Development of Regenerable MgO-Based Sorbent Promoted with

Apr 11, 2008 - To improve their CO2 absorption capacity, alkali-based sorbents prepared by impregnation and wet mixing method of potassium carbonate o...
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Environ. Sci. Technol. 2008, 42, 2736–2741

Development of Regenerable MgO-Based Sorbent Promoted with K2CO3 for CO2 Capture at Low Temperatures SOO CHOOL LEE,† HO JIN CHAE,† SOO JAE LEE,† BO YUN CHOI,† CHANG KEUN YI,‡ JOONG BEOM LEE,§ CHONG KUL RYU,§ AND J A E C H A N G K I M * ,† Department of Chemical Engineering, Kyungpook National University, Daegu, 702-701, Korea, Korea Institute of Energy Research, Daejeon, 305-343, Korea, and Korea Electric Power Research Institute, Daejon, 305-380, Korea

Received October 25, 2007. Revised manuscript received January 4, 2008. Accepted January 8, 2008.

To improve their CO2 absorption capacity, alkali-based sorbents prepared by impregnation and wet mixing method of potassium carbonate on supports such as activated carbon and MgO (KACI30, KACP30, KMgI30, and KMgP30), were investigated in a fixed bed reactor (CO2 absorption at 50–100 °C and regeneration at 150–400 °C). Total CO2 capture capacities of KMgI30-500 and KMgP30-500 were 178.6 and 197.6 mg CO2/g sorbent, respectively, in the presence of 11 vol % H2O even at 50 °C. The large amount of CO2 capture capacity of KMgP30-500 and KMgI30-500 could be explained by the fact that MgO itself, as well as K2CO3, could absorb CO2 in the presence of water vapor even at low temperatures. In particular, water vapor plays an important role in the CO2 absorption of MgO and KMgI30-500 even at low temperatures below 60 °C, in marked contrast to MgO and CaO which can absorb CO2 at high temperatures. The CO2 capture capacity of the KMgI30300 sorbent, however, was less than that of KMgI30-500 due to the formation of Mg(OH)2 which did not absorb CO2. MgO basedsorbents promoted with K2CO3 after CO2 absorption formed new structures such as K2Mg(CO3)2 and K2Mg(CO3)2 · 4(H2O), unlike KACI30 which showed only the KHCO3 crystal structure. The new Mg-based sorbents promoted with K2CO3 showed excellent characteristics in that it could satisfy a large amount of CO2 absorption at low temperatures, a high CO2 absorption rate, and fast and complete regeneration.

Introduction Carbon dioxide (CO2) is a greenhouse gas that is released into the environment due to burning of fossil fuels and it can cause global climate warming, which may be disastrous to the environment. CO2 can be removed from flue gas and waste gas streams by various methods such as membrane separation, absorption with a solvent, and adsorption using molecular sieves (1–5). These methods, however, are costly and consume a lot of energy. * Corresponding author e-mail: [email protected]; tel.: +82-53950-5622; fax: +82-53-950-6615. † Kyungpook National University. ‡ Korea Institute of Energy Research. § Korea Electric Power Research Institute. 2736

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One of the improved techniques for the removal of CO2 is chemical absorption with solid regenerable sorbents. The use of solid sorbents containing alkali metal and alkali earth metal for CO2 absorption has been reported many times in the literature (6–21). In the case of alkali metal, one mole of alkali metal–carbonate can absorb a stoichiometric amount of one mole of CO2 and one mole of H2O at low temperature as the following reaction: M2CO3 + H2O + CO2 h 2MHCO3 (M ) Na, K) (8, 15–18). Water vapor is always necessary in the reaction as shown in the reaction mechanism. In the case of alkali earth metal, however, carbon dioxide is chemically absorbed through the reaction XO + CO2 h XCO3 (X ) Mg, Ca) to form alkali earth metal–carbonate (MgCO3 or CaCO3) in the absence of water vapor at high temperature (19). The sorption of CO2 on the K2CO3-Al2O3 composite sorbent, in the presence of water vapor, was studied by in situ IR spectroscopy and X-ray diffraction analysis (21). Several studies regarding the efficient chemical absorption over K2CO3, supported on carbon (7–9, 15, 16) or other different porous matrices such as silica gel, Al2O3, and vermiculite (12), were also reported by using cyclic fixedbed operations under moist conditions, for the recovery of carbon dioxide from flue gases. Thus, several additives or supports such as activated carbon, SiO2, Al2O3, and others have been used in alkali metal-based sorbents to absorb CO2. Recently, researchers have tried to develop techniques to enhance the uptake capacity and reversibility of alkali earth metal sorbents, like MgO and CaO (6, 13, 19). Those sorbents, however, were applicable at much higher absorption and regeneration temperatures (less than 860 °C). The CO2 capture capacity of the K2CO3/MgO sorbent was larger than those of the potassium-based sorbents supported on activated carbon, Al2O3, and TiO2 even at low temperatures below 60 °C in our previous study (15). In particular, the K2CO3/MgO sorbent showed higher CO2 capture capacity than the theoretical value of the sorbent, which calculated from moles of K2CO3 involved in the sorbent. However, the reason for high CO2 capture capacity and the role of MgO used as support have not been clearly defined. One of the objectives of this work was to develop a new regenerable solid sorbent for use in CO2 absorption at low temperatures below 60 °C. The CO2 capture capacity and regeneration property of several potassium-based sorbents were studied in a fixed-bed reactor using multiple tests. The role of support in CO2 absorption at low temperatures was also investigated. In addition, changes in the physical properties of the sorbents before/after CO2 absorption and its mechanism were investigated with the aid of power X-ray diffraction (XRD; Philips, X’PERT) at the Korean Basic Science Institute (Daegu) and TPD.

