Homogeneous mercury oxidation with bromine species released from HBr-modified fly ash

This research studies the release of bromine species from HBr-modified fly ash and the subsequent homogeneous oxidation of gaseous elemental mercury (Hg-0) by active gaseous bromine species during heating. Tests utilize an innovative bench-scale experimental system specialized for the homogeneous Hg-0 oxidation reaction and confirm that the released bromine species from the fly ash can oxidize Hg-0 in the gas-phase. Low reaction temperatures are believed to favor the formation of intermediate products and effectiveness of Van der Waals forces resulting in improved mercury oxidation efficiency. Higher reactant concentrations improved oxidation performance, most likely because the oxidation reaction occurs via multiple reaction steps. Furthermore, O-2 may not facilitate the Hg-0 oxidation at the specified reaction temperature. Both FT-IR and UV/vis confirmed the release of HBr and Br-2 from brominated fly ash. TG/MS results provided additional evidence of thermal stability and the identity of the released materials. At 140 degrees C, HBr was detected by both FT-IR and TG-MS. UV/Vis detected increasing Br-2 levels with increased heating temperature (especially above 200 degrees C). The suggested mechanism is that the hydrogen atom of the released HBr is attracted to the lattice oxygen, forming active bromine species, such as Br+. The Br+ can then recombine to form Br-2, which desorbs at higher temperatures and improves homogeneous Hg-0 oxidation. (C) 2015 Elsevier Ltd. All rights reserved.