Technical Report

NREL/TP-510-46214

August 2009

Biochemical Production of 

Ethanol from Corn Stover: 2008 

State of Technology Model 

D. Humbird and A. Aden
National Renewable Energy Laboratory

1617 Cole Boulevard, Golden, Colorado 80401-3393

303-275-3000 • www.nrel.gov

NREL is a national laboratory of the U.S. Department of Energy

Office of Energy Efficiency and Renewable Energy

Operated by the Alliance for Sustainable Energy, LLC

Contract No. DE-AC36-08-GO28308

Technical Report

NREL/TP-510-46214

August 2009

Biochemical Production of 

Ethanol from Corn Stover: 2008 

State of Technology Model 

D. Humbird and A. Aden

Prepared under Task No. BB07.2410
NOTICE

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iii

Table of Contents

List of Figures ................................................................................................................... iii

List of Tables .................................................................................................................... iii

1 Introduction ............................................................................................................1

2 State of Technology for FY 2008 ..........................................................................1

2.1 Feedstock ............................................................................................................ 3

2.2 Pretreatment Area ............................................................................................... 4

2.3 Enzymatic Saccharification Area ........................................................................ 6

2.4 Fermentation Area .............................................................................................. 6

3 Progression to $1.49/gal MESP in 2012 ...............................................................7

4 Concluding Remarks .............................................................................................8

5 References ...............................................................................................................9

Appendix ...........................................................................................................................11

List of Figures

Figure 1. SOT MESPs for 2001-2008, corrected for 2007 dollars and feedstock price. 

MESPs beyond 2008 are forecast based on research targets. ............................................. 3

List of Tables

Table 1. Key FY08 Accomplishments in Biochemical Ethanol Research at NREL .......... 2

Table 2. Comparison of MESP (2007$) for Different Pretreatment Scenarios .................. 5

Table 3. Development Targets and Associated MESP (cases from 2007 to 2012 in 2007$, 

cases after 2008 based on FY08 model) ............................................................................. 8

Table 4. Comparison of 2012 MESP Adjusting for Year-dollars and Feedstock Cost 

Assumption ......................................................................................................................... 8
1 

1 Introduction

In 2002, the National Renewable Energy Laboratory (NREL) published a detailed report 

documenting a process design and economic analysis for the biochemical conversion of 

lignocellulosic biomass (corn stover) to ethanol [1]. The overarching process design 

chosen for the report was dilute acid pretreatment of corn stover followed by enzymatic 

saccharification and pentose/hexose cofermentation using recombinant Zymomonas 

mobilis. Aspen Plus chemical process simulation software [2] was used to compute 

material and energy balances for a biorefinery processing 2,000 dry metric tons of corn 

stover per day. Equipment and operating costs were computed from simulation results in 

a spreadsheet framework assuming “nth-plant” economics. Using a discounted cash flow 

rate of return analysis, the minimum ethanol selling price (MESP) required to meet a 

10% internal rate of return was obtained.

The primary function of the design report was to determine the chemical and biochemical 

conversion targets required to attain the ethanol cost goals put forth by the U.S. 

Department of Energy (DOE) in terms of MESP. In the 2002 design report, the DOE goal 

for MESP in 2012 was $1.07/gallon ethanol in 2000 dollars. When inflation was factored 

into capital, chemical, feedstock collection, and labor costs, the updated MESP target 

became $1.49/gal ethanol in 2007 dollars. 

The annual State of Technology (SOT) model is complementary to the design report. 

With minimal change to the unit operations in the Aspen Plus model, the projected

conversions from the design report are replaced by present conversions that have been 

experimentally verified. All data in this report have been demonstrated experimentally, 

with a preference for data obtained in NREL’s ~1 ton/day pilot facility. Not all 

conversion parameters can be practically obtained in the pilot plant, so the SOT contains 

both laboratory-scale data and pilot-scale data. The SOT is arguably a misnomer since no 

commercial cellulosic ethanol plants exist today. The aim of the SOT analysis is not to 

calculate the current commercial cost of cellulosic ethanol. Instead, it is meant to reflect 

NREL’s best estimate of ethanol production costs in a hypothetical “nth plant” using our 

current best slate of demonstrated technical capabilities. By comparing the SOT yearover-year, research advances can be quantified in terms of economic improvements in the 

overall process. As such, progress toward the 2012 goal of $1.49/gal can be tracked. 

