Process for removing carbon dioxide from a gas stream

Abstract

The present invention provides a method for removing carbon dioxide from a gas stream containing carbon dioxide, the method comprising the steps of contacting the gas stream with an aqueous absorbing liquid comprising: (i) a non-volatile amine absorbent; (ii) a non-carbonate hygroscopic metal salt in an amount of at least 10% by weight of the aqueous absorbing liquid; and (iii) water, thereby absorbing carbon dioxide from the gas stream into the aqueous absorbing liquid to produce a carbon dioxide-lean gas and a carbon dioxide-enriched absorbent composition.

Description

[Technical Field]

[0001] This application claims priority to Australian Provisional Patent Application No. 2022903484, filed on 18 November 2022, the contents of which are considered to be incorporated herein by this reference.

[0002] The present invention relates to a method for removing carbon dioxide from a gas stream containing carbon dioxide using an aqueous absorption liquid comprising a non-volatile amine absorbent and a non-carbonate based hygroscopic metal salt. The present invention further relates to an aqueous absorption liquid for carbon dioxide capture and a system for removing carbon dioxide from a gas stream containing carbon dioxide. [Background technology]

[0003] Carbon dioxide (CO2) emissions are considered the primary cause of the greenhouse effect and global warming. In the Paris Agreement, the United Nations set targets for allowable temperature increases, which require significant reductions in greenhouse gas emissions. One way to reduce atmospheric CO2 emissions is to capture CO2 from CO2-rich flue gases produced by industries such as power plants, steel mills, cement kilns, calciners, biogas plants, natural gas processing, methane reforming, and smelters, followed by underground storage. An advanced technology for such applications uses an amine-based aqueous absorbent to absorb CO2 from the gas stream at low temperatures in an absorber and then release the CO2 at high temperatures in a desorber. The CO2-lean absorbent is then recycled to the absorber, while the concentrated CO2 product is liquefied by compression and cooling and injected into an underground storage reservoir. Various amines and alkanolamines have been investigated as reactive absorbents in liquid-based absorbents, with monoethanolamine being the most commonly considered reference material in industrial applications.

[0004] Such approaches remain important tools for mitigating CO2 emissions, particularly from industries that are difficult to decarbonize, as the world strives to achieve a net-zero emissions future. However, the 2021 Intergovernmental Panel on Climate Change report indicated that large-scale removal of CO2 from the atmosphere will also be required to avoid excessive increases in global temperatures compared to pre-industrial levels. Therefore, the development of scalable negative emissions technologies, including efficient capture of CO2 from the air (direct air capture; DAC), is attracting significant interest.

[0005] Amine-based aqueous absorbents can also be used in DAC applications. One challenge that arises here is the loss of amine through evaporation when large amounts of air come into contact with the absorbent in an open environment. Amine loss can be limited by adding a separate water wash section to reabsorb the amine from the CO2-lean air, but this adds capital and operating costs to the process.

[0006] An additional problem with the use of amine-based aqueous absorbents in DAC applications is water loss from the process. Air typically has a relative humidity of less than 100%. Therefore, water evaporates from the absorbent into the air during the CO2 absorption process. Water loss depends on the ambient temperature and relative humidity and varies throughout the day and season. At 25°C, water loss due to evaporation typically ranges from 0 kg / kg CO2 removal (100% relative humidity) to over 10 kg / kg CO2 removal (relative humidity below 30%). Therefore, operation of a DAC system requires a significant water supply. In arid regions, water may not be available or may be available only at a high cost. Water loss can also lead to operational problems, as the solution becomes more concentrated, increasing the risk of precipitate formation, which can lead to blockages and reduced performance. The same problem does not typically occur in post-combustion applications where the inlet gas is saturated with water. It would be desirable to develop a DAC process that can mitigate or avoid water loss, or even maintain a positive water balance, by simultaneously absorbing water and CO2 from the air.

[0007] While the foregoing discussion relates specifically to DAC processes, it will be appreciated that similar considerations apply to other applications where CO2 is absorbed from a gas stream that is not saturated with water, such as environmental control in spacecraft, mining shelters, or submarines. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] U.S. Patent No. 9,409,122 [Non-patent literature]

[0009] [Non-Patent Document 1] Winston et al., Ecology, 1960, 41, pp. 232-237 [Non-patent document 2] Rockland, Anal. Chem. 1960, 32, 0, pp. 1375-1376. [Non-patent document 3] Chiao-Chien Wei, Graeme Puxty, Paul Feron, 2014, Amino acid salts for CO2 capture at flue gas temperatures, Chemical Engineering Science 107, pp. 218-226 [Non-patent document 4] Kiani et al., (Techno-Economic Assessment for CO2 Capture From Air Using a Conventional Liquid-Based Absorption Process. Frontiers in Energy Research, 2020, 8) Summary of the Invention [Problem to be solved by the invention]

[0010] Thus, there is a continuing need for methods for removing carbon dioxide from gas streams that at least partially address one or more of the above-mentioned shortcomings or provide a useful alternative. [Means for solving the problem]

[0011] Reference herein to a patent document or other matter granted as prior art is not an admission that the document or matter was publicly known or that the information it contains was part of the common general knowledge as of the priority date of any claim.

[0012] The inventors have discovered that water loss from amine-based aqueous absorption solutions in DAC and other CO2 capture processes can be mitigated or canceled by including hygroscopic metal salts dissolved in the absorption solution. It has been found that a neutral water balance, or even simultaneous CO2 and water capture, can be achieved using absorbents in which the concentration of hygroscopic metal salts is well below the solubility limit. Thus, nonvolatile amine absorbents can be dissolved in solution at substantially useful concentrations, and the amine absorbent and its reaction products with CO2 remain soluble throughout the capture process. The CO2 mass transfer rate during the absorption step remains within an acceptable range, despite the presence of significant amounts of hygroscopic metal salts and the resulting increase in viscosity of the absorption solution.

[0013] Thus, the present invention provides a method for removing carbon dioxide from a gas stream containing carbon dioxide, the method comprising contacting the gas stream with an aqueous absorption liquid comprising an amine absorbent, a hygroscopic metal salt, and water, whereby carbon dioxide is absorbed from the gas stream into the aqueous absorption liquid to produce a carbon dioxide-lean gas and a carbon dioxide-enriched absorbent composition.

[0014] The amine absorbent may be a non-volatile amine absorbent, thus avoiding significant loss of amine absorbent from the aqueous absorption liquid to the gas feed stream.

[0015] The hygroscopic metal salt may be present in an amount sufficient to significantly reduce or completely prevent water loss to the gas stream during the CO2 absorption process, for example, at least 10% by weight of the aqueous absorption liquid.

[0016] The hygroscopic metal salt may be a non-carbonate hygroscopic metal salt.

[0017] The hygroscopic metal salt may have the property that air in equilibrium with a saturated aqueous solution of the hygroscopic metal salt and water has a relative humidity at 30°C of less than 30%.

[0018] In a first set of embodiments, the method includes contacting a gas stream with an aqueous absorbing solution comprising (i) a non-volatile amine absorbent, (ii) a non-carbonate hygroscopic metal salt in an amount of at least 10% by weight of the aqueous absorbing solution, and (iii) water, thereby absorbing carbon dioxide from the gas stream into the aqueous absorbing solution to produce a carbon dioxide-lean gas and a carbon dioxide-enriched absorbent composition.

[0019] In some embodiments, the aqueous absorbing liquid comprises a non-carbonate hygroscopic metal salt in an amount sufficient to provide zero or negative net water desorption from the aqueous absorbing liquid into the gas stream.

[0020] In some embodiments, the gas stream further comprises water vapor. The gas stream may have a relative humidity of less than 80%, or less than 60%, for example less than 40%.

[0021] In some embodiments, water is absorbed from the gas stream into an aqueous absorption liquid.

[0022] In some embodiments, the non-carbonate hygroscopic metal salt is present in an amount of at least 15%, or at least 20%, such as at least 25%, by weight of the aqueous absorption liquid.

[0023] In some embodiments, the aqueous absorbent composition has the property that air in equilibrium with the aqueous absorbent composition has a relative humidity at 30°C of less than 80%, or less than 60%, for example less than 40%.

[0024] In some embodiments, the aqueous absorbent composition has the property that air in equilibrium with the aqueous absorbent composition, when separated from the carbon dioxide-rich absorbent composition, has a relative humidity of less than 80%, or less than 60%, e.g., less than 40%, at the temperature of the carbon dioxide-lean gas.

[0025] In some embodiments, the non-carbonated hygroscopic metal salt has the property that air in equilibrium with a saturated aqueous solution of the non-carbonated hygroscopic metal salt and water has a relative humidity at 30° C. of less than 60%, or less than 50%, or less than 40%, e.g., less than 30%.

[0026] In some embodiments, the non-carbonate hygroscopic metal salt comprises a cation selected from the group consisting of alkali metals, alkaline earth metals, and nickel. The cation may be selected from the group consisting of lithium, sodium, potassium, calcium, nickel, and magnesium.

[0027] In some embodiments, the non-carbonate hygroscopic metal salt comprises an anion selected from the group consisting of a halide ion, a C1-C6 alkyl or aryl carboxylate ion, a nitrate ion, and a thiocyanate ion.

[0028] In some embodiments, the non-carbonate hygroscopic metal salt is selected from the group consisting of lithium chloride, lithium bromide, lithium iodide, lithium nitrate, lithium thiocyanate, sodium bromide, sodium iodide, sodium thiocyanate, potassium fluoride, potassium formate, potassium acetate, potassium nitrite, potassium thiocyanate, magnesium chloride, magnesium bromide, magnesium iodide, magnesium nitrate, magnesium thiocyanate, calcium bromide, calcium iodide, calcium acetate, calcium nitrate, calcium thiocyanate, strontium iodide, strontium thiocyanate, barium iodide, chromium chloride, manganese chloride, manganese bromide, iron bromide, cobalt bromide, cobalt nitrate, nickel chloride, nickel bromide, copper nitrate, zinc chloride, zinc bromide, zinc iodide, zinc nitrate, cerium chloride, and combinations thereof.

