Isothermal Torrefaction vs. Direct Pyrolysis of Pine Cones: Biochar Texture, Carbon Structure, and Subsequent Electrochemical Evaluation

Pine cones are abundant forestry residues with potential for conversion into carbonaceous materials, yet the influence of torrefaction pretreatment on their particle-size distribution, carbon structure, textural properties, and ash behavior remains insufficiently understood. Here, pine cones were subjected to isothermal torrefaction at 220 °C/120 min or 250 °C/30 min, followed by non-isothermal slow pyrolysis at 500 °C/60 min, and compared with direct pyrolysis. Torrefaction increased fine-particle yield and altered pore development but did not provide an overall textural advantage over direct pyrolysis, demonstrating that improved grindability does not necessarily lead to enhanced biochar textural properties. The directly pyrolyzed biochar was subsequently subjected to H3PO4 activation, resulting in a maximum specific surface area of 561.3 m2 g−1 for the 75–355 µm fraction. Comparative electrochemical testing revealed a particle-size-dependent compromise, with the 63–75 µm fraction providing the highest specific capacitance (34.7 F g−1 at 0.5 A g−1), whereas the 75–355 µm fraction exhibited superior rate retention (56.6% at 5 A g−1). Pine cone ash was K-, Ca-, P-, Si-, and Mg-rich, with a low specific surface area (4 m2 g−1). Ash prepared at 720 °C showed lower subsequent mass loss than ash prepared at 650 °C.