2 State of Technology for FY 2008

During Fiscal Year (FY) 2008, several key biochemical platform accomplishments were 

achieved and documented, spanning all major areas of research including feedstock, 

pretreatment, conditioning, enzymatic hydrolysis, and fermentation. Table 1 presents a 

sample of these accomplishments and their respective importance toward meeting 

programmatic goals. While significant analytical experience was gained on switchgrass, 

corn stover remained the targeted feedstock for this SOT update. Note that several of the 

highlighted accomplishments, particularly in the area of enzymatic saccharification, were 

new findings or new tools. For example, NREL has successfully developed a quantum 

mechanical model that describes how CBH1 molecules in the enzyme cocktail proceed 

down the length of a cellulose chain. Subsequent research plans for FY 2009 will use this 
2 

knowledge to improve the specific activity of enzyme systems, which in turn should 

reduce their overall cost to the process.

Table 1. Key FY08 Accomplishments in Biochemical Ethanol Research at NREL

The key updates to the FY 2008 SOT model were in the pretreatment and fermentation 

areas and in feedstock collection. Advancements in NREL’s Pretreatment and Enzymatic 

Hydrolysis Subtask [3] showed that certain process changes were necessary to meet 

target conversions of xylan to monomeric xylose. In the fermentation area, some positive 

results regarding fermentation of xylose were included in the model. In the feedstock 

collection area, new corn stover cost data provided by Idaho National Laboratory and 

Oak Ridge National Laboratory were incorporated into the overall economics. Figure 1 

Process Area Accomplishment Importance/Significance

Feedstock

Developed and/or adapted dry corn 

stover analytical methods for 

switchgrass

Enables the addition of energy crops to 

the platform research activities

Assessed processability of ensiled corn 

stover

Gives picture of how wet stored 

residues will convert

Pretreatment

Achieved target of 75% xylan 

conversion to xylose at continuous 

reactor scale at 30% solids loading

Critical step towards 2012 pilot scale 

run and cost competitive ethanol

Developed more detailed 

understanding of hydrolysate toxicity 

and its impact on fermentation

Overcoming this technical barrier is a 

key aspect of obtaining high yields and 

successful 2012 demonstration

Confirmed ammonium hydroxide as 

preferred conditioning option vs. 

overliming

Reduces sugar losses and potential for 

fermentative enhancement, also 

begins determining process for 2012

Series of Joule milestones completed 

on accessory enyzmes

First step towards understanding the 

potential of accessory and 

hemicellulase enzymes to improve 

overall yields

First-of-a-kind quantum mechanical 

model of CBH1, from CBM linker to 

catalytic domain

Tells us exactly how this key enzyme 

functions, can be used to develop 

targeted enzyme improvements that 

can be used to enhance the enzyme 

performance

New tools: protein crystallography

can now develop actual 3D structures 

of enzymes to better understand the 

structure/function relationship with 

activity

Testing of current industrial enzymes

Important to understand how they 

perform under process relevant 

conditions (high solids)

Fermentation Successful benchmarking of all 

ethanologen projects

Establishes baseline performance for 

current industry leading organisms, 

provides pathway for obtaining 

organism for use during 2012 demo

Conditioning

Saccharification
3 

shows the MESP computed for the years 2001-2008 (with the exception of 2006, when 

this exercise was not commissioned) and the research targets to 2012. For consistency, 

values in Figure 1 before 2008 were updated to use 2007 dollars and the 2007 feedstock 

cost of $69.60/ton. (The numbers may therefore appear different from previous versions 

of this figure in older publications.) The SOT MESP for FY 2008 was slightly down 

compared with FY 2007 at $2.61/gal due to a reduction in the feedstock cost assumption. 

