Mercury Removal Performance and Mechanism of Biochar Co-Modified with HNO3 and NH4Br under Oxy-Combustion Atmosphere

In order to meet the demand for mercury removal in the oxy-combustion atmosphere, a new mercury adsorbent was prepared by using rice husk char as raw material and co-modification of acid and halogen. The experimental results indicate that the specific surface area of the combined modified char is smaller than those modified by HNO3 and NH4Br alone, but it has better mercury removal performance. X-ray photoelectron spectroscopy (XPS) analysis shows that the co-modification can effectively promote the formation of C-O* and C-Br. The high activity of Br will induce the Hg-0 adsorption, while C-O* will convert the easily decomposed HgBr2 into more stable HgO, which may be the reason why the combined modified char maintains high mercury removal efficiency. When the reaction temperature reaches 150 degrees C, the mercury removal efficiency of RHC (Br+N) is almost 100%. Both above and below the optimum reaction temperature will affect the activity of chemical functional groups and inhibit mercury removal. According to the density functional theory (DFT) results, the addition of O and Br atoms can stably combine with the carbonaceous surface to form covalent compounds (C=O, C-Br), both of which can induce the charge transfer of surrounding C atoms, which makes the C atoms have stronger electronegativity and easier to chemically adsorb Hg-0.