CO2 capture properties of alkaline earth metal oxides

THE JOURNAL OF CHEMICAL PHYSICS 133, 074508 共2010兲

CO2 capture properties of alkaline earth metal oxides and hydroxides: A combined density functional theory and lattice phonon dynamics study Yuhua Duan1,2,a兲 and Dan C. Sorescu1 1

National Energy Technology Laboratory, United States Department of Energy, Pittsburgh, Pennsylvania 15236, USA 2 URS Corp., P. O. Box 618, South Park, Pennsylvania, Pennsylvania 15219, USA

共Received 7 May 2010; accepted 9 July 2010; published online 19 August 2010兲 By combining density functional theory and lattice phonon dynamics, the thermodynamic properties of CO2 absorption/desorption reactions with alkaline earth metal oxides MO and hydroxides M共OH兲2 共where M = Be, Mg, Ca, Sr, Ba兲 are analyzed. The heats of reaction and the chemical potential changes of these solids upon CO2 capture reactions have been calculated and used to evaluate the energy costs. Relative to CaO, a widely used system in practical applications, MgO and Mg共OH兲2 systems were found to be better candidates for CO2 sorbent applications due to their lower operating temperatures 共600–700 K兲. In the presence of H2O, MgCO3 can be regenerated into Mg共OH兲2 at low temperatures or into MgO at high temperatures. This transition temperature depends not only on the CO2 pressure but also on the H2O pressure. Based on our calculated results and by comparing with available experimental data, we propose a general computational search methodology which can be used as a general scheme for screening a large number of solids for use as CO2 sorbents. © 2010 American Institute of Physics. 关doi:10.1063/1.3473043兴 I. INTRODUCTION

Carbon dioxide is one of the major combustion products which, once released into the air, can contribute to the global climate warming effects.1 Solid sorbents containing alkali and alkaline earth metals have been reported in several previous studies to be good candidates for CO2 sorbent applications due to their high CO2 absorption capacity at moderate working temperatures.2,3 In our previous studies,4,5 we have explored the CO2 capture capabilities of alkali metal oxides, hydroxides, and carbonates/bicarbonates systems, and found that M2CO3 / MHCO3 共M = Na, K兲 transformation systems can be used efficiently as CO2 sorbents. In this study, we extend our previous analysis by exploring the CO2 capture capabilities of alkaline earth metal oxides and hydroxides, and then, based on these results, we present a general computational scheme that can be used to screen solid materials for CO2 sorbent development. Relative to the extensive experimental studies of capturing CO2 with alkali and alkaline earth oxides,2,3 capturing CO2 with solid alkaline hydroxides is relatively a new approach which is receiving an increasing attention from scientific community. For example, a novel sodium-based sorbent that can capture CO2 from ambient to 600 ° C was developed by Siriwardane’s research group at National Energy Technology Laboratory 共NETL兲.6 This sorbent consisting of a NaOH/CaO mixture can capture CO2 at 315 ° C and can be regenerated at 700 ° C. Such characteristics make it useful for high-temperature CO2 capture applications in coal gasification systems. Recently, the same group has reported the development of a Mg共OH兲2 sorbent which can be used a兲

Author to whom correspondence should be addressed. Tel.: 412-386-5771. FAX: 412-386-4542. Electronic mail: [email protected].

0021-9606/2010/133共7兲/074508/11/$30.00

for CO2 capture at warm gas temperatures.7 In this case, CO2 capture takes place in the 200– 315 ° C range while regeneration is done above 375 ° C, making it suitable for precombustion capture technologies. Stolaroff et al.8 explored the feasibility of using spray-based NaOH to capture CO2 from atmospheric air and concluded that the cost of CO2 capture using NaOH spray in the full-scale system ranges from $53 to $127 per ton CO2. From Mg-rich mineral, Lin et al.9 extracted Mg共OH兲2 powder, which has domain size as small as 12 nm and apparent surface area of 54 m2 / g. Under 1 atm of 10 vol % CO2 / N2, the carbonation of this Mg共OH兲2 powder can achieve up to 26% of the stoichiometric limit at 325 ° C in 2 h and, moreover, it was found that the amount of CO2 fixation was inversely proportional to the crystal domain size of the Mg共OH兲2 specimens, which suggests that only a monolayer of carbonates was formed on the boundaries of the crystal domains in the gas-solid reaction with little penetration of the carbonates into the crystal domains. Using Ca共OH兲2 as a CO2 sorbent in the gasification of wet biomass, Hu et al.3,10 determined that Ca共OH兲2 powder can function not only as a CO2 sorbent but it also had a catalytic action upon the tar cracking. By contrast to the relative large number of experimental studies of CO2 reactions with alkali and alkaline earth oxides and hydroxides, to date there are only few theoretical studies of the same topics. Based on thermodynamic data,2,3 Feng et al.11 analyzed 11 simple metal oxides and concluded that CaO is thermodynamically the best candidate among them for CO2 capture in zero emission power generation systems. Similarly, Siriwardane and Stevens7 have determined based on the analysis of thermodynamic data that Mg共OH兲2 sorbent system is highly favorable for CO2 capture applications and can be regenerated with high-pressure CO2. Through

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ab initio evolutionary simulations and high-pressure experiments, Oganov et al.12 investigated the high-pressure structures of MgCO3, CaCO3, and CO2 and their roles in the Earth’s lower mantle. Based on density functional theory 共DFT兲, Jensen et al.13 investigated the CO2 adsorption on CaO and MgO surfaces. Their results showed that CO2 adsorbs as monodenate on edge sites and bidentate on corner sites of MgO. In contrast, CO2 adsorbs as monodenate on both edge and corner sites of CaO. Using computational fluid dynamics 共CFD兲, Liu et al.14 simulated the chemical absorption of CO2 from air by NaOH aqueous solution and found that the temperature and the concentration distributions along the height of the column are consistent with measured experimental data. Recently, we proposed a methodology to identify promising solid sorbent candidates for CO2 capture by combining DFT total energy calculations with lattice phonon dynamics.15,16 It was concluded that although pure Li2O can absorb CO2 efficiently, it is not a good solid sorbent for CO2 capture applications because the reverse reaction, corresponding to Li2CO3 releasing CO2, can only occur at very low CO2 pressure or at very high temperature when Li2CO3 is in liquid phase.16 These predicted results are in very good agreement with experimental measurements.17 The same computational methodology has been extended recently by us to describe the phase diagrams of M–C–O–H 共M = Li, Na, K兲 systems.5 We have determined that Na2CO3 / NaHCO3 and K2CO3 / KHCO3 are promising candidates for both precombustion and postcombustion capture in agreement with previous experimental findings. In this study, we apply our analysis based on calculation of the thermodynamic data using first-principles density functional theory and lattice phonon dynamics to screen a larger number of alkaline earth metal oxides, hydroxides, and their corresponding composites, and to explore their CO2 capture properties systematically. We compare our predictions with the available thermodynamic data to assess the accuracy of our approach and outline a general screening scheme that can be used to predict the CO2 capture properties of the new types of materials for which thermodynamic data might not be available. This paper is organized as follows: Sec. II briefly describes the theoretical methods employed. In Sec. III, we present the results of the electronic and phonon properties of alkaline earth metal oxides, hydroxides, and carbonates, followed by the analysis of the thermodynamic properties of the corresponding CO2 capture reactions. A general screening methodology that can be applied to solid materials for which thermodynamic properties might not be available is proposed. Finally, a brief summary and conclusions are indicated in Sec. IV.

