High Temperature CO2 Sorption on Li2ZrO3 Based Sorbents

Article pubs.acs.org/IECR

High Temperature CO2 Sorption on Li2ZrO3 Based Sorbents Chao Wang,† Binlin Dou,*,† Yongchen Song,† Haisheng Chen,*,‡ Yujie Xu,‡ and Baozhen Xie† †

School of Energy and Power Engineering, Key Laboratory of Ocean Energy Utilization and Energy Conservation of Ministry of Education, Dalian University of Technology, Dalian 116023, China ‡ Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100190, China S Supporting Information *

ABSTRACT: In this study, the Li2ZrO3 based sorbents with different compositions were synthesized by the solid-state reaction method from the mixtures of Li2CO3, K2CO3 and ZrO2. CO2 sorption properties of Li2ZrO3 based sorbents were investigated by analyzing the phases and microstructure changes with the help of thermogravimetric analysis, X-ray diffraction and scanning electron microscopy. The thermodynamic calculations were carried out based on the second law of thermodynamics. Li2CO3/ K2CO3-doped Li2ZrO3 sorbent with the composition of 36.23 wt % Li2CO3, 55.12 wt % ZrO2 and 8.65 wt % K2CO3 was considered to achieve excellent capability for high temperature CO2 sorption and presented the maximum sorption rate at 525 °C and 0.15 atm of CO2 partial pressure. The sorbent kept rather stable for multicycles sorption and regeneration, and maintained its original capacity during 12 cycle processes. There were three distinct phases in the nonisothermal CO2 sorption process while the main CO2 sorption occurred during the second phase. An improved iterative Coats−Redfern method was used to evaluate nonisothermal kinetics of the CO2 sorption process, and the kinetic parameters were derived by the MATLAB model. The Fn nth-order reaction model predicted accurately the main phases and differences in the activation energies and the frequency factors for different sorbents in the sorption phases corroborated different mechanism integral functions and reaction orders.

1. INTRODUCTION The most economical route in the production of hydrogen from hydrocarbon feedstock by steam reforming and in situ CO2 sorption during steam reforming has been considered to change the normal equilibrium limits of shift reactions.1,2 Thus, hydrogen generation using a CO2 sorption-enhanced steam reforming process is a promising technology to increase hydrocarbon conversion and achieve CO2 captures. In addition, it is a low-cost hydrogen production method because of the reduction in the number of processing steps required for subsequently separating CO2. Many studies have claimed that CO2 emissions from the utilization of fossil fuels contribute to global warming.3,4 Several methods have been suggested for CO2 capture including dry adsorption, wet absorption, membrane separation and cryogenic distillation, etc. Chemical sorption using solid sorbents has been proposed for CO2 capture for its potential advantages such as reducing energy penalties, avoidance of liquid wastes and the relatively inert nature of solid wastes.5−7 The desired sorbents for sorptionenhanced steam reforming process should be capable of reacting fast toward a maximum capture capacity. They should also maintain a high CO2 sorption capacity, stability and adequate mechanical strength after carrying out a certain number of regeneration cycles in order to control the capital cost.8 Some of the conventional sorbents show certain inherent shortcomings, such as dramatic changing of molar volume during CO2 capture−regeneration, the requirement of high temperature for regeneration and serious decline of CO2 absorption efficiency in terms of uptake rate and capacity in the presence of steam. Among various solid sorbents proposed and developed, a lithium zirconate (Li2ZrO3) based sorbent has been reported to be one of the most promising candidates for © 2014 American Chemical Society

its favorable CO2 sorption characteristic at high temperatures.9−11 Fauth et al. have studied several lithium zirconate sorbents modified by eutectic salts. They found Li2ZrO3 promoted by K2CO3/NaF/Na2CO3 eutectic salt has the fastest CO2 uptake rate and highest CO2 capacity at 600 and 700 °C.9 Iwan et al. reported that the soft-chemistry route for synthesizing the lithium zirconate sorbent resulted in the faster rates of reaction/regeneration with the highest rate 0.83 wt % min−1 and lithium zirconate was found to be stable in consecutive forward−backward reaction cycles.12 Xiong et al. studied the addition of K2CO3 and Li2CO3 in the pure Li2ZrO3 to improve the CO2 sorption rate of the Li2ZrO3 materials and they also gave a double-shell model to explain the mechanism of the CO2 sorption/desorption on both pure and modified Li2ZrO3.13 The forward reaction pathway R1 describes absorption of CO2, whereas the reverse reaction path expresses regeneration of the Li2ZrO3 sorbent. Temperature swing approaches could easily change the direction of the reaction:12 450 ° C < T < 650 ° C

