1

Final Deliverable Report

“Opportunities to add cellulosic feedstocks to the corn dry milling industry”

Kelly Ibsen, PE

Lynx Engineering LLC

Subcontract Number: LFT-9-99189-01

NREL Technical Monitor: Mike Cleary/Andy Aden

Date Submitted: December 2nd

, 2009

Copies To:

Mike Cleary

National Renewable Energy Laboratory

1617 Cole Blvd., MS 3311

Golden, CO 80401

mike.cleary@nrel.gov

One electronic version

Margaret Mortenson

National Renewable Energy Laboratory

1617 Cole Blvd., MS 1533

Golden, CO 80401

One printed copy, including graphics

NREL Publication Services

National Renewable Energy Laboratory

1617 Cole Blvd., MS 1713

Golden, CO 80401

judy.hulstrom@nrel.gov

One electronic version

One printed copy, including graphics
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Opportunities to add cellulosic feedstocks to the corn dry milling industry

Abstract

The DOE/NREL “Bridge to Corn Ethanol” project in the late 1990s was one of the first attempts 

to understand and quantify the similarities and differences between ethanol production using

corn and cellulosic feedstocks. In 2004, NREL and the USDA conducted a study of corn and 

cellulosic processing to understand what the key drivers are between processing corn or 

cellulose. The current study expanded upon the previous studies by exploring three options 1) 

dry mill expansion with cellulosics, 2) dry mill retrofit to cellulosics and 3) greenfield (i.e. a new 

plant) co-processing of corn and cellulosics. All three options showed positive economics when 

a mature biochemical conversion process was paired with today’s dry milling facilities. 

Introduction

The U.S. corn ethanol industry developed in the 1970s in response to rising oil prices and 

increasingly uncertain supplies and has become a significant part of the American rural 

economy. Technology to produce ethanol from corn is primarily of two types, wet and dry 

milling. While both are still used today, new installations tend to be dry mills due in part to lower 

capital costs and decreased demand for high fructose corn syrup, one of wet milling’s primary 

products. To understand the economics of dry milling, models have been developed by several 

groups including the USDA (material and energy balance and economic models), University of 

Minnesota (economic model), and Iowa State University (economic model).

The DOE/NREL “Bridge to Corn Ethanol” project in the late 1990s was one of the first attempts 

to understand and quantify the similarities and differences between ethanol production using

corn and cellulosic feedstocks. DOE challenged companies to find economical ways to 

commercialize cellulose to ethanol, looking to the corn ethanol industry’s development for 

guidance. The results of the five independent studies identified significant opportunities and 

more importantly barriers to not only co-processing, but economic cellulosic processing in 

general, such as the lack of robust enzymatic systems, which became a significant part the 

DOE’s Biomass R&D portfolio. In 2004, NREL and the USDA conducted a study of corn and 

cellulosic technologies (Wallace, 2004) to understand what the key drivers are between 

processing corn and cellulose. That study showed that feedstock price, co-product values and 

utility costs can make cellulosic feedstocks more economically attractive than corn for ethanol 

production. It also concluded that more integrated processing (i.e. combined fermentation and 

product recovery) resulted in poor economics due to the change in DDGS composition from 

addition of cellulosic residue (e.g. lignin) and ultimately its value as an animal feed. The current

study expands upon the previous studies by exploring three options 1) dry mill expansion with 

cellulosics, 2) dry mill retrofit to cellulosics and 3) co-processing of corn and cellulosics.

Some companies have projects underway to add cellulosic feedstocks to their dry milling 

operations. POET’s project LIBERTY, due to be complete in 2011, is expanding an existing 

50MM gallon per year (gpy) dry mill to include corn cob and fiber conversion to produce an 

additional 25MM gpy. They are also expanding the corn ethanol production to make a final 

plant output of 125MM gpy. In 2008, Chippewa Valley Ethanol implemented a phased plan to 
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produce steam from gasification of biomass (corn cobs) followed by production of fuels or 

chemicals from syngas (CVEC News, January 16, 2009). Abengoa Bioenergy has completed 

both a pilot plant and demonstration facility for cellulosic ethanol on two of their existing starch 

to ethanol sites and has an integrated biorefinery project investigating the opportunities for 

converting multiple feedstocks with both biochemical and thermochemical processes to a variety 

of products including ethanol and chemicals. Still other companies are looking at switching dry 

mills from ethanol to chemical production. Gevo Inc. has retrofitted and started up an ethanol

demonstration plant for butanol production and Myriant Technologies LLC has announced it is

investigating retrofitting dry mills to make succinic acid (The Wall Street Journal, Oct. 19, 2009).

