Sodium-Based Dry Regenerable Sorbent for Carbon Dioxide Capture

Jun 5, 2008 - Global EnVironment Group, Korea Electric Power Research Institute, ... Sorb NX30 sorbent satisfied most of the physical requirements for...
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Ind. Eng. Chem. Res. 2008, 47, 4465–4472

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Sodium-Based Dry Regenerable Sorbent for Carbon Dioxide Capture from Power Plant Flue Gas Joong B. Lee,† Chong K. Ryu,*,† Jeom-In Baek,† Ji H. Lee,† Tae H. Eom,† and Sung Hyun Kim*,‡ Global EnVironment Group, Korea Electric Power Research Institute, 103-16 Munji-dong, Yuseong-ku, Daejon 305-380, Korea, and Department of Chemical and Biological Engineering, Korea UniVersity, 1, 5 Ka Anam-dong, Sungbuk-ku, Seoul 136-701, Korea

Dry regenerable sorbent technology is one of the emerging technologies as a cost-effective and energyefficient technology for CO2 capture from flue gas. Six sodium-based dry regenerable sorbents were prepared by spray-drying techniques. Their physical properties and reactivities were tested to evaluate their applicability to a fluidized-bed or fast transport-bed CO2 capture process. Each sorbents contained 20-50 wt% of Na2CO3 or NaHCO3. All sorbents except for Sorb NX30 were insufficient with either attrition resistance or reactivity, or both properties. Sorb NX30 sorbent satisfied most of the physical requirements for a commercial fluidizedbed reactor process along with good chemical reactivity. Sorb NX30 sorbent had a spherical shape, an average size of 89 µm, a size distribution of 38-250 µm, and a bulk density of approximately 0.87 g/mL. The attrition index (AI) of Sorb NX30 reached below 5% compared to about 20% for commercial fluidized catalytic cracking (FCC) catalysts. CO2 sorption capacity of Sorb NX30 was approximately 10 wt % (>80% sorbent utilization) in the simulated flue gas condition compared with 6 of 30 wt % MEA solution (33% sorbent utilization). All sorbents showed almost-complete regeneration at temperatures less than 120 °C. Introduction Carbon dioxide is a major greenhouse gas involved in climate change and is produced in large quantities as a result of fossil fuel use in the industries such as power generation and steel production. Fossil fuel-fired power plants are the main source of carbon dioxide emissions throughout the world, contributing approximately 40% of total global emissions.1 Coal-fired power plants alone contribute approximately 70% of the emissions from fossil-fuel-fired power plants.1 To cap atmospheric CO2 concentrations at less than 550 ppm, a global average emission rate of below 0.055 kg C/(kW h) would need to be achieved by the latter half of the 21st century.2 Therefore, it will be necessary to have diverse approaches such as efficiency improvements, switching to fuels with less carbon emission, as well as carbon dioxide capture and sequestration. The application of carbon capture technology, if it is economically viable, is apparently the first direct step in reducing the CO2 emission rate from fossil-fuel conversion systems instead of a mere dilution of the emitted CO2 through an energy mix. Recently, considerable attention has been directed toward costeffective and energy-efficient CO2 capture and sequestration techniques, which are aimed at capturing CO2 released from fossil-fuel-fired power plants. There are many techniques available for CO2 capture, which all have advantages and limitations. One of the advanced concepts for CO2 capture is absorption process using dry regenerable sorbents.3 White et al. published a critical review on this subject.4 Thermodynamic analysis5 of potential materials for dry regenerable sorbents has shown that alkali metal carbonates are suitable for use in flue gases at temperatures below 200 °C. The CO2 capture process using dry regenerable sorbents consists * Corresponding authors. Phone: +82-42-865-5230. Fax: +82-42865-5708. E-mail address: [email protected] (C.K.R.). Phone: +822-3290-3297. Fax: +82-2-926-6102. E-mail address: [email protected] (S.H.K.). † Korea Electric Power Research Institute. ‡ Korea University.

