Study on Deactivation of Zinc-Based Sorbents for Hot Gas

Apr 20, 2010 - No-Kuk Park, Tae Jin Lee and Si Ok Ryu* .... (19, 20) The flow rate of the simulated coal gas entering the reactor was set at 250 .... ...
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Ind. Eng. Chem. Res. 2010, 49, 4694–4699

Study on Deactivation of Zinc-Based Sorbents for Hot Gas Desulfurization No-Kuk Park, Tae Jin Lee, and Si Ok Ryu* School of Chemical Engineering and Technology, Yeungnam UniVersity 214-1, Dae-dong, Gyeongsan, 712-749, South Korea

The possible reasons that were associated with the reduction of zinc oxide and the deposition of elemental Zn on the zinc-based sorbent surfaces were proposed in this study and examined by monitoring chemical reactivity and physical/chemical properties of the sorbents during the multicyclic tests with the aid of various diagnostics in order to investigate the deactivation of the sorbents that was observed in previous works. The reactivity of the sorbents was investigated using a temperature programmed technique (TPT) with a thermal gravimetric analyzer (TGA). From our experiments, the undesirable features due to the reduction of ZnO that was caused by the reducing components in the gasified coal gas were observed in the chemical reactivity and sorbent properties. In the investigation of the migration of elemental Zn onto the sorbent surface, elemental Zn accumulated on the surface of the sorbents rather than the zinc vaporization. It might have caused the deactivation of the zinc-based sorbents. 1. Introduction These days the possible depletion of the earth’s fossil fuel resources and the associated environmental problems have become international concerns in the area of energy. Many countries expend great efforts toward the development of economically acceptable and environmentally sustainable technologies that utilize low grade fossil fuels such as coal, heavy oil, and Orimulsion. The integrated gasification combined cycle (IGCC) is considered one of the most environmentally acceptable technologies for power generation from low grade fossil fuels. However, harmful gases, such as H2S and COS, are produced during the gasification process because of the sulfur contents in those fuels. The high concentration of H2S in the gasified gas causes serious damages to the gas turbines due to its corrosiveness. Sulfur dioxide emitted after the combustion of fuel gas in the gas turbine also causes acid rain by converting to secondary pollutants, such as sulfuric acid, which are soluble in water. Therefore, a highly efficient desulfurization process is required for the IGCC system in order to reduce the sulfur content of the synthetic fuel gas. Conventional commercialized desulfurization processes are usually based on liquid scrubbing at ambient or lower temperatures, consequently, resulting in not only a low energy conversion efficiency but also an expensive wastewater treatment. The fuel should be desulfurized at elevated temperatures of the synthetic gas leaving the gasifier in order to achieve a maximum thermal efficiency. The newly developed hot gas desulfurization (HGD) process exhibits an improved thermal efficiency over the conventional process. Therefore, many research groups have concentrated on the development of regenerable metal oxide sorbents with high sulfur-removing capacity and long-term durability at high temperatures. Recently, several zinc-based sorbents have been commercialized for HGD because zinc-based sorbents are thermodynamically favorable for the H2S removal at the high temperature processing conditions. They are mostly developed by NETL, RTI, and Phillips and manufactured by Su¨d-Chemie. These zinc-based sorbents have good reactivities and physical properties for the HGD.1-12 In general, the sorbents in the HGD process must * To whom correspondence should be addressed. E-mail: soryu@ ynu.ac.kr.

