Effects of sulfur impregnation temperature on the properties and mercury adsorption capacities of activated carbon fibers (ACFs)

Laboratory studies were conducted to determine the role of sulfur functiona l groups and micropore surface area of carbon-based adsorbents on the adsor ption of Hg-0 from simulated coal combustion flue gases. In this study, raw activated carbon fibers that are microporous (ACF-20) were impregnated wit h elemental sulfur between 250 and 650 degreesC. The resulting samples were saturated with respect to sulfur content. Total sulfur content of the sulf ur impregnated ACF samples decreased with increasing impregnation temperatu res from 250 and 500 degreesC and then remained constant to 650 degreesC. R esults from sulfur K-edge X-ray absorption near-edge structure (S-XANES) sp ectroscopy showed that sulfur impregnated on the ACF samples was in both el emental and organic forms. As sulfur impregnation temperature increased, ho wever, the relative amounts of elemental sulfur decreased with a concomitan t increase in the amount of organic sulfur. Thermal analyses and mass spect rometry revealed that sulfur functional groups formed at higher impregnatio n temperatures were more thermally stable. In general, sulfur impregnation decreased surface area and increased equilibrium Hg-0 adsorption capacity w hen compared to the raw ACF sample. The ACF sample treated with sulfur at 4 00 degreesC had a surface area of only 94 m(2)/g compared to the raw ACF sa mple's surface area of 1971 m(2)/g, but at least 86% of this sample's surfa ce area existed as micropores and it had the largest equilibrium Hg-0 adsor ption capacities (2211-11343 mug/g). Such a result indicates that 400 degre esC is potentially an optima I sulfur impregnation temperature for this ACF . Sulfur impregnated on the ACF that was treated at 400 OC was in both elem ental and organic forms. Thermal analyses and CS2 extraction tests suggeste d that elemental sulfur was the main form of sulfur affecting the Hg-0 adso rption capacity. These findings indicate that both the presence of elementa l sulfur on the adsorbent and a microporous structure are important propert ies for improving the performance of carbon-based adsorbents for the remova l of Hg-0 from coal combustion flue gases.