[0029] In some embodiments, the non-carbonate hygroscopic metal salt is selected from the group consisting of lithium chloride, lithium bromide, lithium iodide, lithium thiocyanate, potassium fluoride, potassium formate, potassium acetate, potassium thiocyanate, magnesium chloride, magnesium bromide, magnesium iodide, calcium bromide, calcium iodide, calcium acetate, calcium thiocyanate, nickel bromide, zinc chloride, zinc bromide, zinc iodide, and combinations thereof.

[0030] In some embodiments, the non-carbonate hygroscopic metal salt is selected from the group consisting of lithium chloride, lithium bromide, potassium formate, and potassium acetate.

[0031] In some embodiments, the non-volatile amine absorbent is selected from the group consisting of an amino acid or a salt thereof, a polyamine containing both quaternized and neutral amine groups, a high molecular weight amine, and combinations thereof. In some embodiments, the non-volatile amine absorbent is an amino acid or a salt thereof.

[0032] In some embodiments, the amino acid is selected from the group consisting of taurine, sarcosine, alanine, glycine, lysine, dimethylglycine, proline, phenyl-alanine, glucosamine, arginine, methyl-taurine, cysteine, tryptophan, hydroxyproline, asparagine, tyrosine, histidine, glutamine, diglycine, serine, methionine, and combinations thereof.

[0033] In some embodiments, the non-volatile amine absorbent is present in the aqueous absorption liquid in an amount between 0.1 mol / L and 6 mol / L, for example, between 0.5 mol / L and 3 mol / L.

[0034] In some embodiments, water is present in the aqueous absorption liquid in an amount of at least 30% by weight.

[0035] In some embodiments, the aqueous absorption liquid further comprises (iv) a base, which may be selected from hydroxides, carbonates, phosphates, additional amines having a higher pKa than the non-volatile amine absorbent, and combinations thereof.

[0036] In some embodiments, the gas stream is air, which may be selected from outside air, air from an enclosed environment, and ventilation air.

[0037] In some embodiments, the gas stream is contacted with the aqueous absorption liquid at a temperature between -5°C and 35°C.

[0038] In some embodiments, the method further comprises removing carbon dioxide from the carbon dioxide-rich absorbent composition to produce a carbon dioxide-lean absorbent composition, and recycling the carbon dioxide-lean absorbent composition to the aqueous absorption liquid. Carbon dioxide can be removed from the carbon dioxide-rich absorbent composition by heating the carbon dioxide-rich absorbent composition to desorb the carbon dioxide.

[0039] In a second set of embodiments, the method includes contacting a gas stream with an aqueous absorbing solution comprising (i) a non-volatile amine absorbent, (ii) a hygroscopic metal salt in an amount of at least 10% by weight of the aqueous absorbing solution, and (iii) water, thereby absorbing carbon dioxide from the gas stream into the aqueous absorbing solution to produce a carbon dioxide-lean gas and a carbon dioxide-enriched absorbent composition, wherein the hygroscopic metal salt has the property that air in equilibrium with a saturated aqueous solution of the hygroscopic metal salt and water has a relative humidity of less than 30% at 30°C.

[0040] Methods according to the second set of embodiments may generally have the features as disclosed above in the context of the first set of embodiments.

[0041] The present invention also provides an aqueous absorption liquid for carbon dioxide capture, the aqueous absorption liquid comprising an amine absorbent, a hygroscopic metal salt, and water.

[0042] The amine absorbent may be a non-volatile amine absorbent, thereby avoiding significant loss of amine absorbent from the aqueous absorption liquid used.

[0043] The hygroscopic metal salt may be present in an amount sufficient to significantly reduce or completely prevent water loss from the aqueous absorption solution used. The hygroscopic metal salt may be present in an amount of a minimum of 10% by weight of the aqueous absorption solution.

[0044] The hygroscopic metal salt may be a non-carbonate hygroscopic metal salt.

[0045] The hygroscopic metal salt may have the property that air in equilibrium with a saturated aqueous solution of the hygroscopic metal salt and water has a relative humidity at 30°C of less than 30%.

[0046] In a first set of embodiments, an aqueous absorption liquid for carbon dioxide capture includes (i) a non-volatile amine absorbent, (ii) a non-carbonate hygroscopic metal salt in an amount of at least 10% by weight of the aqueous absorption liquid, and (iii) water.

[0047] In some embodiments, the non-carbonate hygroscopic metal salt is present in an amount of at least 15%, or at least 20%, such as at least 25%, by weight of the aqueous absorption liquid.

[0048] In some embodiments, the aqueous absorbent composition has the property that air in equilibrium with the aqueous absorbent composition has a relative humidity at 30°C of less than 80%, or less than 60%, for example less than 40%.

[0049] In some embodiments, the non-carbonated hygroscopic metal salt has the property that air in equilibrium with a saturated aqueous solution of the non-carbonated hygroscopic metal salt and water has a relative humidity at 30° C. of less than 60%, or less than 50%, or less than 40%, e.g., less than 30%.

[0050] In some embodiments, the non-carbonate hygroscopic metal salt comprises a cation selected from the group consisting of alkali metals, alkaline earth metals, and nickel. The cation may be selected from the group consisting of lithium, sodium, potassium, calcium, nickel, and magnesium.

[0051] In some embodiments, the non-carbonate hygroscopic metal salt comprises an anion selected from the group consisting of a halide ion, a C1-C6 alkyl or aryl carboxylate ion, a nitrate ion, and a thiocyanate ion.

[0052] In some embodiments, the non-carbonate hygroscopic metal salt is selected from the group consisting of lithium chloride, lithium bromide, lithium iodide, lithium nitrate, lithium thiocyanate, sodium bromide, sodium iodide, sodium thiocyanate, potassium fluoride, potassium formate, potassium acetate, potassium nitrite, potassium thiocyanate, magnesium chloride, magnesium bromide, magnesium iodide, magnesium nitrate, magnesium thiocyanate, calcium bromide, calcium iodide, calcium acetate, calcium nitrate, calcium thiocyanate, strontium iodide, strontium thiocyanate, barium iodide, chromium chloride, manganese chloride, manganese bromide, iron bromide, cobalt bromide, cobalt nitrate, nickel chloride, nickel bromide, copper nitrate, zinc chloride, zinc bromide, zinc iodide, zinc nitrate, cerium chloride, and combinations thereof.

[0053] In some embodiments, the non-carbonate hygroscopic metal salt is selected from the group consisting of lithium chloride, lithium bromide, lithium iodide, lithium thiocyanate, potassium fluoride, potassium formate, potassium acetate, potassium thiocyanate, magnesium chloride, magnesium bromide, magnesium iodide, calcium bromide, calcium iodide, calcium acetate, calcium thiocyanate, nickel bromide, zinc chloride, zinc bromide, zinc iodide, and combinations thereof.

[0054] In some embodiments, the non-carbonate hygroscopic metal salt is selected from the group consisting of lithium chloride, lithium bromide, potassium formate, and potassium acetate.

[0055] In some embodiments, the non-volatile amine absorbent is selected from the group consisting of an amino acid or a salt thereof, a polyamine containing both quaternized and neutral amine groups, a high molecular weight amine, and combinations thereof. In some embodiments, the non-volatile amine absorbent is an amino acid or a salt thereof.

[0056] In some embodiments, the amino acid is selected from the group consisting of taurine, sarcosine, alanine, glycine, lysine, dimethylglycine, proline, phenyl-alanine, glucosamine, arginine, methyl-taurine, cysteine, tryptophan, hydroxyproline, asparagine, tyrosine, histidine, glutamine, diglycine, serine, methionine, and combinations thereof.

[0057] In some embodiments, the non-volatile amine absorbent is present in the aqueous absorption liquid in an amount between 0.1 mol / L and 6 mol / L, for example, between 0.5 mol / L and 3 mol / L.

[0058] In some embodiments, water is present in the aqueous absorption liquid in an amount of at least 30% by weight.

[0059] In some embodiments, the aqueous absorption liquid further comprises (iv) a base, which may be selected from hydroxides, carbonates, phosphates, additional amines having a higher pKa than the non-volatile amine absorbent, and combinations thereof.

[0060] In some embodiments, the aqueous absorbing liquid further comprises absorbed carbon dioxide in a ratio (mol / mol) of carbon dioxide to non-volatile amine absorbent of at least 0.05, such as at least 0.1.

[0061] In a second set of embodiments, an aqueous absorption liquid for carbon dioxide capture comprises (i) a non-volatile amine absorbent, (ii) a hygroscopic metal salt in an amount of at least 10% by weight of the aqueous absorption liquid, and (iii) water, wherein the hygroscopic metal salt has the property that air in equilibrium with a saturated aqueous solution of the hygroscopic metal salt and water has a relative humidity of less than 30% at 30°C.

[0062] The aqueous absorption liquid according to the second set of embodiments may generally have the characteristics as disclosed above in the context of the first set of embodiments.

[0063] The present invention also provides a system for removing carbon dioxide from a gas stream containing carbon dioxide, the system comprising: an aqueous absorption liquid according to any embodiment disclosed herein; an absorption unit for contacting the aqueous absorption liquid with a gas stream, thereby absorbing carbon dioxide from the gas stream into the aqueous absorption liquid to produce a carbon dioxide-lean gas and a carbon dioxide-rich absorbent composition; and a regeneration unit for removing carbon dioxide from the carbon dioxide-rich absorbent composition, thereby producing a carbon dioxide-lean absorbent composition for recycle to the aqueous absorption liquid.

