ARTICLE pubs.acs.org/EF
Removal of CO2 by CaO/MgO and CaO/Ca9Al6O18 in the Presence of SO2 Mingnv Guo,†,‡ Li Zhang,*,†,‡ Zhongqing Yang,†,‡ and Qiang Tang†,‡ † ‡
Key Laboratory of Low-Grade Energy Utilization Technologies and Systems (Ministry of Education), and College of Power Engineering, Chongqing University, Shapingba District, Chongqing 400030, People’s Republic of China ABSTRACT: Sorbents with their components CaO/MgO and CaO/Ca9Al6O18 were produced from calcium D-gluconate monohydrate, magnesium D-gluconate hydrate, and aluminum L-lactate hydrate as precursors with a simple wet mixing method. The effects of sorbents type, mass proportion, SO2 concentration, and calcination temperature on sorbent capability were studied. The results show that CaO/MgO (75:25%, w/w) kept the best CO2-capture capability, while CaO/Ca9Al6O18 (75:25%, w/w) kept the best cyclic stability. The CO2-capture process is drastically blocked when SO2 exists. The CO2-capture capacity drops quickly when the SO2 concentration is promoted. However, the cumulative SO2-capture capacity increases at the same time. The total calcium use rises tenderly after many cycles, and the tendency becomes more obvious with an increasing SO2 concentration. The effect of the calcination temperature on CaO/MgO and CaO/Ca9Al6O18 absorption characteristics shows little difference.
1. INTRODUCTION An effective way to employ calcium-based sorbents is with carbonation/calcination cycles for CO2 near-zero emissions.14 The carbonation/calcination cycle is described in Figure 1. In the carbonator, the carbonation reaction takes place to form CaCO3. The reverse reaction occurs in the calciner to regenerate CaO and produce CO2. However, after many cycles, new fresh sorbents have to be complemented because of sorbent capability decline. For example, for conventional CaO, the capture capability declines quickly after each cycle, even dropping to 8% after hundreds of cycles.2 Obviously, sorbent capability is a key factor to control CO2capture costs. Much attention has recently been directed toward sorbent cyclic capability improvements.57 Some scholars explored synthetic sorbents to improve sorbent cyclic capability. A wet precipitation process was tailored by Gupta and Fan8 to synthesize high-surface-area precipitated calcium carbonate with predominant mesoporous pores (520 nm). The CaO sorbent obtained from it was less susceptible to pore pluggage and attained over 90% conversion. Co-precipitation and hydrolysis of CaO have been employed to produce Ca-based synthetic sorbents by Pacciani et al.9 Although the initial reactivity of their sorbents was less than limestone and dolomite, after 20 cycles, two of the sorbents exceeded both limestone and dolomite. Lu et al.10 have investigated a series of CaO-based sorbents synthesized from various organometallic precursors, and their further work11,12 found that the sorbent prepared from calcium acetate resulted in the best uptake characteristics for CO2. Aihara et al.13 using the conventional powder method and the metal alkoxide method created sorbents composed of CaO and CaTiO3 and found that the reversibility of the sorbents was steady. Sun et al.14 put different chemical compounds into sorbents to improve their sorption capability, such as Al, Si, Ti, Zn, Mg, etc. They indicated that the sorbents performed good capability when the CaO/Al2O3 molar ratio is 1:1. Li et al.15 employed Ca12Al14O33 as an additive to promote calcium-based sorbent capability. Li et al.16,17 modified limestone with ethanol r 2011 American Chemical Society
solution and acetum. They found that the durability of the modified limestone is significantly better than the original limestone during the multiple cycles and acetum solution is better than ethanol solution as a modifier for limestone. Fossil fuels inevitably contain some sulfur, so that the following reactions are bound to occur under combustion conditions:1820 CaO þ CO2 f CaCO3
ð1Þ
CaO þ SO2 þ 1=2O2 f CaSO4
ð2Þ
CaCO3 þ SO2 þ 1=2O2 f CaSO4 þ CO2
ð3Þ
