Ind. Eng. Chem. Res. 2008, 47, 4909–4916
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GENERAL RESEARCH Properties of Nanosize Zinc Titanium Desulfurization Sorbents Promoted with Iron and Cerium Oxides Suk Yong Jung, Soo Jae Lee, Jung Je Park, Soo Chool Lee, Hee Kwon Jun,† Tae Jin Lee,‡ Chong Kul Ryu,§ and Jae Chang Kim* Department of Chemical Engineering, Kyungpook National UniVersity, Daegu 702-701, Korea; GS Fuel Cell Company, 453-2 Seongnae-dong, Gangdong-gu, Seoul 134-848, Korea; School of Chemical Engineering and Technology, Yeungnam UniVersity, Gyongsan 712-749, Korea; and Korea Electric Power Research Institute, Daejon 305-380, Korea
To improve the sulfidation and regeneration properties of Zn-Ti-based desulfurization sorbents, sorbents with 5 wt % of the promoters, iron or cerium oxide, were prepared and tested in a packed reactor under intermediate temperature conditions (H2S absorption at 480 °C, regeneration at 580 °C). These sorbents showed excellent sulfur-removing capacities without deactivation even after 10 cycles of absorption and regeneration. Their regeneration properties were also improved by the iron and cerium promoters. In the case of the iron promoter, heat was released when the metal sulfide was converted to the metal oxide, which improved the regeneration property of the Zn-Ti-based sorbent. In the case of the cerium promoter, there was no reaction with hydrogen sulfide gases, and the spent Zn-Ti-based sorbent was easily regenerated by virtue of the OSC (oxygen storage capacity) property of the cerium promoter, which easily converted the S of ZnS to SO2. Introduction A desulfurization system is necessary in the integrated gasification combined cycle (IGCC), which is considered to be among the most efficient and environmentally acceptable technological methods for the generation of power from coal. The sulfur, which exists in the form of H2S or COS under the highly reducing conditions of a gasifier, must be removed from the hot coal gas, because these species entering the gas turbine are converted into SOx, which is a known precursor of acid rain and whose emission into the atmosphere is limited by strict government regulations. In addition, the H2S gases cause the corrosion of the turbine and the deactivation of the catalyst used in the other processes such as NH3 decomposition process, water gas shift reaction, CO2 absorption process, etc.1,2 Several commercial techniques are available for the removal of hydrogen sulfide and COS, including wet absorption by MDEA, MEA, DEA, DGA, DIPA, sodium carbonate, and potassium carbonate. However, the disadvantage of these commercial techniques for the purification of coal gas is that the hot coal gas must be cooled to ambient temperature and then preheated to a high temperature before it can be used in a gas turbine. To avoid heat loss and save energy, hightemperature desulfurization technology has been widely developed and appears to be the major technique for the removal of hydrogen sulfide from hot coal gas. Therefore, to remove hydrogen sulfide form coal-derived gas, several metal oxides have been studied for the development of regenerable sorbents having high sulfur-removing capacity and long-term stability at both high and intermediate temperatures under highly reducing conditions.3 * To whom all correspondence should be addressed. E-mail: kjchang@ knu.ac.kr. Phone: +82-53-950-5622. Fax: +82-53-950-6615. † GS Fuel Cell Company. ‡ Yeungnam University. § Korea Electric Power Research Institute.
Copper-based sorbents have the most favorable thermodynamics, allowing gas desulfurization down to subppm levels. However, under the strong reducing power of coal gas, they are easily regenerated in an oxidant atmosphere, because copper sulfate becomes unstable at high temperatures. Much effort has been made to achieve copper stabilization in oxidation states 2+ or 1+, through the formation of different mixed oxides with Fe, Al, Mn, Cr, or Ce. In the case of Cu-Mn mixed sorbents, although copper was not stabilized by manganese oxides, the Cu-Mn mixed sorbent showed good performance in multicycle tests without apparent decay at 700 °C.4–10 Because of equipment limitations and other variables, however, the optimum temperature for fuel gas desulfurization 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. Iron-based sorbents have been used in forms such as zinc ferrite. Zinc ferrite sorbents require the lowest possible temperature for complete regeneration without sulfate formation. Therefore, since the regeneration of iron oxide from iron sulfide may be carried out at a lower temperature than that which is required by other metal oxide systems, the use of an iron-based sorbent appears to be the best approach to achieve an energetically viable cyclic sulfidation regeneration process in the temperature range of 350-550 °C.11–17 Zinc-based sorbents are known to be among the best metal oxide sorbents that have the most favorable thermodynamics for H2S removal. However, in the temperature range of 350-550 °C, they have some problems such as their low initial sulfurremoving capacity, low regeneration properties, and deactivation during multiple cycles. As reported in various research papers, to overcome these problems, additives, such as Ti, Al, Si, Zr, Co, Ni, and Fe, can be used as supporters (Ti, Al, Si) and promoters (Co, Ni, Fe) in
10.1021/ie071357f CCC: $40.75 2008 American Chemical Society Published on Web 06/14/2008
4910 Ind. Eng. Chem. Res., Vol. 47, No. 14, 2008
Figure 1. Schema of the experimental apparatus.
