Kinetics of Vapor-Phase Mercury Uptake by Virgin and Sulfur-Impregnated Activated Carbons

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Abstract
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 oC (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/m3 ).

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