Although SO2 formed during combustion is less in flue gas, it has a major influence on sorbent durability for CO2 capture. The adverse effect of SO2 was found by Sun et al.,18,19 who pointed that SO2 appreciably decreases the CO2-capture capacity for multiple cycles. They also found that the CaSO4 layer on the surface of a particle, produced primarily from direct sulfation during the later stage of each cycle, results in the blockage of pores. Ryu et al.20 confirmed that the CO2-capture capacity decreases as the SO2 concentration increases and indicated that the sulfation patterns affect the CO2-capture capacity. For limestones sulfated with the unreacted-core-type mechanism, the total capacity was reduced markedly with an increasing SO2 concentration, whereas the SO2 concentration was found to have little effect on particles that sulfated with the uniform-type mechanism. Co-capture of H2S and CaO has also been tested in a pressurized gasifier-based process by Sun et al.21 The results showed that H2S is much less of a problem than SO2 in impeding CO2 capture. Fan et al.22 suggested that the reaction of the sorbent with SO2 or H2S present will be beneficial to the economics of a calcium looping process because it will negate the requirement for a separate unit for sulfur removal from the flue Received: July 22, 2011 Revised: September 21, 2011 Published: September 27, 2011 5514
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Figure 1. Schematic of the carbonation/calcination process.
Table 1. Chemical Component of Sorbents chemical components (wt %) sample number
CaO
MgO
1
100
2
55
45
3
67
33
4
75
25
5
75
Ca9Al6O18
25
gas. According to the major influence of SO2 on CO2 capture, it is necessary to investigate co-capture CO2/SO2 when new sorbents are produced. In this work, the sorbents with main components CaO/MgO and CaO/Ca9Al6O18 were synthesized with calcium D-gluconate monohydrate, magnesium D-gluconate hydrate, and aluminum Llactate hydrate as precursors in a simple wet mixing method. Cocapture CO2/SO2 experiments were carried out to study effects of the sorbent type, mass ratio, SO2 concentration, and calcination temperature on the sorption capability of new sorbents.
Figure 2. Schematic of the experimental system for CO2 removal from carbonation/calcination cycles by Ca-based sorbents. To ensure the purity of the sorbents, at the beginning of the experiments, the initial calcinations were conducted with a N2 atmosphere in a TGA at 900 °C to remove moisture and CaCO3 after the sorbents were exposed in the air. A typical 650 °C was chosen for sorption cycles with a 15 vol % CO2 and 85 vol % N2 atmosphere under atmospheric pressure. The heating rate was kept at 20 °C/min from room temperature to 650 °C. The total gas flow was 100 mL/min, and the absorption reaction was sustained for 30 min. After the sorption reaction, the temperature was raised to calcination reaction temperature set points with N2 atmosphere. The set points varied from 850 to 950 °C to study the effects of the calcination temperature on cyclic capability. The calcination reaction was sustained for 10 min at a constant temperature. After the regeneration process, the temperature was lowered to 650 °C and a carbonation/calcination cycle was finished. All tests started with 15 ( 1 mg of sorbent, and the particle size range was from 9 to 120 nm. Several parameters are defined as follows: CO2-capture capacity in the nth cycle is described by xc ðnÞ ¼
2. EXPERIMENTAL SECTION Calcium D-gluconate monohydrate (g98%, Sigma-Aldrich), magnesium D-gluconate hydrate (g98%, Sigma), and aluminum L-lactate hydrate (97%, Aldrich) were employed as precursors to produce calcium sorbents in a simple wet mixing method. First, the precursors were dissolved in distilled water. To improve the dissolution rate, the solutions were heated. The solutions containing calcium D-gluconate monohydrate and magnesium D-gluconate hydrate were blended together to obtain mixed solutions for producing CaO/MgO. The solutions containing calcium D-gluconate monohydrate and aluminum L-lactate hydrate were blended together for producing CaO/Ca9Al6O18. The mixed solutions were stirred at 45 °C for 60 min and then desiccated in a drying oven at 110 °C until all distilled water was evaporated. After 30 min of calcination at 900 °C in a muffle furnace, the new sorbents were obtained. The amounts of precursors were pre-determined from the desired calcium oxide component in the final sorbent. The weight ratio between calcium D-gluconate monohydrate and calcium oxide was 9.5, while the ratio between magnesium D-gluconate hydrate and magnesium oxide was 11.5, which were obtained from experiments of decompositions of precursors in a Cahn thermogravimetric analyzer (TGA) at 900 °C in air for 1.5 h. The weight ratio between aluminum L-lactate hydrate and final product Ca9Al6O18 was determined by molecular-weight relations. X-ray diffraction (XRD) analysis confirmed that the sorbents comprised CaO, MgO, or Ca9Al6O18. The chemical components of the final sorbents were listed in Table 1. The experimental system was shown in Figure 2. A TGA (Cahn 121) was used to test sorbent cyclic sorption capability. The sorbent microstructures were observed with an atomic force microscope (NSK, SPI 3800).
