Ind. Eng. Chem. Res. 2008, 47, 7357–7360
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SEPARATIONS Effect of SOx Adsorption on Layered Double Hydroxides for CO2 Capture M. K. Ram Reddy, Z. P. Xu, G. Q. (Max) Lu, and J. C. Diniz da Costa* ARC Centre of Excellence for Functional Nanomaterials, Australian Institute of Bioengineering and Nanotechnology, and The CooperatiVe Research Centre for Greenhouse Gas Technologies, School of Engineering, The UniVersity of Queensland, Brisbane 4072, Australia
In this work, we investigate the effect of SOx on the performance of layered double hydroxide (LDH) derivatives used as adsorbent for CO2 capture. LDH derivatives such as layer double oxide (LDO) have shown great potential for high-temperature CO2 separation from flue gases. We found that even at low flue gas feed concentrations of SOx (0.1%), the sorption values were very high, reaching a maximum sorption capacity equivalent to 11.04% of the sorbent weight. Regeneration of LDO in pure helium resulted in regaining up to 58% of its original sorption capacity. Temperature cycling also revealed the irreversible nature of SOx sorption. In addition, regeneration after CO2/SOx and SOx/CO2 sorption experiments showed that SOx replaces CO2. SOx sorption over CO2 was favored due to the strong acid-base interactions between SOx and LDO, thus forming sulfites and sulfates. Hence, LDH derivatives for CO2 capture require a de-SOx unit operation upstream. Introduction In coal fired power plants, the sulfur content of the feed coal is oxidized during combustion to form sulfur oxides (SO2 and SO3) commonly referred as SOx. Emissions of SOx are proportional to the sulfur content of the fuel, and for the majority of coals currently in use, the sulfur content is in the range of 1-3%.1,2 SOx are released into the atmosphere with flue gases emitted from the power stations causing environmental concerns such as acid rain. Analyses of flue gases produced by power plants burning coal before desulfurization indicate 0.1-0.2% SO2 and ∼0.005% SO3.3,4 Control of these emissions is becoming mandatory due to the increased awareness and strict environmental laws coming into force around the world. Inorganic compounds with high selectivity and good adsorption capacity for CO2 and SOx at relatively high temperature are actively investigated for this purpose.3-7 A layered double hydroxide (LDH) is an interesting class of inorganic compounds receiving growing attention due to their wide range of applications as catalysts, precursors, and adsorbents.8-10 LDH belongs to a large group of anionic or basic clays while Mg-Al LDH is referred as hyrotalcite-like compounds. Their structure consists of positively charged brucite-like (Mg(OH)2) layers with trivalent Al cations substituting for divalent Mg cations. The excess positive charge in the main layers of LDH is compensated by anions and water molecules in the interlayer regions.8 Heat treatment of crystalline LDH at higher temperatures leads to important structure evolutions and phase changes. Crystalline LDH changes to amorphous layered double oxide (LDO) phase upon heating to 400-600 °C.11-14 LDH and in particular their LDO derivatives produced on calcination were found to be very stable even at high temperatures. LDO has potential applications as CO2 and SOx sorption materials to take advantage of acid-base interactions between acidic gas species and basic sites on LDO. Several research groups have investigated CO2 adsorption on LDH and LDO materials and observed an adsorption capacity of 0.5mmol/g at 300 °C and 1 bar.12-17 * To whom correspondence should be addressed. E-mail: j.dacosta@ uq.edu.au. Tel: 61-7-33656960. Fax: 61-7-33656074.
