Co-hydrothermal carbonization of tobacco stems assisted by activated carbon from coal-gasification fine slag: Efficient wastewater purification and enhanced combustion performance of co-hydrochar

This study proposes an integrated co-hydrothermal carbonization (co-HTC) strategy for simultaneously purifying hydrothermal wastewater and improving the combustion performance of co-hydrochar by co-processing tobacco stems (TSs) with chemically activated coal-gasification fine slag (CAC). TSs, a major agricultural residue, generate large volumes of organic-rich wastewater during HTC, while CAC exhibits high porosity and adsorption potential. The co-HTC was performed at 250 °C and 300 °C under varying TS: CAC mass ratios (1:0–1:3). At 250 °C and a TS:CAC ratio of 1:3, COD and TOC values declined from 18,144 mg/L and 8,686 mg/L to 10,383 mg/L and 4,350 mg/L, respectively, achieving removal rates of 42.78 % and 49.92 %. At 300 °C, the removal rates remained above 42 %, despite forming more low-molecular-weight organics. CAC promoted pore development in co-hydrochar, while dichloromethane extraction exposed additional micropores and cracks, enhancing surface reactivity. The introduction of CAC reduced aliphatic and carbonyl groups in both the hydrochar and the bio-oil phases, suggesting CAC's dual role in adsorbing unstable intermediates and catalyzing aromatization. Combustion characterization showed that with increasing CAC ratio, the burnout temperature of hydrochar increased by 40 °C (from 689 °C to 729 °C), while the flammability index and combustion stability index decreased. Dichloromethane extraction improved ignition temperature, maximum combustion rate temperature, and combustion uniformity by 20–45 °C, confirming enhanced reactivity after removal of secondary char. In conclusion, co-HTC of TSs and CAC offers a sustainable route for the integrated treatment of agricultural and industrial residues, providing both wastewater remediation and high-quality carbon materials. Optimization of operating conditions is essential to balance purification efficiency and fuel quality.