CO2 Adsorption Behavior of Activated Coal Char Modified with

Mar 16, 2016 - (3-21) Liu et al. studied the adsorption performance of tetraethylenepentamine (TEPA) and polyethylenimine (PEI)-modified MCM-41 sorben...
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CO2 Adsorption Behavior of Activated Coal Char Modified with Tetraethylenepentamine Xia Wang and Qingjie Guo* Key Laboratory of Clean Chemical Processing of Shandong Province, College of Chemical Engineering, Qingdao University of Science and Technology, Qingdao, Shandong 266042, People’s Republic of China ABSTRACT: The climate deterioration problem mainly caused by excessive CO2 emission from coal-fired power plants has led to heightened concerns, and the reduction of the separation cost of CO2 is essential to realize the industrial application of adsorption technology. The activated char was obtained by passing water vapor through a reactor while pyrolyzing coal at 650 °C, which was further pore-expanded using HCl. Then, the pore-expanded activated char was impregnated with tetraethylenepentamine (TEPA) for the preparation of amine-modified solid sorbents for CO2 capture. The effects of the HCl concentration, coal type, adsorption temperature, and activation time of water vapor on CO2 adsorption were investigated in a fixed-bed reactor, and the regenerability, kinetics, and deactivation rate during the adsorption process for sorbents were also studied. The saturated adsorption capacity of 3.38 mmol/g was obtained for 10 wt % TEPA-modified Ordos coal activated char (PE6-E120-TEPA10%) at 60 °C. After 10 adsorption−desorption cycles, the adsorption capacity for PE6-E120-TEPA10% was 3.19 mmol/g, which was a drop of 5.6%. The experimental breakthrough curve for PE6-E120-TEPA10% was well-fitted by the Avrami−Erofeyev deactivation model, the deactivation rate of which was significantly slower than that of TEPA-modified MCM41 and silica gel (Gel). At the initial breakthrough adsorption stage, the CO2 adsorption rate for PE6-E120-TEPA10% was rapid and then the rate significantly decreased; the external diffusion adsorption was the rate-controlling step. PE6-E120-TEPA10% not only realized effective CO2 capture from coal-fired power plants but also reduced the CO2 separation cost.

1. INTRODUCTION With the rapid development of global industrialization and further increases in energy demand, the climate deterioration problem mainly caused by CO2 emissions has received great attention.1 In its fifth assessment report, the Intergovernmental Panel on Climate Change (IPCC) has predicted that the atmospheric CO2 concentration induced by anthropogenic activity will reach 760 ppm by 2100, which is nearly double today’s CO2 concentration.2 In China, more than 70% of the energy is supplied by the combustion of coal, and realizing effective CO2 capture from coal-fired power plants is essential in reducing the greenhouse gas effect. Solid amine sorbents have the benefits of high adsorption capacity, good regenerability, low corrosion of adsorption devices, and a rapid adsorption rate, demonstrating the potential industrial application of solid amine sorbents.3−21 Liu et al. studied the adsorption performance of tetraethylenepentamine (TEPA) and polyethylenimine (PEI)-modified MCM-41 sorbents on a thermogravimetric analyzer,22 and the TEPA-modified sorbent achieved an adsorption capacity of 2.70 mmol/g at 35 °C when the TEPA loading was 40 wt %. Qi et al. impregnated TEPA and PEI in as-synthesized silica capsules with a well-developed mesoporous structure and studied the adsorption performance and regenerability of the prepared sorbents.23 These results suggested that the prepared sorbent reached an adsorption capacity of 7.9 mmol/g when the CO2 concentration was 10% and showed good thermal stability and regenerability after 50 adsorption−desorption cycles. Chen et al. grafted ethylenediamine (EDA) on a zeolite-like metal organic framework (sod-ZMOF) and studied the adsorption behavior of the prepared sorbents;24 an adsorption capacity of 69 mg/g was obtained at 25 °C at a CO2 concentration of 15% © XXXX American Chemical Society

