Resource-Efficiency Assessment of Spent Coffee Grounds Valorization into Porous Biochars: Effect of KOH Activation Time on Pore Redistribution and Apparent CO2 Gasification Kinetics

Spent coffee grounds (SCGs) are an abundant food-service residue whose conversion into porous carbons can support circular resource use, provided that gains in specific surface area are weighed against material losses during activation. This study examines solid recovery, pore development, and apparent CO2 gasification kinetics during SCG valorization using potassium hydroxide (KOH) activation for 30 or 60 min at 800 °C. Cumulative solid yields relative to dry SCG were 16.5% for pristine biochar (BC), 10.5% for BC-K30, and 9.9% for BC-K60. Corresponding Brunauer–Emmett–Teller (BET) surface areas were 2.6, 1402.8, and 1514.3 m2 g−1. Extending activation from 30 to 60 min increased total pore volume by 10.1% and mesopore volume by 215.1%, while total micropore volume decreased by 5.2%. The volume below 0.7 nm increased by 18.6%, indicating that the micropore response was not uniform across size intervals. Nominal yield-weighted BET areas were 147.3 and 149.9 m2 per gram of dry SCG for BC-K30 and BC-K60. Adsorption was measured once per gas and composite, so textural differences are interpreted descriptively. Kissinger–Akahira–Sunose analysis gave mean apparent activation energies of 214.5, 191.8, and 264.8 kJ mol−1 for BC, BC-K30, and BC-K60; Friedman analysis provided a complementary comparison. Higher gasification peak temperatures for the activated materials indicate that apparent activation energy alone does not rank gasification rates. The results identify pore-size-dependent material recovery trade-offs, without establishing an activation optimum or environmental or economic superiority.