Oxidation of Mercury in Products of Coal Combustion
Laboratory measurements of mercury oxidation during selective catalytic reduction (SCR) of nitric oxide, simulation of pilot-scale measurements of mercury oxidation and adsorption by unburned carbon and fly ash, and synthesis of new materials for simultaneous oxidation and adsorption of mercury, were performed in support of the development of technology for control of mercury emissions from coal-fired boilers and furnaces. Conversion of gas-phase mercury from the elemental state to water-soluble oxidized form (HgCl{sub 2}) enables removal of mercury during wet flue gas desulfurization. The increase in mercury oxidation in a monolithic V{sub 2}O{sub 5}-WO{sub 3}/TiO{sub 2} SCR catalyst with increasing HCl at low levels of HCl (< 10 ppmv) and decrease in mercury oxidation with increasing NH{sub 3}/NO ratio during SCR were consistent with results of previous work by others. The most significant finding of the present work was the inhibition of mercury oxidation in the presence of CO during SCR of NO at low levels of HCl. In the presence of 2 ppmv HCl, expected in combustion products from some Powder River Basin coals, an increase in CO from 0 to 50 ppmv reduced the extent of mercury oxidation from 24 {+-} 3 to 1 {+-} 4%. Further increase in CO to 100 ppmv completely suppressed mercury oxidation. In the presence of 11-12 ppmv HCl, increasing CO from 0 to {approx}120 ppmv reduced mercury oxidation from {approx}70% to 50%. Conversion of SO{sub 2} to sulfate also decreased with increasing NH{sub 3}/NO ratio, but the effects of HCl and CO in flue gas on SO{sub 2} oxidation were unclear. Oxidation and adsorption of mercury by unburned carbon and fly ash enables mercury removal in a particulate control device. A chemical kinetic mechanism consisting of nine homogeneous and heterogeneous reactions for mercury oxidation and removal was developed to interpret pilot-scale measurements of mercury oxidation and adsorption by unburned carbon and fly ash in experiments at pilot scale, burning bituminous coals (Gale, 2006) and blends of bituminous coals with Powder River Basin coal (Gale, 2005). The removal of mercury by fly ash and unburned carbon in the flue gas from combustion of the bituminous coals and blends was reproduced with satisfactory accuracy by the model. The enhancement of mercury capture in the presence of calcium (Gale, 2005) explained a synergistic effect of blending on mercury removal across the baghouse. The extent of mercury oxidation, on the other hand, was not so well described by the simulation, because of oversensitivity of the oxidation process in the model to the concentration of unburned carbon. Combined catalysts and sorbents for oxidation and removal of mercury from flue gas at low temperature were based on surfactant-templated silicas containing a transition metal and an organic functional group. The presence of both metal ions and organic groups within the pore structure of the materials is expected to impart to them the ability to simultaneously oxidize elemental mercury and adsorb the resulting oxidized mercury. Twelve mesoporous organosilicate catalysts/sorbents were synthesized, with and without metals (manganese, titanium, vanadium) and organic functional groups (aminopropyl, chloropropyl, mercaptopropyl). Measurement of mercury oxidation and adsorption by the candidate materials remains for future work.
- Research Organization:
- Univ. of Alabama, Birmingham, AL (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- FG26-04NT42195
- OSTI ID:
- 1013340
- Country of Publication:
- United States
- Language:
- English
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