Al2O3 for Capturing CO2 in Flue Gas from Power Plants. Part 2

Jan 12, 2012 - K2CO3/Al2O3 for Capturing CO2 in Flue Gas from Power Plants. Part 2: ... Southeast University, Nanjing 210096, People's Republic of Chi...
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K2CO3/Al2O3 for Capturing CO2 in Flue Gas from Power Plants. Part 2: Regeneration Behaviors of K2CO3/Al2O3 Chuanwen Zhao, Xiaoping Chen,* and Changsui Zhao School of Energy and Environment, Southeast University, Nanjing 210096, People’s Republic of China ABSTRACT: The present paper is the second part of a series of papers about a systematical investigation on the application of the K2CO3/Al2O3 sorbent for capturing CO2 in flue gas. It was focused on the regeneration behaviors of K2CO3/Al2O3 in a thermogravimetric analyzer coupled with a Fourier transform infrared spectrometer. The effects of operation conditions, including the regeneration temperature, gas composition, and heating rate, on the regeneration process of K2CO3/Al2O3 were thoroughly studied. The regeneration process of K2CO3/Al2O3 consists of three steps, as the temperature increases: KHCO3 is decomposed first when the temperature is lower than 180 °C; then, an intermediate product for the carbonation of K2CO3/ Al2O3 is decomposed; and finally, KAl(CO3)2(OH)2 is decomposed when the temperature is higher than 250 °C. The regeneration conversion of K2CO3/Al2O3 reaches 100% in 15 min when the final temperature reaches 300 °C in N2. The effect of the gas composition of CO2 and CO2/H2O on the regeneration process is not significant. The regeneration conversion decreases from 100 to 92.8% and the reaction time decreases from 72 to 6.5 min when the heating rate increases from 5 to 80 °C/min.

1. INTRODUCTION In the first part of this series of papers (10.1021/ef200725z),1 the carbonation behaviors of K2CO3/Al2O3 were systematically studied by thermogravimetric analysis (TGA). Because most of the energy demand for this process is determined by the regeneration process of K2CO3/Al2O3, it is important to study the regeneration behaviors of this sorbent for CO2 capture. The present work is addressed to investigate the regeneration reaction mechanism of K2CO3/Al2O3 in detail. In previous papers, it was found that the decomposition processes of NaHCO32 and KHCO33 in TGA started at the temperature of 102.3 and 172.3 °C and ended at the temperature of 210.7 and 275.1 °C, respectively. The regeneration behaviors of several potassium-based sorbents were reported.4−13 It was found that K2CO3/activated carbon (AC), K2CO3/TiO2, K2CO3/ZrO2, and a K−Fe sorbent could be completely regenerated at the temperature of 150 °C in a fixed-bed reactor, while K2CO3/MgO, K2CO3/Al2O3, K2CO3/ MgO/Al2O3, and K2CO3/CaO could not be completely regenerated under the same conditions. Because of the formation of KAl(CO3)2(OH)2, K2Mg(CO3)2, K2Mg(CO3)2·4(H2O), MgCO3, and K2Ca(CO3)2, higher temperatures were required for their decomposition. The regeneration behaviors of the analytical reagent alkali metal carbonate are changed when it is loaded on the support, and sorbents with different supports show different regeneration behaviors. The regeneration behavior for the sorbent of K2CO3/Al2O3 is especially complicated. The regeneration temperature was reported to be higher than 350 °C in a fixed-bed reactor because of the formation of KAl(CO3)2(OH)2,6 while it was reported to be lower than 200 °C in recent experiments as the preparation process of the sorbent was improved.14 However, KAl(CO3)2(OH)2 was not observed with an X-ray diffractometry (XRD) in the carbonation product of K2CO3/Al2O3 in our investigation,15 © 2012 American Chemical Society

and the sorbents were completely regenerated before the temperature reached 350 °C in a fluidized-bed reactor.16 To investigate the regeneration behavior of K2CO3/Al2O3 thoroughly and systematically, a thermogravimetric analyzer (TGA92) coupled with a Fourier transform infrared (FTIR) spectrometer was used and the effects of operation conditions, including the regeneration temperature, gas composition, and heating rate, on the regeneration process were thoroughly studied for this sorbent.