mnabs mn 0:7857w0 m0
ð4Þ
Cumulative SO2-capture capacity in the nth cycle is defined as follows: xs ðnÞ ¼
0:7ðmn m0 Þ w0 m 0
ð5Þ
Total calcium use xt(n) can be expressed by xt ðnÞ ¼ xc ðnÞ þ xs ðnÞ
ð6Þ
Cyclic stability in the nth cycle is defined by xcs ðnÞ ¼
xc ðnÞ xmax c
ð7Þ
In these equations, m0 is the sorbent mass after the initial calcination and before the first carbonation reaction, w0 is the CaO mass ratio in sorbents before the first carbonation reaction, mnabs stands for the sorbent mass after the nth carbonation, mn stands for sorbent mass after the nth is the maximum CO2-capture capacity of all cycles. calcination, and xmax c
3. RESULTS AND DISCUSSION 3.1. CO2 Capture without SO2. First, CO2-capture experiments were carried out with a carbonation temperature of 650 °C and a calcination temperature of 950 °C. The atmosphere of 15 vol % CO2 and 85 vol % N2 was adopted for the sorption stage, and 100 vol % N2 was used for the calcination stage. Cyclic capability of the synthetized sorbents and some conventional 5515
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Figure 5. CO2-capture capacity against the reaction time at different cycles of CaO/Ca9Al6O18 sorbent 5 (carbonation conditions, at 650 °C in 15 vol % CO2 and 85 vol % N2; calcination conditions, at 950 °C in 100 vol % N2).
Figure 3. Comparisons of cyclic reaction characters of sorbents and conventional Ca-based sorbents.
Figure 6. Long-term tests for sorbents 1, 4, and 5 (carbonation conditions, at 650 °C in 15 vol % CO2 and 85 vol % N2; calcination conditions, at 900 °C in 100 vol % N2).
Figure 4. CO2-capture capacity against the reaction time at different cycles of CaO/MgO sorbent 4 (carbonation conditions, at 650 °C in 15 vol % CO2 and 85 vol % N2; calcination conditions, at 950 °C in 100 vol % N2).