Li 2ZrO3(s) + CO2 (g) XooooooooooooooooooY Li 2CO3(s) + ZrO2 (s) T > 650 ° C

(R1)

The acceleration of a high temperature CO2 sorption reaction is associated with the formation of an eutectic carbonate such as Li2CO3 and K2CO3. Ida and Lin proposed a comprehensive double-shell model to describe the mechanism of CO2 sorption on pure Li2ZrO3 and potassium-doped Li2ZrO3 sorbents,14 and this model has been proved to be Received: Revised: Accepted: Published: 12744

May 19, 2014 July 25, 2014 July 25, 2014 July 25, 2014 dx.doi.org/10.1021/ie502042p | Ind. Eng. Chem. Res. 2014, 53, 12744−12752

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reliable in some studies.15,16 However, pure Li2ZrO3 is found to be of rather slow CO2 sorption rate while doping Li2CO3/ K2CO3 into Li2ZrO3 has some beneficial effects due to form eutectic molten carbonate at high temperatures.17 The molten carbonate “shell” has the ability to greatly enhance CO2 diffusion speed through the sorbent particle; moreover, the distance from the outer surface to the center of the sorbent for CO2 diffusion is dramatically shortened.17 CO2 sorption rate of the Li2CO3/K2CO3-doped lithium zirconate sorbents can be increased with increasing the Li/K molar ratio at a certain level; however, the equilibrium absorbed amount of CO2 may be decreased with increasing this ratio.11,13,14 In this paper, we studied high temperature CO2 sorption on Li2ZrO3 based sorbents prepared by a solid-state synthesis method. The thermodynamic calculations were carried out based on the second law of thermodynamics. The nonisothermal kinetics for CO2 sorption process in different phases was evaluated using an improved iterative Coats−Redfern method, which allows the kinetic parameters to be estimated iteratively by linear regression and thus enhances the accuracy.18

Lithium carbonate (Li2CO3) and potassium carbonate (K2CO3) were purchased from Shantou Xilong Chemical Co., Ltd., zirconium dioxide (ZrO2) was purchased from Sinopharm Chemical Reagent Co., Ltd. and anhydrous ethanol, as an adhesive, was purchased from Tianjin Fuyu Chemical Co., Ltd. All the reagents were analytical grade and used as received without further purification. The deionized water was obtained from Millipore Milli-Q ultrapure water purification systems with a resistivity larger than 18.2 MΩ. 2.2. Sample Characterization. The specific surface areas of the sorbents were determined with the Brunauer−Emmett− Teller (BET) method with N2 adsorption at 77 K determined by a Micrometric Acusorb 2100E apparatus. The phase structure of the prepared samples was examined by the X-ray powder diffraction (XRD) spectra performed using a Shimadzu XRD-6000 powder diffractometer, where a Cu target Kα-ray (operating at 40 kV and 30 mA) was used as the X-ray source. The XRD patterns were recorded over a 2θ range from 10° to 80°. Phase identification was analyzed by MDI Jade 5.0. The surface morphological details of the obtained sorbent samples were investigated using scanning electron microscopy (SEM) (LEO 1530). Before image capturing, the sample powders were dispersed on a conductive adhesive carbon tab placed on a SEM mount. All SEM images were obtained from secondary electrons with 20 kV of accelerating voltage. 2.3. CO2 Sorption by TGA Apparatus. Thermogravimetric analysis (TGA) of the sorbents for CO2 sorption was carried out using a Stanton-Redcroft thermogravimetric analyzer at a heating rate of 15 °C/min with a dwell time of 10 min for nonisothermal TGA experiments. The sample holder was a platinum basket 1.4 × 10−3 m in diameter and 2.0 × 10−3 m in height. Temperature and sample weight were continuously recorded by a computer. For each run, about 35− 50 mg of sorbent was placed in the sample holder. The total feeding gas flow rate during the test was maintained at 100 mL/ min (0.15 atm CO2, 0.85 atm N2). CO2 partial pressures were set by controlling the composition of the feed (CO2/N2) using a rotor flow controller. The selected partial pressure of CO2 is applicable to the common steam reforming process. Initial experiments were carried out to determine the total gas flow needed to eliminate external diffusion effects around the sample basket.