(with an 85% N2 balance). Our research group prepared (3aminopropyl)trimethoxysilane (APTS) and TEPA co-functionalized MCM-41 sorbents using a two-step method, where APTS was first grafted onto the inner surface of MCM-41 and then TEPA was impregnated in the APTS-grafted MCM-41.25 The results were that the thermal stability, adsorption capacity, and regenerability of the composite sorbents were significantly improved in comparison to those of TEPA-modified MCM-41 as a result of the synergistic effect among groups with steric hindrance in APTS, Si−OH on the surface of MCM-41, and amine groups from both APTS and TEPA. Our group impregnated MCM-41 using a mixture of 2-amino-2-methyl1-propanol (AMP) and TEPA and studied the adsorption behavior of the prepared sorbents.26 The results suggested that, in comparison to TEPA-modified MCM-41, the mixed aminemodified MCM-41 sorbents presented a higher amine efficiency and CO2 adsorption rate. We also prepared the TEPA-functionalized composite supporting material with a hierarchical mesoporous structure and studied its adsorption performance and selectivity of the pore size;27 relatively small mesopores showed a higher selectivity for TEPA, while larger mesopores promoted CO2 diffusion to the active sites, and an adsorption capacity of 4.27 mmol/g was obtained at 55 °C when the weight ratio of silica gel (Gel) to MCM-41 was 1:1 and the TEPA loading was 50 wt %. Despite the good CO2 adsorption performance of the abovementioned sorbents, the high preparation cost and complex Received: December 9, 2015 Revised: March 15, 2016

A

DOI: 10.1021/acs.energyfuels.5b02882 Energy Fuels XXXX, XXX, XXX−XXX

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the temperature was cooled to the desired adsorption temperature, and the feed gas was switched to 15% CO2 at a flow rate of 30 mL/ min. The inlet and outlet gas concentrations were checked using gas chromatography (GC). When the CO2 concentration in the outlet (C) was equal to that in the inlet (C0), the sorbent was deemed to reach saturated adsorption. Afterward, the feed gas was switched to N2 and the temperature was raised to 100 °C to desorb adsorbed CO2. When the CO2 concentration in the outlet was zero, the desorption process was completed. A total of 10 adsorption−desorption cycles were conducted to investigate the cyclic regenerability of the TEPAmodified activated char sorbents. The breakthrough adsorption experiments for each sample were repeated 3 times, and the results showed good repeatability. The CO2 adsorption capacity was calculated using the method mentioned in the previous literature.25,27 2.4. Characterization. The textural properties of the prepared samples were characterized by their physical adsorption of N2 at 77 K using a Quadrasorb SI analyzer (Quantachrome Instruments, Boynton Beach, FL). Before measurement, the samples were degassed at 353 K for 16 h under a vacuum. The specific surface area was calculated by the Brunauer−Emmett−Teller (BET) equation. The total pore volume was obtained according to the adsorption quantity of N2 at a relative pressure of P/P0 = 0.996. The pore size distribution was obtained from the adsorption branch data using the Barrett−Joyner− Halenda (BJH) method. To investigate the effect of HCl pore expansion and TEPA loading on the surface morphology of the prepared sorbents, scanning electron microscopy (SEM) pictures were taken using a JSM-7500F scanning electron microscope (JEOL, Japan) at 5.0 kV. The thermal stability of the activated char, pore-expanded activated char, and TEPA-modified pore-expanded activated char was characterized by thermogravimetric analysis using a NETZSCH STA 409PC (NETZSCH, Germany) in a N2 atmosphere. Before the measurement, the samples were ground to a powder and heated to 625 °C at a heating rate of 10 K/min.

