Experimental study on ZnO-TiO2 sorbents for the removal of elemental mercury

ZnO-TiO2 sorbents synthesized by an impregnation method were characterized through XRD (X-ray diffraction), XPS (X-ray photoelectron spectroscopy) and EDS (Energy dispersive spectrometer) analyses. An experiment concerning the adsorption of Hg0 by ZnO-TiO2 under a simulated fuel gas atmosphere was then conducted in a bench-scale fixed-bed reactor. The effects of ZnO loading amounts and reaction temperatures on Hg0 removal performance were analyzed. The results showed that ZnO-TiO2 sorbents exhibited excellent Hg0 removal capacity in the presence of H2S at 150 °C and 200 °C; 95.2% and 91.2% of Hg0 was removed, respectively, under the experimental conditions. There are two possible causes for the H2S reacting on the surface of ZnO-TiO2: (1) H2S directly reacted with ZnO to form ZnS, (2) H2S was oxidized to elemental sulfur (Sad) by means of active oxygen on the sorbent surface, and then Sad provided active absorption sites for Hg0 to form HgS. This study identifies three reasons why higher temperatures limit mercury removal. First, the reaction between Hg0 and H2S is inhibited at high temperatures. Second, HgS, as the resulting product in the reaction of mercury removal, becomes unstable at high temperatures. Third, the desulfurization reaction strengthens at higher temperatures, and it is likely that H2S directly reacts with ZnO, thus decreasing the Sad on the sorbent surfaces.