Supplementary materials
Techno-economic analysis from corn stover to butanol
1. Feedstock composition
Table S1. Corn Stover Composition from the 2011 Design and the Present Design
Component
Present Design (dry wt %)
Glucan
35.05
Xylan
19.53
Lignin
15.76
Ash
4.93
Acetatea
1.81
Protein
3.10
Extractives
14.65
Arabinan
2.38
Galactan
1.43
Mannan
0.60
Sucrose
0.77
Unknown soluble solidsb
-
Total structural carbohydrate
58.99
Total structural carbohydrate + sucrose
59.76
Moisture (bulk wt %)
20.0
a
Represents acetate groups present in the hemicellulose polymer; converted to acetic acid in
pretreatment.
b
In the 2002 design, unknown soluble solids were calculated by difference to close the mass
balance. This is now included in the extractives component.
2. Feedstock handling
Biomass (corn stover) is stored in a central depot and is preprocessed and homogenized to a
degree before delivery, such that the biorefinery receives feedstock with known, uniform-format
specifications including particle size distribution, moisture content, and bulk density. The
equipment consists of weighing and unloading stations for incoming biomass supply trucks,
short-term queuing storage, and conveyors for feeding bulk feedstock to the pretreatment reactor.
3. Acid pretreatment
The pretreatment reactor is a single horizontal reaction vessel. The reactor is designed for fairly
mild conditions at 158°C (316°F) and 18 mg acid/dry g of biomass (about 0.6 wt % acid
concentration). Acid is metered to the reaction chamber at a rate proportional to the mass flow
rate of feedstock. High-pressure steam is injected into this vessel to maintain temperature. The
reactor pressure is held just at the bubble point for the mixture. Heat losses from the reactor are
not accounted for in the energy balance calculations. The residence time in the pretreatment
reactor is nominally 5 minutes. The reaction conditions are summarized in Table .
Table S2. Pretreatment Reactor Conditions
Sulfuric acid loading
18 mg/g dry biomassa
Residence time
5 minutes
Temperature
158°C
Pressure
5.5 atm (81 psia)
Total solids loading
30 wt %
a
Additional acid is added downstream of the pretreatment reactor.
Table S3 summarizes the reactions and percent conversions that take place in pretreatment.
Glucan contained in the hemicellulose side-chains is converted to glucose along with a small
portion of the cellulose. Minor hemicellulose carbohydrates (arabinan, mannan, galactan) are
assumed to have the same reactions and conversions as xylan. The xylan-to-xylose conversion is
an assumed total hydrolysis that also includes an enzymatic component that will be discussed
later. The sucrose reaction to HMF and glucose reflects 100% hydrolysis of sucrose to fructose
and glucose, followed by complete degradation of the fructose to HMF.
Table S3. Pretreatment Hydrolysis Reactions and Assumed Conversions
Reaction
Reactant
% Converted to
Product
(Glucan)n + n H2O→ n Glucose
Glucan
9.9%
(Glucan)n + n H2O → n Glucose Oligomera
Glucan
0.3%
(Glucan)n → n HMF + 2n H2O
Glucan
0.3%
Sucrose → HMF + Glucose + 2 H2O
Sucrose
100%
(Xylan)n + n H2O→ n Xylose
Xylan
90.0%
(Xylan)n + m H2O → m Xylose Oligomera
Xylan
2.4%
(Xylan)n → n Furfural + 2n H2O
Xylan
5.0%
Acetate → Acetic Acid
Acetate
100%
(Lignin)n → n Soluble Lignin
Lignin
5.0%
a
Sugar oligomers are considered soluble but not fermentable.
The pretreatment reactor is discharged to a flash tank controlled at 130°C. The slurry from the
flash tank goes into the secondary oligomer conversion reaction vessel, where it is held at 130°C
for 20–30 minutes. An additional 4.1 mg/g of sulfuric acid is added in the oligomer conversion
step, bringing the total acid loading to 22.1 mg/g dry biomass. The oligomer conversion reactor
is discharged into another flash tank that operates at atmospheric pressure. After this flash, the
hydrolysate whole slurry containing 30 wt % total solids and 16.6 wt % insoluble solids is sent to
the conditioning tank. In there, the slurry is diluted with water to slightly greater than 20 wt %
total solids to ensure miscibility through enzymatic hydrolysis. Ammonia gas is mixed into the
dilution water to raise the hydrolysate pH to 5, with a residence time of 30 minutes. Then
pretreated slurry is cooled to 75°C. The flash vapors from the flash tanks are condensed, and the
condensate contains volatile, potentially inhibitory organics created in pretreatment and therefore
is routed to wastewater treatment.
4. Enzymatic hydrolysis and fermentation
The process assumed in this design is known as separate (or sequential) hydrolysis and
fermentation (SHF). Enzymatic hydrolysis is initiated while the slurry is still at an elevated
temperature after pretreatment and conditioning. A total 5-day SHF process with batch
fermentation was selected as a more realistic case for the present design. Enzymatic hydrolysis
begins in a continuous, high-solids reactor, saccharified by the enzyme. The viscosity of the
mixture drops dramatically, such that it can be pumped to one of several parallel bioreactors.
Hydrolysis continues in this vessel until complete, then the slurry is cooled and the ethanologen
inoculum is added. The ethanol-containing fermentation broth is emptied to the beer well
(storage tank) before being pumped to distillation. The operating conditions and reactions
assumed for enzymatic hydrolysis are shown in Tables S4 and S5.
