Energy Fuels 2010, 24, 1009–1012 Published on Web 01/20/2010
: DOI:10.1021/ef901018f
Multiple-Cycles Behavior of K2CO3/Al2O3 for CO2 Capture in a Fluidized-Bed Reactor Chuanwen Zhao, Xiaoping Chen,* and Changsui Zhao School of Energy and Environment, Southeast University, Nanjing 210096, China Received September 11, 2009. Revised Manuscript Received January 11, 2010
The sorbent of K2CO3/Al2O3 is a good choice for CO2 capture from flue gas of fossil-fuel-fired power plants. The CO2-capture capacity and regeneration property for K2CO3/Al2O3 were investigated in a bubbling fluidized-bed reactor during multiple cycles. Results show that the carbonation conversion for K2CO3/Al2O3 is greater than 90% during 10 cycles. The microstructure character of K2CO3/Al2O3 is stable. The changes of surface area and pore volume are 8.3 m2/g and 0.036 cm3/g, respectively, after 10 cycles, and there is little change in the pore size distribution. Careful temperature control is important because of the narrow temperature window available for CO2 capture and the highly exothermic nature of the carbonation process. After 10 cycles, the elutriated sorbent is only 2.8 wt % and the reducing rate of average particle size is 0.005 mm/h. The sorbent of K2CO3/Al2O3 shows excellent attrition resistance performance.
CsNaX, and zeolites, were studied in a fixed-bed reactor using multiple tests.11-16 They provided some important suggestions for the choice of supports. However, the experimental conditions were different from industrial applications, for instance, the use of a gas composition with 1% CO2 or the use of a fixed-bed reactor. The proper reactor for this process is a fluidized-bed or transport reactor. In a previous paper by the authors,17 CO2 absorption of K2CO3/AC, K2CO3/Al2O3, and K2CO3/silica gel was studied in thermogravimetric analysis (TGA) and a bubbling fluidizedbed reactor and K2CO3/Al2O3 was found to have the potential to be used as the sorbent for a large-scale CO2-capture process. The important reaction involved in the process is
1. Introduction Recently, to overcome the limits of cost and energy required, dry alkali-metal-based sorbents for CO2 capture were investigated as an innovative concept to treat the massive flue gas from fossil-fuel-fired power plants.1-16 CO2 capture using a dry sodium-based sorbent was reported.1-4 However, the carbonation reaction rate of Na2CO3 was rather slow. Some results were obtained from a fluidized-bed reactor5-7 or a dual circulating fluidized-bed process.8-10 However, the composition of the sorbent was not reported. The CO2 absorption and regeneration of the sorbents with K2CO3 supported on various supports, such as activated carbon (AC), TiO2, MgO, ZrO2, Al2O3, CaO, SiO2, NaX,
K2 CO3 þ H2 O þ CO2 T 2KHCO3 *To whom correspondence should be addressed. Telephone and Fax: þ86-25-83793453. E-mail:
