Environmental Engineering Reference
In-Depth Information
also being used as a replacement for methyl tertiary butyl ether (MTBE), the fuel oxygenate
that is being phased out due to its widespread contamination of groundwater [9].
The majority of ethanol, approximately 62 percent of the world total, is currently
produced in Brazil, primarily from cane sugar (~12.5 billion liters in 2002), and in the United
States, primarily from corn (~5 billion liters per year) [5, 10]. However, these feedstocks are
expensive and are useful as foods, causing a great deal of research to be focused on the
development of biomass such as corn cobs and stalks, sugar cane waste, wheat and rice straw,
other agronomic residues, forestry and paper mill discards, paper municipal waste, and
dedicated energy crops into ethanol [11]. While the use of such non-food substrates helps the
economics of ethanol production substantially, the high cost of production, especially relative
to gasoline, remains the primary obstacle to bioethanol commercialization [5].
Lignocellulosic (non-food) raw materials, such as agricultural, wood chip, and paper
wastes, can yield approximately 100 billion gallons of fuel-grade ethanol per year in the
United States alone [12]. The projected cost of bioethanol has dropped from about $1.22 per
liter to about $0.31 per liter based on consistent improvements in pretreatment, enzyme
application, and fermentation [13]. If additional specific improvement targets are met, this
cost could drop to as low as $0.20-$0.12 per liter by 2015 [14]. For transportation fuel,
therefore, ethanol has real potential to replace gasoline, even in the absence of governmental
support.
2. State of the Science
2.1. Ethanol Biosynthesis: Overview
2.1.1. Feedstocks
. Biological production of ethanol first requires that atmospheric CO
2
be
fixed into organic carbon (biomass) through photosynthesis. While agricultural crops and
residues currently form the vast majority of feedstock for ethanol production, other biomass
sources such as wood chips, sawdust, industrial organic wastes, and municipal organic wastes
are important for the commercial development of fuel bioethanol [11]. Agricultural plant
matter contains approximately 10-15 percent lignin, a polymer of phenolic subunits that is
highly resistant (although far from impervious) to enzymatic attack. Lignin typically
surrounds and protects the more enzymatically-vulnerable components of cellulose and
hemicellulose, which comprise approx. 20-50 percent and 20-3 5 percent of the remaining
plant material, respectively [9].
The next challenge in bioethanol synthesis is therefore the release of fermentable sugars
from the biomass, or conversion of the feedstock into fermentable substrates. This phase
involves both the separation of lignin from cellulosic and hemicellulosic polymers and the
hydrolysis of the polymers into monomeric sugars, primarily glucose and xylose. This phase
is termed pretreatment, and a variety of biotic and abiotic approaches are currently under
investigation; abiotic approaches have been recently reviewed [15].
2.1.2. Mechanical and chemical disruption
. Lignin is typically dissociated from the
carbohydrates by mechanical and/or thermochemical means, including hot water, steam
explosion, and/or acid treatments in either batch or flow-through reactors. Many variations
have been explored. While it was once difficult to compare the performance and economics
of the various approaches due to differences in feedstocks tested, a group of pretreatment
researchers has formed in North America to facilitate such comparisons. This group, the
