Article pubs.acs.org/EF
K2CO3‑Modified Potassium Feldspar for CO2 Capture from Postcombustion Flue Gas Yafei Guo,† Changhai Li,† Shouxiang Lu,*,† and Chuanwen Zhao*,‡ †
State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China ‡ Jiangsu Provincial Key Laboratory of Materials Cycling and Pollution Control, School of Energy and Mechanical Engineering, Nanjing Normal University, Nanjing, Jiangsu 210042, People’s Republic of China ABSTRACT: Capturing CO2 from post-combustion flue gas is one of the major solutions to CO2 abatement in global warming and climate change. Potassium-based solid sorbents are confirmed as promising means for this purpose. To dispose of the substantive flue gas, the CO2 capture process should be cost-effective. In this work, a novel K2CO3/PF sorbent was prepared by impregnation of potassium carbonate (K2CO3) on potassium feldspar (PF). The synthesized sample was characterized by X-ray fluorescence (XRF), field emission scanning electron microscopy (FESEM), and X-ray diffraction (XRD). The CO2 sorption behaviors of K2CO3/PF were evaluated in a fixed-bed reactor in simulated flue gas composition of 60 °C, 5% CO2, and 10% H2O. The sorbent regeneration behaviors were also investigated in a N2 atmosphere at 550 °C with a ramping rate of 10 °C/min. Further insights were focused on the reaction pathway and multiple cycle behaviors of the sorbent in 10 CO2 sorption− desorption tests. The CO2 sorption capacity of K2CO3/PF is calculated as 1.74 mmol of CO2/g. The reaction pathways are revealed as that NaAlSi3O8 and KAlSi3O8 in the support can be converted into NaAlCO3(OH)2, Al2(Si2O5)(OH)4, NaHCO3, and KHCO3 in a humid CO2 atmosphere. The carbonation reaction of K2CO3 to form KHCO3 also contributes considerably to the whole CO2 sorption process. The sorbent regeneration process consists of three steps as the desorption of adsorbed CO2 at 55 °C, the decompositions of NaHCO3 and KHCO3 at 130 °C, and the decompositions of NaAlCO3(OH)2 and Al2(Si2O5)(OH)4 at 550 °C. Besides, the desired sorbent presents excellent regenerability and stability during 10 cyclic CO2 sorption−desorption tests. Considering the high CO2 working capacity, long-term stability, low cost of the supporting materials, and recycling of waste resources, K2CO3/PF can be considered as a new option for flue gas treatment.
1. INTRODUCTION Large amounts of CO2 emissions from energy-intensive industries, such as fossil-fuel-fired power plants, are considered as the main contributors to severe global warming and climate change.1 Carbon capture and storage (CCS), as a cost-effective and energy-efficient process that targets capturing CO2 from these concentrated industrial streams, has been deemed as a promising option for CO2 abatement and has recently received keen interest.2,3 Potential candidates of CO2 sorbents with high availability, large capture capacity, preferable regeneration capability, considerable kinetic performance, and low cost are the key technologies for CCS.4−6 With regard to the economical applicability and resource recycling, particular attention has recently been directed toward capturing CO2 using waste resources.7−10 Recently, the use of mineral materials containing alkali metal oxides or alkaline earth oxides for flue gas treatment has received considerable attention.11−13 The technology has been investigated intensively, and a novel concept of carbon capture and utilization (CCU) has been proposed as a supplementary for CCS.14 Such technologies make it possible to recycle the waste mineral resources and to maintain energy supply from fossil fuel combustion while mitigating their environmental impact. Hydrotalcite-like compounds have recently been suggested as a CO2 capturer as a result of their excellent stability and fast CO2 adsorption and desorption kinetics.11,15−20 Ficicilar and © 2015 American Chemical Society