[0064] In some embodiments, the system is a system for direct air recovery technology. In some embodiments, the gas stream is selected from outside air, air from an enclosed environment, and ventilation air.

[0065] When the terms "comprise", "comprises", and "comprising" are used in the specification (including the claims), these terms are to be interpreted as specifying stated features, integers, steps or components, but not excluding the presence of one or more other features, integers, steps or components or groups thereof.

[0066] Further aspects of the present invention are set forth in the detailed description of the invention below.

[0067] Embodiments of the present invention will now be illustrated, by way of example only, with reference to the accompanying drawings in which: [Brief explanation of the drawings]

[0068] [Figure 1] 1 illustrates a schematic representation of a system for removing carbon dioxide from a gas stream according to some embodiments of the present invention. [Figure 2] 1 is a graph showing the mass transfer coefficient of CO2 into an aqueous solution containing 2 mol / L of taurate and / or potassium carboxylate hygroscopic salt, as measured in Example 2. [Figure 3] 1 is a graph showing the mass transfer coefficient of CO2 to an aqueous solution containing 0.5 mol / L taurinate, or 0.5 mol / L taurinate and a lithium halide hygroscopic salt, as measured in Example 2. [Figure 4] 1 is a graph showing the temperature-dependent viscosity of an aqueous solution containing a taurinate, or a taurinate and a hygroscopic metal salt, measured in Example 2. [Figure 5] 1 is a schematic representation of the apparatus used in Example 3 for measuring water vapor pressure above a hygroscopic salt solution. [Figure 6] 1 is a graph showing the relative humidity (ratio of water vapor pressure above the solution to the water vapor pressure above pure water) of various aqueous solutions of hygroscopic metal salts as a function of salt concentration and a comparison with the predictions of Raoult's Law, as determined in Example 3. [Figure 7] 1 is a graph showing the loss or uptake of CO2 and water recovered by the liquid absorption liquid under direct air capture technology conditions when a 2 M taurate solution in 44 wt % potassium formate solution is used as the liquid absorption liquid and synthetic air of different humidity is used as the gas feed stream, as measured in Example 4. [Figure 8] 10 is a schematic representation of an absorption unit including parameters used to model water loss or uptake in a direct air capture technology process under different climatic conditions in Example 5. [Figure 9]FIG. 1 is a graph showing the predicted water loss or uptake of a direct air capture technology process using an amino acid-based absorbent with (System 2) or without (System 1) the addition of hygroscopic metal salts, modeled for three different climate conditions in Example 5. DETAILED DESCRIPTION OF THE INVENTION

[0069] The present invention relates to a method for removing carbon dioxide from a gas stream containing carbon dioxide. The method comprises contacting the gas stream with an aqueous absorption liquid comprising (i) an amine absorbent, (ii) a hygroscopic metal salt, and (iii) water. Carbon dioxide is thus absorbed from the gas stream into the aqueous absorption liquid to produce a carbon dioxide-lean gas and a carbon dioxide-rich absorbent composition. In a preferred embodiment, the method further comprises removing carbon dioxide from the carbon dioxide-rich absorbent composition to produce a carbon dioxide-lean absorbent composition, and recycling the carbon dioxide-lean absorbent composition to the aqueous absorption liquid. The aqueous absorption liquid is thus repeatedly cycled between the carbon dioxide absorption step and the desorption process step.

[0070] The amine absorbent may be a non-volatile amine absorbent, thus avoiding significant loss of amine absorbent from the aqueous absorption liquid to the gas feed stream.

[0071] The hygroscopic metal salt may be present in an amount sufficient to significantly reduce or completely prevent water loss to the gas stream during the CO2 absorption process, for example, at least 10% by weight of the aqueous absorption liquid.

[0072] The hygroscopic metal salt may be a non-carbonate hygroscopic metal salt.

[0073] The hygroscopic metal salt may have the property that air in equilibrium with a saturated aqueous solution of the hygroscopic metal salt and water has a relative humidity at 30°C of less than 30%.

[0074] Gas flow The gas stream can in principle be any gas stream that contains carbon dioxide and is amenable to treatment with an aqueous amine absorbent to remove a portion of the carbon dioxide, hi some embodiments, the carbon dioxide may be present in an amount less than 10% by weight, for example, less than 1% by weight.

[0075] The gas stream may be air, and thus may contain dinitrogen and dioxygen as the primary components. Typically, carbon dioxide in air may be present in amounts between 350 ppm and 5000 ppm. The short-term exposure limit is equal to 30,000 ppm, and concentrations above 40,000 ppm pose an immediate danger to life or health; breathable air must be controlled so that carbon dioxide levels are well below these limits. For example, the method may be useful for controlling carbon dioxide levels in enclosed environments where carbon dioxide may accumulate or reach undesirable levels, such as vehicles (e.g., spacecraft, submarines), mine shelters, ventilation air, or other locations. Alternatively, the air may be atmospheric air, e.g., for direct air capture technology applications, where the expected carbon dioxide content is between 400 and 450 ppm.

[0076] The gas stream may contain water vapor. In principle, water can be lost from a conventional aqueous absorption solution to the gas being treated, even if the gas stream is initially saturated or nearly saturated with water (i.e., 100% relative humidity). The gas can be heated during contact with the absorbent, thereby increasing its ability to absorb water from the absorption solution. However, the risk and extent of unacceptable water loss is greater when the initial moisture content of the gas stream is low. Thus, in some embodiments, the gas stream has a relative humidity of less than 80%, or less than 60%, less than 50%, less than 40%, e.g., less than 30%, at the temperature of the gas stream fed to the process. In DAC applications, for example, the methods disclosed herein are believed to be particularly useful for locations where the ambient air typically has a humidity of less than 80%, or less than 60%, less than 50%, less than 40%, e.g., less than 30%, at dry-bulb temperature.

[0077] Aqueous absorption liquid The aqueous absorbing liquid may include a non-volatile amine absorbent. The methods disclosed herein are of particular interest in CO2 removal applications where loss of volatile components from the aqueous absorbing liquid to the gas feed stream is a concern. Therefore, a non-volatile amine is required to avoid unacceptable consumption of the amine absorbent in the process.

[0078] As used herein, a nonvolatile amine absorbent refers to a compound containing at least one amine group that is susceptible to chemical reaction with CO in aqueous solution and has negligible vapor pressure under CO absorption conditions. For example, a nonvolatile amine absorbent, as a pure compound, may have a vapor pressure of less than 0.1 Pa at 25° C. Suitable amine absorbents are typically nonvolatile because they are ionic under absorption conditions or because they have a sufficiently high molecular weight.

[0079] In some embodiments, the non-volatile amine absorbent is completely dissolved in the aqueous absorption liquid during the absorption process, although it is not excluded that the non-volatile amine absorbent may partially precipitate during or after absorption.

[0080] In some embodiments, the non-volatile amine absorbent comprises at least one primary or secondary amine group, hi some embodiments, the non-volatile amine absorbent is selected from the group consisting of an amino acid or a salt thereof, a polyamine containing both quaternized and neutral amine groups, a high molecular weight amine, and combinations thereof.

[0081] Suitable polyamines containing both quaternized and neutral amine groups may be polyamine compounds containing: (i) basic amine groups, such as secondary or tertiary amines, that exist in quaternized form (due to protonation) in solution within the expected pH range during absorption, and (ii) less basic amine groups that are at least partially neutral in solution within the expected pH range during absorption and are therefore available for reaction with CO. For example, one non-limiting example of a polyamine compound is 1,4-pentanediamine. Other examples of diamines that are expected to be quaternized under CO absorption conditions are disclosed in U.S. Pat. No. 9,409,122. Optionally, the polyamine compound may be quaternized by adding an acid (e.g., hydrochloric acid or sulfuric acid) to the solution prior to use in carbon dioxide absorption, preferably in an amount sufficient to completely quaternize the basic amine groups. Suitable high molecular weight amines, as pure compounds, have a vapor pressure of less than 0.1 Pa at 25°C.

[0082] Amino acid absorbents are of particular interest. Amino acids generally exist as ionic species in aqueous solutions in the pH range typical for CO2 capture due to ionization of the acidic functional groups, and therefore have low volatility. Furthermore, many simple amino acids offer good CO2 absorption rates and cycling capacities. The anionic forms of many amino acids react rapidly with CO2 in solution to form carbamates, which can then partially hydrolyze to bicarbonate anions. This is shown for the case of taurine in Scheme 1.

[0083] [ka]

[0084] Suitable amino acids may include taurine, sarcosine, alanine, glycine, lysine, dimethylglycine, proline, phenylalanine, glucosamine, arginine, methyl-taurine, cysteine, tryptophan, hydroxyproline, asparagine, tyrosine, histidine, glutamine, diglycine, serine, methionine, etc. The amino acids may be provided as amino acid salts, e.g., potassium or sodium salts, in the aqueous absorption solution.

[0085] The nonvolatile amine absorbent, e.g., an amino acid, can be present in the aqueous absorption solution in any amount sufficient to recover CO2 while preferably avoiding precipitation of the amine species in the present process. In some embodiments, the nonvolatile amine absorbent is present in an amount of at least 0.1 mol / L, or at least 0.3 mol / L, or between 0.1 mol / L and 6 mol / L, e.g., between 0.5 mol / L and 3 mol / L, e.g., between 1.5 mol / L and 2.5 mol / L. The maximum amine concentration may be limited by the presence of a non-carbonate hygroscopic metal salt, e.g., when using a lithium halide hygroscopic salt. However, the inventors have found that an aqueous absorption solution containing a hygroscopic lithium halide salt can still dissolve amino acids at concentrations sufficient to absorb substantially useful amounts of CO2, e.g., about 0.5 mol / L or even higher, while avoiding water loss. Furthermore, other suitable hygroscopic salts, e.g., potassium carboxylate salts, have been found not to impose any practical limits on the solubility of the amino acid absorbent.

