Effect of FeCl3 combined with biochar as dewatering conditioners on sludge pyrolysis products

Owing to the rapid development of cities, sewage treatment plants produce an abundance of sewage sludge (SS). As a new sludge treatment technology, pyrolysis has been widely popularized in the disposal of SS. Sludge must be dewatered before pyrolysis, and FeCl3 combined with biochar has been shown to effectively improve SS dewatering performance in previous studies; however, its influence on subsequent SS pyrolysis disposal has not been studied. Based on the sludge conditioning-dewatering-pyrolysis system process, the effect and mechanism of the SS dewatering process on subsequent sludge pyrolysis disposal were elucidated. In this study, the combined effects of FeCl3 and biochar (FB) on sludge pyrolysis products were investigated using thermogravimetric mass spectrometry (TG-MS) and pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS). Additionally, the mechanism by which FeCl3 and biochar functioned are separately explained. Results showed that FB reduced the release of C/H/O-containing gases to a certain extent, which was mainly related to the biochar conditioner, promoted the S-containing gases, and reduced the release of N- and Cl-containing gases. Thus, FB is beneficial for improving the calorific value and chemical stability of bio-oil, in which the FeCl3 conditioner played a dominant role. Additionally, FB also moderately reduced the formation of N/S/Cl-containing compounds, mainly through the synergistic action of FeCl3 and biochar. Particularly, FB has the catalytic ability to remove tar. Several reaction kinetics models fitted using the Coats−Redfern method showed that FB was beneficial for reducing the energy required during pyrolysis. The results have important guiding significance to SS pyrolysis for the selection of sludge dewatering conditioners.