Mercury transformations in coal combustion flue gas

摘要

Mercury chlorination [i.e.,formation of HgCl_2(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 Hg~0(g) to Hg~(2+)X(g)(e.g.,where X is Cl_2 or O)as well as Hg~0(g) and HgCl_2(g) to particulate mercury, Hg(p). Results from an investigation of Hg~0(g)-O_2(g)-HCl(g) and Hg~(0,2+)(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 Al_2O_3(s)and/or TiO_2(s),and mercury sorption on a calcium-rich (25.0 wt.% CaO) subbituminous coal fly ash. Additons of 50 and 100 ppmv of HCl(g) and approx=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 paricle surface area, respectively.