Pyrolysis Kinetics and Biochar Production of Almond and Pistachio Shells in a Fixed-Bed Pyrolyzer

The effects of pyrolysis temperature (400–500 °C) and time (30–90 min) on the yield and chemical and physical properties of biochar produced from almond and pistachio shells were studied using a fixed-bed pyrolyzer. Thermogravimetric analysis (TGA) was employed to characterize the kinetics of thermal degradation of the shells. This study compared the thermal behavior observed by TGA with biochar yields obtained from a fixed-bed pyrolyzer, providing insight into the agreement between laboratory-scale thermogravimetric measurements and fixed-bed pyrolysis performance. Fourier transform infrared spectroscopy (FTIR) was performed for each type of biochar. Results showed higher biochar yields from almond shells (35.0–41.3% dry basis) than from pistachio shells (26.8–36.7% dry basis). Shell type, pyrolysis temperature, pyrolysis time, and their interactions had significant effects on biochar yield. The Derivative Thermogravimetric (DTG) profiles showed distinct thermal decomposition patterns for almond and pistachio shells. Almond shells exhibited broader decomposition regions, while pistachio shells showed more distinct decomposition stages. FTIR analysis of both shell biochars indicated reduced O–H and oxygen-containing groups with increasing pyrolysis temperature and residence time, suggesting greater carbonization, aromatic enrichment, and formation of carbonaceous compounds. Greater biochar yields were obtained from the fixed-bed pyrolyzer than from TGA. A first-order kinetics model adequately described the thermal decomposition of both shell types. Apparent activation energies were 41.83–44.99 kJ mole−1 for almond shells and 58.19–63.58 kJ mole−1 for pistachio shells. Model validation showed a good agreement between the experimental and predicted conversion values. The results provide a basis for evaluating the potential of TGA-derived thermal behavior to inform biochar production conditions in fixed-bed pyrolysis.