Ind. Eng. Chem. Res. 1998, 37, 4157-4166
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Zeolite-Supported Metal Oxide Sorbents for Hot-Gas Desulfurization Lee D. Gasper-Galvin* Federal Energy Technology Center, U.S. Department of Energy, 3610 Collins Ferry Road, Morgantown, West Virginia 26507
Aysel T. Atimtay Middle East Technical University, Environmental Engineering Department, 06531 Ankara, Turkey
Raghubir P. Gupta† Research Triangle Institute, P.O. Box 12194, Research Triangle Park, North Carolina 27709
This paper describes the results of the development of supported mixed-metal oxide sorbents for hot-gas desulfurization capable of withstanding multiple sulfidation/regeneration cycles at 871 °C (1600 °F). The sorbents consisted of various combinations of copper, molybdenum, and/ or manganese oxides supported on a high silica-containing zeolite. These sorbents were tested in a fixed-bed reactor with simulated coal gas at 205 kPa (15 psig). The combination of all three metal oxides displayed synergism in enhancing efficiency for H2S removal and improved the crush strength on the pellets. Copper oxide was the most active component for reaction with H2S, while molybdenum and manganese oxides appeared to act as catalysts/promoters. During multicycle testing, this ternary metal oxide sorbent retained its reactivity and mechanical strength. Introduction The passage of the Clean Air Act Amendments has presented a major problem to the coal-fired electric utilities: compliance with maximum allowable SO2 emissions of 1.2 lb/million Btu by the year 2000 (Nan, 1994). This provides a primary motivation to develop advanced coal-to-electricity conversion processes that are more economical, environmentally superior, and operationally reliable. An integrated gasification combined cycle (IGCC) system with hot-gas cleanup (HGC) is a process that shows promise for higher thermal efficiency and superior environmental performance than conventional processes. Although thermal efficiencies of 30-35% are typical for conventional pulverized coal plants, efficiencies of 43-46% may be easily attainable by IGCC with HGC. This improvement in efficiency would represent a significant decrease in the cost of electricity along with a reduction in SO2 and CO2 emissions. One of the major barriers in the development of HGC for IGCC plants is the need for an economical regenerable sorbent for desulfurization of coal gases. A twopart article by Swisher and Schwerdtfeger (1992a,b) has reviewed the development of mixed-metal oxide sorbents for high-temperature H2S removal. In this review, binary oxides of Ca, Fe, Zn, Cu, Ni, and Mn were discussed. This review provides a good technical introduction for the reader unfamiliar with the area. Experimental studies of mixed-metal oxide sorbents conducted at the Massachusetts Institute of Technology (MIT) by Tamhankar et al. (1986), Lew et al. (1989, 1992), and Lew (1990); at the Research Triangle Insti* To whom correspondence is addressed. Current address: 6 Country Club Lane, Shenandoah, IA 51601. † Telephone: 919-541-8023. Fax: 919-541-8000. E-mail:
[email protected].
tute (RTI) by Gangwal et al. (1988), Woods et al. (1989), Gupta and Gangwal (1992), and Gupta et al. (1992); at the U.S. Department of Energy/Morgantown Energy Technology Center (DOE/METC) by Grindley and Steinfeld (1981) and Grindley (1991); at General Electric (GE) by Ayala (1991) and Ayala et al. (1992); at Electrochem by Desai et al. (1990); and at the Institute of Gas Technology (IGT) by Anderson and Berry (1987) were evaluated to arrive at possible candidates for H2S removal. These experimental studies, along with the thermodynamic predictions of Westmoreland and Harrison (1976), identified six metal oxides that have the potential to be desulfurization sorbents. These include oxides of iron, vanadium, zinc, copper, manganese, and molybdenum. Iron oxide, which yields equilibrium H2S concentrations in the 100-ppmv range, was researched intensively in the 1970s. Although iron oxide has a high sulfur capture capacity and possesses high reactivity for H2S, its applicability is limited to a low-Btu coal gas and temperatures up to 500 °C. Higher temperatures (>550 °C) lead to severe sorbent decrepitation due to excessive reduction and iron carbide formation (Gangwal et al., 1988; Gupta et al., 1992). Furthermore, at 700 °C in the presence of coal gases, Fe3O4 is reduced to FeO; the reduction to FeO has been shown to have a detrimental effect on sulfidation reactivity (Focht et al., 1988). However, at low temperatures, iron oxide sorbents can be safely used for H2S removal. Ishikawajimi-Harima Heavy Industries (IHI) in Japan has developed a hotgas desulfurization process using raw iron ore from Australia as a sorbent (Sugitani et al., 1987). This process operates at 420 °C with a low-Btu coal gas. Furthermore, Van der Wal (1987) demonstrated that iron oxide sorbents supported on silica can provide enhanced stability for desulfurization over that of unsupported iron oxide at 400 °C.