Experimental Section Preparation of Sorbents. The potassium-based sorbents used in this study were prepared by impregnating K2CO3 on porous supports such as activated carbon (AC, Aldrich) and MgO (Aldrich), and by the wet-mixing method. Impregnation Method. Five (5.0) g of support were added to an aqueous solution containing 2.5 g of anhydrous potassium carbonate (K2CO3, Aldrich) in 25 mL of deionized water. Then, it was mixed with a magnetic stirrer for 24 h at room temperature (7, 11). After stirring, the mixture was dried in a rotary vacuum evaporator at 60 °C. The dried samples were calcined in a furnace under a N2 flow (100 mL/min) for 5 h at 300 and 500 °C. The ramping rate of the temperature was maintained at 3 °C/min. 10.1021/es702693c CCC: $40.75

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Wet-Mixing Method. K2CO3 and MgO were sufficiently mixed with water for 1 h. This slurry was dried for 3 h to remove water from the material at a temperature of 100 °C. The dried samples were calcined in a furnace under the same conditions for 5 h. The weight ratio of MgO to H2O and the amounts of K2CO3 added were fixed at 5:0.2 and 30 wt. %, respectively. The amount of alkali metal of the sorbents prepared was determined by using a Varian Spectra AA 800 atomic absorption spectrophotometer. The sorbents were denoted as KACI30, KACP30, KMgI30300, and KMgP30-500, where K means K2CO3, AC means activated carbon, Mg means MgO, I means the impregnation method, P means the wet mixing method, 30 means the loading or adding amount of K2CO3, and 300 or 500 means the calcination temperature. The composition of sorbent was fixed at 30 wt % of K2CO3 and 70 wt % of activated carbon or MgO. Apparatus and Procedures. CO2 absorption and regeneration processes were performed in a fixed-bed quartz reactor (diameter of 1 cm), which was placed in an electric furnace under atmospheric pressure. One-half (0.5) g of the sorbent was packed into the reactor, and space velocity (SV) was maintained at 3000 h-1 to minimize severe pressure drops and channeling phenomena. All volumetric gas flows were measured under standard temperature and pressure (STP) conditions. The temperature of the inlet and outlet lines of the reactor was maintained above 100 °C to prevent condensation of water vapor being injected into the reactor and GC column. The column used in the analysis was a 1/8 in. stainless tube packed with Porapak Q. When the CO2 concentration of the outlet gases reached the same level as that of the inlet gas (1 vol. %) in the CO2 absorption process, nitrogen was introduced for sufficient time in multiple tests to regenerate the spent sorbents while the CO2 concentration reached 200 ppm. The outlet gases from the reactor were automatically analyzed every 4 min by a thermal conductivity detector (TCD; Donam Systems Inc.), which was equipped with an auto sampler (Valco Instruments CO. Inc.).

