The injection of powdered activated carbon (PAC) into combustion flue gas, with subsequent collection in a particulate control device, and granular activated carbon (GAC) fixed-bed adsorption offer new promise for achieving high-quality air emissions with respect to elemental mercury concentrations. One of the key parameters that governs the applicability of adsorption technology to flue gas cleanup is the rate of vapor-phase mercury removal, which was the main focus of this study. The kinetics of vapor-phase mercury uptake by a virgin bituminous coal-based activated carbon (BPL), a commercially available sulfur impregnated activated carbon (HGR), and a BPL carbon impregnated with sulfur at 600 C (BPL-S) was evaluated as a function of temperature and elemental mercury concentration. For all three carbons, an increase in mercury concentration and a decrease in temperature resulted in an increased overall mercury uptake. The rate of mercury uptake by HGR carbon was slower at higher temperatures due to the change in sulfur structure, which induced a decreased number of terminal sulfur atoms available to react with mercury. For a given flue gas temperature, an increase in mercury concentration resulted in slower mercury uptake kinetics, which suggests that the rate of mercuric sulfide (HgS) diffusion into the sulfur mass is the rate-limiting step. The rate of mercury uptake by BPL-S carbon deteriorated with an increase in temperature, which indicates that the rate of HgS formation is the rate-limiting step in the overall mercury removal process. BPL-S carbon displayed faster uptake kinetics and higher total mercury uptake than HGR carbon, except for very high initial mercury concentrations (e.g.,>1,000 mg/m ).

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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-adsorbate 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 IMPLICATIONS This study suggests that an assessment of the attainment of adsorption equilibrium between activated carbon and elemental mercury is critical when evaluating strategies to improve the removal of mercury from flue gases by powdered sorbent injection. When adsorption equilibrium is achieved within the retention time in the flue gas, carbons with higher adsorptive capacity should be used to achieve a higher percentage of mercury removal for the same mass of carbon applied. If equilibrium is not attained, the system is mass-transfer limited; strategies that can be adopted to improve mercury removal include using smaller carbon particle size, facilitating longer retention time, and using carbons with higher adsorptive capacities.