at least some of the crystalline Y2CO3 and dissolve the at least some of the crystalline M2CO3 and the at least some crystalline Y2CO3 and form the EC feed stream.

[0042] In another aspect combinable with one, some, or all of the previous aspects, the crystallizer includes at least one of: a chiller crystallizer, an evaporative crystallizer, a eutectic freeze crystallizer, a cooling crystallizer, or a membrane distillation crystallizer.

[0043] Another aspect combinable with one, some, or all of the previous aspects includes at least one of: a compression unit, a fuel synthesis system, a syngas generation reactor, or an electrolyzer cell.

[0044] In another aspect combinable with one, some, or all of the previous aspects, wherein the CO2 capture subsystem includes at least one of a: gas-liquid contactor, air contactor, spray tower, liquid-gas scrubber, venturi scrubber, packed tower, single cell air contactor, dual cell air contactor, or multi cell air contactor.

[0045] In another example implementation, a method includes: contacting carbon dioxide from a dilute gas source with a capture solution including a hydroxide (MOH) to form a carbonate-rich capture solution including a carbonate (M2CO3), where M is an alkali metal; crystallizing at least a portion of the M2CO3 to form a crystalline M2CO3; dissolving the crystalline M2CO3 to form an electrochemical (EC) feed stream including a salt of M; in an EC unit including a porous solid electrolyte (PSE) delimited by a first cation exchange membrane and a second cation exchange membrane, a proton-generating compartment between an anode and the first cation exchange membrane, and an alkaline regeneration compartment between a cathode and the second cation exchange membrane: flowing the EC feed stream to the PSE; flowing a water stream to at least the alkaline regeneration compartment; applying an electric potential to the EC unit to form at least two EC productstreams including a first EC product stream including regenerated MOH and a second EC product stream generated by the PSE; and flowing the first EC product stream including the regenerated MOH to use in the contacting the carbon dioxide.

[0046] The details of one or more implementations of the subject matter described in this disclosure are set forth in the accompanying drawings and the description below. Other features, aspects, and advantages of the subject matter will become apparent from the description, the drawings, and the claims.BRIEF DESCRIPTION OF THE DRAWINGS

[0047] FIG. l is a block flow diagram illustrating an example system for capturing carbon dioxide from a dilute gas source.

[0048] FIG. 2 is a block flow diagram illustrating an example regeneration subsystem of the system of FIG. 1.

[0049] FIG. 3 is a block flow diagram illustrating an example regeneration subsystem of the system of FIG. 1.

[0050] FIG. 4 is a block flow diagram illustrating an example regeneration subsystem of the system of FIG. 1.

[0051] FIG. 5 is a block flow diagram illustrating an example regeneration subsystem of the system of FIG. 1.

[0052] FIG. 6 is a block flow diagram illustrating an example regeneration subsystem of the system of FIG. 1.

[0053] FIG. 7 is a block flow diagram illustrating an example regeneration subsystem of the system of FIG. 1.

[0054] FIG. 8 is a schematic flow diagram of a method of the present disclosure.

[0055] FIG. 9 is a schematic flow diagram of a method of the present disclosure.

[0056] FIG. 10 is a schematic diagram of a control system (or controller) for a gas-liquid contactor of the present disclosure.

[0057] FIG. 11A is a side elevational view of an example system for capturing carbon dioxide from a dilute gas source.

[0058] FIG. 1 IB is a top-down view of an example system for capturing carbon dioxide from a dilute gas source comprising multiple contactor walls.DETAILED DESCRIPTION

[0059] Referring to FIG. 1, the present disclosure describes a system 100 and methods for capturing carbon dioxide (CO2) from a dilute gas source such as the atmosphere (e.g., ambient or atmospheric air) or from another fluid source that contains dilute concentrations of CO2. The system 100 may be referred to herein as a direct air capture system 100, or a “DAC” system 100. Concentrations of CO2 in the atmosphere are dilute, in that they are presently in the range of 400-420 parts per million (“ppm”) or approximately 0.04-0.042% v / v. Such concentrations of CO2 in the atmosphere may vary depending on anthropogenic emissions of CO2, and are typically less than 1% v / v. These atmospheric concentrations of CO2 are at least one order of magnitude lower than the concentration of CO2 in point-source emissions, such as flue gases, where point-source emissions can have concentrations of CO2 ranging from 1.5- 15% v / v, or from 5-15% v / v depending on the source of emissions. Thus, design considerations for a CO2 capture subsystem and capture solution regeneration subsystem are different for dilute sources when compared to point sources.

[0060] The present disclosure relates to systems 100 and methods for capturing CO2 from the dilute gas source, such as a stream of CCh-laden air 101, with a CO2 capture solution 144. The present disclosure relates to systems 100 and methods for regenerating the CO2 capture solution 144 and recovering the CO2 using electrochemical processes. The system 100 disclosed herein includes a CO2 capture subsystem 102 coupled to a capture solution regeneration subsystem 164 via a carbonate separation subsystem 162. The carbonate separation subsystem 162 bridges the CO2 capture subsystem 102 to the capture solution regeneration subsystem 164 and allows for the subsystems 102, 164, 162 to be operationally decoupled from one another, which yields several advantages as described in greater detail below.

[0061] Referring to FIG. 1, the CO2 capture subsystem 102 includes one or more gas-liquid contactors 105. The gas-liquid contactors 105 can include air contactors, spray towers, liquidgas scrubbers, membrane contactors, venturi scrubbers, packed towers, and other systems designed to remove at least a portion of the CO2 from the CCh-laden air 101 using the liquid CO2 capture solution 144. The gas-liquid contactors 105 can include single or multi cell contactors, dual cell contactors, or a combination thereof. The gas-liquid contactors 105 can operate in crossflow, countercurrent flow, co-current flow, or a combination thereof. The gas-liquid contactors 105 can include packing or fill to provide a large surface area for the CO2 capture solution 114 to disperse on. Such packing or fill can include structured packing, random packing, or any combination of both.

[0062] In the implementation of FIG. 1, the gas-liquid contactors 105 are operated to capture the dilute CO2 present in ambient air by ingesting the ambient air as a flow of the CO2- laden air 101, and by treating the CCh-laden air 101 so as to transfer CO2 present therein to the CO2 capture solution 144 via absorption. Some or all of the CO2 in the CCh-laden air 101 is removed, and the treated CCh-laden air 101 is then discharged by the gas-liquid contactors 105 as a flow of CCh-lean gas 109 (or, CO2-IOW air). In operating to treat atmospheric air in this manner, the gas-liquid contactors 105 may sometimes be referred to herein as an “air contactors” or “ACs” because they facilitate absorption of CO2 from the atmospheric air into the CO2 capture solution 144. In contrast to water cooling towers which function primarily to transfer heat between water and atmospheric air, the gas-liquid contactors 105 function primarily to achieve mass transfer of CO2 from the atmospheric air to the CO2 capture solution 144. The gas-liquid contactors 105 of FIG. 1 use a liquid sorbent to absorb CO2 from the CO2- laden air 101 and may thus be referred to as a “gas-liquid contactors 105”. In example implementations, a liquid sorbent and a solid sorbent may be used to capture CO2 from the CCh-laden air 101, such that absorption and adsorption are both employed to transfer CO2 from the CCh-laden air 101 into a sorbent.

[0063] In example implementations, and referring to FIG. 1, the CO2 capture solution 144 is a caustic solution. In example implementations, the CO2 capture solution 144 has a pH of 10 or higher. In example implementations, the CO2 capture solution 144 has a pH of approximately 14. In example implementations, and referring to FIG. 1, the CO2 capture solution 144 includes two or more different alkali hydroxide compounds, where the cations of the hydroxide compounds are selected from the group of alkali metals. The cations of the hydroxide compounds can thus include two or more of Lithium (Li), Sodium (Na), Potassium (K), Rubidium (Rb), Cesium (Cs) and Francium (Fr), in any combination. In example implementations, the CO2 capture solution 144 is an alkaline solution including potassium hydroxide (KOH) and sodium hydroxide (NaOH). In another implementation, the CO2 capture solution 144 is an alkaline solution including KOH, NaOH and lithium hydroxide (LiOH). Additional combinations of hydroxides for the CO2 capture solution 144 are possible andwithin the scope of the present disclosure. The implementations of the CO2 capture solution 144 described above can be arranged in any combination, mutatis mutandis.

[0064] In example implementations, the CO2 capture solution 144 can include promoters and / or additives that increase the rate of CO2 uptake. Non-limiting examples of promoters include amines (primary, secondary, tertiary), alcohol (e.g., glycerol), and boric acid. Nonlimiting examples of additives include chlorides, sulfates, acetates, phosphates, surfactants, oxides and metal oxides. For example, a surfactant can be added to the CO2 capture solution 144 to lower the surface tension of the CO2 capture solution 144 to improve the ability of the CO2 capture solution 144 to wet the material of a packing. Non-limiting examples of rateenhancing additives include piperazine, monoethanol amine (MEA), diethanolamine (DEA), zinc triazacycles, zinc tetraazacycles, copper glycinates, hydroxopentaaminecobalt perchlorate, formaldehyde hydrate, saccharose, fructose, glucose, phenols, phenolates, glycerin, arsenite, hypochlorite, hypobromite, or other oxyanionic species.

[0065] In example implementations, at a given reference temperature, the density of the CO2 capture solution 144 is greater than the density of water at the same reference temperature. At comparable reference temperatures, in example implementations, the density of the CO2 capture solution 144 is at least 10% greater than the density of water. In example implementations, at comparable reference temperatures, the density of the CO2 capture solution 144 is approximately 10% greater than the density of water. The density and the viscosity of the CO2 capture solution 144 can vary depending on the composition of the CO2 capture solution 144 and the temperature. For example, at temperatures of 0°C to 20°C, the CO2 capture solution 144 can comprise at least 2 M KOH and at least 0.5 M K2CO3, and at least 1.5 M NaOH and at least .25 M Na2COs, and can have a density of at least 1115 kg / m3and a viscosity ranging from 1.3-2.3 mPa-s. In comparison, water has a density of 998 kg / m3and viscosity of 1 mPa-s at 20°C.

[0066] In example implementations, and referring to FIG. 1, reacting the CO2 from the CO2-laden air 101 with the CO2 capture solution 144 forms a carbonate-rich solution 120. In the implementations where the CO2 capture solution 144 comprises two or more different alkali hydroxides, CO2 is absorbed from the CCh-laden air 101 by reacting the CO2 with the alkali hydroxides to form the carbonate-rich solution 120 including two or more carbonate salts (e.g., K2CO3, Na2CC>3, Li2CC>3, CS2CO3, and / or a combination thereof). The carbonate-rich solution120 is an aqueous mixture comprising carbonate ions, alkaline metal carbonates (e.g., K2CO3, Na2CC>3, Li2CC>3, CS2CO3, and / or a combination thereof), hydroxide, or a combination thereof. In the present disclosure, the CO2 capture solution 144 is sometimes referred to as including two or more alkali hydroxides, including a first hydroxide MOH and a second hydroxide YOH, where “M” represents one of the alkali metals and “Y” represents a different alkali metal. Such a CO2 capture solution 144 reacts with the CO2 of the CO2-laden air 101 to form two or more carbonate compounds in the carbonate-rich solution 120, including a first carbonate M2CO3 and a second carbonate Y2CO3. The carbonate-rich solution 120 can also include other components in smaller amounts, such as hydroxide ions, alkali metal hydroxide (e.g., KOH, NaOH), water, and impurities. For example, the carbonate-rich solution 120 can comprise an aqueous mixture between 0.4 M to 6 M K2CO3 and between 1 M to 10 M KOH, and between 0.2 M to 3 M Na2COs and between 1 M to 5 M NaOH. The term “carbonate-rich,” in some aspects, may mean that a stream contains more CO2than the associated CO2-lean stream (in this case, the CO2 capture solution 144). Therefore, in example implementations, the DAC system 100 provides a CO2 “lean” solution to the gas-liquid contactors 105, and the capture solution regeneration subsystem 164 receives a CO2 “rich” solution from the gas-liquid contactors 105.

[0067] The capture kinetics of capturing CO2 from the CCh-laden air 101 to form carbonate can be improved by the introduction of an additive such as a promoter species in the CO2 capture solution 144. Non-limiting examples of promoters for boosting CO2 capture include amines (primary, secondary, tertiary), alcohol (e.g., glycerol), zwitterionic amino acids, and boric acid. The resulting carbonate-rich solution 120 produced by the gas-liquid contactors 105 includes carbonates and bicarbonates and includes the promoter as well. An example composition of such a carbonate-rich solution 120 can include K2CO3 / KHCO3, Na2CO3 / NaHCC>3 and a promoter. The carbonate-rich solution 120 resulting from such a CO2 capture solution 144 can have a pH in the range of 11-13 and can have little residual hydroxide from the CO2 capture solution 144. In some cases, additives that are not considered promoters can be used to improve the uptake of CO2 in the CO2 capture solution 144.

[0068] The carbonate-rich solution 120 can be processed to recover the captured CO2 for downstream use and to regenerate the alkali hydroxide for re-use in the CO2 capture solution 144, as described in greater detail below. In example implementations, recovered CO2 is aproduct stream (such as CO2 product stream 136, see below) that can be delivered downhole and sequestered in a geological formation, subsurface reservoir, carbon sink, or the like. In example implementations, the recovered CO2 product stream 136 can be used for enhanced oil recovery by injecting the recovered CO2 into one or more wellbores to enhance production of hydrocarbons from a reservoir. In example implementations, the recovered CO2 product stream 136 can be fed to a downstream fuel synthesis system, which can include a syngas generation reactor for generating one or more of syngas, CO, H2, formate, methane, ethylene, or ethanol. In example implementations, the recovered CO2 product stream 136 can be a feedstock for making any suitable carbon-based or carbon-including product, non-limiting examples of which include cement, plastics, and polymers.

[0069] Referring to FIG. 1, it can be advantageous for the CO2 capture solution 144 to include a mixture of potassium-based and sodium-based hydroxide species. In some cases, when the CO2 capture solution 144 includes KOH, the kinetics of CO2 capture from the CO2- laden air 101 are improved compared to if the CO2 capture solution 144 included only NaOH. On the other hand, when the CO2 capture solution 144 includes NaOH, the resulting Na2COs formed after contacting the CO2-laden air 101 with the CO2 capture solution 144 can have a lower solubility compared to K2CO3, which can reduce the duty of units of the system 100 (e.g., crystallizers) that are used for separating carbonates from the carbonate-rich solution 120, as described in greater detail below. Thus, in implementations of the present disclosure where the CO2 capture solution 144 is an alkaline solution including potassium hydroxide (KOH) and sodium hydroxide (NaOH), the CO2 capture solution 144 can be optimized for both capture efficiency and for separation of the “double salt” carbonate compounds (e.g., K2CO3 and Na2CO3). In such implementations, one or both of the CO2 capture solution 144 and the carbonate-rich solution 120 can include a mixture of KOH and NaOH, and double carbonate salts of K2CO3 and Na2CO3. These mixed sodium-potassium CO2 capture solutions 144 can enable tuning of mass transfer kinetics, water balance, and operating temperature. In example implementations, the ratio of the potassium-based components to sodium-based components in the CO2 capture solution 144 is 1 : 1. In example implementations, the ratio of the potassium- based components to sodium-based components in the CO2 capture solution 144 is less than 4:1. Effective ratios of the potassium-based components to sodium-based components can depend on the operating environment.