2. EXPERIMENTAL SECTION 2.1. Samples. The carbonation product of K2CO3/Al2O3 was denoted as Pr-KAl. It was obtained from the reaction of K2CO3/Al2O3 at 60 °C in the gas mixture of 10% CO2, 76% N2, and 14% H2O at a flow rate of 750 L/h for 35 min in a fluidized-bed reactor. The sorbent of K2CO3/Al2O3 was prepared by impregnating K2CO3 on Al2O3. K2CO3 was provided as an analytical reagent, and a special γ-Al2O3 was supplied by the Research Institute of Nanjing Chemical Industry Group. The preparation procedure of the sorbent consisted of three steps: mixing and impregnation, drying at 105 °C for dehydration, and calcination at 300 °C in N2. The K2CO3 loading amount on K2CO3/ Al2O3 was measured to be 28.5%. The particle size of the product PrKAl was chosen as 180−315 μm, with a mean particle size of 250 μm. To understand the decomposition process of Pr-KAl better, analytical reagent KHCO3 was also used to study the regeneration behavior of the main carbonation product, i.e., KHCO3, of the K2CO3/ Al2O3 sorbent. The particle size of KHCO3 used was chosen to be the same as Pr-KAl. 2.2. Apparatus and Procedure. The decompositions of Pr-KAl and KHCO3 were carried out with a TGA92. N2, CO2, and CO2/H2O were used as the carrier gas, respectively. CO2 and N2 with the purity of 99.99% were supplied from high-purity cylinders with mass flow controllers. H2O was fed using a high-precision piston pump and was heated to 200 °C to ensure complete vaporization before mixing with Received: March 1, 2011 Revised: October 2, 2011 Published: January 12, 2012 1406

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other gases. The flow rate of the carrier gas was set to 65 mL/min. Samples of around 20 mg were heated from room temperature to a final temperature with a constant heating rate, and then the temperature kept constant. The evolved gases from TGA were directed to a Bruker Vector 22 FTIR spectrometer with a heated transfer line (about 180 °C). The regeneration process was studied by analyzing the change of the weight of the sorbent with time and evolved gas composition. Detailed information about the TGA−FTIR apparatus and product analysis method were referred to from ref 17. The final temperature was chosen between 150 and 400 °C. The heating rate was chosen between 5 and 80 °C/min. Experiments were repeated 3 times, and the overall results were quite reproducible. The amount of K2CO3 impregnated on Al2O3 was determined by an Advant’XP X-ray fluorescence (XRF). The particle size of the sorbent was determined by the sieving method. The structural change of sorbents before and after the reactions was examined with D/max2500 VL/PC XRD.

4. RESULTS AND DISCUSSION 4.1. Regeneration Processes of Pr-KAl and KHCO3. The regenerations of Pr-KAl and KHCO3 were carried out at a final temperature of 400 °C with a heating rate of 5 °C/min in 100% N2. The changes of dimensionless weight (defined as the ratio of the weight of the sorbent to that of the initial value) and temperature with the time are shown in Figure 1.

3. THEORETICAL ANALYSES 3.1. Calculation of the Regeneration Conversion of KHCO3. On the basis of KHCO3 being completely converted to K2CO3, the regeneration conversion (η) changing with time for KHCO3 is calculated according to eq 1 η=

2M KHCO3(w(0) − w(t )) w(0)(2M KHCO3 − M K2CO3)

Figure 1. Regeneration processes of Pr-KAl and KHCO3.

As shown in Figure 1, the decomposition of KHCO3 starts at 117.2 °C and ends at 219.7 °C. The dimensionless weight decreases from 1 to 0.690. The total regeneration conversion (η) of KHCO3 (calculated from eq 1) was 100%. This regeneration temperature is lower than that reported in ref 3. The reason is that the heating rate and the gas composition in this paper are different from those in ref 3. The regeneration of Pr-KAl starts at the beginning of the heating process and finishes at 384.2 °C. The dimensionless weight decreases from 1 to 0.886. The total regeneration conversion (η) of Pr-KAl (calculated from eq 2) was also 100%. It can be found that the decomposition process of Pr-KAl is more complicated than that of pure KHCO3. In the test results of the FTIR spectrometer, the wave numbers for the strong peaks appearing are similar in the whole process for these two sorbents. The results of these two sorbents are shown in Figure 2 for the absorbance changed with the wave numbers at the same reaction time.

