Ultrasonic pretreatment for low-temperature hydrothermal liquefaction of microalgae: enhancing the bio-oil yield and heating value

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 A degrees 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 A degrees 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.