Conversion costs were flat compared with FY 2007 at $1.71/gal; however, we note that 

some of the key conversion targets were demonstrated at pilot scale in a continuous 

fashion for the first time in FY 2008. In other words, the FY 2008 number is ‘harder’ 

than the FY 2007 number, which was based on laboratory-scale conversion data and 

therefore contained a higher level of uncertainty. It is believed that the technical targets 

needed to reach $1.49/gal by 2012 are still achievable and that the learning from scale-up 

efforts in 2008 will translate directly into economic improvements for 2009 through 

2012. 

Figure 1. SOT MESPs for 2001-2008, corrected for 2007 dollars and feedstock price. 

MESPs beyond 2008 are forecast based on research targets.

2.1 Feedstock

Feedstock, be it corn stover or other biomass, is a significant cost contributor to MESP. 

The cost basis used in previous SOT assessments for washed and milled corn stover 

delivered to the throat of the pretreatment reactor was $60/dry ton through 2008 and 

$0.00 

$1.00 

$2.00 

$3.00 

$4.00 

$5.00 

$6.00 

$7.00 

2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012

Minimum Ethanol Selling Price (2007$ per gallon) 

Conversion Feedstock

$6.47

$2.85 $2.69 $2.61

$2.36

$1.98

$1.68 $1.49

$4.93

$3.89

$3.13
4 

$46/dry ton in the years 2009–2012. Idaho National Laboratory (INL) and Oak Ridge 

National Laboratory (ORNL) performed DOE research in 2008 regarding feedstock 

harvest and logistics and grower payments [3,4]. Their combined numbers were 

significantly higher than the cost assumptions used in previous SOTs, with feedstock cost 

estimated at $69.60/dry ton in 2007 and decreasing modestly each year to reach 

$50.90/dry ton in 2012. This cost increase over the previous assumption was incorporated 

into the 2008 SOT (and previous years, as noted above) and resulted in a higher MESP. 

2.2 Pretreatment Area

In the pretreatment area, significant improvements in the conversion of xylan to xylose 

were demonstrated at the bench scale in FY 2007. For example, 75% conversion of xylan 

to xylose was demonstrated in high-solids experiments performed in bench-scale batch 

pretreatment reactors. The goal for FY 2008 was to demonstrate equivalent conversion to 

xylose (75%) and reduced loss to degradation products (8%) at the pilot scale in 

continuous operation. In other words, the FY 2008 targets for this area were intended to 

enhance process scale-up experience and not to provide a significant reduction in MESP. 

The FY 2008 targets for the pretreatment area were:

• 75% conversion of xylan to monomeric xylose 

• Loss of xylan to degradation products (mostly furfural) of less than 8% 

• >30% solids loading 

• Continuous operation at pilot-scale volume

Continuous pretreatment is more challenging than batch pretreatment because process 

conditions such as heat and mass transfer and residence time cannot be as tightly 

controlled. In their FY 2008 D Milestone [5], the Pretreatment and Enzymatic Hydrolysis 

Subtask reported that pretreatment runs carried out in the horizontal Sunds reactor [6] 

(modified for high solids operation) indicated a limit of conversion of xylan to 

monomeric xylose of ~60% in that reactor. Xylan conversion to the combined products of 

monomeric and oligomeric xylose was around 75%. Loss of xylan to furfural was 11%, 

and the balance (~14%) of xylan was unconverted. The simultaneous presence of high 

furfural and high unconverted xylan led to the hypothesis that poor residence time 

distribution control in the reactor was causing significant fractions of the feedstock to be 

either “undercooked” (unconverted) or “overcooked” (degraded to furfural) as it traveled 

through the reactor. 

Since the targets for monomeric xylose yield and loss to furfural could not be met using 

the horizontal reactor in its current configuration, a subsequent secondary mild hydrolysis 

step was added. This “oligomer hold” step converted a significant fraction of the 

oligomeric xylose to monomeric xylose without generating additional degradation 

products. The oligomer hold step is essentially a hold of the first reactor effluent in an 

agitated tank at 130°C for 20 minutes. With this step, the overall monomeric xylose yield 

met the target of 75%. For the FY 2008 update, a new unit operation was added to the 

SOT model to account for this step. 
5 

In the Aspen Plus model, the new oligomer hold reactor was modeled as a simple 

stoichiometric reactor with a defined conversion. Three reactors were used in parallel, 

following the description of the pretreatment train from the 2002 design report. The 

