Rapid conversion of wet spent coffee grounds into high-calorific biochar via drying-free flame plasma pyrolysis for process intensification

Conventional valorization of high-moisture biomass is economically constrained by energy-intensive pre-drying steps and prolonged residence times required by traditional pyrolysis. Herein, we report a novel process intensification strategy utilizing an atmospheric flame plasma to directly convert wet spent coffee grounds (moisture ~55%) into high-grade biochar within 90 s, entirely eliminating the need for dehydration or de-oiling pre-treatments. The intense heat flux from the LPG-based plasma jet triggered flash evaporation of the inherent moisture, inducing an in-situ steam activation mechanism termed the popcorn effect, which facilitated the development of a highly porous structure with a peak specific surface area of 115.4 m 2 /g. Under the peak phenomenological condition (90 s), the process achieved a mass reduction of 83.3% and yielded a sulfur-free biochar with a higher heating value of 29.0 MJ/kg, surpassing that of standard anthracite. This study demonstrates that flame plasma pyrolysis provides a sustainable, energy-efficient, and ultra-fast pathway for waste-to-energy conversion, effectively turning the intrinsic moisture of biomass from a thermal burden into a functional activation agent. • Ultrafast flame plasma converts wet spent coffee grounds into solid fuel in 90 s • Fixed carbon content triples; calorific value increases by ~33% • Flame plasma creates highly porous structure: BET surface area reaches 115 m 2 /g • Sulfur fully removed, yielding cleaner solid fuel with minimal emissions • Development of rapid, clean, and eco-friendly process for biomass valorization