Impact of Nonoxidized Sulfur Species on Elemental Mercury Removal by SO2 Activated Petroleum Cokes

High-sulfur petroleum coke is potentially one of the most promising materials for developing efficient and low-cost sorbents for Hg0 removal from flue gas. In this study, three distinct SO2-activated high-sulfur petroleum cokes were prepared at 650–850 °C via different sulfurization protocols, including SO2-impregnated petroleum coke (PC-S), SO2-impregnated coke produced with an additional reductive heat-treatment process in N2 (PC-NS), and SO2-impregnated coke prepared with the presence of SO2 in the gas during the cooling process (PC-SC). The SO2-activated cokes were characterized with nitrogen adsorption/desorption measurements, X-ray photoelectron spectroscopy, thermogravimetric analysis, and their mercury removal performances were investigated in a fixed-bed reactor at 80–200 °C with an initial mercury concentration of 35–120 μg/m3. To explore the influence of nonoxidized sulfur species on Hg0 removal, particular focus was given to the comparison of the reduced sulfur, sulfide sulfur, and elemental sulfur formed during different sulfurization processes, as well as the consequent differences in Hg0 adsorption characteristics. It found that the sulfur incorporated into the carbon matrix of PC-S by interaction with SO2 at 650 °C was dominantly organic sulfide. Thermal treatment in N2 at 800 °C led to an addition of reduced sulfur in PC-NS. An increment of 0.74 at. % elemental sulfur was observed in PC-SC after exposure to SO2 during the cooling step. The Hg0 adsorption capacity of PC-SC at low temperatures was significantly enhanced, yielding a mercury capacity of 622 μg/g at 80 °C. However, elemental sulfur suffered from severe temperature sensitivity for mercury binding. The sample PC-NS which contained more reduced sulfur showed the highest Hg0 adsorption capacity at elevated temperatures above 160 °C. Furthermore, the increment in inlet mercury concentration would lead to higher adsorption capacities and faster adsorption rates. According to the RL obtained from Langmuir isotherms, the Hg0 adsorption reactivity can be arranged as follows: PC-NS > PC-S > PC-SC. The mercury temperature-programmed desorption results suggested that sulfur species not only had an effect on Hg0 adsorption capacity but also played an essential role in the thermal stability of the adsorbed mercury compounds. Mercury was principally associated with elemental sulfur in the form of metacinnabar and combined with thermal-stable organic sulfide in cinnabar or organomercuric compounds Hg–SR.