Environmental Engineering Reference
In-Depth Information
produced. SSF has been patented by the Gulf Oil Company and the University of Arkansas
[3]. With the availability of organisms that can ferment both pentoses and hexoses, all
biomass sugars may now be simultaneously fermented in SSF/SSCF processes [23, 24]
Product inhibition of exocellulases is not eliminated, however, unless the glucose dimer
cellobiose is also consumed or hydrolyzed. Since conventional S. cerevisiae strains do not
metabolize cellobiose, and since cellulase preparations with sufficient beta-glucosidase
activity to hydrolyze all the cellobiose are expensive to produce, much research has been
directed toward the use of native cellobiose-utilizing yeast strains in SSF, either
independently or in co-culture with S. cerevisiae [18].
2.1.5. Fermentation following gasification . A radically different approach to preparing
biomass substrates for fermentation is found in biomass gasification. In this process, biomass
is converted to synthesis gas, consisting primarily of CO, CO 2 , and H 2 , in addition to CH4
and N2 (25). After gasification, anaerobic bacteria such as Clostridium ljungdahlii can
ferment the CO, CO 2 , and H 2 into ethanol by an acetogenic process [26-29]. One advantage of
the process is that, unlike acid and enzymatic hydrolysis methods, gasification can convert
essentially all of the biomass, including lignin, to syngas that can be potentially fermented by
bacteria [30]. Higher rates of fermentation are also achieved because the process is limited by
the transfer of gas into the liquid phase instead of the rate of substrate uptake by the bacteria.
2.2. Feedstock Options
Corn and sugar cane are not long-term options for ethanol generation because of their
value as foods. Exploration of various non-food forms of biomass, principally wastes, is
therefore an active area of research.
Worldwide, rice straw has the greatest quantitative potential for bioethanol production,
estimated at 205 gigaliters per year; this potential is concentrated in Asia, which as a region
could produce up to 291 gigaliters per year of ethanol from rice straw in combination with
wheat straw and corn stover. Europe has the next-largest supply of agricultural wastes,
primarily in the form of wheat straw (69.2 gigaliters per year potential ethanol production);
followed by North America, in which corn stover forms the majority of agricultural wastes
and could supply an estimated 38.4 gigaliters per year of ethanol [11]. Bagasse, or waste
derived from sugar cane, is widely available in tropical areas and is being explored by BC
International Corporation (BCI), while municipal solid waste has attracted the attention of
Masada Resources Group, LLC; these two companies are currently planning construction of
unique biomass-to-ethanol plants [31].
Corn stover or fiber, a by-product of the corn wet-milling industry consisting of corn
hulls and residual starch, is the subject of great interest as a possible substrate for ethanol
production in the United States. Conversion of the starch along with the lignocellulosic
components in the corn fiber could increase ethanol yields from a corn wet mill by 13
percent. In a recent study utilizing the bioethanol process development unit at the U.S.
National Renewable Energy Laboratory (NREL), corn fiber was used to support continuous,
integrated operation of the plant. The fiber was pretreated by high-temperature, dilute
sulfuric-acid hydrolysis, and the cellulose was converted to ethanol using simultaneous
saccharification and fermentation using a commercially-available cellulase and conventional
Saccharomyces cerevisiae yeast that did not utilize 5-carbon sugars. Despite difficulties with
bacterial contamination, which are expected to diminish with the use of recombinant, xylose-
and arabinose-utilizing fermentative organisms, the attempt was successful and indicates that
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