of two reactors of carbonation and regeneration. The following reaction5,6 proceeds in each reactors: Carbonation: M2CO3 + CO2 + H2O f 2MHCO3 + heat (1) Regeneration: 2MHCO3 f M2CO3 + CO2 + H2O

(2)

Here, M denotes Na or K and the reaction enthalpy of (1) for Na and K is -31.69 and -33.74 kcal/mol, respectively. A dry regenerable sorbent for the CO2 capture process in a fluidized-bed and transport reactor can be used to treat large volume of flue gases from fossil-fuel-fired power plants. This process has several advantages over other processes: better gas contact with smaller sorbent particles, ease of sorbent makeup and removal, easy control of the exothermic carbonation reaction temperature, and continuous steady operation. The fluidizedbed/transport reactor process requires a sorbent which maintains its structural integrity during repeated use, because the attrition of sorbents can result from both physical attrition such as friction and collision as well as chemical transformations like volume changes caused by reaction. Therefore, a dry regenerable sorbent must have a high attrition resistance, high sorption capacity within a reasonable contact time (2-8 s), and good flow characteristics like high bulk density. It must also be regenerable for multicycle use and be competitive in price.7–9 This paper presents the CO2 sorption capacity, rate, and regenerability as well as the important physical properties of six sodium-based solid sorbents. The sorbents were prepared using a spray-dryer, with a focus on their application to a fast fluidized-bed reactor process and the reactivity was tested by thermogravimeteric analysis (TGA). Experimental 1. Sorbent Preparation. The sorbent was produced using a spray-drying technique that is easily scalable to commercial quantities.10,11 The sorbent was routinely spray dried in batches of approximately 2-30 kg. The sorbent preparation process

10.1021/ie0709638 CCC: $40.75  2008 American Chemical Society Published on Web 06/05/2008

4466 Ind. Eng. Chem. Res., Vol. 47, No. 13, 2008 Table 1. Preparation Parameters for the Slurries sorbents

active ingredient (content in solid wt %) solid content in slurry/wt % pH viscosity/cP

Sorb N2A

Sorb N2B

Sorb N2C

Sorb NX

Sorb NH

Sorb NX30

Na2CO3 (50) 32.4 10.66 380

Na2CO3(30) 32.3 10.55 700

Na2CO3(20) 29.7 10.68 1180

Na2CO3(30) 26.3 10.91 1260

NaHCO3(30) 27.1 8.90 1060

Na2CO3(30) 29.0 10.90 260000

Table 2. TGA Test Conditions and Gas Compositions conditions

carbonation

gas composition (vol %)

CO2 O2 H2O N2

temperature (°C) pressure total gas flow rate (mL/min)

14.4 5.4 10 balance 50, 60, 70 ambient 60

regeneration

Table 4. Specific Surface Area and Porosity by Calcination Temperature BET pore Hg Hg pore calcination BET temperature surface area volume porosity volume (mL/g) (%) (mL/g) (°C) (m2/g)

60 90, 100, 120 ambient 60

Sorb N2A Sorb N2B

Table 3. Physical Properties of the Spray-Dried Sorbents

Sorb N2A Sorb N2B Sorb N2C Sorb NX Sorb NH Sorb NX30

calcination temperature (°C)

avg. particle size (µm)

size distribution (µm)

bulk density (g/mL)

650 750 850 650 750 850 650 750 850 500 550 650 500 550 650 550 600 650

125 119 142 121 112 122 100 101 112 128 120 126 122 122 131 93 89 90

42-303 42-303 49-303 42-303 42-303 49-303 38-196 38-196 49-303 42-303 37-355 42-303 49-303 49-303 57-303 42-231 42-231 42-231

