IMPACT OF TEMPERATURE AND FLUE GAS COMPONENTS ON MERCURY SPECIATION AND UPTAKE BY ACTIVATED CARBON SORBENTS

An experimental setup was built to simulate flue gas representative of coal fired power plants burning sub-bituminious powder river basin (PRB) coal. The impact of different flue gas constituents and bed temperature on mercury uptake capacity and mercury speciation were evaluated using a fixed bed epxerimental system. Two activated carbons were selected for the study: FGD activated carbon (Norit America Inc.) and a novel activated carbon manufactured by Corning Inc. After the experimental setup was tested and validated, evalauation of sorbents' performance was conducted using simulated PRB coal flue gas.A susbstantial increase in the mercury uptake capacity of both sorbents was observed in the absence of SO2 from the flue gas. Temeprature programed desorption (TPD) test on spent FGD sorbent revealed that mercury present on the surface of the spent sorbent was mostly in the elemental form. An instant breakthrough of mercury was observed with both sorbents when HCl was removed from the flue gas. This led to a significant decrease in the mercury adsorption capacity of both sorbents. Absence of water from the flue gas caused an increase in mercury uptake capacity and a decrease in mercury oxidation with both the sorbents. Removal of NO and NO2 had variable impact on different sorbent. Removal of NO or NO2 from the flue gas caused an increase in mercury uptake capacity of FGD sorbent. Removal of NO from the flue gas also led to an increase in mercury oxidation catalyzed by FGD sorbent. On the other hand, removal of NO or NO2 from PRB gas not only caused a decrease in the mercury uptake capacity of the Corning sorbent, but also led to a significant decrease in mercury oxidation catalyzed by this sorbent. A 100 °C increase in bed temperature (from 140 °C to 240 °C) caused an instant breakthrough of mercury with both sorbents under simulated PRB coal flue gas conditions. It also caused a significant decrease in the oxidation of mercury. Based on the findings of the study, a simplistic model explaining the mechanism for mercury uptake and oxidation by activated carbon through competition between Cl and SO2 for active sites on the surface of the activated carbon is proposed