Energy-Efficient CO2 Capture from Flue Gas by Absorption with Amino

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Article Cite This: Ind. Eng. Chem. Res. 2019, 58, 10510−10515

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Energy-Efficient CO2 Capture from Flue Gas by Absorption with Amino Acids and Crystallization with a Bis-Iminoguanidine Kathleen A. Garrabrant, Neil J. Williams, Erick Holguin, Flavien M. Brethome,́ Costas Tsouris, and Radu Custelcean* Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States

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S Supporting Information *

ABSTRACT: We report a hybrid solvent/solid-state approach to CO2 separation from flue gas, consisting of absorption with aqueous glycine or sarcosine amino acids, followed by crystallization of the bicarbonate salt of glyoxalbis(iminoguanidine) (GBIG), and subsequent solid-state CO2 release from the bicarbonate crystals. In this process, the GBIG bicarbonate crystallization regenerates the amino acid sorbent at ambient temperature, and the CO2 is subsequently released by mild heating (120 °C) of the GBIG bicarbonate crystals, which results in quantitative regeneration of GBIG. The cyclic capacities measured from multiple absorption− regeneration cycles are in the range of 0.2−0.3 mol CO2/mol amino acid. The regeneration energy of this hybrid solvent/solid-state approach is 24% and 40% lower than the regeneration energy of benchmark industrial sorbents monoethanolamine and sodium glycinate, respectively. Finally, as the amino acid sorbent is never heated in the hybrid process, its loss through evaporation or degradation is minimized.

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INTRODUCTION Energy-efficient removal of carbon dioxide (CO2) from flue gases and other industrial sources remains a challenging problem with relevance to climate change mitigation. Current CO2 absorbents based on liquid amines, such as monoethanolamine (MEA), suffer from a number of drawbacks, including high volatilities and toxicities, low thermal and chemical stabilities, and high regeneration energies.1 Aqueous amino acid salts have been proposed as superior, environmentally friendly alternatives to conventional amine sorbents, based on their higher loading capacities, faster CO2-sorption kinetics, and lower volatility and corrosivity.2 Like the amine analogues, amino acid salts react with CO2 to generate carbamates, which then hydrolyze to a large extent into (bi)carbonate anions (Scheme 1). The zwitterion of the amino acid is formed as a biproduct, and the reaction stops as the sorbent becomes saturated when the solution pH becomes low enough to

convert most of the amino acid into the inactive zwitterionic form. The overall process can be reversed by heating the loaded amino acid sorbent to release the CO2 gas and regenerate the anionic form of the amino acid, which can be recycled. However, multiple capture/regeneration cycles can lead to degradation of the amino acid sorbent over time. Contrary to general expectations of enhanced chemical stability, amino acid sorbents have been found to have similar or even higher oxidative or thermal degradation rates compared to amine sorbents such as MEA.3,4 Furthermore, the regeneration energy for aqueous amino acid salts, such as sodium glycinate, was found to be significantly higher (5743 kJ/kg CO2) than the regeneration energy of MEA (4503 kJ/kg CO2).5 This is in spite of the reaction enthalpy for CO2 release being slightly lower for sodium glycinate compared to MEA (69 vs 79 kJ/ mol), as sodium glycinate has a sensible heat and latent heat of vaporization that are considerably higher than those for MEA. The source of these issues is the heating and boiling of the aqueous amino acid sorbents required during the regeneration step, which are energy intensive and create a harsh environment leading to amino acid degradation. An improved CO2 capture process based on precipitating amino acid sorbents has been recently proposed.6,7 The regeneration energy for this

Scheme 1. CO2 Absorption by an Aqueous Amino Acid Sorbent,a with the Formation of Carbamate and Bicarbonate Anions

Received: Revised: Accepted: Published:

a

Glycine is used as an example. © 2019 American Chemical Society

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February 18, 2019 April 25, 2019 May 24, 2019 May 24, 2019 DOI: 10.1021/acs.iecr.9b00954 Ind. Eng. Chem. Res. 2019, 58, 10510−10515

