Study on iron-based adsorbents for alternating removal of H2S and O2 from natural gas and biogas

Abstract

This study deals with the development of iron oxide/hydroxide adsorbents for the alternating removal of H2S and O2 from natural gas and biogas. The considered procedure initially implies the chemisorption of H2S. In the second step, O2 is removed by reaction with the H2S-treated substrate accompanied by regeneration of the adsorbent. 35 different iron-based samples were physical-chemically characterized and tested towards H2S/O2 break through behavior and adsorption capacity to evaluate structure-performance correlations. It was derived that high BET surface area, high proportion of meso and micro pores, small average pore diameter, low crystallinity and high number of Bronsted acid sites are crucial properties of the adsorbents. As a consequence, Fe(OH)3 and α-FeOOH revealed highest efficiency for the successive adsorption of H2S and O2, while α-Fe2O3 provided rather low performance.Mechanistic investigations were carried out with α -FeOOH and α -Fe2O3 indicating rapid dissociation of H2S on α -FeOOH, while slow molecular adsorption prevailed on α -Fe2O3. With proceeding H2S exposure, the adsorbents were partially transformed into sulfide and sulfate species with some formation of elemental sulfur. In subsequent reaction with O2, the H2S-exposed samples were re-oxidized under partial restructuring, whereas elemental sulfur and sulfate entities were additionally produced.Finally, the concept of the alternating adsorption of H2S and O2 on best α -FeOOH substrate was technically evaluated in a biogas pilot plant. The investigations showed efficient conversion of H2S for 40–100 h, while subsequent O2 conversion was achieved for ca. 5.5 h.