Thermochemical conversion of polyethylene in a fluidized bed: Impact of transition metal-induced oxygen transport on product distribution

Thermochemical conversion in dual fluidized bed (DFB) systems is a potential alternative to the recycling of abundantly available plastic waste. The development of oxygen transport in DFB systems is in most cases unavoidable due to the transition metal content of the bed material as well as the metal fraction in the waste stream. This work investigates the influence of transition metal oxide-induced oxygen transport on the thermochemical conversion of high-density polyethylene, a model plastic feedstock, in a bubbling fluidized bed reactor. Conversion in the reactor at 700 °C was investigated using four different bed materials that had different concentrations of iron oxide. The share of carbon oxides among the gaseous products increased with an increase in the iron oxide content of the bed material. The yield of light olefinic and paraffinic compounds decreased with increased iron oxide content of the bed. The presence of iron oxide in the bed material significantly increased the formation rates of aromatic compounds and solid carbon deposits on the bed material. The observed shift in the product distribution due to oxygen transport follows a dehydrogenation-type reaction mechanism.