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4158 Ind. Eng. Chem. Res., Vol. 37, No. 10, 1998
Comparative thermogravimetric testing of the oxides of Mn, Ca, Zn, and V by Westmoreland et al. (1977) showed that the reaction rates of V2O3 with H2S were approximately 1 order of magnitude less than those of CaO and ZnO and 2 orders of magnitude less than that of MnO. In addition to the problem of the slow reaction kinetics of V2O3, Case et al. (1978) stated that, because of its toxicity, vanadium cannot be considered for end use applications requiring a gas turbine (such as in an IGCC). Zinc oxide is known as one of the metal oxides that has the most favorable thermodynamics for H2S removal. Unfortunately, despite its favorable thermodynamics, reduction of ZnO at temperatures above 550 °C, followed by subsequent zinc vaporization, presents serious problems in using pure ZnO sorbents at temperatures above 550 °C. Zinc ferrite and zinc titanate sorbents that were prepared by mixing iron oxide and titanium dioxide, respectively, with zinc oxide exhibited better resistance to reduction and vaporization than pure zinc oxide (Lew et al., 1992). Difficulties with zinc titanate have included extensive spalling and cracking of fixed-bed pellets and progressive loss of reactivity during multiple sulfidation/regeneration cycles and significant zinc loss at temperatures above 650 °C (Gupta and Gangwal, 1992). While ZnO-based sorbents may be applicable for temperatures as high as 650 °C, it may be desirable to develop suitable sorbents that can operate at temperatures up to 900 °C without undergoing loss of physical integrity or reactivity. The other three oxides remaining from Westmoreland and Harrison’s (1976) lists oxides of Cu, Mn, and Moswere investigated in this study. The thermodynamic and kinetic limitations of oxides of Cu, Mn, and Mo are briefly discussed to establish the rationale for the reported study. The thermodynamics of copper oxide-H2S and manganese oxide-H2S reactions were given by Atimtay et al. (1993). Copper oxide shows a favorable thermodynamic equilibrium with H2S. In a reducing atmosphere, e.g., coal gas, the dominant species is Cu metal because Cu oxides are easily reduced to metallic Cu or Cu2O at elevated temperatures (Case et al., 1978). The sulfidation equilibria of CuO and Cu2O are vastly superior to that of Cu metal. In a gasifier atmosphere, Mn3O4 is readily reduced to MnO; therefore, MnO is the species that reacts with H2S. It can withstand high temperatures (the melting point of MnO is 1650 °C). Mn metal is not formed in a reducing (producer gas) atmosphere since the equilibrium constant of the reduction reaction is small (Case et al., 1978). Molybdenum is atypical in that its higher oxidation state sulfide, MoS2, is more stable at high temperatures than its lower oxidation state sulfide, Mo2S3. In the presence of a gasifier atmosphere, the dominant Mo-containing sorbent species would be MoO2, not the Mo metal (Case et al., 1978). Copper oxide-containing sorbents in various forms have been studied for hot-gas desulfurization of coal gases by several researchers. Some of the earlier studies involving copper sorbents were reviewed by Jalan (1983). Kyotani et al. (1989) found that the reactivity of pure CuO was lower than that of supported CuO, which was attributed to the formation of a dense sulfide layer during sulfidation. A low initial specific surface area of unsupported Cu oxide led to a low sulfur capacity as found by Jalan (1983). Furthermore, Kyotani
et al. (1989) showed that the reactivity of pure CuO declined greatly on the second and third cycles of sulfidation, due to loss of surface area. The surface area declined from 8 to