[0070] In example implementations, it can be advantageous for the CO2 capture solution 144 to include a mixture of at least three hydroxide species, including KOH, NaOH and Li OH. In some cases, when the CO2 capture solution 144 includes KOH, the kinetics of CO2 capture from the CO2-laden air 101 are improved compared to if the CO2 capture solution 144 included only NaOH. On the other hand, when the CO2 capture solution 144 includes NaOH and LiOH, the resulting Na2COs and Li2COs formed after contacting the CO2-laden air 101 with the CO2 capture solution 144 can have a lower solubility compared to K2CO3, which can reduce the duty of units of the system 100 (e.g., crystallizers) that are used for separating carbonates from the carbonate-rich solution 120, as described in greater detail below. Thus, in implementations of the present disclosure where the CO2 capture solution 144 is an alkaline solution including KOH, NaOH and LiOH, the CO2 capture solution 144 can be optimized for both capture efficiency and for separation of carbonate compounds. In implementations of the present disclosure, the CO2 capture solution 144 is an alkaline solution including a single alkali metal.

[0071] Referring to FIG. 1, the CO2 capture subsystem 102 is fluidly coupled to the carbonate separation subsystem 162 and to the regeneration subsystem 164. Carbonate separation subsystem 162 exploits the solubility differences between carbonate and hydroxide salts in the carbonate-rich solution 120 to enable efficient separation of carbonate. The carbonate separation subsystem 162 includes units to enable the separation of carbonate species from the carbonate-rich solution 120 and form two or more separated carbonate species, including separated M2CO3 and separated Y2CO3. For example, the carbonate separation subsystem 162 includes a caustic evaporator 112 fluidly coupled to a crystallizer 104. In example implementations, crystallizer 104 can by fluidly coupled to a solids separator such as a centrifuge, pressure or vacuum filters, scrapers, cyclones, and the like. Caustic evaporator 112 receives the carbonate-rich solution 120 from CO2 capture subsystem 102.

[0072] In example implementations, caustic evaporator 112 can include a mechanical vapour recompression (MVR) evaporator, a multi-effect evaporator, or a combination thereof. Caustic evaporator 112 removes water from the carbonate-rich solution 120 to form a concentrated carbonate-rich solution 118. Caustic evaporator 112 discharges a water stream 119. The caustic evaporator 112 can increase the ionic concentrations of the carbonate-rich solution 120, thereby moving the composition of the solution in relation to the carbonate saturation curve to lower the solubility of carbonate in the carbonate-rich solution 120 andproducing a concentrated carbonate-rich solution 118. Concentrated carbonate-rich solution 118 can include a higher carbonate concentration and a higher hydroxide concentration than carbonate-rich solution 120. For example, concentrated carbonate-rich solution 118 can comprise between 0.4 M to 6 M K2CO3, between 0.4 M to 6 M Na2COs, between 1 M to 14 M KOH and between 1 M to 14 M NaOH. Thus, caustic evaporator 112 increases the respective concentrations of carbonate and hydroxide such that the carbonate salts in carbonate-rich solution 118 are less soluble, which reduces the crystallizer duty (e.g., evaporative heating or cooling refrigeration) on crystallizer 104. Some examples of caustic evaporators include mechanical vapor recompression (MVR) evaporators and multi-effect evaporators.

[0073] In alternate implementations, the DAC system 100 includes one or more filtration units, for example a nanofiltration unit. In example implementations, the nanofiltration unit is used instead of, or in conjunction with, the caustic evaporator to concentrate carbonate via selective rejection, which can reduce the evaporative load on the crystallizer 104 and can reduce feed flow rates and crystallizer sizing. The nanofiltration unit of the DAC system 100 can include one or more filtration membranes that are impermeable to or select for large divalent ions such as carbonate ions. Nanofiltration membranes can have an inherent surface charge, making them particularly suitable for separating ion mixtures. Rejection of species can depend on size, ionic charge, and membrane affinity. The nanofiltration unit can include membranes that have a wide pH tolerance and are durable enough to operate at a pH ranging from 0 to 14 or hydroxide concentrations of up to 10 M. In example implementations, the nanofiltration unit can include membranes that are operable with hydroxide concentration of up to 10 M. In example implementations, the nanofiltration unit can include membranes that are stable handling hydroxide concentrations up to 10 M. In example implementations, the nanofiltration unit can reject at 85% to 100% of divalent ions (e.g., carbonate ions) to yield a retentate that is carbonate-rich and flowed to the crystallizer 104, and a permeate that is hydroxide rich or carbonate-lean and is flowed to the CO2 capture subsystem 102. The carbonate separation subsystem 162 can have multiple nanofiltration units arranged in stages to get the desired separation, where more nanofiltration units can be added to obtain the desired filtration capacity. The carbonate separation subsystem 162 can have nanofiltration units of various sizes, where the sizing of the nanofiltration unit(s) is based on liquid flowrates and the degree of separation required.

[0074] In example implementations, crystallizer 104 includes an evaporative crystallizer, a eutectic freeze crystallizer, a cooling crystallizer (e.g., vacuum or surface cooled), a membrane distillation crystallizer, or a combination thereof. Crystallizer 104 can be based on forced circulation, draft tube baffle, fluidized bed design, or a combination thereof. Crystallizer 104 increases the hydroxide concentration and thereby decreases the solubility of carbonates in concentrated carbonate-rich solution 118. In some cases, crystallizer 104 evaporates a portion of concentrated carbonate-rich solution 118 to reach supersaturation. This concentration step forms carbonate solids, such as a crystalline carbonate hydrate 122, as well a mother liquor 142, and a water stream 124. Crystallizer 104 discharges water stream 124 for downstream processing (e.g., in filtration system, water treatment system, or disposal system) or use in another application within or beyond the system 100. Crystalline carbonate hydrate 122 is at least partially separated from mother liquor 142 to form a pure or nearly pure carbonate solid that can be used in a feed solution for the regeneration subsystem 164. Mother liquor 142 can include the remaining components of concentrated carbonate solution 118, such as water and hydroxide, after crystalline carbonate hydrate 122 is separated.

[0075] In implementations of the present disclosure where the CO2 capture solution 144 is an alkaline solution including KOH and NaOH, crystalline carbonate hydrate 122 can include potassium carbonate sesquihydrate (K2CO3- I.5H2O) and sodium carbonate decahydrate (Na2CO3- IOH2O). The potassium carbonate sesquihydrate and sodium carbonate decahydrate crystals can be at least partially isolated from the mother liquor 142, which can include a mixture of KOH-K2CO3 and NaOH-Na2CO3. In another example, in implementations where the CO2 capture solution 144 is an alkaline solution including KOH and NaOH, crystalline carbonate hydrate 122 can include potassium sodium carbonate hexahydrate (KNaCO3-6 H2O). In such implementations, the potassium sodium carbonate hexahydrate (KNaCO3-6 H2O) is a double salt. In example implementations, crystalline carbonate hydrate 122 can include a different stoichiometry of water molecules per carbonate unit in the crystalline carbonate such as M2CO3-nH2O or MYCO3-nH2O, where M and Y are alkali metals and n is an integer or fractional value. In example implementations, crystalline carbonate hydrate 122 can include an anhydrous carbonate.

[0076] After separation from mother liquor 142, crystalline carbonate hydrate 122 is sent to the regeneration subsystem 164 and mother liquor 142 is returned to a component of theC02capture subsystem 102, such as the gas-liquid contactors 105. In example implementations, the crystalline carbonate hydrate 122 is a solid phase intermediate product that enables the carbonate to be easily separated from other components of the stream to form a pure or relatively pure carbonate stream for use in the regeneration subsystem 164. In example implementations, the regeneration subsystem 164 and / or the carbonate separation subsystem 162 has additional solids handling or separating componentry, such as a solids separator, to further isolate the crystalline carbonate hydrate 122 from any remaining liquid to form a high solids stream that can flow to other components of the regeneration subsystem 164 and a low solids stream that can return to the crystallizer 104.

[0077] In implementations where the concentrated carbonate solution 118 includes double carbonate salts of K2CO3 and Na2CO3, the solubility of Na2CC>3 in the concentrated carbonate solution 118 is lower than that of the K2CC>3, such that duty of the crystallizer 104 is lower when forming crystalline Na2CC>3 from the concentrated carbonate solution 118. The CO2capture solution 144 can therefore be optimised for efficient CO2capture (with potassium, for example) while also being optimised for lower crystallizer 104 duty (with sodium, for example). By helping to reduce or eliminate the concentration of hydroxides in the feed stream provided to the regeneration subsystem 164, the crystallizer 104 helps to reduce the duty of units of the regeneration subsystem 164, such as an electrochemical unit, for which the hydroxides would be a parasitic load.

[0078] In example implementations, the carbonate separation subsystem 162 includes units other than the crystallizer 104 to enable efficient separation of carbonate species to form two or more separated carbonate species, including separated M2CC>3 and separated Y2CC>3. In example implementations, and as described in greater detail below, the carbonate separation subsystem 162 includes units in addition to the crystallizer 104 to enable efficient separation of carbonate species to form two or more separated carbonate species, including separated M2CC>3 and separated Y2CC>3. In example implementations, the carbonate separation subsystem 162 includes at least one of each of the following components, in any combination: the caustic evaporator 112, the nanofiltration unit and the crystallizer 104, in any combination. In example implementations, the CO2capture subsystem 162 is free of the caustic evaporator 112 and of the crystallizer 104, and instead employs one or more filtration units to separate outa concentrated carbonate stream that is flowed directly to the electrochemical unit of the regeneration subsystem 164, or to another one of its components.

[0079] Referring to FIG. 1, and as described in greater detail below, the regeneration subsystem 164 receives a feed stream of the crystalline carbonate hydrate 122, which embodies the CO2 captured from the CCh-laden air 101. The regeneration subsystem 164 decomposes the crystalline carbonate hydrate 122 and recovers CO2 from the crystalline carbonate hydrate 122 to form the CO2 product stream 136 and generate the CO2 capture solution 144. The CO2 capture solution 144 that has been used to capture CO2 captured from the CCh-laden air 101 is thus “regenerated” by the regeneration subsystem 164, and it can be reused in the CO2 capture subsystem 102. Different configurations of the regeneration subsystem 164 are possible to achieve the functionality ascribed to the regeneration subsystem 164 herein, and some such configurations are now described in greater detail.

[0080] For example, FIG. 2 illustrates an example implementation of the capture solution regeneration subsystem 264. The regeneration subsystem 264 includes a dissolving tank 206 fluidly coupled to an electrochemical (EC) unit 208 and to one or more flash tanks 210. The dissolving tank 206 is fluidly coupled to the carbonate separation subsystem 162 and is configured to receive from the crystallizer 104 some or all of the separated carbonate species of the crystalline carbonate hydrate 122, including at least some of the M2CO3 and at least some of the Y2CO3. The dissolving tank 206 can also receive a water stream 228 and dissolve the M2CO3 and the Y2CO3 to form an electrochemical (EC) feed stream 226. The crystalline carbonate hydrate 122 are in example implementations dissolved in an aqueous solution that is used to feed the EC unit 208. The aqueous solution is primarily water but can include some non-aqueous components.

[0081] In example implementations, the EC feed stream 226 includes different dissolved inorganic carbon (DIC) species. The DIC species embody or carry the CO2 absorbed from the CCE-laden air 101. The DIC species can include various carbonates (CO32) and bicarbonates (HCO31), as well as carbonic acid (H2CO3), dissolved CO2, and / or any combination of the preceding compounds. The DIC species include a first carbonate salt of M, a second carbonate salt of Y, and possibly one or more additional carbonate salts having an alkali metal. For example, in implementations where the crystalline carbonate hydrate 122 includes carbonate salts such as K2CO3 and Na2CO3, the EC feed stream 226 of FIG. 2 can include DIC speciessuch as bicarbonates KHCO3, NaHCCh, and HCCh', as well as carbonates K2CO3, Na2CC>3 and CO32’. The EC feed stream 226 can in example implementations include a bicarbonate HCCh' -rich solution with a mixture of other components such as carbonate and water.

[0082] Other reactions can occur in the dissolving tank 206 of FIG. 2 to form an EC feed stream 226 that has a different composition and is free, or substantially free, of DIC species, as described in greater detail below. In some cases, a polished aqueous solution can be used instead of or in addition to the water stream 228. The water stream 228 and the polished aqueous solution can be substantially free of solids or particulates, and free of dissolved contaminants such as divalent cations. In example implementations, the water stream 228 provided to the dissolving tank 306 can include a slip stream of process solution from elsewhere in the DAC system 100 that is free or substantially free of hydroxide, and that is free of solids, particulates, and dissolved contaminants such as divalent cations.

[0083] In example implementations, and referring to FIG. 2, the regeneration subsystem 264 includes an ion exchanger 241 to reduce or remove at least a portion of undesirable ion species from the EC feed stream 226, such as divalent and multivalent cations (e.g., Ca2+, Mg2+, Sr2+, Ba2+, silicates, borates, iron, zinc, etc.). The ion exchanger 241 can be used to polish or purify the EC feed stream 226 before its components are reacted in the EC unit 208. Trapped ions can be removed from the ion exchanger 241 by using a new column and regenerating the used column by displacing trapped ions with an acid or a base. These columns can be configured to operate in trains (e.g., is series) such that some columns remain in operation while others are being regenerated. Regenerating the used column can produce an ion exchange regenerate waste stream 243 that includes waste salts.

[0084] Although the dissolving tank 206 is disclosed herein in example implementations as a component of the regeneration subsystem 264, in example implementations, the dissolving tank 206 is not a component of the regeneration subsystem 264. In example implementations, the dissolving tank 206 is a component of the carbonate separation subsystem 162 In example implementations, the dissolving tank 206 is not a component of any of the subsystems 102, 162, 164, 264, and is instead another component of the DAC system 100. In such implementations, the dissolving tank 206 is not a component of any of the subsystems 102, 162, 164, 264, and is instead outside of the battery limit of the DAC system 100 and in fluid

[0085] Referring to FIG. 2, the EC feed stream 226 is flowed under pressure from the dissolving tank 206 to the EC unit 208. The capture solution regeneration subsystem 264 includes one or more pump(s) 263 which function to increase the pressure of the EC feed stream 226 provided to the EC unit 208. The EC unit 208 is an electrochemical component of a regeneration subsystem 264 in that the EC unit 208 combines an input of electrical energy to facilitate chemical reactions (e.g., to enable salt-splitting and acid-base recovery). In implementations of the present disclosure where the regeneration subsystem 164, 264 includes an EC unit 208, the regeneration subsystem 164, 264 may be described as an “electrochemical” regeneration subsystem 164, 264 of the DAC system 100.

[0086] The EC unit 208 includes componentry and features that are used to decompose the EC feed stream 226 and generate, directly or indirectly, product streams comprising i) the regenerated CO2 capture solution 144, and ii) the CO2 product stream 136. Referring to FIG. 2, such componentry of the EC unit 208 includes an anode 203, a cathode 207, and a plurality of ion exchange membranes. The EC unit 208 and its components form or include an electric circuit 208C including an electricity source 208P and any other desirable electrical componentry (wires, resistors, capacitors, transistors, etc.) to allow electrons to flow from the anode 203 to the cathode 207 and drive the electrochemical reactions of the EC unit 208. In example implementations, the anode 203 and the cathode 207 are coupled to an intermittent low carbon intensity electricity source 208P (e.g., solar, wind, geothermal) or a low carbon intensity electricity source 208P that is non-intermittent (e.g., hydro, nuclear, biomass, renewable natural gas). In example implementations, the anode 203 and the cathode 207 are coupled to an electrical energy storage unit, such as one or more batteries, which stores electrical energy from the intermittent or non-intermittent low carbon intensity electricity source 208P. The electricity source 208P can be dedicated to the EC unit 208, can be a feature of the broader DAC system 100 (e.g., main power for the DAC system 100), and / or can be coming from an offsite power grid / supply.