190°C effluent of the pretreatment reactor was flashed in an existing blowdown tank to 

about 2.5 atm to bring the temperature down to 130°C for the hold. The effluent was 

directed to the hold reactor, where the only reaction taking place was the conversion of 

xylose oligomers to monomeric xylose with a fractional extent of 70%. This brought the 

overall xylan-to-monomeric xylose yield to 75% between the two reactors. A new 

blowdown tank was added after the oligomer hold reactor to drop the pressure to 1 atm 

before feeding to the solid-liquid separator. The oligomer hold reactor was sized and 

costed using the Aspen Icarus Process Evaluator software [7]. Alloy 20-clad carbon steel 

was chosen for the reactor material due to the potential for corrosion from sulfuric acid. 

With further study on the corrosiveness of the hydrolyzate at the milder temperature of 

130oC, it may be possible to substitute 316 stainless steel or Teflon-lined carbon steel for 

a modest savings. The cost of the new blowdown tank was assumed to be identical to the 

existing one. The total installed equipment cost to implement the oligomer hold step was 

$2.1 million, or approximately 1.2 cents/gal of ethanol. 

Table 2 shows the effect of the oligomer hold step on MESP. When comparing the xylan 

conversion assumptions in FY 2007 with the ones made here (total conversion of xylan to 

xylose of 75% and a loss to degradation products of 11%), the MESP is essentially flat 

when the same feedstock cost is used. The additional expense of the oligomer hold 

equipment is offset by an increase in xylose-to-ethanol conversion demonstrated in the 

fermentation area (discussed in Section 2.4). This is compared with an alternative case 

using the typical performance of the vertical Sunds reactor, in which the initial 

monomeric xylose yield is higher at 70% but no oligomer hold step is used to convert the 

additional 6% oligomers to monomeric xylose. In this case, there would have been an 

increase in MESP of 4 cents/gal over FY 2007 due to the decreased xylose concentration 

entering fermentation, even with the additional ethanol conversion. 

Table 2. Comparison of MESP (2007$) for Different Pretreatment Scenarios 

At the level of xylan conversion discussed here, the oligomer hold step provides minimal 

savings but is presently considered a necessary addition in order to meet the targeted 

monomeric xylose yield at the pilot scale. Planned modifications to the pilot plant reactor 

in FY 2009 will tighten the residence time distribution and should improve the 

FY07 SOT Case 

(FY08 feedstock $)

FY08 SOT

Oligomer Hold

Vertical Sunds

No Oligomer Hold

First Stage Pretreatment

 Xylan to oligomer 6.0% 21.0% 6.0%

 Xylan to xylose 75.0% 60.0% 70.0%

 Xylan to furfural 13.3% 11.0% 11.0%

Second Stage (Oligomer Hold)

 Oligomer to xylose N/A 70% N/A

Overall Xylan to Xylose 75% 75% 70%

MESP ($/gal) $ 2.63 $ 2.61 $ 2.67

Xylose to ethanol in fermentation 76% 80% 80%
6 

conversion of xylan to monomeric xylose. However, even with improved residence time 

distribution, meeting the much more aggressive monomeric xylose (ultimately 90%) and 

xylan degradation (ultimately 5%) targets in the upcoming years seems unlikely without a 

strategy of targeting maximum oligomer yields (e.g., less severe pretreatment conditions) 

and an additional step to convert the oligomers to monomeric xylose. 

It should be noted that the oligomer hold step is not the committed solution for reaching 

higher xylan-to-xylose conversion. Other options for converting the oligomers are being 

explored, e.g., targeted enzymatic conversion. These solutions will all likely contribute 

some additional cost to the process and will be evaluated against each other as more 

information about their cost and efficacy becomes known. In the meantime, the Aspen 

Plus model will be used to investigate different possible modes of operation between the 

pretreatment (first-stage) and oligomer hold (second-stage) reactors. For example, if the 

conditions of the first stage of pretreatment were shifted to favor convertible oligomers

instead of monomers, then the first-stage reactor—currently about $20 million or 12 

cents/gal—could be designed for a lower temperature and pressure, resulting in a 

significant capital savings.

