Catalytic Reduction of Oxidized Mercury to Elemental Form by Transition Metals for Hg CEMS

Abstract

: This study was aimed to develop catalytic system for the dry-based reduction of oxidized mercury (Hg 2+ ) to elemental mercury (Hg 0 ) which is one of the most important components comprising mercury continuous emission monitoring system (Hg-CEMS). Based on the standard potential in oxidation-reduction reaction, transition metals including Fe, Cu, Ni and Co were selected as possible candidates for catalyst proceeding spontaneous reduction of Hg 2+ into Hg 0 . These transition metal catalysts revealed high activity for reduction of Hg 2+ into Hg 0 in the absence of oxygen in reactant gases. However, their activities were greatly decreased in the presence of oxygen, which was attributed to the transformation of transition metals by oxygen to the corresponding transition metal oxides with less catalytic activity for the reduction of oxidized mercury. Hydrogen supplied to the reactant gases significantly enhanced Hg 2+ reduction activity even in the presence of oxygen. It might be due to occurrence of combustion reaction between H2 and O2 causing the consumption of O2 at such high reaction temperature at which oxidized mercury reduction reaction took place. Because the system showed high activity for Hg 2+ reduction to Hg 0 , which was compatible to that of wet-chemistry technology using SnCl2 solution, the catalytic reduction system of Fe catalyst with the supply of H2 could be employed as a commercial system for the reduction of oxidized mercury to elemental mercury.

Reduction of Divalent Mercury to Elemental Mercury by Fe- and Ni-Based Reductants

“…The alkali metals of Fe and Ni are widely used as reductants and exhibit activity in reducing Hg 2+ in experiments. Ham indicated that Fe and Ni can react heterogeneously with HgCl 2 under certain conditions and efficiently reduce Hg 2+ to Hg 0 and generate FeCl 2 and NiCl 2 , respectively. The reaction can be expressed by eqs and . Fe + HgCl 2 → FeCl 2 + Hg 0 Ni + HgCl 2 → NiCl 2 + Hg 0 …”

“…applied HSC thermochemical simulation software to theoretically analyze the Hg 2+ reduction performance of three transition metals (Zn, Cd, and Y), indicating that they can efficiently convert Hg 2+ at 400 °C. Ham et al24 performed experiments on Hg 2+ reduction with transition metals (Fe, Cu, Ni, and Co) and demonstrated that all the four metal elements can reduce Hg 2+ to Hg 0 , but the reduction efficiency is too low to meet the requirements of Hg-CEMS. It is noteworthy that the transition metal material used by Ham et al in the experiment is powdery with a very small particle size, which may cause an agglomeration of the fines and the gas flow in channels, which lowers the utilization rate of the reductant.…”