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International Journal of Greenhouse Gas Control 5 (2011) 1184–1189

Contents lists available at ScienceDirect

International Journal of Greenhouse Gas Control

journa l homepage: www.e lsev ier .com/ locate / i jggc

tudy on the failure mechanism of potassium-based sorbentor CO2 capture and the improving measure

e Wu, Xiaoping Chen ∗, Chuanwen Zhaochool of Energy & Environment, Southeast University, Nanjing 210096, China

r t i c l e i n f o

rticle history:eceived 8 December 2010eceived in revised form 10 May 2011ccepted 28 May 2011vailable online 2 July 2011

a b s t r a c t

With thermogravimetric apparatus (TGA), X-ray diffraction (XRD) and barium sulfate gravimetric meth-ods, the carbonation reactivities of K2CO3 and K2CO3/Al2O3 in the simulated flue gases with SO2 areinvestigated and the reaction equations are inferred. Results show that there are KHCO3 and K2SO3

generated. The generation K2SO3 reduces the utilization ratio of the sorbent. H2O may accelerates thesulfation reaction of AR K2CO3 as K4H2(CO3)3·1.5H2O is generated in the reaction among K2CO3, SO2 andH O. K SO is directly generated from sulfation reaction of K CO /Al O , because there are K CO ·1.5H O

eywords:O2 capture2CO3

2CO3/Al2O3

arbonationO2

2 2 3 2 3 2 3 2 3 2

and K2SO3 generated in the reaction among K2CO3/Al2O3, SO2 and H2O. K2CO3·1.5H2O does not react withSO2, and K2CO3·1.5H2O/Al2O3 reacts with SO2 slowly. Compare with the reaction process without H2Opretreatment, the reaction rates of KAl30 increased after H2O pretreatment and the failure ratio is abouta half of that without H2O pretreatment. So, K2CO3/Al2O3 shows good carbonation and anti-sulfationcharacteristic after H2O pretreatment.

2SO3

. Introduction

Global warming is picking up as a result of emission of green-ouse gases. The CO2 emissions may contribute to increases in sea

evel and increased frequency and intensity of climatic extremes,o, it is thought to be one of the key environmental issues in thearly 21st century. In 2005, Kyoto Protocol reducing the emissionf greenhouse gas became effective. As a result, many countriesay more attention to the research of carbon dioxide emissioneduction. The main fuel in our country is coal, which emits CO2hen burnt out. So, it is extremely important to study and developO2 emission reduction technology in coal-fired power plant. Post-ombustion CO2 separation technology has a wide range of marketrospects in our country. Dry alkali metal-based sorbent (Zhaot al., 2009a,b; Liang et al., 2004; Liang, 2003; Green et al., 2002,001; Hoffman and Pennline, 2001; Yi et al., 2007) for CO2 capture

s one of the research focuses currently.Dry alkali metal-based sorbent such as Na2CO3, K2CO3 are sor-

ents to remove CO2. They are of great concern because they arenexpensive, low energy demand, no corrosion to device, no sec-ndary pollution and have high activity with CO2. Funded by the

OE, the RTI, C & D, LSU (Liang et al., 2004; Liang, 2003; Greent al., 2002, 2001; Hoffman and Pennline, 2001) engaged in theesearch of sodium-based sorbents for removing CO2 from gas. Lee

∗ Corresponding author. Tel.: +86 25 83793453; fax: +86 25 83793453.E-mail address: xpchen@seu.edu.cn (X. Chen).

750-5836/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.oi:10.1016/j.ijggc.2011.05.034

© 2011 Elsevier Ltd. All rights reserved.

and Ryu (Yi et al., 2007, 2006; Lee et al., 2006a,b; Ryu et al., 2006;Seo et al., 2007, 2005) in Korea load K2CO3 in many supports to getthe characteristics of decarburization in the fixed bed, bubbling-bedand circulating fluidized bed. In China, Southeast University (Zhaoet al., 2010a,b,c) has also made a thorough study of the carbonationreaction mechanism of K2CO3. Actually, there is a small amount ofacid gas (SO2, NO and so on) in flue gas of coal-fired power plant,however, the failure mechanism by the acid gas on the process ofCO2 capture using dry potassium-based sorbents has not yet beenreported.

