Formation and chemical speciation of arsenic-, chromium-, and nickel-bearing coal combustion PM2.5

• Published in: Fuel processing technology 2004-06, Vol.85 (6-7), p.701-726
• Main Authors: GALBREATH, Kevin C, ZYGARLICKE, Christopher J
• Format: Article
• Language: English
• Subjects: Air pollution caused by fuel industries; Applied sciences; Energy; Energy. Thermal use of fuels; Exact sciences and technology; Metering. Control
• ISSN: 0378-3820; 1873-7188
• DOI: 10.1016/j.fuproc.2003.11.015

Mode of occurrence analyses indicate that As, Cr, and Ni in the Illinois No. 6 (Her in) coal are generally associated with relatively large discrete mineral grains, whereas these elements are much more strongly associated with macerals and fine-grained minerals in Absaloka subbituminous (McKay or Rosebud seam) coal. The coals were burned using conventional and low-NOx conditions in an = 7-kW combustion system to evaluate the importance of elemental modes of occurrence and combustion conditions on As, Cr, and Ni volatility and speciation. Chemical analyses of size-classified (= 0.4-7.7 mum) Illinois No. 6 fly ash samples indicated that As, Cr, and Ni concentrations and relative enrichment/depletion (RED) factors generally increased with decreasing particle size which is consistent with an elemental vaporization-particle surface deposition process. Similar semivolatile partitioning systematics were noted for Ni in Absaloka fly ashes. Consistent with nonvolatility, As and Cr in Absaloka fly ashes were characterized by relatively uniform particle-size distributions (PSDs) and RED factors. Low-NOx combustion conditions promoted the elemental vaporization-particle surface deposition process, as evidenced by greater elemental concentrations on fine-particle surfaces. The As, Cr, and Ni speciation of PM2.5 samples were determined using X-ray absorption fine-structure spectroscopy (XAFS). 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 PM2.5. Cr3+/Cr6+ is much greater in Illinois No. 6 PM2.5 relative to Absaloka PM2.5. The predominance of maceral-bound Cr3+ and oxygen functional groups in Absaloka coal may have promoted Cr6+ formation. Illinois No. 6 and Absaloka PM2.5 contain similar NiO-containing phases, possibly ferrite spinel.