Research progress of modification method and application of biomass hydrochar

Hydrothermal carbonization is an innovation technology to realize the resource utilization of biomass. It has the advantages of directly treating wet biomass, low reaction temperature, short reaction time and obvious reduction effect. For its well-developed pore structure, rich oxygen-containing functional group, large specific surface area and high heating value, the hydrochar has been widely applied in the fields of environment, energy, electrochemistry and catalysis. Reaction conditions and reaction processes of hydrothermal carbonization technology were briefly described. Conventional modification methods such as the physical activation and chemical activation of hydrochar were also introduced. In particular, the effects of new modification methods, including non-thermal plasma, anaerobic fermentation, magnetization, solid-state microwave, polyethyleneimine and heteroatom doping, on the structure and surface functional groups of hydrochar were emphatically analyzed. The application progresses of hydrochar as adsorbents for heavy metal, organic matter and CO₂, soil amendment, solid fuel, electrode material and catalysts were also reviewed. As adsorbents, hydrochar can remove 82%−99% of heavy metal elements (Cr, Cu, Zn, etc.) from wastewater and adsorb 1.2−8.3 mmol/g of CO₂. As soil amendments, adding hydrochar can reduce the amount of nitrogen fertilizer. As solid fuels, the heating value of hydrochar produced from lignocellulose is as high as 30 MJ/kg. This value is close to the heating value of bituminous coal. As electrode materials, by improving the pore size distribution and increasing the number of oxygen- and nitrogen-containing functional groups, the electrochemical performance of hydrochar can be greatly improved, and the specific capacitance of hydrochar can reach more than 300 F/g. In the field of catalysis, hydrochar can serve as a catalyst or as a catalyst support for active components. Finally, it is pointed out that the future research should focus on the composite modification, the recycling and utilization of hydrochar, and its toxic mechanism to plants and soil microorganisms. Its combustion performance, specific capacitance, energy density and stability should be further improved to realize the commercial application of hydrochar.