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

Abstract

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.

A new mechanism for atmospheric mercury redox chemistry: implications for the global mercury budget

“…Our work suggests that the dominant Hg II species produced in the gas phase are BrHgONO and BrHgOOH, though the actual speciation of Hg II in the atmosphere would likely be modified through cycling in aerosols and clouds (Hedgecock and Pirrone, 2001;Lin et al, 2006). We did not consider heterogeneous Hg 0 oxidation on particle surfaces (e.g., Vidic et al, 1998;Flora et al, 1998;Lee et al, 2004) due to inadequate information to formulate atmospheric rates. Further study of heterogeneous Hg 0 oxidation is needed (Ariya et al, 2015).…”

Modeling Sorbent Injection for Mercury Control in Baghouse Filters: I—Model Development and Sensitivity Analysis

“…The median pore diameter was reported to be 150 Å 18 and was used in estimating D Kn . The activated carbon density ( p ϭ 2.04 g/cm 3 ) and porosity (ε p ϭ 0.67) were provided by the manufacturer. Yang 19 reports that the tortuosity ( p ) for activated carbons ranges from 5 to 65, and Bush et al 20 report that the bed porosity (ε b ) for full-scale coalfired power plants employing a baghouse for particulate control ranges from 0.57 to 0.86.…”

“…Flora et al 3 developed a model for a closed-batch reactor system that accounted for both external mass transfer resistance and intraparticle transport. External mass transfer was modeled using Fick's first law, while the intraparticle transport was modeled using the surface diffusion transport mechanism.…”

Enhanced mercuric chloride adsorption onto sulfur-modified activated carbons derived from waste tires

“…Figure 3 illustrates the pore-size distribution of activated carbons with a high surface area (NH, H-400S, and H-605S) modified by elemental sulfur at temperatures of 400 and 650 C. The elemental sulfur was initially gasified and mainly composed of chain sulfur molecules at a temperature of 400 C, which diffused with relative ease into the mesopores and macropores of activated carbons. Correspondingly, the elemental sulfur was completely gasified and mostly composed of ring sulfur molecules at the temperature of 650 C, which was relatively difficult to diffuse into the mesopores and macropores of activated carbons (Flora et al, 1998;Sandra and Roberto, 1999). The sulfur molecules partially blocked the mesopores and macropores acting as passages to micropores.…”