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