Evaluating mercury transformation mechanisms in a laboratory - scale combustion system

Mercury speciation measurements during injections of 10 mu g/m(3) Hg-0(g) into a 42-MJ/h combustion system containing gaseous O-2-Ar- and O-2-N-2-rich mixtures indicate that 43 and 55% of the Hg-0(g) spike was transformed rapidly(< 0.1 s) to Hg2+ X(g) within a refractory-lined heat exchanger where gas temperatures decrease from approximate to 620 to 200 degrees C. O-2(g) is the probable Hg-0(g) oxidant (i.e. X= O2-). The apparent formation of HgO(g) involves a heterogeneous reaction with adsorbed Hg-0 or O-2 on refractory surfaces or a Hg-0(g)-O-2(g) reaction catalyzed by corundum (Al2O3) and/or rutile (TiO2) components of the refractory. The potential catalytic effects of Al2O3 and TiO2 on Hg-0(g) oxidation were investigated by injecting Al2O3 and TiO2 powders into approximate to 650 degrees C subbituminous coal (Powder River Basin, Montana, USA) combustion flue gas. On-line Hg-0(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 Hg-0(g) oxidation catalysts. (C) 2000 Elsevier Science B.V. All rights reserved.