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-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.

Trace elements in co-combustion of solid recovered fuel and coal

“…Similar low volatility of Cr has been observed in other studies [22,26]. When coal is co-fired with SRF, the RE factor of Cr decreases with increasing share of SRF.…”

“…The trace elements vaporized during combustion would partition to the cyclone ash and filter ash through condensation and/or gas-solid reactions. Both mechanisms would favor the enrichment of trace elements in filter ash [1,[21][22][23], and the RE factor would increase with increasing vaporization degree of trace elements. However, if the vaporization degree of a trace element is fixed, a shift from gas-solid reaction mechanism to condensation mechanism may also result in a higher RE factor.…”

“…However, if the vaporization degree of a trace element is fixed, a shift from gas-solid reaction mechanism to condensation mechanism may also result in a higher RE factor. This is primarily due to that the gas-solid reaction mechanism would lead to trace element concentration proportional to 1/d p , whereas the condensation mechanism would cause the trace element concentration proportional to 1/d p 2 in the continuum regime (larger than 0.5-1 μm) [21][22][23]. Therefore, the condensation mechanism would result in a higher enrichment of trace element in small particles (around 0.5-2.5 μm), compared to the reaction mechanism.…”

“…Typical examples are the reactions between trace elements such as Cd and Pb and aluminosilicates such as kaolinite [20]. Such reactions occur primarily on the particle external surface and may result in trace element concentration in the ash particle being proportional to 1/d p (particle diameter) [1,[21][22][23]. On the other hand, the vaporized trace elements may also react with other gaseous species in the flue gas, such as HCl and SO 2 .…”

“…For ash particles in the continuum regime (supermicron particles with Kn bb 1) condensation may lead to the trace element concentration in the ash particles being proportional to 1/d p 2 , while for particles in the free molecular regime (ultrafine particles with Kn NN 1) the concentration of the condensed trace element may be proportional to 1/d p [21][22][23]. One should be aware that the correlations mentioned earlier are based on several assumptions which may deviate from practical situations, such as perfect spherical and nonporous particles [23].…”