Comprehensive Hg/Br reaction chemistry over Fe2O3 surface during coal combustion

Abstract

A combination of experiments, density functional theory (DFT) and kinetic calculations was used to systematically understand the detailed chemistry of heterogeneous mercury reaction with HBr over Fe2O3 surface. Fe2O3 shows catalytic activity for mercury reaction with HBr. The chemisorption mechanism is responsible for the adsorption of mercury species (Hg0, HgBr and HgBr2) on Fe2O3 surface. Heterogeneous mercury reaction with HBr over Fe2O3 surface follows Langmuir–Hinshelwood mechanism in which adsorbed Hg0 reacts with active surface bromine species derived from HBr decomposition. On the basis of the experimental and DFT calculation results, a new comprehensive heterogeneous reaction kinetic model was established to describe the detailed reaction process of Hg/Br over Fe2O3 surface. This heterogeneous model includes 17 elementary reactions governing the elimination and formation of mercury species on Fe2O3 surface. This kinetic model was validated against the experimental data. The model predictions were found to be in good agreement with the experimental data. X-ray photoelectron spectroscopy (XPS) results, DFT calculations and sensitivity analysis indicate that the dominant reaction pathway of Hg/Br over Fe2O3 surface is a four-step process Hg0 → Hg(s) → HgBr(s) → HgBr2(s) → HgBr2, in which gaseous Hg0 is first adsorbed on Fe2O3 surface and subsequently reacts with brominated iron site to form HgBr(s), HgBr(s) can be further converted to HgBr2(s) and released into flue gas. The proposed dimensionless temperature coefficient can be used to better understand the temperature-dependent relationship between heterogeneous Hg/Br chemistry and mercury transformation.