Activated Carbon Filter Offcuts as Energy-Saving Drying Enhancers to Improve Storability of Spent Coffee Grounds and Enable Microwave Torrefaction into Solid Biofuel

The valorization of spent coffee grounds (SCGs) into solid biofuels represents a promising pathway toward sustainable energy and waste reduction. The objective of this study is to upgrade SCGs into high-quality solid biofuels through an integrated process of activated carbon filter offcuts (ACFO)-assisted drying and microwave torrefaction. The methodology involves employing ACFO as a layered drying enhancer to facilitate moisture removal from SCGs at 85–125 °C, followed by microwave torrefaction at 200–300 °C to evaluate fuel properties and environmental sustainability via life cycle assessment (LCA). Results reveal that the alternating layered ACFO-SCG configuration accelerates drying kinetics and improves heat transfer uniformity. Under optimal conditions (85 °C), the specific power consumption decreased from 3.13 to 2.64 kWh per 1% moisture removal, achieving 15.6% energy savings compared with conventional convective drying. Microwave torrefaction at 300 °C yields high-quality biochar comparable to lignite with a higher heating value of 30.61 MJ·kg−1. Furthermore, LCA indicates that torrefaction at 220 °C minimizes the carbon footprint (0.216 kg CO2-eq per kg), whereas increasing the temperature to 300 °C reverses this advantage due to rising electricity demands. This finding concludes that ACFO can effectively bypass biomass-drying energy barriers, enabling the efficient upcycling of SCGs into high-quality solid biofuels with limited energy input.