Key barriers to corn and cellulose conversion

There are several significant technical, political, economic and environmental barriers to ethanol 

production in the U.S. that affect both the continued growth of the corn ethanol industry and 

commercialization of cellulosic ethanol. They include:

Technical

1. Cellulose conversion technology must be fully developed to an economically viable level –

high yields and competitive operating and capital costs.

2. Corn conversion technology must continue to become more energy and process efficient to 

decrease the effect of energy price fluctuations.

Economic

1. Corn and cellulosics price and availability

2. Natural gas price

3. DDG value and market volume

4. Difficulty in securing financing for cellulosic processes, perceived to be high risk

Policy/Public Perception

1. The current RFS limit of renewable biofuels to 15B gallons in 2015 – effect on corn ethanol

2. The current blend limit of 10%

3. Food vs. fuel debates

4. Concern over genetically modified products – feedstocks and biocatalysts

Environmental

1. Life cycle and emission effects

2. Land use debates

3. Energy density and transportation of cellulosic feedstocks

The technical barriers continue to be addressed by a blend of industry, university and 

government groups funded by state, federal, and increasingly for cellulosics, non-government 

sources. The multiple and complex factors affecting the agricultural and fuel markets make 

forecasting ethanol demand and production difficult. Policy has and will play a large role in the 

outcome of both corn and cellulosic ethanol industries, and environmental issues as steadily 

gaining prominence in debates about the future of ethanol.

Information and assumptions used in this study

The average capacity of corn ethanol plants has almost doubled in recent years, from 32.4MM 

gpy in 2000 to 62MM in 2009 (RFA 2009 Ethanol Industry Outlook, includes wet mill capacity). 
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If the trend continues, 70MM gpy will be the average in just a few years, and many new dry mills

are 100-110MM gpy, which could well be the average by 2022. Since recent past growth has 

been spurred by robust federal and state policies it is difficult to predict what future plant sizes 

might be. 100MM gpy or smaller plant sizes were used for most of the scenarios in this study. 

For operating and capital cost information we compared various sources including models from 

Iowa State University (Hofstrand, 2009), University of Minnesota (Tiffany, 2003), and USDA

(Taylor, 2007). All had similar steam and electricity usage per gallon of corn ethanol, but the 

USDA model included a material and energy balance with the natural gas and electricity inputs 

to various parts of the plant calculated so it was used to calculate dry mill utility requirements for 

expansion and co-processing scenarios. 

Table 1. Corn dry mill natural gas electricity values from various models

Capital cost values were very different among the sources, which may be due in part to the year 

in which the study was done, the recent fluctuations in steel pricing, and the different plant sizes

studied. Capex (defined as total project investment per annual gallon) values included 

$1.53/gal for 40MM (USDA), $2.25/gal for 50MM (U of Minnesota) and to $1.80 or $2.11 for 

100MM (U of Minnesota, ISU). No other recent sources of capital cost were found, so the

100MM gpy ISU model cost, the most recent of the sources, was used and assumed to be 

2007$ (probably 2008/2009, but definitely after the steel spike of 2004/2005). A standard 

scaling exponent of 0.6 was used to calculate total project investment for the 70MM gpy dry mill 

plant size. When specific equipment costs were needed in the co-processing scenarios, the 

USDA model data, which was the only one with discrete installed equipment costs, was used.

The ISU model, which collects price data from the USDA Ag Market News and Energy 

Information Administration (EIA) for Iowa corn, DDGS, and natural gas, (Table 2) was used for 

operating costs. This study used the most recent values (September 2009) for corn, DDGS and 

natural gas, and the ISU model assumed values for other costs (e.g. chemicals and electricity)

for the mature corn dry mill costs. Finally, forecasted natural gas and electricity prices (used in 

the expansion scenarios, Table 8) were taken from EIA’s revised 2009 Annual Energy Outlook. 

Because no dry mill model had the same discounted cash flow rate of return calculation as the 

NREL model, net production cost, defined to include fixed and variable operating costs and 

depreciation, but no interest or return on investment, was used to compare the corn ethanol and 

cellulosic ethanol results more directly. The cellulosic production cost with 10% return (termed 

Minimum Ethanol Selling Price by NREL) is included in the results when available.

It was assumed dry mills will not become significantly more efficient in the future; however 

significant advances like membranes for ethanol dehydration or higher starch corn varieties may 

challenge this assumption. 

