Stanford University

Mercury is a significant contaminant released by coal-fired power plants during electricity generation.  In addition to Hg, coal-fired power plants produce large quantities of fly ash that interact with gaseous Hg contained in flue gas emissions.  Because fly ash is a sink for a significant portion of the Hg produced and is used in products such as cements, it is necessary to understand how Hg is associated with the ash.  Because of the additional potential health hazard of fine ash particles (≤ 100 nm), this work focuses on both bulk and < 100 nm sized fly ash particles reacted with a simulated flue gas stream containing Hg vapor.  Various laboratory- and synchrotron-based techniques were used to determine whether the bulk or fine particle size is responsible for the majority of Hg uptake, identify correlations between Hg and various 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 bulk and fine fraction of coal fly ash were 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.  Numerous synchrotron-based studies were conducted at SSRL to understand how Hg correlates with different constituents of the fly ash. Samples were analyzed by synchrotron x-ray fluorescence (s-XRF) mapping on Beamline 2-3, while extended x-ray absorption fine structure (EXAFS) spectroscopy data collected on Beamline 11-2 were used to identify the Hg-bearing phases present in the samples.  Micro x-ray absorption near edge structure (m-XANES) spectroscopy (Beamline 2-3) was coupled with s-XRF to identify the form of Hg found in Hg hot spots identified during s-XRF mapping.  Bulk 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 coal fly ash.  The mapping also showed that Hg is present in two major regions: areas with high Fe concentrations and Hg hot spots with no associated Fe. Small scale XRF mapping shows a correlation between Fe and Cl in Fe-rich regions, whereas Hg hot spots have associated S and Br with 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 very high sample heterogeneity with some regions of the samples showing a correlation between Hg and C, while other regions showed no association between Hg and C.  m-XANES spectroscopy of the Hg hot spots indicates the presence of cinnabar (a-HgS) with no indication of the higher temperature polymorph, metacinnabar (b-HgS), being present.  Bulk EXAFS indicated extremely complex Hg speciation within the reacted fine fraction sample.  Shell-by-shell fitting of the EXAFS data indicates that Hg phases and pair correlations present include Hg-Br, Hg-Cl, Hg bound to organics, Hg bound to Fe-oxides (presumably hematite), 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).  A portion of Hg-O pathways fit to the data is identical to Hg-O bond distances found in molecules with Hg binding to carboxylic acid functional groups.  FTIR analysis of the fine fly ash samples showed a significant contribution to the signal derived from carboxylic acid functional groups.  Additionally, FTIR analysis of the fine coal fly ash before and after reaction shows a significant decrease in intensity as well as a 2 or 3 wavenumber shift (depending on the absorption band) of the carboxylic acid functional group absorption bands. Computer modeling of the theoretical FTIR sorption bands for carboxylic acid functional groups with and without bound Hg show the same decrease in intensity and 2 or 3 wavenumber shift.  The confirmation of the EXAFS shell-by-shell fitting by FTIR and computer modeling suggests 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 experiments 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.