Evolution of fusion and PCDD/F-signatures of boiler ash from a mechanical grate municipal solid waste incinerator

Abstract

Boiler ash formed at different temperature ranges in a typical mechanical grate incinerator is collected and systemically studied, with the aim of providing a reference for ash disposal and revealing the formation routes and distribution of polychlorinated ρ-dibenzodioxins and dibenzofurans (PCDD/Fs). Key physical and chemical properties are carefully analyzed, including chemical component, ash fusion temperatures (AFTs), crystalline phases, chemical species, and PCDD/Fs. Several fouling and slagging indices are introduced and their relationships with AFT are revealed. The fouling index (Fu) and a slagging index (Rb/a×Na) are well fitted with ash flow temperatures, with correlation coefficient (R2) of 0.82 and 0.82, respectively; these could be better potential indices for disposal applications of municipal solid waste incineration fly ash. CC/C–C/C–H (69.25–80.93%) and inorganic chlorine (94.23–98.68%) are the dominant carbon and chlorine species, respectively. The increasing AFT is mainly attributed to the changing components, the increasing proportions of crystalline CaSO4, NaCl and KCl and the decreasing crystallinity and content of SiO2. Twice as much PCDD/Fs is generated by the low-temperature heterogeneous reaction (6.71–19.22 ng/g) than by the high-temperature homogeneous reaction (0.59–6.71 ng/g). The proportions of highly chlorinated homologues increase and gradually become the main component. Principal component analysis reveals that PCDD/Fs is positively correlated with Cl, Cu, Pb, Sn, Sb, Zn and CC/C–C/C–H but negatively correlated with less volatile elements, e.g., Ni, Mn, Al, Ti, Si, and Cr. These results can benefit further research on boiler ash disposal and PCDD/F formation routes in the post-combustion area of incinerators.