Combination of hydrothermal carbonization and oxy-fuel combustion process for sewage sludge treatment: Combustion characteristics and kinetics analysis

Abstract

Hydrothermal carbonization (HTC) and oxy-fuel combustion were combined for sewage sludge (SS) disposal for the first time. The combustion characteristics of HTC pretreated SS (hydrochar) under air as well as oxy-fuel combustion atmosphere were investigated by thermal-gravimetric analyzer. On the basis of combustion experiments, the kinetics parameters including energy activation, pre-exponential factor and reaction order were evaluated by Flynn–Wall–Ozawa (FWO) model and Avrami’s theory. The results showed that the hydrochars had higher ignition temperature, higher ignition index and narrower combustion stage than SS, which implied more stable combustion behaviors of the hydrochars than SS. The lower activation energy of the hydrochar obtained from HTC at the temperature of 200 °C demonstrated that the hydrochars were more easily combusted in comparison to SS under identical oxy-fuel conditions. In addition, the ash problems of the slagging and fouling were mitigated by HTC treatment. In oxy-fuel atmosphere, the burnout temperature of the hydrochars was lower than that in air. More importantly, the dewaterability of SS was substantially improved by HTC and more than 30% of drying energy consumption was saved prior to combustion. This study indicated that the combination of HTC treatment and oxy-fuel combustion offered a promising alternative for SS disposal with the benefits of energy consumption and energy recovery.