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Ind. Eng. Chem. Res. 2004, 43, 1466-1471
Multicyclic Study on Improved Zn/Ti-Based Desulfurization Sorbents in Mid-Temperature Conditions Si Ok Ryu,†,‡ No Kuk Park,† Chih Hung Chang,‡ Jae Chang Kim,§ and Tae Jin Lee*,† School of Chemical Engineering and Technology, Yeungnam University, Kyongsan 712-749, Korea, Department of Chemical Engineering, Kyungpook National University, Taegu 702-701, Korea, and Department of Chemical Engineering, Oregon State University, Corvallis, Oregon 97331
Conventional zinc titanate sorbent was modified with cobalt oxide, nickel oxide, molybdenum oxide, and iron hydroxide in order to achieve the desirable reactivity and mechanical durability in mid-temperature ranges, and 100 repetitive cycles of sulfidation/regeneration were carried out to investigate its reactivity and attrition resistance. In the attrition tests for the improved sorbents, the results showed that the attrition resistance was 94.08% and that the sulfur capacity was 23 g of S/100 g of sorbent and 15 g of S/100 g of sorbent in the 3- and 100-cycle tests, respectively. The changes in the properties of the ZTG40 sorbent formulated in this study were also investigated with the aid of X-ray diffraction, scanning electron microscopy, Hg porosimetry, and Brunauer-Emmett-Teller surface area measurement during a 100-cycle test. From the analysis, it was concluded that ZTG40 has the desirable physical and chemical properties for desulfurization of coal fuel gases at medium temperatures and is suitable for fluidized-bed coal gas desulfurization systems. Introduction The integrated gasification combined cycle (IGCC) is considered as one of the most thermally efficient, economically attractive, and environmentally acceptable technologies for power generation from coal.1 The main components in an IGCC power plant are a coal gasification unit, a gas cleanup system, and power generation facilities. Hot gas desulfurization (HGD) is a very important process in the gas cleanup system, and its main role is to remove harmful sulfur, which exists in the form of hydrogen sulfide (H2S) under the highly reducing conditions of the gasifier, from the coal-derived fuel gas. The high concentration of H2S in hot coal gas causes two major problems. Not only do its combustion products contribute to the air pollution, but also it is so corrosive that it causes serious damage to gas turbines and fuel cells in the advanced power plants. Therefore, it is necessary to reduce the sulfur content of the gasified fuel gas either from several hundred parts per million levels and down for gas turbines or from 1 ppm and down for fuel cells in power plants.2-4 Study on HGD for coal gas in IGCC has been concentrated on the development of regenerable metal oxide sorbents with high sulfur-removing capacity and long-term durability at high temperatures.5-26 It is known that zinc-based sorbents is thermodynamically favorable for H2S removal.27 Among the zinc-based sorbents, zinc titanate sorbent has been considered to be the best metal oxide sorbent at high temperatures. In recent years, there is a tendency to lower the operational temperatures of the HGD process down below 500 °C to avoid the difficulty in operation and the stability issue at high temperature. Because the conventional zinc titanate sorbents showed poor reactivity at medium temperature as a result of the incomplete regeneration, it is necessary to improve †
Yeungnam University. Oregon State University. § Kyungpook National University. ‡
the conventional zinc titanate based sorbents for a fluidized-bed desulfurization process operating in the medium-temperature ranges. However, only a few studies on multicycle tests were carried out for zinc titanate sorbents in middle-temperature ranges. Therefore, a modification of zinc titanate sorbents with several supports and additives was performed on the basis of our previous studies in order to achieve high sulfur absorption capacity and mechanical strength even at medium temperatures.24-26 Then, 100 cycles of sulfidation/regeneration for zinc titanate sorbents were carried out to assess the durability and attrition resistance of the sorbents in middle-temperature ranges in this study. The modified sorbents were prepared by a