Improvement of the Desulfurization and Regeneration Properties

Crystal Growth & Design, Energy Fuels, Environ. .... SO2 Removal and Regeneration of MgO-Based Sorbents Promoted with ... Industrial & Engineering Che...
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Environ. Sci. Technol. 2005, 39, 9324-9330

Improvement of the Desulfurization and Regeneration Properties through the Control of Pore Structures of the Zn-Ti-based H2S Removal Sorbents SUK YONG JUNG,† HEE KWON JUN,‡ SOO JAE LEE,† TAE JIN LEE,§ CHONG KUL RYU,| AND J A E C H A N G K I M * ,† Department of Chemical Engineering, Kyungpook National University, Daegu, 702-701, Korea, GS Fuel Cell Co., Ltd., 289-15, Daehwa-Dong, Daedeok-Gu, Daejeon, 306-801, Korea, School of Chemical Engineering and Technology, Yeungnam University, Kyongsan, 712-749, Korea, and Korea Electric Power Research Institute, Daejon, 305-380, Korea

To improve the sulfur removing capacity of the conventional Zn-Ti-based H2S removal sorbents, a new Zn-Ti based sorbent (ZT-cp) was prepared by the coprecipitation method and tested in a packed bed reactor at middle temperature conditions (H2S absorption at 480 °C, regeneration at 580 °C). The new Zn-Ti-based sorbent showed excellent sulfur removing capacity without deactivation, even after 10 cycles of absorption and regeneration. The conventional Zn-Ti-based sorbents (ZT-700, ZT-1000), however, that were prepared by physical mixing, were continuously deactivated. In particular, the initial sulfur removing capacity of the ZT-cp sorbent showed a very high absorption value (0.22 g S/g sorbent), which corresponded to 91.6% of theoretical absorption amount. These results can be explained by the difference in physical properties such as pore volume, surface area, and particle size. It was also found that the sulfides formed from the ZT-cp and ZT-1000 sorbents with spinel structure were easily regenerated even at 580 °C. Those from the ZT-700 sorbent, with separated ZnO and TiO2 structures, needed a temperature higher than 610 °C for regeneration.

Introduction The integrated gasification combined cycle (IGCC) is considered to be among the most efficient and environmentally acceptable technological methods for the generation of power from coal. To use this technology, it is necessary to remove pollutants from the coal-derived fuel gas. Among the pollutant gases, sulfur, which exists in the form of H2S or COS under the highly reducing condition of a gasifier, must be removed from the hot coal gas because these species entering the gas turbine are converted to SOx, which are known precursors of acid rain and whose emission into the atmosphere is * Corresponding author e-mail: [email protected]; phone: +82-53-950-5622; fax: +82-53-950-6615. † Kyungpook National University. ‡ GS Fuel Cell Co., Ltd. § Yeungnam University. | Korea Electric Power Research Institute. 9324

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limited by strict government regulation. To remove hydrogen sulfide from coal-derived gas, several metal oxides have been studied for the development of regenerable sorbents in various temperature ranges under highly reducing conditions (1-5). Due to process equipment limitations and other variables, however, the optimum temperature for fuel gas desulfurizaion in this process is estimated to be between 350 and 550 °C. The affinity of many metal oxides for reduced sulfur species improves as the temperature decreases, making many solids potentially suitable for hot gas clean-up applications in this middle temperature range. In the middle temperature range between 350 and 550 °C, use of the Zn-Ti based sorbents that are regarded as lead sorbents in the high-temperature range between 600 and 800 °C brought about the reduction of chemical reactivity and a very low initial sulfur removing capacity, as well as some of the problems for sulfidation and regeneration properties of spent zinc titanate sorbents (6-12). Therefore, copper-based sorbents have received considerable attention by reducing gas desulfurization to sub-ppm levels. Under the strong reducing power of coal gas, however, they are reduced to metallic copper, whose sulfidation thermodynamics are not so favorable (13-16). A lot of effort has been put forth in trying to achieve copper stabilization in the oxidation states 2+ or 1+, through the formation of various oxides mixed with Fe, Al, Mn, Ti, or Cr. On the other hand, suitable additives have been developed to improve the sulfidation and regeneration properties of the Zn-Ti based sorbents even at middle range temperatures (17-24). The low initial sulfur removal capacity of the Zn-Ti-based sorbents, however, promoted with various additives, limited the use in the middle temperature range. In addition, to maintain sulfur removing capacity and excellent regeneration properties with multiple cycles, loadings greater than 20 wt % of such promoters were needed in the case of the conventional preparation method such as physical mixing, which increases the cost of the sorbent (8-12). To solve these problems, the major objectives of this work were to improve the sulfidation and regeneration properties of the Zn-Ti-based sorbents without these promoters, by improving their physical properties such as pore volume, surface area, and particle size.

