Influence mechanism of aqueous organic components on the hydrochar formation reaction during the biomass hydrothermal carbonization wastewater recycling

Reusing the wastewater byproduct of the hydrothermal carbonization (HTC) process as a water source to prepare biomass slurry feedstock for the next HTC run provides a feasible wastewater disposal method. However, the influence of various organic components contained in the hydrothermal wastewater (HW) on the formation routes and characteristics of hydrochar is yet to be clarified. In this study, five representative organic components, phenol, acetic acid, furfural, 5-hydroxymethylfurfural (5-HMF), and 2,5-hexanedione, were selected to study their effects on the hydrochar fuel characteristics and HW components, based on which the hydrochar formation pathways in an organic wastewater environment were proposed. The phenol, furfural, and 5-HMF contained in the wastewater source could polymerize with the active intermediates from biomass degradation, thereby resulting in an increase in the hydrochar mass yield (HCY) by 1.13%–7.07% and an increase in the higher heating value of hydrochar to 27.36 MJ/kg. In contrast, the acetic acid and 2,5-hexanedione contained in the wastewater source resulted in a decrease in HCY, probably because of the catalytic effects of volatile fatty acids on biomass hydrolysis. Moreover, wastewater sources with all five organic components increased the energy recovery efficiency from biomass to a high level of 97.28% under the two effects of deep polymerization and catalytic carbonization.