Hydrothermal processing of macroalgae for producing crude bio-oil

In this study, a green marine macroalga-Enteromorpha prolifera was hydrothermally treated by using a fast-heating rate batch reactor over different temperatures (250-390 °C), retention times (10-120 min), algae/water mass ratios (0.5/8.4-5.5/8.4), and K₂CO₃ loadings (0-30 wt.%). The yields of products (crude bio-oil (CBO), solid residue, gas, and water-soluble products) were affected by all the operating variables. Under conditions of an algae/water mass ratio of 3.5/8.4 and K₂CO₃ loading of 20 wt.%, the highest CBO yield of 34.7 wt.% on dry ash-free basis was achieved at 370 °C for 60 min. The C and H contents of the CBO are much higher than those of the algal biomass feedstock, whereas the O content in the CBO was significantly reduced to 6.9 wt.% (vs. 19.3 wt.% in the feedstock). The typical calorific value of the CBO was estimated at around ca. 39.4 kJ/g. About 48% of the chemical energy in the macroalga biomass could be recovered in the CBO fraction. This study demonstrates the feasibility of applying hydrothermal liquefaction to produce CBO from macroalgae with high ash content.