[0087] The ion exchange membranes of the EC unit 208 serve to delineate cathode and anode reaction chambers in which some of the reactions of the EC unit 208 occur. For example, and referring to FIG. 2, the ion exchange membranes include cation exchange membranes (CEM), such as a first CEM 213 A and a second CEM 213B. The first and second CEMs 213 A, 213B allow for the passage of cations therethrough and block the flow of anions.The first and second CEMs 213 A, 213B can have low anion permeability to mitigate ion leakage which can reduce the efficiency of the EC unit 208. The first CEM 213A and the anode 203 define or delineate a proton-generating compartment 215 of the EC unit 208. The proton-generating compartment 215 includes one or more inlets, including a proton feed inlet 2151, that is configured to receive a source of hydrogen via a hydrogen-containing feed stream (e.g., water, hydrogen gas, etc.). The proton-generating compartment 215 includes one or more outlets, including a proton-generating compartment outlet 2150. The second CEM 213B and the cathode 207 define or delineate an alkaline regeneration compartment 217 of the EC unit 208. The alkaline regeneration compartment 217 includes one or more inlets, including a water feed inlet 2171. The alkaline regeneration compartment 217 includes one or more outlets, including an alkaline regeneration compartment outlet 2170. The first and second CEMs 213A, 213B can be sealed or otherwise attached to a housing of the EC unit 208 to prevent cations from bypassing the CEMs 213 A, 213B.

[0088] Referring to FIG. 2, the electrodes (anode 203 and cathode 207) of the EC unit 208 are operable to apply an electric potential from the electricity source 208P to enable saltsplitting. The componentry of the EC unit 208 also includes a porous solid electrolyte (PSE) 221. The PSE 221 is an electrically conductive solid body which contains multiple pores in which some of the reactions of the EC unit 208 occur. The PSE 221 is a medium which allows for the transport of cations from the anode 203 to the cathode 207. The PSE 221 can maintain electrical conductivity even as gases, such as CO2, are produced in the PSE 221.

[0089] The electric current of the electricity source 208P flows across the PSE 221 causing an electrochemical reaction which generates reaction products, and which allows for the flow of cations into and out of the PSE 221. More particularly, and referring to FIG. 2, the PSE 221 is delimited on both its sides by the first and second CEMs 213 A, 213B. Referring to FIG. 2, the PSE 221 is delimited on one side by the proton-generating compartment 215, and on the other side by the alkaline regeneration compartment 217 (sometimes referred to herein as “the regeneration compartment 217”). Therefore, in the EC unit 208 of FIG. 2, protons are free to flow into the PSE 221 from the proton-generating compartment 215, and protons are free to flow from the PSE 221 to the regeneration compartment 217.

[0090] The PSE 221 includes one or more inlets, including a PSE inlet 2211 which receives the pressurized flow of the EC feed stream 226 from the pump(s) 263 so as to distribute theT1EC feed stream 226 into the pores of the PSE 221. The PSE 221 includes one or more outlets, including a PSE outlet 2210 for flowing some of the reaction products of the PSE 221.

[0091] The PSE 221 can be made of any suitable material, and have any suitable construction or arrangement, to achieve the functionality ascribed to the PSE 221 herein. For example, the PSE 221 can include metals, polymers, ceramics or combinations of the preceding. In example implementations, the PSE 221 is a porous polymer electrolyte that can maintain conductivity even as gases (e.g., CO2) are evolved from the PSE 221. In example implementations, the PSE 221 delimited by the first and second CEMs 213 A, 213B allows for avoiding using bipolar membranes in the EC unit 208.

[0092] In example implementations, and referring to FIG. 2, the EC unit 208 is a single- cell-membrane stack. In example implementations, the EC unit 208 is a multiple-cell- membrane stack. In example implementations, the EC unit 208 includes multiple stacks, where each stack includes multiple cells, and where each cell of the EC unit 208 includes at least the PSE 221 and the first and second CEMs 213 A, 213B. In example implementations, each cell of the EC unit 208 includes at least the PSE 221, the first and second CEMs 213 A, 213B, the anode 203, and the cathode 207. The cells of such a multi-stack EC unit 208 can be fed liquid in a parallel arrangement, and voltage can be applied in series to each cell. In other implementations, the feed for each stack in a multi-stack EC unit 208 is different from the feed from at least one other stack in the multi-stack EC unit 208, for example in implementations where the output from one stack is the feed for another stack of the multi-stack EC unit 208. The inlets and outlets of the EC unit 208 can include multiple inlets and multiple outlets, it being understood that the use of the singular “inlet” or singular “outlet” can also denote multiple inlets and multiple outlets, respectively, when suitable.

[0093] A possible and non-limiting example of the PSE 221 is now described in greater detail. The PSE 221 is a layer of material that serves as a solid ionic conductor. Non-limiting examples of the material which can be included in the PSE 221 include polymeric ion conductors, such as spheres having micron-scale diameters, where such polymeric ion conductors can have different functional groups or inorganic compounds for improved cation or anion conduction. The PSE 221 can be formed by polymeric spheres filling an open compartment and forming a layer between the first and second CEMs 213A, 213B. The polymeric spheres abut against one another such that the surfaces of the spheres are in contact.The abutting polymeric spheres between the first and second CEMs 213 A, 213B allow for proton and cation conduction along their interconnected surfaces and can also allow for liquid to flow between the spheres as well as the release of gaseous products (e.g., CO2). Other possible materials which can be included in the PSE 221 include various solid electrolyte materials such as ceramics, polymer-ceramic hybrids, or solidified gels.

[0094] In example implementations, and referring to FIG. 2, the reactions performed by the EC unit 208 include the following reactions. The EC feed stream 226 includes a solution containing a first salt of M (K, Na, Li, etc.), a second, different salt of Y (K, Na, Li, etc.), and possibly at least one more different salt (K, Na, Li, etc.). In example implementations, the EC feed stream 226 includes DIC species such as bicarbonates KHCO3, NaHCCL, and HCCL', as well as carbonates K2CO3, Na2CC>3 and CCL2'. A water stream 223, such as a stream of deionized water, is the hydrogen-containing feed stream that is flowed into the proton-generating compartment 215 via the proton feed inlet 2151 and is also flowed into the regeneration compartment 217 via the water feed inlet 2171. The water stream 223 can include, or be, a stream of purified water. The hydrogen-containing feed stream can include a different source of protons (H+), as described in greater detail below. When an electric potential is applied across the anode 203 and the cathode 207 at sufficient current density, the EC feed stream 226 is decomposed through multiple reactions. For example, in the proton-generating compartment 215, in implementations where the hydrogen-containing feed stream includes the water stream 223 of purified or deionized water, the water is oxidized to generate protons and oxygen according to Reaction 1.

[0095] Reaction 1 :4e' + O2 + 4H+(water oxidation)

[0096] The anode 203 can include any suitable catalyst to promote the oxygen evolution reaction. Referring to FIG. 2, the gaseous oxygen generated according to Reaction 1 is a component of a third EC product stream 225 which can flow through the proton-generating compartment outlet 2150 to a downstream component separate from the EC unit 208. Referring to FIG. 2, the downstream unit is a degassing unit, such as an oxygen degassing unit (which may be referred to herein as the oxygen flash tank 2100) of the regeneration subsystem 264. The oxygen flash tank 2100 is in fluid communication with the proton-generating compartment 215 via the proton-generating compartment outlet 2150. The oxygen of the third EC product stream 225 can be off-gassed to the atmosphere in the oxygen flash tank 2100, orcollected, processed, compressed and provided to one or more downstream uses as an oxygen product stream 2290 (either onsite or offsite as a feedstock or product). In example implementations, the oxygen of the third EC product stream 225 is off-gassed directly to the atmosphere via the proton-generating compartment outlet 2150. The proton-generating compartment outlet 2150 may thus be referred to as a “gaseous products outlet 2150”.

[0097] In implementations where the EC feed stream 226 includes DIC species, the DIC species are decomposed in the PSE 221 and the products of decomposition react with the protons that have crossed over the first CEM 213A after being generated in the protongenerating compartment 215. The displaced protons from the proton-generating compartment 215 react with the DIC species of the EC feed stream 226 in the PSE 221 according to Reaction 2a to 2f.

[0098] Reaction 2a: NaHCCh+H ^Na +CCh+tEO (sodium bicarbonate to gaseous CO2 and water)

[0099] Reaction 2b: KHCOs+H ^K +CCh+IEO (potassium bicarbonate to gaseous CO2 and water)

[0100] Reaction 2c: I<2CO3+2H ^2K +CO2+H2O (potassium carbonate to gaseous CO2 and water)

[0101] Reaction 2d: Na2CO3+2H ^2Na +CO2+H2O (sodium carbonate to gaseous CO2 and water)

[0102] Reaction 2e: HCCh +H ^tECCh (bicarbonate to carbonic acid)

[0103] Reaction 2f: CO32+H ^HCO3 (carbonate to bicarbonate)

[0104] The water, gaseous CO2, carbonic acid, and bicarbonate reaction products generated in the PSE 221, along with any remaining reactants (e.g., carbonate) according to Reaction 2a-f form a second EC product stream 227 that is flowed through the PSE outlet 2210. In example implementations, the second EC product stream 227 is an aqueous solution of primarily bicarbonate (HCO3 ), which can help to maintain conductivity within the PSE 221 as the cations (Na+, K+, etc.) migrate from the PSE 221 to the regeneration compartment 217, thereby helping to maintain a lower voltage (and thus lower electricity consumption) across the PSE 221. The second EC product stream 227 thus includes carbon dioxide in gaseous form, and / or embodied in DIC species, which is the CO2 originally captured from the CO2- laden air 101 by the gas-liquid contactors 105. In example implementations, and as describedin greater detail below, some or all of the second EC product stream 227 forms a stream that can be used within the regeneration subsystem 264. Different compositions of the second EC product stream 227 are possible depending on the configuration of the regeneration subsystem, as described in greater detail below.

[0105] In example implementations, the PSE 221 is operated under a pH range selected to avoid or reduce the transport of excess protons (H+) across the second CEM 213B and into the regeneration compartment 217 where the protons might react with hydroxide to create water as a parasitic load. In such implementations, it may be desirable to maintain the pH in the PSE 221 relatively high so that protons within the PSE 221 are mostly bound. In example implementations, the pH in the PSE 221 is above 1. In example implementations, the pH in the PSE 221 is above 2. In example implementations, the pH in the PSE 221 is between 2 and 8. In example implementations, the EC unit 208 is operated to maintain a relatively low pH (i.e., acidic environment) in the proton-generating compartment 215, such that the protongenerating compartment 215 provides a large flux of protons (H+) for migrating into the PSE 221. In such implementations, the pH in the proton -generating compartment 215 can be below 2. The pH in the PSE 221 and / or in the proton-generating compartment 215 can be directly measured or can be controlled based on the conductivity of the PSE 221, or on a reference electrode.

[0106] Referring to FIG. 2, the alkali metal cations M+and Y+(e.g., K+, Na+, etc.) formed in the PSE 221 according to Reactions 2a to 2d migrate via the second CEM 213B to the regeneration compartment 217. When an electric potential is applied across the anode 203 and the cathode 207 at sufficient current density, the water stream 223 provided to the regeneration compartment 217 is reduced to generate hydrogen and hydroxyl ions according to Reaction 3:

[0107] Reaction 3 :2OH' + H2 (water reduction)

[0108] The cathode 207 can include any suitable catalyst to enhance or promote the hydrogen evolution reaction of Reaction 3. The displaced alkali metal cations M+and Y+(e.g., K+, Na+, etc.) from the PSE 221 react with the hydroxide in the regeneration compartment 217 according to the Reactions 4a and 4b:

[0109] Reaction 4a: M++ OH'MOH (first alkali metal hydroxide solution)

[0110] Reaction 4b:YOH (second alkali metal hydroxide solution)

[0111] Reactions 3, 4a, and 4b produce a first EC product stream 229 of the EC unit 208 that includes an alkaline solution of MOH and YOH, in addition to hydrogen gas. The first EC product stream 229 includes a regenerated capture solution with alkali hydroxide MOH and YOH, and hydrogen H2. The first EC product stream 229 can be flowed via the alkaline regeneration compartment outlet 2170 of the regeneration compartment 217 to one of the flash tanks 210 in fluid communication with the regeneration compartment 217. The flash tank 210 may be referred to as a hydrogen flash tank 21 OH, which is a degassing vessel that is downstream of, and separate from, the alkaline regeneration compartment outlet 2170 of the regeneration compartment 217.

[0112] To isolate the alkali hydroxide MOH and YOH from the first EC product stream 229 and thus form regenerated CO2 capture solution 144, the first EC product stream 229 can flow from EC unit 208 to a separation unit, such as the hydrogen flash tank 21 OH, where a hydrogen stream 229H is separated from the alkali hydroxide MOH and YOH. Thus, hydrogen can be off gassed from the first EC product stream 229 before flowing the first EC product stream 229 back to CO2 capture subsystem 102 for CO2 capture. The separated alkali hydroxide MOH and YOH is flowed back to the CO2 capture subsystem 102 (such as to the gas-liquid contactors 105) as regenerated CO2 capture solution 144. In example implementations, the separated hydrogen stream 229H can be recycled within the regeneration subsystem 264 as described in greater detail below. The hydrogen flash tank 21 OH is upstream of, and separate from, the CO2 capture subsystem 102.

[0113] In example implementations, such as in FIG. 2, and depending on the Reactions 1 to 4b performed by the EC unit 208, the EC unit 208 may also be referred to as an electrolysis unit 208. For example, Reactions 1 and 3 relate to the splitting of water. Reactions 1 to 4b can be considered electrolysis reactions in that they occur upon application of an electric current to a solution to drive an otherwise non-spontaneous chemical reaction in a liquid. In example implementations, such as in FIG. 2, and depending on the Reactions 1 to 4b performed by the EC unit 208 having the first and second cation exchange membranes 213 A, 213B, the EC unit 208 may also be referred to as an electrodialysis unit 208. For example, Reaction 2a to 2f relates to the transport of ions from one solution through ion-exchange membranes to another solution under the influence of an applied electric potential difference.

[0114] In example implementations, and referring to FIG. 2, the EC unit 208 allows for the direct generation of oxygen gas in the third EC product stream 225 (according to the water oxidation Reaction 1), the direct generation of hydrogen gas in the first EC product stream 229 (according to the water reduction Reaction 3), and the direct generation of CO2 gas in the second EC product stream 227 (according to Reaction 4).

[0115] In example implementations, and referring to FIG. 2, the regeneration subsystem 264 (or the DAC system 100) includes a caustic evaporator 214. The caustic evaporator 214 can include a mechanical vapour recompression (MVR) evaporator, a multi-effect evaporator, or a combination thereof. The caustic evaporator 214 concentrates the first EC product stream 229 by removing water to form the CO2 capture solution 144 and discharging a water stream 231. In such implementations, the CO2 capture solution 144 includes a carbonate-lean mixture and has a higher hydroxide concentration than the first EC product stream 229. In the regeneration subsystem 264 of FIG. 2, the caustic evaporator 214 is downstream of, and separate from, the hydrogen flash tank 21 OH which degasses the hydrogen stream 229H from the first EC product stream 229. In example implementations of the regeneration subsystem 264, the caustic evaporator 214 is upstream of, and separate from, the hydrogen flash tank 21 OH. In example implementations of the regeneration subsystem 264, the caustic evaporator 214 degasses hydrogen in addition to increasing the hydroxide concentration of the first EC product stream 229, such that the regeneration subsystem 264 is free of a dedicated hydrogen gas separating vessel. In example implementations, the caustic evaporator 214 is not a component of any of the subsystems 102, 162, 164, 264, and is instead a component of the DAC system 100. In example implementations, and referring to FIGS. 1 and 2, the CO2 capture subsystem 102 is in fluid communication with the alkaline regeneration compartment outlet 2170. In example implementations, and referring to FIGS. 1 and 2, the CO? capture subsystem 102 is in indirect fluid communication with the alkaline regeneration compartment outlet 2170 via the hydrogen flash tank 210H and the caustic evaporator 214.

