Al2O3 for Capturing CO2 in Flue Gas from Power Plants. Part

Jul 3, 2013 - Yi , C. K.; Jo , S.-H.; Seo , Y.; Park , S. D.; Moon , K. H.; Yoo , J. S.; Lee , J. B.; Ryu .... Seo , Y. W.; Jo , S.-H.; Ryu , C. K.; Y...
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K2CO3/Al2O3 for Capturing CO2 in Flue Gas from Power Plants. Part 5: Carbonation and Failure Behavior of K2CO3/Al2O3 in the Continuous CO2 Sorption−Desorption System Ye Wu,† Xiaoping Chen,†,* Wei Dong,† Chuanwen Zhao,†,‡ Zhonglin Zhang,† Daoyin Liu,† and Cai Liang† †

Key Laboratory of Energy Thermal Conversion and Control, Ministry of Education, School of Energy and Environment, Southeast University, Nanjing 210096, People’s Republic of China ‡ State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei, 230027, People’s Republic of China ABSTRACT: The CO2 capture performance of K2CO3/Al2O3 is investigated in the continuous CO2 sorption−desorption system with three fluidized-bed reactors (two carbonation reactors and one regeneration reactor). A total of 10 Nm3/h of simulated flue gas is treated in this system. The first step of CO2 removal is higher than 75%, and the total CO2 removal reaches 96% after further sorption in the second step, a fairly constant state during the test. The sorbent shows excellent structural stability, and the sulfation percentages of the sorbent in the carbonation and regeneration reactors are 1.3 and 1.1%, respectively. Effects of several operation parameters, such as bed height, solid circulation rate, carbonation temperature, regeneration temperature, fluidization number, and H2O concentration, are investigated to evaluate the CO2 sorption performance of the sorbent when carbonation and regeneration reactors are put to use, and results show that the CO2 removal can reach more than 85% after optimizing the operation parameters. The results from this study prove the concept of the dry sorbent CO2 capture process to be one of the viable methods for capturing CO2 produced by coal-fired power plants.

1. INTRODUCTION A K-based sorbent1−3 for CO2 capture is thought to be an innovative concept because this technology has low energy demands, limited corrosion, and no secondary pollution. Also, the sorbent used has high activity with CO2. Some researchers reported that the energy penalty is 16% less than that of monoethanolamine (MEA).4 It involves the carbonation of K2CO3 and the regeneration of KHCO3. K 2CO3 + CO2 + H 2O → 2KHCO3 (R1) 2KHCO3 → K 2CO3 + CO2 + H 2O

gas have a significant effect on CO2 removal. At a starting concentration of 15%, the sorbent removed 61% of CO2 passing through the entrained-bed reactor. Recently, a continuous operation setup with the function of CO2 sorption−desorption has been developed because the sorbent should be cycled continuously if the technology will be commercialized. Lee and Ryu3,6−14 loaded K2CO3 in many supports to study the carbonation characteristics of the sorbent in a fixed bed, bubbling bed, and circulating fluidized bed, separately. They found that3 the CO2 removal increased as the gas velocity decreased and the solid circulation rate increased. The sorbent by the Korea Institute of Energy Research (KIER) was capable of greater than 80% removal of CO2 in 50 h of operation6 and showed negligible drop-off. Southeast University1,15−19 has also carried out extensive studies to develop suitable sorbent to capture CO2 with a thermogravimetric (TG) apparatus and a bubbling fluidized bed. They found that the carbonation conversion for K2CO3/ Al2O3 was kept steady after a 20 cycle test and the sorbent of K2CO3/Al2O3 showed excellent attrition resistance performance.17 In addition, there is some amount of acid gas, such as SO2 and NO, in the flue gas, and K2CO3 can react with SO2 as reaction R3,19 which will reduce the CO2 capture capacity of the sorbent.

