Kinetic and Thermodynamic Characterization of Enhanced Carbon

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Kinetic and Thermodynamic Characterization of Enhanced Carbon Dioxide Absorption Process with Lithium Oxide-Containing Ternary Molten Carbonate Bowen Deng, Juanjuan Tang, Xuhui Mao, Yuqiao Song, Hua Zhu, Wei Xiao, and Dihua Wang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b02955 • Publication Date (Web): 07 Sep 2016 Downloaded from http://pubs.acs.org on September 12, 2016

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Environmental Science & Technology

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Kinetic and Thermodynamic Characterization of Enhanced Carbon

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Dioxide Absorption Process with Lithium Oxide-Containing Ternary

3

Molten Carbonate

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Bowen Deng, Juanjuan Tang, Xuhui Mao,* Yuqiao Song, Hua Zhu, Wei Xiao, Dihua Wang*

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School of Resource and Environmental Sciences, Hubei International Scientific and

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Technological Cooperation Base of Sustainable Resource and Energy, Wuhan University,

7

430079 Wuhan, China

8

*

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E-mail: [email protected]; Tel: +86-27-6877-4216; Fax: +86-27-6877-5799

10

Corresponding Authors:

E-mail: [email protected]; Tel: +86-27-6877-4216; Fax: +86-27-6877-5799

1

ACS Paragon Plus Environment


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ABSTRACT

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Efficient and high-flux capture of CO2 is the prerequisite of its utilization. Static absorption

13

of CO2 with solid Li2O and molten salts (Li2O-free and Li2O-containing Li–Na–K carbonates)

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was investigated using a reactor with in-situ pressure monitoring. The absorption capacity of

15

dissolved Li2O was 0.835 molCO2/molLi2O at 723 K, larger than that of solid Li2O. For the

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solid Li2O absorbents, formation of solid Li2CO3 on the surface can retard the further

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reactions between Li2O and CO2, whereas the dissociation/dissolution effect of molten

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carbonate on Li2O improved the mass-specific absorption capacity of liquid Li2O. In

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Li2O-containing Li–Na–K molten carbonate, CO2 was mostly absorbed by alkaline oxide ions

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(O2–). The chemical interactions between CO2 and CO32– contributed to CO2 uptake via

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formation of multiple carbonate ions. The mass transfer of these absorbing ions was found as

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the dominating factor governing the rate of static absorption. Higher temperatures reduced

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the thermodynamic tendency of CO2 absorption, but a lower viscosity at elevated temperature

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was conducive to absorption kinetics. Compared with the commonly used CaO absorbent,

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Li2O was much more dissolvable in molten carbonate. The Li2O-containing molten carbonate

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is potentially a promising medium for industrial carbon capture and electrochemical

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transformation process.

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Table of Contents (TOC) art

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INTRODUCTION

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Global climate change caused by increasing emissions of greenhouse gases has recently

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attracted much attention.1,2 Carbon dioxide (CO2 ), as the most concerned greenhouse gas,

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has been increasing from approximately 270 to 400 ppm in the last 200 years. One of the

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effective countermeasures for global warming is capturing CO2 from emission sources, such

40

as power plants and metallurgical industries. Concentrated CO2 streams are captured and

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injected into subsurface with stable geological features to trap CO2 and prevent its subsequent

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emission into the atmosphere.3–6 Alternatively, as an essential carbon feedstock, the captured

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CO2 can be used for production of value-added products, such as carbon powder, syngas, and

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methanol.7,8 These recycling processes, known as carbon capture and utilization (CCU), are

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mainly attractive as a mid-term solution to mitigate the environmental impacts of

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anthropogenic emission of CO2.9,10

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At industrial scale, an efficient and high-flux uptake of CO2 from the exhaust gas stream

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is mandatory for all operations of carbon capture, utilization and storage (CCUS)

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technologies.11 Currently, the most commercially utilized technologies for CO2 removal is the

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chemical

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monoethanolamine, diethanolamine, N-methyldiethanolamine, and others.12 This process,

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however, is energy intensive due to the laborious regeneration and recycling operations, and

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the amine solutions also cause several issues and challenges because of their high volatility

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and poor thermal stability, such as secondary environmental matters, equipment corrosion,