[0116] In example implementations and referring to FIG. 2, the EC unit 208 is a feature of different liquid loops or circuits. A first loop, which can be referred to as a brine loop, circulates liquid between the dissolving tank 206 and the PSE inlet 2211 and the PSE outlet 2210 of the PSE 221 of the EC unit 208. In example implementations, the brine loop carries the absorbed CO2 in the form of the DIC which is protonated to produce the CO2 productstream 136. In other implementations, the brine loop carries a proton-shuttling species that helps to produce the CO2 product stream 136 in the dissolving tank 206. Thus, the DAC system 100 can include process streams flowing to and from the dissolving tank 206, the EC unit 208 and one or more of the flash tanks 210 to form a brine loop in which either the DIC is protonated, and CO2 is released, or a proton-shuttling species helps to produce the CO2 product stream 136.

[0117] A second liquid circulation network that also includes the EC unit 208 circulates liquid between the CO2 capture subsystem 102 and the EC unit 208 and regenerates the CO2 capture solution 144. Thus, the DAC system 100 can include process streams flowing to and from the caustic evaporator 214, the CO2 capture subsystem 102, the carbonate separation subsystem 162, the dissolving tank 206, and the EC unit 208 to form a caustic circulation network in which a capture solution is regenerated.

[0118] An example implementation of the brine loop is now described in greater detail. Reactions 2a to 2f described above, which generates the second EC product stream 227 comprising water, gaseous CO2, carbonic acid, carbonate and bicarbonate reaction products, can be described as a “direct protonation” reaction because protons (H+) from the protongenerating compartment 215 are donated directly to the DIC species of the EC feed stream 226 in the PSE 221. Reactions 2a to 2f thus represent the EC unit 208 employing a pH swing (i.e. a decrease in pH in the PSE 221) to both regenerate the CO2 capture solution 144 and form the CO2 product stream 136.

[0119] Referring to FIG. 2, the second EC product stream 227 is flowed to a CO2 degassing vessel, such as one of the flash tanks 210 (e.g., CO2 flash tank 210C). The carbonic acid (H2CO3) and the gaseous CO2 that can be present in small amounts in the second EC product stream 227 are separated in the CO2 flash tank 210C. At high H2CO3 concentrations, the equilibrium CO2 concentration will be sufficient to off-gas CO2 from the second EC product stream 227. The carbonic acid dissociates into CO2 and water, and the resulting CO2 gas streams are partially or fully released from the CO2 flash tank 210C and sent as the CO2 product stream 136 to one or more downstream processing units described in ensuing implementations (e.g., compression unit, dehydration unit, purification unit, electroreduction subsystem, carbon products manufacturing system, syngas generation reactor).

[0120] In example implementations, the pressure within the CO2 flash tank 210C can be decreased, or vacuum is formed, to further or better facilitate the dissociation of carbonic acid into CO2 and water. This can help to remove as much CO2 as possible from the second EC product stream 227 before flowing the remaining components of the second EC product stream 227 to the dissolving tank 206. In example implementations, the carbonic acid dissociates into CO2 and water, and the resulting gaseous CO2 streams can partially or fully degas in the EC unit 208, such as via the PSE outlet 2210. The remaining components of the second EC product stream 227, which can be an aqueous solution including primarily bicarbonate (HCOs' ), form a brine stream 238 flowable from the CO2 flash tank 210C to the dissolving tank 206, completing the brine loop of the regeneration subsystem 264. The CO2 flash tank 210C is in fluid communication with, and downstream of, the PSE outlet 2210. The CO2 flash tank 210C is in fluid communication with, and upstream of, the dissolving tank 206. Thus, the CO2 flash tank 210C allows for full CO2 degassing to occur away from the EC unit 208 and for bicarbonate to be returned to the EC unit 208 (via the dissolving tank 206) as the brine stream 238. In example implementations, as described in greater detail below, both forming H2CO3, and degassing CO2, can be carried out in a separate tank by employing a proton-shuttle loop that allows the EC unit 208 to indirectly protonate the DIC species.

[0121] Referring to FIG. 2, the crystalline carbonate hydrate 122 in the dissolving tank 206 dissolves in water and mixes with the DIC species in the brine stream 238 (e.g., bicarbonate) received from the CO2 flash tank 210C to form the EC feed solution 226 flowed to the PSE 221. The EC feed solution 226 can itself include DIC species, such as being a bicarbonate (HCO3 ) rich solution, with a mixture of other components such as carbonates and water. For example, in implementations where the crystalline carbonate hydrate 122 includes double carbonate salts such as K2CO3 and NaiCCh, the EC feed stream 226 of FIG. 2 can include DIC species primarily being bicarbonates KHCO3, NaHCCE, and HCCE', as well as small amounts of carbonates K2CO3, Na2CC>3 and CCh2'. In implementations where the brine stream 238 contains DIC species, the brine loop may be referred to as a “recycle loop” because it recycles DIC species produced by the EC unit 208 back to the EC unit 208. The brine loop can also allow for minimising water use by having the aqueous brine stream 238 contribute water to the dissolving tank 206, thereby reducing the freshwater input needed to dissolve the crystalline carbonate hydrate 122.

[0122] In example implementations, the brine loop does not transport DIC species, and instead allows for the protonation of DIC species outside of the EC unit 208. In such implementations and referring to FIG. 3, the regeneration subsystem 364 has a brine loop that carries a “proton-shuttling” species, which is a dissolved ionic species that allows for avoiding or minimizing CO2 degassing inside the cell of the EC unit 308. Non-limiting examples of the proton-shuttling species include sulfates ([SO4]2), bisulfate ([HSO4] ), hydrogen phosphate or biphosphate ([HPO4]2), chlorine (CE), iodine (E), Bromine (Br ), dihydrogen phosphate [H2PO4]1, acetate, and citrate. In example implementations, the proton-shuttling species is any anion which can receive a proton and then subsequently donate a proton. The protonshuttling species is first protonated in the PSE 321 of the EC unit 308 where it receives protons (H+) from the proton-generating compartment 315. The protonated proton-shuttling species then donates, or “shuttles”, these protons to DIC species in an external unit, such as the dissolving tank 306. In one example, the proton-shuttling species is a sulfate [SO4]2, and the EC unit 308 protonates the sulfate in the PSE 321 via the following reaction:

[0123] Reaction s: [SO4]2+H ^[HSO4] (sulfate to bisulfate)

[0124] The second EC product stream produced by the PSE 321 according to Reaction 5 is a brine stream 338 that includes the proton-shuttling species (e.g., a sulfate-bi sulfate mixture). The brine stream 338 is free of DICs. The brine stream 338 is flowed to an external tank that is downstream of, and separate from, the EC unit 308. Referring to FIG. 3, such an external tank is represented as the dissolving tank 306, which receives the sulfate-bi sulfate mixture of the brine stream 338. The dissolving tank 306 also receives the crystalline carbonate hydrate 122 and can also receive the water stream 228. The bi sulfate of the brine stream 338 reacts with the carbonate in the dissolving tank 306 to yield carbonic acid (H2CO3) via the following reactions:

[0125] Reaction 6a: 2KHSO4+I<2CO3^2I<2SO4+H2CO3 (potassium bisulfate to potassium sulfate and carbonic acid)

[0126] Reaction 6b: 2NaHSO4ENa2CO3— >2Na2SO4+H2CC>3 (sodium bisulfate to sodium sulfate and carbonic acid)

[0127] In some cases, a polished aqueous solution can be used instead of or in addition to the water stream 228. The water stream 228 and the polished aqueous solution can be substantially free of solids or particulates, and free of dissolved contaminants such as divalentcations. In example implementations, the water stream 228 provided to the dissolving tank 306 can include a slip stream of process solution from elsewhere in the DAC system 100 that is free or substantially free of hydroxide, and that is free of solids, particulates, and dissolved contaminants such as divalent cations.

[0128] The crystalline carbonate hydrate 122 dissolves in water and reacts with the protonshuttling species in the brine stream 338 according to Reactions 6a and 6b. For example, in implementations of the present disclosure where the CO2 capture solution 144 is an alkaline solution including KOH and NaOH, the crystalline carbonate hydrate 122 can include potassium carbonate sesquihydrate (K2CO3- I.5H2O) and sodium carbonate decahydrate (Na2CO3- 10H2O), or the crystalline carbonate hydrate 122 can include potassium sodium carbonate hexahydrate (KNaCO3-6 H2O). The crystalline carbonate hydrate 122 dissolves in water in dissolving tank 306 to yield potassium carbonate (K2CO3) and sodium carbonate (Na2CO3). In implementations, the brine stream 338 includes a bisulfate-rich solution of potassium bisulfate (KHSO4) and sodium bisulfate (NaHSO4) as the proton-shuttling species. The bisulfate-rich solution can react with carbonate in the dissolving tank 306 according to Reactions 6a and 6b to yield a bisulfate-lean solution and carbonic acid. The carbonic acid will have sufficient equilibrium CO2 partial pressures (e.g., less than 1 bar) to cause dissociation into water and gaseous CO2. As a result, the pH within the dissolving tank 306 will be reduced as CO2 gas is produced. Thus, the bisulfate proton-shuttling species can achieve the same pH swing in the dissolving tank 306 as is achieved with the DIC species in the PSE 221 of FIG. 2 via Reactions 2a to 2f.

[0129] Dissolving tank 306 can partially or fully release a CO2 gas as the CO2 product stream 136. The CO2 product stream 136 can be sent to one or more downstream processing units described below (e.g., compression unit, dehydration unit, purification unit, electroreduction subsystem, carbon products manufacturing system, syngas generation reactor). In some cases, the dissolving tank 306 can operate at a pressure of up to 40 bar. Together, process streams flowing to and from dissolving tank 306 and the EC unit 308 form the brine loop of FIG. 3 in which the proton-shuttling species (e.g., sulfate [SO4]2, bisulfate [HSO4] ) is protonated in the PSE 221, and protons are shuttled to the dissolving tank 306 via the proton-shuttling species. In dissolving tank 306, the proton-shuttling species protonates the DIC species to form carbonic acid. Thus, the brine loop of FIG. 3 allows for the EC unit308 to indirectly protonate the DIC species. The brine loop of FIG. 3 may be referred to as a “recycle loop” because the EC unit 308 recycles the proton-shuttling species. The regeneration subsystem 364 of FIG. 3 allows for the DIC species to be protonated outside of the EC unit 308, which avoids CO2 degassing inside the cells of the EC unit 308. This can extend the lifespan of the PSE 321 by transferring some of the CO2 degassing that would normally occur in the PSE 321 away from the EC unit 308. This can also or alternatively increase the efficiency of the EC unit 308 by reducing electrical resistance and inactive cell area within the EC unit 308 that can be caused by CO2 degassing. Thus, a brine loop including an indirect protonation process, as described in relation to FIG. 3, can be more efficient than a brine loop including a direct protonation system.

[0130] Referring to FIG. 3, Reactions 6a and 6b in the dissolving tank 306 also form the EC feed stream 326. The EC feed stream 326 can include a bi sulfate-lean solution. For example, the EC feed stream 326 can include a solution rich in potassium sulfate (K2SO4) and sodium sulfate (ISfeSCU), with a mixture of other components in dilute concentrations such as potassium bisulfate, sodium bisulfate and water. The dissolving tank 306 is configured to flow the EC feed stream 326 to the EC unit 308. In the example implementation of FIG. 3, the dissolving tank 306 forms DIC species that are protonated, and also produces the EC feed stream 326 which is free, or substantially free, of DIC species. In the PSE 321 of the EC unit 308, the proton-shuttling species of the EC feed stream 326 is protonated with protons (H+) migrating via the first CEM 313 A from the proton-generating compartment 315. The protons combine with the cations (e.g., M, Y, etc.) in the PSE 321 to form the brine stream 338. For example, in implementations where the CO2 capture solution 144 is an alkaline solution including KOH and NaOH, the sulfate ions [SO4]2of the EC feed stream 326 can be protonated and combined with potassium and sodium cations to form potassium bisulfate and sodium bisulfate in the brine stream 338 according to Reactions 7a and 7b.

[0131] Reaction 7a: Na2SO4+H ^NaHSO4+Na (sodium sulfate to sodium bi sulfate)

[0132] Reaction 7b: IGSCU+H ^KHSCU+K (potassium sulfate to potassium bi sulfate)

[0133] The metal alkali cations generated according to Reactions 7a and 7b migrate across the second CEM 313B into the regeneration compartment 317 to form the first EC productstream 329 as described above with reference to the regeneration subsystem 264 of FIG. 2. The first and second CEMs 313 A, 313B of the EC unit 308 help to confine the proton-shuttling species anions to the PSE 321, thereby helping to prevent or reduce the migration of such anions into the regeneration compartment 317 to reduce or eliminate the presence of the protonshuttling species (e.g., sulphur ions) in the regenerated CO2 capture solution 144.

[0134] In example implementations the EC feed stream 326 and the brine stream 338 are free of DICs. In example implementations, brine stream 338 can include between about 1 M to about 2.5 M sulfate and bisulfate. In some cases, the brine stream 338 can include a combination of K2SO4 / KHSO4 and NaSCh / NaHSCh with total concentrations of 7 M or lower. In some cases, the brine stream 338 can include a KNaSCh concentration of 1 M to about 2 M. In some cases, brine stream 338 and EC feed stream 326 can include other sulfate or bi sulfate concentrations (or both), depending on the operating temperature of the EC unit 308 and conversion of sulfate to bisulfate in the EC unit 308. The respective sulfate and bisulfate concentrations of EC feed stream 326 and brine stream 338 can depend on the lowest solubility species for any given operating temperature.

[0135] One or more process streams of FIG. 3 can comprise a mixture of DIC species or proton-shuttling species with concentrations that vary based on reactions partially or fully completing and on process conditions. Ratios of CO2, HCO3 , and CO32, concentrations of HSO4 and SO42, and total ionic strengths in a process stream can depend on the pH. For example, as the pH decreases from 10 to 7.5, the HCO3 " concentration and CO2 concentration can increase while the CO32concentration decreases.

[0136] The description, features, chemical reactions, reference numbers, and advantages of the present disclosure that are associated with the EC unit 208 and with the regeneration subsystem 264 of FIG. 2 apply mutatis mutandis to the EC unit 308 and to the regeneration subsystem 364 of FIG. 3.

[0137] Other configurations of the regeneration subsystem 164, 264, 364 are possible. For example, and referring to FIG. 4, the EC unit 408 of the regeneration subsystem 464 can include a gas diffusion electrode (GDE) 403. The EC unit 408 of FIG. 4 employs the GDE 403 as an anode. The GDE 403 includes a gas diffusion layer 403 G that supports a catalyst layer 403C. Gas diffusion layer 403G is porous and allows movement of gas towards the catalyst layer 403 C. In example implementations, the catalyst layer 403 C can include platinumor non-precious metal catalysts (e.g., lead, nickel, nickel iron, cobalt, metal alloys). In example implementations, the catalyst layer 403C can include precious metals, such as iridium. In example implementations, the GDE 403 is a hydrogen depolarized anode. In example implementations, the gas diffusion layer 403G is hydrophobic, such that its pores are less likely to be blocked by the aqueous electrolyte solution and thus gas transport to the catalyst layer 403C is maintained. In some cases, EC unit 408 having a GDE 403 can operate at a pH ranging between 0 to 14. The first CEM 413A and the GDE 403 define the proton-generating compartment 415.

[0138] The configuration of the EC unit 408 of FIG. 4 can enable high current densities and hydroxide concentrations of up to 35% w / w. This can lower capital costs, reduce water usage, and reduce water treatment costs. The EC unit 408 can be employed for direct protonation or indirect protonation of the DIC species, as described in greater detail below.

[0139] Referring to FIG. 4, the EC unit 408 can employ direct protonation of the DIC species to regenerate the CO2 capture solution 144 and recover the CO2 product stream 136. To regenerate the CO2 capture solution 144, the EC unit 408 functions as described above according to Reactions 3 and 4 to generate the first EC product stream 429 in the regeneration compartment 417. The first EC product stream 429 includes an alkaline solution of MOH, YOH, etc., in addition to hydrogen gas.