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Industrial & Engineering Chemistry Research

pH and output CO2 concentration measurements and ex situ by analyzing the concentration of carbonate and carbamate formed in solution by ion chromatography (IC) and 1H NMR spectroscopy, respectively.9 The loading experiments were run for 2 h, and the results are summarized in Table 1. The 1H and 13 C NMR spectra of the loaded sorbents confirmed the final solutions contained mixtures of unreacted amino acids, their corresponding carbamate ions, as well as bicarbonate anions (Supporting Information, Figures S1−S4). The equilibrium CO2 loading capacities measured for glycine and sarcosine were 0.92 ± 0.06 and 0.85 ± 0.11 mol/mol, with most CO2 absorbed as (bi)carbonate and, to a lesser extent, as carbamate. The CO2 breakthroughs were observed after about 20 min for both the glycine and sarcosine sorbents (Figure 2). Sorbent Regeneration by GBIG-Bicarbonate Crystallization. Regeneration of the CO2-loaded glycine or sarcosine amino acids can be effectively done at ambient temperature by crystallization with GBIG, which gradually dissolves into solution and recrystallizes as GBIGH 2 (HCO 3 ) 2 (H 2 O) 2 (Scheme 2). In this process, the basic guanidine groups of GBIG deprotonate the zwitterionic forms of the amino acids, converting them back into the active anionic forms. The resulting GBIGH22+ cations crystallize with the HCO3− anions, removing them from solution, thereby unloading the CO2 from the sorbent. The reaction reaches equilibrium state when the pH of the solution becomes high enough (≈10−10.2) that the guanidine groups of GBIG are mostly deprotonated and thus unable to crystallize with the bicarbonate anions. The regeneration is mostly driven by the solubility difference between GBIG and GBIGH2(HCO3)2(H2O)2, which is a function of the bicarbonate concentration and the solution pH. The amount of CO2 removed from solution during sorbent regeneration essentially corresponds to the cyclic capacity of the amino acid sorbent. Once removed from solution, the GBIGH2(HCO3)2(H2O)2 crystals are heated at 120 °C to release the CO2 and H2O and regenerate GBIG quantitatively.10 The glycine or sarcosine amino acids were regenerated by adding 0.25 mol equiv of solid GBIG and stirring the resulting slurries at room temperature for 2 h. The concentrations of carbonate and carbamate in solutions were monitored by IC and 1H NMR spectroscopy, respectively. The total amount of CO2 removed as a function of time is plotted in Figure 3. Most of the regeneration occurred in the first 5−20 min, and longer stirring times only resulted in minimal increases in the CO2 removed, which indicates the equilibrium state is reached relatively quickly despite the three-phase reaction. The powder X-ray diffraction patterns of the final isolated solids corresponded to a mixture of GBIG and GBIGH2(HCO3)2(H2O)2 crystalline phases (Supporting Information, Figures S5 and S6). Under these conditions, the corresponding cyclic capacities for glycine and sarcosine were 0.33 ± 0.05 and 0.31 ± 0.05 mol/mol, respectively. CO2 Separation Cycles. Any sustainable CO2 capture process requires efficient sorbent recycling over multiple absorption−release cycles. A complete CO2 separation cycle with amino acid/GBIG consists of loading the glycine or sarcosine sorbent with the bubble reactor, adding the GBIG solid to the loaded sorbent and stirring the suspension for 120 min, and then filtering the solid to separate the GBIG− bicarbonate crystals from the regenerated amino acid solution. The amount of CO2 removed from the sorbent was monitored as described in the previous section, confirming once again that

process, estimated from conceptual design models, was found to be 35% lower than for the MEA benchmark (excluding the energy required to redissolve the precipitates).6 A recent techno-economic analysis of CO2 capture involving precipitating solvents found lower regeneration energies and costs for such phase-changing processes.8 While the predicted energy savings have yet to be validated experimentally, sorbent degradation remains an unresolved issue, as the amino acid solutions still need to be heated during the regeneration process. A potential solution to the high regeneration energy and chemical degradation problems associated with aqueous amino acid sorbents is to prevent heating them altogether by crystallizing the (bi)carbonate salt resulting from the CO2 absorption and removing it from solution before heating the (bi)carbonate solid to release the CO2. Such a hybrid solvent/ solid-state approach to CO2 separation combines the advantages of a liquid sorbent, such as fast absorption rate and easy handling and maintenance, with the benefits of a solid sorbent, such as lower regeneration energy and higher capacity. Furthermore, sorbent loss through evaporation and chemical degradation would be minimized. Practical implementation of this concept requires an effective base that can deprotonate the amino acid zwitterion and regenerate its active anionic form, while also forming a relatively insoluble (bi)carbonate salt.9 Here, we demonstrate a bench-scale approach to CO2 separation from flue gas by using aqueous potassium salts of glycine and sarcosine for CO2 absorption, followed by bicarbonate crystallization with a simple guanidine base, glyoxal-bis(iminoguanidine) (GBIG) (Scheme 2). While Scheme 2. Proposed CO2 Capture Approach by a Hybrid Solvent/Solid-State System, Combining CO2 Absorption by an Aqueous Amino Acid Salt (e.g., Glycine) and Bicarbonate Crystallization with GlyoxalBis(iminoguanidine) (GBIG)