Table S4. Enzymatic Hydrolysis Conditions
Temperature
48°C (118°F)
Initial solids loading
20 wt % total solids (10.6% insoluble/9.4% soluble)
Residence time
3.5 days total (84 h)
Number and size of continuous vessels
8 @ 950 m3 (250,000 gal) each
Number and size of batch vessels
12 @ 3,600 m3 (950,000 gal) each
Cellulase loading
20 mg protein/g cellulose
Table S5. Enzymatic Hydrolysis Reactions and Assumed Conversions
Reaction
Reactant
% Converted to Product
(Glucan)n → n Glucose Oligomer
Glucan
4.0%
(Glucan)n + ½n H2O → ½n Cellobiose
Glucan
1.2%
(Glucan)n + n H2O → n Glucose
Glucan
90.0%
Cellobiose + H2O → 2 Glucose
Cellobiose
100%
The inoculum protocol for Z. mobilis is a direct transfer of cells without a cell concentration step.
In order to provide a required 10% inoculum volume back to the production fermentors, 10% of
the saccharified slurry is split off to seed production. Each seed train consists of five reactors
operating in batch mode with a 24-hour batch time and an additional 12-hour turnaround time,
shown in Table S6. The seed fermentors are cooled with chilled water to maintain the
temperature at 32°C (90°F).
Table S6. Seed Train Specifications
Inoculum level
10 vol % of production vessel size
Batch time
24 h
Fermentor turnaround time
12 h
Number of trains
2
Number of fermentor stages
5
Maximum fermentor volume (F-305)
200,000 gal (757 m3)
Corn steep liquor (CSL) loading
0.50 wt %
Diammonium phosphate (DAP) loading
0.67 g/L fermentation broth (whole slurry)
Table S7 gives the reactions and conversions used in the seed fermentors to describe the
microorganism growth and sugar metabolism.
Table S7. Seed Train Reactions and Assumed Conversions
Reaction
Reactant
% Converted
to Product
Glucose + 0.047 CSLa + 0.018 DAP → 6 E Coli + 2.4 H2O
Glucose
4.0%
Xylose + 0.039 CSL + 0.015 DAP → 5 E Coli + 2 H2O
Xylose
4.0%
a
Corn steep liquor (CSL) and diammonium phosphate (DAP) are both nitrogen sources required
for Z. mobilis growth. The stoichiometry shown above is only used to balance the compositions
assumed for nonstandard components like cell mass.
Besides being fermented to ethanol, sugar may be lost to side products by contaminating
microorganisms, shown in Table S8. A total of 3% of the sugars available for fermentation is
assumed lost to contamination.
Table S8. Co-Fermentation Contamination Loss Reactions
Reaction
Reactant
% Converted to Product
Glucose → 2 Lactic Acid
Glucose
3.0%
3 Xylose → 5 Lactic Acid
Xylose
3.0%
3 Arabinose → 5 Lactic Acid
Arabinose
3.0%
Galactose → 2 Lactic Acid
Galactose
3.0%
Mannose → 2 Lactic Acid
Mannose
3.0%
Fermentation is conducted in a batch system of 950,000-gal vessels. The fermentation residence
time is modeled as 72 hours. Inoculum from the seed train is fed at 10 vol % of the hydrolysate
flow along with the nutrients corn steep liquor (CSL, 0.25 wt %) and diammonium phosphate
(DAP, 0.33 g/L of whole slurry). Table S9 summarizes the conditions in the fermentation vessels
and Table S10 lists the reactions and conversions assumed in fermentation.
Table S9. Fermentation Conditions
Organism
E. coli
Temperature
32°C (96°F)
Initial fermentation solids level
19.8% total solids (14.7% soluble, 5.1% insoluble w/w)
Residence time
3 days (72 h)
Inoculum level
10 vol %
Corn steep liquor (CSL) level
0.25 wt %
Diammonium phosphate (DAP) level
0.33 g/L fermentation broth (whole slurry)
Table S10. Fermentation Reactions and Assumed Conversions
Reaction
Reactant
% Converted
to Product
Glucose → Butanol + 2 CO2
Glucose
85.0%b
Glucose + 0.047 CSLa + 0.018 DAP → 6 E Coli. + 2.4 H2O
Glucose
2.0%
Xylose → Butanol + 5 CO2
Xylose
85.0% b
Xylose + 0.039 CSL + 0.015 DAP → 5 E Coli. + 2 H2O
Xylose
1.9%
a
Corn steep liquor (CSL) and diammonium phosphate (DAP) are both nitrogen sources required
for cell growth. The stoichiometry shown above is only used to balance the compositions
assumed for cell mass. Nutrient requirements have not been optimized and a minimal, low-cost
nutrient formulation has yet to be defined.
b
Sugar yield has been changed for sensitivity analysis to evaluate cost impacts.
The fermentation broth or “beer” has an iso-butanol concentration of 3.25 wt % and is collected
in the beer storage. The butanol purification process was discussed in the manuscript.
5. Downstream processing
Wastewater treatment. Plant wastewater streams are treated by anaerobic and aerobic digestion.
The methane-rich biogas from anaerobic digestion is sent to the combustor, where sludge
from the digesters is also burned. The treated water is suitable for recycling and is returned to
the process.
Storage. This area provides bulk storage for chemicals used and produced in the process,
including corn steep liquor (CSL), ammonia, sulfuric acid, nutrients, water, and ethanol.
Combustor, boiler, and turbogenerator. The solids from distillation and wastewater treatment
and the biogas from anaerobic digestion are combusted to produce high-pressure steam for
electricity production and process heat. The majority of the process steam demand is in the
pretreatment reactor and distillation columns. The boiler produces excess steam that is
converted to electricity for use in the plant and for sale to the grid.
Utilities. This area includes a cooling water system, chilled water system, process water
manifold, and power systems.