Environmental Engineering Reference
In-Depth Information
Startups
producing ethanol from biomass, and by-products
of certain pretreatment approaches have the po-
tential to inhibit enzyme activity (Lynd, 1996).
On February 28, 2007, in response to President
Bush's Twenty in Ten initiative, the U.S. Depart-
ment of Energy (DOE) announced it was funding
six cellulosic ethanol plants, each using different
cellulosic feed stocks and different technologies to
produce fuel ethanol. U.S. DOE Secretary Samuel
Bodman stated, “These biorefineries will play a
critical role in helping to bring cellulosic ethanol
to market, and teaching us how we can produce it
in a more cost effective manner.” (Stevens, 2007)
By January of 2008, the U.S. DOE had announced
over $1 billion in funding for biofuels research
and development projects, including several small
1/10
th
scale pilot plants, research centers for im-
proving enzymes, and alternative approaches to
ethanol production, such as the formation of syn
gas (Ruggiero, 2007b; Ruggiero, 2007a; Barnett,
2007b; Sherwood, 2007; Barnett, 2007a; Rug-
giero, 2007c; Ruggiero, 2008).
The broad range of grant recipients is an
indication of how much work has yet to be ac-
complished in order to develop the technology for
the economic conversion of cellulose into ethanol.
It is also a clear statement of the importance the
United States places on developing an alternative
to fossil fuels in order to increase energy security
and reduce green house gases.
Hydrolysis
There are methods other than enzymes for breaking
apart cellulose into its component sugar molecules,
including treating it with acid. However, chemi-
cal approaches like acid hydrolysis can create
undesirable by-products which decrease the yield
of ethanol (Lee, 1992).
One of the challenges with cellulosic ethanol
is that two types of sugar are produced by the hy-
drolysis reaction: six-carbon sugars (like glucose)
and five-carbon sugars (like xylose). Generally,
yeasts capable of fermenting glucose cannot also
ferment xylose. A great deal of interesting work
has been done to combine enzyme production,
hydrolysis, and fermentation (Ho et al., 1998).
As summarized by Lynd (1996), attempts
have been made to combine cellulase hydrolysis
with the subsequent fermentation of six-carbon
sugars and five-carbon sugars, and to combine
everything—production of the enzyme, hydro-
lysis, and fermentation—into one consolidated
bioreactor. The most likely approach to creating
a microorganism capable of both producing cellu-
lase enzyme and of fermenting the resultant sugars
into ethanol is to genetically modify yeast so that
it also produces cellulase enzymes (Lynd, 1996).
Patent Landscape
Separation
Lignin is the primary non-fermentable component
of biomass, and it must be separated from the
hydrolyzed cellulose, typically by filtration. If no
market can be found for it in the local area, such
as a nearby biomass or coal-fired power plant, an
on-site boiler could provide steam and electricity
for the process. This type of boiler is expensive,
however, and would have to be factored in to the
economics of the separation process (Graf and
Koehler, 2000).
There have been an increasing number of new
patents and patent applications related to cel-
lulosic ethanol in the past few years, focusing
on genetically modified feed stocks, methods of
pretreatment, hydrolysis, separation, and ways
of combining these processes to gain greater ef-
ficiency. Although the technology for converting
cellulose into sugar with enzymes has been known
since cloth-eating fungi were discovered during
World War II, the technology to make cellulosic
ethanol an economically feasible replacement for