II. THEORETICAL METHODS

The complete description of our computational methodology can be found in our previous papers.4,5,15,16 Here, we limit ourselves to provide only the main aspects relevant for

the current study. The CO2 capture reactions by solids in the presence of water vapors can be expressed generically in the form Solid _ A + n1CO2 ↔ Solid _ B + 关Solid _ C兴 ⫾ n2关H2O兴, 共1兲 where the terms given in 关…兴 are optional and n1 and n2 are the numbers of moles of CO2 and H2O involved in the capture reactions. We treat the gas phase species CO2 and H2O as ideal gases. By assuming that the difference between the chemical potentials 共⌬␮0兲 of the solid phases of A, B 共and C兲 can be approximated by the difference in their total energies 共⌬EDFT兲, obtained directly from DFT calculations, the vibrational free energy of the phonons,18 and by ignoring the pressure-volume 共PV兲 contribution terms for solids, the variation of the chemical potential 共⌬␮兲 for reaction 共1兲 with temperature and pressure can be written as15,16,19 ⌬␮共T,P兲 = ⌬␮ 共T兲 − RT ln 0

n1 PCO

2

⫾n2 PH O

,

共2兲

2

where ⌬␮0共T兲 ⬇ ⌬EDFT + ⌬EZP + ⌬FPH共T兲 − n1GCO2共T兲 ⫾ n2GH2O共T兲 − ⌬H0 .

共3兲

Here, ⌬EZP is the zero point energy difference between the reactants and products and can be obtained directly from phonon calculations. ⌬H0 is an empirical correction constant and ⌬EPH is the phonon free energy change between the solids of products and reactants as presented in the following. If reaction 共1兲 does not involve H2O, then the PH2O in Eq. 共2兲关and also in the following Eq. 共13兲兴 is set to P0, which is the standard state reference pressure of 1 bar, and the GH2O term in Eq. 共3兲 is not present. The “+” and “⫺” signs in Eqs. 共2兲 and 共3兲 correspond to the cases when H2O is a product and a reactant, respectively, in the general reaction 共1兲. The free energies of CO2 共GCO2兲 and H2O 共GH2O兲 can be obtained from standard statistical mechanics20 as 4

7 N ah ␯ i − TSCO2共T兲, GCO2 ⬇ RT + 兺 h␯ /kT i 2 e −1 i=1 3

GH2O ⬇ 4RT + 兺 i=1

N ah ␯ i − TSH2O共T兲, eh␯i/kT − 1

共4兲

共5兲

where Na is Avogadro’s constant. The entropy of CO2 关SCO2共T兲兴 and H2O 关SH2O共T兲兴 can be accurately calculated from the Shomate equation.21 The vibrational frequencies 共␯i兲 of CO2 molecule are taken as 673 共␲u兲, 1354 共␴g+兲, and 2397 cm−1 共␴u+兲,22 and the vibrational frequencies 共␯i兲 of H2O molecule are 3657.05 共␯1兲, 1594.75 共␯2兲, and 3755.93 cm−1 共␯3兲.23 The zero point energies for CO2 and H2O molecules calculated using these frequencies are 0.3160 and 0.5584 eV, respectively. In Eq. 共3兲, ⌬EDFT is the total energy change of the reactants and products calculated by DFT. In this work, the Vienna ab initio simulation package24 was employed to calculate the electronic structures of solid oxides, hydroxides,

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Theoretical study on CO2 capture properties

and carbonate materials. In this case, all calculations have been done using the projector augmented wave 共PAW兲 pseudopotentials with the PW91 exchange-correlation functional.25 Plane wave basis sets were used with a cutoff energy of 500 eV and a kinetic energy cutoff for augmentation charges of 605.4 eV. The k-point sampling grids of the general form m1 ⫻ m2 ⫻ m3 were obtained using the Monkhorst–Pack method.26 In our bulk calculations, the indices m1, m2, and m3 were selected to corresponding consistently to a spacing of about 0.028 Å−1 along the axes of the reciprocal unit cells. During optimizations all atoms in the cell as well as the lattice dimensions and angles were fully relaxed to equilibrium. The total energies of CO2 and H2O molecules are ⫺22.994 09 and ⫺14.272 67 eV, respectively, as determined from calculations of the isolated molecules in a cubic box with the length of 20 Å. In the harmonic approximation, the phonon free energy change 共⌬FPH兲 and the entropy change 共⌬SPH兲 between the solid reactants and products can be calculated based on the Helmholtz free energy of the solids Fharm and the entropy of the solids 共Sharm兲

兺

⌬FPH共T兲 =

兺

Fharm共T兲 −

solid_products

Fharm共T兲,

solid_reactants

共6兲

兺

⌬SPH共T兲 =

Sharm共T兲 −

solid_products

兺

Sharm共T兲,

solid_reactants

共7兲 where the Fharm, Sharm, and the total vibrational 共phonon兲 energy 共Etot兲 of the solids are defined as18

冋冉冊册冕再冉冊冋冉冊册

Fharm = rkBT

冕

⬁

ប␻ 2kBT

g共␻兲ln 2 sinh

0

⬁

Sharm = rkB

g共␻兲

0

ប␻ 2kBT

冎

coth

冕

⬁

冉冊

g共␻兲共ប␻兲coth

0

共8兲

ប␻ −1 2kBT

− ln关1 − e−ប␻/2kBT兴 d␻ , 1 Etot = r 2

d␻ ,

ប␻ d␻ . 2kBT

共9兲

共10兲

In the above equations r is the number of degrees of freedom in the primitive unit cell, ␻ is the phonon dispersion frequency, and g共␻兲 is the phonon density of states. It can be seen that the zero point energy 共EZP兲 can be obtained from Eq. 共10兲 by taking T → 0, Ezp = lim 共Etot共T兲兲. T→0

共11兲

In this study, the phonon properties were evaluated using the software package27 in combination to the direct method formalism derived by Parlinski et al.28 Specifically, supercells containing at least 2 ⫻ 2 ⫻ 2 unit cells were used in all phonon calculations. Structures with displacements of 0.03 Å of the nonequivalent atoms were generated from the optimized supercells and DFT calculations were further perPHONON

formed to obtain the forces on each atom due to these displacements. These forces were input into the PHONON package27 to fit the force constant matrix and to compute the phonon density of states. These data were subsequently used in Eqs. 共8兲–共10兲 to calculate the thermodynamic properties. In phonon dispersion calculations, the coordinates of the high symmetry points in the first Brillouin zone of the crystals are consistent with those defined by Bradley and Cracknell.29 The enthalpy change for the reaction ⌬Hcal共T兲 can be derived from the above equations as ⌬Hcal共T兲 = ⌬␮0共T兲 + T共n1SCO2共T兲 ⫿ n2SH2O共T兲 + ⌬SPH共T兲兲.

