Insights into the structure evolution of sewage sludge during hydrothermal carbonization under different temperatures

In this paper, hydrothermal carbonization (HTC) of sewage sludge (SS) under different temperatures was investigated, and the effect of the HTC temperature on the structure evolution and properties of the hydrochar was studied. Here, BET, XPS, Raman and the peak separation and fitting of FTIR were used to analyze the chemical structures of hydrochar during HTC comprehensively. The results indicated that the specific surface area of the hydrochar increased and then decreased with the temperature increasing, and the highest specific surface area was got for hydrochar obtained at 200 ℃, which was 18.30 m2/g. Furthermore, the results showed that raising the temperature could decrease the O/C and H/C ratio of the hydrochars, and the decrease of O/C was mainly attributed to the reduction of CO and COOR rather than C-O during the HTC process. Interestingly, the results also showed that the short chain aliphatic or aromatic substituents ring in SS began to breakage at 160 ℃ and the longer aliphatic chain broke when the temperature was higher than 200 ℃. Increasing the temperature could promote the transformation of the relatively small aromatic ring system to the larger aromatic ring system in hydrochar. The results also showed that the aliphatic carbon content decreased and the degree of aromatization increased with temperature increasing. This work may provide comprehensive structural information of SS during the HTC process, and provide a theoretical basis for the application of SS and its hydrochars.