Zn-based sorbents to improve their sulfur-removing capacity during multiple cycles and their regeneration properties. Cobalt, nickel, and iron promoters were found to improve the sulfidation and regeneration properties of the zinc titanate sorbent in the intermediate temperature range. In particular, the activities of the sorbents to which the iron promoter was added were better than those of the other sorbents promoted with cobalt and nickel. However, the low initial sulfur-removing capacities still deteriorated progressively, and in order to maintain the sulfur-removing capacity and excellent regeneration properties after multiple cycles, more than 20 wt % of the promoters were needed in the case of a conventional preparation method such as physical mixing, which increases the cost of the sorbent.18–28 In the case of cerium oxide, it is known that the catalyst has OSC (oxygen storage capacity) in the intermediate temperature range of 250-1000 °C. Cerium oxide has been used as an important component of three-way catalysts in automotive pollution control and to transform SO2 to SO3 in fluid catalytic cracking (FCC) or RFCC processes. Therefore, cerium oxide is suitable to improve the regeneration property of zinc titanate sorbents, because the OSC of cerium oxide is very helpful for the oxidation of sulfur species.30–32 Among the various compounded sorbents, the initial sulfurremoving capacity of Zn-Ti-based sorbents was improved by a coprecipitation method, as reported in previous research. However, the regeneration properties of the Zn-Ti-based sorbents decreased with increasing number of cycles.29 To use the sorbent in practical processes and ensure the reusability of the sorbent during multicycles, the H2S breakthrough time of the desulfurization process must be longer than the finishing time of the regeneration process. Therefore, to improve the regeneration property of the sorbent, the addition of a small amount (5 wt %) of promoters by a coprecipitation method is needed. In addition, the decrease of the regeneration temperature is necessary to achieve smooth regeneration and to reduce the energy required for the regeneration process.
The objective of this work was to improve the regeneration property of the ZT-cp sorbent by adding promoters such as iron and cerium in small amounts (5 wt %) using a coprecipitation method in the intermediate temperature range, in order to be able to use it in a fixed-bed reactor and to investigate the role of the promoters. Experimental Section Preparation of the Sorbent. The Zn-Ti-based sorbents used in the study were prepared by the coprecipitation method. In the case of the ZT-cp sorbent, zinc nitrate (44.7 g) and titanium sulfate (29.7 g) were dissolved in water (500 mL) and the Zn/ Ti mole ratio of the sorbent was fixed at 1.5. In the case of the ZTFe5-cp and ZTCe5-cp sorbents, iron (7.2 g) and cerium (3.1 g) nitrates, respectively, were added to a solution of dissolved zinc nitrate (42.0 g) and titanium sulfate (28.2 g). Each loading amount of iron and cerium of the sorbent was 5 wt %. Precipitation was carried out by adding 1.5 M solution of NaOH in the metal salts solution with vigorous mixing at 25 °C. The entire product obtained by precipitation was aged at 80 °C and then washed until pH 7 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 and calcined at 700 °C, and the particles from 0.25 to 0.3 mm were used. In addition, in order to identify the 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, as shown in Figure 1. Sorbent (1 g) was packed into the reactor, and the space velocity (SV) was maintained at 5000 h-1 to minimize the severe pressure drop and channeling phenomena. All of the volumetric gas flows were measured under standard temperature and pressure (STP) conditions. The temperatures of the inlet and
Ind. Eng. Chem. Res., Vol. 47, No. 14, 2008 4911 Table 1. Experimental Conditions
temperature (°C) pressure (atm) flow rate (mL/min) gas composition (%)
sulfidation process
regeneration process
480 1 50 H2 ) 11.7 CO ) 9.6 CO2 ) 5.2 H2S ) 1.5 N2 ) balance
580 1 50 O2 ) 3 N2 ) balance
