Page 130 - Advances in bioenergy (2016)

special materials that resist corrosion. The mix of compounds found in pyrolysis oil depends to
some extent on the type of biomass that is processed, but it is not possible to distill the oil in
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the same way traditional petroleum-derived oils have been distilled in the past. When
pyrolysis oil is heated, it reacts and breaks down, driving off gaseous products and forming a
thick, viscous tar that may react further until only coke remains. While the volumetric energy
density of pyrolysis oil is higher than that of biomass, it is approximately half that of

comparable petroleum-derived oils, because it contains large amounts of chemically bonded
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oxygen and water. Pyrolysis oil obtained via fast pyrolysis is thermodynamically and
chemically unstable, and the natural tendency of the oil is to continue to react to form waxes,
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tars, and pitch, as well as gaseous products. The rates of these reactions increase rapidly with
temperature, and, as a result, the viscosity of stored pyrolysis oil may increase significantly
even if it is exposed only to ambient temperatures. The reactive oil also tends to create
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deposits in fuel lines, injectors, and burner components. Microscopic particles of char
entrained in the oil appear to catalyze reactions among various chemical species in the oil. 7
These unavoidable char particles are produced by the rapid abrasion and disintegration of
biomass during the process of fast pyrolysis. Researchers report that the oil undergoes aging
processes, even when great care has been taken to avoid conditions where aging could occur,
and samples have been found to separate into distinct aqueous and hydrophobic phases as the
aging process proceeds to completion. Temperatures as low as −20C may be required to arrest
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the chemical mechanisms involved in the aging of the oil. While technologies that make hot
gas filtration possible for fast pyrolysis have been developed, and appear to make it easier to
remove the char before the oil condenses to a liquid, the char particles in question can be
extremely small. The smallest of these are almost impossible to remove, and will remain in the
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liquid product. It is possible to stabilize pyrolysis oils by adding alcohols such as methanol
and ethanol. As little as 5% by mass of an alcohol can reduce the viscosity of the oil, and
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significantly slow the aging of the oil. However, it should be noted that alcohols can attack
fuel lines and polymer components in fuel delivery systems, and generating these alcohols
requires additional effort and cost. It may never be possible to store and transport pyrolysis oil
as easily as petroleum-derived products.

Recent work has also shown that single-stage catalytic conversion of pyrolysis oil to aromatic
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hydrocarbons is possible. If pyrolysis is carried out in a fluidized bed consisting of an
appropriate catalyst (e.g., a zeolite-based catalyst), then oxygen-containing functional groups
associated with many of the species in the pyrolysis vapors can be largely removed as H O,
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CO, and CO . Because much of the oxygen present in the feedstock is removed by the
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formation of water, a significant amount of hydrogen is also removed from the structure of the
products. For this reason, aromatic hydrocarbons represent the majority of the stable products
when this approach is taken, and these aromatic species may not be usable in every type of
liquid transportation fuel. These aromatic products are primarily useful as gasoline
blendstocks. However, while this pyrolysis oil contains less oxygen and less water than
pyrolysis oil produced in the absence of a catalytically active fluidized bed, at best only about
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30% of the available carbon is converted to aromatic products. Rapid catalyst deactivation