Reduction of inherent mercury emissions in pc combustion. Semi-annual technical211 report, January 1-June 30, 1999

摘要

Mercury emission compliance presents one of the major potential challenges raised211u001eby the 1990 Clean Air Act Amendments. Simple ways of controlling emissions have 211u001enot been identified. The variability in the field data suggest that inherent 211u001emercury emissions may be reduced if the source of this inherent capture can be 211u001eidentified and controlled. The key mechanisms appear to involve the oxidation of 211u001emercury to Hg(sup 2), generally producing the more reactive HgCl(sub 2), followed 211u001eby its capture by certain components of the fly ash or char, or in the air 211u001epollution control equipment. This research focuses on identifying the rate-211u001elimiting steps associated with the oxidation step. Work in this reporting period 211u001efocused on testing of the kinetic mechanism reported in the previous semiannual 211u001ereport, and the interpretation of data (both ours and literature). This model 211u001eyields good qualitative agreement with the data and indicates that mercury 211u001eoxidation occurs during the thermal quench of the combustion gases. The model 211u001ealso suggests that atomic chlorine is the key oxidizing species. The oxidation is 211u001elimited to a temperature window between 700-400 C that is defined by the overlap 211u001eof (1) a region of significant superequilibrium Cl concentration, and (2) a 211u001eregion where oxidized mercury is favored by equilibrium. Above 700 C reverse 211u001ereactions effectively limit oxidized mercury concentrations. Below 400 C, atomic 211u001echlorine concentrations are too low to support further oxidation. The implication 211u001eof these results are that homogeneous oxidation is governed primarily by (1) HCl 211u001econcentration, (2) quench rate, and (3) background gas composition. Work 211u001econducted under the present grant has been the subject of one journal paper that 211u001ewas accepted for publication during the reporting period.