outlet lines of the reactor were 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 autosampler (Valco). The diameter and length of the column used for the gas analysis were 1/8 in. and 1.5 m, respectively. A Teflon tube was packed with Porapak T. The sulfidation and regeneration conditions and composition of the 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 the 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 vol % oxygen was introduced to regenerate the sulfurized sorbents until the SO2 concentration reached 200 ppm. Experimental Method Used for the Characterization of the Sorbents. To investigate the regeneration property of the spent sorbent, the TPO (temperature-programmed oxidation) test of the sorbent was carried out in a fixed-bed reactor. The amount of the sample was 0.5 g, and the ramping rate was 1 °C/min. The temperature range of the TPO test was from 480 to 700 °C. The reactant gases were composed of 3 vol % of O2 and N2 balance. In order to identify the crystalline phases in the sorbents, an XRD (X-ray diffraction) study was performed with a Philips XPERT instrument using a Cu KR radiation source. An FT-Raman study was performed with a Bruker IFS66/ FRP106 instrument using an NdiYAG laser source and InGaAs detector. The particle-size analysis of the sorbents was performed using transmission electron microscopy (TEM) (model H-7100, Hitachi corporation) at the Korea Basic Science Institute at a magnification of 100 000. The pore volumes and surface areas of the sorbents were analyzed by mercury porosimetry (Autopore 9500 series, Micromeritics Corporation) and gas sorption analysis (Autosorb-1, Quantachrome Instruments), respectively, at the Center for Scientific Instruments in Kyungpook National University. The total amount of sorbents used for the measurement of the pore volume was 0.5 g, and highpressure analysis was conducted up to a maximum pressure of 33 000 psi. The total amount of sorbents used for the measurement of the surface area was 0.2 g, and the pretreatment temperature was 150 °C in the vacuum state. Results and Discussion Comparison of Sulfur-Removing Capacities. The general desulfurization reaction may be represented as follows: MeO + H2S f MeS + H2O The metal oxides were converted to metal sulfides when they reacted with H2S gases and produced H2O gas. This reaction is highly exothermic; the standard heat of reaction (298 K) for ZnS oxidation is ∆H ) -106 kcal/mol and is almost independent of temperature. The metal sulfide is reacted with oxygen to regenerate the metal oxide and liberate the sulfur as SO2. So, the regeneration reaction may be represented as follows:
Figure 2. Normalized H2S breakthrough curves of the various Zn-Ti-based sorbents at 480 °C.
Figure 3. Sulfur-removing capacities of the various Zn-Ti-based sorbents.
MeS + (3 ⁄ 2)O2 f MeO + SO2 The breakthrough curves of H2S absorption are the best way to evaluate the sulfur-absorption capacity of sorbents in a typical fixed-bed experiment because 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 an H2S concentration of 200 ppm in the outlet gas. Figure 2 shows the H2S breakthrough curves for the initial cycle of the various Zn-Ti-based sorbents prepared by the coprecipitation method, 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 ratio of H2S emitted from the reactor, respectively. The breakthrough times of the ZT-cp, ZTFe5-cp, and ZTCe5-cp sorbents were about 200 min, and the concentration of the breakthrough curves rose rapidly up to the inlet H2S concentration. These results show the typical breakthrough curves and fast H2S absorption rate of these sorbents. The amounts of sulfur absorbed in the various sorbents before their breakthrough time are defined as a net sulfur-removing capacity and are shown in Figure 3. When both sulfidation and regeneration are considered as one-cycle processes, the horizontal axis indicates the number of cycles repeated. The vertical axis indicates the amount of sulfur absorbed per 1 g of sorbent until the H2S concentration in the outlet gas of the reactor reaches 200 ppmv. Although the sulfurremoving capacities of these sorbents gradually decreased to 0.16 (g of S)/(g of sorbent) by the third cycle, the capacity was subsequently maintained during multiple sulfidation/regeneration cycles beginning with the fourth cycle. From these results, it
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Figure 5. Net time (a) and slope value of the SO2 breakthrough curve (b) of the various Zn-Ti-based sorbents.