[0086] The aqueous absorbing liquid contains a hygroscopic metal salt, preferably a non-carbonate hygroscopic metal salt, in an amount sufficient to significantly reduce or completely prevent water loss to the gas stream during the CO absorption process. Thus, the aqueous absorbing liquid contains a hygroscopic metal salt, preferably a non-carbonate hygroscopic metal salt, in an amount of at least 10% by weight of the aqueous absorbing liquid, optionally at least 15% by weight, or at least 20% by weight, e.g., at least 25% by weight of the aqueous absorbing liquid. As used herein, the concentration of a component of the aqueous absorbing liquid, whether expressed in moles / L or weight percent, refers to the concentration in the aqueous absorbing liquid in the absence of absorbed CO or excluding the contribution of CO. It will be recognized that the maximum amount of the hygroscopic metal salt may be limited by its aqueous solubility and the requirement for the aqueous absorbing liquid to solubilize the non-volatile amine absorbent and its reaction product with CO. Typically, the hygroscopic metal salt is present in pure water at less than its saturation concentration.

[0087] The concentration of the hygroscopic metal salt, preferably a non-carbonate hygroscopic metal salt, may be selected to impart the desired hygroscopicity to the aqueous absorption solution. While it may be desirable to completely prevent water loss from the aqueous absorption solution, this is not always necessary. Indeed, the benefits of reduced water loss may be weighed against other considerations, including the effect of the hygroscopic metal salt on CO2 capture properties and solution viscosity. Thus, in some implementations, even an optimized process will accept some level of water loss, although less than in the absence of the hygroscopic metal salt.

[0088] However, in some embodiments, the aqueous absorbing liquid contains the hygroscopic metal salt in a sufficient amount such that when carbon dioxide is absorbed from a gas stream into the aqueous absorbing liquid, there is zero or negative net water desorption from the aqueous absorbing liquid into the gas stream. Thus, the ratio of water to hygroscopic metal salt in the carbon dioxide-rich absorbent composition is equal to or greater than the ratio in the aqueous absorbing liquid.

[0089] In some implementations, it may be desirable for the aqueous absorbing solution to have approximately zero net water desorption into the gas stream. In other words, the aqueous absorbing solution neither loses nor picks up significant amounts of water as it is used. The hygroscopic metal salt may be present in an amount appropriate to achieve this goal.

[0090] In practice, however, it may not be possible to continuously achieve a perfect continuous water balance, particularly when the relative humidity and / or temperature of the gas stream fed to the process fluctuates (as would be expected for a DAC process). The absorbent composition and the process more generally can be designed to accommodate such fluctuations. For example, the amount of hygroscopic metal salt may be selected so that during some operating periods there is net water desorption from the aqueous absorption solution to the gas stream (e.g., when the temperature is higher or the gas humidity is lower) and during other operating periods there is net water absorption from the gas stream to the aqueous absorption solution (e.g., when the temperature is lower or the gas humidity is higher). In a DAC process, such operating periods may correspond to different parts of the circadian cycle (e.g., day and night). Alternatively, the amount of hygroscopic metal salt may be selected so that net water desorption from the aqueous absorption solution to the gas stream approaches zero but remains positive or negative under all anticipated operating conditions. The composition of the aqueous absorption solution can then be restored, either continuously or intermittently, by controlled addition or removal of water from the solution.

[0091] In some implementations, it may be desirable for the aqueous absorbing liquid to have a negative net water desorption into the gas stream. In other words, the aqueous absorbing liquid absorbs both carbon dioxide and water from the gas stream. This can be useful in a variety of applications, such as air quality control in enclosed areas where both humidity and carbon dioxide need to be controlled, or in applications such as natural gas processing where simultaneous removal of both water and carbon dioxide from a gas stream is desirable. The hygroscopic metal salt may be present in the aqueous absorbing liquid in an amount appropriate to achieve this goal.

[0092] The concentration of hygroscopic salt required to achieve a desired net water desorption (still positive but reduced, near zero, or negative) from the aqueous absorbing liquid into the gas stream can be determined by conventional engineering principles, given the benefit of this disclosure. In particular, it will be recognized that net water desorption from the absorbing liquid will be zero or near zero when the relative humidity of the gas stream (e.g., air) matches the relative humidity of the vapor phase (e.g., air) in equilibrium with the aqueous absorbing liquid. Thus, by measuring the relationship of the relative humidity of a representative gas in equilibrium with an amine-containing aqueous absorbing liquid as a function of its hygroscopic salt concentration, the appropriate concentration of hygroscopic salt can be determined for any particular implementation.

[0093] In some embodiments, the aqueous absorbent composition has a composition, particularly a concentration of hygroscopic salts, such that air in equilibrium with the aqueous absorbent composition at 30°C (and 1 bar pressure) has a relative humidity of less than 80%, or less than 60%, or less than 50%, or less than 40%. In some embodiments, the aqueous absorbent composition has a composition, particularly a concentration of hygroscopic salts, such that air in equilibrium with the aqueous absorbent composition, when separated from the carbon dioxide-rich absorbent composition, has a relative humidity of less than 80%, or less than 60%, or less than 50%, or less than 40% at the expected temperature (and pressure) of the carbon dioxide-lean gas. Such relative humidity values ​​can be routinely measured under controlled laboratory conditions, for example, using the method disclosed in Example 3.

[0094] A hygroscopic metal salt has the property that, when dissolved in an aqueous solution, it reduces the partial pressure of water in the gas phase in equilibrium with the solution significantly below the value predicted by Raoult's law. In principle, any metal salt having this property can be used in the aqueous absorbent composition, provided that the solution is capable of supporting the nonvolatile amine absorbent, absorbing carbon dioxide, and generally still being cycled repeatedly between the absorption and desorption steps without excessive degradation. A wide range of hygroscopic metal salts have been used in saturated solutions to control water content in gases, as disclosed, for example, in Winston et al., Ecology, 1960, 41, pp. 232-237 and Rockland, Anal. Chem. 1960, 32, 10, pp. 1375-1376. Such metal salts can generally be expected to be suitable for the methods disclosed herein.

[0095] The hygroscopic metal salt may be a non-carbonate hygroscopic metal salt. As used herein, a non-carbonate hygroscopic metal salt refers to a hygroscopic metal salt that is not a metal carbonate. Concentrated metal carbonate solutions are used in various CO2 capture applications, such as the Benfield process, in which carbonates absorb CO2 through an acid-base chemical reaction (e.g., K2CO3 + CO2 + HO ⇔ 2KHCO3). A drawback of this approach is the slow reaction rate, which makes carbonate solutions ineffective in applications such as DAC. In contrast, the method of the present invention uses a hygroscopic metal salt to suppress water loss from the absorbent and employs a non-volatile amine absorbent as the primary CO2 absorbent compound. Non-carbonate hygroscopic metal salts suitable for the method disclosed by the present invention typically contribute little or nothing to the CO2 absorption capacity of the aqueous absorption solution.

[0096] In the methods disclosed herein, non-carbonated hygroscopic metal salts are typically present at concentrations significantly below their solubility limit in water (corresponding to the concentration of a saturated solution). However, the properties of saturated metal salt solutions can be conveniently used to characterize the suitability of a component metal salt for use in the methods disclosed herein. Thus, in some embodiments, the non-carbonated hygroscopic metal salt has the property that air in equilibrium with a saturated aqueous solution consisting of (i.e., containing only) the non-carbonated hygroscopic metal salt and water has a relative humidity of less than 60%, or less than 50%, or less than 40%, e.g., less than 30%, at 30°C (and 1 bar pressure). For example, air in equilibrium with saturated lithium chloride and potassium acetate solutions has been reported to have relative humidities of 11.5% and 22%, respectively. Again, this can be routinely measured under controlled laboratory conditions.

[0097] The non-carbonate hygroscopic metal salt may comprise a single metal salt or multiple metal salts. In some embodiments, the non-carbonate hygroscopic metal salt consists essentially of a single metal salt. For example, one metal cation accounts for at least 90 mole percent of its cationic content and one anion accounts for at least 90 mole percent of its anionic content. In some embodiments, the non-carbonate hygroscopic metal salt consists of a single metal salt.

[0098] The hygroscopic properties of metal salts typically depend on the interaction between the cation and anion. However, some common metal cations and anions are expected to provide suitable hygroscopic properties when paired with an appropriate counterion. In some embodiments, the non-carbonate hygroscopic metal salt comprises an alkali metal cation, an alkaline earth metal cation, or nickel, such as selected from lithium, sodium, potassium, calcium, nickel, and magnesium. In some embodiments, the non-carbonate hygroscopic metal salt comprises an anion selected from a halide ion, a C1-C6 alkyl or aryl carboxylate ion, a nitrate ion, and a thiocyanate ion.

[0099] In some embodiments, the non-carbonate hygroscopic metal salt is selected from the group consisting of lithium chloride, lithium bromide, lithium iodide, lithium nitrate, lithium thiocyanate, sodium bromide, sodium iodide, sodium thiocyanate, potassium fluoride, potassium formate, potassium acetate, potassium nitrite, potassium thiocyanate, magnesium chloride, magnesium bromide, magnesium iodide, magnesium nitrate, magnesium thiocyanate, calcium bromide, calcium iodide, calcium acetate, calcium nitrate, calcium thiocyanate, strontium iodide, strontium thiocyanate, barium iodide, chromium chloride, manganese chloride, manganese bromide, iron bromide, cobalt bromide, cobalt nitrate, nickel chloride, nickel bromide, copper nitrate, zinc chloride, zinc bromide, zinc iodide, zinc nitrate, and cerium chloride. The vapor phase (e.g., air) in equilibrium with a saturated aqueous solution of such salts is expected to have a relative humidity of less than about 60% at 30°C. Non-carbonate hygroscopic metal salts may also be combinations of such salts, provided that good hygroscopic properties are retained.

