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.