[0140] The first EC product stream 429 is flowed via the alkaline regeneration compartment outlet 4170 of the EC unit 408 to a separation unit, such as one of the flash tanks 410. The flash tank 410 is a hydrogen degassing vessel and may be referred to as the hydrogen flash tank 410. The hydrogen flash tank 410 is in fluid communication with, and downstream of, the alkaline regeneration compartment outlet 4170. Hydrogen is separated from the first EC product stream 429 in the hydrogen flash tank 410 before the alkali hydroxide solution is flowed to the CO2 capture subsystem 102 as the regenerated CO2 capture solution 144, as described above with respect to FIG. 2. Referring to FIG. 4, the hydrogen flash tank 410 is in fluid communication with, and upstream of, the proton-generating compartment 415. This fluid communication allows some or all of the hydrogen separated in the hydrogen flash tank 410 to be recycled to the EC unit 408 by being flowed (pumped or compressed) to the GDE 403 as a hydrogen feed stream 439. A remaining separated hydrogen stream 429H can be processed as described above with respect to the hydrogen stream 229H.

[0141] In example implementations, the hydrogen-containing feed stream provided to the proton-generating compartment 415 is hydrogen gas. In example implementations, the protongenerating compartment 415 is in fluid communication with the regeneration compartment 417 via the flash tank 410. In example implementations, the proton-generating compartment 415 receives a hydrogen-containing feed stream (hydrogen gas) that is different than the hydrogencontaining feed stream (water) provided to the regeneration compartment 417. In example implementations, some of the hydrogen gas from the flash tank 410 is provided to the GDE 403 as the hydrogen feed stream 439, while a remainder of the hydrogen gas from the flash tank 410 is diverted away from the EC unit 408.

[0142] The hydrogen feed stream 439 can be flowed through the pores of the GDE 403. In example implementations of the EC unit 408, the EC unit 408 is free of the GDE 403 and includes another anode delimiting the proton-generating compartment 415, such that the hydrogen feed stream 439 is fed directly into the proton-generating compartment 415. An electric potential is applied to EC unit 408, and hydrogen of the hydrogen feed stream 439 is oxidized at the GDE 403 according to the following reaction.

[0143] Reaction 8: Eh— > 2H++2e' (oxidation of hydrogen into protons and electrons)

[0144] The protons generated according to Reaction 8 migrate via the first CEM 413A to the PSE 421, where they directly protonate the DIC species of the EC feed solution 426 as described above according to Reaction 2a to 2f, to form the second EC product stream 427. The CO2 product stream 136 and the brine stream 238 of FIG. 4 can be evolved from the second EC product stream 427 as described above with respect to FIG. 2, mutatis mutandis.

[0145] In example implementations, the EC unit 408 can employ indirect protonation to regenerate the CO2 capture solution 144 and to recover the CO2 product stream 136. In such an implementation, the CO2 flash tank 210C of FIG. 4 is absent from the brine loop between the dissolving tank 406 and the EC unit 408, and a proton-shuttling species is used to indirectly protonate the DIC species in the dissolving tank 406, as described above with respect to FIG. 3, mutatis mutandis.

[0146] In example implementations, the water stream 223 is flowed only to the regeneration compartment 417, instead of to both the regeneration and proton-generating compartments 417, 415. The EC unit 408 evolves hydrogen gas in the regenerationcompartment 417 and consumes the produced hydrogen gas in the proton-generating compartment 415. In example implementations, gaseous oxygen is not a product of the EC unit 408. In example implementations of the EC unit 408 employing direct protonation, the primary or only gaseous product that flows from the EC unit 408 is the CO2 present in the second EC product stream 427. By producing only one gaseous product in such implementations, the EC unit 408 can operate at reduced voltages and thereby lower energy consumption. The EC unit 408 including the GDE 403 can be selected or designed to have an applied potential across the entire cell of less than 1.6 V), and a high current density (e.g., current density between 50 mA / cm2and 1000 mA / cm2). In example implementations, the EC unit 408 including the GDE 403 can be selected or designed to have an applied potential across the entire cell of less than 1.2 V.

[0147] The description, features, chemical reactions, reference numbers, and advantages of the present disclosure that are associated with the EC unit 208 and with the regeneration subsystem 264 of FIG. 2, and with the EC unit 308 and with the regeneration subsystem 364 of FIG. 3, apply mutatis mutandis to the EC unit 408 and to the regeneration subsystem 464 of FIG. 4.

[0148] Other configurations of the regeneration subsystem 164, 264, 364, 464 are possible.For example, and referring to FIG. 5, the EC unit 508 of the regeneration subsystem 564 can employ direct protonation of the DIC species in the PSE 521, directly off gases at least some of the CO2 product stream 136 from the PSE 521 and is a component of a brine loop. Referring to FIG. 5, the PSE 521 generates the second EC product stream 527 according to Reactions 2a to 2f described above. The gaseous CO2 of the second EC product stream 527 is off-gassed from the PSE outlet 5210 as some or all of the CO2 product stream 136. The remaining components of the second EC product stream 527 (e.g., water, carbonic acid, carbonate and bicarbonate reaction products) are also flowed through the PSE outlet 5210.

[0149] In example implementations, at high H2CO3 concentrations, the equilibrium CO2 concentration will be sufficient to off-gas CO2 from the second EC product stream 527 in a flash tank 510, forming another portion of the CO2 product stream 136. The remaining components of the second EC product stream 527, which can be an aqueous solution including primarily bicarbonate (HCO3 ), form a brine stream 538 flowable from the flash tank 510 to the dissolving tank 506, completing the brine loop of the regeneration subsystem 564.

[0150] In example implementations of the regeneration subsystem 564, the flash tank 510 is absent, such that the second EC product stream 527 comprising carbonic acid is flowed directly from the PSE outlet 5210 as the brine stream 538 to the dissolving tank 506. In such implementations, the carbonic acid dissociates into CO2 and water in the dissolving tank 506, and the resulting CO2 gas streams form part of the CO2 product stream 136. Referring to FIG. 5, the CO2 product stream 136 can include CO2 generated in each of the PSE 521, the flash tank 510 and the dissolving tank 506.

[0151] In example implementations, the EC unit 508 of the regeneration subsystem 564 employs a GDE. In such implementations, the description, features, reference numbers, and advantages of the present disclosure that are associated with the GDE 403, the EC unit 408 and the regeneration subsystem 464 of FIG. 4 apply mutatis mutandis to a GDE of the regeneration subsystem 564 of FIG. 5. The brine loop of FIG. 5 can allow for minimising water use by having the aqueous brine stream 538 contribute water to the dissolving tank 506, thereby reducing the water needed to dissolve the crystalline carbonate hydrate 122.

[0152] Other configurations of the regeneration subsystem 164, 264, 364, 464, 564 are possible. For example, and referring to FIG. 6, the EC unit 608 of the regeneration subsystem 664 can employ direct protonation of the DIC species in the PSE 621, directly off gases all or substantially all of the CO2 product stream 136 from the PSE 621 and is not a feature of a brine loop. Referring to FIG. 6, the PSE 621 generates reaction products according to Reactions 2a to 2f described above. At high enough concentrations of the carbonic acid (H2CO3) reaction product, the equilibrium CO2 concentration will be sufficient to off-gas CO2 from the carbonic acid in the PSE 621, which combines with the gaseous CO2 reaction product to form the CO2 product stream 136 that flows from the PSE outlet 6210. In such an implementation, the second EC product stream is equivalent to the CO2 product stream 136. The remaining reaction products (e.g., water, carbonate and bicarbonate reaction products) can be returned to the DAC system 100, and / or remain within the PSE 621 and continue to be protonated and react according to Reaction 2a to 2f.

[0153] In example implementations, the EC unit 608 of the regeneration subsystem 664 employs a GDE. In such implementations, the description, features, reference numbers, and advantages of the present disclosure that are associated with the GDE 403, the EC unit 408 andthe regeneration subsystem 464 of FIG. 4 apply mutatis mutandis to a GDE of the regeneration subsystem 664 of FIG. 6.

[0154] In example implementations, and referring to FIG. 6, the presence and operation of the PSE 621 in the EC unit 608 can allow for eliminating a brine loop from the regeneration subsystem 664, which can help to reduce the complexity and capital costs associated with the regeneration subsystem 664. In example implementations, and referring to FIG. 6, the PSE 621 can have enough conductivity to allow for the electrochemical Reaction 2a to 2f to occur within the PSE 621 while simultaneously evolving and flowing CO2 gas through the PSE 621. In example implementations, and referring to FIG. 6, the PSE 621 can allow for the single pass production of the CO2 product stream 136. In example implementations, and referring to FIG. 6, the dissolved crystalline carbonate hydrate 122 can be flowed, as the EC feed stream 226, directly to the EC unit 608 without first mixing with a brine stream.

[0155] In example implementations, and referring to FIG. 7, the operation of the EC unit 708 can allow for forming a hydroxide recycle loop, which can help to reduce water usage by the EC unit 708 and improve conductivity in the alkaline regeneration compartment 217. In example implementations, and referring to FIG. 7, some of the regenerated CO2 capture solution 144 making up the first EC product stream 229 is flowed as a hydroxide stream 775 to the EC unit 708. Referring to FIG. 7, the hydroxide stream 775 is flowed, via the water feed inlet 2171, into the alkaline regeneration compartment 217. The alkaline solution (e.g., MOH and YOH) of the hydroxide stream 775 is reacted in the alkaline regeneration compartment 217 according to Reactions 3, 4a and 4b, to produce the first EC product stream 229. The regeneration subsystem 764 of FIG. 7 allows for some of the hydroxide solution produced by the EC unit 708 to be “recycled” back to the EC unit 708. In providing recycled hydroxide to the EC unit 708 via the hydroxide stream 775, the ionic strength of the catholyte of the alkaline regeneration compartment 217 is increased, which may allow the cathode 207 to split water more efficiently according to Reaction 3. In some implementations, the water stream 223 provided to the alkaline regeneration compartment 217 via the water feed inlet 2171 can be a make-up water stream 223 which supplements the hydroxide stream 775.

[0156] In example implementations, and referring to FIG. 7, the hydroxide stream 775 is a slipstream of the first EC product stream 229. Referring to FIG. 7, the hydroxide stream 775 is flowed from downstream of the hydrogen flash tank 21 OH and upstream of the causticevaporator 214. In example implementations, the hydroxide stream 775 is flowed from downstream of the caustic evaporator 214. In example implementations, the hydroxide stream 775 is flowed from both downstream and upstream of the caustic evaporator 214. In example implementations, during operation of the EC unit 708, the hydroxide stream 775 is flowed continuously to the alkaline regeneration compartment 217. In example implementations, the hydroxide stream 775 is mixed with the water stream 223 to provide a diluted hydroxide solution to the alkaline regeneration compartment 217. In example implementations, the hydroxide stream 775 is flowed undiluted to the alkaline regeneration compartment 217, such that the hydroxide stream 775 provides all or substantially all of the water required for the water reduction reaction of Reaction 3.

[0157] The description, features, chemical reactions, reference numbers, and advantages of the present disclosure that are associated with using a recycle hydroxide stream 775 with the regeneration subsystem 764 of FIG. 7 apply mutatis mutandis to the EC unit 208, 308, 408, 508, 608 and to the regeneration subsystem 164, 264, 364, 464, 564, 664 of the present disclosure. Stated differently, any of the EC unit 208, 308, 408, 508, 608 and of the regeneration subsystem 164, 264, 364, 464, 564, 664 of the present disclosure can use the recycle hydroxide stream 775 during continuous operation thereof, or during start-up thereof. The description, features, chemical reactions, reference numbers, and advantages of the present disclosure that are associated with the EC unit 208 and with the regeneration subsystem 264 of FIG. 2 apply mutatis mutandis to the EC unit 708 and to the regeneration subsystem 764 of FIG. 7. In example implementations, the EC unit 708 can employ indirect protonation to regenerate the CO2 capture solution 144 and to recover the CO2 product stream 136. In such an implementation, the CO2 flash tank 210C of FIG. 7 is absent from the brine loop between the dissolving tank 206 and the EC unit 708, and a proton-shuttling species is used to indirectly protonate the DIC species in the dissolving tank 206, as described above with respect to FIG. 3, mutatis mutandis. In example implementations, the EC unit 708 of the regeneration subsystem 764 employs a GDE. In such implementations, the description, features, reference numbers, and advantages of the present disclosure that are associated with the GDE 403, the EC unit 408 and the regeneration subsystem 464 of FIG. 4 apply mutatis mutandis to a GDE of the regeneration subsystem 764 of FIG. 7.

[0158] The EC unit 208, 308, 408, 508, 608, 708 of the present disclosure can operate at a pH range of approximately 1 to 14. For example, the EC unit 208, 308, 408, 508, 608, 708 can operate with proton concentrations ranging between 0.001 M and 2.5 M. The regeneration subsystem 164, 264, 364, 464, 564, 664, 764 of the present disclosure can include multiple EC units 208, 308, 408, 508, 608, 708. Features shown in some figures without reference numbers can have the same description, reference numbers, and advantages of the present disclosure that are associated with corresponding features numbered in other figures.

[0159] The process streams of the present disclosure can be flowed using one or more flow control systems 999 (see FIGS. 1 to 7) implemented throughout the DAC system 100 or regeneration subsystem 164, 264, 364, 464, 564, 664, 764 of the present disclosure. A flow control system 999 can include one or more flow pumps to pump the process streams, one or more flow pipes through which the process streams are flowed and one or more valves to regulate the flow of streams through the pipes. Control system 999 can include one or more pH monitoring devices and one or more conductivity monitoring devices. In example implementations, control system 999 can include one or more chemical analysis devices (e.g., Fourier transform near-infrared spectroscopy device) to measure DIC species. In example implementations, control system 999 can include one or more temperature sensors (e.g., thermocouples, thermistors, thermometers) and temperature controllers to monitor and control one or more aspects of flow control system 999 in response to heat generated from the one or more elements of the electrochemical system. In example implementations, control system 999 can include one or more power control units, to provide electrical power at constant current or at constant voltage to components of the DAC system 100.

[0160] In example implementations, control system 999 can be operated manually. For example, an operator can set a flow rate for each pump and set valve open or close positions to regulate the flow of the process streams through the pipes in control system 999. Once the operator has set the flow rates and the valve open or close positions for all control systems 999 distributed across the electrochemical system for capturing CO2 and regenerating a capture solution, control system 999 can flow the streams under constant flow conditions, for example, constant volumetric rate or other flow conditions. To change the flow conditions, the operator can manually operate control system 999, for example, by changing the pump flow rate or the valve open or close position.

[0161] In example implementations, flow control system 999 can be operated automatically. For example, the flow control system 999 can be connected to a computer or a computer-readable medium storing instructions (such as flow control instructions and other instructions) executable by one or more processors to perform operations (such as flow control operations). An operator can set the flow rates and the valve open or close positions for all flow control systems 999 distributed across the DAC system 100 for capturing CO2 and regenerating a capture solution using the flow control system 999. In such implementations, the operator can manually change the flow conditions by providing inputs through the flow control system 999. Also, in such implementations, the flow control system 999 can automatically (that is, without manual intervention) control one or more of the flow control systems, for example, using feedback systems connected to flow control system 999. For example, a sensor (such as a pressure sensor, temperature sensor or other sensor) can be connected to a pipe through which a process stream flows. The sensor can monitor and provide a flow condition (such as a pressure, temperature, or other flow condition) of the process stream to flow control system 999. In response to the flow condition exceeding a threshold (such as a threshold pressure value, a threshold temperature value, or other threshold value), control system 999 can automatically perform operations. For example, if the pressure or temperature in the pipe exceeds the threshold pressure value or the threshold temperature value, respectively, flow control system 999 can provide a signal to the pump to decrease a flow rate, a signal to open a valve to relieve the pressure, a signal to shut down process stream flow, or other signals.

