Study of Mercury-Removal Performance of Mechanical-Chemical-Brominated Coal-Fired Fly Ash

The mechanical chemical-modified fly ash (FA-MC) and mechanical-chemical-brominated fly ash (FA-MC-Br) were prepared by omnidirectional planetary ball mill, and impregnated brominated fly ash (FA-I-Br) was also prepared using the same mass ratio of fly ash/NH4Br as a comparison. The mercury-removal efficiency of raw fly ash (FA), FA-MC, FA-MC-Br, and FA-I-Br was evaluated in a fixed-bed reactor. The physical and chemical properties of the four samples were investigated by the Brunauer-Emmet-Teller, scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and mercury temperature-programmed desorption analysis. The results showed that the mercury-removal efficiency of the four samples followed the order of FA-MC-Br > FA-I-Br > FA-MC > FA, the value of which was 67, 30.98, 26.12, and 17.96%, respectively. The mercury-removal performance of the four samples was mainly reflected in the oxidation (>90%), while the adsorption only accounted for a small proportion (<10%). The mechanical-chemical activation improved the content of the surface-adsorbed oxygen and amorphous phase of FA, which promoted the oxidation efficiency of the samples. Due to the generation of Br-containing complexes and further increase in the content of surface-adsorbed oxygen on FA-MC-Br, the mechanical-chemical activation coupled with the addition of NH4Br significantly improved the mercury oxidation and -adsorption efficiency. The mercury-adsorbed species of fresh FA-MC-Br is HgBr2, and that of used FA-MC-Br is HgBr2 and HgO. The reaction between elemental mercury and FA-MC-Br was followed Eley-Rideal reaction. As a comparison, the mercury-removal efficiency of FA-I-Br was far less than that of FA-MC-Br, demonstrating that FA-MC-Br was a cost-effective alternative material to activated carbon for potential industrial applications.