Page 145 - Advances in bioenergy (2016)

components include acetic acid, methanol, aldehydes and ketones, cyclopentenones, furans,
alkyl-phenols, alkyl-methoxy-phenols, anhydrosugars, and oligomeric sugars and water-
insoluble lignin-derived compounds. Nitrogen- and sulfur-containing compounds are also
sometimes found depending on the biomass source.

Chemical Composition

Bio-oil is a complex mixture of oxygenated components containing hundreds of component
types and isomers. The specifics of the component structures are dependent to a significant
degree on the type of biomass feedstock and the biostructures found in the feedstock. With
lignocellulosic biomass the CHO portions are similar with about equal parts of carbon and
oxygen and about 6% hydrogen. The amount of nitrogen and sulfur is directly attributable to the
biomass feedstock with white wood containing near zero and herbaceous biomass having
higher amounts of nitrogen, up to 6%, and sulfur, up to half a percent. There are also trace
elements, which are also directly attributable to the elements in the biomass, particularly when
significant amounts of char remain in the bio-oil. Present in some recovered bio-oils is also a
portion of the fluid bed material (silica or alumina sand/dust).

Essentially all the bio-oil components contain oxygen in one of the functional group types
including phenols, carboxylic acids, dehydrated carbohydrates, carbonyls (both aldehydes and
ketones), ethers, and alcohols. The oxygenated components and the dissolved water component
cause the bio-oil to have a low energy density, like biomass itself and about half that of pure
hydrocarbon. The high level of dissolved water appears to severely complicate the
measurement of flash point. Recent studies have concluded that flash point is not relevant to
bio-oil, and it has been determined as an alternative that bio-oil does not support sustained
combustion as determined in the ASTM method D4260. The acidic nature of some of the
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components causes the bio-oil to have a corrosive nature. It is more of an irritant to the skin
but can corrode metals in long-term exposure or at elevated temperature. The toxicity has been
measured in a few cases and generally been found to be low. The carcinogenic nature of bio-
oil has also been found to be low, but not entirely absent. Biodegradability measurements have
found a significant level of degradability, but the time limits may not be met in all cases. These

results apply broadly to woody biomass-derived bio-oil, but more assessment is required for
bio-oil produced from other biomasses to validate similarities or differences.

Physical Properties

As a result of its chemical composition, bio-oil is not thermally stable. Considering that it is a
nonequilibrium reaction product, recovered by rapid quenching of decomposition products, it
is not surprising that the bio-oil would have a tendency to continue to react. At room
temperature this reaction is barely noticeable. The viscosity of bio-oil will slowly increase,
about doubling in a year's time when stored at room temperature with limited exposure to light
and air. An accelerated aging test has been developed for bio-oil in which it has been found
that 24 h at 80°C has about the same effect on viscosity as a year's time at room temperature. 14
This level of viscosity increase assumes that the bio-oil is kept in a sealed container without
exposure to oxygen. Evaporation of water and volatiles or oxidation will lead to an increase in