Toward Understanding of Bio-Oil Aging: Accelerated Aging of Bio-Oil Fractions

Abstract

Pyrolysis bio-oil from biomass is a promising intermediate for producing transportation fuels and platform chemicals. However, its instability, often called aging, has been identified as a critical hurdle that prevents bio-oil from being commercialized. The objective of this research is to explore the bio-oil aging mechanism by an accelerated aging test of fractionated bio-oil produced from loblolly pine. When water soluble (WS), ether insoluble (EIS), and pyrolytic lignin (PL) fractions were aged separately, the increased molecular weight (Mw) was observed with increasing aging temperature and the presence of acids. WS and EIS fractions had high Mw brown solids formed after aging. Adjusting the pH of WS and EIS fractions from 2.5 to 7.0 significantly reduced the tendency of a Mw increase. Similar Mw rise was also observed on a PL fraction with an elevated temperature and acid addition. Formaldehyde was found to react with the PL fraction in the presence of any acid catalysts tested, i.e., 8-fold Mw increase in acetic acid environment, while other bio-oil aldehydes did not significantly promote lignin condensation. To better understand bio-oil stability, a potential bio-oil aging pattern was proposed, suggesting that bio-oil can be aged within or between its fractions.

Review of recent developments to improve storage and transportation stability of bio-oil

“…Aging experiments have been carried out at various research institutions to assess the fuel stability of bio-oil [15,[45][46][47][48][49][50][51][52]. Nevertheless, no standardized methods for measuring the stability of bio-oils are available.…”

“…Accelerated thermal aging processes have been broadly adopted for thermal stability tests [21,47,48]. In an accelerated thermal aging experiment, the bio-oil is stored at an elevated temperature for a certain time, and the physical and chemical properties prior to and after the aging process are analyzed [53].…”

Characterization of fast-pyrolysis bio-oil distillation residues and their potential applications

“…Owing to the predominance of thermally stable chemical groups, TGRP oil distills with high yields of dry organic non-acidic products (∼60-65%) and significantly less residual bottoms, analogous to heavy vacuum gas oil (HVGO) or vacuum residuum. The 65% yield is a significantly better performance than when traditional bio-oil is distilled (<20% dry organic yield), where excessive repolymerization of the pyrolytic lignin occurs [7,8], resulting in more than 40 wt% of the bio-oil left as bottoms. In both cases, a significant residuum fraction necessitates downstream fractionation or conversion into lighter compounds.…”

Thermal stability of low and high Mw fractions of bio-oil derived from lignin conversion in subcritical water

“…As far as thermal stability is concerned, a few studies on the chemical changes of biomass-pyrolysis bio-oil stored at elevated temperatures (up to 90°C) over extended periods of time (up to 1 week) using different methodology can be found in the literature [3,4,[6][7][8][9][10][11][12]. Boucher et al [7], Hilten et al [3] and Samanya et al [8] argued that 80°C was a suitable temperature for testing thermal stability since, at this temperature, rather large change in the properties of biomass-pyrolysis biooil can be expected.…”

“…Further important properties to study are the variations in the chemical structural changes (gas chromatography-mass spectrometry (GC-MS) and nuclear magnetic resonance (NMR)) and the molecular weight (Mw) during ageing. It has also been shown to be very beneficial to apply a fractionation approach when studying reaction mechanisms and chemical structural changes in bio-oil because it is much more representative of the actual reaction environments than model compounds [11]. According to these authors, the water-insoluble fraction (pyrolytic lignin fraction) showed a significant increase in Mw after accelerated ageing at 80°C and that a higher increase was obtained with a higher ageing temperature (110°C).…”