Sustainable Conversion of Urban Tree Waste Into High Energy Hydrochar via Hydrothermal Carbonization and Process Water Recirculation

The growing demand for renewable energy and sustainable waste management highlights the need for efficient biomass-to-energy technologies. This study investigated the production of solid biofuel from urban tree waste (UTW) in Hanoi, Vietnam, using hydrothermal carbonization (HTC) coupled with process water (PW) recirculation. HTC was optimized at 240 °C for 3 h, yielding hydrochar with a higher heating value (HHV) of 22.95 MJ kg-1 and an energy yield of ~90%. The PW was recirculated for up to ten cycles at a fixed PW-to-freshwater ratio of 1:1. The fifth recirculation cycle resulted in the highest hydrochar quality (HHV: 22.18 MJ kg-1; energy yield: 77.15%), after which further recirculation resulted in a decrease in the HHV. PW analysis revealed that total organic carbon (TOC) and organic acids (acetic and formic acids) significantly influenced hydrochar yield and energy densification through secondary carbon deposition. A predictive model for HHV based on fixed carbon content was developed and validated with independent datasets. The produced hydrochar met or exceeded Vietnamese solid fuel standards regarding HHV, ash, and sulfur content. Overall, HTC combined with PW recirculation is an effective and sustainable approach for valorizing UTW as a high-quality solid biofuel, reducing freshwater consumption, and minimizing wastewater discharge in the environment.