Simulation and advanced modeling of innovative waste-to-chemicals plants in the circular economy: Gasification of Refuse Derived Fuel

This PhD thesis focuses on the advanced study, through modeling and simulation, of innovative high-temperature waste gasification systems, with the overarching goal of promoting circular economy strategies through Waste-to-Chemicals (WtC) processes. The research centers on the analysis of the dynamic behavior of the gasification reactor in an emerging and promising technology known as High Temperature and Direct Melting Gasification (HT&DMG), which enables the efficient thermochemical conversion of municipal solid waste (MSW) and refuse-derived fuel (RDF) into high value syngas. The main objective of the work is the development of accurate and flexible models capable of describing the high-temperature gasification process, by simulating the different process stages (drying, pyrolysis, gasification, combustion, and ash melting) while accounting for the reactor geometry, chemical kinetics, and mass and heat transfer phenomena. To this end, three accurate, complementary and flexible types of modeling approaches have been implemented using different simulation environments:  Kinetic simulation in Aspen Plus: used to evaluate the chemical behavior of the gasifier and predict the syngas composition under varying operating conditions.  One-dimensional dynamic model in COMSOL Multiphysics: developed to analyze the spatial and temporal evolution of the system, capturing the dynamic behavior of the process along the reactor height.  Three-dimensional Computational Fluid Dynamics (CFD) model in Ansys Fluent: employed to characterize the internal flow patterns and thermal distribution within the stabilization zone of the gasifier. These modeling tools have been used to investigate and optimize reactor performance, maximize syngas yield, and minimize the formation of undesired pollutants such as tar and dioxins. In particular, special attention has been devoted to the mechanisms of dioxin formation under the reducing conditions typical of gasification, which significantly limit their production compared to traditional combustion processes. All models have been validated against experimental data collected from an Italian industrial plant adopting the HT&DMG technology. The results confirm that gasification is an advanced and versatile solution for waste valorization, capable of producing high-quality syngas with low pollutant content, and offering a sustainable alternative to conventional waste-to-energy technologies. The HT&DMG process, in particular, has proven to be a reliable, efficient, and environmentally sound technology suitable for treating diverse feedstocks. Overall, this research contributes significantly to the field of thermochemical waste conversion, providing a robust modeling framework that can support future developments aimed at improving efficiency, reducing environmental impact, and fostering the industrial implementation of circular economy principles.