Thermogravimetric and Coats-Redfern Analysis of Loblolly Pine and Kaolin Clay Co-Pyrolysis: From Devolatilization to Char-Mineral Transformation

This study investigates the thermal behavior and kinetic mechanisms governing the co-pyrolysis of loblolly pine (LP) and kaolin clay (KC), with emphasis on how mineral loading and heating rate affect biomass-mineral conversion. Thermogravimetric experiments were conducted under nitrogen atmosphere at 5, 10, and 20 °C min -1 for LP100 (100 wt% loblolly pine), KC100 (100 wt% kaolin clay), and three blends, LP75KC25, LP50KC50, and LP25KC75, resolving devolatilization, Phase II, primary biomass devolatilization, 130°C to 400°C, and both char-mineral transformation, Phase III, 400°C to 900°C. Increasing KC content systematically shifted mass-loss events to higher temperatures, suppressed peak weight-loss rates, and increased residual solids, reflecting thermal stabilization and overlap between biomass charring and kaolinite dehydroxylation. Coats-Redfern analysis showed that in Phase II, LP100 and LP75KC25 were best described by diffusion-limited models, whereas higher KC fractions in LP50KC50 and LP25KC75 led to reaction-order mechanisms, indicating a growing influence of the mineral phase on biomass devolatilization pathways. In Phase III, KC100 followed chemically controlled kinetics associated with aluminosilicate restructuring, while co-feeding LP reduced apparent energy barriers and promoted more diffusion-limited char conversion in biomass-rich blends; while clay-rich blends progressively reverted to mineral dominated, reaction-order behavior. Heat rate further modulated these trends by shifting DTG peaks and inducing mechanism switching at an intermediate blend ratio of LP50KC50. Synergy analysis, FTIR, and XRD further confirmed that LP-KC interactions were not purely additive, with blend-dependent deviations in thermal mass loss and crystallographic evidence of simultaneous biomass carbonization and kaolinite-to-metakaolin transformation. Overall, the combined TG-DTG, Coats-Redfern, synergy, FTIR, and XRD results demonstrate that adjusting LP to KC ratio and thermal ramp provides a practical means to tailor decomposition pathways, char-mineral interactions, and kinetic regimes in biomass-mineral co-pyrolysis systems.