Mercury Oxidation Promoted by a Selective Catalytic Reduction Catalyst under Simulated Powder River Basin Coal Combustion Conditions

Abstract

A bench-scale reactor consisting of a natural gas burner and an electrically heated reactor housing a selective catalytic reduction (SCR) catalyst was constructed for studying elemental mercury (Hg(o)) oxidation under SCR conditions. A low sulfur Powder River Basin (PRB) subbituminous coal combustion fly ash was injected into the entrained-flow reactor along with sulfur dioxide (SO2), nitrogen oxides (NOx), hydrogen chloride (HCl), and trace Hg(o). Concentrations of Hg(o) and total mercury (Hg) upstream and downstream of the SCR catalyst were measured using a Hg monitor. The effects of HCl concentration, SCR operating temperature, catalyst space velocity, and feed rate of PRB fly ash on Hg(o) oxidation were evaluated. It was observed that HCl provides the source of chlorine for Hg(o) oxidation under simulated PRB coal-fired SCR conditions. The decrease in Hg mass balance closure across the catalyst with decreasing HCl concentration suggests that transient Hg capture on the SCR catalyst occurred during the short test exposure periods and that the outlet speciation observed may not be representative of steady-state operation at longer exposure times. Increasing the space velocity and operating temperature of the SCR led to less Hg(o) oxidized. Introduction of PRB coal fly ash resulted in slightly decreased outlet oxidized mercury (Hg2+) as a percentage of total inlet Hg and correspondingly resulted in an incremental increase in Hg capture. The injection of ammonia (NH3) for NOx reduction by SCR was found to have a strong effect to decrease Hg oxidation. The observations suggest that Hg(o) oxidation may occur near the exit region of commercial SCR reactors. Passage of flue gas through SCR systems without NH3 injection, such as during the low-ozone season, may also impact Hg speciation and capture in the flue gas.

Mercury emission and speciation of coal-fired power plants in China

“…Therefore, emissions of Hg 2+ , may be efficiently controlled by typical air pollution control devices (APCD), such as electrostatic precipitators (ESP), fabric filter (FF), and flue gas desulfurization (FGD) systems (US EPA, 1997EPA, , 2000bEPA, , 2002a. However, because the relative proportions of Hg 2+ , Hg p and Hg 0 can vary widely, the corresponding reductions in total mercury achieved by APCD vary (Pavlish et al, 2003;Srivastava et al, 2006;Lee et al, 2008;Cao et al, 2008a). For example, the removal efficiency of Hg from the flue gas by a combination of cold side ESP and wet FGD range from 24 to 70%.…”

Heterogeneous Mercury Reaction Chemistry on Activated Carbon

“…Huggins et al 12,13 investigated various C samples exposed to Hg at simulated coal combustion flue gas conditions and characterized them using X-ray absorption fine structure (XAFS) spectroscopy. The XAFS spectra provided strong indication of Hg chemisorption on C. Their data suggest that the adsorption process occurs through a combination of halide, sulfide, and oxygen anions (either C-bound or C-sulfate-bound) present on the C surface.…”

Oxidation Catalysts for Elemental Mercury in Flue Gases—A Review

“…Laboratory-scale studies proved that the activity of SCR catalysts for Hg el oxidation under simulated flue gas conditions is strongly linked to the halide species present and their concentrations [3,21,23,53,71,78,80]. Since HCl is the dominant halide species in flue gases, Hg el oxidation is considered to proceed according to the following overall equation:…”