Low-temperature catalytic conversion of alkaline sewage sludge bio-oil to biodiesel: Product characteristics and reaction mechanisms

Sewage sludge is a promising biomass alternative for the production of liquid biofuels. In this study, bio-oil derived from the fast pyrolysis of domestic sewage sludge was upgraded in a micro Robinson-Mahoney reactor at 4 MPa and 260 degrees C, and in the presence of hybridized Ni/HZSM-5 catalyst and ethanol. Fourier transform infrared and gas chromatography-mass spectrometry analyses were used to identify the functional groups and chemical components of raw and upgraded sewage sludge bio-oil. Results showed that catalytic upgrading reduced the relative content of acids, aldehydes, phenols, ketones, and nitrogenated compounds in the pyrolytic bio-oil. Thus, the main components of the upgraded bio-oil product included esters (48.59%), alkanes (10.12%), aromatics (9.38%), and alkenes (5.08%). Water and nitriles in the raw pyrolytic bio-oil together with in-situ-produced carbon oxides helped in the esterification of raw bio-oil. Reaction mechanisms such as nitrile hydrolysis, hydrogenation, and esterification were proposed to elucidate the conversion of the non-acidic pyrolytic bio-oil into ester-dominating upgraded bio-oil. Overall, upgrading sewage sludge pyrolytic oil is crucial to produce high-quality diesel-range biofuel. (C) 2020 Elsevier B.V. All rights reserved.