[0100] In some embodiments, the non-carbonate hygroscopic metal salt is selected from the group consisting of lithium chloride, lithium bromide, lithium iodide, lithium thiocyanate, potassium fluoride, potassium formate, potassium acetate, potassium thiocyanate, magnesium chloride, magnesium bromide, magnesium iodide, calcium bromide, calcium iodide, calcium acetate, calcium thiocyanate, nickel bromide, zinc chloride, zinc bromide, and zinc iodide. The vapor phase (e.g., air) in equilibrium with a saturated aqueous solution of such salts is expected to have a relative humidity of less than about 30% at 30°C. The non-carbonate hygroscopic metal salt can also be a combination of such salts, provided that good hygroscopic properties are maintained.

[0101] In some embodiments, the non-carbonate hygroscopic metal salt is selected from the group consisting of lithium chloride, lithium bromide, potassium formate, and potassium acetate. The inventors have demonstrated through experimentation and modeling that a neutral water balance can be maintained in a direct air capture technology process when using an aqueous absorption solution containing less than 35% by weight of such salts over a range of climatic conditions.

[0102] The aqueous absorbing liquid is preferably an alkaline solution, for example, the pH of the aqueous absorbing liquid may be between 8 and 12, such as between 9 and 11, before contacting it with the gas stream to absorb carbon dioxide.

[0103] In some embodiments, the aqueous absorbing solution further comprises a base. The base may be included to maintain the pH of the aqueous absorbing solution within an appropriate range during CO2 absorption. The reaction of CO2 with a primary or secondary amine to form a carbamate salt releases protons, which tend to result in a decrease in the solution pH. The protons can protonate a second non-volatile amine absorbent molecule (as seen in Scheme 1) or another base in the formulation. Including an additional base to accept the released protons can provide several advantages, including a greater availability of the amine for CO2 capture, rather than the non-volatile amine absorbent, ideally due to its higher pKa value. Furthermore, efficient proton recovery can mitigate or avoid undesirable interactions with non-carbonate hygroscopic metal salts. For example, protonation of formate or acetate anions can produce formic acid or acetic acid, with the attendant risk that such species may be lost from the aqueous absorbing solution to the gas stream.

[0104] In some embodiments, the base has a higher pKa than the non-volatile amine absorbent, thereby promoting preferential absorption of protons released during CO absorption by the base rather than the non-volatile amine absorbent. In some embodiments, the base has a higher pKa than the anion of a non-carbonate hygroscopic metal salt, e.g., formate or acetate, thereby promoting preferential absorption of protons released during CO absorption by the base rather than the anion.

[0105] The base may be selected from hydroxides, carbonates, phosphates, additional amines with a higher pKa than the non-volatile amine absorbent, such as tertiary amines or sterically hindered amines, which are compounds containing at least one primary or secondary amino group bonded to either a secondary or tertiary carbon atom, and combinations thereof. For example, the base may be an alkali metal carbonate (e.g., KCO). Preferably, the base is itself non-volatile, thereby avoiding its loss from the aqueous absorption liquid to the gas stream.

[0106] In some embodiments, the aqueous absorbing solution contains a base in an amount less than 10% by weight of the aqueous absorbing solution, preferably less than 5% by weight of the aqueous absorbing solution, e.g., between 1% and 5% by weight. The maximum molar concentration of base required is half the molar concentration of the non-volatile amine absorbent, assuming all of the non-volatile amine absorbent reacts with CO. In practice, CO recovery in DAC applications is limited to approximately 0.3 moles of CO per mole of amine, thereby requiring only 0.15 moles of base per mole of non-volatile amine absorbent. Depending on the molecular weight of the base, an addition of between 1 and 5% by weight is typically sufficient to absorb all protons released under recovery conditions by CO reaction with the non-volatile amine absorbent. In embodiments capable of protonating the anion of the non-carbonate hygroscopic metal salt, sufficient base is preferably added to maintain the pH above the range in which protonation of the anion occurs. This avoids or minimizes the formation of undesirable species, such as formic acid or acetic acid.

[0107] The aqueous absorption liquid comprises water, which may be present in an amount of at least 30% by weight, or at least 40% by weight, such as at least 50% by weight.

[0108] In some embodiments, the aqueous absorbing liquid comprises (i) a non-volatile amine absorbent, preferably an amino acid, in an amount between 1% and 50% by weight of the aqueous absorbing liquid, (ii) a non-carbonate hygroscopic metal salt in an amount between 10% and 60% by weight of the aqueous absorbing liquid, (iii) water in an amount between 30% and 80% by weight of the aqueous absorbing liquid, and optionally (iv) a base in an amount up to 10% by weight of the aqueous absorbing liquid. In some embodiments, the aqueous absorbing liquid comprises (i) an amino acid in an amount between 3% and 40% by weight of the aqueous absorbing liquid, (ii) a non-carbonate hygroscopic metal salt in an amount between 20% and 40% by weight of the aqueous absorbing liquid, (iii) water in an amount between 30% and 70% by weight of the aqueous absorbing liquid, and optionally (iv) a base in an amount up to 5% by weight of the aqueous absorbing liquid.

[0109] In some embodiments, the aqueous absorption liquid comprises absorbed carbon dioxide. Typically, the aqueous absorption liquid is cycled between the absorption step and the desorption step, and the carbon dioxide-lean absorbent composition produced in the desorption step is recycled to form at least a portion of the aqueous absorption liquid for contact with the gas stream. Because only a portion of the absorbed carbon dioxide content is desorbed in the desorption step, the aqueous absorption liquid will contain carbon dioxide even immediately prior to contact with the gas stream.

[0110] The viscosity of the aqueous absorbing solution is a relevant consideration in carbon dioxide capture processes, as excessive viscosity can unacceptably reduce the rates of CO absorption and desorption. However, the inventors have found that the increase in viscosity of the amine absorbent solution caused by functionally significant concentrations of non-carbonate hygroscopic metal salts is relatively minor, and the resulting impact on the CO absorption and desorption mass transfer coefficients is expected to be manageable in commercial-scale processes. In some embodiments, the aqueous absorbing solution has a viscosity at 30°C of less than 20 mPa s, or less than 10 mPa s, e.g., less than 5 mPa s. Viscosity can be measured by conventional analytical methods, including capillary flow viscometry and rotational rheometry. In some embodiments, viscosity is measured by rolling ball viscometry, for example, using a Lovis 2000 ME rolling ball viscometer.

[0111] Absorption of carbon dioxide from gas streams The method includes contacting the gas stream with an aqueous absorbing liquid, thereby absorbing carbon dioxide from the gas stream into the aqueous absorbing liquid to produce a carbon dioxide lean gas and a carbon dioxide rich absorbent composition.

[0112] In some embodiments, the gas stream is contacted with the aqueous absorption liquid at a temperature between -18°C and 50°C, e.g., between -5°C and 35°C or between 15°C and 30°C. In DAC and climate control applications, for example, the gas feed stream is typically air supplied for contact with the aqueous absorption liquid at room temperature conditions. In particular, the presence of a non-carbonate hygroscopic metal salt is expected to depress the freezing point of the absorption liquid, potentially enabling CO2 capture to occur at sub-zero temperatures. In some embodiments, the gas stream is contacted with the aqueous absorption liquid at a pressure of less than 5 bar, e.g., about 1 bar.

[0113] In some embodiments, the gas (air) stream is contacted with the aqueous absorption liquid in an open environment. Because the carbon dioxide-lean gas is thus released directly to the atmosphere, there is no opportunity to recover the volatile components lost from the aqueous absorption liquid. Thus, the method disclosed by this invention is particularly useful for avoiding the loss of water and amine from the process.

[0114] The rate of CO absorption into the aqueous absorbing liquid is an important consideration, especially in applications where the CO concentration in the gas stream is low (e.g., DAC). Therefore, the gas stream can be contacted with the aqueous absorbing liquid in a treatment unit that provides a high surface area gas-liquid interface or significant mixing of the gas and liquid to ensure high mass transfer. Improved mass transfer and high surface area gas / liquid involve a trade-off with energy consumption for fluid movement. For example, gas pressure drop should be minimized to avoid excessive energy consumption for moving gas through the contactor. With this in mind, various gas-liquid contactor designs have been proposed for DAC systems, such as spray towers, countercurrent packed-tower contactors, cross-flow liquid film contactors, and membrane contactors, and it is contemplated that the method disclosed herein can employ any such configuration. The use of a cooling tower system is believed to be particularly advantageous for CO capture from ambient air due to the low cost and mass production of the equipment. The high surface area required to obtain a substantially useful carbon dioxide uptake rate also makes the process prone to water loss, and thus the methods disclosed by this invention are useful in minimizing or avoiding this problem.

[0115] In some embodiments, the net desorption of water from the aqueous absorbing liquid into the gas stream is approximately zero or negative, in accordance with principles previously disclosed herein. In some embodiments, water is absorbed from the gas stream into the aqueous absorbing liquid (i.e., the net desorption of water from the aqueous absorbing liquid into the gas stream is negative).

[0116] The carbon dioxide-rich absorbent composition, after separation from the carbon dioxide-lean gas, may contain absorbed carbon dioxide in a ratio of carbon dioxide to non-volatile amine absorbent (mol / mol) of at least 0.05, e.g., at least 0.1.