selectively remove H2S in the gasified coal and also exhibit a high sulfur absorption capacity and mechanical strength during repetitive operation even at high temperatures. However, the sulfur-removing capacity of most zinc-based sorbents gradually decreases during continuous operation. Gangwal et al.13 at RTI reported that the deactivation of the zinc-based sorbents resulted from the zinc loss at high temperatures. Zinc oxide is reduced into elemental zinc by the carbon monoxide and hydrogen in the coal gas, and then the elemental zinc that is formed during the reduction is vaporized at high temperatures. Mojtahedi and Abbasian reported that deactivation of zinc titanate sorbents might be due to Zn migration to the surface of the sorbent during high temperature and pressure cyclic process.14,15 There is a tendency to lower the operational temperatures of the HGD process down below 500 °C to avoid difficulties in operation and stability issues at high temperatures. Operation in the medium temperature range could prevent the deactivation of the zinc-based sorbents caused by the Zn vaporization. However, in our previous works, the sulfur absorption capacity of the prepared zinc-based sorbents decreased even at temperatures below 500 °C.16-18 Some other factors were believed to directly and/or indirectly exert an effect on the sorbent properties. The sintering of ZnO, the formation of zinc sulfate, the reduction of zinc oxide, or the deposition of zinc onto the sorbent surface due to the zinc migration could all have caused the gradual deactivation of the zinc-based sorbents. Among these possible reasons, intensive studies on the sintering of ZnO and the formation of zinc sulfate were performed in our previous works. These studies confirmed that both processes were responsible for the deactivation of the zinc-based sorbents at high temperature processing conditions.16-18 In this study, the possible reasons associated with the reduction of zinc oxide and the migration of zinc were investigated by monitoring the chemical reactivity and the physical/chemical properties of the sorbents with the aid of various diagnostics. 2. Experimental Section 2.1. Preparation of Sorbents. Three different types of zincbased sorbents that were prepared in previous works were used in this study. All of the sorbents were prepared using a solid mixing method. Zinc oxide, several support materials, and additives were ground and sufficiently mixed with binders. Then

10.1021/ie9017463  2010 American Chemical Society Published on Web 04/20/2010

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Table 1. Experimental Conditions for the Multi-Cyclic Tests of the Prepared Sorbents conditions temperature (°C) pressure (atm) flow rate (mL/min) gas composition (vol.%)

sulfidation a

b

480, 650 1 250 H2S 1.0 H2 11.7 CO 19.0 CO2 6.8 10.0 H2O N2 balance

regeneration 580,a 650b 1 250 O2 5.0 10.0 H2O N2 Balance

a Temperature conditions of the sulfidation/regeneration for the ZNZ series sorbents. b Temperature conditions of the sulfidation/regeneration for the pellet-type ZTG series sorbents.

the sorbents were formulated using either a granulation method or an extrusion method except the zinc titanate sorbents and dried for 24 h at 150 °C in order to remove any moisture from the materials. The dried sorbents were calcined for 4 h at 750 °C in a muffle furnace and ground. The particles with diameters of 150-300 µm were collected through sieving. The collected sorbents were calcined for an additional 4 h. The zinc oxide sorbent was modified with natural zeolite as a support material in order to determine the other causes for the deactivation of the zinc-based sorbents besides the sintering. ZnO sorbents were prepared without any supports and additives in order to investigate the effects of the reduction on the reactivity of the zinc-based sorbents. The zinc titanate sorbents were prepared by mixing ZnO, TiO2, NiO, Co3O4, and MoO3 with binders as described in our previous work and then formulated using a pelletization method in order to investigate the migration of elemental Zn to the sorbent surface.17 2.2. Multicyclic Tests of Sulfidation/Regeneration. Multiple sulfidation/regeneration cycles were carried out using a fixed-bed quartz reactor with a 1 cm diameter in an electric furnace in order to investigate the reactivity of the zinc-based sorbents. A detailed description of the experimental setup was provided elsewhere.19,20 The flow rate of the simulated coal gas entering the reactor was set at 250 mL/min and controlled using mass flow controllers. All of the volumetric flows of the gases were measured at the standard temperature and pressure (STP) conditions. The experimental conditions and the compositions of the simulated coal gases for the sulfidation/regeneration are given in Table 1. The outlet gases from the reactor were automatically analyzed using a gas chromatograph (Donam, DS6200A) that was equipped with a thermal conductivity detector (TCD) and a pulsed flame photometric detector (PFPD, OI Instrument Co). A 1/8 in. Teflon tube that was packed with Chromosil-310(Sufalco) and a GS-GASPRO capillary tube (J&W Scientific) were used as the columns in the analysis. When the H2S concentration of the outlet gases reached 2000 ppm, the inlet stream of the mixed gases was stopped and an inert nitrogen gas was introduced to purge the system until the regeneration temperature was reached. Then the sulfidated sorbents were regenerated by introducing air that was diluted with nitrogen and water. The regeneration process was terminated when SO2 could no longer be detected in the reactor outlet. 2.3. TPR/TPO/TPRe Study. The reactivity of the zinc-based sorbents was investigated using the temperature programmed technique (TPT) with a thermal gravimetric analyzer (TGA, Shimadzu). The temperature programmed reduction (TPR) test was performed in order to study the effects of the reduction characteristics of the coal gas on the sorbents. The changes in the sulfur absorption capacity of the prepared sorbents were monitored before and after the reduction in this study. The

