Sub-Supercritical Hydrothermal Liquefaction of Lignocellulose and Protein-Containing Biomass

Hydrothermal liquefaction (HTL) is an emerging technology for bio-crude production but faces challenges in determining the optimal temperature for feedstocks depending on the process mode. In this study, three feedstocks-wood, microalgae spirulina (Algae Sp.), and hydrolysis lignin were tested for sub-supercritical HTL at 350 and 400 degrees C through six batch-scale experiments. An alkali catalyst (K2CO3) was used with wood and hydrolysis lignin, while e (Algae Sp.) was liquefied without catalyst. Further, two experiments were conducted on wood in a Continuous Stirred Tank Reactor (CSTR) at 350 and 400 degrees C which provided a batch versus continuous comparison. Results showed Algae Sp. had higher bio-crude yields, followed by wood and lignin. The subcritical temperature of 350 degrees C yielded more biocrude from all feedstocks than the supercritical range. At 400 degrees C, a significant change occurred in lignin, with the maximum percentage of solids. Additionally, the supercritical state gave higher values for Higher Heating Values (HHVs) and a greater amount of volatile matter in bio-crude. Gas Chromatography and Mass Spectrometry (GCMS) analysis revealed that phenols dominated the composition of bio-crude derived from wood and hydrolysis lignin, whereas Algae Sp. bio-crude exhibited higher percentages of N-heterocycles and amides. The aqueous phase analysis showed a Total Organic Carbon (TOC) range from 7 to 22 g/L, with Algae Sp. displaying a higher Total Nitrogen (TN) content, ranging from 11 to 13 g/L. The pH levels of all samples were consistently within the alkaline range, except for Wood Cont. 350. In a broader perspective, the subcritical temperature range proved to be advantageous for enhancing bio-crude yield, while the supercritical state improved the quality of the bio-crude.