Co-Pyrolysis Upgrading of High-Ash Biogas Residue with Lignocellulosic Biomass: Synergistic Kinetics, Energy Enhancement, and EBC-Compliant Biochar Production

Abstract

High-ash biogas residue exhibits low reactivity and yields mineral-rich char with limited fuel quality under mono-pyrolysis. This study investigated the macro-isothermal co-pyrolysis of swine digestate (SD) with spent coffee grounds (SCG) or rice husk (RH) at a fixed 50:50 mass ratio using a custom thermogravimetric system with approximately 5 g samples, providing a controlled framework closer to abrupt external heating in a preheated reaction zone than conventional micro-scale TGA. Isothermal experiments at 500–800 °C were analyzed using a first-order model, and the resulting biochars were characterized by elemental analysis, N₂ adsorption, and higher heating value measurements. The first-order model described the main conversion range well (R² > 0.98). The blends showed lower apparent activation energies (21.47–22.06 kJ mol⁻¹) than the corresponding theoretical additive values, indicating positive kinetic synergy under macro-isothermal conditions. Co-pyrolysis also improved char properties relative to digestate mono-pyrolysis. The SD-RH biochar exhibited the highest BET surface area (82.48 m² g⁻¹), while biochar HHV increased from 4.67 MJ kg⁻¹ for SD to 9.75–13.03 MJ kg⁻¹ for the blends. In addition, the blended chars showed low H/Corg and O/Corg ratios (0.34–0.40 and 0.15–0.17), consistent with European Biochar Certificate (EBC) stability thresholds. These results suggest that co-pyrolysis with lignocellulosic biomass can improve the apparent reactivity and quality of digestate-derived biochar, while macro-isothermal testing offers a useful intermediate approach for studying conversion behavior under abrupt external heating.