Hydrothermal liquefaction and in situ supercritical transesterification of algae paste

Abstract

In this manuscript we demonstrate and present the outcome of performing in situ supercritical transesterification (SCT) during hydrothermal liquefaction (HTL) of algal biomass at reaction conditions between 300–380 °C and reaction time (RT) of 5–30 min in the presence of water and solvent methanol. The biocrude obtained was fractioned into hexane soluble and methyl ester (ME) containing light crude (LC) and hexane insoluble heavy crude (HC). The gravimetric yield showed an increase in the LC content at increasing RT with ~50% of the biocrude composed of LC at severe condition. Elemental analysis confirmed deoxygenation and denitrogenation levels similar to HTL produced biocrude for the LC, whereas the HC contained elevated levels of both elements and formed an asphaltene/tar like product to give an energy density for the crudes in the range of 36–39.2 MJ kg⁻¹ and 29.5–34.5 MJ kg⁻¹, respectively. LC formed at 380 °C and 30 min RT had a boiling point distribution (BPD) similar to crude oil and was better than hydrotreated biocrude as deciphered by simulated distillation (SIMDIST) owing to the difference in bonds detected via Fourier transform infrared spectroscopy (FT-IR). ME formation was achieved within the first 10 min of the reaction after which its decomposition observed and subsequent kinetic parameters calculated assuming a first-order reaction. The fractionation of solvent (methanol) processed biocrude infers an alternative method of obtaining algal fuel, however the high nitrogen content of both crudes still persists, necessitating an additional denitrogenation step, albeit at a different extent.