preparation technology for the supporting materials may limit their long-term industrial application. China is rich in coal resources, and cracks are developed during the pyrolysis process of coal. Ramasamy et al. investigated the CO2 adsorption performance of four types of coal and coal char at 45.5 °C and 65 bar using a volumetric method and studied the effects of coal properties, such as the occurrence of vitrinite, ash, volatile matter, and specific surface area, on CO2 adsorption.28 In this study, the activated coal char was obtained by passing water vapor through while pyrolyzing coal at high temperatures. The char was further pore-expanded with HCl to load active sites for CO2 adsorption. TEPA was immobilized in the pore-expanded activated coal char using the impregnation method to prepare a series of CO2 sorbents, and the effects of the HCl concentration, coal type, adsorption temperature, and water vapor activation time on CO2 adsorption were investigated in a fixed-bed reactor. The cyclic regenerability and adsorption kinetics of the prepared sorbents were studied. In addition, the surface activity change of the sorbent during the CO2 adsorption process was also fitted using the Avrami−Erofeyev deactivation model.

2. EXPERIMENTAL SECTION 2.1. Materials. TEPA [commercially pure (CP)] was produced by Tianjin BASF Chemical Co., Ltd. (Tianjin, China), and anhydrous ethanol [analytical reagent (AR)] was obtained from Far Eastern Group: Laiyang Fine Chemical Factory (Yantai, China). HCl (AR, 36.0−38.0 wt %) was provided by Yantai Sanhe Chemical Reagent Co., Ltd. (Yantai, China). N2 (with a purity of 99.999%) and 15% CO2 (with an 85% N2 balance, the mixed gas being prepared by mixing CO2 with a purity of 99.999% and N2 with a purity of 99.999% at a volume ratio of 15/85%) were distributed by the Materials Section in Qingdao University of Science and Technology (Qingdao, China). 2.2. Preparation of TEPA-Modified Pore-Expanded Activated Char. Raw coal was ground to powder and dried in an oven at 100 °C for 16 h to remove adsorbed moisture. A total of 20 g of coal was placed in a fixed-bed reactor. The reactor was suffused with N2, and the temperature then was heated to 650 °C at a heating rate of 20 K/min. Once the temperature reached 650 °C, water vapor was passed through the reactor for a certain time, and the activated coal char was formed, which was labeled as Ex, Qx, and Bx, where E, Q, and B represent the activated char of Ordos coal, Qujing coal, and Beisu coal, respectively, and x represents the water vapor activation time (min). Then, the desired amount of activated char was quickly added to a certain molar concentration of HCl, and the system was stirred for 2 h. The powder was filtered and repeatedly washed with distilled water until that the pH of the solution was approximately 7. The solid powder was then dried in the oven at 100 °C for 16 h to obtain the pore-expanded (PE) activated char, which was denoted as PEy-Ex, PEy-Qx, and PEy-Bx, where y represents the molar concentration of HCl for activated char pore expansion expressed in mol/L (M). The preparation of TEPA-modified pore-expanded activated char used the impregnation method.26,27 A certain amount of TEPA was dissolved in 20 mL of anhydrous ethanol. The mixture was sonicated for 30 min at room temperature to ensure the complete dissolution of TEPA. Then, 1 g of pore-expanded activated char was added, and the mixture was sonicated for another 180 min. The slurry was placed in a vacuum drying oven at 85 °C for 16 h to dry the solid powder. The prepared sorbents were marked as PEy-Ex-TEPA10%, PEy-QxTEPA10%, and PEy-Bx-TEPA10%, where 10% is the weight loading of TEPA on the sorbent. 2.3. CO2 Adsorption−Desorption Experiments. CO2 adsorption and desorption experiments were performed in a self-assembled fixed-bed reactor as previously described.25 Before the adsorption process, 0.9 g of the sample was placed in the reactor supported by the quartz sand and was heated to 100 °C in 30 mL/min of N2 atmosphere for 1 h to remove physically adsorbed moisture. Then,

