Mercury removal from coal combustion flue gas by modified palygorskite adsorbents

Several modified adsorbents were developed by impregnating palygorskite (Pal) with the active substances CuCl2, CuBr2, NaBr, sulfur (S), MnO2 and Co3O4, which were used to conduct experiments via a bench-scale fixed-bed reactor system in simulated flue gas for evaluating their elemental mercury removal capacity. In addition, a variety of characterization methods were applied to understand the physicochemical properties of these adsorbents. Furthermore, adsorbents, namely, CuCl2/CuBr2-impregnated Pal (Cu-Pal), were chosen for deep exploration under various gas conditions. The results showed that the mercury removal capability of Pal was greatly improved after impregnation. At 120 °C in pure N2, the mercury removal efficiency of the adsorbents modified by CuCl2 and CuBr2 could reach 90.9% and 95.2%, respectively, while it could be > 80% for the adsorbents modified by the others. The overall trend showed that O2 and HCl were beneficial to increasing the mercury removal efficiency of Cu-Pal. To be specific, for CuCl2-Pal and CuBr2-Pal, when adding in 8% O2, their efficiencies could be increased by 6.6% and 1.9% respectively, while 50 ppm HCl increased their efficiencies by 2.8% and 2.1%, respectively. Different from O2 and HCl, SO2 and NO had negative effects. The removal efficiencies could be reduced by 6.5% for CuCl2-Pal and 4.7% for CuBr2-Pal with 1200 ppm SO2, while they could be reduced by 4.2% and 2.6% with 300 ppm NO. Compared with CuCl2-Pal, CuBr2-Pal performed better. Combined with the characterization results, Cu2 + was reduced to Cu+ and halogen migrated into new compounds on the surface of Cu-Pal after reaction. Eventually, the mercury removal mechanism of Cu-Pal was analysed and proposed.