SUPPORTING INFORMATION
Energy Consumption and Greenhouse Gas Emissions from Enzyme and Yeast
Manufacture for Corn and Cellulosic Ethanol Production
Jennifer B. Dunn1, 2, Steffen Mueller3, Michael Wang1, and Jeongwoo Han1
1
Center for Transportation Research, Argonne National Laboratory, 9700 South Cass Avenue,
Argonne, IL 60439, USA
2
jdunn@anl.gov (to whom correspondence should be addressed); phone: 630.252.4667; fax:
630.252.3443
3
Energy Resources Center, University of Illinois at Chicago, 1309 South Halsted Street, MC
156, Chicago, IL 60607, USA; muellers@uic.edu
The authors propose this paper for publication in the Biofuels and Environmental
Biotechnology Section of Biotechnology Letters.
1
Supporting Information
To begin, we provide figures that sketch the life-cycle stages of corn and cellulosic ethanol. In
the subsequent sections, we outline our data sources and calculations for key ingredients in
enzyme and yeast production.
Life-Cycle Stages Diagrams for Corn and Cellulosic Ethanol
Supplementary Figures 1and 2 display the stages in the ethanol life cycle. Yeast and enzymes are
fed into the conversion plant.
Supplementary Fig. 1 Life-cycle stages of corn ethanol
2
Supplementary Fig. 2 Life-cycle stages of cellulosic ethanol
Enzyme Manufacturing and Use in Corn and Cellulosic Ethanol Production
Two major enzyme producers provided energy and emissions data for this paper. Supplementary
Table 1 outlines aspects of the enzymes’ life cycle that each manufacturer included in the
reported data. It also describes the scope of MacLean and Spatari’s (2009) work for comparison.
3
Supplementary Table 1 Scope comparison among studies with energy consumption data for
enzyme productiona
Impact
I1
I2
M
Steam
Y
Y
Y
Electricity
Y
Y
Y
Waste
Y
management
Key ingredients
Y
Y
Ingredients to
Y
plant
U.S. plant
a
Y
N
Y
I = industrial enzyme producers; M = MacLean and Spatari (2009).
Only one manufacturer included the impacts of key ingredients in their values for energy
consumption and greenhouse gas (GHG) emissions during enzyme manufacturing, but solely for
AA and GA. We therefore estimated quantities of key nutrients consumed in AA, GA, and
cellulase production and developed material and energy flows for the production of these
nutrients, as described in this supporting information. The key nutrients included in our study are
starch, glucose, corn steep liquor, molasses, NH3, and P2O5. Because one enzyme manufacturer
provided impacts of key ingredients for AA and GA, our values for life-cycle energy
consumption and GHG emissions for the production of these two enzymes are a worst-case
scenario.
4
As Table 1 (in the main journal article body) illustrates, MacLean and Spatari report higher
values for AA and GA loadings than the aggregate manufacturer-reported enzyme dosage.
Additionally, the IRE ethanol plant, which uses Genencor enzymes, provided average enzyme
usage from April 2010 through December 2010 (N. Jakel, IRE, 1900 Stewart Road Rochelle,
IL). The IRE uses 0.29 kg AA/dry metric ton substrate and 0.82 kg GA/dry metric ton substrate.
These values are reasonably close to the dosage data supplied by the manufacturers and
significantly lower than the starch enzyme dosages reported by MacLean and Spatari (2009).
The range of possible cellulase enzyme dosages (10-100 kg/dry metric ton substrate) in Table 1
reflects the inherent complexity and variability of enzyme systems, feedstocks, enzyme specific
activity for a given feedstock, and process conditions used in cellulosic ethanol production. To
emphasize this point, one manufacturer related that the company is currently researching
hundreds of pretreatment and substrate combinations. In our analysis, three sources guided our
choice of cellulase dosage to use in our calculations. First, MacLean and Spatari (2009) used
between 9.2 and 9.6 kg enzyme/dry metric ton substrate. Additionally, the NCERC provided
data on their laboratory-scale work using Novozymes' cellulase enzyme Cellic Ctec2 (S.T.
