Advanced kinetic and thermodynamic analysis of sewage sludge combustion: A study of non-isothermal reaction mechanisms and energy recovery

The management of sewage sludge (SS) is a significant global environmental challenge. Thermochemical conversion methods like incineration, pyrolysis, and gasification offer a way to reduce SS volumes and recover energy, yet their complexity requires a deep understanding of SS reactivity and conversion product properties for effective use and scale-up. This study investigates the non-isothermal combustion kinetics of SS, identifying six reaction stages through deconvolution analysis. Kinetic parameters were estimated using the isoconversional method for each pseudo-component, specifically the Kissinger-Akahira-Sunose (KAS) and Ozawa-Flynn-Wall (OFW) methods were utilized to determine activation energies, while the master plot method identified nth order kinetics (Fn) with n < 1 as the dominant reaction mechanism. The found activation energies were ranging from 71.57 kJ/mol to 267.91 kJ, and the pre-exponential factors from 2.35 × 103 min⁻¹ to 2.35 × 1017 min⁻1. Further thermodynamic assessments provided insights into enthalpy (ΔH = 65.62 to 261.69 kJ/mol), entropy (ΔS = -192.31 J/(mol·K) to 71.68 J/(mol·K)) and Gibbs free energy (ΔG = 149.19 kJ/mol to 208.00 kJ/mol) for the pseudo-components, contributing to a comprehensive kinetic and thermodynamic profile of SS combustion.