Comparison of the Chemical Composition of Liquids from the Pyrolysis and Hydrothermal Liquefaction of Lignocellulosic Materials

Abstract

Although major differences in thermal stability and hydrotreatment behavior of hydrothermal liquefaction (HTL) and pyrolysis oils have been reported, little is known about the variations in the chemical composition of these oils that could explain such differences. Two commercial softwood pyrolysis oils [Pyrovac and Biomass Technology Group (BTG)] and their water-soluble (WS) and water-insoluble (WIS) fractions were analyzed and compared with the aqueous (WSWD-57) and oily (WISWS-57) fractions obtained from HTL of Douglas-fir. The samples were characterized by GC-MS, Karl Fischer titration, carbonyl content, total acid number, elemental composition, calorific value, proximate analysis, Fourier transform infrared spectroscopy, Folin-Ciocalteu (FC), and UV fluorescence. Fourier transform ion cyclotron resonance mass spectrometry (ICR-MS) and electrospray ionization mass spectrometry (ESI-MS) were also used to analyze the fractions. The most prevalent class of compounds in the WIS phases was phenols. WS phases contained mainly oxygenated compounds derived from cellulose and hemicellulose and were richer in the carbonyl functional group. The water content of the resulting aqueous phases was 65 (WBTG) and 96 (WSPyrovac) wt %. The bio-oil from BTG has higher water content and lower high heating value compared to Pyrovac oil. According to the GC-MS results, BTG oil contained more acetic acid and higher TAN number than the Pyrovac oil, while Pyrovac oil was mainly composed of monophenols. The higher number of monophenols in the Pyrovac oil was also confirmed by the FC method. Although the UV fluorescence spectra, ICR-MS, and the ESI-MS analyses showed some minor differences in the molecular weight and chemical make-up of the oligomers soluble and insoluble in water from pyrolysis and HTL, the differences observed were not significant enough to justify the differences in hydrotreatment behavior between these oils reported in the literature.