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and loss of amines.13,14 Room temperature ionic liquids (RTILs)—with advantages of

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extremely low vapor pressure, non-flammability, easy enhancement of structure, and high

absorption

of

CO2

with

amine-based

aqueous

4


solutions,

including

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solvation capacity—have been investigated as potential CO2 absorbents.15–19 However, the

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lack of cost-effective and large-scale production of RTILs has limited their prospects in

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carbon capture application. Calcium oxide-based solid sorbents are derived from natural

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limestone, therefore, they are cheap sorbent precursors and potential CO2 carriers with large

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capacities.20–22 But, the drawbacks of capacity loss and the requirement of high temperature

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regeneration restrict their extensive application for CO2 capture.

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Very recently, we and other scholars developed integrated technologies based on molten

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salts for a simultaneous capture and conversion of CO2, namely the so-called Molten Salt

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Carbon Capture and Electrochemical Transformation (MSCC-ET) process.23–27 The molten

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salts serve as the media for high-flux uptake of CO2 and concurrently as the electrolyte for

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electrochemical conversion of CO2 to value-added zero-valent carbon.23,27,28 Relative to the

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existing solvent-based technologies, molten salts are less toxic, less corrosive, and

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economically attractive. More importantly, molten salts possess favorable CO2 absorption

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capacities, fast absorption rates, and the ability of activating exotic sorbents.29–33 Tomkute et

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al. reported the amount of CO2 uptake by molten calcium dichloride containing 15 wt % CaO

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(~1073 K) as high as 0.541 gCO2/gCaO , while the CaO sorbents could effectively be

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regenerated with nitrogen gas at ~1223 K.29

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In regard to MSCC–ET process, the Li–Na–K ternary carbonate is preferable due to its

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lower melting point and the inherent advantages for CO2 capture and electro-conversion.

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Using external energy sources, such as electricity from solar panels, the seamless coupling

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between CO2 capture and electro-deposition of carbon in Li–Na–K ternary carbonate can be

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achieved through the following reactions:

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Dissociation of Li2CO3 in melts: Electrochemical deposition of carbon: Oxygen evolution on an inert anode: Dissociation of Li2O in melts: Carbonation of Li2O for CO2 capture:

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Li2CO3 ⇆ 2Li+ + CO32–

(1)

CO32– + 4e– = C + 3O2–

(2)

2O2– – 4e– = O2

(3)

2Li+ + O2– ⇆ Li2O

(4)

Li2O + CO2 = Li2CO3

(5)

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The presence of lithium carbonate allows the electrochemical conversion of CO32– to C

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(eqs 1, 2) before the potential deposition of alkaline metals.23,25,27 The produced Li2O can

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absorb CO2 due to its strong alkalinity, resulting in a continuous conversion of CO2 to carbon

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and oxygen (eq 3). Apparently, the carbonation of Li2O (eq 5) is of vital significance as a key

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intermediate reaction. The rate of carbonation (eq 5) should match with the rate of

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electrochemical deposition (eq 2) to achieve a net transformation of CO2. However, the

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previous researches about the carbonation of Li2O were mostly conducted with solid Li2O or

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lithium-based ceramics.34–36 Their results basically showed that both solid Li2O and

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lithium-based ceramics had favorable capability on CO2 absorption over a wide range of

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temperatures and CO2 pressures.34–36 In regard to the molten Li2O, although its capability on

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CO2 uptake was proven in LiCl–Li2O binary molten salts at 923 K,37 the information about

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the kinetics of carbonation reaction of Li2O is very limited.

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Considering that an in-depth understanding of the CO2 absorption in molten salt is

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essential for scaling up the MSCC–ET technologies, we investigated the capture of CO2 by

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Li2O in Li–Na–K molten carbonate using an elaborately designed reactor. The molten ternary

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Li–Na–K carbonate was chosen for the reason that, compared with other molten alternatives,

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its satisfactory melting point of ~ 395 °C was quite conducive to decreasing the operation

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temperature for CO2 capture.29,30,37 Moreover, the molten carbonates showed better CO2

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solubility than molten halides and less corrosion to equipments.38–40 The kinetics and

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thermodynamics properties of CO2 uptake by Li2O-containing Li–Na–K molten carbonate

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were thereby investigated and discussed under varied experimental parameters (e.g.,

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absorption temperature, Li2O content, CO2 pressure, and depth of melts). We hope the results

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obtained in this study not only give instruction for the MSCC–ET process but also bring clues

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to other CCU technologies based on molten salts absorption of CO2.

