Hydrochar from co-hydrothermal carbonization of sewage sludge and sunflower stover: Synergistic effects and combustion characteristics

This study carried out co-hydrothermal carbonization (Co-HTC) on sewage sludge (SS) and sunflower straw (SFS) at various ratios (1:0, 3:1, 1:1, 1:3, 0:1) to prepare hydrochar . The raw samples and produced hydrochar were characterized using ultimate analysis, proximate analysis, Brunauer-Emmett-Teller (BET), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD). The potential synergistic impact during the reaction process were analyzed. As the SFS ratio increased, the fuel ratio of hydrothermal carbons rose from 0.29 to 0.37, while the higher heating value (HHV) improved from 7.44 (MJ/kg) to 20.86 (MJ/kg). The co-hydrothermal synergistic effect resulted in enhanced the hydrochar yield, energy yield, carbon sequestration rate and organic retention rate. Compared to raw sample, the surface of hydrochar exhibits a rougher and more fragmented structure, with an increased number of microspheres and micropores, resulted in an increased specific surface area. Thermogravimetric analysis (TGA) demonstrated that, in comparison to HTC of sludge alone, the Co-HTC of SS with SFS enhances the ignition temperature and delays the completion of combustion. Therefore, Co-HTC with SFS is an effective method for converting SS into clean solid fuel for energy applications. • Co-HTC increased ignition temperature and delay combustion completion of hydrochar. • 12 types of reactions occurring during Co-HTC were identified. • Synergistic effect enhanced hydrochar yield, energy yield, carbon capture efficiency.