aqueous GBIG can function as a CO2 sorbent by itself,10 combining it with amino acid sorbents, as demonstrated in this study, led to superior CO2 capture performances with significantly improved loading capacities and CO2 absorption rates.

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RESULTS CO2 Absorption with Aqueous Glycine and Sarcosine. CO2 absorption experiments from a flue gas simulant (EPA protocol standard, 12.8% CO2 + 87.2% N2) were measured at 25 °C in a bubble reactor using 1 M aqueous solutions of potassium glycinate or sarcosinate (Figure 1). The extent of CO2 absorption as a function of time was monitored in situ by 10511

DOI: 10.1021/acs.iecr.9b00954 Ind. Eng. Chem. Res. 2019, 58, 10510−10515

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Industrial & Engineering Chemistry Research

Figure 1. CO2-loading curves for a flue gas simulant (12.8% CO2) with 1 M potassium glycinate (blue dots) or potassium sarcosinate (red squares) sorbents, using a bubble reactor (shown on the left). The CO2 loading values correspond to the sums of the (bi)carbonate and carbamate concentrations in solution, relative to the molar concentrations of the amino acids. The error bars are defined as the standard deviations from three separate absorption experiments.

Table 1. Equilibrium Loading Values for CO2 Capture with 1 M Aqueous Solutions of Potassium Glycinate or Sarcosinate at 25 °Ca sorbent glycine (1M) sarcosine (1M)

pH (initial/ final)

carbonateb (M)

carbamate (M)

total CO2 (M)

12.2/8.2

0.81 ± 0.07

0.11 ± 0.01

0.92 ± 0.06

12.4/8.4

0.78 ± 0.11

0.07 ± 0.01

0.85 ± 0.11

a Three different absorption experiments were done for each amino acid, and the reported concentrations are the average values for the three runs after 2 h. b As the sum of CO 3 2− and HCO 3 − concentrations.

Figure 3. Time-dependent regeneration of glycine (blue dots) or sarcosine (red squares) by GBIGH2(HCO3)2(H2O)2 crystallization. Solid GBIG was mixed with the loaded amino acid solutions, and the resulting slurries were stirred for 2 h. The amounts of CO2 removed relative to the amino acid concentration in solution (mol/mol) were monitored by measuring the (bi)carbonate and carbamate concentrations left in solution. The error bars are defined as the standard deviations from three separate regeneration experiments.

respectively, and the average cyclic capacities are plotted in Figure 4. While there was some variation from one cycle to another, as expected from such complex crystallization mixtures with multiple phases involved, as well as from oscillations in the ambient conditions, the measured cyclic capacities stayed relatively constant within the 0.2−0.3 mol/ mol range.

Figure 2. CO2 breakthrough curves for glycine (blue dots) and sarcosine (red square) sorbents.

most of the sorbent regeneration occurred in the first few minutes. For each cycle, the amount of CO2 removed and the corresponding error bars were calculated as the average and standard deviation, respectively, from all the values obtained in the 5−120 min stirring time intervals. Finally, the GBIG− bicarbonate crystals were heated in an oven for 2 h at 120 °C to release the CO2 and regenerate GBIG. The recovered amino acid and GBIG were then reused in another cycle. We ran 5 and 4 consecutive cycles with glycine and sarcosine,

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DISCUSSION In this bench-scale study we demonstrated an effective approach to carbon dioxide separation from flue gas based on CO2 absorption by aqueous amino acid salts (i.e., potassium glycinate, potassium sarcosinate), followed by amino acid regeneration and bicarbonate removal by 10512

DOI: 10.1021/acs.iecr.9b00954 Ind. Eng. Chem. Res. 2019, 58, 10510−10515

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Table 2. Minimum Regeneration Energy (kJ/kg CO2) Required for the Hybrid Potassium Glycinate/Sarcosinate + GBIG System Compared to the 30% Aqueous Monoethanolamine (MEA) and 30% Aqueous Sodium Glycinate (SG) Benchmark Sorbents reaction enthalpy sensible heat heat of vaporization total energy a

MEAa

SGb

Gly/Sar + GBIGc

1636 2191 676 4503

1568 3220 955 5743

2761 682 − 3443

Reference 12. bReference 5. cReference 10.

