Pili nutshell (PS), an abundant agro-industrial byproduct in the Bicol Region, Philippines, demonstrates substantial potential as a solid biofuel and bioenergy feedstock. Proximate and ultimate analyses revealed high volatile matter (72.00 ± 0.20 wt%), low ash content (4.33 ± 0.76 wt%), and a higher heating value of 20.60 MJ/kg, indicating strong suitability as a solid fuel for thermochemical conversion and biofuel production. Thermogravimetric analysis (TGA) was conducted from 30 °C to 900 °C at heating rates of 10, 15, and 20 °C/min under nitrogen to examine its thermal decomposition behavior. The process followed three stages: initial moisture loss, active devolatilization, and lignin-rich char formation. The resulting kinetic and thermodynamic parameters are directly relevant for designing fast pyrolysis processes aimed at liquid biofuel production and optimizing downstream fuel utilization of the derived bio-oil and char. Kinetic analysis using the Coats–Redfern method identified third-order reaction (CR03) and diffusion-controlled (DM6) models as best-fitting, with activation energies ranging from 64.03–96.21 kJ/mol (CR03) and 66.98–104.72 kJ/mol (DM6). Corresponding thermodynamic parameters—ΔH (58.67–90.95 kJ/mol), ΔG (201.51–231.46 kJ/mol), and ΔS (−174.57 to −255.08 kJ/mol·K)—indicated an endothermic, non-spontaneous, entropy-reducing reaction pathway. Model-free methods confirmed a highly reactive zone at α = 0.3–0.6, with consistent E_a values (~130–190 kJ/mol). These findings affirm the viability of PS for fast pyrolysis, offering data-driven insights for optimizing advanced fuel and bioenergy systems in line with circular economy objectives.

Waste is increasingly recognized as misplaced biomass, underscoring its potential for reintegration into sustainable environmental management strategies. Biomass pyrolysis has emerged as a promising value-adding process capable of enhancing material properties for diverse applications. In this study, discarded Pili (Canarium ovatum Engl.) nutshells (PS) were utilized as a pyrolysis feedstock to upgrade their fuel characteristics. Pyrolysis conditions were optimized using response surface methodology (RSM) based on a central composite design (CCD) to maximize fixed-carbon content and higher heating value (HHV). The optimized biochar achieved a maximum fixed-carbon content of 86.15% and an HHV of 32.10 MJ/kg at a pyrolysis temperature of 600 °C and a residence time of 60 min, values comparable to those of conventional coal. Under these optimized conditions, the fixed-carbon content and HHV of the precursor biomass were enhanced by up to 254.7% and 58.4%, respectively. Statistical analysis indicated that pyrolysis temperature was the most significant factor influencing both fixed-carbon content and HHV (p < 0.05). The optimized biochar exhibited low volatile matter (8.88%), low ash content (4.97%), and low atomic ratios (H:C = 0.291; O:C = 0.077), indicating a high degree of carbonization and thermal stability. Energy-dispersive X-ray (EDX) analysis identified alkali and alkaline earth metals (Ca, Mg, Na), which contributed to the ash fraction, with minor heavy metals present, predominantly Pb. Hence, these findings enhance understanding of how pyrolysis conditions affect PS–biochar properties, improving fuel quality indicators.