Ultrasonic pretreatment for low-temperature hydrothermal liquefaction of microalgae: enhancing the bio-oil yield and heating value
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We investigated the effect of ultrasonic pretreatment on the bio-oil yield and heating value in the low-temperature hydrothermal liquefaction (HTL) of microalgae. HTL is one of the thermochemical processes for bio-oil production. However, the high pressure of the process is one of the main challenges for commercialization. On the other hand, a decrease in the HTL pressure, and consequently a decrease in the temperature, results in a decrease in the bio-oil yield. In this work, we investigated a new method to increase the bio-oil yield at low pressures and temperatures. The microalgae (Nannochloropsis sp.) were first pretreated by ultrasonic waves for 30, 60, and 90 s at 100 W. After then, the bio-oil was produced using HTL at 210, 230, and 250 °C. According to the results, using ultrasonic-assisted HTL increased the bio-oil yield up to the maximum of 28.9% (90-s sonication time at 250 °C). Moreover, applying ultrasonic pretreatment resulted in a decrease in oxygen content of the bio-oil and consequently an increase in its heating value. However, the average nitrogen content did not change dramatically by using ultrasonic-assisted hydrothermal liquefaction.
KeywordsBio-oil Microalgae Hydrothermal liquefaction Sonication Energy efficiency
We would like to show our gratitude to Professor Serizawa (Department of Chemical Science and Engineering, Tokyo Institute of Technology) for providing Sonicator equipment.