LDH and LDO were also found to be potential SOx adsorbents even though there are very limited studies reported so far in this area. Pinnavaia et al. studied SOx sorption on LDH between 100 and 400 °C and found weight uptakes up to 3-4%.3 It was reported that SOx replaces interlayer CO32- ions from LDH. The sorbent was finally regenerated by calcining at 500 °C even though regeneration levels are not known. In other instance, cerium (CeO2) containing Mg2Al2O5 spinel was used as catalyst in SOx removal from fluid catalytic cracking flue gas.4 Study of sorption of SOx by LDO and its influence on CO2 sorption is very important in the scenario of LDO being currently considered mainly for CO2 capture from flue gas by several researchers. In this work, a detailed investigation of sorption of SOx by LDO was conducted using SOx feed under the conditions of flue gas. In addition, CO2 and SOx sorptions and their competitive affinity toward LDO were studied for comparison purposes. Experimental Section LDH Preparation. LDH was prepared by the coprecipitation method. The 1.0 M solutions of Mg(NO3)2 · 6H2O and Al(NO3)3 · 0.9H2O were mixed together in the required ratio, and 200 mL of this solution was added to the base solution containing NaOH and Na2CO3. The resultant slurry was aged for 24 h at 80 °C with continuous stirring. LDH was separated by centrifuging, and the resultant cake was washed with deionized water 5-6 times in order to completely remove sodium salts from the LDH. LDH was finally dried in an oven at 100 °C overnight. All the LDH samples were calcined in situ at 400 °C for 4 h in an inert helium atmosphere flowing at 80 mL · min-1 before measuring the sorption on the resulting LDO at 200 °C, similar to a flue gas emission temperature. Sorption Measurements. CO2 and SOx sorption was measured using a Shimadzu TG50 thermogravimetric analysis instrument was used for sorption measurements using 0.1% SOx in N2. Experiments were also carried using a synthetic dry gas with 14% CO2, 4% O2, and 82% N2 to study CO2 sorption under
10.1021/ie8004226 CCC: $40.75 Published 2008 by the American Chemical Society Published on Web 08/28/2008
7358 Ind. Eng. Chem. Res., Vol. 47, No. 19, 2008 Table 1. Comparison of CO2 and SOx Sorption on LDO at Different Time Intervals CO2 sorption
Figure 1. (a) SOx (0.1% in N2) sorption at 200 °C and regeneration of the LDO at 400 °C; (b) SOx (0.1% in N2) sorption at 200 °C and regeneration of the LDO at 600 °C.
Figure 2. SOx (0.1% in N2) sorption and desorption pattern after three temperature cycles.
Figure 3. Comparison of CO2 and SOx sorptions by LDO at 200 °C using mixed gas (14% CO2) and 0.1% SOx feeds, respectively.
the influence of SOx in flue gases. A constant flow of feed at 80 mL · min-1 was maintained during all the sorption experiments. Results and Discussion Figure 1a shows the LDO (calcined at 400 °C) sorption behavior of 0.1% SOx in N2 and its regeneration capability. Even at such a low concentration, SOx sorption is significantly high,
SOx sorption
time (min)
weight gained (%)
contribution (%)
weight gained (%)
contribution (%)
2 5 10 20 30 40 50 60
1.98 2.34 2.54 2.72 2.82 2.89 2.95 2.99
66.0 78.2 84.9 91.0 94.3 96.6 98.7 100
0.26 0.60 1.14 2.0 2.78 3.40 3.95 4.45
5.8 13.4 25.6 44.9 62.5 76.4 88.8 100
showing an increase of 6.65% in weight in 2 h. The trend shows that the sorption continues to increase with time and still remains far away from reaching equilibrium after 2 h time. Regeneration of the sample at 400 °C in He atmosphere resulted in desorption of 42%. It is also clear from Figure 1a that more than 90% of desorption takes place during the ramping up of temperature to 400 °C and shows no significant decomposition after that. A similar experiment was also carried out for LDO (calcined at 600 °C) to determine the effect of regeneration at higher regeneration temperature of 600 °C, which are depicted in Figure 1b. Interestingly, the sample shows much higher sorption by 66% contributing to a weight gain of 11.04% against 6.65% observed in the previous case (Figure 1a) in a 2-h time period. Regeneration at 600 °C in He produced only 25% desorption demonstrating poor regenerability. SOx sorption and desorption behavior was also monitored using temperature cycling conducting sorption at 200 °C and desorption at 300 °C in