[email protected]. (1) Liang, Y.; Harrison, D. P.; Gupta, R. P.; Green, D. A.; McMichael, W. J. Energy Fuels 2004, 18 (2), 569–575. (2) Seo, Y.; Jo, S. H.; Ryu, C. K.; Yi, C. K. Chemosphere 2007, 69 (5), 712–718. (3) Lee, J. B.; Ryu, C. K.; Baek, J. I.; Lee, J. H.; Eom, T. H.; Kim, S. H. Ind. Eng. Chem. Res. 2008, 47 (13), 4465–4472. (4) Park, S. W.; Sung, D. H.; Choi, B. S.; Oh, K. J.; Moon, K. H. Sep. Sci. Technol. 2006, 41 (12), 2665–2684. (5) Seo, Y.; Jo, S. H.; Ryu, H. J.; Bae, D. H.; Ryu, C. K.; Yi, C. K. Korean J. Chem. Eng. 2007, 24 (3), 457–460. (6) Seo, Y.; Jo, S. H.; Ryu, C. K.; Yi, C. K. J. Environ. Eng. 2009, 135 (6), 473–477. (7) Park, Y. C.; Jo, S. H.; Park, K. W.; Park, Y. S.; Yi, C. K. Korean J. Chem. Eng. 2009, 26 (3), 874–878. (8) Yi, C. K.; Jo, S. H.; Seo, Y.; Lee, J. B.; Ryu, C. K. Int. J. Greenhouse Gas Control 2007, 1 (1), 31–36. (9) Yi, C. K.; Jo, S. H.; Seo, Y. J. Chem. Eng. Jpn. 2008, 41 (7), 691– 694. (10) Park, Y. C.; Jo, S. H.; Ryu, C. K.; Yi, C. K. Energy Procedia 2009, 1 (1), 1235–1239. (11) Lee, S. C.; Choi, B. Y.; Lee, T. J.; Ryu, C. K.; Soo, Y. S.; Kim, J. C. Catal. Today 2006, 111 (3-4), 385–390. (12) Lee, S. C.; Choi, B. Y.; Lee, S. J.; Jung, S. Y.; Ryu, C. K.; Kim, J. C. Stud. Surf. Sci. Catal. 2004, 153, 527–530. (13) Lee, S. C.; Choi, B. Y.; Ryu, C. K.; Ahn, Y. S.; Lee, T. J.; Kim, J. C. Korean J. Chem. Eng. 2006, 23 (3), 374–379. (14) Lee, S. C.; Kim, J. C. Catal. Surv. Asia 2007, 11 (4), 171–185. (15) Lee, S. C.; Chae, H. J.; Lee, S. J.; Choi, B. Y.; Yi, C. K.; Lee, J. B.; Ryu, C. K.; Kim, J. C. Environ. Sci. Technol. 2008, 42 (8), 2736–2741. (16) Lee, S. C.; Chae, H. J.; Lee, S. J.; Park, Y. H.; Ryu, C. K.; Yi, C. K.; Kim, J. C. J. Mol. Catal. B: Enzym. 2009, 56 (2-3), 179–184. r 2010 American Chemical Society
ΔHr0 ¼ -141 kJ=mol K2 CO3 To investigate the sorbent durability, this paper focused on the CO2-capture capacity of K2CO3/Al2O3 as a function of the cycle number in a fluidized-bed reactor. 2. Experimental Section Sorbent Preparation. The potassium-based sorbent used in this study was prepared by impregnating K2CO3 on Al2O3. The K2CO3 loading amount is 24.5%. The sorbent has a apparent density of 2414 kg/m3, a bulk density of 1250 kg/m3, and a mechanical strength of 314 N/cm2. The surface area and pore volume of K2CO3/Al2O3 are 196.5 m2/g and 0.34 cm3/g, respectively, and the mean pore size is 3.4 nm. The range of particle sizes for K2CO3/Al2O3 is 220-600 μm, and the mean particle size is 420 μm. The minimum fluidization velocity is 0.1 m/s. More information about the sorbent preparation method and its microscopic structure was reported in a previous paper.17 Apparatus and Procedure. Figure 1 is a schematic diagram of the experimental apparatus for the fluidized-bed test.17 The apparatus consists of three sections: a gas injection system, a bubbling fluidized-bed reactor, and a gas post-treatment system. (17) Zhao, C. W.; Chen, X. P.; Zhao, C. S. Energy Fuels 2009, 23 (9), 4683–4687.
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Energy Fuels 2010, 24, 1009–1012
: DOI:10.1021/ef901018f
Zhao et al.
Figure 3. Temperature during the cycle reaction in the fluidized-bed reactor.
Figure 1. Schematic diagram of the experimental apparatus for the fluidized-bed test.
Figure 4. Temperatures in the reactor during carbonation without water cooling.
Figure 2. Changes of the CO2 concentration and reaction temperature during a single carbonation-regeneration cycle.
