The Removal Of Hg~0 In Coal-fired Flue Gas By Metal-modified Activated Carbon: A Theoretical Study

Coal-fired power plant is one of the main sources of mercury pollution, when coal is burned, most mercury is released into atmosphere with the flue gas and brings great damages to animals, plants, human bodies and ecological environment. Among various mercury forms(Hg0, Hg2+, Hgp), the removal of Hg0 is the most challengeable. Therefore, exploring an economic and efficient Hg0 remover is extremely urgent.Considering the large specific surface area and favorable pore size, AC can be used to control mercury pollution in the coal-fired power plants. However, the low efficiency and difficult regeneration of AC result in a high Hg0 removal cost. Based on the fact that metals have the capcity to remove Hg0, metal-modified AC is considered as a Hg0 remover. In this work, the removal of Hg0 by metal-modified AC(non-noble metal representation of Fe, Co, Ni, Cu, Zn and noble metal representation of Pd) is investigated by density functional theory(DFT), along with which is the Cu modulation effect for the purpose of reducing the dosage of Pd and increasing the removal capacity of Hg0.The main conclusions are listed following:1. Hg0 removal mechanism by non-noble metal modified ACNon-noble metals Fe, Co, Ni, Cu and Zn are selected to modify AC, for the study of Hg0 adsorption on the M/AC(M=Fe, Co, Ni, Cu, Zn), it shows that Hg0 physically adsors on the M/AC, indicting a weak interaction of Hg0 on the non-noble metal modified AC. Also, the adsorption of Hg Cl and HgCl2(the oxidation products of Hg0) on the M/AC are studied, it shows that Hg Cl is not a stable intermediate for the preferred adsorption mode of dissociation; HgCl2 inclines to molecular adsorption on the Fe/AC surface, while dissociated adsorptions are primary on M/AC(M= Ni, Cu, Zn) surfaces, but both molecular and dissociated adsorptions exist on the Co/AC surface; For Hg0 oxidation on the M/AC(M=Fe, Co, Ni, Cu, Zn) by Cl2, it accords to the Eley-Rideal mechanism and Fe/AC is the best catalyst.2. Hg0 removal mechanism by noble metal modified ACNoble metal Pd is selected to modify AC, for the study of Hg0 adsorption on the Pd modified AC, it shows that Hg0 physically adsorbs on the Pd/AC, indicating a weak interaction of Hg0 on the Pd/AC and can’t be removed effectively; Among Pdn(n≤38) clusters, Pdn(n=4,6,8,13,19,23,38) are stable magic clusters; the existence of AC can enhance the stability of Pdn(n=4,6,8,13,19,23) clusters, but nearly has no effect on that of Pd38 cluster; the adsorption strength of Hg0 on Pdn/AC(n=4,6,8,13,19,23) shows a zigzag variation, and when loading the icosahedral Pd13 cluster on AC, the adsorption of Hg0 is the strongest.3. Hg0 removal mechanism by Cu modulated Pd13/ACNon-noble metal Cu is selected to modify Pd13/AC, for the study of Hg0 adsorption on the Cu modulated Pd13/AC, it shows that Pd13 cluster can be substituted by eight Cu atoms at most and the Cun Pd13-n(n=1~8) bimetallic clusters have a higher stability than that of Pd13 cluster; substituting one, two and six Pd atoms of Pd13 cluster into Cu atoms is relatively easier, and Cu Pd12, Cu2Pd11 and Cu6Pd7 bimetallic clusters can exist on the AC surface stably; the doping ratio of Cu affects Hg0 adsorption strength greatly, when replacing the center Pd atom of Pd13 cluster into Cu atom, namely, a doping ratio of 7.7%, only a slight improvement for Hg0 adsorption with increased percent of 3.3%, when replacing one more surface Pd atom into Cu, namely, a doping ratio of 15.4%, the adsorption energy increases geratly by 27.1%, in favor of Hg0 adsorption. However, when the number of substituted Cu atoms increases to six, namely a doping ratio of 46.2%, the adsorption energy decreases by 25.1%, unfavorable for Hg0 adsorption.