0.46 0.48 0.61 0.56 0.59 0.75 1.00 0.99 1.05 0.57 0.60 0.54 0.50 0.53 0.54 0.84 0.87 0.82

consists of several steps: (1) mixing the raw materials in water with dispersants, (2) comminution of the raw materials and colloidal slurry preparation with a high-energy bead mill, and (3) spray drying to form a spherical-shaped green body, followed by the predrying and calcination steps. The raw material consisted of an active material, a support, inorganic binders, water as solvent, organic additives as dispersants, a defoamer, and an organic binder. Commercial grade Na2CO3 or NaHCO3 (Dongyang Chemical Co., Korea) was used as the active ingredient. Six formulations containing approximately 20, 30, or 50 wt % of the solid active materials were designed to prepare the dry regenerable CO2 sorbents, which were designated as Sorbs N2A, N2B, N2C, NX, NH, and NX30. The formulations of Sorbs NX and NH were designed based on Sorb N2B in order to obtain a high specific surface area and better water retention on their surface. The formulation of Sorb NX30 was designed based on Sorb NX with the aim of improving the attrition resistance, specific surface area, and reactivity. The slurry preparation is the most important step for obtaining spherically shaped sorbent without defects, such as doughnut, hollow, and dimple shapes. A colloidal slurry must be in the form of a fluid and be homogeneous, dispersed, and stable. These properties are controlled by the concentration, viscosity, and pH of the slurry through the addition of organic additives such as dispersants. Table 1 lists the preparation parameters of the prepared slurries. The average particle sizes of the solids in the slurry were typically in the submicron range. The highest

Sorb N2C Sorb NX Sorb NH Sorb NX30

650 750 850 650 750 850 650 750 850 500 550 650 500 550 650 550 600 650

5.4 4.9 1.6 3.8 4.0 3.7 6.8 3.0 2.1 36.6 33.0 5.8 39.7 29.0 8.6 76.0 63.7 52.5

1.23 1.12 0.36 0.87 0.92 0.84 1.55 0.69 0.48 8.42 7.59 1.33 9.12 6.65 1.97 17.5 14.62 12.07

77.8 78.3 69.1 75.6 78.3 66.2 63.5 69.8 57.5 81.2 66.0 80.8 82.9 79.7 76.3 69.3 68.7 71.9

2.00 2.04 1.67 1.47 1.46 1.26 0.79 0.87 0.81 1.59 1.57 1.70 1.79 1.75 1.71 0.94 0.97 1.07

viscosity was 260 000 cP, which was significantly higher than the normally expected value, thereby causing some difficulties in pumping the slurry into the spray dryer. However, it was possible to pump the slurry into the spray dryer. In this work,

Figure 1. Schematic diagram of the TGA gas supply system: (TIC) temperature indicator and controller, (EB) electronic balance.

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Figure 2. SEM Photographs (300×) of the spray-dried sorbents: (A) Sorb N2A calcined at 650 °C, (B) Sorb N2B calcined at 650 °C, (C) Sorb N2C calcined at 650 °C, (D) Sorb NX calcined at 500 °C, (E) Sorb NH calcined at 500 °C, and (F) Sorb NX30 calcined at 500 °C.

a commercial pump was used and the slurry with high viscosity was successfully spray-dried. This means the sorbents prepared in this work can be produced with the commercial spray drying process. Sorbents that were spray dried before and after the calcination step had spherical shapes. The organic additives were burnt during the calcination step. The sorbents were calcined in a muffle furnace at 500, 600, and 650 °C under an air atmosphere. The reactivity of pure Na2CO3 and NaHCO3 were also tested for comparison. 2. Physical Characterization of the Sorbent. The physical properties of the six sorbents prepared by a spray-drying were characterized. Scanning electron microscopy (SEM) was used to examine the morphology. A tap density meter (ASTM D 4164-88), sieve, and the standard BET and Hg porosimetry were used to determine bulk density, size distribution, specific surface area and pore volume, and pore volume and porosity, respectively. The attrition resistance is one of the critical parameters in the development of a sorbent, because any attrition of the sorbent causes the loss of active material resulting in lower product quality. Attrition can also result in the need for additional filtration and cause plugging as well as affect the fluidization and solid circulation properties.12 The attrition resistance of the catalysts or sorbents for fluidized-bed applications was measured using a modified three-hole air-jet attrition tester based on the ASTM D 5757-95. The attrition index (AI) was determined at 10 slpm over a 5 h period, as described in the ASTM method. The attrition index is the percentage fines generated over 5 h:

