Effect of flue gas component and ash composition on elemental mercury oxidation/adsorption by NH4Br modified fly ash

The mechanism of mercury oxidation/adsorption on an NH4Br modified fly ash (NH4Br-FA) was discussed. The effect of flue gas component including O2, SO2 and NO, and the roles of main fly ash compositions on the Hg0 oxidation/adsorption capability by the NH4Br-FA was evaluated on a fixed-bed reactor. The mercury adsorption species on the spent sorbent were then identified by the temperature programmed decomposition desorption (TPDD) method. The results show that NH4Br modification on the fly ash not only improves Hg0 oxidation, but also promotes Hg0 adsorption. Due to the generation of Br-containing functional groups, HgBr2 is the main adsorption form on the surface of the NH4Br-FA. O2 cannot promote Hg0 oxidation or adsorption, but leads to the generation of little HgO. NO cannot promotes Hg0 oxidation, while significantly improves the Hg0 adsorption on the NH4Br-FA with the adsorbate of HgBr2, HgO or Hg2(NO3)2. SO2 has little effect on Hg0 oxidation, but significantly inhibits Hg0 adsorption on the surface of the NH4Br-FA, because SO2 can destroy the Br activate sites leading to deactivation, in which there is no HgS or HgSO4 formation. The main metal oxides in the NH4Br-FA include Fe2O3, TiO2, CaO, and Al2O3, which display poor mercury removal capability. However, after the modification of NH4Br solution, the NH4Br-Fe2O3 and the NH4Br-TiO2 demonstrate excellent Hg0 oxidation capability with poor Hg0 adsorption performance.