sorbents was compared in Figure 3. It can be seen that cyclic capability of all of the sorbents decreases with the number of cycles. However, either cyclic capture capability or cyclic stability of the synthesized sorbents 15 is obviously higher than conventional CaO, dolomite, and limestone. The CO2-capture capability of sorbent 1, which constitutes pure CaO, declines to 77% after 12 cycles. The sorbents 24 with different mass ratios of CaO and MgO show better cyclic capture capability than sorbent 1 when CO2 is the only sorption gas, presumably because MgO crystalline grains acted as an inert support separating CaO crystalline grains and slowing sorbent sintering.18,21 The cyclic capture capacity of sorbents 2, 3, 4, and 5 are 84, 82, 86, and 71% after 12 cycles, respectively. It is obvious that sorbent 4 has the best cyclic capture capacity when CO2 is the only sorption gas. While sorbent 5 has the minimum decline range of CO2-capture capacity, it shows the best cyclic stability. The existence of Ca9Al6O18, which formed in the sorbent preparation process can effectively prevent CaO from sintering. The reason is probably that Ca9Al6O18 is unreactive with CO2, making the crystalline
grains in sorbents a small shape. The pores are relatively difficult to clog, resulting in the improvement of sorbent cyclic stability. The CO2-capture capacity of sorbents 4 and 5 against the reaction time are shown in Figures 4 and 5, respectively. It is obvious that the reaction of CaO and CO2 can be divided into two stages. The first stage is the chemical reaction controlled process with a rapid reaction ratio, followed by the product layer diffusion stage. In Figure 4, the CO2-capture capacity decreases with the number of cycles and the decline mainly occurred on the chemical reaction controlled stage; however, on the product layer diffusion stage, the number of cycles has little effect on the CO2-capture capacity. This can be attributed to the fact that CaO sintering was intensified with the number of cycles, which means the decrease of the specific surface area and microcrystal conservation in the particle interior (panels a and b of Figure 8). The changes principally affect the capture capacity of sorbents on the chemical reaction controlled stage. Long-term calcination/carbonation tests were investigated for sorbents 4 and 5 over 50 cycles and sorbent 1 over 200 cycles. The results were shown in Figure 6. It is clear that the CO2capture capacity of sorbents 4 and 5 with the structural support is obviously higher than sorbent 1 over 50 cycles, although sorbent 1 has a residual conversion of 0.23 after 200 cycles. Within the 50 cycles, sorbent 4 performs good cyclic stability, while sorbent 5 shows higher CO2-capture capacity, which is in accordance with the results shown in Figure 3. 5516
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Figure 7. Variation of reaction characteristics for CaO/Ca9Al6O18 sorbent 5 with the number of cycles at different calcination atmospheres (carbonation conditions, at 650 °C in 15 vol % CO2 and 85 vol % N2).
As the calcination atmosphere, CO2 is often adopted during the CO2-capture process with calcium-based sorbents to obtain a high concentration of CO2. The effects of the calcination atmosphere on cyclic reaction characteristics of sorbent 5 were compared in Figure 7. In comparison to calcination atmosphere N2, both cyclic stability and CO2-capture capacity decrease when calcination atmosphere CO2 was employed. The decline amplitudes depend upon the calcination temperature. For example, when the calcination atmosphere changed from N2 to CO2, after 12 cycles, the declining values of CO2-capture capacity and cyclic stability were 21% and 0.29 at 950 °C and 3% and 0.04 at 850 °C, respectively. The effects of the calcination atmosphere on cyclic absorption characteristics relate to calcination temperatures. When the calcination temperature is relatively high (e.g., 950 °C), the CO2-capture capacity and cyclic stability are obviously decreased in the CO2 calcination atmosphere. However, when the calcination temperature is relatively low (e.g., 850 °C), the calcination atmosphere has little influence on CO2-capture capacity and cyclic stability. An atomic force microscope (NSK-SPI3800/SPI400) was adopted to observe the microstructure of the sorbent 4. It can be seen in Figure 8 that granulum aggregation occurred after the number of cycles. Therefore, the specific surface area of the sorbent and the number of micropores decrease, resulting in the decline of cyclic capture capability. In comparison to panels b and c of Figure 8, when the calcination atmosphere changed from N2 to CO2, granulum aggregation was accelerated, with sorbent sintering becoming more severe. When SO2 exists in the sorption stage, serious sintered macropores appeared in Figure 8d, which led to the further decline of the CO2-capture capacity of sorbents. 3.2. Simultaneous Capture of CO2/SO2. Four sets of results for the cyclic performance of four sorbents during co-capture are compared in Figure 9. The corresponding baselines of sorbents 4 and 5 for no SO2 are also shown. The CO2-capture capacity for all sorbents dropped sharply to 15% after 12 cycles, especially for sorbent 3, even decreasing to 1%. The presence of SO2 clearly impeded the cyclic CO2 capture of the sorbents. Sorbent 4 appeared
Figure 8. Microcosmic structure of sorbent 4. Carbonation conditions (ac), at 650 °C in 15% CO2 and 85% N2 and (d) at 650 °C in 700 ppm SO2, 15% CO2, N2 balance: (a) after initial calcination, (b) after 12 cycles for CO2 capture (calcination at 950 °C in N2), (c) after 12 cycles for CO2 capture (calcination at 950 °C in CO2), and (d) after 12 cycles for simultaneous CO2/SO2 capture (calcination at 950 °C in N2).