for use with as flue gas under atmospheric pressure. A lower AI or CAI indicates a better attrition resistance of the bulk particles. 3. TGA Reactivity Test. The chemical reactivity of the sorbents was assessed using a simultaneous thermal analyzer (Rheometrics Scientific STA 1500), which has the dual functions of thermogravimetric analysis and differential scanning calorimetry. Carbonation and regeneration was performed at 50-70 and 120 °C, respectively. The temperature was maintained constant during the carbonation and regeneration phases. After the carbonation reaction was completed, the temperature was increased to the regeneration temperature at a heating rate of 5 °C/min. The simulated flue gas compositions were 14.4% CO2, 5.4% O2, and 10% H2O with the remainder being N2. The amount of sample used for the tests was 10 mg. The total flow rate was 60 mL/min (standard), and the dry flow rate was 54 sccm. The TGA test conditions and gas compositions are summarized in Table 2. The regeneration gas was a neat N2 gas with a flow rate of 60 sccm. Electric line heaters along the gas line were used to quantitatively supply water vapor to the sample and to maintain a gas temperature just above the carbonation temperature without any cooling zone. Figure 1 shows a schematic diagram of the TGA system used in this study. The TGA reactivity test was performed at least three times for each sorbent to confirm the reproducibility of the results, and it was found that the reactivities of the carbonation and regeneration were similar in the repeated test.

AI ) total fines collected for 5 h/ amount of the initial sample(50 g) × 100

Results and Discussion (3)

The corrected attrition index (CAI) is the percent of fines generated over 4 h using the following equation: CAI ) (total fines collected for 5 h fines collected for first 1 h)/(amount of initial sample fines collected for first 1 h) × 100 (4) The AI (CAI) of fresh Akzo and Davison FCC catalysts, which were used as references, were 22.5(18)% and 18.4(13.1)%, respectively, under the same conditions. In a fluidized-bed CO2 capture process, materials with an AI < 20% for a transport reactor or a bubbling fluidized-bed reactor would be acceptable

1. Physical Properties. Figure 2 shows SEM photographs of the spray-dried sorbents. Sorbs N2A (50 wt % Na2CO3) and N2B (30 wt % Na2CO3) appear as a wrinkled sphere and a partly fractured sphere with hollow regions, respectively. Sorb NX30 (30 wt % Na2CO3) appeared wrinkled without a hollow region, partly due to the high viscosity of the slurry. Sorbs N2C, NH, and NX had better spherical shape without any hollow regions although some of the particles are fractured. Most of the particles are spherical in shape although some of them are fractured or wrinkled. Some particles have smaller particles on their surfaces. Figure 3 shows the AI and CAI of each sorbent: Sorbs N2A, N2B, and N2C were calcined at 650, 750, and 850 °C,

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Figure 3. AI and CAI of the spray-dried sorbents.

Figure 4. Calcination temperature dependence of the CO2 sorption capacity of Sorb NX 30 (50 °C carbonation, 120 °C regeneration).