Materials and Methods Preparation of the Sorbent. Physical Mixing Method. The ZT-700 and ZT-1000 sorbents used in this study were prepared by the physical mixing method. Zinc oxide and titanium dioxide, with a particle size of about 250-300 mesh, were mixed with an inorganic binder, bentonite, for 1-2 h. Next, a liquid binder, ethylene glycol (EG), was added to the mixture to make a slurry. An extruder was used to form pellets (outer diameter 1 mm) from the slurry. These wet pellets were dried at 150 °C for 4 h to remove ethylene glycol from the material. The dried pellets were calcined in a muffle furnace for 4 h at 700 °C (ZT-700) and 1000 °C (ZT-1000), and then ground to a particle size raging between 250 and 300 Å in diameter. The ramping rate of the temperature was maintained at 3 °C/min. The mole ratio of Zn to Ti was fixed at 1.5:1. Coprecipitation Method. The ZT-cp sorbents used in this study were prepared by the coprecipitation method. The metal salts zinc nitrate and titanium sulfate were dissolved in water. Precipitation was carried out by adding a raw salt solution to NaOH (1.5 mole solution) under vigorous mixing at room temperature. The product of the precipitation was aged for 12 h, and then washed and separated by filtration. An extruder was used to form pellets from the slurry with an outer diameter of 1 mm. The product was dried at 150 °C, 10.1021/es050966g CCC: $30.25

 2005 American Chemical Society Published on Web 10/18/2005

TABLE 1. Experimental Conditions sulfidation process temperature (°C) pressure (atm) flow rate (mL/min) gas composition (%) H2 CO CO2 H2O H2S N2

480 1 50 11.7 9.552 5.2 5-10 1.5 balance

regeneration process

O2 N2

580 1 50 3 balance

calcined at 700 °C, and ground. In addition, to identify crystalline phases in the mixed oxides, an X-ray diffraction (XRD) study was performed with a Philips XPERT instrument using a Cu KR radiation source at the Korea Basic Science Institute. Apparatus and Procedure. Multiple cycles of sulfidation/ regeneration were performed in a fixed-bed quartz reactor with a diameter of 1 cm in an electric furnace. One gram of sorbent was packed into the reactor and the space velocity (SV) was maintained at 5000 h-1 to minimize a severe pressure drop and channeling phenomena. All of the volumetric gas flows were measured under standard temperature and pressure (STP) conditions. The temperature of the inlet and outlet lines of the reactor was maintained above 120 °C to prevent the condensation of water vapor in the sulfidation process. The outlet gases from the reactor were automatically analyzed every 8 min by a thermal conductivity detector (TCD) equipped with an auto-sampler (Valco). The column used in the analysis was 1/8-in. A Teflon tube was packed with Porapak T. The sulfidation and regeneration conditions and the composition of mixed gases are shown in Table 1. When the H2S concentration of the outlet gases reached 15 000 ppm, the concentration of H2S at the inlet stream of mixed gases, an inert nitrogen gas without H2S, was introduced to purge the system until it reached the regeneration temperature. Finally, nitrogen gas mixed with 3% oxygen was introduced to regenerate the sulfurized sorbents until the SO2 concentration reached 200 ppm.

