Ind. Eng. Chem. Res. 2009, 48, 6627–6632
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SO2 Retention by CaO-Based Sorbent Spent in CO2 Looping Cycles Vasilije Manovic,† Edward J. Anthony,*,† and Davor Loncarevic‡ CanmetENERGY, Natural Resources Canada, 1 Haanel DriVe, Ottawa, Ontario, Canada K1A 1M1, and Department of Catalysis and Chemical Engineering, Institute of Chemistry, Technology and Metallurgy, NjegoseVa 12, 11000 Belgrade, Serbia
CaO-based looping cycles are promising processes for CO2 capture from both syngas and flue gas. The technology is based on cyclical carbonation of CaO and regeneration of CaCO3 in a dual fluidized-bed reactor to produce a pure CO2 stream suitable for sequestration. The main limitation of natural sorbents is the loss of carrying capacity with increasing number of reaction cycles, resulting in the need for extra sorbent, and subsequent spent sorbent waste. Use of spent sorbent from CO2 looping cycles for SO2 capture is investigated in this study. Three limestones were investigated: Kelly Rock (Canada), La Blanca (Spain), and Katowice (Poland, Upper Silesia). Carbonation/calcination cycles were performed in a tube furnace with both the original limestones and samples thermally pretreated for different times (i.e., sintered). The spent sorbent samples were sulfated in a thermogravimetric analyzer (TGA). The changes in the resulting sorbent pore structure were then investigated using mercury porosimetry. It has been shown that the sulfation rates of both thermally pretreated and spent sorbent samples are lower in comparison with those of the original samples. However, final conversions of both spent and pretreated sorbents after longer sulfation time were comparable or higher than those observed for the original sorbents under comparable conditions. Maximum sulfation levels strongly depend on sorbent porosity and pore surface area. The shrinkage of sorbent particles during calcination/ carbonation cycling resulted in a loss of sorbent porosity on the order of e48%, which corresponds to maximum sulfation levels of ∼55% for spent Kelly Rock and Katowice. This is ∼10% higher than that seen with the original samples after 15 h of sulfation. By contrast, La Blanca limestone had more pronounced particle shrinkage during pretreatment and cycling, leading to porosities lower than 35%, which resulted in sulfation conversion of spent samples 120 kJ/mol, which is characteristic of ion diffusion. The initial sulfation rate, as well as the shift to the diffusioncontrolled stage, depend on available surface area for reaction. It has been shown12 that the maximum specific surface area of CaO, 110 m2/g, was obtained for very fine limestone particles (2-10 µm), calcined at 700 °C. It was available only for fresh CaO and, with the progress of sintering, asymptotically fell to a much lower level. The final surface area depends on the precursor type and calcination conditions.13-15 It has been shown that temperature, CO2, and H2O concentrations all influence sintering and determine the final surface area.16 The product of sulfation, CaSO4, has a significantly larger molar volume (46 cm3/mol) than that of CaO (17 cm3/mol) or CaCO3 (37 cm3/mol). This is another cause of the decrease in sulfation rate.1 Namely, with increasing sulfation, the pores in sorbent particles are filled by bulky CaSO4,which is especially pronounced in the case of small pores. The final result is the formation of a low-porosity sulfate shell covering an unreacted CaO core. In another limiting case, sulfation continues in sorbent particles with large pores to fill available space with CaSO4.17,18 In summary, the reaction mechanism of sulfation encompasses a number of steps, the most important of which are (a) diffusion of SO2 to the surface of the sorbent particle and into its porous structure and (b) diffusion through the product layer/shell and heterogeneous chemical reaction between CaO and SO2. A schematic representation of changes in the controlling steps with conversion is shown in Figure 1. However, this is an idealized scheme and the real picture depends on limestone type and conditions during the formation of CaO and