Hydrothermal liquefaction of residual microalgae biomass after pulsed electric field-assisted valuables extraction

Abstract

Efficient extraction of valuable products from microalgae and utilization of the residual biomass for biofuel production appear to bring economic benefits for the microalgae biorefineries. The pulsed electric field (PEF) method has been proposed as a promising pre-treatment for microalgae wet extraction. For conversion of microalgae biomass and residues, hydrothermal liquefaction (HTL) is considered a suitable method. Here, a combination of PEF-assisted valuables extraction of microalgae and HTL of the residual biomass is studied for the first time. The microalgae Auxenochlorella protothecoides, Chlorella vulgaris, and Scenedesmus almeriensis were cultivated, harvested, treated by PEF, and then subjected to lipid extraction, protein extraction, or extraction of amino acids after enzymatic protein hydrolysis. The residual biomass obtained from PEF-assisted valuables extraction was subjected to HTL in micro autoclaves at a temperature of 350 °C and pressure of 25 MPa for 15 min holding time. Product yields and analytical results obtained by elemental content measurement, 1H-nuclear magnetic resonance spectroscopy, Fourier-transform infrared spectroscopy, and gel permeation chromatography show that PEF alone has no significant direct influence on microalgae HTL. The harsh HTL conditions decisively determine the product yields and biocrude quality. However, PEF enhances the lipid extraction yield from 4 wt% to 33 wt%. Consequently, the biocrude yield decreases from 58 wt% to 43.2 wt%. Moreover, PEF boosts the protein extraction yield from almost zero to 41.6 wt% of the total protein, resulting in an increased biocrude yield by about 2 wt%. Finally, PEF accelerates the formation of amino acids by enzymatic hydrolysis, improving the extraction efficiency up to 150% in the first 60 min of the extraction. The extracted residue promises to produce a biocrude yield higher by 6 wt% and a better biocrude quality with lower nitrogen content. An overall mass balance of PEF-assisted valuables extraction and HTL is presented.