2.3 Enzymatic Saccharification Area

For the FY 2008 SOT, the assumed cost of cellulase enzymes was updated from 

$0.32/gal to $0.35/gal of ethanol to reflect inflation, since the older value was derived in 

2005 using the cost-metric calculations developed for a series of DOE enzyme research 

subcontracts. DOE has negotiated awards with several enzyme manufacturers and 

developers [8] who, in the coming years, will further improve the cellulase enzymes 

needed for cellulosic biofuels production. 

DOE and NREL have both received verbal feedback from the industry that the equivalent 

purchase price of cellulase enzymes exceeds $0.35/gal. However, more data are required 

before this cost can be confidently updated in the technoeconomic model. Under the 

enzyme award projects, DOE and NREL will be auditing technical and economic 

progress toward meeting program goals. As the performance of these projects is 

validated, those data can be used to better define a more accurate state of technology cost 

for enzymes in the biochemical process.

During FY 2009, NREL will be adding more technical detail to the basis of these enzyme 

costs. Instead of a fixed $/gal ethanol equivalent basis, the technoeconomic model will

begin to incorporate specifics on the price and composition of the actual enzyme product 

and its required feed rate. In this way, the technical progress in this area can be more 

accurately tracked and translated into cost reductions.

2.4 Fermentation Area

The FY 2008 targets for the fermentation area were:

• Increase in xylose-to-ethanol conversion from 76% to 80% 

• Increase minor sugar conversion (arabinose, galactose, mannose) to ethanol from 

0% to 40% 
7 

Research in the fermentation area is currently being performed outside of NREL through 

an ongoing ethanologen strain solicitation project. These data are protected under a 

Cooperative Research and Development Agreement (CRADA) and cannot be shared 

here, but a preliminary review of the results for the SOT report indicated that 80% 

xylose-to-ethanol conversion has been adequately demonstrated. This conversion was 

committed to the model and was used to generate the FY 2008 SOT MESP of $2.61/gal. 

In general, the ethanologen awardee companies had not made significant progress toward 

demonstrating improved conversion of any of the other minor sugars by the time of the 

SOT assessment. Some progress was shown on fermentation of arabinose under idealized

conditions, but we were reluctant to include these results in the technoeconomic model 

until they can be demonstrated under more process-relevant conditions. Minor sugar 

conversion accounted for a significant part of the planned reduction in MESP for FY 

2008. If conversion of all the minor sugars at the target level of 40% were realized, the 

MESP would have been $2.51/gal, including the oligomer hold equipment from the 

previous section. This target has been pushed to FY 2009. However, achieving the more 

aggressive fermentation targets in future years (e.g., 85% conversion of all sugars to 

ethanol on our specific pretreated material) will require a strategy that allows NREL to 

better evaluate the performance of the improved strains on process-relevant hydrolyzates 

throughout the strain development process. 

3 Progression to $1.49/gal MESP in 2012

Table 3 shows the MESP for the FY 2007 and FY 2008 SOT cases along with all planned 

development targets leading up to 2012. The targeted case and the final SOT case are 

both shown for FY 2008. The MESPs were computed in individual Aspen models using 

the conversion targets and feedstock and enzyme costs shown for the interim years. The 

2012 case shown here is a direct progression from the FY 2008 SOT process and is 

different from the 2012 market target process (i.e., the design report process [1]) in a few 

respects, but it still meets the $1.49/gal target. The process additions for oligomer 

conversion discussed here are of course included. Additionally, hydrolyzate conditioning 

with ammonium hydroxide instead of lime has been chosen as the research path going 

forward. Treatment of the whole hydrolyzate slurry with ammonia is ultimately planned, 

which eliminates a solid-liquid separation step. These changes will be reflected in an 

update to the design report scheduled for FY 2009. 
8 

Table 3. Development Targets and Associated MESP (cases from 2007 to 2012 in 2007$, 

cases after 2008 based on FY08 model)

At the end of FY 2008, the projected MESP for 2012 was $1.33/gal, as reported in an 

NREL internal milestone report [9], and this figure may be seen in Biomass Program goal 

documents that were produced around that time. The shift from $1.33/gal to $1.49/gal 

seen here was a result of using (1) the actual cost indices for 2007 instead of the 

estimated ones, (2) updated feedstock cost assumptions ($50.90/ton, up from $46/ton), 

and (3) an inflation correction to the enzyme cost contribution ($0.12/gal, up from 

$0.10/gal). The costs for the 2012 case are summarized in Table 4. The technical targets 

and conversions were the same in each case.