concentration and reaction temperature during a single-cycle reaction. The left part of the figure represents the carbonation reaction, and the right part of the figure represents the regeneration reaction. The CO2 concentration was nearly zero for the first 2 min, indicating that CO2 was completely absorbed. The CO2 concentration then rapidly increased to 14.9% at the time of 4.58 min and remained constant for 20 min. After the gas composition was changed to 100% N2, the temperature was increased to 350 °C and calcination was carried out. Small amounts of CO2 were released at the temperature range of 65-90 °C. We attribute this to the desorption of CO2, which was physically adsorbed by the sorbent. Most CO2 was released at the temperature range of 150-300 °C, and the maximum appears at 200 °C. This part of CO2 was from the decomposition of KHCO3. The calcination finished before reaching 350 °C. A single carbonationregeneration cycle lasted for approximately 60 min. Temperature Change during a Single-Cycle Reaction. The temperatures in the reactor during a single-cycle reaction are shown in Figure 3. During the carbonation stage, although the carbonation reaction was highly exothermic, the bed temperatures remained stable at a range of 60-80 °C because of its fluidized operation and water cooling. In the calcination stage, the calcination reaction is an endothermic reaction and occurs mainly in the bed. The temperature in the bed was lower than that in the freeboard. Temperatures in the different positions reached the same value of 350 °C after 60 min, indicating that calcination was complete. The temperatures in the reactor during carbonation without water cooling are shown in Figure 4. Because of the highly exothermic heat of the reaction for carbonation, the bed temperature increased to 110 °C in 3 min and remained above 80 °C for about 10 min. Most references reported that the proper carbonation temperature
CO2 and N2 were obtained from high-purity cylinders and delivered to the experimental apparatus with mass flow controllers. H2O was fed using a high-precision piston pump and then heated to ensure complete vaporization. The reactor (with an inner diameter of 0.05 m and height of 1.0 m) was made of stainless steel and was set in an electrically heated furnace. A cooling jacket was used to keep the temperature constant. The temperature was controlled by a temperature controller and measured by four thermocouples. These thermocouples were uniformly distributed every 50 mm from the bottom to top in the reactor and defined as T1, T2, T3, and T4. T1-T3 were in the bed, and T4 was in the freeboard. The gas post-treatment system included a filter, a condenser, and a gas analyzer, which analyzed the CO2 concentration every 5 s. A total of 0.25 kg of the sorbent was placed into the reactor, and the height of the material was 0.1 m. A simulated flue gas composition consisting of 13% CO2 (14.9%, dry basis), 74% N2, and 13% H2O at 2.3 m3/h was used for the carbonation reactions. The carbonation temperature was kept constant at 60 °C. After the carbonation reaction was completed, the gas composition changed to 100% N2 at a flow rate of 2 m3/h. The ratio range of actual gas velocity to minimum fluidization velocity was 2.5-3.5. When the CO2 concentration decreased to zero, the temperature was increased to 350 °C. Calcination was carried out. After calcination was completed, the temperature was decreased to 60 °C and the carbonation was carried out again. In this way, 10 reaction cycles were carried out. A micromeritics accelerated surface area and porosimetry 2020 micropore physisorption analyzer was used for surface area and porosity determinations. The structural changes of sorbents before and after the carbonation reaction were examined with D/max2500 VL/PC X-ray diffraction (XRD) and field-emission scanning electron microscopy (SEM). The change of the particle size was determined by the sieving method.
3. Results CO2 Breakthrough Curve of K2CO3/Al2O3 during a SingleCycle Reaction. Figure 2 shows the changes of the CO2 1010
Energy Fuels 2010, 24, 1009–1012
: DOI:10.1021/ef901018f
Zhao et al.
Figure 5. CO2-capture capacity of K2CO3/Al2O3 as a function of the cycle number.
Figure 6. Reaction conversion of K2CO3/Al2O3 for 10 cycles.
for the potassium-based sorbent was 60-80 °C. The carbonation reaction will not occur when the temperature is higher than 80 °C. This result emphasizes the importance of careful temperature control. CO2 Balance Closure. The CO2 amounts were obtained by the numerical integration of the area under the CO2 breakthrough curve. The total amounts of CO2 absorbed at 60 °C, desorbed at 65-90 °C, and released at 150-300 °C were 0.48, 0.04, and 0.44 mol, respectively. They were defined as nA, nD, and nR, respectively. On the basis of the conversion of K2CO3 to KHCO3, the carbonation conversion ηC and calcination conversion ηR were calculated from 5,6,18-20
ηC ¼
Figure 7. Change of the surface area and pore volume with the cycle number.
ðnA - nD ÞMK2 CO3 100% 61:25
ηR ¼
nR MK2 CO3 100% 61:25
where MK2CO3 is the molecular weight of K2CO3 and 61.25 is the weight of K2CO3 in the sorbent (24.5% of 250 g). The carbonation and calcination conversions are 99.5 and 98.8%, respectively. CO2-Capture Capacities of K2CO3/Al2O3 in 10 Cycles. The total amount of CO2 absorbed and released as a function of the cycle number is shown in Figure 5. Figure 5 shows that nA and nR decreased slightly from 0.48 to 0.45 mol and from 0.44 to 0.41 mol, respectively, after 10 cycles and nD remained at around 0.04 mol. The carbonation conversion and calcination conversion are shown in Figure 6. The carbonation conversion and calcination conversion were similar for each cycle. The carbonation conversion decreased from 98.1 to 94%, and the calcination conversion decreased from 97.9 to 93.8%, after 10 cycles. The sorbent retained a high rate of conversion after 10 cycles. It was reported that the carbonation conversion of K2CO3/Al2O3 decreased to 40% after the fourth cycle in a fixed-bed reactor (regeneration at 200 °C in 91% N2 and 9% H2O).11,14 The production of KAl(CO3)2(OH)2 was thought to be the main reason for this phenomena, and KAl(CO3)2(OH)2 could be completely regenerated t 350 °C.11,14 However, in our investigation,17 KAl(CO3)2(OH)2 was not observed and the sorbents were completely regenerated before the temperature reached 350 °C.
