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

Jan 12, 2012 - Kyeongsook Kim , Seugran Yang , Joong Beom Lee , Tae Hyoung Eom , Chong Kul Ryu , Ha-Jin Lee , Tae-Sung Bae , Young-Boo Lee , Se-Jin ...
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K2CO3/Al2O3 for Capturing CO2 in Flue Gas from Power Plants. Part 1: Carbonation 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 first 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 carbonation behaviors of K2CO3/Al2O3 in a thermogravimetric analyzer. The effects of the temperature, gas composition, and pressure on the reactions were studied by analyzing the experimental breakthrough data. It was found that K2CO3/Al2O3 shows a high CO2 capture capacity and its carbonation conversion reaches 68.3−91.8% in 20 min when the reaction temperature is in the range of 55−75 °C, the CO2 concentration is in the range of 5−20%, the H2O concentration is in the range of 12−21%, and the pressure is 0.1 MPa. The total carbonation conversion mainly depends upon the reaction in the first 5 min, and the reaction rate reaches the maximum in about 2 min. The total carbonation conversion increases with the increase of CO2 and H2O concentrations but decreases with the increase of the temperature and pressure. Among the factors studied, the H2O concentration and pressure were found to have a significant impact on the carbonation. Moreover, water pretreatment of the sorbent plays an important role in the carbonation reaction.

1. INTRODUCTION Applying dry alkali-metal-based sorbents for capturing CO2 of flue gas has been recently investigated as an innovative concept.1−3 Various sorbents have been proposed and developed for the application. Previous studies found that CO2 capture capacity was greatly dependent upon the sorbents used and the operation conditions.4−12 Some common problems were confronted with most of the sorbents proposed in the studies. For example, the global carbonation reaction rates of the alkali-metal-based sorbents were generally slow, resulting in slow carbonation conversion of the sorbents. The appropriate operation conditions for carbonation and regeneration reactions were not confirmed. To develop an efficient sorbent and also to solve the problems above, extensive research works were performed in our group. Our previous studies13,14 found that K2CO3 generated from calcining KHCO3 had excellent carbonation capacity. Furthermore, when loading this sorbent as the active component on γ-Al2O3, the resulting K2CO3/Al2O3 sorbent had a fast carbonation reaction rate and achieved high carbonation conversion.15,16 A preliminary study showed that K2CO3/Al2O3 had excellent CO2 capture capacity and regeneration performance in a bubbling fluidized-bed reactor during 10 cycles,17 indicating that K2CO3/Al2O3 has potential to be used as a sorbent for a large-scale CO2-capture process. For the sake of developing this sorbent for industrial application, several key issues should be solved thoroughly. First, understanding the effects of operation conditions, including the temperature, gas composition, heating rate, and pressure, on the carbonation and regeneration reactions are essential for describing the carbonation behaviors and determining appropriate reaction conditions of this sorbent. Second, the loading amount of the active component is a crucial parameter for the sorbent, and the attrition resistance performance of the sorbent is also substantial for its application © 2012 American Chemical Society

in the fluidized-bed reactor. Therefore, an embedded study on them is highly needed. Third, concerning the application of K2CO3/Al2O3 in practice, the CO2 capture capacity and regeneration property in the fluidized-bed reactor should be investigated by long-time and multi-cycle operation. Lastly, the effect of impurity gases, including SO2, NO, and HCl, existing in the flue gas on the carbonation reaction needs to be under consideration. Aiming at clarifying the above issues, a systematical investigation was carried out to investigate CO2-capture behaviors of K2CO3/Al2O3. The carbonation and regeneration behaviors of K2CO3/Al2O3 were investigated with thermogravimetric analysis (TGA). A bubbling fluidized-bed reactor was used to study the effect of the loading amount, SO2, and NO in the flue gas on the CO2-capture behavior, 80 carbonation/ regeneration cycle behaviors, and abrasion characteristics of K2CO3/Al2O3 particles. The results will be reported in a series of papers. The present paper is the first part, which is addressed to the carbonation behaviors of K2CO3/Al2O3, investigated using a pressurized TGA approach.

