Article pubs.acs.org/EF
Improvement in Regeneration Properties and Multicycle Stability for K2CO3/Al2O3 Adsorbents for CO2 Removal from Flue Gas Surajit Sengupta,† Satyanarayana Akuri Reddy,† Rajeshwer Dongara,† Asit Kumar Das,*,† Haripada Bhunia,‡ and Pramod Kumar Bajpai‡ †
Reliance Technology Group, Reliance Industries Limited, Jamnagar, Gujarat 361140, India Department of Chemical Engineering, Thapar University, Patiala, Punjab 147004, India
‡
ABSTRACT: A new adsorbent based on potassium carbonate (K2CO3) supported on modified γ-Al2O3 has been developed in this work. It has shown excellent multicycle stability with adsorption and regeneration at 55 and 130 °C, respectively. The support (γ-Al2O3) is stabilized by thermal treatment and alkali treatment with hydroxide, followed by calcination. The excellent regeneration characteristics are due to minimal interactions between the support and reactants during CO2 adsorption. This is due to acidity reduction of the support responsible for the formation of stable species, for example, KAl(CO3)2(OH)2. Various physicochemical characteristics have been studied to understand the adsorption and regeneration behavior of the adsorbents over multiple cycles. The effects of operating parameters, such as adsorption temperature, thermal dehydration of support material, and gas hourly space velocity (GHSV), during regeneration have also been studied. The CO2 adsorption capacities are found to be in the range of 2.3−2.6 mmol of CO2/g of adsorbent (10−11.4 wt % CO2), which also shows good stability after multicycle tests. The developed adsorbents also show high attrition resistance and, thus, can be effectively used in commercial application for CO2 capture.
1. INTRODUCTION Carbon dioxide (CO2) capture is a major step to control greenhouse gas emission from various industrial sources, including refinery, power plant, cement industries, etc. Various post-combustion capture routes, such as modified amine absorption, solid-based adsorption, and membrane separation, are being evaluated to capture CO2 efficiently in a cost-effective manner. Among these processes, CO2 capture using dry regenerable adsorbent has shown a great interest to the researchers because of lower energy requirements. Alkali-metalcarbonate-based solid adsorbents have a major advantage for adsorption at 50−70 °C and regeneration below 200 °C.1−3 Unlike amine-based absorption, there is no heat demand for vaporization of water or corrosion/solvent degradation issues. Extensive research studies have been reported on CO2 adsorption and regeneration of the solid adsorbents. These mainly include sorbent with alkali carbonate impregnated on various inorganic supports, such as activated carbon, silica gel, TiO2, MgO, Al2O3, etc.5−17 To enhance CO2 capture capacity from flue gases, K2CO3-based alumina-supported adsorbents have been widely studied in fixed-bed and fluidized-bed adsorption systems.14,18−25 The K2CO3-based adsorbents have good adsorption capacity, but its cyclic capacity decreases over multiple adsorption−regeneration.13,20−23 This is mainly due to some stable deactivating compounds, such as KAl(CO3)2(OH)2,22,23 KAl(CO3)2·1.5H2O,25,26 or K4H2(CO3)3· 1.5H2O,22 which are formed during adsorption of CO2 and require a higher regeneration temperature (>300 °C).22,23 However, thus far, very few studies are available addressing this problem. Lee et al. investigated on the improvement in regeneration of sorbents using modified alumina and α-Al2O3,12 and their results showed that K2CO3-based sorbents can be regenerated between 130 and 200 °C. They claim that the © 2014 American Chemical Society
alumina support in the preparation stage in the presence of CO2 forms a stable KAl(CO3)2(OH)2 phase and allows for regeneration of adsorbent at a lower temperature (300 °C), as reported by several authors.22,23,27,28 Keeping this in view, the alumina support has been modified by (i) applying heat treatment and (ii) treatment with alkali hydroxide followed by calcination, etc., to reduce the surface hydroxyl concentration/ acid sites. The modified adsorbents were evaluated in a fixedbed reactor system over a temperature range of 55−75 °C with 8 vol % CO2 in a simulated flue gas mixture. Various physicochemical properties were studied to explain the adsorption as well as regeneration of such adsorbents.