[0162] Although the pump(s) 263 is disclosed herein in example implementations as a component of the regeneration subsystem 164, 264, 364, 464, 564, 664, 764, in example implementations, the pump(s) 263 is not a component of the regeneration subsystem 164, 264, 364, 464, 564, 664, 764. In example implementations, the pump(s) 263 is a component of the carbonate separation subsystem 162. In example implementations, the pump(s) 263 is not a component of any of the subsystems 102, 162, 164, 264, 364, 464, 564, 664, 764, and is instead a component of the DAC system 100.

[0163] One or more of the subsystems 102, 162, 164, 264, 364, 464, 564, 664, 764 can include one or more of piping, valves, fittings, pumps, reservoirs, vessels and othercomponentry that are not explicitly described so as to achieve the functionality ascribed to the regeneration subsystem 164, 264, 364, 464, 564, 664, 764 herein.

[0164] In example implementations, the EC unit 208, 308, 408, 508, 608, 708 can have a low voltage drop (e.g., a voltage drop of less than 1.6 V) and a high current density (e.g., current density between 50 mA / cm2and 2000 mA / cm2). The EC unit 208, 308, 408, 508, 608, 708 can be selected or designed to have desirable characteristics such as low voltage drop (e.g., EC unit voltage drop of less than 2 V). The PSE and the CEMs can be stable in wide ranges of operating temperature, particularly high operating temperatures as they allow for decreased voltage. The PSE and the CEMs can be stable in wide ranges of pH and in high concentration alkaline solutions. This allows regenerated capture solutions that have high hydroxide concentrations, which can improve CO2 capture rates, and reduce evaporator costs.

[0165] Referring to FIG. 8, there is disclosed a method 700. At 702, the method 700 includes contacting the CO2 capture solution 144 with carbon dioxide from a dilute gas source (e.g., the CCh-laden air 101) to form the carbonate-rich solution 120 that includes at least a first carbonate (M2CO3) and a second carbonate (Y2CO3), M and Y being alkali metals. In example implementations, the CO2 capture solution 144 includes at least a first hydroxide (MOH) and a second hydroxide (YOH).

[0166] At 704, the method includes separating at least a portion of the M2CO3 and the Y2CO3 from the carbonate-rich solution 118 to form carbonate solids, including a separated solid M2CO3 and a separated solid Y2CO3.

[0167] At 706, the method 700 includes dissolving the solid M2CO3 and the solid Y2CO3 and forming dissolved inorganic carbon (DIC) species and the EC feed stream 226, 326, 426.

[0168] At 707, the method 700 includes flowing the EC feed stream 226, 326, 426 to the PSE 221, 321, 421, 521, 621. At 710, the method 700 includes flowing the water stream 223 to at least the alkaline regeneration compartment 215, 315, 415. Features 707 and 710 of the method 700 are performed in the EC unit 208, 308, 408, 508, 608, 708.

[0169] At 712, the method 700 includes applying an electric potential to the EC unit 208, 308, 408, 508, 608, 708 to form at least two EC product streams including a first EC product stream 229, 329, 429 comprising hydrogen, regenerated MOH and regenerated YOH and a second EC product stream 227, 338, 427, 527, 538, 136 in the PSE 221, 321, 421, 521, 621.

[0170] At 714, the method 700 includes flowing the first EC product stream 229, 329, 429 comprising the regenerated MOH and the regenerated YOH to use in the contacting the CO2 capture solution 144 of step 702.

[0171] Referring to FIG. 9, there is disclosed another example method 900. At 902, the method 900 includes contacting carbon dioxide from a dilute gas source (e.g., the CO2-laden air 101) with the capture solution 144 to form the carbonate-rich solution 120. The capture solution 144 of FIG. 9 includes, in example implementations, a single alkali metal salt. The capture solution 144 of FIG. 9 comprises a hydroxide (MOH), and the formed carbonate comprises M2CO3, where M is an alkali metal.

[0172] At 904, the method 900 includes crystallizing at least a portion of the M2CO3 to form a crystalline M2CO3 (such as the crystalline carbonate hydrate 122).

[0173] At 906, the method 900 includes dissolving the crystalline M2CO3 to form the EC feed stream 226, 326, 426. The EC feed stream 226, 326, 426 comprises a salt of M.

[0174] At 908, the method 900 includes flowing the EC feed stream 226, 326, 426 to the PSE 221, 321, 421, 521, 621. At 910, the method 900 includes flowing the water stream 223 to at least the alkaline regeneration compartment 215, 315, 415. Features 908 and 910 of the method 900 are performed in the EC unit 208, 308, 408, 508, 608, 708.

[0175] At 912, the method 900 includes applying an electric potential to the EC unit 208, 308, 408, 508, 608, 708 to form at least two EC product streams including a first EC product stream 229, 329, 429 including regenerated MOH and a second EC product stream 227, 338, 427, 527, 538, 136 generated by the PSE 221, 321, 421, 521, 621.

[0176] At 914, the method 900 includes flowing the first EC product stream 229, 329, 429 including the regenerated MOH to use in the contacting the CO2 capture solution 144 of step 902.

[0177] In implementations of the present disclosure, the carbonate separation subsystem 162 bridges the CO2 capture subsystem 102 to the capture solution regeneration subsystem 164, 264, 364, 464, 564, 664, 764, allowing for the subsystems to be operationally decoupled from one another, which can provide advantages. For example, the CO2 capture subsystem 102 can operate in a wider range of ambient conditions because it is decoupled from downstream processes by the carbonate separation subsystem 162. The carbonate separation subsystem 162 forms the crystalline carbonate hydrate 122, and the duty required to form thisproduct is determined by the composition of the CO2 capture solution 144 and its position relative to the saturation curve of the DIC species (e.g., the carbon saturation curve). For instance, in conditions where a high-ionic strength CO2 capture solution 144 is used (e.g., high- hydroxide for faster capture kinetics or high-carbonate for higher crystallizer recovery), the carbonate separation subsystem 162 can require a low duty to reach saturation and form the crystalline carbonate hydrate 122 since the CO2 capture solution 144 is close to the saturation curve. In contrast, in conditions where a dilute CO2 capture solution 144 is needed (e.g., arid climates with high evaporative losses), the carbonate separation subsystem 162 can require a high duty to reach saturation and form the crystalline carbonate hydrate 122. Thus, the CO2 capture subsystem 102 can equilibrate to environmental temperature and relative humidity without significantly affecting the capture solution regeneration subsystem 164, 264, 364, 464, 564, 664, 764 downstream.

[0178] The evaporative load resulting from equilibrating is accommodated by units in the carbonate separation subsystem 162 that bridges the other two subsystems. The solubility of carbonate salt in the carbonate-rich solution 120 dictates the duty on the carbonate separation subsystem 162, as the carbonate separation subsystem 162 is the buffer that provides pure or nearly pure carbonate to the capture solution regeneration subsystem 164, 264, 364, 464, 564, 664, 764. Thus, capture solutions employed in the CO2 capture subsystem 102 can be optimized for capture (e.g., high hydroxide, low carbonate) or for improving water balance and evaporation costs, in consideration of relative humidity equilibria.

[0179] There can also be additional advantages such as adaptability to environmental conditions and cold weather operation. Since the carbonate separation subsystem 162 can carry the load of the CO2 capture subsystem 102, it enables a wide range of operating conditions for the CO2 capture subsystem 102. The CO2 capture subsystem 102 can operate anywhere under the carbonate saturation curve while maintaining a pure or nearly pure carbonate stream (which can be achieved by separating and dissolving the crystalline carbonate hydrates 122, for example) to the EC unit 208, 308, 408, 508, 608, 708. This enables the CO2 capture subsystem 102 to operate at high ionic strengths and at close to the saturation line, where the freezing point of the solution is significantly decreased as a result. For example, the CO2 capture subsystem 102 can operate with a capture solution that includes high hydroxide concentrations which can enable a higher capture rate.

[0180] Another advantage can be load flexibility. For instance, in cases where the EC unit 208, 308, 408, 508, 608, 708 is operating at a limited capacity or is nonoperational (e.g., due to maintenance), the carbonate separation subsystem 162 can include one or more buffer tanks that store the crystalline carbonate hydrate 122. In some cases, a buffer capacity dissolving tank or an overflow tank can be useful for absorbing the load of operational changes since the crystalline carbonate hydrate 122 are highly soluble salts. For example, during non-peak periods, when electricity from intermittent sources (e.g., wind, solar) is scarce, the EC unit 208, 308, 408, 508, 608, 708, the most energy intensive component of the regeneration subsystem 164, 264, 364, 464, 564, 664, 764, can be ramped down but the gas-liquid contactor 105 and the crystallizer 104 can continue to operate, capturing CO2 and producing the crystalline carbonate hydrate 122. The crystalline carbonate hydrate 122 solids can be stored and fed to the EC unit 208, 308, 408, 508 for regeneration when electricity is more readily available.

[0181] Another advantage can be load ramping. In example implementations, main energy driver of the regeneration subsystem 164, 264, 364, 464, 564, 664, 764 is the EC unit 208, 308, 408, 508, 608, 708. Because the EC unit 208, 308, 408, 508, 608, 708 is an electrochemical cell, it can simply have power supply cut, reduced, or ramped up as necessary. This is advantageous over equipment that are sometimes used in a calcium regeneration process, such as fluidized bed reactors and high temperature calciners, which cannot easily be ramped. Further, the EC unit 208, 308, 408, 508, 608, 708 enables elimination of insoluble precipitates. In particular, sodium carbonate and potassium carbonate are highly water soluble, and if these salts were to crystallize in unwanted places or foul equipment, a simple water wash will defoul.

[0182] The EC unit 208, 308, 408, 508, 608, 708 can allow the capture solution regeneration subsystem 164, 264, 364, 464, 564, 664, 764 to be used in conjunction with various different styles of CO2 capture subsystems 102, including air contactors such as gasliquid contactors 105, spray towers, liquid-gas scrubbers, membrane contactors, venturi scrubbers, packed towers, and other systems designed to remove at least a portion of a particular gas component from a larger gas stream using a liquid sorbent.

[0183] For example, and referring to FIGS. 11 A and 1 IB, the CO2 capture subsystem 102 can include multiple gas-liquid contactors 105. Each gas-liquid contactor 105 can be groupedtogether with one or more other gas-liquid contactors 105 to provide the DAC system 100 with one or more wall(s), array(s) or train(s), where each wall, array or train has multiple gas-liquid contactors 105. For example, and referring to FIGS. 11A and 11B, multiple gas-liquid contactors 105 are arranged next to one another to form a contactor wall 1502. The number of gas-liquid contactors 105 composing the contactor wall 1502 may vary (as represented by the ellipsis symbol “[.. .]” in FIG. 11A). The contactor wall 1502 may include a large number of gas-liquid contactors 105, for example between 10 and 100 gas-liquid contactors 105. In some implementations, the number of gas-liquid contactors 105 in the contactor wall 1502 is greater than 1,000. The number of gas-liquid contactors 105 in the contactor wall 1502 may be determined based on a variety of factors, such as a plume of CCh-lean gas 109 generated by the contactor wall 1502 during operation of the gas-liquid contactors 105. The contactor wall 1502 extends along its own wall axis 1509. In the implementation of the contactor wall 1502 of FIG. 11 A, the wall axis 1509 extends along a direction that is perpendicular to a depth of the packing in the cross-flow gas-liquid contactors 105, and perpendicular to the liquid travel dimensions of the gas-liquid contactors 105.

[0184] In implementations where the gas-liquid contactors 105 are positioned (e.g., directly) adjacent each other, and referring to FIG. 11 A, they may be abutted along a dividing wall 1525 which fluidly separates components of one gas-liquid contactor 105 from an adjacent gas-liquid contactor 105. The dividing wall 1525 helps to ensure that the CCh-laden air 101 flowing through an air inlet 1031 of a gas-liquid contactor 105 flows through the packing section(s) of that gas-liquid contactor 105, rather than into an adjacent gas-liquid contactor 105. The dividing walls 1525 extend in an upright or vertical direction, and along a direction parallel to a packing depth. In example implementations, the vertical extent of one or more of the dividing walls 1525 begins at, or below, the liquid level in a liquid collector underlying the packing. This configuration of the dividing walls 1525 can help to minimise or eliminate air bypassing the dividing walls 1525. A plenum 108 of each gas-liquid contactor 105 is separated from the plenum 108 of an adjacent gas-liquid contactor 105 by one or more dividing walls 1525. At least some of the dividing walls 1525 are internal to the contactor wall 1502. Each dividing wall 1525 forms a barrier to airflow between the adjacent plenums 108 delimited by that dividing wall 1525, so as to prevent air from flowing between the plenums 108. The dividing walls 1525 may allow for multiple gas-liquid contactors 105 of the contactor wall 502to remain operational if one of the gas-liquid contactors 105 or its fan is deactivated. The dividing walls 1525 of FIG. 11A are internal to the contactor wall 1502, and it will be appreciated that the contactor wall 1502 can have externally-applied dividing walls 1525 at opposite longitudinal ends of the contactor wall 1502. The plenums 108 are arranged adjacent each other along the length of the contactor wall 1502 defined along the wall axis 1509. In other implementations, the contactor wall 1502 includes a single plenum 108 that is continuous along its length defined parallel to the wall axis 1509, such that the contactor wall 1502 is free of internal dividing walls 1525. In other implementations, the contactor wall 1502 includes multiple plenums 108 delineated by the dividing walls 1525, where two or more gas-liquid contactors 105 of the contactor wall 1502 share a common plenum 108. In some implementations, the dividing walls 1525 include doors or closeable openings, to provide access to an interior of adjacent gas-liquid contactors 105. In example implementations, and referring to FIG. 11 A, the contactor wall 1502 includes multiple plenums 108, where each gasliquid contactor 105 forming the contactor wall 1502 has one plenum 108. Each plenum 108 is separated from an adjacent plenum 108 by one or more dividing walls 1525. In the example implementation of FIG. 11 A, each dividing wall 1525 shown is located between two fan stacks 107 and forms a barrier to airflow between two plenums 108 delimited by that dividing wall 1525, where each plenum 108 is in fluid communication with a respective one of the fan stacks 107.

[0185] The contactor wall 1502 can be part of the DAC system 100. Referring to FIG. 11B, each DAC system 100 can include multiple contactor walls 1502 arranged on a plot of land 1505. Each contactor wall 1502 is spaced apart from another contactor wall 1502. In this disclosure, the terms “train”, “array” and “wall” may be used interchangeably. The DAC system 100 of FIG. 11B is shown with multiple contactor walls 1502 for the purposes of illustration. The DAC system 100 can alternatively have only one contactor wall 1502. Referring to FIG. 1 IB, the DAC system 100 includes a capture solution processing area 1530, which can include one or both of the carbonate separation subsystem 162 and the capture solution regeneration subsystem 164, 264, 364, 464, 564, 664, 764 described above, in fluid communication with the contactor walls 1502. The capture solution processing area 1530 functions to regenerate the CCE-rich sorbent (e.g., the carbonate-rich solution 120) received from the contactor walls 1502, or from other componentry that treats the carbonate-richsolution 120 from the contactor walls 1502. The capture solution regeneration subsystem 164, 264, 364, 464, 564, 664, 764 of the capture solution processing area 1530 forms a regenerated sorbent (e.g., the regenerated CO2 capture solution 114) that is conveyed back to the contactor walls 1502. The capture solution regeneration subsystem 164, 264, 364, 464, 564, 664, 764 of the capture solution processing area 1530 can also function to release CO2 from the CCh-rich sorbent, to produce the CO2 product stream 136. In example implementations, and referring to FIG. 11B, each contactor wall 1502 has a single or common bottom basin. In such implementations, the bottom basin of each contactor wall 1502 is in fluid communication with the capture solution processing area 1530. In example implementations, the process streams from the bottom basin of a contactor wall 1502 flows, or is flowed, to the bottom basin of another contactor wall 1502.

