Effect of temperature and Na2CO3 catalyst on hydrothermal liquefaction of algae

Hydrothermal liquefaction (HTL) has been identified as an innovative technique to convert aquatic or wet biomass such as algae into biofuels. In this study, HTL was performed on three algae strains viz. Nannochloropsis, Pavlova and Isochrysis at three temperatures of 250, 300 and 350 °C, with and without using Na2CO3 as a catalyst and a holding time of 60 min. The effect of temperature on the HTL product yields and their properties were studied for both catalytic and non-catalytic HTL. Maximum bio-oil yield for non-catalytic (48.67 wt.%) and catalytic (47.05 wt.%) HTL was obtained at 350 °C from Nannochloropsis and Pavlova, respectively. Compared to non-catalytic HTL, Na2CO3 increased the bio-oil yield for high carbohydrate containing algae (Pavlova and Isochrysis) at higher temperatures (300 and 350 °C) whereas for high protein containing algae (Nannochloropsis) the yield was higher only at lower temperature (250 °C). Total acid number, pH, density, higher heating value (HHV), ash, moisture and elemental composition were measured for bio-oils produced. The bio-oil obtained had the HHV in the range of 32 to 37 MJ/kg, which was comparable to heavy crude oil. Proximate and ultimate analyses were performed to characterize solid residue, and aqueous fraction was analyzed for acidity, total organic carbon and total nitrogen.