[0117] Regeneration of aqueous absorption liquid After absorption of CO2 into the aqueous absorption liquid, the resulting carbon dioxide-rich absorbent composition is separated from the carbon dioxide-lean gas and is typically then sent for regeneration. Thus, the method can include removing carbon dioxide from the carbon dioxide-rich absorbent composition to produce a carbon dioxide-lean absorbent composition, and recycling the carbon dioxide-lean absorbent composition to the aqueous absorption liquid. In this manner, the aqueous absorption liquid can be repeatedly cycled between the absorption and desorption process steps.

[0118] Carbon dioxide can be removed from the carbon dioxide-rich absorbent composition by any suitable method, typically desorption. Desorption can be accomplished by heating the solution to a temperature between 80°C and 160°C, typically between 100°C and 125°C, when desorption is carried out at atmospheric pressure. Other methods of removing carbon dioxide are also contemplated, such as reducing the pressure, decreasing the solution pH, and crystallizing carbonates (e.g., by adding bis-iminoguanidine or other materials that form insoluble carbonates). In either case, removal of CO2 regenerates the non-volatile amine absorbent in the aqueous absorption solution (e.g., restoring the amino acid in its free amine form).

[0119] In embodiments where water is absorbed from the gas stream into an aqueous absorption liquid, the water can be removed along with the carbon dioxide in the desorption step, thus maintaining an overall water balance in the process.

[0120] The desorbed carbon dioxide, which may be present in a concentrated CO stream, can then be disposed of in a manner consistent with the overall process goals. For example, in a DAC process, the desorbed carbon dioxide can be liquefied by compression and cooling and injected into an underground reservoir for permanent sequestration. Alternatively, in applications where the goal is to control the carbon dioxide content (and possibly the water content) of the air in an enclosed space, the desorbed carbon dioxide stream may simply be vented outside the enclosed space.

[0121] System for removing carbon dioxide from a gas stream containing carbon dioxide - Patent Application 20070122997 The present invention further relates to a system for removing carbon dioxide from a gas stream containing carbon dioxide, the system including an aqueous absorbing liquid as disclosed herein, an absorption unit for contacting the aqueous absorbing liquid with a gas stream, thereby absorbing carbon dioxide from the gas stream into the aqueous absorbing liquid, to produce a carbon dioxide-lean gas and a carbon dioxide-rich absorbent composition, and a regeneration unit for removing carbon dioxide from the carbon dioxide-rich absorbent composition, thereby producing a carbon dioxide-lean absorbent composition for recycle to the aqueous absorbing liquid.

[0122] In some embodiments, the absorption unit comprises a gas-liquid contactor selected from a spray tower, a countercurrent packed tower contactor, a cross-flow liquid film contactor, or a membrane contactor. In a DAC process, the gas-liquid contactor of the absorption unit is preferably low-cost, and in this regard, the use of a standard or custom-designed cooling tower is advantageous. In some embodiments, the regeneration unit is a desorption unit for desorbing carbon dioxide from the carbon dioxide-rich absorbent composition, for example, by increasing the temperature and / or reducing the pressure. The desorption unit can be a countercurrent packed tower type that uses steam, which may be generated in a reboiler, as a stripping gas. The desorption unit can operate at subatmospheric pressure to reduce the temperature of the regeneration process. The desorption unit can also consist of one or more flash units in which the carbon dioxide-rich absorption liquid is heated and injected into a vessel at a lower pressure, releasing steam and CO2.

[0123] Illustrated in Figure 1 is a system 100 for removing carbon dioxide from a gas stream in accordance with some embodiments of the present invention. In particular, system 100 can be a system for direct air capture technology. System 100 includes an absorption unit 110 in the form of a gas-liquid contactor. Absorption unit 110 is configured to receive a gas feed stream 112 (air in a DAC process) and contact it with an aqueous amine absorbent 114 within a gas absorption contacting region 116. Carbon dioxide is thus absorbed into the aqueous amine absorbent 114, and the resulting carbon dioxide-lean gas 118 is separated from the carbon dioxide-rich absorbent 120 and released from absorption unit 110.

[0124] In some embodiments, the aqueous amine absorbent 114 is fed to the top of the absorption unit 110 and, under the influence of gravity, flows as a thin film over packing material located in the gas absorption contacting area 116. This provides a high surface area interface between the gas feed stream 112 and the aqueous amine absorbent 114, facilitating absorption of CO2 into the absorbent. The gas feed stream 112 can suitably flow upward in a countercurrent mode relative to the absorbent flow, or perpendicular to the absorbent flow in a cross-current design. In some embodiments, the gas absorption contacting area 116 is an open environment, thus operating at approximately atmospheric pressure (1 bar) and exposed to the atmosphere. Thus, air can be blown through the gas absorption contacting area 116 by a fan as the gas feed stream 112, contact the aqueous amine absorbent 114, and then vented from the absorption unit 110 to the atmosphere as a carbon dioxide-lean gas 118.

[0125] The aqueous amine absorbent 114 may have a composition according to any of the embodiments disclosed herein in the context of the method of the present invention, and thus includes a non-carbonate hygroscopic metal salt in an amount of at least 10% by weight. Advantageously, the hygroscopic metal salt reduces, and in some embodiments, cancels or even reverses, the loss of water from the aqueous absorption liquid 114 to the carbon dioxide lean gas 118 that would occur in the absence of the hygroscopic metal salt.

[0126] Optionally, the carbon dioxide-rich absorbent composition 120 can be recirculated around the absorption unit 110 via a recirculation loop 122 to increase the overall carbon dioxide uptake before regeneration. In this case, only a slip stream of the carbon dioxide-rich absorbent composition 120 is sent for regeneration via a desorber feed line 124. The advantage of this arrangement is that the recirculation helps cool the inlet temperature of the aqueous amine absorbent 114 so that its temperature during absorption approaches the wet-bulb temperature. Thus, the capacity of the air 118 to hold water is reduced.

[0127] System 100 includes a desorption unit 130 for desorbing carbon dioxide from carbon dioxide-rich absorbent composition 120. The resulting carbon dioxide-lean absorbent composition 132 is recycled via a desorber return line 134 to form part of the aqueous absorption liquid 114 supplied to absorption unit 110.

[0128] The desorption unit 130 can be configured to heat the carbon dioxide-rich absorbent composition 120, thereby driving the desorption of carbon dioxide in a temperature swing process. Preferably, the desorption unit 130 traps the absorbent, thereby allowing the desorbed carbon dioxide stream 136 to be directed to a desired location. In some embodiments, the carbon dioxide-rich absorbent composition 120 is fed to the top of the desorption unit 130 and flows under the influence of gravity on packing material located in the gas desorption zone 138. This provides a high surface area, which promotes the desorption of carbon dioxide from solution and, in some cases, water desorption, if necessary to balance the water content of the system. Steam 139 can be added to the desorption unit 139 as a stripping gas and heat source to drive the desorption of carbon dioxide. The carbon dioxide-lean absorbent composition 132 then exits the bottom of the desorption unit 130.

[0129] In the temperature swing process, both the carbon dioxide lean absorbent composition 132 flowing via the desorber return line 134 and the carbon dioxide rich absorbent composition 120 flowing via the desorber feed line 124 can be passed through a heat exchanger 140, which allows heat transfer from the carbon dioxide lean solution to the carbon dioxide rich solution, thereby improving energy efficiency.

[0130] Thus, during continuous operation, the aqueous absorption liquid 114 supplied to the absorption unit 110 comprises recycled carbon dioxide lean absorbent composition 132, optionally supplemented with absorbent recycled around the absorption unit 110 via recirculation loop 122 and, if necessary, makeup absorbent supplied via makeup line 142. [Example]

[0131] The present invention is described with reference to the following examples, which should be understood as illustrative of the invention described herein and not as limiting the invention.

[0132] Example 1. Solubility Test The solubility limits of different amino acid salts, i.e., equimolar mixtures of amino acids and potassium hydroxide in various hygroscopic salt solutions, were evaluated at room temperature. Amino acid salts and hygroscopic salts were purchased from vendors including Vosun Chemical Co. Aqueous solutions of each hygroscopic salt were first prepared at the concentrations specified in Table 1. A small amount of the amino acid salt was then added to the solution and mixed until dissolved. Additional amounts of the amino acid salt were added periodically until the amino acid salt no longer dissolved. At this point, the concentration of the amino acid salt was noted as the maximum amount of amino acid salt that could be dissolved in the solution as shown in Table 1.

[0133] [Table 1]

[0134] The solubility of amino acids in potassium carbonate solutions was very high. More limited solubility was found in lithium chloride and lithium bromide solutions, although concentrations below 0.5 mol / L are still useful for some CO2 absorption applications.

[0135] Example 2. Mass Transfer Performance A wetted-wall column (WWC) was used to evaluate CO2 absorption rates using a series of absorption liquid formulations. In this system, aqueous mixtures of an amino acid salt (potassium taurate) and different hygroscopic salts were tested at a temperature of 25 °C and a liquid flow rate of 125 mL / min, with gas rates varying from 3 L / min to 6 L / min. Mass transfer coefficients were determined based on the number of moles of CO2 absorbed in the experiment and the CO2 partial pressure difference between the gas and equilibrium conditions (called the driving force). This method is similar to the standard method described in the literature for determining CO2 mass transfer coefficients into various absorption liquids (Chiao-Chien Wei, Graeme Puxty, and Paul Feron, 2014, Amino acid salts for CO2 capture at flue gas temperatures, Chemical Engineering Science 107, pp. 218–226).

