Hydrothermal-Controlled Conversion of Black Liquor Acid Sediment Directly to Phenolics

This study explored the direct conversion of black liquor acid sediment (BLAS) into phenolics by hydrothermolysis. Experiments were successfully performed in a batch-type reactor at temperature of 260 to ∼340 °C over 0–120 min. Depending on the reaction conditions, four products (bio-oil phase containing most of the phenolics, aqueous phase, char, and a small amount of gas) were formed and analyzed by element analysis, Fourier transform infrared spectroscopy (FT-IR), gas chromatography mass spectrometry (GC-MS), gas chromatography (GC), and pyrolysis-gas chromatography mass spectrometry (Py-GC/MS). The total bio-oil yield was 16–26 wt % based on dry BLAS (D-BLAS), in which the main monomeric compounds were 2-methoxyphenol, 2, 6-dimethoxyphenol, and catechol, with yields of 0.74–1.66, 0.28–4.03, and 0–7.08 wt % based on lignin in the BLAS (BLAS-L), respectively. Analysis of char suggested BLAS-L occurred by hydrothermolytic depolymerization to give aryl-ether linkages (β-O-4 and α-O-4 bonds), hydroxyl and carbonyl groups, and aromatic rings. Finally, the reactions were almost complete because the char had no functional groups of lignin at the optimal reaction condition (300 °C and 30 min).