(593b) The Chemistry of Mercuric Chloride Reduction in the Flue Gases of Coal Combustion
AIChE Annual Meeting
2006
2006 Annual Meeting
Environmental Division
Homogenous and Heterogeneous Atmospheric Chemistry
Thursday, November 16, 2006 - 3:28pm to 3:41pm
Understanding the chemical and physical transformation pathways that govern the oxidation states of mercury in flue gases is a key to the development of efficient removal processes for this pollutant. A reduction of oxidized mercury (Hg2+) typically observed downstream of a cold electrostatic precipitator has been reported. However, compared to the relatively large number of efforts to study oxidation mechanisms of elemental mercury (Hg0) to oxidized mercury (Hg2+), a small number of studies report on the reduction mechanisms of Hg2+ to Hg0 in the gas phase. In the present study, mercury speciation, or more specifically reduction of Hg2+ to Hg0, is investigated at flue gas conditions. Experimental data show that SO2 and H2O vapor are the main factors affecting Hg2+ reduction in the gas phase and that the presence of HCl is critical for maintaining Hg2+ in its oxidized form. CO2 and NO reduced the rate of Hg2+ reduction in the absence of HCl gas. Furthermore, the rate of Hg2+ reduction in the absence of HCl increased with an increase in the surface area in direct contact with the flue-gas constituents, due to heterogeneous reactions. Investigations on the effect of functionalized surface ligands, such as mercaptopropyltrimethoxysilane (MPTS), on mercury speciation showed that thermal desorption of Hg2+ bonded to ?SH functional groups on the silica surface yielded Hg0 rather than Hg2+ in the presence of N2 gas.