during sulfation. Some steps can be skipped under specific conditions and/or sorbent type. For example, it is expected that, with decreased sorbent surface area to porosity ratio, the probability of formation of an unreacted core/sulfate shell pattern decreases. On one hand, in general, sulfation kinetics and conversion depend in large measure on limestone type, i.e., geological origin. These differences are consequences of sorbent composition and morphology.19 On the other hand, CaO-based sorbents were recently intensively investigated in carbonation/calcination cycles in connection with their use for CO2 capture.20-25 It has been shown that, during looping cycles, sorbents change their morphology as a result of sintering.26-28 Consequently, sorbent activity decreases with increasing number of reaction cycles,29,30 causing economic penalties for CaO-based CO2 looping cycle technology.31-33 Thus, different possibilities to improve sorbent
utilization in CO2 cycles were investigated.34 It was found that spent sorbent after hydration had excellent sulfation performance, with conversions after a few hours that were typically >70% and sometimes almost quantitative.35-37 Moreover, investigation of the pore structure of the spent sorbent35,36 or sorbent pretreated at high temperatures38 showed that small pores disappeared, resulting in low ratios of pore surface area to porosity. Sulfation tests of spent sorbent36 showed that the reaction rate was slower at the beginning in comparison with that for the original sorbent. However, the shift to the slow reaction stage was less pronounced and occurred later, resulting in higher spent sorbent activity during longer sulfation periods. This spent sorbent performance requires further examination over prolonged sulfation times, which are comparable to sorbent residence times in FBC systems. The use of sorbent spent in CO2 looping cycles for subsequent SO2 retention may be an efficient solution for FBC systems with CO2/SO2 capture. Moreover, total sorbent utilization efficiency for SO2 retention, which is investigated in this study, may be improved in this manner. 2. Experimental Section Limestone samples from three widely different geological locations were used in this study: Kelly Rock (KR) from Canada, particle size 0.300-0.425 mm; La Blanca (LB) from Spain, particle size 0.400-0.600 mm; and Katowice (KT) from Poland (Upper Silesia), particle size 0.400-0.800 mm. The X-ray fluorescence elemental analyses of samples investigated are given elsewhere.38 It should be noted that LB and KT are very pure limestones, whereas KR contains 6% impurities with the highest concentration of SiO2 and Al2O3, indicating the presence of silicates and aluminosilicates. The content of Na2O in LB (1.07%) is unusually high for limestones. For each limestone, the sulfation performance of four samples was examined: (i) original sample, (ii) sample after 30 CO2 cycles in a tube furnace (TF), (iii) sample pretreated at high temperatures, and (iv) sample pretreated in CO2 (in a TF) at 1000 °C for 24 h, followed by 30 CO2 cycles. The samples were obtained in our recent study.38 Calcination was performed in 100% N2 and carbonation in 100% CO2, both for 15 min, isothermally at 800 °C. For the 30 calcination/carbonation cycles, 3.0 g of limestone samples and a gas flow rate of 500 cm3/min were used. Samples (iii) were pretreated in the TF in 100% CO2, for 24 h at 1100 °C and for 6, 24, and 64 h at 1000 °C. The flow rate of CO2 was 200 cm3/min and samples used were 3.0 g. After the pretreatment, samples were cooled under pure N2, to prevent reaction of hot samples with atmospheric CO2 and moisture. The obtained calcines were characterized by nitrogen adsorption/desorption tests38 and by mercury porosimetry. A Mettler Toledo TGA/SDTA851e/LF/1100 °C instrument was used for TGA sulfation runs. Sulfation tests were done at 850 °C, typically for 15 h. The gas mixture used was synthetic flue gas (15% CO2, 3% O2, 0.5% SO2, and N2 balance). The sample masses in the thermogravimetric analysis (TGA) experiments were equivalent to ∼15 mg of CaO. Data on sample mass during the experiments were collected, and the carbonation conversion was calculated on the basis of the mass change, assuming that mass increase occurred only as a result of the formation of CaSO4. The pore size and pore volume distribution were determined by mercury intrusion porosimetry (Carlo Erba 2000 porosimeter with Macropores unit 120, software Milestone 200). Potential difficulties with Hg porosimetry should be mentioned here, as