2. EXPERIMENTAL SECTION 2.1. Adsorbent Development. Using spray-dried γ-Al2O3 and αAl2O3 as support materials (Saint Gobain Norpro, Stow, OH) and K2CO3 (reagent grade, Merck India, Ltd.), the K2CO3-based adsorbents used in this study were prepared following a single-step Received: May 23, 2014 Revised: July 22, 2014 Published: July 22, 2014 5354
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Figure 1. Schematic of the experimental setup.
Table 1. Structural Characteristics of the Modified Support and Prepared Adsorbents adsorbenta
adsorbent name
K2CO3 loading (wt %)
acidity (mmol of NH3/g)
surface area (m2/g)
pore volume (cm3/g)
(a) Modified γ-Al2O3 Supports 0 0.55 188 0.48 Modification of γ-Al2O3 (Calcination of Support Material) γ-Al2O3 calcined at 700 °C A(700) 0 0.53 152 0.45 γ-Al2O3 calcined at 900 °C A(900) 0 0.41 100 0.42 γ-Al2O3 calcined at 950 °C A(950) 0 0.34 86 0.38 Modification of γ-Al2O3 (1% NaOH Treated, Followed by Calcination of Support Material) γ-Al2O3 treated with 1% NaOH followed by A(Na700) 0 0.45 161 0.446 calcination at 700 °C B 0 0.083 8 0.22 fresh α-Al2O3 (as such) (b) Prepared Adsorbents Using Fresh and Calcined Al2O3 40% K2CO3/γ-Al2O3 40KA 40 NDb 22 0.11 K2CO3/Al2O3 Adsorbents (Metal Carbonate Loading on Calcined Al2O3) 34% K2CO3/γ-Al2O3 34KA(700) 34 ND 58 0.18 31% K2CO3/γ-Al2O3 31KA(900) 31 ND 43 0.16 35% K2CO3/γ-Al2O3 35KA(950) 35 ND 31 0.15 46% K2CO3/γ-Al2O3 46KA(950) 46 ND 35 0.13 36% K2CO3/γ-Al2O3 36KA(Na700) 36 ND 53 0.15 (c) Prepared Adsorbents Using Fresh α-Al2O3 K2CO3/Al2O3 Adsorbents (Metal Carbonate Loading on as-Such α-Al2O3) 28% K2CO3/α-Al2O3 28KB 28 ND 4 0.005 33% K2CO3/α-Al2O3 33KB 33 ND 3 0.004 fresh γ-Al2O3 (as such)
A
average pore diameter (Å)
AI (%)
98
4.4
120 170 178
4.8 5.1 5.3
111
6.7
37
5.3
102
7.1
125 155 164 149 120
ND ND 3.8 ND 7.2
62 59
8.2 ND
Abbreviation used for designated adsorbent are Na, Na2CO3; K, K2CO3; A, γ-Al2O3; and B, α-Al2O3. For example, 36KA(Na700) means 36% K2CO3 impregnated on γ-Al2O3, wherein the support is treated 1% NaOH solution, and then calcined at 700 °C prior to impregnation. bND = not determined. a
2.2. Experimental Setup and Procedure. The adsorption− regeneration studies have been carried out in a stainless-steel fixed-bed reactor with a height of 42 cm and shell inner diameter of 3.5 cm. The diameter of the adsorbent zone is 1.7 cm, with a height of 8.3 cm. The temperatures along the adsorber were measured by thermocouples at three different positions: 4, 8, and 12 cm from the bottom. About 8 g of solid adsorbent was used for each experimental run. A simulated flue gas composition of 8 vol % CO2, 15 vol % H2O, and rest N2 was used in all of the adsorption studies. The simulated gas mixture of N2 and CO2 was passed through a temperature-controlled water saturator to saturate the stream at a given temperature. The total flow rate of the feed gas stream was 150 ± 2 mL/min. Then, the simulated gas mixture was passed through the adsorbent bed. The adsorbent was pretreated with water vapor for 30 min to achieve higher adsorption capacity. The
