Pyrolysis and dehalogenation of plastics from waste electrical and electronic equipment (WEEE): A review

Plastics from waste electrical and electronic equipment (WEEE) have been an important environmental problem because these plastics commonly contain toxic halogenated flame retardants which may cause serious environmental pollution, especially the formation of carcinogenic substances polybrominated dibenzo dioxins/furans (PBDD/Fs), during treat process of these plastics. Pyrolysis has been proposed as a viable processing route for recycling the organic compounds in WEEE plastics into fuels and chemical feedstock. However, dehalogenation procedures are also necessary during treat process, because the oils collected in single pyrolysis process may contain numerous halogenated organic compounds, which would detrimentally impact the reuse of these pyrolysis oils. Currently, dehalogenation has become a significant topic in recycling of WEEE plastics by pyrolysis. In order to fulfill the better resource utilization of the WEEE plastics, the compositions, characteristics and dehalogenation methods during the pyrolysis recycling process of WEEE plastics were reviewed in this paper. Dehalogenation and the decomposition or pyrolysis of WEEE plastics can be carried out simultaneously or successively. It could be ‘dehalogenating prior to pyrolysing plastics’, ‘performing dehalogenation and pyrolysis at the same time’ or ‘pyrolysing plastics first then upgrading pyrolysis oils’. The first strategy essentially is the two-stage pyrolysis with the release of halogen hydrides at low pyrolysis temperature region which is separate from the decomposition of polymer matrixes, thus obtaining halogenated free oil products. The second strategy is the most common method. Zeolite or other type of catalyst can be used in the pyrolysis process for removing organohalogens. The third strategy separate pyrolysis and dehalogenation of WEEE plastics, which can, to some degree, avoid the problem of oil value decline due to the use of catalyst, but obviously, this strategy may increase the cost of whole recycling process.     

Kinetic vaporization of heavy metals during fluidized bed thermal treatment of municipal solid waste

Heavy metals volatilization during thermal treatment of model solid waste was theoretically and experimentally investigated in a fluidized bed reactor. Lead, cadmium, zinc and copper, the most four conventional heavy metals were investigated. Particle temperature model and metal diffusion model were established to simulate the volatilization of CdCl₂ evaporation and investigate the possible influencing factors. The diffusion coefficient, porosity and particle size had significant effects on metal volatilization. The higher diffusion coefficient and porosity resulted in the higher metal evaporation. The influence of redox conditions, HCl, water and mineral matrice were also investigated experimentally. The metal volatilization can be promoted by the injection of HCl, while oxygen played a negative role. The diffusion process of heavy metals within particles also had a significant influence on kinetics of their vaporization. The interaction between heavy metals and mineral matter can decrease metal evaporation amount by forming stable metallic species.     

Vaporization of heavy metals during thermal treatment of model solid waste in a fluidized bed incinerator

This paper investigated the volatilization behavior of heavy metals during thermal treatment of model solid waste in a fluidized bed reactor. Four metal chlorides (Cd, Pb, Cu and Zn) were chosen as metal sources. The influence of redox conditions, water and mineral matrice on heavy metal volatilization was investigated. In general, Cd shows significant vaporization especially when HCl was injected, while Cu and Pb vaporize moderately and Zn vaporization is negligible. Increasing oxygen concentration can lower heavy metal vaporization. Heavy metal interactions with the mineral matter can result in the formation of stable metallic species thus playing a negative effect on their behavior. However, HCl can promote the heavy metal release by preventing the formation of stable metallic species. The chemical sorption (either physical or chemical) inside the pores, coupled with the internal diffusion of gaseous metal species, may also control the vaporization process. With SO₂ injected, Cd and Pb show a higher volatility as a result of SO₂ reducing characteristics. From the analysis, the subsequent order of heavy metal volatility can be found: Cd > Cu ? Pb ? Zn.     

An inverse Gaussian plume approach for estimating atmospheric pollutant emissions from multiple point sources

A method is developed for estimating the emission rates of contaminants into the atmosphere from multiple point sources using measurements of particulate material deposited at ground level. The approach is based on a Gaussian plume type solution for the advection–diffusion equation with ground-level deposition and given emission sources. This solution to the forward problem is incorporated into an inverse algorithm for estimating the emission rates by means of a linear least squares approach. The results are validated using measured deposition and meteorological data from a large lead–zinc smelting operation in Trail, British Columbia. The algorithm is demonstrated to be robust and capable of generating reasonably accurate estimates of total contaminant emissions over the relatively short distances of interest in this study.