(R2)

Carbonation−regeneration cycles are realized by means of two interconnected fluidized beds operating under atmospheric pressure. K2CO3 reacts with CO2 in the carbonation reactor to form KHCO3 at temperatures of 60−80 °C. The spent sorbent is then regenerated by heating it at temperatures of 150−300 °C. KHCO3 decomposes to yield K2CO3 for reuse, and concentrated CO2 is obtained after condensing H2O in the exit gas from the regeneration reactor. This process has potential for reducing post-combustion CO2 emissions. Funded by the United States Department of Energy (U.S. DOE), the Research Triangle Institute (RTI), Church and Dwight (C&D), and Louisiana State University (LSU)2,4,5 engaged in the research of sodium-based sorbents for removing CO2 from flue gas. RTI2 found that, in the seven CO2 sorption−desorption cycles, the sorbent performed quite consistently over all seven cycles, was capable of greater than 90% removal of CO2 in every cycle, and showed negligible drop-off in the seven cycles when the sorbent was exposed to a simulated flue gas of 3 vol % CO2 saturated with water vapor (balance N2). Higher initial concentrations of CO2 in the flue © 2013 American Chemical Society

K 2CO3 + SO2 → K 2SO3 + CO2

(R3)

Received: November 29, 2012 Revised: June 24, 2013 Published: July 3, 2013 4804

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active component and 63.2 wt % Al2O3 as a support. The initial inventory of the sorbent is 50 kg (25 kg in the carbonation reactor and 25 kg in the regeneration reactor). On the basis of our previous studies and studies at other locations,1−3 the reaction temperature in the carbonation reactor is maintained in the range of 60−100 °C. The temperature in the regeneration reactor is maintained between 150 and 300 °C. The gas flow rate in the carbonation reactor changes from 8 to 13 m3/h. Because the minimum fluidization velocity (vmin) of the sorbent in the reactor is 0.054 m/s, the fluidization number n, which is calculated as below, is obtained v n= vmin (1)

Therefore, it is significant to investigate long-time continuous operation of the sorbent to prove the suitable concept of the dry sorbent CO2 capture process as a viable method for capturing CO2 and to optimize the work condition to improve the CO2 removal from fossil-fuel-fired power plants. Also, an evaluation of the effect of SOx/NOx in flue gas on CO2 sorption should be considered. The present work attempts to investigate the carbonation and sulfation characteristics of the potassium-based sorbent in the continuous CO2 sorption−desorption system with two bubbling fluidized carbonation reactors and a bubbling fluidized regeneration reactor. In addition, the influence factors, such as bed height, solid circulation rate, carbonation temperature, regeneration temperature, fluidization number, and H2O concentration are also investigated.

where v is the fluidization velocity of the gas in the reactor (m/s). The gas flow rate varies from 8 to 13 m3/h in the carbonation reactor that is corresponding through 1.1−1.7 fluidization numbers. During the test, the gas flow rate in the regeneration reactor is 5.5 m3/h. The solid circulation rate between the carbonation reactor and the regeneration reactor changes from 20 to 50 kg/h by controlling the transducer of the screw conveyor. The content of water vapor and CO2 concentration in the simulated gas change from 8 to 16 vol % and from 3 to 20 vol %, respectively. Also, the bed height in the carbonation reactor changes from 0.3 to 1.1 m. The operating pressure is atmosphere pressure. The CO2 removal RC and the sulfation percentage RS are used to evaluate sorbent performance. RC and RS can be calculated as below

2. EXPERIMENTAL SECTION Figure 1 describes the schematic of the CO2 sorption−desorption system. The typical feature of the process includes two bubbling

RC = RS =

Q inC in − Q outCout Q inC in nK2SO3 nK02CO3

× 100% (2)

× 100% (3)

where Qin is the inlet flow rate of the simulated flue gas (m /h), Qout is the outlet flow rate of the gas (m3/h), Cin is the average CO2 concentration in the inlet of the simulated flue gas (vol %), Cout is the average CO2 concentration in the outlet gas (vol %), nK2SO3 is the conversion amount of K2CO3 that is converted to K2SO3 (mol) and is detected with the barium sulfate gravimetric method, and n0K2CO3 is the initial amount of K2CO3 in the reactor (mol). 3

Figure 1. Schematic representation of the dry sorbent CO2 capture process.

