Hood and 8 other authors reported in 2017 that fly ash was collected from individual mechanical and electrostatic precipitator (ESP) hoppers at a 130-MW pulverized-coal utility unit burning a southeastern Kentucky moderate- to high-S Pennsylvanian Fire Clay coal. The fly ash particle size is generally similar within each row of the mechanical and ESP hoppers, with the mean-particle diameter decreasing from >36 μm in the mechanical hoppers to 6 μm in the last ESP row.
Rare earth element (REE)–bearing phases occur within fine minerals and, possibly, as part of the fly-ash glass phase. Transmission electron microscopy was used to identify Y-Ce-Nd-La–bearing regions within a glassy matrix. Y- and U-bearing zircons were also found in the fly ash. Total REE plus yttrium (REY) occupies a narrow range from 719 to 775 ppm, showing an overall lack of partitioning of the REY elements with respect to particle size and temperature in ash collection systems. The heavy REE increase and the light REE/heavy REE ratio decreases from the mechanical hoppers through ESP rows.
REE are commonly divided into the LREE and HREE fractions from La through Sm and Eu through Lu, respectively. When Y is included in the distributions (REY), the suite can be divided into light, medium, and heavy fractions: LREY (La through Sm plus Y), MREY (Eu through Dy plus Y), and HREY (Ho through Lu plus Y) and further divided the REY into economic categories, based on the known abundances vs. the industrial needs at the time of their writing, as follows: critical (Y, Nd, Eu, Tb, Dy, and Er), uncritical (La, Pr, Sm, and Gd), and excessive (Ce, Ho, Tm, Yb, and Lu) REY.
The total REY occupies a narrow range from 719 to 775 mg/kg, confirming previous observations about the overall lack of partitioning of the REY elements with respect to particle size and temperature in ash collection systems. The HREE increases from the mechanical through ESP hoppers, leading to a decrease in LREE/HREE in the same direction. Similar LREE/HREE trends were noted for another unit burning Fire Clay coal and for several other Kentucky units burning blends of several coals. The critical REY increases from about 241 mg/kg in the mechanical hoppers to 278–298 mg/kg in the ESP hoppers. Particle-to-particle variations in REY content exist.
As noted above, REY concentrations are associated with REE minerals such as monazite, Y-bearing minerals such as zircon, and xenotime (not observed in this study but seen in other Central Appalachian coal-derived fly ashes), and fine minerals, and perhaps as fine particle dispersion in glass. Not all minerals in the fly ash contain REE. Similarly, not all of the glass contains REE, as observed in the LAICP- MS analyses. Semiquantitative La, Ce, and Nd concentrations in the plant W fly ash particles showed a range from 0 mg/kg up to about 75, 120, and 70 mg/kg, respectively (the whole ash contains 744 mg/kg REY; sample SCEGWS01-02; U.S. Department of Energy–National Energy Technology Laboratory, 2015).
Not all of the REE occur within the glass or as free minerals. Amorphous carbon with nanoscale metal inclusions can be found surrounding Al-Si glass particles in bituminous coal–derived fly ashes. Both the carbon and the fine inclusions, here demonstrated to include REE particles, occur in the plant I fly ash. In addition to the Ce and Nd, Y, Pr, and Sm have been observed in the plant I amorphous carbons. As observed in other studies, the total REE do not show much variation with flue gas temperature or with the bulk particle size of the fly ash. The heavy REE do increase from the mechanical hoppers to the ESP hoppers, the direction of decrease in flue gas temperature, leading to a decrease in the LREE to HREE ratio in the same direction. Not all REY-bearing minerals have equal concentrations of the elements, as seen in varying Y concentrations in zircons in this study.
Variations in REE concentrations between glass particles were also noted. In a fly ash from another power plant, semiquantitative measurements of La, Ce, and Nd concentrations in glass particles ranged from 0 ppm to 75, 120, and 70 ppm, respectively. Amorphous carbon, generally best observed using HR-TEM, surrounds Al-Si glass particles in this and other bituminous coal–derived fly ashes. The amorphous carbon contains nanoscale inclusions containing Y, Ce, Nd, Pr, and Sm. The fly ash is dominated by glass and carbons, with quartz and mullite being among the more important mineral phases and lesser amounts of magnetite and hematite.
Titanium minerals were observed in the TEM studies. From investigations of the source coal and from rare views in the fly ash using optical microscopy, REY-bearing minerals such as zircon and monazite are present in the fly ash. Based on thermal gravimetric analyzer experiments, it was demonstrated that monazite will shatter at boiler temperatures (1400°C), accounting for the general lack of monazite observations at the scale of optical microscopy. Although La-/Ce-rich regions of glass and fine minerals were detected in TEM examination of the fly ash, definitive REE-bearing minerals were not found. Yttrium-bearing zircons were detected. Rare earth elements and yttrium, therefore, occur in this fly ash at scales ranging from REY-bearing minerals and glass to fine nanoparticles embedded within the amorphous carbon surrounding the glassy particles.
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