Table 4. Comparison of 2012 MESP Adjusting for Year-dollars and Feedstock Cost 

Assumption 

Year-dollars Feedstock $/dry ton Enzymes $/gal EtOH MESP $/gal

2007 estimated $46/ton $0.10/gal $1.33/gal

2007 actual $46/ton $0.12/gal $1.44/gal

2007 actual $50.90/ton $0.12/gal $1.49/gal

4 Concluding Remarks

In FY 2008, the MESP was slightly down compared with FY 2007 at $2.61/gal due to a 

modest reduction in feedstock cost. Conversion costs were flat from FY 2007 to FY 2008 

at $1.71/gal. Pilot-scale research in the pretreatment area ran into a limit of xylan-toxylose conversion for the continuous horizontal Sunds reactor used. An oligomer hold 

2007 SOT 2008 

Targets 2008 SOT 2009 2010 2011 2012

Minimum Ethanol Selling Price $2.69 $2.50 $2.61 $2.36 $1.98 $1.68 $1.49

 Feedstock Contribution ($/gal) $0.97 $0.86 $0.90 $0.74 $0.65 $0.60 $0.57

 Conversion Contribution ($/gal) $1.72 $1.64 $1.71 $1.62 $1.33 $1.08 $0.92

 Yield (Gallon/dry ton) 72 76 73 78 83 87 90

Technical Targets

Feedstock

 Feedstock Cost ($/dry ton) $69.60 $65.30 $65.30 $57.50 $53.70 $52.00 $50.90

Pretreatment

 Solids Loading (wt%) 30% 30% 30% 30% 30% 30% 30%

 Xylan to Xylose 75%a 75%b 75%b 80% 85% 88% 90%

 Xylan to Degradation Products 13%a 8%b 11%b 8% 6% 5% 5%

Conditioning

 Ammonia Loading (mL of 30wt% per L hydrolyzate) 50 50 50 50 50 35 25

 Hydrolyzate solid-liquid separation yes yes yes yes yes yes no

 Xylose Sugar Loss 2% 2% 2% 2% 2% 1% 1%

 Glucose Sugar Loss 1% 1% 1% 1% 1% 1% 0%

Enzymes

 Enzyme Contribution ($/gal EtOH) $0.35 $0.35 $0.35 $0.35 $0.17 $0.12 $0.12

Saccharification & Fermentation 

 Total Solids Loading (wt%) 20% 20% 20% 20% 20% 20% 20%

 Sugar loss to contamination 7% 7% 7% 7% 7% 5% 3%

 Combined Saccharification & Fermentation Time (d) 7 7 7 7 5 3 3

 Corn Steep Liquor Loading (wt%) 1% 1% 1% 1% 1% 0.6% 0.25%

 Overall Cellulose to Ethanol 85% 85% 85% 85% 85% 85% 85%

 Xylose to Ethanol 76% 80% 80% 80% 80% 85% 85%

 Minor Sugars to Ethanol 0% 40% 0% 40% 80% 85% 85%

Reference Aspen model DW0810R DW0810Q DW0810Z DW0810Y DW0810T DW0810U DW0810V

a = bench scale Targeted Improvements from 2007 to 2008

b = pilot scale/continuous operation Targeted Future Improvements Year-Over-Year
9 

step had to be added to convert xylose oligomers into xylose monomers to reach the 

target conversion of 75% xylan to xylose. This was a relatively inexpensive addition to 

the process and was offset by an increase in conversion of xylose to ethanol (from 76% to 

80%), shown by ethanologen research performed for DOE outside of NREL. The largest 

planned reduction in MESP for FY 2008 was 40% conversion of minor sugars to ethanol, 

but this target was not included in the FY 2008 SOT due to the limited results shown so 

far.