pore volume impregnation method. Prior to the impregnation of K2CO3 loading between 28 and 46 wt %, γ-Al2O3 was pretreated by calcining between 700 and 950 °C for 6 h.27 K2CO3 was dissolved in demineralized water equivalent to a water pore volume of the support. The support was poured into the solution and mixed thoroughly, held up for 1 h at room temperature for equilibration, and followed by drying at 120 °C for 24 h. Another method was adopted to pretreat the support material with 1 wt % NaOH solution and dried at 120 °C for 24 h.15 The dried support was then calcined at 700 °C/other temperatures as specified for 6 h. K2CO3/α-Al2O3 was prepared following the same method, wherein α-Al2O3 was taken as such. Higher loading of K2CO3 (beyond 36 wt %), such as 46 wt %, was performed through dissolution of K2CO3 into water at a higher temperature (70 °C). 5355
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treated effluent stream was passed through a dryer to remove the water, and the outlet concentration of CO 2 was measured continuously by an online infrared (IR) analyzer (measurement limit of 0−10 vol %). After adsorption was completed, the regeneration was carried out at a temperature of 130−150 °C under a N2 atmosphere. The temperature was decreased again to the adsorption temperature, and the multicycle testing was carried out. The schematic flow diagram of this adsorption system is shown in Figure 1. 2.3. Characterization of Adsorbents. The Micromeritics ASAP 2020 apparatus was used to determine the Brunauer−Emmett−Teller (BET) surface area and total pore volume of the adsorbents using nitrogen adsorption−desorption experiments. The morphology of the adsorbent particles was measured by field emission scanning electron microscopy (SEM, JEOL JSM-6100). The structure of the alumina support and adsorbents was investigated by transmission electron microscopy (TEM, Technai-20, Phillips) operated at 200 kV. The identification of crystalline phase of the adsorbent materials was performed by X-ray diffraction (XRD, X’Pert Pro, PANalytical). The presence of chemical bonding of materials was examined using Fourier transform infrared spectroscopy (FTIR, Nicolet 6700 FTIR spectrometer) over the range of 4000−400 cm−1 at a scanning resolution of 2 cm−1. The average particle size was measured using Malvern Mastersizer 2000 and was found between 75 and 80 μm. The thermal stability of selected adsorbents was carried out using a thermogravimetric analyzer (TGA, Universal V4.5A TA Instruments) using N2 as the carrier gas at the heating rate of 10 °C/min. A temperature-programmed desorption (TPD) study was carried out using gas chromatography (GC, PerkinElmer) with a thermal conductivity detector (TCD). The temperature program was set for 40 min with an initial temperature of 30 °C and ramp of 10 °C/min in a N2 atmosphere. Total acidity of γ-Al2O3, modified γ-Al2O3, and α-Al2O3 was also measured in Micromeritics AutoChem II 2920 by the TPD-NH3 method. The sample is preactivated at 600 °C under helium flow. The number of acidic sites was calculated from experimental TPD profiles. Attrition resistance of these adsorbents was measured as the attrition index (AI) by the ASTM D5757 method.
