Catalyst-free production of alkyl esters from microalgae via combined wet in situ transesterification and hydrothermal liquefaction (iTHL)

Abstract

This study introduces a process combining wet in situ transesterification and hydrothermal liquefaction (iTHL) for fatty acid ethyl ester (FAEE) production from intact microalgae, Nannochloropsis gaditana without catalyst at temperatures higher than 160 °C. It is found that the chlorinated hydrocarbon solvents, SolvCl (dichloromethane, chloroform, and dichloroethane (DCE)), enhances the FAEE production by providing hydrogen chloride in an ionized form that can act as an acid catalyst. The SolvCl effect on iTHL is compared to acid catalyst assisted wet in situ transesterification. The most effective solvent is DCE with the FAEE selectivity in biocrude equal to 91.85% (maximum transesterifiable lipid basis). The optimum point for maximizing the FAEE yield is 185.08 °C with 4.69 mL ethanol and 1.98 mL DCE/g of dry algal cells based on the response surface methodology. iTHL with both Nannochloropsis and Chlorella species provides a possibility of the process applicable to the other algal species.