Figure 4. Average cyclic capacities for glycine (top) and sarcosine (bottom) measured for consecutive loading/regeneration cycles. The error bars are defined as the standard deviations from the various amounts of CO2 removed in the 5−120 min regeneration times.

crystallization with a simple bis-iminoguanidine base (GBIG). The measured cyclic capacities of 0.2−0.3 mol CO2/mol amino acid, obtained from multiple absorption/regeneration cycles, are in the same range as the corresponding values reported for aqueous amine sorbents.1 In the final step, the CO2 is released in the solid state by mild heating of the GBIG bicarbonate crystals. By adding the additional crystallization step, this hybrid solvent/solid-state approach circumvents the energy-intensive processes of heating and boiling aqueous solutions, typically involved in traditional solvent-based carbon capture methods.1 To assess the energy efficiency of the CO2 separation approach reported here, we compared the previously determined regeneration energy of GBIG 10 with the regeneration energies of industrial benchmark sorbents, such as aqueous monoethanolamine (MEA)12 or sodium glycinate (SG).5 The former value had been determined from differential scanning calorimetry measurements of solid GBIGH2(HCO3)2(H2O)2, including reaction enthalpy and heat capacity,10 while the corresponding values for the MEA (30 wt %) and SG (30 wt %), determined by standard methodologies involving a combination of CO2 solubilities, heat capacities, and vapor pressure measurements, were retrieved from the literature.5 In this energetic comparison, the total regeneration energy can be broken down into its three components: (1) reaction enthalpy, or the heat required to chemically desorb the CO2 from the sorbent, (2) sensible heat, or the energy required to heat the sorbent from ambient conditions to the CO2-release temperature, and (3) heat of vaporization, or the energy involved in boiling and evaporating the aqueous sorbent.5 These values are summarized in Table 2 and graphically depicted in Figure 5. Despite the higher reaction enthalpy required for the CO2 release, overall the glycine/sarcosine + GBIG system requires 24% and 40% less energy than the MEA and SG benchmarks, respectively. This is a direct consequence of the much lower sensible heat of crystalline GBIGH2(HCO3)2(H2O)2 compared to aqueous MEA and SG and that no solvent evaporation is involved (technically, two equivalents of water

Figure 5. Minimum regeneration energy requirements (kJ/kg CO2) for the glycine/GBIG hybrid system compared to 30% aqueous monoethanolamine (MEA) and 30% aqueous sodium glycinate (SG) benchmarks.

vapor are released from the GBIG−bicarbonate crystals for each equivalent of CO2 released, but that energy is already accounted for as part of the reaction enthalpy). The combined glycine/sarcosine + GBIG system also has major advantages over using aqueous GBIG alone as a sorbent, as reported in a recent publication.10 Although aqueous GBIG can capture CO2 on its own, its capacity is severely limited by the relatively low aqueous solubility of GBIG (≈10−2 M). Specifically, the gravimetric CO2 capacity of a saturated aqueous solution of GBIG is only about 0.08%, 50 times lower than the corresponding CO2 capacity of a 1 M potassium glycinate solution (≈ 4%). The latter can be further increased if higher concentrations of amino acid are used, up to the solubility limit of glycine of about 3.3 M. Furthermore, GBIG reacts relatively slowly with CO2 compared to glycine or sarcosine, whose rates of reaction have been reported to be higher than the corresponding rate of MEA.11 Finally, another potential advantage of the hybrid glycine/ sarcosine + GBIG system is that by avoiding heating and boiling the amino acid solution, sorbent loss through evaporation and degradation is minimized. Instead, the regeneration process involves heating the solid GBIGH2(HCO3)2(H2O)2, which, based on the initial tests, has very high thermal stability with no decomposition detected after 1 week of heating the crystals at 120 °C in air.10 Nevertheless, more extensive tests over hundreds of absorption/regeneration cycles at larger scales and under real-world conditions are necessary to fully evaluate this promising new approach to CO2 separation and develop it into an energy-efficient and cost-effective carbon capture technology. 10513