3. RESULTS AND DISCUSSION Figure 2 shows the CO2 concentration profile of the exit gas from the carbonation reactor (A) for 2 h of continuous

fluidized-bed carbonation reactors (A and B) with an inner diameter of 0.229 m and a height of 2.8 m and a bubbling fluidized-bed regeneration reactor (C) with an inner diameter of 0.257 m and a height of 2.5 m. Cooling coils are placed inside of each carbonation reactor (A and B), and electric heaters are placed around each reactor (A, B, and C). Three cyclones (D) are used in each reactor to make small particles go back to the reactor, and four screw conveyors (I) are used to transfer the sorbent between one of the carbonation reactors (A or B) and the regeneration reactor (C). Eight temperature points are placed in each reactor (A, B, and C), and all of them are measured by a precision digital thermometric instrument, one for each point. In addition, each heater placed around the reactor (A, B, and C) is controlled by one of the eight temperatures, which is placed directly inside the dense phase zone of the reactor. CO2 and SO2 are from high-purity cylinders, and air is from the air compressor. Their flows are monitored using calibrated rotameters and needle valves. H2O is fed using a high-precision piston pump, and the feed lines are heated to ensure complete vaporization before mixing with the permanent gases. A gas composition of CO2, H2O, SO2, and air is used for carbonation reactions. The exit gas of the carbonation reactor is measured with the gas analyzer, and the sorbent sampled every 4 h is detected with the particle size analyzer, N2 adsorption method, and barium sulfate gravimetric method. The solid sorbent, K2CO3/Al2O3 (KAl40), is used to examine the CO2 capture characteristics. It consists of 36.8 wt % K2CO3 as an

Figure 2. Carbon dioxide concentration of carbonation reactor outlet gas when putting the carbonation reactor (A) and the regeneration reactor (C) to use.

operation. The carbonation and regeneration temperatures are 60 and 300 °C, respectively, and the solid circulation rate is 40 kg/h. In the 2 h of continuous operation, 25 kg of KAl40 (0.73 m bed height in the carbonation reactor and 0.6 m bed height in the regeneration reactor) is exposed to a simulated flue gas of 10 vol % CO2 and 12 vol % H2O (balanced with Air) at a flow 4805

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Figure 3. Effects of several parameters on CO2 removal: (a) regeneration temperature range of 150−300 °C, (b) carbonation temperature range of 60−100 °C, (c) CO2 concentration range of 3−20 vol %, and (d) H2O concentration range of 8−16 vol %.

rate of 10 m3/h (1.5 fluidization number). In the continuous CO2 sorption−desorption system, the fresh sorbent in the carbonation reactor (A) partially absorbs CO2 and H2O and moves toward to the regeneration reactor (C) through the screw conveyer (I) and then the carbonated sorbent in the regeneration reactor (C) releases CO2 and water vapor. Thus, solid sorbent is partially rejuvenated and transferred to the carbonation reactor (A) for reuse. The gas residence times in the carbonation reactor (A) and the regeneration reactor (C) are 9.01 and 9.09 s because the gas velocities in the carbonation and regeneration reactors are 0.081 and 0.066 m/s, respectively. The solid residence time in the carbonation reactor (A) and the regeneration reactor (C) is less than 1 h, depending upon the solid circulation rate and the amount of loading sorbent in the reactor. When water vapor is injected into the simulated gas after 30 min of stable operation, the CO2 concentration markedly decreases to about 2.46%, which means that the CO2 removal reached 77.3%. As a result, the CO2 sorption capacity of the sorbent (CCO2) is 0.863 (mol of CO2/kg of sorbent) CCO2 =

