Production of solid fuels by hydrothermal treatment of wastes of biomass, plastic, and biomass/plastic mixtures: A review

Traditional disposal methods of biomass and plastic waste, such as landfill, combustion, and compost, no longer meet the requirements of carbon reduction, carbon neutrality, and sustainable society due to low utilization efficiency and severe pollution. As a green, efficient and environmentally-friendly method, hydrothermal technology has been paid much attention to and has already been applied to recycle or reuse various plastic and biomass wastes. No matter for the single or mixed type of waste, it is expected to achieve efficient recycling and obtain value-added products through the hydrothermal process. This review summarized the basic knowledge of hydrothermal technology and the possible reaction mechanism of biomass and plastics under hydrothermal conditions and listed the previous reports on the application of hydrothermal technology for converting wastes of biomass, plastic, and biomass/plastic mixtures to solid fuels. Moreover, regarding the future of hydrothermal technology, four points related to reaction mechanism, synergistic effect, catalysis, and scaled-up application, were provided for consideration.