The Reduction of Zinc Titanate and Zinc Oxide Solids

The reduction of bulk mixed oxides of zinc and titanium of various compositions and Zn-Ti-0 crystalline phases was studied in a thermogravimetric apparatus in Hz-H,O-N, gas mixtures at 550-1050°C. Comparative reduction experiments with ZnO were also performed. In the absence of water vapor, activation energies of 24 and 37 kcal mol’ were obtained for ZnO and Zn-Ti-0. resnectivelv. The addition of water vapor inhibited reduction and resulted in a change in activation energ; to &I kcal &ol’ for both ZnO and Zn-Ti-0 solids. Similar to H-0, H,S inhibited the initial reduction rate of ZnO and Zn-Ti-0 materials. Based on kinetic experiments, a two-site mode1 is proposed for ZnO reduction. One type of sites is characterized by a rapid reduction rate but is poisoned by water vapor as well as by the presence of titanium atoms in the solid. The other type of sites has a lower reduction rate, is not poisoned by H,O, and is slowly eliminated by the presence of titanium. The efficient removal of H,S from coal-derived gas streams at elevated temperatures is crucial for efficient and economic coal utilization in emerging advanced power generating systems such as the integrated gasification-combined cycle and the gasification-molten carbonate fuel cell. Typically, coal gases contain a mixture of CO, H,, H,O, CO,, N, and CH,. The composition of the gas mixture depends on the type of gasifier used and on the extent of the use of water for quenching. In addition to these gases, minor compounds, most of them undesirable or deleterious, such as H,S and alkali salts, exit the coal gasifier. The relative amounts of these components depend on the type of coal and the type of gasifier used. Previous studies (Jalan and Wu, 1980; Grindley and Steinfeld, 1981; Flytzani-Stephanopoulos et al., 1985) have investigated zinc oxide as a high-temperature, regenerable sulfur sorbent. The thermodynamic equilibrium for ZnO sulfidation is quite favorable, yielding H,S concentrations as low as a few parts per million (ppm) at typical hot gas cleanup temperatures (500-700°C). However, a difficulty with all sorbents containing zinc oxide is some reduction to volatile elemental zinc at temperatures above 600°C. ZnO reduction by H, has been detected at temperatures as low as 160°C (Bonasewicz et al., 1981). The mixed oxide sorbent zinc ferrite, ZnFe,O,, combining ZnO with Fe,O,, has been developed in recent years as an alternative to single zinc oxide sorbent (Grindley and Steinfeld, 1983). Zinc ferrite decomposes into ZnO + Fe304 in the reducing coal gas atmosphere. Hence, it is similarly limited to an operating temperature of approximately 600°C. In recent studies (Lew et al., ‘Present address: ARC0 Chemical Company, Newtown Square, PA 19073, U.S.A. ‘Author to whom correspondence. should be addressed. 1989; Flytzani-Stephanopoulos et al., 1987) the combination of zinc oxide with titanium dioxide was found effective in producing bulk mixed Zn-Ti-0 as alternative, regenerable sulfur sorbcnts which may be more resistive towards reduction to volatile elemental zinc. This paper reports on kinetic and parametric studies of reduction of ZnO and Zn-Ti-0 materials. The reduction of ZnO in the temperature range of 4OO-1500°C by CO and H2 gases has been the subject of many studies over the years. Some of the earlier investigators (Bodenstein, 1927; Maier and Ralston, 1928) were mainly interested in the extraction of zinc from ores in a retort furnace. In more recent investigations (Truesdale and Waring, 1944; Hegedus and Kiss, 1966; Guger and Manning, 1971; Grunze and Hirschwald, 1974, 1975; Gioia et al., 1977), the reduction kinetics of ZnO sintered pellets, powders, and single crystals were examined with H2 as well as CO. Similar activation energies have been obtained for both reduction in H, and CO (Grunze and Hirschwald, 1975). However, the frequency factor of the rate constant for reduction of ZnO with H, is approximately twice as large as that with CO. The first study of the reduction of Zn-Ti-0 mixed oxides was reported by Flytzani-Stephanopoulos et al. (1987). A lower reduction rate by as much as five times was reported for Zn-Ti-0 solids compared to ZnO reduction by hydrogen at 650°C. However, these experiments were carried out in a fixed-bed reactor, where mass transfer resistance might have been limiting. It has been reported by Hirschwald and Noack (1972) that it is possible to decrease the reducibility of ZnO by the inclusion of a small amount of foreign atoms. They found that the reduction of ZnO can be inhibited by the addition of small amount (1 mol %) of L&O, Al,O, or Ga,O,. They attributed this to a change in the electronic state of zinc_oxi$e. However, no such information exists-for-the addition of TiO, into ZnO.