[0136] The CO2 total mass transfer coefficients for potassium carboxylate solutions are shown in Figure 2. The CO2 absorption rate was slightly reduced for the hygroscopic 2M taurinate solution (containing 44% by mass potassium formate or 42% by mass potassium acetate, produced by dissolving taurinate in a 58% by mass potassium formate or potassium acetate solution) compared with the 2M taurinate solution without hygroscopic salts. However, the CO2 absorption rate was still acceptably high and could potentially be increased by increasing the concentration of the amino acid salt, as can be seen from Example 1. The mass transfer coefficient for CO2 absorption into solution increased with gas velocity, indicating that the CO2 mass transfer resistance in the gas phase can also be important, although mass transfer resistance in the gas phase is generally less important in CO2 capture from flue gases.

[0137] The CO2 absorption rate in the hygroscopic 58 wt% potassium formate solution without any amine absorbent was found to be much lower than the solution containing the amino acid salts, thus demonstrating that the amino acids remain the primary CO2 absorbent compounds in the solution. The acetate and formate solutions are slightly alkaline and therefore absorb CO2 at a faster rate than water alone.

[0138] The CO2 total mass transfer coefficients into lithium halide salt solutions are shown in Figure 3. Due to solubility limitations (see Example 1), lower concentrations of amino acids (0.45-0.5 M) were used, such that the solutions contained 0.5 M taurinate and 40 wt% lithium bromide or 0.45 M taurinate and 28 wt% lithium chloride (generated by dissolving taurinate in 43 wt% LiBr or 30 wt% LiCl), resulting in correspondingly lower CO2 absorption rates.

[0139] The mass transfer rate of CO2 into hygroscopic absorbing solutions can be affected by increasing the viscosity of these solutions. Therefore, the viscosities of hygroscopic absorbing solutions (0.5 M taurinate in 28% LiCl or 2 M taurinate in 44 wt% potassium formate) and non-hygroscopic absorbing solutions (0.5 M and 2 M taurinate) were measured using a viscometer (Lovis 2000 ME microviscometer), and the results are shown in Figure 4. The viscosities of the absorbing solutions containing significant amounts of hygroscopic metal salts were higher than those of equivalent amine solutions without hygroscopic metal salts, but were still within acceptable limits. Both hygroscopic solutions had viscosities of less than 5 mPa.s at 30 °C. Furthermore, the increased viscosity observed for the absorbents containing potassium formate had only a limited effect on the CO2 absorption rate, as can be seen in Figure 2.

[0140] Example 3. Water Vapor Liquid Equilibrium The apparatus for measuring water vapor pressure over a hygroscopic salt solution is shown schematically in FIG. 5. For each experiment, solution 302 was placed in a sealed jar 304, and a pump 306 circulated air 308 from above the solution, through a humidity sensor 310, and back into the jar, where it was blown through the solution. Humidity values ​​were electronically recorded on a humidity recording device 312. The entire system, including the piping and relief valve 314, was placed in an oven 316 to maintain the target temperature. After starting the pump, the system was allowed to equilibrate and reach steady-state conditions. Steady state was considered to have been achieved when the measured humidity data showed no more than 1% variation over a 30-minute period.

[0141] Therefore, the humidity of air was measured over solutions of hygroscopic salts: lithium chloride, lithium bromide, potassium formate, and potassium acetate, at salt concentrations between 0 and 72% by mass, at a temperature of 30°C. The results are shown in Figure 6, along with a comparison to the results predicted by Raoult's law. The relative humidity of air in equilibrium with solutions of lithium chloride, lithium bromide, potassium formate, and potassium acetate decreased with increasing salt concentration, deviating significantly from the values ​​predicted by Raoult's law due to the hygroscopic nature of the salts.

[0142] The relative humidity of the air above an aqueous solution containing only potassium taurinate in an amount of 31% by weight was also determined. The results, also shown in Figure 6, are consistent with the predictions of Raoult's law and confirm that the amino acid itself is not a hygroscopic salt.

[0143] The relative humidity of the air above aqueous solutions containing various concentrations of potassium formate as the hygroscopic metal salt, along with 29% by weight potassium taurinate (concentration relative to the total weight of the solution) as the nonvolatile amine absorbent, is also shown in Figure 6. The results indicate that the addition of an amino acid does not inhibit the hygroscopicity of potassium formate in solution. In fact, the addition of potassium taurinate resulted in a further drop in hygroscopicity compared to potassium formate solutions with the same potassium formate concentration.

[0144] Example 4. Water loss / uptake in absorbent under direct air capture technology conditions A wetted-wall column (WWC; also used in Example 2) was used to evaluate water loss or uptake from an aqueous absorption solution containing a nonvolatile amine absorbent and a noncarbonate-based hygroscopic metal salt under direct air capture technology operating conditions. Four experiments were conducted using 2 M taurinate in 44 wt% potassium formate solution (produced by dissolving taurinate in 58 wt% potassium formate solution). Inlet synthetic air (approximately 420 ppm CO2 in nitrogen) with different humidities, 0, 43, 82, and 93%, was fed into the system at a rate of 3.5 L / min and contacted with the absorption liquid at 125 mL / min. Experiments were conducted at atmospheric pressure and room temperature. The results are shown in Figure 7 and Table 2.

[0145] [Table 2]

[0146] In all experiments, CO2 was absorbed from the air feed into the aqueous absorption solution. With a very dry gas feed (0.4% humidity), water was lost from the absorption solution to the treated air. With an air feed of 43% humidity, water was still lost from the absorption solution to the treated air, but the system was nearly balanced. When air feeds with 82.5 and 93% humidity were used, water was recovered from the air feed into the absorption solution. When an air feed with approximately 50% humidity was used, zero water uptake / loss from the absorbent can be expected.

[0147] The results demonstrate, under direct air capture technology conditions using a water-unsaturated air feed: (i) the effectiveness of the non-volatile amine absorbent to capture CO2 despite the presence of hygroscopic metal salts, and (ii) the effectiveness of the hygroscopic metal salts to suppress water loss from the absorbent solution or even to simultaneously capture water and CO2.

[0148] Example 5. Modeling the Water Balance in a Typical DAC System Using the results from Examples 2 and 3, water loss / uptake from the absorber of a typical DAC process detailed in Kiani et al. (Techno-Economic Assessment for CO2 Capture From Air Using a Conventional Liquid-Based Absorption Process. Frontiers in Energy Research, 2020, 8) was modeled for two absorption solutions operating at a scale of 200 t / a of CO2 capture: 2 M taurate in water (System 1) and 2 M taurate in 37% by weight aqueous potassium formate (System 2). Operation of this process was modeled in three different climates: (A) London, (B) Chinchilla, Queensland, Australia, and (C) Singapore. These three climates were chosen because they have characteristic temperature and humidity conditions, the two most important factors affecting the degree of water loss / uptake in a DAC system. Air temperature and humidity data for all these climates over a typical 24-hour period were retrieved from online weather sources and used to model water loss / uptake in this system.

[0149] Figure 7 shows a schematic diagram of a cooling tower-based absorber considered here for CO2 capture from ambient air. From a water loss perspective, the key parameters are the inlet temperature (Ta, in ), outflow temperature (Ta, out ) and humidity of outside air inlet (%RH in ), the temperature of the absorption liquid (TL, in ) and composition, and air flow rate. As discussed in the previous section, the type and concentration of hygroscopic salts used in such systems is important in determining the humidity (%RH) of the exiting air. out Using these parameters and Equation 1, a psychrometric chart was used to determine the water loss at different conditions in this process.

[0150] M loss / gain =V*(W out -Win ) (1) (In the formula, M loss / gain is the water lost or taken up during the process (unit: kg / s), and V is the air flow rate (unit: m 3 / s) and W out and W in is the mass of water per volume of air at the inlet and outlet (unit: kg / m 3 ) is). W out and W in is extracted from the psychrometric chart using the temperature and humidity of the incoming and outgoing airflows.

[0151] Figure 8 shows the tons of water lost from or taken up into the absorbent solution per ton of CO2 absorbed in the DAC process. In all three climates, significant water loss was found when the absorbent of System 1 was used. In contrast, the use of the absorbent of System 2, which contains hygroscopic metal salts, resulted in a net water uptake.

[0152] The concentration of hygroscopic salt required to obtain negligible water loss / uptake was also calculated, and the results are shown in Table 3. It is noted that the salt concentration required for neutral water balance is significantly lower than the concentrations investigated in Example 2, and therefore the effect of hygroscopic salt on the CO2 mass transfer coefficient and absorbent viscosity is less than that measured in that example. Example 2 only considered the concentration of hygroscopic salt that resulted in an approximately 50% reduction in water vapor pressure (outgoing air humidity), which is not necessarily required for most climates.

[0153] [Table 3]

[0154] Those skilled in the art will recognize that the invention described herein is susceptible to variations and modifications other than those specifically described, and it is understood that the invention includes all such variations and modifications that fall within the spirit and scope of the invention. [Explanation of symbols]

[0155] 100 systems 110 Absorption Unit 112 Gas supply stream 114 Aqueous amine absorbent 116 Gas absorption contact area 118 Carbon dioxide-dilute gas 120 Carbon dioxide-rich absorbent / carbon dioxide-rich absorbent composition 122 Recirculation Loop 124 Desorber supply line 130 Desorption Unit 132 Carbon dioxide lean absorbent composition 134 Desorber return line 136 Desorbed carbon dioxide stream 138 Gas Desorption Region 139 Water Vapor 140 Heat exchanger 142 Makeup Line 302 Solution 304 Sealed Jar 306 Pump 308 Air 310 Humidity Sensor 312 Humidity Recording Device 314 Relief valve 316 Oven

Claims

1. 1. A method for removing carbon dioxide from a gas stream containing carbon dioxide, the method comprising contacting the gas stream with an aqueous absorbing liquid comprising: (i) a non-volatile amine absorbent; (ii) a non-carbonate hygroscopic metal salt in an amount of at least 10% by weight of the aqueous absorbing liquid; and (iii) water, thereby absorbing carbon dioxide from the gas stream into the aqueous absorbing liquid to produce a carbon dioxide-lean gas and a carbon dioxide-rich absorbent composition.