Results and Discussion Comparison of CO2 Capture Capacities and the Breakthrough Curves of Various Sorbents. Figure 1 a shows CO2 breakthrough curves of various K2CO3 based sorbents in the presence of 9 vol % H2O and 1 vol % CO2 at 60 °C. The Xand Y-axes indicate reaction time and the CO2 concentration emitted from the reactor, respectively. KACP30 sorbent showed a very short breakthrough time, while KACI30, KMgI30-300, and KMgP30-300 sorbent, which were calcined at 300 °C, showed 24, 40, and 24 min under the same experimental conditions, respectively. The CO2 capture capacities of those sorbents were calculated from the CO2 breakthrough curves, and are shown in Figure 1b. The Y-axis indicates the total CO2 capture capacity and the net CO2 capture capacity. The total CO2 capture capacity describes the amount of CO2 absorbed until the output concentration of CO2 reached 1 vol %, which is the same as that of the inlet. The net CO2 capture capacity is defined as the amount of CO2 absorbed per 1 g of sorbent until the CO2 concentration remains less than 200 ppm. The total CO2 capture capacities of the K2CO3-based sorbents (KACP30, KACI30, KMgI30-300, KMgP30-300, KMgI30-500, and KMgP30-500) were 75.5, 82.0, 103.9, and 73.3 124.8, and 119.1 mg CO2/g sorbent, respectively. The total CO2 capture capacity of the potassium-based MgO sorbents was larger than that of the theoretical value, which was calculated from moles of K2CO3 involved in the sorbent. In particular, the KMgI30-500 sorbent showed about 130% of theoretical value. It was thought that these results were due to the formation of new structure during the CO2 absorption or the CO2 absorption of MgO in the presence of water vapor at low temperature. Until now, alkali earth metals

FIGURE 1. CO2 breakthrough curves (a) and CO2 capture capacities (b) of various alkali metal-based sorbents in the presence of 9 vol % H2O and 1 vol % CO2 at 60 °C. like MgO and CaO have been well-known for the CO2 absorption sorbents at high temperature (6, 13). The potassium-based MgO sorbents, such as KMgI30 and KMgP30, developed in this work, however, have the potential for CO2 absorption at low temperatures below 60 °C. Structural Identification of Sorbents before/after CO2 Absorption by XRD. The structural change of sorbents before/after absorption was examined by XRD. Figure 2 shows the XRD patterns of fresh sorbents before CO2 absorption. In the case of the KACI30 sorbent (a), only K2CO3 phase (JCPDS No. 71-1466) was found before absorption. The XRD patterns of fresh KMgI30-300 and KMgP30-300 showed a K2CO3 phase, a Mg(OH)2 phase (JCPDS No.83-0114), and a MgO phase (JCPDS No. 43-1022). KMgI30-500 and KMgP30500 sorbents did not show the Mg(OH)2 phase, unlike KMgI30-300 and KMgP30-300. Figure 3 shows the XRD patterns of those sorbents after CO2 absorption at 60 °C. The XRD patterns of the KMgI30-300 (b) and KMgP30-300 (c) sorbents showed four phases, which were K2Mg(CO3)2 (JCPDS No. 75-1725), K2Mg(CO3)2 · 4(H2O) (JCPDS No. 83-1955), MgO, and Mg(OH)2, unlike the KACI30 (a) sorbent which included KHCO3 peak only after absorption. The XRD patterns of KMgI30-500 and KMgP30-500 showed three phases, K2Mg(CO3)2, K2Mg(CO3)2 · 4(H2O), and MgO, as shown in Figure 3d and e. It has been noticed that the KHCO3 phase was not observed in the XRD patterns of the KMgI30 and KMgP30 sorbents after CO2 absorption at 60 °C. From the results, it was found that in the case of the KMgI30 and KMgP30 sorbents, new structures such as K2Mg(CO3)2, and K2Mg(CO3)2 · 4(H2O) were formed during CO2 absorption. As stated in a previous section, the K2CO3-based MgO sorbents had a higher CO2 capture capacity than that of the theoretical value. The results, however, could not be explained by the formation of new structures of KMgI30 and KMgP30, as shown in Figure 3. Since 1 mole of K2CO3 absorbs a stoichiometric amount of 1 mole of CO2, the theoretical value of KMgI30 and KMgP30 VOL. 42, NO. 8, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. XRD patterns of fresh sorbents such as KACI30 (a), KMgI30-300 (b), KMgP30-300 (c), KMgI30-500 (d), and KMgP30-500 (e) before CO2 absorption; (b) K2CO3; (4) MgO; (0) Mg(OH)2. is also 95 mg CO2/g sorbent, even though new structures are formed during CO2 absorption. Effects of Water on CO2 Absorption at Low Temperature. As stated in the Introduction, the sorbent containing alkali earth metal can absorb CO2 at high temperature (19). To identify the effects of water on absorption of the K2CO3based MgO sorbent, temperature programmed absorption (TPA) tests of the KMgI30-500 sorbent were performed with/ without water vapor when the ramping rate was 5 °C/min. TPA tests were carried out by measuring the concentration of CO2 during absorption at temperature ranges between 60 and 700 °C. These results are shown as a function of temperature in Figure 4. In the presence of water vapor, the KMgI30-500 sorbent absorbed all the CO2 up to the temperature of 140 °C. In the absence of water vapor, no CO2 was absorbed at the same low temperature range. But CO2 was absorbed at the temperature range between 200 and 320 °C and above 600 °C. In a separate experiment, it was observed that the KMgI30-500 and KACI30 sorbents hardly absorbed CO2 at 60 °C in the absence of water vapor. These results indicated that the CO2 absorption of the K2CO3-based MgO sorbent even at low temperature below 60 °C was due to the presence of water vapor, unlike alkali earth metal. Role of MgO in CO2 Absorption. The amount of CO2 absorbed per 1 g of K2CO3 calculated from the breakthrough curves of Figure 1 for KACI30 and KMgI30-500 sorbents were 273.3 and 416.7 mg CO2/g K2CO3. Considering that all theoretical values of sorbents, which calculated from moles of K2CO3 involved in the sorbent, are about 318.3 mg CO2/g K2CO3, the total amounts absorbed on the KACI30 and KMgI30-500 sorbents are equivalent to 85.9 and 130.9% of the theoretical value. It is clear that these results are not due to the formation of new structures such as K2Mg(CO3)2 and K2Mg(CO3)2 · 4(H2O), but due to the effects of MgO used as support on CO2 absorption, unlike KACI30. These results indicated that the nature of support affected the CO2 capture capacity of the sorbent. 2738