3. RESULTS AND DISCUSSION 3.1. Characterization. The N2 adsorption−desorption isotherm and pore size distribution curves for E120, PE6E120, and PE6-E120-TEPA10% are shown in Figure 1. As shown in Figure 1a, the N2 adsorption−desorption isotherm for E120 was mixed and complex and the desorption branch curve deviated largely from the adsorption branch curve, indicating that there was no developed pore structure for E120. Before and after the TEPA modification, PE6-E120 showed a typical type-IV adsorption isotherm and a H3 hysteresis loop appeared at a high relative pressure, which suggested that a crack-type mesoporous structure existed in the pore-expanded activated char; moreover, the isotherm was unsaturated at nearly 1.0 of relative pressure, suggesting that macropores existed and that the pore structure was irregular. As shown in Figure 1b, the pore size distribution range for PE6-E120 was wide; after 10 wt % TEPA loading, some pores were filled, while partial pores still existed and provided room for TEPA dispersion and CO2 diffusion. Panels a, b, and c of Figure 2 show the SEM images of E120, PE6-E120, and PE6-E120-TEPA10%, respectively. The surface of E120 was covered with tar and was of rough appearance, and a small number of cracks were observed. After pore expansion by 6 M HCl, the tar coating in the pores and on the surface of E120 was largely removed and the pores and cracks became obviously developed. For TEPA-modified PE6-E120, the pores and cracks in PE6-E120 were occupied by TEPA but partial pores could still be observed, which provided channels for CO2 diffusion into the active sites. The thermal stabilities of E120, PE6-E120, and PE6-E120TEPA10% are shown in Figure 3. For both E120 and PE6E120, there was a weight loss peak before 135 °C, which is B

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At HCl concentrations of 3 and 9 M, PE3-E150-TEPA10% and PE9-E150-TEPA10% were not easy to dry in the oven and the TEPA agglomerated on the surface of the sorbents, suggesting that the pore structures of both PE3-E150 and PE9E150 were not sufficiently developed to accept TEPA loading. The reason for this phenomenon was that a concentration of 3 M HCl was too dilute to remove the tar both in the pores and on the surface of E150, while a HCl concentration of 9 M was so large as to corrode the pore structure, thus resulting in the collapse of the pores or cracks. The CO2 adsorption performance of E150 at 70 °C before and after pore expansion using 6 M HCl and TEPA modification is shown in Table 1. The saturated CO2 adsorption capacity for E150 was 1.32 mmol/g, which was the result of both physisorption and chemisorption from the basic functional groups on the surface of E150. In comparison to E150, the adsorption capacity for E150-TEPA10% was not obviously improved, suggesting that the pore structure of E150 was not suitable for TEPA loading and CO2 diffusion. The saturated adsorption capacity for PE6E150 was less than that of E150, which might be caused by the neutralization of the basic active sites when the activated char was pore-expanded by HCl. However, the breakthrough time, breakthrough adsorption capacity, and saturated adsorption capacity for PE6-E150-TEPA10% were significantly improved in comparison to those of the above-mentioned samples, indicating that TEPA dispersed in PE6-E150 increased the active sites for CO2 adsorption. In subsequent experiments, the concentration of HCl for pore expanding the activated char was 6 M. 3.2.2. Effect of the Coal Type. During the pyrolysis process of coal, the ash, volatile matter, and fixed carbon content as well as the coking property of coal all had a great impact on the pore structure, thus influencing the CO2 adsorption performance of the TEPA-modified pore-expanded activated char. The Beisu coal, Qujing coal, and Ordos coal that had great differences in ash (A) content, volatile matter (V) content, fixed carbon (FC) content, and coking property were selected for the investigation of the effects of the coal type. The proximate and elemental analysis results (from the China National Coal Quality Supervision and Testing Center) are shown in Table 2, and the CO2 adsorption capacity for the three types of TEPA-modified pore-expanded activated char at 70 °C is shown in Figure 4. The adsorption capacity for PE6B150-TEPA10% and PE6-Q150-TEPA10% were quite similar, while both the breakthrough adsorption capacity and saturated adsorption capacity for PE6-E150-TEPA10% were much higher than those of PE6-B150-TEPA10% and PE6-Q150-TEPA10%. The Qujing coal had a high volatile matter content, but its high ash content and low fixed carbon content limited the development of the pore structure.29,30 Although the Beisu coal had a low ash content and high fixed carbon content, the coke easily condensed to block the pore and crack development during pyrolysis and activation.28 Ordos coal was a non-coking coal, and the pyrolysis and activation opened originally closed pores, created new pores, and increased the pore size of existing and newly formed pores to form a more-developed pore structure.31 Therefore, Ordos coal was chosen to further investigate the effect of the adsorption temperature on CO2 adsorption performance of the TEPA-modified activated char. 3.2.3. Effect of the Adsorption Temperature. The CO2 adsorption breakthrough curves and adsorption capacity for PE6-E150-TEPA10% at different adsorption temperatures are shown in Figure 5. With the increase of the adsorption

