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.