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FIGURE 3. XRD patterns of sorbents such as KACI30 (a), KMgI30-300 (b), KMgP30-300 (c), KMgI30-500 (d), and KMgP30-500 (e) after CO2 absorption; (9) KHCO3; (4) MgO; (0) Mg(OH)2; (3) K2Mg(CO3)2; (0) K2Mg(CO3)2 · 4(H2O).

FIGURE 4. TPA results of the KMgI30-500 sorbent under the conditions with/without water vapor. To identify the effects of the nature of supports itself such as AC, Mg(OH)2, and MgO, which did not contain K2CO3, the CO2 absorption with 0.5 g of those supports was performed with/without 9 vol % H2O at 60 °C, and the results are shown in Figure 5. AC and Mg(OH)2 hardly absorbed CO2 in spite of the presence of water vapor, while MgO absorbed about 64 mg CO2/g sorbent even at low temperature of 60 °C, as shown in Figure 5a. It is noted that a very low CO2 capture capacity was obtained in the absence of water vapor. In this work, MgO could absorb CO2 in the presence of water vapor even at low temperatures below 60 °C. Another interesting fact is that MgO could not absorb CO2 in the absence of water vapor at 60 °C. Even though the absorption mechanism for this high CO2 capture capacity of MgO even at low temperature is not clear, it is thought that the presence of water vapor is very important in the CO2 absorption process. From these experimental results, it could be concluded that

FIGURE 5. Breakthrough curves for supports such as AC, Mg(OH)2, and MgO under conditions with (a) and without (b) 9 vol % H2O at 60 °C.

FIGURE 6. TPD results of MgO, KACI30, KMgI30-300, and KMgP30-500 after CO2 absorption. MgO played an important role in the high CO2 capture capacities of KMgI30-500 and KMgP30-500, by participating directly in CO2 absorption, as well as by helping K2CO3 transform to K2Mg(CO3)2 or K2Mg(CO3)2 · 4(H2O). In addition, it was found that Mg(OH)2, which could be converted to MgO after treating at 500 °C, did not directly relate to CO2 absorption. TPD of MgO, KACI30, KMgI30-300, and KMgI30-500. Temperature programmed desorption (TPD) tests were carried out by measuring the concentration of CO2 desorbed when the temperature ramping rate was 1 °C/min and these experimental results are shown in Figure 6. The TPD results for the KMgI30-500 sorbent showed three types of CO2 peaks. This indicates that there are three kinds of structures within the KMgI30-500 sorbent after CO2 absorption. The initial peak around 130 °C was consistent with that of KACI30, which was in agreement with KHCO3 (15). The peak around 300 °C

