Investigation on hydrogen-rich syngas preparation from high wet sludge mixed with sawdust based on iron oxygen carrier

•The gasification of high wet sludge mixed with sawdust was explored based on Fe-based oxygen carrier.•The in-situ adsorption of carbon dioxide improved the quality hydrogen-rich syngas.•The role and route of water in sludge in gasification process was investigated. In this study, chemical looping gasification (CLG) of high wet sludge (HWS) mixed with sawdust was carried out based on red mud and Fe/Al composite oxygen carrier (OC) prepared by impregnation method. Further, the optimal gasification conditions in a fixed-bed reactor and the migration path of water from HWS were investigated for the preparation of hydrogen-rich syngas. Results show that the reaction temperature significantly affects gasification, and the higher temperature is conducive to the progress of sludge gasification. Addition of sawdust changes the volatile content of mixture fuel, thus affecting the composition of syngas. Utilization of appropriate amount of OC can promote the gasification rate. The syngas with calorific value of 11.16 MJ⋅Nm−3 was obtained under conditions including temperature of 800 °C, ratio of sludge to sawdust of 3:1, and O/C ratio of 1:10. Owing to the catalyst and adsorbent dual function, addition of CaO into bed material led to the significant increase in the H2/CO ratio of syngas; however, addition of excessive CaO affected the contact between sludge volatile and OC, resulting in the increase of CO2 fraction and the decrease of the calorific value of syngas. A proportion of the vapor released from HWS reacted with carbon and OC and water–gas shift reaction with CO in gas to produce more H2 and improved the syngas quality, which thus proves the advantages of wet sludge CLG to obtain higher energy yield than those of the dried sludge.