ISU Model U of Minn Model USDA Model

Natural gas cu ft/gal corn EtOH 30 34 32.2

Natural gas to DDGS dryer cu ft/gal corn EtOH ----- ----- 13.6

Natural gas to steam cu ft/gal corn EtOH ----- ----- 18.5

Electricity kWh/gal corn EtOH 0.9 0.75 0.92
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Table 2. Iowa Corn, DDGS and utility prices and net production costs (USDA, EIA)

Note: $3.15/bushel is equivalent to $132.40/dry ton for corn

For the cellulosic conversion process, a mature (estimated year 2022) biochemical process was 

used. The model used for this study is a more recent version of the NREL 2022 one reported 

in the 11/3/2008 technical memo. This model returns a Minimum Ethanol Selling Price (which 

includes a ROI of 10%) of $1.25/gal compared to the earlier model at $1.21/gal. A 2,000 dry 

tonne per day feed rate provides 71MM gpy ethanol with a yield of 91.9 gal/dry ton. A 32MM 

gpy plant model (900 dry tonne per day) was also developed to use in the expansion and coprocessing scenarios. Capital costs are 2007$. Refer to the NREL technical memo “Technical 

Modeling to Support the EPA Notice of Proposed Rulemaking (NOPR)”, dated 11/3/2008 for 

information on the detailed process parameters.

The comparison of a mature biochemical lignocellulose process with a dry mill operating with 

September 2009 costs shows that the cellulosic process has potential for significantly lower

operating costs but higher capital costs. Tables 3 and 4 show the process details for 32MM and 

71MM gpy corn and cellulosic plants. There is uncertainty in the corn dry mill capital costs due 

to lack of data across a range of plant sizes from a consistent source.

Table 3. 32MM gpy corn and cellulose process overview

Table 4. 71MM gpy corn and cellulose process overview

One of the questions that arose was how to select the size of facilities to investigate, both on the 

dry mill and cellulose side. The dry mill size was based on current average and emerging sizes, 

Current (9/09) Lowest Date Highest Date

Corn ($/bushel) $3.15 $1.48 October-05 $6.45 June-08

DDGS ($/ton) $81 $68 January-05 $186 July-08

NG ($/1000 cu ft) $4.38 $4.38 August-09 $12.18 October-05

Electricity ($/kWh) $0.05 $0.05 NA $0.05 NA

Net production cost ($/gal) $1.42 $1.06 September-05 $2.49 June-08

Corn Dry Mill Greenfield Cellulosic

Ethanol Production Cost with 10% return ($/gal) NA $1.45

Ethanol Production Cost with 0% return ($/gal) $1.55 $1.02

Annual Production, corn OR cellulosics (MM gal/yr) 30 32

Yield (gal/bu or gal/dry ton) 2.8 91.9

Feedstock Cost ($/bu or $/dry ton) $3.15 $45.90

Total Project Investment ($MM) $97.3 $107.7

TPI per Annual Gallon ($/gal) $3.24 $3.37

Model ISU 30MM GPY KI I0910A30

Corn Dry Mill Greenfield Cellulosic

Ethanol Production Cost with 10% return ($/gal) NA $1.25

Ethanol Production Cost with 0% return ($/gal) $1.45 $0.908

Annual Production, corn OR cellulosics (MM gal/yr) 70 71

Yield (gal/bu or gal/dry ton) 2.8 91.9

Feedstock Cost ($/bu or $/dry ton) $3.15 $45.90

Total Project Investment ($MM) $167 $185

TPI per Annual Gallon ($/gal) $2.39 $2.61

Model ISU 70MM GPY KI I0910A
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70 to 100MM gpy, as discussed earlier. One way to determine cellulosic plant sizes for the 

expansion and co-processing scenarios was to look at the ratio of per acre corn and stover 

yields, assuming that the first cellulosic feedstock supplied to an expanded dry mill would be 

stover from the corn suppliers. For corn yields around 140 bushels per acre (USDA reported

average for 2000 – 2009), the stover production is about 1:1 with the corn grain. Assuming 50% 

of the stover can be sustainably harvested, the ratio of grain to harvested stover is 2:1. If grain 

yield continues to increase, as agricultural experts expect, the ratio could be slightly less, as the 

1:1 ratio does not hold at higher yields (ISU Extension, 2002). Additionally, harvesting practice 

could allow a greater amount to be removed sustainably. A ratio of 1.87:1 corn: stover was 

used for this study based on the analysis and assumptions of 0.8:1 stover to grain grown, and 

67% harvested, as shown in Table 5. This ratio of feed rates translates into a corn ethanol to 

cellulosic ratio of around 2:1, or roughly a 30MM gpy stover plant in conjunction with a 70MM 

gpy dry mill for the expansion and co-processing scenarios. Additionally, a 70MM gpy dry mill 

with a 70 MM gpy cellulosics plant was also investigated in both the expansion and coprocessing scenarios; this assumes the plant is subsidized with additional cellulosic feedstock 

such as switchgrass or poplar, which could be in production by 2022. Table 5 also shows the 

potential ethanol per acre from combined corn and stover for current trends and if estimates of 

increased per acre yield, higher starch varieties and improved harvesting practice become 

reality.

Table 5. Ethanol production per acre cropland – current and forecasted

Notes: Estimate of potential yield with seed improvements = 300 bushel per acre average by 2030 (Monsanto).

