Optimized Hydrothermal Liquefaction for High- and Low-Lipid Algae

Feng Cheng, Zheng Cui, Travis Le-Doux, Kwonit Mallick, Graham Hoffman, Jacqueline Jarvis, Neil Paz, Tanner Schaub, Nagamany Nirmalakhandan, Catherine E. Brewer*

Abstract

Third-generation biofuels produced from hydrothermal liquefaction (HTL) of algae have been developing rapidly. Some advantages of algae-derived oils via hydrothermal liquefaction are attributed to high algae growth rates, strong CO₂-mitigation potential, and avoidance of feedstock drying requirements. In HTL, subcritical water (270-370°C and 60-210bar) depolymerizes lipids, proteins and carbohydrates in algae, leading to high yield of bio-crude oil, which can be upgraded to transportation fuels. In this study, we delineate biocrude composition as influenced by operating condition change for temperatures of 310-350°C, residence time of 5-60min, and solid algae content of 5-10wt.%) for both the high-lipid microalgae Nannochloropsis salina and the low-lipid microalgae Galdieria sulphuraria. The bio-crudes are characterized by Fourier transform ion cyclotron resonance mass spectroscopy (FT-ICR MS), fatty acid methyl ester (FAME) analysis by gas chromatography mass spectroscopy (GC/MS), oxy-combustion calorimetry, and elemental analysis (CHNS). These results are used to optimize species-specific operating conditions for a 1.8 L batch reactor and to predict optimized conditions for new algae strains based on their feedstock compositions and knowledge of compound degradation pathways. In addition, bio-crude oil characterization results will be used to select initial conditions for a pilot-scale continuous flow HTL reactor, designed to produce char-free biocrude oil from low-solid-content feedstock without centrifugation.