Figure 4. SO2 breakthrough curves of the (a) ZT-cp, (b) ZTFe5-cp, and (c) ZTCe5-cp sorbents at 580 °C.
can be seen that 5 wt % of the promoters, iron and cerium, did not affect the sulfur-removing capacity of the ZT-cp sorbent. The total sulfur-removing capacities of the ZT-cp, ZTFe5-cp, and ZTCe5-cp sorbents including the amount absorbed after net breakthrough time were 24.6, 26.3, and 22.7 (g of S)/(g of sorbent), respectively. It was found that the total sulfur-removing capacity of the ZTCe5-cp sorbent was higher than those of the other sorbents. Regeneration Properties. To estimate the regeneration properties of the various Zn-Ti-based sorbents promoted with iron and cerium, their breakthrough curves for SO2 emission during the regeneration process are an important factor. Figure 4 shows the SO2 breakthrough curves of the ZT-cp, ZTFe5-cp, and ZTCe5-cp sorbents for various numbers of cycles. In the case of the ZT-cp sorbent, its maximum SO2 concentration range (19 000-20 000 ppm) was maintained for 96 min, and then the SO2 concentration slowly decreased to less than 100 ppmv with increasing reaction time in the first cycle. However, the time that the maximum SO2 concentration was maintained was decreased with an increasing number of cycles. In the case of the ZTFe5-cp and ZTCe5-cp sorbents, however, their maximum SO2 concentration was maintained for 150 and 120 min,
respectively, and then rapidly decreased to less than 100 ppmv. This phenomenon continued in spite of the increase in the number of cycles. From these results, it was also found that the amounts of sulfur desorbed from the ZT-cp, ZTFe5-cp, and ZTCe5-cp sorbents were different. The differences in the amounts were thought to be due to the differences in their total sulfur-removing capacities shown in Figure 2 rather than those of their net capacities shown in Figure 3. The total sulfurremoving capacity of the ZTFe5-cp sorbent was higher than those of the other sorbents, as shown in Figure 2, because the iron promoter reacted with H2S gas and was transformed into iron sulfide. The total sulfur-removing capacity of the ZTCe5cp sorbent was lower than those of the other sorbents, because the cerium promoter did not react with H2S gas. These results are considered in more detail in the discussion of the XRD and FT-Raman analyses of the iron and cerium promoters in the next section. Figure 5 shows the net time and slope of the breakthrough curve of the Zn-Ti-based sorbents. The net time was defined as the length of time during which the maximum SO2 concentration range (19 000-20 000 ppmv) remained unchanged in the SO2 breakthrough curve. The slope value of the SO2 breakthrough curve was calculated from the ratio of the change of the SO2 concentration to the time interval from the net time to the finish time in the SO2 breakthrough curve. As shown in Figure 4, the net times of the ZT-cp sorbent were 96, 72, and 40 min and the slope values of the SO2 breakthrough curves were 89.8, 64.3, and 50.0 at the 1st, 5th, and 10th cycles, respectively. On the other hand, the net times of the ZTFe5-cp and ZTCe5-cp sorbents were about 150 and 120 min, respectively, and the average slope values of the SO2 breakthrough curves were 200 and 210, respectively. From these results, it can be seen that the net times and slope values of the SO2
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Figure 6. TPO (temperature-programmed oxidation) results of the various Zn-Ti-based sorbents under a gas composition of 3 vol % of O2 and N2 balance.
