Energy Fuels 2011, 25, 591–595 Published on Web 01/14/2011
: DOI:10.1021/ef101358x
Preparation of Sorbents Loaded on Activated Carbon to Remove H2S from Hot Coal Gas by Supercritical Water Impregnation Biao Qiu, Lina Han, Jiancheng Wang,* Liping Chang, and Weiren Bao* Key Laboratory of Coal Science and Technology, Taiyuan University of Technology, Ministry of Education and Shanxi Province, Taiyuan 030024, People’s Republic of China Received October 7, 2010. Revised Manuscript Received December 18, 2010
A series of sorbents were prepared by depositing the oxide particles of Cu, Mn, and Zn onto the activated carbon (AC) using the supercritical water impregnation (SCWI) method. The morphology and structure, amounts of loading metals, pore volume, and surface area of sorbents were characterized by X-ray diffraction (XRD), inductively coupled plasma-atomic emission spectrometry (ICP-AES), and nitrogen sorption measurements. The sulfidation tests of sorbents were performed using a fixed-bed reactor under ambient pressure and simulated coal-derived gas. The results show that the metal-active component, preparation temperature, and impregnation time during SCWI are the main influencing factors of the desulfurization activity of the sorbent. These factors behave mainly by changing the micropore volume and surface area of sorbents and the dispersion of metal oxide particles on the support. The optimal SCWI conditions for preparing Mn-based sorbents are 0.46 mol/L precursor solution concentration, 380 °C preparing temperature, and 30 min impregnation time. The addition of copper component could effectively promote the dispersion of manganese oxide. The Mn-Cu sorbents prepared by SCWI have high desulfurization efficiency and sulfur capacity compared to the single-metal sorbent. The desulfurization efficiency can be maintained at about 100% in the sulfidation time of 910 min, and the corresponding sulfur capacity is 5.58 g S/100 g sorbent for the sorbent with a 3:7 molar ratio of Mn/Cu prepared by SCWI.
significance to promote the coal gas cleaning technology and improve the efficient use of coal. The sorbents must possess both good sulfidation performance and stable mechanical strength during hot coal gas purification. Preparation methods of sorbents are directly correlative with their configuration and structure, further affect their performances of desulfurization. In recent years, supercritical hydrothermal synthesis has received a great deal of attention as a generalized crystallization method for the production of metal oxide particles.10,11 At the supercritical region, a fast reaction rate and low metal oxide solubility led to an extremely high nucleation rate, which allow for the formation of nano-sized particles. The gas-liquid transport properties and very low surface tension of supercritical water have great contribution in the effective impregnation of metal oxides on porous solid materials and the enhanced dispersion of the particles deep inside the pores. More recently, Otsu and Oshima first deposited particles of manganese oxide, silver, and lead oxide on alumina using a supercritical water impregnation (SCWI) method.12 Xu and Teja have deposited RFe2O3 nanoparticles on activated carbon (AC) and obtained egg-shell and uniform dispersions.13,14 As mentioned above, the use of SCWI in preparing sorbent appears to be a promising method, which can not only make particles have the uniform size and stabilizing chemical composition by optimizing the reaction temperature and pressure but also
1. Introduction Gasification technology obviously raises the efficient use of coal. However, H2S, COS, and CS2 (nearly 90% H2S) in coalderived fuel gas not only corrode equipment and cause catalyst poisoning but also seriously endanger the environment and human health. The concentration of sulfur-containing gas must not exceed 20-100 ppm before passing through the gas turbine and less than 0.05-10 ppm for catalysts in synthesis gas.1-5 The desulfurization purification of coal gas is one of the key technologies of the large-scale application of coal gasification technology. Various inorganic sorbents, such as ZnO, Fe2O3, CaO, MnO, CuO, CuMn2O4, and ZnFe2O4, have been investigated for high-temperature removal of H2S.5-9 Considering the efficiency of heat use and the adapted operation in practice, the development of efficient middle temperature (250-450 °C) desulfurization has very important *To whom correspondence should be addressed. Telephone: 86-3516018482. E-mail:
[email protected] (J.W.); Telephone: 86351-6010482. E-mail:
