Lignocellulosic Biomass Processing - Food and Industrial Bioproducts and Bioprocessing - page 295

the extent of the hydrogen treatments. Raw bio-oils, due to the high oxygen contents, cannot
be refined using the current petroleum refining processes.
Commonly, the hydrogen treatment for reducing or eliminating oxygen to the extent of
transforming bio-oils to a liquid hydrocarbon mixture with properties very similar to
petroleum crude oil is called hydrodeoxygenation (HDO) (Scholze, 2002). HDO treatments
of raw bio-oils can be carried out very similarly to the hydrodesulfurization process used in
the petroleum refining industry to reduce the sulfur content of petroleum crude oil. Both
hydrodesulfurization and HDO require catalysts combined with some level of heat and
pressure. Researchers have hypothesized that only minor changes are required for the
current hydrodesulfurization processes and infrastructure of the petroleum industry to be
applied to the HDO of bio-oil. Likewise, HDO treated bio-oils can potentially be refined in
existing petroleum refineries, again with only minor adjustments to the current petroleum
industry refinery infrastructure (Bridgwater and Cottam, 1992).
The carbohydrates, cellulose and hemicellulose components of wood contain oxygen as
primary and secondary alcohols. Oxygen is also present in the phenolic and methyl ether
groups of lignin. During pyrolysis, carbohydrates are dehydrated and broken to form
molecules having various aldehyde, keto and hydroxyacid groups in bio-oil products. Phenol
and a wide array of alkyl and methoxy substituted phenols are derived from the lignin
component of wood.
The combined chemical characteristics of these functional groups are the cause of many
of the undesirable attributes of bio-oil. Immiscibility with petroleum fuels, odor of burned
wood, low heat content, instability, and corrosiveness are examples of some of the more
undesirable properties. The chemical reactivity of different functional groups provides
numerous possible approaches for modifying and improving the characteristics of bio-oil
for fuel or chemical use. Oxygen is also a linkage in the higher molecular weight
carbohydrate-derived polymeric components of bio-oils and removal of the linkage oxygen
will decrease product viscosity from reduction of polymeric components' molecular weights
(Czernik et al ., 2002 ).
Char particles entrained into bio-oil during pyrolysis are known to catalyze polymeriza-
tion reactions, leading to viscosity increases over time. Filtering the bio-oil immediately
after production has been proven to improve bio-oil stability and color (Czernik et al ., 2002 ).
The approximately 25% of water contained in crude bio-oils has been shown to foul the
catalysts applied during the hydrogen treatment step. Previous researchers have fractionated
the bio-oils into water and organic fractions and often the water fractions were discarded
prior to hydrogenating the organic fractions. Researchers have also shown that bio-oils
when mildly hydrogenated do not increase in viscosity with application of heat as compared
to non-hydrogenated bio-oils (Baker and Elliott, 1988; Elliott and Neuenshwander, 1996).
Hydrogenation followed by esterfication will eliminate most of the undesirable
characteristics of bio-oils previously listed. Acetic acid is one of the most common carboxylic
acids present in bio-oil and is a primary cause of the corrosiveness characteristic. Acetic acid
can be reacted with methanol to form methyl acetate. When methyl acetate is hydrogenated
using CuOCuCr 2 O 4 as the catalyst, methanol and ethanol are the products. These chemicals
have value as fuel or can be introduced into another product stream. About 10-20% of the
bio-oil being produced in the pyrolysis unit at MSU can be converted to esters.
The dehydration of carbohydrates also yields furfural, furanone, and other similar
compounds. Mild hydrogenation will reduce the alkenes to alkanes and the aldehyde and
ketone groups to alcohols. The overall positive effect of this reaction is that the heat of
combustion of the products is higher than that of the starting material. The aliphatic and