Trupia, NCERC, University Park Dr., Edwardsville, IL). The dose used for the conversion of
corn kernel fiber to ethanol (10 kg enzyme/dry metric ton substrate) was also at the low end of
the manufacturer-reported range. Finally, Humbird et al. (2011) modeled the cellulase feed in
corn stover hydrolysis as 7.4 kg cellulase/dry metric ton substrate. We adopted a value of 10 kg
cellulase/dry metric ton substrate in our analysis because these three sources did not exceed this
value.
5
Enzyme production energy consumption values for the three enzymes shown in Table 1 varied
among the data sources. MacLean and Spatari (2009) were in good agreement with
manufacturer-reported values for steam and electricity consumption for AA. The industryreported values, however, were significantly lower for GA and cellulase manufacture. This may
reflect plant location effects and process efficiency improvements since 2007 (when MacLean
and Spatari gathered data). It is important to note that on a per mass of enzyme basis within the
plant boundary, cellulase is no more energy intensive to produce than AA or GA.
Yeast Production and Recycling
Knauf and Kraus (2006) describe the yeast production process, which includes propagation,
fermentation, and drying steps. The fermentation medium, which is manufacturer-dependent and
proprietary, includes molasses, oxygen, ammonia, phosphorous pentoxide, and other nutrients.
Molasses, which contains sucrose that fuels yeast growth, is a sugar-mill waste product and may
have contaminants that must be removed before it is fed to the aerobic fermentation step in a
yeast plant. Yeast production generates carbon dioxide, water, and energy in addition to the
desired product. The biogenic CO2 is not included in the greenhouse gas (GHG) emissions that
we report for yeast production.
Ethanol plants can purchase yeast in active dry, fresh cake, or fresh liquid form (Knauf and
Kraus 2006). Of these, active dry yeast (ADY) is the most common form of yeast used at dry
mills. It typically undergoes additional drying in a fluidized bed before packaging. When it
arrives at an ethanol plant, ADY must undergo rehydration in clean, warm water for up to several
hours and advance in its life cycle from the lag phase to the log-growth phase.
6
The effective yeast feed rate to ethanol production processes cannot be reduced by recycling for
several reasons (Voegele 2009). First, corn ethanol plants typically generate ethanol at
concentrations that kill yeast. Second, the substrate in corn ethanol plants has a high amount of
insolubles, thereby complicating yeast separation from the overall medium. Finally, maintaining
yeast quality, purity, metabolism, and genetics over numerous recycling loops proves difficult,
which translates to possible fluctuations in product quality. While interest in recycling yeast in
corn ethanol production exists, technical barriers and low economic incentives (the cost of yeast
is a small fraction of the overall cost of ethanol plant operation) suggest that recycling will not be
common in the industry for some time.
Yeast manufacturing material and energy flow data for the fuel ethanol industry are scarce.
Natural Resources Canada (2006) cites an earlier paper (Finnveden et al. 1994) as a data source
for estimating energy consumption and emissions during yeast manufacturing. Finnveden et al.
(1994) assumed enzyme and yeast manufacturing are equally energy intensive.
In our analysis, we developed an estimate of energy and ingredient consumption during yeast
production from a National Renewable Energy Laboratory (NREL) technical-economic analysis
of lignocellulosic biomass conversion to ethanol via a biochemical process (Humbird et al.
2011). This process, however, uses Z. Mobilis rather than yeast in separate hydrolysis and
fermentation (SHF). Our analysis assumes that the energy intensity of producing Saccharomyces
Cerevisae in a dedicated yeast plant would be similar to that of producing Z.Mobilis as in the
NREL design. However, in a dedicated yeast plant, efficiencies may be gained from the larger
production scale (Ingledew et al. 2009).