Development targets are in place to reach $1.49/gal by 2012. In the pretreatment area, 

learning in FY 2008 has led to some pilot plant modifications designed to improve the 

residence time distribution in the horizontal Sunds reactor. We also have a better 

understanding of the importance of obtaining high oligomeric xylose yields with a 

subsequent step to convert oligomers to monomers. In the saccharification and 

fermentation areas, the minor sugar-to-ethanol conversion target of 40% has been pushed 

to FY 2009, and a strategy involving NREL and DOE is being pursued to allow for better 

evaluation of strain performance on process-relevant material going forward. 

Concurrently, baseline validations of the enzyme solicitation awardee companies in FY

2009 will help to verify the accuracy of the enzyme cost estimates and performance 

capabilities used in the technoeconomic model. 

5 References 

[1] Aden, A., et al. Lignocellulosic Biomass to Ethanol Process Design and Economics 

Utilizing Co-Current Dilute Acid Prehydrolysis and Enzymatic Hydrolysis for Corn 

Stover. NREL/TP-510-32438. Golden, CO: National Renewable Energy Laboratory, June 

2002. http://www.nrel.gov/docs/fy02osti/32438.pdf. 

[2] AspenTech. Aspen Plus. http://www.aspentech.com/products/aspen-plus.cfm. 

[3] Hess, R. “Feedstock Logistics Design.” Presented at the 2009 DOE Biomass Program 

Feedstocks Platform Review. 

http://www.bcsmain.com/mlists/files/biomass/obpreview2009/feedstocks/documents//Fee

dstock_Logistics_Hess_6.2.1.3_v2.pdf. April 8-10, 2009. 

[4] Perlack, B. “Supply Forecast and Analysis.” Presented at the 2009 DOE Biomass 

Program Feedstocks Platform Review. 

http://www.bcsmain.com/mlists/files/biomass/obpreview2009/feedstocks/documents//Fee

dstock_Analysis_Perlack_1.6.1.3.pdf. April 8, 2009. 

[5] Nagle, N., et al. Validate >75% xylan yield and <8% degradation from whole corn 

stover using a high solids continuous pretreatment reactor. NREL FY08 D Milestone 

Report, Sept. 30, 2008, Doc. #10341. http://devafdc.nrel.gov/bcfcdoc/10341.pdf. 

[6] We note that Sunds technology is currently owned by Metso Corporation. 

http://www.metso.com/. 
10 

[7] AspenTech. Aspen Icarus Process Evaluator. 

http://www.aspentech.com/products/aspen-icarus-process-evaluator.cfm. 

[8] “U.S. Department of Energy to Invest up to $33.8 Million to Further Development of 

Commercially Viable Renewable Fuels.” DOE press release, February 26, 2008. 

http://www.energy.gov/news/6015.htm. 

[9] Humbird, D.; Aden A. FY08 State of Technology Update. NREL FY 2008 Milestone 

Completion Report, Oct. 31, 2008, Doc. #10367. 

http://devafdc.nrel.gov/bcfcdoc/10367.pdf. 
11 

Minimum Ethanol Selling Price $2.61

Ethanol Production (MM Gal. / Year) 56.0 Ethanol at 68°F

Ethanol Yield (Gal / Dry US Ton Feedstock) 72.6

Feedstock Cost $/Dry US Ton $65.3

Internal Rate of Return (After-Tax) 10%

Equity Percent of Total Investment 100%

Capital Costs Operating Costs (cents/gal ethanol)

 Feed Handling $0 Feedstock 90.0

 Pretreatment $25,300,000 Biomass to Boiler 0.0

 Neutralization/Conditioning $11,400,000 CSL 15.6

 Saccharification & Fermentation $22,200,000 Cellulase 35.0

 On-site Enzyme Production $0 Sulfuric Acid 4.1

 Distillation and Solids Recovery $27,100,000 Ammonia 20.7

 Wastewater Treatment $5,700,000 Other Raw Materials 2.8

 Storage $4,200,000 Waste Disposal 2.1

 Boiler/Turbogenerator $55,000,000 Electricity -13.2

 Utilities $6,500,000 Fixed Costs 17.2

Total Installed Equipment Cost $157,300,000 Capital Depreciation 24.3

Average Income Tax 18.3

Added Costs $115,200,000 Average Return on Investment 44.4

 (% of TPI) 42%

Operating Costs ($/yr)

