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 ga s. Other potential mechanisms involve mercury interactions with ash particl e surfaces where reactive chemical species, oxidation catalysts, and active sorption sites are available to transform Hg-0(g) to Hg2+X(g) (e.g., where X is Cl-2 or O) as well as Hg-0(g) and HgCl2(g) to particulate mercury, Hg (p). Results from an investigation of Hg-0(g)-O-2(g)-HCl(g) and Hg0.2+(g)-H Cl(g)-CaO(s)-fly ash interactions in a 42-MJ/h combustion system are consis tent with the following mechanisms: mercury chlorination, catalysis of merc ury 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 p pmv of HCl(g) and approximate to 12.6 wt.% of CaO(s) to the subbituminous c oal combustion environment inhibited Hg(p) formation, primarily via a chang e in ash surface chemistry and a decrease in particle surface area, respect ively. (C) 2000 Elsevier Science B.V. All rights reserved.