Investigation of Selective Catalytic Reduction Impact on Mercury Speciation under Simulated NO_x Emission Control Conditions

Abstract

Selective catalytic reduction (SCR) technology increasingly is being applied for controlling emissions of nitrogen oxides (NOx) from coal-fired boilers. Some recent field and pilot studies suggest that the operation of SCR could affect the chemical form of mercury (Hg) in coal combustion flue gases. The speciation of Hg is an important factor influencing the control and environmental fate of Hg emissions from coal combustion. The vanadium and titanium oxides, used commonly in the vanadia-titania SCR catalyst for catalytic NOx reduction, promote the formation of oxidized mercury (Hg2+). The work reported in this paper focuses on the impact of SCR on elemental mercury (Hg0) oxidation. Bench-scale experiments were conducted to investigate Hg0 oxidation in the presence of simulated coal combustion flue gases and under SCR reaction conditions. Flue gas mixtures with different concentrations of hydrogen chloride (HCl) and sulfur dioxide (SO2) for simulating the combustion of bituminous coals and subbituminous coals were tested in these experiments. The effects of HCl and SO2 in the flue gases on Hg0 oxidation under SCR reaction conditions were studied. It was observed that HCl is the most critical flue gas component that causes conversion of Hg0 to Hg2+ under SCR reaction conditions. The importance of HCl for Hg0 oxidation found in the present study provides the scientific basis for the apparent coal-type dependence observed for Hg0 oxidation occurring across the SCR reactors in the field.

Synergistic Mercury Removal by Conventional Pollutant Control Strategies for Coal-Fired Power Plants in China

“…4 The U.S. Environmental Protection Agency (EPA) reported the average Hg capture rates of different air pollution control devices (APCDs), 5,6 especially PM and SO 2 control devices. In some studies, NO x control devices were also mentioned as having certain Hg capturing abilities.…”

Enhanced Elemental Mercury Removal from Coal-Fired Flue Gas by Sulfur−Chlorine Compounds

“…The lower chlorine content of lower rank coals greatly reduces the efficiency of mercury oxidation by SCR catalysts (5,8). Furthermore, for utilities without SCR apparatus installed, it is not obvious that installation for the sole purpose of removing elemental mercury would be cost effective.…”

“…The challenge is to find a cost-effective method to convert the Hg 0 to an oxidized or particle-bound form during its short residence time in the ducts before it encounters the APCDs. The Hg 0 oxidation methods used for this include heterogeneous catalysis and homogeneous gas phase oxidation (7)(8)(9)(10)(11)(12)(13)(14). The catalysts used in Selective Catalytic Reduction (SCR) of NO x have shown significant success in mercury oxidation when the chlorine concentration in flue gas is high, such as is typically the case for bituminous coal combustion (7).…”

Oxidation Catalysts for Elemental Mercury in Flue Gases—A Review

“…Hg el adsorption on the SCR catalyst surface was only observed in the absence of HCl [76,77]. When HCl or NH 3 was added to the simulated flue gas, a rapid mercury desorption was observed [23,76,77].…”