Total Project Investment $272,500,000 Feedstock $50,400,000

Biomass to Boiler $0

Installed Equipment Cost/Annual Gallon $2.81 CSL $8,700,000

Total Project Investment/Annual Gallon $4.86 Cellulase $19,600,000

Other Raw Matl. Costs $15,500,000

Loan Rate N/A Waste Disposal $1,200,000

Term (years) N/A Electricity -$7,400,000

Capital Charge Factor 0.179 Fixed Costs $9,600,000

Capital Depreciation $13,600,000

Denatured Fuel Prod. (MMgal / yr) 58.7 Average Income Tax $10,200,000

Denatured Fuel Min. Sales Price $2.53 Average Return on Investment $24,900,000

Denaturant Cost ($/gal denaturant) $0.739

Excess Electricity (KWH/gal) 3.30

Maximum Yields (100% of Theoretical) Plant Electricity Use (KWH/gal) 2.09

 Ethanol Production (MM Gal/yr) 87.0

 Theoretical Yield (Gal/ton) 112.7 Plant Steam Use (kg steam/gal) 20.2

Current Yield (Actual/Theoretical) 64.4% Boiler Feed -- LHV (Btu/lb) 2,091

Boiler Feed -- Water Fraction 0.501

Specific Operating Conditions

Enzyme Loading (mg/g cellulose) 20.0

Saccharification Time (days) 5.0

Conversion Cellulose --> Glucose 0.9000

Fermentation Time (days) 2.0

All Values in 2007$

Dilute Acid Prehydrolysis with Saccharification and Co-Fermentation

Corn Stover Design Report Case: FY08 State of Technology

Ethanol Production Process Engineering Analysis

Appendix
F1147-E(10/2008) 

REPORT DOCUMENTATION PAGE Form Approved

OMB No. 0704-0188

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PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ORGANIZATION.

1. REPORT DATE (DD-MM-YYYY)

August 2009

2. REPORT TYPE

Technical Report

3. DATES COVERED (From - To)

 

4. TITLE AND SUBTITLE

Biochemical Production of Ethanol from Corn Stover: 2008 State of 

Technology Model

5a. CONTRACT NUMBER

DE-AC36-08-GO28308

5b. GRANT NUMBER

5c. PROGRAM ELEMENT NUMBER

6. AUTHOR(S)

D. Humbird, and A. Aden

5d. PROJECT NUMBER

NREL/TP-510-46214

5e. TASK NUMBER

BB07.2410

5f. WORK UNIT NUMBER

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

National Renewable Energy Laboratory

1617 Cole Blvd.

Golden, CO 80401-3393

8. PERFORMING ORGANIZATION

REPORT NUMBER

NREL/TP-510-46214

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSOR/MONITOR'S ACRONYM(S)

NREL

11. SPONSORING/MONITORING

AGENCY REPORT NUMBER

12. DISTRIBUTION AVAILABILITY STATEMENT

National Technical Information Service

U.S. Department of Commerce

5285 Port Royal Road

Springfield, VA 22161

13. SUPPLEMENTARY NOTES

14. ABSTRACT (Maximum 200 Words)

An update to the FY 2007 assessment of the state of technical research progress toward biochemical process goals, 

quantified in terms of Minimum Ethanol Selling Price (MESP). By comparing the SOT year-over-year, research 

advances can be quantified in terms of economic improvements in the overall process. As such, progress toward the 

2012 goal of $1.49/gal MESP can be tracked.

15. SUBJECT TERMS

Biomass; cellulosic ethanol; state of technology; SOT; minimum ethanol selling price; MESP; biochemical; process 

design

16. SECURITY CLASSIFICATION OF: 17. LIMITATION 

OF ABSTRACT

UL

18. NUMBER 

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a. REPORT

Unclassified

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Unclassified

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