Hydrothermal carbonization and liquefaction for sustainable production of hydrochar and aromatics

Sustainable biorefinery depends on the development of efficient processes to convert locally abundant, energy-rich renewable biomass into fuels, chemicals, and materials. Hydrothermal processing has emerged as an attractive approach for wet biomass conversion with less environmental burden. Although considerable efforts have been made in sustainable biorefinery by unitizing innovative technologies at a laboratory scale, its scaling-up is still impeded by the biomass heterogeneity. This article critically reviews the recent advances in hydrothermal carbonization and liquefaction technologies for the sustainable production of hydrochar and aromatics from different biomass wastes. Three main aspects, including lignocellulose-/lignin-rich feedstock, operating conditions, and design of liquid/solid catalysts, are critically reviewed and discussed to understand the reaction mechanisms and system designs for increasing the yields of aromatics and improving the properties of hydrochar. The latest knowledge and technological advances demonstrate the importance of identifying the physical and chemical properties of feedstock. The science-informed design of hydrothermal technology and optimization of operational parameters with reference to the biomass properties are crucial for the selective production of value-added chemicals and multifunctional hydrochar. This review identifies current limitations and offers original perspectives for advancing hydrothermal processing of biomass towards carbon-efficient resource utilization and circular economy in future applications.