Char and tar formation during hydrothermal gasification of dewatered sewage sludge in subcritical and supercritical water: Influence of reaction parameters and lumped reaction kinetics

Abstract

The hydrothermal gasification of dewatered sewage sludge converts waste into syngas fuel (gas containing H₂, CH₄, CO, and CO₂), but the formation of char and tar by a side reaction restricts practical applications. In this study, the formation of char/tar by the hydrothermal gasification of dewatered sewage sludge at 300-400 °C for 5-90 min with a heating rate of 8-50 °C/min is investigated. The results showed that the formation of tar/char reached equilibrium after 30 min, and an increase in the temperature and heating rate reduced the char yield. However, a higher tar yield was achieved at the highest temperature. Experiments on sludge with different moisture contents were also conducted, and the formation of char/tar was inhibited at a higher moisture content. The addition of NaOH and H₂O₂ can effectively reduce the char and tar yields, respectively. A lumped kinetic model for describing carbon conversion during the hydrothermal gasification of sewage sludge was developed to elucidate the char/tar formation mechanism. The experimental data were used to determine the model parameters, and the fitting results showed that solid-solid conversion from organic matter in sludge was the predominant char/tar formation pathway, with an activation energy of 7.1 kJ/mol. The experiments using the model compounds indicated that humus and protein are the main precursors of char and tar, respectively. This study provides insights for understanding the formation of char/tar from sewage sludge following hydrothermal gasification, which is vital for controlling the formation of char and tar to improve the gasification efficiency.

Keywords: Char; Dewatered sewage sludge; Hydrothermal gasification; Kinetics; Model compound; Tar.