The behavior of different mixed oxides, including zinc titanates (ZT) and zinc ferrites modified with CuO (ZFC) or TiO2 (ZFT), as hot gas desulfurizing sorbents was investigated. The sorbents were prepared by calcination at 650 °C of a mixture of bulk oxides in three different stoichiometries in order to form new phases and modify their textural properties. Tests of stability against reduction were obtained by thermoprogrammed reduction, and kinetic studies of the sulfidation reaction were carried out in a thermobalance in the temperature range of 550−650 °C. Kinetic parameters of the intrinsic reaction were obtained assuming a grain model. The sulfidation behavior of the sorbents as extrudates was investigated in a fixed-bed reactor in terms of breakthrough curves. Fresh and sulfided samples were characterized by Hg porosimetry, X-ray diffraction, and SEM-EDX. The study shows that the addition of TiO2 or CuO to zinc ferrite based sorbents calcined at 650 °C has little effect on the stability against reduction but markedly influences their textural properties. The stabilizing effect of Ti is observed in samples calcined at higher temperature or in non-iron-containing sorbents. The calculated kinetic constants indicate that the Zn content and the incorporation of Cu have an enhancing effect on the kinetics of the sulfidation process. Including H2 in the feed gas decreases the reactivity and increases the activation energy. Extrudated sorbents showed a good performance as desulfurizing agents and maintained the H2S concentration in the outlet gas below 20 ppm. ZT sorbent exhibited a poor efficiency, which makes the addition of Ti questionable.

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The reactivity and performance of different zinc−titanium-based sorbent extrudates for desulfurization of coal gas at high temperature in successive sulfidation−regeneration cycles were studied in both a thermobalance and a fixed-bed reactor. The study showed that the sulfidation and the regeneration of these sorbents as cylindrical extrudates was mostly a diffusion-controlled process. In order to change the degree of dispersion of the active phase and porosity of the sorbent extrudates, some of them were prepared by coprecipitation and others by using graphite as a high-temperature pore-modifier additive. The study showed that the high degree of dispersion of the active phase achieved in the first stages of the coprecipitation process is practically destroyed during sorbent calcination at high temperature. Additionally, coprecipitation enhanced thermal sintering during preparation and the porosity of these fresh sorbents was low. Consequently, the reactivity in thermobalance tests was also low and performance in the reactor was very poor. On the contrary, graphite increased substantially the porosity of the fresh sorbent extrudates and, because it is eliminated at high temperature, this effect prevailed during calcination and successive sulfidation−regeneration cycles. Consequently, graphite notably improved the performance of zinc titanate extrudates as hot temperature sorbents which under the operation conditions used was clearly evidenced by an increase of the sorbent efficiency.

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