Figure 1. Sulfur capacity of the ZnO/natural zeolite sorbents for the 30 cycle test.

reduction was carried out for 5 h at the same temperature as the sulfidation. The temperature programmed oxidation (TPO) test was carried out in order to study the effects of the oxidation characteristics of oxygen on the reduced sorbents, and the temperature programmed regeneration (TPRe) test was performed in order to study the effects of the regeneration temperatures on the sulfidated sorbents. The temperature was increased to 800 °C at a rate of 10 °C/min, and flow rate of the gas was maintained at 250 mL/min. 2.4. Changes of Physical Properties. The changes in the physical properties of the sorbents were investigated before and after the reaction using a scanning electron microscope (SEM, HITACHI, S-4200/4100) and energy dispersive X-ray (EDX, FISONS, KEVEX SIGMA). The weight changes of the sorbents were monitored as a function of time before and after the sulfidation using a Cahn balance (CAHN instrument). 3. Results and Discussion 3.1. Effect of ZnO Reduction on Deactivation of ZnBased Sorbents. Zinc-based sorbents are considered good metal oxide sorbents because of their high reactivity with sulfur compounds, but these sorbents exhibit thermal stability issues at high temperature processing conditions. The reactivity of the sorbents was rapidly decreased in a multiple cycle operation at high temperature. One of the prospective causes for this rapid decrease in the reactivity was the sintering of ZnO in the sorbents at high temperature. The influence of the sintering of ZnO on the deactivation of the sorbents was investigated in our previous study.16 We observed that the sintering of ZnO had a negative effect on the sulfur capacity of the sorbents. Based on these experimental results, we confirmed that the deactivation of the sorbents due to the sintering of ZnO could be inhibited to some extent through the addition of the appropriate support materials into the sorbents. Since natural zeolite acted as a good support of the zinc-based sorbents in a previous study, the ZnO sorbents were modified with natural zeolite (named as ZNZ series hereafter) in order to determine the other factors that led to deactivation besides the sintering.21 A 30 repetitive cycles of sulfidation/regeneration was carried out with these modified sorbents in the midtemperature ranges as shown in Figure 1. The deterioration of the sulfur capacity was observed during the 30 cycle test. Considering the operational temperature of reduction and the existence of natural zeolite, the deactivation in Figure 1 indicated that the sintering was not solely responsible for the deactivation of the zinc-based sorbents. Both the

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Figure 2. H2-TPR curve of ZnO.

Figure 3. Weight changes of the fresh and reduced ZnO sorbents before and after the sulfidation.

reduction of ZnO because of the existence of the reducing gases and the migration of elemental Zn were believed to cause the deactivation of the sorbents. A series of experiments was performed to confirm these assumptions. The reducing ability of the coal gas had a primary effect on the deactivation of the sorbents. Therefore, the reduction characteristics of ZnO were studied in this work. The reactivity of ZnO was assessed using the TPR test with TGA. In the TGA test, the weight change of the sorbent was monitored as a function of temperature for the reduction as