共12兲

As discussed in our previous paper,5 the DFT calculated enthalpy and the chemical potential of reaction are always about 20 kJ/mol higher than the experimental measurements. This is likely due to limitations of DFT in computing the heats of formation and similar differences were also observed for other systems.30,31 In order to empirically correct for this apparent bias, we have subtracted a constant value ⌬H0 from Eq. 共3兲. We have found that using a correction term ⌬H0 = 20 kJ/ mol gives reasonable performance for all systems of oxides and hydroxides considered in this study. By taking the activities of all reactant and product phases to be unity, the equilibrium pressure of the overall reaction can be calculated by setting ⌬␮共T , P兲 = 0 in Eq. 共2兲. Hence, the equilibrium pressure is given by30 PCO2 ⫾n2/n1 PH 2O

冉

= exp

冊

⌬␮0共T兲 . RT

共13兲

By plotting the equilibrium pressures calculated from Eq. 共13兲 as function of temperature T, one can obtain the van’t Hoff plot for CO2 capture reactions. Comparison of the calculated data to the experimental values for the CO2 capture reactions has been done using the 32 HSC CHEMISTRY package. In this last case, the experimental measured thermodynamic data for each compound is fitted using analytic polynomials such as the Kelly equation for the dependence of the heat capacity on temperature, and the resulted polynomial equations are further used to calculate the thermodynamic properties for reactions of interest. III. RESULTS AND DISCUSSIONS A. DFT and phonon calculated results

The optimized lattice constants and the total energies for a set of 15 alkaline earth metal oxides, hydroxides, and carbonates considered in this work are presented in Table I, along with the corresponding experimental structural data. It should be pointed out that each compound might have more than one known phase in the temperature and pressure range investigated. However, in Table I, for each compound, only the most stable structure with the lowest energy is listed. We note that we were unable to find the crystal structure of BeCO3 although its thermodynamic data are available in the literature. From our calculations, the energy for the MgCO3 based on the crystallographic structure of BeCO3 is about 0.5

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TABLE I. Comparison of the experimental and the DFT calculated structural parameters for the list of compounds involved in the CO2 capture reactions studied. All distances are given in angstroms and angles in degrees. Although calculations were performed on several phases of each compound, only the most stable phase, determined by DFT total energy, is listed in this table. The zero point energy and entropy calculated from phonon lattice dynamics as well as the corresponding experimental data are also indicated. Entropy 共J/mol K兲

Calculated Energy 共eV/f.u.兲a

Structural parameters

EDFT

EZP

Phonon 共T = 300 K兲 Expt.b 共T = 298.15 K兲

Space group and reference

Experimental

Calculated

BeO

P63mc 共No.186兲, Ref. 41

a = 2.6979 c = 4.3772 ␤ = 120°

a = 2.710 59 c = 4.400 06 ␤ = 120°

⫺14.366 57 0.225 59

14.713

13.770

MgO

¯ m 共No.225兲, Ref. 42 Fm3 ¯ m 共No.225兲, Ref. 43 Fm3 ¯ m 共No.225兲, Ref. 44 Fm3 ¯ m 共No.225兲, Ref. 45 Fm3

a = 4.2198

a = 4.248 88

⫺12.007 59 0.126 11

33.294

26.950

a = 4.8152

a = 4.819 03

⫺12.987 52 0.110 88

39.374

38.10

a = 5.1326

a = 5.193 50

⫺12.192 23 0.079 03

56.742

55.580

a = 5.539 a = 4.5301 b = 4.621 c = 7.048 a = 3.1425 c = 4.7665 ␤ = 120° a = 3.582 c = 4.904 ␤ = 120° a = 9.8889 b = 6.1202 c = 3.9184 a = 11.03 b = 16.56 c = 7.11 No expt. data available. Taken same as SrCO3

a = 5.660 43 a = 4.508 25 b = 4.650 19 c = 7.011 85 a = 3.168 16 c = 4.749 01 ␤ = 120° a = 3.613 77 c = 4.967 88 ␤ = 120° a = 10.000 19 b = 6.046 30 c = 3.947 27 a = 11.150 59 b = 16.761 30 c = 7.102 24 a = 6.762 93 b = 4.851 91 c = 7.012 98 a = 4.686 49 c = 15.137 95 ␤ = 120° a = 5.039 79 c = 17.126 72 ␤ = 120° a = 6.102 09 b = 5.156 53 c = 8.490 87 a = 5.370 61 b = 9.007 70 c = 6.552 10 rC–O = 1.1755 rO–H = 0.9714 ⬔HOH= 104.2°

⫺11.955 82 0.064 92 ⫺29.293 64 0.909 42

74.184 39.4651

72.00 53.555

⫺27.149 38 0.775 31

65.8659

63.137

⫺28.335 93 0.711 40

82.8147

83.400

⫺27.725 16 0.723 79

93.8210

97.069

⫺27.698 74 0.701 18

109.9148

107.280

⫺36.730 83 0.578 29

50.8681

52.007

⫺35.960 46 0.532 35

69.3531

65.090

⫺37.610 11 0.484 10

95.9951

91.710

⫺37.335 26 0.460 27

104.4270

97.200

⫺37.522 45 0.445 84

116.7325

112.100

Compound

CaO SrO BaO Be共OH兲2

P212121 共No.19兲, Ref. 46

Mg共OH兲2

¯ m1 共No.164兲, Ref. 35 P3

Ca共OH兲2

¯ m1 共No.164兲, Ref. 43 P3

Sr共OH兲2

Pnam 共No.62兲, Ref. 47

Ba共OH兲2

Pnma 共No.62兲, Ref. 48

BeCO3

Pnma 共No.62兲

MgCO3

¯ cH 共No.167兲, Ref. 49 R3

CaCO3

¯ cH 共No.167兲, Ref. 50 R3

SrCO3

Pnma 共No.62兲, Ref. 51

BaCO3

Pmcn 共No.62兲, Ref. 52

CO2 molecule H2O molecule

P1共D⬁h兲 P1共C2v兲

a = 4.6338 c = 15.0192 ␤ = 120° a = 4.991 c = 17.068 ␤ = 120° a = 6.02 b = 5.093 c = 8.376 a = 5.3293 b = 8.9275 c = 6.6076 rC–O = 1.163 rO–H = 0.957 ⬔HOH= 104.4°