Effects of several parameters, such as carbonation temperature, regeneration temperature, and CO2 concentration, are investigated. When each variable above is changed, the other parameters are kept the same as that in Figure 2, and the results are shown in Figure 3. The CO2 removal reaches 44.6% with a regeneration temperature of 150 °C and 77.3% with a regeneration temperature of 300 °C. The CO2 removal increases as the regeneration temperature increases from 150 to 200 °C and reaches a plateau over 200 °C, as shown in Figure 3a. When the bed height is fixed, which means that the gas−solid contact time is fixed, the higher regeneration temperature leads to more regeneration of sorbent for reuse; however, a further increase of the regeneration temperature is useless because of the limitation of the gas−solid contact time in the reactor. Thus, the CO2 removal does not change too much at that working condition when the regeneration temperature is from 200 to 300 °C. The effect of the carbonation temperature on CO2 removal is shown in Figure 3b. The CO2 removal reaches a plateau at about 77% when the carbonation temperature increases from 60 to 70 °C and then decreases to 52.2% when the carbonation temperature increases from 70 to 90 °C. When the carbonation temperature reaches 100 °C, the CO2 removal markedly decreases to about 6.6%. The CO2 sorption performance of the sorbent above is due to the fact that there would be severe competition between carbonation and regeneration, and reaction R2 is the main reaction at this temperature. Figure 3c shows the effect of the CO2 concentration on CO2 removal under the baseline operating conditions. The CO2 removal decreases from 85.4 to 62.5% as the CO 2 concentration increases from 3 to 20%. However, the CO2 sorption capacity of the sorbent increases from 0.33 to 2.23 mol of CO2/kg of sorbent, which is because the greater the CO2

R CO2 R sorbent

(4)

where RCO2 is the CO2 removal rate (m3/h) and Rsorbent is the circulation rare of the sorbent (kg/h). Our previous study1 showed that the conversion ratio of K2CO3 to KHCO3 in the TG apparatus or the single bubbling fluidized bed reactor is more than 90%, which means that the CO2 sorption capacity of the sorbent in the sorption temperature of 60 °C and CO2 concentration of 10 vol % is 2.4 mol of CO2/kg of sorbent. On the basis of the results above, it can be concluded that 77.3% of CO2 capture efficiency can be obtained at that low CO2 sorption capacity when operated in the continuous CO2 sorption−desorption system. 4806

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Figure 4. Effects of several parameters on CO2 removal. (a) effect of the bed height, (b) effect of the solid circulation, and (c) effect of the fluidization number.

Figure 5. CO2 removal of 20 h of operation when putting the carbonation reactor (A and B) and the regeneration reactor (C) to use: (a) first step of CO2 removal and (b) total CO2 removal.

concentration in the flue gas, the more contact area between CO2 and K2CO3. Under the baseline operating conditions, the effect of the water vapor content in the inlet gas on the CO2 removal is also examined. The CO2 removal increases from 65.2 to 82.9% as the water vapor content increases from 8 to 16 vol %, as shown in Figure 3d. The carbonation reaction requires equivalent moles of H2O and CO2. More water causes more K2CO3 and CO2 to dissolve, thus accelerating the sorption process.20 Therefore, an excessive water vapor supply can increase the CO2 removal. Additional testing is conducted to evaluate the sorbent performance as a function of the bed height, solid circulation, and gas velocity. Figure4 shows the results of these tests. The CO2 removal reaches 44.88% with a bed height of 0.3 m and 79.88% with a bed height of 1.1 m, as shown in Figure 4a. As the bed height increases from 0.3 to 0.73 m, the CO2 removal increases to 77.3%, while when the bed height is increased from 0.73 to 1.1 m, the CO2 removal reaches a

plateau at about 78%, because the gas residence time in the carbonation reactor is long enough for the carbonation process. Figure 4b shows the effect of solid circulation. The CO2 removal increases from 68.4 to 78.6% as the solid circulation is increased from 20 kg/h (0.135 kg m−2 s−1) to 50 kg/h (0.337 kg m−2 s−1). Because the theoretical demand of the solid circulation is 25 kg/h (0.169 kg m−2 s−1) at a flow rate of 10 m3/h and the solid residence time in the regeneration reactor is long enough for the regeneration process, the CO2 removal reaches a plateau at about 75% when the solid circulation is over 30 kg/h (0.202 kg m−2 s−1). The study by KIER3 showed that the CO2 removal increased from 26.4 to 52.6% as the solid circulation increased from 10 to 40 kg m−2 s−1. Besides the higher sorption temperature (80 °C), the lower residence time (3 s) of the gas in the carbonation reactor (a fast fluidized bed) is also the main reason for the lower CO2 removal. Figure 4 shows the effect of the fluidization number on the CO2 sorption performance of the sorbent. The CO2 removal 4807