Figure 1. (a) N2 adsorption−desorption isotherm and (b) pore size distribution curves for E120, PE6-E120, and PE6-E120-TEPA10%.

caused by the volatilization of physically adsorbed moisture; after 135 °C, little weight loss could be observed, which is due to the decomposition or volatilization of the tar coated on the activated char. Moreover, because of the relatively welldeveloped pore structure and crack system, the moisture content adsorbed in PE6-E120 was higher than that in E120; therefore, the weight loss for PE6-E120 at temperatures lower than 135 °C seemed more obvious than that for E120. For PE6-E120-TEPA10%, there was a moisture volatilization peak before 135 °C and an obvious weight loss peak after 135 °C, which was mainly caused by the decomposition of TEPA, and these results suggested that TEPA was successfully loaded in the pore-expanded activated char. In addition, the poreexpanded activated char before and after TEPA modification was stable below 100 °C, which is an indication of being suitable for the adsorption−desorption process. 3.2. Dynamic CO2 Adsorption Behavior of Prepared Sorbents. 3.2.1. Effect of the HCl Concentration. To remove the ash and coated tar both in the pores and on the surface of the activated char, E150 was pore-expanded using 3, 6, and 9 M HCl and then the activated char before and after pore expansion was impregnated with TEPA to investigate the effect of the HCl concentration on the CO2 adsorption behavior of the prepared sorbents. C

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Figure 2. SEM images of (a) E120, (b) PE6-E120, and (c) PE6-E120-TEPA10%.

TEPA was well-dispersed in the pores and cracks; moreover, the kinetic energy for the CO2 molecules increased, and the collision probability for CO2 with the active sites also increased. For these reasons, the increase in the adsorption temperature resulted in the increase of the adsorption performance. However, the adsorption of CO2 is exothermic, and a further increase in the temperature caused the adsorption performance of PE6-E150-TEPA10% to decrease, which was controlled by thermodynamics. The same trends were also observed for the PEI-modified SBA-1520 and TEPA-modified MCM-41/Gel.27 The adsorption temperature of 60 °C was adopted to further investigate the effect of the water vapor activation time. 3.2.4. Effect of the Water Vapor Activation Time. As the pyrolysis and activation of the coal proceeded, the pores and cracks in coal changed and the extent of the development of the pore structure directly affected the dispersion of TEPA and diffusion of CO2. The breakthrough curves and adsorption capacity for TEPAmodified pore-expanded activated char from Ordos coal at different activation times are shown in Figure 6. With the increase in the activation time from 60 to 180 min, the breakthrough time, breakthrough adsorption capacity, and saturated adsorption capacity first increased and then decreased, reaching their maximum value when the activation time was 120 min. At the beginning of pyrolysis and activation of the coal, the volatile matter was released and then a great amount of organics decomposed to release gases, during which pores that were originally closed were opened, new pores were created, and the pore size of existing and newly formed pores was increased to form a more developed pore structure, thus promoting the greater dispersion of TEPA and the diffusion of CO2. Therefore, the adsorption performance of the TEPAmodified pore-expanded activated char increased as activation proceeded. However, a further increase in activation time may lead to the coalescence and collapse of pores, thus preventing the dispersion of TEPA and diffusion of CO2 and, thus, the

Figure 3. Thermal stability curves for E120, PE6-E120, and PE6-E120TEPA10%.