Figure 2. PSD of modified alumina supports and adsorbents.
respect to fresh adsorbent, which indicates retention of the active phase inside the pores. Similarly, from parts b and c of Table 1, it is seen that both the surface area and pore volume decreased with the increase in K2CO3 loading because of the blocking of pores during K2CO3 impregnation. The average pore diameter of K2CO3-based adsorbents supported on calcined/alkali-treated calcined Al2O3 increases with the increase in the calcination temperature of the Al2O3 support. At different calcination temperatures, the larger average pore diameter of the support helps to disperse a similar amount of active component (i.e., K2CO3) inside the pores during impregnation. Figure 2 also shows a shift in the average pore diameter in the case of 35KA(950) and 46KA(950), wherein the average pore diameter of 46KA(950) is shifted 15 Å lower (i.e., 149 Å), as compared to 35KA(950). This is due to excess growth of K2CO3 on the pores, resulting in a decrease in the average pore diameter, as shown in Figure 3b. 3.1.2. Acidity. The acidities of γ-Al2O3, modified γ-Al2O3, and α-Al2O3 were measured using TPD of ammonia (NH3), are reported in Table 1a. It is observed that the total acidity (Lewis and Brønsted acidities) decreased significantly with the increase in the calcination temperature. For the A(950) support, the acidity reduced to 0.34 mmol of NH3/g from 0.55 mmol of NH3/g. Because of thermal dehydration, the surface hydroxyl groups and chemically bound internal hydroxyl groups are released as water from γ-Al2O3, which results in reduction of acid sites. These observations are in good agreement with the results reported by Maciver et al.30 During pretreatment, the different phases of transition alumina adsorb water that reduces Lewis acid sites (strong acid) and transforms them into Brønsted sites (weak acid).31 According to Chen et al.,32 during transient phase transformation of γ-Al2O3, the AlV coordinated state is responsible for creating Lewis acid sites. It is interesting to observe that, the lower the acidity of Al2O3, the higher the multicycle CO 2 adsorption capacity for K 2 CO 3 /Al 2 O 3 adsorbents. For example, the acidity of the 35KA(950) adsorbent is 0.34 mmol of NH3/g, and its multicycle CO2 adsorption capacity is found to be 2.1 mmol of CO2/g of adsorbent, which is higher compared to 34KA(700) (capacity of 1.75 mmol/g, as shown in Figure 12). This is due to the reduction of stable component KAl(CO3)2(OH)2 formation during CO2 adsorption. It is in good agreement with inferences from XRD, TGA, and TPD analyses discussed in subsequent sections. It is also found from Table 1a that, at almost the same K2CO3 loading, adsorbents supported on α-Al2O3 show very
3. RESULTS AND DISCUSSION 3.1. Physicochemical Characterization of Adsorbents. 3.1.1. Surface Area, Pore Volume, and Average Pore Diameter. Table 1a shows the BET surface area, total pore volume, and average pore diameter of the fresh/modified support materials, e.g., γ-Al2O3, modified γ-Al2O3, and as-such α-Al2O3. It is seen that, during calcination of γ-Al2O3 alone (without metal carbonate loading) at high temperatures, both the surface area and pore volume decrease significantly and their average pore diameter increases drastically. This is due to the formation of the macrostructure from changes in the wall crystal structure.36 When the calcination temperature is increased, the water, e.g., free water, water of crystallization, interlayer water, etc., is removed from γ-Al2O3, resulting in collapse of pore structures, and produces large pores from small pores. The dehydrated alumina consists of a mix phase of η, δ, and θ alumina.15 Figure 2 shows the decrease in the pore volume and increase in the average pore diameter with the increase in the calcination temperature. A broader pore size distribution (PSD) of mesopores (100−250 Å) is observed from the calcination of the γ-Al2O3 support. These mesopores increase the dispersion of K2CO3 inside the pores, which facilitates the accessibility to CO2 during adsorption. As explained earlier, a shifting in average pore diameter is observed because of collapsing of smaller mesopores into larger mesopores in the γ-Al2O3 support. It is also seen that the PSD pattern of 35KA(950) after 11 cycles is almost similar with 5356
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Figure 3. SEM images of K2CO3-based modified adsorbents: (a) 35KA(950) and (b) 46KA(950).