2. EXPERIMENTAL SECTION 2.1. Samples. K2CO3/Al2O3 used in this study was prepared by impregnating K2CO3 on γ-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 process of the sorbent consisted of three steps: mixed and impregnation, dried at 105 °C for dehydration, and calcined at 300 °C. The loading amount of K2CO3 was determined to be 28.5 wt %. The generated K2CO3/Al2O3 sorbent is in the form of particles in the size range of 10−30 μm, with a mean size of 20 μm. Its surface area and pore volume are 71.4 m2/g and 0.27 cm3/g, respectively, and the mean pore size is 13 nm. More Received: March 1, 2011 Revised: August 16, 2011 Published: January 12, 2012 1401

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information about the method of the sorbent preparation and its microscopic structure characterization were reported in detail in a previous paper.15 The amount of alkali metal impregnated was determined by an Advant’XP X-ray fluorescence (XRF). An accelerated surface area and porosimetry (ASAP) 2020 system with N2 adsorption−desorption was used for surface area and pore structure determinations. 2.2. Apparatus and Procedure. Experimental studies for carbonation of K2CO3/Al2O3 were performed with TherMax 500 TGA in simulating flue gas composed of CO2, H2O, and a balance of N2. The gas flow rate was set to be 500 mL/min. The mass of the sorbent loaded in TGA was about 100 mg. Considering the high water adsorption capacity of K2CO3/Al2O3, the fresh sorbent was first heated to 200 °C for dehydration and then the temperature decreased to a certain value for the carbonation reaction. The details about this experimental system were reported in our previous paper.17 To determine the optimum reaction condition for carbonation, the carbonation of K2CO3/Al2O3 was carried out under a wide range of reaction conditions by varying the carbonation temperature between 55 and 80 °C, the CO2 concentration between 5 and 20%, the H2O concentration between 0 and 21%, and the operation pressure between 0.1 and 0.5 MPa. As confirmed by X-ray diffraction (XRD) analysis in the previous study,15 the main carbonation product of K2CO3/Al2O3 was KHCO3. On the basis of the theoretical value increment corresponding to the complete conversion of K2CO3 to KHCO3, the carbonation conversion (η) and reaction rate (rc) are calculated as

η=

rc =

MK2CO3(w(t ) − w(0)) αw(0)(2MKHCO3 − MK2CO3) MK2CO3

dw dt

αw(0)(2MKHCO3 − MK2CO3)

Figure 1. Effect of the temperature on (a) carbonation conversion and (b) reaction rate.

when the reaction was carried out in 60−80 °C. In contrast, Na2CO3 calcined from NaHCO3 was found to have better reactivity than other sodium-based sorbents.21 Its total conversion was 76% in 200 min with NaHCO3 as the product when the reaction temperature was 60 °C. The conversion was 95% in 200 min with Wegscheider’s salt as the product when the reaction temperature was 70 °C. No carbonation occurred at 80 °C when the CO2 concentration was 5%.3 While the carbonation temperature range of the sodium-based sorbents was generally narrow, that of the potassium-based sorbents was found relatively wider. With 2 h of reaction, the weight of K2CO3 increased 26, 12, and 6% when the carbonation temperatures were 60, 80, and 100 °C, respectively.22 The carbonation reactivity of K2CO3 calcined from KHCO3 was better than K2CO3, and the total conversion was 74−83% in 40 min when the temperature was in the range of 60−70 °C.23 In comparison to the carbonation behaviors of all of the abovementioned sorbents reported in the literature, the carbonation conversion and reaction rate of K2CO3/Al2O3 shown in Figure 1 are higher at the same reaction condition. What is especially significant is that the reaction rate in the first 5 min was much higher. As reported previously,16 after K2CO3 had been loaded on Al2O3, the total surface area and pore volume of the sorbent 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 CO2 capture capacity increased significantly. 3.2. Effect of the CO2 Concentration on Carbonation. Figure 2 shows the effect of the CO2 concentration on the carbonation conversion and reaction rate of K2CO3/Al2O3 in the same condition of 65 °C and 15% H2O at 0.1 MPa. As shown in panels a and b of Figure 2, when the CO2 concentration increases from 5 to 20%, η increases from 67.9 to 76.9% in 25 min and the maximum reaction rate increases from 6.9 to 28.7%/min. It means that the effect of the CO2 concentration is not significant on the total carbonation conversion but is significant on the reaction rate. These are attributed to the change of the concentration driving force.

× 100% (1)

× 100% (2)

where t is the reaction time, w(t) and w(0) are the weights of the sorbent at time t and at the beginning of carbonation, respectively, α is the K2CO3 loading amount of the sorbents, dw/dt is the change of the weight of the sorbent with time, and MK2CO3 and MKHCO3 are the molecular weights of K2CO3 and KHCO3, respectively.

