Residual carbon from coal gasification fine slag for inducing rice straw hydrothermal carbonization to achieve improved reactivity and wastewater decontamination

The large-scale resource utilization of coal gasification fine slag (CGFS) and the decontamination of wastewater through the hydrothermal carbonization (HTC) process of biomass are crucial for the production of clean fuels. In this study, the residual carbon in CGFS was recovered by froth flotation, and the HTC of rice straw was carried out with different addition amounts of residual carbon and hydrothermal temperatures. The physicochemical structure and fuel properties of hydrochar as well as water quality indicators of hydrothermal wastewater were carefully explored. The results showed that the combustion reactivity of residual carbon was significantly improved, implying an obvious synergistic effect after inducing HTC of rice straw. In addition, the wastewater produced by rice straw HTC was dark brown and contained high concentration of complex organic substances. The addition of residual carbon significantly improved the water quality of hydrothermal wastewater by removing organic matter. The abundant pore structure of the residual carbon can adsorb the organic molecules in the aqueous phase and induce their polymerization and conversion into hydrochar, thereby reducing the concentration of organic compounds in the wastewater and improving the water quality. At the same time, it can also adsorb potassium metal ion dissolved in the aqueous phase by rice straw. Hydrochar formed on the surface of residual carbon has lower aromatic condensation degree and more carbon defects, which can provide more reaction sites. In addition, the alkali metal adsorbed on residual carbon has a catalytic effect. All the above factors contribute to the synergistic effect of combustion reaction.