Co-Torrefaction of Sub-Bituminous Coal and Empty Palm Fruit Bunch: A Process Optimization Study Using Response Surface Methodology (RSM)

This study investigates the co-torrefaction of sub-bituminous coal (SBC) and empty fruit bunch (EFB) biomass as a strategy for enhancing the fuel characteristics of biomass-coal blends for sustainable energy applications. A series of experiments were conducted by systematically varying the EFB mass fraction (20–80%), torrefaction temperature (200–300 °C), and residence time (20–60 minutes). The process optimization was performed using Response Surface Methodology (RSM) with a Box–Behnken design to evaluate both the individual and interactive effects of these parameters on the higher heating value (HHV) and energy yield (EY) of the resulting solid fuel. Statistical analysis revealed that torrefaction temperature exerted the most significant influence on both HHV and EY, whereas the effects of EFB proportion and residence time were comparatively less pronounced. Interaction effects among variables were minimal. The optimal torrefaction conditions—20% EFB content, 200 °C processing temperature, and 60-minute residence time—yielded a maximum HHV of 31.98 MJ/kg and an energy yield of 92.18%. Model validation through experimental data confirmed the predictive accuracy and robustness of the RSM model. The findings demonstrate the technical viability of biomass-coal co-torrefaction as a thermal pre-treatment method to improve the calorific and energy retention properties of solid fuels. This work provides a valuable framework for optimizing co-processing parameters in bioenergy applications and contributes to the advancement of cleaner and more efficient energy systems.