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Coal combustion and gasification are an anthropogenic source of mercury emission to the atmosphere. Effectively minimizing the emission and understanding the atmospheric fate and transport of mercury require knowledge of its speciation in flue gases. Hg0(g) is the thermodynamically stable form in the highest temperature regions of combustors and gasifiers. Hg0(g) remains as the dominant form in the relatively reducing conditions of a gasification flue gas, but with decreasing temperature in a combustion flue gas Hg0(g) will react to form Hg2+ compounds. Current mercury speciation analysis results suggest that generally >50% of the Hg0(g) reacts with oxidants in coal combustion flue gases; results for gasification conditions are lacking. Oxidation is beneficial because Hg2+ compounds are generally water-soluble and are therefore more effectively captured by wet scrubber pollution control systems and are more apt to deposit locally or regionally. Conversely, Hg0(g) is difficult to control and is likely to enter the global atmospheric cycle because of its high vapor pressure and low water solubility. The physical and chemical processes governing the interactions of mercury species with flue gas components are poorly known.

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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.