In order to solve this problem, this paper started with the reac-tion characteristics of analytical reagent K2CO3 (AR K2CO3) andfocused on the reaction characteristics of K2CO3/Al2O2 samples,the K2CO3 loading amount is 30%, and found out the two sampleshave different failure mechanisms. From the analysis of compo-sition of the products, detailed studies of the reaction processeswere obtained, and the failure inhibition way of potassium-basedsorbent for CO2 capture is got.

2. Experimental

2.1. Samples

Analytical reagents KHCO3 is 99.5% pure. Analytical reagents

K2CO3 is 99.5% pure with an average particle size of 300 �m. andthe particle size of activated alumina (Al2O3) is 250–350 �m. Thesorbent K2CO3/Al2O3(KAl30) is made up of the two materials, andthe K2CO3 loading amount is 30%.

Y. Wu et al. / International Journal of Greenhouse Gas Control 5 (2011) 1184–1189 1185

Table 1Different working conditions.

Sample CO2 H2O SO2 N2

1 AR K2CO3 15 15 0 70

2

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3K2CO3 + 2.5H2O + SO2 → K4H2(CO3)3 · 1.5H2O + K2SO3 (2)

Table 2Different working conditions.

Sample CO2 H2O SO2 N2

2 AR K2CO3 15 15 0.05 69.953 KAl30 15 15 0 704 KAl30 15 15 0.05 69.95

.2. Apparatus and procedure

The reactions were studied with the TherMax 500 high pressureGA, The calcination tests of KHCO3 samples were in the atmo-phere of pure N2 at 200 mL min−1. The reactor was heated at thepeed of 20 K min−1 from 293 K to 473 K. The carbonation tests of2CO3 and K2CO3/Al2O3(KAl30) samples were processed in theas with the composition of CO2, H2O, SO2 and N2 at a flow of000 mL min−1 and at a temperature of 333 K.

The composition of reaction products were detected with a/max2500 VL/PC X-ray diffractometry, the sulfur content wasetected with barium sulfate gravimetric method.

. Experimental results and discussion

.1. Effects of SO2 on the carbonation process of AR KAl30 andAl30

Based on the regulations of our country, the SO2 emission of theas ranges from 140 to 840 ppm in different areas after FGD. As aesult, the TGA tests conditions are given in Table 1. The results arehown in Fig. 1(a) and (b).

Compared with the processes of cases 1, 2, 3 and 4 in Fig. 1. It cane determined that the weight of the reaction products is changed

n different atmospheres. As a result, the participation of SO2 mayffect the reaction of carbonation of the sorbent.

In order to understand the characterization of the chemical com-osition of the products, the reaction products of cases 2 and 4 areetected with XRD and the result is shown in Fig. 2(a) and (b).

It can be seen from Fig. 2 that K2SO3 and KHCO3 are generated,hich means the generation of K2SO3 reduces the utilization ratio

f the sorbent.The reaction products of cases 2 and 4 are detected by barium

ulfate gravimetric method. And the failure ratio of the sorbent inase 2 is 10.2% after about 20 min, while the sulfur content of theorbent in case 4 is 3.35%, and the failure ratio �S which can bealculated from Eq. (1) is 48.15% after about 30 min.

S = am

wnCO(1)

here nCO is the amount of K2CO3 before reaction, mol; a is theulfur content of the product, %; m is the weight of the product, g;

is the molecular weight of SO2, g/mol.

.2. The formation mechanism of K2SO3

There may be three formation ways of K2SO3. First, it maye generated from K2CO3 and SO2. Second, it may be gener-ted from K2CO3, SO2 and H2O. Third, it may be generated from2CO3·1.5H2O, SO2 and H2O. In order to determine the formationechanism of K2SO3, TGA tests conditions are given in Table 2.It can be seen in Fig. 3 that the reaction rate of the sorbent in

ase 5 is slow. It can be determined that, K2SO3 can be generatedn the reaction between AR K2CO3 and SO2.

