Preparation and Evaluation of Coal-Derived Activated Carbons for Removal of Mercury Vapor from Simulated Coal Combustion Flue Gases

Abstract

Coal-derived activated carbons (CDACs) were tested for their suitability in removing trace amounts of vapor-phase mercury from simulated flue gases generated by coal combustion. CDACs were prepared in bench-scale and pilot-scale fluidized-bed reactors with a three-step process, including coal preoxidation, carbonization, and then steam activation. CDACs from high-organicsulfur Illinois coals had a greater equilibrium Hg 0 adsorption capacity than activated carbons prepared from a low-organic-sulfur Illinois coal. When a low-organic-sulfur CDAC was impregnated with elemental sulfur at 600 °C, its equilibrium Hg 0 adsorption capacity was comparable to the adsorption capacity of the activated carbon prepared from the high-organicsulfur coal. X-ray diffraction and sulfur K-edge X-ray absorption near-edge structure examinations showed that the sulfur in the CDACs was mainly in organic forms. These results suggested that a portion of the inherent organic sulfur in the starting coal, which remained in the CDACs, played an important role in adsorption of Hg 0 . Besides organic sulfur, the BET surface area and micropore area of the CDACs also influenced Hg 0 adsorption capacity. The HgCl 2 adsorption capacity was not as dependent on the surface area and concentration of sulfur in the CDACs as was adsorption of Hg 0 . The properties and mercury adsorption capacities of the CDACs were compared with those obtained for commercial Darco FGD carbon.

Effects of Sulfur, Nitric Acid, and Thermal Treatments on the Properties and Mercury Adsorption of Activated Carbons from Bituminous Coals

“…In our previous study, ACs were prepared from highsulfur coals and had comparable Hg 0 and HgCl 2 adsorption performance to that of a commercial AC (Hsi et al, 1998). This study further discusses the impacts of sulfur impregnation (elemental sulfur or polysulfide), HNO 3 treatment, followed by thermal treatments on the physical/ chemical properties and Hg 0 and HgCl 2 adsorption by ACs derived from high-and low-organic-sulfur bituminous coals.…”

“…As-received coal samples were ground and sieved to between 0.21 and 1.00 mm in diameter. Benchscale production of ACs was performed in a custom 5-cm ID fluidized-bed reactor with oxidation (air, 225°C), carbonization (N 2 , 400°C), and then steam activation (50% H 2 O/50% N 2 , 825°C) (Hsi et al, 1998). Properties of the ACs are presented in Table 2.…”

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.…”

Modeling Sorbent Injection for Mercury Control in Baghouse Filters: II—Pilot-Scale Studies and Model Evaluation

“…Subsequently, the equilibrium constant b, which represents the ratio of the two rate constants, should increase with temperature. At a temperature of 135°C (275°F) and a Hg concentration of 59 g/m 3 , Hsi et al 25 reported the capacity of Norit Darco FGD of 2566 g/g, which is reasonably close (within 17%) to the capacity calculated for that temperature using the data in Figure 4 (3007 g/g). Figure 5 shows the model predictions for all the data sets (including the data sets used for model fitting) accounting for the variation of the isotherm coefficients with temperature.…”