Postcondensation Filtration of Pine and Cottonwood Pyrolysis Oil and Impacts on Accelerated Aging Reactions

Abstract

Chemical and physical effects of postcondensation filtration on pine and cottonwood pyrolysis oil were investigated in conjunction with a 3-week accelerated (80 °C) aging study. Pine and cottonwood pyrolysis oil (produced from clear wood, whole tree, or bark biomass) underwent serial filtration to remove fine particulates. Neat (as produced) and filtered pyrolysis oil samples were then aged at 80 °C for time intervals ranging from 24 to 504 h. Physical characterization included pH, water content, and viscosity measurements. Attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, gel permeation chromatography (GPC), and coupled gas chromatographyÀmass spectroscopy (GC/MS) were used to identify and monitor chemical composition. Filtration alone (without aging) was found to decrease water content and subsequently increase viscosity, an effect most likely due to filter wetting. Water content and average molecular weight increased with aging time in all unfiltered and filtered pyrolysis oil samples, regardless of the starting biomass composition. Upon aging, viscosity of the filtered samples increased in all samples excluding pine whole tree fractionated (PWTF) which decreased in viscosity during aging. Aging-related changes in the viscosity correlated directly with water content and molecular weight changes for all pyrolysis oils studied, with the exception of PWTF. A major finding is that postcondensation filtration did not prevent or retard property changes that typify "aging" in pyrolysis oil, namely, increases in viscosity, water content, and molecular weight. In an attempt to better understand these physical properties in relationship to the woody biomass (sub)types used as feedstocks, the chemical composition of the pyrolysis oils was monitored using FTIR spectroscopy and GC/MS both prior to and during the aging study. Pyrolysis oils produced from clear wood presented more dominant chemical signatures from ketone, aldehyde, and/or carboxylic acid species versus pyrolysis oils produced from barkcontaining biomass, demonstrating pyrolysis oil chemical composition differences due to the tree species and tree parts included in the feedstock. During aging, all pyrolysis oil samples, regardless of feedstock, exhibited decreased alcohol and/or ether content and increased concentrations of aromatics, ketones, and/or carboxylic acids. Chemical similarities were identified in the aging reactions occurring in these oils even when produced from different tree species and woody biomass compositions (i.e., clear wood, bark, whole tree). Potential reactions are discussed that match the observed increases in water content and molecular weight during accelerated aging, including air oxidation of alcohols and aldehydes to form acids and polycondensation.

Chapter 9: Upgrade of Bio-Oil to Bio-Fuel and Bio-Chemical

“…aging process. 32 Compared with freshly generated pyrolysis oil, the contents of hydroxyacetaldehyde, 34, 35 furfurals 34 and guaiacols 34 in the aged pyrolysis oils have been detected to decrease by GC/MS.…”

“…It is well known that the viscosity of pyrolysis oils increases with storage of time, especially when the oils is stored or handled at high temperatures. 23,[30][31][32][33][34][35][36][37] The average molecular weight of pyrolysis oils also increases during the storage 23,31,33,35 and it has been indicated that the formation of larger molecules is one of the reasons for the increasing viscosity of pyrolysis oils during the storage. Since the water content of the pyrolysis oils has been found to increase during the aging process, it is considered one of the by-products of the process.…”

“…Since the water content of the pyrolysis oils has been found to increase during the aging process, it is considered one of the by-products of the process. 23,31,32,34,36,37 By using small-angle neutron scattering (SANS), pyrolysis oils has been shown to be nanostructured fluids, consisting of a complex continuous phase and nanoparticles mainly formed by the association of units of pyrolytic lignin components from biomass. The aggregation of these units producing branched structures is reported to be responsible for the aging.…”

Phase Behavior and Fuel Properties of Bio-Oil/Glycerol/Methanol Blends

“…Peak height ratios were calculated for quantitative analysis according to a method used previously. 32 The rheological property of selected samples was analyzed based on a method stated elsewhere. 35,36 Briefly, steady shear tests were carried out to investigate the flow behavior of samples using a Haake Mars II Rheometer fitted with a cone−plate sensor (C35/4 Ti) and a thermocontroller (Haake TC501).…”

“…Another noticeable change is in the C−C stretch represented by peak at 1646 cm −1 , suggesting the formation of alkene or conjugate ketone. 32 This is likely due to glycerol dehydration (known to occur under acidic conditions and high temperature 57,58 ) that may also contribute to the increase in water content of the fuel blends after aging.…”