Dottorato in ENERGIA E AMBIENTE

Titolo della tesi: Thermo-fluid-dynamic analysis of biomass gasifier systems and SOFC fuel cells: "Contaminants removal and polarization losses"

This thesis investigates three critical elements in the integration of thermochemical conversion of waste/biomass with Solid Oxide Fuel Cells (SOFCs), aiming to improve sustainability and efficiency in power generation. First, a fluidized-bed devolatilization process was examined using polypropylene (PP) particles, focusing on how temperature (650–850 °C) and particle size (8–12 mm) affect product yields. Results showed that elevated temperatures encourage enhanced molecular cracking, producing hydrogen-rich gas but also increasing tar and residual solid carbon. A pseudo–first-order kinetic model incorporating thermal conduction and global reaction kinetics yielded an apparent activation energy of 11.8 kJ/mol, a pre-exponential factor of 0.55 s⁻¹ for the reference particle size, and a size-correction exponent of 0.77. These findings enable more accurate reactor-scale predictions for co-gasifying plastics and biomass in industrial fluidized-bed systems. Next, the issue of hydrogen sulfide (H₂S) in syngas was addressed via high-temperature desulfurization using zinc oxide (ZnO) sorbents in a fixed-bed reactor, with operating temperatures ranging from 400 to 600 °C. Optimal performance was observed at 550 °C, where the sorbent captured 5.4 g of sulfur per 100 g of ZnO over 2.7 hours before breakthrough. By applying a linearized deactivation model, key sorption and deactivation rate constants were extracted and validated under realistic syngas conditions that included steam and secondary reactions (e.g., water–gas shift). Thermodynamic parameters derived through Arrhenius and Eyring–Polanyi treatments confirmed the feasibility of ZnO-based desulfurization at higher temperatures, while a spherical-coordinate model clarified diffusion coefficients and intrinsic reaction constants. Finally, detailed SOFC performance analyses were conducted using button cells with approximately 2 cm² active area at 650–800 °C. Polarization curves, electrochemical impedance spectroscopy (EIS), and Distribution of Relaxation Times (DRT) revealed how hydrogen concentration, temperature, and trace contaminants influence activation, ohmic, and mass-transport losses. A zero-dimensional Butler–Volmer-based approach found dimensionless charge-transfer coefficients of about 2 at the anode and 3.5 at the cathode, with exchange current densities ranging from 0.012 to 0.073 A/cm² for hydrogen oxidation and 0.024 to 0.048 A/cm² for oxygen reduction. The corresponding activation energies—100 kJ/mol (anodic) and 66 kJ/mol (cathodic)—agree with literature values, underscoring strong predictive capability. Collectively, these results form a cohesive framework for designing integrated gasification-SOFC systems that account for feedstock variability, hot gas cleanup requirements, and electrochemical factors to achieve high-efficiency, clean power generation.