Evaluating mercury transformation mechanisms in a laboratory--scale combustion system.

Mercury speciation measurements during injections of 10 microg/m3 Hg0(g) into a 42-MJ/h combustion system containing gaseous O2-Ar- and O2-N2-rich mixtures indicate that 43 and 55% of the Hg (g) spike was transformed rapidly (< 0.1 s) to Hg2+X(g) within a refractory-lined heat exchanger where gas temperatures decrease from approximately 620 to 200 degrees C. O2(g) is the probable Hg0(g) oxidant (i.e. X = O2-). The apparent formation of HgO(g) involves a heterogeneous reaction with adsorbed Hg0 or O2 on refractory surfaces or a Hg0(g)-O2(g) reaction catalyzed by corundum (Al2O3) and/or rutile (TiO2) components of the refractory. The potential catalytic effects of Al2O3 and TiO2 on Hg0(g) oxidation were investigated by injecting Al2O3 and TiO2 powders into approximately 650 degrees C subbituminous coal (Powder River Basin, Montana, USA) combustion flue gas. On-line Hg0(g) and total mercury measurements indicate, however, that Al2O3 and TiO2 injections were ineffective in promoting the formation of additional Hg2+X(g). Apparently, either the chemically complex flue gas hindered the catalytic effects of Al2O3 and TiO2, or these compounds are simply not Hg0(g) oxidation catalysts.