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reaches 87% when the fluidization number is 1.1, because decreasing the fluidization number is available to reduce the amount of bubble in the carbonation reactor to increase the contact area and the residence time in the gas−solid reaction. To improve the CO2 removal and investigate the sulfation performance of the sorbent, the carbonation reactor (B) is put to use. Another 200 mg/m3 (70 ppm) SO2 is fed to the simulated gas. In addition, the flue gas from the carbonation reactor (A) is pumped into the carbonation reactor (B). Also, water vapor must be replenished into the flue gas from the carbonation reactor (B). The CO2 concentration and removal profiles are shown in Figure 5. The carbonation and regeneration temperatures are maintained at 60 and 250 °C, respectively. As shown in Figure 5, the CO2 concentration from the exit gas of the carbonation reactor (A) is about 3 vol %, which means that the CO2 removal is about 75%. In addition, the profile is a little fluctuant, which is attributed to the fluctuation of the regeneration temperature, as shown in Figure 6.

However, the CO2 concentration from the exit gas of the carbonation reactor (B) is normally maintained around 0.4 vol %, which means that the total CO2 removal after the carbonation reactor (B) is about 96%. During the 20 h of operation, the sorbent is sampled for detection every 4 h, and the detection results are shown in Figure 7. The Brunauer−Emmett−Teller (BET) surface area and the pore volume change from 70.9 to 78.3 m2/g and from 0.212 to 0.235 cm3/g, respectively. The aperture distribution changes little during the 20 h of operation. In addition, the average particle size remains constant, which changes from 0.25 to 0.228 mm (Figure 8). The K2CO3/Al2O3 (KAl40) shows excellent structural stability.

Figure 8. Profile of the average particle size every 4 h.

Figure 9 shows the sulfation percentage profile of the sorbent in the carbonation reactor (A) and the regeneration reactor (C) caused by SO2. No SO2 is detected in the outlet gas from both the carbonation and regeneration reactors. The sulfation percentages of the sorbent in the carbonation reactor (A) and the regeneration reactor (C) are around 1.1 and 1.3%, respectively. Our previous study19 showed that K2SO4 was the

Figure 6. Temperature profiles in the carbonation reactor (A and B) and the regeneration reactor (C) during continuous operation.

Figure 7. Structural variation of the sorbent every 4 h: (a) profile of the specific surface area, (b) profile of the specific pore volume, and (c) profile of the aperture distribution. 4808