Estimate of potential yield with high starch varieties = 2-4% increase (Growers for Biotechnology, Inc.)

Scenarios and Results

The 2004 NREL/USDA study only considered a greenfield corn/cellulose co-processing facility. 

The current study looked at expanding or retrofitting dry mills in addition to new installations.

Current Corn Data bu/acre dry ton/acre gal/bu gal/dry ton gal/acre

Corn grain 147 4.88 2.77 408

Corn:Stover, grown (1:1) 4.88

Stover, collected (50%) 2.44 91.9 224

Corn:Stover ratio 2.00

Total ethanol per acre 632

Corn Etoh:Stover Etoh ratio 1.82

Future Corn Estimates

Corn grain 300 8.40 2.85 856

Corn:Stover, grown (1:0.8) 6.72

Stover, collected (67%) 4.50 91.9 414

Corn:Stover ratio 1.87

Total ethanol per acre 1,270

Corn Etoh:Stover Etoh ratio 2.07

Feed Yields Ethanol Yields
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Expansion Scenarios

These scenarios considered the benefits of expanding an existing corn dry mill with the ability to

convert cellulose to ethanol. POET’s LIBERTY project is an example of this scenario, using 

corn cobs and expanding an existing 50MM gpy corn dry mill with 25MM gpy cellulosic ethanol 

capacity. POET also plans to expand the corn ethanol capacity by an additional 50MM gpy, 

making the plant capacity 125MM gpy total. For this study, a 70MM gpy dry mill was expanded 

by adding either 30 or 70MM gpy cellulosic ethanol capacity. It was assumed there was no 

extra capacity in the dry mill’s product recovery, utilities or waste handling systems.

For the cellulosic plant addition, the operating costs are the same as a greenfield plant with the 

added cost of additional biomass to supply steam for the added dry mill burden – for the 

purposes of this study, this cost is passed on to the dry mill and offset by the gas and power 

savings. A bigger combined heat and power (CHP) island (in the form of a fluidized bed 

combustor and steam turbogenerator) adds to the capital cost, but the savings in natural gas 

and electricity from the dry mill are larger than the combined costs of biomass and increased 

CHP capital. There could also be some additional, minor savings in fixed costs, site prep and 

permitting for the expansion (not included). One could also consider a gasifier for supplying the 

steam and power to the processes. The cellulose expansion costs are compared to the 

greenfield costs in Tables 6 & 7. The emissions estimates for the expansion are in Table 11.

Table 6. 32MM gpy greenfield and expansion costs

Cellulosic Process - greenfield Cellulosic Process - expansion

Ethanol Production Cost with 10% return ($/gal) $1.45 $1.65

Ethanol Production Cost with 0% return ($/gal) $1.02 $1.11

Annual Production (MM gal/yr) 32 32

Yield (gal/dry ton) 91.9 91.9

Feedstock Cost ($/dry ton) $45.90 $45.90

Total Project Investment ($MM) $107.7 $131.2

TPI per Annual Gallon ($/gal) $3.37 $4.11

Model I0910A30 I0910DME7030

Capital Costs by Area 

Feed Handling $0 $0

Pretreatment $11,300,000 $11,300,000

Neutralization/Conditioning $800,000 $800,000

Saccharification & Fermentation $5,300,000 $5,300,000

Distillation and Solids Recovery $13,500,000 $13,500,000

Wastewater Treatment $2,000,000 $2,000,000

Storage $1,500,000 $1,500,000

Boiler/Turbogenerator $24,500,000 $38,900,000

Utilities $3,600,000 $3,400,000

Total Installed Equipment Cost $62,500,000 $76,700,000

Added Costs $45,200,000 $54,500,000

(% of TPI) 42% 42%

Total Project Investment $107,700,000 $131,200,000

Operating Costs 

Feedstock $0.500 $0.500

Biomass to Boiler $0.000 $0.178

CSL $0.024 $0.024

Cellulase $0.075 $0.075

Other Raw Materials $0.142 $0.141

Waste Disposal $0.015 $0.020

Electricity -$0.087 -$0.229

Fixed Costs $0.178 $0.194

Capital Depreciation $0.169 $0.207
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Table 7. 71MM gpy greenfield and expansion costs

The dry mill has the same operating costs as without the expansion minus the natural gas to the 

boiler and electricity costs; there are still natural gas costs for the DDGS dryer and thermal 

oxidizer. The natural gas boiler is idled; it could be used for start-up or other intermittent use. 