respectively. Within the calcination triad of each sorbent, the AI and CAI tend to decrease to a lower value with increasing calcination temperature. The attrition resistance of Sorb N2C (AI < 20%) was better than that of the commercial FCC catalysts. However, the CO2 sorption capacity of Sorb N2C was below 2 wt %, as discussed in the chemical reactivity section. Figure 3 also shows the AI and CAI of the sorbents, Sorbs NX, NH, and NX30. The AI and CAI for Sorbs NX and NH were almost proportional to the calcination temperature. The attrition resistance of Sorbs NX and NH were worse than that of Sorb N2B. The AIs of Sorbs N2A, N2B, NX, and NH were over 20 so that their attrition resistances were not sufficient to be used

in fluidized-bed processes. Sorb NX30 showed superior attrition resistance with a high bulk density of 0.87 g/mL The high attrition resistance of Sorb NX30, which is superior to commercial fluidized catalytic cracking (FCC) catalysts, provides a low level of additional sorbent during CO2 removal in a fluidized-bed because loss of sorbent by attrition is decreased. In addition to the basic physical properties, the high attrition resistance and bulk density of Sorb NX30 offer considerable advantages for a process developer wishing to consider the feasibility of the use of dry regenerable sorbent technology for removing CO2 from flue gas. A fluidized-bed process cannot be used if there is too much particle loss due to low attrition

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Figure 5. CO2 sorption capacity and utilization of sorbents.

Figure 6. CO2 sorption profiles of the Sorb N2 sorbents (50 °C carbonation, 120 °C regeneration): (a) without pretreatment of water vapor, (b) with pretreatment of water vapor.

Figure 7. Calcination temperature dependence of the CO2 sorption capacity of Sorb NX and Sorb NH at 60 °C carbonation.

resistance and bulk density. It should be noted that the attrition test used here is only a test of the physical attrition properties of the sorbent particles. In practice, the sorbent undergoes both physical and chemical attrition. Chemical attrition is caused by continuous volume change of the sorbent due to absorption and desorption of CO2 on the sorbent during the repeated carbonation and regeneration reactions. Table 3 summarizes the physical properties of the spray-dried sorbents. The shape, bulk density, size, and size distribution of the sorbent are essential parameters for evaluation of the

fluidization and solid circulation characteristics in a fluidizedbed reactor process. An average particle size of 100-300 µm, and bulk density higher than 0.8 g/mL are recommended for fluidized-bed applications.13,14 The average particle size of the sorbents in this work was 89-164 µm and a size distribution of 40-300 µm. The bulk density of Sorb N2C and Sorb NX30 is therefore appropriate for fluidized-bed applications. The pore characteristics and surface area influence the overall reaction kinetics. The sorbents developed in this study have porosities of over 60%, and their porosities were maintained at different

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Figure 8. Carbonation temperature dependence of the CO2 sorption capacity.

calcination temperatures. The specific surface area and porosity as a function of calcination temperature are presented in Table 4. All sorbents in this work were calcined over 500 °C because the organic binder added during slurry preparation is burned off above this temperature. The pore volume and specific surface area of Sorbs N2A, N2B, and N2C calcined at 850 °C were reduced, indicating secondary grain growth during thermal sintering. The surface area of Sorbs NX, NH, and NX30 were much higher than the surface area of Sorbs N2A, N2B, and N2C. The specific surface area of Sorbs NX and NH was significantly reduced below 10 m2/g when calcined at 650 °C while that of Sorb NX30 continued to be high, over 50 m2/g, even at a calcination temperature of 650 °C. A large surface area, pore volume and high bulk density of Sorb NX30 are good features for potential fluidized bed application. A large surface area and pore volume is closely related to the chemical reactivity of the sorbents. Because a fluidized-bed or transport reactor is designed based on the volume of the sorbent it holds, not the weight of the sorbent, greater sorbent circulation can be achieved between the two reactors when the bulk density of the sorbent is increased. 2. TGA Chemical Reactivity of Sorbents. All the developed sorbents as well as the neat Na2CO3 and NaHCO3 were tested for chemical reactivity. A 30.3 wt % MEA solution was used as the reference. The MEA solution is a well-known commercial wet scrubbing method for capturing CO2 in industrial emissions. Its CO2 sorption capacity of 6 wt %, and its sorbent utilization of 33% were obtained from literature for comparison.15 These values were used as the reference target for developing solid sorbents. The percentage utilization of sorbent is calculated by actual CO2 sorption capacity divided by theoretical maximum CO2 sorption capacity of active material. To obtain the net CO2 sorption capacity from the CO2 + H2O absorption by the solid-alkali-carbonate-based sorbents, the TGA test was divided into four different sequential stages: Region I is the baseline with the neat N2 gas. H2O + N2 mixture gas was introduced at the onset of region II until there was no more TGA weight gain from H2O adsorption. At the beginning of region III, a simulated flue gas was introduced to perform the normal TGA experiment, in which the active component mainly reacts with a stoichiometric amount of CO2 and H2O. Finally, the sorbent was regenerated by dry N2 while releasing CO2 in region IV. These regions are depicted in Figure 4. Division of region was apparent for Sorbs NH, NX, and NX30 which are designed to have better water retention. Figure 5 shows the TGA CO2 sorption capacities of the various sorbents at 50 °C carbonation along with those of a 30.3 wt % MEA solution and pure Na2CO3 and NaHCO3 for