granulation method. In addition, the changes of physical properties for the sorbents before and after the reaction were investigated using X-ray diffraction (XRD; Rigaku D/MAX-2500), scanning electron microscopy (SEM; Hitachi S-4100), Hg porosimetry (Micromeritics AutoPoreIII-9320), and Brunauer-Emmett-Teller (BET) surface area measurement (Micromeritics Gemini 2375). Experimental Section Preparation of Sorbents. Advanced zinc titanate (ZTG) sorbents used in this study were prepared by a solid mixing method. Zinc oxide and titanium dioxide were sufficiently mixed with an inorganic binder, R-Al2O3. Cobalt oxide, nickel oxide, molybdenum oxide, and iron(III) hydroxide were added to the mixture in order to achieve the desirable reactivity and attrition resistance in the medium-temperature ranges.28,29 The particle sizes of metal oxides used in the preparation of sorbent were about 20 µm. The sorbents were formulated by a granulation method and were dried for 24 h to remove moisture from the material at 150 °C. The dried sorbents were calcined at 750 °C for 2 h in a muffle furnace. They were then ground, and the particles of 150-300 µm diameter were collected through sieving. The selected sorbents were calcined for 2 h secondarily. The
10.1021/ie030452v CCC: $27.50 © 2004 American Chemical Society Published on Web 02/18/2004
Ind. Eng. Chem. Res., Vol. 43, No. 6, 2004 1467 Table 1. Compositions of ZTG Series Sorbents sorbent
components
calcination condition
ZTG32 ZTG34 ZTG36
ZnO, TiO2 ) 0.6R-Al2O3, Co3O4, NiO ZnO, TiO2, R-Al2O3, Co3O4 (5 wt %), NiO (5 wt %), MoO3 (3 wt %) ZnO/TiO2 ) 0.6, R-Al2O3, Co3O4 (10 wt %), NiO (10 wt %), MoO3 (3 wt %) ZnO, TiO2, R-Al2O3, Co3O4 (10 wt %), NiO (10 wt %), MoO3 (3 wt %) ZnO, TiO2, R-Al2O3, MoO3 (3 wt %), FeO(OH) (10 wt %) ZnO, TiO2, R-Al2O3, Co3O4 (10 wt %), NiO (10 wt %), MoO3 (3 wt %), FeO(OH) (10 wt %) ZnO, TiO2, R-Al2O3, MoO3 (3 wt %), FeO(OH) (10 wt %)
750 °C for 5 h, 1000 °C for 1 h first at 750 °C for 5 h and second at 850 °C for 0.5 h first at 750 °C for 5 h and second at 850 °C for 0.5 h
ZTG38 ZTG39 ZTG40 ZTG41
Table 2. Experimental Conditions for the ZTG40 Sorbents condition temperature (°C) pressure (atm) flow rate (mL/min) gas composition (vol %) H2S H2 CO CO2 H2O O2 N2
sulfidation
regeneration
480 1 250
580 1 250
1.0 11.7 19.0 6.8 10.0 balance
10.0 5.0 balance
mole ratio of Zn to Ti was fixed to 1.5:1. Compositions of ZTG series sorbents are shown in Table 1. Attrition Resistance. The mechanical strength of the ZTG formulations was investigated by the ASTMD-5757 attrition method. The perforated air-jet plate containing three 0.3-mm-diameter holes was installed at the bottom of the ASTM attrition tester. The sample was fluidized in the tester for 5 h by passing nitrogen gas through the holes. The remaining sample from the tube was collected, and then its weight and particle distribution were measured at the end of the test. The test for attrition resistance in this study was performed in the dry-basis conditions. The loading amount of the fresh ZTG formulations was 50 g, and the flow rates of the nitrogen gas were 7 and 10 L/min. Sulfidation and Regeneration. Multiple cycles of sulfidation/regeneration were performed in a fixed-bed quartz reactor with a 1 cm diameter placed in an electric furnace. A detailed description of the experimental setup has been given elsewhere.24-26 Flow rates of simulated coal gas entering the reactor were set to 250 mL/min and controlled by mass flow controllers. All of the volumetric flows of gases were measured at the standard temperature and pressure (STP) conditions. The outlet gases from the reactor were automatically analyzed by a gas chromatograph (Shimadzu 8A) equipped with a thermal conductivity detector (TCD). The column used in the analysis was a 1/8-in. Teflon tube packed with Chromosil 310 (Sufalco). The detectability of SO2 with the TCD used in this study was about 50 ppm. The experimental conditions and the compositions of mixed gases for sulfidation/regeneration are given in Table 2. 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 it reached the regeneration temperatures. Then, the sulfidated sorbents were regenerated by introducing the air diluted with nitrogen. Regeneration was terminated when SO2 could not be detected at the reactor outlet.