breakthrough curves of the ZTFe5-cp and ZTCe5-cp sorbents were higher than those of the ZT-cp sorbent and did not decrease with increasing number of cycles. It was found that the regeneration property of the ZT-cp sorbent was improved by the addition of the promoters, iron and cerium, to the sorbent. These results are similar to those obtained for sorbents loaded with more than 20 wt % of promoters such as cobalt and iron by the physical mixing method, as reported in previous papers.21–25 Figure 6 shows the TPO (temperature-programmed oxidation) results of the Zn-Ti-based sorbent promoted with the various additives for the gas composition of 3 vol % of O2 and N2 balance. As shown in Figure 5, in the case of the ZT-cp sorbents, the concentration of SO2 gradually increased with increasing temperature starting from 500 °C and the maximum concentration of SO2 was observed at 580 °C. On the other hand, in the case of all the other sorbents, the concentration of SO2 increased starting from a temperature of 490 °C and the maximum concentration of SO2 was observed at 550 °C. As shown in these results, the addition of iron and cerium to the Zn-Ti-based sorbents affected their regeneration properties, effectively increasing their regeneration ability and decreasing their regeneration temperature. Characterization of the Sorbents. The regeneration property of the ZT-cp sorbent was improved by the addition of iron and cerium in the intermediate temperature range. In a previous study, the decrease in the regeneration property of the ZT-cp sorbent with increasing number of cycles was reported to be caused by the decrease of the pore volume.29 Therefore, the physical properties, such as surface area, pore volume, and pore distribution, of the Zn-Ti-based sorbents promoted with the additives, iron and cerium, were measured by porosimetry, BET, and TEM. Figure 7 shows the pore distributions of the various Zn-Ti-based sorbents in the fresh state and after 10 cycles of regeneration. In the case of the fresh state, the average pore sizes of the ZT-cp, ZTFe5-cp, and ZTCe5-cp sorbents were 212.4, 433.5, and 132.4 nm, respectively. After 10 cycles of regeneration, the ZT-cp and ZTFe5-cp sorbents exhibited a bidispersed pore distribution with their average pore sizes of 261 and 509 nm, respectively. In the case of the ZTCe5-cp sorbent, its average pore size was 294 nm. From these results, it was found that the minimum average pore sizes of the sorbents were increased to 50-150 nm after 10 cycles. In addition, the cumulative pore volumes of the various sorbents were about 0.8-0.9 mL/g in the fresh state. After 10 cycles of regeneration, their cumulative pore volumes were decreased to 0.48-0.55 mL/g. The physical properties such as the cumulative pore volume and pore size of the sorbents after 10 cycles of
Figure 7. Variation of the incremental pore volume with the pore diameter for the various Zn-Ti-based sorbents in the fresh state (a) and the regenerated state (b) after 10 cycles. Table 2. Physical Properties of the Various Zn-Ti-Based Sorbents in the Fresh State and Regenerated State after 10 Cycles
sorbents ZT-cp
pore average cumulative pore size particle surface (nm) size (nm) cycle no. area (m2/g) volume (mL/g)
fresh 10 cycles ZTFe5-cp fresh 10 cycles ZTCe5-cp fresh 10 cycles
18.34 11.67 23.60 20.41 19.01 12.31
0.89 0.45 0.85 0.57 0.90 0.48
212.4 261.8 433.5 509.5 132.4 294.5
40-80 40-80 40-80 40-80 40-80 40-80
regeneration significantly varied due to the expansion/contraction of the sorbent during the sulfidation and regeneration process. Therefore, the spalling or cracking of the sorbents was caused by their expansion/contraction, as reported in previous studies.22–27 In addition, the decrease in the surface areas of the sorbents was caused by the decrease in their cumulative pore volumes and the increase in their average pore sizes. Table 2 shows the surface area of the various Zn-Ti-based sorbents in the fresh state and after 10 cycles of regeneration. In the case of the fresh state, as shown in Table 2, the surface areas of the ZT-cp, ZTFe5-cp, and ZTCe5-cp sorbents were 18.34, 23.60, and 19.01 m2/g, respectively. After 10 cycles of regeneration, the surface areas of the ZT-cp, ZTFe5-cp, and ZTCe5-cp sorbents were 11.67, 20.41, and 12.31 m2/g, respectively. The surface areas of all of the sorbents were shown to have similar values. The average particle sizes of all of the sorbents were between 40-80 nm in the fresh state and remained in the same range after 10 cycles of regeneration. From these results, it can be seen that the physical properties, viz. pore size, pore volume, surface area, and particle size, of the Zn-Ti-based sorbents promoted with iron and cerium were similar to those of the ZT-cp sorbents.
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Figure 8. XRD patterns for the various Zn-Ti-based sorbents in the fresh state (a), sulfidation state (b), and regenerated state (after 10 cycles) (c).
