Co-hydrothermal carbonization of sewage sludge and coal slime with sulfuric acid for N, S doped hydrochar

In this study, co-hydrothermal carbonization (co-HTC) of sewage sludge(SS) and coal slime(CS) was studied for preparation of N, S doped hydrochar. The effect of blending ratios and temperature was also investigated. The results showed that there existed a synergistic effect during co-HTC, especially at the ratio of 7:3. Compared with the calculated values, the experimental values of the fixed carbon content increased by 51.00%, the carbon content increased by 21.73%, the mass yield increased by 13.52% and the carbon retention increased by 42.34%; while the H/C and O/C decreased by 32.20% and 31.97%, respectively. Furthermore, Raman spectrum and XPS analysis indicated that adding CS could promote the aromatization degree. XPS and FTIR analysis showed that the sulfuric acid could affect the occurrence mode of sulfur and nitrogen and led to abundant oxygen-containing functional groups such as –SO, O–CO and –OH on the hydrochar surface during the HTC process. This study could provide a theoretical basis for the utilization of SS and CS, and also provide a novel method for the preparation of N, S doped hydrochar. [Display omitted] •The synergistic effect is most significant at the ratio of 7:3 during the co-HTC.•Coal slime can promote aromatization and improve the properties of the hydrochar.•Sulfuric acid could make hydrochar retain more oxygen-containing functional groups.•The N, S doped hydrochar is promising for clean utilization of sewage sludge and coal slime.