Rethinking hydrothermal carbonization: development of lignin-focused biorefinery for generation of high-value hydrochar and lignin spheres

Abstract

This tutorial review aims to provide a comprehensive overview of hydrothermal carbonization (HTC) as a unified platform for biomass valorisation including the most recent developments and future challenges. After the introduction, section 2 distils HTC fundamentals – operating regimes, acid-catalysed pathways (hydrolysis, dehydration, aromatization), and the distribution of solids, liquids and gases under subcritical water conditions. Section 3 examines hydrochar, highlighting how variations in feedstock, temperature, and residence time govern surface functionality, porosity and thermal stability, and enable applications in soil amendment, CO2 capture, heavy-metal adsorption, and solid-acid catalysis. Section 4 surveys HTC-derived carbon spheres, demonstrating that control over dopants, activation protocols, and reaction parameters yields particles with tailored meso-/microporosity, surface chemistry, and electrochemical performance for supercapacitors and water purification. Section 5 introduces lignin-focused strategies that recover structurally preserved lignin directly from hydrochar and convert it into micro-/nanospheres with high carbon content, intact β-O-4 linkages, and tuneable functionality for catalytic and materials applications. Lastly, section 6 is dedicated to the environmental assessment of solvents and chemical reagents used in post-HTC functionalization and activation. Collectively, these developments establish HTC as a versatile, zero-waste biorefinery approach – producing hydrochar, engineered carbon nanostructures, and high-value lignin under green chemistry principles and scalable process conditions.