3. RESULTS AND DISCUSSION 3.1. Effect of the Reaction Temperature on Carbonation. The carbonation of K2CO3/Al2O3 in the simulating flue gas with the composition of 15% CO2, 15% H2O, and N2 balanced at 0.1 MPa was carried out at different temperatures. The carbonation conversion (η) and the reaction rate (rc) changing with the reaction time are presented in Figure 1. Figure 1a shows that the total conversion η increases with the reaction time for all reaction temperatures. However, η decreases with the temperature increasing. It can be seen that η reaches 68.3−91.2% in 20 min when the reaction temperature is in the range of 55−75 °C, while η is only 57.9% in 18 min when the reaction temperature is 80 °C. The low carbonation conversion at a higher temperature is attributed to the reduction in the concentration driving force because the carbonation reaction is reversible and highly exothermic. Figure 1b shows that the total carbonation conversion is mainly dependent upon the reaction in the first 5 min, and the reaction rate reaches a maximum at about 2 min and then decreases at all temperatures. For an alkali-metal-based sorbent, the temperature is an important factor for the carbonation reaction. The studies on sodium-based sorbents reported in the literature indicated that the carbonation reactivity of Na2CO3, Na2CO3·H2O, and Na2CO3 calcined from sodium sesquicarbonate was weak18−20 1402

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Figure 2. Effect of the CO2 concentration on (a) carbonation conversion and (b) reaction rate.

Figure 3. Effect of the H2O concentration on (a) carbonation conversion and (b) reaction rate.

Because the carbonation reaction is fast in the first 5−10 min, the total carbonation conversion is not significantly affected in 25 min. It was reported that24 the carbonation conversion and the reaction rate of Na2CO3 calcined from NaHCO3 increased in 250 min, when the CO2 concentration increased from 5 to 8%. This is consistent with the observation for K2CO3/Al2O3 here. These results emphasize the importance of choosing the CO2 concentration. In previous papers,4−6 the CO2 capture capacities of many potassium-based sorbents were studied in a fixed-bed reactor with the CO2 concentration of only 1% in reaction gas. Because the CO2 concentration is between 10 and 20% in the flue gas from power plants, the difference in the CO2-capture behavior may be caused by the CO2 concentration. 3.3. Effect of the H2O Concentration on Carbonation. The effect of the H2O concentration on the carbonation conversion and reaction rate of K2CO3/Al2O3 in the same condition of 65 °C and 15% CO2 at 0.1 MPa is shown in Figure 3. It can be seen that there was actually no reaction occurring for K2CO3/Al2O3 if no H2O was provided, implying that CO2 could not be adsorbed without the presence of H2O. Figure 3a shows that η increases significantly from 26.1 to 85.1% in 25 min when the H2O concentration in the gas mixture was increased from 3 to 21%. The total carbonation conversion only increases for 9.0% when the CO2 concentration increases from 5 to 20% in Figure 2a, while it increases for 59.0% when the H2O concentration increases from 3 to 21% in the same reaction time of 25 min. It indicates that the effect of the H2O concentration on the total carbonation conversion is more significant than that of the CO2 concentration. Figure 3b shows that the maximum reaction rate increases from 3.4 to 28.6%/ min when the H2O concentration increases from 3 to 15%, and then it remains at about 30%/min when the H2O concentration increases from 15 to 21%. The reason is that, for the carbonation of K2CO3/Al2O3, H2O is first adsorbed on the surface of the sorbent and then CO2 reacts with the adsorbed H2O and K2CO3 to produce KHCO3.16 In these processes, H2O adsorption is considered as the rate-controlling step. As

the H2O concentration increases, the diffusion and adsorption capacities of H2O in the sorbent are improved; therefore, the total carbonation conversion increases as the H2O concentration increases. When the H2O concentration is high enough, the effect of the H2O concentration change on carbonation will become not significant. 3.4. Effect of the Water Pretreatment on Carbonation. It was found that the carbonation capacity decreased after water pretreatment for both Na2CO3 calcined from NaHCO3 and K2CO3 calcined from KHCO3,13,14,19 while the carbonation capacity increased after water pretreatment for the alkali-metalbased sorbents, such as K2CO3/activated carbon (AC), sorbNX35, and sorbKX35.7,9,11 To confirm whether the CO2 capture capacity was excellent after water pretreatment for K2CO3/Al2O3, the effect of the water pretreatment on carbonation is shown in Figure 4.

Figure 4. Effect of the water pretreatment on the carbonation reaction.

