Mercury transformations in coal combustion flue gas

Mercury chlorination [i.e., formation of HgCl2(g)] is generally assumed to be the dominant mercury-transformation mechanism in coal combustion flue gas. Other potential mechanisms involve mercury interactions with ash particle surfaces where reactive chemical species, oxidation catalysts, and active sorption sites are available to transform Hg0(g) to Hg2+X(g) (e.g., where X is Cl2 or O) as well as Hg0(g) and HgCl2(g) to particulate mercury, Hg(p). Results from an investigation of Hg0(g)–O2(g)–HCl(g) and Hg0,2+(g)–HCl(g)–CaO(s)-fly ash interactions in a 42-MJ/h combustion system are consistent with the following mechanisms: mercury chlorination, catalysis of mercury oxidation by Al2O3(s) and/or TiO2(s), and mercury sorption on a calcium-rich (25.0 wt.% CaO) subbituminous coal fly ash. Additions of 50 and 100 ppmv of HCl(g) and ≈12.6 wt.% of CaO(s) to the subbituminous coal combustion environment inhibited Hg(p) formation, primarily via a change in ash surface chemistry and a decrease in particle surface area, respectively.