× 100% (1)

where t is the reaction time, w(t) is the weight of the sorbent at time t, w(0) is the sorbent weight at the beginning of the regeneration, and MKHCO3 and MK2CO3 are the molecular weights of KHCO3 and K2CO3, respectively. 3.2. Calculation of the Regeneration Conversion of Pr-KAl. With the calculation from the total amount of CO2 absorbed, the CO2 sorption capacity of K2CO3/Al2O3 was 90.86 mg/g of sorbent when the carbonation test was carried out at 60 °C in the gas mixture of 10% CO2, 76% N2, and 14% H2O at a flow rate of 750 L/h for 35 min in a fluidized-bed reactor. It is corresponding to the theoretical value (90.87 mg/g of sorbent) of 1 mol of K2CO3 absorbing 1 mol of CO2. KAl(CO3)2(OH)2 was found in the carbonation product of K2CO3/Al2O3 in a fixed-bed reactor.6 However, in our previous paper,15 KAl(CO3)2(OH)2 was not detected with XRD in the carbonation product of K2CO3/Al2O3 in TGA. It was deduced that the amount of KAl(CO3)2(OH)2 in the carbonation product in TGA was so little that it could not to be detected with XRD. The compositions of Pr-KAl and its regeneration product at a final temperature of 400 °C after 90 min were examined with XRD. It was confirmed that KHCO3 and KAl(CO3)2(OH)2 were formed when the carbonation of K2CO3/Al2O3 was carried out in a fluidized-bed reactor. After regeneration, the XRD patterns contained two phases: K2CO3 and Al2O3. Because the carbonation reaction and regeneration reaction are completely finished, η is 100% for Pr-KAl regenerated at a final temperature of 400 °C after 90 min. On the basis of the situation above, η of Pr-KAl changing with time for other reaction conditions is calculated according to eq 2

η=

(w(0) − w(t )) × 100% 0.114w(0)

Figure 2. Absorbance changed with the wave numbers at the same reaction time of 16.99 min for (a) Pr-KAl and (b) KHCO3.

As shown in Figure 2, four strong peaks are present at the wave numbers of 400−4000 cm−1 for these two sorbents. The strongest peak at the wave number of 2358.51 cm−1 and the peak at the wave number of 669.18 cm−1 (the expected bond is CO) are attributed to CO2, while the peaks at the wave numbers of 3631.3 and 3731.58 cm−1 (the expected bond is O− H) are attributed to H2O.

(2)

where 0.114 is the ratio of the decrease of the weight for Pr-KAl regenerated at a final temperature of 400 °C after 90 min to the initial weight of the sorbent. 1407

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energy demand, the regeneration of Pr-KAl and KHCO3 in N2 was carried out at different final temperatures with a heating rate of 20 °C/min. The obtained regeneration conversion η and the release of CO2 changing with time are shown in Figures 4 and 5, respectively.

To observe the release of CO2 and H2O for these two sorbents, the changes of absorbance at the wave numbers of 2358.51 and 3731.58 cm−1 with time are shown in panels a and b of Figure 3, respectively.

Figure 4. Effect of the final temperature in N2 on regeneration conversion for (a) Pr-KAl and (b) KHCO3.

Figure 3. Gas release with time of (a) CO2 and (b) H2O for (I) PrKAl and (II) KHCO3.

Figure 3 shows that most CO2 and H2O are released from the decomposition of KHCO3 within 12.4−29.5 min and there is only one peak appearing at 20.2 min. The releases of CO2 and H2O for Pr-KAl start at the beginning of the process and finish after 75.3 min. Three strong peaks appear at 16.99, 28.52, and 56.32 min, corresponding to the reaction temperatures of 128, 183.6, and 319.6 °C, respectively. Because the decomposition of KHCO3 is in the temperature range of 117.2−189.7 °C, the first CO2 and H2O released peaks are attributed to the decomposition of KHCO3 and the second CO 2 and H 2 O released peaks are attributed to the decomposition of KHCO3 and an intermediate product for the carbonation of K2CO3/Al2O3. The decomposition of KAl(CO3)2(OH)2 was reported in the temperature range of 260−350 °C;7 therefore, the third CO2 and H2O released peaks are attributed to the decomposition of KAl(CO3)2(OH)2. Although only KHCO3 and KAl(CO3)2(OH)2 are examined with XRD in the carbonation product of K2CO3/Al2O3, an intermediate product is assumed to exist. The reason is that the regeneration process is divided as three stages. The decomposition temperature of this intermediate product is assumed to be higher than that of KHCO3 and lower than that of KAl(CO3)2(OH)2. As reported previously,18 after K2CO3 had been loaded on Al2O3, the total surface area and pore volume were greatly increased and the active components were uniformly distributed on the surface of Al2O3 in the form of many small aggregates. As a result, the decomposition temperature of KHCO3 is lower for Pr-KAl than pure KHCO3. 4.2. Effect of the Final Temperature on Regeneration. To choose a proper regeneration temperature to decrease the

Figure 5. Effect of the final temperature in N2 on CO2 release for (a) Pr-KAl and (b) KHCO3.