shown in Figure 2. In this experiment, the particle sizes of ZnO ranged from 150 to 300 µm in diameter. The reduction rate was proportional to the reaction temperature in this test. On the basis of the TGA data, the reduction of zinc oxide began at around 420 °C, and the reduction rate increased sharply at temperatures above 600 °C. At the sulfidation process temperature, which was 480 °C, the reduction of ZnO was observed but proceeded slowly. It indicated that the reactivity of the ZnO sorbents was influenced by the ZnO reduction at temperatures above 420 °C. The compulsory reduction of the ZnO sorbents was performed with the simulated coal gas in order to determine the effects of the ZnO reduction on the sulfidation. In Figure 3, the weight changes of the sorbents were monitored as a function of time before and after the sulfidation using a Cahn balance in order to compare the sulfur capacity and the reaction rate of the fresh ZnO sorbents to the ZnO sorbents containing elemental Zn that were produced during the compulsory reduction. The sulfur absorption capacity of the fresh ZnO sorbent was about 20 g S/100 g sorbent after the first cycle, whereas the sulfur absorption capacity for the ZnO sorbent that was compulsorily reduced for five hours was about 7 g S/100 g sorbent. In addition to the reduction of the sulfur capacity, the reaction rate of sorbent, which was obtained from the slope of the weight change curve, also decreased in this study. The experimental results showed that the reduction of ZnO by the coal gas exerted a negative influence on the reactivity of the sorbents. 3.2. Effect of Zinc Migration on Deactivation of ZnBased Sorbents. Gupta et al.13 reported that the deactivation of the zinc-based sorbents was caused by the zinc loss at high temperatures. Metallic zinc was formed through the reduction of zinc oxide in the sorbents and was gradually diffused to the surface. Consequently, this zinc vaporized to the exterior during the sulfidation/regeneration cycles. A series of experiments were carried out in order to clarify the deactivation of the sorbents because of the Zn loss. At first, SEM and EDX were employed to detect the elemental Zn that was produced through the reduction of ZnO. Figure 4 shows the SEM images of the fresh ZnO sorbent that was mixed with natural zeolite and the reacted sorbent after 30 sulfidation/regeneration cycles. Coagulations were observed on the surface of the reacted sorbents in Figure 4(b). The deposit on the surface might have been caused by the migration of elemental zinc to the surface of sorbent. The amount of zinc on the surface of the ZnO/natural zeolite sorbents was determined using EDX after the 24th and the 30th cycles, and those values were compared to the fresh sorbents

Figure 4. SEM images of the surface of the ZnO/natural zeolite sorbents: (a) fresh and (b) after 30 cycles.

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Figure 5. EDX results that were measured using a 30-cycle test for the ZnO/natural zeolite sorbent.

Figure 6. Transfer characteristics of elemental Zn in the zinc-based sorbent particles.

in order to study the Zn loss that was caused by the possible Zn migration to the surface. The EDX measurements for the ZnO/natural zeolite sorbents are presented in Figure 5. The amount of zinc on the surface of the ZnO/natural zeolite sorbents increased as the test cycles further advanced. The increase in zinc on the sorbent surface could be described by the reaction ZnO(s) + CO, H2(g) f Zn(s) + CO2, H2O(g) as presented in Figure 6. Flynn and Wanke originally proposed the possible mechanism that led to the changes in the properties of the zinc-based sorbents because of the reduction of ZnO.22 According to their proposed mechanism, ZnO was converted into elemental Zn through reaction (2) under the reducing conditions. Then the metallic zinc in the sorbent gradually migrated to the sorbent surface and vaporized to the exterior from the surface during the sulfidation/regeneration cycles. The

increase in the elemental zinc on the surface also caused the progressive deterioration of the reactivity over the repetitive cycles because it blocked the mass transfer of H2S in the coal gas to the active ingredient of the sorbents. Considering that the elemental zinc that formed during the reduction of ZnO melted at temperatures above 419 °C, which was the melting point of Zn, the sorbent deactivation was caused by the deposition of elemental zinc onto the sorbent surfaces, which led to the decrease in the turnover frequency of the active sites on the sorbent surface and the increase in the diffusional resistance to mass transfer. A series of experiments was performed with the pellet-type zinc-based sorbents at a high temperature range in order to investigate the migration of elemental zinc to the sorbent surface. The pellet-type sorbents were prepared by mixing ZnO, TiO2, NiO, Co3O4, and MoO3 with binders and then formulated using the pelletization method as presented in Figure 7. Those sorbents were named the ZTG series throughout the rest of this study. The diameter and thickness of the pellets were 10 and 3 mm, respectively. The formulated sorbents were calcined at 750 °C for 4 h. The sulfidation/regeneration process was repeated for 20 cycles at a temperature of 650/650 °C using a 30 mm diameter fixed-bed quartz reactor in an electric furnace. After every five cycles, a pellet-type sorbent was removed from the reactor, and its cross section was analyzed using EDX

Figure 7. A schematic diagram of the pelletization method and a cross-sectional image of the reacted sorbent after 10 cycles.

