Thermal and Storage Stability of Bio-Oil from Pyrolysis of Torrefied Wood

Abstract

The objective of this paper is to investigate the biomass torrefaction effect on bio-oil stability by comparing the physicochemical and compositional properties of aged bio-oils. Two aging methods, accelerated aging (held at 80°C for 24 h) and long-term natural aging (12-month storage at 25°C), were employed to produce aged bio-oils for such comparison. The results indicate that bio-oils made from heat-treated wood had similar aging behavior in terms of increase of water content, acid content, molecular weight, and viscosity. The increase rate, however, was found to be different and dependent on the aging method. The accelerated method found parallel water and total acidity number (TAN) increments between raw and torrefaction bio-oils, while the natural aging method found torrefaction bio-oils, especially those made from heavily treated wood, had much slower water and acid accumulation than that of raw bio-oil. As a negative effect, both methods identified the viscosity of torrefaction bio-oils increased more significantly than that of raw bio-oil, while their molecular weights were unexpectedly lower. The correlation study showed that bio-oil viscosity is more tied to the content of bio-oil−water insoluble fraction rather than its average molecular weight. In addition, the characterization of aged bio-oils using NMR, GC/MS, and solvent fractionation indicated that torrefaction bio-oils had less compositional alternation after accelerated aging than the raw bio-oil. Also, they were more stable during the first 6 months of storage at room temperature. During the long-term storage, the raw bio-oil completely phase-separated after 6 months. However, two distinct torrefaction bio-oils (LP-280T and LP-330T) had enhanced phase stability, as a stable uniform oil phase without gum formation can be maintained during the entire 12-month storage.

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

“…Moreover, no major structural differences can be detected between the LO in either the aliphatic or aromatic regions after accelerated ageing at 80°C for 1 week. Compared to the results of Meng et al [14] pertaining to pyrolysis bio-oil, the LO obtained by catalytic conversion/HTL of lignin is relatively stable after ageing at 80°C for up to 1 week.…”

“…The yield of insolubles in the aged LO (aLO_EIS_80) and the aged HO (aHO_TIS_80) after 1 week of ageing is 0.5 and 3.1%, respectively, showing that the ageing rate of HO is faster than that of LO at 80°C for 1 week. The formation rate of insolubles (aLO_EIS_80 and aHO_TIS_80) in this study is relatively low compared to the biomass-pyrolysis bio-oil [1,7,[13][14][15]. The LO and HO produced by the HTL of lignin have, thus, a comparably high degree of stability at 80°C.…”

“…They also reported that the accelerated process gave an increase in aromatic C-O and C-C bonds and a decrease in aromatic C-H and aliphatic C-O bonds in the pyrolysis bio-oil through lignin condensation. In another study, Meng et al [14] reported that there was an increase in alkyl carbons (about 9.5%) and aromatic carbons (about 16.1%) but a decrease in methoxyl carbons (about 12.3%) when the biomass-pyrolysis bio-oil was aged at 80°C for 24 h. Consequently, all of the condensation reactions in the biomass-pyrolysis bio-oil consume methoxyl groups and form new aromatic C-C, thus increasing the Mw of the bio-oil components during ageing. This is in agreement with the results of Garcìa-Pèrez et al [13] and Chaala et al [16] obtained in ageing studies of biomass-pyrolysis bio-oil.…”

“…The amount of insolubles in biomasspyrolysis bio-oil was found to increase considerably during accelerating ageing at elevated temperature (i.e. 0.5-26%) in various studies [1,7,[13][14][15]. 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.…”

“…O ne plausible ageing mechanism of the polymerisation/condensation reactions in the biomasspyrolysis bio-oil is the acid-catalysed reactions between the electron-rich aromatic ring and the cationic sites provided by benzyl alcohol and benzyl ether at the α-carbon [14]; another is the radical reactions that could take place under an acid-free condition. These reactions are based on the aromatic substrate substitutions involving the coupling of the aromatic rings from the aromatic free radicals.…”