Figure 8. Pore size distribution of K2CO3/Al2O3.
Microscopic Structure Change of K2CO3/Al2O3. Samples (0.5 g) were taken after the 3rd, 6th, and 10th cycles, respectively, and the microscopic structures were measured with the N2 adsorption method. The surface area and the pore volume of those samples are shown in Figure 7. The surface area and pore volume of K2CO3/Al2O3 decrease as the number of cycles increases. After 10 cycles, the reduction in the surface area and pore volume are 6.2 m2/g and 0.022 cm3/g, respectively. As shown in Figure 8, for 90% pores of the fresh sorbent, the pore size distribution ranged from 1.76 to 22.21 nm. The maximum value of the pore volume appears at 6.2 nm. There is little change in the pore size distribution after 10 cycles. The SEM images for fresh sorbent and sorbent after 10 cycles are shown in Figure 9. The SEM images confirm that the change in the microscopic structure is minimal after 10 cycles. Particle Size Change of K2CO3/Al2O3. The change in the particle size of K2CO3/Al2O3 is the focus of the multiplecycles test. The result is shown in Figure 10. Figure 10 shows that the particle size range for more than 97% of fresh sorbent was 0.3-0.6 mm. The average particle size decreased from 0.42 to 0.37 mm after 10 cycles. After 10 cycles, the amount of elutriated sorbent, which were smaller
(18) Hirano, S.; Shigemoto, N.; Yamaha, S.; Hayashi, H. Bull. Chem. Soc. Jpn. 1995, 68 (3), 1030–1035. (19) Shigemoto, N.; Yanagihara, T.; Sugiyama, S.; Hayashi, H. Energy Fuels 2006, 20 (2), 721–726. (20) Hayashi, H.; Taniuchi, J.; Furuyashiki, N.; Sugiyama, S.; Hirano, S.; Shigemoto, N.; Nonaka, T. Ind. Eng. Chem. Res. 1998, 37 (1), 185–191.
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Energy Fuels 2010, 24, 1009–1012
: DOI:10.1021/ef901018f
Zhao et al.
Figure 10. Change of the particle distribution for K2CO3/Al2O3.
The CO2-capture conversion for K2CO3/Al2O3 was above 90% during the 10 cycles, and the microstructure character remained relatively stable. The changes in the surface area and pore volume after 10 cycles are 6.2 m2/g and 0.022 cm3/g, respectively. The changes in the pore size distribution and surface character are small. Careful temperature control is important because of the narrow temperature window available for CO2 removal and the highly exothermic nature of the carbonation process. After 10 cycles, the elutriated sorbent was 2.8 wt %, and the rate of reduction of the average particle size was 0.005 mm/h. The particle shows excellent attrition resistance performance. K2CO3/Al2O3 could be used as a sorbent for a largescale CO2-capture process with two fluidized-bed reactors. To investigate this technology further, a dual circulating fluidized-bed system will be built in the near future.
Figure 9. SEM images of K2CO3/Al2O3: (a) fresh sorbent and (b) sorbent after 10 cycles.
than 0.052 mm, was only 2.8 wt %. The rate of reduction of the average particle size was 0.005 mm/h. The attrition of the sorbent is small. The particle shows excellent attrition resistance performance.
Acknowledgment. The financial support from the National HighTechnology Research and Development Program of China (2009AA05Z311), the National Natural Science Foundation (50876021), the Foundation of Graduate Creative Program of Jiangsu Province (CX08B_141Z), and the Scientific Research Foundation of Graduate School of Southeast University (YBJJ0825) is sincerely acknowledged.
4. Conclusions The CO2-capture capacity of K2CO3/Al2O3 as a function of the cycle number was investigated in a fluidized-bed reactor.
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