Dogu investigated the high-temperature CO2 capture performances of activated hydrotalcite (HTC), and they found that the sorbent provided a total CO2 capture capacity of 1.16 mmol of CO2/g at 550 °C.15 Ding and Alpay presented the experimental results of HTC and potassium promoted HTC for hightemperature CO2 treatment in flue gas.16,17 The CO2 capture capacity of HTC was 0.58 mmol g−1 in the presence of H2O at 480 °C. A further modification of the sorbent with potassium was found to increase its CO2 capture capacity to 0.8 mmol of CO2/g at a lower temperature of 208−302 °C. Yong and Rodrigues and Reijers et al. also indicated that the CO2 capture capacity of hydrotalcite-like compounds at a high temperature could be greatly enhanced with the promotion of K2CO3.18,19 More recently, Xie et al. proposed that mineral resources of potassium feldspar (PF) could be a promising means for CO2 disposal when pretreated at high temperatures.20 These publications indicated that the use of waste resources could provide an economically feasible option for reducing the energy expenditure and material cost in CCS. Alkaline metal minerals are deemed as economically and technically promising for post-combustion flue gas treatment. However, low-temperature CO2 capture from post-combustion flue gas using these technologies has not been extensively Received: September 24, 2015 Revised: November 2, 2015 Published: November 3, 2015 8151
DOI: 10.1021/acs.energyfuels.5b02207 Energy Fuels 2015, 29, 8151−8156
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pump was applied to propel liquid H2O into the mixtures. Before flowing through the fixed-bed reactor, the mixtures were heated to ensure complete water vaporization. The core part of the experimental system included a fixed-bed reactor (an inner diameter of 0.02 m) made of stainless steel with electric heating encircled. A mass transmitter was equipped to monitor the mass change of the sorbent during CO2 sorption. A temperature indicator, a pressure indicator, and a relative humidity (RH) indicator were equipped for measuring the change of the temperature, pressure, and RH in the reactor, respectively. A gas analyzer was used for online monitoring of the CO2 concentration at the outlet of the reactor. A total of 10 g of the sorbent was filled into the reactor. Prior to the CO2 sorption experiment, the samples were heated to 120 °C in pure N2 to remove the adsorbed water vapor. The reactor was then cooled to 60 °C, and the reactant gases were switched to simulated flue gas mixtures of 5% CO2, 10% H2O, and 85% N2, with a flow rate of 500 mL/min for CO2 sorption. When the outlet CO2 concentration equaled that of the inlet, the CO2 sorption experiment finished. The gas composition was then switched to pure N2 with a flow rate of 1000 mL/min to purge the reactor system until the outlet CO 2 concentration reached zero. Thereafter, the temperature was risen to 550 °C with a ramping rate of 10 °C/min for sorbent regeneration. When the outlet CO2 concentration decreased to zero, the regeneration process finished. In this way, the CO2 sorption, sorbent regeneration, and cyclic sorption−desorption tests were carried out.
reported. In the present work, a novel sorbent of PF promoted with K2CO3 is tried as a low-temperature CO2 capturer. The CO2 capture behaviors and sorbent regeneration performances of the sorbent in a simulated flue gas atmosphere are evaluated in a modified fixed-bed reactor. The pathways for CO2 capture and sorbent regeneration are expounded. Further insights have been focused on the cyclic operating performance of the sorbent in 10 CO2 sorption−desorption tests. The results will provide further insights into large-scale application of this sorbent for flue gas treatment.