to have a better CO2-capture capacity than the other sorbents when SO2 was present. Sorbent 5 dropped less sharply than the other sorbents, showing the best cyclic stability. During simultaneous CO2/SO2 capture, the CO2-capture capacity declined with the ascending SO2 concentration because part of CaO absorbs CO2, while others absorb SO2. Figure 10 shows the CO2-capture capacity of sorbents 4 and 5 versus the number of cycles for the co-capture at different SO2 concentrations. 5517
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Figure 9. Comparisons of cyclic sorption characteristics of sorbent 4 for simultaneous CO2/SO2 capture (sorption, 15 vol % CO2, 400 ppm SO2, 5 vol % O2, balance N2, 30 min for each cycle, at 650 °C; calcination, 100% N2, 10 min for each cycle, at 950 °C).
Figure 11. Effects of the SO2 concentration on sorbent cumulative SO2capture capacity: (a) CaO/MgO sorbent 4 and (b) CaO/Ca9Al6O18 sorbent 5 (sorption, 15% CO2, 0700 ppm SO2, 5 vol % O2, balance N2, 30 min for each cycle, at 650 °C; calcination, 100% N2, 10 min for each cycle, at 950 °C).
Figure 10. Effects of the SO2 concentration on sorbent CO2-capture capacity: (a) CaO/MgO sorbent 4 and (b) CaO/Ca9Al6O18 sorbent 5 (sorption, 15% CO2, 0700 ppm SO2, 5 vol % O2, balance N2, 30 min for each cycle, at 650 °C; calcination, 100% N2, 10 min for each cycle, at 950 °C).
Sorbent 4 shows better CO2-capture capacity, cumulative SO2capture capacity, and total calcium use than sorbent 5. However, the cyclic capture capabilities of two sorbents show similar trends that CO2-capture capacity decreased with an increasing number of cycles and SO2 concentration, which was also pointed out by Ryu et al.20 for the three limestones. Figure 11 portrays effects of increasing SO2 concentrations on cumulative SO2-capture capacity of sorbents 4 and 5. For all concentrations, the cumulative SO2-capture capacity increased with the number of cycles because absorbed SO2 was not desorbed during regeneration intervals. For a given cycle, the cumulative SO2-capture capacity increases with the SO2 concentration. In Figure 12, it is seen that the total calcium use during simultaneous CO2/SO2 capture was lower than that during no SO2
Figure 12. Effects of the SO2 concentration on total calcium use: (a) CaO/MgO sorbent 4 and (b) CaO/Ca9Al6O18 sorbent 5 (sorption, 15% CO2, 0700 ppm SO2, 5 vol % O2, balance N2, 30 min for each cycle, at 650 °C; calcination, 100% N2, 10 min for each cycle, at 950 °C).
capture because the increasing cumulative SO2-capture capacity is not enough to make up the decreasing CO2-capture capacity with the number of cycles. It is worth mentioning that total calcium use began to rise after several cycles, partly owing to the increase of the cumulative SO2-capture capacity with the number of cycles. 5518
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Figure 13. Effects of calcination temperatures on the CO2-capture capacity: (a) CaO/MgO sorbent 4 and (b) CaO/Ca9Al6O18 sorbent 5 (sorption, 15% CO2, 700 ppm SO2, 5 vol % O2, balance N2, 30 min for each cycle, at 650 °C; calcination, 100% N2, 10 min for each cycle).