comparison. It is should be noted that pure Na2CO3 itself has poor CO2 sorption capacity (3.2 wt %) with a sorbent utilization of only 7.8% under the experimental conditions used. On the other hand, pure NaHCO3 has CO2 sorption capacity of 15.9 wt % with 61% sorbent utilization. This was attributed to an increase in active sites because the porosity of the pure NaHCO3 sample increased as a result of CO2 emission during the regeneration process. Although Sorbs N2A, N2B, and N2C have poor CO2 sorption capacities, Sorbs N2A and N2B have higher CO2 sorption capacities than that of pure Na2CO3. This means that the active component works more effectively when used with an appropriate inorganic matrix. Sorbs NX, NH, and NX30 with large surface areas show a remarkably improved CO2 sorption capacity at over 8 wt %. The percentage utilization of the active material for Sorbs NX, NH, and NX30 were 71, 90, and 80%, respectively. Their sorption capacities and sorbent utilization were almost one magnitude higher that of the Sorb N2 series. CO2 sorption capacities of Sorbs NX, NH, and NX30 were superior to the MEA solution. The CO2 sorption profiles of Sorbs N2A, N2B, and N2C calcined at 650 °C were determined at 50 °C carbonation and 120 °C regeneration, respectively. The results are shown in Figure 6. Although the CO2 sorption capacities increased to approximately 6 wt % with increasing content of the active component, they were similar to that of 30 wt % MEA. The low CO2 sorption capacities of the Sorb N2 series can be explained by their low surface area and low H2O retention on the surfaces. It should be noted that the weight gain in region II was not apparent in Figure 6, even though the H2O + N2 mixture gas had been introduced. This means that the Sorb N2 series of sorbents absorb little H2O, which is essential for a high CO2 sorption capacity and reactivity, even though there is a sufficient amount of H2O in the gas phase. As shown in Figures 6a and b, sorbents pretreated with water vapor absorbed more CO2 at the same reaction time than that of sorbent that are not pretreated. In the preliminary test with the pure NaHCO3 sample, it was found that pure NaHCO3 was not activated for CO2 absorption at the low vapor content. As shown in eq (1), in the carbonation reaction, the active component (Na2CO3) requires at least a stoichiometric amount of water for the reaction to proceed. Therefore, the spraydried sorbents must have a significant water sorption capacity under the reaction conditions, e.g., a flue gas stream. The dependence of the CO2 sorption reaction on the calcination, carbonation, and regeneration temperature of the sorbents developed in this study were examined. Figures 7–9 show the effects of calcination temperature on sorption capacity. The carbonation reaction was carried out at 60 °C for Sorbs NH and NX and at 50 °C for Sorb NX30. The results of the reaction for Sorbs NX and NH clearly indicate that the CO2 sorption capacity reached at maximum for the 500 °C-calcined samples and was quite low for the sample calcined at 650 °C. This is consistent with the abrupt reduction in the BET of sorbents calcined at 650 °C. However, a high CO2 sorption capacity for Sorb NX30 was still maintained even with the sample calcined at 650 °C on account of its high surface area and high H2O retention capacity. This means that Sorb NX30 has high thermal stability. Figure 8 shows the carbonation temperature dependence of the CO2 sorption capacity for the 500 °C-calcined Sorbs NX and NH. The TGA CO2 sorption capacity increases as carbonation temperature decrease. The sorption capacity of sodium-based sorbents becomes negligible above a calcina-