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Table 1. Results of Mercury Porosimetry Analyses samplea
SHg (m2/g)
SBET (m2/g)b
porosity (%)
shrinkagec (%)
maximum sulfationd (%)
KR-limestone KR KR-30cyc KR-24h KR-24h-30cyc LB-limestone LB LB-30cyc LB-24h LB-24h-30cyc KT-limestone KT KT-30cyc KT-24h KT-24h-30cyc
1.22 7.18 4.00 1.90 10.78 0.23 22.45 13.35 1.54 12.50 0.39 15.19 8.98 1.92 9.63
n.d. 4.7 2.5 0.8 8.4 n.d. 23.5 10.1 1.8 3.9 n.d. 13.5 6.3 1.0 7.5
4.63 48.80 46.13 49.85 44.66 3.00 47.45 36.43 32.86 34.85 3.45 47.78 45.62 51.03 48.53
n.a. 4.12 9.37 2.06 12.25 n.a. 10.80 31.52 38.23 34.49 n.a. 7.90 12.06 1.63 6.45
n.a. 63.01 56.61 65.72 53.35 n.a. 54.21 34.40 29.38 32.11 n.a. 55.78 51.14 63.53 57.48
a Sample designations: KR, LB, KT: calcined limestone; 30cyc: 30 carbonation/calcination cycles; 24h: pretreatment in CO2 for 24 h; 24h-30cyc: pretreatment in CO2 for 24 h followed by 30 carbonation/ calcination cycles. b Manovic et al.38 c Shrinkage - relative loss of pore volume ) (theoretical porosity - measured porosity)/(theoretical porosity). Theoretical porosity is calculated with assumption that particle dimensions are constant and that pore volume change occurs solely as a result of the difference in molar volumes of CaCO3 (37 cm3/mol) and CaO (17 cm3/mol). d Maximum sulfation: calculated with the assumption that particle dimensions are constant and that product, CaSO4 (46 cm3/ mol), fills the entire space in the sorbent particle.
it may be expected that lower sorbent porosity values than the true total porosity (including the volume from all pores, especially the largest ones) will be determined. Another way around this shortcoming would be use of gas displacement pycnometry and powder displacement pycnometry measurements. However, these methods may have difficulties related to the envelope density measurement. If particles are irregular in shape, which one would expect of calcined limestone, packing efficiency of a fine powder around a sample might be reduced and a lower density than the “true” envelope density observed. Thus, in this study mercury intrusion porosimetry was applied and this approach was verified by good correlation between the obtained porosities and sulfation conversions. 3. Results and Discussion Pore surface area, pore size distribution, and porosity are important properties of solids in reactions. Thus, sample characterization in this study was focused on porous structural properties. Taking into account that the product of sulfation, CaSO4, is voluminous and its formation in large pores may be important for reaction rate and conversion, samples were analyzed by mercury porosimetry. This technique supplies information on pores larger than 1 µm that cannot be analyzed by gas physisorption [Brunauer-Emmett-Teller (BET); Barrett-Joyner-Halenda (BJH)].38 The pore surface area and porosity of the original limestones, corresponding limes, thermally pretreated samples, and samples spent in cycles are given in Table 1. The porosity of limestones was 3.00-4.63%, increasing after calcination to 47.45-48.80% as a result of the lower molar volume of CaO in comparison with that of CaCO3. Carbonation/calcination cycles led to a loss of porosity of ∼2% for KR and KT and 11% for LB. Interestingly, KR and KT samples pretreated at high temperature showed slightly increased porosity, whereas LB lost porosity. Changes in pore volume during pretreatment/cycles can be attributed to particle size changes (shrinkage/growth), as confirmed by helium displacement pycnometry and powder displacement pycnometry.28 With use of data on the original
Figure 2. Pore size distribution (Hg porosimetry) of original, pretreated, and/or cycled samples: (a) Kelly Rock, (b) La Blanca, and (c) Katowice. Sample designations can be seen in Table 1.