The Zn-Ti-based sorbents promoted with iron and cerium had a large surface area and pore volume and contained nanosize particles, as in the case of the ZT-cp sorbent. However, the increase of the regeneration properties of the ZTFe5-cp and ZTCe5-cp sorbents could not be explained by the physical properties alone. To determine the other properties of the sorbents containing the promoters such as iron and cerium, the crystal structures of the various Zn-Ti-based sorbents were determined by XRD. Figure 8 shows the XRD patterns of the ZT-cp, ZTFe5-cp, and ZTCe5-cp sorbents in the fresh state and after the sulfidation and regeneration processes. As shown in Figure 8a, in the case of the fresh state, the main structure of all of the sorbents was observed to be a spinel structure like Zn2TiO4. In the case of the ZTFe5-cp sorbent, besides the ZnTiO4 structure, the ZnFeTiO4 structure was also formed, the peaks for which overlapped with the 2θ values of the Zn2TiO4 structure, as reported in a previous paper.25 In the case of the ZTCe5-cp sorbent, several crystal structures including CeO2 were also observed. After the sulfidation process, the main crystal structures of all of the sorbents were metal sulfides like ZnS and TiO2. In the case of the ZTFe5-cp, a small amount of iron sulfide (FeS) was also formed without any metal oxides.25 In the case of the ZTCe5-cp, no additional crystal structure was observed. After the regeneration process, most of the metal sulfides of all of the sorbents were converted into the initial phases without any sulfate forms. However, in the case of the ZTFe5-cp sorbent, the amount of iron oxides such as Fe2O3 gradually increased with increasing number of cycles. It was concluded that the iron sulfide produced in the sulfidation process was not completely restored to its initial structure. In the case of the ZTCe5-cp, it was difficult to determine crystal
Figure 9. Change of temperature of the various sorbent beds during regeneration at 580 °C.
structures by the XRD pattern, because the reference 2θ values of cerium oxide and sulfide were overlapped with those of ZnS and TiO2. Figure 9 shows the temperature change of the bed for the various Zn-Ti-based sorbents during the regeneration process. In the cases of the ZT-cp and ZTCe5-cp sorbents, no temperature changes were observed during the regeneration process. However, in the case of the ZTFe5-cp sorbent, the temperature of the bed increased to about 14 °C. It was found that the promoter, iron, emitted heat when the metal sulfides were transformed to metal oxides during regeneration. This is in agreement with the result reported in a previous paper.25 The heat emitted during the conversion of metal sulfides to metal oxides was thought to enhance the conversion of ZnS to ZnO. Although, in the previous results, the physical properties such as the pore volume and surface area of the ZTFe5-cp sorbent were decreased, the
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Conclusion
Figure 10. SO2 breakthrough curve of the ZT-cp sorbent at 600 °C.
Figure 11. FT-Raman spectra of the ZT-cp and ZTCe5-cp sorbents after sulfidation at 480 °C.
regeneration property of the ZTFe5-cp sorbent was maintained by the heat emission of the iron promoter during the regeneration reaction, as shown in Figure 4. In fact, the regeneration property of the ZT-cp sorbent was increased at 600 °C. Figure 10 shows the SO2 breakthrough curves of the ZT-cp sorbent with increasing number of cycles at a regeneration temperature of 600 °C. Thus, it was concluded that the regeneration property of the sorbent was improved through the emission of heat by the iron promoter. In the previous XRD result of the ZTCe5-cp sorbent, no cerium oxides or sulfides were observed after the sulfidation process. In addition, the ZTCe5-cp sorbent did not emit heat during regeneration. It was found that the promoter, cerium, has a different role from iron. To investigate the role of cerium, the cerium crystal structure after sulfidation was examined using FT-Raman spectroscopy, and the results are shown in Figure 11. As shown in Figure 11, the wavelengths of ZTCe5-cp were 400, 464, 515, and 634 nm. Here, the value of 464 nm was defined as cerium oxide. It was found that cerium oxide was not transformed into cerium sulfide but was maintained during the sulfidation process. In a previous paper, cerium oxide was reported to have a very high OSC performance, and the gases such as CO and SO2 were very easily converted to CO2 and SO3, respectively, at temperature ranges of 350-1000 °C.30–32 As shown in Figure 3, the maintenance of the regeneration property of the ZTCe5-cp sorbent with increasing number of cycles was explained by the high OSC performance of the cerium oxide. The result agreed well with those of a previous paper.24