2. EXPERIMENTAL SECTION 2.1. Sorbents Preparation. A solid-state reaction method was employed to prepare pure Li2ZrO3 powders with the materials of reagent-grade Li2CO3 and ZrO2 in a 1:1 molar ratio. The materials were first weighed, mixed and ground in an agate mortar with a suitable amount of ethanol as a liquid binder, which permits much closer mixing of the reactants in the aqueous slurry phase. Afterward, the mixtures were dried at 80 °C for 2 h and kept in a crucible, and then calcined under air atmosphere for 12 h at 900 °C. The selection of calcination temperature is based on the fact that the reaction to form lithium zirconate (Li2ZrO3).17 Both temperature increase and decrease ramping rates for calcination process were set to be 60 °C/h. After the calcination process, the products were ground to powder again in the agate mortar for later use. Powders of Li2CO3 and K2CO3 mixtures were also prepared following the same preparation procedure mentioned above. According to the phase diagram, it is known that the mixture of Li2CO3 and K2CO3 can form an eutectic mixture having the melting point of 498 °C.14 Within the temperature range of CO2 sorption, the mixture of Li and K carbonates is partially or totally liquid depending on their composition ratio in the mixture.17−19 Therefore, the molar ratios of the sorbents (Li 2 CO 3 :ZrO 2 :K 2 CO 3 ) considered in this work were 1.0:1.0:0.2, 1.1:1.0:0.2 and 1.2:1.0:0.2, denoted as S1, S2 and S3, respectively. The preparation method is to invoke the reverse reaction describing the typical potassium-doped lithium zirconate (Li2ZrO3), which can be expressed as18,19

3. RESULTS AND DISCUSSION 3.1. Sorbent Characterization. The values of BET surface areas and compositions of the synthesized sorbents are summarized in the Supporting Information (Table S1). It can be found that the BET surface areas of larger than 7 m2 g−1 for the synthesized sorbents in this work are larger than those with the similar compositions reported as those generally under the level of 4 m2 g−1, and are also larger than those of the sorbents prepared by the liquid-phase method.20 The XRD patterns of Li2CO3/K2CO3-doped Li2ZrO3 with 0%, 10% and 20% molar fraction excess of Li2CO3 prepared with ZrO2 are illustrated in Figure 1. More minor reflections are identifiable in these patterns, and most peaks belong to lithium and potassium carbonates. All of the three samples exhibit dominant phases of monoclinic and tetragonal Li2ZrO3. Reflections at around 20° could be assigned to a minor presence of monoclinic lithium zirconate (Li2ZrO3) phase. The reflections at 31° are confirmed to be Li2CO3 and LiKCO3, because the peaks assigned to these two substances overlap or too close to be easily distinguished. Between 26° and 32°, two

(1 + x)Li 2CO3 + 1.0ZrO2 + 0.2K 2CO3 ⇔ 1.0Li 2ZrO3 + (x)Li 2CO3 + 0.2K 2CO3 + CO2 (R2)

It could be noticed from the reaction that x (x = 0%, 10%, 20%) excess of Li2CO3 is used, which implies 0%, 33% and 50% Li2CO3 mole fractions in the carbonate mixture are achieved. There is a fact known as a solid phase presents with the eutectic liquid phase at the operating temperature for S2 and S3. When the sorbent prepared with a 10% excess of Li2CO3 (S2), only a liquid phase presents at the operating temperature over 613 °C, and for S3, the temperature is 503 °C.19 12745

