ethanol selling price is $2.70/gal. To be cost
effective in today’s market, the overall cost
of ethanol must be reduced by a factor of
3–5. The problem has been found to be
more complex than simply finding new
methods for cellulose-to-sugar conversion.
Plants contain a wide variety of other
molecules, whose structural relationships on
the nanoscale are unknown. Understanding
these relationships could lead to the
development of cost-effective methods to
break down cellulose and make the sugars
available for bioconversion to fuels.
Cellulose is stable both chemically and
biologically — a necessary feature in nature,
where plants survive the elements for years.
Work is underway to understand and
develop molecular models of cellulose and
the enzymes that hydrolyze it; however, that
work has not moved from ideal systems
(e.g., ones that do not involve the links to
other plant structural components, such as
hemicellulose and lignin) to those that are
more realistic. The limitations to the rates at
which enzymes break down cellulose are
not understood either. If researchers could develop an understanding of those limitations, the
rates could be increased, allowing shorter residence times and/or reduced enzyme loadings.
The breakdown of cellulose and its related substances leads to mixtures of different sugars. To
make a process economically viable, organisms need to convert all of the available sugars to
ethanol. Using corn (Zea maize) as an example, the ethanol yield could be increased 20% by
converting residual starch and the hemicellulose and cellulose in the remaining corn solids into
ethanol. Researchers have developed several genetically modified organisms that can ferment
multiple sugars; however, the existing organisms are inhibited by other compounds that are
naturally present in biomass or are produced in the cellulose-to-sugar conversion process.
Understanding how microorganisms respond to inhibitors would assist researchers in developing
more robust organisms. Likewise, understanding the metabolic rates within organisms would
help researchers develop organisms that convert sugars to fuels more rapidly.
Biomass can be converted into fuels by using direct thermochemical processes (U.S. Department
of Energy [DOE] 2003). One of those processes involves gasification of the biomass to syngas
and subsequent catalytic conversion of the syngas to produce fuels. Another involves pyrolysis
of the biomass to produce oil that can be reformed to liquid fuels. Gasification is well
understood, and a commercial-scale facility that gasifies biomass and uses the syngas in a
combined-cycle power production process has been in operation for several years. The syngas
produced from biomass is similar to that produced by coal gasification, so the process used to
BIOMASS TO FUELSImproving the yields of fuels from biomass requires
substantial effort to understand the detailed processes
involved in fully utilizing existing resources. For example,
microorganisms that are capable of fermenting sugars to
ethanol cannot act directly on cellulose, the major structural
component of plants, which accounts for a significant fraction
of the total biomass. Research into enhancing the
performance of enzymes, such as cellulase, that break down
cellulose into its component sugar glucose would significantly
enhance the use of the total plant biomass to produce
ethanol for fuel. A molecular model illustrating the action of
the enzyme cellulase on a cellulose molecule is shown
schematically below.