Trace Element Partitioning and Transformations During Combustion of Bituminous and Subbituminous U. S. Coals In a 7-kW Combustion System

Abstract

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-NOx 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-NOx combustion of Illinois No. 6 coal did not significantly affect trace element volatility. However, low-NOx 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 (FA2.5) were determined using X-ray diffraction and absorption methods. Illinois No. 6 and Absaloka FA2.5 contain aluminosilicate glass, quartz (SiO2), ferrite spinel (AB2O4; e.g., where A2+ = Fe, Mg, Ni, Co, Cu and B3+ = Al, Fe, Cr), and mullite (Al6Si2O13). Absaloka FA2.5 is distinguished from Illinois No. 6 FA2.5 by the presence of lime (CaO) and periclase (MgO) and lack of anhydrite (CaSO4). Differences in Illinois No. 6 and Absaloka coal combustion conditions did not significantly affect As, Cr, or Ni speciation. As5+O4-containing phases occur in Illinois No. 6 and Absaloka FA2.5. Presumably, carboxyl-bound As3+ and Ca in Absaloka coal promoted the formation of Ca3(AsO4)2. Cr3+/Cr6+ is much greater in Illinois No. 6 FA2.5, relative to Absaloka FA2.5. The predominance of maceral-bound Cr3+ and oxygen functional groups in Absaloka coal may have promoted Cr6+ formation. Illinois No. 6 and Absaloka FA2.5 contain similar NiO-bearing phases, possibly ferrite spinel.