Hydrothermal carbonization (HTC) has emerged as a pivotal technology in the battle against climate change and fosters circular economies. Operating within a unique reaction environment characterized by water as a solvent and moderate temperatures at self-generated pressures, HTC efficiently converts biomass residues into valuable bio-based products. Despite HTC’s potential—from the management of challenging biomass wastes to the synthesis of advanced carbons and the implementation of biorefineries—it encounters hurdles transitioning from academic exploration to industrial implementation. Gaps persist, from a general comprehension of reaction intricacies to the difficulty of large-scale integration with wastewater treatments, to the management of process water, to the absence of standardized assessment techniques for HTC products. Addressing these challenges demands collaboration to bridge the many scientific sectors touched by HTC. Thus, this article reviews the current state of some hot topics considered crucial for HTC development: It emphasizes the role of HTC as a cornerstone for waste management and biorefineries, highlighting potentialities and challenges for its development. In particular, it surveys fundamental research aspects, delving into reaction pathways, predictive models, analytical techniques, and HTC modifications while exploring HTC’s crucial technological applications and challenges, with a peculiar focus on combined HTC, wastewater integration, and plant energy efficiency.

Hydrothermal carbonization (HTC) is a proven cost-effective and energy-efficient method for waste management and value-added product recovery. There are, however, several issues that require further improvement or research. Identifying the strengths and weaknesses of HTC in comparison to traditional pyrolysis is crucial for scientists to choose between them or use both (complementary) to achieve specific product properties. Additionally, sharing information on diverse modeling approaches and scales is crucial to enhance the robustness and universality of HTC process models. In addition, the study on the applicability of hydrochars on target applications such as soil amendment is crucial to give back nutrients to soils and face the dependence on finite specific feedstocks in this field. Also, proper management of the process by-products, especially process water, must be addressed to improve the carbon and hydric footprint of the process. Reviewing the suitability of HTC to treat specific challenging wastes, whose strength is not related to their calorific value but to their nutrient composition (i.e., manures), is also an appealing topic for HTC research. This paper aims to tackle the above-mentioned issues through an updated review and discussion of research gaps that require further investigation.

The increasing reliance on commercial inorganic fertilizers has raised significant environmental and economic concerns, including soil degradation, nutrient leaching, water pollution, and greenhouse gas emissions. This review critically evaluates biomass-derived chars produced via thermochemical processes, i.e., pyrolysis, gasification, and hydrothermal carbonization, as potential alternatives to synthetic fertilizers. Among the three biomass-derived chars, biochar stands out as the most viable option for soil amendment due to its high stability, nutrient retention capacity, and long-term carbon sequestration benefits. Gasification char, despite its high porosity and adsorption capacity, often lacks bioavailable nutrients, whereas hydrochar, though rich in organic compounds, poses challenges related to stability and phytotoxicity. Biochar application has been shown to significantly reduce N2O emissions, enhance soil water retention, and mitigate nutrient runoff, offering clear environmental advantages over conventional fertilizers. Moreover, biochar has transitioned from an experimental soil amendment to a commercially available product with increasing adoption in agriculture worldwide, further reinforcing its practical viability. However, large-scale implementation still faces economic and logistical constraints, including high production costs, transportation inefficiencies, and regulatory uncertainties. Addressing these challenges through policy incentives such as subsidies and carbon credits can enhance the economic feasibility of biochar production and application. Given these findings, this review focuses on biochar as the most practical and sustainable alternative to commercial inorganic fertilizers.