Figure 4. TEM images of (a) fresh γ-Al2O3, (b) calcined γ-Al2O3, and (c) 35KA(950).
to (400) and (440) reflections at 2θ of 46.3 and 67.2, respectively. The TEM image of A(950) shows an increase in the average pore diameter (>20 nm), as shown in Figure 4b. As explained earlier, the average pore diameter is increased during calcination because of the collapse of the smaller mesostructure framework and increased the crystallinity. These results fully agree with the BET analysis data in Table 1a. Moreover, SAED of A(950) shown in Figure 4b indicates the dots in the concentric rings. These dots in A(950) appeared to be more as compared to fresh γ-Al2O3, which indicates more crystalline structure in calcined Al2O3. The TEM image of 35KA(950) shown in Figure 4c indicates that K2CO3 is dispersed uniformly inside the broader mesopores of calcined γ-Al2O3. The SEM image in Figure 3a confirms this observation. The SAED pattern (shown in the inset) also indicates the disappearance of the dot pattern, which suggests that K2CO3 is dispersed inside the crystalline pores. 3.1.6. XRD Analysis. While modifying γ-Al2O3 supports by the heat treatment method or alkaline treatment followed by heat treatment, the XRD pattern shows that, as the calcination temperature increases, the γ-Al2O3 phase transformed into the mixture of metastable phases γ-Al 2 O 3 , δ-Al 2 O 3 [Joint Committee on Powder Diffraction Standards (JCPDS) number 46-1131], and θ-Al2O3 (JCPDS number 35-0121) at calcination temperatures of 700−900 °C (as shown in Figure 5a).29,30,33 The peaks appeared from the intermediate phases are difficult to identify from the XRD pattern but appeared to be δ phase obtained at 900 °C. The presence of the remaining γ phase can also not be ignored from the XRD. On the basis of XRD results, the estimated percentage value of a mix of δ-Al2O3 and θ-Al2O3 present in modified alumina was found to be nearly 46.5 wt %,
low CO2 adsorption capacity (0.43 versus 2.1 mmol/g capacity), although its total acidity is found to be very low (0.083 mmol of NH3/g). The reduced pore volume and surface area available for K2CO3/α-Al2O3, as seen in Table 1c, result in poor dispersion of active K2CO3 ingredient inside the α-Al2O3 support, which ultimately leads to lower CO2 adsorption capacity. 3.1.3. AI. The attrition resistance for selected adsorbents was measured using the ASTM D5757 method. The AI of modified supports/adsorbents was found within the desired range (300 °C). The adoption of aforementioned strategies for improving regeneration properties not only increased multicycle stability of adsorption capacity but also decreased the regeneration temperature drastically from 300 to 130 °C. From the experimental results, it was concluded that γ-Al2O3 calcined at 950 °C significantly reduced the surface acidity and, hence, decreased the formation of KAl(CO3)2(OH)2 to a great extent. Among all of the K2CO3-based adsorbents, 35KA(950) showed excellent adsorption and regeneration performance. CO2 adsorption capacity of 35KA(950) was 2.1 mmol/g after 11 cycle tests at 55 °C. The adsorbent was almost completely regenerated at 130 °C over multiple cycles. All of these results are consistent with XRD, TPD, and TGA. The specific reason for this improved regeneration is linked with acidity of modified γ-Al2O3 support material of different transient phases. The thermal dehydration of γ-Al2O3 at 950 °C produced a dominant mix of δ and θ phases, resulting in the prevention of stable component formation during CO2 adsorption. CO2 adsorption capacity for 35KA(950) was at a maximum at 55 °C and can be regenerated (to ∼98%) at 150 °C. There was approximately 30% reduction of total regeneration time at GHSV of 750 h−1. Also, the dissolution of K2CO3 in deionized water at an elevated temperature (70 °C) could not permit higher loading to achieve a high adsorption capacity because K2CO3 forms as crystals/aggregates on the outside of the support, which reduce active sites for CO2 adsorption. Even then, all of the K2CO3based adsorbents showed high attrition resistance (
