Understanding the transformation of mercury species (elemental and oxidized forms) in stationary combustion sources is of interest to both those responsible for developing control technologies and those responsible for setting regulations. Elemental mercury (Hg 0 ) poses a challenging flue gas control issue, and oxidized forms of mercury (Hg 2ϩ ) are of concern due to local deposition potential. In a previous study, mercury speciation, or more specifically oxidation of Hg 0 , was studied in simulated flue gases and in the presence of three-component model fly ashes. Gas-phase studies indicated that oxidation of Hg 0 in the presence of hydrogen chloride (HCl) is rather slow, and proceeds at measurable rates only at temperatures Ͼ700 °C and HCl concentrations in the range of 100-200 ppm. The effect of fly ash composition was investigated using a fixed-bed reactor containing different synthetic model fly ash components such as alumina (Al 2 O 3 ), silica (SiO 2 ), ferric oxide (Fe 2 O 3 ), copper oxide (CuO), and calcium oxide (CaO). Transition metal oxides, CuO and Fe 2 O 3 , exhibited significant catalytic activity in the surface-mediated oxidation of Hg 0 in the presence of HCl. The observed Hg 0 oxidation activities of the two oxides are possibly caused by the Deacon process in which chlorine gas (Cl 2 ) is produced via catalytic oxidation of HCl over these two oxides. In the present follow-up study, the effect of sulfur dioxide (SO 2 ) to HCl ratio on Hg 0 oxidation was investigated. The addition of SO 2 to the moist flue gas at high SO 2 :HCl ratios (10:1 to 4:1) caused a decrease in oxidation of Hg 0 . This is attributed to a scavenging effect of SO 2 and H 2 O on Cl 2 . Addition of CaO to the synthetic fly ashes also caused a drop in Hg 0 oxidation. It is possible that due to the partial removal of HCl by reaction with CaO less HCl is available for the catalytic Deacon reaction. The Hg 0 oxidation activity of a cement kiln dust (CKD) sample collected from a full-scale hazardous waste incinerator was also studied. Qualitatively, it exhibited Hg 0 oxidation catalytic behavior sim-221

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.

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.