[0186] Since the subsystems 102, 162, 164, 264, 364, 464, 564, 664, 764 can be easily decoupled from one another and are modular, the DAC system 100 has the benefit of scale flexibility. The subsystems 102, 162, 164, 264, 364, 464, 564, 664, 764 can be sized to accommodate capacities ranging from lab-scale to industrial or commercial scale. They are adaptable to various environmental conditions and low carbon intensity electricity generation, including intermittent sources (e.g., wind, solar).

[0187] The resulting CO2 product stream 136 as a feedstock for downstream products manufacturing can yield other carbon products that are inherently low in carbon emissions. Intermittent electricity sources, such as wind and solar energy, tend to fluctuate and are difficult to control. Intermittent electricity sources typically generate electricity only periodically. The regeneration subsystem 164, 264, 364, 464, 564, 664, 764 has a relatively fast ramp rate to allow for coupling with low carbon intensity electricity sources including intermittent electricity sources and non-intermittent electricity sources such as hydro, nuclear, and biomass.

[0188] The CO2 product stream 136 of the present disclosure can be sent to a processing system that is downstream of the regeneration subsystem 164, 264, 364, 464, 564, 664, 764. In example implementations, the CO2 product stream 136 can be pressurized in a downstream compression unit which can include a single stage or multi-stage gas compressor (e.g., piston compressor, reciprocating compressor). In some cases, the CO2 product stream 136 can be sent to a clean-up unit (e.g., purification unit) that removes at least a portion of residual water and / or other impurities. In some cases, the downstream compression unit can include arefrigeration system that liquefies the CO2 product stream 136 at low pressures, making it pumpable with a liquid pump.

[0189] In some cases, the downstream compression unit can compress the CO2 product stream 136 up to about 40 bar. Compressed CO2 can be delivered downhole and sequestered in a geological formation, subsurface reservoir, carbon sink, and the like. In certain downhole conditions, CO2 can mineralize into a solid product, such as calcium carbonate. In some instances, compressed CO2 can be used for enhanced oil recovery by injection into one or more wellbores to enhance production of hydrocarbons from a reservoir.

[0190] In example implementations, the CO2 product stream 136 can be fed to a downstream fuel synthesis system, which can include a syngas generation reactor. The syngas generation reactor can produce a syngas product stream by a reverse water gas shift reaction, a steam methane reforming reaction, a direct methane reforming reaction, or a combination thereof. The downstream fuel synthesis system can also include a Fischer-Tropsch reactor that can react syngas and hydrogen to produce hydrocarbon products such as fuel. In example implementations, the downstream fuel synthesis system can include electrochemical alternatives to Fischer-Tropsch reactors, such as electroreduction units or gas diffusion electrodes.

[0191] In example implementations, the CO2 product stream 136 can be sent to an electrolyzer cell that carries out one or more of the following reactions:

[0192] Reaction 9: CO2+ 2e' -> CO + O2'

[0193] Reaction 10: H2O + 2e H2+ O2

[0194] Reaction 11 : O2' ’ / 2O2+ 2e

[0195] The electrolyzer cell can form downstream products such as syngas, pure carbon monoxide, or pure hydrogen from feedstocks, such as the CO2 product stream 136, or water (or both). The electrolyzer cell can include a nickel-based catalyst, a silver-based catalyst, or a noble metal-based catalyst. In example implementations, the electrolyzer cell is a solid oxide electrolyzer cell. In example implementations, the electrolyzer cell can yield a syngas ratio suitable for downstream Fischer-Tropsch reactions that form value-added carbon products (e.g., short chain hydrocarbons, FT liquids, waxes, etc.). For example, the electrolyzer cell can yield a syngas ratio of 2.5 or higher.

[0196] Carbon products derived from the DAC system 100 are desirable as these products generally have low or zero net emissions on a lifecycle basis. The regeneration subsystem 164, 264, 364, 464, 564, 664, 764 can be modified or integrated with approaches for generating value-added carbon-based or carbon-including products, non-limiting examples of which include cement, plastics, and polymers, syngas and short chain hydrocarbons, in-situ without necessitating a gaseous CO2 feedstock for an electrochemical cell.

[0197] Any one of the subsystems 102, 162, 164, 264, 364, 464, 564, 664, 764 can include flowing a stream (e.g., carbonate-rich capture solution, crystalline carbonate hydrate, EC feed solution, product streams, etc.) to at least one auxiliary unit or auxiliary equipment, such as one or more buffer tanks, filtration systems, water treatment systems, holding tanks, mixing tanks, settlers, clarifiers, conveyors, or other units that facilitate the performance of the aforementioned functions of these subsystems 102, 162, 164, 264, 364, 464, 564, 664, 764. Additives such as simple salts (monovalent salts) can be included in one or more process streams. Simple salts such as sodium chloride can enhance performance by increasing conductivity and depressing the freezing point for the CO2 capture subsystem.

[0198] The water streams 119, 231 from the caustic evaporators 112, 214 can be partially or fully recycled in other units of the DAC system 100 that require water as an influent stream, such as in the EC unit 208, 308, 408, 508, 608, 708 or in the dissolving tanks 206, 306, 406, 506. For example, the caustic evaporator 112 can discharge the water stream 119 that can replace or combine with the water stream 223 flowing to the EC unit 208, 308, 408, 508, 608, 708. For example, the caustic evaporator 214 can discharge the water stream 231 as a condensate, and the water stream 231 can replace or combine with the water stream 223 flowing to the EC unit 208, 308, 408, 508, 608, 708. In some cases, the discharged water steam can require treatment (e.g., in a filtration system or a water treatment system) before flowing to the downstream unit. In example implementations, the caustic evaporators 112, 214 can each remove up to 20 m3of water per t-CCE.

[0199] In cases where additional water removal from the process is advantageous, water can be removed from the gas-liquid contactor 105 or crystallizer 104, or from any other units of the DAC system 100 (e.g., nanofiltration units, reverse osmosis units, auxiliary units), or a combination thereof to maintain water balance in the process. For example, water removal from the process can be advantageous for adjusting to seasonal or diurnal weather conditions,fresh water needs to operate the EC unit 208, 308, 408, 508, 608, 708, or a combination thereof. The water that is removed can be re-used elsewhere, stored for future use, or exported offsite.

[0200] One or more elements that flow or receive a water stream can flow or receive a water stream comprising a certain amount of suspended solids, dissolved solids, or impurities (or a combination thereof). For example, the EC unit 208, 308, 408, 508, 608, 708 can receive water streams 223 that each comprise varying amounts of suspended solids, dissolved solids, impurities, or combinations thereof. In some cases, the EC unit 208, 308, 408, 508, 608, 708 can receive processed water streams 223. For example, processed water can include demineralized, distilled, filtered, purified, or treated water.

[0201] The water stream 223 can be fed to the EC unit 208, 308, 408, 508, 608, 708 to produce the first EC product stream 229, 329, 429 and the second EC product stream 227, 338, 427, 527. Water can be evaporated from one or more streams to maintain water balance. For example, water can be evaporated from the carbonate-rich capture solution 120. Water can be evaporated using an MVR evaporator, multi-effect evaporator multi-effect evaporator, or membrane filtration unit (e.g., ultrafiltration, nanofiltration, reverse osmosis, and the like). Water can also be evaporated through the air capture unit, with evaporation being dependent at least in part on the ambient environmental conditions.

[0202] In each of the subsystems 102, 162, 164, 264, 364, 464, 564, 664, 764, while a wide range of hydroxide concentrations in the CO2 capture solution 144 can absorb some amount of CO2 from dilute gas sources, the most effective compositions can be tuned for improving capture efficiency and accommodating certain operating environments. For example, the CO2 capture solution 144 can include a mixture comprising KOH concentrations ranging from 2.5 M to 5 M, NaOH concentrations ranging from 1 M to 2.5 M, potassium carbonate K2CO3 concentrations ranging form 0.1 M to 0.7 M, and sodium carbonate Na2COs concentrations ranging from 0.05 M to 0.2 M.

[0203] The subsystems 102, 162, 164, 264, 364, 464, 564, 664, 764 and methods 700 of the present disclosure are modularly scalable and can therefore include a different number of elements and subsystems than the implementations illustrated in FIG. 1 through FIG. 10. For example, the CO2 capture subsystem 102 can include multiple gas-liquid contactors 105; the regeneration subsystem 164, 264, 364, 464, 564, 664, 764 can include multiple EC units 208, 308, 408, 508, 608, 708; and combinations of both, which are fluidly coupled to the carbonateseparation subsystem 162. For example, the DAC system 100 can include one or more caustic evaporators 112 in the carbonate separation subsystem 162 for each gas-liquid contactor 105, or each EC unit 208, 308, 408, 508, 608, 708. Combinations of multiple gas-liquid contactor 105, multiple carbonate separation subsystems 162, and multiple regeneration subsystems 164, 264, 364, 464, 564, 664, 764 and their respective elements can necessitate a distribution system that includes one or more trains that fluidly couple the systems and elements.

[0204] Reference is made to US patent application number 17 / 735,943 entitled “Systems and methods for capturing carbon dioxide and regenerating a capture solution”, the entire contents of which are incorporated herein by reference.

[0205] FIG. 10 is a schematic diagram of a control system (or controller) 800 for a system, such as the DAC system 100 and / or any one of its components or units. The system 800 can be used for the operations described in association with any of the computer-implemented methods described herein, for example as or as part of the control system 999 or other controllers described herein.

[0206] The control system 800 is intended to include various forms of digital computers, such as laptops, desktops, workstations, personal digital assistants, servers, blade servers, mainframes, and other appropriate computers. The system 800 can also include mobile devices, such as personal digital assistants, cellular telephones, smartphones, and other similar computing devices. Additionally, the system can include portable storage media, such as, Universal Serial Bus (USB) flash drives. For example, the USB flash drives may store operating systems and other applications. The USB flash drives can include input / output components, such as a wireless transmitter or USB connector that may be inserted into a USB port of another computing device.

[0207] The control system 800 includes a processor 810, a memory 820, a storage device 830, and an input / output device 840. Each of the components 810, 820, 830, and 840 are interconnected using a system bus 850. The processor 810 is capable of processing instructions for execution within the control system 800. The processor 810 may be designed using any of a number of architectures. For example, the processor 810 may be a CISC (Complex Instruction Set Computers) processor, a RISC (Reduced Instruction Set Computer) processor, or a MISC (Minimal Instruction Set Computer) processor.

[0208] In one implementation, the processor 810 is a single-threaded processor. In example implementations, the processor 810 is a multi -threaded processor. The processor 810 is capable of processing instructions stored in the memory 820 or on the storage device 830 to display graphical information for a user interface on the input / output device 840.

[0209] The memory 820 stores information within the control system 800. In one implementation, the memory 820 is a computer-readable medium. In one implementation, the memory 820 is a volatile memory unit. In example implementations, the memory 820 is a nonvolatile memory unit.

[0210] The storage device 830 is capable of providing mass storage for the control system 800. In one implementation, the storage device 830 is a computer-readable medium. In various different implementations, the storage device 830 may be a floppy disk device, a hard disk device, an optical disk device, or a tape device.

[0211] The input / output device 840 provides input / output operations for the control system 800. In one implementation, the input / output device 840 includes a keyboard and / or pointing device. In example implementations, the input / output device 840 includes a display unit for displaying graphical user interfaces.

[0212] In example implementations, the processor 810 is configured to execute a machine learning model (e.g., an artificial intelligence model) that employs multiple layers of models to generate an output for a received input. A deep neural network is a deep machine learning model that includes an output layer and one or more hidden layers that each apply a non-linear transformation to a received input to generate an output. In some cases, the neural network may be a recurrent neural network. A recurrent neural network is a neural network that receives an input sequence and generates an output sequence from the input sequence. In particular, a recurrent neural network uses some or all of the internal state of the network after processing a previous input in the input sequence to generate an output from the current input in the input sequence. The machine learning model executed by the processor 810 can be, for example, a deep-learning neural network or a "very" deep learning neural network. For example, the machine learning model executed by the processor 810 can be a convolutional neural network or a recurrent network. The machine learning model can have residual connections or dense connections.

[0213] In example implementations, the machine learning model executed by the processor 810 is an ensemble of models that may include all or a subset of the architectures described above.

[0214] In example implementations, the machine learning model executed by the processor 810 is a graph neural network (GNN). GNNs are a designed to process data that can be represented in a graph form and feature pairwise message passing to enable iterative updating of node representation of the graph data.

[0215] In example implementations, the machine learning model executed by the processor 810 can be a feedforward auto-encoder neural network. For example, the machine learning model executed by the processor 810 can be a three-layer auto-encoder neural network. The machine learning model executed by the processor 810 may include an input layer, a hidden layer, and an output layer. In example implementations, the neural network has no recurrent connections between layers. Each layer of the neural network may be fully connected to the next, e.g., there may be no pruning between the layers. The neural network may include an optimizer for training the network and computing updated layer weights. In example implementations, the neural network may apply a mathematical transformation, e.g., a convolutional transformation or factor analysis to input data prior to feeding the input data to the network.

[0216] In example implementations, the machine learning model executed by the processor 810 can be a supervised model. For example, for each input provided to the model during training, the machine learning model can be instructed as to what the correct output should be. The machine learning model executed by the processor 810 can use batch training, e.g., training on a subset of examples before each adjustment, instead of the entire available set of examples. This may improve the efficiency of training the model and may improve the generalizability of the model. In example implementations, the machine learning model executed by the processor 810 may be an unsupervised model. For example, the model may adjust itself based on mathematical distances between examples rather than based on feedback on its performance. In example implementations, the machine learning model executed by the processor 810 can provide suggested additional data that could further improve the output of the machine learning model.

[0217] Certain features described can be implemented in digital electronic circuitry, or in computer hardware, firmware, software, or in combinations of them. The apparatus can be implemented in a computer program product tangibly embodied in an information carrier, e.g., in a machine-readable storage device for execution by a programmable processor; and method steps can be performed by a programmable processor executing a program of instructions to perform functions of the described implementations by operating on input data and generating output. The described features can be implemented advantageously in one or more computer programs that are executable on a programmable system including at least one programmable processor coupled to receive data and instructions from, and to transmit data and instructions to, a data storage system, at least one input device, and at least one output device. A computer program is a set of instructions that can be used, directly or indirectly, in a computer to perform a certain activity or bring about a certain result. A computer program can be written in any form of programming language, including compiled or interpreted languages, and it can be deployed in any form, including as a stand-alone program or as a module, component, subroutine, or other unit suitable for use in a computing environment.

[0218] Suitable processors for the execution of a program of instructions include, by way of example, both general and special purpose microprocessors, and the sole processor or one of multiple processors of any kind of computer. Generally, a processor will receive instructions and data from a read-only memory or a random access memory or both. The essential elements of a computer are a processor for executing instructions and one or more memories for storing instructions and data. Generally, a computer will also include, or be operatively coupled to communicate with, one or more mass storage devices for storing data files; such devices include magnetic disks, such as internal hard disks and removable disks; magneto-optical disks; and optical disks. Storage devices suitable for tangibly embodying computer program instructions and data include all forms of non-volatile memory, including by way of example semiconductor memory devices, such as EPROM, EEPROM, and flash memory devices; magnetic disks such as internal hard disks and removable disks; magnetooptical disks; and CD-ROM and DVD-ROM disks. The processor and the memory can be supplemented by, or incorporated in, ASICs (application-specific integrated circuits).

