Effect of dewatering conditioners on pollutants with nitrogen, sulfur, and chlorine releasing characteristics during sewage sludge pyrolysis

Abstract

Sewage sludge is a harmful solid by-product generated during sewage treatment, and pyrolysis is a technology adopted for its disposal. To reduce the energy consumption during pyrolysis, the sludge is subjected to pretreatments, such as conditioning and dewatering, to reduce the water content. However, the mechanism by which these conditioners affect the pyrolysis products during the pretreatment process has not yet been elucidated. In this study, X-ray photoelectron spectroscopy (XPS) was used to analyze the chemical state changes of N, S, and Cl pollutants in the sludge samples with and without the conditioner before and after pyrolysis. Thermogravimetric-mass spectrometry (TG-MS) and pyrolysis–gas chromatography/mass spectrometry (Py-GC/MS) were used to clarify the release characteristics of condensable and non-condensable gases during the pyrolysis of the sludge samples. The results showed that Fe produced more free radicals of •H and •CH3 to combined with •S• and •SH to form H2S and CH3SH, and promoted the decomposition of macromolecular substances. Moreover, different anion atmospheres caused various pollutant emission effects. The Cl− from the conditioner promoted the release of micromolecular N/Cl-containing pollutants. The SO42− enhanced the release of CH3SH and SO2 but inhibited the emission of N-containing pollutants. Several reaction kinetic models fitted by the Coats-Redfern method showed that the Fe-containing sludge exhibited lower activation energy than the original sludge. A comprehensive analysis of the response of conditioners in the sludge pyrolysis process can provide a sustainable and improved understanding of the integrated treatment and disposal of sludge via conditioning, dewatering, and pyrolysis.