[email protected] (W.B.). (1) Descamps, C.; Bouallou, C.; Kanniche, M. Energy 2008, 33, 874–881. (2) Shinada, O.; Yamada, A.; Koyama, Y. Energy Convers. Manage. 2002, 43, 1221–1233. (3) Swisher, J. H.; Schwerdtfeger, K. J. Mater. Eng. Perform. 1992, 3, 399–408. (4) Sun, J. F.; Liu, J. Z.; Wang, J.; Zhou, J. H.; Hu, Y. X.; Cen, K. F. Proc. CSEE 2009, 35, 83–88 (in Chinese). (5) Atak€ ul, H.; Wakker, J. P.; Gerritsen, A. W.; Van den Berg, P. J. Fuel 1985, 2, 187–191. (6) Fan, H. L.; Xie, K. C.; Shangguan, J.; Shen, F.; Li, C. H. J. Nat. Gas Chem. 2007, 16, 404–408. (7) Ko, T.; Chu, H.; Chaung, L. Chemosphere 2005, 4, 467–474. (8) Alonso, L.; Palacios, J. M.; Garcı´ a, E.; Moliner, R. Fuel Process. Technol. 2000, 62, 31–44. (9) Slimane, R. B.; Abbasian, J. Adv. Environ. Res. 2000, 4, 147–162. r 2011 American Chemical Society
(10) Viswanathan, R.; Gupta, R. B. J. Supercrit. Fluids 2003, 27, 187–193. (11) Hakuta, Y.; Hayashi, H.; Arai, K. Curr. Opin. Solid State Mater. Sci. 2003, 7, 341–351. (12) Otsu, J.; Oshima, Y. J. Supercrit. Fluids 2005, 33, 61–67. (13) Xu, C. B.; Teja, A. S. J. Supercrit. Fluids 2006, 39, 135–141. (14) Xu, C. B.; Teja, A. S. Appl. Catal., A 2008, 348, 251–256.
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uses the water as a reaction medium without the use of toxic or noxious solvents and simplifys the process and post-treatments, such as calcining and drying. In the present work, metal oxide/AC was prepared by SCWI. The effects of the precursor concentration, the molar ratios of Mn/Cu in the mixed metal oxide, the supercritical water temperature, and the impregnation time during sorbent preparation on the removal of H2S in simulated coal-derived gas were investigated. 2. Experimental Section 2.1. Preparation of Sorbents. The AC made from coal with the particle sizes of 8-10 mesh was used as supports. The preparation of metal oxide/AC sorbent was accomplished in a 300 mL stainless-steel autoclave, equipped with a pressure gauge and a thermocouple. The reactor was heated in an external heating furnace equipped with a temperature controller. The precursor solution was prepared by dissolving a certain mass of Cu(NO3)2 3 3H2O, Zn(NO3)2 3 3H2O, and Mn(NO3)2 in 100 mL of deionized water, and the precursor solution of Mn-Cu sorbents was prepared by directly dissolving Cu(NO3)2 3 3H2O and Mn(NO3)2 in 100 mL of deionized water. This precursor solution and 20 g of AC were added to the autoclave for each run. The solution in the reactor was then heated at a rate of 5 °C/min to the predetermined temperature and was maintained at this temperature for a certain time, which was defined as the impregnation time. When the heating for the reactor was stopped and the temperature was below 200 °C, the pressure valve was opened and the gas-liquid-solid products were quickly separated. The solid sample remaining in the reactor was the sorbent, which was marked with a TxtyWz symbol. The series samples prepared were directly used to remove H2S from hot coal gas without any further treatment, such as drying and calcining. In the TxtyWz sorbent, “x” stands for the temperature of the preparation, “y” stands for the impregnation time, “z” is referred to as the mass ratio of metal oxide in the 100 mL solution to 20 g of AC. For example, T380t30W20Mn stands for the Mn-based sorbent prepared in the conditions of 380 °C, 30 min, and the mass ratio of manganese oxide/AC of 20%. 2.2. Sulfidation Test. The sulfidation tests of the metal oxide/ AC sorbent at atmosphere pressure were carried out using a fixed-bed flow microreactor. The typical reactant gas mixture consisted of 52 vol % H2, 33 vol % CO, about 500 ppm H2S, and N2 balance gas. All sulfidation tests were carried out at 350 °C and a space velocity of 2000 h-1. The concentration of H2S was analyzed by a gas chromatograph equipped with a flame photometry detector (FPD). The test was terminated when the outlet concentration of H2S from the reactor was more than 100 ppm and the efficiency of desulfurization was less than 80%. 2.3. Characterization of the Sorbent. X-ray diffraction (XRD) analyses of the prepared samples were investigated with a Rigaku D/max2500 diffractormeter, using a graphite monochromator and Cu KR radiation sources (λ = 0.154 056 nm), tube voltage of 40 kV, and tube current of 100 mA. The scan rate was 8°/min for 2θ values of 20-75°. The X-ray analyses were performed on the powder of metal oxide/AC. The metal content in the samples was analyzed by inductively coupled plasma-atomic emission spectrometry (ICP-AES). The pore structure was determined via nitrogen adsorption at 77 K using a Micromeritics ASAP2000 analyzer. The surface area and micropore volumes were obtained using the BrunauerEmmett-Teller (BET) surface area.