7
Supplementary Table 2 presents yeast dosages during corn and cellulosic ethanol production.
Humbird et al. (2011) report a substantially higher yeast loading during cellulosic ethanol
production then do Finnveden et al. (1994). The IRE and Ingledew et al. (2009) values are for
starch ethanol production and are in reasonably good agreement.
Supplementary Table 2 Yeast dosage during ethanol production (kg yeast/dry metric ton
substrate) and energy consumption during yeast production (MJ/kg yeast)
Dosage
Steam
Electricity
(MJ/kg
(MJ/kg
yeast)
yeast)
13
3.1
(kg yeast/
Feedstock
Reference
dry metric ton
substrate)
Cellulosic Humbird et al. (2011)
3.7
Finnveden et al.
Cellulosic
1.5
6.32a
(1994)
Corn
Ingledew et al. (2009)
0.62
Corn
Illinois River Energy
0.10
a. Total energy consumption
Energy consumption values during yeast production, as estimated by Finnveden et al. (1994) and
Humbird et al. (2011), are also presented in Supplementary Table 2. Humbird et al. predict a
higher energy consumption during yeast manufacturing than do Finnveden et al. (Recall that the
8
latter assumed energy consumption during enzyme and yeast production to be equal.)
Manufacturer-provided energy consumption data for enzyme production (Table 1), however,
exceed the estimate of Finnveden et al. Thus, it is possible that Finnveden et al. also
underestimated energy consumption during yeast production. The Humbird et al. data may
therefore be considered a reasonable estimate of the energy used to produce yeast. We note that
this analysis excludes the final drying and packaging steps of yeast production.
Key Ingredients in Enzyme and Yeast Manufacturing
Supplementary Table 3 contains the amount of each key ingredient that is used in the production
of alpha-amylase (AA), glucoamylase (GA), cellulase, and yeast. In the following subsections,
we outline how we derived these values and developed material and energy flows for the
production of each of these key ingredients.
9
Supplementary Table 3 Dosage rates of key ingredients in the production of enzymes and yeast
included in this study
Dosage
AAa
GAb
Cellulasec
Yeastd
(kg/kg enzyme or yeast)
Starch
Glucose
0.47
7.0 × 10-4
0.47
4.2
Corn steep liquor
0.14
NH3
0.21
0.08
Molasses
3.90
P2O5
0.03
a
Kwiatkowski et al. 2006; Huang et al. 2004.
b
Kwiatkowski et al. 2006; Pavezzi et al. 2008.
c
Humbird et al. 2011.
d
Ingledew et al. 2009.
Starch and Corn Steep Liquor
Corn starch and corn steep liquor are produced during the wet milling of corn, as sketched in
Supplementary Figure 3. We analyzed the material and energy flows of corn wet milling in
Brown et al. (1996). This process, which produces several other products, consumes electricity
and steam, which we assume is provided by a natural gas boiler with an 80% efficiency. We
allocated electricity and steam consumption among co-products on a mass basis. Corn steep
10
liquor is produced when steep water leaving the tank in which corn kernels steep in hot water
and SO2 is concentrated. The electricity and natural gas consumption to produce corn steep liquor
are 0.02 and 2.1 MJ/kg corn steep liquor, respectively. Corn starch is produced after several
subsequent steps that separate the germ, hull, and gluten from the starch. The resulting electricity
and natural gas consumption are 0.33 and 4.02 MJ/kg corn starch, respectively.