The savings to the dry mill are offset by the costs of the additional biomass and CHP capacity 

required to supply the dry mill with steam and electricity. Table 8 shows the savings for several 

forecasted natural gas and electricity prices. For the 30MM gpy cellulose expansion, $0.053 -

$0.124/gal corn ethanol savings are predicted. For the larger 70MM gpy expansion, $0.023 -

$0.094/gal corn ethanol savings are predicted. Another way to look at it is that the average 

ethanol price from the 2 standalone plants (one corn, one cellulosic) would be $1.27/gal for 

100MM gpy (Table 3, ($1.55 x 30 + $1.02 x 32)/62) or $1.18/gal for 140MM gpy while the 

average cost for the expanded plant is $1.23/gal for 100MM gpy (Tables 3, 6 & 8 (2020 values), 

($1.55 – $0.198) x 30 + $1.11 x 32)/62) or $1.11/gal (140MM gpy).

Advantages to an expansion are the savings in natural gas, especially when gas prices are 

high. Of course, a dry mill could just install a biomass-fed CHP system to see the same 

savings. For the cellulosic plant, there could be some advantage to an existing site, 

experienced labor and having some permitting in place. A disadvantage could be that one is 

Cellulosic Process - greenfield Cellulosic Process - expansion

Ethanol Production Cost with 10% return ($/gal) $1.25 $1.33

Ethanol Production Cost with 0% return ($/gal) $0.908 $0.972

Annual Production (MM gal/yr) 71 71

Yield (gal/dry ton) 91.9 91.9

Feedstock Cost ($/dry ton) $45.90 $45.90

Total Project Investment ($MM) $185 $196

TPI per Annual Gallon ($/gal) $2.61 $2.76

Model I0910A I0910DME7070

Capital Costs by Area 

Feed Handling $0 $0

Pretreatment $18,300,000 $18,300,000

Neutralization/Conditioning $1,400,000 $1,400,000

Saccharification & Fermentation $9,300,000 $9,300,000

Distillation and Solids Recovery $23,300,000 $23,300,000

Wastewater Treatment $3,100,000 $3,100,000

Storage $2,400,000 $2,400,000

Boiler/Turbogenerator $43,600,000 $51,100,000

Utilities $6,000,000 $5,100,000

Total Installed Equipment Cost $107,300,000 $113,900,000

Added Costs $77,700,000 $81,900,000

(% of TPI) 42% 42%

Total Project Investment $185,000,000 $195,800,000

Operating Costs 

Feedstock $0.500 $0.500

Biomass to Boiler $0.000 $0.048

CSL $0.024 $0.024

Cellulase $0.075 $0.075

Other Raw Materials $0.151 $0.149

Waste Disposal $0.015 $0.016

Electricity -$0.098 -$0.093

Fixed Costs $0.112 $0.119

Capital Depreciation $0.130 $0.147
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forced to a smaller plant size due to land constraints or feedstock supply, and the benefits of 

economies of scale are lost.

Table 8. Savings from adding a cellulosic process expansion to a dry mill

Retrofit Scenarios

A brief literature search found one retrofitting study (Laser et al, 2007). That study showed that 

the decision to retrofit a dry mill from corn to cellulose depended on a variety of factors including 

corn price, cellulosic operating cost and the cost to retrofit the plant. The study suggested what 

areas of the dry mill could be reused and that information was used in determining the potential 

for equipment re-use in this study.

The retrofit scenarios considered taking an existing 100MM gpy corn dry mill and converting it to 

a 70MM gpy cellulose to ethanol facility. Another retrofitting option that is gaining much interest 

is converting a dry mill from ethanol to chemicals production. For this study, it was found that a

100MM gpy dry mill provides close to the required fermentation and distillation capacity needed 

for a 70MM gpy stover plant using mature biochemical technology. The ethanol concentration is 

the bottleneck – cellulosic feedstocks can provide up to about 10% by volume ethanol; corn can 

make 12-20% by volume (USDA, POET). This means the same equipment could only process 

50-80% cellulosic derived ethanol. Since the NREL model used here is 71MM gpy, the fit was 

good. An optimistic and less optimistic set of assumptions were used to determine the range of 

possible savings, and the cellulosic capital costs were either zeroed out or reduced from a 

starting greenfield scenario. Optimistic assumptions included using the entire product recovery 

system (beer column, rectifier, mol sieve), evaporators, wastewater treatment, utilities (recycle 

water, cooling water, air (added filter press requirements), and ethanol, gas and propane 

storage as is. Some of the dry mill fermentors were assumed suitable for the later stages of 

cellulosic fermentation and as a beer well, so only 50% new fermentors were considered

required. All new feed handling, pretreatment, enzymatic hydrolysis and CHP equipment was 

required. The corn handling and saccharification/liquefaction equipment becomes idle as does 

