Microwave pyrolysis of sewage sludge for bio-oil production: Effects of organic components and mechanisms

The microwave pyrolysis (MWP) of sewage sludge (SS) was conducted to investigate the impact of the organic composition of SS on the yield and composition of the derived bio-oil. The experiments were conducted in a microwave oven at 900 °C with a heating rate of 50 °C/min and achieved the product yield of 43.10 ± 2.23% bio-oil, 48.07 ± 1.26% bio-char, 8.83 ± 1.65% bio-gas. The chemical composition of bio-oil was investigated using gas chromatography–mass spectrometry and 145 species were identified. Protein and lipid contents in SS are the primary source of bio-oil yield, while bio-gas are predominantly derived from lignocellulosic materials. The unique non-thermal effects of microwaves can facilitate the ring-opening of small cycloalkanes to form straight olefins through hydrogen transfer reactions. Additionally, they can promote aldol condensation reactions, Pinacol rearrangements, and methoxy cleavage to form phenolic and aromatic structures with methyl groups. Furthermore, microwaves can aid in the dehydration, condensation, and cyclization reactions of amino acids to produce N-heterocycles while also facilitating lipid depolymerization into fragments for Diels–Alder cyclization. The results of this study will be beneficial for deeply understanding reactant characteristics and the reaction process during the MWP of SS.