Kinetic Study of High-Temperature Removal of H₂S by Novel Metal Oxide Sorbents

Abstract

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.

Retention of Arsenic and Selenium during Hot Gas Desulfurization Using Metal Oxide Sorbents

“…Sorbents of the ZFT(a) series were subsequently calcined at 1100ºC to facilitate the progress of spinel formation. 7 The samples obtained were named ZFT(b). Within each series, mixtures of three different stoichiometries were prepared in order to obtain different chemical compositions and textural properties ( Table 1).…”

“…The use of zinc-titanates improved reductive and thermal stability compared to zinc ferrites. [3][4][5][6][7] Although sulphur retention is considered a major topic in IGCC gas cleaning, other corrosive or toxic species present in gasification flue gases are also of great interest. This is the case of arsenic and selenium, which are classified by the Clean Air Amendments as elements of major environmental concern.…”

Advanced Sorbent Development Program; Development of Sorbents for Moving-Bed and Fluidized-Bed Applications

“…12 Outside of the United States, work on sorbent development has been very prolific, particularly in western European countries such as the Netherlands, 80,81 England, and Spain. 82 In order for a metal oxide to be considered suitable for high-temperature desulfurization, the material must exhibit desirable properties in the following areas: The feasibility of operating a moving-bed high-temperature desulfurization system in an economically competitive and technically feasible manner currently restricts its use to a temperature window between 400-750 °C (752-1472 °F) and pressures above 10 atmospheres 1-8 during absorption. Operation below 400 °C does not take full advantage of higher system efficiency compared to low-temperature cleanup systems, while operation above 750 °C requires a tradeoff between more sophisticated material properties/higher capital costs and reliability of process operation.…”

Cleaner energy production with integrated gasification combined cycle systems and use of metal oxide sorbents for H2S cleanup from coal gas

“…However, in strongly reducing atmospheres and at temperatures above 650 7C, ZnO is reduced to metallic zinc and volatilized (Ayala and Marsh 1991; Garcia et al 1997). To prevent sorbent reduction, ZnO is combined with other metal oxides.…”

“…Additionally, a combination of CuO with zinc ferrite has been studied by Garcia et al(1997) to develop superior sorbents even though the addition of Cu has not shown any stabilization effect on bulk zinc ferrite sorbents. However, it was found by Pineda et al (1997) that Cu addition to zinc ferrite improved the sorbent performance through its migration towards the sorbent surface and through its enhancement of the formation of spinel structures.…”