MODELING POWDERED ACTIVATED CARBON INJECTION FOR THE UPTAKE OF ELEMENTAL MERCURY VAPORS

Batch kinetic experiments were performed to assess the rate of element al mercury uptake by virgin activated carbon at 25 and 140 degrees C, and the homogeneous surface diffusion model (HSDM) was used to obtain Langmuir isotherm constants, the film mass transfer coefficient, and t he surface diffusion coefficient for this adsorbent-adsorbate pair. Th e adsorptive capacity of the carbon decreased, while the adsorption ki netics improved, with an increase in temperature. Simulations showed t hat the adsorptive capacity, particle size, and activated carbon dose, as well as the contact time influenced the removal of elemental mercu ry under conditions that may be encountered in the flue gases of coal- fired power plants. When adsorption equilibrium was achieved, the adso rptive capacity determined the carbon dose required to attain a certai n percentage of mercury removal. When the system was mass-transfer lim ited, smaller particle size resulted in beter mercury removal. Althoug h increasing the adsorptive capacity also led to better mercury remova l for mass-transfer-limited systems, the magnitude of the improvement depended on the carbon particle size. Longer contact times resulted in the system approaching equilibrium and a more efficient use of the ad sorptive capacity of activated carbon. Design nomograms were developed to estimate the carbon dose required to attain 80 and 90% removals of elemental mercury from nitrogen atmosphere for various process condit ions and carbon properties.