Results and Discussion Comparison of Sulfur Removal Capacity. The best way to evaluate the absorption capacity of sorbents is to determine the so-called breakthrough curves for H2S absorption. In a typical fixed-bed experiment, the H2S concentration of the outlet gas from the reactor is negligible until the entire bed is saturated with sulfur. In this study, the breakthrough time is defined as the time necessary to detect a H2S concentration of 200 ppm in the outlet gas. Figure 1 shows the H2S breakthrough curves for the initial cycle of various ZT-based sorbents prepared by the physical mixing and coprecipitation methods, when the inlet H2S gas concentration (C0) is 15 000 ppm at 480 °C. The X-axis and Y-axis indicate the reaction time and the C/C0 of H2S emitted from the reactor, respectively. In the case of the ZT-700 and ZT-1000 sorbents, their breakthrough times were 80 and 50 min, respectively. The concentration of the breakthrough curves rose slowly up to the inlet H2S concentration. In the case of the ZT-cp sorbent, however, the breakthrough time was 200 min and the concentration of the breakthrough curve rose rapidly up to the inlet H2S concentration. These results showed that the H2S absorption rate of the ZT-cp sorbent is faster than those of the ZT-700 and ZT-1000 sorbents. The amount of sulfur absorbed was calculated from the H2S breakthrough curves, and those for the ZT-700, ZT-1000, and ZT-cp sorbents are shown in Figure 2. When both sulfidation and regeneration are considered as one-cycle processes, the horizontal axis

FIGURE 1. Breakthrough curves of Zn-Ti-based sorbents in the first sulfidation process.

FIGURE 2. Sulfur removing capacities of Zn-Ti-based sorbent during multiple cycles. indicates the number of cycles repeated. The vertical axis indicates the sulfur absorbed per 1 g of sorbent until the H2S concentration in the outlet gas of the reactor reaches 200 ppmv. The amounts of sulfur absorbed in the first cycle of the ZT-700 and ZT-1000 sorbents were 0.06 g S/g sorbent and 0.04 g S/g sorbent, respectively. It was found that 20-40% all of zinc sites participated in the H2S absorption reaction at the first cycle. Then, the sulfur removing capacity of these sorbents increased to 0.16 g S/g sorbent and 0.12 g S/g sorbent during the second cycle, but they were gradually decreased after the second cycle. In the case of the ZT-cp sorbent, the sulfur removing capacity, until the breakthrough point in the first cycle, was 0.22 g S/g sorbent. It was found that 91.6% of the total active zinc sites participated in the H2S absorption reaction. Although the sulfur removing capacity of this sorbent gradually decreased to 0.16 g S/g sorbent until the third cycle, the capacity was maintained during multiple sulfidation/regeneration cycles from the fourth cycle. Physical Properties of the Fresh Zn-Ti-Based Sorbents. As mentioned previously, the ZT-cp sorbent showed very high initial sulfur removing capacity and no deactivation was found in the multiple-cycle tests at middle temperatures. To investigate the difference in capacity from other sorbents, physical properties such as pore structure, pore volume, particle size, and surface area of the sorbents before and after the reaction were examined by porosimetry, BET, and TEM. Figure 3 shows the incremental and cumulative pore size distribution of the fresh sorbents prepared by the coprecipitation and physical mixing methods. As shown in Figure 3a, the incremental pore volume of the ZT-cp sorbent showed 0.14, 0.049, 0.050, and 0.039 mL/g with 1503, 2257, 4021, and 12096 Å of pore size, respectively, while those of the ZT-700 and ZT-1000 sorbents showed 0.08 and 0.15 mL/g VOL. 39, NO. 23, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. (a) Incremental and (b) cumulative pore volumes of ZnTi-based sorbents before sulfidation process with pore diameter.

TABLE 2. Physical Properties of Various Zn-Ti-Based Sorbents before Sulfidation Process

sorbent ZT-700 fresh ZT-1000 fresh ZT-cp fresh

pore surface average pore skeletal volume area diameter density porosity (cc/g) (m2/g) (4V/A) (g/mL) (%) 0.26 0.31 0.89