DOI: 10.1021/acs.iecr.9b00954 Ind. Eng. Chem. Res. 2019, 58, 10510−10515

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the end of the regeneration experiment, the final solution was filtered in order to recover the solid GBIG and the amino acid solution. The regeneration of each amino acid was run in triplicate, and the average values with the corresponding standard deviations are reported. CO2 Separation Cycles. For the multicycle experiments, the amino acid solution was bubbled with the flue gas simulant at 4 L/min for 2 h and the final CO2 loading level was measured by IC and 1H NMR as described above. The loaded sorbent was subsequently transferred to a 2 L beaker to which 0.25 mol equiv of GBIG was added. The slurry was stirred at 250 rpm for 2 h at ambient temperature. At the end of stirring, a 50 μL sample was withdrawn with a syringe filter and the carbonate and carbamate concentrations left in solution were measured by IC and 1H NMR as described above. The bulk mixture was then vacuum-filtered, and the regenerated amino acid recovered in the filtrate was reused in the subsequent cycle. The recovered GBIG−bicarbonate solid was placed in an oven and heated at 120 °C for approximately 2 h, and the regenerated GBIG solid was reused in the subsequent cycle.

EXPERIMENTAL SECTION Materials and Characterization. Common reagents, including glycine, sarcosine, glyoxal, and aminoguanidine hydrochloride, were purchased from commercial suppliers and used without further purification. The flue gas simulant (12.8% CO2, EPA Protocol Standard) was purchased from Airgas. GBIG was synthesized according to a previously published procedure.10 All water used was deionized (Milli-Q). NMR spectra were collected using a Bruker Avance III 300 MHz NMR Spectrometer. pH measurements for the loading experiments were taken using an Orion Dual Star pH/SE meter with a Ross Ultra Semimicro probe. pH measurements for the regeneration experiments were taken using an Orion Star A211 pH meter with a Ross Ultra Combination probe. A Dionex ICS-5000+ ion chromatography system with an inline eluent generator was used for IC measurements. The column used in the system was a Dionex Ion Pac AS11-SC IC with a 4 mm internal diameter and 250 mm length, with a bore guard column 50 mm long and 4 mm in diameter placed in front. The concentration of CO2 in the breakthrough measurements was monitored with a CM-0003 CO2 meter. CO2 Absorption with Aqueous Glycine and Sarcosine. The CO2 loading experiments were performed in a VirTis Omni-Culture bubble reactor. The reactor consists of a 1.6 L tank, an impeller for stirring, and a heat exchanger with baffles. A porous Teflon tube (10 μm pore diameter) was attached along the impeller support on the bottom of the reactor to generate the bubbles. The temperature of the reactor was set at 25 ± 1 °C using a chiller with circulating coolant. A 1 L aqueous solution of 1 M potassium hydroxide and 1 M glycine/sarcosine was added to the reactor. The stirrer was set to 450 rpm, and the flue gas flow rate was set at 4 L/min. The pH probe was placed inside the reactor to record the evolution of the solution pH in situ. A temperature probe was also located inside the reactor to monitor the temperature of the reaction. The loading experiments were run for a duration of 2 h, with samples being taken every 5 min for the first hour and every 10 min for the remaining hour. The concentrations of carbonate and carbamate formed were determined by IC and 1 H NMR spectroscopy, respectively. The samples (50 μL) were diluted using 450 μL of D2O for the NMR analysis and then further diluted with 980 μL of deionized water added to 20 μL of the NMR sample. Three loading experiments were done for each amino acid sorbent, and the average concentrations with the corresponding standard deviations are reported. Regeneration of Amino Acid Sorbents with GBIG. Time-dependent sorbent regeneration experiments were carried out in a 2 L beaker using an IKA Eurostar 20 Digital mixer and a Teflon impeller, set at a stirring speed of 250 rpm. All regeneration experiments were conducted at ambient temperature. The CO2-loaded amino acid solution (1 L) was added to the beaker, and the solid GBIG (0.25 mol equiv) was added as a suspension. The resulting slurry was stirred for a total of 2 h, with samples being drawn every 5 min for the first hour and every 10 min for the second. The subsamples (50 μL) were withdrawn using a 1 mL syringe with a 0.22 μm syringe filter to remove solid particulates from solution. The samples were diluted with 450 μL of D2O and then left at room temperature for 24 h prior to being analyzed by 1H NMR spectroscopy. For IC analysis, 20 μL of the NMR solution was further diluted with 980 μL of deionized H2O. At