He in 30-min cycles as shown in Figure 2. It is clearly seen from the graph that sorption rate is much higher compared to the desorption rate. This resulted in an upward movement of the curve showing consistent increase in weight despite three desorption cycles. After a initial sorption period of 60 min, the evacuation at 300 °C during the first 30min cycle yielded only 24.5% desorption. This value subsequently reduced to 12 and 8.8% during the second and third desorption cycles, respectively. Figure 3 shows the comparison of sorption trends of CO2 and SOx. CO2 sorption rate is quite rapid and reaches equilibrium very quickly as 85% of the total CO2 sorption takes place within the first 10 min. On the other hand, SOx sorption is a slow process even though the quantity of SOx taken up by the LDO is much higher than CO2. A comparison of sorption values for CO2 and SOx sorption at different time intervals is provided in Table 1. For example, CO2 sorption in the first 5 min of the experiment shows a weight gain of 2.34% while SOx sorption results in an increase of 0.6% in the same period. After 60 min, the CO2 sorption value remains at 2.99% while SOx increases to 4.45%. CO2 sorption reaches nearly equilibrium in ∼30 min, and SOx shows a strong sorption tendency even after 60 min with a consistent increase of 12-15% for every 10-min interval. In order to understand the influence of SOx on CO2 sorption and vice versa, two different experiments were conducted. In the first experiment, CO2 sorption was performed for the first 60 min using mixed gas followed by SOx (0.1% in N2) for the next 60 min, and the sample was finally regenerated at 400 °C. In the second experiment, SOx sorption was carried out for the first 60 min followed by CO2 (100%) sorption and regeneration. Results obtained from these two experiments are shown in Figure 4a and b. Observed values of CO2/SOx and SOx/CO2 sorptions and regeneration from these two experiments are compared with the values presented in Table 2. CO2 shows a normal trend of sorption (Figure 4a) with a weight gain of 2.92%
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400 °C and observed 3-4% weight gain on saturation.3 In the present case, LDO shows much stronger sorption potential and a maximum weight gain of 11.04%. SOx sorption as seen from Figure 3 did not reach equilibrium quickly as in the case of CO2. The fast CO2 uptake is mainly attributed to adsorption phenomena (i.e., physisorption) as CO2 desorbed in excess of 85% as reported elsewhere.18,19 On the other hand, the slow uptake of SOx is associated with a chemical reaction (i.e., chemisorption), evidenced by a maximum SOx reversible desorption of 42% only. These findings further reiterate the fact that SOx sorption is purely an acid-base reaction, which is the general case of Mg-Al-CO3 LDH derivatives, thus resulting in the formation of sulfites and sulfates as indicated by the reactions below.
Figure 4. (a) Sorption of CO2 followed by SOx at 200 °C and regeneration; (b) sorption of SOx followed by CO2 at 200 °C and regeneration.
as observed earlier in Figure 3. In the next 60 min of SOx sorption, there is an increase of weight by 1.48%, which is much lower when compared to its expected sorption capacity (4.6%) as seen from Figure 3. This suggests that rate of SOx sorption is slowed down significantly as the basic sites present on LDO are mostly occupied by CO2. Regeneration of the samples after CO2/SOx sorption yielded only 51% desorption. This is considerably lower when compared with the expected value of 74.9% as calculated from the regeneration of SOx (42%) in Figure 1a and CO2 (90%) as reported earlier.18,19 This lower regeneration value to some extent supports the fact that SOx replaces CO2 from LDO during its sorption after CO2. Figure 4b illustrates the result of SOx sorption followed by CO2 sorption and regeneration. In this case, SOx shows a normal sorption value of 4.6% while CO2 shows a lower sorption value of 1.57% against its normal value of 2.99% (Table 1). This is also attributed to the availability of less number of basic sites due to SOx molecules prior occupation. Regeneration of this sample also resulted in 50.1% desorption of the total sorption value. Interestingly, this value is quite close when tallied with the expected value (52%) calculated from their individual sorption and regeneration capacities. This suggests that CO2, which followed SOx in the sorption experiment, did not replace any SOx from the LDO during its sorption. Sorption values measured with very low (0.1%) SOx feed concentrations revealed that SOx was very strongly adsorbed by LDO. Even at 0.1% SOx concentration, sorption resulted in the weight gain of 6.65% in 2 h. During the sorption, highly acidic SOx is expected to react with the basic sites of LDO as it normally occurs with metal oxides. Pinnavaia et al. investigated SOx sorption on Mg-Al-CO3 LDH between 100 and
