Modeling Powdered Activated Carbon Injection for the Uptake of Elemental Mercury Vapors

Batch kinetic experiments were performed to assess the rate of elemental mercury uptake by virgin activated carbon at 25 and 140 °C, and the homogeneous surface diffusion model (HSDM) was used to obtain Langmuir isotherm constants, the film mass transfer coefficient, and the surface diffusion coefficient for this adsorbent-adsor-bate pair. The adsorptive capacity of the carbon decreased, while the adsorption kinetics improved, with an increase in temperature. Simulations showed that the adsorptive capacity, particle size, and activated carbon dose, as well as the contact time influenced the removal of elemental mercury under conditions that may be encountered in the flue gases of coal-fired power plants. When adsorption equilibrium was achieved, the adsorptive capacity determined the carbon dose required to attain a certain percentage of mercury removal. When the system was mass-transfer limited, smaller particle size resulted in beter mercury removal. Although increasing the adsorptive capacity also led to better mercury removal 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 adsorptive capacity of activated carbon. Design nomo-grams were developed to estimate the carbon dose required to attain 80 and 90% removals of elemental mercury from nitrogen atmosphere for various process conditions and carbon properties.