Progress and challenges in thermochemical technologies for biomass humification: A comprehensive review

Advancing low-carbon, energy-efficient pathways for biomass conversion is essential to achieving carbon neutrality and sustainable resource management. Thermochemical humification (TCH) has recently emerged as a promising approach for converting biomass waste into artificial humic acids (AHAs), stable carbon-rich materials with significant potential for long-term carbon sequestration, improved soil fertility, and environmental management. Beyond agroecological benefits, TCH technologies offer clear advantages in reducing greenhouse gas emissions and enhancing carbon conversion efficiency compared with conventional humification processes. This review critically synthesizes recent advances in wet, dry, and integrated TCH routes, focusing on the mechanistic and energy–environment nexus governing the transformation of labile organic matter into humified carbon structures. Key pathways, including hydrothermal carbonization, hydrothermal humification, hydrothermal fulvification, and pyrolysis humification, are systematically evaluated with respect to operating conditions, oxidation-assisted strategies, catalytic intensification, and integrated multi-stage designs. Reported AHA yields range from ∼10 to 45 wt% for hydrothermal humification and ∼8–25 wt% for pyrolysis humification, while optimized integrated TCH systems can achieve substantially higher efficiencies (∼12–90 wt%). The techno-economic and environmental aspects of TCH technologies are further examined from the perspectives of life cycle, carbon emissions, carbon conversion efficiency, and energy consumption. Energy requirements across TCH pathways vary widely, from as low as 0.26–1.34 kWh/g to 26–32 MJ/kg depending on feedstock and process configuration. Carbon emissions and conversion efficiencies show similarly broad ranges, with losses below 10 % in hydrothermal pathways but exceeding 20 % in pyrolysis humification. Overall, catalytic, oxidative, and co-humification strategies consistently enhance carbon retention while reducing emissions. This review highlights key research gaps and provides a mechanistic, sustainability-oriented framework to advance TCH as a low-emission technology that support biomass humification and circular bioeconomy goals.