2. EXPERIMENTAL SECTION 2.1. Materials. K2CO3/PF was prepared by impregnation of K2CO3 on PF as follows: Anhydrous K2CO3 was first dissolved in deionized water to form uniform solutions. A certain gram of PF support was added to the prepared K2CO3 solutions and mixed with a magnetic stirrer at ambient temperature for 12 h. The mixture was then dried at 105 °C overnight and calcined at 300 °C under N2 flow in a muffle furnace. The theoretical K2CO3 loading was 30%. The supports of PF with an average particle size of 75 μm were obtained from Guangxi, China. An analytical reagent of K2CO3 (99.9%) was purchased from Shanghai Jiuyi Chemical Co., Ltd., and N2 (99.99%) and CO2 (99.99%) were supplied by Henglong Electrical Co., Ltd. 2.2. Sample Characterization. X-ray fluorescence (XRF, XRF1800, Shimadzu) was used to determine the chemical compositions of the support materials. The morphologies of the support and sorbent were observed by field emission scanning electron microscopy (FESEM) SIRION200 (Philips, Netherlands). The scanning electron microscopy (SEM) images were taken at 2500 times under the accelerating voltage of 10.0 kV. The microstructure change of the sorbent before and after CO2 sorption and desorption was determined by power X-ray diffraction (XRD) X’Pert PRO (Philips, Netherlands). The XRD patterns were recorded using nickel-filtered Cu Kα radiation at 30 kV and 150 mA (2θ angle ranging from 10° to 70°, wavelength λ = 0.154 06 nm, and 0.02° sampling width). 2.3. CO2 Sorption and Sorbent Regeneration Tests. CO2 sorption, sorbent regeneration, and cyclic CO2 sorption−desorption experiments of K2CO3/PF were performed on the basis of a fixed-bed reactor system. The schematic diagram of the experimental setup is illustrated in Figure 1. The high-purity cylinders were to supply the simulated flue gas mixtures of CO2 and N2. The flow rates of the mixtures were manipulated by the mass flow controllers. The high-precision piston
3. RESULTS AND DISCUSSION 3.1. Sample Characterization. 3.1.1. Chemical Compositions. The species and amounts of the chemical compounds
Figure 2. Chemical compositions determined by XRF.
Figure 3. SEM images of PF and K2CO3/PF.
in the supporting materials will affect the pathways and CO2 sorption performance of K2CO3/PF. To determine the chemical compositions in PF, the sample is characterized by XRF and the results are presented in Figure 2. As illustrated in Figure 2, the main chemical compositions in PF are SiO2, Al2O3, K2O, and Na2O. The corresponding mass percentages are calculated as 68.1, 16.9, 9.2, and 3.6%, respectively. Some other trace ingredients observed are assigned as TiO2, CaO, Fe2O3, and MgO, respectively. The active constituents of K2O, Na2O, Al2O3, CaO, Fe2O3, and MgO may affect the pathways and CO2 sorption performance
Figure 1. Schematic of the experimental setup: (1) N2 cylinder, (2) CO2 cylinder, (3) H2O, (4) pressure regulating valve, (5) highprecision piston pump, (6) evaporator, (7) mass flow controller, (8) pressure indicator, (9) fixed-bed reactor, (10) mass transmitter, (11) RH monitor, (12) temperature indicator, (13) mass flow controller, (14) temperature controller, (15) pressure indicator, and (16) gas analyzer. 8152
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As indicated in the left panel of Figure 3, the morphology of PF shows a rather smooth and compact surface. For the morphology of K2CO3/PF, masses of white aggregates with an average size of 5 μm are attached to the surface. These white aggregates are deduced as the crystalline of the K2CO3 compound. Despite the fact that the loading of K2CO3 has given rise to a decrease in the pore volume, the active component of K2CO3 shows uniform dispersion over the support, thus will increase the utilization efficiency of the active sites, and eventually will facilitate the CO2 sorption process. 3.2. CO2 Sorption. To compare the CO2 sorption performance of PF and K2CO3/PF, the samples were tested in 5.0% CO2 and 10% H2O at 60 °C and the CO2 sorption efficiency η and the CO2 sorption capacity q are used to characterize the CO2 sorption performance. The CO2 sorption efficiency η is expressed as
Figure 4. CO2 sorption behaviors of PF and K2CO3/PF.