Corn Production
Corn Wet Milling
From Germ Corn Oil Meal
Refined Oil
From Hull
Gluten Animal Feeds
Corn Steep Liquor
Diesel Fuel
Fertilizers
Herbicides/Insecticides
Electricity
Natural Gas
From Starch
Corn Syrup
Dextrose Sugar
Dry Starches
Dextrins
Supplementary Fig. 3 System boundary for production of corn starch and corn steep liquor
from corn
We accounted for the impacts of producing the corn that is the feedstock for starch and corn
steep liquor production. Dry corn kernels are 70% starch by mass (Kwiatkowski et al. 2006). Of
that starch, 17% is used to make dextrins, and 57% feeds the production of dextrose sugar and
corn syrup. The remaining 26% is used to produce dry starches (Brown et al. 1996). We
therefore assumed that 0.18 kg of corn is required to produce 1 kg of starch. In the case of corn
steep liquor, 0.18 kg of this product is produced for every 1 kg of corn kernels processed. For the
production of corn, we used existing data from Argonne National Laboratory’s Greenhouse
Gases, Regulated Emissions, and Energy Use in Transportation (GREET) model (Wang et al.
2011).
11
Glucose
We developed estimates of energy and sodium hydroxide consumption during glucose
production from a starch slurry based on Synowiecki (2007). In this process, the pH of the starch
slurry is adjusted from 3.85 to 6.5 with sodium hydroxide. To calculate the amount of sodium
hydroxide that would be consumed to achieve this pH change, we evaluated Equation 1 at the
final and initial pH values. The value for pKw at 100 ºC, the temperature at which the pH is 6.5,
is 12.2 (NIST Webbook 2012):
[OH-] = Kw/[H+],
[Eq. 1]
where Kw is the dissociation constant of water;
[H+] is the concentration of H+ (mol/L); and
[OH-] is the concentration of OH- (mol/L).
The resulting amount of sodium hydroxide, 8.0 × 10-5 g NaOH/L, is divided by the starch feed
rate, 3,000 g/L, to yield the NaOH consumption per mass of starch, 2.7 × 10-6 g NaOH/g starch.
We assumed that the yield Synowiecki (2007) provides for the conversion of starch to glucose,
94%, is on a mass basis and calculated a NaOH consumption of 2.82 × 10-7 g NaOH/g glucose.
Although the production of glucose from starch is a multi-step process, we assumed that the bulk
of the energy is consumed in the liquefaction step, which occurs at 100 °C, the highest
temperature in the process. Other process steps include saccharification, decolorization and
purification, filtration, desalting, and concentration. We did not have sufficient process
information to model the energy consumed during these steps.
12
We calculated the energy required to heat 1 L of water from 25 °C to 100 °C by using a water
heat capacity of 4.2 J/g K (NIST Webbook 2012) and the average density of water at these two
temperatures, 0.978 g/mL. At a starch feed rate of 3,000 g/L and a mass conversion of 94%, the
energy consumption is 0.17 MJ/kg glucose, assuming that heat is provided by an 80% efficient
boiler.
Molasses
Molasses fed during yeast production can be from a number of sources. In our analysis, we
assumed that the source of the molasses is sugar cane and calculated the energy intensity of its
conversion to molasses with data from Brown et al. (1996) by using mass allocation. We
assumed that steam consumed in the process is generated from an 80% efficient natural gas
boiler. The calculated energy consumption values are in presented in Supplementary Table 4. We
used existing GREET data for sugar cane production (Dunn et al. 2011) to determine the energy
and environmental burdens of producing the sugar cane used to make molasses.
13
Supplementary Table 4 Energy consumed during sugar cane molasses production
Fuel
Consumption (MJ/kg molasses)
Electricity
0.08
Natural gas
2.74
Total
2.82
Ammonia, Phosphate, and Sodium Hydroxide
Material and energy flows for ammonia and phosphate are described in a prior report (Wang et
al. 2003). Sodium hydroxide is a co-product of chlorine production from sodium brine. The
material and energy flows of chlorine and sodium brine production are described elsewhere
(Dunn et al. 2012). To include sodium hydroxide in this analysis, we allocated total energy and
brine consumption in the production of Cl2 to sodium hydroxide production by mass.