The reaction curve of case 7 is shown in Fig. 4, and the reactionroduct of this case is K2SO3, which is detected by XRD. It can beeen from the curve in Fig. 4 that, the reaction goes on slowly. Afterbout 240 min, the weight increased by 6 mg. The conversion ratio

Fig. 1. (a: AR K2CO3, b: K2CO3/Al2O3) The results of carbonation process.

of the sorbent in case 7 is shown in Fig. 5, the conversion rate ofK2CO3 reaches 90% in 240 min. It can be determined that, K2SO3(Fig. 6) can be generated in the reaction between KAl30 and SO2.

However, the conversion ratio of the sorbent in case 5 is about12% after 90 min. While the failure ratio of the sorbent in case 1is 10.2% after about 20 min. Similarly, the conversion ratio of thesorbent in case 7 is 45% in 80 min shown in Fig. 5, while the failureratio of the sorbent in case 4 is 48.15% after about 30 min. So, theexistent of H2O may affect the sulfation reaction of K2CO3.

Fig. 7 is the reaction result of case 6. The reaction ended afterabout 60 min. Fig. 8 shows the XRD pattern of the productions.K2SO3, K4H2(CO3)3·1.5H2O and K2CO3·1.5H2O are detected in thereaction products of case 6. It is out of question that K2CO3·1.5H2Ois generated by hydration of K2CO3 (Zhao et al., 2009a). Based onthe chemical equilibrium method, K4H2(CO3)3·1.5H2O and K2SO3are generated in the reaction among K2CO3, SO2 and H2O, and thereaction equation is shown below.

5 AR K2CO3 0 0 0.05 99.956 AR K2CO3 0 15 0.05 84.957 KAl30 0 0 0.05 99.958 KAl30 0 15 0.05 84.95

1186 Y. Wu et al. / International Journal of Greenhouse Gas Control 5 (2011) 1184–1189

F

JKi

K

Ai

Ku

Fig. 4. Result of reaction of case 3.

ig. 2. (a: AR K2CO3, b: K2CO3/Al2O3) The XRD pattern of carbonation production.

ust as is referred in the literature (Seo et al., 2007),4H2(CO3)3·1.5H2O reacts with CO2 rapidly. The reaction equation

s shown below.

4H2(CO3)3 · 1.5H2O + CO2 → 4KHCO3 + 0.5H2O (3)

s a result, with the reaction (3) going on, the amount of K2SO3 isncreasing.

Fig. 9 shows the result of reaction of case 8. Except K2SO3 and2CO3·1.5H2O, there is no K4H2(CO3)3·1.5H2O in the reaction prod-cts of cases 8 (Fig. 10). K2SO3 is mainly generated from the reaction

Fig. 3. Result of reaction of case 1.

Fig. 5. the conversion rate of K2CO3 in KAl30.

of KAl30 and SO2. The existence of H2O accelerates the sulfationreaction of K2CO3.

3.3. The effect of H2O pretreatment on the sorbents

Based on the study above, K2CO3·1.5H2O is existed in the prod-ucts of cases 6 and 8. As a result, the generation of K2CO3·1.5H2O

Fig. 6. The XRD result of reaction products.

Y. Wu et al. / International Journal of Greenhouse Gas Control 5 (2011) 1184–1189 1187

Fig. 7. Result of reaction case 2.

mt

Kf

Fig. 10. The XRD result of reaction products.

Table 3Different working conditions.

Sample CO2 H2O SO2 N2

9 AR K2CO3 after H2O pretreatment 0 0 0.05 99.95

Fig. 8. The XRD pattern of productions of case 2.

ay offer inhibition way of potassium-based sorbent for CO2 cap-ure.

In order to explore the reaction condition between2CO3·1.5H2O and SO2, TGA tests of hydration and sulfation

or AR K2CO3 and KAl30 were processed, in the hydration process

Fig. 9. Result of reaction of case 4.