η=
C in − Cout × 100% C in
(1)
The CO2 sorption capacity q is calculated as q=
1⎡ ⎢ w⎣
∫0
t
Q
C in − Cout ⎤ T 1 dt ⎥ 1 − Cout ⎦ T0 Vm
(2)
where η and q are the CO2 sorption efficiency (%) and CO2 sorption capacity (mmol CO2/g), respectively, Cin and Cout are the CO2 concentrations (%) at the inlet and outlet of the reactor, respectively, Q is the gas flow rate (mL/min), w is the weight of adsorbent, t is the sorption time (min), T is the experimental temperature (K), T0 is 273 K, and Vm is 22.4 mL/ mmol. Figure 4 illustrates the change of the CO2 sorption efficiency and CO2 sorption capacity of PF and K2CO3/PF versus time. As indicated, the CO2 sorption efficiency of PF maintains at high values in the first few minutes and then decreases with time expanding. This is corresponding to the adsorption breakthrough in the fixed-bed reactor. The time for keeping 90% CO2 sorption efficiency is 2.87 min for PF. A similar tendency of the variation in the CO2 sorption efficiency can be observed for K2CO3/PF. The time for keeping 90% CO2 sorption efficiency for K2CO3/PF is determined as 14 min, as indicative of the enhanced CO2 sorption performance compared to that of PF. The CO2 sorption capacities of the two samples increase linearly with the advance of CO2 sorption. Once the samples are entirely deactivated, the CO2 sorption capacities remain unchanged as 0.51 and 1.74 mmol of CO2/g for PF and K2CO3/PF, respectively. Figure 5 presents the mass change of samples during the CO2 sorption process. The weight gain of the two samples increases rapidly in the first few minutes and then increases rather more slowly. It is noteworthy that the rate for weight gain of K2CO3/PF is significantly faster than that of PF, which indicates that K2CO3/PF exhibits higher initial CO2 sorption kinetics. The final weight gains for PF and K2CO3/PF are 365 and 1320 mg, respectively. Assuming that the active components in the samples for CO2 sorption are presented in the form of K2CO3 and Na2CO3 that could be eventually converted into KHCO3 and NaHCO3 through carbonation reactions and the physical adsorption amounts are neglected, the final weight gains for PF and K2CO3/PF are calculated as 636 and 1898 mg, respectively. It is found that the calculated final weight gains are higher than the experimentally measured values. This indicates that the reaction paths for the CO2
Figure 5. Mass change of PF and K2CO3/PF during the CO2 sorption process.
Figure 6. Regeneration behaviors of PF and K2CO3/PF.
of K2CO3/PF, because they can be converted into inorganic bicarbonates or compound carbonates in a humid CO2 atmosphere. However, the influencing mechanism depends upon the existing form of these compounds in PF. This will be elaborated from the microstructure changes of the sorbent determined by XRD. Besides, the actual K2CO3 loading of the sorbent is determined as 28.1% using an inductively coupled plasma mass spectrometer (Agilent 7500ce, Japan). 3.1.2. Particle Morphologies. The active components in the sorbent are generally used in the dispersion state, and the distribution behaviors of the active components in the sorbent would affect its CO2 sorption performance. To observe the dispersion characteristics of the active components, the SEM images of PF and K2CO3/PF are taken and the results are shown in Figure 3. 8153
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Figure 7. XRD patterns of (a) PF and (b) K2CO3/PF: (I) fresh sample, (II) after CO2 sorption, and (III) after regeneration.
Na2CO3 might not be the existence form of K and Na in the supports. Thus, the CO2 sorption capacities of the samples could be hardly quantified from the mass change curves. It is speculated that the other active constituents in the supports should also be involved in the CO2 sorption process. 3.3. Sorbent Regeneration. The regenerations of PF and K2CO3/PF were carried out at a final temperature of 550 °C with a ramping rate of 10 °C/min in pure N2. The amounts of CO2 released and change of the temperature are plotted in Figure 6. As indicated in Figure 6, the regeneration processes for PF and K2CO3/PF finish within 90 min. The curves of the released CO2 concentration for the two samples present a three-peak distribution. For the regeneration process of the spent PF support, the temperatures corresponding to the three peaks of the released CO2 concentration are 50, 130, and 530 °C, respectively. For the regeneration process of K2CO3/PF, the three peaks of the released CO2 concentration appear at 55,
Figure 8. Multiple cycle behaviors of K2CO3/PF for CO2 sorption.
