Hydrothermal liquefaction of organosolv lignin to bio-oil

Abstract

Hydrothermal liquefaction (HTL), a thermochemical conversion process for the production of bio-oil from lignin, is described in this thesis. Bio-oil is considered to be a viable source of aromatic compounds as well as a general energy carrier. Nonetheless degradation of lignin during HTL is currently not fully understood due to the complexity and heterogeneity of lignin. This study aims to investigate HTL of lignin under subcritical water conditions (270 °C, 290 °C and 310 °C) and three time levels 10 min, 20 min and 30 min to identify the quantitative formation and qualitative composition of bio-oil. The isolated bio-oil fraction contained a mixture of low molar mass lignin degradation products. A general characterization of this fraction was accomplished by applying a set of analytical methods including Gel Permeation Chromatography, Photoacoustic Infrared spectra, the Folin Ciocalteu method, Karl Fischer titration and elemental analysis. The results from Gel Permeation Chromatography measurement indicated the formation of monomers, dimers and trimers (Mw from 260 to 310 g/mol). The carbon content of bio-oil was slightly higher (65.03%) and its oxygen content slightly lower (28.33%) than in the original lignin sample (C content 64.14% and O content 29.88%) as revealed by elemental analysis. Based on its elemental composition a higher heating value (27.98 kJ/g) for bio-oil than for organosolv lignin (26.33 kJ/g) was calculated, emphazing the potential of bio-oil for being a future energy carrier. The Folin Ciocalteu method indicated a coherency between increasing retention times of HTL and increasing phenolic contents in bio-oil (0.157 g GAE/ g bio-oil (10 min), 0.159 g GAE/g bio-oil (20 min) and 0.191 g GAE/ g bio-oil (30 min)), especially at moderate temperatures (290 °C), outlining bio-oil’s high potential as aromatic source for chemical industry. These achievements indicated a valorization of lignin occurring during hydrothermal liquefaction.