Figure 1. Desulfurization efficiency of sorbents prepared from different concentration precursor solutions.
the manganese oxide content in sorbents on their desulfurization activity, a series of sorbents were prepared at the temperature of 380 °C, impregnation time of 30 min, and different concentrations of precursor solution. Desulfurization results of four sorbents with various manganese oxide contents ranging from 15 to 30% (the mass ratio of metal oxide in the 100 mL solution to 20 g of AC) are shown in Figure 1. In addition, their physical parameters are shown in Table 1. Results show that the maintaining time of desulfurization efficiency trend to 100% is increased from about 450 to 600 min with the increase of the metal oxide content in the T380t30WzMn series sorbents, and the corresponding sulfur capacity is increased from 3.58 to 4.08 g S/100 g sorbent. ICP results show that the loading amount of Mn in sorbent is increased from 7.28 to 14.11% with increasing the concentration of precursor solution from 0.34 to 0.69 mol/L at an impregnation time of 30 min and a temperature of 380 °C. It can be seen that the sulfur capacity increases obviously with the increasing loading amount of Mn, but the use rate of active component, which was defined as the ratio of actual sulfur capacity and theoretical sulfur capacity, is decreased from 59.66 to 35.25%. When the manganese content of the sorbent is low, the manganese oxide is well-dispersed on the AC support and has a good use rate of active component. For the sorbents with a high manganese content, excess manganese oxide tends to migrate and forms oxidic crystal clusters and the use rate of active component is decreased significantly.15 On the other hand, high manganese content leads toward rapid formation of metal sulfides during the sulfidation reaction, the rate of sulfur adsorbed will decrease because of diffusion limitation. When the manganese content reaches 9.26% (precursor concentration is 0.46 mol/L), the increase of the sulfur capacity was not obvious. Therefore, it is preferable to choose 0.46 mol/L as the concentration of the precursor solution to prepare the sorbent. 3.2. Effect of the Preparing Temperature on the Desulfurization Activity of the Sorbent. Near the critical point (374 °C and 22.1 MPa) of water, the variation of the temperature can cause large changes in the property of supercritical water. To elucidate the factors dominating the formation of the particle in supercritical water, Mn-based sorbents were prepared at different temperatures in the range of 360-400 °C with the 0.46 mol/L precursor solution and impregnation time of 30 min. The XRD patterns and sulfidation results of sorbents
3. Results and Discussion 3.1. Effect of the Active Component Content on the Desulfurization Activity of the Sorbent. To investigate the effect of
(15) Liang, B.; Korbee, R.; Gerritsen, A. W.; Van den Bleek, C. M. Fuel 1999, 78, 319–325.
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Table 1. Physical Parameters of Sorbents Prepared Using the SCWI Method sorbent
concentration of precursor solution (mol/L)
metal content in sorbent (%)
metal loading rate (%)
sulfur capacity (g S/100 g)
theoretical sulfur capacity (g S/100 g)
active component use (%)
T380t30W15Mn T380t30W20Mn T380t30W25Mn T380t30W30Mn T360t30W20Mn T400t30W20Mn T380t15W20Mn T380t45W20Mn T380t60W20Mn T380t30W20Zn T380t30W20Cu
0.34 0.46 0.57 0.69 0.46 0.46 0.46 0.46 0.46 0.49 0.5
7.28 9.26 11.81 14.11 9.18 10.07 8.81 9.70 9.88 12.82 11.64
85.40 84.04 89.37 92.51 83.20 92.58 79.38 88.65 90.56 95.07 85.29
3.58 3.91 4.02 4.08 3.37 1.97 3.18 2.64 2.14 0.11 3.61
6.00 7.63 9.73 11.63 7.56 8.30 7.26 7.99 8.14 6.71 6.23
59.66 51.23 41.21 35.25 44.54 23.73 43.80 33.04 26.29 1.64 57.93
Table 2. Structure Properties of Sorbents Obtained at Different Preparing Temperatures and Times sorbent
micropore area (m2/g)
micropore volume (cm3/g)
pore diameter (nm)
AC T360t30W20Mn T380t15W20Mn T380t30W20Mn T380t60W20Mn T400t30W20Mn
549.00 506.32 496.78 488.14 474.42 446.10
0.250 0.232 0.238 0.223 0.212 0.201
2.255 2.214 2.225 2.229 2.262 2.245
Figure 2. XRD patterns of sorbents prepared at different temperatures.