sorption process are rather more complicated that cannot be simplified as the carbonation reactions between Na2CO3/ K2CO3 and CO2 in the presence of H2O. Besides, K2CO3 and
Table 1. Comparison of CO2 Sorption Capacity of the Synthesized Sample to Other Sorbents amine-based sorbent A-SCDa AH-RFSAc
synthesis method sol−gel SCDb sol−gel SCD
RFASd sol−gel SCD TEPA-E-SNTse wet impregnation TEPA-CNTsg TEPA-KIT-6h potassium-based sorbent synthesis method K2CO3/activated carbon
wet impregnation
K2CO3/Al2O3
wet impregnation
K2CO3/titanium
wet impregnation
K2CO3/MgO K2CO3/zeolites K2CO3/PF
wet impregnation wet impregnation wet impregnation
amine content
sorption condition
1% CO2, 1% H2O, and 130 °C 1% CO2, 1% H2O, and 110 °C 7.68 mmol of N/g humid air and 25 °C 8.07 mmol of N/g 1% CO2, 1% H2O, and 30 °C 50 wt % 10% CO2, RHf = 28%, and 75 °C 30 wt % 2% CO2, 2% H2O, and 40 °C 50 wt % 10% CO2, RH = 37%, and 60 °C K2CO3 content (wt %) sorption condition 33.3 13.9 36.8 35.0 30.0 30.0 30.0 16.2−37.5 28.1
1% CO2, 9% H2O, and 60 °C 1% CO2, 2% H2O, and 20 °C 10% CO2, 14% H2O, and 60 °C 15% CO2, 10% H2O, and 60 °C 1% CO2, 9% H2O, and 60 °C 1% CO2, 1% H2O, and 60 °C 1% CO2, 9% H2O, and 60 °C 0.5% CO2, 1.8% H2O, and 20 °C 5% CO2, 10% H2O, and 60 °C
capacity (mmol of CO2/g)
reference
5.55 4.43 2.57 1.92 4.74 3.87 3.20 capacity (mmol of CO2/g)
26 27 28 29 30 31 32 reference
2.09 1.02 2.43 1.61 1.95 1.28 2.84 0.35−0.52 1.74
33 34 21 and 35 36 37 38 39 40 this work
a A-SCD = amine hybrid silica aerogel. bSCD = supercritical drying. cAH-RFSA = amine hybrid resorcinol−formaldehyde/silica. dRFAS = resorcinol−formaldehyde/silica. eTEPA-E-SNTs = tetraethylenepentamine-modified silica nanotubes. fRH = relative humidity. gTEPA-CNTs = tetraethylenepentamine-modified carbon nanotubes. hTEPA-KIT-6 = tetraethylenepentamine-modified KIT-6-type mesoporous silica.
8154
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Energy & Fuels 128, and 530 °C, respectively. The resembling peaks that appear at 55 °C for the two samples should be deduced as the desorption of physically adsorbed CO2. It was reported that K2CO3 could be transformed to KHCO3 in a humid CO2 atmosphere at 60−100 °C and formed KHCO3 could be thermally decomposed at 117.2−219.7 °C.21 The second peak that appears at 130 °C for PF and K2CO3/PF is deduced with the decomposition of KHCO3. The resembling peaks that appear at 550 °C might be attributable to the decomposition of more complicated compounds formed in the CO2 sorption process. 3.4. Reaction Pathways for CO2 Sorption and Sorbent Regeneration Processes. The CO2 sorption and sorbent regeneration performances of the samples have been experimentally elaborated. However, detailed reaction pathways for the CO2 sorption and sorbent regeneration processes are not clear. Thereby, the structure changes of PF and K2CO3/PF before and after CO2 sorption and sorbent regeneration are characterized by XRD. The XRD patterns of the samples are presented in Figure 7. As indicated in Figure 7a, the XRD pattern of the fresh PF shows five phases. The major compounds in the XRD pattern are identified as KAlSi3O8 and NaAlSi3O8, as indicated by the strong intensity of the corresponding peaks. The other peaks present are assigned as CaCO3, SiO2, and Al2O3, respectively. No diffraction peaks of TiO2, Fe2O3, and MgO could be observed in the XRD pattern. This can be deduced as two reasons. On the one hand, the contents of these trace compounds are rather low to be detected, as indicated in Figure 2. On