Evaluation of Catalytic Biomass–Clay Interactions on Co-Combustion Kinetics and Thermodynamics Using Integral Coats–Redfern Model-Fitting and Flynn–Wall–Ozawa Model-Free Analysis

This study investigated the co-combustion behavior of loblolly pine (LP100), kaolin clay (KC100), and LP-KC blends (LP75KC25, LP50KC50, and LP25KC75) using thermogravimetric analysis under air gas flow at heating rates of 5, 10, and 20 °C min−1. The objective was to evaluate how biomass–mineral blending affects thermal degradation, reaction-stage development, kinetic behavior, and thermodynamic properties. The TGA-DTG results were interpreted using Coats–Redfern model-fitting kinetics, Flynn–Wall–Ozawa (FWO) model-free kinetics, and thermodynamic analysis. LP100 and LP-KC blends exhibited two main stages: Phase II oxidative devolatilization and Phase III char oxidation with overlapping kaolinite dehydroxylation, while KC100 showed one dominant high-temperature mineral transformation. In Phase III, the blends’ DTG peak temperatures ranged from 401.7 to 474.3 °C at 5 °C min−1, 427.0 to 500.3 °C at 10 °C min−1, and 478.3 to 509.0 °C at 20 °C min−1. Coats–Redfern Ea values were 58.16–70.50 kJ mol−1 for LP100 in Phase III, and 178.85–181.59 kJ mol−1 for KC100 in Phase III, while the blends’ Ea values ranged from 24.07 to 81.31 kJ mol−1 in Phase II and 30.00 to 59.59 kJ mol−1 in Phase III. FWO analysis confirmed conversion-dependent Ea behavior, with KC100 showing the highest energy barrier. Thermodynamic analysis showed positive ∆G values of 170.59–188.34 kJ mol−1 in Phase II and 197.38–240.84 kJ mol−1 in Phase III. ∆H ranged from 19.67 to 76.55 kJ mol−1 in Phase II and reached 172.53–175.43 kJ mol−1 for KC100 in Phase III, while negative ∆S values of −0.07 to −0.28 kJ mol−1 K−1 indicated ordered activated complexes. In conclusion, LP lowered the apparent kinetic and thermodynamic barriers of KC transformation during co-combustion.