To improve the regeneration properties of Zn-Ti-based sorbents, new sorbents promoted with iron (ZTFe5-cp) and cerium (ZTCe5-cp) were prepared by the coprecipitation method and tested in a fixed-bed reactor in the intermediate temperature range (sulfidation 480 °C, regeneration 580 °C). Their sulfurremoving capacities were similar to those of the ZT-cp sorbent, and their regeneration properties were improved by the addition of the promoters, iron and cerium, without any large change in their surface area, pore volume, or nanorange particle size. In the case of the promoter, iron, iron oxides were converted into iron sulfides during the sulfidation process, and then heat was emitted when the iron sulfides were converted into iron oxides. The well-dispersed iron sulfide in the sorbent containing iron increased the regeneration temperature due to the heat emitted during the regeneration process. In the case of cerium, it did not react with H2S gases during sulfidation and remained as cerium oxide having a very high OSC performance. The role of cerium was as an oxygen supplier, resulting in the easy conversion of ZnS to ZnO. These sorbents prepared by the coprecipitation method can be used in a fixed-bed reactor but not in a fluidized-bed one. Therefore, further study is needed including the preparation technique of the sorbents and the attrition test to examine their use in a fluidized-bed reactor. Also, depending on the operating conditions desired, the effective promoter could be varied. For example, if the control of temperature is more important, Ce is proper. But considering capacity, Fe is a more effective promoter for large absorption capacity. Acknowledgment We gratefully acknowledge the financial support from the National Research Laboratory (NRL) program of the Ministry of Science and Technology of the Korean government. Literature Cited (1) Han, W.; Jin, H.; Xu, W. A novel combined cycle with synthetic utilization of coal and natural gas. Energy 2007, 32, 1334–1342. (2) Gangwal, S. K.; Stogner, J. M.; Harkins, S. M. Bench-scale testing of novel sorbents for H2S removal from coal gas. EnViron. Prog. 1989, 8, 265–269. (3) Lew, S.; Jothimurugesan, K.; Flytzani-Stephanopoulos, M. Hightemperature H2S removal from fuel gases by regenerable zinc oxidetitanium dioxide sorbents. Ind. Eng. Chem. Res. 1989, 28, 535–541. (4) Jothimurugesan, K.; Gangwal, S. K. Regeneration of zinc titanate H2S sorbents. Ind. Eng. Chem. Res. 1998, 37, 1929–1933. (5) Ibarra, J. V.; Cilleruelo, C.; Garcı´a, E.; Pineda, M.; Palacios, J. M. Vibrational spectroscopy study of zinc-containing mixed oxides as regenerable sulphur sorbents at high temperature. Vib. Spectrosc. 1998, 16, 1– 10. (6) Pineda, M.; Fierro, J. L. G.; Palacios, J. M.; Cilleruelo, C.; Garcı´a, E.; Ibarra, J. V. Characterization of zinc oxide and zinc ferrite doped with Ti or Cu as sorbents for hot gas desulphurization. Appl. Surf. Sci. 1997, 119, 1–10. (7) Li, Z.; Flytzani-Stephanopoulos, M. Cu-Cr-O and Cu-Ce-O regenerable oxide sorbents for hot gas desulfurization. Ind. Eng. Chem. Res. 1997, 36, 187–196. (8) Garcia, E.; Palacios, J. M.; Alonso, L.; Moliner, R. Performance of Mn and Cu mixed oxides as regenerable sorbents for hot coal gas desulfurization. Energy Fuels 2000, 14, 1296–1303. (9) Abbasian, J.; Slimane, R. B. A regenerable copper-based sorbent for H2S removal from coal gases. Ind. Eng. Chem. Res. 1998, 37, 2775– 2782. (10) Slimane, R. B.; Abbasian, J. Copper-based sorbents for coal gas desulfurization at moderate temperatures. Ind. Eng. Chem. Res. 2000, 39, 1338–1344.
