Comparison of the chemical composition of liquids from the pyrolysis and hydrothermal liquefaction of lignocellulosic materials

Authors: PIRES, ANAMARIA - Washington State University; GARCIA-PEREZ, MANUEL - Washington State University; OLARTE, MARIEFEL - Pacific Northwest National Laboratory; KEW, WILLIAM - Pacific Northwest National Laboratory; SCHMIDT, ANDREW - Pacific Northwest National Laboratory; ZEMAITIS, KEVIN - Pacific Northwest National Laboratory; DENSON, MELBA - Washington State University; Terrell, Evan; MCDONALD, ARMANDO - University Of Idaho; HAN, YINGLEI - Washington State University

Submitted to: Energy and Fuels
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/10/2023
Publication Date: 5/1/2023
Citation: Pires, A.P.P., Garcia-Perez, M., Olarte, M.O., Kew, W., Schmidt, A., Zemaitis, K., Denson, M., Terrell, E., McDonald, A., Han, Y. 2023. Comparison of the chemical composition of liquids from the pyrolysis and hydrothermal liquefaction of lignocellulosic materials. Energy and Fuels. 37(10):7221-7236. https://doi.org/10.1021/acs.energyfuels.2c03239.
DOI: https://doi.org/10.1021/acs.energyfuels.2c03239

Interpretive Summary: This work explores the difference between two technologies by which crude oil replacements can be generated from renewable materials like grasses, wood, and agricultural residues. The two thermochemical conversion technologies studied here are hydrothermal liquefaction and fast pyrolysis. Both processes generate liquid-phase products with similar applications, but some of their chemical characteristics can be significantly different. The difference in these chemical characteristics has implications ultimately on chemistry and engineering applications for the renewable oils (for example, as bio-fuels or renewable chemicals). The focus of this work is to explore a wide array of analytical chemistry techniques for the thorough characterization of hydrothermal liquefaction and fast pyrolysis oils. Results from this characterization help in the understanding of how and why the two types of renewable oils are both similar and different.

Technical Abstract: Major differences in thermal stability and hydrotreatment behavior of hydrothermal liquefaction (HTL) and pyrolysis oils have been reported in the literature. However, 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 fractions (WIS) 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 (FTIR), Folin-Ciocalteu (FC), and UV fluorescence. Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) and electrospray ionization mass spectrometry (ESI-MS) was also used to analyze the fractions. The most prevalent class of compounds in the water-insoluble phases were phenols derived from lignin. Water-soluble 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. The GC-MS results of BTG oil show the presence of a more prominent acetic acid peak and higher TAN number than the Pyrovac oil. The GC-MS of Pyrovac oil showed mainly mono-phenol peaks. The quantification of this family by the Folin-Ciocalteu method confirmed higher content of monophenolic compounds compared with the BTG oil. The lower thermal stability of pyrolysis oils compared with HTL biocrudes can be partially explained by the fact that pyrolysis oils (BTG and Pyrovac) contain carbohydrates while HTL biocrude (WISWD-57) does not. Thus, we further investigated the chemical differences between the phenolic-rich fractions insoluble in water and the holocellulose-derived compounds soluble in water. Even after water extraction, the acid content of the water-insoluble bit from BTG (WISBTG) was higher than that of the water-insoluble fraction obtained by HTL (WISWSD-57). Likewise, the acid content of the aqueous phases derived from pyrolysis oils (WSPyrovac, WSBTG) was also higher than for the aqueous phase obtained by HTL (WSWSD-57). This result is partly due to using bases in the HTL process that neutralizes the acid formed. Moreover, the starting feedstock may also influence the differences between the oils. 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. These results suggest that the differences between HTL biocrudes and pyrolysis oils were likely partially due to holocellulose-derived products in the pyrolysis oils and higher acid contents.