Additives To Lower and Stabilize the Viscosity of Pyrolysis Oils during Storage

Abstract

The initial development of additives to stabilize the viscosity of biocrude during long-term storage has produced dramatic results. The additives investigated were ethyl acetate, methyl isobutyl ketone and methanol, acetone, methanol, acetone and methanol, and ethanol. These additives represent three chemical families, which all demonstrated the ability to drastically reduce the aging rate of biocrude, as defined by the increase in viscosity with time. Accelerated aging tests were run at 90 °C to screen the additives. The additives not only lowered the initial viscosity at 40 °C by half but also reduced the aging rate of a hot gas filtered pyrolysis oil made from hybrid poplar (NREL run 175) by factors of 1−18 compared to the original pure oil. With the best additive, methanol, at a 10 wt % level in the pyrolysis oil, the modified biocrude was still a single-phase liquid and still met the ASTM No. 4 diesel fuel specification for viscosity even after 96 h exposure to 90 °C. Based on the aging rate at 90 °C recently determined for pure biocrude without additives, the pure biocrude tested would have exceeded the allowable ASTM No. 4 viscosity after only 2.6 h. In addition, the unmodified biocrude formed a waxy precipitate that floated on top of the liquid phase after 8 h exposure to 90 °C. Use of methanol with previously aged oils greatly reduces the resultant viscosity, but not quite as effectively as the use of the methanol shortly after the pyrolysis oil is produced. The cost of the additive, e.g., methanol, may be offset by the heating value it adds to the pyrolysis oil, depending on the local cost of each.

A model for the evaporation of biomass pyrolysis oil droplets

“…The distribution functions for acids and aldehydes were chosen so as to span the molecular weight range of the most frequently encountered compounds: from formic to hexanoic acid for the acid group, with a peak around acetic acid, and from acetaldehyde to 1-hydroxy-2-butanone for the aldehyde/ketone group, with a peak between 14 hydroxyacetaldehyde and hydroxypropanone. The lignin distribution was approximately fitted to an experimental one given by Scholze et al [12]; similar distributions are given by others [8,26,27].…”

“…With time the liquid became more viscous and the droplet surface appeared to "skin over"; this progressively restricted bubbling, leading to a quieter period in which the droplet no longer bubbled but still moved around erratically on the fibre. This suggests polymerization of the liquid, which is known to occur with these fuels [6,9,18,26,27]. When motion stopped altogether the droplet had become a largely solid char "cenosphere" of irregular and highly porous shape.…”

“…Pyrolysis oils may be diluted with methanol to make them easier to atomize or to reduce aging effects [1,17,18,26,28]. The effects of doing this were simulated by adding 10% methanol 22 as a fifth component to the fuel of Table 1.…”

“…These fuels are typically denser and more viscous than petroleum fuels, with a specific gravity typically around 1.2 [5,6,10,[22][23][24][25]. The chemical composition may change over time owing to "aging", in which light components are polymerized [18,26,27], and phase separation is also possible [6,7]. Studies of the combustion behaviour of single droplets have been performed 5 on free-falling droplets in drop tube furnaces [1][2][3][4] as well on suspended droplets [4], and have established the sequence of combustion events described earlier.…”

Large-Scale Pyrolysis Oil Production: A Technology Assessment and Economic Analysis

“…In 1997 Diebold and Czernik [32] investigated the addition of various solvents (including water) to bio-oils in an attempt to chemically interrupt molecular building reactions suspected of occurring between known compounds in bio-oil. Acetalization and esterification reactions induced by alcohol addition in the presence of mineral acids has been reported in the literature by Radlein et al [33] to support this idea.…”