2. 2. The method of claim 1, wherein the aqueous absorption liquid comprises the non-carbonate hygroscopic metal salt in an amount sufficient to provide zero or negative net water desorption from the aqueous absorption liquid into the gas stream.

3. 3. The method of claim 1 or claim 2, wherein the gas stream further comprises water vapor, the gas stream having a relative humidity of less than 80%.

4. 4. The method of claim 3, wherein water is absorbed from the gas stream into the aqueous absorption liquid.

5. 5. The method of claim 1, wherein the non-carbonate hygroscopic metal salt is present in an amount of at least 20% by weight of the aqueous absorption liquid.

6. 6. The method of any one of claims 1 to 5, wherein air in equilibrium with the aqueous absorbent composition, when separated from the carbon dioxide-rich absorbent composition, has a relative humidity of less than 80% at the temperature of the carbon dioxide-lean gas.

7. 7. The method according to claim 1, wherein the hygroscopic metal salt has the property that air in equilibrium with a saturated aqueous solution of the non-carbonate hygroscopic metal salt and water has a relative humidity of less than 60% at 30°C.

8. 8. The method according to claim 1, wherein the hygroscopic metal salt has the property that air in equilibrium with a saturated aqueous solution of the non-carbonate hygroscopic metal salt and water has a relative humidity of less than 30% at 30°C.

9. The non-carbonate hygroscopic metal salt contains a cation selected from the group consisting of alkali metals, alkaline earth metals, and nickel, and the non-carbonate hygroscopic metal salt contains a halide ion, C １ ~C ６ 9. The method of any one of claims 1 to 8, comprising an anion selected from the group consisting of alkyl or aryl carboxylate, nitrate and thiocyanate.

10. 10. The method of any one of claims 1 to 9, wherein the non-carbonate, hygroscopic metal salt is selected from the group consisting of lithium chloride, lithium bromide, lithium iodide, lithium nitrate, lithium thiocyanate, sodium bromide, sodium iodide, sodium thiocyanate, potassium fluoride, potassium formate, potassium acetate, potassium nitrite, potassium thiocyanate, magnesium chloride, magnesium bromide, magnesium iodide, magnesium nitrate, magnesium thiocyanate, calcium bromide, calcium iodide, calcium acetate, calcium nitrate, calcium thiocyanate, strontium iodide, strontium thiocyanate, barium iodide, chromium chloride, manganese chloride, manganese bromide, iron bromide, cobalt bromide, cobalt nitrate, nickel chloride, nickel bromide, copper nitrate, zinc chloride, zinc bromide, zinc iodide, zinc nitrate, cerium chloride, and combinations thereof.

11. 11. The method of any one of claims 1 to 10, wherein the non-carbonate, hygroscopic metal salt is selected from the group consisting of lithium chloride, lithium bromide, lithium iodide, lithium thiocyanate, potassium fluoride, potassium formate, potassium acetate, potassium thiocyanate, magnesium chloride, magnesium bromide, magnesium iodide, calcium bromide, calcium iodide, calcium acetate, calcium thiocyanate, nickel bromide, zinc chloride, zinc bromide, zinc iodide, and combinations thereof.

12. 12. The method of claim 1, wherein the non-carbonate, hygroscopic metal salt is selected from the group consisting of lithium chloride, lithium bromide, potassium formate, and potassium acetate.

13. 13. The method of any one of claims 1 to 12, wherein the non-volatile amine absorbent is selected from the group consisting of an amino acid or salt thereof, a polyamine containing both quaternized and neutral amine groups, a high molecular weight amine, and combinations thereof.

14. 14. The method of any one of claims 1 to 13, wherein the non-volatile amine absorbent is an amino acid or a salt thereof, and the amino acid is selected from the group consisting of taurine, sarcosine, alanine, glycine, lysine, dimethylglycine, proline, phenyl-alanine, glucosamine, arginine, methyl-taurine, cysteine, tryptophan, hydroxyproline, asparagine, tyrosine, histidine, glutamine, diglycine, serine, methionine, and combinations thereof.

15. 15. The method of any one of claims 1 to 14, further comprising (iv) a base selected from hydroxides, carbonates, phosphates, additional amines having a pKa greater than the non-volatile amine absorbent, and combinations thereof.

16. 16. The method of any one of claims 1 to 15, wherein the gas stream is air.

17. An aqueous absorbing solution for the recovery of carbon dioxide, comprising: (i) a non-volatile amine absorbent; (ii) a non-carbonate based hygroscopic metal salt in an amount of at least 10% by weight of the aqueous absorbing solution; and (iii) water.

18. 18. The aqueous absorbing solution of claim 17, wherein the non-carbonate hygroscopic metal salt is present in an amount of at least 20% by weight of the aqueous absorbing solution.

19. 19. The aqueous absorbing liquid of claim 17 or claim 18, wherein air in equilibrium with the aqueous absorbent composition has a relative humidity of less than 80% at 30°C.

20. 20. The aqueous absorbing liquid according to any one of claims 17 to 19, wherein the hygroscopic metal salt has the property that air in equilibrium with a saturated aqueous solution of the non-carbonated hygroscopic metal salt and water has a relative humidity of less than 60% at 30°C.

21. 21. The aqueous absorbing liquid according to claim 17, wherein the hygroscopic metal salt has the property that air in equilibrium with a saturated aqueous solution of the non-carbonate hygroscopic metal salt and water has a relative humidity of less than 30% at 30°C.

22. The non-carbonate hygroscopic metal salt contains a cation selected from the group consisting of alkali metals, alkaline earth metals, and nickel, and the non-carbonate hygroscopic metal salt contains a halide ion, C １ ~C ６ 22. The aqueous absorbing liquid according to any one of claims 17 to 21, comprising anions selected from the group consisting of alkyl or aryl carboxylate ions, nitrate ions and thiocyanate ions.

23. 23. The aqueous absorbing solution of any one of claims 17 to 22, wherein the non-carbonate hygroscopic metal salt is selected from the group consisting of lithium chloride, lithium bromide, lithium iodide, lithium nitrate, lithium thiocyanate, sodium bromide, sodium iodide, sodium thiocyanate, potassium fluoride, potassium formate, potassium acetate, potassium nitrite, potassium thiocyanate, magnesium chloride, magnesium bromide, magnesium iodide, magnesium nitrate, magnesium thiocyanate, calcium bromide, calcium iodide, calcium acetate, calcium nitrate, calcium thiocyanate, strontium iodide, strontium thiocyanate, barium iodide, chromium chloride, manganese chloride, manganese bromide, iron bromide, cobalt bromide, cobalt nitrate, nickel chloride, nickel bromide, copper nitrate, zinc chloride, zinc bromide, zinc iodide, zinc nitrate, cerium chloride, and combinations thereof.

24. 24. The aqueous absorbing solution of any one of claims 17 to 23, wherein the non-carbonate hygroscopic metal salt is selected from the group consisting of lithium chloride, lithium bromide, lithium iodide, lithium thiocyanate, potassium fluoride, potassium formate, potassium acetate, potassium thiocyanate, magnesium chloride, magnesium bromide, magnesium iodide, calcium bromide, calcium iodide, calcium acetate, calcium thiocyanate, nickel bromide, zinc chloride, zinc bromide, zinc iodide, and combinations thereof.

25. 25. The aqueous absorbing solution according to any one of claims 17 to 24, wherein the non-carbonate hygroscopic metal salt is selected from the group consisting of lithium chloride, lithium bromide, potassium formate, and potassium acetate.

26. 26. The aqueous absorbing liquid of any one of claims 17 to 25, wherein the non-volatile amine absorbent is selected from the group consisting of an amino acid or a salt thereof, a polyamine containing both quaternized and neutral amine groups, a high molecular weight amine, and combinations thereof.

27. 27. The aqueous absorbing solution of any one of claims 17 to 26, wherein the non-volatile amine absorbent is an amino acid or a salt thereof, and the amino acid is selected from the group consisting of taurine, sarcosine, alanine, glycine, lysine, dimethylglycine, proline, phenyl-alanine, glucosamine, arginine, methyl-taurine, cysteine, tryptophan, hydroxyproline, asparagine, tyrosine, histidine, glutamine, diglycine, serine, methionine, and combinations thereof.

28. 28. The aqueous absorbing solution of any one of claims 17 to 27, further comprising (iv) a base selected from hydroxides, carbonates, phosphates, additional amines having a pKa greater than the non-volatile amine absorbent, and combinations thereof.

29. 29. The aqueous absorbing solution of any one of claims 17 to 28, further comprising absorbed carbon dioxide in a ratio (mole / mole) of carbon dioxide to non-volatile amine absorbent of at least 0.

05.

30. 1. A system for removing carbon dioxide from a gas stream containing carbon dioxide, comprising:

30. An aqueous absorbing liquid according to any one of claims 17 to 29. an absorption unit for contacting the aqueous absorbing liquid with the gas stream, thereby absorbing carbon dioxide from the gas stream into the aqueous absorbing liquid to produce a carbon dioxide lean gas and a carbon dioxide enriched absorbent composition; a regeneration unit for removing carbon dioxide from the carbon dioxide-rich absorbent composition, thereby producing the carbon dioxide-lean absorbent composition for recycle to the aqueous absorption liquid; Including, the system.

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