For all of the cases shown in Figure 4, the sorbent was pretreated in 15% H2O for 30 min and then the gas composition was changed to 15% CO2 concentration and various H2O concentrations with N2 balance. It can be seen that the dimensionless weight only increases to 1.02 after water pretreatment. Additionally, no K2CO3·1.5H2O was detected in the product of K2CO3/Al2O3 by the XRD analysis. Therefore, the increase in the weight is believed to result from the physical 1403

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3.4%/min. It can be seen that, when the pressure is higher than 0.2 MPa, η is less than 50% in 35 min and the maximum reaction rate is less than 10%/min, implying that increasing the pressure significantly depresses the carbonation reaction. The main reason may be the decrease of the diffusion coefficient of water vapor in the sorbent with increasing the pressure. Because H2O adsorption is the rate-controlling step of the carbonation process of K2CO3/Al2O3, the slow diffusion rate of H2O certainly has a significant effect on the total carbonation conversion. Therefore, it is necessary to keep the pressure at atmospheric pressure for achieving a good carbonation performance. Traditional combustion-based systems for power generation are typically operated at near ambient pressures, while the advanced integrated gasification combined cycle systems are operated at elevated pressures.25,26 This implies that the K2CO3/Al2O3 sorbent may be more suitable for CO2 capture in traditional combustion-based systems.

adsorption of H2O. Although no H2O was continued to supply after water pretreatment, the dimensionless weight increased to 1.089 in 47 min, corresponding to 70.4% conversion of K2CO3 to KHCO3 in CO2 and N2. When the H2O concentration was increased, the dimensionless weight increased from 1.089 to 1.096 in 47 min, corresponding to the increase of η from 70.4 to 75.8%. The change in carbonation curves is less than the result in Figure 4 at the same reaction condition, suggesting that the water pretreatment plays an important role in the carbonation process. The study on the carbonation of K2CO3/AC found that,16 with water pretreatment, the hydration reaction occurred to form K2CO3·1.5H2O, which was rapidly transformed into KHCO3 once CO2 was provided. For K2CO3/Al2O3, H2O is physically adsorbed during water pretreatment and then the carbonation reaction is rapid when CO2 is given. However, after water pretreatment, the carbonation conversions decreased significantly for all analytical reagent samples of alkali-metalbased sorbents.13,14,19 The reason is deduced to be that, after loaded on Al2O3, the microstructure and the distribution behavior of K2CO3 are significantly changed. In the flue gas of power plants, the CO2 concentration is usually in the range of 10−20% and the H2O concentration can be as high as 8−17%. Because the concentrations of CO2 and H2O generally vary with time, it is hard to keep them with a constant stoichiometric ratio. It can be found from Figure 4 that the effect of the H2O concentration in the flue gas on the carbonation becomes insignificant for K2CO3/Al2O3 with water pretreatment. As a result, the sorbent can be regenerated in a H2O atmosphere in real operations. In this way, the CO2 capture capacity of K2CO3/Al2O3 will be improved. 3.5. Effect of the Pressure on Carbonation. The carbonation of K2CO3/Al2O3 was carried out at various pressures in the same reaction condition of 55 °C, 15% CO2, 15% H2O, and N2 balanced. Results are shown in Figure 5.

4. CONCLUSION The carbonation behaviors of K2CO3/Al2O3 were systematically investigated using TGA. K2CO3/Al2O3 shows high CO2 capture capacity, and the carbonation conversion reaches 68.3− 91.8% in 20 min under reaction conditions of simulated flue gas. The total carbonation conversion is mainly dependent upon the reaction in the first 5 min, and the reaction rate reaches the maximum at about 2 min. The total carbonation conversion increases with the increase of CO2 and H2O concentrations but decreases with the increase of the temperature and pressure. Water pretreatment of the sorbent plays an important role in the carbonation reaction. After water pretreatment, the effect of the H2O concentration in the flue gas on carbonation becomes not significant for K2CO3/Al2O3. As a result, the sorbent can be regenerated in a H2O atmosphere in real operations. In this way, the CO2 capture capacity of K2CO3/Al2O3 will be improved. It is necessary to keep the pressure at atmospheric pressure. This sorbent may be more suitable for capture of carbon dioxide in traditional combustion-based systems.

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AUTHOR INFORMATION

Corresponding Author

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

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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.

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Figure 5. Effect of the pressure on (a) carbonation conversion and (b) reaction rate.

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Figure 5 shows that, when the pressure increases from 0.1 to 0.5 MPa, η of K2CO3/Al2O3 decreases from 91.7 to 26.2% in 35 min and the maximum reaction rate decreases from 24.0 to 1404

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