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414.5, 402.0 °C for S1, S2 and S3 sorbents, respectively. It seems that the starting temperatures of sorption decrease with the increasing of the Li/K ratio at the same heating rate (15 °C/min); this could be explained by that a low Li/K ratio may not be favorable to induce the start of the reactions at the phase of partially liquid. On the other side, temperatures above 500 °C could provide sufficient conditions to ensure the formation of a total molten liquid phase of potassium lithium carbonate between sorbent particles during the carbonation process, in this case, the sorption reaction could be fully triggered considering the phase transition temperature of the Li and K carbonates mixture. When the temperature came to around 525 °C, the sorption reaction rate of S2 sorbent reached its maximum value and was maintained high until the temperature rose to about 565 °C (shown in Figure 3b). This fact indicates that the S2 sorbent could obtain a larger reaction rate at lower temperatures. The temperatures of CO2 conversion decrease varied as 691.5, 635.5, and 634 °C for S1, S2 and S3, respectively, and this decrease may be attributed to the presence of secondary phases or impurities which do not absorb CO2.21 These temperatures may be suggested as sorbents regeneration conditions. The three sorption curves for each samples was similar in shapes. CO2 sorption process by Li2CO3/K2CO3-doped Li2ZrO3 sorbents could be divided into three phases, as described in Figure 4, this double-shell model was also proposed by Ida and Lin.14 This model described the reactions of CO2 sorption as three progressive phases for nonisothermal sorption processes. Some facts should be mentioned that the reaction of Li+ and O2− from the Li2ZrO3 core and carbon dioxide molecules leads to the formation of ZrO2 shell. This reaction is considered to be the first period when referring to the schematic diagram (Figure 4a) of the mechanism of pure Li2ZrO3 particle. Outside this ZrO2 shell, the development of Li2CO3 shell caused by the reaction of CO2 molecules with Li+ and O2− from the Li2ZrO3 core penetrating the ZrO2 shell, this process is assumed to be the second period. With respect to the third period, sorption reaction becomes weakened after the double-shell structure is stable, while there is almost a full formation of the Li2CO3 shell. Only a small amount of carbon dioxide could react with Li+ and O2−, which are difficult to diffuse onto the external surface of the dense Li2CO3 shell. On the other hand, the Li2CO3/K2CO3-doped Li2ZrO3 sorption mechanism seems slightly different (see Figure 4b). The molten carbonate region instead of dense Li2CO3 shell shows better performance of reducing the resistance of CO2 molecular diffusion. Those phases for each conversion curve covered about 90% (from 5% to 95%) of the conversion increment, spanning from (461.50−528.75)°C to (614.75−666.75)°C depending on the different Li/K ratios. The conversion increment, initial sorption temperature, maximum sorption temperature and final sorption temperature for each phase in the CO2 sorption are detailed described in the Supporting Information (Table S2) for the three sorbents. When the individual phases are looked at more closely, the initial parts (PH1) for the sorbents are supposed to be the process of carbon dioxide molecules move to the surface of the Li2ZrO3 particles and react with Li2ZrO3 to form a solid ZrO2 shell. The ZrO2 shell formation temperature ranges are 528.75−548.50 °C, 486.00−506.25 °C and 461.50−481.75 °C, and the formation consumed 10.00%, 5.99% and 5.91% CO2 conversion, respectively. There is a fact that the higher the Li/K ratio is, the lower the melting point will be. Therefore, the

Figure 1. XRD patterns of the three sorbents.

peaks situated to the LiKCO3 phase appear. When the lithium amount in the sorbent is increased for S2 and S3, the intensity and width of these peaks assigned to substances with lithium are gradually enhanced, indicating a gradual increase in the content of lithium (e.g., peaks at 20°, 40°, 42°, 59°). Patterns assigned to ZrO2 that appeared may be due to an incomplete conversion during the initial preparation or the low-temperature reaction with atmospheric carbon dioxide.12 In particular, when the composition ratio of Li2CO3:ZrO2:K2CO3 reaches 1.2:1.0:0.1, small peaks of Li6Zr2O7 can be observed together with Li2ZrO3 for S3, as shown in Figure 1. The results suggest that when the amount of excess lithium carbonate (to zirconium oxide) is greater than that of potassium carbonate, the following reaction could occur.15 3Li 2CO3 + 2ZrO2 ⇔ Li6Zr2O7 + 3CO2

(R3)

The microscopic views of Li2CO3/K2CO3-doped Li2ZrO3 were characterized by SEM, and the surface morphologies of these sorbents are illustrated in Figure 2 a−c. Polygonal sorbent particles have relatively uniform size and stick together to form large irregular shaped porous agglomerates, and this morphology may correspond with high temperature thermal treatment. As it is reported, the particle size of Li2ZrO3 was strongly influenced by the size of ZrO2, due to the ZrO2 particles acting as cores. During the calcination in the preparation processes, only ZrO2 particles remained in the solid state due to their high melting point.17 Therefore, the size of the particles could be observed around 1 μm, which is about the same size of the ZrO2 powder used in the preparation. At the same time, the presence of potassium may inhibit the growth of the particles.10,17 Taking the SEM images of the fresh sorbents and the spent ones for comparison in Figure 2a−f, it could be noticed that during the CO2 sorption process, CO2 molecules are supposed to diffuse into the bulk of the particle structure and gradually react with the shell, which causes the breakup and reshaping of the surface morphology by liquid carbonates. This may be the explanation of a relatively smooth surface morphology of particle aggregation for the spent sorbents. 3.2. Sorption Properties. Data for sample mass during the experiments was monitored, and conversions were calculated on the basis of mass change, assuming mass change occurred only caused by the sorption of CO2. It is found in Figure 3a that mass increase started from the temperatures of 506.5, 12746



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Figure 2. SEM images of fresh sorbents: (a) S1; (b) S2; (c) S3.