FIGURE 7. CO2 capture capacities of KMgI30-500 (a) and KMgP30-500 (b) with various concentrations of water vapor (7, 9, and 11 vol % H2O) at 60 °C. agreed well with that of MgO, however, in the case of the KMgI30-300 sorbent, most CO2 was desorbed in the temperature range between 320 and 400 °C, unlike the TPD results of KMgI30-500. It must be noted that the peak around 300 °C, by the CO2 desorption of MgO, was not observed in the TPD result of KMgI30-300. From these results, it was found that there was a additional structure related to the CO2 absorption within KMgI30-500, and that Mg(OH)2, which was shown in the XRD patterns of the KMgI30-300, did not absorb CO2. This was consistent with the experimental results of Mg(OH)2, which hardly absorbed CO2, in spite of the presence of water vapor, in the previous section. Effects of Absorption Temperature and Concentration of Water Vapor on CO2 Capture. Figure 7 shows the CO2 capture capacities of KMgI30-500 and KMgP30-500 with various concentrations of water vapor (7, 9, and 11 vol % H2O) at 60 °C. The CO2 capture capacities of KMgI30-500 and KMgP30-500 increased with various concentrations of water vapor, as shown in Figure 7a and b. In particular, the total CO2 capture capacities of the KMgI30-500 and KMgP30500 calculated from the breakthrough curves were 131.3 and 120.7 mg CO2/g sorbent, respectively, in the presence of 11 vol % H2O at 60 °C. From these results, it was known that concentration of water vapor plays an important role in the CO2 absorption rate and the CO2 capture capacity. Figure 8 shows the CO2 capture capacities of those sorbents with various reaction temperatures between 50 and 100 °C in the presence of 11 vol % H2O. KMgI30-500 and KMgP30-500 showed very low CO2 capture capacity at above 80 °C, while at 50 °C, they showed 178.6 and 197.6 mg CO2/g sorbent, respectively. In particular, it must be noted that the net CO2 capture capacities of these sorbents were 106.8 and 126.6 mg CO2/g sorbent, respectively. The increase of the CO2 capture capacity was thought to be due to an increase in relative humidity resulting from a decrease in the absorption temperature. Also, these results indicate that the CO2 capture capacity of MgO is markedly increased with relative humidity. VOL. 42, NO. 8, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 10. XRD patterns of the KMgI30-500 sorbent after regeneration under nitrogen at various temperatures (150 °C (a), 200 °C (b), 350 °C (c), and 400 °C (d)); (b) K2CO3; (4) MgO; (0) Mg(OH)2.

FIGURE 8. CO2 capture capacities of KMgI30-500 (a) and KMgP30-500 (b) in the presence of 11 vol % H2O at various reaction temperatures between 50 and 100 °C.

K2CO3 crystal structure at high regeneration temperatures (350–400 °C). These results indicated that a sufficiently high temperature was needed to convert new structures into the K2CO3 crystal structure. In conclusion, the KMgI30 sorbent could be used as the sorbent that has the potential for CO2 absorption in that it satisfied the requirements of a large amount of CO2 capture capacity and it exhibited excellent regeneration property at temperatures above 350 °C.

Acknowledgments This research was supported by a grant (DA2-202) from Carbon Dioxide Reduction & Sequestration Research Center, one of the 21st Century Frontier Programs funded by the Ministry of Science and Technology of the Korean government.

Literature Cited

FIGURE 9. CO2 capture capacity of KMgI30-500 sorbent during multiple cycles at various regeneration temperatures. In a separate experiment, it was confirmed that the high CO2 capture capacity of KMgI30-500 and KMgP30-500 was due to the increase of the CO2 capture capacity of MgO. Regeneration Property. The CO2 capture capacity of the KMgI30-500 sorbent was tested during the multiple cycles at various regeneration temperatures and the results are shown in Figure 9. The regeneration process of the KMgI30500 sorbent after absorption in the presence of 9 vol % H2O at 60 °C was performed between 150 and 400 °C. The total CO2 capture capacity of that sorbent decreased during multiple cycles below 350 °C, however, the total CO2 capture capacity was maintained during multiple cycles at higher temperature than 350 °C. These results are in agreement with TPD results. The XRD patterns of the KMgI30-500 sorbent, after regeneration under nitrogen at various temperatures (150, 200, 350, and 400 °C), were observed and the results are shown in Figure 10. New structures, such as K2Mg(CO3)2 and K2Mg(CO3)2 · 4(H2O), which were observed after CO2 absorption, were completely converted into the 2740

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