Critical Review on the Chemical Reaction Pathways Underpinning the Primary Decomposition Behavior of Chlorine-Bearing Compounds under Simulated Municipal Solid Waste Incineration Conditions

Incineration has been proven as one of the most viable and efficient waste-to-energy strategies for the handling of rapidly mounting municipal solid waste (MSW). However, as a generic and key element in MSW, chlorine (Cl) brings about severe corrosion in boilers and the presence of Cl-containing species induces the release of toxic dioxins and furans by acting as chlorine donors. The chlorine release behavior from MSW during incineration has thus drawn plenty of attention. This paper has critically reviewed the origin of different chlorine-bearing species in MSW and their respective thermal decomposition behavior. In general, chlorine in MSW has two primary sources: plastics, mainly polyvinyl chloride (PVC), as the source of organic chlorine and household kitchen and garden waste as the source of inorganic chlorine, mainly chlorides. Distinct chlorine liberation properties have been observed for these two different chlorine groups. Chlorine in PVC converts to HCl gas easily at 300 °C as a result of the thermal decomposition combined with a depolymerization of the polymer chain. In contrast, the release of chlorine from inorganic chloride salts is much lower than that from PVC, mainly attributing to either strong volatilization properties of alkali chlorides (NaCl and KCl) or the high ionic bonding energies between some cations, such as Ca 2+ /Ni 2+ and Cl – . Meanwhile, the decomposition of FeCl 2, CoCl 2, and MgCl 2 is easier with the onset temperature for HCl release being around 180, 300, and 415 °C, respectively. Additionally, the addition of SO 2, O 2, or acidic solids (such as SiO 2 and Al 2 O 3 ) has been found to promote the chlorine release from chlorides appreciably. However, the exploitation of the underpinning mechanism is far from completion, especially for CaCl 2 and CoCl 2 . This requests extra and intensive studies to elucidate the chlorine liberation mechanism from these inorganic species, even from the molecular level in terms of their ionic bonding propensity and breakage at high temperatures.