(421d) Mercury Interaction With the Fine Fraction of Coal Fly Ash in a Simulated Coal Power Plant Flue Gas Stream

Mercury is a potent neurotoxin and a significant contaminant released through the burning of coal in coal-fired power plants.  Besides Hg, coal-fired power plants produce large quantities of fly ash that interact with Hg contained in flue gas emissions.  Because fly ash has the potential to be a sink for Hg and is used in products such as cements, it is necessary to understand how Hg is associated with the ash.  This work focuses on both bulk and < 100 nm sized fly ash particles reacted with a simulated flue gas stream containing Hg vapor.  Fly ash samples were analyzed by a variety of laboratory- and synchrotron-based techniques to determine the size fraction of fly ash responsible for the majority of Hg uptake, determine any correlations between Hg and fly ash components (both organic and inorganic constituents), and to determine the Hg phases present after fly ash reaction in the simulated flue gas stream.

The fine fraction of fly ash from a coal-fired power plant was separated to a particle size ≤ 100 nm.  This fine fraction was reacted in a simulated flue gas containing CO₂, H₂O, O₂, NO_x, SO₂, HCl, and Hg.  These gases were initially passed through a flame at ~1,100^oC to simulate gaseous reactions found in a coal-fired power plant boiler, and then the gas stream was reacted with the fly ash in a packed bed reactor at ~140^oC.  Reacted and unreacted samples were analyzed by a large suite of techniques.  Characterization of the ash was done using scanning electron microscopy (SEM), transmission Fourier transform infrared (FTIR) spectroscopy, x-ray diffraction (XRD), and electron microprobe.  To understand how Hg correlates with different constituents of the fly ash, samples were analyzed by both electron microprobe and synchrotron-based x-ray fluorescence (s-XRF) mapping.  X-ray absorption spectroscopy (XAS) was used to identify the Hg-bearing phases present in the samples.  Micro x-ray absorption near edge structure (μ-XANES) spectroscopy was coupled with s-XRF to identify the form of Hg found in Hg hot spots identified during s-XRF mapping.  Bulk extended x-ray absorption fine structure (EXAFS) spectra of the reacted fly ash were collected by slow cooling to 77K to identify the Hg-phases present in the sample, including elemental Hg if present.

Synchrotron XRF mapping of both the bulk and fine fraction reacted ash samples indicates that the fine fraction (≤ 100 nm) dominates the uptake of Hg in fly ash.  XRF mapping also showed that Hg is present in two major regions: those with high Fe concentrations and Hg hot spots not associated with Fe. XRF mapping shows a correlation between Fe and Cl in Fe-rich regions, whereas Hg hot spots have associated S and Br as well as minor amounts of As and Se.  Mercury was not found to be associated with Ti, alkali metals, alkaline earth metals, or the light transition metals in these samples.  Electron microprobe analyses showed that some regions of the samples show a correlation between Hg and C, whereas in other regions there is no association of Hg with C.  μ-XANES spectroscopy of the Hg hot spots indicates the presence of cinnabar (α-HgS) with no indication of the higher temperature polymorph, metacinnabar (β-HgS), being present.  Bulk EXAFS analysis indicates that Hg phases present include Hg-Br, Hg-Cl, Hg bound to organics, Hg bound to Fe-oxides (presumably hematite), Hg-O (montroydite-like phase), and cinnabar. We did not detect elemental Hg in the fly ash, suggesting that Hg uptake by fly ash involves the oxidation of Hg(0) to Hg(II).  FTIR analysis of the fly ash samples before and after reaction shows a significant intensity decrease of the carboxylic acid functional group absorption bands. This decrease is interpreted as suggesting that the carboxylic groups bind to Hg in the unburned carbon portions of the fly ash.  Overall, these results show a complex interaction between Hg and coal fly ash.  By understanding the speciation of Hg in fly ash, more refined model experiments using synthesized forms of phases present in the sample can be done to determine the stability of the Hg in the fly ash that is either disposed of in landfills or used in cement products.