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(4) Liang, Y.; Harrison, D. P.; Gupta, R. P.; Green, D. A.; McMichael, W. J. Energy Fuels 2004, 18 (2), 569−575. (5) Liang, Y. Carbon dioxide capture from flue gas using dry regenerable sorbents. M.S. Thesis, Louisiana State University, Baton Rouge, LA, 2003. (6) Park, Y. C.; Jo, S.-H.; Ryu, C. K.; Yi, C.-K. Energy Procedia 2009, 1 (1), 1235−1239. (7) Ryu, C. K.; Lee, J. B.; Eom, T. H.; Baek, J. I.; Eom, H. M.; Yi, C.K. Proceedings of the 8th International Conference on Greenhouse Gas Control Technology; Trondheim, Norway, June 19−22, 2006. (8) Yi, C. K.; Jo, S.-H.; Seo, Y.; Park, S. D.; Moon, K. H.; Yoo, J. S.; Lee, J. B.; Ryu, C. K. Stud. Surf. Sci. Catal. 2006, 159, 501−504. (9) Seo, Y. W.; Jo, S.-H.; Ryu, C. K.; Yi, C.-K. Chemosphere 2007, 69 (5), 712−718. (10) Lee, S. C.; Choi, B. Y.; Lee, T. J.; Ryu, C. K.; Ahn, Y. S.; Kim, J. C. Korean Chem. Eng. 2006, 23 (3), 374−379. (11) Seo, Y. W.; Jo, S.-H.; Ryu, C. K.; Yi, C.-K. Korean Chem. Eng. Res. 2005, 43 (4), 537−541. (12) Lee, S. C.; Choi, B. Y.; Lee, T. J.; Ryu, C. K.; Ahn, Y. S.; Kim, J. C. Catal. Today 2006, 111, 385−390. (13) Seo, Y. W.; Jo, S.-H.; Ryu, C. K.; Yi, C.-K. Korean Chem. Eng. Res. 2005, 43 (4), 537−541. (14) Seo, Y. W.; Jo, S.-H.; Ryu, C. K.; Yi, C.-K. Chemosphere 2007, 69 (5), 712−718. (15) Zhao, C.; Chen, X.; Zhao, C. Energy Fuels 2012, 26, 1406−1411. (16) Zhao, C.; Chen, X.; Zhao, C.; Wu, Y.; Dong, W. Energy Fuels 2012, 26, 3062−3068. (17) Zhao, C.; Chen, X.; Zhao, C. Energy Fuels 2012, 26 (2), 1395− 1400. (18) Zhao, C.; Chen, X.; Zhao, C. Ind. Eng. Chem. Res. 2010, 49 (23), 12212−12216. (19) Wu, Y.; Chen, X.; Zhao, C. Int. J. Greenhouse Gas Control 2011, 5 (5), 1184−1189. (20) Zhang, B.-T.; Fan, M.; Bland, A. E. Energy Fuels 2011, 25, 1919−1925.

Figure 9. Sulfation percentage profile of the sorbent in the carbonation reactor (A) and the regeneration reactor (C) caused by SO2.

only byproduct generated in the carbonation process, and it is very stable and cannot be decomposed even at a high temperature. Therefore, it can be indicated that K2SO4 is not generated in the regenerator but transported from the carbonator to the regenerator and the spent sorbent is also transferred between the carbonation reactor and the regeneration reactor in time during the 20 h of operation. All of these results ensure the long-term performance of the sorbent in the continuous CO2 sorption−desorption system.

4. CONCLUSION The CO2 capture process is operated in the continuous CO2 sorption−desorption system. Increasing the solid circulation rate, the regeneration temperature, and the water vapor content and decreasing the fluidization number and the carbonation temperature give rise to the increase of the overall CO2 removal in this system. The 20 h of continuous operation demonstrates that the CO2 removal reaches 96% and K2CO3/Al2O3 (KAl40) shows excellent structural stability. The CO2 sorption capacity of the sorbent does not need to reach to a very high level when operated in a continuous CO2 sorption−desorption system. In addition, the sulfation percentages of the sorbent in the carbonation reactor (A) and the regeneration reactor (C) are around 1.1 and 1.3%, respectively, after 20 h of operation. The results from this work ensure the long-term performance of the sorbent in the continuous CO2 sorption−desorption system.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation (50876021) and the National High Technology Research and Development Program of China (2009AA05Z311) is sincerely acknowledged.

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REFERENCES

(1) Zhao, C.; Chen, X.; Zhao, C. Energy Fuels 2012, 26, 1401−1405. (2) Green, D. A.; Nelson, T. O.; Turk, B. S.; Portzer, J. W.; Gupta, R. P. Carbon dioxide capture from flue gas using dry regenerable sorbents. Quarterly Technical Progress Report; Research Triangle Institute: Research Triangle Park, NC, 2005. (3) Yi, C. K.; Jo, S. H.; Seo, Y. W.; Lee, J. B.; Ryu, C. K. Int. J. Greenhouse Gas Control 2007, 1, 31−36. 4809

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