limestone porosity and CaO content in samples, shrinkage of sorbent particles was calculated (also given in Table 1). Drastic shrinkage of LB samples can be seen during pretreatment/cycles (38.23% in the case of pretreated sample). This shrinkage is unfavorable and, on one hand, reduces the maximum calculated sulfation levels, which were 30-35% for LB, significantly lower than that for the original sample after calcination (54.21%). On the other hand, maximum calculated sulfation levels for pretreated KR and KT samples were higher (65.72 and 63.53%, respectively) in comparison with values for the original samples (63.01 and 55.78%). The pore surface areas determined by mercury porosimetry were somewhat higher than those measured by nitrogen adsorption/desorption (BET).38 However, both methods gave results indicating loss of surface area with carbonation/calcination cycles for the original samples. The loss was especially pronounced for pretreated samples, which recovered their surface area during cycling. Surface area came mainly from small pores and was important for the initial, chemically controlled reaction rate, and during the next stage limited by diffusion through the product layer. Pore size distribution is another parameter that determines sorbent performance during sulfation. The changes in pore size are presented in Figure 2. Some general tendencies can be seen, the most pronounced being the shift of pore size distribution to
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Figure 4. Maximum calculated sulfation conversions vs measured conversions (Figure 3).
Figure 3. Sulfation conversions in TGA at 850 °Cs0.5% SO2, 15% CO2, 3% O2, and N2 balance: (a) Kelly Rock, (b) La Blanca, and (c) Katowice. Sample designations can be seen from Table 1.
larger pores in samples pretreated for 24 h. The peak of pore size distribution after pretreatment was at ∼1000 nm for KR and KT and at ∼300 nm for LB. Similar behavior of KR and KT can be seen for samples subjected to the carbonation/ calcination cycles, where cycling led to bimodal pore size distribution. On one hand, pore size distribution shift to larger pores was especially pronounced for the original KT sample after CO2 cycles. On the other hand, pretreated KR and KT showed the reappearance of smaller pores with a peak at ∼50 nm after cycles, confirming the self-recovery of sorbent surface morphology of pretreated samples during CO2 cycles. It is interesting that, after CO2 cycles, the original LB samples showed neither a shift of pore size distribution to larger pores nor bimodal distribution. However, pretreated LB recovered small pores, which were even smaller than those for the original samples. Our recent work38 presents scanning electron microscopy results on sample morphologies, consistent with mercury porosimetry measurements. The conversions for original and pretreated/spent samples during TGA sulfation runs are shown in Figure 3. There is a clear difference in the behavior of LB samples in comparison with that of KR and KT. Conversions of pretreated/spent LB
samples are one-half of those for the original samples, as a result of porosity loss, i.e., sample shrinkage (Table 1), and densification.28 The conversions are close to the calculated maximum, which is 30-35% because of reduced porosity. This shows that LB samples, deactivated in CO2 cycles, are not suitable for SO2 retention. Similar differences in LB-limestone performance in comparison with other limestones were also noticed earlier when CO2 capture was investigated.39 However, spent KR and KT samples were subjected to significantly lower shrinkage, which resulted in sulfation conversions that were ∼50% and similar to those of the original samples. Moreover, samples pretreated for 24 h showed ∼10% higher conversions. The conversions were also close to the calculated maximum (Table 1), with a somewhat higher discrepancy in the original KR sample, suggesting the formation of unreacted core/sulfate shell pattern. These results showed that these limestones, after use for CO2 capture, are suitable for SO2 retention. The main criterion is that they do not lose porosity as a result of carbonation/calcination cycles. Moreover, the cycles may be favorable for sorbents, which tend to give an unreacted core/sulfate shell pattern.17,18 For a spent sorbent with large pores there would be less tendency for a sulfate shell to form, preventing higher sulfation. The clear influence of sorbent porosity, via maximum calculated conversions, on measured conversions can be seen from Figure 4. There is excellent correlation (R2 ) 0.97) between calculated and measured values. The maximum conversion is never reached, as can be expected because in gas-solid reactions giving a solid product, the theoretical maximum can be approached only asymptotically, i.e., after very long times. Moreover, this may be a result of additional sorbent particle shrinkage during sulfation, local unreacted areas, inactive CaO in the form of other calcium compounds, or other influences, but this issue cannot be resolved or confirmed by results obtained here. However, the large differences between calculated maximum and measured conversions for the original KR sample most likely result from the formation of a sulfate shell. Figure 4 shows that sorbent porosity is the main determinant of conversion after long sulfation periods, which is especially noticeable in the case of spent/pretreated samples with larger pores and lower surface areas. The sorbent surface area determines conversion in the first stage of reaction, as can be seen from Figure 5. Here, conversion tends to increase with increasing surface area and depends on the limestone used. However, the correlations are poor, unlike those presented in Figure 4, and cannot be attributed to all points (samples) considered together, which suggests that reaction rate depends also on limestone type. However, for each group of samples originating from the same limestone, correlations are noticeable.