Table 1. Adsorption Performance of E150 before and after HCl Pore Expansion and TEPA Modification at 70 °C

sorbent

breakthrough time (min)

breakthrough adsorption capacity (mmol/g)

E150 E150-TEPA10% PE6-E150 PE6-E150-TEPA10%

4 4 4 6

0.89 0.89 0.89 1.34

saturated adsorption capacity (mmol/g) 1.32 1.41 1.18 2.75

temperature, the breakthrough time, breakthrough adsorption capacity, and saturated adsorption capacity for PE6-E150TEPA10% first increased and then decreased. All variables reached the optimum values of 10 min, 2.23 mmol/g, and 3.09 mmol/g at 60 °C, respectively. With an increase in the adsorption temperature, the activity for TEPA increased and D

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Energy & Fuels Table 2. Proximate and Elemental Analyses for Three Types of Coal proximate analysis (%, air dry)a

a

elemental analysis (%, air dry)

coal

M

A

V

FC

C

H

N

O

S

Beisu coal Qujing coal Ordos coal

3.38 12.63 10.66

4.80 14.54 5.66

38.27 34.22 28.08

53.55 38.61 55.60

74.00 51.82 65.46

4.96 3.65 3.38

1.29 1.25 0.74

9.13 15.95 13.79

2.44 0.16 0.31

M, moisture; A, ash; V, volatile matter; and FC, fixed carbon.

Figure 4. CO2 adsorption capacity for the TEPA-modified poreexpanded activated char from different coal types.

corresponding decrease in the adsorption performance of prepared sorbents. When the activation time was 120 min, the breakthrough adsorption capacity and saturated adsorption capacity for PE6E120-TEPA10% were 2.68 and 3.38 mmol/g, respectively, which were superior to those of the TEPA-modified MCM-4122 and Gel.32 3.3. Cyclic Regenerability of PE6-E120-TEPA10%. Good regenerability is an important factor in evaluating the industrial application of a sorbent. A total of 10 adsorption− desorption cycles were performed on PE6-E120-TEPA10% when the adsorption temperature was 60 °C and the desorption temperature was 100 °C. As shown in Figure 7, after 10 cycles, the saturated adsorption capacity for PE6-E120-TEPA10% was 3.19 mmol/g, which dropped by 5.6% compared to the adsorption capacity for fresh PE6-E120-TEPA10%. The regenerability of PE6-E120-TEPA10% was superior to the regenerability of the TEPA-modified MCM-41 and Gel, which dropped by 7.4 and 8.0% after 10 cycles, respectively.22,32 Therefore, PE6-E120-TEPA10% showed good regenerability. 3.4. Avrami−Erofeyev Deactivation Model. The adsorption of CO2 on amine-modified solid sorbents is a gas−liquid reaction. As the reaction proceeds, the active sites in the solid are gradually occupied by CO2 molecules, which decrease the CO2 adsorption rate, active surface area, and activity per area of a sorbent. According to the conclusions in sections 3.2 and 3.3, the adsorption capacity and regenerability for the TEPA-modified pore-expanded activated char are superior to those of the TEPA-modified MCM-41 and Gel, respectively. To study the effect of the change of the surface activity on the CO2 adsorption process,33−36 the Avrami− Erofeyev deactivation model was used to model the experimental breakthrough curves of PE6-E120-TEPA10%, MCM-41-TEPA60%, and Gel-TEPA40%, the adsorption capacity of which were optimum for the TEPA-modified

Figure 5. (a) Breakthrough curves and (b) adsorption capacities for PE6-E150-TEPA10% at different adsorption temperatures.