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Figure 9. Regeneration temperature dependence of the CO2 sorption capacity of Sorb NX30 (50 °C carbonation).

tion temperature of 70 °C. However, the introduction of more water vapor might increase the net CO2 sorption capacity at the higher temperature.9 Figure 9 shows the regeneration temperature dependence of the CO2 sorption capacity of the 650 °C-calcined Sorb NX 30. Sorb NX30 was almost completely regenerated at the temperatures between 90 and 120 °C. The regenerations of Sorbs NX and NH were also almost completely occurred at temperatures below 120 °C, because the TGA weight losses during the regeneration with neat N2 gas were greater than the net CO2 weight gains. Low regeneration temperature has the advantage of low energy consumption in the sorbent regeneration process.

utilization of the active materials. Sorb NX 30 satisfied almost all the requirements for a commercial fluidized-bed process in the initial laboratory-scale tests. It had superior attrition resistance, high CO2 sorption capacity, and high bulk density. These characteristics are the most important parameters for the compact design of processes aimed at reducing the total capital and operating cost. In the TGA runs, all the sodiumbased dry regenerable CO2 sorbents were almost completely regenerated at 120 °C. This suggests that dry regenerable sorbent technology should be investigated further as a method for capture of CO2 from flue gas. Acknowledgment

Conclusions Using spray-drying techniques, six sorbent formulations containing 20 to 50 wt % of either Na2CO3 or NaHCO3 were spray-dried to form dry regenerable sorbents for CO2 capture from flue gases. Physical properties of all sorbents, i.e. shape, size, size distribution, bulk density, attrition resistance, etc., were characterized. All sorbents showed average particle sizes in the range of 89-164 µm and a size distribution of 40-300 µm that are appropriate for fluidized-bed application. The bulk density of the sorbents ranged from 0.46 to 1.0 g/mL. The bulk density of Sorb N2C and Sorb NX30 was over 0.6 g/mL which is appropriate for fluidized-bed applications. Sorbs N2A, N2B, and N2C, which had a low surface area and low H2O retention capacity, showed low CO2 sorption capacities. However, from a comparison with pure Na2CO3, it was determined that the active components can work more effectively when used with an appropriate inorganic matrix. Sorbs NX and NH, which had high surface area and high H2O retention capacity, showed remarkably improved CO2 sorption capacities over 8 wt % compared to the Sorb N2 series sorbents. The percentage utilization of the active materials for Sorbs NX and NH were 71 and 90%, respectively, which is much greater than 33% for the MEA solutions (30.3 wt %) used in conventional CO2 capture processes. The attrition resistance or reactivity (or both) of the Sorb N2 series, Sorb NX, and Sorb NH sorbents was insufficient for fluidized-bed or transport reactor. Sorb NX30 with a high surface area, high H2O retention capacity, high attrition resistance, and high thermo stability showed a CAI of 1.2% at 10 slpm, a CO2 sorption capacity of 10 wt % and 80%