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Figure 5. Sulfation conversions after 1 h (Figure 3) vs surface area (Table 1).
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ment duration and temperature on sulfation performance for KR and KT. The best results are obtained with these samples pretreated for 6 h. The conversions are >60%, and ∼12% higher than those for the original samples. Increased duration/temperature of pretreatment reduces sulfation conversion, which means that during pretreatment at high temperature, more pronounced shrinkage takes place. These results show that suitable thermal treatment of CaO-based sorbent can cause morphology changes that enhance sulfation performance. It is important to find a balance between sorbent particle shrinkage, which reduces available space for CaSO4 and is an undesirable effect, and pore size modification, i.e., formation of large pores that are not prone to plugging and provide space for CaSO4, which is a desirable effect. A similar balance between particle shrinkage and formation of larger pores along CO2 cycles should also be considered in the utilization of spent sorbent for SO2 retention. Replacement of spent material from CO2 looping cycles and its use for SO2 retention after an appropriate number of cycles, when suitable porosity to pore surface area and pore size distribution are reached, should also be considered. However, other factors should be taken into account during CO2 cycles in FBC systems, such as attrition, elutriation, and undesirable sulfation in the carbonator and calciner. 4. Conclusions
Figure 6. Sulfation conversions in TGA at 850 °Cs0.5% SO2, 15% CO2, 3% O2, and N2 balancesof samples pretreated for different durations (6, 24, and 64 h) at 1000 °C and for 24 h at 1100 °C: (a) Kelly Rock, (b) La Blanca, and (c) Katowice.
An interesting result from Figure 3 was significantly higher sulfation of KR and KT samples pretreated for 24 h. This led us to investigate sulfation performance of samples pretreated for different durations and at different temperatures. The results are presented in Figure 6 and show that for LB strong deterioration occurs. There is an obvious influence of pretreat-
The sulfation performance of CaO-based sorbents modified during thermal pretreatment and/or CO2 capture carbonation/ calcination cycles38 was investigated. Three limestones with greatly different performance were used in the tests. Mercury porosimetry showed significant pore size changes during CO2 cycles and thermal pretreatment. The porosity of both pretreated and spent samples is a major parameter that limits their sulfation conversion. The porosity of samples was influenced by shrinkage during calcination, pretreatment, and CO2 cycles. It has been shown that the reaction rates for both thermally pretreated and spent sorbent samples are typically lower in comparison with those of the original samples. However, final conversions of both spent and pretreated sorbents after longer sulfation times were comparable or higher than those observed for the original sorbents under similar conditions. These results showed that spent sorbent samples from CO2 looping cycles can be used as sorbents for SO2 retention in cases where significant porosity loss does not occur during CO2 reaction cycles. The higher conversions of spent samples are explained by a shift in pore size distribution toward larger pores that reduce the reaction rate and pore plugging near the particle’s outer surface, with formation of either unreacted core or unreacted network patterns. In the case of spent KR and KT samples, sorbent particles are practically uniformly sulfated, achieving final conversions that are determined by the total pore volume available for the bulky CaSO4 product. The combination of CO2 capture and SO2 retention with spent sorbent, in the case of suitable sorbents for which particles do not significantly shrink during carbonation/calcination cycles, represents one potentially important route that can improve the economics of CaO-based CO2 capture. This option provides an opportunity to use spent sorbent, while reducing disposal problems, reducing makeup of fresh sorbent, and reducing the amount of sorbent needed for SO2 retention as a result of the higher sulfation capacity of spent sorbent.
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ReceiVed for reView February 12, 2009 ReVised manuscript receiVed April 24, 2009 Accepted May 29, 2009 IE9002365
