This study evaluated the oxidation of mercury under simulated coal-fired power plant flue gas conditions. The effect of NO, NO2, HCl and H2O on mercury oxidation catalyzed by the surface of activated carbon as well as on the performance of activated carbon for mercury uptake is evaluated under different experimental conditions. Approximately 17-20 µg/m3 of elemental mercury in a simulated flue gas was fed at a flow rate of 1 L/min to a 20 cm long and 22 mm inner diameter quartz tube used as a fixed-bed reactor. The reactor was kept in a 50 cm long tube furnace, which was used to maintain the temperature of the fixed bed at 140 0C. In every experimental run, 40 mg of activated carbon was charged into the quartz tube along with 4 g of glass beads, which were used to provide support for the sorbent. Prior to use, glass beads were washed with distilled water and heated to 500 0C for 4 hours. An impinger containing 20% NaOH and 10% KCl is used to remove all oxidized mercury from the flue gas so that only elemental mercury is present in the flue gas that is sent to mercury analyzer (atomic fluorescence detector with gols-coated sand trap). On the other side, an impinger containing 20% NaOH and 2% SnCl2 is used to reduce the oxidized mercury to elemental form to facilitate total mercury measurement. NaOH in the above solutions was used to remove acidic gases and prevent their interference with the operation of gold trap in atomic fluorescence unit. A condenser operated at 4 oC was used to remove water vapor before sending the gas to the atomic fluorescence unit to measure mercury concentration.

Different flue gas constituents exhibited different impacts on the fate of mercury in a simulated flue gas or in the fixed-bed of activated carbon. For example, very limited (up to 10%) homogeneous mercury oxidation occurred at 140 oC under the flue gas conditions typical of coal-fired power plants. However, the presence of activated carbon surface lead to significant (above 50%) elemental mercury oxidation under the same conditions. Data also revealed that removing H2O from simulated flue gas decreased the oxidation of mercury in the presence of activated carbon surface (from more than 50% to less than 15%), but the capacity of activated carbon increased by as much as 50%. Similarly, removal of HCl from the simulated flue gas caused mercury oxidation to decrease to less than 10% and lead to a slight decrease in activated carbon adsorption capacity. On the other hand, removing NO from simulated flue gas increased the adsorption capacity as well as mercury oxidation (up to 85%). NO2 did not show any impact on either the adsorption capacity of activated carbon or mercury oxidation. Removal of SO2 from simulated flue gas caused the capacity of activated carbon to increase tremendously (i.e., no breakthrough was observed for 24 hours whereas 100% breakthrough was observed anywhere from 12-20 hours in the experiments mentioned above).

From the results obtained in this study, it can be concluded that the surface of activated carbon catalyzes mercury oxidation under simulated flue gas conditions at 140 0C. The combined presence of HCl and H2O in the flue gas is very important for the oxidation reactions. HCl and H2O present alone in the simulated flue gas cannot induce as much mercury oxidation as when they are present at the same time. NO acts as a reducing agent whose removal from the flue gas leads to significant increase in mercury oxidation. The presence of SO2 leads to a significant decrease in the capacity of activated carbon for mercury uptake. Ongoing experiments are designed to assess the impact of these flue gas constituents under a more realistic process conditions using an entrained flow reactor and the results will be presented at the conference.