Investigation of Elemental Mercury Removal Performance and Mechanism of Rice Straw Biochars from a Fluidized Bed Pyrolysis System Impregnated by Nh4br

To remove Hg-0 from flue gas efficiently and economically, the rice straw biochars prepared in a fluidized bed pyrolysis system and then modified with NH4Br are employed as the sorbents. The mercury adsorption capacity of Br-modified biochar (1786.85 mu g/g) is superior to that of activated carbon (207 mu g/g), which indicates Br-modified rice straw biochar sorbent displays higher Hg-0 adsorption efficiency and can replace high-cost activated carbon. The sorbents are characterized by Brunauer-Emmett-Teller (BET), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). The effects of pyrolysis temperature, pyrolysis atmosphere, reaction temperature, the components of flue gas (O-2, NO and SO2) on mercury removal efficiency, kinetics, thermodynamics and mercury removal mechanism are investigated. The results indicate elevating pyrolysis temperature and oxygen content significantly enhance the Hg-0 removal efficiency. The Hg-0 adsorption process is better fitted by pseudo-second-order kinetic model and proves a spontaneous endothermic process. The presence of O-2 and NO in flue gas enhances Hg-0 adsorption, while the presence of SO2 restrains it. After Hg-0 adsorption, the peak area ratio of C=O (288.5 eV) and C-Br (69.1 eV) by Br-modified rice straw biochars produced under 8 %O-2 and 500. C atmosphere decreases from 17.03 % and 70.14 % to 6.55 % and 50.50 %, respectively, while that of Br (68.1 eV) increases from 29.86 % to 48.50 %, indicating the surface O* and Br* species are responsible for Hg-0 removal, and the products are mainly HgO and HgBr2. This work provides a kind of cost-effective Hg-0 removal sorbent and explores the mechanism of Hg-0 removal by Br-modified biochars.