The effect of ethanol as a homogeneous catalyst on the reaction kinetics of hydrothermal liquefaction of lipids

Abstract Promoting renewable sources of energy and waste management is essential to face environmental challenges such as global warming, and to ensure long-term fuel security also crucial for sustainable development. Hydrothermal liquefaction (HTL) of wet biomass is an emergent and promising waste management technology that can produce renewable crude from wet biomass and bio-waste. However, HTL crude properties are not close enough to fuel-oil like products. Issues such as high viscosity, instability, high acidity, nitrogen content, low energy density, and low crude yields limit its application. Lipids as a key component of biomass contribute with a high percentage of low boiling point fractions in the crude phase. However, lipids increase acidity because of the hydrolysis into free fatty acids (FFA), which cause considerable issues to the oil industry. In the present study, the functional groups' reaction kinetics for the HTL conversion of lipids is elucidated using FT-IR. Furthermore, ethanol under acid pH is integrated as a co-solvent and homogeneous catalyst to promote the alcoholysis of lipids and FFA, to reduce crude oil acidity. Reaction kinetics are used to define composition and equilibrium differences while boiling point distribution and total acid number (TAN) validate conversion and quality improvement of the renewable crude. The reported experiments were conducted in a batch reactor with 20% mass feedstock at 250 °C, 300 °C and 350 °C, using sunflower oil as a model lipid. The activation energy for the hydrolysis of sunflower oil into FFA under HTL conditions is 107.8 kJ/mol. When ethanol and acetic acid were included, the hydrolysis, transesterification, and esterification activation energies are 134.2, 74.1, and 9.1 kJ/mol, with a degree of alcoholysis of around 30% near-equilibrium conditions.