Hydrothermal liquefaction of microalgae for the production of bio-crude oil

Due to the depletion of fossil fuels and climate change, extensive research has performed towards renewable energy production from microalgal biomass. Microalgae have several inherent benefits, such as high photosynthetic efficiencies, fast growth rates, and high lipid contents. In addition, microalgae can be cultivated in the non-arable lands (e.g., saline and waste water), thereby no competition with food crops production. Transesterification is the one of commonly used technologies for converting microalgae into liquid bio-fuels (i.e., biodiesel). Normally, an energy-intensive drying step is required in the transesterification treatment, which accounts for nearly half of the energy input. Hence, if “wet” microalgal biomass can be directly used as the feedstock for bio-fuel production without dewatering/drying, the overall economy of microalgae-to-fuel conversion can be greatly improved. In this thesis work, an alternative pre-treatment by pre-cooled NaOH/urea solution was investigated for microalgae. Subsequently, the pre-treated microalgae were applied as the feedstock for producing bio-crude oil. In addition, hydrothermal liquefaction (HTL) of microalgae was carried out in water, alcohol or water-alcohol mixed solvents under various reaction conditions with aims to improve bio-crude oil yield and quality. More noteworthily, the feasibility of recycling aqueous by-product from HTL of microalgae as a reaction medium for bio-crude oil production was explored, which yields great significance for the large-scale HTL applications. Furthermore, the co-processing of microalgae with other lignocellulosic biomass (i.e., aspen sawdust) was performed in ethanol-water co-solvents, and this work demonstrated the synergistic interactions between microalgae/sawdust and ethanol/water in the co-liquefaction process