(PH3), the conversion increment varies from 10.63% to 14.74%. The core and the shells were structurally stable and the sorption capacity may gradually decrease until it saturated. The SEM images of consumed sorbents (shown in Figure 2d−f) show that the particle surface is heavily sintered compared with the fresh ones (shown in Figure 2a−c). This may create a “shell” around the particle, so that the particle size is observed to be larger.22 As it is known, increasing temperature can increase reaction rate; however, it would also increase the equilibrium CO2 partial pressure, which leads to reduce the driving force for sorption at the constant operation CO2 pressure. The temperature effect on the sorption rate depends on both thermodynamic and kinetic factors. Therefore, a moderate

starting temperature of PH1 decreases as the Li/K ratio increases. Main mass increment of CO2 sorption occurred during the second phase (PH2) from 482.00−548.75 °C to 587.00−644.75 °C, depending on the Li/K ratio. The percentage of the mass increment during PH2 was about 69.87−70.00%. Obviously, a major carbonation process was carried out at PH2. In this phase, solid carbonate transformed into the molten state due to the high temperature; meanwhile, the starting point of the rapid increase in CO2 sorption was reached. And the diffusion of CO2 in the molten carbonate is faster than that in the solid carbonate. Thus, the CO 2 conversion increases dramatically at this phase. During PH3, reaction intensity was in decay, reflected by the decrease of the mass increasing rate. Within the final phase of CO2 sorption 12747



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Figure 4. Illustration of the double-shell model for CO2 sorption on (a) pure Li2ZrO3 and (b) Li2CO3/K2CO3-doped Li2ZrO3.

Figure 3. CO2 sorption profiles of the sorbents in 15 vol % CO2 at the heating rate of 15 °C/min: (a) CO2/sorbent ratio; (b) the rate of reaction.

temperature at around 525 °C would be appropriate. Figure 5 shows the multicycles sorption performance of the best S2 sorbent at 450 and 525 °C with desorption temperature of 850 °C As shown in Figure 5, the sorbent reached its maximum sorption capacity of 42 (CO2 mg/sorbent (g)) at about 60 min for 450 °C and 83 (CO2 mg/sorbent (g)) at about 90 min 525 °C, and the values were reasonable compared to those of other work.23 On the basis of these results, each cycle sorption was performed within 60 min at 525 °C and 40 min at 450 °C while the sorbent was regenerated under 850 °C for 60 min. It can be seen that there were no significant differences in terms of sorption capacity during 12 cycles processes and the decrease in the sorption capacity after 12 cycles is very small; therefore, the sorbent appears to be stable for those conditions. 3.3. Thermodynamic Analysis. For the thermodynamic analysis, it is possible to assess the range of temperature and pressure favoring the forward or reverse reactions given by R1. The sorption reaction involves both the solid and the gaseous species. Therefore, an expression for Gibbs free energy involves partial pressure of CO2 is presented as follows: ΔG =

⎛ pCO ⎞ 2 ⎟⎟ p ⎝ 0 ⎠

∑ ηiΔHi − T ∑ ηiΔSi + ηCO RT ln⎜⎜ 2

i

i

Figure 5. CO2 sorption performance of S2 sorbent at 450 and 525 °C with desorption temperature of 850 °C. 0 ΔH = ΔH298 i + A i t + Bi

ΔS = Ai ln(t ) + Bi t + Ci

E t2 t3 t4 + Ci + Di − i + Fi − Hi 2 3 4 t

E t2 t3 + Di − i2 + Gi , 2 3 2t

t=

(2)

T 1000

(3)

It is possible to derive the dependence of Gibbs free energy on pressure; also the condition of spontaneous reaction (G < 0) could be established as follows:

(1)

where η represents a stoichiometric coefficient of species i in eq 1 and p0 is the standard pressure. ΔHi and ΔSi refer to the enthalpy and entropy of component i formation, given as functions of temperature in the following equations:11,24

ΔG =

⎛ pCO ⎞ 2 ⎟⎟ < 0 ⎝ p0 ⎠

∑ ηiΔHi − T ∑ ηiΔSi + ηCO RT ln⎜⎜ 2

i

i

(4) 12748



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dα = kf (α) dt

where pCO

2

p0

⎛ ∑ η ΔHi ∑ η ΔSi ⎞ ⎟ > exp⎜ i i − i i R ⎠ ⎝ RT

The reaction rate constant, k, is given by the Arrhenius equation:

On the basis of the studies of lithium zirconate (Li2ZrO3), the Gibbs energy for the formation of Li2ZrO3 can be described as a polynomial function of temperature:11,24,25 ΔG Li 2ZrO3 =

0 ΔG298

+ aT + bT

2

⎛ E ⎞ ⎟ k = A × exp⎜ − ⎝ RT ⎠

0 ∑ ηjΔH298 j − ∑ ηj A j t (ln(t ) − 1) j

−

g (α ) =

j

t2 2

1 − 2t

∑ ηjBj − j

t3 6

t4 12

∑ ηjCj − j

j

j

⎛ pCO ⎞ 2 ⎟⎟ − ΔG Li 2ZrO3 + ηCO RT ln⎜⎜ 2 ⎝ p0 ⎠

∫0

α

dα = (A / β ) f (α )

