Adsorption of Elemental Mercury on the Residual Carbon in Coal Fly Ash

The injection of large quantities of pulverized activated carbon is one method used to remove elemental mercury (Hg0) from flue gas streams. The purpose of this project was to determine whether the unburned carbon that remains in coal fly ash could be used as an inexpensive and effective replacement for activated carbon. Bench-scale tests were conducted at conditions representative of those found in the flue gas trains of coal-fired power plants and municipal waste incinerators. The temperatures and concentrations ranged from 121 to 177 °C and from 0.019 to 11.7 mg of Hg/m3. Two types of data were obtained: equilibrium data suitable for obtaining adsorption isotherms and breakthrough data suitable for obtaining adsorption kinetics. Adsorbed-phase concentrations were as high as 600 ppm. Forward adsorption rate constants were ≈0.06−2.3 m3/g/s for particle sizes and carbon contents ranging from 59 to 206 μm and from 2% to 36%. Mathematical models were developed to simulate the capture of Hg0 in flue gas ducts and in baghouses. The results of the simulations indicate that a negligible amount of Hg0 can be adsorbed by a dilute suspension of fly ash. The best option for controlling Hg0 emissions using fly ash appears to be injection in pulses prior to a baghouse.