(398w) Surface Characterization of the Interaction Between Fly Ash Particles and Mercury

Fly ash is a primary by-product of the coal combustion process. Despite capture efficiencies of up to 99.95% by particulate control devices, such as electrostatic precipitators or fabric filters, coal-fired power plants in the United States still emit on average 300 tons of fly ash per day. The release of fly ash into the environment is a public and environmental health concern for two reasons. First, the particle size of released fly ash is generally smaller than 2.5 μm (PM_2.5) and current research suggest that inhalation of small particles can have an adverse health effect. The second concern is because of the presence of toxic trace metals/metalloids, such as mercury (Hg), selenium (Se), and arsenic (As), which may exist as either a component of fly ash, or partition onto fly ash as the flue gas cools. Therefore, it is important to understand the characteristics of fly ash and, specifically, the interaction between trace metals and fly ash components. In this work, various experimental methods were carried out to investigate fly ash interaction with Hg in different particle sizes of fly ash. Bituminous coal ultrafine fly ash particles below 0.1 μm were separated using membrane filtration, and both filtered and non-filtered fly ash samples were exposed to methane combustion flue gas, supplemented with SO₂, NO_x and HCl, that contained Hg in a packed-bed reactor to model Hg sorption. Characterization studies including scanning electron microscopy (SEM) and X-ray diffraction (XRD) were carried out on the fly ash samples to determine changes in their morphologies and mineralogy before and after reaction. Sorption reaction products were characterized using synchrotron-based x-ray fluorescence mapping (XRF) to determine which elements Hg is associated with on the fly ash particulate matter. Mercury hot spots were further analyzed with micro x-ray absorption near edge structure spectroscopy (μ-XANES) to determine the form of Hg contained in these regions. Results indicate that significantly more Hg is associated with the filtered ultrafine fly ash sample when compared to the non-filtered sample. XRF analysis shows that the Hg is partitioned into two major regions in the samples: Fe associated and non-Fe associated. Mercury content in the sample is correlated with Fe, S (reduced), Cl, Br, and Cu, and to a lesser degree with Se and As. There is no discernable association between Hg and the alkali metals, the alkaline earth metals, and the lighter transition metals and the Hg hot spots detected are not associated with the Fe-oxides contained in the fly ash. μ-XANES analysis of the Hg hot spots indicates that these regions are dominated by HgS (cinnabar) crystals. These results indicate that ultrafine fly ash particles could be the primary sorbent of Hg from a gaseous phase.