pore-expanded activated char, MCM-41, and Gel, respectively. The deactivation model is shown in eq 1 ⎡ ⎤ ⎛k W ⎞ ⎢ 1 − exp⎜ Q0 (1 − exp(−kdt ))⎟ ⎥ ⎝ g ⎠ C exp( −kdt )⎥ = exp⎢ ⎢ ⎥ 1 − exp( −kdt ) C0 ⎢ ⎥ ⎣ ⎦ (1)

where t is the adsorption time (min), C0 and C represent the concentration of the reactant gas in the bulk phase at adsorption times of 0 and t (mmol/L), w is the weight of the sorbent (g), Qg represents the mixed gas velocity (mL/min), k0 is the initial adsorption rate constant (mL min−1 g−1), and kd is the deactivation rate constant (min−1). E

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Figure 8. Fitting of the deactivation model to experimental breakthrough curves for PE6-E120-TEPA10%, MCM-41-TEPA60%, and Gel-TEPA40%.

Table 3. Parameters of the Deactivation Model for Fitting the Adsorption of CO2 on PE6-E120-TEPA10%, GelTEPA40%, and MCM-41-TEPA60% sorbent

k0 (mL min−1 g−1)

kd (min−1)

R2

PE6-E120-TEPA10% Gel-TEPA40% MCM-41-TEPA60%

239.3667 290.3880 488.3010

0.5246 1.0795 1.3723

0.9946 0.9915 0.9996

breakthrough curves well and the correlation coefficients (R2) were all larger than 0.99, indicating that the deactivation model could correctly describe the adsorption process of the sorbents studied. In comparison to Gel-TEPA40%, the deactivation rate constant of kd for MCM-41-TEPA60% was 1.3723 min−1, which was 1.27 times the deactivation rate constant for GelTEPA40%, while the initial adsorption rate constant of k0 for MCM-41-TEPA60% was 488.3010 mL min−1 g−1, which was 1.68 times the initial adsorption rate constant for GelTEPA40%. The relatively larger initial adsorption rate and longer breakthrough time showed that the adsorption performance of MCM-41-TEPA60% was superior to the adsorption performance of Gel-TEPA40%. In comparison to MCM-41TEPA60%, k0 for PE6-E120-TEPA10% was 239.3667 mL min−1 g−1, which was 0.49 times that of MCM-41-TEPA60%, while the deactivation rate constant for PE6-E120-TEPA10% was 0.5246 min−1, which was 0.38 times that of MCM-41TEPA60%. The relatively slower deactivation rate and longer adsorption time showed that the adsorption performance of PE6-E120-TEPA10% was superior to the adsorption performance of MCM-41-TEPA60%. 3.5. Adsorption Kinetics of PE6-E120-TEPA10%. The CO2 adsorption rate is an important index in evaluating the adsorption performance of a sorbent. According to a previous study, the interparticle diffusion resistance could be eliminated by changing the influent velocity. An influent velocity of 30 mL/min was suitable and used.25 To investigate the effect of the intraparticle diffusion resistance, the intraparticle diffusion model was used to fit the experimental adsorption capacity and the fitting curves along with the experimental data are shown in Figure 9a. According to Weber−Morris, if the CO2 adsorption process was controlled by intraparticle diffusion, the adsorption capacity of qt should vary linearly with the square root of time.

Figure 6. (a) Breakthrough curves and (b) adsorption capacities for PE6-Ex-TEPA10% at 60 °C. The variable “x” represents the water vapor activation time (min).

Figure 7. Regenerability of PE6-E120-TEPA10% after 10 adsorption− desorption cycles.

