A review of rare earth elements and yttrium in coal ash: content, modes of occurrences, combustion behavior, and extraction methods

Biao Fu, James C. Hower, Wencai Zhang, Guangqian Luo, Hongyun Hu, Hong Yao

Research output: Contribution to journalReview articlepeer-review

58 Scopus citations


Rare earth elements and yttrium (REY) have attracted considerable attention over the last decade because of their vital roles in clean energy, consumer product, national defense and security applications, among other uses. Due to the retention of REY during coal burning, coal combustion ash is considered as potential alternative sources for REY. Understanding the content, speciation, retention and/or transformation behavior of REY during coal combustion not only expands our knowledge of the combustion behavior of the trace elements in coal, but also provides basis for modeling REY partitioning during coal combustion and for developing economically viable REY recovery technologies. This review makes a critical summary of recent progress in the study of REY in coal ash. The contents and the extraction potentials of REY in coal ash derived from 15 major coal-producing countries worldwide were summarized and evaluated. Various analytical methods for determining REY bulk contents and speciation, together with the solid sample pretreatment, analytical accuracy and precision, advantages and disadvantages were summarized and compared. Modern analytical approaches combined indirect methods (e.g., sequential extraction) shed light on the physical distribution, mineralogy, and the chemical state of REY in coal ash. Three types of REY occurrences in coal ash, including Si-Al glassy association, discrete minerals or compounds, and organic association (bound with unburned carbon) were defined in the review. The glassy association can be further divided into REY minerals closely bound to glass phases and dispersed throughout the glassy structure. REY partitioning in various emission streams, the size distribution, and their enrichment behavior in coal ash were discussed. Thermal behavior and transformation of various REY forms in coal during combustion process, including organic-associated REY, REY phosphates, REY carbonates, clay-bound REY and among others were summarized. Two possible retention mechanisms of REY by aluminosilicate glass at boiler temperature were proposed: the incorporation of the individual REY phases into the glass as inclusions and the diffusion of REY phases throughout Si-Al glass structures in the melting process. Feed coal mineral types, mineral-mineral associations, boiler conditions, and other factors control the retention process. After coal combustion, the speciation of REY in fly ash may be modified by the reactions of REY phases with flue gas components. Further, an overview of REY transformation mechanisms during coal combustion was deeply discussed. Finally, current extraction techniques for REY recovery from coal combustion ash were introduced. Future outlooks and research problems were also identified.

Original languageEnglish
Article number100954
JournalProgress in Energy and Combustion Science
StatePublished - Jan 2022

Bibliographical note

Funding Information:
This work was supported by the National Natural Science Foundation of China ( 52006082 ) and Chinese Postdoctoral Science Foundation (2019M662586). This study was completed as part of U.S. Department of Energy contracts DE-FE0027167 and DE-FE0026952 ; U.S. Department of Energy contract DE-FE0029007 to the University of North Dakota Energy & Environmental Research Center with a subcontract to the University of Kentucky; and U.S. National Science Foundation grants CBET-1510965 and CBET-1510861 to Duke University and the University of Kentucky, respectively. This work used shared facilities at the Virginia Tech National Center for Earth and Environmental Nanotechnology Infrastructure (NanoEarth), a member of the National Nanotechnology Coordinated Infrastructure (NNCI), supported by NSF (ECCS 1542100). Additional microbeam analyses were conducted at the University of Kentucky Electron Microscopy Center.

Funding Information:
Hong Yao received his PhD degree in 2002 from the department of Environmental and Life Engineering, Toyohashi University of Technology, Japan. He was a Research Fellow supported by the Japan Society for the Promotion of Science (JSPS) from 2002 to 2004. In 2006, he worked at the University of Alberta, Canada as a visiting professor. He is currently a Distinguished Professor and the director of the State Key Laboratory of Coal Combustion at Huazhong University of Science and Technology. His research fields include coal combustion, waste incineration, gasification, and biomass utilization. He has published more than 200 papers and was an associate editor of Journal of Material Cycles and Waste Management. He received the Excellent Paper Awards from The Combustion Institute in 2010 and 2014. He was elected as Fellows of the Combustion Institute for outstanding contributions to the fundamental research of solid fuels and pollutants control in combustion processes.

Publisher Copyright:
© 2021 Elsevier Ltd


  • Coal combustion ash
  • Extraction technologies
  • Partitioning mechanisms
  • Rare earth elements and yttrium

ASJC Scopus subject areas

  • Chemical Engineering (all)
  • Fuel Technology
  • Energy Engineering and Power Technology


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