Industrial Prices (2007$, April 2009 AEO) 2010 2015 2020 2030

Natural gas $/thou cu ft $5.65 $6.51 $7.68 $8.94

Electricity $/kWh $0.057 $0.059 $0.064 $0.070

Utilities Cost per Gallon Corn Ethanol

Natl Gas to DDGS $/gal Corn EtOH $ 0.075 $ 0.086 $ 0.102 $ 0.119

Natl Gas to steam $/gal Corn EtOH $ 0.102 $ 0.117 $ 0.139 $ 0.161

Electricity $/gal Corn EtOH $ 0.053 $ 0.054 $ 0.059 $ 0.065

NG & Electricity costs $/gal Corn EtOH $ 0.155 $ 0.172 $ 0.198 $ 0.226

Expansion Scenario - 70MM gpy corn ethanol, 70MM gpy cellulosic ethanol

Stover cost actual $/gal Corn EtOH $ 0.049 $ 0.049 $ 0.049 $ 0.049

Increase in capital cost $/gal Corn EtOH $ 0.079 $ 0.079 $ 0.079 $ 0.079

Elec revenue decrease/(inc) $/gal Corn EtOH $ 0.004 $ 0.004 $ 0.004 $ 0.004

NG & Electricity savings $/gal Corn EtOH $ (0.155) $ (0.172) $ (0.198) $ (0.226)

Net cost/(savings) to dry mill $ (0.023) $ (0.040) $ (0.066) $ (0.094)

Expansion Scenario - 70MM gpy corn ethanol, 30MM gpy cellulosic ethanol

Stover cost actual $/gal Corn EtOH $ 0.081 $ 0.081 $ 0.081 $ 0.081

Increase in capital cost $/gal Corn EtOH $ 0.084 $ 0.084 $ 0.084 $ 0.084

Elec revenue decrease/(inc) $/gal Corn EtOH $ (0.064) $ (0.064) $ (0.064) $ (0.064)

NG & Electricity savings $/gal Corn EtOH $ (0.155) $ (0.172) $ (0.198) $ (0.226)

Net cost/(savings) to dry mill $ (0.053) $ (0.070) $ (0.096) $ (0.124)
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the natural gas boiler. Some site development was assumed needed for the new equipment, so 

the ratio of indirect costs to new equipment was kept the same. Less optimistic assumptions

included assuming a new beer column is needed along with all new fermentors and 50% 

additional cooling water capacity. Table 9 shows that the capital costs can be greatly reduced.

Table 9. Cost reductions from a dry mill retrofit to process cellulosic feeds (70MM gpy)

Advantages to a retrofit are the reduced capital investment, already developed site and 

potentially shorter construction time. Disadvantages include idled grain processing equipment, 

and the risk and cost associated with altering equipment designed for one purpose to another. 

Figure 1 compares the production and capital costs of several types of 70MM gpy plants. Less 

mature cellulosic processes (i.e. with higher costs or lower yields) will be higher than the mature 

2022 technology used in this study.

Figure 1. Comparison of greenfield and retrofitting options for corn and cellulose ethanol

Greenfield Cellulosic Optimistic Dry Mill Retrofit Less Optimistic DM Retrofit

Ethanol Production Cost with 10% return ($/gal) $1.25 $1.11 $1.15

Ethanol Production Cost with 0% return ($/gal) $0.908 $0.858 $0.872

Annual Production, Corn/Cellulosics (MM gal/yr) 71 71 71

Yield (gal/dry ton) 91.9 91.9 91.9

Feedstock Cost ($/dry ton) $45.90 $45.90 $45.90

Total Project Investment ($MM) $185.0 $134.5 $148.7

TPI per Annual Gallon ($/gal) $2.61 $1.90 $2.10

Model I0910A I0910A optimistic DM retrofit I0910A less optimistic DM retrofit

Capital Costs by Area

 Feed Handling 0 0 0

 Pretreatment 18,300,000 18,300,000 18,300,000

 Neutralization/Conditioning 1,400,000 1,400,000 1,400,000

 Saccharification & Fermentation 9,300,000 7,100,000 9,300,000

 Distillation and Solids Recovery 23,300,000 6,300,000 9,000,000

 Wastewater Treatment 3,100,000 0 0

 Storage 2,400,000 1,000,000 1,000,000

 Boiler/Turbogenerator 43,600,000 43,600,000 43,600,000

 Utilities 6,000,000 700,000 4,100,000

Total Installed Equipment Cost 107,300,000 78,400,000 86,600,000

Added Costs 77,700,000 56,100,000 62,100,000

 (% of TPI) 42% 42% 42%

Total Project Investment 185,000,000 134,500,000 148,700,000

$0.00

$0.50

$1.00

$1.50

$2.00

$2.50

$3.00

Production Costs CAPEX

$/gal Ethanol

Corn Dry Mill vs. Mature Biochemical Cellulosics Processing

70MM GPY each

Today's Dry Mill costs

2005-2009 Lowest Dry Mill Costs

Greenfield Cellulosic Processing

Optimistic Retrofitted Corn Mill to Cellulosic Feeds
11

Co-processing Scenarios

These scenarios looked at building a highly integrated greenfield corn and stover processing 

facility; effectively a 70MM gpy corn facility combined with either 32 or 71MM gpy cellulose 

process to create roughly 100MM or 140MM gpy facilities. Depending on the RFS limits on 

renewable biofuels (currently capped at 15MM gpy in 2015), this may not be an attractive 

scenario in the future compared to an all cellulosic facility. As in the expansion scenarios, the 