MgO + SO2 f MgSO3 (sulfite)
(1)
MgO - SO3 f MgSO4 (sulfate)
(2)
The resultant Mg-Al sulfites formed in the process are highly stable and require very high temperatures (>600 °C) to dissociate. The sorption regeneration studies conducted using 600 °C calcined LDO (Figure 1b) revealed much higher sorption capacity (11.04%) with 25% of this being reversible at 600 °C. Interestingly, the actual quantities of reversible sorption or desorption obtained at 400 and 600 °C are almost similar in the range of 2.79 and 2.76%, respectively. This very clearly tells that the reversible sorption quantity remains close at both 400 and 600 °C, but sorption quantity increased at 600 °C. Additional sorption observed at 600 °C is mainly attributed to further decarbonation of LDH after calcination at 600 °C, forming LDO and MgO phases that open up more basic sites available for the reaction with SOx.11,12 FTIR and XRD investigations of thermal evolution of LDH reported earlier clearly revealed that LDO still retains considerable amounts of carbonate content at 400 °C and shows further decomposition of carbonate at 600 °C releasing more basic sites in the LDO.18,19 Results of SOx sorption/desorption using temperature cycling (Figure 2) make clear that the sorption rate is much higher than desorption, and on the whole, there is a consistent increase in the sorption value even after three desorption cycles. There is a decline in desorption capacity from the first cycle (24.5%) to the third cycle (8.8%) showing the poor recyclability. On the whole, LDO demonstrates very strong sorption behavior for SOx, but poor recyclability and regenerability are the drawbacks in their application. Alternatively higher temperatures (>600 °C) or chemical treatment may be considered for releasing SOx and regenerating LDO for repeated applications. It is noticed from the sorption curves that CO2 sorption rate is much faster when compared to SOx. This may be useful to some extent for preferential separation of CO2 over SOx, but after a few cycles, LDO gets saturated with SOx leaving no place for CO2 sorption. These results strongly suggest that LDH derivatives for CO2 capture do require a de-SOx unit operation upstream. However, the slow sorption kinetics of SOx will impact upon the engineering capital costs regarding unit
Table 2. Observed and Expected Values of CO2 and SOx Sorptions and Regeneration in Sorption Experiments of CO2 Followed by SOx and Vice Versa CO2 sorption (wt %)
SOx sorption (wt %)
regeneration (%)
sorption experiment
observed
expected
observed
expected
observed
expected
difference
CO2/SOx SOx/CO2
2.92 1.57
2.99 2.99
1.48 4.58
4.6 4.6
51.1 50.1
74.9 52
23.8 1.9
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operation sizing and operating costs related to sorbent replacement, thus ultimately dictating on the viability of the process. Conclusions LDO demonstrated excellent sorption potential for SOx. Even at low feed concentrations of SOx (0.1%) the sorption values were very high, indicating strong affinity of SOx for LDO. Samples calcined at 600 °C showed much higher sorption capacity (11.04%) than that from 400 °C calcined samples (6.65%), indicating the availability of more basic sites due to further decarbonation at 600 °C. Regeneration at 400 and 600 °C revealed poor reversibility of the sorption at these temperatures. Temperatures higher than 600 °C may be considered for releasing SOx from the metal sulfites and sulfates while regenerating LDO. Temperature cycling also revealed the irreversible nature of SOx sorption. Comparison of CO2 and SOx sorption on LDO revealed that CO2 sorption is much faster and reached saturation in ∼30 min. On the other hand, SOx sorption is