∫0

T

⎛ E ⎞ ⎟d T exp⎜ − ⎝ RT ⎠

(9)

where β denotes the constant heating rate (dT/dt), g(α) denotes the integral form of the rate expression. Because there is no exact solution to this integral, the following approximation is introduced by Coats and Redfern:26,27

∑ ηjDj j

∑ ηjEj + ∑ ηjFj − t ∑ ηjGj − ∑ ηjHj j

(8)

where A, E and R are, respectively, the prefactor, the activation energy and the universal gas constant. The integral conversion function could be obtained as

(5)

The Gibbs energy for reaction of CO2 with lithium zirconate (Li2ZrO3) is given by the following equation eventually: ΔGsorption =

(7)

j

g (α ) =

ART 2 ⎛⎜ 2RT ⎞⎟ ⎜⎛ E ⎞⎟ 1− exp − ⎝ E ⎠ ⎝ RT ⎠ βE

(10)

The Coats−Redfern (C&R) method, though widely used, has limitations such as taking average temperature to calculate the constant term instead of accurate temperatures, the lack of descriptions for the reaction mechanism and the use of too few data points. These evidence conclude that the Coats−Redfern method needs to be improved.25,28 Therefore, Urbanovici et al. proposed an improved iterative version of the Coats−Redfern method to evaluate nonisothermal kinetic parameters:26,27

(6)

where η represents a stoichiometric coefficient of species j, i.e., CO2, Li2CO3 and ZrO2. The temperature−pressure envelope for the forward and the reverse reactions was calculated by eq 6, and the results are given in Figure 6. At a partial pressure of

⎛ AR ⎞ ⎛ g (α ) ⎞ ⎡ ⎛ E ⎞⎤ E ⎟ = ln⎜ ln⎜ 2 ⎟ − ln⎢Q ⎜ ⎟− ⎥ ⎣ ⎝ RT ⎠⎦ ⎝ T ⎠ ⎝ βE ⎠ RT

(11)

( ) − ln(Q )) represents the C&R fit

where the term (ln

g (α) T2

E RT

function. The aim of the iterative method is to minimize Sres representing the sum of the squares of the residual terms: 2 ⎛ ⎡ g (α ) ⎤ ⎛ AR ⎞ E ⎞ i ⎟ ⎥ − − + ln[ Q ] ln ⎜ ⎟ ⎟ (E / RTi) RTi ⎠ ⎝ Eβ ⎠ T2 i=1 ⎝ ⎣ i ⎦ N

Sres =

∑ ⎜⎜ln⎢

(12)

with a and b denoted as ln(AR/Eβ) = a and E/R = b, eq 12 could be written as

Figure 6. Dependence of Gibbs free energy as a function of CO2 partial pressure and temperature.

2 ⎛ ⎡ g (α ) ⎤ b⎞ i = ∑ ⎜⎜ln⎢ 2 ⎥ − ln[Q (b / T )] − a + ⎟⎟ i Ti ⎠ T i=1 ⎝ ⎣ i ⎦ N

Sres

CO2 as 1 MPa, the forward reaction of formation of carbonates takes place up to 714 °C. Above this temperature, the reversible reaction is more favorable thermodynamically. And theoretically, the reverse reaction would proceed at temperatures as low as 310 °C, by lowering the CO2 pressure below 1.0 × 10−3 MPa. Therefore, the observed formation of carbonates process of the three lithium zirconate (Li2ZrO3) based sorbents agrees well with thermodynamics calculation. Furthermore, S1 and S2 sorbents are observed to be more readily for the favorable reaction. 3.4. Kinetics for Nonisothermal CO2 Sorption Process. With α defined as the conversion of CO2 sorption, kinetics of CO2 sorption on sorbents by TGA was assumed to comply with the following equation:26,27

(13)

To calculate the minimal residue, the following conditions must be checked: ∂Sres ∂S = res = 0 ∂a ∂b

(14)

The values of A and E can be derived from the calculation of a and b. For iteration 1, the first iteration is conducted with the assumption of Q(b/Ti) ≅ 1, leading to values a1 and b1, from which E1 and A1 are extracted. And for iteration 2, 3,...k, the assumption is made that ln|Q(b/Ti)| ≅ ln|Q(bk−1/Ti)| where ak and bk are calculated for each iteration. Calculation stops when the error is lower than the criterion ea and eb.26,27 12749


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and

|bk − bk − 1| < eb bk

Article

(15)

