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Energy & Fuels 2002, 16, 1378-1386
Estimation of Multiple-Cycle Desulfurization Performance for Extremely Low-Concentration Sulfur Removal with Sorbent Containing Zinc Ferrite-Silicon Dioxide Composite Powder Makoto Kobayashi,*,† Hiromi Shirai, and Makoto Nunokawa Yokosuka Research Laboratory, Central Research Institute of Electric Power Industry, Nagasaka 2-6-1, Yokosuka 240-0196, Japan Received February 14, 2002
A sulfidation reaction model was built to estimate the durability of the deep desulfurization performance of zinc ferrite sorbent in a fixed bed reactor. The model is based on detailed change in sulfur capacity of zinc and iron during multiple desulfurization cycles in pressurized coal gas conditions. The model estimated the relation between the breakthrough time and the sulfur capacities of the sorbent. The relation between the breakthrough time of the sorbent bed and each sulfur capacity was estimated at various sulfur loads to the reactor, which indicated that the breakthrough time strongly depends on the change in zinc sulfur capacity. Extrapolation of experimental data for the initial 20 cycles of desulfurization suggested that zinc sulfur capacity might level off at around 40% of its initial value within 50 cycles. If the stabilization of zinc sulfur capacity is established, breakthrough time is maintained at around 70% of that of fresh sorbent over more than 500 repetitions of multiple desulfurization cycles. It was also suggested that the verification of the stabilization effect require multiple cycle testing over more than 50 cycles.
Introduction Zinc-containing sorbents were extensively investigated because of sufficient affinity of zinc for sulfur compounds at high temperature. It is expected that their achievable sulfur concentrations are as low as the tolerance of a specific application such as high temperature fuel cells. Zinc oxide-based sorbent has been used for sulfur removal at elevated temperature around 300 °C. As the zinc oxide absorbs sulfur compounds strongly, regeneration of spent sorbent requires a significantly high temperature, over 900 °C. Thus, its conventional application was limited to disposal use such as a polishing sorbent for desulfurization of liquefied natural gas. It is sought to reconcile sulfur-removal performance and regenerability by introducing a binary oxide of zinc and another element. Several types of double oxides containing zinc were extensively studied. Many attempts have been made to develop high-temperature desulfurization sorbents using a double oxide, such as zinc ferrite,1-3 zinc titanate,4-7 and other metal oxides.8 * Corresponding author. Phone: +81-468-56-2121. Fax: +81-46856-3346. E-mail:
[email protected]. † Corresponding address: 1-6-1 Ohtemachi, Chiyoda-ku, Tokyo 1008126. Japan. Phone: +81-3-3201-6601. Fax: +81-3-3287-2841. (1) Underkoffler, V. S. Summary and assessment of METC zinc ferrite hot coal gas desulfurization test program, Final report Volume I. U.S. DOE Report DOE/MC/21098-2247, 1986a, p 182. (2) Gangwal, S. K.; Harkins, S. M.; Woods, M. C.; Jain, S. C.; Bossart, S. Bench-scale testing of high-temperature desulfurization sorbent. J. Environ. Prog. 1989, 8, 265-269. (3) Ayala, R. E.; Gal, E.; Gangwal, S. K.; Jain, S. Enhanced durability of high-temperature desulfurization sorbents for movingbed applications Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1990, 35, 120-127.
Recent investigation9,10 is focusing on the zinc-containing sorbents, such as zinc ferrite and series of zinc titanate. Harrison indicated the weakness of zinc ferrite concerning zinc vaporization and overreduction of iron oxide at high temperature, typically higher than 600 °C. Thus, development of desulfurization sorbent has shifted to zinc-based sorbents, together with copper-based sorbents.1 Some meaningful results of the series of studies on zinc titanate are summarized in the literature.11 Sulfidation performance of zinc titanate exhibits affordability at around 650 °C. Their regeneration profile, (4) 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. (5) Lew, S.; Sarofim, A. F.; Flytzani-Stephanopoulos, M. Modeling of the Sulfidation of Zinc-Titanium Oxide Sorbents with Hydrogen Sulfide. AIChE J. 1992, 38, 1161-1169. (6) Lew, S.; Sarofim, A. F.; Flytzani-Stephanopoulos, M. The Reduction of Zinc Titanate and Zinc Oxide Solids. Chem. Eng. Sci. 1992, 47, 1421-1431. (7) Lew, S.; Sarofim, A. F.; Flytzani-Stephanopoulos, M. Sulfidation of Zinc Titanate and Zinc Oxide Solids. Ind. Eng. Chem. Res. 1992, 31, 1890-1899. (8) Anderson, G. L.; Berry, F. O.; Hill, A. H.; Ong, E.; Laurence, R. M.; Shah, R.; Feldkirchner, H. L. Development of hot gas cleanup system. Final report. U.S. DOE Report DOE/MC/22144-2722, Contract DE-AC21-85MC22144; U.S. DOE, Morgantown Energy Technology Center: Morgantown, WV, 1988; pp 1-166. (9) Flytzani-Stephanopoulos, M.; Li, Z. Kinetics of Sulfidation Reactions Between H2S and Bulk Oxide Sorbents. In NATO ASI “Desulphurization of hot coal gas with regenerable metal oxide sorbents: new developments”; Atimtay, A. T., Harrison, D. P., Eds.; Springer-Verlag: Berlin Heidelberg, 1998; Vol. 42, p 179. (10) Harrison, D. P. Performance Analysis of ZnO-Based Sorbents in Removal of H2S From Fuel Gas. In NATO ASI “Desulphurization of hot coal gas with regenerable metal oxide sorbents: new developments”; Atimtay, A. T.; Harrison, D. P., Eds.; Springer-Verlag: Berlin Heidelberg, 1998; Vol. 42, p 213-242.
10.1021/ef020024m CCC: $22.00 © 2002 American Chemical Society Published on Web 10/08/2002
Estimation of Multiple-Cycle Desulfurization Performance
Energy & Fuels, Vol. 16, No. 6, 2002 1379
however, is gradually degraded. Difficulty in regeneration should arise when zinc titanate is applied to a fixed bed reactor because the system does not allow frequent replacement of the sorbent. Although zinc titanate and Z-Sorb B, both zinc-based sorbents, have high performance on sulfur removal, pre-breakthrough concentration of H2S is limited to