This research was supported by a grant (code M102KP01001505K1601-01530) from the Carbon Dioxide Reduction & Sequestration Research Center, a 21st Century Frontier Programs funded by the Ministry of Science and Technology of the Korean government and Korea Electric Power Corporation (KEPCO) and its five independent subsidiary fossil power companies. Literature Cited (1) Energy Technology PerspectiVes; OECD/IEA, 2006. (2) Electric Technical Roadmap; EPRI, 2003. (3) Gupta, H.; Iyer, M.; Sakadjian, B.; Fan, L. S. Separation of CO2 from Flue Gas by High Reactivity Calcium Based Sorbents. In The Proceedings of 19th Annual International Pittsburgh Coal Conference, Pittsburgh, PA, Sep 23-27, 2002. (4) White, C. M.; Strazisar, B. R.; Granite, E. J.; Hoffman, J. S.; Pennline, H. W. Separation and Capture of CO2 from Large Stationary Sources and Sequestration in Geological Formations-Coalbeds and Deep Saline Aquifers. J. Air Waste Manage. Assoc. 2003, 53, 1172. (5) Hoffman, J. S.; Pennline, H. W. Investigation of CO2 Capture Using Regenerable Sorbents. In The Proceedings of 17th Annual International Pittsburgh Coal Conference, Pittsburgh, PA, 2000. (6) Liang, Y.; Harrison, D. P.; Gupta, R. P.; Green, D. A.; McMichael, W. J. Carbon Dioxide Capture Using Dry Sodium-based Sorbents. Energy Fuels 2004, 18, 569. (7) Gupta, R. P.; Turk, B. S.; Vierheilig, A. A. Desulfurization Sorbents for Transport-Bed Applications. In The Proceedings of the AdVanced CoalBased Power and EnVironmental Systems’97 Conference, Pittsburgh, PA, July 22-24, 1997. (8) Ryu, C. K.; Lee, J. B.; Oh, J M.; Yi, C. K. Dry Regenerable Sorbents for CO2 Capture from Flue Gas. In The Proceedings of 20th Annual International Pittsburgh Coal Conference, Pittsburgh, PA, Sep 15-19, 2003. (9) Ryu, C. K.; Lee, J. B.; Eom, T. H.; Oh, J. M.; Yi, C. K. Characterization of Sodium-Based Sorbents for CO2 Capture from Flue Gas.

4472 Ind. Eng. Chem. Res., Vol. 47, No. 13, 2008 In The Proceedings of 21st Annual International Pittsburgh Coal Conference, Osaka, Japan, Sep 15-19, 2004. (10) Ryu, C. K.; Lee, J. B.; Ahn, D. H.; Kim, J. J. Hot Gas Desulfurization Sorbents for Fluidized-bed Applications. In Proceedings of the 18th Annual International Pittsburgh Coal Conference, Newcastle, Australia, December 2001. (11) Ryu, C. K.; Lee, J. B.; Ahn, D. H.; Kim, J. J.; Yi, C. K. Highly Attrition Resistant Zinc Oxide-Based Sorbents for H2S Removal by Spray Drying Technique. In The Proceeding of 5th International Symposium on Gas Cleaning at High Temperature, NETL, USA, Sep 17-20, 2002. (12) Zhao, R.; Goodwin, J. G., Jr.; Oukaci, R. Attrition Assessment for Slurry Bubble Column Rreactor Catalysts. Appl. Catal., A 1999, 189, 99. (13) Gupta, R. P.; Gangwal, S. K.; Ciecero, D. C.; Henningsen, G. B.; Katta, S. Hot Coal Gas Desulfurization in Fluidized-bed Reactors using

Zinc Titanate Sorbents. In High temperature gas cleaning; Schmidt, et al. Eds.; University of Karlsruhe: Germany, 1996; pp 543-556. (14) Gupta, P. K.; Turk, B. S.; Vierheilig, A. A. Desulfurization Sorbents for Transport-Bed Application. In The AdVanced Coal-Based Power and EnVironmental Systems’98 Conference, Morgantown, WV, July 21-23, 1998; 2A.2. (15) Cullimane, J. T.; et al. Aqueous Piperazine/Potassium Carbonate for enhanced CO2 Capture. In Proceeding of 7th International Conference on Greenhouse Gas Control Technology, Vancouver, Canada, 2004.

ReceiVed for reView July 16, 2007 ReVised manuscript receiVed April 12, 2008 Accepted April 29, 2008 IE0709638