Investigation of Mercury Removal Behavior by Modified Biomass Coke in Oxy-fuel Combustion Flue Gas

The presence of circulating flue gas in oxy-fuel combustion leads to mercury pollution. Acid–chlorine-modified biomass coke was used to prepare mercury adsorbents. The influence of individual flue gas components (HCl, NO, SO2, and H2O) and complex flue gas on the behavior of modified straw coke for mercury removal under an oxy-fuel combustion atmosphere was studied. The mechanism of the flue gas reaction with mercury was investigated using temperature-programmed desorption (TPD) characterization. The density functional theory (DFT) was employed to study the mechanism of mercury removal by the adsorbent. The results showed that H2O has a dual effect on mercury removal: the low concentrations of H2O promote mercury removal, while moderate to high concentrations of H2O inhibit it. HCl and NO exhibited promotional effects on mercury removal. SO2 competed with Hg0 for active adsorption sites or adsorbed on the surface of the adsorbent, consuming active oxygen species and chlorinated functional groups to inhibit the adsorption and oxidation of Hg0. Under the atmosphere of complex flue gas, H2O and SO2 inhibited the generation of highly oxidizing intermediate products from HCl and NO. Additionally, the coexistence of H2O and SO2 generated H2SO4, further blocking the surface pore structure and hindering the diffusion of Hg0 on the adsorbent surface. The result of the electron localization function (ELF) indicated that the introduction of O and Cl atoms can form stable covalent compounds (C═O and C–Cl) through bonding with the carbonaceous surface, serving as favorable chemical adsorption sites. The high reactivity of Cl induces Hg0 adsorption, and the presence of O atoms not only acts as a chemical adsorption site but also activates the reactivity of Cl atoms, promoting the efficiency of mercury removal. This study provided a theoretical basis for the application of mercury adsorbents in oxy-fuel combustion fuel gas.