⫺22.994 09 0.315 98 ⫺14.272 67 0.558 41

213.388 188.832

a

The acronym f.u. corresponds to one formula unit. Data taken from HSC CHEMISTRY Package 共Ref. 32兲.

b

eV per formula unit higher than the one for the structure based on SrCO3 symmetry. Therefore, for BeCO3, we assumed that this crystal adopts the same crystallographic symmetry as SrCO3. As the data in Table I indicates, the agreement between the optimized structural constants and the experimental data is generally very good. The calculated electronic structural properties 共such as geometrical structure, band structure, etc.兲 are also in good agreement with other theoretical12,33,34 and experimental findings.35,36 The calculated total energy

共EDFT兲 for each compound at the optimized configuration has been used to estimate the reaction enthalpy for CO2 capture reaction as described in Sec. III B. Phonon calculations were performed for each of the compounds listed in Table I and the finite temperature thermodynamic properties were then computed from these data using Eqs. 共2兲–共12兲. As an example, the variation of the calculated phonon free energy and entropy for M共OH兲2 and MCO3 共M = Ca, Mg兲 as a function of temperature are shown in Figs. 1共a兲 and 1共b兲.

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paper. The calculated zero point energies obtained based on Eq. 共11兲 for each compound are also listed in Table I. From the data in Table I, one can see that our calculated entropies of these oxides, hydroxides, and carbonates are very close to the experimental measured values. These findings indicate that reasonable agreement to experimental data for thermodynamic properties of solids can be obtained using the current DFT-based computational method augmented with phonon modes calculations. We note that our calculated electronic properties and phonon dispersions curves 共not shown in this paper兲 for oxides, hydroxides, and carbonates are also quite close to other similar results reported in the literature.12,33,35,36 Therefore, our results can be used reliably for investigation of the thermodynamic properties of the CO2 capture reactions by these oxides and hydroxides as presented in the next sections.

(a)

B. Thermodynamic properties of CO2 capture reactions 1. CO2 capture by alkaline metal oxides

(b) FIG. 1. The calculated phonon related properties for Mg共OH兲2, Ca共OH兲2, MgCO3, and CaCO3 vs temperature: 共a兲 Phonon free energy; 共b兲 entropy.

From Fig. 1共a兲, one can see that the free energy differences between Mg共OH兲2 / MgCO3 and Ca共OH兲2 / CaCO3 are quite similar with the increase in temperature, but the corresponding entropy differences are more pronounced. The entropy of Mg共OH兲2 and MgCO3 are very close to each other, especially at low temperatures 共⬍600 K兲 as shown in Fig. 1共b兲, but the entropy of CaCO3 is higher than that of Ca共OH兲2. Such differences have important implications upon CO2 capture behavior as will be detailed in Sec. IV of this

The reaction thermodynamics at 300 K computed from DFT and phonon calculations are presented in Table II for the CO2 capture reactions of alkaline metal oxides and hydroxides. In addition, the heats and free energies of reactions computed from the HSC chemistry package at 300 K are also reported for comparison. CaO has been widely used in industry to remove CO2 through carbonation/calcination cycle37 and for this reason, we have used the reaction of CO2 with CaO system as the reference reaction in the analysis of all the other systems provided in this study. The heat of reaction 共⌬H兲, the van’t Hoff P-T plot, and the ⌬␮ as a function of PCO2 and T for the reaction CaO+ CO2 = CaCO3 are plotted in Figs. 2共a兲–2共c兲, respectively. Three different sets of data are plotted in each graph. The curves labeled “DFT+ phonon” were computed as described above using the phonon density of states information. The data identified as “DFT-only” were computed by excluding the phonon free energies and zero point energies of the solids because computing the phonon contributions is computationally very demanding. Finally, the data labeled “Exp. From HSC” were computed from the HSC CHEMISTRY

TABLE II. The calculated thermodynamic properties of CO2 capture reactions by alkaline earth metal oxides and hydroxides. ⌬EDFT and ⌬Ezp were obtained at 0 K, while ⌬Hcal, ⌬Gcal, and corresponding experimental data are shown at T = 300 K. All energies values are in units of kJ/mol. Expt. dataa

Calculated thermodynamic properties CO2wt %

Reactions BeO+ CO2 = BeCO3 MgO+ CO2 = MgCO3 CaO+ CO2 = CaCO3 SrO+ CO2 = SrCO3 BaO+ CO2 = BaCO3 Be共OH兲2 + CO2 = BeCO3 + H2O共g兲 Mg共OH兲2 + CO2 = MgCO3 + H2O共g兲 Ca共OH兲2 + CO2 = CaCO3 + H2O共g兲 Sr共OH兲2 + CO2 + SrCO3 + H2O共g兲 Ba共OH兲2 + CO2 = BaCO3 + H2O共g兲 a

Calculated by

HSC CHEMISTRY

129.35 109.19 78.48 42.47 28.70 102.28 75.46 59.40 36.18 25.68

package 共Ref. 32兲.

⌬EDFT ⫺33.051 ⫺92.509 ⫺161.745 ⫺207.344 ⫺248.216 123.911 ⫺8.650 ⫺57.950 ⫺85.746 ⫺112.048

⌬EZP 3.542 8.709 5.523 6.297 6.266 ⫺8.656 ⫺0.051 1.460 ⫺1.986 ⫺1.246

⌬Hcal

⌬Gcal

⫺77.093 ⫺106.054 ⫺176.751 ⫺222.290 ⫺263.336 98.338 ⫺26.487 ⫺74.012 ⫺104.859 ⫺130.921

⫺19.320 ⫺52.666 ⫺129.532 ⫺172.391 ⫺211.895 102.426 ⫺19.977 ⫺70.410 ⫺100.485 ⫺125.411

⌬H ⫺43.090 ⫺100.891 ⫺178.166 ⫺240.494 ⫺272.491 8.579 ⫺19.665 ⫺69.035 ⫺105.424 ⫺121.733

⌬G 9.565 ⫺48.206 ⫺130.127 ⫺188.850 ⫺220.394 16.532 ⫺12.762 ⫺64.033 ⫺97.990 ⫺115.686

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(a)

FIG. 3. The calculated heats of reactions for the alkaline metal oxides capturing CO2. The calculated results and corresponding data from the HSC package for each reaction were plotted in the same color.

