Mercury removal by bio-char based modified activated carbons

This study investigated the effects of mercury concentration, process temperature, halides impregnating agents, halides precursor, and halides loading concentration on the mercury removal efficiency of the bio-char derived activated carbons from gaseous phase. The mercury was mainly in the elemental state. The activated carbons were prepared using steam activation method in a pilot scale set-up for the bio-char obtained from fast pyrolysis of whitewood. The elemental, porous structure, particle size, ash content, and thermogravimetric analyses were performed for the bio activated carbons. The activated carbons were impregnated using various precursors such as potassium and ammonium halides, with the halide loadings in the range of 0.1–1.0 mol% of carbon content. It was observed that impregnation of the halide ions significantly improved the performance of the activated carbon (AC) for elemental mercury capture in nitrogen flow. For the same molar loading of halide ions, mercury removal efficiency increased in the following order: AC < Cl-impregnated AC < Br-impregnated AC < I-impregnated AC. Also, Hg removal efficiency of ammonium halide impregnated activated carbon was higher compared to potassium halide impregnated activated carbon, which is due to the better access of Hg to active sites in pores for ammonium halide impregnating agent. Changes in Hg removal efficiency with the process temperature showed that possibly the main mechanism for Hg capture by activated carbon changed from physisorption to chemisorption after impregnation. In the present study, the field test was performed for bio-based unimpregnated activated carbon and was observed to significantly reduce the carbon to mercury ratio for mercury removal from actual flue gas compared with that for the commercial activated carbon.