first at 750 °C for 2 h and second at 850 °C for 0.5 h first at 750 °C for 2 h and second at 850 °C for 0.5 h 750 °C for 2 h 750 °C for 2 h
Table 3. Attrition Resistance of the ZTG Sorbents attrition resistance (%) sorbent
total sieving
150-300 µm
flow rate (L/min)
ZTG32 ZTG34 ZTG36 ZTG38 ZTG39 ZTG40 ZTG41
87.36 97.68 92.01 99.69 98.14 98.24 98.02
78.75 92.48 88.04 96.09 95.38 94.08 95.76
10 7 7 7 7 7 7
Result and Discussion Our previous works on the zinc titanate sorbent showed that its reactivity was significantly decreased when the operational temperature was lower than 500 °C.24,25 Its reactivity would decrease further if a coal gas contained water contents. The effects of additives in the zinc titanate based sorbents were also investigated in the previous studies. The modified zinc titanate sorbents (ZTC) with cobalt additive showed very high sulfurremoving capacity and no significant deactivation of sorbents in the multicycle tests at both high and medium temperatures. The XRD and Fourier transform infrared analyses, however, indicated that sulfates might be formed in the ZTC sorbent during the regeneration at the medium-temperature range and those could cause SO2 slippage in the subsequent cycles as a result of the incomplete regeneration. The previous work also showed that the sorbent with nickel additive alone has a tendency to lengthen the regeneration time even though it has a good desulfurization efficiency. Meanwhile, the selected sorbents containing both cobalt oxide and nickel oxide have better performance without the problems of SO2 slippage and regeneration time extension. On the basis of our previous results, we modified the zinc titanate sorbents with several metal oxides as additives to extend their operational conditions into the medium-temperature range. Cobalt oxide (Co3O4) and nickel oxide (NiO) were added to the formulation to enhance the absorption efficiency at the lower temperature. MoO3 was added to improve its attrition resistance. FeO(OH) was also introduced into the sorbent to improve its initial reactivity in the medium-temperature range. Mechanical Durability and Sulfur Absorption Capacity. ASTM attrition tests for all formulations of modified zinc titanate sorbents were carried out to select a formulation for a 100-cycle test. The results of ASTM attrition tests are given in Table 3. It indicated that ZTG38, ZTG39, ZTG40, and ZTG41 formulations had high enough mechanical strength for a multicycle test. ZTG38, ZTG39, ZTG40, and ZTG41 formulations with 150-300-µm-diameter particles showed 96.09%, 95.38%, 94.08%, and 95.76% attrition resistance, respectively. Attrition resistance of commercial FCC as the reference material is about 80% (AI ) 20). If attrition resistance
1468 Ind. Eng. Chem. Res., Vol. 43, No. 6, 2004
Figure 1. Breakthrough curves for the ZTG40 sorbent for a 100cycle test.
Figure 3. XRD patterns of ZTG40 sorbents at five different cycles.
Figure 2. Sulfur capacity of the ZTG40 sorbent for a 100-cycle test.
based on total sieving is converted into the attrition index (AI), the value of AI for the ZTG40 sorbent will be less than 2. Even though the attrition resistance of ZTG40 is slightly lower than that of ZTG38, ZTG39, and ZTG41, it was selected for a 100-cycle test because of its excellent reactivity during the initial stage. In the screening tests, ZTG40 showed better sulfur capacity than any other modified zinc titanate sorbents.25 In this study, 100 repetitive cycles of sulfidation/regeneration were carried out not only to investigate its reactivity and durability but also to assess its compatibility with the hot gas cleanup unit. Figure 1 shows the H2S breakthrough curves for the ZTG40 sorbent for a 100cycle test. The breakthrough point was defined as the concentration of H2S in the outlet gas, which reached 50 ppm in this study. Amounts of sulfur absorbed were calculated from the H2S breakthrough curves presented in Figure 1. It is considered as one cycle when both sulfidation and regeneration were completed. The sulfurremoving capacity for the first 15 cycles was about 23 g of S/100 g of sorbent as shown in Figure 2. It started to decrease after 15 cycles and resulted in a sulfur capacity of around 18 g of S/100 g of sorbent between 45 and 60 cycles. It maintained a sulfur capacity of 15 g of S/100 g of sorbent even after 100 cycles. Because 15 g of S/100 g of sorbent is a general standard of excellence for the sulfur capacity of desulfurization sorbents, it is considered that ZTG40 has an excellent performance for desulfurization and durability. The changes in structure
and the physical properties of the ZTG40 sorbents during a 100-cycle test were investigated with the aid of XRD, SEM, Hg porosimetry, and BET surface area measurement. Structural Changes. As was mentioned previously, ZTG40 showed a very high sulfur-removing capacity resulting from the multicycle test at medium temperatures. Also, deactivation of sorbents proceeded very slowly at medium temperatures. The structural changes of sorbents before and after the reaction were examined by an X-ray diffractometer. In Figure 3, XRD patterns of the ZTG40 sorbent at five different cycles indicated a spinel structure of Zn2TiO4. Strong peaks of Zn2TiO4 were observed in XRD measurements for the fresh ZTG40 sorbent. However, the peak intensity for Zn2TiO4 was gradually decreased along with increasing sulfidation/regeneration cycles, while the peaks for ZnO and TiO2 appeared and their peak intensity increased after 5 cycles. It is considered that Zn2TiO4 was formed in the ZTG40 sorbent during calcination at high temperature and was decomposed into ZnO and TiO2, and then zinc oxide was converted into ZnS during the sulfidation processes. These ZnS and TiO2 should be regenerated into their original Zn2TiO4 structure without the formation of sulfate or pure oxide phases. However, the XRD peaks of oxides imply that these oxides were not completely restored to their initial Zn2TiO4 phase after the regeneration process as a result of the insufficient regeneration temperature. It is believed that these pure oxides are the cause for the progressive deterioration of reactivity over 100 cycles. Change of Physical Properties. SEM morphologies of the fresh ZTG40 sorbent and of those after 26, 50, 80, and 100 cycles of sulfidation/regeneration are shown in Figure 4. Most of the sorbents maintained their
Ind. Eng. Chem. Res., Vol. 43, No. 6, 2004 1469
Figure 4. SEM morphologies of the ZTG40 sorbents before/after reaction: (a) fresh; (b) 26 cycles; (c) 50 cycles; (d) 80 cycles; (e) 100 cycles.
