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 HTC wastewater (HW) on the formation routes and the characteristics of hydrochar has yet to be clarified. In this work, five representative organic components, including 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 organic wastewater environment were proposed. The results showed that the organic components in the wastewater source changed the degradation and carbonization reactions of the biomass and therefore influenced the physicochemical characteristics of hydrochar. 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 of the hydrochar mass yield (HCY) by 1.13%−7.07% and the increase of higher heating value (HHV) of hydrochar to 27.36 MJ/kg. By contrast, the acetic acid and 2,5-hexanedione contained in the wastewater source resulted in a decrease of the HCY, probably due to the catalytic effects of volatile fatty acids on the hydrolysis of the biomass. Moreover, wastewater sources with all the five organic components could increase the energy recovery efficiency from biomass to a high level of 97.28% under the two effects of deep polymerization and catalytic carbonization