CAN HYDROTHERMAL BIOMASS PROCESSING SERVE AS AN ANALOGUE OF MILLENNIA-SCALE NATURAL CARBON MATURATION? A UNIFIED PERSPECTIVE ON CARBONISATION, HUMIFICATION, AND FULVIFICATION

Hydrothermal biomass processing provides a rapid and controllable approach to replicating the natural carbon maturation of organic matter, a process that typically spans millennia. Subcritical water treatment at elevated temperatures and pressures converts biomass into products analogous to those found in peat, lignite, and coal. This review examines three principal hydrothermal pathways, namely carbonisation, humification, and fulvification, which yield hydrochar, artificial humic acids, and fulvic-like acids, respectively. These products share similarities with natural soil organic matter in terms of carbon content, polarity, and stability. The Van Krevelen diagram serves as a valuable tool for comparing the transformation trajectories of these materials with those of natural carbon evolution. Analysing the effects of process variables such as temperature, pH, and feedstock composition facilitates the development of tailored carbon materials for soil amendment, peat substitution, and carbon sequestration. The review also addresses technological challenges and scaleup requirements to advance future bioeconomy applications.