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EXPERIMENTAL SECTION

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Reagents. Li2O powder with 99.9% purity was purchased from Sigma-Aldrich.

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Anhydrous Li2CO3, Na2CO3, and K2CO3 with analytical purity were purchased from

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Sinopharm Chemical Reagent and were selected as the molten salts for dissolving Li2O. The

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composition for Li–Na–K carbonate molten salts mixture of Li2CO3:Na2CO3:K2CO3 was

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43.5:31.5:25 in molar ratio. Prior to the absorption experiments, all the reagents were kept at

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423 K under vacuum for 48 h to remove the moisture. A series of Li2O/Li–Na–K carbonate

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mixtures with various Li2O contents (from 0 to 5 wt %) were prepared for the experiments.

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Figure 1. Schematic of the apparatus for CO2 absorption.

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Apparatus and Procedure of CO2 Absorption Experiments. The CO2 absorption

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process was investigated with a so-called in-situ pressure drop method. The schematic

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illustration of the used apparatus for the experiments is shown in Figure 1. The apparatus

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consisted of two interconnected vessels made of AISI 304 stainless steel (304SS). The vessels

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(3.5 L volume for each) were placed into two furnaces respectively for temperature control

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(±0.01 K accuracy). The pressures in the equilibrium and storage vessels were monitored

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with a digital pressure sensor (0–250 kPa, ±1 Pa precision), and recorded using a

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computerized system (one data point per second frequency). The photo of the apparatus is

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exhibited in Figure S1 in the Supporting Information (SI).

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To conduct the absorption experiments, a known weight (100 g, if not elsewise specified)

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of salt mixture was first transferred into a 7.6 cm diameter crucible in the equilibrium vessel.

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Afterwards, both vessels were purged with argon for 1 h and then evacuated under vacuum

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for at least five times to remove gaseous impurities. The vessels were then heated up to 973 K

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at a heating rate of 5 K/min and kept for 1 h to melt salts and then cooled down to a given 8


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temperature (723, 823, and 923 K, in this study). The parts which exposed to room

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temperature area (e. g. the top of each vessel and the connection tubing), were wrapped with

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thermal insulation material asbesto to prevent the heat loss. Carbon dioxide gas (99.9% purity)

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from the gas tank was then discharged into the gas storage vessel until a target pressure

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reached. When the pressure of the gas storage vessel became stable ( Li2O particle, as shown in Table S1 and Table S2 in the SI). Therefore, CO2

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capture in CaO-containing molten carbonate was much inferior to that in Li2O-containing

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molten carbonate (see Figure S4 in the SI). Overall, the absorption capacity of 5 wt %

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Li2O-containing molten carbonate (approximately 0.7 molCO2/molLi2O) was still larger than

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that of solid Li2O powder (approximately 0.6 molCO2/molLi2O), suggesting the enhancing

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effect of molten carbonate on CO2 absorption.

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Effects of CO2 Pressure and Depth of Molten Salts. The absorption curves at various

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initial CO2 pressures for 0.75 wt % Li2O-containing molten carbonate sample (Figure 5a)

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indicate steady enhancement of absorption rate with increasing the initial CO2 pressure in the

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range of 10–40 kPa. Under the pressure above 40 kPa, no obvious increase in absorption rate

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was observed and the equilibrium absorption amount also slightly changed. In the

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relationship between the mass-specific absorption capacity of Li2O in the melts and the

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equilibrium pressure of CO2 (Figure 5b), the absorption capacity of Li2O sharply improved at

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equilibrium CO2 pressures below ~20 kPa. At equilibrium pressure of CO2 above 20 kPa, the

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capacity only slightly increased with the increase of equilibrium pressure. This observation

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can be explained by thermodynamic analysis at this temperature, as shown in the inset of

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Figure 5b. The theoretical equilibrium concentration of Li2O was calculated using eq 9,

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assuming the Li2CO3 concentration in melts as constant. The equilibrium concentration of

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Li2O significantly decreased when the equilibrium CO2 pressure was below ~20 kPa,

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suggesting a dramatic increase in Li2O carbonation. When the CO2 pressure was above 20

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kPa, the small changes equilibrium Li2O concentration indicated a sluggish Li2O carbonation,

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thereby increase in Li2O mass-specific absorption capacity was not evident.