10 K2CO3/Al2O3 after H2O pretreatment 0 0 0.05 84.9511 K2CO3/Al2O3 after H2O pretreatment 15 15 0.05 69.95

the gas composition is 15% H2O + 85% N2 at a flow of 1000 mL min−1

and at a temperature of 333 K, when the reaction ended, the gascomposition was changed to 15% H2O + 0.05% SO2, with a balanceof N2, detail tests conditions are shown in Table 3, and the resultis shown in Figs. 11 and 12, the products of case10 was detectedwith XRD method, and the XRD result is shown in Fig. 13.ARK2CO3·1.5H2O cannot react with SO2 after H2O pretreatment,which is shown in Fig. 11, while KAl30 after H2O pretreatment canreact with SO2 slowly, and K2SO3 is detected in the products incase 10.

It can be seen from Fig. 12 that the weight increased by 3 mgafter about 240 min, which is 50% lighter than that of case 7, as aresult, the conversion ratio of the sorbent in cases 7 and 10 is shownin Fig. 14.

It can be seen from Fig. 14 that the conversion ratio of the sor-bent in case 10 is 50% of that in case 7 at the same time, whichmeans KAl30 shows good anti-sulfation characteristic after H2O

pretreatment. As a result, tests of KAl30 after H2O pretreatmentwere processed in the gas composed of 15% CO2 + 15% H2O + 0.05%SO2 and N2-balance at a flow rate of 1000 mL min−1 and at a tem-

Fig. 11. Result of action of case 9.

1188 Y. Wu et al. / International Journal of Greenhouse Gas Control 5 (2011) 1184–1189

Fig. 12. Result of reaction of case 10.

pcm

ric

Fig. 13. The XRD result of reaction products.

erature of 333 K. The result is shown in Fig. 15, and the sulfurontent in the products was detected by barium sulfate gravimetricethod.In Fig. 15, the green curve represent case 2 and the red curve

epresent cases 11. (For interpretation of the references to colorn this text, the reader is referred to the web version of this arti-le.) It can be determined that the reaction rates of KAl30 increased

Fig. 14. The conversion ratio of K2CO3·1.5H2O/Al2O3 and KAl30.

Fig. 15. The reaction process of case 11.

after H2O pretreatment. Because the weight increased faster whenthe sorbent is pretreated with H2O vapour. At the top of that, thesulfur content of the sorbent in case 11 is 1.73%, while the sul-fur content of KAl30 without H2O pretreatment is 3.35%. So, thefailure ratio of the sorbent with or without H2O pretreatment is24.86% and 48.15%, respectively, the utilization ratio of the sor-bent is 75.14% and 51.85%, respectively. As a result, K2CO3/Al2O3shows good carbonation and anti-sulfation characteristic after H2Opretreatment.

4. Conclusion

The characteristics of potassium-based sorbents for CO2 capturein the atmosphere of 500 ppm SO2 was investigated with TGA andXRD. The results are shown as follows:

(1) The products of AR K2CO3/KAl30 in the carbonation atmospherewith SO2 are KHCO3 and K2SO3. And the generation of K2SO3reduces the utilization rate of the adsorbent.

(2) In the reaction between K2CO3/KAl30 and SO2, only K2SO3is detected in the products. The existence of H2O acceler-ates the sulfation reaction of K2CO3, because in the reactionamong AR K2CO3, SO2 and H2O, the intermediate active productK4H2(CO3)3·1.5H2O is generated, while in the reaction amongKAl30, SO2 and H2O, only K2SO3 is found in the products. So,the formation mechanisms of K2SO3 of AR K2CO3 and KAl30 aredifferent. K2SO3 is directly generated from sulfation reaction ofK2CO3/Al2O3.

(3) K2CO3/Al2O3 shows good carbonation and anti-sulfation char-acteristic after H2O pretreatment, which can slow down thefailure rate of the sorbent.

