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 functional groups and micropore surface area of carbon-based adsorbents on the adsorption of Hg-0 from simulated coal combustion flue gases. In this study, raw activated carbon fibers that are microporous (ACF-20) were impregnated with elemental sulfur between 250 and 650 degreesC. The resulting samples were saturated with respect to sulfur content. Total sulfur content of the sulfur impregnated ACF samples decreased with increasing impregnation temperatures from 250 and 500 degreesC and then remained constant to 650 degreesC. Results from sulfur K-edge X-ray absorption near-edge structure (S-XANES) spectroscopy showed that sulfur impregnated on the ACF samples was in both elemental and organic forms. As sulfur impregnation temperature increased, however, the relative amounts of elemental sulfur decreased with a concomitant increase in the amount of organic sulfur. Thermal analyses and mass spectrometry revealed that sulfur functional groups formed at higher impregnation temperatures were more thermally stable. In general, sulfur impregnation decreased surface area and increased equilibrium Hg-0 adsorption capacity when compared to the raw ACF sample. The ACF sample treated with sulfur at 400 degreesC had a surface area of only 94 m(2)/g compared to the raw ACF sample's surface area of 1971 m(2)/g, but at least 86% of this sample's surface area existed as micropores and it had the largest equilibrium Hg-0 adsorption capacities (2211-11343 mug/g). Such a result indicates that 400 degreesC 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 elemental and organic forms. Thermal analyses and CS2 extraction tests suggested that elemental sulfur was the main form of sulfur affecting the Hg-0 adsorption capacity. These findings indicate that both the presence of elemental sulfur on the adsorbent and a microporous structure are important properties for improving the performance of carbon-based adsorbents for the removal of Hg-0 from coal combustion flue gases.