32MM gpy cellulose portion was selected to show processing costs for all the corn and stover 

per acre. The 71MM gpy cellulose portion assumes additional cellulosic feeds like switchgrass 

or poplar would be used to supplement the corn feeds.

The earlier NREL/USDA study on co-processing found that mixing the streams before 

fermentation resulted in a low protein, lignin containing solid co-product with dubious value as 

livestock feed, so this study (Figure 2) investigated removing the solids (lignin and some 

unconverted cellulose) after enzymatic hydrolysis and before fermentation. A separation step

was added after enzymatic hydrolysis and then the corn mash and hydrolyzed liquor were 

mixed and fermented together. If the enzyme is still active at fermentation temperature (i.e. an 

SSF), some cellulose conversion will be lost with this design because the cellulose will not be in 

the fermentation vessel. Some sugars are also lost with the solids; however this can be 

reduced by a washing separation process (not studied). 

Figure 2. Integrated co-processing of corn and cellulosics

Corn Stover

Milling

Ethanol

Milling

DDG

Cooking

Liquefaction

Saccharification

Co-Fermentation

Pretreatment

Enzymatic 

Hydrolysis

Ethanol

Purification

DDGS

Recovery

CHP, WWT & Other

Utilities

Ethanol

Stripping Lignin to CHP

S/L Separation

Syrup to CHP
12

Another method to remove the lignin prior to mixing with the corn grain solids are fractionation 

technologies such as ARP or clean fractionation to take the lignin out in a pretreatment step. 

Adding the corn mash resulted in higher sugar and ethanol concentrations in fermentation; the 

70MM gpy corn/32MM gpy cellulosics case has 20 wt% sugars and 11.7 wt% ethanol and the 

70MM gpy corn/71MM gpy cellulosics case has 17 wt% sugars and 10 wt% ethanol. In 

comparison, the corn ethanol industry has had success pushing the ethanol concentration up to 

18 wt%; however, the robustness of the ethanologen comes into play, both in the sugar and 

ethanol concentrations and the glucose/xylose ratio. Current co-fermenting organisms 

preferentially consume glucose and xylose inhibition will be greater with more glucose available. 

The syrup from the evaporators was sent to the burner; however, some of this could also be 

sprayed onto the DDG to make DDGS (distillers dried grains with solubles) which is standard

practice in the dry milling industry. 

The co-processing scenarios were not rigorously modeled; rather the corn mash stream and 

steam demands were added to the cellulose material and energy balance model and all other 

changes were made via the Excel cost model. Electricity, natural gas and other dry mill variable 

costs for grain handling, saccharification and liquefaction and DDG drying/handling were added

as well as the installed equipment costs for these operations from the USDA model. September 

2009 Iowa values for corn and natural gas price and DDG value were used from the ISU model. 

Dry mill variable costs other than corn (enzymes, chemicals) totaled $4.5MM per year or about

$0.04/gal ethanol. Labor and other fixed costs were scaled by the combined corn/stover feed 

rate to plant with a 0.25 exponent.

The DDG drying was not modeled in Aspen, but the solids portion of the DDG stream from the 

model was used to calculate the DDG value in the Excel cost model. The emissions estimate 

for the DDG dryer vent is from the USDA model and the other emissions (Table 12) are from the 

combined model. A much more detailed Aspen model would be required to develop a rigorous 

energy balance; however it can be said that the feedstocks and natural gas to the DDG dryer 

are the primary energy inputs while the ethanol and DDG are the primary outputs. Other 

outputs include the flue gas and vent gas streams and the cooling tower evaporation.

Comparing the 100MM gpy models (Table 10 and Figure 3), the ethanol production cost is lower 

for the combined plant than the dry mill and the capex is virtually the same. A corn to ethanol 

yield of 2.77 is equivalent to 118 gal/dry ton corn, so the yield is lower with the combined plant, 

an effect of the lower yields for cellulosics throughout the process. The larger 140MM gpy 

combined plant benefits from the economies of scale which reduces the capex.