Best fit values of a and b generate the pre-exponential factor A and the activation energy E of the reaction using the following equations: E = b·R and A = b·β·exp(a). The usual mechanisms and the corresponding forms of g(α) are mentioned in the previous work (see the Supporting Information, Table S3).29 The iterative method has been originally implemented in a calculation program using Matlab 7.1. Experimental data of sorbent conversions for CO2 sorption can be exploited for different mechanisms. The conversions obtained from TGA experiment were recalculated as a function of temperature using the values of E and A to find the best fit mechanism function. And the models were evaluated by minimizing the equation for the sum of the squares of residual errors (SSE). The activation energy and prefactor values with SSE are calculated for PH1, PH2 and PH3, respectively (see the Supporting Information, Tables S4−S6). It should be noted that n-values for the models of Mampel power law and Avrami−Erofeev are selected to be small in order to reduce the amount of calculation without reducing their reliability. In particular, Mampel power law and power law share the same form of g(α) when n-value equals to 1. Thus, to take n = 2 of Mampel power law instead of n = 1 seems reasonable. The results of S1, S2 and S3 for the phases are illustrated in Figure 7. As Figure 7 shows, the calculated results fit well with the original experiment data, indicating the reliability of the model. On the basis of the calculated SSE values, the selected models along with the E and A values are listed in Table 1. For the initial phase, the models that best describe the processes for each sorbent are considered far from similar. That could be explained by the influencing factors such as CO2 molecules irregular diffusion on the surface of the sorbent particles to form different shells under the conditions of different materials with their own melting temperatures. The three sorbents are described by different models in PH1, indicating that the mechanisms of shell formation reaction are different at low temperatures. And the activation energy, E, values are found to decrease as the Li/K ratio increases. The Fn nth-order reaction model is found to be the optimal choice for calculating the E and A with different n-values of the three sorbents in PH2 with the sorbent particles growth following the sigmoidal rate equations. Although the activation energy of S3 is the less than that of S2, the dominant reaction at the temperature range of PH2 is undesirable. Power law model with the mechanism of phase boundary reaction appears to be the most appropriate one for PH3 due to its description of CO2 molecular reaction with molten carbonate at high temperatures. As a result, more molten carbonate in the sorbent leads to less activation energy in PH3. The different n-values for the three sorbents with same models in PH2 and PH3 are probably caused by the reaction intensity differences.

4. CONCLUSION High temperature CO2 sorption on different Li2ZrO3 based sorbents synthesized by the solid-state method was investigated by TGA, XRD and SEM; the double-shell sorption mechanisms for CO2 sorption by solid sorbents were discussed. Li2CO3/ K2CO3-doped Li2ZrO3 sorbents presented high absorption capacities of CO2. Sorbent with the composition of 36.23 wt % Li2CO3, 55.12 wt % ZrO2 and 8.65 wt % K2CO3 performed the best capability for high temperature CO2 sorption and the

Figure 7. Experimental (scattered points) and modeled (lines) conversions of TGA nonisothermal kinetics for (a) phase 1; (b) phase 2; (c) phase 3.

maximum sorption rate at the reaction temperature of 525 °C. The nonisothermal thermogravimetric kinetics by an improved iterative Coats−Redfern method showed that the Fn nth-order reaction model predicted accurately the main phases for the three sorbents. The kinetic parameters were estimated 12750



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Table 1. Nonisothermal Kinetics Parameters of CO2 Sorption phase 1 (PH1)

phase 2 (PH2)

phase 3 (PH3)

sorbent

S1

S2

S3

E (kJ mol−1) A (s−1) selected mechanism SSE E (kJ mol−1) A (s−1) selected mechanism SSE E (kJ mol−1) A (s−1) selected mechanism SSE

293.4557 6.8605 × 109 power law, n = 3 7.57 × 10−4 199.8266 9.1900 × 109 Fn nth-order reaction, n = 2 9.14 × 10−4 70.4685 38.0306 power law, n = 2 8.96 × 10−5

84.6009 7.4330 × 102 Mampel power law, n = 2 7.69 × 10−5 299.1090 2.2908 × 1017 Fn nth-order reaction, n = 3 8.72 × 10−4 80.5598 3.8884 × 102 power law, n = 3 9.60 × 10−5

154.5977 5.6240 × 107 Fn nth-order reaction, n = 3 6.65 × 10−4 123.8082 3.1720 × 105 Fn nth-order reaction, n = 1 4.97 × 10−4 82.9994 5.4686 × 102 power law, n = 3 1.79 × 10−4

