Halogenated polymers as a problematic component in the pyrolysis of waste plastics

Abstract

The study deals with the complications associated with the pyrolysis of a polymer releasing highly corrosive compounds and other products that are difficult to be utilized in practice. Poly(vinyl chloride-vinyl acetate) was chosen as a sample material. This copolymer was subjected to slow pyrolysis up to a temperature of 650 °C, which was achieved with a temperature ramp of 10 °C min‒1. Thermal decomposition took place in a batch apparatus in nitrogen atmosphere. The evaluation of the experiments included a mass balance using an online recording of gas and condensate formation, which was followed by an analysis of the obtained products. The tests showed very small yields of condensates (pyrolysis tar) in the range of 3.0 – 5.5% compared to gas production reaching 71.1 – 72.1% of the raw material weight. Within the discussion, the problem with the production of non-flammable gas consisting mainly of HCl in the initial stages of pyrolysis was highlighted. Under the given conditions, the release of acid gases, in which HCl was gradually replaced by also non-flammable CO2, continued up to about 400 °C. Only gases collected from this temperature above and having a calorific value of 33.5 MJ m‒3 (20 °C / 101.32 kPa) can be considered energetically usable. Condensates mainly represented by a mixture of aromatic hydrocarbons, acetic acid and chlorobenzene were also evaluated as problematic for possible use. The possibilities of physical activation of solid pyrolysis residues by steam at temperatures of 850 and 900 °C were also tested. All experiments led to obtaining products with very small specific surface areas in the range of 1 – 6 m2 g–1. The process loss during activation reached 25 – 33% of the pyrolysis residue weight, depending on the applied temperature and contact time with steam. These values show that the reaction of the activation medium with the sample took place more or less only on the outer surface of the particles and did not lead to the development of an internal porous structure. The achieved results were compared with the identically processed samples of waste tires, which were evaluated as promising in tests carried out in the past. The contrast be-tween the two materials clearly spoke against the discussed copolymer. The results of the experiments can be interpreted, among other things, as a certain warning against attempts at industrial pyrolysis processing of this type of plastic.