(b)

(c) FIG. 2. Comparison of the calculated and experimental thermoproperties for our reference reaction CaO+ CO2 ↔ CaCO3. 共a兲 The calculated reaction heats vs temperature without empirical correction; 共b兲 van’t Hoff plot; and 共c兲 counterplotting the variation of the chemical potential vs CO2 pressure and temperature. For a clear representation, only ⌬␮ = 0 curve is plotted explicitly. On the line ⌬␮ = 0, the reaction is reversible; above the line ⌬␮ ⬍ 0, CaO absorbs CO2 and reaction goes forward; below the line ⌬␮ ⬎ 0, CaCO3 releases CO2, and reaction proceeds backward. The pressure values in panels 共b兲 and 共c兲 are plotted in logarithmic scale.

package32 and are assumed to be essentially equal to experimental data. As described in the previous sections, due to the limitations of the DFT method for predicting the heats of formation, the calculated energy change for each reaction 共⌬H兲 has an offset of about 20 kJ/mol relative to the experimental data, as shown in Fig. 2共a兲. This is a common issue in many

gas-solid reactions30 and is unlikely to be solved within traditional DFT scheme. In order to obtain good agreement with experimental data, we found that a quantity of 20 kJ/mol is necessary to be subtracted from ⌬H for all reactions analyzed here. From Fig. 2共a兲, one can see that DFT-only values represent a poor prediction of the ⌬H values and they show a qualitatively wrong dependence with temperature. In contradistinction, the DFT+ phonon data has about 20 kJ/mol difference and the right curvature along the whole temperature range relative to the experimental HSC data. After making the empirical correction described above, the predicted results are in good agreement with experimental data and this finding is also valid for the other systems described later in this paper 共see Figs. 3 and 5兲. Based on these results, it can be concluded that including the phonon contribution is crucial for predicting the correct trends in ⌬H, especially at higher temperatures. Similarly, in Fig. 2共b兲, it is shown that the DFT-only calculated van’t Hoff plot has about the right slope, but there is an offset relative to the HSC data by several orders of magnitude in the relative pressure. The DFT+ phonon data, however, are in good agreement with the corresponding HSC values. The dependence of the chemical potential for the CO2 capture reaction on the CO2 pressure and temperature is shown in Fig. 2共c兲. In this figure, only the equilibrium curve, defined by ⌬␮ = 0 equation, is explicitly plotted. From Fig. 2共c兲, it is convenient to identify the temperature and pressure ranges, where CO2 will be captured by CaO 共⌬␮ ⬍ 0兲, respectively, where CO2 is released 共⌬␮ ⬎ 0兲 from CaCO3 and CaO is regenerated. We define the maximum temperature 共turnover T兲 at which CO2 can be captured at a given pressure of CO2 as Tmax. For example, at PCO2 = 1 bar, Tmax ⬇ 1200 K. This value agrees reasonably well with the reported experimental operating conditions which indicate that CaO absorbs CO2 up to 1123 K and is regenerated above 1173 K.37,38 The heats of reactions as a function of temperature for

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FIG. 4. The contour plots of the calculated chemical potentials vs CO2 pressures and temperatures for the reactions of alkaline earth metal oxides capturing CO2. Y-axis is plotted in logarithm scale. Only ⌬␮ = 0 curve is shown explicitly. For each reaction, above the ⌬␮ = 0 curve, the hydroxide can absorb CO2 and the reaction goes forward 共⌬␮ ⬍ 0 region兲, whereas below the ⌬␮ = 0 curve, CO2 is released and the reaction goes backward to regenerate the hydroxide 共⌬␮ ⬎ 0 region兲.

the CO2 capture reaction by the set of oxides BeO, MgO, CaO, SrO, and BaO, are plotted in Fig. 3, together with the corresponding experimental data from the HSC package.32 From Fig. 3, it can be seen that the calculated ⌬H values for the CO2 capture reactions by MgO, CaO, and BaO are very close to the experimental data, with differences of less than ⫾5 kJ/mol.4,16 SrO sorbent shows somewhat larger discrepancies of about 12 kJ/mol. Among the set of five oxides, BeO has the largest difference of 15 kJ/mol from experimental data. This is probably due to the fact that for BeCO3, we have not used a crystal structure consistent to the experimental one. As indicated above, this is due to the lack of available experimental data for this structure; instead, we used the SrCO3-based structure for BeCO3, as shown in Table I. Differences between the SrCO3-based structure of BeCO3 assumed here and the real experimental structure might be the reason for the observed larger errors in ⌬H values. The dependence of the chemical potentials 共⌬␮兲 for the CO2 capture reactions by the set of five oxides with the CO2 pressure and temperature are shown in Fig. 4. Overall, it can be seen that by combining DFT with phonon dynamics calculations, very good predictions relative to the available experimental data can be obtained. The major exception is, again, the case of BeO sorbent for which experimental crystallographic data for BeCO3 is not available. Our calculated results 共not indicated here兲 show that for all cases analyzed in this study, the DFT-only approach predicted higher CO2 pressures at each temperature than the DFT+ phonon approach and the experimental data. From Fig. 4, one can see that up to about 1200 K, SrO and BaO can still absorb CO2. The CO2 pressure can range dramatically from very low 共⬃10−30 bar兲 to very high 共⬃1010 bar兲 values despite of the reaction kinetics. These results indicate that during the first half cycle, these oxides are very good for CO2 absorption 共in the ⌬␮ ⬍ 0 region兲. However, as Fig. 4 demonstrates, in the second half cycle of sorbent regeneration where CO2 is released, the corresponding carbonates are very stable and can dissociate only at very

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high temperatures where ⌬␮ ⬎ 0. Overall, these strong sorbents are not efficient candidate systems for CO2 capture applications. From Fig. 4, one can see that BeO can absorb CO2 just above room temperature with lowest pressure of 10−2 bar, which corresponds well to the 共T, P兲 requirements of postcombustion capture technologies as will be presented later in Sec. III C. However, although BeO apparently has a favorable thermodynamics, this system is not a good candidate for CO2 capture because the health issues associated to beryllium powder or dust. Finally, relative to CaO, MgO appears to be a good candidate for warm temperature CO2 capture applications because its turnover temperature is around 600 K. Such characteristics make this system a potential sorbent candidate for precombustion technologies.13 2. CO2 capture by solid alkaline earth metal hydroxides