the other hand, their diffraction peaks might be covered by the strong diffraction peaks of KAlSi3O8 and NaAlSi3O8. After CO2 sorption, most of the diffraction peaks of KAlSi3O8 and NaAlSi3O8 disappear. Instead, several new phases, including NaAlCO 3 (OH) 2 , Al 2 (Si 2 O 5 )(OH) 4 , NaHCO3, and KHCO3 are present. The other phases of CaCO3, SiO2, and Al2O3 show no change in this process. This indicates that KAlSi3O8 and NaAlSi3O8 can react with CO2 in the presence of H2O to form new species. On the basis of the XRD patterns, the probable pathways are speculated as follows:22
reaction products can be thermally regenerated to Na2CO3 and K2CO3. The probable reaction schemes are extrapolated as Δ
2NaAlCO3(OH)2 → Na 2CO3 + Al 2O3 + 2H 2O + CO2 (9) Δ
Al 2(Si 2O5)(OH)4 → Al 2O3 + 2SiO2 + 2H 2O Δ
2KHCO3 → K 2CO3 + H 2O + CO2
(4)
H 2O + CO2 ↔ H 2CO3
(5)
2NaAlSi3O8 + 2H 2CO3 + H 2O ↔ 2NaHCO3 + 4SiO2 + Al 2(Si 2O5)(OH)4
(6)
KAlSi3O8 + H 2O + CO2 + NaHCO3 ↔ NaAlCO3(OH)2 + 3SiO2 + KHCO3
(12)
Figure 7b shows the XRD patterns of fresh K2CO3/PF and the sorbent after CO2 sorption and regeneration. In comparison to fresh PF, the XRD patterns of the fresh K2CO3/PF sorbent show the K2CO3 phase in addition to the other phases, as indicative of successful loading of K2CO3 on the support. The XRD patterns of K2CO3/PF after CO2 sorption and sorbent regeneration resemble those for PF. This signifies that the reaction pathways for CO2 sorption and sorbent regeneration of PF and K2CO3/PF are similar. For the XRD patterns of K2CO3/PF after CO2 sorption and sorbent regeneration, the diffraction peaks of KHCO3/NaHCO3 and K2CO3/Na2CO3 are more intensive than those observed for the XRD patterns of PF. This indicates that the carbonation reaction of supported K2CO3 contributes considerably to the whole CO2 sorption process. Besides, the decompositions of KHCO3 and NaHCO3 occur in the regeneration processes of the two samples, and this should be responsible for the resembling peaks that appear at 130 °C, as shown in Figure 6. It was reported that the decompositions of NaAlCO3(OH)2 and Al2(Si2O5)(OH)4 started at 350 and 450 °C, respectively.23−25 The resembling peaks of the released CO2 concentration that appear at 550 °C should be attributable to the decompositions of NaAlCO3(OH)2 and Al2(Si2O5)(OH)4. This is why the temperature required for the regeneration process is as high as 550 °C. 3.5. Multiple Cycle Behaviors of K2CO3/PF. Multiple cyclic CO2 sorption−desorption tests were performed to investigate the long-term stability of the sorbent. The change of the CO2 sorption capacity with the cycle number is shown in Figure 8. The CO2 sorption capacity of K2CO3/PF slightly decreases from 1.74 to 1.63 mmol of CO2/g after 10 cycles, indicating that the sorbent keeps considerable long-term working stability. Considering the high CO2 sorption capacity, stable cyclic behaviors, and cost efficiency of the supporting materials, K2CO3/PF could be deemed as a desired choice for capturing CO2 from post-combustion flue gas. 3.6. Comparison of CO2 Sorption Capacity of the Synthesized Sample to Other Sorbents. The CO2 sorption capacity of the synthesized sample is compared to those of some typical amine-based sorbents and potassium-based sorbents, as shown in Table 1. Fan’s group has focused on the CO2 sorption performance of amine-based sorbents synthesized by the sol−gel supercritical drying (SCD) method. In their contributions, the CO2 sorption capacities of A-SCD,26 AH-RFSA,27,28 and RFAS29 in 1% CO2 + 1% H2O and humid air are in the range of 1.92−5.55 mmol of CO2/g. In the work by Yao et al., Ye et al., and Liu et al, the CO2 sorption capacities of TEPA-E-SNTs,30 TEPA-CNTs,31 and TEPA-KIT-632 in 2− 10% CO2 are in the range of 3.20−4.74 mmol of CO2/g. The CO2 sorption capacities of several potassium-based sorbents, including K2CO3/activated carbon,33,34 K2CO3/Al2O3,21,35,36