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Table 2. Atomic percentage of the Elements That Were Detected Using EDX at Nine Locations in the ZTG Sorbent after 10 Cycles composition of elements, atomic % location

Zn

Ni

Mo

Co

Ti

initial A B C D E F G H I

38.4 51.2 45.3 42.3 40.9 38.8 38.3 43.6 60.7 71.4

10.0 8.1 8.5 9.6 9.1 10.9 11.3 9.2 6.7 6.3

3.0 2.8 4.2 5.6 3.5 2.9 3.1 4.1 2.8 1.8

10.0 9.7 10.1 9.9 11.1 10.5 11.3 9.9 7.9 9.7

38.6 28.2 31.9 32.6 35.4 36.9 36.0 33.2 21.9 10.8

at nine different locations that were labeled A to I in Figure 7. The EDX results of the pellet after 10 cycles are presented in Table 2. In Table 2, the atomic percentage of elemental Zn gradually increased as the location of the EDX analysis approached the sorbent surface. The Zn content at locations A and I were 51.2 atomic % and 71.4 atomic %, respectively, which were the highest values in the EDX analysis. Meanwhile, the zinc content in the center (location E) of the sorbent remained nearly the same as the initial stage. Therefore, the zinc content in the area that was adjacent to the sorbent surface was higher than in the sorbent core. The surfaces of the pellet-type sorbents were also observed using a microscope after every five cycles. Figure 8 presents the microscopic images of the fresh pellet-type sorbent and the reacted sorbents after 5, 15, and 20 cycles. In Figure 8, the sorbent was severely cracked after the repeated sulfidation/ regeneration cycles. The color on the sorbent surface changed as the reactions progressed during the multiple cycle test. However, a noticeable color change in the inner part of the sorbent was not observed in the microscopic view. The observations in Figure 8 corresponded well to the EDX measurements in Table 2. As ZnO reduced to elemental Zn with the reducing gases, zinc migrated to the surface of the sorbents and vaporized to the exterior of the sorbents. The zinc loss that was caused by the vaporization was accompanied by the zinc deposition on the reactor outlet at temperatures above 650 °C. However, nothing indicated that zinc vaporized in the temperature range of this study. It was indicated that the deactivation of the sorbents was not caused by any vaporization loss of zinc at 480 °C.

Figure 8. Microscopic images of the fresh pellet-type sorbent and the reacted sorbents after 5, 15, and 20 cycles.

Figure 9. TPO curves of the weight change of Zn on the left axis and the weight change of ZnS on the right axis.