8.43 3.24 18.34

1243 3789 2124

2.26 2.02 1.06

37.23 38.28 48.55

with only 2001and 4031 Å of pore size, respectively. These results mean that the ZT-700 and ZT-1000 sorbents have one median pore size, and that the ZT-cp sorbent has a number of median pore sizes. The cumulative pore volume of the ZT-cp sorbent was 0.89 mL/g, while those of the ZT-700 and ZT-1000 sorbents were 0.26 and 0.31 mL/g, respectively, as shown in Figure 3b. It was found that the cumulative pore volume of the ZT-cp sorbent was more than twice those of the ZT-700 and ZT-1000 sorbents. Table 2 and Figure 4 show the surface areas, porosities, and particle sizes of the ZT-700, ZT-1000, and ZT-cp sorbents in a fresh state. In the case of the ZT-700 and ZT-1000 sorbents, their surface areas and porosities were 8.43 and 3.24 m2/g and 37% and 38%, respectively. Their average particle sizes were fairly large (300-400 nm), as shown in Figure 4a and b. In the case of the ZT-cp sorbent, its surface area and porosity were 18.35 m2/g and 48.55%, respectively, and the particle size was 6070 nm. This is much smaller than those of the ZT-700 and ZT-1000 sorbents, as shown in Figure 4c. To summarize, the ZT-cp sorbent had a larger pore volume and surface area and smaller particle size than the ZT-700 and ZT-1000 sorbents. It is thought that gases are easily diffused into the active sites through the large pores formed in sorbent and that the H2S gas is easily and rapidly absorbed on the sorbent with large surface area, which resulted mainly from the properties of the nanoparticles. 9326

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FIGURE 4. TEM images of (a) ZT-1000, (b) ZT-700, and (c) ZT-cp sorbents. Change in the Physical Properties of the Zn-Ti-Based Sorbents during Multiple Cycles. Figure 5a and b show the incremental and cumulative pore volumes as a function of pore distribution for the ZT-700, ZT-1000, and ZT-cp sorbents regenerated during multiple cycles. As shown in Figure 5a, in the case of the ZT-700 and ZT-1000 sorbents, the pore size distribution changed from mono type to bimodal type. Their pore sizes showed two median pore sizes. It was also found that the surface areas of the ZT-700 and ZT-1000 sorbents increased through the production of new pores, i.e., 300 Å, during multiple cycles. These results are shown in Table 3. Their surface areas were 18.70 and 10.58 m2/g after the second cycle, respectively. Those values were approximately twice those of the fresh state samples. Therefore, the increase in the sulfur removing capacity of the ZT-700 and ZT-1000 sorbents, as shown in Figure 2, could be explained by the increase in the surface area from the second cycle. The sulfur removing capacity of the ZT-700 sorbent, however, gradually

FIGURE 5. (a) Incremental pore volumes of regenerated Zn-Ti-based sorbents as a function of pore size during multiple cycles. (b) Cumulative pore volumes of regenerated Zn-Ti-based sorbents as a function of pore size during multiple cycles.

TABLE 3. Physical Properties of Regenerated Zn-Ti-Based Sorbents during Multiple Cycles no. of cycle

sorbent ZT-700 ZT-1000 ZT-cp

raw regenerated raw regenerated raw regenerated

2 10 2 10 2 10

cumulative pore volume (cc/g)

surface area (m2/g)

average pore diameter (4V/A)

skeletal density (g/mL)

porosity (%)

0.26 0.27 0.187 0.31 0.14 0.13 0.89 0.72 0.45

8.43 18.70 19.21 3.24 10.58 4.83 18.34 13.52 11.67

1243 965 470 3789 1717 1141 2124 2631 2618

2.26 1.96 1.97 2.02 2.02 2.03 1.06 1.21 1.85

37.23 34.43 26.44 38.28 21.69 20.43 48.55 46.77 45.08

decreased when the cumulative pore volume and porosity of the ZT-700 sorbent reduced from 0.27 mL/g and 34.43%

in the second cycle to 0.187 mL/g and 26.44% in the tenth cycle, as shown in Figure 5b and Table 3. The physical VOL. 39, NO. 23, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 7. SO2 breakthrough curves of Zn-Ti-based sorbents during regeneration after the first sulfidation process.

FIGURE 6. TEM images of regenerated (a) ZT-1000, (b) ZT-700, and (c) ZT-cp sorbents after tenth cycle. property significantly varied due to the expansion/contraction of the sorbent during sulfidation/regeneration. The spalling or cracking of the sorbents was caused by the same expansion/contraction, as reported previously (6, 9, 12). Figure 6 shows the TEM images of the Zn-Ti-based sorbents after regeneration in the second and tenth cycles. As shown in Figure 6b, the ZT-700 sorbents showed an average particle size of 80-120 nm in the second and tenth cycles. Considering the size of the fresh sample (300-400 nm), the particle was thought to be broken by expansion due to sulfidation and contraction during regeneration in every cycle. The ZT-1000 sorbent also showed a tendency similar to that of the ZT-700 sorbent. As shown in Figure 5b, however, the cumulative pore volume and porosity of the ZT-1000 sorbent dramatically decreased by about 40-50% as compared to those in the fresh state. After the second cycle, the surface area gradually was decreased when the small pore (300 Å) was decreased, resulting in the decrease in the sulfur removing capacity. In 9328

