Recent Advances in Hydrothermal Carbonization of Biomass: The Role of Process Parameters and the Applications of Hydrochar

Biomass is a resource whose organic carbon is formed from atmospheric carbon dioxide. It has numerous characteristics such as low carbon emissions, renewability, and environmental friendliness. The efficient utilization of biomass plays a significant role in promoting the development of clean energy, alleviating environmental pressures, and achieving carbon neutrality goals. Among the numerous processing technologies of biomass, hydrothermal carbonization (HTC) is a promising thermochemical process that can decompose and convert biomass into hydrochar under relatively mild conditions of approximately 180°C–300°C, thereby enabling its efficient resource utilization. In addition, HTC can directly process feedstocks with high moisture content without the need for high-temperature drying, resulting in lower energy consumption. Based on a systematic analysis of the critical articles mainly published in 2011–2025 related to biomass, HTC, and hydrochar applications, in this review, the category of biomass was first classified and the chemical compositions were summarized. Then, the main chemical reaction pathways involved in biomass decomposition and transformation during the HTC process were introduced. Meanwhile, the roles of key process parameters, including reaction temperature, residence time, pH, feedstock type, pressure, mass ratio of biomass to water, and the use of catalysts on HTC, were carefully discussed. Finally, the applications of hydrochar in energy utilization, environmental remediation, soil improvement, adsorbent, microbial fermentation, and phosphorus recovery fields were highlighted. The future directions of the HTC process were also provided, which would respond to climate change by promoting the development of the sustainable carbon materials field.