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 (Hg₀) 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 (NO_x), hydrogen chloride (HCl), and trace Hg₀. Concentrations of Hg₀ 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 Hg₀ oxidation were evaluated. It was observed that HCl provides the source of chlorine for Hg₀ 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 Hg₀ 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 NO_x reduction by SCR was found to have a strong effect to decrease Hg oxidation. The observations suggest that Hg₀ 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.