Article pubs.acs.org/EF
Size-Dependent Variations in Fly Ash Trace Element Chemistry: Examples from a Kentucky Power Plant and with Emphasis on Rare Earth Elements Jingjing Liu,†,‡ Shifeng Dai,†,‡ Xin He,†,‡ James C. Hower,*,§ and Tanaporn Sakulpitakphon∥ †
State Key Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology, Xuzhou, Jiangsu 221116, People’s Republic of China ‡ College of Geoscience and Surveying Engineering, China University of Mining and Technology (Beijing), Beijing 100083, People’s Republic of China § University of Kentucky Center for Applied Energy Research, 2540 Research Park Drive, Lexington, Kentucky 40511, United States ∥ AECOM, 1515 Poydras Street, Suite 2700, New Orleans, Louisiana 70112, United States ABSTRACT: 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.
1. INTRODUCTION In general, the partitioning of trace elements in coal combustion is a function of the volatility of the elements, with low volatile elements, such as the rare earth elements (REEs, or REY if Y is included), Sc, Hf, Mn, Rb, Th, and Zr, being distributed evenly across bottom ash and fly ash (group 1 after Clarke and Sloss1 and Meij2), intermediate volatility elements, such as Zn and As, which are enriched in fly ash relative to bottom ash (group 2), and the halogens, Hg, Se, and B, among the most volatile elements (group 3). Among other factors, the concentration of an element in fly ash depends first upon the element abundance in the feed coal then upon combustion conditions, the type of fly ash generated (such as the amount of carbon), the particle size and surface area of the fly ash, and the flue gas temperature at the point of ash collection.1,3,4 Further discussion of trace element partitioning can be found in the studies by Martinez-Tarazona and Spears,5 Vassilev and Vassileva,6 Senior et al.,7−9 Yan et al.,10 Vassilev et al.,11,12 Karayigit et al.,13 Narukawa et al.,14 Li et al.,15 Pires and Querol,16 López-Antón et al.,17 and Hower et al.,18−20 among others. In recent years, REEs in coal and coal combustion products (CCPs) have attracted much attention because (1) the rapidly grown demand for REY as a result of their wide applications as metal catalysts, permanent magnets, light-emitting diodes, © 2016 American Chemical Society
batteries, phosphors, and various components for renewable energy equipment,21−24 (2) the supply crisis of 2010 and the price spike of 2011,25 which was caused by the export restrictions from China and initiated a treasure hunt by way of exploration for REY deposits all over the world,26 (3) highly elevated concentrations of REY in some coals and CCPs that are comparable to or even higher than those in conventional economic deposits,27,28 and (4) preliminary REY extraction experiments (e.g., studies by Taggart et al.29 and Rozelle et al.30) that showed that CCPs may be technically suitable as REE ores. In previous studies of fly ashes from several power plants, Hower et al.31 noted variation in REY between ash collection rows. In general, fly ash particle size decreases from the first to the last row of ash collection systems; thus, there was some consideration of REY versus particle size implicit in their discussion. More detailed particle size studies have been made of fly ash from individual hoppers, including in the original study of the power plant studied here,32 but most of our previous studies emphasized the distribution of As, Hg, and a few other elements. The most notable exception has been the Received: October 11, 2016 Revised: December 13, 2016 Published: December 23, 2016 438
DOI: 10.1021/acs.energyfuels.6b02644 Energy Fuels 2017, 31, 438−447
Article
Energy & Fuels
attempted. If certain size fractions are more enriched in REEs, preliminary sizing of the ash can eliminate the need to process large quantities of less promising fly ash.
study of Dai et al. of the REE distribution among size fractions of economizer ash from the Jungar, Inner Mongolia, China, power plant.33 In this case, there was a notable increase in REY of 120 mesh (>125 μm) to the 90 vol % in all of the
