Systematic study of the formation and chemical/mineral composition of waste-to-energy (WTE) fly ash

By Yixi Tian*, Nickolas J. Themelis, A.C. (Thanos) Bourtsalas*, Shiho Kawashima, Yuri Gorokhovich

Journal: Materials Chemistry and Physics Volume 293 (1 January 2023), 126849

Department of Earth and Environmental Engineering
Fu Foundation School of Engineering & Applied Science
Columbia University

The study was supported by the Earth Engineering Center of Columbia University and Global WtERT Council, Inc. The authors gratefully acknowledge the contribution of the engineers of Covanta for the sample collection.


• The formation and chemical/mineral composition of fly ash (FA) were quantified.

• The effects of moisture (via scrubbing and processing) on FA were compared.

• The empirical formulae for the mineral compounds of FA were determined.

• High-moisture process was found to be preferable due to higher hydration phenomena.

• LEAF Method 1313 and TCLP leachability tests were conducted for comparison.


Waste-to-Energy (WTE) power plants combust post-recycling municipal solid waste (MSW). WTE fly ash (FA) is the air pollution control (APC) residue and is classified as hazardous waste. A systematic study was conducted on the formation, chemical/mineral composition of FA, and the leachability of heavy metals. The impacts of two moisture conditions on FA were examined: high-moisture process (semi-dry scrubbing in high water environment; water was added in the ash processing stream), and low-moisture process (spray dryer and circulating dry scrubbing; with no water addition in the ash processing stream). A methodology was established that included the formation of FA particles from two sources (furnace and formed in the scrubber of the APC system) and estimated the empirical formulae for the FA constituents, based on the chemical composition (ICP, XRF, ion chromatography, TGA) and XRD quantification. The dominant species are CaO (40%), Cl (15%), SO3 (8%), CO2 (8%). The crystalline phases (45–50%) consisted of: hydroxides (<0.5%), carbonates (5–16%), chloride salts (∼14–17%), sulfates (6–11%), sulfides (<2%), aluminates (2–11%), and silicates (5–6%), with 50–55% of amorphous phases. High-moisture (20–30%) process was found to be more attractive because it increases the formation of more stable crystalline hydrates (e.g., ettringite) and amorphous C–S–H phases. The effects of eluate pH on heavy metals leachability were determined using the Leaching Environmental Assessment Framework (LEAF Method 1313-pH) and the effects regarding the RCRA standard were established by applying the TCLP procedure.

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