Ethanol is a bio-based, renewable oxygenated fuel that is currently used as an
oxygenate additive in fuels. If ethanol is to progress further than a fuel additive and become a
player in the liquid fuels market, lignocellulosic feedstocks need to be utilized.10 Ethanol is an
attractive fuel because 77% of the energy contained in the carbohydrates of the feedstock can
be recovered as ethanol. Also, for every gallon of gasoline replaced by lignocellulosic ethanol,
an 86% reduction of greenhouse gas emissions would occur.11 When considering
lignocellulosic feedstock choices and plant location for bio-ethanol production, feedstock
availability and transportation costs must be considered. Transportation costs are one of the
largest costs associated with bio-ethanol production because ligncellulosic feedstocks have low
energy densities which require large amounts of feedstock to be transported. Corn stover was
selected as a feedstock because of its high availability and carbohydrate content. Using this as
a feedstock, Iowa was determined to be an ideal location for the bio-ethanol plant because of
Iowa’s high density of corn fields. These dense corn fields allow for minimization of
transportation costs. A feed stock rate of 2000 MT/day was selected because this rate yielded
an ideal balance between transportation costs and production revenue.1 In terms of economics,
for a 2010 start-up date to be economically feasible, the ethanol must have a selling price of
$1.07 per gallon.1
Pretreatment of the feedstock is necessary to break the feedstock down into usable
products for saccharification. Dilute acid hydrolysis was selected for this process. A diagram of
the entire pretreatment process can be seen in figure _ in appendix A. The first step in the
pretreatment process is to take the corn stover feedstock and put it though a cleaning process
to remove excess dirt and metals that would otherwise decrease the overall efficiency of the
process and cause excess volume. The cleaned corn stover is then put through a shredding
process to increase its reaction area. The shredded product is then fed to a pre-steaming
vessel to further break down the feedstock. The steamed product is then added to the dilute
acid hydrolysis vessel where sulfuric acid is added until its concentration within the reactor is
1.1% . This hydrolysis converts hemicellulose carbohydrates into usable sugars. During this
reaction, side reactions occur that create compounds that can inhibit the fermentation process,
such as acetic acid, furfural, and hydroxymethylfurfural (HMF). To remove as much of these
inhibitors as possible, flash and overliming processes were used. The hydrolyzed product from
the hydrolysis reactor is sent to a flash vessel where 7.8% of the acetic acid and 61% of the
furfural and HMF are removed as vapor.1 The slurry that leaves the flash is then filtered into a
solid stream containing cellulose and lignin and a liquid stream. The liquid stream contains the
inhibitors from the hydrolysis and is then treated in an overliming vessel. In this vessels, lime is
added until the pH reaches 10. This allows for the “overliming reactions” to occur which
substantially reduce the amount of inhibitors in the liquid stream.9 This overlimed stream is then
fed to a pH adjustor vessel where the pH is adjusted to that of the fermentation train and let sit
for 4 hours. This long residence time allows for the gypsum crystals to grow large enough for
easy separation.1 Once the crystals are formed, they are filtered out and the liquid stream is
recombined with the cellulose stream and fed to the saccharification train. Sizes and residence
times for all vessels can be found in table P1.
Vessel
Residence Time
Size
Hours
m3
Pre-Steam Vessel
0.33
609
Hydrolysis Vessel
0.17
2032
Flash
0.25
2159
Overlime Vessel
1.0
324
pH Adjustor Vessel 1
4.0
1396
pH Adjustor Vessel 2
4.0
1396
Table P1: Residence times and sizes of pretreatment vessels.
1.
Reference
Aden, A., M. Ruth, K. Ibsen, J. Jechura, K. Neeves, J. Sheehan, B. Wallace, L. Montague, A. Slayton, and J. Lukas.
Lignocellulosic Biomass to Ethanol Process Design and Economics Utilizing Co-Current Dilute Acid Prehydrolysis and
Enzymatic Hydrolysis for Corn Stover. National Renewable Energy Laboratory, 2002. Print.
11. Lu, Yulin, and Nathan S. Mosier. "Current Technologies for Fuel Ethanol Production from Lignocellulosic Plant
Biomass." Genetic Improvement of Bioenergy Crops. Ed. Wilfred Vermerris. Gainesville, FL: Springer, 2008. 161-77.
Print.
9.
Martinez, A, ME Rodriguez, SW York, JF Preston, and LO Ingram. "Effects of Ca(OH)(2) treatments
("overliming") on the composition and toxicity of bagasse hemicellulose hydrolysates.." PubMed.gov 69.5
(2000): 526-536. Web. 20 Dec 2009. <http://www.ncbi.nlm.nih.gov/pubmed/10898862>.
10. Saha, Badal C., Loren B Iten, Michael A. Cotta, and Y. Victor Wu. "Dilute acid pretreatment, enzymatic saccharification
and fermentation of wheat straw to ethanol." Process Biochemistry 40. (2005): 3693-3700. Web. 23 Nov 2009.
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