Investigation of elemental mercury removal performance and mechanism of rice straw biochars from a fluidized bed pyrolysis system impregnated by NH4Br

Abstract

To remove Hg0 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 μg/g) is superior to that of activated carbon (207 μg/g), which indicates Br-modified rice straw biochar sorbent displays higher Hg0 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 temperatures, the components of flue gas (O2, 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 Hg0 removal efficiency. The Hg0 adsorption process is better fitted by pseudo-second-order kinetic model and proves a spontaneous endothermic process. The presence of O2 and NO in flue gas enhances Hg0 adsorption, while the presence of SO2 restrains it. After Hg0 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 %O2 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 Hg0 removal, and the products are mainly HgO and HgBr2. This work provides a kind of cost-effective Hg0 removal sorbent and explores the mechanism of Hg0 removal by Br-modified biochars.