relatively slow but its sorption levels are much higher than that of CO2. Strong sorption potential of SOx is exhibited even after 2 h. SOx was found to significantly influence CO2 sorption on LDO. Regeneration values observed after CO2/SOx and SOx/CO2 sorption experiments to some extent proved that SOx replaces CO2 due to its strong affinity toward LDO. Hence, it is advisable to use some initial adsorption beds of LDO for de-SOx unit operation followed by CO2 sorption. Acknowledgment The authors acknowledge the CRC for Greenhouse Gas Technologies (CO2CRC) for financial support to this project. They also thank Mr. Barry Hooper, Program Manager, CO2CRC for his advice and valuable discussion. Literature Cited (1) ACE Information Programme on Industrial Emission controls available athttp://www.ace.mmu.ac.uk/Resources/Fact_Sheets/Key_Stage_4/ Air_Pollution/pdf/27.pdf. (2) Ruether, J. A. FETC Programs for Reducing Greenhouse Gas Emissions; Report DOE/FETC-98/1058, 1999. (3) Pinnavaia, T. J.; Amarasekera, J. Layered Double Hydroxides Sorbents for the Removal of SOx from Flue Gas Resulting from Coal Combustion. U.S. Patent 5,116,587, 1992. (4) Bhattacharyya, A. A.; Gerald, M. W.; Jin, S. Y.; John, A. K.; William, E. C. Catalytic SOx Abatement: The Role of Magnesium
Aluminate Spinel in the Removal of SOx from Fluid Catalytic Cracking (FCC) Flue Gas. Ind. Eng. Chem. Res. 1988, 27, 1356–1360. (5) Huesca, R. H.; Dıa´z, L.; Armenta, G. A. Adsorption equilibra and kinetics of CO2, CH4 and N2 in natural zeolites. Sep. Purif. Technol. 1999, 15, 163–173. (6) Kapoor, A.; Yang, R. T. Kinetic separation of methane-carbon dioxide mixture by adsorption on molecular sieve carbon. Chem. Eng. Sci. 1989, 44 (8), 1723–1733. (7) Yong, Z.; Mata, V.; Rodrigues, A. E. Adsorption of carbon dioxide at high temperaturesa review. Sep. Pur. Technol. 2002, 26, 195–205. (8) de Roy, A.; Forano, C.; Besse, J. P. Layered Double Hydroxides: Synthesis and Post-Synthesis Modification. In Layered Double Hydroxides: Present and Future; Rives, V., Ed.; Nova Science Publishers: New York, 2002; Chapter 1 (review). (9) Cavani, F.; Trifiro, F.; Vaccari, A. Hydrotalcite-type anionic clays: preparation, properties and applications. Catal. Today 1991, 11, 173–301. (10) Kagunya, W.; Hassan, Z.; Jones, W. Catalytic properties of layered double hydroxides and their calcined derivatives. Inorg. Chem. 1996, 35, 5970–5974. (11) Constantino, V. R. L.; Pinnavaia, T. J. Basic Properties of Mg2+1-x Al3+x Layered Double Hydroxides Intercalated by Carbonate, Hydroxide, Chloride, and Sulfate Anions. Inorg. Chem. 1995, 34, 883–892. (12) Tsuji, M.; Mao, M.; Yoshida, T.; Tamaura, Y. Hydrotalcites with an Extended Al3+-Substitution: Synthesis, Simultaneous TGA-DTA-MS Study, and their CO2 Adsorption Behaviors. J. Mater. Res. 1993, 8 (5), 1137–1142. (13) Kim, Y.; Yang, W.; Liu, K. T.; Shahimi, M.; Tsotsis, T. T. Thermal evolution of the structure of a Mg-Al-CO3 layered double hydroxide: Sorption reversibility aspects. Ind. Eng. Chem. Res. 2004, 43, 4559–4570. (14) Miyata, S.; Hirose, T. Adsorption of N2, O2, CO2, and H2 on Hydrotalcite-Like System: Mg2+-Al3+-(Fe(CN)6)4-. Clays Clay Miner. 1978, 26 (6), 441–447. (15) Ding, Y.; Alpay, E. Equilibria and Kinetics of CO2 Adsorption on Hydrotalcite Adsorbent. Chem. Eng. Sci. 2000, 55, 3461–3474. (16) Yong, Z.; Mata, V.; Rodrigues, A. E. Adsorption of Carbon Dioxide onto Hydrotalcite-Like Compounds (HTLCs) at High Temperatures. Ind. Eng. Chem. Res. 2001, 40, 204–209. (17) (a) Yong, Z.; Mata, V.; Rodrigues, A. E. Hydrotalcite-like compounds as adsorbents for carbon dioxide. Energy ConVers. Manage. 2002, 43, 1865–1876. (b) Hutson, N. D.; Speakman, S. A.; Payzant, E. A. Structural Effects on the High Temperature Adsorption of CO2 on a Synthetic Hydrotalcite. Chem. Mater. 2004, 16, 4135–4143. (18) Ram Reddy, M. K.; Xu, Z. P.; Lu, G. Q.; Diniz da Costa, J. C. Layered Double Hydroxides for CO2 Capture: Structure Evolution and Regeneration. Ind. Eng. Chem. Res. 2006, 45, 7504–7509. (19) Ram Reddy, M. K.; Xu, Z. P.; Lu, G. Q.; Diniz, da Costa, J. C. Influence of Water on High Temperature CO2 Capture Using Layered Double Hydroxide Derivatives. Ind. Eng. Chem. Res. 2008, 47 (8), 2630– 2635.
ReceiVed for reView March 14, 2008 ReVised manuscript receiVed June 15, 2008 Accepted July 3, 2008 IE8004226
