Mercury transformations in the exhausts from lab-scale coal flames

A laboratory-scale pulverized coal flame generated exhausts from five coals which were processed at realistic quench rates and residence times with typical flyash loadings. Mercury levels were monitored in the coal feed, all particulate streams from the furnace, and in the gaseous effluent. Contributions from elemental and oxidized species to the mercury vapors were also determined. Reported extents of Hg oxidation were not proportional to coal–Cl levels. Only the coal with excessive Cl generated an abundance of oxidized Hg species. Extents of Hg oxidation did not increase for progressively longer residence times in the exhaust system, but were affected by the level of unburned carbon, suggesting an essential role for heterogeneous chemistry. The split between particulate and vapor Hg species shifted toward particulate-Hg for progressively cooler temperatures at the exhaust outlet. The levels of particulate-Hg were generally higher for the coals that generated less oxidized Hg vapors. Appreciable Hg sorption was observed at temperatures as hot as 500 °C.