Usually, the alkaline earth metal hydroxides can absorb CO2 and form carbonates according to reaction N共OH兲2 + CO2 = NCO3 + H2O共g兲共N = Be, Mg, Ca, Sr, Ba兲. The calculated and the experimentally fitted values for ⌬H and ⌬G at 300 K for the reactions of alkaline earth metal hydroxides capturing CO2 are listed in Table II. As one can see, overall, our calculated results are close to the experimental measured values and, in most cases, the corresponding differences are less that 10 kJ/mol. The only exception is Be共OH兲2, where the calculated results for ⌬H and ⌬G are much higher than the fitted experimental values. However, we note that either calculated data or the experimental values for ⌬H and ⌬G are positive within the interested temperature range, which indicates that BeCO3 is unstable even at room temperature and dissociates easily to BeO 共Refs. 4 and 5兲 or Be共OH兲2 when H2O is available. The relative significant differences observed for the case BeCO3 reaction might be due to the unstable nature of this crystal for which no known experimental crystal structure is currently available. The calculated reaction heats for the alkaline earth metal hydroxides absorbing CO2 at different temperatures are depicted in Fig. 5. For comparison, we also indicate in the same figure the experimental fitted enthalpy changes 共⌬H兲 of these reactions based on HSC CHEMISTRY data.32 Similar to the case of alkali hydroxide systems, the observed discontinuities in the fitted curves indicate the existence of different phase transitions which we have not included in the theoretic treatment. From Fig. 5, one can see that overall, our calculated heats of reaction are comparable with the fitted values within a range of about 10 kJ/mol. When moving from Be to Ba, it is found that ⌬H values for CO2 capture reactions become more negative, i.e., heat is released, which indicates that the reverse reactions become more difficult to take place due to larger heat requirements. Finally, the chemical potential 共⌬␮兲 for the reactions of alkaline earth metal hydroxides capturing CO2 can be evaluated based on Eqs. 共2兲 and 共3兲. The dependence of ⌬␮ on T and PCO2 / PH2O variables is depicted in Fig. 6, in which ⌬␮ is represented as a contour plot and only ⌬␮ = 0 curve is shown explicitly. Similar to Fig. 5, the results calculated from HSC

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FIG. 5. The calculated heats of reactions 共⌬H兲 for the alkaline earth metal hydroxides capturing CO2. The calculated results and corresponding data from the HSC package for each reaction were plotted in the same color.

package32 are also plotted in Fig. 6. By comparing Figs. 5 and 6, one can see that similar to the trends observed for the heats of reaction, the calculated ⌬␮ curves are very close to the fitted experimental results. The only main exception is Be共OH兲2 which, as shown above, is a problematic case due to the unstable nature of the product BeCO3 even at ambient conditions. From Fig. 6, it can be seen that N共OH兲2 共N = Ca, Sr, Ba兲 still can absorb CO2 at very high temperatures to form carbonates and the reverse reactions corresponding to N共OH兲2 regeneration can occur only at very low CO2 pressures 共⬍10−10 if PH2O = 1 bar兲. These characteristics indicate that these materials do not represent good sorbent candidates for CO2 capture due to the difficult conditions required for sorbent regeneration. As discussed above, Be共OH兲2 is also not a good candidate due to the unstable nature of the BeCO3. Based on this analysis, it can be concluded that only Mg共OH兲2 represents a good candidate for CO2 capture applications. As indicated in Fig. 6, at CHEMISTRY

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PCO2 / PH2O = 1, this system can absorb CO2 up to 750 K. Above 750 K and in the presence of H2O, MgCO3 dissociates and leads to regeneration of Mg共OH兲2. This prediction is in good agreement with experimental findings.7,9 From Fig. 6, one can also see that regenerating Mg共OH兲2 during the release of CO2 cycle can be done at higher temperature and high pressures of CO2 共30 bar or even higher兲. This pressure range is efficient for practical sequestration applications because using heat to compress CO2 is a much cheaper alternative than using electricity. These energy savings have been also pointed out before by Siriwardane and Stevens.7 Similar to alkali metal hydroxides, when alkaline earth metal hydroxides absorb CO2 and form carbonates, the actual CO2 pressure depends on the steam pressure in the reactor 共see Fig. 6兲. When the steam pressure is increased 共⬎1 bar, for example兲, the PCO2 will be decreased as the ratio of PCO2 and PH2O shown in Fig. 6 is fixed, and vice versa. By comparing the reactions of the alkaline earth metal oxides to capture CO2 with those of their corresponding hydroxides, it can be seen that at low temperatures, the ⌬H values of the former reactions are more negative than the ⌬H of the hydroxide reactions, which means the oxides are much stronger to bind with CO2 than the corresponding hydroxides. When oxides absorb CO2, the entropies are decreased 共⌬S ⬍ 0兲 rapidly. As for hydroxides, when they absorb CO2 to form carbonates, the entropy changes of the reactions do not change much because they also release H2O gas. Therefore, the entropy changes 共⌬S兲 of the CO2 capture reactions by oxides are much larger than the cases of the corresponding hydroxides. When the temperature is increased, the chemical potential change 共which in this case is the same as the Gibbs free energy兲 for CO2 capture reactions by oxides change rapidly relative to the case of the corresponding hydroxides as shown in Figs. 4 and 6, resulting in lower turnover temperatures of the oxide sorbents than the corresponding hydroxides.

3. Applications to precombustion and postcombustion CO2 capture technologies

FIG. 6. The contour plots of the calculated chemical potentials vs CO2 and H2O pressures and temperatures for alkaline earth metal hydroxides capturing CO2. Y-axis is given in logarithmic scale. Only ⌬␮ = 0 curve is shown explicitly. For each reaction, above the ⌬␮ = 0 curve, the hydroxide absorb CO2 and the reaction goes forward 共⌬␮ ⬍ 0 region兲, whereas below the ⌬␮ = 0 curve, CO2 is released and the reaction goes backward to regenerate the hydroxide 共⌬␮ ⬎ 0 region兲.

Under precombustion conditions, after water-gas shifting, the gas stream mainly contains CO2, H2O, and H2. The partial CO2 pressure in this case is around 20–25 bar and the temperature is around 300– 350 ° C. For precombustion technologies, the programmatic goal put forward by the DOE is to capture at least 90% CO2 with an increase in the cost of electricity of no more than 10%.39 In order to minimize the energy consumption, the ideal sorbents should work in the above indicated pressure and temperature ranges to separate CO2 from H2. In this work, we have estimated the turnover temperatures for precombustion conditions of ten capture reactions. These temperatures, denoted with T1 and listed in Table III, represent the values above which the sorbents cannot absorb CO2 and the release of CO2 starts. This indicates that during the first half cycle of the CO2 capture process, the operating temperature should be lower than T1, whereas the operating temperature may be higher than T1 共depending on

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TABLE III. The turnover temperatures 共T1 , T2兲 of the alkaline metal based sorbents capturing CO2 under precombustion and, respectively, postcombustion conditions, assuming PH2O = 1 bar.