Figure 4 shows that η of Pr-KAl increases from 76.6 to 100% in 30 min as the final temperature increases from 150 to 300 °C, while η of KHCO3 reaches 100% in 20 min when the final temperature is higher than 180 °C. η of Pr-KAl increases to 90.9% in 20 min for the final temperature of 200 °C. As shown in Figure 5, for KHCO3, there is only one peak of CO2 released for all reaction conditions. The test results for Pr-KAl are similar to that in Figure 3. Only one peak for CO2 release 1408

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appears when the final temperature is 150 °C, which means that only KHCO3 is decomposed. The second peak appears when the final temperature is 180 °C, which means that the intermediate product assumed above is starting to be decomposed. The third peak appears at the final temperature of 200 °C, and the value increases as the final temperature increases. As a result, it can be concluded that the decomposition of KAl(CO3)2(OH)2 is in the temperature range of 200−300 °C in this reaction condition. 4.3. Effect of the Gas Composition on Regeneration. Because the regeneration of the sorbent is in a pure CO2 or CO2/H2O atmosphere in real operation, it is important to study the effect of CO2 and CO2/H2O atmospheres on the regeneration process. In comparison to that in pure N2, the regeneration of Pr-KAl and KHCO3 in the CO2 atmosphere was carried out at different final temperatures with a heating rate of 20 °C/min. The change of η with time is shown in Figure 6.

significant. For KHCO3, η is only 26.7% in 12 min and then decreases to 23.3% in 17 min when the final temperature is 150 °C. The reason is that the reverse reaction of regeneration occurs in this reaction condition. η is 91.7% in 24 min when the final temperature is 180 °C, and it is lower than that in the N2 atmosphere. The effect of the CO2 atmosphere on the regeneration process is not significant when the final temperature is higher than 200 °C. The regeneration of Pr-KAl and KHCO3 in the 50% CO2/ 50% H2O atmosphere was carried out at different final temperatures with a heating rate of 20 °C/min (the test was carried out in the CO2 atmosphere at the beginning, and then the gas composition was changed to 50% CO2/50% H2O when the temperature was higher than 100 °C). The change of η with the increase of the final temperature in the 50% CO2/50% H2O atmosphere is similar to that in a pure CO2 atmosphere. To compare the regeneration behaviors of KHCO3 and Pr-KAl in different gas compositions, the regeneration conversions in different reaction conditions are listed in Tables 1 and 2. Because the reaction temperature is increased to a final temperature at first and then is kept constant, the total regeneration conversion is divided into two parts for each reaction condition. The first part is in the heating process, and the other part is in the isothermal process. As shown in Table 1, the effect of the CO2 and CO2/H2O atmospheres on the regeneration of KHCO3 is significant when the final temperature is lower than 200 °C. The regeneration conversion decreases in the heating process but increases in the isothermal process for each reaction condition when the gas composition is changed from the N2 atmosphere to the CO2 and CO2/H2O atmospheres. These are attributed to the change of the concentration driving force. The regeneration conversion is nearly 100% in the heating process when the final temperature is higher than 200 °C. Table 2 shows that the regeneration behaviors for Pr-KAl in the CO2 and CO2/H2O atmospheres are similar to the regeneration behaviors for Pr-KAl in the N2 atmosphere. The reaction is mainly carried out in the heating process, and the conversion in the isothermal process is low. The reason is deduced to be that the change in active component distribution behavior leads to the different regeneration behaviors. 4.4. Effect of the Heating Rate on Regeneration. The heating rate is another key factor affecting the regeneration process. The regeneration of Pr-KAl and KHCO3 in pure N2 was carried out at the final temperature of 300 °C with various heating rates in the range of 5−80 °C/min. The changes of η and the CO2 release with time are shown in Figures 7 and 8, respectively. As shown in Figure 7, both η and the reaction time of the sorbents of Pr-KAl and KHCO3 decrease as the heating rate increases. For Pr-KAl, η decreases from 99.4 to 92.7% in the whole regeneration process and the reaction time decreases

Figure 6. Effect of the final temperature in CO2 on regeneration conversion for (a) Pr-KAl and (b) KHCO3.