The advantages of co-processing include utilizing the entire crop from an acre of corn and 

having the ability to run a variety of feedstocks. The disadvantages come from the more 

complex process and potential issues with high glucose in mixed sugar fermentation, and this 

would require some R&D to understand the options and limitations.
13

Table 10. Costs for dry milling of corn vs. integrated co-processing corn and cellulosics

Figure 3. Comparison of greenfield and co-processing options for corn and cellulose

Corn Dry Mill

Ethanol Production Cost with 10% return ($/gal) NA $1.46 $1.35

Ethanol Production Cost with 0% return ($/gal) 1.42 $1.18 $1.09

Annual Production, Corn/Cellulosics (MM gal/yr) 100/0 102.6 137.7

Yield (gal/bu or gal/dry ton) 2.8 104 97.6

Feedstock Cost ($/bu or $/dry ton) $3.15 (corn) $3.15/$45.90 $3.15/$45.90

Ratio of corn to cellulosic feedstock 1:0 1.87:1 0.83:1

Total Project Investment ($MM) $211 $217.0 $277

TPI per Annual Gallon ($/gal) $2.11 $2.12 $2.01

Model ISU 100MM GPY KI I0910D5 I0910C3

Capital Costs by Area 

Feed Handling $0 $0

Pretreatment $11,200,000 $18,200,000

Neutralization/Conditioning $800,000 $1,400,000

Saccharification & Fermentation $9,900,000 $14,300,000

Distillation and Solids Recovery $25,200,000 $30,100,000

Wastewater Treatment $2,400,000 $3,400,000

Balance of Dry Mill $24,400,000 $24,400,000

Storage $2,400,000 $3,200,000

Boiler/Turbogenerator $45,800,000 $60,700,000

Utilities $5,100,000 $6,500,000

Total Installed Equipment Cost $127,100,000 $161,900,000

Added Costs $89,900,000 $115,100,000

(% of TPI) 41% 42%

Total Project Investment $217,000,000 $277,000,000

Operating Costs 

Feedstock $0.156 $0.257

Biomass to Boiler $0.000 $0.000

Corn $0.824 $0.614

CSL $0.015 $0.018

Cellulase $0.023 $0.038

Other Raw Materials $0.087 $0.109

Waste Disposal $0.004 $0.007

Natural Gas $0.044 $0.033

Electricity -$0.063 -$0.085

DDG -$0.102 -$0.081

Fixed Costs $0.086 $0.076

Capital Depreciation $0.105 $0.100

Corn/Cellulose Co-processing

$0.00

$0.50

$1.00

$1.50

$2.00

$2.50

Production Costs CAPEX

$/gal Ethanol

Corn alone vs. Corn & Stover Co-processing

100MM GPY each

Corn only at today's Dry Mill costs

Corn & Cellulosics Co-processing
14

Table 11. Estimate major emissions from cellulosic expansion of 70MM gpy dry mill

Table 12. Estimated major emissions from co-processing corn and corn stover

Fermentation off gas Boiler Ash Flue gas Fermentation off gas Boiler Ash Flue gas

Aspen stream # 308 809 810 308 809 810

Mass flow (kg/hr)

CO2 9,551.0 51,416.0 21,225.0 78,817.0

N2 0.0 164,855.0 0.0 256,512.0

O2 80.0 8,044.0 177.0 12,587.0

Methane 0.9 2.1

NO2 47.2 72.0

SO2 1,011.0 2,588.0

CO 47.0 72.0

Water 191.0 43,692.0 423.0 80,850.0

Ethanol 242.0 0.0 549.0 0.1

Furfural 2.2 0.1 5.0 0.2

HMF 0.8 1.8

Other solubles 2.0 - 605.7 1.1 - 1,332.1

Ash 2,629.0 31.9 4,726.0 57.4

Other solids 191.2 2.3 - 239.6 2.9

32MM gpy cellulosic expansion 71MM gpy cellulosic expansion

Fermentation off gas Boiler Ash Flue gas DDGS Dryer Vent Fermentation off gas Boiler Ash Flue gas DDGS Dryer Vent

Aspen stream # 308 809 810 66VENT 308 809 810 66VENT

Mass flow (kg/hr)

CO2 32,523.0 56,624.0 43,011.0 96,286.0

N2 0.0 181,714.0 0.0 309,064.0

O2 165.0 8,662.0 253.0 14,998.0

Methane 1.1 2.1

NO2 57.3 93.7

SO2 888.0 2,474.0

CO 57.3 93.7

Water 642.0 45,342.0 42,864.0 855.0 108,651.0 42,864.0

Ethanol 1,103.0 0.0 15.5 1,298.0 0.1 15.5

Furfural 2.3 0.3 5.0 0.6

HMF 0.8 1.7

Other solubles 2.0 - 317.4 - 2.3 - 422.9 -

Ash 1,901.0 23.1 4,224.0 51.3

Other solids - 78.1 0.9 - - 166.6 2.0 -

100MM gpy co-processing 140MM gpy co-processing