Low-temperature pyrolysis of sewage sludge and putrescible garbage for fuel oil production

Low-temperature pyrolysis of putrescible garbage (PG) and mixtures of PG with sewage sludge (SS) was investigated with the aim to study the mechanisms and product distribution and to identify optimum process conditions for maximising the fuel oil yield. Experiments were performed in two rotary kiln type reactors in series, where the reaction temperature between 400 and 550 °C and solid retention time of 20 and 60 min were employed. Maximum oil yields were obtained at the highest temperature and lowest time (ca. 550 °C and 20 min) for all the different types of feedstock studied. High temperature and short solid retention time favoured the production of oil while high temperatures and longer retention times reduced the viscosity of the oil. Oils produced from pyrolysis of PG were found to have relatively higher viscosity compared to that from the sewage sludge. The compositions of the different oils were analysed with a GC-MS and 13C NMR, enabling the different types of bonds and organic functional groups in the fuel oils to be identified. The species detected in oils from both PG and SS were found to be rich in oxygenated aromatics, however, the arrangement and type of oxygen in the structures were different between the oils. Ash resulting from the combustion of the sewage sludge was also blended into the mixtures of PG and SS to determine the effect of ash on the viscosity of oil and was found to have a significant promoting effect on the oil viscosity.