Co-torrefaction and synergistic effect of spent coffee grounds and tea waste for sustainable waste remediation and renewable energy

This study states the enhancement of higher heating value (HHV) by blending spent coffee grounds (SCG) and tea waste (TW) during torrefaction. Torrefied biomass mixture is a carbon-neutral solid biofuel that can be used to replace coal. The synergistic effect ratio (SER) exhibits the synergistic interplay between SCG and TW, consequently elevating HHV and carbon content. Results show that the highest SER occurs at TWS25 (from blending 75 % TW and 25 % SCG is 3.26). Ca contained in TW has the catalytic effect of pyrolyzing the lipid of SCG into higher carbon content in a torrefied solid biofuel mixture, thereby increasing its HHV. The lipid and carbon contents contribute TWS25's HHV by 4.71 % and 95.29 %, respectively. This also illustrated the importance of the carbonization degree during torrefaction to the calorific value of solid biofuel mixture. The XRD, water activity, ATR, contact angle, and mildew observation results indicate that TWS25 has good thermal stability and storage properties. The ignition temperature and combustion reactivity of TWS25 are 325 °C and 3.059 wt%∙min⁻¹∙°C⁻¹, respectively. Overall, the torrefied solid biofuel mixture from blending TW and SCG is a potential biofuel alternative to coal to achieve carbon neutrality, circular bioeconomy, waste remediation, and low carbon initiative.