Thermochemical decomposition of sewage sludge in CO2 and N2 atmosphere Original Research Article

In this study, the effect of CO2 on the thermal conversion of sewage sludge was investigated by means of the thermogravimetric analysis and the batch-type thermal process. The results showed that the kinetics of sewage sludge during thermal treatment under both N2 and CO2 atmospheres are quite similar and can be described by a pseudo bi-component separated state model (PBSM). It was, however, noticed that under CO2 atmosphere, the first reaction was significantly accelerated whereas the secondary reaction temperature was shifted to a lower temperature. The apparent activation energies for the first decomposition reaction under both N2 and CO2 atmosphere, corresponding to the main decomposition typically at 305 °C were similarly attained at ca. 72 kJ mol−1, while that of the second decomposition reaction was found to decrease from 154 to 104 kJ mol−1 under CO2 atmosphere. The typical reaction order of the decomposition under both N2 and CO2 atmosphere was in the range of 1.0–1.5. The solid yield was slightly reduced while the gas and liquid yields were somewhat improved in the presence of CO2. Furthermore, CO2 was found to influence the liquid product by increasing the oxygenated compounds and lessening the aliphatic compounds through the insertion of CO2 to the unsaturated compounds resulting in the carboxylics and the ketones formation.