References

Green, D.A., Turk, B.S., Portzer, J.W., et al., 2001. Carbon Dioxide Capture fromFlue Gas using Dry Regenerable Sorbents, Quarterly Technical Progress Report.Research Triangle Institute, North Carolina.

Green, D.A., Turk, B.S., Portzer, J.W., et al., 2002. Capture of carbon dioxide from fluegas using solid regenerable sorbents. In: 19th Annual International PittsburghCoal Conference , Pittsburgh, USA (CD-ROM).

Hoffman, J.S., Pennline, H.W., 2001. Study of regenerable sorbents for CO2 capture.Energy Environment Research 1 (1), 90–100.

Lee, S.C., Choi, B.Y., Lee, T.J., Ryu, C.K., Ahn, Y.S., Kim, J.C., 2006a. CO2 absorption andregeneration of alkali metal-based solid sorbents. Catalysis Today 111, 385–390.

Lee, S.C., Choi, B.Y., Lee, T.J., Ryu, C.K., Ahn, Y.S., Kim, J.C., 2006b. The effect of wateron the activation and the CO2 capture capacities of alkali metal-based sorbents.Korean Journal of Chemical Engineering 23 (3), 374–379.

Liang, Y., 2003. Carbon Dioxide Capture from Flue Gas using Regenerable Sodium-based Sorbents. Louisiana State University, Baton Rouge.

reenh

L

R

S

S

Y

Y. Wu et al. / International Journal of G

iang, Y., Harrison, D.P., Gupta, R.P., et al., 2004. Carbon dioxide capture using drysodium-based sorbents. Energy & Fuels 18 (2), 569–575.

yu, C.K., Lee, J.B., Eom, T.H., Baek, J.I., Eom, H.M., Yi, C.K., 2006. CO2 capture fromflue gas using dry regenerable sorbents. In: 8th International Conference onGreenhouse Gas Control Technology , Trondheim, Norway (CD-ROM).

eo, Y.W., Jo, S.H., Ryu, C.K., Yi, C.K., 2005. Effects of steam and temperature on CO2

capture using a dry regenerable sorbent in a bubbling fluidized bed. KoreanChemical Engineering Research 43 (4), 537–541.

eo, Y.W., Jo, S.H., Ryu, C.K., Yi, C.K., 2007. Effects of steam and temperature on CO2

capture using a sodium-based solid sorbent in a bubbling fluidized-bed reactor.Chemosphere 69 (5), 712–718.

i, C.K., Jo, S.H., Seo, Y., Park, S.D., Moon, K.H., Yoo, J.S., Lee, J.B., Ryu, C.K., 2006.CO2 capture characteristics of dry sorbents in a fast fluidized reactor. Studies inSurface Science and Catalysis 159, 501–504.

ouse Gas Control 5 (2011) 1184–1189 1189

Yi, C.K., Jo, S.H., Seo, Y.W., Lee, J.B., Ryu, C.K., 2007. Continuous operation of thepotassium-based dry sorbent CO2 capture process with two fluidized-bed reac-tors. Greenhouse Gas Control 1, 31–36.

Zhao, C., Chen, X., Zhao, C., 2009a. Carbonation and hydration characteristics of drypotassium-based sorbents for CO2 capture. Energy & Fuels 23 (3), 1766–1769.

Zhao, C., Chen, X., Zhao, C., 2009b. CO2 absorption using dry potassium-based sor-bents with different supports. Energy & Fuels 23, 4683–4687.

Zhao, C., Chen, X., Zhao, C., 2010a. Multiple cycles behaviour of K2CO3/Al2O3 for CO2

capture in a fluidized-bed reactor. Energy & Fuels 24 (2), 1009–1012.Zhao, C., Chen, X., Zhao, C., 2010b. Study on CO2 capture using dry potassium-based

sorbents through orthogonal test method. International Journal of GreenhouseGas Control 4 (4), 655–658.

Zhao, C., Chen, X., Zhao, C., 2010c. Carbonation behavior of K2CO3 with differentmicrostructure used as an active component of dry sorbents for CO2 capture.Industrial & Engineering Chemistry Research 49 (23), 12212–12216.