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Hydrothermal carbonization (HTC) represents a promising thermochemical method for converting wet biomass under moderate aqueous conditions into carbon-rich materials, characterized by specific attributes. Notwithstanding the increasing interest surrounding HTC, the current literature remains fragmented regarding the precise mechanisms by which process parameters influence hydrochar formation, its properties, and sustainable utilization. Consequently, the primary objective of this review is to systematically elucidate the fundamental mechanisms that govern HTC, to identify key parameters impacting hydrochar yield and quality, and to assess the sustainability and prospective contributions of HTC within the context of circular economy principles. This paper elaborates on the reaction pathways of hydrolysis, dehydration, decarboxylation, and aromatization that dictate the structural alterations and carbon densification of hydrochars. It emphasizes the roles of temperature, residence time, solid/liquid ratio, catalysts, and feedstock composition in jointly determining hydrochar yield, elemental composition, aromaticity, porosity, and energy density. Additionally, recent advancements, including microwave-assisted HTC, catalytic modifications, and post-activation techniques, are reviewed to enhance hydrochar functionality for applications in energy, adsorption, catalysis, and soil enhancement. Challenges remain regarding the scale-up of the process, reactor design, standardization of hydrochar properties, and the sustainable management or valorization of process water. This review integrates mechanistic insights with recent technological progress to position HTC as a versatile and sustainable method for producing high-value hydrochars, thereby underscoring its potential role in future biorefineries and circular economy initiatives.
Durak et al. (Sun,) studied this question.