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Figure 15. Effects of calcination temperatures on total calcination use: (a) CaO/MgO sorbent 4 and (b) CaO/Ca9Al6O18 sorbent 5 (sorption, 15% CO2, 700 ppm SO2, 5 vol % O2, balance N2, 30 min for each cycle, at 650 °C; calcination, 100% N2, 10 min for each cycle).
cycle when the SO2 concentration is 700 ppm. When the SO2 concentration reduced to 400 ppm, it began to rise after the ninth cycle. Total calcium use still kept decreasing with the number of cycles, even after 12 cycles, when the SO2 concentration is 200 ppm. Figure 13 compares the CO2-capture capacities of sorbents 4 and 5 for different calcination temperatures. In Figure 13a, the CO2-capture capacity of sorbent 4 calcined at 950 °C was slightly lower than those calcined at 900 and 850 °C at the first several cycles. However, with the increasing number of cycles, the CO2-capture capacity of the sorbent calcined at 950 °C turned to be the same as those calcined at 900 and 850 °C. With the increasing number of cycles, the CO2-capture capacity calcined at 950 °C began to be higher than those calcined at 900 and 850 °C. Figure 13b confirms that the calcination temperatures have nearly no effect on the CO2capture capacities for sorbent 5. As shown in Figure 14, the cumulative SO2-capture capacity of both sorbents 4 and 5 appeared to have the same tendency. It decreased with the increase of the calcination temperature at the same number of cycles, and the amplitude of this decline became larger when the number of cycles increased. Figure 15 indicated that the total calcium use during simultaneous CO2/SO2 capture decreased with an increasing calcination temperature at the same cycle. Figure 14. Effects of calcination temperatures on the cumulative SO2capture capaticy: (a) CaO/MgO sorbent 4 and (b) CaO/Ca9Al6O18 sorbent 5 (sorption, 15% CO2, 700 ppm SO2, 5 vol % O2, balance N2, 30 min for each cycle, at 650 °C; calcination, 100% N2, 10 min for each cycle).
In addition, the uptrend became obvious with an increasing SO2 concentration. That is different from the conclusion by Ryu et al. that the total calcium use decreased with the number of cycles.20 As shown in Figure 12b, total calcium use began to rise after the fifth
4. CONCLUSION Experiments were performed on five sorbents, which were produced from calcium D-gluconate monohydrate, magnesium D-gluconate hydrate, and aluminum L-lactate hydrate as precursors with a simple wet mixing method. In all cases, the CO2capture capability decreased with the number of cycles. CaO/ MgO (75:25%, w/w) kept the best CO2-capture capability, while CaO/Ca9Al6O18 (75:25%, w/w) kept the best cyclic stability for 5519
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Energy & Fuels no SO2 tests. In comparison to calcination atmosphere N2, the CO2-capture capacity decreased when calcination atmosphere CO2 was employed. The CO2-capture process is drastically blocked when SO2 exists. The CO2-capture capacity drops quickly when the SO2 concentration is promoted. However, the cumulative SO2-capture capacity increases at the same time. Total calcium use rises tenderly after many cycles, and the tendency becomes more obvious with an increasing SO2 concentration. The effect of the calcination temperature on CaO/MgO and CaO/Ca9Al6O18 absorption characteristics shows little difference.
’ AUTHOR INFORMATION Corresponding Author
*Fax: +86-23-65111832. E-mail:
[email protected].
’ ACKNOWLEDGMENT The authors thank Dr. Bo Feng and Wenqiang Liu at the University of Queensland for their support of the experimental work. The China Scholarship Council is acknowledged for making Ms. Mingnv Guo’s studies in Australia possible. The Chongqing Science and Technology Commission (CSTC) is also acknowledged for its financial support (Project 2009BA6067). ’ REFERENCES
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