Reactions BeO+ CO2 = BeCO3 MgO+ CO2 = MgCO3 CaO+ CO2 = CaCO3 SrO+ CO2 = SrCO3 BaO+ CO2 = BaCO3 Be共OH兲2 + CO2 = BeCO3 + H2O共g兲 Mg共OH兲2 + CO2 = MgCO3 + H2O共g兲 Ca共OH兲2 + CO2 = CaCO3 + H2O共g兲 Sr共OH兲2 + CO2 + SrCO3 + H2O共g兲 Ba共OH兲2 + CO2 = BaCO3 + H2O共g兲

Precombustion T1 Postcombustion T2 共K兲共K兲 450 690 1350 hTa hT ⬍300 hT hT hT hT

370 540 1010 1200 1400 ⬍300 600 hT hT hT

a

hT acronym means that the turnover temperature exceeds the temperature range of practical interest 共1500 K兲.

the desired CO2 pressure兲 during the second half cycle when the sorbents are regenerated and CO2 is released. For postcombustion conditions, the gas stream mainly contains CO2 and N2. In this case, the partial pressure of CO2 is around 0.1–0.2 bar, while the temperature range can vary significantly. In the current analysis, however, we will consider the case of low-temperature capture 共⬍473 K兲. In this case, the DOE programmatic goal for postcombustion technologies is to capture at least 90% CO2 with an increase in electricity cost of no more than 35%.39 The corresponding turnover temperatures 共denoted as T2兲 for postcombustion capture by alkaline earth metal oxides and hydroxides are also listed in Table III. From Table III, it can be seen that the turnover temperatures 共T1 , T2兲 for SrO, BaO, Ca共OH兲2, Sr共OH兲2, and Ba共OH兲2 are all over 1000 K. This means that these oxides and hydroxides are very good for absorbing CO2, but their corresponding carbonates are very stable and can be dissociated to release CO2 only at very high temperatures. Therefore, none of these systems represent good sorbents for CO2 capture under either precombustion or postcombustion conditions. CaO can work at high temperatures 共around 1030 K兲 for CO2 capture under postcombustion conditions 共PCO2 = 0.1 bar兲. These findings agree with the experimental results, which indicate that at PCO2 = 1 bar, CaO absorbs CO2 up to 1123 K and is regenerated above 1173 K.37,38 Comparing with CaO, from the list of alkaline earth metal oxides and hydroxides analyzed here 共see Table III兲, only BeO, MgO, Be共OH兲2, and Mg共OH兲2 can work at lower temperatures for CO2 capture and release. As mentioned in Sec. III B, although BeO apparently has a favorable thermodynamics, this system is not a viable candidate for CO2 capture because of the health issues associated to beryllium powder or dust. Similarly, Be共OH兲2 is not a good candidate for CO2 capture due to its low operating temperature 共⬍300 K兲. Our results show that MgO could be used for both precombustion and postcombustion capture technologies due to its low regenerating temperature 共T2 = 560 K for postcombustion conditions and T1 = 720 K for precombustion conditions兲 which are close to experimental findings.40 However, Mg共OH兲2 can only be used for postcombustion capture tech-

FIG. 7. The calculated phase diagram of MgO– Mg共OH兲2 – MgCO3 system vs the CO2 pressure at fixed PH2O = 0.01, 0.1, 1.0 and 10.0 bar. For each PH2O, only Mg共OH兲2 can be regenerated from MgCO3 for temperatures under the transition values 共Ttr兲. Above Ttr values, only MgO can be obtained.

nologies with a turnover T2 = 720 K because its turnover temperature 共T1兲 is very high, outside the temperature range of interest for precombustion applications. Among the list of alkaline earth metal oxides and hydroxides analyzed in this study, comparing with CaO, only MgO and Mg共OH兲2 are found to be good sorbents for CO2 capture. Upon absorption of CO2, both of these two systems can form MgCO3. However, the regeneration conditions of the original systems can take place at different conditions as indicated in Fig. 7. In this case, we present the calculated phase diagram of MgO– Mg共OH兲2 – MgCO3 system at different CO2 pressures and under several fixed PH2O values 共0.01, 0.1, 1.0, and 10.0 bar, respectively兲. From Fig. 7, it can be seen that when H2O is present and at low temperatures, MgCO3 can release CO2 to form Mg共OH兲2 instead of forming MgO. For example, at PH2O = 0.01 bar, only for temperatures under the transition temperature 共Ttr兲 420 K, MgCO3 can be regenerated to form Mg共OH兲2. By the increase in the H2O pressure, the transition temperature is increased. As shown in Fig. 7, when PH2O is increased to 10 bar, the corresponding Ttr = 600 K. Above Ttr, MgCO3 is regenerated to MgO. Therefore, when water is present in the sorption/ desorption cycle, no matter whether the initial sorbent is MgO or Mg共OH兲2, and for temperatures below Ttr, the CO2 capture reaction is dominated by the process Mg共OH兲2 + CO2 ↔ MgCO3 + H2O共g兲, whereas above Ttr the CO2 capture reaction is given by MgO+ CO2 ↔ MgCO3. The reason is that between MgO and Mg共OH兲2, there is a phase transition reaction MgO+ H2O共g兲 = Mg共OH兲2 happening at the transition temperature Ttr. Obviously, by controlling the pressure of H2O as shown in Fig. 7, the capture CO2 temperature 共T swing兲 can be adjusted. However, more water in the sorbent system will cost more energy due to its sensible heat; there should be a trade-off to balance them in the practical technology. C. A hierarchical computational approach for rapid screening CO2 solid sorbents

In this section, we outline a computational hierarchy for screening a large number of candidates for suitability as CO2

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FIG. 8. Our modeling scheme for screening solid CO2 sorbents.

sorbents for either precombustion or postcombustion capture applications. We have shown that the predicted heats of reactions from DFT-only calculations are higher than the experimental measured values by about 20–30 kJ/mol at low temperatures and that the curvature of the corresponding curves as a function of temperature is often not consistent with the experimental one 关see Fig. 2共a兲兴. From Fig. 2共c兲, one can see that along a wide range of temperatures, the DFT-only predicted chemical potentials of the reactions are also higher than either the experimental data or the results predicted using both the DFT and the phonon free energy calculations. As described in Sec. II, phonon dispersion calculations have to be done with large supercells and this step is computationally intensive. In order to speed up the screening process, it is reasonable to use the DFT-only energies only as a prescreening tool with appropriate error bars. In particular, we have taken the CaO system 共Fig. 2兲 as our reference capture reaction. If the calculated heat of reaction for a test material is lower than the heat of reaction for the reference system, then we include this solid in the list of so-called “good” candidates. In this way we can rapidly eliminate a large number of reactions at minimal computational cost. We then further refine the screening process by performing phonon calculations for materials belonging only to the list of good candidates. A schematic of our search scheme is presented in Fig. 8. This scheme is based on a set of computational filters summarized below. Filter 共0兲: For each solid, we first conduct basic screening based on acquisition of general data, such as the weight percent of absorbed CO2 in the assumption of the complete reaction, materials safety, materials cost, etc. We also include where available the thermodynamic data from literature and from general software package, such as HSC CHEMISTRY, FACTSAGE, etc. If the necessary data for evaluation of the thermodynamic properties exists, then the use of DFT calculations is not necessary. Otherwise, if the material passes basic screening, continue to the next step.

