Aging stability of bio-oil produced from dewatered sewage sludge in subcritical water

In this study, the stability of bio-oil produced from the hydrothermal liquefaction of dewatered sewage sludge was examined under room-temperature and accelerated-aging conditions at 80 °C over 6 months. No significant change was observed in the fluidity, viscosity, or molecular weight of the room-temperature bio-oil samples. The accelerated-aged bio-oil samples maintained their fluidity for up to 1 week, and substantial increases in the viscosity and molecular weight were observed in the samples aged for more than 1 month. Additionally, the decomposition behaviors of the room-temperature-aged samples were considerably similar to that of fresh bio-oil, whereas the low-temperature decomposition peak at 86 °C progressively decreased and shifted toward the high-temperature direction with increasing accelerated-aging time. The composition analyses of the aged bio-oil samples indicated that low-molecular-weight species, such as ethylbenzene, 1H-indole, 3-methyl-1H-Indole, cetene, 3-hexadecene, 1-decene, 2,4,6-trimethyl-benzonitrile, and 2,5-dimethyl-pyrazine, persisted during room-temperature aging, but disappeared as the accelerated-aging period increased.