Figure 6 shows that the change of η with the increase of the final temperature is similar to that in the N2 atmosphere. For Pr-KAl, η increases from 73.6 to 97.7% in 15 min as the final temperature increased from 150 to 300 °C. In comparison to the result of the same final temperature in the N2 atmosphere, the conversion curves are similar and the total regeneration conversion only decreases for about 2.3−3.0%. The effect of the CO2 atmosphere on the regeneration of Pr-KAl is not

Table 1. Total Regeneration Conversions of KHCO3 in Different Reaction Conditions

N2 atmosphere CO2 atmosphere CO2/H2O atmosphere

in in in in in in

final temperature

150 °C

180 °C

200 °C

250 °C

300 °C

heating process isothermal process heating process isothermal process heating process isothermal process

40.7% 59.3% 20.1% 3.2% 20.6% 3.4%

71.7% 28.3% 54.9% 36.9% 54.6% 37.3%

74.7% 25.3% 66.2% 32.4% 66.3% 32.6%

98.6% 1.4% 98.7% 1.3% 98.6% 1.4%

98.8% 1.2% 99.2% 0.8% 98.7% 1.3%

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Table 2. Total Regeneration Conversions of Pr-KAl in Different Reaction Conditions

N2 atmosphere CO2 atmosphere CO2/H2O atmosphere

in in in in in in

final temperature

150 °C

180 °C

200 °C

250 °C

300 °C

heating process isothermal process heating process isothermal process heating process isothermal process

53.8% 22.8% 51.7% 23.3% 51.6% 23.4%

62.9% 20.6% 59.0% 21.7% 58.9% 21.9%

73.4% 17.5% 71.4% 17.5% 71.3% 17.6%

83.2% 14.0% 75.8% 19.3% 75.6% 19.6%

91.5% 8.5% 86.0% 12.8% 86.1% 12.7%

it is a good choice to increase the heating rate to a proper value. Figure 8 shows that the peak for CO2 release of KHCO3 becomes wider and the value of the peak becomes lower as the heating rate decreases. The absorbance of the peak increases as the heating rate increases. The change of CO2 release curves for Pr-KAl is similar, and the amount of peak decreases from 3 to 2 when the heating rate is higher than 35 °C/min. The reason is deduced that, for the slow heating process, the decomposition of those products is carried out one by one as the temperature increases from a low value to a high value. For the rapid heating process, the decomposition of those products is carried out at the same time as the temperature quickly increases to the value needed. The total amount of CO2 release is limited by the short reaction time.

5. CONCLUSION The regeneration behaviors of K2CO3/Al2O3 were investigated in detail with a TGA−FTIR system. The regeneration process of K2CO3/Al2O3 consists of three steps as the temperature increases, and it can be completely regenerated in the N2 atmosphere when the final temperature is 300 °C. Except for CO2 and H2O, there is no other gas product released in the regeneration process of K2CO3/Al2O3. The regeneration behaviors of K2CO3/Al2O3 in the CO2 and CO2/H2O atmospheres are similar to those in the N2 atmosphere. Because the effect of gas composition on the regeneration process is not significant, the regeneration of sorbent can be carried out in a pure CO2 or CO2/H2O atmosphere in real operation. The regeneration reaction time is greatly reduced, and the total regeneration conversion decreases less than 10% when the heating rate increases from 5 to 80 °C/min. Therefore, it is a good choice to increase the heating rate to a proper value. Furthermore, the regeneration behaviors of K2CO3/Al2O3 should be investigated in a fluidized-bed reactor and a system of CO2 removal with continuous sorbent regeneration in the near future.

Figure 7. Effect of the heating rate on regeneration conversion for (a) Pr-KAl and (b) KHCO3.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-25-83793453. Fax: +86-25-83793453. E-mail: [email protected].

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Figure 8. Effect of the heating rate on CO2 release for (a) Pr-KAl and (b) KHCO3.

ACKNOWLEDGMENTS

Financial support from the National High Technology Research and Development Program of China (2009AA05Z311), the National Natural Science Foundation (50876021), the National Basic Research Program of China (2011CB707301), and the Scientific Research Foundation of Graduate School of Southeast University (YBJJ1001) is sincerely acknowledged.

from 67 to 7.3 min as the heating rate increases from 5 to 80 °C/min. For KHCO3, η decreases from 100 to 95.3% in the whole regeneration process and the reaction time decreases from 35 to 3.2 min. The decrease of η is only 6.7%, but the reaction time is greatly reduced for about 1 h. This implies that 1410

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REFERENCES

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