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the case of the ZT-cp sorbent, its pore size was maintained (2500-3000 Å) during the multiple cycles. As shown in Figure 5b, even though the cumulative pore volume of the ZT-cp sorbent gradually decreased to 0.50 mL/g until the third cycle, the pore volume was maintained thereafter. The surface area and porosity of the ZT-cp sorbent were also maintained (1113 m2/g and 45%) during the multiple sulfidation and regeneration cycles, as shown in Table 3. The average particle size of the ZT-cp sorbent was maintained between 60 and 70 nm, the same size as that of the fresh state. From these results, it could be concluded that the physical properties of the ZT-cp sorbent such as pore volume, porosity, spacious surface area, and nanoparticle size did not vary significantly during multiple cycles, resulting in the high and steady sulfur removing capacity. Regeneration Properties. In addition to the sulfur removing capacity, regeneration characteristics are among the most important factors to be considered. Nitrogen gas mixed with 3 vol. % of oxygen was introduced to regenerate the sorbents sulfided until the SO2 concentration reached a value of less than 200 ppmv. The SO2 breakthrough curves and the amount of O2 used during regeneration after the first sulfidation process at middle temperatures are plotted in Figure 7. From the stoichiometric coefficient of the regeneration reaction equation for the Zn-based sorbents, 2 mol of SO2 were produced when 3 mol of oxygen were consumed during regeneration. In the case of the ZT-700 sorbent, 10 000 ppm of SO2 concentration was maintained during regeneration process, and then the concentration gradually decreased. In the case of the ZT-1000 sorbent, the concentration of SO2 had a maximum value of 20 000 ppm, which was observed approximately between 20 and 60 min, while the ZT-cp sorbent showed approximately the same level of concentration between 0 and 100 min and the concentration rapidly decreased. Although the gradient of the regeneration curve for the ZT-cp sorbent was slightly reduced, as shown in Figure 8, with the reduction of the cumulative pore volume during sulfidation and regeneration, as shown in Figure 5 and Table 3, it was found that the regeneration property of the ZT-cp sorbent also was much improved. Temperature-Programmed Regeneration (TPR). To identify the effect of the preparation methods on the regeneration ability of sorbents, the TPR tests were performed after sulfidation. The tests were carried out by measuring the concentration of SO2 desorbed by the introduction of nitrogen gas containing 3 vol. % of oxygen, when the ramping rate of the temperature was 1 °C/min. The TPR experimental results are shown in Figure 9. In the case of the ZT-700 sorbent, most sulfur was desorbed above 600 °C. In the case of the ZT-1000 and ZT-cp sorbents, the maximum SO2 concentration was found at 550 °C. This phenomenon could not be explained by the physical properties of the Zn-Ti-based

FIGURE 8. SO2 breakthrough curves of ZT-cp sorbent during regeneration process in every cycle.

FIGURE 10. Sulfur removing capacities of ZT-1000 and ZT-cp sorbents under gas composition in the presence and absence of water vapor. into their initial phases without the sulfate formation after regeneration. So far, the Zn2TiO4 spinel structure was restored in the case of ZT-cp and ZT-1000 sorbent and a separate ZnO and TiO2 mixture phase was found in the case of the ZT-700 sorbent. The reaction for the structure change during a cycle of sulfidation and regeneration has not been verified yet. It is clear, however, that the Zn-Ti-based sorbents, with a spinel structure formed during preparation, show better regeneration properties than those with a separate mixture of single oxides (10).

FIGURE 9. Temperature programmed regeneration (TPR) results for Zn-Ti based sorbents.