4916 Ind. Eng. Chem. Res., Vol. 47, No. 14, 2008 (11) Alonso, L.; Palacios, J. M.; Moliner, R. The performance of some ZnO-based regenerable sorbents in hot coal gas desulfurization long-term tests using graphite as a pore-modifier additive. Energy Fuels 2001, 15, 1396–1402. (12) Slimane, R. B.; Abbasian, J. Utilization of metal oxide-containing waste materials for hot coal gas desulfurization. Fuel Process. Technol. 2001, 70, 97–113. (13) Alonso, L.; Palacios, J. M.; Moliner, R. The performance of some ZnO-based regenerable sorbents in hot coal gas desulfurization long-term tests using graphite as a pore-modifier additive. Energy Fuels 2001, 15, 1396–1402. (14) Woods, M. C.; Gangwal, S. K.; Harrison, D. P.; Jothimurugeasan, K. Kinetics of the reactions of a zinc ferrite sorbent in high-temperature coal gas desulfurization. Ind. Eng. Chem. Res. 1991, 30, 100–107. (15) Gupta, R.; Gangwal, S. K.; Jain, S. C. Development of zinc ferrite sorbents for desulfurization of hot coal gas in a fluid-bed reactor. Energy Fuels 1992, 6, 21–27. (16) Pineda, M.; Fierro, J. L. G.; Palacios, J. M.; Cilleruelo, C.; Ibarra, J. V. Kinetic behavior and reactivity of zinc ferrites for hot gas desulfurization. J. Mater. Sci. 1995, 30, 6171–6178. (17) Slimane, R. B.; Abbasian, J. Regenerable mixed metal oxide sorbents for coal gas desulfurization at moderate temperature. AdV. EnViron. Res. 2000, 4, 147–162. (18) Ryu, S. O.; Park, N. K.; Chang, C. H.; Kim, J. C.; Lee, T. J. Multicyclic Study on Improved Zn/Ti-Based Desulfurization Sorbents in Mid-Temperature Conditions. Ind. Eng. Chem. Res. 2004, 43 (6), 1466– 1471. (19) Sasaoka, E.; Sada, N.; Manabe, A.; Uddin, M. A.; Sakata, Y. Modification of ZnO-TiO2 high-temperature desulfurization sorbent by ZrO2 addition. Ind. Eng. Chem. Res. 1999, 38, 958–963. (20) Siriwardane, R. V.; Gardner, T.; Poston, J. A., Jr.; Fisher, E. P.; Miltz, A. Spectroscopic characterization of nickel containing desulfurization sorbents. Ind. Eng. Chem. Res. 2000, 39, 1106–1110. (21) Siriwardane, R. V.; Poston, J. A.; Evans, G., Jr. Spectroscopic characterization of molybdenum-containing zinc titanate desulfurization sorbents. Ind. Eng. Chem. Res. 1994, 33, 2810–2818.
(22) Kang, S. C.; Jun, H. K.; Lee, T. J.; Ryu, S. O.; Kim, J. C. The characterization of Zn-based desulfurization sorbents on various supports. Hwahak Konghak 2002, 40, 289–297. (23) Jun, H. K.; Jung, S. Y.; Lee, T. J.; Kim, J. C. The effect of HCl and H2O on the H2S removing capacities of Zn-Ti based desulfurization sorbents promoted by cobalt and nickel oxide. Korean. J. Chem. Eng. 2004, 21 (2), 425–429. (24) Jun, H. K.; Lee, T. J.; Ryu, S. O.; Kim, J. C. A study of Zn-Tibased H2S removal sorbents promoted with cobalt oxides. Ind. Eng. Chem. Res. 2001, 40, 3547–3556. (25) Jun, H. K.; Lee, T. J.; Kim, J. C. Role of iron oxide in the promotion Zn-Ti-based desulfurization sorbents during regeneration at middle temperatures. Ind. Eng. Chem. Res. 2002, 41, 4733–4738. (26) Jun, H. K.; Jung, S. Y.; Lee, T. J.; Ryu, C. K.; Kim, J. C. Decomposition of NH3 over Zn-Ti-based desulfurization sorbent promoted with cobalt and nickel. Catal. Today 2003, 87, 3–10. (27) Jun, H. K.; Lee, T. J.; Ryu, S. O.; Yi, C. K.; Ryu, C. K.; Kim, J. C. A study of Zn-Ti-based H2S removal sorbents promoted with cobalt and nickel oxides. Energy Fuels 2004, 18 (1), 41–48. (28) Jung, S. Y.; Lee, S. J.; Lee, T. J.; Ryu, C. K.; Kim, J. C. H2S removal and regeneration properties of Zn-Al-based sorbents promoted with various promoters. Catal. Today 2006, 111 (3), 217–222. (29) Jung, S. Y.; Jun, H. K.; Lee, S. J.; Lee, T. J.; Ryu, C. K.; Kim, J. C. The imprvement of the desulfurization and regeneration properties through the control of pore structures of the Zn-Ti-based H2S removal sorbents. EnViron. Sci. Technol. 2005, 39 (23), 9324–9330. (30) Kaspar, J.; Fornasiero, P.; Graziani, M. Use of CeO2-based in the three-way catalysis. Catal. Today 1999, 50, 285–298. (31) Trovarelli, A.; de Leitenburg, C.; Boaro, M.; Dolcetti, G. The utilization of ceria in industrial catalysis. Catal. Today 1999, 50, 353–367. (32) Wang, S.; Wang, W.; Zuo, J.; Qian, Y. Study of the raman spectrum of CeO2 nanometer thin films. Mater. Chem. Phys. 2001, 68, 246–248.
ReceiVed for reView October 9, 2007 ReVised manuscript receiVed April 27, 2008 Accepted May 2, 2008 IE071357F