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Figure 5. (a) Kinetics of CO2 absorption for 0.75 wt % Li2O-containing molten carbonate at

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723 K under various initial CO2 pressures. (b) Correlation between CO2 absorption capacities

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and equilibrium pressures of CO2 in the vessels. Inset: Li2O concentration vs equilibrium

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pressure according to the thermodynamic analysis. (c) Kinetics of CO2 absorption in melts

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(0.75 wt % Li2O-containing molten carbonate) with varying depths at 723 K under 50 kPa

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CO2 pressure. (d) Correlation between CO2 absorption capacities and the depths for 0.75 wt %

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Li2O-containing molten carbonate sample.

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For the study of CO2 absorption kinetics with various depths of molten carbonate (Figure

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5c), 20–200 g melts with 0.75 wt% Li2O were used, and the correlation between weight and

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depth was 100 g cm–1 under molten state. As can be seen, the time to reach the absorption

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equilibrium was prolonged with increasing the depth of melt (see the uini in Table S6 in the

21



363

SI). The corresponding CO2 absorption capacities of Li2O at various depths of melts

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decreased from 0.860 to 0.823 molCO2/molLi2O (Figure 5d). These observations are attributed

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to the fact that larger melt depth would extend the migration distance from the melt bulk to

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the gas–liquid surface for the absorption-related ions (i.e. O2–, CO32–, Cn+1O2n+32–), therefore,

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smaller CO2 absorption capacities were obtained for the melt samples with larger depths. The

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observations here further confirm the importance of mass transfer of absorption-related ions

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for the CO2 absorption under static conditions without forced convection.

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ENVIRONMENTAL IMPLICATIONS

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We demonstrated the efficiency of Li2O-containing molten carbonate for the capture of CO2

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at elevated temperatures. The conversion efficiency for the carbonation of Li2O in molten

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carbonate (Li2O/MxCO3) can be promoted to over 80% with the dissociation/dissolution

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effect of molten carbonate. The present finding paves the way for the industrial scale-up of

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MSCC–ET process, because the carbonation of Li2O and its regeneration (through

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electrochemical reduction) are the key reactions to maintain the sustainable operation in

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MSCC–ET processes. For a practical application, forced convection can be introduced to

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improve the mass transfer rate of ions in the absorbing melts. The electrolysis conditions

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parameters (such as electrolysis current) can be deliberately adjusted to match CO2

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absorption rate for the purpose of net transformation of CO2 to carbon. In comparison with

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the commonly used CaO absorbents, the Li2O is much more dissolvable in molten carbonate

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in a wide range of temperatures, indicating superior CO2 uptake capability with the assistance

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of melts. From an industrial perspective, the Li–Na–K ternary carbonate, as an excellent heat

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absorber and conductor, can be coupled with concentrated solar power systems, which would

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further mitigate the energy demand for CO2 capture and utilization. In the regions with rich

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solar energy, the MSCC–ET technologies based on Li2O-containing molten carbonate can be

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developed for carbon sequestration and simultaneously for the production of value-added

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zero-carbon material, which is a stable pool of carbon that does not cause greenhouse effect.

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ACKNOWLEDGEMENTS

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This work was funded by National Natural Science Foundation of China (51325102,

392

21673162), the International Science & Technology Cooperation Program of China

393

(2015DFA90750), the Program for Creation Team of Hubei Province (2015CFA017) and the

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Young-talent Chenguang Project of Wuhan City.

395 396

ASSOCIATED CONTENT

397

Supporting

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morphological changes of Li2O powder, physicochemical properties of Li–Na–K molten

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carbonate, the solubilities of Li2O/CaO in molten carbonate, standard thermodynamic data

400

and the calculated initial absorption rate constants for various absorption conditions.

401

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Kinetic and Thermodynamic Characterization of Enhanced Carbon

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