Feed Rates of Key Ingredients in Enzyme and Yeast Production
Novozymes reported several key ingredients in enzyme production, including corn starch,
glucose, corn steep powder, sulfuric acid, and others (Nielsen et al. 2007). They did not,
however, quantify consumption rates of these ingredients. Lacking industry data for the amounts
of key ingredients used in enzyme manufacturing, we turned to the technical literature to identify
key ingredients and their rates of consumption in AA and GA production. Huang et al. (2004)
report the consumption levels of glucose in AA production. Thus, 4.5 g/L of glucose were fed to
AA, the yield of which was 41.1 U/mL. (U is a measure of enzyme activity with units of
µmol/min.) The consumption rate of glucose was then 1.09 × 10-4 g glucose/U. Kwiatkowski et
al. (2006) model fuel ethanol production via the corn dry milling process. In this process, AA is
14
used in the liquefaction step, and GA is used in the subsequent saccharification step. We
calculated how much glucose must be fed to the liquefaction step to achieve the enzyme activity
level needed to convert the amount of starch fed to the liquefaction step. Then 10 wet kg of corn,
the starch content of which is 59.5% (wet basis), were fed to this step. In one hour, we assumed
complete conversion of the starch to dextrins by mass. The resulting conversion rate is 18 U/min.
We then calculated that, per kg of dry corn fed to the liquefaction process, 2.32 × 10-4 g glucose
must be fed to achieve the necessary enzyme activity. Knowing the AA loading for this step
from Supplementary Table 5, we calculated a glucose dosage of 7.04 × 10-4 kg glucose/kg AA.
In the case of GA, we adopted the growth medium described in Pavezzi et al. (2008), which is
0.02 g/mL each of starch and glucose. The enzyme activity was approximately 2 U/mL. The
loadings of starch and glucose were then each 0.01 g/U. In the saccharification step modeled by
Kwiatkowski et al. (2006), the conversion of the dextrin leaving the liquefaction stage occurs at a
rate of 283 U. Thus, 2.83 g glucose and starch are necessary to achieve this enzyme activity per
10 kg wet corn fed. With the GA loading from Supplementary Table 5, the glucose and starch
dosage for GA growth is 0.47 kg/kg GA each.
We used material flows for cellulase production as modeled by Humbird et al. (2011). Material
flows for yeast production are from Ingledew et al. (2009).
Key GREET Parameters
Supplementary Table 5 outlines the enzyme and yeast-related GREET simulation parameters we
used.
15
Supplementary Table 5 Dosage and energy consumption parameters for GREET simulations
Dosage
Steam
Electricity
(kg/dry metric ton
(MJ/kg enzyme or
(MJ/kg enzyme or
substrate)
yeast)
yeast)
Alpha-amylase
0.33
3.5
7.3
Glucoamylase
0.71
10
32
Cellulase
10
1.8
4.0
Yeast
2.5a/0.36b
13
3.1
Component
a
Dosage for cellulosic ethanol fermentation. Average of values in Supplementary Table 2.
b
Dosage for starch ethanol fermentation.
We used GREET values for conversion of feedstock to ethanol of 506 L/dry metric ton corn and
381 L/dry metric ton switchgrass. GREET values for energy consumption during corn
conversion to ethanol reflect a dry mill plant with only dry distillers’ grains as a co-product and
natural gas as the process fuel.
In our study, we assumed that all components are transported in heavy heavy-duty trucks.
Supplementary Table 6 contains the transportation distances that we used. Back-haul distances
are equivalent to origin-to-destination distances. GREET assumes a fuel economy of 5.0
miles/diesel gallon for heavy heavy-duty trucks. We assumed payloads of 23 tons. One important
note: given the small number of North American plants that produce yeast for the fuel ethanol
industry, it is likely that yeast will have to travel a fair distance between the production plant and
a fuel ethanol facility.
16
Supplementary Table 6 Transportation distances
Component
Transportation Distance (km)
Starch
80.5
Glucose
80.5
Corn steep liquor
80.5
Sodium hydroxide
161
Molasses
80.5
Enzymes
161
Yeast
161
17
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