Effects of Sulfur Dioxide and Nitric Oxide on Mercury Oxidation and Reduction under Homogeneous Conditions

Abstract

This paper is particularly related to elemental mercury (Hg 0 ) oxidation and divalent mercury (Hg 2ϩ ) reduction under simulated flue gas conditions in the presence of nitric oxide (NO) and sulfur dioxide (SO 2 ). As a powerful oxidant and chlorinating reagent, Cl 2 has the potential for Hg oxidation. However, the detailed mechanism for the interactions, especially among chlorine (Cl)-containing species, SO 2 , NO, as well as H 2 O, remains ambiguous. Research described in this paper therefore focused on the impacts of SO 2 and NO on Hg 0 oxidation and Hg 2ϩ reduction with the intent of unraveling unrecognized interactions among Cl species, SO 2 , and NO most importantly in the presence of H 2 O. The experimental results demonstrated that SO 2 and NO had pronounced inhibitory effects on Hg 0 oxidation at high temperatures when H 2 O was also present in the gas blend. Such a demonstration was further confirmed by the reduction of Hg 2ϩ back into its elemental form. Data revealed that SO 2 and NO were capable of promoting homogeneous reduction of Hg 2ϩ to Hg 0 with H 2 O being present. However, the above inhibition or promotion disappeared under homogeneous conditions when H 2 O was removed from the gas blend.

Transformations of mercury in processes of solid fuel combustion – review

“…It is only after the fl ue gas is cooled down to a temperature below 600°C that elemental mercury vapours react with other fl ue gas components, consequently leading to the formation of oxidized forms (Hg +2 ) and Hg p forms bound with fl y ash , Pavlish et al 2003, Zhuang et al 2004. The mechanism of the mercury oxidation in the presence of chlorine has not yet been unambiguously determined, but numerous studies show both homo-and heterogeneous mechanisms of the reaction between mercury and other fl ue gas components (Niksa et al 2001, Zhao et al 2006. The scheme of individual reactions depends on the coal characteristics (mineral and chemical composition), fl ue gas composition, process conditions, as well as the rate of fl ue gas cooling (Frandsen et al 1994, Ariya et al 2002, Senior and Johnson 2005, Wichliński et al 2014.…”

“…Basic parameters which affect chemical and physical speciation of mercury are the process temperature, the content of chlorine, sulphur, iron, and calcium in coals as well as the concentration of HCl and ammonia in fl ue gas (Sliger et al 2000, Pavlish et al 2003, Zhuang et al 2004, Zhao et al 2006. It is estimated that more than 85% of the mercury in fl ue gas from combustion of coal with low-chlorine content (Cl < 150 ppm) represents elemental mercury.…”

“…Numerous studies showed that percentage emissions of mercury for individual plants varied widely (from 5% to 90% of Hg entering with coal) depending on the coal composition and conditions of combustion process (Chow et al 1994, Jones 1994, Galbreath and Zygarlicke 1996, Widmer et al 1998 Lee et al 2006, Zhao et al 2006, Lopez-Anton et al 2010. They also showed that Hg concentrations in stack gases varied in a wide range between 3 and 70 μg/m 3 .…”

“…R828168, 2004), a direct reaction between Hg and HCl, leading to the formation of HgCl, is impossible (Fuente-Cuesta et al 2012). The mechanisms of HgCl 2 formation via homogeneous reactions of Hg 0 and atomic chlorine are suggested to proceed according to the following equations (Niksa et al 2001, Zhuang et al 2004, Zhao et al 2006:…”

“…It is only after cooling down of the gas that the chlorine atoms react with HCl to form Cl 2 and hydrogen. Reaction (1) is characterized by low activation energy and may take place in room temperature at a constant rate of 1.95 ± 1.05 x 10 13 cm 3 /mol s (Horne et al 1968); the reaction has hampering infl uence on the mercury oxidation (Ariya et al 2002, Pavlish et al 2003, Zhao et al 2006). Sliger and co-workers (Sliger et al 2000) proved that reaction (2) proceeds very slowly within the range of temperatures from 620°C to 1700°C, therefore, its contribution to oxidation of mercury is negligible.…”