Valorization of tree leaves waste using microwave-assisted hydrothermal carbonization process

Fallen leaves of landscape trees, as an emerging biomass waste, were valorized using conventional hydrothermal carbonization (HC) and microwave-assisted hydrothermal carbonization (MHC) pretreatments, and were comparatively characterized for physicochemical properties and thermal degradation kinetics. The results show that MHC is superior to conventional HC operation, because at 200℃, the MHC process not only gives a higher hydrochar yield (45.09 vs. 39.47 wt%) with significantly reduced energy consumption (0.63 vs. 2.74 MJ g⁻¹), but also is more effective in removing K and Si. For isoconversional kinetic analysis, the FWO method provides better results than the KAS method as the latter failed to fit the tree leaves sample (R² < 0.9). The thermal degradation kinetics at high temperatures (>400℃) showed that the hydrochar obtained from the MHC process has a lower average activation energy of ~190 MJ kg⁻¹ than the conventional HC process (~260 MJ kg⁻¹). This study reveals the potential for valorization of the landscape tree wastes via the MHC process.