Pyrolysis of WEEE plastics

Waste electrical and electronic equipments (WEEE) are EEE residues at the end of their life-cycle and show an increasing upward tendency: a recent annual estimation for WEEE production in Europe was of almost 6.5 million tonnes, predicting by 2015 an amount of 12 million tonnes. In most developed countries collection and recycling of WEEE is now subjected to political decisions; concern about environment prompts many governments to issue specific legislation about WEEE management stating high quotas of recycling and recovery. With an average plastics content of about 20-30 %, the recycling and recovery quotas fixed by legislation can only be satisfied by including the plastic fraction in recycling and recovery approaches. Pyrolysis appears to be an emerging option in WEEE recycling technology, allowing recovery of high value potentially accessible products such as precious metals, fuel and chemicals. Theoretically, mixed WEEE with an average gross calorific value of 22-24 MJ/Kg (Boerringter, 2000) are rather good combustibles, however plastics are difficult to burn, because of an almost uncontrollable combustion rate. Pyrolysis converts WEEE by thermal decomposition (in inert ambient) in three main phases: gases, oils and chars which can be used as chemical feedstocks or fuels. Smaller molecules volatiles at elevated temperatures goes to the gases or oil fractions while metals, inorganic fillers and supports generally remain unchanged and accumulate in the residue. Degradation of the organic part of the wastes makes the process of separating metallic and glass fibre fractions much easier and recycling of each fraction more viable. Generally both gaseous and liquid products are mixtures of numerous different compounds. For most plastics pyrolysis begins at around 300 °C. The onset of the pyrolysis reaction is strongly influenced by the presence of additives, such as stabilizers, plasticizers and pigments. Gases and pyrolysis oil comes from thermal degradation of the organic part of the WEEE. As a rule, the pyrolysis of plastics follows intricate routes described by a complex set of chemical reactions. While the detailed mechanisms are of scientific interest, an industrial approach is limited to more general considerations, such as the heat effect and the product distribution resulting under particular reaction conditions. Decomposition modes are driven by the polymer structure, the presence of catalysts and by the pyrolysis temperature. Halogenated substances and flame retardant contained in WEEE are a matter of concern for potential generation of PBDD/F but on the other side, if conveniently recycled inorganic halides issued in pyrolysis gases are value products to be removed from gaseous effluents. Despite the huge amount of work on pyrolysis of model WEEE, there are few studies dealing with the product characteristics from the pyrolysis process of real WEEE plastics. WEEE stream are characterised by a large variance in the composition of the feeding coming from different appliances. Therefore a first obstacle in a bench scale pyrolysis is the selection of the waste representative of the whole waste stream; secondly, the composition of the waste in terms of polymers and flame retardant package is mostly unknown.