Hydrothermal Carbonisation of Waste Biomass: A Review of Combustion Behavior, Kinetics, Thermodynamics and Reaction Mechanisms

The increasing generation of organic waste and the growing demand for sustainable solid fuels have intensified interest in hydrothermal carbonisation (HTC) as a pathway for biomass valorization within circular bioeconomy systems. HTC uses subcritical water to upgrade moist biomass into hydrochar with improved fuel properties and combustion behavior. This review correlates key HTC parameters, including temperature, residence time, pH, and the nature of feedstock, with the chemical evolution and thermal reactivity of different hydrochars. Data synthesis identifies a typical ‘kinetic optimization’ range between 180 and 220 °C for conventional lignocellulosic feedstocks. Within this thermal interval, activation energy (Ea) decreases from 180–260 kJ/mol for raw biomass to 70–180 kJ/mol for hydrochars, while the high heating value (HHV) reaches up to ~28 MJ/kg. The results further demonstrate that feedstock composition strongly influences combustion reactivity and kinetic behavior under similar HTC conditions. The integration of isoconversional methods with thermodynamic parameters (ΔH, ΔG, ΔS) confirms a transition toward more ordered and thermally stable carbon structures. Additionally, Criado’s master plots indicate a shift from diffusion-controlled to reaction-controlled combustion mechanisms with increasing HTC severity. These findings provide valuable insights into the optimizing of HTC conditions for balance energy densification and combustion reactivity, offering a comprehensive understanding to guide future hydrochar-based energy applications and scale-up studies.