Filter 共i兲: Perform DFT calculations for all compounds in the candidate reaction with this solid. If 兩⌬EDFT − ⌬Eref兩 / n1 ⬍ 20 kJ/ mol, where n1 is CO2 molar number in reaction 共1兲 and ⌬Eref is the DFT energy change for the reference capture reaction 共e.g., CaO+ CO2 = CaCO3兲, we add this compound to the list of good candidates. Otherwise, we go back to filter 共0兲 and pick another solid. Filter 共ii兲: Perform phonon calculations for reactant and product solids to obtain the corresponding zero point energies and the phonon free energies for the list of good candidates. Specify the target operating conditions 共temperature, partial pressures of CO2 and H2O兲 and compute the change in chemical potential for the reaction, namely, ⌬␮共T , P兲 from Eqs. 共2兲 and 共3兲. If ⌬␮共T , P兲 is close to zero 共e.g., 兩⌬␮共T , P兲兩 ⬍ 5 kJ/ mol兲 at the operating conditions, then we select this reaction as a member of the “better” list. Only a short list of compounds will likely be left after application of filter 共ii兲. Filter 共iii兲: Additional modeling could be performed to rank the remaining short list of better candidates. One is the kinetics of the capture reactions, which could be done by transport and diffusion calculations as well as experimental measurements. Another necessary and doable modeling task is the behavior of the solid in the reactor, which can be done by CFD methods based on finite element method approach. These simulations are currently underway. Application of these screening filters will ensure that only the most promising candidates will be identified for the final experimental testing. This screening methodology provides a path for evaluating materials for which experimental thermodynamic data are unavailable. One area where this approach could be used to great advantage is in evaluating mixtures and doped materials, where thermodynamic data are generally not available but for which the crystallographic structure is known or can be easily determined. IV. CONCLUSIONS

We have demonstrated that first-principles density functional theory can be used in combination with phonon density of states calculations to provide a reasonable estimate of the thermodynamics of CO2 capture reactions involving alkaline earth metal oxides, hydroxides, and carbonates. Due to the limitations of the DFT method used, our calculated heats of reaction 共⌬H兲 for each solid capturing CO2 is about 20 kJ/mol higher than the corresponding experimental data from the HSC CHEMISTRY software package. Comparing with CaO, a widely used industrial sorbent, among the alkaline earth metal oxides and hydroxides, we found that MgO and Mg共OH兲2 are better sorbents for both postcombustion and precombustion CO2 capture because they require less regenerating amounts of heat and can work at lower temperature. When MgO is used as CO2 sorbent and in the presence of steam, or when Mg共OH兲2 is used as a CO2 sorbent, both of these systems will lead upon CO2 absorption to formation of MgCO3. However, the regenerated sorbents can be different under different experimental conditions. Generally, with high steam pressure, the Mg共OH兲2 will be

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mainly regenerated at low temperatures. Above certain temperatures 共depending on the pressures of steam and CO2兲, only MgO could be regenerated. We have outlined a general screening method that can be useful for assessing potential sorbents for CO2 capture. This screening approach is particularly useful in those cases when no thermodynamic data exist. This methodology involves applying a series of screening filters of increasing computational complexity in order to identify the optimal systems that can perform under specific operating conditions, consistent to either precombustion or postcombustion CO2 capture technologies. Such a scheme has the potential to identify a number of promising solid sorbent candidates that can be explored experimentally for reversible CO2 capture applications. ACKNOWLEDGMENTS

This work was performed in support of the National Energy Technology Laboratory’s Office of Research and Development under Contract No. DE-FE-0004000 with activity number 4000.2.660.241.001. One of us 共Y.D.兲 thanks Dr. S. Chen, Dr. J. K. Johnson, Dr. Y. Soong, Dr. H. W. Pennline, and Dr. R. Siriwardane for fruitful discussions. D. Aaron and C. Tsouris, Sep. Sci. Technol. 40, 321 共2005兲; C. M. White, B. R. Strazisar, E. J. Granite, J. S. Hoffman, and H. W. Pennline, J. Air Waste Manage. Assoc. 53, 645 共2003兲. 2 J. C. Abanades, E. J. Anthony, J. Wang, and J. E. Oakey, Environ. Sci. Technol. 39, 2861 共2005兲; S. C. Lee and J. C. Kim, Catal. Surv. Asia 11, 171 共2007兲; R. Siriwardane, J. Poston, K. Chaudhari, A. Zinn, T. Simonyi, and C. Robinson, Energy Fuels 21, 1582 共2007兲. 3 G. X. Hu, H. Huang, and Y. H. Li, Int. J. Hydrogen Energy 33, 5422 共2008兲. 4 Y. Duan, Proceedings of the Eighth Annual Conference on Carbon Capture and Sequestration, Pittsburgh, PA, 4–7 May 2009. 5 Y. Duan, B. Zhang, D. C. Sorescu, and J. K. Johnson, “CO2 capture properties of M-C-O-H 共M⫽Li, Na, K兲 systems: a combined density functional theory and lattice phonon dynamics study,” J. Solid State Chem. 共2010兲共submitted兲. 6 R. Siriwardane, U.S. Patent No. 7,314,847 共2008兲; R. Siriwardane, C. Robinson, M. Shen, and T. Simonyi, Energy Fuels 21, 2088 共2007兲. 7 R. V. Siriwardane and R. W. Stevens, Ind. Eng. Chem. Res. 48, 2135 共2009兲. 8 J. K. Stolaroff, D. W. Keith, and G. V. Lowry, Environ. Sci. Technol. 42, 2728 共2008兲. 9 P. C. Lin, C. W. Huang, C. T. Hsiao, and H. Teng, Environ. Sci. Technol. 42, 2748 共2008兲. 10 G. X. Hu and H. Huang, Biomass Bioenergy 33, 899 共2009兲. 11 B. Feng, H. An, and E. Tan, Energy Fuels 21, 426 共2007兲. 12 A. R. Oganov, S. Ono, Y. M. Ma, C. W. Glass, and A. Garcia, Earth Planet. Sci. Lett. 273, 38 共2008兲. 13 M. B. Jensen, L. G. M. Pettersson, O. Swang, and U. Olsbye, J. Phys. Chem. B 109, 16774 共2005兲. 14 G. B. Liu, K. T. Yu, X. G. Yuan, and C. J. Liu, Ind. Eng. Chem. Res. 45, 3220 共2006兲. 15 Y. Duan, Proceedings of the Seventh Annual Conference on Carbon Capture and Sequestration, Pittsburgh, PA, 5–8 May 2008. 16 Y. Duan and D. C. Sorescu, Phys. Rev. B 79, 014301 共2009兲. 1

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Theoretical study on CO2 capture properties 17

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