Figure 4. Effect of the impregnation time on the desulfurization efficiency of the sorbent.
obtained at different preparing temperatures are shown in Figures 2 and 3. It is shown that all sorbents present the obvious diffraction peaks of Mn3O4 and the peak intensity does not change essentially under different temperature conditions. However, the results in Figure 3 show that the desulfurization efficiency and breakthrough time of the Mnbased sorbent prepared at 380 °C are obviously better than that of the sorbents prepared at 360 or 400 °C. An increase of the temperature will cause the dielectric of water and the
solubility of metal oxide will be decreased, which is favorable to form the particles.16,17 The amount of Mn loading on AC increases from 9.18 to 10.07% with the temperature rising from 360 to 400 °C, which is agreed with the nucleation theory. The structure properties of sorbents are shown in Table 2. It can be seen that the micropore volume and micropore surface area of the sorbent are decreased from 0.232 cm3/g and 506.32 m2/g to 0.201 cm3/g and 446.10 m2/g, respectively. It can be inferred that metal oxide particles on AC supports prepared at 400 °C are seriously agglomerated and the pore structure of sorbents obtained is destroyed. As mentioned above, the desulfurization capacity of the sorbent can be improved by the increase of the Mn loading amount and restrained by the damage of the pore structure and the agglomeration of active components when the temperature to prepare the sorbent has risen. Therefore, it should be preferable to prepare the sorbent at 380 °C.
(16) Hakuta, Y.; Haganuma, T.; Sue, K.; Adschiri, T.; Arai, K. Mater. Res. Bull. 2003, 38, 1257–1265.
(17) Zhao, D.; Wu, X.; Guan, X.; Han, E. J. Supercrit. Fluids 2007, 42, 226–233.
Figure 3. Effect of the preparation temperature on the desulfurization efficiency of the sorbent.
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3.3. Effect of the Impregnation Time on the Desulfurization Activity of the Sorbent. To understand the effect of the impregnation time in SCWI on the desulfurization activity, the four sorbents subjected in SCW from 15 to 30, 45, and 60 min were tested under the simulated coal gas, respectively. The sulfidation results are shown in Figure 4 and Table 1. It can be seen that the loading rate of Mn, which was defined as the metal loaded accounts for the total amount of metal in solution, increases from 79.38 to 90.56% when the impregnation time is extended from 15 to 60 min. However, the sulfur capacity of sorbents increases first and then decreases with the increase of time. The H2S removal capacity of T380t30W20Mn obtained from 30 min impregnation time is the best in the four sorbents. Manganese oxide particles are deposited more inward of supports and have good dispersion with increasing the impregnation time, while a prolonged impregnation time resulted in destroying the pore structure of sorbents. From Table 2, it can be seen that the micropore surface area and pore volume of the sorbents are decreased from 496.78 m2/g and 0.238 cm3/g to 474.42 m2/g and 0.212 cm3/g when the impregnation time is extended from 15 to 60 min, respectively. These results also implied that the effect of the impregnation time on the desulfurization activity of the sorbent is presented by adjusting the amount of metal loading on the support, micropore surface area, and pore volume.