1470 Ind. Eng. Chem. Res., Vol. 43, No. 6, 2004 Table 4. Physical Properties of Fresh ZTG40 Obtained from Hg Porosimetry particle size (µm)
total pore area (m2/g)
median pore diameter (Å)
bulk density (g/ml)
skeletal density (g/ml)
150-300 150-300
1.761 6.476
3825 1487
2.94 1.61
5.45 1.97
sample ZTG-40 fresh after 100 cycles
cycle test. The changes in the properties of the ZTG40 sorbent during a 100-cycle test were also investigated with the aid of XRD, SEM, and BET. On the basis of these analyses, it was concluded that ZTG40 has the desirable physical and chemical properties for the desulfurization of coal fuel gases and would be an excellent sorbent for the hot gas desulfurization system operated at the medium-temperature range. Acknowledgment
Table 5. BET Surface Area Measurement of ZTG40 surface area (m2/g) reaction cycle number sample
fresh
5
19.5
20
50
99.5
100
ZTG-40
0.77
1.01
0.42
1.10
1.53
0.74
2.37
shapes and demonstrated excellent durability even after 100 cycles. The average particle sizes of fresh sorbents were uniformly distributed in the range of 150-300 µm. However, some of the sorbents cracked and coagulated after many cycles of sulfidation/regeneration. It is considered that the cracking on the surface of the sorbents is a spalling due to either the repeated contraction or expansion of the sorbents or Fe species, which was added to ZTG40 in order to increase the initial reactivity of the sorbents. The physical properties of the fresh ZTG40 obtained from Hg porosimetry are shown in Table 4. The changes of physical properties of the sorbent over a 100-cycle test were studied by evaluation of the BET surface area measurement. Table 5 shows the changes in the BET surface area of ZTG40 at several different cycles. The initial value of fresh sorbents was around 0.77 m2/g. The values of the BET surface areas increased along with the sulfidation/ regeneration cycles, i.e., 1.01 m2/g for 5 cycles, 1.10 m2/g for 20 cycles, 1.53 m2/g for 50 cycles, and 2.37 m2/g for 100 cycles, respectively. The surface area of the sorbents after 100 cycles increased more than 3 times over that of the fresh sorbents. The results obtained from the BET surface area measurement could possibly be explained by several theories. First, in the medium-temperature range, the change of physical properties of the sorbents could take place during the repeated sulfidation/ regeneration cycles as a result of the large molar volume changes of the sorbents.24,30 Volume expansion/contraction could cause the sorbents to spall or crack. Second, the surface area increased because of the newly formed small pores, which were formed because the metallic zinc in the sorbent gradually diffused to the surface and vaporized during the sulfidation/regeneration cycles.26 Third, cracking of the sorbents due to the deterioration of thermal stability over multiple cycles might cause an increase in the surface area. Considering the temperature range of our experiments, it is believed that the increase of the surface area is most likely caused by spalling due to volume expansion/contraction during multiple cycles rather than the other reasons. Conclusion The newly formulated zinc titanate sorbents (ZTG40) with Co3O4, NiO, MoO3, and FeO(OH) were prepared, and the 100-cycle tests for reactivity and attrition resistance were carried out in this study. The results showed that the attrition resistance was 94.08% and the sulfur capacity was 23 g of S/100 g of sorbent for a 3-cycle test and 15 g of S/100 g of sorbent for a 100-
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Received for review May 27, 2003 Revised manuscript received October 17, 2003 Accepted November 26, 2003 IE030452V
