Influence of Different Sulfur Forms on Gas-Phase Mercury Removal by SO2-Impregnated Porous Carbons

Due to widespread global mercury pollution via anthropogenic activities such as coal combustion and its severe toxicity even at low concentrations, it is necessary to remove mercury from power plant flue gas to protect both humans and the ecosystem. Currently, significant efforts are being made to maximize Hg-0 adsorption rates while minimizing the impact on the cost of electricity. The purpose of this study is to explore the influence of different sulfur forms on vapor Hg-0 removal by SO2-impregnated porous carbons. After SO2 impregnation, reductive heat treatment in N-2 and H2O2 oxidation process was used to diverge the sulfur form composition in porous carbons. The ultimate and X-ray photoelectron spectroscopy analysis were used to verify the sulfur species. The pore structures of sorbents were determined by nitrogen adsorption/desorption measurements. Then, their mercury removal performance was investigated in a fixed-bed reactor and the influence of different sulfur forms on equilibrium mercury adsorption capacity and mercury desorption was studied. Finally, the kinetics of mercury adsorption on SO2-impregnated sorbents was explored to identify whether the adsorption process is controlled by chemical adsorption. The results showed that apparent increases in the quantities of reduced sulfur species were observed after heat treatment, which is assumed to be beneficial for mercury adsorption. H2O2 oxidation after SO2 impregnation has caused a loss of reduced and nonoxidized sulfur forms in porous carbons, as well as an increase in insoluble oxidized sulfur species. One interesting finding is that even though the micropore volumes of porous carbons decreased after heat treatment, the Hg-0 adsorption capacities of reduced samples and the thermal stability of adsorbed mercury were both positively raised. After H2O2 treatment, the oxidized SO2 impregnated samples showed an obvious decrease in Hg-0 adsorption capacity. Compared with nonoxidized and oxidized sulfur forms, the reduced sulfur forms have shown a rather significant correlation with the mercury adsorption performance of SO2-impregnated samples. Kinetic analysis illustrates that mercury adsorption on SO2 impregnated porous carbons was mainly controlled by chemical adsorption.