Lignocellulosic Biomass Processing - Food and Industrial Bioproducts and Bioprocessing - page 294

Bio-oil chemical properties vary with the feedstock but woody biomass typically
produces a mixture of 30% water, 30% phenolics, 20% aldehydes and ketones, 15%
alcohols, and 10% miscellaneous compounds. As a fuel, bio-oil has environmental
advantages when compared to fossil fuels, producing half the NO x and no SO x . As a fuel
derived from a renewable resource, bio-oil is considered carbon dioxide neutral
(Mulraney et al ., 2002 ).
Bio-oil can be burned directly in engines or mixed with diesel oil. Electricity has been
produced by diesel engines and turbines have been specially modified to burn bio-oil
(Mulraney et al ., 2002). However, technical issues, such as acidity, immiscibility, viscosity
change over time and other problems, must be solved prior to widespread application for use
in engines (Bridgwater et al ., 1999 ).
In addition to the bio-oil product produced in the condensation stage, the fast pyrolysis
process produces combustible gases as exit gas. The amount produced is less than 10% by
weight of the dry feedstock. These exit gases are most often entrained and utilized as process
gas for feedstock pyrolyzation. Some bio-oil reactors, however, do not utilize this gas as
process gas but remove it from the pyrolysis system (Bridgwater et al ., 1999 ).
Almost all pyrolysis reactors employed worldwide are fluidized-bed reactors.
However, these reactors are known to have relatively high capital requirements. The
Department of Forest Products at Mississippi State University (MSU) houses a 10 kg/h
auger pyrolysis reactor. Auger reactors are estimated to require approximately 30% of
the capital required for fluidized-bed technology for equivalent production capacity. The
MSU auger reactor utilizes a novel pyrolysis vapor condensation system that produces
very high quality bio-oil in that the water content is approximately 20% or lower, versus
25-30% for bio-oils produced by fluidized bed reactors. Analysis of the chemical
composition of bio-oils produced by this reactor is currently underway for both southern
pine and red oak feedstocks produced from bole wood and bark at several temperature
and residence time regimes.
Bio-oil can be burned directly in engines and electricity has been produced by bio-oil
fueled diesel engines; turbines have been specially modified to successfully burn bio-oil.
However, some properties of bio-oil, such as lower octane, acidity, immiscibility, viscosity
change over time and a distinctive odor, have prevented its commercial use to date for other
than pilot and demonstration projects (Bridgwater et al ., 1999 ).
Bio-oil can contain up to 45% oxygen, which is responsible for various negative properties
described. Hydrotreatments (hydrogen treatments) of bio-oils to reduce the oxygen content
have been investigated using various catalysts. They have resulted in reducing or eliminating
some of the negative properties of bio-oils due to the elimination of oxygen and/or reduction
of double bonds and aldehydic and keto groups. Two methods have been investigated:
(1) catalytic treatment of the pyrolysis vapors prior to condensation into bio-oil, and
(2) catalytic treatment of the condensed bio-oils with hydrogen or hydrogen precursors
using typical petroleum refining catalysts, including zeolites. The first method has an
advantage of not using a reducing gas such as hydrogen and the overall cost of the treatment
is lower. The method also produces liquid hydrocarbons directly suitable as fuels but the
yields are low and the catalysts are deactivated relatively rapidly due to the high percentages
of coking of bio-oil components (Bridgwater and Cottam, 1992; Czernik et al ., 2002 ).
The second method, catalytic hydrogen treatment of the condensed bio-oil liquids, has
been investigated using standard petroleum hydrogenating procedures and catalysts by
employing slightly modified technologies currently utilized for petroleum refining and
infrastructure. Fuel grade materials and selected chemicals may be produced depending on