Plastics-Bed Interaction in Industrial Fluidized Bed Reactors during Thermochemical Conversion Processes

Developing economic and environmentally acceptable solutions for managing the ever-increasing volumes of plastic waste that are generated worldwide is one of the greatest challenges of our time. Although mechanical recycling clearly has a critical role in reducing the amount of waste to be disposed, there is further opportunity and environmental benefit in what is called “chemical recycling”. Advanced thermochemical technologies, like pyrolysis and gasification, in fluidized bed reactors have an important role to play in converting these feedstocks into valuable intermediates (e.g., methanol, DME and hydrogen) for manufacturing industries. Nevertheless, there are still unsolved challenges when operating with mixed plastic waste feedstocks. These materials are highly heterogeneous and tend to sink soon after injection to the bed, due to the formation of lumps and agglomerates. This can cause segregation, de-fluidization of the bed and, eventually, early shutdown of the operation.

The aim of this work is to link the thermal decomposition behaviour of plastic to its interaction with bed materials at a range of operating conditions relevant to industrial operations. In particular, non-invasive X-ray imaging and infrared gas measurements have been used to investigate plastic-bed interaction and rate of decomposition of a single polypropylene particle in a lab scale facility. A separate experimental campaign is conducted to record the temperature profile inside the particles during the transformation process by inserting a thermocouple into half-drilled plastic samples. This information is essential to understand the thermal history and hydrodynamic behaviour of plastic materials during thermal degradation in fluidized beds, whose details, unlike traditional thermogravimetric data, are still uncertain. The present study highlights the importance of gathering new observations to better understand the fundamental phenomena which govern the interaction between solid feedstocks and fluidized bed, hence promoting the exploitation of such technology in an industrial framework.