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This paper discusses a recent study of mercury catalytic oxidation by chlorinating reagents. Gold was chosen as the catalyst because of its reluctance to chemisorb some gases such as O2, NO, H2O, and SO2. This property, as demonstrated in this study, is instrumental to mercury oxidation by circumventing some undesired inhibitory reactions such as OH + NO + M --> HONO + M and OH + SO2 + M --> HOSO2 + M, which were recognized under homogeneous situations at high temperatures. In comparison to Cl2, HCl showed weak oxidizing capability but appreciable inhibition in mercury oxidation by Cl2, probably through the competition of active sites with Cl2. Overall, the mercury catalytic oxidation by Cl2 on gold catalyst surfaces was viable, reaching 40-60% in this study under temperatures of 448-498 K, where the thermal decomposition of formed Hg2+ was effectively avoided.

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Mode of occurrence analyses indicate that trace elements (Sb, As, Cd, Cr, Co, Cu, Pb, Hg, Ni, Se, V, and Zn) in the Illinois No. 6 coal are generally associated with relatively large discrete mineral grains, especially pyrite, whereas trace elements in Absaloka coal are much more strongly associated with macerals and fine-grained minerals. These coals were burned using conventional and low-NO x conditions in an ∼7-kW combustion system to evaluate the importance of elemental modes of occurrence, coal properties, and combustion conditions on trace element volatility and As, Cr, and Ni speciation. Chemical analyses of size-classified (∼0.4-7.7 µm) fly ash and flue gas samples indicated that Hg and Se were the most volatile elements in both coals. Occurrences of Cr, Co, and Cu in fly ash were characterized by relatively uniform particle-size distributions and relative enrichment/depletion (RED) factors for all four fly ashes, which is indicative of nonvolatility. As and Sb in Absaloka fly ashes exhibited similar nonvolatile partitioning characteristics. Consistent with an elemental vaporization-particle surface deposition process, Sb, As, Cd, Pb, Ni, V, and Zn concentrations and RED factors for the Illinois No. 6 fly ashes generally increased with decreasing particle size. Similar semivolatile partitioning systematics were noted for Cd, Pb, and V in Absaloka fly ashes. Conventional and low-NO x combustion of Illinois No. 6 coal did not significantly affect trace element volatility. However, low-NO x Absaloka combustion promoted Ni, Zn, and Se volatilization. The inorganic phase composition and As, Cr, and Ni speciation of fly ash particles ∼2.5 µm in aerodynamic diameter (FA 2.5 ) were determined using X-ray diffraction and absorption methods. Illinois No. 6 and Absaloka FA 2.5 contain aluminosilicate glass, quartz (SiO 2 ), ferrite spinel (AB 2 O 4 ; e.g., where A 2+ ) Fe, Mg, Ni, Co, Cu and B 3+ ) Al, Fe, Cr), and mullite (Al 6 Si 2 O 13 ). Absaloka FA 2.5 is distinguished from Illinois No. 6 FA 2.5 by the presence of lime (CaO) and periclase (MgO) and lack of anhydrite (CaSO 4 ). Differences in Illinois No. 6 and Absaloka coal combustion conditions did not significantly affect As, Cr, or Ni speciation. As 5+ O 4 -containing phases occur in Illinois No. 6 and Absaloka FA 2.5 . Presumably, carboxyl-bound As 3+ and Ca in Absaloka coal promoted the formation of Ca 3 (AsO 4 ) 2 . Cr 3+ /Cr 6+ is much greater in Illinois No. 6 FA 2.5 , relative to Absaloka FA 2.5 . The predominance of maceral-bound Cr 3+ and oxygen functional groups in Absaloka coal may have promoted Cr 6+ formation. Illinois No. 6 and Absaloka FA 2.5 contain similar NiO-bearing phases, possibly ferrite spinel.

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Bench-scale testing of elemental mercury (Hg 0 ) sorption on selected activated carbon sorbents was conducted to develop a better understanding of the interaction among the sorbent, flue gas constituents, and Hg 0. The results of the fixed-bed testing under simulated lignite combustion flue gas composition for activated carbons showed some initial breakthrough followed by increased mercury (Hg) capture for up to ϳ4.8 hr. After breakthrough, the Hg in the effluent stream was primarily in an oxidized form (Ͼ90%). Aliquots of selected activated carbons were exposed to simulated flue gas containing Hg 0 vapor for varying time intervals to explore surface chemistry changes as the initial breakthrough, Hg capture, and oxidation occurred. The samples were analyzed by X-ray photoelectron spectroscopy to determine changes in the abundance and forms of sulfur, chlorine, oxygen, and nitrogen moieties as a result of interactions of flue gas components on the activated carbon surface during the sorption process. The data are best explained by a competition between the bound hydrogen chloride (HCl) and increasing sulfur [S(VI)] for a basic carbon binding site.