(3)
K 2CO3 + H 2O + CO2 ↔ 2KHCO3
(11)
Δ
2NaHCO3 → Na 2CO3 + H 2O + CO2
2KAlSi3O8 + 2H 2O + CO2 ↔ Al 2(Si 2O5)(OH)4 + 4SiO2 + K 2CO3
(10)
(7)
NaAlSi3O8 + H 2O + CO2 ↔ NaAlCO3(OH)2 + 3SiO2 (8)
For the spent PF regenerated in a N2 atmosphere, the phases of NaAlCO3(OH)2, Al2(Si2O5)(OH)4, NaHCO3, and KHCO3 disappear. The XRD pattern shows two new phases assigned as Na2CO3 and K2CO3, respectively. This indicates that the 8155
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Article
Energy & Fuels K2CO3/titanium,37,38 K2CO3/MgO,39 and K2CO3/zeolites,40 in 1−15% CO2 are reported as 0.35−2.84 mmol of CO2/g. In comparison to amine-based sorbents, the CO2 sorption capacity of K2CO3/PF is rather low, while the sample is insensitive to the thermal switch during the regeneration process and shows considerable cyclic working stability. The CO2 sorption capacity of the sample is almost comparable to those of potassium-based sorbents. Taking account of the high CO2 sorption capacities of amine-based sorbents and the excellent long-term working stabilities of potassium-based sorbents, a further modification of the sample with amine groups will be considered in future work.
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4. CONCLUSION Potassium-based solid sorbents are promising candidates for capturing CO2 from post-combustion flue gas. K2CO3/PF is prepared by impregnation of K2CO3 on PF. The CO2 sorption behaviors, sorption pathways, sorbent regeneration performances, and multiple cycle behaviors are investigated in a modified fixed-bed reactor. The CO2 sorption capacity of K2CO3/PF is 1.74 mmol of CO2/g in simulated flue gas composition of 60 °C, 5% CO2, and 10% H2O. The main chemical compounds of KAlSi3O8 and NaAlSi3O8 in the support can be transformed to new species, including NaAlCO3(OH)2, Al2(Si2O5)(OH)4, NaHCO3, and KHCO3, in the CO2 sorption process. Besides, the carbonation reaction between K2CO3 and humid CO2 also contributes to the CO2 sorption process of K2CO3/PF. The regeneration pathways are determined to contain three stages: the desorption of physically adsorbed CO2 occurring first at 55 °C, then bicarbonates of NaHCO3 and KHCO3 decomposing at 130 °C, and finally compounds of NaAlCO3(OH)2 and Al2(Si2O5)(OH)4 decomposing at 550 °C. Multiple cycle tests indicate that K2CO3/PF is regenerable and stable during long-term cyclic operations. Therefore, the sorbent shows potential for capturing CO2 from flue gas after combustion. Considerably more work will need to be performed to determine the CO2 sorption behaviors of K2CO3/PF in a more approximate untreated flue gas stream containing impurities, such as NOx, SOx, HCl, and Hg.
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AUTHOR INFORMATION
Corresponding Authors
*Telephone: +86-551-63603141. Fax: +86-551-63601669. Email:
[email protected]. *Telephone: +86-551-63603141. Fax: +86-551-63601669. Email:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (51206155) is sincerely acknowledged. REFERENCES
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DOI: 10.1021/acs.energyfuels.5b02207 Energy Fuels 2015, 29, 8151−8156
