Size-dependent variations in fly ash trace element chemistry: Examples from a Kentucky power plant and with emphasis on rare earth elements

Rare earth elements while not considered to be volatile in coal combustion do show some variation between ash collection rows. To better understand this variation, we investigate the trace element variations (with emphasis on rare earth elements) in size fractions of fly ash from multiple ash collection rows at a single power-generating unit at a southeastern Kentucky power plant. Fly ash samples were investigated using optical microscopy, X-ray fluorescence, and inductively coupled plasma mass spectrometry. The arsenic concentration increases from the hottest to the coolest ends of the ash collection system. The relationship between Hg capture and the flue gas temperature is illustrated by the increase in Hg between the economizer hopper and the cooler mechanical hopper. Although the rare earth elements do not show significant variation between rows of ash collection systems, the ratio of light and heavy rare earth elements (LREE/HREE) decreases from the economizer and mechanical rows to the electrostatic precipitator row. Petrographic differences between the ash sizes may also contribute to the LREE/HREE distributions. The glass fraction that appears somewhat uniform using optical microscopy contains rare-earthelement-bearing minerals, accounting for at least some of the rare earth elements seen in the "glass". The positive Eu anomalies in the ashes could possibly be due to the coal combustion conditions rather than inherited from raw coals, and the rare earth element composition in feed coal is probably responsible for the medium rare earth element enrichment type in different size fractions of fly ash.