Sorption of mercury species by activated carbons and calcium-based sorbents: effect of temperature, mercury concentration and acid gases

Abstract

Bench-scale studies of mercury/sorbent reactions were con ducted to understand mechanistic limitations of field-scale attempts to reduce emissions of mercury from combustion processes. The effects of temperature (60 to 140°C), sulfur dioxide (SO2, 1000 ppm), hydrogen chloride (HCl, 50 ppm), and water vapor (5% molar) on the capture of elemental mer cury (Hg°, 8 to 40 ppb) by two thermally activated carbons, and on the capture of mercuric chloride (HgCl2, 11 to 73 ppb) by two calcium (Ca)-based sorbents were examined in a fixed-bed, bench-scale system. Capture of mercury species is affected by sorbent site activity. HgCl2 is quickly adsorbed by the alkaline sites of Ca-based sorbents; sorption in combus tors will likely be kinetically limited. Hg°, however, is more effectively removed by activated carbons; here the number of active sites appears to limit removal from flue gas. SO2 and HCl were found to either enhance or reduce sorption depend ing on the species of mercury. Enhancement or Hg° capture by activated carbon is associated with formation of active car bon-chlorine or carbon-sulfur sites while reduced capture of HgCl2 is due to loss of alkaline sites through Ca-SO2 and Ca- HCl reactions.