Acid-catalyzed co-hydrothermal carbonization of sewage sludge and mixed straws to produce high-quality solid fuel

To improve the hydrochar fuel quality, acid-catalyzed co-hydrothermal carbonization (co-HTC) of sewage sludge with mixed straws were performed at 220 °C. The impacts of mixing ratio (1:0, 1:3, 1:1, 3:1, and 0:1) and solvent (0.5 M HCl or dionized water) on hydrochar quality were assessed. The acid-catalyzed co-HTC increased the higher heating value and fuel ratio of the hydrochar by decreasing the H/C atomic ratio and increasing the O/C atomic ratio. The devolatilization index (D_i) and combustibility index (S) of the hydrochar from co-HTC at a sludge-to-straws dry-mass ratio of 1:1 reached 64.6 and 26.8 (10⁻⁸ wt%² min⁻² K⁻³), respectively, which was over three times and 2.5 times that from sludge. D_i was affected by the surface functional groups, whereas S was related to the chemical composition and fuel properties. Less energy is required after acid-catalyzed co-HTC for both pyrolysis and combustion processes, because of the increase in O/C atomic ratio. This study not only demonstrates the yields and quality of the hydrochar in response to the variation of mixing ratio and solvent, but also uncovers the relationship between hydrochar quality and pyrolysis/combustion performance, thereby providing valuable information for optimizing the acid-catalyzed co-HTC process.