In a circular economy perspective, solid plastic wastes (SPW) are a valuable source of chemicals, energy carriers and fuels, including hydrogen, providing more efficient and environmentally sound options compared to landfill disposal and incineration. This requires the description of (1) the degradation of the polymer backbone in condensed phase and of possible gas/liquid interactions, (2) the evaporation of volatile fragments and (3) their chemical evolution in gas phase through secondary pyrolysis and oxidation (gasification) reactions. Literature studies highlight the formation of residual char during, for example, PET and PVC pyrolysis, (4) making the description of heterogeneous char formation and oxidation chemistry also necessary. Pioneering research efforts at Politecnico di Milano (POLIMI) have proposed one of the few deterministic approaches to polymer degradation available in the literature, based on the systematic definition of reaction classes and rate rules. Starting from the late 1990s detailed polymer degradation models have been proposed for PE, PP and PS. Despite the capability of such models to reproduce both mass loss profiles and product distributions over a wide range of temperatures and heating rates for both pure polymers and binary mixtures, their large size (105-106 chemical species) prevents any application to reactor scale simulations. A functional group based lumping methodology was later developed to describe PVC pyrolysis, and extended successfully to biomass pyrolysis in the last 10 years. More recently, the same approach allowed the simplification of PP, PE and PS kinetic subsets, while maintaining remarkable capabilities in reproducing mass loss profiles and product distribution (Figure 1). New subsets to describe the degradation of PMMA, PET and PA are under development allowing the description of real complex SPW mixtures pyrolysis. The description of gasification requires two additional steps: 1) the definition of a model for char oxidation and 2) the seamless coupling of gas-phase secondary pyrolysis and partial oxidation reactions. This presentation provides an overview of the current state of the art and of recent developments in this direction performed at POLIMI, including theoretical explorations of condensed phase chemical reactions by using hybrid quantum mechanics / molecular mechanics approaches.

Lumped kinetic approach for pyrolysis and gasification of solid plastic wastes (SPW): Recent developments and perspectives for chemistry modelling of SPW recycling processes

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