Hydrothermal Carbonization Processes for Carbon Material Development

Hydrothermal carbonization (HTC) converts wet biomass into carbon-rich solids, termed hydrochar, under moderate temperatures (180–250 °C) and autogenous pressures in subcritical water. Through a series of hydrolysis, dehydration, decarboxylation and polymerisation reactions, HTC transforms diverse feedstocks—such as agricultural residues, lignocellulosic biomass and biopolymers—into tailored carbon materials. Control of reaction parameters influences the physicochemical properties of hydrochar, including porosity, surface functionalities and degree of aromatisation, enabling applications in energy storage, adsorption, catalysis and soil amendment. Recent innovations have addressed energy efficiency, reaction kinetics and carbon footprint, notably through reactor design modifications that decouple temperature and pressure. Advances in activation methods—chemical, physical or combined—have further enhanced surface area, pore distribution and functional group density, broadening the scope of carbon materials derived from HTC. Ongoing research seeks to integrate life-cycle assessments, optimise feedstock utilisation and develop hydrochar-based composites, emphasising the process’s potential for negative carbon emissions and circular bioeconomy initiatives.

Research from Nature Portfolio

Recent studies have demonstrated a decoupled temperature and pressure hydrothermal system that maintains constant high pressure while heating cellulose feedstock, thereby lowering the carbonisation threshold and accelerating the formation of carbon sub-micron spheres. This approach promotes hydrogen bond cleavage and dehydration reactions at reduced temperatures, yielding uniform carbon microspheres without extended isothermal holds. The produced hydrochar exhibits a high degree of graphitisation, abundant oxygen-containing functional groups and enhanced thermal stability. A preliminary life-cycle assessment indicates substantial carbon emission reductions when replacing conventional fuel or employing hydrochar as a soil amendment, supporting its viability as a sustainable, carbon-negative carbon material.

Hydrothermal Carbonization Processes for Carbon Material Development publication trend

The graph below shows the total number of articles in hydrothermal carbonization processes for carbon material development across all publications each year (not limited to Nature Index journals).

Technical terms

Hydrothermal carbonization (HTC): A thermochemical process converting wet biomass into carbonaceous solids under subcritical water conditions (180–250 °C, autogenous pressure).

Hydrochar: The carbon-rich solid product of HTC, characterised by heterogeneous porosity, surface functional groups and varying degrees of aromaticity.

Graphitisation: The transformation of amorphous carbon domains into ordered graphitic structures, enhancing electrical conductivity and thermal stability.

Decoupled temperature and pressure hydrothermal system: A reactor design that independently controls pressure and temperature to lower the carbonisation threshold and improve reaction kinetics.

References

1. Hydrochar stability: understanding the role of moisture, time and temperature in its physiochemical changes. Biochar (2024).
2. Breaking the temperature limit of hydrothermal carbonization of lignocellulosic biomass by decoupling temperature and pressure. Green Energy & Environment (2023).
3. Decoupled temperature and pressure hydrothermal synthesis of carbon sub-micron spheres from cellulose. Nature Communications (2022).
4. Evidence for a core-shell structure of hydrothermal carbon. Carbon (2020).
5. Recent advances in hydrothermal carbonisation: from tailored carbon materials and biochemicals to applications and bioenergy. Green Chemistry (2020).