TABLE 4. XRD Patterns of Zn-Ti-based Sorbents Before and After Sulfidation and Regeneration sorbent

fresh

sulfidation

regeneration

ZT-700 ZT-1000

ZnO, TiO2 Zn2TiO4 (spinel)

ZnS, TiO2 ZnS, TiO2

ZT-cp

Zn2TiO4 (spinel)

ZnS, TiO2

ZnO, TiO2 Zn2TiO4 (spinel), ZnO, TiO2 Zn2TiO4 (spinel)

sorbents because of the different physical properties of the ZT-cp and ZT-1000 sorbents. Therefore, to explain this phenomenon, the XRD patterns of each sorbent were investigated after sulfidation and regeneration. The XRD patterns for the Zn-Ti-based sorbents, prepared by coprecipitation and physical mixing before and after the reaction (sulfidation and regeneration), are shown in Table 4. The phases of the ZT-1000 and ZT-cp sorbents, before the reaction, were assigned to the Zn2TiO4, having a general spinel structure without an unreacted single oxide such as zinc oxide or titanium dioxide. The ZT-700 sorbent, before H2S absorption, consisted of a separate ZnO and TiO2 phase without spinel structure such as Zn2TiO4, owing to low-temperature pretreatment. The origin of the spinel structure of the ZT-cp sorbent was believed to be due to the relative nanoparticle, even though the pretreatment temperature was low (700 °C) and that of the ZT-1000 sorbent was at a high pretreatment temperature (1000 °C). After the sulfidation process, in the case of the ZT-700 sorbent, most of the ZnO was transformed to ZnS and TiO2 did not participate in the H2S absorption. In the case of the ZT-cp and ZT-1000 sorbents, phases of the TiO2, separated from the spinel structure and ZnS, were observed, while the Zn2TiO4 disappeared into the XRD pattern. It was interesting to note that most sulfides converted

Effect of Water on the Capacity of the Zn-Ti-Based Sorbents. To identify the effect of water on the capacity of the ZT-cp sorbent, their sulfur removing capacity was tested in the presence of 10 vol. % H2O. Figure 10 shows the sulfur removing capacities of various sorbents with cyclic numbers under gas composition with 10 vol. % H2O. The sulfur removing capacity of ZT-1000 sorbent was greatly decreased in the presence of water. The similar effect of water vapor for the ZT-1000 sorbent was already observed previously (8) in that the presence of water reduced the reaction rate between zinc oxide, major active sites for H2S absorption, and H2S at medium temperature. In the case of the ZT-cp sorbent, the sulfur removing capacity was not decreased at all under a gas composition of 10 vol. % H2O. The reason for the result has not been clearly verified yet. But it is thought that the stability of the sorbent during multiple cycles was due to the very stable large pores of ZT-cp sorbent resulted from the nanoparticles even in the presence of water. To improve the sulfur removing capacity of the Zn-Tibased sorbent, a new Zn-Ti-based desulfurization sorbent (ZT-cp) was prepared by coprecipitation method and tested in a packed-bed under middle temperature conditions (H2S absorption at 480 °C and regeneration at 580 °C). The sorbent showed that excellent sulfur removing capacity was maintained without deactivation, even after 10 cycles of sulfidation and regeneration. Those of the conventional Zn-Ti-based sorbents (ZT-700 and ZT-1000), however, prepared by the physical mixing method were continuously deactivated. In particular, the new ZT-cp sorbent showed a very high sulfur removing capacity initially. The H2S absorption rate resulted from the nanoparticle characteristics of a large surface area and pore volume, in addition to the enhanced reactivity, even in the presence of water. In fact, the H2S absorption rate and capacity of the coprecipitated sorbent was not affected by the water vapor and the particle size of the new sorbent was about 60 nm. Those of the conventional sorbents were between 300 and 500 nm. In addition, it was found that the regeneration properties of the ZT-cp and ZT-1000 sorbents with a spinel structure were better than that of the ZT-700 sorbent with a separate mixture of single oxides. VOL. 39, NO. 23, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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Acknowledgments This study has been performed under the National Research Laboratory (NRL) program and we gratefully acknowledge the financial support of both Ministry of Science and Technology (MOST) for this work and a grant (DA2-202) from Carbon Dioxide Reduction & Sequestration Research Center, one of the 21st Century Frontier Programs funded by the Ministry of Science and Technology of Korean government.

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Received for review May 20, 2005. Revised manuscript received September 14, 2005. Accepted September 16, 2005. ES050966G