3.4. Effect of Metal Oxide on the Desulfurization Activity of the Sorbent. The components of sorbents, which are controlled by the operating conditions in preparation, significantly affect their activity of desulfurization. The sorbents with zinc oxide and copper oxide supported onto AC were prepared using the SCWI method. The physical parameters of sorbents prepared are also shown in Table 1. It can be seen that the content of zinc, manganese, and copper on the AC support and the metal loading rate is 12.82, 9.26 and 11.64% and 95.07, 84.04, and 85.29%, respectively. These results suggest that higher transport properties, fast reaction rates, and low metal solubilities lead to an extremely high nucleation rate, which makes the metal-active component supported onto AC have the high loading rate in supercritical water. However, there exist an obvious difference for the sorbents obtained from different metal oxide even at the same SCWI conditions with the 20% mass ratio of metal oxide and AC, 380 °C preparing temperature, and 30 min impregnation time. The sulfidation breakthrough curve of metal oxide/AC prepared by SCWI is shown in Figure 5. The efficiency of T380t30W20Mn removing H2S from hot gas is steadily more than 99.9% within the first 450 min and decreased to 80% when the reaction time is up to 640 min. However, T380t30W20Zn exhibits almost no desulfurization ability, and its breakthrough time is only 40 min. The sequence of sulfur capacity and desulfurization precision of three sorbents was T380t30W20Mn > T380t30W20Cu > T380t30W20Zn. It is unexpected that the content and loading rate of the metal-active component is reached up to 12.82 and 95.07% for the Zn/AC sorbent prepared by SCWI, but it shows almost no desulfurization activity. The XRD patterns of sorbents with different metal-active component are shown in Figure 6. The peaks of manganese oxide are weak, while the peak intensity of zinc oxide is very high. A plausible explanation is that manganese oxide is well-dispersed on the AC, whereas zinc oxide has the highest metal loading rate and serious aggregation in the same preparation conditions. The sulfidation behaviors and XRD patterns of Mn-Cu sorbents with different molar ratios of manganese element and copper element are also shown in Figures 5 and 6. It can be seen that the metal-active component has an obvious influence on the desulfurization efficiency of the sorbent. The bimetal mixed oxide of copper and manganese can promote the
Figure 5. Effect of metal oxide on the desulfurization efficiency of sorbents.
Figure 6. XRD patterns of sorbents with different metal-active components (a) before and (b) after sulfidation.
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removal of H2S from hot coal gas compared to single metal oxide, and the sorbent with the 3:7 molar ratio of manganese/ copper has the optimum desulfurization capacity. XRD results in Figure 6 show that the diffractions ascribed to Cu2O and Cu appear in the Mn-Cu sorbents and weak diffraction peaks of Cu2O become noticeable when the Cu content is increased. No crystalline manganese oxide and manganese-copper oxide peak can be observed. It is presumed that the addition of copper component could effectively promote the dispersion of manganese oxide, which may form MnOx nanoclusters with a diameter below the detection limit of XRD and is favorable for the progress of the sulfidation reaction. Figure 6b shows the diffraction patterns of various metal oxide sorbents after removal of H2S. The Mn3O4 and Cu2O phases were transformed into MnS and Cu2S, respectively; however, a small quantity of Cu remained after the sulfidation process. Additionally, no noticeable peaks, such as ZnS, were observed, indicating no interaction between ZnO and H2S in the sulfidation process. It is coincident with the Zn/ AC sorbent that shows almost no desulfurization activity. In the cases of MnS and Cu2S, the reactions can be described as follows:9,15 Mn3 O4 þ 3H2 S þ H2 f 3MnS þ 4H2 O ð1Þ Cu2 O þ H2 S f Cu2 S þ H2 O
4. Conclusions The sorbents prepared by SCWI can obtain high loading amounts of Cu, Mn, and Zn in AC. The desulfurization activity of the sorbent, the loading amount of metal oxide, the dispersion of particles, and the micropore structure properties of the sorbent can be controlled by changing the preparation temperature, impregnation time, and precursor solution concentration of metal oxide. The optimal SCWI condition for preparing Mn-based sorbents is at 380 °C temperature, 30 min time, and 0.46 mol/L precursor solution concentration. The sorbent T380t30W20Mn obtained in these conditions has the appropriate micropore volume, surface area, and well dispersion of metal oxide particles, which is favorable to remove H2S from hot gas. The addition of copper component could effectively promote the dispersion of manganese oxide and improve the sulfidation capacity of the sorbent. The time of desulfurization efficiency at about 100% is maintained about 910 min, and the corresponding sulfur capacity is 5.58 g S/100 g sorbent for the sorbent with a 3:7 molar ratio of Mn/Cu prepared by SCWI. Acknowledgment. This work was supported by the National Natural Science Foundation of China (20976117), the Shanxi Province Natural Science Foundation (2010011014-3 and 2010021008-1), and the Shanxi Province Basic Conditions Platform for Science and Technology Project (2010091015).
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