Environmental Engineering Reference
In-Depth Information
DID YOU KNOW?
Fermentation is a series of chemical reactions that convert sugars to ethanol.
The fermentation reaction is caused by yeast or bacteria, which feed on the
sugars. Ethanol and carbon dioxide are produced as the sugar is consumed.
The simplified fermentation reaction equation for the 6-carbon sugar glucose is
C
6
H
12
O
6
→ 2 CH
3
CH
2
OH + 2 CO
2
Glucose Ethanol Carbon
dioxide
Standard (RPS) and other state incentives. Over 30,000 hectares of poplars are
being grown in Minnesota, and several thousand hectares are also grown as part of
a DOE-funded project to provide biomass for a power utility company in southern
Minnesota. The Pacific Northwest has large plantations of hybrid poplars, estimated
at 60,000 hectares as of 2007. Most of these plantations are currently used for pulp
wood, with little volume being used for bioenergy. Because SRWCs can be used
either for biomass or as feedstock for pulp and other products, pulp demand will
influence the cost of using it for bioenergy production.
An important consideration for energy crops (e.g., switchgrass, poplar, willow)
is the potential for increasing yields and developing other desirable characteristics.
Most energy crops are unimproved or have been bred only recently for biomass yield,
whereas corn and other commercial food crops have undergone substantial improve-
ments in yield, disease resistance, and other agronomic traits. A more complex under-
standing of biological systems and application of the latest biotechnological advances
would accelerate the development of new biomass crops with desirable attributes.
These attributes include increased yields and processability, optimal growth in specific
microclimates, better pest resistance, efficient nutrient use, and greater tolerance to
moisture deficits and other sources of stress. Agronomic and breeding improvements
of these new crops could provide a significant boost to future energy crop production.
B
iothanol
p
roduction
By
d
ry
c
orn
m
ill
p
rocess
Ethanol production from dry corn milling follows a seven-step process as shown in
Figure 5.6
.
First, corn feedstock is cleaned and milled (ground into corn meal), and
the milled corn is then converted to a slurry. The slurry is liquefied (slurried with
water to form a mash), and the enzymes yield glucose. Yeast is then added to the
mash to convert starch into a simple sugar (dextrose) in a process known as
sacchari-
fication.
. After liquefaction, the mash is cooked in a saccharification tank to reduce
bacterial levels and it then moves through several fermenters where fermentation
takes place (sugar is converted to ethanol by yeast). The resulting “beer” containing
2 to 12% ethanol is then distilled into ethanol at 95% alcohol and 5% water. The
remaining solids (stillage) are collected during distillation, dried, and sold as an ani-
mal feed known as dried distillers' grain (DDG). The final removal of water from the
ethanol to less than 1% (dehydration) allows it to be blended with gasoline.
