In recent years,
growing attention has been devoted to the use of
lignocellulosic (woody) biomass as a feedstock to produce
renewable carbohydrates as a source of energy products,
including liquid alternatives to fossil fuels. The U.S.
Department of Energy has recently estimated a sustainable 1
billion ton (dry basis) annual supply of woody biomass,
which under current technology for production and use of
ethanol for instance, can displace approximately 35% of
gasoline transportation fuels, or roughly 10% of annual
petroleum consumption. Furthermore, life cycle greenhouse
gas analyses of cellulosic ethanol from a variety of
feed-stocks shows a nearly 100% reduction of climate active
emissions compared to fossil fuels.
The use of wood from
forest regions as a feedstock does not compete with food
production for land and therefore avoids food commodity
price inflation, and it does not cause severe land use
change emissions of greenhouse gases.
Thus, wood-based alternative transportation fuels
appear to be very environmentally and socially acceptable,
and there are many rural domestic economic benefits as well.
There are a
variety of other liquid transportation fuels that can be
derived from lignocellulosic sugars, including bio-oil
feedstocks from algae, hydrocarbons from
genetically-modified microorganisms, and synthetic fuels
from catalytic processing of sugars.
Currently, research in our
laboratory is driven by the need to reduce the cost of sugar
production and to minimize production of inhibitors of
fermentation. One of the prominent methods is to
thermochemically hydrolyze (pretreat) the biomass using
dilute acid hydrolysis and subsequently, enzymatically
hydrolyze the pretreated material to fermentable sugars that
can then be converted to biofuels or biochemicals using
specialized microorganisms. A key goal of pretreatment is to
produce fermentable sugars from hemicelluloses, enhance
enzymatic conversion of the cellulose fraction, and,
hopefully, obtain a higher ethanol yield. Along these lines,
there are several research topics that we are currently
investigating.
Most
previous studies on biomass pretreatment have involved pure
species, but mixtures of biomass are likely to be feedstocks
for agricultural and forest-based biorefineries. The primary
goal of this research is to obtain detailed kinetic data for
dilute acid hydrolysis from mixtures of several timber
species from the Upper Midwest region of the United States
(aspen, red maple, basswood, balsam) and switchgrass, and to
determine whether there were any synergistic or antagonistic
effects due to the simultaneous pretreatment of biomass
mixtures. In this research, we are primarily interested in
the dominant sugar from hemicelluloses, xylose, but other
sugars are also studied; glucose, galactose, mannose, and
arabinose. The
key inhibitory from sugar degradation that is studied is
furfural, which is not fermentable to biofuels, and thus
must be avoided.
Sugar oligomers are incomplete
hydrolysis products from hemicelluloses during dilute acid
pretreatment, and unfortunately these intermediates are not
fermentable either.
We have recently completed a series of experiments to
determine the kinetics of oligomer sugar production during
batch dilute acid hydrolysis of woody biomass, using
switchgrass, aspen, and balsam as pure species.
By matching the reactor sugar data with models of
monomer xylose, oligomer xylose, and furfural production, we
have obtained a table of parameters for further numerical
experiments.
The dynamics of production of these reaction sugar products
will be described.
Enzymatic hydrolysis of pretreated
woody biomass is the final step in the conversion of woody
biomass to fermentable sugars, dominantly glucose from the
cellulose fraction of the wood.
A suite of enzymes termed cellulases is responsible
for this conversion.
This final section of the seminar will outline a new
research program in our laboratory to create improved
cellulases from the wild type enzymes obtained from the
fungus Trichoderma
reesei.
Acknowledgements: NSF and General
Motors for funding.
Graduate students; Jill R. Jensen, Juan Morinelly,
and Michael Brodeur-Campbell.
