Comparative analysis of bio-oil quality from macroalgae via microwave and ultrasonic pretreatment subjected to hydrothermal liquefaction.

Hydrothermal liquefaction (HTL) offers a sustainable method for producing bio-oil from algal biomass. Evaluating the microwave- and ultrasonic-pretreatment HTL and examining the quality of bio-oil obtained from HTL of marine macroalgae, namely Sargassum and Caulerpa, are the main objectives and novelty behind this work. Algal feedstocks were pretreated by microwave (500 W and 600 W) or ultrasonic (50 W and 75 W) before heating up to 350 oC for 35 min in the HTL reactor. Results showed that Sargassum and Caulerpa displayed completely different behaviors in pretreatment. The bio-oil yield from Sargassum went up by increasing microwave and ultrasonic pretreatment power. Although the microwave-assisted HTL of Caulerpa yielded more bio-oil than ultrasonication, it did not achieve the bio-oil production levels of the conventional HTL process. The highest yield and HHV of bio-oil were obtained for 22.4 wt% and 35.56 MJ/kg for Sargassum with microwave pretreatment, while the pretreatment power had a negative effect on those from Caulerpa by decreasing quantity and quality. It was also found that 56.41% of the bio-oil components were acids for Caulerpa, and 34.6% were ketones for Sargassum under conventional HTL. Thermal weight loss analysis showed that 70.4% of the bio-oil derived from Sargassum obtained by HTL following ultrasonic pretreatment possesses a boiling point below 243 °C which suggests the higher quality of the bio-oil derived from Sargassum with a significant proportion of lighter compounds. The results indicate that the bio-oil obtained from Sargassum had a higher quantity and quality than that for Caulerpa. Besides, the carbon recovery of bio-oil indicated the highest amount of carbon (70.12 wt%) was achieved through the microwave pretreatment of Sargassum. Also, differences in the cellular structure of the raw materials resulted in different quality and quantity of bio-oil.