Elucidating volatile release behavior, kinetics, and the regulatory role of calcite ash in waste paper pyrolysis: A combined TG-FTIR and ReaxFF-MD study

Understanding the volatile evolution, activation energies, and interactions among cellulose, hemicellulose, lignin, and ash during waste paper pyrolysis is essential for its valorization. In this study, the volatile release, activation energies, and pyrolysis models of four waste paper types (CDB, IB, KP, and OPP) were determined by TG-FTIR, while ReaxFF-MD simulations elucidated the effects of cellulose, hemicellulose, lignin, and calcite (the main ash) on the major gases (CO₂, CO, CH₄, H₂O) and functional groups (C C, C O, C–C, C–O, C–H, H–O). Pyrolysis proceeded via three distinct stages: dehydration below 150 °C, devolatilization between 150 and 600 °C, and calcite decomposition above 600 °C. Among the four paper types, KP, with the lowest ash content, showed the strongest DTG and CO₂ peaks, whereas ash-rich CDB displayed suppressed reactivity, confirming that inorganic constituents inhibit volatile release. Kinetic analysis revealed that ash lowered the apparent activation energy: CDB (150 ± 16 kJ/mol), IB (126 ± 16 kJ/mol), and OPP (137 ± 9 kJ/mol) all fell below KP (152 ± 15 kJ/mol). The D1 diffusion model adequately described CDB pyrolysis, and the A4/5 nucleation-growth model fitted IB well. ReaxFF-MD simulations showed that co-pyrolysis promoted CO and CH₄ formation via secondary cracking and suppressed CO₂. Pathway tracing indicated that CO₂, CO, and H₂O originated from decarboxylation, oxygenate cracking, and dehydration of hydroxyl-rich structures, respectively. Calcite suppressed char aromatization and enriched oxygenates in the volatile fraction. The simulated activation energies decreased upon calcite addition, qualitatively consistent with the experimental trends. These findings clarify the catalytic role of calcite and offer kinetic guidance for waste paper pyrolysis optimization.