The oxidation behavior of ZnS and Zn was investigated in order to determine the causes for the migration of Zn to the sorbent surface and also the accumulation of Zn on the surface instead of the oxidation to ZnO during the regeneration process, in which oxygen was supplied into the reactor. The TPO tests were performed to study the effects of the oxidation characteristics of oxygen on both the zinc sulfide that formed during the sulfidation and the elemental Zn that produced during the coal gas reduction. Figure 9 shows the weight change of elemental Zn as a function of the temperature on the left axis and the weight change of ZnS as a function of temperature on the right axis. Zinc sulfide started to lose weight during the oxidation at 570 °C, but the elemental zinc began to gain weight during the oxidation at 640 °C. Therefore, the oxidation of the elemental zinc proceeded very slowly at the regeneration temperature of 580 °C, whereas the oxidation of zinc sulfide proceeded rapidly. Zinc accumulated on the surface of the sorbent as the sulfidation/ regeneration process was repeated because of this slow conversion to zinc oxide. The results of EDX in Figure 5 can be explained by the reason as stated above. The accumulation of elemental Zn on the surface of the sorbents could have caused the deactivation of the sorbents. 4. Conclusion The possible causes that were associated with the deactivation of the sorbents for the hot gas desulfurization were proposed and investigated in this experiment by monitoring the chemical reactivity and the physical/chemical properties of the sorbent with the aid of various diagnostics. For the study on the influence of the reduction of ZnO, the reduction tests were performed on both the pure ZnO sorbents and the ZnO sorbents that were mixed with natural zeolite. ZnO was reduced under the influence of the coal gas even in the medium temperature range. The sulfur capacity and the reaction rate of the sorbents decreased and the extent of the reduction lowered through the addition of natural zeolite into the pure ZnO. The amount of zinc on the surface of the ZnO/natural zeolite sorbents was determined using EDX. The zinc content increased as the multicyclic test continues. In the investigation of the migration of elemental Zn onto the sorbent surface, the zinc content in the area that was adjacent to the sorbent surface was higher than the sorbent core. In the TPO test, the reduced elemental zinc very slowly converted into zinc oxide during the oxidation at 580 °C, which caused the Zn accumulation on the sorbent surface as the sulfidation/ regeneration process was repeated and, consequently, lead to the deactivation of the sorbents.

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Acknowledgment This work was the outcome of a Manpower Development Program for Energy & Resources supported by the Ministry of Knowledge and Economy (MKE). Literature Cited (1) Lee, Y. W.; Lee, T. J.; Yi, C. K. Technical Trend Analysis of Zinc Titanate Sorbent Development for Hot Gas Desulfurization(I). Chem. Ind. Technol. 1997, 15 (3), 237. (2) Siriwardane, R. V. Fixed Bed Testing of Durable, Steam Tolerant Zinc Containing Sorbents. In Proceedings of the Thirteenth Annual International Pittsburgh Coal Conference: Coal-Energy and EnVironment, 1996. (3) Siriwardane, R. V.; Poston, J. A.; Hammerbeck, K. Testing and Analysis of METC10 Sorbent Proceedings of the AdVanced Coal-Based Power and EnVironmental Systems ’97 Conference, Pittsburgh, 1997. (4) Siriwardane, R. V. Durable Regenerable Sorbent Pellet for removal of Hydrogen Sulfide from Coal Gas. U.S. Patent 5,703,003, 1997. (5) Siriwardane, R. V.; Poston, J. A. Interaction of H2S with zinc titanate in the presence of H2S and CO. Appl. Surf. Sci. 1990, 45 (2), 131. (6) Siriwardane, R. V.; Poston, J. A. Characterization of copper oxides, iron oxides, and zinc copper ferrite desulfruization sorbents by X-ray photoelectron spectroscopy and scanning electron microscopy. Appl. Surf. Sci. 1993, 68 (1), 65. (7) Siriwardane, R. V.; Poston, J. A.; Evans Jr, G. Spectroscopic Characterization of Molybdenum-Containing Zinc Titanate Desulfruization Sorbents. Ind. Eng. Chem. Res. 1994, 33 (11), 2810. (8) Siriwardane, R. V.; Woodruff, S. FTIR Characterization of the Interaction of Oxygen with Zinc Sulfide. Ind. Eng. Chem. Res. 1995, 34 (2), 699. (9) Gupta, R. P.; Turk, B. S.; Vierheilig, A. A. Desulfurization Sorbents for Transport-Bed Applications. Proceedings of the AdVanced Coal-Based Power and EnVironmental Systems ’97 Conference, Pittsburgh, 1997. (10) Velten, T. J.; Demmel, E. J. Method of Incorporating Small Crystalline Catalytic Ingredients into an Attrition-Resistant Matrix. U.S. Patent 4,826,793, 1989. (11) Demmel, E. J. a) Method for Producing Attrition-Resistant Catalyst Binders. U.S. Patent No. 5,288,793. 1994, b) Production of AttritionResistant Catalyst Binders through Use of Delaminated Clay. U.S. Patent 5,190,902, 1993.

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ReceiVed for reView November 4, 2009 ReVised manuscript receiVed February 19, 2010 Accepted March 30, 2010 IE9017463