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

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 (similar to 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.