(8) Dou, B. L.; Rickett, G. L.; Dupont, V.; Williams, P. T.; Chen, H. S.; Ding, Y. L.; Ghadiri, M. Steam Reforming of Crude Glycerol with in Situ CO2 Sorption. Bioresour. Technol. 2010, 101, 2436. (9) Fauth, D. J.; Frommell, E. A.; Hoffman, J. S.; Reasbeck, R. P.; Pennline, H. W. Eutectic Salt Promoted Lithium Zirconate: Novel High Temperature Sorbent for CO2 Capture. Fuel Process. Technol. 2005, 86, 1503. (10) Veliz-Enriquez, M. Y.; Gonzalez, G.; Pfeiffer, H. Synthesis and CO2 Capture Evaluation of Li2‑xKxZrO3 Solid Solutions and Crystal Structure of a New Lithium-Potassium Zirconate Phase. J. Solid State Chem. 2007, 180, 2485. (11) Xiao, Q.; Tang, X. D.; Liu, Y. F.; Zhong, Y. J.; Zhu, W. D. Citrate Route to Prepare K-doped Li2ZrO3 Sorbents with Excellent CO2 Capture Properties. Chem. Eng. J. 2011, 174, 231. (12) Iwan, A.; Stephenson, H.; Ketchie, W. C.; Lapkin, A. A. High Temperature Sequestration of CO2 using Lithium Zirconates. Chem. Eng. J. 2009, 146, 249. (13) Xiong, R. T.; Ida, J.; Lin, Y. S. Kinetics of Carbon Dioxide Sorption on Potassium-doped Lithium Zirconate. Chem. Eng. Sci. 2003, 58, 4377. (14) Ida, J.; Lin, Y. S. Mechanism of High-Temperature CO2 Sorption on Lithium Zirconate. Environ. Sci. Technol. 2003, 37, 1999. (15) Kumar, S.; Saxena, S. K. A Comparative Study of CO2 Sorption Properties for Different Oxides. Mater. Renew. Sustain. Energy 2014, DOI: 10.1007/s40243-014-0030-9. (16) Wang, S.; An, C.; Zhang, Q.H. Syntheses and Structures of Lithium Zirconates for High-temperature CO2 Absorption. J. Mater. Chem. A 2013, 1, 3540. (17) Ida, J.; Xiong, R. T.; Lin, Y. S. Synthesis and CO2 Sorption Properties of Pure and Modified Lithium Zirconate. Sep. Purif. Technol. 2004, 36, 41−51. (18) Chen, H. S.; Dou, B. L.; Song, Y. C.; Xu, Y. J.; Wang, X. J.; Zhang, Y.; Du, X.; Wang, C.; Zhang, X. H.; Tan, C. Q. Studies on Absorption and Regeneration for CO2 Capture by Aqueous Ammonia. Int. J. Greenhouse Gas Control 2012, 6, 171. (19) Pannocchia, G.; Puccini, M.; Seggiani, M.; Vitolo, S. Experimental and Modeling Studies on High-Temperature Capture of CO2 Using Lithium Zirconate Based Sorbents. Ind. Eng. Chem. Res. 2007, 46, 6696. (20) Ochoa-Fernandez, E.; Ronning, M.; Yu, X. F.; Grande, T.; Chen, D. Compositional Effects of Nanocrystalline Lithium Zirconate on its CO2 Capture Properties. Ind. Eng. Chem. Res. 2008, 47, 434. (21) Mejia-Trejo, V. L.; Fregoso-Israel, E.; Pfeiffer, H. Textural, Structural, and CO2 Chemisorption Effects Produced on Lithium Orthosilicate by its Doping with Sodium (Li4‑xNaxSiO4). Chem. Mater. 2008, 20, 7171. (22) Symonds, R. T.; Lu, D. Y.; Hughes, R. W.; Anthony, E. J.; Macchi, A. CO2 Capture from Simulated Syngas via Cyclic Carbonation/Calcination for a Naturally Occurring Limestone: PilotPlant Testing. Ind. Eng. Chem. Res. 2009, 48, 8431.

iteratively by linear regression to enhance the accuracy, and the calculated results in each phase fit well with the experimental data.

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ASSOCIATED CONTENT

* Supporting Information S

Detailed descriptions of compositions and specific surface area of the sorbents, TGA results of CO2 sorption, usual mechanisms and the corresponding forms of g(α), kinetic parameters of the sorbents in phase 1, phase 2 and phase 3. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*B. Dou. Telephone: +86-41184708460. E-mail: bldou@dlut. edu.cn. *H. Chen. Telephone: +86-1082543148. E-mail: chen_hs@ mail.etp.ac.cn. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work is supported by the Natural Science Foundation of China (51276032), the NSFC-RS (51311130126).

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REFERENCES

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High Temperature CO2 Sorption on Li2ZrO3 Based Sorbents

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