The fitting of the deactivation model to experimental breakthrough curves and their corresponding parameters are shown in Figure 8 and Table 3, respectively. For three types of sorbents, the deactivation equation fitted the experimental F

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the active sites. At the initial adsorption stage, the active sites for the adsorption of CO2 in the channel were well-exposed and the diffusion of CO2 to the active sites was unhindered. Thus, the adsorption rate at the breakthrough adsorption stage was rapid. Once breakthrough adsorption was finished, the most exposed and accessibly active sites were surrounded by CO2 molecules and the diffusion of CO2 to the active sites was prevented; therefore, the adsorption rate significantly decreased. With the gradual disappearance of the active sites, the CO2 adsorption reached saturation. When the adsorption temperature was 60 °C, the adsorption capacity for PE6-E120-TEPA10% at the rapid breakthrough stage was 2.68 mmol/g, which is close to 80% of the saturated adsorption capacity. These results indicate that the TEPAmodified pore-expanded activated char showed high adsorption efficiency for potential industrial application.

4. CONCLUSION The activated coal char, which was obtained by passing water vapor through the reactor while pyrolyzing coal at 650 °C, was used as the supporting material, and the pore-expanded activated char was impregnated with TEPA to adsorb CO2 from coal-fired power plants. (1) The CO2 adsorption capacity for PE6-E120-TEPA10% was 3.38 mmol/g at 60 °C. After 10 adsorption−desorption cycles, the adsorption capacity decreased by 5.6%, suggesting that the adsorption performance and regenerability of PE6-E120-TEPA10% were better than those of the TEPA-modified MCM-41 and Gel. (2) In comparison to MCM-41-TEPA60% and Gel-TEPA40%, the deactivation rate for PE6-E120-TEPA10% was significantly slower and the saturated adsorption time was much longer, which resulted in better adsorption performance and regenerability for PE6-E120-TEPA10%. (3) The CO2 adsorption on PE6-E120-TEPA10% was controlled by an external diffusion adsorption step; at the initial breakthrough stage, the rate was rapid and then the rate significantly decreased. The TEPA-modified pore-expanded Ordos coal activated char not only realized an effective CO2 capture from coal-fired power plants but also reduced the CO2 separation cost. The competitive adsorption of the TEPA-modified poreexpanded activated char to other components in the flue gas, especially NOx and SOx, as well as the functional groups for CO2 adsorption on the surface of the activated char will be examined in future studies.

Figure 9. (a) Prediction of the rate-controlling step and (b) CO2 adsorption rate curve for PE6-E120-TEPA10% at 60 °C.

The intraparticle diffusion model for CO2 adsorption generally included three stages: an external diffusion adsorption stage (i.e., boundary layer diffusion), a gradual adsorption stage (i.e., intraparticle diffusion), and an equilibrium adsorption stage. If the adsorption rate at different stages was different, it would appear as multilinear and the stage with the smallest slope would be the rate-controlling step.37−40 The intraparticle diffusion model is shown in eq 2 qt = k idt 1/2 + C

(2)

where t is the adsorption time (min), qt is the CO2 adsorption capacity at time t (mmol/g), kid is the intraparticle diffusion rate (mmol g−1 min−0.5), and C is the intercept. As shown in Figure 9a, the intraparticle diffusion model fitted the experimental adsorption data well and three adsorption stages were identified. Among these stages, the first stage linearly passed through the origin and had the smallest slope, suggesting that the external diffusion adsorption was the rate-controlling step for PE6-E120-TEPA10%, in which CO2 molecules diffused from the bulk phase to the surface of the active sites and were adsorbed on the surface of the active sites. The adsorption rate curve for PE6-E120-TEPA10% was obtained by differentiating the adsorption capacity curve with respect to time, as shown in Figure 9b. At the initial breakthrough adsorption stage, the adsorption rate was rapid and then the rate significantly decreased; the variation trend of which was mainly caused by the difficulty diffusion of CO2 to

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AUTHOR INFORMATION

Corresponding Author

*Telephone/Fax: +86-532-84022757. E-mail: qj_guo@yahoo. com. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

The financial support from the Natural Science Foundation of China (21276129), the Key Project of the Natural Science Foundation of Shandong Province (ZR2015QZ02), and the Qingdao Application Foundation Research Project (14-2-4-5jch) is gratefully acknowledged. G

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DOI: 10.1021/acs.energyfuels.5b02882 Energy Fuels XXXX, XXX, XXX−XXX