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 O-2, SO2 and NO, and the roles of main fly ash compositions on the Hg-0 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 Hg-0 oxidation, but also promotes Hg-0 adsorption. Due to the generation of Br-containing functional groups, HgBr2 is the main adsorption form on the surface of the NH4Br-FA. O-2 cannot promote Hg-0 oxidation or adsorption, but leads to the generation of little HgO. NO cannot promotes Hg-0 oxidation, while significantly improves the Hg-0 adsorption on the NH4Br-FA with the adsorbate of HgBr2, HgO or Hg-2(NO3)(2). SO2 has little effect on Hg-0 oxidation, but significantly inhibits Hg-0 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 Hg-0 oxidation capability with poor Hg-0 adsorption performance.