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.9b00954. 1

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H and 13C NMR spectra from the CO2 loading experiments and powder X-ray diffraction patterns from the sorbent regeneration experiments (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Neil J. Williams: 0000-0002-3159-226X Costas Tsouris: 0000-0002-0522-1027 Radu Custelcean: 0000-0002-0727-7972 Author Contributions

K.A.G, N.J.W., E.H., and F.M.B. performed the experiments and analyzed the data. C.T., N.J.W., and K.A.G. designed the CO2-absorption experiments with the bubble reactor. K.A.G wrote the initial draft. R.C. conceptualized the study and revised and edited the manuscript. All authors approved the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The conceptual design and the initial proof of concept by R.C., N.J.W., and F.M.B. was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division. The subsequent system optimization, scale up, and testing of the CO2 absorption, sorbent regeneration, and development of the CO2 separation cycles was supported by the U.S. Department of Energy, Office of Technology Transitions, through a Technology Commercialization Fund (TCF-17-13299).

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REFERENCES

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Industrial & Engineering Chemistry Research (2) Shariff, A. M.; Shaikh, M. S. Aqueous Amino Acid Salts and Their Blends as Efficient Absorbents for CO2 Capture. Energy Efficient Solvents for CO2 Capture 2017, 117−151. (3) Epp, B.; Fahlenkamp, H.; Vogt, M. Degradation of Solutions of Monoethanolamine, Diglycolamine and Potassium Glycinate in View of Tail-End CO2 Absorption. Energy Procedia 2011, 4, 75−80. (4) Huang, Q.; Thompson, J.; Lampe, L. M.; Selegue, J. P.; Liu, K. Thermal Degradation Comparison of Amino Acid Salts, Alkanolamines and Diamines in CO2 Capture. Energy Procedia 2014, 63, 1882−1889. (5) Song, H.-J.; Lee, S.; Park, K.; Lee, J.; Chand Spah, D.; Park, J.W.; Filburn, T. P. Simplified Estimation of Regeneration Energy of 30 wt % Sodium Glycinate Solution for Carbon Dioxide Absorption. Ind. Eng. Chem. Res. 2008, 47, 9925−9930. (6) Sanchez Fernandez, E.; Heffernan, K.; van der Ham, L. V.; Linders, M. J. G.; Eggink, E.; Schrama, F. N. H.; Brilman, D. W. F.; Goetheer, E. L. V.; Vlugt, T. J. H. Conceptual Design of a Novel CO2 Capture Process Based on Precipitating Amino Acid Solvents. Ind. Eng. Chem. Res. 2013, 52, 12223−12235. (7) Sanchez-Fernandez, E.; Heffernan, K.; van der Ham, L.; Linders, M. J. G.; Brilman, D. W. F.; Goetheer, E. L. V.; Vlugt, T. J. H. Analysis of Process Configurations for CO2 Capture by Precipitating Amino Acid Solvents. Ind. Eng. Chem. Res. 2014, 53, 2348−2361. (8) Raksajati, A.; Ho, M. T.; Wiley, D. E. Understanding the Impact of Process Design on the Cost of CO2 Capture for Precipitating Solvent Absorption. Ind. Eng. Chem. Res. 2016, 55, 1980−1994. (9) Brethomé, F. M.; Williams, N. J.; Seipp, C. A.; Kidder, M. K.; Custelcean, R. Direct Air Capture of CO2 via Aqueous-Phase Absorption and Crystalline-Phase Release Using Concentrated Solar Power. Nature Energy 2018, 3, 553−559. (10) Williams, N. J.; Seipp, C. A.; Brethomé, F. M.; Ma, Y.-Z.; Ivanov, A. S.; Bryantsev, V.; Kidder, M. K.; Martin, H. J.; Holguin, E.; Garrabrant, K. A.; Custelcean, R. CO2 Capture via Crystalline Hydrogen-Bonded Bicarbonate Dimers. Chem. 2019, 5, 719. (11) Xiang, Q.; Fang, M.; Yu, H.; Maeder, M. Kinetics of the Reversible Reaction of CO2(aq) and HCO3− with Sarcosine Salt in Aqueous Solution. J. Phys. Chem. A 2012, 116, 10276−10284. (12) Gottlicher, G. The Energetics of Carbon Dioxide Capture in Power Plants. U.S. Department of Energy, Office of Fossil Energy, NETL: 2004.

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