Chlorophenols in Municipal Solid Waste Incineration: A review

Abstract

Chlorophenols (CP) once was a group of important chemicals and intermediates, before being phased out from most of its traditional applications. CP comprise 19 mono- to pentachlorinated congeners. These are regularly cited as a major precursor of dioxins in chemical and thermal processes, including Municipal Solid Waste Incineration (MSWI), the main theme of this review. Some concentration values are cited for CP, other potential precursors, and dioxins (polychlorinated dibenzo-p-dioxins/dibenzofurans or PCDD/F). These are summarised for different locations and conditions in MSWI and their Air Pollution Control Devices (APCDs) and a fair correlation is found between CP and CBz concentrations.CP are products of incomplete combustion (PIC) and may also be generated through several possible pathways, including oxidative conversion or hydrolysis of chlorobenzene, de novo synthesis from a carbonaceous matrix, decomposition or oxidation combined with chlorination of selected PAH, as well as cyclisation combined with oxidation of reactive small molecules, such as acetylene, ethylene, etc.Numerous studies used CP as precursor related to PCDD/F formation. Their rate of formation is some two orders of magnitude higher than alternative routes, over chlorobenzenes or other precursors, and during most tests CP preferentially formed PCDD, rather than PCDF. Some figures for CP concentrations, rate values, and reaction pathways are gathered and discussed briefly, with further reference to more detailed mechanistic studies. Both homogeneous, high temperature gas phase processes and heterogeneous, surface-based catalytic reactions form PCDD/F as well as their precursors. Heterogeneous reactions proceed at much lower temperature, typically 200–400°C, either from precursors, or along the de novo route. As CP appears both in de novo synthesis and in the precursor route, a unified pathway is proposed.