[0219] To provide for interaction with a user, the features can be implemented on a computer having a display device such as a CRT (cathode ray tube) or LCD (liquid crystaldisplay) monitor for displaying information to the user and a keyboard and a pointing device such as a mouse or a trackball by which the user can provide input to the computer. Additionally, such activities can be implemented via touchscreen flat-panel displays and other appropriate mechanisms.

[0220] The features can be implemented in a control system that includes a back-end component, such as a data server, or that includes a middleware component, such as an application server or an Internet server, or that includes a front-end component, such as a client computer having a graphical user interface or an Internet browser, or any combination of them. The components of the system can be connected by any form or medium of digital data communication such as a communication network. Examples of communication networks include a local area network (“LAN”), a wide area network (“WAN”), peer-to-peer networks (having ad-hoc or static members), grid computing infrastructures, and the Internet.

[0221] The term “couple” and variants of it such as “coupled”, “couples”, and “coupling” as used in this description is intended to include indirect and direct connections unless otherwise indicated. For example, if a first device is coupled to a second device, that coupling may be through a direct connection or through an indirect connection via other devices and connections. Similarly, if the first device is fluidly coupled to the second device, flow may be through a direct connection or through an indirect connection via other devices and connections. In particular, a fluid coupling means that a direct or indirect pathway is provided for a fluid to flow between two fluidly coupled devices.

[0222] Terms used to describe acts or results regarding one or more of the process streams or elements of the aforementioned systems and methods (e.g., “flow”, “form”, “return”, “receive”, “produce”, “release”, “employ”, “apply”, “provide”, “dissolve”, and their respective gerunds) as used in this description are intended to include partial and complete acts or results. For example, flowing a solution to a subsystem can include flowing at least a portion or the entirety of the solution to the subsystem. For example, a subsystem that receives a solution can include a subsystem that receives a portion of the solution or the entirety of the solution.

[0223] A number of implementations of the disclosure have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the disclosure. Accordingly, other implementations are within the scope of the following claims. Further modifications and alternative implementations of various aspectswill be apparent to those skilled in the art in view of this description. Accordingly, this description is to be construed as illustrative only. It is to be understood that the forms shown and described herein are to be taken as examples of implementations. Elements and materials may be substituted for those illustrated and described herein, parts and processes may be reversed, and certain features may be utilized independently, all as would be apparent to one skilled in the art after having the benefit of this description. Changes may be made in the elements described herein without departing from the spirit and scope as described in the following claims.

Claims

WHAT IS CLAIMED IS:

1. A method comprising: contacting carbon dioxide (CO2) from a dilute gas source with a CO2 capture solution comprising at least a first hydroxide (MOH) and a second hydroxide (YOH), to form a carbonate-rich solution comprising at least a first carbonate (M2CO3) and a second carbonate (Y2CO3), M and Y being alkali metals; separating at least a portion of the M2CO3 and the Y2CO3 from the carbonate-rich solution to form carbonate solids, the carbonate solids comprising solid M2CO3 and solid Y2CO3; dissolving the solid M2CO3 and the solid Y2CO3, and forming a dissolved inorganic carbon (DIC) species and an electrochemical (EC) feed stream; in an EC unit comprising a porous solid electrolyte (PSE) delimited by a first cation exchange membrane and a second cation exchange membrane, a proton-generating compartment between an anode and the first cation exchange membrane, and an alkaline regeneration compartment between a cathode and the second cation exchange membrane: flowing the EC feed stream to the PSE; and flowing a water stream to at least the alkaline regeneration compartment; applying an electric potential to the EC unit to form at least two EC product streams comprising a first EC product stream comprising hydrogen, regenerated MOH and regenerated YOH and a second EC product stream in the PSE; and flowing the first EC product stream comprising the regenerated MOH and the regenerated YOH to use in the contacting the carbon dioxide.

2. The method of claim 1, wherein: flowing the water stream comprises flowing a deionized water stream to the alkaline regeneration compartment and to the proton-generating compartment; and applying the electric potential to the EC unit comprises: reducing water at the cathode and forming the first EC product stream; and oxidizing water at the anode and forming a third EC product stream comprising oxygen.

3. The method of claim 2, comprising separating the oxygen from the third EC product stream.

4. The method of any one of claims 1 to 3, comprising separating the hydrogen from the first EC product stream before the flowing the first EC product stream.

5. The method of claim 1, wherein: flowing the water stream comprises flowing a deionized water stream only to the alkaline regeneration compartment; and applying the electric potential to the EC unit comprises reducing water at the cathode and forming the first EC product stream.

6. The method of claim 5, comprising: separating the hydrogen from the first EC product stream and forming a hydrogen feed stream; flowing at least some of the hydrogen feed stream to the proton-generating compartment; and oxidizing the at least some of the hydrogen feed stream at the anode to generate protons for the PSE.

7. The method of claim 6, wherein: the anode comprises a gas diffusion electrode; and flowing the at least some of the hydrogen feed stream comprises flowing the at least some of the hydrogen feed stream through the gas diffusion electrode.

8. The method of any one of claims 1 to 7, comprising flowing at least part of the second EC product stream for mixing with the DIC species to form the EC feed stream.

9. The method of claim 8, wherein the second EC product stream comprises carbonic acid, the method comprising: recovering at least a portion of a carbon dioxide gas stream from the at least part of the second EC product stream and forming a brine stream, wherein flowing the at least part of the second EC product stream for mixing comprises flowing the brine stream for mixing with the DIC species to form the EC feed stream.

10. The method of claim 9, wherein recovering the at least a portion of the carbon dioxide gas stream from the at least part of the second EC product stream comprises: flowing the at least part of the second EC product stream to a vessel; and decreasing a pressure in the vessel to form the at least a portion of the carbon dioxide gas stream.

11. The method of claim 8, wherein: the second EC product stream comprises a proton-shuttling species; and flowing the at least part of the second EC product stream for mixing with the DIC species comprises reacting the proton-shuttling species with the DIC species to form carbonic acid and the EC feed stream.

12. The method of claim 8, wherein the second EC product stream comprises carbon dioxide gas and carbonic acid, the method comprising: recovering at least a portion of the carbon dioxide gas from the PSE and forming a first carbon dioxide product stream; and recovering a second carbon dioxide product stream from the carbonic acid and forming a brine stream, wherein flowing the at least part of the second EC product stream for mixing comprises flowing the brine stream for mixing with the DIC species to form the EC feed stream.

13. The method of any one of claims 1 to 7, wherein the second EC product stream comprises carbon dioxide gas, the method comprising: recovering at least a portion of the carbon dioxide gas from the PSE and forming a carbon dioxide product stream.

14. The method of one any one of claims 1 to 13, wherein M is potassium (K), and Y is sodium (Na).

15. The method of any one of claims 1 to 14, wherein separating the at least a portion of the M2CO3 and the Y2CO3 comprises crystallizing the at least a portion of the M2CO3 and the Y2CO3 to form crystalline M2CO3 and crystalline Y2CO3.

16. The method of any one of claims 1 to 15, comprising removing ion species from the EC feed stream.

17. The method of any one of claims 1 to 3, wherein flowing the water stream comprises flowing a hydroxide stream to at least the alkaline regeneration compartment.

18. The method of claim 17, comprising: separating the hydrogen from the first EC product stream; and subsequently, flowing the hydroxide stream from some of the first EC product stream to at least the alkaline regeneration compartment.

19. An electrochemical (EC) unit for regenerating a CO2 capture solution, the EC unit comprising: a proton-generating compartment between an anode and a first cation exchange membrane, the proton-generating compartment comprising a proton feed inlet configured to receive a hydrogen-containing feed stream; an alkaline regeneration compartment between a cathode and a second cation exchange membrane, the alkaline regeneration compartment comprising: a water feed inlet configured to receive a water feed stream; and an alkaline regeneration compartment outlet; a porous solid electrolyte (PSE) delimited by the first cation exchange membrane and the second cation exchange membrane, the PSE comprising: a PSE inlet configured to receive an EC feed stream comprising at least a first salt of M and a second salt of Y, M and Y being alkali metals; and a PSE outlet; and a circuit that extends between the anode and the cathode and configured to carry an electric current to decompose at least the water feed stream and the EC feed stream and form: a first EC product stream in the alkaline regeneration compartment being flowable through the alkaline regeneration compartment outlet, the first EC product stream comprising a regenerated first hydroxide (MOH) and a regenerated second hydroxide (YOH); and a second EC product stream in the PSE being flowable through the PSE outlet.

20. The EC unit of claim 19, wherein the proton-generating compartment comprises a gaseous products outlet.

21. The EC unit of claim 19, wherein the proton-generating compartment is in fluid communication with the alkaline regeneration compartment, the hydrogen-containing feed stream comprising hydrogen gas from the first EC product stream.

22. The EC unit of claim 21, wherein the anode comprises a gas diffusion electrode.

23. A system for capturing carbon dioxide (CO2) from a dilute gas source, the system comprising: a CO2 capture subsystem configured to generate a carbonate-rich solution comprising at least a first carbonate (M2CO3) and a second carbonate (Y2CO3), M and Y being alkali metals; a carbonate separation subsystem fluidly coupled to the CO2 capture subsystem and operable to receive the carbonate-rich solution, the carbonate separation subsystem configured to separate carbonate solids from the carbonate-rich solution, the carbonate solids comprising solid M2CO3 and solid Y2CO3; and. a regeneration subsystem fluidly coupled to the carbonate separation subsystem, the regeneration subsystem comprising: a dissolving tank fluidly coupled to the carbonate separation subsystem, the dissolving tank configured to dissolve at least a portion of the solid M2CO3 and of the solid Y2CO3, and form an electrochemical (EC) feed stream comprising at least a first salt of M and a second salt of Y; and at least one EC unit fluidly coupled to the dissolving tank, the at least one EC unit comprising: a proton-generating compartment between an anode and a first cation exchange membrane, the proton-generating compartment comprising a proton feed inlet configured to receive a hydrogen-containing feed stream; an alkaline regeneration compartment between a cathode and a second cation exchange membrane, the alkaline regeneration compartment comprising: a water feed inlet configured to receive a water feed stream; and an alkaline regeneration compartment outlet; a porous solid electrolyte (PSE) delimited by the first cation exchange membrane and the second cation exchange membrane, the PSE comprising: a PSE inlet configured to receive the EC feed stream; a PSE outlet; and a circuit extending between the anode and the cathode and configured to carry an electric current to decompose at least the water feed stream and the EC feed stream and form: a first EC product stream in the alkaline regeneration compartment64being flowable through the alkaline regeneration compartment outlet, the first EC product stream comprising a regenerated first hydroxide (MOH), a regenerated second hydroxide (YOH) and hydrogen; and a second EC product stream in the PSE being flowable through the PSE outlet.

24. The system of claim 23, wherein: the proton-generating compartment comprises a proton compartment outlet; the hydrogen-containing feed stream is configured to comprise a proton compartment water feed stream; and the circuit is configured to carry the electric current to decompose the proton compartment water feed stream and form oxygen.

25. The system of claim 24, comprising an oxygen degassing vessel in fluid communication with the proton compartment outlet.

26. The system of any one of claims 23 to 25, wherein the CO? capture subsystem is in fluid communication with the alkaline regeneration compartment outlet.

27. The system of claim 26, comprising a hydrogen degassing vessel downstream of the alkaline regeneration compartment outlet and upstream of the CO? capture subsystem, the hydrogen degassing vessel configured to separate the hydrogen from the first EC product stream.

28. The system of claim 23, wherein: the alkaline regeneration compartment outlet is in fluid communication with the protongenerating compartment; and the hydrogen-containing feed stream is configured to comprise at least some of the hydrogen of the first EC product stream.

29. The system of claim 28, comprising a hydrogen degassing vessel downstream of the alkaline regeneration compartment outlet and upstream of the proton-generating compartment, the hydrogen degassing vessel configured to separate the hydrogen from the first EC product stream.

30. The system of any one of claims 23 to 29, wherein the dissolving tank is in fluid communication with the PSE outlet.

31. The system of claim 30, comprising a CO2 degassing vessel downstream of the PSE outlet and upstream of the dissolving tank, wherein: the circuit is configured to carry the electric current to decompose the EC feed stream and form the second EC product stream comprising carbonic acid; and the CO2 degassing vessel is configured to separate carbon dioxide gas from the second EC product stream and form a brine stream.

32. The system of claim 31, wherein the dissolving tank is configured to react the brine stream with the dissolved at least a portion of the solid M2CO3 and of the solid Y2CO3 and form the EC feed stream.

33. The system of claim 30, wherein: the circuit is configured to carry the electric current to decompose the EC feed stream and form the second EC product stream comprising a proton-shuttling species; and the dissolving tank is configured to: react the proton-shuttling species with the dissolved at least a portion of the solid M2CO3 and of the solid Y2CO3 and form carbonic acid and the EC feed stream; and separate carbon dioxide gas from the carbonic acid.

34. The system of claim 30, comprising a CO2 degassing vessel downstream of the PSE outlet and upstream of the dissolving tank, wherein: the circuit is configured to carry the electric current to decompose the EC feed stream and form the second EC product stream comprising carbon dioxide gas and carbonic acid; the PSE is configured to flow at least a portion of the carbon dioxide gas from the PSE outlet as a first carbon dioxide gas stream; the CO2 degassing vessel is configured to separate a second carbon dioxide gas stream from the carbonic acid of the second EC product stream and form a brine stream; and the dissolving tank is configured to react the brine stream with the dissolved at least a portion of the solid M2CO3 and of the solid Y2CO3 and form the EC feed stream.

35. The system of any one of claims 23 to 29, wherein: the circuit is configured to carry the electric current to decompose the EC feed stream and form the second EC product stream comprising carbon dioxide gas; and the PSE is configured to flow at least a portion of the carbon dioxide gas from the PSE outlet as a carbon dioxide product stream.

36. The system of one any one of claims 23 to 35, wherein M is potassium (K), and Y is sodium (Na).

37. The system of one any one of claims 23 to 36, wherein: the carbonate separation subsystem comprises a crystallizer operable to crystallize the carbonate-rich solution and form a crystalline M2CO3 and a crystalline Y2CO3; and the dissolving tank is configured to receive at least some of the crystalline M2CO3 and at least some of the crystalline Y2CO3 and dissolve the at least some of the crystalline M2CO3 and the at least some crystalline Y2CO3 and form the EC feed stream.

38. The system of claim 37, wherein the crystallizer comprises at least one of: a chiller crystallizer, an evaporative crystallizer, a eutectic freeze crystallizer, a cooling crystallizer, or a membrane distillation crystallizer.

39. The system of one any one of claims 23 to 38, comprising at least one of: a compression unit, a fuel synthesis system, a syngas generation reactor, or an electrolyzer cell.

40. The system of one any one of claims 23 to 39, wherein the CO? capture subsystem comprises at least one of a: gas-liquid contactor, air contactor, spray tower, liquid-gas scrubber, venturi scrubber, packed tower, single cell air contactor, dual cell air contactor, or multi cell air contactor.

41. A method comprising: contacting carbon dioxide from a dilute gas source with a capture solution comprising a hydroxide (MOH) to form a carbonate-rich capture solution comprising a carbonate (M2CO3), where M is an alkali metal; crystallizing at least a portion of the M2CO3 to form a crystalline M2CO3; dissolving the crystalline M2CO3 to form an electrochemical (EC) feed stream comprising a salt of M; and in an EC unit comprising a porous solid electrolyte (PSE) delimited by a first cation exchange membrane and a second cation exchange membrane, a proton-generating compartment between an anode and the first cation exchange membrane, and an alkaline regeneration compartment between a cathode and the second cation exchange membrane: flowing the EC feed stream to the PSE; flowing a water stream to at least the alkaline regeneration compartment; applying an electric potential to the EC unit to form at least two EC product streams comprising a first EC product stream comprising regenerated MOH and a second EC product stream generated by the PSE; and flowing the first EC product stream comprising the regenerated MOH to use in the contacting the carbon dioxide.

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