Trace element partitioning and transformations during combustion of bituminous and subbituminous U. S. coals in a 7-kW combustion system

Mode of occurrence analyses indicate that trace elements (Sb, As, Cd, Cr, C o, 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 con ventional and low-NO, conditions in an similar to7-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 sp eciation. Chemical analyses of size-classified (similar to0.4-7.7 mum) fly ash and flue gas samples indicated that Hg and Se were the most volatile el ements in both coals. Occurrences of Cr, Co, and Cu in fly ash were charact erized by relatively uniform particle-size distributions and relative enric hment/depletion (RED) factors for all four fly ashes, which is indicative o f nonvolatility. As and Sb in Absaloka fly ashes exhibited similar nonvolat ile partitioning characteristics. Consistent with an elemental vaporization -particle surface deposition process, Sb, As, Cd, Pb, Ni, V, and Zn concent rations and RED factors for the Illinois No. 6 fly ashes generally increase d with decreasing particle size. Similar semivolatile partitioning systemat ics were noted for Cd, Pb, and V in Absaloka fly ashes. Conventional and lo w-NO, combustion of Illinois No. 6 coal did not significantly affect trace element volatility. However, low-NO, Absaloka combustion promoted Ni, Zn, a nd Se volatilization. The inorganic phase composition and As, Cr, and Ni sp eciation of fly ash particles similar to2.5 mum in aerodynamic diameter (FA (2.5)) were determined using X-ray diffraction and absorption methods. Illi nois No. 6 and Absaloka FA(2.5) contain aluminosilicate glass, quartz (SiO2 ), ferrite spinel (AB(2)O(4); e.g., where A(2+) = Fe, Mg, Ni, Co, Cu and B3 + = Al, Fe, Cr), and mullite (Al6Si2O13) Absaloka FA(2.5) 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 speci ation. As5+O4-containing phases occur in Illinois No. 6 and Absaloka FA(2.5 ). Presumably, carboxyl-bound As3+ and Ca in Absaloka coal promoted the for mation of Ca-3(ASO(4))(2) Cr3+/Cr6+ is much greater in Illinois No. 6 FA(2. 5), relative to Absaloka FA(2.5). The predominance of maceral-bound Cr3+ an d oxygen functional groups in Absaloka coal may have promoted Cr6+ formatio n. Illinois No. 6 and Absaloka FA(2.5) contain similar NiO-bearing phases, possibly ferrite spinel.